SOLID-STATE BATTERIES: ELECTROCHEMICAL-MECHANICAL COUPLING AND THERMAL SAFETY

Abstract

I address two major challenges faced by solid-state batteries today: (1) electrochemical- mechanical coupling and (2) safety. First, Due to solid-solid interfaces in solid-state batteries, mechanical stresses that can reach as high 10s of GPa can be generated during battery fabrication and cycling, affecting both performance and degradation behaviors. However, experimentally understanding the isolated effects of these stresses on thermodynamics, kinetics, and transport is challenging. To address this challenge and to provide a fundamental understanding of stress-potential coupling through experiments, we developed a thin-film nanoindentation platform that leverages the inherent uniformity of electrodes and interfaces in thin-film batteries. This platform uses a nanoindenter to apply controlled loads while measuring electrochemistry in-operando. Specifically, through this platform, we have measured stress-open circuit voltage (OCV) coupling, which quantifies the change in the thermodynamic state of the electrode-electrolyte interface as a result of applied loads. Second, solid-state batteries are often considered inherently safe from thermalrunaway and fire risk due to the absence of flammable liquid electrolytes. However, safety is more complex than just external fires. Several modeling and experimental studies have demonstrated that solid-state batteries are not ultimately safe. Through various solid-gas reactions and reactions involving lithium metal at elevated temperatures, thermal runaway can still occur at low onset temperatures. This process can release significant heat, with calculated adiabatic temperatures reaching thousands of degrees Celsius due to the high energy density of solid-state batteries. Although not necessarily flammable, the gases evolved during thermal runaway can be toxic. To assess the thermal behavior of emerging solid-state and high energy Li-ion battery materials, we have developed a thermal analysis platform. This platform measures the thermal fingerprint of the anode-cathode-electrolyte (ACE) for a specific battery chemistry, including active and inactive components, at an early stage of its development. By providing quantitative data on reaction thermochemistry, total heat release, and onset temperatures of critical reactions, this platform offers valuable in- sights into the precursors of thermal runaway. This information can inform modeling efforts for large-format cells and guide design choices for thermal management and risk mitigation to enhance the safety of current and future battery chemistries.