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Solvothermal coating LiNi0.8Co0.15Al0.05O2 microspheres with nanoscale Li2TiO3 shell for long lifespan Li-ion battery cathode materials
Journal of Alloys and Compounds 665 (2016) 48e56
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Solvothermal coating LiNi0.8Co0.15Al0.05O2 microspheres with nanoscale Li2TiO3 shell for long lifespan Li-ion battery cathode materials Naiteng Wu, Hao Wu, Heng Liu, Yun Zhang* Department of Advanced Energy Materials, College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 December 2015 Received in revised form 31 December 2015 Accepted 7 January 2016 Available online 11 January 2016
LiNi0.8Co0.15Al0.05O2 (NCA) microspheres covered by a nanoscale Li2TiO3-based shell were synthesized by a facile strategy based on a solvothermal pre-coating treatment combined with a post-sintering lithiation process. The morphology, structure and composition of the Li2TiO3-coated NCA samples were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning scanning electron microscope (SEM) with an energy-dispersive X-ray spectroscope (EDS), and transmission electron microscopy (TEM). Owing to the complete, uniform and nanoscale Li2TiO3 coating shell, the resultant surface-modified NCA microspheres used as Li-ion battery cathode materials manifest remarkably enhanced cycling performances, attaining 94% and 84% capacity retention after 200 and 400 cycles at 0.5 C, respectively, which is much better than the pristine NCA counterpart (60% retention, 200 cycles). More impressively, the surface-modified NCA also shows an intriguing storage stability. After being stored at 30 C for 50 days, the coated NCA-based cells are subjected to be cycled both at room and elevated temperatures, in which the aged cells can still remain 84% capacity retention after 200 cycles at 25 C and 77% capacity retention after 200 cycles at 55 C, respectively. All these results demonstrate that the Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 microsphere is a promising cathode material for Li-ion batteries with long lifespan. © 2016 Elsevier B.V. All rights reserved.
Keywords: Solvothermal Li2TiO3 coating LiNi0.8Co0.15Al0.05O2 Long lifespan Lithium-ion batteries
1. Introduction Lithium ion batteries (LIBs) have been one of the most attractive candidates for electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their high energy and high power density, but these potential applications require both extended cycle life and prolonged calendar life [1,2]. Recently, more attentions have been focused on Ni-based ternary oxides, among which LiNi0.8Co0.15Al0.05O2 (NCA) has been considered as the most promising cathode materials for scalable applications in EVs and HEVs due to its high capacity and low cost compared to other commercial cathode materials such as LiCoO2, LiMn2O4, and LiFePO4 [3,4]. However, the LIBs with the NCA cathode material still suffer from a remarkable decline in capacity retention during a long-term cycle use and/or after a long period of storage [5]. According to previous literature, the capacity fading mechanisms of the LIBs as are
* Corresponding author. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.01.044 0925-8388/© 2016 Elsevier B.V. All rights reserved.
summarized as follow [6]: (i) Degradation of active materials. The degradation of the active materials usually results from the transformation in an inactive phase so reducing the cell capacity, and/or increasing of cell impedance [7]. (ii) Decomposition of electrolytes and film formation. The bare active particles are usually covered by a surface films containing Li2CO3 as a major component, which is destroyed and replaced upon cycling by a variety of solutionrelated compounds including ROLi, ROCO2Li, polycarbonates and salt reaction products [8]. These insoluble salt depositions at the surface may be cause of the increasing polarization. Moreover, in LiPF6 solution contamination with HF plays a very important role to influence the electrochemical performance, and the impedance strongly increase due to precipitation of LiF at the surface [9]. To address aforementioned key issues, surface coating has been an efficient approach to enhance the cycling performance of Nibased oxides. Coating with electrochemically inert oxides [10e12], fluoride [13e15] and phosphates [16e18] have been widely reported, since these coating materials can be considered as protective layers not only to reduce the direct contact between host materials and electrolyte [19], but also to avoid Li2CO3 formation at
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host material surface by shielding the attack of CO2 in the air [20]. However, these coating materials are generally unfavorable for both Liþ-ion conduction of cathode materials and interfacial charge transfer of the electrode not only because they behave as insulators for Liþ-ion conduction, but also because they increase Liþ-diffusion length resulting in deterioration of electrochemical performance [21]. Hence, it still remains a great challenge to better modify the Ni-based cathode materials by employing an ideal coating material, which should possess a dual capability of acting as a robust protective shell and offering an efficient pathway for Liþ-ion diffusion [22]. Monoclinic Li2TiO3, a layered material similar to Li2MnO3, is believed to be one of the eligible coating materials for LIBs cathode materials, because it has an electrochemical inertness in a wide voltage range together with excellent structural stability in organic electrolyte [23]. More importantly, Li2TiO3 has a three-dimensional path for Liþ-ion diffusion [24], and its ionic conductivity can be enhanced further when doped with aliovalent ions [25]. In this regard, it is useful to build a complete and doped Li2TiO3 coating layer on ternary layered NCA oxides, which would help to improve the cycling stability and rate capability of the NCA cathode materials; more than that, Li2TiO3 coating also endows the electrode materials with a robust protective layer to resist the erosion from the HF, CO2 and H2O, thereby giving rise to a longer lifespan towards the NCA-based LIBs cells. As to the coating route, traditional approaches such as mechanical mixing [13,16], solgel [11,26] and atomic deposition methods [27,28] have been developed. However, the common problem of these coating methods is that they cannot establish a uniform, complete, and tight coating layer on the core material [29,30]. Our previous work has reported that a low-temperature hydrothermal coating route efficiently enhances the uniformity, integrate and stability of SmF3 coating layer on Li-rich layered cathode materials [31]. Besides, a pre-coating treatment combined with a post-lithiation at high temperature has also demonstrated to be an efficient way to strengthen the bonding between the host and the coating layer [32,33]. Herein, we introduce a facile synthetic approach based on a solvothermal pre-coating treatment combined with a single-step post-sintering lithiation process. Fig. 1 illustrates the overall synthetic route. First, a mixed Al2O3 and TiO2 pre-coating shell is constructed on the surface of Ni0.8Co0.15(OH)1.9 microsphere precursors by a solvothermal reaction of the precursors with a collosol solution of aluminum isopropoxide and tetrabutyl titanate. Subsequently, a lithiation process is carried out by sintering the precoated precursors with LiOH at 780 C to obtain the final product, Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 microspheres. Such uniform, complete and robust Li2TiO3-based coating shell endows the coated NCA with enhanced Liþ-ion conductivity and structural stability. Hence, when evaluated as Li-ion battery cathode materials, the surface-modified NCA microspheres manifest excellent cycling
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stability and rate capability, as well as remarkably enhanced storage stability. Our works presented here not only illustrate the benefits of Li2TiO3 coating on improving the electrochemical performance of Ni-based layered cathode materials, but also highlight their promising applications towards long-lifespan LIBs. 2. Experimental 2.1. Materials synthesis In this work, all the reagents were analytical grade without further purification and purchased from Chengdu Kelong Chemical Reagents Corporation. 2.1.1. Preparation of Ni0.8Co0.15(OH)1.9 microspheres Ni0.8Co0.15(OH)1.9 microspheres as precursors were first prepared by using a co-precipitation method. Typically, NiSO4$6H2O and CoSO4$7H2O were mixed with a molar ratio of Ni: Co ¼ 80: 15 and with a concentration of 2.0 mol/L in distilled water. The mixed solution was put into a CSTR (continuous stirred tank reactor) with a capacity of 2 L. Simultaneously, 2.0 mol/L solution of NaOH and 0.5 mol/L solution of NH4OH as a chelating agent were separately fed into the continuous stirred tank reactor. The reaction was conducted at 50 C for 10 h, and the pH value in solution was kept at 11. Afterwards, the co-precipitated powders were filtered, washed and finally dried at 140 C for overnight to obtain the precursor to Ni0.8Co0.15(OH)1.9 microspheres. 2.1.2. Solvothermal pre-coating procedure A hydrolysis process of C9H21AlO3 and Ti(OC4H9)4 was carried out to coat Al2O3 and TiO2 on the surface of Ni0.8Co0.15(OH)1.9 microspheres. In a typical preparation of 1 mol% coated sample (Ti: (Ni þ Co þ Al) ¼ 1: 100 in molar ratio), 1 mmol Ti(OC4H9)4 and 5 mmol C9H21AlO3 were firstly dispersed into 70 mL absolute ethanol by ultrasonic cleaner, followed by kept stirring for 1 h. Subsequently, 10 mL distilled water was added dropwise into the above suspension to form a white collosol. Thirdly, 0.1 mol Ni0.8Co0.15(OH)1.9 microspheres were dispersed into the white collosol with continuously stirring for 2 h. And then, the pale green suspension was rapidly poured into a 100 mL Teflon-lined autoclave and solvothermally treated at 150 C for 15 h. After the sample was cooled, the pale green precipitate was washed with distilled water and dried at 80 C. 2.1.3. Synthesis of Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 microspheres After solvothermal coating process, the coated microspheres were mixed with LiOH (The amount of LiOH is calculated by the following equation: mLiOH ¼ 41.96 [1.02n(NiþCoþAl) þ 2nTi]), and heated at 500 C for 5 h, and then calcined at 780 C for 15 h in O2 atmosphere to obtain final product (denoted as TNCA).
Fig. 1. Illustration of the synthetic route to Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 microspheres.
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2.1.4. Synthesis of bare LiNi0.8Co0.15Al0.05O2 microspheres For comparison, the uncoated bare LiNi0.8Co0.15Al0.05O2 microspheres were prepared by using the same procedures as described above without adding 1 mmol Ti(OC4H9)4 (denoted as BNCA). 2.2. Materials characterization The crystal structures of all samples were identified by X-ray diffraction (XRD, Bruker DX-1000 diffractometer with Cu Ka radiation) in the 2q angular range of 10e80 at a scanning rate of 0.02 / s. Small Angle X-Ray Scattering (SAXS, Philips X0 Pert Pro MPD) were used to further verity the crystal structure. The chemical composition of the as-prepared samples was analyzed by inductive coupled plasma atomic emission spectrometry (ICP-AES). The morphology and elementary composition of the obtained powders were observed by using scanning electron microscope (SEM, Hitachi S-4800) with energy dispersive spectroscope (EDS). Transmission electron microscope (TEM, JEOL JEM-2100F) and high resolution transmission electron microscope (HRTEM, JEOL JEM2100F) with an accelerating voltage of 200 kV were used for morphology and structure analysis. X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Kratos) was used to characterize valence state and composition of surface elements in products. 2.3. Electrochemical measurements The working electrodes were fabricated by dispersing 80 wt. % LiNi0.8Co0.15Al0.05O2 powder (active materials), 10 wt. % carbon black (conductive agent), and 10 wt. % polyvinylidene fluoride (PVDF, binder) in N-methyl-2-pyrrolidone (NMP) solvent to form a homogeneous slurry, followed by plastering the slurry onto aluminum foil current collector and dried at 120 C overnight in a vacuum oven. All electrodes were cut into disks with a diameter of 1.4 cm, the average mass loading of which was about 2.0 mg/cm2. For electrochemical measurements, CR-2032 coin-type cells were assembled in an argon-filled glove box by utilizing the above prepared pole pieces as cathodes, metal lithium foils as anodes, polypropylene micro-porous films (Celgard 2400) as separators, and 1.0 mol/L LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1 in volume) as electrolyte. The galvanostatic charge and discharge tests of the cells were performed on a Neware program-control test system (Shen Zhen, CT-3008W) in the potential range between 2.7 and 4.3 versus Li/Liþ at different current densities from 0.1 C (1C ¼ 180 mA g1) to 10 C. The entire chargeedischarge tests were carried out at room temperature and 55 C. The electrochemical impedance spectra (EIS) of the cells after 200 cycles were carried out in the frequency range from 100 kHz to 10 mHz with an AC amplitude of 5 mV using CHI600E (Shanghai Chenhua Co.). 3. Results and discussion The crystal phase of the synthetic BNCA and TNCA were first examined using XRD. As shown in Fig. 2, in which all the peaks of BNCA and TNCA can be well indexed to a hexagonal a-NaFeO2 layered structure with a space group of R3m. The distinct peak splitting of (006)/(102) and (018)/(110) pairs showing in the red rectangular region indicates the well-developed layered structure, and no obvious change in the crystal structure of NCA after coating is observed. Besides, the SAXS analyses were also carried out to verify the layered structure of samples. As shown in Fig. S1, the observable characteristic peaks of the BNCA and TNCA marked by the red arrows at the 2 q < 2 , further demonstrated their typical layered structure. In order to confirm the crystal phase of the coating layer, further increasing the Ti: (Ni þ Co þ Al) content to
15 mol% was synthesized by the same process. It should be noted that a series of characteristic diffraction peaks, indexed to crystal phase of Li2TiO3 (JCPDS: 33-0831) were appeared by increasing the content of Ti. This phenomenon demonstrates that Liþ ion conductive Li2TiO3 is indeed formed and present on the surface of NCA under our synthetic conditions. The morphology, detailed structure and elementary distribution of the as-prepared materials were studied by using SEM and EDS. As shown in Fig. 3a and d, the spherical feature of the precursor particles (Fig. S2) is inherited after solvothermal and lithiation process. The BNCA exhibits the uniform and monodisperse spherical morphology with 7e9 mm in diameter. These secondary spherical particles are composed of a few hundred submicron primary particles. The TNCA display the same uniform and monodisperse spherical feature as the BNCA in the panoramic magnification (Fig. 3d). Notably, in the amplified SEM image, (inset of Fig. 3e) the surface of Li2TiO3-coating NCA exhibits a dense flakelike morphology, which is much different from that of BNCA. This unique surface structure indicates the formation of the coating layer after the solvothermal and pre-sintering process. Elemental dot-mapping for Ni, Co, Al and Ti in Fig. 3c and f shows that these elements are homogeneously distributed in the selected regions (shown in Fig. 3c1 and Fig. 3d) of the BNCA and TNCA products. Size distribution of the as-prepared samples were also been counted based on the panoramic SEM images (Fig. 3a and d), in which the average diameter of the BNCA and TNCA is around 7.69 and 7.77 mm, respectively. The similar average diameter of the BNCA and TNCA would rule out the size influence on the subsequent electrochemical evaluation. Besides, XPS was used to determine the surface elements in the BNCA and TNCA (Fig. 4), from which it is observed that the dominating Ni 2p3/2 peak appearing at 855.5 eV corresponds well to Ni3þ as expected for a typical NCA materials [4,34], while the Al 2p3/2, Ti 2p3/2 and Ti 2p1/2 signal detected within material surface confirms that Al and Ti was successfully coated on the surface of Ni0.8Co0.15(OH)1.9 precursor by using the solvothermal coating method. The chemical composition of the BNCA and TNCA determined by ICP-AES analysis (Table S1) also indicate that the molar of Li, Ni, Co, Al and Ti in the BNCA and TNCA is very close to the added stoichiometric value of 1: 0.8: 0.15: 0.05 and 1.02: 0.08: 0.15: 0.05: 0.01, respectively. As a result, the dense coating shell was certain formed, and it would not only protect the cathode materials from the erosion of HF in the electrochemical evaluation, but also endow the electrode with a better protective ability during the aging process. TEM and HRTEM observations were further carried out to investigate the structural feature of the BNCA and TNCA. Fig. 5a and d shows typical TEM images of the BNCA and TNCA, in which Li2TiO3 is relatively well coated over the surface of the NCA particles, and the thickness of the coating layer is about 30 nm. Further observation of the BNCA by using HRTEM (Fig. 5b) reveals that the interplanar distances in the marked red region shown in Fig. 5a, taken from the outer layer, are estimated to be approximately 0.469 nm. Combined with the corresponding Fast Fourier Transform (FFT) patterns, as shown in Fig. 5c, it can be inferred that these observed planes are ascribed to the (003) plane of hexagonal structure LiNi0.8Co0.15Al0.05O2. Fig. 5e depicts the lattice fringes of the coating layer from the red square region in Fig. 5d, in which the lattice spacing is measured to be about 0.240 nm, together with the analysis of corresponding FFT, it can be attributed to the (113) plane of monoclinic Li2TiO3. Therefore, it could be concluded that a crystalline and ultrathin Li2TiO3-coating layer has been established, which is consistent with the above analyses. In particular, the contents of Al3þ and Ti4þ ions calculated from XPS (Table S2) are higher than the start ratio. The maximum thickness is estimated to be 9e10 nm, however, the thickness of coating layer from the TEM
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Fig. 2. XRD patterns of as-prepared products.
Fig. 3. SEM images, EDS mapping and size distribution of BNCA (a-c, g) and TNCA (d-f, h).
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Fig. 4. XPS spectra of BNCA and TNCA, (a) nickel, (b) Cobalt, (c) aluminum, (d) titanium.
Figs. 5. (a and d) TEM images of BNCA and TNCA; (b and e) HRTEM images taken from the selected square region in (a) and (d), and their corresponding (c and f) FFT patterns derived from the HRTEM images.
result is about 30 nm, indicating that the doping of Al3þ and host
ions (Ni3þ and Co3þ) into the phase of Li2TiO3. The doped Li2TiO3
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coating layer strengthened the interaction between coating layer and the core materials. Thus, enhanced cycling stability and structural stability of coated NCA would be expected. The as-prepared BNCA and TNCA were used as cathode materials to determine the influence of the Li2TiO3 coating layer on the electrochemical performance of NCA materials. Fig. 6a shows the typical initial chargeedischarge curves of BNCA and TNCA in the voltage range of 2.7e4.3 V under a current rate of 0.1 C. The chargeedischarge curves of two cells are smooth with a monotonous voltage plateau. The BNCA and TNCA exhibit a close discharge capacity and reach to about 185 mA h g1, whereas their initial coulombic efficiencies are estimated to be 86% and 90%, respectively. The enhanced initial coulombic efficiency may be attributed to the coating layer, which suppresses the over-charge caused by floating lithium on the surface of cathode, and increases the electrical conductivity of the cathode materials. Fig. 6b shows a comparison of rate performances of the cells evaluated at variable currents rate from 0.1 to 10 C for 5 cycles at each current rate and the subsequent cycling tests under 0.5 C. The Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 electrode can be reversibly cycled at 0.1, 0.2, 0.5, 1, 2, 5 and 10 C with stable discharge capacities of 185, 180, 168, 160, 150, 130 and 107 mA h g1, which are much higher than that of BNCA at 1e10 C. When the current rate is reversed back to 0.1 C, a
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stable discharge capacity of 184 mA h g1 can be recovered for the TNCA and its counterpart. The discharge curves of the BNCA and TNCA at various rates were shown in Fig. S3. It is worth noting that the polarizability of the TNCA is much smaller than that of BNCA. The smaller interfacial polarization in the TNCA may be attributed to the Liþ ion conductive coating layer, which promotes intercalary process and reduces the interface resistance. At the following cyclic evaluation, the TNCA shows an excellent cycling stability during the long-term chargeedischarge tests under 0.5 C. It retains about 94% of the initial capacity (169 mA h g1) after 200 cycles and about 84% retention after 400 cycles. However, the BNCA exhibits relatively poor cycle life because only a capacity of 97 mA h g1 (60% retention) can be delivered at the end of 200 cycles. The mechanism of degradation upon charge/discharge cycles was clarified by electrochemical impedance analysis [6]. The AC impedance studies demonstrated that the interfacial resistance at the cathode was a more predominant factor in a rise of impedance [7]. To reveal the origin of the superior cycling performance of the TNCA, the electrochemical impedance spectra (EIS) measurements of the two cells after the 200 cycles were carried out. As can be seen from Fig. 6c, the diameter of the semicircle at high and middle frequencies of the TNCA is much smaller than that of BNCA, indicating the remarkably decrease of the surface film resistance (Rf) and the
Fig. 6. (a) Initial chargeedischarge curves of BNCA and TNCA; (b) Rate capability under variable current rate and cycling performances under 0.5 C; (c) Nyquist plots, the equivalent circuit used to fit the measured impedance spectra and the relationship between Z0 and u0.5 at low frequency of BNCA and TNCA electrodes at charged state of 4.3 V after rate test and 200 cycles. All the measurements used fresh cells and were conducted at 25 C.
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charge transfer resistance (Rct). The values of Rf, Rct and Liþ diffusion coefficient (DLi) are listed in Table S3. Obviously, the calculated DLi for the TNCA electrode is approximately 2.5 times larger than that for the BNCA, indicating that the Li2TiO3 coating layer can effectively accelerate Liþ diffusion and suppress the increasing of Rct which caused by the side reaction between the electrode and electrolyte, thereby manifesting the excellent cycling stability. To examine the integrity of the microsphere morphology of BNCA and TNCA after the cycling evaluation, we carried out postmortem analyses on the cycled electrodes. Fig. 7a and b presents the SEM images of the BNCA and TNCA electrodes after 200 and 400 cycles under 0.5 C, respectively. Obviously, after long-term cycling, both of the two type electrodes maintain their initial micro spherical morphology. However, a large crack across the BNCA microsphere had been appeared, which may be due to the inferior tolerance to stress from volume expansion during repeated lithiation/delithiation processes. In contrast, there only present some microcracks on the surface of the TNCA microspheres, indicating that the Li2TiO3-coating shell endows LiNi0.8Co0.15Al0.05O2 microsphere a better structural stability than its counterpart. EDS analysis, as shown in Fig. 7c and d, reveals that there is almost no change in chemical composition of the electrode material. Whereas, as shown in Fig. 7c, the signal of fluorine had been detected in the red region of Fig. 7a, indicated that the decomposition of the electrolyte had occurred during the cycling process and resulted in an inferior cycling performance. Unlike the counterpart, EDS analysis of cycled TNCA electrode had not detected the fluorine, inferring that the Li2TiO3-coating layer acts as a protective shell of the active cathode, and suppresses the decomposition of the electrolyte during the long-term cycles. The capacity fading of LIBs after long-term storage process is one of the thorniest issues for the application of EVs and HEVs. According to the aging mechanism of the LIBs, the Li2TiO3-based coating shell would endow the NCA electrode with more protective capability to withstand the degradation of core materials and the erosion from HF and CO2. Hence, the electrochemical evaluations
after the aging process were investigated at room temperature and elevated temperature. The aging process was carried out at 30 C for 50 days. The initial chargeedischarge curves of BNCA and TNCA aged cells at 25 C were shown in Fig. 8a. The chargeedischarge curves of two cells are smooth with a monotonous voltage plateau, whereas, the TNCA exhibits a longer discharge plateau obvious than its counterpart. The discharge capacity of TNCA and BNCA is 178 and 162 mA h g1, and their corresponding initial coulombic efficiencies are 86% and 80%, respectively. Because of the degradation of the electrode and electrolyte during the aging process, the discharge capacity and initial coulombic efficiencies of the aged cells are much lower than that of fresh ones. Fig. 8c presents the rate capability and following cycling evaluation. The TNCA exhibits a small enhanced reversible capacity than that of BNCA. However, the better capacity retention can be achieved in the Li2TiO3-coated NCA electrode, which delivered a capacity retention as high as 84% at the end of 200 charge/discharge cycles. Its counterpart only can be delivered a capacity retention of 48% at the same evaluative process. At elevated temperature, more side reactions between electrode and electrolyte, instability of electrolyte, release of O2 and degradation of Ni3þ result in an inferior cycle life of NCA materials, even more after the aging process [35]. Hence, the cycling stability of TNCA at 55 C was also investigated (Fig. 8b and d). The initial discharge capacities and their corresponding initial cyclonic efficiencies of BNCA and TNCA are 184 (90%) and 180 mA h g1 (92%), respectively. The elevated initial discharge capacities and coulombic efficiencies are attributed to the increased electrochemical activity of Ni-based cathode at the higher temperature. More remarkably, the BNCA exhibits a higher discharge capacity than TNCA, whereas, it shows an inferior cycling stability in the subsequent process. The aged TNCA electrode still retains 93% of the initial capacity (172 mA h g1) after 100 cycles at the current rate of 0.5 C, but the counterpart loses its capacity very rapidly as it only deliveries 58 mA h g1 (50%) after 100 cycles. Furthermore, the TNCA still show a robust thermal stability in the extended cycle number, as a high reversible capacity of 132 (77% retention) and
Fig. 7. SEM images and EDS spectra of the cycled electrodes; (a and c) BNCA and (b and d) TNCA.
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Figs. 8. (a and b) Initial chargeedischarge curves of BNCA and TNCA at 25 and 55 C, respectively; (c and d) Rate capability under variable current rate and cycling performances under 0.5C at 25 and 55 C, respectively. All the measurements used the cells after 50 days aging process.
96 mA h g1 (56% retention) are still retained even after 200 and total 300 cycles. Such an impressive electrochemical performance of the TNCA could be ascribed to the Li2TiO3-coated layer not only suppress the degradation of the electrode and electrolyte in the aging procedure, but also possess a stable conductive protected layer at the long-term charge/discharge cycles. This valued feature promises a wider operating temperature range and more stable
cycle performance for the Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 when served as a cathode material towards high-performance EVs and HEVs applications. 4. Conclusions Ternary layered LiNi0.8Co0.15Al0.05O2 microspheres coated with a
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nanoscale Li2TiO3 shell have been synthesized through a facile strategy based on a solvothermal pre-coating treatment together with a post-sintering lithiation process. Such a Liþ-conductive coating layer not only improves the velocity of Liþ migration on electrode surface, but also protects the host materials from the attack of CO2 from ambient environment, as well as the HF erosion by electrolyte decomposition during the long-term cycling and aging process. The surface-modified NCA electrode exhibits a remarkable cyclic stability both at fresh and aged status. These results indicate that Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 holds a great promise for application in long-lifespan LIBs cathode materials. Acknowledgments The authors acknowledge the financial support from the National Basic Research Program of China (973 program no. 2013 CB934700), A Foundation for the Author of National Excellent Doctor Dissertation of P. R. China (no. FANEDD201435), and the Sichuan Province Science and Technology Support Program (no. 2014GZ0093). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.01.044. References [1] B.C. Melot, J.M. Tarascon, Accounts Chem. Res. 46 (2013) 1226e1238. [2] M. Broussely, S. Herreyre, P. Biensan, P. Kasztejna, K. Nechev, R.J. Staniewicz, J. Power Sources 97e98 (2001) 13e21. [3] B. Xu, D. Qian, Z. Wang, Y.S. Meng, Mater. Sci. Eng. R 73 (2012) 51e65. [4] N.T. Wu, H. Wu, W. Yuan, S. Liu, J. Liao, Y. Zhang, J. Mater. Chem. A 3 (2015) 13648e13652. [5] S. Watanabe, M. Kinoshita, K. Nakura, J. Power Sources 247 (2014) 412e422. [6] M.W. Mehrens, C. Vogler, J. Garche, J. Power Sources 127 (2004) 58e64. [7] M. Broussely, P. Biensan, F. Bonhomme, P. Blanchard, S. Herreyre, K. Nechev, R.J. Staniewicz, J. Power Sources 146 (2005) 90e96.
[8] Doron Aurbach, Kira Gamolsky, Boris Markovsky, Gregory Salitra, Yossi Gofer, Udo Heider, Ruediger Oesten, M. Schmidt, J. Electrochem. Soc. 147 (2000) 1322e1331. ne trier, C. Vaysse, L. Croguennec, C. Delmas, C. Jordy, F. Bonhomme, [9] M. Me P. Biensan, Electrochem. Solid State Lett. 7 (2004) A140eA143. [10] Y. Cho, J. Cho, J. Electrochem. Soc. 157 (2010) A625eA629. [11] Y. Cho, Y. Lee, S. Park, Y. Lee, J. Cho, Electrochim. Acta 56 (2010) 333e339. [12] Y.P. Chen, Y. Zhang, B.J. Chen, Z.Y. Wang, C. Lu, J. Power Sources 256 (2014) 20e27. [13] S. Lee, C.S. Yoon, K. Amine, Y. Sun, J. Power Sources 234 (2013) 201e207. [14] B.C. Park, H.B. Kim, H.J. Bang, J. Prakash, Y.K. Sun, Ind. Eng. Chem. Res. 47 (2008) 3876e3882. [15] C. Lu, H. Wu, Y. Zhang, H. Liu, B.J. Chen, N.T. Wu, S. Wang, J. Power Sources 267 (2014) 682e691. [16] D. Lee, B. Scrosati, Y. Sun, J. Power Sources 196 (2011) 7742e7746. [17] K.S. Ryu, S.H. Lee, B.K. Koo, J.W. Lee, K.M. Kim, Y.J. Park, J. Appl. Electrochem. 38 (2008) 1385e1390. [18] Y.W. Zeng, J.H. He, J. Power Sources 189 (2009) 519e521. [19] A. Mauger, C. Julien, Ionics 20 (2014) 751e787. [20] G.V. Zhuang, G.Y. Chen, J. Shim, X.Y. Song, P.N. Ross, T.J. Richardson, J. Power Sources 134 (2004) 293e297. [21] J.H. Park, J.H. Cho, S.B. Kim, W.S. Kim, S.Y. Lee, S. Lee, J. Mater. Chem. 22 (2012) 12574e12581. [22] H. Deng, P. Nie, H. Luo, Y. Zhang, J. Wang, X. Zhang, J. Mater. Chem. A 2 (2014) 18256e18262. [23] A.M. Kannan, L. Rabenberg, A. Manthiram, Electrochem. Solid State Lett. 6 (2003) A16eA18. [24] M. Vijayakumar, S. Kerisit, Z. Yang, G.L. Graff, J. Liu, J.A. Sears, S.D. Burton, K.M. Rosso, J. Hu, J. Phys. Chem. C 113 (2009) 20108e20116. [25] X. Wu, Z. Wen, X. Xu, J. Han, Solid State Ionics 179 (2008) 1779e1782. [26] S.N. Lim, W. Ahn, S. Yeon, S.B. Park, Electrochim. Acta 136 (2014) 1e9. [27] H. Cheng, F. Wang, J.P. Chu, R. Santhanam, J. Rick, S. Lo, J. Phys. Chem. C 116 (2012) 7629e7637. [28] D. Wang, J. Yang, J. Liu, X. Li, R. Li, M. Cai, T.K. Sham, X. Sun, J. Mater. Chem. A 2 (2014) 2306e2312. [29] L. Zhu, Y. Liu, W. Wu, X. Wu, W. Tang, Y. Wu, J. Mater. Chem. A 3 (2015) 15156e15162. [30] J. Lu, Q. Peng, W.Y. Wang, C.Y. Nan, L.H. Li, Y.D. Li, J. Am. Chem. Soc. 135 (2013) 1649e1652. [31] C. Lu, H. Wu, B. Chen, H. Liu, Y. Zhang, J. Alloy. Compd. 634 (2015) 75e82. [32] Z. Wang, S. Huang, B. Chen, H. Wu, Y. Zhang, J. Mater. Chem. A 2 (2014) 19983e19987. [33] H. Wu, Z.Y. Wang, S.J. Liu, L. Zhang, Y. Zhang, Chem. Electrochem. (2015), http://dx.doi.org/10.1002/celc.201500303. [34] H. Liu, Y. Yang, J. Zhang, J. Power Sources 162 (2006) 644e650. [35] S.M. Bak, K.W. Nam, W. Chang, X.Q. Yu, E.Y. Hu, S. Hwang, E.A. Stach, K.B. Kim, K.Y. Chung, X.Q. Yang, Chem. Mater. 25 (2013) 337e351.
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Solvothermal coating LiNi0.8Co0.15Al0.05O2 microspheres with nanoscale Li2TiO3 shell for long lifespan Li-ion battery cathode materials Naiteng Wu, Hao Wu, Heng Liu, Yun Zhang* Department of Advanced Energy Materials, College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 December 2015 Received in revised form 31 December 2015 Accepted 7 January 2016 Available online 11 January 2016
LiNi0.8Co0.15Al0.05O2 (NCA) microspheres covered by a nanoscale Li2TiO3-based shell were synthesized by a facile strategy based on a solvothermal pre-coating treatment combined with a post-sintering lithiation process. The morphology, structure and composition of the Li2TiO3-coated NCA samples were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning scanning electron microscope (SEM) with an energy-dispersive X-ray spectroscope (EDS), and transmission electron microscopy (TEM). Owing to the complete, uniform and nanoscale Li2TiO3 coating shell, the resultant surface-modified NCA microspheres used as Li-ion battery cathode materials manifest remarkably enhanced cycling performances, attaining 94% and 84% capacity retention after 200 and 400 cycles at 0.5 C, respectively, which is much better than the pristine NCA counterpart (60% retention, 200 cycles). More impressively, the surface-modified NCA also shows an intriguing storage stability. After being stored at 30 C for 50 days, the coated NCA-based cells are subjected to be cycled both at room and elevated temperatures, in which the aged cells can still remain 84% capacity retention after 200 cycles at 25 C and 77% capacity retention after 200 cycles at 55 C, respectively. All these results demonstrate that the Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 microsphere is a promising cathode material for Li-ion batteries with long lifespan. © 2016 Elsevier B.V. All rights reserved.
Keywords: Solvothermal Li2TiO3 coating LiNi0.8Co0.15Al0.05O2 Long lifespan Lithium-ion batteries
1. Introduction Lithium ion batteries (LIBs) have been one of the most attractive candidates for electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their high energy and high power density, but these potential applications require both extended cycle life and prolonged calendar life [1,2]. Recently, more attentions have been focused on Ni-based ternary oxides, among which LiNi0.8Co0.15Al0.05O2 (NCA) has been considered as the most promising cathode materials for scalable applications in EVs and HEVs due to its high capacity and low cost compared to other commercial cathode materials such as LiCoO2, LiMn2O4, and LiFePO4 [3,4]. However, the LIBs with the NCA cathode material still suffer from a remarkable decline in capacity retention during a long-term cycle use and/or after a long period of storage [5]. According to previous literature, the capacity fading mechanisms of the LIBs as are
* Corresponding author. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.01.044 0925-8388/© 2016 Elsevier B.V. All rights reserved.
summarized as follow [6]: (i) Degradation of active materials. The degradation of the active materials usually results from the transformation in an inactive phase so reducing the cell capacity, and/or increasing of cell impedance [7]. (ii) Decomposition of electrolytes and film formation. The bare active particles are usually covered by a surface films containing Li2CO3 as a major component, which is destroyed and replaced upon cycling by a variety of solutionrelated compounds including ROLi, ROCO2Li, polycarbonates and salt reaction products [8]. These insoluble salt depositions at the surface may be cause of the increasing polarization. Moreover, in LiPF6 solution contamination with HF plays a very important role to influence the electrochemical performance, and the impedance strongly increase due to precipitation of LiF at the surface [9]. To address aforementioned key issues, surface coating has been an efficient approach to enhance the cycling performance of Nibased oxides. Coating with electrochemically inert oxides [10e12], fluoride [13e15] and phosphates [16e18] have been widely reported, since these coating materials can be considered as protective layers not only to reduce the direct contact between host materials and electrolyte [19], but also to avoid Li2CO3 formation at
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host material surface by shielding the attack of CO2 in the air [20]. However, these coating materials are generally unfavorable for both Liþ-ion conduction of cathode materials and interfacial charge transfer of the electrode not only because they behave as insulators for Liþ-ion conduction, but also because they increase Liþ-diffusion length resulting in deterioration of electrochemical performance [21]. Hence, it still remains a great challenge to better modify the Ni-based cathode materials by employing an ideal coating material, which should possess a dual capability of acting as a robust protective shell and offering an efficient pathway for Liþ-ion diffusion [22]. Monoclinic Li2TiO3, a layered material similar to Li2MnO3, is believed to be one of the eligible coating materials for LIBs cathode materials, because it has an electrochemical inertness in a wide voltage range together with excellent structural stability in organic electrolyte [23]. More importantly, Li2TiO3 has a three-dimensional path for Liþ-ion diffusion [24], and its ionic conductivity can be enhanced further when doped with aliovalent ions [25]. In this regard, it is useful to build a complete and doped Li2TiO3 coating layer on ternary layered NCA oxides, which would help to improve the cycling stability and rate capability of the NCA cathode materials; more than that, Li2TiO3 coating also endows the electrode materials with a robust protective layer to resist the erosion from the HF, CO2 and H2O, thereby giving rise to a longer lifespan towards the NCA-based LIBs cells. As to the coating route, traditional approaches such as mechanical mixing [13,16], solgel [11,26] and atomic deposition methods [27,28] have been developed. However, the common problem of these coating methods is that they cannot establish a uniform, complete, and tight coating layer on the core material [29,30]. Our previous work has reported that a low-temperature hydrothermal coating route efficiently enhances the uniformity, integrate and stability of SmF3 coating layer on Li-rich layered cathode materials [31]. Besides, a pre-coating treatment combined with a post-lithiation at high temperature has also demonstrated to be an efficient way to strengthen the bonding between the host and the coating layer [32,33]. Herein, we introduce a facile synthetic approach based on a solvothermal pre-coating treatment combined with a single-step post-sintering lithiation process. Fig. 1 illustrates the overall synthetic route. First, a mixed Al2O3 and TiO2 pre-coating shell is constructed on the surface of Ni0.8Co0.15(OH)1.9 microsphere precursors by a solvothermal reaction of the precursors with a collosol solution of aluminum isopropoxide and tetrabutyl titanate. Subsequently, a lithiation process is carried out by sintering the precoated precursors with LiOH at 780 C to obtain the final product, Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 microspheres. Such uniform, complete and robust Li2TiO3-based coating shell endows the coated NCA with enhanced Liþ-ion conductivity and structural stability. Hence, when evaluated as Li-ion battery cathode materials, the surface-modified NCA microspheres manifest excellent cycling
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stability and rate capability, as well as remarkably enhanced storage stability. Our works presented here not only illustrate the benefits of Li2TiO3 coating on improving the electrochemical performance of Ni-based layered cathode materials, but also highlight their promising applications towards long-lifespan LIBs. 2. Experimental 2.1. Materials synthesis In this work, all the reagents were analytical grade without further purification and purchased from Chengdu Kelong Chemical Reagents Corporation. 2.1.1. Preparation of Ni0.8Co0.15(OH)1.9 microspheres Ni0.8Co0.15(OH)1.9 microspheres as precursors were first prepared by using a co-precipitation method. Typically, NiSO4$6H2O and CoSO4$7H2O were mixed with a molar ratio of Ni: Co ¼ 80: 15 and with a concentration of 2.0 mol/L in distilled water. The mixed solution was put into a CSTR (continuous stirred tank reactor) with a capacity of 2 L. Simultaneously, 2.0 mol/L solution of NaOH and 0.5 mol/L solution of NH4OH as a chelating agent were separately fed into the continuous stirred tank reactor. The reaction was conducted at 50 C for 10 h, and the pH value in solution was kept at 11. Afterwards, the co-precipitated powders were filtered, washed and finally dried at 140 C for overnight to obtain the precursor to Ni0.8Co0.15(OH)1.9 microspheres. 2.1.2. Solvothermal pre-coating procedure A hydrolysis process of C9H21AlO3 and Ti(OC4H9)4 was carried out to coat Al2O3 and TiO2 on the surface of Ni0.8Co0.15(OH)1.9 microspheres. In a typical preparation of 1 mol% coated sample (Ti: (Ni þ Co þ Al) ¼ 1: 100 in molar ratio), 1 mmol Ti(OC4H9)4 and 5 mmol C9H21AlO3 were firstly dispersed into 70 mL absolute ethanol by ultrasonic cleaner, followed by kept stirring for 1 h. Subsequently, 10 mL distilled water was added dropwise into the above suspension to form a white collosol. Thirdly, 0.1 mol Ni0.8Co0.15(OH)1.9 microspheres were dispersed into the white collosol with continuously stirring for 2 h. And then, the pale green suspension was rapidly poured into a 100 mL Teflon-lined autoclave and solvothermally treated at 150 C for 15 h. After the sample was cooled, the pale green precipitate was washed with distilled water and dried at 80 C. 2.1.3. Synthesis of Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 microspheres After solvothermal coating process, the coated microspheres were mixed with LiOH (The amount of LiOH is calculated by the following equation: mLiOH ¼ 41.96 [1.02n(NiþCoþAl) þ 2nTi]), and heated at 500 C for 5 h, and then calcined at 780 C for 15 h in O2 atmosphere to obtain final product (denoted as TNCA).
Fig. 1. Illustration of the synthetic route to Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 microspheres.
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2.1.4. Synthesis of bare LiNi0.8Co0.15Al0.05O2 microspheres For comparison, the uncoated bare LiNi0.8Co0.15Al0.05O2 microspheres were prepared by using the same procedures as described above without adding 1 mmol Ti(OC4H9)4 (denoted as BNCA). 2.2. Materials characterization The crystal structures of all samples were identified by X-ray diffraction (XRD, Bruker DX-1000 diffractometer with Cu Ka radiation) in the 2q angular range of 10e80 at a scanning rate of 0.02 / s. Small Angle X-Ray Scattering (SAXS, Philips X0 Pert Pro MPD) were used to further verity the crystal structure. The chemical composition of the as-prepared samples was analyzed by inductive coupled plasma atomic emission spectrometry (ICP-AES). The morphology and elementary composition of the obtained powders were observed by using scanning electron microscope (SEM, Hitachi S-4800) with energy dispersive spectroscope (EDS). Transmission electron microscope (TEM, JEOL JEM-2100F) and high resolution transmission electron microscope (HRTEM, JEOL JEM2100F) with an accelerating voltage of 200 kV were used for morphology and structure analysis. X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Kratos) was used to characterize valence state and composition of surface elements in products. 2.3. Electrochemical measurements The working electrodes were fabricated by dispersing 80 wt. % LiNi0.8Co0.15Al0.05O2 powder (active materials), 10 wt. % carbon black (conductive agent), and 10 wt. % polyvinylidene fluoride (PVDF, binder) in N-methyl-2-pyrrolidone (NMP) solvent to form a homogeneous slurry, followed by plastering the slurry onto aluminum foil current collector and dried at 120 C overnight in a vacuum oven. All electrodes were cut into disks with a diameter of 1.4 cm, the average mass loading of which was about 2.0 mg/cm2. For electrochemical measurements, CR-2032 coin-type cells were assembled in an argon-filled glove box by utilizing the above prepared pole pieces as cathodes, metal lithium foils as anodes, polypropylene micro-porous films (Celgard 2400) as separators, and 1.0 mol/L LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1 in volume) as electrolyte. The galvanostatic charge and discharge tests of the cells were performed on a Neware program-control test system (Shen Zhen, CT-3008W) in the potential range between 2.7 and 4.3 versus Li/Liþ at different current densities from 0.1 C (1C ¼ 180 mA g1) to 10 C. The entire chargeedischarge tests were carried out at room temperature and 55 C. The electrochemical impedance spectra (EIS) of the cells after 200 cycles were carried out in the frequency range from 100 kHz to 10 mHz with an AC amplitude of 5 mV using CHI600E (Shanghai Chenhua Co.). 3. Results and discussion The crystal phase of the synthetic BNCA and TNCA were first examined using XRD. As shown in Fig. 2, in which all the peaks of BNCA and TNCA can be well indexed to a hexagonal a-NaFeO2 layered structure with a space group of R3m. The distinct peak splitting of (006)/(102) and (018)/(110) pairs showing in the red rectangular region indicates the well-developed layered structure, and no obvious change in the crystal structure of NCA after coating is observed. Besides, the SAXS analyses were also carried out to verify the layered structure of samples. As shown in Fig. S1, the observable characteristic peaks of the BNCA and TNCA marked by the red arrows at the 2 q < 2 , further demonstrated their typical layered structure. In order to confirm the crystal phase of the coating layer, further increasing the Ti: (Ni þ Co þ Al) content to
15 mol% was synthesized by the same process. It should be noted that a series of characteristic diffraction peaks, indexed to crystal phase of Li2TiO3 (JCPDS: 33-0831) were appeared by increasing the content of Ti. This phenomenon demonstrates that Liþ ion conductive Li2TiO3 is indeed formed and present on the surface of NCA under our synthetic conditions. The morphology, detailed structure and elementary distribution of the as-prepared materials were studied by using SEM and EDS. As shown in Fig. 3a and d, the spherical feature of the precursor particles (Fig. S2) is inherited after solvothermal and lithiation process. The BNCA exhibits the uniform and monodisperse spherical morphology with 7e9 mm in diameter. These secondary spherical particles are composed of a few hundred submicron primary particles. The TNCA display the same uniform and monodisperse spherical feature as the BNCA in the panoramic magnification (Fig. 3d). Notably, in the amplified SEM image, (inset of Fig. 3e) the surface of Li2TiO3-coating NCA exhibits a dense flakelike morphology, which is much different from that of BNCA. This unique surface structure indicates the formation of the coating layer after the solvothermal and pre-sintering process. Elemental dot-mapping for Ni, Co, Al and Ti in Fig. 3c and f shows that these elements are homogeneously distributed in the selected regions (shown in Fig. 3c1 and Fig. 3d) of the BNCA and TNCA products. Size distribution of the as-prepared samples were also been counted based on the panoramic SEM images (Fig. 3a and d), in which the average diameter of the BNCA and TNCA is around 7.69 and 7.77 mm, respectively. The similar average diameter of the BNCA and TNCA would rule out the size influence on the subsequent electrochemical evaluation. Besides, XPS was used to determine the surface elements in the BNCA and TNCA (Fig. 4), from which it is observed that the dominating Ni 2p3/2 peak appearing at 855.5 eV corresponds well to Ni3þ as expected for a typical NCA materials [4,34], while the Al 2p3/2, Ti 2p3/2 and Ti 2p1/2 signal detected within material surface confirms that Al and Ti was successfully coated on the surface of Ni0.8Co0.15(OH)1.9 precursor by using the solvothermal coating method. The chemical composition of the BNCA and TNCA determined by ICP-AES analysis (Table S1) also indicate that the molar of Li, Ni, Co, Al and Ti in the BNCA and TNCA is very close to the added stoichiometric value of 1: 0.8: 0.15: 0.05 and 1.02: 0.08: 0.15: 0.05: 0.01, respectively. As a result, the dense coating shell was certain formed, and it would not only protect the cathode materials from the erosion of HF in the electrochemical evaluation, but also endow the electrode with a better protective ability during the aging process. TEM and HRTEM observations were further carried out to investigate the structural feature of the BNCA and TNCA. Fig. 5a and d shows typical TEM images of the BNCA and TNCA, in which Li2TiO3 is relatively well coated over the surface of the NCA particles, and the thickness of the coating layer is about 30 nm. Further observation of the BNCA by using HRTEM (Fig. 5b) reveals that the interplanar distances in the marked red region shown in Fig. 5a, taken from the outer layer, are estimated to be approximately 0.469 nm. Combined with the corresponding Fast Fourier Transform (FFT) patterns, as shown in Fig. 5c, it can be inferred that these observed planes are ascribed to the (003) plane of hexagonal structure LiNi0.8Co0.15Al0.05O2. Fig. 5e depicts the lattice fringes of the coating layer from the red square region in Fig. 5d, in which the lattice spacing is measured to be about 0.240 nm, together with the analysis of corresponding FFT, it can be attributed to the (113) plane of monoclinic Li2TiO3. Therefore, it could be concluded that a crystalline and ultrathin Li2TiO3-coating layer has been established, which is consistent with the above analyses. In particular, the contents of Al3þ and Ti4þ ions calculated from XPS (Table S2) are higher than the start ratio. The maximum thickness is estimated to be 9e10 nm, however, the thickness of coating layer from the TEM
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Fig. 2. XRD patterns of as-prepared products.
Fig. 3. SEM images, EDS mapping and size distribution of BNCA (a-c, g) and TNCA (d-f, h).
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Fig. 4. XPS spectra of BNCA and TNCA, (a) nickel, (b) Cobalt, (c) aluminum, (d) titanium.
Figs. 5. (a and d) TEM images of BNCA and TNCA; (b and e) HRTEM images taken from the selected square region in (a) and (d), and their corresponding (c and f) FFT patterns derived from the HRTEM images.
result is about 30 nm, indicating that the doping of Al3þ and host
ions (Ni3þ and Co3þ) into the phase of Li2TiO3. The doped Li2TiO3
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coating layer strengthened the interaction between coating layer and the core materials. Thus, enhanced cycling stability and structural stability of coated NCA would be expected. The as-prepared BNCA and TNCA were used as cathode materials to determine the influence of the Li2TiO3 coating layer on the electrochemical performance of NCA materials. Fig. 6a shows the typical initial chargeedischarge curves of BNCA and TNCA in the voltage range of 2.7e4.3 V under a current rate of 0.1 C. The chargeedischarge curves of two cells are smooth with a monotonous voltage plateau. The BNCA and TNCA exhibit a close discharge capacity and reach to about 185 mA h g1, whereas their initial coulombic efficiencies are estimated to be 86% and 90%, respectively. The enhanced initial coulombic efficiency may be attributed to the coating layer, which suppresses the over-charge caused by floating lithium on the surface of cathode, and increases the electrical conductivity of the cathode materials. Fig. 6b shows a comparison of rate performances of the cells evaluated at variable currents rate from 0.1 to 10 C for 5 cycles at each current rate and the subsequent cycling tests under 0.5 C. The Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 electrode can be reversibly cycled at 0.1, 0.2, 0.5, 1, 2, 5 and 10 C with stable discharge capacities of 185, 180, 168, 160, 150, 130 and 107 mA h g1, which are much higher than that of BNCA at 1e10 C. When the current rate is reversed back to 0.1 C, a
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stable discharge capacity of 184 mA h g1 can be recovered for the TNCA and its counterpart. The discharge curves of the BNCA and TNCA at various rates were shown in Fig. S3. It is worth noting that the polarizability of the TNCA is much smaller than that of BNCA. The smaller interfacial polarization in the TNCA may be attributed to the Liþ ion conductive coating layer, which promotes intercalary process and reduces the interface resistance. At the following cyclic evaluation, the TNCA shows an excellent cycling stability during the long-term chargeedischarge tests under 0.5 C. It retains about 94% of the initial capacity (169 mA h g1) after 200 cycles and about 84% retention after 400 cycles. However, the BNCA exhibits relatively poor cycle life because only a capacity of 97 mA h g1 (60% retention) can be delivered at the end of 200 cycles. The mechanism of degradation upon charge/discharge cycles was clarified by electrochemical impedance analysis [6]. The AC impedance studies demonstrated that the interfacial resistance at the cathode was a more predominant factor in a rise of impedance [7]. To reveal the origin of the superior cycling performance of the TNCA, the electrochemical impedance spectra (EIS) measurements of the two cells after the 200 cycles were carried out. As can be seen from Fig. 6c, the diameter of the semicircle at high and middle frequencies of the TNCA is much smaller than that of BNCA, indicating the remarkably decrease of the surface film resistance (Rf) and the
Fig. 6. (a) Initial chargeedischarge curves of BNCA and TNCA; (b) Rate capability under variable current rate and cycling performances under 0.5 C; (c) Nyquist plots, the equivalent circuit used to fit the measured impedance spectra and the relationship between Z0 and u0.5 at low frequency of BNCA and TNCA electrodes at charged state of 4.3 V after rate test and 200 cycles. All the measurements used fresh cells and were conducted at 25 C.
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charge transfer resistance (Rct). The values of Rf, Rct and Liþ diffusion coefficient (DLi) are listed in Table S3. Obviously, the calculated DLi for the TNCA electrode is approximately 2.5 times larger than that for the BNCA, indicating that the Li2TiO3 coating layer can effectively accelerate Liþ diffusion and suppress the increasing of Rct which caused by the side reaction between the electrode and electrolyte, thereby manifesting the excellent cycling stability. To examine the integrity of the microsphere morphology of BNCA and TNCA after the cycling evaluation, we carried out postmortem analyses on the cycled electrodes. Fig. 7a and b presents the SEM images of the BNCA and TNCA electrodes after 200 and 400 cycles under 0.5 C, respectively. Obviously, after long-term cycling, both of the two type electrodes maintain their initial micro spherical morphology. However, a large crack across the BNCA microsphere had been appeared, which may be due to the inferior tolerance to stress from volume expansion during repeated lithiation/delithiation processes. In contrast, there only present some microcracks on the surface of the TNCA microspheres, indicating that the Li2TiO3-coating shell endows LiNi0.8Co0.15Al0.05O2 microsphere a better structural stability than its counterpart. EDS analysis, as shown in Fig. 7c and d, reveals that there is almost no change in chemical composition of the electrode material. Whereas, as shown in Fig. 7c, the signal of fluorine had been detected in the red region of Fig. 7a, indicated that the decomposition of the electrolyte had occurred during the cycling process and resulted in an inferior cycling performance. Unlike the counterpart, EDS analysis of cycled TNCA electrode had not detected the fluorine, inferring that the Li2TiO3-coating layer acts as a protective shell of the active cathode, and suppresses the decomposition of the electrolyte during the long-term cycles. The capacity fading of LIBs after long-term storage process is one of the thorniest issues for the application of EVs and HEVs. According to the aging mechanism of the LIBs, the Li2TiO3-based coating shell would endow the NCA electrode with more protective capability to withstand the degradation of core materials and the erosion from HF and CO2. Hence, the electrochemical evaluations
after the aging process were investigated at room temperature and elevated temperature. The aging process was carried out at 30 C for 50 days. The initial chargeedischarge curves of BNCA and TNCA aged cells at 25 C were shown in Fig. 8a. The chargeedischarge curves of two cells are smooth with a monotonous voltage plateau, whereas, the TNCA exhibits a longer discharge plateau obvious than its counterpart. The discharge capacity of TNCA and BNCA is 178 and 162 mA h g1, and their corresponding initial coulombic efficiencies are 86% and 80%, respectively. Because of the degradation of the electrode and electrolyte during the aging process, the discharge capacity and initial coulombic efficiencies of the aged cells are much lower than that of fresh ones. Fig. 8c presents the rate capability and following cycling evaluation. The TNCA exhibits a small enhanced reversible capacity than that of BNCA. However, the better capacity retention can be achieved in the Li2TiO3-coated NCA electrode, which delivered a capacity retention as high as 84% at the end of 200 charge/discharge cycles. Its counterpart only can be delivered a capacity retention of 48% at the same evaluative process. At elevated temperature, more side reactions between electrode and electrolyte, instability of electrolyte, release of O2 and degradation of Ni3þ result in an inferior cycle life of NCA materials, even more after the aging process [35]. Hence, the cycling stability of TNCA at 55 C was also investigated (Fig. 8b and d). The initial discharge capacities and their corresponding initial cyclonic efficiencies of BNCA and TNCA are 184 (90%) and 180 mA h g1 (92%), respectively. The elevated initial discharge capacities and coulombic efficiencies are attributed to the increased electrochemical activity of Ni-based cathode at the higher temperature. More remarkably, the BNCA exhibits a higher discharge capacity than TNCA, whereas, it shows an inferior cycling stability in the subsequent process. The aged TNCA electrode still retains 93% of the initial capacity (172 mA h g1) after 100 cycles at the current rate of 0.5 C, but the counterpart loses its capacity very rapidly as it only deliveries 58 mA h g1 (50%) after 100 cycles. Furthermore, the TNCA still show a robust thermal stability in the extended cycle number, as a high reversible capacity of 132 (77% retention) and
Fig. 7. SEM images and EDS spectra of the cycled electrodes; (a and c) BNCA and (b and d) TNCA.
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Figs. 8. (a and b) Initial chargeedischarge curves of BNCA and TNCA at 25 and 55 C, respectively; (c and d) Rate capability under variable current rate and cycling performances under 0.5C at 25 and 55 C, respectively. All the measurements used the cells after 50 days aging process.
96 mA h g1 (56% retention) are still retained even after 200 and total 300 cycles. Such an impressive electrochemical performance of the TNCA could be ascribed to the Li2TiO3-coated layer not only suppress the degradation of the electrode and electrolyte in the aging procedure, but also possess a stable conductive protected layer at the long-term charge/discharge cycles. This valued feature promises a wider operating temperature range and more stable
cycle performance for the Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 when served as a cathode material towards high-performance EVs and HEVs applications. 4. Conclusions Ternary layered LiNi0.8Co0.15Al0.05O2 microspheres coated with a
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nanoscale Li2TiO3 shell have been synthesized through a facile strategy based on a solvothermal pre-coating treatment together with a post-sintering lithiation process. Such a Liþ-conductive coating layer not only improves the velocity of Liþ migration on electrode surface, but also protects the host materials from the attack of CO2 from ambient environment, as well as the HF erosion by electrolyte decomposition during the long-term cycling and aging process. The surface-modified NCA electrode exhibits a remarkable cyclic stability both at fresh and aged status. These results indicate that Li2TiO3-coated LiNi0.8Co0.15Al0.05O2 holds a great promise for application in long-lifespan LIBs cathode materials. Acknowledgments The authors acknowledge the financial support from the National Basic Research Program of China (973 program no. 2013 CB934700), A Foundation for the Author of National Excellent Doctor Dissertation of P. R. China (no. FANEDD201435), and the Sichuan Province Science and Technology Support Program (no. 2014GZ0093). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.01.044. References [1] B.C. Melot, J.M. Tarascon, Accounts Chem. Res. 46 (2013) 1226e1238. [2] M. Broussely, S. Herreyre, P. Biensan, P. Kasztejna, K. Nechev, R.J. Staniewicz, J. Power Sources 97e98 (2001) 13e21. [3] B. Xu, D. Qian, Z. Wang, Y.S. Meng, Mater. Sci. Eng. R 73 (2012) 51e65. [4] N.T. Wu, H. Wu, W. Yuan, S. Liu, J. Liao, Y. Zhang, J. Mater. Chem. A 3 (2015) 13648e13652. [5] S. Watanabe, M. Kinoshita, K. Nakura, J. Power Sources 247 (2014) 412e422. [6] M.W. Mehrens, C. Vogler, J. Garche, J. Power Sources 127 (2004) 58e64. [7] M. Broussely, P. Biensan, F. Bonhomme, P. Blanchard, S. Herreyre, K. Nechev, R.J. Staniewicz, J. Power Sources 146 (2005) 90e96.
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