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The impact of porosity of a negative electrode containing a silicon/carbon/graphite composite on the cycling performance of 18650 cells with a NMC-based cathode was investigated. Anode porosity variation between 30 and 40% led to a significant difference in the cycle life of 18650 cells: 225 and 347 cycles were achieved, respectively, before the drop of a discharge capacity to 75% of the initial value. A detailed postmortem study of 18650 cells with 30 and 40% anodes after formation and cycling steps was conducted including measurements of average electrode mass and thickness changes, electrochemical cycling in half-cells, impedance measurements in symmetrical cells, study of the electrode surface compositions via X-ray photoelectron spectroscopy (XPS), and acquisition of cross-sectional scanning electron microscopy images. It was found that the degradation processes taking place in 18650 cells were different as a function of the negative electrode porosity at the beginning of tests. Thus, Li + ion loss in the solid electrolyte interphase layer was the main reason for the performance loss in the case of 40% anode porosity. Compression of the anode to 30% of porosity resulted in Li metal deposition on the negative electrode surface and within the separator during cycling of the 18650 cell, which was evidenced by visual observations and XPS analysis. Li + ion transport was largely impeded by this deposit leading to poor C-rate performance. The present work contributes to a better understanding of important parameters to take into account for the ongoing practical implementation of prospective Si-containing electrodes for lithium-ion batteries.

Fundamental electrochemical studies require to implement a reference electrode inside the cell to be able to record the potential profile and characterize the kinetics properties and the interface structure of each electrode individually [1]. Obtaining such data proves to be essential to better understand the electrochemical processes and the aging mechanisms that are generated by cycling. Moreover, the integration of reference electrode inside lithium-ion cells must be done by keeping the sealing of the cell. Many studies are reported in the literature that characterize 3-electrodes lithium-ion cells in coffee-bag whom the design allows to exit a third tab easily through the heat-sealed laminated aluminum foil [2, 3, 4]. Such instrumentation can be realized when the cell is assembled or on commercial cell [5]. A few more recent studies gives results obtained in 3-electrodes cylindrical cells but hard packaging is much more delicate to instrument because it implies to overcome experimental difficulties for exit connections and sealing preservation [6, 7]. Several electrochemical couples can be used including metallic lithium (Li+/Li) [8], insertion materials Li(1-x)FePO4/ LiFePO4 (LFP), Li4Ti5O12/ Li(4+x)Ti5O12 (LTO) [4, 9] or alloy (LixAl/Al) [10]. They are identified as possible reference material because their insertion/desinsertion curves show a voltage plateau on a large lithiation range. The two last material families have to be partially delithiated (LFP) or lithiated (LTO, alloy) to place the insertion potential of the material on the plateau. That is performed in situ once the cell is assembled and charged [4]. Moreover, the design of the reference electrode and its placement inside the electrochemical cell are important to avoid potential shifts or artefacts on the impedance spectra [11, 12]. In the present study, the response of two reference electrochemical couples, Li+/Li and Li(1-x)FePO4/LiFePO4 with a similar cell design and placement has been evaluated towards three criteria, (i) the potential profiles in function of the current rates, (ii) the impedance spectra shape and the stability in time. The stability in time appears more and more crucial in the perspective to develop innovative smart-cell. The comparative study that was performed in laboratory pouch-cells has allowed to discriminate the two electrochemical couples. The instrumentation of 18650 cell will be also presented in comparison. REFERENCES: [1] Electroanalytical Methods Guide to Experiments and Applications, Springer; Edition: 2nd ed. 2010 (11 november 2014) [2] M.-S. Wu, P.-C. J. Chiang, J.-C. Lin, J. Electrochem. Soc. 152 (1) A47-A52 (2005) DOI: 10.1149/1.1825385 [3] M. Dollé, F. Orsini, A.S. Gozdz, J.-M. Tarascon, J. Electrochem. Soc., 148 (8) A851-A857 (2001) DOI: 10.1149/1.1381071 [4] I. Jiménez Gordon, S. Grugeon, A. Débart, G. Pascaly, S. Laruelle, Solid State Ionics 237 (2013) 50–55 http://dx.doi.org/10.1016/j.ssi.2013.02.016 [5] B. Pilipili Matadi, S. Genies, A. Delaille, C. Chabrol, E. de Vito, M. Bardet, J.-F.Martin, L. Daniel, Y. Bultel, J. Electrochem. Society, 164 (12) A2374-A2389 (2017) [6] E. McTurk, T. Amietszajew, J. Fleming, R. Bhagat, J. Power Sources, Vol. 379, 2018, 309-316 https://doi.org/10.1016/j.jpowsour.2018.01.060 [7] T. Amietszajew, E.McTurk, J. Fleming, R. Bhagat, Electrochimica Acta, Volume 263, 2018, 346-352 https://doi.org/10.1016/j.electacta.2018.01.076 [8] J. Zhou, P. H. L. Notten, J. Electrochem. Soc., 151 (12) A2173-A2179 (2004) DOI: 10.1149/1.1813652 [9] F. La Mantia, C.D. Wessells, H.D. Deshazer, Yi Cui, Electrochemistry Communications, Vol. 31,2013, 141-144 https://doi.org/10.1016/j.elecom.2013.03.015 [10] I.G. Kiseleva, L.A. Alekseeva, A.V. Chekavtsev, P.I. Petukhova, Soviet Electrochemistry, 18 (1982) 114-117. [11] D.W. Dees, A. N. Jansen, D. P. Abraham, Journal of Power Sources 174 (2007) 1001–1006 doi:10.1016/j.jpowsour.2007.06.128 [12] M. Ender, A. Weber, E. Ivers-Tiffée, J. Electrochem. Soc. 159 (2) A128-A136 (2012) DOI:10.1149/2.100202jes