ABSTRACT
Ni-rich LiNixCoyMn1-x-yO2 (0.5 < x < 1) cathode materials have attracted considerable interest due to their high energy density and low cost. However, they are subject to capacity fading during cycling, such as structural degradation and irreversible oxygen release, especially under high voltage. Herein, we report an in situ epitaxial growth strategy to construct a thin layer of LiNi0.25Mn0.75O2 on the surface of LiNi0.8Co0.1Mn0.1O2 (NCM811). Both of them share the same crystal structure. Interestingly, the LiNi0.25Mn0.75O2 layer can be electrochemically converted into a stable spinel LiNi0.5Mn1.5O4 (LNM) due to the Jahn-Teller effect under high voltage cycling. The derived LNM protective layer can effectively alleviate the harmful side reactions between the electrode and electrolyte and suppress oxygen release as well. Furthermore, the coating LNM layer can enhance Li+ ion diffusion due to its three-dimensional channels for Li+ ion transport. When used as half-cells with lithium as the anode, NCM811@LNM-1% realizes a large reversible capacity of 202.4 mA h g-1 at 0.5 C, with high capacity retention of 86.52% at 0.5 C and 82.78% at 1 C, respectively, after 200 cycles in the voltage range of 2.8-4.5 V. Moreover, the assembled pouch full-cell with NCM811@LNM-1% as cathode and commercial graphite as an anode can deliver 11.63 mA h capacity with a high capacity retention of 80.05% after 139 cycles in the same voltage range. This work demonstrates a facile approach to the fabrication of NCM811@LNM cathode materials for enhancing performance in lithium-ion batteries under high voltage, rendering its promising applications.
ABSTRACT
Owing to the capacity boost from anion redox activities, cation-disordered rock-salt oxides are considered as potential candidates for the next-generation of high energy density Li-ion cathode materials. Unfortunately, the anion redox process that affords ultra-high specific capacity often triggers irreversible O2 release, which brings about structural degradation and rapid capacity decay. In this study, we present a partial chlorine (Cl) substitution strategy to synthesize a new cation-disordered rock-salt compound of Li1.225Ti0.45Mn0.325O1.9Cl0.1 and investigate the impact of Cl substitution on the oxygen redox process and the structural stability of cation-disordered rock-salt cathodes. We find that partial replacement of O2- by Cl- expands the cell volume and promotes anion redox reaction reversibility, thus increasing the Li+ ion diffusion rate and suppressing irreversible lattice oxygen loss. As a result, the Li1.225Ti0.45Mn0.325O1.9Cl0.1 cathode exhibits significantly improved cycling durability at high current densities, compared with the pristine Li1.225Ti0.45Mn0.325O2 cathode. This work demonstrates the promising feasibility of the Cl substitution process for advanced cation-disordered rock-salt cathode materials.
ABSTRACT
Cation-disordered rock-salt cathode materials are featured by their extraordinarily high specific capacities in lithium-ion batteries primarily contributed by anion redox reactions. Unfortunately, anion redox reactions can trigger oxygen release in this class of materials, leading to fast capacity fading and major safety concern. Despite the capability of absorbing structural distortions, high-ratio d0 transition-metal cations are considered to be unfavorable in design of a new cation-disordered rock-salt structure because of their electrochemically inactive nature. Herein, we report a new cation-disordered rock-salt compound of Li1.2Ti0.6Mn0.2O2 with the stoichiometry of Ti4+ as high as 0.6. The capacity reducing effect by the low-ratio active transition-metal center can be balanced by using a Mn2+/Mn4+ two-electron redox couple. The strengthened networks of strong Ti-O bonds greatly retard the oxygen release and improve the structural stability of cation-disordered rock-salt cathode materials. As expected, Li1.2Ti0.6Mn0.2O2 delivers significantly improved electrochemical performances and thermal stability compared to the low-ratio Ti4+ counterpart of Li1.2Ti0.4Mn0.4O2. Theoretical simulations further reveal that the improved electrochemical performances of Li1.2Ti0.6Mn0.2O2 are attributed to its lower Li+ diffusion energy barrier and enhanced unhybridized O 2p states compared to Li1.2Ti0.4Mn0.4O2. This concept might be helpful for the improvement of structural stability and electrochemical performances of other cation-disordered rock-salt metal oxide cathode materials.
