Silicon Negative Electrodes Research Articles

Si particle size blends to improve cycling performance as negative electrode for all-solid-state lithium-ion battery

Silicon negative electrodes dramatically increase the energy density of lithium-ion batteries (LIBs), but there are still many challenges in their practical application due to the limited cycle performance of conventional liquid electrolyte systems. In this study, we clarified that the use of an inorganic solid electrolyte improves the cycle performance of the LIB with the Si negative electrode and the size of Si particles influenced the properties of the all-solid-state LIB. The all-solid-state LIB using micro-sized silicon (mSi) particles as the negative electrode active material showed a large initial discharge capacity (2400 mAh g-1), but the discharge capacity decreased as charge/discharge cycles increased. In contrast, the all-solid-state LIB using nano-sized silicon (nSi) particles showed a low initial discharge capacity (1000 mAh g-1), but the discharge capacity increased for the initial 20 charge/discharge cycles. The cross-sectional images analyzed by scanning electron microscopy showed that for mSi particles, cracks and voids occurred due to volumetric changes during charging and discharging, interrupting the flow path of ions and electrons, whereas for nSi particles, the contact with the solid electrolyte and conductive material was improved. Moreover, the all-solid-state LIB using the negative electrode of nSi: mSi = 7: 3 (mass ratio) showed higher initial discharge capacity and longer cycle performance than that using mSi and nSi.

Novel Binder with Cross-Linking Reconfiguration Functionality for Silicon Anodes of Lithium-Ion Batteries.

Silicon is expected to be used as a high theoretical capacity anode material in lithium-ion batteries with high energy densities. However, the huge volume change incurred when silicon de-embeds lithium ions, leading to destruction of the electrode structure and a rapid reduction in battery capacity. Although binders play a key role in maintaining the stability of the electrode structure, commonly used binders cannot withstand the large volume expansion of the silicon. To alleviate this problem, we propose a PGC cross-linking reconfiguration binder based on poly(acrylic acid) (PAA), gelatin (GN), and β-cyclodextrin (β-CD). Within PGC, PAA supports the main chain and provides a large number of carboxyl groups (-COOH), GN provides rich carboxyl and amide groups that can form a cross-linking network with PAA, and β-CD offers rich hydroxyl groups and a cone-shaped hollow ring structure that can alleviate stress accumulation in the polymer chain by forming a new dynamic cross-linking coordination conformation during stretching. In the half cell, the silicon negative prepared by the PGC binder exhibited a high specific capacity and capacity maintenance ratio, and the specific capacity of the silicon negative electrode prepared by the PGC binder is still 1809 mAh g-1 and the capacity maintenance ratio is 73.76% following 200 cycles at 2 A g-1 current density, indicating that PGC sufficiently maintains the silicon negative structure during the battery cycle. The PGC binder has a simple preparation method and good capacity retention ability, making it a potential reference for the further development of silicon negative electrodes.

Approach for Mitigating Interfacial Degradation in Silicon-Based Electrodes to Enhance Cycling Stability of Lithium-Ion Batteries and Capacitors

The growing demand for high energy density in lithium-ion batteries has led to extensive investigations on silicon negative electrodes. In this study, we propose a novel approach to address the interfacial degradation of electrodes caused by volume expansion during lithium insertion/extraction. The approach involves a sandwich structure consisting of a carbon nanofiber (CNF) undercoating electrode and a CNF-coated separator. The CNF is treated with acid to form carboxyl and hydroxyl groups on its surface, which improves the dispersion of the coating slurry and enhances adhesion to the electrode. Through electrochemical testing, the improved silicon electrode shows a high retention rate of 82% compared to the initial capacity after 100 cycles, and high electrochemical performance when applied to both lithium-ion batteries and lithium-ion capacitors. These results are attributed to the improved adhesion between the electrode and current collector, as well as the formation of a stable solid electrolyte interphase layer on the electrode surface. These findings suggest that the conductive coating process, applied to the separator and current collector, effectively mitigates interfacial issues that are associated with silicon-based electrodes. This approach has the potential to overcome the degradation characteristics of silicon electrodes and significantly contribute to improving the cycling stability of lithium-ion batteries and capacitors.

Silicon Negative Electrodes—What Can Be Achieved for Commercial Cell Energy Densities

Historically, lithium cobalt oxide and graphite have been the positive and negative electrode active materials of choice for commercial lithium-ion cells. It has only been over the past ~15 years in which alternate positive electrode materials have been used. As new positive and negative active materials, such as NMC811 and silicon-based electrodes, are being developed, it is crucial to evaluate the potential of these materials at a stack or cell level to fully understand the possible increases in energy density which can be achieved. Comparisons were made between electrode stack volumetric energy densities for designs containing either LCO or NMC811 positive electrode and silicon-graphite negative electrodes, where the weight percentages of silicon were evaluated between zero and ninety percent. Positive electrode areal loadings were evaluated between 2.00 and 5.00 mAh cm−2. NMC811 at 200 mAh g−1 has the ability to increase stack energy density between 11% and 20% over LCO depending on percentage silicon and areal loading. At a stack level, the percentage of silicon added results in large increases in energy density but delivers a diminishing return, with the greatest increase observed as the percentage of silicon is increased from zero percent to approximately 25–30%.

Si nanomaterials in lithium-ion battery anode

Due to its high specific capacity, silicon anode has gained interest on a global scale as the anode of next-generation lithium-ion batteries (LIBs). However, it is challenging for silicon anodes to replace graphite anodes for widespread use due to the inevitable volume expansion and SEI film development generated by silicon during the lithiation/delithiation process. Among these, the advancement of nanotechnology has sped up the development of silicon anodes. This paper reviews nanotechnology applied in the Si anode to improve the battery performance. The volume expansion of Si is effectively decreased by reducing the volume of the silicon anode in order to increase its specific surface area. Additionally, the smaller negative electrode reduces the distance traveled by lithium ions, greatly increasing the silicon negative electrode's efficiency. At present, the main silicon nano-anode materials include nanoparticles, nanowires, nanosheets, nanotubes, nanoporous materials and so on. It is hoped that this review will provide a deep prospect introduction to the nano-silicon anode. Pure silicon nanoparticles and silicon nanowires, and new nanomaterials composed of them with graphite, graphene, metals, etc.

Graphene confined core-shell Si@Cu nanoparticles as integrated anode with enhanced capacity and high-rate performance for Li-ion batteries

To cope with volume expansion, poor silicon conductivity and capacity decay due to solid electrolyte interface rupture, rational design of silicon negative electrode is an effective means to cope with the above challenges. We designed a graphene confined Si@Cu core-shell structure (Si@Cu@rGO) as integrated anode, in which the copper shell on the Si nanosurface enhances the electronic conductivity between the silicon particles and graphene. Meanwhile, the flexibility of the copper shell and graphene help suppress the change of electrode morphology during cycling and keep good contact between electrode material and collector. The graphene cladding not only effectively prevents the aggregation of Si@Cu nanoparticles, but also provides enough space for the expansion of silicon during lithiation, while the three-dimensional redox graphene layer builds a conductive structure that accelerates the reaction kinetics of the cell. Thanks to this design, Si@Cu@rGO has a satisfactory electrochemical performance. After 70 cycles in the rate performance test, the specific capacity of Si@Cu@rGO reached 2243.1 mAh g−1.

In Situ Thickness Measurements During Cycling of Li-Ion Pouch Cells with Silicon Negative Electrodes

Cell expansion and contraction are major concerns for battery pack and module developers. This work is an effort to understand the behavior of cell expansion due to formation, lithiation and cycling, with measurements made by a system that uses a Linear Variable Differential Transformer (LVDT) sensor. For a NCM622-Silicon cell, impact of factors such as initial compression pressure and calendaring of electrodes on cell expansion were explored. Reversible expansion during charge/discharge in a cycle is a function of cell capacity and reversible expansion can be reduced with increasing initial compression. Irreversible expansion during cycling grows linearly with the number of cycles and can also be reduced with initial compression of the cell. With initial compression of 45 psi, measurements showed 3% cell expansion during formation, 4% reversible expansion in a charge/discharge cycle with C/5 rate, and 12% irreversible expansion over 220 cycles. From the data, a hypothesis was developed suggesting that SEI growth and plastic deformation of the silicon particles could be the main causes for irreversible expansion of the cells with silicon electrodes, with the assumption that cells show less than 20% capacity fade over life.

Enhanced Performance of Silicon Negative Electrodes Composited with Titanium Carbide Based MXenes for Lithium-Ion Batteries

Silicon is considered as one of the most promising candidates for the next generation negative electrode (negatrode) materials in lithium-ion batteries (LIBs) due to its high theoretical specific capacity, appropriate lithiation potential range, and fairly abundant resources. However, the practical application of silicon negatrodes is hampered by the poor cycling and rate performances resulting mainly from the huge volume change during Li+ insertion/extraction. Various composite structures have been investigated to maintain the structural integrity and improve the stability and electric conductivity of silicon-based negatrodes. Of these, 2D transition-metal carbides, also known as MXenes (e.g., Ti3C2Tx), have become increasingly attractive for energy storage applications because of their excellent electric, electrochemical and mechanical properties and potential uses as the matrix for construction of 3D networks with larger buffering spaces and more effective charge carrier conduction in silicon-based negatrodes. This article reviews specifically composite negatrodes of silicon with titanium-carbide-based MXenes for LIBs from the materials perspective. The structures design, preparation method, interface control, and their effects on electrochemical performances are comprehensively elaborated on. It is shown that the recent development of Si/MXene-based negatrodes presents great potential for future applications.

Interfacial Reactivity of Silicon Electrodes: Impact of Electrolyte Solvent and Presence of Conductive Carbon.

Silicon (Si) is a promising high-capacity material for lithium-ion batteries; however, its limited reversibility hinders commercial adoption. Approaches such as particle and crystallite size reduction, introduction of conductive carbon, and use of different electrolyte solvents have been explored to overcome these electrochemical limitations. Herein, operando isothermal microcalorimetry (IMC) is used to probe the influence of silicon particle size, electrode composition, and electrolyte additives fluoroethylene carbonate and vinylene carbonate on the heat flow during silicon lithiation. The IMC data are complemented by X-ray photoelectron and Raman spectroscopies to elucidate differences in solid electrolyte interphase (SEI) composition. Nanosized (∼50 nm, n-Si) and micrometer-sized (∼4 μm, μ-Si) silicon electrodes are formulated with and without amorphous carbon and electrochemically lithiated in ethylene carbonate (EC), fluoroethylene carbonate (FEC), or vinylene carbonate (VC) based electrolytes. Notably, n-Si electrodes generate 53-61% more normalized heat relative to their μ-Si counterparts, consistent with increased surface area and electrode/electrolyte reactivity. Introduction of amorphous carbon significantly alters the heat flow profile where multiple exothermic peaks and increased normalized heat dissipation are observed for all electrolyte types. Notably, the VC-containing electrolyte demonstrates the greatest normalized heat dissipation of the electrode compositions tested showing as much as a 50% increase compared to the EC or FEC counterparts. The results are relevant to the understanding of silicon negative electrode function in the presence of electrolyte additives and provide insight relative to silicon containing cell reactivity and safety.

Room-temperature Operation of Lithium Sulfide Positive and Silicon Negative Composite Electrodes Employing Oxide Solid Electrolytes for All-solid-state Battery

Room-temperature Operation of Lithium Sulfide Positive and Silicon Negative Composite Electrodes Employing Oxide Solid Electrolytes for All-solid-state Battery

Design of ultrafine silicon structure for lithium battery and research progress of silicon-carbon composite negative electrode materials

As demand for high-performance electric vehicles, portable electronic equipment, and energy storage devices increases rapidly, the development of lithium-ion batteries with higher specific capacity and rate performance has become more and more urgent. As the main body of lithium storage, negative electrode materials have become the key to improving the performance of lithium batteries. The high specific capacity and low lithium insertion potential of silicon materials make them the best choice to replace traditional graphite negative electrodes. Pure silicon negative electrodes have huge volume expansion effects and SEI membranes (solid electrolyte interface) are easily damaged. Therefore, researchers have improved the performance of negative electrode materials through silicon-carbon composites. This article introduces the current design ideas of ultra-fine silicon structure for lithium batteries and the method of compounding with carbon materials, and reviews the research progress of the performance of silicon-carbon composite negative electrode materials. Ultra-fine silicon materials include disorderly dispersed ultra-fine silicon particles such as porous structures, hollow structures, and core-shell structures; and ordered ultra-fine silicon, such as silicon nanowire arrays, silicon nanotube arrays, and interconnected silicon nano-films. The article analyzes and compares the composite method of ultrafine silicon and carbon materials with different structural designs, and the effect of composite negative electrode materials on the specific capacity and cycle performance of the battery. Finally, the research direction of silicon-carbon composite negative electrode materials is prospected.

Peering into Batteries: Electrochemical Insight Through In Situ and Operando Methods over Multiple Length Scales

Summary In order to overcome the limitations of static and destructive characterizations of Li-ion battery materials and components, comprehensive investigation peering into batteries over various length and time scales is essential. In this regard, emerging in situ and operando characterization methodologies focusing on multiple size domains become a powerful approach to resolve current challenges and navigate future directions. This perspective provides a series of illustrative examples of in situ and operando characterizations over atomic, crystallite/particle, electrode, and battery system length scales with a focus on two electrode material classes: insertion and conversion materials. The operating principles, features, analysis methods, and information gained from each characterization method are discussed. Herein, we present the necessity and opportunity for in situ and operando characterization of electrochemical energy storage materials and systems. Further, we suggest future directions to gain the insight needed to tackle currently intractable issues on Li-ion battery application, failure, and emerging design concepts.

Preparation and electrochemical performance of silicon@graphene aerogel composites for lithium-ion batteries

Silicon/graphene composites are recently received more attention as promising negative electrode materials for the next generation lithium-ion batteries (LIBs) due to the synergistic effect of silicon and graphene. Silicon can provide high specific charge capacity, relatively low discharge potential, environmental compatibility and considerable abundance, while graphene exhibits excellent electrical conductivity, flexibility and large space for silicon expansion in the charge process. Herein, we adopted a simple hydrothermal method to obtain a silicon@graphene aerogel (Si@GA) composite with highly dispersible morphology and three-dimensional porous structure. The electrochemical characterizations prove that the Si@GA composite has excellent cycle performance and moderate rate performance during charge/discharge process. After 100 cycles, the specific charge capacity remains above 1330 mAh g−1 at 0.2 A g−1, and above 600 mAh g−1 at 2 A g−1, respectively. The excellent electrochemical performance of the composite may be attributed to the existence and specific structure of GA, which can improve the electrical conductivity and act as a buffer matrix to stabilize the composite electrode material. The composite material may promote the further application of silicon negative electrode materials.

Editors’ Choice—Understanding the Superior Cycling Performance of Si Anode in Highly Concentrated Phosphonium-Based Ionic Liquid Electrolyte

Considerable effort has been devoted to improving the cyclability of silicon (Si) negative electrodes for lithium-ion batteries because it is a promising high specific capacity alternative to graphite. In this work, the electrochemical behaviour of Si in two ionic liquid (IL) electrolytes, triethyl(methyl)phosphonium bis(fluorosulfonyl)imide (P1222FSI) and N-propyl-N-methylpyrrolidinium-FSI (C3mpyrFSI) with high and low lithium (Li) salt content is investigated at 50 °C. Results highlight that higher capacity and better cycling stability are achieved over 50 cycles with high salt concentration, the first time for a pyrrolidinium-based electrolyte in the area of Si negative electrodes. However, the Si cycling performance was far superior in the P1222FSI-based high salt content electrolyte compared to that of the C3mpyrFSI. To understand this unexpected result, diffusivity measurements of the IL-based electrolytes were performed using PFG-NMR, while their stability was probed using MAS-NMR and XPS after long-term cycling. A higher apparent transport number for Li ions in highly concentrated ILs, combined with a significantly lower extent of electrolyte degradation explains the superior cycle life of the highly concentrated phosphonium-based system. Si/concentrated P1222FSI-LiFSI/lithium nickel cobalt aluminum oxide (NCA) full cells with more than 3 mAh cm−2 nominal capacity deliver a promising cycle life and good rate capability.

Technological development of key domains in electric vehicles: Improvement rates, technology trajectories and key assignees

Technology innovation in electric vehicles is of significant interest to researchers, companies and policy-makers of many countries. Electric vehicles integrate various kinds of distinct technologies and decomposing the overall electric vehicle field into several key domains allows determination of more detailed, valuable information. To provide both broader and more detailed information about technology development in the EV field, unlike most previous studies on electric vehicle innovation which analyzed this field as a whole, this research decomposed the electric vehicle field into domains, which are power electronics, battery, electric motor as well as charging and discharging subdomains and then further extracted the subdomains. Furthermore, In addition, the improvement rates, technology trajectories and major patent assignees in these domains and key subdomains are determined using patents extracted for each domain from the US patent system. The main findings are: (1) The estimated rates of performance improvement per year are 18.3% for power electronics, 7.7% for electric motors, 23.8% for charging and discharging and 11.7% for batteries. The relatively lower improvement rate for electric motors and batteries suggests their potential to hinder the popularization of electric vehicles. Besides, as for the subdomains, the relatively higher technology improvement rate of lithium-ion battery or permanent magnet motor in its domain supports the current trend of battery or motor type quantitively from a patent analysis view. A possible implication for the policy makers encouraging EV development is to issue more incentive plans for innovations in the battery and electric motor domains, especially for lithium-ion battery and permanent magnet motor. (2) The technology trajectories depict the development of four critical subdomains over time, which quantitively proves the focuses and emerging topics of the subdomains and thereby provide guidance to research topic selection. For example, the silicon negative electrode is a promising topic in the subdomain of lithium-ion battery. (3) The key players in the four critical subdomains appear to be Toyota and Honda in hybrid power electronics, E-One Moli Energy Corp in lithium-ion batteries, Panasonic in Permanent Magnet motors and Toyota in discharging. The key players found by the main path method from the view of innovation are also important players in EV from the market view. Other market participants should pay more attention to the adjustment of business strategy of these companies to monitor the market, and make effort to invent important EV related technologies.

Characterization of a Cross-Linked Polymer Containing Hydroxyl Groups as a Binder for High-Capacity Anodes in Li-Ion Batteries.

In an effort to investigate the capabilities of a cross-linked polymer containing hydroxyl groups (-OH groups) as a binder for silicon (Si) negative electrodes in Li-ion batteries (LIBs), a Si/carbon black (CB)/(poly)pentaerythritoltriacrylate (PPETA) composite (Si/CB/PPETA-composite) electrode was fabricated through a curing process from a Si/CB/pentaerythritoltriacrylate (PETA) multifunctional monomer composite mixture (Si/CB/PETA-composite mixture). As a result, the delithiation of the Si/CB/PPETA-composite was nearly four times higher than that of a Si/CB/poly(vinylidene fluoride) (PVdF) composite (Si/CB/PVdF-composite) using the existing PVdF binder for 15 cycles. The lithiation of the Si/CB/PVdF/PPETA (1:2)-composite (approximately 2846 mAh g-1) using amixed PVdF/PPETA (1:2) binder (PVdF/PPETA (1:2) binder) (mixing ratio, 1.0/2.0) was improved by approximately 487 mAh g-1 compared to that of a Si/CB/PVdF-composite (about 2359 mAh g-1.

Insights into Reactivity of Silicon Negative Electrodes: Analysis Using Isothermal Microcalorimetry.

Silicon offers high theoretical capacity as a negative electrode material for lithium-ion batteries; however, high irreversible capacity upon initial cycling and poor cycle life have limited commercial adoption. Herein, we report an operando isothermal microcalorimetry (IMC) study of a model system containing lithium metal and silicon composite film electrodes during the first two cycles of (de)lithiation. The total heat flow data are analyzed in terms of polarization, entropic, and parasitic heat flow contributions to quantify and determine the onset of parasitic reactions. These parasitic reactions, which include solid-electrolyte interphase formation, contribute to electrochemical irreversibility. Cycle 1 lithiation demonstrates the highest thermal energy output at 1509 mWh/g, compared to cycle 1 delithiation and cycle 2. To complement the calorimetry, operando X-ray diffraction is used to track the phase evolution of silicon. During cycle 1 lithiation, crystalline Si undergoes transformation to amorphous lithiated silicon and ultimately to crystalline Li15Si4. The solid-state amorphization process is correlated to a decrease in entropic heat flow, suggesting that heat associated with the amorphization contributes significantly to the entropic heat flow term. This study effectively uses IMC to probe the parasitic reactions that occur during lithiation of a silicon electrode, demonstrating an approach that can be broadly applied to quantify parasitic reactions in other complex systems.

Optimization of Si/Gr Based Anode Formulation for High Energy Density Li-Ion Batteries

In need of increasing automotive applications, it is necessary to find both higher capacity and higher-capacity retention electrode materials for Li-ion batteries. Among the candidates for negative electrode [1], silicon appears as an attractive alternative to graphite due to its natural abundance, high specific gravimetric capacity (3579 mAh.g-1 vs. 372mAh.g-1 for graphite) and a large volumetric capacity (2081 mAh.cm-3 vs. 779 mAh.cm-3). Silicon based electrodes nevertheless suffer from poor cyclability usually due to dramatic volume expansion of the electrode during the cycling and electrolyte reduction [2]. As a consequence, irreversible capacity losses are not limited to the first few cycles, but may continue to deteriorate cell performance and life [3, 4]. On one hand, previous studies demonstrated that the formulation [5], the processing [6] of the composite electrode and the adhesion to the current collector are critical for Si with respect to the electrochemical performance. The design of a flexible network of shaped conductive additives bonded with an aqueous binder to silicon particles through an optimized ratio of covalent and hydrogen bonds greatly help in facilitating a homogeneous functioning of the electrode that minimizes electrolyte reduction and keep the mechanical integrity of the electrode. On the other hand, using a combined silicon/graphite material also help in mitigating volume variations expected to occur upon cycling, especially for high mass loading electrodes (>3-4 mg). In this study, we use both approaches by tuning the binder incorporation in the case of a 50/50 %wt Silicon/Graphite composite electrodes. We focus here in particular on the electrode formulation, using a polyacrylic acid polymer binder thanks to its grafting mechanism to silicon particles enabling a strong mechanical cohesion even in the case of high-loaded electrodes. [7] pH and viscosity adjustments of the electrode slurry will be discussed, as they seem to have a remarkable effect at the interface between electrode and current collector. In fact, the longer drying process (due to thicker coating) exacerbate different corrosion mechanisms and migration phenomena that were not observed previously for thin electrodes usually studied. Characterizations on this interface were carried out, combining XPS and SEM, in order to offer a further comprehension of Si/Gr cycling performance as a function of electrode preparation. Reference : 1) Kasavajjula, U., Wang, C. & Appleby, A. J. Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells. 163, 1003–1039 (2007). 2) Obrovac, M. N. & Chevrier, V. L. Alloy negative electrodes for Li-ion batteries. Chem. Rev. 114, 11444–11502 (2014). 3) Ryu, J. H., Kim, J. W., Sung, Y. & Oh, S. M. Failure Modes of Silicon Powder Negative Electrode in Lithium Secondary Batteries. Electrochem. Solid-State Lett. 7, 306–309 (2004). 4) Beaulieu, L. Y. et al. Colossal Reversible Volume Changes in Lithium Alloys service Colossal Reversible Volume Changes in Lithium Alloys. Electrochem. Solid-State Lett. 4, A137 (2001). 5) Mazouzi, D. et al. Critical roles of binders and formulation at multiscales of silicon-based composite electrodes. J. Power Sources 280, 533–549 (2015). 6) Nguyen, B. P. N., Chazelle, S., Cerbelaud, M., Porcher, W. & Lestriez, B. Manufacturing of industry-relevant silicon negative composite electrodes for lithium ion-cells. J. Power Sources 262, 112–122 (2014). 7) Mazouzi, D., Lestriez, B., Roué, L. & Guyomard, D. Silicon Composite Electrode with High Capacity and Long Cycle Life. Electrochem. Solid-State Lett. 12, A215 (2009).

Significant Suppression of Silicon Negative Electrode for Lithium-Ion Battery in Ionic-Liquid Electrolyte

Elemental Silicon (Si) has a high theoretical capacity of 3580 mA h g-1 and has attracted attention as an active material of negative electrode for high energy density lithium-ion batteries. However, it shows poor cycle stability, which is mainly due to a massive volume change in Si during lithiation and delithiation. Here, we report that combination an ionic liquid electrolyte with a charge capacity limit of 1000 mA h g-1 significantly suppresses Si volume expansion, improving the cycle life. On the other hand, the Si layer expands largely in an organic electrolyte even with the charge capacity limit and even in an ionic-liquid electrolyte without the limit. We demonstrated that the homogeneously distributed Si lithiation−delithiation, phase-transition control from the Si to Li-rich Li-Si alloy phases, and formation of a surface film with structural and/or mechanical stability contribute to suppressing Si volume expansion.1 A Si working electrode was fabricated by gas-deposition method, which is without any binder and conductive agent. A 2032-type coin cell was constructed, comprising a Si electrode, a glass fiber filter as the separator, and a Li metal sheet as the counter electrode. An ionic liquid electrolyte used was 1 mol dm-3 (M) lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in N-methyl-N-propylpyrrodinum bis(fluorosulfonyl)amide (Py13-FSA). For comparison, 1 M lithium bis(trifluoromethanesulfonyl)amide (LiFSA) dissolved in propylene carbonate (PC) was employed as a conventional organic electrolyte. Galvanostatic charge-discharge testing was performed with a charge capacity limit of 1000 mA h g-1 unless stated otherwise. The current density was set at 0.36 A g-1 (0.1 C) during the first cycle and 1.44 A g-1 (0.4 C) during subsequent cycles. An electrode cross section was observed by field emission scanning electron microscopy (FE-SEM). The cross section was fabricated using a cross-section polisher or focused ion beam. The electrode was not exposed to the atmosphere until it was introduced into the chamber of the FE-SEM using a transfer vessel. A reversible capacity of the Si electrode decayed at around the100th cycle in the organic electrolyte (1 M LiTFSA/PC). Conversely, in the ionic-liquid electrolyte (1 M LiFSA/Py13-FSA), the Si electrode exhibited a better cycle life with a reversible capacity of 1000 mA h g-1 after the 600th cycle. To determine the difference between the cycle lives of Si electrode in organic and ionic-liquid electrolytes, we investigated the thickness of the lithiated Si active material layers by FE-SEM. The thickness of the nonlithiated Si layer before charge-discharge testing was 1.6 ± 0.3 mm.2 In the organic electrolyte, the Si layer expnads with the cycle number, developing several cracks and becoming porous after the 50th and 100th cycles. The thickness reached ca. 25 mm after the 100th cycle when the capacity faded. The amorphous Li1.0Si (a-Li1.0Si) phase may mainly form with a charge capacity limit of 1000 mA h g-1 because it has a theoretical capacity of 950 mA h g-1. The expansion rate of the Si layer was about 1460%, whereas the calculated rate of increase in thickness from Si to a-Li1.0Si phases is 17%. In the ionic-liquid electrolyte, the Si electrode retained a thickness of 2.7 mm over 300 cycles, with an expansion rate of ca. 69%. Unexpectedly, the Si layer did not expand significantly after repeated cycling. There are no reports on the significant suppression of Si volume expansion after such long-term cycling. However, the expansion rate of 69% is still above the calculated rate of 17% for the a-Li1.0Si phase. After the 600th cycle and just before capacity fading, the layer thickness increased to 13.3 mm with some void and crack formation inside the layer. We discuss differences in the performances of the Si-alone electrode in the organic and ionic-liquid electrolytes based on the Li storage distribution, phase transition, and surface film formation. Reference s : [1] Y. Domi, H. Usui, K. Yamaguchi, S. Yodoya, H. Sakaguchi, ACS Appl. Mater. Interfaces,.2019, in press. [2] K. Yamaguchi, Y. Domi, H. Usui, H. Sakaguchi, ChemElectroChem, 2017, 4, 3257-3263.

Applicability of an Ionic Liquid Electrolyte to a Phosphorus‐Doped Silicon Negative Electrode for Lithium‐Ion Batteries

AbstractWe investigated the applicability of an ionic liquid electrolyte to a phosphorus‐doped Si (P‐doped Si) electrode to improve the performance and safety of the lithium‐ion battery. The electrode exhibited excellent cycling performance with a discharge capacity of 1000 mA h g−1 over 1400 cycles in the ionic liquid electrolyte, whereas the capacity decayed at the 170th cycle in the organic electrolyte. The lithiation/delithiation reaction of P‐doped Si occurred a localized region in the organic electrolyte, which generated a high stress and large strain. The strain accumulated under repeated charge‐discharge cycling, leading to severe electrode disintegration. In contrast, the reaction of P‐doped Si proceeded uniformly in the ionic liquid electrolyte, which suppressed the electrode disintegration. The P‐doped Si electrode also showed good rate performance in the ionic liquid electrolyte; a discharge capacity of 1000 mA h g−1 was retained at 10 C.