Electrolyte Development for LFP//Graphite Cells: Attempts to stop Fe dissolution from LFP

Information from the abstract

Abstract Iron dissolution and subsequent deposition on the negative electrode is a major issue in LiFePO4//graphite cells operated at elevated temperatures. Fe dissolution can be caused by Li-alkoxides formed during electrolyte reduction at the graphite electrode. The dissolved Fe then deposits on graphite, further accelerating additive consumption (e.g., vinylene carbonate), solid electrolyte interphase deterioration, lithium inventory loss, and causes accelerated capacity fading, especially at high temperatures. In this work, we sequentially removed the reactants responsible for Li-alkoxide formation by developing linear-carbonate-free, ethylene carbonate-free, and finally carbonate-free electrolytes to produce cells where alkoxides could not form. After this optimization process, we employed sulfolane and toluene as co-solvents in electrolytes for LFP cells, which eliminated alkoxide formation, delayed and almost completely suppressed Fe deposition on graphite even during testing at 70 °C. As well, in an EC-free electrolyte containing 10% fluoroethylene carbonate (FEC), both alkoxide formation and Fe deposition are suppressed until at least 20% capacity loss. Therefore, we propose that 10% FEC acts as an alkoxide-suppressing additive. These resulted in a stable capacity fade rate with no accelerated degradation region caused by Fe deposition on the negative electrode. We believe this approach opens a window for new electrolytes for long-lasting LFP cells.

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