Considering the constantly increasing demands for a large variety of applications for lithium-ion batteries (LIBs), the need for both improved high-energy as well as high-power systems is inevitable. However, when it comes to the application of LIBs in the field of high-power applications the power capability of most commercialized cell systems is very limited excluding them from applications that demand a high rate capability and the mostly corresponding wide temperature stability window. In this regard the spinel-type active material Li₄Ti₅O₁₂ (LTO) as negative active material for LIBs is a very prominent candidate to replace graphite when it comes to applications in high power systems.

Even though there are already promising solutions to overcome the drawbacks of LTO in terms of electronic conductivity [1], lithium diffusion coefficient [2] and low capacity [3], the commercialization of LTO is still hindered by the extensive gas evolution during cyclic and calendric aging [4,5].

In this study it is shown that an increase of the formation temperature during the formation procedure does not only suppress gas evolution upon subsequent charge/discharge cycling but also has a positive effect on the specific discharge capacity and rate capability due to reduced cell impedance.

SEM investigations showed that higher formation temperatures lead to the formation of a homogeneous decomposition layer over the complete LTO particle surface area. Volume measurements of the cells before and after formation showed, that an increase in formation temperature is furthermore 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 C_nH_m 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.

The formed decomposition layer might have a stabilizing and protective effect on the LTO surface, preventing direct contact between the active material and the electrolyte and therefore leading to significantly reduced or (in case of applying sufficiently high formation temperatures) even suppressed gas evolution during cell application after formation.

Moreover, the formation of such a decomposition layer might lead to LTO particle rearrangement allowing access to more LTO surface area and thus increasing the cell capacity. 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.

[1] Yuan et al., Adv. Energy Mat., 2017, 7, 1601625.

[2] Duan et al., J. Phys. Chem. C 2015, 119, 5238.

[3] Griffith et al., Chem. Mater. 2020, 33, 4.

[4] Wu et al., J. Power Sources, 2013, 237, 285.

[5] Wang et al., J. Electrochem. Soc., 2019, 166, A4150.