Regulating Surface Redox Activity in Li-Rich Layered Oxides via Band-Aligned Vanadium Phosphate Coatings

With a specific energy density &gt;250mAhg^-1and average voltage of ~3.6V, Li-rich layered transition metal oxides (LLOs) are a commercially interesting high energy cathode material for the next generation of Li-ion batteries. However, LLOs critically suffer from accelerated capacity and voltage decay caused by a layered-to-spinel phase transition triggered by the involvement of O 2p states and formation of electron holes, causing lattice destabilization during redox[1]. This structural transformation is recognized as a surface-propagating phenomenon, so surface modification to improve chemical bonding stability with oxygen has become a primary approach to restrain structural transformation and its associated side effects (i.e., O₂ evolution, electrolyte side reaction, transition metal dissolution etc.) [2] . However, the behavior of the electronic structure at the interface between surface-modifier and bulk of LLOs is generally less investigated but is potentially an important aspect for oxygen redox charge transfer as well as general structural stability during cycling.<br/><br/>In this work, we systematically investigate the influence of redox-active vanadium phosphate (VP) coatings, Li₃V₂(PO₄)₃ (LVP) and Na₃V₂(PO₄)₃ (NVP) on the electronic structure, surface redox activity and electrochemical performance of a typical LLO (Li_1.2Mn_0.54Ni_0.13Co_0.13O₂) using both experimental investigation and <i>ab initio</i> DFT calculations. Experimentally, XPS, UPS, UV-Vis-NIR and ex-situ O K- and Ni, Mn, Co L-edge soft X-ray absorption spectroscopy (sXAS) were used study the redox reactions of the pristine and VP-LLOs and to map their intrinsic surface and interfacial electronic structures. Ex-situ sXAS comparison of pristine and VP coatings during both initial and long-term cycling shows significant reduction in O 2p-TM 3d hole formation at the surface (PEY, TEY) and better redox reversibility during cycling of VP-coated samples. Electronic structure investigation via DFT and spectroscopic techniques indicate evidence of electronic band alignment and a reduced surface polarization for VP-LLOs compared to pristine sample surfaces. Practically, we find VP-coated LLOs offer improved charge transfer kinetics and exhibit far better capacity and voltage retention than pristine samples with up to 90% capacity retention and -0.18eV median voltage fade after over 100 cycles. These results point to the practical utility of considering the electronic compatibility of surface modification agents at the interface in addition to current ionic transport, phase compatibility and chemical stability considerations for aiding charge transfer and alleviating structural deterioration in high energy Li-rich layered oxide battery cathodes.<br/><br/>[1]W. He, W. Guo, H. Wu, L. Lin, Q. Liu, X. Han, et al., Challenges and Recent Advances in High Capacity Li-Rich Cathode Materials for High Energy Density Lithium-Ion Batteries, Advanced Materials, vol. 33, p. 2005937, 2021.<br/>[2]S. Zhao, Z. Guo, K. Yan, S. Wan, F. He, B. Sun, et al., Towards high-energy-density lithium-ion batteries: Strategies for developing high-capacity lithium-rich cathode materials, Energy Storage Materials, vol. 34, pp. 716-734, 2021.