Atomic-Scale Observations of Oxygen Release Degradation in Sulfide-Based All-Solid-State Batteries with Layered Oxide Cathodes

Additive-Derived Surface Modification of Cathodes in All-Solid-State Batteries: The Effect of Lithium Difluorophosphate- and Lithium Difluoro(oxalato)borate-Derived Coating Layers

Sulfide-based electrolytes, with their high conductivity and formability, enable the construction of high-performance, all-solid-state batteries (ASSBs). However, the instability of the cathode-sulfide electrolyte interface limits the commercialization of these ASSBs. Surface modification of cathodes using the coating technique has been explored as an efficient approach to stabilize these interfaces. In this study, the additives lithium difluorophosphate (LiDFP) and lithium difluoro(oxalato)borate (LiDFOB) are used to fabricate stable cathode coatings via heat treatment. The low melting points of LiDFP and LiDFOB enable the formation of thin and uniform coating layers by a low-temperature heat treatment. All-solid-state cells containing LiDFP- and LiDFOB-coated cathodes show electrochemical performances significantly better than those comprising uncoated cathodes. Among all of the as-prepared coated cathodes, LiDFP-coated cathodes fabricated using a slightly lower temperature than the phase-transition temperature of LiDFP (320 °C) show the best discharge capacity, rate capability, and cyclic performance. Furthermore, cells comprising LiDFP-coated cathodes showed significantly low impedance. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy confirm the effectiveness of the LiDFP coating. LiDFP-coated cathodes minimized side-reactions during cycling, resulting in a significantly low cathode-surface degradation. Hence, this study highlights the efficiency of the proposed coating method and its potential to facilitate the commercialization of ASSBs. Overall, this study reports an effective technique to stabilize the cathode-electrolyte interface in sulfide-based ASSBs, which could expedite the practical implementation of these advanced energy-storage devices.

Design of Cathode Coating Using Niobate and Phosphate Hybrid Material for Sulfide-Based Solid-State Battery

Coating the surface of the cathode active material of all-solid-state batteries with sulfide-based solid electrolytes is key for improving and enhancing the battery performance. Although lithium niobate (LiNbO₃) is one of the most representative coating materials, its low durability at a highly charged potential and high temperature is an impediment to the realization of high-performance all-solid-state batteries. In this study, we developed new hybrid coating materials consisting of lithium niobate (Li-Nb-O) and lithium phosphate (Li-P-O) and investigated the influence of the ratio of P/(Nb + P) on the durability performance. The cathode half-cells, using a sulfide-based solid electrolyte Li₆PS₅Cl/cathode active material, LiNi_0.5Co_0.2Mn_0.3O₂, coated with the new hybrid coating materials of LiP_xNb_1-xO₃ (x = 0-1), were exposed to harsh conditions (60 °C and 4.55 V vs Li/Li⁺) for 120 h as a degradation test. P substitution resulted in higher durability and lower interfacial resistance. In particular, the hybrid coating with x = 0.5 performed better, in terms of capacity retention and interfacial resistance, than those with other compositions of niobate and phosphate. The coated cathode active materials were analyzed using various analytical techniques such as scanning electron microscopy/energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (XAS) to elucidate the improvement mechanism. Moreover, the degraded cathodes were observed using time-of-flight secondary-ion mass spectrometry, TEM/electron diffraction, and XAS. These analyses revealed that the Nb-O-P coordination in the hybrid coating material captured O by P. The coordination suppressed the release of O from the coating layer as a decomposition side reaction to realize a higher durability than that of LiNbO₃.

Origin of the Seriously Limited Anionic Redox Reaction of Li-Rich Cathodes in Sulfide All-Solid-State Batteries

Li-rich layered oxide (LLO) cathode materials with mixed cationic and anionic redox reactions display much higher specific capacity than other traditional layered oxide materials. However, the practical specific capacity of LLO during the first cycle in sulfide all-solid-state lithium-ion batteries (ASSLBs) is extremely low. Herein, the capacity contribution of each redox reaction in LLO during the first charging process is qualitatively and quantitatively analyzed by comprehensive electrochemical and structural measurements. The results demonstrate that the cationic redox of the LiTMO₂ (TM = Ni, Co, Mn) phase is almost complete, while the anionic redox of the Li₂MnO₃ phase is seriously limited due to the sluggish transport kinetics and severe LLO/Li₆PS₅Cl interface reaction at high voltage. Therefore, the poor intrinsic conductivity and interface stability during the anionic redox jointly restrict the capacity release or delithiation/lithiation degree of LLO during the first cycle in sulfide ASSLBs. This study reveals the origin of the seriously limited anionic redox reaction in LLO, providing valuable guidance for the bulk and interface design of high-energy-density ASSLBs.

Interface Engineering of All-Solid-State Batteries Based on Inorganic Solid Electrolytes

All-solid-state batteries (ASSBs) based on inorganic solid electrolytes (SEs) are one of the most promising strategies for next-generation energy storage systems and electronic devices due to the higher energy density and intrinsic safety. However, the poor solid-solid contact and restricted chemical/electrochemical stability of inorganic SEs both in cathode and anode SE interfaces cause contact failure and the degeneration of SEs during prolonged charge-discharge processes. As a result, the increasing interface resistance significantly affects the coulombic efficiency and cycling performance of ASSBs. Herein, we present a fundamental understanding of physical contact and chemical/electrochemical features of ASSB interfaces based on mainstream inorganic SEs and summarize the recent work on interface modification. SE doping, optimizing morphology, introducing interlayer/coating layer, and utilizing compatible electrode materials are the key methods to prevent side reactions, which are discussed separately in cathode/anode-SE interface. We also highlight the constant extra stack pressure applied during ASSB cycling, which is important to the electrochemical performance. Finally, our perspectives on interface modification for practical high-performance ASSBs are put forward.

Microscopic Degradation Mechanism of Argyrodite-Type Sulfide at the Solid Electrolyte-Cathode Interface

Interfacial engineering of sulfide-based solid electrolyte/lithium-transition-metal oxide active materials in all-solid-state battery cathodes is vital for cell performance parameters, such as high-rate charge/discharge, long lifetime, and wide temperature range. A typical interfacial engineering method is the surface coating of the cathode active material with a buffer layer, such as LiNbO₃. However, cell performance reportedly degrades under harsh environments even with a LiNbO₃ coating, such as high temperatures and high cathode potentials. Therefore, we investigated the interfacial degradation mechanism focusing on the solid electrolyte side for half cells employing the cathode mixture of argyrodite-type Li₆PS₅Cl/LiNbO₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ exposed at 60 °C and 4.25 and 4.55 V vs Li/Li⁺ using transmission electron microscopy/electron diffraction (TEM/ED) and X-ray absorption spectroscopy (XAS). The TEM/ED results indicated that the ED pattern of the argyrodite structure disappeared and changed to an amorphous phase as the cells degraded. Moreover, the crystal phases of LiCl and Li₂S appeared simultaneously. Finally, XAS analysis confirmed the decrease in the PS₄ units of the argyrodite structure and the increase in local P-S-P domains with delithiation from the interfacial solid electrolyte, corresponding to the TEM/ED results. In addition, the formation of P-O bonds was confirmed during degradation at higher cathode potentials, such as 4.55 V vs Li/Li⁺. These results indicate that the degradation of this interfacial region determines the cell performance.

Transition-metal interdiffusion and solid electrolyte poisoning in all-solid-state batteries revealed by cryo-TEM

Using scanning transmission electron microscopy, along with electron energy loss spectroscopy, under cryogenic conditions, we demonstrate transition-metal dissolution from a layered Ni-rich oxide cathode material and subsequent diffusion into the bulk of a lithium thiophosphate solid electrolyte during electrochemical cycling. This problem has previously only been considered for liquid-electrolyte-based batteries.

Degradation Analysis by X-ray Absorption Spectroscopy for LiNbO3 Coating of Sulfide-Based All-Solid-State Battery Cathode

The surface coating of cathode active material in all-solid-state batteries using sulfide-based solid electrolytes is well-known to be a fundamental technology, and LiNbO₃ is one of the most representative materials. The half cells using the cathode mixture of Li₆PS₅Cl/LiNbO₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ were exposed in harsh conditions at 60 °C and 4.25-4.55 V vs Li/Li⁺ and analyzed by transmission electron microscope/energy dispersive X-ray spectroscopy (TEM/EDS) and X-ray absorption spectroscopy (XAS). TEM/EDS observation shows that Nb element derived from LiNbO₃ coating had remained at the interface, which means that Nb element had not migrated to the solid electrolyte and active material. On the other hand, the XAS spectra of Nb L₃-edge changed corresponding to cell performance degradation. From the comparison with the spectra of the reference materials of the Li-Nb-O system, the XAS spectral changes were assigned to the decomposition reaction which released Li and O from the LiNbO₃ coating. The side reaction is presumed to cause to the oxidization deterioration of sulfide electrolyte at the interface of Li₆PS₅Cl/LiNbO₃-coated LiNi_0.5Co_0.2Mn_0.3O₂.