Silicon Thin Film Electrodes Research Articles

Operando Nanomechanical Mapping of Amorphous Silicon Thin Film Electrodes in All-Solid-State Lithium-Ion Battery Configuration during Electrochemical Lithiation and Delithiation.

An operando bimodal atomic force microscopy system was constructed to perform nanomechanical mapping of an amorphous Si thin film electrode deposited on a Li6.6La3Zr1.6Ta0.4O12 solid electrolyte sheet during electrochemical lithiation/delithiation. The evolution of Young's modulus maps of the Si electrode was successfully tracked as a function of apparent Li content x in lithium silicide (LixSi) simultaneously with real-time surface topography observation. At the initial stage of lithiation, the average modulus steeply decreased due to the generation of LixSi from intrinsic Si, followed by a moderate modulus reduction until the electrode capacity reached 3300 mAh g-1 (Li content x = 3.46). In the following delithiation, the gradual recovery of the average modulus of LixSi was observed up to 1467 mAh g-1 (Li content x = 1.54) at which delithiation stopped due to the significant volume change induced by phase transformation of LixSi.

(Invited) Lithium Battery Interfacial Engineering with Thin Conformal Polymer Coatings

Coupled chemical and mechanical phenomena at electrode interfaces largely govern the reversibility and long term stability of lithium ion and lithium metal batteries. A well-known example in lithium ion batteries is the formation of the electrically insulating, ionically conductive, continuous solid electrolyte interface (SEI) on graphitic anodes, which passivates the electrode against further electrochemical reduction of the electrolyte. Unfortunately, this stable passivation does not occur in Si and Li metal anodes. High voltage cathodes are also prone to detrimental interfacial processes, resulting in electrolyte oxidation, gas formation, and transition metal dissolution. In response, a myriad of coating technologies has been developed to mitigate and control these interfacial phenomena and enable next-generation electrodes. These technologies must provide thin, conformal coatings over the entire electrochemically active surface area of the electrode; the coatings must also possess the requisite mechanical properties to accommodate volume dilation due to lithium insertion/extraction.This presentation will describe the development of conformal, compliant polymer thin film coatings for lithium battery electrodes. The polymer thin films are prepared by initiated chemical vapor deposition (iCVD), which provides exquisite control over film composition and morphology by facilitating heterogeneous free-radical polymerization of vinyl monomers with thickness precision on the order of 1 nm. Poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) (pV4D4) was deposited onto silicon thin film electrodes using iCVD. 25 nm-thick pV4D4 films on Si electrodes improved initial coulombic efficiency by 12.9% and capacity retention over 100 cycles by 64.9% relative to untreated electrodes. PV4D4 coatings also improved rate capabilities, enabling higher lithiation capacity at all current densities. Post-cycling FTIR and XPS showed that pV4D4 inhibited electrolyte reduction and altered the SEI composition, with enrichment in LiF. In another example, thin conformal coatings of perfluorinated polymer were deposited onto battery-grade Cu current collectors to function as an artificial solid electrolyte interface. The effect of the film thickness on Li cycling reversibility was characterized. By galvanostatically cycling Li in an asymmetric Li||Cu cell using 1M LiFSI in a 1:1 (v/v) mixture of dioxolane:dimethoxyethane at 30°C, it was shown that the average coulombic efficiency averaged over 50 cycles increased from 98.36% for bare, untreated Cu to 99.08% for Cu coated with 25 nm of the fluoropolymer. Moreover, the 25 nm coating suppressed interfacial impedance growth on the current collector; the resistance of the coated Cu was 40% lower than the untreated Cu after 100 cycles. The mechanism by which the polymer coating enhances coulombic efficiency and suppresses side reactions will be discussed, along with discussion of the performance in practical anode-free lithium metal batteries.

Operando Nanomechanical Mapping of Silicon Thin Film Electrode during the First Lithiation and Delithiation Investigated By Bimodal Atomic Force Microscopy

Si is emerging as one of the most prospective anode materials for all-solid-state Li-ion batteries due to its extremely high theoretical capacity density of 4200 mAh g-1 and relatively negative potential for lithiation/delithiation. Towards its widespread use, controlling severe volume expansion/contraction during lithiation/delithiation is a challenge because it may cause cracking, fracture, and pulverization that negatively influence the cycle performance and rate capability of the cells [1]. Such mechanical event is induced by the change in the lattice structure due to the formation of LixSi and successive increasing/decreasing of Li content x in LixSi [2].Bimodal atomic force microscopy (AFM) offers quantitative mapping of nanoscale mechanical property, simultaneously with surface topography imaging [3]. Our group previously applied this technique to the cross-section of a composite electrode consist of active materials, binders, conductive additives, and deposited electrolyte solution and successfully demonstrated nanomechanical mapping [4]. Here, we develop an operando bimodal AFM system to track Young’s modulus changes of electrode materials in an all-solid-state battery configuration during the lithiation and delithiation reactions in real-time.A cell in a Cu/Si/LLZT/In/Li configuration was fabricated as follows; a 3 µm-thick Si thin-film and a 100 nm-thick Cu layer were sputter-deposited on one side of Li6.6La3Zr1.6Ta0.4O12 (LLZT) solid electrolyte, an In layer was sputter-deposited on the other side of LLZT, the Cu/Si/LLZT/In sample was milled with an Ar-ion beam to yield a flat and smooth cross-section suitable for AFM analysis, and then a 50 µm-thick Li metal foil was combined with the pre-coated In thin layer. Operando nanomechanical mapping was performed by mounting the cross-section-exposed cell in the custom-made sample holder coupled with a potentiostat for bias applications, in an Ar-filled glove box to prevent any atmospheric influences.Electrochemical lithiation and delithiation were carried out within a potential window of 0.01-1.20 V vs. Li/Li+. Electrochemical characterization indicated that the capacities of the Si electrode for the first lithiation and delithiation are 3300 and 1833 mAh g-1, respectively. These capacities are equal to the formation of Li3.46Si after the first lithiation of the pristine Si and Li1.54Si after the successive delithiation from Li3.46Si. Throughout the lithiation, the averaged Young’s modulus of Si electrode decreased due to the formation of LixSi and increase in the Li content x in LixSi. In the initial stage of lithiation, it drastically decreased because LixSi was formed by the lithiation of pristine Si. Then, it moderately decreased with increasing Li content x in LixSi. Unfortunately, the Young’s modulus maps were only recorded until the formation of Li1.54Si because the successive delithiation was not completed possibly due to the phase transformation of crystalline LixSi to amorphous LixSi [5], which may cause a significant mechanical strain inside the thin film. The measured Young’s modulus values were consistent with those obtained by first principle calculation [6]. Further details are given at the presentation.

First-principles analysis of electrochemical hydrogen storage behavior for hydrogenated amorphous silicon thin film in high-capacity proton battery

Silicon material electrodes as proton carriers for high-capacity proton battery have only been proposed for such a short period of time that their physicochemical properties and electrochemical hydrogen storage behavior during charge and discharge processes remain nearly uncharted territory. Herein, the hydrogenated amorphous silicon (a-Si:H) thin film electrodes are prepared by radio frequency sputtering followed by ex-situ hydrogenation. The electrochemical properties of a-Si:H electrodes are tested experimentally, and the electrochemical hydrogen storage behaviors of a-Si:H electrodes are analyzed by first-principles calculations. The results show that the hydrogenation process significantly increases the electrochemical capacity of the electrodes and reduces the band gap of the electrode structure. The electrode exhibits weak conductivity during the initial charging, but the instability of the electrode electronic structure during the later charging results in a slight fluctuation of the electrochemical charging process. The a-Si:H electrode have better electrochemical hydrogen storage/release reversibility than non-hydrogenated electrodes, but this reversibility is weakened by oxygen atoms covered on the electrode surface. The electrochemical hydrogen storage process is easier to accomplish than the electrochemical desorption process of hydrogen evolution reaction for the a-Si:H electrodes. The a-Si:H thin film electrode is more stable on the Ni(111) substrate surface and the good conductivity of the electrode/substrate interface provides convenient conditions for the free transport of electrons in the electrochemical charge/discharge processes. We believe that these results perfectly explain the microscopic mechanisms responsible for the electrode reaction and electrochemical behavior of a-Si:H electrodes in this type of proton battery, and have a certain reference value in understanding the physicochemical properties and electrochemical hydrogen storage behavior of silicon material electrodes applied to other types of batteries during charge/discharge processes.

The lithiation onset of amorphous silicon thin-film electrodes

The lithiation onset of amorphous silicon (a-silicon) films up to 10% state of charge (SOC) is characterized by a Li+-uptake region around 0.5 V vs a Li reference electrode. In the literature, this is commonly attributed to surface processes such as the formation of a solid electrolyte interphase layer and/or the reduction of the surface native oxide, and more seldom to bulk processes such as reduction of oxygen contaminations inside the silicon film and to silicon lithiation. This work presents evidence that this process is associated with the lithiation of elemental silicon using electrochemical and non-electrochemical measurements and including a discussion of literature data. Cyclic voltammetry performed on pre-lithiated a-silicon and a-silicon films with different oxygen concentrations and with different film thicknesses reveals a lithiation process that is not associated with the reduction of oxygen. Elemental depth-profiling with secondary ion mass spectrometry (SIMS) reveals a two-phase lithiation mechanism at the lithiation onset, which sharply delimits non-lithiated silicon from a Li-poor phase of constant Li content (Li0.3Si). The published operando neutron reflectometry data suggest that this is also the case for oxygen-free Si single crystal wafers. SIMS measurements further show that the whole a-silicon thin film is converted into the Li-poor phase. This phase remains stable in the vicinity of the current collector even at 100% SOC, indicating that mechanical stress may play an important role.

Lithiation/delithiation of a Silicon Thin Film Electrode for All-Solid-State Batteries Using Operando X-ray Photoelectron Spectroscopy Apparatus

Lithiation/delithiation of a Silicon Thin Film Electrode for All-Solid-State Batteries Using Operando X-ray Photoelectron Spectroscopy Apparatus

Al2O3 protective coating on silicon thin film electrodes and its effect on the aging mechanisms of lithium metal and lithium ion cells

In this work, an investigation of the effect of Al2O3-coating on the aging mechanisms of silicon anode thin films in lithium metal and lithium ion cells is presented. Aging mechanisms, namely: loss of lithium inventory, loss of silicon active material and loss of utilizable capacity due to an increase of cell resistance were determined for both, Li||Si and Si||LiFePO4 cells. Al2O3-coating was shown to be an effective strategy to reduce the loss of lithium inventory, while having a marginal effect on decreasing the loss of silicon active material. Indeed, in case of Si||LiFePO4 cells, where fading is governed by loss of lithium inventory, a 5 nm Al2O3-coating leads to a significant reduction (-64%) of the capacity fade per cycle. On the contrary, in case of Li||Si, where the aging mechanism is governed by the loss of active material, Al2O3-coated and uncoated silicon showed comparable tendencies regarding the capacity fade per cycle. It emerges, also, that loss of silicon active material and loss of lithium inventory are independent of each other. This indicates that the main contribution of loss of lithium inventory is not the lithium trapped in electrically insulated silicon, but rather lithium consumed in the ongoing SEI formation. Al2O3-coating could reduce the latter due the insulating nature of the coating. Ex situ investigations of the SEI by means of X-ray photoelectron spectroscopy confirmed a decrease in solvent decomposition in presence of the Al2O3-coating.

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.

(Invited) Oxide-Encapsulated (Photo)Electrocatalysts for Solar Fuels Production

Encapsulating active electrocatalysts with permeable oxide overlayers offers additional “control knobs” for tuning electrocatalyst selectivity and/or activity in ways that are not possible with conventional electrocatalysts.[1] In particular, nanoscopic oxide overlayers can possess selective transport characteristics that allow them to behave as ultrathin membranes that can block impurities and undesirable reactants while still permitting desired reactants and products to pass between the bulk electrolyte and active sites at the overlayer/catalyst buried interface.[1,2] In this presentation, I will describe recent investigations of silicon and titanium oxide-encapsulated thin film electrodes that allow for quantification of species permeabilities and to establish general design principles that govern the structure-property-performance relationships for nanoscopic oxide overlayers. First, I will show how the porosity and chemical composition of silicon oxide overlayers can be systematically tuned to control the relative permeabilities of protons and molecular oxygen, both of which are of great importance for water splitting.[3] Next, the selective transport properties of these coatings are reported in the presence of redox mediator species that are of interest for photocatalytic H2 production from Z-scheme water splitting.[4] Both silicon and titanium oxide overlayers are shown to be capable of selectively facilitating the desirable oxygen and hydrogen evolution reactions with selectivities of > 80% in the presence of a reversible redox mediator. Importantly, this work highlights the ability of ultrathin oxide overlayers to dramatically alter reaction selectivities, and presents design rules for overlayers that can be extended to many other (photo)electrocatalytic materials and reactions of interest to the solar fuels community.

Electrochemical Reactivity and Passivation of Silicon Thin-Film Electrodes in Organic Carbonate Electrolytes.

This work focuses on the mechanisms of interfacial processes at the surface of amorphous silicon thin-film electrodes in organic carbonate electrolytes to unveil the origins of the inherent nonpassivating behavior of silicon anodes in Li-ion batteries. Attenuated total reflection Fourier-transform infrared spectroscopy, X-ray absorption spectroscopy, and infrared near-field scanning optical microscopy were used to investigate the formation, evolution, and chemical composition of the surface layer formed on Si upon cycling. We found that the chemical composition and thickness of the solid/electrolyte interphase (SEI) layer continuously change during the charging/discharging cycles. This SEI layer "breathing" effect is directly related to the formation of lithium ethylene dicarbonate (LiEDC) and LiPF6 salt decomposition products during silicon lithiation and their subsequent disappearance upon delithiation. The detected appearance and disappearance of LiEDC and LiPF6 decomposition compounds in the SEI layer are directly linked with the observed interfacial instability and poor passivating behavior of the silicon anode.

In Situ Observation of Lithiation and Delithiation Reactions of a Silicon Thin Film Electrode for All-Solid-State Lithium-Ion Batteries by X-ray Photoelectron Spectroscopy.

In situ X-ray photoelectron spectroscopy is applied to electrochemical lithiation/delithiation processes of an amorphous Si electrode sputter-deposited on a Li6.6La3Zr1.6Ta0.4O12 solid electrolyte. After the first lithiation, a broad Li peak appears at the Si surface, and peaks corresponding to bulk Si and Si suboxide significantly shift to lower binding energy. The appearance of the Li peak and shift of the Si peaks confirm the formation of lithium-silicide and lithium-silicates due to the lithiation of Si and native suboxide. The composition of lithium-silicide is estimated to be Li3.44Si by quantitative analysis of electrochemical response and photoelectron spectra. Peak fitting analysis shows the formation of Li2O and Li2CO3 due to side reactions. Upon the following delithiation, the peak corresponding to Li3.44Si phase shifts back to higher binding energy to form Li0.15Si phase, while lithium-silicates, Li2O, and Li2CO3 remained as irreversible species. Thus, electrochemical reactions accompanied with lithiation/delithiation processes are successfully observed.

An in situ system for simultaneous stress measurement and optical observation of silicon thin film electrodes

Here an in situ system is presented to simultaneously study the evolution of both morphology and stress in the silicon thin film electrode during lithiation and delithiation. Owing to the specific design with two observation windows in the in situ cell, both the curvature and color of the silicon thin-film electrodes upon lithiation and delithiation processes can be measured by multi-optical sensor and optical microscope. By such colorimetric method, the color evolution can be used to represent the thickness of silicon thin film electrode, and the quantitative relationship can be obtained by in situ atomic force microscope and optical microscopy experiments. Combining the real thickness with Stoney equation, the accurate stress of the LixSi film can be obtained during the electrochemical cycles.

Ultrathin conformal polycyclosiloxane films to improve silicon cycling stability.

Electrochemical reduction of lithium ion battery electrolyte on Si anodes was mitigated by synthesizing nanoscale, conformal polymer films as artificial solid electrolyte interface (SEI) layers. Initiated chemical vapor deposition (iCVD) was used to deposit poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) (pV4D4) onto silicon thin film electrodes. pV4D4 films (25 nm) on Si electrodes improved initial coulombic efficiency by 12.9% and capacity retention over 100 cycles by 64.9% relative to untreated electrodes. pV4D4 coatings improved rate capabilities, enabling higher lithiation capacity at all current densities. Impedance spectroscopy showed that SEI resistance grew from 50 to 191 ohms in untreated Si and only 34 to 90 ohms in pV4D4-coated Si over 30 cycles. Post-cycling Fourier transform infrared and x-ray photoelectron spectroscopy showed that pV4D4 moderated electrolyte reduction and altered SEI composition, with LiF formation being favored. This work will guide further development of polymeric artificial SEIs to mitigate electrolyte reduction and enhance capacity retention in Si electrodes.

SEI Formation Vs. Lithium Intercalation: In-Situ AFM Experiments on Graphite and Silicon Electrodes

The formation of the solid-electrolyte interphase (SEI) passivation layer on anode materials in lithium ion batteries (LIB) is not yet fully understood at the micro- and nanometer scale. Therefore, experiments under realistic conditions with high spatial resolution are of primary importance. Hence, atomic force microscopy (AFM) is an ideal technique to study in-situ the mechanisms at the electrode-electrolyte interface down to the (sub-) nanometer level. Within this contribution we present a combined AFM and electrochemical (de-) lithiation study of the electrode-interface for two types of anode materials. Highly oriented pyrolytic graphite (HOPG) as a model system has been investigated in order to observe the topographical and electrochemical properties simultaneously. We find a clear influence of defect structures on the SEI formation process. Furthermore, the structural as well as the mechanical behavior of silicon anodes during (de-) lithiation will be presented. Sputtered silicon thin film electrodes [1] have been used to avoid strain and crack formation [2] on the silicon electrode during lithiation. We will introduce AFM measurements that enable a differentiation between SEI formation and expansion of the silicon electrode upon lithiation. We observed significant variations in the elastic modulus during the (de-) lithiation processes on the silicon electrode that can be directly attributed to the formation of an SEI layer as well as the expansion of the electrode due to the lithium insertion. Additionally, the analysis of the surface roughness will be discussed with respect to (ir-) reversible processes on the electrode surface.

An Approach to Characterize and Clarify Hysteresis Phenomena of Lithium-Silicon Electrodes

We clarify the hysteretic behavior observed in lithiated silicon electrodes through the development and implementation of a model that describes the main features of the observed voltage hysteresis. Specifically, slow-scan voltammetry at 0.01 mV/s is used to study hysteresis in lithiated silicon thin-film electrodes. At higher scan rates, the curves U(x), where U is voltage and x is state of charge, depend on scan rate, but no difference is seen in the curves when scanning at slow enough rates. For example, no difference is seen at 0.01 mV/s or at 0.005 mV/s, even though the same hysteretic behavior is observed for both scan rates; i.e., the lithiation curve differs significantly from the delithiation curve. The rate of 0.005 mV/s is the slowest that could be measured with state-of-the-art equipment. However, when a lithiating scan at 0.01 mV/s is suddenly set to open circuit conditions, the voltage slowly decays upwards, whereas voltage decays downwards when delithiation is interrupted. From this it is inferred that the rate invariance seen at 0.005 and 0.01 mV/s would have to break down at some lower scan rate, if it could be measured. A semi-empirical model is used to describe many aspects of this behavior based on the assumption of a region of rate invariance, which breaks down at higher scan rates due to transport and kinetic losses, but which also breaks down at slower scan rates due to transients that are observable but remain unexplained.

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.

An approach to characterize and clarify hysteresis phenomena of lithium-silicon electrodes

We clarify the hysteretic behavior observed in lithiated silicon electrodes through the development and implementation of a model that describes the main features of the observed voltage hysteresis. Specifically, slow-scan voltammetry at 0.01 mV/s is used to study hysteresis in lithiated silicon thin-film electrodes. At higher scan rates, the curves U(x), where U is voltage and x is state of charge, depend on the scan rate, but no difference is seen in the curves when scanning at slow enough rates. For example, no difference is seen at 0.01 mV/s or at 0.005 mV/s, even though the same hysteretic behavior is observed for both scan rates, i.e., the lithiation curve differs significantly from the delithiation curve. The rate of 0.005 mV/s is the slowest that could be measured with state-of-the-art equipment. However, when a lithiating scan at 0.01 mV/s is suddenly set to open circuit conditions, the voltage slowly decays upwards, whereas the voltage decays downwards when delithiation is interrupted. From this, it is inferred that the rate invariance seen at 0.005 and 0.01 mV/s would have to break down at some lower scan rate, if it could be measured. A semi-empirical model is used to describe many aspects of this behavior based on the assumption of a region of rate invariance, which breaks down at higher scan rates due to transport and kinetic losses, but which also breaks down at slower scan rates due to transients that are observable but remain unexplained.

Irreversible lithium storage during lithiation of amorphous silicon thin film electrodes studied by in-situ neutron reflectometry

Abstract Amorphous silicon is a promising high-capacity anode material for application in lithium-ion batteries. However, a huge drawback of the material is that the large capacity losses taking place during cycling lead to an unstable performance. In this study we investigate the capacity losses occurring during galvanostatic lithiation of amorphous silicon thin film electrodes by in-situ neutron reflectometry experiments for the first ten cycles. As determined from the analysis of the neutron scattering length density and of the film thickness, the capacity losses are due to irreversible storage of lithium in the electrode. The amount of stored lithium increases during cycling to 20% of the maximum theoretical capacity after the 10th cycle. Possible explanations are discussed.

A Step toward High-Energy Silicon-Based Thin Film Lithium Ion Batteries.

The next generation of lithium ion batteries (LIBs) with increased energy density for large-scale applications, such as electric mobility, and also for small electronic devices, such as microbatteries and on-chip batteries, requires advanced electrode active materials with enhanced specific and volumetric capacities. In this regard, silicon as anode material has attracted much attention due to its high specific capacity. However, the enormous volume changes during lithiation/delithiation are still a main obstacle avoiding the broad commercial use of Si-based electrodes. In this work, Si-based thin film electrodes, prepared by magnetron sputtering, are studied. Herein, we present a sophisticated surface design and electrode structure modification by amorphous carbon layers to increase the mechanical integrity and, thus, the electrochemical performance. Therefore, the influence of amorphous C thin film layers, either deposited on top (C/Si) or incorporated between the amorphous Si thin film layers (Si/C/Si), was characterized according to their physical and electrochemical properties. The thin film electrodes were thoroughly studied by means of electrochemical impedance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. We can show that the silicon thin film electrodes with an amorphous C layer showed a remarkably improved electrochemical performance in terms of capacity retention and Coulombic efficiency. The C layer is able to mitigate the mechanical stress during lithiation of the Si thin film by buffering the volume changes and to reduce the loss of active lithium during solid electrolyte interphase formation and cycling.

Mechanical measurements on lithium phosphorous oxynitride coated silicon thin film electrodes for lithium-ion batteries during lithiation and delithiation

The development of large stresses during lithiation and delithiation drives mechanical and chemical degradation processes (cracking and electrolyte decomposition) in thin film silicon anodes that complicate the study of normal electrochemical and mechanical processes. To reduce these effects, lithium phosphorous oxynitride (LiPON) coatings were applied to silicon thin film electrodes. Applying a LiPON coating has two purposes. First, the coating acts as a stable artificial solid electrolyte interphase. Second, it limits mechanical degradation by retaining the electrode's planar morphology during cycling. The development of stress in LiPON-coated electrodes was monitored using substrate curvature measurements. LiPON-coated electrodes displayed highly reproducible cycle-to-cycle behavior, unlike uncoated electrodes which had poorer coulombic efficiency and exhibited a continual loss in stress magnitude with continued cycling due to film fracture. The improved mechanical stability of the coated silicon electrodes allowed for a better investigation of rate effects and variations of mechanical properties during electrochemical cycling.