Silicon-based Electrodes Research Articles

Effects of adhesion and cohesion on the electrochemical performance and durability of silicon composite electrodes

Although the choice of binder is crucial in determining the electrochemical performance and durability of silicon-based electrodes, the underlying mechanisms (e.g., mechanical vs. chemical) are unclear. Here, we report a study of the effects of adhesion vs. cohesion on the electrochemical behavior of silicon nanoparticle/polymeric binder/carbon black (CB) electrodes on copper conductor by multiple techniques. Two types of polymeric binders, polyvinylidene fluoride (PVDF) and sodium alginate (SA), were chosen for this study. The results show that because of a sufficiently strong interface between polymer and the copper current collector, both Si/PVDF/CB and Si/SA/CB composite electrode laminates have sufficient adhesive strength with the Cu conductor to cause cohesive failure within the electrode laminate during peel test. However, the interfacial strength between SA and silicon is significantly higher than that between PVDF and silicon, resulting in stronger cohesion within the Si/SA/CB electrode (e.g., peel strength of 78.3 N/m for Si/SA/CB electrode and 8.7 N/m for Si/PVDF/CB electrode, respectively). With a higher cohesive strength provided by a stronger binder-silicon interface, superior cell performance was ensured for Si/SA/CB electrodes. Hydrogen bonding is likely responsible for the stronger SA-Si interface since neither PVDF nor SA bonds covalently with Si according to chemical analysis.

Immunoassay of C-reactive protein by hot electron-induced electrochemiluminescence at polystyrene-carbon black composite electrodes

Aromatic Tb(III) chelates can be detected down to subnanomolar concentrations at polystyrene-carbon black composite electrodes on the basis of hot electron-induced electrochemiluminescence, and used as electrochemiluminescent labels in bioaffinity assays. The excitation mechanism is based on chemiluminescent reactions that are initiated by the field emission/tunnel emission of hot electrons from the composite electrode. The composition and the properties of the novel composite electrodes were studied in detail. Hot electron-induced electrochemiluminescence intensities obtainable with the present composite electrodes are comparable to the previously used metal/insulator-type electrodes without exhibiting considerable long-lived solid state electroluminescence background emission, unlike e.g. oxide-coated silicon and aluminum electrodes.C-reactive protein was finally determined in a heterogeneous sandwich-assay by using the present composite electrodes as a solid support material for the capture antibodies. The detection antibody was labeled with a commercially available Tb(II) chelate. Calibration curve for the determination of C-reactive protein was linear over two orders of magnitude and the detection limit was well below the clinical reference value, approx. 1 μg l−1. Disposable polystyrene-carbon black composite electrodes are superior alternatives for e.g. silicon-based electrodes due to the low cost, easier manufacturing process and a very good performance e.g. in immunoassays.

Design of advanced thick anode for Li-ion battery by inserting a graphite/polymer buffer layer: An in-situ mechanical study

In this paper, silicon containing thick anode electrodes were investigated to provide a higher energy density and capacity for EV/HEV applications. In our study, a facile technique of adding a mechanical buffer between thick active material and current collector is proposed and tested by an in-situ measurement using white light interferometry. The electrodes with a modified structure deliver a significant improvement to mechanical stability as well as battery performance compared to conventional electrodes with the original structural design. Therefore, the methodology demonstrated here can probably be used to mitigate the deteriorating effect of mechanical failure in silicon-based electrodes, in which volume variation is usually considered as a severe issue, of lithium ion batteries.

The Electrochemical Performance of Silicon Nanoparticles in Concentrated Electrolyte.

Silicon is a promising material for anodes in energy-storage devices. However, excessive growth of a solid-electrolyte interphase (SEI) caused by the severe volume change during the (de)lithiation processes leads to dramatic capacity fading. Here, we report a super-concentrated electrolyte composed of lithium bis(fluorosulfonyl)imide (LiFSI) and propylene carbonate (PC) with a molar ratio of 1:2 to improve the cycling performance of silicon nanoparticles (SiNPs). The SiNP electrode shows a remarkably improved cycling performance with an initial delithiation capacity of approximately 3000 mAh g-1 and a capacity of approximately 2000 mAh g-1 after 100 cycles, exhibiting about 6.8 times higher capacity than the cells with dilute electrolyte LiFSI-(PC)8 . Raman spectra reveal that most of the PC solvent and FSI anions are complexed by Li+ to form a specific solution structure like a fluid polymeric network. The reduction of FSI anions starts to play an important role owing to the increased concentration of contact ion pairs (CIPs) or aggregates (AGGs), which contribute to the formation of a more mechanically robust and chemically stable complex SEI layer. The complex SEI layer can effectively suppress the morphology evolution of silicon particles and self-limit the excessive growth, which mitigates the crack propagation of the silicon electrode and the deterioration of the kinetics. This study will provide a new direction for screening cycling-stable electrolytes for silicon-based electrodes.

Deposition characteristics and behaviour of high-pressure cold-sprayed silicon powder

ABSTRACTThe high-pressure cold spraying technology was adopted to deposit silicon particles on the copper substrates for preparing the silicon-based electrode of lithium-ion batteries. The deposition of silicon powder was accomplished at two spray process parameter sets by impacting the particle stream or individual particle. The microstructure and the morphology of the silicon coatings were characterised by scanning electron microscopy. A single-silicon particle represents three different deposition behaviours. The mass gains of as-sprayed samples are negative over multiple spray passes, but the thin silicon coating with the particular covered structures is formed. Increasing the number of spray pass cannot cause the obvious increase in the coating thickness. The interfacial morphology of the silicon coatings indicates that, except for the plastic deformation of the copper substrate, both the mechanical interlock effect among the deposited silicon particles and even the chemical binding contributing to the effective bond of silicon particles to the substrate.

Novel Cell Design for an Operando and in-Situ Study of Li-Ion Battery Electrode Aging Mechanisms through SAXS and SANS Measurements

Due to an impressive and continuous improvement in performance, the use of lithium ion batteries (LiBs) has greatly increased since this technology was first commercialized by Sony in 1990 and is quickly spreading to more demanding applications. Today, LiBs are not only used to power portable electronic devices such as smartphones or laptops but also larger systems such as electric vehicles. One way of contributing to the continuing success of LiBs in the automotive field is to increase their energy density. Silicon has received considerable attention in recent years due to its high theoretical capacity. However, because of (i) the severe volume expansion/shrinkage of this material during cycling, which leads to the pulverization of the electrode and to a rapid capacity fading, and (ii) the first cycle high irreversible capacity, the implementation of silicon-based negative electrodes in commercial LiBs remains challenging. Two possible solutions have been approached to overcome these problems: (i) Reducing the size of the silicon particles, and (ii) using dual-phase (a dispersed active phase in an inactive host matrix) composite electrodes. Pulverization is alleviated by reducing silicon particles to nanosized particles or by using a homogenous inactive host matrix which acts as a buffer against volumetric changes. However, because of poor electrode structural stability during cycling, the use of nanoparticles only cannot reduce the capacity degradation and hence, be a practical solution. Even though the total capacity of silicon-based composite electrodes cannot be as high as that of pure silicon electrodes, composite electrodes allow a low first cycle irreversible capacity and good capacity retention. We have undertaken the study of the aging mechanisms in composite Si based electrodes, by means of small angle X-ray and neutron scattering (SAXS and SANS). These characterization techniques give access to the electrode nanostructure and its evolution upon cycling. Ex-situ measurements provided useful information on the evolution of the morphology of small domains. Indeed, initial results of SANS measurements performed at the Laue Langevin Institute (ILL, Grenoble), presented in Figures a and b, show the existence of two intensity regions at high and low Q values. Those respectively give information on small and large domains within the electrode. Interestingly, upon lithiating the electrode, the intensity in the intermediate and high-Q regions increases, while the low-Q region remains unchanged (Figure b). Moreover, after 100 cycles, the pristine electrode spectrum is recovered whereas it is not the case after 300 cycles (Figure a) thus showing signs of aging from the silicon-based composite electrode. In order to probe in real-time the effects of lithiation/delithiation on the composite anode nanostructure and, avoid any risk of altering the sample in between the opening of the cell and sample preparation, in-situ and operando measurements were conducted. For this purpose new electrochemical cells (Figure c) were designed using respectively beryllium or titanium windows. Particular care has been taken to ensure that the cycling conditions of the electrochemical system are as close as possible to those of a coin cell, with the use of a separator and applied pressure on the electrodes. In this talk we will present the first results on the evolution of the nanostructure of the composite electrodes. Figure 1

Silylated functionalized silicon-based composite as anode with excellent cyclic performance for lithium-ion battery

Inferior cycling stability and rate performance respectively caused by rigorous volume change and poor electrical conductivity were the main challenge of state-of-the-art Silicon-based electrode. In this work, silylated functionalized exfoliated graphite oxide (EGO)/silicon@amorphous carbon (3-APTS-EGO/Si@C) was synthesized by adopting silane as intermediate to connect Si particles with EGO sheets followed by introduction of amorphous carbon. The result suggested that 3-Aminopropyltriethoxysilan connected the EGO sheets and Si nanoparticles via covalent bonds. Owing to the strong covalent interaction and the synergistic effect between the silicon, EGO sheets and amorphous carbon, 3-APTS-EGO/Si@C composite possessed a high capacity of 774 mAh g−1 even after 450 cycles at 0.4 A g−1 with the retention capacity of 97%. This work also provided an effective strategy to improve the long cycling life performance of Si-based electrode.

Silicon-Based Composite Negative Electrode Prepared from Recycled Silicon-Slicing Slurries and Lignin/Lignocellulose for Li-Ion Cells

A large amount of kerf loss silicon slurries has been produced in the photovoltaics industry by direct diamond-wire slicing. The high-purity silicon particles in the slurries are suitable for reutilization as anode materials for lithium-ion batteries. In this study silicon particles from the kerf loss of silicon ingot slicing, coupled with lignin or lignocellulose as carbon precursors, are employed to form carbon–silicon composite materials. A pyrolysis thermal treatment in the presence of argon was applied to carbonize the biomass on the silicon materials in order to increase the conductivity of silicon-based anodes. Due to the different carbonaceous precursors, the composites formed different structures. The lignin-silicon electrode with a carbon-coated structure delivered an initial charge capacity of up to 2286 mAh/g and retained 880 mAh/g after 51 cycles at 300 mA/g. On the other hand, the pyrolyzed lignocellulose formed an interconnected structure with silicon particles, providing extra space to acc...

Silicon-based tool electrodes for micro electrochemical machining

Stray current always causes undesired material dissolution in micro electrochemical machining (ECM). The preparation of reliable insulating films on micro electrodes is a critical technique to restrict the stray current, and then improves the machining localization. To meet the requirements for insulating films like good insulation property, thin & uniform thickness and good adhesive strength, a novel method of silicon-based tool electrode is proposed. Heavily doped monocrystalline silicon is used as the electrode body. Silicon dioxide and silicon nitride are deposited on electrode sidewall as insulating films. Through simulations of electrode conductivity, the feasibility of the silicon-based tool electrode is preliminarily verified. Then, a fabrication process of the silicon-based tool electrode is presented. The electrode body is fabricated by wet etching and insulating films are deposited by low pressure chemical vapour deposition (LPCVD). Consequently, the electrodes with section size of 91 × 52 μm, 1–2 mm length and 800 nm-thick insulating films are obtained. As the silicon-based electrode is installed on a rotating head in ECM experiments, the electrode with non-circular cross section not only achieves the same effect with a cylindrical electrode, but also is beneficial to removing electrolytic products through the turbulent electrolyte flow. In experimental results, micro grooves with steep sidewall and smooth undersurface (Ra 0.42 μm) are machined. Micro holes with inlet diameter of 146 μm and taper angle of 0.58° are obtained. They indicate the feasibility of silicon-based tool electrodes and the effect of insulating films on stray corrosion restriction.

Effect of Binder Architecture on the Performance of Silicon/Graphite Composite Anodes for Lithium Ion Batteries.

Although significant progress has been made in improving cycling performance of silicon-based electrodes, few studies have been performed on the architecture effect on polymer binder performance for lithium-ion batteries. A systematic study on the relationship between polymer architectures and binder performance is especially useful in designing synthetic polymer binders. Herein, a graft block copolymer with readily tunable architecture parameters is synthesized and tested as the polymer binder for the high-mass loading silicon (15 wt %)/graphite (73 wt %) composite electrode (active materials >2.5 mg/cm2). With the same chemical composition and functional group ratio, the graft block copolymer reveals improved cycling performance in both capacity retention (495 mAh/g vs 356 mAh/g at 100th cycle) and Coulombic efficiency (90.3% vs 88.1% at first cycle) than the physical mixing of glycol chitosan (GC) and lithium polyacrylate (LiPAA). Galvanostatic results also demonstrate the significant impacts of different architecture parameters of graft copolymers, including grafting density and side chain length, on their ultimate binder performance. By simply changing the side chain length of GC-g-LiPAA, the retaining delithiation capacity after 100 cycles varies from 347 mAh/g to 495 mAh/g.

Three-dimensional rice husk-originated mesoporous silicon and its electrical properties

A facile and cost-effective process to synthesize nanostructured silicon from rice husk-originated silica is introduced. Rice husk was acid-washed and then calcined at various temperatures in the range of 600–900 °C. Nanostructured silicon was obtained through magnesiothermic reduction process of SiO2. Products of the magnesiothermic process, elemental silicon together with magnesia and magnesium silicate, were subjected to leaching to remove the latter two phases. Based on XRD and XRF results, nanometric silica powder obtained after 3 h calcination at 700 °C was selected for further processing. After reduction process, the formation of a three-dimensional silicon skeleton 70–90 nm in thickness with the specific surface area of 53.02 m2/g was confirmed through HRTEM, SEM and BET observations, in agreement with X-ray pattern. The electrical response of the silicon-based electrode was measured to be 0.0023 Ω m, indicating the employed synthesis route as a promising one for electrode fabrication.

Theoretical optimization of electrode design parameters of Si based anodes for lithium-ion batteries

Silicon is considered one of the most promising anode materials for next-generation lithium-ion batteries. However, dramatic volume expansion during the lithiation of silicon complicates its practical implementation. Literature reports nanostructured electrodes, which are capable to accommodate the volume expansion, reducing associated swelling, degradation and capacity fading. However, several phenomena associated with the volume expansion, such as the reduction of the electrode’s porosity, are inherent to the system and must be carefully considered for targeted engineering of high-energy lithium-ion batteries. Herein, we determine design criteria of silicon based electrodes, taking into account the volume expansion during lithiation. A “deformation threshold” is defined signifying the minimum value of the initial porosity that must be adjusted to avoid plastic deformation and dramatic reduction of the electrode’s porosity during charging. In addition, a “C-rate threshold” is defined, guaranteeing diffusion limited currents not falling below a desired discharge rate. The impact of these theoretical limitations on the electrochemical performance of silicon-based electrodes is analyzed from an engineering point of view. The derived relations are used to optimize the electrode design parameters regarding maximum gravimetric and volumetric capacity.

A comparative study of polyacrylic acid (PAA) and carboxymethyl cellulose (CMC) binders for Si-based electrodes

This work is a comparative study on two well-known binder systems for silicon-based negative composite electrodes for Li-ion batteries: polyacrylic acid (PAA) and carboxymethyl cellulose/citric acid (CMC/CA). The electrode slurry rheological properties and the dry electrode morphology and physical properties are studied as a function of the binder content. The electrochemical behavior of the electrodes is also studied as a function of the active mass loading (from 1 to 4.5 mg cm−2). Increasing the binder content from 4 to 12 wt% improves the 1st cycle efficiency, which is clearly related to its contribution as a pre-formed artificial solid electrolyte interphase (SEI) layer. The enhancement of the mechanical strength of the composites with increasing binder content is reflected in the better cyclability of the electrodes with active mass loading higher than 1 mg cm−2. CMC/CA is more efficient as a binder at low content while PAA is more efficient at high content, which could be related to their molecular structure. Finally, for both binders the influence of a post-processing treatment of the electrodes (called maturation) is shown to improve the electrochemical performance. This is however more significant with CMC/CA than with PAA.

Investigation of the Porosity and Tortuosity of Silicon-Graphite Electrodes in Lithium-Ion Batteries

Silicon-based electrodes are promising candidates for the next generation of Lithium-ion batteries with cell-level volumetric energy densities above 700 Wh L-1 cell.1,2 However, the practical energy density of these electrodes is strongly influenced by the electrode porosity and the volume expansion during repeated charge-discharge.3 In addition, the electrode’s rate capability is largely determined by its tortuosity, which continuously increases upon cycling as a consequence of ongoing electrolyte decomposition at the silicon-electrolyte interface.4 In the present study, we investigate the influence of the porosity and tortuosity on the cycling performance of silicon-graphite electrodes. Hence, we prepared electrodes with a practical areal capacity of 4.0 mAh cm-2, comprising 35 wt% nanometer-sized silicon and 58 wt% micron-sized graphite particles. Lithium poly(acrylic acid) binder and C65 conductive carbon accounted for the remaining 7 wt%. By use of a hydraulic press, the electrode porosity was adjusted to values ranging from 30% to 50%. As electrolyte solution, 1 M LiPF6 dissolved in a 3:7 (w:w) mixture of ethylene carbonate:ethyl methyl carbonate (LP57) with 5 wt% fluoroethylene carbonate (FEC) as additive was used. First, the tortuosity and MacMullin number of the pristine silicon-graphite electrodes were determined by use of impedance spectroscopy on symmetrical cells in blocking conditions.5 The electrode polarization and rate capability of these electrodes were then investigated in coin-cells with lithium metal foil as counter electrode. After 50 cycles, the silicon-graphite electrodes were harvested from the cells and examined in terms of their thickness and morphological changes by cross-sectional scanning electron microscopy. Finally, the FEC consumption upon cycling was quantified by post-mortem 19F-NMR analysis of the residual electrolyte to estimate the pore volume of the electrodes that was occupied by electrolyte decomposition products.6 This contribution concludes with a comparison of silicon-graphite electrodes with different porosities and the quantification of the resulting degradation phenomena. Our research helps to better understand the limiting processes in silicon-graphite electrodes upon cycling and provides useful parameters for the modelling and simulation of these electrodes.

Investigation of cycling-induced microstructural degradation in silicon-based electrodes in lithium-ion batteries using X-ray nanotomography

The microstructural degradation of a composite silicon electrode at different stages in its cycle life was investigated in 3D using X-ray nano-computed tomography. A reconstructed volume of 36μm×27μm×26μm from the composite electrode was imaged in its pristine state and after 1, 10 and 100 cycles. Particle fracturing and phase transformation was observed within the electrode with increased cycling. In addition, a distinct, lower X-ray attenuating phase was clearly resolved, which can be associated with surface film formation resulting from electrolyte breakdown and with silicon particle phase transformation. Changes in quantified microstructural properties such as phase volume fraction and particle specific surface area were tracked. Electrode performance loss is associated with loss of active silicon. These imaging results further highlight the capability of high resolution X-ray tomography to investigate the role of electrode microstructure in battery degradation and failure.

Sea Sand-Derived Magnesium Silicide as a Reactive Precursor for Silicon-Based Composite Electrodes of Lithium-Ion Battery

Recently, it has been clearly elucidated that nanostructured Si-based composites hybridized with protective and conductive materials can present enhanced electrochemical performance as anodes for Li-ion batteries (LIBs). One of remaining issues is to develop a sustainable and economic method to synthesize these composites on a large scale for industrial applications. Herein, we introduce a modified magnesiothermic reaction route to prepare the aforementioned Si-based composite electrodes using sea-sand derived Mg2Si as a reactive precursor. Owing to its reducibility and lability, Mg2Si can readily reduce group IVA oxides, such as Na2CO3, SiO2, GeO2, and SnO2, resulting in macroporous Si surrounded by the reduced forms of the counter reactants (C, Si, Ge, and Sn, respectively), some of which can be electrochemically attractive. Notably, the porous Si-based composite can be synthesized by a simple solid state reaction, so simplicity and scalability can be obtained. Also, the sea sand precursor is naturally–abundant; hence this process can be cost-effective, scalable, and sustainable. Porous Si@C composite can be synthesized from the modified magnesiothermic reaction using a sea sand-derived Mg2Si precursor, showing a specific capacity of 1000 mAh/g at 200th cycle. Potentially this process can be used for practical synthesis of Si-based composites.

Understanding ferroelectric Al:HfO2 thin films with Si-based electrodes for 3D applications

Ferroelectric hafnium oxide is a promising candidate for logic and memory applications as it maintains excellent ferroelectric properties at nm-size ensuring compatibility with state of the art semiconductor manufacturing. Most of the published papers report on the study of this material through Metal-Insulator-Metal capacitors or Metal-Insulator-Silicon transistors. However, for 3D vertical transistors in which both the channel and gate are polysilicon, the case of silicon-based electrodes cannot be ignored. In this paper, we report the fabrication of various ferroelectric capacitors with silicon (S) based conductive layers and titanium nitride metal (M) electrodes using aluminum doped hafnium oxide (I). The ferroelectric device with silicon-based electrodes shows superior polarization and steeper switching. These results pave the way toward 3D integration for potential 3D NAND replacement.

Difec the Rescue? Evaluating Di-Fluoroethylene Carbonate As Additive for Silicon Electrodes

Fluoroethylene carbonate (FEC) is one of the most commonly used electrolyte additives for silicon-based electrodes in Li-ion batteries.1 Several studies demonstrated an improved cyclability with FEC-containing electrolytes and ascribed the effect to the formation of a kinetically stable SEI with an increased LiF and Li2O content.2 The SEI morphology also revealed less fractures and a better passivation of the silicon particles upon cycling.3 Nonetheless, recently it was reported that FEC only suppresses the decomposition of other electrolyte compounds, such as ethylene carbonate (EC), as long as FEC is present in the electrolyte.4,5 As a consequence, the lifetime of silicon-based electrodes in Li-ion batteries is limited by the total amount of FEC in the electrolyte solution. In the present study, we evaluate di-fluoroethylene carbonate (DiFEC) as alternative electrolyte additive for silicon-based electrodes. Taking into account the beneficial effect of FEC on the cycling performance, we seek to enhance the additive’s properties by addition of a second fluorine atom. Recently, Dahn and co-workers investigated DiFEC in regard to its oxidative decomposition in NMC442/graphite cells.6 In the same study, they also demonstrated that DiFEC is a SEI forming additive on the negative electrode. To benchmark the electrochemical behavior of DiFEC on silicon electrodes, we prepared three electrolyte solutions, consisting of (i) 1 M LiPF6 in EC:ethyl methyl carbonate (3:7 w:w) – also referred to as LP57; (ii) 5 wt% FEC in LP57, and (iii) 5 wt% DiFEC in LP57. As working electrode, we prepared silicon-graphite (SiG) composite electrodes with 35 wt% silicon and 45 wt% graphite with an areal capacity of 2.1±0.2 mAh cm-2 (~1.5 mgelectrode cm-2). Lithium poly(acrylate) binder and vapor grown carbon fibers accounted for the remaining 20 wt%. Figure 1 . Differential capacity of Li//SiG coin-cells, incorporating different (fluorinated) EC derivatives (each 5 wt%) in LP57, obtained from a galvanostatic scan at 0.02 h-1. By use of dQdV analysis of galvanostatic scans in Li//SiG cells, as shown in Figure 1, we demonstrate that DiFEC reductively decomposes at more positive potentials compared to FEC and EC. Further, we evaluate the cycling stability in SiG//LiFePO4 cells with capacitively oversized cathodes and quantify the (Di)FEC consumption upon cycling by post-mortem 19F-NMR analysis. Our results reveal a trade-off between capacity retention and irreversible capacity losses within 300 cycles. To understand the corresponding processes at the SiG electrode-electrolyte interface, we conduct impedance measurements to monitor the impedance evolution upon cycling, using a gold-wire reference electrode that allows to separate the impedance contributions from the negative and positive electrode.7 Finally, we discuss our findings with the aid of on-line electrochemical mass spectrometry (OEMS) and propose a reduction mechanism for DiFEC on silicon-based electrodes.

Reducing Electrolyte Consumption of Silicon-Electrodes through Capacity-Limited Cycling

Capacity-limited cycling constitutes a practical approach to improve the cycling stability of silicon-based electrodes in Li-ion batteries.1–3 As the volume expansion of silicon increases with the uptake of lithium, the degree of lithiation (LixSi) displays a trade-off between delivered capacity and detrimental volume changes.4 Composite electrodes, consisting of silicon and graphite active materials, are considered as a compromise between the high capacity of silicon and excellent cycling performance of graphite.5 However, decreasing the amount of silicon in these electrodes also increases the need for a higher degree of lithiation to obtain a commercially reasonable electrode specific capacity in the order of 1000 mAh g-1 el.6 In this study, we investigate silicon-graphite composite electrodes with different active material ratios (20-60 wt% silicon) in regard to their degradation during capacity-limited cycling. Besides silicon nanoparticles (~200 nm diameter) and graphite (~20 µm diameter) as active materials, the electrodes comprise conductive carbon fibers and lithiated poly(acrylic acid) as binder. The areal capacity of all electrodes was set to 2.1 ± 0.2 mAh cm-2. As electrolyte solution, 1 M LiPF6dissolved in a 3:7 (w:w) mixture of ethylene carbonate:ethyl methyl carbonate (LP57), and 5 wt% of fluoroethylene carbonate (FEC) was used. To investigate the degradation behavior, the electrodes are always fully lithiated to 0.01 V vs. Li/Li+ and only partially delithiated until a capacity-limit of 800 mAh g-1 el. As the theoretical capacity of these electrodes strongly depends on the silicon content and increases from ~960 to ~2200 mAh g-1 el, the practical capacity utilization of these electrodes to deliver 800 mAh g-1 el consequently behaves inversely to the silicon content and thus decreases from ~90 to ~40%. By evaluating the cycling performance vs. capacitively oversized LiFePO4 electrodes, we demonstrate that none of the investigated electrodes shows a visible capacity fade within 120 cycles. However, by detailed analysis of the coulombic efficiency and voltage profiles, we show that in spite of a decreasing silicon content the irreversible capacity loss and electrode polarization increase with the capacity utilization of the electrode. In accordance with that, we demonstrate through 19F-NMR analysis of the electrolyte solutions after different number of cycles that the FEC consumption also decreases with the capacity utilization. Supported by theoretical considerations of the surface area changes of silicon particles, we finally compare capacity- and voltage-limited cycling procedures and conclude that the degree of lithiation, and thus the capacity utilization, is a key parameter in the design of commercial silicon-based composite electrodes. Furthermore, we present a practical set of analyses to differentiate the cycling performance of silicon-based electrodes and to quantify the underlying degradation mechanisms.

Impact of a Film Maturation Process on the Mechanical Stability of Silicon-Based Electrodes with High Areal Capacities

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