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Thermoelectric properties of LiCo1− x M x O2 (M = Cu, Mg, Ni, Zn): Comparison with Li y
CoO2 and Na y CoO2 systems

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Japanese Journal of Applied Physics 56, 021101 (2017)https://doi.org/10.7567/JJAP.56.021101

Thermoelectric properties of LiCo1%xMxO2 (M = Cu, Mg, Ni, Zn):
Comparison with LiyCoO2 and NayCoO2 systems
Shu Mizuno1, Hiroyuki Fujishiro1*, Mamoru Ishizawa1, Tomoyuki Naito1, Hirokazu Katsui2, and Takashi Goto2
1

Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan2*E-mail: fujishiro@iwate-u.ac.jp
Received June 26, 2016; accepted October 24, 2016; published online January 5, 2017
LiyCoO2 has a similar layered structure to NayCoO2, which is a typical p-type oxide thermoelectric material, and the average Co valence of 3 + y iscontrolled by the Li content y. We investigated the thermoelectric properties of LiCo1%xMxO2 samples (M = Cu, Mg, Ni, Zn) for the ﬁrst time at hightemperatures, in which Co3+ was substituted by the divalent M2+ ions, and the average Co valence of 3 + x can be controlled similarly to the Licontent y in LiyCoO2. The substitution of the M2+ ions for the Co site was found to show thermoelectric properties similar to those of LiyCoO2 withthe same average Co valence. The Mg-doped sample showed the highest thermoelectric performance at high temperatures in this study; thethermoelectric power factor P is 2.38 ' 10%4 W m%1 K%2 at 1173 K and the dimensionless ﬁgure of merit ZT is 0.024 at 876 K. The thermoelectric
potential of LiCo1%xMxO2 is discussed and compared with those of LiyCoO2 and NayCO2 systems. © 2017 The Japan Society of Applied Physics

1.

Introduction

Thermoelectric technology that uses waste heat for power
generating and cooling devices has been revived in anticipation of the forthcoming energy crisis, together with otherrenewable energy sources. The thermoelectric performance ofmaterials can be evaluated from the dimensionless ﬁgure of
merit ZT (= S 2T=ρκ), and the thermoelectric power factor P(= S 2=ρ), where S is the Seebeck coeﬃcient, T is the absolutetemperature, ρ is the electrical resistivity, and κ is the thermal
conductivity. Conventional thermoelectric bulk materials
with higher ZT values include Bi2Te3 and PbTe, which,however, exhibit thermal and chemical instabilities in air atelevated temperatures, and contain toxic, scarce, or expensive
elements.1–3) Nevertheless, oxide materials could be potential
candidates for thermoelectric applications owing to theiradvantages over the heavy metallic alloys in thermal andchemical robustness values, although their potential for suchapplications has not yet been demonstrated. Typical p-type
thermoelectric oxides are layered cobaltites such as Ti-doped
Ca3Co4O9 (ZT = 0.3 at 1000 K)4) and NayCoO2 (ZT = 0.8 at
1050 K),5) in which a relatively higher Seebeck coeﬃcientand a lower thermal conductivity are achieved owing to theirlayered structure. The crystal structure of NayCoO2 consistsof a single-atomic Na layer sandwiched between two CoO2
layers with edge-shared CoO6 octahedra.6) It is known that
NayCoO2 shows a wide range of Na nonstoichiometry andconsists of the following three types of crystal structure
depending on the Na content y: P3 type ( β phase, 0.55 <y < 0.6), P2 phase (γ phase, 0.6 < y < 0.75), and O3 phase(α phase, 0.9 < y < 1.0).7) As y decreases from one, theaverage valence of cobalt increases from +3 toward +4,
where the concentration of magnetic Co4+ ions [S = 1=2 forlow spin (LS), S = 3=2 for intermediate spin (IS), or S = 5=2
for high-spin (HS) states] increases in a nonmagnetic Co3+
(S = 0 for LS state) matrix and electronic and magnetic properties change depending on y. At about y = 0.7, NayCoO2
exhibits an unusually large thermoelectric power and a
metallic and high electrical conductivity simultaneously.8,9)
To enhance the thermoelectric properties of NayCoO2, theCo-site substitution by 3d transition metal elements such as
Cr, Mn, Fe, and Ni,10–12) or other elements such as Zn, B, Ti,

and Bi,10,13–15) and the Na-site substitution for the alkalineearth element (Ca)16) have been widely investigated. To thebest of our knowledge, the maximum ZT value of 0.68 wasachieved at 965 K for the Nay(Co0.95Zn0.05)O2 sample inpolycrystals.10)
LiyCoO2 has a layered structure similarly to NayCoO2 andis the cathode material most commonly used for Li
rechargeable batteries, since Li ions can be removed fromand inserted into LiyCoO2 repeatedly through an electrochemical method.17) The electrochemical behavior andcrystal structure of the LiyCoO2 system are well established
for 0.5 ≤ y ≤ 1.0,18,19) in which the following three types ofregions exist depending on the Li content y; O3–R1 type(0.5 < y < 0.75), O3–R2 phase (0.94 < y < 1.0), and twophase-coexistence region (0.75 < y < 0.94).20) LiyCoO2 isknown to exhibit a fairly large Seebeck coeﬃcient comparable to that of NayCoO2, although the electrical resistivity
is rather high.20,21) The thermoelectric and magnetic properties of LiyCoO2 have been systematically investigated attemperatures lower than 300 K for polycrystals22,23) and thinﬁlms.24) To extend the cyclability and enhance the capacityof electrochemical Li==LiCoO2 cells, numerous species ofcations have been substituted for the Co site, such as Ni, Mn,
Cr, Al, and Fe.25–29) The electrical conductivity and Seebeck
coeﬃcient of Mg-doped LiCoO2 have been reported, with
Mg-doped Liy(Co1−xMgx)O2 samples showing a higher electrical conductivity than nondoped samples.30) The Co site
substitution by the divalent cations in LiCoO2 introduces themagnetic Co4+ ions in a nonmagnetic Co3+ matrix, which issimilar to the eﬀect in LiyCoO2 obtained by the electrochemical technique. However, all the thermoelectric propertiesof the LiyCoO2 system such as ρ(T ) and S(T ) were only
observed at temperatures lower than room temperature.23,31)
There is no reported investigation of the thermoelectric
properties for the Liy(Co1−xMx)O2 system at higher temperatures. In the present study, we investigated the thermoelectric properties of LiCo1−xMxO2 (M = Cu, Mg, Ni, Zn) up
to 1200 K, and compared them with those of the LiyCoO2 andNayCoO2 systems.
2.

Experimental procedure

To fabricate LiCo0.8M0.2O2 (M = Cu, Ni, Zn) and LiCo1−x-021101-1© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 021101 (2017)

S. Mizuno et al.

MgxO2 (0 ≤ x ≤ 0.3) materials, Li2CO3 (99.9%), Co3O4(99.9%), CuO (99.9%), MgO (98%), NiO (99.9%), and ZnO(99.5%) ﬁne powders were used as raw materials. These
were weighed with a stoichiometoric ratio and mixed for
0.5 h using an automatic mortar in air. They were heated at1173 K for 12 h in air and then furnace-cooled. The obtained
precursor was pulverized and mixed in air for 0.5 h using
an automatic mortar. The precursor powders were sintered by
the SPS apparatus (NJS LABOX-110C) under a uniaxialpressure of 40–50 MPa at 1073 K for 10 min in vacuum. The
obtained pellets were heat-treated at 1173 K for 2 h in air tocontrol the oxygen stoichiometry. The relative density of theobtained pellets (10 mm in diameter and 9 mm in thickness)
was about 90% of the ideal one. In this study, the Li contents
in the sintered materials are supposed to be a nominal
composition, assuming that the evaporation of lithium is
negligibly small.
Powder X-ray diﬀraction (XRD) measurements were
performed (Rigaku Multi Flex) at room temperature using
Cu Kα radiation to conﬁrm the impurity phase in the sinteredbulk materials. The electrical resistivity ρ(T ) and Seebeck
coeﬃcient S(T) were simultaneously measured in the temperature range from 300 to 1200 K for a rectangular bar cut fromthe pellet using an automated measuring system (Ozawa
Science RZ2001i), and the thermoelectric power factor
P = S 2=ρ was calculated. The thermal conductivity κ(T ) was
measured in vacuum by a laser ﬂash method (Ulvac-Riko
TC-7000) from 300 to 873 K. The κ(T ) and ρ(T ) values below
300 K were also measured by a steady-state heat ﬂow method
and a four-probe method, respectively, using a laboratorybuilt measuring system.32) The dimensionless ﬁgure of merit,
ZT = S 2T=ρκ, was estimated using these measured values with
about 20% uncertainty for each measured value.
3.

Results and discussion

Figures 1(a) and 1(b) show the normalized XRD patterns of
the LiCo0.8M0.2O2 samples [M = Cu, Mg, Ni, Zn, and Co(nondoped)] and the magniﬁcation around the 104 peak,
respectively. Although LiyCoO2 was reported to show abiphasic regime at 0.75 ≤ y ≤ 0.94, all the diﬀraction peaks
are readily indexed on the basis of the trigonal space group
 and a small number of impurity phases of raw oxides
R3m,with substituted elements were detected. The substitution
limit of Mg for the Co site was reported to be about 5%.30)
Each M ion seemed to substitute Co3+ ions up to thesolubility limit, since the diﬀraction peak of raw oxides was
detected in each sample. In Fig. 1(b), the peak angle of the
104 diﬀraction in the M = Cu, Zn samples is the same asthat in LiCoO2. On the other hand, that in the M = Mg, Ni
samples shifts toward a lower angle. The ionic radius of the
Co3+ ion is 0.0545 nm, which is smaller than that of all the Mions: Mg2+ (0.072 nm), Cu2+ (0.073 nm), Ni2+ (0.069 nm),and Zn2+ (0.074 nm).33) These results suggest that Mg and Ni
ions are easily substituted for the Co site, but the solubility
limit of Cu and Zn for the Co site is still small.
Figure 2 presents the temperature dependence of the electrical resistivity ρ(T ), Seebeck coeﬃcient S(T), and thermoelectric power factor P(T ) of the LiCo0.8M0.2O2 samples. The
ρ(T ) of all the samples shown in Fig. 2(a) depicts a semiconducting behavior, and the absolute value of ρ changes
depending on the species of the M ion. The Mg-doped

(a)

(b)

Fig. 1. (Color online) (a) XRD patterns of LiCo0.8M0.2O2 samples[M = Cu, Mg, Ni, Zn, and Co (nondoped)]. (b) Enlarged XRD patterns of the104 peak for each sample.

sample shows the lowest ρ(T ), which is about four ordersof magnitude smaller than that of the nondoped sample at
300 K. These results are consistent with the peak shift of the
XRD diﬀraction, as shown in Fig. 1(b), in which M ions,especially Mg2+ ions, substitute for the Co3+ site and create
hole carriers.
In Fig. 2(b), the S(T ) value of the Mg- and Ni-doped
samples is the lowest in this study. Since the Seebeck coeﬃcient is inversely proportional to log n (n: carrier concentration) in a classical wide-gap semiconductor model, thereduced S value might be caused by the enhanced carrier
concentration, and the ρ(T ) and S(T ) values are closely
related to each other. However, overall, all the S(T ) values
slightly increased with increasing T, which is inconsistent
with the above relationship. The thermoelectric power factor
P(T ) (= S 2=ρ) as shown in Fig. 2(c) increases with increasingT, and that of the Mg-doped sample is the highest with
2.38 × 10−4 W m−1 K−2 at 1173 K, which results from boththe lowest ρ(T ) and moderate S(T ) values. As shown in
Figs. 2(a)–2(c), the reported ρ, S, and P values of Li0.75CoO2
at 293 K are also shown,23) in which the average Co valenceis +3.25. These values are almost the same as those of the
LiCo0.8Mg0.2O2 sample with the average Co valence of +3.2.
In this manner, Mg doping eﬀectively creates positive (hole)carriers, and the Mg-doped sample shows similar thermoelectric properties to LiyCoO2.
It was found that Mg doping creates hole carriers eﬀectively and shows better thermoelectric properties, as shown in
Fig. 2. To obtain optimum Mg contents, the thermoelectric
properties of the LiCo1−xMgxO2 samples were investigated.
Figure 3(a) shows the XRD patterns of LiCo1−xMgxO2021101-2© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 021101 (2017)

S. Mizuno et al.

(a)

(a)

(b)

(b)

(c)

Fig. 3. (Color online) (a) XRD patterns of the LiCo1−xMgxO2 samples(0 ≤ x ≤ 0.3). (b) Lattice constants a and c of the LiCo1−xMgxO2 samples as afunction of x. The inset shows the enlarged XRD patterns of the 104 peak foreach sample.

Fig. 2. (Color online) Temperature dependence of (a) electrical resistivity
ρ(T ), (b) Seebeck coeﬃcient S(T ), and (c) thermoelectric power factor P(T )of LiCo0.8M0.2O2 samples [M = Cu, Mg, Ni, Zn, and Co (nondoped)]. Thosefor the Li0.75CoO2 sample in Ref. 23 at 293 K are also shown.

(0 ≤ x ≤ 0.3). Figure 3(b) shows the lattice constants a and cof the LiCo1−xMgxO2 samples as a function of x. The inset
shows the enlarged XRD patterns near the 104 peak for eachsample. For x ≤ 0.1, the peak of the MgO raw powder was
not conﬁrmed, but for x ≥ 0.2, the peak can be detected andincreases in intensity with increasing x. In Fig. 3(b), thediﬀraction angle of the 104 peak decreases gradually withincreasing x up to x = 0.2 and then tends to saturate at
x = 0.3, which suggests that the Mg2+ ion substitutes for
the Co3+ site with x and then the substitution saturates. The
lattice constants of the trigonal crystal structure a and c,which were calculated using the 003 and 104 peaks, changed,
depending on the Mg2+ substitution.
Figure 4 shows the temperature dependence of the electrical resistivity ρ(T ), Seebeck coeﬃcient S(T), and thermoelectric power factor P(T) of LiCo1−xMgxO2 samples (0 ≤x ≤ 0.3). The semiconducting ρ(T) behavior becomes weak
and the absolute ρ value decreases with increasing Mg

content x. The absolute S(T ) value shown in Fig. 4(b)gradually decreases with increasing x owing to the decreasein ρ(T ) and then saturates. These values for the x = 0.2 and0.3 samples are almost the same, which is consistent with thesaturation tendency shown in Fig. 3(b). The power factor
P(T ), which was estimated using ρ(T ) and S(T), increases
with increasing x and then also saturates for x = 0.3. The
maximum power factor was formed to be 2.38 × 10−4
W m−1 K−2 at 1173 K for the x = 0.2 and 0.3 samples. We
calculated the P value of the LiCo0.94Mg0.06O2 at 300 K using
the reported ρ(T ) and S(T ) values,30) which was smaller thanthat of the present sample. The diﬀerence may result from thediﬀerent fabrication processes used.
In the LiyCoO2 system, the magnetic and related ρ(T )anomalies were detected at 170 K at y = 0.7–0.67 and
0.5.23,34,35) Mukai et al. suggested that the transition at
170 K is not magnetic but originates from either chargeordering or a change in spin state.34) Figure 5 shows the ρ(T )of the present LiCo1−xMgxO2 samples at T < 300 K. ρ(T )shows a semiconducting behavior similarly to that of thesamples at higher temperature. There is no anomaly in ρ(T) at
around 170 K for the samples. These results are in clear
contrast to those of the LiyCoO2 system. The characteristic
behaviors in the LiyCoO2 system are not necessarily
reproduced in the LiCo1−xMgxO2 system.
Figure 6 shows the relationship between the power factor
P at 300 K and the average Co valence for the present
LiCo1−xMgxO2 samples (0 ≤ x ≤ 0.3). A similar relationshipfor the LiyCoO2 system at 293 K has also been shown,23)
which was calculated by us. The P value of the LiCo1−xMgx-

021101-3© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 021101 (2017)

S. Mizuno et al.

(a)Fig. 5. (Color online) Temperature dependence of the ρ(T ) of the
LiCo1−xMgxO2 samples (0.05 ≤ x ≤ 0.3) at T < 300 K.

(b)

(c)

Fig. 6. (Color online) Relationship between the power factor P at 300 K
and the average Co valence for the present LiCo1−xMgxO2 samples(0 ≤ x ≤ 0.3). Similar relationships for the LiyCoO2 system in Ref. 23 at
293 K and for the NayCoO2 system (polycrystals) estimated from Refs. 36
and 37 at 300 K are also shown.

(a)

(b)

Fig. 4. (Color online) Temperature dependence of (a) electrical resistivity
ρ(T ), (b) Seebeck coeﬃcient S(T ), and (c) power factor P(T ) of
LiCo1−xMgxO2 samples (0 ≤ x ≤ 0.3). The extracted ρ and S values, and theestimated P values from Refs. 23 and 30 are also plotted for reference.

O2 showed a domelike dependence and reached a maximum
at the Co valence of 3.2–3.3 (x = 0.2–0.3) with a continuouschange, which is the same as for the LiyCoO2 system witha two-phase coexistence region (0.75 < y < 0.94). A similar
relationship for the NayCoO2 polycrystals is also shown inthe ﬁgure,36,37) in which a similar dome-shaped relationshipcan be seen at the center of the Co valence of 3.2. However,the P value of the LiCo1−xMgxO2 system is about two ordersof magnitude smaller than that of the NayCoO2 system, which
results from the relatively large ρ value of the LiCo1−xMgxO2and LiyCoO2 systems, even though comparably large S
values are achieved.
Figure 7 shows the temperature dependence of the thermal
conductivity κ(T ) of the LiCo0.8M0.2O2 samples [M = Cu,
Mg, Ni, Zn, and Co (nondoped)] at low and high temperatures. The κ(T ) of the nondoped sample shows a large peak at
a low temperature, which is in clear contrast to the reported
result;38) the absolute value of κ(T ) is small and κ(T ) increases

Fig. 7. (Color online) Temperature dependence of the thermal conductivity κ(T ) for the LiCo0.8M0.2O2 samples [M = Cu, Mg, Ni, Zn, and Co(nondoped)] at (a) T < 300 K and (b) 300 < T < 900 K.

monotonically with T. The κ(T ) peak of the LiCo0.8M0.2O2
samples was markedly suppressed by the Co-site substitution,
depending on the species of the M element; Mg and Ni doping
processes eﬀectively suppressed the κ(T) value. These results
are closely related to the solubility limit of the M element for
the Co site, as shown in Figs. 1(b) and 2(a). κ is the sum of theelectronic thermal conductivity κe and phonon thermal conductivity κphonon (κ = κe + κphonon). In the present samples, the021101-4© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 021101 (2017)

S. Mizuno et al.

Fig. 8. (Color online) Temperature dependence of the dimensionlessﬁgure of merit ZT(T) of the LiCo0.8M0.2O2 samples [M = Cu, Mg, Ni, Zn,
and Co (nondoped)].

thermal conduction entirely depends on phonons because theelectronic contribution is negligibly small owing to the large
ρ(T ). For example, the κe of the LiCo0.8Mg0.2O2 sample isestimated to be 0.77 mW cm−1 K−1 at 800 K using Fig. 2(a) on
the basis of the Wiedemann–Franz law, which is about 1% of
the measured κ shown in Fig. 7(b). There have been many
reports on the thermal conductivity of the NayCoO2 andNay(Co1−xMx)O2 single crystals,5,39,40) and polycrystals.41)
However, the absolute κ values reported are fairly scatteredand reliable results of the x dependence of κ(T ) have not yet
been reported; the typical in-plane κ(T ) values for single
crystal are 20 mW cm−1 K−1 at 300 K without a low-temperature κ(T) peak39) and 40–80 mW cm−1 K−1 at 300 K with alow-temperature κ(T) peak.39,40) Since the κ(T ) value for apolycrystal is generally smaller than that for a single crystal,
especially for a layered material, the κ(T ) value for NayCoO2
polycrystals seems to be considerably lower than that for
single crystals. In this sense, the κ(T) of the Liy(Co1−xMx)O2
polycrystals shown in Fig. 7 is larger than that of the
NayCoO2 polycrystals even at higher temperatures, which
is disadvantageous for a thermoelectric material from theviewpoint of κ(T).
Figure 8 shows the temperature dependence of the dimensionless ﬁgure of merit ZT(T ) of the LiCo0.8M0.2O2 samples.
Among the samples, the M = Mg sample shows the highest
ZT value of 0.024 at 876 K, which is considerably smaller
than that of the NayCoO2 system, resulting from the relatively
high ρ and κ values. Although LiyCoO2 and NayCoO2
systems have similar crystal structures, a slight diﬀerence
exists. Hertz et al. compared the structures of NayCoO2 andLiyCoO2 and reported that the CoO2 layer changes substantially with the alkali content y in the former, but is relatively
constant in the latter, and that the CoO6 octahedra in LiyCoO2
is less distorted.35) Such a slight diﬀerence may aﬀect theρ(T ), S(T), and κ(T ) values, and also the magnetic properties.
The physical properties for both systems must be investigatedfurther in detail.
4.

Conclusions

The thermoelectric properties of LiCo1−xMxO2 samples(M = Cu, Mg, Ni, Zn) have been investigated for the ﬁrst
time up to 1200 K and the thermoelectric potential of the
LiCo1−xMxO2 material has been discussed and compared withthose of the LiyCoO2 and NayCoO2 systems. The importantresults and conclusions obtained from this study are summarized as follows.
1) In the LiCo0.8M0.2O2 system, the M = Mg sample
displays the most eﬀective thermoelectric properties; thethermoelectric power factor P is 2.38 × 10−4 W m−1 K−2 at1173 K and the dimensionless ﬁgure of merit ZT is 0.024 at
876 K. These values at 300 K are comparable to the reported
values of Li0.75CoO2, suggesting that the manner of Co site
substitution by Mg ions is the same as that of Co4+ creation to
Li nonstoichiometry in the LiyCoO2 system.
2) In the LiCo1−xMgxO2 series (0 ≤ x ≤ 0.3), the electricalresistivity ρ(T ) decreases with increasing Mg contents x up to
0.2, and then saturates at x = 0.3. The better thermoelectricperformance of the P value was achieved at x = 0.2–0.3 athigher temperatures.
3) The thermoelectric properties of the LiCo1−xMgxO2series are lower than those of the NayCoO2 series under an
identical average Co valence because of the higher electricalresistivity ρ(T ) and higher thermal conductivity κ(T ) at
higher temperatures.
Acknowledgment

This work was performed under the Inter-university Cooperative Research Program of the Institute for Materials
Research, Tohoku University (16K0009 and 16K0203).

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