Rechargeable Na/NaFePO 4 cells with a sodium bis(trifluoromethanesulfonyl)imide (NaTFSI)-incorporated butylmethylpyrrolidinium (BMP)-TFSI ionic liquid (IL) electrolyte are demonstrated with an operation voltage of $3 V. High-performance NaFePO 4 cathode powder with an olivine crystal structure is prepared by chemical delithiation of LiFePO 4 powder followed by electrochemical sodiation of FePO 4 . This IL electrolyte shows high thermal stability (>400 C) and non-flammability, and is thus ideal for high-safety applications. The effects of NaTFSI concentration (0.1-1.0 M) on cell performance at 25 C and 50 C are studied. At 50 C, an optimal capacity of 125 mA h g À1 (at 0.05 C) is found for NaFePO 4 in a 0.5 M NaTFSI-incorporated IL electrolyte; moreover, 65% of this capacity can be retained when the charge-discharge rate increases to 1 C. This ratio (reflecting the rate capability) is higher than that found in a traditional organic electrolyte. With a 1 M NaTFSI-incorporated IL electrolyte, a 13% cell capacity loss after 100 charge-discharge cycles is measured at 50 C, compared to the 38% observed in an organic electrolyte under the same conditions.

NaFePO4 with an olivine structure is synthesized via chemical delithiation of LiFePO4 followed by electrochemical sodiation of FePO4. Butylmethylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (BMP-TFSI) ionic liquid (IL) with various sodium solutes, namely NaBF4, NaClO4, NaPF6, and NaN(CN)2, is used as an electrolyte for rechargeable Na/NaFePO4 cells. The IL electrolytes show high thermal stability (>350 °C) and nonflammability, and are thus ideal for high-safety applications. The highest conductivity and the lowest viscosity of the electrolyte are obtained with NaBF4. At an elevated temperature (above 50 °C), the IL electrolyte is more suitable than a conventional organic electrolyte for the sodium cell. At 75 °C, the measured capacity of NaFePO4 in a NaBF4-incorporated IL electrolyte is as high as 152 mAh g(-1) (at 0.05 C), which is near the theoretical value (154 mAh g(-1)). Moreover, 60% of this capacity can be retained when the charge-discharge rate is increased to 1 C.

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The charge transfer resistances of practical sodium-ion battery electrodes in an ionic liquid (IL) electrolyte are unexpectedly low. A Na0.44MnO2-hard carbon full cell with an IL electrolyte has higher capacities (117 mA h g(-1)@0.1 C based on Na0.44MnO2) and greater stability (3% capacity decay after 100 cycles) than its conventional-electrolyte-based counterpart at 25 °C.

MXenes are a class of two-dimensional (2D) transition-metal carbides and nitrides that are currently at the forefront of 2D materials research. In this study, we demonstrate the use of metallically conductive free-standing films of 2D titanium carbide (MXene) as current-collecting layers (conductivity of ∼8000 S/cm, sheet resistance of 0.5 Ω/sq) for battery electrode materials. Multilayer Ti 3 C 2 T x (T x : surface functional groups −O, −OH, and −F) is used as an anode material and LiFePO 4 as a cathode material on 5 μm MXene films. Our results show that the capacities and rate performances of electrode materials using Ti 3 C 2 T x MXene current collectors match those of conventional Cu and Al current collectors, but at significantly reduced device weight and thickness. This study opens new avenues for developing MXene-based current collectors for improving volumetric and gravimetric performances of energy-storage devices.