High Rate Capability and Excellent Thermal Stability of Li+-

Aug 21, 2015 - The cations migration, doping effect, and the reduced cation mixing further contribute to the electrochemical performance enhancement...
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High Rate Capability and Excellent Thermal Stability of Li+‑Conductive Li2ZrO3‑Coated LiNi1/3Co1/3Mn1/3O2 via a Synchronous Lithiation Strategy Jicheng Zhang, Zhengyao Li, Rui Gao, Zhongbo Hu, and Xiangfeng Liu* College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: A novel synchronous lithiation route has been successfully used to coat Li+-conductive Li2ZrO3 on the surface of LiNi1/3Co1/3Mn1/3O2 (Li2ZrO3@LNCMO). In this strategy, Li2ZrO3 layer and LiNi1/3Co1/3Mn1/3O2 host simultaneously form from ZrO2@Ni1/3Co1/3Mn1/3C2O4·xH2O precursor. In compared to bare LNCMO, the reversible capacity, cycling performance, thermal stability, rate capability, and polarization of Li2ZrO3@LNCMO have all been greatly improved. At 0.1 and 10 C, the specific capacity of Li2ZrO3@LNCMO is 192 and 106 mAh g−1, respectively, while they are 178 and 46 mAh g−1 for bare LNCMO. At a current density of 5 C, the capacity retention of Li2ZrO3@LNCMO at 25 and 55 °C after 400 cycles is enhanced to 93.8% and 85.1%, respectively, compared to 69.2% and 37.4% of bare LNCMO. The largely enhanced electrochemical performances of Li2ZrO3@LNCMO cathode can be attributed to the high Li-ion conductivity as well as the proctection of Li2ZrO3 coating. Li+ conductivity of Li2ZrO3@LNCMO is about 20 times higher than that of bare LNCMO. Moreover, the migration of partial Zr4+ to the host LNCMO phase not only benefits Liion or electron conductivity but also alleviates the Li−Ni cation mixing and improves the structure stability. The cations migration, doping effect, and the reduced cation mixing further contribute to the electrochemical performance enhancement. Li2SiO324 have been coated on the surface of some cathodes to improve the electrochemical performance. Among Li-ion conductors, Li2ZrO3 has been reported as a thermodynamic stable Li-ion conductor.25 Monoclinic Li2ZrO3 (space group C2/c) has two structural lithium sites: Li1 and Li2. Li1 sites are in the mixed (Li, Zr) layer while Li2 is located in the middle of 6-fold octahedral oxygen coordinations.26 The unique structure makes Li2ZrO3 possess a three-dimensional path for Li-ion diffusion. Owing to the advantages Li2ZrO3 has attracted great interest as a coating layer.18−20 For examples, Zhou et al. took a postcoating route to coat LiNi0.4Co0.2Mn0.4O2 with Li2ZrO3 using Zr(NO3)4·5H2O and CH3COOLi·2H2O as coating reagents.19 In Zhou’s method, LiNi0.4Co0.2Mn0.4O2 was first prepared, and then Li2ZrO3 was coated on the surface of the cathode. Zhou’s results indicated that a suitable Li2ZrO3 coating layer not only suppressed the dissolution of transition metal and side reaction between electrode and electrolyte but also improved the ion transportation characteristics, which enhanced the discharge capacity, rate performance, and capacity retention.19 Wang et al. coated spherical spinel LiMn2O4 with Li2ZrO3 layer via a sol−gel method, and the electrochemical performances were improved.20

1. INTRODUCTION Lithium-ion batteries are recognized as the most promising energy storage technology for electric vehicles (EVs) and hybrid electric vehicles (HEVs) in the next decades. To meet the requirements of EVs and HEVs, Li-ion batteries need to possess the following characteristics: low cost, large energy intensity, high rate capability, long cycle life, and nontoxic or low toxic.1−6 Among the cathode materials, layered LiNixCoyMnzO2 (NCM, x + y + z = 1) compounds including LiNi0.5Co0.2Mn0.3O2,7 LiNi1/3Co1/3Mn1/3O2 (LNCMO),8 and so on have been considered as one promising cathode material due to the higher capacity and lower cost. However, LNCMO cathode exhibits poor cycling stability and low rate performance;9 especially at an evaluated temperature, severe side reaction between the electrode and electrolyte will happen and cause the large fading of the capacity and battery life. In order to reduce side reaction and improve the capacity and life, surface modification has been extensively explored.10−17 Different coating materials including ZrO2,10 MgO,11 Al2O3,12 SiO2,13 AlF3,14 Al,15 carbon,16 and graphene17 have been reported. However, most of these coating materials, especially oxides, are insulator to Li ion, which influences the overall rate capability due to the low Li-ion diffusion ability in the interface of cathode/electrolyte. An ideal coating material should act not only as a protective layer but also as a Li-ion and even electron conductor. Recently, Li-ion conductors including Li2ZrO3,18−20Li2TiO3,21−23 and © XXXX American Chemical Society

Received: July 16, 2015 Revised: August 13, 2015

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2H2O particles, and Zr(OC4H9)4 was hydrolyzed to form ZrO2 coating on the surface of Ni1/3Co1/3Mn1/3C2O4·xH2O. This product is labeled as ZrO2@Ni1/3Co1/3Mn1/3C2O4·xH2O. Preparation of Li2ZrO3@LNCMO via a Synchronous Lithiation Route. The obtained ZrO2@Ni1/3Co1/3Mn1/3C2O4· xH2O was washed with ethanol and collected by centrifugation. After that the as-prepared ZrO2@Ni1/3Co1/3Mn1/3C2O4·xH2O and stoichiometric amounts of LiOH·H2O were mixed and calcined at 500 °C for 5 h and then at 900 °C for 12 h. The amount of LiOH·H2O was set at molar ratios of nLi = 1.05n(Ni+Co+Mn) + 2nZr. The obtained sample was designed as Li2ZrO3@LNCMO. For comparison, the precursor of Ni1/3Co1/3Mn1/3C2O4· 2H2O was also mixed with LiOH·H2O and calcined at 500 °C for 5 h and then at 900 °C for 12 h. The amount of LiOH·H2O was set at molar ratios of nLi = 1.05n(Ni+Co+Mn). The obtained sample was designed as LNCMO. 2.2. Materials Characterization. The crystal structures of the samples were characterized using X-ray diffractometer (XRD, Persee XD2, Cu Kα) in the 2θ range of 10°−80°. Data were recorded at a step width of 0.01° and a scan rate of 1°/ min. The particle shape and morphology images of the samples were observed with a high-resolution transmission electron microscope (HRTEM, JEOL-2010F, 200 kV). The surface element compositions were characterized by an X-ray photoelectronic spectrometer (XPS, Thermo Scientific ESCALAB 250Xi) using nonmonochromated Al Kα X-ray radiation as the excitation source. 2.3. Electrochemical Measurements. Electrochemical performances of the samples were tested in CR2025 coin cells. The positive electrodes of the active materials (80 wt %), carbon black (10 wt %), and poly(vinylidene fluoride) (10 wt %) were mixed in N-methylpyrrolidinone (NMP). The slurry was spread uniformly on an aluminum foil current collector and dried under vacuum at 80 °C for about 4 h and at 120 °C for about 12 h. The material loading of the cathode electrode is 3− 4 mg cm−2, and the thickness of electrode is about 150 μm. The 2025-type coin cells were assembled in an Ar-filled glovebox. The electrolyte was LiPF6 (1 mol L−1) in a 1:1 mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC). Galvanostatic charge−discharge cycling was performed between 3.0 and 4.5 V (vs Li/Li+) using an automatic galvanostat (NEWARE) at different current rates at 25 and 55 °C. The electrochemical impedance spectroscopy (EIS) was performed on an electrochemical workstation (Metrohm-Autolab, PGSTAT 302N) with an amplitude of 5 mV and a frequency range from 100 kHz to 0.1 Hz.

To the best of our knowledge, the coatings of Li2ZrO3 on the cathodes reported previously all need to first synthesize cathode materials, and then the as-synthesized cathodes are coated with Li2ZrO3, which is also called postcoating route. Through comparing the lattice parameters of bare LiNi0.4Co0.2Mn0.4O2 and [email protected] reported by Zhou,19 we can see that the lattice parameters of LiNi0.4Co0.2Mn0.4O2 almost do not change after Li2ZrO3 coating, which means that the cation migration between Zr and Ni, Co and Mn might not happen during the postcoating procedure. But the cation migration or doping resulting from coating usually benefits to the enhancement of Li-ion, electron conductivity, and the consequent electrochemical performance.21 In addition, as for the postcoating procedure, when the calcination temperature is low, the coating layer of Li2ZrO3 might not form. But if the calcination temperature is high enough, part of Li+ will dissolve out of the host phase due to the duplicated high-temperature treatment, which also results in the drop of electrochemical performance. With respect to the above limitations of the postcoating procedure, we proposed a synchronous lithiation route to coat LiNi1/3Co1/3Mn1/3O2 with Li2ZrO3. In this synchronous lithiation process, ZrO2 was first coated on the surface of the precursor of Ni1/3Co1/3Mn1/3C2O4·xH2O to form ZrO2@ Ni1/3Co1/3Mn1/3C2O4·xH2O using Zr(OC4H9)4 as a coating reactant, and then ZrO2 and Ni1/3Co1/3Mn1/3C2O4·xH2O were simultaneously lithiated to form Li2ZrO3@LNCMO. In addition to cations migration and doping effect, the binding between the surface layer and host phase might also be enhanced through the synchronous lithiation stategy. In comparison to bare LNCMO, the reversible capacity, cycling performance, thermal stability, rate capability, and polarization of Li2ZrO3@LNCMO have been greatly improved, especially for the high rate capability and thermal stability. The enhanced electrochemical performances of Li2ZrO3@LNCMO cathode can be largely attributed to the high Li-ion conductivity as well as the proctection of Li2ZrO3 coating. Moreover, the migration of partial Zr4+ to the host phase might also alleviate the Li−Ni cation mixing which widely exists in LiNi1/3Co1/3Mn1/3O2 and stabilizes the structure especially at an elevated temperature. Therefore, the cations migration, doping effect, and the reduced cation mixing further contribute to the electrochemical performance enhancement, especially the high rate capability, long-term cyclability, and thermal stability.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. Preparation of Ni1/3Co1/3Mn1/3C2O4·2H2O. The precursor Ni1/3Co1/3Mn1/3C2O4·2H2O of LNCMO was synthesized via a coprecipitation method by adding H2C2O4 solution to a solution that contained stoichiometric amounts of nickel sulfate (NiSO4·6H2O), cobalt sulfate (CoSO4·7H2O), and manganese sulfate (MnSO4· 5H2O). The solution was strongly stirred at 400−1000 rpm. After the oxalate precursor was obtained, it was washed with deionized water for three times and ethanol for two times and dried overnight at 80 °C. Preparation of ZrO2@Ni1/3Co1/3Mn1/3C2O4·xH2O. In a typical process, Ni1/3Co1/3Mn1/3C2O4·2H2O (500 mg) was dispersed into absolute ethanol (10 mL), followed by adding a stoichiometric amount of Zr(OC4H9)4. The Zr content was set at mass ratios of Li2ZrO3/LNCMO = 1.5%. The mixtures were maintained at 180 °C for 5 h. During the solvothermal process, some crystal water would come out of Ni1/3Co1/3Mn1/3C2O4·

3. RESULTS AND DISCUSSION The whole schematic illustration for the synthesis of Li2ZrO3@ LNCMO is shown in Figure 1. In this process, ZrO2 was first coated on the surface of the precursor of Ni1/3Co1/3Mn1/3C2O4· xH2O to form ZrO2@Ni1/3Co1/3Mn1/3C2O4·xH2O using Zr(OC4H9 )4 as a coating reactant, and then ZrO2 and Ni1/3Co1/3Mn1/3C2O4·xH2O was simultaneously lithiated to form Li2ZrO3@LNCMO. The crystal structures of bare LNCMO and Li2ZrO3@ LNCMO were analyzed by XRD, as shown in Figure 2a. The diffraction patterns of bare LNCMO and Li2ZrO3@LNCMO can be well identified as α-NaFeO2 structure with space group R3̅m, which indicates that Li2ZrO3 coating does not change the crystal structure. Distinct splitting of the (006), (102) and (108), (110) peaks could be observed in these patterns, which B

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increase to 2.8529(2) and 14.1863(11), respectively. Both a and c show an increase after Li2ZrO3 coating, and c presents a larger increase than a, which is similar to a previous work.31 It is well-known that a diffraction peak corresponding to a smaller 2θ angle in XRD pattern has a larger interplanar spacing. As shown in Figure 2a, the (003) planes of Li2ZrO3@LNCMO shift to the left compared to bare LNCMO, which indicates the expansion of interplanar spacing of the (003) slab. Larger (003) interplanar spacing can significantly facilitate the lithium diffusion and is also favorable to the rate capability.32 By comparing the change of the lattice parameters before and after coating, we can deduce that some Zr4+ have been incorporated into the bulk structure because the radius of Zr4+ (0.79 Å) is larger than other irons (r(Ni2+) = 0.69 Å, r(Co3+) = 0.545 Å, and r(Mn4+) = 0.53 Å).33−35 As mentioned in the Introduction, the parameters of [email protected], which was prepared through a postcoating method, almost did not change after Li2ZrO3 coating,19 which indicated that Zr4+ did not migrate into the bulk structure. In our case of Li2ZrO3@ LNCMO, Zr4+ are incorporated into the bulk structure, and moderate doping could improve the structure stability and even Li+ or electron conductivity. Besides Zr migration, at the same time, there must be some transition metal elements in bulk structure migrating into the coating layer of Li2ZrO3. The elements doping due to the cation migration will further benefit to the enhancement of electrochemical performance. For host material, the moderate doping could lower the electric resistivity and optimize the structure stability, and for coating layer, the doping can enhance the Li-ion conductivity, which is more favorable for high charge−discharge rate.21 In addition, the larger c and increased I(003)/I(104) value of Li2ZrO3@ LNCMO are favorable for a high rate performance.36−38 It should be noted that the phase of Li2ZrO3 was not observed from the XRD pattern of Li2ZrO3@LNCMO sample owing to the tiny amount of Li2ZrO3. To verify that Li2ZrO3 can form during the coating process, we perform the experiment in the same route but only using Zr(OC4H9)4; the XRD pattern (Figure 2b) of the resulting powder perfectly meets the standard peaks of Li2ZrO3, which indicates that the coating layer should be Li2ZrO3. This is also further confirmed by TEM and XPS. In order to observe the surface morphology of Li2ZrO3@ LNCMO, HRTEM measurements are applied. It could be clearly observed in Figure 3a that the sample of Li2ZrO3@ LNCMO has a good spherical shape with the size ranging from 300 to 500 nm. The spherical shape is favorable for a high tap density and energy density. At the edge of the sphere, it can be observed that a thin layer is clinging tightly. In Figure 3b, more specific information could be obtained. It could be seen that the sphere is coated perfectly with a thin layer. As shown in Figure 3b, a homogeneous coating layer grows on the surface of LNCMO, and the thickness of the coating layer is about 7−10 nm. And the d-spacing of the coating layer is 0.2873 nm, which is well matched with the d-spacing of (111) plane for the standard Li2ZrO3 (0.2853 nm). This also further demonstrates that the coating layer is Li2ZrO3. For clarifying the surface

Figure 1. Schematic illustration of the preparation process for Li2ZrO3@LNCMO.

Figure 2. (a) XRD patterns of bare LNCMO and Li2ZrO3@LNCMO samples. (b) XRD patterns of Li2ZrO3 as prepared and Li2ZrO3 standard. (c) Observed/calculated XRD patterns of bare LNCMO. (d) Observed/calculated XRD patterns of Li2ZrO3@LNCMO. The insets in (a) are the magnification of (003), (006), and (102) peaks.

indicates a highly ordered layered structure.27 The former is located at around 38°, and the latter is at 65°. The ratio of the intensity of the two major peaks I(003)/I(104) is always used to indicate the degree of cation mixing, and a higher value of I(003)/I(104) usually means a lower cation mixing.27−29 In this case (as shown in Table 1), the I(003)/I(104) value for bare LNCMO is 1.357, while the value is increased to 1.394 for Li2ZrO3@LNCMO, which indicates that Li2ZrO3 coating can alleviate the cation mixing. This may result from the migration and doping of Zr4+ into the host structure. To investigate the effect of Li2ZrO3 on the lattice parameters of LNCMO, the XRD patterns of bare LNCMO and Li2ZrO3@LNCMO are analyzed by Rietveld refinement as shown in Figure 2c,d and Table 1. Li ions, transition metal ions, and oxygen ions are located at Wyckoff position 3a sites (0, 0, 0), 3b sites (0, 0, 0.5), and 6c sites (0, 0, 0.2410).30 The lattice parameters a and c of bare LNCMO are 2.8516(2) and 14.1806(9), respectively. In contrast, the lattice parameters a and c of Li2ZrO3@LNCMO

Table 1. Cell Parameters Derived from Rietveld Refinement and I(003)/I(104) Value of Bare LNCMO and Li2ZrO3@LNCMO samples

a (Å)

c (Å)

V (Å3)

Rp (%)

I(003)/I(104)

bare LNCMO Li2ZrO3@LNCMO

2.8516(2) 2.8529(2)

14.1806(9) 14.1863(11)

99.86(1) 99.99(1)

8.82 9.95

1.357 1.394

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charge−discharge test at different current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 C, respectively. As shown in Figure 5a, it is

Figure 3. (a) HRTEM images of Li2ZrO3@LNCMO sample. (b) Magnified HRTEM images of Li2ZrO3@LNCMO sample.

elements composition of Li2ZrO3@LNCMO, X-ray photoelectron spectroscopy (XPS) experiments were carried out. As shown in Figure 4a, Li, Zr, O, Ni, Co, Mn, and C elements were detected. The atomic ratio of Zr to (Ni + Co + Mn) determined from XPS in Figure 4b−f is 0.46:(3.07 + 3.63 + 3.02), while the setting starting ratio is 0.098:(3.33 + 3.33 + 3.33). It is known that XPS could only detect the elements on the surface within about 10 nm.39 If the material is completely wrapped and cation migration is not taken into consideration, only the Zr element could be detected, and the elements of Ni, Co, and Mn should not be detected in the layer. But the measured value by XPS is 0.46:(3.07 + 3.63 + 3.02), which indicates that there is a migration of Zr element from layer to the host, and Ni, Co, and Mn elements also migrate from the host to Li2ZrO3 layer. And the detection of Li and Zr on the outer layer could further suggest that the coating layer is Li2ZrO3. To study the effects of Li2ZrO3 coating on the electrochemistry performance of LNCMO, rate capability testing is first measured. For comparison, we tested the rate capability of Li2ZrO3@LNCMO and bare LNCMO. We set the electrochemistry window from 3.0 to 4.5 V and carried out the

Figure 5. (a) Rate performances of bare LNCMO and Li2ZrO3@ LNCMO. (b) Discharge curves of LNCMO at different current density. (c) Discharge curves of Li2ZrO3@LNCMO at different current density. (d) Charge−discharge curves of LNCMO and Li2ZrO3@LNCMO at 10 C.

clearly observed that the capacity of Li2ZrO3@LNCMO is higher than bare LNCMO, especially at a large current density. At 0.1 C, the capacity of bare LNCMO and Li2ZrO3@LNCMO sample is 178 and 192 mAh g−1, respectively. But when it is at 10 C, the specific capacity of the bare sample sharply dropped to 44 mAh g−1 while it is 106 mAh g−1 for Li2ZrO3@LNCMO, which can be greatly attributed to the coating effect of Li+conductive Li2ZrO3. First, a suitable Li2ZrO3 coating layer not only suppressed the dissolution of transition metal and side reaction between electrode and electrolyte but also improved

Figure 4. (a) X-ray photoelectron spectrum of Li2ZrO3@LNCMO sample. (b) X-ray photoelectron spectrum for Li 1s. (c) X-ray photoelectron spectrum for Ni 2p. (d) X-ray photoelectron spectrum for Co 2p. (e) X-ray photoelectron spectrum for Mn 2p. (f) X-ray photoelectron spectrum for Zr 3d. D

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after 400 cycles, it only remains 77.6 mAh g−1 with a capacity retention of 69.2%. In contrary, the initial discharge capacity of Li2ZrO3@LNCMO at 5 C is 125.7 mAh g−1, and it still remains 117.9 mAh g−1 after 400 cycles showing a higher capacity retention of 93.8%, which indicates that Li2ZrO3 enhances the cycling stability and capacity retention. When the testing temperature is evaluated to 55 °C, the difference of the cycling stability and capacity retention becomes very large, as shown in Figure 6c. For the bare LNCMO sample, the initial discharge capacity at 5 C is 100.4 mAh g−1, and after 400 cycles it only remains 37.5 mAh g−1 and the capacity retention is 37.4%. But as for Li2ZrO3@LNCMO sample, the initial discharge capacity at 5 C is 141.9 mAh g−1, and after 400 cycles, it still remains 120.8 mAh g−1 with a much higher capacity retention of 85.1%, which illustrates the excellent structure stability and high cyling performance of Li2ZrO3@LNCMO sample at elevated temperature. At 25 °C, the Coulombic efficiency for bare LNCMO and Li2ZrO3@LNCMO samples is almost 100% (Figure 6b), but when the temperature is raised to 55 °C, there is much difference (Figure 6d). The Coulombic efficiency for bare LNCMO is quite low at the first 10 cycles, which can be largely attributed to the severe side reaction between the electrode and electrolyte. In comparison with bare LNCMO, the Li2ZrO3@ LNCMO sample has a more stable and higher Coulombic efficiency at 55 °C, indicating Li2ZrO3 can alleviate the side reaction between the electrode and electrolyte especially at an elevated temperature. We have also tested the electrochemical performance (cycling performance and Coulombic efficiency) of bare LNCMO and Li2ZrO3@LNCMO at −20 °C at the current density of 1 C for 200 cycles and 3 C for 80 cycles, as shown in Figure 6e,f. Li2ZrO3@LNCMO still shows a higher reversible capacity and better rate capability than LNCMO. It should be noted that the capacity gradually increases with increasing cycles which maybe largely attributed to the electrode materials activation and the raised temperature of the electrode resulting from the charge/discharge process. EIS could give the detailed structures of the electrode/ electrolyte interface by virtue of a wide frequency range. The EIS describes a capacitive loop in the high-frequency range and a straight line with a 45° in the low-frequency range.40−42 Here we use it to investigate the structure changing and calculate Liion conductivity (DLi) of bare LNCMO and Li2ZrO3@ LNCMO. The equivalent circuit is given in Figure 7 which involves Rele, Rct, Cdl, and W. Rele and Rct stand for electrolyte resistance and charge-transfer resistance, respectively. Cdl stands

the Li+ transportation, which enhanced the rate performance. Second, the reduced cation mixing owing to the migration of Zr4+ also plays a positive role on the enhancement of rate capability. Third, the migration of Zr element from layer to the host and the migration of Ni, Co, and Mn elements from the host to Li2ZrO3 layer benefit to the Li+ diffusion. The polarization at different current densities of bare LNCMO and Li2ZrO3@LNCMO could be attained from Figure 5b,c. At 10 C the ΔV of bare LNCMO and Li2ZrO3@LNCMO samples is 0.765 and 0.351 V, respectively. Figure 5d shows a direct comparison between LNCMO and Li2ZrO3@LNCMO at 10 C. As shown in Figure 5d, the charge−discharge overpotential of Li2ZrO3@LNCMO is much smaller than that of bare LNCMO, indicating Li2ZrO3 largely reduces polarization of LNCMO. The polarization comes from the concentration polarization and electrochemical polarization. It could be inferred that by surface modification with Li2ZrO3 layer, the electrode material has better lithium ion conductivity and better electric conductivity. We deduced that the better lithium ion conductivity comes from Li2ZrO3 layer and the broadened parameter c, and the better electric conductivity comes from cation migration and Zr doping, which has been illustrated by XRD data in Figure 2. In the following part, EIS measurement will be applied to support this point. For verifying the structure stability and capacity retention of Li2ZrO3@LNCMO, the galvanostatic charge−discharge cycling tests of bare LNCMO and Li2ZrO3@LNCMO samples are performed at 25 and 55 °C, respectively. The electrochemistry window is set from 3.0 to 4.5 V at 5 C for 400 cycles. Before cycling, the electrodes were activated at 0.1 C for two cycles. As shown in Figure 6a, at the current density of 5 C the initial discharge capacity of the bare sample is 112.1 mAh g−1, and

Figure 6. Cycling performance of bare LNCMO and Li2ZrO3@ LNCMO samples at 25 (a), 55 (c), and −20 °C (e). Coulombic efficiency of bare and Li2ZrO3@LNCMO samples at 25 (b), 55 (d), and −20 °C (f).

Figure 7. Nyquist plots for bare and Li2ZrO3@LNCMO electrodes. E

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The Journal of Physical Chemistry C for the electric double-layer capacitance, and W stands for the Warburg impedance. Calculation results of the impedance spectrum in Figure 7 are shown as follows. Rele is both about 3 Ω. Rct for bare LNCMO and Li2ZrO3@LNCMO electrodes is 914 and 614 Ω, respectively. σ, which is called the Warburg constant, is 1.23 × 107 and 2.61 × 106 respectively for bare LNCMO and Li2ZrO3@LNCMO electrodes. When Li2ZrO3 is coating, Rct decreases a lot (∼300 Ω), which may come from two factors. One is that the Li2ZrO3 layer increases the electric conductivity of the active material, and the other is that Zr element doped into the LNCMO phase and it increases the electric conductivity. Equation 143 could show the relationship of temperature, Warburg constant, molar concentration, and Liion conductivity 1 DLi = [(RT )/(SF 2σC)]2 (1) 2 where T is the absolute temperature, R is the gas constant, S is the electrode area, F is the Faraday constant, and C is the molar concentration. The calculation result of DLi is 3.06 × 10−9 and 6.80 × 10−8 cm2 s−1 for bare LNCMO and Li2ZrO3@LNCMO electrodes, respectively, which means Li2ZrO3 coating layer largely enhances the Li-ion diffusion. Based on the analysis and discussion of XRD data, XPS data, and the electrochemical performance, there are Zr4+ migrating into the bulk structure and at the same time M ion (M = Ni, Co, M) migrate into the coating layer, which benefit the rate performance, cycling performance, specific capacity, and thermal stability. In addition, according to the ratio of the intensity of the two major peaks I(003)/I(104) which is always used to indicate the degree of cations mixing, a higher value of I(003)/I(104) was observed for Li2ZrO3@LNCMO. This means Li2ZrO3 coating can reduce the cation mixing, which is also favorable to the electrochemical performance. We draw a model to explain the influence of Li2ZrO3 coating on the cation disorder and use a sketch to illustrate the mechanism. As shown in Figure 8, there always exists a certain of cation disorder in

LNCMO, Li2ZrO3@LNCMO shows much higher specific capacity, higher cycling stability, higher thermal stability, higher rate capability, and lower charge−discharge overpotential. The enhanced performance of Li2ZrO3@LNCMO cathode can be largely attributed to the protection and Li+ conductivity of Li2ZrO3 coating. In addition, the cations migration, doping effect, and the reduced cations mixing induced by Li2ZrO3 coating also further benefit the electrochemical performance enhancement. The proposed synchronous lithiation strategy might also be applied to other cathodes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86 10 8825 6840 (X.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the State Key Project of Fundamental Research (Grants 2014CB931900 and 2012CB932504), National Natural Science Foundation of China (Grant 11575192) and “Hundred Talents Project” of the Chinese Academy of Sciences.

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REFERENCES

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Figure 8. Sketch illustrating the relationship between layer amount and cations occupancy.

bare LNCMO, and it could be varied with processing environment and preparation method. When LNCMO is coated with a layer of Li2ZrO3 during the lithiation process, Zr4+ will incorporate into the bulk structure which will broaden the c axis parameter and reduce the cation disorder. Moreover, the doped Zr and Li2ZrO3 layer could improve the structure stability when the long cycles are performed.

4. CONCLUSIONS In summary, a uniform thin Li2ZrO3 layer has been successfully coated on the surface of LNCMO via a novel synchronous lithiation method. Li2ZrO3, as a Li-ion conductor, largely improves the velocity of Li migration and reduces the chargetransfer resistance of the electrode. In comparison with bare F

DOI: 10.1021/acs.jpcc.5b06858 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b06858 J. Phys. Chem. C XXXX, XXX, XXX−XXX