It is well known that the mechanical properties of lithium‐ion battery electrodes impact their electrochemical performance. This is especially critical for Si‐based negative electrodes, which suffer from large volume changes of the active mass upon cycling. Here, this study presents a postprocessing treatment (called maturation) that improves the mechanical and electrochemical stabilities of silicon‐based anodes made with an acidic aqueous binder. It consists of storing the electrode in a humid atmosphere for a few days before drying and cell assembly. This results in a beneficial in situ reactive modification of the interfaces within the electrode. First, the binder tends to concentrate at the silicon interparticle contacts. As a result, the cohesion of the composite film is strengthened. Second, the corrosion of the copper current collector, inducing the formation of copper carboxylate bonds, improves the adhesion of the composite film. The great improvement of the mechanical stability of the matured electrode is confirmed by in‐operando optical microscopy showing the absence of film delamination. The result is a significant electrochemical performance gain, up to a factor 10, compared to a not‐matured electrode. This maturation procedure can be applied to other types of electrodes for improving their electrochemical performance and also their handling during cell manufacturing.
Increasing the energy density of Li-ion batteries (LiB) is a key issue for transport applications. A promising approach is to replace the currently used graphite in LIB anodes with silicon since its specific capacity is ten times higher than that of graphite. The main challenge is to deal with the large silicon volume expansion induced by its lithiation, which damages the mechanical integrity (electronic network) of the electrode, and produces an unstable solid electrolyte interphase (SEI). In last years, our group has successfully improved the performance of silicon based anodes by working on various aspects1. First, our ball-milled silicon offers the right nanostructure to limit Si particle cracking in addition to be produced using an industrially viable process. Second, the use of an acid buffer solution to prepare the electrode ink favors the formation of resilient covalent bonds between the carboxymethylcellulose (CMC) binder and the Si particles. Third, adding fluoroethylene carbonate (FEC) to the electrolyte limits the SEI growth. Lastly, the use of carbon nanoplatelets instead of carbon black as conductive additive insures more durable electronic network, especially for Si-based electrodes with high areal capacities2. Recently, we have developed a film maturation process that further improves the performance of our silicon-based anodes3. This simple process consists in storing the electrode in a humid atmosphere for a few days before drying and cell assembling. As seen in Fig.1, this results in an impressive improvement of the electrode cycle life. Such a cycling performance is remarkable considering the high Si areal mass loading of the electrode (2 mg Si cm-2). In this work, we present a comprehensive study that aims at explaining how the maturation process works. On one hand, we have studied the effect of the maturation process on the mechanical properties of our Si-based electrodes using indentation measurement, peeling test, scratch test, in-operando dilatometry and in-operando optical microscopy. Those various experiments confirm that the film adhesion and cohesion strengths are reinforced by the maturation step. On the other hand, we have studied the effect of maturation on the electrochemical performance of various electrode formulations. These results show that maturation works with different types of silicon, binder and conductive agent. They also suggest that the nature of the substrate and the acidic environment play a crucial role in the maturation process. Furthermore, reflectance Fourier transform infrared spectroscopy (ATR - FTIR) and Nuclear Magnetic Resonance (NMR) analyses have been performed in order to evaluate the impact of the maturation step on the chemical bonds between the Si particles and the CMC binder and with the Cu substrate. ). Lastly, focused ion beam scanning electron microscopy (FIB-SEM) tomography shows a better preservation with cycling of the pore and Si particle connectivities for the matured electrode. From the conjunction of these different experiments, a film maturation mechanism is proposed. References M. Gauthier, D. Mazouzi, D. Reyter, B. Lestriez, P. Moreau, B. Lestriez, D. Guyomard, L. Roué. A low-cost and high-performance Si-based electrode for Li-ion batteries. Energy Environ. Sci. 6 (2013) 2145–2155. 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. 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
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
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