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

Li-rich layered oxide cathode materials are regarded as an attractive candidate of next-generation Li-ion batteries (LIBs) to realize an energy density of >300 Wh kg–1. However, challenges such as capacity fade, cycle life, oxygen release, and structural transformation still restrain its practical application. Micro/nanotechnology is one of the effective strategies to enhance its structural stability and electrochemical performance. An in-depth understanding of the relationship between micro/nanostructures and the electrochemical performance of Li-rich layered oxides is undoubtedly important for developing high-performance cathode materials. Herein, Li1.2Ni0.13Co0.13Mn0.54O2 with different micro/nanostructures including irregular particles, microspheres, microrods, and orthogonal particles are synthesized. Starting from the amount of surface oxygen vacancies in the different structures, the influence of oxygen vacancies on every step during the charge–discharge processes is analyzed by experimental characterizations and theoretical calculations. It is indicated that intrinsic oxygen vacancies can enhance the electrical conductivity and decrease the energy barrier for ion migration, which exerts a significant influence on promoting the kinetics and capacity. Among the different micro/nanostructures, microrods with abundant oxygen vacancies can not only promote lithium ion transport but also stabilize a cathode electrolyte interface (CEI) film by adjusting the distribution of surface elements with lower nickel content. The microrods deliver an initial discharge capacity of up to 306.1 mAh g–1 at 0.1C rate and a superior cycle performance with a capacity retention of 91.0% after 200 cycles at 0.2C rate.

For rechargeable lithium−metal batteries (RLBs), gel polymer electrolytes (GPEs) are a very competitive and pragmatic option because the special composite structure could restrain the uncontrolled lithium dendrite in a liquid electrolyte and avoid the poor interface contact for a solid-state electrolyte. However, the difficulty lies in finding a delicate balance between ion transport and interface stability. Herein, a heterostructured GPE, in which a metal−organic framework layer and an ultrathin Al 2 O 3 deposition are coated on the same side of a polymer matrix, is fabricated to homogenize lithium ion transport and stabilize the lithium anode interface. With the heterostructured GPE, the Li + transference number is improved to 0.74, and the lithium metal electrode displays an enhanced cycle stability over 1000 h. Moreover, Li-rich Mnbased layered oxides, the high-capacity cathode material, are matched for the first time with a lithium−metal anode to assemble a quasi-solid-state RLB, which delivers an initial discharge capacity of 257.5 mAh g −1 with a long-cycle capacity retention of 84.6% after 500 cycles at a rate of 0.2C.

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As one of the most promising cathode materials in lithium-ion batteries, nickel-rich cathode materials have been widely studied due to their high specific capacity and high operating voltage to realize the energy density of 300 Wh kg −1 . However, the oxidative decomposition of electrolyte catalyzed by transition metal ions and the crack of secondary particles have brought great challenges to their further development. In order to solve this problem, functionalized electrolyte is often used to stabilize the interface and structure of cathode materials. Herein, it is proposed to add trimethyl borate (TMB) in commercial electrolyte to enhance the interfacial stability of Li-Ni 0.88 Co 0.09 Al 0.03 O 2 cathode material. By adjusting the content of TMB in the electrolyte, it is found that a volume ratio of 10% can achieve the best electrochemical performance. In the voltage range of 3.0−4.3 V, the LiNi 0.88 Co 0.09 Al 0.03 O 2 electrode with the 10% TMB-containing electrolyte can achieve a capacity retention of up to 82% after 300 cycles at 1C rate (1C = 200 mA g −1 ), while the electrode with blank electrolyte is only 61%. Through a systematic analysis toward cycled electrode materials, it is indicated that the cracking degree of secondary particles decreases and a thin, stable, and conductive cathode electrolyte interface film is formed on the electrode with 10% TMB-containing electrolyte. This simple and effective method of improving the interfacial and structural stability of nickel-rich cathode materials via adding TMB in the electrolyte provides a practical strategy for the development of highperformance lithium-ion batteries.