Si Composite Electrode Research Articles

Effects of polymeric binders on the cracking behavior of silicon composite electrodes during electrochemical cycling

Mechanical degradation caused by lithiation/delithiation-induced stress and large volume change is the primary cause of fast capacity fading of silicon (Si)-based electrodes. Although intensive efforts have been devoted to understanding electromechanically induced fractures of electrodes made of Si alone (e.g., Si particles, Si thin films, and Si wafers), the cracking behavior of Si/polymeric binders/carbon black composite electrodes is unclear and poorly understood. Here, we investigate, by in situ and ex situ techniques, the cracking behavior of Si composite electrodes made with different binders, including polyvinylidene fluoride (PVDF), sodium-alginate (SA), sodium-carboxymethyl cellulose (Na-CMC), and Nafion. We found that extensive cracks form during the 1st delithiation process, which periodically open and close during subsequent lithiation/delithiation cycles at the same locations in the Si composite electrodes made with SA, Na-CMC, and Nafion. In contrast, a significantly fewer number of cracks form in the Si/PVDF electrodes after electrochemical cycling. A possible mechanism is proposed to help understand the effects of binders on the cracking behavior (e.g., crack spacing and island size) of Si composite electrodes. We also suggest possible approaches, including reducing the electrode thickness, patterning electrodes, and using highly recoverable binders, to inhibit cracks and improve the mechanical integrity of Si composite electrodes.

Sheet-type Li6PS5Cl-infiltrated Si anodes fabricated by solution process for all-solid-state lithium-ion batteries

All-solid-state lithium-ion batteries (ASLBs) employing sulfide solid electrolytes are considered a promising alternative to conventional lithium-ion batteries (LIBs) from the perspectives of safety and high energy density. From a practical point of view, the development of sheet-type electrodes employing alternative electrode materials by scalable fabrication is of prime importance. While Si has been extensively studied for next-generation LIBs, reports on ASLBs are scarce. Herein, we fabricate sheet-type Si composite electrodes by infiltrating conventional LIB electrodes with solid electrolytes using a homogeneous Li6PS5Cl-ethanol solution. Further, we systematically investigate effects of the particle size (micro- vs. nano-Si) and polymeric binders (polyvinylidene fluoride vs. polyacrylic acid/carboxymethyl cellulose) on the electrochemical performance of ASLBs under varying external pressures (140, 20, and 5 MPa) upon cycling. Owing to intimate ionic contacts enabled by liquefied solid electrolytes, the Li6PS5Cl-infiltrated Si electrodes show higher capacities of over 3000 mA h g−1 at 0.25 mA cm−2 and 30 °C as compared with conventional dry-mixed electrodes. At 20 MPa, the Si electrodes using micro-Si and polyvinylidene fluoride show marginal degradation of performance. The high energy density of 338 W h kg−1 of LiCoO2/Si ASLBs fabricated using the Li6PS5Cl-infiltrated electrodes is demonstrated, highlighting the prospect of high-energy practical ASLBs.

Influence of polymeric binders on mechanical properties and microstructure evolution of silicon composite electrodes during electrochemical cycling

Polymeric binders are a critical component to enhance mechanical integrity, maintain electronic conductivity, and achieve long durability of silicon (Si)-based electrodes. A fundamental understanding of the relationship between binder properties and mechanical degradation of Si electrodes is indispensable to developing durable Si-based electrodes. Using an environmental nanoindentation system, we measured the mechanical properties of Si composite electrodes made with different binders, including polyvinylidene fluoride (PVDF), Nafion, sodium-carboxymethyl cellulose (Na-CMC), and sodium-alginate (SA), as a function of the state-of-charge and cycle numbers under both dry and wet conditions. In contrast to electrodes made of Si alone, both elastic modulus (E) and hardness (H) of Si composite electrodes increase with lithium concentration within each cycle. E and H continuously decrease during long-term cycling. The mechanical property evolution of Si composite electrodes can be correlated with the porosity and irreversible thickness changes, which are largely determined by the mechanical properties of binders, instead of the adhesion between binders and Si. Electrodes under wet conditions have smaller E and H values than those under dry conditions because binders soften in the electrolyte. These findings not only provide useful mechanical parameters for battery modeling, but also may help design high performance and durable Si-based electrodes.

Real-time measurements and experimental analysis of material softening and total stresses of Si-composite electrode

The challenges facing high-performance lithium-ion batteries are not only chemical, but also key mechanical problems. Herein, we perform an experiment to investigate the crucial but difficult task of measuring the elastic modulus and total stress evolutions of a bilayer Si-composite electrode during electrochemical processes. Electrode bending deformation is measured in-situ through an optical system. And four mechanical models featuring Li concentration-dependent elastic modulus are used to quantitatively convert curvature into elastic modulus, normal stresses in electrode material and in current collector, and shear stress along interface. Experiments show that the elastic modulus and the three stresses decrease and increase nonlinearly with capacity, respectively, and all demonstrate hysteresis loops during lithiation and delithiation processes. The significant softening of electrode material indicates that the elastic modulus is not linear with Li content. And further discussions show that all the three stresses are significantly affected by elastic modulus, exhibiting a degree of relief similar to material softening. These results guide the mechanical modeling and provide experimental data for mechano-electro-chemical degradation for Si-composite electrode.

Operando Quantification of (De)Lithiation Behavior of Silicon–Graphite Blended Electrodes for Lithium‐Ion Batteries

AbstractDue to the high lithium capacity of silicon, the composite (blended) electrodes containing silicon (Si) and graphite (Gr) particles are attractive alternatives to the all‐Gr electrodes used in conventional lithium‐ion batteries. In this Communication, the lithiation and delithiation in the Si and Gr particles in a 15 wt% Si composite electrode is quantified for each component using energy dispersive X‐ray diffraction. This quantification is important as the components cycle in different potential regimes, and interpretation of cycling behavior is complicated by the potential hysteresis displayed by Si. The lithiation begins with Li alloying with Si; lithiation of Gr occurs at later stages when the potential dips below 0.2 V (all potentials are given vs Li/Li+). In the 0.2–0.01 V range, the relative lithiation of Si and Gr is ≈58% and 42%, respectively. During delithiation, Li+ ion extraction occurs preferentially from Gr in the 0.01–0.23 V range and from Si in the 0.23–1.0 V range; that is, the delithiation current is carried sequentially, first by Gr and then by Si. These trends can be used for rational selection of electrochemical cycling windows that limits volumetric expansion in Si particles, thereby extending cell life.

In situ fabrication of nanohybrid carbon/polyamide film providing robust binding and conductive network in silicon anode for lithium-ion battery

Engineering of a composite electrode containing Si-based anode materials is important for the practical applications of Si-based lithium-ion batteries. In this work, a hybrid composite anode of silicon/carbon/polymer was fabricated in situ by optimized heat treatment of the coating slurry containing silicon power and polyamic acid polymer, without adding any carbon agent. The resulting electrode has a three-component (silicon-carbon-polyamide) composite structure, wherein the nanohybrid carbon/polyamide matrix is in-situ generated from polyamic acid by optimized heat treatment. The hybrid Si composite electrode exhibits robust binding and conductive properties, and delivers promising battery performance even for high contents of the micron-sized Si powder (∼95 wt%). Consequently, the proposed method for Si anode engineering is another effective way to manufacture advanced lithium-ion batteries.

Utilizing Room Temperature Liquid Metals for Mechanically Robust Silicon Anodes in Lithium‐Ion Batteries

AbstractSilicon has attracted attention as one of the most promising anode materials for future generation energy storage devices, owing to its high theoretical specific capacity. However, its use has been impeded by significant volume expansion during electrochemical lithiation. In this work we utilise liquid galinstan (68.5 % Ga, 21.5 % In and 10 % Sn) to repair the cracking caused by volume expansion in Si thin film and Si composite electrodes. A bridging mechanism is observed, through which the electrical conductivity can be significantly improved. The composite electrode delivers an initial capacity of 2697 mAh g−1 and reaches a coulombic efficiency ∼100 % after 140 cycles. In addition, nanoindentation tests show that a better mechanical behavior is achieved with increased flexibility. We believe that these results may provide a way to prepare mechanically robust high capacity electrodes.

A 1D Mathematical Model to Study the Effects of Adhesive Strength on the Electrochemical Performance of Silicon Electrodes for Advanced Lithium Ion Batteries

In recent times, commercially available lithium ion batteries (LIBs) employ graphite, with a theoretical capacity of 372 mAh g-1, as their anode material. To enhance the energy density of LIBs, alternative anode materials with higher capacities, such as Li metal, Si, and various alloys, have been extensively exploited. With its advantages of a higher theoretical capacity (4200 mAh g-1), environmental friendliness, and terrestrial abundance, Si is particularly well-suited as an anode material for this purpose. Nevertheless, Si suffers from large volume variations (up to 300%) during lithiation/delithiation processes, leading to pulverization, low cycling efficiency, unstable solid electrolyte interphases (SEI), and permanent capacity losses. There have been several scientific investigations and strategies to solve the issues related to the large volume changes in Si. Relevant breakthroughs have been made with the help of advanced nanotechnologies in terms of enhancing the cycle life and improving the cycle rate performance. Notable among them is the use of binding materials with improved adhesive properties, such as copolyimide, alginate, poly(acrylic acid) (PAA), carboxymethyl cellulose, polyamide-imide, polyimide, and polymers that form cross-linked networks. Moreover, enhancing the adhesive strength between the Si composite electrode and Cu current collector by coating the latter’s surface with a very thin polymeric layer has significantly enhanced the electrochemical performance of Si anodes. However, the mechanism underlying this improvement in performance was not clearly addressed. In this presentation, a coupled chemo-mechanical model which considers the contact resistance as well as the influence of the attractive forces inside the contact area between the electrode and current collector was developed to study the effects of the adhesive strength of a binding material on the electrochemical performance of silicon-based lithium-ion batteries. The model predictions were validated with experimental data from coin-type half-cells composed of Li metal, Si electrodes, and Cu current collectors coated with binding materials with different adhesive strengths. The proposed model was used to investigate the effects of the adhesive strength and various cell design parameters on the specific capacity of the Si-based Li-ion cells.

Synthesis and Characterization of Silicon/Reduced Graphene Oxide Composites as Anodes for Lithium Secondary Batteries.

Silicon (Si) is one of the most attractive anode materials for lithium secondary batteries because of its large theoretical capacity, high safety, low cost and environmental benignity. However, Si-based anode material needs to overcome the structural change of the solid-electrolyte interphase due to the large volume change during cycling. To resolve these problems of composites by exploiting the superior conductivity, large specific surface area and flexibility of graphene, we have synthesized reduced graphene oxide (rGO)/Si composite electrode via a simple dip-coating method. Nickel foam is used as a current collector and template for the electrode fabrication. At 0.03 wt%, Si concentration, the rGO/Si composite anode presented the excellent cycle performance with large reversible capacity (778 mAh g-1 after 100 cycles). The characteristics of the rGO/Si composites were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Raman and X-ray photoelectron spectroscopy (XPS). The improved anode performance of the rGO/Si composite anode is ascribed to the rGO serving as a buffer layer, thereby preventing the volume expansion of Si nanoparticles, and provide facile electron pathways.

Low-cost and sustainable corn starch as a high-performance aqueous binder in silicon anodes via in situ cross-linking

In situ cross-linked corn starch is used as a binder in silicon (Si) composite anodes for enhancing the cycling stability of lithium-ion batteries. The corn starch binder is in situ cross-linked by maleic anhydride during the fabrication of Si anodes. The cross-linking reaction of corn starch is investigated by infrared spectroscopy, solid-state 13C nuclear magnetic resonance spectroscopy and thermal gravimetric analysis. The results show that the hydroxy groups of corn starch react with maleic anhydride to form ester linkages, which eventually form a three-dimensional network structure. The cross-linked corn starch binder significantly enhances the adhesion of the Si/conductive carbon composite on the copper current collector. Peeling test experiments show that the adhesion force of the cross-linked corn starch binder is 4.9 times higher than that of the corn starch binder without cross-linking. Moreover, the cell performance tests show that the Si composite cathode with the cross-linked corn starch binder exhibits a high specific capacity of 3720 mAh g−1 and enhances cycle-life performance. This in situ cross-linking approach underscores the potential of corn starch, which is a low-cost and environmentally sustainable material, as a binder for the Si composite electrodes to improve its electrochemical performance.

Mechanical Properties and Microstructure Evolution of Silicon Composite Electrodes

Silicon is considered one of the most promising negative electrode materials for future lithium ion batteries (LIBs) due to its high capacity (3579 mAh g-1 , based on Li15Si4) and its proper delithiation voltage (~ca. 0.4 V vs. Li/Li+). However, the huge volume change (~300 %) of Si during lithiation/delithiation leads to fracture and pulverization and of Si particles, continuous formation of solid-electrolyte interphase (SEI), and, consequently, fast capacity fade. Mechanical degradation remains a major barrier for commercializing Si electrodes. The most likely commercial Si electrodes, i.e., Si composite electrodes consisting of Si particles, conductive additives, polymeric binders, and porosity, are likely to exhibit mechanical properties and volume expansion/shrinkage behavior that are different from that of individual Si particles and Si thin films. Despite of a growing effort on electro-mechanical modeling of Si electrodes, the mechanical behavior and microstructure evolution of Si composite electrodes during lithiation/delithiation are largely unknown. To fill this gap, we developed an environmental nanoindentation system installed inside an argon-filled glovebox [Adv. Energy Mater., 10.1002/aenm.201702578 (2017) and Scr. Mater. 130, 191 (2017)] to measure Young’s modulus (E) and hardness (H) of Si composite electrodes during lithiation/delithiation under both dry and wet (the liquid electrolyte) conditions. In contrast to Si films, E and H values of Si/polyvinylidene fluoride (PVDF) electrodes and Si/Na-carboxymethyl cellulose (Na-CMC) electrodes, under both dry and wet conditions, increase with increasing Li concentration within each electrochemical cycle and decrease with increasing cycle number. The E and H values of the composite electrodes under dry condition are smaller than that under wet condition due to the softening of polymeric binders in the electrolyte. The porosity of both composite electrodes decreases with increasing Li concentration and increases with increasing cycle number. At the same state-of-charge (SOC), Si/Na-CMC electrodes have smaller porosity and better mechanical integrity than Si/PVDF electrodes, which is one of the reasons why Na-CMC works better than PVDF as a binder. The results show that mechanical integrity and microstructural stability play a vital role in improving the performance and durability of Si composite electrodes. The relationships between E, H, porosity, and SOC is useful to develop and validate electro-mechanical models of electrodes. They also provide valuable insight into developing high capacity Si electrodes. Furthermore, the environmental nanoindentation method can be used to measure the mechanical behavior of a wide range of electrochemical energy storage materials under realistic working conditions.

Latex Based Emulsion Functional Conductive Polymer Binder for Si Composite Electrode

Functional conducting polymer binders are a class of stable, elastic, electrically conductive adhesive that makes possible the fabrication of silicon composite electrodes for a new generation of low-cost, high-capacity lithium-ion batteries. The vastly higher storage capacity of Si has not been fully realized because silicon swells up to nearly four-times its volume when the Si electrode draws in lithium ions during the charging process. This mechanical expansion and contraction quickly damage the electrode, rendering Si material useless. Functional conductive polymer binder directly addresses the volume-expansion problem by connecting the volume changing Si particles with an elastic polymer to form the anode electrode of a rechargeable lithium battery. This elastic adhesive polymer not only holds the silicon particles in place, but also stretches as those particles expand and contracts when they shrink back to their original volume. We have advanced the functional conductive polymer binder technology into its 4th generation, which is now a water based latex emulsion similar to industry standard styrene-butadiene rubber (SBR) binder, but electronically conductive. The latex based emulsion system of the 4th generation of functional conductive polymer binder ensures that the binder is low cost, and the battery manufacturing process is compatible with the current industry standard.

Mechanical Property Evolution of Silicon Composite Electrodes Studied by Environmental Nanoindentation

AbstractMechanical degradation is largely responsible for the short cycle life of silicon (Si)‐based electrodes for future lithium‐ion batteries. An improved fundamental understanding of the mechanical behavior of Si electrodes, which evolves, as demonstrated in this paper, with the state of charge (SOC) and the cycle number, is a prerequisite for overcoming mechanical degradation and designing high capacity and durable Si‐based electrodes. In this study, Young's modulus (E) and hardness (H) of Si composite electrodes at different SOCs and after different cycle numbers are measured by nanoindentation under both dry and wet (liquid electrolyte) conditions. Unlike electrodes made of Si alone, E and H values of Si composite electrodes increase with increasing Li concentration. The composite electrodes under wet conditions are softer than that under dry conditions. Both E and H decrease with the cycle number. These findings highlight the effects of porosity, liquid environment, and degradation on the mechanical behavior of composite electrodes. The methods and results of this study on the mechanical property evolution of Si/polyvinylidene fluoride electrodes form a basis for exploring more effective binders for Si‐based electrodes. Furthermore, the evolving nature of the mechanical behavior of composite electrodes should be taken into consideration in future modeling efforts of porous composite electrodes.

Silicon Composite Electrodes with Dynamic Ionic Bonding

Silicon (Si) composite electrodes are developed with increased cycle lifetimes and reliability through dynamic ionic bonding between active Si nanoparticles and a polymer binder. Amine groups are covalently attached to Si nanoparticles via surface functionalization. Si composite electrodes are fabricated by combining the Si nanoparticles with a poly(acrylic acid) (PAA) binder. The formation of ionic bonds between amine groups on Si particles and carboxylic acid groups on the PAA binder is characterized by X‐ray photoelectron spectroscopy and Raman spectroscopy. Si composite anodes with ionic bonding demonstrate long term cycling stability with capacity retention of 80% at 400 cycles at a current density of 2.1 A g−1 and good rate capability. The dynamic ionic bonds effectively mitigate the deterioration of electrical interfaces in the composite anodes as suggested by stable impedance over 300 cycles.

Modeling and in situ characterization of lithiation-induced stress in electrodes during the coupled mechano-electro-chemical process

Abstract Stress is one of the key factors leading to capacity fading and triggering safety issues in lithium-ion batteries. In this paper, the stress in layered electrodes during the electrochemical process is investigated. Considering the electrochemically induced changes in the elastic modulus of the electrode material, a stress model is established to obtain a modified Stoney stress equation. In addition, in situ bending deformation of the Si composite electrode during lithiation and delithiation is measured to be determined as a function of Li concentration. Combined with the established stress model, the lithiation-induced stress in the Si composite electrode material is quantitatively characterized. It is shown that the maximum compressive stress is 8.53 MPa. Furthermore, the differences between the established stress model and the classical Stoney stress equation are compared and discussed. Lithiation results in softening of the Si composite electrode material, which affects the stress during lithiation. The experimental results clearly demonstrate that softening of the Si composite electrode material significantly decreases and alleviates the stress.

3D flexible Si based-composite (Si@Si3N4)/CNF electrode with enhanced cyclability and high rate capability for lithium-ion batteries

Despite extremely high capacity of Si-based anodes in lithium-ion batteries (LIB), Si-based materials have shown a structural collapse caused by a volumetric expansion/contraction during the cycling process. The conventional electrode structure, which consists of active materials, a current collector, a conducting agent, and a binder, actually showed a low loading of active material due to the other heavy components. In this study, we prepared a 3D flexible Si-composite electrode consisting of core (Si)-shell (Si3N4) NPs (Si@Si3N4) and carbon nanofibers (denoted as Si-composite/CNF). The Si-composite/CNF was directly utilized as an anode in the absence of the other components was electrochemically evaluated using a coin-type cell. The Si-composite/CNF showed a high capacity of 665mAhg−1 at a fairly high current density of 10Ag−1 and an extremely low capacity loss for 2000 cycles.

Mussel-inspired Polydopamine-treated Copper Foil as a Current Collector for High-performance Silicon Anodes

A new Cu current collector was prepared by introducing a mussel-inspired polydopamine coating onto a Cu foil surface to improve the electrochemical performance of a Si electrode. The polydopamine coating covalently bonded the polymeric binder (with hydroxyl functional groups) via a condensation reaction. The coating improved the adhesion strength between the Si composite electrode and the Cu current collector (245.5 N m−1, 297.5 N m−1, and 353.2 N m−1 for the Si electrodes based on bare Cu, polydopamine-treated Cu without thermal treatment, and polydopamine-treated Cu with thermal treatment, respectively). We demonstrate that the detachment between the Si composite electrode and the current collector plays an important role in determining the electrochemical performance of the Si electrode. The cycle life and rate capability of the Si electrode improved when the polydopamine surface-treated Cu current collector was used (963.9 mAh g−1, 1361.1 mAh g−1, and 1590.0 mAh g−1 for the Si electrodes based on bare Cu, polydopamine-treated Cu without thermal treatment, and polydopamine-treated Cu with thermal treatment, respectively, at C/2 after 500 cycles).

Improved Electrochemical Performance of Lanthanum Silicide/Silicon Composite Electrode with Nickel Substitution for Lithium-Ion Batteries

The effect of nickel substitution on the electrochemical performance of a lanthanum silicide (LaSi2)/silicon (Si) composite electrode for lithium-ion batteries was studied. The results of X-ray diffraction analysis showed that LaSi2 forms a substitutional solid solution with Ni, and that only the Si site in LaSi2 is substituted by Ni, whereas elemental Si in the crystal structure is not substituted. Although the charge–discharge capacity of a LaNixSi2–x electrode (x = 0.06 and 0.12) was lower than that of a LaSi2 electrode, the LaNixSi2–x electrode exhibited a high-rate performance. A LaNi0.10Si1.90/Si (70:30 wt %) composite electrode showed a large initial discharge capacity and a superior long-term cycle performance compared to electrodes composed of Si alone and LaSi2/Si composite, and suppressed the decrease in the initial Coulombic efficiency of the Si electrode. The LaNi0.10Si1.90/Si electrode also exhibited an excellent high-rate performance with a reversible capacity of 2240 mA h g(Si)−1 at a rate of 10 C. The results of computational chemistry demonstrated that LaNi0.25Si1.75 favors Li migration in the pathway compared to LaSi2. These results indicate that Ni substitution in a LaSi2/Si composite negative electrode significantly improves its electrochemical performance.

Effect of the Heat Treatment on the Dimensional Stability of Si Electrodes with PVDF Binder

Dimensional stabilities of Si-Graphite-PVDF composite electrodes were investigated with respect to heat treatment temperature using in situ swelling measurement during cycling and direct observation of the cycled Si electrodes. In situ measurements of the electrode thickness revealed that volume expansion of Si composite electrodes using PVDF binder was suppressed by the simple annealing of the electrode at elevated temperature. Direct observations of the cycled Si composite electrodes after 50 cycles clearly showed that the adhesion of Si-graphite-PVDF composite layer to the current collector was highly dependent on the heat treatment temperature. These results showed that heat treatment of Si composite electrode with PVDF binder played an important role of enhancing the cohesion between Si and other components in the electrode as well as the adhesion of coated Si-graphite-PVDF composite layer to the current collector, thereby suppressing volume expansion of the electrode and improving the cycle performance of the Si composite electrode.

High-performance lithium-ion battery with nano-porous polycrystalline silicon particles as anode

This study presents a high-performance lithium-ion battery with nano-porous Si anode etched from polycrystalline Si particles. Nano-pores formed in micro-sized particles help relieve volume expansion/shrinkage effectively. Meanwhile, the formation of nano-porous skeleton in micro-sized particles greatly reduces the agglomeration of Si particles in nano-sized units and helps improve the cyclic performance of batteries. Pyrolyzed polyacrylonitrile mixed in anode acts as conductive agent and binder, and the network structure enhances the conductivity. The battery with 1μm porous Si and PPAN composite electrode exhibits a remarkably improved electrochemical performance with a coulombic efficiency ≥99.5% even after 220 cycles. Excellent electrochemical performance is observed under galvanostatic mode with a current density of 0.3A/g. High initial discharge (lithiation) capacity of 3652mAh/g and charge (delithiation) capacity of 3067mAh/g are observed with an initial coulombic efficiency of 84%, which increases to 97.2% at the second cycle. The charge capacity remains as high as 2043mAh/g with a coulombic efficiency of ≥99.5% even after 100 cycles.