Dynamic Cross-Linking of Polymeric Binders Based on Host-Guest Interactions for Silicon Anodes in Lithium Ion Batteries

Electrolyte-Phobic Surface for the Next-Generation Nanostructured Battery Electrodes

Nanostructured electrodes are among the most important candidates for high-capacity battery chemistry. However, the high surface area they possess causes serious issues. First, it would decrease the Coulombic efficiencies. Second, they have significant intakes of liquid electrolytes, which reduce the energy density and increase the battery cost. Third, solid-electrolyte interphase growth is accelerated, affecting the cycling stability. Therefore, the interphase chemistry regarding electrolyte contact is crucial, which was rarely studied. Here, we present a completely new strategy of limiting effective surface area by introducing an "electrolyte-phobic surface". Using this method, the electrolyte intake was limited. The initial Coulombic efficiencies were increased up to ∼88%, compared to ∼60% of the control. The electrolyte-phobic layer of Si particles is also compatible with the binder, stabilizing the electrode for long-term cycling. This study advances the understanding of interphase chemistry, and the introduction of the universal concept of electrolyte-phobicity benefits the next-generation battery designs.

A Pyrene-Poly(acrylic acid)-Polyrotaxane Supramolecular Binder Network for High-Performance Silicon Negative Electrodes

Although being incorporated in commercial lithium-ion batteries for a while, the weight portion of silicon monoxide (SiO_x , x ≈ 1) is only less than 10 wt% due to the insufficient cycle life. Along this line, polymeric binders that can assist in maintaining the mechanical integrity and interfacial stability of SiO_x electrodes are desired to realize higher contents of SiO_x . Herein, a pyrene-poly(acrylic acid) (PAA)-polyrotaxane (PR) supramolecular network is reported as a polymeric binder for SiO_x with 100 wt%. The noncovalent functionalization of a carbon coating layer on the SiO_x is achieved by using a hydroxylated pyrene derivative via the π-π stacking interaction, which simultaneously enables hydrogen bonding interactions with the PR-PAA network through its hydroxyl moiety. Moreover, the PR's ring sliding while being crosslinked to PAA endows a high elasticity to the entire polymer network, effectively buffering the volume expansion of SiO_x and largely mitigating the electrode swelling. Based on these extraordinary physicochemical properties of the pyrene-PAA-PR supramolecular binder, the robust cycling of SiO_x electrodes is demonstrated at commercial levels of areal loading in both half-cell and full-cell configurations.

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.

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.

Recent Advances in Designing High-Capacity Anode Nanomaterials for Li-Ion Batteries and Their Atomic-Scale Storage Mechanism Studies

Lithium-ion batteries (LIBs) have been widely applied in portable electronics (laptops, mobile phones, etc.) as one of the most popular energy storage devices. Currently, much effort has been devoted to exploring alternative high-capacity anode materials and thus potentially constructing high-performance LIBs with higher energy/power density. Here, high-capacity anode nanomaterials based on the diverse types of mechanisms, intercalation/deintercalation mechanism, alloying/dealloying reactions, conversion reaction, and Li metal reaction, are reviewed. Moreover, recent studies in atomic-scale storage mechanism by utilizing advanced microscopic techniques, such as in situ high-resolution transmission electron microscopy and other techniques (e.g., spherical aberration-corrected scanning transmission electron microscopy, cryoelectron microscopy, and 3D imaging techniques), are highlighted. With the in-depth understanding on the atomic-scale ion storage/release mechanisms, more guidance is given to researchers for further design and optimization of anode nanomaterials. Finally, some possible challenges and promising future directions for enhancing LIBs' capacity are provided along with the authors personal viewpoints in this research field.

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.

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.

Compact-Nanobox Engineering of Transition Metal Oxides with Enhanced Initial Coulombic Efficiency for Lithium-Ion Battery Anodes

A novel strategy is proposed to construct a compact-nanobox (CNB) structure composed of irregular nanograins (average diameter ≈ 10 nm), aiming to confine the electrode-electrolyte contact area and enhance initial Coulombic efficiency (ICE) of transition metal oxide (TMO) anodes. To demonstrate the validity of this attempt, CoO-CNB is taken as an example which is synthesized via a carbothermic reduction method. Benefiting from the compact configuration, electrolyte can only contact the outer surface of the nanobox, keeping the inner CoO nanograins untouched. Therefore, the solid electrolyte interphase (SEI) formation is reduced. Furthermore, the internal cavity leaves enough room for volume variation upon lithiation and delithiation, resulting in superior mechanical stability of the CNB structure and less generation of fresh SEI. Consequently, the SEI remains stable and spatially confined without degradation, and hence, the CoO-CNB electrode delivers an enhanced ICE of 82.2%, which is among the highest values reported for TMO-based anodes in lithium-ion batteries. In addition, the CoO-CNB electrode also demonstrates excellent cyclability with a reversible capacity of 811.6 mA h g^-1 (90.4% capacity retention after 100 cycles). These findings open up a new way to design high-ICE electrodes and boost the practical application of TMO anodes.

Mussel-Inspired Coating and Adhesion for Rechargeable Batteries: A Review

A significant effort is currently being invested to improve the electrochemical performance of classical lithium-ion batteries (LIBs) or to accelerate the advent of new chemistry-based post-LIBs. Regardless of the governing chemistry associated with charge storage, stable electrode-electrolyte interface and wet-adhesion among the electrode particles are universally desired for rechargeable batteries adopting liquid electrolytes. In this regard, recent studies have witnessed the usefulness of mussel-inspired polydopamine or catechol functional group in modifying the key battery components, such as active material, separator, and binder. In particular, the uniform conformal coating capability of polydopamine protects active materials from unwanted side reactions with electrolytes and increases the wettability of separators with electrolytes, both of which significantly contribute to the improvement of key battery properties. The wet-adhesion originating from catechol functional groups also largely increases the cycle lives of emerging high-capacity electrodes accompanied by huge volume expansion. This review summarizes the representative examples of mussel-inspired approaches in rechargeable batteries and offers central design principles of relevant coating and adhesion processes.

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.

Towards scanning probe lithography-based 4D nanoprinting by advancing surface chemistry, nanopatterning strategies, and characterization protocols

Biointerfaces direct some of the most complex biological events, including cell differentiation, hierarchical organization, and disease progression, or are responsible for the remarkable optical, electronic, and biological behavior of natural materials. Chemical information encoded within the 4D nanostructure of biointerfaces - comprised of the three Cartesian coordinates (x, y, z), and chemical composition of each molecule within a given volume - dominates their interfacial properties. As such, there is a strong interest in creating printing platforms that can emulate the 4D nanostructure - including both the chemical composition and architectural complexity - of biointerfaces. Current nanolithography technologies are unable to recreate 4D nanostructures with the chemical or architectural complexity of their biological counterparts because of their inability to position organic molecules in three dimensions and with sub-1 micrometer resolution. Achieving this level of control over the interfacial structure requires transformational advances in three complementary research disciplines: (1) the scope of organic reactions that can be successfully carried out on surfaces must be increased, (2) lithography tools are needed that are capable of positioning soft organic and biologically active materials with sub-1 micrometer resolution over feature diameter, feature-to-feature spacing, and height, and (3) new techniques for characterizing the 4D structure of interfaces should be developed and validated. This review will discuss recent advances in these three areas, and how their convergence is leading to a revolution in 4D nanomanufacturing.

Systematic Investigation of the Alucone-Coating Enhancement on Silicon Anodes

Polyvinylidene fluoride (PVDF) is the most popular binder in commercial lithium-ion batteries but is incompatible with a silicon (Si) anode because it fails to maintain the mechanical integrity of the Si electrode upon cycling. Herein, an alucone coating synthesized by molecular layer deposition has been applied on the laminated electrode fabricated with PVDF to systematically study the sole impact of the surface modification on the electrochemical and mechanical properties of the Si electrode, without the interference of other functional polymer binders. The enhanced mechanical properties of the coated electrodes, confirmed by mechanical characterization, can help accommodate the repeated volume fluctuations, preserve the electrode structure during electrochemical reactions, and thereby, leading to a remarkable improvement of the electrochemical performance. Owing to the alucone coating, the Si electrodes achieve highly reversible cycling performance with a specific capacity of 1490 mA h g^-1 (0.90 mA h cm^-2) as compared to 550 mA h g^-1 (0.19 mA h cm^-2) observed in the uncoated Si electrode. This research elucidates the important role of surface modification in stabilizing the cycling performance and enabling a high level of material utilization at high mass loading. It also provides insights for the future development of Si anodes.

Visualization of Si Anode Reactions in Coin-Type Cells via Operando Scanning Electron Microscopy

Understanding the electrochemical behavior and controlling the morphological variations of electrodes are critical for the design of high-capacity batteries. In this article, we describe a newly established operando scanning electron microscopy (SEM) to visualize the battery reactions in a modified coin cell, which allowed the simultaneous collection of electrochemical data and time-resolved images. The investigated silicon (Si)-polyimide-binder electrode exhibited a high capacity (∼1500 mAh g^-1) and a desirable cyclability. Operando SEM revealed that the morphology of the Si anode drastically changed and cracks formed on the electrode because of the lithiation-induced volume expansion of the Si particles during the first charge process. Interestingly, the thickness variation in the Si composite layer was moderated in subsequent cycles. This strongly suggested that cracking caused by the breakage of the stiff binder alleviated the internal stress experienced by Si. On the basis of this finding by the operando SEM technique, patterned Si electrodes with controlled spacing were successfully fabricated, and their improved performance was confirmed.

5L-Scale Magnesio-Milling Reduction of Nanostructured SiO2 for High Capacity Silicon Anodes in Lithium-Ion Batteries

Nanostructured silicon (Si) is useful in many applications and has typically been synthesized by bottom-up colloid-based solution processes or top-down gas phase reactions at high temperatures. These methods, however, suffer from toxic precursors, low yields, and impractical processing conditions (i.e., high pressure). The magnesiothermic reduction of silicon oxide (SiO₂) has also been introduced as an alternative method. Here, we demonstrate the reduction of SiO₂ by a simple milling process using a lab-scale planetary-ball mill and industry-scale attrition-mill. Moreover, an ignition point where the reduction begins was consistently observed for the milling processes, which could be used to accurately monitor and control the reaction. The complete conversion of rice husk SiO₂ to high purity Si was demonstrated, taking advantage of the rice husk's uniform nanoporosity and global availability, using a 5L-scale attrition-mill. The resulting porous Si showed excellent performance as a Li-ion battery anode, retaining 82.8% of the initial capacity of 1466 mAh g^-1 after 200 cycles.

Amphiphilic Graft Copolymers as a Versatile Binder for Various Electrodes of High-Performance Lithium-Ion Batteries

It is known that grafting one polymer onto another polymer backbone is a powerful strategy capable of combining dual benefits from each parent polymer. Thus amphiphilic graft copolymer precursors (poly(vinylidene difluoride)-graft-poly(tert-butylacrylate) (PVDF-g-PtBA)) have been developed via atom transfer radical polymerization, and demonstrated its outstanding properties as a promising binder for high-performance lithium-ion battery (LIB) by using in situ pyrolytic transformation of PtBA to poly(acrylic acid) segments. In addition to its superior mechanical properties and accommodation capability of volume expansion, the Si anode with PVDF-g-PtBA exhibits the excellent charge and discharge capacities of 2672 and 2958 mAh g(-1) with the capacity retention of 84% after 50 cycles. More meaningfully, the graft copolymer binder shows good operating characteristics in both LiN0.5 M1.5 O4 cathode and neural graphite anode, respectively. By containing such diverse features, a graft copolymer-loaded LiN0.5 M1.5 O4 /Si-NG full cell has been successfully achieved, which delivers energy density as high as 546 Wh kg(-1) with cycle retention of ≈70% after 50 cycles (1 C). For the first time, this work sheds new light on the unique nature of the graft copolymer binders in LIB application, which will provide a practical solution for volume expansion and low efficiency problems, leading to a high-energy-density lithium-ion chemistry.

The Effects of Cross-Linking in a Supramolecular Binder on Cycle Life in Silicon Microparticle Anodes

Self-healing supramolecular binder was previously found to enhance the cycling stability of micron-sized silicon particles used as the active material in lithium-ion battery anodes. In this study, we systematically control the density of cross-linking junctions in a modified supramolecular polymer binder in order to better understand how viscoelastic materials properties affect cycling stability. We found that binders with relaxation times on the order of 0.1 s gave the best cycling stability with 80% capacity maintained for over 175 cycles using large silicon particles (∼0.9 um). We attributed this to an improved balance between the viscoelastic stress relaxation in the binder and the stiffness needed to maintain mechanical integrity of the electrode. The more cross-linked binder showed markedly worse performance confirming the need for liquid-like flow in order for our self-healing polymer electrode concept to be effective.