The capacity deterioration model of mechanically alloyed MgXNi100−X amorphous electrodes in charging-discharging cycling

Pergamon SO360-3199(96)00022-S

THE CAPACITY DETERIORATION MODEL OF MECHANICALLY ALLaOYED Mg,,Ni,,,,,_ ,yAMORPHOUS ELECTRODES IN CHARGING-DISCHARGfNG CYCLING WEIHONG Materials

Abstract-In

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

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 mechanism amorphous Energy

data are in good agreement with the deduced relation. Analysis shown that this is the common of hydride electrode degradation when the activation process is ignored. The parameters m and B of some hydrides are calculated on the basis of this relation. ,(’ 1997 International Association for Hydrogen

NOMENCLATURE

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 MgNi 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]:

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

or

INTRODUCTION

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 composition, 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

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, sity, Shanghai 200433. P.R. China.

Fudan

Univer-

99’ 9

1000

WEIHONG LIU

initial stage of electrochemical cycling. Many studies on LaNi,-type hydride electrodes have shown that the oxidation reaction during electrochemical cycling which causes capacity degradation is a nucleation and growth reaction [4]. In this paper, the authors attempt to formulate 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.

As mentioned above, previous investigation [2, 31have discovered that the oxidation of Mg and Ni in amorphous Mg-Ni hydrides is the main cause for the capacity degradation 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 simplest form can be written as [5]: ’ CI= l-exp(-Bt”) ln[-ln(l-cr)]

(3)

= lnB+mlnt

where CIis 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 - -Bt”-‘(1 Tt-

THE PROPOSED MODEL FOR CAPACITY DETERIORATION OF AMORPHOUS Mg-Ni HYDRIDE ELECTRODES

or

et al.

(4)

-a)m

where (1 -m) may be regardedas the allowancefor the impingementeffect; here B is a rate constantparameter andm the time exponent,Hulbert [9] hastabulatedvalues of m for a variety of nucleation-and-growthmodels.Two major types of transformationsmay be treated by the application of Avrami equation. To onecategorybelong the diffusion-controlled transformations such as solid stateprecipitations,and to the other the diffusionless,or cellular, transformationstypified by polymorphic transition. In Table 1, valuesof m are listed for a few types of transformation [lo]. Now definethe effective Mg fraction& as the active Mg remaining after cycling. As the electrochemical capacity isdeterminedby the amountof active Mg in the hydride, we can express&, as,

fMg=G.

(6)

We alsodefinec1asthe fraction of metallicMg changed into Mg(OH), at a certain numberof cycles.If there are no other kinds of Mg consumption,we obtain

fM, = 1-a.

(7)

Combiningequations(3), (6) and (7), we obtain c - = exp(-Bt”) co

or

(8)

In If the oxidation time of hydride for the nth cycle is t,, then t=t,+t,+...t,=nf,.

Fig.

1. SEM morphology of Mg,,NiSo cycles.

after

(10)

Under our experimental conditions, electrons were transmittedto the negativeelectrodeduring chargingand kept the electrodein the reducedstate,thusmetalsin the electrodewould not be oxidized. Yet during discharging, the negativeelectrodeactedasanelectronexport station, with the potential of the electrodeincreasingfrom - 0.93 to -0.60 V vs Hg/HgO, much higher than the equilibrium oxidation potential of Mg (which is - 2.436V vs four electrochemical Hg/HgO). Thus Mg is inevitably oxidized during the dischargingprocess.Let usdenote

MECHANICALLY

ALLOYED

AMORPHOUS

I01)I

ELECTRODES

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

(11) where I 1s the discharge current, nth cycle, and r,=

I.5 2.5 0.5

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

C, is the capacity of the

c,> 7.

(12)

Assuming

c = exp( - KI,Y’ -‘b. C’,,

If the capacity degradation is not serious, or n is a big number. then the following simplification can be made: c7 = /,, := a constant

(13)

<, to be a constant

When K,N’ ’ is numerically order approximation

I,1’))

small. we can make a first

then

I3Oi ((~ = exp [ - B(QmNm] $1 = exp(-

then

K,N'")

(14)

when m = 1. we can obtain c -7 = exp [ - B( QN] ( (I = 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

where N,, was assigned as an empirical Willems. Evidently, ,\(,, = ;

/

parameter

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 conform well to the assumption by Avrami of ditrusioncontrolled 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 equation (20) and (21) is quite reasonable.

by EXPERIMENTAL

(17)

From the SEM and TEM micrographs of LaNi, and LaNi,Cu electrodes after galvanostatic cycling, the formation 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

DETAIL

Mg,Ni,,,, x and Mg,Ni,,,,.. ,M ,. amorphous alloys were synthesized by mechanical alloying of the pure powder 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

,” G -0.5 3 5 L -1.0 3

-1.5

-2.0

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

Ln(t)

0.5 -

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

O-

=

-0.5

-

82

-1.0

-

+ 2 4

-1.5

-

0 Mg50Ni4SMn5 * Mg50Ni45W5

-2.0

-

V Mg50Ni4SFe5 * Mg5pi4p5 A MgS0Ni4sTi5

-2.5

1 4.5

z

1

2

3

4

5

N (cycling

6

7

8

9

number)

I 5.0

I 5.5

I 6.0

I 6.5

I 7.0

I 7.5

LW) 400

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

0 Mg50Ni45Mn5 * Mg50Ni45W5

350

V Mg50Ni45Fe5 300

l

& MgS0Nid5Ti5

Q

a -f -5 3 'n k 5

v Mg5,,Ni45Zr5

250 200 150

50

1

2

3

4 N (cycling

Fig.

M&,&&US

2. The

capacity

5

6

7

8

9

number)

decline of some amorphous . . .. . . nyariae electroaes.

Mg-based

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,, microscopic observations must also be resorted to. By the application of equation (S), from the deterioration 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 __ r

= c( = l-exp(-BP).

MECHANICALLY Table 2.

ALLOYED

AMORPHOUS

100:

ELECTRODES

Valuesof parameters m andBfor theexperimental allo!rs

Alloy Mg50Ni50 Mg55Ni4q

WkJ% &,Ni&o, Mgdi,,Zn, MgsclNi4,Se5 Mg,,Ni,,Mnc MMJiasWi Mg5&5Fe5

Mg,,Ni,,Cu, Mg,,Ni,,Ti, Mg,,Ni,,Zr,

Co (mAh/g) 395.8 206.7 196.7 306.7 388.3 93.3 345 226.7 273.3 343.3 400 239.2

m 1.36 0.75 0.91 0.64 1.36 1.26 0.92 0.83 0.82 1.01 1.01 0.95

Valuesof m and B thus obtained for different alloys are in Table 2. With a definite m, the alloys with higher B valueshave higher deterioration rates after the same cycling number.In addition, the capacity of the electrode alloy alsohassomeeffect on the deteriorationdegree,as a rule, during cycling, the dischargecurrent density is fixed at a certain value (sayat 100mA/g). With the other parametersremainingthe same,the alloy with a large initial capacity has a longer dischargingtime. As discussedabove, the deterioration reaction time equalsthe dischargingtime. Therefore the alloy with a larger initial capacityundergoesa longerdeteriorationreaction,which leadsto a higher degreedeterioration. In order to get both high capacity and a long cycling life, we should choosethosealloyswith smallB andm valuesaspotential electrodematerial. As m is a kinetic exponent and B is the rate constant parameterof the deterioration reaction, any factor that affects the deterioration kinetics will affect B and m. These factors are the specific surface area, diffusion coefficient,thepulverization tendencyetc.A deeperdepth of dischargewhich resultsin a longerdeterioration time, also decreases the anticorrosive propertiesof a hydride electrode. In order to obtain a good alloy with desirablecycling properties,we mustchoosethosealloyswith smallvalues of B and m in equation(8), by eitherchangingthe x value of Mg,Ni,,,,,.-. or adding a ternary element. Ternary additives suchas Fe, Cu, Ti. W etc. may causechanges both in m and B, asshown in Table 2. Theseadditives modify the microstructuresof these alloys, both the matrix and the surface structures. Surface protection methodssuchasNi-plating are alsoeffective in lowering the rate constant B. Adding somereagentto hinder the oxidation of the alloy alsochangesB and m. CONCLUSIONS ( 1) Oxidation of the hydride in KOH solution during cycling is the main causeof capacity deterioration of

B(xl0

“) 8.7

C(‘,,-

(;>I/(‘,,

(X)

5X.‘.

1100 2x5 2190 ! I.1 I 70 136 261 163 60.5

60.‘” 4X..! 66.:: 5X.): ho.4’..’ 40. I 2’) (I 3’

127

66. i

‘20

5i.4

metal hydride electrodes.The rate of capacity deterioration is determinedby the growth rate of hydroxide. the specificsurfaceareaof the hydride particlesand the oxidation time in eachcharging-dischargingcycle. (2) Equation (8) describeswell the cycling behavior of hydride electrodeswhenthe activation processisignored. It is very useful in evaluating capacity and cycling life testson hydride electrodes. (3) In order to get good cycling properties.methods that lower B and m shouldbe used.

REFERENCES 1. Y. Q. Lei, Y. M. Wu, Q. M. Yang, J. Wu and Q. I). Wang. Z. Ph.vs.Chrm. BD183.S379-384 (1994). 2. W. H. Liu, Y. Q. Lei, D. L. Sun, J. Wu and Q. D. Wang, J. Power Souwes, 58(2). 243 -247 (1996). 3. D. L. Sun, Y. Q. Lei, W. H. Liu, J. J. Jiang, J Wu and Q. D. Wang. International Symposium on Metal-H,vdrogen Svstrms, Fundamentals and Applications. Tokyo. Japan, Nov. 6-11 (1994) J. Al10,v.sCompd.. 231, 621-624 (1995). 4. Z. P. Li, Y. Q. Lei, B. H. Liu. J. Wu and Q. I>. Wang,

International Symposium on Metal-Hydrogen System.\. Fundamento1.r and Applications. Uppsala, Sweden, June 8 -17 (1992) Z. Phys. Gem. BD183, S287-295 (1994). 5. J. D. Hancock and J. H. Sharp, J. .Im. (‘cram SW 55. 74 77 (1972). 6. W. A. Johnson and R. F. Mehl, Trans. .4/M6. 135(P). 416 442 (1939). 7. M. Avrami, J. Chem. P/q,.>.7(12), 1103~ It 1939): [hid.. g(2). 212-24 (1940): ibid. 9(2), 177-84(1941). 8. B. V. Erofe’ev, C. R. Doki Acad. Sci. PCRSS. 52. 51 I I4 (1946). 9. S. F. Hulbert, Models for solid-state reactions in powdered compacts: A review. J. Brir. Cwam. Sot. 6(I), I l--Xl (1969). 10. C. N. R. Rao, Phase 7imnition in Solids, p. ‘)3, ‘McGrawHill. New York (1978). II. J. J. G. Willems. Philip\ .I Rex 39( 1). I i 19841