Formulation of Industry-Relevant Silicon Negative Composite Electrodes for Lithium Ion-Cells

Compared with graphite, silicon is a favourable alternative candidate due to a high specific capacity (3572 vs. 372 mAh g^-1) and specific volumetric capacity (2081 vs. 779 mAh cm^-3). However, large capacity fading and low cyclability of Si electrodes is a major issue. Several strategies have been undertaken to overcome these problems. One way is optimising the binder in order to cope at the molecular scale with the expansion and contraction of Si upon alloying and dealloying with Li. Since early works on CMC, several other works proposed alternative binders to CMC, most of them being polysaccharides and/or bearing COOH functional groups. Another way to improve the cycle life is using electrolytes containing a film-forming agent such as VC and/or FEC.

            Besides, electro-conductive additives also display an important influence for the performance of nano Si-based electrodes with high active mass loadings of about 2.5-3.3 mAh per cm². We found a significant improvement of the electrochemical performance (2000 mAh g ^-1 after 100 cycles for 2.5 mg of Si cm ^-2) by using reduced graphene oxide (rGO) or exfoliated graphite nanoplatelets instead of carbon black as the conductive additive [1,2]. The influence of electro-conductive additives is not only to play on the electronic conductivity but also on the micromechanics (stress distribution) of the composite films.

In a further work, we aimed to design the formulation of these nano Si-based electrodes with high active mass loadings to wind some cylindrical cells. This way, Poly (acrylic-co-maleic) acid (PAMA) is used as a dispersant to improve the stability of electrodes slurries. Sedimentation test, electrical measurement, SEM-EDX observations as well as rheological measurements show that a more homogeneity distribution of carbon black (CB) inside the stack of Si particles is reached with presence of PAMA. However, there is an optimal amount of PAMA due to the competition in the adsorption of PAMA and Carboxylmethyl cellulose (CMC) at the surface of the CB particles. Upon cycling with capacity limitation (1200 mAh per g of Si), the optimized electrode formulation at lab scale could achieve more than 400 cycles with surface capacity ~2.5-3.3 mAh cm^-2. At the pilot scale, the improvement of adhesion of the tape to the current collector by using Styrene-co-Butadiene rubber copolymer latex (SB) helps to maintain long cycle life while by calendaring is detrimental to electrochemical properties. [3] On the whole, the pilot-made electrodes show a lower cycle life than the labe-made electrode as a consequence of poorer CB distribution, likely due to the larger volume and longer processing time involved.

Acknowledgements

Financial support provided by the European Commission (EC), through the project EuroLiion (NMP3-SL-2010-265368) is gratefully acknowledged.

References

[1] B.P.N. Nguyen, J. Gaubicher, B. Lestriez, Electrochimica Acta, 2014, 120, 319.

[2] B. P. N. Nguyen, N. A. Kumar, J. Gaubicher, F. Duclairoir, T. Brousse, O. Crosnier, L. Dubois, G. Bidan, D. Guyomard, B. Lestriez, Adv. Energy Mater., 2013, 3, 1351.

[3] B. P. N. Nguyen, S. Chazelle, M. Cerbelaud, W. Porcher, B. Lestriez, submitted to J. Power Sources.