Bioinspired phosphorylated cellulose nanocrystals-based multi-crosslinked binder for enhanced stability and sustainability in silicon anodes.

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

Silicon (Si) anodes have attracted considerable attention for high-energy-density lithium-ion batteries (LIBs), owing to their high theoretical capacity (>3500 mAh g−1 at room temperature), low reaction potential (<0.4 V vs. Li/Li+), and natural abundance. However, the repeated volume changes of Si during the lithiation/delithiation process induce critical complications, such as the pulverization of Si particles, formation of a thick solid-electrolyte-interphase (SEI) layer, and electrode delamination, resulting in poor cycle performance. To mitigate these complications, several strategies have been proposed, including the nano/microstructural design of Si, employing a Si/C composite, and using multifunctional binders. Regarding the choice of the binder, the conventional poly(vinylidene fluoride) (PVDF) is inadequate as it cannot accommodate a large hoop stress because of its weak van der Waals force. Compatible binders, such as carboxymethyl cellulose, poly(acrylic acid) (PAA), and poly(vinyl alcohol), have been generally employed to improve the electrochemical performance of Si anodes. Although binders with abundant functional groups (e.g., –OH, –COOH, –CN, and –NH2) prevent the delamination of Si anodes from the current collector, they suffer from brittleness due to their serious chain-entanglement network and strong interactions, leading to a high glass transition temperature (Tg ). Moreover, polymeric binders with poor stretchability and self-healing ability degrade the structural integrity of the Si anode during repeated volume changes. Further, low ionic conductivity deteriorates the transport of Li ions in the Si anode, thereby impeding the effectiveness of fast-charging systems.Therefore, various physicochemical properties of a polymeric binder are required to improve the electrochemical performance of Si anodes besides adhesion property. First, stretchability and a high elastic modulus are essential for a binder to endure the stress of large volume expansions and recover to the original state after the lithiation process. A Si anode incorporating a rigid and stiff polymeric binder could crack and become damaged, resulting in drastic capacity decay and safety issues. Second, the self-healing ability is required for a binder to spontaneously recover from the damage induced by the significant volume changes. The mechanical fracture of Si particles induces the loss of the active material and exposes a highly reactive surface to the electrolyte. Third, in the LIB system, the rate performance highly depends on the ionic conductivity as well as the electronic conductivity. Thus, a binder with high ionic conductivity is highly desired to enhance the Li-ion diffusion coefficients and achieve a high-rate-performance. A binder design that satisfies the three requirements above is desperately desired.a series of poly(Li[3-sulfopropylmethacrylate]-r-acrylic acid) (PLSA) polymers with different moiety ratios was synthesized from 3-sulfopropyl acrylate lithium salt (Li[SPMA]) and acrylic acid (AA) monomers. Subsequently, glycerol, as a thermally stable plasticizer (high boiling point: 290 °C), was added to the polymer matrix to lower the Tg and maintain the softness of the polymers after water evaporation. The hydroxyl groups in the glycerol interact with the oxygen in the polymers, enhancing ion solvation for rapid Li-ion conduction. The ionic side chain of the polymer, as a mechanical modulator, formed the crosslinking via the electrostatic interaction between the polymers, conferring the stretching and self-healing properties. Furthermore, the mobile Li ions in the side chain enhanced the ionic conductivity of the polymers. Meanwhile, the carboxylic acid group (–COOH) in AA interacted with the current collectors (i.e., copper (Cu) foil) and the silanol groups (–SiOH) from the Si particles via hydrogen bonding, enhancing the adhesion properties. The rationally designed polymers with excellent physicochemical properties (i.e., high stretchability, rapid self-healing ability, and high ionic conductivity) were introduced as polymeric binders in the Si anode in LIBs. The Si anode with the PLSA75 binder (Si–PLSA75) exhibited stable cycling performance, retaining 81.2% of its initial capacity after 300 cycles at 0.5 C (1.5 A g−1) and exhibiting outstanding rate performance (815 mAh g−1 at 5 C (15 A g−1)). Moreover, compared with the Si–PAA, the half-cell with Si–PLSA exhibited lower internal resistance and higher diffusion coefficients of the Li ion during the whole lithiation/delithiation process. Based on the outstanding electrochemical performance in Si anodes, Si/graphite (Si/Gr) blend anode was also tested with PLSA binder which could afford around 840 mAh g-1. The blended anode with PLSA75 also maintained 82.7% of its initial capacity after 200 cycles at 0.5 C. Furthermore, the blended anode was paired with a nickel (Ni)-rich cathode, LiNi0.8Co0.1Mn0.1O2, for the full-cell test, and the full cell still delivered a reversible capacity of 139.2 mAh g−1 after 200 cycles at 0.5 C. This work provides insights into the rational design of polymeric binders for achieving Si anodes with fast-charging and stable cycling performance.

The design of a novel binder is required for high-capacity silicon anodes, which typically undergo significant changes during charge/discharge cycling. Hence, in this study, a stable network structure was formed by combining tannic acid (TAc), which can be cross-linked, and poly(acrylic acid)(PAA) as an effective binder for a silicon (Si) anode. TAc is a phenolic compound and representative substance with antioxidant properties. Owing to the antioxidant ability of the C-PAA/TAc binder, side reactions during the cycling were suppressed during the formation of an appropriate solid–electrolyte interface layer. The results showed that the expansion of a silicon anode was suppressed compared with that of a conventional PAA binder. This study demonstrates that cross-linking and antioxidant capability facilitate binding and provides insights into the behavior of binders for silicon anodes. The Si anode with the C-PAA/TAc binder exhibited significantly improved cycle stability and higher Coulombic efficiency in comparison to the Si anode with well-established PAA binders. The C-PAA/TAc binder demonstrated a capacity of 1833 mA h g−1Si for 100 cycles, which is higher than that of electrodes fabricated using the conventional PAA binder. Therefore, the C-PAA/TAc binder offers better electrochemical performance.

The design of novel binder systems is required for the high capacity silicon (Si) anodes which usually undergo huge volume change during the charge/discharge cycling. Here, we introduce a poly (acrylic acid sodium)-grafted-carboxymethyl cellulose (NaPAA-g-CMC) copolymer as an excellent binder for Si anode in lithium ion batteries (LIBs). The NaPAA-g-CMC copolymer was prepared via a free radical graft polymerization method by using CMC and acrylic acid as precursors. Unlike the linear, one-dimensional binders, the NaPAA-g-CMC copolymer binder is expected to present multi-point interaction with Si surface, resulting in enhanced binding ability with Si particles as well as with the copper (Cu) current collectors, and building a stable solid electrolyte interface (SEI) layer on the Si surface. The NaPAA-g-CMC based Si anode shows much better cycle stability and higher coulombic efficiency than those made with the well-known linear polymeric binders such as CMC and NaPPA.

Synergistically enhanced LiF–rich protective layer for highly stable silicon anodes

Electrochemical capacitors have attracted increasing attention not only for portable electric devices as smart phone and tablet PC, but also for large power sources in electric vehicles. The advantages of electric double layer capacitors (EDLCs) are high-rate capability and long cycle life, which quite fit the recent needs for smart grid and energy harvesting. The good performances of EDLCs are attributed to the fast charge/discharge mechanism by using electric double layer regardless of the Faradaic reaction with electrodes. However, the main drawback of EDLCs is a small energy density compared with the other energy storage devices, i.e. lithium ion batteries (LIBs), nickel-hydrogen batteries, etc [1]. To overcome this weak point, recently lithium ion capacitors (LICs) using Li pre-doped graphite anodes have been developed and partially released as commercial products. Fig. 1 shows the schematic models for the principles of EDLC and LIC. For the EDLC using two active carbon (AC) electrodes as both cathode and anode, the cell voltage was linearly changed by charging and discharging, which limits the energy density of EDLC to a low level. On the other hand, for the LIC system, the cell voltage becomes much higher by using Li pre-doped graphite anode with a low redox potential and the energy density also increased up to about 3 times larger than that of EDLC. In this study, to further enhance the energy density, we tried to fabricate a new LIC system using Li pre-doped silicon (Si) anode, i.e. Si-CAP (Fig. 2), and investigated the better conditions to prepare the Li pre-doped Si anode for the Si-CAP.The Si anode was prepared by coating slurry on Cu foil as a current collector. The slurry was prepared by mixing 83.3 wt% Si nano-powder (Alfa Aesar®, Φ = 50 nm), 5.6 wt% Ketjen Black (KB) as a conductive agent and 11.1 wt% carboxymethyl cellulose sodium (NaCMC) salt as a binder. For optimization, two types of thickness for the Si anode were prepared; one was ca. 20 μm and the other was ca. 12 μm. Li pre-doping of the Si anode was carried out by making a contact with Li foil and immersing in 1 M LiBF4 dissolved in propylene carbonate (Kishida Chemical Co, Ltd.), where the treatment time was set to 1, 2 and 3 hours. Coin-type cells with the Li pre-doped Si anodes, AC cathode and 1 M LiBF4/PC electrolyte were fabricated and the electrochemical properties were evaluated by charge/discharge tests in constant current (CC) mode at 30oC.Fig. 3 shows a typical charge/discharge curve of the Si-CAP with a Li pre-doped Si(thick, 2 h) anode. The cell voltage successfully increased up to 4.2 V vs. Li/Li+ and the charge/discharge curve was bent upward (not linear). These indicates that the Li pre-doped Si anode worked well as low potential redox electrode and increased the cell voltage of Si-CAP. The plots of discharged energy densities vs. cycle number for the Si-CAPs with various Li pre-doped Si anodes were shown in Fig. 4. For the EDLC using the same electrolyte (1 M LiBF4/PC), the energy density was ca. 40 mAh g-1, which was very stable during 50 cycles. On the other hand, all the Si-CAPs exhibited higher energy densities although the energy densities decreased with the charge/ discharge cycling. In comparison of the Si-CAPs with Si(thick) anodes, the energy density increased with an increase in the Li pre-doping time. However, the decrease in energy density was also more rapid and significant. This implies that the extension of Li pre-doping time caused to large volume change of Si nano-powders and gave a bigger damage. Therefore, to improve the method of Li pre-doping we used a thinner Si anode, which was easy to conduct the Li pre-doping in short time (1 h) and made the treatment more homogeneous. As a result, the Si-CAP with Si(thin, 1 h) anode exhibited the highest energy density of over 400 mAh g-1(Si), and 292 mAh g-1(Si) was remained even after 50 cycles, which was more than 7 times larger of that of EDLC. Therefore, the Si-CAP was quite attractive as one of the next-generation energy storage system. Further optimization of Li pre-doped Si anode and the cell performances will be discussed in the meeting.This study was supported by JST “A Tenure-track Program” from MEXT, Japan.[1] K. Naoi et al., Energy & Envirn. Sci., 5, 9363 (2012).

This talk will highlight the use of in situ neutron reflectometry (NR) to explore the effect polymeric binders used in composite slurries have on the composition and formation of the solid electrolyte interface (SEI) on amorphous silicon (aSi) anodes. Due to the high-volume expansion, upwards of 300%, a polymeric binder is needed for added mechanical strength; however, little is known about binder interactions with Si and its effect on SEI formation. Two different binder systems, polyacrylic acid (PAA), the most commonly used binder to date, and PEFM, an electronically conductive binder, were explored using NR at various potential steps. Thin film architectures were used to provide a model electrode surface to better probe interfacial reactions. In this case vapor deposited aSi films were spin coated with low weight percent polymer binder solution to create an aSi/binder interface. NR measurements indicate changes in both the polymer layer and resulting SEI as determined by a change in material scattering length density (SLD), a measure of the composition of a layer, during the initial lithiation stage. Ex situ techniques, in this case x-ray photoelectron spectroscopy, were used in conjunction with NR measurements to confirm models of the data. Preliminary results indicate the formation of a predominantly inorganic SEI layer. Neutrons interact with the nucleus of an atom, unlike x-rays, and as such scattering cross-section is not dependent on Z number. This allows for probing of lighter elements, especially Li, inherent to batteries. Because of their wave-like nature, neutrons follow Snell’s Law in which the beam is reflected and refracted at interfaces separating layers with different indices of refraction providing information on the thickness, roughness, and/or diffuseness of a layer and most importantly the composition described by the SLD. NR is an ideal tool to study interfaces, including buried such as those found in materials for energy storage. Acknowledgement: This research was supported by the Vehicle Technologies Office, Hybrid Electric Systems Program, David Howell (Manager), Battery R&D, Brian Cunningham and Peter Faguy (Technology Managers), at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy. Neutron reflectometry measurements were carried out on the Liquids Reflectometer at the Spallation Neutron Source which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Lithium-conducting phosphates as artificial solid-electrolyte interphase on silicon anode for supreme lithium storage

The volumetric changes and the structural deterioration in Silicon anodes during successive electrochemical lithiation/delithiation cycles limits the utilization of Silicon (Si) anodes in Lithium-based batteries. However, using morphologically modified Si and suitable conducting additives can ensure better electrochemical performance and structural stability even after repeated electrochemical cycles. This research is aimed at comparing the electrochemical performance and solid electrolyte interface (SEI) of different Si anode geometries i.e. powdered Si and Si microwires after cycling. The SEI is analyzed using Transmission Electron Microscope (TEM) methods after cycling in the electrochemical half cell. Si microwires show relatively superior electrochemical performance compared to the powdered Si.

The practical application of silicon (Si) anodes is limited by the large volume change, unstable solid electrolyte interface (SEI), and low intrinsic conductivity. To address these challenges, we designed Prelithiated poly(acrylic acid)-boric acid (LiPAA-BA) a multifunctional binder with a three-dimensional (3D) network structure, using a combined prelithiation and cross-linking strategy. LiPAA-BA showed a high interfacial adhesion force (4.8 N), which was improved by about 2 times compared to conventional PAA due to the increased number of contact sites with the silicon and the 3D network structure. As a result, the Si@LiPAA-BA electrode achieved an excellent capacity retention of 100% after 300 cycles and volume expansion was effectively suppressed. LiPAA-BA contains a Li+ conducting group, which increases the Li+ diffusion coefficient (DLi+) compared to PAA by a 1.6 times and enhances the rate capability. The cross-linker (BA) adopted in this study contains boron atoms with electron-deficient properties, which enabled the formation of a stable SEI layer on the silicon surface through interaction with anions in the electrolyte. A full cell using a SiOx@C anode with a LiPAA-BA and an LFP cathode showed 64.89% capacity retention at 50 cycles, demonstrating potential for practical application.

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

The solid electrolyte interface (SEI) is a passivation layer formed on the surface of lithium-ion battery (LIB) anode materials produced by electrolyte decomposition. The quality of the SEI plays a critical role in the cyclability, rate capacity, irreversible capacity loss and safety of lithium-ion batteries (LIBs). The stability of the SEI is especially important for Si anodes which experience tremendous volume changes during cycling. Therefore, in this review we discuss the effect of the SEI on Si anodes. Firstly, the mechanism of formation, composition, and component properties of solid electrolyte interfaces (SEIs) are introduced, and the SEI of native-oxide-terminated Si is emphasized. Then the growth model and mechanical failure of SEIs are analyzed in detail, and the challenges facing SEIs of Si anodes are proposed. Moreover, we highlight several modification methods for SEIs on Si anodes, including electrolyte additives, surface-functionalization of Si, coating artificial SEIs or protective layers, and the structural design of Si-based composites. We believe that designing a high-quality SEI is of great significance and is beneficial for the improved electrochemical performance of Si anodes.

This work focuses on investigating the role that mesoscopic configuration plays in dictating the solid-electrolyte interface (SEI) mechanical durability on lithiation in silicon (Si) anodes. Due to the large expansion of Si upon lithiation, SEI formation and cohesion at the electrolyte interface of Si based anodes is a principle concern in performance of the overall cell, and enabling a more durable SEI would lead to the ability to employ higher Si percentages in anodes, increasing capacity. However, we are still building fundamental understanding of SEI formation and the resultant meso- and microscopic structure evolved due to this formation. This work seeks not to understand the formation itself, but instead to establish intuition about what type of resultant structure we should target as we learn to control the SEI formation process, by investigating the effect that non-homogenous SEI configurations have upon the overall SEI response when subjected to large strain due to the swelling of the underlying Si anode. Utilizing the material point method (MPM), a continuum mechanics approach akin to the finite element method, but particularly well suited for investigation of materials subjected to large deformation, we model the intrinsic swelling of Silicon under lithiation when coupled to a model SEI. This model SEI, in turn, is constructed of multiple different phases with mesoscopic variability and realistic material response parameterized from atomistic simulation of the mesoscopic phases including both crystalline and glassy inner SEI compounds and ordered/disordered outer SEI compounds derived from liquid carbonate decomposition products. Model mesostructures are constructed and fed into the MPM framework, from which an overall SEI mechanical response may be extracted via numerical simulation. By varying the mesostructures, sensitivity of the mechanical response to the mesostructural features is quantified, providing guidance to possible experimental approaches at improving the SEI capability of protecting the anode during multiple charge/discharge cycles.

The importance of network binder for improving cycling lifespan of silicon (Si) anode needs no further emphasis. However, the linear structure of natural polymer hardly creates a robust network binder. Herein, we propose a facile strategy of establishing a robust network binder by using small molecules of tartaric acid (TA) to locally link sodium carboxymethyl cellulose (CMC). Through hydrogen or covalent bonds, the resultant CMC‐TA binder exhibits improved tensile and adhesive properties. The Si anode using CMC‐TA binder delivers a satisfactory specific capacity of 2213 mAh g−1 after 100 cycles at the rate of 0.2 C, with a capacity retention rate of 68.8%. This result has well confirmed the effectiveness of using small molecules to reinforce hydrogen‐bonding linking between CMC and between Si particles for a high‐performance Si anode.

Development of higher capacity anodes in lithium ion batteries for use in electric vehicles is necessary in order to further enhance their energy density. Silicon anodes are being considered for these next generation lithium ion batteries due to its exceptionally high specific capacity (3579 mAh/g). One main drawback to silicon anodes is the formation of an unstable solid electrolyte interface (SEI), a surface film formed by decomposition of the electrolyte components during cycling of the battery. This unstable SEI continuously consumes Li ions and solvents from the electrolyte and stunts battery life. One major cause of this SEI instability is due to silicon anode volume expansion and contraction of up to 300% during cycling[1]. However, there is still much to learn about the chemical reactions occurring at the silicon surface before, during and after the first cycle of the battery. A useful way to narrow observations to just silicon surface reactions with no convolution from binders or conductive additives is by studying thin film silicon anodes. A neutron reflectivity study done by Veith et al utilized thin film silicon to note that the complexity of the reactions at the surface of silicon is increased by the chemical reactivity of the with the electrolyte at open circuit voltage before the anode was even cycled[2]. This implies that the length of time and composition of the surface of the anode before cycling will have an impact on how the SEI forms during the first cycle. To better understand the nature and evolution of this SEI layer formed prior to any cycling of the silicon anode and how it impacts the performance of the silicon anode, model SEI layers were deposited on clean 50 nm thick silicon thin films using RF magnetron co-sputtering. Thin film chemistries from SiO2 to Li4SiO4 were synthesized in order to model the proposed lithiation of the oxide layer during the first cycle. Model thin films were soaked in 1.2M LiPF6 in EC:EMC 3:7 wt% electrolyte from 30 minutes to 3 days in an inert atmosphere glovebox, removed and rinsed with DMC and studied using ATR IR, XPS, XRR and FIB CS. Once the pre-lithiatied film was studied using these techniques, half cells with these same silicate model films were cycled in order to observe any differences in SEI formation or cell performance. Impedance measurements were taken on model films before and after lithiation in order to better understand lithium transport of these model lithium silicates during cycling. Initial results on these model films after soaking in the electrolyte indicate a dependence on stoichiometry for the time resolved surface reactivity. While electrodes with unlithiated oxide (SiO2) soaked in electrolyte for up to 24 hours before discernable surface reaction peaks presented in their ATR IR spectra, the electrodes with lithium silicates showed significantly faster appearance of reaction peaks. This could indicate that lithium silicates passivate more quickly against the electrolyte. Preliminary FIB CS data also indicates that the SiO2 layer may be consumed with time, perhaps producing gaseous products instead of a passivating surface film. These model systems have started to tease apart the complexity of the surface reactivity and lithiation kinetics that manifest during storage and cycling of silicon anodes and could provide key insight into the low cycling stability of silicon anodes observed in practice. Ko, M., S. Chae, and J. Cho, Challenges in Accommodating Volume Change of Si Anodes for Li-Ion Batteries. Chemelectrochem, 2015. 2(11): p. 1645-1651.Veith, G.M., et al., Direct measurement of the chemical reactivity of silicon electrodes with LiPF6-based battery electrolytes. Chemical Communications, 2014. 50(23): p. 3081-3084. Figure 1