Effect of Formation Temperature on SEI and Performance of Elastomeric Lithium Metal Batteries

The formation of a stable and highly Li+-conductive solid electrolyte interphase (SEI) is key to reliable cycling in lithium metal batteries (LMBs). In this work, we show that the temperature during the formation cycle strongly influences SEI composition and performance, particularly under low-temperature operation. SEI formed at 0 °C exhibits roughly tenfold lower interfacial resistance than that formed at 25 °C, enabling faster Li⁺ transport and more uniform lithium deposition. Consequently, Li||LiNi0.8Co0.1Mn0.1O2 cells formation-cycled at 0 °C maintain 89.7% of their initial capacity after 100 cycles at −10 °C and display stable behavior at ambient temperature. Enhanced performance at low-temperature formation is attributed to fluorinated polymer-induced LiF and highly conductive species such as Li2CO3 and Li3N, confirmed via combined experiments and multiscale simulations. These findings emphasize the critical role of elastomeric electrolyte design and formation protocols in producing robust SEI and improving LMB performance across a wide temperature range.