Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
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Enhanced Electrochemical Performance of Fast Ionic Conductor LiTi2(PO4)3‑Coated LiNi1/3Co1/3Mn1/3O2 Cathode Material Lu-Lu Zhang,† Ji-Qing Wang,† Xue-Lin Yang,*,† Gan Liang,‡ Tao Li,† Peng-Lin Yu,† and Di Ma† †
College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, China Three Gorges University, 8 Daxue Road, Yichang, Hubei 443002, China ‡ Department of Physics, Sam Houston State University, Huntsville, Texas 77341, United States S Supporting Information *
ABSTRACT: Layered LiNi1/3Co1/3Mn1/3O2 (NCM333) is successfully coated by fast ionic conductor LiTi2(PO4)3 (LTP) via a wet chemical method. The effects of LTP on the physicochemical properties and electrochemical performance are studied. The results reveal that a highly layered structure of NCM333 can be well maintained with less cation mixing after LTP coating. LTP of about 5 nm thickness is coated on the surface of NCM333. Such an LTP coating layer can effectively suppress the side reactions between NCM333 and electrolyte but will not hinder the lithium ion transmission. As a result, LTP-coated NCM333 owns an improved capability and cyclic performance, for example, NCM333/LTP delivers an initial capacity as high as 121.0 mA h g−1 with a capacity retention ratio of 82.3% after 200 cycles at 10 C, whereas NCM333 only has an initial capacity of 120.4 mA h g−1 with a very low capacity retention ratio of 66.4%. This method of using a fast ionic conductor like LTP as a coating material may provide a simple and effective strategy to modify those electrode materials with poor cyclic performance. KEYWORDS: lithium ion battery, cathode material, LiNi1/3Co1/3Mn1/3O2, LiTi2(PO4)3, coating
1. INTRODUCTION Because of the serious environmental pollution caused by the use of fossil fuels in automobiles, there is an urgent and expanding demand for people to develop clean and more efficient new power sources such as lithium ion batteries (LIBs).1 Particularly, owing to their high energy density, large power capability, long cycle life, and no memory effect, LIBs are regarded as an ideal power source for a wide range of applications.2 So far, some cathode materials for LIBs, such as layered LiCoO2,3 olivine LiFePO4,4 and spinel LiMn2O4,5 have been widely used as cathode materials for commercial LIBs in electrical vehicles (EVs) or hybrid electric vehicles (HEVs). Among them, the further development of the earliest commercialized LiCoO2 is being restricted because the relatively high cost and the toxicity of the cobalt limit its future applications. 6−8 Thus, in the last two decades, researchers have been working on finding alternative cathode materials to satisfy the required environmental, safety, and cost standards for LIBs.9−11 In comparison with LiCoO2, layered LiNixCoyMnzO2 (x + y + z = 1) has been considered more suitable for EVs or HEVs because of its low cost, high capacity, and high safety. In 1999, Liu et al.12 first proposed different atomic composition ratios of Ni, Co, and Mn including ratios of x/y/z = 7:2:1, 5:2:3, and 6:2:2. In 2001, Ohzuku et al.13 successfully synthesized LiNi1/3Co1/3Mn1/3O2 (abbreviated as NCM333) as a cathode material. Compared with NCM721, NCM523, and NCM622, NCM333 appears easier to prepare © 2018 American Chemical Society
and shows better cycle performance and structural stability. Thus, NCM333 draws much attention in the field of cathode materials for LIBs. Later, Li et al.14 also obtained porous nano− micro hierarchical NCM333 microspheres with a high discharge capacity of 133.2 mA h g−1 at 1 C via a facile nano-etchingtemplate route. However, the poor electronic conductivity, transition-metal dissolution, and some side reactions between NCM333 and electrolyte lead to a poor rate capability and cyclic stability of NCM333,15−18 which seriously plagues its large-scale applications. Since then, researchers worldwide have proposed various ways to overcome these drawbacks. For example, coating could be an easy and effective way to increase the electronic conductivity of NCM333 and inhibit the side reactions between NCM333 and electrolyte.17 Various coating materials including carbon18 and metal oxides (Al2O3,19 Li2TiO3,20 ZrO2,21 TiO2,22 AlF3,23 AlPO4,24 etc.) have been studied. Very recently, Yang et al.18 used a one-step method to prepare carbon-coated NCM (NCM333/C) with active carbon as the reaction matrix, and this NCM333/C cathode material delivered a discharge capacity of 191.2 mA h g−1 at 0.5 C (85 mA g−1). However, the oxidation of carbon in air during a hightemperature sintering process leads to the uncontrollable formation of the coating layer on the surface of NCM333. By Received: December 28, 2017 Accepted: March 16, 2018 Published: March 16, 2018 11663
DOI: 10.1021/acsami.7b19692 ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic illustration of the synthesis route for NCM333/LTP, (b−d) XRD patterns and the corresponding Rietveld refinement results of NCM333 and NCM333/LTP, and (e) schematic microstructure of NCM333. 0.5 h and then added into the above NCM ethanol/acetone mixed solution drop-by-drop and stirred for 9 h. The resulted precipitate was collected by filtration and dried. Finally, the dried precipitate was heated at 500 °C for 5 h in air and then cooled to room temperature to obtain the LTP-coated NCM333 composite (denoted as NCM/LTP). The schematic illustration of this synthesis route is displayed in Figure 1a. The materials characterization and electrochemical measurements sections are detailed in the Supporting Information.
comparison, a metal oxide coating layer can be easily achieved, and the cycling performance could also be enhanced by oxide coating because of the improved structural stability. However, the rate capability may be hindered simultaneously because of the low lithium ion diffusion rate of metal oxide.25 Interestingly, NASICON lithium titanium phosphate LiTi2(PO4)326,27 (abbreviated as LTP) has a three-dimensional open framework and fast ion diffusion rate; thus, it could be used as an ideal coating material for NCM333. In this work, we introduced the fast ionic conductor LTP to modify NCM333 through a facile synthetic route for the first time. The phase, morphology, microstructure, valence state of key elements, and magnetic property of the as-obtained LTPcoated NCM333 cathode material were carefully investigated, and the roles that the coated LTP layer plays in improving the electrochemical performance of NCM333/LTP will be also discussed.
3. RESULTS AND DISCUSSION X-ray powder diffraction (XRD) patterns and the corresponding Rietveld refinement results of the two as-prepared samples are displayed in Figure 1b−d. Figure 1b clearly shows that the two XRD patterns are similar and all the diffraction lines can be indexed with the hexagonal α-NaFeO2 crystal structure29 in Figure 1e. No impurity was observed in the two XRD patterns of NCM333 and NCM333/LTP, indicating that incorporating LTP does not affect the NCM333 crystal structure. Moreover, both samples have well-ordered layered structures, as can be indicated by the splitting of peaks (006)/(102) and (018)/ (110).30,31 The intensity ratios of the (003) to (104) peaks of both NCM333 (∼1.63) and NCM333/LTP (∼1.68) are greater than 1.2, corresponding to a low degree of cation mixing.32,33 Furthermore, Rietveld refinements were carried out by using the GSAS software.34,35 As shown in Figure 1c,d and Table 1, the low error factors (Rp and Rwp) for both samples are indicative of the reliability of the refinement results. The c/a ratios of NCM333 (4.9756) and NCM333/LTP (4.9773) are both larger than 4.9, which further confirm the highly layered structure and lower cation mixing for both samples.28,36 Noting that, according to the measured Ti content (0.48 wt %) by ICP,
2. EXPERIMENTAL SECTION Pristine NCM333 was synthesized by blending the Ni1/3Co1/3Mn1/3(OH)2 precursor prepared by a coprecipitation method and lithium carbonate (Li2CO3) with a molar ratio of 1:1.06.28 The mixture was calcined at 450 °C for 5 h, and then at 900 °C for 12 h in air to obtain the pristine NCM333 sample. The LTPcoated NCM333 sample was prepared by a wet chemical method as follows. First, the as-prepared NCM333 was dispersed in a mixed solution of ethanol and acetone and stirred constantly for 1 h to obtain a suspension. Then, a certain amount of tetrabutyl titanate (C16H36O4Ti) was added to keep the LTP content of 2 wt % and stirred for 0.5 h. Subsequently, stoichiometric lithium hydroxide (LiOH·H2O) and ammonium dihydrogen phosphate (NH4H2PO4) were dissolved in 20 mL of deionized water under magnetic stirring for 11664
DOI: 10.1021/acsami.7b19692 ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
Research Article
ACS Applied Materials & Interfaces Table 1. Lattice Parameters of NCM333 and NCM333/LTP Samples sample
a (Å)
c (Å)
c/a
Rp (%)
Rwp (%)
NCM333 NCM333/LTP
2.866 2.863
14.26 14.25
4.9756 4.9773
3.12 3.21
3.97 4.24
the actual coating amount of LTP can be calculated to be 1.94 wt %, which is very close to the theoretical value of LTP (2.0 wt %). Just owing to the low concentration of LTP in NCM333/ LTP, diffraction lines for LTP are not observed in the XRD pattern of NCM333/LTP. The morphology and size distribution of NCM333 and NCM333/LTP powders were investigated by a scanning electron microscope (SEM) and a Nano Measurer particle size analysis software, respectively. From Figure 2, it is clearly
Figure 3. TEM images of samples: (a) NCM333, (b−d) NCM333/ LTP; and (e,f) EDS mapping of NCM333/LTP.
ion transmission. In addition, the elemental distribution of the NCM333/LTP sample is analyzed by EDS mapping (Figure 3e,f). Obviously, Ti shows a similar distribution as Ni, Co, and Mn, which indicates that besides Ni, Co, and Mn of NCM333, Ti is also uniformly dispersed in NCM333/LTP powders. The XPS measurements were performed to study the composition and valence state of key elements in NCM333 and NCM333/LTP. The full XPS spectra in Figure 4a clearly shows the presence of Ni, Co, and Mn in these two samples, but the Ti and P peaks can only be observed in the spectrum of NCM333/LTP. In Figure 4b−d, the peaks of Ni 3p1/2, Co 2p3/2, and Mn 2p1/2 are observed for NCM333 at 67.30, 780.10, and 654.10 eV, respectively, indicating that the corresponding transition-metal ions are Ni2+, Co3+, and Mn4+, respectively. This result is well matched with previous reports.30,31 In addition, to further verify the existing position of LTP, Ar-ion sputtering was done to strip different depths of NCM333/LTP in the XPS measurements (Figure 4e−h). Because all XPS peaks in the interior deviate slightly to lower binding energy in comparison with those on the surface caused by the chemical reduction of Ar-ion, we use the peaks before sputtering (the black lines in Figure 4e−h) to confirm the valence states of key elements in NCM333/LTP. The banding energies of Ni 3p1/2, Co 2p3/2, and Mn 2p1/2 peaks for NCM333/LTP at 67.50, 780.10, and 654.30 eV confirm that the valence of Ni, Co, and Mn is still +2, +3, and +4, respectively, which demonstrates that the LTP coating does not change the valence of Ni, Co, and Mn in NCM333/LTP. As seen in Figure 4h, the binding energy of the Ti 2p3/2 peak at 458.10 eV before sputtering confirms that the Ti ions in NCM333/LTP are Ti4+.37,38 Moreover, from Figure 4e−h, it can be clearly observed that the peak intensity of Ni 3p1/2, Co 2p3/2, and Mn 2p1/2 on the surface is weaker than that in the interior, but the Ti 2p3/2 peak on the surface
Figure 2. SEM images of samples: (a,b) NCM333 and (c,d) NCM333/LTP and (e) particle size distribution of samples.
seen that both samples exhibit an approximate square shape and have the same size distribution, and the average particle size is about 150 nm. To further study the microstructure of the two samples, transmission electron microscope (TEM) pictures are also presented in Figure 3a,b. Clearly, the average particle size for both samples is about 150 nm, which agrees well with the SEM results. As displayed in the high-resolution transmission electron microscopy (HRTEM) image of NCM333/ LTP (Figure 3c), a coating layer with an average thickness of ∼5 nm is obviously observed on the NCM333 surface. The magnified image in Figure 3d clearly shows a lattice spacing of 0.186 nm, representing the (015) planes of NCM333; meanwhile, on the edge of the NCM333 particle, other clear lattice spacings of 0.348 and 0.269 nm are also observed, which are related to the (202) and (116) planes of LTP, respectively. This observation reveals that the coating material is LTP, and such an LTP layer may effectively suppress side reactions between NCM333 and electrolyte; also, as a fast ionic conductor, the thin LTP coating layer will not hinder lithium 11665
DOI: 10.1021/acsami.7b19692 ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
Research Article
ACS Applied Materials & Interfaces
conductor, the thin LTP coating layer can increase the lithium ion diffusion rate and thus improve the capacity performance for NCM333/LTP. Figure 5 shows the temperature-dependent magnetic susceptibility χ(T) and reciprocal magnetic susceptibility χ−1(T) for NCM333 and NCM333/LTP. χ(T) was measured in both field cooling and zero-field-cooling modes at a magnetic field of H = 1 kOe and in a temperature range of 5 K ≤ T ≤ 300 K. The χ−1(T) curve displayed in Figure 5a for NCM333 exhibits a paramagnetic linear behavior above 150 K and can be fitted to the Curie−Weiss law χ(T) = C/(T − θp) in the whole temperature range of 150 K ≤ T ≤ 300 K with Curie constant C = 1.063 emu·K·Oe−1·mol−1 and the paramagnetic Curie− Weiss temperature θp = −110 K. The value of the effective moment μeff is estimated to be 2.92 μB per formula unit (f. u.) for NCM333 from the formula μeff = (8C)1/2, which is almost the same as the value 2.90 μB reported previously by Hashem et al.39 and quite close to the theoretical value of 2.75 μB calculated by using the theoretical values of effective moment for Ni2+, Mn4+, and Co3+.40 This magnetic result is consistent with our XPS results that the valence values of Ni, Mn, and Co in the NCM333 sample are +2, +4, and +3, respectively. The χ−1 data shown in Figure 5b for NCM333/LTP can also be fit to the Curie−Weiss law in the same temperature range 150 K ≤ T ≤ 300 K, and the fitting result gives C = 1.133 emu·K· Oe−1·mol−1 and θp = −108 K. The effective moment μeff estimated by μeff = (8C)1/2 is 3.01 μB, which is very close to the μeff value (2.92 μB) for NCM333. This result is also in good agreement with our XPS results that Ti in the very low-level coating LTP layer is in the Ti4+ configuration (for which μeff is zero), and thus, the Ti ions basically do not contribute to the magnetic moment of NCM333/LTP. Compared with the value θp = −110 K for NCM333, the slightly less negative value of θp (= −108 K) for NCM333/LTP indicates that the thin LTP coating layer might have a very small surface disturbance to the antiferromagnetic type interaction (or Weiss molecular field) between the transition-metal ions in the NCM333 particles. To investigate the electrochemical reversibility and kinetic behavior, cyclic voltammetry (CV) tests of the NCM333 and NCM333/LTP fresh electrodes were conducted in the voltage range of 2.8−4.5 V with a scan rate of 0.05 mV s−1 for the first three cycles. As shown in Figure 6a,b, the two CV curves exhibit a distinct pair of redox peaks around 3.70/3.80 V during Li+ ion insertion/deinsertion process, corresponding to the reduction and oxidation reactions between the Ni2+/Ni4+ redox couple.17,18 The absence of a characteristic reduction peak of Mn4+ around 3.2 V indicates that the Mn4+ in NCM333 and NCM333/LTP is electrochemically inactive and only improves
Figure 4. (a) Full XPS spectra of NCM333 and NCM333/LTP; the high-resolution XPS spectra of samples: (b−d) NCM333 and (e−h) NCM333/LTP.
has stronger intensity than those at different depths from the NCM333/LTP surface; and with the increase of sputtering times, the XPS intensity of Ti peaks decreases gradually. The two observations support the results from HRTEM images that the LTP layer has been successfully coated on the surface of NCM333 particles. Such an LTP coating layer plays a vital role in inhibiting the side reactions between NCM333 and electrolyte.24,25 This could in turn improve the cyclic performance of NCM333/LTP. Meanwhile, as a fast ionic
Figure 5. Temperature dependence of magnetic susceptibility χ(T) and reciprocal magnetic susceptibility χ−1(T) for samples: (a) NCM333 and (b) NCM333/LTP. 11666
DOI: 10.1021/acsami.7b19692 ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
Research Article
ACS Applied Materials & Interfaces
Figure 6. (a,b) CV curves of NCM333 and NCM333/LTP, respectively; and (c,d) EIS spectra and the corresponding relationships between Z′ and ω−1/2 at a low-frequency region of NCM333 and NCM333/LTP electrodes.
the structural stability of NCM333.41 As we know, the redox peak of Co3+/Co4+ appears at a voltage higher than 4.6 V (the upper limit of voltage in the present study is 4.5 V). Therefore, in the CV curves of NCM333 and NCM333/LTP, no Mn2+/ Mn 4+ and Co 3+ /Co 4+ redox peaks can be observed. Furthermore, the potential difference value (0.101 V) of the redox peaks of NCM333/LTP is smaller than that of NCM333 (0.130 V), which confirms the lower polarization effect during the electrochemical reaction process for the NCM333/LTP electrode. Therefore, we can conclude that LTP coating may have an admirable impact on the electrochemical performance of NCM333/LTP. Electrochemical impedance spectra (EIS) were measured over a frequency range from 0.01 Hz to 100 kHz to get further insights into the kinetic behavior of both NCM333 and NCM333/LTP. As seen in Figure 6c,d, the two Nyquist plots include a semicircle at the high frequency region ascribed to the charge-transfer resistance (Rct) between the electrode and electrolyte interface and a straight line at the lowfrequency region corresponding to lithium ion diffusion through the solid electrode.42 Both EIS curves were fitted by using the Zview software (the corresponding equivalent circuit is shown in the inset in Figure 6d). As listed in Table 2, the Rct
relationship between Z′ (Z′ is the real part of impedance) and ω−1/2 (ω is the angular frequency) at a low frequency according to previous reports.43,44 The Z′ versus ω−1/2 plots are presented in Figure 6d. The calculated DLi+ values for NCM333 and NCM333/LTP are also listed Table 2. It shows that NCM333/ LTP has a higher DLi+ value (1.56 × 10−13 cm2 s−1) than NCM333 (0.93 × 10−13 cm2 s−1). Obviously, the LTP coating layer is highly effective for improving the electrochemical reaction environment and thus enhancing the electrochemical performance of NCM333. To study the effect of LTP on the electrochemical performance of NCM333, galvanostatic charge/discharge measurements were carried out between 2.8 and 4.5 V at room temperature, and the results are displayed in Figure 7. Figure 7a,b present the initial charge/discharge curves and cycle performance, respectively, for the two samples at 0.5 C. As can be seen in Figure 7a, both samples display similar charge/ discharge profiles and only have characteristic charge/discharge platforms for NCM333 between 3.60 and 3.90 V, indicating that LTP is electrochemically inactive during this measured voltage range (2.8−4.5 V).30,45 It can be also found that NCM333/LTP owns a higher initial discharge capacity of 191.5 mA h g−1 with a columbic efficiency of 91.1% than NCM333 (187.9 mA h g−1, 89.3%) at 0.5 C. The corresponding cycle performance of both samples is presented in Figure 7b and Table 3. It is observed that NCM333/LTP exhibits significantly higher capacity and better cycle performance than NCM333. For example, NCM333/LTP delivers a high capacity of 161.8 mA h g−1 after 100 cycles at 0.5 C, corresponding to a desirable capacity retention ratio of 84.5%. By sharp contrast, NCM333 shows a gradual capacity fading along with cycling, and the discharge capacity decreases to 135.6 mA h g−1 after 100 cycles with a low capacity retention ratio of 72.1%. In addition, NCM333 and NCM333/LTP electrodes were also cycled from 0.1 to 4 C and then back to 0.1 C with 10 cycles per C-rate (shown in Figure 7c). Clearly, as compared with NCM333, NCM333/LTP possesses much higher capacity at any C-rate. For instance, NCM333/LTP delivers a higher average specific capacity (204.9 mA h g−1) than NCM333 (192.5 mA h g−1) at
Table 2. EIS Fitting Results and Electronic Conductivity of Samples samples
Rct (Ω)
DLi+ (cm2 s−1)
electronic conductivity (S cm−1)
NCM333 NCM333/LTP
222.40 79.12
0.93 × 10−13 1.56 × 10−13
0.20 × 10−6 4.99 × 10−6
value of NCM333/LTP (79.12 Ω) is smaller than that of NCM333 (222.40 Ω), indicative of a faster charge transfer for NCM333/LTP. The measured electronic conductivity values for NCM333 and NCM333/LTP are 0.20 × 10−6 and 4.99 × 10−6 S cm−1, respectively. The low charge-transfer resistance and the high electronic conductivity are positive in enhancing the rate performance of electrode materials. Furthermore, the lithium diffusion coefficient DLi+ can be evaluated from the 11667
DOI: 10.1021/acsami.7b19692 ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
Research Article
ACS Applied Materials & Interfaces
Figure 7. Electrochemical performance of samples: (a,b) initial charge/discharge profiles and cycle performance at 0.5 C, (c) the rate performance profiles of NCM333 and NCM333/LTP at different rates, and (d) the cycle performance of NCM333 and NCM333/LTP at 10 C.
Figure S3. Obviously, when the quantity is insufficient (NCM333/LTP-0.5), the improvement effect is not obvious. When overdosed (NCM333/LTP-3), on the one hand, the excess electrochemical inactive LTP will reduce the utilization of active material NCM333 and on the other hand, the LTP coating layer will become thicker and hinder ion transmission. When incorporating the appropriate amount of LTP (NCM333/LTP-2), the LTP coating layer with a suitable thickness can inhibit the side reactions between NCM333 and electrolyte and also is favorable for ion transmission and thus improves the performance of NCM333. Therefore, the roles that the LTP plays in improving the electrochemical performance of NCM333/LTP should be ascribed to its property of fast ionic conductor and the protecting effect of the LTP coating layer.
Table 3. Discharge Capacity and Capacity Retention Ratio at 0.5 C of Samples discharge capacity (mA h g−1) sample
1st
100th
capacity retention ratio (%)
NCM333 NCM333/LTP
187.9 191.5
135.6 161.8
72.1 84.5
0.1 C. As the rate increases from 0.5 to 4 C, the specific capacity decreases from 179.5 to 133.4 mA h g−1 for NCM333/ LTP, but each value is much larger than that for NCM333 (from 163.1 to 115.9 mA h g−1). To further evaluate the influence of the LTP coating on the capability and cyclic performance of NCM333 at a high current rate, charge/ discharge measurements at 10 C were performed. As shown in Figure 7d, NCM333/LTP still holds a high initial capacity of 121.0 mA h g−1 with the capacity retention of 82.3% after 200 cycles at 10 C, whereas NCM333 only has a slightly lower initial capacity of 120.4 mA h g−1 but a very low capacity retention ratio of 66.4%. Obviously, the incorporation of LTP can effectively improve the rate capability and cycle performance of NCM333. To help understand the function of LTP in NCM333/LTP, we specially synthesized LTP under the same synthesis condition as NCM333/LTP (described in the Supporting Information section) and test its XRD pattern and CV curve. As clearly seen in the XRD pattern of this asobtained LTP composite (Figure S1), except for three impurity peaks, the main diffraction peaks can be indexed with the standard LiTi2(PO4)3 (JCPDS, no. 35-0754), which demonstrates that LTP can be successfully obtained under the same synthesis condition as NCM333/LTP. In addition, as shown in the CV curves of LTP (Figure S2), there is no redox peak detected, which reveals that LTP is electrochemical inactive, in other words, LTP will not contribute to the capacity of NCM333/LTP. But as a coating layer with the property of fast ionic conductor, LTP may have influence on the performance of NCM333. The electrochemical performance of pristine and modified NCM333 with different LTP contents is shown in
4. CONCLUSIONS The LiTi2(PO4)3-coated NCM333 (NCM333/LTP) composite has been successfully prepared via a wet chemical method. The effects of the LTP coating on the physicochemical properties and electrochemical performance are investigated. The XRD results reveal that both NCM333 and NCM333/ LTP have highly layered property and lower cation mixing, and LTP incorporation does not affect the NCM333 crystal structure. By a combined XPS and TEM analysis, it is found that an LTP layer about 5 nm in thickness is coated on the surface of NCM333. Both the XPS and magnetic susceptibility results confirm that the valence states of the transition-metal elements Ni, Co, Mn, and Ti in NCM333/LTP are +2, +3, +4, and +4, respectively. The electrochemical measurements clearly show that compared with NCM333, the NCM333/LTP sample presents higher capability and better cycling stability. Such an NCM333/LTP electrode exhibits a high initial discharge capacity of 191.5 mA h g−1 with a desirable capacity retention ratio of 84.5% after 100 cycles at 0.5 C, whereas NCM333 only has an initial capacity of 187.9 mA h g−1 with a much lower capacity retention ratio of 72.1%. Even at a high current rate of 10 C, NCM333/LTP still delivers an initial capacity as high as 121.0 mA h g−1 and a desirable capacity retention of 82.3% 11668
DOI: 10.1021/acsami.7b19692 ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
Research Article
ACS Applied Materials & Interfaces
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after 200 cycles at 10 C, whereas NCM333 has a lower initial capacity of 120.4 mA h g−1 with a very low capacity retention ratio of 66.4%. The superior capability and cycling stability of NCM333/LTP may be attributed to the reduced chargetransfer resistance, increased electronic conductivity, increased lithium ion diffusion coefficient, and the protecting effect of the LTP coating.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b19692. Synthesis of LTP, materials characterization, electrochemical measurements, XRD pattern of LTP (Figure S1), CV curve of LTP (Figure S2), charge/discharge curves, and cycle performance of pristine and modified NCM333 with different LTP contents at 0.5 C (Figure S3) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Lu-Lu Zhang: 0000-0003-2277-3920 Xue-Lin Yang: 0000-0001-5626-701X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51572151, 51772169 and 51672158), by the Outstanding Youth Science and Technology Innovation Team Project of Hubei Educational Committee (T201603), and in part by a FRC research grant of Sam Houston State University.
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DOI: 10.1021/acsami.7b19692 ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
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DOI: 10.1021/acsami.7b19692 ACS Appl. Mater. Interfaces 2018, 10, 11663−11670
