Robust interphase derived from a dual-cation ionic liquid electrolyte enabling exceptional stability for nickel-rich layered cathodes†

Nickel-rich layered cathodes suffer from unstable interfaces and structural collapse, leading to poor cycling stability in conventional carbonate-based electrolytes. Ionic liquid electrolytes have the potential to enable high-safety and high-specific energy in lithium metal batteries employing nickel-rich cathodes. However, their practical performance is limited by their low ionic conductivity and unsatisfactory interphase formation, which allow operation only at relatively low current densities. In this work, a dual-cation-IL-based electrolyte was employed comprising NaPF₆ as an additive for tuning the solvation structure. This electrolyte, which exhibited high ionic conductivity (5.06 mS cm⁻¹ at 20 °C), enabled Li||LiNi_0.83Co_0.11Mn_0.05B_0.01O₂ cells operating in the voltage range of 3.0–4.3 V with excellent capacity retention after 500 cycles at 1 C (95.2%) and a 1500-cycle-long lifespan (>80%). Even after reducing the operative temperature to 0 °C, the cells could deliver high discharge capacity (above 150 mA h g⁻¹) at 0.5C without capacity decay. Ex situ X-ray photoelectron spectroscopy and time-of-flight secondary-ion mass spectrometry analyses revealed that the electrode/electrolyte interphase derived from the NaPF₆ additive was more robust and uniform, possibly facilitating sodium co-deposition on the anode surface against Li dendrite growth. Meanwhile, the inorganic-dominated cathode/electrolyte interphase (CEI) considerably protected the cathode structure and inhibited lattice distortion and microcracks, as revealed by atom-level electron microscopy and in situ X-ray diffraction.