Battery formation protocol

Abstract

The present invention relates to an improved formation protocol for NIB hard carbon anodes in super-concentrated ionic liquid electrolytes. In contrast to methods established for carbonate-based solvents, the high current density 2C formation protocol resulted in the highest specific capacity with the lowest EIS resistance during subsequent cycles at 1 / 2C current density, compared to 1C and 1 / 10C formation protocols. Variable cycle conditions (e.g., 1 / 5C in the case of long-term cycling) were not affected by high C-rate treatment, demonstrating the need to apply high-rate formation protocols for conditioning in these electrolytes. XPS and NMR analyses revealed that a thinner SEI layer was formed after high C-rate formation, which can facilitate Na+ charge transfer and diffusion across the electrolyte / electrode interface.

Description

Detailed Description of the Invention

[0001] [Technical Field] The present invention relates particularly to a rapid battery formation method or protocol for a sodium-ion battery including a hard carbon anode.

[0002] [Background Art] Batteries with high energy density, long life, and especially low manufacturing costs are desirable. Rechargeable sodium-ion battery cells are generally assembled in a discharged state or "fresh" state or "initial" state, which means the cell is in a pre-cycle state. The fresh cell must be "formed" as the next step during battery manufacturing. Battery cell formation is a process that performs a set of initial charge / discharge (polarization cycle) operations (typically 2 / 3 polarization cycles) on fresh battery cells in a factory. During charging, the positive electrode active material (e.g., sodium oxide) generates mobile sodium transport ions, and the mobile sodium transport ions diffuse to the negative electrode active material (e.g., carbon, graphite), where they are reduced and "absorbed" into the negative electrode material that does not contain sodium ions before charging (initial state). During discharge, the reverse reaction occurs, and the stored chemical energy is converted into electrical energy used to power the attached load. The charging step and the subsequent discharging step are called "polarization cycles". During the first polarization cycle, an electrochemical electrolyte interface accumulates in the form of a solid electrolyte interface (SEI) on the electrodes, and mainly on the negative electrode, which is a passivation layer formed on the anode surface of all sodium-ion batteries using liquid electrolytes. SEI formation depends greatly on the chemical properties of the cell and the electrolyte components because it is formed from electrolyte component decomposition / reduction products that consume part of the electrolyte in the first few cycles. A useful formation protocol forms a good-quality and robust SEI that is important for battery performance, resulting in improved cycle stability, higher capacity, and / or higher Coulomb efficiency, all of which are evidence of a reduction in anode and electrolyte degradation. The ideal SEI provides rapid transport of transport ions across the interface while being a good electronic insulator, thereby protecting the electrolyte from further degradation that could otherwise rapidly cause capacity loss in the cell. The SEI should be stable both under cycling conditions and under aging conditions. SEI formation results in irreversible consumption of transport ions from the electrolyte during cycling, causing a decrease in achievable capacity and an increase in resistance, and thus an increase in impedance within the cell.Conventionally, the best SEI layer is selectively permeable to the transport ions being used and, being thin, reduces the resistance to ion transport that an optimal battery performance supports, so it is desirable to be as thin as possible, typically only a few nanometers thick. However, if the SEI is too thin, it cannot sufficiently protect the electrolyte from reduction at the anode. A good SEI should be uniformly distributed across the entire surface of the anode, completely prevent contact between the anode and the electrolyte, be mechanically robust / strong, but chemically inert to avoid unwanted side reactions.

[0003] Hard carbon (HC) is a suitable graphite anode alternative for sodium cells but is known to have a more complex sodium ion absorption mechanism and SEI formation process. This is due to the higher surface area and the reactivity of HC resulting from its porosity, which leads to a thick and non-uniform SEI. The thicker SEI hinders transport ion diffusion, thereby degrading battery performance, while the non-uniform SEI supports local side reactions and degrades performance over time. Most HC / SEI research involves conventional film-forming electrolyte additives such as hexafluorophosphate or TFSI salts, various carbonates, fluorinated carbonates, or other solvents. Always, the decomposition products of these additives form part of the HC SEI during formation. However, the decomposition products of state-of-the-art ester solvent-based sodium ion battery electrolytes (e.g., propylene carbonate (PC) and ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), etc.) result in an outermost SEI layer rich in organic species, often making the SEI unstable and having a large charge transfer resistance and a high energy barrier to sodium ion transport. Some groups include ionic liquid / carbonate electrolytes for NIBs, and the improved SEI layer is derived from the ionic liquid electrolyte.

[0004] To form the necessary robust and homogeneous interface, the battery formation / conditioning protocol, which depends on the chemical nature of the cells used in commercial battery manufacturing, typically takes up to one week. Typically, several cycles of low-current constant-current charge and discharge are performed with various rest periods at elevated temperatures. Conventional slow formation is thought to produce the most uniform and stable SEI layer. Typically, to ensure that a uniform and stable SEI layer is formed, a very small current density (e.g., 1 / 10C or 1 / 20C using an equivalent low current density) is used for an extended period to insert the transport metal and thus maximize the charge capacity to the negative electrode during formation charging. Therefore, the formation time is very slow (e.g., 2 / 3 cycles at 20 - 40 hours per polarization cycle means 40 - 120 hours for formation alone without rest, etc.). Therefore, battery production efficiency is low, and conventional protocols incur high capital costs. Current densities above 1 / 1C rate are considered detrimental to cell performance because they are thought to damage the electrodes due to early aging and have thus been avoided during commercial formation. Furthermore, rapid formation has a risk of dendrite formation (via electroplating) during cycling, damaging the cell and degrading its performance under normal operation.

[0005] A new formation protocol that balances electrochemical performance, manufacturing cost, and time efficiency is desirable, especially for cells containing HC anodes. Sodium-ion batteries (NIBs) are considered as next-generation LIB alternatives. The same slow and expensive conventional formation protocols as for LIBs are applied to NIBs. However, the SEI formed for NIBs is less stable than their LIB counterparts. Na + including an increase in size of Na + and Li +The significant differences and reactivity differences with [the comparison object] make it more difficult to form a uniform and stable SEI for NIBs. Finally, the NIB SEI components exhibit an increase in solubility, which is a problem for SEI stability. Therefore, such knowledge of SEI formation in LIBs cannot be directly transferred to NIB formation, and SEI formation in Na cells has been studied empirically. Overall, little is known about the desirable compositional characteristics of SEI for HC in NIBs.

[0006] Rakov et al. (Chem. Mater. 2022, 34; “Rakov 2022”) reported Li deposition / stripping studies in a high-viscosity super-concentrated electrolyte (100IL), where in the SEI composition, Li x (anion) y (x > y) is dominant. Therefore, compared with a low-concentration Li-salt-containing IL electrolyte, a single Li metal anode preconditioning step containing a high current density (≥10.0 mA cm -2 ~1.0 mAh cm -2 of the depth of charge) was found to be beneficial to the SEI composition. However, when using a low-viscosity super-concentrated electrolyte (80IL20DME) containing 20 wt% of an ether co-solvent, a more moderate preconditioning step current density (6.0 mA cm -2 / 1.0 mA cm -2 ) resulted in a more optimally formed Li deposition morphology. Furthermore, 15.0 mA cm -2 / 1.0 mAh cm -2The best cycle performance is achieved at extreme currents. Since the optimized current densities are different for viscous electrolytes and more fluid electrolytes, Rakov 2022 suggested that individual adjusted pretreatment protocols should be applied according to the electrolyte composition, but did not show how such protocols should be derived. However, it should be noted that the focus of Rakov 2022 is limited to SEI formation on the lithium metal anode in a symmetric cell after a single conditioning step, and there is nothing about SEI formation on the HC anode in NIB. As explained, due to the unpredictable nature of SEI formation on HC in NIB, Rakov 2022 teaches that there is no value in re-SEI formation in NIB systems using hard carbon anodes.

[0007] Rakov 2020 (Nature Materials, 19, 2020, 1096 - 1101) applied different cathode polarizations as a conditioning treatment before long-term electrochemical cycling at 50 °C in a sodium symmetric cell based on C3mpyrFSI IL containing 50 mol% NaFSI salt. The different electrode polarizations were tested by applying three different current densities of 0.1, 1.0, and 5.0 mA cm -2 at a depth of charge of 0.1 mAh cm -2 in five short cycles. After conditioning, each test cell was cycled for 20 cycles at current densities of 0.1 and 1.0 mA cm -2 and then for 10 cycles each at 5.0 mA cm -2 . The cell conditioned at 5.0 mA cm -2 resulted in the largest initial polarization potential and the shortest deposition time, and showed the smallest overpotential among the three cells during cycling at both 0.1 mA cm -2 and 1.0 mA cm -2 . During cycling at 0.1 mA cm -2 and 1.0 mA cm -2Cells preconditioned at lower currents either fail due to short circuits or exhibit unstable voltage profiles. There is nothing for SEI formation at the HC anode in NIB. As explained, due to the unpredictable nature of SEI formation on HC in NIB, the teachings of Rakov 2020 including Na metal symmetric cells do not provide information on HC SEI formation in the Na-HC system in NIB cells.

[0008] Recently, Kishore et al. (Chem. Commun., 2020, 56, 12925 - 12928) recognized that formation and conditioning are one of the least understood processes in LIBs and even less known or understood for NIBs. And Kishore reported a formation protocol to shorten the formation time and maximize the cycle life in NIBs with a Na layered oxide cathode and an HC anode. The formation involves a series of low currents (0.12 mA cm -2 , 10 mA g cat -1 ) cycles being applied to five charge-discharge cycles that are carried out over various described voltage windows. Protocols F4 and F5 include narrow voltage subsets of the standard voltage window, are carried out at voltage windows 1.0 - 1.8 and 1.8 - 2.6 V, and recorded minimum formation times of 6 hours and 17 hours respectively. However, F4 and F5 cells showed the worst cell performance with respect to capacity retention after 150 cycles (51.1% and 54.3% respectively). The best results were for F2 formed at low current and a voltage of 3.6 - 3.8 V, which showed a 9.2% capacity drop and a formation time of 42 hours. Kishore has demonstrated the unpredictability of SEI formation on the HC anode in NIBs and the type of complexity involved in many experimental formation protocols, which may not be suitable for commercial / industrial applications.

[0009] As described above, the dependencies on SEI formation in LIBs and from Na / Na symmetric cells cannot be directly transferred to NIBs for various reasons as discussed above. The conventional approach, which involves a low current density over multiple slow steps, is typically applied to ensure the proper formation of the best SEI on the HC anode within the NIB cell.

[0010] [Summary of the Invention] In a first aspect, the present invention is an electrochemical cell comprising a super-concentrated sodium salt-containing ionic liquid electrolyte comprising at least one ionic liquid and at least one sodium salt, wherein the sodium salt concentration in the super-concentrated sodium salt ionic liquid electrolyte is 75% or more of its saturation limit in the ionic liquid electrolyte, a super-concentrated sodium salt-containing ionic liquid electrolyte, a counter electrode or a positive electrode comprising an electrochemically oxidizable material, which releases sodium ions from the material into the super-concentrated sodium salt electrolyte in the cell during the sodiation step or the charging step, a hard carbon working electrode or a negative electrode, which absorbs the sodium ions received at the hard carbon negative electrode from the super-concentrated electrolyte as reduced sodium occluded in the hard carbon during the sodiation step or the charging step, comprising (a) the hard carbon negative electrode substantially free of low-order / short-range sulfur (S8 2- ~S 2- ) species, as identified, for example, by XPS testing and etching investigations, and optionally substantially free of atomic nitrogen species, as identified, for example, by XPS testing and etching investigations, and having a formed solid electrolyte interface (SEI), and / or (b) The solid electrode interface (SEI) / hard carbon electrode has an interfacial resistance (R int ) related to the SEI of less than 200 ohms after the first formation cycle as determined by electrochemical impedance spectroscopy, providing an electrochemical cell.

[0011] In a second aspect, the present invention is the use of a superconcentrated ionic liquid electrolyte for forming an SEI on a hard carbon electrode in a cell comprising a hard carbon working electrode or a negative electrode, wherein the superconcentrated ionic liquid electrolyte contains a sodium salt concentration of 75% or more of its saturation limit in the electrolyte, where (a) the hard carbon negative electrode substantially does not contain low-order / short-range sulfur (S8 2- ~S 2- ) species as identified, for example, by XPS testing and etching investigations, and optionally substantially does not contain atomic nitrogen species as identified, for example, by XPS testing and etching investigations, and comprises a formed solid electrode interface (SEI), and / or (b) the solid electrode interface (SEI) / hard carbon electrode has an interfacial resistance (R int ) related to the SEI of 200 ohms or less after the first formation cycle as determined by electrochemical impedance spectroscopy, providing the use.

[0012] In a third aspect, the present invention is a cell formation method, comprising the step of providing an initial state cell based on sodium ion electrochemistry, wherein the fresh cell comprises a counter electrode or a positive electrode containing an electrochemically oxidizable material that releases sodium metal ions from the material into the superconcentrated electrolyte in the cell during a sodiation step or a charging step, A working hard carbon electrode or a negative electrode that, during the sodiation step or the charging step, absorbs sodium ions received at the hard carbon negative electrode from a super-concentrated electrolyte as reduced sodium occluded in the hard carbon, the working hard carbon electrode or negative electrode, and A super-concentrated sodium salt-containing ionic liquid electrolyte containing at least one ionic liquid and at least one sodium salt comprising wherein the sodium ion concentration in the super-concentrated ionic liquid electrolyte is 75% or more of its saturation limit in the electrolyte, step, and Manufacturing the target cell formed by generating an SEI formed by reduction of the electrolyte on the hard carbon electrode by polarizing the initial cell during up to 5 formation cycles in the range of 1 / 2C to 5C, preferably at a high constant current density of 2C, within the total voltage window of the cell, (a) the hard carbon negative electrode substantially free of low-order / short-range sulfur (S8 2- ~S 2- ) species, as identified by, for example, XPS testing and etching investigations, and optionally, substantially free of atomic nitrogen species, as identified by, for example, XPS testing and etching investigations, comprising a formed solid electrolyte interface (SEI), and / or (b) the solid electrolyte interface (SEI) / hard carbon electrode having an interface resistance (R int ) related to the SEI of 200 ohms or less after the first formation cycle, as determined by electrochemical impedance spectroscopy, step and including, provides a method.

[0013] The inventors have found that high current density-driven SEI formation is surprisingly highly conductive and Na across the electrode / electrode boundary +It has been found that an SEI on hard carbon (HC) is generated that very effectively promotes ion diffusion. The improved SEI and related improved cell performance are demonstrated in some embodiments by stable and high-performance cycling using a C rate of 1 / 2C or slower (e.g., 1 / 5C), and the cell exhibits a Coulombic efficiency of greater than 99.0%, preferably greater than 99.2%, preferably greater than 99.3%, preferably greater than 99.4%, preferably greater than 99.5%, preferably greater than 99.6%, preferably greater than 99.7%, preferably greater than 99.8%, preferably greater than 99.9% during nominal rate cycling (after formation is complete). For example, in some embodiments, the cell during cycling at a nominal 1 / 10C cell rate (after formation) exhibits a CE of greater than 99.5%, preferably greater than 99.6%, more preferably greater than 99.7%, most preferably greater than 99.8% after 50, 100, 200, or 300 cycles.

[0014] Embodiments of the present invention will be described by way of example only with reference to the accompanying drawings.

Brief Description of the Drawings

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[0016] [Mode for Carrying Out the Invention] The present invention relates to an electrochemical cell, a super-concentrated sodium salt-containing ionic liquid electrolyte containing at least one ionic liquid and at least one sodium salt, wherein the sodium ion concentration in the super-concentrated ionic liquid electrolyte is 75% or more of its saturation limit in the electrolyte, a super-concentrated sodium salt-containing ionic liquid electrolyte, a counter electrode or a positive electrode containing an electrochemically oxidizable material, which releases sodium ions from the material into the super-concentrated electrolyte in the cell during the sodiation step or the charging step, the counter electrode or the positive electrode, a hard carbon working electrode or a negative electrode, which reductively absorbs the sodium ions received from the super-concentrated electrolyte at the hard carbon negative electrode as sodium metal occluded in the hard carbon during the sodiation step or the charging step, the hard carbon working electrode or the negative electrode comprising, the hard carbon negative electrode having a formed solid electrode interface (SEI), and the solid electrode interface not completely containing atomic nitrogen species identified by a peak of a binding energy of about 396 eV in XPS tests and etching investigations, and not completely containing low-order / short-distance sulfur (S8 2- ~S 2- ) species identified by peaks of binding energies of about 164 eV to 160 eV in XPS tests and etching investigations relates to an electrochemical cell which is at least one of these.

[0017] The inventors have found that an improved SEI is a thinner and / or more ion-conductive SEI, and / or the formed SEI is related to a favorable (surprisingly low / reduced) interfacial resistance as indicated by a lower impedance compared to a cell formed at a conventional formation rate / current density. The inventors have found low-order / short-distance sulfur (S8 2- ~S 2-) The absence of species is thought to result in a thinner SEI layer that aids faster Na+ diffusion through the SEI layer. Since the XPS studies described herein show some compositional differences between the SEI on hard carbon formed at low current density (1 / 10C) and the SEI on hard carbon formed at high current densities (1C and 2C), it is also thought that the absence of one or more atomic nitrogen species in the SEI composition may be beneficial.

[0018] Conventionally, the inorganic species described herein have a lower diffusion energy barrier for Na + and are expected to increase the Na migration rate. For the reason that the inventors have found that the presence of more reduced sulfur species in the SEI thickens the SEI layer and actually increases the Na + diffusion path and decreases the reaction rate. Other species in the SEI, such as atomic nitrogen species determined by XPS analysis, may also increase the thickness and size of the Na+ diffusion path. The inventors have now found that an SEI without these species actually has a thinner, more porous, or lower density SEI formed on hard carbon, or at least an SEI layer that is more ion-conductive in other ways. As a result, those having shorter sodium ion diffusion paths and / or more ion-conductive characteristics are obtained. The prior art also has no teaching that one or more of the compositional SEI characteristics described herein are important for improving SEI formation and excellent cell performance in sodium hard carbon cells.

[0019] In some embodiments, the thickness of the SEI can be determined or at least approximated by the time of etching in an etching investigation, e.g., to see the XPS signal that coincides with the surface of the electrode in its initial state. Other methods, including electrochemical or electron microscopy methods that can be used, can be used to measure the SEI thickness. Typically, a conventionally formed SEI exhibits a thickness of several hundred nanometers. The inventors have published reports of etching investigations of these compositions (e.g., ACS Appl. Mater. Interfaces 2021, 13, 4, 5706 - 5720), and the estimation of SEI thickness using electrochemical methods is described on page 5710. The relevant part is incorporated herein by reference.

[0020] In some embodiments, the ionic conductivity of the SEI and / or the interfacial resistance of the SEI can be determined or inferred by well-known methods of electrochemical impedance spectroscopy (EIS). A preferred solid electrolyte interface (SEI) / hard carbon electrode has an interfacial resistance (R int ) associated with an SEI of less than 300 ohms, more preferably less than 200 ohms, after the first formation cycle, as determined by electrochemical impedance spectroscopy. At the end of the formation cycle, a preferred solid electrolyte interface (SEI) / hard carbon electrode has an interfacial resistance (R int ) associated with an SEI of less than 75 ohms, and in some cases 50 ohms, indicating the ease of Na + transport through the SEI of the present invention.

[0021] Substantially complete and stable improved SEI formation is indicated by the cell performance characteristics shown above in the methods of the first and second aspects when compared to SEI formed in the conventional manner. Additionally or alternatively, substantially complete and stable improved SEI has a thinner or otherwise more ion-conductive SEI layer formed on the active material surface. Substantially complete and stable improved SEI has a shorter metal ion diffusion pathway and / or more ion-conductive characteristics. The shorter metal ion diffusion pathway is the result of a thinner, more porous or lower density SEI, or a combination of one or more of these factors, and overall, is thought to result in a shorter ion diffusion pathway compared to SEI formed in an equivalent cell where the above conventional formation conditions otherwise apply. Since conventional thinking for SEI formation has indicated that a thick comprehensive SEI layer that is very slowly constructed and / or components from film-forming additives added to the electrolyte are required to ensure optimal battery performance, the generation of such improved SEI was a surprising discovery. These results are completely contrary, and forming with a super-concentrated ionic liquid electrolyte at high current rates and fast formation times promotes improvement in battery performance while reducing the electrolyte used for SEI formation, resulting in a better SEI with improved ion conductivity / shortened ion diffusion pathway (thinner, or lower density, or more porous, or all of these characteristics, etc.). Furthermore, since the cell has less irreversible capacity loss during improved SEI formation, the cells of the present invention are expected to have a longer life as they have more starting electrolyte than conventional cells and thus experience a slower capacity degradation due to aging.

[0022] Suitably, the SEI is formed in accordance with the optimized cell formation conditions described herein. Also described herein are formed target cells that can be obtained or are obtained by the methods of the present invention.

[0023] Desirably, the improved SEI of the formed target cell has a composition having an outer layer containing more C-N species than the SEI of the formed test cell, as indicated by XPS testing and / or etching investigations.

[0024] XPS and etching investigations have shown that the SEI composition of test cells formed at 1 / 10 C current density has features attributable to atomic nitrogen (binding energy of about 396 eV in the tests described herein). Desirably, by the method of the present invention, XPS analysis of the SEI composition of cells formed at 1 C and 2 C shows no features attributable to atomic nitrogen at about 396 eV.

[0025] Preferably, the improved SEI of the formed target cell contains no small sulfur-containing species (low-order / short-range sulfur (S8 2- ~S 2- )) or contains smaller sulfur-containing species than those found in the SEI of the formed test cell. These species are associated with the thicker SEI of the 1 / 10 C formed cell, which increases the Na + diffusion path and decreases the reaction rate.

[0026] Preferably, the improved SEI in the formed target cell contains a C to O ratio of 1 or greater after 40 minutes of etching.

[0027] The above compositional differences in the SEI formed at 1 / 10 C rate and the higher rates of 1 C and 2 C confirm that the SEI differs with respect to the component species present. These differences are related to the performance improvements observed for cells formed at higher current densities.

[0028] Desirably, the improved SEI of the formed target cell is a thinner layer than the SEI of the formed test cell, as indicated by XPS testing and etching investigations and / or EIS measurements.

[0029] Also, in this specification, a method for specifying optimized cell formation conditions for a cell formation method that forms an improved SEI on a hard carbon anode in a sodium ion electrochemical cell in a total time of preferably 20 hours or less, preferably 10 hours or less, preferably 5 hours or less, and more preferably 2.5 hours or less, providing two or more fresh (initial state) cells based on sodium ion electrochemistry, each fresh cell comprising a counter electrode containing an electrochemically oxidizable material capable of releasing sodium ions into a super-concentrated sodium salt / ionic liquid electrolyte in the cell during cell polarization, and a hard carbon working electrode that absorbs sodium metal ions received at the hard carbon working electrode from the super-concentrated electrolyte as reduced sodium occluded in the hard carbon working electrode, comprising a super-concentrated sodium salt / ionic liquid electrolyte containing an ionic liquid and at least one sodium salt, wherein the sodium salt concentration is 75% or more of its saturation limit in the electrolyte, step; manufacturing a test cell formed by generating an SEI on the hard carbon working electrode of the fresh test cell by applying a cell formation process to the first fresh test cell, polarizing the first fresh test cell within the full voltage window of the fresh test cell at a first current density (e.g., a current density equivalent to 1 / 10C or less, or 1 / 20C or less) predetermined to achieve the full discharge capacity of the fresh test cell within a period of at least 10 hours for a single discharge, more preferably at least 20 hours for a single discharge, and optionally, identifying the presence and / or absence of one or more markers in the recorded corresponding charge-discharge cycles or related analysis related to substantially complete and substantially stable SEI formation on the hard carbon electrode of the formed test cell, and optionally, Repeating the polarization cycle step one or more times at the same first current density or at a different, higher or lower predetermined formation current density on the formed test cell until the presence and / or absence of one or more markers in the recorded corresponding charge-discharge cycles or related analysis associated with substantially complete and stable SEI formation on the hard carbon electrode of the formed test cell is observed comprising the step of, Manufacturing a formed target cell having an improved SEI on the working electrode of the formed target cell by applying an improved cell formation process to a fresh first target cell, polarizing the fresh first target cell within the full voltage window of the fresh first target cell at a second current density that is higher than the current density used for the fresh test cell, predetermined to achieve the cut-off discharge potential limit of the cell and the resulting discharge capacity of the fresh first target cell in a period of less than 5 hours for a single discharge, less than 2 hours for a single discharge, preferably less than 1 hour for a single discharge, more preferably less than 30 minutes for a single discharge identifying the presence and / or absence of one or more markers in the recorded corresponding charge-discharge cycles or related analysis associated with substantially complete and substantially stable improved SEI formation on the hard carbon electrode of the formed first target cell, and optionally repeating the polarization step one or more times at the same predetermined second current density or at a different, higher or lower predetermined formation current density on the obtained formed first target cell until the presence and / or absence of one or more markers indicates that substantially complete and substantially stable improved SEI formation has occurred on the hard carbon electrode of the formed first target cell comprising the step of and including, Improvement of the SEI formed on the hard carbon of the formed first target cell is demonstrated by the formed first target cell exhibiting a higher specific capacity and / or a higher Coulombic efficiency during cycling at the nominal C-rate of the formed first target cell as compared to the specific capacity and / or Coulombic efficiency shown for the first test cell formed at the same nominal C-rate. A method is described.

[0030] In addition to one or more of the features of the SEI composition described above, preferably, the improved SEI on the hard carbon of the formed target cell has an SEI composition further defined by: Having an outer layer containing more C-N species than the SEI of the formed test cell, as shown by XPS testing and / or etching investigations at the same etching time. Having a composition containing a ratio of C to O that is one or more after 40 minutes of etching.

[0031] The comparison at the nominal C-rate is made for formed cells where all other things are equal, for example, where the same number of polarization steps are applied to both. The cut-off discharge potential limit of a cell is the defined voltage limit at which cell discharge is considered complete and typically enables achieving the maximum useful capacity of the battery under typical operating conditions that result in the best performance of the cell in question and also depends on the chemistry of the cell. There is a corresponding cut-off charge potential limit of the cell at which cell charge is considered complete. Determining the appropriate cut-off potential limit for any cell is within the ordinary purview of an electrochemist, a person skilled in the art. The C-rate is a measure of the rate at which a battery is charged or discharged relative to its maximum capacity. For example, 1C means that the current required to fully charge or discharge the battery in 1 hour is applied or drawn from the battery, at 1 / 10C the current is applied / drawn in 10 hours, and at 2C the current is applied / drawn in 30 minutes to fully charge or discharge the battery to the selected voltage cut-off point. Here, the C-rate is used because of the small amount of active material used in the electrodes. However, for electrodes using a greater amount of active material, the current density required to effect full charge / discharge can be readily determined electrochemically.

[0032] In some embodiments, the cells according to the present invention can operate at temperatures from -20°C to 150°C, such as from -10°C to 150°C, 0°C to 150°C, 0°C to 125°C, 0°C to 100°C, 0°C to 75°C, 0°C to 50°C, or 0°C to 25°C.

[0033] Preferably, in the optimization method of the present invention, the method is an additional step of manufacturing a further formed second target cell having an improved SEI on the working electrode of the formed second target cell by applying a cell formation process to one or more fresh target cells, Polarizing a fresh second target cell within the full voltage window of the fresh second target cell at a third current density that is higher than a second current density, which is predetermined to achieve a cut-off discharge potential limit of the cells of the fresh second target cell and a resulting discharge capacity, in a period shorter than the period selected for the method of the first aspect, and identifying the presence and / or absence of one or more markers in a recorded corresponding charge-discharge cycle or related analysis related to substantially complete and substantially stable further improved SEI formation on the hard carbon electrode of the formed second target cell, and optionally, repeating the polarization step one or more times on the obtained formed second target cell at the same predetermined higher third current density or at a different predetermined higher or lower formation current density until the presence and / or absence of the one or more markers indicates that substantially complete and stable improved SEI formation has occurred on the hard carbon electrode of the formed second target cell including additional steps may further include The improvement of the SEI formed on the hard carbon of the formed second target cell is evidenced by the formed target cell exhibiting a higher specific capacity and / or a higher Coulombic efficiency during cycling at the nominal C-rate of the formed target cell compared to the specific capacity and / or Coulombic efficiency shown for test cells formed at the same nominal C-rate and the formed first target cell. This comparison at the nominal C-rate is understood to be made for formed cells where all other things are equal, for example, where the same number of polarization steps have been applied to both.

[0034] A further improvement of the method may be achieved by repeating this step, optionally, on a further fresh third target cell or one or more subsequent fresh additional target cells, whereby a further improvement of the formed SEI is indicated by the observation of further improved performance at the nominal C-rate of the cell.

[0035] In some embodiments, up to 5 or 5 formation cycles are required to generate a substantially complete and substantially stable SEI. It will be appreciated that 5 cycles, each containing a period of at least 10 hours for a single discharge stage for each of the 5 polarization cycles, require a total time of 5 × 20 hours, i.e., a total formation time of 100 hours. In contrast, 5 cycles, each containing a period of 5 hours or less for a single discharge stage for each of the 5 polarization cycles, require a total time of 5 × 10 hours, i.e., a total formation time of 50 hours, which is half the total time of conventional formation protocols. Similarly, in contrast, 5 cycles, each containing a period of 30 minutes for a single discharge stage of the 5 polarization cycles, require a total time of 5 × 0.5 hours, which is a total formation time of 2.5 hours, which is 40 times faster than during cycling at the nominal C-rate at which equivalent cells are formed in a total time of 100 hours and at the same time results in good performance.

[0036] Also provided herein is a cell formation method for forming an improved SEI on a hard carbon anode in a sodium ion electrochemical cell in a total time of 20 hours or less, preferably 5 hours or less, more preferably 2.5 hours or less, comprising: providing a fresh cell based on sodium ion electrochemistry, the fresh cell comprising: a counter electrode comprising an electrochemically oxidizable material capable of releasing sodium ions into the super-concentrated electrolyte within the cell during cell polarization, and a hard carbon working electrode that absorbs sodium ions received at the hard carbon working electrode from the super-concentrated electrolyte as reduced sodium occluded in the hard carbon working electrode, comprising: a super-concentrated sodium ion ionic liquid electrolyte comprising an ionic liquid and at least one sodium salt, the sodium salt concentration being 75% or more of its saturation limit in the electrolyte, step, and Polarizing a fresh target cell at a predetermined formation current density that achieves the cut-off discharge potential limit of the cell and the resulting discharge capacity of the fresh target cell for a period of less than 5 hours for a single discharge, less than 2 hours for a single discharge, preferably less than 1 hour for a single discharge, and more preferably less than 30 minutes for a single discharge. Identifying the presence and / or absence of one or more markers in the recorded corresponding charge-discharge cycles or related analyses related to substantially complete and stable SEI formation on the hard carbon electrode, and optionally, Repeating the polarization step one or more times on the obtained formed target cell at the same predetermined formation current density until the presence and / or absence of one or more markers indicates the occurrence of substantially complete and stable improved SEI formation on the hard carbon electrode of the formed target cell. The predetermined formation current density and / or the number of polarization cycle repetitions required to generate the improved SEI are identified by the method of the first aspect. Step Manufacturing a formed target cell having an improved SEI on the working electrode of the formed target cell by applying the cell formation process according to to a fresh target cell. A method is described that includes.

[0037] Preferably, in the method of the present invention, cell polarization is caused by charging and discharging each cell at a constant current.

[0038] Electrochemical impedance spectroscopy (EIS) can be used to show the interfacial resistance of the hard carbon surface at different current densities.

[0039] The formed target cells that can be obtained and / or are obtained by the method of the present invention are also described herein. Preferably, the SEI composition of the cell is defined according to the first aspect above.

[0040] The inventors have developed a new formation protocol for improving SEI formation and electrochemical cell performance in sodium ion cells comprising a hard carbon working electrode (or the negative electrode of a battery) and a super-concentrated sodium salt / ionic liquid electrolyte. The present invention extends to cells manufactured by the formation method described herein, which uses an unprecedentedly high current density in the formation polarization cycle including the full voltage window of the cell in question. Hard carbon is suitable for use in full cell sodium ion batteries, and proof of concept has been demonstrated by half cell studies involving sodiation of the hard carbon negative electrode. Using the method of the present invention, the sodiation of as-received hard carbon involves substantially complete SEI formation during the first formation cycle, and in some cases is completed entirely within the first few formation cycles, typically being completed entirely after 5 formation cycles. The formation process results in a hard carbon electrode having an SEI formed at the interface between the electrode surface and the electrolyte as a result of at least one polarization cycle, and in some embodiments up to 10 polarization cycles, preferably up to 10, up to 9, up to 8, up to 7, up to 6, up to 5, up to 3 formation cycles of the electrochemical cell. It will be appreciated that a method in which fewer polarization cycles are preferred to form a substantially complete and substantially stable SEI is preferred because this results in a shorter overall formation time.

[0041] The inventors have also developed a method for identifying the optimal formation conditions / parameters required for use in a preferred formation method of the present invention, which can achieve a desired degree of SEI formation in an unprecedentedly short total formation period of 20 hours or less, preferably 5 hours or less, more preferably 2.5 hours or less, including 5 complete formation cycles to ensure complete SEI formation. However, in some methods, the formation period is even shorter, where the SEI is completely formed after only one or two high current density polarization cycles as described herein. The improved method can produce a substantially complete and substantially stable SEI formation by applying about one or two to five formation cycles as described herein, and each polarization cycle includes a discharge stage that is 5 hours or less for a single discharge, preferably 2 hours or less for a single discharge, preferably 1 hour or less for a single discharge, more preferably 30 minutes or less, and in some cases 15 minutes or less for a single discharge step. It will be understood that the high current density (1 / 2C to 5C) required to support this high speed cycle reaches the cut-off potential limit of the cell at a corresponding relatively low capacity compared to the total discharge capacity possible using conventional polarization / cycle rates of at least 10 hours for a single discharge, more preferably at least 20 hours for a single discharge. Since 2 / 3 of such low speed cycles are used in conventional formation methods, the total formation time takes about 40 to 120 hours. In contrast, the formation method of the present invention is much shorter, whereby a substantially complete and stable SEI can be formed, for example, in about 5 cycles with a period of 5 hours or less for a single discharge, preferably 2 hours or less for a single discharge, preferably 1 hour or less for a single discharge, more preferably 30 minutes or less for a single discharge, depending on the current density selected for the method, leading to a total formation time of 20 hours or less, preferably 5 hours or less, more preferably 2.5 hours or less.

[0042] Preferably, the method does not include a rest period between consecutive charge and discharge cycles, or consecutive full cycles of a single charge and discharge cycle. In some preferred embodiments, complete and stable SEI formation occurs after one formation cycle, in 4 or fewer formation cycles, preferably 3 or fewer formation cycles, preferably 2 or fewer formation cycles, and in some particularly preferred cycles.

[0043] In some preferred embodiments, the formation protocol described herein reduces the SEI formation and conditioning time on the hard carbon negative electrode, preferably by at least about 1 / 10, preferably by at least about 1 / 15, more preferably by about 1 / 20 (e.g., up to about 20 times faster than conventional formation methods), and in some cases up to 1 / 38, compared to conventional formation protocols (e.g., current densities that give the full discharge capacity at a rate of 1 / 10C or less or 1 / 20C or less, or equivalent current densities). The cost reduction and efficiency improvement resulting from the method of the present invention are evident.

[0044] Accordingly, the present invention provides a novel and unconventional cell formation method that includes, in a NIB cell, the step of forming an improved SEI on a hard carbon electrode in a total time of 20 hours or less.

[0045] A substantially complete, substantially stable, improved SEI formed in an unprecedentedly short time is at least as good a high-performance / improved SEI as the SEI formed in the same cell (same chemical properties, materials, manufacturing, batch, etc.) using a conventional but much slower formation method (e.g., 2 / 3 polarization cycles at 1 / 10C or 1 / 20C rate). "At least as good" means that the SEI has the same ionic conductivity and / or the same impedance as the SEI formed using a conventional formation method (all other things being equal) that includes polarization using at least 10 hours, more preferably at least 20 hours, of a single discharge. However, the preferred formation method of the present invention produces an improved SEI that is substantially complete and substantially stable with a total time of 20 hours or less, which results in an SEI having a higher ionic conductivity and / or a lower impedance than the SEI that would have been formed using a conventional formation method as described above. Therefore, the improved SEI of the present invention is compositionally different, and / or structurally different, and / or different with respect to mechanical properties, and / or chemical properties, such as composition, physical properties, such as thickness, porosity, density, etc., and electrical properties, such as the impedance of the SEI and the ionic conductivity of sodium ions, compared to the SEI of an equivalent cell formed at a much slower rate. The SEI of the present invention is believed to have one or more of the following characteristics: for example, being substantially free of atomic nitrogen species as identified by a peak in the binding energy of about 396 eV, e.g., by XPS, e.g., by XPS testing and etching investigations, and for example, by XPS, e.g., by XPS testing and etching investigations, low-order / short-range sulfur (S8 2- ~S 2-Excluding seeds. Further, the forming method described herein results in a formed cell that exhibits superior performance compared to a cell formed conventionally at the above rate (as a result of improved SEI formation and reduced electrolyte consumption with less associated irreversible capacity loss during formation). Advantageously and surprisingly, the observed improved performance improvement is achieved despite the forming time being a factor up to approximately 20 times faster than conventional forming protocols (e.g., using a forming cycle that includes more than 10 hours for a single discharge, more typically 20 hours for a single discharge cycle, with at least 2 or 3 of those being necessary for good SEI formation). The result is surprising because the method avoids using very high current densities during formation, contrary to conventional thinking in the art. The prior art also has no teaching that the compositional SEI characteristics described herein are important for improving SEI formation and excellent cell performance of sodium hard carbon cells.

[0046] Cell forming conditions This method includes the step of manufacturing a formed test cell by applying a cell formation process to a fresh cell to generate a ( "conventional") SEI on the working electrode of the first fresh cell, whereby the formed test cell typically undergoes a conventional set of low speed / low speed formation method conditions, which, at conventional rates, take more than 10 hours for a single discharge of a single polarization cycle and even more than 20 hours for a single discharge of a single polarization cycle. Performing the conventional cell formation step is an optional but useful step as it provides an indication of SEI behavior at the low current densities (which are slow rates such as 1 / 10C or 1 / 20C) used for typical SEI formation that occurs under conventional conditions. This indication is useful for setting the initial improved formation conditions according to the present invention, which desirably include shorter discharge cycles possible at higher current densities. Thereby, the higher current density used in the formation method of the present invention can be easily determined. For example, an electroanalysis study regarding this first formation process applied to the first fresh test cell can also provide information regarding the nature, stage, and completeness / stability of SEI formation at conventional rates, including identification of one or more markers indicating that SEI formation is occurring and / or the extent of SEI formation, such as information regarding the completeness and / or stability of the formed SEI and the ionic conductivity and / or impedance of the SEI formed in such a manner. These markers can be useful for comparing SEI formation in the target cell to which the formation method is applied and can help identify the formation of a substantially complete and stable SEI formed at the higher rate / current density conditions used in the method of the present invention. They also provide a basis for comparing the characteristics and / or performance of the improved SEI with the conventionally formed SEI and demonstrate the advantages of the present invention. Accordingly, the method of the first aspect may include the step of establishing a baseline for a test cell formed "as conventionally". Accordingly, the method in this case includes the following steps: At a predetermined first current density, polarize a fresh test cell within the full voltage window of the fresh test cell for a period of at least 10 hours for single discharge, more preferably at least 20 hours for single discharge (e.g., a current density corresponding to a 1 / 10C or 1 / 20C discharge step) to achieve the full discharge capacity of the cell, and optionally, identify the presence and / or absence of one or more markers in the recorded corresponding charge-discharge cycles or related analysis related to substantially complete and stable SEI formation on the hard carbon electrode, and optionally, as necessary, until the presence and / or absence of one or more markers in the recorded corresponding charge-discharge cycles or related analysis related to substantially complete and stable SEI formation on the hard carbon electrode of the newly formed test cell is observed, repeat the polarization of the test cell one or more times at the same first current density. Conventional thinking suggests that a 1 / 20C rate produces the best SEI for a particular test cell, but a 1 / 10C is considered to form an acceptable SEI that supports acceptable cell performance. Next, the method of the present invention may include the following steps: A predetermined second current density that is higher than the current density used in step a), wherein the higher current density achieves the full discharge capacity of the cell in a period of 5 hours or less for single discharge, preferably 2 hours or less for single discharge, preferably 1 hour or less for single discharge, more preferably 30 minutes or less for single discharge, and producing a target cell formed by repeating step a) on a target fresh cell (having the same chemical properties and characteristics as the fresh cell used in the first step) until one or more markers provide evidence of the formation of a substantially complete and stable improved SEI on the hard carbon working electrode at the second current density.

[0047] The target cell is a cell to which an unconventional forming process is applied.

[0048] Optionally (a marker, i.e., the presence or absence of which indicates that it is beneficial for further development of the SEI), the method may include repeating the above manufacturing steps one or more times on the same formed target cell at the same second current density until one or more markers provide sufficient evidence of the formation of a substantially complete and stable improved SEI on the hard carbon working electrode.

[0049] Note that the improvement of the SEI formed on the hard carbon of the formed target cell is easily demonstrated when the formed target cell exhibits a higher specific capacity and / or a higher Coulombic efficiency during cycling at the nominal C-rate of the formed target cell compared to the specific capacity and / or Coulombic efficiency shown for the formed test cell. This comparison at the nominal C-rate is understood to be made for formed cells where all other things are equal, for example, where the same number of polarization steps are applied to both. This ensures a direct performance comparison, with the only differences being the particularly high rate / current density formation conditions applied to the formed cell and the number of repeated polarization steps.

[0050] Formation of cell formation under optimal formation conditions The present invention provides a cell formation method for forming an improved SEI on a hard carbon anode in a sodium ion electrochemical cell in a total time of 20 hours or less, preferably 5 hours or less, more preferably 2.5 hours or less. After the improved SEI is formed, the cell exhibits the performance improvements described during cycling at the nominal C-rate of the particular cell. In some embodiments, a set of optimized formation conditions (e.g., as specified according to the method of the first aspect above) can be applied by a formation method applicable to the same fresh (as-new) cell, or a batch of the same fresh (as-new) cells that may be encountered in commercial battery manufacturing. Such a formation method is a first step of providing one or more fresh cells to be formed based on sodium ion electrochemistry, wherein the fresh cells are A counter electrode comprising an electrochemically oxidizable material capable of releasing sodium ions into the super-concentrated electrolyte within the cell during cell polarization, A hard carbon working electrode that absorbs sodium ions received from the super-concentrated electrolyte at the hard carbon working electrode as reduced sodium occluded in the hard carbon working electrode, the hard carbon working electrode Comprising, The super-concentrated sodium ion ionic liquid electrolyte contains an ionic liquid and at least one sodium salt, and the sodium ion concentration is 75% or more of its saturation limit in the electrolyte, Including a first step.

[0051] Next, the forming method is, The cut-off discharge potential limit of the cell of the fresh target cell and the resulting discharge capacity are achieved in a period of 5 hours or less for single discharge, preferably 2 hours or less for single discharge, preferably 1 hour or less for single discharge, more preferably 30 minutes or less for single discharge, and polarizing the fresh target cell at a predetermined forming current density, and Identifying the presence and / or absence of one or more markers in the recorded corresponding charge-discharge cycles or related analyses related to substantially complete and stable SEI formation on the hard carbon electrode, and optionally, Until the presence and / or absence of one or more markers indicates the occurrence of substantially complete and stable improved SEI formation on the hard carbon electrode of the target cell, for the resulting target cell, at the same predetermined second current density, or at a different higher or lower predetermined forming current density, repeating the polarization step one or more times By applying the cell formation process according to to a fresh target cell, including the step of manufacturing a formed target cell having an improved SEI on the working electrode of the formed target cell, Improvement of the SEI formed on hard carbon is demonstrated by the formed target cell exhibiting a higher specific capacity and / or a higher Coulombic efficiency during cycling at the nominal C-rate of the formed target cell compared to the specific capacity and / or Coulombic efficiency shown for the formed test cell. Preferably, (i) the predetermined formation current density and / or (ii) the number of polarization cycles corresponding to the number of polarization steps required to generate the improved SEI are determined by the method for specifying the optimized cell formation conditions described in the first aspect.

[0052] Suitably, the method of the present invention is carried out in a temperature range of 0 °C to 100 °C, preferably 20 °C to 85 °C, more preferably 25 °C to 80 °C, and even more preferably 45 °C to 55 °C. In some preferred embodiments, the method is carried out at 50 °C (±2%). Preferably, the cell polarization comprises charging and discharging a fresh test cell at a constant current within the full voltage window of the fresh test cell at a predetermined first current density that achieves the full discharge capacity of the cell over a period of at least 10 hours for a single discharge, more preferably at least 20 hours for a single discharge. "Charging" the cell refers to polarizing the cell such that ions are reduced at the working electrode (i.e., sodiation or lithiation, etc.), and "discharging" refers to the reverse process of oxidation at the working electrode (i.e., desodiation, delithiation, etc.).

[0053] In some embodiments, during polarization, the current density is selected to reach the cut-off discharge potential limit of the cell and the resulting discharge capacity over a period of less than 5 hours for a single discharge, less than 2 hours for a single discharge, preferably less than 1 hour for a single discharge, and more preferably less than 30 minutes for a single discharge. It will be understood that the desired formation time leads to the level of the current density used to achieve full discharge in the method described above.

[0054] In an embodiment, the formation time means the time required for a substantially complete and substantially stable improved SEI to be formed on the hard carbon working electrode, which is characterized by meeting the above performance requirements of the cell. In short, in some embodiments, a substantially stable improved SEI is such that the formed target cell exhibits a higher specific capacity and / or a higher Coulombic efficiency during cycling at the nominal C-rate of the formed target cell as compared to the specific capacity and / or Coulombic efficiency shown for the formed test cell, and is sufficiently complete to support higher performance. In some preferred embodiments, the higher performance of the formed target cell as compared to the test cell can be observed at the nominal C-rate after at least 50 cycles, at least 100 cycles, at least 200 cycles, at least 500 cycles, at least 1000 cycles, at least 2000 cycles. The C-rate is defined as the charge / discharge current divided by the nominal rated battery capacity. For example, a 5000 mA discharge of a 2500 mAh rated battery results in a 2C rate such that full discharge occurs in 30 minutes. Similarly, a 2500 mA discharge of a 2500 mAh rated battery results in a 1C rate such that full discharge occurs in 1 hour. Similarly, a 250 mA discharge of a 2500 mAh rated battery results in a 1 / 10C rate such that full discharge occurs in 10 hours.

[0055] Desirably, the current density applied during the method is a constant current density.

[0056] Preferably, the hard carbon working electrode contains hard carbon, and more preferably consists essentially of hard carbon.

[0057] In some embodiments, the second current density achieves the cut-off discharge potential limit of the cell in a polarization cycle with a period of 5 hours or less for a single discharge step. In some embodiments, the second current density achieves the cut-off discharge potential limit of the cell in a period of 2 hours or less for a single discharge step of the polarization cycle.

[0058] In some embodiments, the first predetermined current density at the working electrode in the test cell formation step is 200 mA / g or less, 150 mA / g or less, 100 mA / g or less, 50 mA / g or less, 10 mA / g or less. In some embodiments, the first predetermined current density at the working electrode in the test cell formation step is about 50 mA / g to about 100 mA / g. In some embodiments, the second or third or subsequent current density at the working electrode in the first target cell formation step is about 350 mA / g or more, about 375 mA / g or more, about 400 mA / g or more, about 450 mA / g or more, about 500 mA / g or more, about 550 mA / g or more, about 600 mA / g or more, about 650 mA / g or more, about 700 mA / g or more, about 750 mA / g or more, about 800 mA / g or more, about 850 mA / g or more, about 900 mA / g or more.

[0059] In particular, in the case of the C3mpyrFSI-based super-concentrated ionic liquid described in this example, the 1 / 10C rate requires a current density of 30 mA / g for full discharge, the 1C rate requires a current density of 300 mA / g, and the 2C rate requires a current density of 600 mA / g.

[0060] The "super-concentrated" ionic liquid-based electrolyte includes a sodium ion ionic liquid electrolyte suitable for use in the cells of the present invention and is not particularly limited as long as it exists in a liquid form at the operating temperature of the cells. Suitably, the "super-concentrated" ionic liquid-based electrolyte is dry in that it contains less than 50 ppm of water. The ionic liquid of the super-concentrated electrolyte preferably contains a pyrrolidinium cation, preferably an alkylated pyrrolidinium cation, preferably a 1-methyl-1-alkyl-pyrrolidinium cation (C3mpyr + ), most preferably a 1-methyl-1-propyl-pyrrolidinium cation (C3mpyr + ). However, for the sodiation of hard carbon, the preferred electrolyte is the C3mpyrFSI-based super-concentrated ionic liquid particularly described in this example.

[0061] Otherwise, the sodium salt / ionic liquid electrolytes used in accordance with the present invention are 1-butyl-3-methylimidazolium bisulfate ([C4mim][HSO4]), 1-alkyl-3-methylimidazolium bromide ([C n mim][Br]), 1-hexadecyl-3-methylimidazolium chloride ([C 16 mim][Cl]), 1-octyl-3-methylimidazolium chloride ([C8mim][Cl]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]), 1-octyl-3-methylimidazolium tetrafluoroborate ([C8mim][BF4]), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C2mim][Tf2N]), 1-butyl-3-methylimidazolium chloride ([C4mim][Cl]), 1-hexyl-3-methylimidazolium tetrachlorferrate(III) ([C6mim][FeCl4]), 1-propyl-3-methylimidazolium iodide ([C3mim][I]), 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([C2mim][OTf]), 1-alkyl-3-methylimidazolium triflate ([C n mim][Tf]), 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]), N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide (([C4mPyr][Tf2N]) or ([C4mpy][Tf2N])), 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]), 1-dodecyl-3-methylimidazolium bromide ([C 12 mim][Br]), 1-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C8mim][Tf2N]), 1-hexadecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C 161-ethyl-3-methylimidazolium chloride ([C2mim][Cl]), 1-(3-aminopropyl)-3-methylimidazolium bromide ([(3-aminopropyl)mim][Br]), 1,2-dimethyl-3-butylimidazolium bis(trifluoromethanesulfonyl)amide ([C4(2-Ci)mim][Tf2N]), 1-butyl-3-methylimidazolium dicyanamide ([C4mim][N(CN)2]), 1-hexadecyl-3-methylimidazolium tetrafluoroborate ([C 16 mim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([C4mpyr][Tf2N]), 1-butyl-3-methylimidazolium tetrachloroferrate(III) ([C4mim][FeCl4]), 1-ethyl-3-methylimidazolium bromide ([C2mim][Br]), 1-hexadecyl-3-methylimidazolium bromide ([C 16 mim][Br]), 1,2-dimethyl-3-(3-hydroxypropyl)imidazolium bis(trifluoromethanesulfonyl)imide (C2-OH)N-ethyl-tris(2-(2-methoxyethoxy)ethyl)ethaneammonium bis(fluorosulfonyl)imide ([N 2(2O2O1)3 [FSI]) and N-ethyl-tris(2-(2-methoxyethoxy)ethyl)ethaneammonium bis(trifluoromethanesulfonyl)imide ([N 2(2P2O1)3 [TFSI]). It may contain one or more organic salts selected from the group consisting of

[0062] In some embodiments, the sodium salt / ionic liquid electrolyte used according to the present invention may contain one or more organic salts containing a salt selected from bis(trifluoromethanesulfonyl)imide ([Tf2N], or [TFSI]), bis(fluorosulfonyl)imide ([FSI]), or combinations thereof.

[0063] For example, the organic salts include 1-butyl(propyl)-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4C3mpyr][TFSI]) and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ([C3mpyr][FSI]). Suitable organic salts for use in forming the electrolyte of the present invention may include those disclosed in Gebrekidan Gebresilassie Eshetu, Michel Armand, Bruno Scrosati, and Stefano Passerini, Energy Storage Materials Synthesized from Ionic Liquids Angewandte Chemie Int.Ed. 2014, volume 53, page 13342, the content of which is hereby incorporated by reference in its entirety.

[0064] Desirably, the electrolyte contains a phosphorus-based organic salt. That is, the sodium ion ionic liquid electrolyte used according to the present invention may also contain one or more organic salts selected from phosphorus analogs of the organic salts disclosed herein. The phosphorus "analog" of the organic salts disclosed herein means an organic salt that shares the same chemical structure as the organic salts disclosed herein and has a nitrogen atom replaced by a phosphorus atom.

[0065] Thus, the electrolyte used according to the present invention is trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)imide ([P 66614 [Tf2N]), trihexyl(tetradecyl)phosphonium bis(fluorosulfonyl)imide ([P 66614 [FSI]), diethyl(methyl)(isobutyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P 1224 [Tf2N]), diethyl(methyl)(isobutyl)phosphonium bis(fluoromethanesulfonyl)amide ([P 1224 [FSI]), triisobutyl(methyl)phosphonium bis(trifluoromethanesulfonyl)imide ([P 1224[Tf2N]), triisobutyl(methyl)phosphonium bis(fluoromethanesulfonyl)imide ([P 1444 [FSI]), triethyl(methyl)phosphonium bis(trifluoromethanesulfonyl)imide ([P 1222 [Tf2N]), triethyl(methyl)phosphonium bis(fluoromethanesulfonyl)imide ([P 1222 [FSI]), trimethyl(isobutyl)phosphonium bis(trifluoromethanesulfonyl)imide ([P 111i4 [Tf2N]) and trimethyl(isobutyl)phosphonium bis(fluoromethanesulfonyl)imide ([P 111i4 [FSI]) and may contain one or more organic salts selected from the group consisting of

[0066] The electrolyte used according to the present invention may also contain one or more organic salts selected from those described herein that incorporate an alkoxyether functional group into the cation side chain (e.g., by substituting an alkyl chain on the cation).

[0067] In some embodiments, the electrolyte used according to the present invention comprises one or more sodium salts selected from sodium bis(trifluoromethane)sulfonimide (Na[TFSI]), sodium (bis(fluorosulfonyl)imide (Na[FSI]), sodium triflate (NaOTf), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4) and sodium hexafluorophosphate (NaPF6).

[0068] Desirably, the sodium ions are Na +and the electrolyte includes Na[TFSI] and [C3mpyr][TFSI], Na[TFSI] and [C4C3pyr][TFSI], Na[TFSI] and [C3mpyr][FSI], Na[TFSI] and [C4C3mpyr][FSI], Na[FSI] and [C3mpyr][TFSI], Na[FSI] and [C4C3mpyr][TFSI], Na[FSI] and [C3mpyr][FSI], Na[FSI] and [C4C3mpyr][FSI], or combinations thereof.

[0069] Desirably, the electrolyte includes sodium bis(fluorosulfonyl)imide (Na[FSI]) and N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyr[FSI]).

[0070] In other suitable ionic liquids, there are oxazolidinium cations such as [C1moxa] + , [C2moxa] + ; ammonium cations such as [N 111,1O1 , or ammonium having a functional group such as cyanoammonium, morpholinium; charge-diffused cations such as hexamethylguanidinium; pyrrolidinium such as [C2epyr] + , [C (i3) mpyr]. These cations can be used with any suitable anion, particularly TFSI - , FSI - , BF4 - or PF6 - anions. Particularly preferred for the present invention are ILs having cations including HMG (hexamethylguanidinium), oxa (such as dimethyloxazolidinium), and ammonium (such as tetramethyl and methyl, triethyl, etc.), and particularly having FSI and TFSI.

[0071] According to the present invention, the sodium salt concentration in the electrolyte is 75% or more of its saturation limit in the electrolyte. The concentration of the sodium salt in the electrolyte is the mol% of the sodium salt with respect to the total moles of the sodium salt and the organic salt. The "saturation limit in the electrolyte" means the highest concentration of the sodium salt in the electrolyte at which the sodium salt does not precipitate from the electrolyte at a given temperature. In other words, when the additional sodium salt added to the electrolyte does not dissolve, the sodium salt concentration is at its saturation limit in the electrolyte at a given temperature. The saturation limit of the sodium salt ions in the electrolyte at the reference temperature is a concentration that can be conveniently measured according to standard procedures known in the art. According to such a procedure, at the initial temperature, the sodium salt is added to the organic salt while gradually increasing the amount, and the initial temperature is higher than the reference temperature at which the saturation limit is to be determined. The addition of the sodium salt is continued until the formation of a visible precipitate of the undissolved salt, indicating that the saturation limit has been exceeded. Then, the temperature is lowered to the reference temperature, and additional sodium salt is precipitated from the organic salt. When the precipitation of the sodium salt stops, the total amount of the sodium salt precipitate is determined by separating the precipitate from the solution using means known to those skilled in the art. The sodium salt saturation limit at the reference temperature is calculated as the difference between the total sodium salt added to the organic salt and the amount of the sodium salt precipitate.

[0072] "75% or more of its saturation limit in the electrolyte" means a sodium salt concentration within the range of 75% to 100% of the saturation limit in the electrolyte. For example, if the saturation limit of the sodium salt in the electrolyte at a given temperature is 60 mol%, the sodium salt concentration in the electrolyte according to the present invention is 45 mol% or more (45 mol% is 75% of 60 mol%). In other words, in this example, the concentration of the sodium salt in the electrolyte according to the present invention is within the range of 45 mol% to 60 mol%.

[0073] The saturation limit of the electrolyte is the limit at the operating temperature of the cell. The "operating temperature of the cell" means the temperature at which the cell becomes functional, for example, when the SEI layer is formed, during discharge when supplying power to an external load attached to the electrode, and / or during charging. In other words, at the operating temperature of the cell, the sodium salt / ionic liquid electrolyte has a sodium salt concentration that is 75% or more of its saturation limit in the electrolyte.

[0074] Preferably, the sodium salt concentration in the super-concentrated ionic liquid-based electrolyte is 80% or more, 85% or more, 90% or more, 95% or more, or 100% of its saturation limit in the electrolyte. Preferably, the sodium salt concentration in the super-concentrated ionic liquid-based electrolyte is 40 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, or 95 mol% or more. In some embodiments, the concentration of the sodium salt in the electrolyte is 76% or more, 77% or more, 78% or more, 79% or more, 80% or more, 81% or more, 82% or more, 83% or more, 84% or more, 85% or more, 86% or more, 87% or more, 88% or more, 89% or more, 90% or more, 91% or more, 92% or more, 93% or more, 94% or more, 95% or more, 96% or more, 97% or more, 98% or more, or 99% or more of its saturation limit in the electrolyte. In some embodiments, the concentration of sodium ions in the electrolyte is at its saturation limit.

[0075] In addition to being 75% or more of its saturation limit in the electrolyte, in some embodiments, the sodium salt concentration in the electrolyte can be 40 mol%, 50 mol%, or more. For example, the molar concentration of the sodium salt in the electrolyte can be at least 40 mol%, 50 mol%, 55 mol%, 60 mol%, 65 mol%, 70 mol%, 80 mol%, or 90 mol%.

[0076] In some embodiments, the cell is in a half-cell configuration. For example, when the cell is in a half-cell configuration, the counter electrode is sodium metal and the working electrode is a negative electrode material, such as hard carbon. In other embodiments, the cell is in a full-cell configuration. For example, the cell may be in a full-cell configuration, and the counter electrode includes or is composed of a material that can reversibly intercalate / deintercalate sodium ions in their atomic structure, absorb / desorb sodium ions by a reversible oxidation / reduction reaction, or promote an alloying / dealloying reaction with sodium ions.

[0077] In some embodiments, the counter electrode is Na 0.45 Ni 0.22 Co 0.11 Mn 0.66 O2, Na 2 / 3 Fe 2 / 3 Mn 2 / 3 (O3), Na 2 / 3 Fe 2 / 3 Mn 1 / 3 O2(P2), olivine-type NaFePO4, Na x FePO4, silicate, including or composed of a NASICON-type phase of the general formula Na2M2(XO4)3 (M = transition metal and X = P, S).

[0078] In a preferred embodiment, the predetermined first current density in step a) achieves the full discharge capacity of the cell in a period of 10 hours corresponding to the conventional formation process using a 1 / 10C rate for formation. In a preferred embodiment, the predetermined constant second higher current density in step b) achieves the full discharge capacity of the cell in a period of 0.5 hours or less. In a preferred embodiment, the predetermined constant second higher current density in step b) achieves the full discharge capacity of the cell in a period of 0.25 hours or less. When it is desirable for the period to be 0.25 hours or 0.5 hours, it is most preferred to use a period of 10 hours or less in step a).

[0079] The nominal C-rate of the formed target cell is preferably 1 / 2C or 1 / 5C, but any particularly desirable appropriate nominal C-rate can be used as long as the same C-rate is applied to the formed test cell in step a). Preferably, the formed target cell can be further galvanostatically cycled at a lower C-rate between a maximum of 100 cycles, preferably a maximum of 50 cycles, for the purpose of comparing performance with the formed test cell.

[0080] Preferably, during the nominal C-rate cycling (after SEI formation is complete), the formed target cell exhibits a Coulombic efficiency (CE) of 99.2% or more, preferably 99.3% or more, preferably 99.4% or more, preferably 99.5% or more, preferably 99.6% or more, preferably 99.7% or more, preferably 99.8% or more, preferably 99.9% or more after the 20th cycle, more preferably after the 10th cycle, even more preferably after the 6th cycle (in the case of a 5-cycle formation cycle process). Performance is preferably compared with the formed test cell after 10 or more, 50 or more, or 100 or more cycles at the nominal selected C-rate of the cell. As the cell ages, irreversible capacity loss or fade occurs, and a small decrease in CE away from 1 accumulates as the cell is cycled. Therefore, considering that the capacity fade that necessarily occurs in the latter half of the cycle occurs more rapidly initially as the cell ages, the higher the CE after cycle formation at the nominal C-rate, the better, where the starting CE is closer to 99% than 100%. Therefore, considering that the CE is higher during the early nominal C-rate cycling, the cells of the present invention exhibit an overall longer cycle life (number of achievable cycles) before reaching the end of the determined life capacity loss point.

[0081] Desirably, one or more markers associated with substantially complete and stable SEI formation on the hard carbon electrode (e.g., in the recorded corresponding charge-discharge cycles or related analyses) include: (a) Discharge-charge curve markers / features Using the characteristics indicating the progress of SEI development and comparing with previous cycles, it is not clearly observed and / or is observed that there is no substantial static electrolyte reduction voltage gradient in the recorded voltage-capacity cycles, The invariability of characteristics / stability of characteristics in the recorded charge-discharge cycles is observed under comparison of the differences between a specific formation cycle and the preceding or subsequent formation cycles, Under comparison of the differences between a specific formation cycle and the preceding or subsequent formation cycles, there is no "low voltage" characteristic (e.g., less than about 1.5 V after the initial cycle of the exemplified half-cell) in the recorded charge-discharge cycles. Low voltage characteristics are known / determinable to those skilled in the art as being related to the reduction / decomposition of the electrolyte and / or the reduction / decomposition of electrolyte additives. Such characteristics can be determined by separate electrochemical studies performed on the specific electrolyte system used, for example, by CV studies that can identify the redox peak voltages of the ionic liquids (cations and / or anions) and anions of the metal salts used, or by any additives contained, It is observed that the cell cutoff discharge potential limit is reached without the presence / appearance of curve characteristics related to the insertion of sodium ions into the negative active electrode, i.e., intercalation characteristics. That is, for example, in the example of the described half-cell, the insertion of sodium ions is identified by the presence of a flat plateau at about 0.01 V (i.e., there is no insertion peak in the curve during the formation cycle, and the insertion peak is seen in cycles after the substantially completed and substantially stable formed improved SEI), and It is observed that there is a minimum or stable irreversible capacity loss between cycles, (b) Electrochemical impedance spectroscopy (EIS) markers / characteristics Evidence of a low and / or stable EIS span in electrochemical impedance studies including, for example, the recorded Nyquist plot analysis. For example, in the case of an unstable SEI, EIS shows a significant increase in impedance and / or attenuation to noise due to the lack of stability. A stable SEI does not exhibit this behavior, Evidence of the formation of a low-impedance SEI, e.g., arising from the magnitude of the EIS and the amount of polarization during loading, i.e., a decrease or increase in the relative potential during loading of a full cell or symmetric cell under equivalent conditions (current, temperature, etc.) Evidence of a SEI with higher ionic conductivity than the formed test cell. For example, in the provided half-cell studies, the ohmic values in the range of 10 s to 100 s indicate good movement of Na ions through the SEI and the electrode interface. (c) Differential capacitance analysis markers / features Absence of electrolyte reduction / decomposition and / or electrolyte additive reduction / decomposition peaks in the recorded differential capacitance curve (dQ / dV). For example, the absence of an FSI-anion reduction peak in dQ / dV means complete formation of the SEI layer in the examples of half-cells described herein in super-concentrated ILs containing metal FSI salts. Absence of a metal ion insertion peak indicates that complete discharge occurs before metal ion insertion. Na before reaching the full charge capacity of a high-rate cell + Instead of a Na insertion peak, the presence of peaks of adsorption and / or chemisorption of Na ions at the active sites of the hard carbon electrode at high rates, and + The absence of an insertion peak such that an insertion peak becomes visible after sufficient SEI formation.

[0082] ​The detectable difference in the capacity of the sodium ion absorption step compared to the detachment step indicates the charge consumed in SEI formation during any given polarization cycle. As the formation cycle progresses, the SEI develops with respect to properties such as composition, ionic conductivity, thickness, porosity, density, etc., depending on the chemistry of the cell and electrolyte used. The development of the SEI is typically considered to be completed in several formation cycles (e.g., 2 / 3 cycles) carried out at conventional slow charge / discharge rates (1 / 10C or less, 1 / 20C or less), which includes enabling full charge and full discharge of the cell, which was considered essential for the best subsequent performance of the cell. Thus, the decreasing charge consumption as the cycle progresses indicates the degree of completeness of SEI formation for any given cycle. As SEI formation proceeds, the SEI becomes more complete and stable, consuming less additional electrolyte, so the charge loss (irreversible capacity loss) from one formation cycle to the next decreases. Considering that the formed SEI is substantially complete and stable, i.e., optimal SEI formation has occurred and little additional electrolyte decomposition occurs at this stage, the charge lost between the charge step and the discharge step in successive polarization cycles is minimized. Thus, the observation of decreasing irreversible capacity loss between adjacent cycles and / or minimal charge loss between the charge step and the discharge step can also be used to identify the cycle at which there is subsequently substantially complete and substantially stable SEI formation.

[0083] During the first formation cycle performed on a test fresh cell, the method uses a current density that is low enough to achieve the full discharge capacity of the cell within a period of at least 10 hours, more preferably at least 20 hours, for a single discharge. This means that the formation of the test fresh cell becomes slow enough for SEI formation to be slow, and the cycle proceeds naturally over a time sufficient to ensure that the formed SEI is substantially complete. Thus, the analysis of the test fresh cell can identify the presence and / or absence of electrochemical analysis parameters / one or more markers associated with substantially complete and substantially stable SEI formation for the slow formation step.

[0084] A "fresh cell" is a manufactured cell that has not yet been polarized and thus has electrodes in an initial state. References to one or more fresh cells referred to in the various steps of the method described herein mean that fresh unpolarized cells are used, which cells are from the same batch of cells having the same type of chemistry, amount of active material, type of electrolyte, etc. within the experimental and manufacturing control limits.

[0085] In the first formation cycle performed on the target fresh cell, the current density is selected to be higher than the current density used for the test cell so as to reach the cut-off discharge potential limit and the resulting discharge capacity of the target cell in a period of less than 5 hours for a single discharge, less than 2 hours for a single discharge, preferably less than 1 hour for a single discharge, more preferably less than 30 minutes for a single discharge. The higher the selected current density, the faster the cut-off discharge potential limit of the fresh target cell will be reached compared to that of the same step on the test cell. For example, a higher current density used means that the cut-off discharge potential limit may be reached before sodium ion intercalation occurs in the hard carbon electrode. Thus, the preferred current density for the target cell is one that results in a cut-off at a low discharge capacity. Thus, the target cell shows a decrease in discharge capacity compared to the test cell.

[0086] In another aspect, the present invention relates to the use of a super-concentrated ionic liquid electrolyte in a cell in a method of forming an improved SEI on a hard carbon negative electrode, preferably a hard carbon negative electrode in a sodium ion electrochemical cell, in 10 hours or less. The features of the method and the super-concentrated ionic liquid electrolyte have been described above. Preferably, the use is in a sodium ion electrochemical cell comprising a hard carbon working electrode, preferably in a method of forming a hard carbon working electrode, a super-concentrated electrolyte comprising an ionic liquid and at least one sodium salt, wherein the sodium ion concentration is 75% or more of its saturation limit in the electrolyte, and the cell formation is completed at 50 °C in 10 hours or less. Suitably, the use comprises a cell in which the sodium ions are sodium ions, the hard carbon working electrode is hard carbon, and the electrolyte is [C3mpyr][FSI]IL containing about 50 mol% of the NaFSI salt. In this case, about means ±2%.

[0087] Definitions "Electrochemical cell" means a device capable of converting chemical potential energy into electrical energy.

[0088] "Half cell" means that the cell is in a half cell configuration, whereby the cell comprises a working electrode and a counter electrode under study and is controlled by the potential. In a half cell configuration, charge can only be withdrawn during discharge to a negative cell voltage. In the case of a half cell configuration, the cell of the present invention may be suitable for use as a diagnostic or test device that can assist in measuring the electrochemical properties of the electrolyte or identifying a positive electrode suitable for use in a full cell configuration according to the present invention.

[0089] "Full cell" means that the cell is in a full cell configuration controlled by a cell voltage, such as in a battery. In such a configuration, the cell further includes a positive electrode material as a counter electrode and a negative electrode material as a working electrode. As used herein, the expression "full cell configuration" refers to a cell configuration in which, after charging, the positive and negative electrodes support a substantial potential difference (e.g., greater than about 1 V) and charge can be extracted therefrom during discharge at a positive cell voltage. When the electrode pair is a suitable pair of negative and positive active material electrodes, the cell is in a full cell configuration. When the electrode pair supports a high current density at the negative electrode and maintains a large number of polarizations or charge / discharge cycles, this pair is suitable for use in a cell that can function as a high-capacity and cycle-stable metal ion-based rechargeable battery.

[0090] "Intercalation" means the reversible insertion of transport metal ions into the atomic structure of an electrode.

[0091] "Negative electrode" refers to the electrode from which electrons exit the cell during discharge (anode during discharge, cathode during charging). For example, when in electrical contact with a sodium ion ionic liquid electrolyte used according to the present invention, the negative electrode material causes the formation of an SEI layer as a result of undergoing at least one polarization cycle, and electrolyte reduction at the electrode forms SEO. The negative electrode can reversibly intercalate sodium ions within its atomic structure and interact (e.g., absorb / desorb) with sodium ions by promoting reversible oxidation / reduction reactions, or can promote alloying / dealloying reactions with sodium ions and can include (or be made from) a hard carbon material. Examples of materials that the hard carbon negative electrode can include (or be made from) include hard carbon, graphite, activated hollow carbon, expanded graphite, hard carbon composites, and their doped analogs.

[0092] The "positive electrode" refers to the electrode through which electrons enter the cell during discharge (cathode during discharge, anode during charging). For example, the positive electrode can include (or be made from) materials that can reversibly intercalate sodium ions within their atomic structures, absorb / desorb sodium ions through reversible oxidation / reduction reactions, or promote alloying / dealloying reactions with sodium as described herein.

[0093] The "sodium ion ionic liquid electrolyte" refers to an ionic liquid in which a sodium salt is dissolved in an organic salt. In some embodiments, the sodium salt is the sodium salt equivalent of the organic salt (i.e., shares the same anion).

[0094] The "polarization cycle" means a cycle of charging and discharging the cell. The "formation cycle" means the first cycle within the life of the cell or the first few (up to the first 5) polarization cycles that promote the formation of the SEI layer on the working electrode, negative electrode material, or "anode". A cell that has undergone a "polarization cycle" is intended to mean that the cell has undergone a two-step cycle that includes a step in which a current of a specific density flows through the negative electrode in one direction and a step in which the current is switched and flows through the negative electrode in the opposite direction. In some embodiments, the cells according to the present invention can be configured and used such that the current periodically flows through the negative electrode in opposite directions. That is, the cell can undergo a plurality of polarization cycles in which the current flows through the negative electrode in alternating opposite directions. As a result, potentials of alternating signs can be observed.

[0095] A person skilled in the art will know the technical meaning of the expression "charge / discharge cycle" and how to perform such a procedure. For example, a charge / discharge cycle may be a charge / discharge performed to operate a rechargeable battery after assembly. As is known to a person skilled in the art, this refers to a procedure adopted to form a negative electrode by a charge / discharge routine under controlled voltage, temperature, and environmental conditions, which is intentionally performed to induce the formation of a solid electrolyte interface (SEI) layer on the negative electrode. To avoid misunderstanding, it should be understood that in the context of the present invention, a polarization cycle is equivalent to one charge / discharge cycle.

[0096] "Coulombic efficiency (CE)" is defined as the quotient of the discharge capacity and its preceding charge capacity for a given set of operating conditions. This is a measure of how reversible the electrochemical energy storage reaction is, and any value less than 1 indicates a non-productive and often irreversible reaction. A decrease in CE from 1 reflects the number of non-productive reactions per 1000 reactions, resulting in an irreversible loss of reactants per cycle and having a significant impact over hundreds of cycles.

[0097] "Irreversible capacity" is defined as the amount of charge lost between the discharge capacity and its preceding charge capacity for a given set of operating conditions, and reflects the magnitude of the irreversible processes occurring during the polarization cycle.

[0098] "Cumulative irreversible capacity" is defined as the cumulative sum of the irreversible capacities over a plurality of polarization cycles and reflects the progress of the irreversible processes within the cell.

[0099] Description of Preferred Embodiments The present invention will be described with reference to the following examples. It should be understood that the examples are illustrative of the invention described herein and not limiting.

[0100] The general principle of the application underlying the present invention is demonstrated by the following half-cell studies, which include the high-current density polarization (up to five formation cycles) of an initial state cell with a sodium metal counter electrode and a hard carbon working electrode in a super-concentrated IL, which is a 1:1 IL:Na salt, namely a C3mpyrFSI IL containing 50 mol% NaFSI salt. The formed SEI has the desired compositional characteristics described in the present disclosure.

[0101] Materials and Methods Preparation of Electrolytes and Slurry Casting Hard carbon (HC) was purchased from Kuraray (Kuranode, SSA = 4 m 2 / g). Na metal was purchased from Sigma-Aldrich. Battery-grade sodium bis(fluorosulfonyl)imide (NaFSI, 99.7%) and N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (C3mpyrFSI, 99.9%) were purchased from Boron molecular. Dimethyl carbonate (DMC, 99%) was purchased from Sigma-Aldrich and used to wash the cycled electrodes. The electrolytes were dried under vacuum at 50 °C by a Schlenk line before use. The super-concentrated ionic liquid was prepared by dissolving 50 mol% NaFSI in C3mpyrFSI and stirring at 50 °C for 24 h before transferring to the glove box. The HC anode was prepared by mixing hard carbon, carboxymethyl cellulose (CMC, Sigma-Aldrich), and carbon black (Sigma-Aldrich) in an 8:1:1 ratio with water as the solvent. The slurry was then coated onto an Al current collector using a doctor blade. The mass loading of the HC electrode was 1 mg / cm 2 2.

[0102] Electrochemical Measurements The Na / HC cell was prepared in a glove box having O2 and H2O levels of less than 0.1 ppm. The R2032 half cell was assembled using an HC electrode (8 mm in diameter) as the working electrode and Na metal (10 mm in diameter) as the counter and reference electrodes. Solupor® polyethylene 5P09B (19 mm in diameter, 38 μm thick, 86% porosity) was used as the separator. A three-electrode cell was assembled in the same way in a custom setup using two separate Na metals as the counter and reference electrodes, respectively. The assembled cells were left standing at 50 °C for 24 hours for wetting before testing. The Na / HC cells were cycled at different C-rates (2C, 1C, and 1 / 10C, C = 300 mAh / g) within a voltage range of 0.01 - 2 V using a Neware battery cycler and then cycled at the same current density for long-term testing. EIS measurements were performed using a Biologic VMP potentiostat having a frequency range of 1 MHz to 100 mHz.

[0103] Material and Electrode Characterization Post-characterization was performed by X-ray photoelectron spectroscopy (XPS) and solid-state nuclear magnetic resonance spectroscopy (NMR). XPS was carried out on a Thermo Scientific Nexsa spectrometer equipped with a hemispherical analyzer. The incident radiation was monochromatic Al Kα X-rays (1486.6 eV) at 72 W (6 mA and 12 kV, 400×800 μm spot). Survey (wide) and high-resolution (narrow) scans were recorded at analyzer pass energies of 150 and 50 eV, respectively. The survey scan was performed with a step size of 1.0 eV and a dwell time of 10 ms. The high-resolution scan was obtained with a step size of 0.1 eV and a dwell time of 50 ms. The base pressure in the analysis chamber was 5.0×10 -9It was less than mbar. A low-energy dual-beam (ion and electron) flood gun was used to compensate for surface charging. All data were processed using Casa XPS, and the energy calibration was based on the low binding energy component of the C 1s peak at 284.8 eV. The etching depth is based on Ta2O5 (0.30481 nm / s) and is used as an approximation in all depth analyses. Solid-state magic angle spinning (MAS) NMR experiments were collected on a 500 MHz (11.7 T) Bruker Avance III wide-angle spectrometer. Samples formed under different formation protocols were scraped from a cycle cathode inside an Ar glove box. Control samples were also prepared in the same procedure by immersing them in the IL electrolyte for 24 hours. The samples were then rinsed with DMC, dried under vacuum, mixed with boron nitride as a filler material, and then placed in a 1.3 mm MAS NMR rotor. They were then rotated at 40 kHz using dry air. 23 The Na spectrum was obtained by 200 million scans acquired using a single pulse experiment and a recycle delay of 0.5 s. 19 The F spectrum was obtained by 200 million scans acquired using a Hahn echo pulse sequence with an echo delay of 50 μs, a recycle delay of 1 s, and. Both nuclei were referenced using solid NaF ( 23 δ = 7.4 ppm for Na and 19 -224.2 ppm for F). All samples were filled in an argon environment.

[0104] Results and Discussion Example 1 - Constant Current Charge and Discharge SEI Formation - Capacity, Cycle Performance, and Coulombic Efficiency The constant current charge and discharge of the cell (based on the measurement of the potential of a cell polarized at a constant current) reveals the irreversible capacity loss that occurs during SEI formation and further shows, for example, the SEI stability in the case of a slow nominal C rate for a particular cell by considering the capacity retention as a cell cycle.

[0105] As a result of the first sodiation process, the initial SEI formation that occurs on the hard carbon electrode (which occurs during the first formation cycle of the initial cell in a half-cell configuration) is recognizable from features related to the reduction / decomposition of electrolyte components, e.g., the presence of a voltage gradient during the first charge step in full cell measurements. However, in the case of half-cell studies involving a sodium metal counter electrode, it proceeds during the first hard carbon sodiation step / discharge process. The voltage gradient related to the reduction / decomposition of the electrolyte from the electrolyte reaction is a useful marker of incomplete SEI formation. The presence of a similar electrolyte decomposition / reduction voltage gradient in subsequent formation cycles can indicate that the SEI of a particular cycle is not complete or can provide information that the SEI is approaching completion as the voltage gradient shortens over successive sodiation cycles. Similarly, SEI formation / electrolyte reduction peaks can be observed in cyclic voltammetry experiments. Other markers include changes in EIS during cycling, such as the occurrence of significant fluctuations and noise, and other markers are also evident when the SEI is not complete and stable. Conversely, a substantially noise-free EIS during cycling indicates the formation of a stable and complete SEI. For example, an S / N ratio of 10 or less, preferably 5 or less, most preferably 3 or less indicates that the SEI is stable / complete.

[0106] Furthermore, observations that the irreversible charge lost between successive charge and discharge steps is minimal can also be used to identify cycles that result in a substantially complete and substantially stable formation of the SEI.

[0107] In the proof-of-concept experiment, for the chemical properties of the half-cells, within a 0.01 V cut-off voltage for sodiation and a total voltage window of 2 V for the desodiation process, three different formation protocols (1 / 10 C for the test cells, 1 C or 2 C for the target cells) were applied to the hard carbon working electrodes of a fresh half-cell configuration using sodium metal as the counter electrode. Various potential-time curves for the baseline (1 / 10 C, based on C = 300 mAh / g for HC) and alternative formation protocols (1 C and 2 C) can be observed in Fig. 1a. In this study, up to five formation cycles were used to observe the progression / establishment of a complete and stable SEI. However, in the process with higher current density, complete SEI formation occurred immediately after the first or second cycle. With the conventional formation protocol using the conventional slow 1 / 10 C current density step, it takes about 20 hours for one complete formation cycle (about 10 hours for the discharge step), and in total, it takes about 95 hours for the five cycles required for complete and stable SEI establishment in the 1 / 10 C test cells (see Fig. 1b). In contrast, the 1 C and 2 C formation protocols take only about 1 hour and about 0.5 hour per charge or discharge cycle, respectively, and for the five formation cycles of the 1 C and 2 C cells, the total formation time for the five cycles (required to observe complete and stable SEI establishment at 1 / 10 C) is shortened to a very short time of 5 hours and 2.5 hours, respectively. If complete SEI formation occurs immediately after the first or second cycle, this time is even shorter.

[0108] As is apparent from the constant current charge / discharge curve of Fig. 1c, in the first formation cycle, it includes the sodiation of hard carbon in the galvanic half cell. The conventional slow 1 / 10C formation test cell shows a discharge curve with a full discharge capacity of 313 mAh / g at the cut-off sodiation voltage, starting with a decreasing gradient at about 0.75 V, followed by a long flat plateau at about 0.01 V. The decrease in the plateau between 0.85 V and 0.97 V can be observed in the first cycle at 1 / 10C (see Fig. 1(f)), which disappeared in the fifth cycle. The discharge voltage decrease gradient of about 0.75 V is due to the formation of SEI (i.e., decomposition of the electrolyte) on the hard carbon surface, whereby the subsequent long flat plateau starting from about 0.01 V occurs within the hard carbon graphite layer due to the insertion of Na + and contributes to the high capacity achieved at a slow C rate (see Fig. 1(c) and Fig. 1(f)). The 1 / 10C rate reflects the low current density used in the conventional slow formation rate, which is expected to be beneficial for optimal SEI formation to achieve the full discharge capacity during polarization. The CV cycle (see Fig. 1(g)) shows a small reduction peak (about 0.73 V) derived from the decomposition of the electrolyte in the first cycle, which is not observed in the fifth cycle, indicating that the SEI formation was mainly completed by the fifth cycle under 1 / 10C conditions. The broad reaction peak between 0.2 V and 1 V corresponds to the adsorption of Na clusters on carbon defects.

[0109] However, interestingly, in the case of the 1C target cell and the 2C target cell, the sodiation reaction in the first formation cycle is not complete, which is indicated by the short reaction gradients occurring at 0.5 V and 0.35 V at the start, and most of the capacity is due to electrolyte reduction (voltage gradient meaning SEI formation) and Na on the surface +There is no capacity increase due to sodium insertion / intercalation during the first formation cycle dominated by adsorption and observed at 1 / 10C cells. Therefore, the total discharge specific capacity achieved at high rate cells is very low, 121 mAh / g at 1C and 97 mAh / g at 2C, compared to the 1 / 10C cells of 313 mAh / g. Considering the insufficient capacity observed, these cells would not have been expected to generate a useful SEI under these formation conditions.

[0110] The voltage gradient of the 2C target cells is shorter than that of the 1C cells and also shorter than that of the 1 / 10C cells, which indicates that the electrolyte decomposition occurring in the SEI formation of the 2C cells was the least, and at the same time, in both cases of high current density, compared to the same stage of the 1 / 10C cells, less electrolyte decomposition occurred during SEI formation. That is, less electrolyte is consumed during SEI formation. The amount of electrolyte lost corresponds to the irreversible capacity lost between charge and discharge of consecutive cycles. Furthermore, the curve of the fifth cycle shows that the capacity loss between charge and discharge curves is very small (see Fig. 1(d)), indicating that the formed SEI was substantially complete and substantially stable at that stage until the fifth cycle was completed in all cells.

[0111] The charge-discharge curves of the 6th cycle for the test cell formed at 1 / 10C and the target cells formed at 1C and 2C are shown in FIGS. 7(a) and 7(b) (FIG. 7(b) is an enlarged view of a part of FIG. 7(a)). The curve of the target cell formed at 2C in the 6th formation cycle at the 2C rate shows that the sodiumation overpotential and the de-sodiumation overpotential of the 2C sample are lower than those of the other cells. Thus, the potential is not more negative during easier sodiumation and not more positive during easier de-sodiumation compared to the previous (5th) formation cycle and compared to the 6th cycle profiles of the 1C and 1 / 10C formed cells. This is due to the complete and stable SEI formation that occurred by the 5th formation cycle. The presence of a long voltage plateau related to sodium insertion / intercalation into the hard carbon anode, indicating that the cell accommodates and releases much more charge during this cycle, is also interesting.

[0112] FIG. 1(d) clearly shows the subsequent charge / discharge behavior of three cells formed at a nominal C rate of 1 / 2C after completion of the formation protocol considered. Unexpectedly, the 2C formed cell delivered the highest discharge capacity of 242.5 mAh / g, followed by the 1 / 10C cell at 237.0 mAh / g and the 1C cell at 216.8 mAh / g. This reveals that SEI formation due to the high C rate is beneficial for sodium storage in terms of surprisingly good capacity storage considering the formation rate and the severe high current density conditions used.

[0113] Example 2 - Differential Capacity Analysis Delta dQ / dV analysis can provide a comprehensive understanding including peak deconvolution for SEI formation and Na + intercalation / de-intercalation. Thus, the SEI formation procedures under different formation C rates can be distinguished. Two reduction peaks at 0.96V and 0.87V are seen in the initial cycle of the 1 / 10C cell (FIG. 2f), and these are due to FSI in the electrolyte resulting from different coordination environments -It is assigned to the electrochemical reduction of anions. It should be noted that the reduction peak in the first cycle was not detected in the fifth cycle (Figs. 2g - 2i) for all cells, indicating the establishment of a complete and stable SEI up to that stage. When the C-rate was increased to 1C and 2C, it was 0.52V for the 1C cell and 0.45V for the 2C cell, and the peak position shifted to the lower potential side (more negative side) during the first discharge process. These changes suggest differences in the SEI formation mechanism due to the interfacial and mass transport interactions occurring under these conditions.

[0114] Interestingly, when the current density increased from 1 / 10C to 1C and 2C, the peak size decreased significantly and the peak shape broadened. This may be caused by a large amount of charge approaching the carbon surface at a low C-rate, and most of that charge moves to the low plateau region (0.01 - 0.1V), resulting in the total capacity. On the other hand, the peak intensity is limited by Na + ion transport at higher rates related to the concentration gradient, the relaxation time of the phase equilibrium becomes shorter, and a non-uniform Na distribution occurs near the carbon surface. As a result, the non-uniform sodium distribution across the hard carbon surface leads to a higher overpotential and a broader redox peak. On the other hand, the electrolyte decomposition process for forming the SEI layer and the corresponding SEI composition is also affected, which will be explained in the following section. Since the reduction peak in the first cycle is not detected in the next cycle (Figs. 2(d) - 2(i), Figs. 2(j) - 2(q)), this indicates the SEI formation established in the first cycle. The two large peaks at 0.6V in Figs. 2(g) - 2(h) are due to the adsorption / chemisorption of Na + ions onto the active sites of the carbon material, which is different from the insertion peak in Fig. 2(i) where most of the capacity is due to the contribution of diffusion.

[0115] Example 3 - Coulombic Efficiency Next, all formed cells with complete and stable SEI formation (after 5 cycles) were cycled at the same selected nominal C-rates (1 / 2C and 1 / 5C) for the cells for further evaluation after the completion of the 5 formation cycles.

[0116] In such cycles, the formed cells show distinct capacity fluctuations at 1 / 2C (Fig. 3(a), Fig. 3(c), Fig. 3(d) and Fig. 3(g), and the specific capacities at the 40th cycle are 258 mAh / g for 2C, 250 mAh / g for 1C, and 240 mAh / g for 1 / 10C respectively (see Fig. 3(b)). The tendency for the capacity of all Na / HC cells to increase at the 1 / 2C current density is probably due to the wettability of the hard carbon material under the high C-rate of the super-concentrated electrolyte, and it should be noted that this is related to the concentration gradient. The long-term charge capacities of the three cells in Fig. 3 follow the same trend, with the 2C cell having the highest charge capacity and the 1 / 10C cell having the lowest value. The Coulombic efficiency (CE) for the three cells in Fig. 3(b) and Fig. 3(h) is consistent with the cycle data, with the 1 / 10C cell showing the lowest CE of an average of 99.0% during cycling, while the 1C and 2C cells are higher, both close to 99.6%. This is probably related to the SEI reconstruction from the 1 / 10C formation where more irreversible electrochemical by-products are formed. The low CE of the 1 / 10C cell results in a somewhat faster capacity fading, which has an adverse effect on the full cell performance.

[0117] The excellent CE of 99.6% for the target cells formed at 1C and 2C is considered to be related to the formation of a robust, substantially complete, and substantially stable SEI layer on the carbon surface up to the 5th formation cycle under high C-rates. The SEI layer is widely homogeneous and more ion-conductive than that of the 1 / 10C test cell, and thus promotes the diffusion of Na + across the SEI. The stability of the SEI after 5 cycles can also be observed from the EIS study.

[0118] Figure 2b is an inset of Figure 2a where electrolyte decomposition occurs during the first cycle when the SEI is first formed. The three arrows in Figure 2(b) are very small and indicate reduction plateaus that are difficult to see in these curves. Therefore, this feature can be more clearly demonstrated by considering the corresponding dQ / dV curves in Figures 2d - 2f (at 0.45V, 0.52V, and 0.96V respectively), which clearly showed the electrolyte decomposition peaks.

[0119] Furthermore, Figure 2c shows an insertion plateau at approximately 0.01V for the 1 / 10C test cell, but the 1C and 2C target cells do not show the same insertion plateau. This is because the high current used causes a large polarization associated with the ohmic resistance, resulting in a shorter time to reach the cut-off potential compared to the 1 / 10C cell and the insertion reaction not being completed. This is the reason why the 1C and 2C target cells have a much lower capacity than the 1 / 10C cell during the formation cycle. SEI formation is shown in the region of 1V - 0.5V as the SEI is formed through the region of 1V - 0.5V. In particular, as can be seen from Figures 3a and 3b, the capacity observed in 5 formation cycles does not affect the capacity in the long term. Instead, 2C has a better capacity because the SEI formed in the region between 1V and 0.5V is more complete and more ion-conductive, improving the Na + reaction rate. +

[0120] The cumulative irreversible capacity of the three cells is summarized in Fig. 3e. The 1 / 10C cell has the highest irreversible capacity of 59 mAh / g in the initial cycle compared to the 1C cell at 38 mAh / g and the 2C cell at 36 mAh / g, meaning less electrolyte consumption for the faster rate cells. This is confirmed by the reduction plateau at 0.75 V for the 1 / 10C cell related to significant electrolyte decomposition. The high cumulative irreversible capacity of the 1 / 10C cell can be observed in consecutive formation cycles (insert in Fig. 3e), but when the current density is changed to 1 / 2C, the values for the three cells become negligible. Fig. 3(f) demonstrates the cycle stability of the 2C formation cell under a current density of 1 / 2C over 300 cycles. The cell shows a highly reversible capacity of 259 mAh / g after 300 cycles, with a capacity retention rate of 99.7%. The charge / discharge curves at the 50th and 300th cycles in Fig. 3(i) imply a negligible capacity difference between the two cycles, in good agreement with the stable cycle performance. The influence on the rate performance test of the high-speed formation protocol can also be found in Figs. 8(c) and 8(d), where the 2C cell has a higher specific capacity of 260 mAh / g when the current density decreases from 2C to 1 / 10C. The 2C formation cell maintains the advantage of exceeding the 1 / 10C cell at low current densities with average capacity increases of 8 mAh / g and 6 mAh / g at 1 / 5C and 1 / 2C, respectively. However, when a current density of 1 / 5C is applied (Fig. 3g), all cells show a rather high capacity of 280 ± 2 mAh / g, but the charge capacity in Fig. 3d is irreversible Na +It has a relatively low value of 275 ± 2 mAh / g due to intercalation. On the other hand, CE shows a similar value of 98.5 ± 0.2% in Fig. 3h, indicating that the cell performance does not degrade with a fast formation C-rate. To compare with 2C formation, higher C-rates of 3C and 10C were also investigated in the Na / HC cell (Fig. 3(j)). When the current density was changed to 1 / 2C, the 10C cell slowly recovered while the capacity increased from 77.8 mAh / g to 216.7 mAh / g from the 6th cycle to the 50th cycle. Interestingly, the capacities of the 2C and 3C cells increased rapidly to 260 mAh / g and 250 mAh / g, respectively, at the 20th cycle. This can be explained by the formation of an unfavorable SEI on the carbon surface at 10C formation that does not promote Na+ diffusion and thus has a lower capacity than the 2C formation cell. The inset shows an average capacity increase of 6 mAh / g from the 3C cell to the 2C cell between the 20th and 40th cycles. Here, the inventors concluded that under current conditions, 2C pretreatment is the optimized formation protocol in the HC sodium ion half-cell when a super-concentrated ionic liquid electrolyte is used. Apparently, the optimized formation protocol described herein does not limit the cell performance. Instead, unexpectedly, the cells after high-speed C-rate show better performance than conventional formation cells under a specific current density and can reduce the formation time of Na / HC in the super-concentrated ionic liquid electrolyte to 1 / 38.

[0121] Example 4 - Electrochemical Impedance Spectroscopy To correlate the formation protocol with the interfacial properties of the carbon surface, electrochemical impedance spectroscopy (EIS) measurements were carried out. A three - electrode configuration was designed using one Na foil as the counter electrode, one Na roll as the reference electrode, and hard carbon as the working electrode. Thus, Na metal functions as both the reference electrode and the counter electrode in the EIS experiment. The purpose of the three - electrode EIS measurement is to eliminate the contribution of the interfacial properties from the Na counter electrode. The Nyquist plots in FIGS. 4(a) - 4(c) show the various EIS resistances of the three formation cells after the first cycle, the second cycle, and the fifth cycle, clearly showing that the resistance of the 2C cell is the lowest while the 1 / 10C cell has the highest value. The curves were further fitted to three components using an equivalent circuit, and the results are shown in FIGS. 4(d) - 4(f), Table 1, and FIG. 10. A preferred SEI / HC electrode has an R int less than 500 ohms, preferably less than 300 ohms, more preferably less than 200 ohms after the first formation cycle, the second formation cycle, the third formation cycle, the fourth formation cycle, or the fifth formation cycle. A particularly preferred SEI / HC electrode has an R int less than 100 ohms, more preferably less than 75 ohms after the first formation cycle, the second formation cycle, the third formation cycle, the fourth formation cycle, or the fifth formation cycle. In some cases, for example, after more than one formation cycle at 2C, R int can be less than 50 ohms. R int can be determined by setting up the EIS as described above.

[0122]

Table 1

[0123] The 2C formation cell showed the lowest resistance of 45 ohms, while the 1C formation cell had a resistance of 300 ohms and the 1 / 10C formation cell showed the highest resistance of 700 ohms. The 2C cell showed a stable trend in both interfacial resistance and charge transfer resistance during 5 formation cycles. The interfacial resistance from the 2C cell (optimized formation protocol) was significantly smaller than that of the 1 / 10C cell, indicating robust and highly ion-conductive SEI formation under 2C conditions. This can be explained by the occurrence of complete electrolyte decomposition and SEI formation in the 1 / 10C formation cell after the first cycle, thereby forming a thick SEI layer and a long Na + diffusion path is brought about. In contrast, the SEI formation process in the case of the 2C formation cell is very fast, and thus an SEI layer with a different composition that is more ion-conductive is achieved. The SEI of the 2C cell is beneficial for Na + diffusion across the electrode / electrolyte interface and reduces the activation energy. In contrast, the decrease in interfacial resistance after 5 cycles in the case of the 1 / 10C cell in Fig. 4(f) is due to incomplete SEI formation and greater electrolyte consumption under the conventional formation process, resulting in a slow Na + diffusion rate.

[0124] Example 5 - XPS and Etching Investigation XPS tests were performed to analyze the SEI composition formed on the HC anode under different formation protocols. The C1s spectra in Figs. 5a and 5d show that all three HC electrodes are covered by C-N (402 eV) and C=O (288.3 eV) species from the decomposed ionic liquid, while in the 1 / 10C cell, a smaller amount of C-N species is found compared to the 1C and 2C cells (Figs. 5(f) - 5(h)). The C-N peak disappears after 2 minutes of etching, indicating that the C-N containing species of the SEI are located in the outer layer. Interestingly, the C-N peak is hardly observed in the 1 / 10C formation electrode.

[0125] The N 1s spectrum in Fig. 5(b) is consistent with the C 1s spectrum, and Ar +C3mpyr is in the outer layer of SEI that can be removed after etching + N from + (402.8 eV) is present, as demonstrated (Figs. 6(f) and 6(g)). Apart from C3mpyr + There are two main peaks corresponding to NS=O (398 eV) and N=SO (399.8 eV) observed in the SEI layer, which are derived from the decomposition of FSI - Interestingly, an additional peak at 396 eV, which is still obtained even after 40 minutes of Ar + cluster etching, is observed in the 1 / 10C cell. As can be detected from the etching profile in Fig. 6(f), atomic nitrogen exists throughout the SEI layer. The S 2p high-resolution spectra in Fig. 5(c) show the presence of -SO2F- (168.6 eV) and -SO x - (166.3 eV) for all three systems, and the "small" sulfur-containing species below 164 eV (164 - 160 eV) (insert in Fig. 5(c)) distinguish the 1 / 10C hard carbon from its 1C and 2C counterparts. As can be observed in Fig. 5(e), another inorganic SEI composition for all three cells is Na2O. These inorganic species have been reported to have a lower Na + diffusion energy barrier, thereby enhancing the Na migration rate. However, the presence of more reduced sulfur species in the SEI of the 1 / 10C formation cell makes the SEI layer thicker, thereby increasing the Na + diffusion path and decreasing the reaction rate compared to their counterparts.

[0126] The etching depth profiles in Figs. 6(a) - 6(d) provide a clear understanding of the SEI compositions resulting from the three different formation protocols considered. The amounts of sulfur (S), fluorine (F), oxygen (O), and nitrogen (N) were shown to increase continuously over the first 6 minutes of etching, while carbon showed a significant decrease for all three systems. This indicates that in the outer layer of the SEI, there is Na2O, Na2SO xThis is because inorganic species such as / NaNSO2F and NaF exist. In particular, in the case of the 2C-formed HC electrode, the ratio of C to O after 40 minutes of etching exceeds 1 (Figure 6(a)), which is the same trend as the initial-state electrode (Figure 6(d)), indicating that the surface of the 2C hard carbon approaches the bulk surface after 40 minutes of etching. In contrast, the C / O ratio in the case of the 1 / 10C electrode is less than 1, indicating that more Na2O species are present in the inner layer of the SEI and thus contribute to a thicker SEI layer. The C / O ratio in the case of the 1C hard carbon (Figure 6(b)) is between that of the 2C cell and the 1 / 10C cell, following the same trend where the SEI thickness plays an important role in capacity and EIS resistance. The schematic diagram in Figure 6(e) demonstrates the differences in SEI composition from the three formation processes: a thin SEI layer is rapidly formed on the hard carbon surface during 2C formation, with shorter Na + diffusion paths and higher ionic conductivity. Conversely, the 1 / 10C cell experiences more extensive electrolyte decomposition, generating more inorganic species as well as small amounts of sulfur and atomic nitrogen elements in the SEI composition. As a result, the SEI is much thicker than those of the 1C and 2C cells, thereby hindering Na+ diffusion into the carbon interlayer and reducing ionic conductivity. Our previous studies have shown that the influence of SEI formation varies from electrolyte polarity and concentration to the pretreatment protocol in both Li batteries and Na batteries. In particular, the MD simulation results indicate that a higher concentration of Na x FSI y aggregates forms a favorable inorganic-rich SEI for cell cycling. For the low-speed counterpart, the electrode surface has a low concentration of Na x FSI y aggregates, but a significant number of C3mpyr + cations. The involvement of C3mpyr + cation decomposition results in a thick organic-dominated SEI layer.

[0127] The C / O ratio on the 2C hard carbon is consistent with that of the initial state of the electrode (Figure 6d), indicating that the etched surface on the hard carbon of the 2C formation cell is approaching the bulk surface. However, the opposite C / O ratio (greater than 1) of the 1 / 10C formation cell electrode has a thicker SEI layer, which is 40 times of Ar + It is suggested that this is the reason why the etching did not reach the bottom of the SEI.

[0128] The C / O ratio of the 1C formation cell hard carbon (Figure 6b) is between that of the 2C and 1 / 10C cells, following the above-mentioned capacity and EIS resistance trends.

[0129] Under high-current formation treatment, a thin or otherwise more ion-conductive SEI layer gradually forms on the carbon surface, and it can be concluded that the SEI has shorter Na + diffusion paths and more ion-conductive characteristics. The shorter Na + diffusion paths can be the result of one or more combinations of these factors, which result in a thinner, or more porous or lower-density SEI, or an overall shorter ion diffusion path. Since the conventional view for SEI formation from organic electrolytes indicates that a thick and comprehensive SEI layer is required to ensure optimal battery performance, this was a surprising discovery. These results are completely contrary, and when formed using a super-concentrated ionic liquid electrolyte at a high current rate and fast formation time, a better SEI with improved ionic conductivity / shortened ion diffusion paths (thinner, or lower density, or more porous, or all of these characteristics, etc.) is generated, promoting the improvement of battery performance while reducing the electrolyte used for SEI formation.

[0130] XPS studies confirm that the 1 / 10C cells experience complete electrolyte decomposition (consistent with the cycle marker showing more irreversible capacity loss for SEI formation in the 1 / 10C cells), and more sulfur and atomic nitrogen elements are generated among the SEI components. Therefore, the SEI is considered to be much thicker, less porous, or denser than the more ion-conductive SEI of the 1C and 2C forming cells. The SEI from 1 / 10C, as a result of the characteristic of low ion conductivity, hinders the diffusion of Na + into the carbon graphite layer. Furthermore, as mentioned above, the greater the degree of electrolyte decomposition that occurs during the formation of the 1 / 10C cells, the more rapid the capacity decay will ultimately be in the long term due to the resulting electrolyte deficiency.

[0131] Example 6 - Cumulative Irreversible Capacity vs. Number of Cycles Figure 8(a) is a plot of irreversible capacity (mAh / g) versus cycle number for the 1 / 10C test cell and the 1C and 2C target cells for the first 1 - 10 polarization cycles including the first 5 formation cycles. The irreversible capacity data shows that for the 1C and 2C formation cells (cycles 1 - 5), the irreversible capacity loss is much less, but then (cycles 6 - 10), after the formation of a substantially complete and stable SEI is required, the further irreversible capacity loss is very low and very similar for each cell in cycles 6 - 10. This data indicates that for cells formed according to the formation method of the present invention, the charge that enters SEI formation during each formation cycle is less than that of the corresponding cycle of a test cell formed using a current density that matches the conventional low current density used in commercially available formation protocols. The fact that less charge is converted to SEI formation indicates that the SEI formed on the target cells in cycles 1 - 5 is different and / or improved compared to the SEI of the test cell. The difference / improvement can be one or more combinations of one or more of the following factors: the SEI is more ion - conductive (with respect to sodium ion transport); the SEI is thinner, more ion - conductive, and / or porous, and / or less dense, and / or has lower impedance, and / or has higher stability; the SEI has a similar thickness, is more ion - conductive, and / or porous, and / or less dense, and / or has lower impedance, and / or has higher stability, or results in an overall shorter ion diffusion path. A further important feature of the SEI formed by the formation process proposed by the present invention is its subsequent stability in maintaining low impedance (after the required formation cycles are completed), and the continuous charge transfer during subsequent cycles of the cell at the nominal C - rate.

[0132] Figure 9 shows the cumulative irreversible capacity from the first 5 formation cycles, indicating that among all three cells, the 1 / 10C formation test cell has the largest irreversible capacity, which may be related to a large amount of electrolyte decomposition associated with the formation of a thick or high-density SEI layer. In contrast, the 2C and 1C cells demonstrate much less cumulative irreversible capacity and much less overall electrolyte decomposition, suggesting that the formed SEI has shorter Na+ diffusion paths with one or more of the above characteristics (thin, low density, porous, etc.).

[0133] When the cells were cycled at the same lower current density after formation, the cells from the 2C formation showed the highest cumulative irreversible capacity in the 6th to 10th cycles, which may be due to ongoing SEI formation. Nevertheless, after 25 cycles, the cumulative irreversible capacity from the 1 / 10C formation test cell has a maximum value of 70 mAh / g, indicating that the SEI formed by high current density formation and its properties have a significant impact on long-term cycling compared to the low current density formation test cells, and thus result in better capacity utilization for the formed target cells.

[0134] Example 7 - NMR Study To further investigate the interfacial properties of the HC electrodes under different formation protocols, solid-state 23Na and 19F magic-angle spinning (MAS) nuclear magnetic resonance (NMR) measurements were performed. All the electrodes were cycled in a super-concentrated ionic liquid electrolyte under various C-rates (i.e., 1 / 10C, 1C, and 2C). Control samples immersed in the electrolyte (denoted as immersed HC) were also included. All the spectra in Fig. 11a are normalized with respect to the electrolyte residue signal of 23Na, and all the spectra in Fig. 11b are normalized by the electrolyte residue signal (S-F) of the 19F spectrum. This is because it is difficult to quantitatively measure the sample amount due to the very small amount contained and it is difficult to add the boron nitride filler material to a small sample rotor. In Fig. 11a, an IL residue signal at -8.9 ppm was observed for all the electrodes, indicating that the IL residue cannot be easily washed away by an organic solvent. However, the inventors observed another peak at +9.2 ppm for the three formed electrodes, but no signal was seen for the immersed HC. This peak is assigned to NaF from the electrolyte decomposition products according to the literature. Interestingly, the inventors normalized the spectra with the IL residue, and it was found that the S / N ratio was very low for the 2C sample, and 1 / 10C had the highest value. This serves as evidence that the amount of SEI material on the 2C electrode is less, which is in good agreement with the results of EIS and XPS. For further investigation of the SEI composition, 19F MAS spectra were also tested. Fig. 11b shows three fluorine peaks due to the S-F groups from the electrolyte residue, the C-F groups from the MAS rotor material (i.e., the background signal not originating from the sample), and the NaF peak from the electrolyte decomposition. The NaF peak appears only for the three formed electrodes, but not for the HC immersed electrode. On the other hand, the S / N ratio of the 2C cell is the lowest, which is consistent with the 23Na spectrum.

[0135] Conclusion The inventors have developed an optimized formation protocol for hard carbon anodes for sodium-ion batteries when using super-concentrated IL electrolytes. By adjusting the current density within a certain potential range, the total formation time can be shortened to 1 / 38 while maintaining the electrochemical performance. Batteries under high C-rate (2C, C = 300 mAh / g) are, furthermore, more performant than their counterparts formed at low C-rate when cycled at 1 / 2C over long-term tests. Comprehensive analysis of the SEI layer by three-electrode EIS, XPS and NMR techniques has demonstrated that a thinner and more ion-conductive SEI structure has been developed by the inventors' optimized formation protocol. The interfacial resistance from the EIS tests of the high-rate formation cells also confirms the promotion of the Na+ diffusion rate across the SEI layer, which is a result of the formation of a highly ion-conductive SEI layer.

[0136] In conclusion, three formation protocols were investigated for NIB hard carbon anodes in super-concentrated ionic liquid electrolytes. In contrast to the method established for carbonate-based solvents [6,41], the high current density 2C formation protocol resulted in the highest specific capacity with the lowest EIS resistance during subsequent cycles at 1 / 2C current density, compared to the 1C and 1 / 10C formation protocols. Variable cycling conditions (e.g., 1 / 5C for long-term cycling) were not affected by high C-rate treatment, demonstrating the need to apply the high-rate formation protocol for conditioning in these electrolytes. XPS and NMR analysis revealed that a thinner SEI layer is formed after high C-rate formation, enabling the promotion of Na+ charge transfer and diffusion across the electrolyte / electrode interface. A slight but significant difference in the SEI composition (absence of reduced S and N species after high-rate formation) may also contribute to the performance improvement of the 2C sample. The optimized formation protocol is more performant than conventional low-rate formation in sodium-ion batteries. Such a new formation protocol could have a major impact on industrial applications, enabling the use of alternative safe electrolyte materials such as ILs by reducing the formation time and manufacturing costs.

Claims

1. It is an electrochemical cell, A superconcentrated sodium salt-containing ionic liquid electrolyte comprising at least one ionic liquid and at least one sodium salt, wherein the concentration of the sodium salt in the superconcentrated sodium salt / ionic liquid electrolyte is 75% or more of the saturation limit of the sodium salt in the electrolyte, A counter electrode or positive electrode comprising an electrochemically oxidizable material, wherein during a sodiumization step or charging step, sodium ions are released from the material into the superconcentrated electrolyte in the cell, A hard carbon working electrode or negative electrode, wherein during a sodiumization step or charging step, the hard carbon negative electrode absorbs sodium ions received from the ultra-concentrated electrolyte as reduced sodium stored in the hard carbon. Equipped with, (a) The hard carbon anode is For example, low-order / short-range sulfur (S) identified by XPS testing and etching investigations. ８ ２－ ~S ２－ ) Contains virtually no seeds, And optionally, For example, substantially free of atomic nitrogen species, as identified by XPS testing and etching investigations. It comprises a formed solid electrode interface (SEI), And / or (b) The solid electrode interface (SEI) / hard carbon electrode has an interfacial resistance (R) associated with the SEI less than 200 ohms after the first formation cycle, as determined by electrochemical impedance spectroscopy. ｉｎｔ ) has Electrochemical cell.

2. The electrochemical cell according to claim 1, wherein the formed SEI is generated by reducing the electrolyte on the hard carbon electrode by polarizing the corresponding initial state cell for up to five formation cycles at a high constant current density in the range of 1 / 2C to 5C within the cell's entire voltage window.

3. The electrochemical cell according to claim 1, wherein the formed SEI is generated by the reduction of the electrolyte on the hard carbon electrode by polarizing the corresponding initial state cell for up to two formation cycles at a constant current with a current density of 2C within the cell's entire voltage window.

4. The aforementioned SEI composition An outer layer containing more C-N species than the SEI of the formed test cell, as indicated by XPS testing and / or etching investigation, The ratio of C to O after 40 minutes of etching is 1 or greater, and the composition after 40 minutes of Ar+ cluster etching reflects the initial state of hard carbon. An electrochemical cell according to claim 1, having one or more of the following:

5. The electrochemical cell according to claim 1, wherein, after the completion of the SEI formation cycle, during a nominal C-rate cycle using a C-rate of 1 / 2C or slower, the cell exhibits a Coulomb efficiency (CE) of 99.2% or more, preferably 99.3% or more, preferably 99.4% or more, preferably 99.5% or more, preferably 99.6% or more, preferably 99.7% or more, preferably 99.8% or more, preferably 99.9% or more.

6. The ionic liquid of the superconcentrated electrolyte is preferably derived from one or more of the ionic liquid anions and the sodium salt anions. － Anion or TFSI － An electrochemical cell according to claim 1, comprising an anion.

7. The ionic liquid of the super-concentrated electrolyte is a pyrrolidinium cation, preferably an alkylated pyrrolidinium cation, preferably a 1-methyl-1-alkyl-pyrrolidinium cation (C ３ mpyr ＋ ), most preferably a 1-methyl-1-propyl-pyrrolidinium cation (C ３ mpyr ＋ ), and the electrochemical cell according to claim 1.

8. The electrolyte is Na[TFSI] and [C ３ [mpyr] [TFSI], Na [TFSI] and [C ４ C ３ [pyr] [TFSI], Na [TFSI] and [C] ３ [mpyr] [FSI], Na [TFSI] and [C] ４ C ３ [mpyr] [FSI], Na[FSI] and [C] ３ [mpyr] [TFSI], Na [FSI] and [C] ４ C ３ [mpyr] [TFSI], Na [FSI] and [C] ３ [mpyr] [FSI], Na[FSI] and [C] ４ C ３ The electrochemical cell according to claim 1, comprising [mpyr] [FSI], or a combination thereof.

9. The aforementioned electrolyte is [C ３ [mpyr] [FSI] ionic liquid or [C ３ The electrochemical cell according to claim 1, which is an mpyr[TFSI]ionic liquid containing at least about 50 mol% of a NaFSI salt.

10. The electrochemical cell according to claim 1, wherein the ultra-concentrated electrolyte and the formed SEI do not contain carbonate species.

11. The electrochemical cell according to claim 1, wherein the cell and the counter electrode in the full cell configuration include or are composed of a material capable of reversibly intercalating / deintercalating sodium ions within the atomic structure of the cell and the counter electrode, absorbing / desorbing sodium ions by reversible oxidation / reduction reactions, or promoting alloying / dealloying reactions with sodium ions.

12. The counter electrode or positive electrode is Na ０．４５ Ni ０．２２ Co ０．１１ Mn ０．６６ O ２ Na ２／３ Fe ２／３ Mn ２／３ (O3), Na ２／３ Fe ２／３ Mn １／３ O ２ (P2), olivine-type NaFePO ４ Na ｘ FePO ４ , silicate, or general formula Na ２ M ２ (XO ４ ) ３ The electrochemical cell according to claim 1, comprising or composed of a NASICON-type phase (M = transition metal and X = P, S).

13. The use of a superconcentrated ionic liquid electrolyte for forming a SEI on a hard carbon electrode in a cell comprising a hard carbon working electrode or a negative electrode, wherein the superconcentrated ionic liquid electrolyte contains a sodium salt concentration of 75% or more of the saturation limit of the sodium salt in the electrolyte, where, (a) The hard carbon anode is For example, low-order / short-range sulfur (S) identified by XPS testing and etching investigations. ８ ２－ ~S ２－ ) Contains virtually no seeds, And optionally, For example, substantially free of atomic nitrogen species, as identified by XPS testing and etching investigations. It comprises a formed solid electrode interface (SEI), And / or (b) The solid electrode interface (SEI) / hard carbon electrode has an interfacial resistance (R) associated with the SEI of 200 ohms or less after the first formation cycle, as determined by electrochemical impedance spectroscopy. ｉｎｔ ) has use.

14. The electrolyte contains at least about 50 mol% of NaFSI salt [C ３ The use according to claim 13, wherein the mpyr[FSI]IL is

15. A step of providing a cell in an initial state based on sodium ion electrochemistry, wherein the fresh cell is A counter electrode or positive electrode comprising an electrochemically oxidizable material, wherein during a sodiumization step or charging step, sodium ions are released from the material into the superconcentrated electrolyte within the cell. A hard carbon working electrode or hard carbon anode, wherein during a sodiumization step or charging step, sodium ions received at the hard carbon anode from the ultra-concentrated electrolyte are absorbed as reduced sodium stored in the hard carbon, and Ultra-concentrated sodium salt-containing ionic liquid electrolyte containing at least one ionic liquid and at least one sodium salt Equipped with, The sodium ion concentration in the superconcentrated ionic liquid electrolyte is 75% or more of the saturation limit of the sodium ions in the electrolyte. Steps and A step to manufacture a target cell by generating SEI formed by the reduction of the electrolyte on the hard carbon electrode by polarizing the initial state cell for up to five formation cycles at a high constant current density in the range of 1 / 2C to 5C, preferably 2C, within the entire voltage window of the cell, (a) The hard carbon anode is For example, low-order / short-range sulfur (S) identified by XPS testing and etching investigations. ８ ２－ ~S ２－ ) Contains virtually no seeds, And optionally, For example, substantially free of atomic nitrogen species, as identified by XPS testing and etching investigations. It comprises a formed solid electrode interface (SEI), And / or (b) The solid electrode interface (SEI) / hard carbon electrode has an interfacial resistance (R) associated with the SEI of 200 ohms or less after the first formation cycle, as determined by electrochemical impedance spectroscopy. ｉｎｔ ) has Steps and A cell formation method, including the following.

16. The cell formation method according to claim 15, wherein cell polarization is produced by charging and discharging the cell with a constant current.

17. The cell formation method according to claim 15, wherein the cell has a half-cell configuration or a full-cell configuration.

18. The cell formation method according to claim 15, wherein the nominal C rate of the target cell formed is 1 / 2C or 1 / 5C.

19. The cell formation method according to claim 15, wherein the formed target cell is further cycled under constant current conditions at a nominal C rate of 1 / 2 sC or slower for a maximum of 50 cycles, preferably a maximum of 100 cycles, and during the nominal C rate cycle, the formed target cell exhibits a Coulomb efficiency (CE) of 99.2% or more, preferably 99.3% or more, preferably 99.4% or more, preferably 99.5% or more, preferably 99.6% or more, preferably 99.7% or more, preferably 99.8% or more, preferably 99.9% or more.

20. The process further includes recording a charge-discharge cycle during each forming polarization cycle, wherein the observation of one or more markers in the recorded corresponding charge-discharge cycle related to substantially complete and stable SEI formation on the hard carbon electrode indicates complete formation, and the markers are (a) Discharge-charge curve markers / characteristics - Observation of the absence of a significant electrolyte reduction gradient and / or a substantially static electrolyte reduction gradient in the recorded voltage-capacitance cycle. Observation of invariant features / stability of features in the recorded charge-discharge cycle, - Absence of the “low voltage” feature in the recorded charge-discharge cycle (e.g., less than approximately 1.5V after the initial cycle of the exemplified half-cell), - Observation of reaching the cutoff discharge potential limit of the cell and the resulting discharge capacity without the presence / appearance of curve features related to the insertion of sodium ions into the negative active electrode, i.e., in the absence of intercalation features (e.g., sodiumization / lithification), (b) Electrochemical impedance spectroscopy (EIS) markers / characteristics - For example, evidence of low and / or stable EIS spans in electrochemical impedance studies, including recorded Nyquist plot analysis. - Evidence of low impedance SEI formation, - Evidence of SEI having higher ion conductivity than the formed test cell, and (c) Differential volume analysis markers / characteristics - Absence of reduction / decomposition peaks of electrolytes and / or reduction / decomposition peaks of electrolyte additives in the recorded differential capacity curve (dQ / dV), - The absence of a metal ion insertion peak indicates that a complete discharge occurs before metal ion insertion, and - In the case of high-rate cells, Na before reaching full charge capacity ＋ Instead of an insertion peak signal cutoff, Na is used to target the active site of the hard carbon material at a higher rate. ＋ Presence of peaks indicating ion adsorption and / or chemisorption. The cell formation method according to claim 15, including the method described in claim 15.

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