Models are developed for the transport of Li ions in the electrolyte of lithium ion batteries, their diffusion through storage electrode particles, and their kinetics through the surface of the particles between the electrolyte and the particles. As a consequence of the Li ion intercalating in the storage particles, their lattice swells, leading to elastic stress when the concentration of Li ions in the particles is not uniform. The models of transport are based on standard concepts for multi-component diffusion in liquids and solids, but are not restricted to dilute solutions, or to small changes in the concentration of the diffusing species. In addition, phase changes are permitted during mass transport as the concentration of lithium varies from the almost depleted state of the storage particle to one where the material is saturated with its ions. The elastic swelling and shrinkage may involve very large dilatations, which can be allowed for in the formulation of the model. Thus, the models can be suitable for storage particle, where the amount of Li can vary by large amounts depending on the state of charge, for staging as observed in the storage process in graphite, for the enormous swelling that takes place when silicon is used for storage, and for electrolytes in which the concentration of Li ions is high. The model is used to compute the processes of charging and discharging the battery to assess the parameters that influence the development of stress in the storage particles, and to deduce the likelihood of fracture of the storage particle material. The objective is to assess designs of porous electrode microstructures that permit rapid charging and discharging, but obviate the likelihood of fracture and other mechanical damage that limit the performance and reliability of the battery.

This presentation will discuss applications of the phase-field method to microstructural processes during Li-plating and Li-insertion/intercalation into or extraction from electrodes in Li-ion batteries. The focus will be on Li_xFePO₄, one of the most-studied cathode materials in Li-ion batteries. The thermodynamics of the FePO₄-LiFePO₄ two-phase system and the effect of coherent stress on the miscibility gap and two-phase morphology will be discussed. A three-dimensional phase field model for modeling the morphological evolution during the intercalation/extraction of Li-ions into a host electrode will be described. It incorporates the effects of anisotropic diffusional mobility of Li-ions in the electrode host lattice, flux of Li-ions across the electrode/electrolyte interface, and coherency strains arising from the lattice parameter mismatch between the lithiated and unlithiated phases. Implementation of spectral methods to solving the systems of equations under non-periodic boundary conditions will be presented. The microstructural features obtained from the simulations are compared with available experimental observations.

A PNP-type model for the transport of Li-ion resulting from diffusion and electro-migration is studied. The re-distribution of ion results in an evolving eigen-strain that further induces a displacement over the sample. The displacement on the top surface is proposed to be linearly scaled to the ion concentration, which can be measured by electrochemical strain microscopy (ESM). A bias of DC and AC using scanning probe microscopy can be implemented to probe the local ion concentration (n) and the diffusion coefficient (D) representing the thermodynamic and kinetic characteristics of Li-ion. We illustrate a proposed method to measure ion concentration and diffusivity by means of an ESM experiment and mathematical argument with simulation.

The effect of reversible electrochemical reaction on Li-ion diffusion and stress in a cylindrical Li-ion battery is studied. The volumetric change created by the diffusion of Li-ion and formation of reversible reaction product would generate the diffusion-reaction-induced stress in the electrode. The general relation among concentration of Li-ion, reversible reaction product, and mechanical stress is derived, and the numerical solutions of the concentration, stress and reaction product fields are obtained. The forward reaction with a high forward reaction rate can retard Li-ion diffusion and increase the compressive stress at the surface of the active material for potentiostatic charging, which may lead to premature failure of electrode structure; but reduce the compressive stress in the active material for galvanostatic charging. While the backward reaction has little effect on the distribution of Li-ion and stress. As charging goes on Li-ion diffusion and electrochemical reaction with extremely high forward reaction rate would enter an equilibrium state that the distributions of Li-ion concentration and corresponding stress are stable. The higher the forward reaction rate is, the sooner the equilibrium state comes. The effect of reversible electrochemical reaction on Li-ion diffusion and stress can be neglected if the forward reaction rate is relatively low.

The mechanical degradation of electrodes caused by lithiation and delithiation is one of the main factors responsible for the short cycle life of lithium-based batteries employing high capacity electrodes. Recent experiments have revealed a number of interesting phenomena, such as size-dependent delamination of patterned silicon thin film electrodes from a current collector during lithiation and delithiation cycling, which cannot be satisfactorily explained by existing theories in the literature. Here we discuss a combined series of recent theoretical, computational and experimental studies aimed to clarify the mechanisms of fracture, delamination and ratcheting in thin film Si electrodes on substrates during lithiation/delithiation cycles. We show that the observed delamination size effect can be rationalized by modeling thin film delamination in the presence of large scale interfacial sliding. A method is proposed to deduce the critical size for delamination based on the critical conditions for the nucleation and growth of edge or center cracks at the film-substrate interface under plane strain or axisymmetric conditions. We derive the critical film thickness for fracture as a function of both the fracture toughness of the film and the sliding resistance of the interface. Our analysis indicates that a slippery interface due to lithiation could significantly decrease the critical thickness for fracture. It is further shown that ratcheting can occur as soon as one allows the yield stress of Si and/or the friction strength of the interface to vary from lithiation to delithiation half-cycles, and that this important failure mode can be avoided by reducing the lateral size of the islands below a critical length scale.

Due to the emergence of effects such as the growth of the so-called solid electrolyte interface (SEI), the loss of contact of particles to conductive pathways and the complete disintegration of the electrode, fracture of active electrode particles is a crucial mechanism leading to capacity fade and power loss in commercial lithium ion batteries. The appearance of fracture and cracks in active particles is commonly ascribed to mechanical stresses evolving from heterogeneous swelling and shrinkage of the material when lithium is inserted or extracted.
In our work, we approach the problem of fracture in active particles by combining a coupled model for mechanical stressing and transport of lithium ions with a phase field description of an evolving crack. While the mechanics of the particle is described by a linear, elastic, isotropic constitutive law accounting for swelling and shrinkage, the diffusion equation for lithium transport is derived from fundamental thermodynamics and is valid beyond the dilute concentration limit. Furthermore, the generation of mechanical stress due to heterogeneous lithium concentration is not only simulated, but is also taken into account in the driving force for lithium diffusion. To determine the kinetics of crack evolution, the phase field model is coupled to the equations for stress by a Griffith type energy minimization principle, where the relevant energy is compromised of the surface or fracture energy associated with any crack surface that has been created and the elastic potential energy due to the presence of stress.
This novel approach allows us to simultaneously study the evolution of the lithium concentration together with the initiation and growth of a crack in an arbitrary geometry, in two and three dimensions, and without presuming a specific crack path. This also enables us to compare standard two-dimensional assumptions, such as plane strain or stress, to the fully three-dimensional situation. We investigate how the formation of cracks depends on the geometric features of the particle and the electrochemical boundary conditions applied at the particle surface, e.g. the lithium flux through the surface.

We quantified dendrite growth in an optically accessible Li-metal coin cell battery charged under applied temperature gradients normal to the electrodes. We found that average dendrite lengths decrease by ca. 30% upon increasing T-gradients 3.5 times.

Lithium-ion secondary batteries are commonly used to power many consumer devices such as handheld phones, laptops, portable music players, and even electric vehicles. One of the key properties that influence the performance of lithium-ion batteries is the ionic conductivity of the electrolyte (i.e., the movement of Li ions from one electrode to another). This is dependent on the speed at which Li ions diffuse across the cell and related to the solvation structure of the Li ions. The choice of the electrolyte can greatly impact both solvation and diffusivity of Li ions. In this work, we use first principles molecular dynamics to examine the solvation and diffusion of lithium ions in several bulk organic electrolytes. We find that differences in the local environment throughout the liquid can lead to solvation of Li ions by either carbonyl or ether oxygen atoms. In addition, we examine the differences in solvation of associated and dissociated Li(PF₆) salts, showing that the bulky PF₆ group blocks complete solvation of Li⁺ by solvent oxygen atoms. Finally, we calculate Li diffusion coefficients in each electrolyte, finding slightly larger diffusivities in a linear carbonate such as ethyl methyl carbonate (EMC) compared to a cyclic carbonate like ethylene carbonate (EC). Results from this work can be used to design new bulk electrolytes that will improve the performance of current Li-ion batteries.

The proliferation of portable electronics, hybrid electric vehicles (HEVs), and large scale energy storage has drawn a lot of attentions toward the development of new generation-Lithium ion batteries (LIBs) as the most prevailing power sources. Among all potential anode material for LIBs, Silicon (Si), being able to host a large amount of Lithium (Li)-each silicon atom can host up to 4.4 lithium atoms, is the most promising candidate. It is well-known that migration of sharp interface between the crystalline silicon and amorphous LixSi, called reaction front, controls the kinetics of lithiation. Recently, self-limiting lithiation of Si-nanowires (SiNWs) anodes has been observed using in situ transmission electron microscopy (TEM) [1]. However, the kinetics of Li interface reaction and the evolution of diffusion-induced stress resulting in the retardation of lithiation process remain unclear. In this study, systematic Molecular Dynamics (MD) simulation has been performed using ReaxFF reactive potential to investigate the atomistic mechanisms governing the retardation effect of the lithiation-induced stress upon lithiation. Moreover, the ledge flow process producing the amorphous LixSi through layer-by-layer peeling of the {111} Silicon facets has been modeled throughout this simulation. The simulation provides a comprehensive picture about the orientation-dependent mobility of the sharp-interface, and the underlying physics of the self-limiting dynamic lithiation in Si-NW anodes.
[1] Liu, Xiao Hua, et al. "Self-Limiting Lithiation in Silicon Nanowires." ACS nano 7.2 (2013): 1495-1503.

In the expanding world of small scale energy harvesting, the ability to combine thermal and mechanical harvesting is growing ever more important. We have demonstrated the feasibility of using ZnO nanowires grown using chemical vapor deposition to harvest both mechanical and low-quality thermal energy in simple, scalable devices. Nanowires were transferred to a flexible Kapton thin film by dry transfer. This method yielded well-aligned nanowires on the receiving substrate. Following dry-transfer, the Kapton film was evaporated with patterned gold electrodes using microfabrication techniques. The circuit was designed as long, parallel electrode arrays perpendicular to the nanowire axis. While the ZnO nanowires were of good quality, the internal resistance of the wires necessitated measuring the open circuit voltage (OCV) rather than a direct output power. Mechanical harvesting was demonstrated using a periodic application of force, modeling the motion of the human body. Tapping the device from the top of with a wood stick, for example yielded an OCV of 0.2 - 4 V, which is in an ideal range for device applications. To demonstrate thermal harvesting from low quality heat sources, commercially available Nitinol (Ni-Ti alloy) thin film, a phase transition material with transition temperature of ~ 50 C, was attached to the nanowire piezoelectric device. The whole device assembly was bent at room temperature. Upon heating above 50 C, Nitinol slip restored to its original flat shape, which yielded an output voltage/power of nearly 1 V.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. LLNL-ABS-645293

This work studies the influence of temperature on the output performance of triboelectric generators (TEGs). PTFE film and aluminum foil are used as the contact materials for the TEG. A high temperature system and a low temperature system are used to conduct measurements of TEG output voltage and current from 300 K to 500 K and from 77 K to 300 K respectively. Dependence of output performance on temperature is subsequently obtained by statistically analyzing the data at each temperature point. Several LEDs connected in series are successfully lit up by TEG as the sole power source at temperatures as low as 77 K and as high as 500 K. The results of this study indicates that the output performance of TEG tends to degrade with increasing temperature and confirms the applicability of TEG from 77 K to 500 K, spanning a range of 423 K.

The fundamental science and applicable technology for harvesting environmental energy are not only essential in realizing the self-powered systems, but also tremendously helpful in meeting the rapid-growing world-wide energy consumptions. Mechanical energy is one of the most universally-existing, diversely-presenting, but usually-wasted energies in the natural environment, which has attracted a lot of research efforts in developing the harvesting technologies.
The triboelectric effect is a universal phenomenon that can generate electrostatic charges from mechanical contact. Here, a new type of technology—triboelectric nanogenerators (TENGs)—has been developed to efficiently convert mechanical energy into electricity, based on the coupling of triboelectrification and electrostatic induction. In order to develop TENGs of different structural designs for harvesting various types of mechanical energies existing in the natural environment, we established three basic modes/mechanisms of TENGs that serve as the basis for most of the TENG structures: (1) vertical contact-separation mode, in which two triboelectric-charged layers (with an electrode deposited at the back) periodically separate in vertical-to-plane direction [1]; (2) lateral sliding mode, in which the charge separation is achieved along the in-plane direction through the relative sliding of two triboelectric layers (with an electrode deposited at the back) [2]; (3) freestanding-triboelectric-layer based mode, which has two stationary electrodes and one freestanding triboelectric layer that moves in between under the guidance of external mechanical energy [3]. For every mode, we made both theoretical studies and experimental demonstrations to illustrate the basic working principles and the output characteristics. Also, the key factors and parameters determining the performance were identified for each case, which provides the importance guidance for the future design and optimization of device structures. Moreover, the utilized triboelectric materials and surfaces were purposely modified for the improvement of the static charge density. All of the three modes of TENGs are shown to be effective electrical sources that are capable of generating a voltage over hundreds of volts and a power density larger than 10 W/m2, and also instantaneously driving hundreds of electronic devices (such as LEDs). The generated electricity can also be stored in the energy storage units such as Li-ion batteries and capacitors, for the purpose of driving personal electronics, like cell phones. The establishment of the three basic modes opens the path of the research on triboelectric nanogenerators for applications in self-powered systems for personal electronics, environmental monitoring and even large-scale power.
[1] Wang, S. H.; Lin, L.; Wang, Z. L. Nano Lett. 2012, 12, 6339-6346.
[2] Wang, S. H.; Wang, Z. L. et al. Nano Lett. 2013, 13, 2226-2233.
[3] Wang, S. H.; Wang, Z. L. et al. Adv. Mater. Under review.

Energy harvesting from ubiquitous ambient vibrations has enormous potential for small power applications including but not limited to wireless sensors, portable electronics, and medical implants. Piezoelectric energy harvesting devices constitute the simplest means of scavenging power directly from ambient vibrations through the direct conversion of mechanical energy into electrical energy. In particular, nano-piezoelectric energy harvesting devices sensitive to small vibrations can be incorporated into small-scale devices, which is attractive in light of the increasing demand for flexible, wearable and implantable electronics.
Ceramics such as lead zirconium titanate and semiconductors such as zinc oxide are currently the most widely used piezoelectric energy harvesting materials. However, our work focuses on a different class of piezoelectric material, namely ferroelectric polymers, such as poly-vinylidene fluoride (PVDF) and its co-polymers. These are potentially superior energy harvesting materials due to their flexibility, lightweight, ease of fabrication and potentially low cost, as well as being lead-free and bio-compatible. Piezoelectric nanowires of polyvinylidenefluoride-co-trifluoro-ethylene [P(VDF-TrFE)] having diameter 200 nm and length 60 µm were grown within nanoporous anodized aluminium oxide (AAO) membranes using a simple, scalable, low-cost template wetting technique. Detailed characterization using scanning electron microscopy, differential scanning calorimetry, Fourier transform infrared spectroscopy and X-ray diffraction has shown that the nanowires grown are in the ferroelectric phase, without the need of poling. Energy harvesting devices comprising nanoporous AAO membranes filled with aligned P(VDF-TrFE) nanowires were shown to have an output voltage of 3 V and an output current of 5.5 nA when impacted by an aluminium arm oscillating at a frequency of 5 Hz with an amplitude of 1 mm. The energy harvesting performance of the P(VDF-TrFE) nanowires has been enhanced by fabricating devices comprising nanowires freed from the AAO membranes and laterally aligned on substrates with lithographically-patterned inter-digitated electrodes. Strategies to further improve the electromechanical coupling and thus the energy harvesting performance of the P(VDF-TrFE) nanowires will be discussed.

The modern world is largely defined by electricity. Everything that hallmarks the high-tech era, from advanced illumination, through smart appliances to portable and even wearable electronics, depends for their appearance and development on electricity. Although electricity-generating technologies have been developed for some two hundred years, people have never ceased to explore new methods of power generation in order to address the rapidly rising demand on the electric energy in the modern society.
In this work, we report a unique planar-structured approach called triboelectric generator (TEG) that converts mechanical energy into electricity through harnessing triboelectrification between two rotary surfaces. The periodically changing triboelectric potential generates induced alternating currents formed by free electrons. Enabled by a design of two radial-arrayed fine electrodes that are complementary on the same plane, the TEG completely solves the problem of low output current that has limited the use of triboelectrification in electricity generation. Operating at a rotation rate of 3000 revolutions per minute (rpm), a TEG having a diameter of 10 cm could produce an open-circuit voltage of ~850 V and a short-circuit current of ~3 mA at a frequency of 3 KHz. Under a matched load of 0.5 MOmega;, an average effective power of over 1.5 W could be delivered to an external load at an efficiency of over 25 %. The TEG could directly drive multiple types of light bulbs as a power supply and may potentially charge portable electronics with the assistance of a power management circuit. Given its compelling advantages including small volume, light weight, low cost, and proven scalability, not only is the TEG suited to harvest mechanical energy for self-powered electronics, but also it can be potentially applied for energy generation in large scale.

Interfacial debonding of photovoltaic (PV) encapsulants exposed to hostile application environments is not well understood. In particular, the effects of ultraviolet (UV) light and the environment on the debond kinetics of the highly strained material at a propagating crack tip remains unexplored. The damage of solar environments on PV module materials has traditionally been studied by exposing them to "stress parameters" such as elevated temperatures, moist environments and large doses of UV light. After exposure, degradation in the material has been quantified by measuring changes in selected properties such as color, stiffness and chemical composition. However, the mechanical stress and its importance in determining the kinetics of interface adhesive and cohesive cracking has been rarely investigated but remains critically important. The understanding obtained from improved control of multiple "stress parameters" and their effects on the evolution of defects forms the basis for life-time prediction and the design of accelerated tests.
Using a recently developed quantitative mechanics technique, we report the effect of indoor aging on the debond energy of a photovoltaic ethylene-co-vinyl acetate (EVA) encapsulant. Employing a debond-kinetics characterization method with in-situ UV, we report debond growth rates of the EVA encapsulant in terms of the applied mechanical loads in an environment of controlled temperature and relative humidity. We employ a load relaxation technique that allows debonding rates as low as 1 nm/s to be accurately quantified and related to the rupture of molecular bonds at the crack tip. The effect of moisture, temperature and UV exposure on debond growth acceleration is demonstrated. The debond energy of the encapsulant decreased dramatically from 2000 J/m2 at 20 °C to less than 500 J/m2 at 60 °C. The debond growth rate increased up to 1000-fold with small changes of temperature (10 °C). To elucidate the degradation processes leading to envrionmental debonding, the kinetics of the debond growth process are interpreted using a recently developed viscoelastic fracture-mechanics model, including the effects of moisture and UV light. The molecular bond rupture processes investigated underlie the principal causes of degradation in PV materials exposed to terrestrial environments and can be exploited to acquire, not only a fundamental understanding of damage formation and progression, but also to develop accelerated testing techniques and make long term reliability predictions.

The next generation of high-end rechargeable batteries will still rely on lithium-ion host materials. Later, post-lithium-ion systems, at first Li/S, are expected to enter the market. Independently of the technology, understanding the fundamental electrochemical, structural, and mechanical properties of battery materials and the interactions of these materials with their environment will be the key to further improvements in energy density, safety, and life time of batteries. Our approach to answer the related scientific questions starts with the development of various in situ methods for use mainly in the field of nonaqueous solid-state electrochemistry. Then, the physical and electrochemical properties of host materials and electrochemical interfaces are investigated in situ.
When information related to the bulk of a battery material is needed, X-ray diffraction (XRD) is the method of choice. The electrode materials normally accommodate (insert, intercalate) variable quantities of lithium ions. As a rule, the lithium insertion / de-insertion into / from the electroactive material results in lattice changes of the material with associated volume changes inducing strain, which can be followed by analyzing the X-ray diffractograms. X-ray powder diffraction experiments performed at a standard laboratory diffractometer are sufficient to answer most of the scientific questions, however, when the time factor is critical a synchrotron based XRD is advantageous. Furthermore, in contrast to XRD, neutrons are much more sensitive to lithium. Therefore, the combination of results from synchrotron based in situ X-ray diffraction methods and in situ neutron diffraction is essential for deep understanding of the reactions of battery materials. For in situ neutron diffraction experiments a sophisticated electrochemical cell was constructed allowing quantitative analysis using the Rietveld method. In addition, in situ dilatometry on battery electrodes provides valuable information on their periodic expansion and contraction.
In the talk, the experimental background with focus on the construction of the in situ cells will be given followed by discussion of selected results on battery electrodes.

The functionality of energy storage and generation systems like Li-ion batteries or fuel cells is not only based on but also limited by the flow of ions through the device. To understand device limitations and to draw a roadmap to optimize device properties, the ionic flow has to be studied on relevant length scales of grain sizes, structural defects, and local inhomogeneities, i.e. over tens of nanometers. Knowledge of the interplay between the ionic flow, material properties, and microstructure can be used to optimize the device properties, for example to maximize energy density, increase charging/discharging rates, and improve cycling life for Li-ion batteries for applications in electric vehicles and aerospace. Existing solid-state electrochemical methods are limited to a spatial resolution of 10 micron meter or greater, well above the characteristic size of grains and sub-granular defects, and are based on the measurement of current. Because of that, these established techniques cannot be scaled down easily to the nanoscale. However, ionic flows in solid state systems also generates measurable strains which can reach hundreds of percent in the case of Si anodes for Li-ion batteries. By developing Electrochemical Strain Microscopy (ESM), we utilize the correlation between ionic concentration and strain to investigate ionic transport on the nanoscale using Scanning Probe Microscopy.
Here, we present how ESM can be used to measure ionic transport properties in Li-ion batteries and electrochemical supercapacitors. Both systems exhibit very different origins for strain during the electrochemical reactions which will be discussed in detail. For the first, we will show how to use ESM to extract spatially resolved maps of the activation energy and diffusivity of Li-ions in thin film cathodes for Li-ion batteries. We also explore how the ionic transport is influenced by the electrode microstructure such as grain orientation, step edges, and mechanically induced defects. Additionally, the application of ESM to study ionic transport across solid-solid interfaces will be presented and discussed. For the second, we will explore how the transport kinetics of ions from an ionic liquid into a porous carbon electrode is limited by the pore size and effectively determines the rate limitation of the capacitance.
Support was provided by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division through the Office of Science Early Career Research Program, by the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, and by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

The coupling between electrochemical and mechanical behavior is of great importance for battery electrodes, particularly in light of the large volume changes associated with intercalation. Here, we introduce a method for in-situ analysis of micro-mechanical properties during electrochemical cycling of electrode materials. An electrochemical cell is integrated into a Nanoindenter (MTS G200) which allows the real time measurement of the electrode contraction/expansion and the change in elastic constants during dis-/charge and the determination of the mechanical properties like yield stress and toughness at different states of charge. The setup is suitable for studying the local response of bulk crystals, single particles or granular electrodes during electrochemical cycling in an ionic liquid or organic electrolyte.
Different material systems for anode and cathode materials are under investigation. First successful tests have been carried out on graphite materials such as HOPG (single crystal) and MCMB (particle). The observed volume expansion is consistent with the literature and it can be shown that the Li intercalation reversibly changes the elastic behavior and can be attributed to changes in the microstructure. A detailed analysis of the changes in elastic response of these highly anisotropic materials during lithiation will be presented.
In a further step, the setup is also used for probing the coupling between mechanical stress and the chemical potential of Li. In theory it is possible to shift the chemical potential using mechanical stress by as much as 10-100mV/GPa depending on the material system. For anode materials that are exposed to high stresses during the Li reaction and have working potentials just above the Li-plating potential (e.g. Si), the stress induced shift of the chemical potential becomes an important effect and can have a profound influence on battery performance. We present results on graphite and silicon electrode materials.

Rechargeable lithium ion batteries are currently still irreplaceable power sources for a variety of electronic devices. The batteries generally require fast charging and discharging rate, high energy density, long cycle life, and many other properties. The performance assessment of the Li-ion battery is primarily based on the overall behavior of the sintered bulk cathode or anode material. However, the fundamental components, i.e. the individual particles in these materials, have rarely been studied independently due to many technical difficulties, such as fixation and probing technique. In this work, the responses of a single particle under a controlled electric field were observed by the advanced Scanning Probe Microscopy technique. Two types of the cathode particles Li(Li0.2Mn0.54Ni0.13Co0.13)O2 and LiNi1/3Co1/3Mn1/3O2, whose size typically ranges from 100 to 200 nm, were characterized. In this case, the Li-ion diffusion and O2 evolution of the smallest unit in the Li-ion battery, i.e., the isolated single particle, can be observed based on the changes of particle volume and surface area under gradually increased/decreased DC bias. High-resolution three dimensional particle images were observed with tapping-mode Atomic Force Microscopy. These results greatly benefit to our understanding of the various mechanisms in the development of the Li-ion batteries.

Electrochemistry coupled with mechanics dictates the microstructural evolution and service life of many materials in the energy industry, and underlies problems such as stress-corrosion cracking and battery cyclability. While atomistic and first-principles modeling is adept at looking at the finer details of energetics and microstructural evolution, it often needs help from experiments to identity the key performance-limiting processes. The creation of a nanoscale electrochemical and mechanical testing platform [Science 330 (2010) 1515; Nano Letters 11 (2011) 4535; ACS Nano 6 (2012) 9425; Nature Nano. 8 (2013) 277] inside a transmission electron microscope (TEM) enables direct observations of the electrochemical lithiation and delithiation of the nanowires. SnO2, ZnO, Si, Ge, graphene and carbon nanotube anodes and LiFePO4 nanowire cathode have been tested. Lithium embrittlement is found to be a persistent issue. These in situ experiments complement our modeling efforts, and together they provide insight into how materials degrade in service due to combined electrochemical-mechanical actions.

The tremendous volume expansion and physical deformation of silicon as it alloys with lithium underscores the importance of understanding the mechanical properties and stability of lithiated silicon as a prospective material for lithium-ion battery anodes. Furthermore, the constant charging and discharging nature of battery electrodes implies inherent time constraints on the various physical processes occurring during alloying and de-alloying of the host material. In this work we explore time-dependent deformation mechanisms in fully-lithiated silicon nanowires using in situ mechanical testing methods. Creep testing was performed by applying constant force loading to lithium-silicon nanowires inside a scanning electron microscope. Elongation measurements were used to calculate the strain-rate and clear indications of quasi-viscous deformation behavior are present. Measurements of the wire size and microstructure are also used to gain insight into the atomic diffusion pathways which govern the creep behavior in these lithium-silicon alloy materials. Implications regarding the implementation, design and construction of silicon-based electrode materials will be discussed.

The formation of dendrites, i.e. deposits of metallic lithium on the surface of graphite anodes, is a major safety concern for lithium ion batteries (LIBs). In January 2013, two Boeing 787 Dreamliners experienced LIB failures that led to a worldwide grounding of all 787s.[1] Although the root cause for failure has not yet been officially identified, the incidents have been linked to lithium dendrite formation.[2] However, the mechanisms and the conditions of dendrite growth are still not understood. Theoretical efforts are underway [3,4] and experimental studies have already established general conditions for lithium plating. For example, optical microscopy experiments have been conducted that allow tracking lithium plating in real time.[5] However, these studies typically require tailored cells and specific electrode geometries for optical access, and therefore do not fully mimic the conditions in a real battery.
Here we present tomographic analyses visualizing and quantifying the morphology of lithium dendrites occurring in commercial LIBs. We focus on separators and link the three-dimensional shape of dendrites and their spatial location within the separator to electrochemical testing conditions and cell temperature. Insights gained by this approach can (1) provide the basis for understanding the fundamental mechanisms governing dendrite growth, (2) allow identification of safe operating conditions based on direct assessment of the root cause of battery failure, and (3) guide the selection and development of separators that can appropriately handle dendrite induced short-circuits.
[1] Interim Factual Report: Boeing 787-8, JA829J Battery Fire; Case Number: DCA13IA037, National Transportation Safety Board, Office of Aviation Safety, Washington DC, USA, 2013.
[2] A. Heller, The G. S. Yuasa-Boeing 787 Li-ion Battery: Test It at a Low Temperature and Keep It Warm in Flight, Interfaces 2013, 22, 35.
[3] R. Akolkar, Modeling dendrite growth during lithium electrodeposition at sub-ambient temperature, J. Power Sources 2014, 246, 84.
[4] D. R. Ely, R. E. García, Heterogeneous Nucleation and Growth of Lithium Electrodeposits on Negative Electrodes, J. Electrochem. Soc. 2013, 160, A662-A668.
[5] S. J. Harris, A. Timmons, D. E. Baker, C. Monroe, Direct in situ measurements of Li transport in Li-ion battery negative electrodes, Chem. Phys. Lett. 2010, 485, 265-274.

Lithium-ion batteries (LIBs) are the most widely used energy storage device for electric portable applications and power sources due to their outstanding features, such as non-memory effect, long cycle life, small volume, few self-discharging and light weight. However the capacity decay, power fading and the increase of the impedance during cycling still need to be understood. During the operation cycles of LIBs, lithium ions are repeatedly intercalated from cathode into anode upon charge and de-intercalated reversibly from anode into cathode during discharge. It is therefore necessary to study the transport mechanisms of Li-ions, further to understand the aging mechanism and hence contribute to the performance enhancement of LIBs. The newly-developed Electrochemical Strain Microscopy (ESM) technique allows the high frequency periodic bias to be applied on the sample surface of the electrochemically active materials, and the bias will induce the local periodic oscillatory displacement caused by the Li-ions transport and redistribution within the material. The local surface displacement, which is defined as electrochemical strain, can be detected by a highly sensitive photodetector in ESM. Therefore, ESM has been emerging as a powerful technique to investigate the Li-ion preferred transport paths and the local electrochemical reaction mechanisms associated with Li-ion concentration changes. In this study, a promising cathode system, lithium-rich layered oxide material Li2MnO3-LiMO2 (M = Ni, Co, Mn), with high capacity (280mAh/g) which is approximately twice of that of the commercial cathode materials, is studied by using ESM technique. The results indicate that transport phenomenon of the Li-ions has a direct relationship with surface topography variations. As the applied bias increases, two types of deformation are observed, which are closely, related to the different Li-ion transport mechanisms (migration induced by electric potential difference and diffusion induced by concentration gradient) in the Li-rich cathode thin film. In addition, the grain expansion in cathode is found to be the results of pure Li-ions diffusion.

The scale-up of lithium-ion cells to capacities of 5-100 Ah is critical for their successful implementation in vehicle and grid energy storage systems. However, complex current pathways in large cells cause an uneven spatial distribution of reacted species, and ultimately an under-utilization of active materials. As a result, when scaling to larger capacities the effective energy density of each cell diminishes. Furthermore, uneven current distribution compromises cell safety by causing localized overcharge and overdischarge hotspots. Recent modeling efforts have provided insight towards this spatial variation but such efforts should be validated by experimental techniques.
In this work we focus on measuring the spatially resolved state of charge in a commercial 8 Ah lithium iron phosphate cell. For this purpose, energy-dispersive x-ray diffraction experiments are carried out by probing the cell in situ with a high energy synchrotron white beam. Nine different locations are probed over the course of discharge and the localized state of charge is determined by measuring the relative amounts of lithiated and non-lithiated phases. The data show a variation of +/- 15% in state of charge across the cell and a faster rate of discharge through the center of the cell and towards the positive tab.

The non-aqueous lithium-oxygen battery is one of a host of emerging opportunities available for enhanced energy storage [1]. Unlike a conventional battery where the reagents are contained within the cell, the lithium-oxygen cell uses dioxygen from the atmosphere to electrochemically form the discharge product lithium peroxide. Degrees of reversible oxidation and formation of lithium peroxide has been demonstrated in a number of non-aqueous electrolyte classes, mostly notably in dimethysulfoxide based electrolytes [2], thus making the lithium-oxygen cell a potential energy storage device.
This talk will present our groups recent results of the electrochemistry of dioxygen in non-aqueous electrolytes, of which particular electrolytes could have practical application within a lithium-oxygen cell. Discussion will touch upon how the electrochemistry can be related to electrode substrate and will be presented with in situ spectroscopic studies that identify intermediate and surface species during the oxygen reduction reaction.
[1] P.G. Bruce, S. Freunberger, L.J. Hardwick, J.-M. Tarascon, Nature Mater. (2012) 11 19
[2] Z. Peng, S.A. Freunberger, Y. Chen, P.G. Bruce, Science, (2012) 337 563

Metal oxides are considered as potential future anode materials for lithium ion batteries due to their much higher theoretical capacities compared to currently commercial available anode, graphite (372 mAh g-1). Tin oxide (SnO2) and iron oxide (Fe3O4) were chosen due to the abilities to reversibly form alloy with lithium and react with lithium through a process known as conversion reaction, respectively. However, for both SnO2 and Fe3O4, huge volume changes occur during the process of lithium insertion and removal, which will induce a breakdown in electrical contact between adjacent active particles, and eventually a large drop in capacity over charge-discharge cycles. To alleviate such problem, different forms of carbon were introduced to metal oxides, forming composite structures. These carbons, including graphene (G), were able to improve the electrochemical performance of metal oxide anode materials due to their high conductivity, good lithium permeability, and flexibility to hold the structure integrity. Besides, specially designed 3D structures, such as hollow and porous beads, provided void space for volume change during lithium insertion/removal, thus enhancing the cycling performances of these metal oxide anodes.

We demonstrated the synthesis of carbon thin film with ordered nanostructure via nanoimprint lithography starting from thermal plastic polymer polyvinylpyrrolidone (PVP). After the pattern is transferred to PVP thin film during nanoimprinting process, a three-step thermal treatment is performed to fabricate carbon thin film with pattern. Feature size of the pattern ranges from 250 nm up to 2 um with various shapes including 1D line, 2D hole and pillar. Characterized with Raman spectroscopy, the carbon thin film shows a high degree of disorder with a small in-plane crystallite size of about 1.346 nm calculated using the peak intensity of D band and G band of sp2 carbon. Those imprinted carbon thin film exhibits good electrical conductivity. When used as anode of lithium ion battery, it shows an enhanced cyclic performance compared to those carbon films without patterns. Further study with Electrochemical Strain Microscope (ESM) helps to understand the enhanced Li ion capacity locally on nanoscale.

From mechanics perspective, the biggest impediment to improving reliability of Li-ion batteries is the large film stress induced in electrodes upon Li-ion intercalation. Inherent stress fluctuations within electrodes during regular battery life lead to crack initiation and propagation, eventually leading to capacity degradation. In this study, we investigate a strategy to restore conductance across newly formed anode cracks; thus preventing capacity degradation in Li-ion batteries. Within the anode matrix, we incorporate microcapsules with a conductive core (carbon black suspended in solvent). Upon mechanical trigger, microcapsules crack and deliver their core in damage zone. We designed a custom battery cell to observe healing in-situ besides other set ups to assess release, delivery, and functionality of the conductive core outside of the battery. The core is shown to be an effective healing agent outside of the battery. In the custom battery cell, we accelerate crack formation by creating mechanically vulnerably location on a conventional graphite base battery anode, and compare self healing electrodes to control electrodes based on the trend of battery capacity and color variations due to Li-ion intercalation.

“Electrochemical shock”—the electrochemical cycling-induced fracture of materials—contributes to impedance growth and performance degradation in ion-intercalation batteries, such as lithium-ion. Using a combination of micromechanical models and acoustic emission experiments, the mechanisms of electrochemical shock are identified, classified, and modeled in targeted model systems with different composition and microstructure. A particular emphasis is placed on mechanical degradation occurring in the first electrochemical cycle. Three distinct mechanisms of electrochemical shock are identified, and a fracture mechanics failure criterion is derived for each mechanism. In a given material system, crystal symmetry and phase-behavior determine the active mechanisms. A surprising result is that electrochemical shock in commercial lithium-storage materials occurs by mechanisms that are insensitive to the electrochemical cycling rate. This fundamental understanding of electrochemical shock leads naturally to practical design criteria for battery materials and microstructures that improve performance and energy storage efficiency. These microstructure and crystal chemical design criteria are demonstrated experimentally for spinel materials such as LiMn₂O₄ and LiMn_1.5Ni_0.5O₄. A case study of LiMn_1.5Ni_0.5O₄ is presented, in which small changes in composition that have negligible impact on electrochemical properties induce a significant change in phase behavior that allow electrochemical shock at relevant electrochemical cycling rates to be avoided. While lithium-storage materials are used as model systems for experimental study, the physical phenomena are common to other ion-intercalation systems, including sodium- and magnesium-storage compounds.

We demonstrate that nanometer-scale TiN coatings deposited by atomic layer deposition (ALD), and to a lesser extent by magnetron sputtering, will significantly improve the electrochemical cycling performance of silicon nanowire lithium-ion battery (LIB) anodes. A 5 nm thick ALD coating resulted in optimum cycling capacity retention (55% vs. 30% for the bare nanowire baseline, after 100 cycles) and coulombic efficiency (98% vs. 95%, at 50 cycles), also more than doubling the high rate capacity retention (e.g. 740 mAh/g vs. 330 mAh/g, at 5C). We employed a variety of advanced analytical techniques such as electron energy loss spectroscopy (EELS TEM), focused ion beam analysis (FIB) and x-ray photoelectron spectroscopy (XPS) to elucidate the origin of these effects. The conformal 5 nm TiN remains sufficiently intact to limit the growth of the solid electrolyte interphase (SEI), which in turn both improves the overall coulombic efficiency and reduces the life-ending delamination of the nanowire assemblies from the underlying current collector. Our findings should provide a broadly applicable coating design methodology that will improve the performance of any nanostructured LIB anodes where SEI growth is detrimental.

Lithium-ion batteries have revolutionized portable electronics, and will be key to electrifying transport vehicles and delivering renewable electricity. Amorphous silicon (a-Si) is being intensively studied as a high-capacity anode material for next-generation lithium-ion batteries. Its lithiation has been widely thought to occur through a single-phase mechanism with gentle Li profiles, thus offering a significant potential for mitigating pulverization and capacity fade. Here, we discover a surprising two-phase process of electrochemical lithiation in a-Si by using in situ transmission electron microscopy. The lithiation occurs by the movement of a sharp phase boundary between the a-Si reactant and an amorphous LixSi (a-LixSi, x ~ 2.5) product. Such a striking amorphous-amorphous interface exists until the remaining a-Si is consumed. Then a second step of lithiation sets in without a visible interface, resulting in the final product of a-LixSi (x ~ 3.75). We show that the two-phase lithiation can be the fundamental mechanism underpinning the anomalous morphological change of microfabricated a-Si electrodes, i.e., from a disk shape to a dome shape. Our results represent a significant step toward the understanding of the electrochemically-driven reaction and degradation in amorphous materials, which is critical to the development of microstructurally stable electrodes for high-performance lithium-ion batteries.

Silicon has been an attractive candidate for use as the anode material in Li ion batteries. This is because the storage capacity of Si for lithium insertion is over an order of magnitude greater than that of conventional anode materials like graphite. Bulk and thin film Si anodes suffer severe mechanical degradation and loss of capacity when subjected to several charge/discharge cycles because of the large volume change associated with lithiation into Si. Nanostructured Si electrodes, such as nanowires and inverse opal lattices, have greatly improved mechanical characteristics due to increased ductility in nano-sized Si and the ability of pores/free spaces to absorb Si volume change during lithiation cycles.
We present a 3D silicon nano-lattice whose structural geometry is optimized for mechanical toughness during large volume changes. Principles from solid cellular mechanics were used to determine connectivity and angles between lattice members to minimize global volume change and mechanical stress under lithiation. 2-photon laser lithography was used to write a 3D polymer scaffolds in the shape of the optimized lattice structure in which beams are ~300-700 nm wide and 1 to 5 microns in length. Cu and then amorphous Si is deposited onto the polymer lattice using RF magnetron sputtering. Si layer thickness is limited to hundreds of nanometers in order to take advantage of the enhanced ductility in nanoscale Si.
In-situ scanning electron microscopy was used to directly observe volume change, mechanical deformation and failure in the Si anode during cycles of lithiation. A lithium electrode was mounted on a telescoping bar on the wall of the SEM chamber. Electrochemical testing was performed by contacting the lithium electrode, electrolyte and Si nano-architected electrode (located on the SEM sample stage) and applying a bias voltage bias across the electrodes. In-situ SEM observations show that mechanical reliability of Si electrodes can be controlled and improved by constructing them as nano-architected structures.

Design and fabrication of reliable electrodes subject to repeated deformation is one of the most important challenges in flexible energy applications since mechanical and electrical properties of devices degrade gradually because of fatigue damage. Here, we introduce two unique nanostructure designs for flexible thin film electrodes on a polymer substrate to suppress fatigue crack damage during cyclic bending. First, a fatigue damage-free flexible metal electrode was developed. To suppress crack nucleation, we introduced a novel nanostructured fatigue damage-free copper electrode on flexible substrate by creating 2-D nanohole arrays. By Implementing this fatigue damage free cu electrode, we successfully demonstrated a flexible Li ion Batteries. Secondly, we developed a polymer nanofiber/TiO2 nanoparticle composite photo-electrode with high bendability by a spray-assisted electro-spinning method. The composite film structure similar to that of a fiber-reinforced composite is used as the photo-electrode in plastic dye-sensitized solar cells (DSCs). Compared to conventional DSCs, composite-based DSCs show outstanding bending stability because the polymer nanofibers prevent delamination of the electrode by relieving the external stress and effectively retarding crack generation and propagation.

The zinc antimonide has been of interest for years in the search for efficient and low-cost thermoelectric materials. Of primary interest has been the β-Zn₄Sb₃ phase which shows a thermoelectric figure of merit, zT, in excess of 1 in proper temperature ranges. This phase is relatively environmental friendly and earth abundant elements, continues to lead vibrant research. Especially, compound Zn_xSb_y is one of the most efficient thermoelectric materials known in high temperature. High temperature thermoelectric materials are of interest for applications as power generators by using waste heat from flue gas. For waste heat recovery applications such as vehicle exhaust and power plant heat, it is very important to find materials with enhanced power factors at above 150 °C. We investigated the Zn_xSb_y thin films deposited by radio frequency (RF) magnetron sputtering for applications in high temperature thermoelectrics. We maximized thermoelectric properties of Zn_xSb_y thin films by adjusting RF power, deposition time, substrate temperature and so forth. At 321 °C, the measured resistivity and Seebeck coefficient values of the Zn_xSb_y film were 17.2 mu;Omega; m and 146 mu;V K^-1, respectively, yielding a power factor value of about 1.39 mW m^-1 K^-2, which is consistent or little above with the reported results for Zn₄Sb₃ single crystals at 400 °C. We also studied extensively the relationships between phase transformation and thermoelectric properties of Zn_xSb_y thin films by using high temperature X-ray diffraction (XRD) analysis, as a function of operation temperature. According to high temperature XRD analysis, the thermoelectric performanceZn_xSb_y thin films depend primarily on a phase distribution in thin film such as Zn₄Sb₃ and ZnSb, etc. As a result, the highest zT of the Zn_xSb_y film were estimated to be 1.26 at 321°C. This value is conservatively estimated since the thin films should have a much reduced thermal conductivity due to their small crystallite sizes and thus the zT value would be higher. Moreover, the Field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic absorption Spectrophotometer (AAS), and specific resistance measurements showed that Zn_xSb_y thin films is useful candidate for high temperature thermoelectric applications.

Recently, some of the layered MAX phases (Mn+1AXn systems, where n = 1, 2, or 3, "M" is an early transition metal, "A" is A group elements, mostly groups 13 and 14 elements, and "X" is carbon and/or nitrogen) have been exfoliated into two-dimensional single- and multi-layers, so-called MXenes [1,2]. Bulk MAX phases are metallic systems. However, our calculations exhibited that upon appropriate functionalization by F, OH, and O groups, some of the exfoliated MAX phases become narrow-band-gap semiconductors, and therefore, might be potentially good thermoelectric materials [3]. Here, on the basis of a set of first-principles band structure calculations combined with Boltzmann transport theory [4,5], we have studied the transport coefficients (Seebeck, electrical conductivity, and power factor) of various functionalized MXenes. Our calculations predict that a number of MXenes may exhibit high thermoelectric performance.
[1] M. Nagubi et al., Adv. Mater. 23, 4248 (2011).
[2] M. Nagubi et al., ACS Nano 6, 1322 (2012).
[3] M. Khazaei et al., Adv. Funct. Mater. 23, 2185 (2013).
[4] G. K. H. Madsen and D. J. Singh, Comput. Phys. Commun 175, 67 (2006).
[5] J. Yang et al., Adv. Funct. Mater. 18, 2880 (2008).

The materials class of skutterudites is one of the promising thermoelectric materials due to its decent electronic properties and cage-like structural feature that can be filled with guest atoms. In this study, first-principles calculations have been performed in order to investigate electronic band structures and related transport properties of pnictogen-substitued skutterudites filled with alkaline-earth elements (MxCo4X6Te6 where M=Ca, Sr, or Ba, X=Ge or Sn, and x=0.5 or 1). Electronic transport properties related to thermoelectricity, including the Seebeck coefficient and the electrical conductivity, are computed by using the Boltzmann transport formalism within the constant-relaxation-time-approximation. The results are compared against the corresponding properties of the unfilled pnictogen-substitued ternary skutterudites (CoX1.5Te1.5) to identify the effects of filling, based on which the potential of filled pnictogen-substituted skutterudites for thermoelectric applications is evaluated. The possible changes in the ionic character of the interatomic bonding, which was suspected to be an important scattering source in unfilled pnictogen-substitued ternary skutterudites, are probed by analyzing the projected density of states, in order to identify a path for potential improvement of the thermoelectric performance of pnictogen-substitued skutterudites.

Supercapacitors are one of the most promising electrochemical energy storage systems due to rapid charge/discharge, cycle stability, high power density, and applications in many fields from electric devices. However, because low energy density have biggest drawback of the supercapacitors, it is difficult to use of the lithium-ion battery levels. So, the role of the electrode is important to enhance the energy density. The importance of the electrode in supercapacitors has been already reported in the latest literature and it has been applied using many electrode carbon materials such as CNT, graphene. Recently, biomaterials have attracted considerable attention for their applications from environment to electric field. Of biomaterials, a new cellulose source, bacterial celluloses (BC), has emerged. BC is the name given to cellulose that is obtained from bacteria through process, such as biosynthesis from of various microorganisms, enzymatic in vitro synthesis, or even chemosynthesis from a glucose derivative. These materials are not only frequently used as model substances for further research on the structures and reactivity of cellulose but also used to develop new materials because of their specific nano-structures. Nanostructured design of electrode materials is important for electrode as supercapacitors to achieve high performances. Carbon materials obtained from carbonized BC maintain the nano-structure which made efficient diffusion of electrolyte ions and conducting pathway of the electrode. However, the carbonized BC has macroporous structure and deficiency of micropores, which are advantageous in electrical double layer capacitance. This can be improved by activation process using KOH. Also, nitrogen doping on the surface of the carbonized BC can induce increasing capacitance owing to both EDLC-based capacitance and pseudocapacitive effects.
In this study, the nitrogen-enriched hierarchical porous carbons from bacterial cellulose were prepared, and their electrochemical performances for supercapacitors were investigated.

Two-dimensional materials and their functionalized structures have shown interesting promise for thermoelectric applications. For example, by reducing the dimensionality of the structure, a new route for substantially enhancing the figure of merit in graphene has been recently suggested, suppressing the thermal conductivity while at the same time enhancing the power factor. In this work, we explore the potential of stacked two-dimensional materials for thermoelectric applications. These heterostructures, which consist of a combination of different isolated layers, give rise to unusual properties due to the relatively weak Van der Waals interaction between the layers. Because the individual layers intact with strong covalent bonds, they maintain their dimensionality even after stacking. Results will be presented for the power factor and thermal conductivity computed using a combination of quantum mechanical and classical simulation approaches, for a range of stacked heterostructures.

To reduce our dependence on fossil fuels and to reduce greenhouse gas emissions, thermoelectric(TE) technology, which can convert waste heat directly into electricity, is one of the more reliable technologies available. Magnesium silicide (Mg2Si) has been recognized as a promising environmentally-benign thermoelectric material operating in the temperature range from 600K to 900K due to several attractive features, such as its lightweight, the abundance of its consistent elements, and its non-toxicity. Focusing on the appropriate systems for integration into automotive applications, TE power generation modules should be resilient against thermal stresses and vibration during heat-cycling conditions. In order to realize a practical TE power generation module, greater understanding of the mechanical properties of appropriate TE materials is essential to enable device design of structures for TE power generators.
Currently, Sb-doped Mg2Si is one candidate that offers practical durability and useful ZT values at elevated temperatures. However, it has been recognized that it has poor sintering reproducibility due to the undesirable inclusion of cracks. In order to improve the scalability and reproducibility of Sb-doped Mg2Si during sintering to sufficient levels, metallic binders such as Al, Cu, Ni, and Zn have been incorporated during the sintering process, resulting in a successful increase in the productivity of sintered pellets,accompanied by improved stability and enhancement of their TE properties.
In order to design a TE module, appropriate mechanical properties are required for the corresponding TE materials. Thus, the purpose of this report is to understand the mechanical properties of sintered Sb-doped Mg2Sithat can be made using our present fabrication methods. We examined ultrasonic tests, nano-indentation tests,3- or 4-point bending tests, and compression tests on sintered pellets of Sb-doped specimens with or without metallic binders. We evaluated the mechanical properties, such as Young&’s modulus, hardness, bending strength, fracture toughness, compressive strength in order to characterizethe currently available Mg2Si. When we think about mass-production-scale fabrication process for Sb-doped Mg2Si TE chips, good spatial homogeneity of the sintered pellets in terms of TE and mechanical properties is taken as read. Thus, we would like to discuss the effects of the incorporation of metallic binders during sintering in terms of local or spatial variations in those mechanical properties that are correlated with the TE properties. Since the TE materials are expected to be utilized in atmospheres at elevated temperatures, additional mechanical properties such as aging functions were also investigated.

The direct conversion of waste heat to electricity is a large incentive to find viable thermoelectric (TE) materials and efforts worldwide are intensifying [1]. The challenge is that TE materials must meet the environmental friendliness and economical requirements in addition to achieving high TE efficiency. Therefore, designing TE device from non-toxic and earth-abundant materials should be a key strategy. Recently, we have discovered that the mineral chalcopyrite, chemically expressed as CuFeS₂, exhibits good TE properties by carrier doping [2]. The electron doped sample Zn_0.03Cu_0.97FeS₂ and Cu_0.97Fe_1.03S₂ show large negative Seebeck coefficient and relatively low electrical resistivity, resulting in a high power factor of above 1 mW/K²m around room temperature. However, the compounds exhibit high thermal conductivity of 5 W/Km. In addition, the compound is only stable at temperature below 600 K [3].
To overcome these problems, we have focused on Cu₉Fe₉S₁₆, known as mooihoekite. The crystal unit cell of Cu₉Fe₉S₁₆ is twice as large as that of CuFeS₂, involving 11 crystalographically inequivalent sites. The complex crystal structure will have a great effect in reducing lattice thermal conductivity [4]. We have synthesized Cu₉Fe₉S₁₆ samples and measured the properties. We observed a large negative Seebeck coefficient S = -150 mu;V/K with a low electrical resistivity ρ= 6 mOmega;cm at 400 K. The thermal conductivity around room temperature is significantly reduced to 2.0 W/Km. Furthermore, the sample is stable up to 800 K, sustainable at much higher temperatures than CuFeS₂.
[1] Thermoelectric Nanomaterials, Materials Design and Application, ed. K. Koumoto and T. Mori, (Springer, Heidelberg, 2013).
[2] N. Tsujii and T. Mori, Appl. Phys. Exp. 6 (2013) 043001. (Spotlights paper)
[3] N. Tsujii, T. Mori and Y. Isoda, J. Elec. Mater., submitted.
[4] E. S. Toberer et al., Chem. Mater. 22 (2010) 624.

Abstract: We have studied the thermoelectric properties of Pb-doped bulk samples prepared by high -pressure and high-temperature (HPHT) method. Seebeck coefficient, electrical resistivity, power factor were performed at room temperature. As our expected, the Seebeck coefficient has been increased with an increase of the synthetic pressure .This improvement is mainly ascribed to the abnormal increase in hole mobility, which is realized by a decreased degree of disorder with the introduction of Pb. The electrical resistivity increases with the increase of synthetic pressure, however that is not apparent yet. The results indicate that HPHT technique may be helpful for optimizing electrical and thermal transports in relatively independent way. In addition, the HPHT method has been shortened the processing time from several days to half an hour. The present study provides a new platform to prepare β-Zn4Sb3 with enhancing thermoelectric properties.
Key words: β-Zn4Sb3, HPHT, Pb-doped,Thermoelectric Performance