On the Additive Microstructure in Composite Cathodes and Alumina-Coated Carbon Microwires for Improved All-Solid-State Batteries

All-solid-state batteries promise to enable lithium metal anodes and outperform state-of-the-art lithium-ion battery technology. To achieve high battery capacity, utilization of the active material in the cathode must be maximized. Carbon-based conductive additives are known to improve the capacity and rate performance of electrode composites. However, their influence on cathode composites in all-solid-state batteries is yet not fully understood. Here, we study the influence of several carbon additives with different morphologies and surface areas on the performance of an all-solid-state battery cell Li|β-Li3PS4|Li(Ni0.6Co0.2Mn0.2)O2/β-Li3PS4/carbon. Cycling tests and microstructure-resolved simulations show that higher utilization of the cathode active material can be achieved using fiber-shaped vapor-grown carbon additives, whereas particle-shaped carbons show a minor influence. Unfortunately, carbon additives generally lead to an accelerated capacity loss during cycling and an enhanced formation of solid electrolyte decomposition products. The latter was studied in more detail using cyclic voltammetry, X-ray photoelectron spectroscopy, and cycling experiments. The results show that carbon additives with a small surface area and a fiber-like morphology result in the lowest degree of decomposition. To completely overcome electrolyte degradation caused by the use of carbon additives, a protection concept is developed. A thin alumina coating with a few nanometers thickness was deposited on the carbon fibers by atomic layer deposition, which successfully prevents decomposition reactions, reduces long-term capacity fading, and leads to an enhanced overall all-solid-state battery performance.