Colloquium: Nonequilibrium Thermodynamics of Li-ion Batteries

Abstract: Li-ion batteries often involve electrode materials, such as iron phosphate and graphite, that separate into Li- rich and Li-poor phases upon intercalation of lithium. In nanoparticles, this bulk thermodynamic relaxation competes with surface electrochemistry, leading to the fundamental question: What is the reaction rate during a phase transformation? A consistent answer is provided by a theory that unifies and extends the Cahn-Hilliard and Allen-Cahn equations for chemical kinetics and charge transfer. The reaction rate depends on concentration gradients, elastic coherency strain, and other thermodynamic non-idealities. The theory predicts some surprising dynamical phenomena, such as suppression of phase separation in nanoparticles and mosaic instability in porous electrodes, which have since been observed experimentally, not only for Li-ion batteries, but also for other chemical systems.