A three-dimensional dandelion-like Li4Ti5O12@graphene microshpere electrode is designed by using
a simple and scalable solution fabricaiton process. The graphene nanosheets are incorporated into
the porous dandelion-like Li4Ti5O12 microshperes homogenously, which provide a highly conductive
network for electron transportation. When tested as an anode for Li-ion batteries, the dandeion-
like Li4Ti5O12@graphene composite whit 3 wt% graphene exhibits excellent rate capabilities and
superior cycle life between 0.01and 3.0 V.The capacities of Li4Ti5O12@graphene (3 wt%) reach 206
mAhg-1 after 500 cycles between 0.01 and 3.0 V and 166 mAhg-1 after 100 cycles between 0.7 and 3.0 V
at a current density of 0.12 Ag-1, respectively. In addition, Li4Ti5O12-based anode materials at
lower voltage can offer a higher cell voltage and discharge capacity for lithium-ion batteries.
Hence, it is significant to study the electrochemical behaviors of the Li4Ti5O12-based anode in a
wide voltage range of 0.01-3.0 V.This facile and scalable method for Li4Ti5O12@graphene composities
represents an effective strategy to develop advanced electrochemical energy storge systems with long
cycle life and high rate performance.
1.Introduction
Lithium ion batteries (LIBs) have gained great commercial success in the field of portable
electronic devices due to their outstanding properties such as high energy density and environmental
friendliness. With the growing demand for stationary off-peak energy systems and modern electric
vehicles, the development of LIBs with high energy and power density, superior security, and long
cycling performance is highly desirable. However, the present commercial graphite anode materials
of lithium ion batteries cannot simultaneously meet all of the above requirements due to their poor
rate capabilities resulting from their low Li+ diffusion coefficients and serious safety issues
induced by the solid electrolyte interphase (SEI) film.
Spinel Li4Ti5O12 is a promising alternative anode material for high pwoer LIBs due to its several
important features in spite of its lower theoretical capacity (i) excellent reversibility due to
its zero volume change during charge-discharge cycles; (ii) a high and stable operating potential
plateau (1.55V vs. Li/Li+), which circumvents the solid electrolyte interphase (SEI) formation and
prohibits lithium dendrite deposition on the surface of the electrode; (iii) excellent lithium ion
mobility, which offers the potntial for high power applications. However, the poor electrical
conductivity and the sluggish lithium ion diffusion coefficient of Li4Ti5O12 grealty limit its rate
capability. To address these issues, two common and useful strategies are adopted. One is design of
nanostructured Li4Ti5O12 to decrease the lithium diffusion pathway. The other strategy is the
formation of a conductive coating (e.g. carbon, surface nitridation, phosphates, metals, etc.) on
the Li4Ti5O12 surface in order to enhance the electronic conductivity between the particles and
the current collector. For instances, well-aligned Li4Ti5O12 nanowire and nanosheet arrays grown
directly on conductive Ti foil have been reported for LIBs with improved rate capability. Jung et
al. reported the carbon coating of Li4Ti5O12 as the anode for LIBs using a simple sol-gel method
with pitch as the chelating agent and carbon resource. Very recently, Wang et al. reported the
utility of rutile-TiO2 as a carbon-free nanocoating to improve the kinetics of Li4Ti5O12 toward fast
lithium insertion/extraction.
Graphene, which has exceptional electrical, mechanical, optical, and surface properties, is widely
utilized to prepare various hybrid materials. As such, graphene may represent a promising conductive
additive to improve the rate capabilities of Li4Ti5O12 composites. For example, Shi et al. developed
a simple strategy to prepare a hybrid of Li4Ti5O12 nanoparticles well dispersed on electrical
conductive graphene nanosheets as an anode material for high rate LIBs. Zhu et al. used graphene as
a conductive additive to fabricate graphene-embedded Li4Ti5O12 nanofibers, which showed grealty
improved surface conductivity as well as rate capability. However, the integration of graphene and
Li4Ti5O12 with a designed morphology and architecture is still a great challenge.
herein, we designed a novel three-dimensional graphene-wrapped Li4Ti5O12 dandelion-like microsphere
electrode by using a facile and scalable solution route for the first time. This hybrid structure
design process has the following advantages:first, the 3D hollow Li4Ti5O12 dandelion-like
microspheres assimbled by a hollow core and a nanosheet shell can provide an enhanced active surface
area and reduced transport path lengths of lithium ions and electrons. Second, the hollow voids are
beneficial to electrolyte infiltration and offer an increased electroactive interface for transfer
of Li+ during charging and discharging. Third, the integration of the graphene network provides a
fast and conductive electron transport path. When used as an anode material for Li-ion batteries,
the Li4Ti5O12@graphene (3 wt%) composite displays excellent rate performance (an initial capability
of 206 mAhg -1 at 0.12 ag-1 in the voltage range of 0.01-3.0 V and 0.7-3.0 V, respectively) and high
cyclic stability (a capability of 206 mAhg-1 after 500 cycles between 0.01 and 3.0 V and 166 mAhg-1
after 100 cycles between 0.7 and 3.0 V at 0.12 Ag-1, respectivley).Whereas a large number of studies
focus on the voltage range between 0.6 and 3.0 V, no investigation has been reported on the rate
cycling performance of graphene-wrapped Li4Ti5O12 discharged to 0.01 V and synthesized by using the
hydrothermal method. A careful investigation has been carried out to give new insights into the
structural properties, rate performance, over-discharge performance (down to 0.01 V) and the
modifying effect on the conductivity of the material. It is also important to study the
electrochemical behaviors of anode materials at lower voltage because the capacity of anode
materials at lower voltage can offer a higher cell voltage and discharge cpacity for lithium-ion
batteries.