Silicon Electrodes In Lithium Ion Batteries Research Articles

Scalable synthesis of porous silicon electrodes for lithium-ion batteries via acid etching of atomized Al–Si alloy powders

Scalable synthesis of porous silicon electrodes for lithium-ion batteries via acid etching of atomized Al–Si alloy powders

Effect of Stress on the Electrochemical Response of Amorphous Silicon Electrodes

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

Hollow Graphene as an Expansion-Inhibiting Electrical Interconnector for Silicon Electrodes in Lithium-Ion Batteries

Huge volume changes of silicon particles upon alloying and dealloying reactions with lithium are a major reason for the poor cycle performance of silicon-based anodes for lithium-ion batteries. To suppress dimensional changes of silicon is a key strategy in attempts to improve the electrochemical performance of silicon-based anodes. Here, we demonstrate that a conductive agent can be exploited to offset the mechanical strain imposed on silicon electrodes caused by volume expansion of silicon associated with lithiation. Hollow graphene particles as a conductive agent inhibit volume expansion by absorbing the swelling of silicon upon lithiation through flattening the free voids surrounded by the graphene shell. As a result, silicon electrodes with hollow graphene showed a height expansion of 20.4% after full lithiation with a capacity retention of 69% after 200 cycles, while the silicon electrode with conventional carbon black showed an expansion of 76.8% under the same conditions with a capacity retention of 38%. Some of the deflated hollow graphene returns to its initial shape on delithiation due to the mechanical flexibility of the graphene shell layer. Such a robust microstructure of a silicon electrode incorporating hollow graphene that serves as both an expansion inhibitor and a conductive agent greatly improves capacity retention compared with silicon electrodes with the conventionally used carbon black.

Effect and Progress of the Amorphization Process for Microscale Silicon Particles Under Partial Lithiation As Active Material in Lithium-Ion Batteries

Silicon electrodes in lithium-ion batteries undergo large volume changes during (de-)lithiation.[1] This leads to severe morphological changes over extended charge/discharge cycles, resulting in particle pulverization and surface area increase concomitant with a continuous growth of the solid-electrolyte-interphase (SEI).[2] To address this issue and to improve cycle-life, various mitigation strategies have been used in the literature, such as the use of nanostructured particles of silicon.[3] While this reduces particle pulverization, the high surface area of nanostructured silicon leads to high initial losses due to SEI formation. In this work, a concept presented by Jantke et al. is investigated using micro-scale silicon particles operated under capacity-limited conditions (i.e., partial utilization of its capacity) in order to diminish the above described phenomena.[4] Since crystalline silicon is irreversibly transformed into an amorphous phase upon lithiation, the purpose of the partial capacity utilization is to maintain a crystalline core and thus to prevent a full particle pulverization.In the present study, we investigate the amorphization process of micro-scale silicon particles in a silicon dominant anode containing 70 %wt silicon over extended charge/discharge cycling in half-cells with a lithium reference electrode, varying the lower cutoff potential of the Si electrode. While the capacity of the Si electrode after formation remains constant for cutoff potentials of ≥170 mV vs. Li+/Li (see green line in Fig. 1) of the lithiation step, the capacity continuously increases over cycling for cutoff potentials of <170 mV vs. Li+/Li (see purple line in Fig. 1), implying an ongoing amorphization of the crystalline core (cycling was conducted at C/5 with a CV step at the end of lithiation until a current of C/20, C-rates a referenced to the areal capacity of 1.0 mAh/cm2).[5] Interestingly, relaxation experiments show that the open circuit voltage (OCV) of the lithiation- and therefore amorphization reaction for crystalline silicon during the formation cycle is also observed at 170 mV vs. Li+/Li. We further validate the change of the amorphous/crystalline phase fraction over cycling by in-situ XRD measurements of Si/Li pouch-cells with a lithium reference electrode. Generating a specific amount of amorphous silicon during formation is achieved by using a defined capacity for subsequent cycling, which prevents the potential of the Si anode from dropping below the critical potential, and thus maintains the crystalline core. Finally, the cycling stability in full-cells with Si anodes and NMC622 cathodes will be presented. Our results offer insights into the effect and the progress of the silicon amorphization process when Si anodes are operated under capacity-limited conditions in order to avoid silicon particle pulverization and to enhance the performance and lifetime of silicon-based anodes. N. Obrovac, V. L. Chevrier, Chemical Reviews, 114, 11444 (2014).Wetjen, S. Solchenbach, D. Pritzl, J. Hou, V. Tileli, H. A. Gasteiger, J. Electrochem Soc., 165, A1503 (2018).Goriparti, E. Miele, F. De Angelis, E. Di Fabrizio, R. Proietti Zaccaria, C. Capiglia, Journal of Power Sources, 257, 421 (2014).Jantke, R. Bernhard, E. Hanelt, T. Buhrmester, J. Pfeiffer, S. Haufe, J. Electrochem Soc., 166, A3881 (2019).N. Obrovac, L. J. Krause, J. Electrochem Soc., 154, A103 (2007). Figure 1 Delithiation capacity of Si//Li pouch-cells (loading: 1.7 mgSi/cm2; utilized areal capacity: 1.0 mAh/cm2) with a lower cutoff limit for lithiation of either 140 mV vs. Li+/Li (purple) or 170 mV vs. Li+/Li (green). Formation cycles were conducted by cycling at C/10 (with constant-current (CC) lithiation and delithiation), while subsequent cycling was conducted at C/5 with a CV step at the end of lithiation until a current of C/20 (C-rates are referenced to the utilized areal capacity). Experiments were conducted at 25°C with 1M LiPF6 in FEC/DEC (2:8 v:v) and with two glassfiber separators (VWR). Figure 1

Improved adhesion of cross-linked binder and SiO2-coating enhances structural and cyclic stability of silicon electrodes for lithium-ion batteries

Silicon (Si) has been considered as a promising alternative anode material for high energy density lithium-ion batteries because of high theoretical capacity, low cost, environmentally benign, and abundant source. But the huge volume change during cycles greatly hinders the application of Si anodes. Constructing robust electrode structures via effective binders are still a challenge. Herein, a cross-linked polyacrylic acid and glycerol (PAA-GL) binder is synthesized in situ and a surface coating of silicon dioxide is obtained on silicon nanoparticles (Si@SiO2), both of which together construct a strong electrode. The Cross-linked binder with massive hydroxyl and carboxyl groups possesses luxuriant contact sites to Si@SiO2, meanwhile, the free carboxyl groups of binder and hydroxyl of Si@SiO2 offer an enhanced interaction, leading to robust electrode with strong mechanical properties. Except for binder, the SiO2-modified coat acts as a protective layer to buffer volume change of Si. Therefore, the remarkable structural stability is obtained. In addition, the Si@SiO2-PAA-GL electrode exhibits excellent electrochemical performance, a high capacity of 1192.7 mA h g−1 at 2 C after 500 cycles with a teeny capacity decay rate of 0.0337% per cycle.

Effects of current density on fracture behaviors for micron-sized crystalline silicon electrodes

The large volume change during lithiation/delithiation leads the silicon electrodes in lithium-ion batteries to severely degrade the mechanical performance and the silicon electrodes in lithium-ion batteries to further deteriorate electrochemical properties, which limits the commercial applications of silicon electrodes. After several year’s studies, the whole process of fracture for crystalline silicon anodes has been almost understood. However, the relationship between fracture behaviors and the lithiation depth has not been sufficiently studied. In this work, the &lt;i&gt;in-situ&lt;/i&gt; observations of morphological changes (e.g., volume expansion, crack initiation, propagation, and debonding of lithiated silicon) during lithiation at the different current densities are reported for silicon micropillars fabricated by standard photolithography and a deep reactive ion etching process. Also, this work focuses on the relative depth of lithiation of silicon electrodes at the moment of crack initiation, which is one of the crucial parameters representing the utilization of active materials with no crack. The results show that the silicon micropillars are broken faster (i.e., crack initiation and pulverization in a shorter lithiation time) and more seriously at a large current density, exhibiting more prominent symmetry of morphology. However, the relative depths of lithiation at the different current densities have just a slight difference (i.e., 18%–22%), when cracks are initiated. Here in this work, a silicon micropillar fracture is confirmed by the optical observation, while the relative depth of lithiation is calculated according to the capacity data recorded by the charge/discharge battery test system. The small fluctuation of the relative depth of lithiation with the large wave of current density can be ascribed to the dominant role of local stress concentration caused by anisotropic volume change in fracture behavior, which is validated by the results obtained by the finite element model (i.e., the depth of lithiation predicted by numerical simulations is ～ 22.6%). Therefore, the relationship between fracture behavior and the lithiation kinetics is established, providing an effective strategy for estimating the utilization of active materials under crack-free operation. With the help of the theoretical mechanics model considering both volume change and concurrent movement of reaction front, the stress state in the lithiated silicon at the moment of crack initiation is given, showing the tensile hoop stress near the reaction front. Consequently, these results suggest that the fracture behaviors depend on the current density, but the position of crack initiation (i.e., the depth of lithiation with no crack) is unrelated to current density (at least in a relatively broad range) for large micron-sized crystalline silicon electrodes, thereby shedding light on the fracture mechanisms and the design of alloy anodes (e.g., size and structure) in lithium-ion batteries.

Failure mechanisms of single-crystal silicon electrodes in lithium-ion batteries.

Long-term durability is a major obstacle limiting the widespread use of lithium-ion batteries in heavy-duty applications and others demanding extended lifetime. As one of the root causes of the degradation of battery performance, the electrode failure mechanisms are still unknown. In this paper, we reveal the fundamental fracture mechanisms of single-crystal silicon electrodes over extended lithiation/delithiation cycles, using electrochemical testing, microstructure characterization, fracture mechanics and finite element analysis. Anisotropic lithium invasion causes crack initiation perpendicular to the electrode surface, followed by growth through the electrode thickness. The low fracture energy of the lithiated/unlithiated silicon interface provides a weak microstructural path for crack deflection, accounting for the crack patterns and delamination observed after repeated cycling. On the basis of this physical understanding, we demonstrate how electrolyte additives can heal electrode cracks and provide strategies to enhance the fracture resistance in future lithium-ion batteries from surface chemical, electrochemical and material science perspectives.

Exploring the Critical Role of Binders for Silicon Electrodes in Lithium-Ion Batteries

The commercialization of high capacity silicon (3579 mAh g-1, based on Li15Si4) is limited by the large volume change of silicon during lithiation and de-lithiation, resulting in poor cycling performance. The choice of polymeric binders is critical to the electrochemical performance of silicon based electrodes. It is known that the cyclability of silicon electrodes can be much enhanced when using carboxymethyl cellulose (CMC), sodium alginate, Nafion as binders, instead of the conventional polyvinylidene fluoride (PVDF) binder. This work focuses on a comprehensive study of the binding mechanisms of these binders for silicon electrodes. Instrumented nanoindentation is used to measure Young’s modulus, hardness, and viscoelastic behavior of these polymeric binders when the binders are either dry or immersed in the organic electrolyte. Together with characterization tools, such as Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), the role of the mechanical behavior of binders and the interaction between polymeric binders and silicon will be reported.

Effects of stress on lithium transport in amorphous silicon electrodes for lithium-ion batteries

Silicon, as a promising electrode material for high energy density lithium ion batteries, experiences large strains and stresses during lithiation and delithation. The coupling effect between stress and lithium diffusion leads to a grand challenge to optimizing the design of Si electrodes with high capacity and high rate capability, particularly considering the amorphization of Si during initial cycles. In this study, we established a relationship between stress and the diffusion coefficients of Li in amorphous Si by ab initio molecular dynamics calculations (AIMD). The prediction from AIMD was validated by the potentiostatic intermittent titration measurements. Our results showed that two Li diffusion mechanisms can operate depending on the stress state. Specifically, the stress can increase Li diffusion either through increasing free volume under tension or by changing local structure under compression. However, within the range of stress generated during the lithiation and delithation process, diffusion coefficients are expected to vary by only one order of magnitude.

Role of surface oxides in the formation of solid-electrolyte interphases at silicon electrodes for lithium-ion batteries.

Nonaqueous solvents in modern battery technologies undergo electroreduction at negative electrodes, leading to the formation of a solid-electrolyte interphase (SEI). The mechanisms and reactions leading to a stable SEI on silicon electrodes in lithium-ion batteries are still poorly understood. This lack of understanding inhibits the rational design of electrolyte additives, active material coatings, and the prediction of Li-ion battery life in general. We prepared SEI with a common nonaqueous solvent (LiPF6 in PC and in EC/DEC 1:1 by wt %) on silicon oxide and etched silicon (001) surfaces in various states of lithiation to understand the role of surface chemistry on the SEI formation mechanism and SEI structure. Anhydrous and anoxic techniques were used to prevent air and moisture contamination of prepared SEI films, allowing for more accurate characterization of SEI chemical stratification and composition by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling. Additionally, multivariate statistical methods were used to better understand TOF-SIMS depth profiling studies. We conclude that the absence of native-oxide layer on silicon has a significant impact on the formation, composition, structure, and thickness of the SEI.

Real-Time Measurement of Expansion and Contraction of Silicon Electrodes in Lithium-Ion Batteries

Silicon has been the focus of many research studies as the next generation high-capacity anode material for lithium-ion batteries. However, the mechanical stability of the material remains a bottleneck to the commercialization of the material. Much work has been devoted to make nanostructured silicon composites to suppress the volume expansion. Yet, the amount of dimensional change on an electrode has not been directly monitored. Here we used an in-situ electrochemical dilatometer to quantify the thickness change of silicon electrodes during charge and discharge in order to develop techniques to stabilize the electrode.Bulk silicon particles with a size of 10-20mm from Sigma Aldrich was used to make the electrodes. We found that the degree of electrode thickness increase depends on the electrode composition (amount of carbon black and binder in the electrode). An electrode with 10wt% polyvinylidene fluoride binder gives as much as 600% increase in thickness after initial discharge. This means that an electrode that is initially 20mm thick is expanded to as much as 140mm thick after lithiation. The amount of thickness change is less when 20% binder is used. Fig. 1 shows the change in electrode thickness with respect to cumulative capacity with 20% carbon black and 20% carboxymethyl cellulose during the first three cycles. As expected, an increase in electrode thickness is observed during lithiation (discharge), and a decrease in thickness during delithiation (charge). The increase and decrease in electrode thickness are however not linear, and a model is devised to explain the mechanical expansion and contraction behaviors in a composite electrode. In brief, the electrode undergoes three stages of expansion during initial lithiation. At the beginning of discharge, the thickness change is small, as the composite electrode contains space between the particles that can accommodate the volume expansion (Stage I). Beyond a certain point, the particles impinge on each other and the volume expansion of the particles lead to an overall increase in the film thickness (Stage II). The amount of increment during stage II is similar to the theoretical increase in volume (dotted line in Fig. 1) for alloying Li with Si. The binder then losses the ability to hold the particles together, leading to an accelerated increase in thickness at the end of lithiation (Stage III). During delithiation, the contraction behavior is different from that during expansion (Stage IV), as seen from the dilatometer results. This is partly because the particle can contract in all three directions, as opposed to one direction during lithiation. The accuracy of the measurement from the dilatometer is verified by cross-sectional SEM test.After understanding the mechanism during charge and discharge, electrodes with different binders were then tested to compare the role of the binder on cycle stability. Electrodes with polyimide (PI) binder show better cycling stability than those with polyvinylidene fluoride and carboxymethyl cellulose, which is attributed to the ability of the PI binder to hold the particles together after expansion. Our results show that binder breakdown is one of the main causes of electrode degradation for Si electrodes. More experimental details and results will be shown during the presentation. With further optimization, even bulk silicon particle has the potential to be used for commercial applications.

Neutron reflectometry studies on the lithiation of amorphous silicon electrodes in lithium-ion batteries

Neutron reflectometry is used to study in situ the intercalation of lithium into amorphous silicon electrodes. The experiments are done using a closed three-electrode electrochemical cell setup. As a working electrode, an about 40 nm thick amorphous silicon layer is used that is deposited on a 1 cm thick quartz substrate coated with palladium as a current collector. The counter electrode and the reference electrode are made of lithium metal. Propylene carbonate with 1 M LiClO4 is used as an electrolyte. The utility of the cell is demonstrated during neutron reflectometry measurements where Li is intercalated at a constant current of 100 μA (7.8 μA cm(-2)) for different time steps. The results show (a) that the change in Li content in amorphous silicon and the corresponding volume expansion can be monitored, (b) that the formation of the solid electrolyte interphase becomes visible and (c) that an irreversible capacity loss is present.

Lithium-Assisted Plastic Deformation of Silicon Electrodes in Lithium-Ion Batteries: A First-Principles Theoretical Study

Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries. Recent experiments indicate that silicon experiences large plastic deformation upon Li absorption, which can significantly decrease the stresses induced by lithiation and thus mitigate fracture failure of electrodes. These issues become especially relevant in nanostructured electrodes with confined geometries. On the basis of first-principles calculations, we present a study of the microscopic deformation mechanism of lithiated silicon at relatively low Li concentration, which captures the onset of plasticity induced by lithiation. We find that lithium insertion leads to breaking of Si-Si bonds and formation of weaker bonds between neighboring Si and Li atoms, which results in a decrease in Young's modulus, a reduction in strength, and a brittle-to-ductile transition with increasing Li concentration. The microscopic mechanism of large plastic deformation is attributed to continuous lithium-assisted breaking and re-forming of Si-Si bonds and the creation of nanopores.

Influence of Pressure on the Cycling Stability of Silicon-Electrodes in Lithium-Ion Batteries

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