Determination of High‐Temperature Float Charge Failure Mechanisms in Lithium‐Ion Batteries by Quantifying Active Lithium Loss

Abstract

ABSTRACT

Lithium‐ion batteries (LIBs) suffer from float charge failure in the grid‐scale storage market. However, the lack of a unified descriptor for the diverse reasons behind float charge failure poses a challenge. Here, a quantitative analysis of active lithium loss is conducted across multiple temperatures into float charge of Li(Ni_0.5Co_0.2Mn_0.3)O₂–graphite batteries. It is proposed that the active lithium loss can be used as a descriptor to describe the reasons for float charge quantitatively. Approximately 6.88% and 0.96% of active lithium are lost due to solid electrolyte interphase thickening and lithium deposition, which are primary and secondary failure reasons, respectively. These findings are confirmed by X‐ray photoelectron spectroscopy depth profiling, scanning electron microscope, and accelerating rate calorimeter. Titration‐gas chromatography and nuclear magnetic resonance are utilized to quantitatively analyze active lithium loss. Additionally, electrolyte decomposition at high temperatures also contributes to active lithium loss, as determined by Auger electron spectrum and nondestructive ultrasound measurements. Notably, no failure is detected in the cathode due to the relatively low working voltage of the float charge. These findings suggest that inhibiting active lithium loss can be an efficient way of delaying failure during high‐temperature float charge processes in LIBs.

Introduction

The development of an efficient electric energy network is crucial for achieving carbon neutrality goals and integrating renewable energy sources [1]. There have been persistent efforts since the early 19th century to incorporate rechargeable batteries into the modern electric energy network. Rechargeable batteries offer numerous advantages, such as managing peak demands with low costs, offering high energy density, and enabling the integration of renewable energy sources like solar energy, tide, and wind [2]. Among the current rechargeable batteries, lithium-ion batteries (LIBs) stand out for their fast response rate, high energy density, and reasonable cycle life [3–7]. Thus, an innovative electric energy network always involves stabilizing wind/solar power systems with LIB storage as backup power [8]. As the core of backup power, LIBs must always be in a long-term float charge condition to ensure quick response to large fluctuations in the electric energy network [3, 9]. Backup power systems are essential for providing immediate power during interruptions in the main supply. The float charge condition ensures that the battery is always fully charged, allowing it to provide power immediately when needed. Emergency backup power systems, such as uninterruptible power supplies (UPS) used in hospitals and data centers, depend on float charge conditions to ensure that batteries are always ready for use. Thus, maintaining the proper float charge conditions is very important to supporting the long-term stability of the battery. The float charge involves using a small current to slowly increase the depth of battery charging at the end of the charging process or compensate for self-discharge losses [9]. However, LIBs often experience high temperatures when float-charged (Figure 1). Elevated temperatures can accelerate aging and enable fast data collection of capacity decay [10]. Nevertheless, increasing the temperature also leads to an increase in the rate of side reactions in the electrolytes and electrodes, ultimately leading to thermal runaway of LIBs [11, 12]. Consequently, there is an urgent need to investigate the failure mechanisms of batteries after float charge at elevated temperatures and to mitigate the occurrence of unforeseen accelerated aging phenomena during batteries' operational phases.

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Various studies have been conducted to understand capacity decay in LIBs under high-temperature float charge conditions [13–15]. Tsujikawa et al. [16] examined LiMn₂O₄/graphite batteries at high temperatures and identified the thickening of solid electrolyte interface (SEI) as the cause of the float charge failure mechanism. Yin et al. [17] discovered that the float charge failure mechanism of LiFePO₄ pouch cells at different voltages resulted in the dissolution of the anode's active material and extensive lithium deposition. Zhao et al. [18] analyzed the capacity of LiFePO₄/graphite pouch cells at various temperatures and states of charge (SOC) and discovered that electrolyte decomposition and interfacial reaction intensified, leading to gas production. The studies all share a common mechanism: the active lithium in the battery is consumed, leading to battery failure during the float charge process. However, there are various sources of active lithium loss, and the main cause of failure is not clear. A quantitative analysis of the active lithium loss can effectively identify the main causes of failure and ultimately solve the problem [19, 20]. However, for float charge failure mechanisms, active lithium loss has not been quantitatively analyzed to determine the primary causes of the failure.

In this work, we investigated the high-temperature failure mechanism of Li(Ni_0.5Co_0.2Mn_0.3)O₂ (NCM523)–graphite (Gr) pouch cells under float charge by quantitatively analyzing active lithium loss. The NCM523/Gr was chosen as the model cell because the NCM523/Gr cell as a commercial battery system is commonly used in academic and industrial fields due to its advantages of high energy density, good cycle stability, cost-effectiveness, and thermal stability. NCM523 offered a well-rounded balance of energy density, cost, stability, and safety compared with other cathode materials, making it a preferred material for current lithium battery applications in float charge conditions. Gr is used as an anode material because of its low cost, long lifespan, good stability, and excellent thermal stability performance. Titration gas chromatography (TGC) and a 600 MHz solid-state nuclear magnetic resonance (NMR) spectrometer were used on the anode to quantify the active Li and plating Li after float charge. An accelerating rate calorimeter (ARC) was used to determine the thermodynamic behavior during float charge. By setting active lithium loss as a descriptor, we found that the thickening of SEI is the main reason for capacity decay at 65°C float charge, of which the active lithium loss from the SEI accounted for 87.76% of the total loss. We determined that the active lithium loss was associated with the anode's SEI instability and lithium plating. These findings provide a comprehensive understanding of the degradation behavior of float charge. More importantly, active lithium loss is proposed as a descriptor for quantitative assessment of the float charge failure under high temperatures.

Results and Discussion

Electrochemical Properties of Float Charge at Different Temperatures in NCM523/Gr Pouch Cells

Pouch-type NCM523/Gr cells with a design capacity of 570 mAh were assembled using three cathode sheets and four anode sheets (Figure 2A,B). The NCM523/Gr pouch cells at 25°C and 45°C float charge showed stable capacity retention rates of 96.8% and 92.59% after 50 days, respectively. However, at 65°C float charge, the pouch cell's capacity retention rate rapidly decreased to 80.05% after 30 days (Figure 2C). These results indicate that at 65°C, the battery is experiencing an abnormally accelerated decay. Electrochemical impedance spectroscopy (EIS) curves showed significant increases in resistance of SEI film (R_SEI) and charge transfer resistance (R_ct) after 65°C float charge at 30 days (Figure 2D,E), which means that the SEI formation impedance and the charge impedance of the electrode increase. Compared with the EIS curves at three different temperatures for 5 days, 20 days, and 30 days, the instantaneous increase in impedance at 65°C occurred on the 30th day (Figure S1). To distinguish whether cathode or anode degradation is the cause of the battery rollover failure, differential voltage analysis (DVA, dQ/dV) is used (Figure 2F) [21, 22]. The intensity of dQ/dV peaks decreased as the float charge temperature increased, implying rapid capacity decay at 65°C. The first clear and sharp redox signal appears at 3.57 V (C1), attributed to Gr's lithiation, followed by the central peak of the charging reaction at 3.70 V (C2), corresponding to the H1 → M (rhombohedral to monoclinic) phase transition of NCM. The third characteristic peak at 3.85 V (C3) corresponded to the formation of SEI and the monoclinic to rhombohedral crystal system (M → H2). Among them, the peak positions of C1 and C2 did not change, whereas the C3 peak shifted to the right, which may be related to SEI growth in Gr or the cathode phase transition. D1 and D2 corresponded to the reverse reactions of C1 and C2, including Gr de-lithiation and M → H1. At 65°C, no peak of Gr de-lithiation was observed, which may be related to SEI formation or lithium deposition on the anode. The dQ/dV curves for different days of battery float charge at different temperatures are summarized in Figure S2.

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To determine the reason for the shifts of peak C3, XRD tests are conducted to investigate the potential phase transition. The XRD data, as shown in Figure S3A, clearly indicated that the cathode's peak position remained constant at three temperatures. Therefore, SEI growth had been confirmed to be the cause of the C3 peak's shift in dQ/dV tests. Dissolving transition metal atoms from cathode materials is considered one of the primary origins of cathodic failure mechanisms. Energy dispersive spectroscopy (EDS) analysis on the anode surface is conducted to investigate the dissolution of Ni, Co, and Mn atoms. The absence of these elements in the EDS spectrum on the anode surface (Figure S3B) strongly suggested that a phase transition and metal element dissolution in the cathode did not occur. This may be related to the relatively low operating voltage of 4.2 V [23]. The disassembled cathode and anode from cells after 30 days of float charge were assembled into half-batteries to test capacity. The cathode's capacity remained stable at all three temperatures, but the capacity of the Gr anode decreased from 330.39 mAh g^–1 to 119.5 mAh g^–1 after 30 days of float charge at 65°C (Figure 2G). The charge and discharge curves of half-cells are shown in Figure S3C,D. These results suggest that the rapid capacity decline may have less to do with the cathode and more to do with the anode.

To further demonstrate what causes anode capacity to decay, we calculated the Coulombic efficiency of the battery at 25°C, 45°C, and 65°C float charge over different days. Our findings indicated no significant change in Coulombic efficiency after 30 days at 25°C and 45°C float charges, but a notable reduction after 30 days at 65°C (Figure S4A). This reduction suggests more active lithium loss after 30 days of 65°C float charge. Furthermore, heterogeneous lithiation at different locations in the electrode was evident from peak displacement, resulting in a gentle slope at this voltage plateau, as reflected in the capacity–voltage curves and the differential voltage curve (DV, dV/dQ) of the charge process [24]. The difference in lithiation was reflected by the change in the voltage slope near the platform transition of the charge process [25–27]. ΔPeak_D2 represents the gap between the values of the third stage and the peak in dV/dQ as a descriptor for the nonuniformity of lithium in Gr (Figure 2H). Based on the changes in the value of ΔPeak_S2 (Figure S4B,C,D), the 65°C float charge 30-day conditions were classified as heterogeneous degradation and 25°C and 45°C float charge 30-day conditions were classified as homogeneous degradation. The dV/dQ curves suggest that heterogeneous lithiation occurs after the 65°C float charge. These electrochemical test results indicate that the significant capacity loss during elevated temperature float charge is due to the active lithium loss and heterogeneous lithiation of the anode.

In addition, the behavior of commercial large-scale pouch cells was also examined (with a standard capacity of 2.8 Ah) at elevated temperatures. Over a period of 90 days, we analyzed the float charge failure behavior of these commercial pouch cells at 65°C. The capacity retention rate decreased to 84.79% from 2.81 to 2.38 Ah after the 90-day period (Figure S5A), demonstrating significant capacity loss. We conducted dQ/dV and dV/dQ analyses on the charging and discharging curves after float charge (Figure S5B,C). The dQ/dV curve revealed that the peak positions of the commercial pouch cell were consistent with those of the pouch cells in our study, showing no noticeable change. The change of peak positions also represents SEI growth and lithium deposition in a commercial large-scale pouch cell. The dV/dQ curve indicated that the peak of the commercial pouch cell shifted significantly to the left after 90 days of float charge, which suggests active lithium loss in the battery. This finding aligns with the behavior observed in the experimental batteries. Furthermore, after 90 days of float charge, we noticed a slight swelling in the pouch cell, indicating gas production associated with electrolyte decomposition and SEI decomposition during the high-temperature exposure. Therefore, we conclude that the results obtained from the commercial large pouch cell are consistent with those from our experimental batteries.

Reason for Active Lithium Loss in Gr

Active lithium loss has two sources: irreversible loss caused by SEI formation and lithium plating [28, 29]. The formation of SEI is initially investigated. Figure 3A–D illustrates the thickness of the SEI layer on the anode interphase. In cells after three cycles, the anode had a thin SEI layer (maximum 3.5 nm) on the surface (Figure 3A). After the float charge at 25°C, the SEI layer became thicker (maximum 7.6 nm) (Figure 3B), and after the float charge at 45°C, the maximum thickness of the SEI layer increased to 13.3 nm (Figure 3C). A thicker and uneven SEI layer with a maximum thickness of 18.4 nm was observed after the float charge at 65°C compared with that at other temperatures (Figure 3D). The results suggest that the thickness of the SEI increases at high temperatures. Moreover, X-ray photoelectron spectroscopy (XPS) depth profiling was further applied to understand the distribution of elements in SEI. F 1s spectra had three characteristic peaks at 685.0, 686.0, and 687.2 eV, corresponding to LiF, Li_xPO_yF_z, and PF₆⁻, respectively (Figure 3E). The content of LiF increased to 85.43% after 4 nm of sputtering and 88.63% after 6 nm of sputtering, indicating that LiF was the dominant F⁻-containing component in the inner layer of the SEI at 25°C float charge. However, the content of LiF in the anode electrode at 65°C was only 57.70% after 6 nm of sputtering and 81.24% after 2 nm of sputtering, indicating that there was a gradient distribution of LiF across the SEI formed at high temperatures (Figure 3F). It is speculated that thermal decomposition of organic components occurs in SEI (alkyl lithium carbonate, etc.) to a more stable inorganic component at high temperatures on the surface of the anode. Combined with the fact that the electrode at 65°C has a thickness of SEI as high as 18.4 nm, the inhomogeneity in the inorganic composition could potentially affect the float charge performance.

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The C 1s spectra showed four peaks that correspond to C–C/C–H (284.8 eV), C═O (288.7 eV), C–O (286.3 eV), and Li₂CO₃ (290.0 eV) (Figure S6) [30]. The content of C–C/C–H in the anode can indirectly reflect the thickness of SEI. The thicker the SEI, the smaller the C–C/C–H content of the C 1s spectrum of the XPS [31]. The proportion of C–C/C–H was 52.79% at 25°C and 69.19% at 65°C in the outermost layer of SEI (near the electrolyte) (Figure 3G), suggesting that the SEI of the anode electrode at 65°C was thicker than that at 25°C. AES quantitatively detected the Li, C, O, F, and P contents on the electrode surface at different temperatures (Figure 3H). It was found that the Li and F contents on the electrode surface increased with the temperature, whereas the content of O decreased significantly due to the decomposition of the SEI layer at high temperatures (Figure 3I), resulting in by-products and gases (Equation 1–2). 1${\left({\text{CH}}_2\text{OCOOLi}\right)}_2\to {\text{Li}}_2{\text{CO}}_3+{\mathrm{C}}_2{\mathrm{H}}_4+{\text{CO}}_2+12{\mathrm{O}}_2,$ 2$2\text{Li}+{\left({\text{CH}}_2{\text{OCO}}_2\text{Li}\right)}_2\to 2{\text{Li}}_2{\text{CO}}_3{+\mathrm{C}}_2{\mathrm{H}}_4.$

In addition to the interface impact of the anode, we also noticed electrolyte decomposition and lithium deposition on the anode surface. After 30 days of float charge at 25°C, 45°C, and 65°C, we conducted non-destructive ultrasonic scanning directly on the pouch cells (Figure 4A). The ultrasonic scanning revealed red and yellow areas in the image, indicating higher ultrasonic signals and good electrolyte infiltration. Meanwhile, blue and white areas indicated reduced ultrasonic signals due to insufficient electrolyte infiltration [32]. Following 30 days of float charge at 65°C, there were significant blue and white areas in the battery compared to that at other two temperatures, which corresponded to the gas resulting from electrolyte decomposition (Figure 4B–D) [33, 34]. This result indicates that after 30 days of float charge at 65°C, the electrolyte and SEI decompose to produce gas, which reduces ultrasonic signals. Gas extracted from the bulging pouch cells was analyzed using gas chromatography FID detection (Figure S7). It revealed the presence of some alkane gases resulting from the decomposition of the SEI and electrolyte at high temperatures (Equation 3–5) [35]. 3$\text{2EC}+{\text{3e}}^{-}+{\text{3Li}}^{+}\to {\mathrm{C}}_2{\mathrm{H}}_5\cdot +{\text{CH}}_2{\text{OCO}}_2{\text{Li}+\text{CH}}_2{\text{OCO}}_2{\text{Li}}_2,$ 4$\text{EMC}+{\mathrm{e}}^{-}+{\text{Li}}^{+}\to {\text{CH}}_3\cdot +{\text{Li}}_2{\text{CO}}_3+{\mathrm{C}}_2{\mathrm{H}}_5\cdot ,$ 5$\text{EMC}+{\mathrm{e}}^{-}{+\text{Li}}^{+}\to {\text{CH}}_3\cdot +{\mathrm{C}}_2{\mathrm{H}}_5{\text{OCO}}_2\text{Li}.$

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The surface morphologies after float charge were characterized using SEM. The Gr anode displayed a smooth surface without significant morphological features at 25°C and 45°C (Figure 4E,F). However, grayish deposits were observed on the Gr surface when the cell was disassembled after 65°C float charge. These deposits were mainly composed of dendrite-like particles, as revealed by SEM (Figure 4G), indicating the presence of lithium dendrites. The thermal runaway time at 65°C was found to be shorter than that at 25°C and 45°C from the ARC results (Figure 4H). Moreover, the thermal runaway curve showed no significant change in the temperature increase at 25°C and 45°C, but displayed a clear peak at 65°C at 3000 s, indicating that heat escaped at this temperature. The extra heat peaks observed during the thermal runaway are characteristic traits for diagnosing lithium plating from a thermal perspective, indicating a Li stripping reaction and an undermined Li intercalation heat peak [36].

After disassembling the pouch cell, we observed that the lithium plating area was concentrated around the gas production on the electrode (Figure S8A), which corresponded to the ultrasonic test results. We speculated that the gas resulted in the loss of local electrode contact, which led to a significant increase in the current density at the edge of the bubble in the subsequent charge and discharge test. The local polarization of current density was increased, resulting in lithium plating on the anode [37]. The process diagram is shown in Figure 4I. Furthermore, we disassembled the pouch cell that produced gas after 30 days of float charge but it was not tested for charge and discharge (Figure S8B). The ultrasonic test also showed that the cell that was not tested for charge and discharge after float charge produced gas (Figure S8C). However, no lithium plating was found on the anode surface (Figure S8D), indicating that the anode was plated with lithium after the float charge because the gas generated hindered the lithium transmission in the subsequent charge and discharge process (Figure 4J).

The above findings reveal that active lithium loss is the main reason for capacity failure under a high-temperature float charge. The active lithium loss, a complex phenomenon, is caused by SEI thickening and lithium plating of the anode. By quantitatively analyzing the amount of active lithium loss in the SEI and lithium plating, we can effectively determine which factor contributes more to the reduction in battery capacity.

Quantitative Analysis of Anode Active Lithium Loss

The TGC method was based on the principle that Li⁰ reacts with water to produce hydrogen gas (H₂) [38, 39]. The amount of H₂ was measured by injecting it into a gas chromatograph (GC) and calculating the peak area (Figure 5A). As a result, the amount of Li⁰ in the anode can be calculated using the following equation: 6${\text{2Li}}^0+{\mathrm{H}}_2\mathrm{O}={\mathrm{H}}_2+{\text{Li}}_2\mathrm{O}.$

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The standard sample consisted of lithium metal with masses of 2.3, 4.7, and 10.5 mg, and the peak areas of H₂ are shown in Figure S9A. The standard curve of the sample demonstrated that the TGC method was highly accurate in determining the dead lithium content (R² = 0.9935) (Figure 5B). The anode was subjected to float charge and then de-lithiated at 0.2 C because the de-lithiation process was related to the amount of Li⁰. The amount of Li⁰ in the float-charged Gr was investigated by TGC (Figure 5C). Importantly, the mass of H₂ increased after the 65°C float charge, as it increased from 0.064 to 0.194 mg. The 1 Ah battery contained approximately 49 mAh (0.0127 g) lithium metal; the C_dead in the 65°C sample was converted into 5.24 mAh [40]. The gas curves of TGC are shown in Figure S9B.

⁷Li NMR spectra were also useful for quantifying the lithium plating and lithium ions in the SEI in the anode [41, 42]. Two prominent peaks involved the lithium plating metal peak at around 269 ppm and Li⁺ at around 0 ppm [38]. The NMR results show only Li⁺, with no plated lithium in unfloated charge pouch cells at 25°C float charge, but Li⁺ and plated lithium were detected in 65°C float-charged pouch cells (Figure 5D). The content of plating lithium was 1.03%. We quantitatively calculated that C_dead was 5.43 mAh, which is consistent with the result of TGC. Also, the C_SEI was 39.00 mAh. Detailed calculation procedures are provided in the Supporting Information. The dead lithium capacity of the battery after 65°C float charge accounted for 0.96%, and the SEI capacity accounted for 6.88% (Figure 5E). Finally, quantitative analyses of active lithium loss from the anode showed that SEI is the primary cause of cell capacity decay.

The failure of float charge in LIBs at high temperature is summarized in Figure 6. The active lithium content in non-float–charged batteries, with both the anode and the cathode fully saturated at 100%, was determined. After the 25°C float charge, the active lithium loss was due to SEI growth, which accounted for 2.23%, and scarcely any electrolyte decomposition was found. Following the 65°C float charge, there were no significant changes in the cathode, due to the low voltage not reaching the potential for side reaction phase transition and transition metal dissolution change in the cathode [43]. The anode experienced thickening of the SEI, with 6.88% of the active lithium being used to form SEI. Additionally, a proportion of the active lithium was lost due to lithium plating, with the precipitated lithium accounting for about 0.96% (Figure 6A). The analysis shows that the main causes of failure after a high-temperature float charge are concentrated on the anode side and the decomposition of the electrolyte (Figure 6B). Through quantitative analysis of the active lithium loss in the battery, it was found that the SEI growth on the anode surface was the main reason. The active lithium loss was mainly affected by the continuous rupture and growth of SEI at high temperatures, significantly affecting the battery's dynamic performance.

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Conclusion

In summary, a systematic float charge failure study of LIBs was performed in this work using active lithium loss as the descriptor of float charge failure. It was revealed that the main reason for the float charge failure of LIBs is excessive SEI formation, and lithium deposition consumes active lithium, resulting in capacity loss of the NCM523-Gr pouch cells. NCM523-Gr pouch cells were float-charged at different temperatures (25°C, 45°C, and 65°C). 96.86% and 92.59% of the initial capacity could be recovered after 50 days of float charge at 25°C and 45°C, respectively. However, only 80.05% of the initial capacity could be recovered after 30 days of float charge at 65°C. From the electrochemical analysis, it was found that the capacity decay of 65°C was caused by active lithium loss of the anode. TEM and XPS depth profiling proved that the thicker and uneven SEI layer was the reason for capacity decay after float charge at 65°C. Ultrasonic scanning of pouch cells indicated the formation of gas from electrolyte decomposition, which reduces ultrasonic signals, and the thermal spectrum of ARC showed the presence of a clear peak at 65°C at 3000 s, indicating a Li stripping reaction and an undermined peak of Li intercalation. Then, TGC and NMR were applied to the anode to quantify active lithium loss of SEI and Li plating. The active lithium loss owing to the formation of SEI and Li plating was 6.88% and 0.96%, respectively, and the SEI formation is the primary reason for failure in the float charge process. This study reveals the main reason for the failure of float charge at high temperatures and provides guidance for improving float charge performance in future electric energy networks.

Experimental Section

Data that support the findings of this study are available in the Supporting Information of this article.

Acknowledgements

This work was supported by the National Key Research and Development (R&D) Program of China (2022YFB4101600), Key Research and Development (R&D) Projects of Shanxi Province (202102040201003, 202202040201007), the Fundamental Research Program of Shanxi Province (20210302123008), and the ICC CAS, SCJC-XCL-2023-13, CAS Project for Young Scientists in Basic Research (Grant No.YSBR-102).

Conflicts of Interest

The authors declare no conflicts of interest.

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