Weak Ionization Induced Interfacial Deposition and Transformation towards Fast-Charging NaTi2(PO4)3 Nanowire Bundles for Advanced Aqueous Sodium-Ion Capacitors

Aqueous sodium-ion capacitors (ASICs) offer great promise for inexpensive and safe energy storage. However, their development is plagued by a kinetics imbalance at high rates between battery and capacitive electrodes and a narrow voltage window due to water electrolysis. Here a unique nanowire bundles anode is designed that simultaneously affords ultrahigh rate capability and manifests robust Na⁺ insertion to suppress hydrogen evolution, enabling an advanced ASIC. The NaTi₂(PO₄)₃ (NTP) is grown on thin titanium foil by elaborately utilizing the weak ionization chemistry of NaH₂PO₄ (NHP), where single-agent NHP not only partially etches titanium to release TiO²⁺, but also induces the interfacial phase-transformation of pre-deposited orthomorphic Na₄Ti(PO₄)₂(OH)₂ cubes to hexagonal NTP nanowires. This anode has hierarchical architectures to facilitate charge and mass transport, thus working stably at considerably high rates of 15–150 C with high capacities. The first 2.4 V flexible solid-state NTP-based ASIC is designed with high energy densities (5.8–12.8 mWh cm⁻³; 57.9–62.1 Wh kg⁻¹; total mass loading up to 8.1 mg cm⁻²) comparable to NASICON-based devices using organic electrolytes, demonstrating outstanding stability of 10 000 cycles and no performance decay even after continuous bending at 180^o. This work presents a versatile strategy to construct NASICON phosphate electrodes for advanced energy storage.