The development of high-voltage cathode materials is crucial in satisfying the increasing demand for high-energy-density batteries in long-range automobiles and consumer electronic devices. However, several problems are apparent at high voltages. The equilibrium oxygen partial pressure is considerably increased at voltages of >4 V due to the release of oxygen from the lattice. Additionally, the cycle life of the battery is decreased because oxygen release leads to the migration of transition metals, irreversible phase transitions, and electrolyte contamination or depletion. Coatings applied to the surface may aid in mitigating this, but their performances are inconsistent and the coated materials are only slightly superior in terms of withstanding repeated cycling. Cathodes should exhibit high-quality surface designs that do not obstruct Li+ migration or inhibit the oxygen evolution reaction (OER).
Lanthurization is a novel strategy for use in passivating high-voltage cathode surfaces. The strategy is based on a liquid-ion exchange process developed by a team led by Fuqiang Huang of College of Chemistry and Molecular Engineering, Peking University (Beijing, China), Ju Li of the Massachusetts Institute of Technology (Cambridge, MA, USA), and Yanhao Dong of the School of Materials Science and Engineering, Tsinghua University (Beijing, China). This method may significantly enhance the high-voltage stability of the cathode and reaction potential of the OER. Using a highly regulated liquid-ion exchange method, uniform, compressively strained, lattice-coherent perovskite reconstruction layers, with thicknesses of several nanometers, were added to the surfaces of cathode particles. At a high voltage, the layers, with strong electronic and Li+ ionic conductivities but low oxygen ionic conductivities, drastically increase the overpotential of the OER and reversibly absorb oxygen ions into their oxygen vacancies. Nature Energy published the results of the research online on January 12, 2023 (https://doi.org/10.1038/s41560-022-01179-3), with the title "Stalling oxygen evolution in high-voltage cathodes by lanthurization."
Using LiCoO2 (LCO) as an example, Li+ close to the surface is first exchanged with La3+ and Ca2+ (major and minor exchange cations, respectively) in an aqueous solution to form an inserted La/Ca gradient. In this case, the solution completely soaks the solid LCO particles, and the slow ion exchange under ambient conditions enables uniform, customizable insertion. After annealing, the near-surface lattice is reconstituted into a buffer layer with a doped La/Ca gradient and protective shell with a Li-poor perovskite structure, both of which are epitaxial to the bulk LCO. The method is denoted lanthurization because of its similarities to the carburization of steel.
The electrochemical performance of lanthurized LCO (La-LCO) is significantly enhanced. Whereas the capacity of the unmodified material rapidly decreases to 0 within 200 cycles, La-LCO retains 79.8% of its initial capacity after 500 cycles at 1 C and 4.6 V in half cells. La-LCO retains 84.4% of its initial capacity after 500 cycles in high-loading pouch-type full cells, whereas the pristine material loses almost half of its initial capacity after only 300 cycles. Ni- and Li-rich layered cathodes may also benefit from this method. The high-voltage cycling performance of Ni-rich LiNi0.8Co0.1Mn0.1O2 is considerably enhanced via lanthurization, increasing the energy retention rate and capacity retention from 80.2% to 100% and 80.0% to 95.9%, respectively.
Characterization of the treated LCO cathode using structural and spectroscopic methods indicates that its high-voltage stability is due to its reversible oxygen storage and the low electrocatalytic activity of its surface perovskite layer. The oxygen vacancies of the surface perovskite layer, in particular, may be used to store the oxygen generated by the oxidized lattice at high voltages. Owing to the high reversibility of this process, the high-voltage OER may be suppressed by storing oxygen species that may subsequently be released and used to repair flaws in the bulk lattice. Furthermore, owing to the increased charge-transfer overpotential of the strained perovskite surface during electrolyte decomposition, the formation of CO2 and development of unstable cathode-electrolyte interphases are remarkably suppressed. As a result, potentially dangerous side reactions at high voltages are mitigated, enabling the stable operation of the cathodes at high voltages.
In conclusion, this work systematically analyzed the mechanism underlying the oxygen release that initiates degradation in high-energy-density layered cathodes and proposed a universal technique to enable high-voltage cycling of cathodes. The strategy displayed the following characteristics: (1) A highly uniform coating could be prepared via the regulated liquid-ion exchange reaction. (2) An oxygen buffer was formed by adding the compressive nanoscale perovskite coating to the surface. This coating used oxygen vacancies to reversibly absorb reactive oxygen species (ROS) at high voltages. The compressive strain significantly strengthened the ionic interactions between oxygen and the transition metals, preventing the migration of ROS to the surface. (3) A novel strategy for the industrialization of high-voltage Ni- and Li-rich layered cathodes is presented based on the universality of the method, which enables its large-scale application to layered oxide cathodes.
Mingzhi Cai, a Peking University PhD alumnus, and Dr. Yanhao Dong, an assistant professor at Tsinghua University, are the co-first authors of this study. The corresponding authors are Prof. Fuqiang Huang of Peking University and Prof. Ju Li of the Massachusetts Institute of Technology. This study was funded by the National Natural Science Foundation of China, Shanghai Science and Technology Commission, Chinese Academy of Sciences Key Research Program of Frontier Science, and Beijing National Laboratory for Molecular Sciences.
Original link of the study: https://doi.org/10.1038/s41560-022-01179-3
Fig. 1 Lanthurization and the surface architecture of lanthurized LiCoO2.
Fig. 2 Superior electrochemical performances of La-LCO.