Synthesis And Electrochemical Studies On Li-insertion Materials Containing Group VA Elements

Graphite-based carbonaceous materials have been widely used as the anode material of commercial lithium-ion batteries for its excellent cycleability. With the growing demands of high-energy secondary batteries, the low capacity of graphite (theoretical capacity: 372 mAh/g) has been thought to be a limiting factor for the wide applications. Therefore, a great deal of effort has been put into research on high-performance anode materials for lithium ion batteries.In this study, two series of lithium metal nitrides and phosphides were synthesized by high-energy ballmilling technique (mechanochemical synthesis). Their electrochemical properties and reaction mechanisms with lithium were investigated. In addition, the charge and discharge characteristics of some semiconductor materials were also studied.The test results show that nitrogen atmosphere is favorable for the ball-milling synthesis of lithium metal nitrides. Of the many ternary lithium metal nitrides Li3-xMxN (M=Co, Cu, Ni), Li2.6Co0.4N shows the highest specific capacity which is about 880mAh/g in the first 10 cycles. Li2.6Co0.2Cu0.2N and Li2.6Coo.2Fe0.2N have a lower capacity but much better cycle life than Li2.6Co0.4N, in which part of Co is substituted by Cu or Fe. By contrast, Li2.6Ni0.2Cu0.2N presents worse electrochemical reversibility than Li2.6Co0.2Cu0.2N. The structure of all these nitrides changes from the crystalline to the amorphous phase in the first lithium extraction process. The amorphous state is maintained during subsequent cycles, which is the reason for the very large capacity. However, it undergoes the structure rearrangement in the initial cycles. The electrochemical reactivity of lithium metal nitrides is strongly influenced by the micro-structures (e.g. short-range ordering after the first lithium extraction).The first lithium insertion into the CuP2 phase leads to copper reduction and theformation of lithium phosphide . The subsequent lithium extraction presents three voltage plateaus related to the formation of new phases such as Li2CuP. It means that both copper and phosphorus face a change of the oxic ation state for the electrochemical insertion and extraction. Lithium copper phosphide exhibits a similar reaction process. It provides a reversible capacity of 750 mAh/g and faradic yield of 100% at the first cycle.The electrochemical characteristics of GaAs, GaSb, InP and InAs are greatly similar. The charge and discharge behavior probably corresponds to alloying and de-alloying process. The reaction mechanism of SnSe and PbSe with lithium is similar to that of the respective oxides.