Ultra-small nanoparticles of MgTi2O5 embedded in carbon rods with superior rate performance for sodium ion batteries

Article abstract of DOI:10.1039/c4cc10203g

Confinement of ultra-small MgTi₂O₅ nanoparticles in carbon is demonstrated to be an efficient method for fabricating long cycle-life anode material for sodium ion batteries. Superior rate and excellent cyclic capabilities as well as high Coulombic efficiency of the MgTi₂O₅-C nanocomposite, obtained from pyrolysis of a single molecule precursor, are shown. This journal is

Full text of DOI:10.1039/c4cc10203g

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Ultra-small nanoparticles of MgTi2O5 embedded
in carbon rods with superior rate performance
for sodium ion batteries†
Cite this:DOI: 10.1039/c4cc10203g
Received 21st December 2014,
Accepted 20th January 2015
Fangxi Xie,a Yuanfu Deng,*bc Ye Xie,b Hongjie Xud and Guohua Chen*acd
DOI: 10.1039/c4cc10203g
www.rsc.org/chemcomm
Confinement of ultra-small MgTi2O5 nanoparticles in carbon is demon- long-life batteries owing to the large volume change during sodium
strated to be an efficient method for fabricating long cycle-life anode insertion and extraction. Hard carbon shows a high specific capacity
material for sodium ion batteries. Superior rate and excellent cyclic and good cycling performance.20,21 However, most of the capacity is
capabilities as well as high Coulombic efficiency of the MgTi2O5–C located at the voltage below 0.1 V, which is near zero versus Na+/Na,
nanocomposite, obtained from pyrolysis of a single molecule precursor, causing potential safety concerns, especially at a fast charging rate or
are shown.
overcharging.22,23 Besides the materials mentioned above, Ti-based
oxides have attracted much attention because of their low price,
environmental friendliness, good cycling performance and, most
importantly, high safety, which is derived from the high sodium
insertion voltage, for example, 0.91 V for the spinel Li4Ti5O12.23
The spinel Li4Ti5O12, a ‘‘zero-strain’’ anode material for the LIBs,
was recently investigated as anode material for SIBs.23,24 Nanorod
anatase TiO2 with the carbon layers also shows excellent cyclability
and high-rate capability in the insertion and extraction process
of Na+.25 Recent studies suggested that the electrochemical per-
formance of the Ti-based oxides can be optimized by shortening
the diffusion path of sodium ions and improvement of the
conductivity.25–27 It is noted that reducing the particle size of
anode materials to the nanoscale (especially o 10 nm) can endure
much higher strain and effectively mitigate pulverization.25 However,
nano-sized particles tend to aggregate during the cycles, resulting in
a fast capacity decay. Correspondingly, integrating nanomaterial
with a conductive matrix such as carbon can decrease the opportu-
nities for particle aggregation and accommodate volume expansion
for keeping the integrity of the electrode.28 From the points of view
of high specific capacity and high conductivity of hard carbon
material and the safety of the Ti-based oxides, novel Ti-based
oxides–carbon nanocomposites with ultra-small particle sizes should
be desirable anode materials for SIBs.
Lithium ion batteries (LIBs) are considered one of the most mature
candidates for the electrification of transportation and renewable
energy integration due to their long lifetime, high energy density
and low environmental impact.1,2 However, there is increasing
attention on the cost and the limited lithium supply for large-
scale energy storage applications. Therefore, sodium ion batteries
(SIBs), new low-cost and reliable electrochemical energy storage
devices, have attracted significant attention because of the abun-
dant supply of sodium.3–6 It is known that the properties of the SIBs
are mostly determined by the performance of the active electrode
materials. Thus, investigations on the novel materials with good
electrochemical performance are very significant and in urgent
demand. Compared with the fast development of the cathode
materials,7–9 only some of the Na-storage anode materials have
reported to demonstrate certain redox capacity and adequate
duration.5,6 Up to now, amorphous phosphorus, metal phosphides,
hard carbon, and alloys have been successfully tested as anode
materials for SIBs.10–19 Among these materials, amorphous P–C
composites, metal phosphides and alloys exhibit high specific
capacities. These composites however would not be suitable for
a Centre for Green Products and Processing Technologies, Fok Ying Tung Graduate
School, The Hong Kong University of Science and Technology, Clear Water Bay,
Hong Kong, P. R. China
Herein, a novel hybrid MgTi2O5–C (MTO–C) nanocomposite is
reported, for the first time, as a superior anode material for SIBs.
The MgTi2O5–C nanocomposite in the nanorod shape, consisting
of ultra-small MgTi2O5 nanoparticles (NPs) embedded by evenly
distributed carbon, was firstly prepared from an in situ carboniza-
tion process of a cheap single molecule precursor with unique
morphology. Such MgTi2O5–C nanocomposites with porous struc-
ture and ultra-small particle size are designed with four specific
b The Key Laboratory of Fuel Cell Technology of Guangdong Province, School of
Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou, 510640, China. E-mail: chyfdeng@scut.edu.cn
c Centre for Green Products and Processing Technologies, Guangzhou HKUST Fok
Ying Tung Research Institute, Guangzhou, 511458, China
d Department of Chemical and Biomolecular Engineering, The Hong Kong University
of Science and Technology, Clear Water Bay, Hong Kong, P. R. China.
E-mail: kechengh@ust.hk
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc10203g aims in mind: (1) enhancement of the conductivity of MgTi2O5 for
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facilitating good transport of electrons from the poorly conducting D and G bands of amorphous carbon, respectively.32 26.0 wt% of
MgTi2O5; (2) utilization of the high specific capacity of the hard carbon in the MTO-Ar sample was determined from TG analysis
carbon to increase the specific capacity of the composite; (as shown in Fig. S3, ESI†).
(3) decrease in the capacity over the range of 0–0.1 V to improve
The morphology of the as-prepared MgTi2O5–C composite was
the safety of the anode material; and (4) preservation of fast characterized using scanning electron microscopy (SEM) and
transport of sodium ions by shortening their diffusion path and transmission electron microscopy (TEM) images. As shown in
prevention of the aggregation of MgTi2O5 nanoparticles. The facile, Fig. 2(A), the shape of the composite is rod-like with smooth
simple and cost-effective route using a single source precursor surface and various diameters from several micrometers to tens
could be extended to the preparation of other high performance of micrometers, similar to the shape of the precursor (Fig. S4, ESI†).
electrode materials.
The high resolution SEM image (Fig. 2(B)) shows that the MTO-Ar
Two steps are involved in the typical preparation of the sample is of woody appearance with some ultra-small nanoparticles
MgTi2O5–C composite: the PVP-assisted (PVP = Polyvinylpyrro- attached to the surface. The MgTi2O5 NPs (black dots) with a
lidone) crystallization process of a single molecule precursor uniform particle size were homogenously embedded in the carbon
(NH4)2Mg(H2O)6[Ti(C6H6O7)3]2Á6H2O and the calcination of the framework (Fig. 2(C)). The typical size of MgTi2O5 particles is
precursor.29 The detailed procedure can be found in the around 5 nm, which could also be clearly seen from the high
experimental section. The samples obtained from pyrolysis of resolution TEM (HRTEM) image (Fig. 2(D)). The lattice fringes are
the precursor at 600 1C in Ar and air are named MTO-Ar or visible (Fig. 2(D)), with a d-spacing of 0.49 nm, corresponding to the
MTO-Air, respectively.
(200) plane of MgTi2O5. This is also direct proof of the existence
Fig. 1 shows the X-ray diffraction (XRD) patterns of the of MgTi2O5 nanoparticles in the MTO-Ar sample. The energy
as-prepared MTO-Ar, MTO-Air, and the standard MgTi2O5 (JCPDS dispersive spectrometer (EDS) mapping (Fig. 2(E)) of the selection
Card No. 35-0792). All of the peaks of MTO-Air can be readily region (Fig. S5, ESI†) of the as-prepared sample demonstrates that
indexed to crystalline MgTi2O5. However, there are no obvious MgTi2O5 NPs are highly dispersed in the carbon matrix.
peaks found in the XRD pattern of the MTO-Ar sample, which may
The unique morphology and structure of the as-prepared
be attributed to the ultra-small size of MgTi2O5 particles obtained. MgTi2O5–C composite motivate us to further investigate its electro-
And the detailed analysis of XRD (Fig. S1, ESI†) revealed the chemical performance. For comparison, the pure MgTi2O5 (MTO-
existence of crystalline MgTi2O5. To determine whether any impu- Air) obtained from the calcination of the precursor in air at 600 1C
rities, which cannot be detected by XRD, are present in the with a particle size of B30 nm (Fig. S6, ESI†) was also investigated.
as-prepared MTO-Ar sample, XRD data were collected on the Fig. 3(A) shows the typical cyclic voltammograms (CVs) of the
annealed MTO-Ar (500 1C in air for 1 h). If there is any impurity, MgTi2O5–C electrode for the initial three cycles at a scan rate of
then the impurity will crystallize during annealing and then be
detected by XRD. As shown in Fig. 1, no obvious impurity peaks of
TiO2, MgTiO3 or MgO appeared on the annealed sample with all
the peaks matching with the standard MgTi2O5, suggesting that the
as-prepared MTO-Ar sample has no impurity except for amorphous
carbon. A similar method for the characterization of ultra-small
Li4Ti5O12 nanoparticles has been reported previously.30 The
presence of pure MgTi2O5 and amorphous carbon in the MTO-Ar
sample is also supported by the Raman spectrum of the MTO-Ar
sample (Fig. S2a, ESI†), in which the peaks between 100 to 1000 cmÀ1
are in agreement with the Raman signals of pure MgTi2O5.31
The two peaks at 1335 and 1580 cmÀ1 can be assigned to the typical
Fig. 2 (A) Low Magnification SEM image of MTO-Ar; (B) high magnifica-
tion SEM image of MTO-Ar; (C) TEM image of MTO-Ar; (D) HRTEM image
of MTO-Ar and (E) the elemental mapping of the transmission electron
microscope image of MTO-Ar.
Fig. 1 The representative X-ray diffraction (XRD) patterns of different materials.
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over the first few cycles, indicating some capacity fading during the
initial cycling. Fig. 3(B) shows the representative discharge–charge
profiles of the MgTi2O5–C electrode for the 1st, 2nd and 3rd cycles
at a current density of 20 mA gÀ1 between the cut-off potentials of
0.01 to 2.7 V. All the specific capacity reported here is calculated on
the basis of the total mass of the MgTi2O5–C composite. The first
discharge and charge capacities of MgTi2O5–C are 651.5 and
251.5 mA h gÀ1, respectively, corresponding to a Coulombic efficiency
(CE) of 38.6%, which is similar to the previous reported literature on
hard carbon33,37 and carbon-coated TiO2 anode materials.25 The low
initial CE can be mainly attributed to the formation of SEI film by
reduction of the electrolyte on the surface of material33,37 and/or
irreversible sodium insertion into special sites35 during the first
discharge process. This low initial CE might cause the reduction of
the energy density of a full sodium ion battery.38
It is noted that the specific charge capacity below 2.0 V, the
practically useful part of the full cell, comprises B85% of the whole
de-insertion capacity. In addition, the specific discharge capacity
below 0.1 V is only B10% of the whole insertion capacity, which is
much smaller than that of the hard carbon in the reported
literature.21,33,37 This is important for the safety of the practical
application in SIBs. Fig. 3(C) shows the rate performance of MTO-Ar
and MTO-Air electrodes cycled at various current densities from
20 to 2000 mA gÀ1. The corresponding discharge–charge curves of
MTO-Ar electrode are given in Fig. 3(D). It is clearly seen that the
MTO-Ar electrode exhibits a much better rate performance than that
of the MTO-Air electrode. MTO-Ar displays a respective specific
capacity of 200, 180, 165, 146, 128, 109 and 86 mA h gÀ1 at 20,
50, 100, 200, 500, 1000 and 2000 mA gÀ1. However, only a small
capacity of 25 mA h gÀ1 was obtained from the MTO-Air sample at
Fig. 3 (A) Cyclic voltammetry curves of MTO-Ar for SIBs; (B) initial cycle
curves of MTO-Ar for SIBs; (C) the comparison of specific capacities of
MTO-Ar and MTO-Air under different current densities for SIBs; (D) the
typical cycle curves of MTO-Ar under different current densities for SIBs;
(E) the long cycle performance of MTO-Ar for SIBs under selected current
densities and (F) comparison of the Nyquist plots of MTO-Ar and MTO-Air
electrodes for SIBs.
0.1 mV sÀ1 between 0 and 3.0 V. During the first cathodic sweep, 50 mA gÀ1. It should be pointed out that the rate capability obtained
there are three main reduction peak regions located at around 1.1, from the MTO-Ar sample can be comparable to the carbon-coated
0.8 to 0.2 and 0.2 to 0.01 V, respectively. For the reduction peak at anatase,25 a novel anode material for SIBs, and the state-of-the-art
around 1.1 V, it can be assigned to the reaction between the Na ion hard carbon-based anode materials.21,32,35 In addition, the rate
and the surface functional group of carbon.33 The broad reduction performance of the MTO-Ar is much better than the carbon anode
peak at 0.8 to 0.2 V is mainly ascribed to the decomposition of the obtained from the pyrolysis of (NH4)3Hcit, although the carbon
electrolyte to form solid-electrolyte interphase (SEI) film and the anode has the same specific capacity of MTO-Ar at low current
reduction peak of Ti4+/3+ 34,35
which could also be found in the CVs density (Fig. S10, ESI†). The superior rate capability of MTO-Ar
,
(Fig. S7, ESI†) of the MTO-Air and the other two MgTi2O5–C should be related to the beneficial combination of the high electric
samples (MTO-Ar-PC-2h and MTO-Ar-PC-12h). The two samples conductivity offered by the carbon matrix and the short diffusion
with 1.0 and 3.0% carbon content (Fig. S8, ESI†) and ultra-small path for both electrons and ions provided by the ultra-small MgTi2O5
MgTi2O5 nanoparticles (Fig. S9, ESI†) were respectively obtained particles. Besides, the specific capacity resumes to 200 mA h gÀ1
from post-calcination of MTO-Ar under Ar with 5% O2 for 2 and when the discharge–charge current density is lowered to 20 mA gÀ1
,
12 h. The initial abrupt mass reduction of these samples in the TGA indicating that the unique structure of the MgTi2O5–C composite
curves at around 100–250 1C comes from the adsorbed water and could preserve the integrity of the electrode and thus is tolerant to
gases, which agrees with the endothermal peaks of MTO-Ar in this the varied charge and discharge currents, which is another critical
area and the difference in the mass amount might come from property for practical power applications in SIBs. To evaluate the
the different amount of adsorbed water and gases.36 The sharp cycling performance of this novel hybrid material as the anode
reduction peaks appearing in the low potential region from 0.01 to material for SIBs, the electrode was cycled for more than 1000 cycles
0.2 V can be attributed to the insertion of Na ions in carbon.32 under different current densities. Fig. 3(E) displays the cyclability
During the anodic sweep, two main oxidation peak regions located of the MTO-Ar electrode after the tests of the rate performance in the
at 0.01 to 0.2 and 0.2 to 1.2 V are observed, which should be first 80 cycles, then 1000 mA gÀ1 for the subsequent 500 cycles,
attributed to the removal of Na ions from the carbon and MgTi2O5, 2000 mA gÀ1 for another 500 cycles and 20 mA gÀ1 for the final 10
respectively.25,33,34 However, the absence of these peaks in the CV cycles. Importantly, this novel hybrid structure of MgTi2O5–C shows
curves of MTO-Air implies the large irreversible capacity of the a very impressive cycling performance. When cycled at 1000 mA gÀ1
initial cycle of MTO-Air. The intensity of the CV peaks decreases the MTO-Ar exhibits an initial capacity of 113 mA h gÀ1, with high
,
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capacity retention of 92% after even 500 cycles. When cycled at Notes and references
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