UC Berkeley

Solid-state batteries (SSBs) have received considerable attention due to their reduced flammability and the ability to use high-energy-density metal anodes compared with those of liquid-electrolyte battery systems. Among known inorganic solid electrolytes (SEs), thiophosphates offer the advantage of a combination of high ionic conductivity, low cost and ease of processing, making them strong candidates for large-scale applications. However, thiophosphate electrolytes suffer from poor stability, leading to unstable interfaces upon contact with electrodes. The (electro-)chemical stability of various interfaces in SSBs has become the bottleneck for stable cycling of SSBs.

Thiophosphates are subjected to Li extraction under high voltages. In this thesis, we present a methodology that combines computational and experimental tools to determine the voltage windows of the electrolytes (both the thermodynamic-driven lower bound and kinetic-stabilized upper bound). We demonstrate that sufficient contact between the solid electrolyte and electron source(s) is particularly important in identifying the electrolyte stability window using electrochemical methods. The intrinsic instability of solid electrolyte is related to the capacity loss in solid state batteries, in particular during the first cycle. In this thesis, we also develop a methodology based on in situ synchrotron X-ray diffraction techniques to analyze the capacity fade due to electrolyte decomposition in a full-cell SSB.

A thermodynamically stable cathode–electrolyte pair is rare in SSBs. A chemical reaction between the cathode and SE can occur during battery cycling. In this thesis, we employ relatively simple and rapid XRD and SDT experiments to describe the interfacial reactions between electrolytes and electrodes. With the aid of high-throughput density functional theory calculations, we screen the compatibility between SEs and cathodes in a large chemical space and understand the thermodynamics and kinetics behind the reactions between incompatible electrolyte/electrode pairs.

To suppress the reaction between the cathode and thiophosphate SEs, various cathode coating materials have been developed to serve as a buffer layer and prevent direct contact between the cathode and SE. In this thesis, we directly observe the severity of the interfacial reactions between a thiophosphate electrolyte (Li2S-P2S5) and oxide cathode (NCM) during normal battery operations using different microscopy techniques. A new borate (Li3B11O18) is developed and exhibits excellent oxidation stability and chemical stability, leading to substantially improved performance over cells with Li2ZrO3–coated or uncoated cathodes.

The use of Li/Na metal anodes is essential for SSBs to achieve a high energy density. In this thesis, we demonstrate the degradation of the solid electrolyte results in the formation of mixed electronic and ionic conductive decomposition, leading to significantly increased overall cell impedance and poor rate performance. With the aid of first principles computations, we identify passivating products to stabilize the highly reactive interface between a thiophosphate electrolyte (Na3SbS4) and Na metal. A passivating interface is constructed through introduction of elements and/or compounds that beneficially reacts with Na metal and forms the passivating products. A general strategy of interface design to stability the electrolyte/metal interface is proposed.