On the binding mechanism of CMC in Si negative electrodes for Li-ion batteries

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Abstract

Silicon is investigated intensively as a promising anode material for rechargeable lithium-ion batteries. The choice of binder is very important to solve the problem of the large capacity fade observed along cycling. Although carboxymethyl cellulose (CMC) is not an elastomeric binder, it has been shown to vastly improve the cycling performance of Si electrodes. We demonstrate here that the efficiency of CMC can be attributed to its extended conformation in solution that facilitates a networking process of the conductive additive and Si particles during the composite electrode elaboration. Taking advantage of this understanding, we have adjusted the processing conditions and obtained a four times higher reversible capacity of the Si/CMC electrode than that obtained with the same electrode processed with standard conditions.

Introduction

Recent works show that the choice of binder is very important to stabilize the cycling performance of alloy negative electrodes for Li-ion batteries such as Si and Sn, which display large volume changes [1], [2], [3], [4], [5]. Chen et al. [1] early suggested that cycling stability of the electrodes might benefit from increasing the elasticity of the binder material. Because an elastomeric binder has a larger breakage elongation than the standard poly(vinylidene fluoride) (PVdF) binder, it should tolerate a greater extent of volume expansion with reversibility. In agreement with this understanding, Chen et al. [1] successfully synthesized a poly(vinylidene fluoride–tetrafluoroethylene–propylene)-based elastomeric binder system combined with an adhesion promoter that improved the cyclability of amorphous Si–Sn alloy. Liu et al. [2] also reported that the cycle life of Si electrodes was improved using a binder containing the elastomeric styrene butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC). CMC is a linear polymeric derivative of cellulose with varying levels of carboxymethyl substitution (DS). The carboxymethyl groups that dissociate to form carboxylate anionic functional groups are responsible for the aqueous solubility of the CMC relative to the insoluble cellulose. The very different cycling stabilities exhibited by the SBR–CMC and PVdF-electrodes was understood in terms of differences in the macroscopic mechanical properties of the PVdF and SBR binders. However, in a diametrically opposed direction, Li et al. [4] showed that Si electrodes made using CMC binder have better capacity retention than those using a binder consisting of CMC and SBR. Because CMC is very stiff, having a small elongation at break (5–8%) it could therefore not function as an elastomeric binder. Thus, the reasons why a brittle polymer like CMC can give good results for Si-based electrodes must be explored.

Only a very few studies from Yoo et al. [6], [7], [8] and Guy et al. [9], [10] have tried to comprehensively address the issue of the binding mechanism. In concentrated particle/polymer suspensions such as electrode slurries, a three-dimensional network is developed due to bridging of the particles by polymer chains because segments of polymer chain adsorb on different particles or because adsorbed chains on two different particles form entanglements in between them. After solvent evaporation, the dried composite electrode retains the memory of the morphology in the wet state and the particles are tighten altogether by those chains. It is known that the efficiency of the bridging process depends on the conformation and the molecular weight of the polymer chains [11]. Conformation is the shape of the macromolecules when dissolved in their solvent. A high molecular weight and an extended configuration make bridging more likely.

In this work we address the question of the binding mechanism of CMC using mainly granulometry and rheology measurements that are sensitive to the interactions between the constituents of a slurry, and to the resulting microstructure of the slurry [12]. The study of the interactions between the electrode constituents during the electrode processing has recently lead to a substantial improvement of the understanding of the composite electrode architecture and properties [9], [10], [13], [14]. We investigated two new aqueous binder combinations. First, a combination of poly(vinyl pyrrolidone) (PVP), ethylene glycol (EG) and TX100. The PVP–EG blend is a water soluble viscoelastic adhesive [15] and TX100 is a nonionic surfactant conventionally used to disperse CB in water [16]. Second, a combination of CMC and poly(ethylene-co-acrylic acid) (PEAA). PEAA is an amphiphilic copolymer which consists of both hydrophobic (E) and hydrophilic water-soluble (AA) groups. The ethylene groups provides PEAA with a high elongation to break (up to 500%) [17] while the acrylic acid groups provide a high adhesion strength with the materials such as the Si particles and the current collector [18]. Because only the AA groups are soluble in water and due to a fairly low concentration of AA (20 wt.%), PEAA forms bulky core–shell aggregates, with a diameter from 15 to 60 nm, with an E core and an AA (ionic form) shell [19], [20]. PEAA conformation is therefore identical to that of the SBR latex. Results are compared to those obtained with the standard PVdF/N-methyl pyrrolidone (NMP) system.

Section snippets

Materials

Si (99.999%, 1–5 μm, Alfa Aesar) was used as active material and Super P carbon black (noted CB, ERACHEM) as the conductive agent. Four polymer binders were used: (1) CMC (DS = 0.7, Mw = 90,000 Aldrich), (2) a mixture of CMC and PEAA (20 wt.% AA, Aldrich) with varying weight ratios, (3) a mixture of PVP (Mw = 1,300,000, Aldrich), EG (Mw = 400, Aldrich) and TX100 (Aldrich) with 7:7:1 weight ratio, and (4) PVdF (Mw = 180,000, Aldrich).

Characteristics of the electrode slurries

For the measurement of the CB particle-size distribution by low angle

Characteristics of model electrode slurries

Fig. 1 shows the CB particle-size distribution prepared with CMC, PVP/EG/TX100, and CMC/PEAA in water, and with PVdF in NMP. Pure CB powder cannot be dispersed in water due to its hydrophobic character. The very high interfacial free energy between CB and water prevents the liquid from wetting the CB powder [21]. However, adding PVP/EG/TX100 or CMC allows to finely disperse CB. The smallest dispersible CB particles are the primary aggregates that have an average diameter of 100–300 nm. They

Conclusion

In summary, the efficiency of CMC could be attributed to its extended conformation in solution that facilitates the formation of an efficient CB–CMC–Si network. Moreover, the astonishing cyclability improvement of the resulting electrode prepared at pH 3 could result from the physical cross-linking of the CMC chains in the solution at pH 3. Of course, the reason why CMC is able to bear large deformation (up to 400%) although its apparent deformation to break is only 5–8% is not answered yet.

Acknowledgement

This work was supported by the “Ministère des Relations Internationales du Québec”.

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