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

Abstract 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.