Silicon Anodes Research Articles

Si-Drive European Project: Developing Crosslinked Polymer and Composite Membrane Electrolytes for Lithium-Based Batteries

Currently the main application of lithium ion batteries is in portable electronics, but new emerging applications such as electric cars require lower cost, improved cycle life, sustainability, safety and energy density.The ambition of Si-DRIVE European project is proposing next-generation batteries for electric cars built in Europe, designed to ensure recycling above 50%. Si-Drive battery will contain a Silicon anode, which has a high specific capacity, and a high voltage cathode without cobalt to reduce the cost and improve sustainability.As an alternative to conventional organic solvents, ionic liquids have been widely investigated over the years, due to their excellent properties in terms of safety and stability.The 1st generation of electrolytes for Si-Drive includes selected ionic liquids with high ionic conductivity and wide electrochemical stability. The 2nd generation of electrolytes combines the ionic liquids and the precursors of the polymer matrix in the formulation, in order to obtain crosslinked self-standing flexible films. The addition of ceramic super lithium ion conductors to obtain composite polymer electrolytes is also envisaged to improve the electrochemical stability window of the electrolyte.Here we present several different polymer electrolytes and semi interpenetrating polymer networks encompassing ionic liquids and ceramic materials having ionic conductivity > 10 mS/cm at 20 °C, which can be in situ-deposited and crosslinked on top of the electrodes, and combined to form bi-layered electrolyte membranes to tune the compatibility with the electrode materials. The cycling performance with different positive and negative electrodes at ambient temperature is also demonstrated. Acknowledgements: The Si-DRIVE project has received funding from the European Union’s Horizon 2020 Research&Innovation programme, under Grant Agreement 814464. References J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Chem. Soc. Rev. 46, 3529-3614 (2017)M. Falco, C. Simari, C. Ferrara, J.R. Nair, G. Meligrana, F. Bella, I. Nicotera, P. Mustarelli, M. Winter, C. Gerbaldi, Langmuir 35, 8210-8219 (2019)M. Falco, L. Castro, J.R. Nair, F. Bella, F. Bardé, G. Meligrana, C. Gerbaldi, ACS Appl. Energy Mater. 2, 1600-1607 (2019)M. Falco et al., Electrochem. Commun. 118 106807 (2020)S. Saffirio, M. Falco, G.B. Appetecchi, F. Smeacetto, C. Gerbaldi, Journal of the European Ceramic Society, 42(3), 1023-1032 (2022).

(Invited) Utilizing Conjugated Imine Polymers to Stabilize Nanoparticle Silicon Anodes

Silicon anodes provide a compelling route towards delivering high-capacity lithium-ion batteries, however they are notoriously difficult to stabilize in conventional liquid electrolytes that are compatible with high performance cathodes. Furthermore, the drastic changes in volume that occur as silicon is lithiated and delithiated can lead to rapid cell failure and innovative polymeric binders are required to help mitigate mechanical issues. We have recently developed a novel method to synthesize the conjugated binder polyphenylmethanimine (polyPMI) using an imine metathesis reaction. We have demonstrated that this polymer can be used to stabilize silicon nanoparticles to produce a composite lithium-ion battery anode that is electrochemically stable for hundreds of lithiation and delithiation cycles with a Coulombic efficiency that is greater than 99.95% in a traditional carbonate liquid electrolyte.1 This novel polymeric coating passivates silicon nanoparticles by shielding them from direct electrolyte contact while maintaining effective ionic and electronic transport and also imbues the electrodes with excellent mechanical resilience to the volumetric changes experienced during cycling. We will cover the structure/property relationship of polyPMI within the context of the unique mechanical and electrochemical requirements of nanoparticle silicon anodes. We will also discuss the electrochemical behavior of this polymer and how it also offers new opportunities as an organic lithium-coordinating material that can be used to stabilize a variety of other battery electrodes. More broadly, we will posit that this solution-processed, ionically conductive, and electronically conductive polymer is applicable for a variety of energy storage and conversion technologies. Finally, this work will demonstrate how this novel polymer chemistry provides additional opportunities for tuning electrode architecture to target high areal capacity electrodes that enable high gravimetric and volumetric capacity full cells.1. T. R. Martin et al., Adv. Energy Mater., 2203921 (2023).

Dual-Additive Approach for Tuning Si-Based Anode Interphase Realized By Operando ATR-FTIR

Dual-additive approach for tuning Si-based anode interphase realized by Operando ATR-FTIRMasoud Baghernejad1, Matthias Weiling1, Felix Pfeiffer1, Diddo Diddens1, Lars Frankenstein2, Christian Lechtenfeld2 1Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster, Germany 2MEET Battery Research Center, Institute of Physical Chemistry, University of Münster, Corrensstraße 46, 48149 Münster, GermanySilicon is a promising next-generation anode material, offering a high gravimetric capacity. However, silicon-based anodes suffer from significant volume expansion during lithiation that can cause particle pulverization and capacity fade, continuous solid-electrolyte interphase (SEI) (re-)formation, and constant electrolyte consumption. To alleviate these issues, the effect of 2-Sulfobenzoic anhydride (2-SBA) as an SEI electrolyte additive was investigated through complementary electrochemical and operando spectroscopic techniques.The electrochemical performance was evaluated through galvanostatic charge/discharge cycling. The results show that the optimum concentration of 2-SBA improves the cycling stability and capacity retention of NMC811||AG+20% SiOx (4.5 – 2.8 V) cells to more than 100 cycles compared to 70 cycles with the baseline electrolyte (BE) before cell failure. Introducing lithium difluorophosphate (LiDFP), a common additive for high-voltage applications, to the electrolyte containing 2-SBA further increased the cycle life to 160 cycles. Interestingly, LiDFP alone in the electrolyte did not lead to improved electrochemical performance or longer cycle life of the cells compared to BE, proving a beneficial dual-additive approach of combining inorganic and organic compounds for sufficient SEI formation on AG+20% SiOx anodes. The analysis of the electrochemical performance concludes that the improved SEI structure formed by 2-SBA and LiDFP mitigates pulverization. In this regard, the SEI structure is further analyzed using operando ATR-FTIR spectroscopy.Operando ATR-FTIR spectroscopy proves to be a powerful technique in battery research, particularly for investigating SEI. The ATR mode's surface sensitivity enables studying the composition of the SEI without bulk signal interference. In this work, we employed the internal reflection mode of the ATR-FTIR spectroscopy, for which the electrode of interest has to be IR permeable. Furthermore, by variation of the incident angle of the IR beam, the penetration depth can be varied to obtain an interphase profile from electrode to bulk electrolyte. In this study, the SEI composition on the silicon anode in the presence of BE and the corresponding additives, 2-SBA and LiDFP, was investigated under real-working conditions. The results elucidate that 2-SBA is accumulated and decomposed via ring-opening on the silicon surface to form a stable SEI layer. However, in the presence of LiDFP and 2-SBA, a hybrid organic and inorganic SEI layer is formed, which proves to enhance electrochemical performance and cycle life. The findings of this study provide valuable structural insights into the silicon particle surface for the development of high-voltage silicon-based lithium-ion batteries with Ni-rich cathodes.

(Invited) Development of Binders, Surface Coatings and Electrolytes to Stabilize Silicon Anodes for Lithium Batteries

Silicon is a desirable anode material for lithium ion batteries due to the high lithiation capacity of silicon. However, the lithiation/delithiation process results in large volumetric changes to the silicon particles. The volumetric changes result in instability of the solid electrolyte interphase (SEI) on silicon. Investigations of the effect of binders, surface modifying agents and electrolyte formulations on the stability of the SEI on silicon anodes will be presented. Methods to modify the silicon interface to improve cycling performance will also be discussed.

Mechanical and Electrochemical Properties of Highly-Dense Li-Si Alloy Anodes Fabricated by Arc Plasma Deposition

Silicon is one of the most promising anode materials because it has a very high theoretical capacity (4200 mAh g− 1). This capacity is about 10 times higher than that of carbon anode (372 mAh g− 1).[1] However, enormous volumetric change and mechanical stress occur in a silicon anode during electrochemical lithiation-delithiation. Consequently, the specific capacity is degraded during repetition of charge-discharge because of cutting off conductive paths caused by mechanical fracture. It is reported that pre-lithiation by sputtering and electrochemical methods suppresses fracture of silicon anodes by decrease in elastic modulus and yielding stress.[2] However, the reported Li-Si alloy anodes possesses the mechanical properties derived from both the deposited silicon and the voids generated during the synthetic processes. Therefore, a highly-dense alloy is necessary to investigate relationships between mechanical and electrochemical properties of Li-Si itself. We have already fabricated highly-dense amorphous silicon films by an arc plasma deposition (APD) method.[3] Thus, we attempted to fabricate highly-dense Li-Si alloy films by an APD method. The obtained Li-Si alloy films were investigated with focusing on relationships between mechanical properties and electrochemical properties.Li x Si (x = 0-3.56) alloy films were prepared on Al2O3(0001) single-crystal and Cu substrates by an APD method. Metallic lithium and B-doped silicon coated with carbon were used as target materials in the APD apparatus. The physical structures and chemical states of the obtained Li x Si alloy films were investigated with Raman spectroscopy, hard X-ray photoelectron spectroscopy (HAXPES), and neutron reflectometry (NR). Elastic modulus and hardness of the Li x Si alloy films were calculated from load-displacement curves gained by nanoindentation measurement. Constant-current charge-discharge tests were carried out with 2032-type coin cells assembled using the Li x Si films as the working electrodes, Li foils as counter electrodes, and 1 M LiPF6 in EC: DEC 3:7 vol% as electrolytes. The charge-discharge tests were conducted with constant current of 0.01-0.16 mA (1C rate), lower and upper voltages of 20 mV and 1.5 V vs. Li/Li+.In HAXPES spectra, Si 2p peaks derived from Si0 shifted to lower binding energy compared to the Si (x = 0) film as x increased. Therefore, Li x Si alloy films with different compositions were successfully fabricated by the APD method. The peaks because of Si-Si bonds in Raman spectra also shifted to lower wavenumber with the increase in x. The shifts suggest that the length of the Si-Si bonds extended with the increase in x. The NR analyses revealed that Li x Si alloy films possessed the smooth surface with the roughness less than 3 nm. The elastic modulus and hardness of Li x Si alloy films decreased as x increased, meaning that Li x Si alloy films were softer and more easily deformed compared to Si films. In addition, the elastic modulus and hardness of Li x Si alloy films were higher than those of Li x Si films previously reported,[2] suggesting that the APD method more densely made the Li x Si alloy films than the reported sputtering and electrochemical methods. Thus, the objective highly-dense films of Li x Si alloys were successfully obtained. The plateaus corresponding to the reactions of Li x Si ↔︎ Li x ’Si ↔︎ Li y Si (x < x’ < y) were observed in charge-discharge curves of Li x Si alloy films. The first discharge capacities of Li x Si alloy films were lower than the theoretical capacity of silicon. This is due to the existence of lithium in the structure by pre-lithiation. Cycle stabilities of Li x Si alloy films were superior to the Si film. To reveal factors of the cycle stabilizing, the charge-discharge reactions were analyzed with dQ/dV plots. The peaks around 0.5 V in the delithation process decreased about Si film. This decrease in the reaction peak is attributed to the decrease in the active species during the reaction process because of the fracture. In contrast, the peaks of Li x Si alloy films around 0.5 V hardly decreased. This result suggests that delamination and fracture by volumetric change during charge-discharge processes were suppressed in Li x Si alloy films because of their softness. Thus, the analyses of the highly-dense Li x Si alloy films fabricated by the APD method clarified that cycle stabilities of active materials were improved by controlling mechanical properties.[1] Ashuri, M.et al., Nanoscale 2016, 8, (1), 74-103.[2] Sitinamaluwa, H. et al., RSC Adv. 2017, 7, 13487-13497.[3] Asano, S. et al., Highly-dense and smooth amorphous silicon films fabricated by arc plasma deposition and electrochemical properties., ACSSI-17, 3A-06, 9.15.2022.

Thin, Conformal Carbon-Coating of Silicon-Phosphorus Nanoparticle Agglomerates for Lithium-Ion Battery Anodes

Efforts to further enhance the capabilities of silicon as a lithium-ion battery anode are continuing into the realm of chemical composition. Adding varying amounts of other elements can significantly alter the material properties or side reactions, which influence electrochemical performance. Additionally, tuning the chemical composition combined with surface modifications can better enable application of silicon anodes in other devices such as hybrid supercapacitors.Prior work, among others, has shown different performance enhancement regimes depending on how much phosphorus is added to silicon-containing anodes [1, 2]. In particular, the amount of phosphorus from ppm levels to majority atomic fraction can result in enhancement in the conductivity, diffusivity, or forming altogether a wholly new protective matrix [3]. However, while the rate capability has been enhanced only primarily for delithiation, silicon-phosphorus (SiPx) anodes still suffer from similar capacity degradation issues as pure silicon anodes.In the present work, we attempt to address the capacity degradation issue by using a thin, conformal carbon coating to modify the solid-electrolyte interface (SEI) reaction. Firstly, amorphous nanoparticles of SiPx with a few atomic percent of phosphorus are prepared using silane and phosphine co-pyrolysis. Using an oscillating furnace at 800 degrees Celsius under a low flow of acetylene in Ar carrier gas, we are able to produce consistent, uniform carbon films on nanoparticle agglomerates with select thicknesses between 2 and 10 nm, validated using low voltage scanning transmission electron microscopy (LV-STEM), as shown in Figure 1. By comparing Si- and SiPx-containing anodes in electrochemical half cells, we can reproduce the rate capability results while demonstrating the effect of carbon coating on cycle life and capacity retention. Furthermore, the carbon coating’s effect on volume expansion for Si- and SiPx-containing anodes is studied in situ using an electrochemical dilatometer in half cell configuration.[1] Y. Domi, H. Usui, M. Shimizu, Y. Kakimoto, H. Sakaguchi, ACS Applied Materials & Interfaces 2016, 8, 7125-7132.[2] S. Huang, L.-Z. Cheong, D. Wang, C. Shen, ACS Applied Materials & Interfaces 2017, 9, 23672-23678.[3] F. T. Huld, S. Y. Lai, W. M. Tucho, R. Batmaz, I. T. Jensen, S. Lu, O. E. Eleri, A. Y. Koposov, Z. Yu, F. Lou, ChemistrySelect 2022, 7, e202202857. Figure 1

Cost-Effective Milled Silicon and Exfoliated Graphene Anode for High-Performance Li-Ion Batteries

Lithium-ion batteries play a significant role in modern electronics and electric vehicles. However, current lithium-ion battery chemistries are unable to satisfy the increasingly heightened expectations regarding energy demand and reliability. To boost the overall energy density while ensuring the safety of Li batteries, researchers have focused on developing the battery materials [1]. Silicon has an ultrahigh theoretical capacity and has been regarded as the best choice for next-generation lithium-ion battery anodes, but it also suffers from dramatic expansion during cycling [2]. Hence mitigating volume expansion is one of the main challenges towards developing a successful Si based anodes. Herein, a facile air-controlled electrospray method to directly deposit silicon/graphene (Si/Gr) onto current collectors was described. This method is inspired by an electrospinning process and possesses all the advantages that electrospinning has such as low cost, easy scaling up, and simultaneous solvent evaporation during the spraying process. Cost-effective silicon/graphene (Si/Gr) composite electrodes have been investigated for the multiple benefits of Gr in providing mechanical integrity to buffer the silicon volume expansion while maintaining electronic contact with Si to provide a Li-ion transport pathway and to form a uniform solid electrolyte interface (SEI) layer. Our results revealed a superior battery performance including a capacity of 1000 mAh/g at 2C rates and minor capacity loss during long cycling. The details of synthesis, characterization, and lithium-ion battery results of the hybrid anodes will be presented.

Modulation of Prelithiation Solution for Hard Carbon Anodes of Lithium-Ion Batteries

The increasing demand for high-energy and high-power lithium-ion batteries (LIBs) in portable gadgets and electric vehicles has led to exploration of high-performance silicon and hard carbon (HC) anodes to replace traditional graphite anodes. However, their practical application is hindered by low initial coulombic efficiency and large irreversible capacity, which decreases the total energy density. Prelithiation is a strategy to restore energy density by supplying additional Li ions. Chemical prelithiation, involving immersing the electrode in a reductive Li−aromatic hydrocarbon (arene) complex solution, is a standout method due to its safety, high efficiency, and practical feasibility. It not only restores the lost lithium but also aids in maintaining a more stable solid electrolyte interphase layer to guarantee battery cycling stability. The first part of our study addresses the issue of expensive and impractical arene complexes for prelithiation, we focus on using low-cost and commercially available aromatic compounds such as naphthalene and methyl-substituted naphthalene compounds. This study investigates the effects of electron-donating methyl groups on naphthalene on the prelithiation activity toward HC using density functional theory calculations. In the second part of this study, we use low-cost and easily available solvents such as ethylene glycol dimethyl ether, bis(2-methoxyethyl) ether, and triethylene glycol dimethyl ether for chemical prelithiation. Ether solvents have a desirable solvation effect with Li+, and the ether with low solvation power enhances the prelithiation activity. The degree of prelithiation is greatly affected by the reaction time, and the details of this relationship are rarely investigated. In the final part of our study, we conduct a detailed investigation on the effects of various reaction times on the prelithiation activity toward the HC anode. This study provides useful insights for effective prelithiation, in terms of choosing aromatic compounds, solvents, and reaction time.

Liovix® Printable Lithium Technology for Controlled and Scalable Prelithiation of Silicon Anodes

Loss of active lithium in the first cycle permanently lowers the lithium-ion cell energy density. Prelithiation employs an external, sacrificial source of lithium to offset these losses allowing more efficient use of lithium from the cathode and raising the cell energy density.LIOVIX®printable lithium technology is comprised of stabilized lithium metal powder (SLMP®) dispersed in a compatible solvent along with other rheology and performance modifiers to create a formulation that is chemically stable and not prone to separation for extended periods of time. The technology is easily scalable using industry standard coating and printing equipment to apply the formulation to the surface of a prefabricated anode. LIOVIX® can be used in anodes with a range of silicon content, because lithium loading can be accurately controlled based on the applications’ requirements.In this presentation, we will compare LIOVIX® to other prelithiation methods. The electrochemical results will be discussed as well.

Silicon Nanowires Diameter Modification for the Lithium-Ion Batteries

Silicon has been considered a prospective anode material for lithium-ion batteries (LIBs) due to its abundant reserves and high theoretical specific capacity (about 10 times higher than the graphite anode). However, the significant volumetric expansion of bulk silicon anode during cycling, which causes cracking and pulverization, results in irreversible capacity decay and a shorter lifetime compared to other anode materials.[1, 2, 3] The general strategy for performance and stability enhancement is the use of nanostructured silicon materials. Various nanostructured silicon electrodes had been developed, such as nanoparticles, nanorods, nanotubes, and hollow nanostructures.[4, 5] In this work, we demonstrate silicon nanowires (Si NWs) directly grown on current collecting substrates by a vapor-liquid-solid (VLS) approach that is capable of achieving a reversible capacity of over 2200 mAh g-1 with good cycling stability. The improved electrochemical performance, compared to previous studies, may be due to faster electron transport, higher surface reactivity and mechanical stability with the growth substrate. Additionally, in this study, we investigate a feasible approach for controlling Si NWs diameters to optimize electrochemical performance. Figure 1. Si nanowires growth scheme. (I) Bare stainless-steel mesh, which was (II) coated by different size tin (Sn) catalyst before the reaction, (III) the dense Si nanowires were anchored on the substrate.The vapor-liquid-solid (VLS) approach that we used is more economic and safer in producing pure Si NWs compared to the other methods, such as metal-assist etching, chemical vapor deposition, and oxide-assisted/magnesium thermal evaporation.[4] The difficulties for Si NWs synthesis are that it has a high melting point and good mechanical property as a semiconductor, which makes it harder to break it down from bulk Si material to nanowires.[4] On the other hand, various silicide might be formed instead of pure Si in the synthesis reaction, as the decomposition of Si precursor and nanowires nucleation happen at the same time, causing the undesirable specific capacity in the further application.[6] As shown in Figure 1, Si NWs were grown on the Sn-coated stainless-steel mesh after injecting the Si precursor. During the reaction, the specific size Sn catalyst is melted and formed as droplets on the substrate, the silane (vapor) is decomposed from phenyl silane and absorbed into the Sn droplet (liquid), Si NWs (solid) are then gradually extracted from the droplet when it got saturated.The electrochemical performance of the self-supported Si NWs/Stainless-steel mesh (SSM) electrodes show that Si NWs with the bigger diameter (SSM01 ~ 120 nm, SSM02 ~ 80 nm) delivered higher initial specific capacity (3761 mAh g-1 and 2871 mAh g-1 separately) but poor stability and decayed ~ 50% at the 100th cycle. The largely irreversible capacity loss may be due to bigger nanowires losing electronic contact after the initial volume change. The smaller wires (SSM03 ~ 41 nm, SSM05 ~ 45 nm) maintained ~ 80% capacity retention but achieved lower initial specific capacity (2201 mAh g-1 and 1872 mAh g-1 separately). This result illustrates that the stability can be improved by nano-sizing the electrode material, as the smaller diameter nanowires may result in faster ion transfer, while the unique electrode architecture leads to electrodes with higher surface reactivity, conductivity and improved robustness and electrode stability. This approach not only eliminated the use of excess explosive gas and the possibility of silicide impurity production but also avoids the energy-intensive top-down process.

β-cyclodextrin and adamantane polyacrylic acid copolymers as supramolecular binder for silicon anodes: N-methylpyrrolidone or water for preparing the slurries?

Silicon is a promising material for anodes in future generations of LIBs. However, its volume changes during the lithiation/delithiation process that degrades battery performance, making it an impractical alternative. To mitigate this issue, we have found a viable alternate solution, which is the preparation of adamantane-polyacrylic acid (AdEN-AA) and β-cyclodextrin-polyacrylic acid (βCD6A-AA) polymers as supramolecular binders. The advantage of these binders is that polymers leverage host-guest interactions. To carry out this research, we prepared the anode materials in water and in NMP, and then we studied their capacity to crosslink and gel. Additionally, we examined the extent of the supramolecular interaction vs. hydrogen bond. Thermogravimetric analysis showed that the thermal stability of the mixture of complementary polymer increased in the following order: AdEN-AA/βCD6A-AA in NMP < βCD6A-AA/PAA < AdEN-AA/βCD6A-AA, with the latter two in water. Accordingly, the anode material prepared in water shows the highest stability, retaining capacities of over 1000 mAhg-1 after 90 cycles vs. 279 mAhg-1 of the electrode prepared in NMP. The results agreed with the SEM micrographs of the anode material, in which the slurry prepared in water showed better integrity of the silicon nanoparticles and of solid electrolyte interphase than the slurry prepared in NMP.

Phosphonium-Based Ionic Liquid Electrolyte in Combination with Silicon Anodes and LiNi0.5-XMn1.5+XO4 Cathodes for Lithium Ion Batteries

Ionic liquids, like bis(fluorosulfonyl)imide (FSI) ionic liquids, are promising solvents for lithium ion batteries due to their high thermal stability and wide electrochemical window. The performance of ionic liquid electrolytes is however hampered by the low conductivity at room temperature, but the high thermal stability allows for operation at elevated temperatures, at which the conductivity is significantly improved.Electrolytes based on LiFSI and ionic liquids of the cations; pyrrolidinium (PYR13 +), and phosphonium (P111i4 +), with FSI anions, have been shown to exhibit stable cycling at room temperature, and good rate performance for cells composed of silicon anodes and LiFePO4 cathodes at 60 °C [1,2]. The lithium ion mobilities, relative to standard carbonate electrolytes, were determined from the limiting current densities at room temperature and 60 °C. Significant differences in the temperature dependence of the transport properties were revealed, with a considerable increase in the lithium ion mobility from 20 to 60 °C for electrolytes based on P111i4FSI. The best rate performance at 60 °C was demonstrated for the P111i4FSI based electrolyte, with discharge capacities around 1000 mAh/gSi at a 2C rate [2]. The improved performance was attributed to a combination of the improved lithium ion mobility and formation of a more conductive solid electrolyte interphase (SEI) for this electrolyte.In view of the excellent high temperature performance of P111i4FSI, including high thermal stability and a large electrochemical window [1,2], this electrolyte has been selected for further studies as electrolyte for high voltage LiNi0.5-xMn1.5+xO4 (LNMO) cathodes. LiNi0.5-xMn1.5+xO4 (LNMO) is an attractive candidate as a next generation Li-ion battery (LIB) cathode material because it has a high energy density, due to the high operating voltage, and a relatively low cost (Co-free) compared to alternative cathode chemistries. However, the stability of LNMO is not yet satisfactory for commercial applications, especially in full-cell configuration with graphite as the anode material, and at elevated temperatures. One challenge is the transition metal (TM) dissolution, which poisons the graphite anode by degrading the solid electrolyte interface (SEI) [4]. The TM dissolution is affected by the formation of hydrofluoric (HF) acid in carbonate-based electrolytes utilizing the LiPF6 salt [3]. Additionally, the decomposition processes occur with an increased rate at elevated temperatures [4,5]. Another challenge is the side reactions occurring at the high operating voltage, which limits the coulombic efficiency. Previous works have indicated that the carbon conductive additive is more prone to side reactions than the LNMO active material [6].Here, we present a study on the phosphonium-based ionic liquid P111i4FSI, with two different salt concentrations: 0.79 m LiFSI and 3.0 m LiFSI, as an alternative electrolyte for LIBs with an LNMO cathode.The focus of the research is to study the side reactions separately on the carbon conductive additive, and on the LNMO active material. Carbon conductive additives differing in structure and particle size are studied. The electrolyte is tested both in half-cell and full-cell configuration with a graphite anode, as well as at room temperature and elevated temperatures. Using 0.79 m LiFSI P111i4FSI results in similar capacity retention, but higher coulombic efficiencies after 200 cycles in half-cells at room temperature compared to 1 M LiPF6 in 1:1 EC:DEC (v/v). Post-mortem characterization is carried out with SEM, TEM, Raman spectroscopy, and FT-IR.

Understanding Electrochemical Properties for a Bio-Based Silicon Anode Electrode Enhanced with Carbon Nanostructure Conductive Additives and Water-Based Binders

Lithium Ion batteries (LIB) demand has shown an exciting increase lately, particularly looking for light-weight solution for portable devices [1], [2]. Recently, The development and implementation of electrode material production techniques pushed the LIB's current state performance to further improvement. Silicon (Si) anode-based electrodes are considered an attractvie alternative to conventional graphite electrodes due to their high specific capacity [3]. Theoretically, Si could alloy with Li up to Li4.4Si and result in a specific capacity of 4200 mAh/g, where the mechanical stress within the electrode material causes loss in the conductivity and breaking of the cell. To overcome this issue, different methodologies have been tested for further improvement of Si implementation with different nanostructures [4]. On the other hand, the production rate of LIB is very important to meet the high demand for different technologies. and, a commercial Si grade is highly recommended along with the conventional tape-casting production method. Furthermore, water-based binders, such as Polyacrylic Acid and Carboxy methyl Cellulose (cellulose-based binders) plays an important role in electrode preparation, where it is supporting the volume expansion of Si and preserveing electrode morphology [5]. Herein, we focus on producing a high-quality Si electrode with cellulose binder, enhanced by conductive carbon allotrope additives. In this study, different Si grades will be tested with different concentrations along with conductive and binder [6]. Electrochemical characterization of the Si anode half cell , such as cycling, cyclic voltammetry, and Impedance spectroscopy vs Li+ metal shows extended and stabilized cycling performance, inferring a reduced anode pulverization.

Doped Micron Silicon and Mxene Based Composite Anodes for High-Performance Li-Ion Batteries

With the world's energy needs constantly increasing, renewable energy sources must be technologically advanced and integrated with energy storage devices to ensure their uninterrupted operation. In recent decades, researchers have devoted substantial efforts to improve the energy density of lithium-ion batteries (LIBs) [1]. While graphite remains the commercial anode in LIBs, it’s very low theoretical capacity (372 mAh g-1) restricts its use in high-energy applications. Silicon (Si), owing to its high theoretical capacity of 4200 mAh g-1, natural abundance, and appropriate Li uptake potential (0.4 V vs Li/Li+), is a promising anode material [2]. However, its massive volume expansion restricts its practical use.In this study, micron-sized Si particles are combined with graphite. Micron Si is less expensive, and the synthesis process is simpler. The well-ground mixture (Si-G) is mixed with a polymer solution to generate pyrolytic carbon (upon carbonising). This aids in the buffering of mechanical stresses caused by Si volume expansion. Furthermore, direct exposure of Si to the electrolyte is avoided, stabilising the solid electrolyte interphase (SEI). The addition of a dopant improves the intrinsic conductivity of the material, resulting in increased capacity by providing electron and Li-ion transport channels. To further enhance the composite's performance, synthesized Mxene is added to promote the charge transport between micron Si. Using various techniques such as SEM-EDAX, XRD, Raman, and XPS spectroscopy, the composite's morphology and structure is studied which reveals uniformly distributed Si, C, and Mxene sheets that retain the sheet architecture.As an anode, the composite exhibits outstanding electrochemical performance which was evident by its impressive lithium storage specific capacity of 2003 mAh g-1 (based on weight of Si) even after long term cycling (at 1C rate). Additionally, the composite demonstrates superior rate performance with a specific capacity of 2439 mAh g-1 at 10 C rate, highlighting its potential for high-power applications. Moreover, the electrode displayed a low charge transfer impedance and fast electron transport, which improved the electrochemical performance. The reason for this improved performance is the unique structure consisting of Mxene and doping, which significantly increased the Li ion diffusion and the active sites. Also, low-cost micron Si is used which has a high tap density. Thus, this work provides an approach to develop high-capacity LIBs based on silicon. Keywords: Lithium-ion battery, micron silicon anode, Mxene, doping. References Liu, L. Kang, J. Hu, E. Jung, J. Zhang, S.C. Jun, Y. Yamauchi, Unlocking the Potential of Oxygen-Deficient Copper-Doped Co 3 O 4 Nanocrystals Confined in Carbon as an Advanced Electrode for Flexible Solid-State Supercapacitors, ACS Energy Lett. 6 (2021) 3011–3019. https://doi.org/10.1021/acsenergylett.1c01373.Zhou, Y. Liu, C. Du, Y. Ren, T. Mu, P. Zuo, G. Yin, Y. Ma, X. Cheng, Y. Gao, Polyaniline-encapsulated silicon on three-dimensional carbon nanotubes foam with enhanced electrochemical performance for lithium-ion batteries, Journal of Power Sources. 381 (2018) 156–163. https://doi.org/10.1016/j.jpowsour.2018.02.009.

(Invited) Binder Investigation for Silicon Electrode of Lithium-Ion Batteries

Though the binder is not an active material and does not contribute capacity to lithium ion batteries (LIBs), it is an important ingredient for maintaining the integrity of the electrode, which helps to improve electrical contact between the active materials and conductive carbon black, improves the Li+ transport efficiency, reduces the internal resistance of Li-ion batteries and effectively depresses the crack formations and phase separation.Polyvinylidene fluoride (PVDF) is a polymer that is commonly used as a binder for both anode and cathode materials in LIBs. However, it is not suitable for Si electrode due to its poor binding capability and stability. Viewed as a next generation anode material, Si has been extensively studied due to its extremely high theoretical specific capacity of 3578 mAh/g (assuming Li15Si4 as intermediate product). However, silicon undergoes huge volume variations during repeated discharge and charge, which requires a strong binder to maintain the electrode mechanical integrity. Developing a high strength binder for silicon anodes has been considered as an efficient pathway to alleviate various capacity decay pathways.Polyacrylic acid (PAA) and its lithiated version (LiPAA), rich with carboxy function group, can provide stronger binding and are widely used as binder for Si electrode. In this work, we optimized Si electrode formulation by varying the binder composition. The optimized Si electrodes were electrochemically tested in both half cells and full cells. With improvement in electrochemical performance, cracks at the electrode level can still be observed for the cycled electrodes, which suggests that a stronger binder is required to mitigate volume expansion of Si electrodes and hold the particles together.We further studied the polyimide (PI) materials as binders for Si anodes in an attempt to achieve more stable cycling performance. In this work, we explore various PI crosslinking conditions and their effects on electrochemical performance. The optimized silicon electrode exhibits a higher tensile strength than that of conventional PAA binder. The strong adhesion of the PI binder suppresses the structural collapse of the Si negative electrode during lithiation/delithiation, enabling good capacity retention and stable cycle life.

(Invited) Polymer Design for Beyond Lithium Ion Batteries

In this talk, I will discuss our work related to polymer design for silicon anodes, S cathodes and polymer coating for lithium metal anodes.

High Precision Coulometry and Calorimetry Measurements of Side Reactions for Si/NCA and G/NMC811 Cells

Side reactions taking place at both anode and cathode can be investigated via different methods. Common methods include the high precision coulometry [1-3] and the isothermal calorimetry [4, 5]. In this study, pure silicon anode (70 wt%) Si/NCA cells and pure graphite G/NMC811 cells are investigated via the methods mentioned above. These measurements include voltage holds and charge and discharge endpoint slippage analysis, which is described in [6]. Glazier et al. [4] already showed that after sufficient time, the heat flow during a voltage hold is equivalent to the side reactions taking place. The aim of this study is to investigate the dependency of side reaction currents and parasitic heat on different voltages and temperatures. In addition, their behaviour during the transient phase caused via relaxation processes and edge effects [7] comparing silicon and graphite materials.The cells voltage was held at different state-of-charge (SOC) conditions, low, medium, and high as well as at different temperatures. At the beginning and at the end of the experiment, check-ups were performed to compare the capacity loss and to analyse the endpoint slippage. During the measurements, the heat flow and the leakage current of the cells were measured simultaneously.The preliminary results show that for the low and for the high SOCs, a strong opposing transient phase is present at the beginning of the voltage hold. The strong transient phase is expected for silicon-based materials due to their high relaxation characteristics, meaning also higher leakage currents for this material. Additionally, it was found that the different pre-histories of the cells influence the amount of leakage current measured. Meaning that the actual side reaction current is only achieved after edge effects and relaxation processes cease, which can take a long time for pure Si anodes, up to 1000 hours at 25°C. Moreover, the transient phase of the thermal and the electrical signals was correlated for the different materials.The study of side reactions for lithium ion cells is investigated in the project “ExZellTUM III”, funded by the German Federal Ministry of Education and Research (BMBF) under grant number 03XP0255. [1] Smith, A.; Burns, J.; Dahn, J.: A high precision study of the coulombic efficiency of Li-ion batteries, In: Electrochemical and Solid-State Letters 13, pp. A177, 2010 [2] Smith, A.; Burns, J.; Xiong, D.; Dahn, J.: Interpreting High Precision Coulometry Results on Li-ion Cells, In: Journal of The Electrochemical Society 158, S. A1136– A1142, 2011 [3] Burns, J. C.; Jain, G.; Smith, A. J.; Eberman, K. W.; Scott, E.; Gardner, J. P.; Dahn, J. R.: Evaluation of effects of additives in wound Li-ion cells through high precision coulometry, In: Journal of The Electrochemical Society, 158 (3), A255-A261, 2011 [4] Glazier, S. L.; Downie, L. E.; Xia, J.; Louli, A. J.; Dahn, J. R.: Effects of fluorinated carbonate solvent blends on high voltage parasitic reactions in lithium ion cells using OCV isothermal microcalorimetry, In: Journal of The Electrochemical Society, 163 (10), A2131-A2138, 2016 [5] Glazier, S. L.; Odom, S. A.; Kaur, A. P.; Dahn, J. R.: Determining parasitic reaction enthalpies in lithium-ion cells using isothermal microcalorimetry, In: Journal of The Electrochemical Society, 165 (14), A3449-A3458, 2018 [6] Streck, L.; Roth, T.; Keil, P.; Strehle, B.; Ludmann, S.; Jossen, A.: A comparison of voltage hold and voltage decay methods for side reactions characterization, In: Journal of The Electrochemical Society, 170, 040520, 2023 [7] Roth, T.; Streck, L.; Graule, A.; Niehoff, P.; Jossen, A.: Relaxation effects in self-discharge measurements of lithium-ion batteries, In: Journal of The Electrochemical Society, 170, 020502, 2023

Investigation of porous multi-carbon layers constructed on submicron silicon flakes for high-performance Li-ion batteries anode

Low initial coulombic efficiency (ICE) of crystal silicon anode is fatal for lithium-ion batteries, leading to impossibility for commercial application. In this study, the submicron flaky silicon particles recycled from Si waste in photovoltaic industry clustered and formed into new structure through liquid-phase pyrolysis of amorphous carbon. After covered by amorphous carbon layer, the silicon flakes are surrounded by carbon skeleton of mesh-porous to be a Nitrogen-doped submicron silicon/multi-carbon material (SCN). The initial coulombic efficiency of SCN reaches 88.55 % at high discharge/charge capacities of 960.4/850.5 mAh g−1, which owe to the appropriate amorphous carbon on surface of Si for reducing the initial irreversible capacity loss due to solid electrolyte interface layer. The capacity retention of SCN keeps to a high stage up to 74.8 % after 100 cycles. It is realized that the Nitrogen-doped reticulated porous carbon not only improves the transport of particles, but also mitigates the volume expansion of silicon during charging and discharging and improves the cycling stability of the electrode. The stable performance of large-size silicon particles in the anode provides a good guarantee for the future development of lithium-ion batteries.

Investigating the Rheological Properties of Silicon/Carbon (Si/C) Composite Supraparticles Used in Lithium-Ion Battery Anodes

Silicon-based active materials emerged as a very promising replacement for graphite in lithium-ion battery (LIB) anodes [1]. The unfavorable issue of particle pulverization due to inherent volume changes during lithiation / delithiation, which is still a problem hindering the full-scale implementation of silicon anodes in LIB, can be avoided by utilizing nanosized active materials. Our new approach of spray drying the nanometer-sized building blocks to micrometer-sized supraparticles enables additional improvement for Si-based anodes [2]. This approach, however, creates new hurdles for processing and quality control since a stable processing of the supraparticles to final electrodes is a key criterion for enhanced LIB performance.Figure 1: Schematic representation of the deagglomeration/breakage behavior of Si/C supraparticles in a slurry as a factor of particle motion/shear [3].In order to find indicators for supraparticle breakage early in the process chain of Si/C anode manufacturing, we conducted detailed rheological investigations on the slurry level. We parameterized structural kinetic models for slurries prepared with mixtures of supraparticles and nanosized building blocks particles to simulate supraparticle breakage (Fig. 1). With this approach, two indicators were found. First, the thixotropic response showed a strong dependence on the ratio between the spherical supraparticles and the nanosized building blocks. Second, the viscosity decreased with increasing percentage of the fractal-shaped nanosized particles. While further investigating this unexpected effect on viscosity, additional rheological characterizations of the components gave interesting insights into the overall rheological properties of hierarchically structured particle dispersions. The component carbon black and the nanosized/agglomerated active material were individually characterized in aqueous binder solutions to determine their individual contributions to the rheological properties of the full slurry. Here, the focus was laid on shear thinning and thixotropy and how the observed properties relate to the well-known rheology of hard spheres [4].In conclusion, a deeper understanding of anode slurry rheology enables tailored formulations and process conditions. Moreover, our methodology is applicable to other energy-related materials that are a key part of formulation science and particle technology. References Xiao, L.; Sehlleier, Y. H.; Dobrowolny, S.; Orthner, H.; Mahlendorf, F.; Heinzel, A.; Schulz, C.; Wiggers, H., Si-CNT/rGO Nanoheterostructures as High-Performance Lithium-Ion-Battery Anodes, ChemElectroChem 2, 1983-1990 (2015).Amin, A.; Loewenich, M.; Kilian, S. O.; Wassmer, T.; Bade, S.; Lyubina, J. et al, One-Step Non-Reactive Spray Drying Approach to Produce Silicon/Carbon Composite-Based Hierarchically Strucutured Supraparticles for Lithium-Ion Battery Anodes, J. Electrochem. Soc. 170,020523 (2023).Watermann, J.; Amin, A.; Wiggers, H.; Segets, D.; Özcan, F., Connecting structure and rheology of silicon/carbon supraparticle-based lithium-ion battery anode slurries, In Powder Technology, manuscript submitted.De Kruif, C.G.; van Lersel, E. M. F.;Vrij, A., Hard sphere colloidal dispersions: Viscosity as a function of shear rate and volume fraction, J. Chem. Phys. 83, 4717-4725 (1985). Figure 1

Optimizing Si-Dominant Anodes in High-Energy Li-Ion Batteries with a Binary Binder System: Exploring Impacts on Adhesion, Cohesion, and Electrochemical Performance

Silicon anode stands out as a promising game-changer for Li-ion batteries (LIBs) due to its remarkable gravimetric and volumetric capacities, boasting 4200 mAh/g1 and 2200 mAh/cm3, respectively. In addition, its operating voltage of 0.4 V vs Li/Li+, abundant availability, and environmental friendliness make Si a strong candidate to revolutionize energy densities in the next-generation high-performance LIBs. However, challenges arise during lithiation as silicon undergoes an expansion of ~300% of its initial volume, leading to a cascade of detrimental effects on the electrode.1 The micron-sized silicon particles tend to pulverize, and resulting in fragmented silicon particles that become electrically isolated. Such volume variations compromise the adhesion of the Si electrode to the current collector. Furthermore, the repetitive expansion and shrinkage cycles contribute to cohesive failure within the electrode. Collectively, these factors considerably diminish the electrochemical performance and lifespan of Si anodes. Consequently, these challenges have placed major constraints on the utilization of silicon, making it currently viable only at levels below 10 wt% in LIB anodes, thereby impacting the commercial feasibility of Si-dominant anodes in high-energy LIBs.Such challenges in Si anodes impose strict demands on the binder, requiring it to maintain intimate contact between electrode components and to the current collector and preserve the integrity of ion and electron transport channels during frequent volume changes upon cycling. Obviously, the traditional binders like PVDF or carboxymethylcellulose (CMC)/Styrene-butadiene rubber (SBR) are not suitable options for addressing these specific challenges.Many approaches have been explored to address such requirements including increased crosslinking, different binding functionalities, adhesion promoters, etc.2 In this study, we investigate a binder system comprising two key components: (i) an acrylic acid-based polymer enriched with a high concentration of carboxyl groups. This component not only forms effective bonds with the silicon surface but also fosters interactions with other polar species, facilitating the self-healing properties. (ii) Incorporating a styrene-butadiene-based polymer, renowned for its exceptional flexibility, improves adhesion and enhances the tensile properties of the electrode, particularly in response to volume expansion challenges. We evaluated the characteristics of the binders and electrode properties, including adhesion to the current collector, cohesion within the electrode, and binder distribution. These properties were subsequently correlated with the initial columbic efficiencies (IEC) and electrode cycle life in Si-dominant/NMC811 cells. We show that there is an optimal binder ratio that achieves the desired adhesion and significantly enhances cohesive strength, leading to improved cycling stability compared to a single binder system.