Self-healing chemistry enables the stable operation of silicon microparticle anodes for high-energy lithium-ion batteries

Developing a commercially viable silicon anode as a replacement for graphite has been the topic of intense research and discussion. Numerous attempts have been made including the use of nanoscale structures smaller than silicon’s critical fracture size (e.g. nanoparticles, nanowires, nanotubes, etc.). However, nanoscale structures, while sufficiently effective in lab scale, are difficult and expensive to scale up. To address this issue, applying a thin coat of polymer with self-healing chemistries has been reported as an effective remedy to enhance the cycle life of the silicon microparticle anode. The self-healing chemistry enabled mechanical fractures generated during the cycling process to self-heal. As a result, excellent cycle life was obtained compared to conventional microscale anodes. However, there still is room to improve the mechanical and electrochemical performances of the anode, most notably areal capacity and flexibility, while using large scale processes. In this study, a free standing silicon microparticle and self-healing polymer (SiSHP) composite is fabricated demonstrating long cycle life while still retaining high capacity (up to ~2,800mAhg-1 and ~3.5mAhcm-2) without a metal foil current collector. SiSHP composite is prepared by simple, inexpensive, and scalable approach. The SiSHP composite consists of silicon microparticles embedded within a self-healing polymeric matrix providing sufficient space for volume expansion during lithiation. The self-healing chemistry reduces irreversible loss of electric contact due to mechanical degradation prevalent in conventional slurry cast silicon electrode design. In addition, because the SiSHP composite eliminates the need for a metal current collector, adhesion issues between the electrode and current collector that arise with repeated bending are eliminated, thereby ensuring minimal loss in mechanical and electrochemical properties even after undergoing repeated bending. We found that a 1:1 Si/SHP weight ratio with 10 wt. % carbon conductive additive exhibits the optimal balance between high capacity and providing sufficient polymer matrix for stable cycling behavior. The fabricated SiSHP composite is moldable into the required shape and dimension ranging from millimeter to tens of centimeter-scale. The freestanding feature of the SiSHP composite anode eliminates the need for a metal foil current collector thereby reducing the non-active mass, which is beneficial to enhancing capacity and energy density of Lithium-ion cells.

Self-healing is a measure of material's ability to repair damage. Physical and chemical processes have been used to obtain self-healing polymers for various applications. Different approaches are involved for these systems, such as shape-memory effects, covalent-bond reform, heterogeneous systems, diffusion and flow, and the dynamics of supramolecular chemistry. Different approaches are used to achieve self-healing polymers, in particular, the role of the healing system in chemistry is highlighted, which enables thermal transients, damage repairing and reconnection. Moreover, self-healing systems and their energy storage applications are currently getting great importance. Inspired by the dynamic network structure of animal dermis, in which collagen fibril (rigid and strong) and elastin fibril (soft and elastic) crosslink through supramolecular interactions to form a sturdy and flexible material we hypothesized that the combination of a rigid conductive polymer with a soft hydrophilic polymer through proper supramolecular interactions would yield a strong and robust polymer binder.According to the literature, the external stimulus such as heat, light, pH and redox initiate the healing process. This process is known as non-autonomous self-healing when some additional external stimulus is needed. Considering the battery chemistry, it is the most functional method to choose self-healing structures and batteries with the help of hydrogen bonds in order to make the self-healing process occurs autonomously. In the literature, polymerized β-cyclodextrin (β-CD) has been used as an advanced binder for Si-nanoparticle anodes, and cracks in the electrode can be avoided with its reversible properties through supramolecular cyclodextrin or hydrogen bonding compounds[1]. Dynamic bonds such as hydrogen bonds between composites positively affect the durability of the Si anode performance. For example, the Si anode was coated by the researchers with hydrogen bond-oriented polymer binder layers based on polyimide polymer such as Upy, resulting in electrochemical performance ten times longer than conventional silicon anodes[2].Horizon projects such as BAT4EVER and HIDDEN under Battery 2030+ initiative have been investigate the integration and optimization of self-healing polymers in Li-ion battery [3]. In the scope of the BAT4EVER project, supramolecular coupling of polyaniline and polyvinyl alcohol via dynamic boronate bond yields polyaniline-polyvinyl alcohol hydrogel with outstanding tensile strength and electrochemical performance. The self-healing chemistry in the Si anode prevents unbalanced volumetric changes and cracks on the electrode surface. Thus, electrochemical performance losses are decreased. The self-healing functionalized components of lithium-ion batteries, which focus on improving and optimizing properties such as high energy density, high voltage, long life, and cycle stability, are of great importance for next-generation batteries.References Jeong, Y.K., Kwon, T.W., Lee, I., Kim, T.S., Coskun, A., Choi, J.W.: Hyperbranched β-cyclodextrin polymer as an effective multidimensional binder for silicon anodes in lithium rechargeable batteries. Nano Lett. 14, 864–870 (2014). https://doi.org/10.1021/NL404237J/SUPPL_FILE/NL404237J_SI_001.PDFYang, J., Zhang, L., Zhang, T., Wang, X., Gao, Y., Fang, Q.: Self-healing strategy for Si nanoparticles towards practical application as anode materials for Li-ion batteries. Electrochem commun. 87, 22–26 (2018). https://doi.org/10.1016/J.ELECOM.2017.12.023https://battery2030.eu/ This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement 957225 (BAT4EVER).

Si anodes have become popular in battery research due to its high theoretical capacity (3500 mAh/g) and storage capacity of a single silicon atom which is 4.4 Li atoms. This phenomenon increases the volumetric energy density of the battery. However, Si atoms undergo too much volumetric change (%300 expansion) during Li intercalation/deintercalation (charge/discharge) that leads to unstable solid-electrolyte interface (SEI), and pulverization of Si atoms). These disadvantages results with rapid capacity fading [1,2]. Traditionally, the role of binders has been as a soft matrix backbone that allows volume expansion of the anode or cathode while preserving its morphology. These polymeric binders are used to ensure the adhesion of other materials to each other and to the existing collector foil. Carboxymethyl cellulose (CMC), alginate poly (acrylic acid) (PAA), polyvinylidene fluoride (PVdF), polyvinyl pyrrolidone (PVP) and poly (vinyl alcohol) (PVA), which interact strongly with particles, are among the commonly used polymeric binders [3]. Water soluble polymers with hydroxyl or carboxylate groups, such as alginate, carboxymethyl cellulose (CMC) and poly (acrylic acid) (PAA) show greater promise, as their functional groups may help to form strong covalent bonds with siloxyl (SiOH) groups on the Silicon (Si) surface maintaining cohesion during volumetric changes. Poly (acrylic acid) (PAA) is widely used as a polymer binder for high-capacity silicon (Si) anodes in Li-ion batteries [4]. When used, PAA's carboxyl (single bond CO2H) groups facilitate the lamination process, especially for large-scale production. PVDF that water non-soluble polymer is one of the most commonly used binders for cathode of lithium-ion batteries because of its superior electrochemical and thermal stability and good adhesion between the current collectors and electrode films [5]. However, PVDF does not have enough mechanical integrity to support the swelling of the silicon anode upon cycling.Smart materials such as self-healing polymers have been investigated to overcome the disadvantages. It has been observed that the self-healing polymer has a positive effect on hindering the pulverization of Si nanoparticles [6]. Self-healing polymers with hydrogen bonding mechanisms (Figure 1) as the new generation systems for cracking prone silicon anodes of Lithium-ion batteries. When a self-healing mechanism is achieved, especially for silicon anodes which are very promising materials for stabilized nano Si -based anodes with enhanced performance. Figure 1

Lithium ion batteries are one of the most promising secondary battery technologies with high gravimetrical and volumetric energy densities, enabling the path towards a green and environmentally friendly future. To further improve the energy density of the battery, much effort is spent in the development of new electrode materials, like silicon and sulfur, to replace the materials with limited specific capacity and critical environmental impact that are currently used. Silicon, with a specific capacity higher than pure lithium (4200 mAh/g vs. 3862 mAh/g [1]), is one of the most promising anode materials being cheap, earth-abundant and less dangerous compared to pure lithium metal anodes. Silicon offers the chance to replace state of the art graphite anodes, which have low specific capacity (372 mAh/g [1]).However, rechargeable silicon anodes still suffer from the intrinsic problems of silicon during cycling, making it unsuitable for battery applications. The huge volume expansion of 400% between the charged and discharged state lead to a pulverization of the silicon anode and consequently poor cycling stability. Since the 1990s many approaches like Si nanoparticles, nanowires or nanotubes, Si thin films or compounds with different metals, metal oxides or carbon lead to an improvement in cycling stability but so far with limited application in real batteries. [1]Here, we present a scalable top down approach where a defined porous micro-structure is etched into silicon wafers. The pores allow enough space in order to cope for the large volume expansion of Si during charging and discharging of the battery. In previous studies, it was already shown that Si microwire anodes could be cycled with 3150 mAh/g for hundreds of cycles without significant capacity. [2][3]Porous silicon can also be fabricated as thick thin films (>30 µm), possessing a defined and adjustable porosity, for use as electrodes with high areal capacity for lithium ion batteries. This generation of silicon anodes has the advantage that it can be processed with cheap silicon material in much larger scale. By the electrochemical top-down approach, the surface of a silicon wafer is porosified and transferred onto copper, which is used as current collector. The use of chemical and electrochemical copper deposition techniques, create a direct ohmic contact without the need of additional, inactive material that only adds to the weight of the anode.This study shows the performance of these porous silicon anodes. By using these anodes, the performance was shown to be almost constant from the 5th to the 100th cycle, with a capacity of 3150 mAh/g and columbic efficiencies around 99%.This novel electrode design is able to counteract the volume expansion problems, and already lead to promising results in the battery performance.To further increase the energy density of lithium ion batteries, cathode materials, like sulfur, are of great interest. Since sulfur is unlithiated, contrary to NMC, NCA and other cathode materials, lithium needs to be incorporated either in the silicon anode or in the cathode. With silicon present as a film, simple prelithiation strategies can be applied, according to Cui et al.[4]. By simply short-circuiting silicon and lithium in presence of an electrolyte, a silicon anode can be prelithiated. Varying time, concentration and other electrochemical and chemical parameters, it is possible to define a certain state of charge of the silicon anode.First results of this simple prelithation strategy demonstrate high capacities and high columbic efficiencies of the silicon anodes, already possible from the first charging cycle, offering the potential to be combined with lithium-free cathodes.[1] X. Zuo, J. Zhu, P. Müller-Buschbaum and Y. Cheng, "Silicon based lithium-ion battery anodes: A chronicle perspective review", 2019.[2] S. Hansen, E. Quiroga-González, J. Carstensen, R. Adelung and H. Föll, "Size-dependent physicochemical and mechanical interactions in battery paste anodes of Si-microwires revealed by Fast-Fourier-Transform Impedance Spectroscopy", Journal of Power Sources, vol. 349, pp. 1-10, 2017.[3] S. Hansen, E. Quiroga-González, J. Carstensen and H. Föll, "Size-dependent cyclic voltammetry study of silicon microwire anodes for lithium ion batteries", 2019.[4] N. Liu, L. Hu, M. McDowell, A. Jackson and Y. Cui, "Prelithiated Silicon Nanowires as an Anode for Lithium Ion Batteries", ACS Nano, vol. 5, no. 8, pp. 6487-6493, 2011.

The development of next-generation lithium-ion batteries (LIBs) is increasingly focused on enhancing anode materials to achieve higher energy density and longer cycle life. Among the various alternatives to conventional graphite, silicon-based active materials look promising thanks to their high theoretical capacity. However, significant volume expansion during charge and discharge cycles lead to limited cycle stability. The optimization of active materials and electrode processing techniques are needed to overcome these challenges. [1]The choice of silicon-based active material, such as silicon, silicon oxide, silicon carbon coated, plays a critical role in determining the performance of the anode. The active material also influences slurry preparation process, binder interactions, and electrochemical cycling stability. [2]For instance, a material with a larger surface area or more significant volumetric expansion may require different binder concentrations or modifications to enhance the slurry's rheological properties and good adhesion to the current collector. Therefore, optimizing the silicon-based anode composition is essential for achieving both high capacity and improved cycle life, while addressing the mechanical challenges associated with its expansion during cycling. [3] [4]This study highlights the critical role of active material selection in determining the overall performance and processability of silicon-based anodes. A systematic evaluation of these materials was conducted, examining their influence on slurry preparation. Electrochemical testing was performed to compare cycling stability and capacity retention, providing insights into the benefits and drawbacks associated with each material choice.To understand the behaviour of different materials, physicochemical and electrochemical characterizations were performed, providing considerations for advanced high-energy density LIBs with improved cycle life and stability.Acknowledgments: This article was produced while attending the PhD programme in Chemistry at the University of Bologna, Cycle XXXVIII, with the support of a scholarship co-financed by the Ministerial Decree no. 352 of 9th April 2022, based on the NRRP - funded by the European Union - NextGenerationEU - Mission 4 "Education and Research", Component 2 "From Research to Business", Investment 3.3, and by the company Syensqo S.A. - Solvay Specialty Polymers Italy S.p.A.[1] Zhou, Y., et al. (2023). Designing highly packed silicon anode slurries for high capacity and lifespan. Journal of Power Sources, 555, 232304.[2] Liang, Y. et al.. (2017). "Carbon coating of SiOx nanoparticles for improved cycling performance in lithium-ion batteries," Journal of Power Sources, 342, 311-318.[3] Zhao, L., et al. (2024). Advanced binder design for high-performance silicon anodes. Journal of Energy Chemistry, 69, 1-9.[4] Li, J., et al. (2023). Innovative solutions for high-performance silicon anodes in lithium-ion batteries. Journal of Materials Science & Technology, 83, 1-9.

Exploring Advanced Polymeric Binders and Solid Electrolytes for Energy Storage Devices

Silicon has attracted ever-increasing attention as a high-capacity anode material in Li-ion batteries owing to its extremely high theoretical capacity.1 However, practical application of silicon anodes is seriously hindered by its fast capacity fading as a result of huge volume changes during the charge/discharge process.2, 3 The situation becomes even worse when making it for practical application where high mass loading of active-materials (mg-Si/cm2) on current collectors is necessary for high energy density batteries.4, 5 Polymer binder, one of major components in electrode, is used to bind active materials and conducting particles together onto the current collector. The properties of polymer binders play an important role on the electrochemical performance of the electrodes, especially for the cycle life and irreversible capacity losses.6, 7 Poly(vinylidene fluoride) (PVDF) is a conventional binder and widely used in traditional LIBs because of its acceptable adhesion and wide electrochemical window. However, the non-functionalized linear chain structure of the PVDF binder can’t afford sufficient binding to high-capacity anode particles that exhibit huge volume changes.6 This requires improved binding characteristics of binders to enable the integrity of the electrodes for long-cycle life. Thus, novel polymer binders with the ability to accommodate this substantial volume changes during lithiation/delithiation of Si anodes are highly desirable. Here, we report an interpenetrated gel polymer binder for high performance silicon anode through in-situ crosslinking of water soluble poly(acrylic acid) (PAA) and polyvinyl alcohol (PVA) precursors. This gel polymer binder with deformable polymer network and strong adhesion on silicon particles can effectively accommodate the large volume change of silicon anodes upon lithiation/delithiation, leading to an excellent cycling stability and high Coulombic efficiency even at high current densities. Moreover, high areal capacity of ~4.3 mAh/cm2 was achieved based on the silicon anode using the gel PAA-PVA polymer binder with a high mass loading. In view of simplicity in using the water soluble gel polymer binder, it is believed that this novel binder has a great potential to be used for high capacity silicon anode in the next generation Li-ion battery, but also for other electrode materials with large volume change during cycling. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-EE0006447.

Silicon (Si) has been gradually explored as a next-generation anode material to replace traditional graphite anodes in lithium-ion batteries (LIBs) due to its high specific capacity (3579 mAh g–1 at room temperature). In terms of cost and tap density, silicon microparticles (SiMPs) are more advantageous than silicon nanoparticles (SiNPs) in high energy density LIBs, but they are also plagued by the more serious volume effect. Here, we design a silicon/carbon (Si/C) composite anode through the in situ polymerization of phenolic resin (PF) onto SiMPs, and after pyrolysis, SiMPs are tightly coated with pyrolytic carbon layers. When applied in LIBs, the composite anodes (μSi@PF) exhibit excellent cycling performance (1283 mAh g–1 after 400 cycles at 2 A g–1) and rate performance (a reversible capacity of about 1000 mAh g–1 at 8 A g–1). The full cell with lithium iron phosphate cathodes and μSi@PF anodes can maintain 87.7% capacity retention after 100 cycles. The great electrochemical performance can be ascribed to the rational structure design of μSi@PF in which PF pyrolytic carbon as a shell around SiMPs can accommodate the volume change of SiMPs during cycling and reduce the internal impedance. This is the first attempt to construct Si/C composites by in situ polymerizing PF resin onto SiMPs, and the great performance of Si/C anode provides a reference for the practical application of SiMPs.

A novel high-performance 3D polymer binder for silicon anode in lithium-ion batteries

A highly crosslinked polymeric binder for silicon anode in lithium-ion batteries

Building stable and efficient electron and ion transport pathways are critically important for energy storage electrode materials and systems. Herein, a scallop-inspired shell engineering strategy is proposed and demonstrated to confine high volume change silicon microparticles toward the construction of stable and high volumetric capacity binder-free lithium battery anodes. As for each silicon microparticle, the methodology involves an inner sealed but adaptable overlapped graphene shell, and an outer open hollow shell consisting of interconnected reduced graphene oxide, mimicking the scallop structure. The inner closed shell enables simultaneous stabilization of the interfaces of silicon with both carbon and electrolyte, substantially facilitates efficient and rapid transport of both electrons and lithium ions from/to silicon, the outer open hollow shell creates stable and robust transport paths of both electrons and lithium ions throughout the electrode without any sophisticated additives. The resultant self-supported electrode has achieved stable cycling with rapidly increased coulombic efficiency in the early stage, superior rate capability, and remarkably high volumetric capacity upon a facile pressing process. The rational design and engineering of graphene shells of the silicon microparticles developed can provide guidance for the development of a wide range of other high capacity but large volume change electrochemically active materials.

Novel conductive binder for high-performance silicon anodes in lithium ion batteries

Classical rocking chair Li-ion batteries are dominating the market as they are currently the battery concept with the highest energy density, both per weight and volume. As the technology is rather mature, only small increases in energy density are expected without changing the electrode chemistry. Though the positive electrode currently limits the capacity of a Li-ion battery most, the state of the art graphite negative electrodes only deliver a moderate capacity of 300-350 mAh/g and improvements to this are also highly desirable. Silicon is a promising alternative negative electrode material. Under cathodic polarisation it forms a Li rich alloy, delivering a very high specific capacity of about 3600 mAh/g. Alloying, in contrast to an intercalation reaction, is associated with immense volume changes of the material. The currently used binder (PVDF) has been shown to be unsuitable for Si based electrodes as it cannot withstand the volume changes associated with cycling. Lately, a variety of biopolymers have been described as potential binders for Si based electrodes. These usually come with the advantage of being water soluble and hence makes the electrode production more environmentally benign. One class of biopolymers which have shown encouraging results are derivates of alginic acid. Most commonly applied as water soluble Na salts, alginates have shown encouraging results in accommodating the volume changes and hence can improve the cyclability of Si based electrodes. [1-3] Most published studies describe the alginates they are using rather incompletely. Alginic acid consists of the two epimers D-mannuronic acid (M) and L-guluronic acid (G). The content and distribution of the repetition units have a strong impact on the 3D structure and hence the properties. Furthermore, the chain length influences the properties of the corresponding alginates. Another crucial factor is the pH of the binder solution, which influences the degree of protonation of the carboxyl groups of the alginate. Protonated carboxyl groups ease esterification of the alginate with the native oxide layer on the Si surface which has a strong influence on the binding strength. [4] This systematic study addresses the effect of pH, Mw and M/G ratio of Na alginates as a binding agent for high capacity Si anodes using commercially available materials. The examined electrodes consisted of Si as main active material, Graphite and carbon black as conductivity enhancers and different Na alginates as binding agents. Na alginates with different Mw and M/G ratio were supplied by FMC BioPolymer AS (DuPont) and a commercially available battery grade Si (Silgrain ®, e-Si 400) was provided by ELKEM AS Technology. Kovalenko, I., et al., A Major Constituent of Brown Algae for Use in High-Capacity Li-Ion Batteries. Science, 2011. 334(6052): p. 75-79.Erk, C., et al., Toward Silicon Anodes for Next-Generation Lithium Ion Batteries: A Comparative Performance Study of Various Polymer Binders and Silicon Nanopowders. Acs Applied Materials & Interfaces, 2013. 5(15): p. 7299-7307.Zhang, L., et al., A coordinatively cross-linked polymeric network as a functional binder for high-performance silicon submicro-particle anodes in lithium-ion batteries. Journal of Materials Chemistry A, 2014. 2(44): p. 19036-19045.Mazouzi, D., et al., New insights into the silicon-based electrode's irreversibility along cycle life through simple gravimetric method. Journal of Power Sources, 2012. 220: p. 180-184.

Dynamic magnesiothermic reduction of various silica to porous silicon structures for lithium battery anodes

Improving the initial Coulombic efficiency (ICE) of silicon anodes in lithium-ion batteries is a key challenge for enhancing their performance. Traditional prelithiation methods, such as using lithium naphthalenide (Li-Naph), are limited by the low lithiation potential of crystalline silicon, making them less effective for commercial applications. This study demonstrates that amorphous silicon anodes, with a higher lithiation potential, can be effectively prelithiated using Li-Naph. This prelithiation process also forms a robust solid electrolyte interphase, which significantly enhances the anode's cycling stability and overall battery performance. The prelithiated silicon anodes achieved a remarkable ICE improvement from 74.8% to 97.2% in full-cell tests. Furthermore, 27 mAh pouch cells exhibited excellent long-cycle stability and low-temperature performance, retaining 90.1% of their capacity after 800 cycles at 1 C. These findings highlight the potential for scalable prelithiation methods and open new avenues for advancing silicon anode technology in next-generation batteries.