Hierarchical Mesoporous Lithium-Rich Li[Li

Research Article www.acsami.org

Hierarchical Mesoporous Lithium-Rich Li[Li0.2Ni0.2Mn0.6]O2 Cathode Material Synthesized via Ice Templating for Lithium-Ion Battery Yu Li,† Chuan Wu,†,‡ Ying Bai,*,† Lu Liu,† Hui Wang,† Feng Wu,†,‡ Na Zhang,§ and Yufeng Zou§ †

Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P.R. China ‡ Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, P.R. China § EV/HEV R&D Department, Tianjin Lishen Battery Joint-Stock Co., Ltd., Tianjin, 300384, P.R. China S Supporting Information *

ABSTRACT: Tuning hierarchical micro/nanostructure of electrode materials is a sought-after means to reinforce their electrochemical performance in the energy storage field. Herein, we introduce a type of hierarchical mesoporous Li[Li0.2Ni0.2Mn0.6]O2 microsphere composed of nanoparticles synthesized via an ice templating combined coprecipitation strategy. It is a low-cost, eco-friendly, and easily operated method using ice as a template to control material with homogeneous morphology and rich porous channels. The as-prepared material exhibits remarkably enhanced electrochemical performances with higher capacity, more excellent cycling stability and more superior rate property, compared with the sample prepared by conventional coprecipitation method. It has satisfactory initial discharge capacities of 280.1 mAh g−1 at 0.1 C, 207.1 mAh g−1 at 2 C, and 152.4 mAh g−1 at 5 C, as well as good cycle performance. The enhanced electrochemical performance can be ascribed to the stable hierarchical microsized structure and the improved lithium-ion diffusion kinetics from the highly porous structure. KEYWORDS: ice templating, micro/nano hierarchical, mesoporous, Li-rich cathode, lithium-ion battery

1. INTRODUCTION To address CO2-related global warming caused by burning fossil fuels, efforts to replace gasoline engines with the electric motors and electric vehicles (EVs) are under development around the world.1,2 Compared to other energy-storage systems, lithium-ion batteries (LIBs) have received much attention for applications in stationary power storage for its higher energy density.3 The energy density of LIBs is mainly hindered by the relatively lower capacity of cathode material.4 In this regard, lithium-/manganese-rich metal oxides, like Li[Li1/3−2x/3MxMn2/3−x/3]O2 (M = transition metals), are receiving significant interest as popular cathode materials for LIBs at present. When they are charged to above 4.5 V, the discharge capacity can reach up to more than 250 mAh g−1.5−9 However, the fast capacity decay and poor rate property of these materials, which are related to the synthetic method, still impede their wide application.10−13 To date, many researchers have made lots of efforts to improve electrochemical performance of these materials through reducing particles size to nanoscale to shorten Li ions diffusion pathway.14−16 Nevertheless, these nanoscaled materials are vulnerable to more side reactions at solid(electrode)/liquid(electrolyte) interface.17 They also have comparative low pack density and energy density.18 Therefore, packing nanoscaled primary particles into © 2016 American Chemical Society

microsized secondary particles to hierarchical structure, which integrates the superiorities of nanomaterials and micro/ macromaterials, has been a modern trend in controllable synthesis field.19−23 It is noteworthy that porous, especially mesoporous, electrode materials may significantly improve power rate during electrochemical process because of the large surface area, abundant migration channels for Li+, and enhanced accessibility to anchor each functional chemical on the surface.24,25 Tremendous efforts have been made to prepare porous Lirich materials by various template, such as carbon felt,26 graphene,27 and aerogel.28 In addition, NaCl molten-salt method29 and polymer-thermolysis method30 have also been utilized. Aforementioned measures are consumptive and difficult to realize for industrialization. Recently, an ice templating strategy is a generic method to prepare aligned porous materials with microsized morphology for applications.31−33 It is a low-cost, environmentally friendly, and versatile strategy for controlling electrode materials with uniquely sophisticated morphologies.34 Received: April 19, 2016 Accepted: June 30, 2016 Published: June 30, 2016 18832

DOI: 10.1021/acsami.6b04687 ACS Appl. Mater. Interfaces 2016, 8, 18832−18840

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Diagram of Synthetic Route for Hierarchical Mesoporous Cathode Material Li[Li0.2Ni0.2Mn0.6]O2

Figure 1. (a) XRD patterns of cathode materials Li[Li0.2Ni0.2Mn0.6]O2 prepared by ice templating method (sample M1) and conventional method (sample M2). The crystal structures of (b) monoclinic Li2MnO3 (C2/m), (c) layered LiNi0.5Mn0.5O2 (R3m), and (d) Li[Li0.2Ni0.2Mn0.6]O2.

nanohierarchical mesoporous electrode materials via a lowcost, environmentally benign tactic in the energy storage field.

Herein, we adopt the ice templating combined coprecipitation method to fabricate 3D hierarchical mesoporous Li[Li0.2Ni0.2Mn0.6]O2 cathode material. This spherical micro/ nanohierarchical appearance is developed by carbonate precipitant in coprecipitation process, and porosity of electrode material originated from ice templating tuned in precursor freeze-drying process. The carbonate precursor is rapidly frozen, then ice is generated among primary particles. During the freeze-drying process, ice is gradually sublimated and the relative space among primary particles will still be maintained. Therefore, the mesoporous Li[Li0.2Ni0.2Mn0.6]O2 is obtained by subsequent heat treatment. The precursor of another sample dried by conventional vacuum drying is also prepared as a comparison. The as-prepared mesoporous hierarchical composite exhibits a remarkable electrochemical performance. The results show a new insight into designing 3D micro/

2. EXPERIMENTAL SECTION 2.1. Synthesis. The precursor Ni0.2Mn0.6(CO3)0.8 of pristine material Li[Li0.2Ni0.2Mn0.6]O2 was prepared by a typical coprecipitation method at 60 °C under a N2 atomsphere, and the synthesis process is illustrated in Scheme 1. A 2 M mixture of nickel sulfate hexahydrate (NiSO4·6H2O) and manganese sulfate monohydrate (MnSO4·H2O) with a Ni/Mn atomic ratio of 1:3 was dissolved in distilled water, here marked as solution A; 1 M sodium carbonate (Na2CO3) and 0.5 M ammonium hydroxide (NH3·H2O) mixed together with distilled water were used as the starting reagents, here marked as solution B. Then solution A and B were fed into a continuously stirred tank reactor synchronously at a speed of 200 mL h−1, and a stirring speed of 1000 rpm was maintained throughout this process. The pH was kept at 8 during the reaction by coordinating the relative speed of two solutions. After 6 h aging, the precipitates were 18833


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Figure 2. SEM images of (a) sample M2; (b) magnified sample M2; (c) sample M1; (d) magnified sample M1; (e) cross-section of sample M1; (f) primary particles of sample M1. CT2001A, China) at selected current rates (1 C = 200 mA g−1), the voltage range was from 2.0 to 4.8 V vs Li+/Li. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were all carried out on a CHI660e electrochemical workstation. The voltage window of CV was 2.0−4.8 V vs Li+/Li; and the frequency range of EIS was 0.1 MHz to 1 × 10−2 Hz with an alternating current perturbation signal of 5 mV.

separated through centrifugal washing and then divided into two sample. One sample was precooled at −90 °C for 3 h, and then transferred to the freezing dryer to further dry under low-temperature vacuum condition for 12 h. The other sample was dried in a vacuum case at 120 °C for 12 h. After drying, these two sample were wellmixed with LiOH·H2O and subsequently calcined in furnace at 900 °C for 720 min to produce the Li-rich cathode materials, respectively, hereinafter the sample dried with freezing dryer was referred to M1 and the sample dried with vacuum oven was referred to M2. 2.2. Material Characterization. X-ray diffraction (XRD) was recorded on Rigaku X-ray diffractometer (Ultima IV-185, Japan) with Cu Kα radiation, the range was from 10 to 80° 2θ and the scan rate was 8° 2θ min−1. Morphologies of all samples were observed by fieldemission scanning electron microscopy (FE-SEM, HITACHI S-4800, Japan) and high-resolution transmission electron microscope (HRTEM, HITACHI H-800, Japan). Element mappings of samples were investigated by energy-dispersive X-ray detector (EDX), which was connected with the SEM and TEM mentioned above. The specific area and pore size of sample were measured using ASIQM000−1-MP, China. 2.3. Electrochemical Tests. The cathode slurry consisting of 80 wt % as-prepared material, 10 wt % Super P (SP), 10 wt % polyvinylidene fluoride (PVDF) binder and amount of N-methylpyrrolidone (NMP) organic solvent was coated on Al foil uniformly. The electrodes were dried at 80 °C for 24 h in a vacuum oven before use. The average mass loading density of the as-prepared material in each electrode slice was 1.8 mg cm−2. One M LiPF6/ethyl carbonate (EC) and diethyl carbonate (DEC) (volume ratio is 1:1) were used as the electrolyte, lithium was used as the anode and Celgard 2400 was used as the separator. Half coin-cells (CR2025) were assembled in an Ar-filled glovebox (Labmaster130, Germany). Galvanostatic charge/ discharge tests were implemented on a battery tester (LAND-

3. RESULTS AND DISCUSSION Figure 1a exhibits the XRD patterns of both samples. Apart from the weak super lattice peaks between 20 and 24°, all the diffraction patterns are consistent with the well-crystallized layered hexagonal α-NaFeO2 structure and the space group R3m, similar to the typical layered crystal structure of LiNi0.5Mn0.5O2 as shown Figure 1c. The inset on the top right corner of Figure 1a is the weak reflections between 20° and 24°, which corresponds to the monoclinic C2/m phase, are characteristic of a Li2MnO3 component (Figure 1b) with LiMn6 cation arrangements in the transition metal layers.35−37 The atomic structure sketch of layered Li[Li0.2Ni0.2Mn0.6]O2 is shown in Figure 1d. The c/a ratios of two cathodes are higher than 4.9, and this value is generally recognized as material with an explicit layered characteristics.7 In addition, the value of I(003)/I(104) has been adopted to estimate the ordering of the series of layered LiMO2 cathode materials.38 And the ratios I(003)/I(104) and c/a are considered as an indication of the degree of Li+/Ni2+ cation mixing.39 Obviously, these two values of sample M1 are higher than those of sample M2, which reveals that sample M1 has a more perfect layered structure, 18834


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Figure 3. Nitrogen adsorption/desorption isotherms and BJH pore size distributions of (a) sample M1 and (b) sample M2.

Figure 4. TEM images of (a) sample M1, (b) primary particles of sample M1. HRTEM images of (c) sample M1, and (inset) FFT algorithm result of the yellow square region. (d) SAED pattern of the yellow square region in c. (e) EDX elemental distribution mapping and corresponding EDX spectra of sample M1.

better crystallinity and higher ordering of lithium and transition metal compared with sample M2. The morphologies of two cathodes are observed by FE-SEM (Figure 2). In Figure 2a, c, the secondary spherical particles of sample M1 have the apparently more regular globular morphologies and more uniform microsphere distributions. The primary particles of sample M2 exhibit obvious

aggregations (Figure 2b and Figure S1), whereas those of sample M1 are well-defined nanoscaled particles on the surface of microspheres. Fortunately, one of the M1 microsphere crosssections is exposed as shown in Figure 2e; we can clearly find that the interior of the M1 microsphere also consists of uniform nanoparticles about 200 nm (Figure 2f). There is space among secondary nanoparticles, so we successfully achieve mesoporous 18835


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Figure 5. (a) CV curves of sample M1 at 2.0−4.8 V and with a scanning rate of 0.1 mV/s; (b) cycling stability of sample M1 and sample M2; (c) the initial discharge/charge voltage profiles of two samples; (d) cycling performance of two samples at different rates; (e) discharge capacities under rates of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 5 C; and (f) discharge profiles for two samples at different rates.

morphologies and porosity exists among their primary nanoparticles. To further verify the porosity of hierarchical microsphere Li[Li0.2Ni0.2Mn0.6]O2, we carried out nitrogen adsorption/ desorption tests and the curves are shown in Figure 3. The average BJH pore diameter of sample M1 is 2.437 nm, which indicates the pores are mainly mesopores. Compared with sample M2, the specific surface area of sample M1 has a higher value of 18.865 m2 g−1, which means sample M1 may afford better electrochemical performance. In a word, the Li[Li0.2Ni0.2Mn0.6]O2 cathode with porous microsphere structure assembled with nanosized particles is verified, which matches well with our original design. As shown in Figure 4, the HR-TEM and selected area electron diffraction (SAED) images of sample M1 are measured to characterize the morphology and structure. The diameter of M1 microsphere is approximately 4 μm (Figure 4a), which is in accordance with the SEM image of sample M1 (Figure 2d).

microsized materials. This unique morphology of sample M1 is attributed to the freezing drying process. Both primary and secondary particles are tuned by the ice templating method. The precursor of sample M2 with liquid moisture between nanoprimary particles is dried by the traditional vacuum drying process. With continuous and tardy volatilization, two adjacent particles will lead to mutual attraction because of the surface tension on the gas−liquid interface. After moisture evaporated completely, two adjoining nanoparticles tend to be closer and agglomerate ultimately to form compact secondary microparticle.40 The precursor of sample M1 is dried by the freezing drying process, and there is ice as a templating between two nanoparticles. With the sublimation of ice, the relative location between the primary nanoparticles will be maintained. Thus, the severe agglomeration of the primary nanoparticles can be effectively suppressed and many pores are produced by the vacuum freezing drying technology. In a word, the secondary particles of sample M1 have more homogeneous spherical 18836


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Figure 6. (a) Nyquist plots and (b) the relationship between Zre and ω−1/2 at low frequencies of two samples before cycle and after 80 cycles.

The initial capacity loss is generally attributed to the irreversible reaction during the plateau above 4.5 V. Additionally, the electrolyte decomposes and the solid electrolyte interphase film forms in the subsequent cycles, which also cause irreversible lithium loss. The higher capacity retention, Coulombic efficiency and lower initial irreversible capacity of sample M1 benefit from stable microsized structure, thus can not only resist electrolyte corrosion but also build up and maintain continuous ion and electron pathways during repeated cycling. To evaluate the ice templating method endowing better electrochemical performance, the rate cycling abilities of two samples are further cycled at both 2 C and 5 C shown in Figure 5d. Obviously, the cyclic stability of sample M1 at 2 and 5 C is significantly superior to that of sample M2. For sample M1 at 2 C, it has a higher discharge capacity of 207.1 mAh g−1 and then the capacity decreases to 188.7 mAh g−1 after 50 cycles with a capacity retention of 91.1%, whereas for sample M2 it is only 74.8%. At 5 C, the first cycle discharge capacity of sample M1 reaches 152.4 mAh g−1 and it can keep the value of 125.1 mAh g−1 in the 50th cycle, showing much better stable property than that of sample M2. In addition, as shown in Figure 5e, to highlight the effect of the drying route on rate capability, the cells based on two samples are charged at 0.1 C and discharged at different discharge rates (0.1−5 C). It can be seen that sample M1 displays significantly better rate performance than sample M2. At 0.1, 0.2, 0.5, 1, 2, and 5 C, the maximal discharge capacities of sample M1 are 279.6, 245.6, 223.1, 191.2, 175.6, and 148.6 mAh g−1, respectively. Once the current density returns to 0.1 C, the capacity still can reach 244 mAh g−1, suggesting the good rate performance and structure stability of sample M1. The representative discharging profiles of samples M1 and M2 at each C rate are shown in Figure 5f. As a result, the excellent rate capability of sample M1 benefits from the hierarchical mesoporous structure, which exposes amounts of porous channel for infiltration of electrolyte. In addition, the primary nanoparticles ensure rapid Li+ intercalation/deintercalation.

The regions inside the dotted green squares are primary nanoparticles (Figure 4a), and the porous property can be once again confirmed by the light and dark regions inside the dotted yellow circles (Figure 4b). From the HR-TEM image in Figure 4c, lattice spacing of (003) plane of R3m group and (020) plane of C/2m symmetry can be indexed as 4.75 and 4.2 Å. And deeper analysis of the SAED patterns in Figure 4d demonstrates that this is (110) plane for layered Li-rich materials. The composition of sample M1 is examined by energy-dispersive X-ray spectra (EDX) shown in Figure 4e. The elements O, Mn, and Ni are evenly distributed in sample M1 and without any phase separation. To obtain a detailed understanding of electrochemical behavior of as-prepared cathode material during cycling, the first 3 cyclic voltammetry curves of sample M1 are presented in Figure 5a. During the first charge cycle, the peak around 4.0 V is indexed to the Ni2+ oxidation and the second strong peak 4.6−4.7 V is attributed to irreversible electrochemical activation of Li2MnO3 component forming MnO2, which corresponds to the mechanism of Li-rich cathodes for its first charge/discharge process.5,41 During the initial discharge process, the cathodic peaks at approximately 3.7 and 3.3 V are assigned to Ni4+ and Mn4+ reduction, respectively. From the curves of second and third cycles, almost all anodic and cathodic peaks overlap well, which proves the good reversibility of sample M1. Figure 5b indicates the cycling performance of samples M1 and M2 at a 0.1 C rate at ambient temperature. The initial discharge capacities of samples M1 and M2 are 280.1 and 243.4 mAh g−1, respectively. After 80 cycles, the capacity retention of sample M1 is 85.4%, which is higher than that of sample M2 with a value of 72.9%. The initial charge and discharge curves of two samples are shown in Figure 5c, each of them shows a typical initial charge curve of the Li-rich cathode, comprising of a slope between 3.7 and 4.5 V and a plateau around 4.5 V. The second plateau can be ascribed to the electrochemical activation of Li2MnO3 component that extracts Li2O.36 The initial irreversible capacity of sample M1 is only 81.1 mAh g−1, whereas that of sample M2 reaches as high as 131.7 mAh g−1. 18837


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Figure 7. SEM images of (a) sample M1 and (c) magnified sample M1 after 50 cycles (charge and discharge at 1 C). (b) HRTEM image of sample M1 after 50 cycles and SEAD patterns of (d) the green and (e) yellow square region in b. (f−i) EDX elemental distribution mapping of sample M1 after 50 cycles.

cells cycled after 50 cycles at 1 C are dissembled and the electrodes are rinsed by organic solvent in the glovebox. Figure 7a provides SEM image of the electrode after 50 cycles. All of the secondary microsized particles still exist. It is noteworthy that the hierarchical mesoporous morphologies are maintained as shown in Figure 7c, which is also supported by the HR-TEM image and EDX mapping (Figure 7f−i). In Figure 7b, detailed structural evolution of cycled sample M1 is further observed under HR-TEM. Parallel lattices in Figure 7d are similar to the ones in Figure 4c, which indicates the main phase is still layered structure. However, retiform patterns are also founded regionally in original parallel stripe. The lattice spacing is 0.209 nm, which is consistent with the (400) plane of the cubic spinel structure. It suggests that the cation well-ordered layered structure has locally turned to spinel-like phase because of structural transformation after charge/discharge cycles. All these results show that both the mesoporous structure and spherical morphology of sample M1 are well-retained after 50 cycles at 1 C. Undoubtedly, this mesoporous Li-rich cathode material has a superior structural stability and produces a better electrochemical performance.

Electrochemical impedance spectroscopy (EIS) tests are conducted to confirm deeper kinetic information on these two samples. The Nyquist plots of samples M1 and M2 before cycle (at initial voltage) and after 80 cycles (at cutoff voltage) are depicted in Figure 6a, respectively. Nyquist plots consist of a semicircle at high frequencies that is related with the chargetransfer resistance (Rct), and a line at low frequencies that is attributed to the Warburg diffusion of Li ions in the material. A possible equivalent circuit model is presented in the inset of Figure 6a. Furthermore, based on the Warburg diffusion supported by the low-frequency region, the diffusion coefficient (DLi+) of lithium ions is calculated based on the following equations

D Li+ =

R2T 2 2

2n 4F 4C B σ 2A2

zre = RD + RL + σω−0.5

(1) (2)

As shown in eqs 1 and 2, R is the ideal gas constant, T is the absolute temperature, n is the number of electrons per molecule during charge/discharge process (for this reaction, it is 1), F is the Faraday constant, CB is the concentration of Li+ in pre unit cell (for this cathode it is 4.96 × 10−2 mol cm−3), A is surface area of the electrode in cm2 (for this electrode it is 1.131 cm2), σ is the Warburg factor which has relationship with Zre (shown in eq 2). Figure 6b shows the linearity of the Zre and ω−1/2 in the low-frequency region. On the basis of the above informations, the lithium-ion diffusion coefficients of two cathodes before cycles and after 80 cycles are calculated. The table below Figure 6 shows that no matter before cycle or after 80 cycles, sample M1 has lower Rct value and higher DLi+ compared with sample M2, demonstrating that the rate of Li+ diffusion in sample M1 is drastically increased during charge/ discharge processes by designing specific hierarchical mesoporous morphology. The excellent structure stability of sample M1 is further demonstrated by the FE-SEM and HR-TEM images. The coin

4. CONCLUSION In summary, hierarchical mesoporous Li[Li0.2Ni0.2Mn0.6]O2 microsphere composed of nanoparticles via ice templating route combined with coprecipitation method are successfully synthesized. The obtained Li[Li0.2Ni0.2Mn0.6]O2 compound (M1) shows high discharge capacity, superior cycling performance and satisfactory rate property as cathode for LIBs in comparison with the sample synthesized by conventional coprecipitation method (M2). The reversible discharge capacity is tested to be as high as 280.1 mAh g−1 at 0.1 C. The capacity retention could achieve 82.1% at 5 C after 50 cycles. It suggests that ice templating strategy to prepare Li-rich material benefits the outstanding electrochemical performance, which is mainly due to stable hierarchical structure and abundant porous channels for Li+ intercalation/deintercalation kinetics. More 18838


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importantly, the low-cost, environmentally benign, and versatile ice templating technique will be referential for controllably synthesizing other electrode materials for a secondary battery system.

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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.6b04687. SEM image of primary particles of sample M2 (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 acknowledge support from the National Key Basic Research Program of China (No. 2015CB251100), the Program for New Century Excellent Talents in University (NCET-13-0033), the Beijing Co-construction Project (20150939014), and the New Energy Automobile Industry Technological Innovation Project of China (2012-2015).

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REFERENCES

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Hierarchical Mesoporous Lithium-Rich Li[Li

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