Li reaction mechanism of MnP nanoparticles

Article abstract of DOI:10.1149/2.081205jes

MnP nanoparticles (MnPs) with a particle size of <30 nm were prepared by the reaction of dimanganese decacarbonyl and trioctyl phosphine (TOP) at 380°C under Ar atmosphere. Lithium reaction mechanism of the MnPs was investigated by using electrochemical cycling, ex situ X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). When lithium ion was first inserted into the MnP, it started to form LiMnP, LiP and LiP₇ phases. During lithium dealloying (charging to 2 V), LiMnP phase was turned into the LiP and LiP₅ phases.

Full text of DOI:10.1149/2.081205jes

Journal of The Electrochemical Society, 159 (5) A669-A672 (2012)
A669
0013-4651/2012/159(5)/A669/4/$28.00 © The Electrochemical Society
Li Reaction Mechanism of MnP Nanoparticles
Soojin Sim and Jaephil Cho∗,z
Interdisciplinary School of Green Energy and Converging Research Center for Innovative Battery Technologies,
Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea
MnP nanoparticles (MnPs) with a particle size of <30 nm were prepared by the reaction of dimanganese decacarbonyl and trioctyl
phosphine (TOP) at 380◦C under Ar atmosphere. Lithium reaction mechanism of the MnPs was investigated by using electrochemical
cycling, ex situ X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). When lithium ion was first inserted into the
MnP, it started to form LiMnP, LiP and LiP7 phases. During lithium dealloying (charging to 2 V), LiMnP phase was turned into the
LiP and LiP5 phases.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.081205jes] All rights reserved.
Manuscript submitted June 10, 2011; revised manuscript received February 15, 2012. Published March 12, 2012.
P-based compounds, especially metal phosphides (MPy: M = Fe,
Co, Cu, Zn, Sn, V, Ni, Mn, and Ti) have been studied as anode ma-
terial candidates because of their high gravimetric and volumetric
capacities. In addition, these materials showed the dynamic phase
changes depending on the potential window.1–8 MPs are categorized
into two groups depending on their Li insertion mechanism. First
are the compounds showing topotactic lithium reaction that main-
tains their pristine phase.3,9,10 Nazar et al. reported that lithium in-
tercalation in MnP4 occurred by an electrochemical redox process
according to a reversible reaction MnP4 ↔ Li7MnP4. The covalent
P-P bonds in the MnP4 are cleaved on Li insertion to produce crys-
talline Li7MnP4. Atom migration with bond rearrangement during
lithium desertion lead to electrochemical recrystallization reforming
MnP4 between 0.57 and 1.7V.9 SnP0.94 also demonstrated simple in-
tercalation mechanism without changing the Sn oxidation state and
can accept 4.5 mole of Li into the material. (SnP0.94 + 4.5Li ↔
Li4.5SnP0.94).10 Similarly MoP2 prepared by ballmilling of stoichio-
metric amount of Mo and P exhibits similar behaviors to SnP0.94 during
lithium reaction and exhibited capacity of 822 mAh/g (Li5.2MoP2).3
The second group is compounds showing metal alloy-
ing/metallization reaction represented by the following decomposi-
tion reaction: MPn ↔ M0 + LixP. Some examples are GaP,11 FeP2,12
CoP3,13 Cu3P,14 VP4,15 and ZnP2.16 For instance, Li reaction in CoP3
was proposed as follows: CoP3 → Li3P + Co → 3LiP + Co. Ac-
cordingly, the initial uptake of Li forms highly dispersed cobalt clus-
ters embedded in a matrix of Li3P; extraction of Li from this ion-
conductive matrix on charging yields nanoparticles of LiP without
oxidation of Co. LiP is an insulator, and once Li3P changed into LiP
in the above reaction, it had a detrimental effect on the reversibility
of the phosphorus electrode.13 However, ZnP2 have different lithium
reaction mechanism which does not form Li3P. ZnP2 showed simple
topotactic reaction until x = 2.6 in LixZnP2, after which it decom-
posed to Zn, ZnP2, LiZnP and LiP5 phases between 2 and 0V. As a
progress of charge, the LiP5 and Zn are reformed to ZnP2 and P, and
finally they are turned to ZnP, P, LiP, and LiZnP.16
the excess P sources of TOP. Finally, the black powder was vacuum
dried at 90◦C for one day. All the sample preparation was carried out
in the glove box under the Ar atmosphere.
For the electrochemical tests, Coin-type half-cell tests were con-
ducted, using MnP powder, Super P carbon black and poly(vinylidene
fluoride) (PVdF) binder in a weight ratio of 70:20:10. The slurry
was thoroughly mixed with N-methyl 2-pyrrolidinone (NMP).
1.05 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC)
(1:1 vol%) was used as the electrolyte. The cell was assembled in an
Ar-filled glove box. The cell was cycled at discharge and charge rates
of 0.1 C (= 72 mA/g) and 0.05 C, respectively, between 0 and 2 V. The
XRD measurements were performed using a Rigaku D/Max 200 with
Cu Kα radiation at 10 kW. The sample was observed using a scanning
electron microscope (SEM, JSM 6400, JEOL) and a transition elec-
tron microscope (TEM, JEOL 2010F). For the TEM measurement,
lithiated and delithiated MnP samples were detached from the Cu
electrode and was introduced into the glass vials containing ethanol
or acetone. After ultrasonic treatment, a droplet of solvent was placed
in carbon-coated copper grid. All these sample preparation was done
in the glove box.
Results and Discussion
Figure 1 shows the powder X-ray diffraction pattern of MnPs,
which can be indexed to an orthorhombic structure with a Pmna space
group. Figure 2a exhibits the SEM images of the as-prepared MnPs,
and it consists of aggregated nanoparticles with <100 nm. Figure 2b
shows the TEM image of pristine MnPs and their primary particle sizes
range between 5 nm and 30 nm. MnPs are surrounded by amorphous
phase, and expanded images (Figs. 2c and 2d) of Fig. 2b exhibit the
lattice fringes of (011) and (211) planes corresponding to d-spacing
values of 2.74 Å and 1.94 Å of MnP, respectively. Energy dispersive X-
ray spectra (EDXS) at spots 1 and 2 (Fig. 2e) confirm that amorphous
region also consists of MnP phase (Fig. 2f).
Voltage profiles of the MnP electrode between 0 and 2 V at a rate
of 0.1 C for discharge and 0.05 C for charge in coin-type lithium
half cells at 21◦C are illustrated in Figure 3a. The first discharge
and charge capacities of the MnP were 1242 mAh/g and 361 mAh/g,
respectively, showing irreversible capacity ratio of 70%. However,
columbic efficiency of MnPs after 2nd cycle was almost 80% and
displays relatively good capacity retention out to 10 cycles. Based
upon its capacity estimate, the insertion amount of lithium ions is
estimated to 4 mole per MnP (Li4MnP) during the first lithiation to
0 V. Figures 3b and 3c shows the differential capacity plots (DCPs) of
MnPs during the first and second cycles, respectively. DCPs show a
sharp and a broad peak at 0.57 and 0.34 V, and two broad peaks at 0.7
and 1.1 V during the first discharge and charge cycles, respectively.
In order to figure out such phase transitions, ex situ XRD experiments
at selected potentials during the first cycle as indicated in Fig. 3a
were carried out and results are given in Figure 4. Similar to XRD
patterns of the bulk and nano-sized metal phosphides,1,2,5,6 our data
To the best of our knowledge, there have been no reports of lithium
reaction mechanism of MnPs. Here we report the synthesis and lithium
reaction mechanism of MnPs prepared by the reaction of dimanganese
decacarbonyl and TOP at 380◦C under Ar atmosphere.
Experimental
Orthorhombic MnPs were synthesized by a reflux method at 380◦C
in schlenk line. 1.5 g of Mn2(CO)10 (98%, Ardrich) is dissolved in
100 mL of TOP (90%, Ardrich) and heated with stirring for 2–4 h at
380◦C. When the solution color was changed from yellow to black, it
was cooled to a room temperature and that was rinsed with ethanol and
chloroform with centrifuging 2 times at 5000 rpm in order to remove
∗
Electrochemical Society Active Member.
z E-mail: jpcho@unist.ac.kr
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A670
Journal of The Electrochemical Society, 159 (5) A669-A672 (2012)
and LiP7 phases get dominant. Especially, LiP phase formation from
the LiMnP phase should be accompanied by Mn phase according to a
following reaction: LiMnP → LiP + Mn. Although no clear evidence
of Mn nanoparticles are observed from the analyzes, decomposed
LiMnP phase may lend the formation of Mn phase. Other MPs, such
as Cu3P, CuP2, NiP2, VP2, and CoP3 were also proposed the forma-
tion of metal phase in spite of no direct evidence was supplied.13,17–21
During the first charge, complete disappearance of residual LiMnP
phase at 0 V and increasing peaks intensities of LiP5 and LiP7 phases
are noticeable (point 6). This means that LiMnP and LiP7 phases are
decomposed to LiP5 and LiP phases. However, at point 7 (2 V), peaks
intensities of LiP and LiP5 phases get decreased, while any new peaks
are detected. This result suggests the possible particle pulverization
of LiP and LiP5 phases.
20
25
30
35
40
45
50
55
60
Figure 6 shows high resolution TEM images and corresponding
DFTI of the electrode after first discharge to 0 V. DFTI of point 1
confirms the presence of the LiP7 which has (211) and (413) planes
Scattering angle (2θ) /degree
Figure 1. XRD patterns of the MnP nanoparticles. (#JCPDS = 78-0267)
and an inset is orthorhombic MnP structure (pink = phosphorus, white
= manganese).
¯
along the [222] zone axis, and that of point 2 is matched with (220)
¯
and (004) phases along the [880] zone axis, and also correspond to
LiP7. Figures 7a and 7b exhibit TEM images after first charge to
2 V, and compared with TEM image of the pristine sample, particles
are pulverized and some of them were grown into larger ones. DFTI
also exhibit rather poor resolution with approaching higher degree
of lithium intercalation/deintercalation. This phenomena is related to
the formation of amorphous phases and particle pulverization.1,2,5,6
In spite of this, each peak can be clearly identified.
¯
at point 1 shows the presence of (120) and (112) phase along the [423]
zone axis, and those planes correspond to LiP5 phase. DFTIs of points
¯ ¯
¯ ¯
At point 2 (0.8 V), MnP phase start to decompose with concomi-
tant formation of LiMnP phase and at point 3 (0.34 V), and LiMnP
and LiP7 phases get dominant. Digitalized Fourier-transformed image
(DFTI) of MnP electrode after discharging at 0.34 V (point 3) also con-
firms the formation of LiMnP phase. Selected area diffraction pattern
2 and 3 show (111) and (112) planes along the [110] zone axis, and
(110) and (111) along the [112] zone axis, and these planes agree
with those of LiP phase. These results agree with XRD patterns, and
it can be proposed that both DCP peaks at 0.57 and 0.34 V during the
first discharge are related to phase transitions from MnP to LiMnP
and from LiMnP to LiP7, respectively. The peaks at 0.7 and 1.1 V
are believed to be related to phase transitions between LiP5 and LiP7.
During the second cycle, DCP shows an absence of the sharp peak at
¯
¯
(Fig. 5b) of Fig. 5a shows (110) and (102) planes along the [221] zone
axis, corresponding to tetragonal LiMnP phase. When it is further dis-
charged to 0 V (point 4), peaks intensity of LiMnP decreases and LiP
Figure 2. (a) SEM and (b, c, d, and e) TEM images and EDXS of pristine MnPs.
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Journal of The Electrochemical Society, 159 (5) A669-A672 (2012)
A671
x in LixMnP
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
MnP
LiMnP LiP7
LiP5
LiP
*
(a)
2.5
2.0
1.5
1.0
0.5
0.0
Charge
1
7
(7)
6
2
5
(6)
(5)
3
4
*
*
*
0
200
400
600
800
1000 1200
Discharge
*
*
*
*
*
Capacity(mAh/g)
(4)
(b)
*
*
1.1V
0.7V
(3)
0.57V
(2)
(1)
0.34V
1st cycle
0.0
0.5
1.0
1.5
2.0
Potential V vs (Li+/Li)
30
35
40
45
50
55
60
(C)
2θ (degree)
1.1V
Figure 4. Ex situ XRD patterns of the MnP electrodes obtained from Fig. 3a.
0.7V
0.23V
(a)
(b)
2nd cycle
0.0
0.5
1.0
1.5
Potential V vs (Li+/Li)
Figure 3. (a) Voltage curves of the MnP electrode between 2 and 0 V in a
coin-type lithium half cell at rates of 0.1 and 0.05 C during the discharge and
charge, respectively (numbers on the first charge and discharge curves indicates
the voltages XRD patterns were taken) and (b and c) Differential capacity plots
as a function of potential after (a) first and (b) second cycles.
(-110)
10 1/nm
(-102)
Figure 5. (a) High resolution image of MnP after discharging to 0.34 V and
(b) selected area diffraction (SAD) pattern of (a).
Figure 6. (c and d) TEM images of MnPs after discharge to 0 V, and (1 and 2) SAD patterns of (c and d) respectively.
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A672
Journal of The Electrochemical Society, 159 (5) A669-A672 (2012)
account for capacity loss in the metal phosphides because microstruc-
ture control did not affect the capacity enhancement.23 Accordingly,
electrolyte decomposition reactions catalyzed by LiPx decomposition
may enhance the cracking and crumbling of the electrode during cy-
cling, resulting in a continuous generation of new active surfaces that
were previously passivated by the stable surface films.
Conclusions
MnP nanoparticles with particle sizes of <30 nm were prepared
by the simple thermal decomposition method. When lithium ion was
first inserted into the MnP, it started to form LiMnP. However, LiMnP
phase was decomposed to LiP and LiP7 phases with decreasing the cell
potential to 0 V. During lithium dealloying (charging to 2 V), LiMnP
phase was turned into the LiP and LiP5 phases. Low coulombic ef-
ficiency of the MnP nanoparticles during the first cycle was related
to irreversible phase transitions of LiMnP phase to LiPx phases. De-
composition and pulverization of the LixP phase led to continuous
capacity fading of MnP.
Acknowledgments
This research was supported by the MKE (The Ministry of Knowl-
edge Economy), Korea, under the ITRC (Information Technology Re-
search Center) support program supervised by the NIPA (National IT
Industry Promotion Agency) (NIPA-2012-C1090-1200-0002).
Figure 7. (a and b) TEM images of MnPs after charge to 2 V, and (1, 2, and
3) SAD patterns of the areas 1, 2 and 3 in (b).
0.34 V but other peaks intensities are quite similar to that of the first
cycle. This means that phase transition between LiP and LiP7 phases
are relatively reversible. However, it should be noted that irreversible
phase transitions of LiMnP phase to LiPx phases, along with the
formation of inactive LiMnP led to very low coulombic efficiency
during the first cycle.
Figure 8a and 8b show the XRD pattern of the MnP electrodes after
2nd and 10 cycles respectively. After 10 cycles, both LiPx phases are
observed but their peak intensities decreased. This means that such
phases are decomposed into P and Li or are pulverized to nanoparti-
cles, leading to continuous capacity fading of MnP.
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