Synergistic Ti–Mg–Al Co-Doping Enhances Surface Stability and Electrochemical Performance of High-Voltage LiCoO2 Cathodes

Advancing lithium-ion batteries (LIBs) relies on the development of cathode materials that can offer higher specific capacities and able to operate at elevated voltages. Lithium cobalt oxide (LiCoO₂) has high theoretical capacity, excellent volumetric energy density, and robust cycling performance which make it excellent candidate material. However, despite these advantages, the practical use of LiCoO₂ is limited by structural and interfacial instability issues that arise when the material is subjected to high cut-off voltages which lead to performance degradation and safety concerns. When LiCoO₂ is charged beyond 4.5 V, it undergoes detrimental phase transformations such as the transition from the hexagonal O₃ phase to a hybridized O1–O3 hexagonal phase and this transition involves the sliding of CoO₂ layers and partial collapse of the O₃ phase structure and can result in the formation of microcracks within the particles. Additionally, the surface of LiCoO₂ becomes highly unstable at these elevated voltages, with significant oxygen loss which can lead to irreversible phase transitions and worsen safety risks. These structural and interfacial degradations severely impede the practical application of high-voltage LiCoO₂ in LIBs. To overcome these challenges, researchers have investigated various strategies to enhance the structural stability and cycle performance of LiCoO₂ at high voltages by doping the LiCoO₂ structure with different elements to improve its stability and performance with single-element doping showed some success and enhanced the cyclability of LiCoO₂ at moderate voltages (around 4.5 V). However, the effectiveness was found to diminish at higher voltages due to its limited ability to stabilize surface oxygen and inhibit parasitic reactions at the electrode/electrolyte interface. Given these limitations, there is a growing interest in synergistic doping strategies that combine multiple elements to achieve more significant improvements. Among these strategies, Ti–Mg–Al co-doping has emerged as a promising approach. Previous studies have indicated that this combination of dopants can significantly enhance the structural stability and electrochemical performance of LiCoO₂ at high voltages. However, the microscopic mechanisms behind these synergistic improvements remain to be fully understood. To this end, new study published in Physical Chemistry Chemical Physics and conducted by Dr. Hongbin Lin, Dr. Xiumei Kang, Dr.Guigui Xu, Dr. Yue Chen, Dr. Kehua Zhong, Dr. Jian-Min Zhang, and led by Professor Zhigao Huang from the Fujian Normal University investigated the effects of Ti, Mg, and Al co-doping on the surface stability, electronic structure, and Li+ diffusion properties of LiCoO₂ to understand how these dopants interact at the atomic level to stabilize the material and enhance its performance at high voltages.

The researchers evalauted the doping formation energies to determine the stability and distribution preferences of Ti, Mg, and Al within the LiCoO₂ structure and they used density functional theory calculations with the Vienna Ab initio Simulation Package to model the LiCoO₂ (104) surface. The calculations revealed that Ti prefers to occupy the Co sites on the surface layer due to its lower formation energy compared to subsurface layers. This finding aligns with the experimental observations where Ti tends to enrich the surface of co-doped particles. Mg and Al, on the other hand, showed a tendency to diffuse into the inner layers which suggested a more homogeneous distribution throughout the particle. The results indicated that all dopants are energetically favorable with Ti–Mg–Al co-doping exhibit a stable configuration. To assess the impact of doping on surface oxygen stability, the authors calculated the formation energy of oxygen vacancies (EV(O)) in the surface layer. The undoped Li_0.333CoO₂ (104) surface exhibited negative EV(O) values, which indicated a propensity for oxygen loss at high delithiation states. When doped with Ti, the surface showed improved oxygen stability, as evidenced by positive EV(O) values for certain oxygen atoms. This improvement was further enhanced with Ti–Mg and Ti–Al co-doping, which suppressed oxygen loss more effectively. The TiIMgIAlI co-doped configuration was notable, with positive EV(O) values for all surface oxygen atoms which suggested a significant suppression of oxygen loss and enhanced surface stability at high voltages. Moreover, the authors analyzed the electronic density of states (DOS) for undoped and doped LiCoO₂ (104) surfaces to understand the impact of doping on electronic properties and found that doping reduced the band gap which can enhance the electronic conductivity of the material. For instance, the TiI-doped surface exhibited nonzero electron density at the Fermi level which indicated metallic behavior and improved conductivity. The TiIMgIAlI co-doped surface showed a band gap reduction, which is indicative of better electronic conductivity. The presence of unoccupied O 2p states in the undoped surface, which contribute to oxygen activity, was significantly suppressed in the doped surfaces, especially in the TiIMgIAlI co-doped configuration. This suppression reduces charge deficiency and enhances the stability of lattice oxygen which correlated with the improved oxygen vacancy formation energy findings.

The researchers used the climbing image-nudged elastic band method to calculate the energy barriers for Li⁺ diffusion on the LiCoO₂ (104) surface and found that the undoped LiCoO₂ (104) surface had a higher diffusion barrier compared to the bulk phase, which indicated more difficult Li+ hopping on the surface. Doping with Ti reduced the diffusion barrier facilitated Li+ transport. According to the authors, the TiI-doped surface improved Li+ diffusion kinetics with lower energy barriers which makes it more favorable for lithium diffusion compared to the undoped surface. Overall, the TiIMgIAlI co-doped surface did not significantly improve ionic conductivity, but the suppression of high-barrier pathways suggested an overall positive impact on Li+ transport. Furthermore, the authors’ findings highlighted the complex interplay between electronic and ionic properties in determining the overall electrochemical performance of LiCoO₂. While single-element doping with Ti enhances surface Li+ diffusion, it does not fully address the instability issues at high voltages. In contrast, Ti–Mg–Al co-doping significantly stabilizes surface oxygen, reduces the band gap, and improves electronic conductivity, albeit with a negligible impact on overall ionic conductivity. The superior surface stability of the TiIMgIAlI co-doped configuration at high delithiation states is promising and can improve the cycling stability and electrochemical performance of LiCoO₂ at high voltages.

In conclusion, Professor Zhigao Huang and colleagues demonstrated that Ti–Mg–Al co-doping can significantly improve the cycling stability and electrochemical performance of LiCoO₂ at high voltages which is advantageous for developing LIBs with higher energy densities with applications in portable electronics, electric vehicles, and renewable energy storage systems. Moreover, their observation of improved structural stability and reduced phase transition at high voltages mean that batteries using Ti–Mg–Al co-doped LiCoO₂ cathodes can maintain their performance over more extended cycles and this longevity translates to lower maintenance costs and longer service life for batteries with the feasibly of scaled up for commercial production