Next, the respective typical electrode structures, optimization strategies, electrochemical performance and charge storage mechanisms are discussed in detail to establish their chemistry-structure-property relationships. Based on different ion charge carriers, the current cathode materials are classified into four groups, including Al 3+-hosting, AlCl 4 -hosting, AlCl 2 +/AlCl 2+-hosting, and Cl -hosting cathode materials. First, the fundamental chemistry, charge storage mechanisms and design principles of RAB cathode materials are highlighted. Pang Quanquan and and colleagues in Peking University focus on current understandings into the charge storage mechanisms of cathode materials in RABs from a chemical reaction point of view (Figure 1). In addition to nanotechnology-based electrode structure designs, the intrinsic chemical structures and charge storage mechanisms of cathode materials play an equally crucial role, if not more, in revolutionizing the battery performance. During the past years, intensive efforts have been devoted to developing new cathode materials and/or designing engineered nanostructures to greatly improve RABs’ electrochemical performance. However, compared with the aluminum anode side, the cathode materials face more problems including low specific capacity, relatively sluggish kinetics in most host structures and/or limited cycle lifespan, which pose the major challenge for RABs in further practical applications. Rechargeable aluminum batteries (RABs) have attracted great interests as one of the most promising candidates for large-scale energy storage because of their high volumetric capacity, low cost, high safety and the abundance aluminum. Due to the intermittent characteristic and uneven distribution of the renewable energy sources, developing low-cost and reliable large-scale energy storage devices is very important for supplying sustainable power into smart grids and/or building distributed micro-grids. To build a carbon-neutral society, one of the most effective strategies is to increase the proportion of clean and renewable energy (wind, tide, solar, etc.), which can notably lower greenhouse gas (CO 2) emission. In summary, the review highlights how the charge heterogeneity is probed and quantified using various sample environments and sheds light on the potential methods to mitigate or even eliminate the charge heterogeneity for improving battery performance.Image: Charge Storage Mechanisms of Cathode Materials in Rechargeable Aluminum Batteries view more Last but not least, studying the charge heterogeneity involves the use of advanced spectroscopic imaging techniques thus we also discuss the working principles of these techniques throughout the review. Third, the review also provides an in-depth analysis of the factors that govern the charge heterogeneity and summarizes the current efforts to eliminate the heterogeneity. These characteristics determine the ion and electron conducting pathways, which is the underlying mechanism for governing the redox propagation in electrodes. Second, we discuss the electrode-level heterogeneity that is mostly influenced by the electrode characteristics such as electrode porosity and tortuosity. The charge heterogeneity at this length scale is associated with ion reaction mechanisms such as solid solution and phase separation, structural defects such as grain boundaries, and morphological features such as facet termination. First, we discuss the particle-level charge heterogeneity. In this review, we provide a comprehensive discussion of the current research frontier in probing and quantifying the charge heterogeneity in intercalating lithium ion cathode materials and electrodes. The multiscale charge heterogeneity can be caused by many factors, including the intrinsic properties of battery particles, battery electrode formulation, electrochemical protocols, and external environment such as temperature. Significant efforts have been made to homogenize the charge distribution at all length scales to eliminate the local over-charge or over-discharge, and to improve the contribution of individual active particles to the overall capacity. Such non-uniformity, often called charge heterogeneity, negatively impacts the battery performance. Due to the heterogeneous nature of the battery materials and electrodes, the redox reactions take place non-uniformly at all length scales ranging from single particles, to multiple particles, and to battery electrodes. Beyond designing new materials, improving the utilization of current materials is a critical step towards this effort. Increasing the energy density and cycle life has been a continuing effort to improve lithium ion batteries.
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