High-capacity Electrode Materials Research Articles

FeCo-Nx encapsulated in 3D interconnected N-doped carbon nanotubes for ultra-high performance lithium-ion batteries and flexible solid-state symmetric supercapacitors

Lithium-ion batteries and supercapacitors represent the predominant electrochemical power sources and energy storage devices. High-capacity and long-life electrode materials are essential for the next-generation lithium-ion batteries (LIBs) and supercapacitors (SCs) with high power density and high energy density. Herein, FeCo-Nx doped hollow shelled polyhedrons excessively covered with a 3D network of carbon nanotubes (FeCo–Nx@CNTs) have been successfully developed and investigated as one of the most promising materials for lithium-ion batteries and supercapacitors. We developed a self-template strategy to synthesize a series of FeCo–Nx@CNTs via one step carbonization process of Fe doped ZIF-67 MOF. Such a FeCo-Nx doped 3D network of carbon nanotubes effectively tolerates the volume strain and prevents the metal nanoparticles aggregation. In the interim, the 3D network of porous N-doped carbon nanotubes agrees fast electron transport to accomplish remarkably excellent rate capability. Consequently, the FeCo–Nx@CNTs-1.5 shows excellent electrochemical activity for Li-ion hopping in LIBs and SCs. Additionally, this effort also unveils the synergistic effect of the iron doping on the Li-ion storage capability.

Distinct Nanoscale Interphases and Morphology of Lithium Metal Electrodes Operating at Low Temperatures.

While Li-ion batteries are known to fail at temperatures below -20 °C, very little is known regarding the low-temperature behavior of next-generation high-capacity electrode materials. The lithium metal anode is of particular interest for high-energy battery chemistries, but improved understanding of and control over its electrochemical and nanoscale interfacial behavior in diverse conditions is necessary. Here, we investigate lithium deposition/stripping, morphology evolution, and solid-electrolyte interphase (SEI) structure and properties down to -80 °C using an ether-based electrolyte (DOL/DME). As temperature is reduced, we find that the morphology of deposited lithium is significantly altered. Furthermore, cryogenic transmission electron microscopy coupled with vacuum-transfer X-ray photoelectron spectroscopy reveal that the SEI exhibits different structure, chemistry, thickness, and conductive properties at lower temperatures. These results show that Li is promising for batteries operating under extreme conditions, and the distinct nanoscale evolution of Li electrodes at different temperatures must be considered when designing high-energy batteries.

Electrospun SiOC ceramic fiber mats as freestanding electrodes for electrochemical energy storage applications

Polymer-derived-ceramics (PDCs) and PDC-based fibers are being studied as potential high capacity electrode materials for electrochemical energy storage applications. Among these, silicon oxycarbide (SiOC) has shown most promise, especially as electrodes in lithium ion batteries (LIB). The study of PDCs for supercapacitor electrodes is, however, very limited. This work investigates the fabrication of SiOC-based PDC fiber mats via electrospinning and pyrolysis of three commercial pre-ceramic silicone resins (MK, H44, RSN) of varying composition, and their use as supercapacitor and LIB electrodes. Electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy was performed to study the effect of precursor chemical composition, pyrolysis time and temperature on the morphology, polymer to ceramic transformation and free carbon content. As supercapacitor electrodes, the H44 (a methyl-siloxane) derived SiOC showed the best performance with maximum specific capacitance of 50 F g-1 with capacity retention of ~100% after 2000 cycles. Similarly, as LIB electrodes, H44 derived SiOC fiber mat showed highest reversible capacity of 578 mAh g-1 at a current density of 50 mA g-1.

Design and Tuning of the Electrochemical Properties of Vanadium-Based Cation-Disordered Rock-Salt Oxide Positive Electrode Material for Lithium-Ion Batteries.

Disordered rock-salt compounds are becoming increasingly important due to their potential as high-capacity positive electrode materials for lithium-ion batteries. Thereby, a significant number of studies have focused on increasing the accessible Li capacity, but studies to manipulate the electrochemical potential are limited. This work explores the effect of transition-metal substitution on the electrochemistry of ternary disordered rock-salt-type compounds with LiM2+0.5V0.54+O2 stoichiometry (M = Mn, Fe, Co) directly synthesized through mechanochemistry. Rietveld refinements of synchrotron X-ray diffraction patterns confirm the disordered rock-salt structures. First-principles density functional theory study is used to predict the impact of the cation substitution on the expected average voltage and the electronic structures of these materials are used to analyze the underlying redox processes. For LiM2+0.5V4+0.5O2 (M = Mn, Fe, Co), discharge voltages increase in the order of Mn < Fe < Co with 2.28, 2.41, and 2.51 V, exhibiting discharge capacities of 219, 207, and 234 mAh g-1, respectively. In comparison, for the disordered rock-salt Li2VO3, an average discharge voltage of ∼2.2 V with V5+/4+ redox couple has been reported. However, detrimental electrode-electrolyte interactions manifested as transition-metal dissolution has been found to result in severe capacity fading. Thereto, the use of a concentrated 5.5 M LiFSI increased the cycling stability significantly, effectively reducing transition-metal dissolution. The underlying reasons for the capacity fading of disordered rock salts are yet unclear. We stress the importance of cathode-electrolyte interactions, thus opening new directions for the improvement of cation-disordered materials.

Electrolyte Chemistry Enables Simultaneous Stabilization of Potassium Metal and Alloying Anode for Potassium-Ion Batteries.

Alloying anodes are promising high-capacity electrode materials for K-ion batteries (KIBs). However, KIBs based on alloying anodes suffer from rapid capacity decay due to the instability of K metal and large volume expansion of alloying anodes. Herein, the effects of salts and solvents on the cycling stability of KIBs based on a typical alloying anode such as amorphous red phosphorus (RP) are investigated, and the potassium bis(fluorosulfonyl)imide (KFSI) salt-based carbonate electrolyte is versatile to achieve simultaneous stabilization of K metal and RP electrodes for highly stable KIBs. This salt-solvent complex with a moderate solvation energy can alleviate side reactions between K metal and the electrolyte and facilitate K+ ion diffusion/desolvation. Moreover, robust SEI layers that form on K metal and RP electrodes can suppress K dendrite growth and resist RP volume change. This strategy of electrolyte regulation can be applicable to other alloying anodes for high-performance KIBs.

Capacitive Deionization of Saline Water by Using MoS2-Graphene Hybrid Electrodes with High Volumetric Adsorption Capacity.

Capacitive deionization (CDI) has received wide attention as an emerging water treatment technology because of its low energy consumption, low cost, and high efficiency. However, the conventional carbon electrode materials for CDI have low densities, which occupy large volumes and are disadvantageous for use in limited space (e.g., in household or on offshore platforms). In order to miniaturize the CDI device, it is quite urgent to develop high volumetric adsorption capacity (VAC) electrode materials. To overcome this issue, we rationally designed and originally developed high VAC MoS2-graphene hybrid electrodes for CDI. It is interesting that MoS2-graphene hybrid electrode has a much higher NaCl VAC of 14.3 mg/cm3 with a gravimetric adsorption capacity of 19.4 mg/g. It has been demonstrated that the adsorption capacity is significantly enhanced because of the rapid ion transport of MoS2 and high electrical conductivity of graphene. In situ Raman spectra and high-angle annular dark-field scanning transmission electron microscopy tests demonstrated a favorable Faradaic reaction, which was crucial to enhancing the NaCl VAC of the MoS2-graphene hybrid electrode. This work opens a new avenue for miniaturizing future CDI devices.

In Situ Characterization of the Lithium Metal Interface

Lithium ion batteries have become the dominant form of energy storage used in consumer electronics and, recently, electric vehicles. However, high costs have prevented widespread deployment of lithium ion batteries for applications other than portable electronics, and the safety issues associated with liquid organic electrolytes remain to be addressed. In order to enable the greater utilization of electric vehicles, allow for grid scale energy storage, and meet the demands of new electronic applications, new materials for high energy density batteries must be developed. High capacity electrode materials like lithium metal have the potential to facilitate these technologies, but lithium metal electrodes are presently limited by significant side reactions, poor quality deposition, and the potential to form hazardous dendrites. Therefore, it is important to develop a clear understanding of the surface reactivity and growth behavior of the lithium metal at the interface with the electrolyte in order to enable stable long-term cycling. The products that form as a result of electrolyte decomposition reactions at the electrode interface are known to be extremely important in determining the final cell performance. Specifically, fluorinated salts and solvent additives have been shown to enable stable cycling of Li metal anodes. This improvement is ascribed to the formation of LiF in the solid electrolyte interphase (SEI), yet the understanding of how LiF and other SEI compounds are formed and how they affect battery cycling is not complete. In this presentation, in situ spectroelectrochemical techniques including infrared spectroscopy (FTIR), differential electrochemical mass spectrometry (DEMS), and electrochemical quartz crystal microbalance (EQCM), are used to clearly identify components of the Li metal SEI. An understanding of SEI formation with respect to electrochemical potential and time will be discussed. With this understanding we provide new insights into the formation and chemical nature of SEI components that promote stable cycling of lithium metal electrodes.

Development of Novel Binders for Silicon Anodes for Lithium Ion Batteries

High capacity electrode material has been studied over the years due to the constant demand for high energy density applications in li ion batteries. Among others, silicon has been considered a more promising candidate for its high theoretical capacity of ~ 3600 mAh/g, abundant resources, low cost and low toxicity. However, silicon-based electrodes face rapid degradation due to the extensive volume variation (~300%) in the charge discharge process. Binders used in the electrode fabrication plays a crucial role in these high-performance electrodes since it can reduce the mechanical fracture in the cycling process. Utilization of polymeric material as binders to hold the active material has been the most common approach used in the li ion battery electrode preparation. Recent studies have shown the use of small molecular carboxylic acids as binders which showed an improvement in the cell performance for silicon electrodes. We will review various non-polymeric binder for silicon electrodes. Many of these alternative binders afford electrode preparation in water which reduces the introduction of toxic organic solvents such as NMP to the environment. The novel non-polymeric binders have better performance compared to PVDF. Surface morphology and Solid electrolyte interface was analyzed using electron microscopy techniques and spectroscopic methods and the results will be discussed.

(Invited) Prussian Blue Analogues Materials for Energy Storage Applications

The open framework structure of Prussian Blue analogues (PBAs) is fundamentally different from other insertion electrode materials because of its large channels and interstices1. This structure is composed of a face-centered cubic framework of transition metal cations where each cation is octahedrally coordinated to hexacyanometallate groups. Large interstitial “A Sites” within the structure can accommodate zeolitic water and hydrated alkali ions (see Figure). This results in a general chemical formula of AxPR(CN)6nH2O, where A is an alkali cation such as K+or Na+, P is a transition metal cation and R(CN)6is a hexacyanometallate anion. Both the P-site transition metal cation and the R(CN)6 3-hexacyanometallate anion can be electrochemically active in this structure. The Prussian Blue framework structure has wide channels between the A sites, allowing rapid insertion and removal of not only alkali ions but also divalent and trivalent ions from aqueous solutions2. In addition, there is little lattice strain during cycling because the A sites are larger than the hydrated ions that are inserted and removed from them. The result is an extremely stable and rapid electrode: thousands of deep discharge cycles and high capacity retention at high C-rates were demonstrated in aqueous electrolytes3,4 , 5. When operated in organic, Na-ion electrolytes they show high specific capacities, long cycle life and good rate capability6. In my talk, I will show how these exciting properties are linked to the peculiar crystal structure of PBAs and how their chemistry can be easily tuned to adapt their electrochemistry to a variety of energy storage applications. References Hurlbutt, K., Wheeler, S., Capone, I. & Pasta, M. Prussian Blue Analogs as Battery Materials. Joule 2, 1950–1960 (2018).Wang, R.Y. et al.Reversible Multivalent (Monovalent, Divalent, Trivalent) Ion Insertion in Open Framework Materials. Adv. Energy Mater.5, 1–10 (2015).Pasta, M. et al.Manganese–cobalt hexacyanoferrate cathodes for sodium-ion batteries. J. Mater. Chem. A4, 4211–4223 (2016).Pasta, M. et al.Full open-framework batteries for stationary energy storage. Nat. Commun.5, 3007 (2014).Wheeler, S., Capone, I., Day, S., Tang, C. & Pasta, M. Low-Potential Prussian Blue Analogues for Sodium-Ion Batteries: Manganese Hexacyanochromate. Chem. Mater., article ASAP (2019).Lee, H.W. et al.Manganese hexacyanomanganate open framework as a high-capacity positive electrode material for sodium-ion batteries. Nat. Commun.5, 5280 (2014). Figure 1

(Invited) Development of High-Performance Lithium-Ion Batteries in the 21st Century

Development of High-Performance Lithium-ion Batteries in the 21st Century Yangxing Li, Ph.D., Chief ScientistWatt Lab, Central Research Institute, Huawei Technologies Co.,Ltd.li.yangxing@huawei.com High-performance lithium batteries are desirable in the modern society, especially on enhanced safety, improved energy density, increased rate capability and high temperature performance. In order to achieve high energy density for battery, significant efforts have been taken to develop high-capacity electrode materials, high-cut-off voltage cathode, and increased loading density. Some specific technical approaches to achieve high-performance lithium-ion batteries will be discussed here.

Understanding Chemo-Mechanical Degradation in High-Capacity Electrode Materials for Beyond-Lithium-Ion Batteries

In recent years there has been growing interest in the development of large-volume-change electrode materials for use in lithium-, sodium- and potassium-ion based systems to satisfy the demand for batteries with low cost and/or high energy density. Conversion and alloying battery materials are attractive options for such systems due to their high specific capacity. The conventional thought is that the larger volume changes due to reaction with larger ions (e.g., Na+ and K+) cause significant mechanical degradation. However, the mechanisms by which these ions react with high-capacity materials are largely unknown. In this work, in situ transmission electron microscopy (TEM) experiments are used to examine the nanoscale transformation mechanisms of two different high-capacity nanocrystal materials during reaction with various alkali ions. In both cases, we find that unexpected changes in morphology during reaction arise due to significant mechanical stress evolution within the nanocrystals. Overall, these results are important because they provide guidance for engineering high-capacity electrode materials with improved durability and cyclability. In the first set of experiments, cubic FeS2 nanocrystals were examined with in situ TEM as they underwent reaction with Li+, Na+, and K+. In all cases, the FeS2 nanocrystals exhibited a conversion-type reaction, but mechanical fracture during reaction was only observed during lithiation (which caused the smallest volumetric changes). This non-intuitive behavior was found to be due to differently-shaped reaction fronts that formed during reaction with Li+ compared to Na+ and K+. Specifically, lithiation induced a reaction front with sharp corners, while sodiation and potassiation resulted in fronts with blunted corners. Chemomechanical modeling of the reaction-induced expansion showed that the sharp corners during lithiation led to higher tensile stress concentrations and thus particle fracture. These results indicate that previously-dismissed high-capacity materials may in fact exhibit good chemo-mechanical stability during reaction and cycling with Na+ and K+. Next, other in situ TEM experiments were performed to investigate reaction mechanisms of Sb nanocrystals. These experiments showed that Sb nanocrystals naturally evolve to form hollow structures during ion removal. This finding indicates that electrochemically optimized nanoscale structures can evolve naturally in certain alloying anode materials, which may obviate the need for complex and expensive synthesis of hollow nanostructures for maximizing cyclability and Coulombic efficiency in battery anodes. Figure 1

Self-coupled nickel sulfide @ nickel vanadium sulfide nanostructure as a novel high capacity electrode material for supercapattery

Recently, self-coupled, mixed phase metal sulfide-based composites have attracted widespread attention as an electrode material for supercapacitors owing to their superior conductivity, high electrochemical activity and excellent redox capacity. Herein, an incorporated 2-dimensional (2D) polycrystalline phase of NiS2@NiV2S4 (NS@NVS) nanostructure was synthesized via facile hydrothermal technique. The synthesized nanocomposite showed mixed crystalline phases of NiS2 (cubic) and NiV2S4 (monoclinic) with 2D nanoflake morphology. The electrochemical behavior of NS@NVS electrode was investigated by three-electrode cell, which showed a maximum capacity of 520C g−1 at 1 A g−1 in 6 M KOH electrolyte. In addition, a supercapattery was designed with NS@NVS as a positive electrode and N-O enrich activated carbon as the negative electrode materials. The resultant supercapattery operated within a cell voltage of 1.6 V with good specific capacity (Csp) value of 78C g−1 at 0.5 A g−1 and exhibited high specific energy of 19.4 Wh kg−1 at a specific power of 140 W kg−1. Moreover, the hybrid device demonstrated a remarkable cycling stability of 90% for 10,000 charge-discharge cycles at a current density of 6 A g−1. Utilizing the high energy storage performance, the fabricated supercapattery was employed as a power source to drive various portable electronic appliances.

Ultra-high Areal Capacity Realized in Three-Dimensional Holey Graphene/SnO2 Composite Anodes

SummaryNanostructured alloy-type electrode materials and its composites have shown extraordinary promise for lithium-ion batteries (LIBs) with exceptional gravimetric capacity. However, studies to date are usually limited to laboratory cells with too low mass loading (and thus too low areal capacity) to exert significant practical impact. Herein, by impregnating micrometer-sized SnO2/graphene composites into 3D holey graphene frameworks (HGF), we show that a well-designed 3D-HGF/SnO2 composite anode with a high mass loading of 12 mg cm−2 can deliver an ultra-high areal capacity up to 14.5 mAh cm−2 under current density of 0.2 mA cm−2 and stable areal capacity of 9.5 mAh cm−2 under current density of 2.4 mA cm−2, considerably outperforming those in the state-of-art research devices or commercial devices. This robust realization of high areal capacity defines a critical step to capturing the full potential of high-capacity alloy-type electrode materials in practical LIBs.

On the Lithiation Mechanism of Amorphous Silicon Electrodes in Li-Ion Batteries

Amorphous silicon is a high-capacity negative electrode material for use in advanced lithium-ion batteries. We investigated the mechanism of Li incorporation into and removal from this material dur...

Conjugated Carbonyl Polymer-Based Flexible Cathode for Superior Lithium-Organic Batteries.

Conjugated carbonyl compounds are deemed as high theoretical capacity and green electrode materials for lithium-ion batteries (LIBs) but are limited by their high dissolution and poor electronic conductivity. In this paper, we have successfully synthesized a series of multicarbonyl conjugated polymers using the coupling polymerization reaction and then constructed carbonyl-conjugated polymer/carbon nanotube hybrid films by a vacuum-filtration method. Importantly, the hybrid films could serve as a flexible, binder-free, and free-standing organic cathode for LIBs, which could deliver a high reversible discharge capacity of 142.3 mAh g-1 at 50 mA g-1, good cycling stability with a capacity retention of 74.6% at 500 mA g-1 after 300 cycles, and excellent rate capability of 120.6 mAh g-1 at 1000 mA g-1. In addition, the theoretical calculation has proved that the symmetrical conjugated structure can well accommodate four Li+ ions during the electrochemical reaction. Interestingly, the assembled full cell and flexible battery can power the small devices, suggesting its potential to use in bendable or wearable energy-storages devices.

Electrochemomechanical coupled behaviors of deformation and failure in electrode materials for lithium-ion batteries

The growing demands of lithium-ion batteries with high energy density motivate the development of high-capacity electrode materials. The critical issue in the commercial application of these electrodes is electrochemomechanical degradation accompanied with the large volume change, built-in stress, and fracture during lithiation and delithiation. The strong and complex couplings between mechanics and electrochemistry have been extensively studied in recent years. The multi-directional couplings, e.g., (de)lithiation-induced effects and stress-regulated effects, require cooperation in the interdisciplinary fields and advance the theoretical and computational models. In this review, we focus on the recent work with topics in the electrochemomechanical couplings of deformation and fracture of conventional and alloying electrodes through experimental characterization, theoretical and computational models. Based on the point of view from mechanics, the strategies for alleviating the degradation are also discussed, with particular perspectives for component-interaction patterns in the composite electrodes. With interdisciplinary principles, comprehensive understanding of the electrochemomechanical coupled mechanism is expected to provide feasible solutions for low-cost, high-capacity, high-safety and durable electrodes for lithium-ion batteries.

A silicon anode for garnet-based all-solid-state batteries: Interfaces and nanomechanics

High-capacity electrode materials are indispensable for developing high energy density solid-state batteries. The lithium metal anode is attractive because of its high capacity and low electrochemical reduction potential, but its application is hampered by the dendrite issue. The silicon anode is a promising material having high capacity and invulnerability to undergoing dendrite formation, but is limited to the nanometer regime for the thickness of a Si anode. Herein, for the first time, we demonstrate a 1 μm thick solid-state silicon anode (10 times the typical thickness of Si anodes used in organic electrolyte) as an alternative to Li metal anode for solid-state batteries. This Si anode forms good contact with the garnet-type solid-state electrolyte and maintains structural integrity during the Li ion intercalation and extraction. The Si anode with the garnet electrolyte exhibits a high discharge capacity of 2685 mA h g−1 and an excellent initial Coulombic efficiency of 83.2%, higher than that of the Si anodes with an organic electrolyte (77.1%). Our mechanics modeling reveals that the strong nanomechanical constraints by the solid garnet electrolyte enables the substantial increase in the critical thickness of the Si anode from just nanometers to micrometers, toward high-capacity solid-state batteries.

Preparation and electrochemical properties of CoS2/carbon nanofiber composites

Carbon nanocomposites are widely used in hydrogen storage materials and high-capacity electrode materials due to their unique structure and excellent performance. Inspired by Sandwich biscuit, we report a green and convenient method for preparing CoS2-carbon nanofiber composites (CoS2-CNFs). The CoS2-CNFs are prepared by electrospinning technology and hydrothermal process. The structural and morphological properties of CoS2-CNFs were characterized through X-ray diffraction, scanning electron microscopy, transmission electron microscope, Raman spectra, and X-ray photoelectron spectroscopy. Meanwhile, the CoS2-CNFs reaches 378 F g−1, over 75.6% of initial capacitance is retained as the current density improves from 1 to 10 A g−1 and also exhibits an excellent cycling performance with 89.5% capacitance retention after 1000 times charge/discharge cycles. This result suggests that the CoS2-CNFs could yield them potentially promising candidates for future energy storage systems.

Anisotropic Compositional Expansion and Chemical Potential of Lithiated SiO2 Electrodes: Multiscale Mechanical Analysis.

The use of high-capacity electrode materials (i.e., Si) in Li-ion batteries is hindered by their mechanical degradation. Thus, oxides (i.e., SiO2) are commonly used to obtain high expected capacities and long-term cycle performances. Despite extensive studies of the electrochemical-mechanical behaviors of high-capacity energy storage materials, the mechanical behaviors of amorphous SiO2 during electrochemical reaction remain largely unknown. Here, we systematically investigate the stress evolution, electronic structure, and mechanical deformation of lithiated SiO2 through first-principles computation and the finite element method. The structural and thermodynamic role of O in the amorphous Li-O-Si system is reported and compared with that in Si. Strong Si-O bonds in SiO2 show high mechanical strength and brittle behavior, but as Li is inserted, the Li-rich SiO2 phases become mechanically softened. The relaxation kinetics of SiO2, inducing deviatoric inelastic strains under mechanical constraints, is also compared with that of Si. The finite element model including the kinetic model for anisotropic expansion demonstrates that the long-term cycling stability of core-shell Si-SiO2 nanoparticles mainly arises from the reaction kinetics and high mechanical strength of SiO2. These results provide fundamental insights into the chemomechanical behavior of SiO2 for practical use.

A New Formulation of the Pseudo2D Battery Model Coupling Macroscopic and Microscopic Deformations

High-capacity electrode materials such as silicon are promising in improving the capacity and energy densities of lithium ion battery (LIB). However, the broad application of such material is hindered by their large volume change during lithiation/delithiation, which not only causes active particle pulverization, but also leads to macroscale deformation that changes the dimension and porosity distribution of the cell. The objective of this work is to consistently reformulate the Pseudo2D battery model to incorporate the effect of large deformation during battery cycling, at both the particle and electrode levels. In this work, the finite strain theory is used to approximate deformations in electrode by coupling it with the distribution of intercalation fraction within the cell. The porosity variation is approximated assuming conservation of the volume of the solid phase, which is then combined with the electrode deformation to consistently yield the magnitude of particle deformation at each position. The specific particle surface area is also consistently updated, which is ignored in most existing models. All transport equations are also reformulated in the reference frame but with the effect of large deformation incorporated. Preliminary simulation results for a LIB with Si anode and NMC532 cathode show that porosities in both electrodes are notably reduced and become nonuniform when charged at a relatively high C rate. The increase of transport resistances and the gradual build-up of compressive stress due to active material volume change contribute to additional overpotentials that adversely affect the electrochemical performances of the cell, such as its accessible capacity/energy. In the future, this model will be used for cell design optimization in terms of electrode loadings, compromise between cell stress and strain, optimal range of Si SOC and possibly the optimal composition of a composite electrode, etc.