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Electrochemical properties of micron-sized, spherical, meso- and macro-porous Co3O4 and CoO–carbon composite powders prepared by a two-step spray drying process† Jung Hyun Kim and Yun Chan Kang* Micron-sized, spherical, meso- and macro-porous Co3O4 and CoO–carbon composite powders were prepared via a simple two-step spray drying process. The CoO–carbon composite powders, in which homogeneous mixing of the metal oxide and carbon components was achieved using the first spray drying process, were wet milled to produce the slurry for the second spray drying process. Co3O4 and CoO– carbon composite powders with mean particle sizes of 4.4 and 4.7 mm were respectively obtained by spraydrying the slurry after post-treatment at 400 C under air and nitrogen atmospheres. Meso- and macropores were uniformly distributed inside the Co3O4 and CoO–carbon composite powders. The CoO–carbon composite powders exhibited discharge capacities of 882 and 855 mA h g 1 at a high constant current density of 1400 mA g
1
for the 2nd and 100th cycles. The discharge capacities of the Co3O4 powders at the
Received 18th December 2013 Accepted 16th February 2014
2
DOI: 10.1039/c3nr06651g
985 to 698 mA h g 1. The superior rate and cycling performances of the CoO–carbon composite powders
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are ascribed to their meso- and macro-porous structures and carbon components.
nd
th
and 100 cycles were 970 and 644 mA h g 1. With stepwise increment in the current density from 500
to 5000 mA g 1, the discharge capacities of the CoO–carbon composite powders decreased slightly from
Introduction Transition metal oxides have been widely studied as anode materials for lithium-ion batteries (LIBs) because of their high theoretical capacities.1–18 Furthermore, improved cycling stability and enhanced high-rate capability have been reported for carbon composites containing transition metal oxides.19–36 The carbonaceous materials act as buffers to accommodate the mechanical stress induced by volume expansion of the electrodes during battery operation and improve the structural stability of the electrodes. The electrochemical properties of transition metal oxides depend strongly on their morphologies as well as mean sizes. Transition metal oxide–carbon composite materials with ne particle sizes and various morphologies, such as microspheres,27,28 hollow spheres,19,29 nanospindles,31 and nanowires,32 have been synthesized by various methods appropriate for small-scale production. However, LIB applications generally require powders with large particle sizes of several microns for the anode and cathode materials given that three-dimensional mesoporous metal oxide materials of several microns in size can provide a short lithium ion path by
facilitating the electrolyte contact with the pores as well as free space that can accommodate volume changes during cycling.35 Spray drying, which involves rapid drying with a hot gas, is the most widely utilized industrial process for producing a dry powder from a liquid or slurry. This method has already been applied to large-scale production of anode and cathode materials for LIBs.37–46 Composite powders of transition metal oxide and carbon have also been prepared by the spray drying process using a slurry of metal oxide and carbon powders.47–51 Slurries for spray drying have conventionally been prepared by mechanical milling of mixed metal oxide and carbon powders. However, uniform mixing of the metal oxide and carbon components could not be achieved in the conventional spray drying process. In this study, an easy and scalable process for preparation of nano-structured materials in which the transition metal oxide and carbon components are homogeneously mixed is introduced for anode applications in LIBs. The CoO–carbon composite was selected as the rst target material, and its electrochemical properties as an anode material for LIBs are evaluated herein.
Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Korea. E-mail: [email protected]; Fax: +82-2-4583504; Tel: +82-2-2049-6010
Experimental
† Electronic supplementary 10.1039/c3nr06651g
A schematic diagram of the spray drying process is shown in Fig. S1.† Conventionally, a liquid containing a high
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(ESI)
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concentration of the product material is pumped into an atomizing device where it is transformed into a spray of small droplets. These droplets meet a stream of hot air. The moisture evaporates very rapidly while the droplets are still suspended in the air stream. The resulting dry powder is separated from the humid air in a cyclone system by centrifugal forces. The centrifugal separation occurs due to a great increase in the air speed when the mixture of particles and air enters the cyclone system. The particles are forced towards the cyclone walls while the lighter, moist air is directed away through exhaust pipes. Conditions such as the inlet temperature, feeding rate, and atomization pressure are controlled during the spray drying process. In the present study, the temperatures of the inlet and outlet of the spray dryer were set to 350 C and 150 C, respectively. A two-uid nozzle was used as an atomizer and the atomization pressure was 1 bar. The spray solution was prepared by dissolving cobalt nitrate hexahydrate [Co(NO3)2$6H2O, Junsei] and citric acid (as a chelating agent) in distilled water. The overall concentration of cobalt and citric acid in the solution was 0.5 M. The precursor powders obtained from the spray drying process were post-treated under a nitrogen atmosphere at a temperature of 400 C for 3 h. The post-treated powders were dispersed in distilled water and wetmilled in a planetary mill for 3 h. The resulting slurry was then spray dried to form powders of several microns in size. The spray dried composite powders were post-treated at 400 C for 3 h under air and nitrogen atmospheres. The capacities and cycling properties of the powders were measured using 2032type coin cells. The electrode was prepared by mixing 35 mg of active powders with 10 mg carbon black and 5 mg sodium
Scheme 1
carboxymethyl cellulose (CMC) in distilled water. The size of the electrode was 1 cm 1 cm and the mass loading was about 1.6 mg cm 2. Lithium metal and a polypropylene lm were used as the counter electrode and separator, respectively. 1 M LiPF6 dissolved in a mixture of uoroethylene carbonate–dimethyl carbonate (FEC–DMC) with a volume ratio of 1 : 1 was used as the electrolyte (TECHNO Semichem Co.). The entire cell was assembled in a glove box under an argon atmosphere. The charge/discharge characteristics of the samples were determined by cycling in the electric potential range of 0.01–3.0 V at various current densities. Cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s 1 in the range 0.01– 3.0 V. Electrochemical impedance spectroscopy measurements of the electrodes were carried out in the frequency range between 500 kHz and 10 mHz at room temperature.
Results and discussion The mechanism of formation of the nanocrystalline mesoporous Co3O4 and CoO–carbon composite powders using the spray drying process is illustrated in Scheme 1. The commercially applicable spray drying process was used for preparation of nanostructured metal oxide–carbon composite aggregates of several microns in size as well as the precursor materials in which the metal oxide and carbon components were homogeneously mixed. The precursor powders with a hollow, thin-walled structure were prepared by spray drying an aqueous solution of cobalt nitrate hexahydrate and citric acid. Various types of carbon source materials could be utilized in the preparation of the
Schematic diagram of the formation of mesoporous cobalt oxide powders with and without carbon.
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precursor powders. In this study, citric acid was used as the carbon source material as well as the chelating agent. Citric acid dissolved in the spray solution was a key material for the formation of the hollow thin-walled precursor powders. Posttreatment under a N2 atmosphere at a low temperature of 400 C transformed the precursor powders into the cobalt oxide– carbon composite. The post-treated composite powders with homogeneously mixed metal oxide and carbon components were wet milled to obtain the slurry for spray drying. Spray drying of the slurry produced the nano-structured cobalt oxide– carbon composite powders of several microns in size. The nanocrystalline Co3O4 powders were prepared by post-treatment of the spray dried powders at 400 C for 3 h in air. In comparison, nanocrystalline CoO–carbon composite powders were prepared by post-treatment of the spray-dried powders using the same temperature and time conditions, under a nitrogen atmosphere. The internal structure of the composite powders comprised meso- and macro-pores that facilitated the penetration of the electrolyte inside the electrode material. Furthermore, the macro-pores may act as a buffer layer to alleviate volume changes during Li-ion insertion/extraction by absorbing the stress. The morphologies of the precursor powders prepared by a spray drying process before and aer post-treatment at 400 C under a nitrogen atmosphere are respectively shown in Fig. 1a and b. The metal citrates formed by chelation of the Co components with citric acid had good drying properties even when the residence times of the droplets within the spray dryer
Fig. 1 Morphologies of the precursor powders prepared by a spray drying process before and after post-treatment at 400 C under a nitrogen atmosphere: (a) precursor powders prepared by a spray drying process, (b) post-treated powders under a nitrogen atmosphere, and (c–f) TEM and dot-mapping images of the cobalt oxide– carbon composite powders crushed in a planetary mill.
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were short, and exhibited high stability in the presence of water vapor. Consequently, the metal citrate powders could be easily collected by using the cyclone system. The dried precursor powders formed from the metal chelates had poor gas penetration properties during spray drying. Therefore, the resulting precursor powders had large dimensions of several tens of micrometers and a hollow morphology with a thin-walled structure. The hollow morphology with a thin-walled structure of the precursor powders was maintained in the cobalt oxide– carbon composite powders post-treated under a nitrogen atmosphere, as shown in Fig. 1b. Fig. 1c–f show the TEM and dot-mapping images of the cobalt oxide–carbon composite powders crushed in a planetary mill using zirconia balls. The micron-sized powders were easily milled into nanopowders. The high-resolution TEM image in Fig. 1e indicates clear lattice fringes with a separation of 0.24 nm and a crystalline structure. This value corresponds to the (111) lattice plane of cubic CoO. The carbon component was clearly detected in the dot-mapping images of the cobalt oxide–carbon composite powders. The stable slurry of the cobalt oxide–carbon composite formed via the wet-milling process was spray dried to form the powders with dimensions of several microns having a solid structure. The spray-dried powders were post-treated at 400 C for 3 h under nitrogen and air atmospheres, and the XRD patterns of the respective powders are shown in Fig. S2.† The powders post-treated under air had a pure cubic spinel Co3O4 structure (JCPDS card no. 42-1467), with no additional peaks detected. In contrast, the diffraction peaks of the powders posttreated under nitrogen were consistent with the standard crystallographic data of cubic CoO (JCPDS card no. 78-0431). The morphologies of the Co3O4 and CoO–carbon composite powders are shown in Fig. 2. All of the powders comprised individually dispersed particles with spherical morphologies and rough surfaces. The mean sizes of the Co3O4 and CoO– carbon composite powders determined from the SEM images were 4.4 and 4.7 mm, respectively. Fig. 2c and f show the lowand high-resolution SEM images of the cross-sections of the Co3O4 and CoO–carbon composite powders cut by applying a cross-section polisher (CP). From the SEM images of the crosssections, uniformly distributed macro-pores were apparent inside the Co3O4 and CoO–carbon composite powders, as indicated by arrows in Fig. 2c and f. Nitrogen adsorption and desorption isotherms and pore size distribution curves (Fig. S3†) were acquired to elucidate the detailed structures of the Co3O4 and CoO–carbon composite spheres. The BET surface areas of the Co3O4 and CoO–carbon composite powders were 28 and 45 m2 g 1, respectively. The mean pore sizes of the Co3O4 and CoO–carbon composite powders were 36 and 21 nm, respectively. The BET surface area of the CoO–carbon composite powders was larger than that of the Co3O4 powders, which is attributed to restriction of the crystal growth of CoO by the carbon matrix during post-treatment under nitrogen. However, growth of the crystalline Co3O4 phase of the powders during the post-treatment process increased the macro-pore diameter and pore volume as shown in Fig. S3.† The carbon content of the CoO–carbon composite powders obtained from the TG analysis was 7.5 wt%.
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Morphologies of the Co3O4 and CoO–carbon composite powders prepared by the second-step spray drying process: (a–c) Co3O4 powders and (d–f) CoO–carbon composite powders.
Fig. 2
The electrochemical properties of the Co3O4 and CoO– carbon composite powders prepared by the two-step spray drying process are shown in Fig. 3. Fig. 3a shows the initial charge and discharge proles of the Co3O4 and CoO–carbon composite powders at a constant current density of 500 mA g 1 in the voltage range of 0.01–3.0 V. The Co3O4 electrode exhibits a long plateau region in the initial discharge prole at a voltage of ca. 1 V, corresponding to lithium insertion into the crystal structure of Co3O4 and reduction of the Co ions to the Co metal.52 On the other hand, three distinct regions were observed in the initial discharge prole of the CoO–carbon composite electrode. The rst plateau appearing at 0.9 V is associated with lithium storage in CoO.53 The second plateau located at 0.6 V can be ascribed to the conversion reaction between CoO and Li.53,54 The slope, which is observed in the voltage range between 0.5 and 0.01 V, may be attributed to the interaction of Co particles with the electrolyte to form a solid electrolyte interface layer.53–55 The initial discharge capacities of the Co3O4 and CoO– carbon composite powders were 1334 and 1273 mA h g 1, respectively. These values are much higher than the theoretical capacities of 890 and 716 mA h g 1 for Co3O4 and CoO, which can be attributed to the formation of solid electrolyte interphase (SEI) lms on the particles during the discharge process. The rst charge capacities of the Co3O4 and CoO–carbon composite powders were 1018 and 970 mA h g 1, respectively. An irreversible capacity loss of about 300 mA h g 1 occurred during the rst cycles due to incomplete decomposition of SEI and Li2O in addition to other factors such as the intrinsic nature of the material and kinetic limitations due to current density.11
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Fig. S4† shows the rate capabilities of the Co3O4 and CoO– carbon composite powders, acquired by step-wise increment in the current densities from 500 to 10 000 mA g 1 for successive cycles in the voltage range of 0.01–3.0 V. The Co3O4 and CoO– carbon composite powders had respectively high discharge capacities of 738 and 717 mA h g 1 even at a high current density of 10 000 mA g 1. The Co3O4 and CoO–carbon composite powders exhibited excellent rate capabilities derived from their unique structures with well-developed macropores within the micron-sized powders. Fig. 3b shows the cycling performance of the Co3O4 and CoO–carbon composite powders at a constant current density of 1400 mA g 1 in the voltage range of 0.01–3.0 V. The cycling performance of the CoO–carbon composite powders was superior to that of the Co3O4 powders. A slight increase in the discharge capacity was observed over the rst 10 cycles for both samples, primarily owing to the formation of a polymeric gel-like lm on the active material.56 The discharge capacity of the Co3O4 powders faded quickly to a relatively low value of 644 mA h g 1 aer 100 cycles, whereas the CoO–carbon composite powders retained a relatively high discharge capacity of 855 mA h g 1 aer 100 cycles. The capacity retention of the Co3O4 and CoO–carbon composite powders measured from the second cycle was 66 and 97%, respectively. Morphological changes of the Co3O4 and CoO–carbon composite powders aer 100 cycles are shown in Fig. 4. The spherical morphology of the CoO–carbon composite powders was maintained even aer cycling. In contrast, the Co3O4 powders were fragmented into several pieces aer cycling. The formation of cracks, which is one of the main origins of capacity fading, is caused by volume expansion of the active material during cycling. The Nyquist impedance plots for both samples, acquired aer the 1st and 100th cycles, are shown in Fig. 3c. The diameters of the semicircles in the medium-frequency region, which were assigned to the charge-transfer resistance (Rct), obtained aer the 1st cycle for the Co3O4 and CoO–carbon composite powders, were similar. However, the charge-transfer resistance of the Co3O4 powders was larger than that of the CoO–carbon composite powders aer 100 cycles. The chargetransfer resistance of the CoO–carbon composite powders, having a stable structure, was lower than that of the Co3O4 powders aer the 100th cycle. The carbon component acted as a structural buffer to accommodate the volume expansion and contraction of the CoO–carbon composite powders during cycling. Fig. 3d shows the rate performance of the CoO–carbon composite powders, measured by increasing the current density in increments from 500 to 5000 mA g 1 for every 5 successive cycles in the voltage range of 0.01–3.0 V. The discharge capacities of the CoO–carbon composite powders decreased slightly from 985 to 698 mA h g 1 when the current density was increased in increments from 500 to 5000 mA g 1. The superior rate and cycling performances of the CoO–carbon composite powders were ascribed to their meso- and macro-porous structures and carbon components. The electrochemical properties of the prepared CoO–carbon composite powders are compared with those of the particles with various morphologies reported in the previous literature, and the results are summarized in Table S1.† The Co3O4–C
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Fig. 3 Electrochemical properties of the Co3O4 and CoO–C composite powders: (a) initial charge and discharge curves at a constant current density of 500 mA g 1, (b) cycling performances at a constant current density of 1400 mA g 1, (c) Nyquist impedance plots acquired after the 1st and 100th cycles, and (d) rate performance of the CoO–carbon composite powders, at constant current density in increments from 500 to 5000 mA g 1 for every 5 successive cycles.
nanospheres prepared by a hydrothermal method had a discharge capacity of 567 mA h g 1 at a current density of 440 mA g 1 aer 107 cycles.30 The Co3O4–CoO–graphene nanocomposite prepared by an auto-combustion method had a discharge capacity of 801 mA h g 1 at a low current density of 21 mA g 1 aer 30 cycles.56 Graphite, the most widely used anode material for LIBs, has a relatively low theoretical gravimetric capacity (372 mA h g 1) and thus is inadequate for high power applications. Thus, the prepared CoO–carbon composite powders had superior electrochemical properties to those reported in the previous literature. The CoO–carbon composite powders with superior electrochemical properties could be applied as anode materials for LIBs.
Conclusions
Fig. 4 Morphology changes of the Co3O4 and CoO–C composite powders after 100 cycles at a constant current density of 1400 mA g 1.
composite powders synthesized via spray pyrolysis had a discharge capacity of 800 mA h g 1 at a low current density of 30 mA g 1 aer 50 cycles.29 The carbon–Co3O4 composite
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A prospective process for large-scale production of spherical, micron-sized CoO–carbon composite powders with internal meso- and macro-pores for anode applications in LIBs was proposed herein. The electrochemical properties of the CoO– carbon composite powders were superior to those of Co3O4 powders with a similar morphology prepared by the same process. The carbon component functioned to minimize the crystal growth of cobalt oxide during the post-treatment process
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and acted as a buffer layer to negate the impact of volume changes during cycling. The meso- and macro-pores also improved the rate and cycling performance of the CoO–carbon composite powders by providing a short lithium ion path by facilitating the electrolyte contact with the pores as well as providing free space for accommodating volume changes during cycling. The newly developed process has potential for large-scale applications to the production of various compositions of metal oxide–carbon composite powders for a wide range of applications including energy storage.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012R1A2A2A02046367). This work was supported by the Creative Industrial Technology Development Program (10045141) funded By the Ministry of Trade, industry & Energy (MI, Korea).
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Electrochemical properties of micron-sized, spherical, meso- and macro-porous Co3O4 and CoO–carbon composite powders prepared by a two-step spray drying process† Jung Hyun Kim and Yun Chan Kang* Micron-sized, spherical, meso- and macro-porous Co3O4 and CoO–carbon composite powders were prepared via a simple two-step spray drying process. The CoO–carbon composite powders, in which homogeneous mixing of the metal oxide and carbon components was achieved using the first spray drying process, were wet milled to produce the slurry for the second spray drying process. Co3O4 and CoO– carbon composite powders with mean particle sizes of 4.4 and 4.7 mm were respectively obtained by spraydrying the slurry after post-treatment at 400 C under air and nitrogen atmospheres. Meso- and macropores were uniformly distributed inside the Co3O4 and CoO–carbon composite powders. The CoO–carbon composite powders exhibited discharge capacities of 882 and 855 mA h g 1 at a high constant current density of 1400 mA g
1
for the 2nd and 100th cycles. The discharge capacities of the Co3O4 powders at the
Received 18th December 2013 Accepted 16th February 2014
2
DOI: 10.1039/c3nr06651g
985 to 698 mA h g 1. The superior rate and cycling performances of the CoO–carbon composite powders
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are ascribed to their meso- and macro-porous structures and carbon components.
nd
th
and 100 cycles were 970 and 644 mA h g 1. With stepwise increment in the current density from 500
to 5000 mA g 1, the discharge capacities of the CoO–carbon composite powders decreased slightly from
Introduction Transition metal oxides have been widely studied as anode materials for lithium-ion batteries (LIBs) because of their high theoretical capacities.1–18 Furthermore, improved cycling stability and enhanced high-rate capability have been reported for carbon composites containing transition metal oxides.19–36 The carbonaceous materials act as buffers to accommodate the mechanical stress induced by volume expansion of the electrodes during battery operation and improve the structural stability of the electrodes. The electrochemical properties of transition metal oxides depend strongly on their morphologies as well as mean sizes. Transition metal oxide–carbon composite materials with ne particle sizes and various morphologies, such as microspheres,27,28 hollow spheres,19,29 nanospindles,31 and nanowires,32 have been synthesized by various methods appropriate for small-scale production. However, LIB applications generally require powders with large particle sizes of several microns for the anode and cathode materials given that three-dimensional mesoporous metal oxide materials of several microns in size can provide a short lithium ion path by
facilitating the electrolyte contact with the pores as well as free space that can accommodate volume changes during cycling.35 Spray drying, which involves rapid drying with a hot gas, is the most widely utilized industrial process for producing a dry powder from a liquid or slurry. This method has already been applied to large-scale production of anode and cathode materials for LIBs.37–46 Composite powders of transition metal oxide and carbon have also been prepared by the spray drying process using a slurry of metal oxide and carbon powders.47–51 Slurries for spray drying have conventionally been prepared by mechanical milling of mixed metal oxide and carbon powders. However, uniform mixing of the metal oxide and carbon components could not be achieved in the conventional spray drying process. In this study, an easy and scalable process for preparation of nano-structured materials in which the transition metal oxide and carbon components are homogeneously mixed is introduced for anode applications in LIBs. The CoO–carbon composite was selected as the rst target material, and its electrochemical properties as an anode material for LIBs are evaluated herein.
Department of Chemical Engineering, Konkuk University, 1 Hwayang-dong, Gwangjin-gu, Seoul 143-701, Korea. E-mail: [email protected]; Fax: +82-2-4583504; Tel: +82-2-2049-6010
Experimental
† Electronic supplementary 10.1039/c3nr06651g
A schematic diagram of the spray drying process is shown in Fig. S1.† Conventionally, a liquid containing a high
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concentration of the product material is pumped into an atomizing device where it is transformed into a spray of small droplets. These droplets meet a stream of hot air. The moisture evaporates very rapidly while the droplets are still suspended in the air stream. The resulting dry powder is separated from the humid air in a cyclone system by centrifugal forces. The centrifugal separation occurs due to a great increase in the air speed when the mixture of particles and air enters the cyclone system. The particles are forced towards the cyclone walls while the lighter, moist air is directed away through exhaust pipes. Conditions such as the inlet temperature, feeding rate, and atomization pressure are controlled during the spray drying process. In the present study, the temperatures of the inlet and outlet of the spray dryer were set to 350 C and 150 C, respectively. A two-uid nozzle was used as an atomizer and the atomization pressure was 1 bar. The spray solution was prepared by dissolving cobalt nitrate hexahydrate [Co(NO3)2$6H2O, Junsei] and citric acid (as a chelating agent) in distilled water. The overall concentration of cobalt and citric acid in the solution was 0.5 M. The precursor powders obtained from the spray drying process were post-treated under a nitrogen atmosphere at a temperature of 400 C for 3 h. The post-treated powders were dispersed in distilled water and wetmilled in a planetary mill for 3 h. The resulting slurry was then spray dried to form powders of several microns in size. The spray dried composite powders were post-treated at 400 C for 3 h under air and nitrogen atmospheres. The capacities and cycling properties of the powders were measured using 2032type coin cells. The electrode was prepared by mixing 35 mg of active powders with 10 mg carbon black and 5 mg sodium
Scheme 1
carboxymethyl cellulose (CMC) in distilled water. The size of the electrode was 1 cm 1 cm and the mass loading was about 1.6 mg cm 2. Lithium metal and a polypropylene lm were used as the counter electrode and separator, respectively. 1 M LiPF6 dissolved in a mixture of uoroethylene carbonate–dimethyl carbonate (FEC–DMC) with a volume ratio of 1 : 1 was used as the electrolyte (TECHNO Semichem Co.). The entire cell was assembled in a glove box under an argon atmosphere. The charge/discharge characteristics of the samples were determined by cycling in the electric potential range of 0.01–3.0 V at various current densities. Cyclic voltammetry measurements were carried out at a scan rate of 0.1 mV s 1 in the range 0.01– 3.0 V. Electrochemical impedance spectroscopy measurements of the electrodes were carried out in the frequency range between 500 kHz and 10 mHz at room temperature.
Results and discussion The mechanism of formation of the nanocrystalline mesoporous Co3O4 and CoO–carbon composite powders using the spray drying process is illustrated in Scheme 1. The commercially applicable spray drying process was used for preparation of nanostructured metal oxide–carbon composite aggregates of several microns in size as well as the precursor materials in which the metal oxide and carbon components were homogeneously mixed. The precursor powders with a hollow, thin-walled structure were prepared by spray drying an aqueous solution of cobalt nitrate hexahydrate and citric acid. Various types of carbon source materials could be utilized in the preparation of the
Schematic diagram of the formation of mesoporous cobalt oxide powders with and without carbon.
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precursor powders. In this study, citric acid was used as the carbon source material as well as the chelating agent. Citric acid dissolved in the spray solution was a key material for the formation of the hollow thin-walled precursor powders. Posttreatment under a N2 atmosphere at a low temperature of 400 C transformed the precursor powders into the cobalt oxide– carbon composite. The post-treated composite powders with homogeneously mixed metal oxide and carbon components were wet milled to obtain the slurry for spray drying. Spray drying of the slurry produced the nano-structured cobalt oxide– carbon composite powders of several microns in size. The nanocrystalline Co3O4 powders were prepared by post-treatment of the spray dried powders at 400 C for 3 h in air. In comparison, nanocrystalline CoO–carbon composite powders were prepared by post-treatment of the spray-dried powders using the same temperature and time conditions, under a nitrogen atmosphere. The internal structure of the composite powders comprised meso- and macro-pores that facilitated the penetration of the electrolyte inside the electrode material. Furthermore, the macro-pores may act as a buffer layer to alleviate volume changes during Li-ion insertion/extraction by absorbing the stress. The morphologies of the precursor powders prepared by a spray drying process before and aer post-treatment at 400 C under a nitrogen atmosphere are respectively shown in Fig. 1a and b. The metal citrates formed by chelation of the Co components with citric acid had good drying properties even when the residence times of the droplets within the spray dryer
Fig. 1 Morphologies of the precursor powders prepared by a spray drying process before and after post-treatment at 400 C under a nitrogen atmosphere: (a) precursor powders prepared by a spray drying process, (b) post-treated powders under a nitrogen atmosphere, and (c–f) TEM and dot-mapping images of the cobalt oxide– carbon composite powders crushed in a planetary mill.
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were short, and exhibited high stability in the presence of water vapor. Consequently, the metal citrate powders could be easily collected by using the cyclone system. The dried precursor powders formed from the metal chelates had poor gas penetration properties during spray drying. Therefore, the resulting precursor powders had large dimensions of several tens of micrometers and a hollow morphology with a thin-walled structure. The hollow morphology with a thin-walled structure of the precursor powders was maintained in the cobalt oxide– carbon composite powders post-treated under a nitrogen atmosphere, as shown in Fig. 1b. Fig. 1c–f show the TEM and dot-mapping images of the cobalt oxide–carbon composite powders crushed in a planetary mill using zirconia balls. The micron-sized powders were easily milled into nanopowders. The high-resolution TEM image in Fig. 1e indicates clear lattice fringes with a separation of 0.24 nm and a crystalline structure. This value corresponds to the (111) lattice plane of cubic CoO. The carbon component was clearly detected in the dot-mapping images of the cobalt oxide–carbon composite powders. The stable slurry of the cobalt oxide–carbon composite formed via the wet-milling process was spray dried to form the powders with dimensions of several microns having a solid structure. The spray-dried powders were post-treated at 400 C for 3 h under nitrogen and air atmospheres, and the XRD patterns of the respective powders are shown in Fig. S2.† The powders post-treated under air had a pure cubic spinel Co3O4 structure (JCPDS card no. 42-1467), with no additional peaks detected. In contrast, the diffraction peaks of the powders posttreated under nitrogen were consistent with the standard crystallographic data of cubic CoO (JCPDS card no. 78-0431). The morphologies of the Co3O4 and CoO–carbon composite powders are shown in Fig. 2. All of the powders comprised individually dispersed particles with spherical morphologies and rough surfaces. The mean sizes of the Co3O4 and CoO– carbon composite powders determined from the SEM images were 4.4 and 4.7 mm, respectively. Fig. 2c and f show the lowand high-resolution SEM images of the cross-sections of the Co3O4 and CoO–carbon composite powders cut by applying a cross-section polisher (CP). From the SEM images of the crosssections, uniformly distributed macro-pores were apparent inside the Co3O4 and CoO–carbon composite powders, as indicated by arrows in Fig. 2c and f. Nitrogen adsorption and desorption isotherms and pore size distribution curves (Fig. S3†) were acquired to elucidate the detailed structures of the Co3O4 and CoO–carbon composite spheres. The BET surface areas of the Co3O4 and CoO–carbon composite powders were 28 and 45 m2 g 1, respectively. The mean pore sizes of the Co3O4 and CoO–carbon composite powders were 36 and 21 nm, respectively. The BET surface area of the CoO–carbon composite powders was larger than that of the Co3O4 powders, which is attributed to restriction of the crystal growth of CoO by the carbon matrix during post-treatment under nitrogen. However, growth of the crystalline Co3O4 phase of the powders during the post-treatment process increased the macro-pore diameter and pore volume as shown in Fig. S3.† The carbon content of the CoO–carbon composite powders obtained from the TG analysis was 7.5 wt%.
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Morphologies of the Co3O4 and CoO–carbon composite powders prepared by the second-step spray drying process: (a–c) Co3O4 powders and (d–f) CoO–carbon composite powders.
Fig. 2
The electrochemical properties of the Co3O4 and CoO– carbon composite powders prepared by the two-step spray drying process are shown in Fig. 3. Fig. 3a shows the initial charge and discharge proles of the Co3O4 and CoO–carbon composite powders at a constant current density of 500 mA g 1 in the voltage range of 0.01–3.0 V. The Co3O4 electrode exhibits a long plateau region in the initial discharge prole at a voltage of ca. 1 V, corresponding to lithium insertion into the crystal structure of Co3O4 and reduction of the Co ions to the Co metal.52 On the other hand, three distinct regions were observed in the initial discharge prole of the CoO–carbon composite electrode. The rst plateau appearing at 0.9 V is associated with lithium storage in CoO.53 The second plateau located at 0.6 V can be ascribed to the conversion reaction between CoO and Li.53,54 The slope, which is observed in the voltage range between 0.5 and 0.01 V, may be attributed to the interaction of Co particles with the electrolyte to form a solid electrolyte interface layer.53–55 The initial discharge capacities of the Co3O4 and CoO– carbon composite powders were 1334 and 1273 mA h g 1, respectively. These values are much higher than the theoretical capacities of 890 and 716 mA h g 1 for Co3O4 and CoO, which can be attributed to the formation of solid electrolyte interphase (SEI) lms on the particles during the discharge process. The rst charge capacities of the Co3O4 and CoO–carbon composite powders were 1018 and 970 mA h g 1, respectively. An irreversible capacity loss of about 300 mA h g 1 occurred during the rst cycles due to incomplete decomposition of SEI and Li2O in addition to other factors such as the intrinsic nature of the material and kinetic limitations due to current density.11
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Fig. S4† shows the rate capabilities of the Co3O4 and CoO– carbon composite powders, acquired by step-wise increment in the current densities from 500 to 10 000 mA g 1 for successive cycles in the voltage range of 0.01–3.0 V. The Co3O4 and CoO– carbon composite powders had respectively high discharge capacities of 738 and 717 mA h g 1 even at a high current density of 10 000 mA g 1. The Co3O4 and CoO–carbon composite powders exhibited excellent rate capabilities derived from their unique structures with well-developed macropores within the micron-sized powders. Fig. 3b shows the cycling performance of the Co3O4 and CoO–carbon composite powders at a constant current density of 1400 mA g 1 in the voltage range of 0.01–3.0 V. The cycling performance of the CoO–carbon composite powders was superior to that of the Co3O4 powders. A slight increase in the discharge capacity was observed over the rst 10 cycles for both samples, primarily owing to the formation of a polymeric gel-like lm on the active material.56 The discharge capacity of the Co3O4 powders faded quickly to a relatively low value of 644 mA h g 1 aer 100 cycles, whereas the CoO–carbon composite powders retained a relatively high discharge capacity of 855 mA h g 1 aer 100 cycles. The capacity retention of the Co3O4 and CoO–carbon composite powders measured from the second cycle was 66 and 97%, respectively. Morphological changes of the Co3O4 and CoO–carbon composite powders aer 100 cycles are shown in Fig. 4. The spherical morphology of the CoO–carbon composite powders was maintained even aer cycling. In contrast, the Co3O4 powders were fragmented into several pieces aer cycling. The formation of cracks, which is one of the main origins of capacity fading, is caused by volume expansion of the active material during cycling. The Nyquist impedance plots for both samples, acquired aer the 1st and 100th cycles, are shown in Fig. 3c. The diameters of the semicircles in the medium-frequency region, which were assigned to the charge-transfer resistance (Rct), obtained aer the 1st cycle for the Co3O4 and CoO–carbon composite powders, were similar. However, the charge-transfer resistance of the Co3O4 powders was larger than that of the CoO–carbon composite powders aer 100 cycles. The chargetransfer resistance of the CoO–carbon composite powders, having a stable structure, was lower than that of the Co3O4 powders aer the 100th cycle. The carbon component acted as a structural buffer to accommodate the volume expansion and contraction of the CoO–carbon composite powders during cycling. Fig. 3d shows the rate performance of the CoO–carbon composite powders, measured by increasing the current density in increments from 500 to 5000 mA g 1 for every 5 successive cycles in the voltage range of 0.01–3.0 V. The discharge capacities of the CoO–carbon composite powders decreased slightly from 985 to 698 mA h g 1 when the current density was increased in increments from 500 to 5000 mA g 1. The superior rate and cycling performances of the CoO–carbon composite powders were ascribed to their meso- and macro-porous structures and carbon components. The electrochemical properties of the prepared CoO–carbon composite powders are compared with those of the particles with various morphologies reported in the previous literature, and the results are summarized in Table S1.† The Co3O4–C
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Fig. 3 Electrochemical properties of the Co3O4 and CoO–C composite powders: (a) initial charge and discharge curves at a constant current density of 500 mA g 1, (b) cycling performances at a constant current density of 1400 mA g 1, (c) Nyquist impedance plots acquired after the 1st and 100th cycles, and (d) rate performance of the CoO–carbon composite powders, at constant current density in increments from 500 to 5000 mA g 1 for every 5 successive cycles.
nanospheres prepared by a hydrothermal method had a discharge capacity of 567 mA h g 1 at a current density of 440 mA g 1 aer 107 cycles.30 The Co3O4–CoO–graphene nanocomposite prepared by an auto-combustion method had a discharge capacity of 801 mA h g 1 at a low current density of 21 mA g 1 aer 30 cycles.56 Graphite, the most widely used anode material for LIBs, has a relatively low theoretical gravimetric capacity (372 mA h g 1) and thus is inadequate for high power applications. Thus, the prepared CoO–carbon composite powders had superior electrochemical properties to those reported in the previous literature. The CoO–carbon composite powders with superior electrochemical properties could be applied as anode materials for LIBs.
Conclusions
Fig. 4 Morphology changes of the Co3O4 and CoO–C composite powders after 100 cycles at a constant current density of 1400 mA g 1.
composite powders synthesized via spray pyrolysis had a discharge capacity of 800 mA h g 1 at a low current density of 30 mA g 1 aer 50 cycles.29 The carbon–Co3O4 composite
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A prospective process for large-scale production of spherical, micron-sized CoO–carbon composite powders with internal meso- and macro-pores for anode applications in LIBs was proposed herein. The electrochemical properties of the CoO– carbon composite powders were superior to those of Co3O4 powders with a similar morphology prepared by the same process. The carbon component functioned to minimize the crystal growth of cobalt oxide during the post-treatment process
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and acted as a buffer layer to negate the impact of volume changes during cycling. The meso- and macro-pores also improved the rate and cycling performance of the CoO–carbon composite powders by providing a short lithium ion path by facilitating the electrolyte contact with the pores as well as providing free space for accommodating volume changes during cycling. The newly developed process has potential for large-scale applications to the production of various compositions of metal oxide–carbon composite powders for a wide range of applications including energy storage.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012R1A2A2A02046367). This work was supported by the Creative Industrial Technology Development Program (10045141) funded By the Ministry of Trade, industry & Energy (MI, Korea).
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