Study On Film-forming Electrolyte Additives And Solvents Of Lithium Ion Batteries At High Voltage

Lithium-ion batteries?LIBs?have been the dominating power sources for portable electronic devices such as mobile phones and notebook computers since their commercialization in the early 1990s.However,the energy density still cannot satisfy the driving requirements of electric vehicles?EVs?and hybrid electric vehicles?HEVs?.One of the ways to enhance the energy density of the LIBs is to apply cathode materials with higher specific capacities and/or higher operating potentials.Among the developed cathode materials,spinel-structured lithium nickel manganese oxide(LiNi_0.5Mn_1.5O₄)and over-lithiated layered oxide Li_1.17Ni_0.25Mn_0.58O₂?OLO?materials have attracted considerable attentions for their high operating potential and high theoretical specific capacity.Unfortunately,the increased upper voltage limit of the cathode is always accompanied with low coulomb efficiency and poor cycling stability due to the continuous oxidation of the conventional electrolyte and the irreversible structure destruction due to severe dissolution of the transition metals.These concerns originate mainly from the vulnerable interfacial stability between the electrodes and electrolyte.To overcome the interface problems as aforementioned,this paper presents film-forming additives and solvents to improve the cycle performance of lithium-ion batteries.The main contents are as follows:1.Tris?pentafluorophenyl?phosphine?TPFPP?is investigated as a functional additive to improve the cyclic stability of Li_1.17Ni_0.25Mn_0.58O₂?OLO?/graphite full batteries at high voltage.Charge/discharge tests demonstrate that the capacity retention performance of OLO/graphite can be significantly improved by TPFPP.After 200 cycles at 0.3 C rate(1 C=300 mA g^-1),the capacity retention of OLO/graphite is 90.6%for the adding of 0.5 wt.%TPFPP,while only 62.6%for the batteries with the baseline electrolyte.The functional mechanism of TPFPP improving the cyclic stability of OLO/graphite is conducted with electrochemical and ex-situ analysis approaches,including linear sweep voltammetry?LSV?,electrochemical impedance spectroscopy?EIS?,transmission electron microscope?TEM?,scanning electron microscope?SEM?,X-ray diffraction?XRD?,and X-ray photoemission spectroscopy?XPS?.Theoretical calculations together with the above test results demonstrate that TPFPP is oxidized preferentially to the baseline electrolyte and generates robust and uniform film on the surfaces of OLO cathode and graphite anode as well,which simultaneously protects both cathode and anode upon cycling,resulting in the enhanced cyclic stability of OLO/graphite batteries.2.Tris?pentafluorophenyl?borane?TPFPB?is investigated as a functional additive to improve the cyclic stability of Li_1.17Ni_0.25Mn_0.58O₂?OLO?/graphite full batteries at high voltage.The combination of DFT calculation and experimental results demonstrate that TPFPB is susceptible towards oxidation/reduction decomposition and participate in the formation of a compact layers on the both OLO and graphite electrodes surfaces.TPFPB is successfully used to stabilize the electrode/electrolyte interface of OLO/graphite full batteries against degradation at high voltage applications.Addition of 0.5 wt.%TPFPB in the baseline electrolyte obtains better capacity retention,high columbic efficiency,and prolonged cycle life.The TPFPB-derived SEI layers effectively mitigate sustaining electrolyte decomposition and the dissolution of transition metal ions,and resulting in the improved electrochemical performances.3.1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropylether?F-EPE?is investigated as a cosolvent to improve the cyclic stability of LiNi_0.5Mn_1.5O₄ at high voltage and elevated temperature?55°C?.Addition of F-EPE in the baseline electrolyte significantly improves the capacity retention of LiNi_0.5Mn_1.5O₄ at elevated temperature.The high temperature capacity retention was 68.4%after 300 cycles.The theoretical calculation combined with electrochemical test shows that the improved capacity retention rate is derived from the presence of fluorine atoms,reducing the HOMO value of the solvent and improving the antioxidant capacity of the electrolyte.