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Structure and thermal stability of LiNi0.8Co0.15Al0.05O2 after long cycling at high temperature
Journal of Power Sources 450 (2020) 227695
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Structure and thermal stability of LiNi0.8Co0.15Al0.05O2 after long cycling at high temperature Heyi Xia a, b, Cheng Liu a, b, Lu Shen a, Jing Yu a, b, Baohua Li a, Feiyu Kang a, b, Yan-Bing He a, * a b
Shenzhen Geim Graphene Center, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen, 518055, China Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China
H I G H L I G H T S
� Thermal stability of delithiated cycled LiNi0.8Co0.15Al0.05O2 is obviously inferior. � Particle surface of aged LiNi0.8Co0.15Al0.05O2 changes from layered to spinel phase. � Bulk area of aged LiNi0.8Co0.15Al0.05O2 maintains the layer structure. � Drastic structure transformation of aged LiNi0.8Co0.15Al0.05O2 occurs at 250 � C. A R T I C L E I N F O
A B S T R A C T
Keywords: In-situ X-ray diffraction Thermal stability Aged LiNi0.8Co0.15Al0.05O2 cathode material Lithium-ion battery
LiNi0.8Co0.15Al0.05O2 (NCA) is widely used as cathode material in commercialized high energy density lithium ion battery due to its high specific capacity. However, the structure and thermal stability of NCA after long electrochemical cycles are remained unclear. Herein, we investigate the structure and thermal stability of aged NCA at both lithiated and delithiated states using in-situ charge/discharge and high temperature X-ray diffraction and transmission electron microscopy images. The thermal stability of delithiated NCA cathode after 400 cycles at 1C is obviously inferior to the lithiated and pristine one. The particle surface of aged NCA changes from layered to spinel phase, while the bulk area maintains the layered structure. The temperature inducing structure change of aged NCA is much lower than that of pristine NCA and the delithiated state of NCA is more vulnerable at high temperature. The drastic structure transformation of aged NCA occurs at 250 � C during heating, which accompanies with oxygen loss and the formation of intergranular cracks in the NCA secondary particles. Thus, this work provides significant information of structure stability of cycled NCA at high temper ature for optimizing its surface structure to achieve excellent cycling performance under extreme conditions.
1. Introduction Lithium-ion batteries (LIBs) have gained widely application since its commercialization in the 1990s [1]. With the extensive use of energy storage battery in recent years, there is a strong requirement to develop higher energy density and better safety performance LIBs [2]. However, the increasing of energy density results in significant threat to the safety of batteries. One of the most catastrophic failure of LIBs is thermal runaway (TR) [3,4], which would be caused by overcharging [5], short circuiting [6], and mechanical stress [7,8]. TR could produce tremen dous heat, which in turn would trigger a chain exothermic reaction, eventually lead to firing and explosion of LIBs [9]. Therefore, the ther mal stability of battery components has a significant impact on battery
safety [10]. There are many studies on the thermal response of cathode [11], anode [12], electrolyte [13] and separator materials [14]. Among them, the oxide cathode materials are known to generate oxygen during TR [15], which would react with flammable electrolyte to cause cata strophic failure of batteries [16]. For cathode, the LiNi0.8Co0.15Al0.05O2 (NCA) has been successfully commercialized in electric vehicles (EV) application because of its high discharge capacity (~200 mAhg 1), excellent cycling life and low cost [17–19]. The NCA is often charged to a high voltage of 4.3 V and excessive extraction of lithium from NCA would cause structure thermal instability [20]. In addition, the NCA crystal would change from the layered structure (R-3m) to the disor dered spinel structure (Fd-3m), and finally to the rock-salt structure (Fm-3m) with increase in the temperature [21]. The more extensive
* Corresponding author. E-mail address: [email protected] (Y.-B. He). https://doi.org/10.1016/j.jpowsour.2019.227695 Received 19 October 2019; Received in revised form 2 December 2019; Accepted 31 December 2019 Available online 6 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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the NCA cathode and graphite-SiOx anode was 15 mg cm 2 and 7.4 mg cm 2, respectively. 10 Ah soft-packed full batteries were assembled using the NCA cathode, graphite-SiOx anode and liquid electrolyte (1 M LiPF6 solution dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC)/ethyl methyl carbonate (EMC), volume ratio of EC:DEC:EMC ¼ 1:1:1) [38]. The formation and cycle test were performed by a Land 2001A bat tery testing system and the batteries were cycled between 3.0 and 4.2 V at 0.5C (5 A) charge rate and 1C (10 A) discharge rate at 45 � C. The batteries after 400 cycles were chosen as the studied batteries and some of batteries were fully charged (4.2 V) and discharged (3.0 V), respec tively. The pristine batteries were only cycled for several times. The pristine, long cycled charged and discharged batteries were transferred into an argon-filled glove box for disassembly. The cathode materials were thoroughly washed by DEC solvent to remove residual salts for insitu XRD, SEM, TG and TEM measurements.
charge leads to the phase transition of NCA at lower temperature [22]. Tremendous literatures have investigated the thermal stability of cathode materials during TR [23–30]. It was reported that the oxygen release from LixCoO2 cathode crystals occurs at the surface of particles at elevated temperatures [31]. Yang et al. [32] adopted time-resolved synchrotron X-ray diffraction and mass spectroscopy technical to demonstrate the gas revolution related to the phase transition of NCA. They found that the state of charge of NCA affected both the structure changes and gas evolution during TR. Wang et al. [33] also found that the delithiated LiNi0.6Mn0.2Co0.2O2 would trigger the explosive nucle ation and propagation of the intragranular cracks in the lattice with increasing temperature. These cracks were attributed to the thermal stress and internal pressure caused by phase changes and oxygen evo lution. These studies enhanced our understanding of the structure changes underlying the thermal behavior of oxide cathode materials. However, almost all the studies on the thermal stability of cathode materials were focused on initial charged state and delithiated state [34–36]. As we all know that the TR mostly occurs at the aged battery. Therefore, to reveal the thermal stability of aged cathode materials is of great significance for the safety issues of batteries [37]. In this work, we investigated the thermal stability of NCA after 400 cycles in 10 Ah soft-packed full batteries by in-situ high temperature and charge/discharge X-ray diffraction (XRD) and high resolution trans mission electron microscope (HRTEM). The particle surfaces of aged NCA changes from layered to spinel phase, while the buck area main tains the layer structure (Fig. 1). The effect of temperature on the structure change of aged NCA is much larger than that of the pristine NCA. In addition, the delithiated state of cycled NCA is more vulnerable at higher temperature. The drastic structure transformation of aged NCA occurs at 250 � C during heating, which accompanies with the loss of oxygen and the formation of intergranular cracks in the NCA secondary particles. This work gives some significant information of structure stability of cycled NCA at high temperature for optimizing its surface structure to achieve excellent cycling performance at extreme conditions.
2.2. Materials characterization The XRD patterns of NCA cathode were collected using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ ¼ 1.5418 Å). The in-situ high temperature XRD of NCA was conducted from room temperature to 400 � C with a heating rate of ~3.0 � C min 1 in Ar atmosphere, and then backed to room temperature. In addition, the in-situ XRD of examination of NCA during charge-discharge were conducted at room temperature. The XRD patterns of samples were refined by Rietveld methods using Topas program. The TG test was done from room temperature to 400 � C with a heating rate of ~3.0 � C min 1 and the samples were dried at 100 � C in vacuum oven for 2 h to remove the residue water before TG test. For post-mortem analysis, the fresh and soft-packed batteries were transferred to an argon-filled glove box and dissembled to take out the cathode and anode. After that, the electrodes were rinsed by DEC to remove the residue electrolyte. Then, the NCA powders were gently abraded from the current collector. Subsequently, the NCA particles were cut with a focused ion beam (Leica EM RES 102) for the crosssection image acquisition. The surface morphologies and structures were examined using field emission scanning electron microscopy (FESEM, HITACHI SU8010, Japan) and a high resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30). The samples for TEM observation were heated from room temperature to 250 � C and 400 � C at a heating rate of 3 � C min 1 and then keep at 250 � C and 400 � C for 10 min in Ar atmosphere.
2. Experimental 2.1. Cathode material and cell test LiNi0.8Co0.15Al0.05O2 was purchased from Shenzhen BTR New En ergy Materials Co., Ltd and the cathode consists of 91 wt% LiNi0.8 Co0.15Al0.05O2, 4 wt% polyvinylidene fluoride (PVDF), 4 wt% Super-P, and 1 wt% carbon nanotubes (CNTs). The active material loading of
3. Results and discussion The in-situ high temperature XRD patterns from 25 to 400 � C and corresponding Rietveld refinement results of pristine, fully charged (CFC) and fully discharged (CFD) NCA after 400 cycles at 45 � C were shown in Fig. 2. The cycling performance of NCA was presented in our previous paper and the capacity retention was 43.8% after 400 cycles [38]. The primary reason for battery degradation was ascribed to the structure change of graphite-SiOx anode, while the aged NCA can maintain its layered structure [38]. It is seen from Fig. 2b and c that the peak intensity of NCA cathode after 400 cycles at 1C obviously decreases compared with the pristine NCA due to the high temperature cycling. When the temperature increases to 250 � C, the (018) and (110) peaks of CFC-NCA and CFD-NCA gradually merge into one peak, while this transformation occurs at nearly 400 � C for pristine NCA (Fig. 2a), which is corresponding to the structure transformation from trigonal to cubic phase in NCA. In addition, the (015), (009) and (113) peaks in aged NCA gradually disappear as the temperature increase from 250 to 400 � C, while these peak intensity diminish slightly in pristine NCA, indicating that the crystallinity of aged NCA reduces obviously upon heating. These results suggest that the thermal stability of aged NCA is obviously inferior to that of the pristine NCA [39]. To identify the lattice param eter change of NCA during in-situ examination from 25 to 400 � C, we
Fig. 1. Degradation mechanism of LiNi0.8Co0.15Al0.05O2 and the phase trans formation in aged materials upon heating. 2
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Fig. 2. In-situ high temperature XRD patterns of (a) pristine NCA; (b) CFC-NCA and (c) CFD-NCA from 25 to 400 � C. Rietveld refinement results of (d) lattice parameter a, (e) lattice parameter c and (f) cell volume of in-situ high temperature XRD.
conducted the Rietveld refinement of the XRD results (Fig. 2d–f). As shown in Fig. 2d, the CFC-NCA has the smallest lattice parameter a compared to that of pristine and CFD-NCA. Upon heating, the lattice parameter a of all the three samples increases but that of the CFC-NCA has a more drastic increase from 250 � C. This result suggests a notably increase of Ni–Ni distances in NCA [40]. In addition, there is an exten sion in the c axis in aged NCA at room temperature (Fig. 2e), which could be attributed to the loss of lithium ions in the lattice after long cycles [40]. Upon further heating, the lattice parameter c of CFC-NCA experi ences a sudden drop when the temperature reaches to 250 � C, and then rises as temperature increases. Since c axis is associated with electro static repulsion between adjacent oxygen layers [41], the larger decrease of the c value suggests that the electrostatic repulsion between slabs of CFC-NCA decreases sharply around 250 � C, which indicates a phase transition from the layer structure. It also can be seen that the cell volume of CFC-NCA increases obviously in Fig. 2f, which leads to the
volume change of primary particle and thus formation of the inter granular microcracks inside the secondary particles. These characteris tics indicate that the drastic structure transformation of aged NCA occurs at 250 � C during heating. Moreover, the layered structured CFC-NCA more easily transforms to NiO-like rock salt structure upon further heating [33]. In-situ charge/discharge XRD measurements of pristine NCA were performed to study the structure evolution during the charge/discharge process of NCA. It is seen from Fig. 3c that the (003) peak shifts consecutively to lower angles when the Liþ is extracted from the lattice. This phenomenon implies that the c axis expands and electrostatic repulsion between adjacent oxygen layers increases during charge. At the end of the charge, the oxygen in the lattice is likely to be activated, which would lead to structure instability. It is worth noting that the (003) peak shifts slightly back to initial angle when the voltage near cutoff voltage. This may because the Ni2þ atoms nearby would remove to 3
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Fig. 3. (a) In-situ XRD patterns collected during the first charge/discharge of the pristine NCA at room temperature at 0.1C between 2.7 and 4.3 V; Voltage curve of charge and discharge program(b). Enlarged XRD patterns of pristine NCA between 17� and 20� (c) and 35� –39� (d). TG curve of three samples from 25 to 400 � C (e); Element contents of CFD-NCA (f) and CFC-NCA (g) samples before and after heating to 400 � C, characterized by inductively coupled plasma (ICP).
unoccupied Li sites in Li layers due to the similar radius between Ni2þ (0.69 Å) and Liþ (0.76 Å) atom [42]. The (003) diffraction peak would slightly deviate from its original position after discharge. Similarly, the (101) peak shifts to higher angles during charge and shifts back to lower angles during discharge, but this peak deviates from its origin position to higher angles after the first cycle (Fig. 3d). The above microstructure change of NCA is accumulated during cycling. Therefore, many peaks have obviously deviated from their original position after 400 cycles (Fig. S1). This would lead to structure degradation and further capacity decay. To examine gas evolution behavior accompanying the structure change during heating, we conducted the thermogravimetry (TGA) analysis on pristine, CFC-NCA and CFD-NCA from 25 to 400 � C. It has been reported in previous literature that the PVDF binder and con ducting carbon materials are quite stable to temperatures as high as 450 � C and 700 � C under inert gas flow, respectively [25,43,44]. Therefore the weight loss during heating is mainly caused by the cathode material. As shown in Fig. 3e, the pristine and CFD-NCA are both stable until 250 � C, after which the weight of those samples decreases obviously. The weight losses of pristine and CFD-NCA are 4.82% and 8.79% when the temperature reaches 400 � C. In contrast, the dramatic weight loss of CFC-NCA occurs at 210 � C, and its weight loss is 12.21% when the temperature reaches 400 � C. Therefore, the CFC-NCA is much less tolerant of thermal abuse than pristine and CFD-NCA. In order to iden tify the components of the weight loss, the ICP-AES of CFC-NCA and CFD-NCA was performed. Fig. 3f and g show the element content of Ni, Co, Al, and Li of CFD-NCA and CFC-NCA before and after heating. It is seen that the CFC-NCA sample presents much lower content of Li element than that of CFD-NCA, which is because the CFC-NCA was derived from the cycled battery that charged to 4.2 V and CFD-NCA was from the one that discharged to 3.0 V. During the charge process, the
lithium-ions transfer from NCA to anode to form a delithiated NCA. In contrast, during discharge process, the lithium-ions are inserted into NCA. Therefore, the lithium content of CFC-NCA was much lower than that of CFD-NCA. It is also seen that the contents of these elements all remain the same after heating. Therefore, the weight loss from the TGA during heating can be interpreted as the weight loss of oxygen. These results indicate that aged NCA go through a phase transition accompa nied by oxygen species release on heating [32]. The mechanical degradation of NCA secondary particles upon heat ing was confirmed by SEM images. Although the particle surface has little change either after long cycles or after heating treatment (Fig. S2), severe degradation occurs inside the particles. As shown in Fig. 4, the microcrack numbers of CFC-NCA (Fig. 4c) and CFD-NCA (Fig. 4d) are almost the same with pristine NCA (Fig. 4a), indicating that long elec trochemical cycles even at 45 � C cannot lead to the microcrack gener ation. However, as temperature reaches to 400 � C, the crack numbers of CFC-NCA and CFD-NCA (Fig. 4d and f) increase greatly while that of pristine NCA (Fig. 4b) have a slight increase. More specially, the CFCNCA secondary particle undergoes a crack explosion and its crystalline grains are disconnected with each other after heating to 400 � C (Fig. 4d), which is probably caused by stress accumulation induced by structure transformation and oxygen evolution inside the particles [33]. As shown in Fig. 2f, the cell volume expansion of CFC-NCA was obvious, which increased from 99.5 to 105.5 Å3 after heating to 400 � C. This large in crease of cell volume would cause the expansion of single crystalline grain eventually, which leads to the more disconnected among the crystalline grains. To probe the structure transformation upon heating, the TEM, highresolution electron microscopy (HRTEM) and corresponding fast Fourier transformation (FFT) of the selected area examination were conducted for the NCA and CFC-NCA. Fig. 5a shows that the pristine NCA presents a 4
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Fig. 4. Cross section SEM images of pristine NCA (a, b), CFC-NCA (c, d) and CFD-NCA (e, f) particles before and after 400 � C heat treatment.
well-ordered crystalline layered structure. Whereas, the CFC-NCA pre sents obvious structure change that is mainly localized on the surface although the bulk region remains the rhombohedral phase. In addition, the diffraction spots of the surface area are weaker than those of the bulk area, and dots in FFT which represent (104)R planes are almost invisible (Fig. 5b). The reduction in the number of diffraction spots indicates the formation of a high symmetry phase [45]. Moreover, the emergence of a diffraction spot in the red circle from the FFT of the CFC-NCA surface area in Fig. 5b could be indexed as (111)s plane, which is corresponding to the spinel phase. The spinel phase is continuously observed over distances 10–15 nm from the surface. This result indicates that long electrochemical cycles (45 � C, 400 cycles) would produce a passivation
spinel layer around the particle surface, which is caused by the etching of electrolyte during charge and discharge process [46]. The HRTEM image for CFC-NCA after heating to 250 � C shows that the bulk and surface areas are both transformed to the rock-salt structure (Fig. 5c). This result indicates that the heating-induced structure change is ho mogenous among aged material particles, and the whole particle of the CFC-NCA would transform to the rock-salt structure after heating to 250 � C. Furthermore, the atomic-resolution STEM and corresponding FFT image of the CFC-NCA heated to 400 � C was conducted as shown in Fig. 5d. It is seen that the whole particle still maintains rock-salt phase but some pits are observed (white circles in Fig. 5d). It is well-known
Fig. 5. HRTEM images and FFTs of the selected area acquired in (a) pristine NCA, (b) CFC-NCA and (c) CFC-NCA after heating to 250 � C. (d) Atomic-resolution STEM-HAADF image of CFC-NCA after heating to 400 � C. Inset displays the whole particle image (upper right) and the FFT of the selected area. R, S and C denote the layered, spinel, and rock-salt structures, respectively. 5
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that the bright-dots in STEM-HAADF images represent the position of TM(Ni, Mn, and Al) columns [47]. Therefore, the appearance of pits indicates the loss of atoms, which are mainly O elements. It is also seen that the pits are homogeneously dispersed in the particle, indicating that the heating-induce pits and the structure transformation are uniformly generated throughout the particle. Combined with previous in-situ XRD and TG examination results, it is demonstrated that some bonding of oxygen atoms is destroyed and leads to more dangling oxygen anions in aged materials upon heating, which accelerates the diffusion of oxygen and induces more oxygen release [48]. As a result, the structure of CFC-NCA would change enormously accompanying with the oxygen release.
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4. Conclusions The thermal stability of aged commercial NCA cathode materials was systematically investigated. The particle surface of aged NCA would change from layered to spinel phase, while the bulk area maintains its layer structure. Upon heating, the effects of high temperature on the structure change of aged NCA is much lower than that of pristine NCA, and delithiated state of aged NCA is more vulnerable at high tempera ture, whose structure would transform to rock-salt phase after heating to 250 � C accompanying with the loss of oxygen due to the sharp decrease of the electrostatic repulsion between adjacent oxygen layers. The ox ygen in delithiated state of aged NCA is easily generated during heating and the heating-induced structure change is homogenous throughout the particle. Furthermore, the oxygen release and the structure change during heating result in intergranular cracks generated inside secondary particles. This study sheds light on the internal causes of thermal sta bility of aged commercial NCA and helps fundamentally understand the safety issue of cathode materials. Declaration of competing interest 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. Acknowledgment This work was supported by the National Natural Science Foundation of China (51672156), Guangdong Special Support Program (2015TQ01N401), Guangdong Province Technical Plan Project (2017B010119001 and 2017B090907005), Shenzhen Technical Plan Project (JCYJ20170412170706047, JCYJ20170307153806471 and GJHS20170314165324888), and Shenzhen Graphene Manufacturing Innovation Center (201901161513). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227695. References [1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367, https://doi.org/10.1142/9789814317665_ 0024. [2] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008) 652–657, https://doi.org/10.1038/451652a. [3] X. Feng, X. He, M. Ouyang, L. Lu, P. Wu, C. Kulp, S. Prasser, Thermal runaway propagation model for designing a safer battery pack with 25 Ah LiNiCoMnO2 large format lithium ion battery, Appl. Energy 154 (2015) 74–91, https://doi.org/ 10.1016/j.apenergy.2015.04.118. [4] G.L. Woods, K.L. White, D.K. Vanderwall, G.P. Li, K.I. Aston, T.D. Bunch, L. N. Meerdo, B.J. Pate, A mule cloned from fetal cells by nuclear transfer, Science 301 (2003) 1063, https://doi.org/10.1126/science.1086743.
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Structure and thermal stability of LiNi0.8Co0.15Al0.05O2 after long cycling at high temperature Heyi Xia a, b, Cheng Liu a, b, Lu Shen a, Jing Yu a, b, Baohua Li a, Feiyu Kang a, b, Yan-Bing He a, * a b
Shenzhen Geim Graphene Center, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen, 518055, China Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China
H I G H L I G H T S
� Thermal stability of delithiated cycled LiNi0.8Co0.15Al0.05O2 is obviously inferior. � Particle surface of aged LiNi0.8Co0.15Al0.05O2 changes from layered to spinel phase. � Bulk area of aged LiNi0.8Co0.15Al0.05O2 maintains the layer structure. � Drastic structure transformation of aged LiNi0.8Co0.15Al0.05O2 occurs at 250 � C. A R T I C L E I N F O
A B S T R A C T
Keywords: In-situ X-ray diffraction Thermal stability Aged LiNi0.8Co0.15Al0.05O2 cathode material Lithium-ion battery
LiNi0.8Co0.15Al0.05O2 (NCA) is widely used as cathode material in commercialized high energy density lithium ion battery due to its high specific capacity. However, the structure and thermal stability of NCA after long electrochemical cycles are remained unclear. Herein, we investigate the structure and thermal stability of aged NCA at both lithiated and delithiated states using in-situ charge/discharge and high temperature X-ray diffraction and transmission electron microscopy images. The thermal stability of delithiated NCA cathode after 400 cycles at 1C is obviously inferior to the lithiated and pristine one. The particle surface of aged NCA changes from layered to spinel phase, while the bulk area maintains the layered structure. The temperature inducing structure change of aged NCA is much lower than that of pristine NCA and the delithiated state of NCA is more vulnerable at high temperature. The drastic structure transformation of aged NCA occurs at 250 � C during heating, which accompanies with oxygen loss and the formation of intergranular cracks in the NCA secondary particles. Thus, this work provides significant information of structure stability of cycled NCA at high temper ature for optimizing its surface structure to achieve excellent cycling performance under extreme conditions.
1. Introduction Lithium-ion batteries (LIBs) have gained widely application since its commercialization in the 1990s [1]. With the extensive use of energy storage battery in recent years, there is a strong requirement to develop higher energy density and better safety performance LIBs [2]. However, the increasing of energy density results in significant threat to the safety of batteries. One of the most catastrophic failure of LIBs is thermal runaway (TR) [3,4], which would be caused by overcharging [5], short circuiting [6], and mechanical stress [7,8]. TR could produce tremen dous heat, which in turn would trigger a chain exothermic reaction, eventually lead to firing and explosion of LIBs [9]. Therefore, the ther mal stability of battery components has a significant impact on battery
safety [10]. There are many studies on the thermal response of cathode [11], anode [12], electrolyte [13] and separator materials [14]. Among them, the oxide cathode materials are known to generate oxygen during TR [15], which would react with flammable electrolyte to cause cata strophic failure of batteries [16]. For cathode, the LiNi0.8Co0.15Al0.05O2 (NCA) has been successfully commercialized in electric vehicles (EV) application because of its high discharge capacity (~200 mAhg 1), excellent cycling life and low cost [17–19]. The NCA is often charged to a high voltage of 4.3 V and excessive extraction of lithium from NCA would cause structure thermal instability [20]. In addition, the NCA crystal would change from the layered structure (R-3m) to the disor dered spinel structure (Fd-3m), and finally to the rock-salt structure (Fm-3m) with increase in the temperature [21]. The more extensive
* Corresponding author. E-mail address: [email protected] (Y.-B. He). https://doi.org/10.1016/j.jpowsour.2019.227695 Received 19 October 2019; Received in revised form 2 December 2019; Accepted 31 December 2019 Available online 6 January 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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the NCA cathode and graphite-SiOx anode was 15 mg cm 2 and 7.4 mg cm 2, respectively. 10 Ah soft-packed full batteries were assembled using the NCA cathode, graphite-SiOx anode and liquid electrolyte (1 M LiPF6 solution dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC)/ethyl methyl carbonate (EMC), volume ratio of EC:DEC:EMC ¼ 1:1:1) [38]. The formation and cycle test were performed by a Land 2001A bat tery testing system and the batteries were cycled between 3.0 and 4.2 V at 0.5C (5 A) charge rate and 1C (10 A) discharge rate at 45 � C. The batteries after 400 cycles were chosen as the studied batteries and some of batteries were fully charged (4.2 V) and discharged (3.0 V), respec tively. The pristine batteries were only cycled for several times. The pristine, long cycled charged and discharged batteries were transferred into an argon-filled glove box for disassembly. The cathode materials were thoroughly washed by DEC solvent to remove residual salts for insitu XRD, SEM, TG and TEM measurements.
charge leads to the phase transition of NCA at lower temperature [22]. Tremendous literatures have investigated the thermal stability of cathode materials during TR [23–30]. It was reported that the oxygen release from LixCoO2 cathode crystals occurs at the surface of particles at elevated temperatures [31]. Yang et al. [32] adopted time-resolved synchrotron X-ray diffraction and mass spectroscopy technical to demonstrate the gas revolution related to the phase transition of NCA. They found that the state of charge of NCA affected both the structure changes and gas evolution during TR. Wang et al. [33] also found that the delithiated LiNi0.6Mn0.2Co0.2O2 would trigger the explosive nucle ation and propagation of the intragranular cracks in the lattice with increasing temperature. These cracks were attributed to the thermal stress and internal pressure caused by phase changes and oxygen evo lution. These studies enhanced our understanding of the structure changes underlying the thermal behavior of oxide cathode materials. However, almost all the studies on the thermal stability of cathode materials were focused on initial charged state and delithiated state [34–36]. As we all know that the TR mostly occurs at the aged battery. Therefore, to reveal the thermal stability of aged cathode materials is of great significance for the safety issues of batteries [37]. In this work, we investigated the thermal stability of NCA after 400 cycles in 10 Ah soft-packed full batteries by in-situ high temperature and charge/discharge X-ray diffraction (XRD) and high resolution trans mission electron microscope (HRTEM). The particle surfaces of aged NCA changes from layered to spinel phase, while the buck area main tains the layer structure (Fig. 1). The effect of temperature on the structure change of aged NCA is much larger than that of the pristine NCA. In addition, the delithiated state of cycled NCA is more vulnerable at higher temperature. The drastic structure transformation of aged NCA occurs at 250 � C during heating, which accompanies with the loss of oxygen and the formation of intergranular cracks in the NCA secondary particles. This work gives some significant information of structure stability of cycled NCA at high temperature for optimizing its surface structure to achieve excellent cycling performance at extreme conditions.
2.2. Materials characterization The XRD patterns of NCA cathode were collected using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ ¼ 1.5418 Å). The in-situ high temperature XRD of NCA was conducted from room temperature to 400 � C with a heating rate of ~3.0 � C min 1 in Ar atmosphere, and then backed to room temperature. In addition, the in-situ XRD of examination of NCA during charge-discharge were conducted at room temperature. The XRD patterns of samples were refined by Rietveld methods using Topas program. The TG test was done from room temperature to 400 � C with a heating rate of ~3.0 � C min 1 and the samples were dried at 100 � C in vacuum oven for 2 h to remove the residue water before TG test. For post-mortem analysis, the fresh and soft-packed batteries were transferred to an argon-filled glove box and dissembled to take out the cathode and anode. After that, the electrodes were rinsed by DEC to remove the residue electrolyte. Then, the NCA powders were gently abraded from the current collector. Subsequently, the NCA particles were cut with a focused ion beam (Leica EM RES 102) for the crosssection image acquisition. The surface morphologies and structures were examined using field emission scanning electron microscopy (FESEM, HITACHI SU8010, Japan) and a high resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30). The samples for TEM observation were heated from room temperature to 250 � C and 400 � C at a heating rate of 3 � C min 1 and then keep at 250 � C and 400 � C for 10 min in Ar atmosphere.
2. Experimental 2.1. Cathode material and cell test LiNi0.8Co0.15Al0.05O2 was purchased from Shenzhen BTR New En ergy Materials Co., Ltd and the cathode consists of 91 wt% LiNi0.8 Co0.15Al0.05O2, 4 wt% polyvinylidene fluoride (PVDF), 4 wt% Super-P, and 1 wt% carbon nanotubes (CNTs). The active material loading of
3. Results and discussion The in-situ high temperature XRD patterns from 25 to 400 � C and corresponding Rietveld refinement results of pristine, fully charged (CFC) and fully discharged (CFD) NCA after 400 cycles at 45 � C were shown in Fig. 2. The cycling performance of NCA was presented in our previous paper and the capacity retention was 43.8% after 400 cycles [38]. The primary reason for battery degradation was ascribed to the structure change of graphite-SiOx anode, while the aged NCA can maintain its layered structure [38]. It is seen from Fig. 2b and c that the peak intensity of NCA cathode after 400 cycles at 1C obviously decreases compared with the pristine NCA due to the high temperature cycling. When the temperature increases to 250 � C, the (018) and (110) peaks of CFC-NCA and CFD-NCA gradually merge into one peak, while this transformation occurs at nearly 400 � C for pristine NCA (Fig. 2a), which is corresponding to the structure transformation from trigonal to cubic phase in NCA. In addition, the (015), (009) and (113) peaks in aged NCA gradually disappear as the temperature increase from 250 to 400 � C, while these peak intensity diminish slightly in pristine NCA, indicating that the crystallinity of aged NCA reduces obviously upon heating. These results suggest that the thermal stability of aged NCA is obviously inferior to that of the pristine NCA [39]. To identify the lattice param eter change of NCA during in-situ examination from 25 to 400 � C, we
Fig. 1. Degradation mechanism of LiNi0.8Co0.15Al0.05O2 and the phase trans formation in aged materials upon heating. 2
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Fig. 2. In-situ high temperature XRD patterns of (a) pristine NCA; (b) CFC-NCA and (c) CFD-NCA from 25 to 400 � C. Rietveld refinement results of (d) lattice parameter a, (e) lattice parameter c and (f) cell volume of in-situ high temperature XRD.
conducted the Rietveld refinement of the XRD results (Fig. 2d–f). As shown in Fig. 2d, the CFC-NCA has the smallest lattice parameter a compared to that of pristine and CFD-NCA. Upon heating, the lattice parameter a of all the three samples increases but that of the CFC-NCA has a more drastic increase from 250 � C. This result suggests a notably increase of Ni–Ni distances in NCA [40]. In addition, there is an exten sion in the c axis in aged NCA at room temperature (Fig. 2e), which could be attributed to the loss of lithium ions in the lattice after long cycles [40]. Upon further heating, the lattice parameter c of CFC-NCA experi ences a sudden drop when the temperature reaches to 250 � C, and then rises as temperature increases. Since c axis is associated with electro static repulsion between adjacent oxygen layers [41], the larger decrease of the c value suggests that the electrostatic repulsion between slabs of CFC-NCA decreases sharply around 250 � C, which indicates a phase transition from the layer structure. It also can be seen that the cell volume of CFC-NCA increases obviously in Fig. 2f, which leads to the
volume change of primary particle and thus formation of the inter granular microcracks inside the secondary particles. These characteris tics indicate that the drastic structure transformation of aged NCA occurs at 250 � C during heating. Moreover, the layered structured CFC-NCA more easily transforms to NiO-like rock salt structure upon further heating [33]. In-situ charge/discharge XRD measurements of pristine NCA were performed to study the structure evolution during the charge/discharge process of NCA. It is seen from Fig. 3c that the (003) peak shifts consecutively to lower angles when the Liþ is extracted from the lattice. This phenomenon implies that the c axis expands and electrostatic repulsion between adjacent oxygen layers increases during charge. At the end of the charge, the oxygen in the lattice is likely to be activated, which would lead to structure instability. It is worth noting that the (003) peak shifts slightly back to initial angle when the voltage near cutoff voltage. This may because the Ni2þ atoms nearby would remove to 3
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Fig. 3. (a) In-situ XRD patterns collected during the first charge/discharge of the pristine NCA at room temperature at 0.1C between 2.7 and 4.3 V; Voltage curve of charge and discharge program(b). Enlarged XRD patterns of pristine NCA between 17� and 20� (c) and 35� –39� (d). TG curve of three samples from 25 to 400 � C (e); Element contents of CFD-NCA (f) and CFC-NCA (g) samples before and after heating to 400 � C, characterized by inductively coupled plasma (ICP).
unoccupied Li sites in Li layers due to the similar radius between Ni2þ (0.69 Å) and Liþ (0.76 Å) atom [42]. The (003) diffraction peak would slightly deviate from its original position after discharge. Similarly, the (101) peak shifts to higher angles during charge and shifts back to lower angles during discharge, but this peak deviates from its origin position to higher angles after the first cycle (Fig. 3d). The above microstructure change of NCA is accumulated during cycling. Therefore, many peaks have obviously deviated from their original position after 400 cycles (Fig. S1). This would lead to structure degradation and further capacity decay. To examine gas evolution behavior accompanying the structure change during heating, we conducted the thermogravimetry (TGA) analysis on pristine, CFC-NCA and CFD-NCA from 25 to 400 � C. It has been reported in previous literature that the PVDF binder and con ducting carbon materials are quite stable to temperatures as high as 450 � C and 700 � C under inert gas flow, respectively [25,43,44]. Therefore the weight loss during heating is mainly caused by the cathode material. As shown in Fig. 3e, the pristine and CFD-NCA are both stable until 250 � C, after which the weight of those samples decreases obviously. The weight losses of pristine and CFD-NCA are 4.82% and 8.79% when the temperature reaches 400 � C. In contrast, the dramatic weight loss of CFC-NCA occurs at 210 � C, and its weight loss is 12.21% when the temperature reaches 400 � C. Therefore, the CFC-NCA is much less tolerant of thermal abuse than pristine and CFD-NCA. In order to iden tify the components of the weight loss, the ICP-AES of CFC-NCA and CFD-NCA was performed. Fig. 3f and g show the element content of Ni, Co, Al, and Li of CFD-NCA and CFC-NCA before and after heating. It is seen that the CFC-NCA sample presents much lower content of Li element than that of CFD-NCA, which is because the CFC-NCA was derived from the cycled battery that charged to 4.2 V and CFD-NCA was from the one that discharged to 3.0 V. During the charge process, the
lithium-ions transfer from NCA to anode to form a delithiated NCA. In contrast, during discharge process, the lithium-ions are inserted into NCA. Therefore, the lithium content of CFC-NCA was much lower than that of CFD-NCA. It is also seen that the contents of these elements all remain the same after heating. Therefore, the weight loss from the TGA during heating can be interpreted as the weight loss of oxygen. These results indicate that aged NCA go through a phase transition accompa nied by oxygen species release on heating [32]. The mechanical degradation of NCA secondary particles upon heat ing was confirmed by SEM images. Although the particle surface has little change either after long cycles or after heating treatment (Fig. S2), severe degradation occurs inside the particles. As shown in Fig. 4, the microcrack numbers of CFC-NCA (Fig. 4c) and CFD-NCA (Fig. 4d) are almost the same with pristine NCA (Fig. 4a), indicating that long elec trochemical cycles even at 45 � C cannot lead to the microcrack gener ation. However, as temperature reaches to 400 � C, the crack numbers of CFC-NCA and CFD-NCA (Fig. 4d and f) increase greatly while that of pristine NCA (Fig. 4b) have a slight increase. More specially, the CFCNCA secondary particle undergoes a crack explosion and its crystalline grains are disconnected with each other after heating to 400 � C (Fig. 4d), which is probably caused by stress accumulation induced by structure transformation and oxygen evolution inside the particles [33]. As shown in Fig. 2f, the cell volume expansion of CFC-NCA was obvious, which increased from 99.5 to 105.5 Å3 after heating to 400 � C. This large in crease of cell volume would cause the expansion of single crystalline grain eventually, which leads to the more disconnected among the crystalline grains. To probe the structure transformation upon heating, the TEM, highresolution electron microscopy (HRTEM) and corresponding fast Fourier transformation (FFT) of the selected area examination were conducted for the NCA and CFC-NCA. Fig. 5a shows that the pristine NCA presents a 4
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Fig. 4. Cross section SEM images of pristine NCA (a, b), CFC-NCA (c, d) and CFD-NCA (e, f) particles before and after 400 � C heat treatment.
well-ordered crystalline layered structure. Whereas, the CFC-NCA pre sents obvious structure change that is mainly localized on the surface although the bulk region remains the rhombohedral phase. In addition, the diffraction spots of the surface area are weaker than those of the bulk area, and dots in FFT which represent (104)R planes are almost invisible (Fig. 5b). The reduction in the number of diffraction spots indicates the formation of a high symmetry phase [45]. Moreover, the emergence of a diffraction spot in the red circle from the FFT of the CFC-NCA surface area in Fig. 5b could be indexed as (111)s plane, which is corresponding to the spinel phase. The spinel phase is continuously observed over distances 10–15 nm from the surface. This result indicates that long electrochemical cycles (45 � C, 400 cycles) would produce a passivation
spinel layer around the particle surface, which is caused by the etching of electrolyte during charge and discharge process [46]. The HRTEM image for CFC-NCA after heating to 250 � C shows that the bulk and surface areas are both transformed to the rock-salt structure (Fig. 5c). This result indicates that the heating-induced structure change is ho mogenous among aged material particles, and the whole particle of the CFC-NCA would transform to the rock-salt structure after heating to 250 � C. Furthermore, the atomic-resolution STEM and corresponding FFT image of the CFC-NCA heated to 400 � C was conducted as shown in Fig. 5d. It is seen that the whole particle still maintains rock-salt phase but some pits are observed (white circles in Fig. 5d). It is well-known
Fig. 5. HRTEM images and FFTs of the selected area acquired in (a) pristine NCA, (b) CFC-NCA and (c) CFC-NCA after heating to 250 � C. (d) Atomic-resolution STEM-HAADF image of CFC-NCA after heating to 400 � C. Inset displays the whole particle image (upper right) and the FFT of the selected area. R, S and C denote the layered, spinel, and rock-salt structures, respectively. 5
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that the bright-dots in STEM-HAADF images represent the position of TM(Ni, Mn, and Al) columns [47]. Therefore, the appearance of pits indicates the loss of atoms, which are mainly O elements. It is also seen that the pits are homogeneously dispersed in the particle, indicating that the heating-induce pits and the structure transformation are uniformly generated throughout the particle. Combined with previous in-situ XRD and TG examination results, it is demonstrated that some bonding of oxygen atoms is destroyed and leads to more dangling oxygen anions in aged materials upon heating, which accelerates the diffusion of oxygen and induces more oxygen release [48]. As a result, the structure of CFC-NCA would change enormously accompanying with the oxygen release.
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