Overcharge Study in Li₄Ti₅O₁₂Based Lithium-Ion Pouch Cell

Abstract

Overcharge tolerance has often been studied from a safety standpoint (e.g. thermal runaway), but rarely from a durability standpoint. A quantitative battery diagnosis was developed to analyze an overcharge event in a commercial Li 4 Ti 5 O 12 || LiNi 1/3 Mn 1/3 Co 1/3 O 2 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.

Overcharge Study in Li4Ti5O12Based Lithium-Ion Pouch Cell

“…Two of these cells, the O/C cell and the baseline cell, were subjected to the same cycle aging conditions, with and without the overcharge event respectively. 1 In order to validate the formulated hypothesis regarding the evolution and propagation of gas within the O/C cell, a third cell, the CT cell, has been used to replicate the overcharge event while monitoring gas evolution via CT scanning. The overcharge event was imposed to the CT and the O/C cells in an identical fashion: following a routine C/3 cycle aging to discharge the cell to 1.8 V, both cells were recharged using a CC-CV charging schedule (CC = C/4 to 3.6 V and CV = 3.6 V with a cutoff at C/25) to create the intended overcharge event.…”

“…In order to verify that the significant capacity recovery was induced by the reconnection of portions of NMC and LTO electrodes disabled by the presence of gas bubbles, we performed a dQ/dV analysis 1,5 on the C/25 data recorded, Figure 7b. To facilitate the discussion, the ABCD markers from the first part of this work 1 have been re-used. First, the region 'A' did not move as a result of the massage, which tends to indicate that the offset was not affected by the massage and its associated reconnection of AM.…”

“…Therefore, to solve this equation, we need to estimate pre SOC P E and post SOC P E . With the help of the model developed for the full cell in the first part of this study, 1 we can relate the OCV measured across the full cell before and after recovery (see. Fig.…”

“…This hypothesis is described in details in the first part of this study 1 and summarized in the next section. In this follow up study, we investigated this hypothesis using X-ray computerized tomography (CT scan), a non-invasive, in-situ imaging investigation technique.…”

“…1 Apart from the controlled overcharge, the reference and overcharged (O/C) cells had been cycled under identical voltage, current and temperature conditions. While the reference cell exhibited an extremely small capacity fade over the course of 700 cycles, the overcharged cell lost about 23% of its initial capacity after 700 cycles (15% shortly after overcharge and an additional 8% from the 150 th cycle to the 700 th cycle).…”