Deformation and failure mechanisms of electrochemically lithiated silicon thin films

A fundamental understanding of mechanical behavior of a Li–Si system is necessary to address the poor mechanical integrity of amorphous silicon (a-Si) electrodes, in order to utilize their enormous capacity in Li-ion batteries. In this work, deformation and failure mechanisms of electrochemically lithiated a-Si thin films were investigated using nanoindentation and molecular dynamics simulation techniques. The cracking observed in the a-Si thin films after the initial lithiation–delithiation cycle is associated with the tension stress developed when constrained by the substrates. The MD simulations provide an atomistic insight on the origin of plasticity and transition of fracture mechanisms with increasing lithium concentration in the electrode. Both experiment and the MD simulations indicate reduced strength, elastic modulus but increased ductility in the a-Si films after the full lithiation–delithiation cycle, as a result of increased disorder in the microstructures. Also, the mapping of void nucleation and growth indicates different failure modes in pristine and delithiated a-Si

Characterization of mechanical and electrochemical properties of silicon based electrodes for Li-ion batteries

This work aims to understand the electrochemical and mechanical behaviour of silicon thin film electrodes in Lithium-ion batteries. The evolution of microstructures, mechanical stresses and material damage have been investigated via combined experimental and molecular modelling approaches. Possible mechanisms responsible for electrochemical behaviour, volume change and material failure during charging/discharging processes have been proposed. The outcome of this work will benefit the development of novel electrode materials for high-capacity Lithium-ion batteries

Deformation and failure of lithiated Si electrodes of Li-ion batteries

Silicon (Si) is widely regarded as one of the most promising anode materials for Li ion battery due to its high theoretical specific capacity. However, the volume change (~300%) during lithiation/delithiation results in poor mechanical integrity, which impedes its commercialization. To address this issue, a better understanding of the atomistic mechanisms responsible for the volume expansion is necessary. In this work, the atomic structure and stress evolution during lithiation/delithiation was investigated on a Si film (2.7 nm). Steep stress gradients were observed when the lithium content in the anode is very low (<Li0.5Si). Plastic flow and stress relaxation are dominant at higher Li concentrations. The microstructure evolution during lithiation process is associated with the break of covalent amorphous structure into small clusters, when Li concentration exceeds ~Li0.5Si. At a low Li concentration, the sharp stress increase is due to the elastic deformation of the covalent Si lattice. The radial distribution function (RDF) analysis indicates a defective amorphous structure in silicon film after a complete lithiation/delithiation cycle, contributing to the capacity loss. These results are believed to be able to help improve the materials selection and design of Li ion batteries

Development of a polymer-modified bitumen specification based on empirical tests – case study for Sri Lanka

Pavements with polymer modification exhibit greater resistance to permanent deformation, less thermal cracking, less fatigue damage and less temperature susceptibility. Implementation of polymer-modified bitumen (PMB) is currently taking place in developing countries and the absence of PMB specification has always been a constraint. This research was aimed at developing a testing procedure for PMB, based on test methods that are currently available in Sri Lankan laboratories. The test methods were selected considering the adequate control of binder properties during application and usage. Penetration test is included to control the intermediate temperature properties and identify binder grades. Softening point test controls the high-temperature properties while viscosity test controls the mixing and compaction temperatures. Elastic recovery test and solubility test were employed in order to identify the presence of polymer in PMB. Storage stability test determines the separation tendency of polymer from bitumen. Flash point limits are set for the application safety. Thus all the essential parameters of bitumen are controlled by the proposed specification. The acceptance limits are determined considering different PMB specifications of several other countries, past research outcomes and laboratory test results. The proposed specification which is based on empirical test methods facilitates adequate quality control of PMB and it would be a useful guideline for the implementation of PMB for hot mix asphalt in Sri Lanka

Structural transformations and stress of amorphous silicon anodes during initial lithiation cycle: A molecular dynamics study

Silicon (Si) is widely regarded as one of the most promising anode materials for Li ion battery (LIB) due to its highest known theoretical specific capacity (4200mAhg-1). Particularly, amorphous silicon has recently attracted great interest due to its robust lithiation behavior. Silicon electrodes are subjected to charge-discharge via an alloying-dealloying (lithiation-delithiation) mechanism. It enables the host silicon to store up to 4.4 lithium atoms per silicon atom, which gives more than ten times greater capacity compared with conventional graphite based electrodes. However, this enormous capacity comes with the expense of significant structural changes to the host silicon, which results in poor mechanical integrity. To address this issue, it is important to understand the transitional structural and mechanical properties of silicon during charge-discharge process. In this study, molecular dynamic simulation is employed to study the structure and mechanics of an amorphous silicon thin film (2.7nm) during a complete lithiation-delithiation cycle. The microstructure evolution during lithiation process is associated with the break of covalent amorphous structure into small clusters, when Li concentration exceeds ~Li0.25Si. Also, it is evident that Li induced stress is largely dependent on lithium concentration in silicon. Steep stress gradients were observed when the lithium content in the anode is very low (<Li0.25Si). Plastic flow and stress relaxation are dominant in higher Li concentrations. Furthermore, we show that the rate of si-si bond breaking is lithium concentration dependent. Also, the Radial Distribution Function (RDF) analysis revealed a defective amorphous structure in silicon film after a complete lithiation/delithiation cycle, which is attributed to breaking down of covalent silicon network and subsequent plastic flow. These results are useful in understanding lithiation-delithiation mechanisms of silicon and the favourable charge-discharge depths to avoid extreme stresses. Furthermore, these results are believed to be able to help improve the materials selection and design of next generation Li ion batteries

Coating Fe2O3 with graphene oxide for high-performance sodium-ion battery anode

Sodium-ion batteries (SIBs) have recently shown the potential to meet the demands for large scale energy storage needs as an attractive alternative to lithium-ion batteries due to the high abundance of sodium resources around the world. The major hurdle of SIBs resides in developing viable anode materials with a high energy density and an appropriately long cycle life. Here a simple and low-cost method for synthesizing Fe2O3/graphene oxide (Fe2O3/GO) composites made out of Fe2O3 nanoparticles sandwiched between graphene oxide (GO) layers is reported. The unique structure of the Fe2O3/GO composites served a synergistic effect to alleviate the stress of Fe2O3 nanoparticles, prevent nanoparticles aggregation, maintain the mechanical integrity of the electrode, and facilitate mass transfer of Na ions during batteries operating. Consequently, the Fe2O3/GO composites as anode for SIBs attained a reversible specific capacity of ca. 420 mAh g-1 after 100 cycles at 0.1C (1C=1007 mA g-1) and a good rate capability at various current densities. Moreover, the Coulombic efficiency of the SIBs could rapidly increase in the early cycles. Due to the facile synthesis method and high electrochemical performance, the Fe2O3/GO composites would have a significant potential as anode materials for rechargeable SIBs

Carbon-based silicon nanohybrid anode materials for rechargeable lithium ion batteries

Silicon has demonstrated great potential as anode materials for next-generation high-energy density rechargeable lithium ion batteries. However, its poor mechanical integrity needs to be improved to achieve the required cycling stability. Nano-structured silicon has been used to prevent the mechanical failure caused by large volume expansion of silicon. Unfortunately, pristine silicon nanostructures still suffer from quick capacity decay due to several reasons, such as formation of solid electrolyte interphase, poor electrical contact and agglomeration of nanostructures. Recently, increasing attention has been paid to exploring the possibilities of hybridization with carbonaceous nanostructures to solve these problems. In this review, the recent advances in the design of carbon-silicon nanohybrid anodes and existing challenges for the development of high-performance lithium battery anodes are briefly discussed

Cu nanoparticles supported on graphitic carbon nitride as an\ud efficient electrocatalyst for oxygen reduction reaction

High active and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) are essential components of renewable energy technologies, such as fuel cells and metal/air batteries. Herein, we propose that ORR active Cu/graphitic carbon nitride (Cu/g-CN) electrocatalyst can be prepared via a facile hydrothermal reaction in the present of the ionic liquid (IL) bis(1-hexadecyl-3-methylimid- azolium) tetrachlorocuprate[(C₁₆mim)₂CuCl₄] and protonated g-CN. The as-prepared Cu/g-CN showed an impressive ORR catalytic activity that a 99 mV positive shift of the onset potential and 2 times kinetic current density can be clearly observed, comparing with the pure g-CN. In addition, the Cu/g-CN revealed better stability and methanol tolerance than commercial Pt/C (HiSPECTM 3000, 20%). Therefore, the proposed Cu/g-CN, as the inexpensive and efficient ORR electrocatalyst, would be a potential candidate for application in fuel cells

Numerical investigation on structural evolution and mechanical behaviour of amorphous silicon Li ion battery anodes

Silicon (Si) is widely regarded as one of the most promising anode materials for Li ion battery (LIB) due to its high theoretical specific capacity. However, the volume change (~300%) during lithiation/delithiation results in poor mechanical integrity, which impedes its commercialization. To address this issue, a better understanding of the atomistic mechanisms behind lithiation is necessary. In this work, the atomic structure and stress evolution during lithiation/delithiation was investigated on an amorphous silicon film (2.7 nm) using molecular dynamics. Steep stress gradients were observed when the lithium content in the anode is very low (<Li0.5Si). Plastic flow and stress relaxation are dominant in higher Li concentrations. The microstructure evolution during lithiation process is associated with the break of covalent amorphous structure into small clusters, when Li concentration exceeds ~Li0.5Si. At a low Li concentration, the sharp stress increase is due to the elastic deformation of the covalent Si lattice. The Radial Distribution Function (RDF) analysis revealed a defective amorphous structure in silicon film after a complete lithiation/delithiation cycle, which is attributed to breaking down of covalent silicon network and subsequent plastic flow. These results are believed to be able to help improve the materials selection and design of next generation Li ion battery

Structural transformations and mechanics of amorphous silicon anodes during initial lithian-delithiation cycle

Silicon (Si) is widely regarded as one of the most promising anode materials for Li ion battery (LIB) due to its highest known theoretical specific capacity (4200mAhg-1). Particularly, amorphous silicon has recently attracted great interest due to its robust lithiation behavior. Silicon electrodes are subjected to charge-discharge via an alloying-dealloying (lithiation-delithiation) mechanism. It enables the host silicon to store up to 4.4 lithium atoms per silicon atom, which gives more than ten times greater capacity compared with conventional graphite based electrodes. However, this enormous capacity comes with the expense of significant structural changes to the host silicon, which results in poor mechanical integrity. To address this issue, it is important to understand the transitional structural and mechanical properties of silicon during charge-discharge process. In this study, molecular dynamic simulation is employed to study the structure and mechanics of an amorphous silicon thin film (2.7nm) during a complete lithiation-delithiation cycle. The microstructure evolution during lithiation process is associated with the break of covalent amorphous structure into small clusters, when Li concentration exceeds ~Li0.25Si. Also, it is evident that Li induced stress is largely dependent on lithium concentration in silicon. Steep stress gradients were observed when the lithium content in the anode is very low (<Li0.25Si). Plastic flow and stress relaxation are dominant in higher Li concentrations. Furthermore, we show that the rate of si-si bond breaking is lithium concentration dependent. Also, the Radial Distribution Function (RDF) analysis revealed a defective amorphous structure in silicon film after a complete lithiation/delithiation cycle, which is attributed to breaking down of covalent silicon network and subsequent plastic flow. These results are useful in understanding lithiation-delithiation mechanisms of silicon and the favourable charge-discharge depths to avoid extreme stresses. Furthermore, these results are believed to be able to help improve the materials selection and design of next generation Li ion batteries