MXenes exhibit excellent capacitance at high scan rates in sulfuric acid aqueous electrolytes, but the narrow potential window of aqueous electrolytes limits the energy density. Organic electrolytes and room‐temperature ionic liquids (RTILs) can provide higher potential windows, leading to higher energy density. The large cation size of RTIL hinders its intercalation in‐between the layers of MXene limiting the specific capacitance in comparison to aqueous electrolytes. In this work, different chain lengths alkylammonium (AA) cations are intercalated into Ti3C2Tx, producing variation of MXene interlayer spacings (d‐spacing). AA‐cation‐intercalated Ti3C2Tx (AA‐Ti3C2), exhibits higher specific capacitances, and cycling stabilities than pristine Ti3C2Tx in 1 m 1‐ethly‐3‐methylimidazolium bis‐(trifluoromethylsulfonyl)‐imide (EMIMTFSI) in acetonitrile and neat EMIMTFSI RTIL electrolytes. Pre‐intercalated MXene with an interlayer spacing of ≈2.2 nm, can deliver a large specific capacitance of 257 F g−1 (1428 mF cm−2 and 492 F cm−3) in neat EMIMTFSI electrolyte leading to high energy density. Quasi elastic neutron scattering and electrochemical impedance spectroscopy are used to study the dynamics of confined RTIL in pre‐intercalated MXene. Molecular dynamics simulations suggest significant differences in the structures of RTIL ions and AA cations inside the Ti3C2Tx interlayer, providing insights into the differences in the observed electrochemical behavior.

ranging from 140 to 200 mAh g −1 , which is the capacity limiting electrode that determines the energy density of a cell. Therefore, it is essential to develop high-energy-density cathode materials with long cycling stability and high capacity retention for the longer driving ranges that promote vehicle electrification.To revolutionize electromobility, extensive research is being carried out on the high-capacity cathodes, amongst other cell components, with new approaches identified to optimize their performances. [4][5][6] LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811 (80% Ni)) is one of the most promising cathode materials and has been touted as the next generation high capacity cathode for vehicle electrification because of its low Co content and simultaneous high practical energy density (specific capacity of ≈200 mAh g −1 and high discharge potential of ≈3.8 V vs Li/Li + ), [7][8][9] rate capability, and relatively lower cost. However, these high-energy cathodes with higher Ni content have suffered from structural instability, rapid capacity fading, and voltage evolution upon cycling. The instability of high voltage, high capacity cathodes originates from the reversible/irreversible phase transitions, particle cracking, oxygen evolution, transition metal cation dissolution, and detrimental side reactions with electrolytes at the electrode-electrolyte interphase leading to thick cathode-electrolyte interphase (CEI). [10][11][12][13][14][15] Similarly, the rapid capacity fading of NMC811, when cycled High-capacity cathodes (LiNi 0.8 Mn 0.1 Co 0.1 O 2 ) that can boost the energy density of lithium-ion batteries are promising candidates for vehicle electrification. However, several factors specific to high energy density materials entailing electrode reactions inhibit their application. Fluorination has shown a promising ability to combat the detrimental electrochemical performances of cathode materials, however, it remains difficult to achieve the desired functionality. Herein, a novel electrochemical fluorination (ECF) that demonstrates a promising electrochemical performance enhancement via stabilization of the cathode-electrolyte-interphase (CEI) by forming conformal LiF is proposed. Besides LiF surface layer formation, ECF reduces the degree of fluorinationinduced Ni/Li disordering and enhances the layered structural stability as probed by X-ray diffraction. Because of the robust CEI, ECF-NMC811 cathodes deliver 203.0 mAh g −1 first discharge capacity at the current rate of C/10, with ≈98% capacity retention up to 100 cycles. Similarly, it delivers ≈180 mAh g −1 capacity at a 1 C rate with 86.4% capacity retention up to 200 cycles with average coulombic efficiency of > 99.5%. Comprehensive characterization with a multitude of probes reveals that ECF enhances the cycling stability of the electrode without altering bulk structure and morphology.

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