【Background】
In lithium-ion batteries, layered materials have demonstrated the potential for high energy density and are the material of choice for the development of high-range electric vehicles, but the bottleneck limiting high energy density has been unresolved. Taking layered NCM materials as an example, in the state of high delithiumization, the layered structure evolves violently, the structural stability and ion diffusion process are intricate, which involves the generation of a variety of defects and the occurrence of side reactions, such as cation mixing, oxygen vacancy formation, gas precipitation, transition metal dissolution, and electrolyte decomposition. From the level of electronic structure, the charging process of the cathode material corresponds to the extraction of electrons from the Fermi level under the action of external voltage, resulting in the redistribution of electron density in real space and the relaxation of the crystal structure until the system reaches a new stable state. When the system is highly unstable, the material has a strong tendency to seek a more stable energy point on the potential energy surface, resulting in transition metal migration, oxygen-oxygen polymerization, etc., at this time, the crystal structure and the corresponding electronic structure may undergo substantial changes, and gradually intensify during the cycling process, causing serious voltage hysteresis and capacity attenuation of the material. For the "outside-in" modification strategy of materials, it is often necessary to use the "inside-out" scientific understanding as the basis to seek the possibility of breaking through the bottleneck of high energy density. The key here is to identify the nodes where the electronic structure of the material is unstable, as well as the main path characteristics of the material decay, and find ways to thermodynamically suppress or kinetically slow down the occurrence.
For traditional layered cathode materials, such as NCM ternary materials, the existing studies have clearly shown that there is a dynamic evolution of cation mixing in the bulk phase, as well as the accumulation of transition metals and serious irreversible phase transformation near the surface, which indicates that the actual layered materials are always in a state of partial disorder associated with the mixing in the working process, and there is a break of crystal symmetry. It can be expected that compared with the layered ordered state, the electronic structure will change significantly, that is, there is a phenomenon of electronic recombination, which may contain a new redox process. The determination of this form of change is crucial because it acts as a bridge for the material to change from layered order to partial disorder and then degenerate to a more symmetrical halite facies, dominating the generation and evolution of layered defects.
【Brief Introduction】
Recently, the research group of Professor Lu Xia of Sun Yat-sen University has systematically studied the electron recombination phenomenon induced by cation mixing in traditional layered cathodes based on first-principles calculations. From the perspective of coordination environment and molecular orbitals, the basic form of cation mixing-induced electronic structure recombination was determined, as well as the essential reasons for this phenomenon. During the disordering process, cation mixing introduces local TM-rich region and Li-rich region into the layered structure, where the TM-rich region has an electron trapping center due to site distortion, and the Li-rich domain is characterized by the appearance of O:2p lone pairs. In the high delithiumation state, degeneracy and coupling can occur between these two electron states, resulting in the excitation of additional lattice oxygen activity and the inhibition of transition metal redox, which belongs to a state of energy excitation. For structural reversibility, this electronic recombination increases the likelihood of oxygen dimer or oxygen generation in the system, and establishes a mutually reinforcing thermodynamic relationship between cation mixing and oxygen loss. It is predicted that there will be a "self-sustaining" decay path in the material, with cation mixing as the nucleation site extending outward, and eventually an irreversible phase transition will be derived. These results suggest that electronic recombination is a key step and node in the severe degradation of traditional layered materials. By manipulating the band structure, the recombination process of the electronic structure can be adjusted and standardized, which is conducive to improving the electrochemical stability of the material. The above related content was published in the international journal Chemistry of Materials with the title of "Regulating Cation Disorder Triggered-Electronic Reshuffling for Sustainable Conventional Layered Oxide Cathodes", and Chen Weixin, a doctoral student at Sun Yat-sen University, is the first author of this paper.
【Main content】
Fig.1 Characteristics of electronic structure recombination induced by cationic mixing
Point 1: Based on the SCAN functional, the electronic structures before and after cation mixing in materials under different states of charge are compared in detail, and the general reaction equation of Li/Ni mixing-induced electron recombination is determined, namely
After the transition metal migrates to the lithium layer, spontaneous reduction will always occur and electron holes will be left behind, and the electron states near the Fermi level will receive additional holes and be oxidized, thus maintaining electrical neutrality. This electronic recombination behavior occurs throughout the charge-discharge process, and its specific form is related to the state of charge. Moreover, different types of mixing have similar equations, and there is a universality in layered materials, which leads to a rich variety of redox behaviors. Especially in the high delithiumization state, the electron recombination behavior can cause the excitation of lattice oxygen activity in the traditional layered cathode.
Fig. 2 Source of electron recombination in the highly delithiumated state
Point 2: The source of the electronic structure reorganization was identified through local coordination environment analysis. The cation mixing introduces two additional special regions of Li-rich and TM-rich in the layered ordered system. Due to the site distortion, the transition metal has a low coordination environment during migration, and the energy level of the antibond orbital decreases, which increases the ability of the transition metal to obtain electrons from the antibond holes, and on the other hand, the Lone Pair of O:2p electrons appears in the Li-rich environment, and the energy level of oxygen rises. That is, the two electronic states move in opposite directions. In the disordered process, when the energy levels of the two states can be degenerate or even reversed, electron recombination will occur, resulting in the reduction of transition metals and the stimulation of lattice oxygen. From a redox perspective, this indicates that the activity of the transition metal is partially limited, while the lattice oxygen activity is promoted. This electronic reorganization is a self-adjusting process in a layered ordered framework to adapt to local disorders, and is an energetically excited state.
Fig.3 Relationship between electron recombination and lattice oxygen reversibility
Point 3: Based on the restriction molecular dynamics simulation, the potential energy surface of oxygen ions forming oxygen dimers was explored. It was determined that after the recombination of electrons, the crystal structure would be significantly unstable compared to the layered ordered state. The electron holes on the reactive oxygen ions can be partially stabilized by the π bonding of TM-O, maintaining a certain degree of lattice oxygen redox reversibility, but there is a strong tendency to form oxygen dimers, increasing the risk of oxygen loss of materials, especially in the surface region.
Fig. 4 Decline path of electronic reorganization and "self-sustaining".
Point 4: Thermodynamic calculations show that there is a reciprocal thermodynamic relationship between lattice oxygen stability and cation mixing due to electronic recombination. The cation mixing will cause the instability of the lattice oxygen of the material, promote the loss of oxygen and the formation of oxygen vacancies, and this process is in turn conducive to the continuation of the cation mixing and further electronic recombination. It is predicted that there is a "self-sustaining" degradation path in the process of decay, with cation mixing as the nucleation site, and through alternating mixing and oxygen loss processes, the material is gradually extended outward for irreversible phase transformation. During the long cycle, the unstable Li-rich region gradually decomposes, prompting it to transform into a more stable TM-rich region, and finally forms a defective halite facies. If lattice oxygen can maintain reversibility, it will support a dynamically stable cation mixing change (equilibrium between generation and death) without significant irreversible phase transitions. This "self-sustaining" process can be exacerbated by external conditions (e.g., temperature increases), phase boundaries of heterogeneous systems (e.g., solid-liquid interface reactions), or lattice defects (e.g., grain boundaries, cracks, surface overhang bonds). These results reveal that electronic recombination is a key step and node for severe structural degradation and performance degradation of traditional layered materials.
Fig.5. Regulation of electronic recombination to achieve cyclic reversibility
Point 5: The key to improving the reversibility of traditional layered materials is to adjust the form of electronic recombination to cut off the "self-sustaining" path. The results of electronic recombination based on multiple systems show that the form of electronic recombination is closely related to the band center of the layered oxide. By adjusting the TM:d and O:p band centers of the layered oxides, the material has a deeper O:2p band, such as improving ionic properties and high-valent ion doping, which can effectively limit the energy level degeneracy of transition metal antibond holes and oxygen lone pairs in the cation mixing process, so as to avoid the overactivation of lattice oxygen, so as to ensure the good electrochemical reversibility of the material.
Paper Links:
https://pubs.acs.org/doi/10.1021/acs.chemmater.3c02219
For articles on the research progress of high energy density cathode materials, please see: Wu, X.; Muhtar, D.; Zhang, D.; Lu, X.Y.; Lu, X., Research Progress on High Energy Density Cathode Materials for Lithium-ion Batteries. Chinese Science Bulletin 2024, doi: 10.1360/TB-2023-1088.
Link: https://doi.org/10.1360/TB-2023-1088
Note: Most of the articles reproduced on this site are collected on the Internet, and the copyright of the article belongs to the original author and the original source. The views in the article are only for sharing and exchange, if it involves copyright and other issues, please let us know, and I will deal with it in a timely manner.