### Code and analysis for "Nonlinear electrochemical impedance spectroscopy of lithium-ion batteries: Experimental approach, analysis, and initial findings" ------------------------------------------------------------------------------------------------------ [This repository](https://github.com/mdmurbach/nleis-battery-manuscript) contains all of the data, code, and figures, used in the manuscript. To refer to the code or analysis, please use the ECSarXiv DOI: https://osf.io/tdnwa/ **Please cite the following project if you use this code:** Murbach, M. D., Hu, V. & Schwartz, D. T. Nonlinear electrochemical impedance spectroscopy of lithium-ion batteries: Experimental approach, analysis, and initial findings. (2018). Available at: osf.io/tdnwa. ------------- #### Abstract ------------- Nonlinear electrochemical impedance spectroscopy (NLEIS) is a moderate-amplitude extension to linear EIS that provides a sensitive and complementary whole-battery diagnostic for charge transfer kinetics, mass transport, and thermodynamics. We present the first full-frequency, second harmonic NLEIS spectra for lithium-ion batteries using commercially available, 1.5 Ah LiNMC|C cells. The mathematical framework for NLEIS shows, and experiments confirm, that moderate-amplitude input modulations can generate a second harmonic output that does not intrinsically corrupt the linear EIS response. Experimental measurements at varied states-of-charge (SoC) and states-of-health (SoH) are used to illustrate and compare NLEIS and EIS data. At low frequencies, the second harmonic NLEIS spectrum is shown to produce a much more distinct response to SoC dependent thermodynamic and diffusion processes than linear EIS. By combining NLEIS and EIS, we are able to characterize degradation in early cell cycling (where cells lost <1% of initial capacity). We also show that NLEIS complements the characterization of charge transfer kinetics of linear EIS through the second harmonic sensitivity to symmetry. For example, NLEIS shows that fresh cells have high symmetry charge transfer (α_a = α_c = 0.5) on both electrodes, whereas early in the cycling there is a shift toward kinetics that favor oxidation on the positive electrode (α_a,pos > 0.5, α_c,pos < 0.5). Combined analysis of EIS and NLEIS spectra shows promise for improved parameter estimation and model validation. All experimental data and analysis code for this manuscript can be found on ECSarXiv. --------------------------- #### Software dependencies --------------------------- The code in this repository has been tested with the following versions of software: - Python 3.6.3 [https://www.python.org/](https://www.python.org/) - Conda 4.3.30 (through Miniconda 3) [https://conda.io/miniconda.html](https://conda.io/miniconda.html) - Git Bash on Windows The environment.yml file can be used to recreate the same Conda environment with the following commands: `conda env create -f environment.yml` `source activate nleis-manuscript` ------------------------- #### Repository Structure ------------------------- **comsol:** This folder contains the COMSOL model used to simulate the linear EIS and second harmonic NLEIS spectra of a lithium-ion battery using the pseudo-two-dimensional (P2D) model. **data:** This folder will need to be added once the repository has been cloned (see below). For minimal changes to the supplementary notebook, put all data downloaded through the OSF page here. **figures:** This folder is where the figures generated by the Python code are located. **jupyter:** This folder contains the Jupyter notebook (Supplementary Notebook.ipynb) and utility module that are used to compile the dataset, perform any analysis, and generate the manuscript figures. **matlab:** This folder contains the MATLAB .m files for running the COMSOL model using LiveLink for MATLAB. **supplementary-files:** This folder contains the input files required for reproducing the simulations found in the manuscript. ---------- #### Data ---------- All of the data files are stored with OSF Projects: https://osf.io/tdnwa/ The data directory is organized into experimental data (`/raw`) and simulation data (`/simulation`). The raw experimental data is further organized by: - `/new`: contains the the data files and the corresponding Kramers-Kronig validation for the EIS/NLEIS experiments done on three fresh LiNMC|C cells. Each cell subdirectory has experimental data for 10, 30, 40, 50, and 60% state of charge and the open-circuit potentials the experiments were done at. - `/aged`: contains the the data files and the corresponding Kramers-Kronig validation for the EIS/NLEIS experiments done on three LiNMC|C cells that were aged for 100 cycles at 2C. Each cell subdirectory has experimental data for 10, 30, 40, 50, and 60% state of charge and the open-circuit potentials the experiments were done at. - `resistor`: contains the data files for the EIS/NLEIS experiment done of the purely resistive wire to determine the instrument offset To reproduce the figures: - Clone the [GitHub repository](https://github.com/mdmurbach/nleis-battery-manuscript) - Download/unzip the folder from the [OSF Project](https://osf.io/tdnwa/) into the main repository directory - Create and activate conda environment from environment.yml (see above) - Open Jupyter Lab (`jupyter lab`) to run the [Supplementary Notebook.ipynb](https://github.com/mdmurbach/nleis-battery-manuscript/blob/master/jupyter/Supplementary%20Notebook.ipynb)