Spinel Heterostructured Special Morphology Cathode

Oct 10, 2017 - Li-rich material 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 with a layered/spinel heterostructure is synthesized by a simple strategy. On the b...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 11005-11015

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Li-Rich Layered/Spinel Heterostructured Special Morphology Cathode Material with High Rate Capability for Li-Ion Batteries Lanhua Yi,*,† Zhongshu Liu,† Ruizhi Yu,† Caixian Zhao,*,‡ Hongfeng Peng,§ Meihong Liu,† Bing Wu,† Manfang Chen,† and Xianyou Wang*,† †

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Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Xiangtan 411105, P. R. China ‡ College of Chemical Engineering, Xiangtan University, Xiangtan 411105, P. R. China § Gold Shine Energy Material Co., Ltd., Changsha 410211, P. R. China ABSTRACT: Li-rich material 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 with a layered/spinel heterostructure is synthesized by a simple strategy. On the basis of structure and morphology analyses, it is revealed that the as-prepared Li-rich material possesses both porous micronano structure and integral layered-spinel heterostructure. Moreover, the obtained layeredspinel cathode material possesses prominent electrochemical characteristics, especially its rate capability. The initial discharge capacity of the as-prepared material is 269 mAh g−1 with a high Coulombic efficiency of 90.3%. The material delivers discharge capacities of 239 mAh g−1 at 0.5C, 195 mAh g−1 at 5C, and 175.8 mAh g−1 even at 10C. Also, the capacity retention of the cell is still as high as 80% at high current density (5C) after 200 cycles. The addition of spinel can inhibit the collapse of the material structure and voltage fading upon cycling. The 3D spinel Li4Mn5O12 phase in the Li-rich compound could provide a fast Li-ion diffusion pathway and a porous micronano structure which are key parameters for the remarkable excellent electrochemical performance of the as-prepared cathode material. KEYWORDS: Lithium-ion batteries, Cathode material, Layered/spinel heterostructure, Porous-rod-like micronano structure, Electrochemical performance

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INTRODUCTION With the rapid decrease of fossil fuels and the urgent need to resolve the environmental problems associated with fossil energy depletion, global warming, and environmental pollution, renewable energy sources have received a great deal of worldwide attention.1,2 Presently, developing new green power sources is becoming increasingly important.3 Li-ion batteries (LIBs) have the merits of high power density, high energy density, and environmental friendliness, and have been regarded as one of the most promising energy storage devices for developing hybrid electric vehicles and electric vehicles. However, the major issues of their low rate performances and poor cycle life need to be addressed prior to large-scale commercialization applications.4 Improving the properties of the active cathode materials can promote the development of LIBs.5 Li-rich layered oxide materials, xLi2MnO3·(1−x)LiMO2 (M = Mn, Ni, Co, etc. 0 < x < 1), have been regarded as one of the most promising candidates for the next generation cathode materials. They have alreadly attracted significant interest because of their high capacities (>250 mAh g−1) under high operating voltages of above 4.6 V and excellent electrochemical and thermal stabilities.6 However, Li-rich materials have faced new © 2017 American Chemical Society

fundamental challenges of poor rate capability, signficant initial irreversibility capacity loss, and severe decrease in the discharge voltage plateau upon cycling.7 Generally speaking, during the activation of the Li2MnO3 process in the first cycle, the component appears to experience lattice breakdown and phase transformation which could cause the poor rate capability and voltage fading of the materials.8,9 The large irreversibility capacity loss in the first cycle is attributed to the elimination of the oxygen atom and Li+ vacancies from the layered lattice, which decreases the number of sites for insertion and extraction of Li+ in the subsequent cycles.7 Moreover, because of the similar radius of Li+ (0.076 nm) and Ni2+ (0.069 nm), the intensive removal of Li+ causes Li/Ni site-exchange during cycling.10,11 This termed cation mixing can obstruct the channel of Li+ transport, resulting in capacity loss, voltage fading, and poor rate capability.3 Recently, it has been reported that the electrochemical performance of Li-rich materials can be improved by surface modification and element doping.12 Coating with oxides (ZnO, Received: August 21, 2017 Revised: October 5, 2017 Published: October 10, 2017 11005

DOI: 10.1021/acssuschemeng.7b02906 ACS Sustainable Chem. Eng. 2017, 5, 11005−11015

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic diagram for the synthesis of PR-LLNCM.

ZrO2, and Al2O3),9,13 fluoride (AlF3)14 and phosphate (FePO4),15,16 conductive polyaniline,17 and so forth have been regarded as the most effective approaches to enhance the electrochemical performance of the Li-rich material, including capacity, rate capability, Coulombic efficiency, and cycling stability.18 In addition, the protective layer can reduce the side reactions of the electrode/electrolyte interface, suppress the appearance of the solid electrolyte interface film, and protect against HF attack in the electrolyte, therefore, the cycling performance and rate capability could be improved.6 It is worth noting that the Li+ diffusion kinetics can only be improved in the surface areas of these materials but not inside.19 In addition, doping cations (e.g., Mg2+, Al3+, Ti3+, and Cr3+) might minimize the voltage fade and preserve the crystal structure.20 However, most of the cationic dopants are electrochemically inactive during the charge and discharge processes, and it is difficult to control the side reactions between foreign ions and the electrolyte.20 Therefore, the above methods cannot well solve the inherent drawbacks of the low first cycle efficiency, voltage decay, problems of cycling performance, and rate capability. Apart from above-mentioned strategies, porous micronanostructures have received extensive concern in a great deal of fields.21 The materials with micronanostructures not only have the advantages of the nanosized primary particles which can significantly shorten the Li+ diffusion pathway and enhance the rate performance, but also retain their outstanding structural stability that can provide excellent cycle performance and reduce side reactions.6 In addition, numerous extra active sites in the hollow cavities of the porous electrode material are beneficial for Li+ storage and hence increase the specific capacity.22 Most importantly, the outstanding structural stability is attributed to porous structures which can alleviate the local volume change upon charge/discharge.21 Moreover, the 3D spinel Li4Mn5O12 phase could enhance the Li+ intercalation/deintercalation process and stabilize the structure,

which effectively improves the rate capability.23 In addition, the spinel/layered heterostructure in Li-rich material could significantly further improve the rate capability because the 3D interstitial space of the spinel structure permits fast Li+ diffusion, and the layered structure enhances the high Li+ storage capacity.24 In this paper, a novel facile approach to synthesize the porous-rod-like micronanostructured Li-rich spinel/layered oxide 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 (PR-LLNCM) for LIB applications was developed. The synthetic method is very simple, and the material is obtained at room temperature in contrast to our previously reported oxalate-precursor route. In addition, the material possesses the merits of spinel/layered heterostructure and micronanostructures at the same time. Furthermore, the morphologies, structures, and electrochemical properties of PR-LLNCM were systematically studied.

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EXPERIMENTAL SECTION

Material Synthesis. The [Ni0.2Co0.2Mn0.6](OH)2 precursor was synthesized by a coprecipitation method. A proportion of MnCl2· 4H2O (1 mmol), Ni(NO3)2·6H2O (0.33 mmol), and CoCl2·6H2O (0.33 mmol) were dissolved in 10 mL of ethylene glycol as an A solution. Meanwhile, 2.5 mmol of ammonium oxalate was used as the precipitating agent and dissolved in 10 mL distilled water as a B solution. Then, the B solution was slowly poured into the A solution under magnetic stirring at room temperature. After stirring for 12 h, the white precipitation was generated. Then, the as-obtained [Ni0.2Co0.2Mn0.6]C2O4 materials were washed with ethanol and water. After drying, the as-produced oxalate precursor was heated at 500 °C for 6 h, and homogeneously mixed with a moderate amount of Li2CO3 and then calcined at 750 °C for 12 h to form PR-LLNCM. The growth process of PR-LLNCM particle is shown in Figure 1. For comparison, the 0.4Li 2 MnO 3 ·0.6LiNi 1/3 Co 1/3 Mn 1/3 O 2 (LLNCM) cathode material was synthesized by solvothermal method. The same amounts of MnCl2·4H2O, Ni(NO3)2·6H2O and CoCl2· 6H2O were dissolved in 10 mL of ethylene glycol. Then, the same ammonium oxalate solution was slowly added into the above solution. 11006

DOI: 10.1021/acssuschemeng.7b02906 ACS Sustainable Chem. Eng. 2017, 5, 11005−11015

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. XRD diffraction patterns of (a) PR-LLNCM and (b) LLNCM. The crystal structures of (c) layered LiMO2 (space group: (R3̅m), (d) monoclinic Li2MnO3 (space group: C2/m), and (e) Li4Mn5O12 spinel phase (space group: (Fd3̅m).

Figure 3. SEM images of (a, b, and c) PR-LLNCM and (d) LLNCM. Next, the mixed solution was poured into a sealed Teflon at 180 °C for 12 h to obtain precipitate. Finally, the precipitate was washed with ethanol and water, then dried at 60 °C. The lithium mixing process of LLNCM was the same as that of PR-LLNCM. Material Characterization. The crystalline phases of the samples were detected by the powder X-ray diffraction (XRD) on a

diffractometer (D/Max-3C, Rigaku), equipped with Cu Ka radiation (λ = 1.54178 Å, 40 mA, and 40 kV) and a graphite monochromator, in a range of 10° to 80° (2θ) at a scan rate of 4° min−1. The morphology, atomic concentration, and elemental distribution of the samples were characterized by scanning electron microscopy (SEM) in cooperation with energy-dispersive X-ray spectroscopy (EDXS) (JSM-6100LV, 11007

DOI: 10.1021/acssuschemeng.7b02906 ACS Sustainable Chem. Eng. 2017, 5, 11005−11015

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. (a) TEM images of PR-LLNCM; (b and d) HRTEM image of the circled region in (a); (c and e) FFT pattern correspond to the monoclinic phase of (b) and (d); and (f) EDS spectrum of PR-LLNCM.

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JEOL, Japan). The particle size and morphologies of the PR-LLNCM samples were observed by field emission scanning electron microscopy (FESEM, Quanta FEG 250) and transmission electron microscopy (TEM, JEOL JEM-2100F). The oxide states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS) (K-Alpha 1063, Thermo Fisher Scientific). Electrochemical Measurements. In order to prepare the positive electrode, the as-prepared Li-rich cathode material was mixed with acetylene black, graphite, and polyvinylidene fluoride (PVDF) binder at a ratio of 80:5:5:10 (in mass, wt %), and then N-methyl-2pyrrolidone (NMP) was added to obtain a uniform slurry. Next, the as-prepared slurry was uniformly coated on the aluminum foil. The asprepared films were cut into wafers of 10 mm diameter and dried under vacuum at 110 °C overnight. The typical mass loading of active material is 3−4 mg cm−2. Metallic lithium foil (Φ16 × 1 mm, 0.1 g) was used as the counter and reference electrode. The electrolyte was 1 mol L−1 LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1, V: V), and porous polypropylene based membrane (Celgard 2400) was used as the separator. Finally, 2025 coin cells were assembled in glove boxes with controlled concentrations of water and oxygen below 1 ppm. The galvanostatic charging and discharging test was done at different current densities in a battery test system (CT3008, Neware Technology Co. Ltd., China) between 2.0 and 4.6 V (vs Li+/Li). The electrochemical impedance spectroscopy (EIS) measurements were accomplished with an AC voltage of 5 mV amplitude in the frequency range of 105 to 10−2 Hz using a VersaSTAT3 electrochemical workstation (Princeton Applied Research, U.S.A.).

RESULTS AND DISCUSSION

The crystallographic structure and crystallinity of the of PRLLNCM and LLNCM were studied by X-ray diffraction (XRD), and the result is shown in Figure 2.25 The XRD patterns were analyzed based on the mixture of LiNi1/3Co1/3Mn1/3O2 (R3̅m) and Li2MnO3 (C2/m) for all samples. The result indicates that the Li4M5O12 ((Fd3̅m)) phase was only detected in the PR-LLNCM sample. Both the layered (R3̅m) and (C2/m) structure and spinel structure exist in the PR-LLNCM sample, but only a layered structure can be noticed in LLNCM (Figure 2). There is a super lattice peak which corresponds to the arrangement of Li and Mn atoms in the monoclinic Li2MnO3 component with the layered phase (C/2m) between 20° and 25°.26,27 Meanwhile, a wellcrystallized layered structure can be indicated by observing the distinct segregation between peaks of (006)/(102) and (018)/(110).28,29 The intensity ratio of (003)/(104) for PRLLNCM cathode material (1.10) is obviously higher than that of LLNCM (1.01), indicating that PR-LLNCM possesses a better layered structure and less cation mixing.30,31 Additionally, combining the patterns of both the hexagonal layered structure and the spinel peaks, it can be well indexed to cubic Li4Mn5O12 with a space group of Fd3m ̅ which is obtained from (311) and (400) in Figure 2a. In the range of 35° to 40°, a clear hump besides the (311) peak is easily identified in PR-LLNCM (in Figure 2b), which indicates the formation of the spinel 11008

DOI: 10.1021/acssuschemeng.7b02906 ACS Sustainable Chem. Eng. 2017, 5, 11005−11015

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. XPS spectra of (a) Ni 2p, (b) Co 2p, and (c) Mn 2p for PR-LLNCM and LLNCM.

phase. Therefore, the PR-LLNCM could offer 3D Li+ diffusion channels which are beneficial for Li+ diffusion kinetics and structural stability. The SEM images in Figure 3 show the morphologies of PRLLNCM and LLNCM at low and high magnifications. As shown in Figure 3a−c, the rod-like PR-LLNCM calcined at 750 °C appears with a homogeneous porous morphology with widths of 500 nm and lengths of 3−4 μm. It is obvious that the porpous large agglomerated PR-LLNCM particles are made up of primary particles. The formation of pores is attributed to the rod-like precursor of PR-LLNCM which has occurred based on the Kirkendall Effect and Ostwald Ripening in the calcining process.32,33 However, as shown in Figure 3d, it can be observed that the morphology of LLNCM is different from PRLLNCM, and the LLNCM particles are broken and stacked together. TEM and HRTEM techniques are further utilized to study the detailed morphological and crystal structures of PRLLNCM (Figure 4). Figure 4a shows the TEM images of PR-LLNCM. The comparison between the dark edges and the light center in Figure 4a reflects that PR-LLNCM is porous. Consistent with the SEM images, the TEM images of PRLLNCM confirm that the materials are actually composed of interconnected nanosized primary particles constituting the walls of the porous-rod-like structure which provides a shorter Li+ diffusion pathway with well-defined crystallographic facets. As shown in Figure 4b, the interplanar spacing of the lattice fringes are 0.205 and 0.243 nm which correspond well to the (400) and (311) planes of (Fd3̅m) Li4Mn5O12 structure, respectively (Figure 4c). Furthermore, the apparent lattice fringe (0.47 nm) apperceived in the HRTEM image (Figure 4d) is ascribed to the (003) planes in layered phase (C2/m) of

Li2MnO3 (Figure 4e). Thus, combined with the XRD observation, the conformity of the (003) lattice spacing of the rhombohedral phase, and the (400) plane for the monoclinic structure of Li4Mn5O12 (Fd3̅m), the formation of spinel/layered phase in PR-LLNCM is confirmed. Moreover, the atomic ratio of the Mn, Ni, and Co is close to 3:1:1 which is confirmed by the EDXS spectrum data, coincide with the ratio in the initial mixture (Figure 4f). In order to investigate the oxidation states of Co, Mn, and Ni in the heterogeneous material, XPS measurements were conducted (Figure 5). As shown in Figure 5, the Ni 2p3/2 peaks at 854.58 and 860.98 eV indicate the coexistence of Ni2+ and Ni3+. In addition, the binding energy at 779.98 eV for Co 2p3/2 peak is assigned to trivalent Co species (Figure 5b). The Ni 2p3/2 and Co 2p3/2 peaks of PR-LLNCM concur with that of LLNCM. However, compared with LLNCM, a slight shift toward lower binding energy for both Mn 2p3/2 and Mn 2p1/ 2 spectra are obtained in PR-LLNCM (Figure 5c). The binding energy of 2p spectra for Mn3+ is lower than that for Mn4+, indicating that a higher amount of Mn4+ exists in PR-LLNCM which further demonstrates the existence of the spinel. The initial charge curves of PR-LLNCM and LLNCM electrode at a current density of 20 mA g−1 between 2.0 and 4.6 V at room temperature are shown in Figure 6a. The initial discharge capacity of PR-LLNCM is 269 mAh g−1 with 90.3% Coulombic efficiency, which is higher than that of the LLNCM electrode (initial discharge capacity of 245.8 mAh g−1 with 81.8% Coulombic efficiency). Figure 6a profiles exhibit two plateaus during the first charge to 4.5 V, which is a representative characteristic of the Li-rich layered cathode material due to the existence of two different processes for Li+ deinsertion from the R3̅m and C2/m phase region.34 Li+ ions 11009

DOI: 10.1021/acssuschemeng.7b02906 ACS Sustainable Chem. Eng. 2017, 5, 11005−11015

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6. (a) The first charge−discharge curve of PR-LLNCM and LLNCM electrodes at 0.1C between 2.0 and 4.6 V, and the voltage profiles of (b) PR-LLNCM and (c) LLNCM for the initial three cycles between 2.0 and 4.6 V (vs Li/Li+) at 0.1 mV s−1.

tetragonal phase.40 In the process of delithiation, Mn3+ is oxidized, resulting in rearrangement of the structure,41 and transition metal ions may be able to shift into the tetrahedral sites. These could generate a spinel structure of the threedimensional tetrahedral (8a)−eoctahedral (16c) interstitial network of M2O4 (space group Fd3 m).42 Compared with the CV profiles of LLNCM (Figure 6c), the consequence shown for PR-LLNCM reveals that the composite material is a spinel/layered heterostructured Li-rich material. The discharge performance of PR-LLNCM and LLNCM was characterized at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C rates between 2.0 and 4.6 V with 3 cycles per step, and then the current densities return from 10C to 0.1C (Figure 7a). The PRLLNCM delivers discharge capacities of 239 mAh g−1, 195 mAh g−1, 175.8 mAh g−1 at 0.5C, 5C, and 10C, respectively. But for the LLNCM, discharge capacities at 0.5C, 5C, and 10C are only 233 mAh g−1, 163 mAh g−1, and 134 mAh g−1, respectively. The rate capability of PR-LLNCM is much better than that of LLNCM, and such good rate capability of PRLLNCM with both Li-rich layered and spinel Li4Mn5O12 phases is attributed to the porous-rod-like morphology and 3D fast Li+ diffusion channel furnished by the spinel structure. Under the influence of the layered and spinel components, the velocity of Li+ exchange is dramatically enhanced by relieving barriers and shortening distance of Li+ diffusion.43 For deeper research the cycling performance at high current density, the cells are tested at 5C with 200 cycles between 2.0 and 4.6 V after the rate capability test (Figure 7b). Besides, the charge−discharge curves of the PR-LLNCM and LLNCM electrodes at 5C in the voltage range of 2.0 to 4.6 V with selected cycles are shown in

extract from the transition metal oxide layer leading to the generation the initial sloping voltage region (