Synthesis And Electrochemical Property Of Functional Electrolyte Additives For Lithium Ion Batteries

Due to the good electrochemical performance,such as high power and energy densities,long cycling life,lithium ion batteries have been widely applied in the field of portable devices like mobile phones and tablets,new energy electric vehicles（EV）,aerospace,and power grid energy storage,and so on.But recently,the safety issue of lithium ion batteries has been attracted the attentions of the scientists as a lot of news related to the battery fire and explosion have been reported.The mostlyused electrolyte for commercial lithium-ion batteries is carbonate based electrolyte,which has low vapor pressures and low flash points.Therefore,the fire or explosion of batteries from the heat and pressure accumulation by internal component reactions under abuse conditions,such as overcharge,overdischarge,short-circuit and so on.Thus,how to enhance the safety of lithium-ion batteries has become a hot topic.In order to enhance the safety of lithium-ion batteries,in this thesis,I designed and synthesized a new kind flame retardant additive,a new kind film additive and a new kind organic electrolyte:Firstly,flame retardant additive tris（2,2,2-trifluoroethyl）phosphate（TFP）was synthesized and added to 1.0 mol L^-1 LiPF₆/EC+DEC,and the effects of TFP on the combustibility of the electrolyte were investigated.When 20%TFP is added into the electrolyte,the fire resistence of the electrolyteis increasing.However,the electrochemical stability of Graphite/LiCoO₂ full cells becomes poor.In order to improve the electrochemical performances of the Graphite/LiCoO₂ cells,the fluoroethylene carbonate（FEC）is added and conjuncted with TFP.The discharge capacity of LiCoO₂ is128.4 mAh g^-1 after 70 cycles,which is higher than 120.7 mAh g^-1 in the base electrolyte.The second work of this thesis is to design and synthesize a kind of new film formation additive,tris（2,2,2-thiophene ethyl）phosphate（TTEP）,and to add itinto the base electrolyte of 1.0 mol L^-1LiPF₆/EC+DEC.Moreover,the effects of TTEP on the electrochemical performance and thermal stability of the LiCoO₂ cathode were investigated.The result shows that the oxidation potential of TTEP is 4.27 V,which is much lower than that of the base electrolyte of 4.45 V.It means TTEP can be oxidized to form CEI film before the base electrolyte,to protect LiCoO₂.Moreover,the electrochemical performance of LiCoO₂ is significantly improved as well.Compared with the LiCoO₂ cell with base electrolyte,the capacity retention rate of the cell with TTEP-electrolyte increased from 40.19%to 71.18%after 150 cycles at 55 ^oC.And the cell with TTEP-electrolyte exhibits the discharge capacities of 140.0 mAh g^-1 and 120.0mAh g^-1 at 5C and 10 C,which are much higher than the cell with base electrolyte.The result of C80 test presents that after the introduction of TTEP,the temperature corresponding to the exothermic main peak of LiCoO₂ cathode material decreased from198.5^oC to 195.9^oC,and the heat release decreased from 141.3 mW g^-1 to 76.21 mW g^-1.Finally,organic compound tri（ethylene glycol）-substituted trimethylsilane（1NM3）was synthesized in this thesis,and LiPF₆,LiTFSI,LiBOB were chosen as lithium salt and added into the prepared 1NM3 respectively to form series of electrolyte system.The electrochemical performance and thermal stability of 1NM3 was further investigated.The result shows that the oxidation potential of 1NM3 is 5.23 V.The electrochemical performance of Li/LiNi _0.6Co_0.2Mn_0.2O₂ cells in the prepared electrolytes was tested.It is found that LiPF₆ in 1NM3 and LiTFSI in 1NM3 were applied as electrolyte,the Li/LiNi_0.6Co_0.2Mn_0.2O₂ cells exhibit poorelectrochemical stability.However,,when the Li/LiNi_0.6Co_0.2Mn_0.2O₂ cell was cycled in 0.6 mol L^-11 LiTFSI+0.4 mol L^-1 LiBOB,it shows a discharge capacity of 121.3 mAh g^-1 and maintain a capacity retention rate of 72.11%after 200 cycles.And when 0.6 mol L^-1 LiPF₆+0.4 mol L^-1 LiBOB was used as the electrolye,the Li/LiNi _0.6Co_0.2Mn_0.2O₂ cell delievers a discharge capacity of 148.0 mAh g^-1 and maintain a capacity retention rate of 83.91%after 200 cycles.