Understanding Multiscale Lithium Transport Dynamics in Composite Si‐C Electrodes: An Asymptotic Framework for GITT Experiments

Solid phase diffusivity Ds is a key parameter in Lithium-Ion cell models because solid phase diffusion typically dominates the voltage transients. The Galvanostatic Intermittent Titration Technique (GITT) is easy to implement and universally accepted as the standard for diffusivity measurement, but the accuracy of GITT diffusivity measurement is unknown. This paper develops a Least Squares GITT (LS-GITT) that uses all of the voltage data from a GITT test to optimally tune the diffusivity in a reduced order solid phase diffusion model. The accuracies of the GITT and LS-GITT diffusivity measurements are evaluated using the RMS error between the model predicted and experimentally measured voltages. Based on experimental results from a NCM half cell, LS-GITT is more accurate than GITT, often by an order of magnitude. The GITT test overestimates Ds because the underlying model neglects the effects of bulk capacity on the voltage transients. LS-GITT gives results accurate to 1 mV RMS from 15%–100% SOC where GITT provides the same level of accuracy over less than half that SOC range. Neither technique provides accurate Ds measurements below 10% SOC.

Origin of anomalous high-rate Na-ion electrochemistry in layered bismuth telluride anodes

Electrochemical performance of Fe2(SO4)3 as a novel anode material for lithium-ion batteries

Lithium transport within cathode material is driven by diffusion. While fast cathode diffusion is desired, solid-state diffusivity is one of the most difficult cell material properties to measure. However, it is an important property to understand as cathode capacity is often limited by poor diffusivity. GITT (Galvanostatic Intermittent Titration Technique) is a common diffusivity measurement method. This technique is accomplished by applying a series of constant current pulses to a cell and analyzing the voltage response. While easy to use, this method only measures near the surface of the active material by assuming that it takes the geometry of a semi-infinite plane. This approximation means that GITT is only accurate when measuring the very initial voltage response. However, the voltage response cannot be analyzed too early as this may amplify error produced by CDL (double layer capacitance) and RI (interface resistance) found at the cathode-electrolyte interface which GITT does not account for. EIS (Electrochemical Impedance Spectroscopy), an AC method, is better suited for distinguishing CDL/RI from diffusivity but also struggles to evaluate diffusivity beyond the surface of active material within a reasonable time frame1. In addition, instruments capable of quality EIS are often cost prohibitive compared to DC cell testers.The voltage responses after the first few moments of a pulse are relevant when evaluating cell behaviour in real-world applications and should be considered to produce comprehensive measurements of diffusivity and its performance impact. AMID (Atlung Method for Intercalant Diffusion) is a multi-rate pulse method produced by this lab which uses a series of pulses, both short/fast and long/slow, and an approximation-free analytical model (developed by Sven Atlung) for diffusion in a spherical particle, not a semi-infinite plane2. However, AMID cannot differentiate diffusion impedance from resistance and therefore requires that resistance be considered negligible. In addition, while AMID mathematics are free of approximations, the typical multi-rate pulse protocol does require that series of pulses be approximated as singular pulses3. Lastly, the multi-rate pulse protocol limits the state-of-charge resolution of diffusivity measurements.AMIDR (Atlung Method for Intercalant Diffusion and Resistance) incorporates a GITT-style single pulse protocol with AMID-style mathematics modified to account for resistance as shown in Figure 1b). AMIDR collects pulse data with a pseudo-logarithmic time distribution. This means that the same number of data points is collected in 0.1-1.0 seconds, 1.0-10 seconds, 10-100 seconds and so on. Unlike GITT which measures diffusivity at an early arbitrary point in a pulse, AMIDR considers the entire length of a pulse comprehensively. Fractional capacity is calculated for each datapoint by dividing the actual capacity with the ideal “impedance-free” capacity expected at that voltage as shown in Fig 1c). These fractional capacity values, normalized from 0-1, are fitted to the modified Atlung model. This modified Atlung model is nearly the same as the original Atlung model except an additional term is added for capacity limited by resistance. Unlike AMID, AMIDR measures both diffusivity and resistance simultaneously by using both as fitting parameters. Lastly, while CDL is usually small enough for RI to behave simply like ohmic resistance on relatively short time scales (>0.1 seconds), AMIDR has the option to account for CDL/RI when relevant such as near the bottom of a complete discharge.AMIDR is a new diffusivity measurement method tailored for cathode material in Li-ion batteries. By combining the advantages of various previous diffusivity methods, AMIDR eliminates many of their concerns and sources of error such as sampling of only the material surface, alternate confounding sources of impedance, and expensive equipment as seen in Figure 1a). When paired with proper cell design (i.e. ultra-low single layer cathode loading to remove pore impedance and reference electrodes to remove anode impedance), AMIDR can produce high resolution diffusivity measurements with improved accuracy and applicability to real-world cell performance.References C. Deng and W. Lu, Journal of Power Sources, 473, 228613 (2020).A. Liu et al., J. Electrochem. Soc., 168, 070503 (2021).M. Doyle, J. Newman, and J. Reimers, Journal of Power Sources, 52, 211–216 (1994). Figure 1. (a) Comparison of various Diffusivity measurements. (b) The modified Atlung model for diffusion in a spherical particle. (c) How pulse data is converted to the unitless Atlung model. Figure 1

The solid-state diffusion coefficient of the electrode active material is one of the key parameters in lithium-ion battery modelling. Conventionally, this diffusion coefficient is estimated through the galvanostatic intermittent titration technique (GITT). In this work, the validity of GITT and a faster alternative technique, intermittent current interruption (ICI), are investigated regarding their effectiveness through a black-box testing approach. A Doyle-Fuller-Newman model with parameters for a LiNi0.8Mn0.1Co0.1O2 electrode is used as a fairly faithful representation as a real battery system, and the GITT and ICI experiments are simulated to extract the diffusion coefficient. With the parameters used in this work, the results show that both the GITT and ICI methods can identify the solid-state diffusion coefficient very well compared to the value used as input into the simulation model. The ICI method allows more frequent measurement but the experiment time is 85% less than what takes to perform a GITT test. Different fitting approaches and fitting length affected the estimation accuracy, however not significantly. Moreover, a thinner electrode, a higher C-rate and a greater electrolyte diffusion coefficient will lead to an estimation of a higher solid-state diffusion coefficient, generally closer to the target value.

Mechanism transition of cell-impedance-controlled lithium transport through Li 1− δMn 2O 4 composite electrode caused by surface-modification and temperature variation

Investigation of the diffusion phenomena in lithium-ion batteries with distribution of relaxation times

Lithium transport through lithium cobalt dioxide thin film electrode prepared by rf-magnetron sputtering was investigated in IM solution of LiClO4 in propylene carbonate using cyclic voltammetry, galvanostatic intermittent titration technique (GITT) and potentiostatic current transient technique. The cathodic and anodic current transients exhibited the non-Fickian behaviour of lithium transport when the applied potential steps encountered potential plateaus near 3.91 V Li/Li+' determined from GITT. It is suggested that the occurrence of this abnormal behaviour during lithium intercalation and deintercalation is accompanied by a ‘diffusion-controlled’ α/β phase transformation and ‘cell-impedance-controlled’ phase transformation, respectively. Lithium transport through the electrode comprising two phases of α and β has been theoretically considered by a numerical analysis of the phase transformation.

In this study, the fabrication of Al-4 wt% Mg-graphite and/or silicon carbide (SiC) particulate composites is described. Composites of Al-4 wt% Mg alloy with different volume percentage values of graphite and/or SiC particles are prepared using the compocasting technique. In this technique, graphite particles and/or SiC particles are added into a semi-solid slurry during the stirring of the melt. A pitched blade stirrer is used to stir the reinforcement particles in the slurry. The slurry is then poured in a metallic mold to obtain the cast composites. The effect of the addition of graphite and/or SiC particles and porosity content on the Rockwell hardness and machinability is studied in the present work. The results show that the porosities increase with increase in particles content while Rockwell hardness decreases with increase of graphite particles. But SiC particles are found to increase the hardness of composites. Surface roughness improves with graphite content in the cast composite, and poor surface finish is obtained when SiC particles are added.

Lithium transport through the Li 1− δCoO 2 film electrode prepared by RF magnetron sputtering

• Spray-coating is a facile and scalable method to produce alternating Si/C thin films. • LiPAA is a suitable binder system for these spray-coated electrodes. • High electrochemical accessibility of Si via alternating layered structures. • Decreasing Si interlayer thickness favors stable and high electrode capacity utilization. Silicon's high specific capacity makes it one of the most promising new materials for anode applications. However, its performance is limited by its cycling stability. Approaches to remedy the various degradation mechanisms (pulverization, delamination and excessive solid electrolyte interphase (SEI) formation) include the use of silicon-carbon (Si/C) composites or the manufacturing of thin layers. In this study, two approaches were combined by producing alternating silicon and reduced graphene oxide (rGO) layers using a spray-coating process. This allowed us to draw important conclusions regarding the relationship between the silicon layer thickness and the total silicon content of the electrode and the resulting degradation behavior. Moreover, this study examined the suitability of prelithiated polyacrylic acid (LiPAA) as binder for spray-coating and its electrochemical performance. Using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and optical microscopy cross-sections, electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT) and galvanostatic cycling, it could be demonstrated that the silicon layer thickness is a limiting factor for a stable cycling performance and can therefore result in an inhomogeneous charge distribution within the electrode. Understanding the correlation between the layer morphology and degradation behavior is essential to allow for the realization of composite electrodes with a high capacity retention.

Physico-chemical modeling is considered as a very interesting way of simulating the behavior of a Li-ion battery cell by linking its internal state to its performance and aging. However, an accurate parametrization of such models is necessary for a truly representative outcome. In the case of commercial batteries, the parametrization procedure is hardly ever entirely undergone, leaving room for parameters tweaking and inaccurate simulation results. In the framework of this work, over 70% of the parameters were measured for a Samsung SDI 37Ah Li-ion prismatic battery (NMC111/Graphite) for PHEV (Plug-in Hybrid Electric Vehicle) applications. A Doyle-Fuller-Newman (DFN) Pseudo 2D model is chosen based on the work of Dufour et al.1. The major electrochemical characterization technics and measurement results for every component in the cell will be presented. The cells were opened in the ante-mortem procedure2, materials were identified, geometrical parameters were measured and samples were taken for coin-cell experiments. A small portion of the electrolyte was recovered and studied to conclude that it was made of EC-DMC-EMC at 1:1:2 portions. A similar solution was synthetized in order to conduct ionic conductivity measurements at temperatures ranging from -30 to 60°C. Samples from the separator were taken for both effective ionic conductivity and porosity measurements. The tortuosity of the electrodes was calculated via an EIS (Electrochemical Impedance Spectroscopy) with a blocking-electrodes configuration, and finally the diffusion factor in the active particles was estimated via GITT (Galvanostatic Intermittent Titration Technique) experiments. A very slow current regime was used to lithiate and de-lithiate the active materials in a half-cell experiment for electrode balancing purposes. The model was compiled using COMSOL Multiphysics 5.5, and results were compared to validation experiments conducted on the full-setup prismatic cell. The charge curve was validated under different current and power regimes at 25°C, and the Incremental Capacity (IC) curve showed a reasonable accuracy. With this in hand, the following step would be to couple this performance model to an aging model and to compare the evolution of the IC peaks with the experimental aging.

An investigation of intercalation-induced stresses generated during lithium transport through Li 1 − δCoO 2 film electrode using a laser beam deflection method

Si-based anodes are being explored for use in advanced lithium ion batteries due to their capacity and energy density, which exceeds that of current graphite anodes. However, there are several important factors that influence and often limit the cycling performance of these electrodes including large silicon volume changes and extensive SEI formation. Nanostructured electrodes have been shown to effectively reduce the negative impact of volume expansion; however, the use of nanostructures can greatly increase the electrode/electrolyte surface area and lead to greater SEI formation. Consequently, several strategies have been developed to reduce the area available for SEI formation while maintaining the benefits of a nanostructured material. In this study, we use silicon-coated vertically aligned carbon nanotube (Si-VACNT) electrodes to examine the role of Li transport in a system where encapsulation has been used to reduce the surface area of the nanostructured electrode that is exposed to the electrolyte. This system is ideal for studying the impact of an electrolyte-blocking layer due to its well-defined geometry and high aspect ratio. Since transport plays an essential role in the cycling performance of these electrodes, it is desirable to understand the factors that govern the transport of Li into and out of the silicon anode in the absence of electrolyte. Two different transport directions and length scales are relevant—1) radial transport of Li in/out of each silicon-coated nanotube (~50nm diameter) and 2) lithium transport along the length of the nanotubes (~100 micron height). Experimental results indicate that the height of the Si-VACNT electrodes does not appear to limit Li transport, even though that height was orders of magnitude greater than the diameter of the tubes. This result has important implications for a variety of encapsulation strategies. Additional tests were performed to characterize transport in the radial direction, which appeared to be a limiting factor. These tests included GITT (galvanostatic intermittent titration technique) measurements and EIS measurements on differently sized electrodes at different conditions. Hysteresis was also examined. Finally, a model was developed to help explain the experimental observations.

We report a comparative study of the electrochemical lithium diffusion properties within the olivine structure of LiMn0.5Fe0.5PO4, LiFePO4,
\nand LiMnPO4 materials prepared by the solvothermal pathway. The study includes careful analysis performed by potentiodynamic cycling with
\ngalvanostatic acceleration (PCGA), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and the galvanostatic intermittent titration technique (GITT), carried out in order to investigate the features of the Fe3+/Fe2+ and Mn3+/Mn2+ redox processes and the lithium ion transport within the olivine structure. The electrochemical investigation reveals a shift of the redox potential of Fe3+/Fe2+ and Mn3+/Mn2+ couples toward higher and lower values, respectively, in LiMn0.5Fe0.5PO4 with respect to the bare materials. Interestingly, the study shows the dependence of the lithium diffusion coefficients on the state of charge of the materials as well as on the adopted technique. Accordingly, CV leads to lithium diffusion coefficients of the order of 10−12 cm2 s−1 for LiMnPO4, 10−9 cm2 s−1 for LiFePO4, and 10−11 cm2 s−1 for LiMn0.5Fe0.5PO4. EIS mainly indicates lower values of lithium diffusion coefficients, i.e., 10−13 cm2 s−1 for LiMnPO4, 10−12 cm2 s−1 for LiFePO4, and 10−13 cm2 s−1 for LiMn0.5Fe0.5PO4. GITT provides a wide range of Li+ diffusion coefficient, depending on the Li1−xMePO4 stoichiometry, that is, 10−14−10−10 cm2 s−1 for LiMnPO4 and LiFePO4 and 10−13−10−10 cm2 s−1 for LiMn0.5Fe0.5PO4. The wide diffusion coefficient window obtained by changing the state of charge and the adopted technique sheds light on the complex trend of the lithium diffusion in olivines and indicates that the technique may actually influence the materials evaluation.