Ni-substitution spinel LiNixMn2−xO4 (x = 0, 0.1, 0.2) materials were synthesized by the sol--gel method. The structure and  morphology of the samples were characterized by the X-ray diffraction (XRD)  and the scanning electron microscopy. The ac conduction of the materials was  investigated by electrochemical impedance spectroscopy (EIS) measurements.  The refinement results showed that the substitution of Ni decreased the  lattice constant and Mn--O distance, while increased Li--O bond length and  16c octahedral volume. The EIS results confirmed the decrease of  conductivity with increasing Ni substitution content. Based on XRD and EIS  results, the relationship between the crystal structure and electrochemical  behavior of the materials was discussed and explained

Anh Tan, Ta

Dinh Trong, Le

Duy Long, Pham

Ha Chi, Le

Si Hieu, Nguyen

Tran Chien, Dang

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 26, No. 4 (2016), pp. 361-368
DOI:10.15625/0868-3166/26/4/8953
STRUCTURE AND ELECTROCHEMICAL IMPEDANCE OF LiNixMn2−xO4
TA ANH TAN1,4, NGUYEN SI HIEU1, LE HA CHI1, DANG TRAN CHIEN2
LE DINH TRONG3 AND PHAM DUY LONG1,†
1Institute of Materials Science, Vietnam Academy of Science and Technology,
18-Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
2Hanoi University of Natural Resources and Environment,
41A-Phu Dien, Bac Tu Liem, Hanoi, Vietnam
3Hanoi Pedagogical University No. 2,
32-Nguyen Van Linh, Xuan Hoa, Phuc Yen, Vinh Phuc, Vietnam
4Hanoi Metropolitan University, 98-Duong Quang Ham, Cau Giay, Hanoi, Vietnam
†E-mail: longphd@ims.vast.ac.vn
Received 01 December 2016
Accepted for publication 27 February 2017
Abstract. Ni-substitution spinel LiNixMn2−xO4 (x = 0,0.1,0.2) materials were synthesized by
the sol–gel method. The structure and morphology of the samples were characterized by the X-ray
diffraction (XRD) and the scanning electron microscopy. The ac conduction of the materials was
investigated by electrochemical impedance spectroscopy (EIS) measurements. The refinement re-
sults showed that the substitution of Ni decreased the lattice constant and Mn–O distance, while
increased Li–O bond length and 16c octahedral volume. The EIS results confirmed the decrease of
conductivity with increasing Ni substitution content. Based on XRD and EIS results, the relation-
ship between the crystal structure and electrochemical behavior of the materials was discussed
and explained.
Keywords: lithium ion battery, spinel manganate cathode material, Rietveld refinement, Electro-
chemical impedance.
Classification numbers: 82.47.Aa, 61.05.cp, 61.72.-y.
I. INTRODUCTION
The efforts for the development of energy storage systems have been forced rapidly due
to the increase in the demand of clean and renewable energy sources. Lithium ion batteries have
been considered as the most attractive energy storage system due to its advantages including high
energy density and long cycle life. However, the issues of cost, safety and capacity of conventional
c©2016 Vietnam Academy of Science and Technology
362 STRUCTURE AND ELECTROCHEMICAL IMPEDANCE OF LiNixMn2−xO4
cathode materials such as LiCoO2 required the development of new cathode materials. Recently,
LiMn2O4 spinel cathode materials attracted a lot of interest due to its low cost, environmental
friendliness and safety [1, 2]. However, this cathode material faced some problems including
Mn dissolution in electrolytes [3], Jahn-Teller distortion at deeply discharge [4], and oxygen de-
ficiency [5]. The substitution of Mn by other elements such as Ni, Al, Fe, Co resulted in the
improvement in the electrochemical performance of the mother material [6]. Excellent electro-
chemical performance of LiNi0.5Mn1.5O4 cathode material has confirmed by many group [7, 8].
Nevertheless, this material is quite expensive due to the high amount of nickel. In this circum-
stance, the studies of LiNixMn2−xO4 at low substitution levels are very necessary.
It is recognised that the electrochemical properties of manganate spinels depend on the
preparation method [9–11]. In addition, published studies showed controversial results about the
effect of Ni substitution on the conductivity of LiNixMn2−xO4. For example, while J. Molenda
et al. [12] showed the decrease of the conductivity by Ni substitution, M. A. Kebede et al. [13]
announced opposite results. Especially, there has not been adequate attention paid to studying the
effect of structure on the conductivity of LiNixMn2−xO4 materials. In this work, LiNixMn2−xO4
(x = 0,0.1,0.2) was synthesized by the sol-gel method. The effect of Ni substitution level on
the structure of the material was investigated. Based on the experiment results, the relationship
between the structure and the conductivity of the material was discussed. In addition, the study
of structure also contributed to explain the effect of Ni substitution on the electrochemical perfor-
mance of LiNixMn2−xO4 cathode materials
II. EXPERIMENTAL
LiMn2−xNixO4 (x = 0,0.1,0.2) materials have synthesized by the sol-gel method. The
stoichiometric amounts of analytical grade of lithium acetate (CH3COOLi.2H2O), manganese II
acetate (2(CH3COO)Mn.4H2O) and nickel II acetate (2(CH3COO)Ni.4H2O) from Sigma Aldrich
were separately dissolved in deionized distilled water followed by the continuous stirring at 80˚C.
The mixtures were stirred for 1h to form a clear homogeneous solution. The solutions then were
added by citric acid (CH2COOH–C(OH)COOH–CH2COOH, Sigma Aldrich, 99.9%) with a molar
ratio of 1 lithium acetate: 3 citric acid followed by the continuous stirring for 8 h. The final
solutions were dried at 120˚C for 12 h and then calcined in air at 800˚C for 15h to obtain the
materials.
The SEM images of the materials were obtained by using Hitachi S-4800 FESEM. The X-
ray powder diffraction analyses of the materials were performed by Siemens D5000 X-ray diffrac-
tometer using CuKα radiation at room temperature. The filament bias voltage and the filament
current were set at 35 kV and 40 mA respectively. The angle of diffraction (2θ) was changed
from 10˚ to 70˚ at a step of 0.03˚/sec. The Rietveld refinement of X-ray diffraction data was
conducted by the FullProf software package [14]. The crystal structures were drawn by VESTA
programs [15].
For the measurements of electrochemical impedance, the powders were uniaxially cold
pressed at 40 MPa to form cylindrical pellets with the diameter of ca. 12 mm and the thickness of
ca. 1.5 mm. The electrodes were prepared by the evaporation of Au on the pellets. The diameter
and the thickness of the electrodes were ca. 8mm and 1µm, respectively. The impedance measure-
ments were carried out by EIS-Zahner IM 6 electrochemical impedance analyzer at amplitude of
TA ANH TAN, NGUYEN SI HIEU, LE HA CHI, DANG TRAN CHIEN, LE DINH TRONG AND PHAM DUY LONG 363
10 mV and frequency range of 10−1–106 Hz. The fitting of EIS data was performed by the Zview
software.
III. RESULTS AND DISCUSSION
Fig. 1. SEM images of the LiMn2O4 (a), LiMn1.9Ni0.1O4 (b) and LiMn1.8Ni0.2O4 (c)
powders (a, b, c) that were calcined at 800˚C.
The SEM observations of the LiMn2−xNixO4 (x = 0,0.1,0.2) powders were showed in
Fig. 1. Generally, the particle size of the un-doped and doped samples was quite similar. The
particle sizes of the samples were in the range of 300 nm – 1µm. It is easy to see that the shape of
the particles changed from round to sharp with increasing substitution content. This fact showed
that Ni substitution affected on the morphology of the crystalline-particle shape.
Fig. 2. The crystal structure of LiMn2O4 material (a). And the illustration of Li+ diffu-
sion via 16c site (b). Black arrows showed the diffusion pathway of Li+ ion.
The structure of the materials was investigated by XRD measurements. Rietveld refinement
was used to refine the patterns. At room temperature, LiMn2O4 has cubic-spinel structure with the
space group of Fd-3m [16]. The structure of LiMn2O4 was showed in Fig. 2A, where Li, Mn and
O atoms respectively locate at 8a, 16d and 32e sites. The exchange of positions between Mn and
Li atoms in the LiMn2O4 lattice was confirmed by several groups. Bjork et al. reported that 9%
of the lithium ions at the tetrahedral 8a sites can be substituted by Mn2+ ions [17]. Martinez et
364 STRUCTURE AND ELECTROCHEMICAL IMPEDANCE OF LiNixMn2−xO4
al. found about 10% of the lithium ions presented at the octahedral 16d sites [18]. Because of the
low X-ray scattering factor of Li, the content of Li at Mn positions was ignored in the refinement
process. To keep things simple, the existence Ni in the crystal lattice was ignored in the refinement
model. In addition, the oxidation number of Mn at 8a sites was set at +2 for all samples.
Fig. 3. X-ray powder diffraction patterns and refinement results of LiNixMn2−xO4 (x =
0,0.1,0.2) materials. The inset shows (440) peak at high magnification.
Figure 3 showed the X-ray diffraction patterns of LiNixMn2−xO4 samples. All samples
were single phase with cubic-spinel structure. It is clear that the X-ray diffraction intensity of
the materials increased with the Ni content. This result pointed out the improvement in the crys-
tallinity degree of the materials by substitution effect. Rietveld refinement results of XRD patterns
were showed in Table 1. The calculated model was in a good agreement with the experimental
results. The doping resulted in the decrease of the lattice constant from 8.2321 A˚ (LiMn2O4) to
8.2137 A˚ (LiMn1.8Ni0.2O4). These results were similar with the reported data [16,19]. In contrast
to the increase of Li–O bond length, Mn–O distance was confirmed to decrease with increasing
x-value. Due to the average Mn–O bond length of the Mn3+ ion is larger than that of the Mn4+
ion [4], the decrease in Mn–O distance can be attributed to the suppression of Jahn–Teller dis-
tortion. The substitution of Ni also resulted in the decrease of the content of transition metal at
8a sites, which represents for the structural disorder of LiMn2O4 crystal. The interesting thing
here is that although 8a tetrahedral volume increased by ca. 15%, the content of Mn2+ ion at 8a
TA ANH TAN, NGUYEN SI HIEU, LE HA CHI, DANG TRAN CHIEN, LE DINH TRONG AND PHAM DUY LONG 365
site decreased from 10% to 2% with increasing Ni substitution content. The diffusion of Li+ ion
via 16c octahedral sites was confirmed by many authors [20, 21]. The diffusion mechanism was
clarified in Fig. 2B. As it can be seen from Table 1, volume of 16c octahedron was confirmed
to increase with decreasing Ni content. The increase of both 8a tetrahedral and 16c octahedral
volumes favored for the diffusion of Li+ ions.
Table 1. Rietveld refinement results of LiNixMn2−xO4 (x = 0,0.1,0.2) materials.
LiMn2O4 LiNi0.1Mn1.9O4 LiNi0.2Mn1.8O4
a (A˚) 8.2321 8.2228 8.2137
x (O position) 0.2571 0.2614 0.2638
Li–O bond length (A˚) 1.8850 1.9458 1.9755
Mn–O bond length (A˚) 2.0003 1.9672 1.9467
8a tetrahedral volume (A˚3) 3.4375 3.7778 3.9523
16c octahedral volume (A˚3) 12.6202 13.2014 13.4483
Mn2+ content at 8a site 0.10 0.07 0.02
GOF (Goodness of fit) 1.968 1.664 1.529
The effect of Ni content on the electrical conductivity of the materials was examined by
the electrochemical impedance analysis. In this work, the cylindrical pellets can be treated as
blocking electrodes where Li+ ions cannot move in and out of the electrodes. The equivalent
circuit of the electrodes is shown in the inset in Fig. 4 where the electrode/current collector con-
tact, the grain boundaries and the bulk material are respectively modeled by R1//C1, R2//CPE2
and R3//CPE3 circuits. The impedance of a constant phase element (CPE) can be expressed as
following equation [24]:
ZCPE =
1
T (iw)p
(1)
Here, w and T respectively represent for the angular frequency and the numerical value of the
admittance of the CPE. The exponent factor P indicates capacitor behavior of the CPE. The value
of P is in the range of 0–1. When P equals to 1, Eq. (1) is identical to the impedance of a capacitor.
The electrochemical impedance spectra of the electrodes have shown in Fig. 4. The fit-
ting results of the electrochemical impedance spectra have been shown in Table 2. It is clear that
the increase of Ni content resulted in the increase of R2 and R3 value, which respectively repre-
sented for the grain boundary and the bulk resistivities of the materials. This result is consistent
with the observation of J. Molenda et al. who confirmed the decrease of the conductivity by dop-
ing effect [12]. At present, there were experimental and theoretical confirmations that LiMn2O4
is a mixed ionic-electronic conductor with dominating electronic conduction, and the electronic
conduction of the material was closely related to the structural stability of the material [12, 22].
366 STRUCTURE AND ELECTROCHEMICAL IMPEDANCE OF LiNixMn2−xO4
Fig. 4. Nyquist plots of LiNixMn2−xO4 (x= 0, 0.1, 0.2) samples. The inset shows the
equivalent circuit using for the fitting process.
Table 2. The fitting results of electrochemical impedance data.
R1 R2 R3 T3 P3
LiMn2O4 101.7 2326.8 41399 8.01E-6 0.8706
LiNi0.1Mn1.9O4 510.0 2498.1 150157 4.22E-7 0.9312
LiNi0.2Mn1.8O4 282.8 2652.1 242857 1.23E-6 0.9323
Generally, the electronic conductivity of a semiconductor will be increased along with the increase
of the defect concentration. In other words, the decrease in structural disorder of a semiconductor
will result in the decrease of the electronic conductivity of the material. Based on these discus-
sions, it can conclude that the substitution of Ni decreased structural disorder of LiMn2O4 crystal
and resulted in the decrease in the electronic conductivity of the material. In the case of the CPE,
which shows the capacitance behavior of the materials, the substitution resulted in the increase of
the exponent factor P. It is well known that the exponent factor of a CPE depends on the shape
and the morphology of the electrode. In this work, these influences can be eliminated due to the
similarity in shape of electrodes and the morphology of the samples. Several groups confirmed
TA ANH TAN, NGUYEN SI HIEU, LE HA CHI, DANG TRAN CHIEN, LE DINH TRONG AND PHAM DUY LONG 367
the effect of the heterogeneity of material on the exponent factor of the CPE [25–27]. Based on
the data presented herein, it is clear that the substitution of Ni resulted in the improvement of the
heterogeneity of the cathode material, which revealed by the increase of the exponent factors of
the bulk CPE.
In another approach, the improvement in the conductivity of LiNixMn2−xO4 with increasing
Ni content was reported by several groups. R. Holze et al. showed that the substitution of Ni in
the range of x=0–0.1 caused the increase in the conductivity of LiNixMn2−xO4 cathode [23]. The
same results are also reported by M. A. Kebede et al. in the range of x=0–0.2 [13]. These results
seem to contradict with the impedance results in this work. However, it should be noted that the
present authors performed the experiments on a half cell configuration where the conductivity of
the cathode limited by Li+ ionic conductivity and affected by other elements including conducting
agent and binder in the cathode. As discussed above, the substitution of Ni not only improved
the structural stability of LiNixMn2−xO4 but also resulted in the extension of 8a tetrahedron and
16c octahedron which supported for the Li+ diffusion. Of course, this substitution will result in
the improvement of Li+ ionic conductivity and electrochemical performance of LiNixMn2−xO4
cathode materials.
IV. CONCLUSIONS
The effect of Ni substitution on the structure and the ac conduction of LiNixMn2−xO4
(x = 0,0.1,0.2) was investigated. The Rietveld refinement results showed that the substitution
decreased Jahn–Teller distortion and improve the structural stability of the materials. As a re-
sult, these effects caused the decrease of the electronic and total conductivities of the materi-
als. In contrast, the substitution increased Li+ conductivity and electrochemical performance of
LiNixMn2−xO4 which arise from the extension of 8a tetrahedron and 16c octahedron. Due to the
electrochemical performance of LiNixMn2−xO4 depends on both electronic and ionic conductions,
the results from this work suggested the improvement of electronic conduction as a solution for
the design of high performance LiNixMn2−xO4 cathode materials.
ACKNOWLEDGEMENTS
This work was supported by the Institute of Materials Science, VAST under research grand
No. CSTX 04.16.
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Structure and Electrochemical Impedance of LiNi\(_{x}\)Mn\(_{2 - x}\)O\(_{4}\)

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Structure and Electrochemical Impedance of LiNi $_{x}$ Mn $_{2 - x}$ O $_{4}$

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Structure and Electrochemical Impedance of LiNix_{x}x​Mn2−x_{2 - x}2−x​O4_{4}4​

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Structure and Electrochemical Impedance of LiNi $_{x}$ Mn $_{2 - x}$ O $_{4}$