In Operando Detection of the Physical Properties Change of the

Interfacial Electrolyte during Li-Metal Electrode Reaction by

Atomic Force Microscopy

AUTHOR NAME

Mitsunori Kitta*

AUTHOR ADDRESS

Research Institute of Electrochemical Energy, Department of Energy and Environment,

National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31,

Midorigaoka, Ikeda, Osaka 563-8577, Japan

KEYWORDS

Li-metal electrode, Interfacial electrolyte, Physical properties, Operando investigation, Atomic

force microscopy, Force curve analysis, Energy dissipation recording

ABSTRACT

The physical properties of the interfacial electrolyte near the electrode surface

essentially affect the electrochemical behavior of the Li metal negative electrode. Therefore,

probing the interfacial electrolyte under in-operando conditions is highly desired to determine

the true electrochemical interface and electrode performance. In this study, dissipation

recording by force-distance analysis based on atomic force microscopy was applied for the first

time to address these challenges and a notable performance was observed during this study. The

energy dissipation of the cantilever during the force curve motion is an important indicator to

evaluate the condition of the interfacial electrolyte because the solution drag is based on

physical properties of electrolyte. In-operando electrochemical experiments of a Li metal

electrode with a tetraglyme-based electrolyte, the dissipation energy clearly changed

corresponding to the charge-discharge reaction. Recording the dissipation based on the force-

distance analysis coupled with electrochemical operation improved the understanding of the

actual characteristics of the electrochemical interface, based on the direct measurement of

physical properties.

INTRODUCTION

Li metal is one of the most attractive negative electrodes for next-generation batteries

because of its high energy density (3860 mAh g⁻

) and extremely negative electrochemical

potential (−3.04 V vs. standard hydrogen electrode (SHE)).

1,2

Li metal negative electrodes are

also extending the usage of various positive electrode materials without Li pre-doping. However,

their instability during the charge-discharge reaction, such as dendrite growth or poor

coulombic efficiency and poor cycle performance, restrict their commercial applications.

3–7

These reactions are controlled by the electrolyte/electrode interface during electrochemical

operation.

8–11

Indeed, the dendrite growth that occurs during the charging process (Li metal

deposition process) is dominated by factors such as the homogeneity of the Li metal surface as

well as surface films. These electrode surface reactions were investigated in detail by various

in-operando techniques, such as optical spectroscopy,

12,13

electron microscopy,

atomic force

microscopy,

15–18

and other related techniques.

19–21

On the other hand, electrochemical reactions

significantly change the physical or chemical properties of the “interfacial electrolyte”. This is

defined as the region that is less than 10% away from the electrode surface, located within the

overall diffusion layer. With respect to the Li metal electrode reaction, the Li-ion concentration

of the electrolyte should increase near the electrode surface during discharging (Li metal

dissolution) and decrease during charging. The Li-ion concentration of the solution should affect

the physical and chemical properties, such as the conductivity (σ), the self-diffusion coefficients

(D), the viscosity (η), and the mass density (ρ).

22–28

Generally, these properties influence the

electrochemical reaction of the electrodes, and especially the metal deposition is significantly

affected.

29–31

Furthermore, the physical properties of the electrolyte provide the key to

understand the solvation of Li-ions, which influence the electrochemical characteristics of the

various negative electrodes as well as that of the Li metal.

32–34

Thus, the investigation of the

interfacial electrolyte is important to understand the fundamental electrochemical reaction of

the Li metal electrode.

Contrary to the electrode surface, present characterization techniques to study the

interfacial electrolyte are limited because of the difficulty to assess the solution close to the

electrode surface using in-operando conditions. Radiation techniques such as X-ray absorption,

X-ray, and neutron reflectometry were applied in various electrochemical interface studies.

35–

However, the low spatial resolution of these methods makes it difficult to retrieve specific

information of the interface. Further, the signal from the interfacial electrolyte is generally weak

even when using advanced energy and brightness sources because the mass volume of the

solution close to the electrode is relatively small. Owing to these signal detection limitations,

in-operando investigations with adequate time resolution are restricted because the

measurements are time consuming. Optical interferometry is a well-known technique for

imaging the gradient of solution concentrations near the electrode surface,

39,40

and was applied

to study Li-ion battery reactions.

41–44

However, the spatial resolution is restricted to the

diffraction limit of the wavelength, and cannot be approaching the µm region of the interface.

Moreover, the physical information of the electrolytes, such as viscosity or elasticity, cannot be

directly accessed. The electrochemical quartz-crystal microbalance (EQCM) technique is a

well-known sensing technique to identify the mass change of electrodes during various

electrochemical reactions, such as corrosion, plating, surface film formation,

45,46

electrode

dissolution and Li metal deposition.

47–50

Additionally, the resonance resistance (R

res

) of

oscillations are closely related to viscosity (η) and mass density (ρ) of electrode surface

compounds and the surrounding electrolyte.

50–53

Serizawa et al. studied in detail the resonance

resistance change during Li-insertion and extraction operations of Li

-coated EQCM

electrodes.

The stability of Li

materials that experience no volumetric or surface

morphological alterations enabled to quantitatively detect both the changes in electrode mass

and physical properties of the surrounding electrolyte during electrochemical operation.

However, in the case of Li metal electrode reactions, EQCM analyses are difficult to apply. A

well-defined electrode surface is required for accurate validation using the Sauerbrey

equation.

52,53

Li metal deposition and surface film formation was essentially inevitable,

affecting the morphology of the electrode surface, preventing a more detailed EQCM analysis.

This is because the oscillator itself operates as electrode. Therefore, the other in-operando

techniques are highly needed.

Recently, atomic force microscopy (AFM) was developed as an advanced technique to

probe electrode reactions, enabling to investigate the Li metal electrode at in-operando

conditions by applying a fast scanning and environmentally controlled technology.

16–19,55

Significant progress was achieved in fast imaging of electrode surfaces studying not only the

morphology but also mechanical properties, such as elasticity, deformation, and adhesion, based

on force curve measurements.

56–58

Noticeably, the AFM cantilever is continuously receiving

the solution drag during force curve motion in the electrolyte, leading to kinetic energy losses,

which is called energy dissipation (W). Here, the solution drag is closely related to the physical

properties of the solution, such as viscosity (η) and mass density (ρ). Therefore, the energy

dissipation of the cantilever in the force curve motion can be used as an indicator to detect

changes in the physical properties of the solution, similar to the resonant parameter shift.

59–62

Contrary to the resonance method, the energy dissipation recording of the force curve motion

can easily assess the changes in the physical properties of the electrolyte because it does not

require a high-quality factor (Q) and resonance oscillation within the solution environment.

Moreover, the AFM-based system has many noticeable advantages for probing the interfacial

electrolyte, such as (1) can easy approach of the closest region of the electrolyte/electrode

interface, (2) can easy access of various Z height positions in the diffusion layer, and (3) only

detect the physical property changes of the interfacial electrolyte solution because the cantilever

is independent of the electrochemical system, as opposed to the EQCM oscillator. Despite these

advantages, AFM-based techniques were not considered as probing tools to analyze physical

properties of interfacial electrolytes. Generally, AFM is used as an imaging tool for solid

surfaces; thus, energy dissipation and the solution drag against the cantilever motion are

strongly regarded as unfavorable disturbances of the experiment.

In this study, the AFM-based technique is applied for the first time to probe the changes

in physical properties of the interfacial electrolyte during the electrochemical reaction of a Li

metal electrode. A glyme-based electrolyte (Tetraglyme, G4) was used for the experiments

because of its stability to Li metal electrode.

31,63

Note that the electrochemical interface of the

G4-based electrolyte/Li metal electrode assembly is a well-studied battery system. The

dissipation energy change was synchronously recorded with the electrochemical voltage

profiles of the Li /Li symmetrical cell during multiple constant-current step operations, which

responded well to the physical property changes of the interfacial electrolyte with the

electrochemical reaction of the Li metal electrode.

EXPERIMENTAL

In this experiment, a Li metal wire (Honjo Metal, Japan) with φ = 2 mm was used as

the working electrode, and another piece of Li metal was used as the counter electrode. The

Li/Li symmetrical cell for in-operando AFM observations was constructed using a designed

sample cell, as described elsewhere.

A 1 mol dm⁻

solution of lithium

bis(trifluoromethanesulfonyl)amide (LiTFSA) dissolved in bis[2-(2-methoxyethoxy) ethyl]

ether (tetraglyme, G4), denoted as 1 M LiTFSA-G4, was used as electrolyte. Before the AFM

scanning, the Li wire was cut with a precision scissor within the electrolyte to prepare a fresh

and flat metallic Li surface.

The PeakForce Tapping mode imaging with 2 nN of contact force

was performed on the Li metal surface in the electrolyte solution using a commercial AFM

system (iCON, Bruker, USA) installed at glove box (MBRAUN, Germany) containing an inert

argon atmosphere (O

, H

O < 0.1 ppm). A commercially available silicon nitride cantilever

probe (ScanAsyst-Fluid, Bruker) with 5 μm of averaged tip height (h), 0.7 N m

–1

of spring

constant (k), and 150 kHz of resonant frequency (f

) was used. Here, the cantilever assessed the

region 5 μm away from the Li metal surface because the tip height was approximately 5 μm.

Additional information on this cantilever tip can be retrieved elsewhere.

A typical

experimental drawing of the overall configuration of the cantilever and the electrochemical

interface is provided in Figure S1 of the Supporting Information. During the imaging, the

PeakForce Tapping frequency (equal to the frequency of the force curve motion, f

) and

amplitude (A

) were kept at 2 kHz and 100 nm, respectively. As the tapping frequency (f

) is

considerably smaller than the resonant frequency of the cantilever (f

), the force curve motion

induced during the approaching and retracting process (200 nm distance) was not affected by

the resonant motion of the cantilever itself.

The analog voltage output signal (V) of the

electrochemical device and the AFM energy dissipation signal (W) were simultaneously

acquired as 1024×1024 pixel images 500 nm

in scan range at a 1.00 Hz line⁻

raster scan rate.

With the raster direction, these acquired images provided information about cell voltage (V)

and dissipation (W) as a function of time for every 1.00 s. Therefore, correlation plots of voltage

and dissipation are presented by cross-section profile of these images along the raster direction.

In the cross-sectional profiles, the data were averaged within each raster line. Thus, the

sampling rate of the dissipation recording was identical to that of the raster scan rate used for

the dissipation imaging, namely 1 Hz.

RESULTS AND DISCUSSION

Detecting physical property changes of the solution by energy dissipation

The detailed principle of this experiment is as follows. Figure 1 shows a schematic of

the force curve motion coupled with the electrochemical operation of the Li metal electrode. In

the force curve motion, the cantilever received solution drag (f) at both approaching (blue

arrow) and retracting (red arrow) motion, as f

and f

, respectively. In the open circuit condition,

as shown in Figure 1(a), these solution drags are generally corrected automatically with

electronic data processing by the AFM system. Therefore, the force-distance plot shows typical

force curve spectra accompanied with a corrected non-contact base line (F = 0), even in the

solution condition. This is defined as an energy dissipation of zero (W = 0). At Li metal

dissolution condition (discharge reaction), as shown in Figure 1(b), the Li-ion concentration of

the interfacial electrolyte increases, and the solution viscosity (η) and mass density (ρ) also

increase. Therefore, the solution drag increased and each base-line of force spectra shifted to a

positive value at approaching and a negative one at retracting motions. Thus, the energy

dissipation positively increased (W > 0). In contrast, the physical properties of the interfacial

electrolyte (η and ρ) decrease with decreasing Li-ion concentration at Li metal deposition

(charge) conditions, as presented in Figure 1(c). In this circumstance, both solution drags also

decreased, and each non-contact baseline of force spectra was shifted in contrast to the case

depicted in Figure 1(b). Therefore, the energy dissipation decreases to a negative value (W <

0). Note that the absolute value of solution drag and dissipation energy were always positive,

but these values were negative compared to the open circuit condition (W = 0). Thus, the

physical property changes of the interfacial electrolyte can be detected by the increase or

decrease in energy dissipation. Of course, the detectable force of the above-mentioned

experiment should depend on the shape and spring constant of the cantilever. The shape of the

cantilever immersed in the electrolyte solution significantly affects the strength of the drag force.

Further, the force sensitivity of AFM is generally determined by the spring constant of the

cantilever. Therefore, the measurement sensitivity could be enhanced by optimizing the basic

properties of the cantilever. A cantilever with a small spring constant and a large area should be

suitable to detect a weak drag force. Further, a smaller tip height would be beneficial in

accessing more closely the region to the electrode surface, as discussed in Figure S1 of the

Supporting Information.

Here, the energy dissipation by solution drag is presented as follows.

  





  









 

where z is the moving vector of the cantilever towards the solid surface. The solution drag (f)

can be approximately represented by the following equation.

  











 











where A and B are constant parameters determined by the morphology and properties of the

cantilever. η and ρ are the viscosity and mass density of the solution, respectively. ω = (2πf

and T are the cycle frequency and period of the force curve motion with sine wave function,

respectively. As shown in equation (2), the drag force is proportional to the velocity (dz/dt) and

acceleration (d

z/dt

) terms of the cantilever motion. Thus, a larger moving distance (z) of the

cantilever increased the magnitude of these terms, leading to a strong drag force. Indeed, the

force-distance plots changed with different moving distances of the cantilever, as shown in

Figure S2 of the Supporting Information. A small moving distance provides a relatively weak

drag force. Further, a larger frequency of force curve motion should also increase the magnitude

of the above time derivative terms. Namely, the drag force would also increase with the

PeakForce Tapping frequency. Therefore, an experiment with a larger frequency may enhance

the detection sensitivity of the dissipation change. From equations (1) and (2), the energy

dissipation is expressed as follows:

  



















 



  

















where the force curve frequency (ω) was set to be constant during the experiment. Therefore,

all time-dependent terms in the above equation become constant because the force curve period

(T) is also constant. Thus, the energy dissipation can be simplified as follows:

   



 

The constant parameters A' and B' are determined by the experimental condition of the

cantilever operation. Namely, cantilever properties, moving distances of the cantilever and

PeakForce Tapping frequency can be summarized into device constant. As for the quantitative

analysis, these constant parameters would possibly be determined by the initial calibration

experiment. Therefore, the physical properties of the solution were directly related to the energy

dissipation value of the force curve motion at the regulated AFM operating condition. If the

force curve motion can operate with the triangle wave function, the acceleration term (dz

/ dt

)

of equation (3) is zero, and the dissipation can be simplified as follows.

  



 

Therefore, the triangle wave function is promising for this experiment, however, this operation

is not typical in imaging and cannot be performed so far. Here, viscosity (η), mass density (ρ),

and corresponding dissipation (W) are closely related to the Li-ion concentration of the

electrolyte solution. However, these physical parameters are not simply proportional to the Li-

ion concentration.

22–28

Therefore, the direct estimation of the Li-ion concentration by measuring

the corresponding dissipation (W) should be difficult in the present experiment. In this regard,

some additional quantitative experiments such as the calibration curve analysis of the Li-ion

concentration as a function of dissipation might be promising to evaluate the Li-ion

concentration by dissipation analysis using the AFM technique.

As shown in Figure 2, the force spectra acquired within the electrolyte were actually

changed by the electrochemical operation. As mentioned above, in this experiment, the

cantilever should be sensing the drag of the electrolyte solution, which is approximately 5 μm

away from the Li metal surface because the average tip height was 5 μm. (see Supporting

Information Figure S1) Generally, the thickness of the diffusion layer extended up to several

hundred μm or higher within the organic electrolyte solution.

39–44

Therefore, the cantilever was

operating at the region of the Li metal electrode surface which was less than 10% of the overall

thickness of diffusion layer, defined as the “interfacial electrolyte”. The cyclic-voltammetry

experiment was performed with a Li / Li symmetric cell at a 5 mV sec⁻

scan rate. A typical

linear slope appeared in the positive and negative potential regions, indicating reversible Li

metal dissolution and deposition on the working electrode surface, respectively. The

corresponding force spectra acquired from each point of A to C on the cyclic-voltammetry

profile are shown in the panels below. The force spectra of the open circuit condition (point A)

showed a typical force curve at both approaching (blue) and retracting (red) conditions,

respectively. The information of electrolyte solution drag was automatically corrected by

electrical processing, therefore, the non-contact baseline showed F = 0 and the energy

dissipation W was also defined as 0. At the Li metal dissolution condition, point B, the force

spectra differed considerably from point A. Especially, the non-contact base lines were shifted

positively and negatively in the approaching and retracting periods, respectively. This

corresponds well to the increase in solution drag based on the increase in viscosity or mass

density of the electrolyte by Li metal dissolution, and thus the energy dissipation shifted

positively (W > 0). In contrast, the above feature of the non-contact baseline was opposite in

case of Li metal deposition, denoted as point C. Here, the absolute value of the solution drag

was not negative, but the value was indicated as negative compared to the open circuit condition

because the Li-ion concentration of the electrolyte decreased due to Li metal deposition (point

C). Therefore, in this case, the energy dissipation was calculated as a negative value (W < 0).

Based on the above principle and the experimental results, the energy dissipation value can be

considered as a useful indicator to detect physical property changes in the electrolyte solution.

The actual results coupled with electrochemical operation are discussed below.

Direct detection of physical property changes in the interfacial electrolyte at galvanostatic

charge-discharge operation of the Li metal electrode.

The charge (Li metal deposition) and discharge (Li metal dissolution) reaction of the

Li metal electrode changes the physical properties of the interfacial electrolyte. This can be

detected by dissipation recording coupled with electrochemical operation. The voltage profiles

of the various galvanostatic charge-discharge reactions (blue) and their corresponding energy

dissipation (red) as a function of time (s) are shown in Figure 3. The numbers next to the voltage

steps indicate current densities (mA cm⁻

) for charge (negative) and discharge (positive),

respectively. There, the waveform of energy dissipation and voltage profiles corresponded well,

which suggested that the physical property changes of the interfacial electrolyte were

adequately detected at in-operando conditions. At the open circuit condition (0 to 100 s region),

the dissipation energy showed ground (0) because the solution drag was corrected by the data

calculation process of the device. Then, during the first charge process at i = -1 mA cm⁻

current density (100 to 200 s region), the voltage significantly shifted to the negative region and

stayed constant, indicating Li metal deposition. With respect to this, the dissipation clearly

decreased to a negative value, which corresponded well with the decrease in solution drag for

the interfacial electrolyte by Li metal deposition. In the following first discharge process (i =

+1 mA cm⁻

within the 200 to 300 s region), the voltage profile flipped, indicating Li metal

dissolution. The dissipation quickly responded and considerably increased in the positive

direction, indicating an increase in the solution drag due to Li ion dissolution into the interfacial

electrolyte. In the following electrochemical operation, the constant current pulse was doubled

(i = ±2 mA cm⁻

) with respect to both charge and discharge regions. The amplitude of the

dissipation change increased, corresponding to the magnitude of current densities. Therefore,

the depth of the dissipation change directly responded to the scale of the Li metal deposition-

dissolution reaction. As for the sensitivity of this experiment, shown in Figure S3 of the

Supporting Information, the dissipation energy change was detected even in the ± 0.1 mA cm⁻

range of the current density charge-discharge operation. This current density should be regarded

as small for general electrochemical experiments. Thus, a relatively sensitive characterization

was performed at present cantilever condition. As shown in Figure 3, the dissipation value itself

was asymmetric during the charge and discharge processes, and therefore the center of

dissipation fluctuation seems not to be the dissipation baseline (W = 0) but shifts to the rather

negative side. This was likely caused by the first charge reaction in the 100 to 200 s region.

Namely, the Li-ion concentration decreased from the open circuit condition during the first

charge (Li-ion deposition) process. Thus, the following discharge (Li metal dissolution) process

was affected by this initial decrease in Li-ion concentration. In this way, the following charge-

discharge process was affected by the initial or preceding electrochemical operation.

During continuous charge-discharge operation, the history of the preceding operation

should noticeably affect the solution properties near the Li metal surface, which complicates

understanding the Li-metal deposition and dissolution mechanism. This is because continuous

operation lacks an adequate relaxation process based on Li-ion diffusion. Li-ion diffusion is

essential for the solution relaxation of the interfacial electrolyte, as observed in the open circuit

condition for the higher than 500 s region in Figure 3. To understand the solution relaxation

and related Li-ion diffusion, a single current pulse experiment was performed, as presented in

Figure 4. A single constant current pulse (i = 1 mA cm⁻

) was applied for 64 s after 64 s of open

circuit condition, as displayed in Figure 4(a). The dissipation value gradually increased during

positive constant current conditions (Li metal dissolution), as discussed above. Then, it

gradually decreased during the second open circuit condition (after t = 128 s), indicating the

relaxation process of the interfacial electrolyte. The solution drag should decrease with

decreasing Li-ion concentration due to diffusion. The dissipation energy decay curve of the

open circuit condition was well fitted by the logarithm approximation (black solid line) using

the general formula of W = a ln (Δt) + b, where Δt is defined as (t − 128) s. Here, the physical

meaning of this approximation was not obvious, since the dissipation energy did not show a

linear relation to the Li-ion concentration. However, the relaxation should be almost complete

within Δt = 70 s, which is the time at which the dissipation W is 0 in this approximation formula.

Assuming a simple self-diffusion model, the diffusion length (L) of Li-ions is expressed as

follows:

 







 

where D

Li+

is a self-diffusion coefficient for Li-ions in the solution, which is estimated to be

about 10⁻

s⁻

in a tetraglyme-based electrolyte.

Therefore, the Li-ion diffusion length

should reach 10

μm or larger within Δt = 70 s. This is considerably far from the Li metal

electrode surface. Thus, the interfacial electrolyte should be sufficiently relaxed within that time

scale. Of course, the quantitative analysis of the Li-ion concentration as a function of time

should be used to understand the interfacial diffusion phenomena directly, and it would provide

more detailed information on this interfacial relaxation. However, the quantitative analysis of

the Li-ion concentration will be studied in future work, as mentioned above.

The fast Li-ion diffusion also affected the response feature of the dissipation change

corresponding to the constant current step, as presented in Figure 4(b). The feature of the

observed dissipation decay seems to be considerably well synchronized with the voltage shift

based on the current step. Therefore, the physical property changes of the interfacial electrolyte

should respond quickly to the electrochemical operation. In this experiment, the cantilever was

mainly sensing the physical property changes of the solution about 5 μm away from the

electrode surface. According to equation (5), the Li-ion should diffuse this distance within 0.1

s. The sampling rate of the dissipation recording was 1 Hz, indicating that the time resolution

of 1 s for this experiment was far below the diffusion time. Therefore, the dissipation response

would be synchronously observed with the electrochemical operation in this experiment. Of

course, a certain time delay may exist between the electrochemical operation and the dissipation

response because the Li-ion diffusion rate is finite. This time delay based on the Li-ion diffusion

rate will be investigated by enhancing the sampling rate of the dissipation recording, exceeding

0.1 s. According to the PeakForce Tapping AFM technology, the frequency of the force curve

motion have reached to the kHz scale.

Therefore, the sampling rate of the dissipation

recording can also be enhanced to the kHz scale if each force-distance data is analyzed with

every force curve cycle. There, the time resolution of the dissipation recording will reach ms.

Moreover, electronic data processing also restricts the sampling rate, while, the state-of-art

digital technique will balance this limitation.

At present, the AFM system is designed to focus on imaging. Therefore, further

instrumental modification is needed for the above-mentioned dissipation recording. The energy

dissipation analysis with force curve motion was promising with respect to the direct in-

operando detection of the physical property changes of the interfacial electrolyte. Further, it

should be beneficial to investigate the physical property changes of other electrolytes using this

method. Indeed, the dissipation change corresponding to the charge-discharge reaction of the

Li metal electrode was also confirmed in a propylene carbonate (PC)-based electrolyte (1 M

LiPF

-PC) as an example of a conventional electrolyte, as shown in Figure S4 of the Supporting

Information. Recently, ionic liquids,

66,67

super-concentrated electrolytes,

68,69

and solvated ionic

liquids

70,71

were presented as promising electrolytes for Li metal electrodes. However, the

actual interface properties of these electrolyte systems are yet not well understood. Therefore,

the present method should be suitable to investigate the various interfaces of above electrolytes

and Li metal electrodes, and contribute to the further development of Li metal battery

applications.

CONCLUSIONS

The in-operando detection of physical property changes of the interfacial electrolyte

during the Li metal electrode reaction was well demonstrated by the dissipation analysis of

cantilever force curve motion using an atomic force microscopy-based technique. The solution

drag of the cantilever depends on the physical properties of the electrolyte, such as viscosity

and mass density, leading to the kinetic energy dissipation during force curve motion. During

the galvanostatic charge-discharge reaction of the Li metal electrode in the tetraglyme-based

electrolyte, the dissipation energy was clearly altered, corresponding to the voltage shift of the

electrochemical cell. The energy dissipation in the interfacial electrolyte, at a distance 5 μm

away from the electrode surface, increased during discharging of the Li metal electrode. This

corresponds well to the increase in the viscosity and mass density of the solution caused by the

Li-ion dissolution into the electrolyte. These physical property changes of the electrolyte cannot

be ignored with respect to the continuous charge-discharge reaction. As for the open circuit

condition, the changed dissipation value is relaxed within ~ 70 s owing to the self-diffusion of

Li-ions. The fast diffusion of Li-ions at the interfacial region of the electrolyte will be clarified

by enhancing the time resolution of energy dissipation recording. As a further modification of

the present AFM system, the force-distance analysis of every force curve motion should

improve the sampling rate of dissipation recording. The direct detection and investigation of

these interfacial phenomena at the Li metal electrode surface should contribute to the

understanding of the electrochemical operation of Li metal batteries with various electrolytes.

FIGURES

Figure 1. Principle for the detection of Li metal electrode reaction by energy dissipation of

cantilever motion. (a) In the open circuit condition, the solution drag of approaching motion

) and retracting motion (f

) were placed on the cantilever. The force (F) and displacement

(Z) plot presented at the bottom, shows a typical force curve with a corrected non-contact

baseline by the data processing of the AFM system. (b) At the discharge (Li metal dissolution)

condition, the solution drag increased with increasing Li-ion concentration, and the dissipation

(W) changed to the positive side. (c) At the charge (Li metal deposition) condition, the solution

drag decreased and the dissipation was shifted vice versa of (b).

(a)

App-

roach

Retract

(b)

App-

roach

Retract

(c)

App-

roach

Retract

Approach

Retract

Force

Displacement (Z)

Contact

force (F)

Force

Displacement (Z)

0 0

W > 0

Force

Displacement (Z)

W = 0 W < 0

Figure 2. The cyclic voltammetry performed at a 5 mV sec⁻

scan rate and measured force

curves corresponding with A to C of the voltammogram. The features of the force curves were

consistent with those explained in Figure 1.

-0.4

-0.2

0.2

0.4

-1.2 -0.8 -0.4 0 0.4 0.8 1.2

-2

0 100 200

Distance Z (nm)

Force (

nN)

Approach

Retract

-2

0 100 200

Distance Z (nm)

Force (

nN)

Approach

Retract

-2

0 100 200

Distance Z (nm)

Force (

nN)

Approach

Retract

Potential (V vs. Li

/Li)

Current (mA)

Li-metal

deposition

Li-metal

dissolution

A B C

Figure 3. Plot of energy dissipation (red) and cell voltage (blue) during galvanostatic charge-

discharge reaction of the Li metal electrode as a function of time. The number next to the

voltage profile is the current density (mA cm⁻

-0.3

0.3

0 300 600 900

-1.5

-1

-0.5

0.5

W (keV)

V (vs. Li

/Li)

Time t (sec)

Cell Voltage

Dissipation

-1

-2

+2 mA cm

-2

100 sec

Figure 4. Plot of energy dissipation and cell voltage as a function of time at single constant

current pulse measurement. The current pulse was applied for 64 s with i = 1.0 mA cm⁻

current density. (a) Overall features of this experiment. The black solid line shows the logarithm

approximation for the relaxation region after 128 s, where Δt is defined as t-128 s. (b) Zoomed

plot of (a) in the 110 to 140 s region. Dissipation and cell voltage are shown in red squares and

blue circles, respectively.

0.6

1.4

1.8

110 120 130 140

-1

-0.3

0.3

0.9

0 50 100 150 200 250

i = 0

Voltage

shift

Dissipation

decay

Time t (sec)

W (keV)

V (vs. Li

/Li)

W = a ln (Δt) + b

a = -0.435

b = 1.831

Δt = t - 128 (sec)

W (keV)

Time t (sec)

(a)

(b)

64 (sec)

i = 0

i = 1

ASSOCIATED CONTENT

Supporting Information

A typical model of cantilever on the electrochemical interface, Force-distance plots with

different cantilever displacement, Dissipation recording with less current density operation,

Dissipation recording with 1 M LiPF

-PC electrolyte. (PDF)

AUTHOR INFORMATION

Corresponding Author

Mitsunori Kitta*

E-mail: [email protected]

TEL: +81-72-751-8703

FAX: +81-72-751-9714

Research Institute of Electrochemical Energy, Department of Energy and Environment,

National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31,

Midorigaoka, Ikeda, Osaka 563-8577, Japan

ACKNOWLEDGMENT

M. K. thanks Dr. Zyun Siroma (AIST) and Dr. Hikaru Sano (AIST) for discussion and

comments for the experimental results.

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SYNOPSIS TOC

Appro

ach

Strong solution drag

Retract

Appro

ach

Retract

Li-metal electrode

In discharge In charge

Electrolyte

Weak solution drag