Energy storage mechanism of monocrystalline layered FePS3 and FePSe3 as active materials for Mg batteries and pseudocapacitors

•Mg2+ ions can be intercalated/deintercalated into/out of layered FePS3 reversibly.•Both Faradaic and non-Faradaic processes contribute to energy storage.•FePS3 behaves a high specific capacity (232.7 mAh g–1) at 0.2 A g–1.•FePS3 retains layered structure after cycles irrespective of the (de)intercalation. [Display omitted] We report the reversible intercalation of Mg2+ ions into monocrystalline layered FePS3 in a tetrahydrofuran electrolyte containing 0.8 M PhMgCl and 0.4 M AlCl3. Mg2+ ions were not intercalated into FePSe3 because of the lower interlayer spacings (FePS3: 1.24 Å, FePSe3, 1.00 Å). The contribution of the non-Faradaic (double layer formation) and Faradaic (electrochemical intercalation/deintercalation, adsorption/desorption) processes toward the energy storage mechanism of FePS3 and FePSe3 were quantified. The maximum specific capacity displayed by FePS3 was 232.7 mAh g–1 at a current density of 0.2 A g–1, which was higher than that of FePSe3 (153.1 mAh g–1). Even at high current density (2 A g–1), FePS3 exhibited a specific capacity of 100.7 mAh g–1 after 450 cycles; the specific capacity of the less robust FePSe3 was just 26.3 mAh g–1.