Crystal Engineering of Nanomaterials To Widen the Lithium Ion Rocking

Sep 20, 2012 - Crystal Engineering of Nanomaterials To Widen the Lithium Ion Rocking “Express Way”: A Case in LiCoO2. Hui Juan Zhang†‡, Chee C...
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Crystal Engineering of Nanomaterials To Widen the Lithium Ion Rocking “Express Way”: A Case in LiCoO2 Hui Juan Zhang,†,‡ Chee Cheong Wong,‡ and Yu Wang*,†,§ †

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, People's Republic of China School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore § Institute of Chemical and Engineering Sciences (ICES), 627833, Singapore ‡

S Supporting Information *

ABSTRACT: Hexagonal LiCoO2 nanomesh is fabricated for the first time based on the low crystal mismatch strategy. From the precursor of (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet to Co3O4 nanomesh and finally to LiCoO2 nanomesh, the crystal mismatch ranges from 0 to 13%, which ensures that the feature of single crystallinity remains in the prepared samples. LiCoO2 nanomesh exhibits combined properties of single crystallinity, mesoporosity, and ultrathin thickness. To the best of our knowledge, no similar results have been reported before in selectively exposing (100) and (010) crystal planes, the rapid rocking planes of the Li ions, for LiCoO2 at almost 100% surface ratio. The enhanced performance in rate capability enables the LiCoO2 nanomesh to be an excellent cathode in Li ion batteries (LIBs).

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INTRODUCTION Historically electrochemical energy storage has always been an eternal topic since human beings stepped into the modern and electrified world in the early part of the last century.1−3 With the development of miniaturization processes in electronics, especially in portable electronic devices, and the impressive growth in the electric vehicles (EVs) market, the slow advance of electrical-energy storing technology is often criticized because of its not being able to keep pace with the progress of practical needs, which critically require both high energy density and high power rate from power sources.4,5 Among all the electrical energy storage mediums, the lithium-ion battery (LIB) is evidenced to be a promising candidate considering its design flexibility, shape versatility, and abundance in material sources, although some difficulties have to be circumvented in terms of cost and safety issues.6,7 Layered LiMO2 (M = Ni, Co, Mn, etc.) compounds and their derivatives have stimulated a great deal of research interest because of their inherently high theoretical capacity and wonderful reproducibility.8−11 Olivine LiFePO4 is conventionally regarded as another hopeful candidate due to its advantages on cost-effectiveness, nontoxicity, and high reversibility in application.11,12 With respect to improving the power rate in LIBs, nanomaterialization of active electrodes definitely is favorable for Li+ diffusion and transport across the overall dimension; however, another crucial criterion must be taken into account simultaneously. To our knowledge, Li ions can only be intercalated/ deintercalated rapidly along some specific crystal directions, whereas for some others, the behavior is substantially blocked.13,14 So in the research field of enhancing the rate capability in LIBs, construction and maximization of these Li ion rocking “express ways” provide another alternative route © 2012 American Chemical Society

and also have attracted more and more attention consequently. Herein, we will refer to LiCoO2, the well-known cathode material in LIBs, to visualize this viewpoint. LiCoO2 is the currently commercialized cathode material or key component in LIBs and most used in portable entertainment, telecommunication, and mobile computing devices. As with other Li-based transition-metal oxides (LiNiO2, LiMnO2), hexagonal LiCoO2 possesses a layered structure, which markedly provides ideal accommodation for Li ion insertion and extraction.15,16 If LiCoO2 is entirely lithiated and delitiated upon cycling, the unique capacity can reach up to 280 mA·h/g. However, aiming at preventing the α-NaFeO2 phase from collapsing, generally 0.5 Li+ at most is allowed to be utilized.1 Of course, well-crystallized structure is highly qualified to liberate and accommodate more Li ions so that higher capacity is thereby expected in some cases.17,18 By analyzing the atomic modeling of LiCoO2 as shown in Figure 1, we can see that along the specific directions of [100] and [010], Li ions can diffuse inward and outward the bulk of material without big hurdles. As a comparison, it is hard for Li ions to break though the frameworks and barriers along [001] since a large amount of oxygen and cobalt atoms block the pathway. In light of this observation, it would be a feasible approach to improve the rate capability of LIBs by increasing mobility of Li ions as long as we can design and fabricate nanostructured LiCoO2 with many enough (100) and (010) planes preferentially exposed. Received: August 8, 2012 Revised: September 10, 2012 Published: September 20, 2012 5629

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Figure 1. Schematic illustration to introduce the Li ions rocking express way along [100] and [010].

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RESULTS AND DISCUSSION In this article, we will present a totally new cathode material for LIBs, LiCoO2 nanomesh. So far it has been claimed that LiCoO2 is an extensively studied material and we have already learned enough knowledge about it so that there is no need for the further research on it. However, by our elaborate analyses of LiCoO2 nanomesh from morphology, structure, and electrochemical properties, the material exhibits completely distinguishable features compared with other previous LiCoO2 samples. The LiCoO2 nanomesh demonstrates the combined property of mesoporous and single crystal features. Its front and back planes are both indexed and determined as (010) or the equivalent crystal planes of LiCoO2 with α-NaFeO2 phase. The thickness is restricted within the range of 20 nm down to 10 nm. In the subsequent electrochemical properties’ investigations, the as-fabricated LiCoO2 nanomesh exhibits high capacity of around 135 mA·h/g at 100 mA/g, enhanced rate capability of 90 mA·h/g at a rate of 3 A/g, improved cycleability with no capacity fading after 100 cycles, and wonderful reversibility upon a series of galvanostatic rate steps. All these findings verify the LiCoO2 nanomesh as a promising cathode candidate in high-rate LIBs for the ever-increasing demand from today’s information-rich and mobile society. To the best of our knowledge, no similar results have been reported before in selectively exposing (100) and (010) crystal planes for LiCoO2 at almost 100% surface ratio. Figure 2 is the general characterization of the as-prepared LiCoO2. Figure 2a is the scanning electron microscopy (SEM) image to describe the free-standing sheet-like LiCoO2 samples. From Figure 2a, we can detect that these LiCoO2 have dimensions from several hundreds of square nanometers to tens of square micrometers; in the meantime, the integrity of these samples implies that solid architectural framework will be readily available during the galvanostatic cycling, which is one of the important prerequisite conditions to sustain the improved reversibility, cycleability, and stability. Moreover, the fact that the thickness is estimated within 20 nm from the magnified SEM image (Supporting Information) reduces the lithium ion rocking pathway to a certain extent compared with those of the conventionally synthesized LiCoO2 nanoparticles.19,20 So the rapid response to the external requirement of high power rate is believed to be easily achieved. Figure 2b is the corresponding powder X-ray diffraction (XRD) patterns,

Figure 2. (a) SEM image and (b) XRD peaks to show the general morphology and crystal phase as α-NaFeO2. (c) TEM and (d, e) the enlarged TEM images to describe the uniformity and mesoporosity in LiCoO2 nanomesh. HRTEM image verifies that the frontal plane is (100) or the equivalent plane of (010).

where all the peaks are indexed as α-NaFeO2 phase (JCPDS No. 44-0145) and in good agreement with the reflections from layered LiCoO2,19,20 even though the synthesis temperature is as low as 500 °C.19,20 Figure 2c is the transmission electron microscopy (TEM) image to show the monodisperse LiCoO2 nanosheets and further verify the uniformity in the samples. The individually magnified nanosheet is demonstrated in Figure 2d, from which some void spaces are visible under electron beam irradiation, indicating the potential presence of mesoporous structure in the sheet-like samples. Figure 2e is the locally enlarged TEM image to further disclose the detailed morphological and structural information about the LiCoO2 samples. From Figure 2e, we can find some void spaces are unevenly distributed across the sheet-like LiCoO2, solidifying the fact that the obtained LiCoO2 nanosheet is mesoporous and therefore can be identified as LiCoO2 nanomesh; meanwhile, the continuous membranous structure signifies the extreme possibility of single-crystal nature in these LiCoO2 nanomesh. High-resolution TEM (HRTEM) image as shown in Figure 2f provides powerful evidence for the assumption of single crystal for these nanomesh, in which two perpendicularly crossing lattice interdistances are calculated to be 0.46 and 0.25 nm, respectively, and correspond to those of (003) and (100) in sequence. The clear and regularly extended crystal lattices further verify the LiCoO2 nanomesh is single crystal. More importantly, by indexing the crystal lattices, the most exposed planes, in other words, the front and back planes, are fixed as (010) or the equivalent plane of (100), both of which finally account for almost 100% surface ratio in LiCoO2 nanomesh. In a departure from our previous research, single crystal Co3O4 nanomesh had been successfully fabricated before with single crystal (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet as precursor (Supporting Information). The synthesis strategy is rationally based on the tiny crystal mismatch below 2% between them (Figure 3). Since LiCoO2 nanomesh succeeds the single crystal 5630

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conversion while retaining the characteristic of single crystal.21−24 In order to investigate LiCoO2 nanomesh in some crucial aspects of surface area, pore size distribution, status of valence, and chemical compositions, Brunauer−Emmett−Teller (BET) method, X-ray photoelectron spectroscopy (XPS), and energydispersive X-ray spectroscopy (EDX) are used for further measurements. Figure 4a,b shows the magnified TEM images to describe the mesopore distribution in LiCoO2 nanomesh, from which some void spaces aligned between the striped crystals are visible. This mesoporous structure is believed to originate from the mesopores in Co3O4 naomesh, which previously developed from thermal decomposition of (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet. Figure 4c and the inset picture are the nitrogen absorption−desorption isotherm and the induced pore size distribution profiles, respectively, to reveal that the surface area of LiCoO2 nanomesh shrinks to 53.5 m2/g from the original value of 382.0 m2/g for Co3O4 nanomesh (Supporting Information); in the meantime, the pore size is expanded to around 3.80 nm instead of 3.27 nm in Co3O4 nanomesh. Variation in surface area and pore size is nested in changes of chemical compositions and temperature from 250 to 500 °C. It is inevitable that higher temperature will result in negative influence on surface areas; however, it will simultaneously create a positive effect on crystallization, which is the most important for lithiation/delithiation processes in cathode materials. Figure 4d is the EDX analysis and confirms the presence of Co element in the samples. The inset to Figure 4d is the XPS measurement, and the binding energies at 780.12 and 795.00 eV are attributed to Co 2p3/2 and Co 2p1/2, respectively. No apparent satellite shakeup peaks available around these core levels further verify that the LiCoO2 nanomesh is mainly composed of Co3+, which substantially

Figure 3. The schematics to introduce the mechanism through which the (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet is converted into Co3O4 nanomesh, then to LiCoO2 nanomesh while maintaining the single crystal feature. The crystal mismatch between (NH4)2Co8(CO3)6(OH)6·4H2O and Co3O4 is below 2% from the calculation based on the crystal planes of (200) in (NH4)2Co8(CO3)6(OH)6·4H2O and (1̅11̅) in Co3O4. For the crystal planes of (012) in (NH4)2Co8(CO3)6(OH)6·4H2O and (220) in Co3O4, the crystal mismatch is close to 0%. Formation of void spaces in Co3O4 nanomesh can compensate the gas emission after (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet is calcined at 250 °C and is favorable for maintaining the single crystallinity throughout the chemical conversion process. However, for the conversion from Co3O4 to LiCoO2 nanomesh, the corresponding crystal mismatch ranges from 0 to 13%, and the exact crystal match along one direction still enables the formation of single crystal in LiCoO2 nanomesh.

feature from (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet and Co3O4 nanomesh subsequently via crystal mismatch of 0−13% (Supporting Information), it is actually a thermodynamic equilibrium product without any necessity to conquer the energy gap, which is usually required when conventionally transformed from thermodynamically stable (001) to kinetically dominated (010) in LiCoO2 crystal. Such a situation is also involved in some previous reports so that to scientifically certify the feasibility and validity of structural and compositional

Figure 4. (a, b) Further magnified TEM images show the mesoporous structure in the LiCoO2 nanomesh. (c) BET measurement demonstrates 53.5 m2/g for the LiCoO2 nanomesh, and the inset picture indicates the pore size focused at 3.8 nm. (d) EDX and XPS (the inset) analyses reveal that the LiCoO2 nanomesh is mainly composed of Co3+. 5631

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Figure 5. (a) Galvanostatic measurement within the voltage window of 4.3−3.0 V is presented at a rate of 200 mA/g, (b) cyclic voltammetry (CV) is applied to confirm the charge/discharge plateaus, (c) the variation of specific capacity upon the designed current densities is to verify the enhanced reversibility and rate capability for the LiCoO2 nanomesh, and (d) data extracted from the galvanostatic measurement shows the Coulombic efficiency and stability at 200 mA/g within 50 cycles.

differs from (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet but is similar to Co3O4 nanomesh (Supporting Information). Figure 5 provides the results of the electrochemical investigation on LiCoO2 nanomesh. The galvanostatic measurement profiles are described in Figure 5a. From Figure 5a, we can find that the voltage plateau of discharge is almost parallel to 3.8 V vs Li +/Li, same as the conventional LiCoO 2 nanostructures.15,16,19,20 As a result, the same situation happens to the charge stage as well (Figure 5a) to locate anodic potential at 4.0 V. In order to confirm the reliabilityof the results, cyclic voltammetry (CV) measurement was applied to trace the charge−discharge processes as shown in Figure 5b. From Figure 5b, we can see that the voltage plateau of discharge is really at 3.82 V; meanwhile, the voltage plateau of charge is at 3.97 V. Figure 5c is the galvanostatic result of LiCoO2 nanomesh upon the designed current densities. Normally the capacity falls into the range of 85−135 mA h/g at rates from 100 mA/g to 3000 mA/g, and the decrease of capacity is unavoidable with the increase of current densities. Originally the capacity of 135 mAh/g can be reversibly delivered from LiCoO2 nanomesh at 100 mA/g within 50 cycles, and then with the increase of the cycling current densities, LiCoO2 nanomesh still can provide steady capacity of 90 mAh/g when run at a very high rate of 3000 mA/g. More importantly, when cycled back to the current rate of 100 mA/g, the specific capacity of 135 mAh/g survives the long time running, which signifies the greatly improved rate capacity and cycleability for LiCoO2 nanomesh. The electrochemical performance for the as-synthesized LiCoO2 nanomesh is superior to that of many previous reports, especially in the aspect of rate capability.25,26 Normally LiCoO2 nanostructures exhibit good specific capacities; however, when cycled at high rates, the specific capacities would decay to 50% of their

original values for most of them and a very high rate up to 3 A/ g was seldom applied either.27,28 Figure 5d is the galvanostatic measurement at a rate of 200 mA/g to determine Coulombic efficiency, specific capacity, and reliability upon 50 cycles. From Figure 5d, we can detect that LiCoO2 nanomesh exhibits enhanced stability and Coulombic efficiency up to 100%. During cycling, LiCoO2 nanomesh is capable of steadily delivering no less than 130 mA h/g. Thus, within the voltage window from 4.3 to 3.0 V, the specific capacity is very close to the theoretical value of 140 mA h/g for LiCoO2, implying approximately 0.5 Li+ can be reversibly utilized to carry charges and deliver energy as cathode in LIBs.29,30 In contrast, LiCoO2 nanoparticles exhibit poor performance in rate capability and Coulombic efficiency.19,20 In practical applications, LIBs should permit different power supplies for a long time cycling required by diverse devices, for example, electric vehicles, portable computing equipment, and mini-telecommunication mobiles. So the rate capability, specific capacity, and cycleability in LIBs become the crucial criteria to evaluate batteries’ comprehensive performance. All the above findings echo and verifie the fact that the morphological and structural novelty of LiCoO2 nanomesh plays a unique and irreplaceable role in its functionalization in LIB cathodes. In summary, LiCoO2 nanomesh exposes its surface area as (010) or the equivalent plane (100) at almost 100% ratio, over which Li ion rocking express routes are considerably distributed, and the thickness limited to 10 or 20 nm is inherited from the precursor of Co3O4 nanomesh; meanwhile, LiCoO2 exhibits mesoporous structure, which is favorable for electrolyte overall flooding into electrode material. All the described features above enable the realization of reducing the lithiation/delithiation distance and widening the Li ion rocking express way so that greatly enhanced rate capability is expected from LiCoO2 nanomesh cathode. As a 5632

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electron microscopy (TEM, Philips, Tecnai, F20, 200 kV) coupled with energy-dispersive X-ray spectroscopy analyzer (EDX), power Xray diffraction (XRD, Bruker D8 Advance X-ray diffractometer with Cu Kα radiation), Brunauer−Emmett−Teller surface area measurement (BET, Quantachrome Autosorb-6B surface area and pore size analyzer), and X-ray photoelectron spectrometery with a ESCALAB250 analyzer (XPS) were employed to characterize the obtained samples. Electrochemical Characterization. A homogeneous mixture composed of active material, carbon black, and poly(vinyl difluoride) (PVDF) using 1-methyl-2-pyrrolidinone (NMP) as solvent in weight ratio of 90:5:5 was prepared under strong magnetic stirring for at least 3 days and then extracted, and some samples were spread to Al foils. Before and after the samples were spread, the Al foils had to be weighed in a high-precision analytical balance (Sartorius, max weight 5100 mg, d = 0.001 mg). The reading difference was the exact mass for the coated samples on Al foils. Normally the mass loading is around 2−3 mg/cm2. The obtained pieces of Al covered with samples were then used as working electrodes with 1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC-DEC, v/v = 1:1) as electrolyte. Celgard 2400 was used as the separator film to isolate the two electrodes. Pure Li foil was accepted to serve as counter electrode and reference electrode. The cell was assembled in an argon-filled glovebox where moisture and oxygen concentrations were strictly limited to below 0.1 ppm. The galvanostatic cyclying was performed on Neware battery testing system in model of 5 V, 1−5 mA. Li foil was used as counter electrode (anode) and reference electrode. Cyclic voltammetry (CV) was collected using Autolab (model AUT71740) in a three-electrode cell.

comparison, LiCoO2 nanoparticles performed poorly as shown in the Supporting Information.

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CONCLUSION In conclusion, layered-structure LiCoO2 nanomesh has been successfully fabricated for the first time based on chemical conversion strategy using Co3O4 nanomesh as precursor. The as-made LiCoO2 nanomesh possesses combined properties of mesoporosity, single crystallinity and limited thickness. Moreover, it is worth highlighting that Li ion rapid diffusion planes of (010) and (100) overwhelmingly dominate the exposed surface areas with approximately 100% ratio, which build up a large amount of express routes for Li ions rocking in a reversible way. Whereafter the electrochemical performance for the LiCoO2 nanomesh verifies the importance of the morphological and structural properties, where promoted rate capability, cycleability and reliability are achieved over long time cycling and variable current rates, to make the LiCoO2 nanomesh a promising cathode in LIBs to meet diverse needs from modern portable devices and electrical equipment. Besides the applied levels, the achieved LiCoO2 nanomesh also will be useful in fundamental research. For example, combining ultrahigh vacuum scanning tunneling microscope (STM) technique, perhaps lithium ions can be investigated to insert/extract one by one into/from LiCoO2 via the highly uniform planes of (010) so that the right intercalation/deintercalation mechanism and process underlying the Li-ion battery energy storage will be exactly studied. Undoubtedly, the research will be of great significance in improving LIB efficiency and exploiting the next generation of Li-based electrochemical energy storing devices. Now the related research is in progress.

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ASSOCIATED CONTENT

S Supporting Information *

SEM image to determine the thickness of the LiCoO2 nanomesh, typical characterizations of (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet, morphological and structural characterizations of Co3O4 nanomesh, Brunauer−Emmett−Teller (BET) surface area measurement and size distribution for Co3O4 nanomesh, X-ray photoelectron spectroscopy (XPS) analysis of (NH4)2Co8(CO3)6(OH)6·4H2O nanosheet and Co3O4 nanomesh, SEM image showing the morphology of LiCoO2 nanoparticles, and XRD patterns and galvanostatic measurements of LiCoO2 nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.

EXPERIMENTAL SECTION

Materials. All chemicals or materials were used directly without any further purification prior to usage: ethylene glycol (Fisher Chemical, 99.99%), ammonium hydroxide (NH3·H2O, 28−30 wt %, J.T.Baker), cobalt nitrate (Co(NO3)2, 99.9%, Aldrich), sodium carbonate (Na2CO3, 99.9%, Aldrich), lithium hydroxide (LiOH, 99.9%, Aldrich), aluminum foil (0.5 mm thick, annealed, 99.999%, General Research Institute for Nonferrous Metals, Beijing), and metallic Li foil (99.9%, Charslton Technologies). Preparation of (NH4)2Co8(CO3)6(OH)6·4H2O Nanosheets. In a typical synthesis, ethylene glycol (12.5 mL), concentrated NH3·H2O (12.5 mL, 28 wt ratio %), 1 M Na2CO3 aqueous solution (5 mL), and 1 M Co(NO3)2 aqueous solution (5 mL) were mixed step by step under vigorous stirring with intervals of 2 min. After that, the mixture was stirred for another 20 min, and finally the mixture turned into a homogeneous solution with a deep dark color. Once the precursor was transferred into a Teflon-lined stainless steel autoclave at a volume of 45 mL, a thermal treatment was performed for the Teflon-liner in an electric oven at 170 °C for 18 h. After the autoclave was cooled naturally to room temperature in a fumehood, samples deposited at the bottom were collected and washed by centrifugation for at least three cycles using deionized water (D.I. water) and one cycle using pure ethanol. The as-synthesized samples were then dried in a vacuum oven at 40 °C overnight to remove the absorbed water for the subsequent characterizations. Preparation of Co 3 O 4 and LiCoO 2 Nanomeshes. (NH4)2Co8(CO3)6(OH)6·4H2O nanosheets were annealed at 250 °C for 200 min in air to be oxidized into Co3O4 nanomesh. Then Co3O4 nanomesh was converted to LiCoO2 nanomesh when reacted with overstoichiometric molar ratio of LiOH at 500 °C for 300 min in air. The tap density for the final LiCoO2 nanomesh is around 2.6 g/ cm3. Characterization of the Samples. Field emission scanning electron microscope (FESEM, JEOL, JSM-7600F), transmission

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Hundred Talents Program at Chongqing University and research finding from Institute of Chemical and Engineering Sciences (ICES) under Agency for Science, Technology and Research (A*STAR) in Singapore.

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