Electrochemical performance of Na/NaFePO4 sodium-ion batteries with ionic liquid electrolytes

Abstract

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.

Rechargeable Na/Na0.44MnO2 cells with ionic liquid electrolytes containing various sodium solutes

“…9(b) reveals that the capacity retentions after 100 chargeedischarge cycles for the cells with the NaBF 4 -, NaClO 4 -, NaTFSI-, and NaPF 6 -incorporated IL electrolytes are 70, 80, 75, and 45%, respectively, at 50 C. The increased cyclability of the IL electrolyte cells at the elevated temperature (compared to that at 25 C in Fig. 5(c)) is ascribed to the promoted Na þ transport in the ILs [24], minimizing the unfavorable intercalation/deintercalation of BMP þ and TFSI e into/from Na 0.44 MnO 2 , which can lead to electrode deterioration. As also shown in Fig.…”

“…As shown in Fig. 10(a) and (b), while the maximum capacity and high-rate performance in the organic electrolyte decrease at above 50 C, those for the IL electrolyte continuously increase with temperature and become superior to those found for the former electrolyte at 75 C. The measured capacity of Na 0.44 MnO 2 in the IL electrolyte was as high as 115 mAh g À1 (at 0.05 C) at 75 C; moreover, 85% of this capacity can be retained when the chargeedischarge rate was increased to 1 C. At high temperature, the organic electrolyte is volatile and unstable (see TGA data); in contrast, the IL (with a decomposition temperature of~350 C) becomes less viscous and has increased Na þ mobility [24]. As a result, the use of the IL electrolyte is favorable for operations at above 50 C. Fig.…”

Vanadium oxychloride as electrode material for sodium ion batteries

“…The positive electrode materials of polyanion compounds (NaMPO 4 , M = Fe, Mn, etc.) [8][9][10] or layered oxide compounds (NaMO 2 , M = Ni, Co, Mn, etc.) have been widely investigated for the feasibility of Na intercalation [11][12][13].…”

Electrolytes for electrochemical energy storage

“…20,247,248 Additionally, it is necessary to design appropriate liquid electrolyte compositions to minimise unwanted interface reactions and to enhance the electrochemical performance and safety in SIBs. Among various aqueous, 249, 250 organic 251 and ionic liquid based choices [252][253][254] , organic electrolytes are more promising owing to their high ionic conductivity, wide electrochemical window and good electrochemical performance. 255 The most common electrolyte formulations for SIBs include NaClO4 or NaPF6 dissolved in carbonate ester solvents, particularly EC and/or PC.…”