High-Rate and Cycling-Stable Nickel-Rich Cathode Materials with

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High-Rate and Cycling-stable Nickel-rich Cathode Materials with Enhanced Li Diffusion Pathway +

Jun Tian, Yuefeng Su, Feng Wu, Shaoyu Xu, Fen Chen, Renjie Chen, Qing Li, Jinghui Li, Fengchun Sun, and Shi Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09641 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on November 29, 2015

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ACS Applied Materials & Interfaces

High-Rate and Cycling-stable Nickel-rich Cathode Materials with Enhanced Li+ Diffusion Pathway Jun Tian,† Yuefeng Su,*,†‡ Feng Wu,*,†‡ Shaoyu Xu,§ Fen Chen,§ Renjie Chen,†‡ Qing Li,† Jinghui Li,† Fengchun Sun,‡ Shi Chen†‡ †

School of Material Science and Engineering, Beijing Institute of Technology, Beijing

Key Laboratory of Environmental Science and Engineering, Beijing, 100081, P. R. China ‡

Collaborative Innovation Center for Electric Vehicles in Beijing, Beijing, 100081, P.

R. China §

China North Vehicle Research Institute, Beijing, 100072, P. R. China

KEYWORDS: Lithium-ion batteries, Nickel-rich layered material, Li+ transportation, Cycling stability, Rate capability

ABSTRACT: The Nickel-rich LiNi0.7Co0.15Mn0.15O2 material was sintered by Li source with the Ni0.7Co0.15Mn0.15(OH)2 precursor, which was prepared via hydrothermal treatment after co-precipitation. The intensity ratio of I(110)/I(108) obtained from

X-ray diffraction patterns and high-resolution transmission

electronmicroscopy confirm that the particles have enhanced growth of (110), (100) and

(010)

surface

planes,

which

supply

superior

inherent

Li+

deintercalation/intercalation. The electrochemical measurement shows that the

ACS Paragon Plus Environment


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LiNi0.7Co0.15Mn0.15O2 material has high cycling stability and rate capability, along with fast charge and discharge ability. Li+ diffusion coefficient at the oxidation peaks obtained by cyclic voltammogram measurement is as large as 10-11 (cm2 s-1) orders of magnitude, implying that the Nickel-rich material has high Li+ diffusion capability.

1 Introduction

Rechargeable lithium-ion batteries (LIBs) play an important role human’s life nowadays, especially in portable electronics, electric vehicles (EVs) and large-scale energy storage devices.1-4 In order to expand these applications, the development of cathode materials which supply high energy density and high power density is urgently needed.5 Among the cathode materials, Nickel-rich layered materials have been researched vastly because of their high capacity (> 200 mAh g-1) with a high charge voltage over 4.5 V, which contributing a high energy density. However, their commercialization has been impeded for that these materials undergo poor rate capability and cycling performance. 6-12 In recent years, many efforts have been made to improve the electrochemical performance of cathode materials, such as surface modification13-15 or fabrication of nanoarchitecture paticles16-18, which afford a short Li+ transportation pathway owing to their nanoscale dimensions. Up to now, it has been widely reported that Li+ preferably moves along the direction parallel to the Li+ layers for an α-NaFeO2 structure layered material.19-21 As elucidated in Figure 1, As elucidated in Figure 1, (001) plate and (010) plate grow perpendicular to c axis and a (or b) axis, respectively.


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((010) plate is equivalent to (100) plate) However, only the (010) lattice planes including (010), (100) and (110) nanoplates supply unimpeded Li+ diffusion path. In the present research, we report a Nickel-rich LiNi0.7Co0.15Mn0.15O2 cathode material, the crystal nanoplates dominated by exposed Li+ active surface planes contributing to rapid Li+ deintercalation/intercalation, resulting in excellent electrochemical properties for the material.

Figure 1. (001) and (010) plates and the Li+ diffusion pathway.

2 Experimental

2.1

Synthesis method

The Ni0.7Co0.15Mn0.15(OH)2 precursor was prepared via hydrothermal method



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after

co-precipitation.

Ni(CH3COO)2·4H2O,

Co(CH3COO)2·4H2O

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and

Mn(CH3COO)2·4H2O with the stoichiometric ratio of 0.70: 0.15: 0.15 were dissolved into a transparent solution. At the same time, a stoichiometric amount of NaOH (precipiting agent) solution was pumped into the above solution under Ar atmosphere. The pH value was kept at 11 constantly by pumping NH3·H2O aqueous solution. After reaction, the mixed solution was transfered into a Teflon stainless reactor with 80% volume filling, adding polyvinylpyrrolidone (PVP) as the surface-directing agent22 in the hydrothermal process. The Teflon reactor was heated in an air-circulating oven for 4 h at 150 °C and naturally cooled down to room temperature afterwards. The hydroxide precursor was then seperated via vacuum filtration and washed several times with deionized water, and finally dried in the vacuum oven at 70°C. The dried Ni0.7Co0.15Mn0.15(OH)2 was mixed with LiOH·H2O (5% excess amount), then calcinated for 6 h at 450 °C preliminarily and fired for 12 h at 800°C in air to obtain the final product.

2.2 Material Characterization

The crystalline phases of the precursor and cathode material were characterized by X-ray powder diffraction (XRD) at the scan rate of 0.2° 2θ/min (Rigaku UltimaIV-185 Instrument). Field emission scanning electron microscopy (FESEM) was adopted to characterize the morphology of the material particles (FEI QUANTA 250 Instrument). Transmission electronmicroscopy (TEM) and high-resolution transmission electronmicroscopy (HRTEM) were operated upon a JEOL JEM-2100


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instrument.

2.3 Electrochemical measurement

A slurry coating procedure was adopted to prepare the working electrodes. The active material, conductive agents (carbon black), polyvinylidene fluoride (PVDF) with 80 wt.%, 10 wt.%, 10 wt.% mass fraction were dissolved in N-methyl pyrrolidone (NMP) solvent. The slurry was uniformly coated onto the aluminum foil and dried at 80°C and punched into discs of with 14 mm diameter. The average areal density of the active material is 9.132×10-2 mg cm-2. The electrodes were assembled into CR2025 coin-type cells with Li electrodes and electrolyte in an Ar-filled glove box. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate (EC : EMC : DMC = 1 : 1 : 1 in volume). The charge/discharge tests were performed galvanostatically at desired potential (vs. Li+/Li) and current densities using CT2001A Land instrument at room temperature. Cyclic voltammograms (CV) were carried out at different scan rate between 2.5 and 4.6 V to determine the lithium ion diffusion coefficient.

3 Results and discussion

The Ni0.7Co0.15Mn0.15(OH)2 XRD patterns exhibited in Figure 2a confirm that the precursor is typical M(OH)2 oxide (M= Ni, Co or Mn).23, 24 The SEM images of Ni0.7Co0.15Mn0.15(OH)2 precursor are presented in Figure 2b,c. The precursor particles are nanosheets with an average diameter around 300 nm. PVP drives the precursor



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regrowth into nanosheets for its adsorbate-directed effect during the hydrothermal process. PVP molecules adsorb on its negatively charged (001) surface, reducing the plane growth parallel to c-axis direction and contributing to the formation of nanoplates dominated by (001) plane.22 Energy dispersive X-ray spectroscopy (EDX) (Fig. S1) reveal that the Ni, Co, Mn atoms are uniformly distributed and the atom ratio is 0.72: 0.15: 0.13, which is close to the designed ratio 0.7:0.15: 0.15. Figure 2d shows the XRD patterns of the as-prepared LiNi0.7Co0.15Mn0.15O2 material. Rietveld method is adopted to refine the XRD patterns. It is observed that the LiNi0.7Co0.15Mn0.15O2 material has a layered hexagonal α-NaFeO2 structure with R 3 m space group. The (006)/(102) and (018)/(110) peaks are clearly separated, demonstrating that the layered structure is highly crystallized25, 26. It is obviously that the intensity of (110) peak is stronger than the (108) peak. According to the calculated results from the refined XRD patterns, the ratio values of I(110)/I(108) is 1.18. This clearly imply the enhanced growth of the (110) planes.27 The ratio of I(110)/I(108) obtained from the XRD patterns is not sufficient. Because the (100) and (010) peaks cannot be observed for the α-NaFeO2 structure (space group, R 3 m) from the XRD patterns due to the crystal extinction. Therefore, the HRTEM analysis was performed to provide further more experimental evidence as described later. The SEM images of the LiNi0.7Co0.15Mn0.15O2 material exhibited in Figure 2e,f show that the LiNi0.7Co0.15Mn0.15O2 particles are nanoplate-like which have an average diameter around 300 nm with a little agglomeration.


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Figure 2. XRD patterns of (a) the precursor and (d) as-prepared material; SEM images of (b,c) the precursor and (e,f) LiNi0.7Co0.15Mn0.15O2 material.

HRTEM characterization of the LiNi0.7Co0.15Mn0.15O2 was performed and the results are shown in Figure 3 to elucidate the specific characteristics of the nanoplates. Several nanoplates can be obviously seen in Figure 3b from high magnification. HRTEM images from two different plates (region I and region II) are displayed in Figure 3c−f. In both regions, the apparent lattice fringes with the interplanar distance of 0.475 nm can be clearly observed, which corresponds to the planar distance between (003) planes of R 3 m structure, implying that the presented image planes of the nanoplates grow along the c axis, i.e., parallel to (010), (100), or (110) planes. The as-obtained Ni-rich material with enhanced growth of (100) and (010) surface planes has superior inherent Li+ deintercalation/intercalation, thus we assume the as-prepared material has great rate capability, stable cycling perfromance and high power density.



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Figure 3. (a,b) TEM images of the as-prepared material; (c-f) HRTEM images of the as-prepared mateial: (c) the magnification of region I, (d) the magnification of the white rectangle region in (c), (e) the magnification of region II and (f) the magnification of the white rectangle region in (e) The cycling performance of the LiNi0.7Co0.15Mn0.15O2 material (1 C = 200 mA g-1) obtained under the 2.7~4.3 V, 2.7~4.5 V and 2.7~4.6 V ranges and the relevant charge/discharge

profiles

are

exhibited

in

Figure

4.

The

corresponding

electrochemical data of are shown in Table 1. We can conclude that both the initial charge capacity and initial discharge capacity increase by enhancing the upper cut-off voltage. However, the initial coulombic efficiency decreases from 92.5% (2.7~4.3 V) to 76.3% (2.7~4.6 V), this is reasonable for that the reversibility of Li+


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deintercalation/intercalation is worse at higher upper cut-off voltage. The material reveal a capacity retention of 92.5%, 88.1%, 76.3% after 80 cycles under the voltage ranges of 2.7~4.3 V, 2.7~4.5 V and 2.7~4.6 V, respectively. More severe structural change occurs during cycling under a higher upper cut-off voltage, leading to a more serious capacity decay. The charge/discharge voltage profiles at 0.2 C between 2.7 and 4.3, 4.5, 4.6 V, respectively, after the 1st, 30th, 50th and 80th cycles are exhibited in Figure 4b~d. The voltage of layered cathode material will decrease due to polarization caused by structural change during cycling.28, 29 The average working voltage decrease more rapidly under higher upper cut-off voltage upon cycling, which further indicates that a high charge voltage will cause severe structural change. However, the as-prepared LiNi0.7Co0.15Mn0.15O2 material has an advantageous cycling performance even under a high charge voltage compared with the previous reports for Ni-rich materials.6, 12, 30 This is owing to the easy Li+ transportation during cycling.

Table 1. Electrochemical data of the as-prepared LiNi0.7Co0.15Mn0.15O2 at 0.2 C charge/discharge cycling Initial charge Voltage (V)

Initial discharge

Initial

80th discharge

Capacity

capacity

capacity

coulombic

capacity

retention after

(mAh g-1)

(mAh g-1)

efficiency (%)

(mAh g-1)

80 cycles (%)

2.7~4.3

206.9

179.0

86.5

165.5

92.5

2.7~4.5

225.8

192.0

85.0

169.2

88.1

2.7~4.6

255.4

203.7

79.8

155.5

76.3



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Figure 4. (a) Cycling performance of the as-prepared LiNi0.7Co0.15Mn0.15O2 at 0.2 C under different upper cut-off voltages; Corresponding charge/discharge profiles under the potential range of (b) 2.7~4.3 V, (c) 2.7~4.5 V and (d) 2.7~4.6 V

Figure 5 shows the corresponding differential capacity vs. voltage (dQ/dV) curves of the 1st, 30th, 50th and 80th cycles charged to 4.5 V at 0.2 C. Phase transitions occur can be predicted from the dQ/dV curves. In the initial charge curve, rhombohedral 1 (R1) to monoclinic (M) phase transition appears at ∼3.63 V (peak I), monoclinic (M) to rhombohedral 2 (R2) phase transition occurs at ∼3.78 V ( peak II), and rhombohedral 2 (R2) to rhombohedral 3 (R3) phase transition occurs at ∼4.32 V (peak III).6 The phase transitions is reversed in the initial discharge process, and the reduction peaks are a little lower than the oxidation peak because of polarization. For the 30th to 80th dQ/dV curves, the transition peaks are all a little lower compared with that of the intial charge/discharge curves. However, the curves are compact to


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each other, which means that the structure is relatively stable during the cycles.

Figure 5. Differential capacity vs. voltage curves of 1st, 30th, 50th and 80th between 2.7 and 4.5 V at 0.2 C

EVs and renewable energy storage application require lithium-ion batteries with high rate capability.31,

32

Figure 6a shows the rate discharge capacities of the

as-prepared LiNi0.7Co0.15Mn0.15O2 sample at the different current densities. The cathode are charged at 0.2 C and discharged at 0.2 , 0.5 , 1 , 2 , 5 and 10 C, respectively for each 10 cycles between 2.7 and 4.5 V, and finally 0.2 C for 5 cycles. It presents the initial discharge capacities of 196.0, 185.6, 177.9, 168.6, 155.6, 143.0 mAh g-1 for 0.2, 0.5, 1, 2, 5, 10 C, respectively. The discharge capacity still stably maintains at 140.4 mAh g-1 after 10 C discharge. The discharge profiles at different rates are exhibited in Figure 6b. The superior rate capabilty is ascribed to the rapid Li+



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deintercalation/intercalation caused by enhanced growth of (010), (100) and (110) surface planes.

Figure 6 The rate performance of LiNi0.7Co0.15Mn0.15O2 material: (a) discharge capacities under rates of 0.2 , 0.5 , 1 , 2 , 5 , 10 C, respectively between 2.7 and 4.5 V and finally 0.2 C; (b) the corresponding discharge profiles

The rate cycling ability has been further determined by 1 C cycling between 2.7 and 4.5 V (as shown in Figure 7). It delivers initial discharge capacity of 175.1 mAh g-1 with coloumbic efficiency of 81.9%. The discharge capacity maitains at 147.8 mAh g-1 (84.4% capacity retention) after 100 cycles. The morphology of the LiNi0.7Co0.15Mn0.15O2 particles maintains well after cycling as shown in Fig. S2 compared with that of the pristine material, except for that some little cracks occur. This implies that the as-prepared LiNi0.7Co0.15Mn0.15O2 has fast charge and discharge capability as cathode material.


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Figure 7 1C charge/discharge cycling ability of the as-prepared LiNi0.7Co0.15Mn0.15O2 The Li+ diffusion coefficient is determined by CV measurement at various scan rates ranging from 0.1 to 1 mV s−1 in a potential range of 2.5~4.6 V versus Li+/Li for the first circle (Figure 8a). The obtained data were used to analyze the diffusion coefficient at the peak positions via the Randles–Sevcik equation33, 34: ip = k n3/2 A D1/2 C v1/2

(1)

ip stands for the peak current (A), the constant k = 2.69×105 C mol−1 V-1/2 under standard conditions (temperature of 25 °C), n represents the number of electrons involved in the electrochemical process (n = 1), A is the electroactive area (1.095×104 cm2 g-1). Here Li+ concentration C = 1.631×10-2 mol cm−3 (one Li+ occupies in average unit cell, and the unit cell volume = 101.79 Å3 from cell refinement using jade), v represents the potential scan rate (V s-1). The peak current ip increases in correlation with the square root of the scan rate v.



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By plotting the slope of the peak current ip versus the square root of the scan rate v, the apparent diffusion coefficient can be determined from the slope of this linear curve (Figure 8b). For the linear curve: y = a + bx a≈0,

b = k n3/2 A D1/2 C

(2)

The calculation result is that the Li+ diffusion coefficient at the oxidation peak I DOI= 3.029×10-11cm2 s-1, and that at the reduction peak I DRI=4.255×10-12cm2 s-1. These are relatively large apparent Li+ diffusion coefficients for the layered cathode mateial35-38, implying that the as-prepared layered LiNi0.7Co0.15Mn0.15O2 has high Li+ diffusion capability.

Figure 8 (a) Cyclic voltammograms at scan rate of 0.1 to 1 mV s-1 between 2.5 and 4.6 V, (b) maximum peak currents in relation to the square root of scan rate

4 Conclusion

In our work, Ni0.7Co0.15Mn0.15(OH)2 precursor was prepared via a hydrothermal method after co-precipitation, after sintering with Li source, the LiNi0.7Co0.15Mn0.15O2 material was successfully obtained. The LiNi0.7Co0.15Mn0.15O2 particles have enhanced


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growth of (010), (100) and (110) surface planes which supply enhanced Li+ diffusion pathway. Therefore, the material has the merits of stable cycling performance and high rate capability, as well as fast charge and discharge ability as cathode materials in lithium-ion batteries. Thus we conclude that the LiNi0.7Co0.15Mn0.15O2 material as reported is prospective to be applied in EVs and high-power storage applications.

AUTHOR INFORMATION

Corresponding Author *Corresponding

Author:

Yuefeng

Su.

Tel:

+86-10-6891-8099;

Fax:

+86-10-6891-8200; Email address: [email protected] & Feng Wu, Tel: +86-10-6891-2508;

Email address: [email protected].

ACKNOWLEDGMENT This work was funded by the Chinese National 973 Program (2015CB251100), National Natural Science Foundation of China (51472032, 51202083), Program for New Century Excellent Talents in University (NCET-13–0044), the Special Fund of Beijing Co-construction Project and BIT Scientific and Technological Innovation Project (2013CX01003), National Key Technology R&D Program (2013BAG10B00), Major achievements Transformation Project for Central University in Beijing.



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SUPPORTING INFORMATION The Supporting Information contains the following contents:

Figure S1. Energy dispersive X-ray spectroscopy mapping of the as-prepared LiNi0.7C00.15Mn0.15O2. The testing results reveal that the atom ratio Ni: Co :Mn = 0.72 :0.15: 0.13. Figure S2. SEM images of the LiNi0.7Co0.15Mn0.15O2 material after 80 cycles at 0.2 C between 2.7 and 4.5 V. (a) ×50000 magnified; (b) ×100000 magnified.


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Table of Contents Graphic


High-Rate and Cycling-Stable Nickel-Rich Cathode Materials with

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