The Common Compatibility Of Electrolyte With Both Positive And Negative Electrode Materials In Lithium Ion Secondary Batteries

Both negative and positive electrodes in lithium ion batteries have their own most suitable (compatible) electrolyte. However, it is impossible for us to use two different kinds of electrolytes in the same battery system. Therefore, the selected electro1~te must have common compatibility with both positive and negative electrode materials. We prepared 3 kinds (4 sorts) of widely used carbon negative electrode materials (coal tar pitch based mesocarbon microbeads, petroleum pitch based mesocarbon microbeads, petroleum coke and natural flake graphite with pyrolytic carbon coating) which have excellent structure for storing lithium ions, selected 2 kinds of widely used positive electrode materials (spinel LiMn2O4 and LiCoO.) and chose 6 kinds of electrolytes. The compatibilities of each kind electrode materials with the 6 kinds of electrolytes were investigated. The common compatibilities of electrolytes with both positive and negative electrode materials was concluded, analyzed, and summarized. The followings are the main conclusions. The charging-discharging performances of coal tar pitch-based mesocarbon microbeads(C-MCMB) and petroleum pitch-based mesocarbon microbeads(P- MCMB) relate to the maximum heat-treatment temperature (HTTmax). The mechanism of storing lithium ions of C-MCMB and P-MCMB samples heat- treated at temperatures below 2000℃ is storing lithium ions in micropores due to the existence of a large amount of micropores in the samples vhich have relatively high initial capacities of storing lithium ions and fast capacity fading, but with no flat charging-discharging plateaus. The mechanism of storing lithium ions of C- MCMB and P-MCMB samples heat-treated at temperatures above 2000℃ is the intercalation of lithium ions into the layers of graphite rnicrocr stals due to the existence of many large graphite microcrvstals in the samples which have high charging-discharging capacities, flat charging-discharging plateaus and excellent cyclabilities. When HTTmaX is equal to 2800℃, the discharging capacities of CMAh/g, and PM2/l, samples in 1 mol/L LiClO4/EC+DEC(1 :1) electrolyte are 285. lrnAh and 289.2mAh/g respectivel and the charging-discharging efficiencies are 90.7% and 96.9% respectively. The charging-discharging performances of these two kinds of MCMB have reached to the advanced level in the world. The charging-discharging performance of petroleum coke (FPC) relates to the maximum heat-treatment temperature (HTTmax). With increasing HTTmax, FPC changes from turbostructure to highly ordered graphite microcrystalline structure. When HITmax is equal to 2750℃ the discharging capacity of FPC1750 sample in I mol,L LiClO4/EC+DEC(1:1) electrolyte is 342.2mAh/g and the charinu- discharging efficiency of it is 93.2%. The charging-discharging performance of FPC2750 has reached to the advanced level in the world. When HTT.OO is equal to 3000℃, the raphitization degree is higher, but the charging-discharging capacity and efficiency decrease since the exfoliation of graphite mocrocrystals occurs due to the disappearance of SP3 bond which can prevent the solvated lithium ions from intercalating into graphite layers. The coating process of pyrolytic carbon can not change the microstructure of NFG. But the texture of solid electrolyte interphase (SEI film) can be improved, which can prevent solvent molecules from strong reduction on the surface of NFG and prevent NFG from exfoliation due to the intercalation of solvated lithium ions. Too thick or too thin coating laye...