LiMn2O4/CNTs and LiNi0.5Mn1.5O4/CNTs Nanocompositesas High-Performance Cathode Materials for Lithium-IonBatteries

Thang Van Le • Ha Tran Nguyen • Anh Tuan Luu • Man Van Tran • Phung Loan My Le

Received: 12 February 2014 / Revised: 24 August 2014

� The Chinese Society for Metals and Springer-Verlag Berlin Heidelberg 2014

Abstract The demand of higher energy density and higher power capacity of lithium (Li)-ion secondary batteries has led

to the search for electrode materials whose capacities and performance are better than those available today. Carbon

nanotubes (CNTs), with their unique properties such as 1D tubular structure, high electrical and thermal conductivities, and

extremely large surface area, have been used as materials to prepare cathodes for Li-ion batteries. The structure and

morphology of CNTs were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission

electron microscopy (TEM). The functional groups on the purified CNT surface such as –COOH, –OH were characterized

by Fourier Transform infrared spectroscopy. The electrode materials were fabricated from LiMn2O4 (LMO), doped spinel

LiNi0.5Mn1.5O4, and purified CNTs via solid-state reaction. The structure and morphology of the electrode were charac-

terized using XRD, SEM, and TEM. Finally, the efficiency of the electrode materials using CNTs was evaluated by cyclic

voltammetry and electrochemical impedance spectroscopy.

KEY WORDS: Nanocomposites; Carbon nanotubes (CNTs); Spinel materials; Lithium-ion batteries (LIBs)

1 Introduction

Today, mobile society provides the energy storage media

for powering increasingly diverse applications which is

highly efficient, low cost, and environmentally friendly [1,

2]. Among various energy and power technologies,

rechargeable Li-ion batteries (LIBs) are one of the most

efficient energy storage devices based on electrochemical

energy storage and conversion [3–8]. Commonly, a typical

LIB includes an anode made from graphite, and a positive

electrode (cathode) made from LiCoO2, and a Li-ion

conducting electrolyte. When the cell is charged, Li ions

are extracted from the cathode, pass through the electro-

lyte, and are inserted into the anode. Upon discharging, the

Li ions are released by the anode and move back to the

cathode. The electrons pass around the external circuits in

opposite directions. Compared to traditional rechargeable

batteries such as lead–acid and Ni–Cd batteries, recharge-

able LIBs provide many advantages including high voltage,

high energy-to-weight ratio, i.e., high energy density, long

cyclic life, no memory effect, and slow loss of charge [1–

3]. For these advantages, LIBs are currently the most

popular battery type of powering portable electronic devi-

ces. Although LIBs have shown remarkably commercial

success, the electrodes and their component materials are

still the target of interesting research for enhancement of

electrochemical performance of battery devices. The main

disadvantages of LIBs are low power density as well as low

Available online at http://link.springer.com/journal/40195

T. Van Le (&) � H. T. Nguyen � A. T. Luu

Faculty of Materials Technology, Ho Chi Minh University of

Technology, 268 Ly Thuong Kiet, Ward 14, District 10, Ho Chi

Minh City, Vietnam

e-mail: vanthang@hcmut.edu.vn

M. Van Tran � P. L. M. Le

Department of Physical Chemistry, University of Science, VNU-

HCM, 227 Nguyen Van Cu Street, District 5, Ho Chi Minh City,

Vietnam

123

Acta Metall. Sin. (Engl. Lett.)

DOI 10.1007/s40195-014-0181-9

ion diffusion and charge transfer during the process of

charge–discharge at high rates as a consequence of high

polarization [9–11]. Therefore, the development of new

electrodes having a large surface area, a short diffusion,

and high electric/thermal conductions, is necessary to

overcome the disadvantages of conventional materials.

Carbon nanotubes (CNTs), having high electronic con-

ductivity, good lithium permeability, and electrochemical

stability, are particularly attractive for LIB applications

[12–14]. The hybrid nanostructures obtained by incorpo-

rating CNTs into Li-storage compounds as novel electrode

materials have been developed utilizing the aforemen-

tioned attractive properties of CNTs [15–19]. The use of

CNTs in electrodes results in many advantages because of

their high-specific surface area as well as mechanical and

transport properties. Recently, there have been many

reports presenting the structures combining CNTs and

nanoparticles such as V2O5 and SnO2 [20] via a hydro-

thermal process. Such obtained V2O5–SnO2/CNT com-

posites display fast Li? transformation and high electronic

conductivity. More recently, Zhu et al. has reported Cu-

CrO2–CNT nanocomposites, which were fabricated by an

in situ hydrothermal method. This material exhibits

excellent reversible specific capacity and cyclic perfor-

mance. Even at high charge/discharge rates (1 C), the

specific capacity retention was still as high as 530 mAh/g

after 40 cycles [21].

In this study, the LMO, doped spinel LNMO, and CNT

nanocomposite materials were successfully prepared via a

mixing method. The structure and morphology of these

nanocomposites were analyzed by X-ray diffraction (XRD)

and scanning electron microscopy (SEM). These nano-

composites were obtained as cathode materials having high

diffusion of lithium ions compared to LMO (LNMO)/

Vulcan-carbon composites materials.

2 Experimental

2.1 CNTs Purification

150 mg of CNTs was immersed in 100 mL of the mixture

of H2SO4 (95%) and HNO3 (65%). The reaction was per-

formed in sonication bath for 4 h. The CNTs were washed

with deionized (DI) water several times, filtered and then

dried to constant weight.

2.2 Synthesis of Spinel LiMn2O4 (LMO)

and LiNi0.5Mn1.5O4 (LNMO) Materials

Li2CO3 (2.5 g, 67.6 mmol) and MnO2 (5.8 g, 67.6 mmol)

were mixed together and annealed at 600 �C for 24 h. The

mixture was made as pellets, which were annealed at

830 �C for 24 h to obtain LMO.

Li2CO3 (2.5 g, 67.6 mmol), MnCO3 (23.32 g, 0.202 mol),

and Ni(NO3)2�6H2O (19.65 g, 67.6 mmol) were prepared by

mixing and decomposed at 600 �C for 24 h. Then, the mixture

was grounded and made into pellets, which were heated at

900 �C for 24 h to obtain LNMO.

2.3 Synthesis of LMO (LNMO)/CNT Nanocomposites

Spinel of LMO (LNMO) (9.8 g) materials and purified

CNTs (0.2 g) were mixed together. Then, the binder of

60% PTFE dispersed in water (1 g) and 15 mL of absolute

ethanol were added to the mixture. The mixture was stirred

for 15 min, and the solvent was evaporated at 80 �C for

24 h. Then, the mixture was coated on aluminum substrates

of about 0.1 mm in thickness as cathode materials, which

were dried at 130 �C under vacuum for 24 h.

2.4 Structure and Morphology Analysis

FTIR (thermo fisher scientific-model Nicolet 6700, fre-

quency range of 4000–500 cm-1). spectra were recorded

using BIO-RAD Excalibur spectrometer equipped with an

ATR Harrick Split PeaTM. Wide-angle powder X-ray

diffraction (XRD) patterns were recorded at room tem-

perature on a Bruker AXS D8 Advance diffractometer

using CuKa radiation (k = 0.15406 nm), at a scanning rate

of 0.05�/s. SEM images were acquired using a field-emis-

sion scanning electron microscope (NOVA NANO-

SEM450, FEI) equipped with an Oxford energy dispersive

X-ray (EDX) analysis system. TEM images were per-

formed on a FEI Tecnai F20 microscope equipped with an

Oxford EDX analysis system and with an acceleration

voltage of 200 kV.

2.5 Electrochemical Characterization

Cyclic voltammetry (CV) was performed on MGP2 (Bio-

logic, France) using ECLab software. The electrochemical

cell including three electrode cells: a platinum electrode

covered with a composite material as a working electrode

(WE), an Ag/0.1 mol/L AgNO3/0.01 mol/L TBAP in aceto-

nitrile as reference electrode (RE), and a Pt wire as counter

electrode (CE). The electrolyte solution consists of 0.75 mol/

L lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dis-

solved in 1:1 mixture of ethylene carbonate (EC) and dime-

thyl carbonate (DMC) [22, 23, 24]. The standard voltage of

the reference electrode is 0.542 V versus SHE at 25 �C.

The impedance measurement (EIS) was performed in

Swagelok-type batteries at room temperature. The cell

consisting of two electrode films as positive and negative

electrodes was assembled. The electrolyte was a 1 mol/L

T. Van Le et al.: Acta Metall. Sin. (Engl. Lett.)

123

solution of LiClO4 in EC-DMC (1:1). The EIS was carried

out using an Autolab 302 N (MetroOhm).

3 Results and Discussion

Purified CNTs were first treated with a mixture of H2SO4

(95%)/HNO3 (65%) to clean the surface. Moreover, this

treatment process also functionalized reacting groups such

as hydroxyl, carboxyl, and carbonyl groups on the surface

of CNTs. These functional groups not only facilitated the

dispersion of the CNTs in water but also acted as centers

for incorporation of metal-oxide particles. The structures

and morphologies of CNTs were characterized by SEM.

Figure 1 shows the SEM images of CNTs before (a) and

after (b) the purification process using H2SO4 (95%)/HNO3

(65%). Moreover, the morphology of CNTs was confirmed

by TEM. Figure 2 shows the TEM images of purified

CNTs (Fig. 2b), which are much clearer compared with the

as-prepared one (Fig. 2a). These results suggest that the

CNTs were successfully purified using the acid treatment

process. It is observed that the acid treatment process

causes the peeling of graphite layers on surface of

untreated CNTs, and therefore, the diameter of purified

CNTs is quite smaller than the diameter of untreated CNTs.

The functional groups such as hydroxyl and carboxyl

groups of CNTs were characterized by FTIR. The presence

of hydroxyl groups and carbonyl groups corresponds to the

appearance of broad peaks at 3425 and 1736 cm-1,

respectively. It should be noted that the presence of

hydroxyl and carbonyl groups can enhance the dispersion

of CNTs in media such as polar solvents. Spinels of

LiMn2O4 (LMO) and LiNi0.5Mn1.5O4 (LNMO) were syn-

thesized by solid-state reaction. The morphology of the

LMO particles shows a well-faceted octahedral structure,

while the morphology of the LNMO particles exhibits

Fig. 1 SEM images of CNTs before a, after b acid treatment

Fig. 2 TEM images of CNTs before a, after b acid treatment

T. Van Le et al.: Acta Metall. Sin. (Engl. Lett.)

123

irregular cubes. Both LMO and LNMO particles exhibit

grain sizes from 0.5 to 1 lm, which were analyzed by SEM

in Fig. 3. Furthermore, The LMO and LNMO structures

were characterized by XRD analysis, as shown in Fig. 4.

The diffraction peaks at 2h = 18�, 36.5�, 38�, 44�, 48�,

58�, and 64� relate to the doped spinel structure of LMO

and LNMO materials [24, 25].

The schematic illustration of the formation of the LMO

(LNMO)/CNT nanocomposites via the mixing process is

shown in Fig. 5. CNTs were first treated in a mixed acid of

HNO3/H2SO4 to clean the CNTs surface and functionalize

reacting groups such as hydroxyl, carboxyl, and carbonyl

groups. After acid treatment, the CNTs and LMO (LNMO)

materials were stirred in ethanol for 15 min, and the sol-

vent was evaporated under vacuum. Finally, the obtained

mixture was coated on aluminum substrates of about

0.1 mm in thickness as cathode materials, which were then

dried at 130 �C under vacuum for 24 h to form LMO

(LNMO)/CNT nanocomposites.

The morphologies of LMO (LNMO)/CNT (2 wt%)

nanocomposites are characterized by SEM in Fig. 6. The

SEM results show a 1D tubular structure of the LMO

(LNMO)/CNT nanocomposites after mixing process.

A typical high-magnification transmission electron

microscopy (TEM) image (Fig. 7) further shows that LMO

(LNMO) distributed randomly on the CNTs surface. It

should be noted that the particle size distribution of LMO

and LNMO is much smaller than the original particle size

of LMO and LNMO. This phenomenon can be explained

that the preparation of LMO (LNMO)/CNT nanocompos-

ites via mixing process eliminated the particles aggrega-

tion. Therefore, the obtained particle size distribution of

LNO and LMNO in their nanocomposites is around 50 nm.

These results confirm that LMO and LNMO particles were

deposited on the surface of the LMO (LNMO)/CNT

nanocomposites. More interestingly, the LMO (LNMO)/

CNT nanocomposites exhibit an interpenetrating network

structure that offers a benefit of charge transfer in cathode

materials.

The structure of LMO (LNMO)/CNT (2 wt%) nano-

composites was also determined by XRD. The XRD dia-

gram in Fig. 8 shows the most important peaks related to

CNTs at 25�, and all characteristic peaks corresponding

LMO/LNMO at 18�, 36.5�, 38�, 44�, 48�, 58�, and 64�,

respectively.

Li? intercalation/de-intercalation in LMO/CNT and

LNMO/CNT nanocomposites as cathode electrode

Fig. 3 SEM images of LMO particles a, LNMO particles b

Fig. 4 XRD patterns of the doped spinel structure of LMO and

LNMO materials

T. Van Le et al.: Acta Metall. Sin. (Engl. Lett.)

123

materials was characterized by cyclic voltammetry (CV).

Figures 9 and 10 show the CV curves of LMO (LNMO)/

CNTs (2 wt%) compared to LMO (LNMO)/VC (Vulcan

Carbon *2 wt%) as a standard material for comparison.

The CV curves were carried out with a potential region of

0.1–1.3 V versus Ag/AgNO3 and a scanning rate of

0.2 mV/s in an 0.75 mol/L LiTFSI/EC:DMC (1:1 by vol-

ume) electrolyte solution. In the case of LMO/CNTs, the

Fig. 5 Schematic illustration of the synthesis of LMO/CNT and LNMO/CNT nanocomposites

Fig. 7 TEM images of LMO/CNT (2 wt%) a and LNMO/CNT (2 wt%) b nanocomposites

Fig. 6 SEM images of LMO/CNT (2 wt%) a, LNMO/CNT (2 wt%) b nanocomposites

T. Van Le et al.: Acta Metall. Sin. (Engl. Lett.)

123

CV curve shows double oxidation–reduction peaks on the

forward and backward scanning. As seen in Fig. 9, the

oxidation and reduction peaks in those curves relate to the

Mn3?/Mn4? redox couple, indicating the lithium inter-

calation/de-intercalation via the oxidation–reduction pro-

cess. The intensity of Mn3?/Mn4 oxidation and reduction

peaks of the LMO/CNT nanocomposites is higher than that

of the LMO/Vulcan Carbon materials. Similar to the CV

results of LMO/CNTs, the intensity of Ni?4/Ni?2 oxidation

and reduction peaks in LNMO/CNT nanocomposites is

also higher than that in LMO/Vulcan Carbon materials.

These results proved that the higher efficiency of LMO

(LNMO)/CNT nanocomposites is better than LMO

(LMNO)/Vulcan materials.

Fig. 8 XRD diagrams of pure CNTs, LMO/CNT (2 wt%), and

LNMO/CNT (2 wt%) nanocomposites

Fig. 9 CV curves of LMO/CNT (2 wt%) nanocomposites and LMO/

VC (Vulcan Carbon *2 wt%)

Fig. 10 CV curves of LNMO/CNT nanocomposites with 2 wt%

CNTs and LNMO/VC with 2 wt% Vulcan Carbon

Fig. 11 Nyquist plot of LMO/10 wt% CNT and LMO/10 wt%

Vulcan-Carbon nanocomposite electrode at room temperature

T. Van Le et al.: Acta Metall. Sin. (Engl. Lett.)

123

Last but not the least, the electronic conductivity of

LMO (LNMO)/CNTs as cathode nanocomposites com-

pared to LMO (LNMO)/Vulcan Carbon was characterized

by electrochemical impedance spectroscopy (EIS). As seen

in the Nyquist plot spectra, at a high content of Vulcan

Carbon, the charge transfer resistance (Rct) of LMO/

10 wt% Vulcan Carbon decreases significantly and the

diffusion resistance corresponding Warburg (W) at low

frequencies seems to dominate (Fig. 11). On the other

hand, the Warburg value of LMO (LNMO)/10 wt% CNTs

is much lower than that of LMO (LNMO)/10 wt% Vulcan

Carbon (Fig. 12). These results indicate that the electro-

chemical reaction of LMO/CNTs and LNMO/CNT nano-

composites is better than that of LMO (LNMO)/VC

materials as a result of high diffusion of lithium ions in the

charge–discharge process.

4 Conclusions

We have successfully purified and functionalized CNTs by a

mixture of H2SO4/HNO3. The LMO (LNMO)/CNT nano-

composites were synthesized successfully with well disper-

sion of LMO (LNMO) particles on the CNTs surface.

Finally, the cathode materials based on LMO/CNT and

LNMO/CNT nanocomposites were fabricated via mixing

process. The LMO/CNT and LNMO/CNT nanocomposites–

cathode materials have proved a remarkably higher effi-

ciency of Mn3?/Mn4? and Ni2?/Ni4? redox couples in

comparison to traditional cathodes based on LMO (LNMO)/

Vulcan Carbon. With this proof of concept, LMO/CNT and

LNMO/CNT nanocomposites could act as promising cath-

ode materials for the development of high-power lithium

rechargeable batteries.

Acknowledgments This research was supported by a Grant

Research of Vietnam National University, Ho Chi Minh City (No.

B2012-20-10TÐ). We would like to thank the supporting research of

National Laboratory of Polymer and Composite Materials for helpful

discussion.

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Fig. 12 Nyquist plot of LNMO/10 wt% CNTs and LNMO/10 wt%

Vulcan-Carbon nanocomposite electrode at room temperature

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123