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Understanding the charge storage mechanism of MnCO3 is essential to improve its capacitance performance. Herein, we report the charge storage mechanism of MnCO3 in aqueous Na2SO4 and Mg(ClO4)2 electrolytes studied systematically by using ex‐situ X‐ray diffraction and X‐ray photoelectron spectroscopy. Theoretical specific capacitance of MnCO3.H2O 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(ClO4)2 and 0.1 M Na2SO4 electrolytes, respectively, which suggests that only a limited fraction of MnCO3 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 Mg2+‐ions into/from MnCO3 accompanied by redox reaction between Mn2+ and Mn1+ during charge/discharge are reversible and do not result in phase transformation in aqueous 0.1 M Mg(ClO4)2 electrolyte. In contrast, lattice expansion and contraction by insertion and extraction of Na+‐ions into/from MnCO3 result in a gradual transformation of MnCO3 into α‐MnO2 in aqueous 0.1 M Na2SO4 electrolyte. [ABSTRACT FROM AUTHOR]