Modification And Electrochemical Properties Of Lithium Cobalt Oxide For Cathode Materials

Developing high energy density cathode materials,as well as improving the cycle stability,safty and performance in extreme environment,is one of the research interests for Li-ion batteries.In the thesis,Micro-size hollow spherical LiCoVO₄ cathode materials were synthesized by solid-state reaction method.Single phase inverse spinel structure LiCoVO₄ materials were obtained at 300℃⁵00℃for 4 hours.The specific discharge capacity of the LiCoVO₄ prepared at 300℃was 60.9 mAh/g,which was highest compared with that of the samples obtained at400℃（60 mAh/g）and at 500℃（57.6 mAh/g）,respectively.After 10 cycles,the retention of specific discharge capacity of the LiCoVO₄ prepared at 400℃was 71.7%,which higher than that of both samples obtained at 300℃（62.2%）and 500℃（62.1%）,respectively.LiCo_1-xNi_xVO₄（x=0、0.25、0.5、0.75）cathode materials were prepared at 400℃.It was found that the highest first specific discharge capacity was achieve when x=0.25.As x≥0.5,the first specific discharge capacities decreased with the Ni content increasing,which even lower than that of the pristine LiCoVO₄.After 20 cycles,the specific discharge capacity retention of LiCo_0.75Ni_0.25VO₄was 67.4%,which was higher than that of the pristine LiCoVO₄（60%）.By EIS analysis,the Li⁺diffusion coefficient of LiCo_0.75Ni_0.25VO₄ was 3.78 10^-13cm²/s,which was one order of magnitude higher than that of the pristine LiCoVO₄(7.81 10^-14cm²/s).It proved that partical Ni²⁺substitution of Co²⁺could increase the Li⁺diffusion coefficient of the LiCoVO₄,then improve the electrochemical performance.inverse spinel structure nano-size LiCoVO₄（LCV,50-100 nm）was synthesized by hydrothermal method at 220℃for 24h.LiCoVO₄/graphene was prepared,using graphene oxide as precursor,by microwave heating.It was found that the electrical resistivity of LCV/G was9.3513×10^-7S/cm,which was 6.5 times of that of LCV(1.5028×10^-7 S/cm).The first specific discharge capacity of LCV/G was 118.2 mAh/g,which was 1.35-fold increase of that of LCV.After 10 cycles,the specific discharge capacity of LCV/G was 41 mAh/g,which was 1.24 times of that of LCV.It shows that grapheme could effectively raise the electronic conductity of LCV to improve the specific discharge capacity and cycle stability of LCV.