Research Progress of Functional Binders in Silicon-Based Anodes for Lithium-Ion Batteries

Abstract

Silicon (Si) has a high theoretical gravimetric capacity (3579 mAh·g⁻¹ for Li₁₅Si₄), which is almost ten times higher than that of graphite (372 mAh·g⁻¹) anode. Besides, it has low electrochemical potentials (0.4 V vs. Li⁺/Li), and abundant reserves. Thus, Si becomes a key anode material for the development of high-energy lithium-ion batteries. Nano-Si, typically compounded with graphite, has opened its commercialization. But the specific capacity of commercial Si/graphite composites is generally below 600 mAh·g⁻¹, which is far below the theoretical specific capacity of Si. In the meanwhile, the high cost, high specific surface area and low tap density of nano-Si limit its volumetric energy density and large-scale production further. Compared to the above materials, micro-Si (1–10 μm) is gaining industry attention for its low cost, as it does not require high-energy ball milling to reduce the particle size. Also, low specific surface area and high tap density conduce to reducing interfacial side reactions and increasing volumetric energy density. Therefore, micro-Si has a particular advantage over application in high volumetric energy density storage devices. However, due to the huge stress caused by significant volume change (300%), there are more severe problems such as particle pulverization, electrode disintegration, conductive network failure and uncontrolled growth of solid electrolyte interphases, which greatly hinder its commercialization. Binders are essential in adapting to Si volume changes to ensure the integrity of the electrode and keeping the tight contact among the active material, conductive additive and current collector to provide a stable conductive network. The development of high-capacity and high-stability micro-Si-based anodes poses greater challenges to the design of binders. In this review, we first clarify the binding mechanism of binders, factors that influence the bonding forces, and design strategies of binders for relieving the volume change of Si electrodes. As a major part, we systematically discuss the strategies and corresponding mechanisms of functional binders for silicon-based anodes from aspects of self-healing binders, conductive binders, ion-conductive binders, and the facilitating effect of functional binders on the stable solid electrolyte interphase (SEI) formation. Finally, the existing problems and challenges are pointed out in terms of long-cycle stability, initial Coulombic efficiency (ICE) and binder ratio under commercial loading. We put forward the promising directions for developing functional binders towards the practical use of micro-Si anode: an ideal binder should be multifunctional and helpful to robust electron/ion conductive networks and stable SEI throughout the long cycling life of micro-Si, where the polymer molecular structure of functional binders can be systematically designed by artificial intelligence and machine learning technologies.

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