Si Composite Electrode Research Articles

Influence of mechanochemical reactions between Si and solid electrolytes in the negative electrode on the performance of all-solid-state lithium-ion batteries

All-solid-state lithium-ion batteries that employ Si negative electrodes are the most promising candidates for next-generation batteries because of their high safety performance and energy density. However, the contact between Si and the solid electrolyte becomes insufficient because of the volume changes of Si associated with charging and discharging, resulting in a significant drop in battery performance. Although mechanical milling forms good interparticle contacts, the reaction between Si and a sulfide solid electrolyte increases the resistance, thereby decreasing battery performance. Therefore, in this study, we investigated the effects of the mechanochemical reactions between several solid electrolytes and Si on battery performance. A decrease in electronic conductivity was observed from the reaction between Si and sulfide or oxide solid electrolytes, whereas no significant decrease was observed with halide solid electrolytes. Consequently, an Si composite electrode constructed with a halide solid electrolyte showed high reversibility, achieving a high area capacity of 4.6 mA h cm−2 and specific energy density of 470 Wh kg−1 (masses of positive and negative composite electrodes) in a full-battery cell with an Li2S positive composite electrode at 0.25 mA cm−2 and 25 °C. The study contributes to understanding the important factors in the advancement of all-solid-state lithium-ion batteries.

Investigating Plastic Deformation Between Silicon and Solid Electrolyte in All-Solid-State Batteries Using Operando X-ray Tomography

Si anodes in all-solid-state batteries are expected to achieve high energy density and durability because large volume changes in Si can be mechanically suppressed by the hardness of solid electrolytes. However, the effects of volume changes on the mechanical interface between Si and solid electrolytes during charge/discharge reactions have not been investigated. In this study, operando X-ray computed tomography was used to determine the microstructure of an all-solid-state battery comprising Si active materials and a solid sulfide electrolyte, Li10GeP2S12, during charge/discharge reactions. To evaluate the volume expansion/contraction effects on the charge/discharge properties, the tortuosity of the ion conduction path and the contact area fraction between Si and the solid electrolyte during the charge/discharge reactions were quantitatively estimated. Shell-shaped voids around the Si particles were observed after Si shrinkage owing to the plastic deformation of the solid electrolyte. This characteristic resulted in poor charge/discharge efficiency and incomplete delithiation in the battery. These results will facilitate the design optimization of Si composite electrodes, which will be highly beneficial to the development of effective all-solid-state batteries.

Preparation of Co/Ni nanoparticles doped g-C3N4 for improving the electrochemical hydrogen storage performance of Co0.9Cu0.1Si alloy

A facile liquid-phase reduction strategy was used to prepare cobalt and nickel nanoparticles doped graphitic carbon nitride (g-C3N4). Co0.9Cu0.1Si alloy was manufactured by mechanical alloying. Subsequently, a series of g-C3N4, Co-g-C3N4 and Ni-g-C3N4 modified Co0.9Cu0.1Si composites were fabricated via ball milling. The electrochemical hydrogen storage behaviors of metal doped g-C3N4/Co0.9Cu0.1Si composite electrodes were studied for the first time. Ultimately, Co0.9Cu0.1Si + Co-g-C3N4 and Co0.9Cu0.1Si + Ni-g-C3N4 revealed preferable discharge capacity of 575.8 mA h/g and 561.0 mA h/g than Co0.9Cu0.1Si + g-C3N4 and conventional Co0.9Cu0.1Si electrodes. The g-C3N4 offered more electrochemical active sites and beneficial interface for charge transfer. The metal particles within g-C3N4 further improved the electrocatalytic activity. The metal nanoparticles and g-C3N4 played synergistic effect in enhancing the hydrogen diffusion and electrochemical behavior of Co0.9Cu0.1Si alloy. Moreover, Co-g-C3N4 and Ni-g-C3N4 modified Co0.9Cu0.1Si also demonstrated better capacity retention, high-rate dischargeability (HRD) and kinetics properties. Accordingly, Co/Ni nanoparticles doped g-C3N4 can be considered as effective material for hydrogen storage alloy modification.

Interlayer Engineering and Prelithiation: Empowering Si Anodes for Low-Pressure-Operating All-Solid-State Batteries.

Silicon (Si) anodes, free from the dendritic growth concerns found in lithium (Li) metal anodes, offer a promising alternative for high-energy all-solid-state batteries (ASSBs). However, most advancements in Si anodes have been achieved under impractical high operating pressures, which can mask detrimental electrochemo-mechanical issues. Herein, we effectively address the challenges related to the low-pressure operation of Si anodes in ASSBs by introducing an silver (Ag) interlayer between the solid electrolyte layer (Li6PS5Cl) and anode and prelithiating the anodes. The Si composite electrodes, consisting of Si/polyvinylidene fluoride/carbon nanotubes, are optimized for suitable mechanical properties and electrical connectivity. Although the impact of the Ag interlayer is insignificant at an exceedingly high operating pressure of 70MPa, it substantially enhances the interfacial contacts under a practical low operating pressure of 15MPa. Thus, Ag-coated Si anodes outperform bare Si anodes (discharge capacity: 2430vs 1560mAhg-1). The robust interfacial contact is attributed to the deformable, adhesive properties and protective role of the in situ lithiated Ag interlayer, as evidenced by comprehensive ex situ analyses. Operando electrochemical pressiometry is used effectively to probe the strong interface for Ag-coated Si anodes. Furthermore, prelithiation through the thermal evaporation deposition of Li metal significantly improves the cycling performance.

A strategy of boosting the effect of carbon nanotubes in graphite-blended Si electrodes for high-energy lithium-ion batteries

High capacity silicon (Si) has been added in graphite electrodes to overcome the theoretical capacity limit of the carbonaceous anode. However, different active materials that store Li+ ions via intercalation and alloying mechanisms are prone to suffer electrical disconnection due to the large gap derived from their different volume shrinkage ratios during delithiation. Herein, to address this poor compatibility on graphite-blended Si composite electrode (Gr-SC), a strategic usage of carbon nanotubes (CNTs) as electric bridge between graphite and Si is investigated through the comparison of three different types of Si blended graphite anodes (Gr-SC, Gr-SC with CNTs as additive, and Gr-MSC having implanted multi-walled CNTs on Si). Among the comparison group, the selectively located MW-CNT around Si in Gr-MSC exhibits outperformed capacity retention (98.4 %) and faster charge transfer kinetics after 50 cycles in a half cell, demonstrating a great compatibility between graphite and Si. This study presents a comprehensive understanding on the role of CNTs as an electric bridge on the electrode exploiting multiple Li storage mechanisms which undergo dissimilar volume change.

Investigation of Xanthan Gum and Carboxymethyl Cellulose Binders for the Silicon Anode of Lithium-Ion Batteries

The binder is known to play an important role in the cycle stability of silicon-based anodes for lithium-ion batteries. Nature-derived biopolymers such as sodium carboxymethyl cellulose (NaCMC) and xanthan gum (XG) are a promising class of binders that offer several advantages over traditional polyvinylidene fluoride (PVDF). Advantages include better contact between silicon particles and the ability to process the electrodes using water as a solvent. While many studies have explored the fundamental properties of these biopolymer binders and their interaction with silicon, there has been little research on the use of these binders under practical loadings (such as ∼2 mg Si cm−2 and <10 wt% binder). Herein, we compare the electrochemical performance of both NaCMC and XG-based silicon electrodes with a low binder content. Si-binder interactions and their role in electrode performance are revealed with XPS, SEM, and EDX. In addition, we report the results of both a high-silicon (80 wt% Si) and a practical low-silicon (20 wt% Si) composite electrode while using silicon nano powder prepared by industrial-scale synthesis. It is found that NaCMC consistently outperforms XG as a binder, which is attributable to superior cohesion within the electrode.

Understanding the Electrochemical Performances of Si Anodes Incorporating Mechanically Interlocked Binders Prepared from α-Cyclodextrin-Based Polyrotaxanes

Rechargeable Li-ion batteries with larger autonomy are needed to meet increasing market demands, hence the intensive research effort to switch from graphite to silicon (Si) anodes, despite their detrimental massive volume changes upon cycling. Many elegant polymer chemistries have addressed this issue by designing smart binders capable of buffering Si electrodes fracturing and maintaining their overall structure. In this sense, self-healing PR–PAA binders relying on the α-cyclodextrin (α-CD) supramolecular chemistry and prepared by cross-linking poly(acrylic acid) (PAA) with α-CD based polyrotaxanes (PR) have recently been proposed. We herein further explore this binder chemistry to understand the proper function of such mechanically interlocked networks. We successfully synthesized a wide range of PR–PAA binders by varying their structural parameters: the doping ratio of PR, the cross-linking density, as well as the polymer molecular weight and the PR ring coverage. Then, their electrochemical performances were tested in nano-sized Si composite electrodes, and a structure/property correlation was evidenced. By promoting the α-CD sliding motion through both an increase of the PR doping fraction and a decrease of the PR ring coverage, we succeeded in making a PR−PAA-based Si electrode having an initial capacity of >3000 mA h/g and showing 82% capacity retention after 100 cycles as opposed to only 42% for PAA-based Si electrodes. A resulting better stress dissipation was evidenced by ex situ scanning electron microscope analysis as well as operando internal stress monitoring experiments via optical sensors. Altogether, this work emphasizes the benefits that CD-based supramolecular architectures can offer to the battery community for designing self-healing binders.

Influence of compositing conditions for Si-composite negative electrodes in sulfide-type all-solid-state lithium-ion batteries

The battery performances of Si-composite electrodes are strongly influenced by the compositing process strength caused by the reaction between Si and the solid electrolyte for all-solid-state batteries.

Real time characterization of the current collector's role on the electro-chemo-mechanical coupling performance of Si composite electrode

The electro-chemo-mechanical properties of the electrode play an essential role in the lifetime and performance of lithium-ion batteries (LIBs). Large diffusion-induced stress causes degradation of mechanical properties, resulting in the fading of capacity and cycle performance. Current collector, an important component of the batteries, acts as a crucial part to the structural stability and mechanical integrity of the electrode. In this paper, the effect of current collector's thickness on the cyclic performance of silicon (Si) composite electrodes is investigated experimentally. The results demonstrate that a thicker current collector can hugely enhance the cyclic performance of Si composite electrodes. Besides, an in-situ measurement is conducted to analyze the relationship between mechanical response and cycle performance. A cantilever model is employed to extract the stress evolution within the Si composite electrode with different current collectors. Obviously, the thicker collector has strong restrictions on the electrode bending caused by the volume expansion of silicon particles, thus induce larger compressive stress. This can help maintain the structural stability of the composite electrodes and further enhance the mechanical performance and electrochemical performance of the Si based electrodes.

Perfluoroinated Ionomer as an Artificial SEI for Silicon Nano-Flake Anode in LiTFSI/Tetraglyme Solvate Ionic Liquid

Cycle performance of silicon nano-flake powder (LeafPower® Si) anodes was improved by coating of perfluorosulfonic acid (PFSA) polymers (Nafion® and Aquivion®) as an artificial solid electrolyte interphase (SEI). The Si electrodes were coated with PFSA ionomer by addition of PFSA ionomer solution to aqueous slurry of the Si composite electrode. H+-type Nafion®-coated Si electrodes demonstrated improved capacity retentions and Coulombic efficiencies owing to the formation of a LiF-rich SEI composed of reductive decomposition products of Nafion®. The Nafion® coating suppressed the increase in polarization and the electrode swelling upon cycling because the LiF-rich SEI effectively suppressed electrolyte decomposition. Li+ and Cs+-type Nafion® and H+-type Aquivion®, which has a shorter side chain than Nafion®, provided similar positive effects on cycle performance of the Si electrodes. Furthermore, the Si electrodes achieved good cycleability up to 500 cycles owing to synergy effects of the Nafion® coating and an SEI-forming additive of fluoroethylene carbonate (FEC).

Engineering Lignin-Derived Carbon–Silicon Nanocomposite Electrodes: Insight into the Copyrolysis Mechanism and Process–Structure–Property–Performance Relationships

As a renewable source, available in a large quantity, it is rewarding to find applications for lignin with high added value. We report the use of lignin in synthesizing a three-dimensional, interconnected carbon/silicon nanoparticle (C/Si NP) composite material as a low-cost replacement to conventional anode materials synthesized using expensive and toxic solvents and binders such as polyvinylidene in today’s lithium-ion battery (LIB) manufacturing process. To understand how lignin pyrolysis chemistry and processing conditions affect the structure, mechanical property, and electrochemical performance of the synthesized electrode materials, the thermochemical conversion process was, for the first time, quantitatively investigated using analytical pyrolysis–gas chromatography–mass spectrometry (GC–MS) along with a suite of other analytical tools. Results suggest that the surface bonding interaction of the C/Si NPs was evolved from pristine Si to −Si–O–C–, to −O═Si═O–, with the increase of pyrolysis temperature. The −Si–O–C– bond plays a key role in enhancing the cohesive strength and thus improving the electrochemical performance of the Si composite electrode. The pyrolysis–GC–MS can serve as a useful tool to predict the optimal pyrolysis temperature or tailor the properties of the synthesized composite electrodes by controlling the pyrolysis conditions. This study elucidates the processing–structure–property–performance relationships among lignin pyrolysis chemistry, carbon material structure and properties, and the electrochemical performance of the resulting electrode materials. Such knowledge serves as a basis for designing lignin-derived composite materials for electrochemical energy storage applications.

Experimental measurement of electro-chemo-mechanical properties of a composite silicon electrode in lithium ion batteries.

The applications of silicon (Si)-based electrodes in lithium ion batteries have been impeded by mechanical degradation caused by lithiation/delithiation-induced volume changes. Understanding the evolution of mechanical behavior and properties of Si composite electrodes during electrochemical cycling is indispensable to develop coping strategies and predict battery life. In this study, we optimized an in situ method for measuring electro-chemo-mechanical properties, including partial molar volume, elastic modulus, and electrochemical reaction-induced stress, based on the curvature changes of cantilever electrodes. We found that the swell strain and partial molar volume of Si electrodes increase with the Li concentration. The elastic modulus generally decreases with the Li concentration. The in-plain stress transforms from tensile stress to compressive stress and showed an increasing tendency during further lithiation, while it shows a reverse trend during delithiation. The stress evolution correlates well with the "opening" and "closing" of micro-cracks in Si composite electrodes during cycling. These findings provide not only input parameters for battery modeling but also help understand the capacity fading of Si electrodes. Furthermore, the in situ measurement methodology developed in this study is readily applied to other battery electrodes.

Watermelon-like texture lithium titanate and silicon composite films as anodes for lithium-ion battery with high capacity and long cycle life

Buffering the volume expansion of Si particles is considered to be an effective method for preparing Si based electrode with high capacity and prominent cycling performance. Here, watermelon-like texture Li4Ti5O12 (LTO) and Si composite film electrode was synthetized by magnetron co-sputtering using spinel LTO powder and crystalline Si wafer as the sputtering source. In the composite film electrode, LTO acts as watermelon pulp and Si acts as watermelon seed. The obtained composite film electrode endows a high reversible capacity of ~800 mAh g−1 at 100 mA g−1 with a capacity retention ~76% after 1000 cycles. The extraordinary electrochemical characteristics of the composite film electrode could be attributed to the role of LTO in watermelon pulp which prevents electrolyte from contacting with Si nanoparticles directly, easing the stress caused by periodic volume change and the multi-interface between LTO and Si as well as supplying efficient and rapid transport routes for Li-ions. The outcomes reveal the watermelon-like texture composite film is a promising high energy density and splendid long cycle stability anode material for lithium-ion batteries.

Novel synthesis of porous Si-TiO2 composite as a high-capacity anode material for Li secondary batteries

Porous Si materials can be considered as a candidate for and an alternative to commercial graphite anodes for Li-ion batteries. In this study, we propose a modified magnesiothermic method to increase the materials’ yield and a design strategy to improve their electrochemical properties. For magnesiothermic solid-state reactions, Mg2Si was used instead of metallic Mg and amorphous nanoscale SiO2 was adopted. The two starting materials were converted into Si and MgO via a high-temperature solid-state reaction, resulting in a high-yield percentage of Si. Then, amorphous TiO2 layers were formed and the MgO was removed by chemical etching, forming a porous structure. The materials at each step were thoroughly characterized by using various analytical tools. Electrochemical test results demonstrated that the amorphous TiO2 coated porous Si composite electrode exhibited a high reversible capacity of 1965 mA h g−1 after 100 cycles and a greatly improved initial Coulombic efficiency of 90.1%. These results can be attributed to the controlled porous structure of the composite and amorphous TiO2 coating layer. The modified magnesiothermic method and porous material design could attract commercial use of Si-based materials as Li-ion battery anodes.

Effects of fluoroethylene carbonate addition to Li-glyme solvate ionic liquids on their ionic transport properties and Si composite electrode performance

The design of battery electrolytes with superior transport, thermal, and electrochemical properties is a key consideration in the development of high-energy-density batteries. To this end, an equimolar mixture of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraglyme (G4) is of interest because it forms a thermally and electrochemically stable solvate ionic liquid (SIL), [Li(G4)][TFSA]. In this study, fluoroethylene carbonate (FEC) was added to [Li(G4)][TFSA] in an attempt to improve transport properties of the SIL and to modify the chemical composition of the solid electrolyte interphase (SEI) formed on Si electrodes. The addition of FEC had a negligible impact on the Li ion coordination structure in [Li(G4)][TFSA] while enhancing its ionic conductivity nearly twofold. The Si electrode in [Li(G4)][TFSA] with the FEC additive showed a stable and reversible capacity in long-term cycling with high Coulombic efficiency. X-ray photoelectron spectroscopy revealed that the chemical composition of the SEI formed in [Li(G4)][TFSA] with the FEC additive was significantly different from that in the electrolyte without FEC. The FEC-addition-derived SEI most likely consists of carbonates and LiF, retarding electrolyte decomposition and, thus, improving the electrochemical performance of the Si electrode therein.

Ultra-efficient polymer binder for silicon anode in high-capacity lithium-ion batteries

As a highly promising anode material for high-capacity lithium-ion batteries (LIBs), the low electronic conductivity and large volume variation of silicon (Si) make the slurry-coating Si based electrode requiring high content of “inert” materials and suffering rapid capacity fading. Herein, a polyimine, synthesized via one-step condensation reaction, has been demonstrated as an ultra-efficient polymer binder that can resolve the above issues. The polyimine binder containing Si electrode delivers superior electrochemical performance: a delithiation specific capacity of 804.4 mAh g −1 with capacity retention of 82.4% after 1000 cycles at the current density of 2 A g −1 . The high efficiency of polyimine binder for Si electrode has also been demonstrated with ultrahigh weight ratio of “active” material to “inert” material (R A/I ). The electrode with 95 wt% of Si (95Si/Polyimine, R A/I = 19) reveals a reversible delithiation capacity of 2114 mAh g −1 (capacity retention ~ 80.4%) over 200 cycles at the current density of 400 mA g −1 . Even at the high current density of 2 A g −1 , a delithiation capacity of 1087.8 mAh g −1 after 500 cycles can be obtained. Molecular simulations and atomic force microscopy (AFM) indentation are utilized to investigate the ultra-efficiency of polyimine binder. With simple manufacturing process and ultra-efficient binder performance, the designed polyimine binder will be definitely meaningful in achieving low-cost and high-capacity LIBs with prolonged cycle life. An ultra-efficient polymer binder enables the slurry-coated Si electrode with unprecedented highest weight ratio of “active” materials to “inert” materials (R A/I ) and prolonged cycle life is reported. Traditional Si based electrodes always require high content of “inert” materials and suffer rapid capacity fading. A polyimine, synthesized via one-step condensation reaction, can efficiently resolve the above issues, and the electrochemical performance of Si electrodes can be significantly improved using a small amount of polyimine binder. Molecular simulations and atomic force microscopy (AFM) indentation are utilized to investigate the ultra-efficiency of polyimine binder, and the current design will be useful in achieving the Si (or Si composite) electrodes with high cost efficiency and long cycle life. • An ultra-effcient polymer binder for high-performance Si anode. • Silicon electrode with unprecedented highest weight ratio of “active/inert” materials. • Significantly improved binder performance than state-of-art binders. • Towards a cost-efficient process and energy-efficient battery via developed polyimine binder.

Short-time and ultrasensitive electroanalytical technique for electrode active materials used in secondary batteries

In this article, a short-time and ultrasensitive electrochemical approach capable of detecting subtle phase changes during the charge-discharge process is introduced. It is attained through the use of a Si microparticle electrode specially prepared using the electrophoretic deposition (EPD) method. The features of the electrode are that it is binder-free and the active material is sparsely deposited onto the electrode. Despite the scan rate being 5 to 60 times faster than the usual scan rates, the cyclic voltammograms recorded at the binder-free Si microparticle electrode in the 83.3–16.7 mol% 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide−lithium bis(fluorosulfonyl)amide ionic liquid electrolyte indicate several redox waves related to the lithiation reactions with phase transition, although it is difficult to see similar waves when using common Si composite electrodes. Co-deposition of both a Si active material and acetylene black onto the current collector using the EPD method significantly improves the sensitivity and performance of the electrode. Applying this electrode to operando scanning electron microscopy visually enables a deeper understanding of the impact of the morphology variation of the Si materials on the electrode performance within a short time.

In situ experimental measurement of softening of material modulus and stresses evolution of Si-composite electrodes during the electrochemical process

In situ experimental measurement of softening of material modulus and stresses evolution of Si-composite electrodes during the electrochemical process

A green strategy for the preparation of a honeycomb-like silicon composite with enhanced lithium storage properties.

The high-performance silicon (Si) composite electrodes are being widely developed due to their considerable theoretical capacity. Coating with carbon-based materials is an efficient way to solve the common issues of Si-based materials. Currently, most of the reported strategies are complicated, pollutive, or uneconomic, which hamper their practical applications. Herein, a honeycomb-like Si-based composite was prepared to address these issues via a facile and green reduction approach at room temperature. The pre-anchored Si nanoparticles could be packed and interconnected through a three-dimensional graphene network to further enhance the electrochemical properties of the active materials. As an electrode, this composite shows good rate capabilities upon lithium storage and cycling stability. The continued cycling measurement delivers a -0.049% capacity decay rate per cycle within 600 cycles. A direct comparison further exhibits the obviously improved performance between the as-designed Si-based composite and naked Si, suggesting a potential application of this convenient strategy for other high-performance electrode materials.

Modeling of Concurrent Reaction Driven Stress at Heterogeneous Composite Electrode-Collector Interface in Li-Ion Batteries: Elastic and Elasto-Plastic Analysis

A model is developed to study the concurrent reaction driven interfacial stress for an elastically heterogeneous, isotropic bi-layer electrode free for deformation at one end. The slow and fast electrochemical charging reaction process and stiffness induced elasto-plastic deformation characteristics often arise during lithiation of composite electrode with high capacity. The analytical formalism of the stress generated at electrode-collector interface incorporates elasto-plastic behavior of the material and relative change in resistivity in detail through regional material heterogeneity sensitivity exponent. The elasto-plastic behavior and stiffness of the anode material (i.e., Si composite electrode) has been accounted as a non-linear function of Li-ion concentration and validated. The insight for initiation of cracking due to interfacial stress and state-of-health of the battery electrodes has also been discussed. Finally, unified lithiation-induced stress model unravels the effect of embedded material heterogeneity parameter coupled with resistivity and stiffness and its anomalous dynamics at composite electrode-collector interface. The verification with the available experimental data in the literature has also been made and hence providing a better insight into the origin of degradation and the evaluation of advanced battery electrodes.