Niobium-doped titanium oxide anode and ionic liquid electrolyte for a safe sodium-ion battery

• Nb-doped rutile TiO2 was studied as anode material for Na-ion batteries.
• The anode performance was evaluated in an ionic liquid electrolyte.
• Excellent electrode cycle performance was attained at 60 °C.
• The non-flammability was demonstrated by a closed-system ignition test.

The anode properties of Nb-doped rutile TiO2 electrodes were investigated in an ionic liquid electrolyte comprised of N-methyl-N-propylpyrrolidinium cation and bis(fluorosulfonyl)amide anion for use in a safe Na-ion battery. Although the electrolyte's conductivity was lower than that of a conventional organic electrolyte at 30 °C, it showed high conductivity comparable to that of the organic electrolyte at 60 °C. The Nb-doped TiO2 electrode showed excellent cyclability in the ionic liquid electrolyte at 60 °C: a high capacity retention of 97% was observed even at the 350th cycle, which is comparable to value in the organic electrolyte (91%). In a non-flammability test in a closed system, no ignition was observed with the ionic liquid electrolyte even at 300 °C. These results indicate that combination of a Nb-doped TiO2 anode and ionic liquid electrolyte gives not only an excellent cyclability but also high safety for a Na-ion battery operating at a temperature below the sodium's melting point of 98 °C.

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