Materials Research Bulletin 70 (2015) 748–752

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Materials Research Bulletinjournal homepage: www.elsevier.com/locate/matresbu

Synthesis and lithium insertion properties of ramsdellite LixTiO2 anodematerials
Isao Tsuyumoto* , Takumi Moriguchi
Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 921-8501, JapanA R T I C L E I N F O

A B S T R A C T

Article history:
Received 3 May 2015
Received in revised form 2 June 2015
Accepted 6 June 2015
Available online 9 June 2015

Ramsdellite LixTiO2 (x = 0.47) with reduced form is successfully prepared as a single phase by heating an
amorphous lithium peroxo-polytitanate in 4.8% H2 at 1100  C for 0–1 h. The XRD data are well reﬁned
with the space group Pnma with a = 0.96076 nm, b = 0.29514 nm and c = 0.50276 nm. The ramsdellite
LixTiO2 transforms into a rutile–ramsdellite intergrowth phase by extending the heating time to 1.25–4 h.
The capacity of the ramsdellite LixTiO2 in the region of 1.0–2.5 V (vs Li) is 208 mAh g1 at 0.8 C rate
(0.13 mA mg1), substantially larger than that of the ramsdellite Li2Ti3O7 with oxidized form. The lithium
insertion and deinsertion proceeds in two stages: a sloping region at higher potential (1.4–2.3 V) and aplateau region at lower potential (1.3–1.4 V).
ã2015 Elsevier Ltd. All rights reserved.

Keywords:
A. Oxides
B. Sol–gel chemistry
C. Electrochemical measurements
C. X-ray diffraction
D. Electrochemical properties

1. Introduction
Lithium titanates have been widely investigated as anodematerials for lithium ion batteries since early 1990s, because they
allow safer operating conditions compared to graphite electrodes
due to higher working potential which does not give rise to Li
electroplating (Li+ reduction) [1–5]. Spinel-type Li4/3Ti5/3O4 shows
good cycle characteristics with little capacity loss due to no
noticeable lattice dimension change during lithium insertion anddeinsertion processes [4] and is already in practical use incommercially available lithium ion batteries [2].
Ramsdellite Li2Ti3O7 and LixTiO2 (0 2 x 2 0.57) have also
attracted much attention as anode materials for lithium ion
batteries, because in the ramsdellite structure the double columnsof TiO6 octahedra form open framework with tunnels along c axis
favorable for lithium insertion (Fig. 1) [2,6–8]. The ramsdellite
Li2Ti3O7 with oxidized form is colorless and shows great lithium
ionic conductivity around 1.2  106 S cm1 and negligible electronic conductivity at 25  C [6], while the ramsdellite Li0.5TiO2 with
reduced form is black [9,10] and shows room-temperature
resistivity of 60 Vcm [11] due to electronic conduction via mixed
valence state between Ti3+ and Ti4+. The ramsdellite Li2Ti3O7 andLixTiO2 belong to the same space group Pbnm, but the distribution
situation of Li and Ti in the framework differs to compensate thedifference in the Ti/O ratio. In the structure of LixTiO2, the* Corresponding author. Fax: +81 76 294 6713.
E-mail address: tsuyu@neptune.kanazawa-it.ac.jp (I. Tsuyumoto).
http://dx.doi.org/10.1016/j.materresbull.2015.06.014
0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.

occupancy of Ti in the 4c site at the center of the TiO6 octahedra isunity, and all Li atoms are located in the tunnels [9,10,12]. On theother hand, in the structure of Li2Ti3O7, one-seventh of Ti in the 4csite is replaced by Li to adjust the Ti/O ratio at 3/7 [13–18], and thus
the compositional formula is more properly represented by
Li3/7[Li1/7Ti6/7]O2. Since this atomic replacement is considered tocause structural instability and to impair the reversibility oflithium insertion, we can expect better charge–discharge performance of the reduced-form ramsdellite LixTiO2.
While a substantial number of reports have appeared on the
preparation, structure and electrochemistry of the oxidized form,
Li2Ti3O7 [6,13–24], there have been few reports on the reduced
form, LixTiO2 [7,8]. Soares et al. prepared delithiated LixTiO2(0 2 x 2 0.57) from a homogeneously ground mixture of Li2CO3,
TiO2 and sucrose through a carbothermal synthesis route andreported that the delithiation of the ramsdellite LixTiO2 wasaccompanied by the growth of a secondary spinel phase [8].
Homogeneous starting materials and controlled synthetic conditions were suggested to be essential for the preparation of theramsdellite LixTiO2.
New synthetic routes of metal oxides from peroxo-metallic acidformed by reaction of metallic powder with concentratedhydrogen peroxide have been developed in our previous study[25–27]. For example, peroxo-polytitanic acid (Ti-iPA where iPA
stands for isopolyacid) solution was formed by reaction of metallic
titanium powder with 30–70 wt% hydrogen peroxide solution, andnanocrystalline anatase TiO2 was prepared by heating a dry residueof the Ti-iPA solution at 300–500  C [28–31]. The thus—preparedanatase TiO2 showed good lithium insertion and deinsertion

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749

Osaka, Japan) was added to the Ti-iPA solution at a molar mixing
ratio of Li/Ti = 0.75. The mixed solution was dried at 115  C for 24 hin air to yield a pale yellow amorphous powder, lithium peroxopolytitanate. The products were obtained by heating the amorphous powder at 1100  C in a reducing atmosphere of 4.8% H2 in Ar
in a quartz tube, where the temperature was increased at
24  C min1 to 600  C, at 11  C min1 from 600  C to 1000  C, and1.7  C min1 from 1000 to 1100  C. The ﬁnal products were rapidly
quenched from 1100  C in 4.8% H2 in order to suppress theformation of byproduct. For comparison, the product using theconventional solid state reaction of a well-ground mixture of
Li2CO3 and rutile TiO2 (Li/Ti = 0.75) under the same heating
condition was also investigated.
2.2. Characterization

Fig. 1. Perspective view of the ramsdellite LixTiO2 crystal structure projected alongb axis.

performance [32]. Nanosized tetragonal BaTiO3 was also preparedby heating a precipitate obtained by mixing Ti-iPA solution with Ba(OH)2 solution at 600–650  C [33]. We considered such syntheticroutes from Ti-iPA allowing uniform mixing of the constituent
elements of raw materials at atomic level were appropriate forpreparation of lithium titanates, because the conventional solidstate reactions required repeated heating and grinding for long
periods. In this study, we investigated synthetic conditions for thelithium titanates and successfully developed a new synthetic routeof ramsdellite LixTiO2 as a single phase from the mixture of Ti-iPA
and lithium carbonate solutions. We present here its synthetic
procedure, crystal structure, and lithium insertion and deinsertion
properties as an anode for lithium ion batteries.
2. Experimental
2.1. Synthesis procedure
Ti-iPA was prepared according to our previously reported
method [32,33]. In short, metallic titanium powder (0.17 g, 45 mm
particle size, as received from Wako Pure Chemical Industries Ltd.,
Osaka, Japan) was dissolved in 150 mL of 30 wt% aq. H2O2 solution(Wako Pure Industries Ltd., Osaka, Japan) at 20  C with stirring, anda yellow transparent Ti-iPA solution was obtained in 24 h. After
ﬁltration to remove insolubles, Li2CO3 (Wako Pure Industries Ltd.,The Li/Ti ratios of the products were determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES). Sample
solutions for ICP-AES were prepared by dissolving 8.5 mg of the
products into 20 mL of 36 wt% HCl aq. and diluting them withultrapure water from Milli-Q system (Millipore, USA). The X-ray
diffraction (XRD) patterns of the products were measured with adiffractometer (XD-D1, Shimadzu, Kyoto, Japan) using graphitemonochromatized Cu Ka radiation (l = 0.15406 nm) at 30 kV and
30 mA. The X-ray proﬁles were collected between 2 and 120 of 2u
angles with a step interval of 0.02 and a count time of 1 s/step. The
XRD samples were mixed with parafﬁn at weight ratio of 1:1 inorder to avoid oxidation by air during the XRD measurement. The
crystalline parameters were reﬁned by the Rietveld method using
the RIETAN-FP program [34,35], and the crystallite sizes were
estimated by the Scherrer equation. The morphology was observed
by a scanning electron microscope (SEM, JSM-5610, JEOL, Tokyo,
Japan) with an accelerating voltage of 10 kV. Electrochemicalmeasurements were performed using three electrode type cells
assembled in an Ar-ﬁlled glove box. For a working electrode, eachsample of 5.02–5.04 mg was intimately ground with acetylene
black (AB) as conductive auxiliary agent, and the composite, after
mixing with polytetraﬂuoroethylene (PTFE) as a binder, waspressed onto a copper mesh. The weight ratio of sample:AB:PTFE
was 40:40:20. Li metal was used as reference and counterelectrodes. The electrolyte used was a 1 M solution of LiPF6 in
ethylene carbonate (EC)/diethyl carbonate (DEC) solution (1:1 byvolume, Kishida Chemical, Osaka, Japan). Cyclic voltammetricproﬁles were recorded at scan rates of 0.20–3.33 mV s1 in therange of 0.8–3.0 V (vs Li), and charge–discharge measurements

Fig. 2. XRD patterns of the products obtained by heating lithium peroxo-polytitanate in 4.8% H2 along with that by the solid state reaction. (a) lithium peroxo-polytitanate
dried at 115  C in air, (b) heating lithium peroxo-polytitanate at 950  C for 1 h in 4.8% H2, (c) heating lithium peroxo-polytitanate at 1100  C for 1 h in 4.8% H2, (d) heating
lithium peroxo-polytitanate at 1100  C for 4 h in 4.8% H2, and (e) heating the mixture of Li2CO3 and TiO2 at 1100  C for 1 h in 4.8% H2.

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I. Tsuyumoto, T. Moriguchi / Materials Research Bulletin 70 (2015) 748–752

were carried out in the galvanostatic mode at 0.653–3.26 mA(0.130–0.649 mA mg1) in the range of 0.8–3.0 V (vs Li).
3. Results and discussion
3.1. Synthesis
A black ramsdellite lithium titanate with reduced form,
LixTiO2, was obtained as a single phase by heating at 1100  C for
0–1 h in a reducing atmosphere of 4.8% H2 in Ar (Fig. 2c). At 950–
1000  C the ramsdellite LixTiO2 was formed with a considerable
amount of the spinel Li4Ti5O12 (Fig. 2b), the fraction of the spinel
phase was decreased with elevating the heating temperature,and the ramsdellite phase was obtained as a single phase at1070–1100  C. By extending the reducing time to 1.25–4 h at
1100  C, the products transformed into a rutile–ramsdelliteintergrowth phase, in which the tunnels were composed ofalternating double and single columns (Fig. 2d) [36]. The Li/Ti
ratios of the products reduced at 1100  C for 0, 0.75, 1.00 and 1.25 h
were 0.49, 0.47, 0.47 and 0.37, respectively, from ICP analysis. Thissuggests that the ramsdellite LixTiO2 is stable when x 3 0.47 andthat it transforms to the intergrowth phase when x falls below 0.47
due to the volatilization of lithium at 1100  C. In the present case,the transformation to the intergrowth phase seems to alsoenhance the volatilization of lithium because the Li/Ti ratio
abruptly decreases between the heating time 1.00 and 1.25 h. It
was also found that the ramsdellite LixTiO2 (x = 0.47–0.49) withdark blue color was oxidized in air at room temperature to its
oxidized form with gray color. The conventional solid state
reaction between Li2CO3 and TiO2 at 1100  C for 1 h in a reducing
atmosphere of 4.8% H2 in Ar resulted in the formation of the
mixture of the ramsdellite LixTiO2 and the spinel Li4Ti5O12 (Fig. 2e).
This suggests that homogeneous starting materials in whichconstituent elements are uniformly mixed at atomic level are
advantageous for preparing complex oxides in a short reaction
time.

Fig. 3. SEM image of the ramsdellite Li0.47TiO2 obtained by heating lithium peroxopolytitanate in 4.8% H2 at 1100  C for 1 h.

cell, i.e., b0 = 12b, and the peak at 5.02 corresponds to 010 reﬂection
when b0 = 6b. This suggests that the lithium ions and theirvacancies in the tunnels are arranged periodically every twelve
unit cells along the b axis. Fig. 3 shows the morphology of theramsdellite Li0.47TiO2. Plate-like particles with diameter ranging
from submicron to micron scale were observed in the SEM image,while the crystallite size was estimated at 69.2 nm using theScherrer equation from the XRD pattern. Secondary particles
consisting of several tens nanometer scale crystallites are
considered to be observed in the SEM image, and this suggeststhat intimate mixing with conductive auxiliary agent at several
tens nanometer scale is essential for preparation of the working
electrode to extract optimal performance as lithium insertion
electrodes [37–39].

3.2. Structural characterization
The structural parameters of the single phase ramsdellite
LixTiO2 (x = 0.47) obtained from the Rietveld reﬁnement aresummarized in Table 1. The crystal structure was similar to theramsdellite Li0.5TiO2 reported by Akimoto et al. [9]. All the XRD
data were well reﬁned with the orthorhombic space group Pnma
with a = 0.96076 nm, b = 0.29514 nm, and c = 0.50276 nm. The
structure has four Ti, eight O, and 1.88 Li atoms per unit cell.
Distorted TiO6 octahedra form double columns along b axis bysharing their edges, which give rise to open framework withtunnels running along the b axis. Lithium ions are located in thetunnels, in which two Li sites at the same y coordinate are occupied
by 47% Li ions on average. Interestingly, as shown in Fig. 2c, longrange periodicity was ﬁrst observed along the b axis. The peak at
2.50 corresponds to 010 reﬂection when the lattice constant of theb axis becomes twelve times larger than that of the original unit
Table 1Reﬁned crystallographic data for LixTiO2 obtained by heating the amorphouslithium peroxo-polytitanate in 4.8% H2 at 1100  C.
Orthorhombic; Space group; Pnma (62) Z = 4
a = 0.9608 nm b = 0.2951 nm c = 0.5028 nm Crystallite size: 69.2 nm
Atom

Site

x

y

z

Occupancy

Li
Ti
O(1)
O(2)

4c
4c
4c
4c

0.417
0.141
0.274
-0.035

0.25
0.25
0.25
0.25

-0.061-0.024
0.691
0.186

0.47
1.0
1.0
1.0

Fig. 4. (a) ﬁrst three CV proﬁles of the ramsdellite Li0.47TiO2 at a scan rate of
0.2 mV s1, and (b) second CV proﬁles of the ramsdellite Li0.47TiO2 at different scanrates from 0.2 to 3.3 mV s1.

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751

3.3. Electrochemical properties
Fig. 4a shows the CV proﬁles of the ramsdellite Li0.47TiO2 at ascan rate of 0.20 mV s1. Two large cathodic peaks at 2.15 and 1.31 V
and two large anodic peaks at 1.48 and 2.26 V were observed
during the reversible lithium insertion and deinsertion process in
the range of 0.8–3.0 V (vs Li). This indicates that the insertion anddeinsertion proceed in two stages at higher and lower potential(stages H and L after this, respectively). In reported studies, the CV
proﬁles of the oxidized-form ramsdellite Li2Ti3O7 showed only an
extra shoulder peak on the principal 1.5 V peak, while the phases inthe Li1+xTi22xO4 system (x = 0–0.1) showed additional smaller
peaks in the range of 1.5–2 V [7]. The additional peaks at higher
potential seem to be peculiar to the reduced-form ramsdellitetitanates. Although the possibility of the presence of several Li
insertion sites was mentioned in the report, details on the two
peaks have not been clariﬁed so far.
Fig. 4b shows the CV proﬁles of the ramsdellite Li0.47TiO2 atdifferent scan rates. The peak current values were increased withincreasing the scan rate, suggesting diffusion-controlled insertionand deinsertion. The relationship between the peak current values
and the scan rates are described byIp¼ 2:69  105 n3=2 Acv1=2 D1=2 ;m(1)where Ip is the peak current, m is the mass of electrode, n is thenumber of transferred electrons, A is an electrode surface area perunit mass, c is the concentration of lithium in Li0.47TiO2, v is thescan rate, and D is the apparent diffusion coefﬁcient thatincorporates both Li diffusion in the bulk and Li transfer throughthe interface [40,41]. c is calculated as 0.0219 mol cm3 because
Li0.47TiO2 has a bulk density of 3.88 g cm3, a molar mass of
83.2 g mol1, and a Li composition of 0.47. As for Li0.47TiO2, onethird of the total electrode surface area should be used as theelectrode surface area A, because the diffusion occurs in the onedimensional channels along b axis and only (0 1 0) plane is effective
for diffusion. A is calculated as 7.46  104 cm2 g1 from the mean
crystallite diameter of 69.2 nm from the XRD pattern and the bulk
density. The apparent diffusion coefﬁcients, D, for the stages H and
L were determined from the slope of Ip/m vs v0.5 plot at 6.0  1015
and 8.0  1015 cm2 s1 for anodic diffusion, and 6.3  1015 and6.3  1015 cm2 s1 for cathodic diffusion, respectively.
Charge–discharge curves of the reduced-form ramsdellite
Li0.47TiO2 in the range of 0.8–3.0 V (vs Li) at 0.8 C rate (0.4Li/1 h)are shown in Fig. 5. Two-stage processes were observed in both of
the insertion and deinsertion curves: a sloping region at higher
potential (1.4–2.3 V) and a plateau region at lower potential(1.3–1.4 V). The capacities were 201–208 mAh g1 in the region of
1.0–2.5 V (vs Li) except for the ﬁrst charge process presumably

Fig. 5. Charge–discharge curves of the ramsdellite Li0.47TiO2 at a current density of
0.13 mA mg1 (0.8 C).

Fig. 6. The second charge–discharge curves of the reduced form—ramsdellite
Li0.47TiO2 at different current densities of 0.13, 0.26, and 0.65 mA mg1 (0.8, 1.6, and
4.0 C), along with that of the oxidized-form ramsdellite Li2Ti3O7 at a current densityof 0.10 mA mg1.

partly consumed for the formation of the SEI (solid electrolyte
interface) layer, and these values are comparable to the spinel
phase Li4Ti5O12. The second charge–discharge curves of the
reduced-form ramsdellite Li0.47TiO2 at rates of 0.8, 1.6, and 4.0 C
are shown in Fig. 6, along with that of the oxidized-form
ramsdellite Li2Ti3O7 at a rate of 1.0 C obtained by heating in Ar
at 1100  C for 0 h for comparison. With increasing the rate, thecapacities of the plateau region at lower potential were decreased
more rapidly than those of the sloping region at higher potential.
The sloping and plateau regions correspond with the stages H and
L, respectively, observed in the CV proﬁles, and the ratedependence of each region can be related to an accommodation
mechanism for Li ions. Furthermore, the capacities of the reducedform ramsdellite were increased with increasing the heating timefrom 0 to 1 h at 1100  C probably due to the improvement ofcrystallinity, while the intergrowth phase formed as a byproductby heating for 1.25 h or more brought about the reduction of thecapacities. This suggests that the rutile–ramsdellite intergrowth
type is unfavorable for lithium insertion and deinsertion, though
half of the channels are composed of the double columns. The
oxidized-form ramsdellite Li2Ti3O7 showed similar behavior tothose of the conventional ramsdellite Li2Ti3O7 reported in theearlier research, differently from those of the reduced-form
ramsdellite Li0.47TiO2, and the capacities were about 100–120 mAhg1 in the region of 1.0–3.0 V (vs Li), much smaller than those of the
reduced form. The two-stage curves were peculiar to the reduced
form. The plateau regions can be explained by assuming a twophase equilibrium in the reduced-form ramsdellite Li0.47TiO2.
When there are two phases, e.g., LiaTiO2–LibTiO2 and LibTiO2–
LicTiO2 (a < b < c), and an equilibrium is established between
Lix1TiO2 in the former phase and Lix2TiO2 in the latter phase atcertain values of x1 and x2 (a < x1 < b, b < x2 < c), the lithiumcontent change in the range from x1 to x2 occurs through phasetransitions between Lix1TiO2 and Lix2TiO2. In other words, thelithium insertion and deinsertion bring about the change in thefraction of each phase, not in the lithium content of each phase. In
this case, the electrode potential remains constant in spite of thelithium content change, because Lix1TiO2 and Lix2TiO2 have thesame chemical potential. Similar plateau behavior has beenreported for the ramsdellite TiO2, and a comparable mechanism
involving two two-phase equilibria has been proposed [7].
Considering the experimental fact that such plateau behavior
was not observed for the oxidized-form ramsdellite Li2Ti3O7, thetwo phase equilibrium is expected to be established when Ti is
deeply reduced and close to the trivalent state. It is highly desirable
to extend the plateau region capacity to apply it to the lithium ion
batteries, and we expect that this will be possible by investigating
precisely phase diagrams of the reduced-form ramsdellite anddesigning an oxide with optimum composition and valence.

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4. Conclusions
We developed a new synthesis method of the reduced-form
ramsdellite LixTiO2 as a single phase using an amorphous lithium
peroxo-polytitanate. We found that the lithium insertion anddeinsertion performance as anode materials of the lithium ion
batteries was much higher than the oxidized-form ramsdellite
Li2Ti3O7 and comparable to the spinel Li4Ti5O12. The capacity was
208 mAh g1 at 0.8 C in the region of 1.0–2.5 V (vs Li). The reducedform LixTiO2 showed peculiar two-stage processes in both of theinsertion and deinsertion curves: a sloping region at higher
potential and a plateau region at lower potential. It is highly
desirable to extend the plateau region at lower potential forpractical use, and the plateau region is presumably due to a twophase equilibrium. Our future challenge is to elucidate the twophases in equilibrium and to prepare a ramsdellite LixTiyO2z with
an optimum composition.
Acknowledgements
The authors are sincerely grateful to Prof. Yujiro Watanabe ofKanazawa Institute of Technology for assistance in ICP analysis.
This work was supported by a Grant-in-Aid for Scientiﬁc Research(C), No. 25410244, from the Japan Society for the Promotion of
Science (JSPS).
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