State of charge dependent degradation effects of lithium titanate oxide batteries at elevated temperatures: An in-situ and ex-situ analysis

Abstract

Aging effects of Li4Ti5O12 (LTO)-based lithium-ion batteries are highly controversial and still not fully understood. Known degradation effects of LTO such as surface layer formation or gas formation are state of charge (SOC) dependent and strongly accelerated at high temperatures. However, very few long-term studies have investigated the SOC dependent calendar lifetime of LTO-based cells so far. To close this gap, commercialized 10Ah LiNi1-y-zMnyCozO2 (NMC)|LTO cells were calendrically aged at 60°C and 80°C at different SOCs. In addition to a non-invasive incremental capacity (IC) analysis, post-mortem examinations were performed that include cell opening, coin cell assembly with aged electrode material, and surface examinations by X-ray photoelectron spectroscopy and scanning electron microscopy. At 60°C calendar aging, the cells exhibit high thermal stability with dominant reversible capacity effects. 80°C storage temperature leads to intense gas formation and a superposition of different degradation modes. These modes are identified in the IC analysis as predominantly loss of lithium inventory and loss of active material at the positive electrode. At low and medium SOC, the residual cell capacity increases, which is attributable to both added capacity at higher cell voltages caused by extension of the LTO potential plateau and a pronounced passive electrode effect. Irreversible degradation of cell materials is most severe at high SOCs. Surface layer formation is observed on both LTO and NMC, though its composition differs and its thickness rises with increasing SOC.