How the electrolyte limits fast discharge in nanostructured batteries and supercapacitors

Abstract

Examples of LiFePO4 composite electrodes are shown in which solid state and interfacial processes are not the principal rate limiting process during fast discharge. Rate dependence on electrode thickness, electrolyte concentration, lithium transference number, and dilution of the active material is explained by a simple salt diffusion model. A discharge to 25 % capacity (0.3 mA h) was obtained on a 40 micrometre thick electrode after only 4 s in an optimised electrolyte -aqueous Li2SO4. Publication of this paper is extrem ely urgent because it provid es an alternative m od el w ith evid ence for an alternative explanation for the recent results in a published in N ature by Kang and Ced er, concerning ultrafast d ischarge of LiFePO4. Several prom inent w orkers in the field , includ ing Ced er, are aw are of the w ork d escribed in the present m anuscript because I gave an outline of it at recent conferences. H ow ever, the significance of the w ork w as not fully appreciated until the Kang paper cam e out in March . Since then I have received countless m essages asking w here I published the m od el so they can reference the w ork. Of course, I have had to reply "com ing asap!". School of Chemistry

A solid with a hierarchical tetramodal micro-meso-macro pore size distribution

“…The superior rate capability of tetramodal α-MnO 2 over microporous and bimodal forms may be assigned to better Li + transport in the electrolyte within the hierarchical pore structure of tetramodal α-MnO 2 . The importance of electrolyte transport in porous electrodes has been discussed recently354748 and the results presented here reinforce the beneficial effect of a hierarchical pore structure.…”

Phase-transforming electrodes

“…LiFePO 4 was first described as a material for low-power batteries because it is virtually an electrical insulator in its pristine form. Several years of research followed to speed up the delivery of charge into LiFePO 4 particles: carbon coatings ( 5), other coatings, or dopants were added to assist electron transport; likewise, optimized electrolyte conductance in the composite electrode structure facilitated ion transport ( 6), and the particles themselves were reduced to nanometer size to minimize the distance from the surface to the center of each particle. The result was a material that could discharge its stored electrical energy in just a few seconds.…”

Multiscale modelling and analysis of lithium-ion battery charge and discharge

“…Since each electrode particle has a given capacity to absorb lithium, this results in a decreased fraction of utilised stored energy that can be achieved by a battery operating at high currents. In this context, we note the recent study of Johns et al [25], which treats this aspect of lithium ion battery operation in which a simple model is used, in conjunction with experiments, to illustrate the effects of salt starvation at high rates of discharge. For comparison with the results of [25], we have included plots of the fraction of the utilised stored energy (as a function of the discharge current) calculated based on our model.…”