Optimization of Silicon-Graphite Anode and Polymeric Binders for Lithium-Ion Batteries

Abstract

Graphite (G) is considered as the state-of-the-art anode material in commercial Li-ion batteries (LIBs), attributed due to its long cycle life, high Coulombic/energy efficiency and low cost. However, its low theoretical specific capacity (372 mAh g-1/ 840 mAh cm−3 for LiC6) is a limiting factor towards the large scale integration of emerging applications such as electromobility (electric vehicles, xEVs). Therefore, it is essential to search for other anode active materials with higher theoretical capacity.1 Silicon (Si) is one of the most appealing anode active materials with an extra-high capacity of 4200 mAh g-1 /9660 mAh cm−3 (Li4.4Si) and 3590 mAh g-1/8360 mAh cm−3 (Li3.75Si) at 415 oC and 25 oC respectively, suitable operating potential (0.2-0.4 V vs. Li/Li+), high abundancy (second abundant element in the earth crust) and thus low cost, superior safety and, environmental friendliness. However, in spite of all the resilient features, Si shows an extreme volume change ( ≥ 280%) upon (de-)lithiation, low electric conductivity, low initial Coulombic efficiency, electrode swelling and, electrolyte drying. These challenges limit the usage of Si in large-scale commercial LIBs. Many strategies have been suggested to overcome the above-mentioned difficulties, and two of the most promising strategies are: (1) use of Si-G composite anode active material, resulting in a synergistic beneficial effect of G and Si. The composite electrode shows longer cycle life and lower volume change with respect to Si anode and higher theoretical capacity compared to G anode. Moreover, G also enhances the electrical conductivity of the blended anode electrode. However, it is crucial to optimize the composition of the Si to G ratio so as to obtain desirable performances. (2) Use of new tailored binders for Si-based anodes. Binders are electrochemically inactive components of LIBs, nevertheless, they play a major role in the mechanical stability of the electrodes, irreversible capacity losses and stabilizing the cycling performance. Suitable binder for Si-based anodes should accommodate the volume change of Si particles. These binders usually contain carboxylate or imide functional groups, which interact strongly with the surface of the Si during charge/discharge of the battery.2 This implies also that the choice of binders for Si may not be necessarily the same as that of G.In this work, we have investigated and optimized the composition of Si-G composite anode using a mixture of state-of-the-art carboxymethyl cellulose (CMC) and poly (acrylic acid) (PAA) binders. Furthermore, the composition of the binders was optimized for the corresponding different ratio of the Si-G blends. Afterward, the benchmarked electrodes were further studied with different designer bio-polymeric binders such as chitosan, tragacanth gum, alginates, etc. The electrochemical properties (long-term cycling, rate capability, cyclic voltammetry,...) for the optimized Si to G ratio electrodes were investigated, followed by surface characterization and thermal reactivity.