A High-Capacity Anode for Lithium Batteries Consisting of

Article pubs.acs.org/EF

A High-Capacity Anode for Lithium Batteries Consisting of Mesoporous NiO Nanoplatelets A. Caballero, L. Hernán,* and J. Morales Dpto. Química Inorgánica, Instituto Universitario de Investigación en Química Fina y Nanoquímica, Campus de Rabanales, Universidad de Córdoba, Córdoba 14071, Spain

Z. González, A.J. Sánchez-Herencia, and B. Ferrari Instituto de Cerámica y Vidrio, CSIC, c/Kelsen 5, Madrid 28049, Spain S Supporting Information *

ABSTRACT: The two-dimensional (2D) nanostructures of NiO obtained after calcination at 450 °C of β-Ni(OH)2, which is a product of the sonocrystallization of Ni precursors with the aid of an organic modifier, was studied as an electrode for Li-ion batteries. Two outstanding properties that have not yet been reported for this oxide are described: (i) an unusual high specific capacity (ca. 1100 mAh g−1 at the second cycle), which notably exceeds its theoretical capacity (718 mAh g−1); and (ii) a continuous increase of the capacity on cycling (ca. 1500 mAh g−1 at the 30th cycle). We assign this extra capacity to the reversibility of side reactions undergone by the electrolyte on forming the solid electrolyte interface (SEI). We believe that the mesoporous texture of the NiO-platelet-like particles and their special microstructural properties are responsible for this unexpected electrochemical behavior. In fact, electrodes made of commercial nonporous NiO, with higher crystallite size and low microstrain content, lack these unusual properties. Its capacity at the second cycle was only 460 mAh g−1, and it was significantly reduced during cycling, with the value at the 30th cycle barely reaching 25 mAh g−1.

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disappointing, as the capacity delivered faded after the first few cycles, being notably lower than the theoretical value, in most cases. To our knowledge, only Varghese et al.7 reported values above the theoretical capacity of NiO after 20 cycles (ca. 750 mAh g−1). In general, capacity fading has been ascribed to the volume changes of the Ni-based species during charge and discharge processes, leading to the fragmentation and the associated electrical disconnection of the active material from the current collector.23 Different groups have considered mixing NiO with conductor species, particularly carbon-based species, which can also help to avoid aggregation and fragmentation of the active particles.16,17,22 This strategy has improved the cycling stability of the cell, but the registered capacities are still lower than the theoretical value. So far, the graphene irruption in the field of LIB has been the approach that has clearly improved the electrochemical response of the NiO-based electrodes. The electrochemical properties of different NiO/graphene fashioned composites have recently been described.19−21 In these reports, the capacities exceeded 1000 mAh g−1, with the values being referred to as the composite weight. The capacity delivered by the NiO particles in those composites is difficult to evaluate since the graphene itself reacts reversibly with Li and synergistic effects cannot be ruled out. Moreover, these composites are prepared by complex procedures and the synthesis requires multiple steps.

INTRODUCTION New features of hybrid electric vehicles and electric vehicles have boosted the development of storage devices, such as the Li-ion batteries (LIB) (see refs 1−3 (and references therein). For this application, the electrodes should be able to deliver higher capacities and energies than those commonly used in commercial batteries, such as LiCoO2 and graphite, which are materials that act as either the cathode or anode, respectively. The transition-metal oxides stand out among the materials that could satisfy the requirements to act as an anode. Their reversible reactivity in lithium cells was first reported by Tarascon et al.4 NiO is one of the best choices, because of its high theoretical capacity (718 mAh g−1, which is nearly 2-fold the capacity of graphite) and a lower cost, compared to other alternatives, such as Co-based oxides. This has led to many reports in the last years focusing on the improvement of the performance of the Ni-based electrodes in lithium cells, paying special attention to obtaining high specific capacities with cycling. Table 1 summarizes the available references that reported on the electrochemical behavior of NiO in Li halfcells. The table has been divided into two blocks, following synthetic procedures: (i) pure NiO or NiO grown on Ni substrate and (ii) NiO-based composites synthesized with different types of carbon. The indicated values for the capacities correspond to those obtained at the lowest current density (if different current densities were tested). The study of the various strategies of single-phase synthesis of NiO nanostructures with specific morphologies, and frequently deposited on Ni substrates, has been described.5−15 Published results have been somewhat © XXXX American Chemical Society

Received: April 30, 2013 Revised: July 31, 2013

A

dx.doi.org/10.1021/ef400797r | Energy Fuels XXXX, XXX, XXX−XXX

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Table 1. Electrochemical Properties of NiO in Lithium Cells Reported in the Literature initial capacity (mAh g−1)

final capacity (mAh g−1)

last cycle

current (mA g−1)

600

200

20

25

1152 1353 997 1100 1006 1020 500 914

630 757 646 560 518 746 387 680

50 20 65 45 50 10 100 50

100 448 156 200 718 143 286 71

1033 1150

543 638

100 50

359 40

950

429

40

71.8

760 1083 1650 1641 1398

540 800 1031 1041 (60) 1098

50 50 40 60 50

718 50 71 100 100

1112

698

10

100

synthesis method Pure NiO or NiO Grown on Ni Substrates NiO nanotubes obtained from AAO template in two steps: Ni(OH)2 formation, template elimination by NaOH, and heating at 300 °C NiO-Ni nanocomposites obtained by heating Ni(OH)2 at 700 °C in air NiO nanowalls prepared by plasma assisted method, grown on Ni thermal oxidation of Ni foams in air at 500 °C; three-dimensional porous structures (150 nm) NiO hollow microspheres NiCl2/RFgel calcined at 700 °C (ps 200 nm) hierarchically ordered porous NiO film; electrodeposition on PS sphere template thermal oxidation of Ni foams in air at 400 °C porous NiO from nickel acetate and porous Ni foam as substrate at 300 °C highly ordered mesoporous NiO; synthesis by template of mesoporous SiO2 (KIT-6) and P123 as surfactant Ni immersed in NH3, followed by heating at 500 °C (nanoporous NiO grown on Ni foam) porous NiO fibers prepared by electrospinning from PAN/Ni(NO3)2·6H2O fiber composite Carbon−NiO Composites net-structured NiO-C composite prepared by urea−mediated hydrolysis of Ni-acetate; glucose used as a carbon source Ni foam-supported porous NiO/polyaniline film NiO mixed with MWCNT Ni(OH)2 grown on graphene nanosheets (GNS) calcined at 360 °C RGO−NiO microspheres via coprecipitation of Ni(OH)2 and GO reduced with hydrazine ultrathin porous NiO nanosheets/graphene hierarchical structure (complex synthetic procedure) biotemplated fabrication of hierarchically NiO/C composites from lotus pollen; complex preparation

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

200 kV and equipped with an Orius Gatan CCD Camera. N2 adsorption/desorption measurements were carried out with a Micromeritics ASAP 2020 system. Electrochemical measurements were made on 2032 coin-type cells. The electrodes were prepared by mixing the oxide with 20% carbon black and then deposited onto copper foil using the doctor blade technique. Lithium metal was used as the counter electrode. Solutions of LiPF6 (1 M) in an EC-DMC or EC-DEC mixture (1:1 w/w) were used as the electrolyte. Cells were assembled in an M-Braun glovebox under an argon atmosphere. Constant current tests were performed on a multichannel potentiostat−galvanostat system (Arbin BT2000). All measurements were made in duplicate to ensure reliability in the electrochemical tests.

Graphene has also been used to increase the Li storage capacity of β-Ni(OH)2 for LIB, and a capacity of 507 mAh g−1 after 30 cycles was registered.24 Moreover, monophasic electrodes, made of synthetic β-Ni(OH)2 nanoplatelets, prepared by combining the effects of high energy ultrasound and surfactants, are able to deliver capacities as high as 1450 mAh g−1 after 25 cycles.25 In this work, we report the electrochemical properties of NiO obtained by calcination of βNi(OH)2 fashioned in this particular two-dimensional (2D) nanostructure. At moderate rates, it has the ability to deliver specific capacity values higher than those shown in Table 1. Moreover, these values notably exceed the theoretical capacity.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

The thermogravimetry (TG) curve of the β-Ni(OH)2 plateletlike powders is shown in Figure 1S in the Supporting Information. A small loss of weight was observed at 0.95. The crystallite size and strains, calculated from the intercept and the slope values, respectively, are shown in Table 2. As expected, the c-NiO sample was highly crystalline (the β cos θ = 2⟨e⟩ sin θ +

Table 2. Textural Properties, Crystallite Size, and Microstrain Content of h-NiO and c-NiO SBET sample (m2 g−1) h-NiO c-NiO

35 41

pore volume (cm3 g−1)

micropore areaa (m2 g−1)

dBET (nm)

D (nm)

⟨e⟩ (× 10−4)

0.398 0.053

26 3

25 21

26 61

29 7

b

Figure 1. (a) N2 adsorption/desorption isotherm and (b) pore distribution of NiO.

contribution was very small. As shown below, all these features were consistent with the TEM images, where a mesoporous system was observed in the h-NiO 2D nanostructure. Moreover, there was a significant discrepancy between the particle size determined from surface area and XRD data for the c-NiO sample. The higher crystallinity of the c-NiO sample, and hence the greater size of the coherent diffraction domains, as deduced from X-ray broadening analysis, could be the cause of this discrepancy. Figure 2 shows the FE-SEM images of both h-NiO and cNiO powders. Figure 2a shows that c-NiO powders are

Values calculated using the α−t plot method. bValues calculated by assuming spherical particle shape and using the expression dBET = 6/ (ρSBET), where ρ is the particle density (6.8 g cm−3). a

lower intercept and slope values reflect a large crystallite size and a low strain) when compared with h-NiO. The crystallite size of c-NiO was about two times lower and the lattice distortion was four times lower than those of h-NiO. Figure 1a shows the adsorption isotherms for c-NiO and hNiO powders. The shape of the h-NiO isotherm exhibited mixed type I and type II isotherms, respectively, for low and high relative pressure (P/P0). The rise after P/P0 > 0.9 represented the integral porosity of h-NiO contributed by the interspaces between the nanosheets, offering abundant channels for the liquid electrolyte. A depletion of the hysteresis loop was found in the c-NiO sample, indicating the lack of the slit-shaped pores produced by the platelet particles. Table 2 shows selected textural properties of the two oxides. As shown, while their specific surface areas were relatively low, they were somewhat higher for c-NiO compared to h-NiO, which was unexpected. By contrast, the total pore volume for h-NiO notably exceeded that of c-NiO. Figure 1b shows the pore size distribution calculated by the Barrett, Joyner, and Halenda (BJH) model31 applied to the adsorption branch of isotherm. The h-NiO sample had a heterogeneous mesopore distribution, with a predominance of mesopores ∼9 nm in size. No peaks were observed for c-NiO (the upward trend of the plot at very low pressure, P/P0 < 0.05, is meaningless due to the pressure errors). For h-NiO, the t-plot analysis31 revealed a significant contribution of the pore area to the BET surface (micropore area in Table 2). By contrast, for the c-NiO sample, this

Figure 2. FE-SEM micrographs of (a) c-NiO and (b) h-NiO.

nanometric in size (100 nm. The morphology of c-NiO is consistent with the surface area data reported in Table 2. The FE-SEM image of h-NiO shows the platelet-like morphology of this powder. After careful inspection of the particles, we were able to determine their nanostructure. The h-NiO powder is also agglomerated, forming flakes 1−2 μm in diameter, while the thickness is in the nanometric rage (30−20 nm). C


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The morphology and complementary information about hNiO particles were clarified in the TEM images shown in Figure 3. Indeed, the TEM micrograph in Figure 3a verified

Figure 3. TEM images of (a,b) h-NiO, and (c,d) c-NiO.

that the h-NiO particles (Figure 3a) maintain the platelet-like structure of its precursor, β-Ni(OH)2.25,26 However, the h-NiO nanostructures were 1−5 nm thick and were characterized by clusters of nanosheet-like particles with a diameter of ∼30 nm. The HRTEM image (Figure 3b) revealed the crystalline nature of the h-NiO nanosheets, which was consistent with the rather low microstrain content, despite the low temperature of calcination and the absence of a post-annealing process.32 Moreover, several holes that had a tendency to adopt a hexagonal shape with diameters of 5−20 nm formed in the nanoplatelets. This mesoporous morphology meets the N2 adsorption measurements discussed above and it will play a significant role on NiO reactivity, promoting contact between the active particles and the electrolyte. The TEM image of cNiO (Figure 3c) revealed the absence of the mesoporosity, consistent with the N2 adsorption data. Also, the particle morphology of this oxide is different, as the particles showed a less anisotropic growth instead of the nanoplatelet morphology of the h-NiO 2D nanostructures. The HRTEM images (Figure 3d) also revealed the crystalline nature, as fringes in two directions were clearly observed. The calculated spacings are consistent with those of NiO. The plots in Figure 4a and 4b show the selected discharge/ charge curves for h-NiO and c-NiO, respectively, recorded at 100 mA g−1 (0.17C, C = 718 mA g−1) within the voltage range 3.0−0.0 V. The shape of discharge/charge curves for h-NiO was similar to those for c-NiO, except the pseudoplateau lengths. Moreover, they were similar to the discharge/charge curves reported for this oxide, independent of the particle morphology and size.6−9,11,12,14,15 The curve for the first discharge of h-NiO exhibited a well-defined plateau at ca. 0.7 V, with an associated capacity of ca. 1000 mAh g−1. Then, the voltage dropped slowly and the total capacity delivered by the electrode was ∼1500 mAh g−1, which was twice the theoretical

Figure 4. Galvanostatic discharge/charge curves for NiO, as recorded during different cycles: (a) h-NiO and (b) c-NiO. The inset shows the differential capacity plots of h-NiO. The current density was 100 mA g−1 and the voltage range was 3.0−0.0 V. (c) Cyclic voltammetry curves of h-NiO. Scan rate = 0.1 mV s−1.

value (718 mAh g−1) if a complete reduction of Ni2+ to Ni0 is assumed. This capacity excess was assigned to the solid electrolyte interface (SEI) formation.4,32,33 The charge curve was strongly polarized and its shape was consistent with that expected for NiO. For a better description of the curve profiles, the inset on Figure 4a and 4b shows the corresponding differential capacity plots, where the plateaus appear as peaks (stronger and sharper peaks when the plateau was better defined). The first charge curve exhibited two broad peaks at ca. 1.6 and 2.3 V, which have been assigned to the oxidation of the SEI layer and Ni nanograins, respectively.33−36 The quasiamorphous nature of NiO that was formed electrochemically is reflected by a greater reduction in potential, showing less plateau definition and, thus, a weaker and broader peak. No significant changes were detected in the subsequent charge/ discharge curves, except a slight decrease in the lower potential oxidation peak (mainly assigned to the SEI decomposition) and, specifically, an increase in its intensity. Despite the similar shape of the discharge/charge curves of c-NiO, we noticed that the initial capacity was lower (ca. 750 mAh g−1) and very close to the theoretical value. Its performance was also worse, as shown below. As expected, the CV curves (Figure 4c) were similar to the differential capacity plots and reflected the same processes the compound underwent. D


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surface area is generated by the porosity against only the 7% calculated for c-NiO nanostructure. Moreover, the pore size distribution clearly revealed the mesoporous texture of h-NiO compared with the nonporous texture of c-NiO (Figure 1b), and was further confirmed by the TEM images (see Figures 3a and 3c). Thus, the textural properties, particularly the pore system, must play a relevant role in its electrochemical properties. The porous structure of h-NiO offered the following advantages: (i) Ease of the electrolyte impregnation of the active sites of the electrode, increased by the larger internal surface area. (ii) The mesoporosity system provides a short and easier pathway for the mass transport of Li ions and endows a high ionic conductivity to the electrode. Moreover, the variety of pore sizes may be beneficial, as the small pores can interconnect the large pores and allow for fast transport of Li ions. (iii) Ease of the electrolyte molecules trapped within the pore to undergo reversible reduction and oxidation processes. The nanosheet morphology and the low crystallite size of h-NiO, which also promote accessibility of the electrolyte and diffusion of the species, can buffer the strains induced by the volume changes during the charge and discharge processes. Moreover, the reversibility of the SEI formation and decomposition has been related to the catalytic activity of the Ni formed.34 In this context, the nanostructure of h-NiO, including its higher microstrain content, could promote the formation of highly disordered Ni nanograins and, therefore, implement the catalytic activity of the reduction and oxidation of the electrolyte as a plausible explanation for the capacity increase on cycling. All these arguments offer explanations for the superior electrochemical performance of the h-NiO sample. The Li-electrolyte reactivity catalyzed by the nanoplatelet was checked by substituting the solvent DMC with diethyl carbonate (DEC). Although a capacity increase was also observed in the first cycles (see Figure 4S in the Supporting Information), after the 15th cycle, this trend inverts and the capacity significantly decreases over the following cycles. This was a somewhat unexpected result, since DEC is usually recommended for testing anodic materials.38 We tentatively explain this behavior by assuming a polymer-like process for the SEI formation.39 The molecule reduction should cause ringopening followed by the monomer combination. The smaller volume of the methyl group versus the ethylene group should enhance this polymerization process, thus facilitating the SEI formation. The rate capability of h-NiO (from 200 mA g−1 to 1000 mA −1 g ) was also investigated, and the results are shown in Figure 6. When the electrode was cycled at 200 mA g−1 for 10 cycles, the final discharge capacity was 935 mAh g−1, with a slight tendency to increase with cycling. This tendency was enhanced at 400 mA g−1, and after the next 10 cycles, the capacity value that was achieved was 1000 mAh g−1. At 800 mA g−1, the electrode started to degrade and a continuous capacity fading was detected after the first six cycles. The capacity fading was very pronounced at 1000 mA g−1, since the capacity after 10 cycles barely exceeded 200 mAh g−1. When current density was lowered to 200 mA g−1, the capacity increased to 700 mAh g−1, but it drastically decreased during cycling. Thus, the NiO

Figure 5 shows the discharge/charge capacity values as a function of the number of cycles. Two outstanding features are

Figure 5. Discharge/charge capacity values of h-NiO (black square) and c-NiO (red circle) as a function of the number of cycles. Rate = 100 mA g−1; electrolyte: LiPF6; EC = DMC.

worth noting for the h-NiO sample. First, after an initial drop, the capacity gradually increased during the cycling. This phenomenon has been well documented for CoO,32,33,36 but different explanations have been proposed: (i) the reversibility of the processes, which involve the electrolyte;33 (ii) the irreversible formation of Li2O in the first discharge step;35 and (iii) the increase of the Co valence,37 where, according to the reaction Li 2O + 3CoO → Co3O4

the spinel formation has been confirmed by ex situ X-ray diffraction. For NiO, this last explanation cannot be applied, since there are no experimental probes for the existence of the Ni3O4 spinel. The second noted feature is related to the capacity values. In fact, the electrode exceeded its initial capacity at the 20th cycle (1500 mAh g−1), achieving a value higher than 2-fold the theoretical capacity. None of these characteristics were found in the capacity evolution of c-NiO during cycling. For this electrode, the capacity did not exceed the theoretical value (except in the initial capacity), and it continuously faded during cycling. None of the numerous studies on the electrochemical properties of NiO prepared by multiple ways and synthetic procedures describe the uncommon behavior of the h-NiO.5−15 As explained above, only graphene/NiO composites delivered capacities which exceed the theoretical value, but in those electrodes the role of graphene hinders an accurate knowledge of the real electrochemical response of the oxide particles. The cause of the significant differences in the performance of the two NiO samples is mainly associated with their textural properties, with a minor role played by their structure. In fact, the XRD patterns were quite similar (see Figure 2S in the Supporting Information), with small differences in the broadening of the diffraction lines (related to their microstructure). Regarding the textural properties, there was not a direct correlation between the reactivity toward Li and the BET surface area. In fact, the nanostructure that delivered a higher capacity, h-NiO, had a somewhat lower BET area than c-NiO (15% less). However, the pore volume of h-NiO is 7.5 times greater than that of c-NiO (see Table 1). As consequence of this result and according to the t-plot, nearly 75% of the h-NiO E


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AUTHOR INFORMATION

Corresponding Author

*Fax: 0034957218621. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding from Spain’s Ministry of Science and Innovation (Project Nos. MAT2009-14408-C02-01, MAT2008-03160, and MAT2011-27110) and the Andalusian Regional Government (Group No. FQM-175).

Figure 6. Discharge capacity values of h-NiO cycled at different current densities.

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nanosheets perform quite well at low and medium current densities. Under these conditions, the reduction and oxidation processes the electrolyte underwent were enhanced, and the electrode maintained its integrity during cycling. It should be noted that the oxide texture and microstructure play a significant role in its electrochemical response, and the modification of these properties (e.g., using the strategy described by Hu et al.40), can improve its performance to increase the cycle life and the rate capability. Studies with this goal are in progress.

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CONCLUSIONS After a low temperature calcination of the sonochemically prepared β-Ni(OH)2 powders, h-NiO retains the nanoplatelet morphology of the pristine compound, harboring a mesoporous nanostructure. When h-NiO is tested as the electrode in lithium cells, its electrochemical response showed that the delivered specific capacity (ca. 1500 mAh g−1) after 35 cycles at a current density of 100 mA g−1 doubles the theoretical value. Even at 400 mA g−1, the delivered capacity was quite high (1000 mAh g−1), and notably exceeded its theoretical capacity (718 mAh g−1). By contrast, the delivered capacity of an electrode made of commercial NiO nanoparticles, c-NiO, under identical experimental conditions was only 25 mAh g−1. The better electrochemical performance of h-NiO is ascribed to the specific architecture of h-NiO (2D platelet-like nanostructures, with a random mesoporosity, fashioned by sheetlike nanocrystals of NiO 30 nm in diameter and with a thickness of 1−5 nm) determined from N2 adsorption measurements, XRD, FE-SEM, and TEM. These morphological, crystallographic, and nanostructural characteristics supply the electrode with an elevated number of active sites and/or particle/electrolyte contacts, providing numerous paths for Li ion diffusion and catalyzing the reduction/oxidation reactions, as well as improving electrical interparticle connection and the accommodation of volume changes.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

TG curve of Ni(OH)2; X-ray diffraction (XRD) patterns of NiO samples; plots of the Williamson and Hall equation of NiO sample, and discharge/charge capacity values of h-NiO. This material is available free of charge via the Internet at http://pubs.acs.org. F


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A High-Capacity Anode for Lithium Batteries Consisting of

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