Biochar is considered as key anode material for alkali metal (lithium, sodium, and potassium) ion batteries (AIBs) owing to its rich microstructural features, high specific surface area, active sites, excellent conductivity, and mechanical strength. The multidimensional structures and diverse functional groups of biochar make it enable easy modification to improve ion transport, interface deposition behavior, and electrolyte stability. In addition, biochar-based derivatives, such as silicon/biochar composite anode materials, combine the advantages of high-energy density and low lithiation potential of silicon materials, as well as the superior conductive ability and outstanding mechanical qualities of biochar. In this review, the microstructure, properties, and synthesis methods of biochar materials are systematically clarified, and then, their applications in AIBs are presented followed by summarizing the energy storage mechanism and advanced physicochemical characterizations. Common structural configurations and preparative technique for biochar/silicon-based composites are summarized, such as core–shell, yolk–shell, and embedded coating structures with improved electrochemical and mechanical stability. Finally, toward practical application of biochar and biochar-based derivatives in future AIBs, the issues and challenges are outlined.

State-of-the-art evolution of biochar in alkali metal ion (Li, Na, K) batteries’ applications
    Abstract
        Graphical abstract
    1 Introduction
    2 Micro–nano-structures, properties, and synthesis methods of biochar
        2.1 Dimensional designing of biochar
            2.1.1 Zero-dimensional
            2.1.2 One-dimensional
            2.1.3 Two-dimensional
            2.1.4 Three-dimensional
        2.2 Microstructural characteristics of biochar
            2.2.1 Effective specific surface area
            2.2.2 Pore size distribution
            2.2.3 Surface functional groups
        2.3 Approaches for generating biochar
            2.3.1 High-temperature solid-state sintering method
            2.3.2 Hydrothermal carbonization
            2.3.3 Chemical vapor deposition (CVD) method
        2.4 Energy storage process and mechanism
            2.4.1 Lithiumsodiumpotassium storage mechanism in biochar
            2.4.2 Advanced physicochemical characterizations
    3 Utilization of biocharsilicon-based materials in AIBs
        3.1 Structural design of biocharsilicon-based composites
            3.1.1 Core–shell structure
            3.1.2 Yolk–shell structure
            3.1.3 Embedded coating structure
        3.2 Preparation methods of biocharsilicon composites
            3.2.1 Hydrothermal synthesis
            3.2.2 Mechanical ball milling method
            3.2.3 Thermal reduction method
    4 Conclusion and outlook
    Acknowledgements 
    References

• Bin Yang(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China)
• Liuliu Liu(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China)
• Keqi Chen(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China)
• Keyu Zhang(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China) Corresponding author
• Xinyu Jiang(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China)
• Rui Yan(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China)
• Shaoze Zhang(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China)
• Yin Li(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China)
• Junxian Hu(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China)
• Yaochun Yao(National Engineering Research Center of Vacuum Metallurgy, Faculty of Metallurgy and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China, National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China)