Hierarchical Mesoporous Lithium-Rich Li[Li0.2Ni0.2Mn0.6]O2

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Hierarchical Mesoporous Lithium-Rich Li[Li Ni Mn ]O 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04687 • Publication Date (Web): 30 Jun 2016 Downloaded from http://pubs.acs.org on July 5, 2016

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ACS Applied Materials & Interfaces

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,§ Yufeng Zou§ †

Beijing Key Laboratory of Environmental Science and Engineering, School of Materials

Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡

Collaborative Innovation Center of Electric Vehicles in Beijing, Beijing 100081, PR

China §

EV/HEV R&D Department, Tianjin Lishen Battery Joint-Stock Co., Ltd., Tianjin,

300384, PR China

ACS Paragon Plus Environment


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ABSTRACT Tuning hierarchical micro/nano structure of electrode materials is a sought-after means to reinforce their electrochemical performance in the energy storage filed. Herein, we introduce a type of hierarchical mesoporous Li[Li0.2Ni0.2Mn0.6]O2 microsphere composed of nanoparticles synthesized via an ice templating combined co-precipitation strategy. It is a low-cost, eco-friendly and easily-operated method using ice as a template to control material with homogenous 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 co-precipitation method. It has satisfactory initial discharge capacities of 280.1 mAh g-1 at 0.1C, 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 micro-sized structure and the improved lithium ions diffusion kinetics from the highly porous structure. KEYWORDS ice templating, micro/nano hierarchical, mesoporous, Li-rich cathode, lithium ion battery


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1. INTRODUCTION To address CO2-related global warming caused by burning fossil fuels, much efforts paid 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 nano-scaled 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 nano-scaled primary particles into micro-sized secondary particles to hierarchical structure, which integrates the superiorities of nano-materials and micro/macro-materials, 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 due to large surface area, abundant migration channel for Li+ and enhanced accessibility to anchor each functional chemical on the surface.24-25 Tremendous efforts have been made to prepare porous Li-rich materials by various template, such as carbon felt,26 graphene27 and aerogel28. In addition, NaCl molten-salt method29 and polymer-thermolysis method30 have also been



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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 micro-sized morphology for applications.31-33 It is a low-cost, environmentally-friendly and versatile strategy for controlling electrode materials with uniquely sophisticated morphologies.34

Scheme 1. Schematic diagram of synthetic route for hierarchical mesoporous cathode material Li[Li0.2Ni0.2Mn0.6]O2. Herein, we adopt the ice templating combined co-precipitation method to fabricate 3D hierarchical mesoporous Li[Li0.2Ni0.2Mn0.6]O2 cathode material. This spherical micro/nano-hierarchical

appearance

is

developed

by

carbonate

precipitant

in

co-precipitation 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


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designing 3D micro/nano-hierarchical mesoporous electrode materials via low-cost, environmentally benign tactic in energy storage filed. 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 co-precipitation method at 60 °C under N2 atomsphere, and the synthesis process is illustrated in Scheme 1. 2M mixture of nickel sulfate hexahydrate (NiSO4·6H2O) and manganese sulfate monohydrate (MnSO4·H2O) with 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 ageing, the precipitates were separated through centrifugal washing and then divided into two sample. One sample was pre-cooled at -90 °C for 3 h, and then transferred to the freezing dryer to further dry precursor 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 well mixed 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 field-emission scanning electron microscopy (FE-SEM, HITACHI S-4800, Japan) and high-resolution transmission electron microscope (HR-TEM, HITACHI H-800, Japan).



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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. 1 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 glove box (Labmaster130, Germany). Galvanostatic charge/discharge tests were implemented on a battery tester (LAND-CT2001A, China) at selected current rates (1C=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 10-2 Hz with an alternating current perturbation signal of 5 mV. 3. RESULTS AND DISCUSSION Figure 1(a) 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 1(c). The inset on the top right corner of Figure 1(a) is the weak reflections between 20° and 24°, which corresponds to the monoclinic C2/m phase, are characteristic of a Li2MnO3 component (Figure 1(b)) with LiMn6 cation arrangements in the transition metal layers.35-37 The


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atomic structure sketch of layered Li[Li0.2Ni0.2Mn0.6]O2 is shown in Figure 1(d). 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 more perfect layered structure, better crystallinity and higher ordering of lithium and transition metal compared with sample M2.

Figure 1. XRD patterns (a) 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. The morphologies of two cathodes are observed by FE-SEM (Figure 2). In Figure 2(a) and (c), the secondary spherical particles of sample M1 have the apparently more



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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. regular globular morphologies and more uniform microsphere distributions. The primary particles of sample M2 exhibit obvious aggregations (Figure 2(b) and Figure S1), whereas those of sample M1 are well-defined nanoscaled particles on the surface of microspheres. Fortunately, one of the M1 microsphere cross section is exposed as shown in Figure 2(e), we can clearly find the interior of M1 microsphere is also consisted of uniform nanoparticles about 200 nm (Figure 2(f)). There is space among secondary nanoparticles, so we successfully achieve mesoporous micro-sized materials. This unique


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morphology of sample M1 is attributed to freezing drying process. Both primary and secondary particles are tuned by ice templating method. Precursor of sample M2 with liquid moisture between nano-primary particles, which is dried by the traditional vacuum drying process. With continuous and tardy volatilization, two adjacent particles will lead to mutual attraction owing to 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 micro-particle. 40 Precursor of sample M1 is dried by the freezing drying process, there is ice as a templating between two nanoparticles. With the sublimation of ice, the relative location between the primary nanoparticles will be kept. 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 morphologies and porosity exists among their primary nanoparticles.

Figure 3. Nitrogen adsorption/desorption isotherms and BJH pore size distributions of (a) sample M1 and (b) sample M2. To further verify the porosity of hierarchical microsphere Li[Li0.2Ni0.2Mn0.6]O2, nitrogen adsorption/desorption tests are carried out and the curves are shown in Figure 3. The average of 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



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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.

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


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and corresponding EDX spectra (e) of sample M1. 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 4(a)), which is in accordance with the SEM image of sample M1 (Figure 2(d)). The regions inside the dotted green squares are primary nanoparticles (Figure 4(a)), and the porous property can be once again confirmed by the light and dark regions inside the dotted yellow circles (Figure 4(b)). From the HR-TEM image in Figure 4(c), lattice spacing of (003) plane of R3m group and (020) plane of C/2m symmetry can be indexed as 4.75 Å and 4.2 Å. And deeper analysis of the SAED patterns in Figure 4(d) 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 4(e). 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 5(a). 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 V 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 5(b) 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



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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. 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 5(c), 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 ascribe to the electrochemical activation of Li2MnO3 component that extracts Li2O.36 The initial


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irreversible capacity of sample M1 is only 81.1 mAh g-1, while that of sample M2 reaches as high as 131.7 mAh g-1. 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 micro-sized structure, thus can not only resist electrolyte corrosion but also build up and maintain continuous ion and electron pathways during repeated cycling.

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. In order to evaluate ice templating method endowing better electrochemical performance, the rate cycling abilities of two samples are further cycled at both 2 C and 5C shown in Figure 5(d). Obviously, the cyclic stability of sample M1 at 2 C 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



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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 5(e), 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 5(f). 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/de-intercalation. Electrochemical impedance spectroscopy (EIS) tests are conducted to confirm deeper kinetic information of these two samples. The Nyquist plots of samples M1 and M2 before cycle (at initial voltage) and after 80 cycles (at cut-off voltage) are depicted in Figure 6(a), respectively. Nyquist plots consist of a semicircle at high frequencies that is related with the charge-transfer 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 6(a). 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: =

(1) 2 = + + . (2)

As shown in Eq. (1) and (2), R is the ideal gas constant, T is the absolute


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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 6(b) shows the linearity of the Zre and the ω-1/2 in the low frequency region. Based on 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.

Figure 7. SEM images (a) of sample M1 and (c) magnified sample M1 after 50 cycles (charge and discharge at 1 C). HRTEM image (b) of sample M1 after 50 cycles and SEAD patterns of (d) the green and (e) yellow square region in (b). EDX elemental distribution mapping (f)-(i) of sample M1 after 50 cycles. The excellent structure stability of sample M1 is further demonstrated by the FE-SEM and HR-TEM images. The coin cells cycled after 50 cycles at 1 C are dissembled and the electrodes are rinsed by organic solvent in the glove box. Figure 7(a)



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provides SEM image of the electrode after 50 cycles. All of the secondary micro-sized particles still exist. It is noteworthy that the hierarchical mesoporous morphologies are maintained as shown in Figure 7(c), which is also supported by the HR-TEM image and EDX mapping (Figure 7(f-i)). In Figure 7(b), detailed structural evolution of cycled sample M1 is further observed under HR-TEM. Parallel lattices in Figure 7(d) are similar to the ones in Figure 4(c), 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 (400) plane of the cubic spinel structure. It suggests that the cation well-orderedly layered structure has locally turned to spinel-like phase due to 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 1C. Undoubtedly, this mesoporous Li-rich cathode material has a superior structural stability and produces a better electrochemical performance. 4. CONCLUSION In summary, hierarchical mesoporous Li[Li0.2Ni0.2Mn0.6]O2 microsphere composed of nanoparticles via ice templating route combined with co-precipitation 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 co-precipitation

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/de-intercalation kinetics. More importantly, the low-cost, environmentally benign and versatile ice templating technique will be


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referential for controllably synthesizing other electrode materials for secondary batteries system. ASSOCIATED CONTENT Supporting Information Available: SEM image of primary particles of sample M2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] (Y. Bai). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge support from the National Key Basic Research Program of China (No. 2015CB251100), the Program for New Century Excellent Talents in University (No. NCET-13-0033), the Beijing Co-construction Project (No. 20150939014), and the New Energy Automobile Industry Technological Innovation Project of China (2012-2015).



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TOC GRAPHIC Hierarchical mesoporous Li[Li0.2Ni0.2Mn0.6]O2 microsphere composed of nanoparticles is synthesized via an ice templating combined co-precipitation strategy. It is a low-cost, eco-friendly and easily-operated method using ice as a template to control material with homogenous morphology and rich porous channels. The as-prepared material exhibits remarkably enhanced electrochemical performances.


Hierarchical Mesoporous Lithium-Rich Li[Li0.2Ni0.2Mn0.6]O2

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