The effect of cation mixing controlled by thermal treatment duration on the electrochemical stability of lithium transition-metal oxides

Abstract

Lithium cathode materials have been considered as promising candidates for energy storage applications because of their high power/energy densities, low cost, and low toxicity. However, the Li/Ni cation mixing limits their application as practical electrode materials. The cation mixing of lithium transition-metal oxides, which was first considered only as the origin of performance degeneration, has recently been reconsidered as a way to stabilize the structure of active materials. Here we find that as the duration of the post-synthesis thermal treatment (at 500 °C) of LiNiCoMnO (NCM) was increased, the Li/Ni molar ratio in the final product was found to decrease, and this was attributed to the reduction in nickel occupying lithium sites; the cation mixing subtly changed; and those subtle variations remarkably influence their cycling performance. The cathode material with appropriate cation mixing exhibits a much slower voltage decay and capacity fade during long-term cycling. Combining X-ray diffraction, Rietveld analysis, the Fourier transform infrared technique, field-emission scanning electron microscopy, and electrochemical measurements, we demonstrate that an optimal degree of Ni occupancy in the lithium layer enhances the electrochemical performance of layered NMC materials and that this occurs through a "pillaring" effect. The results provide new insights into "cation mixing" as a new concept for material design utilization of layered cathodes for lithium-ion batteries, thereby promoting their further application in lithium-ion batteries with new functions and properties.

Ti‐Gradient Doping to Stabilize Layered Surface Structure for High Performance High‐Ni Oxide Cathode of Li‐Ion Battery

“…Consequently, the amount of Ni 2+ increases and results in more Li/Ni anti-sites at the particle surface, stabilizing Li layers during Li insertion/extraction. [11] The strategy of constructing Mn-rich surface, including core-shell and concentration-gradient structure, [9] can also effectively protect the particle surface of high-Ni layered oxides, and deliver excellent electrochemical performance. Nevertheless, such doping strategy could not completely prevent the formation of the detrimental surficial residues, including rock-salt phase and Li 2 CO 3 at the particle surface.…”

Inducing Favorable Cation Antisite by Doping Halogen in Ni‐Rich Layered Cathode with Ultrahigh Stability

“…Most studies think Ni at Li sites block the Li diffusion path. [18][19][20][21] The optimal degree of Ni 2+ occupancy in the Li + sites can improve the electrochemical performance of layered Li(Ni x Co y Mn z )O 2 (NMC) materials and offer an electrostatic repulsion force to prevent more transition metal migration. [12][13][14][15] In order to reduce antisite concentration, many approaches are utilized to improve the electrochemical performance, such as adjusting synthetic process, surface coating, and element doping.…”

“…Moreover, the Ni 2+ takes active Li + place, which result in first-cycle reversible capacity loss and poor layered structure stability. [19] Cho et al found that the coating Ni-based material with a thin antisite layer demonstrated an excellent cycling property under elevated temperature test, which is mainly ascribed to the unique pillar layer including moderate antisite concentration. Zhou and co-workers reduced the antisite of LiNi 0.8 Co 0.15 Al 0.05 O 2 materials via control of oxygen flow rate during sintering.…”

“…Zhang and co‐workers reported a novel multishelled Ni‐rich Li(Ni x Co y Mn z )O 2 hollow fibers with low antisite as high‐performance cathode materials . On the contrary, few believe that antisite can play a good role in enhancing the stability of layered Ni‐rich cathode material . The optimal degree of Ni 2+ occupancy in the Li + sites can improve the electrochemical performance of layered Li(Ni x Co y Mn z )O 2 (NMC) materials and offer an electrostatic repulsion force to prevent more transition metal migration .…”

“…On the contrary, few believe that antisite can play a good role in enhancing the stability of layered Ni‐rich cathode material . The optimal degree of Ni 2+ occupancy in the Li + sites can improve the electrochemical performance of layered Li(Ni x Co y Mn z )O 2 (NMC) materials and offer an electrostatic repulsion force to prevent more transition metal migration . Cho et al found that the coating Ni‐based material with a thin antisite layer demonstrated an excellent cycling property under elevated temperature test, which is mainly ascribed to the unique pillar layer including moderate antisite concentration .…”