Resilient Binder Network with Enhanced Ionic Conductivity for High-Areal-Capacity Si-Based Anodes in Lithium-Ion Batteries
Authors: Kim Sungryong , Park Taiho 
Published: December 2024

Silicon (Si) anodes have attracted considerable attention for high-energy-density lithium-ion batteries (LIBs), owing to their high theoretical capacity (>3500 mAh g−1 at room temperature), low reaction potential (<0.4 V vs. Li/Li+), and natural abundance. However, the repeated volume changes of Si during the lithiation/delithiation process induce critical complications, such as the pulverization of Si particles, formation of a thick solid-electrolyte-interphase (SEI) layer, and electrode delamination, resulting in poor cycle performance. To mitigate these complications, several strategies have been proposed, including the nano/microstructural design of Si, employing a Si/C composite, and using multifunctional binders. Regarding the choice of the binder, the conventional poly(vinylidene fluoride) (PVDF) is inadequate as it cannot accommodate a large hoop stress because of its weak van der Waals force. Compatible binders, such as carboxymethyl cellulose, poly(acrylic acid) (PAA), and poly(vinyl alcohol), have been generally employed to improve the electrochemical performance of Si anodes. Although binders with abundant functional groups (e.g., –OH, –COOH, –CN, and –NH2) prevent the delamination of Si anodes from the current collector, they suffer from brittleness due to their serious chain-entanglement network and strong interactions, leading to a high glass transition temperature (Tg
). Moreover, polymeric binders with poor stretchability and self-healing ability degrade the structural integrity of the Si anode during repeated volume changes. Further, low ionic conductivity deteriorates the transport of Li ions in the Si anode, thereby impeding the effectiveness of fast-charging systems.

Therefore, various physicochemical properties of a polymeric binder are required to improve the electrochemical performance of Si anodes besides adhesion property. First, stretchability and a high elastic modulus are essential for a binder to endure the stress of large volume expansions and recover to the original state after the lithiation process. A Si anode incorporating a rigid and stiff polymeric binder could crack and become damaged, resulting in drastic capacity decay and safety issues. Second, the self-healing ability is required for a binder to spontaneously recover from the damage induced by the significant volume changes. The mechanical fracture of Si particles induces the loss of the active material and exposes a highly reactive surface to the electrolyte. Third, in the LIB system, the rate performance highly depends on the ionic conductivity as well as the electronic conductivity. Thus, a binder with high ionic conductivity is highly desired to enhance the Li-ion diffusion coefficients and achieve a high-rate-performance. A binder design that satisfies the three requirements above is desperately desired.

a series of poly(Li[3-sulfopropylmethacrylate]-r-acrylic acid) (PLSA) polymers with different moiety ratios was synthesized from 3-sulfopropyl acrylate lithium salt (Li[SPMA]) and acrylic acid (AA) monomers. Subsequently, glycerol, as a thermally stable plasticizer (high boiling point: 290 °C), was added to the polymer matrix to lower the Tg
and maintain the softness of the polymers after water evaporation. The hydroxyl groups in the glycerol interact with the oxygen in the polymers, enhancing ion solvation for rapid Li-ion conduction. The ionic side chain of the polymer, as a mechanical modulator, formed the crosslinking via the electrostatic interaction between the polymers, conferring the stretching and self-healing properties. Furthermore, the mobile Li ions in the side chain enhanced the ionic conductivity of the polymers. Meanwhile, the carboxylic acid group (–COOH) in AA interacted with the current collectors (i.e., copper (Cu) foil) and the silanol groups (–SiOH) from the Si particles via hydrogen bonding, enhancing the adhesion properties. The rationally designed polymers with excellent physicochemical properties (i.e., high stretchability, rapid self-healing ability, and high ionic conductivity) were introduced as polymeric binders in the Si anode in LIBs. The Si anode with the PLSA75 binder (Si–PLSA75) exhibited stable cycling performance, retaining 81.2% of its initial capacity after 300 cycles at 0.5 C (1.5 A g−1) and exhibiting outstanding rate performance (815 mAh g−1 at 5 C (15 A g−1)). Moreover, compared with the Si–PAA, the half-cell with Si–PLSA exhibited lower internal resistance and higher diffusion coefficients of the Li ion during the whole lithiation/delithiation process. Based on the outstanding electrochemical performance in Si anodes, Si/graphite (Si/Gr) blend anode was also tested with PLSA binder which could afford around 840 mAh g-1. The blended anode with PLSA75 also maintained 82.7% of its initial capacity after 200 cycles at 0.5 C. Furthermore, the blended anode was paired with a nickel (Ni)-rich cathode, LiNi0.8Co0.1Mn0.1O2, for the full-cell test, and the full cell still delivered a reversible capacity of 139.2 mAh g−1 after 200 cycles at 0.5 C. This work provides insights into the rational design of polymeric binders for achieving Si anodes with fast-charging and stable cycling performance.