Delayed Rapid Anode Potential Decrease During Fixed Resistive Load Overdischarge of LiFePO4/Graphite Lithium-Ion Cells

Abstract

A lithium-ion 0.7 mAh LiFePO4/graphite pouch cell with 1.0 M LiPF6 ethylene carbonate:ethyl methyl carbonate 1:1 v/v electrolyte was overdischarged by application of a fixed resistive load for 336 h after cell conditioning. A rapid (<5 min) decrease in the anode potential was observed after the anode reached 3.3–3.4 V vs Li/Li+ for about 6 h during the overdischarge. Non-uniform depositions of fluorine, oxygen, phosphorous, lithium and copper were observed on the surface of the anode from the overdischarged cells by SEM/EDS and XPS. Three-electrode measurements show lithium inventory in the cell increased after overdischarge, increasing cell discharge capacity. Tests on additional LiFePO4/graphite cells show that 1) the rapid anode potential decrease doesn’t occur if the anode potential doesn’t increase to >1.8 V vs Li/Li+ during overdischarge, although oxygen deposition on the anode does occur, 2) fluorine, oxygen and phosphorous increases on the anode surface are not observed when the fixed resistive load is removed before the rapid anode potential decrease and 3) fluorine, copper and oxygen deposition occur without cell recharge. Collectively, results indicate the rapid anode potential decrease is an oxidative breakdown of the electrolyte on the surface of the anode that is initiated by copper oxidation.