High performance sputtered LiCoO2 thin films obtained at a ...

Electrochimica Acta 60 (2012) 121–129 

Contents lists available at SciVerse ScienceDirect 

Electrochimica Acta 

jou rn al hom epa ge: www.elsevier.com/locate/electacta 

High performance sputtered LiCoO 2 thin films obtained at a moderate annealing 

treatment combined to a bias effect 

Sophie Tintignac a,b , Rita Baddour-Hadjean a,∗ , Jean-Pierre Pereira-Ramos a , Raphaël Salot b 

a Institut de Chimie et des Matériaux Paris Est, ICMPE/GESMAT, UMR 7182 CNRS-Université Paris Est Créteil (UPEC), 2 rue Henri Dunant 94320 Thiais, France 

b CEA Grenoble, DRT/LITEN/DTNM/LCMS, 17 rue des Martyrs 38054 Grenoble, France 

a r t i c l e i n f o 

Article history: 

Received 8 June 2011 

Received in revised form 

29 September 2011 

Accepted 4 November 2011 

Available online 16 November 2011 

Keywords: 

LiCoO 2 thin films 

Lithium microbattery 

Raman spectroscopy 

RF sputtering 

a b s t r a c t 

LiCoO 2 thin films have been successfully deposited by radio frequency magnetron sputtering onto aluminum 

substrate and post-annealed at a lower temperature of 500 ◦ C compared to usual annealing 

treatments. We demonstrate that an appropriate combination of key deposition parameters including a 

high working pressure and a substrate bias enables for the first time the reproducible synthesis at this 

moderate temperature of high performance layered HT-LiCoO 2 thin films. The morphology, the structure 

and the electrochemical behaviour of the deposits have been examined as a function of the working 

gas pressure ranging from 0.55 Pa to 3 Pa. Raman microspectrometry has been thoroughly used to characterize 

the structure of the thin films: a mixture of the layered HT-LiCoO 2 and the cubic LT-LiCoO 2 

phases is systematically revealed in the absence of substrate bias and for the first time, the access to 

the relative amount of both compounds is allowed. A promoting effect of increasing gas pressure on the 

formation of the layered HT-LiCoO 2 phase is demonstrated, the HT/LT relative amount increasing from 

1:3 at 0.55 Pa to 3:4 at 3 Pa. In a second step, we show that the effect of substrate bias combined with high 

values of working gas pressure and a moderate heat treatment (500 ◦ C) leads to the formation of pure 

HT-LiCoO 2 thin films at 3 Pa, with very attractive electrochemical properties: a high specific capacity of 

67 A h cm −2 m −1 at C/5 rate, a good adherence to the substrate and the best capacity retention known 

for a LiCoO 2 film in liquid electrolyte (4.2–3 V). 

© 2011 Elsevier Ltd. All rights reserved. 

1. Introduction 

The development of all-solid-state lithium microbatteries has 

been widely investigated over the last decade. Indeed, they can 

fit with the requests of portable devices and microelectronic components 

and especially with the ability of the power source to 

be integrated directly on/in the component as a backup power 

source in microelectromechanical system (MEMS), smart cards and 

biochips for instance. These microbatteries consist in stacks of thin 

film materials elaborated by physical vapour deposition techniques 

and their total thickness must not exceed few tens of micrometers. 

High quality thin film cathodes with high electrochemical performance 

are one of the key points for microbatteries. Among cathodic 

materials for Li-ion batteries, LiCoO 2 is the most commonly used in 

commercial devices due to its high specific capacity (140 mA h g −1 ), 

high operating electrode potential (≈ 4 V), long cycle life and ease 

∗ Corresponding author. Tel.: +33 149 781 195; fax: +33 149 781 195. 

E-mail address: baddour@icmpe.cnrs.fr (R. Baddour-Hadjean). 

of preparation involving a heat treatment at temperature above 

700 ◦ C. 

LiCoO 2 thin films can be prepared by sputtering techniques 

[1–24], pulsed laser deposition (PLD) [25–30], chemical vapour 

deposition CVD [31–34], and chemical routes including the electrostatic 

spray pyrolysis and sol–gel processes [35–40]. DC or RF 

magnetron sputtering is the most widely used technique for LiCoO 2 

preparation investigated in liquid or solid electrolytes. A major 

characteristic of the sputtered films is their low crystallinity and 

low electrochemical efficiency when they are used as-prepared 

in Li batteries. The synthesis of high performance thin film, in 

general, requires a post-annealing treatment at high temperature 

(600–800 ◦ C) to improve the crystallinity of the LiCoO 2 deposit. 

However, such a high temperature process is often not compatible 

with devices requiring the direct integration of the battery and 

many desirable substrates such as flexible polymer materials and 

low-temperature substrate materials, which can limit the application 

and fabrication of the thin film batteries. Therefore, significant 

efforts must be undertaken to optimize the film crystallinity and 

electrochemical properties through a low temperature process by 

controlling some of the deposition parameters. Nevertheless, only 

a few works focused on the influence of deposition parameters 

0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. 

doi:10.1016/j.electacta.2011.11.033

122 S. Tintignac et al. / Electrochimica Acta 60 (2012) 121–129 

Fig. 1. SEM surface observations of films deposited on Al substrate at 0.55 Pa (a), 1 Pa (b), 1.5 Pa (c), 2.2 Pa (d) and 3 Pa (e); SEM cross section observation of a thin film 

deposited at 2.2 Pa on a Si substrate (f). 

of the sputtering process. Several ways of improvements have 

been proposed such as the use of a moderate annealing treatment 

(

S. Tintignac et al. / Electrochimica Acta 60 (2012) 121–129 123 

Table 1 

Atomic positions, site symmetries and optical vibrational modes expected for R-3m, 

Fd3m LiCoO 2 and Co 3O 4. 

* 

* 

(101) 

* 

* 

Atoms 

Wyckoff 

positions 

IR active 

modes 

Raman active 

modes 

LiCoO 2 (space group R-3 m) 

Li (0, 0, 1/2) 3b 2A 2u + 2E u A 1g + E g 

Co (0, 0, 0) 3a 

O (0, 0, .26) 6c 

LiCoO 2 (space group Fd3 m) 

Li (0, 0, 0) 16c 5F 1u A 1g + E g + 2F 2g 

Co (1/2, 1/2, 1/2) 16d 

O (1/4,1/4,1/4) 32e 

Co 3O 4 (space group Fd3 m) 

Co (1/2, 1/2, 1/2) 16d 5F 1u A 1g + E g + 3F 2g 

Co (1/8, 1/8, 1/8) 8a 

O (.26, .26, .26) 32e 

Intensity / a.u. 

(003) * 

(012) 

* * 

* * 

* 

3 Pa 

2.2 Pa 

1.5 Pa 

1 Pa 

0.55 Pa 

* 

(110) 

10 

20 

30 

40 50 

2 θ / degree 

60 

70 

With the objective of lowering the global thermal budget by 

using a temperature below 700 ◦ C, the deposition conditions to 

get well crystallized and high performance LiCoO 2 thin films have 

been optimized. Our approach consists in the appropriate combined 

use of a high power value, a substrate bias, an optimized 

value of the working pressure and a moderate heat treatment 

temperature. The present work reports the investigation of the 

influence of working pressure and substrate bias on the structural 

and electrochemical properties of LiCoO 2 thin films. By 

using Raman spectroscopy, a quantitative analysis of electroactive 

deposited LiCoO 2 phases as a function of working pressure 

and bias is allowed for the first time. We will show that the 

optimization of these deposition parameters leads to excellent performances 

when associated with a process temperature as low as 

500 ◦ C. 

2. Experimental 

LiCoO 2 films with a thickness of 500 nm were deposited by 

radio frequency magnetron sputtering, using an ALCATEL SCM 600 

reactor. The 150 mm target was made of 99.9% pure LiCoO 2 (SCI 

engineered materials). Aluminum foils (Goodfellow, purity 99.0%) 

with thickness of 200 m were used as substrates. The target power 

was set to 500 W, the Ar/O 2 ratio to 3 and the total amount of gases 

Fig. 2. XRD measurements of films deposited at 0.55 Pa, 1 Pa, 1.5 Pa, 2.2 Pa, 3 Pa and 

post-annealed at 500 ◦ C. * substrate peaks. 

was set to 53 sccm. The distance between target and substrate was 

9.5 cm. The working pressure was varied between 0.55 Pa and 3 Pa. 

The deposition was made either with or without a negative substrate 

bias voltage of 50 V. The deposited films were post-annealed 

in air at 500 ◦ C for 2 h with a rate of 5 ◦ C/min and cooled to room 

temperature with a rate of 2 ◦ C/min. The film thickness was determined 

with a Tencor AlphaStep 500 profilometer. Morphology and 

surface aspects of the films were observed with a Scanning Electron 

Microscope (SEM) LEO 1530 FEG. X-Ray diffraction experiments 

were performed using a Brüker D8 Advance diffractometer with Cu 

K radiation. 

Raman microspectrometry was also performed to gather structural 

information with a LaBRAM HR 800 (Jobin-Yvon-Horiba) 

Raman micro-spectrometer including Edge filters and equipped 

for signal detection with a back illuminated charge coupled device 

detector (Spex CCD) cooled by Peltier effect to 200 K. A He:Ne laser 

(632.8 nm) was used as the excitation source. The spectra were 

measured in back-scattering geometry. The resolution was about 

0.5 cm −1 . A 100× objective lens was used to focus the laser light on 

607 

LiCoO 2 

Co 3 O 4 


450 

485 

+ 

487 

+ Co 3 O 4 

591 

597 

+ 

Fd3m 

R-3m 


197 

484 

525 

694 

621 

300 

400 

500 600 

Raman shift / cm -1 

700 

800 

200 

400 600 


800 

Fig. 3. Raman scattering spectra for spinel Fd3m LT-LiCoO 2, hexagonal R-3m HT-LiCoO 2 and Co 3O 4. Excitation with 514.5 nm radiation. From [43].


sample surface to a spot size of 1 m 2 . To avoid local heating of the 

sample, the power of the laser beam was adjusted to 0.2–0.5 mW 

with neutral filters of various optical densities. Raman spectra have 

been recorded on 15 different spots of each thin film electrode. The 

Raman spectra have been examined using deconvolutions analysis 

of data sets exhibiting multiple peaks. 

Electrodes of 14 mm in diameter were assembled in cointype 

cells (CR2032), with metallic lithium as counter electrode, 

filled in with a commercial grade LP30 electrolyte (Merck) 1 M 

LiPF 6 (EC:DMC = 1:1). The separator is constituted by a Viledon ® 

membrane soaked with electrolyte and Cellgard ® . Galvanostatic 

experiments were made with a multichannel VMP3 apparatus (Bio- 

Logic Science Instruments). 

3. Results and discussion 

SEM images of the films grown on aluminum substrates at different 

working pressures from 0.55 Pa to 3 Pa and then annealed at 

500 ◦ C are shown in Fig. 1. All the thin films exhibit homogeneous 

grain size distribution and morphology. However, for working pressure 

values in the range 0.55–1.5 Pa, some cracks and large size 

particles appear. The particle size, as well as the porosity, is seen 

to increase with the pressure. For instance, porous deposits are 

achieved at 2.2 and 3 Pa with well-defined grains of 50 nm against 

dense deposits of 20 nm particles for lower pressures. The SEM 

image of the cross section for a thin film deposited at 2.2 Pa on a Si 

substrate shows a columnar growth perpendicular to the substrate 

surface (Fig. 1f). 

XRD experiments were carried out in order to check the crystallinity 

of the samples annealed at 500 ◦ C as a function of the 

working pressure (Fig. 2). In a first approach, all the diffraction 

patterns can be indexed on the basis of an hexagonal symmetry 

with unit cell parameters a = b = 2.82 Å and c = 14.05 Å of the 

LiCoO 2 phase (R-3m space group). The broadness of the lines indicates 

small crystallite size. These polycrystalline thin films exhibit 

a preferential orientation which is clearly influenced by the working 

pressure. Indeed, two sets of diffraction patterns are observed: 

in the 0.55/2.2 Pa range (0 0 3) oriented films are obtained. At 3 Pa, 

the (0 0 3) line disappears while a strong (1 0 1) line emerges as 

well as the (1 1 0) diffraction peak. The preferential (1 1 0) orientation 

was reported to be more favourable for fast Li diffusion than 

the (0 0 3) one [6]. However, as the R-3m and Fd3m XRD patterns 

are very close (0 1 8)/(1 1 0) and (0 0 6)/(0 1 2) peaks splitting are 

the only evidence for the presence of the R-3m HT-LiCoO 2 phase, 

and due to the broadness of the peaks, X-ray diffraction does not 

enable a clear assignment of the LiCoO 2 structure in the deposited 

films. Moreover, the presence of Co 3 O 4 impurity phase cannot 

be discarded. Conversely, vibrational spectroscopy can be used to 

solve the problem of phase determination of LiCoO 2 . Indeed, from 

a spectroscopic viewpoint, the factor group analysis determines 

the optical vibrational modes expected for Fd3m and R-3m LiCoO 2 

(Table 1). Hence, one should expect to observe two Raman bands 

Fig. 4. Atomic displacements of the IR and Raman active modes of hexagonal R-3m 

LiCoO 2. 

for the R-3m phase and four Raman bands for the Fd3m one. This 

feature makes Raman microspectrometry a convenient and powerful 

technique for the qualitative microstructural analysis of LiCoO 2 

[41]. As shown in Fig. 3, each LiCoO 2 crystal structure gives rise to 

a specific Raman fingerprint, consistent with the theoretical predictions, 

i.e., four Raman bands are observed at ca. 450, 485, 591 

and 607 cm −1 for Fd3m LiCoO 2 , whereas two Raman bands at 487 

and 597 cm −1 , attributed to the A 1g and E g modes, respectively, are 

observed for R-3m LiCoO 2 [42,43]. The Raman active modes A 1g and 

E g involve vibrations from oxygen atoms only. They correspond to 

relative displacements of neighbouring oxygen layers with respect 

to each other, the displacement occurs along the c axis for the A 1g 

mode and is perpendicular to c for the E g mode (Fig. 4). Furthermore, 

Raman spectroscopy is able to detect the secondary product 

Co 3 O 4 even as traces (Table 1). This sensitivity is due to the particularly 

strong scattering intensity of this oxide which exhibits 

five Raman bands consistent with the theoretical predictions 

(Fig. 3). 

The Raman spectra obtained for the samples deposited at different 

pressures and post-annealed at 500 ◦ C are reported in Fig. 5. 

All the experimental spectra exhibit two broad bands centered 

respectively at 490 cm −1 and 599 cm −1 , which could be in a first 

approach consistent with the presence of the HT-LiCoO 2 phase. The 

broadness of these peaks is ascribed to the nanometer size of films 

particles and also to more or less pronounced shoulders due to the 

presence of several phases in the samples. The shape and intensity 

of Raman bands is seen to be strongly dependent on the working 

pressure. 

Table 2 

Raman wavenumbers obtained from the deconvolutions of the spectral profiles of LiCoO 2 thin films deposited at different working pressures, with their asssignments to 

HT-LiCoO 2, LT-LiCoO 2 and Co 3O 4. 

Raman shift (cm −1 ) 

LT LT HT Co 3O 4 LT HT LT Co 3O 4 Co 3O 4 

From [43] 449 484 487 524 590 597 605 622 692 

0.55 Pa 456 480 490 539 573 597 616 645 – 

1 Pa 456 480 490 539 573 597 616 645 – 

1.5 Pa – 480 490 539 573 597 616 645 – 

2.2 Pa 459 480 490 539 573 597 616 645 667 

3 Pa – – 490 – 571 599 – 636 675


490 599 

0.55 Pa 

LT 

HT 


3 Pa 

2.2 Pa 

1.5 Pa 

1 Pa 

HT 

LT 

LT 

1 Pa 

Co 3 O 4 

LT 

Co 3 O 4 

0.55 Pa 

HT 

HT 

400 

500 

600 


700 

Fig. 5. Raman spectra obtained from thin films deposited at 0.55 Pa, 1 Pa, 1.5 Pa, 

2.2 Pa and 3 Pa and post-annealed at 500 ◦ C. 

The Raman spectra were fitted assuming the coexistence of both 

HT- and LT-LiCoO 2 structures and cobalt oxide Co 3 O 4 . The fitting 

results are gathered in Fig. 6. Table 2 summarizes the Raman band 

wavenumbers obtained from the deconvolution of the spectral profiles 

for each pressure. Whatever the working pressure, the Raman 

spectra account for a mixture of the Fd3m and R-3m fingerprints 

consisting in four Raman bands located at ca. 456, 480, 573 and 

616 cm −1 and two bands at 490 and 597 cm −1 respectively. Additional 

broad Raman features at 539, 636/645 and 667/675 cm −1 

indicate the presence of low crystallized residual Co 3 O 4 in all the 

samples, the very weak contribution observed for the highest working 

pressure of 3 Pa revealing probably only some traces for this 

impurity. The systematic presence of residual Co 3 O 4 in the samples 

raises the question of the compositional evolution in sputter 

deposited LiCoO 2 thin films. Actually, this is a difficult task not 

yet resolved because sputtering from a multielemental target like 

LiCoO 2 is largely influenced by the process parameters involved 

during deposition. Recent studies based upon MC simulations deal 

with some of these complex aspects and put forward the role of 

inhomogeneous distribution of Li and Co atoms, differential sputtering 

yield of individual components but also angular ejection 

of the sputtered atoms and differential transport of the sputtered 

atoms [44]. 

On the other hand, the intensity of Raman bands associated with 

the R-3m structure strengthens along with the pressure increase, 

whereas the shoulders assigned to the Fd3m phase undergo a significant 

decrease. Due to the spectroscopic similarity of both phases, 

this trend can be quantitatively depicted from the analysis of the 

integrated intensities related to the Fd3m and R-3m Raman features 

respectively which provides a good estimation of the Fd3m/R-3m 

relative amounts as a function of the working pressure (Fig. 7). This 

leads to an amount of ca. 30% R-3m phase in the deposit obtained at 

0.55 Pa. Then, when the pressure increases from 1 Pa to 2.2 Pa, the 

R-3m amount significantly increases to ≈50% to reach 75% at 3 Pa. 

It comes out that the R-3m phase appears from a pressure as low as 

0.55 Pa, but that 25% Fd3m phase still coexists with the R-3m phase 

at 3 Pa. 


LT 

LT 

1.5 Pa 

2.2 Pa 

3 Pa 

HT 

LT 

HT 

LT 

LT 

HT 

Co 3 O 4 

LT 

HT 

LT 

Co LT 

3 

O 4 

Co 3 O 4 

HT 

LT 

Co 3 O 4 

LT 

LT 

Co 3 O 4 

Co 3 O 4 Co 3 O 4 

LT 

HT 

Co 3 O 4Co3 O 4 

400 500 600 700 


Fig. 6. Raman spectra of thin films deposited at 0.55 Pa, 1 Pa, 1.5 Pa, 2.2 Pa and 3 Pa 

and post-annealed at 500 ◦ C, with the deconvolutions shown below.


100 

HT HT bias -50V 

LT LT bias -50V 

a 

4.2 

i= 10 µA cm -2 

HT/LT relative amount / % 

80 

60 

40 

20 

0 

0 

1 

Working pressure / Pa 

Fig. 7. Relative amount of HT/LT-LiCoO 2 phases in thin films deposited at different 

working pressure with or without a negative substrate bias of 50 V and 

post-annealed at 500 ◦ C. 

All the thin films were cycled at 10 A cm −2 between 3 and 

4.2 V versus Li/Li + to investigate their charge–discharge properties. 

As shown in Fig. 8a, the electrochemical results are consistent 

with the previous structural characterization afforded by Raman 

analysis. Indeed, the film deposited at 3 Pa and containing the highest 

ratio of HT-LiCoO 2 phase leads to the best charge capacity of 

52 A h cm −2 m −1 while the low pressure sample is barely cycled, 

giving 22 A h cm −2 m −1 , i.e. less than half the capacity of the 3 Pa 

thin film. The higher the working pressure for deposition, the higher 

the HT-LiCoO 2 content and the larger the specific capacity. This 

capacity value, lower than that expected, can be explained by the 

coexistence of the cubic phase but also by some residual Co 3 O 4 . The 

derivative curves of the chronopentiometric signals highlight the 

following features (Fig. 8b): the electrochemical fingerprint clearly 

marked for the HT-LiCoO 2 phase at 3.9 V at 3 Pa is accompanied by 

another couple of peaks at 3.75 V and 3.6 V due to the LT-LiCoO 2 

phase [45,46]. This film corresponds to an optimized composition 

since the contribution of the LT-LiCoO 2 phase significantly 

decreases with the pressure to the benefit of the R-3m phase. This 

trend is well consistent with the Raman analysis. The cycling properties 

of these films are illustrated in Fig. 8c. Starting from different 

specific discharge capacities depending on the working pressure, 

the same regular capacity decline is observed over 25 cycles for the 

thin films prepared in the 1–3 Pa range. For instance, the discharge 

capacity changes at 3 Pa from 45 to 34 A h cm −2 m −1 after 25 

cycles. This slow capacity decrease with cycles can be ascribed to 

the presence of the LT-LiCoO 2 phase which is known to be poorly 

rechargeable. 

A DC negative polarization (50 V) has been applied to the substrate 

during the film deposition at 2 Pa and 3 Pa. This aims at 

increasing the local temperature by an ion bombardment of the 

sample surface. Indeed, effect of Bias during deposition has been 

previously described [18,47]. It has been reported that the ion bombardment 

of growing films strongly influences their microstructure 

by increasing the mobility of adatoms on the surface, which results 

in increasing the rearrangement probability of adatoms. 

The surface morphology of the thin film deposited with a substrate 

bias at 2.2 Pa and post-annealed at 500 ◦ C (Fig. 9) indicates an 

increase in the mean grain size from 50 to 100 nm. As a result, for 

the same deposition conditions and annealing treatment (500 ◦ C), 

the Raman spectra of the films indicate also a strong influence of 

the bias polarization on the film composition (Fig. 10). Indeed, as 

2 

3 

E vs. (Li/Li + ) / V 

b 

c 

Discharge capacity / µAh cm -2 µm -1 

4.0 

3.8 

3.6 

3.4 

3.2 

dQ dE -1 / a.u. 

0.55 Pa 

1 Pa 

1.5 Pa 

2.2 Pa 

3 Pa 

100 

3.0 

0 20 40 60 80 

i = 10 µA cm -2 

Capacity / µAh cm -2 µm -1 0.55 Pa 

1 Pa 

1.5 Pa 

2.2 Pa 

3 Pa 

3.2 3.4 3.6 3.8 4.0 4.2 

E vs. (Li/Li + ) / V 

70 

60 

i = 10 µA cm -2 

3 Pa 

2.2 Pa 

1.5 Pa 

50 

1 Pa 

0.55 Pa 

40 

30 

20 

10 

0 

0 5 10 15 20 25 

Cycles 

Fig. 8. Galvanostatic experiments at 10 A/cm 2 in EC:DMC = 1:1/1 M LiPF 6. Cell 

potential versus capacity (a), derivative signal (b) and discharge capacity versus 

cycle number (c). 

evidenced from the Raman spectra fitting reported in Fig. 11, the 

Fd3m contribution is strongly reduced and even removed: from 40% 

to 10% and from 25% to 0% for the LiCoO 2 film deposited at 2.2 Pa 

and 3 Pa respectively (Fig. 7). 

As a consequence, the corresponding electrochemical features 

are significantly improved, as shown in the derivative curve of 

discharge–charge curves (Fig. 12a). The intensity of the peak 

located at 3.90 V, characteristic of the HT-LiCoO 2 phase, rises


Fig. 9. Comparison of the surface morphology for films deposited at 2.2 Pa with or without a negative substrate bias of 50 V and post-annealed at 500 ◦ C. 


* Co 3 

O 4 

* 

200 

3 Pa / bias 

2.2 Pa / bias 

* 

3 Pa 

2.2 Pa 

400 

487 

597 

600 


* 

* * 

* 

800 

Fig. 10. Raman spectra of the 2.2 Pa and 3.3 Pa thin films deposited with or without 

a negative substrate bias of 50 V and post-annealed at 500 ◦ C. 

tremendously when the substrate bias of −50 V is applied, corroborating 

the Raman spectra results. The LT-LiCoO 2 peaks at 

3.75 V and 3.6 V disappear from the electrochemical signal as 

soon as a substrate polarization is used. The electrochemical 

cycling performances of these samples are reported in Fig. 12b. 

The substrate polarization leads to a large increase by 40% of 

the capacity value as well as an improvement of cycling properties. 

The achievement of 67 A h cm −2 m −1 (133 mA h g −1 ) for 

the optimized 3 Pa biased sample, close to the theoretical capacity 

of HT-LiCoO 2 can be explained by the Raman analysis giving 

a composition of pure HT-LiCoO 2 phase with some traces of 

Co 3 O 4 impurity. The cycling properties are also clearly influenced 

by the HT/LT ratio. Indeed, the 100% HT-LiCoO 2 phase 

obtained for the 3 Pa/bias film shows an excellent cyclability with 

reduced capacity loss of 0.2% per cycle with a slow decrease 

from 67 A h cm −2 m −1 (133 mA h g −1 ) for the first cycle to still 

61 A h cm −2 m −1 (121 mA h g −1 ) after 50 cycles. In comparison, 

the remaining 10% of LT-LiCoO 2 phase contained in the 2.2 Pa/bias 

layer is responsible for a faster capacity fading over 50 cycles, the 

capacity diminishing from 63 A h cm −2 m −1 (125 mA h g −1 ) to 

54 A h cm −2 m −1 (107 mA h g −1 ). In other respect, Fig. 13 shows 

there is little change in the discharge–charge curves of the 3 Pa/bias 

film which superimpose during cycling. The capacity decreases very 

slowly and no polarization phenomenon appears, which reflects 

good cycling properties. Conversely, a fast capacity decline accompanied 

by a strong polarization phenomenon is observed for the 

deposit obtained at the same working pressure without the bias 

procedure. The efficiency of the synthesis conditions proposed in 


HT 

2.2 Pa / bias = -50V 

HT 


HT 

3 Pa / bias = - 50V 

HT 

* 

LT 

* * 

* * 

* 

* 

* 

400 

500 

600 

700 

400 500 600 700 

Raman shift / cm -1 Raman shift / cm -1 

Fig. 11. Raman spectra of the 2.2 Pa/bias and 3 Pa/bias thin films post-annealed at 500 ◦ C, with their deconvolutions shown below. * Co 3O 4 peaks.


the present work is demonstrated by the theoretical specific capacity 

reached here in the initial discharge while only ≈66% of this 

value is usually obtained with an annealing treatment at 500 ◦ C 

without the bias procedure [20,21] (Fig. 8). We have checked that 

an increase in the applied bias voltage (90 V instead of 50 V) induces 

a decrease in discharge capacity and an increase in the polarization 

of the electrodes. Furthermore, measurements of the residual 

compressive stress in the films indicate an increase by a factor 2 

when the applied bias voltage is increased from 50 to 90 V, which is 

detrimental towards the all-solid-state microbatteries application. 

It must be outlined that a thermal treatment at a minimum 

temperature of 700 ◦ C is usually required to get nearly the theoretical 

capacity [1,20–22]. One study reports a discharge capacity 

of 130 mA h g −1 in liquid electrolyte for a sputtered LiCoO 2 thin 

film deposited at an elevated working pressure of 10 Pa and postannealed 

at 600 ◦ C [17]. However these films are found to exhibit 

a capacity loss of 0.7% per cycle, a decrease from 130 mA h g −1 for 

the first cycle to 109 mA h g −1 being reported after 25 cycles [17]. 

With a 90% capacity retention (61 A h cm −2 m −1 after 50 

cycles in liquid electrolyte), the present cycling performance 

achieved with the 3 Pa/bias film significantly exceeds all the results 

reported in the 4.2–3 V potential range for a sputtered LiCoO 2 film 

annealed at 500 ◦ C [20,21]. Indeed, a high capacity loss of ≈40% is 

usually observed over 50 cycles for thin films annealed at temperature 

as low as 500 ◦ C. This outlines the relevance of the combined 

use of optimized bias and working pressure values to get a high 

crystallinity of the hexagonal LiCoO 2 compound without any presence 

of the disordered cubic LiCoO 2 phase. Even compared with 

thin films heat-treated at higher temperatures, our results offer a 

better cycling behaviour since a large capacity loss from 25 to 50% 

is usually recorded for films annealed in the temperature range 

650–900 ◦ C [20–23]. A better adherence of the film to the substrate 

provided by the bias procedure cannot be discarded to explain the 

better behaviour in addition to the promoting effect on the formation 

of HT-LiCoO 2 . 

The understanding of the capacity loss of LiCoO 2 thin films in 

liquid electrolyte is not solved at present time but the faster capacity 

decline compared to that achieved in an all-solid-state battery 

is thought to be related with the degradation of the LiCoO 2 crystal 

structure [24] while this phenomenon would be suppressed when 

the oxide film is covered by a solid electrolyte. Therefore, further 

experiments are under progress in our laboratory to investigate the 

cause of the capacity fading by comparing the electrochemical and 

structural responses of the 500 ◦ C heat-treated 3 Pa/bias films in 

liquid and solid electrolyte. 

Discharge capacity / µAh cm -2 µm -1 

a 

dQ dE -1 / a.u. 

b 

70 

60 

50 

40 

30 

20 

10 

0 

0 

3 Pa bias 

2.2 Pa bias 

3 Pa 

2.2 Pa 

i=10µA cm -2 

3.6 3.8 4.0 4.2 

E vs. (Li/Li + ) / V 

3 Pa bias 

2.2 Pa bias 

3 Pa 

i = 10µA cm -2 2.2 Pa 

50 

10 

20 

Cycles 

30 

40 

Fig. 12. Derivative curve of the galvanostatic discharge–charge curve (a) and 

capacity versus cycling (b) for the 2.2 Pa, 2.2 Pa/bias, 3 Pa and 3 Pa/bias thin films 

post-annealed at 500 ◦ C. Current density: 10 A cm −2 , EC:DMC = 1:1/1 M LiPF 6. 

50 

4.2 

3Pa 

3Pa / bias 

4.0 

E vs. (Li/Li+) / V 

3.8 

3.6 

3.4 

3.2 

Cycle 2 

Cycle 5 

Cycle 10 

Cycle 25 

Cycle 2 

Cycle 5 

Cycle 10 

Cycle 25 

3.0 

0 

20 40 60 

Capacity / µAh cm -2 µm -1 

0 

20 40 60 

Capacity / µAh cm -2 µm -1 

Fig. 13. Comparison of galvanostatic cycling curves at 10 A cm −2 for the 3 Pa and 3 Pa/bias thin films post-annealed at 500 ◦ C.


4. Conclusions 

Using a high power of 500 W and a moderate annealing treatment 

at 500 ◦ C in air, the influence of the working pressure in the 

0.55–3 Pa range has been investigated to get crystalline sputtered 

LiCoO 2 thin films with attractive electrochemical properties. The 

morphology and the structure of the films are seen to be controlled 

by the working pressure. In particular, a thorough Raman 

analysis based upon systematic Raman spectra fittings indicates 

the presence of a LT-LiCoO 2 /HT-LiCoO 2 mixture in the deposits. 

Furthermore, this technique turned out to be very useful as it 

allowed for the first time to check quantitatively the Fd3m/R-3m 

relative amount in the sputtered thin films: a promoting effect 

of the increasing pressure is demonstrated, in good accord with 

the corresponding electrochemical properties in the 4.2/3 V potential 

region, the HT-LiCoO 2 content increasing from 30% at 0.55 Pa 

to 75% at 3 Pa. Enhanced electrochemical properties in terms of 

capacity and cycle life have been obtained by applying a negative 

polarization to the substrate. Indeed, the combined use of substrate 

bias (−50 V) with a high working pressure of 3 Pa during rf magnetron 

sputtering has allowed to get rid of the cubic phase and 

then to achieve high performance HT-LiCoO 2 films with specific 

capacity of 67 A h cm −2 m −1 close to the theoretical value and 

a capacity retention of ≈90% over 50 cycles at C/5 rate without 

any polarization phenomenon during cycling. These electrochemical 

characteristics are the best known for a LiCoO 2 thin film in liquid 

electrolyte. The use of a high pressure, high power, bias substrate 

and moderate heat-treatment at 500 ◦ C has ensured the high crystallinity 

of the deposit, the exclusive formation of the hexagonal 

HT-LiCoO 2 phase, the decrease of the Co 3 O 4 amount and probably 

a good adherence of the LiCoO 2 film to the aluminum substrate. 

Of course, additional measurements such as kinetic studies and 

extended cycling experiments are under progress in our group as 

well as the integration of these HT-LiCoO 2 thin films in all-solidstate 

Li batteries. 

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