LiBforSecUse processed data for machine learning modelling and uncertainty propagation

Original data 

Original data source

The raw data have been downloaded from Zenodo:

LiBforSecUse Data Release - Impedance spectra of life cycle tests of commercial 18650 cells (v2)

DOI link to the data : https://doi.org/10.5281/zenodo.6418665

CC BY 4.0 license (allows re-distribution and re-use on the condition that the creators of the dataset are appropriately credited)

Users are asked to pay tribute to the original project and the researchers providing the data by citing the following data source.

Citation : Chan, H. S., Dickinson, E., Fedorovskaya, E., Gaberšček, M., Heins, T. P., Meddings, N., Lee, Y. Y., Moškon, J., Park, J., Ruiz, V., & Seitz, S. (2022). LiBforSecUse Data Release - Impedance spectra of life cycle tests of commercial 18650 cells (Version V2) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.6418665

Description of the original dataset

Dataset release from the EMPIR project LiBforSecUse. It includes regular impedance spectroscopy (EIS) and capacity measurements made during life cycle tests of commercial 18650 (graphite/NMC) cells. 

Measurements were performed by 6 separate institutions using different setups (hardware and software). However, during the LiBforSecUse project, the analysis of the data revealed some discrepancies for Institution 5 and 6.

In total, 34 cells were measured, at different fixed SOC (range 20-100%) after several charge-discharge cycles (up to 5900 cycles), resulting in 5479 experiments. For each experiment, EIS measurements are performed for different frequencies ranging from $10^{-2}$ to $10^{4}$ Hz. From an experiment to another, the number of discretisation points of the frequency spectrum can vary form 39 to 61. However, a vast majority of experiments are recorded at 49, 58 and 61 points.

For details on BUTs, setups, experimental design, description of measured variables and original file format, please refer to the documentation of the original dataset. 

Data processing by METAS

Exploratory data analysis and data cleaning

According to the original documentation, Institution 5 and 6 were removed from the dataset (corresponding to 8 cells and 1146 experiments).

Some SOH measurements are missing. As it is one of the principal quantities of interest, these were removed (corresponding to 33 experiments)

NFRA measurements were excluded.

Constant variables, such as temperature during EIS tests (23 °C) and cycling current (4 Amperes).

3 experiments have no data at low frequencies. They have been removed.

Potential outliers disclaimer

Some data points exhibit physically unrealistic behavior (e.g., increasing SOH with more charge-discharge cycles). However, they have been retained, as an appropriate handling (correction/removal) of these points depends on the context of use.

Interpolation of the frequency spectrum

After data cleaning, the majority of experiments are measured at 61 points descretizing the frequency spectrum from $10^{-2}$ to $10^{4}$ Hz, (10 points per decade, with logarithmic spacing).All data were aligned on the 61 frequencies measured for the experiments conducted by Institution 1, Cell 4, at SOC 100 before cycling. This is done by interpolating the measurements with Cubic Splines on the log-transformed frequency axis, with the constraint that the second derivative of each Spline curve vanishes at its bounds. When extrapolations were required, missing values (Nan) were introduced. Processing was done in Python using scipy.interpolate.CubicSpline. 

Data wrangling

Original tabular data contained in 3 separate files were merged into a unified dataset. EIS data, originally stored as triplets (frequency, resistance, reactance) were expanded into 122 variables : 61 log-regular points on the frequency axis between $10^{-2}$ to $10^{4}$ Hz, multiplied by the 2 dimensions of the couple (resistance, reactance). In this format, each line corresponds to an experiment for given cell, SOC, and number of charge-discharge cycle. A unique experiment identifier was defined as "exp_ID" = "Cell_ID"_"nCycle"_"SOC".Variable names for EIS data follow the format {Z_real, Z_imag}@{10^x_Hz}, where 

the first part of the name can represent the real part of impedance (i.e. resistance) or the imaginary part of the impedance (i.e. reactance),  

the last part is the corresponding frequency, $10^x$ Hz, for $x = -2, -1.9, ... 3.9, 4$.

The final dataset after this processing has the following properties :

26 cells

4297 experiments

133 variables (incl. 122 for EIS)

## 原始数据
### 原始数据来源
原始数据下载自Zenodo平台：

LiBforSecUse 数据发布——商用18650电池循环寿命测试阻抗谱（v2版）
数据DOI链接：https://doi.org/10.5281/zenodo.6418665

本数据集采用知识共享署名4.0（CC BY 4.0）许可协议（允许再分发与再使用，但需正确标注数据集创作者）。

使用者需通过引用以下数据源致敬原始项目与提供数据的研究人员：
引用格式：Chan, H. S., Dickinson, E., Fedorovskaya, E., Gaberšček, M., Heins, T. P., Meddings, N., Lee, Y. Y., Moškon, J., Park, J., Ruiz, V., & Seitz, S. (2022). LiBforSecUse Data Release - Impedance spectra of life cycle tests of commercial 18650 cells (Version V2) [数据集]. Zenodo. https://doi.org/10.5281/zenodo.6418665

## 原始数据集说明
本数据集由EMPIR项目LiBforSecUse发布，包含商用18650（石墨/镍锰钴（NMC））电池循环寿命测试过程中采集的常规电化学阻抗谱（Electrochemical Impedance Spectroscopy, EIS）与容量测试数据。

测试由6家独立机构使用不同软硬件设备完成，但在LiBforSecUse项目的数据分析过程中，发现机构5与机构6的数据存在部分不一致。

本次测试共涉及34颗电池，在多次充放电循环（最多5900次）后，于不同固定荷电状态（State of Charge, SOC，范围20%~100%）下开展测试，总计得到5479组实验数据。每组实验的测试频率范围为$10^{-2}$至$10^{4}$Hz，阻抗谱的离散采样点数介于39至61之间，其中绝大多数实验的采样点数为49、58与61。

如需了解待测设备（BUTs）、实验装置、实验设计、测量变量说明与原始文件格式等细节，请参阅原始数据集文档。

## METAS 数据处理
### 探索性数据分析与数据清洗
根据原始文档说明，本数据集移除了机构5与机构6的全部数据（对应8颗电池与1146组实验）。
部分健康状态（State of Health, SOH）测量值存在缺失，由于SOH是核心关注量之一，因此移除了对应33组实验。
移除了NFRA测量数据。
保留了恒定测试参数，如电化学阻抗谱测试温度（23℃）与循环充放电电流（4A）。
另有3组实验的低频段无有效数据，已予以移除。

### 潜在异常值声明
部分数据点存在不符合物理规律的表现（例如随着充放电循环次数增加，健康状态（SOH）反而上升），但予以保留，因为此类数据点的处理方式（修正或移除）需结合具体应用场景确定。

### 阻抗谱插值
数据清洗完成后，绝大多数实验的阻抗谱采样点数为61，频率范围为$10^{-2}$至$10^{4}$Hz（采用对数间隔，每十倍频程包含10个采样点）。
所有数据均以机构1、第4颗电池在循环前SOC为100%时采集的61个频率点为基准进行对齐：通过对对数变换后的频率轴使用三次样条（Cubic Splines）插值，约束每条样条曲线在端点处的二阶导数为零，对其余实验数据进行插值。若需外推，则用缺失值（NaN）填充。该处理过程使用Python的scipy.interpolate.CubicSpline工具完成。

### 数据整理
原始存储于3个独立表格文件中的数据被合并为统一数据集。原始的电化学阻抗谱数据以（频率、电阻、电抗）三元组形式存储，被扩展为122个变量：频率轴上$10^{-2}$至$10^{4}$Hz区间内的61个对数均匀分布采样点，乘以阻抗的实部（电阻）与虚部（电抗）两个维度。
在此数据格式下，每一行对应一颗电池、某一荷电状态与某一充放电循环次数下的一组实验。定义唯一实验标识符"exp_ID" = "Cell_ID"_"nCycle"_"SOC"。
电化学阻抗谱数据的变量名格式为"{Z_real, Z_imag}@{10^x_Hz}"，其中：
- 变量名前半部分可表示阻抗实部（即电阻）或阻抗虚部（即电抗）；
- 变量名后半部分为对应频率$10^x$Hz，x的取值范围为-2、-1.9、…、3.9、4。

## 最终数据集属性
经上述处理后的最终数据集具备以下属性：
- 26颗电池
- 4297组实验
- 133个变量（包含122个EIS相关变量）