Influence of Lithium Precursors and Calcination Atmospheres on Graphene Sheets-Modified Nano-Li4Ti5O12 Anode Material

With the advantageous features include suitable lithium insertion position and "zero strain" structure, Li₄Ti₅O₁₂ has become one of the ideal anode materials for lithium-ion battery and battery-capacitor system. However, the obstacle of its poor electronic and Li⁺ conductivity restrict its rate capacity. Many researches proved that the one-pot preparation process, which is mixing the carbon precursor with lithium and titanium precursor and then thermal treating in an inert or reducing atmosphere together, not only can form a carbon coating layer to improve the electronic property of the materials easily, but also inhibits the particle size growth of LTO by a barrier effect of carbon. Among kinds of carbon source, graphene presents excellent unique properties, which has been considered as an ideal carbon source to improve the electrochemical performance of LTO and many other active materials. However, the intersheet π-π attractions of GS easily resulted in its restacking and agglomeration. Therefore, an oxygenated graphene (GO) with a special amphiphilic molecular structure is adopted as GS precursor to prepare GS modified composites. Due to the amphiphilic characteristic of GO, the choice of an appropriate Li precursor is extremely important to obtain a high quality product.

In this study, we systematically investigated the influence of lithium precursor and calcination atmosphere on the reaction mechanisms, phase formation, particulate morphology, surface properties and electrochemical performance of graphene sheets-modified nano-Li₄Ti₅O₁₂ composite. The results shown that the lithium precursor containing carboxyl anion such as LiAc and Li₂CO₃ would connect with oxygen groups of GO by strong hydrogen bonds to restrict the morphology of composites and the phase formation of pure spinel Li₄Ti₅O₁₂. Furthermore, the oxygen ratio in the molecular structure of lithium compound is proportional to the consumption of graphene. In addition, the reducing atmosphere facilitates the partial reduction of Ti⁴⁺ to promote the interfacial charge transfer kinetics of the product.

Through optimization the conditions of the reaction, the sample adopting LiOH precursor with adjustment GS ratio calcined under Ar/H₂ (5 %) atmosphere delivers 172.8 mAh g^-1 at 1 C and maintained a discharge capacity of 98.0 mAh g^-1 at a high rate of 40 C. This sample also showed fairly stable cycling performance. After 800 deep cycles at a high charge/discharge rate of 40 C, the capacity can hold 97.7 % of its second discharge value. This study not only provides an optimization of Li precursor and calcination condition for GS modified LTO material, but also has significant reference value for any combination reaction with GO participation.