Effects Of Trivalent Phosphorus Compound Additives On The Performance Of High-voltage Lithium Ion Batteries

Lithium-ion batteries are considered one of the most promising forms of electrochemical energy storage,due to the rapid development of electronic devices,the requirements for the energy density of lithium-ion batteries are constantly increasing.An important way to increase the energy density is to increase the working potential of the battery.However,the electrochemical performance of lithium ion battery is directly reduced due to the serious oxidation decomposition of traditional carbonate electrolyte at high voltage.On the one hand,the development of new electrolytes with high voltage stability are not yet perfect.On the other hand,adding a certain amount（<5%）of additives can improve the stability of the electrolyte under high voltage and improve the interface structure between the electrode and electrolyte,which is an effective,economical and convenient solution.In this paper,two trivalent phosphorus compounds,triallyl phosphite（TAPi）and triphenylphosphine（TPP）are used as additives.Using 1M Li PF₆-EC/DMC/EMC（1:1:1 by volume）as the baseline electrolyte,explore the effect of additives on Li Ni_0.5Co_0.2Mn_0.3O₂/Li half-cell,graphite/Li half-cell and Li Ni_0.5Co_0.2Mn_0.3O₂/graphite full-cell system.Through a series of electrochemical tests and physical characterizations,such as charge/discharge tests,cyclic voltammetry（CV）,linear sweep voltammetry（LSV）,electrochemical impedance spectroscopy（EIS）,scanning and transmission electron microscopy（SEM/TEM）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）,to study the effects and mechanisms of trivalent phosphorus compound additives at high voltage.The main conclusions are as follows:1.The effects of TAPi as an electrolyte additive on lithium-ion batteries（1）Theoretical calculations combined with LSV tests demonstrate that TAPi tends to be preferentially oxidized and suppresses the decomposition of traditional carbonate-based electrolytes.In addition,the results of ¹⁹F NMR spectra proved that TAPi can effectively inhibit the hydrolysis process of the electrolyte salt Li PF₆.（2）0.2%TAPi can effectively improve the specific discharge capacity and cycle stability of Li Ni_0.5Co_0.2Mn_0.3O₂/Li half-cell and Li Ni_0.5Co_0.2Mn_0.3O₂/graphite full-cell under high voltage.At the same time,the cycle performance of graphite/Li half-cell shows that TAPi has good compatibility with the graphite anode.（3）The mechanisms of TAPi are studied by a series of physical characterizations,the results indicate that TAPi forms a protective interphase film on the cathode surface after cycling.The TAPi-derived film protects the cathode structure from destruction to improve the electrochemical performance of the Li Ni_0.5Co_0.2Mn_0.3O₂ cathode at high voltage.2.The effects of TPP as an electrolyte additive on lithium-ion batteries（1）The Li Ni_0.5Co_0.2Mn_0.3O₂/Li half-cell containing 1%TPP has the best cycle performance.The CV results show that TPP is beneficial to the insertion and extraction of lithium ions in the electrodes.（2）The addition of TPP significantly improves the capacity retention ratio of Li Ni_0.5Co_0.2Mn_0.3O₂/Li half-cell and Li Ni_0.5Co_0.2Mn_0.3O₂/graphite full-cell.In addition,the specific discharge capacity of the graphite/Li half-cell has also been improved to a certain extent.（3）The mechanisms of TPP on the Li Ni_0.5Co_0.2Mn_0.3O₂ surface are investigated by SEM,TEM,XRD and XPS.It is found that TPP participates in the formation of a thin and uniform cathode surface film,which effectively protects the electrode structure.