Enhanced Electrochemical Performance of Li1.2Mn0.54Ni0.13Co0

Nov 25, 2015 - Enhanced Electrochemical Performance of Li1.2Mn0.54Ni0.13Co0.13O2 Cathode with an Ionic Conductive LiVO3 Coating Layer. Xiaoyu Liu ...
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Research Article pubs.acs.org/journal/ascecg

Enhanced Electrochemical Performance of Li1.2Mn0.54Ni0.13Co0.13O2 Cathode with an Ionic Conductive LiVO3 Coating Layer Xiaoyu Liu, Qili Su, Congcong Zhang, Tao Huang, and Aishui Yu* Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200438, China S Supporting Information *

ABSTRACT: With the aim to enhance the Li+ ion conductivity, an ionic conductor, LiVO3, has been successfully coated on the surface of lithium-rich layered Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials for the first time. After combining with LiVO3, significantly improved high-rate capability and cyclic stability of the Li-rich cathode have been achieved due to the enhanced lithium ion diffusion and stabilized electrode/electrolyte interface. Moreover, a stable three-dimensional spinel phase has been generated in the surface region during the coating process, which mitigates the structure deterioration and suppresses the voltage decay and energy density degradation. After optimization, 5 wt % LiVO3-coated−Li1.2Mn0.54Ni0.13Co0.13O2 exhibits superior electrochemical performance with a higher reversible capacity of 272 mA h g−1, increased initial Coulombic efficiency of 92.6%, and an excellent high-rate capability of 135 mA h g−1 at 5 C, respectively. The coexistence of an ionic conductor coating layer and the locally transformed spinel structure generated in a one-step approach provides a novel design concept for surface modification on Li-rich Mn-based cathode materials toward high-performance lithium-ion batteries. KEYWORDS: Lithium-ion battery, Lithium-rich cathode material, Surface modification, Phase transformation

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INTRODUCTION

In order to solve these problems, many efforts have been made all around the world. Among them, surface modification, an effective strategy to solve the problems of high irreversible capacity and cycle deterioration, has been widely utilized because the coating layer hinders direct contact and the resulting side reactions between the electrode and the electrolyte. Metal oxides,9−13 fluorides,14,15 and phosphates16,17 are commonly used in the surface modification approach; however, the rate performance and reversible capacity of the modified Li-rich cathodes are still unsatisfying due to the poor ionic and electronic conductivity of these surface coating materials. In addition, this kind of traditional surface modification has little effect on the voltage decay upon longterm cycling since the structure of the active materials remains unchanged after surface modification. Therefore, it is important to develop a novel surface modification method to solve the above-mentioned issues. One route is to construct spinel structures with three-dimensional Li+ diffusion channels in the surface region, facilitating the Li+ transfer in the interface.18−21 Additional surface coating outside the surface spinel structure is required to prevent direct contact with the electrolyte to improve cyclic stability.

Along with the rapid development of electric vehicles (EVs) and hybrid electric vehicles (HEVs), lithium-ion batteries are regarded as one of the most advanced electrochemical energy storage systems.1,2 However, the limitation in the energy density of the current lithium-ion batteries restricts their application on EVs and HEVs. As the source of lithium ions, cathode materials determine the performance of lithium-ion batteries; however, conventional cathode materials, such as LiFePO4, LiCoO2, and LiMn2O4, cannot satisfactorily meet requirements due to their insufficient specific capacity. In recent years, Li-rich Mn-based layered oxides with a composition of xLi2MnO3·(1 − x)LiMO2 (0 < x < 1, M = Ni, Co, Mn1/2Ni1/2, Mn1/3Ni1/3, Co1/3, etc.) have drawn considerable attention for their higher capacities over 250 mA h g −1 with significantly reduced cost and toxicity. 3−5 Unfortunately, several drawbacks hinder the practical utilization of Li-rich cathode materials: (1) a huge irreversible capacity loss during the first charge−discharge cycle derived from the irreversible removal of the net Li2O; (2) poor rate capability resulting from slow lithium ion diffusion in Li 2 MnO 3 component; (3) inferior cyclic performance due to the electrode/electrolyte interface deterioration at higher potential (>4.5 V); (4) severe discharge voltage decay upon long-term cycling caused by structural transformation.6−8 © 2015 American Chemical Society

Received: September 15, 2015 Revised: November 7, 2015 Published: November 25, 2015 255

DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263

Research Article

ACS Sustainable Chemistry & Engineering In this study, for the first time, a good Li+-conductor, LiVO3, has been successfully coated on the surface of Li1.2Mn0.54Ni0.13Co0.13O2 through a facile sol−gel method. The LiVO3 coating layer not only suppresses the surface deterioration originating from the electrode−electrolyte side reaction but also facilitates lithium ion transfer at the interface. Meanwhile, structure transformation from the two-dimensional layered phase to the three-dimensional spinel phase in the surface region has been induced during the wet chemical coating process and the following heat-treatment, which is beneficial to lithium ion diffusion and structure stability during charge−discharge cycles. Therefore, the electrochemical properties of Li1.2Mn0.54Ni0.13Co0.13O2 cathode, in terms of reversible capacity, initial coloumbic efficiency, cyclic stability, rate capability, and voltage stability have been significantly improved by LiVO3 coating. The results presented here illustrate the benefits of the LiVO3 surface modification for Li-rich Mn-based layered oxides. This novel modification strategy provides an opportunity for Li-rich Mn-based layered oxides to be a promising cathode candidate for advanced lithium ion batteries upon future application.

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current density at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (Zahner IM6e).

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RESULTS AND DISCUSSION The phase purity and crystal structure of pristine and LiVO3coated Li1.2Mn0.54Ni0.13Co0.13O2 were examined by XRD. The XRD profiles exhibited in Figure 1 show characteristic patterns

EXPERIMENTAL SECTION

Preparation of Samples. The layered Li-rich Li1.2Mn0.54Ni0.13Co0.13O2 was synthesized via a precipitation method as we reported in a previous paper.22 Then the obtained cathode materials were modified with various mass ratios of LiVO3 via a citric acid assisted sol−gel method in the following steps. According to different contents of LiVO3 to Li1.2Mn0.54Ni0.13Co0.13O2, stoichiometric amounts (1:1) of LiCH3COO·H2O and NH4VO3 were dissolved in hot distilled water and then Li1.2Mn0.54Ni0.13Co0.13O2 was ultrasonically dispersed in the solution. After adding appropriate amount of citric acid as a chelating agent, the pH value is adjusted to 7−8 with ammonium hydroxide. Then the solution was evaporated at 80 °C until a viscous gel emerged. The gel was dried in a vacuum oven at 120 °C for 12 h and then annealed at at 400 °C for 5 h. As a reference, pure LiVO3 was was also synthesized through a sol−gel method under the same conditions. Characterization. The crystalline structure and phase purity were characterized by X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα radiation, λ = 1.5406 Å). The morphology images were collected by scanning electron microscopy (SEM, JEOL JSM-6390), energydispersive X-ray spectrometer (EDS, FE-SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEM-2100F) and selected area electron diffraction (SAED, TEM, JEM-2100F). Furthermore, Xray photoelectron spectroscopy (XPS) measurements were carried out on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Al Kα radiation (hυ = 1486.6 eV). Binding energies were calibrated using the containment carbon (C 1s = 284.6 eV). Raman spectra were obtained on a Raman spectrometer (Renishaw inVia Reflex) coupled with microscope in a reflectance mode with a 514.5 nm excitation laser source. Electrochemical Measurements. The working electrodes were made up of 80 wt % as-prepared powders, 10 wt % carbon conductive agents (Super P), and 10 wt % polytetrafluoroethylene (PTFE) and compressed onto the aluminum nets. The total mass of each active electrode material is 4−5 mg, and the surface area of each electrode is 0.785 cm2. The electrodes were dried overnight at 80 °C in a vacuum oven before use. Coin-type (CR2016) half-cells were assembled in an argon-filled glovebox (Mikarouna, Superstar 1220/750/900). In the half-cell, metallic lithium foil served as an anode, 1 M LiPF6 in ethylene carbonate (EC)−dimethyl carbonate (DMC)−diethyl carbonate (DEC) (1:1:1 in volume) was used as the electrolyte, and a polypropylene microporous film (Cellgard 2300) served as the separator. The galvanostatic discharge−charge performance were measured on the battery test system (Land CT2001A, Wuhan Jinnuo Electronic Co. Ltd.) between 2.0 and 4.8 V (vs Li/Li+) at different

Figure 1. XRD patterns of Li1.2Mn0.54Ni0.13Co0.13O2 coated by different amounts of LiVO3 and the pure LiVO3.

corresponding to α-NaFeO2 hexagonal type structure with a space group symmetry of R3̅ m. Meanwhile, the weak peaks between 2θ = 20−25° indicate the presence of monoclinic Li2MnO3 phase with a space group symmetry of C2/m, ascribed to short-range Li−Mn cation ordering in the transition metal layers.23,24 For LiVO3-coated samples, a spinel-related peak appears at 36.5° (marked by arrows) and becomes stronger with the increase of LiVO3 amount, attributed to the surface structure transformation induced by chemical dilithiation during the coating process. The pure LiVO3 phase can be obtained at this annealing temperature through a sol−gel method, as demonstrated by characteristic XRD patterns indexed to LiVO3.25−27 However, on account of low quantity and poor crystallinity of the coating LiVO3, there is little noticeable difference among the pristine, 2, and 5 wt % LiVO3 coated samples. With the ratio of the coating LiVO3 reaching 10 wt %, a small amount of LiVO3 phase has been detected, as illustrated by extra weak peaks located between 27° and 32°, due to the formation of crystalline LiVO3. Meanwhile, the broader diffraction peaks, as well as the obscure splitting of (006)/(012) and (018)/(110) peaks, indicate that the layered structure of Li1.2Mn0.54Ni0.13Co0.13O2 has been damaged after 10 wt % LiVO3 coating. SEM images of pristine and 5 wt % LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2 were collected to study the morphology variation after LiVO3 coating. As exhibited in Figure 2a and b, both samples exist as loose spherical aggregates of several microns and each aggregate consists of a large amount of primary particles with a diameter of 100−200 nm. At a higher magnification, it can be seen that the surface of 5 wt % LiVO3 coated samples is quite rougher than that of pristine samples (Figure 2c and d). In order to determine the presence of LiVO3, EDS measurements were carried out on both samples at various areas. EDS spectra accompanied by the real time images of pristine and 5 wt % LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2 in the area scan mode are shown in Figure 2e and f, 256

DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. SEM images of pristine (a, c) and 5 wt % LiVO3 coated (b, d) Li1.2Mn0.54Ni0.13Co0.13O2 and EDS patterns of pristine (e) and 5 wt % LiVO3 coated (f) Li1.2Mn0.54Ni0.13Co0.13O2.

Li2VO3 which was reported to have a rock salt structure with a strong amorphization.25 Meanwhile, cations rearrangement occurs in the surface region of active materials after lithium ions leaching, leading to the formation of the spinel structure. Interestingly, the Li2VO3 structure has a space group of Fd3̅m, similar to the generated spinel structure. Thus, it is difficult to discern the boundary between the active materials and the coating layer. The structure transition from the inner region to the surface region can be illustrated by the enlarged images in the panels A−C of Figure 3b. The atomic composition of coated sample was analyzed by EDS and the results further reflect the presence of vanadium, as presented in Figure 3c. To gain insight into the surface chemistry of pristine and 5 wt % LiVO3 coated samples, XPS measurements were conducted and the results are exhibited in Figure S2. No obvious change has been detected on the valences of Mn, Co, and Ni in active materials after LiVO3 coating, which is supported by the evidence that the Ni2+, Co3+, and Mn4+ 2p3/2 peaks remain at their original binding energies after coating. However, the intensities of these peaks decrease with respect to those of the pristine materials, indicating the existence of surface coating species. It is worth noting that the V 2p3/2 peak of the LiVO3 coated sample can be fitted by two components at 516.9 and 517.9 eV. The former one is attributed to the V4+ cations while the latter one is ascribed to the V5+ cations in the coating layer.28,29 As discussed above, part of surface lithium

respectively, demonstrating the presence of V element besides O, Mn, Ni, and Co, elements for the coated sample. Figure S1 shows the element distribution of 5 wt % LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2, indicating the homogeneous distribution of coating LiVO3 on the surface of pristine particles. Further details on the composition and structure of pristine and 5 wt % LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2 were acquired via TEM and the images are offered in Figure 3. The pristine Li1.2Mn0.54Ni0.13Co0.13O2 exhibits clear lattice fringes with interlayer spacing of 0.203 nm in Figure 3a, corresponding to the (104) plane of the hexagonal layered structure. These fringes remain straight and uninterrupted to the surface of the particle, revealing a well-crystalline layered structure. The structural regularity is further certified by the same Fast Fourier Transformation (FFT) images at different areas. However, for LiVO3-coated sample, the surface region shows distinctive lattice fringes from the inner region with lower crystallinity. The interlayer spacing of inner fringes is measured to be 0.244 nm, corresponding to the (101) plane of the hexagonal layered structure while the interlayer spacing of fringes in the surface region is measured to be 0.249 nm, indexed to the (311) plane of the cubic spinel structure. This type of variation is attributed to the LiVO3 coating along with the surface reaction between the active material and the coating LiVO3. During the process of heating treatment, some leached lithium ions from the active materials may diffuse into the coating LiVO3 layer to form 257

DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. TEM images of pristine (a) and 5 wt % LiVO3 coated (b) Li1.2Mn0.54Ni0.13Co0.13O2 with FFT images in the different region and corresponding EDS patterns of 5 wt % LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2 (c).

Figure 4. Initial charge−discharge curves (a) and corresponding dQ/dV plots (b) for Li1.2Mn0.54Ni0.13Co0.13O2 coated by different amounts of LiVO3.

C = 250 mA g−1). It can be seen that all the samples display a plateau at around 4.5 V during the first charge process, related to the oxygen removal from the surface of particles.30−32 With the amount of the coating LiVO3 increasing, this irreversible plateau is suppressed, leading to smaller initial charge capacities and larger first Coulombic efficiencies (FCE). This phenomenon can be attributed to the stronger V−O bonds (644.21 kJ mol−1) which contribute to reduce the activity of surface oxide ions, resulting in less oxygen removal and less electrolyte oxidation during the initial charging.33,34 Moreover, an additional plateau below 3.0 V, which is characteristic of spinel

ions may diffuse into the coating LiVO3 layer during the heating treatment of the coating process and vanadium tends to be in the V4+ oxidation state in the new environment. The integrated area of the V4+ peak is smaller than that of the V5+ peak, suggesting that the main component of coating material is LiVO3. Galvanostatic charge/discharge experiments were used to investigate the effects of LiVO3 coating on the electrochemical properties. Figure 4a compares the performance of the initial charge/discharge curves of Li1.2Mn0.54Ni0.13Co0.13O2 modified with different amounts of LiVO3 at the current rate of 0.1 C (1 258

DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. Cyclic performance of Li1.2Mn0.54Ni0.13Co0.13O2 coated by different amounts of LiVO3 at 0.1 (a) and 1 C (b) and various rates (c).

Figure 6. Continuous charge−discharge curves of pristine (a) and 5 wt % LiVO3 coated (b) Li1.2Mn0.54Ni0.13Co0.13O2 and average discharge voltage (c) and energy density (d) of both samples upon cycling.

change in voltage profiles is clearly evident in the dQ/dV profiles of Figure 4b. The characteristic peak for the spinel structure below 3.0 V (enlarged in the inset) gradually increases

structure, emerges at the end of the initial discharge curves and progressively becomes distinct with the increase of coating LiVO3, consistent with the XRD results in Figure 1. This 259

DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263

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ACS Sustainable Chemistry & Engineering

Figure 7. Nyquist plots of pristine (a) and 5 wt % LiVO3 coated (b) Li1.2Mn0.54Ni0.13Co0.13O2 after various cycles and the used equivalent circuit (c). The symbols and lines correspond to the experimental and simulated data, respectively.

respectively. Besides, 2 wt % LiVO3 coated sample presents a better rate performance than the pristine one but does not appear as much improvement as the 5 wt % LiVO3 coated sample while the 10 wt % LiVO3 coated sample shows more severe capacity decay at higher rates. The significantly improved rate capability results from a LiVO3 coating layer with good ionic conductivity and surface spinel structure with a threedimensional interstitial space for lithium ion diffusion. Voltage decay is one of the major challenges for Li-rich Mnbase cathode materials, which is caused by the structure conversion and always leads to gradual decrease in energy density.35−37 Therefore, we have studied the voltage profiles upon cycling and the results are depicted in Figure 6. It can be observed that the pristine Li1.2Mn0.54Ni0.13Co0.13O2 shows a much faster voltage decline along with rapid capacity fading (Figure 6a), electrochemically revealing dramatic changes of local Li environments during cycling. Comparatively, the voltage decay has been obviously mitigated for the 5 wt % LiVO3 coated sample, as shown in Figure 6b. To clearly reveal the trend of discharge voltage, Figure 6c compares the average discharge voltages of both electrodes. After 80 cycles at a low current of 0.1 C, the 5 wt % LiVO3 coated sample presents a voltage drop of 0.31 V, much smaller than that of 0.51 V for the pristine sample. The alleviated voltage fading can be attributed to the decrease of average resistance and the suppression of local structure deterioration caused by LiVO3 coating and surface spinel structure. Figure 6d demonstrates energy density loss of the cells upon cycling which comprises the contributions from capacity fading and voltage decay. Accordingly, the superior stability of capacity and voltage for the 5 wt % LiVO3 coated sample ensures the better cyclic performance on the energy density. After 80 cycles, the 5 wt % LiVO3 coated sample delivers an energy density of 768 W h kg−1, compared with 585 W h kg−1 for the pristine sample. To get insight into the origin of the improvement in electrochemical performance after LiVO3 surface modification, EIS is applied to identify the charge transfer resistance and the evolution of electrode/electrolyte interface. The Nyquist plots of the pristine and 5 wt % LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2 electrodes after the 5th, 50th, and 100th cycles are exhibited in Figure 7.

in intensity with the coating LiVO3 amount varying from 2 to 10 wt %. However, this spinel-related peak was not observed in the pristine Li1.2Mn0.54Ni0.13Co0.13O2 electrode, indicating a structural transformation from the layered phase to the spinel phase induced by the LiVO3 coating. Due to the precondition chemical activation, larger initial discharge capacities of 255.3 and 272 mA h g−1 were maintained for 2 and 5 wt % LiVO3 coated samples respectively, compared with 249.8 mA h g−1 for the pristine sample. However, the excess LiVO3 coating (10 wt %) leads to more severe polarization with a much lower discharge potential, which is ascribed to higher transmission resistance for lithium ions. To investigate the effects of the LiVO3 coating on the cycling stability, the pristine and LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2 were cycled at different charge− discharge current rates. As demonstrated in Figure 5a, the discharge capacity of the pristine Li1.2Mn0.54Ni0.13Co0.13O2 at 0.1 C drops to 197.8 mA h g−1 after 80 cycles with the capacity retention of 78.7%. For 2 and 5 wt % LiVO3 coated samples, the discharge capacities at 0.1 C show a more stable trend after initial several cycles. Finally, 2 wt % LiVO3 coated sample retains a discharge capacity of 219.8 mA h g−1 with 84.2% of the initial discharge capacity while 5 wt % LiVO3 coated sample delivers a discharge capacity of 246.8 mA h g−1 with a retention rate of 90.3%. Significantly enhanced cycling stability of LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2 can be attributed to the protection of coating layer which prevents the inner active material from reacting with acidic species in the electrolyte. However, only 184 mA h g−1 is obtained for 10 wt % LiVO3 coated sample after cycling, due to the surface damage after excessive coating. At a higher charge−discharge current rate of 1 C (shown in Figure 5b), the 5 wt % LiVO3 coated sample still delivers a high discharge capacity of 215 mA h g−1 with an excellent cycling stability, superior to other samples. The rate capability further highlights the advantage of the LiVO3 coating with a weight ratio of 5 wt %, as presented in Figure 5c. The 5 wt % LiVO3 coated sample exhibits a discharge capacity of 245 mA h g−1 at 0.5 C, 181 mA h g−1 at 2 C, 156 mA h g−1 at 3 C, 135 mA h g−1 at 5 C and 111 mA h g−1 at 10 C. By contrast, the discharge capacities of the pristine sample at 0.5, 2, 3, 5, and 10 C are only 215.6, 136.2, 100, 65, and 31.2 mA h g−1, 260

DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263

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Figure 8. XPS O 1s spectra of pristine (a) and 5 wt % LiVO3 coated (b) Li1.2Mn0.54Ni0.13Co0.13O2 after 100 cycles and TEM images of pristine (c) and 5 wt % LiVO3 coated (d) Li1.2Mn0.54Ni0.13Co0.13O2 particles after 100 cycles.

spectra reflect two oxygen environments: the peak at 529.3 eV is assigned to oxide ion (O2−) in the pristine crystal lattice whereas the peak at 531.9 eV corresponds to carbonate species (CO32−) derived from side reaction.32,38,39 It can be observed that there is a significant amount of Li2CO3 on the surface of the pristine electrode while the 5 wt % LiVO3 coated electrode shows a much smaller amount of Li2CO3 on the surface after long-term cycling. Thus, the stable electrode−electrolyte interface of the coated sample can be partly attributed to the buffer effect of the surface LiVO3 layer which reduces the activity of extracted oxygen species and consequent electrolyte oxidation. The interface structure changes upon cycling were further monitored by TEM and the images of the cycled electrodes are shown in Figure 8c and d. Many microcracks are observed at the surface of the pristine Li1.2Mn0.54Ni0.13Co0.13O2 particles, as arrowed in Figure 8c, which originate from the large lattice strain due to serious structural evolution and the erosion from acidic species in the electrolyte.40,41 On the contrary, the LiVO3 coated electrode has a clear grain edge without the presence of microcracks, as exhibited in Figure 8d. These results indicate that the LiVO3 coating with the appropriate amount can effectively suppress the deterioration of the particle surface during long-term cycling, leading to a slower increase of interface charge transfer resistance and obviously improved cyclic stability. Based on the above experimental results and discussion, we proposed a schematic diagram for the reaction mechanism of LiVO3 surface modification, as illustrated in Figure 9. First, reagents are coated on the surface of the pristine Li1.2Mn0.54Ni0.13Co0.13O2. During the process of heat treatment, the reaction of reagents induces the extraction of lithium ions from the surface region of electrode materials. Subsequently, the atomic rearrangement in the surface region leads to the formation of the spinel structure with fast three-dimensional Li+ diffusion channels. Since the spinel phase can share the oxygen

All the EIS curves consist of a suppressed semicircle in the high-frequency range and a sloping line in the low-frequency region. The suppressed semicircle results from the overlap of two different semicircles, with a high-frequency semicircle related to the resistance (Rsl) of the solid electrolyte interface (SEI) and an intermediate-frequency semicircle corresponding to charge transfer resistance (Rct). Additionally, intercept on the x-axis in the highest-frequency region represents the resistance to the lithium ion conduction in the electrolyte (Rs) while the sloping line in the low-frequency is related to the diffusion kinetics of lithium ion in the bulk of the electrode. Accordingly, the experimental results were fitted with the equivalent circuit in Figure 7c and the fitting results are listed in Table S1. The increase of Rsl and Rct values during cycling is expected because of the growth of the SEI layer and the deterioration of the particle surface. After initial 5 cycles for full electrochemical activation, the Rct value of the 5 wt % LiVO3 coated electrode is much lower than that of the pristine electrode, attributed to the LiVO3 coating layer which facilitates the lithium ion transfer at the surface of particles. Upon subsequent charge−discharge cycling, the growth of the Rct value is also slower for the 5 wt % LiVO3 coated electrode than that for the pristine electrode. After 100 cycles, the Rct value of the pristine electrode increases to about four times of the initial value while that of the 5 wt % LiVO3 coated electrode increases to only twice of the initial value. Furthermore, the increase of the Rsl value has been significantly suppressed by LiVO3 coating. These results indicate that the degradation of the electrolyte/electrode interface and the growth of the undesired SEI layer have been alleviated by LiVO3 coating, which leads to significant improvement in cyclic stability and rate capability. In order to further understand the improvement of the electrolyte/electrode interface after LiVO3 coating, the surface compositions of both electrodes after cycles were studied via XPS measurements, as illustrated in Figure 8a and b. The O 1s 261

DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263

Research Article

ACS Sustainable Chemistry & Engineering

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Figure 9. Schametic diagram of LiVO3 surface modification on Li1.2Mn0.54Ni0.13Co0.13O2.

framework with the layered phase, they are structurally compatible in the grain with a continuous transition from the surface to the bulk. Meanwhile, the particles are further coated by a lithium-ion conductive LiVO3 layer, which also stabilizes the interface between the cathode and the electrolyte upon cycling.

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CONCLUSION A convenient strategy has been developed to modify the surface of Li-rich Mn based cathode materials with LiVO3 coating. Besides the lithium-ion conductive coating layer, a stable spinel structure with facile lithium-ion diffusion channels is simultaneously generated in the surface regions. The improved electrochemical performance of LiVO3 modified Li-rich Mnbased cathode can be attributed to the enhanced ionic conductivity and suppressed deterioration of the particle surface. Meanwhile, the formation of the stable spinel structure is beneficial to retarding the undesired phase transformation in the grain, leading to the mitigation of voltage decay during cycles. The results of this study provide new insight into surface modification on Li-rich Mn-based layered oxides, which can be used to achieve large-scale commercial application for Li-rich Mn-based layered oxides as appealing cathode materials of high-energy lithium ion batteries.

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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/acssuschemeng.5b01083. EDX mapping of 5 wt % LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O2, XPS spectra of pristine and 5 wt % LiVO3 coated Li1.2Mn0.54Ni0.13Co0.13O, and simulated impedance parameters of the equivalent circuit for EIS plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the partial financial support from 973 Program (2013CB934103) and Science & Technology Commission of Shanghai Municipality (12dz1200402 & 08DZ2270500), China.

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REFERENCES

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DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263

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DOI: 10.1021/acssuschemeng.5b01083 ACS Sustainable Chem. Eng. 2016, 4, 255−263