Research Progresses of Garnet-Type Solid Electrolytes for Developing All-Solid-State Li Batteries

Structural Complexities in Sodium Ion Conductive Antiperovskite Revealed by Cryogenic Transmission Electron Microscopy

We use low-dose cryogenic transmission electron microscopy (cryo-TEM) to investigate the atomic-scale structure of antiperovskite Na2NH2BH4 crystals by preserving the room-temperature cubic phase and carefully monitoring the electron dose. Via quantitative analysis of electron beam damage using selected area electron diffraction, we find cryogenic imaging provides 6-fold improvement in beam stability for this solid electrolyte. Cryo-TEM images obtained from flat crystals revealed the presence of a new, long-range-ordered supercell with a cubic phase. The supercell exhibits doubled unit cell dimensions of 9.4 Å × 9.4 Å as compared to the cubic lattice structure revealed by X-ray crystallography of 4.7 Å × 4.7 Å. The comparison between the experimental image and simulated potential map indicates the origin of the supercell is a vacancy ordering of sodium atoms. This work demonstrates the potential of using cryo-TEM imaging to study the atomic-scale structure of air- and electron-beam-sensitive antiperovskite-type solid electrolytes.

Effect of grain boundary resistance on the ionic conductivity of amorphous xLi2S-(100-x)LiI binary system

Solid-state electrolytes (SSEs) hold the key position in the progress of cutting-edge all-solid-state batteries (ASSBs). The ionic conductivity of solid-state electrolytes is linked to the presence of both amorphous and crystalline phases. This study employs the synthesis method of mechanochemical milling on binary xLi2S-(100-x)LiI system to investigate the effect of amorphization on its ionic conductivity. Powder X-ray diffraction (PXRD) shows that the stoichiometry of Li2S and LiI has a significant impact on the amorphization of xLi2S-(100-x)LiI system. Furthermore, the analysis of electrochemical impedance spectroscopy (EIS) indicates that the amorphization of xLi2S-(100-x)LiI system is strongly correlated with its ionic conductivity, which is primarily attributed to the effect of grain boundary resistance. These findings uncover the latent connections between amorphization, grain boundary resistance, and ionic conductivity, offering insight into the design of innovative amorphous SSEs.

Evaluation and Improvement of the Stability of Poly(ethylene oxide)-based Solid-state Batteries with High-Voltage Cathodes

Solid-state batteries (SSBs) with high-voltage cathode active materials (CAMs) such as LiNi_1-x-y Co_x Mn_y O₂ (NCM) and poly(ethylene oxide) (PEO) suffer from "noisy voltage" related cell failure. Moreover, reports on their long-term cycling performance with high-voltage CAMs are not consistent. In this work, we verified that the penetration of lithium dendrites through the solid polymer electrolyte (SPE) indeed causes such "noisy voltage cell failure". This problem can be overcome by a simple modification of the SPE using higher molecular weight PEO, resulting in an improved cycling stability compared to lower molecular weight PEO. Furthermore, X-ray photoelectron spectroscopy analysis confirms the formation of oxidative degradation products after cycling with NCM, for what Fourier transform infrared spectroscopy is not suitable as an analytical technique due to its limited surface sensitivity. Overall, our results help to critically evaluate and improve the stability of PEO-based SSBs.

Accelerated Discovery of Novel Garnet-Type Solid-State Electrolyte Candidates via Machine Learning

All-solid-state batteries (ASSBs) have attracted considerable attention because of their higher energy density and stability than conventional lithium-ion batteries (LIBs). For the development of promising ASSBs, solid-state electrolytes (SSEs) are essential to achieve structural integrity. Thus, in this study, a machine-learning-based surrogate model was developed to search for ideal garnet-type SSE candidates. The well-known Li₇La₃Zr₂O₁₂ structure was used as a base material, and 73 chemical elements were substituted on La and Zr sites, leading to 5329 potential structures. First, the elasticity database and machine learning descriptors were adopted from previous studies. Subsequently, the machine-learning-based surrogate model was applied to predict the elastic properties of potential SSE materials, followed by first-principles calculations for validation. Furthermore, the active learning process demonstrated that it can effectively decrease prediction uncertainty. Finally, the ionic conductivity of the mechanically superior materials was predicted to suggest optimal SSE candidates. Then, ab initio molecular dynamics simulations are followed for confirmation of diffusion behavior for materials classified as superionic; 10 new tetragonal-phase garnet SSEs are verified with superior mechanical and ionic conductivity properties. We believe that the current model and the constructed database will become a cornerstone for the development of next-generation SSE materials.

Mapping and Modeling Physicochemical Fields in Solid-State Batteries

The safety and energy density of solid-state batteries can be, in principle, substantially increased compared with that of conventional lithium-ion batteries. However, the use of solid-state electrolytes instead of liquid electrolytes introduces pronounced complexities to the solid-state system because of the strong coupling between different physicochemical fields. Understanding the evolution of these fields is critical to unlocking the potential of solid-state batteries. This necessitates the development of experimental and theoretical methods to track electrochemical, stress, crack, and thermal fields upon battery cycling. In this Perspective, we survey existing characterization techniques and the current understanding of multiphysics coupling in solid-state batteries. We propose that the development of experimental tools that can map multiple fields concurrently and systematic consideration of material plasticity in theoretical modeling are important for the advancement of this emerging battery technology. This Perspective provides introductory material on solid-state batteries to scientists from a broad physical chemistry community, motivating innovative and interdisciplinary studies in the future.

Approaches to Combat the Polysulfide Shuttle Phenomenon in Li–S Battery Technology

Lithium–sulfur battery (LSB) technology has tremendous prospects to substitute lithium-ion battery (LIB) technology due to its high energy density. However, the escaping of polysulfide intermediates (produced during the redox reaction process) from the cathode structure is the primary reason for rapid capacity fading. Suppressing the polysulfide shuttle (PSS) is a viable solution for this technology to move closer to commercialization and supersede the established LIB technology. In this review, we have analyzed the challenges faced by LSBs and outlined current methods and materials used to address these problems. We conclude that in order to further pioneer LSBs, it is necessary to address these essential features of the sulfur cathode: superior electrical conductivity to ensure faster redox reaction kinetics and high discharge capacity, high pore volume of the cathode host to maximize sulfur loading/utilization, and polar PSS-resistive materials to anchor and suppress the migration of polysulfides, which can be developed with the use of nanofabrication and combinations of the PSS-suppressive qualities of each component. With these factors addressed, our world will be able to forge ahead with the development of LSBs on a larger scale—for the efficiency of energy systems in technology advancement and potential benefits to outweigh the costs and performance decay.

Enhancing Electrochemical Performances of Rechargeable Lithium-Ion Batteries via Cathode Interfacial Engineering

Lithium-ion batteries (LIBs) have transformed modern electronics and rapidly advancing electric vehicles (EVs) due to their high energy and power densities, cycle-life, and acceptable safety. However, the comprehensive commercialization of EVs necessitates the invention of LIBs with much enhanced and stable electrochemical performances, including higher energy/power density, cycle-life, and operational safety, but at a lower cost. Herein, we report a simple method for improving the high-voltage (up to 4.5 V) charge capability of LIBs by applying a Li⁺-ion-conducting artificial cathode-electrolyte interface (Li⁺-ACEI) on the state-of-the-art cathode, LiCoO₂ (LCO). A superionic ceramic single Li⁺ ion conductor, lithium aluminum germanium phosphate (Li_1.5Al_0.5Ge_1.5(PO₄)₃, LAGP), has been used as a novel Li⁺-ACEI. The application of Li⁺-ACEI on LCO involves a scalable and straightforward wet chemical process (sol-gel method). Cycling performance, including high voltage charge, of bare and LAGP-coated cathodes has been determined against the most energy-dense anode (lithium, Li metal) and state-of-the-art carbonate-based organic liquid electrolyte (OLE). The application of an LAGP-based Li⁺-ACEI on LCO displays many improvements: (i) reduced charge-transfer and interfacial resistance; (ii) higher discharge capacity (167.5 vs 155 mAh/g) at 0.2C; (iii) higher Coulombic efficiency (98.9 vs 97.8%) over 100 cycles; and (iv) higher rate capability (143 vs 80.1 mAh/g) at 4C. Structural and morphological characterizations have substantiated the improved electrochemical behavior of bare and Li⁺-ACEI LCO cathodes against the Li anode.

On the feasibility of all-solid-state batteries with LLZO as a single electrolyte

Replacement of Li-ion liquid-state electrolytes by solid-state counterparts in a Li-ion battery (LIB) is a major research objective as well as an urgent priority for the industry, as it enables the use of a Li metal anode and provides new opportunities to realize safe, non-flammable, and temperature-resilient batteries. Among the plethora of solid-state electrolytes (SSEs) investigated, garnet-type Li-ion electrolytes based on cubic Li₇La₃Zr₂O₁₂ (LLZO) are considered the most appealing candidates for the development of future solid-state batteries because of their low electronic conductivity of ca. 10⁻⁸ S cm⁻¹ (RT) and a wide electrochemical operation window of 0–6 V vs. Li⁺/Li. However, high LLZO density (5.1 g cm⁻³) and its lower level of Li-ion conductivity (up to 1 mS cm⁻¹ at RT) compared to liquid electrolytes (1.28 g cm⁻³; ca. 10 mS cm⁻¹ at RT) still raise the question as to the feasibility of using solely LLZO as an electrolyte for achieving competitive energy and power densities. In this work, we analyzed the energy densities of Li-garnet all-solid-state batteries based solely on LLZO SSE by modeling their Ragone plots using LiCoO₂ as the model cathode material. This assessment allowed us to identify values of the LLZO thickness, cathode areal capacity, and LLZO content in the solid-state cathode required to match the energy density of conventional lithium-ion batteries (ca. 180 Wh kg⁻¹ and 497 Wh L⁻¹) at the power densities of 200 W kg⁻¹ and 600 W L⁻¹, corresponding to ca. 1 h of battery discharge time (1C). We then discuss key challenges in the practical deployment of LLZO SSE in the fabrication of Li-garnet all-solid-state batteries.