Molten salt synthesis of spherical LiNi0.5Mn1.5O4 cathode materials

Article abstract of DOI:10.1016/j.electacta.2005.12.018

This paper describes a novel simple redox process for synthesizing monodispersed MnO₂ powders and preparation of spherical LiNi_0.5Mn_1.5O₄ cathode materials by molten salt synthesis (MSS) method. Monodispersed MnO₂ powders have been synthesized by using potassium permanganate and manganese sulfate as the starting materials. By using this redox method, it was found that monodispersed MnO₂ powders with average particle size ～5 μm can be easily obtained. Resultant MnO₂ and LiOH, Ni(OH)₂ was then used to synthesis LiNi_0.5Mn_1.5O₄ cathode materials with retention of spherical particle shape by MSS method. The discharge capacity was 129 mAh g^-1 in the first cycle and 127 mAh g^-1 after 50 cycles under an optimal synthesis condition for 12 h at 800 °C.

Full text of DOI:10.1016/j.electacta.2005.12.018

Electrochimica Acta 51 (2006) 4388–4392
Molten salt synthesis of spherical LiNi Mn O
0
.5
1.5 4
cathode materials
∗
Lei Wen , Qi Lu, Guoxiang Xu
Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Received 10 October 2005; received in revised form 12 December 2005; accepted 14 December 2005
Available online 26 January 2006
Abstract
This paper describes a novel simple redox process for synthesizing monodispersed MnO
2
powders and preparation of spherical LiNi0.5Mn1.5O
4
cathode materials by molten salt synthesis (MSS) method. Monodispersed MnO powders have been synthesized by using potassium permanganate
and manganese sulfate as the starting materials. By using this redox method, it was found that monodispersed MnO powders with average particle
and LiOH, Ni(OH) was then used to synthesis LiNi0.5Mn1.5
2
2
size ∼5 ␮m can be easily obtained. Resultant MnO
2
2
O
4
cathode materials with retention
−
1
−1
of spherical particle shape by MSS method. The discharge capacity was 129 mAh g in the first cycle and 127 mAh g after 50 cycles under an
◦
optimal synthesis condition for 12 h at 800 C.
©
2006 Elsevier Ltd. All rights reserved.
Keywords: Spinel; LiNi0.5Mn1.5O4; Cathode; Molten salt synthesis; Spherical
1
. Introduction
ingofmulticomponentoxidesprovidesconsiderableadvantages
of good mixing of starting materials and excellent homogene-
ity of the final product. So, various wet-chemical methods have
been employed to synthesis LiNi0.5Mn1.5O4 cathode materials,
such as sol–gel [6,7], composite carbonate process [2,8] and
spray pyrolysis [9]. One main disadvantage of these methods
is the complicated synthetic procedure. Molten salt synthesis
(MSS) is a simple technique for preparing complex oxides with
the desired composition in a low-melting point flux [10]. Prior
to us, preparation of LiNi0.5Mn1.5O4 have been conducted by
Kim et al. and in his paper, synthetic variable, such as syn-
thetic atmosphere and temperature, LiCl salt amount have been
intensively investigated [11]. But in his paper, resultant cathode
materials does not have spherical particle shape, and it is mainly
composed of irregular particles. It has been found that regu-
lar quasi-sphericity of particles could significantly enhance the
performance of spinel LiMn2O4. Spherical particles of cathode
materials could lead to high tap-density, which affects the energy
density of the battery [12]. Therefore, in this paper, we have
used potassium permanganate as the precipitant to synthesize
monodispersed MnO2, and then MnO2 was used to synthesis
LiNi0.5Mn1.5O4 cathode materials with retention of spherical
particle shape by MSS and solid-state reaction method. Elec-
trochemical performance of the resultant materials produced by
MSS and solid-state reaction method has also been studied.
LiMn2O4 is one of the candidates for the cathode mate-
rials of lithium-ion batteries. Substitution of manganese with
other metals can dramatically enhance electrochemical perfor-
mance of LiMn2O4. When doped with extensive amount of other
metals, such as Ni, Cr, Co, Cu and Fe to form solid solution
LiMxMn2 − xO4, the dopant reduces the initial capacity at 4 V
plateau, whereas the lost capacity reappears at ∼5 V plateau, i.e.
form high-voltage cathode materials series [1]. Among them,
LiNi0.5Mn1.5O4 materials have received significant attention as
a high-voltage cathode material due to the high-energy density
and stable cyclability with considerable capacity [2–4]. Com-
pared to Ni substitution, Cr, Fe and Co doping shows a dramatic
capacity fading and low-initial capacity. So, Ni doping mate-
rial will become a promising cathode material for lithium-ion
batteries [5]. LiNi0.5Mn1.5O4 could be traditionally produced
by a conventional solid-state reaction between the component
oxides which requires repeated mechanical mixing and exten-
sive heat treatment to eliminate intermediate phase, such as NiO
and LixNi1 − xO. It is well recognized that wet-chemical process-
∗
Corresponding author.
E-mail address: wenleicn1974@163.com (L. Wen).
0
013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.12.018

L. Wen et al. / Electrochimica Acta 51 (2006) 4388–4392
4389
2
. Experimental
In such a reaction process, potassium permanganate reacts
immediately with manganese sulfate to form black MnO2 pre-
cipitation.
Potassium permanganate (>99%, purity) and manganese sul-
fate (>99%, purity) were used as starting materials. The stock
solution of salts for MnO2 synthesis was made by dissolving
potassium permanganate and manganese sulfate into distilled
water. The concentration of the salts was selected as 0.3 M.
pH value of the manganese sulfate solution was adjusted to
about 0.1 by adding excess H2SO4. To prepare MnO2 pow-
ders, potassium permanganate was added to manganese sulfate
solution and the slow addition rate of 0.5 ml/min was chosen.
Then, the precipitate slurry was aged for 3 h with mild agitation
by a magnetic stirrer, and the reaction and aging temperature
In the present work, it was found that pH value of the man-
ganese sulfate influences the morphology and particle size of
the resultant MnO2 products, which in turn affects tap-density
and electrochemical performance of the final LiNi0.5Mn1.5O4
powders.
Fig. 1 shows SEM morphologies of the MnO2 powders
obtained at different initial pH value of MnSO4 solution. A
common feature of the resultant MnO2 powders is that they
all have quasi-spherical structure. However, different synthetic
conditions caused some differences in the morphology of these
powders. MnO2 powders synthesized at high pH value (pow-
ders c and d) are composed of a large number of fine primary
particles, which are packing loosely to form a quasi-spherical
structure. With the decrease of pH value (powders a and b),
more uniform spherical particles were formed and the particle
has smooth surface.
◦
was chosen at 50 C. As-synthesized powders were washed
several times with hot de-ionized water to remove potassium
ions.
For the fabrication of LiNi0.5Mn1.5O4 cathode materials,
the resultant MnO2 powder, commercial spherical Ni(OH)2
powder and LiOH were weighted in order to ensure a nominal
composition of the LiNi0.5Mn1.5O4 material. Five percent
excess LiOH was added to compensate the loss of LiOH
during calcing. In molten salt synthesis, an equal mole ratio
of NaCl and KCl is used. The mixed starting powders were
mixed with salt at a 1:5 (LiOH + MnO2 + Ni(OH)2/NaCl + KCl)
weight ratio with a mortar and pestle for 1 h, and then
According to Qiu et al. [14] and Levi et al. [15], uniform
spherical particles with high tap-density are essential to the fab-
rication of LiNi0.5Mn1.5O4 powders with excellent electrochem-
ical performance. So X-ray diffraction patterns of the powder (a)
is shown in Fig. 2. The as-synthesized powders are essentially
␥-MnO2 phase.
◦
calcined at 800 C for 12 h. The calcined material was
washed four times with de-ionized water to remove Na,
K and Cl ions until no free chloride ions were detected
by silver nitrate solution. After washing, the powders was
annealed at 750–800 m for about 6 h in air to yield the final
product.
3.2. Characterization of LiNi0.5Mn1.5O4 cathode materials
Two kinds of MnO2 have been used to synthesize
LiNi0.5Mn1.5O4 cathode materials by MSS and solid-state reac-
tion method, one is the present MnO2 obtained by redox method,
and the other is commercial chemically MnO2. In the present
work, commercial spherical Ni(OH)2 has also been used as start-
ing materials.
Fig. 3 shows the SEM morphology of the commercial chem-
ically manganese dioxide (CMD). It is mainly composed of
irregular shape particles with sharp edges rather than monodis-
persed MnO2 particles like powder (a) in Fig. 1.
Phase identification was performed via X-ray diffraction
(
XRD) (Multiflex X-ray diffractometer, Rigaku), using nickel-
◦
filtered Cu K␣ radiation and a scanning speed of 2θ = 4 /min. A
scanning electron microscope (SEM, JEOL JSM-5600LV) was
used to observe the morphologies of the MnO2 and resultant
LiNi0.5Mn1.5O4 powders.
The performance of the LiNi0.5Mn1.5O4 as cathode was
evaluated using a test cell with a lithium metal anode.
The cathode was a mixture of cathode materials/acetylene
black/polyvinylidene fluoride (PVDF) with weight ratio
Fig. 4 shows the SEM morphology of the commercial
Ni(OH)2 particles used as starting materials. The particles are
spherical with the particle size of about 5 ␮m, and are made up
of a large number of fine crystalline grains.
8
5/5/10, and the electrolyte was 1 M LiPF in a 1:1 (v/v) mix-
6
ture of ethylene carbonate (EC) and diethyl carbonate (DEC).
The galvanostatic charge–discharge experiment was performed
between 3.5 and 5.0 V at 25 C.
Fig. 5 shows the morphologies of the LiNi0.5Mn1.5O4 cath-
ode materials synthesized by MSS method, using commercial
CMD and MnO2 obtained by redox method, respectively. As
shown in Fig. 5a, when using commercial CMD as starting mate-
rials, the LiNi0.5Mn1.5O4 powders are composed of irregular
particles with broad particle size distribution, whereas when
using the present MnO2 as starting materials, the resultant
LiNi0.5Mn1.5O4 cathode materials retain the spherical particle
shape of the starting materials. Average particle size is about
◦
3
. Results and discussion
3
.1. Preparation of MnO2 powders
In acidic and neutral reaction system, when potassium per-
2
+
manganate was added to the manganese sulfate solution, Mn
∼
10 ␮m.
will be oxidized to Mn4 , the following chemical reactions were
+
According to Ying et al. [16], particle morphology is a very
expected [13]:
important factor on the tap-density of the materials. The spher-
ical powders have high tap-density because of the excellent
fluidity, so only a small quantity of space exists within the pow-
ders after long period of tapping. In fact, the tap-density of the
2
KMnO4 + 3MnSO4 + 2H2O → 5MnO2 ↓
K2SO4 + 2H2SO4
+

4
390
L. Wen et al. / Electrochimica Acta 51 (2006) 4388–4392
Fig. 2. XRD spectra of the resultant MnO2 powders.
Fig. 3. SEM image of the commercial CMD.
intermediate phases have been detected. The XRD analysis
shows there is negligible difference among the crystalline
phase and lattice parameters between the two LiNi0.5Mn1.5O4
materials.
Fig. 1. SEM morphologies of the MnO2 powders: (a) pH 0.1; (b) pH 0.2; (c)
pH 0.6; (d) pH 0.8.
powder Fig. 5a is only about 1.76 g cm 3, which is much lower
−
−
3
than 2.05 g cm of the powder Fig. 5b.
Fig. 6 shows the X-ray diffraction patterns of the
LiNi0.5Mn1.5O4 materials prepared with different MnO2 as
◦
starting materials. These materials calcined at 800 C for
Fig. 4. SEM image of the commercial Ni(OH)2.
1
2 h are well crystallized into spinel phase without any

L. Wen et al. / Electrochimica Acta 51 (2006) 4388–4392
4391
Fig. 7. XRD spectrum of the LiNi0.5Mn1.5O4 prepared with different starting
materials: (a) commercial CMD as starting materials, solid-state reaction method
and (b) MnO2 obtained by redox method as starting materials, solid-state reac-
tion method.
In the present work, due to much higher diffusion rates
between reaction components in the molten media, single spinel
phase can be easily obtained at relatively lower temperature and
short calcining time.
When using conventional solid-state reaction method to syn-
thesize LiNi0.5Mn1.5O4 materials, impurity phases, such as NiO
and LixNi1 − xO are often be detected. Fig. 7 shows the X-ray
diffraction patterns of the LiNi0 Mn O4 materials using the
.5
1.5
same starting materials except without using the molten salt, and
impurity phases was indicated by arrows.
Fig. 8 shows the initial charge/discharge curves. Cells were
operated at a specific current of 130 mA g in the voltage range
of 3.5–5 V. When employing molten salt method, for the cath-
ode materials using commercial CMD and present MnO2 as
starting materials, the initial charge/discharge capacity at room
Fig. 5. SEM images of the LiNi0.5Mn1.5O4 prepared by MSS method: (a) com-
mercial CMD and (b) MnO2 obtained by redox method as starting materials.
−1
Fig. 8. Initial charge and discharge curves of the LiNi0.5Mn1.5O4 cathode
materials obtained by using different MnO2 as starting materials: (a) MnO2 syn-
thesized by redox method, molten salt method; (b) commercial CMD, molten
salt method; (c) MnO2 synthesized by redox method, solid-state reaction; (d)
commercial CMD, solid-state reaction.
Fig. 6. XRD spectrum of the LiNi0.5Mn1.5O4 prepared with different starting
materials: (a) commercial CMD as starting materials, MSS method and (b)
MnO2 obtained by redox method as starting materials, MSS method.

4
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L. Wen et al. / Electrochimica Acta 51 (2006) 4388–4392
cal LiNi0.5Mn1.5O4 cathode materials by molten salt synthesis
MSS) method. Using potassium permanganate and manganese
(
sulfateasstartingmaterials, quasi-sphericalMnO2 powdershave
been synthesized by redox method. It was found that pH value
of the manganese sulfate influences the morphology and particle
size of the resultant MnO2 products. Uniform spherical particles
with smooth surface have been synthesized when the initial pH
value of manganese solution is about 0.1. Using molten salt syn-
thesis and solid-state reaction method, LiNi0.5Mn1.5O4 cathode
materials have been prepared by using different MnO2 and com-
mercial Ni(OH)2 as starting materials. Due to much higher dif-
fusion rates between reaction components in the molten media,
single spinel phase LiNi0.5Mn1.5O4 can by obtained, which does
nothaveanyimpurityphase. Comparedwithsolid-statereaction,
electrochemical performance of LiNi0.5Mn1.5O4 cathode mate-
rials have been obviously improved when employing molten salt
method and MnO2 obtained by redox method as starting mate-
rials.
Fig. 9. Cyclic performances of the LiNi0.5Mn1.5O4 cathode materials: (a) MnO2
synthesized by redox method; molten salt method; (b) commercial CMD, molten
salt method; (c) MnO2 synthesized by redox method, solid-state reaction; (d)
commercial CMD, solid-state reaction.
temperature is 125/120 and 135/129 mAh g 1, respectively. For
the presence of impurity phases, such as NiO and LixNi1 − xO,
the sample prepared by the solid-state reaction method delivered
lower discharge capacity.
−
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