Invited Presentation: New Na Rechargeable Battery Based on Non-Flammable Inorganic Liquid Electrolyte

In order to address recent concerns on the limited resources of lithium and the localized reserves, Na rechargeable batteries have gained much attention as alternative power sources to replace Li rechargeable batteries. Although Na and Li have similar physicochemical properties, Na rechargeable batteries have not enjoyed the success that Li batteries have, due to their low energy density and poor cycle performance. Herein we introduce a new Na rechargeable battery with high energy density using an SO₂-based inorganic liquid electrolyte as both (i) a Na⁺-conducting medium and (ii) cathode material. In addition to the high capacity and the long cycle-life, the Na–SO₂ battery also has an advantage of the use of a highly conductive electrolyte, e.g., up to 0.1 Scm^-1 at room temperature. This excellent conductivity of Na⁺ensures a high reversibility and rate capability.

Fig. 1a shows the first and second voltage profiles of the Na–SO₂ battery that delivers a discharge capacity of ~1800 mAhg^-1 based on the carbon cathode at a rate of 0.1C. The Na–SO₂ battery also gave an encouraging rate capability, where a high capacity of 897 mAhg^-1 is observed even at a significantly high current density of 5C (= 17 mAcm^-2) (Fig. 1b). The Na–SO₂ battery also showed relatively good capacity retention, i.e. 83% of the initial capacity after 50 cycles (Fig. 1c), even under full-discharge condition, accompanied by high columbic efficiencies during cycling (average of 99%). As another unique performance of the Na–SO₂ battery, there is a self-regeneration mechanism removing residual discharge products from the cathode. When the cell was relevantly overcharged, a recombination reaction to regenerate NaAlCl­₄ took place and the cathode was recuperated, and finally, the capacity of the Na–SO₂ battery was restored and remained with cycling without significant capacity fade (Fig. 1c). For the cell chemistry, we performed an ab inito calculation, combined with various experimental analyses, and identified the highly reversible redox reaction of SO₂ with tetrachloroaluminate. Considering the many favorable features displayed in this report, the Na–SO₂ battery can be a viable system for post-LIB energy storage devices.