(a) The first and second galvanostatic voltage profiles of a Na–SO2 cell at 0.1C (=150 mA g−1 and 0.34 mA cm−2). The cutoff voltage for charge is 4.05 V. (b) Discharge rate capability of a Na–SO2 cell. The charging rate is fixed at 0.2C. (c) Cycle performance of a Na–SO2 cell at a rate of 0.5C discharge and 0.2C charge for 100 cycles. Jeong et al. Click to enlarge.

Here we report a Na–SO₂ rechargeable battery system using NaAlCl₄⋅2SO₂ electrolyte. We found that the optimized carbon cathode enables a reversible reaction of the catholyte with high capacity, good rate capability, a long life-span over 300 cycles, and an estimated theoretical energy density of 407 Wh kg⁻¹ (based on the discharged product including carbon cathode). This value is comparable with those of other high-energy Na rechargeable batteries. Moreover, non-flammability, low vapor pressure, and unique self-regeneration mechanism of the inorganic electrolyte presented here would be noteworthy merits of Na–SO₂ system over other Na rechargeable battery systems.

—Jeong et al.

For their study, the researchers constructed a 2032 coin-type Na–SO₂ cell using a Na-metal anode and a porous carbon cathode with NaAlCl₄⋅2SO₂ as a catholyte.

The cell delivers a discharge capacity of ~1800 mAh g⁻¹ based on the carbon cathode at a rate of 0.1C (=150 mA g⁻¹ or 0.34 mA cm⁻²). This corresponds to an areal capacity of 4.1 mAh cm⁻²—comparable to typical values of commercial Li-ion batteries (3–5 mAh cm⁻²) and much higher than those of reported Li–O₂ and Na–O₂ batteries.

The Na–SO₂ cell also showed a high capacity of 897 mAh g⁻¹ is observed even at a significantly high current density of 5C (7500 mA g⁻¹ or 17 mA cm⁻²).

Given that the rate capability is one of the most challenging issues in NAS, ZEBRA, and Na–O₂ batteries, the excellent power capability could give the Na–SO₂ battery a critical edge over other Na rechargeable batteries previously reported.

—Jeong et al.

However, they authors noted in their paper, a low round-trip energy efficiency (~80%) needs to be further ameliorated. The Na–SO₂ cell showed relatively good capacity retention during cycling, with 75% of the initial capacity after 100 cycles, even under full depth-of-discharge condition, accompanied by high columbic efficiencies during cycling (average of ~99%).

They demonstrated that the cell chemistry is based on the highly reversible redox reaction of SO₂ with tetrachloroaluminate and the use of the NaAlCl₄⋅2SO₂ inorganic electrolyte enables the highly reliable Na–SO₂ system in terms of long cycle life as well as safety.

Considering the many favorable features and promises discussed in this report, the Na–SO₂ battery can be a viable system for next cost-effective energy storage system. Further, the SO₂-based inorganic electrolyte can be widely applied to battery systems adopting other metallic anodes like Ca, K, Al, and Mg, which paves the way for the development of various non-lithium metal-based battery systems.

—Jeong et al.

Resources

• Goojin Jeong, Hansu Kim, Hyo Sug Lee, Young-Kyu Han, Jong Hwan Park, Jae Hwan Jeon, Juhye Song, Keonjoon Lee, Taeeun Yim, Ki Jae Kim, Hyukjae Lee, Young-Jun Kim & Hun-Joon Sohn (2015) “A room-temperature sodium rechargeable battery using an SO2-based nonflammable inorganic liquid catholyte” Scientific Reports 5, Article number: 12827 doi: 10.1038/srep12827