Mechanistic Insights Into The Pseudocapacitive Performance And Modification Strategies Of Bronze-type Vanadium Dioxide With Mono/multi-valent Cations Intercalation

The transition metal oxides-based pseudocapacitors have received extensive attention recently due to the increasing demands for energy storage and conversion.Through"（near）surface redox reactions",the pseudocapacitors can provide much higher energy density than double-layer capacitors,however there still exists problems with respect to limited utilization of bulk phase.Through introducing"intercalation pseudocapacitance",the fast ion diffusion channels can be constructed inside the materials,which is expected to further improve its energy density.The two-dimensional materials are typical intercalation pseudocapacitors,however,its practical applications are still hindered by the low cycling stability.Bronze-type vanadium dioxide（VO₂（B））,which comprising sheets of distorted edge-sharing VO₆ octahedra that are linked to adjacent sheets by corner sharing along the c-direction,is a promising intercalation pseudocapacitive material because the corner and edge sharing motif of VO₂（B）is resistant to lattice shearing upon cation intercalation,thus leading to a quite stable structure during electrochemical cycling.Meanwhile,the utilization of multivalent cations as charge carriers has been considered as an effective strategy to further improve its charge storage properties since they can induce multi-electron transfers upon cycling compared to that of monovalent cations.However,little information about the intercalation behaviors of multivalent cations into VO₂（B）-based pseudocapacitive electrode is available until now.（1）Herein,via combined structure/spectroscopic characterizations,theoretical calculations and electrochemical analysis,we have shown that only ion（de-）intercalations into VO₂（B）are occurred in Na₂SO₄ and Mg SO₄electrolytes upon cycling,and their distinct charge storage performance is considered due to the synergistic effects between ionic radius of electrolyte cations and their polarizing powers.In contrast,part of VO₂（B）is reversibly converted to Zn₃（OH）₂V₂O₇·2H₂O in Zn SO₄ electrolyte,followed by Zn²⁺（de-）intercalation into both phases upon cycling,thus enables full utilization of the bulk electrode and realizing maximization of the specific capacitance（460F/g at 1 A/g current density）.When cycled in Al₂（SO₄）₃ electrolyte,the large VO₂（B）nanobelts are collapsed into small pellets due to the strong electrostatic force between Al³⁺ions and host structure,thereby resulting in serious structural instability and inferior pseudocapacitive properties.（2）Moreover,we have discussed the modification strategies that can improve VO₂（B）’s pseudocapacitive performance via aliovalent metal ion doping,polymer intercalation and employment of a novel one-step exfoliation-flocculation synthesis process.Our results showed that with increasing the doping amounts of Na and Zn,the specific capacitances of VO₂（B）increased first and then decreased.In contrast,the Co doping significantly increased VO₂（B）’s specific capacitances,whereas the Mg-doping mainly improves the rate performance of VO₂（B）.Adding a certain amount of stearic amine in the hydrothermal preparation process can effectively improve the specific capacity and rate performance of VO₂（B）,which is a promising modification method of VO₂（B）.Besides,the single/few-layers VO₂（B）nanosheets have been obtained by ultrasonic exfoliation process,meanwhile the flocculation with metal ions has been used to adjust its morphology/structure,thus greatly improving the as-prepared sample’s rate performance.In general,this work provides valuable insights in understanding the intercalation behaviors of mono/multi-valent cations into VO₂（B）and effective modification strategies,which will enable rational design of more layered oxides with excellent charge storage properties or extend to other applications.