Side-Chain Conducting and Phase-Separated Polymeric Binders for High-Performance Silicon Anodes in Lithium-Ion Batteries

Synthesis of Si/C Composites by Silicon Waste Recycling and Carbon Coating for High-Capacity Lithium-Ion Storage

It is of great significance to recycle the silicon (Si) kerf slurry waste from the photovoltaic (PV) industry. Si holds great promise as the anode material for Li-ion batteries (LIBs) due to its high theoretical capacity. However, the large volume expansion of Si during the electrochemical processes always leads to electrode collapse and a rapid decline in electrochemical performance. Herein, an effective carbon coating strategy is utilized to construct a precise Si@C_PPy composite using cutting-waste silicon and polypyrrole (PPy). By optimizing the mass ratio of Si and carbon, the Si@C_PPy composite can exhibit a high specific capacity and superior rate capability (1436 mAh g^-1 at 0.1 A g^-1 and 607 mAh g^-1 at 1.0 A g^-1). Moreover, the Si@C_PPy composite also shows better cycling stability than the pristine prescreen silicon (PS-Si), as the carbon coating can effectively alleviate the volume expansion of Si during the lithiation/delithiation process. This work showcases a high-value utilization of PV silicon scraps, which helps to reduce resource waste and develop green energy storage.

Intrinsic Self-Healing Chemistry for Next-Generation Flexible Energy Storage Devices

The booming wearable/portable electronic devices industry has stimulated the progress of supporting flexible energy storage devices. Excellent performance of flexible devices not only requires the component units of each device to maintain the original performance under external forces, but also demands the overall device to be flexible in response to external fields. However, flexible energy storage devices inevitably occur mechanical damages (extrusion, impact, vibration)/electrical damages (overcharge, over-discharge, external short circuit) during long-term complex deformation conditions, causing serious performance degradation and safety risks. Inspired by the healing phenomenon of nature, endowing energy storage devices with self-healing capability has become a promising strategy to effectively improve the durability and functionality of devices. Herein, this review systematically summarizes the latest progress in intrinsic self-healing chemistry for energy storage devices. Firstly, the main intrinsic self-healing mechanism is introduced. Then, the research situation of electrodes, electrolytes, artificial interface layers and integrated devices based on intrinsic self-healing and advanced characterization technology is reviewed. Finally, the current challenges and perspective are provided. We believe this critical review will contribute to the development of intrinsic self-healing chemistry in the flexible energy storage field.

N-Type Polyoxadiazole Conductive Polymer Binders Derived High-Performance Silicon Anodes Enabled by Crosslinking Metal Cations

The dilemma of employing high-capacity battery materials and maintaining the electrodes' electrical and mechanical integrity requires a unique binder system design. Polyoxadiazole (POD) is an n-type conductive polymer with excellent electronic and ionic conductive properties, which has acted as a silicon binder to achieve high specific capacity and rate performance. However, due to its linear structure, it cannot effectively alleviate the enormous volume change of silicon during the process of lithiation/delithiation, resulting in poor cycle stability. This paper systematically studied metal ion (i.e., Li⁺, Na⁺, Mg²⁺, Ca²⁺, and Sr²⁺)-crosslinked PODs as silicon anode binders. The results show that the ionic radius and valence state remarkably influence the polymer's mechanical properties and the electrolyte's infiltration. Electrochemical methods have thoroughly explored the effects of different ion crosslinks on the ionic and electronic conductivity of POD in the intrinsic and n-doped states. Attributed to the excellent mechanical strength and good elasticity, Ca-POD can better maintain the overall integrity of the electrode structure and conductive network, significantly improving the cycling stability of the silicon anode. The cell with such binders still retains a capacity of 1770.1 mA h g^-1 after 100 cycles at 0.2 C, which is ∼285% that of the cell with the PAALi binder (620.6 mA h g^-1). This novel strategy using metal-ion crosslinking polymer binders and the unique experimental design provides a new pathway of high-performance binders for next-generation rechargeable batteries.

Hollow Porous N and Co Dual-Doped Silicon@Carbon Nanocube Derived by ZnCo-Bimetallic Metal-Organic Framework toward Advanced Lithium-Ion Battery Anodes

Silicon (Si) has been recognized as a promising alternative to graphite anode materials for advanced lithium-ion batteries (LIBs) owing to its superior theoretical capacity and low discharge voltage. However, Si-based anodes undergo structural pulverization during cycling due to the large volume expansion (ca. 300-400%) and continuous formation of an unstable solid electrolyte interphase (SEI), resulting in fast capacity fading. To address this challenge, a series of different amounts of silicon nanoparticles (Si NPs)-encapsulated hollow porous N-doped/Co-incorporated carbon nanocubes (denoted as p-CoNC@SiX, where X = 50, 80, and 100) as anode materials for LIBs are reported in this paper. These hollow nanocubic materials were derived by facile annealing of different contents of Si NPs-encapsulated Zn/Co-bimetallic zeolitic imidazolate frameworks (ZIF@Si) as self-sacrificial templates. Owing to the advantages of well-defined hollow framework clusters and highly conductive hollow carbon frameworks, the hollow porous p-CoNC@SiX significantly improved the electronic conductivity and Li⁺ diffusion coefficient by an order of magnitude higher than that of Si NPs. The as-prepared p-CoNC@Si80 with 80 wt % Si NPs delivered a continuously increasing specific capacity of 1008 mAh g^-1 at 500 mA g^-1 over 500 cycles, excellent reversible capacity (∼1361 mAh g^-1 at 0.1 A g^-1), and superior rate capability (∼603 mAh g^-1 at 3 A g^-1) along with an unprecedented long-life cyclic stability of ∼1218 mAh g^-1 at 1 A g^-1 over 1000 cycles caused by low volume expansion (9.92%) and suppressed SEI side reactions. These findings provide new insights into the development of highly reversible Si-based anode materials for advanced LIBs.

Establishing a Resilient Conductive Binding Network for Si-Based Anodes via Molecular Engineering

ConspectusSilicon-based anode materials have become a research hot spot as the most promising candidates for next-generation high-capacity lithium-ion batteries. However, the irreversible degradation of the conductive network in the anode and the resultant dramatic capacity loss have become two ultimate challenges that stem from inherent characteristics of the Si-based materials, including poor conductivity and massive volume changes (up to 300%) during cycling. Apart from optimization of the active materials, one effective way to stabilize high-capacity Si-based anodes is by designing polymeric binders to reinforce the conductive networks during repeated charge and discharge processes. As an inactive component in the electrode, the binder not only holds other components (e.g., active materials, conductive agents, and current collectors) together to maintain the mechanical integrity of the electrode but also serves as a thickener to facilitate the homogeneous distribution of particles. Therefore, binders play a key role in Si-based anodes by maintaining the integrity of conductive networks in the electrode.In this Account, on the basis of the extensive binder-related work on Si-based anodes since the 2000s, efforts made on maintaining the conductive network can be categorized into two main strategies: (1) stabilization of the primary conductive network (which generally refers to conductive agents) by enhancing the binding strength and resilience of the binding between electrode components (i.e., Si particles, conducting agents, and current collectors) via various interactions (e.g., dipolar interactions and covalent bonds) and (2) construction of the secondary conductive network by employing conductive binders, which serve as a molecular-level conductive layer on active materials. In this sense, functional groups in binders can be divided into two categories: mechanical structural units and conductive structural units. On the one hand, functional groups with strong polarities (e.g., -OH, -COOH, -NH_2, and -CONH-) generally serve as binding structural units because of their bonding tendencies; on the other hand, exhibiting high electronic conductivity, conjugated functional groups (e.g., -C₄H₄O₂S-, -C₁₆H₉, -C₁₃H₈-, and -C₁₂H₈N-) are commonly found in conductive binders. Through establishing the correlation between structural units and their corresponding properties, we systematically summarize the optimization strategies and design principles of binders to achieve a robust conductive network in Si-based anodes. In addition, integration of desirable mechanical properties and high conductivity into the binder in order to achieve a multidimensionally stable conductive network is proposed. Through an insightful retrospective and prospective on binders, a key electrode component, we hope to provide a fresh perspective on performance optimization of Si-based anodes.

Anion π-π Stacking for Improved Lithium Transport in Polymer Electrolytes

Polymer electrolytes (PEs) with excellent flexibility, processability, and good contact with lithium metal (Li°) anodes have attracted substantial attention in both academic and industrial settings. However, conventional poly(ethylene oxide) (PEO)-based PEs suffer from a low lithium-ion transference number (T_Li⁺), leading to a notorious concentration gradient and internal cell polarization. Here, we report two kinds of highly lithium-ion conductive and solvent-free PEs using the benzene-based lithium salts, lithium (benzenesulfonyl)(trifluoromethanesulfonyl)imide (LiBTFSI) and lithium (2,4,6-triisopropylbenzenesulfonyl)(trifluoromethanesulfonyl)imide (LiTPBTFSI), which show significantly improved T_Li⁺ and selective lithium-ion conductivity. Using molecular dynamics simulations, we pinpoint the strong π-π stacking interaction between pairs of benzene-based anions as the cause of this improvement. In addition, we show that Li°∥Li° and Li°∥LiFePO₄ cells with the LiBTFSI/PEO electrolytes present enhanced cycling performance. By considering π-π stacking interactions as a new molecular-level design route of salts for electrolyte, this work provides an efficient and facile novel strategy for attaining highly selective lithium-ion conductive PEs.

Bio-Inspired Binder Design for a Robust Conductive Network in Silicon-Based Anodes

Due to the severe volume variations during electrochemical processes, Si-based anodes suffer from poor cycling performance as the result of a collapsed conductive network. In this regard, a key strategy for fully exploiting the capacity potential of Si-based anodes is to construct a robust conductive network through rational binder design. In this work, a bio-inspired conductive binder (PFPQDA) is designed by introducing dopamine-functionalized fluorene structure units (DA) into a conductivity enhanced polyfluorene-typed copolymer (PFPQ) to enhance its mechanical properties. Through constructing hierarchical binding networks and resilient electron transportations within both nano-sized Si and micro-sized SiOx electrodes via interweaved interactions, the PFPQDA successfully suppresses the electrode expansion and maintains the integrity of conductive pathways. Consequently, owing to the favorable properties of PFPQDA, Si-based anodes exhibit improved cycling performance and rate capability with an areal capacity over 2.5 mAh cm-2 .

Key Factors for Binders to Enhance the Electrochemical Performance of Silicon Anodes through Molecular Design

Silicon is considered the most promising candidate for anode material in lithium-ion batteries due to the high theoretical capacity. Unfortunately, the vast volume change and low electric conductivity have limited the application of silicon anodes. In the silicon anode system, the binders are essential for mechanical and conductive integrity. However, there are few reviews to comprehensively introduce binders from the perspective of factors affecting performance and modification methods, which are crucial to the development of binders. In this review, several key factors that have great impact on binders' performance are summarized, including molecular weight, interfacial bonding, and molecular structure. Moreover, some commonly used modification methods for binders are also provided to control these influencing factors and obtain the binders with better performance. Finally, to overcome the existing problems and challenges about binders, several possible development directions of binders are suggested.

In Situ Room-Temperature Cross-Linked Highly Branched Biopolymeric Binder Based on the Diels-Alder Reaction for High-Performance Silicon Anodes in Lithium-Ion Batteries

Silicon (Si) is an auspicious anode material in next-generation lithium-ion batteries due to its exceptional theoretical gravimetric capacity, environmental friendliness, and high natural abundance. However, the practical application of Si anodes remains a "must-solve" challenge because of its drastic capacity fading that results from the inherent property of drastic volume expansion of Si during repeated lithiation and delithiation. Developing binders employed in robust electrodes has been considered an economical and practical method to affect the electrochemical performance of Si-based electrodes. Some natural polymers have demonstrated good adhesive properties with Si-active materials. However, they have limited capacity to keep the structural integrity of electrodes because the network structures solely based on weak hydrogen bonds are susceptible to deformation during cycling. Herein, we develop an in situ covalently cross-linked three-dimensional (3D) supramolecular network and apply it to the Si electrode to improve cycling performance. This network architecture is constructed using furan-modified branched arabinoxylan of corn fiber gum (CFG) and an ionically conductive cross-linker of maleimido-poly(ethylene glycol) (PEG) through the Diels-Alder reaction. The maleimide groups in PEG can react spontaneously with the furan groups in CFG at room temperature without any other stimulation, thus forming strong covalent bonds in the network. The cross-linked CFG-PEG binder has demonstrated robust adhesive properties with Si-active materials and the current collector. The branching of CFG and functional groups of PEG are conducive to improving the lithium-ion conductivity in the silicon anode, resulting in excellent rate performances. The Si anode with a cross-linked CFG-PEG binder exhibits superior cycling stability. As a result, an in situ cross-linking 3D network as a novel binder has a great potential for fabricating an advanced Si anode in next-generation Li-ion batteries.

Constructing a Resilient Hierarchical Conductive Network to Promote Cycling Stability of SiOx Anode via Binder Design

Despite exhibiting high specific capacities, Si-based anode materials suffer from poor cycle life as their volume change leads to the collapse of conductive network within the electrode. For this reason, the challenge lies in retaining the conductive network during electrochemical processes. Herein, to address this prominent issue, a cross-linked conductive binder (CCB) is designed for commercially available silicon oxides (SiO_x ) anode to construct a resilient hierarchical conductive network from two aspects: on the one hand, exhibiting high electronic conductivity, CCB serves as an adaptive secondary conductive network in addition to the stiff primary conductive network (e.g., conductive carbon), facilitating faster interfacial charge transfer processes for SiO_x in molecular level; on the other hand, the cross-linked structure of CCB shows resilient mechanical properties, which maintains the integrity of the primary conductive network by preventing electrode deformation during prolonged cycling. With the aid of CCB, untreated micro-sized SiO_x anode material delivers an areal capacity of 2.1 mAh cm^-2 after 250 cycles at 0.8 A g^-1 . The binder design strategy, as well as, the relevant concepts proposed herein, provide a new perspective toward promoting the cycling stability of high-capacity Si-based anodes.

Binary Network of Conductive Elastic Polymer Constraining Nanosilicon for a High-Performance Lithium-Ion Battery

Silicon-based anodes are attracting more interest in both science and industry due to their high energy density. However, the traditional polymeric binder and carbon additive mixture cannot successfully accommodate the huge volume change and maintain good conductivity when cycling. Herein, we report a multifunctional polymeric binder (PPTU) synthesized by the cross-linking of conducting polymer (PEDOT:PSS) and stretchable polymer poly(ether-thioureas) (PETU). The multifunctional polymeric binder could be curved on the surfaces of nanosilicon particles, forming an interweaving continuous three-dimensional network, which is beneficial to electron transfer and the mechanical stability. Furthermore, the binder is elastic and adhesive, and which can accommodate the huge volume change of silicon to keep its integrity. Utilizing this multifunctional polymeric binder instead of commercial poly(acrylic acid) binder and carbon black mixtures, the nanosilicon anode demonstrates enhanced cycling stability (2081 mAhg^-1 after 300 cycles) and rate performance (908 mAhg^-1 at 8 Ag^-1). The multifunctional polymeric binder has high conductivity, elasticity, and self-healing properties is a promising binder to promote progress toward a high performance lithium-ion battery.

Effect of Conductivity on In Situ Deactivation of Catechol-Boronate Complexation-Based Reversible Smart Adhesive

To reduce the need for elevated electrical potential to deactivate catechol-based smart adhesive and preserve its reversibility, conductive 1-pyrenemethyl methacrylate (PyMA) was incorporated into a catechol and phenylboronic acid-containing adhesive coating immobilized on aluminum (Al) discs. Electrochemical impedance spectroscopy (EIS) indicated that incorporation of 26 mol % of PyMA reduced ionic resistance (R_s) and charge-transfer resistance (R_c) of the coating from over 22 Ω/mm² to 5.9 and 1.2 Ω/mm², respectively. A custom-built Johnson-Kendall-Roberts (JKR) contact mechanics test setup was used to evaluate the adhesive property of the coating with in situ applied electricity using a titanium (Ti) sphere both as a test substrate as well as the cathode for application of electricity and the Al disc as the anode. The adhesive coating demonstrated over 95% reduction in the adhesive property when electricity (1-2 V) was applied while the adhesive was in direct contact with the Ti surface. The addition of PyMA enables the deactivation of the adhesive using a voltage as low as 1 V. Both cyclic voltammetry (CV) and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra confirmed the formation of catechol-boronate complexation through electrochemical stimulation. Breaking the complex with an acidic buffer (pH 3) recovered the catechol for strong wet adhesion and the coating could be repeatedly deactivated and reactivated using low electrical potential for up to five cycles. Incorporation of both conductive PyMA and boronic acid as the temporary protecting group was required to achieve rapidly switchable adhesive that could be deactivated with low applied voltage.

Challenges and prospects of nanosized silicon anodes in lithium-ion batteries

Batteries are commonly considered one of the key technologies to reduce carbon dioxide emissions caused by the transport, power, and industry sectors. We need to remember that not only the production of energy needs to be realized sustainably, but also the technologies for energy storage need to follow the green guidelines to reduce the emission of greenhouse gases effectively. To reach the sustainability goals, we have to make batteries with the performances beyond their present capabilities concerning their lifetime, reliability, and safety. To be commercially viable, the technologies, materials, and chemicals utilized in batteries must support scalability that enables cost-effective large-scale production. As lithium-ion battery (LIB) is still the prevailing technology of the rechargeable batteries for the next ten years, the most practical approach to obtain batteries with better performance is to develop the chemistry and materials utilized in LIBs-especially in terms of safety and commercialization. To this end, silicon is the most promising candidate to obtain ultra-high performance on the anode side of the cell as silicon gives the highest theoretical capacity of the anode exceeding ten times the one of graphite. By balancing the other components in the cell, it is realistic to increase the overall capacity of the battery by 100%-200%. However, the exploitation of silicon in LIBs is anything else than a simple task due to the severe material-related challenges caused by lithiation/delithiation during battery cycling. The present review makes a comprehensive overview of the latest studies focusing on the utilization of nanosized silicon as the anode material in LIBs.

Chemical Coupled PEDOT:PSS/Si Electrode: Suppressed Electrolyte Consumption Enables Long-Term Stability

Huge volume changes of Si during lithiation/delithiation lead to regeneration of solid-electrolyte interphase (SEI) and consume electrolyte. In this article, γ-glycidoxypropyl trimethoxysilane (GOPS) was incorporated in Si/PEDOT:PSS electrodes to construct a flexible and conductive artificial SEI, effectively suppressing the consumption of electrolyte. The optimized electrode can maintain 1000 mAh g^-1 for nearly 800 cycles under limited electrolyte compared with 40 cycles of the electrodes without GOPS. Also, the optimized electrode exhibits excellent rate capability. The use of GOPS greatly improves the interface compatibility between Si and PEDOT:PSS. XPS Ar⁺ etching depth analysis proved that the addition of GOPS is conducive to forming a more stable SEI. A full battery assembled with NCM 523 cathode delivers a high energy density of 520 Wh kg^-1, offering good stability.

Critical barriers to the large scale commercialization of silicon-containing batteries

Silicon has received a considerable amount of attention in the last few years because of its large lithiation capacity. Its widespread utilization in real-life lithium-ion batteries has so far been prevented by the plethora of challenges presented by this material. This review discusses the most promising technologies that have been put forward to address these issues. While silicon is now much closer to being compatible with commercial-grade storage devices, some critical barriers still deserve further attention. Most importantly, device performance is strongly dependent on particle size and size distribution, with these parameters strongly controlled by the particle synthesis technique. Moreover, the nanoparticle synthesis technique ultimately controls the material manufacturing cost and compatibility with large-scale utilization. These issues are discussed in detail, and recommendations to the community are provided.

Scalable Route to Electroactive and Light Active Perylene Diimide Dye Polymer Binder for Lithium-Ion Batteries

Developing multifunctional polymeric binders is key to the design of energy storage technologies with value-added features. We report that a multigram-scale synthesis of perylene diimide polymer (PPDI), from a single batch via polymer analogous reaction route, yields high molecular weight polymers with suitable thermal stability and minimized solubility in electrolytes, potentially leading to improved binding affinity toward electrode particles. Further, it develops strategies for designing copolymers with virtually any desired composition via a subsequent grafting, leading to purpose-built binders. PPDI dye as both binder and electroactive additive in lithium half-cells using lithium iron phosphate exhibits good electrochemical performance.

Supremely elastic gel polymer electrolyte enables a reliable electrode structure for silicon-based anodes

Silicon-based materials are promising anodes for next-generation lithium-ion batteries, owing to their high specific capacities. However, the huge volume expansion and shrinkage during cycling result in severe displacement of silicon particles and structural collapse of electrodes. Here we report the use of a supremely elastic gel polymer electrolyte to address this problem and realize long-term stable cycling of silicon monoxide anodes. The high elasticity of the gel polymer electrolyte is attributed to the use of a unique copolymer consisting of a soft ether domain and a hard cyclic ring domain. Consequently, the displacement of silicon monoxide particles and volume expansion of the electrode were effectively reduced, and the damage caused by electrode cracking is alleviated. A SiO|LiNi_0.5Co_0.2Mn_0.3O₂ cell shows 70.0% capacity retention in 350 cycles with a commercial-level reversible capacity of 3.0 mAh cm^-2 and an average Coulombic efficiency of 99.9%.

A Comprehensive Study of Hydrolyzed Polyacrylamide as a Binder for Silicon Anodes

Silicon anodes have a high theoretical capacity for lithium storage, but current composite electrode formulations are not sufficiently stable under long-term electrochemical cycling. The choice of polymeric binder has been shown to impact stability and capacity of silicon anodes for electrochemical energy storage. While several promising polymeric binders have been identified, there is a knowledge gap in how various physicochemical properties-including adhesion, mechanical integrity, and ion diffusion-impact electrochemical stability and performance. In this work, we comprehensively investigate the physical properties and performance of a molecular-weight series (3-20 × 10⁶ g/mol) of partially hydrolyzed polyacrylamide (HPAM) in silicon anodes. We quantify the mechanical strength, electrolyte uptake, adhesion to silicon, copper, and carbon, as well as electrochemical performance and stability and find that HPAM satisfies many of the properties generally believed to be favorable, including good adhesion, high strength, and electrochemical stability. HPAM does not show any electrolyte uptake regardless of any molecular weight studied, and thin films of mid- and high-molecular-weight HPAM on silicon surfaces suppress lithiation of silicon. The resulting composite electrodes exhibit an electrochemical storage capacity greater than 3000 mAh/g initially and 1639 mAh/g after 100 cycles. We attribute capacity fade to failure of mechanical properties of the binder or an excess of the solid electrolyte interphase layer being formed at the Si surface. While the highest-molecular-weight sample was expected to perform the best given its stronger adhesion and bulk mechanical properties, we found that HPAM of moderate molecular weight performed the best. We attribute this to a trade-off in mechanical strength and uniformity of the resulting electrode. This work demonstrates promising performance of a low-cost polymer as a binder for Si anodes and provides insight into the physical and chemical properties that influence binder performance.

Protic Imidazolium Polymer as Ion Conductor for Improved Oxygen Evolution Performance

Improving the electrocatalytic performance of oxygen evolution reaction (OER) is essential for oxygen-involved electrochemical devices, including water splitting and rechargeable metal–air batteries. In this work, we report that the OER performance of commercial catalysts of IrO₂, Co₃O₄, and Pt-C can be improved by replacing the traditional Nafion^® ionomer with newly synthesized copolymers consisting of protonated imidazolium moieties such as ion conductors and binders in electrodes. Specifically, such an improvement in OER performance for all the tested catalysts is more significant in basic and neutral environments than that under acidic conditions. We anticipate that the results will provide new ideas for the conceptual design of electrodes for oxygen-involved electrochemical devices.

Mussel-Inspired Self-Healing Metallopolymers for Silicon Nanoparticle Anodes

Polymers with supramolecular functionality are receiving increasing attention for rechargeable battery binders as strong supramolecular interactions can facilitate self-healing effects via bond recovery of the cleaved cross-links. Such self-healing capability is useful for emerging high-capacity battery materials that undergo large volume changes. Benchmarking mussel's sticky byssus cuticle, herein we report a copolymer binder with Fe³⁺-(tris)catechol coordination cross-links, which can induce a self-healing effect for silicon anodes. The high strength of the Fe³⁺-(tris)catechol coordination bond can lead to recovery of the dissociated bond that occurs from the large volume expansion of silicon. The given copolymer also contains monomer units with sufficient flexibility to increase the interchain motions for efficient Fe³⁺-(tris)catechol bond recovery. The superior electrochemical performance of the Si electrodes based on the proposed copolymer binder indicates that metallopolymers which utilize metal-organic ligand coordination for interchain cross-linking are promising binders with self-healing effects for sustainable cycling of high capacity battery electrodes.

Confronting the Challenges of Next-Generation Silicon Anode-Based Lithium-Ion Batteries: Role of Designer Electrolyte Additives and Polymeric Binders

Silicon has emerged as the next-generation anode material for high-capacity lithium-ion batteries (LIBs). It is currently of scientific and practical interest to encounter increasingly growing demands for high energy/power density electrochemical energy-storage devices for use in electric vehicles (xEVs), renewable energy sources, and smart grid/utility applications. Improvements to existing conventional LIBs are required to provide higher energy, longer cycle lives. This is attributed to its unparalleled theoretical capacity (4200 mAh g^-1 for Li_4.4 Si), which is approximately 10 times higher than that of a state-of-the-art graphitic anode (372 mAh g^-1 for LiC₆ ), with a suitable operating voltage, natural abundance, environmental benignity, nontoxicity, high safety, and so forth. However, despite the overwhelming beneficial features, the practical integration of LIBs containing a silicon anode beyond the commercial niche is hampered by unavoidable challenges, such as excessive volume changes during the (de-)alloying process, inherently low electrical and ionic conductivities, an unstable solid-electrolyte interphase, and electrolyte drying out. Among various extenuating strategies, non-electrode factors encompassing electrolyte additives and polymeric binders are regarded as the most economical, and effective approaches towards circumventing these disadvantages are in short supply. With the aim of providing an in-depth insight into rapidly growing accounts of electrolyte additives and binders for use with silicon anode-based LIBs, this Review assesses the current state of the art of research and thereby examines opportunities to open up new avenues for the practical realization of these silicon anode-based LIBs.

High capacity silicon anodes enabled by MXene viscous aqueous ink

The ever-increasing demands for advanced lithium-ion batteries have greatly stimulated the quest for robust electrodes with a high areal capacity. Producing thick electrodes from a high-performance active material would maximize this parameter. However, above a critical thickness, solution-processed films typically encounter electrical/mechanical problems, limiting the achievable areal capacity and rate performance as a result. Herein, we show that two-dimensional titanium carbide or carbonitride nanosheets, known as MXenes, can be used as a conductive binder for silicon electrodes produced by a simple and scalable slurry-casting technique without the need of any other additives. The nanosheets form a continuous metallic network, enable fast charge transport and provide good mechanical reinforcement for the thick electrode (up to 450 µm). Consequently, very high areal capacity anodes (up to 23.3 mAh cm⁻²) have been demonstrated.

Developing thick electrodes could enable high-energy-density Li-ion batteries, however, above a critical thickness, the mass transport issues become dominating. Here the authors show that MXene can serve as a conductive binder leading to thick silicon anodes (up to 450 µm) with high areal capacity.

Strategies for Building Robust Traffic Networks in Advanced Energy Storage Devices: A Focus on Composite Electrodes

The charge transport system in an energy storage device (ESD) fundamentally controls the electrochemical performance and device safety. As the skeleton of the charge transport system, the "traffic" networks connecting the active materials are primary structural factors controlling the transport of ions/electrons. However, with the development of ESDs, it becomes very critical but challenging to build traffic networks with rational structures and mechanical robustness, which can support high energy density, fast charging and discharging capability, cycle stability, safety, and even device flexibility. This is especially true for ESDs with high-capacity active materials (e.g., sulfur and silicon), which show notable volume change during cycling. Therefore, there is an urgent need for cost-effective strategies to realize robust transport networks, and an in-depth understanding of the roles of their structures and properties in device performance. To address this urgent need, the primary strategies reported recently are summarized here into three categories according to their controllability over ion-transport networks, electron-transport networks, or both of them. More specifically, the significant studies on active materials, binders, electrode designs based on various templates, pore additives, etc., are introduced accordingly. Finally, significant challenges and opportunities for building robust charge transport system in next-generation energy storage devices are discussed.

Size-dependent kinetics during non-equilibrium lithiation of nano-sized zinc ferrite

Spinel transition metal oxides (TMOs) have emerged as promising anode materials for lithium-ion batteries. It has been shown that reducing their particle size to nanoscale dimensions benefits overall electrochemical performance. Here, we use in situ transmission electron microscopy to probe the lithiation behavior of spinel ZnFe₂O₄ as a function of particle size. We have found that ZnFe₂O₄ undergoes an intercalation-to-conversion reaction sequence, with the initial intercalation process being size dependent. Larger ZnFe₂O₄ particles (40 nm) follow a two-phase intercalation reaction. In contrast, a solid-solution transformation dominates the early stages of discharge when the particle size is about 6–9 nm. Using a thermodynamic analysis, we find that the size-dependent kinetics originate from the interfacial energy between the two phases. Furthermore, the conversion reaction in both large and small particles favors {111} planes and follows a core-shell reaction mode. These results elucidate the intrinsic mechanism that permits fast reaction kinetics in smaller nanoparticles.

Reducing particle size of electrode materials to nanoscale dimensions is believed responsible for their enhanced reaction kinetics and electrochemical performance. Here, the authors use in situ transmission electron microscopy to study the dynamic process of the spinel zinc ferrite nanoparticles as a function of size, finding that the intercalation reaction pathway changes below a critical particle size.

A long-cycling anode based on a coral-like Sn nanostructure with a binary binder

We report a facile one-pot displacement reaction for the synthesis of a 3D coral-like Sn nanostructure towards a long-cycling Li-ion battery anode.

Surface-Functionalized Silicon Nanoparticles as Anode Material for Lithium-Ion Battery

An epoxy group was successfully attached to the surface of silicon nanoparticle (SiNPs) via a silanization reaction between silanol-enriched SiNPs and functional silanes. The epoxy-functionalized SiNPs showed a much improved cell performance compared with the pristine SiNPs because of the increased stability with electrolyte and the formation of a covalent bond between the epoxy group and the polyacrylic acid binder. Furthermore, the anode laminate made from epoxy-SiNPs showed much enhanced adhesion strength. Post-test analysis shed light on how the epoxy-functional group affects the physical and electrochemical properties of the SiNP anode.

Toward Mechanically Stable Silicon-Based Anodes Using Si/SiO x@C Hierarchical Structures with Well-Controlled Internal Buffer Voids

Low conductivity and structural degradation of silicon-based anodes lead to severe capacity fading, which fundamentally hinders their practical application in Li-ion batteries. Here, we report a scalable Si/SiO _x@C anode architecture, which is constructed simultaneously by sintering a mixture of SiO/sucrose in argon atmosphere, followed by acid etching. The obtained structure features highly uniform Si nanocrystals embedded in silica matrices with well-controlled internal nanovoids, with all of them embraced by carbon shells. Because of the improvement of the volumetric efficiency for accommodating Si active spices and electrical properties, this hierarchical anode design enables the promising electrochemical performance, including a high initial reversible capacity (1210 mAh g^-1), stable cycling performance (90% capacity retention after 100 cycles), and good rate capability (850 mAh g^-1 at 2.0 A g^-1 rate). More notably, the compact heterostructures derived from micro-SiO allow high active mass loading for practical applications and the facile and scalable fabrication strategy makes this electrode material potentially viable for commercialization in Li-ion batteries.

Robust Micron-Sized Silicon Secondary Particles Anchored by Polyimide as High-Capacity, High-Stability Li-Ion Battery Anode

Silicon is an attractive high-capacity anode material for lithium-ion battery. With the help of nanostructures, cycling performance of silicon anode has improved significantly in the past couple of years. However, three major shortcomings associated with nanostructures still need to be addressed, namely, their high surface area, low tap density, and poor scalability. Herein, we present a facile and practical method to produce micron-sized Si secondary particle cluster (SiSPC) with a high tap density and a low surface area from bulk Si by high-energy ball-milling. By coupling SiSPC with a mechanically robust polyimide binder, more than 95% of the initial capacity is retained after 500 cycles at 3500 mA g^-1 (1C rate). Reversibility of electrode thickness change is confirmed by in situ dilatometry. In addition, the polyimide binder suppresses the surface reaction of the particles with electrolyte, resulting in a high Coulombic efficiency of 99.7%. Excellent cycling performance is obtained even for thick electrodes with an areal capacity of 3.57 mAh cm^-2, similar to those in commercial lithium-ion batteries. The presented Si electrode system has a high volumetric capacity of 598 mAh cm^-3, which is higher than that of the commercial graphite anode materials.

Exploring Chemical, Mechanical, and Electrical Functionalities of Binders for Advanced Energy-Storage Devices

Tremendous efforts have been devoted to the development of electrode materials, electrolytes, and separators of energy-storage devices to address the fundamental needs of emerging technologies such as electric vehicles, artificial intelligence, and virtual reality. However, binders, as an important component of energy-storage devices, are yet to receive similar attention. Polyvinylidene fluoride (PVDF) has been the dominant binder in the battery industry for decades despite several well-recognized drawbacks, i.e., limited binding strength due to the lack of chemical bonds with electroactive materials, insufficient mechanical properties, and low electronic and lithium-ion conductivities. The limited binding function cannot meet inherent demands of emerging electrode materials with high capacities such as silicon anodes and sulfur cathodes. To address these concerns, in this review we divide the binding between active materials and binders into two major mechanisms: mechanical interlocking and interfacial binding forces. We review existing and emerging binders, binding technology used in energy-storage devices (including lithium-ion batteries, lithium-sulfur batteries, sodium-ion batteries, and supercapacitors), and state-of-the-art mechanical characterization and computational methods for binder research. Finally, we propose prospective next-generation binders for energy-storage devices from the molecular level to the macro level. Functional binders will play crucial roles in future high-performance energy-storage devices.

A Quadruple-Hydrogen-Bonded Supramolecular Binder for High-Performance Silicon Anodes in Lithium-Ion Batteries

With extremely high specific capacity, silicon has attracted enormous interest as a promising anode material for next-generation lithium-ion batteries. However, silicon suffers from a large volume variation during charge/discharge cycles, which leads to the pulverization of the silicon and subsequent separation from the conductive additives, eventually resulting in rapid capacity fading and poor cycle life. Here, it is shown that the utilization of a self-healable supramolecular polymer, which is facilely synthesized by copolymerization of tert-butyl acrylate and an ureido-pyrimidinone monomer followed by hydrolysis, can greatly reduce the side effects caused by the volume variation of silicon particles. The obtained polymer is demonstrated to have an excellent self-healing ability due to its quadruple-hydrogen-bonding dynamic interaction. An electrode using this self-healing supramolecular polymer as binder exhibits an initial discharge capacity as high as 4194 mAh g-1 and a Coulombic efficiency of 86.4%, and maintains a high capacity of 2638 mAh g-1 after 110 cycles, revealing significant improvement of the electrochemical performance in comparison with that of Si anodes using conventional binders. The supramolecular binder can be further applicable for silicon/carbon anodes and therefore this supramolecular strategy may increase the choice of amendable binders to improve the cycle life and energy density of high-capacity Li-ion batteries.

Highly Graphitized Carbon Coating on SiO with a π⁻π Stacking Precursor Polymer for High Performance Lithium-Ion Batteries

A highly graphitized carbon on a silicon monoxide (SiO) surface coating at low temperature, based on polymer precursor π–π stacking, was developed. A novel conductive and electrochemically stable carbon coating was rationally designed to modify the SiO anode materials by controlling the sintering of a conductive polymer, a pyrene-based homopolymer poly (1-pyrenemethyl methacrylate; PPy), which achieved high graphitization of the carbon layers at a low temperature and avoided silicon carbide formation and possible SiO material transformation. When evaluated as the anode of a lithium-ion battery (LIB), the carbon-coated SiO composite delivered a high discharge capacity of 2058.6 mAh/g at 0.05 C of the first formation cycle with an initial Coulombic efficiency (ICE) of 62.2%. After 50 cycles at 0.1 C, this electrode capacity was 1090.2 mAh/g (~82% capacity retention, relative to the capacity of the second cycle at 0.1 °C rate), and a specific capacity of 514.7 mAh/g was attained at 0.3 C after 500 cycles. Furthermore, the coin-type full cell composed of the carbon coated SiO composite anode and the Li[Ni_0.5Co_0.2Mn_0.3O₂] cathode attained excellent cycling performance. The results show the potential applications for using a π–π stacking polymer precursor to generate a highly graphitize coating for next-generation high-energy-density LIBs.

A "Sticky" Mucin-Inspired DNA-Polysaccharide Binder for Silicon and Silicon-Graphite Blended Anodes in Lithium-Ion Batteries

New binder concepts have lately demonstrated improvements in the cycle life of high-capacity silicon anodes. Those binder designs adopt adhesive functional groups to enhance affinity with silicon particles and 3D network conformation to secure electrode integrity. However, homogeneous distribution of silicon particles in the presence of a substantial volumetric content of carbonaceous components (i.e., conductive agent, graphite, etc.) is still difficult to achieve while the binder maintains its desired 3D network. Inspired by mucin, the amphiphilic macromolecular lubricant, secreted on the hydrophobic surface of gastrointestine to interface aqueous serous fluid, here, a renatured DNA-alginate amphiphilic binder for silicon and silicon-graphite blended electrodes is reported. Mimicking mucin's structure comprised of a hydrophobic protein backbone and hydrophilic oligosaccharide branches, the renatured DNA-alginate binder offers amphiphilicity from both components, along with a 3D fractal network structure. The DNA-alginate binder facilitates homogeneous distribution of electrode components in the electrode as well as its enhanced adhesion onto a current collector, leading to improved cyclability in both silicon and silicon-graphite blended electrodes.

Mussel-Inspired Conductive Polymer Binder for Si-Alloy Anode in Lithium-Ion Batteries

The excessive volume changes during cell cycling of Si-based anode in lithium ion batteries impeded its application. One major reason for the cell failure is particle isolation during volume shrinkage in delithiation process, which makes strong adhesion between polymer binder and anode active material particles a highly desirable property. Here, a biomimetic side-chain conductive polymer incorporating catechol, a key adhesive component of the mussel holdfast protein, was synthesized. Atomic force microscopy-based single-molecule force measurements of mussel-inspired conductive polymer binder contacting a silica surface revealed a similar adhesion toward substrate when compared with an effective Si anode binder, homo-poly(acrylic acid), with the added benefit of being electronically conductive. Electrochemical experiments showed a very stable cycling of Si-alloy anodes realized via this biomimetic conducting polymer binder, leading to a high loading Si anode with a good rate performance. We attribute the ability of the Si-based anode to tolerate the volume changes during cycling to the excellent mechanical integrity afforded by the strong interfacial adhesion of the biomimetic conducting polymer.

The emerging era of supramolecular polymeric binders in silicon anodes

Silicon (Si) anode is among the most promising candidates for the next-generation high-capacity electrodes in Li-ion batteries owing to its unparalleled theoretical capacity (4200 mA h g^-1 for Li_4.4Si) that is approximately 10 times higher than that of commercialized graphitic anodes (372 mA h g^-1 for LiC₆). The battery community has witnessed substantial advances in research on new polymeric binders for silicon anodes mainly due to the shortcomings of conventional binders such as polyvinylidene difluoride (PVDF) to address problems caused by the massive volume change of Si (300%) upon (de)lithiation. Unlike conventional battery electrodes, polymeric binders have been shown to play an active role in silicon anodes to alleviate various capacity decay pathways. While the initial focus in binder research was primarily to maintain the electrode morphology, it has been recently shown that polymeric binders can in fact help to stabilize cracked Si microparticles along with the solid-electrolyte-interphase (SEI) layer, thus substantially improving the electrochemical performance. In this review article, we aim to provide an in-depth analysis and molecular-level design principles of polymeric binders for silicon anodes in terms of their chemical structure, superstructure, and supramolecular interactions to achieve good electrochemical performance. We further highlight that supramolecular chemistry offers practical tools to address challenging problems associated with emerging electrode materials in rechargeable batteries.