Defective　layered　Mn-based　materials　were　synthesized　by　Li/Na　ion　exchange　to　improve　their　electro-chemical　activity　and　Coulombic　efficiency.　The　annealing　temperature　of　the　Na　precursors　was　impor-tant　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]Od　cath-ode　made　from　a　P3-type　precursor　calcined　at　700　degrees　C　was　analyzed　using　X-ray　photoelectron　spectrom-etry　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　dif-fusion　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　envi-ronment　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.(c)　2023　Science　Press　and　Dalian　Institute　of　Chemical　Physics,　Chinese　Academy　of　Sciences.　Published　by　ELSEVIER　B.V.　and　Science　Press.　All　rights　reserved.