Patent Application
20250096276
The invention is directed to the field of solid state rechargeable batteries.
Thermodynamically stable interface voltage windows of solid electrolytes are often narrower than the operational voltage range necessary in a commercial battery. When batteries with solid electrolytes are utilized outside of their stable interface voltage windows, interfacial decomposition reactions occur that reduce the operational lifetime of the batteries. Previous attempts to solve this problem have focused on finding and incorporating coating materials for the solid electrolytes that are thermodynamically stable within the desired operational voltage range. Such searches have produced few materials containing all of the desired characteristics for a commercial battery.
Thus, there is a need for improved solid state batteries incorporating solid electrolytes.
The invention provides compounds that can be used as cathode interface coating layers in solid state batteries. The compounds disclosed herein provide enhanced dynamic stability.
In one aspect, the invention provides a compound selected from Table 1:
An aspect of the invention provides a solid state battery including an anode and a cathode including cathode particles and solid state electrolyte particles, a solid state electrolyte separating the anode and cathode, and an interface coating layer between the cathode particles and solid state electrolyte. The interface coating layer includes a compound of Table 1.
In another aspect, the invention provides a method of storing energy including applying a voltage across the anode and cathode and stably cycling any rechargeable battery disclosed herein. In another aspect, the invention provides a method of providing energy including connecting a load to the anode and cathode and discharging any rechargeable battery disclosed herein.
The invention provides a solid state battery with an interface coating layer between cathode particles and solid electrolyte particles in the cathode that, under applied voltage, evolves from unstable to stable, providing a dynamic voltage stability for advanced battery performance. A constrained ensemble computational approach systematically evaluated and compared dynamic stability voltage windows in response to the mechanical constriction effect. High-throughput calculations screened coating materials for different interfaces between sulfide, halide, and oxide electrolytes and typical cathode materials with enhanced dynamic voltage stability. A demonstration with an assembled battery containing cathode particles with an interface coating layer to solid state electrolyte in the cathode layer shows the value of these computations to confirm the validity of predicted compounds described.
Solid state batteries are one of the most promising next-generate energy storage technologies, due to the potential to apply lithium metal anode for high energy density and much improved safety by preventing lithium dendrite penetration.[1-4] For battery applications, the Li ion conductivity, voltage stability window, and mechanical properties are three key electrolyte parameters. Mechanical properties of solid electrolytes are of particular interest in solid state batteries. Low modulus of sulfides enables better contacts between the particles in the electrolyte and cathode mixture by a simple cold-press calendaring procedure.[5-7] More importantly, these three parameters are often strongly coupled in a solid-state battery to greatly influence electrochemical behaviors.
In theory, the strictest definition of voltage stability window refers to the voltage range that the electrolyte can work without any electrochemical decompositions thermodynamically. Precise calculations of such intrinsic voltage windows of various types of solid electrolytes have been performed previously.[8-12] However, in practice those intrinsic voltage windows are often narrower than the operational voltage range needed by a full battery, thus various decomposition reactions can still happen. This is especially true for sulfide solid electrolytes, where the intrinsic voltage window is only around 1.7˜2.3 V. Even considering the delithiation capacity in sulfide electrolyte, the effect can only widen the electrolyte voltage window to 2.5˜3 V.[13-14]
In stark contrast, sulfide electrolyte-based solid state batteries can cycle well in experiment in a wide voltage range with Li metal anode and 4V cathodes, up to high current densities around 50 mA/cm2, and in a wide operational pressure range from several hundred MPa down to a few MPa[2, 4, 15-20]. These experimental facts suggest that certain stabilization mechanism must play a critical role here to widen the practical operational voltage window of solid-state batteries beyond the intrinsic voltage stability predicted by the standard convex hull computational approach.
It was found that for all-solid electrolyte batteries, although small decomposition could happen beyond the intrinsic voltage window, they often show self-limiting decomposition, meaning that the decomposition can stop quickly at a certain stage, giving the wide operational voltage stability in practice. This is in drastic difference to the case when the solid electrolyte is immersed in a liquid electrolyte, where the electrolyte decomposes deeply[5]. This is because in the former case any volume expansion decomposition reaction has to overcome the mechanical constriction imposed at the solid-solid interface by the all-solid environment, which is a critical factor lacks in the latter case with liquid being added.
The local effective mechanical constriction modulus, Keff, on the order of the bulk modulus of electrolytes has been proposed to strongly correlate with operational electrochemical stability through interactions with such positive reaction strains,[4-5, 15, 21-23] where the reaction strain has been observed experimentally together with advanced battery performance. Solid-solid interface under mechanical constriction was shown to be able to penalize decomposition reaction nuclei with an energy scale on the order of KeffεV, where E is the local reaction strain and V is a reference volume. That is, the effect can lead to a dynamic evolution from interface instability to stability, giving the so-called dynamic voltage stability for advanced performance of solid-state batteries with greatly widened operational voltage window of sulfide electrolytes in contact with 4 V cathode and 0 V Li metal anode.[5, 16, 24]
In this work, we first articulate our state-of-the-art perspective on the thermodynamic and kinetic constitution of the dynamic voltage stability. We broaden the meaning of Keff to include the kinetic stability, which allows the KeffεV energy penalty to effectively stabilize interface reactions when the local stress is smaller than the fracture limit. This is a critical development of our constrained ensemble description for interface reactions in solid-state batteries, since most solid electrolyte materials do not have a high fracture toughness[25], but many of them can exhibit operational interface voltage stability way beyond the predicted limit of their thermodynamic voltage stability. We then further investigate the dynamic voltage stability for all the main types of Li solid electrolytes, including chalcogenides, oxides, halides and borohydrides, as well as their interface stability with coating materials for classic oxide cathodes. Here we apply our constrained ensemble computations across these solid-state electrolytes (SSEs) to systematically evaluate and compare their dynamic stability voltage windows in response to the mechanical constriction effect. High-throughput calculations based on pseudo-binary approach are used to search for coating materials for different interfaces between electrolyte and cathode materials with enhanced dynamic stability.
A comparison with experiment is given based on a readily available coating procedure for LiNbO3 to demonstrate the unique prediction capability of our computational approach to design dynamic voltage stabilities by interface coatings. The detailed agreement between computation and experiment further highlights the potential value of the ˜150 new cathode interface coating materials predicted in this work. Our work thus will speed up the solid-state battery development by providing a promising list of candidate coating materials to the field with a potential to significantly stabilize the cathode interface reactions during the battery cycling.
Batteries of the invention include an anode, a cathode including cathode particles and solid state electrolyte particles, a solid state electrolyte, and an interface coating layer between the cathode particles and the solid state electrolyte particles in the cathode layer. The interface coating layer can include any compound from Table 1.
The interface coating between the cathode particles and solid state electrolyte particles includes a compound of Table 1. Interface coating layers may also include additional materials, such as polymers from the coating procedure as described herein. Upon battery cycling, the interface coating layer will react with either cathode or solid state electrolyte, or both, so that chemical elements from both cathode and solid state electrolyte may be mixed into the interface coating layer through interface reactions or forming new phases. These interface reactions, however, will be self-limited to stop quickly, so that the thickness of interphase reaction layer is well limited. The as-formed interphase layer will provide both electrochemical voltage stability in the following cycles and sufficient Li ion conductivity due to the limited thickness.
The thickness of the interface coating layer surrounding the cathode particles or solid electrolyte particles can be 0.1 nm to 1 μm, e.g., about 1-100 nm (e.g., about 1-10 nm, 1-25 nm, 10-20 nm, 20-30 nm, 25-50 nm, 30-40 nm, 40-50 nm, 50-60 nm, 50-75 nm, 60-70 nm, 70-80 nm, 75-100 nm, 80-90 nm, or 90-100 nm, e.g., about 1 nm, 5 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, or 100 nm), e.g., about 100-1000 nm (e.g., about 100-110 nm, 100-125 nm, 100-200 nm, 200-300 nm, 250-500 nm, 300-400 nm, 400-500 nm, 500-600 nm, 500-750 nm, 600-700 nm, 700-800 nm, 750-1000 nm, 800-900 nm, or 900-1000 nm, e.g., about 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm).
Anodes of the invention may be any suitable anode known in the art, such as Li metal. For example, lithium metal foil, e.g., Li metal foil on a current collector, e.g., of stainless steel. The lithium metal can also mix or alloy with Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Ag, Cd, In, Sn, Sb, Bi, Cs, Te, or a combination thereof to form one single layer.
Anodes may be deposited on an appropriate substrate, e.g., a fluoropolymer or carbon. For example, liquefied polytetrafluoroethylene (PTFE) has been used as the binder when making solutions of electrode materials for deposition onto a substrate. Other binders are known in the art. The anode materials described herein can be used without any additives. Alternatively, the anode material may have additives to enhance its physical and/or ion conducting properties. For example, the anode materials may have an additive that modifies the surface area exposed to the solid electrolyte, such as carbon. Other additives are known in the art.
Cathode materials can be chosen to have optimum properties for ion transport. For example, the cathode may preferably be LiNi0.8Mn0.1Co0.1O2 (NMC811) due to its high capacity, energy density, and also cost effectiveness due to the decreased composition of the expensive Co element. Other materials for use as electrodes in solid state electrolyte batteries are known in the art.
The cathodes may be a solid piece of the material, or alternatively, may be deposited on an appropriate substrate, e.g., a fluoropolymer or carbon. For example, liquefied polytetrafluoroethylene (PTFE) has been used as the binder when making solutions of cathode materials for deposition onto a substrate. Other binders are known in the art. The cathode material can be used without any additives. Alternatively, the electrode material may have additives to enhance its physical and/or ion conducting properties. For example, the cathode materials may have an additive that modifies the surface area exposed to the solid electrolyte, such as carbon. Other additives are known in the art.
In some embodiments, the cathode can include, e.g., LiNi0.8Mn0.1Co0.1O2 (NMC811), LiNi0.33Mn0.33Co0.33O2 (NMC111), LiNi0.5Mn0.3Co0.2O2 (NMC532), LiNi0.6Mn0.2Co0.2O2 (NMC622), LiNi0.9Mn0.05Co0.05O2 (NMC955), LiNixMnyCo(1-x-y)O2 (0≤x,y≤1), LiNixCoyAl(1-x-y)O2 (0≤x,y≤1), LiMn2O4, LiMnO2, LiNiO2, Li1-zNixMnyCo(1-x-y-z)O2 (0≤x,y,z≤1), Li1+zNixMnyCo*Al(1-x-y-z-s)O2 (0≤x,y,z,s≤1), Li1-zNixMnyCosW(1-x-y-z-s)O2 (0≤x,y,z,w≤1), V2O5, selenium, sulfur, selenium-sulfur compound, LiCoO2 (LCO), LiFePO4, LiNi0.5Mn1.5O4, Li2CoPO4F, LiNiPO4, Li2Ni(PO4)F, LiMnF4, LiFeF4, or LiCo0.5Mn1.5O4. The cathode can be coated with LiNbO3, LiTaO3Li2ZrO3, LiNbXTa1-XO3 (0≤x≤1), yLi2ZrO3-(1-y)LiNbXTa1-xO3 (0≤x, y≤1), Al2O3, TiO2, ZrO2, AlF3, MgF2, SiO2, ZnS, ZnO, Li4SiO4 Li3PO4. Li3InCl6, Li1+xAlxTi2-x (PO4)3(0<x<2), LiMn2O4, LiInO2—LiI, Li6PS5Cl, LiAlO2, a polymer, or carbon. In some embodiments, the cathode includes a polymer and/or carbon black, or the first and/or second solid electrolytes include a polymer.
The cathode can be mixed with polymer and/or carbon. Examples of polymers may include polyethylene oxide, polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene), poly(ethyl methacrylate), or poly(vinylidene fluoride-co-trifluoroethylene). The particle size of cathode materials can be 1 nm-30 μm.
The loading of the cathode can be 0.1-100 mg/cm2. The thickness of the cathode can be 5 μm-2000 μm. The cathode may be mixed with solid state electrolyte materials to provide increased cathode capacity.
Other cathode materials such as selenium or sulfur that exhibit promising high capacity and energy density also show much better cycling performance in our multilayer design than the single layer design.
In certain embodiments, the cathode can be mixed with polymer and carbon black, solid electrolytes can be mixed with polymer. Examples of polymers may include polyethylene oxide, polyvinylidene fluoride, poly(vinylidene fluoride-co-hexafluoropropylene), poly(ethyl methacrylate), or poly(vinylidene fluoride-co-trifluoroethylene). The thickness of the solid electrolyte layer is 5-1000 μm. The thickness of the cathode can be 5-2000 μm.
Suitable solid state electrolytes that may be used in the invention include inorganic solid electrolytes, e.g., crystalline or glassy inorganic lattices with high ionic conductivity, in which ions (e.g., Li+ ions) can diffuse through the lattice. SSEs may be, for example, oxides, halide, chalcogenides, borohydrides, phosphates, or sulfides of lithium (e.g., LGPS, LiSiPS, LiPS, Li5.5PS4.5Cl1.5 (LSPCl1.5), Li6PS5Cl1.0 (LPSCl1.0).
Other solid state electrolyte materials that may be suitable include sulfide solid electrolytes, e.g., SixPySz, e.g., SiP2S12, or β/γ-PS4. Other solid state electrolytes include, but are not limited to, germanium solid electrolytes, e.g., GeaPbSc, e.g., GeP2S12, tin solid electrolytes, e.g., SndPeSf, e.g., SnP2S12, iodine solid electrolytes, e.g., P2Sal crystals, glass electrolytes, e.g., alkali metal-sulfide-P2S5 electrolytes or alkali metal-sulfide-P2S5— alkali metal-halide electrolytes, or glass-ceramic electrolytes, e.g., alkali metal-PgSh-i electrolytes. Other solid state electrolyte materials are known in the art. The solid state electrolyte material may be in various forms, such as a powder, particle, clay, or solid sheet. An exemplary form is a powder.
Solid electrolytes may be deposited or cast on an appropriate substrate, e.g., a Polyester (PET) film, a cathode, an anode, or other layers of solid electrolytes. For example, Nitrile rubber (NBR), Acrylate rubber (ABR), Polyisobutene (PIB) have been used as the binder when making solutions of electrolyte materials for deposition onto a substrate. Other binders are known in the art. Solid electrolytes may be mixed with solvents (e.g., p-xylene, isobutyl isobutyrate or a mixture thereof, e.g., anhydrous p-xylene and isobutyl isobutyrate (1:1 vol/vol)) and binders (e.g., a polymer, e.g., an arylate polymer, e.g., from 0.5% to 5 wt %) to prepare a slurry for layer formation. Multiple layers of electrolytes can be deposited or cast or transferred to a substrate layer-by-layer. The multilayer may contain ‘n’ layers of solid state electrolytes (where n=e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, etc.).
Advantageously, the solid state electrolyte may adopt a core-shell particle structure, e.g., core-shell LPSCl-X (where X is a halide) or LGPS (Li10GeP2S12) (see, WO 2019/104181, WO 2020/112843, and WO2022/094412). LGPS (Li10GeP2S12) may also adopt a core-shell particle structure. The shell can include a compound of Table 1. Solid state electrolyte particles, e.g., core-shell particles, may have a cross sectional dimension, e.g., diameter, of between about 1 nm and about 30 μm, e.g., about 1-100 nm (e.g., about 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, or 100 nm), e.g., about 100-1000 nm (e.g., about 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm), e.g., about 1-10 μm (e.g., about 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm), or, e.g., about 10-30 μm (e.g., about 10 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, or 30 μm). In core-shell particles, the shell may make up from about 0.1% to about 99.9% of the particle, e.g., about 1-10%, about 10-20%, about 20-30%, about 25-50%, about 40-60%, about 50-75%, about 60-80%, about 75-90%, or about 80-99% of the particle, by, e.g., volume or mass. Stability may be determined experimentally.
In some embodiments, the battery interface is under local mechanical constriction. Mechanical constriction at the interface between the solid state electrolyte and the cathode can limit the extent of chemical or electrochemical decomposition of solid state electrolyte materials by volumetric constraint. Local stress on the order of a few GPa up to the fracture limit of solid electrolyte, may be generated from mechanical constriction. Local compressive strain at reaction front can indue a diffusion limiting process to limit interface reaction, contributing a kinetic part to the local effective constriction modulus beyond the fracture limit. One condition to implement the mechanical constriction can be an external pressure applied to the battery cell of at least 0.1 MPa up to several hundred MPa. The level of external pressure for a battery is determined by the battery material, material processing, and battery assembly methods. Mechanical constriction may be provided by a formation pressure from cold and/or hot and/or warm isotropic and/or anisotropic press and/or rolling with the external pressure on the order of 0.1 MPa to 1000 MPa and temperature at 25° C.-500° C. Examples of suitable assembly methods include, but are not limited to, warm isotropic pressing (WIP), cold isotropic pressing (CIP), and hydraulic cold pressing of the battery cell or pouch. The mechanical constriction may result from an applied pressure of at least 0.05 MPa, e.g., at least 0.1 MPa, 0.5 MPa, 1 MPa, 5 MPa, 10 MPa, 15 MPa or 20 MPa, e.g., about 0.05 MPa to about 50 MPa, e.g., about 0.05 MPa to about 0.1 MPa, about 0.075 MPa to about 0.15 MPa, about 0.1 MPa to about 1 MPa, about 0.1 MPa to about 10 MPa, about 1 MPa to about 30 MPa, about 20 MPa to about 40 MPa, about 30 MPa to about 50 MPa, about 40 MPa to about 60 MPa, about 50 MPa to about 70 MPa, about 60 MPa to about 80 MPa, about 70 MPa to about 90 MPa, or about 80 MPa to about 100 MPa, about 100 MPa to about 200 MPa, about 200 MPa to about 400 MPa, about 300 MPa to about 500 MPa, about 400 MPa to about 600 MPa, about 500 MPa to about 700 MPa, about 600 MPa to about 800 MPa, about 700 MPa to about 900 MPa, or about 800 MPa to about 1,000 MPa, e.g., about 70 MPa, about 75 MPa, about 80 MPa, about 85 MPa, about 90 MPa, about 95 MPa, about 100 MPa, about 150 MPa, about 200 MPa, about 250 MPa, about 300 MPa, about 350 MPa, about 400 MPa, about 450 MPa, about 500 MPa, about 550 MPa, about 600 MPa, about 650 MPa, about 700 MPa, about 750 MPa, about 800 MPa about 850 MPa, about 900 MPa, about 950 MPa, or about 1,000 MPa. Greater mechanical constriction may be applied during battery fabrication. After providing the formation pressure, the porosity of the cathode, and/or multilayer may be 0%-50%. In some embodiments, the mechanical constriction is sufficient to raise the local effective modulus above Keff, thereby preventing decomposition, or such that a local stress field caused by decomposition of the solid state electrolyte raises Keff above Kcrit, thereby arresting decomposition.
When the battery is operating, the local stress can be maintained by applying an operational stack pressure on the order of between 0 MPa and 1000 MPa. Alternatively, the local stress may be maintained without applying an operational stack pressure.
Methods Methods of storing and releasing electrical energy involve using electrical energy to charge a solid state battery by applying a voltage across the battery that causes Li to migrate as Li+ ions from a cathode to an anode, where the Li deposited, thereby storing the electrical energy as chemical energy. In discharge, the Li metal is oxidized to Li+ and migrates back to the cathode.
To release (discharge) the stored chemical energy as electrical energy, a load is electrically connected between the anode and cathode in a circuit to allow the Li+ ions to migrate from the anode via the solid state electrolyte to the cathode.
Methods of the invention may involve repeating the above cycle multiple times, e.g., greater than 1000 times, e.g., 1000-20,000 times (e.g., 1,000-1,500 times, 1,250-1,750 times, 1,500-2,000 times, 1,500-2,500 times, 2,000-3,000 times, 2,500-5,000 times, 5,000-10,000 times, 5,000-15,000 times, 10,000-20,000 times, or 15,000-20,000 times).
The method may include first allowing a portion of the interface coating layer to form a surface layer on the cathode, e.g., in an initial charge-discharge cycle. Alternatively, the cathode may already have an interface coating layer. All suitable techniques can be used to apply the compounds of Table 1 to cathode particles, e.g., chemical synthesis, atomic layer deposition, chemical vapor decomposition, sputtering, pulsed laser decomposition, etc.
Here, we first explain the physical meaning of the dynamic voltage stability using our state-of-the-art understanding and formula description, which forms the foundation for further high-throughput search and design of interface coating materials with enhanced such stability. The local effective modulus Keff and the effective stress σeff=εKeff describe the level of local mechanical constriction. It is important to note that the effective stress can often be larger than the actual local stress σlocal (
The plastic local strain field ε together with the actual local stress σlocal from the initial local decomposition provide the common strain energy Estrain∝σlocalεV for thermodynamic metastability for any interface voltage reaction. More precisely, Estrain=∫σlocal Vdε. However, the magnitude of the strain energy is limited by the plastic deformation and the fracture limit of electrolyte materials, beyond which there is no local mechanical constriction. Simultaneously, at the reaction front, the same local inhomogeneous strain field ε can significantly decrease the ionic interdiffusion in the electrolyte by orders of magnitude to kinetically prevent further decomposition propagation, giving an ionic passivation effect from local reaction strain induced diffusion limiting process. Interface reactions will thus feel the significant effect from an additional energy stabilization term from the kinetic stability, Ekinetic, for much wider interface voltage stability than what can be provided by thermodynamic metastability.
Since both energy terms of Estrain and Ekinetic share the same local reaction strain term e, we have Ekinetic=∫σeffkineticVdε, which defines the kinetic part of the effective stress σeffkinetic. The total stabilization energy that includes both thermodynamic metastability and kinetic stability for interface reactions is thus Etotal=Estrain+Ekinetic=∫(σlocal+σeffkinetic) Vdε, which defines the total effective stress σeff=σlocal+σeffkinetic. Since σeff=εKeff, we thus also have Etotal=ϵKeffV in the simplest format as the effective mechanical constriction energy.
More specifically, as illustrated in
This kinetic stability mainly requires a positive reaction strain at a local reaction interface under mechanical constriction (i.e., low porosity at micrometer scale as in
Thus, in liquid electrolyte batteries, the reaction front can propagate deeply to consume the electrolyte[5], as the reaction strain field is flat with small curvature to be more easily released to the liquid environment, giving little effective stress to self-limit the decomposition. In contrast, in solid state batteries, reaction strain was found to build up plastically without a release. This is due to the inhomogeneous μLi(x), giving large local curvatures of the strain field (
The effective stress σeff=ϵKeff thus could be larger than both the actual local stress σlocal and the fracture limit σfrac of the electrolyte materials without forming any actual fractures, as long as σeff>σfrac>σlocal is satisfied. In practice it will need the initial local decomposition to be suppressed quickly so that σlocal is maintained at a low level, which is a property of interface reaction that can be designed. In addition, it also needs the electrolyte material to exhibit sufficient plastic deformation capability.
Therefore, in solid state batteries there could be an important dynamic evolution of electrochemical process, where the local compressive strain at the reaction front induced by the tensile strain from initial decomposition will kinetically shut down the ionic diffusion locally by encapsulating the local decomposition by the ionically passivated reaction front layer, preventing further decomposition and crack formation. To design such interface reactions, technically, for any electrolyte material or its interface with electrodes, there is a critical effective modulus, Kcrit[26] or K*[15], beyond which the local reaction can be fully suppressed. This critical modulus can be calculated by making KeffϵV equal the decomposition hull energy Ehull, thus a smaller K* or Kcrit is preferred, as it suggests that the decomposition is easier to be suppressed by Keff.
Importantly, we also point out here that since for a given decomposition hull energy Ehull, a larger local reaction strain ϵ will give smaller K* from Ehull=K*ϵV, and simultaneously, larger e also indicates stronger ionic passivation at the reaction front, looking for interfaces with smaller critical modulus K* thus also forms one important aspect to design the kinetic stability induced by the ionic passivation effect.
The above description forms our state-of-the-art understanding of the so-called dynamic voltage stability or simply dynamic stability that was proposed previously in an experimental work,[4] where advanced battery performance was demonstrated by utilizing the effect. This interpretation of dynamic voltage stability also goes beyond our previous works[5, 15, 26] by clearly stating that first, significant portion of the kinetic stability energy Ekinetic is already included in the term of KeffϵV; second, the inhomogeneous local lithium chemical potential μLi(x) at the solid-solid interface is critical to the formation of the plastic reaction strain; and third, the quantitative condition of σeff>σfrac>σlocal needs to be satisfied to avoid fractures with sufficiently small local reaction stress, which however shares the same local reaction strain to simultaneously prevent further decomposition by sufficient effective stress. This interpretation forms an indispensable foundation for our computational approach to design dynamic stability presented in the following sections regarding intrinsic voltage stability window and interface coating materials that is of importance to the performance of solid state batteries.
The dynamic voltage stability window of solid electrolytes is calculated by the minimization method (see Methods). We systematically calculated the voltage window and reaction strain in response to mechanical constriction for three mainstream types of electrolytes, including sulfides, halides and oxides (
Outside the stability window at a given voltage, the thermodynamic decomposition energy decreases with increasing Keff. With increasing Keff the reaction strain also decreases and approaches zero eventually, giving the limit of voltage window opened by this thermodynamic metastable process. However, since the thermodynamic driving force for decomposition also decreases with increasing Keff, it makes other nonequilibrium decomposition processes with larger reaction strains become more competitive and thus more likely to happen. Those nonequilibrium reactions thus may override the thermodynamic metastable evolution pathway, which is an effect that could give an even wider operational voltage window in a properly designed practical battery than what can be predicted by the minimization method here.
As shown in
For Uw0, the values for sulfides are the lowest, while oxides and halides with stronger chemical bonding due to higher electronegativity of O2−, Cl− and Br− show much higher Uw0. Therefore, in the (kox−kre) vs. Uw0 plot (
Since kre is zero only for oxide, for a fair comparison with halide and sulfide, we plot kox vs. Uw0 in
The results of 7 representative sulfides are shown by the blue bars. Most of their oxidative limits can be opened from ˜2.5 V to larger than 3V. LGPS and Li5.5PS4.5Cl1.5 (LPSCl) can be opened to larger than 4V. The effect of mechanical constriction on glass sulfides, glass-ceramic sulfides, thio-LISICON, including Li3PS4, Li7P3S11, LGPS, LSPS have been retrospectively reviewed[23], suggesting that constriction induced voltage stability should have been a key concept in sulfides since the first glassy-ceramic sulfides.
Noted that other than LGPS and Li5.5PS4.5Cl1.5, the oxidative limits do not or barely further increase with the increase of Keff from 10 GPa to 20 GPa. This is due to the existence of decomposition with negative reaction strains (defined to be 0 in calculation, as discussed in Methods) shown by the light-blue color above the oxidative limit, causing no thermodynamic stability in response to constriction. In these cases, dynamic voltage stability can be further adjusted by using the strategies of coating or more generally the interface composition modification, as we will discuss in part B. Briefly, the requirement of Keff can be lowered for a given oxidative limit, as long as higher reaction strain can be designed to such an interface so that the decomposition can be more easily suppressed (i.e., reducing the critical effective modulus K*[15,26]).
The result of six representative oxides is shown by the green bars in
Noted that Li3OCl and Li3OBr are with low enough modulus to be cold pressed into a dense pellet and have a 4.8 V oxidative limit at Keff≈0.5 Kv. Together with high Li conductivity on the order of 10−3-10−2 mS/cm[39, 43-44], they are also very promising high voltage electrolytes. In computation, the decomposition products are Li, LiCl, LiClO4 beyond oxidative limit at Keff from 0 GPa to 20 GPa. Experimentally, trace amount Ba doped Li3ClO has shown high RT Li conductivity of 25 mS/cm, and its oxidative limit is measured to be higher than 8 V even at 130° C.[43]. The much wider 8 V stability than the 4.8 V stability in our computation could be due to the high kinetic barrier in forming the high valence Cl7+ in the decomposition product of LiClO4. Other potential decomposition products in Li—Cl—O system could be non-solid ClxOy that become energetically competitive at RT than the OK products, which result in larger reaction strain and thus a dynamic voltage window beyond 4.8 V. Therefore the 8 V stability of Li3ClO could be due to such kinetic stability beyond the constriction induced thermodynamic metastability at RT.
Six halide electrolytes of Li3MX6 (M=Y, In, Er, Sc, X=Cl, Br) are considered. Their oxidative decomposition products are always Li, MX3 and X2 and the increase of reaction strain in a few tens of volts above the oxidative limit is due to the increasing portion of partial decomposition of Li3MX6 in the multi-phase equilibrium region of Li-M-X grand potential phase diagram. If there is any Cl2 gas release in such a battery, the contribution from the volume of Cl2 to reaction strain will disappear, causing negative reaction strain of ˜−15%, thus probably no broadening of voltage window above 4.3 V. As the data in Materials Project are calculated at 0 K, Cl2 is crystalized molecules cluster. Removing Cl2 from potential decomposition products in computation will give infinite oxidation limit since other compounds in the Li-M-X systems cannot compose a balanced reaction equation due to the lack of Cl-rich phases. Note that Cl-rich NaxCly phases, i.e. NaCl3 and NaCl7 can form under GPa order of pressure[45], Cl-rich LixCly counterparts may also exist and can form under GPa level mechanical constriction. As there is no report on Cl2 gas release experimentally, it is possible that halide electrolytes are either well constricted in batteries so that it is stable, or CI-rich LixCly phases are formed during decomposition. In such a case, the predicted voltage window is shown at the bottom of
The interface electrochemical stability is calculated by pseudo-phase approach[12] within the perturbation method framework (see Methods and
Lastly, we examine the electrochemical stability of the interfaces by unconstrained and constrained ensemble pseudo-binary interface simulations: If the interface electrochemical decomposition energy at 4 V is <50 meV/atom at Keff=20 GPa, the material will become a coating candidate to help stabilize cathode interface reaction. We choose 20 GPa because it is a value roughly half of the bulk modulus of sulfide or halide electrolyte materials, which is around the maximum mechanical constriction that can be provided at the solid-solid interface based on the inclusion model[22]. Our goal is thus to predict cathode coating materials that can bring the critical effective modulus K* below 20 GPa, where a lower K* below 10 GPa or 5 GPa is in general more preferred, as they make the interface reaction easier to be suppressed by local mechanical constriction Keff.
There are 91 Li containing materials that passed all the screening steps. We selected 49 of them within ICSD id to present in
Without coating, the direct interface between cathode and electrolyte cannot be electrochemically stabilized even at 23 GPa due to small response to constriction as indicated by the large K* value in
For the LiNbO3 and cathode interfaces, the decomposition products are just Li, O2 and Nb2O5, so the interface is in principle stable. However, LiNbO3 itself is thermodynamically unstable at 4V at 0 GPa. Nevertheless, we find that the 4V decomposition can be stabilized by a modest 2 GPa constriction. Note that such a small constriction might also be provided by Nb diffusion[46-47] induced surface tension at the cathode particle level in a liquid electrolyte battery, so that although unconstrained at the cell device level, Li—Nb—O coating can still improve the cycling performance of NCM811. Comparing LiNbO3 with other predicted coatings in
Here we use LiNbO3|LiCoO2 (LCO) interface as a model system to experimentally demonstrate the importance of dynamic voltage stability in predicting coating materials for solid state batteries. For solid state battery with LGPS as cathode electrolyte (
Since it is difficult to remove mechanical constriction in a solid-state battery to make Keff=0, we tested liquid electrolyte batteries without mechanical constriction and thus Keff=0 (
Although coating is a general strategy to improve the interface stability, we found that LCO and Li(Ni0.8Mn0.1Co0.1)O2 (NMC811) can sometimes cycle with sulfide electrolytes even without cathode coating[4, 15-16]. For such an interface, the initial chemical interphase formed by the direct contact between the cathode and electrolyte during powder mixing in mortar and cathode film formation in battery assembly before the application of voltage for electrochemical reaction should be considered.[26]
We further tested uncoated LCO and NMC811 cathode with sulfide cathode electrolytes in solid-state batteries. For LCO-LGPS cathode composite, it can barely cycle directly at room temperature with less than 50 mAh/g capacity and a fast capacity decay, but surprisingly, we found that it can cycle at 55° C. (
A classic garnet type LLZO is used as an example of oxide electrolyte to explore the interface reaction by constrained ensemble (
Similar screening procedure is used here for the interface between cathode and halide electrolyte. Most of the predicted coating materials for halide electrolytes are still oxides. When unconstrained (Keff=0,
In this work, we systematically reinvestigated the voltage window response of mechanical constriction of different solid-state electrolytes. The oxidative limit of sulfide electrolytes can be opened to ˜4V, where kinetic stability can play an important role below 4V for the voltage stabilization. Oxide electrolytes can be opened to more than 6V if dense pellet can be achieved, where the additional role of plastic deformation in comparison with sulfides can be better evaluated in experiment. Halide materials have the highest window opening slope upon mechanical constriction, and LixCly phases may form during decomposition instead of the gas phase Cl2.
By applying constrained ensemble to high-throughput search, we predicted several lists of coatings for Li cathode/electrolyte interfaces for sulfide, oxide, halide electrolytes. For interfaces between sulfide electrolytes and oxide cathodes, coatings with lower critical mechanical modulus K* than LiNbO3 are predicted. LiNbO3—LiCoO2 interface shows better cyclability in solid state battery than in liquid battery in our experiment, giving experimental evidence of dynamic interface stability related to coating at the solid-solid interface. The fact that some batteries can cycle without coating can be explained by the more stable interfaces with chemically formed interphase during materials mixing. For oxide materials, potential coating materials that can act as electrochemically stable binder between cathode and garnet electrolyte are in the prediction list, including the existing B-doped Li2CO3 and Li3PO4. For halide materials, we discussed the possibility of forming LixCly at the interface with cathodes. Our work will shed light on the future design of electrolyte and electrode interface reaction by explicitly considering the effect of dynamic voltage stability. An application of the new design strategy in future experiments will further advance the performance of solid-state batteries.
The computational modeling for constriction induced voltage stability is illustrated in
In eqn. (1), Deci denotes the ith decomposition product and di is its stoichiometry. In eqn. (2), the reaction strain is defined as the fraction of the difference between the final volume and the initial volume. Note that the reactant volume of Li metal in the reduction reaction at anode is not counted in the calculation of reaction strain, but Li metal product volume in the oxidative reaction at cathode is counted. Below the reduction limit, Li metal reactant is absorbed into the SSE to form the interface reaction decomposition, which contributes to the local volume expansion of the decomposition and the formation of the plastic strain field surrounding the decomposition reaction front that is of importance to the dynamic voltage stability of interest here. Since Li metal reactant initially is not inside the region of SSE, the volume of lithium metal reactant is thus not counted in the calculation of reaction strain that is inside SSE. Outside the region of the reaction front, Li+ ion and electron can still leave the anode region from the ion and electron reservoir to complete the reduction reaction as also required by the eU term in eq (3), which, however, does not influence the local positive reaction strain of the interface reaction, as all decomposition products are encapsulated by the ionically passivated reaction front layer. Similarly, beyond oxidation limit, although electron should go through outside circuit from cathode to anode side and then combine with a Li+ that is migrated from the Li reservoir at cathode to the anode, these electron and Li ion left the cathode are not from the reaction product of Li metal but from the reservoir outside the decomposition region encapsulated by the reaction front. Li+ ions as reaction product are trapped in the cathode decomposition region encapsulated by the ionically passivated reaction front layer, which then could combine with a neighboring electron to form Li metal. Li metal product in oxidative reaction thus contributes to the positive reaction strain.
In eqn. (3), GDec=ΣdiGDec
At Keff>0, if ΔGC-RXN(U) is moved from negative to positive values due to the positive KeffVSSEε, then the decomposition will not happen anymore, and the voltage U will be included into the expanded voltage window. Thus, the new voltage ranges from Ure (Keff>0) to Uox(Keff>0) defined by the two new reactions in
The perturbation method only considers reactions with the largest decomposition energy, so that it is computationally effective. However, there are often more than one decomposition reactions competing in the reaction space with various reaction strains, which means other decomposition reactions with smaller positive reaction strains or even negative reaction strains that are neglected in the perturbation method could happen at voltages between Ure (0) and Ure (Keff>0), and between Uox (0) and Uox (Keff >0).
A more robust approach named direct minimization method is used to consider all reactions in the reaction space.[23]
The x-interceptions of the reaction energy straight lines are the stability voltage limits, which can be expressed by equating eqn. (4) to 0:
The slope is the negative stoichiometry of Li (−n) in the eqn. (3), and the positive increase of the decomposition energy by mechanical constriction is proportional to reaction strain ε, therefore the horizontal shifts are proportional to their ε/n. If the reaction strain is negative, void will form and the constriction effect will disappear locally, thus the KeffVSSEε term will become 0 instead of being negative. This means the voltage window will not change, or equivalently we can define ε=0 in this case.
Reactions 1 and 2 are oxidative decompositions, and reactions 3 and 4 are reductive decompositions. Reactions 1 and 3 decide the voltage window at Keff=0 GPa, and the reactions 2 and 4 decide the voltage window at Keff=10 GPa. Other reactions in the reaction space besides these four are not discussed here since it turns out that they do not determine the voltage window for LGPS here. But we should keep in mind that they do exist in the energy landscape, and solid-state electrolyte can decompose into other products if different voltage and Keff values are applied, or nonequilibrium decomposition pathways override such thermodynamic metastable decompositions.
For this specific case illustrated in
When increasing Keff to 10 GPa, the energy of all reactions will increase proportionally to their reaction strain (if positive) as described by eqn. (3) and indicated by the 4 arrows in
With the illustration discussed above, we can more generally define Ure (Keff) and Uox (Keff). Each decomposition reaction has a n value, so we can scan n to scan all decompositions:
A pseudo-phase[12] composed of x coating and (1−x) SSE is denoted as pp(x). DFT phase energy Gpp(x), composition x, and volume Vpp(x) of the pp(x) are interpolated from coating and SSE. Equation (3) can then be rewritten as
Let equation (8) equal to zero, it gives the critical Keff, i.e., Kcrit, that prohibits the electrochemical decomposition of the interface, and the maximum value of Kcrit(x) to prevent decomposition at all x composition is defined as K*:
More detailed illustration regarding pseudo-phase approach and critical modulus K* by a computational example can be found in
As shown in Table 2, without mechanical constriction, LGPS decomposition leads to 30.2% reaction strain with 0.988 eV/atom decomposition energy. When we assign KeffεVLGPS energy penalty to each reaction with different reaction strain ε, the sequence of the decomposition energy magnitude in the reaction space changes. Reactions with larger ε will have larger KeffεVLGPS penalty, thus increasing Keff will make reactions with smaller ε have larger decomposition energy in the reaction space, leading to a change of ground state reaction toward smaller ε with increasing Keff. At Keff=20 GPa, LGPS does not decompose, suggesting a critical Keff (i.e., Kcrit or K*) between 10 GPa and 20 GPa, beyond which there is no oxidative decomposition reaction for LGPS.
Technically, when Keff is small, perturbation method (see Methods) is approximately equal to minimization method. We know that in perturbation method, only the decomposition reaction with the highest reaction energy at 0 GPa is considered, so we take the derivative of equation (5) in Methods with respect to the Keff for both oxidation reaction and reduction reaction:
Equations. (A) and (B) show that the volume of the electrolyte, the reaction strain, and the number of charge (or Lithium) transferred together decide the kox and kre. For example, comparing the kox between Li3YCl6 and LGPS, Li3YCl6 shows 13% larger atomic volume, 28% larger reaction strain and 33% larger 1/nox that in the end gives 92% larger kox, which is very close to the 100% larger kox calculated by the minimization method (
[27]
[28]
[29]
[30]
[31]
[13]
[33]
[6]
[34]
[7]
[33]
[35]
[36]
[37]
[38]
[39]
In calculating the stability of interface and the effect of mechanical constriction, the pseudo-phase method[12] is adopted to interpolate the phase energy, composition and volume of two phases. The solid black curve in
Treating pseudo phase at each composition x (pp(x)) as a solid-state electrolyte and using ΔGEC-RXN=GDec+n(GLi−eU)−GSSE+KeffVSSEε (eqn. 2 in Methods), we can evaluate the constriction induced voltage stability of the interface. Each pp(x) has its own GDec, n, Gpp(x), Vx and εx.
At composition x, the particular decomposition reaction is suppressed if and only if Keff >Kcrit
Li5.5PS4.5Cl1.5, was prepared by ball milling and solid state reactions. Stoichiometric amounts of Li2S (99.9% purity, Alfa Aesar), P2S5(99% purity, Sigma Aldrich), and LiCl (>99% purity, Alfa Aesar) were milled for 16 hours under argon protection. The precursor was transferred into a quartz tube and annealed at 550° C. for 1 hour with a temperature increasing rate of 5° C./min in an argon gas flow. LiNi0.83Mn0.06Co0.11O2 (NMC811) and LGPS (325 mesh) are purchased from MSE. LiCoO2 is purchased from Sigma Aldrich.
Solid state batteries were made with the configuration of Li/graphite-solid electrolyte layer(s)-cathode matrix. The Li metal foil of 0.63 cm diameter and 25 μm thickness (0.42 mg, 1.62 mAh, 5.2 mAh/cm2) was covered by a graphite thin film of 0.95 cm diameter to act as the anode. The graphite layer was made by mixing 95 wt % graphite (BTR, China) with 5 wt % PTFE, and the capacity ratio of lithium to graphite is 2.5:1. 30 mg LPSCl (120 μm thickness) and 100 mg central layer powder (400 μm thickness) were applied as the electrolyte. A 60 mg separating layer (240 μm) of the same electrolyte powder in the cathode matrix is added when the central layer is different from that in the cathode matrix. LiNbO3 (LNO) is coated on LiNi0.83Mn0.06Co0.11O2 (NMC811) or LiCoO2 (LCO) by 1.9 wt % following previous report[46]. 70 wt % bare 811, bare LCO, or LNO coated 811 or LiCoO2 was mixed with 30 wt % LPSCl or LGPS to serve as the cathode with an additional 3% PTFE to make a cathode film. The loading of the cathode is kept at 2 mg/cm2 for all the battery tests. The battery was initially pressed at 460 MPa and a stack pressure of 150 MPa was maintained by a pressurized cell. The batteries were cycled at 55° C. or room temperature on an Arbin battery testing station in an environmental chamber with the humidity controlled <10% inside Memmert hpp110, 1 C-rate=150 mA/g in this work. Liquid electrolyte batteries used glass fiber as separator and 1 M LiPF6 in EC/DMC (v:v=1:1) as electrolyte. Li metal is used as anode. The powder of cathode active material, carbon black, and PTFE are mixed with weight ratio of 85:10:5 and then roll into a thin film with diameter of 5/16″, and then assembled in a Swagelok cell.
Other embodiments are in the claims.
| Number | Date | Country | |
|---|---|---|---|
| 63539467 | Sep 2023 | US |
