Investigation of space group effects of High-Voltage spinel LiNi0.5Mn1.5O4: Unveiling the influences of fluorinate benzimidazole salt additive

Spinel LiNi0.5Mn1.5O4 (LNMO) is one of the attractive cathodes due to several advantages such as Co-free for low cost and high operating voltage (∼5 V). Normally, there are Fd3¯m and P4332 space group types of LNMOs. However, both LNMOs suffer phase segregation at intragranular side which affects the lithium diffusion pathway and structural degradation during cycles. On the other hand, Jahn-Teller distortion on Mn3+ as well as HF effects from electrolyte oxidation at high voltage window triggered lead Mn dissolution from LNMO that causes performance fade. In this work, a new salt is developed, lithium 1-fluoro benzimidazole (Li-1FB) for LNMO with two different space groups. Ex-situ XRD results show the Li-1FB postpones the formation of intermediate phase transition on both LNMOs at different states of charge. In addition, 1H NMR analysis reveals the Li-1FB salt can prevent HF formation by the N-Li functional group of imidazole ring thus inhibits Mn dissolution on both LNMOs after cycling. Furthermore, the Li-1FB salt is suitable to improve cycle performance of both LNMOs in half-cell (Li metal as anode) and full cell (graphite as anode) applications. This new salt provides dramatic improvements, and it is suitable for further high-energy applications.