Investigation the Effect of a Phosphorus Based Flame Retardant on the Safety and Electrochemical Properties of Lithium-Ion Battery

Lithium-Ion Batteries (LIBs) are attractive candidates as a power source in hybrid and electric vehicles due to their high specific energy, large energy density and good cycling stability. Safety concern of lithium ion batteries, in the recent years, have been the key problem in their practical applications because organic solvents which are used in their electrolyte component such as Ethylene carbonate (EC) Dimethyl carbonate (DMC) Diethyl carbonate (DEC) Ethyl methyl carbonate (EMC) are very flammable and can be ignited in the situations of various types of accidents and abuses, which has imparted the state-of-the-art electrolyte systems with potential fire hazards. The efforts aiming to improve the thermal stability of electrolytes have been intensified in the recent years, especially driven by the more stringent safety requirements for large industry lithium-ion batteries intended for electric vehicles. The most efficient and economical ways use flame retardants (FRs) as electrolyte additives or co-solvents. The performance and stability of the electrolyte are highly dependent on the presence of impurities such as chlorine, water and HF. Phosphonate compounds are known as FRs that function in the vapor phase by a radical mechanism. Since the high content of flame retardants in many cases reduce the cells performance, it is important to optimize the additives content in the electrolyte of LIBs. The objectives of this work are to investigate the effect of Dimethyl methyl Phosphonate (DMMP) electrolyte additive on the flammability, thermal stability and electrochemical performance in the ۱۸۶۵۰ commercial cells. DMMP was used as a flame retardant to reduce the flammability of the electrolyte with minimum negative impact on the electrochemical performance of the electrodes. The flammability and thermal stability of prepared electrolyte in the presence of DMMP by different contents were evaluated using self-extinguishing time (SET), differential scanning calorimeter (DSC). Linear Sweep voltammetry (LSV) and galvanostatic charge and discharge tests were carried out to investigate the effect of DMMP on the electrochemical properties. When a sample containing ۵ vol.% DMMP is used as electrolyte in ۱۸۶۵۰ cell, the cell exhibits quite good cycling stability; like the Blank electrolyte with capacity fade after ۵۵ cycles at ۰.۱ C is ۹۲.۶۸ and ۹۶.۵۹%, respectively. The cathode electrode interface is characterized with EIS, SEM, and XRD, DSC after several charge/discharge cycles which demonstrate stable and liner protective film is formed on the surface of cathodes in the presence of ۵ vol.% DMMP. The results imply that DMMP is a promising option as highly flame-retardant and electrochemically compatible electrolyte additive for safe lithium-ion batteries.