Quantifying the factors limiting rate-performance in battery electrodes

TL;DR

This paper introduces a quantitative model that fits capacity versus rate data in batteries, identifying key physical parameters and rate-limiting processes to improve design and predict maximum charging speeds.

Contribution

The authors develop a novel equation that describes rate-performance in batteries, linking it to measurable physical parameters and enabling analysis of experimental data.

Findings

Excellent fit to ~200 data sets from ~50 papers

Parameters like diffusion coefficients can be extracted

Model predicts upper speed limits for Li/Na batteries

Abstract

A significant problem associated with batteries is the rapid reduction of charge-storage capacity with increasing charge/discharge rate. For example, improving this rate-performance is required for fast-charging of car batteries. Rate-performance is related to the timescales associated with charge or ionic motion in both electrode and electrolyte. However, no quantitative model exists which can be used to fit experimental data to give insights into the dominant rate-limiting processes in a given electrode-electrolyte system. Here we develop an equation which can be used to fit capacity versus rate data, outputting three parameters which fully describe rate-performance. Most important is the characteristic time associated with charge/discharge which can be expressed by a simple equation with terms describing each rate-limiting process, thus linking rate-performance to measurable physical parameters. We have fitted these equations to ~200 data sets from ~50 papers, finding exceptional agreement, and allowing parameters such as diffusion coefficients or electrolyte conductivities to be extracted. By estimating relevant physical parameters, it is possible to show which rate-limiting processes are dominant in a given situation, facilitating rational design and cell optimization. In addition, this model predicts the upper speed limit for Li/Na ion batteries, in agreement with the fastest electrodes in the literature.