N-functionalized graphene quantum dots: Charge transporting layer for high-rate and durable Li₄Ti₅O₁₂-based Li-ion battery

Spinel Li₄Ti₅O₁₂ can replace carbon in Li-ion battery anodes due to its high voltage, preventing decomposition of the electrolyte and formation of Li metal dendrites. However, Li₄Ti₅O₁₂ has a low electronic conductivity and Li-ion diffusion coefficient, limiting its charge/discharge properties at high rate capacities, and also suffers from gassing during cycling. Here, we used N-functionalized graphene quantum dots interfacial layer, which (1) protects Li₄Ti₅O₁₂ from ambient degradation, (2) forms a thin and smooth solid-electrolyte interphase layer on the Li₄Ti₅O₁₂ surface, (3) acts as a charge transfer layer, (4) protects the Li₄Ti₅O₁₂ electrode from reactions with the electrolyte, and (5) suppresses gassing during cycling. Consequently, the Li-ion diffusion coefficient increased by ∼19%. The effectiveness of the N-functionalized graphene quantum dots is manifested in the specific capacity of 161 mAh/g at 50C, which is improved by ∼23% compared to pure Li₄Ti₅O₁₂ electrode and maintained for over 500 cycles. Unlike graphene, N-functionalized graphene quantum dots themselves work as a stable charge transporting and protecting layer. Our strategy successfully obtained a good cycling performance and long cycling life of Li₄Ti₅O₁₂ at high C-rates.