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Templated spinel Li4Ti5O12 Li-ion battery electrodes combining high rates with high energy density
Electrochemistry Communications 35 (2013) 124–127
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Templated spinel Li4Ti5O12 Li-ion battery electrodes combining high rates with high energy density Deepak P. Singh, Fokko M. Mulder, Marnix Wagemaker ⁎ Department of Radiation Science and Technology, Delft University of Technology, Mekelweg 15, Delft 2629JB, The Netherlands
a r t i c l e
i n f o
Article history: Received 7 August 2013 Received in revised form 15 August 2013 Accepted 15 August 2013 Available online 23 August 2013 Keywords: Li-ion batteries Li4Ti5O12 spinel Templating High charge rates Microstructure
a b s t r a c t High power and high energy density electrodes for rechargeable lithium-ion batteries are required for electrical mobility applications. Though nano-structuring of electrode materials generally improves the kinetics of the charge transport, thereby increasing the power density, the drawback is the low density of these electrodes compromising the energy density. Combining high power density with high energy density requires dense electrodes with optimal ionic and electronic wiring throughout the electrode microstructure. Here we present a facile and low cost templating method using carbonate salts creating 3D interconnected ionic pathways that improve the ionic charge transport without compromising the electrode density significantly. The method was demonstrated for C/Li4Ti5O12 electrode material resulting in excellent capacity retention reaching ~90% at 5 C and ~50% at 200 C rate combined with high active material electrode densities around 1.45 gm/cm3. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Inspired by sustainable mobility, Li-ion battery research has focused towards improving the energy and power density, under the conditions of safety, long cycle life and the use of low cost materials. Since Hearing et al. [1] and Ferg et al. [2] have demonstrated lithium insertion in defect spinel Li4Ti5O12 (Li8a[Li1/3Ti5/3]16dO4), this material has received considerable attention owing to the combination of high Li-ion capacity with facile Li-ion diffusion and excellent cycleability. The latter is due to its “zero” strain property having only 0.2% volume change upon lithiation from the Li4Ti5O12 (Li8a[Li1/3Ti5/3]16dO4) to Li7Ti5O12 ([Li2]16c[Li1/3Ti5/3]16dO4) composition remaining in the Fd3m cubic symmetry. The extremely flat (dis)charge voltage plateau at 1.55 V vs Li+/Li is due to the coexistence of Li4Ti5O12 and Li7Ti5O12 [1].The low coherency strain and interface energy between these two coexisting compositions [3] result in nanosized domains that appear as a solid solution to diffraction [4,5]. The disadvantage of the relatively high negative electrode operating voltage (1.55 V vs Li+/Li) is the lower battery voltage compared to graphite (0.15 V) vs. Li+/Li). However, the advantage is that it operates within the stability window of most organic lithium electrolytes. Numerous strategies have been adopted to improve the (dis)charge transport properties of Li4Ti5O12 electrodes, including the crystallite size reduction [6], doping with high valence metal ions [7,8], coating with conducting phases [9–11]. Amongst all, combination with carbon coating and nanosizing of electrode particles proved the most successful ⁎ Corresponding author. Tel.: +31 152783800. E-mail address: [email protected] (M. Wagemaker). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.08.014
strategy [4,12]. The challenge is to combine high rate performances with high electrode densities which requires knowledge of what charge transport phenomena are rate limiting under what conditions. The rate limiting charge transport step dominates the internal resistance of the battery resulting in a voltage polarization that scales with the (dis) charge current. It has been long recognized that in many cases the ionic transport through the electrolyte and through the porous electrode structure may be rate limiting and not the electrode material itself [13–24]. This is convincingly illustrated by the decreasing capacity at the same (dis) charge rate while increasing the electrode thickness [19,25]. In an elegant study Fongy et.al. demonstrated that increasing the porosity initially leads to improved kinetics owing to the diffusion through the electrolyte in the porous electrode matrix [23]. However, above a certain porosity the rate performance decreases due to the reduced electronic wiring that relies on the interconnectivity of the electrode. Clearly, the complete electrode microstructure is decisive for the charge transport, and for this reason increasing research effort is aimed at controlling the microstructure to improve battery performance [16,19–24,26,27]. This has resulted into various methods to control the electrode nanomorphology [13,28–30]. Though these nano-structuring strategies generally enhance the electrode rate performance they considerably compromise the electrode density, and consequently the volumetric energy density, additionally being relatively expensive for commercial applications. Here we report on a generally applicable, low cost method, using ammonium bicarbonate (NH4HCO3) salts as template enhancing the interconnectivity of the available porosity in the electrode matrix, realizing high rate performance in combination with high electrode densities.
D.P. Singh et al. / Electrochemistry Communications 35 (2013) 124–127
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2. Electrode templating The templated Li4Ti5O12 electrode was prepared by adding 20% to 50 wt% of NH4HCO3 in mixture of 90% C/Li4Ti5O12 (Phostech), 5% PVDF, 5% SuperP in NMP solvent. The resulting slurry was casted onto Al foil and dried in a vacuum oven at 50 °C for 48 to 72 h. Dried electrode was further calendared to enhance good electronic contact through active material and heated at 90 °C to decompose NH4HCO3 template into CO2, N2 and H2O which most likely attributes to the interconnectivity of the porous network. The presence and removal of NH4HCO3 in the Li4Ti5O12 electrode matrix are confirmed by X-ray diffraction shown in Fig. 1(a). All electrodes were tested against lithium metal in 1 M LiPF6 in EC/DMC electrolyte. 3. Result and discussion Fig. 1(b) shows the charge voltage profiles (constant discharge at C/5) of the templated electrode with 3.5 mg/cm2 active material (AM) loading. The capacity strongly depends on the charge rate, decreasing from ~165 mAh/g at C/10 up to an exceptionally high (for the relatively high AM loading) 50 mAh/g at 100C. Fig. 1(c) demonstrates that a thinner templated Li4Ti5O12 electrode (loading 0.8 mg/cm2) results in even higher capacity retentions from ~170 mAh/g at C/10 up to 85 mAh/g at 200C. As expected the decrease in AM loading increases the (dis)charge capacity in particular at high C rates, consistent with other studies [6,13,19,23,25]. More interesting is the impact of the templating, at high rates retaining approximately twice the capacity when compared to the non-templated electrodes with a comparable AM loading. Strikingly, up to 10 C rate the 3.5 mg/cm2 templated electrode outperforms the 0.7 mg/cm2 non-templated electrode. This illustrates that improved kinetics induced by the templating can also be applied to increase the electrode thickness (by increasing the AM loading) without compromising (dis)charge rate compared to non-templated electrodes. In practice this increases the amount of active material in a battery, resulting in a higher overall battery energy density. To the best of our knowledge the highest rate performances reported for Li4Ti5O12 include 120 mAh/g at 10 C using TiN coating [9], 110 mAh/g at 60 C using rutile TiO2 coating [10], 85 mAh/g at 160 C with carbon coating [11], 65 mAh/g at 100 C using nanomaterials [30], and 90 mAh/g at 40 C for coaxial CNT-Li4Ti5O1 [29]. The presently templated LTO electrodes exceed these performances reaching up to 120 mAh/g at 100 C for the thinnest electrodes. However, great care should be taken comparing rate performances because of the difference in AM loading are in many cases not reported. Also of large significance in this context is the electrode packing density which for nano-structured materials is relatively small (leading to a higher electrode porosity). The present templating method applied to the commercial C/Li4Ti5O12 powder results in a small increase of the electrode porosity (free volume), approximately 32% for the template electrodes compared to 30.5% for the non-templated electrodes, resulting in a minor decrease in loading density (templated 1.43 g/cm3; non-templated 1.47 g/cm3). Clearly, the approximately 2% increase in the porosity of the templated electrodes, does not explain the improved rate performance [23]. Therefore the template electrodes combine high density with excellent rate performance. To gain insight in the improved performance of the templated electrodes we adopted Prosini's [31] and Fongy's [24] approach by introducing two variables, Q0 and k, extracted from the relation between the capacity and the charge current Q ¼ Q 0 −kreal I
ð1Þ
Here Q is the (dis)charge capacity at current I, Q0 is the equilibrium (dis)charge capacity and kreal is a time constant (hours) characterizing the rate performance of electrode extracted from the capacity retention at increasing rates (See Ref [24,32]). The resulting kreal values for
Fig. 1. (a) XRD pattern of templated Li4Ti5O12 electrode before and after NH4HCO3 template removal. (b) Charging voltage profiles for different C rates (at constant discharge C/5) of the templated electrode with 3.5 mg/cm2 AM loading, (c) Charge capacities as a function of the specific current with different loading densities for templated and nontemplated electrodes.
templated and non-templated electrodes are shown as a function of the squared electrode thickness in Fig. 2(a). As expected, increasing the electrode thickness (i.e. AM loading) results in increasing kreal values representing the poorer rate performance of thicker electrodes. The linear relationship between kreal and L2, also observed by Fongy [24], is characteristic for a diffusion limited systems, indicating that the diffusion of Li-ions through the electrode (either through the solid state
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D.P. Singh et al. / Electrochemistry Communications 35 (2013) 124–127
Fig. 2. (a) kreal as a function of the squared electrode thickness. The lines represent linear fits based on a diffusion length dependent model (See text) where the slope determines the Bruggeman exponent and the tortuosity of the electrodes. The non-templated fit requires a thickness independent component indicating that the diffusion is limited by the charge transport through the solid state. (b) Schematic impact of templating. (i): Non-templated electrode with high tortuosity and resulting on average in long Li-ion diffusion pathways or even absence of pathways towards the active material. (ii): Electrode including the solid NH4HCO3 template. (iii): Templated electrode upon removal of the NH4HCO3 template with decreased tortuosity resulting in shorter Li-ion diffusion pathways towards the active material.
or through the electrolyte present inside porous electrode) is rate limiting. The most interesting observation in Fig. 1(d) is the impact of templating with the NH4HCO3 that reduces kreal significantly, the consequence of the faster response of the system upon charge. Assuming one-dimensional diffusion under the condition of constant ion flux at the surface of the electrode and simplified macroscopic diffusion through a porous system we can assume the following approximation of kreal [32–34] kionic ¼
L2 τ þ θks þ ke 3D0
ð2Þ
The first term is the contribution of ionic diffusion through the electrolyte in the pores of the electrode where, L is electrode thickness, D0 the bulk diffusion coefficient of the Li-ions in the electrolyte and τ the tortuosity taking into account the porous morphology of the electrode. θ is the fraction of ionic transport through the solid state in case of a non-templated (θ = 1) or templated electrode (θ b 1) [32]. The electronic contribution ke depends on the percolation of the Carbon Black (CB) network [35], the contact between the CB and electrode particles [36] and current collector [37], all depending strongly on the electrode processing [38]. Eq. (2) is used to fit the kreal values of both non-templated and templated electrodes, shown in Fig. 1(d), assuming an average porosity 30.5% for the non-templated and 32% for the templated electrodes, and a bulk diffusion coefficient D0 = 1.5 10− 6 cm2/s for the electrolyte [39]. The result is a smaller time constant θkS + ke and a smaller tortuosity for the templated electrodes (τ = 1.38 for templated; τ = 2.55 for non-templated electrodes). The first effect indicates that in the templated electrode a larger fraction of the ionic diffusion passes through the electrolyte rather than through the solid state (θtemplated b θnon-templated) making the charge transport easier due to the larger diffusion coefficient of the electrolyte. The lower tortuosity indicates approximately 46% reduction in the path length of the Li-ions towards the electrode material compared to the non-templated electrode. This is consistent with a better connectivity of the pores and enhanced ionic wiring as schematically shown in Fig. 2(b). 4. Conclusion To enhance the power density of Li-ion battery electrodes without compromising the loading density a simple and cost effective templating technique is developed using ammonium bicarbonate salt. The solid templating material makes compaction possible, as
required for establishing good electronic contacts in the electrodes, while its subsequent removal creates 3D interconnected network with only a small (~ 2%) increase in electrode porosity. Applying this templating method to Li4Ti5O12 results in high density electrodes with excellent capacity retention at high rates explained by improved electrolyte accessibility through the interconnected network in the electrode matrix. The present templating approach is expected to be generally applicable to other types of Li-ion insertion materials enabling the improvement of electrode rate performance. Acknowledgments Financial support from The Shell Netherlands is acknowledged for Shell Sustainable Mobility funding to D.P.S. The Netherlands Organization for Scientific Research (NWO) is acknowledged for the CW-VIDI grant of M.W. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007–2013)/ERC Grant Agreement n. [307161] of M.W. References [1] K.M. Colbow, J.R. Dahn, R.R. Haering, J. Power Sources 26 (1989) 397. [2] E. Ferg, R.J. Gummow, A. Dekock, M.M. Thackeray, J. Electrochem. Soc. 141 (1994) L147. [3] S. Ganapathy, M. Wagemaker, ACS Nano 6 (2012) 8702. [4] M. Wagemaker, E.R.H. van Eck, A.P.M. Kentgens, F.M. Mulder, J. Phys Chem. B 113 (2009) 224. [5] M. Wagemaker, D.R. Simon, E.M. Kelder, J. Schoonman, C. Ringpfeil, U. Haake, D. Lützenkirchen-Hecht, R. Frahm, F.M. Mulder, Adv. Mater. 18 (2006) 3169. [6] L. Kavan, J. Prochazka, T.M. Spitler, M. Kalbac, M.T. Zukalova, T. Drezen, M. Gratzel, J. Electrochem. Soc. 150 (2003) A1000. [7] C.H. Chen, J.T. Vaughey, A.N. Jansen, D.W. Dees, A.J. Kahaian, T. Goacher, M.M. Thackeray, J. Electrochem. Soc. 148 (2001) A102. [8] J. Wolfenstine, J.L. Allen, J. Power Sources 180 (2008) 582. [9] K.S. Park, A. Benayad, D.J. Kang, S.G. Doo, J. Am. Chem. Soc. 130 (2008) 14930. [10] Y.Q. Wang, L. Guo, Y.G. Guo, H. Li, X.Q. He, S. Tsukimoto, Y. Ikuhara, L.J. Wan, J. Am. Chem. Soc. 134 (2012) 7874. [11] H.G. Jung, S.T. Myung, C.S. Yoon, S.B. Son, K.H. Oh, K. Amine, B. Scrosati, Y.K. Sun, Energy Environ. Sci. 4 (2011) 1345. [12] W.J.H. Borghols, M. Wagemaker, U. Lafont, E.M. Kelder, F.M. Mulder, J. Am. Chem. Soc. 131 (2009) 17786. [13] E.M. Sorensen, S.J. Barry, H.-K. Jung, J.M. Rondinelli, J.T. Vaughey, K.R. Poeppelmeier, Chem. Mater. 18 (2005) 482. [14] M. Doyle, J. Newman, J. Appl. Electrochem. 27 (1997) 846. [15] M. Doyle, T.F. Fuller, J. Newman, J. Electrochem. Soc. 140 (1993) 1526. [16] P.A. Johns, M.R. Roberts, Y. Wakizaka, J.H. Sanders, J.R. Owen, Electrochem Comm. 11 (2009) 2089. [17] J. Zhou, D. Danilov, P.H.L. Notten, Chem. Eur. J. 12 (2006) 7125. [18] M. Gaberscek, J. Jamnik, Solid State Ionics 177 (2006) 2647. [19] D.Y.W. Yu, K. Donoue, T. Inoue, M. Fujimoto, S. Fujitani, J. Electrochem. Soc. 153 (2006) A835.
D.P. Singh et al. / Electrochemistry Communications 35 (2013) 124–127 [20] J. Jamnik, R. Dominko, B. Erjavec, M. Remskar, A. Pintar, M. Gaberscek, Adv. Mater. 21 (2009) 2715. [21] S. Pejovnik, R. Dominko, M. Bele, M. Gaberscek, J. Jamnik, InternationalBattery-Material-Association Meeting, CHINA, 2007. 593. [22] M. Gaberscek, M. Kuzma, J. Jamnik, Phys. Chem. Chem. Phys. 9 (2007) 1815. [23] C. Fongy, S. Jouanneau, D. Guyomard, J.C. Badot, B. Lestriez, J. Electrochem. Soc. 157 (2010) A1347. [24] C. Fongy, A.C. Gaillot, S. Jouanneau, D. Guyomard, B. Lestriez, J. Electrochem. Soc. 157 (2010) A885. [25] M. Thunman, K. Marquardt, R. Hahn, D. Kober, O. Goerke, H. Schubert, ECS Trans. 41 (2012) 147. [26] Y.S. Hu, Y.G. Guo, R. Dominko, M. Gaberscek, J. Jamnik, J. Maier, Adv. Mater. 19 (2007) 1963. [27] R. Dominko, M. Bele, J.M. Goupil, M. Gaberscek, D. Hanzel, I. Arcon, J. Jamnik, Chem. Mater. 19 (2007) 2960. [28] F. Cheng, Z. Tao, J. Liang, J. Chen, Chem. Mater. 20 (2008) 667. [29] L. Shen, C. Yuan, H. Luo, X. Zhang, K. Xu, F. Zhang, J. Mater. Chem. 21 (2011) 761.
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[30] E. Kang, Y.S. Jung, G.H. Kim, J. Chun, U. Wiesner, A.C. Dillon, J.K. Kim, J. Lee, Adv. Funct. Mater. 21 (2011) 4349. [31] P.P. Prosini, J. Electrochem. Soc. 152 (2005) A1925. [32] D.P. Singh, F.M. Mulder, A.M. Abdelkader, M. Wagemaker, Adv. Energy. Mater. 3 (2013) 572. [33] V. Srinivasan, J. Newman, J. Electrochem. Soc. 151 (2004) A1517. [34] I.V. Thorat, D.E. Stephenson, N.A. Zacharias, K. Zaghib, J.N. Harb, D.R. Wheeler, J. Power Sources 188 (2009) 592. [35] D. Guy, B. Lestriez, R. Bouchet, D. Guyomard, J. Electrochem. Soc. 153 (2006) A679. [36] W. Porcher, B. Lestriez, S. Jouanneau, D. Guyomard, J. Electrochem. Soc. 156 (2009) A133. [37] M. Gaberscek, J. Moskon, B. Erjavec, R. Dominko, J. Jamnik, Electrochem. Solid-State Lett. 11 (2008) A170. [38] R. Dominko, M. Gaberscek, J. Drofenik, M. Bele, S. Pejovnik, Electrochem. Solid-State Lett. 4 (2001) A187. [39] S.G. Stewart, J. Newman, J. Electrochem. Soc. 155 (2008) F13.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Templated spinel Li4Ti5O12 Li-ion battery electrodes combining high rates with high energy density Deepak P. Singh, Fokko M. Mulder, Marnix Wagemaker ⁎ Department of Radiation Science and Technology, Delft University of Technology, Mekelweg 15, Delft 2629JB, The Netherlands
a r t i c l e
i n f o
Article history: Received 7 August 2013 Received in revised form 15 August 2013 Accepted 15 August 2013 Available online 23 August 2013 Keywords: Li-ion batteries Li4Ti5O12 spinel Templating High charge rates Microstructure
a b s t r a c t High power and high energy density electrodes for rechargeable lithium-ion batteries are required for electrical mobility applications. Though nano-structuring of electrode materials generally improves the kinetics of the charge transport, thereby increasing the power density, the drawback is the low density of these electrodes compromising the energy density. Combining high power density with high energy density requires dense electrodes with optimal ionic and electronic wiring throughout the electrode microstructure. Here we present a facile and low cost templating method using carbonate salts creating 3D interconnected ionic pathways that improve the ionic charge transport without compromising the electrode density significantly. The method was demonstrated for C/Li4Ti5O12 electrode material resulting in excellent capacity retention reaching ~90% at 5 C and ~50% at 200 C rate combined with high active material electrode densities around 1.45 gm/cm3. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Inspired by sustainable mobility, Li-ion battery research has focused towards improving the energy and power density, under the conditions of safety, long cycle life and the use of low cost materials. Since Hearing et al. [1] and Ferg et al. [2] have demonstrated lithium insertion in defect spinel Li4Ti5O12 (Li8a[Li1/3Ti5/3]16dO4), this material has received considerable attention owing to the combination of high Li-ion capacity with facile Li-ion diffusion and excellent cycleability. The latter is due to its “zero” strain property having only 0.2% volume change upon lithiation from the Li4Ti5O12 (Li8a[Li1/3Ti5/3]16dO4) to Li7Ti5O12 ([Li2]16c[Li1/3Ti5/3]16dO4) composition remaining in the Fd3m cubic symmetry. The extremely flat (dis)charge voltage plateau at 1.55 V vs Li+/Li is due to the coexistence of Li4Ti5O12 and Li7Ti5O12 [1].The low coherency strain and interface energy between these two coexisting compositions [3] result in nanosized domains that appear as a solid solution to diffraction [4,5]. The disadvantage of the relatively high negative electrode operating voltage (1.55 V vs Li+/Li) is the lower battery voltage compared to graphite (0.15 V) vs. Li+/Li). However, the advantage is that it operates within the stability window of most organic lithium electrolytes. Numerous strategies have been adopted to improve the (dis)charge transport properties of Li4Ti5O12 electrodes, including the crystallite size reduction [6], doping with high valence metal ions [7,8], coating with conducting phases [9–11]. Amongst all, combination with carbon coating and nanosizing of electrode particles proved the most successful ⁎ Corresponding author. Tel.: +31 152783800. E-mail address: [email protected] (M. Wagemaker). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.08.014
strategy [4,12]. The challenge is to combine high rate performances with high electrode densities which requires knowledge of what charge transport phenomena are rate limiting under what conditions. The rate limiting charge transport step dominates the internal resistance of the battery resulting in a voltage polarization that scales with the (dis) charge current. It has been long recognized that in many cases the ionic transport through the electrolyte and through the porous electrode structure may be rate limiting and not the electrode material itself [13–24]. This is convincingly illustrated by the decreasing capacity at the same (dis) charge rate while increasing the electrode thickness [19,25]. In an elegant study Fongy et.al. demonstrated that increasing the porosity initially leads to improved kinetics owing to the diffusion through the electrolyte in the porous electrode matrix [23]. However, above a certain porosity the rate performance decreases due to the reduced electronic wiring that relies on the interconnectivity of the electrode. Clearly, the complete electrode microstructure is decisive for the charge transport, and for this reason increasing research effort is aimed at controlling the microstructure to improve battery performance [16,19–24,26,27]. This has resulted into various methods to control the electrode nanomorphology [13,28–30]. Though these nano-structuring strategies generally enhance the electrode rate performance they considerably compromise the electrode density, and consequently the volumetric energy density, additionally being relatively expensive for commercial applications. Here we report on a generally applicable, low cost method, using ammonium bicarbonate (NH4HCO3) salts as template enhancing the interconnectivity of the available porosity in the electrode matrix, realizing high rate performance in combination with high electrode densities.
D.P. Singh et al. / Electrochemistry Communications 35 (2013) 124–127
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2. Electrode templating The templated Li4Ti5O12 electrode was prepared by adding 20% to 50 wt% of NH4HCO3 in mixture of 90% C/Li4Ti5O12 (Phostech), 5% PVDF, 5% SuperP in NMP solvent. The resulting slurry was casted onto Al foil and dried in a vacuum oven at 50 °C for 48 to 72 h. Dried electrode was further calendared to enhance good electronic contact through active material and heated at 90 °C to decompose NH4HCO3 template into CO2, N2 and H2O which most likely attributes to the interconnectivity of the porous network. The presence and removal of NH4HCO3 in the Li4Ti5O12 electrode matrix are confirmed by X-ray diffraction shown in Fig. 1(a). All electrodes were tested against lithium metal in 1 M LiPF6 in EC/DMC electrolyte. 3. Result and discussion Fig. 1(b) shows the charge voltage profiles (constant discharge at C/5) of the templated electrode with 3.5 mg/cm2 active material (AM) loading. The capacity strongly depends on the charge rate, decreasing from ~165 mAh/g at C/10 up to an exceptionally high (for the relatively high AM loading) 50 mAh/g at 100C. Fig. 1(c) demonstrates that a thinner templated Li4Ti5O12 electrode (loading 0.8 mg/cm2) results in even higher capacity retentions from ~170 mAh/g at C/10 up to 85 mAh/g at 200C. As expected the decrease in AM loading increases the (dis)charge capacity in particular at high C rates, consistent with other studies [6,13,19,23,25]. More interesting is the impact of the templating, at high rates retaining approximately twice the capacity when compared to the non-templated electrodes with a comparable AM loading. Strikingly, up to 10 C rate the 3.5 mg/cm2 templated electrode outperforms the 0.7 mg/cm2 non-templated electrode. This illustrates that improved kinetics induced by the templating can also be applied to increase the electrode thickness (by increasing the AM loading) without compromising (dis)charge rate compared to non-templated electrodes. In practice this increases the amount of active material in a battery, resulting in a higher overall battery energy density. To the best of our knowledge the highest rate performances reported for Li4Ti5O12 include 120 mAh/g at 10 C using TiN coating [9], 110 mAh/g at 60 C using rutile TiO2 coating [10], 85 mAh/g at 160 C with carbon coating [11], 65 mAh/g at 100 C using nanomaterials [30], and 90 mAh/g at 40 C for coaxial CNT-Li4Ti5O1 [29]. The presently templated LTO electrodes exceed these performances reaching up to 120 mAh/g at 100 C for the thinnest electrodes. However, great care should be taken comparing rate performances because of the difference in AM loading are in many cases not reported. Also of large significance in this context is the electrode packing density which for nano-structured materials is relatively small (leading to a higher electrode porosity). The present templating method applied to the commercial C/Li4Ti5O12 powder results in a small increase of the electrode porosity (free volume), approximately 32% for the template electrodes compared to 30.5% for the non-templated electrodes, resulting in a minor decrease in loading density (templated 1.43 g/cm3; non-templated 1.47 g/cm3). Clearly, the approximately 2% increase in the porosity of the templated electrodes, does not explain the improved rate performance [23]. Therefore the template electrodes combine high density with excellent rate performance. To gain insight in the improved performance of the templated electrodes we adopted Prosini's [31] and Fongy's [24] approach by introducing two variables, Q0 and k, extracted from the relation between the capacity and the charge current Q ¼ Q 0 −kreal I
ð1Þ
Here Q is the (dis)charge capacity at current I, Q0 is the equilibrium (dis)charge capacity and kreal is a time constant (hours) characterizing the rate performance of electrode extracted from the capacity retention at increasing rates (See Ref [24,32]). The resulting kreal values for
Fig. 1. (a) XRD pattern of templated Li4Ti5O12 electrode before and after NH4HCO3 template removal. (b) Charging voltage profiles for different C rates (at constant discharge C/5) of the templated electrode with 3.5 mg/cm2 AM loading, (c) Charge capacities as a function of the specific current with different loading densities for templated and nontemplated electrodes.
templated and non-templated electrodes are shown as a function of the squared electrode thickness in Fig. 2(a). As expected, increasing the electrode thickness (i.e. AM loading) results in increasing kreal values representing the poorer rate performance of thicker electrodes. The linear relationship between kreal and L2, also observed by Fongy [24], is characteristic for a diffusion limited systems, indicating that the diffusion of Li-ions through the electrode (either through the solid state
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D.P. Singh et al. / Electrochemistry Communications 35 (2013) 124–127
Fig. 2. (a) kreal as a function of the squared electrode thickness. The lines represent linear fits based on a diffusion length dependent model (See text) where the slope determines the Bruggeman exponent and the tortuosity of the electrodes. The non-templated fit requires a thickness independent component indicating that the diffusion is limited by the charge transport through the solid state. (b) Schematic impact of templating. (i): Non-templated electrode with high tortuosity and resulting on average in long Li-ion diffusion pathways or even absence of pathways towards the active material. (ii): Electrode including the solid NH4HCO3 template. (iii): Templated electrode upon removal of the NH4HCO3 template with decreased tortuosity resulting in shorter Li-ion diffusion pathways towards the active material.
or through the electrolyte present inside porous electrode) is rate limiting. The most interesting observation in Fig. 1(d) is the impact of templating with the NH4HCO3 that reduces kreal significantly, the consequence of the faster response of the system upon charge. Assuming one-dimensional diffusion under the condition of constant ion flux at the surface of the electrode and simplified macroscopic diffusion through a porous system we can assume the following approximation of kreal [32–34] kionic ¼
L2 τ þ θks þ ke 3D0
ð2Þ
The first term is the contribution of ionic diffusion through the electrolyte in the pores of the electrode where, L is electrode thickness, D0 the bulk diffusion coefficient of the Li-ions in the electrolyte and τ the tortuosity taking into account the porous morphology of the electrode. θ is the fraction of ionic transport through the solid state in case of a non-templated (θ = 1) or templated electrode (θ b 1) [32]. The electronic contribution ke depends on the percolation of the Carbon Black (CB) network [35], the contact between the CB and electrode particles [36] and current collector [37], all depending strongly on the electrode processing [38]. Eq. (2) is used to fit the kreal values of both non-templated and templated electrodes, shown in Fig. 1(d), assuming an average porosity 30.5% for the non-templated and 32% for the templated electrodes, and a bulk diffusion coefficient D0 = 1.5 10− 6 cm2/s for the electrolyte [39]. The result is a smaller time constant θkS + ke and a smaller tortuosity for the templated electrodes (τ = 1.38 for templated; τ = 2.55 for non-templated electrodes). The first effect indicates that in the templated electrode a larger fraction of the ionic diffusion passes through the electrolyte rather than through the solid state (θtemplated b θnon-templated) making the charge transport easier due to the larger diffusion coefficient of the electrolyte. The lower tortuosity indicates approximately 46% reduction in the path length of the Li-ions towards the electrode material compared to the non-templated electrode. This is consistent with a better connectivity of the pores and enhanced ionic wiring as schematically shown in Fig. 2(b). 4. Conclusion To enhance the power density of Li-ion battery electrodes without compromising the loading density a simple and cost effective templating technique is developed using ammonium bicarbonate salt. The solid templating material makes compaction possible, as
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