As electrolytes for sodium secondary batteries operating over a wide temperature range, Na[FSA]-[C 3 C 1 pyrr][FSA] (FSA = bis(fluorosulfonyl)amide, C 3 C 1 pyrr = N-methyl-Npropylpyrrolidinium) ionic liquids have been investigated. The effects of Na ion concentration (0-60 mol% Na[FSA]) and operation temperature (253-363 K) on the viscosity and ioncic conductivity and charge-discharge performance of Na/Na[FSA]-[C 3 C 1 pyrr][FSA]/NaCrO 2 cells are studied. Results show that Na ion concentration strongly affects the rate capability of the cells, and that the best rate capability at 363 K is obtained at 40 mol% Na [FSA]. The 2 operation temperature also significantly influences the charge-discharge performance, especially at low temperatures. At operation temperatures below 273 K, 25 mol% Na[FSA] is found to be the optimum Na ion concentration. There exist different optimum ranges of Na ion concentration depending on the operation temperatures.
The electrochemical properties of a Na 2 FeP 2 O 7 positive electrode were investigated in the ionic liquid sodium bis(fluorosulfonyl)amide (NaFSA)-N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (C 3 C 1 pyrrFSA). A stable charge-discharge behavior was obtained over the temperature range 253-363 K. The cell offered nearly a one-electron theoretical capacity of about 90 mAh g −1 at 298-363 K. The rate capability showed considerable enhancement with increasing temperature, delivering 50 mAh g −1 at a very high rate of 4000 mA g −1 (ca. 41 C) at 363 K. Furthermore, the Na 2 FeP 2 O 7 positive electrode demonstrated excellent cyclability, exceeding 300 cycles, in terms of both capacity retention and coulombic efficiency at 298-363 K, as only negligible capacity fade (<1%) was observed. This good performance over a wide temperature range will meet the requirements of the versatile applications expected for Na secondary batteries.promising platform for the broad spectrum of applications expected for Na secondary batteries. 4 5 6 7 8
Please cite this article as: Changsheng Ding, Toshiyuki Nohira, Rika Hagiwara, Atsushi Fukunaga, Shoichiro Sakai, Koji Nitta, Electrochemical performance of hard carbon negative electrodes for ionic liquid-based sodium ion batteries over a wide temperature range, Electrochimica Acta http://dx.Abstract Sodium ion batteries (SIBs) have been attracting much attention as promising next-generation energy storage devices for large-scale applications. The major safety issue with SIBs, which arises from the flammability and volatility of conventional organic solvent-based electrolytes, is resolved by adopting an ionic liquid (IL) electrolyte. However, there are only a few reports on the study of negative electrodes in ILs. Here, we report the electrochemical performance of a hard carbon (HC) negative electrode in Na[FSA]-[C 3 C 1 pyrr][FSA] (FSA = bis(fluorosulfonyl)amide, C 3 C 1 pyrr = N-methyl-N-propylpyrrolidinium) IL over a wide temperature range of −10 ºC to 90 ºC.High-temperature operation, which is realized for the first time by using an IL, can take full advantage of the high capacity of HC even at a very high discharge rate of 1000 mA (g-HC) -1 : the discharge capacity is 230 mAh (g-HC) -1 at 90 ºC and 25 mAh (g-HC) -1 at 25 ºC. Moreover, surprisingly stable cycleability is observed for the HC electrode at 90 ºC, i.e. a capacity retention ratio of 84% after 500 cycles. Finally, a high full-cell voltage of 2.8 V and stable full-cell operation with Coulombic efficiency higher than 99% are achieved for the first time when using NaCrO 2 as the positive electrode at 90 ºC.
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