Flexible energy-storage devices have attracted growing attention with the fast development of
bendable electronic systems. Thus, the search for reliable electrodes with both high mechanical
flexibility and excellent electron and lithium-ion conductivity has become an urgent task. Carbon-
coated nanostructures of Li4Ti5O12(LTO) have important applications in high-performance lithium ion
batteries (LIBs). However, these materials still need to be mixed with a binder and carbon black and
pressed onto metal substrates or, alternatively, by be deposited onto a conductive substrate before
they are assembled into batteries, which makes the batteries less flexible and have a low energy
density. Herein, a simple and scalable process to fabricate LTO nanosheets with a N-doped carbon
coating is reported. This can be assembled into a film which can be used as a binder-free and
flexible electrode for LIBs that does not requrire any current collectors. Such a flexible electrode
has a long life. More significantly, it exhibits an excellent rate capability due to the thin carbon
coating and porous nanosheet structures, which produces a highly conductive pathway for electrons
and fast transport channels forlithium ions.
1. Introduction
Flexible electronics is an emerging and promising technology for the next-generation electronic
devices such as roll-up displays, smart electronics, and wearable devices. To meet the needs for the
future portable electronic industry, the development of flexible, lightweight and environmentally-
friendly energy storage devices, in which electrodes are key components, are required. Though
several kinds of flexible composite electrodes have been reported, it is still a great challenage
to fabricate highly flexible energy storage devices with high energy and power densities, and
excellent cyclic stability, while maintaining low cost. Searching for high performance flexible
electrodes is thus becoming one of the key issues.
Spinel Li4Ti5O12(LTO) has attracted significant attention in the past few years as a suitable
anode material for lithium ion batteries (LIBs) because of its outstanding atility to tolerate
abuse and its excellent cycle life. LTO exhibits an extremely flat charge/discharge plateau at
1.55 V vs. Li/Li+, which is above the potential range where most types of electrolytes or solvents
are reduced. It can accommodate up to threee lithium ions per molecule without volume change as a
zero-strain insertion material. These features make LTO a promising candidate for the anode of LIBs.
However, the main disadvantage that restricts the use of LTO is its low electronic conductivity
and moderate Li+ diffusion coefficient. To address the above issues, various nano/micro structured
LTO materials have been synthesized for use as high performance anodes. Among the nanostructures,
nanosheets have been evaluated in an attempt to achieve a shorter transport distance for both
lithium ions and electrons. However, the synthesis of ultrathin LTO nanosheets in quite challenging.
To the best of our knowledge, all avaiable LTO sheets show a thickness larger than 10 nm.
Another strategy for improving the electrochemical performance involves carbon coating the LTO
materials, which can improve the electron transfer through the interface of the electrode material
particles and suppress the interfacial reaction. Carbon-coated spinel LTO nanostructures have
attained more than 90% of the theoretical capacity and exhibited excellent rate capability. However,
these carbon-coated LTO nanostructures still need to be mixed with a binder and conductive additive
such as carbon black and pressed onto a metal substrate as a current collector or, alternatively,
deposited onto a conductive substrate before they are assembled into batteries, which makes the
batteries have poor flexibility and a low energy density. Therefore, it is expected that advancement
in LIB technology can be achieved by combining both a flexible design approach and carbon-coated
nanostructures, leading to a superior rate performance. By using flexible current collectors such as
carbon nanotubes and fibers, flexible LTO composite electrodes have been obtained. However only
moderate charge/discharge rates have been achieved. Recently, we have developed a flexible
LTO/graphene foam electrode which simultaneously provides high charge/discharge rates, but its
energy density needs to be increased.
In this work, we report a simple and scalable process to fabricate LTO nanosheets with N-doped
carbon coating (denoted C-LTO nanosheets). Using this material, we developed a flexible film which
can be directly used as a flexible electrode for LIBs. This flexible electrode shows an excellent
rate capabliity and a significantly improved cycling performance.
2. Results and discussion
2.1 Fabrication and Characterization of C-LTO Nanosheets
The schematic shown in Figure 1 illustrates the synthesis of C-LTO nanosheets. Hydrous lithium
titanate nanosheets were formed by the chemical lithiation of H0.68Ti1.83O4 nanosheets in a LiOH
solution. An annealing treatment in argon is used to convert the precursor into LTO nanosheets with
N-doped carbon coating. Tetrabutylamonium hydroxide (TBAOH) was used as a carbon and nitrogen source
to form a thin N-doped carbon layer on the surface of the LTO nanosheets. According to the
thermogravimetric(TG) result (Figure S1, Supporting Information), the percentage of pyrolytic carbon
in C-LTO nanosheets was 4 wt%. The morphology and microsturcture of the C-LTO nanosheets were
investigated using scanning electron microcopy (SEM) and transmission electron microcopy (TEM), as
shown in Figure 2. Representative nanosheets appear size of tens of micrometers(Figure 2a). The
low-magnification TEM image in Figure 2b shows that the LTO sheets are quite similar in appearance
to graphene. Inthis image, the light regions sugggest planar or curled thin sheets lying on the
substrate, with an edge length more than 400 nm. Figure 2c reveals that the LTO nanosheets really
have a porous structure composed of small nanoparticles instead of single-crystal. Between the
small crystalls, wormhole-like pores were formed and provide a large volume for electrolyte storage
and ensure Li+ diffusion in channels across the LTO nanosheet film anode. This provides remarkable
rate capability and cycling performance. From the HRTEM image in Figure 2d, it can be observed that
a thin uniform amorphous carbon layer is formed on the surface. A lattice spacing of 0.48 nm was
observed, in good agreement with the d-spacing of 0.484 nm associated with the (111) direction of
spinel LTO.
Atomic force microcopy (AFM) analyses(Figure 2e,f) display the 2D features of C-LTO nanosheets with
a thickness of ≈2.5 nm. X-ray diffraction (XRD) paterns (Figure 3a) reveal that the H0.68Ti1.83O4
nanosheets completely convert to spinel LTO after chemical lithiation and postannealing . The Raman
spectrum of the C-LTO nanosheets shows two bands at 1350 and 1590 cm-1, which correspond to the D
and G bands of carbon, respectively (Figure S2, Supporting Information). The specific surface area
of the C-LTO nanosheets is 256 m2g-1 (Figure 3b). The high surface area is the result of not only
the nanosheet structure but also the presence of inner nanopores. A narrow peak in pore size
distribution indicates that the main pore size (inset in Figure 3b) in the C-LTO nanosheets is 4 nm,
which confirms a uniform pore size distribution. A typical high-resolution XPS spectrum of N 1s for
the C-LTO nanosheets is shown in Figure 3c. Peaks at binding energies of 398.5 and 400.5 eV can be
attributed to pyridinic-N (C-N bond) and graphitic-N (C=N bond), respectivley. Furthermore, an
additional peak at ca.396 eV was observed in N 1s spectrum, which can be assigned to atomic β-N,
nanely the substitution of O with N. High-angle annular dark-field STEM elemental mapping in Figure
3d confirms the homogeneous districution of C and N on/inside the nanosheets.
2.2 Fabrication and Characterization of C-LTO Nanosheets Electrode
Self-standing C-LTO nanosheets films were prepared by a vacuum filtration process (Figure S3,
Supporting Information). The film electrodes have a robust mechanical stability because of the
flexbile nanosheet building blocks, which is similar to a piece of graphene paper. Notably, the
electrode is flexible (Figure 4a), and can be bent into arbitrary shapes. SEM images (Figure 4b-d)
reveal that it has a uniform thickness (≈70μm) throughout its cross-section, and the LTO nanosheets
align parallel to form a layered structure. The porous nanosheet structure of this electrode
provides a large specfic volume for the fast transfer of Li+. The electrical conductivity is
significantly improved by the thin N-doped carbon coating on the surface. In the N-doped carbon,
N-doping can enhance the reactivity and electronic conductivity. Moreover, this unique electrode is
a monolithic block. All of these factors contribute to the effective ambipolar diffusion of Li+ and
electrons into/out of LTO in the C-LTO nanosheet film electrode enabling remarkable rate capability
and cycling performance.
2.3 Electrochemical Performance of C-LTO Nanosheets Electrode
Electrochemical studies of the electrode were conducted using two-electrode cells with lithium metal
as a counter electrode. The C-LTO nanosheet film electrode features a gravimetric capacity of 170
mAh g-1 at a rate of 1 C and can deliver up to 72% of the theoretical capacity at high rate of 100 C
(corresponding to 36 s charge/discharge) without deterioration over 100 cycles (Figure 5a).
Moreover, the discharge curve of the electrode at high rates (up to 100 C) showw a long flat
potenial plateau, which ensures a constant power output, a very important factor for LIBs. Although
there is only 40 wt% N-doped carbon in the electrode and no ancillary materials, the rate
performance of the electrode is much better than that of nanoporous LTO, carbon-coated LTO,
TiN-coated LTO, rutile-TiO2-LTO, LTO-graphene and LTO-carbon nanotube composites. The performance of
the C-LTO nanosheet thin film electrode could be attributed to the following two factors. First, the
porous nanosheet structure possesses a large specific volume that facilitates the fast transfer of
Li+. Second, the electronic conductivity of the electrode is significantly improved by the thin
N-doped carbon layer on the nanosheet surface, which ensures all the nanosheets have an ultrafast
rate of electrochemical reaction.
For comparison, we also performed electrochemical experiments for LTO nanosheet film electrodes
without N-doped carbon layer (Figure S4, Supporting Information) and nano-LTO (LTO particle size <
20 nm, Figure S5, Supporting Information). Figure 5a shows the charge/discharge voltage profiles of
the C-LTO nanosheet, LTO nanosheet film electrodes and nano-LTO cycled at various current rates from
1 to 100 C. At a low discharge/charge current rate of <10 C, the capacities of the C-LTO nanosheet
and LTO nanosheet film electrodes are comparable, which is in good agreement with the very recent
reports that carbon-free LTO electrodes have shown excellent electrochemical performance without any
conductive additives. However, the C-LTO nanosheet film electrode delivers a discharge capacity of
131 mAh g-1 at a high rate of 100 C, which is 77% of the specific capacity at 1 C (Figure 6a). In
contrat, the discharge capacity of the LTO nanosheet film electrode at the current rate of 100 C is
32% ofthe specific capacity at 1 C. The higher rate capability of the C-LTO nanosheet film electrode
demonstrates that the N-doped carbon layer improves the high rate performance of LTO at discharge/
charge current rates of > 10 C. It is remarkable that the LTO nanosheet film electrode without a
carbon layer exhibits a higher rate performance than that of nano-LTO. Another excellent property of
the C-LTO nanosheet film electrode is its superior cycling performance with very slight capacity
decay. After 100 discharge/charge cycles at 10 C, the C-lto nanosheet film electrode delivers a
specific capacity as high as 152 mAh g-1 (Figure 5b). Note that the capacity decreases less than 1%
of the initial value after 100 cycles, demonstrating the excellent electrochemical stability of this
self-standing flexible electrode. TEM images of the C-LTO nanosheet film electrode after 100 charge/
discharge cycles at 10 C show that the nanosheet structure and nanoporous internal structure
remained intact (Figure S6, Supporting Information).
The thin N-doped carbon layer on the nanosheet surface contributes to the good conductivity of the
C-LTO nanosheets, which directly correlates to the excellent electrochemical performance and is
confirmed by electrochemical impedance spectroscopy measurements. These cells were measured after
the first cycle at a current rate of 1 C in the charged state (2.5 V vs Li+/Li)(Figure S7,
Supporting Information). The elements in the equivalent circuit include ohmic resistance of the
electrolyte and cell components (Re), and charge-transfer resistance at the interface between the
electrode and electrolyte (Rct). The parameter Rct for the C-LTO nanosheets electrode (98.6Ω) is
much lower than that of the LTO nanosheets (155.4Ω) and nano-LTO electrode (209.5Ω), which means
that the C-LTO nanosheet electrode has faster charge transfer than the others. Figure 6b shows the
polarization of ΔE versus rate plots of the C-LTO nanosheet, LTO nanosheet film electrodes and the
nano-LTO electrode. The value of ΔE is defined as the difference between the potentials of the
charge and discharge plateaus. We can observe an increase of ΔE as a function of C-rate. This
behavior is certainly due to the poor ionic conductivity of the electrode materials which limits
the diffusion of the lithium ions. Both plots show dependence on the discharge/charge rates, while
the ΔE of the C-LTO nanosheet film electrode is much smaller than that of the LTO nanosheet film
electrode and nano-LTO electrode, suggesting a better rate capability.
3. Conclusions
LTO nanosheets were fabricated by a simple and scalable process. They consist of a nanoporous
interior and a N-doped carbon-coated surface. The C-LTO nanosheets are assembled by vacuum
filtration into yield mechanically robust self-standing films of ≈70μm thickness. When the flexible
LTO film was tested as a sefl-standing LIB anode, it showed an excellent rate performance and a good
cycling stability derived from the thin uniform carbon layer and the porous structure, producing a
highly conductive pathway for electrons and fast transport channels for lithium ions. The
self-standing LTO electrodes without any ancillary materials could open up new opportunities for LTO
to power flexible electronic devices.
4. Experimental Section
4.1 Sample Preparation
Synthesis of C-LTO Nanosheets: A titanate precursor of H0.68Ti1.83O4 was prepared according to a
procudure previously reported. H0.68Ti1.83O4 (1.2g) was dispersed in TBAOH solution (300 mL, 0.2
mol L-1) and was shaken for 7 days at room temperature until it is exfoliated to a white colloidal
suspension of H0.68Ti1.83O4 nanosheets. Lithium hydroxide (LiOH·H2O, 63 mg) was added to the above
solution (50 mL) at ambient temperature under magnetic stirring (10 min) and the mixture was freeze-
dried to form a white powder. Finally, the white powder was heated at 600 ℃ for 6 h under an argon
atmosphere. After heat treatment, the color of the initial white powder changed to dark gray. Here,
TBAOH was used as the carbon and nitrogen source to from thin N-doped carbon coating on the surface
of the LTO nanosheets.
Synthesis of LTO Flexible C-LTO Nanosheet Film Electrode: The C-LTO nanosheets were dispersed in N-
methylpyrolidone (NMP) at a concentration of 25 mg L-1, and were sonicated for 10 h to enhance the
dispersion. A self-standing film was synthesized by vacuum-filtration on a Whatman polycarbonate
track-etched membrane (0.2 mm in pore diameter), which results in aninterwoven and mechanically
robust LTO nanosheet film attached to the membrane. Then a self-standing electrode was obtained by
removing the air-dried LTO nanosheet film and further dried for 12 h at 70 ℃ in air, and then cut
for punched to the desired size. The typical thickness of the C-LTO nanosheet film electrode was ≈
70 μm.
Synthesis of Flexible LTO Nanosheet Film Electrode: The procedure was the same as that for the C-LTO
nanosheet film electrode except for using LTO nanosheets.
Synthesis of LTO Nanoparticles (Nano-LTO): Nano-LTO was prepared according to a procedure previously
reported.
4.2 Characterization
XRD patterns of samples were recorded on a Rigaku diffractometer using Cu Kα irradiation. SEM, TEM
and AFM images were obtained on a Nova NanoSEM 430 at 15 kV, Tecnai F20 at 200 kV and Veeco
MultiMode/NanoScope Ⅲa, respectively. High-angle annular dark-field STEM mapping was carried out
using a Tecnai F30 TEM with an accelerating voltage of 300 kV. TG measurements were carried out on
a Netzsch-STA 449C from 30 to 1000 ℃ at a heating rate of 10 ℃ min-1 in air. The specific surface
areas were determined by a Micromeritics ASAP 2010M, and pore size distribution was determined from
the desorption branch of the isotherm using the DFT model. Raman spectra were collected using a
632.8 nm laser with JY HR800.
4.3 Electrochemical Measurements
For electrochemical measurements of the self-standing C-LTO nanosheet and LTO nanosheet film
electrodes, coin cells were used. In both cases, a lithium metal foil was used as a counter
electrode,1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 vol) as electrolyte, and
a polypropylene film (Celgard 2400) as a separator. The electrochemical performance was measured at
different rates at room temperature. All tests on the self-standing C-LTO nanosheet and LTO
nanosheet film electrodes were performed without current collectors, carbon black conducting
additives and binders. A reference nano-LTO electrode was prepared by mixing the nano-LTO, carbon
black (Super-P) and poly(vinyl difluoride) at a weight ratio of 80:10:10. Cells were assembled in an
argon-filled glove-box with oxygen and water contents below 1 and 0.1 ppm, respectively. All the
capacities and C-rate currents were calculated based on the LTO active materials (1C corresponding
to 175 mAh g-1).
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This work was supported by Ministry of Science and Technology of China (No. 2011CB932604) and
National Science Foundation of China (Nos.50921004 and 51172239).