High Temperature Formation – Supressing gas evolution in LTO||NCM111 Pouch Cells

Summary:

The spinel-type negative electrode active material Li4Ti5O12 (LTO) is a prominent candidate with regard to its application in lithium-ion batteries (LIBs) for high-power applications. While commercially used graphite-based LIBs struggle with high current densities and the mostly corresponding wide temperature stability window, LTO offers a high C-rate capability, long cycle life, high thermal stability and thus, excellent safety properties, amongst others due to its de-/lithiation plateau at 1.55 V vs. Li/Li+ excluding the risk of lithium plating. However, although most of the challenges associated with the application of LTO (low electronic conductivity, low lithium diffusion coefficient and low specific capacity) have already been successfully compensated, gas evolution remains the main issue hindering its broad commercialization.
In this study a high temperature formation approach is presented to suppress gas evolution in LTO||NCM111-based pouch cells during subsequent charge/discharge cycling. Scanning electron microscopy (SEM) revealed that after 20 °C formation only isolated spots of the smooth LTO particle surface are covered by a decomposition layer. However, by increasing the formation temperature up to 60 °C a homogeneous decomposition layer has formed over the complete LTO particle surface area. Volume measurements of the cells before and after formation showed, that an increase in formation temperature is associated with more gas evolution, suggesting that gas evolution and decomposition layer formation are directly correlated to each other. The analysis of the evolved gases within the cell via GC-BID/WLD revealed that H2 is the main gas species for every formation temperature, which most probably originates from the reduction of residual moisture within the electrodes. The increasing CO2 and CnHm content indicate more LiPF6 decomposition via Lewis acidic PF5 and more electrolyte decomposition at higher temperatures, which is in good agreement with the SEM investigations.
After final degassing all cells were cycled with the same charge/discharge procedure at 40 °C. The results clearly show that higher formation temperatures significantly reduce the gas evolution rate during charge/discharge cycling and furthermore lead to an enhanced specific discharge capacity and capacity retention. For a formation temperature of 60 °C, gassing could be completely suppressed. It is assumed, that higher formation temperatures contribute to the formation of a stable and protective decomposition layer on the LTO surface, preventing direct contact between the active material and the electrolyte leading to significantly reduced or even suppressed gas evolution.
Since neither particle pretreatment nor the addition of film-forming electrolyte additives were necessary to suppress severe gas evolution, this high temperature formation approach could be a milestone for a cost-efficient and straight forward commercialization of LTO-based cells.