Single-Crystalline LiFePO
4
Nanosheets for High-Rate Li-Ion Batteries
Yu Zhao,
†
Lele Peng,
†
Borui Liu, and Guihua Yu*
Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin,
Texas 78712, United States
*
S
Supporting Information
ABSTRACT: The lithiation/delithiation in LiFePO
4
is highly anisotropic
with lithium-ion diffusion being mainly confined to channels along the b-
axis. Controlling the orientation of LiFePO
4
crystals therefore plays an
important role for efficient mass transport within this material. We report
here the preparation of single crystalline LiFePO
4
nanosheets with a large
percentage of highly oriented {010} facets, which provide the highest pore
density for lithium-ion insertion/extraction. The LiFePO
4
nanosheets
show a high specific capacity at low charge/discharge rates and retain
significant capacities at high C-rates, which may benefit the development
of lithium batteries with both favorable energy and power density.
KEYWORDS: Lithium iron phosphate, nanosheets, batteries, crystal orientation, energy storage
T
he olivine orthophosphate lithium iron(II) phosphate
(LiFePO
4
) is a promising cathode material for use in
lithium-ion batteries owing to its high operating voltage (∼3.4
V vs Li/Li
+
), large theoretical capacity (∼170 mA h g
−1
), and
thermal stability, as well as being inexpensive, nontoxic, and
environmentally benign.
1
The low electronic conductivity and
one-dimensional (1D) diffusion channel for lithium ions are
two major obstacles to achieve high gravimetric/volumetric
power density of this material.
2,3
The former obstacle is
extensively studied with success by modification of the interface
of LiFePO
4
particles with electrolyte through coating/doping to
lower the charge transfer resistance.
4−6
For the latter obstacle,
both the size and orientation control of LiFePO
4
is necessary
because lithium-ion diffusion in LiFePO
4
can only occur along
[010] direction unlike layered or spinel cathode materials,
which possess multiple diffusion directions for lithium ions.
The insertion/extraction reaction proceeds via a two-phase
process, and a reversible loss in capacity with increasing current
density is found to be associated with a diffusion-limited
transfer of lithium across the two-phase interface.
7−9
In this
regard, it appears that controlling the crystal structure of
LiFePO
4
with shortened lithium-ion diffusion pathways as well
as oriented {010} facets can serve as an e ffective solution to
improve the rate capability of LiFePO
4
.
10,11
Decreasing the size
of LiFePO
4
particles to the nanoscale enables higher power
density,
12−14
which is mainly attributed to the shortened ion
diffusion pathways inside the 1D channel of LiFePO
4
.
However, the decrease in particle size adversely affects the
tap density and volumetric energy density, and the increased
surface area increases the undesirable electrode/electrolyte
reactions, which lead to a poor cycling performance.
2,3
The 1D
Li diffusion pathways along the [010] direction, as shown in
Figure 1a, suggest the use of thin, unagglomerated LiFePO
4
particles with large {010} surface area to increase the active area
and decrease the diffusion distances for Li-ions.
15
Indeed,
calculations based on its electronic structure indicate the low-
energy surfaces, such as {010} facets, can dominate in the
equilibrium shape of the total surface area according to the
Wulff construction.
16
Similarly, the potential based calculation,
which allows a larger number of particles and surfaces to be
treated, also suggests that equilibrium morphology of LiFePO
4
takes on a more rounded, isometric appearance with {010},
{201}, {011}, and {100} facets prominent as presented in
Figure 1b.
17
Unfortunately, single crystalline LiFePO
4
nano-
sheets with almost exclusive {010} facets, to the best of our
knowledge, have not yet been achieved either through high-
temperature solid reactions or solution-based coprecipitation
methods, although several groups have reported plate-like
LiFePO
4
crystals showing enhanced electrochemical perform-
ance with increased percentage of {010} facets in the total
surface area.
18−20
For a crystal in its thermodynamic equilibrium shape, the
relative area of each facet on a particle depends on its surface
free energy. In the case of LiFePO
4
, a preference of the {010}
facet on the crystal implies that the {010} surface is
thermodynamically favored over other facets.
16
Though there
is little theoretical prediction of the solvent effects on the
stabilization of {010} facets of LiFePO
4
, most {010}-oriented
plate-like LiFePO
4
have been prepared under solvothermal
conditions, typically using ethylene glycol as the solvent.
19−22
Though ethylene glycol is supposed to bond with Fe(II) in the
reactive FeO
5
group at the {010} facets due to its strong
chelating ability,
23
we have found that diethylene glycol also
Received: March 6, 2014
Revised: April 6, 2014
Published: April 14, 2014
Letter
pubs.acs.org/NanoLett
© 2014 American Chemical Society 2849 dx.doi.org/10.1021/nl5008568 | Nano Lett. 2014, 14, 2849−2853