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Enhanced electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode materials by ultrasonic-assisted co-precipitation method
Journal of Alloys and Compounds 644 (2015) 607–614
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Enhanced electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode materials by ultrasonic-assisted co-precipitation method Xiaobo Zheng a, Xinhai Li a, Zhenjun Huang a, Bao Zhang a,⇑, Zhixing Wang a, Huajun Guo a, Zhihua Yang b,⇑ a
School of Metallurgy and Environment, Central South University, Changsha 410083, PR China Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Technical Institute of Physics & Chemistry of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, PR China b
a r t i c l e
i n f o
Article history: Received 10 January 2015 Received in revised form 9 April 2015 Accepted 25 April 2015 Available online 29 April 2015 Keywords: Lithium ion battery LiNi0.6Co0.2Mn0.2O2 Rate performance First-principles calculations Ultrasonic-assisted co-precipitation method
a b s t r a c t Homogenous nanoscale LiNi0.6Co0.2Mn0.2O2 (NCM) cathode material powder was successfully synthesized by the ultrasonic-assisted co-precipitation (UC) method. Compared with traditional co-precipitation (TC) method, the obtained NCM by UC method has better layered structure and lower level of cation mixing, which is confirmed by the XRD and Rietveld refinement results. First-principles calculations also verify that NCM material with better structure stability can be obtained by UC method as the ultrasonic catalysis reduces the formation energy of NCM. The electrochemical test shows that NCM materials synthesized by UC method exhibit enhanced cycling capability (improved from 72.1% to 84.3% after 100 cycles at 1 C) and rate performance (deliver 119.5 mA h g1 at 5 C). Moreover, the CV and EIS results demonstrated that UC method is beneficial to improve the transfer kinetic behavior of Li ions of NCM material. Ó 2015 Published by Elsevier B.V.
1. Introduction LiCoO2, due to its inherent excellent cycle performance advantages, has been broadly used in the lithium-ion battery materials field. Nevertheless, its expensive cost, and the finite capacity limit its wider application, which promotes the development alternative materials [1–3]. Layered LiNiO2 [4], spinel LiMn2O4 [5,6] and olivine LiFePO4 [7,8] are the potential cathode materials. However, stoichiometric LiNiO2 is difficult to synthesize for Li/Ni cations displacement [9,10]. LiMn2O4 shows a tremendous capacity fading for Jahn–Teller distortion [11] and LiFePO4 restrict its wider range of applications because of poor intrinsic electronic conductivity [7]. The most meaningful and effectual method is to import Ni, Co, and Mn ions together in the layer structure. The LiNi12xCoxM nxO2 series has the advantage of high capacity, excellent cycling stability and good safety behavior, which have aroused the interest of many researchers and launched detailed study [12–19]. Among LiNi12xCoxMnxO2 series, LiNi0.6Co0.2Mn0.2O2 has been studied as a potential battery material. A wide variety of approaches have been applied to obtain LiNi0.6Co0.2Mn0.2O2 cathode materials, such as co-precipitation method [12,20–22], solid-state method [23] and spray-drying ⇑ Corresponding authors. Tel./fax: +86 731 88836633 (B. Zhang). Tel.: +86 991 3859931; fax: +86 991 3838957 (Z. Yang). E-mail addresses: [email protected] (B. Zhang), [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.jallcom.2015.04.173 0925-8388/Ó 2015 Published by Elsevier B.V.
method [24,25]. Among these methods, the products obtained by solid-state method usually contain large irregular particle with a larger particle size distribution and have an impurity phase. Spray drying method has some drawbacks of complex synthetic route and high cost, which results in difficulty for mass production. The component of the product obtained by co-precipitation method is relatively uniform and co-precipitation method is easy to scale up. Therefore it is widely used to synthesize various materials. However, co-precipitation method still exists some shortcoming, such as particle size inhomogeneity, low tap density. Hence, searching for an economical way that can synthesize uniform precursors and product is extremely significant. Ultrasonic has been applied to synthesize lithium ion cathode materials, such as LiNi0.5xMn1.5+xO4, LiAl0.05Mn1.95O4, LiCrxM n2xO4 [26–28]. It is well known that micrometer-sized bubbles form during the rarefaction phase and collapse during the compression phase in the water system under ultrasonic conditions. This process leads to a dramatic increase in temperature and pressure. It can form acoustic cavitation phenomenon, which can cause physical and chemical reactions. Acoustic cavitation of ultrasound can achieve a reduced particle size, eliminating the uneven local concentration and shearing the reunion. At the same time, ultrasonic vibration can enhance mass and heat transfer to promote a chemical reaction. In the process of co-precipitation, ultrasound can achieve uniform mixing of the reactant, homogenous distribution of the precipitate composition, good dispersibility of fine
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sediment particle, and facilitate to promote the synthesis of the target product at high-temperature. With this consideration, we introduce ultrasonic-assisted co-precipitation method to acquire nanoscale layered LiNi0.6Co0.2Mn0.2O2 cathode materials with uniform particle size. In this study, LiNi0.6Co0.2Mn0.2O2 was synthesized by UC and TC method (Named UC-NCM and TC-NCM respectively). The structural properties, morphology, electrochemical performance of LiNi0.6Co0.2Mn0.2O2 powders prepared by two methods were investigated. What is particularly worth mentioning is that first-principle method is used to calculate the formation energy of LiNi0.6Co0.2Mn0.2O2 because it determines the structural stability of the materials [29]. 2. Experimental 2.1. Materials preparation NiCl26H2O, MnCl24H2O, CoCl26H2O and NaOH were employed as the raw materials to obtain the precursor. A moderate aqueous solution of the transition metal chloride was slowly dripped into a continuous stirred tank reactor (2 L) under argon atmosphere. In the meantime, 2 M NaOH solution (aq) and moderate amount of NH4OH solution (aq) as chelating agent were also pumped into the reactor. The pH of the mixed solution was hold 11.5 and the agitation speed of the mixture was kept at 800 rpm. After shocking at 55 °C for 12 h in an ultrasonic cleaner (800 W, 80 kHz), the redundant water was wiped off and the precipitated precursor was obtained. The obtained precursor was washed for six times with distilled water to get rid of the residual sodium salt by centrifugal machine. Then the precipitate was dried at 120 °C for 12 h. The Ni0.6Co0.2Mn0.2(OH)2 and LiOHH2O was preheated at 500 °C for 5 h and then re-sintered at 800 °C for 15 h in oxygen atmosphere. 2.2. Materials characterization The synthesized compounds were characterized by using X-ray diffraction (XRD) (Riguku h/h diffractometer with Cu Ka radiation (r = 1.54056 Å). XRD patterns were obtained at 2h = 10–80°, with a scan speed of 10° min1. X-Ray Rietveld refinement were performed using GSAS software [30], the data was collected 2h = 10–80°, with scan speed of 2° min1. Scanning electron microscope (SEM, Hitachi X-650) and transmission electron microscope (TEM, JEM-2100F) was used to characterize the morphological and particle sizes of materials. 2.3. Electrochemical measurements and calculations The electrodes consisted of the LiNi0.6Co0.2Mn0.2O2 (80% wt.%), super p (10% wt.%) and poly (10 wt.% polyvinylidene fluoride, PVDF). Then being mixed in N-methyl pyrrolidinone and the mixed slurry was coated uniformly on a thin aluminum foil. Subsequently, the electrodes were dried under vacuum at 120 °C. Assembling of the battery was completed in an argon-filled glove box. The counter electrode was a lithium foil and a celgard 2400(polypropylene) was as the separator. The electrolyte was consisted of LiPF6 (1 M) dissolved in a mixture of ethylene carbonate and dimethyl carbonate. First-principles calculations were performed in the generalized gradient approximation (GGA) to density-functional theory (DFT) as implemented in the CESTEP package (Accelrys, Inc.) The nuclei and core electrons are represented by Ultrasoft pseudo-potentials and the PerdeweWang (PW91) density functional was used for the exchange correlation [31,32]. The cutoff energy of plane wave was set at 380 eV and a 6 6 2 k-point grid was used. All calculations were performed in reciprocal space. The virtual crystal approximation (VCA) method proposed by Bellaiche [33] is utilized here to calculate the formation energy. Herein, solid solution elements were not as a whole virtual atom but the weight of the different atoms a with their own weights
3. Results and discussion
Fig. 1. XRD pattern for TC-NCM and UC-NCM (a); Rietveld refinement XRD patterns of TC-NCM (b), and UC-NCM (c).
3.1. XRD structure characterization Fig. 1(a) exhibit the XRD patterns for the UC-NCM and TC-NCM. The XRD pattern shows both materials are the a-NaFeO2 structure symmetry. As seen in Fig. 1(a), no additional diffraction and R3m peaks indicate it not exists related secondary phases and impurities. The major peaks for these synthesized materials are labeled with h k l indexes. The prepared powders have good crystallinity
followed all fundamental sharp peaks. In addition, proper cation ordering is demonstrated by the clear splitting of the (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) peaks, as shown in Fig. 1(a). The splitting of the (0 0 6)/(1 0 2) of UC-NCM are clearer than the TC-NCM, which demonstrates that UC-NCM has well-defined layered structure.
X. Zheng et al. / Journal of Alloys and Compounds 644 (2015) 607–614 Table 1 at Final results of Rietveld refinements for TC-NCM and UC-NCM in space group R3m room temperature. Atom
Site
x
y
z
Li1 Ni1 Li2 Ni2 Co Mn O
3a 3a 3b 3b 3b 3b 6c a c c/a I(0 0 3)/I(1 0 4) Rp Rwp R-factor
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0.5 0.5 0.5 0.5 0.241028
Site occupancy (%) TC-NCM
UC-NCM
0.9482 0.0518 0.0598 0.5413 0.2007 0.1982 1 2.87279 14.22880 4.95296 1.506 8.9 11.71 0.736
0.9697 0.0303 0.0303 0.5697 0.2 0.2 1 2.86556 14.21078 4.97653 1.727 8.88 11.69 0.383
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In order to investigate the effect of the ultrasonic treatment in more detail, a Rietveld analysis was used to analyze the crystal structure. The typical crystal structure was assumed that in the space group R3m, which Li was on the 3a site (0, 0, 0), oxygen was at the 6c site (0, 0, z), and transition metal was at the 3b site (0, 0, 1/2). Fig. 1(b) and (c) shows the Rietveld refinement results for the UC-NCM and TC-NCM and the refined crystallographic data are listed in Table 1. As presented in Fig. 1(b) and (c), a satisfactory agreement was obtained to compare the observed and calculated patterns. The lattice parameters a and c of TC-NCM are given as 2.87279 Å and 14.22880 Å, while the lattice parameters a and c of UC-NCM are 2.86556 Å and 14.21078 Å, respectively. These results identify with values reported previously (i.e. a = 2.8714 Å and c = 14.2191 Å [14]). The structural ordering of the materials are usually characterized by the value of I(0 0 3)/I(1 0 4) and the ratios of c/a [34]. The intensity ratio of I(0 0 3)/I(1 0 4) is an extremely important parameter, which is used to determine the cation mixing in the lattice. That a value of R < 1.2 is a sign of high degree
Fig. 2. SEM images of Ni0.6Co0.2Mn0.2(OH)2 TC-precursor (a), UC-precursor (b); LiNi0.6Co0.2Mn0.2O2 sample TC-NCM (c), and TC-NCM (d); TEM images of TC-NCM (e) and UCNCM (f).
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of cation mixing, while the ratio is greater than 1.2, the higher the ratio is, the lower level of cation mixing and the excellent hexagonal layered structure [35,36]. The intensity ratio of the two peaks, I(0 0 3)/I(1 0 4), are given as 1.727 (UC-NCM) and 1.506 (TC-NCM), which both the ratio are more than 1.2, indicating both have well-ordering layered a-NaFeO2 structure and low level cation mixing. The ratio of UC-NCM is greater than the TC-NCM, which indicate ultrasonic could suppress occupancy of Li+ layers by Ni2+ and be conducive to better electrochemical performance during electrochemical cycling than the TC-NCM. In addition, the c/a ratio of both materials, which also reflect the layered characteristics of the material [37], is greater than 4.9. As seen in the Table 1, the c/a ratio of TC-NCM is 4.95296, while the UC-NCM is 4.97653, which indicates UC-NCM have better layered characteristics. The values of the R factor (R = (I(0 0 6) + I(1 0 2))/(I(1 0 1))) for the TC-NCM is 0.736 and the R factor of UC-NCM is 0.385. The UC-NCM exhibits the lowest the R factor values. Therefore, it can be concluded that UC-NCM show better layered characteristics for ultrasonic-assisted. Table 1 also list final results of Rietveld refinements for TC-NCM at room temperature, respecand UC-NCM in space group R3m tively. As presented in Table 1, the Ni occupancy of TC-NCM at the 3a site is 0.0518, whereas, the UC-NCM is 0.0303. This verifies that UC-NCM has a lower degree of cation mixing. It is obvious that ultrasonic contribute to decrease the Li/Ni mixing, which is consistent with the above result. It is well established that low R values indicate satisfactory refinement. From Table 1, it concludes that both refined results are reliable.
solution was studied and the interactions of the transition metal, oxygen, and lithium ions were investigated. An effective method that measuring Ni–Co–Mn interactions could be acquired by comparing the energy of LiNi0.6Co0.2Mn0.2O2 to the average energy of LiNiO2, LiCoO2, and LiMnO2, according to general alloy theory, was used to analyze the model. In other words, D E mix = E(LiNi 0.6 Co 0.2 Mn 0.2 O 2 ) 0.6E(LiNiO 2 ) 0.2E(LiCoO 2 ) 0.2E(LiMnO2). If DEmix is negative, it suggests that Ni, Co, and Mn have an equal opportunity to interact and the system will be casually mixed or ordered, which depend on the strength of the interaction. However, a positive DEmix demonstrates that it hardly forms the solid solution due to likely local phase segregation [38]. By calculating the formation energy in the above model, it was found that DEmix for LiNi0.6Co0.2Mn0.2O2 synthesized by UC and TC method is 13.519 eV and 13.471 eV, respectively, indicating an attractive tendency between Ni, Co, and Mn. It shows that the synthesis of both materials is thermodynamically favorable and the material obtained by UC is more stable than synthesized by TC method for the lower formation energy. 3.4. Electrochemical performance Fig. 3(a) illustrates initial charge–discharge curves of LiNi0.6Co0.2Mn0.2O2 synthesis by UC and TC at 0.1 C (16 mA g1) rate in a voltage of 2.8–4.3 V current at 25 °C. As shown in Fig. 3(a), the UC-NCM exhibit the higher discharge capacity of 187.1 mA h g1 and initial coulombic efficiency of 88.0%.
3.2. SEM and TEM morphological analysis Specific SEM and TEM photographs of the LiNi0.6Co0.2Mn0.2O2 and its precursor prepared by two methods are illustrated in Fig. 2. It is noteworthy that there exist apparent differences in the morphology and particle size between the two materials. The images in Fig. 2(a) indicate that the precursor of TC-NCM particles are secondary particles consist of primary nanoparticle. The nanoscale primary particles agglomerate with each other to form micron particles. The precursor of TC-NCM has a wider range of particle size distribution for a greater extent of agglomeration of particles and the surface of the particles is relatively rough. In contrast, the UC-NCM particles, shown in Fig. 2(b), provide relatively more homogeneous particle size distribution. The agglomeration phenomenon is much less in the precursor of UC-NCM particles. It can be observed that the surface of the UC-NCM (Fig. 2(d)) powder is smoother and the particles are slightly smaller than the TC-NCM (Fig. 2(c)) sample. The results suggest that the ultrasonic process can be conducive to material dispersion and effective suppression material reunion [26]. This shows that the UC method is an extremely effective method to synthesize the narrow and uniform particle size distributions materials. From the TEM images in Fig. 2(e) and (f), it confirms that the primary particles of both materials are nanoscale. The TC-NCM materials are secondary particles that consist of nanoparticle while the UC-NCM materials are well dispersed primary nanoparticles. Therefore, the UC-NCM provide more interface areas to contact the liquid electrolyte. It can conducive lithium ions into the layered structure, which is supposed to enhance the rate performance of the materials. 3.3. First-principles calculations The calculations are initiated by establishing a layered structure with parameters that obtain from the experimental results. (a = 2.87279 Å, c = 14.22880 Å (TC-NCM) and a = 2.86556 Å, c = 14.21078 Å (UC-NCM)). The formation energy of the solid
Fig. 3. (a) The initial charge–discharge curves at 0.1 C and (b) cycling performance of the TC-NCM and UC-NCM materials.
X. Zheng et al. / Journal of Alloys and Compounds 644 (2015) 607–614
Fig. 4. Rate performance of the TC-NCM and UC-NCM materials at 0.1 C, 0.2 C,0.5 C, 1 C,2 C, and 5 C and back to 1 C for every ten cycles.
However, the TC-NCM show the discharge capacity of 171.9 mA h g1 and initial coulombic efficiency of 82.9%. Obviously, both the initial discharge capacity and coulombic efficiency of the NCM are enhanced by UC method. Nevertheless, it should be considered that ultrasonic-assisted not only can improve initial discharge capacity of NCM, but also enhance the structural stability of the material for suppressing the reaction between active material and electrolyte. According to pre-report, UC method could enhance the discharge capacity of LiNi0.5xM n1.5+xO4 [26] and LiCrxMn2xO4 [28] and Li1.3Ni0.61M n0.64O2+d [39], moreover, including structural stability and rate performance. In addition, the increase reversible capacity demonstrated that the layered structure is improved by ultrasonic process, which facilitates to migration of lithium ions and electron transfer, this is well consistent with the results of the Rietveld refinements analysis that the Li/Ni mixing for UC-NCM is less than the TC-NCM, Because the increase in discharge capacity is consistent with the increase of ratio I(0 0 3)/I(1 0 4) [2]. In order to further study the influence of ultrasonic-assisted method on the cycle performance of LiNi0.6Co0.2Mn0.2O2, the result
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of the cycling performance of both electrodes at 1 C is shown in Fig. 3(b). The cells were initially cycled 2 times at 0.1 C, 0.2 C, 0.5 C between 2.8 V and 4.3 V, respectively. The TC-NCM sample show more capacity fades, from 153.1 mA h g1 to 110.4 mA h g1 after 100 cycles with retention of only 72.1%. For the UC-NCM sample, the capacity retention is 84.3% from 170.9 mA h g1 to 144 mA h g1. Compared with TC-NCM sample, the UC-NCM sample has a higher initial discharge capacity and capacity retention at 1 C rate after 100 cycles, which may relate to its better layered structure and regular morphology. The rate capabilities of the TC-NCM sample and UC-NCM sample electrodes at various rates between 2.8 and 4.3 V (vs. Li/Li+) are presented in Fig. 4. Both the electrodes were cycled between 2.8 and 4.3 V at 0.1 C, 0.2 C, 1 C, 2 C, and 5 C and back to 1 C for each ten cycles, respectively. When the current density increases, the discharge capacity of the TC-NCM and the UC-NCM electrodes all decrease due to polarization. It can be seen that the UC-NCM electrodes present a better rate capability than the TC-NCM at various rates, especially at high rates. For instance, the UC-NCM electrodes show a discharge capacity of 119.5 mA h g1 at 5 C, whereas the TC-NCM electrodes deliver a discharge capacity of only 98.8 mA h g1. It is well known that Li+ diffusion in the layered oxide is an important factor that determines the rate performance which a battery can be charged and discharged. Synthesis of nanoparticles is considered an effective way to improve rate capability. Ultrasonic-assisted method contributes to get nanoscale level particle, which can provide better lithium ion diffusion pathways and facilitate Li+ intercalation kinetics. Moreover, small particles have a relatively large surface area and favor the active material contact with the electrolyte [40]. According to the pre-report [41], there is a tight relevance between cation disorder and rate capability in the layered LiNi1yzCoyMnzO2 systems. Combining the rate performance and the results of XRD, it can be speculated that the reduction of the cation disorder and the particle size are important factors in the enhancement of the rate capability. Electrochemical impedance spectroscopy (EIS) tests are executed for the TC-NCM and UC-NCM electrodes after 1th, 10th, 50th, 100th cycles at 1 C between 2.8 and 4.3 V. The cells were initially cycled 2 times at 0.1 C, 0.2 C, 0.5 C between 2.8 V and 4.3 V, respectively. Fig. 5 indicates the Nyquist plots of the UC-NCM
Fig. 5. Electrochemical impedance spectra of TC-NCM (a) and UC-NCM (b); equivalent circuit used for fitting the impedance spectra (c).
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Table 2 The electrochemical impedance fitting result of the TC-NCM and UC-NCM electrode. Sample
Cycle times
Re (X)
Rsf (X)
Rct (X)
TC-NCM
1th 10th 50th 100th
4.266 5.237 5.273 5.937
58.86 76.76 109.3 143.7
36.68 51.22 189.1 559.8
UC-NCM
1th 10th 50th 100th
2.914 3.233 3.251 7.973
10.33 14.24 18.18 23.16
7.439 17.38 19.01 42.09
and TC-NCM in the open circuit state with the frequency range from 0.1 Hz to 100 kHz. The EIS measurements were performed at the fully charged state at 1 C at room temperature. As illustrated in Fig. 5(a) and (b), the semicircle at high frequency range represents resistance of surface film; another semicircle at the middle frequency region indicates the charge transfer resistance (Rct); and the sloping line at low frequency range is the Warburg impedance, which was connected with the lithium ions diffusion through the solid particles [42,43]. A straightforward equivalent circuit model (Fig. 5(c)) was used to investigate the impedance spectra. It is believed that the impedance of lithium-ion batteries mainly rest with the charge transfer resistance (Rct). The Zview software was used to simulate the EIS data. As described in Table 2 and Fig. 6, with the increasing of the cycle number, the values of Re, Rct and Rsf are all increase. The Rct value of the TC-NCM electrode at the 1th cycle is 36.68 X and it reached to 51.22 X, 189.1 X, 559.8 X at the 10th, 50th, 100th cycle, separately, while the Rct value of the UC-NCM electrode is 7.439 X, 17.38 X, 19.01 X, 42.09 X at the 1th, 10th, 50th, 100th cycle respectively. Obviously, ultrasonic-assisting reduce the Rct value of the LiNi0.6Co0.2Mn0.2O2, which indicate that the impedance growth is
Fig. 7. The relationship plot between Zre and x1/2 at low frequency region.
drastically suppressed by ultrasonic-assisting co-precipitation method. EIS results suggest that the improvement of cycling performance and rate capability is due to prevent the possible reaction of the electrode material and the electrolyte which substantially reduce the impedance growth by ultrasonic-assisting co-precipitation method. The lithium ion diffusion coefficient in LiNi0.6Co0.2Mn0.2O2 electrode is calculated on account of the following Eq. (1):
D¼
R2 T 2 2A2 n4 F 4 C 2
r2
Fig. 6. The electrochemical impedance fitting result of the TC-NCM and UC-NCM electrode.
ð1Þ
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be concluded that the UC method significantly enhances the electrochemical performances of LiNi0.6Co0.2Mn0.2O2. 4. Conclusions Nanoscale LiNi0.6Co0.2Mn0.2O2 cathode material powder was successfully synthesized by UC and TC method. From XRD and the refinement results indicate that the LiNi0.6Co0.2Mn0.2O2 obtained by UC has better layered structure and the lower level of cation mixing. The SEM and TEM results show that the materials synthesized by UC method produced homogenous nanoscale particles. First-principles calculations also confirmed that it has a better structural stability of UC-NCM material. The electrochemical test indicated that UC-NCM material exhibits a higher capacity retention and initial discharge capacity. The rate performance showed that the UC-NCM show better rate capability. The CV curves of UC-NCM show UC method has both better reversibility and cycle capability. Therefore, the UC method is a better alternative method than the TC method to obtain lithium-ion battery cathode materials. Acknowledgments The authors gratefully acknowledge financial support from the National Basic Research Program of China (973 Program 2014CB643406). References
Fig. 8. Cyclic voltammograms of TC-NCM (a) and UC-NCM (b) samples for the first three cycles at a scan rate of 0.1 mV s1.
In here, R represents the gas constant, T represents the absolute temperature, A represents the surface area of the cathode, n represents the number of electrons per molecule during oxidization, F represents the Faraday constant, C represents the concentration of lithium-ion (herein, C = 4.936 102 mol cm3). The graph of Zre against x1/2 corresponding to Eq. (2) were illustrated in Fig. 7 and r is the slope of the diagonal.
1
Z re ¼ Rsf þ Rct þ rx2
ð2Þ
The calculated lithium ion diffusion coefficient of the TC-NCM is 3.09 1013 cm2 s1, while the UC-NCM is 2.28 1012 cm2 s1. Obviously, the DLi+ value of UC-NCM is far bigger than the TC-NCM and it indicates that the DLi+ value increase markly by ultrasonic assisting. The CV curves of the TC-NCM electrode and the UC-NCM electrode for the first three cycles with a scan rate of 0.1 mV s1 are shown in Fig. 8. The TC-NCM and UC-NCM material both shows one redox couple in CV, which means that there is no other reactions occur during electrochemical cycling within the limits of 2.8–4.3 V. The clear-cut reaction peaks of the UC-NCM at 3.802/3.713 V can be attributed to the Ni2+/Ni4+ redox couple transformation. It is obvious that the redox potential gap of the UC-NCMO is 0.089 V, which is less than that of the TC-NCMO (0.259 V). It is indicated the electrode reversibility is improved by ultrasonic process according to the decrease in the redox potential gap [14]. Given all of the above analysis and discussion, it can
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode materials by ultrasonic-assisted co-precipitation method Xiaobo Zheng a, Xinhai Li a, Zhenjun Huang a, Bao Zhang a,⇑, Zhixing Wang a, Huajun Guo a, Zhihua Yang b,⇑ a
School of Metallurgy and Environment, Central South University, Changsha 410083, PR China Key Laboratory of Functional Materials and Devices for Special Environments of CAS, Xinjiang Technical Institute of Physics & Chemistry of CAS, Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, PR China b
a r t i c l e
i n f o
Article history: Received 10 January 2015 Received in revised form 9 April 2015 Accepted 25 April 2015 Available online 29 April 2015 Keywords: Lithium ion battery LiNi0.6Co0.2Mn0.2O2 Rate performance First-principles calculations Ultrasonic-assisted co-precipitation method
a b s t r a c t Homogenous nanoscale LiNi0.6Co0.2Mn0.2O2 (NCM) cathode material powder was successfully synthesized by the ultrasonic-assisted co-precipitation (UC) method. Compared with traditional co-precipitation (TC) method, the obtained NCM by UC method has better layered structure and lower level of cation mixing, which is confirmed by the XRD and Rietveld refinement results. First-principles calculations also verify that NCM material with better structure stability can be obtained by UC method as the ultrasonic catalysis reduces the formation energy of NCM. The electrochemical test shows that NCM materials synthesized by UC method exhibit enhanced cycling capability (improved from 72.1% to 84.3% after 100 cycles at 1 C) and rate performance (deliver 119.5 mA h g1 at 5 C). Moreover, the CV and EIS results demonstrated that UC method is beneficial to improve the transfer kinetic behavior of Li ions of NCM material. Ó 2015 Published by Elsevier B.V.
1. Introduction LiCoO2, due to its inherent excellent cycle performance advantages, has been broadly used in the lithium-ion battery materials field. Nevertheless, its expensive cost, and the finite capacity limit its wider application, which promotes the development alternative materials [1–3]. Layered LiNiO2 [4], spinel LiMn2O4 [5,6] and olivine LiFePO4 [7,8] are the potential cathode materials. However, stoichiometric LiNiO2 is difficult to synthesize for Li/Ni cations displacement [9,10]. LiMn2O4 shows a tremendous capacity fading for Jahn–Teller distortion [11] and LiFePO4 restrict its wider range of applications because of poor intrinsic electronic conductivity [7]. The most meaningful and effectual method is to import Ni, Co, and Mn ions together in the layer structure. The LiNi12xCoxM nxO2 series has the advantage of high capacity, excellent cycling stability and good safety behavior, which have aroused the interest of many researchers and launched detailed study [12–19]. Among LiNi12xCoxMnxO2 series, LiNi0.6Co0.2Mn0.2O2 has been studied as a potential battery material. A wide variety of approaches have been applied to obtain LiNi0.6Co0.2Mn0.2O2 cathode materials, such as co-precipitation method [12,20–22], solid-state method [23] and spray-drying ⇑ Corresponding authors. Tel./fax: +86 731 88836633 (B. Zhang). Tel.: +86 991 3859931; fax: +86 991 3838957 (Z. Yang). E-mail addresses: [email protected] (B. Zhang), [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.jallcom.2015.04.173 0925-8388/Ó 2015 Published by Elsevier B.V.
method [24,25]. Among these methods, the products obtained by solid-state method usually contain large irregular particle with a larger particle size distribution and have an impurity phase. Spray drying method has some drawbacks of complex synthetic route and high cost, which results in difficulty for mass production. The component of the product obtained by co-precipitation method is relatively uniform and co-precipitation method is easy to scale up. Therefore it is widely used to synthesize various materials. However, co-precipitation method still exists some shortcoming, such as particle size inhomogeneity, low tap density. Hence, searching for an economical way that can synthesize uniform precursors and product is extremely significant. Ultrasonic has been applied to synthesize lithium ion cathode materials, such as LiNi0.5xMn1.5+xO4, LiAl0.05Mn1.95O4, LiCrxM n2xO4 [26–28]. It is well known that micrometer-sized bubbles form during the rarefaction phase and collapse during the compression phase in the water system under ultrasonic conditions. This process leads to a dramatic increase in temperature and pressure. It can form acoustic cavitation phenomenon, which can cause physical and chemical reactions. Acoustic cavitation of ultrasound can achieve a reduced particle size, eliminating the uneven local concentration and shearing the reunion. At the same time, ultrasonic vibration can enhance mass and heat transfer to promote a chemical reaction. In the process of co-precipitation, ultrasound can achieve uniform mixing of the reactant, homogenous distribution of the precipitate composition, good dispersibility of fine
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sediment particle, and facilitate to promote the synthesis of the target product at high-temperature. With this consideration, we introduce ultrasonic-assisted co-precipitation method to acquire nanoscale layered LiNi0.6Co0.2Mn0.2O2 cathode materials with uniform particle size. In this study, LiNi0.6Co0.2Mn0.2O2 was synthesized by UC and TC method (Named UC-NCM and TC-NCM respectively). The structural properties, morphology, electrochemical performance of LiNi0.6Co0.2Mn0.2O2 powders prepared by two methods were investigated. What is particularly worth mentioning is that first-principle method is used to calculate the formation energy of LiNi0.6Co0.2Mn0.2O2 because it determines the structural stability of the materials [29]. 2. Experimental 2.1. Materials preparation NiCl26H2O, MnCl24H2O, CoCl26H2O and NaOH were employed as the raw materials to obtain the precursor. A moderate aqueous solution of the transition metal chloride was slowly dripped into a continuous stirred tank reactor (2 L) under argon atmosphere. In the meantime, 2 M NaOH solution (aq) and moderate amount of NH4OH solution (aq) as chelating agent were also pumped into the reactor. The pH of the mixed solution was hold 11.5 and the agitation speed of the mixture was kept at 800 rpm. After shocking at 55 °C for 12 h in an ultrasonic cleaner (800 W, 80 kHz), the redundant water was wiped off and the precipitated precursor was obtained. The obtained precursor was washed for six times with distilled water to get rid of the residual sodium salt by centrifugal machine. Then the precipitate was dried at 120 °C for 12 h. The Ni0.6Co0.2Mn0.2(OH)2 and LiOHH2O was preheated at 500 °C for 5 h and then re-sintered at 800 °C for 15 h in oxygen atmosphere. 2.2. Materials characterization The synthesized compounds were characterized by using X-ray diffraction (XRD) (Riguku h/h diffractometer with Cu Ka radiation (r = 1.54056 Å). XRD patterns were obtained at 2h = 10–80°, with a scan speed of 10° min1. X-Ray Rietveld refinement were performed using GSAS software [30], the data was collected 2h = 10–80°, with scan speed of 2° min1. Scanning electron microscope (SEM, Hitachi X-650) and transmission electron microscope (TEM, JEM-2100F) was used to characterize the morphological and particle sizes of materials. 2.3. Electrochemical measurements and calculations The electrodes consisted of the LiNi0.6Co0.2Mn0.2O2 (80% wt.%), super p (10% wt.%) and poly (10 wt.% polyvinylidene fluoride, PVDF). Then being mixed in N-methyl pyrrolidinone and the mixed slurry was coated uniformly on a thin aluminum foil. Subsequently, the electrodes were dried under vacuum at 120 °C. Assembling of the battery was completed in an argon-filled glove box. The counter electrode was a lithium foil and a celgard 2400(polypropylene) was as the separator. The electrolyte was consisted of LiPF6 (1 M) dissolved in a mixture of ethylene carbonate and dimethyl carbonate. First-principles calculations were performed in the generalized gradient approximation (GGA) to density-functional theory (DFT) as implemented in the CESTEP package (Accelrys, Inc.) The nuclei and core electrons are represented by Ultrasoft pseudo-potentials and the PerdeweWang (PW91) density functional was used for the exchange correlation [31,32]. The cutoff energy of plane wave was set at 380 eV and a 6 6 2 k-point grid was used. All calculations were performed in reciprocal space. The virtual crystal approximation (VCA) method proposed by Bellaiche [33] is utilized here to calculate the formation energy. Herein, solid solution elements were not as a whole virtual atom but the weight of the different atoms a with their own weights
3. Results and discussion
Fig. 1. XRD pattern for TC-NCM and UC-NCM (a); Rietveld refinement XRD patterns of TC-NCM (b), and UC-NCM (c).
3.1. XRD structure characterization Fig. 1(a) exhibit the XRD patterns for the UC-NCM and TC-NCM. The XRD pattern shows both materials are the a-NaFeO2 structure symmetry. As seen in Fig. 1(a), no additional diffraction and R3m peaks indicate it not exists related secondary phases and impurities. The major peaks for these synthesized materials are labeled with h k l indexes. The prepared powders have good crystallinity
followed all fundamental sharp peaks. In addition, proper cation ordering is demonstrated by the clear splitting of the (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) peaks, as shown in Fig. 1(a). The splitting of the (0 0 6)/(1 0 2) of UC-NCM are clearer than the TC-NCM, which demonstrates that UC-NCM has well-defined layered structure.
X. Zheng et al. / Journal of Alloys and Compounds 644 (2015) 607–614 Table 1 at Final results of Rietveld refinements for TC-NCM and UC-NCM in space group R3m room temperature. Atom
Site
x
y
z
Li1 Ni1 Li2 Ni2 Co Mn O
3a 3a 3b 3b 3b 3b 6c a c c/a I(0 0 3)/I(1 0 4) Rp Rwp R-factor
0 0 0 0 0 0 0
0 0 0 0 0 0 0
0 0 0.5 0.5 0.5 0.5 0.241028
Site occupancy (%) TC-NCM
UC-NCM
0.9482 0.0518 0.0598 0.5413 0.2007 0.1982 1 2.87279 14.22880 4.95296 1.506 8.9 11.71 0.736
0.9697 0.0303 0.0303 0.5697 0.2 0.2 1 2.86556 14.21078 4.97653 1.727 8.88 11.69 0.383
609
In order to investigate the effect of the ultrasonic treatment in more detail, a Rietveld analysis was used to analyze the crystal structure. The typical crystal structure was assumed that in the space group R3m, which Li was on the 3a site (0, 0, 0), oxygen was at the 6c site (0, 0, z), and transition metal was at the 3b site (0, 0, 1/2). Fig. 1(b) and (c) shows the Rietveld refinement results for the UC-NCM and TC-NCM and the refined crystallographic data are listed in Table 1. As presented in Fig. 1(b) and (c), a satisfactory agreement was obtained to compare the observed and calculated patterns. The lattice parameters a and c of TC-NCM are given as 2.87279 Å and 14.22880 Å, while the lattice parameters a and c of UC-NCM are 2.86556 Å and 14.21078 Å, respectively. These results identify with values reported previously (i.e. a = 2.8714 Å and c = 14.2191 Å [14]). The structural ordering of the materials are usually characterized by the value of I(0 0 3)/I(1 0 4) and the ratios of c/a [34]. The intensity ratio of I(0 0 3)/I(1 0 4) is an extremely important parameter, which is used to determine the cation mixing in the lattice. That a value of R < 1.2 is a sign of high degree
Fig. 2. SEM images of Ni0.6Co0.2Mn0.2(OH)2 TC-precursor (a), UC-precursor (b); LiNi0.6Co0.2Mn0.2O2 sample TC-NCM (c), and TC-NCM (d); TEM images of TC-NCM (e) and UCNCM (f).
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of cation mixing, while the ratio is greater than 1.2, the higher the ratio is, the lower level of cation mixing and the excellent hexagonal layered structure [35,36]. The intensity ratio of the two peaks, I(0 0 3)/I(1 0 4), are given as 1.727 (UC-NCM) and 1.506 (TC-NCM), which both the ratio are more than 1.2, indicating both have well-ordering layered a-NaFeO2 structure and low level cation mixing. The ratio of UC-NCM is greater than the TC-NCM, which indicate ultrasonic could suppress occupancy of Li+ layers by Ni2+ and be conducive to better electrochemical performance during electrochemical cycling than the TC-NCM. In addition, the c/a ratio of both materials, which also reflect the layered characteristics of the material [37], is greater than 4.9. As seen in the Table 1, the c/a ratio of TC-NCM is 4.95296, while the UC-NCM is 4.97653, which indicates UC-NCM have better layered characteristics. The values of the R factor (R = (I(0 0 6) + I(1 0 2))/(I(1 0 1))) for the TC-NCM is 0.736 and the R factor of UC-NCM is 0.385. The UC-NCM exhibits the lowest the R factor values. Therefore, it can be concluded that UC-NCM show better layered characteristics for ultrasonic-assisted. Table 1 also list final results of Rietveld refinements for TC-NCM at room temperature, respecand UC-NCM in space group R3m tively. As presented in Table 1, the Ni occupancy of TC-NCM at the 3a site is 0.0518, whereas, the UC-NCM is 0.0303. This verifies that UC-NCM has a lower degree of cation mixing. It is obvious that ultrasonic contribute to decrease the Li/Ni mixing, which is consistent with the above result. It is well established that low R values indicate satisfactory refinement. From Table 1, it concludes that both refined results are reliable.
solution was studied and the interactions of the transition metal, oxygen, and lithium ions were investigated. An effective method that measuring Ni–Co–Mn interactions could be acquired by comparing the energy of LiNi0.6Co0.2Mn0.2O2 to the average energy of LiNiO2, LiCoO2, and LiMnO2, according to general alloy theory, was used to analyze the model. In other words, D E mix = E(LiNi 0.6 Co 0.2 Mn 0.2 O 2 ) 0.6E(LiNiO 2 ) 0.2E(LiCoO 2 ) 0.2E(LiMnO2). If DEmix is negative, it suggests that Ni, Co, and Mn have an equal opportunity to interact and the system will be casually mixed or ordered, which depend on the strength of the interaction. However, a positive DEmix demonstrates that it hardly forms the solid solution due to likely local phase segregation [38]. By calculating the formation energy in the above model, it was found that DEmix for LiNi0.6Co0.2Mn0.2O2 synthesized by UC and TC method is 13.519 eV and 13.471 eV, respectively, indicating an attractive tendency between Ni, Co, and Mn. It shows that the synthesis of both materials is thermodynamically favorable and the material obtained by UC is more stable than synthesized by TC method for the lower formation energy. 3.4. Electrochemical performance Fig. 3(a) illustrates initial charge–discharge curves of LiNi0.6Co0.2Mn0.2O2 synthesis by UC and TC at 0.1 C (16 mA g1) rate in a voltage of 2.8–4.3 V current at 25 °C. As shown in Fig. 3(a), the UC-NCM exhibit the higher discharge capacity of 187.1 mA h g1 and initial coulombic efficiency of 88.0%.
3.2. SEM and TEM morphological analysis Specific SEM and TEM photographs of the LiNi0.6Co0.2Mn0.2O2 and its precursor prepared by two methods are illustrated in Fig. 2. It is noteworthy that there exist apparent differences in the morphology and particle size between the two materials. The images in Fig. 2(a) indicate that the precursor of TC-NCM particles are secondary particles consist of primary nanoparticle. The nanoscale primary particles agglomerate with each other to form micron particles. The precursor of TC-NCM has a wider range of particle size distribution for a greater extent of agglomeration of particles and the surface of the particles is relatively rough. In contrast, the UC-NCM particles, shown in Fig. 2(b), provide relatively more homogeneous particle size distribution. The agglomeration phenomenon is much less in the precursor of UC-NCM particles. It can be observed that the surface of the UC-NCM (Fig. 2(d)) powder is smoother and the particles are slightly smaller than the TC-NCM (Fig. 2(c)) sample. The results suggest that the ultrasonic process can be conducive to material dispersion and effective suppression material reunion [26]. This shows that the UC method is an extremely effective method to synthesize the narrow and uniform particle size distributions materials. From the TEM images in Fig. 2(e) and (f), it confirms that the primary particles of both materials are nanoscale. The TC-NCM materials are secondary particles that consist of nanoparticle while the UC-NCM materials are well dispersed primary nanoparticles. Therefore, the UC-NCM provide more interface areas to contact the liquid electrolyte. It can conducive lithium ions into the layered structure, which is supposed to enhance the rate performance of the materials. 3.3. First-principles calculations The calculations are initiated by establishing a layered structure with parameters that obtain from the experimental results. (a = 2.87279 Å, c = 14.22880 Å (TC-NCM) and a = 2.86556 Å, c = 14.21078 Å (UC-NCM)). The formation energy of the solid
Fig. 3. (a) The initial charge–discharge curves at 0.1 C and (b) cycling performance of the TC-NCM and UC-NCM materials.
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Fig. 4. Rate performance of the TC-NCM and UC-NCM materials at 0.1 C, 0.2 C,0.5 C, 1 C,2 C, and 5 C and back to 1 C for every ten cycles.
However, the TC-NCM show the discharge capacity of 171.9 mA h g1 and initial coulombic efficiency of 82.9%. Obviously, both the initial discharge capacity and coulombic efficiency of the NCM are enhanced by UC method. Nevertheless, it should be considered that ultrasonic-assisted not only can improve initial discharge capacity of NCM, but also enhance the structural stability of the material for suppressing the reaction between active material and electrolyte. According to pre-report, UC method could enhance the discharge capacity of LiNi0.5xM n1.5+xO4 [26] and LiCrxMn2xO4 [28] and Li1.3Ni0.61M n0.64O2+d [39], moreover, including structural stability and rate performance. In addition, the increase reversible capacity demonstrated that the layered structure is improved by ultrasonic process, which facilitates to migration of lithium ions and electron transfer, this is well consistent with the results of the Rietveld refinements analysis that the Li/Ni mixing for UC-NCM is less than the TC-NCM, Because the increase in discharge capacity is consistent with the increase of ratio I(0 0 3)/I(1 0 4) [2]. In order to further study the influence of ultrasonic-assisted method on the cycle performance of LiNi0.6Co0.2Mn0.2O2, the result
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of the cycling performance of both electrodes at 1 C is shown in Fig. 3(b). The cells were initially cycled 2 times at 0.1 C, 0.2 C, 0.5 C between 2.8 V and 4.3 V, respectively. The TC-NCM sample show more capacity fades, from 153.1 mA h g1 to 110.4 mA h g1 after 100 cycles with retention of only 72.1%. For the UC-NCM sample, the capacity retention is 84.3% from 170.9 mA h g1 to 144 mA h g1. Compared with TC-NCM sample, the UC-NCM sample has a higher initial discharge capacity and capacity retention at 1 C rate after 100 cycles, which may relate to its better layered structure and regular morphology. The rate capabilities of the TC-NCM sample and UC-NCM sample electrodes at various rates between 2.8 and 4.3 V (vs. Li/Li+) are presented in Fig. 4. Both the electrodes were cycled between 2.8 and 4.3 V at 0.1 C, 0.2 C, 1 C, 2 C, and 5 C and back to 1 C for each ten cycles, respectively. When the current density increases, the discharge capacity of the TC-NCM and the UC-NCM electrodes all decrease due to polarization. It can be seen that the UC-NCM electrodes present a better rate capability than the TC-NCM at various rates, especially at high rates. For instance, the UC-NCM electrodes show a discharge capacity of 119.5 mA h g1 at 5 C, whereas the TC-NCM electrodes deliver a discharge capacity of only 98.8 mA h g1. It is well known that Li+ diffusion in the layered oxide is an important factor that determines the rate performance which a battery can be charged and discharged. Synthesis of nanoparticles is considered an effective way to improve rate capability. Ultrasonic-assisted method contributes to get nanoscale level particle, which can provide better lithium ion diffusion pathways and facilitate Li+ intercalation kinetics. Moreover, small particles have a relatively large surface area and favor the active material contact with the electrolyte [40]. According to the pre-report [41], there is a tight relevance between cation disorder and rate capability in the layered LiNi1yzCoyMnzO2 systems. Combining the rate performance and the results of XRD, it can be speculated that the reduction of the cation disorder and the particle size are important factors in the enhancement of the rate capability. Electrochemical impedance spectroscopy (EIS) tests are executed for the TC-NCM and UC-NCM electrodes after 1th, 10th, 50th, 100th cycles at 1 C between 2.8 and 4.3 V. The cells were initially cycled 2 times at 0.1 C, 0.2 C, 0.5 C between 2.8 V and 4.3 V, respectively. Fig. 5 indicates the Nyquist plots of the UC-NCM
Fig. 5. Electrochemical impedance spectra of TC-NCM (a) and UC-NCM (b); equivalent circuit used for fitting the impedance spectra (c).
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Table 2 The electrochemical impedance fitting result of the TC-NCM and UC-NCM electrode. Sample
Cycle times
Re (X)
Rsf (X)
Rct (X)
TC-NCM
1th 10th 50th 100th
4.266 5.237 5.273 5.937
58.86 76.76 109.3 143.7
36.68 51.22 189.1 559.8
UC-NCM
1th 10th 50th 100th
2.914 3.233 3.251 7.973
10.33 14.24 18.18 23.16
7.439 17.38 19.01 42.09
and TC-NCM in the open circuit state with the frequency range from 0.1 Hz to 100 kHz. The EIS measurements were performed at the fully charged state at 1 C at room temperature. As illustrated in Fig. 5(a) and (b), the semicircle at high frequency range represents resistance of surface film; another semicircle at the middle frequency region indicates the charge transfer resistance (Rct); and the sloping line at low frequency range is the Warburg impedance, which was connected with the lithium ions diffusion through the solid particles [42,43]. A straightforward equivalent circuit model (Fig. 5(c)) was used to investigate the impedance spectra. It is believed that the impedance of lithium-ion batteries mainly rest with the charge transfer resistance (Rct). The Zview software was used to simulate the EIS data. As described in Table 2 and Fig. 6, with the increasing of the cycle number, the values of Re, Rct and Rsf are all increase. The Rct value of the TC-NCM electrode at the 1th cycle is 36.68 X and it reached to 51.22 X, 189.1 X, 559.8 X at the 10th, 50th, 100th cycle, separately, while the Rct value of the UC-NCM electrode is 7.439 X, 17.38 X, 19.01 X, 42.09 X at the 1th, 10th, 50th, 100th cycle respectively. Obviously, ultrasonic-assisting reduce the Rct value of the LiNi0.6Co0.2Mn0.2O2, which indicate that the impedance growth is
Fig. 7. The relationship plot between Zre and x1/2 at low frequency region.
drastically suppressed by ultrasonic-assisting co-precipitation method. EIS results suggest that the improvement of cycling performance and rate capability is due to prevent the possible reaction of the electrode material and the electrolyte which substantially reduce the impedance growth by ultrasonic-assisting co-precipitation method. The lithium ion diffusion coefficient in LiNi0.6Co0.2Mn0.2O2 electrode is calculated on account of the following Eq. (1):
D¼
R2 T 2 2A2 n4 F 4 C 2
r2
Fig. 6. The electrochemical impedance fitting result of the TC-NCM and UC-NCM electrode.
ð1Þ
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be concluded that the UC method significantly enhances the electrochemical performances of LiNi0.6Co0.2Mn0.2O2. 4. Conclusions Nanoscale LiNi0.6Co0.2Mn0.2O2 cathode material powder was successfully synthesized by UC and TC method. From XRD and the refinement results indicate that the LiNi0.6Co0.2Mn0.2O2 obtained by UC has better layered structure and the lower level of cation mixing. The SEM and TEM results show that the materials synthesized by UC method produced homogenous nanoscale particles. First-principles calculations also confirmed that it has a better structural stability of UC-NCM material. The electrochemical test indicated that UC-NCM material exhibits a higher capacity retention and initial discharge capacity. The rate performance showed that the UC-NCM show better rate capability. The CV curves of UC-NCM show UC method has both better reversibility and cycle capability. Therefore, the UC method is a better alternative method than the TC method to obtain lithium-ion battery cathode materials. Acknowledgments The authors gratefully acknowledge financial support from the National Basic Research Program of China (973 Program 2014CB643406). References
Fig. 8. Cyclic voltammograms of TC-NCM (a) and UC-NCM (b) samples for the first three cycles at a scan rate of 0.1 mV s1.
In here, R represents the gas constant, T represents the absolute temperature, A represents the surface area of the cathode, n represents the number of electrons per molecule during oxidization, F represents the Faraday constant, C represents the concentration of lithium-ion (herein, C = 4.936 102 mol cm3). The graph of Zre against x1/2 corresponding to Eq. (2) were illustrated in Fig. 7 and r is the slope of the diagonal.
1
Z re ¼ Rsf þ Rct þ rx2
ð2Þ
The calculated lithium ion diffusion coefficient of the TC-NCM is 3.09 1013 cm2 s1, while the UC-NCM is 2.28 1012 cm2 s1. Obviously, the DLi+ value of UC-NCM is far bigger than the TC-NCM and it indicates that the DLi+ value increase markly by ultrasonic assisting. The CV curves of the TC-NCM electrode and the UC-NCM electrode for the first three cycles with a scan rate of 0.1 mV s1 are shown in Fig. 8. The TC-NCM and UC-NCM material both shows one redox couple in CV, which means that there is no other reactions occur during electrochemical cycling within the limits of 2.8–4.3 V. The clear-cut reaction peaks of the UC-NCM at 3.802/3.713 V can be attributed to the Ni2+/Ni4+ redox couple transformation. It is obvious that the redox potential gap of the UC-NCMO is 0.089 V, which is less than that of the TC-NCMO (0.259 V). It is indicated the electrode reversibility is improved by ultrasonic process according to the decrease in the redox potential gap [14]. Given all of the above analysis and discussion, it can
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