Silicon Electrodes for Li-ion Batteries. Addressing the Challenges

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Silicon Electrodes for Li-ion Batteries. Addressing the Challenges through Coordination Chemistry. Thomas Devic, Bernard Lestriez, and Lionel Roué ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b02433 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Silicon Electrodes for Li-ion Batteries. Addressing the Challenges through Coordination Chemistry Thomas Devic, a* Bernard Lestriez,a Lionel Rouéb** a Institut

des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS UMR 6502, 2 rue de la

Houssinière, BP 32229, 44322 Nantes cedex 3, France. b

Institut National de la Recherche Scientifique (INRS), Centre Énergie, Matériaux,

Télécommunications (EMT), 1650, Boulevard Lionel Boulet, Varennes, QC J3X 1S2, Canada.

Corresponding Authors E-mail: *[email protected] E-mail: **[email protected]

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ABSTRACT. Silicon is considered as a promising negative electrode active material for Li-ion batteries, but its practical use is hampered by its very limited electrochemical cyclability arising from its major volume change upon cycling, which deteriorates the electrode architecture and the solid electrolyte interphase. In this perspective, we aim at critically discussing the opportunities offered by coordination chemistry to tackle these challenges. More precisely, we will show how the characteristics of the coordination bonds, notably their tunability, medium strength and dynamic character can be exploited to offer alternative paths for binding, templating and coating Si particles, in order to ultimately improve the cycle life of Si electrodes in Li-ion batteries.

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The advantages of silicon as negative electrode active material for Li-ion batteries, such as its impressive theoretical capacity when compared to graphite (3600 mAh g-1 and 2200 mAh cm3 versus 350 mA g-1 and 720 mAh cm-3),1 low Li uptake voltage (~0.4 V vs. Li), high natural abundance and low toxicity, are well established. However, its practical use in devices is impaired by notorious drawbacks, notably its extreme volume variation upon cycling which induces electrical disconnections and destabilizes the solid electrolyte interphase (SEI), leading to a very rapid capacity fading upon cycling. Numerous strategies are developed to tackle these challenges, such as the precise control of the Si particles size, shape and arrangement, as well as the engineering of their surfaces and interfaces, allowing to increase notably the cycle life of Si electrodes.1-4 In this perspective, we aim at specifically addressing the opportunities offered by coordination chemistry. Based on recent literature, we will discuss how the coordination bond can be exploited in the fields of binding, templating and coating of Si electrodes to circumvolve their intrinsic limitations and further lead to enhanced electrochemical performance (Figure 1).

Figure 1. Overview of the potential of coordination chemistry in the field of Si electrodes for Liion batteries.


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The coordination bond is usually classified as of intermediate strength between purely covalent (strong) and supramolecular (weak) bonds, although its nature (covalent to ionic) and strength is strongly dependent on the chemical nature of both the cation and the complexing group. Indeed, considering the broad variety of metallic cations (arising from the s, p, d or f blocks) and complexing groups (either neutral: amine, pyridine, nitrile… or anionic: carboxylate, phosphonate, azolate, phenolate…) available, these bonds are indeed highly tunable. Among their characteristics, the following ones are of prime importance once if one wants to combine them with Si electrodes: (i) Coordination bonds can be formed in a very broad range of solvent, ranging from pure water to fully anhydrous, and even under solvent-free conditions. Soft conditions, especially low temperature (room temperature to 200°C), are usually required, as well as an adjustable pH, which depends on the nature of the organic and inorganic moieties in play. It is thus feasible to find conditions of formation which are compatible with the preparation of Si electrodes (even the complex ones). (ii) These bonds are highly dynamic.5 First, specific ligand-cation pairs could afford labile bonds, such a reversibility allowing for instance the quantitative preparation of very complex molecular architectures under thermodynamic control without any kinetic by-product. Second, even with more robust bonds (i.e. irreversible under the working conditions), the coordination shells often present a certain level of flexibility induced, e.g., by rotation of the ligands or partial decoordination. Such geometrical rearrangements could lead to an easy adaptation to external stimuli (variation of temperature, humidity, pressure) both during the preparation of the electrodes or under electrochemical working conditions. As an example, under a mechanical stress induced by a volume change associated with the formation of LixSi, coordination networks


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are prone to deform, but also to recover their initial state easily, eventually inducing self-healing properties in the electrodes. (iii) It is usually possible to break irreversibly coordination bonds, again under soft conditions, notably through the introduction of competitive complexing agents or through the modification of the pH. Applied to a polymeric coordination network, such a treatment would lead to a complete dissolution, making this solid attractive as an easy-to-remove template to prepare i.e. hollow particles. (iv) Well-defined, crystalline materials with targeted structures and (micro)porosity are accessible. Although porous coordination polymers or Metal Organic Frameworks (MOFs) have been primarily considered for sorption-related applications because of their unique adsorption features (high surface area and pore volume, tunable porosity), they are nowadays also considered in field of electrochemical energy storage, mainly as active materials or precursors of active materials (structured oxides or carbonaceous species).6–9 Eventually, such a controlled porosity could also be exploited in other components of the batteries, e.g., as a separator in which optimized pore size could favor fast Li+ diffusion 10 or as a coating avoiding close contacts between the electrolyte and the silicon particles. Strengthening binders through coordination chemistry Although being the minor component of batteries electrodes, the role of the binder in the electrochemical performance of the Li-ion batteries is nowadays well acknowledged.11,12 The binder insures the cohesion of the constituents of the electrode (active material and conducting agent), as well as its adhesion to the current collector (typically a copper foil at the anode side), both during the preparation of the electrodes and under working conditions. Notably, in the case of Si electrodes, the binder should mitigate the very large volume variation (~ 300%) associated


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with the formation of LixSi without significant delamination, fractures and collapsing which would lead to the loss of the electronic wiring of the active mass. Polar organic polymers bearing carboxylate/ic groups, such as polyacrylic acid (PAA), sodium carboxymethylcellulose (CMC), sodium alginate (Alg) or arabic gum were first found to be more efficient than the conventionally used polyvinylidenefluoride (PVDF).12 Such improvements were attributed to the presence of strong interactions within the binder (hydrogen bonds, ionic interactions) and between the binder and the Si particles (notably through the formation of covalent Si-O-C(=O)R bonds and Si-OH••••O hydrogen bonds), all leading to a stiffening and better resistance towards a mechanical stress. Here, the medium strength of most coordination bonds, coupled with their easiness of formation, can be exploited to further improve the mechanical properties of the binders likely to increase the cohesion and adhesion strength of the Si electrodes. It is possible to exploit one the key characteristics of the carboxylate groups found in many polar binders, i.e. their notorious ability to bind to metallic cations, to produce directly a coordination-driven cross-linkage (also quoted ionic crosslinking). In 2014, three research groups independently reported that the addition of calcium chloride to sodium alginate gives rises to a clear improvement of the cyclability of Si and Si-C materials, with a net improvement of the capacity upon long term cycling.13–15 As an example, Zhang et al. studied the effect of the addition of calcium chloride (0.05-0.15 equivalent per alginate) to an electrode made of submicronic Si particles (average size 200 nm), super P carbon black and sodium alginate in a 70:15:15 weight ratio.15 The comparison of the reversible cycling capacity for the pristine Na-Alg-Si electrode and the Ca-Alg-Si electrode is shown in Figure 2. Similar capacities were observed for the first cycle (ca. 3500 mAh g-1), with a slightly more pronounced coulombic loss in the presence of calcium. Nevertheless, capacity fading was significantly reduced in the presence of calcium: for a mass loading of 1.1 mg cm-2 and a cycling


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rate of 0.1 C (0.42 A g-1), a capacity retention of ca. 86% after 200 cycles was observed in the presence of calcium alginate, while this value only reaches 21% for the sodium alginate based electrode. This effect was found to be less marked for higher Si loading, but still noticeable (capacity retention of 54% for a mass loading of 4 mg cm-2, see Figure 2).

Figure 2. Reversible charge capacity vs. number of cycles for various electrodes made of Si particles, super P carbon black and sodium alginate in a 70:15:15 weight ratio, in the absence (black) and presence of 0.15 equivalent of Ca ion per alginate (green and pink). Cycling range: 0.01-1V vs. Li, cycling rate: 0.1C. Reprinted from Ref. 15, copyright 2014, with permission from The Royal Society of Chemistry. The improvement of the cyclability was attributed to a higher level of crosslinkage, allowing the electrode tolerating large volume changes without impairing its mechanical and electrical integrity. Such a relationship between the cycling performance and the mechanical behavior was supported by peeling tests, which suggested a 4 times higher mechanical strength for the Ca-Alg-Si electrode than the Na-Alg-Si one. As evidenced by infrared (IR) spectroscopy, this behavior directly relies on the coordination ability of alginate chains, which are cross-linked through the formation of very


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well defined Ca-alginate egg-box like motifs, leading to both self-healing properties and toughness, as already reported in alginate biohybrids.16 From a practical standpoint, one of the strength of this method, when compared to e.g. covalent (organic) crosslinking, is that it does not require noticeable changes of the process of preparation: calcium is simply added in the slurry containing the constituents or the electrode,14,15 or further sprayed on an already deposited Si-carbon-Na alginate film.13 While electrodes are intrinsically very complex systems (multicomponent and multiscale) and hence sensitive to a large number of experimental parameters, this method of crosslinkage relies on the modification of a single parameter, allowing the fast evaluation of its effectiveness. Quite logically, the effect of the nature of the cation on the electrochemical performance was further explored.17,18 Wu et al. examined cations of various sizes (Ca2+, Ba2+, Zn2+, Mn2+) and charges (Al3+). They showed that alginate-cation gels presenting the higher viscosity and hardness, here obtained from Al3+ and Ba2+ ions, ultimately lead to a better retention of capacity upon cycling.17 The amount of cations was also found to be critical: a ratio of 0.1 to 0.15 cation per alginate seems to optimal to achieve a high capacity retention.15,18 Such an approach could also be extended to other polar binders: we demonstrated that upon maturation under humid atmosphere of an electrode made of ball-milled silicon, carbon black (CB) and CMC in a 80:12:8 molar ratio in a pH 3 citrate buffer solution, a discharge capacity of 1700 mAh g-1 was maintained after 200 cycles, while this value reaches less than 200 mA h g-1 after only 100 cycles for the non-maturated electrode (Si mass loading: 2 mg cm-2).19 Such an improvement is directly related to the slow corrosion of the copper current collector in the acidic medium, leading to the release of Cu2+ ions in the electrode, which are further able to coordinate to the carboxylate groups of the CMC, hence strengthening the cohesion and adhesion of the electrode and further impairing mechanical failures


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and rapid capacity fading. This process can also be directly induced upon the deliberate addition of metallic salts during the preparation of the electrode.20 Although it is as the moment difficult to fully rationalize these results, its appears clearly that the coordination environment (nature of the cation and complexing groups, number of cationbinder interactions, geometry of the first and second coordination spheres, length and strength of the cation-ligand bonds,…) has a direct impact on the electrode performance. Here, precise local information arising from e.g. IR, Raman or X-ray absorption (XAS) spectroscopies, possibility compared with model compounds of well-established structures, would help to draw (local) structure-properties relationships. Although the previous examples showed that increasing mechanical strength of the electrodes afforded improved electrochemical performance, one should not run for too rigid binders, that shows a high stress but a low strain at break as observed with conventional covalent cross-linking. As already pointed out by Kwon et al., medium strength binding is prone to dynamic behavior inducing self-healing properties, whereas strong binding would lead ultimately to irreversible rupture.12 Hence, an intermediate hardness is probably required to achieve optimal electrochemical performance, a regime which can be easily attained, as mentioned above, with coordination bonds. The addition of cations to the alginate binder was found to impact not only the mechanical properties, but also the nature and the stability of the SEI. Similarly to Ca2+, Ni2+ was found to have a beneficial impact on the cyclability of electrodes composed of silicon nanoparticles, acetylene black and Ni2+-alginate in a 70:15:15 weight ratio (Si mass loading = 0.45 mg cm-2).18 By combining electrochemistry, X-ray photoelectron spectroscopy (XPS) analyses and microscopy, the authors also revealed that only ca. 10% on the Ni2+ was reduced to Ni0 after cycling, suggesting that the coordination network is almost unaffected by the redox processes. The


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reduced Ni is likely involved in the SEI, which was furthermore shown to contain less lithium carbonate that in the nickel free electrode, suggesting a limited decomposition of the electrolyte in the presence of Ni2+ alginate. Ultimately, the incorporation of Ni2+ leads to a reduction of the SEI and charge transfer resistance, explaining the retention of the capacity of the electrodes even at high cycling rates (10 to 20 C). Extension to even more complex electrode materials, such as hybrid silicon-graphite electrodes involving two binders (CMC and styrene-butadiene rubber (SBR)) has been recently proposed21, as well as the combination of coordination and covalent crosslinking of alginate derivatives (with Ca2+ ions and polyacrylamide) to further increase the mechanical strengh.22 Eventually, coordination-based binders were found to be beneficial for other electrode materials than Si, notably cathode materials such as lithium rich manganese oxides23 and sulfur.24,25 Considering the variety of coordination motifs available, there is clearly room for enlarging the scope of accessible binders. On the inorganic side, it appears reasonable to focus mainly on low weight, electrochemically inert cations. On the organic side, it is tempting to expand this approach to other complexing groups that carboxylate, notably catecholate, which are both available in various polymers (catechol functionalized alginate,26 naturally occurring polymers such as tannins,…) and prone to coordinate to afford mussel-like crosslinked polymers.27,28 As the strength of the coordination bonds is known to increase with the charge of the cation and the pKa of the ligands, there is here a clear room to drastically modulate the mechanical properties of the binder; the next step will be to correlate quantitatively the coordination state to the mechanical properties.29 With the aim at a priori controlling the structure of the final coordination network, the use of prebuilt secondary building units, i.e. assemblies of cations and ligands of precise composition and well defined geometry,30 is also appealing. Si electrodes are mainly prepared in


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water, and a subtle variation of the pH might also affect the coordination sphere of the cation; this parameter should probably be considered with a special care, knowing that it probably evolves during the last step of preparation (drying of the Si – conductive carbon – binder mixture), and that it might not be homogenous. Such a tedious screening will be of poor interest if not coupled with suitable characterization tools, which should help to gain a better understanding of the coordination environments in the complete electrode, in order to attain a certain level of control, and even design for coordinationdriven binders. Here, the knowledge issued from the studies of molecular model compounds will be highly beneficial.

Coordination polymers as templating agents Mastering the Si particles size, shape and arrangements is another strategy to mitigate the volume change of Si electrodes upon cycling. Hollow Si particles are here particularly interesting, as the internal void space can in principle easily buffer the variation of the volume. Nevertheless, the preparation of such hollow particles relies on time consuming, multistep processes, which could hamper their practical uses. Here again, coordination chemistry presents some interesting features: first, coordination networks can be formed but also destroyed, in very mild conditions (notably low temperature). Second, MOFs particles can further present very different chemical groups at their surface (hydrophilic and hydrophobic, organic and inorganic) rendering them rather easy to functionalize with a broad set of components, including silicon precursors. Hence, such solids could act as easy-to-remove templates for hollow silicon particles with an appropriate morphology (size, porosity) for limiting their pulverization upon cycling. In this vein, Yoon et al. proposed to use MOFs nanoparticles as template, following the process schematized Figure 3.31 60 nm


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nanoparticles of the zinc methylimidazolate ZIF-8 (ZIF = Zinc Imidazolate Framework), which can be produced easily in solution with a narrow polydispersity, were first treated with tetraethylorthosilicate (TEOS) under alkaline solution, giving rise to an homogenous coating of SiO2. These core shell ZIF-8@SiO2 particles were further calcined to afford ZnO@SiO2 particles, which were then treated with magnesium to lead to ZnO@Si-MgO. Ultimately, oxides were etched through to an acid treatment, resulting in hollow Si cubic particles of ca. 60 nm size and with a wall thickness of 15 nm.

Figure 3. Preparation of Si hollow particles through MOF templating. Adapted from reference 31. When incorporated in electrodes made of Si, PAA-CMC and carbon Super P in a 70:15:15 weight ratio (mass loading 1 mg cm-2), the hollow particles exhibited upon cycling at 0.05 C a moderate initial capacity (< 2000 mAh g-1), but a capacity retention of more than 70% after 300 cycles. Scanning electron microsopy (SEM) analyses revealed that the variation of the electrode thickness after cycling was less than half of the one measured for a Si reference electrode, thus supporting the beneficial effect of the void space on the mitigation of the variation of volume.


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Although this method of preparation suffer from intrinsic limitations (multistep reaction, "costly" template, and a reduction step sensitive to experimental parameters as highlighted by the authors31), further developments could be proposed: - upon playing with the composition and synthetic conditions, it is possible to grow monodisperse particles of MOFs of controlled size in the 10-100 nm range with various shapes (cube, octahedron, platelet, rods, needles),32 offering potentially access to numerous Si hollow particles. Given MOFs crystals could further present exposed faces of different polarities (from very polar to very apolar), which will interact differently with Si precursors, ultimately offering the possibility to grow space-controlled Si coatings. - as mentioned in the introduction, the etching step may be performed on MOFs themselves rather than on oxides, hence simplifying the process of preparation.

Coatings and controlled SEI The destabilization of the SEI associated with the formation of cracks upon cycling is another parameter explaining to the poor cycling stability of silicon electrodes. Such an effect could be counterbalanced by the use of prebuilt, stable coatings on Si particles, that hamper the contact between silicon and the electrolyte while allowing charge (Li+ and electron) transport. Here, the sieving properties of MOFs could be exploited: upon playing with the pore size and polarity, it should be possible with a MOF coating to favor the diffusion of lithium while avoiding the one of the bulkier constituents of the electrolyte (solvent, anion). Two main methods, both involving low temperature wet processes, are currently available to achieve MOF coating on Si surfaces : the first one relies on their direct exposure to a solution containing the ligand and the metallic salt, and can be applied either to covalently functionalized 33,34 or non-functionalized35 Si surfaces, and the


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second one is based on the deposition of pre-formed MOF nanoparticles.36 Both methods were used to prepare Si electrodes for Li-ion batteries. Han et al. first reported the preparation of a sandwich Si electrode, which was obtained by casting preformed ZIF-8 nanoparticles on a prebuilt electrode made of successive layers of copper foil, carbon SP and Si micro- or nano-particles.37 For a Si mass loading capacity of 0.4-0.7 mg cm-2 and a thickness of 50 µm, a residual areal capacity of 42% was achieved after 50 cycles, while this value is close to 14% for the Cu-carbon-Si-carbon sandwich electrode analogue. Unfortunately, thicker electrodes (100-150 µm) gave rise to more pronounced capacity drop. The improvement of the cyclability in the presence of the MOF coating was attributed to both (i) the formation of a more stable SEI, by preventing the direct contact of the electrolyte with the Si and (ii) a facilitated diffusion of the Li+ ions through the porosity of the MOF, in line with the lower charge transfer resistance deduced from alternating current (AC) impedance measurements. This approach was extended to MOFs of various compositions and pore sizes. While a beneficial effect was always detected, this set of experiments suggests that the pore size is the key parameter: a pore aperture close in size to Li+ ensures to a good cyclability, whereas larger pores lead to rapid capacity fading (Figure 4). However, the worse behaving MOFs (namely MOF-5 and HKUST-1, where HKUST stand for Hong-Kong University of Science and Technology) are also the less chemically stable ones;38 this parameter could also affect the capacity retention.


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Figure 4. Capacity and capacity retention of copper-carbon super P-Si microparticles-MOF sandwich electrodes for various MOF materials. HKUST-1: Cu(BTC); MOF-5: Zn4O(BDC)6, ZIF-8: Zn(MeIm), ZIF-67: Co(MeIm), MIL-53: Al(OH)(BDC); NH2-MIL-53: Al(OH)(NH2BDC) with BTC: 1,3-5-benzenetricarboxylate, BDC : 1, 4-benzenedicarboxylate, NH2-BDC : 2amino-1, 4-benzenedicarboxylate, MeIm: 2-methylimidazolate, MIL : Materials Institut Lavoisier. Reprinted from Ref. 37. The similar approach was applied to TiN/Ti coated Si nanorods: the coating with ZIF-8 was this time realized by exposing directly an array of nanorods to a solution containing the Zn2+ salt and 2-methylimidazole, and a noticeable capacity retention upon cycling was again observed when compared to the non-coated nanorods.39 By playing with the experimental conditions (time and concentration), Yu et al. were also able to grow from a solution of Zn2+ and imidazole a broad variety of coordination coatings on arrays of Si nanorods.40 Whereas an amorphous, but continuous coating gives rise to an improved cyclability, a highly crystalline, but incomplete coating has a very limited impact. Knowing that the amorphous form of zinc imidazolate is very likely less porous than the crystalline one, this suggests that the main positive effect of the coating is to


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prevent the formation of an unstable SEI, rather that favoring the diffusion of ions through the pores. Whereas MOF coatings appear promising, few points need to be clarified: as mentioned above, the exact role of the coating remains under debate, as well as its evolution upon cycling. The beneficial or detrimental impact of the microporosity could be addressed more deeply, and the optimal thickness for the coating needs to be determined. Coordination coatings could also present a strong drawback: many MOFs, and at least those mentioned so far, are notorious electrical insulators, and might obiter electron transport. To circumvolve this problem, the afore mentioned MOF coatings could be used as intermediates to prepare conductive carbon coated Si particles by pyrolysis,41–43 although this transformation is likely accompanied by a loss of the textural properties of the MOFs. A more appealing method relies on the use of a judicious combination of organic and inorganic constituents which will favor the electrical conduction.44 Using Fe and polypyrole, Zhou et al. were able to produce an amorphous, but electrically conducting, homogeneous coordination coating on Si particles.45 Here, the improvement of the capacity retention upon cycling does not only rely on the formation of a stable SEI and mitigation of the volume change, but also on a proper electrical connection of the particles (and carbon) through the electrode.

Summary and perspectives Coordination networks are characterized by their diversity, arising from the chemical nature of the organic and inorganic moieties, in terms of geometry, bond strength, connectivity, reversibility, and so on, offering a very large panel of materials to be incorporated in Si electrodes for Li-ion batteries. Up to now, only few compositions have been considered and there is thus plenty of room


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for a broader screening. As mentioned above, coordination networks have been primarily considered as mechanical strengthener and as protective coating, but many questions remain open: - What is the exact behavior of these species upon cycling? For example, ZIF-8, which was used in numerous studies,37,39,40 although highly stable towards water, is prone to transform to complex zinc carbonates in the presence of carbon dioxide and water;46 could it also happen upon the direct exposure to carbonate arising from the degradation of standard carbonate electrolytes? Moreover, what about the stability of the coordination networks during the discharging step and when close to highly reducing materials (LixSi), could they interfere with the redox processes in play, especially when coordination networks are involved in the conduction paths? Up to now, this issue was only scarcely addressed both in the case of binding18 and coating,39 and clearly deserves a deeper attention. - As mastering the volume variation of the electrode is one of key factor of an enhanced cyclability, what about the use of intrinsically flexible MOFs,37 which can sustain very large volume change without any network disruption?47 - Most of the examples reported so far deal with low Si mass loading corresponding to low areal capacities ( typically

Silicon Electrodes for Li-ion Batteries. Addressing the Challenges

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