Lithium polyacrylate as a binder for tin–cobalt–carbon negative electrodes in lithium-ion batteries
Introduction
Sn-based alloys have been intensively studied as negative electrode candidates for Li-ion batteries in the last decade because of their much higher specific and volumetric capacity compared with graphite [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. However, they suffer from large-volume expansion during lithiation, which presents a huge challenge for capacity retention. Amorphous, nanoscale or nanostructured materials have been proposed to mitigate the capacity fading problem because they can eliminate the inhomogeneous expansion caused by the insertion of lithium atoms that occurs in crystalline materials [1]. Such candidates include nanostructured Si [2], amorphous Si1−xSnx [3], carbon-coated nanoparticles of Sn [4], Si/graphite nanocomposite [5], amorphous Si–Al–Fe [6], tin oxide nanocomposites [7], Sn2Fe/SnFe3C nanocomposites [8], “amorphous” tin–cobalt–carbon (Sn–Co–C) [9], [10], [11], [12], [13], [14] and so on. Among these materials, “amorphous” Sn–Co–C has been used in current commercial Li-ion cells [9] due to its excellent cycling performance and relative low-cost of mass production. It was suggested by Dahn et al. [10] that the addition of Co and C to Sn produces a nanostructured material that shows a stable structure on the atomic scale and thereby leads to a promising cycle life. Nanostructured Sn–Co–C can be prepared in bulk by mechanochemical synthesis [11], which makes it commercially attractive.
From a practical point of view, Sn–Co–C alloy materials ultimately need to combine with carbon black and binder to make commercially viable negative electrodes. Although the optimization of alloy materials contributes significantly to overcome the problem of capacity fading due to particle cracking, capacity fading caused by huge volume changes is still present for composite electrodes with amorphous alloy materials because of the significant motions of particles within a composite electrode during cycling. For example, amorphous Si64Sn36 materials have good cycle life as binder-free sputtered films while amorphous Si64Sn36 composite electrodes using conventional PVDF binder show poor capacity retention [15]. It is believed that this problem is not caused by the active material itself, but by the polymeric binder that holds the active materials together. Therefore, the impact of binder selection on the performance of electrodes with alloy materials is significant.
Wagner et al. [16] first showed the importance of the binder choice on the cycling behavior of metallic alloys. Chen et al. [15], [17] modified a poly(vinylidene fluoride-tetrafluoroethylene-propylene) elastomeric binder system to improve the capacity retention of amorphous Si64Sn36 composite electrodes. Liu et al. [18] then showed an enhanced cycle life of Si-based electrodes using sodium carboxymethyl cellulose (CMC)/styrene butadiene rubber (SBR) mixed binder compared to electrodes made using PVDF binder and attributed this success to the better mechanical properties of elastomeric SBR binder over PVDF binder. Li et al. [19] reported promising cycle life of Si electrodes made from crystalline Si powder (−325 mesh) and CMC binder. Based on the fact that CMC is extremely stiff and brittle, it was suggested that mechanical properties are not the only factor that determines the performance of binders and other properties of binders such as surface modification need to be considered. Chen et al. [20] improved the cycling performance of 20% nanoscale Si-containing Si/C composite and nanoscale Si electrodes by replacing PVDF with acrylic adhesive binder. The performances were further enhanced by adding CMC because CMC was believed to help the distribution of the acrylic adhesive in the coating slurry, thereby improving the adhesion strength.
The interesting fact that brittle polymers like CMC can work well as binders for large-volume change alloy material attracted attention [5], [21]. Guyomard and co-workers [21] studied the CMC and poly(ethylene-co-acrylic acid) blended-binder system for Si electrodes and claimed that the extended conformation of CMC in solution facilitates an efficient networking process between the conductive agent and Si particles. Winter and co-workers [5] reported that the chemical bonding between CMC binder and Si particles contributes to the enhanced capacity retention of Si/C composite electrodes.
Recently, electrochemically active polyimide-type binder was reported for Si-based electrodes [22], [23]. It was suggested that an electrochemically active binder helps to maintain the electron conduction network during cycling [23]. Garsuch et al. [24] reported a promising cycling performance of Si electrodes by using a lithium-exchanged Nafion as a binder. Given that these binders are not elastomeric, it is believed that ionic conducting binders would be beneficial for the formation of a more effective solid electrolyte interphase (SEI) layer, which helps to improve the capacity retention of the alloy negative electrodes.
Not only the nature of the binder but also binder-related processing of the electrode such as electrode heat-treatment [24] and the solvent content of the electrode slurry [25] influence electrode cycling performance. These reports suggest that the composite electrode architecture, especially the distribution of the binder–carbon black network on the active material surface helps determine the electrochemical performance of the electrodes made from alloy materials.
The studies above clearly show that the choice of binder has a critical impact on the performance of composite electrodes with alloy materials. Recently, Dinh Ba Le at 3M Company developed a lithium polyacrylate (Li-PAA) binder which is useful in electrodes incorporating alloy anode materials [26]. Similar to CMC, polyacrylic acid (PAA) and its derivatives are often used as dispersants, thickeners and flocculants depending on their molecular weight [27]. They have been typically studied as a component of solid polymer electrolytes for alkaline batteries including Zn/MnO2, Ni/MH, Ni/Cd, Ni/Zn and Zn/air batteries [28], [29]. As for lithium-ion batteries, ammonium polyacrylate was proposed as a binder for LiCoO2 positive electrodes to improve the slurry dispersion behavior and the rate capability [30]. In this work, Li-PAA was used to prepare negative electrodes made with Sn–Co–C material, prepared by mechanical attrition. Here the composition of Sn–Co–C material is Sn30Co30C40, similar to that of the negative electrode used in new commercial lithium-ion cells (Nexelion, Sony [9]). A comparison of the Li-PAA binder with CMC and PVDF binders is presented and discussed.
Section snippets
Active materials
The Sn30Co30C40 samples were alloyed mechanically using a vertical-axis attritor (Union Process 01-HD attritor). Samples were prepared from CoSn2, Co (Sigma–Aldrich, <150 μm, 99.9+%) and graphite (Fluka, purum). The combination of these starting materials was selected due to the poor mechanical properties of elemental Sn powder in a high energy milling process [11]. If elemental Sn is used, chunks of materials are formed on the walls of the attritor can instead of a uniform powder. CoSn2 was arc
Results and discussion
Fig. 1 shows the XRD pattern of the Sn30Co30C40 material as prepared by mechanical attrition. The diffraction pattern appears amorphous with two broad humps at about 32° and 43°, corresponding to an amorphous CoSn phase. This diffraction pattern is consistent with the model presented by Todd et al. [31] where amorphous CoSn grains are within a disordered carbon matrix. Fig. 2 shows SEM images of the attrited Sn30Co30C40 used in our experiments. Fig. 2 shows that the average particle size of Sn30
Conclusion
In summary, the electrochemical performance of Sn30Co30C40 electrodes made using Li-PAA binder developed by 3M Company was compared to CMC or PVDF binders. This comparison clearly shows the impact of binder selection on the performance of negative electrodes made from alloy materials. Sn30Co30C40 electrodes using different binders have an increasing order of capacity retention: PVDF < CMC < Li-PAA. In particular, the electrodes using Li-PAA binder retain over 450 mAh/g for 100 cycles.
These results
Acknowledgments
The authors acknowledge the support of this research by NSERC and 3M Canada under the auspices of the Industrial Research Chair program.
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