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The International Conference on Materials Research and Innovation (ICMARI 2021) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 1234 (2022) 012029 doi:10.1088/1757-899X/1234/1/012029
Solid-state reaction synthesis and characterization of Mn-doped
LiFePO4 cathode material
A M M Kyaw1,2, G Panomsuwan1,2 and R Munprom1,2, *
1
Department of Materials Engineering, Faculty of Engineering, Kasetsart University,
Bangkok 10900, Thailand
2
International Collaborative Education Program for Materials Technology, Education, and
Research (ICE-Matter), ASEAN University Network/Southeast Asia Engineering
Education Development Network (AUN/SEED-Net), Bangkok, Thailand
*Corresponding author : fengrtpm@ku.ac.th
Abstract. Olivine type LiFePO4 has great advantages for Li-ion batteries due to its non-toxicity, high
safety, and good cycle life performance. However, its low-rate capability and low energy density
make some challenges for this LiFePO4. Several methods like doping with transition metals were
used, and Mn ion was used in this work to improve the overall electrochemical properties. LiMnxFe1-
xPO4 is promising cathode material owing to high voltage, structural and chemical stability. However,
the electrochemical performance of these materials depends on phases and structures obtained from
synthesis. In this work, the effect of solid-state reaction conditions, including calcination temperature
and duration, on morphology, structure, and electrochemical properties of LiMnxFe1-xPO4 cathode
materials with the composition of x = 0.5 was investigated. The morphology, crystallography and
local structure of the synthesized materials were examined by field emission scanning electron
microscope (FE-SEM), X-ray diffractometer (XRD) and Fourier-transform infrared spectrometer
(FTIR), respectively. The surface area was also determined by the Brunauer-Emmett-Teller (BET)
model. The effect of calcination temperature and reaction time upon the morphology, structures of
the synthesized cathode materials were studied and discussed. The results could be essential for
further development and employment of LiMnxFe1-xPO4 in Li-ion batteries.
1. Introduction
Olivine-type lithium-transition metal phosphates (LiMPO4) (M = Fe, Mn, Co, Ni, etc…) are taken into
account for potential cathode materials due to high theoretical specific capacity, superior safety capacity,
environmental friendliness and low cost [1]. Among these LiMPO4, LiFePO4 (theoretical capacity of 170
mAhg-1 and operation potential of 3.4 V vs. Li/Li+) is well known for the aforementioned advantages as well
as has been investigated intensively and successfully commercialized [2]. However, this material possesses
drawbacks of low electronic conductivity (<10-9 Scm-1) and poor Li+ ion diffusion. In addition, its low
working potential (3.4 V vs. Li/Li+) restricts to use in high power applications, such as electric vehicles
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
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Published under licence by IOP Publishing Ltd 1
The International Conference on Materials Research and Innovation (ICMARI 2021) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 1234 (2022) 012029 doi:10.1088/1757-899X/1234/1/012029
[3-5]. Several methods can be used to tackle the drawbacks such as tailoring morphology, coating conductive
carbon on primary particles and doping with some transition metal ions (e.g., Mn+, Co+, Ni+, Mg2+, Cr3+ ,
Ti4+, etc.) [6, 7].
Owning the same structure as LiFePO4, lithium manganese phosphate (LiMnPO4), is also a promising
alternative cathode material [8]. Containing Mn element gives benefits in terms of material cost and
environmental friendliness. Compared with LiFePO4, it exhibits a higher potential of 4.1 V vs. Li/Li+, which
leads to being more suitable for high power applications [9, 10] Nevertheless, structural change during
intercalation and deintercalation of Li ions, Jahn-Teller distortion of unstable Mn3+ ion and poor electronic
conductivity make this cathode material imperfect. Similar improvement methods can be implemented to
enhance the properties of LiMnPO4 [11, 12]. For example, Jo et al. reported that the composition of
LiMn0.71Fe0.29PO4 cathode material prepared by polyol method showed the high volumetric capacity of 243
and 128 mAh cm-3 at 0.1C and 7C, respectively, and excellent cycling performance [13]. Moreover,
LiMn0.5Fe0.5PO4 prepared by a solvothermal route exhibited long term cycling stability and good high rate
performance up to 1000 cycles [9].
Various synthesis methods, such as sol-gel and solid-state reaction, can be used to prepare LiMnxFe(1-x)PO4
and the synthesis condition can strongly influence electrochemical performance [6, 12, 14]. By adjusting
synthesis parameters, the morphology of the materials can be tailored to nanostructure. Among all
techniques, a solid-state reaction is often applied due to simplicity and low cost. Many research works have
applied this method to synthesize the cathode materials, but most of the works used long two-step calcination
at high temperatures, which is quite energy and time consuming [8, 14]. Only a few works demonstrate a
synthesis condition using short calcination time. Therefore, in this work, LiMnxFe(1-x)PO4 with x = 0.5 was
synthesized by a solid-state reaction method and, then thermally treated at 450℃ and 550℃ for a given time
(i.e., 2, 6 and 12 h). The particle morphology, structure, local structure, and specific surface area obtained
from different calcination times was investigated and discussed. This relationship could emphasize the
importance of synthesis condition optimization for performance improvement.
2. Experimental
2.1. Synthesis of LiMn0.5Fe0.5PO4 Materials
LiMn0.5Fe0.5PO4 cathode materials were synthesized by a solid-state reaction method. Manganese (II)
carbonate (Aldrich, 99.9%), iron (II) oxalate dihydrate (Aldrich, 99%) and lithium phosphate monobasic
(Aldrich, 99%) were used as starting materials and ethanol was used as a dispersant. The stoichiometric
ratio of Li: Mn: Fe (1: 0.5: 0.5) and an appropriate amount of ethanol were mixed by jar rolling with yttrium-
stabilized zirconia balls as grinding media for 18 h. Then, the mixture was dried overnight at 80℃ to
evaporate the ethanol. After that, the dried powder was ground. Finally, the materials were calcined at 450℃
and 550℃ for 2, 6 and 12 h in argon (Ar) atmosphere to obtain the final powder of LiMn0.5Fe0.5PO4.
2.2. Materials Characterization
The crystal structure and phase of the synthesized materials were examined using an X-ray diffractometer
(Philips X’Pert) with Cu Kα radiation (λ= 0.154 nm) over the range of 10° to 60° at a step size of 0.02°. The
surface morphology and particle size of the particles were analyzed using a field emission scanning electron
microscope (JEOL-JSM-7600F). The composition of the LiMn0.5Fe0.5PO4 powder was analyzed by energy
dispersive spectroscopy (EDS). The Fourier transform infrared (FT-IR) spectrum was obtained with KBr
pellets using Bruker Alpha-E spectrometer. The specific surface areas were measured with Micromeritics
3Flex at 77 K using the Brunauer-Emmett-Teller method.
2
The International Conference on Materials Research and Innovation (ICMARI 2021) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 1234 (2022) 012029 doi:10.1088/1757-899X/1234/1/012029
3. Results and discussion
The X-ray diffraction pattern of the cathode materials for each synthesis condition is shown in Figure 1. For
all conditions, some peaks were well indexed with the orthorhombic structure with the space group of Pnmb
(62) (ICDD-00-042-0580). Such peaks can be identified as LiMn0.5Fe0.5PO4. Some impurity phases, namely
LiPO3, Li3PO4 and Li4P2O7 were also found. However, the impurity phases decreased when the temperature
increased. This is possibly caused by an incomplete reaction at low temperatures and short calcination times.
The crystallite sizes of these materials were estimated using the Scherrer’s equation and shown in Table 1.
The crystallite sizes of all synthesis conditions were in the range of 25-30 nm and increased with the
calcination time.
Figure 1. XRD patterns of LiMn0.5Fe0.5PO4 calcined at (a) 450℃ (b) 550℃ for 2, 6 and 12 h.
The morphology and structure of the synthesized materials were characterized by FE-SEM and the images
are shown in Figure 2(a - f). From the results, the particles of all conditions were similar. Specifically,
particles were agglomerated with a primary size of 20 – 52 nm. However, the particle sizes became larger
with increasing temperatures. On the other hand, increasing calcination time can slightly decrease the
particles sizes. The box plot of particle size analyses of LiMn0.5Fe0.5PO4 is shown in Figure 2(g).
Figure 2. FESEM images of LiMn0.5Fe0.5PO4 (a - c) synthesized at 450℃ for 2, 6 and 12 h, respectively
(d - f) synthesized at 550 ℃ for 2, 6 and 12 h, respectively and (g) box plot of particle size (yellow and
blue color represents calcination at 450℃ and 550℃, respectively).
The functional group vibration modes of LiMn0.5Fe0.5PO4 prepared at different conditions were examined
by FTIR spectroscopy in the region of 4000 - 400 cm-1 and are shown in Figure 3. Both spectra have similar
vibrational modes. The bands at 1092 and 1052 cm-1 belong to the LiMnPO4 and LiFePO4 respectively. A
3
The International Conference on Materials Research and Innovation (ICMARI 2021) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 1234 (2022) 012029 doi:10.1088/1757-899X/1234/1/012029
single broad band found between these two bands belongs to a solid solution of LiMn0.5Fe0.5PO4 [15]. Such
a solid solution began to form at high temperatures for a long period of time. As a result, it can only be
found in the condition of 550 ℃ for 12 h, and not found in the other conditions due to the incomplete
reaction. In addition, all synthesized materials exhibited the vibrational mode of PO43-: three bands between
1140 and 1043 cm-1 corresponding to (υ3) antisymmetric stretching mode, two bands between 978 and 749
cm-1 corresponded to (υ1) symmetric stretching mode, three bands between 636 and 552 cm-1 corresponded
to (υ4) bending mode and the band around 460 cm-1 corresponding to (υ2) bending mode.
Figure 3. FTIR spectra of LiMn0.5Fe0.5PO4 calcined at (a) 450 ℃ and (b) 550 ℃ for 2, 6, and 12 h.
The specific surface area of these LiMn0.5Fe0.5PO4 materials were measured with Brunauer-Emmett-Teller
(BET) model and is showed in Table 1. The materials synthesized at 550℃ possessed a higher specific
surface area. Increasing the reaction time can also increase the surface area which was correlated well with
the primary particle size analyses. In other words, the smaller the size of particles, the greater is the specific
surface area.
Table 1. Crystallite size, particle size and specific surface area of LiMn0.5Fe0.5PO4 at different conditions.
Temperature Time Crystallite size Particle size Specific surface area
(℃) (h) (nm) (nm) (m2/g)
2 24.3 29 5.38
450 6 28.7 24 5.54
12 29.6 21 6.24
2 23.4 52 7.31
550 6 26.8 37 12.06
12 28.3 33 12.36
4. Conclusions
Mn-doped LiFePO4 cathode materials were successfully synthesized by a solid-state reaction method at
different temperatures and reaction times. XRD patterns showed that materials for all conditions had
identified with the reference data of LiMn0.5Fe0.5PO4, but impurity peaks were found at low synthesis
temperatures. The particle became aggregate at high temperature and the smaller particle size was achieved
when increasing the reaction time. LiMn0.5Fe0.5PO4 synthesized at 550℃ possessed a higher specific surface
4
The International Conference on Materials Research and Innovation (ICMARI 2021) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 1234 (2022) 012029 doi:10.1088/1757-899X/1234/1/012029
area than 450℃. The results can provide the optimal conditions for Mn-doped LiFePO4 synthesis, which
positively influences the electrochemical performance of the cathode material for Li-ion batteries.
Acknowledgements
This work was financially supported by International Collaborative Education Program for Materials
Technology, Education, and Research (ICE-Matter). In addition, the authors gratefully acknowledge the
facilities of the Department of Materials Engineering, Faculty of Engineering, Kasetsart University.
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