The correlation between charge/discharge rates and morphology, surface chemistry, and performance of Li electrodes and the connection to cycle life of practical batteries

We investigated the correlation among surface chemistry, morphology, and current densities of the charge-discharge processes and the performance of Lithium electrodes in Li vs. Li half-cell testing and practical rechargeable AA Li-Li2MnO2 batteries (Tadiran Batteries, Limited). The electrolyte system was LiAsF6/tributylamine (stabilizer)/1,3 dioxolane solution. It was found that the performance of the lithium anodes in practical batteries depends on the current densities at which the batteries are operated. These determine the surface chemistry of the anodes in the following manner: at sufficiently high discharge rates (Li dissolution) the native films which cover the active metal are replaced completely and rapidly by surface films which originate from solvent-reduction processes. These films induce uniform, dendrite-free Li deposition. At too-low discharge rates, part of the native films remains, and thus the surface films are too heterogeneous. This leads to dendritic Li deposition. Charging the batteries at too-high rate (Li deposition) leads to the exposure of fresh Li to the solution, which reacts predominantly with the salt anion (AsF6-). The surface films thus formed (comprised of LiF, LixAsFy species, etc.) lead to nonuniform Li deposition. It is possible to adjust charging rates which lead to lithium deposition with a very minor exposure of fresh lithium, and thereby change the Li surface chemistry to that dominated by solvent reduction. This leads to an extended cycle life of the Li anodes due to the uniform Li deposition that the surface films thus formed induce.