Recently, needs to rechargeable batteries have been diversified into various kinds due to expansion of their application range. Not only future high-performance batteries exceeding Li-ion batteries (LiBs), but also new concept ones, which are composed solely of common elements and have performance comparable to current LiBs, have become one of the important recent research targets in this area. Focusing rich abundance and very-high theoretical capacity (2980 mAh g^-1, 8046 mAh cm^-3) of aluminum (Al) metal, we have created new concept rechargeable batteries with the following anode reaction in chloroaluminate ionic liquids (ILs):^1,2

4[Al₂Cl₇]^- + 3e^- ⇌ Al + 7[AlCl₄]^-

Many research groups have also reported similar Al metal anode batteries.³ Herein, some sulfur (S) composite cathodes that enable the use of S(IV)/S electrode reaction are designed to greatly enhance the capacity of Al metal anode battery.

The preparation and purification processes for 60.0-40.0 mol% AlCl₃–[C₂mim]Cl and 61.0-26.0-13.0 mol% AlCl₃–NaCl–KCl IL electrolytes were identical with those described in our previous articles.^1,2 Three types of cathode active materials, S-coated multi-walled carbon nanotube (S-MWCNT), sulfurized polyethylene glycol (SPEG),⁴ and sulfurized polyacrylonitrile (SPAN),⁵ were employed. All the S composite electrodes were prepared by pressing the mixtures of 50 wt% S-based cathode active materials, 45 wt% conductive additive (MWCNT), and 5 wt% polytetrafluoroethylene onto Mo plate current collectors. Al metal plate was employed as the anode active material. Electrochemical experiments were carried out with a commonly used three-electrode cell or a two-electrode sealed cell in an Ar gas-filled glove box with O₂ and H₂O < 1 ppm.

Figure 1 shows multiple cyclic voltammograms recorded at a S-MWCNT composite electrode in a three-electrode cell with 60.0-40.0 mol% AlCl₃–[C₂mim]Cl at 298 K. When the potential scan was initiated from the rest potential to the negative direction, a pair of reduction and oxidation waves appeared with large over potential. These waves gradually varied with an increase in the number of cycles. Considering the article reported by Gao et al.,⁶ the redox waves observed in Fig. 1a can be regarded to be the following electrochemical reaction (S/S^2-):

3S + 8[Al₂Cl₇]^- + 6e^- ⇌ Al₂S₃ + 14[AlCl₄]^-

As for the positive scan from the rest potential (Fig. 1b), several oxidation and reduction waves were observed. Overpotential obviously diminished compared to that for the S/S^2-. Similar electrode behavior for S in a Lewis acidic 63-37 mol% AlCl₃–NaCl molten salt is reported by Marassi, et al.⁷ Given that the electrochemical reaction process is the same, the number of electrons involved in the reaction is estimated to be 4. Then, the electrochemical reaction (S(IV)/S) is:

[SCl₃]⁺ + 3[Al₂Cl₇]^- + 4e^- ⇌ S + 6[AlCl₄]^-

If we can apply this S(IV)/S reaction to the Al metal anode-S cathode rechargeable battery, S cathode capacity becomes double and higher working voltage is expected relative to conventional S/S^2- one. Unfortunately, the redox waves for the reaction decrease sharply with increase in the cycle number. The waves almost disappeared at the 5^th cycle. Analogous results were also obtained in the 61.0-26.0-13.0 mol% AlCl₃–NaCl–KCl. We concluded that the electrode reaction for S(IV)/S on the S-MWCNT composite cathode is not suitable for battery application. However, the use of S-combined active materials, SPEG and SPAN, substantially improved the electrode reaction in the 61.0-26.0-13.0 mol% AlCl₃–NaCl–KCl IL at 393 K. Thus, we carried out the charge-discharge test for the Al | SPEG and Al | SPAN batteries with the inorganic IL electrolyte using both S/S^2- and S(IV)/S electrode reactions. If SPEG was used, the discharge capacity was ca. 1050 mAh (g-S)^-1 at 50^th cycle. When using only S/S^2- reaction, such high capacity was not attained.² The S(IV)/S electrode reaction should be directly involved in the capacity increase. Interestingly, the use of SPAN made further improvement possible. One of the typical results is shown in Fig. 2. At the first cycle, the discharge capacity reached ca. 4700 mAh (g-S)^-1 and showed ca. 2600 mAh (g-S)^-1 even after 50^th cycle. These results suggest that the S-combined active material, i.e., SPEG and SPAN, composite electrodes can work as the high-capacity cathodes for Al metal anode rechargeable battery.

This research was supported by JST-MIRAI program (JPMJMI17E9). SPAN was provided by ADEKA corporation.

References

1. Tsuda, et al., J. Electrochem. Soc., 161, A908 (2014).
2. Tsuda, et al., Chem. Commun., 58, 1518 (2022).
3. Ru, et al., J. Mater. Chem. A, 7, 14391 (2019).
4. Kojima, et al., ECS Trans., 75, 201 (2017).
5. S. Ahmed, et al., Adv. Sci., 8, 2101123 (2021).
6. Gao, et al., Angew. Chem. Int. Ed., 55, 9898 (2016).
7. Marassi, et al., J. Electrochem. Soc., 126, 231 (1979).