A combination of experimental measurements and numerical simulations are used to characterize the mechanical and electrochemical response of thin film amorphous Si electrodes during cyclic ...lithiation. Parameters extracted from the experiment include the variation of elastic modulus and the flow stress as functions of Li concentration; the strain rate sensitivity; the diffusion coefficient for Li transport in the electrode; the free energy of mixing as a function of Li concentration in the electrode; the exchange current density for the Lithium insertion reaction; as well as reaction rates and diffusion coefficients characterizing the rate of formation of solid-electrolyte interphase layer at the electrode surface. Model predictions are compared with experimental measurements; and the implications for practical Si based electrodes are discussed.
Charge lost per unit surface area of a silicon electrode due to the formation of solid-electrolyte-interphase (SEI) layer during initial lithiation was quantified, and the species that constitute ...this layer were identified. Coin cells made with Si thin-film electrodes were subjected to a combination of galvanostatic and potentiostatic lithiation and delithiation cycles to accurately measure the capacity lost to SEI layer formation. While the planar geometry of amorphous thin films allows accurate calculation of surface area, creation of additional surface by cracking was prevented by minimizing the thickness of the Si film. The cycled electrodes were analyzed with X-ray photoelectron spectroscopy to characterize the composition of the SEI layer. The charge lost due to SEI formation measured from coin cell experiments was found to be in good agreement with the first-cycle capacity loss during the initial lithiation of a Si(100) crystal with planar geometry. The methodology presented in this work is expected to provide a useful practical tool for battery-material developers in estimating the expected capacity loss due to first cycle SEI layer formation and in choosing an appropriate particle size distribution that balances mechanical integrity and the first cycle capacity loss in large volume expansion electrodes for lithium-ion batteries.
► A method to quantify capacity loss per unit area due to initial SEI formation on Si. ► Identify species that constitute the SEI layer. ► Reported data can be used to predict first-cycle capacity loss on any geometry.
•Thin layer of SAC305 solder was sandwiched between Cu bars to make DCB specimens.•DCBs were fractured under various temperatures to measure mode-I Jc.•The critical energy release rate of solder ...joints decreased with temperature.•The crack path shifts to SAC305-IMC interface at low temperature.
Double cantilever beam (DCB) specimens consisting of Sn96.5Ag3.0Cu0.5 (SAC305) solder sandwiched between two copper bars were fabricated under standard surface mount processing conditions. Mode-I fracture tests were performed on the DCB specimens under various temperatures. The load–displacement behavior and critical loads corresponding to crack initiation were recorded and used in a finite element model of the DCB to evaluate critical energy release rates. Fracture surfaces were analyzed to understand the effect of temperature on crack path and fracture mechanism. The critical energy release rate decreased from 603 at −50 °C to 93 J/m2 at 100 °C and the crack path shifted from solder/intermetallic compound interface to bulk solder.
Durability of hard carbon (HC) electrodes is becoming important owing to their emergence as potential sodium-ion battery anodes. The volume expansion/contraction during electrochemical cycling of HC ...against sodium may result in significant mechanical degradation and capacity fading. From this perspective, the real-time stress evolution in composite HC electrodes was measured as a function of capacity for several sodiation/de-sodiation cycles using substrate-curvature method. The effect of polymer binder on the stress evolution was investigated with two different binder systems: 1) Carboxymethyl cellulose/Styrene-butadiene rubber (CMC/SBR) and 2) polyvinylidene fluoride (PVdF). The HC electrodes exhibited similar stress evolution pattern upon electrochemical cycling irrespective of the binder. Compressive stress was generated during sodiation which increased to a peak value of approximately – 6.8 MPa in case of HC-CMC/SBR and – 5.9 MPa in case of HC-PVdF at the end of sodiation. Upon desodiation there was elastic unloading and the stress became tensile with cycling reaching a peak value of ∼3.3 MPa and ∼1.5 MPa in case of HC-CMC/SBR and HC-PVdF, respectively, at the end of desodiation. Insights were provided about the effect of stress hysteresis on the cell performance. Some preliminary observations on sodiation mechanism were also made based on the stress measurements.
•In-situ stress evolution in hard carbon was measured during sodiation/desodiation.•Compressive stress in hard carbon composite anodes was observed during sodiation.•Effect of polymeric binders on stress response of HC was investigated.•Stress-capacity showed hysteresis suggesting inelastic deformation of electrode.
The strain and temperature responses of commercial high capacity (3.4 Ah) 18650 lithium-ion cells were investigated under normal operation and overcharge conditions. Strain measurements were carried ...out on the casing of the 18650 cells using surface-mounted strain gauges while cycling the cells in a temperature-controlled environment. During a normal C/10 charge/discharge cycle, the hoop strain at the midsection of the cylindrical cell reached a maximum value at 100 % state-of-charge (4.2 V). Strain returned to a baseline value upon discharge. Differential capacity (dQ/dV) data showed several characteristic peaks corresponding to a full cell with a Ni-rich layered oxide cathode and graphite-Si anode. A differential strain analysis (dε/dV) showed peaks in strain response that coincided with the dQ/dV peaks. In all the cycling scenarios, the peaks associated with the electrochemical reactions in the cell coincided with change in strain response. Repeated measurements across multiple cycles and multiple cylindrical cells have produced similar strain behavior. Finally, an overcharge test was conducted at a C/5 rate to measure strain up to CID activation. During the overcharge, strain increases significantly, i.e., more than 2.5 times the peak strain of a normal C/5 charge process. The results from this study indicate that strain measurements on a cell's surface are directly correlated to the reversible electrochemical behavior of a cell during normal cycling, and therefore can be used as a diagnostics tool for battery operation to detect irreversible phenomenon.
Composite cathode coatings made of a high energy density layered oxide (Li1.2Ni0.15Mn0.55Co0.1O2, theoretical capacity ∼377 mAh-g−1), polyvinylidene fluoride (PVdF) binder, and electron-conduction ...additives, were bonded to an elastic substrate. An electrochemical cell, built by pairing the cathode with a capacity-matched graphite anode, was electrochemically cycled and the real-time average stress evolution in the cathode coating was measured using a substrate-curvature technique. Features in the stress evolution profile showed correlations with phase changes in the oxide, thus yielding data complementary to in situ XRD studies on this material. The stress evolution showed a complex variation with lithium concentration suggesting that the volume changes associated with phase transformations in the oxide are not monotonically varying functions of lithium concentration. The peak tensile stress in the cathode during oxide delithiation was approximately 1.5 MPa and the peak compressive stress during oxide lithiation was about 6 MPa. Stress evolution in the anode coating was also measured separately using the same technique. The measured stresses are used to estimate the internal pressures that develop in a cylindrical lithium-ion cell with jelly-roll electrodes.
In situ electrochemical cells were assembled with an amorphous germanium (a-Ge) film as working electrode and sodium foil as reference and counter electrode. The stresses generated in a-Ge electrodes ...due to electrochemical reaction with sodium were measured in real-time during the galvanostatic cycling. A specially designed patterned a-Ge electrode was cycled against sodium and the corresponding volume changes were measured using an AFM; it was observed that sodiation/desodiation of a-Ge results in more than 300% volume change, consistent with literature. The potential and stress response showed that the a-Ge film undergoes irreversible changes during the first sodiation process, but the subsequent desodiation/sodiation cycles are reversible. The stress response of the film reached steady-state after the initial sodiation and is qualitatively similar to the response of Ge during lithiation, i.e., initial linear elastic response followed by extensive plastic deformation of the film to accommodate large volume changes. However, despite being bigger ion, sodiation of Ge generated lower stress levels compared to lithiation. Consequently, the mechanical dissipation losses associated with plastic deformation are lower during sodiation process than it is for lithiation.
The majority of existing battery models that simulate composite electrode behavior assume the binder as a linear elastic material due to lack of a thorough understanding of time-dependent mechanical ...behavior of binders. Here, thin films of polyvinylidene fluoride binder, prepared according to commercial battery manufacturing method, are subjected to standard monotonic, load-unload, and relaxation tests to characterize the time-dependent mechanical behavior. The strain in the binder samples is measured with the digital image correlation technique to eliminate experimental errors. The experimental data showed that for (charging/discharging) time scales of practical importance, polyvinylidene fluoride behaves more like an elastic-viscoplastic material as opposed to a visco-elastic material; based on this observation, a simple elastic-viscoplastic model, calibrated against the data is adopted to represent the deformation behavior of binder in a Si-based composite electrode; the lithiation/delithiation process of this composite was simulated at different C rates and the stress/strain behavior was monitored. It is observed that the linear elastic assumption of the binder leads to inaccurate results and the time-dependent constitutive behavior of the binder not only leads to accurate prediction of the mechanics but is an essential step towards developing advanced multi-physics models for simulating the degradation behavior of batteries.
•Time-dependent mechanical behavior of PVdF was thoroughly characterized.•PVdF behaves more like an elastic-viscoplastic material for time-scales of interest.•Lithiation/delithiation of Si-PVdF composite was simulated with Abaqus FE package.•Linear elastic assumption of binder leads to inaccurate results.•Elastic-viscoplastic model predicts mechanics of composite electrode accurately.
An in situ study of deformation, fracture, and fatigue behavior of silicon as a lithium-ion battery electrode material is presented. Thin films (100-200 nm) of silicon are cycled in a half-cell ...configuration with lithium metal foil as counter/reference electrode, with 1M lithium hexafluorophosphate in ethylene carbonate, diethylene carbonate, dimethyl carbonate solution (1:1:1, wt%) as electrolyte. Stress evolution in the Si thin-film electrodes during electrochemical lithiation and delithiation is measured by monitoring the substrate curvature using the multi-beam optical sensing method. The stress measurements have been corrected for contributions from residual stress arising from sputter-deposition. An indirect method for estimating the potential errors due to formation of the solid-electrolyte-interphase layer and surface charge on the stress measurements was presented. The films undergo extensive inelastic deformation during lithiation and delithiation. The peak compressive stress during lithiation was 1.48 GPa. The stress data along with the electron microscopy observations are used to estimate an upper bound fracture resistance of lithiated Si, which is approximately 9-11 J/m2. Fracture initiation and crack density evolution as a function of cycle number is also reported.
Double cantilever beam (DCB) fracture specimens were made by joining copper bars with both continuous and discrete SAC305 solder layers of different lengths under standard surface mount (SMT) ...processing conditions. The specimens were then fractured under mode-I and various mixed-mode loading conditions. The loads corresponding to crack initiation in the continuous joints were used to calculate the critical strain energy release rate,
J
ci
, at the various mode ratios using elastic–plastic finite element analysis (FEA). It was found that the
J
ci
from the continuous joint DCBs provided a lower bound strength prediction for discrete 2
mm and 5
mm long joints at the various mode ratios. Additionally, these
J
ci
values calculated from FEA using the measured fracture loads agreed reasonably with
J
ci
estimated from measured crack opening displacements at crack initiation in both the continuous and discrete joints. Therefore, the critical strain energy release rate as a function of the mode ratio of loading is a promising fracture criterion that can be used to predict the strength of solder joints of arbitrary geometry subject to combined tensile and shear loads.
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