Lithiation Of Si Research Articles

Lithium Redistribution Mechanism within Silicon-Graphite Electrodes: Multi-Method Approach and Method Validation

Li redistribution processes within Si-graphite composite (SiG) electrodes are analyzed using in situ and operando X-ray diffraction (XRD), ex situ light microscopy (LM), in situ optical microscopy of cross-sectioned full cells (CS-IOM), and 3D microstructure-resolved simulations of full cells. First, the lithiation behavior of graphite and SiG full cells (Si content 20.8 wt.-%) is analyzed. The results are used as validation of the methods (XRD, LM, CS-IOM, simulation). Second, the Li redistribution between the graphite component and Si component within SiG electrodes is investigated: By operando XRD measurements during charging in comparison with relaxed cells, a higher lithiation degree in the graphite component is found during charging compared to the relaxed state, indicating Li redistribution from graphite to Si during relaxation. The Li redistribution is directly observed by in situ and ex situ optical microscopy, where the golden LiC6 phase disappears during a 24 h relaxation period. The results are supported by simulations showing the variation in the Li concentration, not only in graphite but also within the Si component. Furthermore, all methods find that the Li redistribution is more pronounced at a higher C-rate of 0.5 C, suggesting a preference for graphite lithiation over Si lithiation.

Localisation of stress-affected chemical reactions in solids described by coupled mechanics-diffusion-reaction models

Chemical reactions in solids can induce chemical expansion of the solid that causes the emergence of the mechanical stresses, which, in turn, can affect the rate of the reaction. A typical example of this is the reaction of Si lithiation, where the stresses can inhibit the reaction up to the reaction locking. The reactions in solids can take place within some volume (bulk reactions) or localise at a chemical reaction front (localised reactions). These cases are typically described by different thermo-chemo-mechanical theories that contain the source/sink terms either in the bulk or at the propagating infinitely-thin interface, respectively. However, there are reactions that can reveal both regimes; hence, there is a need to link the theories describing the bulk and the localised (sharp-interface) reactions. The present paper bridges this gap and shows that when a certain structure of the Helmholtz free energy density is adopted (based on the ideas from the phase-field methods), it is possible to obtain (in the limit) the same driving force for the chemical reaction (hence, the same reaction kinetics) as derived within the theory of the sharp-interface chemical reactions based on the chemical affinity tensor.

Operando NMR Characterization of Cycled and Calendar Aged Si Anodes

Replacing graphite anodes with Si anodes can greatly increase the capacity of current Li-ion batteries. Detailed characterization of Si lithiation reactions, solid electrolyte interphase (SEI) formation, and reversibility are therefore active areas of research. Solid-state nuclear magnetic resonance (NMR) spectroscopy is useful for characterizing different local environments within Si anodes as well as differentiating surface and bulk environments. Furthermore, non-invasive and non-destructive NMR methods can reveal metastable LixSi phases or SEI species forming on (dis)charge that may be too reactive to detect via ex situ methods. Here, we use an in situ and in operando NMR methodology to characterize full pouch cells comprising of Si anodes and NMC cathodes. We can observe changes in the relative amounts of Li in different environments within the same Si anode before/during/after charge & discharge and with various amounts of cycling or calendar aging using in situ 7Li NMR. The pouch cells used in our in situ NMR study are comparable to pouch cells made using mid- to large-scale fabrication methods as opposed to laboratory scale; in other words, challenges of cell-to-cell variability are minimized. We hypothesize “trapped” lithiated phases formed in the Si anode no longer contribute to electrochemical cycling and thus limit capacity in subsequent cycles/aging. We also characterized the SEI/surface species in situ with aging via 13C NMR measurements, identifying reactions of lithium silicides with electrolyte solvents and their subsequent decomposition.

Silicon-stabilized lithium metal powder (SLMP) composite anodes for fast charging by in-situ prelithiation

Advanced battery properties, such as high specific energy density and fast charging, are currently in demand owing to the expansion of the electric vehicle (EV) market. Si and Li metals have attracted research attention as anode materials because of their large theoretical capacities and fast reaction rates with Li ions. However, extreme volume changes and solid electrolyte interphase (SEI) issues hinder their applications in EVs. Herein, we mix Si with stabilized lithium metal powder (SLMP) to achieve a composite anode with complementary characteristics of Si and SLMP (SiLP) that does not require carbon additives. Prelithiation is performed before applying an external current to contribute to a fast reaction rate because of the exclusion of the Si lithiation process and the plating process conducted during the initial charging state. Furthermore, a stable lithium fluoride layer is formed to mitigate SEI issues by adding a fluorinated polymer during electrode fabrication. Capacity characteristics are improved by more than double and a high SiLP anode initial Coulombic efficiency (ICE) of 70% is attained. In addition, the diminished SiLP adsorption energy is provided via density functional theory (DFT) calculations.

A two-scale framework for coupled mechanics-diffusion-reaction processes

There is a wide range of industrially-relevant problems where mechanical stresses directly affect kinetics of chemical reactions. For example, this includes formation of oxide layers on parts of micro-electro-mechanical systems (MEMS) and lithiation of Si in Li-ion batteries. Detailed understanding of these processes requires thermodynamically-consistent theories describing the coupled thermo-chemo-mechanical behaviour of those systems. Furthermore, as the majority of materials used in those systems have complex microstructures, multiscale modelling techniques are required for efficient simulation of their behaviour. Hence, the purpose of the present paper is two-fold: (1) to derive a thermodynamically-consistent thermo-chemo-mechanical theory; and (2) to propose a two-scale modelling approach based on the concept of computational homogenisation for the considered theory. The theory and the two-scale computational approach are implemented and tested using a number of computational examples, including the case of the reaction locking due to mechanical stresses.

MAS NMR and DFT study of electrochemical performance of silicon nanoparticle as anodes for Li-ion batteries

In this work we have investigated the effect of slurry pH on the lithiation mechanism of silicon nanoparticle (SiNP) anodes in lithium-ion batteries. To this end, we have used a combination of lithium (7Li) and silicon (29Si) magic angle spinning nuclear magnetic resonance (MAS NMR), density functional theory (DFT), and electrochemical charge–discharge cycling. The NMR results show that different lithiation mechanisms are present depending on the pH. An acidic slurry facilitates the lithiation of SiNP, whereas a basic slurry oxidises the SiNP to form a shell of SiO/SiO2, which must be cracked before Si lithiation becomes relevant. In a basic slurry, the leading process during the first cycle is the reaction of Li+ and SiO/SiO2 to form lithium silicates. Consequently, this intermediate step leads to a greater capacity loss in the basic media than in the acidic one.

Carbon Nanotube-Reinforced Dual Carbon Stress-Buffering for Highly Stable Silicon Anode Material in Lithium-Ion Battery.

Silicon (Si) anode suffers from huge volume expansion which causes poor structural stability in terms of electrode material, solid electrolyte interface, and electrode, limiting its practical application in high-energy-density lithium-ion batteries. Rationally designing architectures to optimize the stress distribution of Si/carbon (Si/C) composites has been proven to be effective in enhancing their structural stability and cycling stability, but this remains a big challenge. Here, metal-organic frameworks (ZIF-67)-derived carbon nanotube-reinforced carbon framework is employed as an outer protective layer to encapsulate the inner carbon-coated Si nanoparticles (Si@C@CNTs), which features dual carbon stress-buffering to enhance the structural stability of Si/C composite and prolong their cycling lifetime. Finite element simulation proves the structural advantage of dual carbon stress-buffering through significantly relieving stress concentration when Si lithiation. The outer carbon framework also accelerates the charge transfer efficiency during charging/discharging by the improvement of lithium-ion diffusion and electron transport. As a result, the Si@C@CNTs electrode exhibits excellent long-term lifetime and good rate capability, showing a specific capacity of 680 mAh g-1 even at a high rate of 1 A g-1 after 1000 cycles. This work provides insight into the design of robust architectures for Si/C composites by stress optimization.

Link between anisotropic electrochemistry and surface transformations at single‐crystal silicon electrodes: Implications for lithium‐ion batteries

AbstractSilicon is a promising negative electrode material for high‐energy‐density Li‐ion batteries (LiBs) but suffers from significant degradation due to the mechanical stress induced by lithiation. Volume expansion and lithiation in Si are strongly anisotropic but associated early interfacial transformations linked to these phenomena and their implications for electrode performance remain poorly understood. Here we develop a novel correlative electrochemical multi‐microscopy approach to study local interfacial degradation at the early stages for three different surface orientations of Si single crystals: Si(1 0 0), Si(1 1 0) and Si(3 1 1), after Li‐ion electrochemical cycling. The experimental strategy combines scanning electrochemical cell microscopy (SECCM) measurements with subsequently recorded scanning transmission electron microscopy images of high‐quality cross sections of Si electrodes, extracted at selected SECCM regions, using a novel Xe+ plasma‐focused ion beam procedure. These studies reveal significant surface orientation–dependent nanoscale degradation mechanisms that strongly control electrode performance. Si(1 0 0) was immune to interfacial degradation showing the best lithiation reversibility, whereas local nanoscale delamination was observed in Si(1 1 0) leading to a lower Coulombic efficiency. Continuous electrochemical deactivation of Si(3 1 1) was associated with delamination across the whole interface, Li trapping and formation of thick (ca. 60 nm) SiO2 structures. These results demonstrate surface crystallography to be a critical factor when designing Si‐based battery materials and strongly suggest that promoting Si(1 0 0) facets could potentially provide longer cycling life and performance due to a higher resistance to degradation.

Limiting anode utilization: A strategy to increase Si content and useable capacity in Si/C composite anode without compromising cycle life

Despite having a high theoretical capacity, silicon-based anodes fall short in providing practical and cycle-stable specific capacity close to the theoretical potential. The main challenge, namely particle pulverization and loss of mechanical integrity due to a large volume expansion, is commonly addressed by nano-structuring and blending with graphite in a Si/C composite anode. However, a maximum Si content of 15% in Si/C composite anodes has been realized, leaving scope for much improvement. Herein we discuss and theoretically benchmark a new strategy to increase Si content and achievable specific capacity by limiting anode utilization to minimize degradation. Through careful consideration to differential rates of lithiation of Si and C, particle fracture, and electrode swelling limit, our physical analysis suggests that this strategy could provide specific capacities of greater than 1100 Ah/Kg while eliminating degradation. These benchmarks are arrived at though a comprehensive analysis of lithiation of Si and C on an individual component level and the associated propensity for Si particle fracture as well as reduction in electrode porosity due to volume expansion and arriving at safe lithiation limits for a given set of electrode parameters under consideration. Since this safe range of operation is dependent on particle size, we show that we can optimize the Si% in Si/C to achieve maximum utilizable specific capacity for a given particle size.

Understanding Substrate Mechanics and Chemo‐Mechanical Behavior of Columnar Silicon Films to Enable Deformation Free Anodes for High‐Energy Li‐Ion Batteries

AbstractColumnar silicon (col‐Si) is a great candidate as anode material to achieve volumetric energy density, outperforming state‐of‐the‐art lithium‐ion batteries. The utilization of col‐Si in industrial scale is mainly restricted by poor cyclic life originating from immense volumetric expansion of Si, resulting in mechanical failure of the electrode. Understanding the mechanic and breathing behavior of col‐Si films coupled with copper current collectors (Cu‐CC) is therefore crucial to mitigate the above‐mentioned issues. In this study, structure‐mechanical changes of col‐Si anodes, which are induced by stress evolution upon (de‐)lithiation of Si, are investigated via operando electrochemical dilatometry and in situ thickness monitoring on material and stack level depending on Cu‐CC thickness (10 and 18 µm) and state‐of‐charge/balancing factor (SoC / N/P ratio). Employing moderately thick Cu‐CC (18 µm) prohibits electrode deformation significantly, achieving energy densities of 1101 and 873 Wh L−1 in multilayered‐pouch cells for N/P ratios of 1.1 and 2.0, respectively. The volume uptake that takes place after the first cycle can strongly be reduced from ∆Vcell stack: 55% to 25% by adapting N/P: 2.0 instead of 1.1. Even after volumetric growth (lithiated state), energy densities around 700 Wh L−1 are still achievable, confirming the feasibility of the col‐Si approach.

Insights into Electrolytic Pre-Lithiation: A Thorough Analysis Using Silicon Thin Film Anodes.

Pre-lithiation via electrolysis, herein defined as electrolytic pre-lithiation, using cost-efficient electrolytes based on lithium chloride (LiCl), is successfully demonstrated as a proof-of-concept for enabling lithium-ion battery full-cells with high silicon content negative electrodes. An electrolyte for pre-lithiation based on γ-butyrolactone and LiCl is optimized using boron-containing additives (lithium bis(oxalato)borate, lithium difluoro(oxalate)borate) and CO2 with respect to the formation of a protective solid electrolyte interphase (SEI) on silicon thin films as model electrodes. Reversible lithiation in Si||Li metal cells is demonstrated with Coulombic efficiencies (CEff ) of 95-96% for optimized electrolytes comparable to 1 m LiPF6 /EC:EMC 3:7. Formation of an effective SEI is shown by cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). electrolytic pre-lithiation experiments show that notable amounts of the gaseous product Cl2 dissolve in the electrolyte leading to a self-discharge Cl2 /Cl- shuttle mechanism between the electrodes lowering pre-lithiation efficiency and causing current collector corrosion. However, no significant degradation of the Si active material and the SEI due to contact with elemental chlorine is found by SEM, impedance, and XPS. In NCM111||Si full-cells, the capacity retention in the 100th cycle can be significantly increased from 54% to 78% by electrolytic pre-lithiation, compared to reference cells without pre-lithiation of Si.

Density functional theory assessment of the lithiation thermodynamics and phase evolution in si-based amorphous binary alloys

Development of novel alloy-based anodes has the potential to increase the energy storage capacity of current Li-ion based energy storage technology. In particular, Si-based anodes are of interest due to their high theoretical capacity, but suffer from poor cycle and calendar life stemming from large volumetric expansion and a non-passivating solid-electrolyte interface. The addition of amorphous components to the Si anode has been shown to improve the mechanical and chemical stability during lithiation. In this study, we use density functional theory (DFT) to probe the thermodynamics of amorphous alloy formation in a range of binary Si-X alloy systems, where X constitutes any element from periodic table groups 1-17. The alloying elements are classified as active or inactive components based on the reactivity with Li, where active elements form stable binary compounds with Li and inactive elements do not. We find that when alloying inactive elements, most inactive components do not fully reduce and hence result in the extrusion of metallic phases. Formation of Si-X compounds with no reactivity to Li results in deactivation of Si and decreased capacity. Alloying with Li-inactive elements also bypasses early Si lithiation stages and decreases the onset potential for lithiation. Most of the Li-active elements do not form stable Si-X binaries or Li-Si-X ternaries, resulting in lithiation potentials composed of voltage steps matching those of the base elements (Li x X and Li x Si), while the others may not be of much practical use due to their high lithiation potentials or preciousness, but may buffer against volumetric expansion.

Model-Informed Si Electrode Design Considering Dynamic Pore-Closure and Stack Pressure Effects

Silicon is novel Li-ion battery anode chemistry with exceptional theoretical energy densities. However, this alloying material has significant challenges with non-passivating solid-electrolyte interface (SEI) formation and significant chemo-mechanics issues. These issues have been studied extensively at the particle-level. However, extensive SEI formation and dynamic particle chemo-mechanics need to be accounted for when designing the overall electrode microstructure. For example, Si particle expansion can result in electrolyte pore-closure. During charging (Si lithiation), Si particles near the separator lithiate faster than Si near the current-collector. Thus, Si particles near the separator begin to expand faster and close-off electrolyte access to particles near the current collector. This heterogeneous through-plane lithiation then results in low overall electrode utilization, especially at high rates (i.e.,>1 C). The “choking off” effect is a result of a strong feedback between heterogeneous lithiation, Si expansion, and electrolyte pore closure.To aid in Si electrode design and account for dynamic particle expansion/contraction, the standard pseudo-2D battery model is reformulated to account for finite-strain chemo-mechanics. This model explores the trade-offs between initial electrode porosity, electrode thickness, rate-capability, and external pressure. Additionally, the model implements simplified SEI growth dynamics to explore how SEI growth effects should be accounted for in electrode design.The Figure illustrates polarization responses for a 1C charge for a Si/NMC532 cell. The different colors indicate different initial anode porosities. As shown in Figure a, the low initial porosity cases results in significant polarization, but additional gains after 50% initial porosity are significantly diminished. These results are shown for an external pressure of 15 psi. Figure b illustrates the cell thickness as a function of time. As illustrated, the lower porosity case starts at a shorter initial thickness. For all cases, the cell thickness increases as Si lithiates/expands. Figure c illustrates the anode porosity at the cut-off voltage as a function of anode thickness. As shown, the lower initial porosity case has significant pore-closure on the right-side at the separator interface. The higher initial porosity cases have increased anode lengths because the Si utilization is improved due to the reduced pore-closer effects. The graphic illustrates the main features of the reformulated pseudo-2D battery model. Figure 1

Toward Elucidation of Li Pre-Doping Reactions between Si Electrodes and Li-Naphthalenide Solutions

Next-generation batteries using Li negative electrodes have been attracting much attention recently. They still have many tough problems to be challenged, such as instability of surface chemistry and anomalous deposition of Li. Therefore, Si is regarded as a promising alternative for Li because of its relatively stable surface chemistry. Simple substitution of Si for Li induces two issues: one is large irreversible capacity of Si electrodes, and the other is lack of Li source in itself because some of cathode materials of next-generation batteries cannot supply. However, Li pre-doping enables us to straighten out these issues. We have already reported that the Li pre-doped capacities by Li-naphthalenide (Li-NTL) solution depended on the solvent of the Li-NTL solution [1]. In this study, we analyzed the effect of Li concentration in Li-NTL solution on Li doped capacities and equilibrium potentials by evaluating the solvation structures around Li+ ions and pairing structure of the solvated Li+ ions with [NTL]・⁻ in Li-NTL solutions by IR and UV-vis spectra. The results allowed us to discuss on the mechanism of the lithiation of Si in Li-NTL pre-doping solution. Si electrodes were formed on Cu foil substrate with a NMP slurry containing Si powder, Ketjen Black, and polyimide binder in a mass ratio of 80: 5: 15. Li-NTL pre-doping solution was prepared with 2-methyltetrahydrofuran (MeTHF) as solvent, naphthalene fixed at 2.5 mmol, and Li of 2.5, 5.0 mmol or “saturated Li” which meant to supply Li enough to leave a certain amount of it intact. Another Li-feeding method is continuous supply of Li by immersing Li foil in the solution during pre-doping, named “Li-supply.” The pre-doping time of the Si electrode was fixed at 24 h, and the pre-doped capacity was measured by a constant current discharge of 0.05 mA cm−2 between OCV and 1.5 V at 30℃. We employed spectroscopic method to analyze the solvation structure of the pre-doping solutions by FT-IR and UV-vis spectra. Figure 1 shows discharge voltage profiles for the Si electrodes pre-doped in Li-NTL solutions of different Li concentrations, which demonstrates clear dependency of pre-doped capacities on the Li concentrations. Li-supply delivered the largest pre-doped capacity among the four solutions. Figure 2 shows the UV-vis spectra of Li-NTL solutions with different Li concentrations. We focus two absorption bands at 320 nm and 550 nm in the spectra. The former is assigned to ion pair structure formed by radical anion of naphthalene and solvated Li+ ion [2], whose intensity decreased as Li concentration increased. The latter is assigned to the naphthalene dianion [3], whose intensity increased as the Li concentration increased. Thus, the results presented above evidently elucidate that the dianion plays a crucial role in Li pre-doping reactions.On our presentation, we will discuss the Li pre-doping reaction mechanism by adding more data on the characterization of the pre-doping solutions.[1] M. Saito et al., s of PRiME 2020, A02-0410 (2020).[2] M. Szwarc et al., J. Am. Chem. Soc., 98, 5707 (1976).[3] D. Vofsi et al., J. Polym. Sci. Polym. Chem. Ed., 20, 901 (1982). Figure 1

Lithiation Pathway Mechanism of Si-C Composite Anode Revealed by the Role of Nanopore usingIn SituLithiation

Lithiation kinetics of a Si-C composite anode for high-capacity lithium (Li)-ion batteries were investigated through in situ lithiation and electrochemical C–V measurements using a focused ion beam (FIB). Here, we found in the lithiation procedure that Li migrates sequentially into carbon (C), nanopores, and silicon (Si) in the Si-C composite. In the first lithiation step, Li was intercalated inside C particles while spreading over the surface of the C particles. The second lithiation process occurred through the filling of nanopores existing between electrode particles that consisted of the Si-C composite. The nanopores acted as a Li reservoir during the pore-filling process. Finally, the Si particles were lithiated with a volume expansion of ∼70%, corresponding to a 300% volume expansion of 25 wt % Si particles included in the composite anode. The nanopores did not accommodate a large volume expansion of Si particles, because pore-filling lithiation occurred before the Si lithiation in the charging process. We suggest a design rule related to the role of the nanopores of the Si-C composite anode in LIB systems.

Elucidating in-Situ Lithiation Pathway of Si-C Composite Anode in Lithium Ion Battery

Lithiation kinetics of a Si-C composite anode for high-capacity lithium-ion batteries were investigated through in-situ lithiation with electrochemical C-V measurements using a focused ion beam. Here, we found in the lithiation procedure that Li migrates sequentially into carbon (C), nanopores, and silicon (Si) in the Si-C composite. In the first lithiation step, Li was intercalated inside C particles while spreading over the surface of the C particles. The second lithiation process occurred through the filling of nanopores existing between electrode particles that consisted of the Si-C composite. The nanopores acted as a Li reservoir during the pore-filling process. Finally, the Si particles were litheated with a volume expansion of 173%, corresponding to a 400% volume expansion of 25wt.% Si particles included in the composite anode. The nanopores did not accommodate a large volume expansion of ~400% in Si particles, because pore-filling lithiation occurred before the Si lithiation in the charging process.

Constructing Robust Cross-Linked Binder Networks for Silicon Anodes with Improved Lithium Storage Performance.

Despite the high specific capacity of silicon as a promising anode material for the next-generation high-capacity Li-ion batteries (LIBs), its practical applications are impeded by the rapid capacity decay during cycling. To tackle the issue, herein, a binder-grafting strategy is proposed to construct a covalently cross-linked binder [carboxymethyl cellulose/phytic acid (CMC/PA)], which builds a robust branched network with more contact points, allowing stronger bonds with Si nanoparticles by hydrogen bonding. Benefitting from the enhanced mechanical reliability, the resulting Si-CMC/PA electrodes exhibit a high reversible capacity with improved long-term cycling stability. Moreover, an assembled full cell consisting of the as-obtained Si-CMC/PA anode and commercial LiFePO4 cathode also exhibits excellent cycling performance (120.4 mA h g-1 at 1 C for over 100 cycles with 88.4% capacity retention). In situ transmission electron microscopy was employed to visualize the binding effect of CMC/PA, which, unlike the conventional CMC binder, can effectively prevent the lithiated Si anodes from cracking. Furthermore, the combined ex situ microscopy and X-ray photoelectron spectroscopy analysis unveils the origin of the superior Li-ion storage performance of the Si-CMC/PA electrode, which arises from its excellent structural integrity and the stabilized solid-electrolyte interphase films during cycling. This work presents a facile and efficient binder-engineering strategy for significantly improving the performance of Si anodes for next-generation LIBs.

Lithiation of the crystalline silicon as analyzed using soft X-ray emission spectroscopy and windowless energy dispersive X-ray spectroscopy

There is no clear consensus on the exact compounds that are generated during the room temperature lithiation of Si, which may include amorphous LixSi or a crystalline Li15Si4 alloy. Here, the lithiation behavior of a single-crystal Si(111) anode was studied using scanning electron microscopy (SEM), soft X-ray emission spectroscopy (SEXS), and windowless energy dispersive X-ray spectroscopy (EDS). A Li-Si alloy generated electrochemically via potentiostatic lithiation was found to have a layered structure that contained the 1st layer of crystalline Li15Si4 (c-Li15Si4) alloy pyramids, the 2nd layer of amorphous Li13Si4 (a-Li13Si4) alloy, and the 3rd layer of Li-diffused LixSi alloy. A maximum cathodic current appeared during this process, and both the c-Li15Si4 and Li-diffused LixSi alloys were observed to grow during the initial lithiation and to maintain almost constant thicknesses prior to reaching the maximum current, while the 2nd layer began to grow after the maximum current. Comparing the experimental and theoretical currents showed that the lithiation of crystalline Si is diffusion-controlled. The low Li diffusion coefficients in the bulk LixSi alloy and bulk Si evidently result in a low current density during Si lithiation.

A high-performance lithiated silicon–sulfur battery with pomegranate-structured electrodes

Lithiated silicon-sulfur (Si–S) batteries are promising next-generation energy storage systems because of their high theoretical energy density, low cost, and high safety. However, the unstable solid-electrolyte interphase (SEI) on the Si anode and its side reactions with highly soluble polysulfides limit the lifespan of lithiated Si–S batteries. To simultaneously address both the issues, this work report on a new lithiated Si–S full battery by developing pomegranate-structured hosts for both the anode and cathode. The pomegranate-like sulfur host with titanium nitride-carbon dual-layer hollow nanospheres (Pome-TiN@C) not only effectively suppresses the polysulfides diffusion by multiple layers of chemical and physical barriers, but also facilitates their conversion reactions. In the meantime, Si nanoparticles are encapsulated in an integrated pomegranate-like carbon framework (Pome-Si@C), which accommodates the large volume variation of Si and guides the formation of stable SEI to prevent undesired side reactions. As a result, the newly developed lithiated Si–S full battery achieves a high reversible capacity (940 mAh g−1 at 300 mA g−1), a superior rate capability (537 mAh g−1 at 2 A g−1), and long cycle life (508 mAh g−1 remains after 300 cycles at 500 mA g−1).

Probing the Reactivity of the Active Material of a Li-Ion Silicon Anode with Common Battery Solvents.

Calculations and modeling have shown that replacing the traditional graphite anode with silicon can greatly improve the energy density of lithium-ion batteries. However, the large volume change of silicon particles and high reactivity of lithiated silicon when in contact with the electrolyte lead to rapid capacity fading during charging/discharging processes. In this report, we use specific lithium silicides (LS) as model compounds to systematically study the reaction between lithiated Si and different electrolyte solvents, which provides a powerful platform to deconvolute and evaluate the degradation of various organic solvents in contact with the active lithiated Si-electrode surface after lithiation. Nuclear Magnetic Resonance (NMR) characterization results show that a cyclic carbonate such as ethylene carbonate is chemically less stable than a linear carbonate such as ethylmethyl carbonate, fluoroethylene carbonate, and triglyme as they are found to be more stable when mixed with LS model compounds. Guided by the experimental results, two ethylene carbonate (EC)-free electrolytes are studied, and the electrochemical results show improvements with graphite-free Si electrodes relative to the traditional ethylene-carbonate-based electrolytes. More importantly, the study contributes to our understanding of the significant fundamental chemical and electrochemical stability differences between silicon and traditional graphite lithium-ion battery (LIB) anodes and suggests a focused development of electrolytes with specific chemical stability vs lithiated silicon which can passivate the surface more effectively.