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Amorphous Zr(OH)4 coated LiNi0.915Co0.075Al0.01O2 cathode material with enhanced electrochemical performance for lithium ion batteries
Accepted Manuscript
Amorphous Zr(OH)4 coated LiNi0.915 Co0.075 Al0.01 O2 cathode material with enhanced electrochemical performance for lithium ion batteries
http://www.journals.elsevier.com/ journal-of-energy-chemistry/
Zhen Zhang , Pengfei Zhou , Huanju Meng , Chengcheng Chen , Fangyi Cheng , Zhanliang Tao , Jun Chen PII: DOI: Reference:
S2095-4956(16)30264-9 10.1016/j.jechem.2016.12.003 JECHEM 249
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
22 October 2016 9 December 2016 12 December 2016
Please cite this article as: Zhen Zhang , Pengfei Zhou , Huanju Meng , Chengcheng Chen , Fangyi Cheng , Zhanliang Tao , Jun Chen , Amorphous Zr(OH)4 coated LiNi0.915 Co0.075 Al0.01 O2 cathode material with enhanced electrochemical performance for lithium ion batteries, Journal of Energy Chemistry (2016), doi: 10.1016/j.jechem.2016.12.003
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Amorphous Zr(OH)4 is uniformly coated on the surface of NCA through a dry method.
Zr(OH)4 coating can effectively decrease the polarization and improve the
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cycling stability of NCA. 0.50 wt% Zr(OH)4 coated NCA exhibits excellent electrochemical
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performance.
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Amorphous Zr(OH)4 coated LiNi0.915Co0.075Al0.01O2 cathode material with enhanced electrochemical performance for lithium ion batteries Zhen Zhang, Pengfei Zhou, Huanju Meng, Chengcheng Chen, Fangyi Cheng, Zhanliang Tao*, Jun
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Chen Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and
Collaborative Innovation Center of Chemical Science and Engineering, College of Chemistry, Nankai University, Tianjin 300071, China
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E-mail: [email protected]
Abstract
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LiNi0.915Co0.075Al0.01O2 (NCA) with Zr(OH)4 coating is demonstrated as high performance cathode
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material for lithium ion batteries. The coated materials are synthesized via a simple dry coating method of NCA with Zr(OH)4 powders, and then characterized with scanning electron microscopy
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(SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).
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Experimental results show that amorphous Zr(OH)4 powders have been successfully coated on the surface of spherical NCA particles, exhibiting improved electrochemical performance. 0.50 wt%
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Zr(OH)4 coated NCA delivers a capacity of 197.6 mAh/g at the first cycle and 154.3 mAh/g after 100 cycles with a capacity retention of 78.1% at 1 C rate. In comparison, the pure NCA shows a capacity of 194.6 mAh/g at the first cycle and 142.5 mAh/g after 100 cycles with a capacity retention of 73.2% at 1 C rate. Electrochemical impedance spectroscopy (EIS) results show that the coated material exhibits a lower resistance, indicating that the coating layer can efficiently suppress transition metals dissolution and decrease the side reactions at the surface between the
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electrode and electrolyte. Therefore, surface coating with amorphous Zr(OH)4 is a simple and useful method to enhance the electrochemical performance of NCA-based materials for the cathode of LIBs. Keywords
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Ni-rich cathode material; Surface modification; Dry coating method; Zr(OH)4 powders; Electrochemical performance 1. Introduction
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Lithium ion batteries (LIBs) have been widely used in portable electronic devices (PEVs) since their first commercialization by Sony in 1991 [1-7]. In a battery system, the cathode materials play a key role in a battery’s cycling life and theoretical capacity [8-10].Among various cathode
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materials, LiCoO2 is the most commonly used in commercialized LIBs due to its good cycling performance and easy preparation [11]. While the high cost and toxicity of cobalt and relatively
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low practical capacity (140 mAh/g) limit its further application with the increasing demand of
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high-energy storage devices, thus it is urgent to develop new cathode materials with high capacity and low cost [12-14]. Recently, Co and Al doped Ni-rich layered LiNi1-x-yCoxAlyO2 (NCA,
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0
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investigated due to the high capacity (200 mAh/g) and relatively low cost [15-18]. Nevertheless, there still remain some crucial problems to be solved including low thermal stability, rapid capacity fading and poor cycling performance. These problems can be mainly ascribed to the unstable Ni4+ ions tending to migrate from the transition metal layer to the lithium layer. Thus, the formative cation mixing layer and rock salt NiO phase on the cathode surface increase the interfacial resistance and lead to an unstable structure [19, 20]. From another aspect, the strong
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oxidative Ni4+ ions tend to react with the electrolyte, speeding up the degradation of the cathode materials and resulting in poor electrochemical performance [21, 22]. Surface coating has been considered as an effective method to improve the electrochemical properties of cathode materials [23-28]. A proper coating layer can effectively avoid HF attack.
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This suppresses the dissolution of transition metals and decrease side reactions at the surface between the electrode and electrolyte [29]. Surface coating materials such as SiO2 [30], TiO2 [31], ZrO2 [32, 33], Al(OH)3 [34], metal fluoride (AlF3) [35], and metal phosphates of Ni3(PO4)2 [36],
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have been studied. Notably, Jang et al. firstly reported amorphous Al(OH)3 coated LiNi1/3Co1/3Mn1/3O2 via a wet method without subsequent calcination, showing good thermal stability [34]. Li et al. and Hu et al. prepared ZrO2 coated LiNi1/3Co1/3Mn1/3O2 through a wet
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method with subsequent calcination, exhibiting improved cycling stability at a high cut-off voltage [32, 33]. ZrO2 coating layer can effectively suppress the polarization effects during
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charge-discharge process and lower the activity of oxygen with strong Zr-O bonds [37]. The above
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coating processes contain some moisture or need relatively complicated procedures such as pH adjustment and calcination. Furthermore, these surface coating techniques are not applied to NCA
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materials with extremely high Ni content (e.g., >90%). Previous studies have shown that Ni-rich
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cathode materials are sensitive to humidity due to their rapid H2O and CO2 uptake when exposed in air, forming the residual lithium salts (LiOH, Li2CO3, and et al) on the surface [38]. Thus, to coat Ni-rich cathode materials, anhydrous system and simple process should been taken into consideration. Based on the previous literatures, Zr(OH)4 coated NCA is firstly reported in this work without introducing moisture and calcination process. In our experiment, LiNi0.915Co0.075Al0.01O2 (NCA) with a theoretical capacity of 275mAh/g was
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chosen as the raw material. As the Ni content (> 90%) is much higher than ever reported, this material exhibits appealing capacity but requires more strict modification processes. In this respect, we firstly report a facile dry method to coat such material with an amorphous Zr(OH)4 layer at room temperature. Schematic diagram for the effects of Zr(OH)4 coating layer on the NCA surface
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are shown in Scheme 1. The coating layer can effectively reduce the direct contact of NCA surface with H2O and CO2. This also acts as a protective layer to suppress the side reactions at the surface between the electrode and electrolyte during cycling, thus improving the structure stability of the
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active materials and the cycling life of the electrodes/batteries.
2. Experimental
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Scheme 1. Schematic diagram for the effects of Zr(OH)4 coating layer on the surface of NCA.
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2.1. Zr(OH)4 coated NCA synthesis
The pristine NCA sample supplied from Samsung SDI (Tianjin) was used as the raw material.
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To prepare Zr(OH)4 coated NCA samples, amorphous Zr(OH)4 powders were synthesized as
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follows: a certain amount of zirconium(Ⅳ) tetrapropoxide (Zr(OC3H7)4, Aladdin) was added into ethanol to form a uniform solution. Afterwards, distilled water and ethanol mixed with a volume ratio of 1:10 were slowly added into the above solution dropwise under continues ultrasonic agitation for 1 h to prevent Zr(OH)4 nanoparticles from aggregating. After filtrated with ethanol for several times, the obtained powders were dried at 80 °C overnight. Zr(OH)4 coated NCA samples were prepared through a simple dry method. Different amounts
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of Zr(OH)4 powders were mixed with NCA in a microtube homogenizer (USA, D1030-E) with a speed of 3500 r/min for 2 h to obtain the Zr(OH)4 coated NCA samples. The treatments of the coated Zr(OH)4 powders were 0.25-2.00 wt% of the pristine and 0.50 wt% is the optimum amount in our experiment.
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2.2. Characterization methods The crystalline structures of the pristine and coated samples were characterized by X-ray diffraction (XRD, Rigaku MinFlex600, Cu Kα radiation, λ=1.5406 Å) at a scanning rate of 8°
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min−1 from 15 to 80°. The morphologies (with particle size) and structures of the samples were observed using scanning electron microscopy (SEM, JEOL JSM7500F) equipped with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM, Philips Tecnai F20).
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Moreover, the surface chemical states of the elements were identified by X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI 1600). The vibration bands of hydroxyl (-OH) were
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analyzed by Fourier transform infrared spectroscopy (FTIR, Bruker Tensor Ⅱ).
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2.3. Electrochemical measurements
The electrochemical properties of the samples were tested via CR2032 coin cells. The cathode
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electrode was fabricated by mixing active material, Denka carbon and polyvinylidene fluoride
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(PVdF) binder dissolved in N-methylpyrrolidone (NMP) together with a weight ratio of 92:4:4. The cathode slurry was coated onto aluminum foil current collector. After dried at 110 °C for 10 h in a vacuum oven, the electrode was punched into circular tablets with a diameter of 10 mm and pressed under 4 MPa using a tablet machine. The active material loading in the electrode was 8-10 mg/cm2. The coin cells were assembled in an argon-filled glovebox. Lithium foil was used as the counter electrode and celgard polymer was used as the separator. The electrolyte consisted of 1.15
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M LiPF6 dissolved in the mixture solution of ethyl carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) with a volume ratio of 1:2:2. The charge-discharge tests were performed on a Land CT2001A cell testing system over the voltage range of 3.0-4.3 V at different rates of 0.1 C, 0.2 C, 0.5 C,1 C, 2 C, and 5 C (1 C corresponds to 180 mA/g). Electrochemical
(AMETEK) at a frequency ranged from 5 mHz to 100 kHz. 3. Results and discussion
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3.1. Characterization of the materials
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impedance spectroscopy (EIS) was conducted using Parstat 2273 electrochemical workstation
FTIR spectra and XRD patterns of the synthetic and commercial Zr(OH)4 powders are shown in Figure 1(a). In the FTIR spectra, the synthetic and commercial Zr(OH)4 powders exist an obvious peak at 3246.70 cm-1 and 3270.97 cm-1, respectively, belonging to the stretching absorption of
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hydroxyl (-OH). Other peaks between 1633.34 cm-1 and 1339.23 cm-1 can be assigned to the
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bending absorption of hydroxyl (-OH) [39]. Combined with the inset XRD patterns, the as
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prepared Zr(OH)4 powders are amorphous. From Figure 1(b), it is clearly observed that the as prepared Zr(OH)4 powders consist of nanometer-sized particles with an average particle size of
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120 nm.
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Fig. 1. (a) FTIR spectra and XRD patterns (inset) of the synthetic and commercial Zr(OH)4 powders; (b) SEM images and particle size distribution (inset) of the synthetic Zr(OH)4 powders.
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Figure 2 shows XRD patterns of the pristine and Zr(OH)4 coated NCA samples with different
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amounts of coating. All the diffraction peaks are indexed to a hexagonal α-NaFeO2 structure belonging to space group R3m. Compared with the pristine, no impurities and secondary phases
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were observed in the patterns of the coated samples, suggesting that the thin coating layer does not influence the crystal structure of bare material. The clear splitting of (006)/(012) and (018)/(110) peaks (magnified region) reveals that a well-ordered layered structure remains after coating. Table 1 summarizes the lattice parameters of the pristine and Zr(OH)4 coated samples. Modification with amorphous Zr(OH)4 powders does not change the lattice constants as the coating amount is very small, indicating that Zr has not been doped into the internal structure of the pristine.
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Fig. 2. XRD patterns of the pristine and different amounts of Zr(OH)4 coated NCA samples. Table 1. Lattice parameters of the pristine and Zr(OH)4 coated NCA samples. a (Å)
Pristine 0.25 wt%
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2.00 wt%
c/a
2.8791
14.2283
4.942
2.8750
14.2008
4.939
2.8724
14.1930
4.938
2.8727
14.1944
4.941
2.8748
14.2016
4.940
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0.50 wt% 1.00 wt%
c (Å)
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Samples
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Figure 3(a) and 3(b) show SEM images of the pristine and 0.50 wt% Zr(OH)4 coated NCA. Both samples have a good spherical morphology with a particle size of about 15 μm in diameter.
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Nanometer-sized primary particles constitute these spherical micro-sized secondary particles,
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leading to a high tap density which influences the volumetric capacity of the commercial lithium ion batteries [33]. It can be seen that the surface and edge of the pristine and Zr(OH)4 coated powders are clean and smooth. As the amount of coating is very small, there is no obvious difference in both samples. In order to demonstrate the presence of the Zr(OH)4 coating layer directly, TEM was performed to investigate the surface of particles in Figure 3(c) and 3(d). As shown in figure 3(c), the d-spacing of 0.240 nm is the characteristic of (101) plane for NCA
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structure and the pristine does not have an extra layer on its surface. Compared with the smooth surface of NCA (Figure 3(c)), a uniform coating layer with a thickness of 15-20 nm is clearly observed after surface modification with Zr(OH)4 in Figure 3(d). From the inset in Figure 3(d), the pristine has a well-defined crystal structure with a crystal spacing of 0.204 nm, which can be
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indexed to the (104) crystal plane of NCA layered structure. In comparison, the coating layer of Zr(OH)4 is amorphous. Based on TEM results, it is known that Zr(OH)4 powders have been
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successfully coated on the surface of the pristine.
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Fig. 3. SEM images of (a) the pristine and (b) 0.50 wt% Zr(OH)4 coated NCA; TEM images of (c) the pristine and (d) 0.50 wt% Zr(OH)4 coated NCA.
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Mapping analysis and EDS spectra and were carried out in Figure 4(a)-(e). It can be known that
Zr(OH)4 powders have been successfully coated on the surface of the pristine and Zr element distributes homogeneously as the innate elements of Ni and Co. The surface chemical states of Zr in the coated sample, were characterized by XPS as shown in Figure 4(f). The signals corresponding to Zr(3d5/2, 3d3/2) doublet are clearly observed in the coated material derived from Zr(OH)4 coating layer, which are related to Zr4+ [40]. On the basis of the above results, it can be
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further suggested that Zr(OH)4 powders have been successfully coated on the surface of the
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pristine.
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Fig. 4. (a) SEM image of 0.50 wt% Zr(OH)4 coated NCA; EDS mapping of (b) Ni, (c) Co, and (d) Zr; (e) EDS spectra of 0.50 wt% Zr(OH)4 coated NCA; (f) XPS spectra of Zr 3d for the pristine
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and 0.50 wt% Zr(OH)4 coated NCA.
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3.2. Electrochemical performance of the materials Cyclic voltammetry (CV) was performed to characterize the phase transformations of the
electrode during charge-discharge process. Figure 5 compares the CV curves of the pristine and 0.50 wt% Zr(OH)4 coated NCA at 0.1 mV/s between 3.0 V and 4.3 V for the first four cycles at 25 °C. A significant difference is observed between the first cycle and subsequent cycles in both samples, which is mainly due to the formation of the SEI film on the surface of the electrode
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during cycling [41]. There are three pairs of peaks in both samples. The peaks around 3.60/3.75 V can be attributed to the oxidation/reduction of Ni3+/Ni4+, while the peaks around 3.93/4.02 V and 4.22/4.23 V correspond to the phase transitions of the monoclinic phase (M) to the second hexagonal phase (H2) and the hexagonal phase (H2) to a third hexagonal phase (H3), respectively
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[42]. As shown in Figure 5, the intensity of the second and third oxidation peaks are slightly weakened after coating during cycling in reflect of the reduction of phase transformations [43]. In addition, there exists some potential difference between the two samples. The major oxidation
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peak potential (Epo) and reduction peak potential (Epr) are listed in Table 2. From the first four cycles, the potential difference between the major cathodic and anodic peaks of 0.50 wt% Zr(OH)4 coated NCA is lower than the pristine. This means that Zr(OH)4 coating can effectively decrease
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the polarization and improve the cycling reversibility of the pristine electrode [44].
Fig. 5. Cyclic voltammetry curves of (a) the pristine and (b) 0.50 wt% Zr(OH)4 coated NCA at
0.1 mV/s between 3.0-4.3 V for the first four cycles at 25 °C. Table 2. Oxidation peak potential (Epo) and reduction peak potential (Epr) of the pristine and 0.50 wt% Zr(OH)4 coated NCA powders.
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Pristine
Value (V
0.50 wt%
1st
2nd
3rd
4th
1st
2nd
3rd
4th
Epo
3.557
3.586
3.583
3.617
3.640
3.599
3.606
3.614
Epr
3.885
3.757
3.745
3.734
3.907
3.760
3.748
3.730
0.328
0.171
0.162
0.117
0.267
△E= Epr Epo
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vs. Li+/Li
0.161
0.142
0.116
Figure 6(a) and (b) show the initial charge-discharge curves and cycling performance of the
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pristine and different amounts of Zr(OH)4 coated NCA electrodes at 1 C from 3.0 to 4.3 V at 25 °C. As can be seen in Figure 6(a), there is no significant difference on the shapes of the charge-discharge curves between the pristine and Zr(OH)4 coated NCA electrodes, indicating that
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various amounts of Zr(OH)4 coating have not changed the structure of the pristine, which is in agreement with the results of XRD. Among all the samples, 0.50 wt% Zr(OH)4 coated NCA
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delivers a higher initial discharge capacity of 197.6 mAh/g compared with that of 194.6 mAh/g for
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the pristine sample at 1 C. Furthermore, the discharge voltage of 0.50 wt% Zr(OH)4 is slightly higher than that of the pristine. This means the appropriate amount of Zr(OH)4 powders can
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effectively reduce the polarization. Small amount of Zr(OH)4 powders is unable to coat uniformly
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and excess amount can suppress the diffusion of ions, thus leading to a low cycling capacity. For further investigation, cycling performance of the as prepared samples was investigated over 100 cycles at 1 C between 3.0 and 4.3 V at 25 °C. In Figure 6(b), 0.50 wt% Zr(OH)4 shows a higher discharge capacity and capacity retention than other amounts of Zr(OH)4 coated samples and the pristine. After 100 cycles, the pristine delivers a capacity of 142.5 mAh/g and a capacity retention of 73.2%, In comparison, 0.50 wt% Zr(OH)4 coated NCA shows 154.3 mAh/g and 78.1%
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retention after 100 cycles. As coulombic efficiency is a key factor to evaluate an electrode, figure 6(b) shows that coulombic efficiency of all the samples can reach approximately 99% in the overall battery operation, demonstrating lower irreversible capacities during cycling and relatively good cycling stability. Capacity degradation is a common problem in NCA materials on account of
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the instable layered structure of Ni-rich materials [45]. Zr(OH)4 coating layer on the surface of the pristine can protect the active materials from HF attack and reduce side reactions, thus improving the structural stability and cycling performance.
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Figure6 (c) and (d) show the rate capability of the pristine and 0.50 wt% Zr(OH)4 coated NCA evaluated with different current rates of 0.1 C to 5 C over 3.0-4.3 V at 25 °C. 0.50 wt% Zr(OH)4 coated NCA delivers higher capacity than that of the pristine at all current rates. For example, the
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discharge capacity of 0.50 wt% Zr(OH)4 coated NCA is 109.0 mAh/g at 5 C, while in case of the pristine, the discharge capacity is 103.2 mAh/g, indicating that rapid litigation/delithiation at high
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current rates have not damaged the crystal structure after coating. The improvement of the
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discharge capacity and structural stability, especially at relatively high current rates, is due to the lower polarization and the decrease of side reactions between the electrode and electrolyte by
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coating a layer of amorphous Zr(OH)4. Based on the above results, amorphous Zr(OH)4 layer on
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the surface of the electrode plays a positive role in enhancing the structural stability and rate capability of the pristine.
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Fig. 6. (a) The initial charge-discharge curves of the pristine and Zr(OH)4 coated NCA electrodes
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at 1 C between 3.0 and 4.3 V; (b) The cycling performance of the pristine and Zr(OH)4 coated NCA electrodes at 1 C between 3.0 and 4.3 V; (c) Rate capacity of the pristine and 0.50 wt%
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Zr(OH)4 coated NCA electrodes at various discharge rates over 3-4.3 V; (d) The discharge
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capacity of the pristine and 0.50 wt% Zr(OH)4 coated NCA electrode at various current densities over 3-4.3 V.
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The electrochemical impedance spectra of the pristine and 0.50 wt% Zr(OH)4coated NCA after
1, 30, and 50 cycles at 1 C are shown in Figure 7. Both Nyquist plots of the two samples are similar with two semicircles and a short linear portion. The semicircle occurred at high frequency is related to the resistance of Li ion migration through surface film (Rsf) and the semicircle at medium to low frequency is attributed to the resistance of charge transfer (Rct) at the interface between the electrolyte and electrode [46]. The inclined line at low frequency represents Warburg
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impedance (W) which is connected with Li ion diffusion in bulk materials. Figure 7(c) shows the Nyquist plots fitted with the equivalent and the values of Rsf and Rct are summarized in Table 3. There is no large difference on Rsf, which is associated with a stable solid electrolyte interface (SEI) film, indicating that the coated Zr(OH)4 powders are not dissolved into the electrolyte and
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the composition and conductivity have not been changed [47]. It can be seen that the Rct value of the pristine increases rapidly during cycling, while the Rct value of 0.50 wt% Zr(OH)4 coated NCA shows a slow growth. After 30th and 50th cycle, the Rct value of the pristine is 219.8 Ω and 262.3
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Ω, respectively. In comparison, after 30th and 50th cycle, the Rct value of 0.50 wt% Zr(OH)4 coated NCA is 128.9 Ω and 156.5 Ω, respectively. Based on the above results, it can be concluded that Zr(OH)4 coating layer can effectively decrease the resistance of charge transfer, which is due
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to the reduction of side reactions between the electrolyte and electrode. In addition, the coating layer can prevent the dissolution of the active materials, thus improving the electrochemical
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properties of the samples.
Fig. 7. Nyquist plots of (a) pristine and (b) 0.50 wt% Zr(OH)4 coated NCA electrodes during cycling at 1 C. (c) Equivalent circuit model for the electrochemical impedance fitting of the
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electrode. Table 3. Impedance fitting parameters of surface film resistance (Rsf) and charge transfer resistance (Rct) for pristine and 0.50 wt% Zr(OH)4 coated NCA electrodes. 0.50 wt%
Rsf (Ω)
Rct (Ω)
Rsf (Ω)
1
26.2
93.5
12.5
30
12.6
219.8
18.3
50
18.1
262.3
23.6
Rct (Ω) 125.8 128.9 156.5
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number
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Pristine
Cycle
Fig.8 shows the SEM images of (a) the pristine and (b) 0.50 wt% Zr(OH)4 coated NCA after 50 cycles at 1C. After cycling, the surface of NCA becomes rough and the edge of the particle breaks
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to small fragments. Moreover, some cracks are observed on the surface of the pristine as shown in Fig. 8(a). The structural damages of the pristine might be caused by the internal stress
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accompanied by degradation of structure and the dissolution of transition metal elements in the
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electrolyte [46]. In contrast, the particle of 0.50 wt% Zr(OH)4 coated NCA keeps integrity and the surface is relatively smooth, indicating the coating layer can efficiently decrease HF attack and
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reduce transition metal elements dissolution. The above results better demonstrate the excellent
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structural stability of 0.50 wt% Zr(OH)4 coated NCA during cycling.
Fig. 8. SEM images of (a) the pristine and (b) 0.50 wt% Zr(OH)4 coated NCA after 50 cycles at 1C.
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4. Conclusions LiNi0.915Co0.075Al0.01O2 with amorphous Zr(OH)4 coating has been synthesized through a simple dry method at room temperature in consideration of the sensitivity of NCA sample to water and temperature. After modified with Zr(OH)4 powders, the coated sample exhibits excellent
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electrochemical performance. The coating of 0.50 wt% Zr(OH)4 is chosen as the optimum coating quantity with a thickness of 15-20 nm. After 100 cycles at 1 C, 0.50 wt% Zr(OH)4 coated NCA sample delivers a capacity of 154.3 mAh/g with a capacity retention of 78.1%. In comparison,
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after 100 cycles at 1 C, NCA without coating shows the capacity of 142.5 mAh/g with a capacity retention of 73.2%. In addition, the coated sample shows a lower charge transfer resistance than the pristine during cycling. The improved electrochemical performance can be attributed to the
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reduced side reactions between the electrolyte and electrode. Furthermore, Zr(OH)4 coating layer can effectively prevent cathode active materials from decomposing and thus enhance the structural
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stability of the electrode. Therefore, surface coating with amorphous Zr(OH)4 is a simple and
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useful method to enhance the electrochemical performance of NCA-based materials for the cathode of LIBs.
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Acknowledgements
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This work was supported by National Projects of NSFC (21322101 and 21231005) and MOE
(B12015 and IRT13R30).
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Graphic abstract
Amorphous Zr(OH)4 coated LiNi0.915 Co0.075 Al0.01 O2 cathode material with enhanced electrochemical performance for lithium ion batteries
http://www.journals.elsevier.com/ journal-of-energy-chemistry/
Zhen Zhang , Pengfei Zhou , Huanju Meng , Chengcheng Chen , Fangyi Cheng , Zhanliang Tao , Jun Chen PII: DOI: Reference:
S2095-4956(16)30264-9 10.1016/j.jechem.2016.12.003 JECHEM 249
To appear in:
Journal of Energy Chemistry
Received date: Revised date: Accepted date:
22 October 2016 9 December 2016 12 December 2016
Please cite this article as: Zhen Zhang , Pengfei Zhou , Huanju Meng , Chengcheng Chen , Fangyi Cheng , Zhanliang Tao , Jun Chen , Amorphous Zr(OH)4 coated LiNi0.915 Co0.075 Al0.01 O2 cathode material with enhanced electrochemical performance for lithium ion batteries, Journal of Energy Chemistry (2016), doi: 10.1016/j.jechem.2016.12.003
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Amorphous Zr(OH)4 is uniformly coated on the surface of NCA through a dry method.
Zr(OH)4 coating can effectively decrease the polarization and improve the
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cycling stability of NCA. 0.50 wt% Zr(OH)4 coated NCA exhibits excellent electrochemical
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performance.
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Amorphous Zr(OH)4 coated LiNi0.915Co0.075Al0.01O2 cathode material with enhanced electrochemical performance for lithium ion batteries Zhen Zhang, Pengfei Zhou, Huanju Meng, Chengcheng Chen, Fangyi Cheng, Zhanliang Tao*, Jun
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Chen Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) and
Collaborative Innovation Center of Chemical Science and Engineering, College of Chemistry, Nankai University, Tianjin 300071, China
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E-mail: [email protected]
Abstract
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LiNi0.915Co0.075Al0.01O2 (NCA) with Zr(OH)4 coating is demonstrated as high performance cathode
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material for lithium ion batteries. The coated materials are synthesized via a simple dry coating method of NCA with Zr(OH)4 powders, and then characterized with scanning electron microscopy
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(SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS).
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Experimental results show that amorphous Zr(OH)4 powders have been successfully coated on the surface of spherical NCA particles, exhibiting improved electrochemical performance. 0.50 wt%
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Zr(OH)4 coated NCA delivers a capacity of 197.6 mAh/g at the first cycle and 154.3 mAh/g after 100 cycles with a capacity retention of 78.1% at 1 C rate. In comparison, the pure NCA shows a capacity of 194.6 mAh/g at the first cycle and 142.5 mAh/g after 100 cycles with a capacity retention of 73.2% at 1 C rate. Electrochemical impedance spectroscopy (EIS) results show that the coated material exhibits a lower resistance, indicating that the coating layer can efficiently suppress transition metals dissolution and decrease the side reactions at the surface between the
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electrode and electrolyte. Therefore, surface coating with amorphous Zr(OH)4 is a simple and useful method to enhance the electrochemical performance of NCA-based materials for the cathode of LIBs. Keywords
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Ni-rich cathode material; Surface modification; Dry coating method; Zr(OH)4 powders; Electrochemical performance 1. Introduction
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Lithium ion batteries (LIBs) have been widely used in portable electronic devices (PEVs) since their first commercialization by Sony in 1991 [1-7]. In a battery system, the cathode materials play a key role in a battery’s cycling life and theoretical capacity [8-10].Among various cathode
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materials, LiCoO2 is the most commonly used in commercialized LIBs due to its good cycling performance and easy preparation [11]. While the high cost and toxicity of cobalt and relatively
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low practical capacity (140 mAh/g) limit its further application with the increasing demand of
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high-energy storage devices, thus it is urgent to develop new cathode materials with high capacity and low cost [12-14]. Recently, Co and Al doped Ni-rich layered LiNi1-x-yCoxAlyO2 (NCA,
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0
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investigated due to the high capacity (200 mAh/g) and relatively low cost [15-18]. Nevertheless, there still remain some crucial problems to be solved including low thermal stability, rapid capacity fading and poor cycling performance. These problems can be mainly ascribed to the unstable Ni4+ ions tending to migrate from the transition metal layer to the lithium layer. Thus, the formative cation mixing layer and rock salt NiO phase on the cathode surface increase the interfacial resistance and lead to an unstable structure [19, 20]. From another aspect, the strong
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oxidative Ni4+ ions tend to react with the electrolyte, speeding up the degradation of the cathode materials and resulting in poor electrochemical performance [21, 22]. Surface coating has been considered as an effective method to improve the electrochemical properties of cathode materials [23-28]. A proper coating layer can effectively avoid HF attack.
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This suppresses the dissolution of transition metals and decrease side reactions at the surface between the electrode and electrolyte [29]. Surface coating materials such as SiO2 [30], TiO2 [31], ZrO2 [32, 33], Al(OH)3 [34], metal fluoride (AlF3) [35], and metal phosphates of Ni3(PO4)2 [36],
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have been studied. Notably, Jang et al. firstly reported amorphous Al(OH)3 coated LiNi1/3Co1/3Mn1/3O2 via a wet method without subsequent calcination, showing good thermal stability [34]. Li et al. and Hu et al. prepared ZrO2 coated LiNi1/3Co1/3Mn1/3O2 through a wet
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method with subsequent calcination, exhibiting improved cycling stability at a high cut-off voltage [32, 33]. ZrO2 coating layer can effectively suppress the polarization effects during
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charge-discharge process and lower the activity of oxygen with strong Zr-O bonds [37]. The above
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coating processes contain some moisture or need relatively complicated procedures such as pH adjustment and calcination. Furthermore, these surface coating techniques are not applied to NCA
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materials with extremely high Ni content (e.g., >90%). Previous studies have shown that Ni-rich
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cathode materials are sensitive to humidity due to their rapid H2O and CO2 uptake when exposed in air, forming the residual lithium salts (LiOH, Li2CO3, and et al) on the surface [38]. Thus, to coat Ni-rich cathode materials, anhydrous system and simple process should been taken into consideration. Based on the previous literatures, Zr(OH)4 coated NCA is firstly reported in this work without introducing moisture and calcination process. In our experiment, LiNi0.915Co0.075Al0.01O2 (NCA) with a theoretical capacity of 275mAh/g was
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chosen as the raw material. As the Ni content (> 90%) is much higher than ever reported, this material exhibits appealing capacity but requires more strict modification processes. In this respect, we firstly report a facile dry method to coat such material with an amorphous Zr(OH)4 layer at room temperature. Schematic diagram for the effects of Zr(OH)4 coating layer on the NCA surface
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are shown in Scheme 1. The coating layer can effectively reduce the direct contact of NCA surface with H2O and CO2. This also acts as a protective layer to suppress the side reactions at the surface between the electrode and electrolyte during cycling, thus improving the structure stability of the
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active materials and the cycling life of the electrodes/batteries.
2. Experimental
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Scheme 1. Schematic diagram for the effects of Zr(OH)4 coating layer on the surface of NCA.
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2.1. Zr(OH)4 coated NCA synthesis
The pristine NCA sample supplied from Samsung SDI (Tianjin) was used as the raw material.
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To prepare Zr(OH)4 coated NCA samples, amorphous Zr(OH)4 powders were synthesized as
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follows: a certain amount of zirconium(Ⅳ) tetrapropoxide (Zr(OC3H7)4, Aladdin) was added into ethanol to form a uniform solution. Afterwards, distilled water and ethanol mixed with a volume ratio of 1:10 were slowly added into the above solution dropwise under continues ultrasonic agitation for 1 h to prevent Zr(OH)4 nanoparticles from aggregating. After filtrated with ethanol for several times, the obtained powders were dried at 80 °C overnight. Zr(OH)4 coated NCA samples were prepared through a simple dry method. Different amounts
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of Zr(OH)4 powders were mixed with NCA in a microtube homogenizer (USA, D1030-E) with a speed of 3500 r/min for 2 h to obtain the Zr(OH)4 coated NCA samples. The treatments of the coated Zr(OH)4 powders were 0.25-2.00 wt% of the pristine and 0.50 wt% is the optimum amount in our experiment.
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2.2. Characterization methods The crystalline structures of the pristine and coated samples were characterized by X-ray diffraction (XRD, Rigaku MinFlex600, Cu Kα radiation, λ=1.5406 Å) at a scanning rate of 8°
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min−1 from 15 to 80°. The morphologies (with particle size) and structures of the samples were observed using scanning electron microscopy (SEM, JEOL JSM7500F) equipped with energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM, Philips Tecnai F20).
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Moreover, the surface chemical states of the elements were identified by X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI 1600). The vibration bands of hydroxyl (-OH) were
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analyzed by Fourier transform infrared spectroscopy (FTIR, Bruker Tensor Ⅱ).
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2.3. Electrochemical measurements
The electrochemical properties of the samples were tested via CR2032 coin cells. The cathode
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electrode was fabricated by mixing active material, Denka carbon and polyvinylidene fluoride
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(PVdF) binder dissolved in N-methylpyrrolidone (NMP) together with a weight ratio of 92:4:4. The cathode slurry was coated onto aluminum foil current collector. After dried at 110 °C for 10 h in a vacuum oven, the electrode was punched into circular tablets with a diameter of 10 mm and pressed under 4 MPa using a tablet machine. The active material loading in the electrode was 8-10 mg/cm2. The coin cells were assembled in an argon-filled glovebox. Lithium foil was used as the counter electrode and celgard polymer was used as the separator. The electrolyte consisted of 1.15
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M LiPF6 dissolved in the mixture solution of ethyl carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) with a volume ratio of 1:2:2. The charge-discharge tests were performed on a Land CT2001A cell testing system over the voltage range of 3.0-4.3 V at different rates of 0.1 C, 0.2 C, 0.5 C,1 C, 2 C, and 5 C (1 C corresponds to 180 mA/g). Electrochemical
(AMETEK) at a frequency ranged from 5 mHz to 100 kHz. 3. Results and discussion
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3.1. Characterization of the materials
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impedance spectroscopy (EIS) was conducted using Parstat 2273 electrochemical workstation
FTIR spectra and XRD patterns of the synthetic and commercial Zr(OH)4 powders are shown in Figure 1(a). In the FTIR spectra, the synthetic and commercial Zr(OH)4 powders exist an obvious peak at 3246.70 cm-1 and 3270.97 cm-1, respectively, belonging to the stretching absorption of
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hydroxyl (-OH). Other peaks between 1633.34 cm-1 and 1339.23 cm-1 can be assigned to the
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bending absorption of hydroxyl (-OH) [39]. Combined with the inset XRD patterns, the as
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prepared Zr(OH)4 powders are amorphous. From Figure 1(b), it is clearly observed that the as prepared Zr(OH)4 powders consist of nanometer-sized particles with an average particle size of
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120 nm.
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Fig. 1. (a) FTIR spectra and XRD patterns (inset) of the synthetic and commercial Zr(OH)4 powders; (b) SEM images and particle size distribution (inset) of the synthetic Zr(OH)4 powders.
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Figure 2 shows XRD patterns of the pristine and Zr(OH)4 coated NCA samples with different
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amounts of coating. All the diffraction peaks are indexed to a hexagonal α-NaFeO2 structure belonging to space group R3m. Compared with the pristine, no impurities and secondary phases
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were observed in the patterns of the coated samples, suggesting that the thin coating layer does not influence the crystal structure of bare material. The clear splitting of (006)/(012) and (018)/(110) peaks (magnified region) reveals that a well-ordered layered structure remains after coating. Table 1 summarizes the lattice parameters of the pristine and Zr(OH)4 coated samples. Modification with amorphous Zr(OH)4 powders does not change the lattice constants as the coating amount is very small, indicating that Zr has not been doped into the internal structure of the pristine.
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Fig. 2. XRD patterns of the pristine and different amounts of Zr(OH)4 coated NCA samples. Table 1. Lattice parameters of the pristine and Zr(OH)4 coated NCA samples. a (Å)
Pristine 0.25 wt%
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2.00 wt%
c/a
2.8791
14.2283
4.942
2.8750
14.2008
4.939
2.8724
14.1930
4.938
2.8727
14.1944
4.941
2.8748
14.2016
4.940
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0.50 wt% 1.00 wt%
c (Å)
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Samples
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Figure 3(a) and 3(b) show SEM images of the pristine and 0.50 wt% Zr(OH)4 coated NCA. Both samples have a good spherical morphology with a particle size of about 15 μm in diameter.
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Nanometer-sized primary particles constitute these spherical micro-sized secondary particles,
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leading to a high tap density which influences the volumetric capacity of the commercial lithium ion batteries [33]. It can be seen that the surface and edge of the pristine and Zr(OH)4 coated powders are clean and smooth. As the amount of coating is very small, there is no obvious difference in both samples. In order to demonstrate the presence of the Zr(OH)4 coating layer directly, TEM was performed to investigate the surface of particles in Figure 3(c) and 3(d). As shown in figure 3(c), the d-spacing of 0.240 nm is the characteristic of (101) plane for NCA
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structure and the pristine does not have an extra layer on its surface. Compared with the smooth surface of NCA (Figure 3(c)), a uniform coating layer with a thickness of 15-20 nm is clearly observed after surface modification with Zr(OH)4 in Figure 3(d). From the inset in Figure 3(d), the pristine has a well-defined crystal structure with a crystal spacing of 0.204 nm, which can be
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indexed to the (104) crystal plane of NCA layered structure. In comparison, the coating layer of Zr(OH)4 is amorphous. Based on TEM results, it is known that Zr(OH)4 powders have been
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successfully coated on the surface of the pristine.
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Fig. 3. SEM images of (a) the pristine and (b) 0.50 wt% Zr(OH)4 coated NCA; TEM images of (c) the pristine and (d) 0.50 wt% Zr(OH)4 coated NCA.
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Mapping analysis and EDS spectra and were carried out in Figure 4(a)-(e). It can be known that
Zr(OH)4 powders have been successfully coated on the surface of the pristine and Zr element distributes homogeneously as the innate elements of Ni and Co. The surface chemical states of Zr in the coated sample, were characterized by XPS as shown in Figure 4(f). The signals corresponding to Zr(3d5/2, 3d3/2) doublet are clearly observed in the coated material derived from Zr(OH)4 coating layer, which are related to Zr4+ [40]. On the basis of the above results, it can be
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further suggested that Zr(OH)4 powders have been successfully coated on the surface of the
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pristine.
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Fig. 4. (a) SEM image of 0.50 wt% Zr(OH)4 coated NCA; EDS mapping of (b) Ni, (c) Co, and (d) Zr; (e) EDS spectra of 0.50 wt% Zr(OH)4 coated NCA; (f) XPS spectra of Zr 3d for the pristine
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and 0.50 wt% Zr(OH)4 coated NCA.
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3.2. Electrochemical performance of the materials Cyclic voltammetry (CV) was performed to characterize the phase transformations of the
electrode during charge-discharge process. Figure 5 compares the CV curves of the pristine and 0.50 wt% Zr(OH)4 coated NCA at 0.1 mV/s between 3.0 V and 4.3 V for the first four cycles at 25 °C. A significant difference is observed between the first cycle and subsequent cycles in both samples, which is mainly due to the formation of the SEI film on the surface of the electrode
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during cycling [41]. There are three pairs of peaks in both samples. The peaks around 3.60/3.75 V can be attributed to the oxidation/reduction of Ni3+/Ni4+, while the peaks around 3.93/4.02 V and 4.22/4.23 V correspond to the phase transitions of the monoclinic phase (M) to the second hexagonal phase (H2) and the hexagonal phase (H2) to a third hexagonal phase (H3), respectively
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[42]. As shown in Figure 5, the intensity of the second and third oxidation peaks are slightly weakened after coating during cycling in reflect of the reduction of phase transformations [43]. In addition, there exists some potential difference between the two samples. The major oxidation
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peak potential (Epo) and reduction peak potential (Epr) are listed in Table 2. From the first four cycles, the potential difference between the major cathodic and anodic peaks of 0.50 wt% Zr(OH)4 coated NCA is lower than the pristine. This means that Zr(OH)4 coating can effectively decrease
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the polarization and improve the cycling reversibility of the pristine electrode [44].
Fig. 5. Cyclic voltammetry curves of (a) the pristine and (b) 0.50 wt% Zr(OH)4 coated NCA at
0.1 mV/s between 3.0-4.3 V for the first four cycles at 25 °C. Table 2. Oxidation peak potential (Epo) and reduction peak potential (Epr) of the pristine and 0.50 wt% Zr(OH)4 coated NCA powders.
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Pristine
Value (V
0.50 wt%
1st
2nd
3rd
4th
1st
2nd
3rd
4th
Epo
3.557
3.586
3.583
3.617
3.640
3.599
3.606
3.614
Epr
3.885
3.757
3.745
3.734
3.907
3.760
3.748
3.730
0.328
0.171
0.162
0.117
0.267
△E= Epr Epo
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vs. Li+/Li
0.161
0.142
0.116
Figure 6(a) and (b) show the initial charge-discharge curves and cycling performance of the
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pristine and different amounts of Zr(OH)4 coated NCA electrodes at 1 C from 3.0 to 4.3 V at 25 °C. As can be seen in Figure 6(a), there is no significant difference on the shapes of the charge-discharge curves between the pristine and Zr(OH)4 coated NCA electrodes, indicating that
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various amounts of Zr(OH)4 coating have not changed the structure of the pristine, which is in agreement with the results of XRD. Among all the samples, 0.50 wt% Zr(OH)4 coated NCA
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delivers a higher initial discharge capacity of 197.6 mAh/g compared with that of 194.6 mAh/g for
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the pristine sample at 1 C. Furthermore, the discharge voltage of 0.50 wt% Zr(OH)4 is slightly higher than that of the pristine. This means the appropriate amount of Zr(OH)4 powders can
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effectively reduce the polarization. Small amount of Zr(OH)4 powders is unable to coat uniformly
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and excess amount can suppress the diffusion of ions, thus leading to a low cycling capacity. For further investigation, cycling performance of the as prepared samples was investigated over 100 cycles at 1 C between 3.0 and 4.3 V at 25 °C. In Figure 6(b), 0.50 wt% Zr(OH)4 shows a higher discharge capacity and capacity retention than other amounts of Zr(OH)4 coated samples and the pristine. After 100 cycles, the pristine delivers a capacity of 142.5 mAh/g and a capacity retention of 73.2%, In comparison, 0.50 wt% Zr(OH)4 coated NCA shows 154.3 mAh/g and 78.1%
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retention after 100 cycles. As coulombic efficiency is a key factor to evaluate an electrode, figure 6(b) shows that coulombic efficiency of all the samples can reach approximately 99% in the overall battery operation, demonstrating lower irreversible capacities during cycling and relatively good cycling stability. Capacity degradation is a common problem in NCA materials on account of
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the instable layered structure of Ni-rich materials [45]. Zr(OH)4 coating layer on the surface of the pristine can protect the active materials from HF attack and reduce side reactions, thus improving the structural stability and cycling performance.
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Figure6 (c) and (d) show the rate capability of the pristine and 0.50 wt% Zr(OH)4 coated NCA evaluated with different current rates of 0.1 C to 5 C over 3.0-4.3 V at 25 °C. 0.50 wt% Zr(OH)4 coated NCA delivers higher capacity than that of the pristine at all current rates. For example, the
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discharge capacity of 0.50 wt% Zr(OH)4 coated NCA is 109.0 mAh/g at 5 C, while in case of the pristine, the discharge capacity is 103.2 mAh/g, indicating that rapid litigation/delithiation at high
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current rates have not damaged the crystal structure after coating. The improvement of the
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discharge capacity and structural stability, especially at relatively high current rates, is due to the lower polarization and the decrease of side reactions between the electrode and electrolyte by
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coating a layer of amorphous Zr(OH)4. Based on the above results, amorphous Zr(OH)4 layer on
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the surface of the electrode plays a positive role in enhancing the structural stability and rate capability of the pristine.
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Fig. 6. (a) The initial charge-discharge curves of the pristine and Zr(OH)4 coated NCA electrodes
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at 1 C between 3.0 and 4.3 V; (b) The cycling performance of the pristine and Zr(OH)4 coated NCA electrodes at 1 C between 3.0 and 4.3 V; (c) Rate capacity of the pristine and 0.50 wt%
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Zr(OH)4 coated NCA electrodes at various discharge rates over 3-4.3 V; (d) The discharge
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capacity of the pristine and 0.50 wt% Zr(OH)4 coated NCA electrode at various current densities over 3-4.3 V.
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The electrochemical impedance spectra of the pristine and 0.50 wt% Zr(OH)4coated NCA after
1, 30, and 50 cycles at 1 C are shown in Figure 7. Both Nyquist plots of the two samples are similar with two semicircles and a short linear portion. The semicircle occurred at high frequency is related to the resistance of Li ion migration through surface film (Rsf) and the semicircle at medium to low frequency is attributed to the resistance of charge transfer (Rct) at the interface between the electrolyte and electrode [46]. The inclined line at low frequency represents Warburg
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impedance (W) which is connected with Li ion diffusion in bulk materials. Figure 7(c) shows the Nyquist plots fitted with the equivalent and the values of Rsf and Rct are summarized in Table 3. There is no large difference on Rsf, which is associated with a stable solid electrolyte interface (SEI) film, indicating that the coated Zr(OH)4 powders are not dissolved into the electrolyte and
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the composition and conductivity have not been changed [47]. It can be seen that the Rct value of the pristine increases rapidly during cycling, while the Rct value of 0.50 wt% Zr(OH)4 coated NCA shows a slow growth. After 30th and 50th cycle, the Rct value of the pristine is 219.8 Ω and 262.3
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Ω, respectively. In comparison, after 30th and 50th cycle, the Rct value of 0.50 wt% Zr(OH)4 coated NCA is 128.9 Ω and 156.5 Ω, respectively. Based on the above results, it can be concluded that Zr(OH)4 coating layer can effectively decrease the resistance of charge transfer, which is due
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to the reduction of side reactions between the electrolyte and electrode. In addition, the coating layer can prevent the dissolution of the active materials, thus improving the electrochemical
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properties of the samples.
Fig. 7. Nyquist plots of (a) pristine and (b) 0.50 wt% Zr(OH)4 coated NCA electrodes during cycling at 1 C. (c) Equivalent circuit model for the electrochemical impedance fitting of the
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electrode. Table 3. Impedance fitting parameters of surface film resistance (Rsf) and charge transfer resistance (Rct) for pristine and 0.50 wt% Zr(OH)4 coated NCA electrodes. 0.50 wt%
Rsf (Ω)
Rct (Ω)
Rsf (Ω)
1
26.2
93.5
12.5
30
12.6
219.8
18.3
50
18.1
262.3
23.6
Rct (Ω) 125.8 128.9 156.5
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number
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Pristine
Cycle
Fig.8 shows the SEM images of (a) the pristine and (b) 0.50 wt% Zr(OH)4 coated NCA after 50 cycles at 1C. After cycling, the surface of NCA becomes rough and the edge of the particle breaks
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to small fragments. Moreover, some cracks are observed on the surface of the pristine as shown in Fig. 8(a). The structural damages of the pristine might be caused by the internal stress
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accompanied by degradation of structure and the dissolution of transition metal elements in the
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electrolyte [46]. In contrast, the particle of 0.50 wt% Zr(OH)4 coated NCA keeps integrity and the surface is relatively smooth, indicating the coating layer can efficiently decrease HF attack and
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reduce transition metal elements dissolution. The above results better demonstrate the excellent
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structural stability of 0.50 wt% Zr(OH)4 coated NCA during cycling.
Fig. 8. SEM images of (a) the pristine and (b) 0.50 wt% Zr(OH)4 coated NCA after 50 cycles at 1C.
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4. Conclusions LiNi0.915Co0.075Al0.01O2 with amorphous Zr(OH)4 coating has been synthesized through a simple dry method at room temperature in consideration of the sensitivity of NCA sample to water and temperature. After modified with Zr(OH)4 powders, the coated sample exhibits excellent
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electrochemical performance. The coating of 0.50 wt% Zr(OH)4 is chosen as the optimum coating quantity with a thickness of 15-20 nm. After 100 cycles at 1 C, 0.50 wt% Zr(OH)4 coated NCA sample delivers a capacity of 154.3 mAh/g with a capacity retention of 78.1%. In comparison,
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after 100 cycles at 1 C, NCA without coating shows the capacity of 142.5 mAh/g with a capacity retention of 73.2%. In addition, the coated sample shows a lower charge transfer resistance than the pristine during cycling. The improved electrochemical performance can be attributed to the
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reduced side reactions between the electrolyte and electrode. Furthermore, Zr(OH)4 coating layer can effectively prevent cathode active materials from decomposing and thus enhance the structural
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stability of the electrode. Therefore, surface coating with amorphous Zr(OH)4 is a simple and
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useful method to enhance the electrochemical performance of NCA-based materials for the cathode of LIBs.
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Acknowledgements
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This work was supported by National Projects of NSFC (21322101 and 21231005) and MOE
(B12015 and IRT13R30).
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Graphic abstract