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

Low-Temperature Ion Exchange Synthesis of Layered LiNiO₂ Single Crystals with High Ordering

Layered Ni-rich oxide cathode materials are being explored in an effort to boost the energy density of lithium-ion batteries, especially for automotive applications. Among them, the ternary-phase LiNiO2 (LNO) is a promising candidate but brings along various issues, such as poor structural stability. The material is prone to disordering (Li off-stoichiometry) when prepared by conventional solid-state synthesis, leading to the presence of Ni2+ in the Li layer. These point defects negatively affect the utilization of the Li inventory, thereby limiting the practical specific capacity. In this work, we report on a two-step synthesis approach that avoids the formation of nickel substitutional defects. First, NaNiO2 (NNO) is prepared, showing no such defects due to larger differences in ionic radii between Ni2+/Ni3+ and Na+. NNO is then subjected to Na+/Li+ exchange under mild conditions. In so doing, monolithic LNO particles free of NiLi• defects can be produced at relatively low temperatures. Notably, this route allows for tailoring of the grain size, a strategy that may be used to gain insights into the structure–size–property relations in single-crystalline LNO

Interface and Electrode Microstructure Engineering for Optimizing Performance of the LiNiO₂ Cathode in All-Solid-State Batteries

Solid-state batteries (SSBs) utilizing superionic thiophosphate solid electrolytes (SEs), such as argyrodite Li6PS5Cl, are attracting great interest as a potential solution for safe, high-energy-density electrochemical energy storage. However, the development of high-capacity cathodes remains a major challenge. Herein, we present an effective design strategy to improve the cyclability of the layered Co-free oxide cathode active material (CAM) LiNiO2, consisting of surface modification and electrode microstructure engineering. After optimization, the SSB cells were found to deliver high capacities (qdis ≈ 200 mAh/gCAM) and to cycle stably for hundreds of hours. A combination of operando and ex situ characterization techniques was employed to reveal the mechanism of optimization in overcoming several issues of LiNiO2, including poor SE compatibility, outgassing, and state-of-charge heterogeneity. Tailoring the microstructure of the composite cathode and increasing the CAM|SE interface stability enable superior electrochemical performance