Phase relations of Li2O-MnO-P2O5 system and the electrochemical properties of Li1+xMn1-xPO4 compounds

Article abstract of DOI:10.1016/j.jallcom.2015.02.143

The phase relations of Li₂O-MnO-P₂O₅ ternary system under reducing atmosphere have been systematically investigated by means of X-ray diffraction. Inferior to what we expected, no other new lithium manganese phosphates exist within the Li₂O-MnO-P₂O₅ ternary system under the reducing atmosphere. A high-pressure phase Mn₃(PO₄)₂ with graftonite Fe₃(PO₄)₂-type structure can be easily obtained in the MnO-P₂O₅ system under the ordinary solid-state reaction conditions in H₂/Ar atmosphere and its detail structure is presented. In addition, the solid solubility of Li_1+xMn_1-xPO₄ is determined as -0.05 ≤ x ≤ 0.03. The lattice parameters and electrochemical properties of Li_1+xMn_1-xPO₄ with x content are investigated. The electrochemical test results show that excess Li-ion (x > 0) or the excess Mn-ion (x < 0) in LiMnPO₄ has an unfavorable effect on the electrochemical properties caused by the deterioration of the lithium diffusion along the one-dimensional tunnels.

Full text of DOI:10.1016/j.jallcom.2015.02.143

Accepted Manuscript
Phase relations of Li O-MnO-P O system and the electrochemical properties
2
2
5
of Li1+ x Mn1- x PO compounds
4
Danlin Yan, Yanming Zhao, Youzhong Dong, Zhiyong Liang, Xinghao Lin
PII:
S0925-8388(15)00583-6
DOI:
http://dx.doi.org/10.1016/j.jallcom.2015.02.143
JALCOM 33520
Reference:
To appear in:
Journal of Alloys and Compounds
Received Date:
Revised Date:
Accepted Date:
11 December 2014
17 February 2015
18 February 2015
Please cite this article as: D. Yan, Y. Zhao, Y. Dong, Z. Liang, X. Lin, Phase relations of Li O-MnO-P O system
2
2
5
and the electrochemical properties of Li1+ x Mn1- x PO compounds, Journal of Alloys and Compounds (2015), doi:
4
http://dx.doi.org/10.1016/j.jallcom.2015.02.143
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Phase relations of Li O-MnO-P O system and the electrochemical
2
2
5
properties of Li Mn PO compounds
1+x
1-x
4
a
a,b,
a
a
a
Danlin Yan , Yanming Zhao *, Youzhong Dong , Zhiyong Liang , Xinghao Lin
a. School of Material Science and Engineering, South China University of Technology
b. School of Physics, South China University of Technology
Guangzhou, 510640, China
*
Corresponding author. Tel.:+86-20-87111963，Fax: +86-20-85511266
E-mail address: zhaoym@scut.edu.cn
Abstract
The phase relations of Li
2
O-MnO-P
2
O
5
ternary system under reducing
atmosphere have been systematically investigated by means of X-ray diffraction.
Inferior to what we expected, no other new lithium manganese phosphates exist
within the Li
high-pressure phase Mn
obtained in the MnO-P
/Ar atmosphere and its detail structure is presented. In addition, the solid solubility
is determined as -0.05 ≤ x ≤ 0.03. The lattice parameters and
electrochemical properties of Li1+xMn1-xPO with x content are investigated. The
electrochemical test results show that excess Li-ion (x ＞ 0) or the excess Mn-ion (x
0) in LiMnPO has an unfavorable effect on the electrochemical properties caused
2
O-MnO-P
(PO
system under the ordinary solid-state reaction conditions in
2
O
5
ternary system under the reducing atmosphere. A
3
4
)
2
with graftonite Fe (PO -type structure can be easily
3
4 2
)
2
O
5
H
2
of Li1+xMn1-xPO
4
4
<
4
by the deterioration of the lithium diffusion along the one-dimensional tunnels.
1

Keywords: Solid-state reactions; Li
2
O-MnO-P
2
5
O phase diagrams; X-ray diffraction;
Li1+xMn1-xPO
4
solid solutions.
1. Introduction
Since rechargeable lithium ion batteries became the most widely used and most
advanced energy storage system, electrode materials are causing special concern
[
1-3]. For cathode materials, cobalt-based oxides are still utilized as the most
common cathode material in commercial cells until now. However, its cost,
toxicity, and poor thermal stability have become hinders of LiCoO in large-scale
2
applications in electrical vehicle (EV) and hybrid electric vehicles (HEV), which
are considered as the tendency of the future application of lithium batteries. To
satisfy the need for safer, environmental friendly and favorable performance
cathode materials, a worldwide search for LiFePO
4
with three-dimensional
tetrahedra and FO octahedra has been
carried out. The good thermal stability, comparable power density and flat voltage
profile make LiFePO to be a great potential candidate of the lithium battery
3
-
polyanionic structures built of PO
4
6
4
cathode materials. In searching for new cathode materials, much work has also
been performed on other lithium transition metal phosphate system [4-6]. To be
noticed, typical studies have been reported on phosphates LiMPO
Mn, Cu) with isostructure of LiFePO [7-10], where LiMnPO
4
(M=Co, Ni,
4
4
appeals more
interest due to its high redox potential which matches with the electrolyte window
2

in the present Li-ion batteries. Actually, much effort has been done to optimize
LiMnPO by coating with conductive materials or doping in Mn-site or Li-site
11-13]. Since its first reported by Adam.L et al. [14], manganese pyrophosphate
Li MnP ) has also been widely investigated in both experimental and
4
[
(
2
2
O
7
theoretical calculation way [15, 16]. The manganese phosphates attract great
2
+
3+
attentions not only due to its high Mn /Mn redox potential, but also its multi
valence states which show possibility to achieve beyond one-electron reaction just
like Li
2
MnSiO [17].
4
Besides the two manganese phosphates mentioned above, only a few
manganese phosphates have been investigated, and ones still expecting there exist
other new lithium manganese phosphates which may be as a candidate cathode
material. Phase diagram is very important in searching for new materials and
material synthesis [18]. To our knowledge, no phase diagram of the
Li
2
O-MnO-P
2
5
O system is available yet. To find the new lithium manganese
phosphates, it will be very interesting to research the
Li
2
O-MnO-P
2
O
5
under reducing atmosphere. Here, the phase
relation of Li
2
O-MnO-P
2
O
5
ternary system has been systematically investigated
by means of X-ray diffraction (XRD). In order to avoid the appearance of trivalent
manganese, 95%Ar + 5%H reducing atmosphere was used in the synthesis
process of all samples. It will offer a better understanding for the phase relations
of the Li O-MnO-P system and provide an experimental reference in searching
2
2
2
O
5
of other new lithium manganese phosphates which may be as a candidate cathode
3

material. In addition, due to its much more attractive interests with high redox
potential for Li-ion batteries, the solid solution region with excess lithium doping
into manganese sites and reversely manganese doping into lithium sites in
LiMnPO
4
(nominated as Li1+xMn1-xPO ) was determined as -0.05 ≤ x ≤ 0.03 and
4
their corresponding lattice parameters were also reported here. Furthermore, the
influence of manganese sites doping with excess lithium and lithium sites doping
with excess manganese on the electrochemical performance of LiMnPO
also investigated.
4
were
2. Experimental
A total of 50 specimens with different compositions were prepared by two-step
solid-state method. There are 27 specimens for the two binary subsystems and 23
specimens for the ternary system.
Analytical grade Li
2
CO
3
, MnCO
3
and NH
4
H
2
PO
4
were used as starting
materials. The appropriate amounts of starting materials were grounded in an
agate mortar for 30 min. The mixtures of the samples at P
2
5
O poor regions within
o
the Li
expel NH
were reground again in the agate mortar for 30 min, pressed into pellets of about
0 mm in diameter and 2-3 mm in thickness, and then heated at 500-750 ℃ for 12
2
O-MnO-P
2
O
5
system were preheated at 350 C for 5 h in Ar atmosphere to
3
, H O and CO
2
2
. After cooling down to room temperature, the samples
1
h in an alumina boat, depending on the samples composition. The samples finally
cooled down to room temperature naturally. For the samples containing higher
4

mole ratio of P
2
5
O with lower melting temperatures of the reactants, the
preheating was performed in a slow heating rate to prevent the precursor
corroding with the alumina boat. In order to prevent forming of trivalent
manganese, all samples were heated under the 95% Ar + 5% H atmosphere.
2
The phase identification of these products was carried out by XRD using a
D/Max-2400(Rigaku) diffractometer with Cu Kα radiation. A graphite
monochromator was used for diffracted beams. The Rietveld refinement of
Li1+xMn1-xPO
4
(-0.05 ≤ x ≤ 0.03) samples and Mn
GSAS program.
The electrochemical characterization of Li1+xMn1-xPO
3
(PO
4
)
2
were performed by
4
(-0.05 ≤ x ≤ 0.03)
powders as cathode of the two-electrode electrochemical cells was measured by
an automatic battery tester system (Land®, China). The cathode of the
two-electrode electrochemical cells were fabricated by blending the prepared
powder with acetylene black and polyvinylidiene fluoride (PVDF) binder in a
weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP). The obtained slurry
was coated on Al foil, dried at 90 ◦C for 24 h and pressed (5MPa), respectively.
The electrodes fabricated were dried again at 90 ◦C for 12 h in vacuum and cut
into 0.8 cm×0.8 cm in size. Two-electrode electrochemical cells were assembled
in a Mikrouna glove box filled with high-purity argon, where the lithium metal
foil were used as anode, Celgard® 2320 as separator, and 1M LiPF6 in EC:DMC
(
1:1 vol.%) were used as an electrolyte. The electrochemical capacity
measurements were performed at the current rate of 5 mA/g between the voltage
5

range of 2.0 and 4.5 V.
Cyclic voltammetric (CV) tests and electrochemical impedance spectroscopy
(
EIS) experiments were performed on an AUTOLAB PGSTAT100 type
electrochemical workstation (Metrohm AG company). CV tests were carried out
-1
+
at a scan rate of 0.05mVs on the potential interval 2.5-4.5 V (vs. Li /Li). All tests
were performed at room temperature. Galvanostatic intermittent titration
technique (GITT) was conducted by applied a series of constant current step
(
discharge or charge at 0.01 mA for 30 min) followed by an interrupt step (rest at
current for 240 min) to the cell.
0
3
3
3
. Results and discussion
.1. Binary systems
.1.1. Li O-P O system
2
2
5
Three compounds have been reported in the Li
2
O-P
2
O
5
system: LiPO
3
, Li
4
P
2
O
7
and Li PO . These three compounds have been verified by our group during the
3
4
investigation of Li
2
O-P
2
5
O -FeO ternary system [19], and here they were also
confirmed again.
3
.1.2. MnO-P O system
2
5
Manganese phosphates are among the most wide spread minerals in the class
of phosphates, and four compounds with manganese valence of +2 were reported, i.e.
MnP 11[20-22], Mn (PO [23,24], Mn [25], Mn (PO [26-28]. Only three of
4
O
2
3
)
4
2
P
2
O
7
3
)
4 2
6

them, Mn
reported the new modification of manganese ultraphosphate MnP
heating MnC with polyphosphoric acid (Mn:P = 2:11) at 550℃ for 5 hours [20].
However, MnP 11 cannot exist under our synthesis condition, i. e., MnCO and
NH PO were used as the starting materials and solid-state method was adopted
under the 95% Ar + 5% H atmosphere. Mn (PO has a monoclinic system with the
space group of C2/c (Z=4) or I2/a ( Z=8) [23, 24]. Here we obtained Mn (PO with
a space group of C2/c. And we observed Mn existed under our experiment
2
(PO
3
)
4
, Mn
2
P
2
O
7
and Mn
3
(PO
4
)
2
, were confirmed here. Murashova et al.
4
O11 compound by
2
O
4
4
O
3
4
H
2
4
2
2
)
3 4
2
)
3 4
2
P
2
O
7
condition with a monoclinic symmetry belonging to the space group of C12/m1 as the
report [25].
For Mn
thermodynamically stable one at standard pressure, known as β’-phase, a
high-pressure phase with graftonite Fe (PO -type structure, called as α-phase[27],
and γ-phase that can be synthesized under hydrothermal conditions[28]. All these
three polymorphs adopt monoclinic system with same space group of P2 /n or P2 /c
SG No. 14) but different structural topology (crystallographical position) [28]. It has
been reported that pure manganese graftonite (high-pressure α-phase) seems never to
be prepared and solid solutions of the form (Mn1-x (PO with graftonite-type
structure can be formed, like a naturally occurring mineral with the chemical
composition of (Fe, Mn, Ca, Mg) (PO )[27]. Under high pressure, the β’-phase can be
transformed to be α-phase. A. G. Nord et al. obtained the α-phase Mn (PO by
at 600℃ for 24
3
(PO
4
)
2
, a monoclinic system with three modifications was reported: a
3
)
4 2
1
1
(
M
x
)
3
)
4 2
3
4
3
)
4 2
applying an extremely high pressure (2.5Gpa) to β’-phase Mn
3
(PO
)
4 2
7

h, where the β’-phase Mn
MnO and Mn in an evacuated silica tube at 1100℃ for two weeks[27].
Interestingly, Mn with the space group of P2 /c (crystallized in a high-pressure
α-phase) can be observed under our experiment condition, i.e., the mixture of MnCO
and NH PO heated in Ar atmosphere for a few hours firstly, pressed into pellets
and annealed under the 95% Ar + 5% H flow at 700 ℃ with intermediate grindings,
and then furnace cooled to room temperature. Here, the existing of high-pressure
α-Mn (PO in the Li O-MnO-P three-phase regions suggests that atmosphere can
3
(PO
4
)
2
were obtained by heating equimolar amounts of
2
P
2
O
7
3
(PO
4
)
2
1
3
4
H
2
4
2
3
4
)
2
2
2
O
5
be somehow as an important factor which causing the formation of high-pressure
phase under the ordinary solid-state reaction conditions.
In the literature, it only offered the index results (cell parameters) of x-ray powder
diffraction data, and the detail structure of α-Mn
27,28]. Fig. 1 illustrates the X-ray diffraction patterns of α-Mn
where the XRD patterns of β-Mn (PO , γ-Mn (PO , and graftonite Fe
which is isostructured with α-Mn (PO are also given for comparison. Here, the
structure of α-Mn (PO was characterized, and the XRD patterns could be indexed
to monoclinic structure. The number of chemical formulas per unit cell of the
3
(PO
4
)
2
has not been reported yet
(PO obtained here,
(PO
[
3
)
4 2
3
4
)
2
3
4
)
2
3
)
4 2
3
)
4 2
3
)
4 2
α-Mn
3
(PO
4
)
2
compound is 4. The cell parameters of a=8.817(7) Å, b=11.455(6) Å,
3
c=6.244(5) Å, β=98.96(7) °, and V=155.7(4) Å for α-Mn
3
(PO
4
2
) phase obtained from
Rietveld refinement is shown in Table 1. In addition, the cell parameters obtained
from the reference for Fe (PO [30], α-Mn (PO [27], ´-Mn (PO [26],
3
4
)
2
3
4
)
2
3
)
4 2
γ-Mn
3
(PO
4
)
2
[28] are also given in Table 1 for comparison.
8

Preliminary refinements of the tetragonal structure were performed by Rietveld
method [29] for α-Mn (PO compound, and the results are shown in Fig.2. The
weighted pattern R factor, RWP, is 2.54%. As illustrated in Table 2, all atoms of
α-Mn (PO occupy the same Wyckoff site(4e), but different crystallographically
3
)
4 2
3
)
4 2
sites. Table 3 contains interatomic distances computed from the nuclear positions
listed in Table 2. Each Mn1 atom is surrounded by a polyhedron of 6 oxygen atoms.
Mn2 and Mn3 are surrounded by 5 atoms respectively. The Mn1-O bond lengths
extend from 1.99(4) Å to 2.581(25) Å, Mn2-O from 1.947(26) Å to 2.529(34) Å, and
Mn3-O from 1.989(34) Å to 2.362(31) Å. Figure 3(a) shows the structure of
α-Mn
3
(PO
4
2
) , projected along the c-aixs direction, where three manganese atoms form
three distinct polyhedra with surrounding oxygen atoms. Mn2 and Mn3 are irregularly
coordinated by five oxygen atoms, and Mn1 has six nearest oxygen atoms in the
shape of highly distorted octahedron. As shown in Fig. 3 (b) of the assignment of
coordination for phosphorus and manganese atoms, each oxygen atom is bonded to
one phosphorus and two manganese atoms respectively.
3
.1.3. MnO-Li O system
2
The MnO-Li
2
O system in reducing atmosphere has been studied. In the system,
MnO , where manganese presents in
only one compound was reported in literature: Li
2
2
the +2 oxidation state. It should be noted that without any other literatures reported
about its existence under chemical condition except David’s report [31], in which
Li
2
MnO
2
can be synthesized by using butyllithium and LiMn
2
4
O through a chemical
9

way in room temperature. In our experiment condition, there is no evidence of the
existence of Li MnO
2
2
.
3
.2. Ternary Li O–MnO–P O system
2
2
5
3
.2.1 Ternary compounds
In this system, 4 kinds of ternary compounds containing Mn ion were verified in
the literatures. These are LiMnPO , Li MnP , Li Mn(PO , LiMn(PO . Here, all
four compounds can be obtained by solid-state method. LiMnPO was reported to
have an orthorhombic system with space groups of Pmnb [32] or Pbnm [33] or
Pnma[34,35]. In our experiment condition, LiMnPO was observed with the space
group of Pnmb. For Li MnP , we obtained it with the space group of P2 /a as
reported in Ref. [36]. Li Mn(PO was observed to have an monoclinic system with
the space group of Pnam in work [37], which is confirmed by our experimental result.
Murashova et al [38] reported that LiMn(PO adopt an orthorhombic system with
the space group of P2 , and our X-ray diffraction analysis verified this conclusion.
It should be noted that, inferior to what we expected, no other new lithium manganese
phosphates exists within the Li O-MnO-P ternary system under the reducing
atmosphere.
2
+
4
2
2
O
7
2
3
)
4
)
3 3
4
4
2
2
O
7
1
2
3 4
)
)
3 3
1
2
2
1 1
2
2
O
5
3
.2.2 The subsolidus phase relations
No experimental work has been reported for ternary Li
diagram. We have prepared 50 different compositions samples by two-step solid-state
2
O–MnO–P
2
5
O phase
10

method under the reduction atmosphere (95% Ar + 5% H
ternary Li O–MnO–P system were systematically investigated and shown in Table
. The ternary LiO -MnO-P system can be divided into 15 three-phase regions
under our experiment condition, as shown in Fig. 4, where the solubility of
(-0.05 ≤ x ≤ 0.03) is also labeled.
2
). The phase relations in the
2
2
O
5
4
2
2
O
5
Li1-xMn1+xPO
4
3
.3 The solid solutions of Li Mn PO
1+x
1-x
4
3
.3.1 Phase identification
Fig.5 shows the X-ray diffraction patterns of Li1+xMn1-xPO
4
compounds, and all
the peaks can be indexed with single phase orthorhombic Pmnb. No impurity phase
can be detected in the doping range of -0.05 ≤ x ≤ 0.03. Furthermore, the Rietveld
refinememt of the XRD data was carried out and a space group of Pmnb was chosen
as refinement model. In the hypothesis that the formulas were as prepared
stoichiometric Li1+xMn1-xPO (x = 0.03, 0.02, 0.00, -0.02, -0.04, -0.05) respectively.
4
The occupancy of lithium and manganese sites were constrained not to exceed 100%,
and the exceed lithium ions (or manganese ions) were set to occupy manganese site
(
or lithium site). The refinement results of Li1.03Mn0.97PO
compounds were shown in Fig.6. The reasonably small refinement factors indicate
that single phases of Li1+xMn1-xPO (-0.05 ≤ x ≤ 0.03) compounds were obtained in
our work. These results indicate that there exists a solubility of -0.05 ≤ x ≤ 0.03 for
samples. Comparing our results with that of Lin et al. [19], where the
has much larger solid solubility with -0.15 ≤ x ≤ 0.06 than that of -0.05 ≤ x ≤
4
and Li0.95Mn1.05PO
4
4
Li1+xMn1-xPO
4
LiFePO
4
11

0
.03 in the Li1+xMn1-xPO
4
compounds, the smaller solubility of LiMnPO may be
4
+
attributed to the relative larger difference of ionic radius between Li ion (0.76 Å) and
2
+
+
2+
Mn ion (0.83 Å) if one compare it of 0.76 Å to Li ion and 0.78 Å to Fe ion.
Actually, the situation of solid solution in LiMPO compounds is similar to the cation
mixing phenomenon which was detected in hydrothermally prepared LiFePO [39]
and LiMnPO [40]. Fang et al. [40] reported a suitable formula of
for hydrothermally synthesized LiMnPO sample, which exhibits
a much large solubility of manganese ions substitution onto Li-site. The great
difference in solubility of Li1+xMn1-xPO between the solid state method and
4
4
4
Li0.828Mn0.086MnPO
4
4
4
hydrothermal method may be owing to the lower synthesis temperature and the liquid
synthesized environment. However the cation mixing between lithium and manganese
ions on the crystallographic 4a and 4c site of the olivine structure can be often
observed which may be unfavorable for the electrochemical performance of the
LiMnPO
4
sample. The olivine structure LiMnPO consists of a distorted hexagonal
4
close-packed oxygen framework with Li and Mn occupy octahedral site and P in the
tetrahedral site [4]; this crystallographic environment makes one-dimensional tunnels
+
2+
through which lithium ions migrate. Partial occupation of Li lattice sites by Mn
will block the one-dimensional tunnels of lithium diffusion. Conversely, a partial
2
+
+
occupation of Mn lattice sites by Li , in order to insure local charge neutrality, the
+
stronger electrostatic attraction between oxygen and Li (4a) ions will be unfavorable
for lithium diffusion. In addition, the obviously difference of the ionic radius between
+
2+
Li and Mn will lead to the distortions of the one-dimensional tunnels which can
12

hinder lithium diffusion.
Fig. 7 shows the corresponding lattice parameters as a function of content x
obtained by the refinement results. It is noted that with the increasing of amount of
excess Mn-ion (x＜0) in the structure, the lattice parameters show elongation of the c
axis and shrinkage in the a and b axes and a consequent augment of the unit-cell
volume. While, the inversed tendency of lattice parameters with the content x can be
observed for the increase amount of excess Li-ion (x＞0). The difference gradient of
the lattice parameters change between x＞0 and x＜0 may due to the different
electronic environment caused by the different valence stage between the excess
2
+
+
Mn -ion and Li -ion, which will change the structure dependently.
3
.3.2 Electrochemical properties of Li1+xMn1-xPO
4
-10
-1
As we know that the low electronic conductivity(＜10 S cm ) and low ionic
-15
2
-1
diffusion coefficient(～10 cm s ) have been major obstacles hindering applications
of LiMnPO . Strategies such as doping and conductive carbon coating that employed
to LiFePO have also been resolved to LiMnPO , expecting improvement in
4
4
4
electrochemical performance. In the previous report [41], Chung et al. reported
+
appropriate doping metal supervalent Nb to Li in LiFePO
4
enable forming black
P-type conductors which leading to a significant improvement in electrochemical
properties.
Here, the electrochemical capacities of Li1+xMn1-xPO
4
(x = -0.05, -0.02, 0.00, 0.02,
0
.03) solid-solution were compared. The electrochemical tests were performed at a
13

current density of 5 mA/g between the voltages of 2.0 V and 4.5 V. Their initial
charge and discharge curves are shown in Fig.8. All Li1+xMn1-xPO samples exhibited
4
charge plateaus at around 4.2 V and the discharge plateau at around 4 V, showing an
+
average voltage of 4.1 V vs. Li/Li . In addition, the initial discharge capacity of 93
−
1
mAhg was obtained for pristine LiMnPO
Pieczonka’s result [42]. As shown in Fig.8, a decrease of initial discharge capacity for
solid solution Li1+xMn1-xPO compounds were observed. The discharge capacity of 76
4
compound, which agrees well with that of
4
−
1
−1
-1
−1
−1
mAhg , 42 mAhg , 72 mAhg , 61 mAhg and 35 mAhg can be obtained with the
x = 0.02, 0.03, -0.02, -0.04 and -0.05, respectively. For Li1+xMn1-xPO (x = 0.02, 0.03)
compounds, the loss capacity may due to the decrease of the active plot of Mn in
4
2
+
2
+
+
Li1+xMn1-xPO after partial Mn ion was substituted by Li , which will decrease the
4
2
+
3+
capacity contribution of the Mn ↔Mn conversion compared to the pristine. In
addition, the charge/discharge plateaus indicate that the solid solutions keep the phase
transformation between LiMnPO
4
and MnPO phase during the charge-discharge
4
process. No matter excess Mn-ion doping in Li-site or excess Li-ion doping into
Mn-site, the electrochemical curves of compounds show the same trend, which are
gradual decrease in discharge capacity and more unconspicuous platform. The change
of the platform in charge/discharge curves can be reflected more clearly in CV curves.
Fig. 9 shows the CV curves of the Li1+xMn1-xPO
4
(x = -0.05, 0.00, 0.03) samples at
−
1
+
a scan rate of 0.05 mVs on the potential interval 2.5-4.5 V (vs. Li /Li). We can find
that the electrochemical response of doped samples, especially the deoxidization
peaks, were weaker than the pristine one. It means the substitution in Li-site and
14

Mn-site can not improve the electrochemical activity.
The cycle performance of the Li1+xMn1-xPO (-0.05 ≤ x ≤ 0.03) samples are given in
4
Fig. 10. In order to investigate the rate capability during cycling, various current rates
of 0.03C, 0.1C, 0.5C and 1C were applied to the working electrode. Here, the rate
performance of Li1+xMn1-xPO
4
samples in Fig. 10 indicated a remarkable degradation
for the doping samples. It can be ascribed to the blocking effect caused by the cation
2
+
+
mixing between Mn and Li . In order to understand influence of block effect on
olivine phosphate LiMnPO , Electrochemical Impedance spectra (EIS) test have been
(-0.05 ≤ x ≤ 0.03), as shown in Fig. 11. The relevant
4
performed for Li1+xMn1-xPO
4
parameters, including the Warburg impedance coefficient σ, the diffusion coefficient
D, the double layer capacitance Cdl and the exchange current density i°, are calculated
based on A. Y. Shenouda et. al’s report and listed in table 5 [43,44]. With the
increasing of the doping content, an obvious decrease of the value of the D can be
observed for doped sample. Here, for the LiMnPO
4
sample, the value of the D is
samples, the values of
the D are 3.25×10 and 6.43×10 cm s respectively. In order to confirm the result
of the Li ion diffusion coefficient D, the GITT curves of Li1+xMn1-xPO in the voltage
of 2.0-4.5V are also performed. The typical GITT results for Li1+xMn1-xPO samples
-13
2 -1
2
.62×10 cm s . For the Li1.03Mn0.97PO
4
and Li0.95Mn1.05PO
4
-14
-14
2 -1
4
4
with x = 0.00 and x = 0.03 are shown in Fig. 12. The diffusion coefficients obtained
through GITT results are shown in Table 6 [45]. Compared with the EIS results, the
same decreasing trend was observed. To conclude, the poor capacity for
Li1+xMn1-xPO (x = -0.02, -0.04, -0.05) compounds in our experiment can be ascribed
4
15

to the blocking effect of Mn-ion after it occupied the Li-ion position, which will
+
impede the diffusion of Li during the electrochemical process, ultimately reduce the
electrochemical activity.
4. Conclusions
The phase diagram of Li
2
O-MnO-P
2
O
5
ternary system under reduction
atmosphere (95% Ar + 5% H
verified and the solid solution ranges of Li1+xMn1-xPO
determined. Inferior to what we expected, no other new lithium manganese
phosphates exist within the Li O-MnO-P ternary system under the reducing
atmosphere. In MnO-P system, a high-pressure phase Mn (PO with graftonite
Fe (PO -type structure is easily obtained under our experiment condition and its
detail structure is presented. In addition, the Rietveld refinement of the crystal
structure reveals that the lattice parameters of Li1+xMn1-xPO solid solutions vary
2
) has been investigated. Four ternary compounds are
4
(-0.05 ≤ x ≤ 0.03) is
2
2
O
5
2
O
5
3
)
4 2
3
)
4 2
4
linearly with the increase amount of excess Li-ion (x＞0) or with the increase amount
of excess Mn-ion (x ＜ 0). In addition, the electrochemical properties of
C-Li1+xMn1-xPO
4
(-0.05 ≤ x ≤ 0.03) compounds are also compared at a current density
of different current rates between 2.0 V and 4.5 V, and the results show that excess
Li-ion (x＞0) or the excess Mn-ion (x＜0) in LiMnPO have an unfavorable effect on
4
the electrochemical properties caused by the deterioration of the lithium diffusion
along the one-dimensional tunnels.
16

Acknowledgments
This work was funded by NSFC Grant (No. 51172077 & 51372089) supported
through NSFC Committee of China.
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Captions for Figures and Tables
Table 1 Crystal data for α -Mn (PO and related compounds
Table 2 Atomic sites and coordinates of α-Mn (PO sample determined from
Rietveld refinement of power X-ray Diffraction at 300K
Table 3 Bond distances for manganese and phosphor polyhedral
Table 4 Phase regions and phase compositions in the Li O-MnO-P
Table 5 the related parameters calculated data from EIS plots
Table 6 the Li-ion diffusion coefficient D calculated from GITT test results.
Fig. 1 X-ray diffraction patterns of α-Mn (PO and related compounds.
3
)
4 2
3
)
4 2
2
2
5
O ternary system
3
)
4 2
20

Fig. 2 Rietveld refinement results of α-Mn
Fig. 3 (a) Structure of α-Mn (PO , projected from the c-aixs direction; (b) Plot of
α-Mn (PO crystal structure showing polyhedra linkage.
Fig. 4 Subsolidus phase relations of the LiO -MnO-P
-0.05 ≤ x ≤ 0.03); (B) Li MnP ; (C) Li Mn(PO
phase; square: two phases; triangle: three phases.
Fig. 5 XRD patterns of Li1+xMn1-xPO (-0.05 ≤ x ≤ 0.03) solid solutions.
Fig. 6 Rietveld refinement results of Li1. 03Mn0.97PO
3
(PO
4
)
2
sample.
3
)
4 2
3
)
4 2
2
2
5
O system: (A) Li1+xMn1-xPO4
(
2
2
O
7
2
3
)
4
; (D) LiMn(PO
3
3
) ; dots: single
4
4
and Li0.95Mn1.05PO
4
samples.
Fig. 7 Lattice parameters of Li1+xMn1-xPO
4
(-0.05 ≤ x ≤ 0.03) samples as a function of
x content.
Fig.8 The initial charge-discharge curves of Li1+xMn1-xPO
a) x = 0; (b) x = -0.02; (c) x = 0.02; (d) x = -0.04; (e) x = 0.03; (f) x = -0.05.
Fig.9 CV curves of the Li1+xMn1-xPO4 (-0.05, 0.00, 0.03) samples
Fig.10 the cycle performance of the Li1+xMn1-xPO (-0.05 ≤ x ≤ 0.03) samples
Fig.11 Impedance spectra for Li1+xMn1-xPO (-0.05 ≤ x ≤ 0.03)
Fig. 12 the typical GITT results for Li1+xMn1-xPO samples with x = 0.00 and x = 0.03
4
(-0.05 ≤ x ≤ 0.03) samples.
(
4
4
4
21