In Situ Electron Holography Analysis of Electric Potentials inside a Lithium-ion Conductive Solid-state Electrolyte

Yuka Aizawa1,*, Kazuo Yamamoto1, Takeshi Sato1,*, Hidekazu Murata2, Ryuji Yoshida1, Craig A. J.

Fisher1, Takehisa Kato3, Yasutoshi Iriyama3, Tsukasa Hirayama1

1Nanostructures Research Laboratory, Japan Fine Ceramics Center, Nagoya, 456-8587, Japan

2Faculty of Science and Technology, Meijo University, Nagoya, 468-8502, Japan 3Department of Electronics, Nagoya University, Nagoya 464-8601, Japan

*Now with Hitachi High-Technologies Corporation

In situ electron holography (EH) has been applied to observe electric potential

distributions in all-solid-state lithium (Li)-ion batteries during a charge-discharge cycle

[1]. In general, electron holography detects phase shifts of electron waves modulated by

electric potential, and the phase shift is proportional to the potential. However, it is still

not easy to interpret the phase distribution obtained by EH, because the potential

distribution in battery materials depends on a variety of factors, for example, Li-ion and

Li-vacancy distributions, and IR losses induced by transport of ions. Furthermore, when

a voltage is applied to the cathode and anode of the thin specimen in a TEM, the electric

potential is generally spread 3-dimensionally around the specimen. The electrons pass

through the 3-dimensional (3D) potential, and thus the detected phase images include

the influence of the projected potential. In order to measure the potential distribution

inside the battery precisely, we need to determine the influence of the 3D potential on

the phase images. To do this, in this study, we measured the phase distribution in a

simple model specimen with a Cu-electrode/solid-electrolyte/Cu-electrode

configuration, by applying a voltage between the electrodes. We then ran a computer

simulation to calculate the 3D potential around the specimen and obtained the projected

phase distribution. By comparing the experimental and simulation results, we succeeded

in observing the precise potential distribution in the solid-electrolyte and electric double

layer (EDL) at the electrode/solid-electrolyte interfaces.

Figure 1 shows a schematic of the TEM specimen. A lithium phosphorus oxynitride

(Li3.3PO3.8N0.22: LiPON) was used as the solid electrolyte, and the Cu/LiPON/Cu layers

were formed on a glassy-carbon (GC) substrate by Pulsed laser deposition and

sputtering methods. The biasing TEM specimen was prepared by FIB and low-voltage

Ar-ion milling. A constant voltage, VCu-Cu, of -2, -1, 1 or 2 V was applied to the Cu

electrode on the tungsten protection layer, and the other Cu electrode on the GC

substrate was kept grounded. The reconstructed phase profile along the Cu/LiPON/Cu

(red-boxed region in Fig. 1) was obtained by EH at each voltage.

The Cu/LiPON/Cu model used for simulation is shown in Fig. 2(a). The thickness of

the thin region and the height of the protruding wall are the same as those of the actual

specimen shown in Fig. 1. We assumed that the potentials in the Cu electrodes and the

LiPON electrolyte were flat and the EDL had a linear distribution, as shown in Fig. 2(b).

We also assumed that the potential value in the LiPON, VLiPON, was half of VCu-Cu and

the width, t, of the EDL was 25 nm. To calculate the 3D potential around the model, we

used a 3D boundary-charge method [2].

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AMTC Letters　Vol. 5 (2016) ©2016 Japan Fine Ceramics Center

Figure 3(a) shows the experimental

(black and red curves) and simulated (green

curves) phase profiles across the

Cu/LiPON/Cu specimen. The phase

distributions in the Cu and LiPON regions

are sloped, although flat potentials were

assumed in the simulation. It can also be

noticed that the simulated phase curves are

slightly above the measured curves. We

re-calculated the phase profiles with lower

potential values, VLiPON, as indicated in Fig.

3(b), and compared them with the measured

profiles (Fig. 3(b)). The results are in much

better agreement. We conclude from these

comparisons that the actual potential in the

LiPON is flat or slightly sloped, and the

potential values are less than half of the

applied voltage, as shown in Fig. 4(b), while

those in a liquid electrolyte are commonly

half the voltage (Fig. 4(a)). Our results

suggest that the local density of Li ions at the

Cu (GC side)/LiPON interface is lower than

that of Li vacancies (VLi-) at the other Cu (W

side)/LiPON interface, as illustrated in Fig.

4(b). Details of the EDL profile at the

interface are reported in ref.3. We will report

that in the conference.

Reference

[1] K. Yamamoto et al., Angew.Chem. Int.Ed. 49 (2010) 4414-4417.

[2] H. Murata, T. Ohye, H. Shimoyama, Proc. SPIE 4510 (2001) 107-118.

[3] Y. Aizawa et al., Ultramicroscopy (submitted).

This work was supported by the RISING project of the NEDO in Japan.

Figure 1 Schematic of the TEM specimen for in situ

electron holography.

A

BCu

CuLiPON

W

GC

22 m

5 m

100 nm

x

yz

Z = 0

19 m

Figure 2 Model used in the 3D simulation. (a) Model

shape and dimensions. (b) Internal potential profile

assumed in Cu/LiPON/Cu.

(b) (a)

VCu-Cu

0

VLiPON

Vo

ltag

e /

V

Cu (GC side) LiPON Cu (W side)

Distance/nmA B

t

t

400 28000 3200

Figure 3 Experimental and simulated phase profiles

(a) assuming that the VLiPON is 1/2VCu-Cu.

(b) the VLiPON is slightly lower than 1/2 VCu-Cu.

Figure 4 Schematic diagrams of potential distributions

in (a) liquid electrolyte and (b) solid electrolyte. Local

density of Li ions at the Cu (GC side)/LiPON

interface is lower than that of Li vacancies (VLi-) at

the Cu (W side)/LiPON interface in (b), in contrast to

(a).

(a) (b)

WCuLiPON

CuGC

4002400

400100

Ga+

Ar+

nmnm

nm

nm

Observed region

22 m

Ga+

Ar+

(a) (b)

-40

-30

-20

-10

0

10

20

30

40

0 1000 2000 3000

-40

-30

-20

-10

0

10

20

30

40

0 1000 2000 3000

Phase /

rad

LiPON

2 V

-2 V

1 V

-1 VPha

se /

rad

simulationexperiment

Cu (GC side) Cu (W side)

1.0 V

0.5 V

-0.5 V

-1.0 V

VLiPON =

VCu-Cu =

Distance/nm

-40

-30

-20

-10

0

10

20

30

40

0 1000 2000 3000

Phase /

rad

LiPON

2 V

-2 V

1 V

-1 V

-40

-30

-20

-10

0

10

20

30

40

0 1000 2000 3000

Cu (GC side) Cu (W side)

simulationexperiment

0.8 V

0.4 V

-0.5 V

-1.1 V

VLiPON =

VCu-Cu =

Distance/nm

Cu (GC side) Cu (W side)

0

VLiPON

VCu-Cu

Vo

lta

ge

/ V

Electric potential distribution

VLiPON = VCu-Cu / 2

anionLi+

Liquid electrolyte

Li+ VLi-

Cu (GC side) Cu (W side)LiPON

0

VLiPON

VCu-Cu

Vo

lta

ge

/ V

Electric potential distribution

VLiPON＜ VCu-Cu / 2

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AMTC Letters　Vol. 5 (2016) ©2016 Japan Fine Ceramics Center