Diethyl n-hexynyl Phosphonate (DEHEX): A Novel Flame Retardant Additive for Lithium-Ion Battery Electrolyte

Organic phosphates are commonly used as flame-retardant additives or co-solvents in lithium ion battery electrolytes, such as trimethyl phosphate (TMP), dimethyl methyl phosphonate (DMMP), and so on. TMP and DMMP are highly promising for the battery industry for their high phosphonic content and excellent physicochemical characteristics. However, most of organic phosphates like DMMP and TMP have the bad compatibility with the graphite anode. For example, the decomposition products from TMP and DMMP reductions, which occur before the formation of a stable solid electrolyte interface (SEI) layer, will cause the exfoliation of graphite anode and then cause the capacity loss of batteries. In order to improve the compatibility between the organic phosphates and the graphite anode, we synthesize the diethyl n-hexynyl phosphonate (DEHEX). DEHEX have an alkynyl functional group, which may be reduced before the other functional groups in the compound to form a stable SEI layer. In this work, 1M LiPF6/EC/DMC/DEHEX with different ratio of DEHEX and 1M LiPF6/EC/DMC/DMMP with different ratio of DMMP are used as the testing electrolytes. The self-extinguishing time (SET) of these electrolytes are firstly tested. The SET of 20% DEHEX electrolyte is 208 s/g while the SET of baseline electrolyte is 434 s/g. Fig.1 shows the first charge and discharge curves of CMS/Li cells containing various electrolytes at a current rate of 0.2C. During the first charge and discharge, there is a plateau around 0.5V in cells with DMMP-based electrolytes which may be corresponding to the graphite exfoliation. And there is no such 0.5V-plateau in cells with DEHEX-based electrolyte. The first cycle irreversible capacity of cells containing DEHEX-based electrolyte are much smaller than that of cells containing DMMP-based electrolyte, especially in cells containing 20% flame retardant electrolytes.