Entropy Assisted Cathode Design for Solid‑State Battery

Abstract

The increasing demand for higher energy density and improved safety in electrochemical energy storage has driven significant interest in solid‑state battery technologies as successors to conventional lithium‑ion batteries. While the replacement of flammable liquid electrolytes with solid electrolytes offers clear safety advantages, practical performance gains remain limited by system‑level constraints and materials‑level instabilities. In particular, cathode active materials continue to represent the primary bottleneck governing achievable energy density, cost, and long‑term stability in both liquid‑ and solid‑state battery systems. This thesis presents a comprehensive investigation of cathode‑driven energy density limitations and introduces a new thermodynamic framework for cathode material design based on entropy change during cycling during electrochemical cycling. First, a cell‑level energy density model is developed to evaluate realistic performance limits of all‑solid‑state batteries under practical design constraints. This analysis demonstrates that widely cited claims of doubling energy density are overly optimistic and establishes that maximizing reversible lithium utilization in the cathode is the most impactful pathway toward meaningful energy density improvements. Building on this insight, the thesis reframes entropy as a dynamic thermodynamic variable rather than a static material descriptor. Entropy is treated within the framework of mixing entropy, with configurational disorder providing the dominant composition‑dependent contribution under solid‑solution assumptions. Analytical entropy modeling combined with density functional theory calculations reveals that cathode with moderate entropy, particularly layered oxides containing approximately 65–70% nickel, exhibit minimized entropy change during delithiation, enabling deep reversible lithium extraction while preserving structural stability. These results demonstrate that entropy change during cycling, rather than absolute entropy magnitude or compositional complexity, constitutes a rational design constraint for high‑energy cathode materials. Experimental validation is provided through temperature‑dependent open‑circuit potential measurements, representing the first systematic comparison of mixing entropy among commercial layered oxide cathodes. The experimentally observed entropy trends are consistent with analytical predictions, confirming entropy change during cycling as a meaningful indicator of cathode stability. Collectively, this work establishes an entropy‑based design paradigm that bridges thermodynamics, electrochemical performance, and system‑level battery design, offering clear guidance for the development of stable, high‑energy cathode materials and advancing the practical realization of safe, high‑performance solid‑state batteries.