Lithiation Of Silicon Research Articles

Silicon Nanoparticles in Li-Ion Batteries: Understanding the Role or Particle Size, Surface Oxidation, and Processing Conditions on Composite Electrode Longevity

The high theoretical gravimetric charge density (4200 mAhr g-1), low lithium storage voltage (0 – 0.4 V vs Li metal), elemental abundance, and existing expertise in manufacturing make silicon a promising candidate as an anode material for next generation lithium ion battery (LIB) technology. However, broad adoption of silicon is limited by the rapid electrode failure associated with irreversible process related to repeated charge and discharging. Because of the extreme volume changes of the silicon active material with lithium alloying (~300% at Li3.75Si) the electrode experiences large mechanical stressors. In addition, the highly reducing environment upon lithium silicide formation forces chemical reactions between the active material and electrolyte and other electrode components which result in a heterogeneous interfacial layer called the silicon-electrolyte-interphase (SEI). The role and composition of the SEI is complex, however, the SEI is hypothesized to passivate the Si surface against additional parasitic reactions but is also prone to mechanical degradation from repeated cycling. These aspects of lithium storage in silicon slow the on-ramp of silicon in LIB anodes.To begin to alleviate these irreversible processes in silicon-based electrodes we study the effect of silicon particle size, processing conditions, and electrolyte formulation on the electrode’s cycle stability, rate capability, and calendar life. We find that upon exposure to water, all silicon in our single-nanometer scale particles are oxidized to SiO2, which, we show, has no reversible lithiation capacity in the electrochemical potential window of normal silicon lithiation. We find a very strong relationship between the silicon particle size and the cycle stability, where, smaller silicon particles show a profound improvement on the capacity retention, but, due to the high surface area-to-volume ratio, also have an increased rate of reaction with the electrolyte. These results help to build a more comprehensive understanding on the role the electrode processing conditions and silicon particle size, and provide a foundation for additional work to improve the longevity of silicon in lithium ion battery devices.

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.

Unveiling the intrinsic reaction between silicon-graphite composite anode and ionic liquid electrolyte in lithium-ion battery

Due to the excellent theoretical capacity, silicon-based electrodes have been extensively studied to develop high energy-density lithium-ion batteries (LIBs). Nevertheless, the large volume expansion and unfavorable interface seriously hamper its application, and the underlying physics behind are still unclear. In this work, using in-situ environment scanning electron microscopy (ESEM), the morphological evolution of composite silicon-graphite electrodes with different ionic liquids as electrolytes are compared in the range of 20 °C–60 °C. Surprisingly, combining the microstructure and surface reaction products from transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS), it finds out that the fully reaction of lithium-silicon does not generate huge volume expansion, while the side reaction between graphite and electrolyte decomposition products causes dramatic volume expansion and hinders the further lithiation of silicon. On the other hand, the electrolyte which is capable of rapidly releasing F− to form LiF-rich solid electrolyte interphase (SEI) is favorable to maintain the structure integrity and cycling performance. This work casts new understanding from the perspective of intrinsic reaction between electrode and ionic liquid electrolyte, which suggests that the practical application of silicon-based electrodes with appropriate electrolyte is a promising route for high energy-density LIBs.

Silicon/Biogas-Derived Carbon Nanofibers Composites for Anodes of Lithium-Ion Batteries

The electrochemical performance of novel nano-silicon/biogas-derived carbon nanofibers composites (nSi/BCNFs) as anodes in lithium-ion batteries was investigated, focusing on composition and galvanostatic cycling conditions. The optimization of these variables contributes to reduce the stress associated with silicon lithiation/delithiation by accommodating/controlling the volume changes, thus preventing anode degradation and therefore improving its performance regarding capacity and stability. Specific capacities up to 520 mAh g−1 with coulombic efficiency > 95% and 94% of capacity retention are achieved for nSi/BCNFs anodes at electric current density of 100/200 mA g−1 and low cutoff voltage of 80 mV. Among the BCNFs, those no-graphitized with fishbone microstructure, which have a great number of active sites to interact with nSi particles, are the best carbon matrices. Specifically, a nSi:BCNFs 1:1 weight ratio in the composite is the optimal, since it allows a compromise between a suitable specific capacity, which is higher than that of graphitic materials currently commercialized for LIBs, and an acceptable capacity retention along cycling. Low cutoff voltage in the 80–100 mV range is the most suitable for the cycling of nSi/BCNFs anodes because it avoids formation of the highest lithiated phase (Li15Si4) and therefore the complete silicon lithiation, which leads to electrode damage.

Coupling effects of self-limiting lithiation, reaction front evolution and free volume evolution on chemical stress in amorphous wire-based electrodes

The lithiation of silicon (Si) involves the evolution of reaction front, self-limiting lithiation, and visco-plastic deformation. During the lithiation of crystalline Si, solid-state amorphization occurs to lower Gibbs free energy, and lithiated Si-electrode in lithium-ion battery is mainly present in amorphous phase. In this work, we develop a viscoplastic constitutive relationship for the lithiation-induced deformation of amorphous materials from the theory of free volume, and establish a chemo-mechanical model for the lithiation-induced deformation of a-Si electrode from the frameworks of phase-field theory, stress-assisted thermal activation process and the viscoplastic constitutive relationship. The chemo-mechanical model takes into account three important chemophysical phenomena of the self-limiting lithiation, evolution of reaction front/interphase zone and plastic flow. Using the newly developed chemo-mechanical model, we investigate the lithiation-induced deformation of an a-Si nanowire. The numerical results reveal that both the stress-assisted thermal activation process and plastic flow retard the motion of the reaction front from free surface to the center of the a-Si nanowire. The annihilation and creation of free volume significantly reduces the Cauchy stress.

Probing Silicon Lithiation in Silicon-Carbon Blended Anodes with a Multi-Scale Porous Electrode Model

Blended negative electrodes with a mix of graphite and silicon are attractive solutions to extend the energy density of lithium-ion batteries. Given the huge volume expansion of silicon, the relative lithiation/delithiation of both active materials upon cycling is crucial to determine the mechanical behavior of the electrode. It is complicated however by the potential hysteresis displayed by silicon. We focus on a blended anode with 16 wt% of silicon carbon composite (SiC-C) at a gravimetric capacity of 1500 mAh.g−1 mixed with 84 wt% graphite. A multi-scale porous electrode model with three different hierarchical units has been developed to account for the morphology of the SiC-C composed of nanoflakes of silicon embedded in a carbon matrix. The developed physics-based model is parametrized with appropriate experiments and validated for two electrodes loadings and various operating conditions. The validated model allows to probe the lithiation competition between the silicon and the carbon within the composite and the graphite. It is able to capture the streaking features revealed by synchrotron experiments. In particular, we show that at higher C-rates, the lithiation of the silicon is delayed and reduced compared to the graphite one. Quantifying this relative competition is valuable as aging effects are significantly accelerated by the expansion of the silicon.

A Comprehensive Study of Hydrolyzed Polyacrylamide as a Binder for Silicon Anodes.

Silicon anodes have a high theoretical capacity for lithium storage, but current composite electrode formulations are not sufficiently stable under long-term electrochemical cycling. The choice of polymeric binder has been shown to impact stability and capacity of silicon anodes for electrochemical energy storage. While several promising polymeric binders have been identified, there is a knowledge gap in how various physicochemical properties-including adhesion, mechanical integrity, and ion diffusion-impact electrochemical stability and performance. In this work, we comprehensively investigate the physical properties and performance of a molecular-weight series (3-20 × 106 g/mol) of partially hydrolyzed polyacrylamide (HPAM) in silicon anodes. We quantify the mechanical strength, electrolyte uptake, adhesion to silicon, copper, and carbon, as well as electrochemical performance and stability and find that HPAM satisfies many of the properties generally believed to be favorable, including good adhesion, high strength, and electrochemical stability. HPAM does not show any electrolyte uptake regardless of any molecular weight studied, and thin films of mid- and high-molecular-weight HPAM on silicon surfaces suppress lithiation of silicon. The resulting composite electrodes exhibit an electrochemical storage capacity greater than 3000 mAh/g initially and 1639 mAh/g after 100 cycles. We attribute capacity fade to failure of mechanical properties of the binder or an excess of the solid electrolyte interphase layer being formed at the Si surface. While the highest-molecular-weight sample was expected to perform the best given its stronger adhesion and bulk mechanical properties, we found that HPAM of moderate molecular weight performed the best. We attribute this to a trade-off in mechanical strength and uniformity of the resulting electrode. This work demonstrates promising performance of a low-cost polymer as a binder for Si anodes and provides insight into the physical and chemical properties that influence binder performance.

Modeling the Mesoscopic Influence of SEI Mechanical Response to Electrode Swelling

This work focuses on investigating the role that mesoscopic configuration plays in dictating the solid-electrolyte interface (SEI) mechanical durability on lithiation in silicon (Si) anodes. Due to the large expansion of Si upon lithiation, SEI formation and cohesion at the electrolyte interface of Si based anodes is a principle concern in performance of the overall cell, and enabling a more durable SEI would lead to the ability to employ higher Si percentages in anodes, increasing capacity. However, we are still building fundamental understanding of SEI formation and the resultant meso- and microscopic structure evolved due to this formation. This work seeks not to understand the formation itself, but instead to establish intuition about what type of resultant structure we should target as we learn to control the SEI formation process, by investigating the effect that non-homogenous SEI configurations have upon the overall SEI response when subjected to large strain due to the swelling of the underlying Si anode. Utilizing the material point method (MPM), a continuum mechanics approach akin to the finite element method, but particularly well suited for investigation of materials subjected to large deformation, we model the intrinsic swelling of Silicon under lithiation when coupled to a model SEI. This model SEI, in turn, is constructed of multiple different phases with mesoscopic variability and realistic material response parameterized from atomistic simulation of the mesoscopic phases including both crystalline and glassy inner SEI compounds and ordered/disordered outer SEI compounds derived from liquid carbonate decomposition products. Model mesostructures are constructed and fed into the MPM framework, from which an overall SEI mechanical response may be extracted via numerical simulation. By varying the mesostructures, sensitivity of the mechanical response to the mesostructural features is quantified, providing guidance to possible experimental approaches at improving the SEI capability of protecting the anode during multiple charge/discharge cycles.

Effect of Polymer Capping Layers on Initial Fracture Mechanism of Silicon Anodes

Using silicon nanomaterials is a widely accepted strategy for mitigating the extreme volume expansion associated with the lithiation of silicon and the accompanying capacity fade. However, the high surface area of silicon nanomaterials provides more opportunity for electrolyte decomposition which leads to high irreversible capacity loss and low Coulombic efficiency. Silicon microparticles area available in large scale at a low cost and offer an alternative with lower surface area that have been shown to have high initial Coulombic efficiency. However, to make silicon microparticles viable, the drastic volume change and high stress of these larger particles must be mitigated. In order to develop a more thorough understanding of the initial lithiation mechanics of the silicon crystalline microparticles, this work studies the initial lithiation mechanism through model crystalline electrodes. We have discovered that the thickness of the crystalline silicon wafer itself affected the initial crack formation mechanism. The next step for this study was to modify the mechanical properties of the top surface layer of the crystalline silicon substrates by adding a polymer capping layer and observe how these polymer layers affect the electrochemically induced fractures in crystalline silicon anodes through TEM, SEM and ellipsometry analysis. Polymer films of typical binder materials such as PVDF, PAA and CMC as well as novel modified PAA polymer binders were deposited on crystalline silicon substrates and lithiated to the same extent as an uncoated silicon anode to directly compare crack formation under the same level of silicon lithiation. The results of this work provide important analysis targeted at the initial crack formation of silicon and how polymer capping layers affect the lithiation depth and cracking formation of crystalline silicon. These findings may be applied to create a viable capping layer to reduce cracking in larger, more economical, silicon particles. Figure 1

Direct-Write Lithiation of Silicon Using a Focused Ion Beam of Li.

Electrochemical processes that govern the performance of lithium ion batteries involve numerous parallel reactions and interfacial phenomena that complicate the microscopic understanding of these systems. To study the behavior of ion transport and reaction in these applications, we report the use of a focused ion beam of Li+ to locally insert controlled quantities of lithium with high spatial resolution into electrochemically relevant materials in vacuo. To benchmark the technique, we present results on direct-write lithiation of 35 nm thick crystalline silicon membranes using a 2 keV beam of Li+ at doses up to 1018 cm-2 (104 nm-2). We confirm quantitative sub-μm control of lithium insertion and characterize the concomitant morphological, structural, and functional changes of the system using a combination of electron and scanning probe microscopy. We observe saturation of interstitial lithium in the silicon membrane at ≈10% dopant number density and spillover of excess lithium onto the membrane's surface. The implanted Li+ is demonstrated to remain electrochemically active. This technique will enable controlled studies and improve understanding of Li+ ion interaction with local defect structures and interfaces in electrode and solid-electrolyte materials.

Spatially Resolving Lithiation in Silicon-Graphite Composite Electrodes via in Situ High-Energy X-ray Diffraction Computed Tomography.

Optimizing the chemical and morphological parameters of lithium-ion (Li-ion) electrodes is extremely challenging, due in part to the absence of techniques to construct spatial and temporal descriptions of chemical and morphological heterogeneities. We present the first demonstration of combined high-speed X-ray diffraction (XRD) and XRD computed tomography (XRD-CT) to probe, in 3D, crystallographic heterogeneities within Li-ion electrodes with a spatial resolution of 1 μm. The local charge-transfer mechanism within and between individual particles was investigated in a silicon(Si)-graphite composite electrode. High-speed XRD revealed charge balancing kinetics between the graphite and Si during the minutes following the transition from operation to open circuit. Subparticle lithiation heterogeneities in both Si and graphite were observed using XRD-CT, where the core and shell structures were segmented, and their respective diffraction patterns were characterized.

In Situ Measurement of Phase Boundary Kinetics during Initial Lithiation of Crystalline Silicon through Picosecond Ultrasonics

Studying the kinetics of phase transformation and phase boundary propagation during initial lithiation of silicon electrodes in lithium ion batteries is relevant to understanding their performance. Such studies are usually challenging due to the difficulties in measuring the phase boundary velocity in the interior of the sample. Here we introduce a non-invasive, in situ method to measure the progression of the phase boundary in a planar specimen geometry while maintaining well-controlled lithium flux and potential. We developed an apparatus integrating an electrochemical cell with picosecond ultrasonics to probe the propagating phase boundary in real time. Phase propagation during initial lithiation of crystalline silicon, which is an example of a high capacity anode, is investigated. The primary objective of this manuscript is to report on the experimental technique development and some preliminary results. For lithiation normal to the (100) plane, we observe the phase boundary velocity to be approximately 12 pm/s and x to be 3.73 in LixSi under galvanostatic lithiation with a current density of 40 μA/cm2. The growth rate of the lithiated phase and the reaction rate coefficient are examined using a Deal-Grove type model.

Stretched to the Limit for Better Batteries

Conventional polymeric binders used within Li-ion battery electrodes cannot withstand the large volume changes exhibited by high-capacity alloying anode materials such as silicon, resulting in fast capacity decay with cycling. In this issue of Joule, Xu and coworkers report a new cross-linked polymer binder that enables excellent capacity retention over hundreds of cycles in silicon microparticle-based electrodes. To achieve this performance, this binder strongly adheres to the silicon while also exhibiting substantial mechanical elasticity and self-healing functionality.

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.

Theoretical optimization of electrode design parameters of Si based anodes for lithium-ion batteries

Silicon is considered one of the most promising anode materials for next-generation lithium-ion batteries. However, dramatic volume expansion during the lithiation of silicon complicates its practical implementation. Literature reports nanostructured electrodes, which are capable to accommodate the volume expansion, reducing associated swelling, degradation and capacity fading. However, several phenomena associated with the volume expansion, such as the reduction of the electrode’s porosity, are inherent to the system and must be carefully considered for targeted engineering of high-energy lithium-ion batteries. Herein, we determine design criteria of silicon based electrodes, taking into account the volume expansion during lithiation. A “deformation threshold” is defined signifying the minimum value of the initial porosity that must be adjusted to avoid plastic deformation and dramatic reduction of the electrode’s porosity during charging. In addition, a “C-rate threshold” is defined, guaranteeing diffusion limited currents not falling below a desired discharge rate. The impact of these theoretical limitations on the electrochemical performance of silicon-based electrodes is analyzed from an engineering point of view. The derived relations are used to optimize the electrode design parameters regarding maximum gravimetric and volumetric capacity.

A study of evolution of residual stress in single crystal silicon electrode using Raman spectroscopy

Silicon is a promising anode material for lithium ion batteries. However, lithiation of silicon generates stress that is known to be the primary reason for the failure of the anode. This study explored the existence of residual stress in single crystalline silicon electrodes after full de-lithiation, i.e., under no mechanical or electrical load. The magnitude of residual stress and its evolution with the number of lithiation-delithiation cycles is measured by Raman spectroscopy and a simple mechanics based approach. It is shown that the residual stress is tensile in nature and increased from 69 ± 11 MPa after the 1st cycle to ∼291 ± 56 MPa after 50 cycles of lithiation and de-lithiation. Concurrently, microstructural studies were performed to demonstrate the consequence of the evolution of residual stress on failure by the fracture of the silicon anode. These results provide understanding on the progressive failure mechanism of single crystalline-based silicon electrodes upon lithiation-delithiation.

Stress effects on lithiation in silicon

Recent experiments have revealed that external bending breaks the symmetry of lithiation in germanium nanowires. However, the effects of external stress on lithiation in silicon and the associated underlying mechanisms remain unclear. Here, we have performed a series of large-scale atomistic simulations based on a newly developed reactive force field (ReaxFF) to investigate the effects of external stress on the interfacial reactions and diffusion (two dominant processes during lithiation) of silicon anodes. The simulation results quantitatively show the variations in the migration velocity of the phase boundary (i.e., the reaction front during lithiation) and the diffusivity of lithium (in crystalline and amorphous lithiated silicon) as a function of the external stress and indicate that the tensile stress accelerates the lithiation rate while the compressive stress retards it. Furthermore, comparing the large-scale ReaxFF-based simulations and previous ab initio molecular dynamics (AIMD) simulations indicated that large samples and long duration times are crucially important and indispensable for accurately calculating the diffusivity of amorphous lithiated silicon under external stress. These results not only provide a fundamental understanding of the intimate coupling between mechanical stress and lithiation kinetics but also open avenues for optimizing batteries to control/alter the lithiation rate of silicon anodes by applying external mechanical stresses.

In-Situ Measurements of Volume Change and Constraint Forces of C-Si Pouch Cells during Cycling

Silicon has been targeted as an anode material that can deliver higher capacity for lithium ion batteries than current prevailing graphite. One strategy to minimize the detrimental effects of volume expansion during lithiation of silicon is to use a limited amount of silicon in combination with graphite. Even in these cases, however, cell volume expansion is a key concern when considering battery packaging for applications like electric vehicles, where cells are cycled over a wide state-of-charge window and packaged energy density is critical to delivering long range. The first step towards understanding the implications of expansion to cell design and packaging is to characterize the volume expansion of silicon-containing cells during use. In the current work, in-situ volume and pressure responses of pouch cells were carefully measured with specially designed in-house fixtures (Figure 1). Volume expansion of pouch cells was measured by tracking the buoyancy force with a high precision balance, giving volume change precision to 1 micro liter with a long-term drift of < 10 microliter/day. Cell stack loading pressure was investigated by constraining cells between two fixed plates and a floating plate under a load cell. We found that the volume and pressure fixtures provided very similar patterns during pouch cell charging and discharging. Commercially available NMC/Graphite pouch cells showed symmetric and reversible P/V responses, while in-house cells with silicon-composite anodes showed an asymmetric response and long-term drift associated with continued SEI formation and gas generation. [1] L. Lu, X. Han, J. Li, J. Hua, M. Ouyang, J. Power Sources, (226) 2012, pp 272 - 288 Figure 1

A Comparative Study on Continuum-Scale Modeling of Elasto-Plastic Deformation in Rechargeable Ion Batteries

Continuum-scale modeling techniques are frequently used to simulate the elasto–plastic deformation of rechargeable battery's electrode materials during intercalation. The primary objective of this paper is to explore and compare the mathematical formulations, the ease of implementations and parametrization of two such techniques that are extensively used to represent the physics of ions intercalation in rechargeable battery electrodes. The first technique is a finite element coupled diffusion/structure mechanics (CDSM) model, and the second is a phase-field model (PFM). In the present work, the two models are compared quantitatively utilizing an example based upon lithiation of silicon. It is observed that the two models provide essentially similar results; however, their implementations and parameters clarification are significantly diverse. The detailed discussion in the paper identifies the merits and limitations of each model in the context of the assumptions in their theoretical formulations and their influence on the physics of diffusion/intercalation and deformation. Hence, the paper delivers new insights regarding the ranges of applicability and helps to understand similarities and differences between the two models.

Shoring Up the Lithium Ion Batteries with Multi-Component Silicon Yolk-Shell Anodes for Grid-Scale Storage Systems: Experimental and Computational Mechanical Studies

The focus of the current study is to investigate the role of different carbon materials on the mechanical integrity and electrochemical stability of silicon anodes during cycling tests. Because, the electrochemical performance of the lithium ion batteries (LIBs) strongly depends on the mechanical integrity of Si during the lithium insertion process with huge volume change of silicon (∼300%) causes undesirable mechanical pulverization of electrodes that results in electrical disconnection between the active materials and the current collector, and eventual fast capacity fading. To this aim, multi-component anode materials were designed with various combinations of Si, Amorphous Carbon (C), reduced Graphene Oxide (rGO), and Carbon Nanofibers (CNFs) for high capacity and long stability LIBs. The stress generations in Si/rGO and Si/C/rGO were calculated after full lithiation of silicon and comparison of the contribution of the carbon shell to the deformation and stress in silicon was evaluated using finite element method in ANSYS design modeler. According to the experimental findings, Si/C/rGO/CNFs electrode has exhibited the highest reversible capacity of about 1750 mAhg−1 after 200 cycles and the fairly good integral stability against lithium insertion-induced expansion of a silicon.