The cathodic decomposition of propylene carbonate in lithium batteries

273

J. Eiec?wanal, Chenq 219 (1987) 273-280 Ekvier Sequoia S.A., Lausatme - Printed in The Netherlands

THE CATHODIC DEC~~S~ON LZTHlUMBA-

OF PROMO

CARBONATEIIN

MXSAYASU ARAKAWA and JUN-ICHI YAMAKI Nippon Teiegraph ad Telephone Corpwation, Tokai, Rmraki-ken, 319-11 (Japm)

NTI” &r&i

Electrical Co~~‘c~t~~

Laboratories,

(Received 29th January 1986; in revised form 20th October 1986)

ABSTRACT

Cathodic decomposition of propylene carbonate (PC) on the graphite electrode in lithium batteries is investigated galvanostatically and potentiostatically. A new reaction process of PC decomposition is proposed, in wbicb the PC decomposition reaction completes with the formation of the lithium intercalated compound (GIC). The process is also argued theoretically and experimentally. Many experimental results support this process.

INTRODUCTION

Recent ~v~tigations of lithium secondary batteries indicate that they may explode during extensive multicycle testing [1,2]. For lithium batteries using propylene carbonate (PC) as a solvent, the explosion hazard caused by PC decomposition was foreseen in a report by Dey and Sullivan [3]. They identified the main component of the decomposition products as propylene gas. They also indicated that an electrochemical decomposition process occurs at the graphite electrode. Dousek et al. indicated that PC was decomposed on a lithium amalgam [4]. They assumed that PC would be decomposed chemically at the graphite electrode through its surface catalytic effect. Eichinger proposed a process of PC decomposition due to the chemical reaction of lithium intercalated graphite compounds [5]. These investigations assumed 100% efficiency of the PC decomposition against the quantity of electricity. The present authors suggest that there are some reactions in addition to the PC decomposition. In this work, PC decomposition at the graphite cathode is investigated more closely. 0022-0728/87/$03.50

@ 1987 Elsevier Sequoia S.A.

274

The test cell used for all experiments was constructed as described in a previous paper [6]. Graphite powder (Nippon Carbon, SP-2) was used as a cathode material. Graphite powder was mixed with polytetrafluoroethylene at a ratio of 94:6 weight percent and pressed at 2.94 x 10’ Pa to form I3 mm diameter disks which were covered with titanium nets having welded nickel wire lea& The lithium electrode was formed by pressing 13 mm diameter and 0.43 mm thick lithium {LITHCOA) on the nickel net. Lithium metal was also used for the reference electrode. PC containing 1 M LiClQ, (Tomiyama Pure Chemical) was used as received. Water contents were less than 20 ppm. The test cell was set in a vacuum line arid evacuated to a pressure of 1 X RF2 Pa. The volume of gas that evolved during discharge tests was measured by an oil manometer in the vacuum line. It was expected that the gas volume would be measured more precisely by this method than by Dey’s method [3]. The components of the gas were identified by a mass spectrometer (ULVAC, MSQ-150) connected to the vacuum line. X-ray diffraction analysis of the cathodes was carried out with a diffractometer (Rigaku) using CuKo radiation. All cathodes were dried at room temperature in vacua (1 x lo-* Pa) for several weeks before the analysis. RESULTS AND DISCUSSION

There are two proposals regarding the PC decomposition process of lithium batteries using PC as a solvent. One is the electrochemical decomposition process proposed by Dey and Sullivan [3]. It goes as follows: PC+2

e=+C,H,+CG,Z”-

0)

The other is the EC process via graphite intercalation compound (GIC) formation proposed by Eichinger [a], which proceeds as follows: k,

2 Li+ + 2 e- f C,dC,Li, C,Li,+PC~~H,+2Li++CO:-~C,

(3)

These proposed processes suggest 100% efficiency of gas evolution in relation to the quantity of electricity at a steady state. IIowever, new experimental results, which indicate the efficiency is less than IOtX& were obtained in this work_ Galvanostatic discharge tests of the lithium battery using graphite cathodes were carried out at current densities of 0.?5 mA/cm=, 2.26 r&/cm2 and 4.52 mA/cm2. The discharge voltage dropped sharply to a relatively steady plateau voltage of approximately 0.9 V in each case. Remarkable gas evolution resulting from PC

275

evdved gp

Fig. 1. Mass spectrum of gas evolved from the test cell and the standard spectrum of propylene.

d~rn~siti~ was observe& during the discharge tests. A typical mass spectrum of the gas that evolved from the cathode during the discharge tests is shown in Fig. 1. The main component was identified from the mass peaks (M/Z = 39,41 and 42) as propylene. Upon comparison with a standard spectrum of propylene, the mass spectrum also suggested the existance of a small percentage of other gas species. This result was consistent with previous reports [3,5]. The relationships between gas v&me and specific capacities in each of the current densities are shown in Fig. 2. The coulombic efficiencies were calculated to be 54.7% for 0.75 mA/cm2 and 68.1% for both 2.26 mA/cm2 and 4.52 mA/cm*. In each case, two-electron transition reactions were assumed for the PC decomposition reaction. There is other evidence against eqns. (l), (2) and (3). The ~lations~ps of the evolved gas column and the discharge voltage, plotted as a function of the quantity of electricity, are shown in Fig. 3. In this experiment, the discharge was continued after the plateau voltage had been reached. Most earlier investigators stopped measurements at the plateau voltage, which was regarded as the decomposition voltage of PC. The voltage dropped gradually, and reached 0 V at 1.37 X lo3 A h kg-‘. When the galvanostatic charge was applied after the discharge to 0 V, the voltage immediately increased to about 5 V. This result suggests that cathode discharge to 0 V is electrochemically irreversible.

216

capacity

I nt?ti’

Fig. 2. The dependence of the gas column on discharge capacity at different discharge current densities.

If electrochemical reactions occurred on the electrode as in eqn. (l), and if all GIC decomposed PC accompanied re-formation of graphite as in eqn. (3), then the discharge voltage would remain steady and the gas volume evolved would be consistent with the quantity of electricity. However, the ratio between the gas volume and that equivalent to the quantity of electricity passed was 65.2%. Figure 3 suggests the formation of some lithium compounds which do not decompose PC. Consequently, we propose a new PC decomposition reaction scheme. This

Fig. 3. The relationships of evolved gas column and discharge voltage against discharge capacity when galvanostatic discharge was carried out, up to 0 V (vs. Li).

211

involves an intermediate, which consists of carbon atoms and solvated lithium ions, as follows: 2 Li+(PC),

+ C, + 2 e- 2 [Li(PC),],C,

[Li(PC),I&:

C, + C,EI, + LisCOs + (2m - 1) PC k5 Li&

(4) 0)

+ 2m PC

(61

where k,, k, and k, are rate constants of the intermediate formation, PC decomposition and GIC formation, respectively. C,, represents a quantity of carbon atoms which depends on the voltage. Also, k, is the electrochemical reaction constant: k, = k; exp(-2aFV/lW). As an intermediate, [Li(PC),],C, will be formed at the surface of the electrode, and it will decompose PC through its catalytic effects. Equations (5) and (6) proceed competitively, and some of the lithium atoms on the cathode diffuse (with a rate constant = k,) into the cathode to form the GIC (Li&), which does not decompose PC. In order to confirm our proposal, reaction kinetics are considered. If eons. (4)-(6) are correct, the system may be described by the following equations: x

+ k,y

dx/dr

= -k,x

dy/dt

= k,x - (k, + k,)y

(8)

dz/dt

= k, y

(9)

(7)

where x, y and z are the quantities of C,, intermediate and propylene gas, respectively. Consequently, the gas evolution rate (dzfdt) can be described as follows: dz/dt

00

= k3k4c,[e(b-a)’ - e-@+@‘]/26

1

2

3

4

5

6

7

8

9

(10)

10 11 12 13 14

Time/h

Fig. 4. The dependence of the gas evolution rate on time at 0.8 V potentiostatic discharge. The broken line was calculated using eqn. (11). The solid line was calculated using eqn. (10).

278

where a = (k3 + k, + k&‘2, b = /(k, + k, + k,)’ - 4k,k, /2, and ce is the initial quantity of C, at a certain voltage (V). Equation. (10) suggest that the gas evolution rate will decrease after reaching its maximum. However, if eqn. (1) is correct, the gas evolution rate will be constant at all times. Also, if eqns. (2) and (3) are correct, the gas evolution rate (dz/dt) will be described as follows: dz/dt = k,k,c,[l - e(-k~-k~)i]/( k, + k,) 01) where k, and k, are the rate constants shown in eqns. (2) and (3). Equation (11) indicates that the gas evolution rate will reach a constant value after some time. The dependence of the gas evolution rate on time at 0.8 V potentiostatic discharge was examined. The results are shown in Fig, 4. As can be seen in the figure, the gas evolution rate increased at first and gradually decreased after reaching a quasi-steady state, which strongly supports eqn. (10). Characteristics of PC decomposition Gas evolution rates in potentiostatic discharge tests were investigated to understand the PC decomposition reaction better. The gas evolution rate at quasi-steady states in each ~~tiostatic discharge was regarded as that ~~es~nd~g to a certain voltage. The dependenoes of gas evolution rate, total current and net current on voltage are shown in Fig. 5. The voltage was dropped step by step from 1.0 V to 0.7 V. The gas evolufion rate was converted into current, assuming a two-electron transition. The total current indicates the overall current flow in the cell. The net current was

ld”

) 03

I I 09I 0.6 Wtage

IV

.a,1.0

0

1 0.7

I 0.8

I 0.9

1 1.0

J

VdtaqeiY

Fig. 5. The depcndcncc of gas evolution rate, total current aud net current on voltage. Fig. 6. The dependence of the ratio of gas evolution to the quantity of electricity on the voltage.

279

derived by subtracting the gas evolution rate from the total current, thus indicating the net amount of lithium which reacted with graphite. The figure shows that the gas evolution rate and total current increase logarithmically with a decrease of voltage, and that the gradient of each plot decreases at lower voltages. In particular, the net current scarcely changes between 0.9 and 0.7 V. In eqn. (6), the rate constant of lithium-graphite compound formation (k,) involves a ~ff~ion process of lithium atoms into the graphite cathode. The net current in Fig. 5 suggests that the rate of Li&s formation will be controlled by the diffusion rate of lithium atoms. This is because the net current is limited. On the other hand, the PC decomposition rate will increase logarithmically with a decrease in voltage because C, will increase with a decrease in voltage. Consequently, the ratio of gas evolution to the quantity of electricity will increase with a decrease in voltage, as shown in Fig. 6. From this point of view, the 100% efficiency in Dey’s report [3] may have resulted from some difficulties in the diffusion of lithium atoms. This is probably because they used graphite rods as the cathode, in which the surface area was smaller than that of the cathode used in this work. Litfiium-graphite compound

The lithium-graphite compound (Li&) formed by discharge is also of interest. Li& is usually considered to be GIC [7] and is in this work, too. However, Li,Cs was electrochemically irreversible after the discharge to 0 V. The result shown in Fig. 3 may help to explain this. According to one report concerning lithium intercalation, 1 lithium atom reacts with 6 carbon atoms of graphite (1 carbon atoms reacts with 0.17 lithium atom) at most [8]. Nevertheless, the results of this work indicate that at least 1 carbon atom reacts with 0.21 lithium atom. This result suggest that a lithium-graphite compound discharged to 0 V is no longer an intercalation compound. It is probable that lithium atoms are inserted into the

5 I.

10

15

20

25

28 (deg.)

30 Cu Ka

35

40

1

45

Fig. 7. X-ray diffraction pattern of the cathode discharged up to 0 V (vs. Li). Asterisks (*) mark the peaks which appeared in the ex-situanalysis.

280

graphite cathode to~he~c~y to form GIC, and that lithium atoms in topochemical excess form a chemical compound with carbon atoms, accompanying the decomposition of GIC. Thus, the graphite discharged to 0 V is thought to be a chemical compound. Also, if a lithium-graphite compound discharged to 0 V is a chemical compound, it is reasonable to expect that a cathode discharged to 0 V is electrochemically irreversible over the voltage range adopted in this work. An X-ray diffraction analysis of the graphite cathode discharged to 0 V is shown in Fig. 7. The intensity of the (002) peak decreased to about one-fortieth of the undischarged one. Unfortunately, other peaks which indicate the formation of some kinds of lithium-graphite compounds were not observed, except for many unidentified peaks which appeared during the ex-situ X-ray diffraction analysis. CONCLUSION

The decomposition of PC in lithium batteries was investigated galvanostatically and potentiostatically. Gas evolution, resulting from the decomposition of PC, was observed during the discharge. The gas evolution efficiency was less than 1008, though Dey and Sullivan had reported it to be 100%. Furthermore, when the galvanostatic discharge was continued after reaching the voltage plateau (which had been considered the voltage of PC decomposition), discharge voltage dropped until it reached 0 V. The cathode discharged to 0 V has little charge capacity. To explain these experimental results, a new scheme was considered, in which PC decomposition by an intermediate competes with GiC formation and GIC does not decompose PC. ~guments using the reaction kinetics supported the new scheme. Results concerning the gas evolution rate dependence on time in the potentiostatic discharge test also supported this. The results of potentiostatic discharge tests are also explained well by the new scheme. It was also suggested that lithium in topochemical excess formed an electrochemically irreversible compound with carbon atoms of graphite. ACKNOWLEDGEMENTS

The authors wish to thank Chikao Uemura and Takeshi Okada for their helpful guidance and suggestions during the course of this work. REFERENCES 1 2 3 4 5 6 7 8

G.H. Newman, R.W. Fraszis, L.H. Gaines and M.L. Rao, J. Electrochem. Sot., 127 (1980) 2025. L.P. Klemaun and G.H. Newman, J. Electrochem. Sot., 128 (1981) 13. A.N. Dey and B.P. Sullivan, J. Electrochem. Sot., 117 (1970) 222. F.P. Dousek, J. Jansta and J. Riha, J. Electroaual. Chem., 53 (1974) 329. G. Eichinger, J. Electroanal. Chem., 74 (1976) 183. M. Arakawa, I. Yamaki aud T. Okada, I. Ebrochem. Sot., 131 (L984) 2605. J.O. Besenhard, Carbon, 14 (1976) 111. D. Guerard and A. Herold, Carbon, 13 (1975) 337.