Suppressing Gas Evolution in Li4Ti5O12||LiNi1/3Co1/3Mn1/3O2 Pouch Cells By High Temperature Formation

Abstract

The increasing demands for a wide range of lithium-ion battery (LIB) applications requires the development of both improved high-energy as well as high-power systems. However, for high-power applications, the performance and thermal stability window of most commercial LIB systems is very limited. To meet these requirements, the spinel-type active material Li4Ti5O12 (LTO) is a very prominent candidate as an alternative to conventionally used graphite as a negative active material for LIBs. However, despite the excellent lifetime and safety properties of LTO [1], its commercialization is still hindered by severe gas evolution during cyclic and calendar aging [2,3].This study demonstrates that elevated temperatures during the formation procedure do not only suppress gas evolution upon subsequent charge/discharge cycling but also have a positive impact on the specific discharge capacity and rate capability. It was shown by same-spot SEM investigations that higher formation temperatures lead to the formation of a homogeneous decomposition layer over the entire electrode surface area. Volume measurements of the cells showed, that an increase in formation temperature is furthermore associated with more gas evolution, suggesting that gas evolution and decomposition layer formation are directly correlating.It is assumed that the formation of this decomposition layer on the LTO electrode surface during the formation procedure essentially serves to reduce or (in case of applying sufficiently high formation temperatures) even suppress surface catalytic gassing reactions during subsequent cell application.In this context, the impact of the electrode surface area on the qualitative and quantitative gassing behavior during cell aging is highlighted. In contrary to the assumption that LTO is solely responsible for gas evolution in LTO-based LIBs, it could be shown that gas evolution is primarily determined by the specific electrode surface area. Accordingly, when LTO is used as negative active material, a protective layer for suppression of gas evolution should be formed not only on the active material particles but also on the inactive surface are, hence, the entire electrode surface area.Therefore, the high-temperature formation approach presented in this study is ideally suited for formation of an effective decomposition layer over the entire electrode. Since neither particle pretreatment nor the addition of film-forming electrolyte additives were necessary to suppress severe gas evolution, the high temperature formation approach could be the cornerstone for a cost-effective and easy commercialization of Li4Ti5O12-based cells.[1] C. Han, Y.B. He, M. Liu, B. Li, Q.H. Yang, C.P. Wong, F. Kang, J. Mater. Chem. A, 2017, 5, 6368–6381[2] K. Wu, J. Yang, Y. Liu, Y. Zhang, C. Wang, J.Xu, F. Ning, D. Wang, J. Power Sources, 2013, 237, 285.[3] S. Wang, J. Liu, K.Rafiz, Y. Jin, Y. Li, Y. S. Lin, J. Electrochem. Soc., 2019, 166, A4150.