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Register a new userElectrochemical Modeling of GITT Measurements for Improved Solid-State Diffusion Coefficient Evaluation
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Galvanostatic Intermittent Titration Technique (GITT) is widely used to evaluate solid-state diffusion coefficients in electrochemical systems. However, the existing analysis methods for GITT data require numerous assumptions, and the derived diffusion coefficients typically are not independently validated. To investigate the validity of the assumptions and derived diffusion coefficients, we employ a direct pulse fitting method for interpreting GITT data that involves numerically fitting an electrochemical pulse and subsequent relaxation to a one-dimensional, single-particle, electrochemical model coupled with non-ideal transport to directly evaluate diffusion coefficients that are independently verified through cycling predictions. Extracted from GITT measurements of the intercalation regime of FeS2 and used to predict the discharge behavior, our non-ideal diffusion coefficients prove to be two orders of magnitude more accurate than ideal diffusion coefficients extracted using conventional methods. We further extend our model to a polydisperse set of particles to show the validity of a single-particle approach when the modeled radius is proportional to the total volume-to-surface-area ratio of the system.
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Characterizing electrochemical energy conversion devices during operation is an important strategy for correlating device performance with the properties of cell materials under real operating conditions. While operando characterization has been used extensively for low temperature electrochemical cells, these techniques remain challenging for solid oxide electrochemical cells due to the high temperatures and reactive gas atmospheres these cells require. Operando X-ray diffraction measurements of solid oxide electrochemical cells could detect changes in the crystal structure of the cell materials, which can be useful for understanding degradation process that limit device lifetimes, but the experimental capability to perform operando X-ray diffraction on the fuel electrodes of these cells has not been demonstrated. Here we present the first experimental apparatus capable of performing X-ray diffraction measurements on the fuel electrodes of high temperature solid oxide electrochemical cells during operation under reducing gas atmospheres. We present data from an example experiment with a model solid oxide cell to demonstrate that this apparatus can collect X-ray diffraction spectra during electrochemical cell operation at high temperatures in humidified H2 gas. Measurements performed using this apparatus can reveal new insights about solid oxide fuel cell and solid oxide electrolyzer cell degradation mechanisms to enable the design of durable, high performance devices.
Interfacial deposition stability between Li metal and a solid electrolyte (SE) is important in preventing interfacial contact loss, mechanical fracture, and dendrite growth in Li-metal solid-state batteries (SSB). In this work, we investigate the deposition and mechanical stability at the Li metal/SE interface and its consequences (such as SE fracture and contact loss). A wide range of contributing factors are investigated, such as charge and mass transfer kinetics, the plasticity of Li metal and fracture of the SE, and the applied stack pressure. We quantify the effect of the ionic conductivity of the SE, the exchange current density of the interfacial charge-transfer reaction and SE surface roughness on the Li deposition stability at the Li metal/SE interface. We also propose a mechanical stability window for the applied stack pressure that can prevent both contact loss and SE fracture, which can be extended to other metal-electrode (such as Sodium) SSB systems.
We report the development of redox-active conjugated polymers with potential application to electrochemical energy storage. Side chain engineering enables processing of the polymer electrodes from solution, stability in aqueous electrolytes and efficient transport of ionic and electronic charge carriers. We synthesized a 3,3 dialkoxybithiophene homo polymer (p type polymer) with glycol side chains and prepared naphthalene 1,4,5,8-tetracarboxylic-diimide-dialkoxybithiophene (NDI gT2) copolymers (n type polymer) with either a glycol or zwitterionic side chain on the NDI unit. For the latter, we developed a post-functionalization synthesis to attach the polar zwitterion side chains to the polymer backbone to avoid challenges of purifying polar intermediates. We demonstrate fast and reversible charging of solution processed electrodes for both the p- and n type polymers in aqueous electrolytes, without using additives or porous scaffolds and for films up to micrometers thick. We apply spectroelectrochemistry as an in operando technique to probe the state of charge of the electrodes. This reveals that thin films of the p-type polymer and zwitterion n-type polymer can be charged reversibly with up to two electronic charges per repeat unit (bipolaron formation). We combine thin films of these polymers in a two-electrode cell and demonstrate output voltages of up to 1.4 V with high redox stability. Our findings demonstrate the potential of functionalizing conjugated polymers with appropriate polar side chains to improve specific capacity, reversibility and rate capabilities of polymer electrodes in aqueous electrolytes.
Aluminum-germanium nanowires (NWs) thermal activated solid state reaction is a promising system as very sharp and well defined one dimensional contacts can be created between a metal and a semiconductor, that can become a quantum dot if the size becomes sufficiently small. In the search for high performance devices without variability, it is of high interest to allow deterministic fabrication of nanowire quantum dots, avoiding sample variability and obtaining atomic scale precision on the fabricated dot size. In this paper, we present a reliable fabrication process to produce sub-10 nm Ge quantum dots (QDs), using a combination of ex-situ thermal annealing via rapid thermal annealing (RTA) and in-situ Joule heating technique in a transmission electron microscope (TEM). First we present in-situ direct joule heating experiments showing how the heating electrode could be damaged due to the formation of Al crystals and voids at the vicinity of the metal/NW contact, likely related with electro-migration phenomena. We show that the contact quality can be preserved by including an additional ex-situ RTA step prior to the in-situ heating. The in-situ observations also show in real-time how the exchange reaction initiates simultaneously from several locations underneath the Al contact pad, and the Al crystal grows gradually inside the initial Ge NW with the growth interface along a Ge(111) lattice plane. Once the reaction front moves out from underneath the contact metal, two factors jeopardize an atomically accurate control of the Al/Ge reaction interface. We observed a local acceleration of the reaction interface due to the electron beam irradiation in the transmission electron microscope as well as the appearance of large jumps of the interface in unpassivated Ge wires while a smooth advancement of the reaction interface was observed in wires with an Al2O3 protecting shell on the surface. Carefully controlling all aspects of the exchange reaction, we demonstrate a fabrication process combining ex-situ and in-situ heating techniques to precisely control and produce axial Al/Ge/Al NW heterostructures with an ultra-short Ge segment down to 8 nanometers. Practically, the scaling down of Ge segment length is only limited by the microscope resolution.
Solid state electrolytes are widely considered as the enabler of lithium metal anodes for safe, durable and high energy density rechargeable lithium ion batteries. Despite the promise, failure mechanisms associated with solid state batteries are not well-established, largely due to limited understanding of the chemomechanical factors governing them. We focus on the recent developments in understanding the solid state aspects including the effects of mechanical stresses, constitutive relations, fracture, void formation and outline the gaps in literature. We also provide an overview of the manufacturing and processing of solid state batteries in relation to chemomechanics. The gaps identified provides concrete directions towards the rational design and development of failure resistant solid state batteries.
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