Reversible oxygen price mechanism of positive electrode of sodium ion battery

The embedded transition metal layered oxide (AMO2, A=Li+ or Na+, M=transition metal) is an important lithium ion/sodium ion battery cathode material. In the conventional concept, the redox reaction of the transition metal provides charge compensation during the ion deintercalation process, so the capacity of the positive electrode material is limited by the redox ability of the transition metal in the layered oxide material. However, this conventional concept has been challenged with the discovery of lithium-ion battery rich lithium layered oxide cathode materials (O3 structure Li[LixM1-x]O2). Lithium-rich materials have an ultra-high reversible specific capacity (300 mAh/g), but this source of capacity cannot be explained by only transition metal redox. Numerous studies have shown that lattice oxygen in lithium-rich layered materials participates in the electron-loss process, providing additional capacity. In fact, not only lithium-rich materials, many layered oxide materials can achieve an electrochemical process in which oxygen participates in charge compensation to provide additional capacity. However, how lattice oxygen participates in charge compensation and how to achieve reversible oxygen price change has always been a hot topic in the debate. The relationship between crystal structure and oxygen ion price change process is the key to explain its reaction mechanism.

[Introduction]

Recently, Associate Professor Yan Xiqian and Researcher Hu Yongsheng of the Institute of Physics of the Chinese Academy of Sciences published the latest research result "Structure-Induced Reversible Anio nic Redox Activity in Na Layered Oxide Cathode" in Joule, the new flagship journal of the energy field of Cell Press. The researchers collaborated with researchers at the Oak Ridge National Laboratory, Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, and the Stanford Linear Accelerator Center to conduct detailed research through advanced characterization methods such as neutron scattering and synchrotron radiation. The mechanism of sodium storage of P3-Na0.6[Li0.2Mn0.8]O2 sodium ion battery cathode material, which is involved in redox reaction and highly reversible, clarifies the structural reasons for the material to achieve reversible oxygen price.

[Graphic introduction]

Fig.1 Electrochemical performance of P3-Na0.6[Li0.2Mn0.8]O2

(a) Typical charge and discharge curves at 0.1 C and 2.0 C rates;

(b) Cyclic performance at 0.1 C and 2.0 C rates.

Fig.2 Comparative analysis of initial and charged states of P3-Na0.6[Li0.2Mn0.8]O2 nPDF and xPDF

The research team used neutron pair distribution function (nPDF) combined with X-ray and neutron diffraction techniques to study the crystal structure and oxygen-related oxygen of the P3 cathode oxide material before and after the electrochemical reaction. The short-range structural changes confirm the reversible bulk phase structure changes caused by the oxygen reversible valence. This research method is the first to study the crystal structure changes related to oxygen charge compensation.

Fig. 3 Analysis of the state of charge based on the neutron diffraction refinement result

The neutron powder diffraction refinement results show that the material structure after charging is still a P-phase layered structure, but accompanied by a large number of stacking faults. The oxygen occupancy obtained after the refinement was still 1, indicating that there was almost no loss of lattice oxygen after charging. By analyzing the above results, it is found that the crystal structure of the material has a key regulatory effect on its reversible oxygen valence behavior: the P structure has a large interlayer spacing (relative to the O3 phase) and can tolerate the lattice caused by the change in the O-O bond length. Distortion; at the same time, the larger layer spacing can effectively inhibit the migration of cations into the alkali metal layer during charging (the layered to spinel structure phase transition occurs in the lithium-rich material), maintaining a stable layered structure, thereby making oxygen ions The redox reaction is reversible.

【summary】

This study clarifies the mechanism of reversible redox reaction of oxygen ions from the perspective of material structure, and provides a new idea for designing high voltage and high capacity lithium/sodium ion cathode materials with stable and reversible oxygen pricing behavior. Class research introduces a powerful tool for neutron pair distribution functions (nPDF), which broadens the research dimension.

The research work was supported by the 973 project of the Ministry of Science and Technology (2014CB932300), the National Science Fund for Distinguished Young Scholars (51725206), the National Natural Science Foundation Innovation Research Group (51421002), and the Chinese Academy of Sciences Hundred Talents Program.

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