A safe and high-rate negative electrode for sodium-ion batteries: Hard carbon in NaFSA-C1C3pyrFSA ionic liquid at 363 K

• Hard carbon (HC) negative electrodes were examined in several ionic liquids.
• A Na/NaFSA-C1C3pyrFSA/HC cell exhibited good cycle performance at 363 K.
• The discharge capacity was 260 mAh (g-HC)−1 at 50 mA (g-HC)−1.
• The cell showed high rate capability up to 1000 mA (g-HC)−1.
• The coulombic efficiency was higher than 99.9% for 40 cycles.

Charge–discharge properties of hard carbon (HC) for sodium secondary batteries using NaFSA-C1C3pyrFSA (FSA: bis(fluorosulfonyl)amide, C1C3pyr: N-methyl-N-propylpyrrolidinium) ionic liquids were investigated. A Na/NaFSA-C1C3FSA/HC cell showed reversible capacity of 260 mAh (g-HC)−1 at a constant rate of 50 mA (g-HC)−1 at 363 K, maintaining cell capacities of approximately 95.5% of the initial capacity after 50 cycles. Coulombic efficiency was higher than 99.5% except for the initial few cycles, reaching 99.9% after 40 cycles. The Na/NaFSA-C1C3pyrFSA/HC cell also exhibited a high rate capability of 211 mAh (g-HC)−1 at a rate of 1000 mA (g-HC)−1, which corresponds to a charge rate of approximately 4C.