Low Temperature/high Voltage Electrolyte For Lithium Ion Battery

Carbonate electrolyte is the most commonly used system for the commercial lithiumion battery (LIB), with high conductivity, superior compattability with electrodes, it isconvenient to prepare. However, at low temperature (≤-20○C), a dramatic conductivitydrop might happen due to the high melting point of some carbonate solvents, leading to apoor low temperature performance of the LIB. Moreover, in the high voltage battery system,large amounts of electrolyte are oxided as the anodic stability of the carbonate electrolyte islower than4.5V, then severe capacity decay is caused. Electrolyte modifications arepreceded in this paper to address the above problems, so that to enhance theelectrochemical performance of LIB at low temperature and high voltage. The details are asfollows:1) Mass triangle model is applied to LiODFB-EC/PC/EMC for electrolyteconductivity forecasting and solvents ratio optimization, to obtain the highest conductivityin this system. According to the variation tendency of calculated conductivity isolines, theforecasting region is narrowed, EC/PC/EMC=0.19/0.22/0.59is finally obtained with aconductivity of0.745mS cm-1at-40○C, which is51.4%higher than the one ofEC/PC/EMC(1/1/1, without optimization). The discharge test of LiCoO2/Li at-40○C showsthat cell using the optimized electrolyte delivers a capacity of18mAh g-1higher than theone without optimization.2) Solvent ratio for LiPF6-EC/PC/EMC is optimized to be (0.14/0.18/0.68) afterconductivity forecasting. Base on the ratio, additive (Li2CO3) is added to improve the filmformation ability of electrolyte, so that the compatibility between electrolyte and electrodecan be further promoted. LiFePO4electrode is used for room/low temperature discharge test.With the addition of Li2CO3, its capacity increased by6%and63.6%at-20○C and-30○C,respectively. The kinetic properties of electrode at room/low temperature are confirmed tobe enhanced by Li2CO3through CV and exchange current density (j0) tests. The result fromXPS and XRD measurements show the SEI film is modified by Li2CO3.3) Electrolyte that performs superior at low temperature always behaves poor at roomtemperature. In order to relieve the problem, novel electrolyte additive is explored. Thecommon salt NaCl is added into LiPF6-EC/PC/EMC, the transference number of Li+increases by one time as tested. LiFePO4/Li and MCMB/Li cells are assembled forcharge/discharge (C/D) test under room/low temperature. The result indicates a goodcompatibility of NaCl with the two electrodes. Moreover, capacity increases by21%for LiFePO4/Li at2C, and it is140%higher for MCMB/Li after52cycles. SEM, XRD andXPS are conducted to investigate the component and properties of the SEI film, it is foundthat the SEI film is modified by NaCl additive.4) In order to widen the application of the low temperature electrolyte on the highvoltage electrode, optimized additive is added into the carbonate electrolyte, with expect toinhibit electrolyte oxidation at high voltage, so that improved cycling stability could beachieved. Electrochemical performance at high voltage is performed on cell with LiCoO2(4.5V) or LiNi0.5Mn1.5O4(5V) as cathode, the inorganic insoluble lithium salt (Li2CO3) isadded into LiPF6-EC/PC/DEC(1/1/3). The result turns out that capacity decay of the twocathodes was both obviously suppressed by Li2CO3. SEM, XRD and XPS are performed toinvestigate the component and properties of the SEI film, DFT calculation is conducted tostudy the effect of Li2CO3on the oxidability of Co4+and Ni4+to make the mechanismclearer.5) In order to combine the advantages of the solid and liquid electrolytes, inorganicsolid particles (Li2SiO3) and the carbonate electrolyte are mixed to make slurry electrolyte.First,4%Li2SiO3is added with the aim to study its compatibility with LiNi0.5Mn1.5O4cathode, through C/D, CV, XPS measurements. Then the addition content increases to30%to form slurry. Properties of the slurry electrolyte on safety, conductivity, anodic andstorage stability are examined, which are testified to be all enhanced comparing to the basecarbonate electrolyte. Cycled with this slurry electroylte, the performance of5VLiNi0.5Mn1.5O4cathode at high voltage and high rate are both improved, too. TEM imagesdisplaye that the SEI film is thinner in the presence of Li2SiO3. Density functional theory(DFT) calculations turn out that in the presence of Li2SiO3, the oxidizability of Ni4+oncathode surface is weakened, and LiPF6is stabilized against decomposition. A possible pathfor the reaction between Li2SiO3and PF5is proposed through exploring transition statesusing DFT method.