Polycrystalline　Ni-rich　layered　lithium　transition　metal　oxides　are　one　of　the　most　promising　cathode　materials　for　next-generation　high　energy　density　lithium-ion　batteries,　yet　they　are　still　facing　many　challenges,　especially　for　the　cycling　induced　structural　degradations.　Intergranular　cracking　has　been　identified　as　one　of　the　most　crucial　degradations,　and　grain　boundary　(GB)　engineering　has　been　demonstrated　to　be　an　effective　countering　strategy.　Herein,　we　report　a　GB　modification　protocol　that　can　realize　not　only　improved　GB　stability　but　also　interfacial　reaction　kinetics,　realizing　much　improved　cycling　performance　of　NCM811.　The　simple　and　effective　solution　method　can　incorporate　Ti-dopant　into　GBs　and　secondary　particle　surface,　realizing　the　increase　of　the　capacity　retention　from　79.5%　to　93.5%　at　3.0-4.5　V　after　100　cycles,　and　its　high　voltage　(4.7　V)　and　high　temperature　(55　°C)　cycling　stability　are　also　significantly　improved.　Comprehensive　microstructure　and　electrochemical　characterizations　of　the　samples　before　and　after　cycling　are　conducted　to　reveal　the　underlying　mechanisms,　validating　that　both　interfacial　degradations　and　bulk　failures　have　been　effectively　mitigated.　This　work　provides　an　effective　protocol　in　the　modification　of　GBs　and　interfaces　of　polycrystalline　battery　materials,　which　is　promising　and　feasible　for　industrial　mass-production　application.　©　2024　American　Chemical　Society