A0-Allb 790 EIC LABS INC NEWTON MA pso,LITHIUM CYCLING IN POLYMETHOXYMETHANE SOLVENTS. F/ 1/

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TECHNICAL REPORT NO. 7

LITHIUM CYCLING IN POLYMETHOXYMETHANE SOLVENTS

!W-4 by

N J. S. Foos and J. McVeigh

Prepared for Publication

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Journal of the Electrochemical Society

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June 1982 JUL 1 2 1982

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Solutions of LiAsF6 in dimethoxymethane (DM1) and trirnethoxymethane(TMM) are shown to be conductive and capable of cycling Li in high effi-ciency in half-cells. Inspection of electrolytes stored at 700C in thepresence and absence of Li indicated that TIE' is more stable than DM11.However, DM11 cycles Li much better than TMM after storage.

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LITHIUM CYCLING IN POLYMETHOXYMETHANE SOLVENTS

J. S. Foos and J. McVeighEIC Laboratories, Inc., 67 Chapel Street, Newton, MA 02158

Introduction

During the past few years several Li battery electrolytes have been

reported which allow the plating and stripping of Li in greater than 95%

efficiency at 250C. Newman reported the use of LiClO4 in dioxolane in

Li/TiS 2 cells (1). The cycling efficiency in these cells is very good but

unfortunately they are susceptible to detonation (2). Koch has reported

the use of 2-methyltetrahydrofuran (2Me-THF) (3) and diethyl ether blends

(4) as solvents for LiAsF6 in Li half-cells. The conductivity of these

electrolytes is low.

The solution of LiAsF6 in dioxolane is an electrolyte with good

conductivity but is reported to polymerize readily (5). Ring strain may

contribute to the reactivity of dioxolane. It might be anticipated that

the open-chain analogues of dioxolane might be less reactive. Dimethoxy-

methane (DMM)I and trimethoxymethane (TMM)II were investigated as solvents

for possible use with LiAsF 6 in Li batteries.

CH2 (OMe)2 CH(OMe) 3

I II

We wish to report that these solvents produce conductive solutions

with LiAsF6 , show good stability at 70'OC, and cycle Li well in half cell

tests.

Experimental

The solvents, DMM and TMM (trimethyl orthoformate), were obtained

from the Aldrich Chemical Company. Before use the solvents were purified

first by preliminary drying using MgSO4 and/or molecular sieves (3A or 4A)

for DM4; and molecular sieves (3A or 4A) only for TMM. Subsequently they

were distilled from Na benzophenone ketyl (6). Some difficulty was en-

countered in forming the blue ketyl in TMM. This was attributed to surface

films on the Na which can be disrupted by placing the reaction flask in an

ultrasonic bath.

The electrolytes for cycling and storage tests were prepared using

LiAsF 6 (U.S. Steel, as-received) with cooling using Cu shot previously

cooled by dry ice. The solvents, if treated with alumina, were passed

through a column of activated alumina (Woelm N-Super 1) and the first 10%

discarded. Where applicable, pre-electrolyses were done by passing a cur-

rent (1 mA/cm2) between two Li metal electrodes (-30 cm2) suspended in the

electrolyte (100 ml) for -16 hrs.

Conductivities were obtained using dried distilled DMM and TMM. The

conductivities were determined in conductivity cells with cell constants

determined using aqueous KCl. An AC conductivity bridge (YSI Model 31) was

used to measure the cell resistance. The conductivities were determined

at, or corrected to, 250C with a precision of -5%.

The storage tests were done in screw cap (Teflon-lined) glass vials.

These were stored at 700 C.

2 .- -6-- - -- .--.

The Li cycling was done in prismatic glass cells containing 15-20 ml

electrolyte. This experiment, described previously (7), involves making a

pre-plate of 4.5 C/cm2 of Li on a Ni substrate (Ni shim) followed by strip-

ping and then re-plating 1.1 C/cm2 on the pre-plated Li. The average effi-

ciency per cycle E, is calculated in Eq. (1):

Qex% s - -n (-Qs

where n is the number of apparent "100%" cycles, Qs is the charge stripped

(1.1 C/cm2 ), and Qex is the charge in the excess Li (3.4 C/cm2 ) at the

start of the experiment.

Solution preparation and the electrochemical experiments themselves

were conducted at room temperature under an Ar atmosphere in a Vacuum-

Atmospheres Corporation dry box equipped with a Model HE-493 Dri-Train.

Results

Conductivity of Electrolytes. In order to minimize electrolyte IR

drop, the electrolyte in a battery should show maximum conductivity. The

solvents I and II give electrolytic solutions with greater conductivities

than most other non-cyclic ethers.

The conductivities of solutions of LiAsF 6 in ethers are generally

high as compared to solutions of other salts such as LiClO 4, LiBF4 , etc.

For instance, the maximum conductivity of LiAsF6 in THF is 0.017 Q-1 cm-1

(7) as compared to 0.006 and 0.004 for LiCIO4 (8) and LiBF4 (9), respec-

tively. The ethereal solutions of LiAsF 6 previously known to cycle Li with

high efficiency, i.e., 2Me-THF and DEE blends, have maximum specific con-

ductivities in the 0.002-0.004 S- 1 cm-1 range.

3' '4

Conductivity vs. concentration profiles for LiAsF6 in DM and TM

are shown in Figure 1. The conductivity profiles are typical for solutions

of LiAsF6 in ethers and show maximum conductivities of -0.0065 Q-1 cm-1

for DMM and -0.008 1- 1 cm-1 for TMM.

Lithium Cycling in Half-Cells. The plating and stripping of Li at

250C is a rigorous test of electrolyte stability. Electrolytes were eval-

uated for half-cell cycling efficiency using electrolytes prepared using

distilled solvent (D), distilled solvent treated with alumina (DA), and

electrolytes that were additionally pre-electrolyzed (DP).

The freshly plated Li, generated during pre-electrolysis, should be

especially reactive. It was hoped that this Li would react with impurities

in the electrolyte giving insoluble reaction products and thereby purify

the electrolyte. The results for DMM and TM electrolytes are shown in

Table 1. This table shows representative cycling efficiencies which illus-

trate the effect of different purification on Li cycling. In these tests,

both electrolytes show high cycling efficiency.

Electrolyte Stability at 700C. The cycling test above allows the

evaluation of room temperature stability of the electrolyte towards freshly

plated Li over the period of the experiment, one or two days. An accel-

erated test for long term stability is the storage of electrolytes at 700C

in the presence and absence of Li. A solvent, 2Me-THF, which has proved

useful in Li/TiS 2 cells shows excellent stability when stored with Li at

700C (7).

414 I 4

Stability tests were conducted by storing samples of 1.5M LiAsF6 in

DMU and TM in the presence and absence of Li at 700C. The results are

shown in Table 2. The TMM appears more stable with no apparent reaction

noted at 20 days at 700C. These tests may be somewhat misleading. Al-

though no reaction was observed visually, the TM electrolyte stored. at

700C with Li subsequently failed completely to cycle Li. The DMM results

were variable,.with the results reported typical of several tests. Despite

the apparent lack of stability in DMM electrolytes, the samples stored with

Li retained moderate cycling ability.

Discussion

The conductivity of the electrolytes studied make them interesting

candidates for Li battery use. However, the stability of the electrolytes

under actual cell conditions is of primary concern. This includes stabi-

lity towards reduction by Li and to thermal decomposition in the bulk elec-

trolyte. The half-cell cycling results indicate a relatively high degree

of stability of electrolyte towards Li. However, this test is somewhat

insensitive to electrolyte decomposition due to the large excess of elec-

trolyte present. The variation of the cycling efficiency with purification

appears to be reproducible. it is attributed to variations of the levels

of impurities. What is not clear is the nature of the impurities and

whether they improve half-cell and storage test behavior. it may be that

the electrolyte containing only pure solvent and salt would give the best

results. On the other hand, perhaps small amounts of impurities may scav-

enge undesirable reactive intermediates. Differentiating between these

possibilities is difficult.

In this work, electrolytes which were optimized in half cell work

were chosen for storage tests and cycling after storage. The DMM elec-

trolytes (D and DA) gave similar cycling efficiencies before storage and

similar, though decreased, efficiencies after storage. The electrolyte

purified with alumina (DA) gave contradictory results in that it appeared

more stable than the DMM (D) in the presence of Li and less stable in the

absence of Li. The TMM electrolyte also gave contradictory results by

showing no apparent reaction in stability tests and yet failing to cycle

after storage.

The above observations illustrate the difficulty in evaluating elec-

trolytes for Li batteries half-cells and the appearance of storage tests.

Further investigation of these electrolytes in Li/TiS2 test cells as well

as a more thorough chemical analysis of the storage tests is in progress.

We wish to acknowledge the support of the Office of Naval Research

for this research.

*1*

References

1. G. H. Newman in Proceedings of the Workshop on Lithium NonagueousBattery Electrochemistry, Vol. 80-7, the Electrochemical Society, Pen-nington, NJ, 143 (1980).

2. G. H. Newman, R. W. Francis, L. H. Gaines and B. M. L. Rao, J. Elec-

trochem. Soc., 127, 2025 (1980).

3. V. R. Koch and J. H. Young, Science, 204, 499 (1979).

4. V. R. Koch, J. L. Goldman, C. J. Mattos and M. Mulvaney, J. Electro-chem. Soc. 129, 1 (1982).

5. P. G. Glugla in Proceedings of the Symposia on Power Sources for Bio-medical Implantable Applications and Ambient Temperature Lithium Bat-

teries, B. B. Owens and N. Margalit, eds., Vol. 80-4, The Electro-chemical Society, Pennington, NJ, 407 (1980).

6. V. R. Koch, J. Electrochem. Soc., 126, 181 (1979).

7. J. L. Goldman, R. M. Mank, J. H. Young and V. R. Koch, J. Electrochem.Soc., 127, 1461 (1980).

8. P. Jagodzinski and S. Petrucci, J. Phys. Chem., 78, 917 (1974).

9. S. D. James, J. Chem. Eng. Data, 23, 313 (1978).

7

FIGURE CAPTION

Fig. 1. Conductivity of LiAsF6 solutions in DMM and TMM (250C).

8I

W~b~kI

(SAT'D)6*

ux

0

-o0. .0152. .

MOA COCNRTO

TABLE 1

HALF-CELL CYCLING TESTS USING D14Z AND TMM ELECTROLYTES

1.5M LiAsF6 in DMM (D) 97.3%

(DA) 97.5%

(DP) 96.5%

1.5M LiAsF6 in TMM (D) 93.2%

(DP) 95.1%

10

TABLE 2

STORAGE TESTS AND CYCLING AFTER STORAGE USING DMM AND THM

ELECTROLYTES AT 70 0 C

Solvent With Li Without Li

DMM (D) Electrolyte clear with a small Electrolyte clear >10 daysamount of Li corrosion at 10 then darkened rapidly.days.

DMM (D) Cycled after 10 days storagewith Li -1 87.5%.

DMM (DA) Electrolyte clear with a small Electrolyte dark at 6 days.amount of Li corrosion, > 1month.

DMM (DA) Cycled after 10 days storage

with Li -* 86.9%.

TMM (D) No reaction at 20 days. No reaction at 20 days.

TMM (DP) Cycled after 14 days with Li

(no reaction) - 0%.

11

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* - -4 - - . .- . - = - - -