Preparation And Electrochemical Properties Of LiMnPO 4 And S / SnO 2 Composites

Olivine structured lithium manganese phosphate is considered to be one of the most promising cathode materials in hybrid electric vehicles (HEVs) and electric vehicles (EVs) because of its relatively high working voltage and matching with the stable voltage window of the electrolyte. But the biggest disadvantage is the very low electronic conductivity and lithium ion diffusion rate.The transmission direction of lithium ion is along [010]. So we shorten the transmission path of lithium ion by changing the morphology of LiMnPO4. The results indicate that we successfully synthesized six different morphologies of LiMnPO4. However, the initial discharge specific capacities are all very low, less than 5 mA h g-1, which is a far cry from the theoretical specific capacity of 171 mA h g-1. It is probably due to the low electrical conductivity. The sample added to 0.6 g of CTAB has higher capacity probably because of its small size reducing the transmission path of lithium ion. Compared with the sample without CTAB, the capacity and coulomb efficiency of the other samples have improved within 50 cycles.Compared with the traditional lithium-ion batteries, lithium sulfur battery has higher theoretical specific capacity and specific energy. Its theoretical specific capacity can reach to 1672 mA h g"1 and the theoretical specific energy is as high as 2600 W h kg"1, which is far more than the cathode materials of traditional lithium-ion batteries. So lithium sulfur battery is the most promising battery of high capacity and high energy at present. Lithium sulfur battery has three problems:the insulation of sulfur; the long chain polysulfides are soluble in the electrolyte during charge and discharge; the volume of sulfur will change thus leading to the damage of electrode. Focusing on the last two problems, we prepared hollow SnO2 spheres to alleviate the dissolution of polysulfides and volume expansion of sulfur. The research contents and results are as follows:(1) We discussed the cycle performance of S/SnO2 composites with different sulfur content and sulfur electrode at 0.5C within 50 cycles. The capacity retention of the S/SnO2 composites with 66% sulfur is 62.6%. Upon increasing the sulfur content of the S/SnO2 composites to 74% and 85%, the initial discharge capacity is respectively measured at 825.8 mA·h·g-1 and 744.7 mA·hg-1, both of which are lower than 66% sulfur content cathode. However, the initial discharge specific capacity of the composites with 66% sulfur is as high as 1176 mA·h·g-1 and the capacity is higher than the composites with 74% and 85% sulfur at every cycle within 50 cycles. Moreover the capacity retentions of the S/SnO2 composites with three different sulfur content are all very high indicating that the S/SnO2 composites can effectively improve the cycle performance of the battery.(2) We compared the impact of the three different conductive additives to the performance of the battery. The electrochemical impedance spectroscopy test indicates that the conductive sequence of three kinds of conductive additives is acetylene black< xc-72< ketjenblack. Both of the initial discharge capacity and the capacity retention increase with the increase of electrical conductivity.(3) We discussed the impact of the SnO2 doped fluorine (FTO) to the performance of the battery. TEM images showed that doped fluorine didn’t damage the core-shell structure of SiO2@ SnO2.The proportion of doped fluorine is about 1.53 wt%. The electrical conductivity increased slightly after doping fluorine. Though the capacity retention is slightly lower, the specific capacity of every cycle is higher than the composites without doping fluorine.(4) We discussed the impact of current collector to the performance of the battery. The result of the electrochemical impedance spectroscopy indicates that the impedance of the aluminum foil is greater than that of nickel foam. The initial discharge specific capacity of nickel foam is 4.5 times as high as aluminum foil at the same condition.