Guest Editorial

J. Electrochem. En. Conv. Stor.. 2016;13(3):030301-030301-1. doi:10.1115/1.4035311.

In recent years, a critical need has emerged to accelerate innovation of energy storage devices with the goal of improving device performance (energy and power), safety, and reliability for diverse applications ranging from vehicle electrification to renewable energy integration and grid storage. Lithium-ion batteries, for example, are leading the race for electric drive vehicles, while alternative chemistries like sodium-ion batteries are receiving renewed attention for grid applications. A commonality among these energy storage devices is that they are complex, dynamic systems composed of mixed functional materials. These materials support a multitude of coupled physicochemical processes encompassing electronic, ionic, and diffusive transport in electrode and electrolyte phases, electrochemical and phase-change reactions, and stress generation in multiscale porous electrodes. The performance and lifetime of such electrochemical energy storage devices are therefore dependent on complex reaction and transport processes spanning across multiple length and time scales. Continued improvement of electrochemical energy storage devices for vehicle electrification, renewable energy integration, and grid storage depends on understanding the underlying multiscale, multiphysics processes. Computational models and characterization of mechanical, thermal, and electrochemical processes play an important role in providing insight into coupled multiphysics interactions.

Commentary by Dr. Valentin Fuster

Review Article

J. Electrochem. En. Conv. Stor.. 2016;13(3):030801-030801-13. doi:10.1115/1.4034413.

The performance, safety, and reliability of electrochemical energy storage and conversion systems based on Li-ion cells depend critically on the nature of heat transfer in Li-ion cells, which occurs over multiple length scales, ranging from thin material layers all the way to large battery packs. Thermal phenomena in Li-ion cells are also closely coupled with other transport phenomena such as ionic and charge transport, making this a challenging, multidisciplinary problem. This review paper presents a critical analysis of recent research literature related to experimental measurement of multiscale thermal transport in Li-ion cells. Recent research on several topics related to thermal transport is summarized, including temperature and thermal property measurements, heat generation measurements, thermal management, and thermal runaway measurements on Li-ion materials, cells, and battery packs. Key measurement techniques and challenges in each of these fields are discussed. Critical directions for future research in these fields are identified.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2016;13(3):030802-030802-5. doi:10.1115/1.4034415.

The lithium-ion battery (LIB) has emerged as a key energy storage device for a wide range of applications, from consumer electronics to transportation. While LIBs have made key advancements in these areas, limitations remain for Li-ion batteries with respect to affordability, performance, and reliability. These challenges have encouraged the exploration for more advanced materials and novel chemistries to mitigate these limitations. The continued development of Li-ion and other advanced batteries is an inherently multiscale problem that couples electrochemistry, transport phenomena, mechanics, microstructural morphology, and device architecture. Observing the internal structure of batteries, both ex situ and during operation, provides a critical capability for further advancement of energy storage technology. X-ray imaging has been implemented to provide further insight into the mechanisms governing Li-ion batteries through several 2D and 3D techniques. Ex situ imaging has yielded microstructural data from both anode and cathode materials, providing insight into mesoscale structure and composition. Furthermore, since X-ray imaging is a nondestructive process studies have been conducted in situ and in operando to observe the mechanisms of operation as they occur. Data obtained with these methods has also been integrated into multiphysics models to predict and analyze electrode behavior. The following paper provides a brief review of X-ray imaging work related to Li-ion batteries and the opportunities these methods provide for the direct observation and analysis of the multiphysics behavior of battery materials.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2016;13(3):030803-030803-9. doi:10.1115/1.4035310.

A Li-ion battery is a system that dynamically couples electrochemistry and mechanics. The electrochemical processes of Li insertion and extraction in the electrodes lead to a wealth of phenomena of mechanics, such as large deformation, plasticity, cavitation, fracture, and fatigue. Likewise, mechanics influences the thermodynamics and kinetics of interfacial reactions, ionic transport, and phase transformation of the electrodes. The emergence of high-capacity batteries particularly enriches the field of electrochemomechanics. This paper reviews recent observations on the intimate coupling between stresses and electrochemical processes, including diffusion-induced stresses, stress-regulated surface charge transfer, interfacial reactions, inhomogeneous growth of lithiated phases, instability of solid-state reaction front (SSRF), as well as lithiation-modulated plasticity and fracture in the electrodes. Most of the coupling effects are at the early stage of study and are to be better understood. We focus on the elaboration of these phenomena using schematic illustration. A deep understanding of the interactions between mechanics and electrochemistry and bridging these interdisciplinary fields can be truly rewarding in the development of resilient high-capacity batteries.

Commentary by Dr. Valentin Fuster

Research Papers

J. Electrochem. En. Conv. Stor.. 2016;13(3):031001-031001-10. doi:10.1115/1.4034755.

Particle size plays an important role in the electrochemical performance of cathodes for lithium-ion (Li-ion) batteries. High energy planetary ball milling of LiNi1/3Mn1/3Co1/3O2 (NMC) cathode materials was investigated as a route to reduce the particle size and improve the electrochemical performance. The effect of ball milling times, milling speeds, and composition on the structure and properties of NMC cathodes was determined. X-ray diffraction analysis showed that ball milling decreased primary particle (crystallite) size by up to 29%, and the crystallite size was correlated with the milling time and milling speed. Using relatively mild milling conditions that provided an intermediate crystallite size, cathodes with higher capacities, improved rate capabilities, and improved capacity retention were obtained within 14 μm-thick electrode configurations. High milling speeds and long milling times not only resulted in smaller crystallite sizes but also lowered electrochemical performance. Beyond reduction in crystallite size, ball milling was found to increase the interfacial charge transfer resistance, lower the electrical conductivity, and produce aggregates that influenced performance. Computations support that electrolyte diffusivity within the cathode and film thickness play a significant role in the electrode performance. This study shows that cathodes with improved performance are obtained through use of mild ball milling conditions and appropriately designed electrodes that optimize the multiple transport phenomena involved in electrochemical charge storage materials.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2016;13(3):031002-031002-10. doi:10.1115/1.4034412.

Understanding interfacial phenomena such as ion and electron transport at dynamic interfaces is crucial for revolutionizing the development of materials and devices for energy-related applications. Moreover, advances in this field would enhance the progress of related electrochemical interfacial problems in biology, medicine, electronics, and photonics, among others. Although significant progress is taking place through in situ experimentation, modeling has emerged as the ideal complement to investigate details at the electronic and atomistic levels, which are more difficult or impossible to be captured with current experimental techniques. Among the most important interfacial phenomena, side reactions occurring at the surface of the negative electrodes of Li-ion batteries, due to the electrochemical instability of the electrolyte, result in the formation of a solid-electrolyte interphase layer (SEI). In this work, we briefly review the main mechanisms associated with SEI reduction reactions of aprotic organic solvents studied by quantum mechanical methods. We then report the results of a Kinetic Monte Carlo method to understand the initial stages of SEI growth.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2016;13(3):031003-031003-10. doi:10.1115/1.4034697.

Present research deals with multiphysics, pore-scale simulation of Li–O2 battery using multirelaxation time lattice Boltzmann method. A novel technique is utilized to generate an idealized electrode–electrolyte porous media from the known macroscopic variables. Present investigation focuses on the performance degradation of Li–O2 cell due to the blockage of the reaction sites via Li2O2 formation. Present simulations indicate that Li–air and Li–O2 batteries primarily suffer from mass transfer limitations. The study also emphasizes the importance of pore-scale simulations and shows that the morphology of the porous media has a significant impact on the cell performance. While lower porosity provides higher initial current, higher porosity maintains sustainable output.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2016;13(3):031004-031004-5. doi:10.1115/1.4034483.

This perspective paper underscores the importance of coupled electro-mechanical studies in lithium battery systems with a specific example given of the interaction between temperature-dependent dendrite morphologies and polymer separators. Polymer separators are passive components in lithium battery systems yet play a critical role in cell safety. Separators must maintain dimensional stability to provide electronic isolation of the active electrodes and resist puncture and penetration from lithium dendrites. The polyolefin class of polymers has been used extensively for this application with mixed success. Recent research efforts to characterize lithium dendrite formation and growth have shown distinct temperature-dependent dendrite morphologies: rounded blunt mushroom-shaped, sharp jagged needle-like, and granular particulates. Each of these dendrite morphologies will induce a difference physical interaction with the polymer separator. Anticipating this interaction is difficult since the mechanical properties of the polymer separator itself are largely temperature dependent. This paper describes the anticipated physical interaction of the three different dendrite morphologies listed above as a function of temperature and the local physical properties of the commercial polymer separator. A discussion is also provided on the utility of estimating local mechanical properties in the electrochemical battery environment from traditional mechanical and thermomechanical measurements made in the laboratory.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2016;13(3):031005-031005-10. doi:10.1115/1.4034410.

Battery performance, while observed at the macroscale, is primarily governed by the bicontinuous mesoscale network of the active particles and a polymeric conductive binder in its electrodes. Manufacturing processes affect this mesostructure, and therefore battery performance, in ways that are not always clear outside of empirical relationships. Directly studying the role of the mesostructure is difficult due to the small particle sizes (a few microns) and large mesoscale structures. Mesoscale simulation, however, is an emerging technique that allows the investigation into how particle-scale phenomena affect electrode behavior. In this manuscript, we discuss our computational approach for modeling electrochemical, mechanical, and thermal phenomena of lithium-ion batteries at the mesoscale. We review our recent and ongoing simulation investigations and discuss a path forward for additional simulation insights.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2016;13(3):031006-031006-13. doi:10.1115/1.4035198.

The lithium-ion battery (LIB) electrode represents a complex porous composite, consisting of multiple phases including active material (AM), conductive additive, and polymeric binder. This study proposes a mesoscale model to probe the effects of the cathode composition, e.g., the ratio of active material, conductive additive, and binder content, on the electrochemical properties and performance. The results reveal a complex nonmonotonic behavior in the effective electrical conductivity as the amount of conductive additive is increased. Insufficient electronic conductivity of the electrode limits the cell operation to lower currents. Once sufficient electron conduction (i.e., percolation) is achieved, the rate performance can be a strong function of ion-blockage effect and pore phase transport resistance. Even for the same porosity, different arrangements of the solid phases may lead to notable difference in the cell performance, which highlights the need for accurate microstructural characterization and composite electrode preparation strategies.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2016;13(3):031007-031007-10. doi:10.1115/1.4035245.

Lithium-ion batteries are the most commonly used portable energy storage technology due to their relatively high specific energy and power but face thermal issues that raise safety concerns, particularly in automotive and aerospace applications. In these environments, there is zero tolerance for catastrophic failures such as fire or cell rupture, making thermal management a strict requirement to mitigate thermal runaway potential. The optimum configurations for such thermal management systems are dependent on both the thermo-electrochemical properties of the batteries and operating conditions/engineering constraints. The aim of this study is to determine the effect of various combined active (liquid heat exchanger) and passive (phase-change material) thermal management techniques on cell temperatures and thermal balancing. The cell configuration and volume/weight constraints have important roles in optimizing the thermal management technique, particularly when utilizing both active and passive systems together. A computational modeling study including conjugate heat transfer and fluid dynamics coupled with thermo-electrochemical dynamics is performed to investigate design trade-offs in lithium-ion battery thermal management strategies. It was found that phase-change material properties and cell spacing have a significant effect on the maximum and gradient of temperature in a module cooled by combined active and passive thermal management systems.

Commentary by Dr. Valentin Fuster

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