Lithium-ion Battery Overcharge Protection Additives

Lithium-ion batteries has been widely used as reliable energy sources for kinds of portable electronic devices because of their high voltage and energy density. However, as lithium-ion batteries have some safety problems, their application has been limited. Therefore, to resolve and improve the safety of lithium-ion batteries is very important. Overcharge is the prominent problem of the safety for lithium-ion batteries, which can cause explosion or combustion in batteries. But after adding suitable electrolyte additive, it can effectively improve the safety of lithium-ion batteries. The recent research is just at outset both domestic and abroad.In this paper, the three kinds of new electrolyte additives, xylene, 4-bromoanisole and 4-Bromo-2-Fluoromethoxybenzene, were firstly used as overcharge protection additives of lithium-ion batteries. The property of their overcharge protection and the effect on lithium-ion batteries were mainly studied. The overcharge protection mechanism was also discussed, which provided a strong guarantee for practical application in future.On this basis,4-Bromo-2-fluoro-anisole of flame-retardant properties were explored to find an additive with overcharge protection and flame-retardant effect to provides a new solution for the future.Xylene as overcharging additive was added into lithium-ion battery electrolyte. When overcharged, xylene in electrolyte can electrochemically polymerization at 4.54 V (vs.Li/Li+). Xylene in the system of electrolyte can produce insulating polymer film, and the lithium ion batteries can be much safer when overcharged. Meanwhile, during the normal charge-discharge process of the battery system, the addition of 5% by weight percentage of xylene didn't have negative effect on the capacity character of lithium-ion batteries. Therefore, xylene can be a promising overcharge protection additive in lithium ion batteries.In order to improve the ability of overcharge protection,4BA was added as a novel electropolymerizable additive to the electrolyte of lithium-ion batteries. Proved by overcharge of batteries, cyclic voltammetry, infrared spectra, AC impedance tests, and discharge capacity measurements in this research, the overcharge protection ability of this novel additive was confirmed. When the external potential was 4.4 V,4BA (5% by mass) in the system of electrolyte 1 mol·L-1 LiPF6/EC+DEC +DMC (1/1/1, m/m/m) started to electrochemically polymerize and produced a layer of polymer film, which increased the internal resistance and prevented the voltage from dramatically rising up. Therefore, the lithium-ion batteries can be much safer. During the normal charge-discharge processes of the battery system, the addition of the proper weight percentage of 4BA does not have negative effect on the capacity of lithium-ion batteries, and the mechanism of overcharge protection is an interdiction mechanism.4-Bromo-2-Fluoromethoxybenzene (BFMB) was used as a novel electrolyte additive for lithium-ion batteries. Studied by cyclic voltammetry, charge-discharge measurements, overcharge and flammability tests, differential scanning calorimetry (DSC) analysis, the results showed that this additive could electrochemically polymerize at 4.6 V (vs. Li/Li) to form a polymer film, which can prevent the voltage runaway at overcharge. Moreover, this molecule can also lower the flammability of the electrolyte, and thereby increase the thermal stability of the Lithium-ion batteries. meanwhile, the use of BFMB as an electrolyte additive did not influence the normal performance of lithium-ion batteries. Thus, it is possible to use this electrolyte additive to provide both overcharge protection and flame retardancy for lithium-ion batteries.