Silicon Electrodes Research Articles

Sulphur-template method for facile manufacturing porous silicon electrodes with enhanced electrochemical performance

Sulphur (S)-template method based on conventional slurry-casting method has been developed to produce porous silicon (Si) electrodes. The facile fabrication technology is suitable for current production line and expected to be widely applied to various electrode materials under large volume change during operation. Specifically, S particles as template agent are mixed with active material Si, carbon conductor and binder forming uniform slurry. After casting and drying, the electrodes are immersed in carbon disulfide solution to remove S particles rapidly, generating pores in-situ at the original position of S particles. Electrochemical analysis shows that the pores inside electrodes are able to shorten lithium ion diffusion paths, reduce normal expansion rate and decrease formation of cracks in the Si electrode (2 mgSi/cm2), demonstrating a reversible capacity of 951 mAh/g at 0.5 A/g after 100 cycles (with a capacity retention of 99.5%) and a capacity of ∼826 mAh/g at 2 A/g.

Silicon-Based Anode and Its Full-Cell Performance Test Using a High-Capacity Pouch Cell

Despite impressive theoretical capacity, silicon (Si) electrodes suffer from severe pulverization due to enormous volume change. The pulverization leads to low electrical conductivity and reduced diffusivity of lithium ions, resulting in poor rate and cycling ability. In this work, the electrical conductivity of Si electrodes is improved by incorporating carbon nanotubes (CNTs) to enhance the rate and cycling ability. The incorporation of CNTs in the Si electrode is optimized systematically based on the electrode conductivity and cycling performance. Furthermore, the importance of nano-sized conductive carbon in the electrode for achieving an integrated conductive hybrid network across the entire bulk electrode that meets short-range and long-range conduction requirements for the enhancement of electrochemical reactions is explained. The SiCNTs (SiCNTs-30) assessed using coin-type half cells exhibited a higher specific capacity of ∼980 mAh g−1 after 100 cycles at the current density of 0.25 A g−1. Also, the cyclability tests of the SiCNTs-30 anode, paired with the commercial cathode, at the pouch cell level revealed an impressive specific capacity of more than 556 mAh g−1 at 1 C after 500 cycles. Hence, the method shown in this work can pave the way for commercializing SiCNTs hybrid anodes for high-energy and high-power lithium-ion batteries.

From biomass to batteries: the contribution of silicon–carbon composites prepared from high-nitrogen egg whites and micron-sized silica powder to lithium-ion battery performance

This research uses egg white proteins as a carbon source to create silicon/nitrogen-doped composites, addressing silicon electrode volume expansion in lithium-ion batteries.

Characterization of pitch carbon coating properties affecting the electrochemical behavior of silicon nanoparticle lithium-ion battery anodes

700 °C is the optimal heat treatment temperature for pitch-coated silicon electrodes. Both the amorphous carbon/pitch and silicon are active materials but store Li+ ions through different ion storage mechanisms and contribute to overall performance.

Implantation of Solid Electrolyte Interphase Stabilizer within High-Capacity Silicon Electrode Enabling Enhanced Battery Performance

The commercial application of high-capacity silicon (Si) anode in lithium-ion batteries is limited by the marked volume expansion and continuous interface side reactions between the active material and the electrolyte. To address the issues, one popular strategy is to induce functional salt additives to the electrolyte, which could help to construct a robust solid electrolyte interphase (SEI) to resist the undesirable parasitic reactions and fast electrode failure. However, there exists the shortness of the dependency in the solubility of the additive salt and the possible homogeneity of the SEI. In light of this, we propose an innovative method of incorporating an SEI stabilization regent, exemplified by lithium difluorooxalate borate (LiDFOB), in the Si anode. This approach facilitates the effective utilization of the functional SEI stabilizer and impressively enhances the presence of inorganic compounds within the SEI. The resultant stable SEI effectively impedes interfacial side reactions, mitigates substantial expansion/contraction, and promotes the transport of Li + ions. As a result, the Si electrode incorporated with LiDFOB displays superior long cycle life and enhanced rate capability, indicating the advancement of planting LiDFOB in the electrode in promoting the development of advanced high-energy-density lithium-ion batteries.

Effect of H2/Ar Ratio on the Photoanodic Currents of Graphene/MoS2 Films

Enhancing advanced solar energy storage in rechargeable batteries is one of the most critical challenges in clean energy technology aimed at reducing air pollution and dependence on fossil fuels. It has been demonstrated that dye-sensitized electrodes, silicon electrodes, and transition metal-based photoelectrodes can be applied in solar-charged rechargeable batteries to effectively capture visible light. However, potential pollution and cost issues limit their large-scale applications. This study aims to reduce the increased charging potential, caused by the high overvoltage due to the dissolution of compounds such as lithium peroxide and lithium carbonate, through photo-assisted charging. To achieve this goal, efficient graphene/MoS2 composites are synthesized with chemical vapor deposition (CVD), and their photoelectrochemical properties are characterized to facilitate efficient photocharging. In this context, by positively altering the carrier gas ratio towards H2 (from 30% to 60%), samples are synthesized at different H2/Ar ratios to investigate the varying ratio's impact on photoanodic currents.

Approach for Mitigating Interfacial Degradation in Silicon-Based Electrodes to Enhance Cycling Stability of Lithium-Ion Batteries and Capacitors

The growing demand for high energy density in lithium-ion batteries has led to extensive investigations on silicon negative electrodes. In this study, we propose a novel approach to address the interfacial degradation of electrodes caused by volume expansion during lithium insertion/extraction. The approach involves a sandwich structure consisting of a carbon nanofiber (CNF) undercoating electrode and a CNF-coated separator. The CNF is treated with acid to form carboxyl and hydroxyl groups on its surface, which improves the dispersion of the coating slurry and enhances adhesion to the electrode. Through electrochemical testing, the improved silicon electrode shows a high retention rate of 82% compared to the initial capacity after 100 cycles, and high electrochemical performance when applied to both lithium-ion batteries and lithium-ion capacitors. These results are attributed to the improved adhesion between the electrode and current collector, as well as the formation of a stable solid electrolyte interphase layer on the electrode surface. These findings suggest that the conductive coating process, applied to the separator and current collector, effectively mitigates interfacial issues that are associated with silicon-based electrodes. This approach has the potential to overcome the degradation characteristics of silicon electrodes and significantly contribute to improving the cycling stability of lithium-ion batteries and capacitors.

Multimodal Operando Analysis of High-Capacity Electrodes with Photons and Neutrons

High-capacity electrodes such as sulfur cathodes and silicon anodes are promising candidates for sustainable electrochemical energy storage in a post-lithium-ion era. While lithium-sulfur (Li/S) batteries have five times higher theoretical gravimetric energy density (about 2500 Wh/kg) than modern lithium-ion batteries [1], silicon is an interesting anode material for lithium-ion batteries because its specific capacity is ten times higher than that of commercially used graphite anodes. However, despite decades of research, the sharp drop in capacity with the increasing number of cycles is still a major obstacle to widespread technical use of both electrodes. Operando techniques [2,3] are well suited to gaining a mechanistic understanding of the degradation processes. In particular, the simultaneous combination of several independent measurements (multimodal) during the operation of the electrodes allows deep insights into the degradation mechanisms. It enables a more advanced mechanistic understanding of the two complex chemical processes.This conference contribution presents the results of novel experimental setups in which multiple independent measurements were performed simultaneously. Electrochemical impedance spectroscopy (EIS), temperature measurement and X-ray imaging or neutron techniques such as neutron small angle scattering and neutron reflectometry were performed over several cycles. Simultaneously, cells were charged and discharged galvanostatically or potentiostatically. Structural changes at the macroscopic and microscopic levels and chemical compositions can be correlated with characteristic signals in the EIS and charge-discharge curves.In particular, imaging and spectroscopic techniques on button cells with X-rays, especially at a synchrotron source, allowed essential insights into the processes in lithium/sulfur batteries [4–8]. But also, applying this characterization method on industrially relevant pouch cells could provide important information about processes in this cell format [9].Neutron reflectometry is particularly well suited for analyzing electrochemical interfaces on silicon electrodes [10], which was also performed in operando mode [11–13]. Furthermore, for this electrode material, important insights into electrode processes could be obtained using operando X-ray spectroscopy and X-ray imaging at the synchrotron [14,15].

(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.

Electrochemical Aptamer-Based Biosensor Developed to Monitor PCA3 and PSA Released by Prostate Cancer Cells

1. Introduction Detection of prostate cancer (PCa) in the early stage of the disease is fundamental for the prevention of metastases and for the initiation of treatment, which may bring greater chance of patient cured; so several research groups have sought the manufacture of a simple and non-invasive methodology [1]. The early diagnosis relies on prostate specific antigen (PSA) serum test, even if it showed clear limits [2]. Recently, prostate cancer gene 3 (PCA3), a non-coding mRNA which is overexpressed in tumor cells when compared with noncancerous prostate tissue [3]. Here, we report a new method for detection of prostate cancer-specific antigen 3 (PCA3) and prostate specific antigen (PSA) released by cancer cells using an aptasensor. Electrochemical impedance spectroscopy (EIS), SEM and EDS were used to evaluate biosensor surface. 2. Experimental Part The aptamers were modified through Biosearch Technologies at 5′-terminus in C6-disulfide linker to have it bond to the gold electrode; and at 3′-end in methylene blue (MB) (as redox probe) to perform the electrochemical analysis, which was followed by the HPLC purification process to remove truncated-synthesis products and to enrich the purity of the aptamer sequences. Aptamer immobilization was done by incubation of 1 μM thiolated aptamer on the top of the gold-coated silicon electrode overnight, in the dark, at 4°C. Subsequently, the electrodes were washed with PBS and a backfilling consisted of 3 mM of MCH aqueous solution for 30 minutes was done to displace the nonspecifically adsorbed aptamer molecules and to passivate the electrode surface [26]. Finally, the electrodes were once more rinsed with PBS to remove MCH excesses. The electrochemical characterization was done in a potentiostat μAutolabIII (Metrohn, NL), using a three-electrode system, which consisted of Ag/AgCl (3M KCl), as reference electrode; Pt wire, as a counter electrode; and gold-coated silicon wafer, as a working electrode. The prostate cell lines used were normal human prostate cells (RWPE-1; ATCC® CRL-11™), androgen-sensitive human prostate cancer cells (LNCaP; ATCC® CRL-1740™), and androgen-independent human prostate cancer cells (PC-3; ATCC® CRL-1435™). The three lineages were chosen to better understand PCA3 and PSA production patterns by different cell types. 3. Results and Discussion The gold-coated silicon wafe electrode was analyzed by SEM and EDS, which acts as working electrode and substrate to anchor the thiolated aptamer on the surface. By using square wave voltammetry, the present aptasensor was able of monitoring the PCA3 and PSA released from prostate cells (normal and cancerous). The PSA and PCA3 detection limits were 1 ng/mL and 9.2 ng/mL, respectively, at linear range up to 150 ng/mL, and it was also able to detect both biomarker even in presence of several interfering compounds (UA, AA, BSA and MUC1). In addition, the herein developed aptasensor was able to verify, at accurate and robust response, the patterns of the PCA3 and PSA released by different cell lines (RWPE-1, LNCaP and PC3). The detection of PSA and PCA3 by aptasensor agree with the cell lineage type, the LNCaP is an androgen-dependent lineage. In this way, an overexpressed both biomarkres was release was verified [49,50] and it can be compared to early-stage patients. On the other hand, the PC3 cells PSA and PCA3 releasing was nearly background signal, demonstrating almost absent production of studied targets, the PC3 lineage is androgen-independent which can be compared to advanced-disease patients. Funding This work has been supported by a grant from FAPESP-Brazil, CNPq and Capes.

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.

Revealing the Quantitative Connection between Electrode-level Cracks and Capacity Fading of Silicon Electrodes in Lithium-ion Battery

Revealing the Quantitative Connection between Electrode-level Cracks and Capacity Fading of Silicon Electrodes in Lithium-ion Battery

Multilevel Gradient-Ordered Silicon Anode with Unprecedented Sodium Storage.

While cost-effective sodium-ion batteries (SIBs) with crystalline silicon anodes promise high theoretical capacities, they perform poorly because silicon stores sodium ineffectively (capacity <40 mAh g-1 ). To address this issue, herein an atomic-order structural-design tactic is adopted for obtaining unique multilevel gradient-ordered silicon (MGO-Si) by simple electrochemical reconstruction. In situ-formed short-range-, medium-range-, and long-range-ordered structures construct a stable MGO-Si, which contributes to favorable Na-Si interaction and fast ion diffusion channels. These characteristics afford a high reversible capacity (352.7 mAh g-1 at 50mA g-1 ) and stable cycling performance (95.2% capacity retention after 4000 cycles), exhibiting record values among those reported for pure silicon electrodes. Sodium storage of MGO-Si involves an adsorption-intercalation mechanism, and a stepwise construction strategy of gradient-ordered structure further improves the specific capacity (339.5 mAh g-1 at 100mA g-1 ). Reconstructed Si/C composites show a high reversible capacity of 449.5 mAh g-1 , significantly better than most carbonaceous anodes. The universality of this design principle is demonstrated for other inert or low-capacity materials (micro-Si, SiO2 , SiC, graphite, and TiO2 ), boosting their capacities by 1.5-6 times that of pristine materials, thereby providing new solutions to facilitate sodium storage capability for better-performing battery designs.

Broadband Tunable Infrared Light Emission from Metal-Oxide-Semiconductor Tunnel Junctions in Silicon Photonics.

Broadband near-infrared light emitting tunnel junctions are demonstrated with efficient coupling to a silicon photonic waveguide. The metal oxide semiconductor devices show long hybrid photonic-plasmonic mode propagation lengths of approximately 10 μm and thus can be integrated into an overcoupled resonant cavity with quality factor Q ≈ 49, allowing for tens of picowatt near-infrared light emission coupled directly into a waveguide. The electron inelastic tunneling transition rate and the cavity mode density are modeled, and the transverse magnetic (TM) hybrid mode excitation rate is derived. The results coincide well with polarization resolved experiments. Additionally, current-stressed devices are shown to emit unpolarized light due to radiative recombination inside the silicon electrode.

Polyolefin-derived substrate-grown carbon nanotubes as binder-free electrode for hydrogen evolution in alkaline media

Switching from a linear mode of waste management to a circular loop by transforming plastic waste into carbon nanotubes (CNTs) is a promising approach to current plastic waste treatment. One of the many applications of CNTs is its use for electrocatalytic water splitting for hydrogen evolution. Existing methods of CNTs-based hydrogen evolution reaction (HER) electrode fabrication involve additives like polymeric binders and additional steps to improve CNT dispersion, which are detrimental to the CNT structure and properties. The in-situ fabrication approach can potentially be a one-pot solution to HER electrode synthesis. In this study, polyolefins pyrolysis gas and a Co:Ni:Mg catalyst were used to fabricate binder-free CNTs-based electrodes on different substrates for HER. The study assessed CNT quality on conductive carbon paper, semiconductive silicon, and dielectric glass substrates, evaluating their HER performance in 1 M KOH. A mixture of hollow-core, bamboo-like, and cup-stacked arrangement nanotubes were synthesized on the substrates, with CNTs on glass and carbon paper substrates possessing better graphitization than CNTs grown on silicon. This is in agreement with HER performance, whereby the as-prepared electrodes required overpotentials of 267 mV, 241 mV, and 216 mV for silicon, glass, and carbon paper, respectively, to achieve 10 mA/cm2. Despite being poorly conductive, the glass substrate electrode achieved a lower overpotential than the silicon electrode. Additionally, the as-prepared silicon electrode faced a delamination issue likely attributed to the lower surface energy of the silicon substrate surface, demonstrating the weaker adhesion between the CNTs and silicon surface. The proposed approach thus showed that the in-situ fabricated electrodes performed better than separately synthesized CNTs prepared into electrodes by 27.4% and 14.2% for carbon paper and glass substrates, respectively. The improved performance of the as-prepared, binder-free electrodes can be linked to the lower charge-transfer resistance and reduced contact resistance between the CNTs and substrate.

Metal Carbide Additives in Graphite‐Silicon Composites for Lithium‐Ion Batteries

AbstractThe pathway for improving lithium‐ion batteries′ energy density strongly depends on finding materials with enhanced performance. Although great efforts have been done, on the anode‐side, graphite is still the best choice. In the last decade, silicon elements are attracting growing attention as anode since their use can theoretically increase specific capacity of the negative electrode side. However, as the electrochemical mechanism involves the alligation of a large amount of Li, the silicon electrode experiences huge volume changes (more than 300 % of its initial volume), leading to fractures and pulverizations of the electrode. Herein, we propose for the first time using Molybdenum and Chromium Carbides as additive to stabilize graphite/silicon composites. Spark plasma sintering technology is used to sinter the electrode powders. We demonstrated that the presence of molybdenum or chromium carbides promotes the performance of C/Si electrodes, improving the cycling stability compared to pristine graphite/silicon electrodes.

New Insights into the Behaviour of Commercial Silicon Electrode MaterialsviaEmpirical Fitting of Galvanostatic Charge‐Discharge Curves

AbstractSilicon (Si) materials for use in Lithium ion batteries (LIBs) are of continued interest to battery manufacturers. With an increasing number of commercially available Si materials, evaluating their performance becomes a challenge. Here, we use an empirical fitting function presented earlier to aid in the analysis of galvanostatic charge‐discharge data of commercial Si half‐cells with relatively high loading. We find that the fitting procedure is capable of detecting dynamic changes in the cell, such as reversible capacity fade of the Si electrode. This fading is found to be due to the highly lithiated Li2SiLi3.5Si phase and that the behaviour is strongly dependent on the potential of this phase. EIS reveals that the Si electrode is responsible for the reversible behaviour due to progressive loss of Li+leading to increasing resistance. SEM/EDX and XPS characterization are also employed to determine the origin of the irreversible resistance growth on the Si electrodes.

Structural and functional changes of deep layer pyramidal neurons surrounding microelectrode arrays implanted in rat motor cortex.

Devices capable of recording or stimulating neuronal signals have created new opportunities to understand normal physiology and treat sources of pathology in the brain. However, it is possible that the tissue response to implanted electrodes may influence the nature of the signals detected or stimulated. In this study, we characterized structural and functional changes in deep layer pyramidal neurons surrounding silicon or polyimide-based electrodes implanted in the motor cortex of rats. Devices were captured in 300µm-thick tissue slices collected at the 1 or 6 week time point post-implantation, and individual neurons were assessed using a combination of whole-cell electrophysiology and 2-photon imaging. We observed disrupted dendritic arbors and a significant reduction in spine densities in neurons surrounding devices. These effects were accompanied by a decrease in the frequency of spontaneous excitatory post-synaptic currents, a reduction in sag amplitude, an increase in spike frequency adaptation, and an increase in filopodia density. We hypothesize that the effects observed in this study may contribute to the signal loss and instability that often accompany chronically implanted electrodes. STATEMENT OF SIGNIFICANCE: Implanted electrodes in the brain can be used to treat sources of pathology and understand normal physiology by recording or stimulating electrical signals generated by local neurons. However, a foreign body response following implantation undermines the performance of these devices. While several studies have investigated the biological mechanisms of device-tissue interactions through histology, transcriptomics, and imaging, our study is the first to directly interrogate effects on the function of neurons surrounding electrodes using single-cell electrophysiology. Additionally, we provide new, detailed assessments of the impacts of electrodes on the dendritic structure and spine morphology of neurons, and we assess effects for both traditional (silicon) and newer polymer electrode materials. These results reveal new potential mechanisms of electrode-tissue interactions.

Silicon Hysteresis and Voltage Relaxation Phenomena: Implications on Overpotential Analysis

Currently, many strategies are being developed to enable a successful implementation of silicon anode active material into commercial lithium-ion cells with the goal of increasing their specific capacities and energy densities. Compared to the commonly used graphite active material, silicon gives rise to a number of challenges owing to its large voltage hysteresis. Next to a decrease in energy efficiency and a significant corresponding heat evolution during cycling,1 the path-dependency of the potential and the resulting ambiguous voltage vs. SOC correlation complicates the development of battery control systems. While the origin of the voltage hysteresis is not yet fully understood, several studies suggest that it can, to a large part, be explained by compressive and tensile stress on the silicon material induced by its lithiation and delithiation respectively.2, 3 Another hysteresis effect which may also be explained by changes in mechanical stress is the unusual voltage relaxation of silicon at open-circuit voltage (OCV) conditions: the voltage undergoes significant changes at open-circuit and the voltage relaxation is still significant over extended periods of time.4 The implications of this voltage relaxation/hysteresis behavior on the commonly used experimental methods for the analysis of overpotentials (EIS, GITT, DCIR) have, to the best of our knowledge, not been analyzed in the literature.In this study we characterize the hysteresis and voltage relaxation behavior of a silicon-dominant electrode containing 70 wt% micro-scale silicon particles (Wacker Chemie AG) for which the amorphous silicon fraction is reversibly lithiated to the composition Li2Si (reversible capacity: 1200 mAh/gSi).5, 6 We find an offset of ~60 mV between the voltage after a 2 h OCV phase compared to the voltage at a very slow rate of C/20, which is surprisingly large compared to the voltage differences observed between different C-rates (e.g., ~10 mV between C/20 and C/2, see Fig. 1). This suggests that a form of structural relaxation may occur in the silicon material once there is no ongoing (de-)lithiation. As a result of this, the offset between the potential under load and at open-circuit cannot be used to calculate a resistance or a diffusion coefficient, as would typically be done for other active materials.7 By comparing charge/discharge potential curves recorded at different C-rates for thin silicon electrodes (1.0 mAh/cm2), effective cell resistances are estimated. Based on these results, methods used for the characterization of the kinetics of silicon are critically analyzed for their significance. We compare galvanostatic impedance (GEIS) measurements performed under load to potentiostatic impedance (PEIS) measurements performed at OCV. Further, we compare ΔE/ΔI values calculated at different points in time after the current has been turned off or on in order to analyze the impact and time-scale of the current-independent OCV relaxation. All results are compared to the same measurements performed for a graphite electrode. We ultimately show that the analysis of overpotentials of silicon active materials, when done incorrectly, can yield parameters which are physically meaningless and misleading for the characterization of the kinetics of silicon electrodes. Acknowledgements We gratefully acknowledge the German Federal Ministry of Education and Research (BMBF) for its financial support within the AQua HysKaDi project (03XP0321B). We also thank Wacker Chemie AG for kindly providing the silicon material. References V. L. Chevrier, Z. Yan, S. L. Glazier, M. N. Obrovac and L. J. Krause, 168, 030504 (2021).V. A. Sethuraman, V. Srinivasan, A. F. Bower and P. R. Guduru, 157, A1253 (2010).B. Lu, Y. Song, Q. Zhang, J. Pan, Y.-T. Cheng and J. Zhang, Physical chemistry chemical physics : PCCP, 18(6), 4721–4727 (2016).K. Pan, F. Zou , M. Canova and Y. Zhu, 413, 20-28 (2019).D. Jantke, R. Bernhard, E. Hanelt, T. Buhrmester, J. Pfeiffer, and S. Haufe, 166(16), A3881 (2019).M. Graf, C. Berg, R. Bernhard, S. Haufe, J. Pfeiffer and H. A. Gasteiger, 169(2), 020536 (2022).D. W. Dees, S. Kawauchi, D. P. Abraham, and J. Prakash, 189, 263-268 (2009). Figure 1 Silicon electrode potentials measured against a lithium reference electrode in a half-cell (Swagelok® T-cell setup) plotted against the estimated composition of lithium in the amorphous silicon fraction. (De-)lithiation potentials are shown for the C-rates of C/20 and C/2 and are compared to potentials recorded after 2 hours at OCV. The gray inset shows the offset between the two C-rates and the OCV at the estimated composition of ~Li1Si. Loading: 0.8 mgSi/cm2; areal capacity: 1.0 mAh/cm2; electrolyte: 1M LiPF6 in FEC/DEC (2:8 v:v); separator: two glassfiber separators (VWR); cycling: CC capacity limited lithiation, CCCV delithiation to 1.0 V. Figure 1

Effect of Stress on the Electrochemical Response of Amorphous Silicon Electrodes

Silicon electrodes for lithium-ion batteries exhibit a theoretical capacity ten times higher than that of the graphite electrodes currently used in commercial systems. When lithiated/delithiated, silicon experiences stresses on the order of one GPa, inducing the damage of the material, which results in poor cyclability. Concurrently, irreversible continuum thermodynamics predicts that these stresses affect the lithiation itself in different ways for monophasic and biphasic lithiation (progressive invasion of the electrode by a lithiated phase of given composition) [1,2]. When lithiation takes place homogeneously (without phase transformation), mechanical stresses affect lithiation through their effect on the chemical potential of diffusing lithium (Larché-Cahn theory). On the other hand, when lithiation is biphasic, mechanical stresses affect lithiation through an effect on both chemical potential of lithium and thermodynamic driving force acting on the phase boundary. This coupling has first been probed experimentally in homogeneous lithiation using a method where the stress state of silicon is modified indirectly through incremental delithiation [3].In this work, we investigate the interplay between the electrochemical state (electrode potential and current density) of an amorphous silicon electrode and its stress state. The electrochemical response of the amorphous silicon thin film deposited on a stainless steel substrate has been recorded under varying mechanical loading during lithiation and delithiation cycles. The mechanical loading is applied by deforming elastically the steel substrate of the amorphous silicon thin film, which allows us to directly probe the contribution of mechanics to the lithiation/delithiation processes. When carried out under galvanostatic conditions, incremental changes in stress lead to instantaneous change in the electrode potential. Experimental results from uniaxial tensile tests show that an applied strain of 0.3%, resulting in changes in stress in silicon of ~ 200-300 MPa, induces a potential variation on the order of 2.5 mV. Stress-induced changes in the potential are measured for the biphasic first lithiation, as well as subsequent monophasic lithiation and delithiation sequences at different states of charge. These results are discussed within the context of continuum thermodynamic theories of the coupling between transport of chemical species and mechanical stress.[1] A. F. Bower, E. Chason, P. R. Guduru, B. W. Sheldon, Acta Mater. 98 (2015) 229-241.[2] A. F. Bower, P. R. Guduru, E. Chason, Int. J. Solids Struct. 69 (2015) 328-342.[3] V. A. Sethuraman, V. Srinivasan, A. F. Bower, P. R. Guduru, J. Electrochem. Soc. 157 (2010) A1253.