High Cathode Loading and Low-Temperature Operating Garnet-Based All-Solid-State Lithium Batteries - Material/Process/Architecture Optimization and Understanding of Cell Failure.

All-solid-state lithium batteries (ASSLBs) are prepared using garnet-type solid electrolytes by quick liquid phase sintering (Q-LPS) without applying high pressure during the sintering. The cathode layers are quickly sintered with a heating rate of 50-100 K min-1 and a dwell time of 10 min. The battery performance is dramatically improved by simultaneously optimizing materials, processes, and architectures, and the initial discharge capacity of the cell with a LiCoO2 -loading of 8.1 mg reaches 1 mAh cm-2 and 130 mAh g-1 at 25 °C. The all-solid-state cell exhibits capacity at a reduced temperature (10 °C) or a relatively high rate (0.1 C) compared to the previous reports. The Q-LPS would be suitable for large-scale manufacturing of ASSLBs. The multiphysics analyses indicate that the internal stress reaches 1 GPa during charge/discharge, which would induce several mechanical failures of the cells: broken electron networks, broken ion networks, separation of interfaces, and delamination of layers. The experimental results also support these failures.

Garnet-Type Solid-State Electrolytes: Materials, Interfaces, and Batteries.

Solid-state batteries with desirable advantages, including high-energy density, wide temperature tolerance, and fewer safety-concerns, have been considered as a promising energy storage technology to replace organic liquid electrolyte-dominated Li-ion batteries. Solid-state electrolytes (SSEs) as the most critical component in solid-state batteries largely lead the future battery development. Among different types of solid-state electrolytes, garnet-type Li7La3Zr2O12 (LLZO) solid-state electrolytes have particularly high ionic conductivity (10-3 to 10-4 S/cm) and good chemical stability against Li metal, offering a great opportunity for solid-state Li-metal batteries. Since the discovery of garnet-type LLZO in 2007, there has been an increasing interest in the development of garnet-type solid-state electrolytes and all solid-state batteries. Garnet-type electrolyte has been considered one of the most promising and important solid-state electrolytes for batteries with potential benefits in energy density, electrochemical stability, high temperature stability, and safety. In this Review, we will survey recent development of garnet-type LLZO electrolytes with discussions of experimental studies and theoretical results in parallel, LLZO electrolyte synthesis strategies and modifications, stability of garnet solid electrolytes/electrodes, emerging nanostructure designs, degradation mechanisms and mitigations, and battery architectures and integrations. We will also provide a target-oriented research overview of garnet-type LLZO electrolyte and its application in various types of solid-state battery concepts (e.g., Li-ion, Li-S, and Li-air), and we will show opportunities and perspectives as guides for future development of solid electrolytes and solid-state batteries.

Toward Understanding the Reactivity of Garnet-Type Solid Electrolytes with H2O/CO2 in a Glovebox Using X-ray Photoelectron Spectroscopy and Electrochemical Methods.

Chemical stability of garnet-type lithium ion conductors is one of the critical issues in their application in all-solid-state batteries. Here, we conducted quantitative analysis of impurity layers on the garnet-type solid electrolytes, Li6.5La3-xAExZr1.5-xTa0.5+xO12 (x = 0 and 0.1; AE = Ca, Sr, and Ba), by means of X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Two complimentary XPS techniques were employed: (i) background analyses by Tougaard's method and (ii) relative intensity analyses of La 3d/La 4d spectra to determine the surface chemical composition. XPS revealed that even after cleaning by annealing and polishing, the surface is covered by LiOH- and Li2CO3-based compounds with a thickness of 4-6 nm within 30 min as a result of the reaction with traces of H2O (<0.5 ppm) and CO2 (<5 ppm) in an Ar-filled glovebox. The sensitivity to H2O and CO2 depends on the basicity of dopants. Ba-doped solid electrolytes exhibited the thickest impurity layers compared to Sr- and Ca-doped compounds. A surface cleaning process, consisting of annealing and polishing, effectively reduces the charge-transfer resistance to 10-15 Ω cm2 because of negligible impurity layers. Highest short-circuit tolerance is obtained for a 700 °C annealed specimen (critical current density: 0.5 mA cm-2), which is possibly due to the strengthened grain boundaries by Li2CO3 among grains around its melting point.

Efficient Synthesis and Characterization of Robust MoS2 and S Cathode for Advanced Li-S Battery: Combined Experimental and Theoretical Studies.

Here, we report that in situ MoS2 and S cathodes (MGC) prepared by simple decomposition of (NH4)2MoS4 facilitate direct formation of Li2S and suppress the long-term problem associated with polysulphide shuttling in Li-S batteries. For comparison, we prepared ex situ MoS2 and S cathodes (EMS) with a similar S/MoS2 mole ratio to that of in situ-prepared cathodes. Discharge capacity of EMS cathodes dropped by 80% after first few cycles, while assembled MGC cells demonstrated an initial discharge capacity of 1649 mA h/g, achieving close to theoretical capacity of elemental sulfur (1675 mA h/g) at C/3 and a reversible capacity of 1500 mA h/g was obtained in further cycles. The MoS2 nanostructure evolution after initial discharge helped in extending the cycle life of assembled cells even at a high C rate. Density functional theory (DFT) calculation was performed to understand the structural stability of intermediate MoS3 and possible electrochemical reactions pertaining to Li+ insertion in MoS2 and S. Based on DFT studies, MoS3 undergoes stoichiometric decomposition to stable MoS2 and S. Furthermore, electrochemical analysis confirmed the redox activity of MoS2 and S at 1.3 and 1.8 V against Li/Li+, respectively.