Introduction
What are Architected Materials?
Nature’s Hierarchical Designs
Remarkable Properties of Architected Materials
What is Electrochemistry?
Tailor-Making Functional Architected Materials by Design
Design of Functional Architected Materials
Despite the fruitful characterization of new properties in architecture materials, practical demonstrations of functional architected materials are still scarce. Only after relevant materials and efficient fabrication techniques have been carefully planned can the architectural design of functional architectural materials be designed, tested and optimized.
Additive Manufacturing Methods for Architected Materials
The area of the Si thin film on the substrate that came into contact with the ionic liquid also participated in the lithiation reaction. Point Γ to point Χ corresponds to waves propagating in the x or y direction (due to symmetry) of the microlattices.
Post-processing Methods to Define Functionalities
Architected Material Design for Electrochemical Systems
Focused ion beam is used to cut cross-sections of the horizontal beams in the SEM (Fig. 4.11g, h). Cracks are found in the Ni-polymer core of the beams, but not in the Si layer (Fig. 4.11h).
3D-Architected Electrodes for Li-ion Batteries
Redesigning the Battery Architecture
After decades of intensive research and development, the energy density of state-of-the-art Li-ion batteries has gradually improved to approximately 200 Wh/kg, less than half of theoretical values [98]. At the material level, state-of-the-art electrode materials for both anode and cathode have the intercalation-based Li storage mechanism.
Mechanical Challenges due to Volume Expansion in Si-based Anodes
The development of three-dimensional, nano-architected electrodes is a promising approach to extend the mechanical robustness of nanoscale Si to device-scale electrodes. The periodic pore space allowed such nano-architected electrodes to accommodate a large increase in Si volume, but the active material loading was low and the electrode structure was limited to the inverse opal geometry.
Fabrication of Cu-Si Core-shell Nanolattices as Battery Electrodes
The fabrication process along with SEM and TEM characterization of the Cu–Si core–shell nanolattices are summarized in Fig. Inset: a selected area electron diffraction pattern with concentric rings characteristic of amorphous microstructure of the a-Si shell. f) TEM image of a void at the Cu-Si interface.
In situ Observation of Electrode Deformation in a Scanning Electron Microscope
The observed volumetric changes imply that not all of the inserted Li was extracted from the Li–Si alloy. Via the combined movement of the sample stage and the nanomechanical arm, the suspended ionic liquid droplet was fine-tuned to immerse the Cu-Si nanogrid structure with minimal contact between the substrate and the ionic liquid droplet to reduce the influence of the Si thin film surrounding the nanogrid on measured electrochemical behavior.
Finite Element Modeling of Lithiation-induced Volume Expansion
Distribution profiles of (c) the interfacial normal stress and (d) the interfacial shear stress at the Cu-Si interface at three different times. The fracture energy of the Cu-Si interface was measured by Maranchi et al. as 𝛤 = 7.9𝐽/𝑚2.
Outlook and Summary
Red-colored rays on the left side of the extended unit cell shown in the inset of Fig. A.2b below, where we compare 𝜎𝑧𝑧 and 𝜎1 for three points in the middle of the ray span. The initial undeformed coordinate of the beam is chosen as a parametric variable in this work.
Electrochemically Reconfigurable Architected Materials
Reconfiguration of Architected Materials After Fabrication
Most architectural materials reported so far are passive in the sense that they have a prescribed geometry that fulfills a single functionality. It has recently been shown that the structure of architected materials can be reconfigured by mechanical deformation [36] and instabilities, hydration-induced swelling as well as magnetic actuation [69] . Understanding and controlling defects provides a path to drive the dynamic response of architected materials along a defined trajectory.
Design of Electrochemically Reconfigurable Si Microlattices
The theoretical capacity of the Si microlattice samples is estimated to be 30μAh by calculating the C rate. The lower parts of the vertical posts appear to be loosely connected to the substrate, especially in the outer layers of Ni and Si (marked by red arrows in Fig. 4.11i). In this section we would like to discuss the physical origin of the energy fluctuation 𝑄𝐸𝐶.
The secured ends on either side will create an axial compressive force in the beam, which can ultimately lead to buckling. Interfacial properties of the a-Si∕Cu: active-inactive thin-film anode system for lithium-ion.
Fabrication of Si-coated Tetragonal Microlattices
Comparison with Lattice Geometries with Higher Symmetry
Horizontal beams with circular cross-sections and similar diameters would still undergo cooperative buckling in an almost identical manner, because at the structural level the in-plane buckling deformation is caused by the constraints imposed by the vertical uprights that are free to rotate but cannot rotate. translational movement laterally. We learned that these higher symmetry lattices were more sensitive to manufacturing errors, for example sewing inaccuracies during manufacturing, as evidenced by the periodic distortions in zoomed-out SEM images in Figure. Narrower vertical posts would actually snap into the bottom layer when driven by lithiation. due to the greater degree of rotation.
Sn Microlattice Fabrication and Comparison
Electrochemical Testing Method
A Li foil is punched into a ring shape to unblock the top-down view of the Si microgrid during in situ observation. During electrochemical lithiation/delithiation, Keyence VW-9000 digital microscope records the dynamics of cooperative buckling/unbending in the Si microlattices. The first cycle Coulombic efficiency is ∼70% with the 0.6V delithiation cutoff voltage, indicating that about 30% of inserted Li remains in the Si microlattices.
In situ Observation of Lithiation-induced Cooperative Buckling
The Si-Li bonding reaction is a spontaneous discharge process, which means that the alloy has a lower free energy than that of the two electrodes combined. Supplementary Video 6 shows lithiation and thermodynamic bending of a pull-current Si microlattice from the junction reaction for Joule heating of the 2000Ω resistor. Supplementary Video 7 shows stable and reversible structural transformations of 3rd charge, 4th discharge, 4th charge and 5th discharge with a high degree of lithiation/delithiation of the same sample as in Video Supplementary 6 with a 150X playback speed.
Electrochemical Characterization and Cycling of Si Microlattices
A cyclic voltammogram of a representative Si microgrid with a Li counter electrode at a scan rate of 0.1 mV/s between 0.01 V and 1.5 V. The red arrow shows the current fluctuation starting around 0.55V in the 3rd delithiation. f) Voltage profiles of the 1st and the 10th cycles with 1.5 V and 0.6 V delithiation cut-off voltages in modified coin cells. The Si microgrid has a relatively stable capacity above 2000mAh/g-Si in the first 50 cycles, and then the capacity starts to decrease slowly to 1030mAh/g-Si in the 100th lithiation. In all galvanostatic cycling tests, the Coulombic efficiency stabilized around 95%, possibly due to the relatively significant side reactions in the modified coin cells.
Coupled Chemo-Mechanical Finite Element Analysis of Individual Beams
Colored contours represent the Li concentration normalized by the maximum possible molar concentration of Li in Li-Si alloys based on the theoretical specific capacitance. Furthermore, the tendency for buckling instability is determined by the slenderness ratios of the beams and contributes to the dominant deformation mechanism at different stages of lithiation. This is visualized by the phase map of beam distortion mechanisms at different slenderness ratios and SOCs in Fig.
Role of Defects in Cooperative Buckling and Domain Formation
-f) SEM images of typical defects in microlattices: (c) seam distortion at a node, (d) pre-existing beam bending caused by residual stresses and (e, f) periodically arranged artificial defects of 5 μm long, 100 nm-thick spot added to one side of the polymer core of the horizontal beams, with Ni and Si layers following the morphology. h) SEM image of a single domain sinusoidal network formed as a result of defect engineering. i) SEM image of a Caltech icon outlined by domain boundaries that appeared after lithiation by pre-engineering artificial defects. Each artificial defect was a 5 µm long, 100 nm thick chip added to one side of the polymer beam during two-photon lithography (Fig. 4.13f). The subsequently deposited Ni and Si layers follow the surface morphology of the polymer beams.
Lithiation Rate Dependence of Domain Size Distribution
Two samples are shown for C/6 to show nominally identical Si microlattices at the same lithiation conditions produce different domain patterns. To quantify the average domain size, we calculated the correlation function for each domain map. 4.16g, h are correlation functions at different lithiation rates with two samples per rate at different zooms, demonstrating that despite the significant difference in the shapes of domains across the two samples at the same lithiation rate, the statistical correlation functions are comparable. 4.16, the domains are larger at the same lithiation rates at 37°C than those at room temperature.
Statistical Mechanics Analysis of Domain Formation Dynamics
Inset: Simulated variations in correlation length 𝜉 with node coupling rate 𝑅 in Monte Carlo simulations for four different electrochemical energy fluctuations 𝑄𝐸𝐶. m) Fraction of domain boundaries, represented by the ratio of Mode-II buckled beams out of all beams, as a function of the nodal rotation coupling 𝐽 for five different coupling slopes 𝑅, indicating that a higher rate of increase leads to a higher stabilized domain boundary fraction indicative of smaller domains. The relationship between the coupling ramp rate 𝑅 and the correlation length 𝜉 is shown in the inset of figure. This shows that higher coupling slope rates in Monte Carlo simulations lead to smaller correlation lengths for each 𝑄𝐸𝐶, while higher 𝑄𝐸𝐶 shifts this relationship to larger correlation lengths. .
Implementation and Details of Monte Carlo Simulations
Increasing the temperature in the experiments or describing larger electrochemical energy fluctuations in the simulations would shift the velocity–correlation length relation toward larger correlation lengths. First, in Monte Carlo simulations the domains are able to reach smaller sizes with a shorter correlation length at high merger ramp rates. We have not observed this in experiments due to the additional bending distortion of the vertical pillars surrounding the domain boundaries, which cannot be accounted for in the Monte Carlo simulations.
Origin of Electrochemical Energy Fluctuations
For lithiation-induced cracking of a Si beam, the stochastic local electrochemical nucleation events occur on the two opposite surfaces of the Si beam and constantly change the local stress distribution. The mechanistic nature of the electrochemical energy fluctuation 𝑄𝐸𝐶 should be pursued more deeply and is beyond the scope of this paper. For the purpose of the discussion here, our only assumption is that there exists an energy fluctuation 𝑄𝐸𝐶 related to electrochemical reactions during lithiation-induced cooperative cracking.
Application and Outlook
Through the interactions between adjacent beams, cooperative buckling transforms a simple tetragonal lattice into a sinusoidal lattice with intriguing mechanical metamaterial properties, which is discussed here as an example of the possibilities of electrochemically reconfigurable architectural materials. With partial delithiation, the center frequencies of the band gaps shift by 6 MHz and 9 MHz, respectively. Furthermore, complex tailor-made reconfigurability can be designed for non-periodic architectures by controlling the relative ratio of the polymer scaffold and active material, as well as mechanical constraints. 4.20d-k illustrate example building blocks with rotation, bending, out-of-plane buckling, and extended degrees of freedom induced by electrochemical lithiation.
Comparison of Reconfiguration Mechanisms for Architected Materials
Electrochemically reconfigurable architectural materials have the unique advantage of being electrically controlled and thus directly compatible with miniaturized electronic circuits. The most distinctive advantage of electrochemically reconfigurable architectural materials is the stability and maintenance of their structural transformation after removal of external stimuli. In terms of implementation, the electrochemically reconfigurable architectural materials in this work have feature sizes on the order of 1 µm and contain a significantly larger number of repeating unit cells (see Table 4.1), giving rise to .
Summary
An imperfection is introduced by considering a pin which is displaced by an amount 𝑒/𝐿 from the center of the beam. We consider the critical buckling load to be equal to the state of deformation where the bending energy is 1% of the beam's total energy. SOC (normalized concentration) of the beam under elastic-plastic or buckling deformations. vi) Results of the reduced-order model.
Summary and Outlook
Summary
This thesis investigates the dynamic behavior of architectural materials undergoing electrochemical reactions and aims to provide an in-depth understanding of the underlying mechanisms as well as design principles that can be generalized to other functional systems of architectural materials. We begin by introducing a toolkit of state-of-the-art techniques for the production of rationally designed architectural materials of complex geometries and diverse chemical compositions. At a more fundamental level, the flexible and reconfigurable architectural materials studied in this thesis reveal an interesting analogy between architectural and classical materials.
Open Questions and Future Work
Therefore, we believe that the polymer core is attached to the Si/Ni layers and contributes to the stiffness of the entire beam. We now present the results of the reduced-order model, starting from the force-stress curves. Then, as a measure of deflection, we consider the point in the deformation of the beam, where the bending energy of the beam is 1% of the total elastic energy.
For as-fabricated Si microlattices, we used the same geometry as the experimental samples described above. In situ SEM observation of the Si-negative electrode reaction in an ionic liquid secondary lithium-ion battery.