First Author: Lin Ying, Hu Wenxuan
Corresponding Author: Yang Yong
Correspondence unit: Xiamen University
【Background】
Lithium-ion batteries (LIBs) have been widely used in the field of new energy due to their excellent energy and power density. However, lithium evolution may occur in graphite-based anode materials under harsh working conditions such as fast charging and low temperature. Uncontrolled lithium evolution can lead to the rapid accumulation and growth of "dead lithium" and SEI, while the continuous growth of dendrite may puncture the separator and cause a short circuit in the positive and negative electrodes of the battery, causing thermal runaway. Many in-situ/in-situ detection methods have been proposed for when lithium evolution occurs, but they are limited to giving the starting point of lithium evolution and cannot effectively describe the subsequent evolution process of lithium evolution. In this work, the researchers used a hyphenated analysis method using in-situ dynamic electrochemical impedance spectroscopy (DEIS) combined with thickness measurement to comprehensively study the evolution of lithium evolution, including the occurrence of lithium evolution and the change of lithium deposition state. The results of this paper will help to understand the evolution process of graphite lithium evolution, and provide new insights for promoting the operation of lithium-ion batteries under harsh working conditions and battery safety management.
【Job Profile】
Recently, Professor Yang Yong's team from Xiamen University has comprehensively studied the evolution process of lithium evolution on the surface of graphite in graphite/lithium iron phosphate pouch batteries under harsh working conditions (low temperature/normal temperature fast charging) by using in-situ dynamic electrochemical impedance spectroscopy (DEIS) combined with thickness measurement, and expanded the application of impedance method and thickness measurement in the detection of lithium evolution. Combined with mass spectrometry titration (MST) and electron microscopy, it was confirmed that these three stages correspond to three different evolution processes of lithium evolution, lithium nucleation & lithium nucleation growth, and dendrite growth, respectively, and analyzed in detail the effects of lithium evolution and different lithium deposition states on battery capacity decline. The related work was published in the international authoritative journal Advanced Energy Materials as "Unveiling the Three Stages of Li Plating and Dynamic Evolution Processes in Pouch C/LiFePO4 Batteries", and Lin Ying and Hu Wenxuan, Ph.D. students from the School of Chemistry and Chemical Engineering of Xiamen University, are the first authors of this paper.
【Content Description】
1. In-situ impedance-thickness testing combined analysis techniques
In this work, the evolution of lithium evolution in graphite/lithium iron phosphate pouch batteries was comprehensively studied using in-situ dynamic electrochemical impedance spectroscopy (DEIS) coupled with thickness measurement (Fig. 1). The DEIS method is to continuously apply AC disturbance signals to the battery during the charging process to obtain EIS spectra of different SOCs, and the low-frequency information is discarded in order to improve the SOC resolution of the EIS spectrum, and the frequency range of the disturbance signal is 50 kHz ~ 5 Hz, and a EIS spectrum with a resolution of about 33 s is obtained<1% SOC。 Combined with the relaxation time distribution (DRT), the negative electrode load transfer impedance Rct, a with the charging process was quantitatively analyzed. In terms of thickness measurement, the researchers first decoupled the variation of the positive and negative electrode thicknesses by combining graphite and lithium iron phosphate electrodes with lithium titanate (LTO), a zero-strain material, respectively (Fig. 3). Through experiments, it is concluded that the thickness variation law of the whole battery is dominated by the negative electrode. According to the volume expansion caused by lithium evolution is significantly greater than that of lithium intercalation, the capacity is differentiated (dT/dQ) by real-time thickness increment, and the lithium evolution threshold is set, once the dT/dQ during the charging process exceeds the threshold, the occurrence of lithium evolution is indicated.
Figure 1. Schematic diagram of an in-situ dynamic electrochemical impedance-thickness measurement device (consisting of an electrochemical workstation and an in-situ expansion test system) for lithium evolution detection experiments, and a method for thickness measurement to detect the occurrence of lithium evolution.
Figure 2. Negative EIS spectra (Nyquist plot and DRT) for different SOCs and temperatures.
Figure 3. LTO was used to decouple the expansion variation of graphite and lithium iron phosphate.
2. Characteristics of pouch cell thickness-impedance change in lithium evolution process
Through the analysis technique of impedance and thickness testing proposed above, the researchers can analyze the impedance and thickness changes in the lithium separation process of pouch batteries at the same time, and compare the accuracy difference between the two in the indication of lithium separation. The researchers charged the pouch cells at different rates at low temperatures (0 °C) to induce different levels of lithium evolution (Figure 5). In terms of impedance, under the condition of 0.1C charging, Rct,a decreases linearly with the charging process. When charging at 0.2C, Rct,a undergoes a linear decline and then suddenly accelerates the decline, and this inflection point indicates the occurrence of lithium evolution (the negative electrode load transfer impedance Rct, a will become smaller after lithium evolution occurs); However, when charging at 0.5C, Rct,a showed a three-stage change law, and the additional third stage showed platform characteristics, which was reported for the first time in the literature. The researchers believe that the reaction of the negative electrode in stage II gradually transitions from lithium intercalation to lithium evolution, while the reaction of the negative electrode in stage III is basically dominated by lithium evolution. In terms of thickness measurement, the thickness/capacity differential curves (dT/dQ) under the 0.1C and 0.2C charging conditions did not exceed the lithium evolution threshold, that is, the thickness measurement indicated that no lithium evolution occurred under the charging conditions. At 0.5C, the dT/dQ exceeds the lithium evolution threshold after charging about 1200 mAh, i.e., the thickness measurement indicates that the lithium evolution signal has been detected at this SOC. The results show that DEIS can indicate lithium evolution (lithium evolution initiation) earlier than the thickness method. This is because DEIS, as an electrochemical method, will be more sensitive to the occurrence of lithium evolution (charge transfer process), but it cannot give specific physical information; Thickness measurement is a macroscopic test method, which is not as sensitive to lithium evolution as electrochemical methods, but it can reflect the morphology and severity of lithium evolution to a certain extent. It is important to note that the onset of Stage III and the thickness indicate that the SOC of lithium evolution is highly consistent, indicating that both indicate the same evolution of lithium evolution, which will be discussed in more detail in the next section.
Taken together, both impedance and thickness measurements can detect lithium evolution processes, but the information and potential physical significance they provide are different, and they are complementary and cross-validated. Multi-dimensional descriptors composed of electrochemical information (Rct,a) and structural (volume) information (dT/dQ) to determine the precise state of lithium deposition can help to understand the evolution of lithium more comprehensively.
Figure 4. Changes in EIS spectra (Nyquist plot and DRT) during charging at different rates of pouch cells.
Figure 5. Characteristics of pouch cell thickness-impedance change in lithium evolution process (from top to bottom, battery voltage curve, negative electrode load transfer impedance/capacitance change, thickness change).
3. Mechanism analysis of the three-stage evolution process of lithium evolution
The researchers first verified the accuracy of Rct,a indicating the start of lithium evolution by incremental capacity analysis (ICA) and voltage relaxation analysis (dOCV) (Figure 6). Subsequently, mass spectrometry titration (MST) and scanning electron microscopy (SEM) were used to disassemble and characterize the cells charged to three different Rct,a stages under low temperature (0°C/0.4C) and normal temperature and high rate charging conditions (25°C/3C). Together, MST and SEM analyses indicate that stage I is lithium intercalation in graphite, stage II is lithium nucleation and lithium nucleation growth, and stage III is characterized by dendrite growth (Figure 7). This conclusion is correct at different temperatures and magnifications, and the analytical technique used in combination with impedance and thickness testing can help to comprehensively elucidate the evolution of lithium.
Figure 6. Incremental Capacity Analysis (ICA) and Relaxation Voltage Profile (dOCV) analysis were performed to determine the accuracy of the DEIS method indicating lithium evolution.
Figure 7.(a) Graphite electrode mass spectrometry titration results at three different Rct stages. and (b) capacity loss due to "dead lithium" and different organic SEI components. (c) SEM and optical images of three representative cells at different RCT,a change stages. (d) Schematic diagram of lithium evolution behavior on graphite surface when Rct,a changes in stage II./III.
In addition, the researchers obtained the variation law of Rct,a at different charging rates at 0°C, and drew a boundary map of lithium intercalation and lithium evolution according to the inflection point of Rct,a at different charging rates, and the three regions correspond to three different stages of lithium evolution (Fig. 8). By conducting experiments to create similar images, researchers can accurately avoid the occurrence of lithium precipitation or large dendrite growth during battery cycling.
Figure 8. The in-situ impedance-thickness measurement method monitors a schematic diagram of the lithium evolution process and the boundaries at which the different stages of lithium evolution occur.
4. The effect of lithium evolution on battery performance degradation
During the charging process of three consecutive cycles, the researchers found that the second cycle of stage II (lithium nucleation & lithium core growth) and stage III (a large number of dendrite growth) occurred earlier than the first circle, and it was believed that the presence of unstripped lithium on the surface of the discharge graphite in the first circle reduced the energy barrier of subsequent lithium nucleation, which in turn led to the early occurrence of lithium precipitation. In addition, the researchers cycled the three cells in different SOC ranges, which corresponded to the three stages of the proposed evolution of lithium evolution (no lithium evolution, lithium core growth, and dendrite growth), and analyzed the state of health (SOH) and active lithium loss (LLI) of the batteries with different cycle times using the electromotive force curve (EMF) measurement method developed within the research group. The results show that the early occurrence of lithium evolution and the accumulation of SEI and "dead lithium" caused by dendrite growth will accelerate the loss of battery capacity and even lead to capacity "diving", which once again emphasizes that the in-depth analysis and monitoring of the evolution process of lithium evolution are necessary to promote the application of lithium-ion batteries under extreme conditions.
Figure 9. (a) Rct, the change in a period of three cycles, and (b) the starting SOC of phase 2/3 in these cycles. (c) The change in the discharge capacity of the three batteries at different SOC intervals with the number of cycles. Change in (d) state of health (SOH) and (e) lithium inventory loss (LLI) of the battery with the same total charge throughput.
【Conclusion】
In this study, the evolution process of lithium evolution is studied simultaneously from the aspects of electrochemistry and thickness change through the combined analysis technique of in-situ impedance-thickness measurement, which avoids the limitations of a single method and emphasizes the significance of studying the problem from different perspectives. While both impedance and thickness can detect lithium evolution processes, the information they provide and the underlying physical significance are different, and they are complementary. The main conclusions of the article are as follows:
(1) When RCT,A is used as an indicator of lithium evolution, it exhibits a three-stage pattern of variation, namely a slow linear downward trend (Phase 1), an accelerated decline (Phase 2), and a plateau (Phase 3). These three phases correspond to lithium intercalation, the transition from lithium intercalation to the mixing zone of lithium evolution, and the almost complete lithium evolution reaction, respectively. The first inflection point indicates the start of lithium evolution, and the second inflection point indicates that lithium evolution dominates the interfacial reaction. When dT/dQ is used as an indicator of lithium evolution, it captures the moment when lithium nuclei grow into massive dendrite growth, but ignores the nucleation and nuclear growth stages. And the point it gives is very close to the beginning of the Rct, a change III. stage.
(2) Through disassembly characterization analysis (MST and SEM), the researchers confirmed that the negative electrode load transfer impedance Rct, a three change stages correspond to three different lithium evolution processes, namely lithium non-evolution, lithium nucleation & lithium nucleus growth, and dendrite growth.
(3) The aging state of the battery at different SOC cycles shows that lithium dendrite growth and the formation of a large amount of SEI result in significant capacity loss compared to the case without lithium evolution or dendrite growth. In addition, without precise monitoring of the lithium deposition status, lithium evolution can easily become uncontrollable, which can lead to a rapid decline in capacity or even a capacity "dive".
【Literature Details】
Ying Lin, Wenxuan Hu, Meifang Ding, Yonggang Hu, Yufan Peng, Jinding Liang, Yimin Wei, Ang Fu, Jianrong Lin, Yong Yang. Unveiling the Three Stages of Li Plating and Dynamic Evolution Processes in Pouch C/LiFePO4 Batteries. Advanced Energy Materials. 2024, 2400894.
https://doi.org/10.1002/aenm.202400894
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