Recent Advances in 3D Array Anode Materials for Sodium-Ion Batteries

Lithium-ion batteries have achieved tremendous success in the fields of portable mobile devices, electric vehicles, and large-scale energy storage owing to their high working voltage, high energy density, and long-term lifespan.However, lithium-ion batteries are ultimately unable to satisfy increasing industrial demands due to the shortage and rising cost of lithium resources.Sodium is another alkali metal that has similar physical and chemical properties to those of lithium, but is more abundant.Therefore, sodium-ion batteries (SIBs) are promising candidates for next-generation energy storage devices.Nevertheless, SIBs generally exhibit inferior electrochemical reaction kinetics, cycling performance, and energy density to those of lithiumion batteries owing to the larger ion radius and higher standard potential of Na+ compared to those of Li+.To address these issues, significant effort has been made toward developing electrode materials with large sodiation/desodiation channels, robust structural stability, and high theoretical capacity.As electrode performance is closely related to its architecture, constructing an advanced electrode structure is crucial for achieving high-performance SIBs.Conventional electrodes are generally prepared by mixing a slurry of active materials, conductive carbon, and binders, followed by casting on a metal current collector.Electrodes prepared this way are subject to shape deformation, causing the active materials to easily peel off the current collector during charge/discharge processes.This leads to rapid capacity decay and short cycle life.Moreover, binders and other additives increase the weight and volume of the electrodes, which reduces the overall energy density of the batteries.Therefore, binder-free, three-dimensional (3D) array electrodes with satisfactory electronic conductivity and low ion-path tortuosity have been proposed.In addition to solving the aforementioned issues, this type of electrode significantly reduces contact resistance through the strong adhesion between the array and the substrate.Furthermore, electrolyte infiltration is greatly facilitated by the abundant interspacing between individual nanostructures, which promotes fast electron transport and shortens ion diffusion, thus enabling the electrode reaction.The array structure can also readily accommodate substantial volume variations that occur during repeated sodiation/desodiation processes and release the generated stress.Therefore, it is of great interest to explore binder-free array electrodes for sodium-ion storage applications.This review summarizes the recent advances in various 3D array anode materials for SIBs, including elemental anodes, transition metal oxides, sulfides, phosphides, and titanates.The preparation methods, structure/morphology characteristics, and electrochemical performance of various array anodes are discussed, and future opportunities and challenges from employing array electrodes in SIBs are proposed.