Beijing Institute of Technology Associate Professor Zhao Yongjie EnSM's view: Anion and cation co-substitution effectively improves the energy density and electrochemical performance of
[Article information]
Based on the charge balance theory, enhances the electrochemical performance of NASICON type sodium ion positive electrode material
First author: Liu Yang
Corresponding Author: Zhao Yongjie *
Communication unit: Beijing University of Technology, Beijing Institute of Technology Yangtze River Delta Research Institute
[Research background]
sodium superion conductor (NASICON) type positive electrode material is considered to be one of the most forward-looking sodium ion battery positive electrode materials due to its open and stable 3D framework, rich adjustability and relatively high Na+ interpolation voltage. Its open and stable 3D framework provides a good diffusion channel for Na+, providing a strong guarantee for cyclic stability and rate performance. The high Na+ interpolation voltage gives it huge application potential. Although limited energy density limits their practical application, the abundant adjustability of active motifs brings more possibilities to it.
So far, the modification research on Na3V2(PO4)3 (NVP) is mainly based on coating, morphological design, metal cation substitution and polyanionic group substitution. Among them, metal cation substitution can stimulate V4+/V5+ redox reaction , effectively improving the energy density of the material (as shown in Figure 1a). Research on metal cation substitution has been very extensive. Various new positive electrode materials such as Al3+, Cr3+, Mg2+, Ti4+ substitution have been found, but the research on the replacement of anion groups is slightly insufficient.
This research is based on the team's previous research (Energy Storage Materials, 2022, 49, 291-298; Advanced Functional Materials, 2020, 30, 1908680), based on the charge equilibrium theory, Al3+ part replaces V3+, and low-priced and isomorphic SiO44-substituted PO43-, to study the electrochemical properties of NVP-based sodium ion cathode materials under the co-substitution of anion and cation, and explore the impact of different SiO44-substitution amounts on the electrochemical properties of materials. This paper provides new ideas for improving the electrochemical properties of NASICON type positive electrode materials, which helps accelerate the research and practical application of the positive electrode materials in sodium ion battery.
[Article Introduction]
Recently, the research group of Associate Professor Zhao Yongjie from Beijing Institute of Technology was published in the internationally renowned journal Energy Storage Materials"Enhanced electrochemical performance of NASICON-type solar ion cathode Research article based on charge balance theory”. This article synthesized Na+ structure-rich Na3+xV1.5Al0.5(PO4)3-xV1.5Al0.5(PO4)3-x(SiO4)x(SiO4)x (0≤x≤0.5) The positive electrode material was clarified through series of analysis and characterization and electrochemical tests, and the impact of SiO44-substitution on the electrochemical properties of the material was also discussed in detail.
[Big points of this article]
Key points 1: Synthesis and characterization of materials
Figure 1. Phase analysis of materials
(a) Schematic diagram; (b) The XRD map of Na3+xV1.5Al0.5(PO4)3-x(SiO4)xx (0≤x≤0.5); (c) Detailed lattice parameter evolution obtained by Rietveld refinement; (d) XRD map of Na3.3V1.5Al0.5(PO4)2.7(SiO4)0.3 obtained by Rietveld; (e) crystal structure +xV1.5Al0.5(PO4)3-x(SiO4)x(SiO4)xhxhtml.
First, Na3+ 1 xV1.5Al0.5(PO4)3-x(SiO4)x(SiO4)x (0≤x≤0.5) Precursor, the precursor was annealed in the tube furnace in Ar atmosphere to obtain Na3+xV1.5Al0.5(PO4)3-x(SiO4)x(SiO4)xx(SiO4)x (0≤x≤0.5) Positive electrode material. The resulting positive electrode material was subjected to XRD analysis test and it was clearly observed that all these diffraction peaks were well indexed to the NASICON structure with R-3c spatial group. And as the SiO44- content increases, the characteristic peaks and peaks such as (012), (104) and (113) shift toward low angles, which is attributed to changes in lattice parameters.
Figure 2. Plain representation of material
(a) SEM images and particle size distribution maps (insert); (b) and (c) TEM images; (d) high-resolution TEM images and SAED images (insert); (e) SEM-EDS images.
Then the morphology and composition of the material are characterized. It can be observed that the positive electrode material particles exhibit irregular shapes and the surface is wrapped with a thin layer of carbon. High resolution TEM and SAED tests can see clear lattice stripes and diffraction spots on the material's surface. In addition, EDS energy spectrum analysis can detect elemental signals of Na, V, Al, Si, P, O, and C, indicating that Si is well dissolved in the matrix structure.
Key points 2: Test of electrochemical properties
Figure 3. Cyclic voltammetry curve
CV curve obtained by scanning speed of 0.2 mV s-1: (a) NVAP. (b) NVAP-Si1. (c) NVAP-Si2. (d) NVAP-Si3. (e) NVAP-Si4. (f) NVAP-Si5.
uses metal sodium as the counter electrode and a buckle battery is assembled for testing. The results show that NVAP, NVAP-Si1, NVAP-Si2, NVAP-Si3, and NVAP-Si4 showed two redox reaction pairs around 3.5V and 4.0V, corresponding to the redox reaction of V3+/V4+/V5+. It is worth noting that with the increase of SiO44- content, the redox peak located around 4.0V shows signs of weakening until the redox peak of NVAP-Si5 completely disappears. In addition, it can be observed that with the increase of SiO44- content, the CV curve shows signs of smoother, and there are signs of a decrease in the degree of separation of the redox peak. The existence of SiO44- on the surface inhibits polarization and has a positive effect on improving the charge and discharge performance.
Figure 4. Charging and discharging performance of the material at 2.5V-4.4V.
(a) Charge discharge curve at 0.1C ratio; (b) Rate performance; (c) cyclic performance of 1C; (d) cyclic performance of 2C; (e) long cycle performance of NVAP-Si3 at 10C ratio; (f) impedance of NVAP-Si3 at different cycles; (g) values corresponding to (f) of Rs and Rcts and Rct; (h) XRD curve corresponding to (f).
was subjected to a constant current charge and discharge test at 2.5-4.4V. The charge and discharge curves of NVAP, NVAP-Si1, NVAP-Si2, NVAP-Si3, NVAP-Si4 and NVAP-Si5 at 0.1C can match CV tests well. Their average discharge capacity tends to increase first and then decrease with the increase of SiO44- content, which shows that the introduction of more Na+ can enable the system to achieve higher capacity. However, when the SiO44- content is too high, the discharge capacity shows a sharp decline, which is related to the suppression of V4+/V5+ redox reaction. All six materials showed excellent rate performance. But at the same time, it is noted that as the magnification increases, the capacity loss of NVAP-Si3 is minimal. While maintaining the highest capacity, NVAP-Si3 also showed minimal polarization, which is mainly related to the improvement of structure and inhibition of polarization by a moderate amount of SiO44. NVAP-Si3 exhibits excellent ring stability and rate performance.
Figure 5. Charging and discharging performance of NVAP-Si3 at 1.4V-4.4V.
(a) CV curve; (b) Charge and discharge curves in the first three turns; (c) Rate performance; (d) Charge and discharge curves at different magnifications; (e) 1C cycle performance; (f) 10C cycle performance; (g) Radar diagram comparing the performance of positive electrode materials of different sodium ion batteries.
In addition, we evaluated the charge and discharge performance of NVAP-Si3 at 1.4-4.4 V vs. Na+/Na. As shown in Figure 5a, the CV curve shows that at 1.4-4.4 V, NVAP-Si3 has a redox reaction peak corresponding to V2+/V3+, V3+/V4+ and V4+/V5+ at 1.6/1.4 V, 3.5/3.2 V and 4.0/3.9 V, respectively. This can correspond well to the charge and discharge curve in Figure 5b. In addition, due to the higher open circuit voltage (2.0~3.0 V), the initial Coulomb efficiency of NVAP-Si3 exceeds 100%, and the first discharge capacity reaches an astonishing 181.5 mAh g-1.
Because of the superiority of the Na+ rich structure, the Coulomb efficiency remained at around 99% during the subsequent charging and discharging process. NVAP-Si3's excellent rate performance and cycle stability can still be maintained well under the three-electron reaction. Finally, by comparing with other NASICON type positive electrode materials and typical layered materials (Fig. 5g), NVAP-Si3 has a good competitive advantage in cycling performance, rate performance and discharge specific capacity.
Point 3: Discussion on charge and discharge mechanism
Figure 5. Discussion on charge and discharge mechanism of NVAP-Si3 in 1.4V-4.4V.
(a) 2D contour map of non-in-situ XRD; (b) non-in-situ XRD curve; (c) Schematic diagram of structural changes during the cycle; (d) non-in-situ XPS map; (e) Evolution of lattice parameters under different charge and discharge voltage states refined by Rietveld.
In order to study the sodium ion storage mechanism of NVAP-Si3, an in-situ XRD test was performed. Figures 6a and b show the out-of-site XRD map of the NVAP-Si3 cathode during the first charge/discharge process in the 1.4–4.4 V voltage range. Overall, the Na+ storage process of NVAP-Si3 involves solution reactions and biphasic reaction mechanisms. valence state of V in the material was analyzed by non-in-situ XPS. . Figure 6e shows the lattice parameter changes of NVAP-Si3 during charging and discharging. It is worth noting that NVAP-Si3 showed very small cell volume changes, lower than other reported NASICON type positive electrode materials. At the same time, the volume changes of unit cell with small electrode materials are crucial for cyclic stability.
【Article link】
Enhanced electron chemical performance of NASICON-type sodium ion cathode based on charge balance theory
https://doi.org/10.1016/j.ensm.2022.10.011
[ Corresponding Author Introduction]
Associate Professor Zhao Yongjie School of Materials, Beijing Institute of Technology,
is mainly engaged in the development of functional ceramic materials and electronic ceramic components.Up to now, as the first author and corresponding author, more than 380 SCI papers have been published in Advanced Functional Materials, Energy Storage Materials, ACS Catalysis, Nano Letters, Nano Energy, Small and other papers. More than 10 invention patents have been applied for. He chaired the National Natural Science Foundation of China Youth and Projects , Outstanding Young Teachers of Beijing Institute of Technology, Tsinghua University New Ceramics and Fine Crafts National Key Laboratory Open Fund, Enterprise-entrusted Technology Development and other projects. He serves as a member of the teaching steering committee of the "National Experimental Teaching Demonstration Center for Advanced Materials" of School of Materials, a youth editor of the domestic journal Rare Metals, and a guest editor of the journal Batteries and Materials.
