N-functionalized graphene quantum dots: Charge transporting layer for high-rate and durable Li 4 Ti 5 O 12 -based Li-ion battery

Abstract

Spinel Li 4 Ti 5 O 12 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, Li 4 Ti 5 O 12 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 Li 4 Ti 5 O 12 from ambient degradation, (2) forms a thin and smooth solid-electrolyte interphase layer on the Li 4 Ti 5 O 12 surface, (3) acts as a charge transfer layer, (4) protects the Li 4 Ti 5 O 12 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 Li 4 Ti 5 O 12 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 Li 4 Ti 5 O 12 at high C-rates. © 2019 Elsevier B.V.