Multifunctional Separator Enabled by a High Phosphorus Content Additive for Ni-Rich Transition Metal Oxide Batteries

Ni-rich transition metal oxide batteries, generally represented as LiNixCoyMn1 – x – y (Ni-rich NMC), where x ≥ 0.5 and y < 0.5, have drawn much attention recently due to their high energy density and high discharge capacity (∼200 mAh g – 1). However, the Ni-rich NMC battery frequently suffers from the interfacial issues and safety concerns attributed to the reduction of the Ni element during cycling. In this work, a multifunctional separator employing a high phosphorus content additive, i.e., dimethyl methylphosphonate (Fyrol DMMP), is prepared using a phase inversion method. The synthesized separator reveals a superior electrolyte uptake of 2213%, which is nearly 10 times higher than that of the commercial polypropylene separator. A key feature of the synthesized separator is the capability to generate a uniform and stable cathode-electrolyte interphase with a thickness of ∼5 nm, consisting of LiF, on subsequent cycles. Consequently, the cycling stability of the NMC cell is enhanced exhibiting a high and consistent Coulombic efficiency of ∼99%, and the capacity fading is decreased from 18 to 9.5% during 100 cycles (at C-rate). Furthermore, the stabilized interphase contributes to a decrease in impedance from ∼154 to ∼59.2 Ω. The synthesized separator also displays strong thermal stability and flame-retardant property. Ignitions were attempted during the combustion test, but any flames were promptly extinguished ( < 1.0 s). This study presents an effective approach to tackle both the interfacial issues and safety concerns of Ni-rich NMC batteries at the same time.