Cathode Microstructure Research Articles

Constructing Favorable Microstructures in Solid-State Organic Cathodes Via Mechanical Property Manipulation

Organic electrode materials, due to their low environment footprint and high material-level specific capacities, have become competitive alternatives to inorganic materials for solid-state batteries (SSB) in recent years. Additionally, the soft nature of organic compounds ensures consistent and intimate contact with solid sulfide electrolytes during cycling, which is beneficial for battery longevity. However, the low-modulus organic materials and high-modulus sulfide electrolytes, upon mixing and compression, would form unfavorable composite microstructure where sulfide particles are encapsulated by organics and cannot form an efficient ion conduction path. This mismatch in mechanical property prevents a high fraction of organic compounds to be used in a solid-state cathode, limiting the energy density of organic solid-state batteries.Here we report the formation of favorable microstructures of organic cathodes by “softening” the sulfide electrolytes. Solvent treatment of the sulfide electrolyte Li6PS5Cl more than halves its modulus from 28.6 ± 8.5 GPa to 16.0 ± 1.6 GPa. Thermal gravimetric analysis, Raman spectroscopy, and X-ray diffraction were used to elucidate the evolution of the sulfide electrolyte during the softening and recovering process. The organic cathode formed by mixing organic materials with this softened electrolyte shows a favorable microstructure where the electrolyte forms a percolated domain. As a result, the utilization of an organic material, pyrene-4,5,9,10-tetraone (PTO), is increased by 133.6% and 90.8% compared with cells with a re-hardened and the pristine Li6PS5Cl, respectively. The mass fraction of PTO can be improved from 20 to 40wt% while maintaining high utilization (85.7%). Our exploration of softened electrolyte builds the correlation among structure, mechanical property, microstructure engineering and battery performance, and the strategy is applicable to other active materials with low modulus.

(Invited, Digital Presentation) Heterogeneities at Solid/Solid Interfaces

Solid-state lithium metal batteries hold the potential to enhance the energy density, power density and safety of conventional lithium-ion batteries. Interface stability and performance of solid-state batteries is predicated on the coupled set of electrochemical-mechanical-transport interactions that underlie the solid-solid interfaces within the system. Microstructure of the solid-state cathode dictates the distribution of percolation pathways, reaction kinetics and evolution of distinct internal resistance modes. Morphological response of the anode-electrolyte interface is influenced by various aspects including the presence of defects, grain boundaries and mechanical/transport properties of the solid electrolyte and lithium. In this presentation, the mechanistic role of heterogeneities on the morphological stability of the anode, onset of mechanical/thermal hot spots and electrochemical response of the cathode will be analyzed. The implications of heterogeneous kinetic-transport interactions and chemo-mechanics interplay on the performance and failure modes in solid-state batteries will be delineated.

(Digital Presentation) Understanding the Impact of High-Nickel Cathode Microstructure on Battery Safety and Cycling Performance

To meet the increasing energy demands of portable devices and electric vehicles, high-nickel lithium-ion cathode materials with the general formula Li(NixMnyCoz)O2 (NMC) have been extensively researched. Currently NMC811 is used commercially for high-energy applications. The energy density of NMC also comes with concerns over cycle life and safety1,2. To improve the cycle life of NMC-based cells, single-crystal materials have recently gained attention to tackle the particle cracking issues found in polycrystalline cathodes3. However, for successful introduction to the lithium-ion battery market, inherent safety over a material’s lifetime also needs to be proven. Failure and degradation mechanisms both need to be fully understood to improve the stability of future cathode materials. Abusive testing, such as overheating, overcharge and nail penetration, has been used in conjunction with in-situ and ex-situ X-ray computed tomography (CT) 3D imaging to perform post-mortem studies and understand the relationship between thermal failure and cathode microstructure4,5. However, the interplay between safety characteristics, microstructural properties and material degradation remains unclear.This work first aims to compare the safety performance of polycrystalline and single-crystal NMC811 in 200 mAh pouch cells. Accelerating rate calorimetry (ARC) with a heat-wait-search (HWS) technique is used to heat cells and determine the onset of self-heating, onset of thermal runaway and the peak thermal runaway temperature. Laboratory-based pre- and post-mortem in-situ and ex-situ X-ray CT is also used for non-destructive imaging at multiple length scales to determine how failure propagates through the cells and the impacts on the electrodes and microstructure. Pouch cells containing polycrystalline and single-crystal NMC811 cathode and graphite anode are electrochemically cycled to induce material degradation. EIS measurements and diagnostic cycles are performed to identify prevalent degradation modes in both types of cathode materials. Finally, the same ARC and X-ray CT characterisations are performed on the aged cells to determine how degradation and changes to the material structure affect the safety performance in high-nickel cathode materials.The results of this work will improve the current understanding of capacity fade in high-nickel cathodes and the safety behaviour over the lifetime of a battery cell. This information can then be used to inform future materials development and strategies for mitigating thermal runaway in batteries. References L. Ma, M. Nie, J. Xia, and J. R. Dahn, J. Power Sources, 327, 145–150 (2016).H. J. Noh, S. Youn, C. S. Yoon, and Y. K. Sun, J. Power Sources, 233, 121–130 (2013).J. Langdon and A. Manthiram, Energy Storage Mater., 37, 143–160 (2021).D. Patel, J. B. Robinson, S. Ball, and D. J. L. Brett, (2020).D. P. Finegan et al., Phys. Chem. Chem. Phys., 18, 30912–30919 (2016).

Garnet-Based Composite Cathodes for All-Solid-State Lithium Batteries

Garnet-based all-solid-state lithium batteries (ASSLB) provide high intrinsic safety, extended operational temperature range, and high energy density. As the first two are intrinsic to the materials system, one prerequisite to obtain high energy densities with such ASSLBs is the manufacturing of composite cathodes. Preferably, cathode active material (CAM) and electrolyte should form an intertwined 3D-network with an intimate extended contact between two phases. The interface between the CAM and the electrolyte should feature high total surface area with a low impedance to enable an efficient charge transfer and transport and minimize the resistance losses in a battery.Composite solid cathode microstructures can be formed via a co-sintering of solid electrolyte and CAM powders. However, many CAMs such as Li[Ni1-x-yCoxMny]O2 (NMC), Li[Ni1-x-yCoxAly]O2 (NCA) or Li2NiMn3O8 (LMO) cannot withstand sintering temperatures required, for oxide class solid electrolytes as Li7La3Zr2O12 (LLZ) or Li1+xAlxTi2-x(PO4)3, to obtain sintered composite cathodes. Furthermore, the thermodynamic stability of CAMs significantly decreases in the presence of solid electrolytes. Still, several groups have demonstrated that thermodynamic limitations can be significantly lessened via a kinetic control of the process such as reduction of reaction times and a careful control of powder morphology during ceramic sintering process.Following this work, we demonstrate that Field Assisted Sintering Technique / Spark Plasma Sintering (FAST/SPS) enables fabrication of different functional solid-state battery components within minutes such as dense LLZ separator layers with a high ionic conductivity, thick LiCoCO2 (LCO)/LLZ composite cathodes without any interfacial reactions, and integrated cathode half-cells consisting of a composite cathode and a LLZ separator layer at temperatures as low as 675 °C. . The analysis of these composite cathodes revealed well sintered pure phases and a homogenous distribution of cubic LLZ and cathode active material. Electrochemical tests showed promising electronic and ionic conductivity (0.4 mS cm-1) values and an increased capacity (4 mA cm-2) compared to cathodes with pure active material.The structural analysis after the electrochemical characterization shows the mechanical integrity of the LLZ/LCO interface, which means that a mechanically stable rigid LLZ/LCO interface can be obtained.Such a mechanically stable LLZ/LCO cathode allows to analyze possible electrochemistry induced degradation, which is suggested by theoretical work but lacks experimental validation. A detailed analysis of the LLZ/LCO interface by SEM, TEM, Raman, and XRD provide insights into the processes occurring during electrochemical cycling. The findings can pave the way to increase the cycling stability of ceramic all-solid-state batteries via interface optimization Figure 1

High performance finite element simulations of infiltrated solid oxide fuel cell cathode microstructures

To better understand the effects of infiltration on local electrochemistry and transport in solid oxide fuel cell (SOFCs) electrodes, high-throughput, high-performance finite element simulations are presented within dozens of SOFC cathodes containing synthetically generated nanoscale infiltrates. The computational approach retains the complex microstructural morphologies of cathodes, including those of the three backbone phases (gas, ion, and electron conductors) and the infiltrates (an electron conductor), in meshed domains and computes distributions of local electrochemical quantities within the domains. Simulations were implemented on a supercomputer and converged for 48 distinct microstructural subvolumes, with varying backbone heterogeneities and infiltrate loadings. Analyzing both the ensemble (averaged over subvolumes) and the local (evaluated within subvolumes) performance metrics indicate that infiltration of an electron conductor significantly improves the electrochemical performance of each backbone in a linear fashion with the increase of triple phase boundary content, but the essential ionic transport pathways of the backbone are unchanged. These results shed light into the design and fabrication of optimal electrodes in fuel cells.

A Review on the Process-Structure-Performance of Lanthanum Strontium Cobalt Ferrite Oxide for Solid Oxide Fuel Cells Cathodes

Perovskite-structured La1-xSrxCo1-yFeyO3-δ (LSCF) is a promising mixed ionic/electronic-conducting material that exhibits excellent electro-catalytic activity toward oxygen reduction and oxygen evolution reactions. LSCF offers potential applications in many processes, such as electrodes for solid oxide fuel cells (SOFCs), oxygen sensors, and dense membrane for oxygen separation and thus have been studied extensively in various fields. However, its physical and electrochemical properties are substantially influenced by dopant concentration, dopant type and processing conditions (synthesis methods, composite cathode effect, fabrication conditions, and chromium poisoning). Understanding and correlating the effect of LSCF composition, its synthesis methods, fabrication conditions, and its parameters are essential to enhance the performance of LSCF cathode for high- to- intermediate temperature SOFC applications. This review emphasizes the importance of enhancing the performance of LSCF cathode by optimizing the influential factors to facilitate and expedite research and development efforts for SOFC commercialization in the near future. Various synthesis and fabrication methods used to prepare and fabricate LSCF and LSCF-based composite cathodes are discussed in detail. Moreover, their pros and cons in optimizing the microstructure of LSCF cathodes are highlighted. Finally, the strategies to improve the long-term microstructural stability and electrochemical performances of the LSCF cathode are discussed.

Recent progress in design and fabrication of SOFC cathodes for efficient catalytic oxygen reduction

Solid oxide fuel cells (SOFCs) are highly promising energy conversion devices. However, the performance and efficiency of current SOFC devices are still limited by inadequate oxygen reduction activity in the cathode, which has been recognized as a great challenge for SOFCs to achieve high power output at low operating temperatures. This has stimulated intensive efforts to develop more efficient cathodes for enhanced catalytic oxygen reduction. This article reviews recent advances in design and fabrication of cathodes for efficient SOFCs, with emphasis on (1) engineering cathode microstructures, (2) fabrication techniques to prepare cathode layers, and (3) strategies to address segregation and poisoning issues.

Tailoring shape and exposed crystal facet of single-crystal layered-oxide cathode particles for all-solid-state batteries

The design of composite cathode microstructures is important for improving the performance of all-solid-state batteries (ASSBs). The microstructural properties can provide efficient ionic/electronic percolation, good cathode/solid electrolyte (SE) contact, and minimal void spaces. Although the effects of the size of cathode and SE particles have been extensively investigated, the shape effects of cathode particles remain unexplored. Here, we demonstrate that the shape and exposed crystal facet of cathode particles affect the electrochemical performance of ASSBs using crack-free single-crystalline LiNi0.6Co0.2Mn0.2O2 (NCM) as the cathode and oxidation-tolerant Li3YCl6 as the SE. Systematic studies using five particle shapes (octahedra, plates, rods, spherical single crystals, and spherical polycrystals) reveal the important effects of the shape and exposed facets on the solid–solid contact as well as Li+ ion diffusion in composite cathodes. Single-crystalline octahedral NCM particles have a higher rate capability than that of their counterparts because their wide planar surface promoted plane-to-plane contact with the SE and the (012) facets provided straightforward 3D Li+ transfer channels between the surface and the center. These results suggest that single-crystal NCM cathodes with advanced shapes and exposed facets can enable the fabrication of ASSBs with superior performance.

Evolution of a Radially Aligned Microstructure in Boron-Doped Li[Ni0.95Co0.04Al0.01]O2 Cathode Particles.

Boron (B) (1.5 mol %) is introduced into Li[Ni0.95Co0.04Al0.01]O2 (NCA95) to create a radially oriented microstructure with a strong crystallographic texture. The cathode microstructure allows dissipation of the abrupt lattice strain near the charge end and improves the cycling stability of the NCA95 cathode (88% capacity retention after 100 cycles at 0.5 C). Transmission electron microscopy (TEM) analysis of the B-doped NCA95 cathode during lithiation reveals that the highly oriented microstructure is provided by a hydroxide precursor. Boron prevents random agglomeration of the primary particles and keeps them elongated through (003) faceting. The selected-area electron diffraction analysis shows that the structure of the lithiated oxide undergoes subtle structural changes even after the crystal structure is fully converted from P3̅m1 to R3̅m at 600 °C. Li+/Ni2+ intermixing is prevalent due to the slow oxidation of Ni2+ to Ni3+. Li+ and Ni2+ do not randomly occupy the Ni and Li layers; instead, these ions occupy their sites in an ordered pattern, forming a superlattice. The superlattice gradually disappears as the lithiation temperature is increased. One peculiar structural feature observed during lithiation is the prevalence of twin defects that preexist in the hydroxide precursor as growth twins. The twin defects, which could serve as nucleation sites for intraparticle cracks, also gradually anneal out during lithiation. TEM analysis substantiates the importance of the hydroxide precursor microstructure in a coprecipitation process and provides a basis for choosing the appropriate lithiation temperature and soaking time to obtain the desired cathode structure and primary particle morphology.

In-situ surface reconstruction induced-significant performance promotion of Ca3Co4O9+δ cathode for solid oxide fuel cells

Surface morphology of cathode has a significant impact on the oxygen reduction reaction (ORR) activity as well as the performance of solid oxide fuel cells (SOFCs). Up to now, great efforts have been devoted to decorate the microstructure of SOFC cathode, such as changing synthesis method, infiltration, electrospinning and so on. Herein, we report an effective and feasible method to reconstruct the surface of Ca3Co4O9+δ (CCO) cathode by reduction and re-oxidation process, during which remarkable morphology change is observed as the surface is wrinkled with the formation of numerous small particles. ORR activity of CCO cathode is notably improved after surface reconstruction, since area specific resistance (ASR) of CCO cathode significantly decreases from 0.48 Ωcm2 to 0.19 Ωcm2 at 700 °C. Specific surface area of CCO increased and calcium and cobalt oxide nanoparticles are generated after the redox treatment. The oxygen adsorption ability of the CCO is largely enhanced, which contributes to the performance promotion of CCO cathode after the redox treatment. Overall, we propose a feasible method to reconstruct CCO surface to promote the ORR activity effectively and study the corresponding mechanism thoroughly, all of which make valuable improvements on the development of SOFC cathode.

Influence of carbon binder domain on the performance of lithium‐ion batteries: Impact of size and fractal dimension

AbstractA lithium‐ion battery (LIB) cathode comprises three major components: active material, electrical conductivity additive, and binder. The combination of binder and electrical conductivity additive leads to the formation of composite clusters known as the carbon binder domain (CBD) clusters. Preparation of a LIB cathode strongly influences the dispersion of the above‐mentioned constituents leading to the formation of distinct pore and electrical conduction networks. The resulting structure thus governs the performance of LIBs. The presence of CBD is essential for the structural integrity and sufficient electrical conductivity of the LIB cathode. However, CBD abundance in LIB cathodes leads to unfavorable gravimetrical and volumetrical consequences owing to its electrochemical inertness. Increasing CBD content adds to the weight of the LIBs, thus negatively impacting the energy density. Furthermore, increased electrical conductivity is won at a cost of ionic conductivity as CBD clusters breach the pore networks in the cathode microstructure. The following study establishes a link between the various possibilities of CBD cluster size and fractal dimension that may eventualize during the mixing process of slurry preparation to the resulting microstructural properties and hence to the performance of LIBs by means of idealized cathode geometries. Since the performance determining processes occur at the microstructural scale, which are often very tedious to study via experimental research, the study makes use of spatially resolving microstructural, numerical, simulations. The results demonstrate that the CBD cluster size has a strong influence on the cathode microstructure. The CBD cluster fractal dimension on the other hand displayed a minor influence on the structural properties of the cathode, and the size of the cluster primary particles was shown to be the dominant factor. Finally, performance evaluation simulations confirmed the trends seen in structural properties with changing cluster size and fractal dimension.

Densification and charge transport characterization of composite cathodes with single-crystalline LiNi0.8Co0.15Al0.05O2 for solid-state batteries

Solid-state batteries (SSBs) hold great promise for high-performance and safe energy storage solutions. Several challenges, however, including the standardization of cell/electrode formats and interface instabilities between electrode components, remain to be overcome. Here, the densification effect of composite cathodes on the electrochemical properties in SSBs was studied. By varying the pressure during uniaxial pressing, composite cathode pellets consisting of single-crystalline LiNi0.8Co0.15Al0.05O2 and Li6PS5Cl particles are fabricated without conductive agents, and the microstructural evolution, charge transport kinetics, and electrochemical performance are examined. The densification process results in a close connection between the component particles while reducing the pore spaces within the composite cathodes. The cathode compacted with a nominal pressure of 1500 MPa shows excellent capacity retention (≥ 99%) with discharge capacities of ∼160 mAh g−1 (areal capacity of ∼3.68 mAh cm−2) at 0.1 C (∼0.46 mA cm−2) over 100 cycles and improved rate capability. These improvements are attributed to the enhanced effective conductivities and diffusion coefficients of Li+ ion and electron, based on impedance and DC polarization measurements. These findings will enhance an understanding of the correlation between the microstructure and charge carrier transport properties of composite cathodes for SSBs based on sulfide-based SEs.

Effects of cathode thickness and microstructural properties on the performance of protonic ceramic fuel cell (PCFC): A 3D modelling study

Protonic Ceramic Fuel Cells (PCFCs) are promising power sources operating at an intermediate temperature. Although plenty of experimental studies focusing on novel material development are available, the design optimization of PCFC through numerical modelling is limited. In this study, a 3D PCFC model focusing on the cathode thickness and microstructure design is developed due to the high overpotential loss of the cathode. Unlike the 1D/2D models, the rib-size effects on the PCFC performance are fully considered when optimizing the cathode structure. Different from 1D/2D models suggesting thin cathode thickness, this study finds that the optimal cathode thickness is about 120–200 μm. In a thin cathode, weak O2 diffusion from the channel to the rib-covered cathode can lead to O2 depletion under the rib and very low local cell performance. By adjusting the cathode porosity from 0.3 to 0.5, nearly 9% performance improvement and 22.5% improvement in gas distribution uniformity can be achieved. When the cathode particle size changes from 0.1 μm to 0.2 μm, the O2 concentration under the rib increases nearly 50%. The optimal electronic phase volume fraction is suggested to be around 50–60% for achieving a balance between ohmic resistance and reaction sites. This model elucidates the relationship between cathode microstructure and PCFC performance comprehensively and can serve as a guiding tool for cell fabrication and future novel interconnect structure design.

High-Energy Cathodes via Precision Microstructure Tailoring for Next-Generation Electric Vehicles

With the prevalence of electric vehicles (EVs), Ni-rich layered cathodes have been extensively studied to increase their capacities. The use of a Ni-rich core encapsulated by a shell with concentration gradients (CSG) is the only field-proven strategy that is able to tap the potential capacity of Ni-rich cathodes. Herein, it was demonstrated that doping a CSG cathode with an average composition of Li[Ni0.9Co0.05Mn0.05]O2 with 0.5 mol% Sb substantially improved its cycling stability while providing manufacturing flexibility. Sb doping allowed precise tailoring of the cathode microstructure through the retardation of cation migration and the inhibition of coarsening by pinning particle boundaries. The Sb-doped CSG cathode retained ∼80% of its initial capacity for 2500 cycles, while the pristine CSG90 cathode showed similar capacitive deterioration over only 1500 cycles. The proposed Sb-doped CSG90 cathode for use in EVs represents an ideal high-energy-density cathode with a composition engineered to maximize capacity; its modified microstructure ensures a long battery life and ease of manufacturing, enabling cost reduction.

Optimizing the Microstructure of Composite Cathodes in Garnet-Based All Solid-State Batteries By Continuum Modeling and Simulation

Due to their potentially higher energy densities and advantages in safety, all solid-state batteries (ASSBs) are being considered to replace conventional Li-ion batteries in a number of applications. Ceramic garnet-type electrolytes are promising candidates for the solid electrolyte in ASSBs because they provide high ionic conductivity and a wide electrochemical stability window.To improve battery performance, the optimization of the composite cathode in ASSBs is crucial. The microstructure of the cathode as well as interface phenomena occurring between active material particles and electrolyte grains are performance-limiting factors which need to be improved.In our contribution, we present microstructure resolved simulations in the 3D simulation framework BEST [1] demonstrating the effects of characteristic properties of the microstructure on the cell performance. By simulation studies using composite cathode microstructures with different particle size, active material fraction, and sinter density, we are able to identify advantageous properties which can serve as a target for the cell production.In the experiments, a large polarization of garnet-based ASSBs is observed which is commonly attributed to secondary phase formation between the active material and electrolyte in the production process [2]. Therefore, a new electrochemical interface model is introduced to account for the hindered transport of Li and electrons due to secondary phases. This model is based on physical parameters of the interface and, thus, can be used to get insights into the interface properties which are still subject of intensive research [3].[1] Latz, A.; Zausch, J. Thermodynamic consistent transport theory of Li-ion batteries. J. Power Sources 2011, 196, 3296-3302.[2] Finsterbusch, M.; Danner, T.; Tsai, C.-L.; Uhlenbruck, S.; Latz, A.; Guillon, O. High Capacity Garnet-Based All-Solid-State Lithium Batteries: Fabrication and 3D-Microstructure Resolved Modeling. ACS Applied Materials & Interfaces 2018, 10, 22329-22339.[3] Vardar, G.; Bowman, W. J.; Lu, Q.; Wang, J.; Chater, R. J.; Aguadero, A.; Seibert, R.; Terry, J.; Hunt, A.; Waluyo, I.; Fong, D. D.; Jarry, A.; Crumlin, E. J.; Hellstrom, S. L.; Chiang, Y.-M.; Yildiz, B. Structure, chemistry, and charge transfer resistance of the interface between Li7La3Zr2O12 electrolyte and LiCoO2 cathode. Chem. Mater. 2018, 30, 6259–6276.

Role of Cathode Microstructure Heterogeneity in All-Solid-State Battery Performance

Achieving fast charging in all-solid-state lithium metal batteries is a critical challenge. In this regard, the composition and spatial arrangement of constituent phases (e.g., active material, binder, conductive diluent, solid electrolyte) in the porous cathode play a major role in determining the ionic/electronic percolation pathways and internal resistances. The underlying heterogeneity in the cathode microstructure can result in non-uniform kinetic-transport processes, and thereby necessitates a detailed understanding of its influence on the electrochemical performance. In this study, the mesoscale underpinnings of the cathode microstructural heterogeneity will be examined in the context of fast charge extremes for all-solid-state lithium metal batteries.

Effect of Cathode Microstructure on Electrochemical Properties of Sodium Nickel-Iron Chloride Batteries.

Sodium metal chloride batteries have become a substantial focus area in the research on prospective alternatives for battery energy storage systems (BESSs) since they are more stable than lithium ion batteries. This study demonstrates the effects of the cathode microstructure on the electrochemical properties of sodium metal chloride cells. The cathode powder is manufactured in the form of granules composed of a metal active material and NaCl, and the ionic conductivity is attained by filling the interiors of the granules with a second electrolyte (NaAlCl4). Thus, the microstructure of the cathode powder had to be optimized to ensure that the second electrolyte effectively penetrated the cathode granules. The microstructure was modified by selecting the NaCl size and density of the cathode granules, and the resulting Na/(Ni,Fe)Cl2 cell showed a high capacity of 224 mAh g−1 at the 100th cycle owing to microstructural improvements. These findings demonstrate that control of the cathode microstructure is essential when cathode powders are used to manufacture sodium metal chloride batteries.

Three-dimensional optimization of La0.6Sr0.4Co0.2Fe0.8O3 cathode microstructure with particle radius constraint

A numerical model is developed to optimize the La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode porous microstructure of solid oxide fuel cell (SOFC), in which the local radius constraints are applied. During the optimization, the LSCF porous microstructure is deformed and the total reaction current is maximized. A level-set method is utilized to parametrize the 3D microstructure, and an adjoint method is applied for the sensitivity analysis. In addition, in order to improve the computational accuracy and to make the optimized microstructure manufacturable, a radius constraint is imposed to ensure that the local radii are above the target value, where the interface is updated only when the local radius value is larger than the target radius. Computational results show that the sphere-shaped LSCF solid particles are preferred in terms of performance improvement. In addition, the computational accuracy is improved with the imposed radius constraint, and the total electrochemical reaction current in the optimal microstructure is improved comparing with the experimentall fabricated microstructure obtained with 3D recontruction technique. Furthermore, the optimized microstructure and performance of the LSCF cathode are independent from the initial cathode structures. Finally, the relation between Thiele modulus and the optimal microstructure is investigated by varying the ionic conductivities and exchange current densities.

Reconstruction and optimization of LSCF cathode microstructure based on Kinetic Monte Carlo method and Lattice Boltzmann method

Solid phase sintering is a critical process for fabricating mixed ionic and electronic conductivity (MIEC) electrodes. In this study, the microstructures of MIEC electrodes are numerically reconstructed by a Kinetic Monte Carlo method. The performance of the reconstructed MIEC electrodes is then evaluated by a pore scale Lattice Boltzmann model. The present study provides the first comprehensive assessment of local O2 partial pressure on electrode performance. It is found that ohmic loss tends to play remarkable roles at a low O2 partial pressure of pO2<0.1bar. As insufficiency of O2 is almost unavoidable in the SOFC stack, the influence of local O2 partial pressure on ionic conductivity should be considered in LSCF modeling. Another important finding is that the initial states of compact powder have a profound impact on the electrode performance. Small initial grain size and irregular particles both contribute to generate large reaction area after sintering thereby decrease activation loss. It is also found that compact powder consistency even plays a more important role in electrode performance than particle size. The study also provides deep insight into influence of sintering process. The effective conductivity of electrode is mainly controlled by the enhancement of electrode connectivity. Subsequently, nanostructured SOFC electrodes by infiltration/impregnation are reconstructed evaluated numerically. The infiltrated electrodes demonstrate improved performance and significantly promote uniformity of reaction rates. The present study forms a solid foundation for optimization of the fabrication procedures to improve the fuel cell performance.

Optimization of SOFC Electrode Microstructures with Long Term Performance Modeling and Machine Learning

Performance degradation over time is a major barrier to the commercialization of solid oxide fuel cells (SOFCs). A major driver of this degradation is grain coarsening in the 3D microstructure of the porous, composite electrodes. The rate and extent of this microstructural degradation depends on the initial microstructure itself, raising the question of whether an optimal electrode microstructure can be determined for long-term performance. The high dimensionality of parameters that determine the initial microstructure makes parametric optimization challenging. In this work, we present a fully resolved 4D model for simulating microstructural and performance changes over 1,000 hours of cell operation by combining a phase-field coarsening model with spatially resolved microstructural analysis and continuum-level multiphysics performance modeling. The integrated model framework was run in a highly parallelized fashion to simulate the long-term performance of hundreds of LSM-YSZ cathodes and Ni-YSZ anodes. These electrode microstructures were synthetically generated to intentionally span 11 independent initial microstructural parameters. The results of the long-term performance model included electrochemical performance every 100 hours throughout the entire 1,000 hours of operation. Together with a novel figure of merit that accounts for both the initial and long-term performance, this high-dimensional bank of results was used to train a machine learning regression model. The machine learning model was trained to predict the long term performance based on the 11 independent initial microstructural parameters, allowing interpolation to values not included in the fully resolved models, as well as a more robust analysis of each parameter’s influence in this highly dimensional parameter space. A SHAP analysis [1] was performed on the trained machine learning model to determine the relative impact of each of the 11 independent initial microstructural parameters. This demonstrated that, for example, the LSM/YSZ ratio was the most impactful parameter in the cathode microstructure, with lower LSM/YSZ ratios generally leading to more energy produced over the cell’s lifetime. The fully trained model and SHAP analysis are also able to make specific recommendations for changes that would improve existing electrodes, providing a valuable tool for SOFC electrode developers and manufacturers.[1] S. M. Lundberg, S-I. Lee. A Unified Approach to Interpreting Model Predictions. NIPS 2017. Figure 1