Defective　layered　Mn-based　materials　were　synthesized　by　Li/Na　ion　exchange　to　improve　their　electrochemical　activity　and　Coulombic　efficiency.　The　annealing　temperature　of　the　Na　precursors　was　important　to　control　the　P3-P2　phase　transition,　which　directly　affected　the　structure　and　electrochemical　characteristics　of　the　final　products　obtained　by　ion　exchange.　The　O3-Li0.78[Li0.25Fe0.075Mn0.675]Oδ　cathode　made　from　a　P3-type　precursor　calcined　at　700　°C　was　analyzed　using　X-ray　photoelectron　spectrometry　and　electron　paramagnetic　resonance.　The　results　showed　that　the　presence　of　abundant　trivalent　manganese　and　defects　resulted　in　a　discharge　capacity　of　230　mAh/g　with　an　initial　Coulombic　efficiency　of　about　109%.　Afterward,　galvanostatic　intermittent　titration　was　performed　to　examine　the　Li+　ion　diffusion　coefficients,　which　affected　the　reversible　capacity.　First　principles　calculations　suggested　that　the　charge　redistribution　induced　by　oxygen　vacancies　(OVs)　greatly　affected　the　local　Mn　coordination　environment　and　enhanced　the　structural　activity.　Moreover,　the　Li-deficient　cathode　was　a　perfect　match　for　the　pre-lithiation　anode,　providing　a　novel　approach　to　improve　the　initial　Coulombic　efficiency　and　activity　of　Mn-based　materials　in　the　commercial　application.　©　2023　Science　Press　and　Dalian　Institute　of　Chemical　Physics,　Chinese　Academy　of　Sciences