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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A high-voltage LiNi 0.5 Mn 1.5 O 4 cathode material with a cubic spinel structure is synthesized using a citricacid-assisted sol-gel process. Butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI)based ionic liquids (ILs) with various kinds of Li salts, namely LiTFSI, LiPF 6 , and their mixtures, are used as electrolytes for Li/LiNi 0.5 Mn 1.5 O 4 cells. The IL electrolytes show high thermal stability (>400 C) and nonflammability, and are thus ideal for high-safety applications. At 25 C, LiTFSI is more suitable than LiPF 6 as an IL electrolyte in terms of cell capacity, rate capability, and cyclic stability. The IL electrolytes clearly outperform the conventional organic electrolytes at 50 C, since the latter decomposes at high voltage and corrodes both the Al current collector and LiNi 0.5 Mn 1.5 O 4 , degrading the electrode performance. At such an elevated temperature, using LiPF 6 to partially substitute LiTFSI in the IL electrolyte can effectively suppress Al pitting corrosion and thus improves the cell performance. In the 0.4 M LiTFSI/0.6 M LiPF 6 mixed-salt IL electrolyte, an LiNi 0.5 Mn 1.5 O 4 discharge capacity of 115 mA h g À1 (at 0.1 C) is obtained at 50 C with a high cell voltage of $4.7 V.