Study On The Safety Additive Of Lithium Ion Battery Electrolyte

The safety of Li-ion batteries is greatly related to flammability of electrolyte, since electrolyte typically consists of LiPF6salt and carbonates solvents. Once used in abnormal conditions, lithium ion batteries are possibly conducted to thermal runaway and serious hazards, such as leakage, fire and explosion. Overcharge is the most common form of abusement. In this work, we study on the flame-retardant and overcharge protection electrolyte to improve the safety of lithium ion batteries.The flame retardant additive, diphenyloctyl phosphate (DPOF) was proposed based on the property requirement of a flame retardant for lithium ion batteries. We studied anisole,2-bromoanisole,3-bromoanisole,1,2-dimethoxy-4-nitro-benzene and1,4-dimethoxy-2-nitro-benzene as overcharge protection additives. Cyclic voltammetry (CV), electrochemical impedancespectra (EIS), charge-discharge cycle performance, scanning electron microscope (SEM) were used to research on the influence of additives on batteries. The results are presented as follows:1. Lithium ion batteries with DMNB1and DMNB2achieve improved overcharging tolerance than those without additives. DMNB1shows better overcharge protective performance than DMNB2, and DMNB1has less bad effect on specific capacity of batteries. It indicates that the structure of the two dimethoxy locating on ortho and nitro on alignment of benzene ring is more suitable to be used as overcharge additive, than that the two dimethoxy locating on alignment and nitro on ortho of benzene ring.2.2-bromoanisole is the most excellent overcharge additive, among anisole,2-bromoanisole and3-bromoanisole; however, it leads a high capacity loss of LiNii/3Co1/3Mn1/3O2/Li and Li/AG batteries. The bad effect of anisole on the cycling performance of the battery is least. Although at low rate, overcharge protection performance of the battery with anisole is worse than that of2-bromoanisole, the effect of anisole is quite close to2-bromoanisole at high rate. It indicates that the anisole with bromine in the ortho shows better overcharge protective performance, than that of bromine in the alignment, however, anisole without bromine on the benzene ring achieve the minimal impact on cycle performance of lithium ion batteries, than the other two additives.3. DPOF can effectively improve the thermal stability of the electrolyte. DPOF has no distinct effect on SEI film formation of artificial graphite, however, the cycling stability of LiNi1/3Co1/3Mn1/3O2/Li cells becomes worse with the increasing ratio of DPOF. After50cycles, the capacity rentention of LiNi1/3Co1/3Mn1/3O2/Li cells with high content of DPOF is very low, and the values of Rct and RSEI become larger. Therefore, the capacity loss of LiNi1/3Co1/3Mn1/3O2/artificial graphite cell may be mainly related to the LiNi1/3Co1/3Mn1/3O2cathode electrode in DPOF flame-retardant electrolyte.