Metastable gamma-MnS and stable alpha-MnS were synthesized via a one-step facile hydrothermal method and investigated as anode materials in Li-ion batteries and capacitors for revealing the essence of their electrochemical difference. Comparing with octahedrally coordinated [MnS6] in stable alpha-MnS, the Mn in gamma-MnS has a tetrahedral coordination with smaller crystal field splitting and the Mn-S bond with a stronger covalent character, resulting in a lower discharge potential that enables assembled devices with a higher energy density. Density functional theory (DFT) calculation corroborates that the metastable gamma-MnS possesses high charge density overlapping in the chemical bonds and decreased energy barriers for the reactive electrons deviating from 3d orbitals. Metastable gamma-MnS delivers a larger storage capacity of 705 mAh g(-1) at a current density of 100 mA g(-1), and also demonstrates superior rate capability (362 mAh g(-1) at 2 A g(-1)) and cycling stability (550 mAh g(-1) remaining after 200 cycles at 0.5 A g(-1)) in Li-ion half cells. Moreover, the gamma-MnS//AC hybrid capacitor with a wide voltage window of 0.1-4 V demonstrates a maximum energy and power density of 220 W h kg(-1) and 10133 W kg(-1) respectively, and cycles at 5 A g(-1) without detectable capacity fading over 5000 cycles.