Aluminum-ion　batteries　(AIBs)　are　considered　as　a　promising　alternative　to　traditional　rechargeable　batteries　due　to　their　high　theoretical　capacity,　low　cost,　and　multivalent-ion/multi-electron　transfer,　but　are　hindered　from　commercialization　by　the　lack　of　suitable　cathode　materials.　Herein,　an　aluminum-selenium　(Al-Se)　battery　that　operates　at　room　temperature　with　high　energy　efficiency　is　reported.　This　Al-Se　battery　exhibits　high　selenium　utilization　with　a　discharge　capacity　of　607　mAh　g(-1),　a　reduced　overpotential,　and　high　volumetric　capacity　for　over　100　cycles.　Furthermore,　ex-situ　spectroscopic　and　microscopic　investigations　of　the　Se　cathodes　indicated　the　existence　of　an　aluminum　selenide　component　as　the　discharge　product.　Additionally,　in-situ　transmission　electron　microscopy　provided　not　only　insights　into　understanding　the　Al-Se　electrochemistry,　but　also　a　real-time　characterization　technique　to　evaluate　the　electrochemistry　of　AIBs　in　general.