Spinel　LiNi0.5Mn1.5O4　(LNMO)　is　a　promising　high　voltage　cathode　material　for　lithium　ion　batteries　(LIBs).　However,　dissolution　of　Mn　and　unwanted　side　reactions　between　LNMO　and　the　electrolyte　raises　several　safety　issues　while　also　resulting　in　deteriorated　electrochemical　performance　of　LIBs　at　high　working　voltages.　Here,　we　report　the　use　of　ultrathin　atomic　layer　deposited　(ALD)　AlPO4　thin　film　as　a　coating　material　for　LNMO　electrodes　to　circumvent　the　stated　issues.　The　as-prepared　AlPO4　coated　LNMO　demonstrates　excellent　capacity　retention　with　prolonged　cycle　life　compared　to　the　bare　one.　Synchrotron　based　X-ray　spectroscopy　was　employed　to　understand　how　ultrathin　coating　layer　improve　the　cycle　life,　and　then　develop　a　detailed　mechanism　for　the　effect　of　coating　layer.　Our　studies　revealed　that　using　atomic　scale　coating　layer　with　improved　thermal　stability　effectively　impede　the　side-reactions　occurrence　at　high　voltage,　resulting　in　significantly　improved　safety　and　electrochemical　performances.