| Lithium-ion batteries are considered as most widely used rechargeablebatteries with much of advantages, like high specific energy, no memory effect,long life, high voltage, no pollution to circumstances etc. Because of the energysources crisis and environmental pollution, many researchers focus on thedevelopment of new energy vehicles. Lithium-ion batteries as the power of theelectric vehicles must meet the requirements of excellent safety, long life, highenergy and high power. Lots of research groups focus on new electrode materials,but relatively short attention has been placed on improving the performance ofelectrolyte of lithium-ion battery. This paper focuses on improving the life,stability and flammability of current commercial electrolyte, and developing anew lithium-ion electrolyte with wide temperature range, good rate capability,good flame retardancy and safety.Currently, most commercial high-voltage electrolyte use LiTFSI as lithiumsalt, but LiTFSI can cause Al collector corrosion, which seriously affect thestability and life of the battery. Two measures have been taken to solve thisproblem. One is adding a perfluorinated inorganic anion salt to protect Alcollector, the other is using a kind of low viscosity ether to replace higherviscosity solvent of electrolyte. Following the two measures configurationelectrolyte2#:0.9M LiTFSI+0.1M LiBOB+0.1M LiPF6/EC+EMC+ADN(Vol.1:2:1) and electrolyte3#:1.0M LiTFSI+0.1M LiBOB/THF+EMC+AND (Vol.1:2:1).This two electrolytes can protect Al collector from corroding.The Al collector corrosion potential of the electrolyte2#and electrolyte3#are3.9V and4.0V, which are higher than unimproved electrolyte1#3.7V. After5laps cycle, Al collector corrosion potential increased to4.2V owning to theformation of passivation film. The self extinguishing time of Electrolyte1#,2#,3#is220.87s/g,192.82s/g,225.27s/g respectively, it shows that electrolyte2#has the best flame retardancy. In different environment temperatures, LiFePO4inelectrolyte3#perform best, which is at25℃, after100cycles the dischargespecific capacity remains93.4%of the initial capacity, higher than73.4%and88.9%of the1#and2#. The capacity remains81.2%of the initial capacity whenthe temperature reaches55℃. As for LiMn2O4and Li3V2(PO4)3, they performbetter in electrolyte2#. At25℃, the discharge specific capacity of LiMn2O4remains103.4mAh/g after100cycles, which corresponds to capacity retentionof90.7%. The capacity remains76.7mAh/g, which corresponds to capacityretention of67.3%at55℃. At25℃, the discharge specific capacity ofLi3V2(PO4)3remains125.7mAh/g after100cycles, which corresponds to capacity retention of80.5%, higher than68.4%and76.8%of the electrolyte1#and2#. The capacity remains75.6%, which corresponds to capacity retention ofat55℃.Secondly, due to the structure and properties of carbonate, itselectrochemical performance could not always be promoted. So tetramethylenesulfone (TMS) is selected as main solvent, which has wider electrochemicalwindow and higher conductivity. Because TMS does not infiltrate into batteryseparator, with large viscosity and high melting point, we need a silane withflame resistance, good stability, small viscosity and low melting point to modifythe single TMS electrolyte. It was found that methyltrimethoxysilane (MTMS)has wider electrochemical window than methyltriethoxysilane (MTES) andmethyltripropoxysilane (MTPS). The conductivity of MTMS was0.64mS/cmand the electrochemical window reached5.2V, at25℃. When MTMS and TMSmixed with the volume ratio of4:6, the electrochemical window was5.2V, theconductivity was2.35mS/cm and a lower self extinguishing time183.02s/g, theAl collector corrosion potential was3.9V, which increased to4.4V after5lapscycle. All these proved that the new hybrid electrolyte has excellent flameresistance and it can protect the aluminum collector effectively. For the threekinds of positive electrode materials, the new hybrid electrolyte has goodcompatibility with out-side reaction. At55℃, the discharge specific capacity ofLiFePO4，LiMn2O4and Li3V2(PO4)3remains126.1mAh/g,95.7mAh/g,126.2mAh/g after100cycles respectively. The new hybrid electrolyte has excellentrate discharge ability, at5C LiFePO4, LiMn2O4and Li3V2(PO4)3remains20.0mAh/g,30.0mAh/g,66.7mAh/g respectively. At the same rate, the commercialcarbonate electrolyte could not discharge any capacity.In conclusion, the modified commercial electrolyte can improve the life andsafety of the bettery. When sulfolane clectrolyte modified with methyltrimethoxysilane, batteries can obain better performance than single sulfolane electrolyte. |
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