Recently, Yu Hong, Du Chengfeng of Northwestern Polytechnical University and Wu Xinglong of Northeast Normal University published on ACS Nano a study on in-situ synthesis of Cr substituted Na3V2 (PO4) 3 (NVCP) high voltage and high magnification cathode materials by solid solution MXene. The MXene two-dimensional sheet of Cr and V bimetallic solution not only provides the raw materials of metal and carbon, but also serves as the NVCP growth template, finally forming an excellent structure of double carbon network coated NVCP. In situ X-ray diffraction (in situ XRD) and density functional theory (DFT) were used to study the charge discharge phase transition, electronic structure and sodium ion (Na+) migration before and after Cr substitution.
Fig. 2 In situ XRD study of charge discharge process: (a) and (c) are NVP; (b) And (d) are NVCP.
It is found that Cr substitution changes the Na+ordering of the Na2 site and forms an additional intermediate phase during charging/discharging, thus reducing the energy barrier of Na+migration near the Cr site. Therefore, the diffusion of Na+in NVCP is 2-3 orders of magnitude higher than NVP. In NVCP, even at ultra-high magnification (200 C) and low temperature (− 20 ° C), two stable oxidation platforms of 3.4 V and 4.0 V can be obtained. Compared with NVP, NVCP has significantly improved the magnification and circulation performance. When the current density of NVCP is 1 C, the specific capacity is 119 mA g − 1, and the energy density reaches 410 Wh kg − 1 (the power density is 407 W kg − 1), breaking through the theoretical energy density limit of NVP (≈ 394 Wh kg − 1); When the current density is 200 C, the power density is 68975 W kg − 1 (energy density reaches 230 Wh kg − 1); At a current density of 10 C, 1500 cycles can be stabilized. At the same time, NVCP also shows excellent low-temperature performance and full battery performance.
Figure 3. DFT study of NVP and NVCP: (a) and (b) are density of states; (c) - (e) is the migration path and energy barrier of Na+.