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.

Automated summary

This study presents a unique nanowire bundles anode design for aqueous sodium-ion capacitors, demonstrating high rate capability, high capacity, stable performance, and outstanding energy density. The use of NASICON phosphate electrodes in a flexible solid-state NTP-based ASIC shows excellent stability over 10,000 cycles and no performance decay even after continuous bending at 180 degrees.