Insights into Li‐Rich Mn‐Based Cathode Materials with High Capacity: from Dimension to Lattice to Atom

Abstract

of electrode reaction by suppressing the Mn 4+ /Mn 3+ reduction reaction. [3c] Although a series of effective coating, doping and structural design strategies previously reported have been put into practice and significantly improved the electrochemical performance and structural stability from the macro perspective, plenty of the underlying and fundamental reaction mechanisms have not yet been elucidated clearly. [6][7][8] From a more microscopic dimension, root of the decline in electrochemical performance include escape of oxygen atoms, redox of cations and anions, and phase transformation, in which such microscopic characterizations require cutting-edge technologies. Recently, tremendous efforts have been devoted to investigating complex reaction mechanisms behind the structure with the help of state-of-the-art characterization techniques. [8] In general, the superior electrochemical behavior of Li-rich Mn-based cathode materials is not only derived from the contribution of transition metal cations, but also closely related to the anionic redox activity, especially reflected in the unique first charge process. [9] The initial charge process usually consists of two regions: sloping region and plateau region. The first stage occurs below 4.4 V, and lithium ions are extracted from the lithium layer along with the oxidation of transition metal ions to high valence state. [10,11] While the second stage presents a long plateau that appears above 4.4 V and stands for the activation of Li 2 MnO 3 phase. During the plateau region, lithium ions are continuously extracted and interact with oxygen in the form of Li-O-Li configuration as charge compensation. [12] Whereas, the specific role and mechanism of anionic redox activity in electrochemical reactions are still in dispute. This is because oxygen can participate in both reversible and irreversible redox reactions. What kind of intermediate form does oxygen take part in the reversible process? Where does the irreversible oxygen loss originate from, bulk lattice or electrolyte oxidation? How do these two parts contribute to capacities independently and influence mutually? These issues seriously challenge further development and need to be clarified in detail. In addition, the original structure is prone to transition metal migration and phase transformation due to anionic redox activities. [13,14] It is worth noting that not only the intrinsic structure of Li-rich Mn-based cathode materials is very complicated, but also its evolution mechanism of phase transformation during cycling is even more controversial. Furthermore, the lack of powerful characterization tools

Significant Enhancement of the Capacity and Cycling Stability of Lithium-Rich Manganese-Based Layered Cathode Materials via Molybdenum Surface Modification

“…All peak potentials are higher than those of the LMR sample, indicating that the Mo modification strategy effectively increased the reversibility of the Ni 2+ / 4+ redox and hindered the phase transition from layer to spinel. In the next two CV curves, the same trend can be observed, indicating the reduction in irreversible capacity loss, as well as oxygen release, at the activation of Li 2 MnO 3 [ 52 , 53 , 54 , 55 , 56 , 57 ], which resulted in a better initial coulombic efficiency and cycling stability for this lithium-rich cathode material.…”

Redox Hyperactive MOF for Li+, Na+ and Mg2+ Storage

“…The comparison manifests that the designed MOF exhibits twice as high energy density in respect to both Li + and Mg 2+ . Moreover, the energy density of MOF-S21 towards Li + exceeds that of Li-rich oxides, which are now considered as next-generation electrode materials for LIBs (i.e., energy density is higher than 400 W h/kg) [ 55 ].…”

Multi-dimensional correlation of layered Li-rich Mn-based cathode materials

“…Based on the traditional view and recent research progress into LRMOs [22][23][24] , this review elaborates the intrinsic structure, electrochemical properties and their relationship in the scope of multiple spatial scales (atomic scale, local structure, particle morphology and so on). The temporal clues of these materials in electrochemical processes also demonstrate the structure and energy band changes that cause inconsistency in the initial charge-discharge behavior, the influence of the local structural evolution on the electrochemical performance during cycling and the restrictive factors of the sluggish electrode reaction kinetics.…”