Coprecipitation-Gel Synthesis and Degradation Mechanism of

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 23018−23028

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Coprecipitation-Gel Synthesis and Degradation Mechanism of Octahedral Li1.2Mn0.54Ni0.13Co0.13O2 as High-Performance Cathode Materials for Lithium-Ion Batteries Wenxiang He,†,§ Jianguo Liu,*,†,‡,∥ Wei Sun,*,§ Wuwei Yan,‡,∥ Liang Zhou,‡,∥ Congping Wu,‡,∥ Junsheng Wang,§ Xinliang Yu,§ Haimin Zhao,§ Tianren Zhang,§ and Zhigang Zou†

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†

Jiangsu Key Laboratory for Nano Technology, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, 22 Hankou Road, Nanjing 210093, China ‡ Kunshan Sunlaite New Energy Co., Ltd., Kunshan Innovation Institute of Nanjing University, Kunshan, 1699# South Zuchongzhi Road, Suzhou 215347, China § R&D Department, Zhejiang Tianneng Energy Technology Co., Ltd., Changxing 313100, Zhejiang, China S Supporting Information *

ABSTRACT: The octahedral core−shell Li-rich layered cathode material of Li1.2Mn0.54Ni0.13Co0.13O2 can be synthesized via an ingenious coprecipitation-gel method without subsequent annealing. On the basis of detailed X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and electron energy loss spectroscopy characterizations, it is suggested that the as-prepared material consists of an octahedral morphology and a new type of core− shell structure with a spinel-layered heterostructure inside, which is the result of overgrowth of the spinel structure with {111} facets on {001} facets of the layered structure in a single orientation. The surface area of Li1.2Mn0.54Ni0.13Co0.13O2 crystals where the spinel phase is located possesses sufficient Li and O vacancies, resulting in the reinsertion of Li into position after the first charge and maintenance of the interface stability via the replenishment of oxygen from the bulk region. Compared to that synthesized by the traditional coprecipitation method, the Li1.2Mn0.54Ni0.13Co0.13O2 synthesized by the coprecipitation-gel method exhibits higher discharge capacity and Coulombic efficiency, from 73.9% and 251.5 mAh g−1 for the spherical polycrystal material to 86.2% and 291.4 mAh g−1. KEYWORDS: Li-rich layered cathodes, Li-ion batteries, octahedral, coprecipitation-gel methods, spinel phase

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materials.14 However, these methods cannot solve the problem of initial loss of oxygen from the crystal lattice during the first charging process corresponding to a phase transition, resulting in a large irreversible capacity loss and poor rate capability, thereby preventing the material from being used in mass applications. Armstrong et al. proposed that the irreversible capacity loss of Li-rich layered materials could be attributed to O loss from the surface accompanied by diffusion of transitionmetal (TM) ions from the surface to the inner part of the lattice.15 Therefore, much research has been conducted to solve the problem of Li-rich layered materials, such as materials compositing,16 surface modification,17 coating,18 etc.19,20 As an effective surface modification approach, introducing a spinel phase into the crystal lattice can result in the retention of large numbers of O vacancies, which can reduce the irreversible

INTRODUCTION

With increasing awareness of the need for environmental protection and the reinforcement of environmental legislation, the demand for green energy resources is on the rise. Thus, new-energy vehicles have attracted much more attention than ever before. As the key component in electric vehicles, batteries are very important vis-a-vis the driving distance. Therefore, it is of great significance to develop a battery with a high energy density; however, the capability of cathode materials has become the bottleneck. Li-Rich layered cathode materials have become the most promising of various cathode materials for future generations of lithium-ion batteries due to their high capacity.1−3 Various methods have been used to prepare Lirich layered materials, including the sol−gel method,4,5 coprecipitation method,6,7 combustion method,8,9 hydrothermal method,10,11 and so on.12,13 The preparation methods have significant effects on the composition, homogeneity, morphology, and size of the materials. Subsequently, they strongly affect the electrochemical properties of cathode © 2018 American Chemical Society

Received: March 11, 2018 Accepted: June 18, 2018 Published: June 18, 2018 23018

DOI: 10.1021/acsami.8b04023 ACS Appl. Mater. Interfaces 2018, 10, 23018−23028

Research Article

ACS Applied Materials & Interfaces

Figure 1. SEM images of the precursor (a, b), C-LMNCO (c, d), and CG-LMNCO (e, f).

capacity loss of materials in the first cycle.21−23 Qiu et al. created oxygen vacancies to form a spinel/rock-salt-like phase on the surface of Li-rich layered oxides, which can provide a favorable ionic diffusion path in the bulk structure and suppress gas release from the surface. They concluded that the “Li−Li dumbbell” can be broken by introducing oxygen vacancies. The trapped Li ions have a higher chance of escaping from the tetrahedral sites and continuing their diffusion process.24 This is an effective and insightful theory to explain the increased performance during the initial stage. However, most of these reports involved tedious synthesis processes and impurity phases that are easily introduced into the bulk structure via subsequent modification processes,25 in which utilization of a reducing atmosphere23 or leaching with acid accompanied by heat treatment is attempted.26,27 Most xLi2MnO3·(1 − x)LiMO2 and LiMO2 (M = Ni, Co, Mn) layered materials with perfect structures commonly need to be calcined in air or in pure oxygen at high temperatures. The undesirable resulting diminished rate capability and cycling stability are inevitable if there is any deviation and inaccuracy during the synthesis process. Under these conditions, acid or carbon formed from any added organic precursors in a posttreatment step is likely to corrode or reduce the elemental metals,28 which could destroy the original stable structure of the material and diminish the electrochemical capability. To reduce this negative effect on the structure of the material, a novel and well-designed method that leads to uniform spinel phase growth on the surface of contemporaneous Li-rich layered oxides has been proposed. Finally,

Li1.2Mn0.54Ni0.13Co0.13O2 consisting of an octahedral shape, as well as a type of core−shell structure with a spinel-layered heterostructure inside, was synthesized. In this paper, the combined approach of the coprecipitation and sol−gel methods is first presented to prepare high-performance Lirich layered materials with a spinel-layered heterostructure. Most importantly, the as-prepared cathode material delivers a much higher specific capacity and enhanced cycling stability in comparison with general Li-rich layered cathode materials. Meanwhile, the degradation model for understanding the layered → spinel → rock-salt phase transformation in layered oxide cathodes during electrochemical cycling is widely acknowledged,29,30 but few studies have explored how this special spinel-layered heterostructure affects the inner structural transformation of Li-rich cathode materials during cycling. Therefore, the effect of the introduced spinel phase on the degradation mechanism of the as-prepared materials was investigated using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analyses. A rare “anti-core−shell phase transformation” was first observed in the as-prepared Li-rich layered materials. This abnormal phenomenon provides a new way to explore the role of the spinel-layered heterostructure in the degradation mechanism of Li-rich cathode materials.

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EXPERIMENTAL SECTION

Preparation of [Mn0.675Ni0.1625Co0.1625]CO3 Precursor. The [Mn0.675Ni0.1625Co 0.1625]CO3 precursor was synthesized via a coprecipitation route in a glass reaction vessel with the addition of 2 M sodium carbonate solution to a 2 M mixed solution of manganese 23019


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Figure 2. XRD patterns of the precursor, C-LMNCO and CG-LMNCO samples (a), enlarged views (b, c), and Rietveld refinement results (d, e) of C-LMNCO and CG-LMNCO, respectively. sulfate monohydrate, nickel sulfate hexahydrate, and cobalt sulfate heptahydrate (metal ratio of Mn/Ni/Co = 0.54:0.13:0.13) at 60 °C with continuous magnetic stirring. At the same time, an appropriate amount of ammonia solution was also fed into the reactor as a chelating agent and the pH of the reaction system was fixed at 7.8. The precipitated mixed carbonate of [Mn0.675Ni0.1625Co0.1625]CO3 was filtered and washed with deionized water thoroughly and then dried at 120 °C for 10 h. Preparation of Li-Rich Layered Materials Li1.2Mn0.54Co0.13Ni0.13O2. The as-prepared [Mn0.675Ni0.1625Co0.1625]CO3 was ground with lithium hydroxide in a molar ratio of Li/Mn of 1.2:0.54 and then this mixture was dispersed in ethanol and ball milled for 2 h at the speed of 300 rpm; the mass ratio of balls to powder was 10:1. After suitable grinding, the mixture was precalcined at 450 °C for 6 h. Finally, the resulting powders were calcined at 900 °C in air for 10 h under a heating rate of 2 °C min−1. The sample was labeled C-LMNCO. The coprecipitation-gel method was carried out as follows. First, stoichiometric amounts of [Mn0.675Ni0.1625Co0.1625]CO3 were dispersed in deionized water. Then, lithium hydroxide with citric acid (CA) was also dissolved in distilled water. The molar ratio of Li/Mn/ CA was 1.2:0.54:1. Second, the mixed solution of lithium hydroxide and CA was added slowly into the suspension prepared above under vigorous stirring. At the same time, the pH of the suspension was regulated from 7.5 to 8.0 with ammonia solution. Third, the suspension was heated at 80 °C with continuous stirring until a sol−gel was obtained. Then, the dried precursor was annealed in air under the same conditions described above. The sample was labeled CG-LMNCO. Materials Analysis Techniques. X-ray powder diffraction (XRD) characterization of the samples was performed on a Japan Rigaku D/MAX2500V diffractometer using Cu Kα radiation. Rietveld refinement of the collected XRD data was performed using the GSAS/EXPGUI software. The chemical compositions of the samples

were measured by a PerkinElmer OPTIMA 8000 inductively coupled plasma optical emission spectrometer (ICP-OES). The morphologies of the samples were investigated using a Helios 600i dual beam scanning electron microscope (SEM). TEM and EELS measurements of the samples were carried out on a TECNAI F20 field emission transmission electron microscope equipped with a GIF 200 electron energy loss spectroscopy spectrometer. Cell Fabrication and Characterization. An assemble CR2032 coin cells were used for testing the electrochemical performances of the as-prepared materials. The positive electrodes were manufactured by mixing the as-prepared materials, Super P and poly(vinylidene fluoride) (8:1:1 in weight) dissolved in an appropriate amount of Nmethyl-2-pyrrolidone and coated on Al foil circular flakes. Then, the electrodes were dried at 120 °C for 10 h. Li-Metal foil was used as negative electrodes. Electrodes of the samples with the same thickness were obtained under the same pressure, and the loading density of the cathode electrodes was approximately 15 mg cm−2. The separator was a microporous polypropylene membrane (W-SCOPE 20UM). The electrolyte was composed of 1 M LiPF6 in 1:1 (in volume) ethylene carbonate and diethyl carbonate electrolyte. The coin cells were assembled in a glovebox filled with highly pure argon gas, where the O2 and H2O levels were less than 0.1 ppm. Charge/discharge tests of the coin cells were conducted using the LANHE CT2001A battery testing system at 25 ± 2 °C after 10 h of standing. Electrochemical impedance spectroscopy (EIS) measurements were carried out on a CHI-660B electrochemical workstation in the frequency range between 100 kHz and 0.01 Hz with an amplitude of the input alternating current signal of 5 mV.

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RESULTS AND DISCUSSION Morphology Characterization by Scanning Electron Microscopy. Figure 1a,b shows the SEM images of precursor particles of [Mn0.675Ni0.1625Co0.1625]CO3 under different 23020


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Field Emission Transmission Electron Microscopy. To understand the interior structure of Li1.2Mn0.54Ni0.13Co0.13O2 after introducing the spinel phase, CG-LMNCO and CLMNCO were analyzed by high-resolution TEM (HRTEM) and EELS measurements, and the results are displayed in Figure 3. Figure 3a is a low-magnification TEM view showing a representative pristine C-LMNCO microparticle, and its selected area electron diffraction (SAED) along the [11̅0]L zone axis of the layered structure shown in Figure 3b confirms that it is the layered phase, with no diffraction spots from other phases observed (at the region marked by a red circle in Figure 3a). Local structure information extracted from the HRTEM image area marked by red boxes using the fast Fourier transform (FFT) is shown in Figure 3c. As displayed, the lattice arrangement on the surface of the C-LMNCO sample is the same as that in the bulk structure. The lattice fringes of one plane with a spacing of 0.475 nm coincide with the interplanar distances of the (003) plane of Li1.2Mn0.54Ni0.13Co0.13O2.14,3514,35 In contrast, the lattice arrangement in the surface domain of the CG-LMNCO sample is completely different from that in the bulk structure according to the TEM results, as shown in Figure 3d−f. The SAED image of the CG-LMNCO sample (Figure 3e), for which the zone axis applied is [010]L, shows two different diffraction spots. This finding presents clear evidence of the coexistence of layered and spinel structures.36 The weak spots indicated by the red lines and arrows in Figure 3e correspond to a spinel configuration along the [41̅1̅]S zone axis of the spinel structure. By magnifying the region of Figure 3d marked by a red circle, the HRTEM image (Figure 3f) shows a few layers of the structure with structural orientations in the surface region (∼2 nm depth), which is different compared to the layered phase in the bulk structure. The FFT pattern from the surface region confirms a spinel component, whereas the FFT pattern from the inner region shows only the reflections from the layered structure, as supported by our XRD results. The presence of a spinel-like phase on the surface of the layered bulk structure is easily detected in another region, as shown in Figure S1 (Supporting Information). The results prove that a core−shell structured Li1.2Mn0.54Ni0.13Co0.13O2 material with a spinel-layered heterostructure was successfully synthesized. It is well established that a spinel phase forms on the surface of Li-rich layered material under a certain growth mechanism, so understanding the mechanism of crystal growth and the inherent structure of as-prepared material is necessary. HRTEM and SAED images along the [001]L zone axis are shown in Figure S2 (Supporting Information). The results further demonstrate an overgrowth of the spinel structure on {001}L facets of the layered structure with {111}S facets, and these two phases share a similar 2-fold symmetry with a [010]L//[41̅1̅]S parallel orientation. It is reasonable to deduce that such a highly ordered LixM2O4 spinel phase growing on {001}L facets of the layered structure is attributed to the structural compatibility in view of the (003)L and (111)S lattice fringes. It is very strange that both the sol−gel and general coprecipitation methods do not trigger a similar integrated LiMO2−LiM2O4 (M = Mn, Co, and Ni) heterostructure. Therefore, the growth mechanism of CG-LMNCO under different temperatures was studied through a combination of XRD and SEM and the results are shown in Figure S3 (Supporting Information). These results indicate that a tetragonal spinel phase is introduced in the early stages of

magnifications. The precursor particles have a spherical and dense morphology with particle sizes in the range of 15−20 μm. By examining the highly magnified images of the precursor and C-LMNCO in Figure 1c,d (marked with red boxes in Figure 1a,c), it is concluded that the spherical morphology of the precursor is preserved after calcination to produce CLMNCO. On the other hand, an observed difference is that the CO2 emission and O2 absorption from the thermal decomposition of the precursor results in the porous and loose morphology of C-LMNCO. However, as shown in Figure 1e,f, the spherical morphology disappears, and instead, an octahedral crystal structure with smooth edges appears for CG-LMNCO. This type of morphology of Li-rich layered material is rarely reported in the literature.31 Powder X-ray Diffraction. Figure 2a displays the XRD patterns of the precursor, C-LMNCO and CG-LMNCO samples. The XRD pattern (black curve) of the precursor shows a pure phase corresponding to MCO3 (M = Mn, Ni, and Co; JCPDS no. 44-1472). The strong intensity of these diffraction peaks indicates the high crystallinity and perfect crystals of the precursor. The XRD patterns of C-LMNCO and CG-LMNCO can be indexed to the α-NaFeO2 structure with the R3̅m space group. The inserted XRD pattern in the top right corner presents weak peaks between 20 and 25° for CLMNCO and CG-LMNCO, respectively, which can be indexed to the monoclinic structure (C2/m). This can be ascribed to the existence of a Li2MnO3 phase and the cation ordering of Li and TM cations (M = Mn, Ni, and Co) in the TM layers. Moreover, clear splits of the (006)/(102) and (108)/(110) peaks of the XRD patterns indicate a layered structure, such as LiCoO2, and good crystallinity. However, the unexpected and weak peaks at 19.1° (Figure 2b) and 36.8° (Figure 2c), which are similar to the diffractions from (111) and (311) in the LixMn2O4 spinel structure, indicate the presence of a spinel phase in CG-LMNCO crystals.21 Figure 2d,e and Table 1 demonstrate the XRD refinement patterns Table 1. Lattice Parameters of C-LMNCO and CG-LMNCO sample

R(003)/(104)

a (Å)

c (Å)

c/a

Vcell (Å3)

C-LMNCO CG-LMNCO

1.750 1.327

2.8467 2.8523

14.2194 14.2454

4.9950 4.9943

99.791 100.371

and refined lattice parameters of C-LMNCO and CGLMNCO, respectively. The high c/a ratios (greater than 4.98) of the two samples indicate a good layered structure.32 However, it is worth noting that the integrated intensity ratios of R(003)/(104) for the C-LMNCO and CG-LMNCO samples are 1.750 and 1.327, respectively, suggesting cation mixing of TM (M = Mn, Ni, and Co) in Li-layer sites4,33 for the CGLMNCO sample. This can be attributed to the TM cations migrating to octahedral sites in the Li layer and then occupying the Li sites of CG-LMNCO. In addition, the cell volume of CG-LMNCO is slightly higher than that of C-LMNCO, which indicates that a larger lattice volume change occurs because of the presence of vacancies in the Li layer.34 The abovementioned results indicate that a spinel phase has been successfully introduced into the Li-rich layered material. All of the measured TM compositions (Mn/Ni/Co) of the precursor, C-LMNCO and CG-LMNCO samples, by inductively coupled plasma optical emission spectroscopy (ICP-OES) were close to 0.54:0.13:0.13; the analysis is shown in Table S1. 23021


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Figure 3. TEM image (a), SAED pattern (b), and HRTEM image (c) of C-LMNCO. TEM image (d), SAED pattern (e), and HRTEM image (f) of CG-LMNCO. EELS spectra obtained from the surface to the bulk structure of C-LMNCO (g) and CG-LMNCO (h). Mn L3, L2 energy, and L3/ L2 intensity ratio of C-LMNCO (i) and CG-LMNCO (j). 23022


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Figure 4. Charge/discharge patterns and corresponding dQ/dV plots of C-LMNCO (a, c) and CG-LMNCO (b, d); the cyclic performance comparison of C-LMNCO (e) and CG-LMNCO (f).

the bulk structure to the surface of C-LMNCO and CGLMNCO (represented by the blue boxes in Figure 3a,d, respectively). As shown in Figure 3g,h, the O K edge profiles and Mn L3 and L2 profiles of C-LMNCO are not altered much over the entire region. However, the peaks of the O K edge and Mn L3 and L2 become gradually weaker and shift to a lowerenergy region from the bulk structure to the surface for CGLMNCO, which can be ascribed to the reduced O 2p hybridization with TM 3d levels and the reduction of the Mn oxidation state, respectively.37 The oxidation state of Mn ions can be estimated from the integrated intensity ratio of the L3 and L2 edge peaks. As shown in Figure 3i,j, the intensity ratio of L3/L2 for CG-LMNCO in the surface domain, approximately 10 nm from the interface, is greater than 2.5, thus indicating the existence of Mn3+ compounds. Therefore, as confirmed by both the TEM and EELS results, Li and O vacancies have been successfully introduced on the surface without a noticeable interruption in the bulk structure through the coprecipitation-gel approach. As shown in the structure

synthesis under the effect of CA combined with adsorption and decomposition, with the material undergoing the reaction MnCO3 + O2 → Mn3O4. Finally, a small amount of residual spinel phase is preserved on the surface of crystals with a high percentage of exposed {111} S facets because of the thermodynamic stability of spinel Mn3O4. Coincidently, the burning carbon produces a large amount of heat accompanied by emission of CO2, removing O and Li from the surface structure of immature particles, which generates a sufficient number of vacancies for Li and O. Mn, which should be in the octahedral sites of the TM layer, migrates to octahedral sites in the Li layer to form a spinel framework on the surface of the layered structure. On the basis of the XRD and SEM results above, the mechanism of an octahedral core−shell structure forming in layered oxide cathodes through the coprecipitationgel method can be well explained. The EELS analysis also supports the explanation described above. To explore the electronic states of oxygen and TM from these samples, EELS was collected at different positions from 23023


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Figure 5. EELS mapping images of C-LMNCO (a) and CG-LMNCO (d), HRTEM images of C-LMNCO (b) and CG-LMNCO (e), and SAED patterns of C-LMNCO (c) and CG-LMNCO (f) after 10 cycles.

modeling diagram for the CG-LMNCO sample presented in Figure 3d, when sufficient numbers of Li vacancies are formed in the Li layer, the Mn in the octahedral sites of the TM layer migrates to octahedral sites in the Li layer to form a spinel framework on the surface of CG-LMNCO oxides during the synthesis process.34 Electrochemical Properties. Figure 4a,b shows the charge/discharge curves of C-LMNCO and CG-LMNCO, respectively, charged and discharged at a rate of 0.1C (20 mA g−1) in the voltage range of 2.0−4.8 V. The initial charge curves of these two samples show a typical and similar slope potential region (3.7−4.5 V) and a plateau potential region (>4.5), which is related to the oxidation of Ni2+ and Co3+ to the oxygen loss, respectively.38 In the first cycle, C-LMNCO can realize a charge and discharge capacity of 340.5 mAh g−1 and 251.5 mAh g−1, respectively, corresponding to an initial Coulombic efficiency of 73.9%. However, CG-LMNCO exhibits a higher charge capacity of 338.1 mAh g−1 and a discharge capacity of 291.4 mAh g−1 in the 1st cycle, corresponding to an initial Coulombic efficiency of 86.2%, and good rate capabilities for CG-LMNCO cells are also obtained. The CG-LMNCO sample exhibits a higher capacity than that of the C-LMNCO sample at rates of 0.2, 0.5, 1, 5, and 10C. More details about the rate capabilities of CLMNCO and CG-LMNCO can be found in Figure S4 (Supporting Information). The dQ/dV plots corresponding to the charge/discharge curves of C-LMNCO and CG-LMNCO in the 1st, 2nd, 20th, 50th, and 100th cycles are shown in Figure 4c,d, respectively. During the oxidation process in the initial cycle (1st and 2nd) for both samples, the two strong peaks located at approximately 4.0 and 4.5 V can be ascribed to the oxidation of Ni2+/Co3+ in the layered LiMO2 (M = Mn, Ni, and Co) phase and the loss of Li2O from the Li2MnO3 phase, respectively. Correspondingly, the gentle reduction peaks ranging from 4.5 to 3.0 V are assigned to the reduction of Ni4+/Co4+ and MnO2 during the following cathodic process.39

However, as shown in the partial enlarged view within Figure 4c, a weak reduction peak located at approximately 2.7 V emerges in subsequent cycles (20th, 50th, and 80th), which can be attributed to surface lattice degradation of C-LMNCO crystals induced by the transition-metal migration. This result indicates that the well-ordered layered structure has transformed into the disordered spinel/rock-salt phase accompanied by the degradation of the electrochemical performance of the cathode materials.40 By comparison, the reduction peak can be observed in CG-LMNCO approximately 2.5 V during the initial cycle (1st and 2nd), which is believed to be due to reinsertion of Li into the spinel LiMn2O4 structure.41 This suggests that LiMnO4 allows the reinsertion of redundant Li ions to form Li2Mn2O4 when a proper potential is applied. However, the peak vanishes in followed cycles and a new higher reduction potential gradually emerges at approximately 2.7 V from the 50th cycle (the partial enlarged view within Figure 4d). This interesting observation may be explained by an interaction between surface region and the inner bulk that differs from C-LMNCO during cycling. Finally, by contrasting the dQ/dV plots corresponding to the 100th cycle, both of the samples are quite similar since one gentle reduction peak ranging from 4.5 to 3.0 V transforms into two separate reduction peaks at approximately 2.7 and 2.9 V. This result indicates that regardless of whether the structure is the optimized spinel-layered heterostructure or the well-ordered layered structure, it tends to transform into a similar irreversible structure as a result of its intrinsic thermodynamic properties42,43 but the former has the effect of restraining the phase transformation. In consideration of the stability of the electrolyte, CLMNCO and CG-LMNCO were cycled at a rate of 0.5C in two voltage ranges of 2.0−4.8 and 2.5−4.4 V. The results are shown in Figure 4e,f. In the voltage range of 2.0−4.8 V, the initial discharge capacity of CG-LMNCO was 246.0 mAh g−1, higher than the value of 223.8 mAh g−1 for C-LMNCO. After the 100th cycle, 220.9 and 179.8 mAh g−1 of retained discharge 23024


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Figure 6. Schematic illustration of the formation processes and reaction mechanism of Li1.2Mn0.54Ni0.13Co0.13O2 synthesized by the coprecipitationgel method.

introduced spinel phase could lead to the acceleration of performance degradation of the as-prepared material. Although we fully understand that introducing a spinel phase into a layered structure to form a spinel-layered heterostructure can improve the discharge capacity and Coulombic efficiency of Lirich layered materials, there is controversy about what the mechanism for improvement of other electrochemical properties is, especially for the cycle life.21,24,25,45 To better understand the electrochemical behavior of the octahedral core−shell structure, we further investigated C-LMNCO and CG-LMNCO samples after 10 cycles in the range of 2.0−4.8 V at 0.1C by TEM and EELS analysis and the results are shown in Figure 5. Figure 5a presents EELS mapping of the local region of the C-LMNCO part after 10 cycles, showing the Mn, Ni, Co, O, and color-mix mapping of the domain from the bulk structure to the surface. According to the oxygen map, oxygendeficient areas are presented from the bulk structure to the surface for C-LMNCO, whereas the other elements’ contents in the entire region remain almost unchanged. Figure 5b,c shows a HRTEM image and SAED views along [11̅ 0]L corresponding to the oxygen-deficient areas, revealing the extensive formation of the spinel phase from the surface to the bulk structure. A very thin layer of rock-salt-like phase and amorphous phase is observed on the nonuniform surface of the domain marked by a blue dotted line, which can be attributed to full occupancy of the Li sites by TM and residual poly(vinylidene difluoride) binder, respectively. The SAED image of the C-LMNCO sample along the [11̅0]L/[2̅10]S zone axis also shows two different diffraction spots, which can be separated into two different identified phases. In contrast, the O map of the CG-LMNCO sample in Figure 5d shows a narrow and dark belt from the surface approximately 20 nm wide, which indicates a decrease of O concentration within this region. According to the HRTEM and SAED views along [55̅1]L shown in Figure 5e,f, the core−shell structure of the spinel phase around the surface of pristine CG-LMNCO

capacities corresponding to CG-LMNCO and C-LMNCO, respectively, could be obtained, which were 89.8 and 80.3% of their initial discharge capacities. In the voltage range of 2.5− 4.4 V, the initial discharge capacities of CG-LMNCO and CLMNCO were 196.7 and 175.6 mAh g−1, respectively. At the end of the 100th cycle, the capacity retention was 94.2 and 86.3% of their initial discharge capacities. Comparing the cyclic performances of the two samples in the different voltage ranges, both had better cyclic performance in the range of 2.5− 4.4 V because of electrolyte and cathode stability at low anodic potentials. The results above indicate that octahedral Li1.2Mn0.54Ni0.13Co0.13O2 synthesized by the coprecipitationgel method has better electrochemical performance than that synthesized by the coprecipitation method and certainly better than that synthesized by the sol−gel method previously reported.5,9,44 The performance benefits from the octahedral core−shell structure Li1.2Mn0.54Ni0.13Co0.13O2 with a spinellayered heterostructure, which can retain a sufficient number of O vacancies in the lattice and allow Li ions to reinsert back into position to improve the Coulombic efficiency of the material after the first charge. EIS results (Figure S5 and Table S1, Supporting Information) further verify that more Li vacancies in the CG-LMNCO sample are created at the electrode/electrolyte interface compared to the C-LMNCO sample, as supported by the above results. This also indicates reduced polarization and quicker transfer of Li ions on the interface of the electrode/electrolyte for the CG-LMNCO sample. Mechanism Analysis. According to the reported “layered → spinel → rock-salt” phase transformation degradation model, the layered phase tends to transform into an ionically insulating rock-salt phase with deintercalation and intercalation of Li ions during charging and discharging. The spinel phase as a metastable structure can reduce the energy barrier for the phase transformation, offering a kinetically feasible pathway for the layered → rock-salt transformation.43 This implies that the 23025


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failure of materials caused by interface stress, which is associated with the volume change of the crystal upon Li-ion extraction and reinsertion,50 and maintains the stability of the electrode/electrolyte interface.

particles disappeared and was replaced by the layered structure.46 Furthermore, according to the view of the intermediate zone, as shown in the magnified HRTEM image with a red border, the original layered and spinel structure both within nanodomains can be easily identified. This view shows an abnormal phenomenon in which the spinel structure disappears from the surface and emerges again in the adjoining bulk structure. To our knowledge, this anti-core− shell phase transformation was first observed in Li-rich layered materials but several reports about its application have been reported in Zhang 47 and Yan et al.’s 48 study on LiNixCoyMn1−x−yO2 layered materials. The formation of the spinel and rock-salt domain requires the removal of oxygen from the layered phase.36 Therefore, the formation of the spinel phase in the bulk structure while retaining the surface as the layered phase requires the outward diffusion of oxygen anions from the bulk structure to the surface region. The oxygen loss at pristine surface regions is dynamically replenished from the bulk region, which, accordingly, results in a lower oxygen concentration in the bulk structure while relatively maintaining the oxygen content in the surface regions. The EELS analysis of C-LMNCO and CG-LMNCO samples after 20th cycles in the range of 2.0−4.8 V at 0.1C also supports the conclusion described above, and the results are shown in Figure S6. On the basis of the above analysis, the proposed reaction mechanism of the structural transformation process of CGLMNCO synthesized by the coprecipitation-gel method can be explained. A schematic illustration of the crystal formation and degradation mechanism of CG-LMNCO is presented in Figure 6. In the precipitation stage, the concentrations of TM ions can be precisely controlled by the coprecipitation method to ensure the final homogeneity of the TM in the Li1.2Mn0.54Ni0.13Co0.13O2 structure. In the later stage, the Li ions are dispersed in the organic gel and become inserted into the structure of the precursor uniformly, thus making it easier for Li ions to enter into the structure of Li 1.2Mn 0.54Ni 0.13Co 0.13O 2 at 3a sites via a subsequent calcination process.49 Under dual promotion of heat and CA, more stable Mn3O4 is formed and a sufficient number of vacancies for Li and O are generated within the same region, leading to the formation of a spinel structure in the surface region of the layered structure, which is composed mainly of {111}S facets along the [111]S directions. Therefore, octahedral core−shell structure Li1.2Mn0.54Ni0.13Co0.13O2 with a spinel-layered heterostructure was successfully synthesized by the coprecipitation-gel method. The core−shell structure can retain sufficient O vacancies in the lattice and allow Li ions to reinsert back into position to improve the Coulombic efficiency and cycle life of the material after the first charge. Meanwhile, oxygen vacancies provide a favorable ionic diffusion environment in the bulk structure and reduce the partial pressure of oxygen on the surface, resulting in superior electrochemical performance for the CG-LMNCO sample. Furthermore, an anti-core−shell phase transformation in which the spinel structure disappears from the surface and emerges again in the adjoining bulk structure was observed for the first time by TEM−EELS analysis in Li-rich cathode materials. The oxygen loss at pristine surface regions is dynamically replenished from the bulk region, which, accordingly, results in a lower oxygen concentration in the adjoining bulk structure while relatively maintaining the oxygen content in the surface regions. This phase transition can reduce the mechanical

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CONCLUSIONS We have reported a novel method to introduce a spinel phase on the surface of Li-rich layered material using a simple chemical approach to achieve better electrochemical performance, thus avoiding any other modification and coating treatments involving tedious synthesis processes or a reducing/inert atmosphere. By combining the coprecipitation and sol−gel methods, the as-prepared octahedral Li1.2Mn0.54Ni0.13Co0.13O2 with a spinel-layered heterostructure shows a higher discharge capacity, better rate capability, and cycling performance in comparison with the material prepared by the coprecipitation method. On the basis of the structural and kinetic elaboration of crystal growth in the synthesis stage, the facile coprecipitation-gel method offers an effective technique to solve irreversible capacity loss and to prepare Li-rich layered cathode materials with excellent electrochemical properties.

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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.8b04023.

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ICP-OES result of samples, TEM measurements of CGLMNCO, XRD patterns and simultaneous SEM images of CG-LMNCO under different temperatures, rate performances, Nyquist plots, and EELS analysis (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (W.S.). ORCID

Jianguo Liu: 0000-0002-9229-4936 Author Contributions ∥

J.L., W.Y., L.Z., and C.W. contributed equally to this work.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Key Science and Technology project for Zhejiang province (2015C01054), Leading Innovative and Entrepreneur Team Introduction Program (ZWR [2016] No. 1), Natural Science Foundation for Distinguished Young Scholars of Jiangsu Province (BK20150009), and Jiangsu Province Natural Science Foundation (BK20171247 and BK20171245). The authors also thank the support of PAPD of Jiangsu Higher Education Institutions, “Six Talent Peaks Program” of Jiangsu Province and Fundamental Research Funds for the Central Universities, China. 23026


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Coprecipitation-Gel Synthesis and Degradation Mechanism of

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