Cathode with coated disordered rock salt material

Abstract

The cathode comprises a disordered rock salt phase material and a coating disposed on a surface of the disordered rock salt phase material. The coating may comprise one or more of an oxide, a phosphate, a phosphide, or a fluoride. Ideally, the coating comprises a phosphate, a phosphide, or a combination thereof.

Description

Background Art

[0001] The present invention belongs to the field of battery technology, and more particularly to the field of high-energy materials for cathodes of electrochemical cells.

[0002] Lithium metal oxides are high energy density materials commonly used in lithium-ion batteries. One form or category of lithium metal oxides is a disordered rock salt structure. The compound represented by formula (1):

[0003] xLi3NbO4·(1-x)LiMO2 (1)

[0004] Where M is a trivalent cation, it has shown promise as a cathode in lithium-ion batteries. Disordered rock salt compositions, such as those represented by formula (1), typically have a random atomic arrangement of lithium and transition metal ions densely packed in a cubic structure. Compared to conventional lithium-excess layered materials, these disordered rock salt compositions can have more lithium atoms per formula unit.

[0005] Disordered rock salt structures are attractive cathode materials for next-generation lithium-ion batteries due to their high specific energy density. For example, some materials with disordered rock salt structures have a theoretical gravimetric energy density of approximately 1120 Wh / kg. Disordered rock salt materials can also be formed using relatively low-cost raw materials such as manganese. In this way, disordered rock salt materials can achieve relatively high energy density at a relatively low material cost. To achieve comparable energy density, other types of known cathode materials use higher-cost raw materials such as cobalt and / or nickel.

[0006] One challenge of using disordered rock salt materials in secondary batteries is the poor capacity retention during cycling. The poor capacity retention may be attributed to the instability of anion redox and high voltage cycling, which leads to side reactions during charging. Side reactions may involve residual lithium salts, such as lithium hydroxide (LiOH) and lithium carbonate (Li2CO3), which may be present on the surface of the lithium-rich material and are detrimental to the electrochemical performance of the battery. Based on the pH value of the cathode material, the disordered rock salt material may also have residual lithium salts. The byproducts of the side reactions may increase the battery resistance, and the large battery resistance may shorten the cycle life of the disordered rock salt material due to the overpotential. The true state of charge (SOC) and depth of discharge (DOD) ranges shrink as the number of cycles increases, which leads to reduced cycle capacity. Summary of the Invention

[0007] Embodiments of the inventive subject matter described herein are directed to improving the cycling performance of electrochemical cells having a disordered rock salt cathode. For example, the embodiments described herein can improve the capacity retention of an electrochemical cell by stabilizing the disordered rock salt material. The disordered rock salt material can be stabilized by applying a coating to the surface of the disordered rock salt material.

[0008] In one or more embodiments, a cathode is provided that includes: a disordered rock salt phase material, and a coating disposed on a surface of the disordered rock salt phase material. The coating can include one or more of an oxide, a phosphate, a phosphide, or a fluoride. Desirably, the layer includes a phosphate, a phosphide, or a combination thereof.

[0009] Optionally, the disordered rock salt phase material is represented by Chemical Formula (i):

[0010] LixNyMzOb-aFa (i)

[0011] where 1.0 < x < 1.65; 0 ≤ y < 0.55; 0.1 < z < 1; 0 ≤ a < 0.8; 1 ≤ b ≤ 3; N is one of Nb, Ti, Ta, Zr, W, Sb, or Mo; and M is one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Y, Mo, Ru, Rh, or Sb. Optionally, b = 2.

[0012] Optionally, the coating is present in an amount of not less than 0.05 wt% and not greater than 10 wt% relative to the total weight of the disordered rock salt phase material and the coating. Optionally, the coating is present in an amount of 5 wt% relative to the total weight of the disordered rock salt phase material and the coating. Optionally, the coating is present in an amount of 2 wt% relative to the total weight of the disordered rock salt phase material and the coating.

[0013] Optionally, the coating includes one or more of LiAlO2, Li3Al2(PO4)3 / AlPO4, Li3PO4, or Li2TiO3 / TiO2.

[0014] Optionally, the disordered rock salt phase material has an Fm-3m crystal structure.

[0015] In one or more embodiments, a method for forming a cathode is provided. The method includes synthesizing a disordered rock salt phase material and mixing the disordered rock salt phase material with one or more coating precursors to form a mixture. The precursors may be precursors that desirably form lithium phosphate, lithium metal phosphate, lithium phosphide, lithium metal phosphide, or a combination thereof. The one or more coating precursors may include one or more of Al(CH3CO2)3, AlPO4, H3PO3, NH4H2PO4, or TiO2. The method may also include annealing the mixture to form a coated disordered rock salt powder having a coating disposed on the surface of the disordered rock salt phase material. Exemplarily, the precursors may form phosphoric acid, phosphine, or a combination thereof during annealing, which may then react with surface species typically present on lithium metal, such as lithium carbonate, lithium oxide, and lithium hydroxide, to form lithium phosphate, lithium phosphide, or a combination thereof. Another example may be the use of phosphine gas with or without other liquid or solid precursors to adjust the final desired coating composition. When lithium phosphate and lithium phosphide are present, they may be present in the coating in a desired ratio and typically range from 0.01, 0.1, 0.5 to 100, 90, 5 or 2 lithium phosphate to lithium phosphide.

[0016] The coating may comprise one or more of LiAlO2, Li3Al2(PO4)3 / AlPO4, Li3PO4, Li3P and Li2TiO3 / TiO2.

[0017] Optionally, the annealing comprises heating the mixture at a temperature not lower than 200° C. and not higher than 800° C. for a time not less than 0.5 hours and not more than 24 hours. Optionally, the annealing comprises heating the mixture in the presence of an argon (Ar) gas flow.

[0018] Optionally, the method further comprises grinding the coated disordered rock salt powder with one or more carbon precursors to generate a cathode active material after the annealing.

[0019] Optionally, synthesizing the disordered rock salt phase material includes grinding a suspension of a rock salt precursor in a solvent to form a precursor mixture, drying the precursor mixture, and annealing the precursor mixture after drying to generate the disordered rock salt phase material. The rock salt precursor may illustratively include one or more of MnO, Mn2O3, Li2CO3, LiF, LiOH, Nb2O5, or NbF5.

[0020] In one or more embodiments, a secondary battery is provided, the secondary battery comprising: an anode, a cathode, and an electrolyte. The cathode is capable of reversibly exchanging lithium ions. As described above, the cathode comprises a disordered rock salt phase material having a coating. The electrolyte comprises an organic solvent and a lithium salt.

[0021] Optionally, the disordered rock salt phase material has an Fm-3m crystal structure.

[0022] Optionally, the disordered rock salt phase material is represented by chemical formula (i):

[0023] LixNyMzOb-aFa (i)

[0024] where 1.0 < x < 1.65; 0 ≤ y < 0.55; 0.1 < z < 1; 0 ≤ a < 0.8; 1 ≤ b ≤ 3; N is one of Nb, Ti, Ta, Zr, W, Sb or Mo; and M is one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Y, Mo, Ru, Rh or Sb. Optionally, b = 2.

[0025] The coating may comprise one or more of LiAlO2, Li3Al2(PO4)3 / AlPO4, Li3PO4, Li3P and Li2TiO3 / TiO2. The layer may comprise lithium phosphate, lithium phosphide or a combination thereof.

[0026] The coating may be present in an amount of about 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt% or 1 wt% to 10 wt%, 7 wt%, 5 wt% or 3 wt% of the weight of the disordered rock salt phase material and the coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a flow chart of a method for forming a cathode according to one embodiment.

[0028] Figure 2A is a graph plotting the initial discharge capacity of the eighteen test electrochemical cells determined in Tables 1 and 2.

[0029] Figure 2B is a graph plotting the Coulombic efficiency (CE) of the test cells (also referred to as test cell units) determined in Tables 1 and 2.

[0030] Figure 3A is a graph plotting the discharge capacity versus the number of cycles for the candidate test cells and control cells (which were post-annealed at 300 °C) shown in Table 1.

[0031] Figure 3Bis a graph of cycling capacity retention versus cycle number plotted for the candidate test cells and control cells shown in Table 1 and FIG3a.

[0032] Figure 4A is a graph of discharge capacity plotted against cycle number for the candidate test cells shown in Table 2 and a control cell (which was post annealed at 600°C).

[0033] Figure 4B is a graph of cycling capacity retention versus cycle number plotted for the candidate test cells and control cells shown in Table 2 and FIG. 4a.

[0034] Figure 5A is a graph of CE plotted against cycle number for the candidate test cells shown in Table 1 and a control cell (which was post-annealed at 300°C).

[0035] Figure 5B is a graph of the cell resistance growth rate versus cycle number for the candidate test cells and control cells shown in Table 1 and FIG5a.

[0036] Figure 6 Graphed are the capacity versus cycling and the capacity retention versus cycling of a half-cell with the coated disordered rocksalt of the present invention and a control without the coated disordered rocksalt of the present invention.

[0037] Figure 7 Graphed are the capacity versus cycling and the capacity retention versus cycling of a full cell with the coated disordered rocksalt of the present invention and a control without the coated disordered rocksalt of the present invention. DETAILED DESCRIPTION

[0038] The following definitions apply to various aspects described with respect to one or more embodiments of the subject matter of the present invention. These definitions may also be described in detail herein. Each term is further explained and illustrated in the specification, figures, and examples. Any interpretation of a term in this specification should be considered in light of the complete specification, figures, and examples provided herein.

[0039] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, a reference to an object may include a plurality of objects unless the context clearly dictates otherwise.

[0040] The term "active material" refers to a material in an electrode, particularly in a cathode, that contributes, releases, or otherwise supplies a conductive species during an electrochemical reaction in an electrochemical cell.

[0041] The term "metal" refers to alkali metals, alkaline earth metals, transition metals, lanthanides, and actinides, as these terms are understood by one of ordinary skill in the art or as defined herein. The term "alkali metal" refers to any chemical element in Group 1 of the periodic table, including lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). The term "alkaline earth metal" refers to any chemical element in Group 2 of the periodic table, including beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The term "transition metal" refers to chemical elements in Groups 3 to 12 of the periodic table, including scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury (Hg), (Rf), (Db), (Sg), (Bh), (Hs) and (Mt). The term "late transition metal" refers to aluminum (Al), gallium (Ga), germanium (Ge), indium (In), tin (Sn), antimony (Sb), thallium (Tl), lead (Pb), bismuth (Bi), and polonium (Po).

[0042] Rate "C" means (depending on the context) a discharge current that is a fraction or multiple of a "1C" current value at which a battery (in a substantially fully charged state) would be substantially fully discharged in one hour, or a charge current that is a fraction or multiple of a "1C" current value at which a battery (in a substantially fully discharged state) would be substantially fully charged in one hour.

[0043] To the extent that certain battery characteristics may vary with temperature, such characteristics are specified at 30 degrees Celsius (° C.) unless the context clearly indicates otherwise.

[0044] The ranges provided herein are inclusive of their endpoints. Thus, for example, the range 1 to 3 includes the values ​​1 and 3, as well as intermediate values ​​between the endpoints.

[0045] Embodiments of the present subject matter provide disordered rock salt compositions and morphologies (e.g., structures) for preparing cathodes of electrochemical cells. The electrochemical cell employing the disordered rock salt material disclosed herein can be a secondary (e.g., rechargeable) battery. The secondary battery can be a lithium ion battery. The lithium ion battery comprises an electrolyte formulation having a lithium salt present in a concentration suitable for conducting lithium ions through the electrolyte formulation between electrodes (e.g., cathode and anode) during discharge and recharge operations. For example, the discharge and recharge of the lithium ion battery can be accomplished by the exchange of lithium ions into and out of the cathode and anode. The exchange can be characterized by the intercalation and deintercalation of lithium ions and / or via conversion. For example, some lithium ions can be exchanged via intercalation, while other lithium ions can be exchanged via conversion. According to at least one embodiment, the cathode comprises a disordered rock salt material.

[0046] In the disordered rock salt composition, both lithium and transition metals occupy a cubic close-packed lattice of octahedral sites. The disordered rock salt phase can have an Fm-3m rock salt crystal structure. In the electrochemical reaction, lithium diffusion occurs by lithium ions jumping from one octahedral site to another octahedral site via an intermediate tetrahedral site. The lithium in the intermediate tetrahedral site is the activated state in lithium diffusion. The activated tetrahedral lithium ion shares a face with the following four octahedral sites: (i) the site previously occupied by the lithium ion itself; (ii) the vacancy into which the lithium ion will move; and (iii & iv) two sites that can be occupied by lithium, transition metal, or a vacancy.

[0047] The disordered rock salt phase material of the cathode described herein can have various compositions. The disordered rock salt chemical composition generally comprises lithium, a transition metal, and oxygen. One or more of the transition metal or oxygen sites can be doped with another element for improved electrochemical performance. In one non-limiting example, the oxygen sites are doped with fluorine. The chemical formula of the disordered rock salt phase material that may be doped at the oxygen site is formula (i):

[0048] Li x N y M z O b-a F a (i)

[0049] Where, 1.0 < x < 1.65; 0 ≤ y < 0.55; 0.1 < z < 1; 0 ≤ a < 0.8; 1 ≤ b ≤ 3; N is one of Ti, Ta, Zr, W, Nb, Sb or Mo; and M is one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Y, Mo, Ru, Rh or Sb. In one embodiment, y > 0.01. In one embodiment, b = 2. In a non-limiting example, x + y + z = b, so the atomic ratio of metals (e.g., Li, N and M) to non-metals (e.g., O and F) is invariant. Recognizing that defects and / or anion deficiencies may occur, which will cause deviations in the sum. These compositions have demonstrated excellent specific capacity or energy density, such as ~350 mAh / g at 55 °C and C / 40, and ~300 mAh / g at 30 °C and C / 15.

[0050] When a > 0, the presence of fluorine dopants at the oxygen sites in the disordered rock salt can improve the electrochemical performance of the lithium-ion battery cell. Without being bound by a particular theory or mechanism of action, the anion substitution of fluorine for oxygen (forming fluorides) can improve the cycling performance due to greater resistance to attack by hydrogen fluoride from electrolyte decomposition at high voltages. Alternatively, the higher ionic nature of the metal-fluorine bond relative to the metal-oxygen bond can result in less transition metal leaching from the cathode to the electrolyte, thereby further stabilizing the structure. Several disordered rock salt material compositions are disclosed in U.S. Application No. 15 / 222,377 (now U.S. Patent 10,280,092), which is incorporated herein by reference in its entirety.

[0051] Optionally, the disordered rock salt composition can be free of niobium (Nb). Instead of, or in addition to, the oxygen sites, the N and / or M metal sites can optionally be doped.

[0052] The cathode according to one or more embodiments has a coating on the surface of the disordered rock salt phase material. The coating modifies the surface of the disordered rock salt material to improve the cycling performance of the battery. For example, the coating provides a more stable and / or more conductive surface of the disordered rock salt material (relative to formulations lacking a coating on the surface of the disordered rock salt material), which can reduce side reactions with lithium salts and other compounds in the electrolyte. The coating can also be stabilized by reducing the overpotential. Thus, the coating improves the cycling stability (e.g., capacity retention) within a certain operating temperature range, including room temperature.

[0053] The coating is formed on the surface of the disordered rock salt phase material using oxide and / or phosphate coating precursors. For example, the coating precursor may include one or more phosphorus-containing compounds (e.g., phosphates, phosphides, and phosphites), oxides, fluorides, silicates, nitrides, carbonates, nitrates, borates, sulfates, acetates, and the like. Specific examples of coating precursors include, but are not limited to, aluminum acetate ("AlAc") (Al(CH3CO2)3), aluminum phosphate (AlPO4), phosphorous acid (H3PO3), ammonium dihydrogen phosphate (NH4H2PO4), and titanium oxide (TiO2). For example, the coating may be formed using only one of the listed coating precursors or using a combination of multiple of the listed precursors. Additionally, a gas that reacts with surface species on the lithium metal surface, such as phosphine (PH3), may be introduced during annealing to react with one or more species present in or on the lithium metal to form lithium phosphide. The gas may also be generated by the decomposition of a solid or liquid precursor (e.g., the decomposition of phosphorous acid). The precursor can react with the lithium ions present on the surface of the disordered rock salt material to form a thin coating covering the surface. The lithium ions can be residually present due to the synthesis of the disordered rock salt phase material below. The specific type and amount of the one or more coating precursors used can be selected based on various factors (such as the specific precursor for synthesizing the disordered rock salt phase). For example, some coating precursors can be more compatible with specific disordered rock salt compositions compared to other disordered rock salt compositions. Compatibility in this article refers to the ability of the coating precursor to form and maintain a coating on the surface of the disordered rock salt phase. Other factors can include the reactivity of one or more coating precursors to the disordered rock salt phase, the conductivity of one or more coating precursors, etc. For example, the coating precursor can be selected so that the resulting coating stabilizes the disordered rock salt phase (for example, by reducing side reactions), but does not significantly increase the resistance to ion transport and / or electron transport to and from the cathode.

[0054] The composition of the coating is based on the specific coating precursor or precursors used during the synthesis process. The coating composition may include or represent oxides, fluorides, phosphates, silicates, nitrides, carbonates, nitrates, borates, and / or sulfates. Non-limiting examples of coating compositions include: lithium aluminate (LiAlO2), aluminum phosphate-doped aluminum-stabilized NASICON structural materials (Li3Al2(PO4)3 / AlPO4), lithium phosphate (Li3PO4), and / or titanium oxide-doped lithium titanate (Li2TiO3 / TiO2). For example, an AlPO4 precursor can be used to produce a coating comprising Li3Al2(PO4)3 / AlPO4. An Al(CH3CO2)3 precursor can produce a coating comprising LiAlO2. An NH4H2PO4 precursor can produce a coating comprising Li3PO4, and a TiO2 precursor can produce Li2TiO3 / TiO2 in the coating. Optionally, multiple coating precursors can be used to produce a coating comprising multiple compounds. For example, the coating may comprise more than one of LiAlO2, Li3Al2(PO4)3 / AlPO4, Li3PO4, and Li2TiO3 / TiO2. In other embodiments, the coating may have other compositions, such as, instead of one or more of the listed precursors, or in addition to one or more of the listed precursors, using precursors other than Al(CH3CO2)3, NH4H2PO4, AlPO4, and TiO2. Another ideal example is the use of phosphorous acid, which can decompose to form phosphoric acid and phosphine, which can produce a coating comprising lithium phosphide, lithium metal phosphide, lithium metal phosphate, and lithium phosphate. The metal of the phosphide and phosphate can be any suitable metal, such as transition metals and aluminum, and can include those metals described herein, including those metals that can be used to prepare disordered rocksalts.

[0055] The amount of the coating material relative to the disordered rock salt phase may also affect the cycle performance of the battery. In one or more embodiments, the coating is present in an amount of 0.05 wt % to 10 wt % (for example, not less than 0.05 wt % and not more than 10 wt %) relative to the gross weight of the disordered rock salt phase material and the coating. For example, the content of the coating can be 0.05 wt %, 0.1 wt %, 0.5 wt %, 1 wt %, 2 wt %, 3 wt %, 4 wt %, 5 wt %, 6 wt %, 7 wt %, 8 wt %, 9 wt %, 10 wt % or any non-zero intermediate amount (0-10 wt %). In the first non-limiting example, the coating is present in an amount of 2 wt %. In the second non-limiting example, the coating is present in an amount of 5 wt %.

[0056] The coated disordered rock salt material can be used as the active material of the cathode in a battery. For example, the coated disordered rock salt can be formed as a composite cathode film on a metal current collector. As described herein, the coated disordered rock salt cathode generally provides improved initial coulombic efficiency (CE) and cycle capacity retention relative to a control cell with an uncoated disordered rock salt material, without significantly reducing the initial capacity. The beneficial initial CE and capacity retention results indicate that the coating helps protect the disordered rock salt phase from side reactions with compounds in the electrolyte during charging.

[0057] Figure 1 is a flow chart of a method 100 for forming a cathode according to one embodiment. The method can be performed using laboratory and / or industrial equipment. More specifically, the method is used to prepare a cathode active material having a coated disordered rock salt crystal structure. Figure 1 The method may include more steps, fewer steps, and / or different steps than shown in FIG.

[0058] At 102, a disordered rock salt phase material is synthesized. The disordered rock salt material can be synthesized by solid-state chemical synthesis. For example, a suspension of a rock salt precursor can be prepared by mixing a rock salt precursor into a solvent. The rock salt precursor includes a metal compound, such as manganese oxide (Mn2O3), lithium carbonate (Li2CO3), lithium fluoride (LiF), lithium hydroxide (LiOH), niobium pentoxide (Nb2O5) and / or niobium fluoride (NbF5). The solvent can include deionized water. The precursor can be added in a specific stoichiometric amount according to the desired composition of the disordered rock salt phase material. The suspension is then ground to form a precursor mixture. Grinding can be performed using a planetary ball mill to reduce the particle size and homogenize the mixture. In other embodiments, grinding can be performed using other equipment.

[0059] After grinding, the precursor mixture is dried and then annealed. For example, the precursor mixture can be dried at 100°C for 12 hours in the presence of an air flow. The annealing process can include heating the dried mixture at a temperature of 750°C to 900°C for 6 to 24 hours. The annealing process generates a disordered rock salt phase. For example, heating reacts the precursors and forms a uniform phase. The conditions of the annealing step are selected to provide the desired crystal structure during calcination. For example, the conditions are selected so that the resulting disordered rock salt phase material has an Fm-3m crystal structure. Annealing at a temperature outside the specified range and / or for a duration outside the specified time range may not ideally affect the formed crystal structure. Optionally, the annealing process can be carried out in the presence of a gas flow, such as argon (Ar). Alternatively, the annealing environment can be nitrogen (N2) or air instead of Ar.

[0060] The synthesized disordered rock salt phase material may have the following chemical formula (i):

[0061] Li x N y M z O b-a F a (i)

[0062] Where 1.0 < x < 1.65; 0 ≤ y < 0.55; 0.1 < z < 1; 0 ≤ a < 0.8; 1 ≤ b ≤ 3; N is one of Ti, Ta, Zr, W, Nb, Sb or Mo; and M is one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Y, Mo, Ru, Rh or Sb. In one embodiment, y > 0.01.

[0063] At 104, the disordered rock salt phase material is then mixed with one or more coating precursors to form a mixture. In one non-limiting example, the one or more coating precursors include Al(CH3CO2)3, AlPO4, NH4H2PO4, and / or TiO2. Based on the desired composition and properties (e.g., thickness) of the coating to be formed, the one or more coating precursors can be added to the mixture in a specific amount.

[0064] At 106, the mixture of the disordered rock salt phase material and the one or more coating precursors is annealed to form a coated disordered rock salt powder having a coating disposed on the surface of the disordered rock salt phase material. The annealing step (e.g., post-annealing) can include heating the mixture at a temperature of 200 °C to 800 °C for a time of 0.5 hours to 24 hours. A more narrow range of conditions includes heating the mixture at a temperature of 300 °C to 600 °C for 10 hours to 14 hours. In one non-limiting example, the mixture can be heated at 300 °C for 12 hours. In another non-limiting example, the mixture can be heated at 600 °C for 12 hours. Heating can cause the coating precursors to react with the surface of the disordered rock salt phase material. For example, the coating precursors can react with lithium ions present by residual lithium hydroxide, residual lithium carbonate, or present within the disordered rock salt phase itself, thereby forming a thin coating material layer covering the surface of the disordered rock salt phase. Even if limited reaction occurs, the annealing step can cause physical deposition of the coating precursor layer on the surface of the disordered rock salt phase. Annealing can be carried out in the presence of an Ar gas stream or other gas streams such as N2 or air. The coating on the surface can have a composition of phosphate, oxide, and / or fluoride. For example, based on the one or more coating precursors used in the mixture, the coating composition can include: oxides such as LiAlO2 or Li2TiO3 / TiO2, or phosphates such as Li3Al2(PO4)3 / AlPO4 or Li3PO4.

[0065] In one embodiment, after the annealing step, the coated disordered rock salt powder is ground together with a carbon precursor to produce a cathode active material. The carbon precursor may include acetylene black, carbon black, carbon fiber, graphite, carbon nanotubes, KJ600, etc. In one embodiment, the carbon precursor and the coated disordered rock salt powder are ground in a ratio in which the disordered rock salt powder accounts for the majority. For example, the ratio of the coated disordered rock salt powder to the carbon precursor may be 60:40, 70:30, 80:20, 90:10 or greater. In one embodiment, the ratio of the coated disordered rock salt powder to the carbon precursor exceeds 90:10, such as 95:5, 96:4 or 97:3. In a non-limiting example used for experimental testing, the ratio is 96:4.

[0066] The active material is then formed into a composite cathode film. For example, active material powder (including coated disordered rock salt powder and carbon precursor) can be mixed with one or more solvents to form a slurry. Non-limiting examples of the one or more solvents include polyvinylidene fluoride ("PVDF") and 1-methyl-2-pyrrolidone ("NMP"). The resulting slurry is deposited on a metal current collector. The metal current collector can be stainless steel. The slurry on the metal current collector is then dried to form a composite cathode film.

[0067] The composite cathode film prepared by the above method has a coated disordered rock salt crystal structure (e.g., Fm-3m structure). The composite cathode film can be used as a cathode in a secondary battery cell (e.g., a lithium-ion battery cell). The battery cell includes a cathode and an anode separated by a polymer separator. The battery cell includes an electrolyte that enables lithium ions and electrons to be transferred between the cathode and the anode. The cathode active material described herein can be used in combination with anodes and electrolytes of various types and compositions.

[0068] In one non-limiting example, a secondary battery is formed comprising a composite cathode material as a cathode, an anode, a separator, and an electrolyte. The battery can be formed in a glove box (M-Braun, O2 and humidity content <0.1 ppm) filled with high purity argon. Examples of suitable anode materials include conventional anode materials used in lithium-ion batteries, such as lithium, graphite ("Li x C6”), silicon, and other carbon, silicate or oxide based anode materials, as well as composite alloys combining multiple anode materials. In one non-limiting example, the anode is a lithium anode such that lithium accounts for at least 50 weight percent of the total weight of the anode active material. Optionally, the lithium content can be greater than 75 weight percent, such as greater than 90 weight percent. For example, the anode active material can be at least 95 weight percent lithium. In an alternative embodiment, the secondary battery can be anode-free such that the battery completely lacks a lithium or graphite anode. In this alternative embodiment, the cathode can provide lithium ions for battery function.

[0069] The electrolyte may include at least one organic solvent and at least one lithium salt. The one or more organic solvents may include one or more carbonates, such as ethylene carbonate ("EC"), ethyl methyl carbonate ("EMC"), diethyl carbonate ("DEC"), fluoroethylene carbonate ("FEC"), trifluoropropylene carbonate ("TFPC"), propylene carbonate ("PC"), etc. The lithium salt may include lithium hexafluorophosphate ("LiPF6"), lithium bis(fluorosulfonyl)imide ("LiFSI"), lithium difluoro(oxalato)borate ("LiDFOB") (LiBF2(C2O4), etc. The electrolyte may be in a liquid phase, a solid phase, a gel phase, or other non-solid phase. In one example, the electrolyte is a liquid electrolyte. In another example, the electrolyte is a solid electrolyte. The separator may be a polymer, such as polypropylene.

[0070] In one non-limiting example composition, the electrolyte includes LiPF6 and additives in a mixture of EC and EMC solvents. The cell is then sealed and cycled. The cell can be cycled between 1.5V and 4.6V at 30°C.

[0071] experiment

[0072] To test the performance of the coated disordered rock salt material as a cathode active material in a secondary battery cell, multiple test cells were prepared, cycled, and then analyzed. The compositions and processes described in these experiments are non-limiting examples. The test cells were formed according to the above-described method, with only specific properties being changed. For example, stoichiometric amounts of rock salt precursors (Mn2O3, Li2CO3, Nb2O5, and other dopants in the form of oxides or fluorides) were mixed in deionized water to prepare a suspension, which was then ball-milled using a planetary ball mill to reduce the particle size and prepare a uniform mixture of all precursors. The mixture was dried at 100°C under an air flow for 12 hours (h) and then annealed at 750-900°C under an argon flow for 6-24 hours to obtain the disordered rock salt phase.

[0073] The disordered rock salt phase material is then mixed with a specific amount of a coating precursor (such as Al(CH3CO2)3, AlPO4, NH4H2PO4, and TiO2), followed by post-annealing at 300°C for 12 hours or at 600°C for 12 hours under an argon flow, depending on the test cell, to form a coated disordered rock salt powder. The coated disordered rock salt powder is then ground together with a carbon precursor at a ratio of 96:4 of coated disordered rock salt powder to carbon precursor to generate an active material for cathode tape casting. The active materials used in the test cells only vary in the type of coating precursor used, the amount of coating precursor (e.g., 2 wt% or 5 wt% relative to the total weight of the active material), and the annealing temperature (e.g., 300°C or 600°C).

[0074] The battery cells are formed in a high-purity argon-filled glove box (M-Braun, O2 and humidity content <0.1ppm). The cathode is prepared by mixing the active material (e.g., coated disordered rock salt powder and carbon precursor) with solvents polyvinylidene fluoride (SigmaAldrich) and 1-methyl-2-pyrrolidone (Sigma Aldrich). The resulting slurry is deposited on a stainless steel current collector and dried to form a composite cathode film. For the anode, a thin Li foil is cut into the required size. Each battery cell includes a composite cathode film, a polypropylene separator, and a lithium foil anode. An electrolyte of LiPF6 contained in a mixture of EC and EMC is used. The battery cell is sealed and cycled between 1.5-4.6V at 30°C.

[0075] Two control test cells and sixteen candidate test cells were prepared. The control test cells had no coating material on the disordered rocksalt material. Each of the sixteen test cells contained a coating based on Al(CH3CO2)3, AlPO4, NH4H2PO4, or TiO2. Four of the test cells had an Al(CH3CO2)3 precursor, another four had an AlPO4 precursor, another four had an NH4H2PO4 precursor, and a final set of four had a TiO2 precursor. The sixteen test cells generally had an amount of coating precursor to produce a coating that comprised 2 wt% of the total weight of the disordered rocksalt phase material and the coating precursor. The remaining test cells had a higher amount of coating precursor, resulting in a coating that comprised 5 wt% of the cathode active material. Finally, post-annealing conditions were varied to heat the test cells at two different temperatures for the same amount of time. For example, half of the test cells were annealed at 300°C, while the other half were annealed at 600°C. The test cells are identified and characterized in Tables 1 and 2 below. The test cells all had disordered rocksalt cathode material and lithium anode and were cycled between 1.5-4.6 V at 0.1 C charge (0.05 C CV) / 0.1 C discharge at 30°C.

[0076] Table 1 shows the cycling results of test cells with coated disordered rock salt cathodes according to various embodiments compared to control test cells, wherein all test cells in Table 1 were annealed at 300° C. for 12 hours. Table 2 shows the cycling results of test cells with coated disordered rock salt cathodes according to various embodiments compared to control test cells, wherein all test cells in Table 2 were annealed at 600° C. for 12 hours. These tables show experimental results including the first cycle capacity (“Cyl discharge capacity”) and the first cycle coulombic efficiency (“Cyl CE”). The unit of the data for the first cycle capacity is mAh / g, and the coulombic efficiency is in percentage (%).

[0077] Table 1. Cycling results of various test cells post-annealed at 300°C for 12 hours.

[0078]

[0079] Table 2. Cycling results of various test cells post-annealed at 600°C for 12 hours.

[0080]

[0081] The data in Tables 1 and 2 are plotted in Figures 2a and 2b. Figure 2a is a graph 200 that plots the initial (first cycle) discharge capacity of the 18 test cells determined in Tables 1 and 2. In Figure 2, the data from Table 1 (300°C annealing) is shown on the left half 202 of the graph, while the data from Table 2 (600°C annealing) is shown on the right half 204 of the graph. Graph 200 shows that most of the candidate test cells have similar or only slightly reduced initial discharge capacities relative to the control test cells. Only four candidate cells containing Al(Ac)3 as the coating precursor showed significantly reduced discharge capacities relative to the control cells.

[0082] FIG2b is a graph 250 plotting the coulombic efficiency (CE) of the 18 test cells identified in Tables 1 and 2. As shown in FIG2a, the data from Tables 1 and 2 were split in half. FIG250 shows that, with the exception of several compositions containing Al(CH3CO2)3, almost all candidate test cells had improved initial CE percentages relative to the control cell. The cycling performance of these 18 test cells was tested, and the results are shown in FIG3a, 3b, 4a, 4b, 5a, and 5b.

[0083] Figure 3a is a graph 300 of discharge capacity plotted against cycle number for the candidate test cells and control cells shown in Table 1, which were post-annealed at 300°C. Figure 3b is a graph 350 of cyclic capacity retention plotted against cycle number for the candidate test cells and control cells shown in Table 1 and Figure 3a. Figure 4a is a graph 400 of discharge capacity plotted against cycle number for the candidate test cells and control cells shown in Table 2, which were post-annealed at 600°C. Figure 4b is a graph 450 of cyclic capacity retention plotted against cycle number for the candidate test cells and control cells shown in Table 2 and Figure 4a.

[0084] Figures 3a, 3b, 4a, and 4b show that all candidate test cells with coated disordered rock salt cathode materials provided improved cycling performance relative to the control test cell. Candidate test cells containing phosphate precursors AlPO4 and NH4H2PO4 were observed to generally perform better compared to oxide coating precursors. As shown in Figures 3b and 4b, candidate test cells with 5 wt% coating were observed to have better cycling capacity retention compared to 2 wt% coating under both post-annealing conditions.

[0085] Figure 5a is a graph 500 of CE versus cycle number plotted for the candidate test cells shown in Table 1 and the control cells (which were post-annealed at 300°C). Figure 5b is a graph 550 of cell resistance growth rate versus cycle number plotted for the candidate test cells and the control cells shown in Table 1 and Figure 5a. The resistance growth rate is expressed as a percentage of the resistance at a given cycle number relative to the resistance at the first or initial cycle ("Cyx / Cy1"). The trends plotted for the cyclic CE in Figure 5a and the cell resistance growth rate in Figure 5b are consistent with the trends for the cyclic capacity retention shown in Figure 2b. That is, the candidate test cells showed improved cycling performance relative to the control test cells in terms of CE and limiting cell resistance growth rate. The cyclic CE and cell resistance growth rate for the candidate cells post-annealed at 600°C were not plotted because the results were similar to the trends shown in Figures 5a and 5b.

[0086] Experimental results show that candidate test cells containing a coated disordered rock salt cathode material according to embodiments described herein exhibit superior cycling performance compared to control cells lacking a coating on the disordered rock salt cathode material. For example, it is believed that the coating stabilizes the disordered rock salt surface, which leads to better cycling performance. The fact that the candidate cells exhibited greater initial and cycled CEs compared to the control cells while achieving similar cycling capacities suggests that fewer side reactions occurred between the disordered rock salt phase material and the electrolyte during charging. While slightly lower initial reversible capacities were observed for the candidate test cells compared to the control, with the exception of the cell with Al(CH3CO2)3, the differences were insignificant and masked by the cycling performance gains. While all coating precursors performed better during cycling compared to the control cells, the phosphate precursors AlPO4 and NH4H2PO4 generally performed slightly better than the oxide precursors and were therefore preferred. The TiO2 precursor generally performed better than the Al(CH3CO2)3 precursor and was therefore preferred over Al(CH3CO2)3.

[0087] Additional experiments using disordered rock salts of the same composition but different batches were coated in the same manner as described above with both a control and a coated disordered rock salt formed using a phosphorous acid (H3PO3) precursor. The amount of phosphorous acid was 1% and 3% of the weight of the disordered rock salt and phosphorous acid. Similarly, for the disordered rock salts coated with phosphorous acid at the same concentration, the same disordered rock salt and process were repeated with ammonium dihydrogen phosphate (ADP). The phosphorous acid coated disordered rock salt was examined by scanning electron microscopy with energy dispersive X-ray analysis, showing a relatively uniform distribution of phosphorus. The cycling performance of the uncoated half-cell (control) and the phosphorous acid coated disordered rock salt cathode and lithium metal anode was Figure 6As shown in . It is apparent that these coated disordered rock salt cathode cells have significantly improved cycle life without sacrificing significant initial capacity. The ADP coated disordered rock salt half-cell improves cycling performance, but not to the same extent as the phosphorous acid disordered rock salt coated half-cell. Without limitation, it is believed that phosphorous acid decomposes to form phosphoric acid and phosphine, which react to form lithium phosphate (Li3PO4) and lithium phosphide (Li3P), which may explain the improved performance. That is, without being limited in any way, it is believed that the presence of phosphine may further benefit the coated disordered rock salt cathode cell.

[0088] A full cell of phosphorous acid-coated disordered rocksalt was prepared, and as Figure 7 As shown, the full cell exhibits improved cycling performance compared to the half-cell, achieving approximately twice the cell lifespan of an uncoated disordered rocksalt cathode (control).

[0089] As used herein, various modifiers such as "about," "substantially," and "approximately" inserted before a numerical value indicate that the value may represent other values ​​above and / or below the specified value within a specified threshold range, such as a value within 5%, 10%, or 15% of the specified value.

[0090] It should be understood that the above description is intended to be illustrative, not restrictive. For example, the above embodiments (and / or their various aspects) can be used in combination with each other. In addition, without departing from the scope thereof, various changes can be made to adapt specific situations or materials to the teachings of the various embodiments of the present disclosure. Although the sizes and types of materials described herein are intended to limit the parameters of the various embodiments of the present disclosure, the embodiments are by no means restrictive, but rather exemplary embodiments. After reading the above description, multiple other embodiments will be obvious to those skilled in the art. Therefore, the scope of the various embodiments of the present disclosure is determined by reference to the appended claims and the full scope of equivalents to which such claims are entitled. In the appended claims and the detailed description herein, the terms "including" and "in which" are used as the plain English equivalents of the corresponding terms "comprising" and "wherein". In addition, the terms "first", "second", and "third" are used only as labels and are not intended to impose numerical meaning requirements on their objects. Furthermore, the appended claim limitations are not drafted in a means-plus-function format and are not intended to be interpreted under 35 U.S.C. §112(f) unless and until such claim limitations expressly use the phrase "means for" and are not followed by a description of the functionality of the further structure.

[0091] This written description uses examples to disclose various embodiments of the disclosure, including the best mode, and to enable any person skilled in the art to practice the various embodiments of the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the various embodiments of the disclosure is defined by the claims, and may include other embodiments that occur to those skilled in the art. Such other embodiments are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements and do not differ substantively from the literal language of the claims.

Claims

1. A cathode, the cathode comprising: a disordered rock salt phase material, and a coating disposed on a surface of the disordered rock salt phase material, the coating comprising one or more of titanium oxide, lithium metal phosphate, lithium phosphate, LiAlO2, Li2TiO3, phosphide, and fluoride, wherein the coating comprises Li3PO4 and Li3P.

2. The cathode according to claim 1, wherein the disordered rock salt phase material is represented by chemical formula (i): Li x N y M z Oh b-a F a (i) in, 1.0 < x < 1.65; 0 ≤ y < 0.55; 0.1 < z < 1; 0 ≤ a < 0.8; 1 ≤ b ≤ 3; N is one of Nb, Ti, Ta, Zr, W, Sb, or Mo; and M is one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Zr, Y, Mo, Ru, Rh, or Sb.

3. The cathode according to claim 2, wherein b = 2.

4. The cathode according to any one of the preceding claims, wherein the coating is present in an amount of not less than 0.05 wt% and not more than 10 wt% relative to the total weight of the disordered rock salt phase material and the coating.

5. The cathode according to claim 4, wherein the coating is present in an amount of 1 wt% to 5 wt% relative to the total weight of the disordered rock salt phase material and the coating.

6. The cathode according to claim 4, wherein the coating is present in an amount of 2 wt% to 4 wt% relative to the total weight of the disordered rock salt phase material and the coating.

7. The cathode according to claim 1, wherein the coating comprises one or more of Li3Al2(PO4)3 and TiO2.

8. The cathode according to claim 4, wherein the disordered rock salt phase material has a Fm-3m crystal structure.

9. A method for forming a cathode, the method comprising: mixing a disordered rock salt phase material with one or more coating precursors to form a mixture, the one or more coating precursors comprising one or more of the following: Al(CH3CO2)3, a phosphate that decomposes during annealing to form lithium phosphate or lithium metal phosphate, lithium phosphide, lithium metal phosphide, and TiO2; and annealing the mixture to form a coated disordered rock salt powder having a coating disposed on a surface of the disordered rock salt phase material, wherein phosphine is present during annealing.

10. The method according to claim 9, wherein the one or more coating precursors comprise one or more of the following: a phosphate that decomposes during annealing to form lithium phosphate or lithium metal phosphate, lithium phosphide, lithium metal phosphide, and TiO2, and the phosphate decomposes to form Li3PO4.

11. The method according to claim 9, wherein the phosphate decomposes to form phosphine.

12. The method according to claim 11, wherein the phosphine reacts to form phosphide.

13. The method according to any one of claims 9 to 12, wherein the coating formed comprises one or more of LiAlO2, Li3Al2(PO4)3, Li3PO4, Li3P and Li2TiO3 / TiO2.

14. The method according to any one of claims 9 to 12, wherein the annealing comprises heating the mixture at a temperature of not less than 200°C and not more than 800°C for a time of not less than 0.5 hours and not more than 24 hours. The method of claim 9 , wherein the annealing comprises heating the mixture in an inert gas.

16. The method of claim 15, wherein the inert gas is flowing argon.

17. The method according to any one of claims 9 to 12, 15 and 16, further comprising grinding the coated disordered rock salt powder with one or more carbon precursors to produce a cathode active material after the annealing.

18. The method of claim 17, wherein the carbon precursor comprises carbon nanotubes. 19 . A secondary battery comprising the cathode according to claim 1 . 20 . A secondary battery comprising a cathode produced by the method according to claim 9 .

21. The cathode of claim 1, wherein the phosphide is lithium metal phosphide, lithium phosphide, or a combination thereof.

22. The cathode of claim 21, wherein the metal in the lithium metal phosphate and lithium metal phosphide is a transition metal.

23. The cathode of claim 22, wherein the metal is a metal present in the disordered rock salt.

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