Three Electron Reversible Redox Reaction in Sodium Vanadium Chromium Phosphate as a High‐Energy‐Density Cathode for Sodium‐Ion Batteries

Abstract

A sodium‐ion battery operating at room temperature is of great interest for large‐scale stationary energy storage because of its intrinsic cost advantage. However, the development of a high capacity cathode with high energy density remains a great challenge. In this work, sodium super ionic conductor‐structured Na3V2−xCrx(PO4)3 is achieved through the sol–gel method; Na3V1.5Cr0.5(PO4)3 is demonstrated to have a capacity of 150 mAh g−1 with reversible three‐electron redox reactions after insertion of a Na+, consistent with the redox couples of V2+/3+, V3+/4+, and V4+/5+. Moreover, a symmetric sodium‐ion full cell utilizing Na3V1.5Cr0.5(PO4)3 as both the cathode and anode exhibits an excellent rate capability and cyclability with a capacity of 70 mAh g−1 at 1 A g−1. Ex situ X‐ray diffraction analysis and in situ impedance measurements are performed to reveal the sodium storage mechanism and the structural evolution during cycling.

Surface Potential Regulation Realizing Stable Sodium/Na3Zr2Si2PO12 Interface for Room‐Temperature Sodium Metal Batteries

“…Based on the satisfactory Na/NZSP interface, all-solidstate sodium batteries were assembled using a NASICONtype Na 3 V 1.5 Cr 0.5 (PO 4 ) 3 (NVCP) cathode reported by our group. [18] The phase structure and morphology are analyzed in Figure S11 and S12, Supporting Information, showing a monoclinic phase structure and a micron-scale aggregate with uniform mapping of Na, V, Cr, P in consistence with the literature. [18] As shown in Figure 5a, a composite cathode was prepared by adding a plastic-crystal electrolyte of NaClO 4succinonitrile as a interphase agent to form good contact Figure 2.…”

“…Two pairs of voltage plateau at 3.4 and 3.9 V versus Na/Na + are observed, corresponding to the redox reactions of V 3+ /V 4+ and V 4+ / V 5+ , respectively. [18] The charge/discharge voltage plateaus and capacities are well retained against the increasing current densities, suggesting good rate performance. Moreover, the Na//NVCP battery shows excellent cycling stability with a highly reversible capacity of 103 mA h g −1 under 100 mA g −1 and a capacity retention of 73% is achieved after 400 cycles.…”

“…The discharge capacity at 30 mA g −1 is 114.6 mA h g −1 , which is close to the theoretical capacity of 115.3 mA h g −1 of NVCP. [18] An initial Coulombic efficiency (CE) of 91.0% is achieved which is much higher than that of the Na//NVCP batteries using liquid electrolyte (less than 80%). [18] The enhanced CE indicates that the irreversible interfacial reactions is effectively suppressed in solid-state batteries.…”

“…[18] An initial Coulombic efficiency (CE) of 91.0% is achieved which is much higher than that of the Na//NVCP batteries using liquid electrolyte (less than 80%). [18] The enhanced CE indicates that the irreversible interfacial reactions is effectively suppressed in solid-state batteries. Similar phenomenon has been reported on a Na//Na 2 MnFe(CN) 6 battery operated at 60 °C using NZSP.…”

“…The cathode material of Na 3 V 1.5 Cr 0.5 (PO 4 ) 3 (NVCP) was synthesized via a sol-gel method according to the literature. [18] Typically, raw chemicals of C 6 H 5 Na 3 O 7 , V 2 O 3 , Cr(NO 3 ) 3 and NH 4 H 2 PO 4 with a stoichiometric ratio were first dissolved in deionized water. After stirring at 80 °C for 12 h, a dried precursor gel was formed and subsequently calcinated at 800 °C for 10 h in Ar flow to obtain the NVCP cathode material.…”