Synergistic effect of inorganic Mg(OH)2 and organic triphenyl phosphate based coating layers on flame-retardant separator for high-voltage Li||LiNi0.8Co0.1Mn0.1O2 cell

Abstract

Improving the safety of Li-ion battery becomes an urgent problem along with the widespread use of nickel-rich cathode materials. The common method to enhance the safety is adding flame retardant into liquid electrolyte at the expense of electrochemical performance. Taking into account of safety and electrochemical performance, inorganic/organic composites of Mg(OH)2/TPP(triphenyl phosphate) are co-coated onto polyethylene (PE) separator to obtain the flame-retardant separator. Contributed by Mg(OH)2 with excellent anti-thermal shrinkage and TPP with outstanding flame retardancy, the synergistic effect happens in MT31/PE separator with a weight ratio of 3:1 for Mg(OH)2:TPP. MT31/PE separator presents better flame-retardant property with self-extinguish time that reduces from 216.7 s g−1 (PE separator) to 133.3 s g−1. Due to superior compatibility with lithium anode, the Li||MT31/PE separator||Li cell maintains lower over potential after 250 h cycled under a current density of 1 mA cm−2. Compared with 5.0 V for PE separator, the oxidative decomposition potential for MT31/PE separator also extends to 5.8 V. Furthermore, higher capacity retention of 74% in Li||LiNi0.8Co0.1Mn0.1O2 coin cell between 3.0 V and 4.35 V is achieved for MT31/PE separator (55% for PE separator) after 100 cycles at room temperature. Therefore, the flame-retardant separator gives great practical value for high-energy-density and high-safety Li-ion battery.