Low‐Electronegativity Cationic High‐Entropy Doping to Trigger Stable Anion Redox Activity for High‐Ni Co‐Free Layered Cathodes in Li‐Ion Batteries

Abstract

LiNi0.8Co0.1Mn0.1O2 (NCM‐811) exhibits the highest capacity in commercial lithium‐ion batteries (LIBs), and the high Ni content (80%) provides the only route for high energy density. However, the cationic structure instability arisen from the increase of Ni content (>80%) limits the further increase of the capacity, and inevitable O2 release related to anionic structure instability hinders the utilization of anion redox activity. Here, by comparing various combinations of high‐entropy dopants substituted Co element, we propose a low‐electronegativity cationic high‐entropy doping strategy to fabricate the high‐Ni Co‐free layered cathode (LiNi0.8Mn0.12Al0.02Ti0.02Cr0.02Fe0.02O2) that exhibits much higher capacity and cycling stability. Configurational disorder originated from cationic high‐entropy doping in transition metal (TM) layer, anchors the oxidized lattice oxygen ((O2)n‐) to preserve high (O2)n‐ content, triggering the anion redox activity. Electron transfer induced by applying TM dopants with lower electronegativity than that of Co element, increases the electron density of O in TM‐O octahedron (TM‐O6) configuration to reach higher (O2)n‐ content, resulting in the higher anion redox activity. With exploring the stabilization effect on both cations and anions of high‐entropy doping and low‐electronegativity cationic modified anion redox activity, we propose an innovative and variable pathway for rationally tuning the properties of commercial cathodes.

Regulating the Charge Distribution of Oxygen by High-Entropy Doping to Stabilize Single-Crystal Ni-Rich Cathode Materials

“…23 A new peak at around 530.3 eV emerged after charging to the 4.3 V stage, which indicated the oxidation of O 2− in a highly delithiated state. 23 The proportion of oxidized lattice oxygen in HE811 (21.03%) is higher than that in 811 (11.22%), indicating that highentropy doping effectively promotes the reversible redox of lattice oxygen, thereby providing more capacity. After discharging to 2.8 V, the peak of oxidized lattice oxygen disappears and a P−O−F signal appears at 533.5 eV, 23 which is attributed to the decomposition reaction of LiPF 6 .…”

“…21 Due to high nickel content being the only way for Ni-rich cathodes to achieve high specific capacity, traditional near-equimolar strategies are not feasible. 22,23 Recently, high-entropy doping provides a new opportunity to solve the dilemma of high specific capacity and stability of Ni-rich cathodes. 24 Xin et al achieved a zero strain state of polycrystalline high-nickel materials during charge and discharge processes through high-entropy doping, confirming the enormous potential of this strategy and further developed by other researchers.…”

Microstructures of layered Ni-rich cathodes for lithium-ion batteries

“…A key question is how will the number and type of introduced foreign elements influence NRLO cathode's stability? For example, high-entropy NRLO cathodes have recently shown a possible advantage of high stability, 309,310 while more mechanistic investigations are necessary to realize their full potential.…”

Recent advances and understanding of high-entropy materials for lithium-ion batteries

“…The incorporation of cationic high-entropy doping in transition metal layer demonstrates higher cycling stability and capacity in comparison with the commercial NMC-811, originating from the relatively electronegative cations which increase the electron density of oxygen in transition metal to achieve higher oxidized lattice oxygen content, thereby triggering anion redox activity. Furthermore, the high-entropy doping by low-electronegative cations stabilizes the structure of the improved transition metal layer with high-Ni and Co-free, leading to capacity of 203.6 mAh g −1 at voltage of 2.5-4.5 V [100].…”