# Revealing the Impact of High Current Overcharge/Overdischarge on the Thermal Safety of Degraded Li-Ion Batteries

Authors: Liqin Qian; Yahui Yi; Wenjing Zhang; Chenlong Fu; Chengyu Xia; Tiancai Ma
Journal: International Journal of Energy Research
 (2023)

---

## Abstract

To analyze the impact of two commonly neglected electrical abuse operations (overcharge and overdischarge) on battery degradation and safety, this study thoroughly investigates the high current overcharge/overdischarge effect and degradation on 18650-type Li-ion batteries (LIBs) thermal safety. Based on the temperature-voltage behavior and induced thermal runaway (TR) mechanisms, the overcharge and overdischarge-triggered TR processes are divided into four and three stages. Furthermore, the degradation effect is analyzed by analyzing the incremental capacity-differential voltage curves. During the high current cycling process, lithium inventory decreases significantly. Besides, the active material decreases when the battery degrades to a certain level. Lithium plating is the primary reason for lithium inventory loss; the plated lithium grows with the increment of degraded/overcharged level. Besides, the dissolution and deposition affect the internal short circuit degree, which can be observed from the electrode potential and cell voltage value. Moreover, battery cells undergo different degradation degrees, and different current rates of charging/discharging exhibit similar temperature-rising trends during the adiabatic TR tests. However, with the degradation degree increase, battery capacity fades, TR becomes easier to be triggered by the high current rate, and TR reactions are severe. This study guides early quantitative detection, safer battery cell design, and enhanced thermal safety management.


---

## Body

## 1. Introduction

The carbon neutrality proposal has accelerated the electric transportation transition in recent years, promoting electric vehicle (EV) development [1–3]. Li-ion battery (LIB) cell, with the distinct merits of higher power density and long cycling lifespan, has received wide application and commercialization in the transportation industry, electronic products, and energy storage stations [4–6]. As the primary power source of EVs, LIB safety and duration performance have a crucial impact on the popularization and application of EVs [6–9]. However, the degradation effect and safety issues limit the LIBs’ further application [10]. LIB degradation is determined by many factors, such as cycling operation, ambient temperature, and large current rates [11–13]. The thermal characteristics and stability change during the aging process, far from the fresh battery cell [14–16].The LIB degradation mechanism and thermal safety issues are complicated [17]. As the most commonly utilized LIB type, graphite is widely applied as the anode material for LIBs [18]. With the increasing charging speed requirement, fast charging technology has attracted public attention [19]. However, the thermal safety accident number also increases with the popularization of fast charging technology [20]. Besides, large current rates accelerate the aging process [21, 22].There are many previous studies conducted on the degradation mechanism of LIBs. The vacant lattice structure is proved to be intercalated by lithium ions while charging [23]. A protective solid electrolyte interphase (SEI) layer is generated between the anode and the electrolyte through the electrolyte decomposition, which is lithium-ion conductive but electrically insulating [5]. The cathode is the primary lithium-ion source after cell assembly. Two-dimensional layered LiMO2 (M=Ni, Mn, Co, Al) is commonly utilized as the cathode material. However, local volume expansion occurs during the aging process because active material is covered by plated lithium, increasing mechanical stress. Higher mechanical stress leads to the active material cracking during the charging-discharging cycles, which causes the loss of active material (LAM). The LAM effect limits the active sites and reduces the anode capacity [24]. Due to the structural tension of the oxygen lattice, the cathode is prone to degradation, especially during the charging state (delithiated) [25–28]. A similar protective cathode electrolyte interphase (CEI) film is formed on the cathode surface [29, 30]. The SEI and CEI formation are different types of favorable electrolyte aging reactions that prevent uncontrolled electrolyte/electrode decomposition [31, 32].Thermal analysis has been conducted to evaluate the thermal stability, thermally initiated reactions, reaction products, and material compositions [33]. Thermal profiling was analyzed based on the thermally induced weight loss (thermogravimetric analysis, TGA) and the decomposition products detection (evolved gas analysis-mass spectrometry, EGA-MS) while continuously heating a sample [33]. Exothermic and endothermic reactions can be detected by analyzing the heat flow with differential scanning calorimetry (DSC), which helps assign the mass loss and intrinsic reactions [34]. Furthermore, the mass spectrometer is essential to obtain general information about decomposition zones. Bak et al. [35] utilized the intrinsic oxygen loss of the cathode or the CO2 detection to indicate the organic material loss considering CO2 as the thermal decomposition product.Apart from the change in electrochemical characteristics, the thermal characteristics of the LIBs also change during the degradation process. Thermal characteristic changes during the aging process are affected by the degradation paths, degradation mechanism, and battery material [33, 36, 37]. It has been proved that LIBs degraded under high-temperature storage exhibited enhanced thermal stability (higher self-heat generation temperature T1 and lower temperature rate during the thermal runaway test) [38, 39]. After cycling at ambient or high temperatures, lower T1 and thermal runaway triggering temperature T2 and slightly increased self-heat generation rate were found [37, 40]. The dramatic deterioration of thermal stability could be observed after low-temperature cycling with decreased T1 and T2 and increased self-heating rate [37, 41–44].Based on the above analysis, thermal characteristic change during the degradation process varies on the aging history and environment. However, previous studies primarily focus on thermal performance under a specific operation. To fully acquire the thermal mechanism under complex operations for battery safety design, thermal performance under various paths should be investigated.In this study, the aging effect on the heat generation of a Li-ion battery during fast charging is investigated. A series of heat insulation fast charging tests on the cells degraded under different degradation paths are conducted. The correlations between the degradation mechanism and the thermal characteristic change are revealed through postmortem analysis of the aged electrodes. Besides, the thermal runaway behavior of the aged battery cells induced by slight overcharge is analyzed. This study comprehensively explains various side reaction effects on the battery thermal characteristic. It guides the safety design and management of Li-ion batteries during their lifespan in real applications. Besides, the temperature-voltage changing trend analysis guides TR warning.


## 2. Experiments

This study tests and analyzes a commercial 18650-type cylindrical lithium-ion battery cell with a 3.5 Ah nominal capacity. The cathode material of the fresh cell is tested as Li0.92 (Ni0.84, Mn0.05, and Co0.11) O2 through inductively coupled plasma- (ICP-) optical emission spectrometry during the discharging process. The carbon-hydrogen nitrogen analyzer determines the anode material as 97 wt% carbon, 2 wt% silicon, and negligible hydrogen, sulfur, and nitrogen. The liquid absorption approach measures the cathode/anode loading as 0.0486 and 0.0271 g/cm2, respectively. Gas chromatography-mass spectrometry (GC-MS) determines the electrolyte and electrolyte additives, which are described in Table 1.Table 1
Electrolyte composition of the tested cells.
Electrolyte componentCompositionSoluteEthylene carbonate/dimethyl carbonate/ethyl methyl carbonate (EC/DMC/EMC)SolventLithium hexafluorophosphate (LiPF6)AdditivesVinylene carbonate and propylene sulfite


### 2.1. Accelerating Aging Tests

An accelerating aging test is designed to characterize the battery capacity degradation evolution. The cycling process is set within the operational voltage range (2.7-4.2 V). Besides, a reference performance test is carried out to measure the battery capacity. The discharging capacity at the third cycle is defined as the battery capacity. All the tests are carried out using the battery test machine (Neware BTS4000). These tests are demonstrated in Table2:Table 2
Test procedures settings.
Test typeTest proceduresReference performance test (25°C)(1) Constant current (CC) charging to 4.2 V under 1/3C and then constant voltage (CV) charging under 4.2 V until current decreased to <1/20C(2) Rest for 30 mins(3) Discharging to 2.5 V at 1/3C(4) Rest for 30 minsNote: The reference performance test lasts for three cycles.2C cycling at 25°C(1) Constant current (CC) charging to 4.2 V under 2C and then constant voltage (CV) charging under 4.2 V until current decreased to <1/20C(2) Rest for 30 mins(3) Discharging to 2.5 V at 2C(4) Rest for 30 minsThe state of health (SOH) value is defined as(1)SOH=QQ0×100%,where Q represents the discharging capacity, and Q0 represents the nominal capacity.


### 2.2. Adiabatic Charging/Discharging Tests

To analyze the thermal characteristics evolution, battery cells are charged/discharged with different current rates and various degradation states under an adiabatic environment (with 50 mm thick heat insulation cotton covered).As demonstrated in Figure1, three thermocouples are attached on the cathode tab side, anode tab side, and the middle surface part of the cell to measure the real-time temperature change during the adiabatic charging/discharging process.Figure 1
Adiabatic charging/discharging test setup.


### 2.3. Overcharge/Overdischarge-Induced Thermal Runaway Tests

With the continuous cycling degradation process, battery capacity fades. Slight overcharge/overdischarge tests are carried out to trigger thermal runaway (TR) for thermal stability evaluation analysis and define the constant current charging/discharging profiles based on the theoretical capacity, which is closer to the triggering factor in real EV accidents. The overcharge/overdischarge tests are carried out under a fully charged (4.2 V)/fully discharged (2.7 V) state, which is also conducted under an adiabatic environment (with 50 mm thick heat insulation cotton covered).


## 2.1. Accelerating Aging Tests

An accelerating aging test is designed to characterize the battery capacity degradation evolution. The cycling process is set within the operational voltage range (2.7-4.2 V). Besides, a reference performance test is carried out to measure the battery capacity. The discharging capacity at the third cycle is defined as the battery capacity. All the tests are carried out using the battery test machine (Neware BTS4000). These tests are demonstrated in Table2:Table 2
Test procedures settings.
Test typeTest proceduresReference performance test (25°C)(1) Constant current (CC) charging to 4.2 V under 1/3C and then constant voltage (CV) charging under 4.2 V until current decreased to <1/20C(2) Rest for 30 mins(3) Discharging to 2.5 V at 1/3C(4) Rest for 30 minsNote: The reference performance test lasts for three cycles.2C cycling at 25°C(1) Constant current (CC) charging to 4.2 V under 2C and then constant voltage (CV) charging under 4.2 V until current decreased to <1/20C(2) Rest for 30 mins(3) Discharging to 2.5 V at 2C(4) Rest for 30 minsThe state of health (SOH) value is defined as(1)SOH=QQ0×100%,where Q represents the discharging capacity, and Q0 represents the nominal capacity.


## 2.2. Adiabatic Charging/Discharging Tests

To analyze the thermal characteristics evolution, battery cells are charged/discharged with different current rates and various degradation states under an adiabatic environment (with 50 mm thick heat insulation cotton covered).As demonstrated in Figure1, three thermocouples are attached on the cathode tab side, anode tab side, and the middle surface part of the cell to measure the real-time temperature change during the adiabatic charging/discharging process.Figure 1
Adiabatic charging/discharging test setup.


## 2.3. Overcharge/Overdischarge-Induced Thermal Runaway Tests

With the continuous cycling degradation process, battery capacity fades. Slight overcharge/overdischarge tests are carried out to trigger thermal runaway (TR) for thermal stability evaluation analysis and define the constant current charging/discharging profiles based on the theoretical capacity, which is closer to the triggering factor in real EV accidents. The overcharge/overdischarge tests are carried out under a fully charged (4.2 V)/fully discharged (2.7 V) state, which is also conducted under an adiabatic environment (with 50 mm thick heat insulation cotton covered).


## 3. Results and Discussion

As demonstrated in Figures2 and 3, adiabatic battery temperature profiles under different degradation degrees (100%, 90%, 80%, 70%, and 60%) are acquired. Besides, thermal responses under various fast charging and fast discharging current rates (1C, 2C, and 4C) are compared and analyzed.Figure 2
Adiabatic temperature profiles of the degraded cells under various fast charging rates ((a) Fresh cell, (b) 90% SOH, (c) 80% SOH, (d) 70% SOH, and (e) 60% SOH).
(a)(b)(c)(d)(e)Figure 3
Adiabatic temperature profiles of the degraded cells under various fast discharging rates ((a) Fresh cell, (b) 90% SOH, (c) 80% SOH, (d) 70% SOH, and (e) 60% SOH).
(a)(b)(c)(d)(e)It can be observed that a high current rate causes a more apparent temperature rising under the same degradation state. Furthermore, TR is easier to be triggered by high current rates since the electrochemical reaction temperatures are more likely to be reached under severe heat generation. Moreover, fast charging increases the temperature peak level and the thermal accident percentage compared with the high current discharging operations, which results from the lithium plating issue and more energy released from the battery cell under a high SOC state.To statistically analyze the thermal characteristics of the tested cells during the adiabatic charging/discharging, maximum temperature (Tmax) and the mean temperature rising rate (vT) are defined in this study, which is described as the following equations:
(2)Tmax=MaxTi,i=1,2,3,(3)vT=Tmax−TinitialΔt,where Ti represents the temperature value of the ith measuring position; Tinitial represents the initial temperature value; ∆t represents the period between the initial and maximum temperature time.


### 3.1. Degradation Effect

Adiabatic temperature rising curves of the different degraded cells under different charging/discharging current rates are compared in Figures4 and 5. With the degradation process, the capacity fades. Overcharge and overdischarge tend to occur, which even induce thermal runaway (TR).Figure 4
Comparison of the adiabatic temperature rising curves between the different degraded cells under different charge currents ((a) 1C, (b) 2C, and (c) 4C).
(a)(b)(c)Figure 5
Comparison of the adiabatic temperature rising curves between the different degraded cells under different discharge currents ((a) 1C, (b) 2C, and (c) 4C).
(a)(b)(c)With a large current rate (≥1C), the heat generation of the battery can be described as [45]
(4)MCpdTdt≈I⋅U−E−I⋅TdEdT+K⋅I3−Q̇dis,where M, Cp, T, U, and I represent the mass weight, heat capacity, temperature, open-circuit voltage, and current, respectively. The current is defined as a positive value when the battery is charged. Q̇dis represents the heat dissipation power of the battery.The thermal stability of the battery material fades during the degradation process, and battery capacity also fades, which induces overcharge and overdischarge. As described in Equation (4), when the battery cells are charged/discharged with large current rates, rapid heat generation quickly reaches the SEI film decomposition temperature, which induces the chain thermal-electrochemical reactions, finally triggering TR.Incremental capacity-differential voltage (IC-DV) curves of the tested cells are shown in Figure6.The available charging/discharging capacity decreases during the aging process due to the active lithium ion loss. Besides, it can be observed that the operational voltage also fades with a more degraded state, which results from the active material loss of the electrodes. Some significant peaks and valleys can be observed in Figures 6(a) and 6(b), reflecting the redox reactions and phase change. The IC curves exhibit similar shapes. However, the voltage shifts to a lower level, representing slight CL. The corresponding valleys shift to lower voltage, and the magnitude decreases, representing the LAM degradation mode.Figure 6
IC-DV analysis after different high current degraded degrees. (a, b) IC analysis during the charging/discharging process; (c, d) DV analysis during the charging/discharging process.
(a)(b)(c)(d)Similarly, DV curves shift to a lower capacity magnitude indicating that LLI happens. Moreover, apart from the DV curves shifting to lower capacity, the peak magnitude slightly rises. Therefore, LAM degradation mode also occurs in this process. With the overcharge process, battery voltage gradually increases. All the cells are overcharged under almost the same trend. Finally, the cells reach different SOCs since the capacity fades. Moreover, with the current rate increase, a rapid voltage drop occurs with higher frequency due to the large-scale internal short circuit (ISC) during the TR process.The relationship between the curve parameter change and the degradation mode can be identified by comparing the peak/valley position and magnitude in the IC and DV curves, which can be depicted in Table3:Table 3
Relationship between the curve changes and the degradation mode.
Curve parametersChangeDegradation modeICShifting to a lower voltageConductivity loss (CL)DVShifting to a lower capacityLoss of lithium inventory (LLI)ICDecrement of the valley magnitudeLAMDVIncrement of the peak magnitudeLAM


### 3.2. High Current Rate Effect

Characteristic parameters (maximum temperature and mean temperature rising rate) between the different degraded cells under different charge/discharge current rates are depicted in Figures7 and 8. It is apparent that the current rate increment significantly increases the temperature rising rate. Besides, the fast charging current rate promotes the Li dendrite growth and the inserted active lithium-electrolyte reaction rate.Figure 7
Comparison of the characteristic parameters between the different degraded cells under different charging current rates ((a)Tmax and (b) vT).
(a)(b)Figure 8
Comparison of the characteristic parameters between the different degraded cells under different discharging current rates ((a)Tmax and (b) vT).
(a)(b)Some characteristic parameters are acquired to analyze the influence of high current overcharge/overdischarge on the degraded battery cells’ thermal safety. Considering the open experimental environment, TR triggering temperature (T2) and highest temperature (T3) are evaluated in this study. The separator collapse temperature mainly determines T2; T3 represents the total thermal energy released from the battery failure, calculated by Equation (5). However, due to the significant temperature difference between the surface and the center, the temperature value acquired in the experiments is not precisely the accurate inner temperature [46]. Besides, self-heat generation temperature T1 cannot be detected in the experiments. Furthermore, rapid temperature rises do not demonstrate apparent TR triggering temperature when the battery cells are charged/discharged with a high current rate.
(5)Q=Cp⋅M⋅T3−T1.As Figure4(a) and Figure 5(a) demonstrate, a large current rate of overcharge/overdischarge does not have a noticeable impact on T2,which represents a large current rate that cannot affect the polymer separator collapsing and internal short circuit, inside side reactions have a negligible effect on the separator melting temperature. Besides, under the same degraded state, TR is easier to be triggered by overcharge. With decreased thermal stability, overcharge induces lithium plating, and more energy is released during the TR process compared with the overdischarge.As depicted in Figures7(a) and 8(a), T3 during the overcharge process is generally higher than in the overdischarge process. However, for the overcharge/overdischarge process, the electrolyte is continuously consumed during the electrolyte decomposition and the electrolyte-metal lithium reactions. However, the total energy of the overcharged battery cell is higher than the overdischarged cell. Therefore, overcharge-induced TR is more severe compared with the overdischarge behavior.


### 3.3. Overcharge Behavior

A slight overcharge triggers complicated side reactions of the batteries, affecting the aging states and TR characteristics. The corresponding relationship is demonstrated in Figure9. Based on previous characterization and analysis of battery components, slight overcharge induces complex side reactions, including the plated lithium-electrolyte reaction, lithium plating, transition metal dissolution, and the electrolyte, CEI, and SEI decomposition. Active lithium is continuously consumed during the overcharging process, causing the LLI degradation mode. Meanwhile, the active materials are constantly consumed, commonly accompanied by gas generation in the side reactions, and the byproducts accumulate in this process [24].Figure 9
Relationship between the aging effect and thermal runaway induced by overcharge.Degraded battery cells exhibit lower thermal stability and self-heat generation temperature, mainly resulting from lithium plating and the plated lithium-electrolyte reactions. The plated metal lithium reacts with the electrolyte and generates massive heat at a lower temperature level, which is reactive compared with the lithium intercalated into the anode. As mentioned above, the lithium inventory and active material are consumed during the side reactions, which limit the total energy stored. Thus, the T3 of a more degraded or overcharged cell can be lower. Furthermore, continuous gas generation and accumulation inside significantly increase the internal pressure. The current interrupt device (CID) works when the internal pressure increases to the threshold. Therefore, as the overcharging or degraded state rises, the sharp voltage drop occurs earlier, and the temperature decreases.Taking the overcharging process of a 60% SOH state cell as a sample, the temperature-voltage curves are shown in Figure10. The cell voltage increases rapidly, mainly caused by lithium plating on the anode. With the continuous accumulation of the plated lithium, the anode potential significantly shifts; the cathode-anode potential difference increases, and the cathode is excessively delithiated. When the voltage value reaches 4.4698 V, it immediately drops. Under a high voltage state, oxidation and electrolyte decomposition occur [47], and the TR side reactions excessively consume the electrolyte and cannot support lithium-ion transportation between the electrodes. Therefore, the external current stimulation cannot maintain the battery voltage.Figure 10
Temperature-voltage curve of the overcharged cell (60% SOH).Overcharge-induced TR mechanism is briefly introduced in Figure11.The TR process can be divided into four stages:Figure 11
Scheme of overcharge-induced thermal runaway mechanism for LIBs.Stage I: overcharge increases the cell temperature and slightly decreases cell capacity, mainly caused by the metal ion (Li+, Mn2+, Co2+, and Ni2+) dissolution under high temperatures.Stage II: when the self-heating starts, the SEI film gradually decomposes. However, the active anode material cannot provide SEI film protection. The intercalated lithium reacts with the electrolyte. Side reactions accelerate heat generation. Besides, some gases are also generated during side reactions.Stage III: continuous gas generation increases the internal pressure to the threshold and the CID functions, and the cell voltage drops drastically. Chain side reactions and temperature rise promote each other; self-heat and gas generation becomes severe. When the threshold is reached, the valve opens, exhausting the gases, vaporized solvents and particles. Venting takes away part of the heat and slightly decreases the temperature. However, due to the violent exothermic side reactions, battery temperature rapidly recovers and rises to the separator melting temperature; the separator collapses and triggers a large-scale ISC.Stage IV: heat accumulation and higher temperature cause more violent exothermic reactions. The electrodes, electrolytes, and the other components decompose and induce drastic reactions, causing a sharp temperature rise. The TR process cannot be stopped at this stage. Moreover, massive smoke and combustion reactions may occur.


### 3.4. Overdischarge Behavior

Due to the unavoidable voltage inconsistency between the battery cells, the BMS cannot monitor the voltage of each cell. The cell with the lowest voltage level tends to be overdischarged, a commonly underestimated type of electrical abuse. Different aged state cells are tested with forcible discharge to analyze the overdischarge hazard for the degraded cells.During the overdischarge process, the pole reverses, and the voltage may become negative, causing abnormal heat generation. Besides, the overdelithiation of the anode results in the SEI decomposition, generating gases such as CO or CO2, which may cause the cell to swell [48]. Once recharged after overdischarging, new SEI regenerated on the anode surface will change the anode electrochemical properties [49]. Consequently, the resistance increases and capacity degrades [50]. Furthermore, overdischarge causes the copper collector dissolution, and the dissolved copper migrates and deposits on the anode surface [49], accelerating the capacity fading and causing an internal short circuit (ISC). Figure 12 illustrates the TR mechanism induced by overdischarge. When the anode potential reaches the copper dissolution potential (3.5 V), the copper can be electrolyzed as Cu2+, transmitted from the current collector to the electrolyte [51, 52]. The dissolved Cu2+ migrates through the membrane, leading to the copper dendrite growth on the cathode side, with a lower potential. Without suppression, the dendrite may penetrate the membrane, causing severe ISC. Finally, massive heat generation and accumulation trigger TR.Figure 12
Scheme of overdischarge-induced thermal runaway mechanism for LIBs.Figure13 depicts the 1C overdischarge process of a 60% SOH state cell, and the overdischarge process is divided into three stages according to the voltage variation characteristics:Figure 13
Temperature-voltage curve of the overdischarged cell (60% SOH).Stage I: overdischarge causes lithium-ion deintercalation from the anode and insertion into the cathode, resulting from the increasing and decreasing anode potential.Stage II: anode copper collector corrosion occurs, and the anode potential reaches an electrochemical reaction platform for the copper dissolution. Cu2+ dissolved in the electrolyte travels through the separator and deposits on the cathode; the reduction of Cu2+ leads to increased cathode potential.Stage III: with the continuous electrochemical reactions of copper dissolution and deposition, the internal short becomes more severe, with a decrease in RISC. The voltage value decreases and approaches zero due to the continuous overdischarge current and lower RISC.


## 3.1. Degradation Effect

Adiabatic temperature rising curves of the different degraded cells under different charging/discharging current rates are compared in Figures4 and 5. With the degradation process, the capacity fades. Overcharge and overdischarge tend to occur, which even induce thermal runaway (TR).Figure 4
Comparison of the adiabatic temperature rising curves between the different degraded cells under different charge currents ((a) 1C, (b) 2C, and (c) 4C).
(a)(b)(c)Figure 5
Comparison of the adiabatic temperature rising curves between the different degraded cells under different discharge currents ((a) 1C, (b) 2C, and (c) 4C).
(a)(b)(c)With a large current rate (≥1C), the heat generation of the battery can be described as [45]
(4)MCpdTdt≈I⋅U−E−I⋅TdEdT+K⋅I3−Q̇dis,where M, Cp, T, U, and I represent the mass weight, heat capacity, temperature, open-circuit voltage, and current, respectively. The current is defined as a positive value when the battery is charged. Q̇dis represents the heat dissipation power of the battery.The thermal stability of the battery material fades during the degradation process, and battery capacity also fades, which induces overcharge and overdischarge. As described in Equation (4), when the battery cells are charged/discharged with large current rates, rapid heat generation quickly reaches the SEI film decomposition temperature, which induces the chain thermal-electrochemical reactions, finally triggering TR.Incremental capacity-differential voltage (IC-DV) curves of the tested cells are shown in Figure6.The available charging/discharging capacity decreases during the aging process due to the active lithium ion loss. Besides, it can be observed that the operational voltage also fades with a more degraded state, which results from the active material loss of the electrodes. Some significant peaks and valleys can be observed in Figures 6(a) and 6(b), reflecting the redox reactions and phase change. The IC curves exhibit similar shapes. However, the voltage shifts to a lower level, representing slight CL. The corresponding valleys shift to lower voltage, and the magnitude decreases, representing the LAM degradation mode.Figure 6
IC-DV analysis after different high current degraded degrees. (a, b) IC analysis during the charging/discharging process; (c, d) DV analysis during the charging/discharging process.
(a)(b)(c)(d)Similarly, DV curves shift to a lower capacity magnitude indicating that LLI happens. Moreover, apart from the DV curves shifting to lower capacity, the peak magnitude slightly rises. Therefore, LAM degradation mode also occurs in this process. With the overcharge process, battery voltage gradually increases. All the cells are overcharged under almost the same trend. Finally, the cells reach different SOCs since the capacity fades. Moreover, with the current rate increase, a rapid voltage drop occurs with higher frequency due to the large-scale internal short circuit (ISC) during the TR process.The relationship between the curve parameter change and the degradation mode can be identified by comparing the peak/valley position and magnitude in the IC and DV curves, which can be depicted in Table3:Table 3
Relationship between the curve changes and the degradation mode.
Curve parametersChangeDegradation modeICShifting to a lower voltageConductivity loss (CL)DVShifting to a lower capacityLoss of lithium inventory (LLI)ICDecrement of the valley magnitudeLAMDVIncrement of the peak magnitudeLAM


## 3.2. High Current Rate Effect

Characteristic parameters (maximum temperature and mean temperature rising rate) between the different degraded cells under different charge/discharge current rates are depicted in Figures7 and 8. It is apparent that the current rate increment significantly increases the temperature rising rate. Besides, the fast charging current rate promotes the Li dendrite growth and the inserted active lithium-electrolyte reaction rate.Figure 7
Comparison of the characteristic parameters between the different degraded cells under different charging current rates ((a)Tmax and (b) vT).
(a)(b)Figure 8
Comparison of the characteristic parameters between the different degraded cells under different discharging current rates ((a)Tmax and (b) vT).
(a)(b)Some characteristic parameters are acquired to analyze the influence of high current overcharge/overdischarge on the degraded battery cells’ thermal safety. Considering the open experimental environment, TR triggering temperature (T2) and highest temperature (T3) are evaluated in this study. The separator collapse temperature mainly determines T2; T3 represents the total thermal energy released from the battery failure, calculated by Equation (5). However, due to the significant temperature difference between the surface and the center, the temperature value acquired in the experiments is not precisely the accurate inner temperature [46]. Besides, self-heat generation temperature T1 cannot be detected in the experiments. Furthermore, rapid temperature rises do not demonstrate apparent TR triggering temperature when the battery cells are charged/discharged with a high current rate.
(5)Q=Cp⋅M⋅T3−T1.As Figure4(a) and Figure 5(a) demonstrate, a large current rate of overcharge/overdischarge does not have a noticeable impact on T2,which represents a large current rate that cannot affect the polymer separator collapsing and internal short circuit, inside side reactions have a negligible effect on the separator melting temperature. Besides, under the same degraded state, TR is easier to be triggered by overcharge. With decreased thermal stability, overcharge induces lithium plating, and more energy is released during the TR process compared with the overdischarge.As depicted in Figures7(a) and 8(a), T3 during the overcharge process is generally higher than in the overdischarge process. However, for the overcharge/overdischarge process, the electrolyte is continuously consumed during the electrolyte decomposition and the electrolyte-metal lithium reactions. However, the total energy of the overcharged battery cell is higher than the overdischarged cell. Therefore, overcharge-induced TR is more severe compared with the overdischarge behavior.


## 3.3. Overcharge Behavior

A slight overcharge triggers complicated side reactions of the batteries, affecting the aging states and TR characteristics. The corresponding relationship is demonstrated in Figure9. Based on previous characterization and analysis of battery components, slight overcharge induces complex side reactions, including the plated lithium-electrolyte reaction, lithium plating, transition metal dissolution, and the electrolyte, CEI, and SEI decomposition. Active lithium is continuously consumed during the overcharging process, causing the LLI degradation mode. Meanwhile, the active materials are constantly consumed, commonly accompanied by gas generation in the side reactions, and the byproducts accumulate in this process [24].Figure 9
Relationship between the aging effect and thermal runaway induced by overcharge.Degraded battery cells exhibit lower thermal stability and self-heat generation temperature, mainly resulting from lithium plating and the plated lithium-electrolyte reactions. The plated metal lithium reacts with the electrolyte and generates massive heat at a lower temperature level, which is reactive compared with the lithium intercalated into the anode. As mentioned above, the lithium inventory and active material are consumed during the side reactions, which limit the total energy stored. Thus, the T3 of a more degraded or overcharged cell can be lower. Furthermore, continuous gas generation and accumulation inside significantly increase the internal pressure. The current interrupt device (CID) works when the internal pressure increases to the threshold. Therefore, as the overcharging or degraded state rises, the sharp voltage drop occurs earlier, and the temperature decreases.Taking the overcharging process of a 60% SOH state cell as a sample, the temperature-voltage curves are shown in Figure10. The cell voltage increases rapidly, mainly caused by lithium plating on the anode. With the continuous accumulation of the plated lithium, the anode potential significantly shifts; the cathode-anode potential difference increases, and the cathode is excessively delithiated. When the voltage value reaches 4.4698 V, it immediately drops. Under a high voltage state, oxidation and electrolyte decomposition occur [47], and the TR side reactions excessively consume the electrolyte and cannot support lithium-ion transportation between the electrodes. Therefore, the external current stimulation cannot maintain the battery voltage.Figure 10
Temperature-voltage curve of the overcharged cell (60% SOH).Overcharge-induced TR mechanism is briefly introduced in Figure11.The TR process can be divided into four stages:Figure 11
Scheme of overcharge-induced thermal runaway mechanism for LIBs.Stage I: overcharge increases the cell temperature and slightly decreases cell capacity, mainly caused by the metal ion (Li+, Mn2+, Co2+, and Ni2+) dissolution under high temperatures.Stage II: when the self-heating starts, the SEI film gradually decomposes. However, the active anode material cannot provide SEI film protection. The intercalated lithium reacts with the electrolyte. Side reactions accelerate heat generation. Besides, some gases are also generated during side reactions.Stage III: continuous gas generation increases the internal pressure to the threshold and the CID functions, and the cell voltage drops drastically. Chain side reactions and temperature rise promote each other; self-heat and gas generation becomes severe. When the threshold is reached, the valve opens, exhausting the gases, vaporized solvents and particles. Venting takes away part of the heat and slightly decreases the temperature. However, due to the violent exothermic side reactions, battery temperature rapidly recovers and rises to the separator melting temperature; the separator collapses and triggers a large-scale ISC.Stage IV: heat accumulation and higher temperature cause more violent exothermic reactions. The electrodes, electrolytes, and the other components decompose and induce drastic reactions, causing a sharp temperature rise. The TR process cannot be stopped at this stage. Moreover, massive smoke and combustion reactions may occur.


## 3.4. Overdischarge Behavior

Due to the unavoidable voltage inconsistency between the battery cells, the BMS cannot monitor the voltage of each cell. The cell with the lowest voltage level tends to be overdischarged, a commonly underestimated type of electrical abuse. Different aged state cells are tested with forcible discharge to analyze the overdischarge hazard for the degraded cells.During the overdischarge process, the pole reverses, and the voltage may become negative, causing abnormal heat generation. Besides, the overdelithiation of the anode results in the SEI decomposition, generating gases such as CO or CO2, which may cause the cell to swell [48]. Once recharged after overdischarging, new SEI regenerated on the anode surface will change the anode electrochemical properties [49]. Consequently, the resistance increases and capacity degrades [50]. Furthermore, overdischarge causes the copper collector dissolution, and the dissolved copper migrates and deposits on the anode surface [49], accelerating the capacity fading and causing an internal short circuit (ISC). Figure 12 illustrates the TR mechanism induced by overdischarge. When the anode potential reaches the copper dissolution potential (3.5 V), the copper can be electrolyzed as Cu2+, transmitted from the current collector to the electrolyte [51, 52]. The dissolved Cu2+ migrates through the membrane, leading to the copper dendrite growth on the cathode side, with a lower potential. Without suppression, the dendrite may penetrate the membrane, causing severe ISC. Finally, massive heat generation and accumulation trigger TR.Figure 12
Scheme of overdischarge-induced thermal runaway mechanism for LIBs.Figure13 depicts the 1C overdischarge process of a 60% SOH state cell, and the overdischarge process is divided into three stages according to the voltage variation characteristics:Figure 13
Temperature-voltage curve of the overdischarged cell (60% SOH).Stage I: overdischarge causes lithium-ion deintercalation from the anode and insertion into the cathode, resulting from the increasing and decreasing anode potential.Stage II: anode copper collector corrosion occurs, and the anode potential reaches an electrochemical reaction platform for the copper dissolution. Cu2+ dissolved in the electrolyte travels through the separator and deposits on the cathode; the reduction of Cu2+ leads to increased cathode potential.Stage III: with the continuous electrochemical reactions of copper dissolution and deposition, the internal short becomes more severe, with a decrease in RISC. The voltage value decreases and approaches zero due to the continuous overdischarge current and lower RISC.


## 4. Conclusions

In this study, the impact of high current overcharge/overdischarge and aging on the thermal safety of 18650-type batteries has been thoroughly investigated, guiding the safer battery cell design and thermal management.Voltage behaviors of the selected cells in the overcharging/overdischarging processes are investigated. Based on the voltage and induced TR behavior, the overcharging triggered TR process can be classified into four stages, and the mechanisms under each stage are clarified. Further, the aging effect is analyzed by analyzing capacity-voltage curves. During the overcharge/overdischarge process, LLI degradation mode exists, while LAM exists when the battery is overcharged to a certain level. Lithium plating is the primary reason for lithium inventory loss; the plated lithium grows with the overcharging level increment. Besides, the dissolution and deposition affect the internal short degree, which can be observed from the electrode potential and cell voltage value, which guides early quantitative detection and warning. With the degradation process, thermal stability, and overcharge/overdischarge resistance of the batteries worsening, the battery thermal management system should cooperate with the BMS to provide suitable real-time thermal management strategies.Adiabatic thermal response during the overcharge/overdischarge process and thermal runaway behaviors are investigated. Battery cell undergoes different degradation degrees, and the different current rate of charging/discharging exhibit a similar temperature rising trend. However, with the increase of the degradation degree and current rate, battery capacity fades, and TR becomes easier to be triggered.TR reactions are severe. Therefore, battery thermal stability is seriously attenuated.



---

*Source: 8571535-2023-02-11.xml*