ISSN 19950780, Nanotechnologies in Russia, 2011, Vol. 6, Nos. 11–12, pp. 705–710. © Pleiades Publishing, Ltd., 2011. Original Russian Text © S.N. Beznosov, M.G. Pyatibratov, O.V. Fedorov, T.L. Kulova, A.M. Skundin, 2011, published in Rossiiskie Nanotekhnologii, 2011, Vol. 6, Nos. 11–12.
ARTICLES
Electrochemical Properties of Nanostructured Material Based on Modified Flagella of Halophilic Archaea Halobacterium salinarum for Negative Electrode of LithiumIon Battery S. N. Beznosova, M. G. Pyatibratova, O. V. Fedorova, T. L. Kulovab, and A. M. Skundinb a
Institute of Protein, Russian Academy of Sciences, ul. Institutskaya 4, Pushchino, 142290 Russia b Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 117071 Russia email:
[email protected] Received February 18, 2011; in final form, August 11, 2011
Abstract—Modified flagella of halophilic archaea Halobacterium salinarum mineralized by cobalt oxide and applied to a conductive nickel grid support have been tested as a material for the negative electrode of a lith iumion battery. It is found that the reversible capacity of such nanostructured samples exceeded 400 mAh/g and their stability in cycling increased significantly when flagella were fragmented by sonication prior to min eralization with cobalt oxide. DOI: 10.1134/S1995078011060061
INTRODUCTION The development of modern lithiumion batteries is related to the development of new electrode materi als, including nanocrystalline and nanostructured materials [1]. A new (though still not comprehensive) and intensively developing trend for obtaining such materials is the application of biological polymers for the directed assembly of inorganic components to larger structures [2]. This approach was used by employees of the laboratory of A. Belcher (Massachu setts Institute of Technology, United States) for the manufacture of functional cobalt oxide nanowires on the basis of a filamentous M13 bacteriophage [3, 4]. A genetically modified bacteriophage was obtained. Its protein subunits contained an additional peptide loop of four glutaminic acid residues that were capable of binding positively charged ions. The insertion site was chosen so that the inserted loop did not affect the bac teriophage assembly and was exposed on its surface. The thus modified virus was incubated in an aqueous cobalt chloride solution, and nanowires of cobalt oxide (Co3O4) were obtained as a result of interaction with NaBH4 and the further spontaneous oxidation. Such nanowires were used as a basis for experimental samples of the anode [3] and cathode [4] for lithium ion batteries that featured high characteristics. According to the authors of these works, the improve ment of such materials may result in the commission
ing of the serial production of lithiumion batteries in the immediate future [5]. Cobalt oxides are under consideration as a material for the negative electrode of lithiumion batteries as an alternative to graphite and having a higher lithium intercalation capacity [6–17]. While the standard capacity of a graphite anode of commercial batteries is within 250–300 mA h/g at the theoretical limit of 372 mA h/g, the theoret ical limit of the capacity of a cobalt oxide anode is 890 mA h/g. At the same time, it is known that the degradation of such electrodes under cycling is too high and that it can be decreased upon a transition to nanosized preparations of cobalt oxides [9, 18]. Viruses and other biological objects can stabilize nanosize particles (prevent their aggregation) [3, 4]. Biopolymers of extremophilic microorganisms preserving structural integrity under a wide range of external conditions can be of special interest for nano biotechnology. In particular, such objects may include the flagella of galophilic archaea, which are elongated multisubunit protein polymers with lengths of several micrometers and thicknesses of about 10 nm. Earlier, we showed the possibility in principle of using the fla gella of extremophilic archaea as nanomatrices capa ble of binding the earlier provided substances [19]. A genemodified H. salinarum strain was obtained. It synthesized flagella capable of binding metal ions (Co2+) because the flagella contained a group of four
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Characteristics of the synthesized materials Deno tation
Material
Active substance mass on the elec trode, mg
A
Co3O4
7.4
B
Wild flagella mineralized by cobalt oxide
7.3
C
Modified flagella mineralized by cobalt oxide
12.5
D
Modified flagella fragmented by sonication and mineralized by cobalt oxide
10.0
negatively charged amino acids on their surface. This made it possible to obtain a new nanostructured mate rial representing particles of nanosized cobalt oxide (Co3O4) attached to genetically modified flagella of galophilic archaea H. salinarum. In this work a similar nanostructured material was used to develop the negative electrode of a lithiumion battery. The effect of flagella modification by a metal binding peptide on the electrochemical capacity of electrodes was studied. Apart from that, the effect of flagella fragmentation through sonication up to the mineralization stage on the electrochemical charac teristics of the material was studied. EXPERIMENTAL Materials and Methods Used Flagella of H. salinarum. Wild flagella (without modifications) of galophilic archaea H. salinarum and genemodified flagella were used. Builtin (addi tional) peptides of four negatively charged asparagic acid residues were exposed on the surface. Flagella were obtained, isolated in preparative amounts, and purified according to [19]. Fragmentation of Flagella by Sonication. The iso lated and purified flagella were subjected to sonication at a frequency of 44 KHz using an UZDN disperser at the active power output rating of the device generator in a working frequency range of 400 W. The treated sample was placed into a 1.5ml centrifuge tube (Eppendorf) which, in its turn, was placed into a spe cial addon for the treatment of samples in tubes. The tube with the sample and the addon were placed into a cooling glass filled with ice water. Sonication contin ued for 40 min in periods of 1 min with pauses of 1 min to cool the treated sample. Experiments in binding cobalt ions. Experiments were carried out according to the modified technique described in [3]. The flagella at a concentration of 0.1 g/l were conditioned for 1 h in the presence of
10 mM CoCl2 in a 1.5 M NaCl solution, to which a NaBH4 solution was further added up to the concen tration of 25 mM, and the whole thing was incubated for 1 h until the reaction ended. The insoluble black fraction was deposited by centrifugation at 8000 g for 10 min, rinsed with water, and dried at room tempera ture. The calculated ratio of flagella mass to that of cobalt oxide in the obtained substance was approxi mately 1 to 10. Further, the obtained substance under went electrochemical tests. Electron microscopy. A JEM100c microscope (JEOL, Japan) was used to obtain electron micro graphs. To obtain electron microscopy samples, prep arations were applied onto electron microscopy cop per grid with a formvar support, conditioned for 1 min, and treated by filter paper to remove the solution, after which the grids were placed onto a 2% uranyl acetate solution, conditioned for 30 s, treated by filter paper to remove the solution, and dried. Electrode manufacturing. The electrodes were man ufactured according to a standard paste technique including the active material preparation, its applica tion on a conducting support, initial drying in a drying chamber, pressing, and final vacuum drying using a forvacuum pump. A mixture of the active substance (75 wt %) and acetylene carbon black (15 wt %) was thoroughly ground in an agate mortar to manufacture the active material. Then a binder was added to this mixture (polyvinylidene fluoride, 10 wt %, dissolved in advance in Nmethylpyrrolidone). The resulting liq uid mixture was treated at an ultrasonic disperser for 30 s. The conducting support for the electrodes was cut out of a nickel grid with a thickness of 50 µm. A nickel foil current lead of similar thickness was welded to the support using contact welding. The active material was applied on one side of the support using a scalpel. Fur ther, the electrodes were dried in a drying chamber at the temperature of 80°C until constant weight. After drying, the electrodes were pressed at the force of 1000 kg/cm2 for 15 s. The final electrode vacuum dry ing was carried out using a forvacuum pump at the temperature of 120°C for 8 h. The active substance mass on the electrodes was 7–10 mg/cm2. Cell assembly and electrochemical tests. The cells were assembled in a box with a dry argon atmosphere. Sealed cells contained a working (studied) electrode, an auxiliary electrode, and a reference electrode. The auxiliary electrode and reference electrode were man ufactured using lithium rolling on a nickel grid sup port. The electrodes were separated using a 25µm thick polypropylene separator. The electrolyte used was 1 M LiPF6 in an ethylene carbonate–diethyl car bonate–dimethyl carbonate mixture (1 : 1 : 1). The water content in electrolytes did not exceed 50 ppm. The studied electrodes (electrode tests at repeated and generally continuous charge–discharge cycles) were cycled in a galvanostatic mode at a current of 20 mA/g
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of the active substance (Co3O4). In this experiment, pauses between the cycles never exceeded 20 s and each cycle corresponded to electrode charging and discharging. The cycling limits were 3.0–0.01 V.
E, V (Li/Li+)
RESULTS AND DISCUSSION Description and denotations of the synthesized materials are given in the table. Figure 1 shows the first cycle of charge–discharge curves for samples A, B, C, D. All these curves have a characteristic shape generally coinciding with the curve shape described in the literature [6, 9, 10, 12–16]. A pla teau is observed in the cathodic branch of the first cycle at a potential of about 1 V from 500 to 800 mA h/g (as per cobalt oxide mass). After this plateau in the cathodic curve branch, a smooth potential drop to 0 V is registered. This section corresponds to a charge capacity of 600–800 mA h/g. Thus, the overall cathodic charge in the first cycle is 1200 mA h/g (for sample C) to 1550 mA h/g (for sample B). The nature of the cathodic process in the first cycle is still being discussed. Most authors believe that the first cathodic process on nanostructured Co3O4 sam ples consists of oxide reduction to metallic cobalt (in the form of nanosize formations) within a lithium oxide matrix: Co3O4 + 8Li+ + 8e– 3Co + 4Li2O. (1) A more detailed analysis [14] shows that process (1) can occur via two parallel (or alternative) routes with different intermediates. Under charging at a suffi ciently high current density (as per the true surface area of cobalt oxide), the LixCo3O4 intercalation inter mediate is formed (which is later decomposed to Co and Li2O): Co3O4 + xLi+ + xe LixCo3O4 (2a)
2
D
3 B
LixCo3O4 + (8 – x)Li+ + (8 – x)e (2b) 3Co + 4Li2O. CoO is formed as an intermediate in the course of charging at a low current density: 3CoO + Li2O (3a) Co3O4 + 2Li+ + 2e + 3Co + 4Li2O. (3b) 3CoO + Li2O + 6Li + 6e The theoretical specific capacity corresponding to process (1) is 890 mA h/g. Therefore, a high percent age of electricity in the first cathodic semicycle is con sumed in some parallel processes, most likely in the irreversible reduction of components of electrolyte, including the formation of a passive film. This conclu sion is also confirmed by a comparison of the electric ity amount in the cathodic and further anodic semicy cles. The electricity amount in the first anodic semicy cle in the range of potentials from 0 to 3 V for samples A, B, C, and D was 486, 523, 637, and 584 mA h/g, accordingly. Also, the mechanism of the first anodic process cannot be considered wholly elucidated. In the opin NANOTECHNOLOGIES IN RUSSIA
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C A
1
0 −1500
−1000
−500 Q, mA h/g
0
500
Fig. 1. First cycles of charge–discharge curves of the elec trodes based on samples A, B, C, and D. The electrolyte was 1 M LiPF6 in the EC–DEC–DMC mixture. The current density was 20 mA/g.
ion of most researchers, the first anodic process was the process opposite that of Eq. (1): Co3O4 + 8Li+ + 8e–. (4) 3Co + 4Li2O In this case the reversible process in the further cycles is described by the equation of Co3O4 + 8Li+ + 8e– 3Co + 4Li2O. (5) Some authors (see, e.g., [6, 17]) believe, however, that CoO instead of Co3O4, is formed in the first anodic process: Co + Li2O CoO + 2Li+ + 2e (6) and the following reversible process occurs under fur ther cycling CoO + 2Li Co + Li2O. (7) The theoretical capacity corresponding to process (7) is 715 mA h/g. It is characteristic that the occurrence of electrode processes on cobalt oxides via a particular mechanism correlates in no way with the shape of discharge and charging curves. In many papers (e.g., [10–13]), the first cycle of the anodic curves or, sometimes, several initial cycles are shown in which a more or less extensive plateau is observed at a potential of about 2 V. There is no such plateau in the curves obtained in this work, like in [6, 7]. The cause for such discrepancies remains unclear. The difference between the electricity amount in the cathodic and anodic semicycles is irreversible capacity. The ratio of the electricity amount in the anodic semicycle to the electricity amount in the cathodic semicycle is denoted as cyclic efficiency. In the series A, B, C, D, this value was 0.37, 0.30, 0.43, 0.5, accordingly. A certain increase in cyclic efficiency may point to a decrease in the percentage of irrevers ible processes related to the reduction of electrolyte. Figure 2 shows the second cycle of the charge–dis charge curves for samples A, B, C, and D. The shape of
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D BA
1.5 1.0 0.5 0 −600
−400
−200
0 Q, mA h/g
200
400
600
Fig. 2. Second cycles of charge–discharge curves of the electrodes based on samples A, B, C, and D. The electrolyte was 1 M LiPF6 in the EC–DEC–DMC mixture. The current density was 20 mA/g.
the charge–discharge curves for all samples was prac tically the same, which points to a similar mechanism of reversible lithium intercalation into various synthe sized cobalt oxide samples. The cycling efficiency in the second cycle increases to 0.86–0.88 for all the studied samples. A gradual decrease in the discharge capacity occurred under further cycling. Thus, e.g., the dis charge capacity of a cobalt oxide electrode (sample A) in cycle 20 did not exceed 200 mA h/g (Fig. 3). Elec trodes of wild flagella mineralized by cobalt oxide showed practically the same characteristics as elec trodes of pure cobalt oxide (sample B). The replace ment of wild flagella by modified flagella resulted in a significant improvement of the electrochemical char acteristics of the electrode (sample C). Thus, the reversible capacity in the first cycle increased to 637 mA h/g and the degradation rate decreased con
siderably. The sonication of modified flagella used for the manufacturing of Dtype electrodes resulted in a certain decrease in reversible capacity in the first cycle. However, there was practically no degradation starting from the third cycle. The absolute values of discharge capacity obtained for all the studied samples generally agree with most of
B
Q, mA h/g 600 600 D C
C
400
A 200
0
B
5
10
15
Cycle number Fig. 3. Change in the discharge capacity of the electrodes on the basis of samples A, B, C, and D under cycling. The electrolyte was 1 M LiPF6 in the EC–DEC–DMC mix ture. The current density was 20 mA/g.
D Fig. 4. Microphotographs of samples B, C, and D. The scale is 300 nm.
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the literature data. However, the stability of sample D under cycling proved to be much better. The obtained characteristics of sample D are close to the values obtained for a similar anode material based on the modified M13 virus (about 600 mA h/g) [3]. A decrease in the reversible capacity under cycling may be related to several reasons, including aggrega tion, cracking, and the coloration of cobalt oxide par ticles under cycling [20, 21]. The smaller the synthe sized cobalt oxide particles are and the more uniform their distribution in any matrix is, the lower their deg radation under cycling must be. Figure 4 shows micro photographs of the initial samples B, C, and D. Wild flagella (sample B) bind cobalt oxide particles weakly, and microphographs show mostly flagella in the form of aggregates, with certain inclusions of elec trondense particles, while the bulk of cobalt oxide is in the free state in the form of larger particles. The size of cobalt oxide particles was about 50–100 nm. In the case of sample C, the bonding between flagella and cobalt oxide particles is stronger. The size of the latter is also 50–100 nm. In the case of sample D, the size of cobalt oxide particles is on average lower than that of particles of samples C and B. This is apparently due to the fact that flagella were fragmented by sonication before sample D was manufactured, which affects the process of cobalt oxide particle formation in the min eralization procedure of the flagella. Large aggregates in sample D are easily transformed into smaller clus ters consisting of flagella fragments coated by particles of cobalt oxide. CONCLUSIONS Nanostructured samples were manufactured on the basis of flagella of galophilic archaea H. salinarum modified and mineralized by cobalt oxide. Samples were studied as negative electrodes of a lithiumion battery. It was found that samples could have a revers ible lithium intercalation capacity of more than 400 mA h/g. Herewith their stability under cycling was considerably enhanced after the ultrasonic fragmenta tion of the flagella prior to their mineralization. REFERENCES 1. A. S. Rudyi, T. L. Kulova, and A. M. Skundin, “Nano materials in Thin Film LithiumIon Batteries,” Inte gral, No. 1, 19 (2010). 2. Ch. E. Flynn, S. Lee, B. R. Peelle, and A. M. Belcher, “Viruses as Vehicles for Growth, Organization, and Assembly of Materials,” Acta Mater. 51, 5867 (2003). 3. K. T. Nam, D. Kim, P. J. Yoo, C. Chiang, N. Mee thong, P. T. Hammond, Y. Chiang, and A. M. Belcher, “VirusEnabled Synthesis and Assembly of Nanowires for Lithium Ion Battery Electrodes,” Science (Wash ington) 312, 885 (2006). 4. Y. J. Lee, H. Yi, W.J. Kim, K. Kang, D. S. Yun, M. S. Strano, G. Ceder, and A. M. Belcher, “Fabricat NANOTECHNOLOGIES IN RUSSIA
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