Understanding the role of mechanics in energy materials: A perspective
Mechanical integrity of solution-processed perovskite solar cells
Keywords:Perovskite solar cells;Thermomechanical reliability;Grain boundaries;Degradation modes;Fracture processes;
Abstracts:Low-cost solar technologies such as perovskite solar cells are not only required to be efficient, but durable too, exhibiting chemical, thermal and mechanical stability. To determine the mechanical stability of perovskite solar cells, the fracture resistance of a multitude of solution-processed organometal trihalide perovskite films and cells utilizing these films were studied. The influence of stoichiometry, precursor chemistry, deposition techniques, and processing conditions on the fracture resistance of perovskite layers was investigated. In all cases, the perovskites offered negligible resistance to fracture, failing cohesively below 1.5 J/m2. The solar cells studied featured these perovskites and a variety of organic and inorganic charge transporting layers and carrier-selective contacts. These ancillary layers were found to significantly influence the overall mechanical stability of the perovskite solar cells and were repeatedly the primary source of mechanical failure, failing at values below those measured for the isolated fragile perovskite films. A detailed insight into the nature of perovskite and perovskite solar cell fracture is presented and the influence of grain size, device architecture, deposition techniques, environmental variables, and molecular additives on these fracture processes is reported. Understanding the influence of materials selection, deposition techniques and processing variables on the mechanical stability of perovskite solar cells is a crucial step in their development.
Crash analysis of a conceptual electric vehicle with a damage tolerant battery pack
Keywords:Battery;Electric vehicle;Energetic materials;Multifunctionality;Crash simulation;Finite element analysis;
Abstracts:In current electric vehicles, batteries fulfill only the role of power source and are stored within the passenger cabin, protected from external impact loads. This study considers a multifunctional, damage tolerant battery system which combines the energetic material with mechanically sacrificing elements that control mechanical stresses and dissipate energy. With such a multifunctional battery system in place it is proposed to place the battery pack into the secondary safe zone of a unibody-type vehicle. Full-vehicle crash analyses via finite element simulations are conducted for several battery pack configurations, thereby comparing the multifunctional battery system to battery packs with batteries alone and battery packs where cellular solids are used as energy absorbers. The analysis demonstrates the use of a multifunctional (damage tolerant and energy storage capable) battery system to ensure battery safety and aid in the energy absorption in a crash overall. The use of the multifunctional battery systems can aid in solving technology limitations of electric vehicles.
Poromechanical effect in the lithium–sulfur battery cathode
Keywords:Lithium–sulfur battery cathode;Precipitation induced volume expansion;Poromechanical effect;Microcrack formation;
Abstracts:In lithium–sulfur (Li–S) batteries, during discharge, solid sulfur (S) gets dissolved and undergoes successive reduction and finally precipitates as lithium sulfide (LiS) in a typical carbon-based, porous cathode. Deposition of LiS leads to 80% volume expansion compared to solid S. During the dissolution–precipitation process, the total volume change of the electrolyte in the pore space can be attributed to two factors: (a) precipitation/dissolution of the solid sulfur phase; and (b) the cathode microstructure shrinks or swells to accommodate the changes in the pore volume resulting from the electrolyte induced hydrostatic pressure. Current lithium–sulfur performance models neglect this contribution. In this work, a computational methodology has been developed to quantify the impact of precipitation induced volume change, pore morphology and confinement attributes in a Li–S cathode. Impact of volume expansion on cell voltage has also been analyzed using a performance model. It is found that the poromechanical interaction significantly affects the second voltage plateau. Cathode microstructures with relatively smaller pores tend to experience less volume expansion, for the same operating conditions. It has been found that non-uniform precipitation may lead to significant pore confinement, which has the potential to cause microcrack formation in the pore walls of a typical carbon-based cathode microstructure.
Enhanced thermoelectric performance of P-type BixSb2−xTe3 nanowires with pulsed laser assisted electrochemical deposition
Keywords:Thermoelectric Nanowires;AAO template;Electrochemical deposition;Laser irradiation;
Abstracts:Crystalline thermoelectric nanowires with well controlled chemical composition, defects and grain structures are desired for their thermoelectric performance. Here, P-type thermoelectric (TE) nanowires, Bi x Sb2−x Te3, are deposited in anodized aluminum oxide (AAO) templates at room temperature by pulsed laser assisted electrochemical deposition (ECD). Evident differences in the ECD processes resulting from pulse laser irradiation are monitored by cyclic voltammetry (CV) and current–time (I-t) curves, where instant current developments are captured. Variations in the crystal structure due to laser assisted ECD are examined using high-resolution transmission electron microscope (HR-TEM). We find that after laser assisted ECD, nanowires are deposited in the highly oriented crystallographic direction with enhanced crystallinity. Simultaneous enhancements in the electrical conductivity and the Seebeck coefficient are observed for those nanowires treated by laser assisted ECD, while the thermal conductivity remains almost the same. Theoretical calculations based on the Boltzmann transport equations suggest that the reduction of charge carrier concentration by the reduced anti-site defect densities after the laser treatment is responsible for the large enhancement of the Seebeck coefficient for the nanowires. The reduced defect densities also increase the carrier mobility substantially, which results in the enhanced electrical conductivity despite the reduced carrier concentration. This work highlights the beneficial impacts of the laser treatment for the thermoelectric performances of electrochemically grown semiconductor nanowires.
Practical considerations for reliable stress and oxygen surface exchange coefficients from bilayer curvature relaxation measurements
Keywords:Oxygen surface exchange;Curvature relaxation;Chemical strain;Chemical stress;Mechano-chemical coupling;Thin film;Mechano-chemically active;
Abstracts:The curvature relaxation technique is a new electrode-free method for simultaneously measuring a material’s chemical oxygen surface exchange coefficient and stress state under controlled atmosphere and temperature conditions. Provided certain conditions are met, this in situ/in operando technique can be used to accurately measure the oxygen surface exchange coefficients and stress states of dense, porous, thin, or thick film oxygen exchange materials. The present paper provides a detailed, practical discussion of these conditions and compares the curvature relaxation technique to alternative oxygen surface exchange coefficient measurement approaches.
Internal stress due to solvent co-intercalation in graphite electrodes for Li ion batteries
Keywords:Battery;Graphite;Thin film stress;Delamination;Solid electrolyte interphase;
Abstracts:Thin film graphite electrodes were investigated in Li half-cells, to investigate the stresses induced by propylene carbonate (PC) additions to a standard liquid electrolyte. In situ wafer curvature measurements indicate that substantial compressive stress occurs above 0.5 V, before there is significant Li intercalation into the graphite. Transmission electron microscopy shows that this cycling produces voids throughout the film, via delamination between graphene layers. To explain these observations, a model based on interlaminar debonding and the subsequent buckling of graphite layers is proposed. Existing mechanics models of these phenomena are in good agreement with the experimental observations. Based on this analysis, it appears that the PC additions lead to a very low value of the interlaminar fracture resistance.
Polarized light emission from grain boundaries in photovoltaic silicon
Abstracts:Some crystalline defects in photovoltaic silicon have deleterious effects on the energy conversion efficiency of the material. Distinguishing the harmful defects from the benign defects is a critical problem in the mechanics of materials for solar energy conversion. Interestingly, the visible light absorbed by silicon in the same part of the solar spectrum that is used to generate photocurrent, can also excite photoluminescence, which may be used to generate images of the microstructure. Slightly longer wavelengths in the near infrared (IR) may be used to measure strain in the material via photoelastic (PE) imaging. These two imaging modalities have recently been combined in a single instrument, and we show here the additional capability to identify and categorize defects directly by capturing the narrow band of photoluminescence emitted by regions of high dislocation density. We use this method to show that dislocations arranged in low angle grain boundaries emit polarized light, while dislocation structures in neighboring high angle grain boundaries do not emit polarized light. This capability may form the basis for next-generation, full-field optomechanics-based characterization of materials for solar energy conversion.
Chemical modification of polymer surfaces for advanced triboelectric nanogenerator development
Keywords:Triboelectric nanogenerator;Surface modification;Polymer surfaces;Mechanical energy harvesting;
Abstracts:Triboelectric nanogenerator (TENG) is a newly developed technique for harvesting mechanical energy from ambient environment with sparkly high output and extremely flexible structural designs. The operation of TENGs is based on the combined effects of triboelectrification and electrostatic induction. The charge density on triboelectric surfaces (mostly polymers) sets the foundation of TENG output. Meanwhile, the charge density on polymer surface is closely related to the surface chemical property. Therefore, engineering the surface chemical environment by appropriate functionalization is the most fundamental approach in controlling the TENG outputs. This article systematically reviews recent processes of chemical modifications of triboelectric polymers for advanced TENG developments. According to different functionalization techniques, four categories of chemical modifications, including fluorinated surface, ion injection, sequential infiltration synthesis and molecular-targeting functionalization are thoroughly reviewed, and their contributions to TENG performance are discussed.
Molecular dynamics simulations of plasticity and cracking in lithiated silicon electrodes
Keywords:Silicon electrode;Plasticity;Failure;Molecular dynamics;Interatomic potential;
Abstracts:First-principle calculations have provided critical insights into the deformation behavior of lithiated silicon electrodes in high-capacity lithium ion batteries, but quantitative interpretations have been limited by the size scales of these calculations. Here, we show that large-scale molecular dynamics (MD) simulations, based on the modified embedded atom method (MEAM) potential, are capable of describing the elastic softening and plastic behavior of LiSi alloys. In particular, our MD simulations at 0 K correctly reproduce the stress–strain response of LiSi alloys from Density Functional Theory (DFT) calculations across all Li concentrations, while matching the corresponding yield strength data from existing experiments at 300 K. Results from these MD simulations reveal a sharp transition in the atomic-scale plasticity mechanisms for LiSi with increasing Li content: from the breaking of Li–Si bonds at , to the breaking of Li–Li bonds at . The associated transition in the fracture behavior from brittle to ductile is due to the high stretchability of Li–Li bonds compared to Li–Si and Si–Si bonds.