Weak Ionization Induced Interfacial Deposition and Transformation towards Fast‐Charging NaTi 2 (PO 4 ) 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 NaTi 2 (PO 4 ) 3 (NTP) is grown on thin titanium foil by elaborately utilizing the weak ionization chemistry of NaH 2 PO 4 (NHP), where single‐agent NHP not only partially etches titanium to release TiO 2+ , but also induces the interfacial phase‐transformation of pre‐deposited orthomorphic Na 4 Ti(PO 4 ) 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.