Dual modulation in electrode and electrolyte enabling ultra-stable NaTi2(PO4)3 anode toward advanced quasi-solid-state sodium-ion capacitors

NASICON-type NaTi2(PO4)3 (NTP) holds enormous potential for aqueous sodium-ion capacitors (ASICs) as an anode candidate by virtue of its high theoretical capacity/ionic conductivity, and suitable voltage platform. However, the inherent deficiencies of NTP itself including low electronic conductivity and disgusting side reactions always lead to its poor cycling stability and rate properties, which severely limits its practical applications. For this, herein, a synergistic modulation methodology in both electrode material and electrolyte is first devised to guarantee the single-crystal NTP submicro-cubes coated with nano carbon (S-NTP@C) efficiently operating in an aqueous Na2SO4-SiO2 "Soggy-Sand" gel electrolyte. The single-crystal feature and carbon coating cooperatively create a fast electron/ion "expressway" and physically stabilize robust structure and sur-/interfaces, while the "Soggy-Sand" electrolyte acts as an "escort" to enhance the benefits of NTP by reducing dissolved oxygen content and water activity. Benefiting from such appealing merits, the achieved S-NTP@C exhibits competitive high-rate and long-cycle properties, completely surpassing other reported NTP anodes. Moreover, the constructed S-NTP@C-based quasi-solid-state ASICs achieve a negligible capacitance decay of only 0.9% over up to 18,000 cycles, along with a maximum energy density of 71.2 Wh kg- 1. More essentially, our contribution here provides original design philosophy for the next-generation ASICs and beyond.