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Toward a high-voltage fast-charging pouch cell with TiO2 cathode coating and enhanced battery safety
Journal Pre-proof Toward a High-Voltage Fast-Charging Pouch Cell with TiO2 Cathode Coating and Enhanced Battery Safety Yan Li, Xiang Liu, Dongsheng Ren, Hungjen Hsu, Gui-Liang Xu, Junxian Hou, Li Wang, Xuning Feng, Languang Lu, Wenqian Xu, Yang Ren, Ruihe Li, Xiangming He, Khalil Amine, Minggao Ouyang PII:
S2211-2855(20)30200-7
DOI:
https://doi.org/10.1016/j.nanoen.2020.104643
Reference:
NANOEN 104643
To appear in:
Nano Energy
Received Date: 8 January 2020 Revised Date:
16 February 2020
Accepted Date: 21 February 2020
Please cite this article as: Y. Li, X. Liu, D. Ren, H. Hsu, G.-L. Xu, J. Hou, L. Wang, X. Feng, L. Lu, W. Xu, Y. Ren, R. Li, X. He, K. Amine, M. Ouyang, Toward a High-Voltage Fast-Charging Pouch Cell with TiO2 Cathode Coating and Enhanced Battery Safety, Nano Energy, https://doi.org/10.1016/ j.nanoen.2020.104643. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
CRediT author statement Yan Li: Conceptualization, Methodology, Investigation, Formal analysis, Writing - Original Draft Xiang Liu: Conceptualization, Methodology, Investigation Dongsheng Ren: Resources, Formal analysis Hungjen Hsu: Resources, Formal analysis Gui-Liang Xu: Formal analysis, Investigation Junxian Hou: Formal analysis Li Wang: Methodology, Writing - Review & Editing Xuning Feng: Supervision Languang Lu: Supervision Wenqian Xu: Investigation Yang Ren: Investigation Ruihe Li: Formal analysis Xiangming He: Writing - Review & Editing Khalil Amine:
Conceptualization,
Supervision,
Funding
acquisition
Conceptualization, Supervision, Project administration, Funding acquisition
Minggao
Ouyang:
Toward a High-Voltage Fast-Charging Pouch Cell with TiO2 Cathode Coating and Enhanced Battery Safety Yan Li,1,= Xiang Liu,1,2,= Dongsheng Ren,1,3 Hungjen Hsu,1 Gui-Liang Xu,2 Junxian Hou,1 Li Wang,1,3 Xuning Feng,3 Languang Lu,1 Wenqian Xu,4 Yang Ren,4 Ruihe Li,1 Xiangming He,3
Khalil Amine,2,5 Minggao Ouyang1,*
1. State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, 100084, China; 2. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA; 3. Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China; 4. X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA 5. Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA =
These authors contribute equally to this work.
*
Corresponding author.
E-mail: [email protected] (Prof. Minggao Ouyang)
Abstract: Nickel-rich layered lithium transition metal oxides, LiNixCoyMn1-x-yO2, are key cathode materials for high-energy lithium-ion batteries owing to their high specific capacity. However, the commercial deployment of nickel-rich oxides has been hampered by their poor thermostability and insufficient cycle life. Here full batteries with uncoated and TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathodes and graphite anodes are compared in terms of electrochemical performance and safety behavior. The battery using a TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathode exhibited better cyclic performance at high cutoff voltage. Electrochemical impedance spectroscopy analysis indicated that the TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathode gave the battery a more stable charge transfer resistance. Transmission electron microscopy demonstrated that TiO2 coating reduced accumulation of the cathode electrolyte interface layer on the particle surface. Time-of-flight secondary ion mass spectrometry demonstrated that TiO2 coating markedly enhanced the interface stability of the cathode particle and protected the particle from serious etching by the electrolyte. Accelerating rate calorimetry revealed that the trigger temperature of thermal runaway for the battery using TiO2-coated LiNi0.5Co0.2Mn0.3O2 as cathode material was 257°C, which was higher than that of the battery with the uncoated LiNi0.5Co0.2Mn0.3O2 cathode (251°C). In situ X-ray diffraction during heating demonstrated that this enhanced safety can be attributed to the suppressed phase evolution of the coated cathode material. Keywords: Lithium-ion batteries; Cathode; TiO2-coated LiNi0.5Co0.2Mn0.3O2; High Voltage; Safety
Introduction Layer-structured Ni-rich materials, with the general formula and abbreviation of LiNixCoyMn1-x-yO2 and NCM, respectively, have been considered the most promising future cathode materials for lithium-ion batteries (LIBs) [1–3]. In recent years, the capacity failure mechanism [4–6] and structure degradation mechanism [7–10] of LIBs have been thoroughly investigated, and consequently, their electrochemical performance has significantly improved. High specific energy LIBs depend on cathode materials of high specific capacity or high voltage [11]. Layer-structured Ni-rich materials can provide more specific energy than LiFePO4 and LiCoO2. However, their higher nickel content leads to lower stability [12]. Previous research has shown that increasing the charging cutoff voltage is a practical approach for increasing energy density [13]. For example, LiNi0.5Co0.2Mn0.3O2 (denoted as NCM523) materials have been widely commercialized in electric vehicles (EVs) with good cyclic stability and acceptable safety performance. NCM523 materials are reported to have a reversible capacity of 160 mAh g−1 at 4.3 V (vs. Li/Li+) and deliver 190 mAh g−1 at 4.5 V (vs. Li/Li+) [14]. Unfortunately, the extraction of more lithium ions from NCM materials would initiate additional parasitic reactions in their interface during charging or storage [15–18]. It has been reported that in the fully charged state, Ni4+-O bonds are unstable and tend to turn to a more stable form of Ni2+ with oxygen release [6], which could react with the electrolyte and even trigger the battery thermal runaway [19]. In addition, irreversible transition metal (TM) migration and severe phase transformation could be accelerated at a higher potential, resulting in particle cracking, which could expose the new reaction
location inside the particles to the electrolyte and accelerate mechanical degradation [20]. Many studies have been devoted to overcoming the abovementioned problems, employing techniques such as surface coating [21], cation or anion doping [22–24], concentration gradient structures [25], and placing additives in the electrolyte [26–27]. Surface coating, which can serve as a protective barrier in the particle interface, has been considered one of the most effective ways to improve the performance of NCM materials. NCM materials coated with metal oxides (e.g., TiO2, ZrO2, and Al2O3), metal fluorides (e.g. LiF and AlF3), metal phosphates (e.g., Li-Mg-PO4, LiFePO4, and Li3PO4), and organic polymers have been reported to exhibit greatly improved electrochemical performance and thermal stability [21, 28, 29]. Titanate materials contain wider interlayer spaces and greater surface areas, which can facilitate the transportation of Li+ [30, 31]. For example, TiO2-coated LiNi0.6Co0.2Mn0.2O2 has been reported to present reduced electron-transfer resistance at the delithiated state and improved high-voltage electrochemical performance [28]. It has been reported that possible Ti4+ doping at the interface layer, as introduced by TiO2 coating preparation, resulted in more electrons being shared with the surrounding oxygen, and the bond strength of Ti-O was stronger than that of Ni-O [29]. Thus, both the Ti4+ doping effect and the robust TiO2 structure support a Ti-coated LiNi0.8Co0.1Mn0.1O2 material with enhanced thermostability [29]. In addition to inorganic coatings, PEDOT (poly(3,4-ethylenedioxythiophene)) conductive polymer coating on LiNi0.5Co0.2Mn0.3O2 was reported to improve conductivity and ionicity, suppress undesired phase transformation, and stabilize the interface. The
protective skin enhanced capacity and thermostability at high cutoff voltage [21]. However, most of these published studies used coin cells to investigate electrochemical performance or material-based thermal stability to deduce safety performance of a full battery. Given that measurement on the materials level is still far away from applicable performance, a study on electrochemical and safety performance in full batteries is necessary. In
this
study,
two
LiNi0.5Co0.2Mn0.3O2-based
kinds
battery
of
full
(denoted
as
batteries,
namely,
TiO2-NCM523)
TiO2-coated and
uncoated
LiNi0.5Co0.2Mn0.3O2-based battery (denoted as NCM523), were studied in terms of electrochemical performance and safety to elucidate the critical role of coating layers. Combining electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM) with time-of-flight secondary ion mass spectrometry (TOF-SIMS) characterization, we revealed an unprecedented correlation between the coating layer and parasitic reactions on the cathode under high voltage. By using in situ synchrotron high-energy X-ray diffraction (XRD) with heating and accelerating rate calorimetry, the correlation between coating layer and phase transformation under thermal driving forces was revealed. The TiO2-NCM523 battery showed less TM dissolution, fewer parasitic reactions, no obvious cathode electrolyte interface (CEI) layer, and a higher phase transformation temperature, in contrast with the NCM523 battery. As a result, the battery employing TiO2-NCM523 exhibited better electrochemical and safety performance at high voltage. This work can shed light on designing high energy density LIBs with high safety performance.
Experimental Pouch cells. Pouch cells (200 mAh) with graphite anodes and TiO2-NCM523 or NCM523 cathodes were purchased from LiFun Technology (Hunan Province, China, 412000).
Morphology characterization. The morphology of the cathode materials was characterized by scanning electron microscopy (SEM, Zeiss Merlin), TEM, JEOL JEM2010), and TOF-SIMS (ION-TOF GmbH, Münster, Germany).
Electrochemical measurements. To evaluate cycling and rate performance, the full batteries were charged and discharged between 2.7 and 4.5 V.
Differential scanning calorimeter (DSC) test. The DSC test was used to examine the thermostability of the cathode, which was performed on a Netzsch DSC214 Polyma. The samples were obtained by disassembling a fully charged cell in a glovebox. The cathode powder was scratched off from electrode after having been rinsed several times with dimethyl carbonate (DMC) to minimize lithium salt (LiPF6) in the porous electrode. To reduce oxygen and moisture contamination, the test samples were prepared in an argon atmosphere. For the DSC measurement, the powder (3.0 mg) was mixed with or
without the electrolyte (3.0 µL) in an aluminum crucible and the samples were tested from 30°C to 600°C (10°C min−1).
High-energy X-ray diffraction (HEXRD). In situ HEXRD during charge/discharge was obtained at Beamline 11 ID-C of the Advanced Photon Source (APS), Argonne National Laboratory. The wavelength was 0.1173 Å. The in situ HEXRD was conducted in half coin cells with NCM523/TiO2-NCM523 cathodes and lithium metal anodes. Charge/discharge was in the voltage range (2.8–4.5 V) at C/10 (1C = 200 mAh g-1). The in situ heating HEXRD was carried out at 17 BM at APS with a wavelength of 0.24125 Å. The charged cathode was collected from the fully charged battery. A homemade capillary furnace was used to heat the cathode powder from 30°C to 600°C at a heating rate of 10°C min−1. The obtained 2D diffraction patterns from a PerkinElmer amorphous silicon detector were converted to 1D patterns. Rietveld refinement with the GSAS II program was used to refine the obtained high-energy diffraction patterns and so monitor in situ lattice parameter evolution.
ARC test. The EV+ARC system (Thermal Hazard Technology) was used to evaluate the safety of the full cell. For each ARC test, two batteries were wrapped together, and type K thermocouples were used to monitor battery temperature. The ARC tests were performed with the heat-wait-seek method to ensure an adiabatic condition, with a heating step of 5°C, waiting time of 40 min, and self-heating rate of 0.02°C min−1.
Results and Discussion
The SEM images of NCM523 and TiO2-NCM523 electrodes are compared in Figure S1. Both NCM523 and TiO2-NCM523 cathodes consisted of spherical secondary particles, with sizes ranging from 2 µm to 10 µm. The particle dispersity of the two electrodes was quite similar, implying that particle morphology was not affected by the surface coating process. The inset of Figure S1b shows that a uniform coating layer was detected on the particle surface, with a thickness of about 8 nm. The crystallinity of the TiO2 layer was beneficial to lithium-ion transport on the particle interface, which is consistent with previous reports [32–34]. To understand the function of the TiO2 layer on the electrochemical performance of the cathode, in situ HEXRD was applied. Figures S2a and S2b show the voltage profiles alongside in situ diffraction peak evolution during charge/discharge for NCM523 and TiO2-NCM523. When charged to 4.5 V, the (003) peak of TiO2-NCM523 nearly returned to its original 2θ position with a moderate shift (Fig. S2b). By comparison, the shift of NCM523 deviated from its original 2θ position (Fig. S2a). Figure S3 shows the evolution of the unit cell lattice parameters of NCM523 (Fig. S3a) and TiO2-NCM523 (Fig. S3b). Anisotropic lattice distortion is regarded as one of the most important factors effecting the capacity degradation [35]. It is obvious that TiO2-NCM523 had a smaller volume contraction (1.1%) than NCM523 (1.53%). The smaller volume contraction can be attributed to the coating layer. The surface TiO2
coating suppressed the lattice “breathing,” which would restrain capacity fading, especially at high cutoff voltage [36].
Figure 1. The (a) cycling and (b) rate performances of NCM523- and TiO2-NCM523-based full batteries. The voltage windows for cycling and rate measurements are 2.7–4.5 V. Impedance spectrum evolution with an inset image of the equivalent circuit model of (c) TiO2-NCM523- and (d) NCM523-based full batteries. Both batteries were at 4.5 V after 1/10 C rate charging with voltage window of 2.7–4.5 V.
To explore capacity and cyclability, TiO2-NCM523- and NCM523-based full batteries were tested under a high rate of 1 C with voltage window of 2.7–4.5 V at room temperature. The NCM523-based battery presented a rapid capacity drop as it cycled. Retention was only 30% after 120 cycles (Figure 1a). However, the retention of the TiO2-NCM523-based battery was 80% at the 200th cycle, which indicates that the TiO2 coating layer improved cycling performance at high cutoff voltage, similar to results
that have been previously reported [37]. In addition to cycling performance, the voltage plateau of TiO2-NCM523 also significantly improved, as it declined more slowly than NCM523 (Fig. S4). The rate performances of NCM523- and TiO2-NCM523-based batteries were investigated between 2.7 and 4.5 V. As shown in Figure 1b, the two batteries were similar in rate capability, and the introduction of the TiO2 coating layer blocked the parasite reactions between NCM523 and the electrolyte while maintaining rate performance. The electrochemical impedance variations of NCM523- and TiO2-NCM523-based batteries were deconvoluted and quantified using an EIS test. Figures 1c and 1d show the Nyquist plots of NCM523- and TiO2-NCM523-based batteries at 4.5 V. Nyquist plots generally consist of two semicircles, which are assigned to the surface film resistance (Rs) and charge transfer resistance (Rct) [38], and the following tail is interpreted as an indicator for Li+ diffusion [39]. The EIS fitting data are listed in Table S1 and were obtained by using an equivalent circuit (inset image in Figure 1). The charge transfer (Rct) resistance of the uncoated NCM523-based battery showed an obvious increase during charge/discharge, which may be related to the formation of poorly conductive CEI on the particle surface [40]. However, the TiO2-NCM523-based battery showed a much smaller increase in Rct, which indicates that the TiO2 coating isolated electrode and the electrolyte. The isolation effect of the TiO2 coating restrained the side reactions on the interface and maintained the ionic and electronic conductivity during long-term charge/discharge. This result coincided with the cycling performance.
Figure 2. TEM images of (a) NCM523 and (b) TiO2-NCM523 cathodes after 200 cycles within a voltage between 2.7 and 4.5 V. (c) Depth profiles of NiF4−, CoF4−, and MnF4− collected on NCM523 and TiO2-NCM523 cathodes. Illustrative TOF-SIMS chemical mapping on (d) NCM523 and (e) TiO2-NCM523 cathodes after 200 cycles.
Figure 2a shows the TEM characterization results of NCM523 particles scraped from the battery after 200 cycles. A thick CEI layer (10–50 nm) formed on the cathode particle surface, caused by parasitic reactions at the interface between the transition metal ions and the electrolyte, especially at high voltage [41]. The conductivity of CEI
is very poor, and the accumulation of CEI created a vicious cycle, leading to the polarization increase and capacity fading of the NCM-based battery [26]. Unlike the NCM523, where the particle surface was covered with a thick CEI layer, the surface of the TiO2-NCM523 particle was much cleaner and the TiO2 coating layer well-preserved after 200 cycles, as shown in Figure 2b. TOF-SIMS was used to illustrate the chemical species on the cathode surface after extensive cycling by making use of its high sensitivity, ultrahigh selectivity, and quantitative advantages [42, 43]. Figure 2c shows the corresponding depth profiles of NiF4−, CoF4−, and MnF4− on NCM523 and TiO2-NCM523 electrodes disassembled from the pouch-type full batteries after 200 cycles. The detected species are largely caused by the surface parasitic oxidation and TM dissolution [44]. Figure 2d illustrates the TOF-SIMS chemical maps on a 100 µm × 100 µm cathode electrode disassembled from a NCM523 full battery after 200 cycles. TM species (NiF4−, CoF4−, and MnF4−) are scattered all over the cathode particle surface. However, for the TiO2-NCM523 electrode, the signal of the TM species is weak because the TiO2 coating isolated the cathode and electrolyte, and the parasitic reactions between them were largely inhibited (Figure 2e). The dissolved TM ions may react with ethyl/diethyl carbonates to form a metal complex surface film that can be stripped from the surface of the cathode during long-term cycling [45]. A portion of these metal complexes can migrate from cathode to anode [46–48]. This phenomenon is once again proved by the ICP result. Figure S5 compares the amount of TM ions on the anode surface after the battery was cycled 200 times. As shown, TM ions were detected in the anode, and thus TM dissolution can be
mitigated by TiO2 coating.
Figure 3. DSC curves of NCM523 and TiO2-NCM523 cathodes, disassembled from full batteries charged to 4.5 V, (a) without and (c) with electrolyte. (b) O2 release of delithiated NCM523 and TiO2-NCM523 during heating.
The abovementioned findings demonstrate that the isolation effect of TiO2 coating on interface reactions is helpful for improving electrochemical performance. It is natural to associate with the safety performance of the battery. The thermostability of active materials should be the first consideration in evaluating the safety of a battery [19, 49]. A highly delithiated cathode will experience structure change and release oxidizing species (O2, O2-, O-, etc.) during heating [50]. The reactions between the released highly reactive oxygen species and flammable organic electrolyte are highly exothermic, leading to severe thermal runaway [21]. For this reason, a robust surface layer that can isolate surface reactions should improve the safety of the battery’s chemistry. Therefore, to determine whether TiO2 could improve the thermostability of NCM523 toward the electrolyte, the highly delithiated bare NCM523 and TiO2-NCM523 cathodes were
characterized by DSC, as shown in Figure 3. The DSC curves and O2 spectra of the NCM523 and TiO2-NCM523 cathodes are shown in Figures 3a and 3b. The exothermic and oxygen release peaks corresponded with the phase transformation. The NCM523 sample exhibited a lower exothermal initial temperature around 223°C. The last exothermic peak at 510°C corresponded with the phase transition from spinel-type to rock-salt structure [51], while the TiO2-NCM523 sample shifted to a higher temperature (537°C). However, the exothermal behavior of the cathode with an electrolyte addition was quite different. For the NCM523 cathode, the exothermic peak initiated at 290°C, with an enthalpy change (∆H) of 110 J g−1 (Fig. 3c). The released oxidizing species (O2, O2-, O-, etc.) from the cathode were reduced by the electrolyte, which accelerated phase transformation [21]. For the TiO2-NCM523 cathode, the exothermic peak delayed at 295°C, with ∆H decreasing to 29 J g−1, indicating that the side reactions on the cathode surface were restrained by the isolation effect of the TiO2 layer.
Figure 4. Thermally induced phase transformation of charged (a) NCM523 and (b) TiO2-NCM523 cathode materials.
In Figure 4, phase transformation during the thermal decomposition process of NCM523 and TiO2-NCM523 cathode materials at the highly delithiated state is shown with in situ temperature-resolved XRD data. As shown in Fig. 4a, two splitting peaks (108 and 110) of the fully charged NCM523 cathode began to disappear at 240°C and disappeared completely at 320°C, corresponding with the change from layered (R3m) to spinel (Fd3 m) structure [53]. Subsequently, when the temperature rose to 420°C, a new peak appeared (the green zone in Fig. 4a), indicating irreversible TM cation
migration from layered (R3m) to spinel (Fd3 m) to rock-salt (Fm3 m) structure [54]. For highly delithiated TiO2-NCM523, the characteristic peak (108 and 110) of the layered structure began to disappear at 260°C and disappeared completely at 360°C, while the new peak appeared at 480°C. The phase transformation temperatures were higher than those of NCM523. TiO2 coating is thus helpful for enhancing NCM523 thermostability, and the postponed phase transformation is critical for improving cathode safety [19].
Figure 5. Thermal runaway characterization of NCM523 and TiO2-NCM523. Measured by ARC.
According to the DSC and in situ XRD results, the TiO2 layer improved safety performance at the raw material level. However, did the coating layer truly enhance the safety of a full battery? To answer this question, ARC tests were conducted on NCM523 and C-NCM523 full batteries charged to 4.5 V. T1, T2, and T3 stand for the self-heating
onset, thermal runaway (TR) ignition, and maximum temperature [55, 56]. The reason for battery self-heating is primarily the failure of the solid-electrolyte interphase (SEI). As the ARC system provides an adiabatic environment, the failure of SEI triggers continuous reactions between the electrolyte and anode, forming a chain reaction until triggering TR. At the T2 temperature, the heating rate of the battery is fastest during TR. Therefore, T2 is a critical index for evaluating the safety of the battery, and a higher T2 means better overall thermal safety. As shown in Figure 5, The T1s for NCM523 and TiO2-NCM523 were 55°C and 56°C, respectively (nearly the same, as the initial heating was already 5°C/step). The maximum temperature, T3, was 660°C for TiO2-NCM523, 72°C higher than for NCM523, indicating that fewer side reactions occurred for TiO2-NCM523 during the adiabatic stage. By connecting the abovementioned exothermic behavior of delithiated NCM materials with the presence of an electrolyte (Fig. 3c), the fewer side reactions of the TiO2-NMC523 battery can be mainly attributed to the isolation effect of TiO2 coating on interface reactions. The most important improvement for the TiO2-NMC battery was the delay of T2 at 257°C for TiO2-NCM523 compared with 251°C for NMC523 (Fig. 5b). Combined with the above discussion, the postponement of T2 can be attributed to two factors: (1) the TiO2 coating isolated the cathode surface from the electrolyte and inhibited parasitic reactions, and (2) the phase transformation temperature increased and the TR trigger, the release of oxidizing species (O2, O2-, O-, etc.), was suppressed. As a result, the TR of TiO2-NCM523 occurred 167 min later and at a temperature 6°C higher than that of the NCM523 battery (Fig. 5).
Conclusion We have demonstrated a new mechanism for a TiO2 coating layer on the cathode surface in full batteries under both electrochemical and thermal driving forces. This TiO2 coating layer isolated the active material from the electrolyte, thus slowing down parasitic oxidation and TM dissolution. The TiO2 coating layer postponed phase transformation during heating, significantly improving battery safety. This work paves the way to improve safety for battery chemistries of higher energy density. Acknowledgement This research is supported by the Ministry of Science and Technology of China under the Grant No. 2016YFE0102200. Research at the Argonne National Laboratory was funded by the U.S. Department of Energy (DOE), Vehicle Technologies Office. Support from Tien Duong of the U.S. DOE’s Office of Vehicle Technologies Program is gratefully acknowledged. Use of the Advanced Photon Source (APS), Office of Science user facilities, was supported by the U. S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Declaration The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] A. Manthiram, J.C. Knight, S.-T. Myung, S.-M. Oh, Y.-K. Sun, Adv. Energy Mater. 6 (2016) 1501010. [2] M. Li, J. Lu, Z.W. Chen, K. Amine, Adv. Mater. 30 (2018) 1800561. [3] S.-T. Myung, F. Maglia, K.-J. Park, C.S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett. 2 (2017) 196-223. [4] J. Lu, T.P. Wu, K. Amine, Nat. Energy 2 (2017) 17011.
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[56] D.S. Ren, H.J. Hsu, R.H. Li, X.N. Feng, D.X. Guo, X.B. Han, L.G. Lu, X.M. He, S. Gao, J.X. Hou, Y. Li, Y.L. Wang, M.G. Ouyang, eTransportation 2 (2019) 100034.
Dr. Yan Li received the Ph.D. degree in the College of Chemical Engineering from Nanjing Technology University, Nanjing, China, in 2016. He is currently a Post-Doc Researcher in Automotive Department in Tsinghua University. His main research interests focus on the fields of battery safety.
Dr. Xiang Liu is now work as postdoctoral researcher in Argonne National Laboratory. He obtained his PhD from the University of Hong Kong in 2016. Prior to his current position, he worked as a postdoc in Automotive Department in Tsinghua University. His research focus includes cathode material design and synthesis for lithium-ion and sodium-ion batteries, as well as the in situ/operando synchrotron X-ray based characterizations for the understanding of battery safety and degradation mechanisms.
Dr. Dongsheng Ren is now a postdoctoral researcher in Tsinghua University. He received his B.E.(2014) and Ph.D.(2020) degree in School of Vehicle and Mobility, Tsinghua University. His research focuses on understanding the degradation and thermal runaway mechanism of lithium-ion battery, as well as electrochemical/thermal models for lithium-ion battery.
Mr. Hungjen Hsu is a master student in Battery Safety Lab under Powertrain System Group in Tsinghua University. He mainly focuses on multi-scale safety of lithium-ion battery from material to cell level. Mr Hsu graduated from Energy Engineering of National Cheng Kung University in 2013 for his bachelor degree.
Dr. Gui-Liang Xu is currently an assistant chemist in the Electrochemical Energy Storage group under the division of Chemical Sciences and Engineering at Argonne National Laboratory. His researches focus on both fundamental understanding and materials development for lithium-ion batteries and beyond. Dr. Xu earned his Bachelor (2009) and PhD (2014) degree in the department of Chemistry of Xiamen University. After three years postdoc research at Argonne, he was promoted to permanent staff scientist. Dr. Xu has authors/co-authored 67 peer-reviewed research articles and has 1 U.S. patent and several patent applications.
Dr. Junxian Hou is now work as postdoctoral researcher in the Battery Safety Lab of Tsinghua University. She obtained his PhD from Beijing University of Technology in 2018. Her researches focus on much safer electrolyte development as well as thermal runaway mechanisms of different electrolytes in lithium-ion batteries.
Dr. Li Wang is an associate professor of the vice director of the Advanced Energy and Material Chemistry Lab in Tsinghua University. Her research is focused on the failure analysis on lithium ion and lithium metal batteries, aiming to enhance both the performances and safety of the batteries by exploring advanced materials and battery design.
Xuning Feng received the B.E. Degree and the Ph.D. degrees in the Department of Automotive Engineering from Tsinghua University, Beijing, China, in 2011 and 2017, respectively. He is currently a Post-Doc Researcher with Tsinghua University. His
research interests include battery management system and battery safety.
Dr. Languang Lu is an associate professor of School of Vehicle and Mobility Tsinghua University. His interesting research fields include the integration, optimization, and control of automotive new powertrains system and battery management system. He has more than 100 academic papers, 1 book and 83 patents. He has received 5 Technology Awards. The latest achievement “performance optimization control and system integration design and application of lithium-ion power battery for vehicle” won 2016 China automobile industry technology invention first prize.
Dr. Wenqian Xu is a beamline scientist at Advanced Photon Source at Argonne National Laboratory. He operates Beamline 17-BM, an X-ray powder diffraction instrument, and is interested in improving and applying in situ X-ray diffraction and total elastic scattering techniques to materials research. He received Ph. D. in Geosciences from Stony Brook University, and was a postdoctoral research associate at Brookhaven National Laboratory prior to his current position.
Yang Ren received his M.S. degree in condensed matter physics from the Institute of Physics, Chinese Academy of Science, China, and his Ph.D. degree in Chemical Physics from the University of Groningen, The Netherlands. He is currently a physicist and lead beamline scientist at the Advanced Photon Source, Argonne National Laboratory, USA. His research interests focus on the structure-property studies of materials by utilizing synchrotron X-ray and neutron scattering and other techniques. His research activities include the investigation of phase transition, correlated electron systems, engineering materials, nanoparticles and nanocomposites, energy storage and conversion materials.
Ruihe Li is currently a master student in the School of Vehicle and Mobility at Tsinghua University. He obtained his Bachelor’s degree from Tongji University. His research is mainly about the electrochemical-mechanical coupled effects of lithium ion battery, such as volume deformation of aged cells and resulting effects on battery degradation, effects of external stress on cycle life, state estimation via mechanical signal, etc. He has published some papers and patents and given several presentations in international academic conferences. He will graduate at Feb. 2021 and he is now looking for a PhD position on related fields.
Xiangming He is a Professor in Institute of Nuclear and New Energy Technology (INET) at Tsinghua University. He is the director of New Energy and Materials Chemistry Division in INET. In the R&D of lithium ion batteries and their key materials over 20 years, focusing on the pivotal scientific fundamental of the electrical performance and safety of lithium ion batteries. As the principal investigator, his research projects are granted by National Natural Science Foundation of China, Chinese Government and world-famous companies. He has authored/Edited 4 books. He has published over 500 papers and has been awarded over 400 patents.
Dr. Khalil Amine is an Argonne Distinguished Fellow and the leader of the Advanced Battery Technology team at Argonne National Laboratory, where he is responsible for directing the research and development of advanced materials and battery systems for HEV, PHEV, EV, and satellite applications. He is an adjunct professor at Stanford University. Among his many awards, Dr. Amine is the 2019 reception of the prestigious Global Energy Prize. He is a six-time recipient of the R&D 100 Award, which is considered as the Oscar of technology and innovation. He is an ECS fellow, and
associate editor of the journal of Nano-Energy.
Dr. Minggao Ouyang is Changjiang Distinguished Professor and the leader of Advanced Powertrain System team at Tsinghua University, where he is responsible for directing the research and development of Lithium-ion Battery Safety Design and Management, PEM Fuel Cell Powertrain and Hydrogen Systems, Engine Control and Hybrid Powertrains, Energy Storage and Smart Energy Systems. From 2007 to now, Prof. Ouyang Minggao has been the chief scientist of the China National Key R&D Program of. He got many national and international awards. He is Member of the Chinese Academy of Sciences and Editor-in-chief of the international journal of eTransportation.
Highlights The TiO2 coating layer isolated the active material from the electrolyte, thus slowing down parasitic oxidation and transition metal dissolution Owing to the isolation effect of the TiO2 layer, the reaction between the charged cathode and the electrolyte is effectively inhibited, which greatly reducing the exothermic during thermal runaway; The TiO2 coating layer postponed phase transformation from layered to spinel to rock salt structure during heating, and the oxidizing species (O2, O2-, O-, etc.) release is suppressed, which is the thermal runaway trigger; Thermal runway of the TiO2-coated LiNi0.5Co0.2Mn0.3O2 battery occurred 167 min later and 6 oC higher than pristine LiNi0.5Co0.2Mn0.3O2 based battery.
1
Declaration The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S2211-2855(20)30200-7
DOI:
https://doi.org/10.1016/j.nanoen.2020.104643
Reference:
NANOEN 104643
To appear in:
Nano Energy
Received Date: 8 January 2020 Revised Date:
16 February 2020
Accepted Date: 21 February 2020
Please cite this article as: Y. Li, X. Liu, D. Ren, H. Hsu, G.-L. Xu, J. Hou, L. Wang, X. Feng, L. Lu, W. Xu, Y. Ren, R. Li, X. He, K. Amine, M. Ouyang, Toward a High-Voltage Fast-Charging Pouch Cell with TiO2 Cathode Coating and Enhanced Battery Safety, Nano Energy, https://doi.org/10.1016/ j.nanoen.2020.104643. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
CRediT author statement Yan Li: Conceptualization, Methodology, Investigation, Formal analysis, Writing - Original Draft Xiang Liu: Conceptualization, Methodology, Investigation Dongsheng Ren: Resources, Formal analysis Hungjen Hsu: Resources, Formal analysis Gui-Liang Xu: Formal analysis, Investigation Junxian Hou: Formal analysis Li Wang: Methodology, Writing - Review & Editing Xuning Feng: Supervision Languang Lu: Supervision Wenqian Xu: Investigation Yang Ren: Investigation Ruihe Li: Formal analysis Xiangming He: Writing - Review & Editing Khalil Amine:
Conceptualization,
Supervision,
Funding
acquisition
Conceptualization, Supervision, Project administration, Funding acquisition
Minggao
Ouyang:
Toward a High-Voltage Fast-Charging Pouch Cell with TiO2 Cathode Coating and Enhanced Battery Safety Yan Li,1,= Xiang Liu,1,2,= Dongsheng Ren,1,3 Hungjen Hsu,1 Gui-Liang Xu,2 Junxian Hou,1 Li Wang,1,3 Xuning Feng,3 Languang Lu,1 Wenqian Xu,4 Yang Ren,4 Ruihe Li,1 Xiangming He,3
Khalil Amine,2,5 Minggao Ouyang1,*
1. State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing, 100084, China; 2. Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA; 3. Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China; 4. X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA 5. Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA =
These authors contribute equally to this work.
*
Corresponding author.
E-mail: [email protected] (Prof. Minggao Ouyang)
Abstract: Nickel-rich layered lithium transition metal oxides, LiNixCoyMn1-x-yO2, are key cathode materials for high-energy lithium-ion batteries owing to their high specific capacity. However, the commercial deployment of nickel-rich oxides has been hampered by their poor thermostability and insufficient cycle life. Here full batteries with uncoated and TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathodes and graphite anodes are compared in terms of electrochemical performance and safety behavior. The battery using a TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathode exhibited better cyclic performance at high cutoff voltage. Electrochemical impedance spectroscopy analysis indicated that the TiO2-coated LiNi0.5Co0.2Mn0.3O2 cathode gave the battery a more stable charge transfer resistance. Transmission electron microscopy demonstrated that TiO2 coating reduced accumulation of the cathode electrolyte interface layer on the particle surface. Time-of-flight secondary ion mass spectrometry demonstrated that TiO2 coating markedly enhanced the interface stability of the cathode particle and protected the particle from serious etching by the electrolyte. Accelerating rate calorimetry revealed that the trigger temperature of thermal runaway for the battery using TiO2-coated LiNi0.5Co0.2Mn0.3O2 as cathode material was 257°C, which was higher than that of the battery with the uncoated LiNi0.5Co0.2Mn0.3O2 cathode (251°C). In situ X-ray diffraction during heating demonstrated that this enhanced safety can be attributed to the suppressed phase evolution of the coated cathode material. Keywords: Lithium-ion batteries; Cathode; TiO2-coated LiNi0.5Co0.2Mn0.3O2; High Voltage; Safety
Introduction Layer-structured Ni-rich materials, with the general formula and abbreviation of LiNixCoyMn1-x-yO2 and NCM, respectively, have been considered the most promising future cathode materials for lithium-ion batteries (LIBs) [1–3]. In recent years, the capacity failure mechanism [4–6] and structure degradation mechanism [7–10] of LIBs have been thoroughly investigated, and consequently, their electrochemical performance has significantly improved. High specific energy LIBs depend on cathode materials of high specific capacity or high voltage [11]. Layer-structured Ni-rich materials can provide more specific energy than LiFePO4 and LiCoO2. However, their higher nickel content leads to lower stability [12]. Previous research has shown that increasing the charging cutoff voltage is a practical approach for increasing energy density [13]. For example, LiNi0.5Co0.2Mn0.3O2 (denoted as NCM523) materials have been widely commercialized in electric vehicles (EVs) with good cyclic stability and acceptable safety performance. NCM523 materials are reported to have a reversible capacity of 160 mAh g−1 at 4.3 V (vs. Li/Li+) and deliver 190 mAh g−1 at 4.5 V (vs. Li/Li+) [14]. Unfortunately, the extraction of more lithium ions from NCM materials would initiate additional parasitic reactions in their interface during charging or storage [15–18]. It has been reported that in the fully charged state, Ni4+-O bonds are unstable and tend to turn to a more stable form of Ni2+ with oxygen release [6], which could react with the electrolyte and even trigger the battery thermal runaway [19]. In addition, irreversible transition metal (TM) migration and severe phase transformation could be accelerated at a higher potential, resulting in particle cracking, which could expose the new reaction
location inside the particles to the electrolyte and accelerate mechanical degradation [20]. Many studies have been devoted to overcoming the abovementioned problems, employing techniques such as surface coating [21], cation or anion doping [22–24], concentration gradient structures [25], and placing additives in the electrolyte [26–27]. Surface coating, which can serve as a protective barrier in the particle interface, has been considered one of the most effective ways to improve the performance of NCM materials. NCM materials coated with metal oxides (e.g., TiO2, ZrO2, and Al2O3), metal fluorides (e.g. LiF and AlF3), metal phosphates (e.g., Li-Mg-PO4, LiFePO4, and Li3PO4), and organic polymers have been reported to exhibit greatly improved electrochemical performance and thermal stability [21, 28, 29]. Titanate materials contain wider interlayer spaces and greater surface areas, which can facilitate the transportation of Li+ [30, 31]. For example, TiO2-coated LiNi0.6Co0.2Mn0.2O2 has been reported to present reduced electron-transfer resistance at the delithiated state and improved high-voltage electrochemical performance [28]. It has been reported that possible Ti4+ doping at the interface layer, as introduced by TiO2 coating preparation, resulted in more electrons being shared with the surrounding oxygen, and the bond strength of Ti-O was stronger than that of Ni-O [29]. Thus, both the Ti4+ doping effect and the robust TiO2 structure support a Ti-coated LiNi0.8Co0.1Mn0.1O2 material with enhanced thermostability [29]. In addition to inorganic coatings, PEDOT (poly(3,4-ethylenedioxythiophene)) conductive polymer coating on LiNi0.5Co0.2Mn0.3O2 was reported to improve conductivity and ionicity, suppress undesired phase transformation, and stabilize the interface. The
protective skin enhanced capacity and thermostability at high cutoff voltage [21]. However, most of these published studies used coin cells to investigate electrochemical performance or material-based thermal stability to deduce safety performance of a full battery. Given that measurement on the materials level is still far away from applicable performance, a study on electrochemical and safety performance in full batteries is necessary. In
this
study,
two
LiNi0.5Co0.2Mn0.3O2-based
kinds
battery
of
full
(denoted
as
batteries,
namely,
TiO2-NCM523)
TiO2-coated and
uncoated
LiNi0.5Co0.2Mn0.3O2-based battery (denoted as NCM523), were studied in terms of electrochemical performance and safety to elucidate the critical role of coating layers. Combining electrochemical impedance spectroscopy (EIS) and transmission electron microscopy (TEM) with time-of-flight secondary ion mass spectrometry (TOF-SIMS) characterization, we revealed an unprecedented correlation between the coating layer and parasitic reactions on the cathode under high voltage. By using in situ synchrotron high-energy X-ray diffraction (XRD) with heating and accelerating rate calorimetry, the correlation between coating layer and phase transformation under thermal driving forces was revealed. The TiO2-NCM523 battery showed less TM dissolution, fewer parasitic reactions, no obvious cathode electrolyte interface (CEI) layer, and a higher phase transformation temperature, in contrast with the NCM523 battery. As a result, the battery employing TiO2-NCM523 exhibited better electrochemical and safety performance at high voltage. This work can shed light on designing high energy density LIBs with high safety performance.
Experimental Pouch cells. Pouch cells (200 mAh) with graphite anodes and TiO2-NCM523 or NCM523 cathodes were purchased from LiFun Technology (Hunan Province, China, 412000).
Morphology characterization. The morphology of the cathode materials was characterized by scanning electron microscopy (SEM, Zeiss Merlin), TEM, JEOL JEM2010), and TOF-SIMS (ION-TOF GmbH, Münster, Germany).
Electrochemical measurements. To evaluate cycling and rate performance, the full batteries were charged and discharged between 2.7 and 4.5 V.
Differential scanning calorimeter (DSC) test. The DSC test was used to examine the thermostability of the cathode, which was performed on a Netzsch DSC214 Polyma. The samples were obtained by disassembling a fully charged cell in a glovebox. The cathode powder was scratched off from electrode after having been rinsed several times with dimethyl carbonate (DMC) to minimize lithium salt (LiPF6) in the porous electrode. To reduce oxygen and moisture contamination, the test samples were prepared in an argon atmosphere. For the DSC measurement, the powder (3.0 mg) was mixed with or
without the electrolyte (3.0 µL) in an aluminum crucible and the samples were tested from 30°C to 600°C (10°C min−1).
High-energy X-ray diffraction (HEXRD). In situ HEXRD during charge/discharge was obtained at Beamline 11 ID-C of the Advanced Photon Source (APS), Argonne National Laboratory. The wavelength was 0.1173 Å. The in situ HEXRD was conducted in half coin cells with NCM523/TiO2-NCM523 cathodes and lithium metal anodes. Charge/discharge was in the voltage range (2.8–4.5 V) at C/10 (1C = 200 mAh g-1). The in situ heating HEXRD was carried out at 17 BM at APS with a wavelength of 0.24125 Å. The charged cathode was collected from the fully charged battery. A homemade capillary furnace was used to heat the cathode powder from 30°C to 600°C at a heating rate of 10°C min−1. The obtained 2D diffraction patterns from a PerkinElmer amorphous silicon detector were converted to 1D patterns. Rietveld refinement with the GSAS II program was used to refine the obtained high-energy diffraction patterns and so monitor in situ lattice parameter evolution.
ARC test. The EV+ARC system (Thermal Hazard Technology) was used to evaluate the safety of the full cell. For each ARC test, two batteries were wrapped together, and type K thermocouples were used to monitor battery temperature. The ARC tests were performed with the heat-wait-seek method to ensure an adiabatic condition, with a heating step of 5°C, waiting time of 40 min, and self-heating rate of 0.02°C min−1.
Results and Discussion
The SEM images of NCM523 and TiO2-NCM523 electrodes are compared in Figure S1. Both NCM523 and TiO2-NCM523 cathodes consisted of spherical secondary particles, with sizes ranging from 2 µm to 10 µm. The particle dispersity of the two electrodes was quite similar, implying that particle morphology was not affected by the surface coating process. The inset of Figure S1b shows that a uniform coating layer was detected on the particle surface, with a thickness of about 8 nm. The crystallinity of the TiO2 layer was beneficial to lithium-ion transport on the particle interface, which is consistent with previous reports [32–34]. To understand the function of the TiO2 layer on the electrochemical performance of the cathode, in situ HEXRD was applied. Figures S2a and S2b show the voltage profiles alongside in situ diffraction peak evolution during charge/discharge for NCM523 and TiO2-NCM523. When charged to 4.5 V, the (003) peak of TiO2-NCM523 nearly returned to its original 2θ position with a moderate shift (Fig. S2b). By comparison, the shift of NCM523 deviated from its original 2θ position (Fig. S2a). Figure S3 shows the evolution of the unit cell lattice parameters of NCM523 (Fig. S3a) and TiO2-NCM523 (Fig. S3b). Anisotropic lattice distortion is regarded as one of the most important factors effecting the capacity degradation [35]. It is obvious that TiO2-NCM523 had a smaller volume contraction (1.1%) than NCM523 (1.53%). The smaller volume contraction can be attributed to the coating layer. The surface TiO2
coating suppressed the lattice “breathing,” which would restrain capacity fading, especially at high cutoff voltage [36].
Figure 1. The (a) cycling and (b) rate performances of NCM523- and TiO2-NCM523-based full batteries. The voltage windows for cycling and rate measurements are 2.7–4.5 V. Impedance spectrum evolution with an inset image of the equivalent circuit model of (c) TiO2-NCM523- and (d) NCM523-based full batteries. Both batteries were at 4.5 V after 1/10 C rate charging with voltage window of 2.7–4.5 V.
To explore capacity and cyclability, TiO2-NCM523- and NCM523-based full batteries were tested under a high rate of 1 C with voltage window of 2.7–4.5 V at room temperature. The NCM523-based battery presented a rapid capacity drop as it cycled. Retention was only 30% after 120 cycles (Figure 1a). However, the retention of the TiO2-NCM523-based battery was 80% at the 200th cycle, which indicates that the TiO2 coating layer improved cycling performance at high cutoff voltage, similar to results
that have been previously reported [37]. In addition to cycling performance, the voltage plateau of TiO2-NCM523 also significantly improved, as it declined more slowly than NCM523 (Fig. S4). The rate performances of NCM523- and TiO2-NCM523-based batteries were investigated between 2.7 and 4.5 V. As shown in Figure 1b, the two batteries were similar in rate capability, and the introduction of the TiO2 coating layer blocked the parasite reactions between NCM523 and the electrolyte while maintaining rate performance. The electrochemical impedance variations of NCM523- and TiO2-NCM523-based batteries were deconvoluted and quantified using an EIS test. Figures 1c and 1d show the Nyquist plots of NCM523- and TiO2-NCM523-based batteries at 4.5 V. Nyquist plots generally consist of two semicircles, which are assigned to the surface film resistance (Rs) and charge transfer resistance (Rct) [38], and the following tail is interpreted as an indicator for Li+ diffusion [39]. The EIS fitting data are listed in Table S1 and were obtained by using an equivalent circuit (inset image in Figure 1). The charge transfer (Rct) resistance of the uncoated NCM523-based battery showed an obvious increase during charge/discharge, which may be related to the formation of poorly conductive CEI on the particle surface [40]. However, the TiO2-NCM523-based battery showed a much smaller increase in Rct, which indicates that the TiO2 coating isolated electrode and the electrolyte. The isolation effect of the TiO2 coating restrained the side reactions on the interface and maintained the ionic and electronic conductivity during long-term charge/discharge. This result coincided with the cycling performance.
Figure 2. TEM images of (a) NCM523 and (b) TiO2-NCM523 cathodes after 200 cycles within a voltage between 2.7 and 4.5 V. (c) Depth profiles of NiF4−, CoF4−, and MnF4− collected on NCM523 and TiO2-NCM523 cathodes. Illustrative TOF-SIMS chemical mapping on (d) NCM523 and (e) TiO2-NCM523 cathodes after 200 cycles.
Figure 2a shows the TEM characterization results of NCM523 particles scraped from the battery after 200 cycles. A thick CEI layer (10–50 nm) formed on the cathode particle surface, caused by parasitic reactions at the interface between the transition metal ions and the electrolyte, especially at high voltage [41]. The conductivity of CEI
is very poor, and the accumulation of CEI created a vicious cycle, leading to the polarization increase and capacity fading of the NCM-based battery [26]. Unlike the NCM523, where the particle surface was covered with a thick CEI layer, the surface of the TiO2-NCM523 particle was much cleaner and the TiO2 coating layer well-preserved after 200 cycles, as shown in Figure 2b. TOF-SIMS was used to illustrate the chemical species on the cathode surface after extensive cycling by making use of its high sensitivity, ultrahigh selectivity, and quantitative advantages [42, 43]. Figure 2c shows the corresponding depth profiles of NiF4−, CoF4−, and MnF4− on NCM523 and TiO2-NCM523 electrodes disassembled from the pouch-type full batteries after 200 cycles. The detected species are largely caused by the surface parasitic oxidation and TM dissolution [44]. Figure 2d illustrates the TOF-SIMS chemical maps on a 100 µm × 100 µm cathode electrode disassembled from a NCM523 full battery after 200 cycles. TM species (NiF4−, CoF4−, and MnF4−) are scattered all over the cathode particle surface. However, for the TiO2-NCM523 electrode, the signal of the TM species is weak because the TiO2 coating isolated the cathode and electrolyte, and the parasitic reactions between them were largely inhibited (Figure 2e). The dissolved TM ions may react with ethyl/diethyl carbonates to form a metal complex surface film that can be stripped from the surface of the cathode during long-term cycling [45]. A portion of these metal complexes can migrate from cathode to anode [46–48]. This phenomenon is once again proved by the ICP result. Figure S5 compares the amount of TM ions on the anode surface after the battery was cycled 200 times. As shown, TM ions were detected in the anode, and thus TM dissolution can be
mitigated by TiO2 coating.
Figure 3. DSC curves of NCM523 and TiO2-NCM523 cathodes, disassembled from full batteries charged to 4.5 V, (a) without and (c) with electrolyte. (b) O2 release of delithiated NCM523 and TiO2-NCM523 during heating.
The abovementioned findings demonstrate that the isolation effect of TiO2 coating on interface reactions is helpful for improving electrochemical performance. It is natural to associate with the safety performance of the battery. The thermostability of active materials should be the first consideration in evaluating the safety of a battery [19, 49]. A highly delithiated cathode will experience structure change and release oxidizing species (O2, O2-, O-, etc.) during heating [50]. The reactions between the released highly reactive oxygen species and flammable organic electrolyte are highly exothermic, leading to severe thermal runaway [21]. For this reason, a robust surface layer that can isolate surface reactions should improve the safety of the battery’s chemistry. Therefore, to determine whether TiO2 could improve the thermostability of NCM523 toward the electrolyte, the highly delithiated bare NCM523 and TiO2-NCM523 cathodes were
characterized by DSC, as shown in Figure 3. The DSC curves and O2 spectra of the NCM523 and TiO2-NCM523 cathodes are shown in Figures 3a and 3b. The exothermic and oxygen release peaks corresponded with the phase transformation. The NCM523 sample exhibited a lower exothermal initial temperature around 223°C. The last exothermic peak at 510°C corresponded with the phase transition from spinel-type to rock-salt structure [51], while the TiO2-NCM523 sample shifted to a higher temperature (537°C). However, the exothermal behavior of the cathode with an electrolyte addition was quite different. For the NCM523 cathode, the exothermic peak initiated at 290°C, with an enthalpy change (∆H) of 110 J g−1 (Fig. 3c). The released oxidizing species (O2, O2-, O-, etc.) from the cathode were reduced by the electrolyte, which accelerated phase transformation [21]. For the TiO2-NCM523 cathode, the exothermic peak delayed at 295°C, with ∆H decreasing to 29 J g−1, indicating that the side reactions on the cathode surface were restrained by the isolation effect of the TiO2 layer.
Figure 4. Thermally induced phase transformation of charged (a) NCM523 and (b) TiO2-NCM523 cathode materials.
In Figure 4, phase transformation during the thermal decomposition process of NCM523 and TiO2-NCM523 cathode materials at the highly delithiated state is shown with in situ temperature-resolved XRD data. As shown in Fig. 4a, two splitting peaks (108 and 110) of the fully charged NCM523 cathode began to disappear at 240°C and disappeared completely at 320°C, corresponding with the change from layered (R3m) to spinel (Fd3 m) structure [53]. Subsequently, when the temperature rose to 420°C, a new peak appeared (the green zone in Fig. 4a), indicating irreversible TM cation
migration from layered (R3m) to spinel (Fd3 m) to rock-salt (Fm3 m) structure [54]. For highly delithiated TiO2-NCM523, the characteristic peak (108 and 110) of the layered structure began to disappear at 260°C and disappeared completely at 360°C, while the new peak appeared at 480°C. The phase transformation temperatures were higher than those of NCM523. TiO2 coating is thus helpful for enhancing NCM523 thermostability, and the postponed phase transformation is critical for improving cathode safety [19].
Figure 5. Thermal runaway characterization of NCM523 and TiO2-NCM523. Measured by ARC.
According to the DSC and in situ XRD results, the TiO2 layer improved safety performance at the raw material level. However, did the coating layer truly enhance the safety of a full battery? To answer this question, ARC tests were conducted on NCM523 and C-NCM523 full batteries charged to 4.5 V. T1, T2, and T3 stand for the self-heating
onset, thermal runaway (TR) ignition, and maximum temperature [55, 56]. The reason for battery self-heating is primarily the failure of the solid-electrolyte interphase (SEI). As the ARC system provides an adiabatic environment, the failure of SEI triggers continuous reactions between the electrolyte and anode, forming a chain reaction until triggering TR. At the T2 temperature, the heating rate of the battery is fastest during TR. Therefore, T2 is a critical index for evaluating the safety of the battery, and a higher T2 means better overall thermal safety. As shown in Figure 5, The T1s for NCM523 and TiO2-NCM523 were 55°C and 56°C, respectively (nearly the same, as the initial heating was already 5°C/step). The maximum temperature, T3, was 660°C for TiO2-NCM523, 72°C higher than for NCM523, indicating that fewer side reactions occurred for TiO2-NCM523 during the adiabatic stage. By connecting the abovementioned exothermic behavior of delithiated NCM materials with the presence of an electrolyte (Fig. 3c), the fewer side reactions of the TiO2-NMC523 battery can be mainly attributed to the isolation effect of TiO2 coating on interface reactions. The most important improvement for the TiO2-NMC battery was the delay of T2 at 257°C for TiO2-NCM523 compared with 251°C for NMC523 (Fig. 5b). Combined with the above discussion, the postponement of T2 can be attributed to two factors: (1) the TiO2 coating isolated the cathode surface from the electrolyte and inhibited parasitic reactions, and (2) the phase transformation temperature increased and the TR trigger, the release of oxidizing species (O2, O2-, O-, etc.), was suppressed. As a result, the TR of TiO2-NCM523 occurred 167 min later and at a temperature 6°C higher than that of the NCM523 battery (Fig. 5).
Conclusion We have demonstrated a new mechanism for a TiO2 coating layer on the cathode surface in full batteries under both electrochemical and thermal driving forces. This TiO2 coating layer isolated the active material from the electrolyte, thus slowing down parasitic oxidation and TM dissolution. The TiO2 coating layer postponed phase transformation during heating, significantly improving battery safety. This work paves the way to improve safety for battery chemistries of higher energy density. Acknowledgement This research is supported by the Ministry of Science and Technology of China under the Grant No. 2016YFE0102200. Research at the Argonne National Laboratory was funded by the U.S. Department of Energy (DOE), Vehicle Technologies Office. Support from Tien Duong of the U.S. DOE’s Office of Vehicle Technologies Program is gratefully acknowledged. Use of the Advanced Photon Source (APS), Office of Science user facilities, was supported by the U. S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Declaration The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] A. Manthiram, J.C. Knight, S.-T. Myung, S.-M. Oh, Y.-K. Sun, Adv. Energy Mater. 6 (2016) 1501010. [2] M. Li, J. Lu, Z.W. Chen, K. Amine, Adv. Mater. 30 (2018) 1800561. [3] S.-T. Myung, F. Maglia, K.-J. Park, C.S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett. 2 (2017) 196-223. [4] J. Lu, T.P. Wu, K. Amine, Nat. Energy 2 (2017) 17011.
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Dr. Yan Li received the Ph.D. degree in the College of Chemical Engineering from Nanjing Technology University, Nanjing, China, in 2016. He is currently a Post-Doc Researcher in Automotive Department in Tsinghua University. His main research interests focus on the fields of battery safety.
Dr. Xiang Liu is now work as postdoctoral researcher in Argonne National Laboratory. He obtained his PhD from the University of Hong Kong in 2016. Prior to his current position, he worked as a postdoc in Automotive Department in Tsinghua University. His research focus includes cathode material design and synthesis for lithium-ion and sodium-ion batteries, as well as the in situ/operando synchrotron X-ray based characterizations for the understanding of battery safety and degradation mechanisms.
Dr. Dongsheng Ren is now a postdoctoral researcher in Tsinghua University. He received his B.E.(2014) and Ph.D.(2020) degree in School of Vehicle and Mobility, Tsinghua University. His research focuses on understanding the degradation and thermal runaway mechanism of lithium-ion battery, as well as electrochemical/thermal models for lithium-ion battery.
Mr. Hungjen Hsu is a master student in Battery Safety Lab under Powertrain System Group in Tsinghua University. He mainly focuses on multi-scale safety of lithium-ion battery from material to cell level. Mr Hsu graduated from Energy Engineering of National Cheng Kung University in 2013 for his bachelor degree.
Dr. Gui-Liang Xu is currently an assistant chemist in the Electrochemical Energy Storage group under the division of Chemical Sciences and Engineering at Argonne National Laboratory. His researches focus on both fundamental understanding and materials development for lithium-ion batteries and beyond. Dr. Xu earned his Bachelor (2009) and PhD (2014) degree in the department of Chemistry of Xiamen University. After three years postdoc research at Argonne, he was promoted to permanent staff scientist. Dr. Xu has authors/co-authored 67 peer-reviewed research articles and has 1 U.S. patent and several patent applications.
Dr. Junxian Hou is now work as postdoctoral researcher in the Battery Safety Lab of Tsinghua University. She obtained his PhD from Beijing University of Technology in 2018. Her researches focus on much safer electrolyte development as well as thermal runaway mechanisms of different electrolytes in lithium-ion batteries.
Dr. Li Wang is an associate professor of the vice director of the Advanced Energy and Material Chemistry Lab in Tsinghua University. Her research is focused on the failure analysis on lithium ion and lithium metal batteries, aiming to enhance both the performances and safety of the batteries by exploring advanced materials and battery design.
Xuning Feng received the B.E. Degree and the Ph.D. degrees in the Department of Automotive Engineering from Tsinghua University, Beijing, China, in 2011 and 2017, respectively. He is currently a Post-Doc Researcher with Tsinghua University. His
research interests include battery management system and battery safety.
Dr. Languang Lu is an associate professor of School of Vehicle and Mobility Tsinghua University. His interesting research fields include the integration, optimization, and control of automotive new powertrains system and battery management system. He has more than 100 academic papers, 1 book and 83 patents. He has received 5 Technology Awards. The latest achievement “performance optimization control and system integration design and application of lithium-ion power battery for vehicle” won 2016 China automobile industry technology invention first prize.
Dr. Wenqian Xu is a beamline scientist at Advanced Photon Source at Argonne National Laboratory. He operates Beamline 17-BM, an X-ray powder diffraction instrument, and is interested in improving and applying in situ X-ray diffraction and total elastic scattering techniques to materials research. He received Ph. D. in Geosciences from Stony Brook University, and was a postdoctoral research associate at Brookhaven National Laboratory prior to his current position.
Yang Ren received his M.S. degree in condensed matter physics from the Institute of Physics, Chinese Academy of Science, China, and his Ph.D. degree in Chemical Physics from the University of Groningen, The Netherlands. He is currently a physicist and lead beamline scientist at the Advanced Photon Source, Argonne National Laboratory, USA. His research interests focus on the structure-property studies of materials by utilizing synchrotron X-ray and neutron scattering and other techniques. His research activities include the investigation of phase transition, correlated electron systems, engineering materials, nanoparticles and nanocomposites, energy storage and conversion materials.
Ruihe Li is currently a master student in the School of Vehicle and Mobility at Tsinghua University. He obtained his Bachelor’s degree from Tongji University. His research is mainly about the electrochemical-mechanical coupled effects of lithium ion battery, such as volume deformation of aged cells and resulting effects on battery degradation, effects of external stress on cycle life, state estimation via mechanical signal, etc. He has published some papers and patents and given several presentations in international academic conferences. He will graduate at Feb. 2021 and he is now looking for a PhD position on related fields.
Xiangming He is a Professor in Institute of Nuclear and New Energy Technology (INET) at Tsinghua University. He is the director of New Energy and Materials Chemistry Division in INET. In the R&D of lithium ion batteries and their key materials over 20 years, focusing on the pivotal scientific fundamental of the electrical performance and safety of lithium ion batteries. As the principal investigator, his research projects are granted by National Natural Science Foundation of China, Chinese Government and world-famous companies. He has authored/Edited 4 books. He has published over 500 papers and has been awarded over 400 patents.
Dr. Khalil Amine is an Argonne Distinguished Fellow and the leader of the Advanced Battery Technology team at Argonne National Laboratory, where he is responsible for directing the research and development of advanced materials and battery systems for HEV, PHEV, EV, and satellite applications. He is an adjunct professor at Stanford University. Among his many awards, Dr. Amine is the 2019 reception of the prestigious Global Energy Prize. He is a six-time recipient of the R&D 100 Award, which is considered as the Oscar of technology and innovation. He is an ECS fellow, and
associate editor of the journal of Nano-Energy.
Dr. Minggao Ouyang is Changjiang Distinguished Professor and the leader of Advanced Powertrain System team at Tsinghua University, where he is responsible for directing the research and development of Lithium-ion Battery Safety Design and Management, PEM Fuel Cell Powertrain and Hydrogen Systems, Engine Control and Hybrid Powertrains, Energy Storage and Smart Energy Systems. From 2007 to now, Prof. Ouyang Minggao has been the chief scientist of the China National Key R&D Program of
Highlights The TiO2 coating layer isolated the active material from the electrolyte, thus slowing down parasitic oxidation and transition metal dissolution Owing to the isolation effect of the TiO2 layer, the reaction between the charged cathode and the electrolyte is effectively inhibited, which greatly reducing the exothermic during thermal runaway; The TiO2 coating layer postponed phase transformation from layered to spinel to rock salt structure during heating, and the oxidizing species (O2, O2-, O-, etc.) release is suppressed, which is the thermal runaway trigger; Thermal runway of the TiO2-coated LiNi0.5Co0.2Mn0.3O2 battery occurred 167 min later and 6 oC higher than pristine LiNi0.5Co0.2Mn0.3O2 based battery.
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Declaration The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.