Improved Performances of LiNi0.8Co0.15Al0.05O2 Material

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 38567-38574

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Improved Performances of LiNi0.8Co0.15Al0.05O2 Material Employing NaAlO2 as a New Aluminum Source Ming Liang,† Dawei Song,*,† Hongzhou Zhang,† Xixi Shi,† Qiang Wang,‡ and Lianqi Zhang*,† †

Tianjin Key Laboratory for Photoelectric Materials and Devices, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China ‡ Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609, United States S Supporting Information *

ABSTRACT: To prepare a high-performance LiNi0.8Co0.15Al0.05O2 material (LNCA) for Li-ion batteries, a new aluminum source, NaAlO 2 , is employed in the coprecipitation step for the first time, and the effect of aluminum sources on the performances is systematically investigated. Different from the traditional preparation process using Al(NO3)3 as the aluminum source, the preparation process of the Ni0.8Co0.15Al0.05(OH)2.05 precursor from NaAlO2 is a hydrolysis process, during which the fast precipitation of Al3+ and the formation of a flocculent precipitate can be effectively avoided. As expected, stoichiometric LNCA with uniform element distribution, low cation mixing and well-ordered layered structure is obtained from NaAlO2, which is designed as LNCA-NaAlO2. The characterization and electrochemical measurements show that LNCA-NaAlO2 exhibits significantly improved performances (such as tap density, initial discharge capacity and volumetric energy density, rate performance, cycle performance, electrochemical stability, microstructure stability, and storage stability) compared to the performances of those prepared from Al(NO3)3 (LNCA-Al(NO3)3), indicating that it is an effective strategy to preparing highperformance LNCA employing NaAlO2 as the aluminum source. KEYWORDS: Li-ion battery, LiNi0.8Co0.15Al0.05O2, coprecipitation, NaAlO2, electrochemical performance

1. INTRODUCTION The requirement of a high-performance Li-ion battery cathode material is becoming stronger with the rapid development of portable electronic products, electric vehicles (EVs), and hybrid electric vehicles (HEVs).1−6 As the isomorphic solid solution of LiNiO2, LiCoO2, and LiAlO2, LiNi0.8Co0.15Al0.05O2 combines the advantages of LiNiO2 (high capacity, low cost, and low toxicity),7−9 LiCoO2 (good cycle performance and high conductivity),10,11 and LiAlO2 (good thermal stability and light weight)12 and is regarded as one of the promising cathode materials for the next-generation Li-ion battery.13−16 To further improve the performance of the LiNi0.8Co0.15Al0.05O2 material, many strategies have been explored up to now.17,18 However, among them, the research on the preparation method is rarely concerned. At present, Ni0.8Co0.15Al0.05(OH)2.05 as a precursor of LiNi0.8Co0.15Al0.05O2 has been prepared via a coprecipitation method in commerce. To obtain a spherical Ni0.8Co0.15Al0.05(OH)2.05 precursor, a continuously stirred tank reactor (CSTR) that allows metal salts (Ni2+, Co2+, and Al3+) to be added simultaneously (coprecipitation) is usually used16,19−31 and Al2(SO4)3 or Al(NO3)3 is used as the traditional aluminum source to prepare the hydroxyl precursor.23−30 However, from the equilibrium reactions and constants between transition metal ions and ligands in the aqueous solution (Table 1), it is obviously observed that the Ksp of Al(OH)3 is much less than that of © 2017 American Chemical Society

Ni(OH)2 and Co(OH)2, meaning that Al(OH)3 sol will be formed prior to the coprecipitation reaction of preparing the Ni0.8Co0.15Al0.05(OH)2.05 precursor. This will inhibit the continuous formation of precursor cores and the continuous growth of secondary particles and in turn affect the performances of the LiNi0.8Co0.15Al0.05O2 material.32 Herein, to solve the above problem, NaAlO2 is employed as a new aluminum source to prepare the Ni0.8Co0.15Al0.05(OH)2.05 precursor via a coprecipitation step for the first time in this article. Different from the traditional preparation process using Al(NO3)3 as the aluminum source, the preparation process of the Ni0.8Co0.15Al0.05(OH)2.05 precursor from NaAlO2 is a hydrolysis process, during which the fast precipitation of Al3+ and the formation of a flocculent precipitate can be effectively avoided. As excepted, stoichiometric LNCA with uniform element distribution, low cation mixing, and well-ordered layered structure is obtained from NaAlO2 (designed as LNCANaAlO2), which contributes to the improvement of electrochemical performances. In addition, the improved electrochemical stability, microstructure stability, and storage stability of LNCA-NaAlO2 are also further verified. Received: August 16, 2017 Accepted: October 13, 2017 Published: October 13, 2017 38567

DOI: 10.1021/acsami.7b12306 ACS Appl. Mater. Interfaces 2017, 9, 38567−38574

Research Article

ACS Applied Materials & Interfaces Table 1. Equilibrium Reactions and Constants between Transition Metal Ions and Ligands in the Solution equilibrium reaction 2+

−

Ni + 2OH = Ni(OH)2 Co2+ + 2OH− = Co(OH)2 Al3+ + 3OH− = Al(OH)3 AlO2− + 2H2O = Al(OH)3 + OH−

Ksp

C (M2+)

C (OH−)

pH (begin to precipitate)

0.80 0.15 0.05 0.05

5.00 × 10−8 1.03 × 10−7 4.50 × 10−11

6.70 7.01 3.65

−15

2.00 × 10 1.58 × 10−15 4.57 × 10−33

2. EXPERIMENTAL SECTION

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of LNCA from NaAlO2 (LNCA-NaAlO2) and Al(NO3)3 (LNCA-Al(NO3)3). All of the

2.1. Preparation of LNCA. The spherical Ni0.8Co0.15Al0.05(OH)2.05 precursor (NCA) was prepared via a coprecipitation step as previously reported by our group.32,33 In a typical synthesis process, 1.9 mol L−1 NiSO4·6H2O and CoSO4·7H2O mixed solution (Ni/Co = 80/15, molar ratio) was used as the starting solution and 0.1 mol L−1 Al(NO3)3 or NaAlO2 solution was used as the aluminum source, respectively. The Al(NO3)3 solution was prepared by dissolving Al(NO3)3·6H2O in deionized water, and the NaAlO2 solution was prepared by dissolving NaAlO2 in deionized water and adding 10 mol L−1 NaOH solution simultaneously until a settled mixed solution was formed. The total Ni/Co/Al molar ratio of NCA is designed as 80/15/ 5. To prepare the spherical NCA precursor, the starting solution and the Al(NO3)3 or NaAlO2 solution were simultaneously pumped into a continuously stirred tank reactor (CSTR, 30 L) and reacted with 10 mol L−1 NaOH solution and 1.5 mol L−1 NH3·H2O solution for 30 h, and the NaOH solution and NH3·H2O solution acted as the pH control agent and the chelating agent, respectively. The reaction temperature, pH value, and stirring rate were controlled sternly by an automatic control system. Then, the NCA precursor was obtained after filtration, washing, and drying at 120 °C for 12 h. Finally, the obtained NCA precursor was mixed in stoichiometric ratio with Li2CO3 (Li/M = 1.05, molar ratio) and sintered at 700 °C for 12 h and then at 750 °C for 12 h under the flowing oxygen atmosphere in a tube furnace to form LNCA. The flow diagram of preparing NCA and LNCA from Al(NO3)3 and NaAlO2 is shown in Figure S1. 2.2. Compositional and Structural Characterization of LNCA. The structure of LNCA was examined by X-ray powder diffraction using Cu Kα radiation (XRD, Rigaku D/MAX-2500, Japan). The start and finish angles of XRD patterns were 10 and 80°, respectively, and the scanning step was 0.02°. The morphologies of NCA and LNCA were obtained by a scanning electron microscope (SEM, MERLIN Compact, ZEISS, Germany). The microstructure of LNCA was observed with a transmission electron microscope (TEM, Tecnai G12). Particle size distributions of NCA and LNCA were collected by a laser particle analyzer (OMEC, LS-POP(6), China). The tap density of LNCA was examined by a tap density meter (ZS-201, LIRI, China). The specific surface area (SSA) of LNCA was analyzed with the Brunauer−Emmett−Teller (BET) method using a Quantachrome Autosorb MP instrument. The total chemical composition of LNCA was analyzed by an inductively coupled plasma spectrometer (ICPOES, VISTA-MPX). Energy-dispersive spectroscopy (EDS, VERIOS 460L, FEI) was used to analyze the cross section chemical composition of the precursor and the cathode materials. 2.3. Electrochemical Tests of LNCA. For fabricating the working electrodes, a uniform slurry containing LNCA, carbon black, PVDF (80/10/10, mass ratio), and N-methylpyrrolidinone was coated onto Al foil. Then, the coated Al foil was dried at 120 °C for several hours in an oven and then rolling-pressed, cut, and weighted. Before using, the electrodes were dried at 120 °C for 2 h in a vacuum oven. The dried electrodes were assembled into 2032-type coin cells to characterize the electrochemical performance. Among them, the anode of the coin cells was lithium metal and the electrolyte was 1 mol L−1 LiPF6 in ethylene carbonate/diethyl carbonate (1/1, volume ratio). The LAND CT2001A test system was employed to test the charge/discharge capacity, rate, and cycle performance of the electrodes. The charge/ discharge voltage range is 3.0−4.3 V (vs Li/Li+). The cycle progress of the cells is 0.1 C (1 C = 200 mA g−1) for the initial five cycles and 0.2 C for the rest of cycles at 25 °C. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured using an electrochemical workstation (Princeton Applied Research).

Figure 1. XRD patterns of LNCA-Al(NO3)3 and LNCA-NaAlO2.

diffraction peaks of both LNCA materials can be indexed to a well-defined hexagonal α-NaFeO2-type structure with a space 34,35 group of R3m No extra impurity peak is observed, ̅ . indicating the high purity of both LNCA materials. Strong peak splitting of (006)/(012) and (018)/(110) peak pairs indicates the highly ordered layered structure of the two kinds of LNCA materials.36 The lattice parameters of the two kinds of LNCA calculated on the basis of the XRD patterns are listed in Table 2. It is well known that the c/a and I(003)/I(104) values can reflect the layered structure and the degree of cation mixing, respectively. Generally, the higher c/a and I(003)/I(104) values represent a well-layered structure and a less degree of cation mixing.36−39 As shown in Table 2, LNCA-NaAlO2 exhibits higher c/a and I(003)/I(104) values, implying less cation mixing and a well-ordered layered structure. As reported before,30,34−39 less cation mixing and well-ordered layered structures are usually beneficial for the electrochemical performance. To further qualitatively confirm degree of cation mixing, Rietveld refinement is conducted using Fullprof software, and the full profile refinement results of LNCA-Al(NO3)3 and LNCA-NaAlO2 are shown in Figure 2. All Bragg positions can be indexed to the hexagonal α-NaFeO2-type structure with a space group of R3m ̅ . The result analysis of Li+/Ni2+ mixing degree and R factors of both LNCA materials are listed in Table 2. The refinement results are reliable because of the small difference between the observed and calculated curves and the acceptable value of the Rwp factor. The concentration of Ni2+ in the Li+ layer decreases from 2.56 to 1.21% by using NaAlO2 to replace Al(NO3)3 as the aluminum source, indicating that the cation mixing of LNCA-NaAlO2 is lower than that of LNCA38568


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ACS Applied Materials & Interfaces

Table 2. Lattice Parameters and X-ray Rietveld Refinement Result Analysis of LNCA-Al(NO3)3 and LNCA-NaAlO2 lattice parameters LNCA

a (Å)

c (Å)

c/a

V (Å3)

I003/I104

Li+/Ni2+ mixing

Rp (%)

Rwp (%)

LNCA-Al(NO3)3 LNCA-NaAlO2

2.8596 2.8594

14.1671 14.1682

4.95 4.96

100.3 100.5

1.30 1.35

2.56 1.21

10.7 10.0

11.8 11.4

Figure 2. Rietveld refinement results of LNCA-Al(NO3)3 (a) and LNCA-NaAlO2 (b).

Al(NO3)3. It is in good agreement with the above result concluded from c/a and I(003)/I(104) values. High-resolution transmission electron microscopy (HRTEM) images of LNCA-Al(NO3)3 and LNCA-NaAlO2 are shown in Figure 3. It can be observed that both LNCA

Figure 4. SEM images of NCA-Al(NO3)3 (a), NCA-NaAlO2 (b), LNCA-Al(NO3)3 (c), and LNCA-NaAlO2 (d).

dense for LNCA-NaAlO2. Figure 5 displays the particle size distribution curves of both LNCA materials. Through laser particle size distribution measurements, it is clear that LNCANaAlO2 has a narrower particle size distribution. From Table 3, LNCA-NaAlO2 displays higher D50 of 8.45 μm and tap density of 2.363 g cm−3 than that of LNCA-Al(NO3)3, which further Figure 3. HRTEM images of LNCA-Al(NO3)3 (a) and LNCANaAlO2 (b).

materials show clear lattice fringes, indicating high crystallization. The interplanar distances of both LNCA materials are about 0.47 nm, corresponding to the planar distance of the (003) plane in XRD patterns, which is the strongest peak in the XRD patterns. It is corroborative that the LNCA material can be synthesized by employing NaAlO2 as a new aluminum source. Figure 4 shows the SEM images of NCA (a, b) and LNCA (c, d) from different aluminum sources. For both NCA materials, the primary particles are needle-shaped and the secondary particles are spherical. However, the secondary particles of NCA-NaAlO2 are composed of thinner needlelike primary particles, and it has a more dense and smooth surface, as shown in Figure 4b, which may result in a higher tap density of LNCA-NaAlO2. As shown in Figure 4c,d, the morphology and size of both LNCA materials are similar, but the primary particles are more regular and the secondary particles are more

Figure 5. Particle size distribution curves of LNCA-Al(NO3)3 and LNCA-NaAlO2. 38569


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ACS Applied Materials & Interfaces Table 3. D50, Tap Density, and BET Specific Surface Area of LNCA-Al(NO3)3 and LNCA-NaAlO2 LNCA

D50 (μm)

tap density (g cm−3)

SSA (m2 g−1)


7.64 8.45

2.1 2.4

0.20 0.31

efficiency (87.8%) than those of LNCA-Al(NO3)3, which may be attributed to the less cation mixing and well-ordered layered structure. In addition, LNCA-NaAlO2 also has a higher volumetric energy density (1840.541 Wh L−1) than that of LNCA-Al(NO3)3, which are bound up with the higher tap density, discharge median potential, and discharge specific capacity. From Figure 7b, LNCA-NaAlO2 displays a higher rate performance than that of LNCA-Al(NO3)3, which is related to the morphology and microstructure of the secondary particles. An exciting result can be found that the discharge specific capacity of LNCA-NaAlO2 can still remain ∼125 mAh g−1 at 20 C, which exceeds the highest value of LNCA reported in the present literature. Figure 7c displays the cycle performance of both LNCA materials at 0.1 C for initial five cycles and at 0.2 C for subsequent cycles at 25 °C in the potential range of 3.0−4.3 V. As can be seen, LNCA-NaAlO2 also displays a better cycle performance (capacity retention of 74.1% and volumetric energy density retention of 71.9% after 200 cycles) than that of LNCA-Al(NO3)3 (capacity retention of 69.2% and volumetric energy density retention of 67.2% after 200 cycles). The above investigations demonstrate that LNCA-NaAlO2 delivers significantly improved electrochemical performance (initial discharge capacity, volumetric energy density, rate performance, and cycle performance) compared to that of LNCA-Al(NO3)3. Because of the well-ordered layered structure and less cation mixing, LNCA-NaAlO2 displays significantly improved electrochemical performance, indicating that employing NaAlO2 as the aluminum source is beneficial to keep the layered structure stability of LNCA during repeated lithium insertion and extraction. To further verify the electrochemical stability of LNCA-NaAlO2, CV curves of both LNCA materials at 1st, 2nd cycles at 0.1 C, and 200th cycles at 0.2 C are shown in Figure 8. From Figure 8a, three anodic peaks are observed at 3.72, 4.00, and 4.21 V and three cathodic peaks, which match with the anodic peaks appearing at 3.67, 3.96, and 4.18 V. Generally, the peaks of the CV curves match with the redox reactions caused by lithium insertion and extraction. In the anodic process, the peak appearing at 3.72 V corresponds to the transformation from the hexagonal phase to the monoclinic phase (H1 to M),

confirms the above conclusion from SEM images. In addition, BET specific surface areas (SSAs) of both LNCA-Al(NO3)3 and LNCA-NaAlO2 displayed in Table 3 are approximate to that of commercial LNCA. Table 4 presents the ICP elemental compositions of both LNCA materials. The molar ratios of Ni/Co/Al in LNCATable 4. ICP Chemical Elemental Compositions of LNCAAl(NO3)3 and LNCA-NaAlO2 LNCA

Ni

Co

Al


0.802 0.800

0.155 0.154

0.043 0.045

NaAlO2 and LNCA-Al(NO3)3 are 0.802/0.155/0.043 and 0.800/0.154/0.045, respectively. The elemental compositions of both LNCA materials are approximate to the designated value. Ni, Co, and Al elemental distributions on the cross section of NCA and LNCA particles are further examined by an energy-dispersive spectrometer (EDS). As shown in Figure 6, Ni, Co, and Al elements are distributed uniformly from the center to the border of all NCA and LNCA particles. The ICP and EDS results indicate that the Ni, Co, and Al elemental composition and distribution of LNCA-NaAlO2 can reach the similar level as that of LNCA-Al(NO3)3. The initial charge/discharge curves of both LNCA materials in the potential range of 3.0−4.3 V at 0.1 C are displayed in Figure 7a. The detailed data of initial charge/discharge capacities, Coulombic efficiencies, and volumetric energy densities are listed in Table 5. It is observed that LNCANaAlO2 displays a higher initial discharge median potential, discharge specific capacity (204.7 mAh g−1), and Coulombic

Figure 6. Cross-sectional EDS elemental mapping of NCA-Al(NO3)3 (a), NCA-NaAlO2 (b), LNCA-Al(NO3)3 (c), and LNCA-NaAlO2 (d) particles. 38570


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Figure 8. Cyclic voltammetry curves of LNCA-Al(NO3)3 (a) and LNCA-NaAlO2 (b) with the scanning rate of 0.1 mV s−1 from 3.0 to 4.3 V.

A larger peak area at first and second cycles represents a higher initial specific capacity, which is in good agreement with Figure 7a. Furthermore, the major anodic peak located at 3.70 V at the second cycle shifts to about 3.75 V at the 200th cycle. However, the peak shift is smaller compared to that for LNCA-Al(NO3)3, indicating better Li ion reversibility during charge and discharge of LNCA-NaAlO2. This result is concordant with the cycle performance shown in Figure 7c. In addition, SEM images of LNCA-NaAlO2 and LNCAAl(NO3)3 electrodes after 200 charge/discharge cycles are compared in Figure 9. The surfaces of both LNCA materials

Figure 9. SEM images of LNCA-Al(NO3)3 (a) and LNCA-NaAlO2 (b) electrodes after 200 charge/discharge cycles.

Figure 7. Initial charge/discharge (a), rate performance (b), and cycle performance (c) curves of LNCA-Al(NO3)3 and LNCA-NaAlO2.

after 200 cycles change much in contrast with the original surfaces, as shown in Figure 4c,d. However, the spherical morphology of LNCA-NaAlO2 is still maintained and the primary particles are compact after 200 cycles. In contrast, the secondary particles of LNCA-Al(NO3)3 are fragmentized. In conclusion, the microstructure of LNCA-NaAlO2 is more stable than that of LNCA-Al(NO3)3 even after long cycles, which may contribute to the improved cycle performance. Storage stability is also important for LNCA. At present, the storage stability of LNCA is still unsatisfying because electrical dielectric Li2CO3 and LiOH on the surface of LNCA can react with CO2 and H2O in damp air to form a continuous amorphous layer. Figure 10a,b shows the surface HRTEM images of LNCA exposed to moist air with 80% relative humidity for 24 h. It is found that the LNCA-NaAlO2 particle

Table 5. Initial Charge/Discharge Capacities, Coulombic Efficiencies, and Volumetric Energy Density of LNCAAl(NO3)3 and LNCA-NaAlO2 LNCA

charge/discharge capacity (mAh g−1)

Coulombic efficiency (%)

volumetric energy density (Wh L−1)


218.4/175.3 233.2/204.7

80.3 87.8

1399.686 1840.541

the peak at 4.00 V corresponds to the transformation from the monoclinic phase to a new hexagonal phase (H2 to M), and the peak at 4.21 V corresponds to the transformation from the hexagonal phase to another hexagonal phase (H2 to H3). Similar CV curves of LNCA-NaAlO2 are observed in Figure 8b. 38571


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Figure 10. HRTEM images of LNCA-NaAlO2 (a) and LNCA-Al(NO3)3 (b) after exposure to moist air with 80% relative humidity for 24 h, and the EIS curves (c) of the electrodes before and after storage.

much better spherical structure, and more dense and regular secondary particle. As expected, LNCA-NaAlO2 displays significantly improved tap density (2.363 g cm−3), discharge capacity (204.7 mAh g−1 at 0.1 C), volumetric energy density (1840.541 Wh L−1), rate performance (∼125 mAh g−1 at 20 C), and cycle performance (capacity retention of 74.1% and volumetric energy density retention of 71.9% after 200 cycles) compared to those of LNCA-Al(NO3)3. In addition, the improved electrochemical stability, microstructure stability, and storage stability of LNCA-NaAlO2 are further verified. It is believed that the new strategy employing NaAlO2 as the aluminum source to prepare high-performance LNCA for Liion batteries is feasible and suitable for application.

surface is covered by a continuous amorphous layer of about 12.93 nm thickness, which is thinner than that observed on the LNCA-Al(NO3)3 particle surface. This is related to the obvious difference between the storage stabilities of LNCA-Al(NO3)3 and LNCA-NaAlO2. The AC impedance method was used to investigate the degradation possibility of the LNCA electrode interface.40 From EIS in Figure 10c and the fitted EIS data calculated on the basis of the corresponding equivalent circuit model in Table 6, there is clear evidence for a larger Table 6. Fitted EIS Data of LNCA-Al(NO3)3 and LNCANaAlO2 before and after Storage LNCA

Rf (Ω)

Rct (Ω)

LNCA-Al(NO3)3, fresh LNCA-NaAlO2, fresh LNCA-Al(NO3)3, exposed 24 h LNCA-NaAlO2, exposed 24 h

9.45 8.40 13.84 10.14

67.13 78.79 122.42 102.50

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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.7b12306. Flow diagram of preparing NCA and LNCA (PDF)

electrochemical polarization of LNCA electrodes exposed to moist air for 24 h. LNCA-NaAlO2 has smaller initial Rf and significantly reduced deterioration degree of Rf and Rct after exposure for 24 h. These data indicate that the content of electrically insulating Li2CO3 and LiOH on the surface of LNCA-NaAlO2 particles is lower and the reaction rate with CO2 and H2O in the moist air is also significantly suppressed compared to that in LNCA-Al(NO3)3. The HRTEM and EIS results are accordant and confirm that LNCA-NaAlO2 has better storage stability.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.S.). *E-mail: [email protected] (L.Z.). ORCID

Lianqi Zhang: 0000-0003-2165-7995 Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS To prepare high-performance LNCA, a new strategy employing NaAlO2 as the aluminum source has been introduced for the first time. Compared with LNCA-Al(NO3)3, LNCA-NaAlO2 presents less cation mixing, well-ordered layered structure,

ACKNOWLEDGMENTS This work was sponsored by National Key Research and Development Program of China (2016YFB0100500), National Natural Science Foundation of China (21503148), and Tianjin 38572


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Sci. & Tech. Program (15ZCZDGX00660, 16JCQNJC03300, 16YFZCGX00250).

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Improved Performances of LiNi0.8Co0.15Al0.05O2 Material

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