Correlating Gas Composition and Electrochemical Data from First Formation Cycle of Lithium-Ion Batteries

Abstract

The solid electrolyte interface (SEI) plays a crucial role for the performance and lifetime of lithium-ion batteries. It consists of decomposition products from electrolyte components (salt, solvent and additives) and is initially formed during the first charging process called the formation cycle. The chemical processes creating the SEI are a matter of current research. A better understanding of these reactions and their implications on cell performance can provide new ways to optimise the formation process. This can help reducing costs while increasing lifetime and performance of lithium-ion batteries.In this contribution we apply statistical correlation analysis on a large set of cells to gain new insights on the connection between formation and cell-performance. Cells (NMC622|Graphite, 1 Ah, ordered from LiFun) are divided in three groups. The groups differ only by the electrolyte. An electrolyte solution with EC:EMC (3:7 by wt.) and 1 M LiPF6 serves as a baseline. The other two groups use the same solvent and salt concentration but contain an additional 5% fluor ethylene carbonate and vinylene carbonate, respectively.All cells underwent the same formation procedure. It consists of a formation cycle with C/5 at 80°C, followed by a degassing-step and 9 subsequent cycles with the same current rate at 45°C. After these ten cycles a reference performance test (RPT) is conducted, including qOCV, capacity and pulse-resistance tests. During the degassing step gas samples from half of the cells were analysed using GC-MS.An incremental capacity analysis of the formation cycle shows clear distinctions between the three different electrolytes. Distinct peaks below 3 V are found for each group. The gas composition demonstrates that cells containing additives contain a higher share of carbondioxide and carbonmonoxid amid a substantially lower share of ethylene, compared to the cells without additives.The large number of over 100 cells in total enables a systematic correlation analysis. We investigated the correlation between features extracted from the first formation cycle and the RPT. In addition, correlations between electrical data from the first cycle and the relative amount of carbon-containing gasses in the samples are analysed.Correlations differ for the three groups of cells, however some common trends were observed. We found a negative correlation between the average voltage during formation and the difference between the capacity of the first discharge and the capacity measured in the RPT. This difference represents the loss over the 9 cycles at 45°C. It is positively correlated with the first discharge capacity indicating that lower lithium-losses in the first cycle are countered by higher losses in subsequent cycles.Correlations between gasses and electrical data from the formation cycle provide new insights into the chemical processes. If the shares of two gasses are positively correlated, we assume that the reactions producing these gasses are linked. We found such strong positive correlations for the shares of CH4, C2H6, C2H4 and CO in the case of VC-containing electrolyte. In this group the share of carbondioxide is negatively correlated to these gas-shares.Correlating the different gas-shares with the charge throughput in different voltage windows (during wetting at 1.5V, during the formation charge cycles from 1.5 to 3V, 3 to 3.8V, and 3.8 to 4.2 V, respectively), we found very different patterns for the three groups. VC-containing cells show weak or very weak correlations with all four charge-throughput values, with the exception of a moderate negative correlation between CO. Cells without additives show a moderate correlation between the wetting charge and the CO2-share. Cells containing FEC exhibit a moderate positive correlation between the share of CO2 and the charge throughput between 1.5 and 3.8 V.The combination of electrical data with the gas composition and statistical analysis provides a new way to improve our understanding of the SEI formation and the complex chemical reactions. Our results indicate that gasses generated by the decomposition of VC are not influenced by small changes of charge throughput during formation, whereas a higher charge-throughput at lower voltages in cells containing FEC correlates with an increased share of CO2.