Carbon coated transition metal borates as anode materials for Na-ion batteries

•Two transition metal borates are proposed as new promising anode materials for NIBs.•The conversion reaction can be thoroughly realized for transition metal borates.•Na insertion into transition metal borates are energetically more favorable than oxides.•Fe3BO5/Na2V3(PO4)2F3 full cell exhibits a high energy density of 180.3 Whkg−1. Development of anode materials with high performance is crucial for the successful application of Na-ion batteries. In this study, we provide a novel type of conversion-type anode materials, transition metal borates, as promising candidates for Na-ion storage. The transition metal borates (Fe3BO5 and Ni3(BO3)2) are successfully fabricated by a facile sol-gel route. When they are firstly evaluated for Na-ion storage, they deliver remarkably better Na-ion insertion kinetics and high reversible capacity than transition metal oxides. It’s notable that the conversion reaction can be thoroughly realized for transition metal borates during sodiation/desodiation process accompanying with the change of B-O coordination. First-principles calculation indicates that Na insertion into transition metal borate are energetically much more favorable compared with transition metal oxides. Furthermore, carbon coated borates are designed and firstly prepared, which exhibit good rate capability and excellent cycling stability. Moreover, the Fe3BO5/Na2V3(PO4)2F3 full cell exhibits a high energy density of 180.3 Wh kg−1 at a high power density of 150.1 W kg−1. This study demonstrates a class of new promising conversion-type anodes and also verifies the practical application in Na-ion batteries with high energy/power density.