Current storage cell with electrolyte volume adapted to pore volume at high states of charge

Abstract

The invention relates to a current storage cell, designed as a lithium-ion storage cell or sodium-ion storage cell, comprising an anode, a cathode, a separator located between the anode and the cathode, and an electrolyte solution which is arranged in pores of the anode, cathode and separator, wherein the electrolyte solution comprises a conducting salt and at least one organic solvent, wherein the storage cell has, at a state of charge of 75% to 100%, a volume factor fv between 0.9 and 1.05, preferably 0.95 to 1.00, wherein the volume factor fv is defined as the ratio (volume of the electrolyte solution) / (sum of the volumes of the pores in the anode, the volumes of the pores in the cathode and the volumes of the pores in the separator at the state of charge of 75% to 100%). Such a current storage cell has reduced ageing compared to conventional current storage cells.

Description

[0001] Power storage cell with electrolyte volume adapted to pore volume at high charge states

[0002] The present invention relates to a power storage cell with an electrolyte volume adapted to the pore volume at high charge levels. The present invention further relates to a use of the power storage cell and a method for determining the pore volume of the electrodes of a power storage cell at high charge levels.

[0003] In a power storage cell designed as a lithium-ion storage cell or a sodium-ion storage cell, ion transport between the anode and the cathode is enabled by an electrolyte solution comprising a lithium conducting salt or a sodium conducting salt and at least one solvent. This electrolyte solution is present in the pores of the anode and cathode, as well as in the pores of the separator located between the anode and the cathode. When filling a power storage cell with an electrolyte solution, the electrolyte solution is added in excess of the available pore volume in the power storage cell to obtain a power storage cell with high capacity and a long service life.A known mechanism for the aging of energy storage cells is the deposition of lithium at the anode, known as "lithium plating" in lithium-ion storage cells, or the deposition of sodium at the anode, known as "sodium plating" in sodium-ion storage cells. This deposition of metal at the anode leads to a rapid loss of the energy storage cell's capacity.

[0004] The object of the present invention is to provide a power storage cell, designed as a lithium-ion storage cell or sodium-ion storage cell, which is improved with respect to the above-mentioned disadvantages. A further object of the present invention is to specify a use of the power storage cell according to the invention for a rapid charging method. The present invention also provides a method for determining the pore volume of a power storage cell at high charge states of 75% to 100%.

[0005] One aspect of the present invention provides a power storage cell designed as a lithium-ion storage cell or as a sodium-ion storage cell. The power storage cell comprises an anode, a cathode, a separator located between the anode and the cathode, and an electrolyte solution present in the pores of the anode, the cathode, and the separator. The electrolyte solution comprises a conductive salt and at least one organic solvent. At a charge state of 75% to 100%, the storage cell has a volume factor f v between 0.9 and 1.05, preferably between 0.95 and 1.00. Most preferably, the volume factor f v 1. The volume factor f v is the ratio of the volume of the electrolyte solution to the total sum of the pore volumes in the anode, cathode and separator at a state of charge of 75% to 100% of the storage cell according to the formula: where electrolyte volume is the total volume of the electrolyte solution in the storage cell, anode pore volume is the pore volume of the anode, cathode pore volume is the pore volume of the cathode and separator pore volume is the pore volume of the separator.

[0006] The pore volumes of the anode, cathode, and separator can be determined using helium pycnometry, for example. Helium pycnometry yields the skeletal density of the respective materials, without taking the pores into account. This skeletal density can be compared with the bulk density, the quotient of the sample mass and the sample volume, to determine the empty volume and the respective pore volume of the anode, cathode, and separator. The volumes of the various materials can be determined, for example, using a thickness gauge, and the mass using a precision balance. To determine both the skeletal density and the bulk density and volume of the sample, three independent measurements can be carried out at 10 different locations on the electrode.The skeletal density, bulk density, and volume of the sample can then be determined as the average of the measurement results. According to the invention, pore volumes, particularly of the anode and cathode, are determined at a high charge state of 75% to 100%.

[0007] The state of charge of a storage cell (SOC) indicates the capacity available in a storage cell in relation to a fully charged state with a charge level of 100% and an uncharged state with a charge level of 0%. In particular, a charge level of 0% occurs at a defined lower cut-off voltage and a charge level of 100% occurs at a defined upper cut-off voltage. In the case of a lithium-ion storage cell, for example, the lower cut-off voltage can be 2.8 V and the upper cut-off voltage 4.2 V. The lower cut-off voltage and the upper cut-off voltage of a storage cell can be specified in particular by a battery management system.

[0008] The pore volumes in the anode, cathode, and separator can refer in particular to a formed storage cell in which the storage cell has already been charged and discharged after production, so that boundary layers form, particularly on the anode and cathode. On the anode, this boundary layer is referred to in particular as SEI ("solid electrolyte interphase") and on the cathode as CEI ("cathode electrolyte interphase"). During formation, an irreversible volume expansion of the electrodes also occurs, in particular through the formation of the SEI or CEI and through a partial rearrangement of the particles, particularly in highly compressed electrodes.

[0009] Due to volume changes of the electrochemically active materials in the anode and, where applicable, the cathode, the pore volume of the electrodes can change during the charging process. The inventors have discovered that, due to the change in the pore volumes of the electrodes, electrolyte solution with a lower conducting salt concentration than the originally used conducting salt concentration can be displaced from the electrodes. In particular, the pore volume of the anode can decrease during charging of the storage cell depending on the state of charge. This can be attributed in particular to an increase in the volume of the electrochemically active anode materials during charging of the storage cell. As a result, different concentrations of the conducting salt can develop in the electrolyte solution in different regions of an electrode during extended operation of the storage cell after several charging and discharging processes.These different concentrations of conducting salt in the electrodes can lead to premature aging of the storage cell and, in particular in the case of a lithium-ion storage cell, to the deposition of lithium and, in the case of a sodium-ion storage cell, to increased deposition of sodium on the anodes.

[0010] These different concentrations of conducting salt can develop, in particular, when an excess of electrolyte solution is added compared to the available pore volume of the electrodes and the separator, as is normally the case. The pore volume of the electrodes and the separator is conventionally determined in the uncharged state. With a large excess of electrolyte solution, particularly large amounts of electrolyte solution with a reduced conducting salt concentration compared to the original concentration can be displaced from the electrodes, in particular from the anode, during charging of the storage cell, as the inventors have discovered. To reduce or prevent this, the volume factor f vadjusted so that it moves within a range of 0.9 to 1.05, preferably 0.95 to 1.00, with a lower pore volume, particularly of the anode, at high charge states of 75% to 100%.

[0011] According to the invention, the sum of the pore volumes of the anode, the pore volumes of the cathode, and the pore volumes of the separator are determined at high charge levels of 75% to 100%. At these high charge levels, the pore volume of the anode, in particular, is greatly reduced compared to a charge level of 0%, since at these high charge levels the electrochemically active material of the anode, for example graphite, silicon oxide, or silicon, expands particularly strongly, and therefore the porosity of the anode at these charge levels differs significantly from the porosity of the anode at a charge level of 0%. The high charge level is particularly preferably 80% to 100%, further preferably 90% to 100%.

[0012] With such a small volume factor f v During charging of the storage cell, less or no electrolyte solution with reduced conducting salt concentrations can be displaced from the electrodes, especially from the anode. Therefore, the formation of different conducting salt concentrations in different areas of the electrode, especially the anode, during operation of the storage cell is reduced or prevented. In particular, with such a small volume factor f v a large part of the electrolyte solution or the entire electrolyte solution remains in the electrodes even at high charge levels of 75% to 100% of the storage cell, although the pore volumes of the electrodes, in particular the anode, decrease during charging and therefore less pore volume is available for the absorption of the electrolyte solution.

[0013] The volume factor f is particularly preferred vset to 1. This means that the power storage cell has exactly as much volume of electrolyte solution as the sum of the pore volumes of the anode, cathode, and separator at high charge states of 75% to 100%. This makes it particularly effective to reduce or prevent the displacement of electrolyte solution depleted of conducting salt during charging of the power storage cell. Furthermore, it is preferably possible to determine experimentally, for example by means of helium pycnometry, at which charge state the smallest sum of the pore volumes of the anode, cathode, and separator is achieved within the range of high charge states of 75% to 100%. The volume factor can then be set to this minimum sum of the pore volumes of the anode, cathode, and separator.

[0014] The power storage cell can comprise an electrode coil or an electrode stack. The electrode coil or the electrode stack can be housed in a housing, wherein the electrode coil and the electrode stack comprise the above-described anode, cathode, and separator. The electrode coil or the electrode stack can extend in the housing along a longitudinal axis of the power storage cell. In an electrode coil, in particular a strip-shaped anode, a strip-shaped cathode, and a first strip-shaped separator located between the strip-shaped anode and the strip-shaped cathode, together with a second further strip-shaped separator, can be wound in layers around a winding core. In an electrode stack, individual anodes, cathodes, and separators located therebetween can be layered one on top of the other as plate-shaped elements.

[0015] Such power storage cells can achieve particularly high capacities.

[0016] Preferably, the electrode coil or the electrode stack has an extension of at least 3 cm along the longitudinal axis of the power storage cell. Power storage cells with such extensions along their longitudinal axis have particularly high capacities due to their size. Due to the volume factor f set according to the invention, vThe formation of different areas in the electrode, especially the anode, with different concentrations of conductive salt along the longitudinal axis of the power storage cell occurs only to a minor extent or not at all. Due to the large extension of at least 3 cm along the main axis of the power storage cell, different concentrations of conductive salt along this main axis cannot normally be compensated by diffusion within a very short time during operation of the power storage cell and therefore lead to premature aging of the power storage cell.

[0017] Examples of energy storage cells with a length of at least 3 cm along the longitudinal axis include round cells of the 18650 type with a length of 6.5 cm along the longitudinal axis. Other examples of energy storage cells with such a length are cells of the 4680 type with a length of 8 cm, cells of the 4695 type with a length of 9.5 cm along the longitudinal axis, and cells of the 46120 type with a length of 12 cm along the longitudinal axis. Such energy storage cells can be used particularly in battery-electric vehicles.

[0018] The housing of the energy storage cell can be arranged circumferentially along the longitudinal axis around the circumference of the electrode coil or electrode stack. The housing can contact the main surfaces of the anode and cathode all the way around. The housing can circumferentially delimit the electrode stack or electrode coil. The housing can span the electrode stack or electrode coil. In particular, a separator layer can be present between the housing and the respective electrodes for electrical insulation. Such a housing, in which the electrodes or separators of the electrode coil or electrode stack are contacted by the housing, allows for a particularly compact design of the energy storage cells.

[0019] The housing can, in particular, comprise a metallic outer shell or a plastic outer shell. The housing can, in particular, be cylindrical, with poles for electrically contacting the power storage cell located at the opposite ends of the cylinder. The housing can also comprise a flexible foil, for example, made of aluminum, in which the electrode stack is firmly clamped. Such housings are used, for example, in pouch cells.

[0020] Since the electrode stack or electrode coil is clamped in the housing, there is no room for the electrodes to expand perpendicular to the longitudinal axis of the power storage cells during charging. In such power storage cells, conventional charging of the power storage cell with an excess of electrolyte solution therefore particularly easily leads to a displacement of the electrolyte solution from the electrode stack or electrode coil, thus leading to the formation of regions with different conducting salt concentrations during operation of the power storage cell. This problem can be avoided due to the volume factor f set according to the invention. v reduced or prevented.

[0021] In a power storage cell according to the invention, at a state of charge of 75% to 100%, at most 4%, preferably at most 3%, more preferably at most 1% of the total volume of the electrolyte solution in the housing can be located outside the electrode coil or the electrode stack. With such a small amount of free electrolyte solution outside the electrode coil or the electrode stack, the formation of regions with different conducting salt concentrations in the electrodes, in particular the anode, only occurs to a minor extent. Particularly preferably, there is no electrolyte solution outside the electrode coil or the electrode stack. This can particularly reliably prevent the formation of different regions in the electrodes which have different conducting salt concentrations. In particular, no electrolyte solution will then leak from the electrode stack or the anode at high states of charge.Electrode coils can be pressed if the volume factor is a maximum of 1. The presence and quantity of electrolyte solution in the housing outside the electrode coil or the electrode stack can be determined, for example, using computed tomography (CT).

[0022] In the power storage cell, the anode, the separator, and the cathode can have a first and a second edge region extending along the longitudinal axis, and a central region extending between the first and second edge regions along the longitudinal axis. The first edge region, the second edge region, and the central region located therebetween can each have the same extent along the longitudinal axis. Furthermore, the first edge region, the second edge region, and the central region located therebetween each have equal areas. The inventors have discovered that in conventional power storage cells, after several charging and discharging processes, the electrolyte solution in the pores of the first edge region, in the pores of the second edge region, and in the pores of the central region, for example the anode, have different concentrations of conductive salt.This can be achieved by adjusting the volume factor f according to the invention. v reduced or prevented.

[0023] An embodiment of a power storage cell according to the invention can, after at least 50 charging and discharging processes (average C-rate during charging and discharging at least 1C, SOC range 0-100% each, temperature 25 °C) after discharging to the cut-off voltage and resting for at least 1 hour and at most one day, have a concentration of Li + in the first edge region and in the second edge region, which differ by at most 30%, preferably by at most 20%, more preferably by at most 10% with respect to the original concentration of Li + in the electrolyte solution. The average C-rate can be determined, in particular, by the average values ​​of the C-rates occurring during charging over a period of 1 hour.

[0024] According to a further embodiment of a power storage cell according to the invention, this can, after at least 50 charging and discharging processes (average C-rate during charging and discharging at least 1C, SOC range 0-100% in each case, temperature 25 °C) after discharging to the cut-off voltage and resting for at least 1 hour and at most one day, have a concentration of Li + in the first and second edge region, which differ by a maximum of 50%, preferably a maximum of 40%, more preferably a maximum of 20% from the concentration of Li + in the middle range. Preferably, the power storage cell can contain the above-mentioned concentrations of Li + after at least 130 charging and discharging cycles.

[0025] The pore volume of the anode of the power storage cell can decrease, particularly during the charging process, depending on the state of charge. The anode can, in particular, comprise an electrochemically active anode material. An electrochemically active anode material is understood, in particular, to be a material capable of absorbing and releasing lithium ions if the power storage device is a lithium-ion power storage device. The pore volume of the anode of the power storage cell can decrease, particularly during the charging process, depending on the state of charge. This can be attributed, in particular, to the fact that the volume of the electrochemically active anode material increases during the charging process due to intercalation / alloy formation of lithium into the electrochemically active anode material, for example, graphite or silicon oxides, silicon, or mixtures thereof.In particular, the electrochemically active anode material can be selected from a group consisting of: synthetic graphite, natural graphite, graphene, mesocarbon, doped carbon, hard carbon, soft carbon, fullerene, silicon-carbon composites, silicon, surface-coated silicon, silicon suboxide, silicon alloys, aluminum alloys, indium, tin alloys, cobalt alloys, and mixtures thereof. Preferably, the electrochemically active anode material can be selected from a group consisting of: graphite, silicon oxides, and silicon, or mixtures thereof. Such electrochemically active anode materials are particularly suitable for providing lithium-ion storage cells with high capacity.

[0026] In a sodium-ion storage cell, the anode can comprise an electrochemically active anode material. The electrochemically active anode material in a sodium-ion storage cell can be designed to absorb and release sodium ions during charging and discharging. The electrochemically active anode material in a sodium-ion storage cell can, in particular, comprise hard carbon, carbon, for example in the form of graphite, which, similar to a lithium-ion storage cell, can absorb and release sodium ions through intercalation. Similar to the lithium-ion storage cell, this can lead to a change in the pore volume of the anode in the sodium-ion storage cell.

[0027] In the power storage cell, the pore volume of the cathode can increase during the charging process depending on the state of charge. The cathode can, in particular, comprise an electrochemically active cathode material. In a lithium-ion power storage cell, an electrochemically active cathode material is understood to mean, in particular, a cathode material capable of releasing and reabsorbing lithium ions during charging and discharging. In a lithium-ion power storage cell, the electrochemically active cathode material can be selected from a group consisting of: lithium transition metal oxides such as lithium cobalt oxide (UCOO2), lithium nickel cobalt manganese compounds (known by the abbreviation NCM or NMC), for example UCOO2, LiNiO,33Coo,33Mno,33O2, lithium nickel cobalt aluminum oxides (NCA), lithium olivines such as lithium iron phosphate (LFP), lithium spinels such as lithium manganese oxide spinel (LMO).Lithium manganese nickel spinel (LNMO) or combinations thereof, further preferably wherein the electrochemically active cathode material is selected from a group consisting of: lithium nickel cobalt manganese compounds.

[0028] As cathode material, for example, materials containing sodium ions such as phosphates and diphosphates, for example sodium iron phosphates or compounds such as Na2 / 3Fei / 2Mni / 2O2 can be used in a sodium ion power storage cell.

[0029] In addition to the electrochemically active anode and cathode materials, the materials for the anode and cathode can also contain other materials, such as binders and conductive additives. Binders can include, for example, carboxymethylcellulose and / or polyvinylidene fluoride. The electrolyte solution for a lithium-ion energy storage cell can comprise a lithium salt as the conductive salt and at least one organic solvent.

[0030] The electrolyte solution for a sodium-ion power storage cell may comprise a sodium salt as a conductive salt and at least one organic solvent.

[0031] In a lithium-ion power storage cell, the lithium salt can preferably be selected from a group consisting of: LiPFe, LiAsFe, LiCl, ÜCF3SO3, lithium bis(trifluoromethylsulfonyl)amide or combinations thereof.

[0032] In a sodium ion power storage cell, the sodium salt may preferably be selected from a group consisting of: NaPFe, NaCIO4, Na-bis(trifluoromethane)sulfonimide, Na-bis(fluorosulfonyl)imide, Na-difluoro(oxalato)borate, Na-bis(oxalato)borate or combinations thereof.

[0033] The organic solvent may, in particular, comprise an organic polar solvent. The organic solvent may, in particular, be selected from a group consisting of: C2 to C4 cyclic esters of carbonic acid, for example propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, lactones, for example γ-, δ-, and ε-lactone, or combinations thereof.

[0034] At a 0% state of charge, the porosity, the ratio of pore volume to total volume of the electrode material, can range from 20% to 30% for the anode and from 20% to 26% for the cathode. The separator typically has a higher porosity of 30% to 60%.

[0035] The present invention also relates to the use of a power storage cell according to the invention, as described above, for rapid charging. Rapid charging is defined as charging at an average C-rate of at least 1C. The C-rate is the ratio of the charging current of a power storage cell in amperes (A) to the capacity of the power storage cell (Ah). Due to the volume factor set according to the invention, power storage cells according to the invention can be charged particularly quickly using rapid charging methods without causing excessive aging of the power storage cells.

[0036] The present invention further relates to a method for determining the pore volume of a power storage cell at charge levels of 75% to 100%. The power storage cell comprises an anode, a cathode, and a separator located between the anode and the cathode, with electrolyte solution located in the pores of the anode, cathode, and separator. The electrolyte solution comprises a conductive salt and at least one organic solvent. The method comprises the following steps:

[0037] A) Filling a plurality of power storage cells each with a volume of electrolyte solution that is greater than the pore volume of the power storage cell at charge states of 75% to 100%,

[0038] B) charging and discharging the plurality of power storage cells to form the electrodes, and then,

[0039] C) charging the plurality of power storage cells to different charge levels in the range of 75% to 100%, and

[0040] D) Opening the plurality of charged power storage cells and determining the pore volumes of the anode and cathode at different charge states.

[0041] The multitude of energy storage cells have the same design and are identical, in particular, in terms of the properties and chemical compositions of the anode, cathode, and separator. In particular, all energy storage cells used as test cells have the same current collector materials for the electrodes and the same loading of the current collector foils with electrode material.

[0042] This makes it particularly easy to determine a range of pore volumes for different charge states of the power storage cells of the same design.

[0043] In particular, the pore volume of a wide variety of power storage cells with different electrochemically active materials can be determined particularly easily by means of the method according to the invention.

[0044] In order to determine the state of charge as accurately as possible within the range of 75% to 100%, at which the sum of the pore volumes of the anode, the cathode and the separator assume a minimum value, the values ​​for the state of charge within the range of 75% to 100% can be staggered more closely.

[0045] In process step D), the pore volumes of the anode, the separator, and the cathode can be determined, for example, using helium pycnometry. In particular, the pore volumes can be determined as described in the publication by Beuse et al., "Comprehensive Insights into the Porosity of Lithium-Ion Battery Electrodes: A Comparative Study on Positive Electrodes Based on LiNi0.6Mn0.2Co0.2O2 (NMC622)," Batteries 2021, 7(4), 70, which is hereby incorporated by reference. In process step A), the power storage cells can be filled as test cells, in particular, with a volume of electrolyte solution that is in any case greater than the sum of the pore volumes of the anode, the cathode, and the separator at charge states of 75% to 100%. This can be done, for example, by adding approximately 2 g of electrolyte solution per 1 Ah of cell capacity to the power storage cell.

[0046] In particular, in process step B), a defined volume of electrolyte solution can be added to a test cell. After formation in process step B), the cell can be opened and the solvent can be evaporated and extracted in a sealed box, for example at 45 °C and under a pressure reduced compared to normal conditions, for example 1 mbar, over a longer period of time, for example one month. The extracted electrolyte solution can be collected in a cold trap and its volume determined directly. Alternatively, the filled and subsequently emptied test cell can be weighed and the difference can be used to determine the density of the electrolyte solution (typically approx. 1.25 g / cm 3) the volume of electrolyte solution originally present in the formed test cell can be determined. The difference between the original volume and the volume of electrolyte solution present in the test cell after formation can be used to determine the volume of electrolyte solution consumed during cell formation for a specific test cell type.

[0047] Once the volume of electrolyte solution lost during formation and the sum of the pore volumes of the anode, the cathode and the separator at a state of charge of 75% to 100% is known, further power storage cells of the same type can be particularly easily filled before formation with a volume of electrolyte solution that is a sum of the electrolyte volume lost during formation with the sum of the determined pore volumes of the electrodes and the separator at the high states of charge.

[0048] The present invention also relates to a power storage cell, designed as a lithium-ion storage cell or sodium-ion storage cell, comprising an anode, a cathode, a separator located between the anode and the cathode, and an electrolyte solution arranged in pores of the anode, cathode, and separator, wherein the electrolyte solution comprises a conductive salt and at least one organic solvent. At a charge state of 75% to 100% of the power storage cell, at most 4 percent, preferably at most 3 percent, of the total volume of the electrolyte solution in the housing is located outside the electrode coil or the electrode stack. The anode, the cathode, and the separator located between the anode and the cathode are components of an electrode coil or electrode stack. The power storage cell further comprises a housing in which the electrode coil or electrode stack is located.Particularly preferably, when the power storage cell is at a charge level of 75% to 100%, there is no electrolyte solution outside the electrode coil or the electrode stack.

[0049] With such a power storage cell, the formation of areas of varying conducting salt concentrations during cell operation can be particularly effectively reduced or prevented, since only small amounts of electrolyte solution, or no electrolyte solution at all, are forced out of the electrode coil or electrode stack during cell operation. The high state of charge is particularly preferably between 80% and 100%, and more preferably between 90% and 100%.

[0050] The amount of electrolyte solution or the presence of electrolyte solution outside the electrode coil or the electrode stack can be particularly easily detected using a CT scan.

[0051] In the following, aspects of the present invention will be explained in more detail with reference to figures and exemplary embodiments. They show:

[0052] Figures 1a) to 1e) are schematic cross-sectional drawings of an anode and a cathode of a conventional power storage cell with exemplary conductive salt concentrations in the electrolyte solution during a charging process and a discharging process with the formation of regions of different conductive salt concentrations in the electrodes as determined by the inventors,

[0053] Figure 2 shows a graph with the different conducting salt concentrations in the edge regions and in a central region of the anode flanked by the edge regions,

[0054] Figure 3 shows a graph with a course of the conducting salt concentration in an edge region of the anode with a time course over a large number of charging and discharging cycles,

[0055] Figures 4a) and 4b) are photographs of an anode from a conventional power storage cell and an anode from a power storage cell according to the invention after 130 cycles, 50% of which were fast charge cycles, after opening the cells,

[0056] Figure 5 is a schematic cross-sectional drawing of a power storage cell with an electrode coil and electrolyte solution pressed out of the electrode coil,

[0057] Figure 6 shows a schematic top view of an unrolled anode after opening a power storage cell with the positioning of the samples in the first edge area, second edge area and in the middle area of ​​the anode to determine a spatially resolved conductive salt concentration within the anode, and Figure 7 shows a CT image of a conventional power storage cell with a volume factor above 1.05 at a high state of charge with electrolyte solution pressed out of the coil, and

[0058] Figure 8 shows a CT image of a power storage cell according to the invention with a volume factor of maximum 1 at a high state of charge, wherein no electrolyte solution can be seen being squeezed out of the winding.

[0059] In the following, elements with the same function are given the same reference symbols.

[0060] Figures 1a) to 1e) show schematic cross-sectional drawings of exemplary concentrations of the conducting salt in the electrolyte solution in the pore volumes of the anode 2 and the pore volumes of the cathode 3 of an electrode coil of a conventional lithium-ion energy storage cell prior to charging at a state of charge of 0%. The anode 2 and the cathode 3 are separated from each other by a separator layer 4. An excess of electrolyte solution with a conducting salt was introduced into the lithium-ion energy storage cell compared to the available pore volume of the anode, the cathode, and the separator layer, so that the volume factor f vabove the volume factor set according to the invention is at a value of 1.08. Due to the excess of electrolyte solution, a small portion 27 of the electrolyte solution is already located outside the electrode coil. As shown in Figure 1a), the concentration of the conducting salt is 1 mol / l. The anode 2 has a first edge region 2A and a second edge region 2C at the respective poles of the lithium-ion storage cell, which flank a central region of the anode 2B located in the middle (poles not shown in Figures 1a) to 1e)). Analogously, the cathode 3 has a first edge region of the cathode 3A and a second edge region of the cathode 3C, which flank a central region of the cathode 3B. The respective edge regions and the central regions extend along a longitudinal axis 7 of the power storage cell 1.Before the charging process begins, the concentration of the lithium conducting salt is 1 mol / l in all areas of the electrodes of the electrode coil and in the part 27 of the electrolyte solution located outside the electrode coil. A distance 26 perpendicular to the longitudinal axis 7 between the anode 2 and the cathode 3 across the separator layer 4 is typically a few hundred microns, for example, 200 microns. In contrast, the anode 2, the cathode 3, and the separator 4 extend along the longitudinal axis 7 over a larger distance 25, which is usually at least a few centimeters, for example, at least 3 cm.

[0061] During the rapid charging process, which is shown in Figure 1b), the volume of the electrochemically active anode material, for example, graphite, expands, as lithium is intercalated into the anode during the charging process. This expansion of the volume of the electrochemically active anode material leads to a reduction in the pore volume in the anode, so that the electrolyte solution located in the pores of the anode is displaced from the anode 2, as indicated by the arrows 5. Since Li +is taken up from the electrolyte solution into the anode, the electrolyte solution present there is depleted in lithium conducting salt, the concentration being, for example, 0.5 mol / l, as shown in Figure 1b). As the inventors of the present invention were able to show, the electrolyte solution 6 displaced from the anode is therefore also depleted in lithium conducting salt and has a concentration of, for example, 0.5 mol / l. In particular, the electrolyte solution depleted in lithium conducting salt can be displaced from the electrode coil at both ends of the anode 2, so that the electrolyte solution 6 displaced at both ends of the electrode coil collects outside the electrode coil. In contrast, at the cathode 3, lithium ions are released from the electrochemically active cathode material, resulting in an increased concentration of lithium conducting salt in the electrolyte solution adjacent to the cathode.The concentration of lithium conducting salt there, for example, is 1.5 mol / l. Since the permeability of the electrolyte solution along the longitudinal axis 7 is greater for the anode than for the cathode, and since the pore volumes of the pores in the anode are primarily reduced, electrolyte solution 6 depleted of lithium conducting salt is predominantly displaced from the anode.

[0062] Figure 1c) shows the electrode coil with the anode 2 and the cathode 3 after completion of the charging process shown in Figure 1b) and after a rest period of at least one hour at a charge state of 100%. The concentration of lithium conducting salt in the anode 2 and in the cathode 3 has equalized again over the short distance 26 between the anode and cathode through diffusion and is slightly higher than the original lithium conducting salt concentration and is, for example, 1.1 mol / l. In contrast, the concentration of lithium conducting salt in the electrolyte solution 6 displaced from the electrode coil, as described in Figure 1b), is lower than the original conducting salt concentration and is, for example, approximately 0.5 mol / l.

[0063] Figure 1d) shows the discharge process of the lithium-ion storage cell after the rest period shown in Figure 1c). During the discharge process, the reverse processes occur compared to the charging process. In particular, the anode releases 2 Li + so that the concentration of lithium conducting salt increases adjacent to anode 2, while Li +from the electrolyte solution at the cathode 3 into the cathode, so that the lithium conducting salt concentration adjacent to the cathode 3 decreases. Since the volume of the electrochemically active anode material decreases during the discharge process and thus the pore volume of the anode increases, electrolyte 6 located outside the electrode coil can be drawn into the anode 2 during discharge, as indicated by the arrows 8. Since this electrolyte 6 located outside the electrode coil has only a low lithium conducting salt concentration, regions with different lithium conducting salt concentrations form along the longitudinal axis 7 over the distance 25. For example, the lithium conducting salt concentration in the first edge region 2A and in the second edge region 2B of the anode can be 1.2 mol / l, while in the central region 2C of the anode 2 it is 1.5 mol / l.

[0064] Figure 1e) shows the discharged lithium-ion power storage cell after at least 1 hour of rest at a state of charge of 0%. Due to the short distance 26 between the anode and cathode, the different lithium conducting salt concentrations horizontally to the longitudinal axis 7 have equalized by diffusion. However, due to the significantly greater distance 25, a conducting salt gradient exists along the longitudinal axis 7, with the first and second edge regions of the anode 2 and the cathode 3 each having lower conducting salt concentrations of, for example, 0.85 mol / l, while the central regions of the anode and cathode have increased conducting salt concentrations of, for example, 1.15 mol / l. Depending on the size of the distance 25, the conducting salt concentration gradient built up along the longitudinal axis 7 may only equalize by diffusion after many days or, at greater distances of 6 or 9 cm, as is the case with larger round cells, only within months.The uncharged lithium-ion power storage cell shown in Figure 1e) is then subjected to a charging process during normal operation, for example, a rapid charging process as shown in Figure 1b). As a result, with the increasing number of charge and discharge cycles, an increasing gradient of conducting salt concentration builds up along the longitudinal axis 7 in the conventional power storage cell. This increasing gradient of conducting salt concentration leads to accelerated aging of the power storage cell and can, in particular, also lead to lithium plating.

[0065] According to the invention, by specifically adjusting the volume factor f vthe amount of electrolyte 6 displaced from the electrode coil can be reduced compared to the conventional power storage cell or displacement of the electrolyte solution from the electrode coil can be completely prevented. According to the invention, the sum of the volumes of the pores in the anode, the volumes of the pores in the cathode and the volumes of the pores in the separator at a state of charge of 75% to 100% is used to calculate the volume factor. In such a range of state of charge, the pore volume in the anode available for the electrolyte solution is significantly smaller than the pore volume available in the anode in the uncharged state at a state of charge of 0%. Based on this calculation, displacement of the electrolyte solution from the electrode coil at high states of charge of the power storage cell can therefore be particularly reliably reduced or prevented.Figure 2 shows a graph with the experimentally determined lithium conducting salt concentrations along the longitudinal axis of a lithium-ion power storage cell in the anode of conventional power storage cells with an unadjusted volume factor f. v and power storage cells according to the invention with a correspondingly adjusted volume factor f v after approximately 130 charge and discharge cycles. Graphs 10 and 11 show the slightly different concentrations of the conducting salt in the electrolyte solution of a power storage cell according to the invention, where the volume factor f v was set to a value between 0.9 and 1.05. In contrast, the two graphs 12 and 13 show very different electrolyte concentrations in conventional power storage cells, where the respective volume factor f vabove the range set according to the invention. The values ​​designated 14 and 16 for graphs 10, 11, 12, and 13 respectively show the conducting salt concentrations in the first and second edge regions of the anode, while the values ​​designated 15 show the lithium conducting salt concentrations in the central region of the respective anodes.

[0066] Figure 3 shows, in graph 17, the modeled course of the lithium-ion conducting salt concentration in an edge region of an anode of a conventional power storage cell over a period of 60 hours with a large number of charge and discharge cycles. At the beginning of the charge and discharge cycles, the area of ​​graph 17A shows an initial drop in the lithium conducting salt concentration during charging of the power storage cell and then a rapid increase in the lithium conducting salt concentration in the anode during discharging of the power storage cell. It can be observed that with each charge and discharge cycle, the concentration of the lithium conducting salt in the edge region of the anode successively decreases from a concentration of over 1000 mol / m 3 to below 200 mol / m 3 Such a decrease in the lithium conducting salt concentration over a sequence of many charge and discharge cycles can be achieved by adjusting the volume factor f according to the invention. vreduced or prevented.

[0067] Figures 4a) and 4b) show photographs of anodes 2 of a conventional lithium-ion power storage cell (Figure 4a) and a lithium-ion power storage cell according to the invention (Figure 4b), in which the volume factor f v was set. In the anode 2 of the conventional power storage cell with a volume factor of 1.08, a dark layer can be seen in the edge regions 2A, 2C, which is attributable to lithium plating. In contrast, the anode 2 in the power storage cell according to the invention with a volume factor of 1 (Figure 4b) shows no lithium plating.

[0068] Figure 5 shows a schematic cross-section of a conventional lithium-ion storage cell 1 with a housing 20 that contacts the anodes 2 and the cathodes 3 of the electrode coil around the circumference. A mandrel 24 is located in the center of the electrode coil. The separator layer 4 is arranged between the anodes 2 and the cathodes 3. Due to the mechanically rigid housing 20, the electrode coil cannot expand perpendicular to the longitudinal axis 7 during charging of the storage cell 1. Due to this tension of the electrode coil in the housing, electrolyte solution 23 with a depleted concentration of lithium conducting salt is displaced from the electrode stack, in particular from the edge regions of the anodes, and collects above and below the electrode stack.The lithium-ion storage cell 1 further comprises poles 21 and 22, which are each electrically contacted by the current collector foils 18 of the anodes 2 or the current collector foils 19 of the cathodes 3.

[0069] Figure 6 shows a schematic plan view of an unrolled anode 2 or cathode 3 after opening a power storage cell. The unrolled anode 2 can have a length of up to 2.5 m in the electrode coil of the power storage cell. Figure 6 also shows the distance 25 over which the first edge region 2A, the second edge region 2C, and the central region 2B of the anode 2 located therebetween extend. The width of the anode 2, the distance 25, can in particular be approximately 8 cm. In order to determine the different concentrations of the conductive salt, for example LiPFe, with spatial resolution along the distance 25, round samples 30A can be punched out from the first edge region 2A, as well as round samples 30B from the central region 2B, and round samples 30C of the anode from the second edge region 2C. The samples 30A, 30B, and 30C each have the same area.The solvent in the electrolyte solution, for example, ethylene carbonate, can be determined from these samples using liquid chromatography / mass spectrometry (LC-MS). Furthermore, the amounts of conductive salt can also be determined from the samples using ion chromatography.

[0070] For the spatially resolved determination of the amount of conducting salt in the electrode coil, the cell is discharged to its lower cut-off voltage after cycling and left to rest for one day. The cell is then opened in an argon-filled glove box, the electrode coil is removed and unrolled approximately halfway. Round electrode samples (called "coins") are then taken using a hole punch (e.g. 12 mm in diameter). Coins are taken from the outer areas in the first and second edge zone at a distance of approximately 1 to 2 mm from the edge of the electrode coating and from the middle of the electrode coating in the central zone. 15 coins are taken from each zone and each placed together in previously dried glass vessels. In principle, sampling can be carried out in either the anode or the cathode, but sampling in the cathode is preferable because no falsification of the result is to be expected due to SEI formation.5 mL of dried dimethyl carbonate (DMC) or acetonitrile (ACN) is then added to each coin as the extraction agent, and the glass vials are tightly closed. The coins are left in the extraction agent for at least 16 hours, ideally on a shaker plate, to ensure complete extraction of the conducting salt. The conducting salt concentration in the extraction agent is then determined. The mean value of the conducting salt concentrations determined for the 15 individual coins is taken. The ion chromatography results can be directly compared for a relative comparison of the conducting salt amounts in the various positions.

[0071] Figure 7 shows a CT image of an area inside the housing of a conventional power storage cell with a volume factor of 1.08 outside the electrode coil at a state of charge of 100%. The CT images were taken using a laboratory CT system from General Electric. A horizontal section is taken through the area directly beneath the electrodes, where there is still protruding separator and protruding copper foil as the current collector foil. The horizontal sections through the cell can be taken in particular along lines 31 A and 31 B, as shown in Figure 5. In the image processing program, the electrolyte solution is optically highlighted using gray value modification; materials with a higher X-ray absorption capacity (e.g. steel or active material) are shown darker in Figures 7 and 8, materials with a lower X-ray absorption capacity (e.g. gas) are shown lighter.This creates an image contrast that shows the electrolyte solution 23 outside the electrode coil within the protruding separator. CT images can thus easily detect the presence of electrolyte solution. Using optical integration analysis, the volume of electrolyte solution outside the electrode coil can also be determined.

[0072] The CT image in Figure 8 shows a horizontal section through a power storage cell according to the invention with a volume factor of 0.93 at a state of charge of 100%. In contrast to the conventional power storage cell in Figure 7, no electrolyte solution outside the electrode coil can be detected.

[0073] Example:

[0074] Two different lithium-ion energy storage cells of type 4695 with a diameter of 4.6 cm and a length (along the longitudinal axis) of 9.5 cm with a cylindrical housing were built. The electrochemically active anode material consisted of a mixture of graphite and silicon, with the weight fraction of silicon in the anode active material being less than 10 weight percent. The electrochemically active cathode material consisted of a typical nickel-cobalt-manganese layered oxide with a nickel content of greater than 85 atomic percent. A copper foil was used as the current collector foil for the anode, with the thickness of the coating with the electrochemically active anode material being 70 μm. An aluminum foil was used as the current collector foil for the cathode, with the thickness of the coating for the cathode being 50 μm. Both the anode current collector foil and the cathode current collector foil were coated on one side.The separator is made of a polymer and contains a ceramic coating. The anode porosity at a 0% state of charge was 25%, and the cathode porosity was 23% at a 0% state of charge for the already formed energy storage cell. The electrolyte solution consisted of ethylene carbonate with a lithium conducting salt LiPFe at a concentration of 1.2 mol / l. Both energy storage cells have lower cutoff voltages of 2.8 V (0% state of charge) and upper cutoff voltages of 4.2 V (100% state of charge).

[0075] One of the cells was filled with a volume factor f v of 0.93 and the other cell with a volume factor f vof 1.08. The sum of the pore volumes of the anode, cathode, and separator at a 100% state of charge for both cells was 34.9 ml. The volume of electrolyte solution used up during formation was 2.1 ml. The cell with a volume factor of 0.93 was filled with 34.5 ml of electrolyte solution: (34.5 - 2.1) / 34.9 = 0.93. The cell with a volume factor of 1.08 was filled with 39.7 ml of electrolyte solution: (39.7 - 2.1) / 34.9 = 1.08.

[0076] After formation, both energy storage cells were subjected to a fast-charge test. The fast-charge test took place at a test chamber temperature of 35°C. A typical step-charge profile was used to charge the cells from a state of charge of 10% to 80% (Adam et al., “Application of the differential charging voltage analysis to determine the onset of lithium-plating during fast charging of lithium-ion cells,” Journal of Power Sources, Volume 495, 31 May 2021, 229794). The charging time was between 17 and 21 minutes. 50% of the cycles were fast-charge cycles, the other 50% were normal charge cycles with a C-rate of C / 2 and an SOC range of 10 to 100%. All discharge steps were performed at a C-rate of C / 2. A total of 130 cycles were performed, of which 50% were fast charging cycles and 50% were normal charging cycles.

[0077] After completing the 130 cycles, the energy storage cells were opened in an argon-filled glove box, and their anodes were visually examined for lithium plating. The energy storage cell with a volume factor f v of 1.08 showed significant lithium plating at both edge regions (see Figure 4a), while the power storage cell with a volume factor of 1 showed no lithium plating at all (see Figure 4b). Furthermore, the electrode coils of both power storage cells were half-unwound, and the corresponding cathodes of both power storage cells were analyzed by ion chromatography and liquid chromatography-MS by removing and punching out samples, as shown in Figure 6. The results are shown in Figure 2.

[0078] The invention is not limited by the description based on the exemplary embodiments. Rather, the invention encompasses any novel feature and any combination of features, including, in particular, any combination of features in the patent claims, even if this feature or combination itself is not explicitly stated in the patent claims or exemplary embodiments.

Claims

Patent claims 1. A power storage cell, designed as a lithium-ion storage cell or sodium-ion storage cell, comprising an anode, a cathode, a separator located between the anode and the cathode, and an electrolyte solution arranged in pores of the anode, cathode, and separator, wherein the electrolyte solution comprises a conductive salt and at least one organic solvent, wherein the storage cell at a charge state of 75% to 100% has a volume factor f v between 0.9 and 1.05, preferably 0.95 to 1.00, wherein the volume factor f v is defined as the ratio (volume of electrolyte solution) / (sum of the volumes of the pores in the anode, the volumes of the pores in the cathode and the volumes of the pores in the separator at the state of charge from 75% to 100%).

2. Power storage cell according to the preceding claim, comprising an electrode coil or an electrode stack, wherein the electrode coil or the electrode stack is accommodated in a housing and comprises the anode, the cathode, the separator and the electrolyte solution, wherein the electrode coil or the electrode stack extends in the housing along a longitudinal axis of the power storage cell.

3. Power storage cell according to the preceding claim, wherein the electrode coil or the electrode stack has an extension of at least 3 cm along the longitudinal axis.

4. A power storage cell according to one of the preceding claims 2 or 3, wherein the housing is arranged circumferentially along the longitudinal axis around the circumference of the electrode coil or the electrode stack and wherein the housing circumferentially contacts the main surfaces of the anode and cathode.

5. Power storage cell according to one of the preceding claims 2 to 4, wherein at a charge state of 75% to 100% of the power storage cell, at most 4 percent, preferably at most 3 percent of the total volume of the electrolyte solution in the housing is located outside the electrode coil or the electrode stack.

6. Power storage cell according to one of the preceding claims 2 to 5, wherein the anode, the separator and the cathode have a first and a second edge region extending along the longitudinal axis and a central region extending between the first and second edge region along the longitudinal axis, and wherein after at least 50 charging and discharging processes (average C-rate during charging and discharging at least 1C, SOC range 0-100% each, temperature 25 °C) after discharging to the cut-off voltage and resting for at least 1 hour and at most 1 day, the concentrations of Li +in the electrolyte solution in the first edge region, in the second edge region and in the middle region by a maximum of 30 percent, preferably by a maximum of 20 percent, more preferably by a maximum of 10% in relation to the original concentration of Li + in the electrolyte solution.

7. Power storage cell according to one of the preceding claims, wherein the pore volume of the anode decreases during the charging process depending on the state of charge, preferably wherein the anode comprises an electrochemically active anode material selected from a group consisting of: synthetic graphite, natural graphite, graphene, mesocarbon, doped carbon, hard carbon, soft carbon, fullerene, silicon-carbon composites, silicon, surface-coated silicon, silicon suboxide, silicon alloys, aluminum alloys, indium, tin alloys, cobalt alloys and mixtures thereof, further preferably wherein the electrochemically active anode material is selected from a group consisting of: graphite, silicon oxide and silicon and mixtures thereof.

8. Power storage cell according to one of the preceding claims, wherein the pore volume of the cathode increases during the charging process depending on the state of charge, preferably wherein the cathode comprises an electrochemically active cathode material which is selected from a group consisting of: lithium transition metal oxides such as lithium cobalt oxide (UCOO2), lithium nickel cobalt manganese compounds (known by the abbreviation NCM or NMC), for example LiCoO2, LiNiO,33Coo,33Mno,33O2, lithium nickel cobalt aluminum oxides (NCA), lithium olivines such as lithium iron phosphate (LFP), lithium spinels such as lithium manganese oxide spinel (LMO) and lithium nickel manganese spinel (LNMO) or combinations thereof, further preferably wherein the electrochemically active cathode material is selected from a group consisting of: lithium nickel cobalt Manganese compounds.

9. Power storage cell according to one of the preceding claims, wherein the lithium salt is selected from a group consisting of: LiPFe, LiAsFe, UCIO4, UCF3SO3, lithium bis(trifluoromethylsulfonyl)amide or combinations thereof and / or wherein the organic solvent comprises an organic polar solvent, preferably wherein the organic solvent is selected from a group consisting of: C2 to C4 cyclic esters of carbonic acid, for example Propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, lactones, for example γ-, δ- and ε-lactone or combinations thereof.

10. Use of a power storage cell according to one of the preceding claims for rapid charging, wherein rapid charging is charging at an average C-rate of at least 1C.

11. A method for determining the pore volume of a power storage cell at charge states of 75% to 100%, wherein the power storage cell comprises an anode, a cathode, a separator located between the anode and the cathode, and an electrolyte solution arranged in the pores of the anode, cathode, and separator, and wherein the electrolyte solution comprises a lithium salt as a conductive salt and at least one organic solvent, comprising the method steps: A) Filling a plurality of power storage cells each with a volume of electrolyte solution that is greater than the pore volume of the power storage cell at charge states of 75% to 100%, B) charging and discharging the plurality of power storage cells to form the electrodes, and then, C) charging the plurality of power storage cells to different charge levels in the range of 75% to 100%, and D) Opening the plurality of charged power storage cells and determining the pore volumes of the anode and cathode at different charge states.

12. A power storage cell, designed as a lithium-ion storage cell or sodium-ion storage cell comprising an anode, a cathode, a separator located between the anode and the cathode and an electrolyte solution in an electrode coil or electrode stack, wherein the electrolyte solution is arranged in pores of the anode, cathode and the separator, wherein the electrolyte solution comprises a conducting salt and at least one organic solvent, wherein the electrode coil or electrode stack is located in a housing, and wherein in the storage cell at a charge state of 75% to 100% of the power storage cell, at most 4 percent, preferably at most 3 percent of the total volume of the electrolyte solution, more preferably no electrolyte solution in the housing is located outside the electrode coil or the electrode stack.

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