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Electrochemical dilatometry experiments were performed on silicon/carbon/carboxymethylcellulose (Si/C/CMC) composite electrodes prepared with pH7 and buffered pH3 slurries. It was shown that the pH3 electrode better accommodates the severe volume change of the micrometric Si particles, inducing a much better capacity retention with cycling (70% after 10 cycles compared to only 6% for the pH7 electrode). During the first discharge (lithiation), a maximum electrode thickness expansion of ∼170% was observed for the pH3 electrode compared to ∼330% for the pH7 electrode. A lower irreversible expansion was also observed at the end of the 1 st cycle (∼50% compared to ∼180% for the pH7 electrode). It was explained by the fact that the pH3 of the slurry, which is known to favor the formation covalent bonds between the Si particles and the CMC chains, greatly improves the cohesive strength of the electrode as supported by the higher hardness and elastic modulus of the pH3 electrode. When the discharge capacity was limited to 1200 mAh g −1 , a progressive and irreversible swelling of the pH3 electrode was observed upon prolonged cycling, which was attributed to the accumulation of solid electrolyte interface (SEI) products. Silicon is a very attractive active material for Li-ion battery anodes due to its ∼10 times higher gravimetric capacity and ∼3 times higher volumetric capacity than conventional graphite anode (i.e., 3579 mAh g −1 and 2190 mAh cm −3 for Li 15 Si 4 compared to 372 mAh g −1 and 719 mAh cm −3 for LiC 6 ). However, obtaining commercially viable Sibased anodes is very challenging due to the large volume expansion of Si during its lithiation (∼280% from Si to Li 15 Si 4 ).1 This leads to the fracture and rearrangement of the Si particles, inducing the rupture of the electrical network in the composite electrode. As a result, a rapid capacity decay with cycling is observed.2 The large Si volume change also induces an instability of the solid electrolyte interface (SEI), which continuously grows with cycling, decreasing the coulombic efficiency (CE) and increasing the electrode impedance.3 To address these issues, numerous strategies have been investigated for several years as reviewed in Refs. 4-8 with a few significant successes in the improvement of the cycle life of Si-based electrodes (>1000 cycles, at least in half-cell).9-15 Actually, further work is still required to tackle the issue of SEI stability and to obtain low-cost Si-based electrodes with practical relevant surface, gravimetric and volumetric capacities.Considering that the volume change of Si-based electrodes largely contributes to their degradation, monitoring their expansion and contraction with cycling is highly relevant to evaluate the impact of the composite electrode formulation and morphology, and cycling conditions on this process. For this purpose, a simple and efficient method consists of integrating a non-contact gap sensor 16 or a contact displacement transducer 17 to the electrochemical cell, which permits a measurement of any ver...

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The evolution with cycling of the three-dimensional (3D) microstructure of a silicon/carbon/carboxymethylcellulose (Si/C/CMC) electrode for Li-ion batteries is investigated by combined focused ion beam (FIB) / scanning electron microscopy (SEM) tomography. Using appropriate image processing methods, a volume of 20 x 8 x 11 µm 3 is reconstructed in which the Si and pore phases are clearly identified. Their respective morphological characteristics (volume fraction, spatial distribution, size, connectivity, and tortuosity) are determined before and after 1, 10 and 100 cycles. The Si particles (37 vol.%, median size = 0.37 µm ) and pores (57 vol.%, median size = 0.40 µm) are homogeneously distributed and fully connected in the pristine electrode. Major changes in the electrode morphology occur upon cycling due to electrode cracking and the growth of the solid electrolyte interface (SEI) layer. It also appears that the size and shape of the Si particles change upon cycling. They display a non-spherical, stringy morphology after 100 cycles with a median size of 0.14 µm.

Over the last ten years, considerable efforts have been devoted to solve the problem of the low cycle life of Si-based electrodes for Li-ion batteries. This mainly originates from the huge volume variation (up to ~300%) of silicon during its lithiation/delithiation, which induces a rupture of the electrode conductive network and an instability of the solid electrolyte interphase (SEI). The use of nanosized Si materials (nanoparticles, nanowires, thin films…) able to better accommodate large strain without extensive electrode cracking has been successively developed for improving the Si electrode cycling stability. However, in most of these studies, the active mass loading is low, resulting in a low areal capacity, typically less than 1 mAh cm-². This is much lower than that of commercial graphite-based negative electrodes, which can reach up to 5 mAh cm-². Obtaining stable Si-based electrodes with high areal capacities is very challenging as the increase of the Si areal mass loading accentuates the mechanical strain generated by the Si volume change1. We have recently shown that the storing of a Si/C/CMC electrode in humid atmosphere for a few days before drying and cell assembling has a very positive impact on its cycling performance2. With such a ‘’maturated’’ electrode, an areal capacity higher than 4 mAh cm-2 can be achieved for more than 100 cycles compared to less than 3 cycles for a no matured electrode. The precise mechanism of this maturation process is still unclear Here, the impact of the maturation step on the mechanical properties of Si/C/CMC electrodes is investigated by means of indentation, peeling and scratch tests. They confirm the higher adhesion and cohesion strengths of the maturated electrode. Its more reversible expansion/contraction behavior upon cycling is also demonstrated from electrochemical dilatometry measurements and in-operando optical microscopy observations (Fig. 1). In addition, focused ion beam scanning electron microscopy (FIB-SEM) tomography shows a better preservation of the pore and Si particle connectivities with cycling for the matured electrode. Lastly, reflectance Fourier transform infrared spectroscopy ( ATR - FTIR) complemented by Nuclear Magnetic Resonance (NMR) analyses indicate that the nature and distribution of the Si-CMC bonds are modified by the maturation step. On the basis of these different analyses, a film maturation mechanism is proposed, which opens up new avenues for optimizing the manufacture process of high-performance Si-based electrodes. 1. Z. Karkar, D. Mazouzi, C. Reale Hernandez, D. Guyomard, L. Roué, B. Lestriez. Threshold-like dependence of silicon-based electrode performance on active mass loading and nature of carbon conductive additive. Electrochim. Acta 215 (2016) 276-288. 2. C. Real Hernandez, Z. Karkar, D. Guyomard, B. Lestriez, L. Roué. A film maturation process for improving the cycle life of Si-based anodes for Li-ion batteries. Electrochem. Comm. 61 (2015) 102-105. Figure 1

The replacement of graphite by silicon as the active material in negative electrodes of Li-ion batteries is very attractive since the specific capacity of Si is 10 times higher than that of graphite. However, Si suffers from huge volume variation (up to ~300% vs 10% for C) during its lithiation. This leads to the electrode cracking which induces electrical disconnections in addition to cause an instability of the solid electrolyte interface (SEI), resulting in poor cycle life and low coulombic efficiency. A precise evaluation of the electrode volume variation and cracking upon cycling is thus crucial to develop more efficient Si-based anodes. To date, the study of their morphological changes is usually limited to post mortemexaminations by microscopy. This does not allow a detailed analysis of the morphological degradation process, which can significantly vary depending on the electrode composition and processing, and the charge/discharge conditions. In the present study, the volume change with cycling of Si-based anodes is monitored by operando dilatometry experiments. For that purpose, a specific displacement transducer is used, which allows measuring expansion or shrinkage of the electrode during cycling down to the sub-micrometer range. Operando acoustic emission (AE) measurements are also performed to study the electrode cracking [1]. The AE technique is based on the detection and analysis of transient elastics waves generated by stress events. The influence of the cycling conditions and electrode formulation on the volume variation and cracking of Si-based electrodes is highlighted, and correlated to their electrochemical performance. [1] A. Tranchot, A. Etiemble, P-X. Thivel, H. Idrissi, L. Roué In-situ acoustic emission study of Si-based electrodes for Li-ion batteries, J. Power Sources 279 (2015) 259-266.