Combining ReaxFF Simulations and Experiments to Evaluate the Structure-Property Characteristics of Polymeric Binders in Si-Based Li-Ion Batteries

Establishing a Resilient Conductive Binding Network for Si-Based Anodes via Molecular Engineering

ConspectusSilicon-based anode materials have become a research hot spot as the most promising candidates for next-generation high-capacity lithium-ion batteries. However, the irreversible degradation of the conductive network in the anode and the resultant dramatic capacity loss have become two ultimate challenges that stem from inherent characteristics of the Si-based materials, including poor conductivity and massive volume changes (up to 300%) during cycling. Apart from optimization of the active materials, one effective way to stabilize high-capacity Si-based anodes is by designing polymeric binders to reinforce the conductive networks during repeated charge and discharge processes. As an inactive component in the electrode, the binder not only holds other components (e.g., active materials, conductive agents, and current collectors) together to maintain the mechanical integrity of the electrode but also serves as a thickener to facilitate the homogeneous distribution of particles. Therefore, binders play a key role in Si-based anodes by maintaining the integrity of conductive networks in the electrode.In this Account, on the basis of the extensive binder-related work on Si-based anodes since the 2000s, efforts made on maintaining the conductive network can be categorized into two main strategies: (1) stabilization of the primary conductive network (which generally refers to conductive agents) by enhancing the binding strength and resilience of the binding between electrode components (i.e., Si particles, conducting agents, and current collectors) via various interactions (e.g., dipolar interactions and covalent bonds) and (2) construction of the secondary conductive network by employing conductive binders, which serve as a molecular-level conductive layer on active materials. In this sense, functional groups in binders can be divided into two categories: mechanical structural units and conductive structural units. On the one hand, functional groups with strong polarities (e.g., -OH, -COOH, -NH_2, and -CONH-) generally serve as binding structural units because of their bonding tendencies; on the other hand, exhibiting high electronic conductivity, conjugated functional groups (e.g., -C₄H₄O₂S-, -C₁₆H₉, -C₁₃H₈-, and -C₁₂H₈N-) are commonly found in conductive binders. Through establishing the correlation between structural units and their corresponding properties, we systematically summarize the optimization strategies and design principles of binders to achieve a robust conductive network in Si-based anodes. In addition, integration of desirable mechanical properties and high conductivity into the binder in order to achieve a multidimensionally stable conductive network is proposed. Through an insightful retrospective and prospective on binders, a key electrode component, we hope to provide a fresh perspective on performance optimization of Si-based anodes.

Cross-Linked γ-Polyglutamic Acid as an Aqueous SiOx Anode Binder for Long-Term Lithium-Ion Batteries

Silicon oxide (SiO_x) has outstanding capacity and stable lithium-ion uptake/removal electrochemistry as a lithium-ion anode material; however, its practical massive commercialization is encumbered by unavoidable challenges, such as dynamic volume changes during cycling and inherently inferior ionic conductivities. Recent literature has offered a consensus that binders play a critical role in affecting the electrochemical performance of Si-based electrodes. Herein, we report an aqueous binder, γ-polyglutamic acid cross-linked by epichlorohydrin (PGA-ECH), that guarantees enhanced properties for SiO_x anodes to implement long-term cycling stability. The abundant amide, carboxyl, and hydroxyl groups in the binder structure form strong interactions with the SiO_x surface, which contribute strong interfacial adhesion. The robust covalent interactions and strong supramolecular interactions in the binder ensure mechanical strength and elasticity. Additionally, the interactions between lithium ions and oxygen (nitrogen) atoms of carboxylate (peptide) bonds, which serve as a Lewis base, facilitate the diffusion of lithium ions. A SiO_x anode using this PGA-ECH binder exhibits an impressive initial discharge capacity of 1962 mA h g^-1 and maintains a high capacity of 900 mA h g^-1 after 500 cycles at 500 mA g^-1. Meanwhile, the assembled SiO_x||LiNi_0.6Co_0.2MnO_0.2 full cell shows a reversible capacity of 155 mA g^-1 and displays 73% capacity retention after 100 cycles.