Textural and structural studies on nickel hydroxide electrodes

Reactivity of Solids, 2 (1986) 223-233 Elsevier Science Publishers B.V., Amsterdam

223 - Printed

in The Netherlands

TEXTURAL AND STRUCTURAL STUDIES ON NICKEL HYDROXIDE ELECTRODES I. CRYSTALLIZED NICKEL HYDROXIDE TO CHEMICAL AND ELECTROCHEMICAL

A. DELAHAYE-VIDAL,

B. B~UDOIN

MATERIALS SUBMIlTED REDOX CYCLING

and M. FIGLAR_Z *

lJniversit6 de Picardie, Laboratoire de Rgaetivit6 et de Chimie des Solides (U.A. C.N.R.S. 33, rue Saint Leu, F-80039 Amiens Cedex (France) (Received

March

17th, 1986; accepted

12il j,

May 2nd, 1986)

ABSTRACT The mechanism of operation of the crystallized nickel(I1) hydroxide electrode used in the ~ckel-cadmium battery is a matter of considerable study and controversy. In this paper particular emphasis is placed on the textural and structural m~ifications of the phases involved, as a new approach to the chemical and electrochemical redox behaviour of the nickel electrode. X-ray diffraction, electron microscopy and diffraction were performed on two kinds of /3(E)-nickel hydroxide materials, during chemical and electrochemical cycling, in an effort to determine the oxidation-reduction mechanism. The existence of important textural modifications was established during the first oxidation cycle. Starting from ji’(II)nickel hydroxide with a monolithic texture, it consists in the formation of a mosaic texture. This mosaic texture is preserved during the first reduction and further cycling. A model based on the pseudomorphous and topotactic characteristics of the reactions involved was used to explain the formation of the mosaic texture. The reactions that take place in the solid state by oriented growth of &HI) or ~(111) on p(I1) induce strains within the particles; they are due to the large differences in the Ni-Ni distances in the oxidized and reduced phases. Strain relaxation brings about the formation of a mosaic texture, which gives the particles elastic properties that minimize the further strains and make their relaxation easier. The formation of this mosaic texture can be related to the well known forming process of the electrode and its preservation during cycling can be related to the life cycle performance of the system.

INTRODUCTION

Nickel(I1) hydroxides and their oxidation products constitute the active materials of the positive electrode of nickel-cadmium alkaline cells. The mechanism of operation of the crystallized Ni(OH), electrode has been the subject of a considerable number of studies and much controversy; because of the numerous aspects to be taken into consideration, its mode of 0168-7336/86,‘$03.50

0 1986 Elsevier Science Publishers

B.V.

224

operation still remains an enigma. Many studies have been devoted to improving the understanding of the mechanisms involved during the charge and discharge of the cell [l-9]. Most of the investigations have dealt with the electrochemical behaviour of the electrode; experimental methods such as potential sweeps and potentiostatic pulses are usually employed to perform kinetic and thermodynamic approaches. Nevertheless, an important feature has often been neglected, viz., the textural characterization of the phases involved in nickel hydroxide electrodes and their evolution during cycling. The electrode reactions are complex both in structural types and in chemical formulae. Bode et al. [lo] proposed initially the following scheme: /3-Ni(OH) z % P-NiOOH t 1 a-Ni( OH) 2 + y-NiOOH The a-phase has never really been exploited industrially but electrochemically deposited a-phase is used in a few applications. In fact, the commercial Ni-Cd cells are cycled between the two phases ,8-Ni(OH)2 and pNiOOH. In dilute electrolytes and in the absence of overcharge the redox reaction can be written as ,&Ni(OH),

+ P-NiOOH

+ H+ + e-

So far, no correlation has been shown to exist between textural development and electroactivity of the phases. The purpose of this paper is to describe the textural and structural evolution of the phases during both the chemical and the electrochemical cycling. Indeed, such investigations may enable us to enhance our understanding of the nickel hydroxide electrode behaviour [ll]. This first paper examines the behaviour of P-type hydroxides as starting active materials. After recalling the structure of nickel hydroxides phases, we shall first describe the preparation and the characterization of the starting active materials. Second, we shall examine the results of chemical redox cycling, and last the electrochemical redox cycling. The chemical cycling was performed as an initial study in order to make it easier to understand the electrochemical redox process. It should be noted that there is no equivalence between the chemical and electrochemical kinetics of the involved reactions.

NICKEL

HYDROXIDE

PHASES

We do not consider the a-nickel hydroxide phase with a turbostratic structure; this will be examined in Part II [12]. The P(II)-Ni(OH), crystallized hydroxide has a lamellar structure. It crystallizes in the hexagonal system with a = 3.126 A and c = 4.605 A.

225

The oxidized nickel hydroxides present two types of phases, both with a lamellar structure which differ in interlayer spacing and composition [13]; both show more or less imperfectly organized structures with randomly oriented layers. The structure of the P(III)-NiOOH phase is closely related to that of of one proton /3( II)-Ni(OH) 2 f rom which it derives owing to deintercalation and one electron. It crystallizes in the hexagonal system with a = 2.82 A and c = 4.85 A. The ~(111) phases are lamellar with layers of the same composition as /?(III)-NiOOH but water and alkali metal ions in variable amounts are intercalated between the layers, leading to an interlamellar spacing of about 7 A. The unit cell is tripled with a = 2.82 A and c = 21 A.

PREPARATION MATERIALS

AND CHARACTERIZATION

OF THE /3(11) STARTING

Structural and textural investigations were carried out by X-ray diffraction and electron microscopy. Transmission electron microscopic observations were made with a Philips EM 301 microscope. The X-ray diffraction patterns were obtained with a Philips PW 1710 diffractometer or a Guinier camera using cobalt XKa radiation. In order to facilitate electron microscopic studies the hydroxide chosen for chemical cycling was a hydroxide with good crystallinity and a well defined form and textural characteristics. Its preparation has been described by Fievet and Figlarz [14]. The preparation proceeds in two steps. First, an ammonia solution is added to a nickel nitrate solution at room temperature; the precipitate is then washed and centrifuged several times until the pH becomes neutral. After rapid drying at ambient temperature a green hydroxide of the a-type is obtained. This has been called turbostratic nickel(I1) hydroxide by the authors because of the random orientation of the nickel hydroxide layers along the c axis [15]. In a second step, this turbostratic hydroxide is suspended in water and treated at 200°C under hydrothermal conditions. The crystallized hydroxide thus obtained presents an X-ray diffraction pattern with very sharp lines, which show a high degree of crystallinity (Fig. la). Transmission electron microscopic studies showed that this P-hydroxide appears as non-porous, thin hexagonal platelets (Fig. 2). The selected area electron diffraction pattern of an isolated particle corresponds to a single crystal lying on the (001) plane (Fig. 3b). The diameter of the particles is several thousand angstroms and their mean thickness is in the range 300-500 A; this hydroxide will be called Cl. For the electrochemical cycling we used an industrial P-hydroxide; this hydroxide consists of smaller hexagonal particles which are about 300-500

226

b--+

;

i : j : ....... / : .:’ :.. ‘. .... ‘:. .i j ,... ‘... ....... -..............d’. 2.1

2.3

2.5

3

3.5

4.5

d(i)

Fig. 1. X-ray diffraction patterns for: (a) Uncycled well crystallized P(II)-Ni(OH) Z prepared by hydrothermal treatment from an aqueous suspension of turbostratic nickel hydroxide: Cl starting material. (b) /3(111)-NiOOH obtained by chemical oxidation of the previous Cl hydroxide. The X-ray diffraction pattern shows a very broad and complex group of lines centred around 2.5 A which cannot be explained by the overlapping of the corresponding hkl lines of /3(111)-NiOOH. It is likely that /3(111) presents an imperfectly organized structure with some misorientation between the NiOOH planes due to the strong interactions of nickel(II1) ions of adjacent layers. (c) P(II)-Ni(OH), obtained by reduction of the previous P(III)-NiOOH. The broadening of the diffraction lines is an indication of the mosaic texture of the particles.

Fig. 2. Hexagonal platelets of Cl /3(11)-Ni(OH), prepared by hydrothermal treatment of turbostratic hydroxide. The Bragg extinction contours indicate fairly good crystalline perfection.

227

Fig. 3. (a) Bright field image of a particle of Cl P(II)-Ni(OH), with Bragg extinction contours. (b) SAED showing the particle lies on the (01) plane. (c) Dark field image of the same particle; the grain is homogeneously illuminated, indicating that the particle is monolithic. (d) Bright field image of an oxidized P(III)-NiOOH particle with a homogeneous contrast as opposed to the Bragg extinction contours that characterized Cl Ni(OH), [see (a)]. (e) Electron diffraction pattern of the ~(III)-NiOO~ particle lying on the (001) plane; the diffraction spots appear as small arcs. (f) Dark field image of the same particle showing the mosaic texture of the grain.

Fig. 4. Small hexagonal

platelets

of C2 /3(11)-Ni(OH),.

A in diameter and about 100 A thick (Fig. 4); this second type of crystallized hydroxide will be called C2. Although such a compound is less suitable for a textural study by transmission electron microscopy, we chose it as the starting material for electrochemical cycling because the Cl hydroxide is inactive for electrochemical cycling owing to its textural characteristics. The two ~-hydroxides have the same structure but differ in their textural properties (grain size, thickness. crystallite size).

CHEMICAL

REDOX

CYCLING

The redox chemical cycling of the Cl hydroxide was carried out under the following conditions. This P(II)-nickel hydroxide was suspended in sodium hypochlorite solution with or without potassium hydroxide, either at room temperature or at 90°C in order to increase the oxidation kinetics. Such a treatment led to oxidized phases, which were reduced by means of hydrogen peroxide in alkaline solution. Further cycling was performed in the same way. All these reactions were carried out in an inert atmosphere to avoid carbonation. Samples were removed during the course of redox chemical cycling and submitted to X-ray diffraction and electron microscopic examinations. In the absence of potassium hydroxide, the oxidation product is the p(II1) phase. In the presence of potassium hydroxide a mixture of the oxidized phase p(II1) and of y(II1) appears; the transformation towards ~(111) is promoted by a high potassium hydroxide concentration. The oxidation reaction is pseudomo~hous, i.e., the shape and size of the I-Nip starting particles are retained during the reaction, but oxida-

229

textural change, as can be established by X-ray tion induces an important diffraction and by electron microscopy and diffraction. of the diffraction lines X-ray diffraction shows a significant broadening for the oxidized p(II1) and ~(111) phases compared with those of the /3(11)-Ni(OH), starting material (Fig. lb). Such a broadening effect indicates that a textural modification occurs during the first chemical oxidation. This textural change consists of a decrease in the crystallite size and the textural modifications involved in the active phases can be described in detail and visualized by electron diffraction and microscopic studies. As indicated previously, the particles of p(I1) appear as non-porous hexagonal platelets (Fig. 3a) and the selected area electron diffraction (SAED) pattern of an isolated grain corresponds to a single crystal lying on the (001) plane (Fig. 3b). The diameter and thickness of the particles determined by electron microscopy compared with the crystallite size inferred from X-ray diffraction allow us to conclude that the p(I1) hydroxide platelets are built up only by a single layer of crystallites in the [OOl] direction and by a small number of crystallites, possibly only one, in the (001) plane. The particles may be considered either as monolithic single crystals or as mosaic single crystals with good crystallinity, as indicated by the sharpness of the electron diffraction spots (Fig. 3b). This is confirmed by the Bragg extinction contours, as can be seen in Fig. 3a; this feature implies that the platelets are often slightly bent and present good crystalline perfection. Samples removed during the course of the oxidation consist of a mixture of nickel(I1) and nickel(II1) phases. Electron microscopic studies showed that the reaction is pseudomorphous (Fig. 3a and d). SAED on a single platelet indicated that the p(II1) or ~(111) oxidized phases are in a definite and reproducible orientation relative to the P(II)-Ni(OH), (Fig. 5). The reaction is topotactic with (001) P(III)//(OOl)

/WI>

(110) P(IIIM(lI0)

P(II>

b

Fig. 5. SAED on a partly oxidized Cl P(II)-Ni(OH)z orientation relationship between /3(11) and /3(111).

particle,

showing

the structural

230

but the reflections corresponding to p(II1) are broad arcs as opposed to the very sharp spots associated with p(II). Study of the oxidized phases at the end of the reaction leads to the following observations. After oxidation the platelets do not show any Bragg extinction contours but a homogeneous contrast (Fig. 3d); this is an indication of the textural change inside the particle. As already noted, SAED (Fig. 3e) shows that the diffraction spots appear as small arcs, which indicates a small misorientation of the crystallites inside the particle, which can nevertheless be considered as a single crystal lying on the (001) plane. The crystallite size inferred from X-ray diffraction, which is about 10 A, is clearly much smaller than the size of the hexagonal platelets. All these features indicate that the texture of the p(II1) or y(M) phases consists of a great number of small crystallites that are slightly misoriented with respect to each other. In other words, the oxidized particles are monocrystalline mosaic grains. The dark field electron microscopic images confirm these results more explicitly. With /3(11)-Ni(OH) 2 the particle shows a contrast (Fig. 3c) which indicates that the particle is monolithic. In contrast, the dark field image of oxidized particles (Fig. 3f) shows a large number of small white spots whose size is about the same order of magnitude as that of the mean crystallite size inferred from X-ray diffraction; this is consequently a kind of materialization of the crystallites and of the mosaic texture of the oxidized platelets. We can now summarize the essential features which characterize the textural modifications occuring in the oxidation reaction: (i) P(II)-Ni(OH), starting material: hexagonal platelets, each platelet contains only a large crystallite, i.e., monolithic single monocrystal (narrow X-ray diffraction lines, pointed electron diffraction spots, Bragg extinction contours, homogeneous illumination of the dark field image). (ii) P(II1) or ~(111) oxidized phases: each platelet contains a large number of small crystallites fairly well oriented (broad X-ray diffraction lines, curved electron diffraction spots, no Bragg extinction contour but homogeneous contrast, heterogeneous dark field image which reveals the crystallites). This mosaic texture of the oxidized phases is preserved during the reduction (Fig. lc) and its character is enhanced by further cycling. The results as a whole enable us to reach a conclusion about the mechanism of the oxidation reaction of the crystallized P(II)-Ni(OH), hydroxide. The reaction is pseudomorphous, i.e., the particles of the oxidized phases retain the habit of the starting hydroxide and the reaction is topotactic, i.e., the oxidized phases are in a definite and reproducible orientation relative to the p(I1) hydroxide. This reaction, which takes place in the solid state by oriented growth of p(II1) or ~(111) on p(II), induces strains within the particles. These strains result from the mismatch of the lattices of the nickel(I1) and nickel(II1) phases; they are due to the large difference in the Ni-Ni distances in the oxidized (2.82 A) and reduced (3.12

231 First

Further

Reduction

Cycling

First Oxidation

Monolithic

texture

Y

Mosaic

\

texture

Fig. 6. Scheme illustrating the textural modifications electrochemical redox cycling of p(II)-Ni(OH) 2.

that

occur

in the chemical

and

A) phases. Strain relaxation brings about the formation of a mosaic texture. This mosaic texture, which occurs during the first oxidation cycle, confers on the particles elastic properties that will minimize the further strains and make their relaxation easier. Thus, this texture is preserved during further reductions and oxidations (Fig. 6).

ELECTROCHEMICAL

REDOX

CYCLING

As the well crystallized Cl hydroxide exhibits very poor charging ability, we chose another hydroxide for electrochemical cycling, viz., commercial C2 hydroxide. Although this material is less suitable for electron transmission microscopic studies than Cl hydroxide, its use as a starting material for electrochemical cycling will allow us to simulate the behaviour of the positive electrode of a Ni-Cd pocket cell. This C2 hydroxide was cycled by means of a micro-cell that simulates the working of pocket cells. After mixing the hydroxide with graphite, in order to improve the conductivity inside the electrode, the powder is placed between two nickel-plated steel discs, 12 mm in diameter, and compressed at 1 ton/cm2. The pellet thus obtained is placed in a plexiglass support with a current collector. The nickel hydroxide electrode is then plunged into 4.5 M potassium hydroxide solution, between two cadmium counter electrodes, and is cycled at the C/5 rate. In order to form the electrode, the two first charges were performed for 20 and 15 h, respectively (compared with 7.5 h for the normal cycling conditions). After variable numbers of charges and discharges some electrodes were removed, and the samples were studied by X-ray diffraction and electron microscopy. As for chemical redox cycling, the most significant result is the broadening of the X-ray diffraction line that appears after the first oxidation cycle and is preserved during further cycling. The widths at half-maximum inten-

232 TABLE

1

Width at half-maximum intensity of different /3(H)-Ni(OH), after various numbers of cycles hkl lines

Width at half-maximum

001 100 101 102 110 111

intensity

hkl lines of the discharged

(arbitrary

active

material

units)

C2 starting p(I1) hydroxide

After 1 cycle

After 30 cyles

After 59 cycles

After 99 cycles

115 62 175 210 70 85

130 102 220

145 110 240 330 140 160

130 113 210 320 150 170

130 115 240 360 160 170

120 138

sity for different hkl lines of p(I1) hydroxide phases removed after various numbers of discharges are reported in Table 1 and illustrate this phenomenon. After a few cycles, the widths remain almost constant. As indicated previously, a complete textural study by electron microscopy and diffraction is not possible. However, electron microscopic investigations of the different samples indicate that during the electrochemical redox cycling the habit of the starting C2 hydroxide remains unchanged. This result and the observed broadening of the X-ray diffraction lines allow us to conclude without any ambiguity that the textural modification induced by the electrochemical redox cycling is, once again, the formation of a mosaic texture as opposed to the monolithic one of the starting C2 hydroxide. The mechanism of the formation of this mosaic texture is the same as previously described for chemical redox cycling. It is interesting to compare these textural characteristics of the active material with the discharge capacities of the C2 nickel hydroxide electrode. The data are reported in Table 2. It is clear that the capacity falls only slightly in the initial stage of cycling, then remains constant for at least a 100 cycles. This fall in discharge capacity of the nickel hydroxide electrode may be related to the overcharging conditions imposed on the system in order to form the electrode. Except for this formation period, the electrode is characterized by quasi-stability of the capacity delivered during discharge; this may be closely connected to the textural features we have established.

TABLE

2

Mean capacity charge/discharge

of the C2 nickel cycles

hydroxide

Number of cycles Mean capacity (mA h/g Ni) Number of electrons exchanged during discharge per mol of Ni(OH) z

electrode

1 326 0.71

as a function

3 316 0.69

of the number

30 316 0.69

99 316 0.69

of

233 ACKNOWLEDGEMENTS

The authors who sponsored

thank the MinistGre de la Recherche et de la Technologie this study and the SAFT for permission to publish this work.

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