Effect Of Fluoride-type Flame Retardant Additive On The Electrochemical Performance Of Lithium-ion Battery

Rechargeable lithium-ion batteries are regarded as ideal green batteries and have been widely used in portable electronic devices as well as electric vehicles(EVs) and hybrid electric vehicles(HEVs) because of their high energy density, rapid charge/discharge, no memory effect, stable cycling life and non-pollution. But security is an important factor restricting the commodification of high-capacity lithium-ion battery. Highly flammable electrolyte is one of the most important schemes for the security of the batteries, then finding a suitable flame retardant additive is an effective method of solving this problem.Flame retardation of additive HRF on electrolyte has been studied by methods of self-propagating rate test, self-extinguishing time test, et al., also, LSV, CV and EIS are used to study the impact of HRF on graphite anode and Li Ni0.5Co0.2Mn0.3O2 cathode, which the purpose is studying the compatibility of HRF with typical electrode materials of lithium-ion battery. The main researches contents and conclusions are as follows:(1) The pristine electrolyte is 1 mol·L-1 Li PF6/EC+DMC+DEC, different additive amounts are 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0% and 5.0%, respectively. results show that the conductivity falls down from 8.21 m S·cm-1 to 6.85 m S·cm-1; Self-propagating rate test and self-extinguishing time test are using to study the flammability of electrolyte, with the addition of HRF, longer time is taken for burning with unit quality of electrolyte to and for firing with unit length, results show that flame retardant additive HRF can reduce the flammability of electrolyte, what’s more, combustion process become more gentle and flame spread rate become slower, this can prevent the electrolyte burning in a short period of time but releasing quantity of heat which can cause the explosion of lithium ion battery. With the HRF further increase of the amounts of 6%, the electrolyte can not be ignited in the air already. The results of LSV show that HRF can improve the oxidation resistance of the electrolyte and partly inhibit the oxidation of solvent molecules. And the oxidation resistance becomes stronger and stronger as the amounts of HRF increase.(2) Effects of additive HRF on graphite anode are researched. The SEM results show that SEI film in electrolyte with 0.5% HRF is less meticulous and uniform than that in pristine electrolyte and there are ravines on it, which make the intercalation and deintercalation of lithium ions more intensively, cause the damage on SEI film then lead to bad cycle performance; The electrochemical performance is studied by constant current charge and discharge, CV and EIS, results show that HRF can suppress the reduction process of EC(one of the electrolyte components) in the first charge/discharge process; The charge transfer resistance is also bigger in electrolyte with 0.5% HRF. Which make the cycle performance get a little worse.T(3) Choosing Li Ni0.5Co0.2Mn0.3O2 as the typical cathode materials, researches of the impact of HRF on the electrochemical performance are shown here. The results of constant current charge and discharge test show that Li Ni0.5Co0.2Mn0.3O2 electrode has good cycle performance in pristine electrolyte and electrolyte only 0.5% or 1% HRF, and the capacity fading is very slow. When the additive amounts increase to 3% even 5%, the capacity fluctuates tempestuously, because the structure is destroyed by the lithium ion intercalation and deintercalation; CV results show that HRF can weak the second pair of redox peaks indicating that the process of Co3+/Co4+ transformation has been restrained. The peaks attribute to Ni2+/Ni3+ transformation process become more obvious with the addition of HRF; EIS results show that in electrolyte with 0.5% HRF, RSEI of Li Ni0.5Co0.2Mn0.3O2 electrode is bigger than that in pristine electrolyte, reflecting thicker SEI film has formed in electrolyte with 0.5% HRF, which makes the intercalation and deintercalation of lithium ions more difficult; EIS results of Li Ni0.5Co0.2Mn0.3O2 electrode after CV test show that the diameters of HFS and MFS both increase as the cycle numbers of CV increase.