CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201400092

Challenges of “Going Nano”: Enhanced Electrochemical Performance of Cobalt Oxide Nanoparticles by Carbothermal Reduction and In Situ Carbon Coating Dominic Bresser,[a] Elie Paillard,*[a] Philip Niehoff,[a] Steffen Krueger,[a] Franziska Mueller,[a] Martin Winter,[a] and Stefano Passerini*[a, b] The electrochemical performance of nano- and micron-sized Co3O4 is investigated, highlighting the substantial influence of the specific surface area on the obtainable specific capacities as well as the cycling stability. In fact, Co3O4 materials with a high surface area (i.e. a small particle size) show superior specific features, which are, however, accompanied by a rapid capacity fading, owing to the increased formation of an insulating polymeric surface film that results from transition-metalcatalyzed electrolyte decomposition. The simultaneous coating with carbon of Co3O4 nanoparticles and in situ reduction of the

Co3O4 by a carbothermal route yields a CoOCoC nanocomposite. The formation of this material substantially enhances the long-term cycling stability and coulombic efficiency of the lithium-ion active material used. Although the metallic cobalt enhances the electronic conductivity within the electrode and remains electrochemically inactive (as revealed by in situ powder X-ray diffraction analysis), it might have a detrimental effect on the long-term cycling stability by catalytically inducing continuous electrolyte decomposition.

1. Introduction In 1969, Cairns and Shimotake[1] claimed the need for highenergy, compact, and environmentally friendly energy-storage devices for applications in vehicle propulsion and off-peak energy storage for central stations, which are nowadays more necessary than ever, owing to the increasing importance of renewable energies and their intrinsic fluctuating nature. However, back in 1979, Murphy and Christian[2] reported the same envisioned applications for new high-energy battery systems, focusing their research activities on ambient-temperature lithium batteries based on displacement reactions of inorganic compounds, according to Reaction (1) : þ



2 MO2 þ 2 Li þ 2 e ! M2 O3 þ Li2 O

ðM ¼ MetalÞ

ð1Þ

Nevertheless, these displacement reactions were assumed to be irreversible at room temperature, owing to the extensive bond cleavage, atomic reorganization, and formation of new bonds.[2] In agreement with the mentioned work, Thackeray et al.[3] reported, in 1985, the electrochemical insertion of lithium into the spinel Co3O4, in which they discovered that the spinel structure was preserved up to a lithium content of [a] D. Bresser, Dr. E. Paillard, P. Niehoff, S. Krueger, F. Mueller, Prof. Dr. M. Winter, Prof. Dr. S. Passerini Institute of Physical Chemistry & MEET Battery Research Centre University of Muenster, Corrensstr. 28/30 & 46 48149 Muenster (Germany) E-mail: [email protected] [email protected] [b] Prof. Dr. S. Passerini Helmholtz Institute Ulm, Karlsruhe Institute of Technology Albert Einstein Allee 11, 89081 Ulm (Germany)

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LiCo3O4, though upon further lithiation they observed structural degradation. The greatest breakthrough so far in regard to the electrochemical reaction of such metal oxides (specifically transition-metal oxides) was reported 15 years later by Tarascon and co-workers, showing that such oxides can host multiple lithium ions reversibly by undergoing a conversion reaction.[4] The general formula for this new lithium storage is shown in Reaction (2): MOx þ 2x Liþ þ 2x e $ x Li2 O þ M0 ðM ¼ Co, Ni, Cu, FeÞ

ð2Þ

Such transition-metal oxides generally offer specific capacities in the range of 700–1000 mAh g1, roughly twice that of graphite (372 mAh g1), and with a three-fold specific gravity, which results in a six-fold volumetric capacity. Since 2000, a tremendous amount of research has been reported, investigating the electrochemical reaction of these transition-metal oxides.[5–8] Initially, most of these studies focused on cobalt oxides, because these have shown the most stable cycling performance in the work published by Tarascon and co-workers.[4] Following the suggestion of Tarascon et al.,[5] researchers focused their attention on nanostructured materials or active material composites comprising electronically conductive carbon. Such efforts had the aim to overcome kinetic issues related to the substantial structural rearrangement upon (de)lithiation,[4, 9] along with the insulating issues, owing to the nature of most of the explored transition-metal oxides.[10–14] In fact, the direct utilization of nanostructured materials resulted frequently in rather rapid capacity decay and poor longterm cycling stability,[15–23] presumably related to the increasing resistance caused by the partially reversible formation of a polyChemPhysChem 0000, 00, 1 – 10

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CHEMPHYSCHEM ARTICLES meric layer on the nanosized active material particles, resulting from electrolyte decomposition.[9, 24–27] Nevertheless, several studies, including the first report on conversion materials published by Tarascon and co-workers,[4] revealed that not only the particle size (i.e. the surface area), but also the particle morphology, crystallinity, and the synthesis conditions played an important role in the electrochemical performance of different cobalt oxide particles.[28–36] To follow the approach of forming secondary, electronically conductive heterostructures by using carbon-derived materials, nanotubes,[37, 38] graphene,[39–42] hollow carbon spheres,[43] or carbonaceous coatings/carbon composites[44, 45] became a promising approach, resulting in a substantial improvement of the electrochemical performance. Carbonaceous coatings, which consist of a continuous carbonaceous layer covering the particle surface, offer particular advantages as they prevent the intimate contact between the active material and the electrolyte, thus reducing the superficial (catalytic) activity of cobalt oxide particles towards electrolyte decomposition.[45] Herein, we present a comparison of the electrochemical performance of micron- and nano-sized Co3O4 particles. Although Co3O4 nanoparticles show a higher initial specific capacity, their cycling stability is worse than that of micron-sized particles. However, application of a carbonaceous coating accompanied by the reduction of Co3O4 to CoO and metallic Co results in significant enhancement of the cycling stability and capacity retention, due to the formation of an electronically conductive, percolating network of amorphous carbon and metallic cobalt within the obtained composite. Interestingly, the size of the initial Co3O4 particles does not only influence the electrochemical performance, but also the reduction behavior upon the composite preparation. The obtained results indicate that an enhanced electrochemical performance might not be achieved by simply decreasing the particle size, but by combining the two approaches of nanostructuring and embedding these particles in secondary host matrices, which warrant the electronic contact to the current collector, buffer volume changes upon continuous (delithiation), and prevent or at least reduce the intimate contact between the nanostructured active material particles and the electrolyte.

2. Results and Discussion 2.1. Comparison of Co3O4nano with Co3O4micro Scanning electron microscopy (SEM) analysis of nano- and micron-sized Co3O4 particles revealed an almost spherical shape and a particle size of about 20–30 nm for the first (Figure 1 a), whereas the second showed a highly crystalline morphology and an average particle diameter in the range of 1– 2 mm (Figure 1 b). The substantially smaller particle (i.e. crystallite) size is also apparent from the X-ray diffraction (XRD) patterns of both samples (Figure 1 c, d). Apart from the confirmation that both samples are pure phases of spinel-structured Co3O4 (space group Fd-3m, JCPDS reference 00-043-1003), the reflections for Co3O4micro (Figure 1 d) are more intense and less broad than those for the nanosized sample (Figure 1 c), in  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 1. SEM images of a) nano-sized (100 kx) and b) micron-sized (10 kx) Co3O4 particles, as well as the corresponding XRD patterns of c) Co3O4nano and d) Co3O4micro. The JCPDS reference pattern for spinel Co3O4 (00-0431003) is shown below for both XRD patterns.

agreement with the SEM images. These results are in line with the Brunauer–Emmett–Teller (BET) specific surface area of the two pristine samples, 41.4 and 0.84 m2 g1 for Co3O4nano and Co3O4micro, respectively. Considering a density of about 6.1 g cm3 and the almost spherical shape of the nanoparticles, the average diameter can be calculated to be around 24 nm, which is also in agreement with the SEM image (Figure 1 a). For the electrochemical characterization of both materials, electrodes were prepared using conductive carbon and polyvinylidene fluoride (PVdF) as a binder. Such electrodes were subjected to galvanostatic cycling to study their long-term (de)lithiation stability (Figure 2). Based on a series of experiments applying different cut-off potentials, the most stable cycling performance was obtained by initially discharging (lithiation) the electrodes down to 0.01 V to fully activate the material and subsequently limit the cathodic cut-off potential to 0.2 V. As expected from previous results on such materials, electrodes based on Co3O4nano showed higher reversible capacities than those based on Co3O4micro. In the first cycle, a reversible capacity of 1114 mAh g1 was observed for Co3O4nano, which exceeded significantly the theoretical capacity of 890 mAh g1, whereas for Co3O4micro, a specific capacity of 877 mAh g1 was obtained. As mentioned above, this extra capacity is considered to result from the partially reversible formation of a polymeric layer on the active material particles, resulting from electrolyte decomposition at potentials below 0.8 V.[24, 27] In fact, downsizing the active material particles led to an increasing amount of (catalytically) active sites at the particles surface (i.e. generally a larger surface area), resulting in an increased electrolyte decomposition and solid electrolyte interphase (SEI) formation;[46, 47] such an increase is also reflected in the reduced first-cycle coulombic efficiency of Co3O4nano (66.2 %) compared with Co3O4micro (77.2 %). More severely, the SEI formation is only partially reversible, as indicated by the significantly lower coulombic efficiency of Co3O4nano upon subsequent galvanostatic ChemPhysChem 0000, 00, 1 – 10

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Figure 2. Electrodes based on a) Co3O4nano and b) Co3O4micro subjected to galvanostatic cycling. The corresponding potential profiles for selected cycles (2, 10, 20) for c) Co3O4nano and d) Co3O4micro. e) Comparison of the potential profiles for the first cycle, Co3O4micro in black, and Co3O4nano in grey, dashed line.

cycling, which reached about 95 % at its maximum, whereas Co3O4micro-based electrodes showed coulombic efficiencies of up to 99.2 %. In line with the inferior coulombic efficiency for Co3O4nano, the capacity faded rather rapidly upon continuous cycling (Figure 2 a), whereas the capacity for Co3O4micro was stable at about 800 mAh g1 for 25 cycles before it started to decrease (Figure 2 b). Looking at the potential profiles for selected cycles, it is obvious that the internal resistance (IR) within the cell increased steadily for Co3O4nano, leading to an increasing IR drop (indicated by the arrow in Figure 2 c), which likely contributed to the observed capacity fading. These observations are in accordance with a study of Vidal-Abarca et al.,[19] who performed impedance spectroscopy to measure the inner resistance of several samples to explain the capacity fading of nanosized Co3O4 samples prepared by a reverse-micelle procedure. It should be kept in mind that the increasing IR drop as well as the capacity fading might also originate (partially) from the lithium metal counter electrode, which is also affected by the dissolved electrolyte decomposition products.[48] For micron-sized Co3O4, such an increase in IR drop was not observed for the same cycles (Figure 2 d), indicating that the lower surface area resulted in a decreased formation of insulating electrolyte decomposition layer. Interestingly, a direct comparison of the potential profiles for the first cycle revealed another major difference between Co3O4nano and Co3O4micro : a short initial plateau-like potential feature for Co3O4nano prior to the main potential plateau, which was absent for Co3O4micro (marked by the grey dashed arrow in Figure 2 e). According to Larcher et al.,[49] this additional plateaulike feature is frequently observed for nanosized particles, that  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

is, the application of a rather low current density per unit surface area. Furthermore, they have shown that the electrochemical reaction pathway of Co3O4 and lithium is dependent on exactly these parameters, following principally two competing reaction mechanisms, including two different intermediately formed products, which indicates that the formation of reaction intermediates can be kinetically controlled [Reactions (3)– (5) for Co3O4nano and Reactions (6)–(8) for Co3O4micro]: Co3O4nano : Co3 O4 þ 2 Liþ þ 2 e ! 3 CoO þ Li2 O

ð3Þ

3 CoO þ 6 Liþ þ 6 e ! 3 Co0 þ 3 Li2 O

ð4Þ

in total :

Co3 O4 þ 8 Liþ þ 8 e ! 3 Co0 þ 4 Li2 O

ð5Þ

Co3O4micro : Co3 O4 þ x Liþ þ x e ! Lix Co3 O4 ðx  2Þ

ð6Þ

Lix Co3 O4 þ ð8xÞ Liþ þ ð8xÞ e ! 3 Co0 þ 4 Li2 O

ð7Þ

in total :

Co3 O4 þ 8 Liþ þ 8 e ! 3 Co0 þ 4 Li2 O

ð8Þ

However, both materials were unable to provide sufficient long-term cycling stability. In particular, Co3O4micro also showed a substantial capacity fading, most likely associated with the severe SEI formation (see Figure 3). For such reasons, we followed the approach of embedding the cobalt oxide nanoparticles in secondary, electronically conductive matrices by applyChemPhysChem 0000, 00, 1 – 10

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Figure 3. Ex situ SEM images of a Co3O4micro-based electrode at different magnifications: a) 50 kx and b) 100 kx after being subjected to a full (dis)charge cycle (discharge to 0.01 V and subsequent charge to 3.0 V).

ing a carbonaceous coating on the nanoparticles, which has already proven to be effective for ZnFe2O4 nanoparticles.[50, 51] 2.2. Preparation and Characterization of the CoOCoC Composite Carbon coating the cobalt oxide nanoparticles was performed by using sucrose as a carbon source, according to a previously described method.[50, 51] Co3O4nano was chosen as precursor because of the general advantages of nanostructured active materials such as, for instance, reduced Li + and e transport pathways, a decreased current density per unit surface area, and an improved strain accommodation upon (de)lithiation.[52–54] More importantly, however, was the idea of proving the concept that a coating layer homogenously covering the particle surface could prevent (or at least substantially reduce) the superficial (catalytic) activity of the cobalt nanograins with respect to the electrolyte decomposition. This effect would presumably be more evident by investigating nanosized rather than micron-sized particles, for which this phenomenon is less severe. At first glance, SEM analysis of the obtained material showed no obvious changes on particle morphology (Figure 4). The particle size and shape are basically preserved despite the additional thermal treatment at 500 8C for 4 h under argon. The BET surface area is slightly increased to  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 4. SEM images of cobalt oxide nanoparticles after applying a carbon coating by using sucrose as precursor at different magnifications: a) 100 kx and b) 200 kx.

about 52.3 m2 g1, which is supposedly related to the presence of remaining carbon within the composite.[55, 56] A major change, however, occurs on the carbon-coated material as evidenced by XRD analysis (Figure 5), which revealed that the initial spinel-structured Co3O4 particles are reduced to rock salt-structured CoO and, to a minor extent, to metallic cobalt having a cubic or hexagonal structure. The formation of the cubic, rock-salt-structured CoO phase (space group Fm-3m)

Figure 5. XRD pattern of nano-sized cobalt oxide after applying a carbonaceous coating. The JCPDS reference for rock salt-structured CoO (00-0431004) is shown at the bottom of the spectrum. Reflections corresponding to cubic and hexagonal metallic cobalt, JCPDS 00-015-0806 for cubic (*), and JCPDS 01-071-4239 for hexagonal Co0 (+).

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rather than a mixture of the cubic and the hexagonal CoO is, indeed, thermodynamically expected.[57–60] However, the presence of both metallic phases is a little surprising, because the hexagonal close-packed Co phase (space group Fm-3m) is considered to be, in principle, thermodynamically more stable than the face-centered cubic Co phase (space group: P63/mmc), although mixed phases were already reported.[61, 62] Nevertheless, the stability of different Figure 7. XPS of Co 2p, O 1s, and C 1s for CoOCoC nanocomposite. transition-metal and transitionmetal oxide phases is highly dependent on the particle size.[63–65] Hence, to fully clarify the middle panel). Finally, the rather small peak present on C 1s presence of the different phases and to vary their amount can be attributed to the carbon-coating layer on the particle’s within the obtained composite, a series of experiments, varysurface. Based on a series of three measurements on different ing, for instance, the applied temperature or the duration of samples, a mean coating thickness of about 0.9  0.3 nm was the thermal treatment, might shed further light on this issue. calculated using the XPS Multiquant software, indicating that However, this is certainly beyond the scope of the present the carbonaceous coating is extremely thin. study. Basically, it can be concluded that nanoparticulate Co3O4 For the electrochemical characterization, electrodes of CoOwas reduced to cubic CoO and metallic cobalt upon thermal Co-C nanocomposite were prepared analogously to those treatment using sucrose as carbon source for the in situ creabased on Co3O4nano and Co3O4micro. As shown in Figure 8 a, the tion of a carbonaceous layer on the nanoparticles surface. cycling stability is substantially improved and more than 80 The carbonaceous coating was further characterized by (dis)charge cycles were obtained. The first-cycle coulombic effiRaman spectroscopy (Figure 6), which revealed the characterisciency was significantly increased to 74.2 %, compared to tic D (1350 cm1) and G (1585 cm1) bands, attributable to the induced disorder along the c-axis of graphitic carbons, and the

Figure 6. Raman spectrum of CoOCoC nanocomposite.

graphitic carbon-carbon stretching along the graphene layer plane, respectively.[66, 67] Accordingly, the comprised carbon can be considered as partially graphitic but essentially amorphous. For an estimation of the average coating layer thickness, not readily accessible using SEM, X-ray photoelectron spectroscopy (XPS) was performed (Figure 7). On the Co 2p spectrum (Figure 7, left panel), a doublet accompanied by two satellite peaks is observed, and was assigned to CoO,[26, 68] along with the single peak observed in the O 1s spectrum (Figure 7,  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 8. a) CoOCoC-based electrode subjected to galvanostatic cycling, and b) corresponding potential profiles for selected cycles.

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CHEMPHYSCHEM ARTICLES 66.2 % for Co3O4nano, almost reaching the first-cycle reversibility of Co3O4micro (77.2 %). This advanced electrochemical performance is seemingly related to the presence of a carbonaceous coating, improving the electronic conductivity within the composite and preventing particle agglomeration, surface cracking, particle breakage, as well as primary particle coalescence as reported by Jayaprakash et al.[44] for submicron-sized Co3O4. We propose that the presence of metallic cobalt contributes to the enhanced electronic conductivity within the composite, which is particularly important considering the insulating nature of Li2O formed during lithiation. Moreover, it facilitates the reversible formation of the latter, leading to improved capacity retention upon cycling. Huang et al.[69] have reported recently a beneficial effect of metallic nickel in a NiO–Ni nanocomposite, on both, reversible capacity and cycling stability. The reversible capacity of 800 mAh g1 obtained for the first recharge still exceeds the theoretical value of 715 mAh g1. This is remarkable since the specific capacity for the material is calculated based on the total weight of the CoOCoC nanocomposite. Subsequently, the capacity dropped to about 700 mAh g1, when the C-rate as well as the cathodic cut-off potential were increased to C/20 (around 45 mA g1) and 0.2 V, respectively. Then, the capacity reached a maximum value of about 810 mAh g1 before it declined again to about 700 mAh g1 at the 80th cycle. The increasing capacity detected in the initial 40 cycles is presumably again related to the partially reversible formation of the polymeric layer originating from electrolyte decomposition. However, the coulombic efficiency in this cell is enhanced to about 99 % (ca. 95 % for Co3O4nano). Additionally, no significant increase in polarization was observed from the potential profiles recorded at the 2nd, 10th, and 20th cycles (Figure 8 b). However, for higher cycle numbers a slight capacity fading was observed, likely associated with the increasing internal resistance (indicated by the black arrow in Figure 8 b), which strongly suggests the carbon coating is unable to grant an everlasting protection of the nanoparticles. Inhomogeneity in the carbon-coating thickness might, for example, result in weakly shielded areas. Considering the initial carbon precursor distribution as rather homogenous, it is proposed that the coating layer is particularly thin on metallic cobalt nanoparticles, because the reduction of Co3O4 to metallic cobalt requires a larger amount of carbon than the reduction to CoO. In this case, the formation of the polymeric layer should basically take place at the metallic cobalt surface, as reported already for iron[70] and nickel,[71] resulting in an increasing specific capacity. This would occur without a detrimental effect on the (de)lithiation of the CoO particles, at least upon the initial cycles. However, once the amount of formed polymeric layer exceeds a certain volume, it might have covered also the electrochemically active CoO nanoparticles, which hinders their proper (de)lithiation by blocking the lithium-ion transport. Finally, in order to confirm that the comprised metallic cobalt remained electrochemical inactive, in situ XRD analysis coupled with cyclic voltammetry was performed on the CoO CoC nanocomposite. The voltammogram of the first cycle  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org (Figure 9 a) shows the characteristic reduction and oxidation peaks.[25] The major cathodic peak, occurring at about 0.5 V, is related to the conversion reaction (CoO + 2 Li + + 2 e ! Co0 + Li2O), whereas the two features at higher potentials— a peak at about 0.9 V with a shoulder at about 0.8 V—are presumably related to electrolyte decomposition processes. In fact, the SEI formation on carbonaceous materials commonly takes place at about 0.8 V.[72, 73] It is important to note that the cobalt oxide reduction, as apparent from its XRD analysis, already starts at potentials of about 1.4 V, that is, at the 14th scan (scan rate: 0.05 mV sec1, duration of each scan: 30 min; Figure 9 b). This is the onset potential of the evolving cathodic current, indicating that the conversion reaction and the electrolyte decomposition process (at least on the carbonaceous surface) are proceeding simultaneously. In fact, the formation of the polymeric layer on lithiated transition-metal oxides is considered to occur at lower potentials, after the conversion reaction.[24, 27] Nevertheless, once the reflections attributed to CoO had vanished, no new reflections were detected upon further lithiation of the composite, indicating the amorphous nature of the formed cobalt nanograins in a surrounding Li2O matrix.[4, 9, 74] Despite the clear redox peaks in the cyclic voltammograms (Figure 9 a, c), no new reflections were observed in the subsequent anodic and the second cyclic sweep (Figure 9 c, d). Thus, the once reduced transition-metal oxide maintained its amorphous nature also upon re-oxidation.[4, 26, 75, 76] The only stable reflection withstanding both discharge and charge processes was the most intense one, assigned to both metallic cobalt phases (marked by an asterisk in Figure 9 b, d). This confirmed that the comprised metallic cobalt is not affected by—and does not affect—the reversible (de)lithiation process.

3. Conclusions Transition-metal oxides or, more generally, conversion materials such as cobalt oxide, are scientifically interesting materials with respect to their unique electrochemical reaction with lithium. However, for practical applications, usually requiring from hundreds to thousands of continuous charge/discharge cycles, these materials still suffer several issues. One particular challenge is the high (catalytic) activity towards commonly utilized organic carbonate-based electrolytes, leading to the partially reversible formation of insulating electrolyte decomposition layers on the active material particles, which hinders continuous particle (de)lithiation, resulting in a rather rapid capacity fading. Accordingly, the use of (preferably) electronically conductive materials such as carbon for particle surface passivation results in a substantially improved electrochemical performance of an in situ carbon-coated and carbothermally reduced CoOCoC nanocomposite, compared to Co3O4 nanoparticles. The obtained specific capacity is mainly derived from the comprised CoO, but the metallic cobalt remains electrochemically inactive within the composite, presumably warranting an enhanced electronically conductive, percolating network. Such effect seems to be possible by the combination of carbon coating, metallic cobalt, and upon the electrode prepaChemPhysChem 0000, 00, 1 – 10

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Figure 9. In situ XRD analysis of the CoOCoC nanocomposite. a) Cyclic voltammogram of the first cyclic sweep and b) corresponding XRD patterns. c) Cyclic voltammogram of the second cyclic sweep and d) corresponding XRD patterns. The main reflection for metallic cobalt (indicative for hexagonal and cubic Co0) is marked by an asterisk in (b) and (d).

ration added conductive agent. However, to complete from hundreds to thousands of cycles, it has to be ensured that also the metallic cobalt remains coated by a “passivating” surface layer as, for instance, carbon.

Experimental Section Preparation of the Cobalt Oxide–Carbon Composite Sucrose (0.15 g ,  99.5 %, Sigma-Aldrich) was dissolved in deionized water (2 mL, Millipore) by magnetic stirring. Subsequently, Co3O4 (0.85 g, Sigma-Aldrich) was added and the mixture was homogenized by using a planetary ball-milling set (Vario-Planetary Mill Pulverisette 4, Fritsch) at 800 rpm for 1.5 h. The obtained mixture was dried at 60–80 8C under air, followed by thermal treatment at 500 8C for 4 h under argon atmosphere.

Morphological Characterization Analysis of the starting materials and the prepared composite was performed through XRD analysis by using a Bruker D8 Advance (CuKa radiation, l = 0.154 nm), high-resolution SEM carried out on a ZEISS Auriga microscope, and BET surface area analysis using an ASAP 2020 (accelerated surface area and porosimetry analyzer) apparatus (Micromeritics). The density was determined by means of an AccuPyc II 1340 Gas Pycnometer (Micromeritics), by using helium as the working gas. The amount of residual carbon within the composite was determined by elemental analysis (Elementar Vario EL III), resulting in 1.5 wt % of the CoOCoC nanocomposite. The carbon was further  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

characterized by using Raman spectroscopy utilizing a Senterra Raman spectrometer (Bruker Optics) equipped with a 532 nm laser of a 2 mW output power, and XPS carried out on an Axis Ultra DLD (Kratos) by using a monochromatic AlKa source (hu = 1486.6 eV; 12 mA filament current, 15 kV filament voltage energy source). The measurement was performed at a 0 8C emission angle in an electrostatic-lens mode with a pass energy of 20 eV. The analyzed area was about 1  2 mm. For the subsequent fitting of the obtained spectra, the software CasaXPS was used (fitting function: GL(30); Shirly background type). The binding energy (BE) was calibrated by utilizing the energy of the C 1s peak (BE = 285 eV) as an internal reference. The carbon-coating layer thickness on the cobalt oxide particles was calculated based on the area beyond the Co 2p, O 1s, and C 1s peaks, using the software Multiquant and postulating a spherical particle shape. The average particle diameter of the starting material (Co3O4nano) was determined by the following equation: 2*(3/BET surface area/density)*1000, hereby also assuming spherical particle morphology. For the cross sections, Scofield factors were used. The inelastic mean free paths were calculated using the Cumpson–Seah equation.

Electrode Preparation The active materials (Co3O4nano, Co3O4micro, CoO-Co-C) were mixed with Super C45 conductive carbon (Timcal) and PVDF-HFP copolymer (Kynarflex 2801, Arkema), dissolved in N-methylpyrrolidone (Sigma-Aldrich). The weight ratio of the three components was 80:15:5 (active material/conductive carbon/binder). The obtained mixture was homogenized using the ball mill mentioned above, at 800 rpm for 1 h. The resulting slurry was then cast on dendritic copper foil (Schlenk) with a wet-film thickness of 120 mm, using ChemPhysChem 0000, 00, 1 – 10

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CHEMPHYSCHEM ARTICLES a laboratory doctor blade. The electrodes were dried in air for 2 h at 80 8C first, then for 12 h at room temperature. Disk electrodes (Ø = 12 mm) were punched and dried for 12 h at 120 8C under vacuum. The active material mass loading was comprised between 1.5 and 1.6 mg cm2 (Co3O4nano), 2.3 to 2.5 mg cm2 (Co3O4micro), and 2.2 to 2.4 mg cm2 (CoO-Co-Cnano). A C rate of 1C corresponds to an applied specific current of 890 mA g1 (theoretical capacity of Co3O4). For comparability reasons the same definition of 1C was used for all electrochemically investigated samples.

Electrochemical Characterization Electrochemical studies were carried out in three-electrode Swagelok-type cells, utilizing lithium metal foils (Rockwook Lithium, lithium battery grade) as counter and reference electrodes. Cells were assembled in an MBraun glove box with oxygen and water contents below d = 0.1 ppm. A stack of polypropylene fleeces (Freundenberg, FS2190) drenched with a 1 m solution of LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (EC/DEC; 3:7 by volume, UBE), serving as the electrolyte, was used as the separator. As lithium foil was used as counter and reference electrode, all potential values given in this manuscript refer to the Li/Li + reference couple. All electrochemical studies were performed at 20  2 8C. For galvanostatic cycling and cyclic voltammetry, a Maccor Battery Tester 4300 and VMP3 potentiostat (Biologic) were used, respectively. For the ex situ SEM investigation of a Co3O4micro-based electrode, the cell was discharged and charged only once (0.01 to 3.0 V) and then disassembled in a glove box under argon atmosphere. The cobalt oxide electrode was briefly rinsed with dimethyl carbonate (DMC) and then mounted on a self-designed sample holder for the SEM, which enabled the investigation of cycled electrodes without any contact to air and moisture.

In situ XRD Analysis In situ XRD analysis of the CoOCoCnano composite was performed by potentiodynamic (de)lithiation of the composite accompanied by simultaneous XRD scans using a self-designed in situ cell.[50] The stainless-steel cell body was covered internally by a sheet of Mylar foil for electrical insulation. The working electrode was prepared by dissolving carboxymethyl cellulose (CMC; 0.01 g) in deionized H2O (0.7 mL). Subsequently, CoOCoCnano (0.065 g) and conductive carbon (0.025 g, Super C65) were added. The obtained mixture was homogenized by ball milling for 120 min and then cast on a beryllium disc (thickness: 250 mm, Brush Wellman), which served as the current collector and a window for the X-ray beam. The coated Be disc was dried at 80 8C for 1 h in the first step before it was dried at 60 8C under vacuum overnight. In a previous study, it was shown that CMC itself does not show any reflection within the investigated 2 q range. Accordingly, all observed reflections could be assigned to the CoOCoCnano composite or the Be window. Metallic lithium foil was used as counter and reference electrode. Two sheets of Whatman glass fiber, serving as the separator, were drenched with 500 mL of 1 m LiPF6 in EC/DEC (3:7 by volume). Subsequently, the cell was allowed to rest for 3 h prior to the experiment. Potentiodynamic cycling was then performed by means of a Solartron 1287 potentiostat. The scan rate and reversing potentials were set to 0.05 mV s1 and 0.01–3.0 V, respectively. Simultaneously, XRD analysis was carried out, investigating a 2 q range of 15–908, with a step size of 0.02898 and a time per step of 0.5 s. Accordingly, a complete scan lasted 30 min, including a rest period of 440 s at the beginning of each scan.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemphyschem.org Acknowledgements Financial support from the European Commission in the ORION project (229036) under the Seventh Framework Programme (7th FWP) is gratefully acknowledged. Furthermore, the authors would like to thank Dr. Ren Schmitz for performing Raman spectroscopy. Keywords: carbon · coatings · cobalt oxide · lithium-ion batteries · nanoparticles

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ARTICLES D. Bresser, E. Paillard,* P. Niehoff, S. Krueger, F. Mueller, M. Winter, S. Passerini* && – && Challenges of “Going Nano”: Enhanced Electrochemical Performance of Cobalt Oxide Nanoparticles by Carbothermal Reduction and In Situ Carbon Coating

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Not just a carbon layer! Carbon coating of an active electrode material based on cobalt oxide reduces the impact of electrolyte decomposition and enhances the electronic conductivity within the electrode material composite, which results in a substantial improvement of the cycling stability and coulombic efficiency. This aspect is particularly important for high-surface-area, transition-metal-oxide nanomaterials, herein proposed as advantageous alternative lithium-ion anodes.

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Challenges of "going nano": enhanced electrochemical performance of cobalt oxide nanoparticles by carbothermal reduction and in situ carbon coating.

The electrochemical performance of nano- and micron-sized Co(3)O(4) is investigated, highlighting the substantial influence of the specific surface ar...
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