Studies On The SEI Film-forming Additives To Improve The Properties Of Graphite Anode

Lithium ion batteries are exhibiting their increasing importance in recent years,especially since they can be used as power supplies for electric vehicles. The solidstate interface (SEI) film formed during the first charging and discharging process hassignificant effects on the properties of lithium-ion batteries. Choosing favorableelectrolyte additives for lithium-ion battery so as to help form better SEI film is one ofthe most simple and economic methods to improve the properties of lithium-ionbatteries. In this paper, K2CO3, Na2CO3, LiF and Vitamin E are investigated aselectrolyte additives for lithium-ion batteries. Electrochemical performances ofgraphite electrode in electrolytes with different additives are studied and themechanism for improvements is explored. Testing methods used in this paper includeconstant-charge/discharge test (GC), cyclic voltammetry (CV), electrochemicalimpedance spectroscopy (EIS) and scanning electron microscopy (SEM). The mainresearch contents and the conclusions obtained are as follows:(1) Add saturated K2CO3to1mol dm-3LiPF6-EC:DMC electrolyte and GC, CV,EIS and SEM are used to study the effects of K2CO3on the graphite electrodeperformances. Through SEM, it is seen that SEI film in electrolyte added K2CO3showdifferent morphologies compared in the pristine electrolyte, the SEI components mayalso changed. CV results show that, the addition of K2CO3can suppress the reductionprocess of EC, which is one of the electrolyte components, therefore improving theelectrochemical performances of graphite electrode; EIS results indicate that, both theSEI resistance and the whole resistance of the graphite electrode are reduced afteradding K2CO3.(2) The effects of Na2CO3as film-forming additive on graphite electrode for Li-ionbattery are researched. The surface morphology and structure of SEI film formed ongraphite electrode are observed by SEM. The influences of Na2CO3on theelectrochemical performances of graphite electrode are investigated using CV and EISmeasurements. The results indicate that, the addition of Na2CO3in1mol/L LiPF6-EC:DEC electrolyte can help to form a smoother and denser SEI film, increase thereversibility of electrode reaction, effectively suppress the reduction of electrolyteduring the first lithiation process, thus improving the cyclic performance of graphiteelectrode.(3) VE and LiF are used as electrolyte additives for lithium-ion batteries. SEM, GC,and CV methods are combined to investigate their effects on the morphology and composition of SEI film, as well as on the electrochemical performances of graphiteelectrode. The GC results show that, the addition of Vitamin E and LiF cansignificantly improve the discharge capacity and columbic efficiency of graphiteelectrode; The CV research results show that, adding VE and LiF into LiPF6-EC:DEC:DMC electrolyte can suppress the first reduction process of EC, improve thereversibility of lithium ion intercalation and deintercalation, thus enhance the cycleperformance of lithium ion battery; SEM observation indicates that, bothLiF and vitamin E can help to form better SEI film on the graphite surface, improvethe structure and mophorlogy of SEI, thus more effectively protect the graphiteelectrode.