10 research outputs found
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The lithiation process and Li diffusion in amorphous SiO2 and Si from first-principles
Silicon is considered the next-generation, high-capacity anode for Li-ion energy storage applications, however, despite significant effort, there are still uncertainties regarding the bulk Si and surface SiO2 structural and chemical evolution as it undergoes lithiation and amorphization. In this paper, we present first-principles calculations of the evolution of the amorphous Si anode, including its oxide surface layer, as a function of Li concentration. We benchmark our methodology by comparing the results for the Si bulk to existing experimental evidence of local structure evolution, ionic diffusivity as well as electrochemical activity. Recognizing the important role of the surface Si oxide (either native or artificially grown), we undertake the same calculations for amorphous SiO2, analyzing its potential impact on the activity of Si anode materials. Derived voltage curves for the amorphous phases compare well to experimental results, highlighting that SiO2 lithiates at approximately 0.7 V higher than Si in the low Li concentration regime, which provides an important electrochemical fingerprint. The combined evidence suggests that i) the inherent diffusivity of amorphous Si is high (in the order 10−9cm2s−1 - 10−7cm2s−1), ii) SiO2 is thermodynamically driven to lithiate, such that Li–O local environments are increasingly favored as compared to Si–O bonding, iii) the ionic diffusivity of Li in LiySiO2 is initially two orders of magnitude lower than that of LiySi at low Li concentrations but increases rapidly with increasing Li content and iv) the final lithiation product of SiO2 is Li2O and highly lithiated silicides. Hence, this work suggests that - excluding explicit interactions with the electrolyte - the SiO2 surface layer presents a kinetic impediment for the lithiation of Si and a sink for Li inventory, resulting in non-reversible capacity loss through strong local Li–O bond formation
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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 (LixX and LixSi), while the others may not be of much practical use due to their high lithiation potentials or preciousness, but may buffer against volumetric expansion
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Evaluation of Amorphous Oxide Coatings for High-Voltage Li-Ion Battery Applications Using a First-Principles Framework.
Cathode surface coatings are widely used industrially as a means to suppress degradation and improve electrochemical performance of lithium-ion batteries. However, developing an optimal coating is challenging, as different coating materials may enhance one aspect of performance while hindering another. To elucidate the fundamental thermodynamic and transport properties of amorphous cathode coating materials, here, we present a framework for calculating and analyzing the Li+ and O2- transport and the stability against delithiation in such materials. Our framework includes systematic workflows of ab-initio molecular dynamics calculations to obtain amorphous structures and diffusion trajectories coupled with an analysis of critical changes of the active-ion local environment during diffusion. Based on these data, we provide an estimate of room-temperature diffusivities, including statistical error bars, and the evaluation of the coating suitability in terms of its ability to facilitate Li+ transport while blocking O2- transport. Finally, we add the thermodynamic stability analysis of the coating chemistry within the operating voltage of common Li-ion cathodes. We apply this framework to two commonly used amorphous coating materials, Al2O3 and ZnO. We find that (1) in general, a higher Li+ content increases both Li+ and O2- diffusivities in both Al2O3 and ZnO. Also, Li+ and O2- diffuse much faster in ZnO than in Al2O3. (2) However, neither Al2O3 nor ZnO is expected to retain a significant concentration of Li+ at high charge. (3) ZnO performs much more poorly in terms of O2- blocking, and hence, Al2O3 is preferred for high-voltage cathode applications. These results will help to quantitatively evaluate amorphous materials, such as metal oxides and fluorides, for different performance metrics and facilitate the development of optimal cathode coatings
Evaluation of Amorphous Oxide Coatings for High-Voltage Li-Ion Battery Applications Using a First-Principles Framework
Cathode surface coatings are widely used industrially as a means to suppress degradation and improve electrochemical performance of lithium-ion batteries. However, developing an optimal coating is challenging, as different coating materials may enhance one aspect of performance while hindering another. To elucidate the fundamental thermodynamic and transport properties of amorphous cathode coating materials, here, we present a framework for calculating and analyzing the Li+ and O2- transport and the stability against delithiation in such materials. Our framework includes systematic workflows of ab-initio molecular dynamics calculations to obtain amorphous structures and diffusion trajectories coupled with an analysis of critical changes of the active-ion local environment during diffusion. Based on these data, we provide an estimate of room-temperature diffusivities, including statistical error bars, and the evaluation of the coating suitability in terms of its ability to facilitate Li+ transport while blocking O2- transport. Finally, we add the thermodynamic stability analysis of the coating chemistry within the operating voltage of common Li-ion cathodes. We apply this framework to two commonly used amorphous coating materials, Al2O3 and ZnO. We find that (1) in general, a higher Li+ content increases both Li+ and O2- diffusivities in both Al2O3 and ZnO. Also, Li+ and O2- diffuse much faster in ZnO than in Al2O3. (2) However, neither Al2O3 nor ZnO is expected to retain a significant concentration of Li+ at high charge. (3) ZnO performs much more poorly in terms of O2- blocking, and hence, Al2O3 is preferred for high-voltage cathode applications. These results will help to quantitatively evaluate amorphous materials, such as metal oxides and fluorides, for different performance metrics and facilitate the development of optimal cathode coatings
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Tin Metal Improves the Lithiation Kinetics of High-Capacity Silicon Anodes
Si-based anodes present a great promise for high energy density lithium-ion batteries. However, its commercialization is largely hindered by a grand challenge of a rapid capacity fade. Here, we demonstrate excellent cycling stability on a Si-Sn thin film electrode that outperforms pure Si or Sn counterpart under the similar conditions. Combined with the first-principles calculations, in situ transmission electron microscopy studies reveal a reduced volume expansion, increased conductivity, as well as dynamic rearrangement upon lithiation of the Si-Sn film. We attribute the improved lithiation kinetics to the formation of a conductive matrix that comprises a mosaic of nanostructured Sn, LiySn (specifically, Li7Sn2 develops around the lithiation potential of Si), and LixSi. This work provides an important advance in understanding the lithiation mechanism of Si-based anodes for next-generation lithium-ion batteries
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Mechanical Properties and Chemical Reactivity of Li xSiO y Thin Films.
Silicon (Si) is a commonly studied candidate material for next-generation anodes in Li-ion batteries. A native oxide SiO2 on Si is often inevitable. However, it is not clear if this layer has a positive or negative effect on the battery performance. This understanding is complicated by the lack of knowledge about the physical properties of the SiO2 lithiation products and by the convolution of chemical and electrochemical effects during the anode lithiation process. In this study, Li xSiO y thin films as model materials for lithiated SiO2 were deposited by magnetron sputtering at ambient temperature, with the goal of (1) decoupling chemical reactivity from electrochemical reactivity and (2) evaluating the physical and electrochemical properties of Li xSiO y. X-ray photoemission spectroscopy analysis of the deposited thin films demonstrate that a composition close to previous experimental reports of lithiated native SiO2 can be achieved through sputtering. Our density functional theory calculations also confirm that the possible phases formed by lithiating SiO2 are very close to the measured film compositions. Scanning probe microscopy measurements show that the mechanical properties of the film are strongly dependent on lithium concentration, with a ductile behavior at a higher Li content and a brittle behavior at a lower Li content. The chemical reactivity of the thin films was investigated by measuring the AC impedance evolution, suggesting that Li xSiO y continuously reacts with the electrolyte, in part because of the high electronic conductivity of the film determined from solid-state impedance measurements. The electrochemical cycling data of the sputter-deposited Li xSiO y/Si films also suggest that Li xSiO y is not beneficial in stabilizing the Si anode surface during battery operation, despite its favorable mechanical properties
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Intrinsic chemical reactivity of solid-electrolyte interphase components in silicon-lithium alloy anode batteries probed by FTIR spectroscopy
The chemical reactivity of silicon surface species with LiPF6/carbonate electrolyte are detailed via FTIR spectroscopy and verified by MD/DFPD simulations