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Quantifying the factors limiting rate-performance in battery electrodes

01 December 2019

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A significant problem associated with batteries is the rapid reduction of chargestorage capacity with increasing charge/discharge rate. For example, improving this rateperformance 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 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 ratelimiting process, thus linking rate-performance to measureable 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 optimisation. In addition, this model predicts the upper speed limit for Li/Na ion batteries, in agreement with the fastest electrodes in the literature.