Sodium-ion　batteries　(SIBs)　have　been　regarded　as　the　most　promising　candidates　for　the　next-generation　energy　storage　devices　owing　to　their　low　price　and　high　abundance.　However,　the　development　of　SIBs　is　mainly　hindered　by　the　instability　of　cathode　materials.　Here,　we　report　a　new　P2-type　manganese-rich　cathode　material,　Na0.66Li0.18Mn0.71Mg0.21Co0.08O2　(P2-NaLiMMCO)　with　uniform　spherical　structure　prepared　via　a　simple　solvothermal　method　and　subsequent　solid-state　reaction.　This　P2-NaLiMMCO　cathode　material　with　uniform　microsize　secondary　spheres　and　nanosize　primary　crystalline　particles　delivers　a　high　initial　discharge　capacity　of　166　mA　h　g(-1)　and　superior　capacity　retention,　which　are　superior　to　most　previously　reported　results.　The　improved　stability　of　the　cathode　material　was　further　investigated　by　the　in　situ　X-ray　diffraction　technique,　which　suggests　an　enhanced　reversibility　of　the　cathode　material　during　the　desodiation/sodiation　process.　With　the　superior　electrochemical　performance　and　stable　structures,　this　new　P2-NaLiMMCO　can　serve　as　a　practical　cathode　material　for　SIBs.