Article

DOI: 10.1557/jmr.2019.299

ADVANCES IN BATTERY TECHNOLOGY: MATERIAL INNOVATIONS IN DESIGN AND FABRICATION

Annealing of LiCoO

ﬁlms on ﬂexiblestainlesssteel for thin

ﬁlm lithium batteries

Yibo Ma

, Mu Chen

, Yue Yan

1,a)

, Youxiu Wei

, Weiming Liu

, Xiaofeng Zhang

, Jiaming Li

Ziyi Fu

, Jiuyong Li

, Xuan Zhang

Beijing Engineering Research Center of Advanced Structural Transparencies for the Modern Trafﬁc System, Beijing Institute of Aeronautical Materials,

Beijing 100095, China

Address all correspondence to this author. e-mail: [email protected]

Received: 27 June 2019; accepted: 18 September 2019

The LiCoO

ﬁlms were directly deposited on stainless steel (SS) using medium-frequency magnetron sputtering,

and the effects of annealing paramete rs, such as ambiences, temperatures, holding times, and heating rates,

were systematically compared based on surface morphologies, crystal structures, and electrochemical

properties. The results demonstrate that an aerobic atmosphere with 3.5 Pa is the most important parameter to

maintain the performance of LiCoO

ﬁlms. The in ﬂuence of the annealing temperature (>550 °C) ranks second

because the formed (101) or (104) planes of LiCoO

facilitate Li

migration. A short holding time of 20 min and

a moderate heating rate of 3 °C/min are selected to reduce the oxidation or inter-diffusion between the LiCoO

ﬁlms and the SS substrate. Finally, the optimal annealing process is conﬁrmed and corresponds to the initial

discharge capacity of 37.56 lA h/(cm

lm) and the capacity retention of 83.81% at the 50th cycle.

Introduction

Researchers now pay more attention to all-solid-state thin ﬁlm

lithium batteries (TFLBs) due to their potential applications

such as main power in identiﬁcation cards, metal–oxide–

semiconductors, and ﬂexible electronic paper displays [1].

The fabrication of LiCoO

ﬁlms as cathodes in TFLBs is

a successful choice due to their excellent electrochemical

properties and maturity of manufacturing [2]. Compared to

the traditional hard substrates, such as Si or mica, TFLBs on

stainless steel (SS) are ﬂexible; that is, they can be charged/

discharged in a bending state [3] and, thus, can be applied in

wearable devices [4, 5]. Since the as-grown LiCoO

ﬁlms

prepared by magnetron sputtering (MS) at room temperature

are in an amorphous state, a post-annealing process is indispens-

able to obtain crystalline LiCoO

. Traditional post-annealing

methods for LiCoO

ﬁlms include tube furnace heating [6], rapid

thermal annealing [7, 8, 9, 10, 11], in situ heating [12], two-step

heating [13], and plasma-assisted treatment [14, 15]. In this

article, annealing is performed in the tube furnace due to the

uniform temperature distribution and low cost.

The deposition of LiCoO

ﬁlms by MS is always in an

anoxic state; in order to keep the ideal stoichiometric ratio of

Co:O and an ideal layered structure, an aerobic environment is

required in the post-annealing process. But the SS substrate is

inevitably oxidized and the oxidation layer formed on both

sides of SS increases the battery internal resistance [16, 17, 18].

These oxidation layers need to be polished before battery

assembly. However, the shortcomings do not obscure the

potential of SS as a substrate. SS can directly act as current

collectors, and thereby simplify the manufacturing process and

reduce the material cost [19]. Compared with ﬂexible sub-

strates such as Cu or Al foil, SS has better chemical stability, is

more suitable for annealing at a high temperature, and is easily

compatible with ﬂexible electronic devices [20, 21].

Failures in LiCoO

ﬁlms, such as delaminations, wrinkles,

or cracks, often occur after post-annealing. The cracks cause

short circuits when the solid electrolyte and lithium anode are

deposited subsequently during TFLB fabrication. So it is

obvious to optimize annealing parameters of the LiCoO

ﬁlms

to improve the discharge capacity or cycle life of TFLBs. In

addition, some annealing works on LiCoO

in Table I in-

troduce the following four annealing parameters [6, 8, 21, 22,

23, 24, 25, 26, 27, 28]. First, both pure oxygen and air

environments are preferred annealing ambiences. Second,

a low annealing temperature (,600 °C) can reduce ﬁlm cracks

or by-products; however, insufﬁcient heating (,400 °C) leads

ª Materials Research Society 2019 cambridge.org/JMR 1

j Journal of Materials Research j www.mrs.org/jmr

FOCUS ISSUE

Downloaded from https://www.cambridge.org/core. La Trobe University, on 27 Oct 2019 at 05:00:57, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/jmr.2019.299

to poor crystallization, which results in poor cycle ability. Last,

a time-saving method is adopted to reduce the interface side

reaction for holding time and heating rate. All of the articles

mentioned in Table I is beneﬁcial to the optimal annealing of

LiCoO

ﬁlms on ﬂexible SS. However, each article only

discusses one annealing parameter, while a reasonable anneal-

ing process considering the combined effects of four annealing

parameters is performed here.

This article introduces a systematic investigation of the

effects of the post-annealing process on LiCoO

ﬁlms grown by

MS. The effects of annealing ambiences, temperatures, holding

times, and heating rates are compared in terms of composi-

tions, surface morphologies, crystal structures, and charge/

discharge performances. Then, an optimum annealing process

for LiCoO

ﬁlms is obtained and this work provides compre-

hensive support for the annealing of LiCoO

ﬁlms.

Results and discussion

Annealing atmosphere

The compositions of these LiCoO

ﬁlms ann ealed u nder

different atmospheres are detected by X-ray photoelectron

spectroscopy (XPS). The contaminated carbon (C 1s,284.8

eV) in Fig. 1(a) is used to c alibrate the positions of other

elements. In Fig. 1(b), Li signals at the binding en ergy of 60.9

eV represent LiC

[27], which is a compound with surface-

contaminated carbon; this p roduct exists in other annealing

conditions too. However, Li signals under rough v acuum exist

in the form of Li

at 55.6 eV, and Li

may be formed

by the reaction between Li

O precipitated from ﬁlm and H

and CO

in air. The other Li signals are LiOH at 54.2 eV,

formed by the reaction between Li

Oprecipitatedfrom

LiCoO

and H

O in air [24]. Figure 1(c) expresses the same

Co s ignal s of Co 2p

3/2

(BE 5 77 9.9 eV) and Co 2p

1/2

(BE 5

796.4 eV) in any an nealing atmosphere, and the correspond-

ing products are Co

and CoO impurity phases [29, 30].

The peaks of O 1s signals for different samples are also the

same in Fig. 1(d). One peak corresponds to LiCoO

at 531.1

eVandtheotherisCo

at 529.4 eV, with the removal of Li

element d uring annealing, and th e signal of Co

increases

[31, 32]

The SEM images of the annealed LiCoO

ﬁlms are shown

in Fig. 2. The surface of any annealed LiCoO

ﬁlms [as shown

in Figs. 2(a)–2(s)] exhibits the network cracks, and the

average width of the crack is ;100 n m. Particularly, sample

A4 annealed under pure vacuum [Fig. 2(d)] expresses more

obvious c rack topography with a width more than ;400 nm.

In fact, all the cracks are introduced by the thermal stress

thermal

) during the cooling down process, and r

thermal

calculated by the difference in thermal expansion coefﬁcient

between the LiCoO

ﬁlm and SS according to formula (1) [33,

34, 35]. Once the thermal stress exceeds the load limitation,

more interface cracks are generated to release the excessive

thermal stress [21, 36, 37]. For example, r

thermal

in the

LiCoO

ﬁlm annealed at 650 ° C is equal to 0.78 GPa

(compressive stress); the ﬁlm’ sstressis0.18GPalargerthan

that of the sample annealed at 500 °C. Especially, the un-

annealed ﬁlm is uniformly continuous without any cracks, but

several large particles with diameter of ;500 nm distribute on

the surface according to Fig. 2(t). Moreover, Fig. 3 shows

a cross-sectional image of the LiCoO

ﬁlms on SS; the

annealed LiCoO

ﬁlms possess the traditional columnar

crystal structure and the ﬁlm thickness is ;1.09 lm, consistent

with the measured thickness of 1.1 lm using a proﬁlerTencorP

7, KLA (keeping looking ahead) Co., Chandler, Arizona.

thermal

¼ Y

LiCoO

=1  m

LiCoO

 a

LiCoO

 a

ðÞDT ; ð1Þ

where the thermal expansion coefﬁcient of LiCoO

LiCoO

ðÞis

1.3  10

5

1

, a

is 1.8  10

5

1

, and Young’s modulus

LiCoO

ðÞand Poisson’s ratio m

LiCoO

ðÞof LiCoO

are 191 GPa

and 0.24, respectively [38].

All the LiCoO

ﬁlms annealed in pure oxygen ambiences,

such as A1, A2, and A5 in Figs. 2(a), 2(b), and 2(e), form

triangular-shaped grains and the average grain size is

;100 nm. This triangular shape is probably due to the strong

(001) preferential plane [39]. For the LiCoO

ﬁlm annealed

under air (A3) in Fig. 2(c), grains are uniformly granular

shaped and the average size also approaches 100 nm. However,

sample A4 annealed in rough vacuum shows loosely packed

worm-like particles and large amounts of voids or cracks

[Fig. 2(d)]. These are not typical morphologies of the crystal-

lized LiCoO

structure.

TABLE I: Previous annealing parameters for LiCoO

thin ﬁlms.

Annealing parameters Type or range Function Reference

Annealing atmosphere Ar/O

mixture, air, O

, Ar,

rough vacuum

Inﬂuencing the ﬁlm composition. Pure O

—better LiCoO

performance;

pure Ar—dehydrated LiCoO

; Air—poor LiCoO

performance

[3, 13, 21]

Annealing temperature 400–750 °C T . 700 °C—Li is missing, Co

impurity phase, and a serious side

reaction occurs; T , 400 °C— a spinel type (LT-LiCoO

);

T 5 550 °C–600 °C—an ideal layered structure

[13, 14, 15, 16]

Holding time 0–120 min Determining the crystal structure [6, 17]

Heating rate 1–600 °C/min Controlling the interface side reactions [5, 18, 19]

Article

ª Materials Research Society 2019 cambridge.org/JMR 2

j Journal of Materials Research j www.mrs.org/jmr

Figure 4(a) presents the XRD patterns of the LiCoO

ﬁlms

annealed at various atmospheres. The diffraction peaks are

located at 2h 5 19.2°, 37.3°, 39.0°, 45.3°, and 66.1°, corre-

sponding to the (003), (101), (012), (104), and (110) crystal

planes of the LiCoO

ﬁlm [7]. Here, (003) stands for a layered

structure and (104) identiﬁes a basic unit Co–O–Co in the

layered LiCoO

structure. Additional four positions particularly

marked by the asterisk (*) stand for MoNi

(PDF 65-5480)

from SS during the annealing process [21]. The sample

annealed in air shows (003) preferred plane especially; the

other samples are polycrystalline. The relative intensity of the

(003) peak gradually increases and the intensity of the (104)

peak begins to appear with the enhancement of O

content,

which declares that the oxygen-rich environment contributes to

LiCoO

crystallization. Due to the formation of oxygen

vacancies during the annealing process, the increase in oxygen

vacancies is against the formation of dense (003) grains, but

tends to form (101) and (104) grains. So the efﬁcient oxygen

content in the annealing environment is necessary because

excess oxygen atoms on the LiCoO

surface are ﬁrst taken away

and then a large number of oxygen atoms at the grain

boundary position are driven into the ﬁlm surface to form an

oxide layer, causing an increase in oxygen vacancies inside the

ﬁlms.

It should be mentioned that the initial open-circuit

potential is ;2.8 V when the LiCoO

half-cells are assembled.

Charge/discharge curves and cycle performance are presented

in Fig. 5. The annealed LiCoO

ﬁlms show the same trend: the

discharge capacity decays abruptly during the initial four cycles

and then decreases slowly, which is testiﬁed by the columbic

efﬁciency values. In the case of two samples annealed in O

5 3

sccm and O

5 6 sccm, the discharge curves in Figs. 5(a) and

5(c) show a typical discharge platform in the range of 3.8–4.0

V, and the discharge capacity decreases with ;0.1 lA h/(cm

lm) per cycle as shown in Figs. 5(a9) and 5(c9), the sample

treated with O

5 3 sccm has the 1st discharge capacity of

41.06 lA h/(cm

lm) and the 50th capacity retention ratio of

64.34%, and the sample treated with O

5 6 sccm has the

corresponding values of 38.96 lA h/(cm

lm) and 46.25%.

However, the discharge curve slopes down without any

plateaus in Fig. 5(b) for the sample annealed under rough

vacuum, its 1st discharge capacity is only 5.51 lA h/(cm

lm),

Figure 1: Surface composition of LiCoO

ﬁlms under different annealing atmospheres: (a) C 1s, (b) Li 1s, (c) Co 2p, and (d) O 1s signals. Atmosphere types are

added next to the curves.

Article

ª Materials Research Society 2019 cambridge.org/JMR 3

j Journal of Materials Research j www.mrs.org/jmr

and the 50th discharge capacity is 1.21 lAh/(cm

lm); this low

capacity can be explained by decomposing of the formed LiCoO

to an Li-deﬁcient phase (Li

0.62

CoO

) or to the spinel hetero-

phase due to the lack of oxygen during annealing. The presence

of the above phases makes LiCoO

capacity to attenuate fast

during charging in a wide range of voltages (3.6–4.2 V) [24].

Furthermore, the LiCoO

ﬁlms annealed in Ar/O

mixed gas or

air atmosphere are unable to charge to 4.2 V because an unstable

secondary phase of LiCoO

forms and this disturbs Li

migration. In addition, the formation of the Co

impurity

phase during the annealing process also deteriorates the electro-

chemical properties of LiCoO

[22, 40]. According to the above

charge/discharge curves, the annealing atmosphere of pure O

conﬁrmed prior to the other process parameters.

Annealing temperature (T)

The effects of different T on the surface topographies are

shown in Figs. 2(f)–2(j). Sample A6 that annealed at 500 °C

contains uniformly granular grains with a small size of ;30 nm

Figure 2: SEM images of the LiCoO

ﬁlms annealed with different atmospheres, temperatures, holding times, and heating rates from (a) to (s), and (t) represents

an un-annealed LiCoO

ﬁlm.

Article

ª Materials Research Society 2019 cambridge.org/JMR 4

j Journal of Materials Research j www.mrs.org/jmr

as shown in Fig. 2(f); LiCoO

grains become triangular once

the T reaches 550 °C and the corresponding grain size increases

to ;50 nm as shown in Fig. 2(g). Moreover, the average

LiCoO

grain size increases by ;35 nm for every 50 °C

increment between 550 and 650 °C. Some micro-cracks with

a length of 200–500 nm appear as the T increases. Thermal

stress in sample A6 (500 °C) is compressive and the stress value

is 0.66 GPa, 0.13 GPa less than that in A10 (650 °C). This

excessive stress may be released in the form of micro-cracks.

Figure 4(b) presents the XRD diagrams of the LiCoO

ﬁlms

annealed at different T.Alloftheseﬁlms are crystallized and

a hexagonal phase with a space group of R



3m is obtained. The

(003) plane disappears when the T exceeds 550 °C, while the other

peaks exist in the range of 500–650 °C. In addition, the intensities

of (101) and (104) peaks relative to substrate signals increase

slightly with the elevated T, which indicates the LiCoO

ﬁlms

tend to be crystallized in (101) and (104) facets and the

crystallization degree becomes better on a SS substrate [12,

19]. The preferred textures block Li

diffusion s ince no Li

channel is accessible on the ﬁlm surface [41]. Especially, the

Figure 3: SEM image of the fractured cross section of the LiCoO

ﬁlm grown

on SS substrate, annealed at 550 °C under a pure oxygen environment.

Figure 4: XRD patterns of LiCo O

ﬁlms annealed at different (a) annealing atmospheres, (b) temperatures, (c) holding times, and (d) heating rates. Crystal patterns

in LiCoO

are marked with the black dashed line and substrate signals are marked by the red dotted line. Different annealing conditions are added next to the

curves.

Article

ª Materials Research Society 2019 cambridge.org/JMR 5

j Journal of Materials Research j www.mrs.org/jmr

sample annealed at 650 °C shows a slight splitting at the (104)

peak position, which indirectly reﬂects that the grain is

distorted here, and the Li

lacking phase may occur due to

severe evaporation of Li

atoms when T is too high [21, 29].

Moreover, Jeon et al. point out that the interface side reaction

between LiCoO

ﬁlms and SS occurs using XRD di agrams of

different T.WhenT increases to 700 °C, the reﬂections of

,Fe

,andCoCrO

compounds are observed [12, 21].

An AES spectrum and an auger depth pro ﬁle in Fig. 6 are

used to indicate the inter-diffusion between LiCoO

and SS

[42]. Using SS as substrates, elements from both LiCoO

and SS

can diffuse into each other. The LiCoO

ﬁlms become semi-

conducting due to Fe, Cr doping [17, 18], while SS is lightly

oxidized by O atoms from LiCoO

. Figure 6(a) shows the AES

spectrum obtained from the LiCoO

ﬁlm surface. It consists of

Li, Co, and O at the surface. The auger peak of Li1 (39 eV) is

particularly affected by Fe

(50 eV) and Co

(57 eV), and the Li

element signal cannot be deducted from Co and Fe during

post-data processing. An auger depth proﬁle of the annealed

LiCoO

ﬁlms is shown in Fig. 6(b); it clearly shows that the

stoichiometric ratio of Li:Co:O is stable at ;1:1:2 in the LiCoO

ﬁlm zone. Fe, Cr elements also have a steady concentration in

the SS substrate zone. However, Li, Co, O, Fe, and Cr in the

transition zone between LiCoO

and SS vary with the

Figure 5: The (a)–(i) charge/discharge curves and corresponding (a‘)–(i‘) cycle performance of LiCoO

ﬁlms annealed at different atmospheres, temperatures,

holding times, and heating rates, and these annealing parameters are marked at the lower left corner.

Article

ª Materials Research Society 2019 cambridge.org/JMR 6

j Journal of Materials Research j www.mrs.org/jmr

sputtering depth. The content of Li

1 Co

1 Fe

has

a complex diversiﬁcation; the decline curve in the initial stage

is mainly caused by Li, Co reduction; and the latter sharp

increment is caused by the Fe content from SS. The Co

atomic

content decreases linearly from 15 to 0% with the increased

sputtering depth. The O

signal decreases slowly from 41 to

37% at the initial 2/3 stage and then decays to nearly 0. Fe

and

increase linearly from 2% to 15% and 0% to 4% with

sputtering depth, respectively.

So the diffusion of impurity from the SS substrate into

LiCoO

is the main reason for the destruction of the compo-

sition of LiCoO

and it also reduces LiCoO

discharge capacity.

The ﬁlms obtained at 500 °C in Fig. 5(d) express a short

discharge platform near the 3.8 V position, which may

correspond to an unexpected Li

phase, whose structure

falls between the ideal layered structure and the ideal spinel

structure [13]. In addition, cycle stability of the sample at

500 °C only lasts 37 times because the large LiCoO

surface

roughness results in uneven current density distributions,

which may lead to lithium dendrite and short circuits. The

1st and 37th discharge capacities are 32.53 lA h/(cm

lm) and

13.37 lA h/(cm

lm), respectively. This short-circuit is ﬁgured

out according to its columbic efﬁciency in Fig. 5(d9). The half-

cell assembled by the sample A6 can only cycle 37 times

normally; it stays in a charge state but do not discharge when

reaching the cutoff voltage at the 38th cycle. In addition, the

columbic efﬁciency of this half-cell remains at 97% for 31

cycles and it decays to 24% from the 31st cycle to 38th cycle.

The formation of lithium dendrite may cause short-circuits

because the declination of columbic efﬁciency along with the

cycle numbers reﬂects the formation of lithium dendrite [5].

Furthermore, both the short discharge platform and limited

cycle life indicate that 500 °C cannot meet the perfect

crystallization of LiCoO

ﬁlms. It is necessary to increase T

or extend the holding time further. As T increases to 550 or

600 °C, the initial discharge capacity is equal to 41.06, 32.05 lA

h/(cm

lm) separately, and the highest discharge capacity

retention ratio equals to 64.34%, 87.69% at the 50th cycle, as

shown in Figs. 5(a), 5(a9), 5(e), and 5(e9). Particularly, the

LiCoO

ﬁlms annealed at T . 625 °C have the discharge

capacity of ;0; this may be a result of the LiCoO

component

destruction caused by inter-diffusion or severe side reactions.

Holding time and heating rate

The interface diffusion between LiCoO

and SS is inevitable

during the annealing process. This partially changes the LiCoO

composition and impairs the discharge capacity of the LiCoO

cathodes severely. However, the interface diffusion can be

controlled by tuning the holding time and heating rate. The

surface particles on the sample A11 without heat preservation (0

min) in Fig. 2(k) are in an irregular shape and are packed-up

together. While the sample A12 annealed at 550 °C, holding for

10min[asshowninFig.2(l)]formsaregulargranulargrain

shape, but the average grain size is smaller than 30 nm. Besides,

thegrainshapetransformsintoatriangleshapewhentheholding

time exceeds 30 min and the grain size becomes more uniform

with the extended holding time, as shown in Figs. 2(m) and 2(n).

ThesampleA15withthelongestholdingtimeof;120 min

[Fig. 2(o)] corresponds to an average grain size of 100 nm.

Especially, the selected heating rate range (2–13 °C/min) corre-

sponding to Figs. 2(p)–2(s) have no obvious effect on the LiCoO

surface morphologies, all of which are uniform and loosely

packed, and the average crack width is ;100 nm [30].

The lattice orientation of LiCoO

ﬁlms annealed with

different holding times and heating rates is given in Figs. 4(c)

and 4(d) individually. Both holding time and heating rate

mostly affect the intensities of the (003) diffraction peak. For

example, the samples with no holding time (0 min) or with

higher heating rates (5 °C/min and 10 °C/min) express distinct

(003) grains, and the intensity of the (003) peak attenuates as

the holding time increases or the heating rate decreases.

In the case of the ﬁlms grown at different holding times

and diverse heating rates, there exists a holding time or

Figure 6: (a) AES spectrum of LiCoO

ﬁlms surface and (b) auger depth proﬁle of the LiCoO

ﬁlms grown on SS. Different element types are marked with letters A,

B, C, D, and E.

Article

ª Materials Research Society 2019 cambridge.org/JMR 7

j Journal of Materials Research j www.mrs.org/jmr

a heating rate that corresponds to the best capacity. For the

sample annealed in 10 min in Figs. 5(f) and 5(f9), the discharge

capacity at 1st/50th is equal to 34.31/11.08 lA h/(cm

lm),

respectively. Once the holding time exceeds more than 30 min,

the discharge curves quickly reach 3.9 V with cycle numbers.

Battery internal resistance increases with holding time possibly;

the reason for the increased impedance is the loss of Co and the

dislodged structures of LiCoO

, and black traces of LiCoO

material on the separator in the literature support the claim

[32]. In Figs. 5(g) and 5(g9), the LiCoO

sample with a ramping

rate of 3 °C/min has the 1st discharge capacity of 37.56 lAh/

(cm

lm) and capacity retention of 83.81% at the 50th cycle,

and the sample annealed with 5 °C/min has the equivalent

values of 41.48 lA h/(cm

lm) and 70.07%, as shown in

Figs. 5(h) and 5(h9). The sample annealed with 13 °C/min in

Figs. 5(i) and 5(i9) has the 1st discharge capacity of 38.07 lAh/

(cm

lm) and capacity retention of 70.82% at the 50th cycle.

Comparison of the parameters

Since the properties of the LiCoO

ﬁlms strongly correlate with

their compositions, morphologies, and structures, it is neces-

sary to take the synthetic effects into account. Figure 7

summarizes several important parameters: surface roughness,

peak intensity ratio [I

(101)

1 I

(104)

1 I

(110)

]/I

(003)

, the 1st

discharge capacity, and capacity retention rate at the 50th cycle

for selection of the optimum annealing process.

In Figs. 7(a)– 7( d), the average roughness of all the

annealed LiCoO

ﬁlms lies between 10 and 60 nm; this

roughness can be acceptable because the separator thickne ss

(30 lm) is thicker than the roughness. Annealing atmosphere

has the most obvious impact on roughness. The sample

annealed under rough vacuum has the largest roughness of

56.68 6 12.69 nm; the high roughness corresponds to

a large activation area, but more likely leads to battery failure.

The other annealing conditions lead to an average roughness

of ;30 nm.

The value of c/a lies among 4.95–5.00 and reﬂects the layer

spacing for Li

transfer; however, the peak intensity ratio of

(101)

1 I

(104)

1 I

(110)

]/I

(003)

lies between 1.20 and 3.42 with

varied annealing processes, as shown in Figs. 7(e) and 7(f).

Generally, the (101) and (104) planes facilitate de-intercalation

of Li

and correspond to a high initial discharge capacity; the

(003) plane can maintain structure stability during the Li

transfer process and a long cycle life can be achieved [43, 44,

45], so the ratio of [I

(101)

1 I

(104)

1 I

(110)

]/I

(003)

can reﬂect the

comprehensive electrochemical performance of LiCoO

cath-

odes directly. The maximum c/a value of 4.98 and maximum

Figure 7: Summary of the inﬂuences of annealing atmospheres, temperatures, holding times, and heating rates on (a)–(d) surface roughness, (e)–(h) peak

intensity ratio [I

(101)

1 I

(104)

1 I

(110)

]/I

(003)

, and (i)–(l) the 1st discharge capacity (solid point) and the 50th capacity retention ratio (hollow point).

Article

ª Materials Research Society 2019 cambridge.org/JMR 8

j Journal of Materials Research j www.mrs.org/jmr

ratio [I

(101)

1 I

(104)

1 I

(110)

]/I

(003)

of 3.42 are preferred for the

LiCoO

crystal structure.

Electrochemical performance of the LiCoO

ﬁlmsisthe

combined effect of the ﬁlm’s composition and structure. The 1st

discharge capacity and 50th capacity retention are two key

indicators for LiCoO

annealing works, and their change trends

arerelatedtothechangeruleoftheratioof[I

(101)

1 I

(104)

(110)

]/I

(003)

. In Figs. 7(i)–7(l), the normal value of the 1st

discharge capacity is at 32.05–41.48 lAh/(cm

lm), which is

lower than the theoretical value of 69 lAh/(cm

lm) [46]; this is

mainly caused by the diffusion and contamination from SS

substrates. The 50th capacity retention varies between 32.28 and

87.69%; some annealed samples can be conﬁrmed to behave

a long cycle life according to the index of 80% capacity retention.

An optimum annealing process (pure O

, 550 °C, 20 min, and

3 °C/min) for large-scale LiCoO

ﬁlmsisproposedthrough

comparison with these characteristic values (as shown in Fig. 7),

corresponding to the initial discharge capacity of 37.56 lAh/(cm

lm) and capacity retention at the 50th cycle of 83.81%.

Conclusions

The crystal LiCoO

cathode on a ﬂexible SS has been successfully

obtained by annealing with a traditional tube furnace. According

to the different annealing conditions, a pure oxygen annealing

atmosphere is conﬁrmed ﬁrst because the LiCoO

composition

ratio is the basis. T is the second important factor for an ideal

layered form of LiCoO

cathode because T can directly change the

crystal structures. After T ensures LiCoO

crystallization, the

holding time needs to be chosen to be as short as possible to limit

the SS oxidation and inter-diffusion, and it is equivalent to cutting

down the holding time or accelerating the heating rate to optimize

the LiCoO

annealing process.

In the end, LiCoO

ﬁlms annealed at 550 °C for 20 min

under pure oxygen (O

5 3 sccm, 3.5 Pa) and with a heating rate

of 3 °C/min show the optimum electrochemical performance in

our system. The initial discharge capacity is 37.56 lAh/(cm

lm)

and the capacity retention at the 50th cycle is 83.81%.

Experimental

LiCoO

ﬁlm deposition and annealing

A roll of SS (SUS304) with dimension of 2.0  0.5 m was, ﬁrst,

wiped with ethanol to remove surface contaminants and release

residual stress. The nominal thickness equaled to 0.03 mm and

the density was 7.93 g/cm

. The preparation and annealing

process are shown in Fig. 8; the LiCoO

ﬁlms were fabricated

by MS with a power density of 4 W/cm

and the working

pressure during the whole deposition was 1.0 Pa. A uniform

ﬁlm thickness of 1.1 lm was achieved by controlling the

dynamic deposition rate and deposition time. In fact, the ﬁlm

thickness actually equals to 1.09 lm by calibration of the cross-

section image, and the deposited LiCoO

ﬁlm had an areal

loading of 0.28 mg/cm

. The as-grown LiCoO

ﬁlms were

annealed according to the annealing parameters shown in

Table II. Among them, the annealing experiment marked with

A1 acted as reference and the other experiments changed only

one parameter once based on A1.

Half-cell assembly and electrochemical

performance test

The half-cells were assembled with an Li foil as a counter-

electrode and 0.5 M LiPF

in an EC and DMC mixture solvent

(volume ratio is 1:1). The used polyethylene (PE) separator was

purchased from Linyi Gelon New Battery Material Co., Ltd.,

Shandong, China. This separator was prepared by a dry stretching

method, the holes on its surface were elliptical shapes with the long

Figure 8: Schematic diagram of LiCoO

ﬁlm deposition and annealing.

TABLE II: LiCoO

ﬁlm thermal treatment parameters.