Optimization of Von Mises Stress Distribution in Mesoporous α-Fe₂O₃/C Hollow Bowls Synergistically Boosts Gravimetric/Volumetric Capacity and High-Rate Stability in Alkali-Ion Batteries

Hollow structures are often used to relieve the intrinsic strain on metal oxide electrodes in alkali-ion batteries. Nevertheless, one common drawback is that the large interior space leads to low volumetric energy density and inferior electric conductivity. Here, the von Mises stress distribution on a mesoporous hollow bowl (HB) is simulated via the finite element method, and the vital role of the porous HB structure on strain-relaxation behavior is confirmed. Then, N-doped-C coated mesoporous α-Fe₂O₃ HBs are designed and synthesized using a multistep soft/hard-templating strategy. The material has several advantages: (i) there is space to accommodate strains without sacrificing volumetric energy density, unlike with hollow spheres; (ii) the mesoporous hollow structure shortens ion diffusion lengths and allows for high-rate induced lithiation reactivation; and (iii) the N-doped carbon nanolayer can enhance conductivity. As an anode in lithium-ion batteries, the material exhibits a very high reversible capacity of 1452 mAh g⁻¹ at 0.1 A g⁻¹, excellent cycling stability of 1600 cycles (964 mAh g⁻¹ at 2 A g⁻¹), and outstanding rate performance (609 mAh g⁻¹ at 8 A g⁻¹). Notably, the volumetric specific capacity of composite electrode is 42% greater than that of hollow spheres. When used in potassium-ion batteries, the material also shows high capacity and cycle stability.