Study On In Situ Electrochemical Preparation And Li-storage Properties Of Cu₂S-based Cathode Materials

With a theoretical capacity of 337 m Ah·g-1, good electronic conductivity（ca. 104 S cm-1）, and flat discharge curves,abundant resources in nature, Cu₂ S is one of the promising cathode materials for the next generation high energy density lithium secondary battery. However, there have limited number of research papers in Cu₂ S basics composites as a lithium-ion cathode material. The Cu₂S-Li battery systems have some persistent problems. One is its poor cycle stability,which hindering the application of Cu₂ S in LIBs, mainly caused by the high solubility of products lithium polusulfides（Li2Sx, x>2） in organic electrolyte, which leads to the loss of the active material in the cathode and a rapid fading of the reversible capacity. In this paper, sulfur-based precursor composite materials（S-C and S-PPy composites） were prepared in-situ composite treatmen, their morphology, microstructure were investigated. Then, a copper foil is employed as not only the cathode current collector（instead of an aluminum foil）, but also reaction raw material, and during charge-discharge processes, the copper foil can release copper ions（the corroded is moderately）, thus, gradually generating CuxS, and become into final product of Cu₂ S composites. The electrochemical performances were investigated, appropriate electrolyte were looking for, Cu₂ S cathodes cycled on different current collectors. The experimental results are as follows:（1） S-C composite is prepared in a pan, and Cu₂S-C composite is generating by a insitu electrochemical process, which exploited the charge-discharge processes of a S-C composite on a copper-foil current collector. The electrochemical performance results show that the as-prepared Cu₂S-C composite can deliver a reversible capacity of 255.4 m Ah·g-1 at the first cycle, and a high reversible capacity of 252.3 m Ah·g-1 remained even after 100 cycles, with the negligible capacity loss. Furthermore, the composite show a superior rate capacity of 232.8, 207.6 and 183.8 m Ah·g-1 at a current density of 0.2 C, 0.5 C and 1 C（1 C=337 m A·g-1）. The SEM test results show that different carbon contents and different H2SO4 concentration have influence on morphology of the Cu₂S-C materials, the electrochemical performance are influenced by the carbon contents and the concentration of H2SO4, the carbon design content for 5 wt.% and 75 % H2SO4 concentration of the S-C composite material precursor has the best electrochemical properties at a current density of 0.2 C.（2） Cu₂S-PPy composites have prepared in-situ generation methode by using the charge-discharge processes of S-PPy composites precursor, the S-PPy composites precursor is prepared by in-situ oxidation method. The Cu₂S-PPy exhibits long cycle stability and high reversible capacity of 270.2 m Ah·g-1 at 0.2 C over 100 cycles, with the negligible capacity loss, when Cu₂S-PPy composites is used as cathode for lithium-ion batteries. Furthermore, the S-PPy composites precursor deliver different voltage plateau on copper and aluminum foils, and different voltage plateaus mean different electrochemical reactions.Using an ether-based electrolyte（Li TFSI/DOL/DME） can significantly improves the cycling stability of Cu₂S-based composites cathode materials.