Binder is a key component for enabling good cycle life of Si and Si-based negative electrode materials in Li-ion cells, despite comprising only 2-10 wt% of the electrode coating. In previous research, thermal carbonization of polymer binders, e.g. polyvinylidene fluoride (PVDF),¹ cyclized polyacrylonitrile (PAN),² and poly(amide imide),³ was shown to enhance the cycling performance of high energy density alloy anodes greatly. For instance, sintered Si electrodes with PVDF binder can maintain a reversible capacity of 1150 mA h g^-1 over 500 cycles at 1C rate.¹ Among various binders, sodium carboxymethyl cellulose (Na-CMC) has some favorable characteristics: abundance, low cost, high water solubility, good thickening properties, and a high carboxmethyl group content, which is responsible for surface binding interactions with Si.^4,5 To our best knowledge, the effect of heat treatment on Na-CMC binder for Si-alloy containing anodes has not been explored yet.⁶

Here, electrodes were composed of 60 wt% Si alloy, 28 wt% graphite, 2 wt% carbon black (CB), and 10 wt% Na-CMC. Some of electrodes were sintered under Ar flow by heating up to 300 or 600 °C at a rate at 10 ºC/min, and then held at 300 or 600 °C for three hours. Figure 1(a) shows the TGA curve of Na-CMC polymer in the temperature range of 25-800 ºC under Ar gas. After being heated at 600 ºC in Ar, Na-CMC has a mass residue of 38.6 wt.%. The pyrolysis product consists of amorphous carbon (42 wt.%) and Na₂CO₃ (58 wt.%). The cycling capacity retention of Si-alloy/graphite electrodes with Na-CMC binder is shown in Figure 1(b). The unheated electrodes suffer from significant capacity fade, having only 78% capacity left after 100 cycles. The capacity retention is improved when the thermal treatment is applied. After heating at 300 ºC, the capacity retention after 100 cycles was improved to 87%. A significant improvement of capacity retention results when 600 ºC thermal treatment is applied: over 95% capacity is retained. The formation of Na₂CO₃ in a network of amorphous carbon contributes to this high capacity retention.

Reference

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Figure 1 (a)TGA curves of Na-CMC collected at a heating rate of 5 ºC/min under Ar gas. (b) Capacity retention of three V6/SFG6L/CB/Na-CMC electrodes: untreated, heated at 300 ºC in Ar, and heated at 600 ºC in Ar.