The Charge Storage Mechanism of MnCO 3 in Aqueous Electrolytes

Understanding the charge storage mechanism of MnCO 3 is essential to improve its capacitance performance. Herein, we report the charge storage mechanism of MnCO 3 in aqueous Na 2 SO 4 and Mg(ClO 4 ) 2 electrolytes studied systematically by using ex‐situ X‐ray diffraction and X‐ray photoelectron spectroscopy. Theoretical specific capacitance of MnCO 3 .H 2 O in the potential window of 0.1 to 1.0 V is 806 F g −1 , however, it delivers a specific capacitance value of only 95 and 66 F g −1 in 0.1 M Mg(ClO 4 ) 2 and 0.1 M Na 2 SO 4 electrolytes, respectively, which suggests that only a limited fraction of MnCO 3 is participating in the charge storage. The ex‐situ X‐ray diffraction and X‐ray photoelectron spectroscopic studies reveal that the insertion and extraction of Mg 2+ ‐ions into/from MnCO 3 accompanied by redox reaction between Mn 2+ and Mn 1+ during charge/discharge are reversible and do not result in phase transformation in aqueous 0.1 M Mg(ClO 4 ) 2 electrolyte. In contrast, lattice expansion and contraction by insertion and extraction of Na + ‐ions into/from MnCO 3 result in a gradual transformation of MnCO 3 into α‐MnO 2 in aqueous 0.1 M Na 2 SO 4 electrolyte.