Structure and Charge Regulation Strategy Enabling Superior Cyclability for Ni‐Rich Layered Cathode Materials

Abstract

Ni‐rich layered oxides are significantly promising cathode materials for commercial high‐energy‐density lithium‐ion batteries. However, their major bottlenecks limiting their widespread applications are capacity fading and safety concerns caused by their inherently unstable crystal structure and highly reactive surface. Herein, surface structure and bulk charge regulation are concurrently achieved by introducing high‐valence Ta5+ ions in Ni‐rich cathodes, which exhibit superior electrochemical properties and thermal stability, especially a remarkable cyclic stability with a capacity retention of 80% for up to 768 cycles at a 1C rate versus Li/Li+. Due to the partial Ta enrichment on surface, the regulated surface enables high reversibility of Li+ insertion/extraction by preventing surface Ni reduction in deep charging. Moreover, bulk charge regulation that boosts charge density and its localization on oxygen remarkably suppresses microcracks and oxygen loss, which in turn prevents the fragmentation of the regulated surface and structural degradation associated with oxygen skeleton. This study highlights the significance of an integrated optimization strategy for Ni‐rich cathodes and, as a case study, provides a novel and deep insights into the underlying mechanisms of high‐valence ions substitution of Ni‐rich layered cathodes.

Unblocking Oxygen Charge Compensation for Stabilized High‐Voltage Structure in P2‐Type Sodium‐Ion Cathode

“…Starting from a Li-intercalated layered structure (Figure 1a), the framework exhibits a continuously evolved interlayer contraction when deeply charged, which is sustainable for long-term cycling. [25,26] A charge compensation mechanism between highly hybridized TM-3d and O-2p states has been thought to be responsible for this moderate solid solution behavior. [27,28] In detail, when most of the lithium ions are extracted from the lattice at high voltage, the interlayer distance is governed heavily by the O-O electrostatic force between the layers.…”

The Relationship between Initial Coulombic Efficiency and Transition Metal Ion Redox in P2-Na0.85[Cu0.1FexMn1–x]O2 Cathodes

“…However, when the Fe content was more than 0.30, the initial valence state of Mn ions showed a downward trend, as shown in Figure b. It is speculated that the abnormal phenomenon may be because high Fe content makes Fe ions locally clustered together, providing local additional electrons, thereby reducing the adjacent Mn oxidation state. , It may also be that high Fe doping leads to Mn ions occupying both octahedral and tetrahedral positions, while Mn ions in a tetrahedral may be a mixture of divalent ions and trivalent ions, thereby resulting in the decrease of the average valence state of Mn . The initial valence states of Fe and Mn ions are trivalent and divalent, respectively.…”

Understanding of the Irreversible Phase Transition and Zr-Doped Modification Strategy for a Nickel-Rich Cathode under a High Voltage

“…Based on the above concerns, in response, extensive studies have been carried out to address the existence of nickel-based cathodes, including an insight into the structural degradation mechanism and further modification. Many reports have provided evidence of the structural failure mechanism in nickel-based cathodes from an atomic-scale perspective. − In the process of excessive lithium deintercalation (increased formation of lithium vacancies), the transition metal (TM) migrates from its octahedral sites (TMO 6 ) to the alternating lithium octahedral sites (LiO 6 ), which is considered to be the first step for the structural degradation of Ni-rich cathodes. , Among three metal ions, nickel ions are the first cation that migrates to the lithium layer in a charged NCM-layered cathode, so a higher proportion of Ni would be more susceptible to structural instability . Additionally, the continued migration of Ni 4+ ions into Li octahedral sites after deeply charging causes the formation of a LiMn 2 O 4 -type spinel phase and even an MO-type rock-salt structure .…”

“…Among the proposed strategies, lattice doping is one of the simplest and most effective methods to solve the abovementioned problem. Bulk doping with Na + , Mg 2+ , Al 3+ , Zr 4+ , Nb 5+ , and W 6+ in a transition metal layer or F – , S 2– , BO 3 3– , and PO 4 3– in an oxygen layer has been widely reported. ,,− Despite the failure mechanism, a modification strategy has been proposed and reported, and the connection between the electrochemical behaviors and the structural degradation mechanism of the nickel-rich cathode in a wide voltage window of 3–4.6 V has rarely been studied. Understanding the electrochemical reaction mechanism of nickel-rich cathode materials under different voltage windows and looking for the corresponding modification strategies are the key to enhancing comprehensive performance.…”