Silicon Thin Film Anodes Research Articles

Cobalt-free cathodes and silicon thin-film anodes towards high-capacity solid-state batteries

This study provides a comprehensive analysis of a novel battery system, which integrates a high-loading (∼5 mAh/cm2) cobalt-free cathode composed of lithium nickel manganese aluminum oxide (LiNi0.9Mn0.05Al0.05O2, NMA) into an all-solid-state cell for the first time. The argyrodite (Li6PS5Cl) solid electrolyte is used in conjunction with a 99 wt% silicon thin-film anode. Room temperature discharge capacities of >210 mAh/gNMA and > 170 mAh/gNMA were achieved at cycling rates of 0.05C and 0.25C, respectively. Electrochemical impedance spectroscopy measurements, taken during the first cycle detail onset of electrolyte degradation, lithiation of the silicon anode, and the change in charge transfer kinetics as a function of cell voltage. Raman, Fourier Transform Infrared, and X-ray photoelectron spectroscopy are used to identify the argyrodite degradation products that form in the catholyte on cycling, unveiling lithium carbonate as a potential source of oxygen-related degradation commonly alluded to in literature. Furthermore, high cell stack pressure, 350 MPa during fabrication, led to fracturing and pulverization of some cathode particles.

Enhancing Methylated Amorphous Silicon Anode Performances in Li-Ion Batteries By Boron Doping

Methylated amorphous silicon has already demonstrated some advantages as compared to silicon in terms of mechanical properties and cyclability for Li-ion batteries (LIB) [1]. However, the conductivity of methylated amorphous silicon drops by several orders of magnitude when increasing the methyl content in the material, which prevents investigating methyl contents higher than 10%. It is well known that doping significantly improves the conductivity of crystalline or amorphous silicon, and has sometimes been reported to improve the material cyclability when used as a LiB anode [2].Here, boron-doped methylated amorphous silicon electrodes with different methyl contents were investigated as anodes for LiB in half-cell configuration. Boron doped methylated silicon thin films (100nm thick) with various methyl content were cycled in the range 0.025V – 1V at C/2 rate (electrolyte: LP30 with 5%FEC). 10% methylated amorphous silicon with 2% boron doping performs a capacity retention of 77% after 1000 cycles (exhibiting a capacity around 1800 mAh.g-1) of full lithiation/delithiation. Figure 1a shows a clearly improved performance as compared to the undoped material. The stability upon cycling of higher methyl contents (15% and 20%) electrodes is further increased, with a capacity retention exceeding 80% over 1000 cycles of full lithiation/delithiation.The evolution of the SEI thickness can be quantitatively determined from IR spectra during cycling. The thickness in the delithiated state is found to increase from cycle to cycle, but with a lower rate for the doped 10% methylated electrodes (thickness lower than 25 nm after 40 cycles and 30 nm after 80 cycles) than for the undoped ones (thickness larger than 35 nm after 40 cycles), see Figure 1b. The SEI evolution upon cycling for higher methyl content is currently under investigation. Reference [1] L. Touahir, A. Cheriet, D. A. D. Corte, J.-N. Chazalviel, C. H. Villeneuve, F. Ozanam, I. Solomon, A. Keffous, N. Gabouze et M. Rosso, «Methylated silicon: A longer cycle-life material for Li-ion batteries,» Journal of Power Sources, vol. 240, pp. 551-557, 10 2013. [2] M. Salah, C. Hall, P. Murphy, C. Francis, R. Kerr, B. Stoehr, S. Rudd et M. Fabretto, «Doped and reactive silicon thin film anodes for lithium ion batteries: A review,» Journal of Power Sources, vol. 506, p. 230194, 9 2021. Figure 1

Fine-Tuning Intrinsic and Doped Hydrogenated Amorphous Silicon Thin-Film Anodes Deposited by PECVD to Enhance Capacity and Stability in Lithium-Ion Batteries.

Silicon is a promising alternative to graphite as an anode material in lithium-ion batteries, thanks to its high theoretical lithium storage capacity. Despite these high expectations, silicon anodes still face significant challenges, such as premature battery failure caused by huge volume changes during charge-discharge processes. To solve this drawback, using amorphous silicon as a thin film offers several advantages: its amorphous nature allows for better stress mitigation and it can be directly grown on current collectors for material savings and improved Li-ion diffusion. Furthermore, its conductivity is easily increased through doping during its growth. In this work, we focused on a comprehensive study of the influence of both electrical and structural properties of intrinsic and doped hydrogenated amorphous silicon (aSi:H) thin-film anodes on the specific capacity and stability of lithium-ion batteries. This study allows us to establish that hydrogen distribution in the aSi:H material plays a pivotal role in enhancing battery capacity and longevity, possibly masking the significance of the conductivity in the case of doped electrodes. Our findings show that we were able to achieve high initial specific capacities (3070 mAhg-1 at the 10th cycle), which can be retained at values higher than those of graphite for a significant number of cycles (>120 cycles), depending on the structural properties of the aSi:H films. To our knowledge, this is the first comprehensive study of the influence of these properties of thin films with different doping levels and hydrogen distributions on their optimization and use as anodes in lithium-ion batteries.

Progress in the application of silicon-based anode nanotechnology in lithium batteries

With the development of technology, graphite materials in traditional lithium batteries can no longer meet people’s needs due to their relatively low specific capacity, limited charging and discharging rates, and poor safety. Silicon has a very high theoretical specific capacity, far exceeding traditional graphite negative electrode materials, making silicon nanoparticles an ideal choice for improving the energy density of lithium-ion batteries. In this paper, we first introduce the silicon nanoparticle anode and its preparation methods: mechanical ball milling, and thermal cracking, and introduce the application of binders in it. Secondly, the silicon nanowire anode and the chemical deposition method for its preparation are introduced, and the high-performance silicon nanowire lithium battery of Amprius is introduced. Thirdly, the preparation of silicon thin film anode and two types of composite film was introduced. Finally, the three types of silicon nano anodes are summarized and prospected. This paper has reference significance for the future research of silicon-based lithium-ion batteries.

Pre-Lithiation of Silicon-Based Anode Materials: Concepts and Realization

The capacity in lithium ion battery (LIB) relies on active lithium, i.e., lithium-ions and electrons, which separately shuttle between the electrodes during charge and discharge. Given state-of-the art active materials for anodes, loss of active lithium and capacity remains a major challenge during LIB operation, in particular in the first charge/discharge cycle due to electrolyte decomposition and formation of solid electrolyte interphase (SEI).Pre-lithiation, i.e., artificial increase in active lithium amount into the battery is a promising strategy to counteract active lithium loss. Electrochemical(1) and chemical(2,3) methods of pre-lithiation can be distinguished. While electrochemical methods are based on the use of an external power source, the driving force of the chemical reaction is a negative change in the Gibb´s free energy which in the case of pre-lithiation is provided by the use of highly reactive chemical compounds (e.g. lithium-organic complex solutions or lithium metal). The use of lithium metal allows for a wide range of technological processes, including lamination of lithium foil, gas-phase deposition, application of lithium melts or suspensions containing lithium powder. In this contribution, various concepts of pre-lithiation will be discussed from experimental and up-scaling point of view on the example of Si-based anodes.(1) Haneke, L.; Pfeiffer, F.; Bärmann, P.; Wrogemann, J.; Peschel, C.; Neumann, J.; Kux, F.; Nowak, S.; Winter, M.; Placke, T., Insights into Electrolytic Pre‐Lithiation: A Thorough Analysis Using Silicon Thin Film Anodes. Small 2022, 2206092.(2) Adhitama, E.; Bela, M. M.; Demelash, F.; Stan, M. C.; Winter, M.; Gomez‐Martin, A.; Placke, T., On the Practical Applicability of the Li Metal‐Based Thermal Evaporation Prelithiation Technique on Si Anodes for Lithium Ion Batteries. Advanced Energy Materials 2023, 13 (3), 2203256.(3) Bärmann, P.; Mohrhardt, M.; Frerichs, J. E.; Helling, M.; Kolesnikov, A.; Klabunde, S.; Nowak, S.; Hansen, M. R.; Winter, M.; Placke, T., Mechanistic Insights into the Pre‐Lithiation of Silicon/Graphite Negative Electrodes in “Dry State” and After Electrolyte Addition Using Passivated Lithium Metal Powder. Advanced Energy Materials 2021, 11 (25), 2100925.

Preparation of pure hydrogenated amorphous silicon film via PECVD method as anode materials for high-performance lithium-ion batteries

In this paper, plasma-enhanced chemical vapor deposition (PECVD) is used to prepare a nano-scale pure hydrogenated amorphous silicon thin film anode, which improves the problem of large volume effect of traditional silicon-based anode materials during the charge and discharge process of lithium-ion batteries. Among them, the thinnest 120 nm pure silicon film still has a high specific capacity of 1254.6 mAh g−1 after 200 cycles at 0.5C (1C = 4.2 A g−1), and also has an excellent performance of 848.2 mAh g−1 after a large rate of charge and discharge of 5C. To give consideration to the performance and obtain a certain mass load at the same time, 170 nm pure silicon film was selected for vacuum annealing treatment at different temperatures to release the non-uniform compression stress during the deposition of hydrogenated amorphous silicon, and a more stable film structure was constructed, which further improved the performance of silicon film. Among them, the best performance is held at 100 °C for 5 h, and the reversible specific capacity of 1172.7 mAh g−1 is still maintained at 200 cycles at 0.5C, and the reversible specific capacity of 1131.6 mAh g−1 is also available at a super high rate of 10C.

Lifetime Optimization of Amorphous Silicon Thin-Film Anodes for Lithium-Ion Batteries

Lifetime Optimization of Amorphous Silicon Thin-Film Anodes for Lithium-Ion Batteries

Insights into Electrolytic Pre-Lithiation: A Thorough Analysis Using Silicon Thin Film Anodes.

Pre-lithiation via electrolysis, herein defined as electrolytic pre-lithiation, using cost-efficient electrolytes based on lithium chloride (LiCl), is successfully demonstrated as a proof-of-concept for enabling lithium-ion battery full-cells with high silicon content negative electrodes. An electrolyte for pre-lithiation based on γ-butyrolactone and LiCl is optimized using boron-containing additives (lithium bis(oxalato)borate, lithium difluoro(oxalate)borate) and CO2 with respect to the formation of a protective solid electrolyte interphase (SEI) on silicon thin films as model electrodes. Reversible lithiation in Si||Li metal cells is demonstrated with Coulombic efficiencies (CEff ) of 95-96% for optimized electrolytes comparable to 1 m LiPF6 /EC:EMC 3:7. Formation of an effective SEI is shown by cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). electrolytic pre-lithiation experiments show that notable amounts of the gaseous product Cl2 dissolve in the electrolyte leading to a self-discharge Cl2 /Cl- shuttle mechanism between the electrodes lowering pre-lithiation efficiency and causing current collector corrosion. However, no significant degradation of the Si active material and the SEI due to contact with elemental chlorine is found by SEM, impedance, and XPS. In NCM111||Si full-cells, the capacity retention in the 100th cycle can be significantly increased from 54% to 78% by electrolytic pre-lithiation, compared to reference cells without pre-lithiation of Si.

Revealing the Role, Mechanism, and Impact of AlF 3 Coatings on the Interphase of Silicon Thin Film Anodes (Adv. Energy Mater. 41/2022)

Solid Electrolyte Interphases In article number 2201859, Aurora Gomez-Martin, Tobias Placke, and co-workers discuss the role, mechanism, and impact of AlF3 coating on Si thin film anodes. The observations were done in different (de-)lithiation states. AlF3 transforms to Li-Al-F compounds with higher ionic conductivity, thus, increasing cycling stability. This study provides significant insights into the solid electrolyte interphase behavior with regard to the functional coating layer.

Revealing the Role, Mechanism, and Impact of AlF3 Coatings on the Interphase of Silicon Thin Film Anodes

AbstractSilicon (Si) holds great promise as an anode material for high energy density lithium ion batteries owing to its theoretical capacity of up to 3579 mAh g−1. However, this potential comes at the expense of major challenges, because the solid electrolyte interphase (SEI) at Si anodes hardly provides long‐term protection due to severe volume expansion. Yet, when it comes to the SEI, the formation mechanism is not thoroughly understood. Here, thin AlF3 coatings are deposited on Si thin film to stabilize the SEI. To evaluate the SEI, systematic observation utilizing X‐ray photoelectron spectroscopy is performed at different (de‐)lithiation states, allowing stage‐by‐stage analysis to reveal the role, mechanism, and impact of AlF3 coating. Results show that the capacity retention is significantly improved for 90% after 100 cycles. The transformation of AlF3 into Li‐Al‐F compounds, as confirmed by ion chromatography, is responsible for an enhanced performance due to its high ionic conductivity. Moreover, the SEI of coated Si thin films is rich in inorganic species (i.e., LiF) which is beneficial to prevent electrons to pass through. This work will deepen the understanding of SEI on Si anodes with respect to the coating approach, suggesting future directions to improve coating layers on Si.

Pre‐Lithiation of Silicon Anodes by Thermal Evaporation of Lithium for Boosting the Energy Density of Lithium Ion Cells (Adv. Funct. Mater. 22/2022)

Pre-Lithiation In article number 2201455, Aurora Gomez-Martin, Tobias Placke, and co-workers develop Li thermal evaporation as a novel pre-lithiation technique for silicon thin film anodes to achieve the most uniform Li distribution possible. In this study, the impact of addition of the electrolyte on the reaction mechanism is explored. The terms “dry-state” and “wet-state” pre-lithiation are revisited.

Effect of vinylene carbonate electrolyte additive and battery cycling protocol on the electrochemical and cyclability performance of silicon thin-film anodes

The use of silicon as a replacement for graphite, the commonly utilised anode material, would help increase the energy density of lithium-ion batteries, as it has a significant specific capacity of 4200 mAh/g compared to only 372 mAh/g for graphite. However, the high electronic resistivity and low mechanical stability of silicon have hindered its commercial uptake. In this contribution, we have employed a multifaceted approach in order to enhance the capacity retention of pure silicon anodes. The effect of adding 5 vol. % vinylene carbonates to a standard electrolyte has been examined with respect to the performance of physical vapour deposited pure Si thin films. Changes in battery cycling parameters (i.e., lower and upper cut-off voltages and depth of discharge were examined using charge/discharge cycling, cyclic voltammetry, and electrochemical impedance spectroscopy, indicating that cycling stability and electrochemical performance of the anodes were heavily influenced by these changes. The effect of each procedure (i.e., electrolyte additive and battery cycling protocol) on the initial discharge capacity, initial coulombic efficiency, initial irreversible capacity, capacity retention, and the lithium-ion diffusion coefficient into the silicon anode was examined. The Si film with optimised: deposition conditions, electrolyte additives, and battery testing protocol had a discharge capacity of 1740 mAh/g and capacity retention of 92 % at a charge/discharge rate of C/2 after 1000 cycles.

Quantifying lithium loss in amorphous silicon thin-film anodes via titration-gas chromatography

Silicon with a high theoretical capacity (3,579 mAh/g) is a promising anode candidate for lithium-ion batteries. However, commercialization is still impeded by low Coulombic efficiency, caused by solid electrolyte interphase (SEI) formation and trapped lithium (Li)-silicon (Si) alloy during repeated volume change. Quantifying capacity losses from each factor is crucial to formulate rational design strategies for further improvement. In this work, titration-gas chromatography and cryogenic transmission electron microscopy are applied to characterize the evolution of trapped Li-Si alloy and SEI growth in a silicon thin-film anode. It is found that continuous growth of the SEI is the dominant factor for lithium inventory loss during cycling, with only a marginal increase in trapped Li-Si alloy. This study offers a quantitative approach to differentiate Li in the SEI from trapped Li in Li-Si alloy through a silicon thin-film anode, providing unique insights into identifying critical bottlenecks for developing Si anodes. Li loss in silicon anode is a combination of SEI formation and trapped Li-Si alloy TGC quantifies the amount of trapped Li-Si alloy Cryo-TEM reveals the trapped Li-Si alloy intermix with SEI Growth of SEI is the dominant factor for capacity decay in Si thin-film anodes Silicon-based anodes for use in lithium-ion batteries are promising, but their fast degradation is not fully understood. Bao et al. apply gas titration chromatography for trapped Li-Si alloy quantification, revealing that interphase-consumed lithium is the primary reason for inferior cycle stability.

Quantification of aging mechanisms of carbon-coated and uncoated silicon thin film anodes in lithium metal and lithium ion cells

In this work, a comprehensive investigation of the effect of carbon-coating on the aging mechanism of silicon thin films in lithium metal and lithium ion cells is presented. In Li||Si cells with sufficient lithium excess, 92% of the total capacity loss of the silicon film was attributed to a loss of active material and 8% was attributed to an increased cell resistance. Carbon-coating reduces the loss of active material by improving the mechanical integrity of the silicon thin film, leading to a 67% reduction of capacity loss per cycle. In Si||LiFePO4 lithium ion cells, 86% of the total capacity loss was attributed to a loss of lithium inventory and 14% attributed to an increase in cell resistance. Furthermore, a loss of 69% of silicon active material was observed. Carbon-coating reduces the capacity loss per cycle by 28%. After aging of the lithium ion cells, the negative electrode of the carbon-coated silicon retained double the capacity compared to the uncoated silicon electrode. Hence, carbon-coating is an effective measure to improve mechanical stability of silicon thin film electrodes, but it must be coupled with additional strategies to reduce lithium consumption in order to increase the overall effectiveness of the coating in lithium ion cells.

Compressively Stressed Silicon Nanoclusters as an Antifracture Mechanism for High-Performance Lithium-Ion Battery Anodes

Silicon has been considered a good candidate for replacing the commonly used carbon anodes for lithium-ion batteries (LIBs) due to its high specific capacity, which can be up to 11 times higher than that of carbon. However, the desirable advantage that silicon brings to battery performance is currently overshadowed by its stress-induced performance loss and high electronic resistivity. The induced stress arises from two sources, namely, the deposition process (i.e., residual stress) during fabrication and the volume expansion (i.e., mechanical stress) associated with the lithiation/delithiation process. Of the two, residual stress has largely been ignored, underestimated, or considered to have a negligible effect without any rigorous evidence being put forward. In this contribution, we produced silicon thin films having a wide range of residual stress and resistivity using a physical vapor deposition technique, magnetron sputtering. Three pairs of silicon thin-film anodes were utilized to study the effect of residual stress on the electrochemical and cyclability performance as anodes for LIBs. Each set consisted of a pair of films having essentially the same resistivity, density, thickness, and oxidation amount but distinctly different residual stresses. The comparison was evaluated by conducting charge/discharge cycling and cyclic voltammetry (CV) experiments. In contrast to the fixed belief within the literature, higher compressive residual-stress films showed better electrochemical and cycle performance compared to lower residual-stress films. The results, herein, present an informed understanding of the role that residual stress plays, which will help researchers improve the development of silicon-based thin-film anodes.

Unraveling the Nanoscale Heterogeneity of Solid Electrolyte Interphase Using Tip-Enhanced Raman Spectroscopy

Summary We employ tip-enhanced Raman spectroscopy (TERS) to study model amorphous silicon (a-Si) thin film anodes galvanostatically cycled for different numbers. For the 1× cycled a-Si, TERS shows good correlation between solid electrolyte interphase (SEI) topography and chemical mapping, corresponding to distribution of lithium ethylene dicarbonate (LEDC) and poly (ethylene oxide) (PEO)-like oligomer species. Subsequent electrochemical cycling makes the SEI relatively thick and rough with the chemical composition heavily dominated by LEDC monomer-dimer for 5× cycled a-Si. For 20× cycled a-Si, the TERS signal is dominated by carboxylate (RCO2Li) compounds of various conformations and fluorinated species (LixPOyFz). A nanomosaic-multilayer hybrid SEI model on top of the a-Si anode is proposed. The significance of this work is applicable not only to silicon, where SEI plays a dominant role in determining the cycle life performance and reversibility, but also for a number of other relevant battery chemistries such as Na-ion and multivalent redox systems.

High performance sandwich structured Si thin film anodes with LiPON coating

The sandwich structured silicon thin film anodes with lithium phosphorus oxynitride (LiPON) coating are synthesized via the radio frequency magnetron sputtering method, whereas the thicknesses of both layers are in the nanometer range, i.e. between 50 and 200 nm. In this sandwich structure, the separator simultaneously functions as a flexible substrate, while the LiPON layer is regarded as a protective layer. This sandwich structure combines the advantages of flexible substrate, which can help silicon release the compressive stress, and the LiPON coating, which can provide a stable artificial solid-electrolyte interphase (SEI) film on the electrode. As a result, the silicon anodes are protected well, and the cells exhibit high reversible capacity, excellent cycling stability and good rate capability. All the results demonstrate that this sandwich structure can be a promising option for high performance Si thin film lithium ion batteries.

Investigation of the degradation mechanisms of silicon thin film anodes for lithium-ion batteries

Silicon is a promising material for negative electrodes in lithium-ion batteries. The degradation of silicon was investigated using thin films between 5 and 50 nm deposited on substrates of varying roughness. We used differential capacity plots to examine the degradation behavior of thin films during cycling. We found that the delamination of the thin films from the substrate depends on the surface roughness and film thickness. The delamination changes the morphology of the thin film and is characterized through the formation of Li15Si4. The morphology change is also accompanied by reduced coulombic efficiency and an increasing cell polarization. The transient occurrence of Li15Si4 is explained by the processes during cycling. It may be prevented by stress induced potential suppression which was observed in very thin films as the polarization increases with decreasing layer thickness.

A step towards understanding the beneficial influence of a LIPON-based artificial SEI on silicon thin film anodes in lithium-ion batteries.

In this work, we present a comprehensive study on the influence of lithium phosphorus oxynitride (LIPON) as a possible "artificial SEI layer" on the electrochemical performance of pure silicon (Si) thin film electrodes for a possible application in microbatteries or on-chip batteries. Si thin film anodes (140 nm) with and without an additional amorphous LIPON surface layer of different thicknesses (100-300 nm) were prepared by magnetron sputter deposition. The LIPON surface coating was characterized thoroughly by means of electrochemical impedance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. In situ electrochemical dilatometry and ex situ cross-section analysis of the electrodes after cycling could prove that the LIPON coating greatly diminishes the volume expansion of the Si electrode and, therefore, significantly improves the cycling stability and capacity retention. Furthermore, the LIPON coating remarkably reduces parasitic electrolyte decomposition reactions that originate from the Si volume expansion and contribute to the overall electrode volume expansion, as observed by the enhanced Coulombic efficiency over ongoing charge/discharge cycling. Overall, this article focuses on the preparation of optimized Si-based thin film electrodes in combination with LIPON solid electrolyte coatings for use in high-energy lithium ion batteries.

Kinetic Study of the Initial Lithiation of Amorphous Silicon Thin Film Anodes

The mechanisms and kinetics of lithiation and delithiation of amorphous silicon were investigated using potentiostatic techniques and thin films of different thickness, with a focus on the initial lithiation process that occurs in the first cycle. In potentiostatic tests, distinct kinks were observed in the current vs. time curves, and the time at which the kink occurred increased for thicker films. This behavior can be explained using a model in which a sharp interface between an amorphous LixSi phase and Li-saturated amorphous Si propagates through the film. Using this model, the rate-limiting process was determined to be diffusion of Li in the LixSi phase rather than reaction at the lithiation front. The Li diffusivity in the lithiated phase was determined to be in the 10−13 cm2/s range, independent of film thickness above 135 nm. The thin-film potentiostatic technique used in this study should prove useful in investigation of the mechanisms and rate parameters for other phase transitions that occur during lithiation of silicon and for kinetic studies of other electrode materials.