中科大章根强教授团队， Advanced Functional Materials观点

中科大章根强教授团队， Advanced Functional Materials观点：非金属硼间隙位点掺杂诱导P2/O3复合相结构实现近乎零应变钠离子电池正极

【Article Information】

The non-metallic boron interstitial site doping induces the P2/O3 composite phase structure to achieve a cathode of sodium-ion batteries with near-zero strain

First author: Yu Lai, He Xiaoyue, Peng Bo

Corresponding Authors:WANG Yi-fei*,ZHANG Gen-qiang*

Unit: Hefei Guoxuan Hi-Tech Power Energy Co., Ltd., University of Science and Technology of China

【Background】

Sodium-ion batteries (SIBs) are becoming popular candidates for large-scale energy storage devices in the future due to their low cost and similar working principle to lithium-ion batteries (LIBs), and have attracted extensive research attention. However, SIBs still face multiple challenges on the way to practical application, such as relatively low energy/power density, limited cycle life, and unresolved cost issues. In order to break through these bottlenecks, the selection of appropriate cathode materials has become the top priority of research. Recently, layered transition metal oxide cathode materials for sodium-ion batteries have attracted the attention of many researchers due to their high theoretical capacity, significant cost-effectiveness, and simple synthesis methods.

In particular, O3-type oxide cathode materials show great potential to match hard carbon (HC) anodes due to their high capacity and abundant sodium content. Among the many O3 oxides, the cobalt-free O3-NaNi0.5Mn0.5O2 (NNMO) material is particularly eye-catching, and its high specific capacity indicates its broad prospect as a cathode material. However, due to the complex phase transition and weak Na+ ion diffusion kinetics of this material during the electrochemical cycle, it often leads to irreversible structural changes and rapid capacity decay, which greatly limits its practical application. Therefore, the search for more effective solutions to overcome these challenges has become an urgent need for current research.

【Introduction】

近日，中国科学技术大学章根强课题组，在国际知名期刊Advanced Functional Materials上发表题为“Nonmetal Substitution in Interstitial Site of O3-NaNi0.5 Mn0.5O2 Induces the Generation of a Nearly Zero Strain P2&O3 Biphasic Structure as Ultrastable Sodium-Ion Cathode”的观点文章。 该工作通过一种优化的硼协同策略成功构建了P2&O3双相结构（NNMBO），材料具备了多种结构优势从而实现了优异的电化学储钠性能。 通过理论计算证实了轻质级硼掺杂在材料间隙位点，并且降低了P2和O3之间的形成能，从而诱导了P2和O3双相的形成。

The in-situ XRD test further demonstrated the structural change process in the cycle, and the volume structure change in the whole process was only 1.3%, and the overall structure remained stable. Based on this reasonable and effective design strategy, the cathode still has a capacity retention rate of 85.2% after 1000 cycles at a current density of 5C (1C=150 mA g-1) in the half-cell test. More importantly, the pouch battery assembled from the cathode and the commercial hard carbon anode can cycle stably for 150 cycles at a current density of 0.1 C.

【Main points of the text】

Point 1: Theoretical calculations explore the causes of the formation of composite structures

First, with the help of density functional theory (DFT) calculations, the authors explored the potential doping position of boron in NNMO materials to reveal the specific form of boron atoms in the materials. By comparing the system energies at different boron doping positions, the calculated results show that boron at the interstitial position has a more negative formation energy, which clearly indicates that the boron atoms are more inclined to occupy the interstitial position of the material. In order to further understand the inducing effect of boron doping on the formation of composite structures, the research team also calculated the formation energies of P2 and O3 NNMO models before and after boron doping, respectively. The graphical representation of the calculated results clearly shows that the energy difference between the boron doped and post-boron systems is significantly reduced, which strongly demonstrates the great potential of boron-doped NNMO cathode materials to achieve composite structures.

Figure 1: Theoretical computational study of the layered cathode of NNMBO

Point 2: Exploration of the microscopic morphology and structure of electrode materials

On the basis of theoretical research, the authors successfully prepared P2/O3 composite phase electrode materials by sol-gel method. As shown in Figure 2, the phase structure of the material was revealed by XRD analysis, and the boron(B) doping successfully induced a duplex structure compared to the undoped O3 phase NNMO cathode material. In terms of morphology, the synthesized material presents a uniform and complete block structure. To gain a deeper understanding of the crystal structure of the material, the authors performed high-resolution testing. The test results show that the lattice spacing of the material corresponds to the P2 phase (102) and O3 phase (101) crystal planes, respectively, which further confirms the formation of the composite phase structure after B doping. In addition, through the element mapping technology, the uniform distribution of each element in the material was revealed, showing the uniformity of the material.

Figure 2: Characterization of the microscopic morphology and structure of the NNMBO layered cathode

Point 3: Excellent storage performance of sodium-ion batteries

After in-depth exploration of the microscopic morphology and structure of NNMBO materials, the NNMBO materials were assembled into half-cells to comprehensively evaluate their electrochemical properties. The test results show that NNMBO exhibits high electrochemical reversibility capacity and long-term cycling stability when used as a battery cathode material. As shown in Figure 3, the charge-discharge curves of the NNMBO electrode material are smoother compared to the comparison samples, which is attributed to the restricted phase transition reaction, resulting in significantly improved cycling stability. Specifically, at a current density of 1.0 C, the NNMBO material exhibits an electrochemical capacity of up to 97.8 mAh g⁻¹ and remains as high as 90.5% after 200 cycles. In addition, at a high current density of 5.0C, the NNMBO cathode material still maintains 85.2% capacity after 1000 cycles, which fully proves its excellent long-cycle stability. In order to explore the potential of commercial applications, it is combined with commercial HC anodes to assemble full-cell devices. This is shown in Figure 4.

In rate performance tests, the NNMBO material exhibited an electrochemical capacitance of up to 65.9 mAh g⁻¹ at a current density of 0.1 C, maintaining a capacity of 32.5 mAh g⁻¹ (based on the total mass of the cathode and anode materials) even at a high current density of 5.0 C. In addition, the all-cell device achieves an energy density of up to 192.8 Wh kg⁻¹ at a power density of 26.1 W kg⁻¹ and a power density of 90.4 Wh kg⁻¹ at an increased power density of 1226.75 W kg⁻¹. After evaluating the cycling performance of the full-cell device, we found that the capacity retention rate was as high as 86.0% after 100 cycles at a current density of 0.5C. More remarkably, a single pouch cell successfully assembled with NNMBO cathode and HC anode can maintain 70.8% capacity after 150 cycles at a current density of 0.1C, which not only verifies the great potential of NNMBO materials in practical applications, but also provides strong support for its commercialization process.

Figure 3: Characterization of the electrochemical properties of the NNMBO layered cathode in sodium-ion half-cells

Figure 4: Application of NNMBO layered cathode in full-cell devices

Point 4: In-situ testing explores the evolution of material structure in the cycle

To gain a better understanding of how the NNMBO composite cathode works, we utilize in-situ XRD testing to monitor the structural evolution of the material during electrochemical cycling. As shown in Figure 5, during the first charge, the characteristic peak of the O3 phase of the NNMBO gradually shifts towards a low angle, and a new peak appears, which has the same position as the P2 phase and is preliminarily judged to be the P3 phase. As the charging process continues, the O3 phase gradually disappears through the sliding of the TMO2 layer, and finally completely transforms into the P3 phase. During the subsequent charging, the peak value of the P3 phase remains stable until a high voltage of 4.0 V is reached.

When the cell is fully discharged to 2.0 V, we observe that the P3 phase of the NNMBO can be directly reversed to the O3 phase without going through other intermediate phases. It is worth noting that the P2 phase of NNMBO did not undergo significant peak shift during the whole cycling process, indicating that the presence of the P2 phase is essential to maintain the structural stability of the material, thereby enhancing the cycling performance of the NNMBO cathode. After calculating the volume change during cycling, we found that the volume change of NNMBO during charging was only about 1.3%, which was lower than the 3.5% of the NNMO of the comparison sample. The results show that NNMBO has almost zero strain structural change characteristics, which gives NNMBO a more stable material structure.

Figure 5: In-situ mechanism study of the layered cathode of NNMBO during cycling

【Article Link】

Nonmetal Substitution in Interstitial Site of O3-NaNi0.5Mn0.5O2 Induces the Generation of a Nearly Zero Strain P2&O3 Biphasic Structure as Ultrastable Sodium-Ion Cathode

https://doi.org/10.1002/adfm.202406771

【About the Corresponding Author】

Professor Zhang Genqiang's profile: Professor of University of Science and Technology of China, doctoral supervisor, dual-employed researcher of Hefei National Scientific Research Center for Microscale, national overseas high-level talent, Elsevier China Highly Cited Researcher, member of the Advanced Ceramics Branch of the Chinese Materials Science Society, and young editorial board member of eScience, Infomat, SusMat and Nano Research. The research group is committed to the optimal synthesis of advanced functional nanomaterials and their application in energy devices, including the application of electrode materials for energy storage devices, the design and synthesis of high-performance electrocatalysts, and the application of novel composite nanostructures in the field of energy storage and conversion. To date, in Nat. Commun.、Sci. Adv.、Adv. Mater.、Angew. Chem. Inter. Ed and other internationally renowned academic journals have published more than 140 SCI research papers, with more than 11,000 citations and an H-factor of 55.