Effects of crystal structure and electronic properties on lithium storage performance of artificial graphite

Abstract

Graphite is nowadays commonly used as the main component of anode materials of lithium-ion batteries (LIBs). It is essential to deeply investigate the fundamentals of artificial graphite to obtain excellent anode, especially crystal structure and electronic properties. In this report, a series of graphite with different crystal structure were synthesized and used for anodes of LIBs. Meanwhile, a concise method is designed to evaluate qualitatively the conductivity of lithium ion (σ_Li) and a profound mechanism of lithium storage was revealed in terms of solid state theory. The conductivity analysis demonstrates that the graphite with longer crystal plane and lower stacking layers possesses higher conductivity of electron (σ_e). On the other hand, lower initial charge/discharge voltage indicates the graphite with lower L_a and higher L_c holds higher conductivity of lithium ion (σ_Li). According to the solid state theory, graphite is considered to be a semi-conductor with zero activation energy, while the lithium intercalated graphite is like a conductor. The conductivity of graphite mainly depends on the σ_e, while the conductivity of lithium intercalated graphite can be determined by the summation of σ_e and σ_Li. In lower charge/discharge rate, Li⁺ have enough time to insert into the graphitic layer, making the special capacity of graphite primarily determined by σ_e. However, with the increase of charge/discharge rate, Li⁺ insertion/extraction will become more difficult, making σ_Li become the mainly factor of the graphite special capacity. Therefore, the graphite with longer crystal plane and lower stacking layers owns higher specific capacity under slow charge/discharge rate, the graphite with shorter crystal plane and higher stacking layers shows relatively lower specific capacity under rapid charge/discharge rate. These results provide important insights into the design and improvement of graphite's electrochemical performance.