Engineering Multiscale Coupled Electron/Ion Transport in Battery Electrodes

Coupled electron/ion transport is a defining characteristic of electrochemical processes, for example, battery charge/discharge. Analytical models that represent the complex transport and electrochemical processes in an electrode in terms of equivalent electrical circuits provide a simple, but successful framework for understanding the kinetics of these coupled transport phenomena. The premise of this review is that the nature of the time-dependent phase transitions in dynamic electrochemical environments serves as an important design parameter, orthogonal to the intrinsic mixed conducting properties of the active materials in battery electrodes. A growing body of literature suggests that such phase transitions can produce divergent extrinsic resistances in a circuit model (e.g., R_e, describing electron transport from an active electrode material to the current collector of an electrode, and/or R_ion, describing ion transport from a bulk electrolyte to the active material surface). It is found that extrinsic resistances of this type play a determinant role in both the electrochemical performance and long-term stability of most battery electrodes. Additionally, successful suppression of the tendency of extrinsic resistances to accumulate over time is a requirement for practical rechargeable batteries and an important target for rational design. We highlight the need for battery electrode and cell designs, which explicitly address the specific nature of the structural phase transition in active materials, and for advanced fabrication techniques that enable precise manipulations of matter at multiple length scales: (i) meso-to-macroscopic conductive frameworks that provide contiguous electronic/ion pathways; (ii) nanoscale uniform interphases formed on active materials; and (iii) molecular-level structures that promote fast electron and/or ion conduction and mechanical resilience.