(Glassy) Carbon, a Promising Material for Sodium Ion Battery Anodes

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Amorphous (Glassy) Carbon a Promising Material for Sodium Ion Battery Anodes: a Combined First Principles and Experimental Study Fleur Legrain, Jonas Sottmann, Konstantinos Kotsis, Sandeep Gorantla, Sabrina Sartori, and Sergei Manzhos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03407 • Publication Date (Web): 26 May 2015 Downloaded from http://pubs.acs.org on June 2, 2015

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The Journal of Physical Chemistry

Amorphous (Glassy) Carbon, a Promising Material for Sodium Ion Battery Anodes: a Combined FirstPrinciples and Experimental Study AUTHOR NAMES. Fleur Legraina, Jonas Sottmannb, Konstantinos Kotsisa, Sandeep Gorantlac, Sabrina Sartoric and Sergei Manzhosa*. AUTHOR ADDRESS. a

Department of Mechanical Engineering, National University of Singapore, Block EA #07-08, 9

Engineering Drive 1, Singapore 117576, Singapore. b

Department of Chemistry, University of Oslo, Blindern, P.O. Box 1033, 0315 Oslo, Norway.

c

Department of Physics, University of Oslo, Blindern, P.O. Box 1048, 0316 Oslo, Norway.

KEYWORDS. sodium ion batteries; lithium ion batteries; amorphous carbon; density functional theory.

ABSTRACT. We present a comparative, combined ab initio and experimental study of sodium and lithium storage in amorphous (glassy) carbon (a-C) vs. graphite. Amorphous structures are obtained by fitting stochastically generated structures to a reference radial distribution function.

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Li insertion is thermodynamically favored in both graphite and a-C. While sodium insertion is thermodynamically unfavored in graphite, a-C possesses multiple insertion sites with binding energies stronger than Na cohesive energy, making it usable as anode material for Na-ion batteries. Binding energy of Na is predicted to be stronger than the Na cohesive energy for Na concentrations corresponding to a capacity of about 200 mAh/g. These results are confirmed by experimental measurements using highly amorphous carbon, in which a specific capacity of 173 mAh/g for Na is obtained after 100 cycles.

1. Introduction The perspective of a widespread use of clean but intermittent sources of electricity (wind and solar) as well as of hybrid electric vehicles calls for alternatives to Li-ion batteries, as Li resources may be insufficient for massive deployment of electrochemical batteries1, 2. Sodium being abundant, cheap, and a relatively light and small atom, Na-ion batteries have recently attracted much interest2, 3. However, while most studies of Na-ion batteries focus on the positive electrode, the negative electrode remains less investigated and an efficient anode material providing all a good capacity, a high cycle life, and a descent rate of charge/discharge, is still not available. Some efficient electrode materials for Li, in particular diamond silicon and graphite, have been shown to not allow the intercalation of Na4, 5. Computational studies report positive intercalation energies6, 7 and therefore suggest that the insertion of Na into the crystalline framework of graphite and Si is thermodynamically not favored: Na atoms prefer to cluster rather than to intercalate into the crystalline phase. Absence of significant Na insertion in graphite is also confirmed experimentally8. Na insertion into graphite has only been achieved by co-insertion with some electrolytes9.


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Amorphization of Si has recently been shown to improve substantially the interaction between Si and Na, and amorphous Si (a-Si) has been predicted to allow Na insertion by two independent studies6, 10. We hypothesize that the same should hold for amorphization of carbon. To the best of our knowledge, the effect of amorphization of carbon on Na and Li storage has not been investigated. Its knowledge is needed for rational design of electrodes for Na-ion batteries as well as for Li-ion batteries. Indeed, carbon is widely used as storage medium and/or conducting binder in both types of batteries11. Specifically, nano-sized graphite (which is also sometimes called “amorphous carbon”, as opposed to truly amorphous or “glassy” C12) has been used as anode for Na-ion batteries13. Amorphous carbon as anode material has been investigated for Li insertion14. In Ref. 15, amorphous carbon-coated TiO2 nanocrystals were used in a lithium-ion battery, while amorphous carbon-coated Na7Fe7(PO4)6F3 has recently been investigated experimentally as a cathode material in sodium ion batteries, and it was shown that the coated material exhibits twice the capacity of the uncoated sample16. Glassy carbon in nanocomposites can be a component of electrodes. An increasing interest lies in the combination of different materials, composite materials, e.g. amorphous phosphorous/carbon composites17 and nanocomposite Sb/C18 were investigated as promising anode materials for Na-ion batteries. In this paper, we therefore investigate the effect of amorphization of carbon on the energetics of insertion of Li and Na and show that, contrary to graphite, amorphous (glassy) carbon favors sodium insertion and can therefore work as anode for Na-ion batteries. This is a combined ab initio computational – experimental study. In Section 2, we introduce the methodologies used to make the amorphous structures and to do ab initio calculations as well as the experimental setup. In Section 3, we present calculations of two different amorphous carbon structures and the number of the most stable insertion sites into these two structures (Section 3.1) in comparison to


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the known lowest site in graphite, as well as the energetics of Li and Na in a-C versus graphite (Section 3.2). We show that amorphization leads to stronger binding of Li and Na to carbon. In Section 3.3, we present experimental results confirming the computed effect of amorphization of carbon. Section 4 concludes that amorphous carbon is suitable as anode material for Na-ion batteries and may also be advantageous for use in Li-ion batteries.

2. Methods 2.1. Simulations of the amorphous carbon structure Amorphous structures were generated by randomly sampling distributions of 64 carbon atoms placed in a box of size 8x8x8 Å3, with periodic boundary conditions. The initial density of about 2.5 g/cm3 is near that of previously reported amorphous structures19, 20. A large number (>106) of distributions were sampled, for which the radial distribution functions (RDF) were compared to the experimental RDF of Ref. 19. The structures giving a good fit of the RDF to the experimental RDF19 were then optimized ab initio including optimization of lattice vectors (to zero pressure), which did not result in significant changes of the RDF or of the density. While the method bears similarity to that used in Ref. 12 in that initial random structures are used, we introduce significant improvements in that we fit to the experimental RDF and ensure that the structure is stable under cell vector optimization to target pressure. 2.2. Ab initio calculations Structures were optimized with density functional theory (DFT)21, 22 using the SIESTA code 23. The PBE exchange correlation functional24 and a double-ζ polarized basis set were used. The basis sets were tuned to reproduce the cohesive energies of Li, Na, and diamond carbon (computed values of, respectively, 1.67, 1.14, and 7.65 eV are in good agreement with reference


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values25-27). Core electrons were replaced with Troullier-Martins pseudopotentials28. Spinpolarized calculations were performed. The DFT-D2 approach of Grimme29 is used to model the van der Waals interaction between the C atoms. This is important for comparison with graphite. No correction was used between the C and Li/Na atoms due to a significant ionicity of the bonds (large charge donation, see below). The Grimme parameters were tuned to reproduce the spacing of layers of graphite (the computed value of 3.35 Å is in good agreement with reference values 30, 31

). This was achieved with Grimme parameter values of s6=1.0, D=20, C6= 1.75 J nm6 mol−1,

r=1.725 Å29. Nearly cubic supercells of 64 and 128 C atoms with periodic boundary conditions were used to model the intercalation of Li and Na in a-C and in graphite, respectively. The Brillouin zone was sampled with 3×3×3 (4×4×4) Monkhorst-Pack point grid32 for amorphous (graphite) structures, and a 100 Ry cutoff was used for the Fourier expansion of the electron density. All atomic positions and the lattice vectors were allowed to relax, until forces were below 0.03 eV/Å and stresses below 0.1 GPa, respectively. The energetics of Na and Li insertion in the amorphous and crystalline (graphite) phases of C were analyzed based on the defect formation energies per Li/Na atom Ef,

𝐸𝑓 =

𝐸(𝐶 − 𝑀) − 𝐸(𝐶) − 𝑛𝑛(𝑀) , 𝑛

where M stands for the inserted metal (M=Li/Na), n represents the number of inserted metal atoms of type M, E(C-M) designates the energy of the supercell with the Li/Na-inserted C structures, E(C) represents the energy of the pure carbon structures (graphite or amorphous), and E(M) represents the energy of one atom of Li/Na in bulk (i.e. bcc Li/Na). Negative values of Ef


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indicate therefore thermodynamic preference for Li/Na insertion, and positive values indicate thermodynamics preference for bulk metal formation (“plating”)33, 34. 2.3. Experiments Ball-milling was conducted using the planetary mill Fritsch Pulverisette 7 (P7). Conductive carbon black (Super P Timcal) was ball milled for 24 h under Ar at 720 rpm with a ball-topowder ratio of 20:1. Transmission electron microscopy (TEM) was carried out on FEI Titan 60-300 G2 TEM equipped with DCOR probe-Cs corrector and operated at 300 kV. Conventional TEM bright field imaging and HRTEM (High-Resolution TEM) imaging techniques are used to investigate the microstructure of the samples. For the electrochemical characterization coin cells (2032) were assembled in a glove box with water and oxygen levels lower than 1 ppm. The working electrode was prepared by coating a slurry 80 wt% of ball milled Super P and 20 wt% PAA binder (Sigma Aldrich) on Cu foil. The so obtained electrodes had a mass loading of active material of about 1.3 mg/cm2. The working electrode was separated from the Li/Na metal disk as counter electrode by electrolyte soaked glass fiber. The electrolyte used was a 1M solution of LiPF6 in EC/DMC and NaPF6 in EC/DEC (1:1 in vol.) solution. The currents applied during galvanostatic cycling were 225 mA/g for Li and 75 mA/g for Na which correspond to similar C-rates.

3. Results and Discussion 3.1. Amorphous carbon structure and insertion sites Two amorphous structures were generated and are shown in Fig. 1(a, b). We use two different structures to ensure that the results are not skewed due to a particular generated structure. Their


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RDF are shown in Fig. 1(c) together (and in good agreement) with that from neutron diffraction data19. Both amorphous structures have a mass density of ~2.5 g/cm3 and a sp3 fraction of ~0.5, in agreement with amorphous structures generated by melting and quenching (MD) using the DFTB scheme20. The fractional sp3 character is related to a coordination number of ~3.535, 36. The amorphous structures are less stable than the graphite phase by 0.80 and 0.92 eV per atom for structures 1 and 2, respectively. To find Li/Na insertion sites in a-C, we performed a k-means clustering37, 38 analysis of a uniform three-dimensional grid of points covering each structure spaced by 0.2 Å and excluding points closer than 1.5 Å to C atoms. This allowed us to identify 17 and 19 potential insertion sites in structures 1 and 2, respectively. These sites were used as initial guesses for the insertion sites and further optimized by DFT-D. The positions of optimized unique insertion sites are also shown in Fig. 1 (a, b). A total of 13/12 unique Li/Na sites were found in both structures. The known lowest energy site in graphite is also shown in Fig. 1(d) and is used for comparison31, 39, 40.


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Figure 1. Structures 1 (a) and 2 (b) of a-C (brown) with Li/Na insertion sites (green), and their radial distribution functions compared to experiment19 (c). Visualization by VESTA44. The lowest energy insertion site in graphite is also shown in panel (d). Li sites are shown, and Na sites are visually similar. 3.2. Insertion energetics of Li and Na into a-C vs. graphite The defect formation energies of Li and Na in all structures are listed in Table 1 and are plotted in Fig. 2(a). The defect formation energy for Li insertion in graphite is -0.09 eV and for Na insertion +0.76, in good agreement with observed anodic voltages for Li34, 8, with previous


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calculations7, and with the fact that Na does not intercalate in graphite4, 5. In a-C, there is a distribution of Ef values, with lowest Ef values stabilized by 1.48 eV for Li and 1.99 eV for Na, vs. graphite. This behavior is similar to what we previously observed for Li and Na insertion in amorphous vs. crystalline Si6 and TiO2 41. All Li sites except one have a negative Ef. More importantly, the amorphization of carbon makes Na insertion thermodynamically favored, with half the sites showing binding energies stronger than the cohesive energy of Na. a-C will therefore operate as an anode for Na-ion batteries, while graphite does not. Amorphization of carbon could also be used to increase the anodic voltage in Li-ion batteries by up to 1.5 V, which could be useful e.g. to match it with the redox window of the electrolyte and limit electrolyte decomposition34, 42.

Table 1. Defect formation energies Ef of Li and Na in graphite and in a-C. Zero corresponds to the cohesive energy of Li and Na, respectively. host

Li Ef, eV

Na Ef, eV

Graphite

-0.085

0.760

-1.563

-1.234

-1.516

-1.106

-1.196

0.701

-0.507

1.232

-0.306

1.313

-0.197

1.568

a-C 1


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a-C 2

-1.539

-0.891

-1.053

-0.845

-0.697

-0.153

-0.613

0.407

-0.543

1.247

-0.137

1.666

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0.285

In Fig. 2(b), we plot Ef as a function of the effective coordination number N (number of neighbors of the Na atom with a cutoff distance rc=2.5 Å). The correlations with Pearson’s R2 values of 0.86 (0.53) for Na (Li) are statistically significant. As expected, the inserted Li and Na atoms donate charge to the host, ranging 0.2-0.6 (0.5-0.7) |e| based on Mulliken (Voronoi) charges for Li and 0.4-0.7 (0.4-0.6) |e| for Na in a-C. To compare, in graphite, the charge donation is 0.5 (0.7) |e| for Li and 0.5 (0.6) |e| for Na. However, no significant correlation of Ef to the charges was found.


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Figure 2. (a) Filled symbols: the defect formation energies Ef of Li and Na with respect to bulk Li and Na, respectively, in a-C. Circles and rhombuses are for the two amorphous structures. The corresponding Ef values in graphite are also shown as empty black circles. (b) Dependence of Ef on the coordination number N. We also investigated the insertion energetics of Li/Na in a-C for higher concentrations x 2

( ,

3

,

4

,

5

6

, ) than that of x =

64 64 64 64 64

1

64

considered above (corresponding the insertion of a single

Li/Na atom in the supercell), x being the number of Li/Na atom inserted per C atom. Because of the prohibitive computational cost of the exhaustive screening of all combinations of two, three, and six occupied sites, only the 2 to 6 lowest energy sites are computed for each amorphous structure (which means that the curve of Fig. 3 is the upper limit estimate). The plot of the defect formation energies against Li/Na concentration is given in Fig. 3. The negative defect formation energies for x =

2

,

3

64 64

,

4

64

and

5

64

(and also

6

64

in Li) suggest that the insertion of 2, 3, 4 and 5

Li/Na (and 6 Li) per 64 atoms of C is favored in the amorphous phase. For 6 Na atoms, the configurations modeled give a favored insertion in one of the two amorphous structures, and a


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slightly positive Ef in the other. The defect formation energies increase with metal concentration, as expected; we note that the negative of the defect formation energy, -Ef, is equal to the average voltage, see Ref. 43. These results suggest that Na intercalation could happen for a higher concentration than the very dilute concentration of x = concentration of about x =

5

…

64

6

64

1

. The calculations suggest that a Na

64

is where the voltage should drop to 0, which corresponds to a

specific capacity of about 170…200 mAh/g.

Figure 3. The defect formation energies per dopant atom (Ef) of Li/Na with respect to bulk Li/Na in a-C against Li/Na concentration x (x being the number of Li/Na metal atom per C atom). Circles and rhombuses are for the two amorphous structures. 3.3. Experimental confirmation The high resolution TEM images of the as purchased and the ball-milled carbon black in Fig. 4 indicate that the partial ordering in the purchased carbon black (Fig. 4(a)) is destroyed by ball-


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milling. This is further confirmed by the absence of weak spots and sharp rings in FFT (Fast Fourier transform) of the microstructure of the carbon after ball-milling and shows the effect of amorphization induced by ball-milling (Inset Fig. 4(b)).

Figure 4. TEM images of the as purchased carbon black (a) and after ball milling (b). Insets = FFT diffractogram of the respective samples. Electrochemical investigations of the Li/Na insertion behavior were carried out on the amorphous carbon (a-C) obtained after ball-milling carbon black and are shown in Fig. 5. The galvanostatic measurements show sloped voltage profiles in the voltage range from about 1.6 V down to 0.01 V as exemplarily shown for the initial ten galvanostatic cycles in Fig. 5(a-b). In Fig. 5(c) the voltage profiles of the 2nd galvanostatic cycle of a-C with Li and Na are compared. A slightly bigger overpotential is observed for Li which is most likely due to the slightly higher C-rate. During the first lithiation/sodiation high capacities of 783 mAh/g and 338 mAh/g, while during the first delithiation/desodiation only 490 mAh/g and 180 mAh/g are obtained. These irreversible capacities are commonly observed in the first cycle and could be accounted for by


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SEI formation and other passivation processes including the possibility of Li/Na insertion in sites where the metal ions remain trapped. Within the first ten cycles the columbic efficiency improves to almost 100 % and in the 100th cycle 390 mAh/g and 173 mAh/g of reversible capacity are retained which correspond to exchange of 11 Li and 5 Na ions per 64 C (Fig. 5(d)). The sloped voltage profile indicates that the electrochemical insertion/extraction of Li/Na ions proceeds through a large number of successive a-C phases without any formation of crystalline phases. The defect formation energies calculated above correspond well to the obtained voltage profile considering a simple approximation for the average voltage, V = -Ef /z (z = 1 for Li/Na)43. We note that the lowest computed concentration of Li/Na of x =

1

64

corresponds to a capacity of

35 mAh/g. An accuracy of the order of 0.1 eV is expected for DFT estimates of Ef. The obtained reversible capacities are consistent with the Na concentrations predicted by the calculations and also for Li assuming a similar linear trend of the defect formation energies with increasing Li concentration.


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Figure 5. Discharge and charge profiles for the first ten cycles of a-C for Li and Na (a,b), for the second cycle (c) and cycling performance of a-C for Li and Na over the first 100 cycles (d). In (c) M stands for the metal, Li and Na, respectively.

4. Conclusions We presented a comparative, combined ab initio and experimental study of the effect of amorphization of carbon on sodium and lithium storage. Specifically, we have shown in a comparative computational study that while the lowest-energy insertion site in graphite does not favor Na intercalation, the amorphous phase (a-C) provides insertion sites with a wide distribution of energies, including sites with binding energies of Na stronger than Na cohesive


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energy. a-C can therefore operate as an anode for Na-ion batteries, while graphite cannot. Insertion of both Li and Na is stabilized by amorphization, by up to 1.5 and 2.0 eV, respectively. Amorphization of carbon could also be used to increase the anodic voltage in Li-ion batteries to e.g. limit electrolyte decomposition. This prediction has been confirmed in an experimental anode using highly amorphous carbon. Specifically, the measured specific capacity of Na in a-C of 173 mAh/g after 100 cycles, as well as the voltage range of 0-1.5 V are in good agreement with theoretical estimates. We also show that it is possible to obtain a reliable amorphous structure in a computationally efficient way by optimizing randomized structures pre-selected to satisfy the desired (e.g. experimental) radial distribution function.

AUTHOR INFORMATION Corresponding Author E-Mail: [email protected]; Tel.: +65-6516-4605; Fax: +65-6779-1459 Funding Sources This work was supported by the Ministry of Education of Singapore via an AcRF grant (R-265000-494-112). ABBREVIATIONS RDF, radial distribution function; DFT, density functional theory; TEM, transmission electron microscopy; HRTEM, high-resolution transmission electron microscopy; PAA, polyacrylic acid; EC, ethylene carbonate; DMC, dimethyl carbonate; DEC, diethyl carbonate; MD, molecular dynamics; DFTB, density functional tight binding; FFT, fast Fourier transform.


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17. Kim, Y.; Park, Y.; Choi, A.; Choi, N.-S.; Kim, J.; Lee, J.; Ryu, J. H.; Oh, S. M.; Lee, K. T. An amorphous red phosphorus/carbon composite as a promising anode material for sodium ion batteries. Adv. Mater. 2013, 25, 3045-3049. 18. Qian, J.; Chen, Y.; Wu, L.; Cao, Y.; Ai, X.; Yang, H. High capacity Na-storage and superior cyclability of nanocomposite Sb/C anode for Na-ion batteries. Chem. Commun. 2012, 48, 7070-7072. 19. Gilkes, K. W. R.; Gaskell, P. H.; Robertson, J. Comparison of neutron-scattering data for tetrahedral amorphous carbon with structural models. Phys. Rev. B 1995, 51, 12303-12312. 20. Frauenheim, T.; Jungnickel, G.; Köhler, T.; Sitch, P. K.; Blaudeck, P. Amorphous carbon: state of the art; World Scientific: Singapore, 1998, pp. 59-72. 21. Hohenberg, P.; Kohn, W. Inhomogeneous electron gas. Phys. Rev. 1964, 136, B864-B871. 22. Kohn, W.; Hohenberg, P. Self-consistent equations including exchange and correlation effects. Phys. Rev. 1965, 140, A1133-A1138. 23. Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. The SIESTA method for ab initio order-N materials simulation. J. Phys. Condens. Matter 2002, 14, 2745-2779. 24. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 25. Gschneidner Jr., K. R. Solid State Physics; Vol. 16, New York: Academic, p. 276.


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26. Maezono, R.; Towler, M. D.; Lee, Y.; Needs, R. J. Quantum Monte Carlo study of sodium. Phys. Rev. B 2003, 68, 165103. 27. Schimka, L.; Harl, J.; Kresse, G. Improved hybrid functional for solids: the HSEsol functional. J. Chem. Phys. 2011, 134, 024116. 28. Troullier, N.; Martins, J. L. Efficient pseudopotentials for plane-wave calculations. Phys. Rev. B 1991, 43, 1993-2006. 29. Grimme, S. Semiempirical GGA-type density functional constructed with a long-range dispersion correction. J. Comput. Chem. 2006, 27, 1787-1799. 30. Nicklow, R.; Wakabayashi, N.; Smith, H. G. Lattice dynamics of pyrolytic graphite. Phys. Rev. B 1972, 5, 4951-4962. 31. Tasaki, K. Density functional theory study on structural and energetic characteristics of graphite intercalation compounds. J. Phys. Chem. C 2014, 118, 1443-1450. 32. Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188-5192. 33. Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research development on sodium-ion batteries. Chem. Rev. 2014, 114, 11636−11682. 34. Goodenough, J. B.; Kim, Y. Challenges for rechargeable Li batteries. Chem. Mater. 2010, 22, 587-603. 35. McKenzie, D. R. Tetrahedral bonding in amorphous carbon. Rep. Prog. Phys. 1996, 59, 1611-1664.


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36. Frauenheim, Th.; Jungnickel, G.; Köhler, Th.; Stephan, U. Structure and electronic properties of amorphous carbon: from semimetallic to insulating behavior. J. Non-Cryst. Solids 1995, 182, 186-197. 37. Seber, G. A. F. Multivariate Observations; John Wiley & Sons: Hoboken NJ, 1984. 38. Spath, H. Cluster Dissection and Analysis: Theory, FORTRAN Programs, Examples; Halsted Press: New York, 1985. 39. Lee, E.; Persson, K. Li Absorption and intercalation in single layer graphene and few layer graphene by first principles. Nano Lett. 2012, 12, 4624-4628. 40. Liu, Y.; Artyukhov, V. I.; Liu, M.; Harutyunyan, A. R.; Yakobson, B. I. Feasibility of lithium storage on graphene and its derivatives. J. Phys. Chem. Lett. 2013, 4, 1737-1742. 41. Legrain, F.; Malyi, O. I.; Manzhos, S. Insertion energetics of lithium, sodium, and magnesium in crystalline and amorphous titanium dioxide: a comparative first-principles study. J. Power Sources 2015, 278, 197-202. 42. Komaba, S.; Murata, W.; Ishikawa, T.; Yabuuchi, N.; Ozeki, T.; Nakayama, T.; Ogata, A.; Gotoh, K.; Fujiwara, K. Electrochemical Na insertion and solid electrolyte interphase for hard-carbon electrodes and application to Na-ion batteries. Adv. Funct. Mater. 2011, 21, 3859-3867. 43. Aydinol, M. K.; Kohan, A. F.; Ceder, G.; Cho, K.; Joannopoulos, J. Ab initio study of lithium intercalation in metal oxides and metal dichalcogenides. Phys. Rev. B 1997, 56, 1354-1365.


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44. Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Cryst. 2011, 44, 1272-1276.


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TOC graphic

Sodium insertion into glassy carbon.


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. Structures 1 (a) and 2 (b) of a-C (brown) with Li/Na insertion sites (green), and their radial distribution functions compared to experiment19 (c). Visualization by VESTA44. The lowest energy insertion site in graphite is also shown in panel (d). Li sites are shown, and Na sites are visually similar. 135x141mm (96 x 96 DPI)


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(a) Filled symbols: the defect formation energies Ef of Li and Na with respect to bulk Li and Na, respectively, in a-C. Circles and rhombuses are for the two amorphous structures. The corresponding Ef values in graphite are also shown as empty black circles. (b) Dependence of Ef on the coordination number N. 165x82mm (96 x 96 DPI)



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The defect formation energies per dopant atom (Ef) of Li/Na with respect to bulk Li/Na in a-C against Li/Na concentration x (x being the number of Li/Na metal atom per C atom). Circles and rhombuses are for the two amorphous structures. 106x100mm (96 x 96 DPI)


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TEM images of the as purchased carbon black (a) and after ball milling (b). Insets = FFT diffractogram of the respective samples. 157x79mm (96 x 96 DPI)



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Discharge and charge profiles for the first ten cycles of a-C for Li and Na (a,b), for the second cycle (c) and cycling performance of a-C for Li and Na over the first 100 cycles (d). In (c) M stands for the metal, Li and Na, respectively. 165x120mm (96 x 96 DPI)


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Sodium insertion into glassy carbon. 84x84mm (96 x 96 DPI)


(Glassy) Carbon, a Promising Material for Sodium Ion Battery Anodes

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