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THE CAPACITY DETERIORATION MODEL OF MECHANICALLY ALLaOYED Mg,,Ni,,,,,_ ,y AMORPHOUS ELECTRODES IN CHARGING-DISCHARGfNG

CYCLING

WEIHONG LIU,* YONGQUAN LEI, JING WU and QIDONG WANG Materials Science and Engineering Department, Zhejiang University, Hangzhou 310027. P.R. (‘ha

Abstract-In this paper, a relationship between the electrochemical capacity of mechanically alloyed Mg,YNl,:,,, ! amorphous hydride electrodes and discharging time is deduced on the basis of oxidation of Mg and Ni. The experimental data are in good agreement with the deduced relation. Analysis shown that this is the common mechanism of hydride electrode degradation when the activation process is ignored. The parameters m and B of some amorphous hydrides are calculated on the basis of this relation. ,(’ 1997 International Association for Hydrogen Energy

NOMENCLATURE

Discharge capacity (mAh/g) Intrinsic maximum capacity (mAh/g) Capacity of the nth cycle Empirical parameters Cycling number Fraction reacted in time I Constant Rate constant parameter Time Effective Mg fraction Oxidation time of hydride for the nth cycle Arithmetical mean of oxidation time per cycle Discharge current density Willems empirical parameter Arithmetical mean capacity per cycle

INTRODUCTION

It was recently found that some amorphous Mg-Ni alloys prepared by mechanical alloying (MA) can absorb and desorb a large amount of hydrogen electrochemically at room temperature [l-3]. This makes them promising materials for Ni-metal hydride rechargeable batteries. However, the degradation of its discharge capacity is

* Present address: Department of Chemistry, Fudan Univer- sity, Shanghai 200433. P.R. China.

much faster than that of ABS-, AB,-, and AB-type metal hydride electrodes. XRD and XPS experimental results indicated that serious oxidation of magnesium and nickel occurred during cycling, which was regarded as the main reason for the capacity degradation of’ amorphous Mg- Ni alloy prepared by MA [2, 31.

Recently, considering the oxidation of magnesium and nickel as the main cause for capacity degradation of the electrodes, we deduced a relation between the discharge capacity C and cycling number N of mechanic&y alloyed MgxNi, 0,, _ x amorphous eiectrodes [3]:

or

where C, stands for the intrinsic maximum capacity. For amorphous electrodes, as no activation processes are needed, the initial capacity is the maximum capacity, thus C, = C, * K is an empirical parameter associated with the intrinsic properties of the surface oxide (such as the com- position, structure, particle size, the diffusitity of oxygen in the oxide layer and specific surface area of the sample etc.). However, in these equations, many factors have not been considered, including the oxide ~uele0ttil-m process and the morphology of the electrode particles and oxide layer, and thus mismatches between the experimental data and the catculated values are produced during the

99’ 9

1000 WEIHONG LIU et al.

initial stage of electrochemical cycling. Many studies on LaNi,-type hydride electrodes have shown that the oxi- dation reaction during electrochemical cycling which causes capacity degradation is a nucleation and growth reaction [4]. In this paper, the authors attempt to for- mulate a physical mode1 for the cycling behavior of hydride electrodes and try to elucidate the mechanism of capacity deterioration of the Mg-Ni amorphous hydride electrodes.

THE PROPOSED MODEL FOR CAPACITY DETERIORATION OF AMORPHOUS Mg-Ni

HYDRIDE ELECTRODES

As mentioned above, previous investigation [2, 31 have discovered that the oxidation of Mg and Ni in amorphous Mg-Ni hydrides is the main cause for the capacity degra- dation of the hydride electrodes. We found from our experiments on the charging-discharging cycling that an oxide layer was growing on the surface of the Mg-Ni amorphous hydride particles as shown in Fig. 1. The layer was identified as Mg(OH)2 and Ni(OH)* by XRD and XPS. We believe the consumption of Mg in this manner is the main cause of capacity degradation of the hydrides.

The classical method of analysing the reaction kinetics of a reaction on the basis of nucleation and grain growth in condensed systems is to compare the kinetic data for solid state reactions with an eauation. which in its sim- plest form can be written as [5]: ’

CI = l-exp(-Bt”) or

ln[-ln(l-cr)] = lnB+mlnt

(3)

(4)

Fig. 1. SEM morphology of Mg,,NiSo after four electrochemical cycles.

where CI is the fraction reacted in time t, B is a constant which depends both on the nucleation frequency and linear rate of grain growth and m is a constant that varies according to the geometry of the system. This equation was derived almost simultaneously by Johnson and Mehl [6] and by Avrami [7], and subsequently, Erofe’ev [8] proposed it as a generalized equation for the kinetics of solid-state chemical reactions. The theoretical value of m varies according to the derivation (rate expression):

da Tt-

- -Bt”-‘(1 -a)m

where (1 -m) may be regarded as the allowance for the impingement effect; here B is a rate constant parameter and m the time exponent, Hulbert [9] has tabulated values of m for a variety of nucleation-and-growth models. Two major types of transformations may be treated by the application of Avrami equation. To one category belong the diffusion-controlled transformations such as solid state precipitations, and to the other the diffusionless, or cellular, transformations typified by polymorphic tran- sition. In Table 1, values of m are listed for a few types of transformation [lo].

Now define the effective Mg fraction& as the active Mg remaining after cycling. As the electrochemical capacity is determined by the amount of active Mg in the hydride, we can express&, as,

fMg=G. (6)

We also define c1 as the fraction of metallic Mg changed into Mg(OH), at a certain number of cycles. If there are no other kinds of Mg consumption, we obtain

fM, = 1-a.

Combining equations (3), (6) and (7), we obtain

(7)

or

c - = exp(-Bt”) co

(8)

In

If the oxidation time of hydride for the nth cycle is t,, then

t=t,+t,+...t,=nf,. (10)

Under our experimental conditions, electrons were transmitted to the negative electrode during charging and kept the electrode in the reduced state, thus metals in the electrode would not be oxidized. Yet during discharging, the negative electrode acted as an electron export station, with the potential of the electrode increasing from - 0.93 to -0.60 V vs Hg/HgO, much higher than the equi- librium oxidation potential of Mg (which is - 2.436 V vs Hg/HgO). Thus Mg is inevitably oxidized during the discharging process. Let us denote

MECHANICALLY ALLOYED AMORPHOUS ELECTRODES I 01) I

Table 1. Values of exponent m for the Avrami equation in various cases [IO]

Polymorphic or diffusionless or cellular transformations: (a) Nucleation only at the start of transformation (b) Nucleation at constant rate (c) Nucleation at increasing rate (d) Nucleation at start plus continuing nucleation at grain edges (e) Nucleation at start plus continuing nucleation at grain boundaries

Diffusion-controlled transformations: (a) Initial growth of particles nucleated only at start of transformation (b) Initial growth of particles nucleated at constant rate (c) Growth of isolated plates or needles of finite size (d) Thickening of plates after their edges have impinged

I.5 2.5

0.5

(11)

where I 1s the discharge current, C, is the capacity of the nth cycle, and

c,> r,= 7. (12)

I f the capacity degradation is not serious, or n is a big number. then the following simplification can be made:

c7 = /,, := a constant (13)

then

((~ = exp [ - B(QmNm] $1

= exp(- K,N'")

when m = 1. we can obtain

(14)

c -7 ( (I

= exp [ - B( QN]

= exp(-K,N). (15)

This is the relation obtained by Willems [I I] from the results of deterioration tests of LaNi, and LaNi,Cu; expressed as

in this case. In the case m -= 1,2, we get. from equation (14):

Assuming <, to be a constant

c = exp( - KI,Y’ -‘b.

C’,, I, 1’))

When K,N’ ’ is numerically small. we can make a first order approximation

then

I3Oi

c =exp(-K,W’J

( ‘~, ill I )

which is the equation (2) we obtained originally. based on the assumption that oxidation is diffusion controlled. and the nucleation process can be ignored. These con- form well to the assumption by Avrami of ditrusion- controlled transformation, and thickening of plates after their edges have impringed. In this case the value of the exponent m for the Avrami equation is exactly 12. In addition, equation (2) was deduced with ,lr < 9, and Kin the order of 0.01, and hence KN a quite small number. As (&IV,‘) = (KAq’ ‘, the approximation used in equa- tion (20) and (21) is quite reasonable.

where N,, was assigned as an empirical parameter by Willems. Evidently,

,\(,, = ; (17) / From the SEM and TEM micrographs of LaNi, and

LaNi,Cu electrodes after galvanostatic cycling, the for- mation of La(OH), needles during cycling was observed, and the needle dimensions did not change noticeably with the cycling number. Thus, the formation of La(OH), belongs to the diffusion-controlled transformation, and the growth of needles of finite size. As listed in Table 1, the value of exponent m for the Avrami equation is exactly 1

EXPERIMENTAL DETAIL

Mg,Ni,,,, x and Mg,Ni,,,,.. ,M ,. amorphous alloys were synthesized by mechanical alloying of the pure pow- der mixtures at designed compositions in a planetary ball mill. To prevent the alloys from being oxidized during milling, the powder mixtures were sealed under argon in tightly closed steel vials. The amorphous states were identified by X-ray diffraction analysis: with only one amorphous large band for each MA alloy.

The discharge capacity of each sample was measured by the galvanometric method described elsewhere 12. 31.

1002 WEIHONG LIU et al.

RESULTS AND DISCUSSION

Figure 2 shows the capacity deterioration results of some mechanically alloyed amorphous Mg-based hydride electrodes. In the present study, the capacity degradation of the amorphous electrode is large, thus the approximate form of equation (14) cannot be used to analyse the test data. The relation between ln[ - In (C/C,)] vs In(t) of the mechanically alloyed Mg-Ni amorphous hydride electrodes is shown in Fig. 3. It can be seen that the mathematical representation of equations (8) or (9) agrees reasonably well with the experimental data.

The values of B and m are tabulated in Table 2, based on the calculation from equation (9) and experiment data.

The deterioration kinetics of the electrode during cycling can be roughly deduced by comparing the m value of the electrode with the exponent in Table 1. For example,

0 M&,0Ni45Se5 * Mg60Ni4,, v M&,,&,,

1 2 3 4 5 6 7 8 9

N (cycling number)

400 0 Mg50Ni45Mn5

350 * Mg50Ni45W5

V Mg50Ni45Fe5

300 l M&,&&US

Q & MgS0Nid5Ti5

a -f 250

v Mg5,,Ni45Zr5

-5 3 'n 200 k 5

150

50 1 2 3 4 5 6 7 8 9

N (cycling number)

Fig. 2. The capacity decline of some amorphous Mg-based . . . . . .

0

,” G -0.5 3 5 L -1.0

3

-1.5

-2.0 4.0

0.5 -

O-

= -0.5 -

z

8 2 -1.0 -

+ 2 4 -1.5 -

-2.0 -

4.5 5.0 5.5 6.0 6.5 7.0 7.5

Ln(t)

0 Mg50Ni4SMn5

* Mg50Ni45W5

V Mg50Ni4SFe5

* Mg5pi4p5 A MgS0Ni4sTi5

-2.5 1 I I I I I I 4.5 5.0 5.5 6.0 6.5 7.0 7.5

LW)

Fig. 3. The relation between In [-ln(C/CJ] vs In t of some amorphous Mg-based hydride electrodes.

the m value of a Mg,,Ni,&o, electrode is 0.64, not far from the exponent 0.5 in Table 1. Thus, we regard the degradation kinetics of this electrode as belonging to diffusion controlled transformation and thickening of plates after their edges have impinged. The tabulated m value of Mg,,Ni&u, electrode is 1 .Ol, from Table 1 we believe that there are two possible ways of degradation that lead to an exponent m value of 1. Thus in order to define the deterioration kinetics of Mg,,Ni,,Cu,, micro- scopic observations must also be resorted to.

By the application of equation (S), from the deterio- ration degree of an electrode after certain cycles, c( = C,-C,/C,,, the values of B and m can be obtained from the following relation, if the value of C, at two different n values is determined experimentally.

G-G __ = c( = l-exp(-BP). r nyariae electroaes.

MECHANICALLY ALLOYED AMORPHOUS ELECTRODES 100:

Table 2. Values of parameters m and B for the experimental allo!rs

Alloy Co (mAh/g) m B(xl0 “) C(‘,,- (;>I/(‘,, (X)

Mg50Ni50 395.8 1.36 8.7 5X.‘. Mg55Ni4q 206.7 0.75 1100 60.‘” WkJ% 196.7 0.91 2x5 4X..!

&,Ni&o, 306.7 0.64 2190 66.:: Mgdi,,Zn, 388.3 1.36 ! I.1 5X.): MgsclNi4,Se5 93.3 1.26 I 70 ho.-

Mg,,Ni,,Mnc 345 0.92 136 4’..’ MMJiasWi 226.7 0.83 261 40. I Mg5&5Fe5 273.3 0.82 163 2’) (I Mg,,Ni,,Cu, 343.3 1.01 60.5 3’ Mg,,Ni,,Ti, 400 1.01 127 66. i Mg,,Ni,,Zr, 239.2 0.95 ‘20 5i.4

Values of m and B thus obtained for different alloys are in Table 2. With a definite m, the alloys with higher B values have higher deterioration rates after the same cycling number. In addition, the capacity of the electrode alloy also has some effect on the deterioration degree, as a rule, during cycling, the discharge current density is fixed at a certain value (say at 100 mA/g). With the other parameters remaining the same, the alloy with a large initial capacity has a longer discharging time. As dis- cussed above, the deterioration reaction time equals the discharging time. Therefore the alloy with a larger initial capacity undergoes a longer deterioration reaction, which leads to a higher degree deterioration. In order to get both high capacity and a long cycling life, we should choose those alloys with small B and m values as potential electrode material.

As m is a kinetic exponent and B is the rate constant parameter of the deterioration reaction, any factor that affects the deterioration kinetics will affect B and m. These factors are the specific surface area, diffusion coefficient, the pulverization tendency etc. A deeper depth of discharge which results in a longer deterioration time, also decreases the anticorrosive properties of a hydride electrode.

In order to obtain a good alloy with desirable cycling properties, we must choose those alloys with small values of B and m in equation (8), by either changing the x value of Mg,Ni,,,,,.-. or adding a ternary element. Ternary additives such as Fe, Cu, Ti. W etc. may cause changes both in m and B, as shown in Table 2. These additives modify the microstructures of these alloys, both the matrix and the surface structures. Surface protection methods such as Ni-plating are also effective in lowering the rate constant B. Adding some reagent to hinder the oxidation of the alloy also changes B and m.

CONCLUSIONS

( 1) Oxidation of the hydride in KOH solution during cycling is the main cause of capacity deterioration of

metal hydride electrodes. The rate of capacity deteriora- tion is determined by the growth rate of hydroxide. the specific surface area of the hydride particles and the oxi- dation time in each charging-discharging cycle.

(2) Equation (8) describes well the cycling behavior of hydride electrodes when the activation process is ignored. It is very useful in evaluating capacity and cycling life tests on hydride electrodes.

(3) In order to get good cycling properties. methods that lower B and m should be used.

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