Performance Investigations Of Electrolyte Containing Flame-retardants Of Phosphate For Lithium Ion Batteries

Lithium ion batteries have higher safety than lithium battery. However, lithium ion batteries will ignite or explode under extreme conditions, such as overcharge, excessive heating, crushing and so on. At present, with the development of power battery for hybrid electric vehicle or electric vehicle, the research of high-safety lithium ion batteries is becoming more urgent. Because the frequently used electrolyte of lithium ion battery is flammable, the thermal stability of electrolyte is very important to the safety of batteries. In this dissertation, the safety mechanism and methods of improving battery safety were reviewed. Effect of phosphate flame-retardant on performance of electrolyte was studied by self-extinguishing time method (SET), differential scanning calorimetry (DSC), and linear sweep voltammetry (LSV). The compatibility between phosphate and cathodes and anodes was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), charge-discharge test and X-ray photoelectron spectroscopy (XPS).The effects of trimethyl phosphate (TMP), dimethyl methyl phosphate (DMMP) and methylene diphosphate isopropoxide (TPPP) on the thermal stability of electrolyte were investigated and the results showed that DMMP have the highest flame-retardant capability. The compatibility of DMMP-contained electrolyte with cathodes (LiFePO₄, LiCoO₂) and anode (graphite) materials was investigated. The results showed that the LiFePO₄ had better compatibility with DMMP than that of LiCoO₂. On the other hand, DMMP had poor compatibility with graphite. The first coloumbic efficiency was 86.1% in the electrolyte without DMMP, but the efficiency was decreased to 41.1% with addition of 10% DMMP due to the decomposition of DMMP on the graphite surface.The highest unoccupied molecular orbit (HOMO) and lowest occupied molecular orbit (LUMO) of flame-retardants, LiBOB and FEC were investigated by the density functional theory. The results showed that LiBOB and FEC have higher reduction ability than DMMP and can form passivation film preferentially. Furthermore, electrochemical properties of graphite electrode were studied in 1mol·L-1 LiPF6 / EC: DEC: EMC (1:1:1, vol) +10%DMMP with LiBOB or FEC. The compatibility of graphite and DMMP was improved by LiBOB or FEC. The first irreversible capacity loss decreased significantly and the discharge capacity increased obviously. Analysis of the electrodes after passivation film formation revealed that FEC and LiBOB inhibited the reduction of DMMP on the graphite surface.The effect of triphenyl phosphite (TPPi) and triphenyl phosphate(TPP) on the performance of electrolyte was investigated. At the same time, the compatibility of electrolyte containing TPPi or TPP with cathodes and anode materials was researched. These results showed that the electrolyte with TPPi have high thermal stability and low conductivity. TPPi can only be used for battery employed LiFePO₄ as cathode due to the decreased anodic potential. TPPi has excellent compatibility with graphite and LiFePO₄. The cyclic performance of LiFePO₄/graphite cells was not affected by 10%TPPi and the potential of LiFePO₄ electrode was restrained during overcharged. The passivation film was formed on the surface of LiFePO₄ electrode during overcharged. The electrochemical window was not affected by TPP. Charge-discharge performance of LiCoO₂/graphite was still excellent in the electrolyte containing 6% TPP during 250 cycles, while the safety of LiCoO₂/graphite cells was improved by TPP. Sb₂O₃ coated LiMn2O₄ was prepared by alkoxide hydrolysis method. X-ray diffraction exhibited that Sb₂O₃ coating did not affect the structure of LiMn2O₄. The electrochemical test showed that the cyclic performance of LiMn2O₄ was enhanced by Sb₂O₃ but the first discharge capacity decreased. The thermal stability of LiMn2O₄ with Sb₂O₃ and TPP was investigated by means of differential scanning calorimetry. The results showed that the thermal stability of LiMn2O₄ can be elevated by synergistic effect of Sb₂O₃ and TPP.