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Article

Effect of High-Rate Cycle Aging and Over-Discharge on NCM811 (LiNi0.8Co0.1Mn0.1O2) Batteries

by
Tao Yin
1,2,3,
Longzhou Jia
1,2,3,
Xichao Li
1,2,3,4,
Lili Zheng
1,2,3,* and
Zuoqiang Dai
1,2,3,*
1
College of Mechanical and Electrical Engineering, Qingdao University, Qingdao 260071, China
2
Engineering Technology Center of Power Integration and Energy Storage System, Qingdao University, Qingdao 266071, China
3
National and Local Joint Engineering Technology Center for Intelligent Power Integration Technology for Electric Vehicles (Qingdao), Qingdao 266071, China
4
Energy Storage Business Department, CRRC Qingdao Sifang Rolling Stock Research Institute Co., Ltd., Qingdao 266031, China
*
Authors to whom correspondence should be addressed.
Energies 2022, 15(8), 2862; https://doi.org/10.3390/en15082862
Submission received: 15 February 2022 / Revised: 25 March 2022 / Accepted: 4 April 2022 / Published: 14 April 2022
(This article belongs to the Topic Safety of Lithium-Ion Batteries)
Browse Figures
Versions Notes

Abstract

:
Inconsistencies in a monomer battery pack can lead to the over-discharge of a single battery. Although deep over-discharge can be avoided by optimizing the battery control system, slight over-discharge still often occurs in the battery pack. The aging behavior of cylindrical NCM811 batteries under high-rate aging and over-discharge was studied. By setting the end-of-discharge of 1 V, the battery capacity rapidly decayed after 130 cycles. Additionally, the temperature sharply increased in the over-discharge stage. The micro short-circuit was found by the discharge voltage curve and impedance spectrum. Batteries with 100%, 79.6% and 50.9% SOH (state of health = Q_now/Q_new × 100%) as a result of high-rate aging and over-discharging were subjected to thermal testing in an adiabatic environment. The battery without high-rate aging and over-discharge did not experience thermal runaway. However, severe thermal runaway occurred in the 79.6% and 50.9% SOH batteries. Regarding the cyclic aging of the 50.9% SOH battery, the fusion temperature of the separator decreased by 22.3 °C, indicating a substantial degradation of the separator and thus reducing battery safety. Moreover, the results of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses revealed that the particles of the positive material were broken and detached, and that large-area cracks and delamination had formed on the negative material. Furthermore, Ni deposition and the uneven deposition of P and F on the negative surface were observed, which increased the risk of short-circuit in the battery. Positive and negative materials were attached on both sides of the separator, which reduced the effective area of ionic transportation.

1. Introduction

With the emergence of a new energy era, batteries are a power source for several electronic devices. Particularly, lithium-ion batteries which are widely used owing to their high energy density and long cycle life, account for a significant market share in the energy industry. Therefore, it is imperative to ensure their safety [1,2]. Previous studies have analyzed electrical [3,4], mechanical [5], and thermal [6] abuses on lithium-ion batteries. Among these, over-discharge is a common issue that may lead to safety problems, such as temperature abnormalities and short-circuits. Over-discharge occurs when the discharge voltage is less than the normal operating voltage range, resulting in an abnormal increase in the potential of the negative electrode [7]. Furthermore, the increasing application of lithium-ion batteries in high-voltage and power electrical systems has an increased risk of over-discharge.
Martin [8] noted that over-discharge leads to the breakdown of the solid electrolyte interphase (SEI), electrolyte reduction, copper foil oxidation, and the reversal of the voltage to a negative value. In addition, they found that the copper ions penetrated the separator, resulting in a shunt between the positive and negative electrodes, and increasing the battery temperature. Qian [9] over-discharged NCM batteries, thereby producing a decreased stability of the NCM structure. Electrolyte degradation was found to be the primary reason for the observed capacity degradation. In addition, Rb (the ohmic impedance) and Rct (transfer impedance) of the battery increased. Zheng [10] over-discharged LiFePO4 batteries to different voltages for 110 cycles. They found that a decrease in the cycle voltage corresponded to the decay of the battery capacity at a faster rate. The poor cycling performance was primarily attributed to the dissolution and breakdown of the SEI at low voltage. Daniel et al. [4] studied the over-discharge and cyclic aging behavior of LCO (LiCoO2) batteries. The increased thickness of the SEI thickening was noted to be the dominant factor for aging at the end-off voltages of 2.7 and 1.5 V. Under the conditions of 0 and −0.5 V, the structure of the LCO particles could break, and the negative collector dissolved. Moreover, they also observed that the SEI and electrolyte decomposed to form gas. Their last significant finding was that the deposition of copper and lithium created a micro-short circuit or a short-circuit. Guo [11] found that a short circuit was induced when the battery was over-discharged to −12% SOC (state of charge). They found this phenomenon to be attributable to the dissolution and deposition of the copper in the collector. Simultaneously, the over-discharge severely damaged the electrode material and dissolved the SEI. Conner [7] over-discharged an 18,650 lithium-ion battery to 200% DOD (deep of discharge), at voltages of −1.3 and −1.5 V, they observed the onset of copper dissolution, and the maximum negative potential to be approximately 4.8 V. Copper deposition was observed for all of the over-discharged cell structures investigated. At the rate of 1 C, the cell surface temperature reached up to 79 °C. The concentration polarization and SEI rupture observed to occur prior to copper dissolution were determined to be the primary causes of the heat generation during over-discharge. Wang et al. [12] used different rates of 0.2, 0.5, 1, and 2 C to over-discharge batteries in an adiabatic environment. When the discharge rate was increased, the batteries reached the highest temperature. However, fire, explosion, or thermal runaway did not occur in their study. Ma et al. [13] found that electrode deformation resulted in structural expansion after 15 over-discharge cycles. In their study, increasing the number of over-discharge cycles correspondingly increased the electrode deformation, which consequently increased the battery resistance and short-circuit risk. In addition, they found that the average strength of Li decreased by 13%, and that the half-peak width of Li distribution increased by 2%, suggesting the increased randomness of the lithium distribution, which reduced the utilization rate of active lithium. Kim [14] found that the large current near the short-circuit point generated a large amount of Joule heat, causing the temperature to rapidly increase. The voltage rapidly declined and slowly recovered, suggesting the occurrence of a recoverable micro short-circuit. Lastly, they found that the high local temperatures reduced the internal polarization of the battery.
A BMS (battery management system) can balance battery discharge to avoid intense over-discharge. However, if the batteries have an inconsistency, it will cause a capacity difference of the batteries in the battery pack and the slight over-discharge will still occur. The effect of high-rate cycle aging and over-discharge with a current of 2 C was studied. The voltage, capacity, temperature, and impedance of the battery were studied continuously throughout the cycling process. The aged battery was disassembled, and the internal structure of the battery was observed using SEM. Additionally, the safety of batteries with different aging degrees was studied using an adiabatic accelerated calorimeter.

2. Experiments

2.1. Samples

In the experiments, 18,650 lithium-ion batteries from BAK (standard capacity: 2.9 Ah) were utilized. The positive material was NCM, and the negative material was graphite. The main component of the electrolyte was LiPF6, and that of the separator was PP/PE/PP. According to the battery specifications provided by the manufacturer, the standard operating voltage range was 2.5–4.2 V. The battery had a capacity greater than 80% after 1000 normal cycles, and the maximum charge and discharge rates were 1 C and 3 C, respectively. In total, three discharge–charge cycles were carried out at a rate of 0.5 C, and batteries with similar capacities were selected for parallel experiments.

2.2. Cycles

The cycle current was set at 2 C to investigate the effects of high-rate aging and over-discharge. First, the batteries were charged by applying a constant current–constant voltage method (CC–CV) at a current of 5.8 A. The cutoff voltage and current were 4.2 V and 0.25 A, respectively. After being allowed to rest for 30 min, the battery was discharged at 5.8 to 2.5 V, which is considered to be 100% DOD. The initial capacity at a high discharge rate was 2.3980 Ah. Thereafter, the discharge end-off voltage was set to 1 V for every cycle, and the discharge capacity was recorded after each cycle. This aging test was conducted as the temperature was maintained at 25 °C over 130 cycles.

2.3. Impedance

An electrochemical workstation (Zahner, Germany) was used to measure the battery with 0% SOC. The EIS measurements were conducted under the conditions of a frequency range from 10 mHz to 100 KHz, scanning from high to low frequency. To ensure high measurement accuracy, each battery was discharged and allowed to rest before being subjected to the EIS test.

2.4. Thermal Runaway

An adiabatic accelerated calorimeter (ARC) (Hel Limited, UK) was used to conduct the thermal runaway experiment. The design of the thermal runaway experimental platform is shown in Figure 1 [15]. The ARC “Adiabatic Test” mode was applied to detect the amount of heat released from the battery. The ARC heated the battery to the initial temperature. Then, following a period of calibration, the fully charged battery was discharged to 1 V, and the system was set to detection mode.
The battery was determined to be self-heating if the detected rate of battery temperature increase exceeded the preset temperature sensitivity. The temperature of the ARC chamber was synchronized with the battery temperature to prevent heat loss and provide an insulated environment. All changes in the rate of heat release and temperature of the battery were recorded. The parameter settings for the “Adiabatic Test” mode are listed in Table 1.

2.5. Material Characterization

The battery was disassembled after 130 cycles. The micro-morphology of each battery was analyzed by SEM (Zeiss, Germany), and EDS was used to analyze the material components. A thin gold layer was sputtered onto the surface of the material to increase its electrical conductivity and obtain better imaging quality.

3. Results and Discussion

3.1. Effects of Aging on Battery Performance

A total of 130 cycles were applied to each battery as the voltage, capacity, impedance, and temperature data were collected. The results of the measurement are discussed in this section.

3.1.1. Voltage

The voltage curves during cycles are shown in Figure 2a,b. The positive and negative electrode of the battery became increasingly polarized as the number of cycles increased, this caused the initial charge (discharge) voltage of the battery to gradually increase (decrease). Significant differences between the charge and discharge voltage curves can be observed. The charge voltage platform gradually shifted to the left, and the duration of the CV phase was longer, indicating reduced battery-charging efficiency and capability. A shift to the left can also be observed in the discharge voltage curve, in which the rapidly decreasing discharge time indicates deteriorating battery discharge capability. The discharge can be divided into the following two stages: normal discharge and over-discharge. After the aging cycles, the voltage decreased at a faster rate during the normal discharge stage; during the over-discharge stage, the voltage rapidly decreased to 1 V, indicating that the over-discharge of the aged battery prevents the release of more electricity. After 130 cycles, the discharge curve was found to have a new voltage plateau at approximately 1.95 V; this is an abnormal signal that may indicate the occurrence of a micro-short circuit in the battery [11,16]. The voltage plateau changed because the micro-short circuit induced the current flow through the battery. However, the continuous discharge did not result in battery failure, indicating that the micro-short circuit was recoverable [8,17,18]. Similar results were obtained from the experimental data of the parallel groups.

3.1.2. Discharge Capacity

The relationship between the battery discharge capacity and number of cycles is shown in Figure 3. Stage I included the first five cycles wherein the discharge capacity gradually increased, reaching a maximum discharge capacity of 2.867 Ah. The discharge capacity increased at a rate of approximately 19.5%, implying an increase in the rate at which active lithium ions were released [19]. During Stage II, i.e., from the 6th to 55th cycles, the discharge capacity began to decay, following an approximately linear trend. Owing to the self-healing ability of the battery, which allows it to recover its capacity [20,21], the discharge capacity was 2.404 Ah after the 55th cycle. The reduced capacity can be attributed to the irreversible damage to the structure of the electrode materials, which occurred in forms such as particle breakage and the delamination of the electrode materials that increased in severity as the cycle number increased [4,22,23]. It should also be noted that the continuous dissolution and generation of the SEI throughout the cycling period contributed to the loss of active lithium ions [24].
Additionally, the dehydration of the electrolyte increased the resistance [25] and rate of capacity decay. Stage III of the discharge decay was defined to include the cycles associated with when the capacity was less than the initial capacity of 2.3980 Ah, i.e., after the 56th cycle. By this cycle, the self-healing ability of the battery decreased and had deteriorated, resulting in the decrease in its capacity by approximately 0.42% after each cycle. After 32 cycles, the battery reached 80% SOH. The high rate of over-cycles accelerated the capacity attenuation and reduced the cycle life of the battery. However, setting a low platform voltage platform for discharge in a short time could restore the battery capacity [26] and enhance its performance, which is an aspect worth taking into consideration.

3.1.3. Impedance Spectrum

The electrochemical impedance results for the over-discharge cycling experiment are presented in Figure 4. In the high-frequency region, the point of intersection between the curve and real axis represents the ohmic impedance (Rb) of the battery. The high-frequency semicircle, mid-frequency semicircle, and low-frequency diagonal represent the impedance of lithium ions through the solid electrolyte (Rsei), the transfer impedance (Rct), and the coefficient for lithium-ion diffusion through the electrode material, respectively [27,28,29,30]. As shown in Figure 4, as the discharge cycle increased, the battery impedance initially increased and then decreased. This trend differs from that observed in previous studies, which demonstrated that the impedance continuously increased [31].
In the early cycles, electrolyte side reaction occurred, resulting in electrolytic reduction [9,32], lithium-ion consumption [33], and electrode-material particle rupture [28,34,35]. This reaction also hindered electron and ion transport, which ultimately increased Rb. Interestingly, the change in Rct was similar to that in Rb. With increasing battery circulation, the SEI became thicker, and the resistance to ions passing through the SEI increased [4,36]. The gradual decreases in Rb and Rct in the later cycles suggest the occurrence and increasing influence of the micro-short circuit in the battery [18]. Additionally, the electrode material fell off as a result of accumulating mechanical stress, consequently exposing the collector and reducing Rb. Micro-short circuits were also found to increase the battery temperature. These results indicate that at low voltages and high temperatures, the SEI can form or transform into relatively conductive components. Inorganic SEI components have been shown to be more conductive than their organic counterparts [25,37]; this characteristic contributed to reduce Rb and Rct during the final stage (i.e., Stage III). Thus, over-discharge can be presumed to have led to further degradation, SEI breakdown, and dissolution [10,38]. In addition, the battery temperature and the rate of electrolyte decomposition increased. The LiPF6 that was used in the electrolyte solution is prone to producing acidic substances (e.g., HF) that react with the SEI, thus accelerating the dissolution of the SEI [39] and reducing the Rsei. The extent to which the impedance was reduced after the aging experiment was found to differ from those previously reported. This is believed to be because of the strong effects of the micro-short circuits on the impedance. Thus, it is necessary to analyze the EIS of the battery in combination with the voltage measurement results to obtain an accurate understanding of the battery health and ensure its safe use.

3.1.4. Temperature

Figure 5a,b shows the typical thermal behavior of a lithium battery during the cycling process. Particularly, the figure shows the changes in the surface temperature and voltage during the charge and discharge periods. The battery degradation deteriorated the battery performance and intensified its thermal behavior. For example, as the cycle number increased during the CC stage, the maximum temperature significantly increased from 28.7 to 33.5 °C during the charging period, indicating more ion movement and embedded obstruction. During the discharge stage, the battery temperature significantly increased from 33.5 to 54.5 °C with increasing cycle number. During the first stage of over-discharge, the surface temperature increased at a rate of 0.33 °C/min, reaching a peak temperature of 33.5 °C. As the number of cycles increased, the damage to the electrode structure increased, consequently increasing the internal resistance. The increase in discharge temperature was attributed to the electrolyte and SEI decomposition [16,38]. After 120 cycles, the battery temperature was 40 °C. The battery was observed to exhibit severe thermal behavior once a new voltage plateau appeared; this was attributable to the generation and aggravation of micro-short circuits in the battery [14,40,41]. By the end of the 130th cycle, the temperature was increasing at a rate of 3.94 °C /min, and the voltage had dropped to 1 V. Additionally, the maximum temperature of the battery was 54.5 °C, but no dangerous thermal behavior was observed. The measured temperature results served to confirm the occurrence of micro-short circuits. Although over-discharge is generally not considered to be significant, its long-term effect at high power rates during battery operation should be taken into account.

3.2. Influence of the High-Rate Aging and Over-Discharge Cycle on Thermal Safety

Increasing the number of cycles was found to correspond to a sharp increase in discharge temperature, which can significantly reduce battery safety. Thus, an ARC was used to evaluate the over-discharge behavior of a battery based on its safety. This adiabatic over-discharge experiment was applied to batteries with 100%, 79.6% and 50.9% SOHs. The results are respectively shown in Figure 6a–c. The characteristic parameters of the battery high-rate aging and over-discharge thermal runaway are listed in Table 2, where the discharge end temperature is T1, the voltage drop temperature is Td, the maximum temperature of the thermal runaway is Tm, and the rupture temperature of the safety valve is T.
Under the adiabatic conditions, the non-over-discharged battery did not exhibit thermal runaway or short-circuit failure. However, the thermal runaway phenomenon occurred in the batteries with 79.6% and 50.9% SOHs; this resulted in battery fire and explosion. When the voltage of the 100% SOH battery dropped to 2.5 V, the temperature was 78.88 °C; the voltage further dropped to 1 V rapidly. The discharge end temperature was 86.1 °C. The rate of temperature increase (dT/dt), which can provide information on the intensity of the battery explosion, rapidly increased during the over-discharge stage, reaching a maximum of 4.72 °C/min; additionally, the maximum temperature was 88.69 °C under adiabatic conditions. Consequently, the battery temperature decreased, and no thermal runaway was observed; these results indicate that a new battery is safe in an over-discharged state.
In the case of the battery with 79.6% SOH, the temperature was 77.31 °C at a normal discharge of 2.5 V. With the over-discharge of 1 V, the battery temperature was 82.88 °C, and the rate of temperature increase was 5.11 °C/min. The voltage increased after the battery discharge. After 5405 s, the voltage dropped from 3.364 to 0.809 V. As the temperature increased to the melting point of the separator, the localized contact between the anode and cathode prevented the reversal of the internal short circuit. At this point, the temperature was 120.66 °C, the voltage subsequently fluctuated until it became constant at 0 V. This fluctuation reflected the process of separator fusion. Additionally, the short circuit caused the battery to generate a large amount of heat, which rapidly increased the temperature [42] and promoted the occurrence of thermal runaway. The maximum temperature of the thermal runaway, Tm was 594.38 °C, and the maximum rate of temperature increase was 436.18 °C/min. The primary sources of heat for the thermal runaway were (1) SEI decomposition [6] resulting from a reaction between the negative electrode and electrolyte [43], (2) the Joule heat released by the short circuit, and (3) the decomposition of the positive electrode reacting with the electrolyte [44]. During thermal runaway, when T increased to 153.39 °C, the temperature of the battery slightly decreased. Large amounts of gas were also produced by the battery at high temperatures [45], resulting in the rupture of the safety valve.
The discharge end temperature (T1) of the battery with 50.9% SOH was 76.77 °C. The deterioration of the battery health and shorter discharge period served to reduce the discharge end temperature of the over-discharged battery operated under adiabatic conditions. However, the rate of temperature increase was significantly higher, at 6.87 °C/min. After 3406 s, the voltage dropped from 3.528 to 0.336 V, and the separator (fuse) temperature was 98.36 °C, which is 22.3 °C lower than that of the battery with 79.6% SOH. Once the temperature reached 138.35 °C, the safety valve broke; furthermore, the maximum temperature of the battery reached 595.35 °C. These results provide insight into the deterioration of the separator over the course of the high-rate over-discharge cycling and relatively easy dissolution of the diaphragm in an adiabatic condition, which generated a short-circuit and increased the risk of thermal runaway.

3.3. Material Analysis Results

After the over-discharge cycling experiment, each battery was disassembled, and the changes in its internal components were analyzed. Figure 7a shows the upper and lower edges of the separator, which were severely deteriorated because of the advanced depletion of the electrolyte at the edges. Figure 7b shows the damage of the positive electrode material near the winding center. As can be seen in Figure 7c, there was clear evidence of shedding near the winding center and upper and lower edges of the battery; moreover, this shedding directly resulted in the exposure of the copper collector. The damage position of the negative material was similar and more severe than that of the positive material. When the battery was turned on, the copper collector was directly exposed, and the negative material was observed to be brittle. On the separator-facing negative side, a large amount of negative material exfoliation adhesion was observed. Additionally, brownish spots were observed on the non-adhered part; this is primarily attributable to the occurrence of inconsistent lithium plating, as shown in Figure 7d, which led to the generation of a micro-short circuit. The idea that over-discharging a battery could result in the generation of lithium deposits may seem to be counterintuitive because lithium deposits often occur during charging. However, over-discharge-induced degradation is known to increase the internal resistance of the cell. Furthermore, the increased overpotential generated during the charging process can drive the anode potential below zero and facilitate lithium plating [4]. Alternatively, the anode-facing part of the separator was observed to have inhomogeneity and discoloration, as shown in Figure 7e; these characteristics were more evident at the winding center. These characteristics are indicative of the thinning of this separator part. Thus, the conditions of over-discharge cycling at a high rate led to the battery components incurring severe damage, as well as degeneration at the winding center and edges. Therefore, samples of this part of the structure and material were extracted and cleaned with dimethyl carbonate for further SEM observation.

3.3.1. Positive Electrode

SEM was applied to the disassembled NCM anode. Figure 8 shows how the material was partially peeled off of the surface, which is indicative of structural damage to the anode. This damage mode also increased the surface area, suggesting that the battery consumed more active lithium ions to form a CEI. The magnified image presented in Figure 8b shows that the NCM particles became loose, and that some NCM particles were broken. This was found to be due to (1) the conditions of over-discharging contributing to increase the number of active lithium ions in the electrolyte, and (2) more lithium ions being embedded into the NCM particles during charging. The particles began to over-expand, which eventually led to rupture. In addition, the decomposition of the NCM particle structure increased the solid-phase transport resistance and reduced the capacity retention of the battery [46]. The breakage of particles contributed to the establishment of direct contact between the electrolyte and electrode, which, in turn increased the temperature during cycling. Moreover, some lamellar material was found to be covering the surface of the NCM electrode. EDS analysis was used to determine that this material was Al-layered material, as shown in Figure 8c,d. This Al-based material is believed to have been Al2O3 material that had fallen off of the separator, which suggests that the separator incurred significant damage and that there was a higher short-circuit risk.

3.3.2. Negative Electrode

Figure 9a shows the severe damage to the graphite cathode. As can be seen, there were numerous cracks distributed across the surface of the electrode, and most of the surface material was completely peeled off; delamination was also observed to occur. These changes reduced the thickness of the electrode material, ultimately exposing the copper collector. The SEI layer consisted of a mixture of different compounds, such as lithium fluoride (LiF), lithium hydroxide (LiOH), lithium carbonate (Li2CO3), lithium oxide (Li2O), and lithium alkyl carbonate (ROCO2Li, RCOLi). The formation of these compounds increased the oxygen and fluorine contents and decreased the carbon content [47]. EDS was used to analyze the elemental composition; the results are illustrated in Figure 9b and summarized in Table 3. The carbon and oxygen contents were 20.96% and 69.53%, respectively, indicating that the over-discharge cycling process increased the thickness of the SEI on the surface of the cathode. Moreover, large particles were found to be deposited on the surface of the cathode. The results of the EDS analysis presented in Figure 9c show that the large particles were primarily composed of Ni. This indicates that the NCM particles were broken, and that the transition metal nickel was dissolved, transported though the separator, and deposited on the cathode surface. EDS analysis also confirmed significant increases in the P and F contents, the mixture of both elements after the high-rate over-discharge cycle. EDS analysis was deemed to be necessary for this study even though EDS cannot be used to detect lightweight elements such as H, He, and lithium. This is because lithium can be generated as a result of an oxidation reaction between lithium and air [4], which increases the oxygen content and reduces the carbon content. Thus, the higher oxygen content observed in the results confirmed the existence of lithium. Simultaneously, F, P, and O elements overlapped with boundaries, as shown in Figure 9d. Such conditions promoted the formation of lithium mixture deposits on the surface of the cathode, as was observed by Abe et al. [48]. The formation of these deposits indicates the loss of active lithium ions, which is known to accelerate battery capacity decay. Most importantly, these findings suggest reduced separator puncture resistance and short-circuit protection.

3.3.3. Separator

Figure 10a shows that a large amount of material was attached to the anode-facing part of the separator. The EDS image presented in Figure 10b shows that this material was from broken NCM particles; thus, the breakage of the particles of the anode material did not only affect the performance of the electrode but also obstructed the holes of the separator. Figure 10c shows the deposits that were observed on the cathode-facing part of the separator. Additionally, the graphite anode was found to be adhered to the separator, which further reduced the ion-passing area on the separator. Several cracks were observed on the surface of the exposed separator, which were formed as a direct consequence of the adhesion of graphite, leading to further stress on the separator and existing cracks. The separator was also found to be closed, confirming the occurrence of a short circuit in the battery. The results of the EDS analysis presented in Figure 10d show that this is the same as the deposition on the cathode, which is a mixture of P and F. In addition, the distribution of the deposition, including P and F, was only uniform across the exposed surface of the separator.

4. Conclusions

In this study, the behavior of cylindrical NCM811 batteries after high-rate aging and over-discharge was evaluated. The following conclusions can be drawn:
  • When the high-rate and over-discharge cycle was initiated, more active lithium ions were released, this contributed to the increased discharge capacity of the battery. However, the capacity decreased rapidly during the sixth cycle. After 130 cycles, the state of health was 50.9%, and a new voltage plateau appeared due to micro short-circuits in the battery. As the cycle increased, the battery impedance first increased and then decreased, and the occurrence of internal short circuits has an important impact. The maximum temperature of battery charging was 33.5 °C, the maximum temperature of battery discharging was 54.5 °C, and the temperature rise rate from 2.5 V discharge to 1 V was 3.94 °C/min.
  • In thermal runaway experiments under adiabatic conditions, thermal runaway did not occur on the normal discharged battery. However, after the high-rate over-discharge cycles, thermal runaway occurred in the batteries with 79.6% and 49.7% SOH. The voltage drop temperature Td was 120.66 °C and 98.36 °C, the separator melt-off temperature dropped 22.3 ℃, and fusing time was also shortened to 1999s. The rupture temperature of the safety valve T was 153.39 °C and 138.35 °C, with the SOH value decreases, the battery is more prone to gassing at high temperatures. The maximum temperature of the thermal runaway Tm was 594.38 °C and 595.33 °C, which indicates that the battery state of health has declined, and the safety valve has ruptured prematurely, but the severity of thermal runaway has not changed. A long-term high-rate over-discharge is harmful to the battery safety and increase the risk of thermal behavior.
  • After the high-rate aging and over-discharge cycle, the NCM811 battery was disassembled. The upper and lower edges and winding center of the battery were seriously degraded. NCM particles were broken. The positive and negative material will break and layer, and the copper collector will be exposed. It is also noteworthy that Ni deposits and deposits of a mixture of P and F were observed on the negative electrode surface. The Al2O3 coating of the separator was stripped. The pores of the separator were closed, which confirmed the occurrence of an internal short circuit. The damage to the battery material after the high-rate over-discharge cycle is irreversible, which also causes a potential safety hazard to the battery while reducing its service life.

Author Contributions

Conceptualization: L.J., X.L., L.Z. and Z.D.; methodology, software, validation, formal analysis, investigation, resources, data curation, writing—original draft preparation, writing—review and editing: T.Y.; supervision: L.Z. and Z.D. All authors have read and agreed to the published version of the manuscript.

Funding

National Key Research and Development Program of China (Grant No. 2017YFB0102004); Natural Science Foundation of Shandong Province (Grant No. ZR2020ME019).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental platform for thermal runaway [15].
Figure 1. Experimental platform for thermal runaway [15].
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Figure 2. (a,b) The charge/discharge voltage and capacity curve of a -type NCM811 battery during 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V.
Figure 2. (a,b) The charge/discharge voltage and capacity curve of a -type NCM811 battery during 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V.
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Figure 3. The discharge capacity changes of a -type NCM811 battery during 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V.
Figure 3. The discharge capacity changes of a -type NCM811 battery during 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V.
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Figure 4. The impedance variation of a -type NCM811 battery during 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V.
Figure 4. The impedance variation of a -type NCM811 battery during 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V.
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Figure 5. (a,b) Voltage and battery surface temperature curve of a -type NCM811 battery during 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V.
Figure 5. (a,b) Voltage and battery surface temperature curve of a -type NCM811 battery during 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V.
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Figure 6. Temperature, voltage, and temperature rise rate (dT/dt) variation of -type NCM811 batteries with (a) 100% SOH, (b) 79.6% SOH and (c) 50.9% SOH in the adiabatic environment.
Figure 6. Temperature, voltage, and temperature rise rate (dT/dt) variation of -type NCM811 batteries with (a) 100% SOH, (b) 79.6% SOH and (c) 50.9% SOH in the adiabatic environment.
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Figure 7. Disassemble image of a -type NCM811 battery after 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V. (a) Disassembled battery. (b) Positive electrode. (c) Negative electrode. (d) Separator facing the negative electrode. (e) Separator facing the positive electrode.
Figure 7. Disassemble image of a -type NCM811 battery after 130 cycles at the cycle ratio of 2 C with the cut-off voltage of 1 V. (a) Disassembled battery. (b) Positive electrode. (c) Negative electrode. (d) Separator facing the negative electrode. (e) Separator facing the positive electrode.
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Figure 8. (a–c) SEM micrographs with different magnifications obtained from the NCM electrode of a -type NCM811 battery after 130 cycles. (d) The EDS diagram of the marked part from (c).
Figure 8. (a–c) SEM micrographs with different magnifications obtained from the NCM electrode of a -type NCM811 battery after 130 cycles. (d) The EDS diagram of the marked part from (c).
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Figure 9. (a–c) SEM micrographs with different magnifications obtained from the negative electrode of a -type NCM811 battery after 130 cycles. (d) The EDS diagram of the marked part from (c).
Figure 9. (a–c) SEM micrographs with different magnifications obtained from the negative electrode of a -type NCM811 battery after 130 cycles. (d) The EDS diagram of the marked part from (c).
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Figure 10. SEM micrographs of electrodes from the disassembled battery (a) obtained from the separator facing the positive electrode, (b) the EDS diagram of the marked part from (a). (c) Obtained from the separator facing the negative electrode, (d) the EDS diagram of the marked part from (c).
Figure 10. SEM micrographs of electrodes from the disassembled battery (a) obtained from the separator facing the positive electrode, (b) the EDS diagram of the marked part from (a). (c) Obtained from the separator facing the negative electrode, (d) the EDS diagram of the marked part from (c).
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Table
Table 1. ARC parameter settings for the “Adiabatic Test” mode.
Table 1. ARC parameter settings for the “Adiabatic Test” mode.
NumberParameterValue
1Initial temperature25 °C
2Calibration time120 min
3The sensitivity0.01 °C/min
4The maximum temperature to track250 °C
Table
Table 2. Thermal runaway characteristic parameters of NCM811 battery with different SOH.
Table 2. Thermal runaway characteristic parameters of NCM811 battery with different SOH.
100%79.6%50.9%Annotation
T2.5 (°C)78.8878.8859.85temperature at voltage = 2.5 V
T1 (°C)86.182.8876.77temperature at voltage = 1 V
Td (°C)/120.6698.36temperature at voltage = 0 V
T (°C)/153.39138.35Safety valve rupture temperature
Tm (°C)88.69594.38595.35Maximum temperature
t (s)/54053406Time at voltage = 0 V
dT/dt4.725.116.87Temperature rise rate at end discharge
dT/dt (max)4.72436.18416.29Maximum rate of temperature rise
Table
Table 3. The element content obtained from the EDS at the marked negative position in Figure 9b.
Table 3. The element content obtained from the EDS at the marked negative position in Figure 9b.
Elementwt%
C20.96
O69.53
F7.24
P1.32
S0.31
Ni0.54
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Yin, T.; Jia, L.; Li, X.; Zheng, L.; Dai, Z. Effect of High-Rate Cycle Aging and Over-Discharge on NCM811 (LiNi0.8Co0.1Mn0.1O2) Batteries. Energies 2022, 15, 2862. https://doi.org/10.3390/en15082862

AMA Style

Yin T, Jia L, Li X, Zheng L, Dai Z. Effect of High-Rate Cycle Aging and Over-Discharge on NCM811 (LiNi0.8Co0.1Mn0.1O2) Batteries. Energies. 2022; 15(8):2862. https://doi.org/10.3390/en15082862

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Yin, Tao, Longzhou Jia, Xichao Li, Lili Zheng, and Zuoqiang Dai. 2022. "Effect of High-Rate Cycle Aging and Over-Discharge on NCM811 (LiNi0.8Co0.1Mn0.1O2) Batteries" Energies 15, no. 8: 2862. https://doi.org/10.3390/en15082862

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