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Muon spin relaxation in Li0.6TiO2 anode material
Solid State Ionics 177 (2006) 145 – 147 www.elsevier.com/locate/ssi
Muon spin relaxation in Li0.6TiO2 anode material P.C.M. Gubbens a,*, M. Wagemaker a, S. Sakarya a, M. Blaauw b, A. Yaouanc c, P. Dalmas de Re´otier c, S.P. Cottrell d a
Department of Radiation, Radionuclides and Reactors, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands b Reactor Instituut Delft, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands c Commissariat l’Energie Atomique, De´partement de Recherche Fondamentale sur la Matie`re Condense´e, F-38054, Grenoble Cedex 9, France d ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire OX11 0QX, UK Received 18 April 2005; received in revised form 17 August 2005; accepted 20 September 2005
Abstract In Li0.6TiO2 the longitudinal muon spin relaxation function has been measured for temperatures between 10 and 600 K. The ASR spectra were analyzed with a Markov process for multiple collisions. The time scale found for the Li+ diffusion is of the order of the microsecond or shorter. Above T = 100 K the magnetic field distribution at the muon is decreasing with increasing temperature. D 2005 Elsevier B.V. All rights reserved. PACS: 76.75+i; 66.30Hs Keywords: Lithium-ion batteries; Muon spin relaxation; Li-titanate
1. Introduction TiO2 anatase is capable of hosting a high concentration of Li-ions that can be inserted chemically or electrochemically. The lithium insertion involves a number of changes. In the first place the crystallographic structure of the TiO2 anatase host progressively changes to a structure that we refer to as Lititanate [1]. Until the original microsized anatase particle is completely converted into Li-titanate, the two phases coexist as sub-micron domains [2,3], the original anatase phase with composition Liå 0.026TiO2 and the Li-titanate phase with composition Liå 0.52TiO2. In order to maintain charge neutrality, the inserted Li-ions are accompanied by electrons which results in a change in the electric and optical properties [4]. This is directly observed as the original white powder turns dark blue, whilst in thin film from the initial transparent film it becomes partially reflecting. The optical changes are applicable in electrochromic devices [5], whereas the storage of lithium ions in the TiO2 anatase
* Corresponding author. Tel.: +31 152785574; fax: +31 152788303. E-mail address: [email protected] (P.C.M. Gubbens). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.014
lattice makes it a candidate for electrode material in Li-ion batteries [6]. In a battery ionic transport within the electrode is of great importance as it will determine the maximum current and the capacity. In addition, the constant electrochemical potential is maintained by the equilibrium Li-ion flux between the two coexisting phases. Using high field solid state nuclear magnetic resonance (NMR) this equilibrium flux was observed on millisecond time scale and 100 nm length scale at room temperature [7]. According to the same NMR technique, on the much faster microsecond time scale, diffusion takes place at room temperature within each of the separate phases [2]. Using neutron diffraction on a Li0.12TiO2 sample with mixed anatase and titanate phases, it appeared that, due to the distortion of the oxygen octahedra and the small Li-ion size, there are two possible positions for the Li-ions in each octahedron. At room temperature Li is hopping between these two sites inside one octahedron on a picosecond time scale (quasi-elastic neutron scattering results and molecular dynamics simulations) [8]. In this study the lithium ion mobility is investigated with the muon spin relaxation technique (ASR). The muon depolarization offers a sensitive probe for processes that take place on the microsecond time scale, such as the Li-ion mobility in Li-titanate. Therefore, it offers valuable complementary infor-
P.C.M. Gubbens et al. / Solid State Ionics 177 (2006) 145 – 147
Asymmetry
146
0.25
3. Results and discussion
0.20
In Figs. 1 and 2 ASR spectra measured at different temperatures and external fields are shown. For analysis of these spectra we have used the strong collision approximation. In this model it is assumed that the local field at the muon site changes orientation and magnitude with a single fluctuation rate m, and a probability distribution of exp( mt). After collision, the field takes a new value randomly chosen from a field distribution without any correlation to the field before the collision. This process is also known as a Markov process. For the distribution we shall assume a Gaussian distribution of field with a width D / c A. Therefore, before the first collision the polarization should follow the static Kubo – Toyabe function (see e.g. Ref. [11]). After the first collision the ensemble will depolarize, following a Kubo –Toyabe function p Z (t), but with the initial time taken zero at the time of the collision. This process can be written down for multiple collisions and leads to [13]: Z t PZ ðt Þ ¼ emt pZ ðt Þ þ m pZ ðt1 ÞpZ ðt t1 Þdt1
0.15
Li 0.6 TiO 2
0.10
zero-field
0.05
10 K 300 K
0.00 0
5
10
15
Time (µs) Fig. 1. Typical ASR spectra, measured at T = 10 and 300 K in zero field. The lines are fit to a model explained in the main text.
mation, which can be compared with previous NMR results. The results of the ASR measurements on the Li0.6TiO2 titanate sample will be discussed in connection with earlier published results on Lix [Mn1.96Li0.04]O4 [9].
0
2. Experimental
þ m2
0
t2
pZ ðt1 ÞpZ ðt2 t1 ÞpZ ðt t2 Þdt1 dt2 þ N : ð1Þ
0
mt
e p Z (t) represents the product of the static Kubo –Toyabe function with the probability of having no field change between 0 and t. The subsequent terms in the right hand side of the equation represent the contribution to the depolarization function in the case of 1, 2 and more field changes between 0 and t. It can be shown that P Z (t) is solution of the following equation PZ ðt Þ ¼ pZ ðt Þexpð mt Þ Z t þ m PZ ðt tVÞpZ ðt VÞexpð mt V Þdt V:
ð2Þ
0
This expression in general can not be expressed analytically and needs to be solved numerically. With Eq. (2) we have analyzed the spectra shown in Figs. 1 and 2. As shown in Fig. 2 a common value of D / c A for all fields at one certain temperature could be determined. The frequency correlation m = 0.20 (20) MHz is very nearly constant over the whole temperature range. The
0.25
Asymmetry
Microcrystalline anatase TiO2 (99%) was obtained from Janssen Chemica. The Li0.6TiO2 sample was prepared by chemical intercalation of the pure powder with n-butyllithium (1.6 M Aldrich) [10]. The powder was mixed with hexane, and n-butyllithium was added while stirring the mixture to obtain a homogeneously intercalated sample. All sample preparations were carried out in a glove box in an inert argon atmosphere to prevent reaction of Li with air or humidity. After preparation, the sample was subjected to wet-chemical inductively coupled plasma spectroscopy (ICP) analysis, which confirmed the intended Li/Ti ratio of 0.6. This means that our sample is a titanate single phase. The ASR technique uses the positive muon as a local magnetic probe. The muon is a spin 1/2 particle with a gyromagnetic ratio c A = 851.6 Mrad s1 T1. The polarized muons are implanted into the sample where their polarization evolves in the local magnetic field until they decay (the muon lifetime is 2.2 As). The decay positron is emitted preferentially along the muon spin direction. By collecting several million positrons as a function of the evolution time, one can construct the time dependence of the muon spin polarization, which, in turn, reflects the magnitude of the magnetic field at the muon site. We used the longitudinal geometry, where an external magnetic field (B ext) can be applied on the sample along the direction of the initial muon beam polarization taken as the Z direction. The positrons are collected with detectors placed in the forward and backward directions relative to the Z direction. With this setup, the depolarization in the Z direction, the timedependent asymmetry aPZ(t), is measured. More detailed information about the ASR technique can be found in Refs. [11,12]. The ASR measurements were performed on the EMU experimental setup at the ISIS facility of the Rutherford Appleton Laboratory, Didcot, England. We have used a He cryostat and a furnace. The sample was mounted in a closed Ti sample holder under Ar atmosphere to avoid contact with air.
Z tZ
0.20
Li 0.6 TiO 2
0.15
300 K
0.10
zero-field 0.5 mT 1 mT
0.05 0.00 0
5
10
15
Time (µs) Fig. 2. Spectra recorded at T = 300 K as a function of the intensity of the longitudinal field. The different lines are fits obtained by simply varying the magnitude of the external field while keeping D/c A and m constant.
Field width ∆/γµ (mT)
P.C.M. Gubbens et al. / Solid State Ionics 177 (2006) 145 – 147
0.25
Li 0.6 TiO 2
0.20 0.15 0.10 0.05 0.00 0
100
200
300
400
500
600
Temperature (K) Fig. 3. Temperature dependence of the field width at the muon site in Li0.6TiO2. The dashed line was drawn to guide the eyes.
temperature dependence of the field width D / c A is shown in Fig. 3. A decrease starts around 100 K. The highest measured temperature (600 K) could not be analyzed with confidence. However, after heating Li0.6TiO2, unlike for Li[Mn1.96Li0.04]O4 [9], the results at 300 K could be reproduced, which means that there is no sign of degradation. We now recall some results obtained for Li[Mn1.96Li0.04]O4. It was found that, above 200 K, D / c A decreases as the temperature of the sample is increased [9]. In the case the Li-ions move locally between a limited number of fixed sites, simulations show that the spectra are qualitatively unchanged. The effect of a rapid motion of Li between these sites leads only to a change in the value of D / c A. A numerical study was undertaken for a large variety of local (short range) movements between different sites and resulted in a variation of at most 0.024 mT compared to the case of a static Li [14]. This value is far too small to explain the experimental change in D / c A of about 0.08 mT. In fact this change is very well explained if some Liions do not contribute any longer to the field distribution at the muon site. This can occur if the Li-ions diffuse over the 8a and 16c sites on a microsecond time scale [9,15]. The aforementioned assignments of Li sites are in agreement with the results of an earlier neutron diffraction study [16]. The same site assignments were also found with NMR [17]. However, in this case, the time scale of the diffusion is proposed to be in the millisecond range. Until now this contradiction between ASR and NMR in time scale is unexplained. In a similar way, the decrease of D / c A above about 100 K observed for Li0.6TiO2 (shown in Fig. 3) proves that the Li-ions start to escape the muon surrounding above that temperature. That means that the correlation time of these ions is in the microsecond range or faster. It is consistent with the NMR results above 250 K [2]. However, especially at low temperatures the results obtained here for Li0.6TiO2 are different with those of the NMR study in Li0.12TiO2. The latter system consists of anatase and titanate phases with in the last phase the
147
same amount of Li. The rather small and constant correlation frequency m means that the muon is quasistatic in the whole temperature range. When the sample is single phase as in the case investigated here, the NMR and ASR results seem to be inconsistent at low temperatures. For ASR above 100 K the measured time scale is in the microsecond range or faster. Although the origin of the discrepancy between time scales measured with NMR and ASR for Lix[Mn1.96Li0.04]O4 is also still not understood. The other results, the temperature range of the Li diffusion and the sites between which the Li-ions diffuse, agree very well. Acknowledgements This work is a contribution from the Delft Institute for Sustainable Energy (DISE). Financial support from The Netherlands Organization for Scientific Research (NWO) is gratefully acknowledged. The muon spectroscopy measurements were performed at the EMU setup of the ISIS facility of the Rutherford Appleton Laboratory in England. References [1] R.J. Cava, D.W. Murphy, S. Zahurak, A. Santoro, R.S. Roth, J. Solid State Chem. 53 (1984) 64. [2] M. Wagemaker, R. van de Krol, A.P.M. Kentgens, A.A. van Well, F.M. Mulder, J. Am. Chem. Soc. 123 (2001) 11454. [3] R. van de Krol, A. Goossens, J. Schoonman, J. Phys. Chem., B 103 (1999) 7151. [4] R. van de Krol, A. Goossens, E.A. Meulenkamp, J. Appl. Phys. 90 (2001) 223. [5] C. Bechinger, S. Ferrere, A. Zaban, J. Sprague, B.A. Gregg, Nature 383 (1996) 608. [6] S.Y. Huang, L. Kavan, I. Exnar, M. Gratzel, J. Electrochem. Soc. 142 (1995) L142. [7] M. Wagemaker, A.P.M. Kentgens, F.M. Mulder, Nature 418 (2002) 397. [8] M. Wagemaker, G.J. Kearley, A.A. van Well, H. Mutka, F.M. Mulder, J. Am. Chem. Soc. 125 (2003) 840. [9] C.T. Kaiser, V.W.J. Verhoeven, P.C.M. Gubbens, F.M. Mulder, I. de Schepper, A. Yaouanc, P. Dalmas de Re´otier, S.P. Cottrell, E.M. Kelder, J. Schoonman, Phys. Rev., B 62 (2000) R9236. [10] M.S. Wittingham, M.B. Dimes, J. Electrochem. Soc. 124 (1977) 1387. [11] P. Dalmas de Re`otier, A. Yaouanc, J. Phys.: Condens. Matter 9 (1997) 9113. [12] E.B. Karlsson, Solid State Phenomena as Seen by Muons, Protons and Excited Nuclei, Clarendon Press, Oxford, 1995. [13] R.S. Hayano, Y.J. Uemura, J. Imazato, N. Nishida, T. Yamazaki, R. Kubo, Phys. Rev., B 20 (1979) 850. [14] C.T. Kaiser, Thesis 2001, Delft University of Technology. [15] M.J. Ariza, D.J. Jones, J. Rozie`re, J.S. Lord, D. Ravot, J. Phys. Chem., B 107 (2003) 6003. [16] H. Berg, E. Kelder, J.O. Thomas, J. Mater. Chem. 9 (1999) 427. [17] V.W.J. Verhoeven, I.M. de Schepper, G. Nachtegaal, A.P.M. Kentgens, E.M. Kelder, J. Schoonman, F.M. Mulder, Phys. Rev. Lett. 86 (2001) 4314.
Muon spin relaxation in Li0.6TiO2 anode material P.C.M. Gubbens a,*, M. Wagemaker a, S. Sakarya a, M. Blaauw b, A. Yaouanc c, P. Dalmas de Re´otier c, S.P. Cottrell d a
Department of Radiation, Radionuclides and Reactors, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands b Reactor Instituut Delft, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands c Commissariat l’Energie Atomique, De´partement de Recherche Fondamentale sur la Matie`re Condense´e, F-38054, Grenoble Cedex 9, France d ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxfordshire OX11 0QX, UK Received 18 April 2005; received in revised form 17 August 2005; accepted 20 September 2005
Abstract In Li0.6TiO2 the longitudinal muon spin relaxation function has been measured for temperatures between 10 and 600 K. The ASR spectra were analyzed with a Markov process for multiple collisions. The time scale found for the Li+ diffusion is of the order of the microsecond or shorter. Above T = 100 K the magnetic field distribution at the muon is decreasing with increasing temperature. D 2005 Elsevier B.V. All rights reserved. PACS: 76.75+i; 66.30Hs Keywords: Lithium-ion batteries; Muon spin relaxation; Li-titanate
1. Introduction TiO2 anatase is capable of hosting a high concentration of Li-ions that can be inserted chemically or electrochemically. The lithium insertion involves a number of changes. In the first place the crystallographic structure of the TiO2 anatase host progressively changes to a structure that we refer to as Lititanate [1]. Until the original microsized anatase particle is completely converted into Li-titanate, the two phases coexist as sub-micron domains [2,3], the original anatase phase with composition Liå 0.026TiO2 and the Li-titanate phase with composition Liå 0.52TiO2. In order to maintain charge neutrality, the inserted Li-ions are accompanied by electrons which results in a change in the electric and optical properties [4]. This is directly observed as the original white powder turns dark blue, whilst in thin film from the initial transparent film it becomes partially reflecting. The optical changes are applicable in electrochromic devices [5], whereas the storage of lithium ions in the TiO2 anatase
* Corresponding author. Tel.: +31 152785574; fax: +31 152788303. E-mail address: [email protected] (P.C.M. Gubbens). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.09.014
lattice makes it a candidate for electrode material in Li-ion batteries [6]. In a battery ionic transport within the electrode is of great importance as it will determine the maximum current and the capacity. In addition, the constant electrochemical potential is maintained by the equilibrium Li-ion flux between the two coexisting phases. Using high field solid state nuclear magnetic resonance (NMR) this equilibrium flux was observed on millisecond time scale and 100 nm length scale at room temperature [7]. According to the same NMR technique, on the much faster microsecond time scale, diffusion takes place at room temperature within each of the separate phases [2]. Using neutron diffraction on a Li0.12TiO2 sample with mixed anatase and titanate phases, it appeared that, due to the distortion of the oxygen octahedra and the small Li-ion size, there are two possible positions for the Li-ions in each octahedron. At room temperature Li is hopping between these two sites inside one octahedron on a picosecond time scale (quasi-elastic neutron scattering results and molecular dynamics simulations) [8]. In this study the lithium ion mobility is investigated with the muon spin relaxation technique (ASR). The muon depolarization offers a sensitive probe for processes that take place on the microsecond time scale, such as the Li-ion mobility in Li-titanate. Therefore, it offers valuable complementary infor-
P.C.M. Gubbens et al. / Solid State Ionics 177 (2006) 145 – 147
Asymmetry
146
0.25
3. Results and discussion
0.20
In Figs. 1 and 2 ASR spectra measured at different temperatures and external fields are shown. For analysis of these spectra we have used the strong collision approximation. In this model it is assumed that the local field at the muon site changes orientation and magnitude with a single fluctuation rate m, and a probability distribution of exp( mt). After collision, the field takes a new value randomly chosen from a field distribution without any correlation to the field before the collision. This process is also known as a Markov process. For the distribution we shall assume a Gaussian distribution of field with a width D / c A. Therefore, before the first collision the polarization should follow the static Kubo – Toyabe function (see e.g. Ref. [11]). After the first collision the ensemble will depolarize, following a Kubo –Toyabe function p Z (t), but with the initial time taken zero at the time of the collision. This process can be written down for multiple collisions and leads to [13]: Z t PZ ðt Þ ¼ emt pZ ðt Þ þ m pZ ðt1 ÞpZ ðt t1 Þdt1
0.15
Li 0.6 TiO 2
0.10
zero-field
0.05
10 K 300 K
0.00 0
5
10
15
Time (µs) Fig. 1. Typical ASR spectra, measured at T = 10 and 300 K in zero field. The lines are fit to a model explained in the main text.
mation, which can be compared with previous NMR results. The results of the ASR measurements on the Li0.6TiO2 titanate sample will be discussed in connection with earlier published results on Lix [Mn1.96Li0.04]O4 [9].
0
2. Experimental
þ m2
0
t2
pZ ðt1 ÞpZ ðt2 t1 ÞpZ ðt t2 Þdt1 dt2 þ N : ð1Þ
0
mt
e p Z (t) represents the product of the static Kubo –Toyabe function with the probability of having no field change between 0 and t. The subsequent terms in the right hand side of the equation represent the contribution to the depolarization function in the case of 1, 2 and more field changes between 0 and t. It can be shown that P Z (t) is solution of the following equation PZ ðt Þ ¼ pZ ðt Þexpð mt Þ Z t þ m PZ ðt tVÞpZ ðt VÞexpð mt V Þdt V:
ð2Þ
0
This expression in general can not be expressed analytically and needs to be solved numerically. With Eq. (2) we have analyzed the spectra shown in Figs. 1 and 2. As shown in Fig. 2 a common value of D / c A for all fields at one certain temperature could be determined. The frequency correlation m = 0.20 (20) MHz is very nearly constant over the whole temperature range. The
0.25
Asymmetry
Microcrystalline anatase TiO2 (99%) was obtained from Janssen Chemica. The Li0.6TiO2 sample was prepared by chemical intercalation of the pure powder with n-butyllithium (1.6 M Aldrich) [10]. The powder was mixed with hexane, and n-butyllithium was added while stirring the mixture to obtain a homogeneously intercalated sample. All sample preparations were carried out in a glove box in an inert argon atmosphere to prevent reaction of Li with air or humidity. After preparation, the sample was subjected to wet-chemical inductively coupled plasma spectroscopy (ICP) analysis, which confirmed the intended Li/Ti ratio of 0.6. This means that our sample is a titanate single phase. The ASR technique uses the positive muon as a local magnetic probe. The muon is a spin 1/2 particle with a gyromagnetic ratio c A = 851.6 Mrad s1 T1. The polarized muons are implanted into the sample where their polarization evolves in the local magnetic field until they decay (the muon lifetime is 2.2 As). The decay positron is emitted preferentially along the muon spin direction. By collecting several million positrons as a function of the evolution time, one can construct the time dependence of the muon spin polarization, which, in turn, reflects the magnitude of the magnetic field at the muon site. We used the longitudinal geometry, where an external magnetic field (B ext) can be applied on the sample along the direction of the initial muon beam polarization taken as the Z direction. The positrons are collected with detectors placed in the forward and backward directions relative to the Z direction. With this setup, the depolarization in the Z direction, the timedependent asymmetry aPZ(t), is measured. More detailed information about the ASR technique can be found in Refs. [11,12]. The ASR measurements were performed on the EMU experimental setup at the ISIS facility of the Rutherford Appleton Laboratory, Didcot, England. We have used a He cryostat and a furnace. The sample was mounted in a closed Ti sample holder under Ar atmosphere to avoid contact with air.
Z tZ
0.20
Li 0.6 TiO 2
0.15
300 K
0.10
zero-field 0.5 mT 1 mT
0.05 0.00 0
5
10
15
Time (µs) Fig. 2. Spectra recorded at T = 300 K as a function of the intensity of the longitudinal field. The different lines are fits obtained by simply varying the magnitude of the external field while keeping D/c A and m constant.
Field width ∆/γµ (mT)
P.C.M. Gubbens et al. / Solid State Ionics 177 (2006) 145 – 147
0.25
Li 0.6 TiO 2
0.20 0.15 0.10 0.05 0.00 0
100
200
300
400
500
600
Temperature (K) Fig. 3. Temperature dependence of the field width at the muon site in Li0.6TiO2. The dashed line was drawn to guide the eyes.
temperature dependence of the field width D / c A is shown in Fig. 3. A decrease starts around 100 K. The highest measured temperature (600 K) could not be analyzed with confidence. However, after heating Li0.6TiO2, unlike for Li[Mn1.96Li0.04]O4 [9], the results at 300 K could be reproduced, which means that there is no sign of degradation. We now recall some results obtained for Li[Mn1.96Li0.04]O4. It was found that, above 200 K, D / c A decreases as the temperature of the sample is increased [9]. In the case the Li-ions move locally between a limited number of fixed sites, simulations show that the spectra are qualitatively unchanged. The effect of a rapid motion of Li between these sites leads only to a change in the value of D / c A. A numerical study was undertaken for a large variety of local (short range) movements between different sites and resulted in a variation of at most 0.024 mT compared to the case of a static Li [14]. This value is far too small to explain the experimental change in D / c A of about 0.08 mT. In fact this change is very well explained if some Liions do not contribute any longer to the field distribution at the muon site. This can occur if the Li-ions diffuse over the 8a and 16c sites on a microsecond time scale [9,15]. The aforementioned assignments of Li sites are in agreement with the results of an earlier neutron diffraction study [16]. The same site assignments were also found with NMR [17]. However, in this case, the time scale of the diffusion is proposed to be in the millisecond range. Until now this contradiction between ASR and NMR in time scale is unexplained. In a similar way, the decrease of D / c A above about 100 K observed for Li0.6TiO2 (shown in Fig. 3) proves that the Li-ions start to escape the muon surrounding above that temperature. That means that the correlation time of these ions is in the microsecond range or faster. It is consistent with the NMR results above 250 K [2]. However, especially at low temperatures the results obtained here for Li0.6TiO2 are different with those of the NMR study in Li0.12TiO2. The latter system consists of anatase and titanate phases with in the last phase the
147
same amount of Li. The rather small and constant correlation frequency m means that the muon is quasistatic in the whole temperature range. When the sample is single phase as in the case investigated here, the NMR and ASR results seem to be inconsistent at low temperatures. For ASR above 100 K the measured time scale is in the microsecond range or faster. Although the origin of the discrepancy between time scales measured with NMR and ASR for Lix[Mn1.96Li0.04]O4 is also still not understood. The other results, the temperature range of the Li diffusion and the sites between which the Li-ions diffuse, agree very well. Acknowledgements This work is a contribution from the Delft Institute for Sustainable Energy (DISE). Financial support from The Netherlands Organization for Scientific Research (NWO) is gratefully acknowledged. The muon spectroscopy measurements were performed at the EMU setup of the ISIS facility of the Rutherford Appleton Laboratory in England. References [1] R.J. Cava, D.W. Murphy, S. Zahurak, A. Santoro, R.S. Roth, J. Solid State Chem. 53 (1984) 64. [2] M. Wagemaker, R. van de Krol, A.P.M. Kentgens, A.A. van Well, F.M. Mulder, J. Am. Chem. Soc. 123 (2001) 11454. [3] R. van de Krol, A. Goossens, J. Schoonman, J. Phys. Chem., B 103 (1999) 7151. [4] R. van de Krol, A. Goossens, E.A. Meulenkamp, J. Appl. Phys. 90 (2001) 223. [5] C. Bechinger, S. Ferrere, A. Zaban, J. Sprague, B.A. Gregg, Nature 383 (1996) 608. [6] S.Y. Huang, L. Kavan, I. Exnar, M. Gratzel, J. Electrochem. Soc. 142 (1995) L142. [7] M. Wagemaker, A.P.M. Kentgens, F.M. Mulder, Nature 418 (2002) 397. [8] M. Wagemaker, G.J. Kearley, A.A. van Well, H. Mutka, F.M. Mulder, J. Am. Chem. Soc. 125 (2003) 840. [9] C.T. Kaiser, V.W.J. Verhoeven, P.C.M. Gubbens, F.M. Mulder, I. de Schepper, A. Yaouanc, P. Dalmas de Re´otier, S.P. Cottrell, E.M. Kelder, J. Schoonman, Phys. Rev., B 62 (2000) R9236. [10] M.S. Wittingham, M.B. Dimes, J. Electrochem. Soc. 124 (1977) 1387. [11] P. Dalmas de Re`otier, A. Yaouanc, J. Phys.: Condens. Matter 9 (1997) 9113. [12] E.B. Karlsson, Solid State Phenomena as Seen by Muons, Protons and Excited Nuclei, Clarendon Press, Oxford, 1995. [13] R.S. Hayano, Y.J. Uemura, J. Imazato, N. Nishida, T. Yamazaki, R. Kubo, Phys. Rev., B 20 (1979) 850. [14] C.T. Kaiser, Thesis 2001, Delft University of Technology. [15] M.J. Ariza, D.J. Jones, J. Rozie`re, J.S. Lord, D. Ravot, J. Phys. Chem., B 107 (2003) 6003. [16] H. Berg, E. Kelder, J.O. Thomas, J. Mater. Chem. 9 (1999) 427. [17] V.W.J. Verhoeven, I.M. de Schepper, G. Nachtegaal, A.P.M. Kentgens, E.M. Kelder, J. Schoonman, F.M. Mulder, Phys. Rev. Lett. 86 (2001) 4314.