In the race to develop a >400 Wh/kg, <$200/kWh lithium-ion battery, the lithium-manganese-rich (LMR) oxide cathode has drawn much attention. Boasting a high voltage cycling operating window with capacities exceeding 250 mAh g^-1, the LMR cathode was once considered the key to enabling a market-viable electric vehicle. Despite its major advantages and the potential for massive technological impact, worldwide research has struggled to enable the high-energy LMR material due to its signature drawbacks: oxygen evolution during initial cycling and the lowering of cell operating voltage over cycling life. In an attempt to alleviate oxygen evolution and mitigate phase transformation-induced voltage fade in the LMR material, myriad efforts have focused on surface modification, ion substitution or doping, and morphological control of particles and grains, all with limited success in enhancing long term cell energy retention. In this study we discuss a less common approach to enabling stable LMR cycling, developing a modified ionic liquid (IL) electrolyte capable of forming a favorable cathode-electrolyte interface (CEI). We present the long-term cycling of LMR half-cells in this unique electrolyte, demonstrating the highest degrees of voltage fade mitigation to-date with >70% energy retention in 950 cycles at C/2 rate (100% depth-of-discharge). We investigate the CEI formed on the LMR surface and demonstrate its unprecedented ability to stabilize the LMR crystal structure using a comprehensive suite of characterization methodology spanning XRD, Raman, XAS, XPS, simulation, TEM, and electrochemical testing. Coincidentally, the same modified IL electrolyte enables high performance of our previously published nano-wire silicon anode. The dual functionality of this electrolyte composition provides the opportunity to fabricate high-energy full-cell Li-ion batteries. Our study culminates in the demonstration of Si/LMR full-cells capable of retaining >90% energy over early cycling and 90.84% capacity over more than 750 cycles at the 1C rate (100% depth-of-discharge). While much work remains to enable a commercially viable Si/LMR battery, this study presents the effectiveness of an electrolyte-centric approach while at the same time shedding light on the critical electrolyte-electrode interplay that affects LMR cycling.