Synthesis And Properties Of Large-scale Li-ion Battery Materials

Lithium-ion batteries are the most widely used secondary batteriescurrently because of their large energy density, long cycle life and nomemory effect and so on. The cathode and anode materials are the mostcritical materials; LiCoO₂and MCMB are the two curretlycommercialized materials. However, the high cost, low rate capacity andpoor safety performance seriously hampered the further development oflithium-ion battery in a large-scale power. It is an urgent need to developlow-cost, long-life, good rate capacity and good safety materials to meetthe demand of3G network, the wind and solar energy storage, electricvehicles and other application. This thesis focused on studyingLi₄Ti₅O₁₂/C composite anode materials, cheap LiNixCoyMn1-x-yO₂cathode material and LiCo_0.75Al_0.25O₂coated LiNiO₂cathode materialinstead of LiCoO₂to improve the cycle life, rate capability and safety ofLi-ion batteries at room temperature and high temperature. Theconclusions have been summarized as following:1. Spinel Li₄Ti₅O₁₂have been synthesized via a solid state reactionwith TiO₂-anatase and Li2CO3as the starting materials. The effects ofLi/Ti molar ratio and the calcinaton temperature on the morphology, structure and electrochemical performance have been investigated. Itwas found that the best Li/Ti molar ratio is1.05and the best secondarycalcining temperature is800oC.2. Li₄Ti₅O₁₂/C composites have been synthesized via a solid statereaction with TiO₂-anatase, Li2CO3and different carbon sources, such asconductive graphite KS-6and sucrose, as the starting materials. It wasfound that the carbon layer from sucrose was homogeneously coated onthe Li₄Ti₅O₁₂surface and the KS-6was embedded among the Li₄Ti₅O₁₂particles as a conductive bridge without affecting the major spinelstructure of Li₄Ti₅O₁₂. Moreover, it was demonstrated that the sucroseand KS-6played different roles in improving the electrochemicalproperties of Li₄Ti₅O₁₂/C composite. Compared with samples preparedby solely KS-6or sucrose as the carbon source, the Li₄Ti₅O₁₂/Ccomposite (LTO-1) with KS-6and sucrose as carbon sources togetherrevealed the optimal electrochemical performance. It showed a highinitial specific capacity of152.5mAh·g^-1at0.2C and an excellentcycling performance with96.8%capacity retention after1000cycles at25oC at1C. Furthermore, the (LTO-1)/LiMn2O4full batterydemonstrated a good cycling performance at55oC and could pass the5C-20V overcharge test, external short-circuit and nail-puncture test.3. Li₄Ti₅O₁₂/C composites have been synthesized via a solid statereaction with TiO₂-anatase, Li2CO3, sucrose and VGCF as the starting material. VGCF has a larger length-diameter ratio than KS-6and goodconductive properties. The Li₄Ti₅O₁₂/C composite (LTO-b) with1%sucrose and0.5%VGCF showed good cycling performance at25and55oC. It showed an initial capacity of157.5mAh·g^-1at1C and anexcellent cycling performance with97.8%capacity retention after100cycles at55oC. Moreover, the (LTO-b)/LiMn2O4full batteries havesimilar cycling performance at0.1-3V and1-3V, which shows goodanti-redischarge properties. In addition, the capacity recovery ratio of(LTO-b)/LiMn2O4full batteries are99.8%(to the initial capacity) after150days at55oC, which shows excellent storage performance.4. Li_1+x[Ni_3/8Co_1/8Mn_4/8]O₂cathode materials have been prepared bycalcination of LiOH·H2O and [Ni_3/8Co_1/8Mn_4/8]CO3under O₂atmosphere.The structure and morphology have been studied, and theelectrochemical behavior and safe characteristic have been tested by14500R-type MCMB/Li_1+x[Ni_3/8Co_1/8Mn_4/8]O₂batteries. The capacityretention ratio of Li_1.11[Ni_3/8Co_1/8Mn_4/8]O₂is95.7%after300cycles in2.75-4.2V at25oC. At55oC, in2.75-4.2V and2.75-4.5V, thedischarge specific capacities are150.3mAh·g^-1and189.3mAh·g^-1, withthe capacity retention ratios are90.6%and88.2%after100cycles,respectively. The MCMB/Li_1.11[Ni_3/8Co_1/8Mn_4/8]O₂batteries can pass the3C-5V overcharge test and short-circus experiment. Compared withLiCoO₂,the Li_1.11[Ni_3/8Co_1/8Mn_4/8]O₂material has higher capacity and better cycling performance in2.75-4.5V at55oC, with better rate andanti-recharged property, and with an about50%price of LiCoO2.5. CoAl-LDH or Co(OH)₂coated spherical Ni(OH)₂precursorswere obtained via a coprecipitation method at a constant pH. After theprecursors and LiOH H2O were mixed, the mixtures were annealed athigh temperature in O2atmosphere, and then the0.08LiCo_0.75Al_0.25O₂-0.92LiNiO₂,0.08LiCoO2-0.92LiNiO2and LiNiO2cathode materials were synthesized. Effects of the coating layer werealso studied. The results showed that the0.08LiCo_0.75Al_0.25O₂-0.92LiNiO2material owned the best rate andcycle-life. The0.1C,0.5C and3C discharging capacities were211.0mAh g^-1,195.6mAh·g^-1and161.0mAh g^-1respectively, and thecapacity retention ratio after30cycles at0.5C was93.2%. Theseresults were much better than both pure LiNiO2and0.08LiCoO2-0.92LiNiO2.