Ultrastable Sodium Storage in MoO₃ Nanotube Arrays Enabled by Surface Phosphorylation

Molybdenum trioxide (MoO₃) has been considered as an appealing choice of anode for sodium-ion batteries because of its high theoretical capacity (1117 mA h g^-1). However, the large volume change upon Na⁺ storage results in poor cycling stability and capacity fade of MoO₃. Here, we demonstrate a surface phosphorylation strategy to mitigate the degradation of three-dimensional MoO₃ array electrodes. Such a phosphorylation strategy allows MoO₃ arrays to sustain a capacity of 265 mA h g^-1, or ∼90% of the initial value, at a rate of 2 A g^-1 over 1500 cycles, outperforming most reported MoO₃ electrodes. Moreover, kinetic analysis unveils a capacitance-dominated Na⁺ storage feature of MoO₃ arrays, owing to the enhanced electron mobility imparted by oxygen vacancies that are simultaneously introduced by phosphorylation. Hence, surface phosphorylation might offer new possibilities to bypass multiple materials challenges facing current sodium electrodes.