Exposing the {010} Planes by Oriented Self-Assembly with

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6407−6414

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Exposing the {010} Planes by Oriented Self-Assembly with Nanosheets To Improve the Electrochemical Performances of Ni-Rich Li[Ni0.8Co0.1Mn0.1]O2 Microspheres Yuefeng Su,†,‡,§,⊥ Gang Chen,†,⊥ Lai Chen,*,† Weikang Li,† Qiyu Zhang,† Zhiru Yang,† Yun Lu,†,‡,§ Liying Bao,†,‡,§ Jing Tan,∥ Renjie Chen,†,‡,§ Shi Chen,†,‡,§ and Feng Wu†,‡,§ †

Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, and ∥School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, PR China ‡ Collaborative Innovation Center for Electric Vehicles in Beijing, Beijing 100081, PR China § National Development Center of High Technology Green Materials, Beijing 100081, PR China S Supporting Information *

ABSTRACT: A modified Ni-rich Li[Ni0.8Co0.1Mn0.1]O2 cathode material with exposed {010} planes is successfully synthesized for lithium-ion batteries. The scanning electron microscopy images have demonstrated that by tuning the ammonia concentration during the synthesis of precursors, the primary nanosheets could be successfully stacked along the [001] crystal axis predominantly, self-assembling like multilayers. According to the high-resolution transmission electron microscopy results, such a morphology benefits the growth of the {010} active planes of final layered cathodes during calcination treatment, resulting in the increased area of the exposed {010} active planes, a well-ordered layer structure, and a lower cation mixing disorder. The Li-ion diffusion coefficient has also been improved after the modification based on the results of potentiostatic intermittent titration technique. As a consequence, the modified Li[Ni0.8Co0.1Mn0.1]O2 material exhibits superior initial discharges of 201.6 mA h g−1 at 0.2 C and 185.7 mA h g−1 at 1 C within 2.8−4.3 V (vs Li+/Li), and their capacity retentions after 100 cycles reach 90 and 90.6%, respectively. The capacity at 10 C also increases from 98.3 to 146.5 mA h g−1 after the modification. Our work proposes a novel approach for exposing high-energy {010} active planes of the layered cathode material and again confirms its validity in improving electrochemical properties. KEYWORDS: Li[Ni0.8Co0.1Mn0.1]O2, Ni-rich cathode material, active {010} planes, self-assembly, high-rate performance EVs.8 These applications usually call for sufficient energy and long calendar life.9 Increasing the Ni content in Li[NixCoyMn1−x−y]O2, named as Ni-rich cathode materials (x > 0.6), could achieve higher capacity.10 Li[Ni0.8Co0.1Mn0.1]O2, as a representative Ni-rich cathode material, has much higher capacity compared with the commercial cathode materials such as LiCoO2 and LiFePO4. However, the higher the Ni content is, the severer the problems of Ni-rich cathodes are. For example, because of the similar

1. INTRODUCTION The exhaustion of energy resources has led to a significant development of new energy storage devices. Lithium-ion batteries (LIBs) have attracted much attention because of their high energy density, high capacity, good cycling stability, and environmentally friendly property.1,2 They have been successfully applied in 3C (computer, communication, and consumer electronics) products.3 Nowadays, layered Li[NixCoyMn1−x−y]O2 (0 < x < 1, 0 < y < 1, 0 < 1 − x − y < 1) as the cathode material for LIBs has been widely studied4−7 because of its low cost and high capacity, which has been considered as one of the most promising cathode materials to meet the requirements of electric vehicles (EVs) and hybrid © 2018 American Chemical Society

Received: December 13, 2017 Accepted: January 31, 2018 Published: January 31, 2018 6407

DOI: 10.1021/acsami.7b18933 ACS Appl. Mater. Interfaces 2018, 10, 6407−6414

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic diagrams for the precursors and their corresponding final pristine/modified Li[Ni0.8Co0.1Mn0.1]O2. solution, NaOH solution, and NH3·H2O solution were separately pumped into a continuous-stirred tank reactor (CSTR) under the atmosphere of argon. The pH value was kept at 11.5 ± 0.2. The reaction was operated at 55 °C in an aqueous bath. After the reaction, the obtained precursor was filtered and washed with deionized water and then dried in the vacuum oven at 80 °C overnight. The prepared Ni 0.8 Co 0.1 Mn 0.1 (OH) 2 was mixed with 5% excess of Li 2 CO 3 thoroughly and precalcined in air at 550 °C for 3 h and then heated at 750 °C for 15 h in the muffle furnace to obtain Li[Ni0.8Co0.1Mn0.1]O2. The sample sintered from the precursor synthesized by highconcentration (4 M) NH3·H2O was labeled as “HLNCM”, and the sample from the low-concentration (2 M) NH3·H2O one was labeled as “LLNCM”. The compositions of materials are similar, and the detailed information can be found in the Supporting Information (Table S1). 2.2. Material Characterization. The crystalline phases of the prepared material were characterized by powder X-ray diffraction (XRD) at the scan rate of 0.2° 2θ/min and ranged from 10° to 80° (Rigaku UltimaIV-185 instrument). The morphologies of the samples were characterized by field-emission scanning electron microscopy (FEI Quanta 250 instrument). The transmission electron microscopy (TEM) and the high-resolution TEM (HRTEM) measurements were carried out by using a JEOL JEM-2100 instrument. The compositions of the samples were detected by inductively coupled plasma optical emission spectrometry (ICP−OES, Agilent ICPOES730). The valence states of Ni, Co, and Mn ions were examined by X-ray photoelectron spectroscopy (XPS). XPS was performed on a PHI Quantera (ULVAC-PHI, Inc.) spectrometer with monochromatic Al Kα radiation (hν = 1486.7 eV). During XPS measurement, the base pressure of the sample chamber was kept below 3.0 × 10−10 mbar. 2.3. Electrochemical Measurements. The electrochemical performances of the cathodes were tested by CR2025 coin-type halfcells, which were assembled in a glovebox under argon atmosphere. The electrodes were made up of the as-prepared material powder, carbon black, and polyvinylidene fluoride with 80:10:10 wt %. All these materials were dissolved in N-methyl pyrrolidone solvent, and the slurry was then spread onto an aluminum foil and dried in a vacuum oven at 120 °C for 24 h and punched into pellets with 14 mm diameter. The loading of the active material was in an average of 2.14 mg cm−2. The anode of the cell was lithium metal and was separated by a porous polypropylene film (Celgard 2400). The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (1:1:1 in volume ratio). The galvanostatic charge/ discharge tests of the cells were performed at 2.8−4.3 V (vs Li+/Li) at different rates (1 C = 200 mA g−1) by using a CT2001A Land Instrument at room temperature (25 °C). Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 105−10−2 Hz with an amplitude of 5 mV via an Electrochemical Workstation (CHI660E, Shanghai, China). The EIS measurements were performed after 100 cycles (1 C/1 C) at the charge state of 4.3 V to assess the

radius of Li+ (0.076 nm) and Ni2+ (0.069 nm), the high Ni content would suffer severe Li+/Ni2+ ion exchange, known as cation mixing.11 The existence of cation mixing will seriously influence Li+-ion diffusion, causing unsatisfied electrochemical performances. Besides, Ni-rich materials also suffer low initial Coulombic efficiency (CE) and poor rate performance. To solve these problems, many efforts have been expended, such as doping cations to enlarge the Li slab space,12,13 adding functional additives in electrolytes to suppress oxygen release,14,15 modifying the surface of the material to stabilize the structure,1,16−19 and designing a special structure to shorten the Li+ diffusion path.20−22 As for rate capability, it is found that the surface microstructure of the cathode material is critical to Li+ transport. Winter et al.23 proved that the overall specific capacity loss is induced by kinetic limitations, and Chen et al.3 provided that it is useful to improve the rate performance by affording unimpeded Li+ migration. Our previous works also confirmed that the rate performance of layered cathode materials can be enhanced by increasing the areas of exposed {010} active planes.24−26 Nevertheless, Guo et al. have shown that the (001) plane-dominated primary particles would be preferentially formed during the coprecipitation reaction.27 Worse still, the {010} planes tend to disappear during calcination as they are high-energy facets in the hexagonal crystal system.26 Therefore, increasing the areas of exposed {010} planes is still a challenging task. Here, we propose a facile way to increase the areas of exposed {010} active planes in the Li[Ni0.8Co0.1Mn0.1]O2 material (as shown in Figure 1). By tuning the ammonia concentration during the coprecipitation reaction, the primary nanosheets can be self-assembled regularly, stacking along the [001] crystal axis.27,28 According to the testing results, such an oriented arrangement has been confirmed to be in favor of achieving an increased area of {010} active planes during calcination because of which the rate performance would be significantly improved.29

2. EXPERIMENTAL SECTION 2.1. Synthesis of Pristine and Modified Samples. The Ni0.8Co0.1Mn0.1(OH)2 precursor was prepared via the coprecipitation method. NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O with the stoichiometric ratio of 8:1:1 were dissolved in deionized water to form a 4.0 mol L−1 solution. A stoichiometric amount of NaOH (as a precipitating agent) solution and NH3·H2O (as a chelating agent) aqueous solution was prepared in the same way. The sulfate salt 6408


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Figure 2. XRD patterns of the as-prepared Li[Ni0.8Co0.1Mn0.1]O2 materials for (a) HLNCM and (b) LLNCM. “R” refers to the “reference peak”.

Table 1. Rietveld Refinement Results of HLNCM and LLNCM Based on the XRD Patterns in Figure 2 sample

a (Å)

b (Å)

c (Å)

Vol (Å3)

wRp

Rp

I(003)/I(104)

HLNCM LLNCM

2.874134 2.874250

2.874134 2.874250

14.208426 14.202621

101.646 101.613

0.1255 0.1110

0.0950 0.0847

1.31 1.02

Figure 3. SEM images of (a) precursors of LLNCM; (b) precursors of HLNCM; (c) LLNCM; and (d) HLNCM.

means that HLNCM has lower cation mixing than LLNCM.36 Detailed data are listed in Table 1. The scanning electron microscopy (SEM) images of the precursors and final cathode materials are presented in Figure 3. Figure 3a,b shows that the secondary precursor particles are assembled by primary nanosheets with an average thickness of around 25 nm. However, the rectangular regions of Figure 3a,b clearly illustrate the differences between these two precursors. The HLNCM precursors comprise of self-assembled nanosheets with an orderly arrangement, whereas the LLNCM precursors are assembled with different packing degrees and form a flowerlike microsphere.37 These two different stacking styles of precursors are then proved to have a huge effect on the morphology of final cathodes. Figure 3c,d shows that the thickness of the exposed planes in the HLNCM particle is about 130 nm, whereas that of LLNCM is around 93 nm. The huge difference in the thickness suggests that the oriented stacking of primary nanosheets benefits maintaining the lateral planes during calcination treatment, although faint contraction

impedance changes of the cathode electrode after long cycles. The potential steps of the potentiostatic intermittent titration technique (PITT) experiments were 20 mV, and the current was measured as a function of time for the initial charge process. The current threshold limit was set at 0.01 mA, and the potential step would advance to the next level when the measured current fell below 0.01 mA. The voltage window was set at 3.44−4.30 V.

3. RESULTS AND DISCUSSION The refined XRD patterns are depicted in Figure 2, which shows that both HLNCM (Figure 2a) and LLNCM (Figure 2b) have the well-ordered layer structure based on a hexagonal 30 α-NaFeO2 with a space group of R3m ̅ . The peak splitting of HLNCM between I(006)/I(102) and I(018)/I(110) is clearer than that of LLNCM, indicating a better layer structure.31−33 The I(003)/I(104) intensity ratio usually reflects the degree of the cation mixing. If the ratio is higher than 1.2, it is believed that the material has desirable cation mixing degree.34,35 According to the refinement results, the ratio value of I(003)/I(104) is 1.31 in HLNCM, which is much higher than 1.02 in LLNCM. This 6409


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Figure 4. TEM images of (a) HLNCM particles; (b) single LLNCM particle; high-resolution TEM images and FFT graphs of (c) HLNCM particle and (d) LLNCM particle; and (e) schematic diagrams of Li+ diffusion in {010} active planes.

Figure 5. Electrochemical performances of LLNCM and HLNCM. (a) Rate performance; (b) discharging profiles of samples at different rates; (c) cycling performances at 1 and 10 C; (d) cycling performance and CE at 0.2 C; (e) discharging profiles in 1st, 10th, 30th, and 50th cycles at 0.2 C; and (f) initial charge and discharge voltage profiles at 0.2 and 1 C.

{010} planes, the FFT analysis was performed. The FFT images from region (i) and region (ii) are placed in the corresponding figures. The indexed bright spots in FFT images mainly correspond to (003), (104), and other diffractions viewed down from the [010] zone axis. These algorithm results confirm that the lateral plane observed in region (i) and region (i) belongs to {010} planes. Such clear fringes can only be observed from the lateral planes of primary nanosheets in both HLNCM and LLNCM particles. Therefore, combined with the results of Figure 3, we can conclude that the increase in the thickness of lateral planes for HLNCM means its areas of the exposed active {010} planes are 40% larger than that of LLNCM. Therefore, we can conclude with confidence that tuning the concentration of NH3·H2O during coprecipitation contributes to the self-assembled primary particles with an orderly arrangement and the enhanced growth of {010} active planes. As mentioned above, as for layered materials, Li ions only diffuse along the two-dimensional pathway perpendicular to {010} planes (as illustrated in Figure 4e). Hence, the

is inevitable. Specifically, the thickness of the HLNCM particle increased to about 40% with the help of orderly arrangement of primary nanosheets. TEM and HRTEM with the fast Fourier transform (FFT) graphs were applied to analyze the microstructure of the materials. Figure 4a,b shows the TEM images of the HLNCM and LLNCM particles. The diameters of both two particles are much larger than those marked in Figure 3c,d, implying that the marked planes in Figure 4a,b are different from those marked in Figure 3. Logically, the magnified HRTEM images (region I and region II in Figure 4a,b) shown in Figure 4c,d, that is, the lateral planes of the particles, should correspond to the marked regions in Figure 3c,d. As shown in Figure 4c,d, the edge area of the LLNCM particle is disordered (region III), whereas the edge of the HLNCM particle is clear. Furthermore, the separation between well-defined lattice fringes in Figure 4c,d is 4.75 Å, which is in accordance with the distance between transition-metal layers along the [003] direction of layered materials. To further verify that the observed planes belong to 6410


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ACS Applied Materials & Interfaces Table 2. Initial Discharge Capacities of HLNCM and LLNCM at Various Rates 0.1 C

0.2 C

0.5 C

1C

HLNCM LLNCM

2C

5C

10 C

174.8 151.3

163.6 123.8

146.5 98.3

mA h g−1

samples 201.4 190.2

200.0 180.2

198.5 179.4

185.9 168.1

Figure 6. (a) Nyquist plots of HLNCM and LLNCM after 100 cycles at 1 C charge/discharge and (b) partially magnified region from (a); (c) equivalent circuit model of the EIS test; (d) calculated Li-ion diffusion coefficient from PITT tests, which is illustrated in (e) HLNCM and (f) LLNCM.

electrochemical performances of HLNCM are expected to be improved. The electrochemical performances of HLNMO and LLNMO were then tested, and the results are shown in Figure 5. The comparison of high-rate performances of HLNMO and LLNCM materials is evidenced in Figure 5a. The specific capacities of HLNCM are always better than that of LLNCM under all different rates; especially at the 10 C rate, the specific discharge capacity of HLNCM is around 146 mA h g−1, whereas that of LLNCM is about 98 mA h g−1. The representative discharge profiles at various rates of HLNCM and LLNCM are illustrated in Figure 5b. For HLNCM and LLNCM, the initial discharge capacities at various rates are listed in Table 2. From Figure 5a,b and Table 2, it is easy to conclude that the rate performances of HLNCM are greatly improved with the enhanced {010} active planes, especially at high rates. To test the cycling stability of materials at high rates, the cells were cycled at 1 and 10 C for 100 cycles as shown in Figure 5c. The capacity and the cycling stability of HLNCM are both higher than those of LLNCM. The initial discharge capacities of HLNCM/LLNCM are 186.3/159.6 and 156.2/ 101 mA h g−1 at 1 and 10 C, respectively. Even after 100 cycles, the discharge capacity of HLNCM is still higher than the initial discharge capacity of LLNCM at both 1 and 10 C rates, let alone the 100th discharge capacity of LLNCM. The gap in discharge capacities between HLNCM and LLNCM is much higher at 10 C than that at 1 C rate, which points out that the high-rate performance of the modified material has been improved significantly. However, the degradation still exists, which may be due to the structural change during cycling.26,30,38

The excellent rate performance of HLNCM reveals the advantages of exposed {010} planes in improving the rate capability of the layered cathode material. Nevertheless, increasing exposed areas of {010} planes also means increasing electrolyte contact areas. Therefore, it is necessary to check the cycle performance at a low rate. Figure 5d shows the capacity and cycling stability of HLNCM and LLNCM at 0.2 C. It can be seen that the first discharge capacities of HLNCM and LLNCM are 201.6 and 193.1 mA h g−1, respectively. The discharge capacity of LLNCM at 0.2 C is similar to the average discharge capacity of commercial Li[Ni 0.8 Co 0.1 Mn 0.1 ]O 2 materials, stating that the electrochemical properties of LLNCM are representative.39,40 After 100 cycles, the discharge capacity of HLNMO is maintained at about 183.6 mA h g−1, whereas that of LLNMO dropped to 160.7 mA h g−1. The capacity retention is as high as 91% after 100 cycles for HLNCM, whereas that of LLNCM is maintained at only 87%. As shown in Table S2, the electrochemical performance of HLNCM is also comparable with some reported results. The superior cycling stability of HLNCM indicates that the increased exposure of {010} planes can improve the rate capability of layered cathodes without suffering a faster fade. Figure 5e indicates the charge/discharge curves of both samples at the 0.2 C rate. The changes between curves of both materials are ascribed to polarization that is caused by the structural change during long cycling. However, the modified material has smaller changes and advantageous cycling performance than the traditional material. This result implies that the increased {010} planes have no adverse impact on the low-rate cycling performance; instead, benefiting from the exposed active planes, the faster Li+ transport helps to alleviate the increasing polarization during cycling. 6411


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LLNCM materials. These results agree well with the discussion of Figure 5, indicating that the increased exposed {010} planes are beneficial to the improvement of the DLi+ value, and finally enhance the electrochemistry performances of HLNCM compared to that of LLNCM.

However, the valence variation of elements could also help in increasing the capacity.41 Therefore, to preclude the influence of the variation of elements, the X-ray photoelectron spectroscopy (XPS) measurements were performed to confirm the oxidation states of Ni, Co, and Mn in both materials. As shown in Figure S1, there is almost no difference between HLNCM and LLNCM samples, indicating that the oxidation states of transition elements are similar, which implies that the capacity increase should be ascribed to the exposed {010} planes. We also compared the 1st cycle voltage profiles of HLNCM and LLNCM at 0.2 and 1 C rates in Figure 5f. The initial CE of the HLNCM/LLNCM materials is similar at the 0.2 C rate (90.6%/90.8%). However, the result is different at the 1 C rate: the initial CE of HLNCM is 87.84%, whereas that of LLNCM is 79.18%. The low initial CE of Ni-rich cathodes may result from the structural transition and unimpeded transport channels for the insertion of Li ions during the first cycle.42,43 The similar CE of the HLNCM/LLNCM materials at low rate precludes the influence from structural transition. Consequently, the enhanced CE of HLNCM at a high rate should be attributed to the increased exposed {010} planes, which affords efficient ion transport for fast Li+ transport kinetics. To support this conclusion, the EIS analysis and PITT test were utilized to evaluate the electrochemical kinetics.44−46 From Figure 6a−c, it can be found that every plot includes two semicircles and a slope. The first semicircle in the highfrequency region refers to the resistance of solid electrolyte interphase film (Rint), and the second semicircle at an intermediate frequency corresponds to charge-transfer resistance (Rct). The slope represents the Li-ion diffusion in the bulk material.27,44,45,47 The SEM images of both samples before/ after cycling are exhibited in Figure S2. It is obvious that after cycling, the surface of the LLNCM electrode became smoother than that of HLNCM. The smoother surface means a thicker cathode electrolyte interphase film on the cathode electrolyte, which is believed to cause higher Rint. This is in accord with EIS results. Besides, the second semicircle of HLNCM, which is smaller than that of LLNCM, suggests that the ordered selfassembly of primary particles can decrease the charge-transfer resistance of the materials. Thus, it is reasonable to conclude that both the Li-ion diffusion coefficient (DLi+) and the Li-ion diffusion rate of HLNCM would be higher than those of LLNCM. To further measure the DLi+ and the Li-ion diffusion rate, the PITT test was carried out. The cells were charged to 4.3 V with a voltage step of 20 mV. The Li-ion transport in the electrode obeys Fick’s second law because it is a semi-infinite system,48−50 so the DLi+ can be calculated by the following equation DLi

+

4L2 dln(I ) =− 2 d(t ) π

4. CONCLUSIONS In summary, we successfully synthesized a hierarchical micro-/ nanostructured Li[Ni0.8Co0.1Mn0.1]O2 material via a simple coprecipitation method by controlling the NH3·H2O concentration. We found out that increasing the NH 3 ·H 2 O concentration during the coprecipitation reaction can induce the primary nanosheets to stack along the [001] crystal axis predominantly, self-assembling like multilayers. Such a morphology has been determined to facilitate the growth of high-energy {010} active planes during the following calcination treatment. The resulting modified Ni-rich material with exposed {010} active planes (HLNCM) shows a wellordered layer structure and a lower cation mixing disorder. Electrochemical results demonstrate that HLNCM exhibits both higher reversible capacity and better rate capability than the pristine one (LLNCM). The reversible capacity of HLNCM is measured as high as 201.6 mA h g−1 with a CE of 90.53% and the discharge capacity is still maintained at 183.6 mA h g−1 at 0.2 C rate after 100 cycles. Besides, HLNCM also shows superior rate performance (165 mA h g−1 at 5 C and 146 mA h g−1 at 10 C) and excellent cyclic stability (capacity retentions of 90 and 90.6% after 100 cycles at 0.2 and 1 C, respectively). This study provides a new facile approach for exposing high-energy {010} active planes, which can be applied to other layered cathode materials synthesized by the CSTR tanker.

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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.7b18933. XPS patterns of the survey spectrum, Ni 2p, Co 2p, and Mn 2p for LLNCM and HLNCM materials; SEM image of LLNCM and HLNCM before/after cycling tests with low magnification; Li/Ni/Co/Mn molar ratios calculated from ICP−OES for LLNCM and HLNCM samples; and electrochemical performance comparison between HLNCM and other reported results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-010-68918099. ORCID

Yuefeng Su: 0000-0002-5144-2832 Lai Chen: 0000-0002-8226-3140 Yun Lu: 0000-0002-7783-1561 Renjie Chen: 0000-0002-7001-2926

(1)

where L refers to the thickness of the active material on the electrode, I means the step current, and t refers to the step time during the test procedure. Figure 6d shows the calculated Li-ion diffusion coefficient curves of HLNCM and LLNCM. The details of PITT tests for HLNCM and LLNCM are described in Figure 6e,f, respectively. Figure 6e,f illustrates the potential and current curves obtained from a voltage step size of 20 mV, and the linearity of ln(I) versus t is good enough for eq 1. As shown in Figure 6d, in the range of 3.50−4.30 V, HLNCM monotonously exhibits much higher DLi+ compared to the

Author Contributions ⊥

Y.S. and G.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by National Key R&D Program of China (2016YFB0100301), National Natural Science Foundation of 6412


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China (51472032, 21573017, and U1664255), and the Major achievements Transformation Project for Central University in Beijing.

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