Symposium ST05—Mechanics of Energy Storage Materials

The production of free-standing carbonized electrospun lignin carbon fibers (ELCF) is similar to that used in large scale paper making. This makes ELCF highly prospective for commercially viable supercapacitor electrodes, being a low-cost and bio-renewable starting material. However, untreated ELCF -based electrodes for supercapacitors face several challenges due to their poor mechanical performance and limited conductivity. Thermal treatments applied for carbonization and activation vastly improve both electrochemical performances and mechanical properties by giving rise to fiber crystallization. Yet as supercapacitor electrodes, their microporous structure faces the issue of electrolyte ion accessibility. The charge transfer impedance is known to increase due to the inaccessibility of electrolyte ions to the porous structures, resulting in decreased charge storage ability [1]. Understanding the correlation between the electrode physiochemical properties and the charge storage is key to guide the production and commercialization of ELCF materials for supercapacitor applications. In this work, ELCF were fabricated by electrospinning and carbonized at 1000 °C. The ELCF performance was evaluated in a coin cell configuration with two symmetric electrodes in 6M KOH electrolyte over 5k, 10k, 20k, and 50k cycles. There was a constant increase in performance up to 2-fold from 3.8 mF/g to 7.7 mF/g after 50k cycles. The capacitance increase is accompanied by an interfacial resistance decrease that is caused by the interaction between the bulk electrolyte and the electrode’s interface as inferred from the Nyquist plots. This improvement behaviour with cycling is attributed to the better accessibility of the alkaline KOH electrolyte ions to outer pores over repeated cycling. Another contribution to the increased capacitance could be related to lignin being a complex natural biopolymer incorporated with various functional groups [2]; more reactive functional groups could be made available during the consecutive cycling for the charge transfer to take place [3]. The K⁺ electrolyte ions are adsorbed on the exposed reactive sites in the larger lignin chains over their repeated intercalation and deintercalation during redox reactions resulting in slower and steady growth in capacitances with highly stable cycling behaviour. This can also be explained by the common behaviour during electrochemical analysis that the unreactive constituents remain inactive to capacitive behaviour initially and begin electroactivity additionally after the reaction. To further corroborate these explanations, the specific surface area, pore size, and functional groups of the ELCF before and after cycling will be investigated by using Brunauer-Emmett-Teller (BET) and X-ray Phototelectron Spectroscopy(XPS) characterization analysis. Additionally, a thorough electrochemical investigation will be carried out regarding the improved capacitive performance with cycling, alongside with the quantification of electrostatic (pore) and pseudocapacitive (functional group) charge storage of ELCF material.
Keywords: lignin, supercapacitors, cycling, pseudocapacitance, pore accessibility
[1] Inchan Yang, Sang-Gil Kim, Soon Hyung Kwon, Myung-Soo Kim, Ji Chul Jung, “Relationships between pore size and charge transfer resistance of carbon aerogels for organic electric double-layer capacitor electrodes”, Electrochimica Acta, 2017
[2] José Luis Espinoza-Acostaa , Patricia I. Torres-Chávez , Jorge L. Olmedo-Martínez , Alejandro Vega-Rios , Sergio Flores-Gallardo, E. Armando Zaragoza-Contreras , “ Lignin in storage and renewable energy applications: A review”, Journal of Energy Chemistry 27 (2018) 1422-1438
[3] Manohar D. Mehare, Abhay D. Deshmukh , S. J. Dhoble , “Preparation of porous agro-waste-derived carbon from onion peel for supercapacitor application”, Journal of Materials Science 55 (2020)pages 4213–4224

3D structuring significantly enhances the cycle life performance of silicon anodes by accommodating large volume changes during de/lithiation and enables fast charging. The accumulation of mechanical stress induced by large volume changes and understanding its impact on cell performance is key toward optimizing the cycle life of these structured anodes. In this presentation, we report on the contribution of mechanical degradation to capacity fading in Si-coated 3D Ni scaffolds. We propose a novel approach to investigate the mechanical properties of the electrode materials decoupling the geometry evolution. The mechanical resilience of the anodes, including the active material and metallic support, the SEI, and the associated interfaces, are probed throughout cycling using nanoindentation. Additionally, the microstructural evolution and structural disintegration of Si and the metallic scaffold were investigated at different stages of cycling. Results revealed a reduction of strength and connectivity in the Ni scaffold during cycling and provided critical insight into the design optimization of durable high energy density anodes. A fundamental understanding of the coupling between mechanics and electrochemistry in lithium-ion battery electrodes can be advantageous in developing reliable next-generation energy storage technologies.

Acceptor-doped BaZrO₃ is a promising electrolyte for protonic ceramic fuel cells as it combines high bulk proton conductivity with good chemical stability. The protonic conductivity is achieved by dissociative water incorporation into oxygen vacancies formed by acceptors on Zr⁴⁺ sites. We have investigated the influence of dopants, oxygen vacancies, and protons on the macroscopic elastic and electromechanical properties of acceptor-doped BaZrO₃ ceramics.
Ceramics of BaZr_1-xX_xO_3-x/2+δH_2δ with X = Ga, Sc, In, Y, Eu and 0.05 ≤ x ≤ 0.2 were prepared by solid state reactive sintering and hydration. Ultrasonic pulsed echo time of flight measurements were used to infer the Young’s and the shear moduli. Both moduli decrease by up to ~20% due to the presence of dopants and oxygen vacancies that cause local lattice distortions [1,2]. Water incorporation into the vacancies decreases the moduli even further. An unexpectedly large electrostriction coefficient (M₃₃ ~10^-15 m²/V²) was detected with a capacitive proximity sensor for all dopants introducing a new class of non-classical electrostrictors. M₃₃ of the hydrated ceramics exhibits a Debye-type relaxation with the relaxation frequency exponentially increasing with the ionic radius closely matching dielectric relaxation measured by impedance spectroscopy. This implies that the protons are associated with the dopants, and the binding strength decreases from Ga to Y.

[1] M. F. Hoedl, E. Makagon, I. Lubomirsky, R. Merkle, E. A. Kotomin, J. Maier, Acta Mater. (2018) 160, 247
[2] E. Makagon, R. Merkle, J. Maier, I. Lubomirsky, Solid State Ionics (2020) 344, 115130

In this work, rGO/Pd/rGO nanolaminates for metal-hydride hydrogen storage material was studied for the first time that indicated an enhancement in the hydrogen storage capacity due to the mechanical strain imposed on the Pd nanoparticles. Pd nanoparticles were encapsulated by the graphene layers to fabricate rGO/Pd/rGO nanolaminates and reduced graphene oxide (rGO) wrapping induced strain on Pd nanoparticles were then analyzed using high resolution transmission electron microscope (HRTEM). The average strain induced on the Pd nanoparticles in the as synthesized rGO/Pd/rGO nanolaminates was 3.30% in tension, and this value of strain increased to 5.44% upon hydrogenation. The strain increase to 4.94% in Pd and 7.12% in PdH_x nanoparticle if the contracted lattice parameter by nanosize effect was considered. Atomistic simulations were performed to provide further analysis on the effect of surface tension on the reference lattice parameter of Pd nanoparticles to refine the calculated strain from the HRTEM analysis. Hydrogen storage performance indicates a significant improvement, where both kinetics and the hydrogen storage capacity are significantly improved without sacrificing nano-size effects, and this enhancement is due to the tensile strain on the Pd nanoparticles causing a reduction in the activation energy of hydride formation.

Owing to their high theoretical capacity to incorporate substantial amounts of lithium, silicon anodes in lithium-ion batteries undergo significant volumetric changes during charge-discharge cycles, leading to battery failure due to pulverization and fracture of the anode material. We have constructed an analytical model based on concentration-driven diffusion and linear elasticity. This comprehensive model incorporates diffusion-induced stress (DIS), electrochemical (forward) reaction induced stress (RIS), and surface effects associated with a hollow silicon spherical electrode under potentiostatic operation. Also included are various forms of stress - radial, hoop and hydrostatic, coupled with various functional parameters to study variations in stress profiles for low lithium concentrations. Two surface effects are accounted for: a) surface energy , which is the Laplace pressure arising from lithium concentration difference at the silicon interface, and surface elastic modulus. To illustrate the importance of surface effects in this model, we carried out numerical solutions of the coupled problem in its natural geometry - spherical polar coordinates, to study a hollow silicon sphere with inner radius of 200 nm and outer radius of 500 nm. When aforementioned surface effects are ignored, the maximum value of radial stress is found to be 5.3 MPa (tensile) at 318 nm (close to the center of the hollow sphere). When surface energy and surface modulus are included in the solution, the maximum value of radial stress reduces dramatically by nearly 59% and the location of the maximal stress moves inwards to ~285 nm. This consequently leads to the stress at the outer surface becoming compressive in nature. Further, with a systematic increase in surface energy parameters, radial as well as hoop stress at the outer interface becomes more compressive. Physically, in a manner somewhat analogous to hydrophobic liquids, this shift can be understood as the effect of enhanced surface energy countering the tendency of the system to expand and relieve its increased surface stress by propagating cracks. Unsurprisingly, as the surface to volume ratio increases for smaller hollow silicon spheres, the effect becomes even more discernible for low surface lithium-concentrations ([Li]~ 1 mol / m³). This suggests that it may be advantageous to use slightly larger electrodes (at the cost of lower capacity), or to employ lower working lithium concentrations. We are currently studying the effect of suitable barrier layers for rational design of anodes with engineered stress profiles, while maintaining high capacity.

Existing all-solid-state Li-metal batteries suffer attacks by the chemically aggressive and mechanically stressful Li metal. Li metal is a soft crystal and may exhibit either displacive or diffusive deformation. Here, we describe a class of all-solid-state Li-metal batteries enabled by 3D porous Li-metal hosts made of electrochemically stable mixed Li-ion and electronic conductor (MIEC) and electronic and Li-ion insulators (ELI). Within 3D open porous MIEC/ELI structure, Li metal advances and retracts via interfacial diffusional creep as an ``incompressible working fluid'' with fast stress relaxation and minimal contact with a solid electrolyte (SE), thereby significantly improving the electrochemomechanical stability. In situ transmission electron microscopy corroborated with thermodynamic analyses offers design principles in materials, sizes, and interfaces of the 3D porous MIEC/ELI structures, which are applicable to other alkali-metal batteries. The successful construction of a creep-enabled battery engine opens a new avenue toward high-density, electrochemically and mechanically robust all-solid-state Li-metal batteries. [Nature 578 (2020) 251]

High-energy-density silicon oxide-based/ graphite composite anodes are promising to become the next-generation anode material in the lithium-ion battery industry. However, as the vital raw material of the newly designed anodes, micron-sized silicon oxide-based particles exhibit poor cycling stability, which limits its industrialization promotion. Due to the complex amorphous microstructure inside the silicon oxide-based particles, its evolution and failure mechanism during the electrochemical cycling process remains poorly understood.
Accurate sample preparation for the silicon oxide-based particles will be performed by FIB-SEM dual-beam system, so that the chemical compositions and structure in the internal space of silicon oxide-based particles would be characterized. Combining with the different results during the whole electrochemical cycling process, we would obtain the formation and evolution principles of the internal microstructure for the silicon oxide-based particles.
Moreover, the dramatic volume change, the damage of ion pathway and conductive network inside the particles, the continuous growth and reconstruction of SEI, and the Si-ion loss to the silicon oxide-based particles during hundreds of electrochemical cycles would be comprehensively investigated, in order to summarize the negative factors of the particle failure and capacity loss.
Benefited from the results in this report, more effective strategies would be designed, which could be generalized for the preparation of other silicon oxide-based anode materials. Therefore, the next-generation anodes of Li-ion batteries with excellent cycling stability, high reversible capacity and superior rate performance would be obtained.

Pathways to batteries with higher energy density necessarily involve the use of electrode materials that undergo substantial chemo-mechanical transformations in both liquid and solid-state electrolyte environments. Here, I discuss my group’s efforts to use in situ and operando characterization to investigate the interplay between chemistry and mechanics during reactions of high-capacity battery electrode materials. First, our work on using in situ transmission electron microscopy (TEM) to investigate reversible void nucleation and growth during delithiation of antimony (Sb) alloy nanocrystals is presented. Sufficiently small Sb alloy nanocrystals were found to form single voids during dealloying rather than undergoing shrinkage, as in larger particles. This voiding behavior during dealloying was a result of the impact of stiff native oxide surface layers that retained their shape. This effect was found to be size dependent, and an analytical model was developed to show that small nanocrystals do not exhibit enough total strain energy during shrinkage of the Li-Sb phase to drive mechanical buckling of the shell. These observations translated to improved electrochemical behavior in battery cells, where the small nanocrystals were observed to exhibit the highest average Coulombic efficiency compared to larger particles. Next, a study on chemo-mechanical degradation of FeS₂ electrode materials reacting with different ions (lithium, sodium, and potassium) is presented. Despite larger volume changes during reaction with Na⁺ and K⁺, only reaction with Li⁺ was observed to cause fracture. Modeling showed that this surprising result was found to be due to the shape of the reaction front as it evolves during reaction, with sharp corners during lithiation resulting in stress concentration and fracture. Finally, our efforts on understanding interphase formation and mechanical contact at solid-state interfaces in all-solid-state batteries is discussed. The reaction of L_1+xAl_xGe_2-x(PO₄)₃ (LAGP) solid-state electrolyte with lithium is found to involve amorphization and volume expansion of ~130%. In situ X-ray tomography experiments of operating LAGP-based cells reveal that the growth of the interphase causes fracture, which is the underlying cause of impedance increase within the cell. Together, these studies highlight the controlling influence of chemo-mechanics in high-capacity battery electrolyte materials when used in both liquid and solid-state battery systems.

The non-uniform growth of the microstructures in dendritic form inside the battery during prolonged charge-discharge cycles causes short-circuit as well as the capacity fade. We develop a feed-back control framework for the real-time minimization of such microstructures. Due to accelerating nature of the branched evolution, we focus on the early stages of growth, identify the critical ramified peaks and compute the effective time for the dissipation of ions from vicinity of those branching fingers. The control parameter is a function of the maximum interface curvature (i.e. minimum radius) where the rate of run-away is the highest. The minimization of total charging time is performed for generating the most packed microstructures which correlate closely with those of considerably higher charging periods, consisting of constant and uniform square waves. The developed framework could be utilized as a smart charging protocol for the safe and sustainable operation the rechargeable batteries, where the branching of the microstructures could be correlated to the sudden variation in the current/voltage.

Low cycle efficiency and dendrite growth are two critical barriers for rechargeable batteries using Li metal as negative electrodes, mainly due to the coupled mechanical/chemical degradation of the SEI layer formed on Li metal surface. We have developed a comprehensive set of in situ diagnostic techniques combined with atomic/continuum modeling schemes to investigate and understand the coupled mechanical/chemical degradation of the SEI layer/Li system including fundamentally understanding the surface and interface phenomena. We have found that the mechanical incompatibility between SEI and soft Li leads to the complicated mechanical behaviors of the lithium metal electrode during the plating and stripping process. We systematically investigated the relationship between surface morphology and current density distribution which results in an inhomogeneous Li plating/stripping process. We also investigated the mechanical properties of Li metal electrode at different cycle period, including the stress evolution and deformation mechanism which have been significantly impacted by SEI formation. Based on the fundamental understanding from the integrated in-situ diagnostics, atomic simulation, and continuum framework, we have developed a new coating design strategy to achieve high cycle efficiency/dendrite free and extend the cycle life of lithium rechargeable batteries.

Na-ion and Na-metal batteries are attractive alternatives to Li-ion, as they have the potential to reduce cost and add supply chain stability for energy storage portfolios. The raw materials for Na-based batteries are more readily sourced: Na is much more earth-abundant than Li, Na batteries can utilize aluminum for both current collectors (instead of copper), and they can also readily use cobalt-free cathodes. In order to advance the state-of-the-art for Na-ion and Na-metal batteries, reliable mechanical property data for Na is needed. However, few reports have characterized the mechanical properties of Na, especially in terms of its viscoplastic and creep properties [1]–[3].

In this study, we report the first temperature-dependent measurements of power-law creep of Na. While prior works on the viscoplastic and creep deformation of Na have reported values at room temperature, this work measured the tensile response of Na in inert environments between 273 and 348 K. Furthermore, this work revealed insights about sodium deformation in general that have implications for Na batteries. For example, tests at room temperature utilizing digital image correlation (DIC) were performed inside of an argon glovebox [4] to map the local deformation of Na tensile samples. Local strain and rotation maps from DIC revealed millimeter-scale inhomogeneity that was attributed to plastic grain rotation, due to the large (~1 mm) grain size in the Na. The full calibration of power-law creep behavior from this work is valuable for modeling efforts to understand the fundamental research of Na-based batteries.

[1] P. M. Sargent and M. F. Ashby, “Deformation mechanism maps for alkali metals,” Scr. Metall., vol. 18, pp. 145–150, 1984, doi: 10.1016/0036-9748(84)90494-0.
[2] C. D. Fincher, D. Ojeda, Y. Zhang, G. M. Pharr, and M. Pharr, “Mechanical properties of metallic lithium: from nano to bulk scales,” Acta Mater., vol. 186, pp. 215–222, 2020, doi: 10.1016/j.actamat.2019.12.036.
[3] M. J. Wang, J. Chang, J. B. Wolfenstine, and J. Sakamoto, “Analysis of Elastic, Plastic, and Creep Properties of Sodium Metal and Implications for Solid-State Batteries,” Materialia, p. 100792, 2020, doi: 10.1016/j.mtla.2020.100792.
[4] W. S. LePage et al., “Lithium Mechanics: Roles of Strain Rate and Temperature and Implications for Lithium Metal Batteries,” J. Electrochem. Soc., vol. 166, no. 2, pp. A89–A97, 2019, doi: 10.1149/2.0221902jes.

High energy Ni-rich cathode will play a key role in advanced Li-ion batteries, but it suffers from moisture sensitivity, side reactions and gas generation. Single crystalline Ni-rich cathode has a great potential to address the challenges present in its polycrystalline counterpart by reducing phase boundaries and materials surfaces. However, synthesis of high-performance single crystalline Ni-rich cathode is very challenging, not mentioning a fundamental linkage between over potential, microstructure and electrochemical behaviors in single crystalline Ni-rich cathodes. This talk will discuss the planar gliding and microcracking along the (003) plane in a single crystalline Ni-rich cathode by using in situ AFM and TEM. The formation of microstructure defects is correlated with the localized stresses induced by a concentration gradient of Li atoms in the lattice, providing clues to mitigate particle fracture from synthesis modifications.

Multiscale electro-chemo-mechanical interplay governs the local structural and chemical evolution and is fundamental to the global electrochemical behavior of batteries. Prolonged battery operation under realistic conditions induces state-of-charge (SoC) heterogeneity, builds up mechanical stress, and provokes morphological breakdown. These processes prevail in most battery electrodes and ultimately determine the device-level battery performance. In lithium-rich nickel-manganese-cobalt (LirNMC) layered material, a promising cathode for lithium-ion batteries with superior energy density however suffers from capacity and voltage fade, the coexisting cationic and anionic redox activities further complicate the redox heterogeneity, urging for a comprehensive investigation of at the nano- to meso- scales. In this talk, we will present a systematic study on the mesoscale degradation of the layered battery cathode material. We look into the intertwined lattice, redox, and micro-morphology properties as the cathode is cycled under different conditions. Our results suggest that the cathode particle engineering could be a viable approach for further improving the structural and chemical robustness of the Li-ion battery.

In modern layered cathodes for lithium-ion batteries, the sizes and morphologies of the primary particles play a key role in governing the electrochemical dynamics during cycling, since the desired charge-transfer reactions and any competing solid-electrolyte interphases rely on the exposed particle surfaces. Synthesis of layered cathodes is a two-step process. The secondary structure is determined largely by the conditions used in the initial preparation of the transition metal precursor material. The subsequent high-temperature calcination transforms the precursor to the final lithiated cathode while largely preserving the structure of secondary particle agglomerates. Recent research has characterized the crystal structure during this transformation, but has been largely silent on the accompanying morphological changes in the primary particles. Through a combination of in-situ X-ray diffraction and X-ray tomography, the morphology and structure of layered NMC primary particles were tracked during calcination. Comparison with first principles calculations and continuum modeling provided additional insights into the changes in both phase and morphology. These results provide a foundation for a more deliberate design of cathode materials with the necessary performance characteristics to help meet the growing demand for electrochemical energy storage.

Extremely fast charging and discharging, along with long cyclability, require energy storage materials that respond with minimum mechanical deformation while at the same time storing significant amounts of charge. The coupling of mechanics with electrosorption and insertion leads to fundamental limits in the power capability of materials. During this presentation, I will discuss our use of operando atomic force microscopy to probe the real-time deformation of ion insertion materials at fast (minute and second) timescales. The nanoscale dimensions of the AFM tip allow us to probe not only the deformation but also the heterogeneity of the insertion process. Furthermore, I will share our insights into how understanding mechanical deformation can lead to the design of high power energy storage materials such as tungsten oxides with confined interlayer water.

Sn is one of promising anode materials with high theoretical capacity for Li ion batteries and has attractive mechanical properties for effective stress relaxation during lithiation. Soft, ductile nature as well as creep at room temperature allow for Sn to effectively relax the generated stress due to the volumetric strain as a result of lithiation. In this study, finite element simulation was used to study the evolution of diffusion induced stresses in Sn micropillar anode with the focus on the effect of concentration-dependent material properties in the diffusion induced stresses. Simulation results using concentration-dependent material properties indicated a different failure mode in comparison to that from simulations results using concentration-independent material properties. When modeled using constant material properties that are independent of Li composition, compressive hoop stress is developed at surface of pillar. If we assume elastic deformation, max hoop stress at the surface is 1000 MPa, but this reduced down to 150 MPa and 40 MPa in compression when plasticity and creep, respectively, is incorporated in simulation model. On the other hand, when concentration-dependent material properties were incorporated in the model, concentration profile of Li in Sn becomes homogeneous as lithiation is proceeded and compressive hoop stress at surface is transformed into tensile hoop stress in the early stage of lithiation. This hoop stress transition at surface is found to be originated from the increase of diffusivity of Li in the lithiated phases; outer region near the surface that has already transformed to Li rich phases is pushed out as the core of the pillar undergoes lithiation reaction to result in tensile hoop stress at the surface of the micropillar. Development of tensile hoop stress at surface can then trigger the crack formation at the surface of Sn micropillar. In order to experimentally determine the critical dimension for Sn micropillar as a result of lithiation, Sn micropillars of varying sizes were studied which was determined to be 5.3 μm when charged at rate of C/10.

We assess the heterogeneous electrochemistry and mechanics in a composite electrode of commercial batteries using synchrotron X-ray tomography analysis and microstructure-resolved computational modeling. Morphological defects are visualized at multi-scales ranging from the macroscopic composite, particle ensembles, to individual single particles. Particle fracture and interfacial debonding are identified in a large set of tomographic data of active particles. Mechanical failure in the regime near the separator is more severe than toward the current collector. The active particles close to the separator experience deeper charge and discharge over cycles and thus are more mechanically loaded. The difference in the Li activity originates from the polarization of the electrolyte potential and the non-uniform distribution of the activation energy for the charge transfer reaction. We model the kinetics of intergranular fracture and interfacial degradation to confirm that the various Li activities are the major cause of the heterogeneous damage. The interfacial failure may reconstruct the conductive network and redistribute the electrochemical activities that render a dynamic nature of electrochemistry and mechanics evolving over time in the composite electrodes. We further quantify the influence of the mechanical damage on the metrics of battery performance. We simulate the electrochemical impedance profile to build a relationship between the interfacial debonding and the impedance of electron transport and surface charge transfer. The mechanical failure disrupts the conduction path of electrons and results in significant polarization and capacity loss in batteries.

Structural energy storage devices combine energy storage and structural functionalities in a single unit, leading to lighter and more efficient electric vehicles. However, conventional electrodes for batteries and supercapacitors are optimized for high energy but suffer from poor mechanical properties. Here, we use Kevlar® nanofibers to fabricate mechanically strong Li-ion battery anodes and cathodes. Specifically, we report on free-standing electrodes consisting of lithium iron phosphate (LFP, cathode) or silicon (Si, anode) particles, reduced graphene oxide (rGO), and branched aramid nanofibers (BANFs). LFP and Si have high theoretical capacity values. rGO has high electrical conductivity, high surface area, high capacitance, and excellent mechanical properties. Recently developed branched aramid nanofibers, nanoscale Kevlar® fibers, are of great interest due to their exceptional mechanical properties, such as ultimate strength and stiffness. rGO and BANFs interact with each other through hydrogen bonding and π-π stacking interactions allowing for load transfer within the electrodes. The composite electrodes were fabricated via vacuum-assisted filtration to achieve highly layered structures. Electrical, mechanical, and electrochemical properties for all composite electrodes were evaluated as a function of composition. Overall, we obtained up to two orders of magnitude improvements in Young’s modulus and tensile strength compared to commercial battery electrodes while maintaining energy storage capability. The role of BANFs was further investigated using scanning electron microscopy (SEM), x-ray photoelectron microscopy (XPS), and electrochemical impedance spectroscopy (EIS) before and after electrochemical cycling. This work demonstrates an efficient route for designing structural Li-ion battery cathodes and anodes with enhanced mechanical properties. This work is supported by AFOSR under Grant No. FA9550-19-1- 0170.

Volumetric hydrogen storage was investigated on compacted monoliths of zeolite-templated carbon, a material with unique mechanical properties allowing for extreme densification. Pellets were prepared by a hot-pressing method between 433-573 K and 50-345 MPa, with densities of up to ~1 g/mL. Hydrogen adsorption measurements were carried out on the so-obtained monolithic samples between 40-298 K and 0.0003-10 MPa, with a focus on volumetric storage and delivery metrics at 77 and 298 K. Under optimal pelletization conditions, a sample was prepared which demonstrates near-ideal stored and delivered quantities of hydrogen when compared to that of ZTC powder compacted to “crystalline” (periodic model) density. The improvement in hydrogen delivery is notable at room temperature, exhibiting a 10% advantage over compression alone. Higher capacities and deliveries could not be achieved with activated carbons owing to mechanical properties prohibitive to the preparation of pellets without the use of excessive binder, highlighting the unique advantage of ZTC as a soft, sponge-like material for high density energy storage applications.

Two-dimensional materials (2DM) such as graphene, transition metal dichalcogenides (TMD), MXenes, and their heterostructures are among the most promising energy materials for radically advanced batteries. In this presentation, two important computational aspects of 2DM-based batteries are addressed – (i) 2DM and its heterostructures as anode materials, and (ii) 2DM as van der Waals (vdW) slippery interface. The conventional anode materials have several problems, such as low gravimetric capacity (e.g., graphite – 372 mAh/g) and high volume expansion (e.g., silicon – 300%). Our computational modeling shows that topologically modified 2DM can be utilized as high-capacity anode materials for ion batteries with capacity as high as 1000 mAh/g. However, despite enormous opportunities in 2DM anode, several challenges need to be addressed, such as trapping of adatoms at the defect sites, the effect of defects on the diffusivity of adatoms, mechanical degradation at defect sites during charging/discharging, etc. The second part of the presentation discusses the interface of anode and current-collector (e.g., silicon anode and copper current-collector in Li-ion battery). To combat the issue of high-stress development at the anode-current collector interface during charging/discharging, we propose the usage of the graphene layer over the current collector as a vdW slippery interface that reduces the interfacial stress and enhances the cycle life of batteries. Our computational results are in excellent agreement with the experimental findings.

Wearable energy storage devices are indispensable cornerstones for future wearable electronics. Current energy storage technologies are based on materials and devices that are rigid, bulky, and heavy, making them difficult to wear. On the other hand, fibers are flexible and lightweight materials that can be assembled into different textiles and have been worn by human beings thousands of years. Different from conventional two-dimensional thin films and foils, the three-dimensional fiber and textile structures not only provide superior wearing ability but also much larger surface areas. This talk will introduce how our research group makes use of the attributes of fibers for high-performance wearable energy storage devices. We will demonstrate the strategies and discuss the perspectives to modify fibers and textiles for making wearable capacitors and batteries with excellent mechanical durability, electrochemical stability, and high energy/power density.

Polypyrrole (PPy)-based electrochemical energy storage electrodes have been widely investigated due to its desired pseudocapacitive charge storage capabilities. However, with the inflexible nature of the conducting polymer, its utilization in fully flexible and stretchable supercapacitor electrodes has been hindered. Herein, a surface-modified thermoplastic polyurethane (TPU) was utilized as the flexible substrate, coated with a thin surface layer of carbon nanotubes (CNT) to provide the desired electrical conductivity and allowed better adhesion of the PPy micro-foam (PPyMF) structures. The fabrication process involved density difference induced alignment of surface CNTs for an even surface coating of the conducting carbon layer suitable for PPy polymerization. With the formation of high specific surface area PPyMF, the flexibility of the sandwiched structure was retained, while providing a highly conducting network with extremely porous active PPyMF for charge storage. The as-fabricated TPU/CNT/PPyMF electrodes were shown to produce a high 345 F/g gravimetric capacitance at a scanning rate of 5 mV/s, an exceptional electrical conductivity of 713 S/m, and a retained capacity of 71% after 3000 charge/discharge cycles, which was better than previously reported polypyrrole based electrode systems. The retained flexibility and charge storage capability during bending was tested to show that 78% of capacity was retained at a bending angle of 90 degrees. At the same time, the supercapacitor electrode was also designed to be self-sensing with an ability to sense strain based on resistance changes, allowing further monitoring of supercapacitor stretching and flexing during usage.

Energy storage materials that also provide structural integrity are in need for the weight reduction of electrical automobiles. Structural supercapacitor electrodes consisting of aramid nanofiber (ANFs) and reduced graphene oxide (rGO) nanosheets have shown promising experimental combinations of mechanical properties and capacitance. Further, electrodes including chemically functionalized rGO nanosheets (frGO) has demonstrated even higher experimental moduli than that of the rGO/ANF nanocomposite. The purpose of the present work is to develop a micromechanics model for elastic moduli to analyze the influence of different functional groups and support the development of multifunctional composite electrodes. A three-step micromechanics model was developed to capture the influence of the functional groups on the overall elastic modulus of frGO/ANF composite electrode, as well as the waviness (or curvature) of the respective nanomaterials. The functional groups on rGO were modeled as a separate phase surrounding the rGO. The interphase was introduced in the nano scale representative volume element (RVE) to form the effective frGO phase. In the microscale RVE, waviness and random orientation was introduced to both ANFs and the frGO. Then, the effective ANF and frGO phases were combined in the mesoscale RVE. Parametric studies on the interphase were carried out. The influence of the interphase thickness and interphase modulus was considered. The in-plane moduli increase significantly with initial interphase modulus increase. However, the increase of in-plane moduli of the composite films will reach a plateau after 30 GPa. Functionalized rGO/ANF composites have lower waviness and a wider waviness distribution per film compared to rGO/ANF composites according to scanning electron microscopy images, which was also captured by the model. Based on the modeling results, the sensitivity of elastic moduli to waviness is higher at lower waviness because of the nonlinear relationship between elastic moduli and waviness which led to the higher standard error of the elastic moduli observed in the experimental results.

Engineering materials that can store electric energy in structural load paths can revolutionise lightweight design across transport modes. Contemporary electric vehicles and aircraft use traditional lithium ion batteries for electrical energy storage. The battery is only providing an electrical energy storage function - adding weight to the system but does not contribute to its structural performance. This paper will present research aimed at the development and demonstration of a multifunctional material that can simultaneously store electrical energy and carry mechanical loads. We have coined this material as the structural battery composite. Structural battery composite materials will allow radical weight savings for any electrically powered structural systems, from laptops to cars and aircraft. We foresee that access to structural batteries would allow that practically all the weight of the current monofunctional battery is removed in satellites and significantly reduced in electric cars. Use of structural batteries will provide more energy efficient transport solutions and hence increase competitiveness and save money. As an example, for satellites, a 250 kg weight reduction corresponds to a cost reduction for launch of at least $3.8M.

Over the last decade, Chalmers and KTH have performed research to realise structural battery composites. By this work, the Swedish team has established a position as leaders in the field [1]. Current structural battery composites have demonstrated an energy density of 24 Wh/kg at a Young's modulus of 25 GPa. We are currently in the process of developing and demonstrating the second-generation structural battery composite with an anticipated energy density of at least 50 Wh/kg and a modulus of 70 GPa. This is slightly lower than traditional composites and Li-ion batteries but combined into a multifunctional material providing significant mass savings.

Structural battery composites are made from carbon fibres in a structural battery electrolyte (SBE) matrix material. Neat carbon fibres are used as a structural negative electrode, exploiting their high mechanical properties, excellent lithium insertion capacity and high electrical conductivity. Lithium iron phosphate coated carbon fibres are used as the structural positive electrode. Here, the lithium iron phosphate is the electrochemically active substance and the fibres carry mechanical loads and conduct electrons. The surrounding SBE is lithium ion conductive and transfers mechanical loads between fibres. With these constituents, structural battery cells are realised.

The presented research relates to recent activities funded by EU's Clean Sky II and USAF. It further comprises ongoing research activities to realise the second-generation laminated structural battery composite full cells. The work comprises Multiphysics models for design and analysis of structural battery devices; separators to allow increased charge/discharge rates; highly multifunctional carbon fibre-reinforced positive electrodes; and design and characterisation of multifunctional electrodes/SBE and separator/SBE interfaces.

1. Our paper Fredi G, et al. Multifunctional Materials, 1, 2018, 015003 was appointed among the top-ten breakthroughs in physics in 2018 by the Physics World magazine.

Structural power composites is an important emerging technology which has the potential to address challenges associated with both lightweighting and electrical energy storage for a range of platforms, but particularly transportation. The focus of the work reported here are structural supercapacitors. Although these devices do not offer the high energy densities associated with batteries, they provide a research vehicle to understand the generic development and implementation hurdles associated with structural power. Structural supercapacitors consist of two electrodes (woven carbon fibre plies impregnated with carbon aerogel) which sandwich an electrically insulating (but ionically conducting) separator (spread tow woven glass fibre). This assembly is immersed in a structural electrolyte, which is a biphasic interconnecting network of a structural phase (typically epoxy) and an ionically conducting phase (typically ionic liquid).
Predictive modelling is discipline which could potentially accelerate the progress, but there is a dearth of work into such studies for structural power. This paper describes the modelling efforts which have been undertaken at Imperial College London, focusing on studies at three different lengthscales: micro, meso and macro.
Firstly, at the micro lengthscale, topology optimization (TO) methods are being adopted to identify the optimum microstructure of the structural electrolyte and hence maximize the multifunctional performance of the resulting structural power devices. The structural electrolyte needs to provide a balance between mechanical load-bearing and ionic transport functions. TO has been used to find solutions for the ideal microstructures under different loading cases (tension and shear) whilst maximizing ionic conductivity. This has led to the optimum microstructures for given volume fractions of each phase, over a range of weightings (ranging from dominated by mechanical loading requirements to dominated by ionic transport) having been identified. These predictions have been validated against 3D printed unit cells, the mechanical and electrochemical performance of which has been characterized.
At the meso lengthscale, a multiphysics numerical model has been developed, using a commercially available finite element code (ABAQUS), to predict both the mechanical and electrochemical response of our devices. Key to this is predicting the consolidation of the device during manufacture, since this will dictate the spacing between the electrodes (hence the electrochemical performance), but also the reinforcement volume fraction (hence the mechanical performance). The consolidated device geometries have been then been modelled to predict their mechanical response but also, using a modified user element, their electrochemical response. The future aspiration is to permit coupling in these models, such that the influence of mechanical cycling or damage on the electrochemical performance can be predicted. Ultimately this model will provide a means to support the certification of structural power components for transport applications.
Finally, at the macro lengthscale, is the challenge of multifunctional design: identifying how best to design with and adopt structural power composites. Methodologies have been developed to provide a clear comparison between conventional compartmentalized systems (such as load-bearing structure and conventional batteries) and structural power components. Hence, the energy and power densities need to be achieved in such multifunctional materials to offer a weight saving over existing systems have been determined. Such studies have considered a range of platforms, and provided an insight into target applications for future structural power materials.
It is anticipated that development of predictive models at these different lengthscales will underpin and accelerate the maturation of this emerging technology, and hasten its adoption by end-users.

In the past decades, lots of efforts are made by researchers to promote the development of structural lithium ion batteries, which endow energy storage ability to structural parts for vehicle lightweighting. However, the two strategies people normally use nowadays still have major challenges. One method is introducing thick or heavy packaging and mechanically supporting parts to or inside cells, which intrinsically decreases the energy densities of these energy storage devices. The other one is replacing conventional cell parts like electrodes and electrolytes with materials which have strong mechanical properties but poor electrochemical cycling stability, which sacrifice cycling performance of batteries.
By finite element analysis, we find that the reason of unsatisfying flexural behavior of conventional lithium ion batteries is the sliding of separator and electrode layers relative to each other, and it will greatly increase if we can suppress this sliding. The reason is that if layers are bonded together, all components will participate in the resistance of bending rather than slide relatively, which inspires us with a new idea to improve the mechanical behavior of batteries.
Here we propose a straightforward and rational interfacial design to make cell parts of lithium ion batteries robust by applying suitable and strong “binders” to each interface inside. With a thin coating layer of porous PVDF-HFP, which is previously shown without detriment to cycling performance, electrodes can be bonded with PVDF-coated separators tightly by hot pressing. With these techniques, all components in cells can contribute to the overall mechanical strength, which allows the battery with greatly enhanced flexural strength than conventional batteries. On the other side, on account of the chemical simplicity of this strategy, the structural battery in this work can deliver a comparable specific capacity with conventional cells. This work comes up with a new and simple way to design structural batteries, with both mechanical performance and energy density satisfied, and the method can be easily transferred to large-scale production in industry. Thanks to the wide electrochemical stability of PVDF-HFP, nonflammable and fire-retardant electrolytes can also be utilized, once they are compatible with electrodes, to improve the safety of batteries for their practical application as structural parts in vehicles.
In addition, we apply finite element analysis to conduct simulations to compare the contribution of materials' intrinsic mechanical properties and interfacial adhesion strength in flexural behaviors of batteries, which provides a valuable insight into feasible structural batteries with balanced mechanical and electrochemical properties.

Structural energy and power devices combine the mechanical properties of structural composites with the energy storage properties of capacitors or batteries. These multifunctional devices promise to reduce the mass and volume of electric vehicles, aircraft, and spacecraft. Carbon fiber mats have been proposed as platforms due to their high stiffness, but it is a challenge to achieve high specific energy and power because the carbon fiber occupies a large volume/mass and is not as electrochemically active due to its low surface area. In response to this challenge, we have explored composite electrodes containing reduced graphene oxide (rGO) nanosheets and Kevlar aramid nanofibers (ANFs). The rGO nanosheets are conductive, mechanically stiff, and have a high surface area; the ANFs are mechanically stiff and provided bridging among the rGO nanosheets via noncovalent interactions.

This talk will provide an overview of the rGO/ANF structural supercapacitor platform, experimentally emphasizing the role of non-covalent interfacial interactions and computationally emphasizing electrode architecture and interphase effects. We compare nacre-like and open-cell electrode morphologies, in which rGO/ANFs electrodes are made via vacuum filtration and hydrothermal synthesis, respectively. Experiments and COMSOL multi-physics modeling demonstrate tradeoffs among ion transport. Generally, the open cell structure results in poorer mechanical properties but satisfactory capacitance. We also compare different functional groups on chemically functionalized rGO, including dopamine, tannic acid, amine, carboxylic acid, and chelating ions. It is found that the mechanical stiffness increases with increasing non-covalent interaction strength, but this results in denser electrodes and poor ion transport. Further, micromechanical modeling shows modulus and conductivity variations depending on electrode arrangement and waviness (i.e., curviness) of the rGO nanosheets and ANFs. The interphase, as described by modeling as a proxy for the non-covalent interacting layer, affects modulus and conductivity, as well. Together, these results indicate that rGO/ANF electrodes with good combinations of mechanical stiffness and capacitance should have a layered structure with flattened nanomaterials (not wavy, wrinkled, or curvy) with strong and numerous noncovalent interactions, but these electrodes should also have sufficient porosity to allow for ion access to the electrode’s surfaces. Lastly, in comparison to the carbon fiber platform, the rGO/ANF capacitors have higher capacitance, but lower stiffness.

Looking to the future, structural energy and power devices bear tradeoffs among mechanical properties and energy storage performance that still need to be addressed with new materials and platforms. The effect of electrochemomechanical coupling is needed to understand failure modes and long term stability.

This work is supported by AFOSR under Grants No. FA9550-16-1-0230 and FA9550-19-1- 0170.

Lithium-ion batteries (LIBs) are ubiquitous in modern society with a wide spectrum of applications from consumer electronics to electric vehicles and the future electric aircraft. The basic structure of a commercial LIB cell consists of alternating porous layers (electrodes and separator) and metal foils (current collectors). These solid materials are saturated with a liquid electrolyte. The first part of this talk will cover a comprehensive physics-based modeling program of the LIB structure, based on several key publications of our team. We will then show that conventional computational tools such as finite element simulations gradually lose their effectiveness as the dimensionality of the system increases. We will use the second part of the talk to demonstrate the applicability of physics-guided neural network approaches, both PDE-based and energy-based, in the mechanical modeling of LIBs in a few real-world scenarios.

Maintaining the physical contact at the electrolyte/electrode interface and preventing mechanical failure are critical to the performance of all-solid-state batteries (ASSB). The ceramics based solid electrolyte and electrode interface tends to have imperfect contact, which can be worsened due to cycling and improved due to cell stack pressure. The solid electrolyte is also subject to electrochemical decomposition at the interface, causing volume change, in addition to the well-known volume change in the electrodes during lithiation-and-delithiation. These chemical strains generate mechanical stress, which leads to fracture in the highly compliant all-solid-state batteries, especially in 3D ASSBs. In this presentation, new continuum models were developed to capture the highly coupled electrochemical-mechanical phenomena. First, we will present a 1D Newman battery model for a film-type Li|LiPON|LiCoO₂ ASSB that incorporated the effect of imperfect contact area with battery performance by assuming the current and Li concentration will be localized at the contacted area. To establish the relationship between the applied pressure and the contact area, we applied Persson’s contact mechanics theory as it uses self-affined surfaces to simplify the multi-length scale contacts in ASSLBs. The model is then used to suggest how much pressures should be applied to recover the capacity drop due to contact area loss. Furthermore, we will introduce a 3D Newman battery model for the experimentally-made 3D Si|LiPON|LiCoO₂ ASSB to incorporate the chemo-mechanical strains due to interfacial decomposition of LiPON and the lithium concentration gradient while cycling. This electrochemo-mechanical model could be used for predicting whether and where the fractures would be produced in order to guide the architecture design of 3D ASSB with balanced energy density and mechanical integrity.

All-solid-state lithium batteries (ASSLBs) are pursued intensively as a promising next-generation energy storage technology due to their high potentials to achieve superior safety and high energy/power densities.¹ Sulfide solid-state electrolytes (SSEs) are arguably more viable for bulk-type ASSLBs than oxide and polymer-based SSEs, thanks to their higher ionic conductivities at room temperature and lower elastic modulus, which allows fast Li⁺ transportation and intimate contact with active materials.² However, deployment of sulfide solid-state electrolytes in practical bulk type ASSLBs are hindered by the processing of highly conductive and ultra-thin solid electrolyte films. Most ASSLBs are evaluated using thick sulfide SSE pellets (>300 microns) for appropriate mechanical strength. This method is not realistic since thick SSE pellet causes a serious penalty to energy density at the cell level.³ Therefore, it is urgent to develop highly conductive and ultra-thin sulfide SSE film with good mechanical properties for the deployment of ASSLBs.
Here, we report an up-scalable method for the preparation of highly conductive sulfide SSE films with a lean amount of polymeric binder. A new polymeric binder/solvent combination, which is compatible with sulfide SSEs, was identified to prepare the ultra-thin SSE film. Under processing, the polymeric binder is well-controlled into filament-shape and builds a 3D-matrix, providing superior confinement for SSE particles with decent flexibility. Effects of the polymeric binder on SSE film have been investigated in terms of film mechanical property, ionic conductivity, and electrochemical properties of Li/SSE/Li symmetric cells. As a result, ultra-thin SSE film with a thickness of 75-100 um was prepared with 2 wt. % of the polymeric binder and demonstrates an overall conductivity of about 1 mS/mcm at room temperature. More details of this research will be discussed at the symposium.

1. Randau, S. et al. Benchmarking the performance of all-solid-state lithium batteries. Nat. Energy 5, 259–270 (2020).
2. Wang, Y. et al. Superionic conduction and interfacial properties of the low temperature phase Li 7 P 2 S 8 Br 0.5 I 0.5. Energy Storage Mater. (2019).
3. Lee, Y. G. et al. High-energy long-cycling all-solid-state lithium metal batteries enabled by silver–carbon composite anodes. Nat. Energy (2020).

Mechanical constriction induced various stabilities greatly widen the operational voltage window of solid-state batteries. Bulk and interface stabilities can be modulated to operate at extreme voltages of high voltage cathodes and Li metal anode. Ab-initio simulations and experimental characterizations are used to unveil chemical, electrochemical and microstructural details to support the unique physical picture of mechanical constriction modulated electrochemical behavior. Based on the design principle, up to 10 mA/cm² current density on lithium metal anode and up to 10 V voltage stability of sulfide electrolytes are demonstrated. The potential impact of this effect on the design of high voltage solid state batteries beyond the commercial level will also be discussed.

Metallic anodes have the potential to enable batteries with enormous capacities. Indeed, lithium metal has the highest theoretical capacity, lowest density, and most negative electrochemical potential of known anode materials for rechargeable batteries. However, dendrites of Li can form during cycling, which lead to significant safety issues that have precluded their practical deployment. Sodium metal anodes have similar safety concerns but have recently received increased attention due to their natural abundance, relatively low cost, and potential for grid-scale energy storage. Potassium metal anodes are still very much in a developmental phase but have promise stemming from their potential for simple cell design with cheap fabrication, earth abundance, and relatively low cost. Magnesium metal anodes have enormous volumetric capacities but readily form passivating layers during recharging, and recent studies suggest dendrite formation can occur under certain conditions. While the electrochemistry of these systems has received extensive study, at the heart of many practical issues in these systems lies a mechanics of materials problem. Specifically, as atoms are deposited and stripped during electrochemical cycling, the material deforms, generating stresses under constraint. These stresses can result in fracture, delamination, detachment, and/or unstable deformation (e.g., the formation of dendritic structures) of the electrodes, diminishing their capacity or leading to severe safety issues. Likewise, the stresses in turn may affect the electrode kinetics, the growth morphology under cycling, and/or the integrity of the contact at the anode/solid-state-electrolyte interface. Studies that provide deeper understanding of mechanics in these systems may thus play a key role in mitigating or even preventing many of these issues. For instance, modelling studies have suggested that the mechanical properties of the anode materials (relative to the separators or solid-state electrolytes with which they are in contact) are key in guiding the suppression of dendritic growth during electrochemical operation. Experimental studies have shown that the morphology of Li during electrochemical deposition depends on external pressures applied to battery stacks, as does the propensity for maintaining interfacial contact in all-solid-state batteries. As such, prior to real applications, a comprehensive understanding of the mechanical properties of these materials is vital. To this end, through nanoindentation, microhardness, and bulk testing, this talk will present experimental studies of the mechanical properties of metallic Li, Na, K, and Mg anodes, including how these properties vary with loading rate, representative size scale, and temperature. These properties will be connected to implications in terms of potential battery performance, e.g., in preventing dendritic structures, detachment between solid-state electrolytes and metallic anodes, etc. Overall, this talk will provide insight into guiding the design of battery materials, architectures, and electrochemical (e.g., charging) conditions that mitigate unstable growth of metal anodes during electrochemical cycling.

Bioinspired ion conductive membranes have been widely investigated for beyond lithium-ion battery applications. The high theoretical specific energy density (2600Wh/kg) and high specific capacity (1675mA/g) along with the natural abundance and low toxicity of sulfur have been attracting significant attention for the development of an alternative battery system to replace traditional lithium-ion batteries which suffer from safety and capacity/energy density limitations in various applications. However, challenges such as polysulfide dissolution and shuttling prevent mass commercialization of metal sulfur batteries. Inspired from biological ion transport mechanisms, we show a practical yet comprehensive approach for the development of high-performance metal sulfur batteries. Aramid nanofiber (ANF) based composite ion conductive membranes not only prevent dendrite formation but also confine polysulfides on the cathode side. ANF composite battery separators provide diverse and opposing properties including high mechanical properties, high ionic conductivity, and high thermal/chemical stability. Moreover, the highly selective ion sieving properties of these strong biomimetic separators provide safe and high-performance structural batteries. Fabrication of such biocompatible, affordable, flexible, and high energy density structural battery is quite crucial in powering next-generation electric vehicles, robotics and electronics including but not limited to portable, wearable, and implantable biomedical devices.