In Situ Neutron Powder Diffraction of a Nickel Hydroxide Electrode

According to Barnard's work,1-3 it was generally assumed34 that the reversible transformation of β-II into β-III occurs in three definite steps: an ...
0 downloads 0 Views 540KB Size
Download PDF
3936

Chem. Mater. 2004, 16, 3936-3948

In Situ Neutron Powder Diffraction of a Nickel Hydroxide Electrode F. Barde´,*,† M. R. Palacin,‡ Y. Chabre,§ O. Isnard,|,⊥ and J.-M. Tarascon† Laboratoire de Re´ activite´ et de Chimie des Solides, Universite´ de Picardie Jules Verne, CNRS UMR 6007, 80039 Amiens, France, Institut de Ciencia de Materiales de Barcelona, ICMAB-CSIC, Campus UAB, 08193 Bellaterra, Catalonia, Spain, Laboratoire de Spectrome´ trie Physique, Universite´ Joseph Fourier, CNRS, BP87, 38402 Saint Martin d’He` res, France, Laboratoire de Cristallographie, Universite´ Joseph Fourier, CNRS, BP166, 38042 Grenoble, France, and Institut Laue Langevin, BP 156X, F-38042 Grenoble, France Received April 1, 2004. Revised Manuscript Received July 13, 2004

The redox processes occurring at the nickel oxyhydroxide electrodes were followed by in situ neutron diffraction. The aim was to get a deeper insight into the existing phases and the reactivity mechanisms involved in the reduction process, paying special attention to the so-called “second plateau” phenomenon, occasionally appearing during electrochemical reduction at a potential of 0.8 V vs Hg/HgO. Chemically prepared protonated or deuterated nickel hydroxides, having different phase compositions, oxidation state, and particle size were studied to serve as reference samples. The electrochemically driven structural evolution of four samples upon discharge and charge was followed by in situ neutron powder diffraction using a specially designed cell. For both γ and β-NiOOH phases, the neutron diffraction results evidenced a direct and continuous structural transformation into the β-Ni(OH)2 phase upon reduction, on both the first and the second plateau, with no discontinuity when encountering the second plateau. This confirms that the second plateau phenomenon is not due to any intrinsic structural properties of the active material but is related to its surface properties being prone to be strongly dependent upon the electrode preparation.

Introduction Nickel-based alkaline batteries (Ni/Cd, Ni/MH, or Ni/ H2) are still widely used nowadays, despite the emergence of the lithium technology. Their domain of application is extremely broad and ranges from power tools to portable electronics, electric vehicles, and space applications. The performances of these batteries appear to be limited by the efficiency of their positive electrode, which is governed by the Ni(OH)2 T NiOOH couple. Although the first patents involving this electrode go back to the end of the 19th century, our understanding of the mechanisms and phases involved in its oxidationreduction process (charge-discharge of the battery) is still not complete. For example, the crystal structure of the oxidized phase is not known. Moreover, there exists a phenomenon termed “second plateau” corresponding to a partial transfer of the usual 1.2 V capacity to a 0.8 V potential plateau. The origin of this phenomenon, known to be detrimental to the electrochemical performances of this nickel oxyhydroxide electrode (NOE), is still controversial. Several explanations to account for such behavior were put forward. Among them are (i) the formation of a barrier insulating layer at the active material/electron collector interface as deduced from †

Universite´ de Picardie Jules Verne. ICMAB-CSIC. Laboratoire de Spectrome´trie Physique, Universite´ Joseph Fourier. | Laboratoire de Cristallographie, Universite ´ Joseph Fourier. ⊥ Institut Laue Langevin. ‡ §

electrochemical studies performed by Barnard et al.,1-3 namely the presence of an ohmic drop; (ii) the presence of γ-NiOOH phase in the oxidized electrode, as deduced from chemical/electrochemical investigations performed by Sac-Epe´e et al.;4 or (iii) the existence of an insulating almost stoichiometric phase, Ni(OH)2-, in the vicinity of the current collector, also deduced from chemical/ electrochemical investigations performed by Le´ger et al.5 None of these explanations seems to be completely satisfactory. On one hand explanations i and iii fail to account for the amount of capacity delivered on the second plateau, contrary to hypothesis ii that correlated the amount of the second plateau to the amount of γ-NiOOH. But on the other hand, hypothesis ii is also dismissed by the fact that it has been confirmed that the second plateau can also be observed in the absence of the γ-NiOOH phase in the electrode. Thus, the origin (or origins!) of this phenomenon remains an open question that requires further investigation explaining the motivation of the present study. (1) Barnard, R.; Crickmore, G. T.; Lee, J. A.; Tye, F. L. J. Appl. Electrochem. 1980, 10, 61-70. (2) Barnard, R.; Randell, C. F.; Tye, F. L. J. Appl. Electrochem. 1980, 10, 109-125. (3) Barnard, R.; Randell, C. F.; Tye, F. L. J. Appl. Electrochem. 1980, 10, 127-141. (4) Sac-Epee, N.; Palacin, M.-R.; Beaudoin, B.; Delahaye-Vidal, A.; Jamin, T.; Chabre, Y.; Tarascon, J.-M. J. Electrochem. Soc. 1996, 144, 11 3896-3907. (5) Le´ger, C.; Tessier, C.; Me´ne´trier, M.; Denage, C.; Delmas, C. J. Electrochem. Soc. 1999, 146, 3, 924-932.

10.1021/cm0401286 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/08/2004

In Situ NPD of a Ni(OH)2 Electrode

Chem. Mater., Vol. 16, No. 20, 2004 3937

for NOE electrodes crystallizing either in an R-, β-, or γ-form, each having different oxidation states and particle sizes, with the aim to gain further insight in the phenomenon of the second plateau as well as in the reaction mechanisms taking place in the NOE in general. Because the crystal structure for some of the nickel hydroxide/oxyhydroxide phases is still not fully determined, batches of reduced and oxidized “model phases” were chemically prepared to serve as references for comparison purposes. Investigations were made mainly on cells containing β-Ni(OH)2 and γ-NiOOH to try to ascertain the influence of the latter on the second plateau phenomenon, as previously proposed by Sac Epe´e et al.11

Figure 1. Bode’s diagram.

Commercial Ni alkaline cells utilize the β-II/β-III redox couple with therefore the possibility of having the γ-III-phases as a result of overoxidation. The intrinsic complexity of the NOE is due to the interdependence of multiple structural, chemical, and electrochemical parameters that have been described in many papers, among which those written by Oliva et al.6 and by Mc Breen et al.7 deserve a special mention. Four main phases (Figure 1) can appear during the redox processes of the NOE, as described by Bode et al.8 All of them present layered structures formed by slabs of edgesharing NiO6 octahedra, between which are located protons as well as, in some cases, water molecules or other cationic or anionic species. The two phases existing in the reduced state are denoted R-Ni(OH)2 and β-Ni(OH)2, commonly referred to as R-II and β-II. The chemical formula Ni(OH)2 is only true for the β-type phase, which solely contains protons in the interlayer spacing. The R-type phase has a turbostratic-disordered structure with a larger interlayer distance that can host variable amounts of water molecules and anionic species. Two phases are also known in the oxidized state: β-NiOOH and γ-NiOOH (denoted also β-III and γ-III), the chemical formula being true for the former only. The γ-type phase, as the R-type one, presents a large interlayer spacing that contains cations and water molecules. Typically, the R-II-phase is unstable in alkaline medium, evolving into β-II upon aging or into γ-III upon oxidation. The latter might also appear as a result of overoxidation of β-III. Besides these structural aspects, other factors that determine the electrochemical performances of nonsubstituted nickel hydroxide are the particle and crystallite size and the presence of defects. According to the literature,9,10 the best electrochemically optimized β-II materials are those with small particle size presenting a mosaic structure and high defects content. In this paper, we present an in situ neutron powder diffraction (NPD) study of the charge/discharge process (6) Oliva, P.; Leonardi, J.; Laurent, J.-F.; Delmas, C.; Braconnier, J-J.; Figlarz, M.; Fievet, F.; de Guibert, A. J. Power Sources 1982, 8, 229-255. (7) McBreen, J.; White, R. E.; Bockris, J. O’M.; Conway, B. E. Modern aspects of electrochemistry; Plenum Press: New York, 1990; Vol. 21, pp 29-63. (8) Bode, H.; Dehmelt, K.; Witte, J. Electrochem. Acta 1966, 11, 1079. (9) Tessier, C.; Haumesser, P. H.; Bernard, P.; Delmas, C. J. Electrochem. Soc. 1999, 146, 6, 2059-2067. (10) Delmas, C.; Tessier, C. J. Mater. Chem. 1997, 7, 8, 1439-1443.

Experimental Section Samples were synthesized in protonated or deuterated media using products from Aldrich and C/D/N Isotopes (99% purity). Deuterated media was used to lower the incoherent scattering contribution due to protons when performing NDP experiments. Furthermore, owing to the rapid protondeuterium exchange upon electrochemical cycling in deuterated electrolyte during the in situ experiments of the protonated samples, NPD were collected as references for fully deuterated samples. X-ray powder measurements were performed on a Philips diffractometer PW1710 with Cu KR radiation (λ ) 1.54059 Å). The morphology and particle size of the samples were determined by transmission electron microscopy (TEM) using a Philips CM12. Infrared spectra of different samples were collected on a 510 FT-IR Nicolet apparatus in the transmittance mode in the 4000-400 cm-1 domain with a 4 cm-1 resolution. Typically, the pellets were prepared by compacting to 8 tons 200 mg of KBr that contains about 1% of the sample. Electrochemical studies were performed in both galvanostatic and potentiodynamic modes (GITT, PITT) in threeelectrode configurations, using a 5 N KOH electrolyte solution, a Ni wire as counter electrode, and an Hg/HgO electrode as reference. The positive electrode consisted of a mixture of the active material (i.e. the nickel hydroxide or oxyhydroxide to be tested) and MCMB 25-28 carbon (Meso-Carbon MicroBeads) in a 1:5 ratio. The PITT data have been obtained with a -5 mV potential step, the potential stepping occurring once the amplitude of the reduction current has decreased to -0.3 mA/g (i.e. close to an equivalent C/1000 galvanostatic regime). Neutron diffraction experiments were performed on either D1B or D20 high flux powder diffractometers at the Institut Laue Langevin (ILL). On D1B, wavelengths λ ) 1.28 or 2.52 Å were used to collect data over 2θ domains ranging from 6° to 86° and 10° to 90°, respectively; on the D20, diagrams were collected at λ ) 2.41 Å on a 2θ domain ranging from 5° to 155°. Data acquisition times of 10 and 20 min were sufficient enough to obtain good statistics for analysis at large and small wavelengths, respectively. For such in situ neutron diffraction experiments, a specially designed cell was used. It is made of a silica vessel, with nickel grid cylinders and Cd/Cd(OH)2 as counter and reference electrodes, as shown in Figure 2.12 To decrease the large incoherent scattering contribution due to the proton, a 5 M NaOD electrolyte, rather than the classical NaOH electrolyte, was used. The positive electrodes were prepared as follows: 3-4 g of active material were mixed with Printex carbon and Teflon binder in an 80:15:5 weight ratio. The obtained paste was spread into 3 × 5 cm sheets about 1 mm thick. Two of these sheets were pressed at 1 ton/cm2 against a nickel foam sheet. Then this assembly was rolled into a hollow cylinder of 10 mm diameter, 5 cm high, to fit (11) Sac-Epee, N.; Palacin, M.-R.; Delahaye-Vidal, A.; Chabre, Y.; Tarascon, J.-M. J. Electrochem. Soc. 1998, 145, 5, 1434-1441. (12) Latroche, M.; Chabre, Y.; Percheron-Gue´gan, A.; Isnard, O.; Knosp, B. J. Alloys Compd. 2002, 787, 330-332.

3938

Chem. Mater., Vol. 16, No. 20, 2004

Figure 2. Electrochemical cell used for the in situ neutron diffraction experiments.

Barde´ et al.

Figure 3. X-ray diffraction patterns of reference samples synthesized in hydrogenated or deuterated media: (a) reduced phases of type R-II and β-II and (b) oxidized phases of type β-III and γ-III. H stands for hydrogenated, D for deuterated, and the last number refers to the average diameter of the platelets.

into the in situ NPD cell. The electrochemical monitoring was performed with a MacPile potentiostat/galvanostat.

Results and Discussion For reason of clarity, this section will consist of three parts: first the syntheses and characterization of the Ni phases (protonated and deuterated) will be detailed, and then their electrochemistry reported, and finally their structural aspects and evolution as deduced by NPD when charged and discharged in Ni alkaline in situ NPD electrochemical cells explained. In each section, the different phases will be treated separately. Finally, to make it easier for the reader, a systematic nomenclature has been used to name the various samples. Within such a nomenclature, letter R, β, or γ stands for the R-, β-, or γ-type phase, while the number 2 or 3 denotes the nickel average oxidation state. The capital letter H or D indicates if the samples were prepared in protonated or deuterated media. Finally, the number 1200, 250, or 125 refers to the average diameter (in angstroms) of the platelets of the active material. For example, β2D1200 corresponds to a β-Ni(OD)2 phase having an average size particle diameter of 1200 Å. 1. Syntheses and Characterization of Reference Nickel Samples. 1.1. R-II-Phases. These phases were synthesized according to Le Bihan’s method,13 for which 500 mL of a 1 M nickel nitrate solution prepared from Ni(NO3)2‚6H2O was added to 65 mL of ammonia solution under vigorous stirring at room temperature. After 4 h of reaction time, the obtained green precipitate was copiously washed with water and acetone prior to being dried at room temperature and characterized for its structural and textural properties. A similar protocol, but substituting H2O and NH4OH for D2O and ND4OD, respectively, was used for prepar-

ing deuterated samples in order to avoid the proton contamination. Furthermore, prior to being dissolved in heavy water, the hydrated nickel nitrate was dried at 55 °C in order to remove the water molecules. Heavy water was used for the washings to avoid H-D exchange. From the X-ray diffraction patterns (Figure 3a), the above products were both identified to crystallize into an R-II-Ni(OH)2 type structure, and were denoted R2H and R2D for R-II-Ni(OH)2 and R-II-Ni(OD)2, respectively. The Bragg reflections are broad, indicative of small grain sizes and eventually poor crystallinity. Furthermore, some of them are also highly asymmetric, highlighting the turbostratic character of these phases. From TEM observations (Figure 4(a)), the R2D sample appears as aggregates of very thin tangled fibers without any particular morphology, as commonly observed for the protonated R2H sample.13 The samples were further characterized by means of infrared spectroscopy. As expected, a broad band at 3450 cm-1 (indicating the presence of interlayer water molecules14) appears clearly on the infrared spectra of both R2H and R2D (Figure 5a). For the deuterated phase

(13) Le Bihan, S.; Figlarz, M. C. R. Acad. Sci. 1970, 270, 21312133.

(14) Le Bihan, S.; Guenot, J.; Figlarz, M. Thermochim. Acta 1973, 6, 319-326.

Figure 4. Transmission electron microscopy photos performed on several nickel deuterioxides: (a) R-II-Ni(OD)2 denoted R2D, (b) β-II-Ni(OD)2 denoted β2D125, and (c) β-II-Ni(OD)2 denoted β2D1200.

In Situ NPD of a Ni(OH)2 Electrode

Figure 5. Infrared spectra of R-II-type (a), or β-II-type nickel hydroxides and deuterioxides presenting either 125 Å (b) or 1200 Å (c) average particle diameter. β125′ and β21200′ are the samples prepared by exchange.

(R2D), the additional broad band at 2518 cm-1 was interpreted as the consequence of the presence of D2O within the structure. This line position in wavenumber is in good agreement with that calculated for the band corresponding to (2520 cm-1) O-D bonding, assuming that the ratio of O-H versus O-D frequencies equals 1.37.15 1.2. β-II-Phases. To prepare well-crystallized β-IIphases, 6 g of R-II-Ni(OH)2 precursor (i.e. R2H specimens) were treated for 6 h under 1.2 atm pressure at 120 °C in the presence of 500 mL of H2O16 or D2O. The resulting products, denoted β2H1200 and β2D1200 were characterized by means of X-ray diffraction (Figure 3a). They both present very sharp Bragg reflections that are easily indexed in the P-3m1 space group according to JCPD 14-0117. TEM micrographs (Figure 4c) of the deuterated hydroxide indicate that it consists of hexagonal platelets with an average diameter and thickness of 1200 Å and 350 Å, respectively. To prepare β-II-phases with smaller size particles, another synthesis route was explored. It consists of adding a 1 M nickel sulfate solution prepared from NiSO4‚7H2O to 125 mL of 2 N NaOH solution at 70 °C for 2 h.17 After being washed several times and dried for 15 h at 55 °C, the green precipitate was characterized. Its X-ray diffraction pattern is shown in Figure 3a. According to TEM observations (not presented here), this sample, denoted β2H125, appears as made of 125 Å average diameter and 30 Å thick hexagonal platelets. The same experiment was conducted under similar conditions starting from deuterated precursors (NaOD, D2O) and previously dried nickel sulfate. The resulting product, washed with heavy water and denoted β2D125, exhibits almost the same characteristics as β2H125 from (15) Hair, L. M. Infrared spectroscopy in surface chemistry; 1967; pp 28-29. (16) Le Bihan, S.; Figlarz, M. J. Cryst. Growth 1972, 13-14, 458435. (17) Tessier, C.; Haumesser, P. H.; Bernard, P.; Delmas, C. J. Electrochem. Soc. 1999, 146, 6, 2059-2067.

Chem. Mater., Vol. 16, No. 20, 2004 3939

a structural and morphologic point of view, as shown in Figures 3a and 4b. Finally, a sample (denoted β2H250) with intermediate particle size was synthesized by adding 250 mL of 1 M nickel nitrate solution to 28 mL of 28% NH4OH in the presence of 5 N KOH at 60 °C. Its X-ray diffraction pattern (Figure 3a) indicates a crystallinity degree intermediate between those of β2H1200 and β2H125. This was confirmed by TEM measurements that have indicated platelet particles having an average diameter of about 250 Å and thickness of about 100 Å. Being aware that an exchange between deuterium and a proton takes place instantaneously,18 an alternative synthesis of deuterioxides by a simple ion exchange reaction was considered. More specifically, a few grams of β2H1200 and β2H125 were placed and stirred in D2O for 8 h at room temperature. X-ray diffraction patterns (not presented here) of the exchanged samples, denoted β21200′ and β2125′, were similar to those of the precursors’, suggesting solely that if an ion exchange did take place, it did not enroll a phase transformation/decomposition. Since X-rays are meaningless to distinguish between the protonated or deuterated nature of the β-II-phase, an IR study was conducted on β-II-type prepared samples over the 4000 to 2000 cm-1 wavenumber domain (Figure 5c) for the β2H1200, β2D1200, and β21200′ samples. The IR spectra display drastic differences, namely a single vibration band at 3640 cm-1 for β2H1200 as compared to a single vibration band at 2690 cm-1 for β2D1200. On the basis of previous bibliography reports,19,20 this finding indicates that β2H1200 and β2D1200 contain only O-H or O-D bonds within their structure, respectively. Interestingly, the IR spectrum of the sample β21200′, prepared by ion exchange, presents both types of bands at 3640 and 2680 cm-1, implying that the synthesis method enables solely a partial rather than a complete proton/deuterium exchange. IR experiments performed on β2H125, β2D125, and β2125′ (Figure 5b) show that their spectra present the two bands characteristic of free O-H and O-D vibrations, except for β2H125, which possesses only an O-H band. From the above results and comparing the IR spectra for β2125′ and β21200′, one can deduce, as expected, that the exchange is easier for smaller than for larger particles, since the observed OD/OH ratio is greater for β2125′ than for β21200′. Now, considering the samples prepared in deuterated media exclusively, β2D1200 (large particle size) presents only O-D bands, whereas β2D125 (small particle size) presents both O-D and O-H bands, the latter being probably due to air humidity adsorption, which provides O-H species to replace O-D ones within the sample. Again, this second observation is in agreement with a correlation between the particle size and the success of exchanging D and H. 1.3. β-III- and γ-III-Phases. To obtain β-III nickel oxyhydroxides phases, the previously prepared β2H1200, β2H250, and β2H125 samples were successively oxidized as follows: 150 mL of 13% NaClO solution was (18) Feitknecht, W.; Wyttenbach, A.; Buser, W. Proc. Intern. Symp. Reactivity Solids 1960, 234-239. (19) Kober, F. P. J. Electrochem. Soc. 1965, 112, 1064-1067. (20) Kober, F. P. J. Electrochem. Soc. 1967, 114, 215-218.

3940

Chem. Mater., Vol. 16, No. 20, 2004

Barde´ et al.

Figure 6. Voltage-composition curves obtained from GITT first discharge on β3H125 (a) and γ3H125 (b) samples. The cells were alternately discharged at C/20 and relaxed for 2 h upon all reduction.

added to 1.5 g of β-II-Ni(OH)2 and 150 mL of H2O at 65 °C for 4 h.4,21,22 The resulting washed and dried products denoted β3H1200, β3H250, and β3H125, were identified as pure β-III-NiOOH phases by means of X-ray diffraction (Figure 3b). A different oxidation method was conduced to prepare γ-III-NiOOH. It consists of treating 1.5 g of precursor β-II-Ni(OH)2 (i.e., β2H1200, β2H250, or β2H125) in a 13% NaClO solution in basic media at 80 °C for 4 h. The resulting products were centrifuged and washed several times until neutral pH before being dried at 55 °C. X-ray diffraction patterns indicate that the materials are well-crystallized and exhibit a powder pattern similar to that of the γ-III-NiOOH phase, so that these phases are denoted γ3H1200, γ3H250, and γ3H125, respectively (Figure 3b). Finally, to complete our set of reference samples, we also tried to synthesize nickel oxydeuteroxides by treating 1.5 g of β2D1200 with an oxidizing LiClO/D2O solution at 70 °C for 4 h. (LiClO rather than NaClO was chosen as oxidizing agent since solid LiClO is commercially available and easily dissolved in heavy water.) The experiment was repeated twice, but no evidence of oxidation was deduced from the X-ray and neutron diffraction patterns of the final products. The same synthesis protocol in the presence of KOD allowed partial oxidation of the β2D1200 precursor to yield a mixture of β-II-, β-III- and γ-III-type phases that will (21) Gmelin, Nickel Teil B Lieferung 2 1966, B2, 434-436 and 452. (22) Besson, J. J. Ann. Chim. 2 1947, 527-546.

Figure 7. Chronoamperometric responses to stepwise potential scanning during first discharge PITT experiments on β3H125 (a) and γ3H125 (b) samples applying -5 mV potential steps with a limit current of 0.3 mA h/g.

be denoted mixD1200 (Figure 9b). The tiny amount of the precursor β-II-Ni(OD)2 phase left in the mixture was confirmed by infrared measurements (not presented here) with the presence of an additional peak characteristic of free O-D bond at 2680 cm-1. Finally, attempts to prepare deuterated oxidized phases from either β-III-NiOOH or γ-III-NiOOH phases by cationic exchange in D2O were unsuccessful. 2. Electrochemical Studies. The protonated phases were studied both by GITT and PITT, to check for the presence and the extent of the second plateau phenomenon. Parts a and b of Figure 6 present the voltage composition curve obtained by GITT on the β3H125 and the γ3H125 samples, respectively. The cells were alternately discharged at C/20 and relaxed for 2 h. One can see the occurrence of the second plateau, at about 0.4 V underneath the first one, whose amplitude reaches 44% and 55% of the available capacity, respectively. The fact that the total capacity does not reach one electron per mole (only 65% of that theoretical one for the γ sample) is most likely rooted in technical parameters, such as the difficulty in achieving an electronic percolation within the electrode.

In Situ NPD of a Ni(OH)2 Electrode

Chem. Mater., Vol. 16, No. 20, 2004 3941

Figure 8. Neutron diffraction patterns of selected protonated reference samples collected using the D20 diffractometer with λ ) 2.41 Å. Asterisks indicate NaCl and KCl impurities resulting from the synthesis procedure used for γ3H1200, circles denote β-II impurities in β3H1200, and crosses mark β-III peaks that are not indexed with JCPDS file # 6-0075.

As previously observed,11 one can also see that, when the cell potential traces the second plateau domain under usual pulses, it always relaxes toward the same open circuit potential of the first plateau, implying that whatever the reduction process occurring in the first or second plateau domain, it leads to the same final reduced phase. Parts a and b of Figure 7 present the chronoamperograms of the first stepwise potentiodynamic reduction for the β3H125 and γ3H125 samples, respectively. In both cases, at the early stage of the reaction, the current decays in a monotonic way while the potential decreases noticeably, implying a diffusioncontrolled process in a solid solution domain. Then, as we reach the middle of the first potential plateau, very long duration potential levels occur, with chronoamperograms that no longer correspond to a diffusion process but rather to a two-phase process, as deduced from the time dependence of the current that is characteristic from line reaction displacement interfaces.23 For the two studied samples, the two-phase behavior domain ends up when one reaches the limit of the first plateau. Then the second plateau occurs in practically a single step, with a chronoamperogram corresponding to that of a nucleation and growth process.24,25 Thus, because a potential plateau does correspond to a twophase equilibrium, these chronoamperograms represent the growth of the reduced phases in the second plateau domains. Rather than clarifying the reduction mechanism of the oxyhydroxide phase, the above observations raise the following questions. Is the reduction process the succession of two qualitatively distinct two-phase mechanisms, implying different phase transformations? Or is it the same kind of phase transformation, occurring (23) Puyn and Yoon, Mol. Crystal Liq. Cryst. 1998, 311, 123-128. (24) Kim, S.; Tryk, D. A.; Antonio, M. R.; Carr, R.; Scherson, D. J. Phys. Chem. 1994, 98, 10269. (25) Chabre, Y. Phys. Intercalation II 1993, 181.

Figure 9. (a) Comparison between neutron diffraction patterns of β2H1200 hydroxide and β2D1200 deuterioxide using the D1B diffractometer with λ ) 1.28 Å. (b) Neutron diffraction patterns of selected deuterated reference samples collected using the D20 diffractometer with λ ) 2.41 Å. Asterisks (/) indicate “fictitious” peaks derived from incomplete subtraction of the incoherent scattering contribution from the silica tube used as sample container.

in distinct parts of the electrode material, due to different external conditions (technology-derived parameters). The former is supported by our previous results on γ-phases11 showing that the β-II-phase obtained from reduction of a γ-III-phase has a textural memory of a precursor phase and reoxidizes preferably in a γ-III-phase rather than a β-III one. The latter is supported by the facts that the second plateau may also be observed on pure β-III-phases5 and that the open circuit equilibrium potential does not show any anomaly at the first f second plateau transition, a fact that is indicative of a single type of reduced phase. To answer the above questions, we decided to embark into a thorough study of the electrochemically driven phase changes occurring during the reduction process for various samples by in situ neutron powder diffraction. 3. Neutron Diffraction Studies. 3.1. Reference Samples. Powder neutron diffraction patterns were taken for all the prepared samples to serve as a reference and thus facilitate the interpretation of the in situ neutron diffraction studies. Furthermore, in the cases where the crystal structure of the phases was known (for β-II and also γ-III), calculated patterns were

3942

Chem. Mater., Vol. 16, No. 20, 2004

Barde´ et al.

Figure 10. (a) Selected neutron diffraction patterns corresponding to the beginning and the end of the second and the third reduction of β2H125. The experiments were performed on the D1B at different complementary angular domains. (b) Selected neutron diffraction patterns corresponding to the beginning and the end of the in situ reduction of γ3H1200 performed on the D20 diffractometer. Asterisks (*) denote in both cases a low angle peak due to the cell and a large peak centered at 50° due to NaOD electrolyte and the quartz of the cell.

produced with the use of the Fullprof program.26 For the latter, such pattern calculations are not unique, as the γ-III-phase can exhibit diverse compositions as well as three different sets of cell parameters/symmetries as indicated by the three reported JCPDS files, JCPDS # 6-0075, # 27-764, and # 23-1407, that refers to hexagonal R-3m or monoclinic C2/m unit cell symmetries, respectively.27-31 Moreover, because the above(26) Rodrı´guez-Carvajal, J.; Roisnel, T. FullProf 98 and WinPLOTR: New Windows 95/NT Applications for Diffraction, Commission For Powder Diffraction, International Union for Crystallography, Newsletter No. 20 1998. (27) Glemser, O.; Einerhand, J. Z. Anorg. Chem. 1950, 261, 2642. (28) Glemser, O.; Einerhand, J. Z. Anorg. Chem. 1950, 261, 4351. (29) Bode, H.; Dehmelt, K.; Witte, J. Z. Anorg. Allg. Chem. 1969, 366, 1-21. (30) Bartl, H.; Bode, H.; Sterr, G.; Witte, J. Electrochim. Acta 1971, 16, 615-621. (31) Butel, M.; Gautier, L.; Delmas, C. Solid State Ionics 1999, 122, 271-284.

reported structures were exclusively deduced from powder X-ray diffraction data, proton positions were not available. A pattern simulation tentative of our X-ray and neutron diffraction data for γ-III-phase was done using two models. Since the X-ray pattern of our γ-IIIsamples had more similarities with JCPDS # 6-0075 than with the others, we used this indexation for the experimental patterns, even if no definite structural model has yet been clearly established. Below, we discuss in detail and separately for each type of phase the neutron diffraction patterns recorded (Figures 8 and 9) on the various R-II, β-II, β-III, and γ-III reference samples. To simplify their description and facilitate their comparison, the Bragg peaks were indexed. 3.1.1. R-II-Phases. The powder neutron diffraction patterns for this phase are similar to the X-ray diffraction ones. Four wide asymmetric peaks are observed and can be indexed as (001), (002), (111), and (300) according

In Situ NPD of a Ni(OH)2 Electrode

Figure 11. (a) Number of exchanged electrons versus potential for the first and third in situ reductions of β2H125 performed on the D1B at C/7 rate. (b) Number of exchanged electrons versus potential for the first and second in situ reductions of β2H250 performed on the D20 at C/10 and C/15 rates, respectively.

to the JCPDS card number 22-0444. Therefore, the substitution of proton by deuterium does not significantly affect the peak position but alters their intensity, implying experimental difficulties to perform meaningful in situ NPD studies with this phase. Luckily, the (001) and (300) peaks were found to show up in an angular domain where no additional contributions of the cell are seen. Thus, in the following, the detection of the R-II-phase will mainly rely on the observation of the (001) peak. 3.1.2. β-II-Phases. The neutron diffraction patterns of the β-II reference (Figure 9a) samples (β2D1200 and β2H1200) indicate, as expected from calculations and literature,32,33 that the presence of deuterium highly influences the relative intensity of the peaks (001), (100), and (111). Indeed, the calculated intensity is zero for (001) and (111) and weak for (100). Thus, monitoring the intensity of the (111) peak, which does not coincide with any other peak of the phases involved in the NOE, can be used as a “measure” of the deuterium content of the samples. Such a trick turns out to be quite useful in our NPD experiment to monitor the proton content (32) Greaves, C.; Thomas, M. A. Acta Crystallogr., Sect B 1986, 42 51-55. (33) Eriksson, L.; Palmqvist, U.; Rundlo¨f, H.; Thuresson, U.; Sjo¨vall, R. J. Power Sources 2002, 107, 34-41.

Chem. Mater., Vol. 16, No. 20, 2004 3943

Figure 12. Evolution of (a) the neutron diffraction patterns and (b) the difference patterns, throughout the second reduction of β2H250, performed on the D20 at a C/15 rate with a wavelength of 2.41 Å. A gradual biphasic transformation of β-III into β-II is seen, and no anomalies are seen upon transition to the second plateau. Asterisks (*) denote a low angle peak due to the cell and a broad peak centered at 50° due to both the NaOD electrolyte and the silica of the cell. The sharp peaks are due to the Ni metal used for the collector of the working electrode (NiOOH/Ni(OH)2) and for the counter electrodes.

electrode evolution, because cycling was carried out in an electrochemical cell made as a working electrode of a protonated β(II)-phase (for cost reasons) immersed in deuterated electrolyte medium so that as the DfH ion exchange reaction proceeds, the electrode proton content is progressively reduced. Let us note as expected that the samples with the larger particle sizes give the narrower peaks. Therefore, due to cationic diffusion problems such samples poorly performed electrochemically. Thus, there is the need to find a compromise, as will be discussed later, between large particles that give sharp Bragg peaks and small particles that are suitable for efficient electrochemical processes. 3.1.3. β-III and γ-III-Phases. We should recall (section 1) that we could not prepare pure deuterated samples but only a mixture of the β-II-, β-III-, and γ-III-phases denoted mixD1200. However, the comparison between patterns of β3H1200, γ3H1200 (Figure 8), and mixD1200 (Figure 9b) indicates that the presence of deuterium has no effect whatsoever. In both cases, the most intense peaks appear at low angles (001 for β-III and 003 for

3944

Chem. Mater., Vol. 16, No. 20, 2004

Figure 13. (a) Evolution of the integrated intensity of the β-II (101) and (110) peaks upon the number of exchanged electrons for the experiment performed on the D1B using β2H125, (b) Evolution of the integrated intensity of the β-II and β-III (110) peaks upon the number of exchanged electrons for the experiment performed on the D20 using β2H250.

γ-III). Considering the mixed sample mixD1200, the peaks are consistent with those reported for electrochemically synthesized deuterated β-III,33 with therefore two remaining nonindexed large peaks denoted “x” in Figure 8, the second one being close in position to the (111) β-II peak. The presence of the 2θ peak around 19° in the in situ patterns will be a way to spot the presence of γ-III in the electrode. In contrast, detecting the β-IIIphase is more difficult to ascertain, since its (001) peak falls at the same position as the (001) peak from protonated β-II. Thus, to determine the β-III-phase we will rely more on the tracking of the (110) peak. However, it should be emphasized that if the electrode contains traces of the γ-III-phase, it will be almost impossible to detect the β-III-phase, as the (112) peak of the former appears close to the (110) peak of the latter. 3.2. In Situ Experiments. Such in situ experiments to study the NOE are far from being routine but are rather complicated by diverse engineering- or materialstype difficulties. On one hand, the cell used (see Figure 2) is not electrochemically optimized but designed to hold electrodes with the maximum possible load of active material, and therefore, the capacities observed never attain 100%. On the other, the silica used as separator and the NaOD electrolyte both result in a large band falling in the same angular zone of the (100)

Barde´ et al.

Figure 14. Plots of the position of the β-II (110) peak upon the number of exchanged electrons for the first (a) and second (b) reduction of β2H250 performed on the D20 at C/10 and C/15 rates. The position of the peak does not vary significantly.

and (101) β-II peaks, and finally, the nickel metal used as current collector yields two narrow peaks with positions close to the (105) γ-III peak and the (102) β-II peak. The latter inconvenience proved to be also an advantage, as the positions of the nickel peaks were used for correction of the possible shift in the 2θ angle. In addition to the cell, the characteristics of the specific sample under study do also play a significant role. Indeed, the starting samples are protonated, and thus the deuterium content of the NOE increases upon cycling, inducing changes in the intensity of β-II peaks. Selecting the specific samples for the in situ studies was also rather difficult, as the bigger the particle diameter, the higher the observed intensity of the peaks, but also the worse the electrochemical performances. This is especially dramatic in the case of β-II, because this phase is electronically insulating. The compromise solution was to perform experiments using medium grain size samples such as β2H125 and β2H250, because β2H1200 yielded too low an electrochemical capacity. With respect to the oxidized phases, γ3H250 and γ3H1200 were examined because even the latter yielded significant electrochemical capacities, certainly due to their higher electronic conductivity. The choice of the diffractometer and wavelength is also important. The first experiments were performed on the D1B diffractometer, which can be operated at

In Situ NPD of a Ni(OH)2 Electrode

Figure 15. (a) Number of exchanged electrons versus potential for the in situ reduction of γ3H250 performed on the D1B at C/10 rate. (b) Number of exchanged electrons versus potential for the in situ reduction of γ3H1200 performed on the D20 at C/10-C/20 rates with an intermediate 16.5 h OCV period.

1.28 or 2.52 Å wavelengths, the first having the advantage of allowing a larger area of the reciprocal space within the same angular domain but with a lower neutron flux. Preliminary tests indicated that, to obtain exploitable data, experiments had to be performed at 2.52 Å. This implies that the reciprocal space domain covered is narrower; therefore, the obtainable information is limited because very few peaks are observed. Performing the same experiment twice while observing two different angular domains can only compensate for this. See for instance Figure 10a showing the patterns corresponding to a cell with an electrode fabricated with β2H125 in the reduced state after three cycles (and thus containing mainly deuterated β-II) and in the oxidized state (containing deuterated β-III) in two angular domains. This situation was improved with the use of the D20 diffractometer at 2.41 Å, which has the possibility of wider d-domain and higher fluxes (see Figure 10b corresponding to patterns of an electrode fabricated with γ3H1200 in the initial charged state and the final discharged state). 3.2.1. Charge-Discharge in Situ Studies on Electrodes Prepared with β-II. Two experiments were performed, one with β2H125 using the D1B diffractometer (λ ) 2.52 Å) and the other with β2H250 using the D20 diffractometer (λ ) 2.41 Å). The electrochemical protocol was

Chem. Mater., Vol. 16, No. 20, 2004 3945

similar in both cases, the cell was subsequently galvanostatically charged and discharged at C/7 and C/10, and three cycles were performed in each case, respectively. Discharges were made also at C/7 in the first case and both at C/10 and C/15 in the second. Figure 11a,b shows the first and third cycles for the experiment with β2H125 and the first and second cycles for β2H250. As could be expected, the first sample delivers higher capacity at rapid rates due to its lower crystallinity. The influence of the regime can also be seen, as the capacity at C/15 for β2H250 is almost double that at C/10. In both cases, the potential at which the second plateau is observed is higher upon the first cycle, in agreement with results previously reported by Sac-Epe´e et al.4 Due to limited beam time, the in situ experiments carried out on β-II were coupled with those using γ-III described in the next section. Continuous in situ powder neutron diffraction monitoring was done for all the reduction steps, being obviously more interesting due to the presence of the second discharge plateau, but only for one or two oxidation steps. The results of the experiments carried out on the D1B using β2H125 and on the D20 using β2H250 are coincident. However, due to the more limited amount of information that can be extracted from the former experiment, both due to the small particle size and the narrower angular domain (see Figure 10a), we will present here mostly the results obtained on the D20. The evolution of the neutron diffraction patterns throughout the second reduction of this phase performed at C/15 is shown in Figure 12. Complete charging of the cell could not be achieved and there was always some remaining reduced β-II in the charged state, which is consistent with the fact that the NEE (number of exchanged electrons) is always lower than 1. As was expected, a direct reduction of β-III to β-II was observed with the absence of any additional phases. The fact that no differences in the evolution of the diffraction pattern are seen upon the transition to the second plateau suggests that it is not related to a definite structural transition. Indeed, for both experiments, a gradual increase in the intensity of the β-II peaks and a gradual decrease in the intensity of the β-III peaks is observed throughout the reduction process. This can be seen from Figure 13a,b. According to Barnard’s work,1-3 it was generally assumed34 that the reversible transformation of β-II into β-III occurs in three definite steps: an initial β-II solid solution region up to NEE ) 0.25, a biphasic domain up to NEE ) 0.75, and a final β-III solid solution region up to NEE ) 1. Such assumptions were later confirmed by other techniques, such as thermal analysis and positron lifetime spectroscopy.35 However, later on, by studying chemically prepared samples with controlled oxidation degree, Tessier et al.17 pointed out that at least the β-II solid solution region could be much smaller than expected. Our results confirm the latter, and in fact, we observe a two-phase behavior throughout the reduction of β2H125 and β2H250, and the positions of the peaks do not change significantly throughout the experiment. This can be seen in Figure 14a,b, where there is no significant evolution of the lattice parameters through(34) Huggins, R. A.; Prinz, H.; Wohlfahrt-Mehrens, M.; Jo¨rissen, L.; Witschel, W. Solid State Ionics 1994, 70/71, 417-424. (35) Suvegh, K.; Horanyi, T. S.; Vertes, A. Electrochim. Acta 33 1988, 1061.

3946

Chem. Mater., Vol. 16, No. 20, 2004

Barde´ et al.

Figure 16. Evolution of the (a) neutron diffraction patterns and (b) difference patterns throughout the reduction of γ3H1200 performed on the D20. A gradual biphasic transformation of γ-III into β-II is seen and no anomalies are seen upon transition to the second plateau. Asterisks (/) denote a low angle peak due to the cell and a large peak centered at 50° due to NaOD electrolyte and the quartz of the cell.

out the reduction of the β2H phases investigated here. The fact that complete charge of the electrodes could not be achieved may have hindered the observation of the β-III solid solution region. But as complete discharge seems to have been reached at least for β2H125 and for the second reduction of β2H250 [the large (110) β-III peak diminishes in intensity and disappears in the background of the pattern before the end of reduction; see Figure 13b], our neutron diffraction investigation is in agreement with the work of Tessier et al.,17 who

showed that the β-II solid solution domain, if existing, is very narrow. 3.2.2. Discharge in Situ Studies on γ-III-Based Electrodes. In this case, two experiments were also performed, the first with γ3H250 on the D1B diffractometer (λ ) 2.52 Å) and the second with γ3H1200 on the D20 (λ ) 2.41 Å). The electrochemical protocol started with an initial charge step to compensate for partial reduction of the γ-III-phase into R-II during the electrode preparation process that was impossible to avoid (all

In Situ NPD of a Ni(OH)2 Electrode

Figure 17. Evolution of the integrated intensity of the β-II (110) and γ-III (003) peaks upon the number of exchanged electrons (NEE) for (a) the experiment using γ3H250 on the D1B and (b) the experiment using γ3H1200 on the D20. The discontinuity observed on the latter is due to re-equilibration of the system during the OCV period.

the tests made with less reducing solvents for the electrode paste preparation resulted in lower adhesions to the current collector and thus electrochemical failure). After this initial oxidation step, the cells were discharged. In the first case, the reduction was performed at a C/10 regime, whereas the second followed a more complicated protocol due to technical adjustments that had to be done in the course of the experiment: it was first discharged at C/20 for about 4 h and then at C/10 for 3 h more followed by 16.5 h of relaxation in open circuit voltage (OCV) and final reduction at C/20 that lasted about an additional 7 h. Both electrochemical discharge curves are depicted in Figure 15a,b. As in the case described in the previous section, the results of both experiments are also in agreement. However, more information could be extracted from the experiment done using D1B with γ-III-based electrodes than with β-II-based ones due to both the higher crystallinity of the samples and the fact that the oxidized phase involved (γ-III instead of β-III) presents more intense peaks. The first interesting result of these experiments was that we observed a direct reduction of γ-III into β-II. This fact is somewhat surprising, as it is generally assumed and proved that the reduction of γ-III yields

Chem. Mater., Vol. 16, No. 20, 2004 3947

Figure 18. Plots of the position of the γ-III (003) (a) and β-II (110) (b) peaks upon the number of exchanged electrons for the experiment using γ3H1200 on the D20. The small change observed in the position of the latter for low NEE would account for a variation of 0.07 Å in the interlayer space, which might be due to a change in the amount of intercalated water occurring in the earlier steps of the process below ca. 0.1 NEE.

R-II, and we indeed observed this phenomenon during the preparation of our electrodes. We had already observed such a direct reduction of γ-III into β-II in the solid state, from coupled ex situ powder X-ray diffraction, transmission electron microscopy, electron diffraction, and electrochemical measurements.11 As stated before, neither R-II nor other phases, different than γ-III and β-II, were detected through the whole reduction process. The direct reduction of γ-III into β-II was observed, as the positions of the peaks do not change during the experiments and only a continuous decrease in the intensity of the γ-III peaks is seen to occur at the expense of that of the β-II peaks (see Figures 16 and 17). This result is common to both experiments, but the evolution could be more clearly seen during the experiment on the D20 with γ3H1200 due to both the higher flux of the diffractometer and the much better crystallinity of the sample. In Figure 17 is depicted the integrated intensity of the γ-III (003) peak during the γ3H250 and the γ3H1200 experiments, which is seen to gradually diminish as the intensity of the β-II (110) peak increases. These peaks have been chosen as most representative of the phases because of higher intensity and no overlapping with other reflections, but the trend

3948

Chem. Mater., Vol. 16, No. 20, 2004

is the same for every γ-III peak and β-II peak, respectively. The plot of the evolution is somewhat disrupted for γ3H1200 in the region corresponding to the 16.5 h stop at OCV. This must be attributed to balancing the phases during the rest period, modifying significantly the diffraction pattern (better homogeneity leads to narrower peaks). An additional proof for the biphasic character of the transformation is the fact that the peak positions do not evolve during the process, as can be noted in Figure 18. A slight displacement of the γ-III (003) peak is seen at the beginning of reduction that would correspond to a decrease of 0.07 Å in the interlayer space. The origin of this change is unknown, but given the fact that the γ-III-phase can contain diverse amounts of intercalated water molecules in this interlayer spacing, it is possible that the evolution is due to a small change in this amount at the beginning of reduction. This continuous biphasic transformation throughout the whole reduction process partially contradicts our previous assumptions (based on the fact that solely samples containing the γ-phase were showing the second plateau) that the second plateau was due to the reduction of γ-III into β-II, because this is a common phenomenon gradually taking place during both the first and the second plateau. Thus, it looks as though the origin of the second plateau is not related to the existence of a definite phase transformation. We believe that the origin of the second plateau is rooted in technological parameters (the nature of and assembly with binder and current collector); therefore, more

Barde´ et al.

research is needed to establish a definite hypothesis explaining both the existence of this plateau as well as its amplitude. Conclusion By means of GITT and PITT experiments on both oxidized phases (β-III and γ-III) we have evidenced a two-phase mechanism for both the first and the second plateau. To ascertain if the phases involved were the same, in situ neutron powder diffraction studies were undertaken. The results obtained unambiguously show that the redox processes taking place in the first and the second plateau are the same, and consist of a direct reduction of β-III or γ-III into β-II. This finding seems to prove that the second plateau does not have a “chemical” origin and is not related to an intrinsic property of the material. The cause of this phenomenon seems to be nested in technology-derived parameters related to the electrode formulation (percentage, distribution, and nature of the binder and conductive additive) and the interface with the current collector, putting Barnard’s explanations back on the spot. Acknowledgment. The authors are especially indebted to G. Rousse for precious assistance and discussions during the in situ experiments performed on the D20 and to A. Delahaye-Vidal for enlightening scientific exchanges. The authors are grateful to the Institut Laue Langevin and the CNRS for the use of the high flux powder diffractometer D20 and CRG-D1B, respectively. CM0401286