graphite electrode scrap electrode paste
In-depth understanding of lithium analysis of graphite electrode scrap electrode paste in lithium-ion batteries: detection, quantification and mechanism
Under normal working conditions, lithium ions will be embedded in the graphite anode intercalation. However, due to kinetic or thermodynamic limitations, such as fast charging, low temperature, overcharge or design defects, metallic lithium will precipitate on the graphite anode surface, causing the graphite potential to drop to 0 V vs. Li/Li+. Long-term lithium deposition on the graphite surface will cause capacity degradation, and in severe cases, it can pierce the diaphragm and cause an internal short circuit, which will cause thermal runaway and cause harm to battery safety. However, how to detect, quantify and clarify the mechanism of lithium has not yet a perfect solution.
Research content of graphite electrode scrap electrode paste
In view of this, Wang Qingsong’s research group at the State Key Laboratory of Fire Science, University of Science and Technology of China took graphite/lithium half-cells as the research object, and obtained the graphite electrode scrap electrode paste for lithium-ion batteries through overlithiation (corresponding to half-cell overdischarge). Analyze the in-depth understanding of lithium. The content of the article includes five parts combining experiment and numerical simulation: 1) extracting the lithium signal from the graphite voltage curve; 2) identifying the lithium exfoliation process based on the in-situ heat generation curve; 3) quantifying the reversible efficiency of lithium deposition/exfoliation during cycling 4) The mechanism of lithium analysis is clarified through disassembly and characterization; 5) The lithium analysis process is reconstructed based on the finite element model.
Graphite electrode scrap electrode paste research results and discussion
1) Extract and analyze the lithium signal from the graphite voltage curve
Figure 1 shows the extraction of the lithium signal from the voltage curve of the graphite electrode scrap electrode paste and the IC (dQ/dV) curve. It can be found that there is an obvious voltage plateau during the charging process of the lithium-extracted battery, which corresponds to the peak of the IC curve and precipitates The more lithium contained, the higher the peak of the IC curve. This stage is the reversible process of lithium extraction-lithium stripping (Li), and the stripped lithium will be re-embedded in the negative electrode without causing capacity degradation.
Figure 1 Graphite electrode scrap electrode paste Voltage curve and IC curve to extract and analyze lithium signal. (A) Graphite lithium insertion voltage, (b) Graphite delithiation voltage, (c) IC curve in Figure b, (d) Partial magnification of Figure b and c, (e) Correspondence between delithiation voltage and IC curve, ( f) Correspondence between Lithium removal voltage and IC curve
2) Identify the lithium stripping process based on the in-situ heat generation curve
Based on the button cell calorimeter, we obtained the changes in the heat production curves of batteries with different SOCs (see Figure 2). It can be found that there are three heat production peaks in the batteries without lithium separation, which correspond to the delithiation platform; During the charging process, the lithium-evolving battery has an extra heat production peak “*”, which corresponds to the peeling platform, and the more lithium is precipitated, the more lithium can be reversibly peeled off, and the corresponding heat generation peak is also higher. In addition, we also found that the heat generation peak of lithium stripping will weaken the first peak of lithium insertion, and it can also be used as a sign for detecting lithium evolution.
Figure 2 In-situ heat production and voltage change curve and the proportion of each peak heat production during the 1C delithiation process
3) Quantify the reversible efficiency of lithium deposition/stripping during cycling
We first calculated the reversible lithium (stripped lithium) content through the method of differential voltage, and then obtained the total coulombic efficiency and the coulombic efficiency of lithium deposition/stripping. We found that the reversible efficiency of lithium deposition/stripping after 20 cycles of lithium separation The decrease is the reason for the decrease of the total coulombic efficiency, indicating that the irreversible capacity loss has caused the decrease of the cycle performance
Figure 3 Determination of the reversible efficiency of lithium deposition/stripping.
4) Identify the mechanism of lithium analysis through disassembly and characterization
Based on the above experimental research, we have obtained the mechanism and process diagram of lithium analysis as shown in Figure 4.
Figure 4 Diagram of Lithium Analysis Process
5) Reconstruction of lithium analysis process based on finite element model
Finally, a 2D finite element model was established for the graphite/lithium half-cell, and it was found that the lowest graphite potential, the highest lithium evolution current and the thickest lithium evolution thickness all appeared at the graphite/diaphragm interface.
Figure 5 The results of the finite element model of 0.2C and lithium evolution: from top to bottom are the current of lithium evolution, graphite voltage, and the thickness of the lithium layer
The research results have been published in the journal, with Mei Wenxin as the first author and Wang Qingsong as the corresponding author.