Si Anodes In Li-ion Batteries Research Articles

Silicon-Based Anodes for Li Batteries: Thermodynamics, Structural Analysis, and Li Diffusion.

Quantum mechanical and machine learning models are used to analyze the properties of silicon composite materials and their impact on anode performance. The analysis focuses on addressing challenges related to significant volume expansion during lithiation and provides valuable insights into the Gibbs free energy, chemical potentials, and relative stability of Li0 and Li+ species. Furthermore, the study explores how Li+ ions behave in the primary and secondary phases of the anode, assessing the impact of their formation on ion diffusion. This work highlights the fundamental significance of secondary phases in shaping microstructural features that impact anode properties, elucidating their contribution to the Li diffusion pathway tortuosity, which is the primary cause of the fracture of Si anodes in Li-ion batteries.

Understanding the Electrolyte Chemistry Induced Enhanced Stability of Si Anodes in Li-Ion Batteries based on Physico-Chemical Changes, Impedance, and Stress Evolution during SEI Formation

The volume expansion/contraction of Si-based anodes during electrochemical lithiation/delithiation cycles causes a loss in mechanical integrity and accrued instability of the solid electrolyte interphase (SEI) layer, culminating into capacity fade. Electrolyte additives like fluoroethylene carbonate (FEC) improve SEI stability, but the associated causes still under debate. This work reveals some of the roles of FEC via post-mortem observations/analyses, operando stress measurements and a comprehensive study of the impedance associated with the formation/evolution of SEI during lithiation/delithiation. Usage of 10 vol.% FEC as electrolyte additive leads to significant improvements in cyclic stability, Coulombic efficiency and facilitates smoother/compact/crack-free surface/SEI, in contrast to the cracked/pitted/uneven surface upon non-usage of FEC. Operando stress measurements during SEI formation reveal compressive stress development, followed by loss in mechanical integrity, upon non-usage of electrolyte additive, in contrast to insignificant stress development associated with SEI formation upon usage of FEC. The EIS model proposed here facilitates good fit with the impedance data at all states-of-charges, with the SEI resistance and capacitance exhibiting expected variations with cycling and the SEI resistance progressively decreasing with cycle number in the presence of FEC. By contrast, in the absence of FEC, severe fluctuations observed with the SEI resistance and capacitance indicate instability.

Gas Additives for Silicon Anode Lithium-Ion Batteries

While many organic liquid and electrolyte salt formulations have been explored to improve the stability of lithium-ion batteries, less research has been done to determine the role of evolved gasses in the performance of commercial Li-ion battery chemistries, let alone novel combinations of gas additives that may affect the solid-electrolyte interphase (SEI) formation and evolution.[1,2] Many gases are generated during Li-ion battery cycling, any of which could have possible beneficial or harmful effects on performance. Currently, only the effect of carbon dioxide (CO2) gas addition to Li-ion batteries with Si anodes has been studied.[3-4] In addition to CO2, ethylene gas (C2H4) is a promising additive for Si anode systems because it is a precursor in polyethylene polymerization reactions, nCH2=CH2 --> [-CH2-CH2-]n. In addition, polyethylene oxide (PEO) has been observed in the Si-anode SEI and contributes to its passivation while maintaining flexibility.[5] Therefore, creating polyethylene and increasing PEO concentration in situ via ethylene gas doping may yield an improved SEI and more-stable battery.This work investigates the potential of gas-phase additives, including CO2 and C2H4, to improve the performance of Si anodes for Li-ion batteries. The pressure decay of the gasses at different starting pressures dissolving in GenF3 electrolyte (1.2 M LiPF6 in 3:7 wt:wt ethylene carbonate to ethyl methyl carbonate + 3 wt% fluoroethylene carbonate) was monitored to determine the saturation concentration of dissolved gas. The experimental pressure decay curve was fit to a model and extrapolated to predict the final pressure at equilibrium.[6] The relationship between partial pressure and concentration of dissolved gas in GenF3 at equilibrium was plotted and a curve was drawn to determine the Henry’s law constant. Varying volumes of gas were injected into battery pouch cells containing Si nanoparticle anodes and LiFePO4 (LFP) cathodes to pressurize them to different pressures. Electrochemical cycling and subsequent multi-phase analyses, including vibrational spectroscopy and X-ray characterization of the SEI surface layer and GC-MS/FID were conducted to determine the impact of gas doping on the capacity, SEI, and gas phase reaction products.

Microstructure dependent behavior of the contact interface on porous graphene wrapped Si anodes for Li-ion batteries

Si/carbon composites are attractive anode materials for rechargeable Li-ion batteries (RLIBs), which combine the advantages of Si and graphene. Designing special Si/carbon structure with intimate contact interface and abundant void space for fast ion transmission is an effective strategy for enhancing cyclability and rate performance of the electrode materials. Inspired by the structure of fishnet, here we fabricate a binder-free and self-standing Si@porous-graphene (Si@PGN60) composite with nano silicon particles encapsulated in porous graphene sheets. The obtained Si@PGN60 sample exhibits many favourable features for high performance Si/carbon anodes, including improved Si/graphene contact interface, and abundant void space for fast ion transmission. Tanks to the unique structure, the resultant Si@PGN60 anode presents remarkably improved capacity reaching 1839.9 mA h g−1 at 300 mA g−1, and excellent cycling performance (734 mA h g−1 at a high current density of 3000 mA g−1).

Nano-FTIR Spectroscopy of the Solid Electrolyte Interphase Layer on a Thin-Film Silicon Li-Ion Anode

Si anodes for Li-ion batteries are notorious for their large volume expansion during lithiation and the corresponding detrimental effects on cycle life. However, calendar life is the primary roadblock for widespread adoption. During calendar life aging, the main origin of impedance increase and capacity fade is attributed to the instability of the solid electrolyte interphase (SEI). In this work, we use ex situ nano-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy to characterize the structure and composition of the SEI layer on amorphous Si thin films after an accelerated calendar aging protocol. The characterization of the SEI on non-washed and washed electrodes shows that brief washing in dimethyl carbonate results in large changes to the film chemistry and topography. Detailed examination of the non-washed electrodes during the first lithiation and after an accelerated calendar aging protocol reveals that PF6- and its decomposition products tend to accumulate in the SEI due to the preferential transport of PF6- ions through polyethylene oxide-like species in the organic part of the SEI layer. This work demonstrates the importance of evaluating the SEI layer in its intrinsic, undisturbed form and new strategies to improve the passivation of the SEI layer are proposed.

Amorphization-induced energy loss of amorphous Si anodes for Li-ion batteries

The thermodynamic stability and energy loss (kinetic overpotential) of amorphous Si (a-Si) anodes during electrochemical cycling are investigated by intensive free energy analysis based on molecular dynamics simulations. We first demonstrate the trade-off between energy loss and specific capacity that the enhancement of maximum Li concentration (specific capacity) is compensated with the increase of kinetic overpotential. The effects of external pressure and reaction rate on the formation enthalpy and kinetic overpotential of a-Si anodes are also explored. We find that both effects destabilize amorphous Li-Si (a-LixSi) phases with raised formation enthalpies, and result in the increased kinetic overpotential of a-Si anodes during electrochemical cycling. Our thermodynamic analysis predicts the constant value of overpotential, due to the neglect of ohmic contribution and possible formation of intermediate crystalline LixSi phases. These computational results provide a better understanding of the amorphization effect on the energy loss of alloying-type anodes.

(Digital Presentation) Do Silicon-Based Composite Electrodes for Li-Ion Batteries Need High Molecular Weight Polymeric Binder to Well Perform?

Among all potential active materials for the anodes of Li-ion batteries, silicon is considered as one of the most promising candidate because of its high specific and volumetric capacities, about 3600 mAh.g-1 and 2200 mAh.cm-3 respectively,1 as well as its low operation voltage (0.4 versus Li/Li+), abundant resources (the second largest in the earth’s crust), and environmental benignity (non-toxic). However, during the lithiation (formation of LixSi, x ~ 3.75) the silicon undergoes an expansion of about 280% of its initial volume2 which induces numerous damages to the electrode. The silicon micrometer particles tend to be pulverized when the size of the particles are upper than 150 nm.3 However, nanosizing the materials leads to more electrolyte degradation due to a higher specific surface developed and requires more binder and eventually more conducting additive in the electrode formulation. The pulverized Si particles are scattered and become electrically isolated and the adhesion of the Si electrode to the current collector are also damaged by these volume variations4. This effects drastically reduces the electrochemical performance and lifetime of the Si electrode. As a result, these challenges have seriously restricted the commercialization of Si anodes in high energy LIBs.5 To deal with these problems many researches are developing advanced binders. This later has a crucial role since it reinforces the mechanical strength of the electrode and thus helps to preserve the electrode architecture upon cycling against the large Si volume change.6 It is generally accepted that binders of very high molar masses (polymers) are necessary to allow cycling of silicon-based electrodes, as they favor the formation of more robust molecular bridges and therefore are a priori more capable of maintaining particle-to-particle contacts and therefore cohesion in the electrode. Quite surprisingly, we have discovered that it is possible to obtain good cyclability of silicon-based electrodes by using an organic binder of low molar mass (molecule). This calls into question the understanding of the mechanism by which the binder operates in silicon-based electrodes.This low molecular weight binder is a natural polyphenol, namely tannic acid. Here we will highlight that Silicon-based composite electrodes of high areal capacity (~7 mAh.cm-2) when prepared with tannic acid as small binder show a stable cycling like the one obtained with one state-of-the-art binder such as carboxymethyl cellulose. We will report in-depth characterization of the interactions between tannic acid and the silicon particles surface as well as of the rheological behavior of the electrode slurries and the electrodes properties and electrochemical performances.References(1) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for Li-Ion Batteries. Chem. Rev. 2014, 114 (23), 11444–11502.(2) Jung, S. C.; Choi, J. W.; Han, Y.-K. Anisotropic Volume Expansion of Crystalline Silicon during Electrochemical Lithium Insertion: An Atomic Level Rationale. Nano Lett. 2012, 12 (10), 5342–5347.(3) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. Size-Dependent Fracture of Silicon Nanoparticles During Lithiation. ACS Nano 2012, 6 (2), 1522–1531.(4) Hernandez, C. R.; Etiemble, A.; Douillard, T.; Mazouzi, D.; Karkar, Z.; Maire, E.; Guyomard, D.; Lestriez, B.; Roué, L. A Facile and Very Effective Method to Enhance the Mechanical Strength and the Cyclability of Si-Based Electrodes for Li-Ion Batteries. Adv. Energy Mater. 2018, 8 (6), 1701787.(5) Ko, M.; Chae, S.; Cho, J. Challenges in Accommodating Volume Change of Si Anodes for Li-Ion Batteries. ChemElectroChem 2015, 2 (11), 1645–1651.(6) Eshetu, G. G.; Figgemeier, E. Confronting the Challenges of Next-Generation Silicon Anode-Based Lithium-Ion Batteries: Role of Designer Electrolyte Additives and Polymeric Binders. ChemSusChem 2019, 12 (12), 2515–2539.

Crosslinked poly(acrylic acid) enhances adhesion and electrochemical performance of Si anodes in Li-ion batteries

Water-soluble binders such as poly (acrylic acid) (PAA) possess many advantages in the slurry and electrode preparation due to their low-cost and environmental friendliness. However, due to the linear nature of these binders, they are susceptible to slide under the continuous volume variation of Si-containing anodes during cycling. Therefore, a three-dimensional (3D) interconnected polymeric network is required to provide robust mechanical adhesion with the Si particles to maintain the electrode integrity for excellent cycle stability. Here, pentaerythritol (PER) is used as a crosslinking agent to connect the linear PAA binder to enhance its adhesion strength for Si anodes, which is systematically confirmed using Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric (TG) measurements. Si electrodes with crosslinked PAA-PER binder show enhanced adhesion and elasticity, exhibiting a more robust electrode integrity than purely PAA-based Si electrodes. Galvanostatic cycling shows that Si-PAA-5%PER electrodes maintain a higher discharge capacity of 514.3 mAh g-1 (after 10 cycles) for micro-sized and 1502.1 mAh g-1 (after 105 cycles) for nano-sized Si particles compared to 257.6 mAh g-1 and 1413.9 mAh g-1 for micro- and nano-sized Si in Si-PAA electrodes, respectively. XPS analyses on cycled electrodes confirmed that crosslinked PAA-PER binder has no negative effects on the SEI formation and its functionality in Si electrodes. SEM cross-sections reveal that Si-PAA-5%PER electrodes show reduced electrode thickness variation (micro-/nano-: 114.2%/182.2%) than that of Si-PAA electrodes (micro-/nano-: 134.1%/212.0%) after cycling, which indicates that crosslinked PAA-PER binder can enhance the electrode integrity due to its 3D interconnected network. This work provides meaningful insight into the exploration of novel binders and their impact on the SEI formation and functionality, especially for high-capacity alloy-type anode materials.

An interface-enhanced continuous 2D-carbon network enabling high-performance Si anodes for Li-ion batteries

A continuous interface-enhanced 2-D carbon network is introduced in the Si anode by a templating method, which enhances the Li-ion diffusion and electron transport within the Si anode and stabilizes the interface between the Si anode and electrolyte.

Stress-Dependent Chemo-Mechanical Performance of Amorphous Si Anodes for Li-Ion Batteries upon Lithiation

Alloying-type anodes are significantly governed by their chemo-mechanical performance during the electrochemical cycling. The reaction-induced huge volumetric change of these anodes may cause material degradation and failure under mechanical constraints. Here, we investigate the stress-dependent lithiation behavior of amorphous Si (a-Si) anodes using molecular dynamics simulations. It is indicated that a-Si anodes can sustain higher hydrostatic stress than biaxial/uniaxial ones without the occurrence of mechanical failure. Thermodynamic and electrochemical calculations demonstrate that although the lithiation procedure also affects the thermodynamic stability of a-Si anodes, it is mainly dominated by the external mean stresses. Compressive stress is confirmed to destabilize a-Si anodes and further trigger their capacity fading. Compared with our atomistic simulations, previous continuum models underestimate the open-cell potentials of a-Si anodes, due to their ignored large volumetric deformation at higher stresses and Li concentrations. This computational study provides the intensive atomic-level understanding of the stress-dependent lithiation behavior of a-Si anodes.

Combining ReaxFF Simulations and Experiments to Evaluate the Structure-Property Characteristics of Polymeric Binders in Si-Based Li-Ion Batteries.

High energy capacity silicon (Si) anodes in Li-ion batteries incorporate polymeric binders to improve cycle life, which is otherwise limited by large volume and stress fluctuations during charging/discharging cycles. Several properties of the polymeric binder play a role in achieving optimal battery performance, including interfacial adhesion strength, mechanical elasticity, and lithium-ion conduction rate. In this work, we utilize atomistic simulations with the ReaxFF force field and complementary experiments to investigate how these properties dictate the performance of Si/binder anodes. We study three C/N/H-based polymer binders with varying structures (pyrolyzed polyacrylonitrile (PPAN), polyacrylonitrile (PAN), and polyaniline (PANI)) to determine how the structure-property characteristics of the binder affect performance. The Si/binder adhesion analysis reveals some counter-intuitive results: although an individual PANI chain has a stronger affinity to Si compared to PPAN, the PANI bulk binds weaker to the Si surface. Interfacial structural analyses from simulations of the bulk phase show that PANI chains have poor stacking at the interface, while PPAN chains exhibit dense and highly ordered stacking behavior, leading to stronger adhesion. PPAN also has a lower Young's modulus compared to PANI and PAN owing to its ordered and less entangled bulk structure. This added elasticity better accommodates volume changes associated with cycling, making it a more suitable candidate for Si anodes. Finally, both simulations and experimental measurements of Li-ion diffusion rates show higher Li mobility through PPAN than PAN and PANI because the ordered stacking of PPAN chains creates channels that are favorable for Li diffusion to the Si surface. Galvanostatic charge-discharge cycling experiments show that PPAN is indeed a highly promising binder for Si anodes in Li-ion batteries, retaining a capacity of ∼1400 mAh g-1 for 150 cycles. This work demonstrates that the orientation and structure of the polymer at and near the interface are essential for optimizing binder performance as well as showcases the initial steps for binder evaluation, selection, and application for electrodes in Li-ion batteries.

Investigating Ternary Li–Mg–Si Zintl Phase Formation and Evolution for Si Anodes in Li-Ion Batteries with Mg(TFSI)2 Electrolyte Additive

Investigating Ternary Li–Mg–Si Zintl Phase Formation and Evolution for Si Anodes in Li-Ion Batteries with Mg(TFSI)₂ Electrolyte Additive

Influence of Binder Crystallinity on the Performance of Si Electrodes with Poly(vinyl alcohol) Binders

Silicon is a highly promising electrode material for Li-ion batteries because of its high theoretical capacity, but severe volume changes during cycling leads to pulverization and rapid capacity fading. The use of alternative and water-soluble polymer binders such as poly(vinyl alcohol) (PVA) or poly(acrylic acid) (PAA) can improve the cycling performance of Si-based Li-ion batteries. Here, we investigate the effect of the substitution of the hydroxyl groups of PVA chains by carboxylic acid and acetate groups on the electrochemical performance of Si anodes in Li-ion batteries. Using modified PVAs, a model system is created spanning the chemical space between PVA and PAA, and the role of different Si-adhering functionalities is investigated. When comparing the electrochemical performance of Li-ion battery cells using Si anodes and the investigated binder systems, PVA with the highest degree of hydrolysis exhibits a superior performance (100 cycles with 1019 mAh g–1) compared to modified PVAs and PAA as a binder for Si anodes. An increased degree of hydrolysis of PVA is also seen to be beneficial for high capacity retention. These effects can be largely explained by the crystallinity of the binder system, which renders an improved electrode integrity during cycling and less swelling of the Si particles.

Construction of a secondary conductive and buffer structure towards high-performance Si anodes for Li-ion batteries

Nowadays, Silicon (Si) is known as the next generation anode of lithium ion batteries (LIBs), owing to its high theoretical capacity, low lithium storage potential and natural abundance. However, the huge volume expansion and poor conductivity of silicon anodes restrict their practical application due to their poor cyclability and unsatisfied rate performances. Herein, to improve the electrochemical performance of silicon anodes, a facile one-step injection pyrolysis was used to construct a novel secondary conductive and buffer structure where Si nanoparticles are conformally coated by the interconnected onion-like carbon and then are anchored in graphene nanosheets (GNS) skeleton. The synergy of interconnected ordered onion-like carbon coating layer and GNS favors for promoting fast electron transfers and improving electrochemical kinetics, which benefits for achieving an excellent rate performance. Additionally, layer-by-layer strain relaxation by onion-like carbon and flexible GNS hierarchically buffer the substantial variation in volume of Si and then endow the composite with good cyclability. Specifically, the anode of GNS@OLC-Si shows high reversible capacity of 1129.3 mAh g−1 after 300 cycles at a current density of 0.5 A g−1. In rate performance test, the capacity retention of the GNS@OLC-Si is high especially at high current density (66.2% at 2 A g−1). Hence, a new secondary structure is successfully designed to accelerate the diffusion kinetics and buffer the volume change of Si, thereby achieving stable cyclabilities and rate performances.

Covalently Bonded Silicon/Carbon Nanocomposites as Cycle-Stable Anodes for Li-Ion Batteries.

Carbon coating is a popular strategy to boost the cyclability of Si anodes for Li-ion batteries. However, most of the Si/C nanocomposite anodes fail to achieve stable cycling due to the easy separation and peeling off of the carbon layer from the Si surface during extended cycles. To overcome this problem, we develop a covalent modification strategy by chemically bonding a large conjugated polymer, poly-peri-naphthalene (PPN), on the surfaces of nano-Si particles through a mechanochemical method, followed by a carbonization reaction to convert the PPN polymer into carbon, thus forming a Si/C composite with a carbon coating layer tightly bonded on the Si surface. Due to the strong covalent bonding interaction of the Si surface with the PPN-derived carbon coating layer, the Si/C composite can keep its structural integrity and provide an effective surface protection during the fluctuating volume changes of the nano-Si cores. As a consequence, the thus-prepared Si/C composite anode demonstrates a reversible capacity of 1512.6 mA h g-1, a stable cyclability over 500 cycles with a capacity retention of 74.2%, and a high cycling Coulombic efficiency of 99.5%, providing a novel insight for designing highly cyclable silicon anodes for new-generation Li-ion batteries.

Heteroatom-doped carbon networks enabling robust and flexible silicon anodes for high energy Li-ion batteries

Flexible Si anodes for Li-ion batteries were enabled by heteroatom-doped carbon networks, the mechanisms by which heteroatom-doped carbons enhance the Si anodes performance were detailed explored and provided.

Ni–Sn intermetallics as an efficient buffering matrix of Si anodes in Li-ion batteries

High-capacity Si-based anodes with good coulombic efficiency and long-cycle life are achieved by embedding silicon nanoparticles in dual Ni3Sn4/Ni3Sn2 active/inactive intermetallic matrix.

B-Doped Si@C Nanorod Anodes for High-Performance Lithium-Ion Batteries

B doping plays an important role in improving the conductivity and electrochemical properties of Si anodes for Li-ion batteries. Herein, we developed a facile and massive production strategy to fabricate C-coated B-doped Si (B-Si@C) nanorod anodes using casting intermediate alloys of Al-Si and Al-B and dealloying followed by C coating. The B-Si@C nanorod anodes demonstrate a high specific capacity of 560 mAg-1, with a high initial coulombic efficiency of 90.6% and substantial cycling stability. Notably, the melting cast approach is facile, simple, and applicable to doping treatments, opening new possibilities for the development of low-cost, environmentally benign, and high-performance Li-ion batteries.

Chitosan-grafted-polyaniline copolymer as an electrically conductive and mechanically stable binder for high-performance Si anodes in Li-ion batteries

We developed chitosan-grafted-polyaniline copolymers (CS-g-PANIs) as new water-soluble electrically conductive and mechanically robust polymer binder materials for silicon anodes. Unlike most other electrically conductive polymer binders, the CS-g-PANIs were easily prepared by grafting electrically conductive PANI onto the chitosan, which adheres well to Si. Various CS-g-PANI copolymers were prepared with different grafting degrees of PANI to investigate the effect of PANI on the electrochemical and physical properties of the prepared electrodes. The copolymer whose composition was 50% PANI and 50% chitosan showed the best cell performance, due to the well-balanced electrical conductivity and mechanical properties. It exhibited a specific capacity of 1091 mAh g−1 at the 200th cycle with 99.4% Coulombic efficiency.

(Invited) Si Anodes for Li-Ion Batteries: Experimental Studies on Li-Inventory and Pre-Lithiation Approaches

Li inventory and pre-lithiation methods for Si-anode containing Li-ion batteries are evaluated via a number of avenues, and are summarized briefly in this abstract. The capacity fade of the Si-graphite electrode was tracked using a capacity over-sized LiFePO4 cathode in a diagnostic cell. With essentially limitless Li inventory the intrinsic capacity fade was monitored over hundreds of cycles, without limitations/complications from Li metal in a half cell. The introduction of lithium inventory to the cell was accomplished in three ways. To date we have (1) investigated in detail the synthesis and electrochemistry of Li5FeO4 as a prelithiation source. Approach #1. We are currently working towards a blended LFO-NMC cathode to understand the cross-reactivity, impedance changes, and capacity changes in the resultant electrode. (2) Introducing lithium inventory to the cell #2. Overlithiated NCM, i.e., Li1+xNCM523O2, was found to be a viable way to introduce lithium to the cathode with minimal weight increase and no residual byproduct. However, we observed that the electrochemical cycling stability of overlithiated NCM is compromised, likely a result of the irreversible anisotropic structural changes. Work is ongoing to assess the nature of the irreversibility and we are developing strategies to minimize its impact. (3) Introducing lithium inventory to the cell #3. We will report that the 5 V spinel cathode, or Li1+xNi0.5Mn1.5O4, could be further lithiated (x>0) via an ammonia-based chemical lithiation reaction. This material was paired with a Si-graphite anode in a full cell. The excess lithium is released during the first charge and accounts for the first cycle irreversible capacity loss. Subsequent cycling only at 3 to 4.7 V shows a higher capacity and similar fade rate compared to the baseline, un-lithiated cathode.