Superionic Solid Electrolytes Research Articles

Lithium Metal-Compatible Antifluorite Electrolytes for Solid-State Batteries.

Lithium (Li) metal solid-state batteries feature high energy density and improved safety and thus are recognized as promising alternatives to traditional Li-ion batteries. In practice, using Li metal anodes remains challenging because of the lack of a superionic solid electrolyte that has good stability against reduction decomposition at the anode side. Here, we propose a new electrolyte design with an antistructure (compared to conventional inorganic structures) to achieve intrinsic thermodynamic stability with a Li metal anode. Li-rich antifluorite solid electrolytes are designed and synthesized, which give a high ionic conductivity of 2.1 × 10-4 S cm-1 at room temperature with three-dimensional fast Li-ion transport pathways and demonstrate high stability in Li-Li symmetric batteries. Reversible full cells with a Li metal anode and LiCoO2 cathode are also presented, showing the potential of Li-rich antifluorites as Li metal-compatible solid electrolytes for high-energy-density solid-state batteries.

High‐Entropy Lithium Argyrodite Solid Electrolytes Enabling Stable All‐Solid‐State Batteries

AbstractSuperionic solid electrolytes (SEs) are essential for bulk‐type solid‐state battery (SSB) applications. Multicomponent SEs are recently attracting attention for their favorable charge‐transport properties, however a thorough understanding of how configurational entropy (ΔSconf) affects ionic conductivity is lacking. Here, we successfully synthesized a series of halogen‐rich lithium argyrodites with the general formula Li5.5PS4.5ClxBr1.5‐x (0≤x≤1.5). Using neutron powder diffraction and 31P magic‐angle spinning nuclear magnetic resonance spectroscopy, the S2−/Cl−/Br− occupancy on the anion sublattice was quantitatively analyzed. We show that disorder positively affects Li‐ion dynamics, leading to a room‐temperature ionic conductivity of 22.7 mS cm−1 (9.6 mS cm−1 in cold‐pressed state) for Li5.5PS4.5Cl0.8Br0.7 (ΔSconf=1.98R). To the best of our knowledge, this is the first experimental evidence that configurational entropy of the anion sublattice correlates with ion mobility. Our results indicate the possibility of improving ionic conductivity in ceramic ion conductors by tailoring the degree of compositional complexity. Moreover, the Li5.5PS4.5Cl0.8Br0.7 SE allowed for stable cycling of single‐crystal LiNi0.9Co0.06Mn0.04O2 (s‐NCM90) composite cathodes in SSB cells, emphasizing that dual‐substituted lithium argyrodites hold great promise in enabling high‐performance electrochemical energy storage.

High-Entropy Lithium Argyrodite Solid Electrolytes Enabling Stable All-Solid-State Batteries.

Superionic solid electrolytes (SEs) are essential for bulk-type solid-state battery (SSB) applications. Multicomponent SEs are recently attracting attention for their favorable charge-transport properties, however a thorough understanding of how configurational entropy (ΔSconf ) affects ionic conductivity is lacking. Here, we successfully synthesized a series of halogen-rich lithium argyrodites with the general formula Li5.5 PS4.5 Clx Br1.5-x (0≤x≤1.5). Using neutron powder diffraction and 31 P magic-angle spinning nuclear magnetic resonance spectroscopy, the S2- /Cl- /Br- occupancy on the anion sublattice was quantitatively analyzed. We show that disorder positively affects Li-ion dynamics, leading to a room-temperature ionic conductivity of 22.7 mS cm-1 (9.6 mS cm-1 in cold-pressed state) for Li5.5 PS4.5 Cl0.8 Br0.7 (ΔSconf =1.98R). To the best of our knowledge, this is the first experimental evidence that configurational entropy of the anion sublattice correlates with ion mobility. Our results indicate the possibility of improving ionic conductivity in ceramic ion conductors by tailoring the degree of compositional complexity. Moreover, the Li5.5 PS4.5 Cl0.8 Br0.7 SE allowed for stable cycling of single-crystal LiNi0.9 Co0.06 Mn0.04 O2 (s-NCM90) composite cathodes in SSB cells, emphasizing that dual-substituted lithium argyrodites hold great promise in enabling high-performance electrochemical energy storage.

Effective regulation towards electrochemical stability of superionic solid electrolyte via facile dual-halogen strategy

Several halide electrolytes have come back into the spotlight with their high ionic conductivity, soft texture and compatibility with high-voltage electrodes. However, the instability with lithium metal has hindered their application in all-solid-state lithium-metal batteries (ASSLMBs). Thereby, it is urgent to find an efficient way to tackle this issue. Herein, we have simultaneously enhanced the kinetic stability towards lithium metal and intrinsic stability towards high-voltage cathodes for Li2ZrCl6 electrolytes via effective dual-halogen strategy while the ionic conductivity of electrolyte is not sacrificed due to the effects of morphology modification. Specially, the symmetric Li/Li2ZrCl5.2F0.8/Li cell can cycle stably for over 2000 h (0.1 mA cm-2, 0.5 mA h cm-2) and the modified electrolyte Li2ZrCl5.2F0.8 also shows higher oxidative stability than the original Li2ZrCl6. The in-situ formed fluoride interphase (LiF, ZrFx) can account for the excellent kinetic stability at Li/Li2ZrCl5.2F0.8 interface while the improved oxidative stability can be attributed to the higher electronegativity of F element. Finally, the Li2ZrCl5.2F0.8 is also proved appliable in the Li/Li2ZrCl5.2F0.8/LiNi0.8Co0.1Mn0.1O2 battery in the form of stable and satisfied discharge capacity (∼106.9 mAh g-1, the 20th - 65th cycle) due to the achievement of kinetic stability at Li/Li2ZrCl5.2F0.8 interface in the initial stage (the first ∼ 19 cycles). The clever dual-halogen strategy proposed in this work can effectively enhance the electrochemical stability of electrolytes and foster the application of cost-effective Li2ZrCl6 halides in ASSLMBs.

Tailoring the LiNbO3coating of Ni-rich cathode materials for stable and high-performance all-solid-state batteries

The research and development of advanced nanocoatings for high-capacity cathode materials is currently a hot topic in the field of solid-state batteries (SSBs). Protective surface coatings prevent direct contact between the cathode material and solid electrolyte, thereby inhibiting detrimental interfacial decomposition reactions. This is particularly important when using lithium thiophosphate superionic solid electrolytes, as these materials exhibit a narrow electrochemical stability window, and therefore, are prone to degradation during battery operation. Herein we show that the cycling performance of LiNbO₃-coated Ni-rich LiNi<i>_x</i>Co<i>_y</i>Mn<i>_z</i>O₂ cathode materials is strongly dependent on the sample history and (coating) synthesis conditions. We demonstrate that post-treatment in a pure oxygen atmosphere at 350 ℃ results in the formation of a surface layer with a unique microstructure, consisting of LiNbO₃ nanoparticles distributed in a carbonate matrix. If tested at 45 ℃ and C/5 rate in pellet-stack SSB full cells with Li₄Ti₅O₁₂ and Li₆PS₅Cl as anode material and solid electrolyte, respectively, around 80% of the initial specific discharge capacity is retained after 200 cycles (~ 160 mAh·g⁻¹, ~ 1.7 mAh·cm⁻²). Our results highlight the importance of tailoring the coating chemistry to the electrode material(s) for practical SSB applications.

Single- to Few-Layer Nanoparticle Cathode Coating for Thiophosphate-Based All-Solid-State Batteries.

Bulk-type solid-state batteries (SSBs) composed of lithium thiophosphate superionic solid electrolytes (SEs) and high-capacity cathode active materials (CAMs) have recently attracted much attention for their potential application in next-generation electrochemical energy storage. However, compatibility issues between the key components in this kind of battery system are difficult to overcome. Here, we report on a protective cathode coating that strongly reduces the prevalence of detrimental side reactions between CAM and SE during battery operation. This is demonstrated using preformed HfO2 nanoparticles as a secondary particle coating for a layered Ni-rich oxide CAM, LiNi0.85Co0.1Mn0.05O2 (NCM85). The preparation of a stable dispersion of the HfO2 nanoparticles enabled the deposition of a uniform coating of thickness ≤11 nm. When incorporated into Li6PS5Cl-based, pellet-stack SSBs, the coated NCM85 showed superior performance in terms of reversibility, cell capacity, longevity, and rate capability over its uncoated counterpart. The effectiveness of the protective coating in mitigating electro-chemo-mechanical degradation was investigated using a suite of physical and electrochemical characterization techniques. In addition, the adaptability to wet processing of the coated NCM85 is demonstrated in slurry-cast SSBs and liquid-electrolyte-based Li-ion cells.

A High-Entropy Multicationic Substituted Lithium Argyrodite Superionic Solid Electrolyte

Bulk-type solid-state batteries (SSBs) constitute a promising next-generation technology for electrochemical energy storage. However, in order for SSBs to become competitive with mature battery technologies, (electro)chemically stable, superionic solid electrolytes are much needed. Multicomponent or high-entropy lithium argyrodites have recently attracted attention for their favorable material characteristics. In the present work, we report on increasing the configurational entropy of the Li6+aP1–xMxS5I solid electrolyte system and examine how this affects the structure-conductivity/stability relationships. Using electrochemical impedance spectroscopy and 7Li pulsed field gradient nuclear magnetic resonance (NMR) spectroscopy, multicationic substitution is demonstrated to result in a very low activation energy for Li diffusion of ∼0.2 eV and a high room-temperature ionic conductivity of ∼13 mS cm–1 (for Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I). The transport properties are rationalized from a structural perspective by means of complementary neutron powder diffraction and magic-angle spinning NMR spectroscopy measurements. The Li6.5[P0.25Si0.25Ge0.25Sb0.25]S5I solid electrolyte was also tested in high-loading SSB cells with a Ni-rich layered oxide cathode and found by X-ray photoelectron spectroscopy to suffer from interfacial side reactions during cycling. Overall, the results of this study indicate that optimization of conductivity is equally important to optimization of stability, and compositionally complex lithium argyrodites represent a new playground for a rational design of (potentially advanced) superionic solid electrolytes.

Designing Cathodes and Cathode Active Materials for Solid‐State Batteries

AbstractSolid‐state batteries (SSBs) currently attract great attention as a potentially safe electrochemical high‐energy storage concept. However, several issues still prevent SSBs from outperforming today's lithium‐ion batteries based on liquid electrolytes. One major challenge is related to the design of cathode active materials (CAMs) that are compatible with the superionic solid electrolytes (SEs) of interest. This perspective, gives a brief overview of the required properties and possible challenges for inorganic CAMs employed in SSBs, and describes state‐of‐the art solutions. In particular, the issue of tailoring CAMs is structured into challenges arising on the cathode‐, particle‐, and interface‐level, related to microstructural, (chemo‐)mechanical, and (electro‐)chemical interplay of CAMs with SEs, and finally guidelines for future CAM development for SSBs are proposed.

The interplay between (electro)chemical and (chemo)mechanical effects in the cycling performance of thiophosphate-based solid-state batteries

Abstract Solid-state batteries (SSBs) are a promising next step in electrochemical energy storage but are plagued by a number of problems. In this study, we demonstrate the recurring issue of mechanical degradation because of volume changes in layered Ni-rich oxide cathode materials in thiophosphate-based SSBs. Specifically, we explore superionic solid electrolytes (SEs) of different crystallinity, namely glassy 1.5Li2S-0.5P2S5-LiI and argyrodite Li6PS5Cl, with emphasis on how they affect the cyclability of slurry-cast cathodes with NCM622 (60% Ni) or NCM851005 (85% Ni). The application of a combination of ex situ and in situ analytical techniques helped to reveal the benefits of using a SE with a low Young’s modulus. Through a synergistic interplay of (electro)chemical and (chemo)mechanical effects, the glassy SE employed in this work was able to achieve robust and stable interfaces, enabling intimate contact with the cathode material while at the same time mitigating volume changes. Our results emphasize the importance of considering chemical, electrochemical, and mechanical properties to realize long-term cycling performance in high-loading SSBs.

Superionic Solid Electrolyte Li7La3Zr2O12 Synthesis and Thermodynamics for Application in All-Solid-State Lithium-Ion Batteries.

Solid-state reaction was used for Li7La3Zr2O12 material synthesis from Li2CO3, La2O3 and ZrO2 powders. Phase investigation of Li7La3Zr2O12 was carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) methods. The thermodynamic characteristics were investigated by calorimetry measurements. The molar heat capacity (Cp,m), the standard enthalpy of formation from binary compounds (ΔoxHLLZO) and from elements (ΔfHLLZO), entropy (S0298), the Gibbs free energy of the Li7La3Zr2O12 formation (∆f G0298) and the Gibbs free energy of the LLZO reaction with metallic Li (∆rGLLZO/Li) were determined. The corresponding values are Cp,m = 518.135 + 0.599 × T − 8.339 × T−2, (temperature range is 298–800 K), ΔoxHLLZO = −186.4 kJ·mol−1, ΔfHLLZO = −9327.65 ± 7.9 kJ·mol−1, S0298 = 362.3 J·mol−1·K−1, ∆f G0298 = −9435.6 kJ·mol−1, and ∆rGLLZO/Li = 8.2 kJ·mol−1, respectively. Thermodynamic performance shows the possibility of Li7La3Zr2O12 usage in lithium-ion batteries.

Paradigms of frustration in superionic solid electrolytes.

Superionic solid electrolytes have widespread use in energy devices, but the fundamental motivations for fast ion conduction are often elusive. In this Perspective, we draw upon atomistic simulations of a wide range of superionic conductors to illustrate some ways frustration can lower diffusion cation barriers in solids. Based on our studies of halides, oxides, sulfides and hydroborates and a survey of published reports, we classify three types of frustration that create competition between different local atomic preferences, thereby flattening the diffusive energy landscape. These include chemical frustration, which derives from competing factors in the anion–cation interaction; structural frustration, which arises from lattice arrangements that induce site distortion or prevent cation ordering; and dynamical frustration, which is associated with temporary fluctuations in the energy landscape due to anion reorientation or cation reconfiguration. For each class of frustration, we provide detailed simulation analyses of various materials to show how ion mobility is facilitated, resulting in stabilizing factors that are both entropic and enthalpic in origin. We propose the use of these categories as a general construct for classifying frustration in superionic conductors and discuss implications for future development of suitable descriptors and improvement strategies.This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

Influence of synthesis parameters on crystallization behavior and ionic conductivity of the Li4PS4I solid electrolyte

Superionic solid electrolytes are key to the development of advanced solid-state Li batteries. In recent years, various materials have been discovered, with ionic conductivities approaching or even exceeding those of carbonate-based liquid electrolytes used in high-performance Li-ion batteries. Among the different classes of inorganic solid electrolytes under study, lithium thiophosphates are one of the most promising due to their high Li-ion conductivity at room temperature and mechanical softness. Here, we report about the effect of synthesis parameters on the crystallization behavior and charge-transport properties of Li4PS4I. We show that thermally induced crystallization of Li4PS4I (P4/nmm), starting from the glassy phase 1.5Li2S–0.5P2S5–LiI, adversely affects the material’s conductivity. However, both conductivity and crystallization temperature can be significantly increased by applying pressure during the preparation.

Insight into the activation energy for the interfacial stability evaluation in all-solid-state Li-ion batteries

While all-solid-state Li-ion batteries (ASSLIBs) by using non-flammable and superionic solid electrolytes (SEs) exhibit improved safety and higher energy density, a major challenge that remains to be overcome is the dissatisfactory rate performance and capacity retention due to the undesirable reaction at electrode/SE interfaces. Reaction energy is generally taken as the indicator of interfacial reactivity, i.e. reaction degree and reaction rate, although it only represents whether the reaction is thermodynamically favorable. In the physical origin, activation energy (reaction barrier) instead of reaction energy should be the determinant of reaction rate. Here, first principles calculations with the Arrhenius relationship show a difference of several orders of magnitude in reaction rates due to the distinct activation energies although the reaction energies are similar. Then, by combining the activation energy with the reaction energy and conductive ability of products, which representing kinetic feasibility, thermodynamic favorability, and continuity of a reaction respectively, we propose an evaluation strategy for interfacial stability and correspondingly classify the interfaces. This study manifests that the usually unheeded activation energy can provide a kinematic perspective for the interfacial reactions.

Effect of surface carbonates on the cyclability of LiNbO3-coated NCM622 in all-solid-state batteries with lithium thiophosphate electrolytes

While still premature as an energy storage technology, bulk solid-state batteries are attracting much attention in the academic and industrial communities lately. In particular, layered lithium metal oxides and lithium thiophosphates hold promise as cathode materials and superionic solid electrolytes, respectively. However, interfacial side reactions between the individual components during battery operation usually result in accelerated performance degradation. Hence, effective surface coatings are required to mitigate or ideally prevent detrimental reactions from occurring and having an impact on the cyclability. In the present work, we examine how surface carbonates incorporated into the sol–gel-derived LiNbO3 protective coating on NCM622 [Li1+x(Ni0.6Co0.2Mn0.2)1–xO2] cathode material affect the efficiency and rate capability of pellet-stack solid-state battery cells with β-Li3PS4 or argyrodite Li6PS5Cl solid electrolyte and a Li4Ti5O12 anode. Our research data indicate that a hybrid coating may in fact be beneficial to the kinetics and the cycling performance strongly depends on the solid electrolyte used.

Impact of Solvent Treatment of the Superionic Argyrodite Li6PS5Cl on Solid‐State Battery Performance

With growing interest in solution‐based processing of electrolytes for all‐solid‐state batteries comes the need to more deeply understand potential detrimental effects of the solvent on electrolyte materials, as well as effects on the cell performance that may not have been evident by structural characterization alone. Herein, the superionic solid electrolyte Li6PS5Cl is treated with five organic solvents selected for a range of different physical and chemical properties. The electrolytes treated with solvents that do not lead to obvious degradation are used in cathode composites of solid‐state batteries In/LiIn│Li6PS5Cl│NCM‐622:Li6PS5Cl. After treatment in some solvents, the solid electrolyte remains seemingly unaffected, but a strong influence on the solid‐state battery performance is observed, revealing underlying effects that warrant deeper study.

Synthesis and electrical properties of Ag16I12P2O7

Superionic solid electrolyte Ag16I12P2O7 was prepared using solid state synthesis. The ionic conductivity of this compound was studied by the complex impedance spectroscopy method in a wide temperature range. It is shown that Ag16I12P2O7 is characterized by a high ionic conductivity at room temperature, comparable to the conductivity of liquid electrolytes.

Studying of superionic solid electrolyte Li7La3Zr2O12 stability by means of chemical thermodynamics for application in all-solid-state batteries

Li7La3Zr2O12 is a promising solid electrolyte for high-energy lithium and lithium-ion power sources, including all-solid-state batteries, so its stability to electrode materials and different environments over a wide temperature range attracts researchers’ attention. Thermodynamic simulation can be considered an express and convenient technique to estimate the thermodynamic stability of substances. Thermodynamic simulations of the Li7La3Zr2O12 stability against some electrode materials (LiCoO2, LiFePO4, LiMn2O4, Li4Ti5O12 and Li) and environment components have been carried out. According to the simulation results, the Li7La3Zr2O12 solid electrolyte is stable against metallic lithium in a wide temperature range. However, it has high reactivity with Li4Ti5O12 and all the studied cathode materials, with the exception of LiMn2O4 at room temperature. In addition, according to thermodynamic simulation data, Li7La3Zr2O12 reacts with carbon dioxide and water to form carbonates and hydroxides, respectively. The results of thermodynamic simulation are in good agreement with the experimental data, and it can be concluded that the proposed methodology is quite reliable and can be applied to the reactivity estimation of solid electrolytes.

Comparison of Correlated Li+ Diffusion from Molecular Dynamics of Promising Solid Electrolytes

Li-ion batteries (LIBs) have been steadily improving since the first LIBs were commercially developed in the 1990s. Batteries with higher volumetric and gravimetric energy density using all-solid-state materials could revolutionize the LIB market. However, known superionic solid electrolytes are limited by a number of materials properties. Engineering better materials is especially limited by the lack of understanding of diffusion mechanisms, demanding novel analysis of diffusion using molecular dynamics. Molecular dynamics (MD) provides insight into correlated motion between Li-ions and correlated motion with the lattice. Correlated motion of Li-ions is common in the most promising superionic electrolytes. The results presented will be a broad survey of correlated motion using novel analysis tools developed by the authors and their collaborators. Diffusion in some of the most promising electrolytes, such as lithium borohydrides, lithium germanium thiophosphate, amorphous lithium thiophosphate, lithium lanthanum zirconium oxide, and the lithium rich anti-perovskites will be compared. Molecular dynamics of these systems will be performed either by ab-initio MD or classical MD. The Haven ratio and the Distinct Van Hove Analysis will be presented for each material. The novel analysis will focus on computational atomistic insights into the diffusion mechanisms of Li-ions, especially correlated motion.

(Battery Division Student Research Award sponsored by Mercedes-Benz Research & Development) Revisiting the Electrochemical Stability Window of Solid Electrolytes

The development of next-generation batteries has raised higher requirements on safety and energy density, both of which can be potentially improved by developing solid-state batteries (SSBs). Significant advances have been made in achieving superionic solid electrolytes (SEs) with a wide electrochemical stability window (ESW) from 0 to 5 V. The ESW of solid electrolytes was usually measured from cyclic voltammetry of Li/SE/inert metal semi-blocking electrode. Because of the wide ESW, solid electrolytes hold great promise for high energy density batteries with high columbic efficiency and long cycle life. In this presentation, we challenge the claimed ESW of solid electrolytes. The conventional method to measure ESW provides an overestimated value because the kinetics of the electrochemical decomposition reaction is limited in the semi-blocking electrode. A novel experimental method using Li/SE/SE+carbon cell is proposed to approach the intrinsic stability window of solid electrolytes. The ESWs of Li10GeP2S12 (LGPS) and Li7La3Zr2O12 (LLZO), the most promising SE for sulfide and oxide electrolytes respectively, are examined using the novel experimental method. The results suggest that both SEs have much narrower electrochemical stability window than claimed (Figure 1). The cathodic and anodic decomposition products for both electrolytes are also characterized. The measured stability window and the decomposition products agree well with the calculated results from first principles. The reversible decompositions of LGPS at both high and low voltages enable the realization of a battery made from a single material. The electrochemical decompositions of the SEs in SSBs can lead to large interfacial resistances between electrode and electrolyte, depending on the properties of the decomposition products. The interfacial resistances arising from the decomposition of SEs have rarely been considered in previous research because the batteries are cycled within the “claimed” stable window of SEs. Suppressing the (electro)chemical reactions between LiCoO2 cathode and LLZO electrolyte by engineering their interphase enables a high performance all-ceramic lithium battery. By taking advantage of the electrochemical decomposition of SEs, an effective approach to suppress Li dendrite formation in sulfide electrolyte is also demonstrated.

Conductivity and stability of Li3/8Sr7/16-3x/2LaxZr1/4Ta3/4O3 superionic solid electrolytes

Oxide solid electrolytes Li3/8Sr7/16-3x/2LaxZr1/4Ta3/4O3 (LSLZT, x = 0, 0.025, 0.05) with different A-site vacancy were synthesized using conventional solid-state reaction procedure at 1300 °C. Approximately single-phase perovskite-type was obtained which was analyzed by X-ray diffraction. Moreover, scanning electron microscope, AC impedance spectroscopy and potentiostatic polarization measurement methods were adopted to study the microstructure, Li+ conductivities and electronic conductivities of the samples, respectively. Among these samples, the optimal composition of Li3/8Sr7/16-3x/2LaxZr1/4Ta3/4O3 (x = 0.025) was selected with bulk conductivity of 1.26 × 10−3 S cm−1, total conductivity of 3.30 × 10−4 S cm−1, electronic conductivity of 6.60 × 10−9 S cm−1 at 30 °C and activation energy of 0.28 eV. Furthermore, the cyclic voltammogram analysis indicated the stability of this solid electrolyte at voltages higher than 1.3 V against metallic lithium. The solid electrolyte as a separator in LiFePO4/Li half-cell showed good cycle performance that comprises 98.7% of original values at 0.2 C charge-discharge rates after 50 cycles.