Among several energy storage technologies, supercapacitors have achieved considerable attention because of their rapid charge-discharge rates, high power density, and excellent cycling stability, but low energy density and fast self-discharge are some of serious disadvantages for the supercapacitors. Electrodes with combination of battery and pseudocapacitance behaviors offer the advantages of both supercapacitors and the advanced batteries, it shows higher energy density and the same time maintains the extended cycle life and fast charge capability. In this study, a new type of tetragonal Li_xMn₂O₄ (LMO) electrode exhibiting combined pseudocapacitance and battery behaviors in aqueous electrolytes has been synthesized by electrochemical cation-exchange conversion from ZnMn₂O₄ (ZMO) [1]. It is shown that the Zn⁺ ion in ZMO can be replaced by the Li⁺ ion, resulting in the formation of LMO in aqueous Li₂SO₄ electrolytes by cyclic voltammetry. The resulting LMO electrode is in tetragonal structure but yet exhibits redox potentials essentially the same as those of the cubic LMO (Fig. a), delivering a redox capacity over 100 mAh/g under low current rate or a specific capacitance of nearly 300 F/g when behaving as a pseudocapacitor at higher current rates. Despite its tetragonal structure, the electrode shows outstanding cycle stability, exhibiting an interesting self-healing structure transformation process to the cubic structure and retaining 100% capacity after 40, 000 cycles. Using the high-concentration LiTFSI electrolyte enables the expansion of operating potential window to 1.55 V. The material synthesis process and charge-transfer mechanism are characterized in detail.

[1] M. Abdollahifar, S.S. Huang, Y.H. Lin, Y.C. Lin, B.Y. Shih, H.S. Sheu, Y.F. Liao, N.L. Wu, J. Power Sources 378, 90, 2018.