Overcharge Study in Li4Ti5O12 Based Lithium-Ion Pouch Cell I. Quantitative Diagnosis of Degradation Modes

Overcharge tolerance has often been studied from a safety standpoint (e.g. thermal runaway), but rarely from a durability standpoint. A quantitative battet'y diagnosis was developed to analyze an overcharge event in a commercial Li4Ti5O12 parallel to LiNi1/3Mn1/3Co1/3O2 lithium-ion pouch cell and the subsequent cycle aging behavior. Using an electrochemical inference technique and the degradation emulation 'alawa toolbox, quantitative diagnoses of two cells enduring the same cycle aging conditions, with or without the overcharge event respectively, were performed. The baseline cell, which suffered from loss of active material in the positive electrode along with loss of lithium inventory in side-reactions, demonstrated excellent capacity retention (with 99.6% after 700 cycles). In contrast, the overcharged cell suffered from a larger capacity fade (15% immediately after the overcharge, and 23% after 700 cycles). We identified the loss of active material in the negative electrode as the culprit for capacity fade by the overcharge. Loss of active material in the positive electrode as well as loss of lithium inventory were also identified but did not contribute to the capacity fade. A hypothesis of localized blockage of the ionic conduction pathway due to gas accumulation was proposed as the mechanism driving the observed degradation. (C) The Author(s) 2015. Published by ECS. All rights reserved.