Phosphonium ionic liquid-based electrolyte for high voltage Li-ion batteries: Effect of ionic liquid ratio

Continuing the exploration of safe phosphonium-based electrolytes for lithium-ion battery (LIB) applications, we present herein the results of a nonflammable electrolyte containing methyl tributylphosphosphonium Bis(trifluoromethanesulfonyl) imide (Bu3MeP-TFSI) ionic liquid (IL) EC/EMC (1:1, wt) with 1 M LiPF6 on lithium manganese oxide (LMO), lithium nickel manganese oxide (LMNO), graphite, and lithium metal. A high wt% (10-30%) of IL propagates dead Li formation as opposed to 5% wt IL where superior performance to a standard commercial electrode was observed for 150 h of continuous cycling at 0.5 mAh cm(-2). For the LMO//Li half-cell, the presence of the IL allowed cathode operation at high potentials (>4.3 V vs. Li/Li+) with moderate capacity retention (90%). Despite the poor interface that arises from the viscosity of the phosphonium IL, at 5% wt, the LMNO//Li half-cell performance was reversible (DLi+ = 4.83 x 10(-13) cm(2) s(-1)) with a capacity retention of 86%, coulombic efficiencies above 98%, and an energy density of 294 Wh kg(-1) for 50 cycles. Finally, the Gr//LMNO full-cell moderate performance (Q(discharge) 100 mAh g(-1) at C/10) proves the IL's efficacity as an additive but at the same time leaves room for further improvement.