Metal-free high-voltage cathode for lithium-ion or alkali-ion secondary batteries

Abstract

An alkali ion battery is provided that includes an anhydrous alkali salt as a cathode active material, which can be, for example, a lithium sulfate, a sodium sulfate, or a potassium sulfate salt. In some such batteries, sulfate to persulfate interconversion occurs during charging and discharging of the battery, respectively.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Non-provisional Application No. 17 / 148,446, filed January 13, 2021, and is a continuation application entitled to the benefit of the filing date under 35 U.S.C. §119(e), the entire contents of which are incorporated herein by reference. [Background technology]

[0002] Lithium has long been the subject of much attention as a promising anode material for rechargeable batteries. This interest in alkali metals stems from the fact that (1) it is the most electronegative metal (-3.0 V vs. SHE) and (2) it is the lightest metal (0.534 g / cm). 3 This is due to the combination of two unique properties: alkali metals have a negative potential that, when combined with certain cathodes, translates into high cell voltages, and alkali metals have a high specific capacity (3.86 Ah / g). Commercial lithium-ion batteries use lithium ions intercalated in a graphite anode, while the cathode is composed of layered oxides of various compositions, such as lithiated cobalt oxide (LiCoO) or LiNiMnCo (NMC), or spinels such as LiMnNiO, or lithiated metal phosphates. During charging, lithium ions from the lithiated metal oxide flow into the graphite anode, resulting in the intercalation of lithiated graphite (C6Li). The lithium in both electrodes is present in ionic form, hence the name Li-ion battery. The capacity of the graphite anode is approximately 372 mAh / gm, while the capacity of the cathode material (LiCoO2) is approximately 170 mAh / gm, resulting in a significant mismatch between the two electrodes. Spinels and phosphates have even lower capacities, less than 110 mAh / gm. Over the past few decades, several other cathode materials have been investigated, ranging from nickel oxide, manganese oxide, polysulfides, and iron phosphates, as well as combinations thereof, but cathode capacity has only incrementally improved.

[0003] Lithium metal is not used solely as an anode material in batteries due to safety concerns. The failure of lithium as an anode due to dendrite formation led to the search for ways to avoid the dramatic morphological changes of the anode during cell cycling. As a result, "host-guest" chemistry was developed. This reversible chemistry concept, also known as intercalation or insertion electrodes, has been previously applied to cathode materials for lithium batteries, as exemplified by the pioneering work of Whittingham and significant improvements by Goodenough et al. and others. Most host materials are transition metal oxides or chalcogenides with stable crystal lattices, whose layered or tunnel structures provide pathways for guest ions, such as lithium ions, to diffuse. By injecting or extracting electrons, redox reactions occur on the host lattice, while mobile guest ions intercalate into or deintercalate from the host matrix to compensate for local electroneutrality. There is no faradaic change in the "guest ion" throughout the intercalation and deintercalation cycles.

[0004] A similar intercalation host is graphite, which is used as an anode material for commercial lithium-ion batteries. Such intercalation cathodes and anodes allow lithium ions to shuttle between electrodes without the presence of lithium metal. In the charged state of these carbonaceous anodes, lithium exists in an ionic state rather than a metallic state, thus eliminating the possibility of lithium dendrites. The advantages of this new host are highlighted by the low cost of carbon and the high lithium-ion activity of the intercalated compound, which allows the anode potential to approach that of lithium metal, minimizing the energy penalty and possessing a high specific energy capacity of 3.70 Ah / g.

[0005] The adoption of new materials and novel engineering designs has led to the development of more promising 4.0 V cathode materials (Li xThe application of MO2, where M=Mn, Ni, or Co) has improved the cycle life, energy, and power density of lithium-ion battery technology to over 2000 cycles, 160 Wh / kg, and 5000 W / kg, respectively. Lithium-ion batteries have become the dominant rechargeable energy storage devices due to their high energy density and long life. Layered LiCoO2 and LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 The most studied cathode materials, such as O2 (NMC111), are primarily based on the cobalt redox process. However, due to the limited availability, uneven distribution, and toxicity of cobalt, it is desirable to explore a new generation of cathodes. In this regard, iron-based polyanion compounds are attractive cathode materials for large-scale energy storage applications, considering that using naturally abundant iron as the redox center effectively alleviates the limitations of limited resources and reduces energy costs. More importantly, oxygen atoms are bound to polyanions (e.g., (PO4)) through strong covalent bonds. 3- ), which can significantly reduce the risk of oxygen evolution and improve cycling stability. The successful use of lithium iron phosphate (LiFePO4) as a cathode for lithium-ion batteries has stimulated extensive research into various polyanionic compounds for cathode materials due to its low cost, high safety, and long cycle life. Summary of the Invention

[0006] In some embodiments of the present invention, an alkali ion battery is provided that includes an anhydrous alkali salt as a cathode active material, where the anhydrous alkali salt is insoluble in an organic electrolyte. In some embodiments of the alkali ion battery, the alkali salt includes a lithium salt, and lithium ions are extracted and lithiated into the anode. In some embodiments, the anode includes graphite. In some embodiments, the lithium salt includes lithium sulfate. In some embodiments, the interconversion of lithium sulfate to lithium persulfate, and vice versa, occurs during charging and discharging of the battery, respectively. In some embodiments, the alkali ion battery further includes a high-voltage electrolyte. In some embodiments, the electrolyte includes sulfolane.

[0007] In some embodiments of alkali ion batteries, the alkali salt comprises either sodium sulfate or potassium sulfate as the cathode active material, and sodium or potassium ions are extracted from the sulfate salt for insertion into the anode. In some embodiments, the interconversion of sulfate to persulfate occurs during charging and discharging of the battery, respectively.

[0008] In some embodiments of the present invention, a high voltage cathode is provided that includes an anhydrous alkali sulfate layer embedded on a porous current collector. In some embodiments, the alkali sulfate layer includes a lithium sulfate layer. In some embodiments, the porous current collector includes either a porous carbon cloth or a porous carbon paper.

[0009] The foregoing objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of the detailed description of the preferred embodiment when considered in conjunction with the following drawings. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a table reflecting the theoretical capacity of lithium sulfate. [Figure 2]FIG. 1 illustrates the undesired tramp reactions of proposed lithium-ion batteries using sulfate / persulfate chemistry, some of which are irreversible. [Figure 3] 1A-1D show various experimental results for fabricating a lithium-ion rechargeable battery system. [Figure 4] FIG. 1 illustrates one embodiment of a lithium ion battery structure of the present invention that was tested. [Figure 5] FIG. 1 illustrates the electrochemical conversion of lithium sulfate to lithium persulfate. [Figure 6] FIG. 1 shows the reversible interconversion of sulfate to persulfate, as studied using cyclic voltammetry techniques. [Figure 7] FIG. 1 shows the discharge profile of an assembled lithium sulfate battery system tested over multiple cycles. DETAILED DESCRIPTION OF THE INVENTION

[0011] In an embodiment of the present invention, an alkali ion battery is provided in which an alkali salt is used as a cathode active material that exhibits high-voltage chemistry and rechargeability. In one embodiment, the cathode active material comprises a lithium salt anion component, but may also comprise other salts, as described below. In one embodiment, the cathode active material comprises a sulfate salt of lithium, which is electrochemically converted to lithium peroxodisulfate (also known as lithium persulfate) during lithium ion extraction, which is loaded into a graphite or lithium / alloy anode. Lithium ion storage and subsequent charge / discharge are achieved, for example, by interconversion of the solid sulfate / persulfate matrix as the cathode active material on a graphite / lithium alloy substrate. High-voltage lithium ion batteries can be fabricated using lithium sulfate / lithium peroxydisulfate (also known as lithium persulfate) reversible electrochemistry. The detailed electrochemical charge / discharge reactions are: [ka] and the resulting cell voltage was 4.85 V. The complete electrochemical reaction is:

[0012] Charging Mode: Anode: 2Li2SO4 → Li2S2O8 + 2Li + +2e - 2.01V Cathode: 2Li + +2C6+2e - →2C6Li 2.84V Cell: 2Li2SO4 + 2C6 → Li2S2O8 + 2C6Li 4.85V Discharge Mode: Anode: 2C6Li → 2C6 + 2Li + +2e - Standard potential=2.84V Cathode: Li2S2O8+2Li + +2e - →2Li2SO4 standard potential = 2.01V Battery: Li2S2O8 + 2C6Li → 2Li2SO4 + 2C6Cell voltage: 4.85V

[0013] Lithium persulfate and lithium sulfate are poorly soluble in aprotic organic solvents. Therefore, lithium sulfate powder can be impregnated onto porous carbon paper with a suitable binder and solvent (such as n-methylpyrrolidine (NMP), or polyvinylidene fluoride (PVDF) dissolved in a cellulose triacetate or carboxymethylcellulose binder). During charging, lithium ions are extracted from the lithium sulfate electrode and intercalated into the graphite counter electrode, simultaneously converting the sulfate to persulfate. During discharging, lithium ions are deintercalated from the graphite electrode and reinserted into the crystalline matrix of the lithium persulfate electrode, converting it back to lithium sulfate.

[0014] Lithium persulfate (or peroxodisulfate) salts are non-hygroscopic, non-flammable, and can be stored for long periods (approximately 10 years) as salts or concentrated solutions. The oxygen in persulfates (peroxide state) can be readily released to aid oxidation. Persulfates' higher standard potential (2.010 V vs. SHE) and their strong oxidizing power (intermediate between hydrogen peroxide and ozone, without their inherent stability issues and short half-life) provide one of the highest operating voltages in aqueous and / or non-aqueous systems.

[0015] By replacing layered metal oxide cathodes (such as lithiated cobalt oxide) with lithium sulfate on a suitable catalytic current collector (such as porous platinum-carbon paper) or non-catalytic carbon cloth, the cathode capacity limitations of lithiated metal oxide cathodes can be overcome, resulting in high energy densities. The potential cell voltage is much higher than that of current lithium-ion batteries (3.25-3.5 V), resulting in high power densities for integrated cells. As reflected in Figure 1, the theoretical capacity of lithium sulfate is 240 mAh / gm, significantly higher than that of lithiated metal oxide cathodes (approximately 170 mAh / gm) and much better than that of lithium iron phosphate systems.

[0016] Lithium sulfate and lithium persulfate have a monoclinic wedge-shaped crystal structure. The lithium sulfate crystal has edge lengths of a = 8.23 ​​Å, b = 4.95 Å, c = 8.47 Å, and β = 107.98°. The sulfur atoms are coordinated in a tetrahedral configuration, with lithium atoms at the vertices, allowing for easy extraction of lithium ions when electrochemically converted from the sulfate structure to the persulfate structure during charging. Similarly, lithium ions are easily inserted into the sulfate structure during discharging, converting the persulfate structure to the sulfate structure. The strong bonds between the sulfur and oxygen atoms allow lithium ions to be loosely bound within the crystal matrix. The weaker lithium bonds allow lithium ions to move easily from site to site without the need to break strong bonds. The monoclinic structure of lithium sulfate and lithium persulfate, with lithium atoms at the vertices, further reduces the energy required for lithium ions to move from site to site. These two factors allow lithium ions to diffuse quickly and easily through the crystal structure.

[0017] In the proposed lithium-ion battery, it is desirable to prevent tramp reactions. Some tramp reactions can occur in protic solvents, or even in the presence of traces of moisture or binders with hydrogen-bonding structures. Referring to Figure 2, undesired tramp reactions, some of which are irreversible, are shown for the proposed lithium-ion battery using sulfate / persulfate chemistry.

[0018] Similarly, trace metal impurities in the cathode structure must be very strictly controlled due to the strong oxidizing effect of persulfates. This precludes the use of conventional cathode structures and current collectors that use stainless steel or copper mesh. Carbon- and graphite-based current collectors and powders have been found to have sufficient efficiency and inertness during the conversion of sulfate to persulfate, while also enhancing the stability of the persulfate that is formed.

[0019] The proposed Li-SO4 lithium-ion battery's high charge voltage, exceeding 4.85 V, requires an electrolyte / solvent mixture with a large electrochemical window (e.g., -3.0 V to +2.25 V), preferably greater than 5.25 V, to prevent degradation during charge and discharge operations. One solvent with such a large electrochemical window is sulfolane. Similarly, acetonitrile also has an electrochemical window of -3.45 V to +2.35 V, but in testing, it was found to be susceptible to decomposition due to persulfates generated during charging. Other possible solvents include ethylene carbonate, propylene carbonate, and dimethyl carbonate, and mixtures thereof, with or without sulfolane (-4.0 V to +2.3 V).

[0020] The lithium electrolyte suitable for the proposed high-voltage battery contains various lithium salts soluble in the aprotic solvents mentioned above, preferably lithium perchlorate with a molar concentration of 1-2 M in solution to enhance electrochemical conductivity within the battery. Figure 2 shows the various electrolyte / salt mixtures investigated, based on which the final electrolyte of lithium perchlorate dissolved in a mixture of sulfolane with ethylene carbonate and propylene carbonate was selected for optimal performance and long cycle life. The lithium sulfate and lithium persulfate active materials were found to be insoluble in these electrolyte solvents, allowing the lithium sulfate / lithium persulfate to be incorporated into the cathode structure without loss due to dissolution in the electrolyte used.

[0021] The insolubility of lithium sulfate / lithium persulfate in the tested electrolyte solvents also significantly reduces the self-discharge of the proposed high-voltage battery. Li-ion batteries self-discharge approximately 5 percent in the first 24 hours, then lose 1–2 percent per month, with protection circuits adding another 3 percent per month. A defective separator can create a current path, generating heat and resulting in elevated self-discharge, which in extreme cases can initiate thermal breakdown. The self-discharge of the proposed lithium-ion battery can be significantly reduced by applying a negative voltage bias to the graphite anode after charging at a microampere current level (preferably 2–5 μA) and a DC voltage level (preferably 4.85–4.95 VDC) between the peak charge voltage and the maximum discharge voltage, as well as cathodic protection, using a secondary DC power source.

[0022] The solid-state ionic structure of sulfate and persulfate materials, when properly incorporated into a mesoporous carbon matrix, also enhances the ionic conductivity of the cathode structure, improving fast charge / discharge rates. The delocalization and distribution of charge within the persulfate crystalline structure also improves charge storage and interconversion reaction rates. As tested, the activation barrier for the conversion of sulfate to persulfate during charging is low, thus eliminating the need for a catalyst.

[0023] Preferred anode materials for the proposed lithium sulfate battery chemistry include graphite anodes or materials that use silicon, tin, and other materials for metal anodes, preferably as amorphous or nanocrystalline materials.

[0024] A similar alkali-ion battery can be constructed with sodium sulfate / persulfate or potassium sulfate / persulfate as the high-voltage cathode material and a suitable anode material. The corresponding electrochemical charge / discharge reactions for the entire cell are as follows: [ka] or [ka]

[0025] Anode materials suitable for use with sodium-ion battery embodiments of the present invention include Na2Ti3O7 or Na4Ti5O, having a hexagonal or monoclinic crystal structure. 12 Sodium titanium oxide (Na x Ti y O z ). Other suitable anode materials include rubidium, strontium, gallium, calcium, or silver, used as microcrystalline or nanocrystalline powders in a suitable anode matrix, given that these elements form minimal alloy phases with sodium, minimizing crystallographic stress and disruption when sodium is inserted or extracted. Anode materials suitable for use with potassium-ion battery embodiments of the present invention include sodium metal and rubidium metal.

[0026] The proposed lithium sulfate battery embodiment can also be fabricated into a solid-state battery by using a suitable polymeric cationic membrane after exposure and saturation with a lithium salt in an aprotic solvent. Lithium ions are conducted between the anode and cathode through the cationic membrane, preventing the self-discharge commonly observed in conventional liquid electrolyte-containing lithium-ion batteries. One typical polymeric cationic membrane contains polyethylene oxide (PEO) with a suitable lithium salt for ionic conductivity, with or without nanosilica particles to enhance amorphousity. Similarly, polydimethylsiloxane (PDMS) can also be used for the polymeric cationic membrane with added lithium salt. The use of such a membrane also reduces the self-discharge of the battery.

[0027] Other suitable lithium-conducting membranes include glass membranes such as LiSICON, lithium superconducting ionic membranes, or lithium nitride membranes. Such membranes, particularly due to their amorphous nature, have very high ionic conductivity for lithium and are inert to sulfate / persulfate exposure in the proposed battery system. Similar cationic membranes can also be used in sodium-ion and potassium-ion sulfate / persulfate battery systems. The use of such membranes significantly reduces the self-discharge of lithium-ion battery systems during storage.

[0028] Figure 3 shows various experiments conducted to fabricate a lithium-ion rechargeable battery system using lithium sulfate as the cathode active material. After experimenting with several solvents, binders, and membranes, the final cell structure was finalized and tested. Testing was performed at room temperature with 1-2.5 mA charge and discharge for a 1-3 mAh coin cell battery system. The coin cells were assembled under a nitrogen blanket in a glove box with humidity levels below 25 ppm.

[0029] Low-molecular-weight polydimethylsiloxane was found to be an effective binder for lithium sulfate microparticles mixed with microscale carbon powder for electronic conductivity, compared to the traditionally used PVDF binder. It is speculated that the siloxane converts to microsilica and then to lithium silicate during the first charge, creating a fine sealant and binder for the lithium sulfate salt. Sulfolane mixed with propylene carbonate and ethylene carbonate in an 80:10:10 v / v ratio was found to be the best electrolyte for a high-voltage battery system using lithium perchlorate as the lithium salt for ionic conductivity. Lithium perchlorate is oxidation-resistant, unlike the other lithium salts tested (lithium trifluoromethanesulfonylimide, LiTFSI, and lithium hexafluorophosphate, LiPF6). A microporous PVDF membrane (0.2 micron pore size) was a highly efficient separator between the two electrodes while maintaining sufficient solvent and lithium ion transport during multiple charge / discharge cycles.

[0030] Referring to Figure 4, in one embodiment, the cathode comprises a finely ground anhydrous lithium sulfate layer on a porous carbon cloth or porous carbon paper substrate (which also serves as the cathode current collector). A nanoporous polyvinylidene fluoride (PVDF) membrane serves as a separator and electrical insulator between the cathode and anode. The anode comprises a commercially available graphite layer deposited on a copper foil (which also serves as the anode current collector). Sulfolane was used as the ionic solvent in which a lithium salt, such as lithium perchlorate, lithium trifluoromethanesulfonylimide, or lithium hexafluorophosphate, was dissolved. During charging, lithium ions are removed from the lithium sulfate matrix and transferred to the graphite by the electrolyte for intercalation into the graphite. Typical charging voltages are above 4.85 V. Alternatively, a solid electrolyte containing a lithium ion-conducting cationic membrane can be used in place of the PVDF membrane and sulfolane solvent.

[0031] Referring to Figure 5, the electrochemical conversion of lithium sulfate to lithium persulfate results in the release of lithium ions for intercalation in the graphite anode. During charging, lithium ions and electrons are released from the lithium sulfate cathode, converting it to lithium persulfate layers. The lithium ions migrate across the lithium-containing electrolyte to the graphite anode, where they preferentially insert (intercalate) between the graphite layers. The electrons migrate in an external circuit to the graphite electrode, completing the ionic / electrical circuit. During discharge, the reverse occurs, with lithium deintercalating from the charged graphite and returning to the lithium persulfate layers, converting it back to lithium sulfate for the next battery charging cycle.

[0032] Referring to Figure 6, the reversible interconversion of sulfate to persulfate is demonstrated using cyclic voltammetry techniques. Tests were performed in two different modes: one as a continuous sweep and the other with a 600 second hold at 2.4 V. The latter sweep exhibited a typical electron transfer dip, indicating the conversion of sulfate to persulfate.

[0033] Referring to Figure 7, the discharge profile of the assembled lithium sulfate battery system tested over multiple cycles is shown. The discharge voltage over all three cycles begins at over 4.5 V, well above that of current commercially available lithium-ion batteries. Furthermore, the discharge curves demonstrate the effectiveness of using sulfolane as a high-voltage lithium battery electrolyte.

[0034] Those skilled in the art should appreciate that numerous design configurations may be possible for achieving the functional benefits of the system of the present invention. Accordingly, in view of the wide variety of configurations and arrangements of embodiments of the present invention, the scope of the present invention is not narrowed by the embodiments described above, but is reflected by the breadth of the following claims.

Claims

[Claim 1] A solid cathode active material comprising a crystalline ionic alkali sulfate compound; an electrode matrix of said alkali sulfate compound configured to be inert to oxidation by alkali persulfate; An alkaline ion battery comprising: alkali ions of the alkali sulfate compound are extracted and inserted into the anode during charging of the battery, resulting in conversion of alkali sulfate to alkali persulfate at the cathode; the electrode matrix stabilizes the alkali persulfate generated in the electrode matrix; The alkali ion battery, wherein the alkali ions include lithium, sodium, or potassium.