A biopolymer network for lean binder in silicon nanoparticle anodes for lithium-ion batteries

Abstract

Binders play an important role in enhancing the cycling lives of Si anodes. Herein, we propose a new three-dimensional network binder constructed by weaving the dual biopolymers (i.e., κ-carrageenan gum and konjac gum) through the intermolecular interactions. Due to the synergistic effect of κ-carrageenan gum and konjac gum, the proposed binder (N-KCG-KG) demonstrates the enhanced adhesion strength and moderate mechanical properties, which is benefit for accommodating the huge volume change of Si during the repeated deep cycling. As a consequence, a positive long-term cycling performance of 500 cycles at 1 A g⁻¹ is achieved by the Si anode using the N-KCG-KG binder (Si@N-KCG-KG). Moreover, the relatively stable cycling performance of full-cell using the Si@N-KCG-KG anode confirms that the N-KCG-KG binder can work well in the practical cell setting.

Introduction

Driven by ever-increasing power demands for customer electronics, electric vehicles and stationary energy storage systems, colossal efforts are continuously dedicated to lithium-ion batteries (LIBs) with high energy density in the past three decades. The use of silicon (Si) as anode material in high-energy-density LIBs has attracted considerable attention because of its ultrahigh theoretical capacity (4200 mAh g⁻¹ for Li_4.4Si) [[1], [2], [3], [4], [5], [6]]. However, commercialization of Si nanoparticle (SiNP) anodes in high-energy-density LIBs has been impeded by electrode delamination and unstable solid electrolyte interphase (SEI), caused by huge volume change (＞300%) of SiNPs during repeated lithiation-delithiation processes [[7], [8], [9]]. These inherent failure mechanisms have been predominantly addressed through nanostructuring of SiNPs or combination with carbon buffers [3,[10], [11], [12], [13]]. Although exciting progress has been achieved, nanomaterial designs suffer from complex fabrication process and intricate production setups, and the introduction of carbonaceous materials sacrifice high capacity of SiNPs. Alternatively, polymeric binders have been recognized to play a critical role in maintaining sustainable operation of SiNP anodes [[14], [15], [16], [17], [18]].

Polyvinylidene difluoride (PVDF), a traditional binder for LIBs electrodes, is inappropriate for SiNP anodes since it cannot either alleviate large volume changes or offer strong binding strength [19,20]. In addition, processing of PVDF binder toward electrode fabrication requires a toxic organic solvent such as N-methyl-2-pyrrolidone, which is detrimental to human beings and environment [[21], [22], [23]]. Thus, plentiful advanced aqueous biopolymer binders for SiNP anodes have been developed to replace the PVDF binder, such as guar gum [19], konjac glucomannan [24], DNA-polysaccharide [25], carboxymethylated gellan gum [26], gum Arabic [27], chitosan [28], xanthan gum [29], karaya gum [30], agarose [31], and starch [32]. With the introduction of these binders, the cycling stabilities of SiNP anodes have been enhanced substantially. Nonetheless, these biopolymer binders used in SiNPs anodes only exhibit stable cycling performance under high binder content (≥15 wt%) and low mass loading since individual biopolymer binder cannot provide strong scaffold to confine SiNPs. The acquisition of stable cycling performance of SiNPs anode under low binder content remains a big challenge.

It has been beautifully exemplified that construction of 3D network is an auspicious strategy to design binder for anodes materials suffering from large volume change and unstable SEI formation: 1) binder with 3D network shows high mechanical properties, facilitate multidimensional interactions with active materials, and significantly adapt the huge volume variation of the active materials [33]; 2) binder with 3D network can be well coated onto the active material particles which prevents further decomposition of electrolyte during charging/discharging, and then a stable SEI layer could be obtained [34]; 3) 3D network provides continuous pathways for fast electron and ion transport [35]. Hence, we propose that a network binder can be rationally designed through the interactions between the biopolymers to achieve stable cycling performances of SiNP anodes under lean binder content. Herein, a new biopolymer network for lean binder in SiNP anodes is constructed via intermolecular interactions between κ-carrageenan gum (KCG) and konjac gum (KG). Due to the synergistic effect, the as-prepared 3D network binder exhibits strong adhesion and moderate elastic modulus. Consequently, decreasing the biopolymers binder content toward lean amount (10 wt%) provides stable cyclability of SiNP anodes with high Si content in both half-cell and full-cell.