155

J. Electroanal.

JEC 02475

Impedance spectroscopy of lithium electrodes

Part 1. General behavior in propylene carbonate solutions and the correlation to surface chemistry and cycling efficiency

Doron Aurbach l and Arie Zaban

Department of Chemistry, Bar-llan University, Ramat Gan 52900 (Israel)

(Received 30 June 1992; in revised form 26 August 1992)

Abstract

This study involves impedance spectroscopy of Li electrodes in propylene carbonate solutions as a function of solution composition. The influences of the salt used, its concentration and the presence of contaminants such as H,O, Or, CO, and propylene glycol, on the interfacial properties (as reflected by impedance spectroscopy) was measured.

The advantage of this work over previous similar studies is that the electrode surfaces were prepared in situ in the solutions. The interfacial impedances measured in the various solutions were correlated to the previously studied Li surface chemistry and cycling efficiency in the same solutions. The ageing processes of the Li/solution interphase that could be followed by this technique are discussed in light of the Li surface chemistry in solutions.

Modeling the electrical properties of the Li/sblution interface by equivalent circuits is in agreement with the assumption that the interphase formed on lithium in solutions has a multilayer structure including a porous part on the solution side.

INTRODUCTION

It is generally accepted that lithium electrochemistry in polar aprotic systems is controlled by its surface chemistry in solutions [l].

Lithium, which is thermodynamically reactive with all polar aprotic systems including solvents (e.g. alkyl carbonates, ethers, sulfur and nitrogen-containing

l To whom correspondence should be addressed.

0022-0728/93/$06.00 0 1993 - Elsevier Sequoia S.A. All rights reserved

156

species), anions (e.g. CIO,, AsF;) and contaminants (e.g. H,O, O,, N,, CO,), reduces solution species and the products precipitate on the metal as surface films. These surface films passivate the electrodes and, therefore Li corrosion is stopped and the rate determining step of the electrochemical Li deposition or dissolution becomes Li cation migration through these surface films [2].

Hence, the Li/solution interface becomes a complicated interphase with metal/film and film/solution interfaces having different types of capacitive and resistive elements and, therefore, impedance spectroscopy seems to be an ideal tool for the study of lithium electrodes in polar aprotic systems.

Indeed, many research groups have used this tool for the study of lithium electrochemistry in Li battery electrolyte systems [3-91. Special attention has been given to the solvent propylene carbonate (PC>. This solvent is non-toxic, non-vola- tile and highly polar, dissolving all Li salts of practical importance to form solutions of high salt concentration (therefore, high conductivity), and has been widely used in Li battery systems [lo].

Reviewing the literature related to impedance spectroscopy of Li electrodes in PC + LiClO, solutions [4-91 one discovers that each group obtained different results. Usually, the most popular way to display impedance properties of elec- trodes in solutions is the Nyquist (Cole-Cole) plot. All the Nyquist plots shown in refs. 4-9 related to Li/PC + LiCIO, systems obtained by different groups under “similar” conditions differ considerably from each other. This is not surprising, since PC is not too stable a solvent and may decompose even on commonly used desiccants such as Al,O, [ll].

In recent years we have studied Li surface chemistry in PC solutions intensively. In principle, PC is reduced on lithium or on noble metal electrodes at low potentials to ROCO,Li compounds which are the major constituents in the surface films formed [12,13,14] on Li in this solvent [12-141.

It was found that the surface chemistry of Li in PC is strongly affected by trace amounts of contaminants such as H,O, O,, CO, and propylene glycol all of which may be unavoidably present in PC solutions [11,15,161. The presence of these contaminants also ~ strongly affects the cycling efficiency of Li electrodes in charge-discharge cycling (consecutive deposition-dissolution) [11,15,16].

This work is aimed at studying the electrochemical behavior of lithium elec- trodes in PC solutions using impedance spectroscopy, benefiting from the extensive background that we have obtained regarding the surface chemistry of lithium in PC solutions [12-161. The impedance of Li/solution interfaces is correlated to the previously determined surface chemistry and Li cycling efficiency. The effect of all expected contaminants (e.g. H,O, O,, CO,, propylene glycol) and the salt used (L&F:, LiClO, and L.iBr) on the impedance of Li electrodes in PC solution has been explored rigorously.

The interphase formed on Li in solutions was modeled by equivalent resis- tance-capacitance (RC) circuits using the software developed by MacDonald [17]. The proposed models give us greater insight into the basic structure and properties of the lithium/solution interface in propylene carbonate.

157

EXPERIMENTAL

Material9

Three sources of PC were used. That obtained from Aldrich (analytical grade) was distilled twice at reduced pressure under high purity argon. PC (Li battery grade) was obtained from both Tomiyama and Burdick & Jackson. Most of the work reported here is related to the PC obtained from Burdick & Jackson since this solvent was of the highest quality and could be used as received (tested by GCMS and NMR). LiAsF, (Lithco), LiCIO, (Tomiyama) and LiBr (high purity, Aldrich) were dried in a vacuum oven for several days. L&F, was heated to 90°C the other salts could be heated to 150-200°C for drying.

The solvent was dried several times over activated molecular sieves (Aldrich 3& until the water contamination was below the detection limits of the Carl Fisher coulometer (below 10 ppm). The initial water content of the solutions was about lo-15 ppm (for 1.5 M salt concentration). 0, and CO, (high purity, Israel Oxygen Center) saturated solutions were prepared as previously described [16,18]. The saturation concentration of oxygen in PC is estimated as 10-3-10-4 M [181 and the CO, saturation concentration in PC is around 0.1 M (measured by weighing the amount of CO, absorbed by PC during gas bubbling).

Preparations, instrumentation and celLF

The preparation of solutions, glove box operation, etc. were as previously described [ll-161. Impedance spectroscopy was performed using a Schlumberger 1286 electrochemical interface and 1255 frequency response analyzer. The system is computerized (a 286 computer) using the Z-plot software (Scribner Associates). Analysis and modeling of the data (using a 386 computer) was performed using the Z-sim software (Scribner Associates) based on the CNLS software written by MacDonald [17]. Impedance spectroscopy of Li electrodes was obtained around open circuit voltage (OV vs. Li/Li+), the amplitude of the alternating current (ac) was 5 mV and its frequency range was 100 kHz-0.01 Hz.

The reproducibility of measurements of Li electrodes prepared ex situ under a glove box atmosphere was poor. In spite of the high quality of the glove box atmosphere, active atmospheric contaminants at the ppm level are sufficient to cover Li surfaces with surface films, even during a few minutes of exposure.

Therefore, each electrode prepared ex situ reaches the solution already covered with different surface films depending on the occasional exposure to the glove box atmosphere. This led us to develop the procedure for in situ electrode preparation in solution.

The cells used for these measurements and the electrode preparation in solution are shown in Fig. 1. All parts are made of polyethylene, one of the few materials which is inert to lithium.

158

SS Wire 02mm # RE stage 3

LLI wire) (cell is ready)

Fig. 1. Cell used for impedance spectroscopy and the procedure of in situ electrode surface preparation in :olutions. (All parts are of polyethylene.)

The three parts of the cell were put together in the proper solutions and a lithium rod (9 mm diameter) connected the WE and CE compartments (as shown in Fig. .l>. The working and counter electrode surfaces were prepared in solution by shearing the Li rod using a stainless steel wire (0.2 mm thick). Very uniform and smooth Li surfaces were thereby formed. The RE compartment was then placed between the WE and CE compartments (the reference electrode was a Li wire), the cells were closed with a polyethylene cover and put into aluminum vessels. All the above preparations took place in the glove box. The inside surface of the aluminum vessels was dehydrated by plating it with lithium foils. The aluminum vessels were hermetically sealed, taken out of the glove box and placed in a thermostat built especially for this purpose. The thermostat could contain 10 cells and the temperature was regulated and controlled at an accuracy of f O.l”C.

The use of a reference electrode (as shown in Fig. 1) enabled the interface of a single working electrode to be measured.

The experiments were performed in series. In a typical series, cells with solutions of different salt concentration and of different contaminants (e.g. H,O, CO,, O,, propylene glycol), prepared from the same mother solution, were stored simultaneously. The working temperature was 25 + O.l”C. Using these cells and experimental procedures, very reproducible results were obtained.

RESULTS AND DISCUSSION

General behavior

Figures 2 and 3 present typical Nyquist plots obtained from Li electrodes after 3 h of storage in PC + LiAsFs and PC + LiCIO, solutions respectively. Each figure is related to an experiment in which eight cells containing different solutions were

159

% 0

2 N

'N

'N

1 .g N

=N

% 0

_e N

‘N

‘N

STORED OVER AI,O,

Fig. 2. Typical Nyquist plots obtained from Li electrodes stored for 3 h in various LiiF, + PC solutions (as indicated). All solutions were prepared from a 1.5 M LiAsF, +PC mother solution. The diagrams relate to eight experiments (eight cells) conducted simultaneously in the same thermostat.

prepared on the same day, stored simultaneously in the thermostat and measured after the same time intervals. The same experiments and measurements were repeated several times and quite similar results were obtained.

Figures 2(a-c) and 3(a-c) relate to solutions of 0.25 M, 1 M and 1.5 M salt concentration, and Figs. 2(d-h) and 3(d-h) relate to 1 M salt solutions (prepared

160

-2

14 STORED OVER AI,O,

-z h 02 Sot.

‘N zo?,,o _.

0

Z’/Cl.cma Fig. 3. Typical Nyquist plots obtained from Li electrodes stored for 3 h in various LiClO, + PC solutions (as indicated), prepared from a 1.5 M LiCIO, +PC mother solution. The diagrams relate to eight experiments (eight cells) conducted simultaneously in the same thermostat.

from the same mother solutions) with added contaminants as indicated. The solutions related to Figs. 2(g) and 3(g) were stored over activated neutral Al,O, for 24 h followed by decantation.

All the plots of Figs. 2 and 3 are characterized by a large depressed semicircle at high to medium frequencies (100 kHz-100 Hz) and structures such as straight

161

PC+LiAsFG

::u 0 I2345676910

time/days Fig. 4. Variations of the interfacial resistance of Li electrodes as a function of storage time and electrolyte concentration in uncontaminated LiiF, solutions. Salt concentration is indicated in each graph. The resistances were calculated from the diameters of the semicircles in the Nyquist plots.

lines, small depressed semicircles or a combination of both, at low frequencies. However, all the plots relating to the different solutions shown in Figs. 2 and 3 differ from each other in the diameter of the large semicircle, the degree of its depression, w,,,, and the features related to low frequencies. Hence, the different plots of Figs. 2 and 3 reflect the strong influence of the salt used, its concentration and the presence of H,O, 0, and CO, in solutions, even as trace amounts. This may explain the difference in the Nyquist plots related to Li/PC systems obtained by different groups mentioned in the introduction [3-91.

The Nyquist plots of Figs. 2 and 3 obviously reflect the quite complicated structures at Li/solution interfaces. As generally accepted [3-91, the depressed large semicircles should be related to several time constants and are tentatively attributed to the impedance of the major and important part of the interphase formed on the lithium surface by reaction with solution components.

The diameters of these semicircles are therefore the main resistance of the surface films, which is related to migration of Li ions through the interphase [ll. The features related to low frequencies are tentatively attributed to the impedance of the outer part of the interphase (film/solution side) and to a Warburg type impedance, related to diffusion processes, as will be discussed in the next section.

Figures 4 and 5 show the dependence of the main resistance of the interface (the diameters of the big semicircles) on storage time for uncontaminated LiAsF, and LiClO, solutions (respectively) of different salt concentrations.

162

PCtLiCl04

l

160 -

. . . 60-, 0.25M

0 I23 43 6 7 6910

time /days Fig. 5. Variations of the interfacial resistance of Li electrodes as a function of storage time and electrolyte concentration in uncontaminated LiClO, solutions. The resistances were calculated from the diameters of the semicircles in the Nyquist plots.

Figures 6 and 7 show the effect of additives in LiAsF, and LiCIO, 1 M solutions (respectively) on the resistance-storage time behavior of the Li/solution interface. In addition to changes of the interfacial resistance during storage time (as indicated by changes in the size of the semicircles), w,, and the degree of depression of the large semicircles also changed gradually during storage time.

Since the depressed semicircles obtained belong to several time constants related to different parts of the Li/solution interphase (as will be discussed), it is very difficult to interpret changes in w,,. However, the degree of depression is a measure of the separation of the various time constants related to the interface. As the semicircles are more depressed, the separation between the various time constants is more pronounced. This means that the different parts of the Li/solu- tion interphase related to the various time constants differ more in their physical properties. Therefore, changes in the degree of depression in storage time obvi- ously reflect changes in the structure of the Li/solution interphase.

Figure 8 shows the variations in the degree of depression of the semicircles, calculated by dividing their maximum height by their diameter as a function of storage time. This figure clearly shows that the degree of depression changed considerably during storage time and that each solution behaved differently.

163

PctlM LiAsFGt. . . . .

100 - 0.03M H$

../@.-(

Stored Over AIf13.

time/days Fig. 6. Variations of the interfacial resistance of Li electrodes as a function of storage time and the additives present in contaminated 1 M LiiF, PC solutions (the additives present are indicated in each graph).

The impedance spectroscopic behavior of these systems as presented in Figs. 2-8 is summarized below. (1) In the absence of impurities and additives, the interfacial resistance in LiAsF, solutions was higher than in LiClO, solutions. (2) The salt concentration remarkably influences the interfacial impedance. In general, as the salt concentration decreased the interfacial resistance increased and had stronger fluctuations as a function of storage time, as shown in Figs. 4 and 5. These fluctuations in R(t) reflect the complicated ageing processes of the Li/solution interphase, as will be further discussed. (3) When the solutions were contaminated with water, the resistance of the interphase formed initially was considerably lower compared with dry solutions. However, the interfacial resistance in H,O-contaminated solutions increased con- siderably during storage and became much higher in comparison to the dry solutions. (4) The interfacial resistance in O,-saturated solutions and its storage time dependence were quite different in comparison to uncontaminated solutions. It was low initially and increased with storage time (beyond that obtained in uncon- taminated solutions).

164

PC+lM LiClO4+.....

120

100

“E 6C 0

6: 2 6C

,-

l-

)-

J )&

)L 0

I I 6

1234567 time /days

Fig. 7. Variations in the interfacial resistance of Li electrodes as a function of storage time and the additives present in contaminated 1 M LiCIO, + PC solutions (the additives used are indicated in each graph).

Li in PC+ LiAsFs

\ lM,O.O3M Hz0 x-.4- J ---A-+--c--1

0.66- I I I I I I 0 I 2 3 4 5 6 7 8

time /days Fig. 8. The degree of depression of the semicircles in the Nyquist plots, obtained from Li electrodes in various LiAsF, solutions as a function of storage time. The degree of depression is expressed as the height of the semicircles divided by their diameter. The graphs are related to uncontaminated solutions (1 M and 0.25 M, as indicated) and to a few contaminated solutions. The contaminant is indicated in each graph.

165

TABLE 1

Typical Li cycling efficiency obtained in PC solutions

Salt Concentration/M

LiAsF, 0.5 LiAsF, 1.0 LiAsF, 1.5 LiCIO, 1.0

Additives, solution history

Li cycling efficiency

/%

12k3 82k2 85k3 72zt2

LiAsFi or LiC104 LiAsF, LiiF, LiAsF, LiCIO, LiiF, LiClO, LiAsF, LiiF,

l-l.5

1.5 0.5-1.5 0.5-1.5 1.0 l-l.5 0.5-1.0 0.5-1.0 1.0

0, sat. 90*3

H,O 200 ppm 91*1

-4l203 a 96kl

Al,O, --) H,O b 96kl

N2o3 93+1 CO, sat. 96+3 CO2 sat. 96i3 PG > 100 ppm Decline Al,Os --) PG 1000 ppm b 95fl

a Solution was stored over activated neutral alumina prior to the experiment. b Same as a followed by addition of the contaminant before the experiment.

(5) Solution storage over alumina or saturation with CO, considerably lowered the interfacial resistance and kept it stable upon storage. This effect is clearly shown in the figures related to LiAsF, solutions [2,4,6]. In the case of LiClO,, since the Li interfacial resistance measured in 1 M LiClO, solutions was relatively low, the effect of the presence of CO, is pronounced only in solutions of low salt concentration (0.5 M and below). However, in spite of the different electrolytes present, the behavior of CO,-saturated solutions and solutions stored over Al,O, was quite similar in both LiAsF, and LiClO, solutions, as shown in Figs. 6 and 7.

Table 1 presents selected data of Li cycling efficiency measured in different PC solutions obtained in previous work in standard experiments which were already described in detail [11,15,16]. As shown in this table, the salt used, its concentra- tion and the presence of H,O, CO,, 0, or storage over alumina remarkably affects Li cycling efficiency in PC. This is very significant since the same factors influence considerably the impedance of the Li/solution interface as measured in this work. The basic surface reactions of the solvent, salts and contaminants, as well as a few secondary reactions of the surface species, are summarized in Scheme 1 (on the basis of refs. 11-13, 15, 16).

In the following sections, the variation of Li/solution impedance as a function of solution composition is discussed in the light of previously known surface chemistry (Scheme 1) and correlated to Li cycling efficiency (Table 1). Each aspect (e.g. salt, concentration, additives) is discussed separately.

166

SCHEME 1

Basic surface chemistry of lithium in PC solutions (proven by FTIR, XPS and X-ray microanalysis)

Solvent reduction

R (PC)

CHs-CH-CH,

2PC’ - CH -CIk-& 3 2 + CH,-CH=CH, t

2Li +

(Li alkylcarbonate) CH,CH(OCO,Li) CH,(OCO,Li) J

Salt reduction

LiCIO, + xe- + 2xLi+ - xLi,OJ +LiClO, J or LiCIO, 1 or LiCIO 1 or LiCl J

LiiF, +2e-+2Li+ - 3LiF 1 + AsF,

AsF, +LiF - LiAsF., J ?

AsFs+2xe-+2xLi - Li,AsF,_, J +LiFJ?

Contamination reduction

;0,+2e-+2Li+ - Li,OJ

H,O+e-+Li+ - LiOH .I+ ;H, 1

2C02+2e-+2Li+ - Li,CO, +COf

Secondary reactions (proven by FIIR)

2ROCO,Li+H,O - Li,CO, 1 + CO, f + ROH

Li,O+H,O - 2LiOH J

LiOH+H,O - LiOH.H,O monohydrate

Li chlorides + H,O - Hydrated surface species

Li,O+CO, - Li,CO, 1

2LiOH + CO, - Li,CO, J +H,O Decomposition

PC+Al,O, - CO, + propylene oxide derivatives

Impedance of Li/solution interfaces in uncontaminated solutions and the influence of the salt

When the lithium surface is exposed to the solution, the active metal reacts simultaneously with solvent and salt. On the basis of the spectroscopic studies

167

summarized in Scheme 1, the surface films are a matrix of ROCO,Li species, which are organic salts, in which Li halides, formed by salt reduction, and trace Li,CO,, are embedded.

The measured impedance is related, therefore, to several elements in series which are Li/film and solution/film interfaces, and the bulk of the film which may also contain different zones of different physical properties. The main resistance, measured as the diameter of the big semicircle which characterizes the Nyquist plots (Figs. 2 and 31, is attributed to Li+ migration through the surface films [l].

It took about 10 min from the surface preparation to completion of the first measurement. It is clear that within this time, the main resistance of the interface, which is the surface film, has already built up. This is our conclusion since we do not observe a considerable increase in the resistance, with storage time, but rather a decrease (except for L&F, solutions of high salt concentration), as demon- strated in Figs. 4 and 5. The organic salts are crystalline in nature [13] and are not expected to be very good Li+ conductors. The improvement in the Li+ conduc- tance through the films is obviously due to the defects and their inhomogeneity resulting from doping by Li salt reduction products. Indeed, at salt concentrations below 1 M the resistance of the Li/solution interface increased as the salt concentration decreased.

The Li+ conductivity of the surface films depends on a delicate balance between their organic and inorganic species, as at high salt concentration (> 1 M) the film resistance increased with increasing salt concentration (Figs. 4 and 5).

Figure 9 presents the resistance of the Li/solution interface in LiBr solutions. Bromide cannot be reduced on the Li surface and, therefore, one would not expect

Li in PCtLiBr

J-=T-q

time /days Fig. 9. Variations in the interfacial resistance of Li electrodes as a function of storage time and the salt

concentration in PC + LiBr solutions.

168

the resistance of the interface to be dependent on LiBr concentration. Moreover, since salt reduction does not occur and the dominant surface species formed are believed to be solvent reduction products, one would expect that the behavior observed with LiBr solutions would be similar to that of LiAsF, or LiCIO, solutions of very low salt concentration.

In contrast, the interfacial resistance in LiBr solutions was lower than that measured with LiAsF, solutions of the same salt concentration, and, like L&F, or LiCIO, solutions (Fig. 41, depended on salt concentration.

Hence, it can be concluded that the salt anion influences the Li ion conductivity in the surface films not only due to doping by its reduction products but also via doping of the films with ions of the unreduced form. It is quite possible that during the formation of the surface film by a massive solvent reduction, salt anions are also trapped inside the matrix formed.

The change in resistance as a function of salt concentration and storage time shown in Figs. 4,5 and 9 proves that the situation on the Li surface is dynamic and the composition of the film (e.g. the degree of doping by inorganic, salt reduction products), changes with time.

We assume that inorganic species such as LiF, LiCl or Li,O formed by salt reduction (Scheme 1) are less soluble in PC than ROCO,Li (PC reduction products). Therefore, the surface films may be enriched with the inorganic species during storage due to a partial dissolution of the organic species, which enables further formation and irreversible precipitation of salt reduction species. This may explain the pronounced decrease in film resistance recorded during the first days of storage in LiAsF, or LiClO, solutions of low salt concentration (Figs. 4 and 5).

However, there is another contribution to the ageing processes of the Li/solu- tion interphases formed in PC, which complicates the picture. H,O reacts with ROCO,Li species to form Li,CO,, ROH and CO, 1191. Li,CO, formation on the surface changes considerably in the interfacial properties, as will be discussed later. Even in the driest solutions, containing water at the ppm level, the water content is sufficient to influence the film structure via the above secondary reaction. Therefore, the variation of the interfacial resistance with time (as shown in Figs. 4, 5 and 9) also reflects the effect of trace water.

Since the Li/solution interphases are formed by simultaneous reaction of lithium with solvent and salt, followed by ageing processes which change the film composition, one would expect that the interphase formed on Li in solutions has a multilayer structure. The film structure close to the lithium side might be different from that close to the film solution interface.

The simplest equivalent circuits of the Li/solution interface are, therefore, RC circuits in series. Figure 10 presents a typical fitting of experimental results (Li surface in L&F, 1 M solution, 4 days) with a model of RC circuits in series showing both the Nyquist and the Bode plots. The optimized equivalent circuit composed of five RC circuits, one of which also includes a Warburg type element is also presented in Fig. 10 with the values of the resistors and the capacitors. It was impossible to obtain a good fit using the commonly used finite length Warburg

169

Lithium in PC i- IM LiAsF, four days

is 135

r/n~cd

The Model

Fig. 10. Presentation of a typical matching between the Nyquist and Bode plots obtained from the Li/solution interface and an equivalent circuit of five RC circuits in series, including a Warburg type element. The plots relate to Li electrodes stored in 1 M LiiF, solution for 4 days. Dotted line = experimental data, solid line = calculated plot for the model, using the Z-sim software.

element which has the general formula [7,17]

z FLw =A tanh[ (ioB)“‘] /( iWB)“2 (1)

where i = ,/ - 1, A and B are constants that depend on diffusion coefficient, diffusion length, temperature and concentration.

In order to fit the model to the experimental data, it was necessary to use a more generalized diffusion element in circuit e (Fig. 10) of the type [17]

2, =A’ tanh[(iwB’)*]/(ioB’)” (2)

where A’ and B’ are constants as above, and (Y is a number between 0 and 1. All the data obtained in this work related to all the solutions at different salt

concentrations and additives could be well modeled to five RC circuits in series including in one, the diffusion element 2,. Table 2 summarizes typical C and R values calculated for models of five RC circuits in series and the power cy in the

TABLE 2

Summary of R and C values obtained by fitting of a model of five RC circuits in series (Fig. 10) to experimental data related to Li electrodes in several LiAsF, solutions

Solution R,/ C,/ Rb/ cb/ R,/ Cc/ Rd/ cd/ 4/ Ce/ ffa

R cm2 PF cm-2 fl cm2 PF cmm2 R cm2 PF cmm2 fl cm2 FF cmm2 fl cm2 PF cme2

Uncontaminated 1.0 M sol. 3.50 2.95 14.91 4.44 29.73 7.08 3.35 395.05 1.83 3847.5 0.2389 Uncontaminated 0.25 M sol. 4.14 2.41 16.88 3.32 38.23 5.10 7.72 88.13 1.59 3158.3 0.2181 Uncontaminated 1.5 M sol. 4.11 1.48 11.79 3.54 35.06 4.80 6.42 96.41 2.86 1891.2 0.2400 1M sol. sat. with 0, 4.09 1.89 13.84 3.23 21.47 6.53 2.57 350.72 1.44 2215.1 0.1767 1M sol. cont. H,O 0.01 M 4.09 1.70 13.22 2.60 15.31 6.44 1.13 655.01 1.03 3536.4 0.1849 1 M sol. sat. with CO, 2.34 4.43 11.79 4.45 4.77 35.15 2.07 1612.86 0.00 5122.3 0.1527 1M sol. stored over Al,O, 2.81 3.91 13.09 4.16 12.57 11.54 1.77 650.02 1.60 2731.8 0.2369

a (Y is the power calculated in Zd (Warburg type element) where Z, = A tanh [(itiB)n]/(iWB)OL, A and B are constants related to diffusion parameters.

171

expression for Z, fitted to data obtained from Li electrodes stored in various PC solutions for 3 h.

The various resistances and capacitances are related to different layers of the surface films and to the two interfaces (Li/film and film/solution). Tentatively, we attribute circuits a-c with the low capacitances and the higher resistances to the inner part of the film. Circuits d and e with the higher capacitances are attributed to the outer part of the film (solution side). The very high capacitances calculated by the fitting procedure might be attributed to a porous, high area surface which is expected in the solution film interface for several reasons.

When Li is exposed to the solution it is immediately covered with surface species which are electrical insulators. This primary film does not leave any bare lithium. The Li/solution interphase continues to grow mostly by injection of electrons from the metal to the solution through defects in the insulating surface films. Hence the film cannot grow in a homogeneous way, covering all the surface, but rather in certain locations where electrons can reach solution species and reduce them to surface compounds. Such a growth process obviously leads to a porous structure.

This idea that the surface films formed on lithium in PC are composed of compact layers on which a porous layer is built up, is in line with previously published work in which the Li/solution interface in PC was studied using micropolarization and ellipsometry techniques 121.

Hence, circuit e (Fig. 10, Table 2) may be attributed to the film solution interface. R, is related to the transfer of Li cation across the interface and the high capacitance C, reflects the high surface area of this interface. The power of the ‘Warburg’ type (Y element calculated which is around 0.15-0.25 reflects a more complicated diffusion process of the Li cations than that modeled by the more common, finite length Warburg element (power of 0.5) [7,17].

However, the model suggested is preliminary, and there are other alternatives for modeling our data (e.g. the use of circuits with several constant phase elements, CPE [20]. A detailed discussion of possible models and more rigorous verification of their relevance to the experimental results is beyond the scope of this work and will be reported elsewhere [21].

Comparing the Li cycling efficiency in the various solutions (Table 1) to Li/solution interface resistance, it seems that there is no simple correlation between the two parameters. Li cycling efficiency in LiAsF, is much higher than in LiClO, solutions, while the interfacial resistance in LiClO, solutions is much lower compared to LiAsF,. In addition, Li cycling efficiency increases as LiAsF, concen- tration increases, while the interfacial resistance in 1 M L&F, solutions is lower than in both 1.5 M and 0.25 M L&F, solutions.

Hence, general properties of the surface films cannot be correlated directly to the cycling efficiency of Li anodes in charge-discharge cycling. As previously discussed [11,15,16], the major factor determining Li cycling efficiency is to what extent the surface films are homogeneous, enabling a smooth, uniform Li deposi- tion and dissolution to occur.

172

Solution Change I I I I I I *

loo PC/N LiA,F6/ l

“E 80- ”

? 60- PWIM i_iAsFs

b + a a

Stored

-3 -2 -I 0 I 2 3 4

(a) time/days’

Solution Change to C02- Cont. I ’ ’ ’ ’ ’ ’ 1

loo PC/l.Wl LiAsF6 4

I# - 4) 80 -

60-

PWIM LiAsF6/ 40-

* CO2 Sat.-

W 41

20 I I I I I,

-3 -2 -I 0 I 2 3 4

(b) time/days

Fig. 11. (a) Variations in the interfacial resistance of lithium electrodes in 1 M LiAsF, solutions in experiments in which the solutions were changed during storage. Lithium electrodes were prepared and stored for 8 days in a CO,-saturated solution and in a solution stored over Al,O, (before the experiment). Then, at t = 0, both solutions in both experiments were changed to uncontaminated 1 M LiAsF, + PC solutions. (b) Variations in the interfacial resistance of a Li electrode in an experiment in which the solution was changed during storage. The Li electrode was prepared and stored in a 1.5 M LiAsF, +PC solution, after which (at t = 0) the solution was changed to a CO,-saturated solution.

Infiruence of storage over alumina and the presence of CO,

It was found in earlier work that PC decomposes when stored over activated Al,O, to form CO, and, probably, derivatives of propylene oxide [ll]. In spite of the reactivity of PC towards lithium, CO,, when dissolved in PC even well below saturation, reacts with lithium to form Li,CO, [ll]. The presence of these species on the Li surface was found to improve Li cycling efficiency considerably (see Table 1).[11,15,16]. The present work proves that the presence of CO, in solution influences the impedance of the Li/solution interface considerably.

As presented in Fig. 6, the interfacial resistance is much lower when CO, is present in solutions. This is attributed to the formation of Li,CO, on the Li surfaces due to CO, reaction, as discussed above. In contrast to the ROCO,Li surface species, Li,CO, has no secondary reaction with solution species. In addition, from all the various surface species that may be formed on Li in PC (e.g. Li halides, Li oxides, Li hydroxide, ROCO,Li), Li,CO, is the least hygroscopic 1161. Therefore, the influence of trace H,O on the structure of the Li/solution interphase should be minor at high content of Li,CO, in the surface films.

Figure 11(a) shows results of experiments in which Li electrodes were prepared and stored in CO,-containing solutions. After 8 days of storage the solutions were replaced by uncontaminated solutions. Figure 11(b) presents results of an experi- ment in which Li surfaces were prepared and stored in PC + 1.5 M LiAsF, solutions (8 days), after which the solution was replaced by CO,-saturated 1 M LiAsF, solution.

173

In both figures the interfacial resistances are plotted as a function of storage time where t = 0 relates to solution change. As shown, when the solution is changed after the interphase is built up in a CO,-containing solution, its resistance remains unchanged (Fig. 11(a)), while in the opposite case, the interfacial resis- tance drops drastically when the solution is changed to a CO,-contaminated solution.

In several experiments Li surfaces were prepared under a CO, atmosphere (in gas phase) or in pure PC, followed by storage in 1 M LiAsF, solutions and impedance measurements. In both cases, the initial interfacial resistance after preparation and 3 h of storage in 1 M LiAsF, + PC solution was low, of the order of 15-20 51 cm2. However, the interfacial resistance of the Li prepared in CO, stayed constantly low, while the interfacial resistance of the lithium prepared in pure PC increased gradually, and after a few days of storage reached the typical values measured for lithium prepared in 1 M L&F, solutions.

These results further prove that the situation on the Li surface is dynamic. The surface films formed in pure solvent may be changed to those formed in electrolyte solutions. The surface fiims formed on lithium in uncontaminated electrolyte solutions also change considerably when exposed to CO, as a result of Li,CO, formation which stabilizes the Li/solution interphase (and decreases its resistance).

During formation, ROCO,Li species may follow continuous precipitation-dis- solution processes, allowing some interaction between solution species and the lithium in spite of the presence of an interphase between the metal and the solution.

Li,CO, is thought to be more insoluble than the organic salts (ROCO,Li). Therefore, when it is formed by CO, reaction on the active surface (e.g. via electron injection through the interphase), it precipitates and cannot be removed. This correlates well with the high Li cycling efficiency measured for CO,-contain- ing PC solutions [11,15]. The improvement in Li cycling efficiency obtained as a result of Li,CO, formation on the Li surface is not because it reduces the interfacial resistance, but rather because it probably passivates the lithium better. This is proven by the fact that the same improvement due to the presence of CO, was measured for 1 M LiClO, solutions, although the interfacial resistance due to the presence of CO, in this case is similar to that measured in uncontaminated solutions.

Znfhence of 0,

In previous work it was found that the presence of 0, in solutions considerably increased Li cycling efficiency (Table 1, ref. 16). However, since the saturation concentration of oxygen in PC is supposed to be very low (10-3-10-4 M [221) this result was surprising. Spectral studies of Li surfaces stored in O,-containing solutions were not conclusive since the IR spectrum of Li oxides has bands mostly in the range 650-500 cm-’ which is masked by other surface species [16]. The present work proves that in spite of its low concentration, 0, dissolved in PC is

174

reactive with lithium and its reduction to form Li oxides contributes to the build-up of the Li/solution interphase, as proven by Figs. 6 and 7. Moreover, while in uncontaminated 1 M solutions of LiAsFa or LiClO,, the Li/solution interphase is stabilized within three days of storage, when oxygen is present, it takes much more time to stabilize. Hence, it seems that 0, continues to react with lithium in spite of the presence of a passivating interphase and the films formed on lithium in the presence of oxygen in solutions are permeable to oxygen. The Li oxides formed in this case may include L&O, Li,O, and LiO, [231. The Li peroxide and Li superoxide are supposed to be strong nucleophiles and react with PC molecules which are highly electrophilic. Evidence of this reaction has already been reported [14,16].

Hence, the change in the resistance of the interphase formed on Li, in O,-containing PC during storage, shown in Figs. 6 and 7, may reflect a build-up of surface films containing Li oxides by diffusion of oxygen through the interphase, followed by some secondary reactions in which the Li oxides partly react with PC.

These results further prove that Li cycling efficiency does not depend on the magnitude of the interfacial resistance but rather on the uniformity of Li dissolu- tion deposition. The presence of certain surface species such as Li oxides may induce uniform migration of Li ions through the interphase due to uniform dispersion of defects in the crystal structure of the surface films. Indeed, SEM observation of Li electrodes treated in O,-saturated PC revealed that Li deposition in the presence of 0, is less dendritic compared with uncontaminated solutions Ml.

Znfuence of water

There are several paths, both primary and secondary, in which trace water in solution may react on the lithium surface. Direct reduction forms LiOH which can be further reduced to L&O. LiOH, if formed, reacts nucleophilically with PC to form species of the type CH,CH(OH)CH,OCO,Li [12] (which are somewhat similar to those formed by direct PC reduction by lithium).

Water also reacts with ROCO,Li to form Li,CO,, CO, and ROH (Scheme 1, refs. 12 and 13). It also hydrates all the possible surface species formed on lithium (e.g. Li halides, Li oxides, LiOH or LiOR), except for L&CO,, which is the least water-sensitive compound of all the possible surface species formed on lithium in PC [16].

Spectral studies of Li surfaces stored in H,O-contaminated PC proved that Li,CO, is one of the major components in the surface films formed in solutions due to the aforementioned secondary reaction of Hz0 and ROCO,Li [12,13].

Figure 12(a) shows Nyquist plots obtained from Li surfaces, prepared in PC solutions containing 18, 48 and 250 ppm of water, after 3 h and 8 days of storage. In Fig. 12(b), the interfacial resistance (the diameters of the big semicircles in Fig. 12(a)) measured in the three solutions is plotted as a function of storage time. These figures prove that the impedance of lithium electrodes in PC is strongly

175

Z/Q- cm2

PC/IM t LiASF6/X,,, H,O

012345670

(b) time /days

Fig. 12. (a) Typical Nyquist plots obtained from Li electrodes prepared and stored in 1 M LiiF, solutions containing different amounts of water (indicated in each plot). Dotted lines = 3 h of storage, solid lines = 8 days of storage. (b) Variations of the inter-facial resistance of Li electrodes prepared and stored in 1 M LiAsF, solutions containing different amounts of water, as a function of storage time.*

affected by the presence of water, even at the ppm level. As shown in Fig. 12, during the first hours of storage the interfacial resistance of lithium in water-con- taminated solutions is much lower than that measured for dry solutions. This correlates very well with the high content of Li,CO, in the surface films due to the reaction of ROCO,Li and water, as proven by the spectral studies [12,13,15].

However, in contrast in CO,-containing solutions where the Li interfacial resistance is also low, as a result of Li,CO, formation, the inter-facial resistance measured in H,O-contaminated solutions increased rapidly during storage, as shown in Figs. 6, 7 and 12. This increase was more pronounced in LiClO, than in LiAsF, solutions (Figs. 6 and 7).

This might be explained as follows: since water hydrates most of the surface species formed on lithium in solutions (except for Li,CO,), water diffuses through the interphase to the lithium surface and reacts there to form LiOH-Li,O [24,25]. Hence, there is no efficient passivation against water reaction with lithium. Water diffusion is supposed to be faster through the interphase formed in LiClO, solutions (compared to LiAsF,) because LiClO, reduction produces both Li halides and L&O which are highly hygroscopic and therefore may serve as very good water conductors in the interphase (cf. Figs. 6 and 7).

Figure 13 shows the interfacial resistance measured in experiments in which lithium surfaces were prepared and stored for 8 days in dry solutions (until the

176

Solution Chonge- Wetting 90.

5M _.He-*--

7. 1.5M 1

. PC/M LiAsF,#

_-..o----_-,

30-

PC/LiAs Fs

IOl’ * s n ’ ’ * -3 -2 -I 0 I 2 3 4

time /days Fig. 13. Variations in the interfacial resistance of Li electrodes in L&F, + PC solutions, in experiments in which the solutions were changed during storage. Li electrodes were prepared and stored for 8 days in dry solutions (0.25 M, 1 M and 1.5 M in salt concentration). Then, at t = 0, the solutions were changed to a 0.01 M H,O + 1 M LiAsF, + PC solution.

resistance measurements reached steady values). The solutions were then changed to water-contaminated solutions.

As demonstrated in Fig. 13, the interfacial resistance drops considerably owing to the solution change and remains constant, or changes only moderately upon further storage. These results further clarify the influence of water on the interfa- cial properties of lithium solutions. When the surface films are formed on lithium initially in dry solutions, they are composed mostly of ROCO,Li. These films, when exposed to water, react to form Li,CO,. This reaction obviously takes place on the film/solution side and in fact seals the interphase for further water diffusion because Li,CO, is not hygroscopic. In contrast, when the surface films are formed initially in water-contaminated solutions, there are enough water conductors such as LiOH, Li,O and derivatives of propylene glycol embedded in the interphase (in addition to Li halides) to promote water diffusion as discussed above.

Effect of propylene glycol

Propylene glycol (PG) is a common contaminant in PC formed by hydrolysis of PC and is difficult to remove [26]. Contamination of PC by 50-100 ppm of PG was enough to cause considerable deterioration of the coulombic efficiency in lithium charge-discharge (deposition-dissolution) cycling [ll].

Addition of CO, to PG-containing PC solutions canceled the detrimental effect of PG on Li cycling efficiency (Table 1) [ill. Figure 14 shows the interfacial resistance of lithium electrodes in PG-containing PC solutions as a function of storage time. At low PG concentration the Li interfacial resistance in solutions was lower than that measured in uncontaminated solutions and above 250 ppm of PG

177

PC/IM tLiA,F,/Xppm tRG.added

I IO

t lOOOppm/*

5 90- /

OPF - . . . * . . . ” . . . . . . . .

1OOppm )--o-_~____* --A,

,A9 4-m---‘)

A1205+ IOOOppm

0 I 2 345676

time/days Fig. 14. Variations of the interfacial resistance of Li electrodes stored in 1 M + LiiF, PC solutions containing propylene glycol (PG), as a function of storage time. The amount of PG in ppm is indicated in each graph. In one of the experiments (the lower graph), PG-contaminated solutions WOO ppm) were stored for 24 h over Af,O, prior to electrode preparation.

in PC it was higher. As shown in Fig. 14, the interfacial resistance of lithium measured in PG-containing PC solutions which were stored over alumina (Al,O,) before the experiments was very low and stable even at high PG concentrations. The effect of storage over Al,O, was quite similar irrespective of whether the solution was stored over alumina while containing PG, or PG was added after the pure solution was stored over Al,O, and the alumina was removed before PG addition.

This behavior of the interfacial impedance of lithium in PG-contaminated PC solutions may be better understood in light of the previous studies of Li surface chemistry in these solutions [ll].

PG is reduced on lithium to give the Li alkoxide derived from propylene glycol’ [ll]. This reacts nucleophilically with PC to form species of the type CH,CH(OLi)CH,-0-CH,-CH(OCO,Li)cH,. However, at high PG concentra- tion its reaction on the Li surface in solutions is fast enough to form the corresponding Li alkoxide which accumulates faster than its secondary reaction with the solvent. Accumulation of these alkoxide surface species in the Li/solu- tions interphase probably causes the increase in the interfacial resistance mea- sured as the PG concentration increased (Fig. 14). When the solutions contain CO, (e.g. due to storage over Al,O,, [11,15]) Li,CO, formation competes with every other surface reaction, including PG reduction, and therefore the typical behavior of the interfacial impedance of lithium in CO,-containing solutions is observed.

178

CONCLUSION

Impedance spectroscopy of Li electrodes in PC solutions correlates well with the previously studied Li surface chemistry and cycling efficiency (in repeated dissolution-deposition processes) as they are all affected strongly by the same factors.

The interfacial impedance of lithium electrodes in PC solutions is influenced by the salt used, its concentration and the presence of contaminants such as H,O, O,, CO, and propylene glycol, even at the ppm level. For instance, the interfacial resistance decreased as the salt concentration increased (up to 1 M) and was generally higher in LiAsF, than in LiClO, solutions. When the surface films contain Li,CO,, either because of a reaction of lithium with CO, (when contami- nated solutions are used) or a secondary reaction between H,O (when present) with PC reduction products (ROCO,Li), the interfacial resistance is fairly low.

In any event, there is no simple correlation between the coulombic efficiency of Li electrodes in charge-discharge cycling and the magnitude of the interfacial resistance.

The electrical properties of the interphase formed on lithium in the various solutions can be modeled very well by equivalent circuits composed of several RC circuits in series (including, also, a Warburg type element in one of them). These equivalent circuits with the calculated resistances and capacitances seem to reflect a multilayer structure of the surface films formed on lithium in solutions, in which the outer part (solution side) is porous and has high surface area. However, within the framework of this study, modeling of the Li/solution interphase based on impedance spectroscopy was done on a preliminary basis. More rigorous models based on the influence of storage time and temperature on the impedance picture, are being developed at present.

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