Crystal Structure Modification Enhanced FeNb 11 O 29 Anodes for Lithium‐Ion Batteries

The recently explored FeNb 11 O 29 is an advanced anode material for lithium‐ion batteries, owing to its high specific capacity and safety. However, it suffers from poor rate capability. To tackle this issue, a crystal structure modification is employed. Defective FeNb 11 O 29 (FeNb 11 O 27.9 ) is fabricated by using a one‐step solid‐state reaction method in N 2 . FeNb 11 O 27.9 has the same orthorhombic shear ReO 3 crystal structure ( Amma space group) as FeNb 11 O 29 , but a larger unit‐cell volume and 3.8 % O 2− vacancies (vs. all O 2− ions), which improve the Li + ‐ion diffusion coefficient by a factor of 88.3 %. The contained Nb 4+ ions with free 4 d electrons significantly increase the electronic conductivity by three orders of magnitude. Consequently, FeNb 11 O 27.9 shows improved pseudocapacitive behavior and electrochemical properties. In comparison with FeNb 11 O 29 , FeNb 11 O 27.9 exhibits a higher reversible capacity of 270 mAh g −1 with a higher first‐cycle coulombic efficiency of 90.6 % at 0.1 C. At 10 C, FeNb 11 O 27.9 still retains a high capacity of 145 mAh g −1 with low capacity loss of 6.9 % after 200 cycles, in contrast to the values of 99 mAh g −1 and 11.1 % obtained for FeNb 11 O 29 .