Study On Electrochemical Properties And Mechanism Of Copper Sulfides Cathode Materials For Lithium Batteries

Lithium sulfur batteries (Li-S) are one of the most promising secondary batteries due to their low cost, environment-friendly, high energy density. However, the insulating nature of electrode material and the loss intermediate of polysulfides in the liquid organic electrolyte result in very low utilization of positive active materials and poor cyclic stability for lithium sulfur batteries. How to prevent the sulfur from dissolving into the electrolyte, how to improve the utilization of active material, and how to increase the electrical conductivity of electrode materials and improve the cycle stability, those are the most important problems to solve necessarily.Elemental sulfur (S) and transition metal (M) can react with each other to form stable metal sulfides (MS). It is an effective strategy of sulfur fixation by adding transition metal to sulfur cathode material or directly using metal sulfides as the cathode materials for lithium batteries. Metal sulfides, CU2S is a promising cathode material for lithium battery due to its excellent electrical conductivity, which is of the same crystal structure and the equal molar volume with Li2S, and excellent conductivity of its reduction product of nano Cu.In this thesis, we have studied the electrochemical performance and mechanism of Cu2S used as cathode material for lithium battery. The main experiments and results of the thesis are given as the following three parts.(1) The commercial Cu2S was used as the electrode material and its electrochemical performance was investigated by half-cell system. It was found that Cu2S, a mixture of Cu, CuS and Cu2S, had a specific capacity of335mAh g-1and good cycleability. Even in the200th cycle, its capacity retention was99.5%. But the material showed a plurality of discharge plateau at earily cycle stage and the discharge plateau varied together with cycle. But the material showed a plurality of discharge plateau at earily cycle stage and the discharge plateau varied together with cycle. The electrochemical reaction mechanism of Cu2S during discharge-charge process was investigated by XRD, XPS and TEM methods. It is most likely that all of S element in the commercial Cu2S would transfer into a crystal of Cu2S during charge-dinscharge cycle, which corresponded to a single electrochemical reaction, Cu2S+Li (?) Li2S+Cu, characterized by two very flat voltage platform at1.7V and1.85V in the process of charging and discharging.(2) The synthesis process of Cu2S was also developed by the reaction between S powder and different size of Cu powder at different proportions. The results showed that the nano-copper powder and sulfur powder reacted to form the high purity Cu2S when the mole ratio of Cu/S was3/1. That material had a good cyclic stability and excellent effects of sulfur fixation, and the capacity retention was83.3%for100th cycle. When the mole ratios of Cu/S were2/1and1/1, the products synthesized by nano-copper powder and sulfur powder contained CuS and they had bad cyclic stability and poor effects of sulfur fixation, and the capacity retention was33.3%and14.5%for100th cycle, respectively.(3) By the adoption of copper foil current collector and adding copper powder to the sulfur electrode materials for lithium sulfur batteries, the electrochemical properties of Cu2S in-situ synthesized by Cu and S were investigated. The first discharge specific capacity of the composite materials was up to1614.1mAh g-1at the rate of0.5C (weight of sulfur), meaning the utilization rate of sulfur was as high as91%. We also found that the material had excellent cycleability and rate performance, for example, its capacity retention is95.5%at the1000th cycle even at the rate of2C.The results of this thesis revealed that during the charge-discharge process of lithium sulfur battery, the sulfur was able to be fixed by transition metal Cu to generate a Cu2S compound with good conductivity and then an electrode with high utilization rate of sulfur, high specific capacity and good cycle stability was received ultimately, suggesting copper sulfides are a kind of potential enengy-storage materials.