Si Thin Film Electrodes Research Articles

Electrophoretic Deposition as a Versatile Low-Cost Tool to Construct a Synthetic Polymeric Solid-Electrolyte Interphase on Silicon Anodes: A Model System Investigation.

The cycling of next-generation, high-capacity silicon (Si) anodes capable of 3579 mAh·g-1 is greatly hindered by the instability of the solid-electrolyte interphase (SEI). The large volume changes of Si during (de)lithiation cause continuous cracking of the SEI and its reconstruction, leading to loss of lithium inventory and extensive consumption of electrolyte. The SEI formed in situ during cell cycling is mostly composed of molecular fragments and oligomers, the structure of which is difficult to tailor. In contrast, ex situ formation of a synthetic SEI provides greater flexibility to deposit long-chain, polymeric, and elastomeric components potentially capable of maintaining integrity against the large ∼350% volume expansion of Si while also enabling electronic passivation of the surface for longer cycling and calendar life. Furthermore, polymers are amenable to structural modifications, and the desired elasticity can be targeted by selection of the SEI polymer feedstock. Herein, electrophoretic deposition (EPD) is used to apply chitosan as a synthetic SEI on model Si thin film electrodes. Comparison of synthetic SEIs obtained without (Si/Chit) and with CH3COOLi (Si/Chit+CH3COOLi) added during EPD is performed to demonstrate a facile route to tuning of the polymer SEI chemistry. Atomic force and scanning electron microscopy reveal that addition of CH3COOLi at EPD assists in conformal deposition of the synthetic SEI. During electrochemical cycling, the Chit+CH3COOLi coating nearly doubles the capacity retention versus the reference bare Si thin film. X-ray photoelectron and Fourier transform infrared spectroscopy reveal that CH3COOLi caps the -NH2 groups of chitosan through amidation during EPD, which suppresses the catalytic reduction of the electrolyte. The presented approach demonstrates and validates EPD as a low-capital route to achieving and chemistry-tuning synthetic SEIs on Si electrodes. More broadly, the method is a promising avenue toward controlled and tailored polymeric SEIs on various conversion-type electrodes with high particle volumetric expansion.

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.

The Effect of the SEI Layer Mechanical Deformation on the Passivity of a Si Anode in Organic Carbonate Electrolytes

The solid electrolyte interphase (SEI) on a Si negative electrode in carbonate-based organic electrolytes shows intrinsically poor passivating behavior, giving rise to unsatisfactory calendar life of Li-ion batteries. Moreover, mechanical strains induced in the SEI due to large volume changes of Si during charge-discharge cycling could contribute to its mechanical instability and poor passivating behavior. This study elucidates the influence that static mechanical deformation of the SEI has on the rate of unwanted parasitic reactions at the Si/electrolyte interface as a function of electrode potential. The experimental approach involves the utilization of Si thin-film electrodes on substrates with disparate elastic moduli, which either permit or suppress the SEI deformation in response to Si volume changes upon charging-discharging. We find that static mechanical stretching and deformation of the SEI results in an increased parasitic electrolyte reduction current on Si. Furthermore, attenuated total reflection and near-field Fourier-transform infrared nanospectroscopy reveal that the static mechanical stretching and deformation of the SEI fosters a selective transport of linear carbonate solvent through, and nanoconfinement within, the SEI. These, in turn, promote selective solvent reduction and continuous electrolyte decomposition on Si electrodes, reducing the calendar life of Si anode-based Li-ion batteries. Finally, possible correlations between the structure and chemical composition of the SEI layer and its mechanical and chemical resilience under prolonged mechanical deformation are discussed in detail.

Solid Electrolyte Interphase Layer Formation on the Si-Based Electrodes with and without Binder Studied by XPS and ToF-SIMS Analysis

The formation and evolution of the solid electrolyte interphase (SEI) layer as a function of electrolyte and electrolyte additives has been extensively studied on simple and model pure Si thin film or Si nanowire electrodes inversely to complex composite Si-based electrodes with binders and/or conductive carbon. It has been recently demonstrated that a binder-free Si@C-network electrode had superior electrochemical properties to the Si electrode with a xanthan gum binder (Si-XG-AB), which can be principally related to a reductive decomposition of electrolytes and formation of an SEI layer. Thus, here, the Si@C-network and Si-XG-AB electrodes have been used to elucidate the mechanism of SEI formation and evolution on Si-based electrodes with and without binder induced by lithiation and delithiation applying surface analytical techniques. The X-ray photoelectron spectroscopy and time-of-flight ion mass spectrometry results demonstrate that the SEI layer formed on the surface of the Si-XG-AB electrode during the discharge partially decomposes during the subsequent charging process, which results in a less stable SEI layer. Contrarily, on the surface of the Si@C-network electrode, the SEI shows less significant decomposition during the cycle, demonstrating its stability. For the Si@C-network electrode, initially, the inorganic and organic species are formed on the surface of the carbon shell and the silicon surface, respectively. These two parts of species in the SEI layer gradually grow and then fuse when the electrode is fully discharged. The behavior of the SEI layer on both electrodes corroborates with the electrochemical results.

Mechanical stress and morphology evolution in RuO2 thin film electrodes during lithiation and delithiation

RuO2 is a promising candidate for high-performance electrodes in thin film lithium-ion batteries, due to its large volumetric charge capacity, high rate capability, excellent cyclability and compatibility with silicon integrated circuit fabrication techniques. The evolution of mechanical stress associated with the lithiation and delithiation of RuO2 was investigated using in situ stress measurements, and the behavior was found to be fundamentally different from that of previously studied Si and Ge thin film electrodes. Most significantly, the tensile stress during delithiation of RuO2 was found to be very small, especially after the first cycle. The differences can be ascribed to the reversibility of SEI formation on RuO2, which does not occur in the case of Si and Ge. The decomposition of the SEI layer during delithiation allows freer interfacial sliding compared to Si or Ge, which accommodates the stress induced by significant volume changes during cycling. This might explain the relatively better cyclability of RuO2 films compared to Si or Ge films.

Electrochemical Lithiation and Delithiation in Amorphous Si Thin Film Electrodes Studied by Operando X-ray Photoelectron Spectroscopy.

The electrochemical lithiation/delithiation in amorphous Si thin film electrodes deposited on a L6.6La3Zr1.6Ta0.4O12 are dynamically analyzed by operando X-ray photoelectron spectroscopy. In the initial lithiation, the Si 2p peak corresponding to bulk Si0 significantly shifts to a lower binding energy due to the formation of LixSi and then monotonically with increasing capacity, i.e., the Li content x in LixSi. When the lithiation stops at capacity of 2200 mAh g-1 (x = ∼2.3), the peak recovers monotonically to a higher binding energy throughout the successive delithiation. When Li is inserted into LixSi up to 3400 mAh g-1 (x = 3.5), however, the peak drastically shifts in the capacity range of 1520-1920 mAh g-1 (x = 1.6-2.0) in the successive delithiation. This shift is attributed to the phase transition of crystalline Li15Si4 formed in the preceding lithiation to the amorphous phase. The mechanism of initial lithiation/delithiation at each step is summarized on the basis of the state of charge, Li content x in LixSi, and positions of XPS peaks.

Deposition and characterization of silicon thin film on stainless steel by electron beam evaporation

Silicon (Si) is a promising material as anode electrode because of its high theoretical capacity of 3579 mAh g−1 and low electrode potential (< 0.4 V vs. Li/Li+). In this paper, the Si thin film electrodes were fabricated by using electron beam evaporation on the stainless steel (SS) foil at room temperature. The structural and electrochemical characterization were investigated for the Si thin film electrodes before and after annealing at 700 ˚C. X-ray photoelectron spectroscopy analysis revealed that the annealed film layer consisted of SiOx containing about 30 at.% oxygen. The annealed Si thin film electrode resulted in improved electrochemical performance compared to as-deposited Si thin film electrode. The Si thin film electrode after annealing exhibited an initial discharge capacity of 2270 mAh g−1, and sustained excellent cycle stability with retention capacity about 95 % for 100 cycles at 0.5 C-rate. The improvement in cyclability for the annealed Si thin film electrode can be attributed to the enhanced adhesion of Si thin film to SS substrate through interdiffuion at the Si/SS interface owing to the post-annealing process.

Instrumentation for tracking electrochemical reactions by x-ray photoelectron spectroscopy under conventional vacuum conditions

A bias application system for tracking electrochemical reactions by x-ray photoelectron spectroscopy (XPS) is constructed using a laboratory-based apparatus. A specialized sample holder and stage with three electrical terminals provide contacts to a potentio-galvanostat on the outside, allowing bias application to an electrochemical device in the analysis chamber under vacuum conditions. The application of a direct current bias voltage to the sample holder is confirmed using metal Au samples. The Au 4f peaks from the Au grounded to the analyzer are identical, while those from the Au insulated from the analyzer shift due to the applied bias voltage. Furthermore, galvanostatic lithiation/delithiation processes and in situ XPS measurements are performed on a Si thin-film electrode after transferring the sample into the XPS apparatus without exposure to open air using a newly developed transfer vessel. After the initial lithiation process, a Li 1s peak containing a lithium-silicide component appears, and the Si 2p peaks shift to a lower binding energy due to the lithiation of the Si electrode. After the subsequent delithiation process, the Si 2p peak from the lithium silicide partially shifts back to a higher binding energy. As a result, in situ XPS measurements of the lithiation/delithiation reactions are successfully performed using the developed system.

Mg-Coated Si Thin Film As Model Electrode for Mechanistic Study of Li-Mg-Si Zintl Phase Formation in Lithium-Ion Batteries

Silicon (Si) is a promising anode material for high energy density lithium-ion batteries (LIBs) but its poor cycling performance prevents its large-scale adoption. Introducing Mg salt into the electrolyte has shown to form a ternary Li-Mg-Si Zintl phase upon lithiation of Si and improve the cycling stability; however, its formation mechanism and impacts on the solid electrolyte interphase (SEI) are not yet well understood. Herein, we demonstrate the formation of a ternary Li-Mg-Si Zintl phase via a Magnesium (Mg) coated Si thin film electrode, where Mg diffuses into the Si film upon deposition and in the lithiation process. The Zintl phase alters the nature of SEI, suppresses the excess decomposition of electrolyte and improves the capacity retention of the Si anode. This study provides insights into the formation mechanism of ternary Zintl phase and guidelines for the future design of Si anodes.

Completing the Picture of the Solid Electrolyte Interphase

Although solid electrolyte interphase (SEI) formation is an important factor in battery performance, difficulties in characterizing the SEI mean that it is still not well understood. In this issue of Joule, Nanda and coworkers utilize tip-enhanced Raman spectroscopy to paint a more complete picture of the SEI on silicon electrodes.

Polyimide capping layer on improving electrochemical stability of silicon thin-film for Li-ion batteries

Silicon (Si) is a promising anode material for LIBs due to its high theoretical gravimetric capacity of 3570 mAh g−1, which is about ten times higher than that of graphite. However, the large volume expansion of Si during lithiation gives rise to non-trivial mechanical challenges and constraints which hinder widespread commercial adoption. In particular, electrode cracking and delamination destroy the intrinsic electrical connectivity in the Si electrodes, leading to rapid capacity degradation upon cycling. Herein, we demonstrate a novel approach of capping Si thin-film electrodes with a high strength polyimide by electro-spraying to overcome the pulverization and mechanical issues during cycling. The polyimide layer provides mechanical support to the Si thin-film and maintains its capacity during cycling, with a high capacity of 2610 mAh g−1 at 100 mA g−1 and no capacity loss after 300 cycles at 3500 mA g−1. Furthermore, in situ dilatometry tests reveal that the polyimide capping layer enables reversible electrode thickness change during lithiation and de-lithiation, thus improving the cycling performance. This work provides a practical manufacturing strategy for commercializing silicon-based electrode films.

Carbon Dioxide As Electrolyte Additive for Silicon-Based Lithium Ion Full Cells

Following up the great success of lithium ion batteries (LIBs) in the field of consumer electronics in the recent decades, LIBs are today also widely used as energy storage devices for various types of electric vehicles (EVs). However, it is believed that a substantial breakthrough of the electromobility can only take place, if EVs are able to achieve driving ranges of more than 500 km and, therefore, gaining greater consumer acceptance. To achieve these driving ranges, the energy content of the used LIBs needs to be increased to values of 350 Wh/kg and 750 Wh/L at the cell level. These improvements can be achieved by replacing the state-of-the-art graphite anode by high-capacity anode materials like silicon (Si), which is able to provide a nearly 10 times higher specific capacity compared to graphite.[1] However, due to huge volume changes of up to 300% upon lithiation/de-lithiation, Si-based anodes often suffer from poor cycling performance related to cracking and pulverization of the Si particles and continuous solid electrolyte interphase (SEI) re-formation. Typically, these issues are addressed by decreasing the size of the active material particles to the nanoscale (i.e. nanoparticles, nanowires or thin films) or the fabrication of composites like intermetallic Si phases or Si/carbon composite materials to decrease the overall volume changes.[2] Although these strategies are reported to be effective to improve the performance of Si electrodes, it was revealed that the dominating failure mechanism of full cells containing Si-based negative electrodes lies in the consumption active lithium from the cathode related to continuous electrolyte reduction at the Si negative electrode. The addition of appropriate electrolyte additives such as fluoroethylene carbonate (FEC) or vinylene carbonate (VC) can significantly improve the capacity retention and Coulombic efficiency of Si-based full cells by forming a more stable SEI on the Si negative electrode.[3, 4] Recently, Krause et al. demonstrated that the addition of carbon dioxide in the form of dry ice significantly improved the performance of Si-based full cells and even outperformed the most commonly used additive FEC.[5] However, the exact working mechanism of the carbon dioxide additive and especially the reductive decomposition reactions including the resulting morphology and/or chemical composition of the formed SEI layer are still uncertain. Within this study, thin film Si electrodes prepared via magnetron sputtering are employed as model electrodes (i.e. no binder or conductive additive) therefore, possible effects of the carbon dioxide can be correlated to the Si active material. These Si thin film electrodes are coupled with NMC-111 positive electrodes to Si/NMC-111 full cells, which are used for the electrochemical investigations. As the solubility of carbon dioxide in the organic carbonate based electrolytes is rather low, diethylpyrocarbonate is added to the baseline electrolyte, as pyrocarbonates are known as carbon dioxide generators and, therefore, the carbon dioxide is released in-situ within the assembled full cell. The addition of the diethylpyrocarbonate electrolyte additive leads to significantly improved performance of the Si/NMC-111 full cells in terms of Coulombic efficiency and capacity retention full cells during prolonged cycling compared to the baseline electrolyte. The chemical composition of the SEI layer formed on the Si negative electrodes is investigated by means of X-ray photoelectron spectroscopy (XPS). The XPS investigations reveal that the improved performance of the carbon dioxide containing cells is related the improved passivation of the negative electrode by the formation of mainly lithium carbonate at the Si surface. Reference s : [1] D. Andre, H. Hain, P. Lamp, F. Maglia, B. Stiaszny, Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A, 5 (2017) 17174-17198. [2] M.N. Obrovac, V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews, 114 (2014) 11444-11502. [3] R. Nölle, A.J. Achazi, P. Kaghazchi, M. Winter, T. Placke, Pentafluorophenyl Isocyanate as an Effective Electrolyte Additive for Improved Performance of Silicon-Based Lithium-Ion Full Cells, ACS Applied Materials & Interfaces, 10 (2018) 28187-28198. [4] M. Klett, J.A. Gilbert, S.E. Trask, B.J. Polzin, A.N. Jansen, D.W. Dees, D.P. Abraham, Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li1.03(Ni0.5Co0.2Mn0.3)0.97O2/Silicon-Graphite Full Cells, Journal of The Electrochemical Society, 163 (2016) A875-A887. [5] L.J. Krause, V.L. Chevrier, L.D. Jensen, T. Brandt, The Effect of Carbon Dioxide on the Cycle Life and Electrolyte Stability of Li-Ion Full Cells Containing Silicon Alloy, Journal of The Electrochemical Society, 164 (2017) A2527-A2533.

Utilizing Room Temperature Liquid Metals for Mechanically Robust Silicon Anodes in Lithium‐Ion Batteries

AbstractSilicon has attracted attention as one of the most promising anode materials for future generation energy storage devices, owing to its high theoretical specific capacity. However, its use has been impeded by significant volume expansion during electrochemical lithiation. In this work we utilise liquid galinstan (68.5 % Ga, 21.5 % In and 10 % Sn) to repair the cracking caused by volume expansion in Si thin film and Si composite electrodes. A bridging mechanism is observed, through which the electrical conductivity can be significantly improved. The composite electrode delivers an initial capacity of 2697 mAh g−1 and reaches a coulombic efficiency ∼100 % after 140 cycles. In addition, nanoindentation tests show that a better mechanical behavior is achieved with increased flexibility. We believe that these results may provide a way to prepare mechanically robust high capacity electrodes.

Pentafluorophenyl Isocyanate As Effective Electrolyte Additive for Improved Performance of Silicon-Based Lithium Ion Full Cells

Lithium ion batteries (LIBs) are considered as key technology for stationary energy storage systems and especially as power supply for electric vehicles (EVs). Even though LIBs are already used in EVs, there is a need of further improvements of the LIBs to achieve driving ranges of more than 500 km, what is considered to be a value for greater consumer acceptance of EVs. To reach this goal, the specific energy and energy density of LIBs need to be increased to approximately 350 Wh kg-1 and 750 Wh L-1 at cell level, respectively. The use of alternative anode materials to replace the state-of-the-art graphite anode, is considered as an efficient strategy to increase the energy density of LIBs. Silicon (Si) turns out to be the most promising material for advanced anodes in LIBs, as it offers a nearly 10 times higher specific capacity compared to graphite. However, the implementation of electrode materials containing contents of more than 5-10% Si in commercial LIBs is still hampered by huge volume changes leading to a continuous solid electrolyte interphase (SEI) re-formation, loss of active lithium and, therefore, to a poor capacity retention.[1] The application of nano-sized Si materials like nanoparticles, nanowires or thin films is reported to significantly improve the performance of Si based anodes, due to better accommodation of the huge volume changes upon lithiation/de-lithiation. Additionally, Si thin film electrodes exhibit improved specific capacities in comparison to typical composite-based electrodes, because of the absence of inactive components like binder and conductive additives.[2] An additional approach to improve the performance of Si based electrodes is the addition of additives to the electrolyte. Electrolyte additives like fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are commonly known to enhance the capacity retention of Si electrodes by forming a more stable SEI, thus, preventing ongoing electrolyte decomposition and continuous active lithium loss.[3] Isocyanate compounds are able to undergo reductive polymerization and, therefore, may be considered as effective electrolyte additives for Si anodes. Actually, several isocyanates were reported to function as effective film-forming additives for graphite-based negative electrodes.[4] Within this work, magnetron sputtering was utilized for the preparation of thin film Si anodes, which contain neither a binder nor a conductive agent. Therefore, the effect of the electrolyte additive can be directly related to the Si active material. Since it was recently reported, that lithium consumption related to SEI reformation is the main failure mechanism of lithium ion full cells containing a Si anode[5], the electrochemical performance of these Si thin film electrodes was investigated in Si/NMC-111 full cells using different electrolyte formulations. DFT calculations (HOMO/LUMO energies) were performed prior to electrochemical investigations for reductive and oxidative stability predictions of the electrolyte solvent and additive molecules. The addition of the pentafluorophenyl isocyanate electrolyte additive leads to an increased Coulombic efficiency and a significantly enhanced capacity retention of the LIB full cells during prolonged cycling, in comparison to the baseline electrolyte. Post-mortem investigations of the negative Si electrodes by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were performed to study the SEI layer, formed in the different electrolyte formulations. The enhanced cycling performance of the full cells can be correlated to an improved SEI formation. Reference s : [1] D. Andre, H. Hain, P. Lamp, F. Maglia, B. Stiaszny, Future high-energy density anode materials from an automotive application perspective, Journal of Materials Chemistry A, 5 (2017) 17174-17198. [2] M.N. Obrovac, V.L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chemical Reviews, 114 (2014) 11444-11502. [3] S. Dalavi, P. Guduru, B.L. Lucht, Performance Enhancing Electrolyte Additives for Lithium Ion Batteries with Silicon Anodes, Journal of The Electrochemical Society, 159 (2012) A642-A646. [4] C. Korepp, W. Kern, E.A. Lanzer, P.R. Raimann, J.O. Besenhard, M.H. Yang, K.C. Möller, D.T. Shieh, M. Winter, Isocyanate compounds as electrolyte additives for lithium-ion batteries, Journal of Power Sources, 174 (2007) 387-393. [5] M. Klett, J.A. Gilbert, S.E. Trask, B.J. Polzin, A.N. Jansen, D.W. Dees, D.P. Abraham, Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li1.03(Ni0.5Co0.2Mn0.3)0.97O2/Silicon-Graphite Full Cells, Journal of The Electrochemical Society, 163 (2016) A875-A887.

Micro-patterned 3D Si electrodes fabricated using an imprinting process for high-performance lithium-ion batteries

To overcome the volumetric expansion of Si used as an anode in lithium-ion batteries (LIBs), we propose 3D Si electrode structures formed on patterned Cu current collectors designed from metal molds fabricated using wire electrical discharge machining (WEDM). The line- and check-patterned Cu current collectors with microscale periods for LIBs are prepared using an imprinting technique with patterned metal molds fabricated using the WEDM process. The line- and check-patterned Si thin-film and powder-type electrodes as anodes are fabricated using radio frequency magnetron sputtering deposition method and conventional slurry casting process, respectively. The morphology of the Si electrodes before and after the cycling process is characterized using optical microscopy and scanning electron microscopy. The electrochemical properties of the Si electrodes are evaluated using a multi-channel battery tester and electrochemical impedance analyzer. In particular, the check-patterned Si electrodes exhibit relatively high-capacity and enhanced cycling performance due to the stress relief of the Si anode.

N-Type Doped Silicon Thin Film on a Porous Cu Current Collector as the Negative Electrode for Li-Ion Batteries.

This work reports the preparation of a three‐dimensional Si thin film negative electrode employing a porous Cu current collector. A previously reported copper etching procedure was modified to develop the porous structures inside a 9 μm thick copper foil. Magnetron sputtering was used for the deposition of an n‐type doped 400 nm thick amorphous Si thin film. Electrochemical cycling of the prepared anode confirmed the effectiveness of utilizing the approach. The designed Si thin film electrode retained a capacity of around 67 μAh cm−2 (1675 mAh g−1) in 100th cycle. The improved electrochemical performance resulted in an enhancement of both areal capacity and capacity retention in contrast with flat and rough current collectors that were prepared for comparison.

Graphene Coated Ni Foam As a Current Collector for N-Type Doped Si Thin Film Anode for Li-Ion Batteries

Thin film silicon (Si) has gained a significant attention as an anode material for Li-ion batteries (LIBs), since its nanostructure retains structural integrity through minimizing the volume variation. However, 2D Si thin film experiences delamination from the current collector, and loss of electrical contact leads to further capacity fading. To overcome this issue, recent scientific investigations have shown that the 3D structure of electrodes (porous, rough-surface, etc.) is capable of suppressing volume changes upon lithiation/delithiation and providing large contact area between active material and current collector. Here, we propose a novel electrode material for LIBs that comprises of Si on graphene coated Ni foam (G-Ni). Porous Ni foam serves as 3D structured highly electroconductive network, and graphene layer was employed with the purpose of improving adhesion and electrochemical stability of Si thin film electrode. Magnetron sputtering was used to deposit n-type doped amorphous Si thin film on G-Ni. Chemical vapor deposition (CVD) was applied to obtain a few layer graphene (FLG) on catalytic Ni foam. The as-grown anode material was tested in a half-cell configuration with metallic lithium as a counter and reference electrode. The results were compared with n-type doped amorphous Si thin film on pristine Ni foam. Galvanostatic cycling performance was evaluated in the voltage range from 0.1 to 1.5 V at a current density of ~ 32 µA cm-2. Si-G-Ni anode exhibited the initial areal capacity of around 141 µA cm-2 and the stable reversible areal capacity of around 80 µA cm-2 over 100 cycles. Si-Ni anode shows an initial capacity of 91 µA/cm-2 with a rapid capacity fading to approximately 30 µA cm-2 at 100th cycle. The results of characterization and electrochemical tests will be presented at the conference. Figure 1

Mechanical Reliability of All-Solid-State Lithium-Ion Batteries

All-solid-state batteries (ASSBs) are being pursued as potentially safe high energy density storage systems. The replacement of an organic liquid electrolyte with a non-flammable and more reliable inorganic solid electrolyte (SE) is expected to improve Li-ion battery (LIB) safety. In recent years, several solid electrolytes having level of conductivity comparable to organic liquid electrolytes have been discovered and tested with many active materials [1, 2, 3, 4]. However, many challenges remain in manufacturing and fundamental understanding of this technology. We previously developed a fully coupled electro-chemo-mechanical model that contributes to the mesoscale optimization of composite solid-state electrodes [5]. In particular, it guides the understanding of how to overcome the inherent mechanical degradation of ASSBs [6]. Degradation phenomena are not well-understood or predicted in LIBs, in part because they require detailed modeling of the microstructure. Local features and defects are the primary drivers of failure in most materials and systems. By modeling inhomogeneities at different length scales we push the understanding of electrochemical systems beyond the limits achieved with macro-homogeneous models. Electro-chemo-fracture-mechanics (ECFM) and its implications for mechanical and device reliability is the primary focus of this research [6]. Intercalation-induced strains and stress-driven diffusion are phenomena observed in most electrode materials and are a tangible effect of the electro-chemo-mechanical coupling. For instance, the moderate (10% in graphite) or large (300% in silicon) volume changes associated with Li intercalation in negative electrode materials lead to mechanical stress and fracture. Mechanical degradation is more severe in systems where the chemical expansion is large [7], or where it is mechanically constrained, such as in all-solid-state batteries [8, 6]. Our finite element model predicts that solid-state composite negative electrodes will fracture during charging of the battery. Even when electrodes are, on average, under compression they are still vulnerable to mechanical degradation. Microstructure inhomogeneities, such as particle misalignment and non-smooth surfaces, are sufficient to cause tensile and shear stress to arise within the SE matrix. This result has strong implications for the transport properties of the electrode. Fracture within a solid Li-ion conductor has the effect of cutting off diffusion paths for lithium. Consequences of mechanical damage on an electrode’s transport properties will also be discussed. In particular we illustrate via random-walk analysis how micro-cracking of the solid electrolyte region increases the tortuosity by a significant amount, especially in densely packed microstructures. Acknowledgments The work was supported by the grant DE-SC0002633 funded by the U.S. Department of Energy, Office of Science. Keywords Lithium ion batteries, All-solid-state batteries, Stress-potential coupling, Fracture mechanics, Finite element modeling References [1] K. Takada, “Progress and prospective of solid-state lithium batteries”, Acta Materialia, vol. 61, no. 3, pp. 759-770, 2013. [2] J. Li, C. Ma, M. Chi, C. Liang, and N. J. Dudney, “Solid electrolyte: The key for high-voltage lithium batteries”, Advanced Energy Materials, vol. 5, no. 4, pp. 1-6, 2015. [3] J. G. Kim, B. Son, S. Mukherjee, N. Schuppert, A. Bates, O. Kwon, M. J. Choi, H. Y. Chung, and S. Park, “A review of lithium and non-lithium based solid state batteries”, Journal of Power Sources, vol. 282, pp. 299-322, 2015. [4] V. Thangadurai, S. Narayanan, and D. Pinzaru, “Garnet-type solid-state fast Li ion conductors for Li batteries: critical review”, Chem. Soc. Rev., vol. 43, pp. 4714-4727, 2014. [5] G. Bucci, Y.-M. Chiang, and W. Carter, “Formulation of the coupled electrochemical-mechanical boundary-value problem, with applications to transport of multiple charged species”, Acta Materialia, vol. 62, pp. 33-51, 2016. [6] G. Bucci and W. Carter, “Mechanics of Materials: Micro-mechanics in electrochemical systems”, Springer (in production), 2016. [7] G. Bucci, S. P. Nadimpalli, V. A. Sethuraman, A. F. Bower, and P. R. Guduru, “Measurement and modeling of the mechanical and electrochemical response of amorphous Si thin film electrodes during cyclic lithiation”, Journal of the Mechanics and Physics of Solids, vol. 62, pp. 276-294, 2014 [8] G. Bucci, T. Swamy, S. Bishop, B. W. Sheldon, Y.-M. Chiang, and W. C. Carter, “The effect of stress on battery-electrode capacity”, Journal of The Electrochemical Society (in review), 2016 Figure 1

Preparation of Si Thin Film Electrode on Patterned Cu Current Collector and Its Electrochemical Properties

Preparation of Si Thin Film Electrode on Patterned Cu Current Collector and Its Electrochemical Properties

In Situ AFM Observation of Amorphous Si Thin-Film Electrode in Ethylene Carbonate-Based Electrolyte Solutions with Vinylene Carbonate

Si has drawn much attention as a promising next-generation negative electrode material for lithium ion batteries (LIBs) because Si has a high theoretical capacity of ca. 4200mAh g-1, which is approximately 11 times higher than that of graphite. However, several technical issues, such as large volumetric changes of Si upon alloying/dealloying with Li and a large irreversible capacity particularly in the initial cycles, remain to be solved for practical use. Hence, the morphological changes need to be controlled to improve the charge/discharge performance. In this study, an amorphous Si thin-film was used as a model electrode and its morphological changes, which originate from the formation of solid electrolyte interphase (SEI) and volumetric changes upon alloying/dealloying with Li, was observed by in situ atomic force microscopy (AFM). The effects of electrolyte additives on morphological changes of Si were discussed. Amorphous Si thin-films were deposited on 500-μm-thick Cu sheet by electron beam evaporation. The thickness of Si was ca. 100 nm. The electrochemical properties were investigated by cyclic voltammetry (CV) (SP-300, Biologic) at a sweep rate of 0.5 mV s-1 between 3.0 and 0.02 V, using a laboratory-made three-electrode cell. The effective electrode surface area was limited to 1.77 cm2 by an O-ring. Both reference and counter electrodes were Li wire. All potentials are referred to as V vs. Li+/Li. The electrolyte solution used was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and ethylmethylcarbonate (EMC) (1:1, by volume) with and without 10 wt.% vinylene carbonate (VC). Electrochemical AFM observations coupled with CV were conducted using a 5500 AFM/SPM Microscope (Agilent) in a contact mode with pyramidal silicon nitride tips (spring constant: 0.02 N m-1, Olympus). The scan area was set to 10 × 10 μm2. AFM observation was carried out at a room temperature of about 25 oC in an Ar-filled glovebox with a dew point of less than −60 oC. Figure. 1 shows AFM images of amorphous Si thin-film electrodes during the potential scan from 0.48 to 0.02 V in 1 M LiPF6 / EC+EMC (a) without and (b) with 10 wt.% VC additives. In Fig.1a, a very large blister appeared on the Si surface at potentials below 0.35 V. Li1.7 1Si is known to form at around 0.35 V [1], and hence the formation of the blisters should be closely related to alloying reaction of Li-Si. The AFM observation was interrupted below 0.20 V due to the significant morphological changes. The blisters appeared below 0.35 V even when VC was used, while the sizes was relatively small and they were uniformly distributed. We observed the fact that a very thin surface film was formed evenly on a Si electrode at higher potentials. The alloying/dealloying reactions with Li seem to proceed in a relatively uniform way on the whole surface of a Si thin-film electrode through the VC-derived effective surface film. This assumption was proved by ex situ Raman spectroscopy. Figure 2 shows Raman spectra of an amorphous Si thin-film electrode after charging (lithiation) in VC-containing electrolyte solution, together with that of an as-prepared film as a reference. The line that peaks at around 480 cm-1, which is identified as an amorphous Si, decreased upon charging particularly at a blister (B in Fig.2), as compared with a smooth region (A in Fig.2). These results indicate that the blister consisted of Li-Si alloy; Si significantly expands upon alloying with Li and its mechanical strength lowers with an increase in Li content [2], and hence the blister should form. Such non-uniform morphological changes of Si upon lithiation can be suppressed by using VC additives, which results in high cycleability of Si electrode.