Lithiation Of Si Research Articles

(Invited) Extend Calendar Life of Si Based Li-Ion Batteries

Although significant progress has been made in increasing the specific energy and cycle life of silicon (Si) based Li-ion batteries (Si-LIBs), calendar life of these batteries is still less than two years, which is far less than the 10 year life-time required for electrical vehicle applications.1 Graphite anodes undergo only ~10% volume change during cycling, so SEI formed during the initial process does not experience significant mechanical stress during the subsequent cycles. However, the SEI layer formed on Si anodes experiences tremendous mechanical stress during repeated cycles. Most SEI layers formed on the Si surface in conventional electrolytes break down, exposing fresh, lithiated Si or SiO to electrolyte. Thus, significant Si corrosion and electrolyte consumption continuously occur during battery storage, especially in the fully lithiated conditions and elevated temperatures required for the accelerated calendar life test.2 As a result, the impedance of the Si anode will increase much faster than a graphite anode and shorten its calendar life, as observed in nearly all Si-LIBs. In this work, we will report the results of our recent investigation on the fundamental mechanism behind the limited calendar life of Si-LIBs. Several approaches that alleviated degradation of SEI layer and increasing cycle life of Si-LIBs will be discussed, including the nanostructured Si designs that can minimize the external size expansion, minimized particle surface area, and formation of stable SEI layers by localized high concentration electrolyte tailored for Si anode. The combination of these methods can largely extend the calendar life of Si-LIBs and accelerate the application of these batteries for large-scale electrical vehicles and consumer electronics applications.

A diffusion–reaction–deformation coupling model for lithiation of silicon electrodes considering plastic flow at large deformation

Silicon (Si) is extensively used as an electrode in lithium (Li) ion batteries for its superiority in the charging capacity. However, the lithiation of Si would induce large deformation and significant stress jump across a two-phase interface in Si electrodes and might eventually lead to structure failure of batteries. To improve the performance and life of Li ion batteries, it is of great importance to exactly model the lithiation process. In the past decade, although many models have been developed for the lithiation, most of them just relate the deformation with the diffusion, without considering the effect of electrochemical reactions, so that they cannot simulate the formation of the two-phase interface. In this paper, we aim to develop a fully coupling diffusion–reaction–deformation model for large deformation cases and apply it to simulate the lithiation process, where a reaction barrier effect is proposed to describe the formation of the two-phase interface due to fast reaction and the mechanical behaviors of Si electrodes such as large plastic flow during lithiation process are predicted.

Mg-Coated Si Thin Film As Model Electrode for Mechanistic Study of Li-Mg-Si Zintl Phase Formation in Lithium-Ion Batteries

Silicon (Si) is a promising anode material for high energy density lithium-ion batteries (LIBs) but its poor cycling performance prevents its large-scale adoption. Introducing Mg salt into the electrolyte has shown to form a ternary Li-Mg-Si Zintl phase upon lithiation of Si and improve the cycling stability; however, its formation mechanism and impacts on the solid electrolyte interphase (SEI) are not yet well understood. Herein, we demonstrate the formation of a ternary Li-Mg-Si Zintl phase via a Magnesium (Mg) coated Si thin film electrode, where Mg diffuses into the Si film upon deposition and in the lithiation process. The Zintl phase alters the nature of SEI, suppresses the excess decomposition of electrolyte and improves the capacity retention of the Si anode. This study provides insights into the formation mechanism of ternary Zintl phase and guidelines for the future design of Si anodes.

Yolk-shell structured CuSi2P3@Graphene nanocomposite anode for long-life and high-rate lithium-ion batteries

Abstract Silicon-based anode materials enable the development of commercial lithium-ion batteries (LIBs) with higher gravimetric energy densities than are currently available. However, the inherently low electronic and ionic conductivity as well as large volume expansion upon lithiation of Si hinder their use in practical applications. Here we report a cation-disordered CuSi2P3 material, synthesized using high-energy ball milling, that shows improved stability, larger capacity, and higher ionic and electronic conductivity than pure Si. When used as an anode for LIBs, CuSi2P3 demonstrates a high reversible capacity of 2069 mA h g−1 with an initial Coulombic efficiency of 91% and a suitable working potential of 0.5 V (vs. Li+/Li). Further, after a two-step ball milling of CuSi2P3 with graphite, a yolk-shell structured carbon-coated CuSi2P3@graphene nanocomposite is formed that shows enhanced long-term cycling stability (1394 mA h g−1 after 1500 cycles at 2 A g−1; 1804 mA h g−1 after 500 cycles at 200 mA g−1) and rate capability (530 mA h g−1 at 50 A g−1), surpassing those for other Cu-Si, Cu-P, and Si-P compounds or single-component Si- and P-based composites. When coupled with a LiNi0.5Co0.2Mn0.3O2 (NCM) cathode in a full cell, the NCM//CuSi2P3 @graphene battery exhibits a high capacity of 140 mA h g−1 after 200 cycles, demonstrating the potential of CuSi2P3 anodes for the next-generation high-performance LIBs.

In Situ Observation of Lithiation and Delithiation Reactions of a Silicon Thin Film Electrode for All-Solid-State Lithium-Ion Batteries by X-ray Photoelectron Spectroscopy.

In situ X-ray photoelectron spectroscopy is applied to electrochemical lithiation/delithiation processes of an amorphous Si electrode sputter-deposited on a Li6.6La3Zr1.6Ta0.4O12 solid electrolyte. After the first lithiation, a broad Li peak appears at the Si surface, and peaks corresponding to bulk Si and Si suboxide significantly shift to lower binding energy. The appearance of the Li peak and shift of the Si peaks confirm the formation of lithium-silicide and lithium-silicates due to the lithiation of Si and native suboxide. The composition of lithium-silicide is estimated to be Li3.44Si by quantitative analysis of electrochemical response and photoelectron spectra. Peak fitting analysis shows the formation of Li2O and Li2CO3 due to side reactions. Upon the following delithiation, the peak corresponding to Li3.44Si phase shifts back to higher binding energy to form Li0.15Si phase, while lithium-silicates, Li2O, and Li2CO3 remained as irreversible species. Thus, electrochemical reactions accompanied with lithiation/delithiation processes are successfully observed.

Empirical potential optimization for the investigation of lithiation-delithiation cycles of amorphous Si nanowires

The atomistic mechanisms during lithiation and delithiation of amorphous Si nanowires ($a$-SiNW) have been investigated over cycles by molecular dynamics simulations. First, the Modified Embedded Atom Method (MEAM) potential from Cui et al. [J. Power Sources. 2012, (207) 150] has been further optimized on static (Li$_x$Si alloy phases and point defect energies) and on dynamic properties (Li diffusion) to reproduce the lithiation of small crystalline Si nanowires calculated at the {\it ab initio} level. The lithiation of $a$-SiNW reveals a two-phase process of lithiation with a larger diffusion interface compared to crystalline Si lithiation. Compressive axial stresses are observed in the amorphous Si$_x$Li alloy outer shell. They are easily released thanks to the soft glassy behavior of the amorphous alloy. Conversely, in crystalline SiNW, the larger stress in the narrow crystalline lithiated interface is hardly released and requires a phase transformation to amorphous to operate, which delays the lithiation. The history of the charge-discharge cycles as well as the temperature appear as driving forces for phase transformation from amorphous Li$_x$Si alloy to the more stable crystalline phase counterpart. Our work suggest that a full delithiation could heal the SiNWs to improve the life cycles of Li-ion batteries with Si anode.

Nonpassivated Silicon Anode Surface.

A stable solid electrolyte interphase (SEI) has been proven to be a key enabler to most advanced battery chemistries, where the reactivity between the electrolyte and the anode operating beyond the electrolyte stability limits must be kinetically suppressed by such SEIs. The graphite anode used in state-of-the-art Li-ion batteries presents the most representative SEI example. Because of similar operation potentials between graphite and silicon (Si), a similar passivation mechanism has been thought to apply on the Si anode when using the same carbonate-based electrolytes. In this work, we found that the chemical formation process of a proto-SEI on Si is closely entangled with incessant SEI decomposition, detachment, and reparation, which lead to continuous lithium consumption. Using a special galvanostatic protocol designed to observe the SEI formation prior to Si lithiation, we were able to deconvolute the electrochemical formation of such dynamic SEI from the morphology and mechanical complexities of Si and showed that a pristine Si anode could not be fully passivated in carbonate-based electrolytes.

Surface SiO2 Thickness Controls Uniform-to-Localized Transition in Lithiation of Silicon Anodes for Lithium-Ion Batteries.

Silicon is a promising anode material for lithium-ion batteries because of its high capacity, but its widespread adoption has been hampered by a low cycle life arising from mechanical failure and the absence of a stable solid-electrolyte interphase (SEI). Understanding SEI formation and its impact on cycle life is made more complex by the oxidation of silicon materials in air or during synthesis, which leads to SiOx coatings of varying thicknesses that form the true surface of the electrode. In this paper, the lithiation of SiO2-coated Si is studied in a controlled manner using SiO2 coatings of different thicknesses grown on Si wafers via thermal oxidation. SiO2 thickness has a profound effect on lithiation: below 2 nm, SEI formation followed by uniform lithiation occurs at positive voltages versus Li/Li+. Si lithiation is reversible, and SiO2 lithiation is largely irreversible. Above 2 nm SiO2, voltammetric currents decrease exponentially with SiO2 thickness. For 2-3 nm SiO2, SEI formation above 0.1 V is suppressed, but a hold at low or negative voltages can initiate charge transfer whereupon SEI formation and uniform lithiation occur. Cycling of Si anodes with an SiO2 coating thinner than 3 nm occurs at high Coulombic efficiency (CE). If an SiO2 coating is thicker than 3-4 nm, the behavior is totally different: lithiation at positive voltages is strongly inhibited, and lithiation occurs at poor CE and is highly localized at pinholes which grow over time. As they grow, lithiation becomes more facile and the CE increases. Pinhole growth is proposed to occur via rapid transport of Li along the SiO2/Si interface radially outward from an existing pinhole, followed by the lithiation of SiO2 from the interface outward.

Effects of HF on the Lithiation Behavior of the Silicon Anode in LiPF6 Organic Electrolyte Solution.

As one of the major impurities in the organic electrolyte, HF can react with the alkali components in the solid electrolyte interphase (SEI), such as lithium alkoxide and lithium carbonate, to form more LiF-rich SEI. Here, the effects of HF on the lithiation behavior of the single crystal Si(111) anode were studied using scanning electron microscopy, soft X-ray emission spectroscopy, and windowless energy-dispersive X-ray spectroscopy. When the Li–Si alloy is formed in 1.0 M LiPF6 in the propylene carbonate solvent, it has a layered structure that contained the first layer of crystalline Li15Si4 (c-Li15Si4) alloy pyramids, the second layer of amorphous Li13Si4 (a-Li13Si4) alloy, and a third layer of Li-diffused LixSi alloy. When the more concentrated HF is in the electrolyte solution, less amount of the c-Li15Si4 alloy is formed in the first layer. It suggests that the Si lithiation can form only amorphous LixSi alloy relative to the components in the electrolytes. The study also explains why only amorphous LixSi alloy formation was observed in some previous studies.

Electrochemical Lithiation and Delithiation of Si(100) Single-crystal Surface

By the electrochemical lithiation of a Si(100) single-crystal substrate, regular quadrangular pyramidal shaped three-layered structures on Si(100) were observed in scanning electron microscopic ima...

The lithiation process and Li diffusion in amorphous [formula omitted] and Si from first-principles

Silicon is considered the next-generation, high-capacity anode for Li-ion energy storage applications, however, despite significant effort, there are still uncertainties regarding the bulk Si and surface SiO2 structural and chemical evolution as it undergoes lithiation and amorphization. In this paper, we present first-principles calculations of the evolution of the amorphous Si anode, including its oxide surface layer, as a function of Li concentration. We benchmark our methodology by comparing the results for the Si bulk to existing experimental evidence of local structure evolution, ionic diffusivity as well as electrochemical activity. Recognizing the important role of the surface Si oxide (either native or artificially grown), we undertake the same calculations for amorphous SiO2, analyzing its potential impact on the activity of Si anode materials. Derived voltage curves for the amorphous phases compare well to experimental results, highlighting that SiO2 lithiates at approximately 0.7 V higher than Si in the low Li concentration regime, which provides an important electrochemical fingerprint. The combined evidence suggests that i) the inherent diffusivity of amorphous Si is high (in the order 10−9cm2s−1 - 10−7cm2s−1), ii) SiO2 is thermodynamically driven to lithiate, such that Li–O local environments are increasingly favored as compared to Si–O bonding, iii) the ionic diffusivity of Li in LiySiO2 is initially two orders of magnitude lower than that of LiySi at low Li concentrations but increases rapidly with increasing Li content and iv) the final lithiation product of SiO2 is Li2O and highly lithiated silicides. Hence, this work suggests that - excluding explicit interactions with the electrolyte - the SiO2 surface layer presents a kinetic impediment for the lithiation of Si and a sink for Li inventory, resulting in non-reversible capacity loss through strong local Li–O bond formation.

First-Principles Studies of the Lithiation and Delithiation Paths in Si Anodes in Li-Ion Batteries

Understanding the lithiation of Si and the resulting formation of amorphous lithium silicides (a-LixSi) has been the subject of numerous studies due to the importance of silicon anodes for next generation, high energy density lithium batteries. Experimental studies have shown that a-LixSi with 2 ≤ x ≤ 3.75 appears as a prominent phase in the transformation of Si during lithiation and delithiation, but computational studies have yet to elucidate why. In this work, first-principles molecular dynamics computations were performed to simulate the various immediate steps associated with lithiation and delithiation of Si anodes in a Li-ion half-cell battery. The energy of formation and unit-cell volume for relevant LixSi phases and Si have been computed for both crystalline and amorphous states. The first-principles results are utilized to construct lithiation and delithiation pathways and compute the corresponding changes in the voltage, capacity, and cell volume of the anode. These results indicate that multip...

Co-axial fibrous silicon asymmetric membranes for high-capacity lithium-ion battery anode

Silicon as a promising candidate for the next-generation high-capacity lithium-ion battery anode is characterized by outstanding capacity, high abundance, low operational voltage, and environmental benignity. However, large volume changes during Si lithiation and de-lithiation can seriously impair its long-term cyclability. Although extensive research efforts have been made to improve the electrochemical performance of Si-based anodes, there is a lack of efficient fabrication methods that are low cost, scalable, and self-assembled. In this report, co-axial fibrous silicon asymmetric membrane has been synthesized using a scalable and straightforward phase inversion method combined with dip coating as inspired by the hollow fiber membrane technology that has been successfully commercialized over the last decades to provide billions of gallons of purified drinking water worldwide. We demonstrate that ~ 90% initial capacity of co-axial fibrous Si asymmetric membrane electrode can be maintained after 300 cycles applying a current density of 400 mA g−1. The diameter of fibers, size of silicon particles, type of polymers, and exterior coating have been identified as critical factors that can influence the electrode stability, initial capacity, and rate performance. Much enhanced electrochemical performance can be harvested from a sample that has thinner fiber diameter, smaller silicon particle, lower silicon content, and porous carbon coating. This efficient and scalable approach to prepare high-capacity silicon-based anode with outstanding cyclability is fully compatible with industrial roll-to-roll processing technology, thus bearing a great potential for its future commercialization.

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.

Electrochemical Properties of Amorphous Silica As an Anode Material for Li-Ion Batteries

After conquering the scene of portable electronics (i.e. mobile phones and laptop computers) since their introduction to the marketplace in 1991, lithium ion batteries (LIBs) are set to further enable wide-scale deployment of electric vehicles (EVs) and hybrid electric vehicles (HEVs). However, a series of challenges still remain with respect to a number of important factors, such as the operational safety of LIBs, raw material supply, cost, recycling and environmental aspects. In order to fulfill future demands for large-scale applications, significant improvements and deeper understanding are required in these areas. As of today, graphite is the anode (negative electrode) material primarily used in commercial LIBs, mainly due to its charge-discharge capacity retention capability and an attractively low, flat operational voltage as well as a manageable voltage hysteresis upon (de)intercalation which altogether maximizes both the energy density and energy efficiency of the cell in which it is employed. However, recent projections regarding the future cost and availability of graphite have prompted researchers to explore alternatives in the context of cheap and sustainable materials for electrochemical energy storage. Silica (SiO2) is currently being explored as a possible alternative candidate material in this context. SiO2 was long disregarded as a potential electrode material due to its apparent electrochemical inactivity. However, studies in recent years have sparked renewed interest in SiO2 after it was demonstrated that modified nano-SiO2 may indeed be electrochemically active. SiO2 is a conversion-type anode material which irreverisbly produces various silicates (e.g. Li4SiO4 and Li2Si2O5) as well as elemental Si during electrochemical lithiation. Although the precise reaction mechanism is still a topic for debate, it is believed that the by-products formed in-situ during electrochemical cycling can mechanically buffer the stress and strain associated with continued (de)lithiation of Si and thereby extending the cycle life of the electrode material. The present study systematically investigates the electrochemical properties of amorphous SiO2 as a negative electrode material for Li-ion batteries. The SiO2 materials in question include spherical nano-particulate SiO2 recovered as an industrial by-product or naturally nano-sized 3D-architectured SiO2 extracted from a renewable resource of biological origin – the main motivation here clearly being the study of dirt-cheap, highly abundant materials and the possible prospect of their implementation in electrochemical storage devices. By virtue of its electronically insulating properties, SiO2 needs to be carefully engineered in order to activate the material. Important factors in this regard include control of particle morphology, surface area and porosity as well as enhancement of electronic conductivity. Consequently, various carbon sources (i.e. glucose, sucrose, starch) have been utilized in varying amounts in order to achieve a conductive carbon coating on the SiO2 particles via high-temperature pyrolysis (i.e. carbonization), ultimately yielding SiO2/C composites with improved electrochemical properties in relation to bare SiO2. Here, these variations in carbon precursor, carbon content and coating thickness etc. are directly related to the electrochemical activity of the different SiO2 materials. These aspects are additionally highlighted by means of a series of advanced characterization techniques. The influence of particle size and morphology is further made possible by the direct comparison of spherical SiO2 nanoparticles to the intricate 3D-architecture of the SiO2 derived from harvested biomass. Preliminary results indicate it appears possible to electrochemically activate (i.e. displace Si followed by its electrochemical lithiation at low voltages vs. Li+/Li) the latter via careful considerations in terms of electrode design and electrochemical testing procedures which results in gravimetric capacities of >1000 mAh g-1 at an average (de)lithiation voltage in the range of 0.2-0.4 V vs. Li+/Li. By comparison, the spherical nanoparticles exhibit significantly lower electrochemical activity, i.e. a reversible gravimetric capacity of 300-400 mAhg-1 with highly sloping voltage profiles in an electrochemical cycling windowof ~0-2 V vs. Li+/Li.

Elucidating the interfacial evolution and anisotropic dynamics on silicon anodes in lithium-ion batteries

Silicon (Si) has been considered as one of the most promising anodes in secondary high-energy rechargeable lithium-ion batteries (LIBs) owing to its high theoretical specific capacity and abundant reserves, whereas it suffers from an unstable solid electrolyte interphase (SEI) film and dramatic volume expansion. Herein, in situ electrochemical atomic force microscopy (AFM) and visual videos were used to reveal the interphasial properties of SEI film and anisotropic dynamics upon lithiation/delithiation processes on the single-crystalline Si electrodes with different crystal plane orientations. Lithiation of Si (111) prefer to grow along the <121> direction in the XY plane with the shape of oriented rods, in contrast, that of Si (100) form cone-like structure with the axial direction of <100> perpendicular to the (100) surface. Quantitative dynamic processes elucidate the preferable reversibility in Si (111) system, revealing the corresponding structure-reactivity correlations.

A bioinspired high-modulus mineral hydrogel binder for improving the cycling stability of microsized silicon particle-based lithium-ion battery

Silicon with high specific capacity is deemed an ideal anode material for lithium ion batteries, which, however suffers from low cycling life due to its dramatic volume changes. Water-soluble polymer binders recently gain increasing attention by providing an eco-friendly and low-cost way in improving the cycling stability of Si-based anodes. Herein, a novel bioinspired supramolecular mineral hydrogel binder consisting of polyacrylic acid (PAA) physically crosslinked with amorphous calcium carbonate (ACC) nanoparticles is designed for high-performance anodes made from low-cost microsized Si particles. Owing to its organic-inorganic hydrophilic nature, ACC-PAA hybrid binder exhibits the reported highest modulus (~ 22 GPa) for polymer binders in electrolyte, even higher than lithiated Si species (Li15Si4, ~ 12 GPa). Together with its excellent adhesion and electrochemical stability, ACC-PAA binder can effectively suppress the pulverization of Si particles and maintain the mechanical integrity of electrodes during cycling. Therefore, even with a low binder content, the anode still shows an initial discharge capacity of 2,973 mAh·g−1 and Coulombic efficiency of 81.5%, and retains 75% at a current density of 600 mA·g−1 after 100 cycles. The present organic-inorganic hybrid mineral binder may open a new approach for designing more effective polymer binders for Si-based lithium-ion batteries.

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.

Probing the Reaction between PVDF and LiPAA vs Li7Si3: Investigation of Binder Stability for Si Anodes

The chemical interaction of the highly reactive lithiated Si anode with other cell components, such as binders and electrolytes, is one of the most critical issues in batteries using Si electrodes. In this work, crystalline lithium silicide (LS) with Li7Si3 composition is synthesized to study the direct chemical reactions between the lithiated Si anodes and two typical binders, polyvinylidenefluoride (PVDF) and lithiated polyacrylic acid (LiPAA). The solid-state nuclear magnetic resonance (NMR) and Raman characterization experiments show that LiPAA binder is chemically stable against LS. On the contrary, PVDF binder reacts more readily with the LS, resulting in significant loss of Li from the bulk. 13C NMR shows the C-H bonds and C-C PVDF backbone were affected, whereas 19F MAS NMR shows the C-F bonds are relatively more stable. No LiF is detected, however HF release is confirmed by gas chromatography/mass spectrometry. The characterization results are consistent with electrochemical tests, which show the Si electrode using LiPAA as the binder has a much higher cyclability than when compared with PVDF. This work demonstrates the using of LS as an effective approach to evaluate and investigate the reactions between lithiated Si and different cell components, in order to stabilize Si anodes.

Robust Pitch on Silicon Nanolayer–Embedded Graphite for Suppressing Undesirable Volume Expansion

AbstractA significant volume expansion exhibited by high‐capacity active materials upon lithiation has hindered their application as Li‐ion battery anode materials. Although tremendous progress has been made in the development of coating methods that improve the stability of high‐capacity active materials, suitable coating sources that are both strong and economical to use are yet to be discovered. Pitch is reported here as a promising coating source for high‐capacity anodes owing to the high mechanical strength and low‐cost process. Using in situ transmission electron microscopy, it is found that pitch can withstand the severe volume expansion that occurs upon Si lithiation owing to its high mechanical strength, originating from the long‐range graphitic ordering. Notably, pitch‐coated silicon nanolayer–embedded graphite (SG) exhibits superior capacity retention (81.9%) compared to that of acetylene‐coated SG (66%) over 200 cycles in a full‐cell by effectively mitigating volume expansion, even under industrial electrode density conditions (1.6 g cc−1). Thus, this work presents new possibilities for the development of high‐capacity anodes for industrial implementation.