Characterization of LiNi0.5Mn1.5O4 Thin Film
Cathode Prepared by Pulsed Laser Deposition
Hui XIA1, Li LU1, 2, and Gerbrand CEDER1, 3
1
2
Singapore-MIT Alliance, E4-04-01, 4 Engineering Drive 3, Singapore
Department of Mechanical Engineering, National University of Singapore, Singapore 119260
3
Department of Material Science and Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Abstract—LiNi0.5Mn1.5O4 thin films have been grown by
pulsed laser deposition (PLD) on stainless steel (SS)
substrates. The crystallinity and structure of thin films were
investigated by X-ray diffraction (XRD). The microstructure
and surface morphology of thin films were examined using a
field-emission scanning electron microscope (FESEM). The
electrochemical properties of the thin films were studied with
cyclic voltammetry (CV) and galvanostatic charge-discharge
in the potential range between 3.0 and 4.9 V.
The
electrochemical behavior of LiNi0.5Mn1.5O4 thin films showed
reversible capacity above 4.7 V and good cycle performance
up to 50 cycles.
Keywords—Cathode;
LiNi0.5Mn1.5O4;
deposition; Thin film batteries
Pulsed
laser
I. INTRODUCTION
L
AYERED LiCoO2, LiNO2 and spinel LiMn2O4 are the
most important cathode materials for Li-ion batteries
because of their high voltage (4 V vs. Li/Li+) and good
cycleability. Among them, the spinel LiMn2O4 is the most
favored one due to its lower cost and toxicity [1-3].
However, LiMn2O4 is not stable during cycling especially
at elevated temperature, which results in a rapid capacity
fade and limits its practical use [4-6]. Therefore, much
research has been performed to improve its charge and
discharge cycle performance.
One excellent method for improving the cycle
performance has been the substitution of other transition
metals for Mn to make LiMxMn2-xO4 (M = Co [7], Cr [8],
Ni [9], Fe [10], Cu [11], etc.). Interestingly, it has been
found that this approach is also accompanied by a higher
voltage plateau at about 5 V as a result of the redox system
provided by the transition metal substituted. These
materials are now attractive candidates as cathode materials
for lithium-ion batteries, because they can increase the cell
voltage to 5 V from the present 4 V. Among these
materials, LiNi0.5Mn1.5O4 is the most attractive material for
the practical preparation of 5 V cathodes due to its good
stability on repeated Li-ion extraction and insertion [1214].
Thin film lithium batteries now are attracting more and
more interest. It is an interesting research field, not only
because miniaturization of electronic devices makes it
possible to use thin film microbatteries as power sources,
but also because the thin film electrode without any
polymeric binder and carbon black is perfect sample to
investigate the electrochemical properties of the material.
As for the thin film cathodes, LiCoO2, LiNiO2 and
LiMn2O4 are fully researched using different deposition
methods. However, few papers have been published for this
5 V thin film cathode [15, 16] due to the novelty of this
type of cathode materials.
The aim of this work is to prepare thin films of
LiNi0.5Mn1.5O4 on SS substrates by pulsed laser deposition.
The structure and microstructure of thin films were
measured by XRD and FESEM. Electrochemical properties
of the thin film cathodes were investigated by CV and
charge-discharge cycling test. Two high voltage plateaus at
about 4.7 V were observed on both charge-discharge
voltage profiles. The Li/LiNi0.5Mn1.5O4 cell showed very
good cycle performance for 50 cycles.
II. EXPERIMENTAL
A. Thin Film deposition
LiNi0.5Mn1.5O4 target was prepared by a solid-state
reaction using MnO2 99.9% (Alfa Aeser), NiO 99% (Alfa
Aeser) and LiOH 98% (Merck). The mixture powders were
reacted at 750°C for 24 h, then pressed into a pellet and
sintered at 900°C for 2 h. Target and stainless steel
substrates were placed inside a vacuum chamber with a
turbo-molecular pump yielding a pressure less than 1 × 10-5
Torr. The target-substrate distance was kept at 40mm.
During deposition, the target is rotated at 10 to 20 rpm to
avoid depletion of material at any given spot. A Lambda
Physik KrF excimer laser beam (λ = 248nm, pulse width =
25ns, laser energy = 150-160 mJ/pulse) was incident on the
target at an angle close to 45° at a repetition rate of 10 Hz.
The focused spot size is 2 × 5 mm at the target resulting in
a laser fluence of 2 J/cm2. Films deposition was carried out
at a substrate temperature of 600°C in an oxygen
atmosphere of 100mTorr for 40 min. The amount of
LiNi0.5Mn1.5O4 was estimated by weighing the electrode
substrate before and after the film deposition using a
microbalance (A&D, GR-202).
B. Microstructure Analysis
Structure and crystallinity of thin films were measured by
a Shimadzu XRD-6000 X-ray diffractometer with Cu Kα
radiation. The data were collected in the 2θ range of 10-70°
at a scan rate of 2° per min. Surface morphology of thin
films was characterized using a Hitachi S-4100 Field
Emission Scanning electron microscope (FESEM).
C. Electrochemical Measurements
Electrochemical measurements were carried out on
Li/LiNi0.5Mn1.5O2 cells using the LiNi0.5Mn1.5O2 thin film as
cathode and a lithium metal foil as both anode and the
reference voltage electrode. All experiments were
conducted in an Ar-filled glove box with the H2O and O2
levels less than 0.1 ppm. A Solartron 1287 two terminal
cell test system was used to perform all electrochemical
measurements. The electrolyte solution was 1 M LiFP6 in a
1:1 (by volume) ethylene carbonate (EC)-diethylene
carbonate (DEC) solution. Cyclic voltammetry was carried
out between 3.5 and 5 V versus Li/Li+ at a slow sweep rate
of 0.2 mV s-1. Galvanostatic charge-discharge cycling test
was carried out in the potential range between 3.0 and 4.9
V using a constant current density of 20 µA/cm2.
Fig. 1. XRD θ/2θ spectra for the synthesized LiNi0.5Mn1.5O4 powder
and deposited LiNi0.5Mn1.5O4 thin film on SS substrate.
III. RESULTS AND DISCUSSION
The crystal structure of the synthesized LiNi0.5Mn1.5O4
powder and deposited LiNi0.5Mn1.5O4 thin film on SS
substrate were investigated by XRD in Fig.1. From the
XRD spectrum of the synthesized powder, we can see the
calcination of mixture at 750 for 24 h provided us a single
phase of LiNi0.5Mn1.5O4. All peaks appeared in the XRD
spectrum of LiNi0.5Mn1.5O4 powder are very sharp,
indicating a high crystallinity of the powder. The Miller
index for each peak was determined based on a spinel
structure having Fd3m space group. From the XRD
spectrum of LiNi0.5Mn1.5O4 film on SS substrate, only three
peaks are identified as (111), (311) and (222) reflections
from LiNi0.5Mn1.5O4 thin film. Among these reflections,
(111) is the strongest one just as the XRD spectrum of
LiNi0.5Mn1.5O4 powder. However, very weak peaks from
other reflections mean the film is mainly (111) texture. No
impurity phase was observed from the XRD spectrum of
LiNi0.5Mn1.5O4 film deposited on SS substrate.
The surface morphology and crystal shape of the spinel
LiNi0.5Mn1.5O4 thin film deposited on SS substrate at 600°C
was investigated using the FESEM. Fig. 2 shows two
FESEM images of LiNi0.5Mn1.5O4 thin film with low
magnification and high magnification, respectively. From
Fig. 2 (a), it can be seen that the film is very dense without
any crack or pinhole. However, the film is not very smooth
(a)
(b)
Fig. 2. FESEM images for LiNi0.5Mn1.5O4 thin film deposited on SS
substrate at 600°C, (a) image with low magnification (b) image with high
magnification.
due to some big droplets clustered on the surface. From
Fig.2 (a), it can be seen that the average grain size is about
100-200 nm and grains exhibit polyhedral shapes, which
are usually observed in cubic crystal system. The XRD and
FESEM results showed that a well-crystallized and highly
dense LiNi0.5Mn1.5O4 thin film was successfully prepared by
PLD. The advantage of PLD to produce high quality oxide
thin films was also proved by our previous research of
LiCoO2 thin films by PLD.
Fig. 3 displays a typic cyclic voltammogram of a
LiNi0.5Mn1.5O4 thin film cathode cycled between 3.5 and 5
V vs. Li/Li+ at a slow sweep rate of 0.2 mVs-1 in the
LiPF6/EC-DEC electrolyte. As shown in Fig. 3, there pairs
of peaks were observed with two pairs of strong peaks at
high voltages about 4.7 V and one pair of weak peaks at
about 4 V. The high voltage peaks at 4.68 (A1) and 4.79 V
(A2) on anodic scan, and at 4.65 (C1) and 4.75 V (C2) on
cathodic scan correspond to the redox reactions of
Ni2+/Ni3+ and Ni3+/Ni 4+ according to references [12, 13].
Dahn et al. [12] suggested that the electrode potential of
LiMn2O4 can be enhanced from 4.1 to 4.7 V by a
substitution of a part of Mn ions in LiMn2O4. They have
discussed an appearance of the 4.7 V electrode potential
region by using UV photoelectron spectroscopy and have
claimed that an energy of 3d level for Ni2+ ion and Ni3+ ion
in a low spin state is lower than that of Mn3+ ion in a high
spin state. Such a difference between energies for 3d levels
leads to a higher electrode potential of LiNi0.5Mn1.5O4.
However, the presence of peaks at about 4 V,
corresponding to a redox reaction of Mn3+/Mn4+, suggests
that the film may be nickel deficient. Dahn et al. [12] also
suggested that the Ni deficiency in their LiNi0.5Mn1.5O4
powder was due to the oxygen loss in their samples heated
above 650°C. In our case, there is probably some Ni loss
during the high temperature material synthesis and high
vacuum deposition process. LiNi0.5Mn1.5O4 thin film was
also prepared by Mohamedi et al. by electrostatic spray
deposition (EDS) using a precursor solution. No redox
couple at 4 V was seen in their CV result, which means
their spinel film has desired Ni content.
To investigate the battery performance of this thin film
cathode, 50 charge/discharge cycles of a lithium battery
employing LiNi0.5Mn1.5O4 thin film cathode were
performed between 3 and 4.9 V at a constant current of 20
µAcm-2. The charge/discharge voltage profiles at 1, 10, 20,
30, 40 and 50 cycles were shown in Fig. 4. As expected
from the CV results, three plateaus were observed on both
charge/discharge curves. The small plateau at about 4 V
corresponds to the redox reaction of Mn3+/Mn4+, while the
two big plateaus at about 4.7 V correspond to the redox
reactions of Ni2+/Ni3+ and Ni3+/Ni 4+. Except for the first
cycle, charge and discharge are highly reversible for the
subsequent cycles. There is no obvious electrolyte
decomposition reaction observed in the charge process at
high voltage near 5 V, which is very obvious for LiCoO2
thin film cathode. There is only a small difference between
the voltage plateaus of charge and discharge, which means
the Li/LiNi0.5Mn1.5O4 cell has a small polarization. It also
Fig. 3. Cyclic voltammogram of the LiNi0.5Mn1.5O4 thin film cathode
cycled between 3.5 and 5 V vs. Li/Li+ at a sweep rate of 0.2 mVs-1 in
LiPF6/EC-DEC electrolyte.
Fig. 4. Charge/Discharge voltage curves of LiNi0.5Mn1.5O4 as a 5 V
cathode material investigated in the potential range 3.0 – 4.9 V vs.
Li/Li+ with a constant current of 20 µAcm-2.
Fig. 5. Cycle performance of the LiNi0.5Mn1.5O4 thin film cathode
cycled between 3.0 and 4.9 V vs. Li/Li+ at a constant current of 20
µAcm-2 in LiPF6/EC-DEC electrolyte.
can be seen that these voltage plateaus for both charge and
discharge don’t change much even at the 50th cycle, which
means the cell resistance doesn’t change much with cycling.
The theoretical capacity of LiNi0.5Mn1.5O4 is calculated to
be 146.6 mAhg-1 when all Li+ ions can be extracted from
this material. In our test, the first charge capacity is about
120 mAhg-1, after that, a reversible capacity about 100
mAhg-1 is maintained. The utilization of the film calculated
from the first charge is about 80%. The correctness of this
calculation could be affected by the weighing of the thin
film. The charge/discharge capacities vs. cycle number
were shown in Fig. 5. The Li/LiNi0.5Mn1.5O4 cell showed a
very good cycle performance for 50 cycles. The discharge
capacity fades with a small rate of about 0.06% per cycle,
which is similar to the result of Kanamura et al. [13]using a
composite electrode but better than the result of Mohamedi
et al. [15] also using a thin film electrode prepared by ESD.
Except for the first several cycles, the coulombic efficiency
is as high as 98%.
Fig. 6 shows the discharge curves of a Li/LiNi0.5Mn1.5O4
cell at different current densities, which was used to test the
rate-capability of the cell. As the current density is
increased, the cell voltage is lowered and the useful
capacity of the cell is decreased. This is mainly due to the
cell resistance which increases the cell polarization as the
current density increases. The largest contributions to the
battery resistance are lithium-ion transport into and through
the cathode.
IV. CONCLUSION
LiNi0.5Mn1.5O4 thin film cathode as a promising
candidate for 5 V cathode materials of lithium batteries
were successfully prepared by PLD. The CV results
showed two pair of peaks at about 4.7 V corresponding to
the two redox reactions of Ni2+/Ni3+ and Ni3+/Ni 4+. The
presence of peaks at 4 V means that the film is a little bit
deficient of Ni which results in some Mn3+ in the film. The
LiNi0.5Mn1.5O4 thin film cathode showed excellent cycle
performance between 3 and 4.9 V, exhibiting a stable and
highly reversible capacity of about 100 mAhg-1 for 50
cycles.
ACKNOWLEDGMENT
This research was supported by Advanced Materials for
Micro- and Nano- System (AMM&NS) programme under
Singapore-MIT Alliance (SMA) and by National University
of Singapore. We would like to thank Dr. Y. S. Meng from
the department of Materials Science and Engineering,
Massachusetts Institute of Technology, Cambridge and Dr.
Songbai Tang from the department of Mechanical
Engineering, National University of Singapore for useful
discussions.
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