Res.Rep.Fac.Eng.Mie.Univ.,Vol.30,pp. 1-12 (2005)

1

Original Paper

All solid-state Li-ion batteries based on intercalation electrodes
and poly (ethylene oxide)-LiX electrolytes
Y. Liua, Y.Onob, T. Matsumurab, A. Hiranob, T. Ichikawab, N. Imanishib and Y. Takedab(a Satellite Venture Business Laboratory, b Department of Chemistry for Materials)(Received September 20, 2005)
AbstractAll-solid state rechargeable Li-ion batteries based on poly (ethylene oxide) electrolytes arepromising power sources for large application such as electric vehicles (EV), which could be likely
to require safety and reliability as well as high energy density and long cycle life. However, so far,state-of-the-art electrode materials are limited by the performance factors such as electrochemical
instability to lithium insertion/extraction, particularly at the solid/solid interface between theelectrodes and the PEO electrolytes. The efforts on conquering above dilemma have been carriedout over several decades. Recently, we developed novel or modified insertion type electrodes,
which demonstrate good electrochemical behavior with the PEO electrolytes at the elevatedtemperature. In this paper, preparation and characterization of the electrodes proposed are presented
and discussed in detail.
Key words: Power sources; Rechargeable lithium ion batteries; All solid-state PEO electrolytes;
Insertion electrodes; Energy density; Thermal safety.

1. Introduction
Solid-state rechargeable batteries, especially, lithium ion batteries, are principle and promising powersources for a wide variety of electronics. PEO electrolyte formed by dissolving a lithium salt in a host poly
(ethylene oxide) (PEO) shows high ionic conductivity (σ >10−4 S cm−1) at the elevated temperature. Rechargeable
lithium batteries based on the PEO electrolytes have been proposed to overcome low energy density of previously
used batteries such as lead acid and nickel metal hydride and to improve safety of conventional lithium-ion
batteries with an inflammable liquid electrolyte [1]. The PEO type cells have several benefits such as feasible
design and ductile morphology which can absorb volume change for Li insertion/extraction in electrodes.
Although the high temperature limitation (>60 ℃) of the PEO cell is a drawback for applications in the consumer
electronic market, it is probably an advantage for numerous other applications such as for electric vehicles (EV).
For these large size batteries in EV, safety and reliability are the most important issues. Previously developed
lithium polymer batteries contain lithium metal anodes. A lithium battery incident at Los Angeles International
Airport on 28 April, 1999 suggests that safety of a lithium battery with a large amount of lithium metal isquestionable [2]. In addition, an improvement on the ionic conductivity and Li+ transfer number has to be done toachieve a relatively high charge rate. Therefore, so far, research has been extensively concentrated on two major
points such as 1) the improvement of ionic conductivity of PEO-LiX complex at the relatively low temperatureand 2) searching for novel or modified electrodes which can deliver good interfacial compatibility with the solid
PEO electrolytes. By this motivation, in the anode side, researchers have undertaken to replace the lithium metal
by a safer one, such as insertion anode, with a high capacity and an excellent reversibility. Lithium-insertion hosts
such as carbon, Li-alloy and Li-M-O are promising alternative anodes [3, 4]. Unfortunately, carbon still cannot be
adopted successfully with the PEO electrolytes due to the interfacial incompatibility, regardless of its wonderfulelectrochemical manners in the liquid system. On the other hand, some Li-M-O materials, e.g., spinel Li4Ti5O12,
demonstrate zero-strain effects for Li-insertion, resulting in long cycling life with the solid PEO electrolytes.
However, relatively low capacity and high Li reactive potentials of lithium titanates make them incomparable in
the energy density. On the other hand, in the cathode side, the most common cathodes such as LiCoO2, LiNiO2,
LiCoxNi1−xO2, and LiMn2O4 all suffer from side reaction with PEO electrolytes and detrimental phase transitions
above 70 ℃ that inevitably cause to capacity loss and fade while being oxidized at a high potential. Meanwhile,state-of-the-art electrode materials are limited by the performance factors such as electrochemical instability to2

Y. Liu, Y.Ono, T. Matsumura, A. Hirano, T. Ichikawa, N. Imanishi and Y. Takeda

lithium insertion/extraction, particularly at the interface between the electrodes and the PEO electrolytes.
Therefore, development in the electrode materials in associated with the PEO electrolytes would be urgent.
2. Experimental
2.1 Materials preparations
To produce the hexagonal lithium transition-metal nitrides, a given ratio of Li3N and powders of transition
metals was homogeneously mixed in an Ar atmosphere. The mixture was pressed into a tablet with 8 mm indiameter and 5-8 mm in thickness and heated at 700℃ for 12 h under N2 stream with heating rate of 35 ℃ min-1.
The reactions were allowed cool down to room temperature normally. For the compounds containing Fe, theheating temperature was increased to 800 ℃. The resulting products were ground in a glove box and further
treated by high-energy mechanical milling (HEMM) at a rotational speed of 500 rpm for 20 h. The modification
for the graphitic carbon was as follows: a given amount of graphitic carbon was dried under vacuum at hightemperature, and then was mixed with metallic lithium and selected organic solvent under a certain ratio underinert atmosphere. The mixture was treated by high energy mechanical milling under inert atmosphere. The milled
product was further dried under high temperature to entirely remove the residual organic solvent. Ultrafine SnSb
alloy powder (particle size < 0.2 µm) was prepared by chemical precipitation from aqueous solutions containing
the reductive agent NaBH4 and respective metallic salts in the presence of complexants such as citrates. The
Co3O4 powder (< 2 µm) was obtained by a thermal decomposition of CoCO3 at 800 ℃ in air. LiNi0.8Co0.2O2 (ca.
10 µm) was prepared from the mixture of Li2O2 (Aldrich) and (Ni0.8Co0.2)O precursor which was obtained fromthe decomposition of the hydroxide at 300 ℃ for 1 h. The mixture was heated at 700 ℃ for 24 h under O2 gas
flow. LiFePO4 and LiFePO4/C (PVC as carbon sources) was prepared as follows: a certain amount of Li2CO3,
poly (vinyl chloride) (PVC), NH4H2PO4 and FeC2O4 was homogeneously mixed by the aid of THF. The mixture
was heated firstly at 350 ℃ for 3-6 h and smashed at room temperature. The smashed products were further
heated at 700-900 ℃ for 16 h under Ar. The final samples were ground and sieved. Powder X-ray diffraction(XRD) patterns were obtained using automated powder diffractometer with Cu Kα radiation (Rotaflex RU-200B,
Rigaku-denki Corporation). The morphological performance of the materials was characterized with scanningelectron microscopy (SEM) and Electron Probe Micro-Analysis (EPMA).
2.2 PEO Electrolytes and cathode film
PEO electrolytes (Li/O ratio: 1/18) were prepared under a casting technique. All the procedure was protectedby Ar atmosphere in the glove box. A given weight of PEO (MW = 6 × 105) and LiN(CF3SO2)2 was dissolved
completely in anhydrous acetonitrile (AN). BaTiO3 (ca. 0.1 µm) was dispersed homogeneously in the solution as
filler. After strong stirring overnight the viscous solution was cast into a Teflon dish. After AN was slowly andcompletely evaporated under a N2 flow, the obtained film was further dried at 90 ℃ under vacuum at least 8 h.
The conductivities of the composite polymer electrolyte were observed as high as 1.7x10-3 S cm-1 at 80 ℃ and
0.82x10-3 S cm-1 at 65 ℃ and the decomposition potential was estimated to be more than 4.2 V (vs. Li/Li+). The
cathode electrode consisting of 52 wt.% active hosts (LiFePO4 or LiNi0.8Co0.2O2), 10 wt.% acetylene black(AB)and 38 wt.% PEO-LiN(CF3SO2)2 (Li/O ratio: 1/18) were prepared by the casting method similar to PEO film andthe thickness was about 200 µm.
2.3 Anode preparations
For all the electrodes, a given weight of the electrode components was homogeneously mixed in an agate
mortar in a glove box and further pressed onto a 300-mesh stainless steel grid, which served as a current collector.
The geometric area of the electrodes was 0.55 cm2, and the typical thickness was 100~160 µm.
2.4 Electrochemical and thermal stability (DSC) measurements
To evaluate the electrochemical properties of the electrodes, a half-cell containing Li metal as the counterand LiPF6 / EC+DMC (Ethylene carbonate plus diethyl carbonate as 1:1 in volume) electrolytes (in case of thecells with the liquid electrolytes) was used. Basically all the three layers, including test electrode, separator and Li
metal, were stacked in a 2025 coin type cell in a glove box. For the cells based on the PEO electrolyte, theseparator was replaced by the PEO film, which also served as electrolyte. The full solid-state cell was constituted
by replacing lithium metal with the cathode films. A small constant pressure was kept inside cells by means of theNi foam as filler. Before the electrochemical test, the PEO based cells were preheated for 2 hrs at a temperature of
75℃. Unless stated elsewhere, the cells were carried out at a constant current density of 0.1-0.15 mA cm-2. The
rest time between charge and discharge was 1 min.

All solid-state Li-ion batteries based on intercalation electrodes and poly (ethylene oxide)-LiX electrolytes

3

3. Results and discussion
3.1 Anodes

2.0a) Cycle 1
2 43

1.5

1

1.0

1) Li2.6Co0.4N2) Li2.6Co0.2Cu0.2N3) Li2.6Co0.2Ni0.15Fe0.05N4) Li2.6Co0.2Cu0.15Fe0.05N

0.5

Voltage / V

0.0
0

200

400

600

800

1000

2.0

b) Cycle 10
1.5

2

31 4

1.0

0.5

0.0
0

200

400

600

800

1000

-1

Capacity / mAh g

Fig.1. Charge and discharge curves of the electrodes
based on lithium transition-metal nitrides at the first andtenth cycle, voltage cutoff: 0-1.4 V, vs. Li/Li+. Electrodecomposition: 20 wt.% AB, 70 wt.% lithium metal
nitrides and 10 wt.% PVDF.
1200

Dis. capacity / mAh g

-1

1000

800

600

Li2.6Co0.4N
Li2.6Co0.2Cu0.2N
Li2.6Co0.2Ni0.15Fe0.05N
Li2.6Co0.2Cu0.15Fe0.05N

400

200

0
0

10

20

30

40

50

Cycle number

Fig.2. Cycling performance of the electrodes based onlithium transition-metal nitrides. Electrode compositions
are the same as in Fig.1.

3.1.1 Electrochemical behavior and thermal stabilityof the composite electrodes.
Lithium metal (transition) nitrides withhexagonal symmetry, P6/mmm, are composed of M
(M=Co, Cu, Ni) substituting lithium between the
Li2+N3- layers of Li3N [5,6]. Fig.1 shows theelectrochemical
behaviorofthelithiumtransition-metal nitrides prepared from solid-state
reaction and high-energy mechanical milling(HEMM). Due to the vacant sites introduced by the
doped transition metals, the fully lithiated nitrides
can still allow a small amount of lithium intercalationin the first cycle. The compounds gradually undergo
an irreversible transformation from a crystalline
phase to an amorphous one in the first Li-extraction
stage, as shown in Fig.1a. Compared with Li2.6Co0.4N,
the slight increase of the co-doped nitrides in the
charge potentials indicates that part Co substituted by
Cu, Ni and Fe may be obstructive to the lithium
extraction [7,8]. In the subsequent cycle shown in
Fig.1b, the charge potentials become slope and alarge amount of lithium can be reversibly
re-intercalated and extracted, resulting in highcapacities of 700-950 mAh g-1 which are about 2-3
times over that of the commercial graphite. The
discharge potential curves, on the other hand,demonstrate a hystersis of ca. 0.5 V compared withthe charge ones, indicating the different
electrochemical kinetics for Li intercalation andextraction. A comparison with Li2.6Co0.4N in thecycling performance reveals that the co-doped
compounds have a remarkably enhanced cyclability
shown in Fig.2. The significant improvement in thecycling performance for the co-doped nitrides can be
attributed to the improved structural stability inassociation with the Li extraction degree and theenhanced interfacial compatibility [7,8]. Lithium
metal nitrides are very attractive in the packing
density and volumetric capacity due to the density
similar to the current graphite. A major hindrance tothe nitrides as promising anode alternatives forgraphite is that they possess Li-rich structure andtherefore cannot directly combine with the typicallyhigh-potential cathodes such as LiCoO2 and LiMn2O4
to constitute Li-ion cells. This deterrent can beovercome by introducing an appropriate amount of
Co3O4 into the electrodes containing above lithium
metal nitrides.

Fig.3 shows charge and discharge curves of theb)
Li2.6Co0.2Cu0.2N
and
c)
a)
Co3O4,
Co3O4-Li2.6Co0.2Cu0.2N based electrodes vs. lithium
under a potential of 0-1.4 V with liquid electrolytes at room temperature. The Co3O4 electrode has a large
insertion capacity of 1150 mAh g-1 along with a potential plateau of about 1.05 V in the first cycle. However, its
capacity retention in the subsequent cycle shown in Fig.3a is below to 180 mAh g-1 under the potential range of

4

Y. Liu, Y.Ono, T. Matsumura, A. Hirano, T. Ichikawa, N. Imanishi and Y. Takeda

0-1.4V. This is attributed that this material has to be electrochemically activated under a very wide potential as 0-3
V [9]. The electrochemical behavior of the Li2.6Co0.2Cu0.2N, which has a high extraction capacity as 800 mAh g-1
with a potential plateau of 1.0 V but an insertion capacity low as 230 mAhg-1, is obviously in contrast to that of
the Co3O4, as shown in Fig.3b. In the case of Li2.6Co0.2Cu0.2N plus Co3O4, a very high first charge recovery of ca.
100% shown in Fig3c is remarkable while the cycling is started from an insertion process. Moreover, thecomposite electrode does not show an obvious potential plateau in the charge stage that reflects the coexistence oftwo phases in the active hosts similar to that of the Li2.6Co0.2Cu0.2N. Thus, it comes to conclusion that
Li2.6Co0.2Cu0.2N in the composite electrode might be discharged from a delithiated or an amorphous state. The
Li-intercalation potential of Co3O4 (ca.1.1V) is slightly higher than the Li-extraction potential of lithium metal
nitride (ca.1.0V); therefore Co3O4 can thermodynamically extract lithium from the nitrides. XRD measurements
confirm this point because lithium transfer into Co3O4 would be evidenced by a change in the peak intensities forboth the hosts. We proposed the reaction mechanism of the composite electrode as follows: Li2.6Co0.2Cu0.2N +(1)
xCo3O4 → Li2.6-8xCo0.2Cu0.2N + 4xLi2O + 3xCo
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5

2.5
2.0

a. Co3O4

Cycle 1
Cycle 2Li extraction

1.5
1.0
0.5

Li insertion

0.0
0

200

400

600

800

1000

1200

Li extraction

I / mA

Voltage / V

b. Li2.6Co0.2Cu0.2N

1.5
1.0
0.5

Li insertion

0.0
0

200

400

600

800

1000

1200

2.5
2.0

c. Li2.6Co0.2Cu0.2N-Co3O4
Li extraction

1.5
1.0
0.5

Li insertion

0.0
0

200

400

600

800

1000

1200

cycle 2cycle 1
0.0

2.5
2.0

c. Co3O4

2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0

0.4

0.6

b. Li2.6Co0.2Cu0.2N

0.8

1.0

1.2

1.4

1.0

1.2

1.4

1.0

1.2

1.4

Cycle 1

Cycle 2
0.0

2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0

0.2

0.2

0.4

0.6

0.8

c. Li2.6Co0.2Cu0.2N-Co3O4

Cycle 2

Cycle 1
0.0

0.2

0.4

-1

Capacity / mAh g

0.6

0.8

E/V

(Left) Fig.3. Charge and discharge curves of different cells based on a) Co3O4, b)
Li2.6Co0.2Cu0.2N and c) Co3O4-Li2.6Co0.2Cu0.2N, voltage cutoff: 0-1.4 V, vs. Li/Li+. Electrodecompositions: a) 80 wt.% Co3O4, 10 wt.% AB and 10 wt.% PVDF; b) 70 wt.%
Li2.6Co0.2Cu0.2N, 20 wt.% AB and 10 wt.% PVDF and c) 50 wt.% Li2.6Co0.2Cu0.2N, 30 wt %Co3O4, 10 wt.% AB and 10 wt.% PVDF.
(Right) Fig.4. Cyclic voltammogram of the electrode based on a) Co3O4, b) Li2.6Co0.2Cu0.2N
and c) Co3O4-Li2.6Co0.2Cu0.2N at the scan rate of 0.05 mV S-1, voltage cutoff: 1.4-0. V, vs.
Li/Li+. Electrode composites are the same as in Fig.3.
Li2.6-8xCo0.2Cu0.2N + zLi+ + ze- = Li2.6-8x+zCo0.2Cu0.2N(2).

Reaction (1) is a chemical reaction and reaction (2) is an electrochemical one. If Li2.6-8xCo0.2Cu0.2N is thecomposition of the fully charged state, then reaction (2) becomes reversible during charge and discharge even at
the first cycle. From the experimental results shown in Fig. 3, we estimated x and z as 0.16 and 1.7, respectively.
These values correspond to 60/40 weight ratio of Li2.6-8xCo0.2Cu0.2N/Co3O4 and the reversible capacity of 477
mAh g-1. The capacity was calculated by including Li2.6-xCo0.2Cu0.2N and Co3O4 as active hosts. We observed afirst charge efficiency of 100% and a reversible capacity of ca. 520 mAh g-1 for the composite electrodecontaining of 62.5 wt. % Li2.6-xCo0.2Cu0.2N plus 37.5 wt % Co3O4, as shown in Fig. 3c. The high first cycle
efficiency proposed can greatly extend the capacity utilization for the cathode. It is very remarkable because thelow charge efficiency of the anodes in the first cycle might consume extra capacity from cathodes and thereby

All solid-state Li-ion batteries based on intercalation electrodes and poly (ethylene oxide)-LiX electrolytes

5inevitably reduce the total energy density of the cells. Cyclic voltammogram was further conducted on theelectrodes based on different active hosts with a scan rate of 0.05 mV s-1 in the potential scale of 0-1.4 V vs.
Li/Li+, as shown in Fig.5. There is one cathodic peak at about 0.95 V in the first Li-insertion process for Co3O4(Fig.4a. However, this peak does not repeat in the subsequent cycle, indicating that Co3O4 turns into inert tolithium. In the case of Li2.6Co0.2Cu0.2N, an obvious anodic peak shown in Fig.4b appears at 1.2 V in the first cyclethat corresponds to the two-phase transformation. In the subsequent cycle, one cathodic peak at 0.18 V and two
anodic peaks at 0.78, 1.12 V become visible and remain to be stable. However, the large hystersis in the cathodic
and anodic processes seems a drawback and has to be further conquered. In comparison with Li2.6Co0.2Cu0.2N andCo3O4, obviously, the behavior of the composite electrode shown in Fig.4c is highly consistent with that of
Li2.6Co0.2Cu0.2N, suggesting that the active host in the composite is mainly associated with lithium metal nitrides.
Furthermore, a weak cathodic peak at 0.95 V corresponding to the Co3O4 indicates that the thermodynamicallyspontaneous reaction may not entirely drive lithium into the Co3O4 before the electrochemical cycling. The
composite electrodes demonstrated high cycling stability, due to that they could be benefited from the low volume
effects of the lithium metal nitrides upon Li insertion and extraction. As measurement, the charge yield of thecomposite electrode during cycling is always at 100%, indicating that Co3O4 remains inactive and stable in theelectrode while lithium in the nitrides can be electrochemically intercalated and extracted with high reversibility
under a suitable potential window. The composite electrode consisting of Li2.6-xCo0.2Cu0.2N-Co3O4 was found toshow good electrochemical behavior with the solid PEO electrolytes. A high charge efficiency of 99.6 % and alarge capacity of 520 mAh g-1 shown in Fig.5 become feasible, indicating that lithium can be electrochemically
2.5

2.5

c) η1=103%

1.5b) η1=99.6%

2.0
1.5

Voltage/ V

Voltage/ V

2.0

Cycle1
Cycle2
Cycle31.0
0.5

Cycle 1
Cycle 2
Cycle 3

1.0
0.5
0.0

0.0
0

100

200

300

400

500

600

700

0

100

200

300

400

500

600

700

-1

-1

Capacity / mAhg

Capacity/ mAhg

(Left) Fig.5. Charge and discharge profiles of the SnSb-Li2.6Co0.4N electrode with PEO at 65 oC.
(Right) Fig.6. Charge and discharge profiles of the Co3O4-Li2.6Co0.2Cu0.2N electrode with PEO at
65 oC.

2.6

0.4

Capacity / mAh g-1

intercalated into and extracted from the nitrides withhigh reversibility while operating with the PEO
1000electrolytes.
900
Filled: Li insertion
On the other hand, the composite electrode based
800
Open: Li extractiona. SnSb-Li Co Non
SnSband Li2.6Co0.4N also demonstrates a high first700
chargeefficiencyof 103 %, which is attributed to the fact
600
that
lithium
metal
nitrides can make a capacity
500
compensationfortheultrafine alloy hosts in the first
400
cycle
[10].
As
can
be
seen,
the charge-discharge behavior
300
CoN
composite
electrode reflects theoftheSnSb-Li200
2.6
0.4
b. Co O -Li Co Cu N
mixingelectrochemical
characteristicsof these two types
100
of active hosts, as shown in Fig.6. The capacity changes,
0
0
10
20
30
40
50
as a function of cycling, for different anodes with the
Cycle number
solid PEO electrolytes are shown in Fig. 7. The
SnSb-Li2.6Co0.4N anode has a large capacity of over 600
Fig.7. Cycling performance of the composite mAh g-1 in the cycling beginning. However, a gradualanodes with PEO electrolytes.
deterioration in the cycling performance is always
observed. A replacement of Li2.6Co0.4N with theco-doped nitrides proposed does not bring any progress, indicating that the volume effect from the Li-alloying
reaction possesses a morphological instability at the interface while operating with the solid-state PEO electrolytes.
3

4

2.6

0.2

0.2

6

Y. Liu, Y.Ono, T. Matsumura, A. Hirano, T. Ichikawa, N. Imanishi and Y. Takeda

Specific reaction heat / J mAh

Reaction heat / J g-1-1

Heat Flow / mW mg-1

By using the Li2.6-xCo0.2Cu0.2N-Co3O4 anode, asignificantly improved cycling performance was obtained
20with a high capacity of 500 mAh g-1. The
a. SnSb-Li2.6Co0.4N
A:charged to 0.6 V
charge-discharge efficiency is almost 100% and the15
capacity fade during cycling is estimated to be only about
B:charged to 0.3 V
0.37%/cycle. The capacity retention can be further
10
improved by decreasing moisture in the PEO electrolytes
C:charged to 0.1 V
since hexagonal lithium metal nitrides suffer from a high
5 D:charged to 0.01 V
sensitivity with H2O.
The safety of small size lithium-ion cells under
E: Li/polymer
normal use is well established. In contrast, the safety of0
the large size lithium-ion batteries is still questionable,
100
200
300
400
500
especially in cases of abusive use. Safety of lithium ion
batteries is mainly related to the thermal reactivity of the16b. Co3O4-Li2.6Co0.2Cu0.2N
components. Fig.8 shows differential scanning
A:charged to 0.6 V
calorimetric curves (DSC) for the composite anodes of12
Li2.6Co0.4N-SnSb (a) and Li2.6Co0.2Cu0.2N-Co3O4 (b) with
B:charged to 0.3 V
8the PEO electrolytes, respectively, as a function of the
C:charged to 0.1 V
charged state along with that for Li metal with the PEO
4 D:charged to 0.01 V
electrolytes (1:1 weight ratio) as a reference. For theLi2.6Co0.4N-SnSb anode, the reaction heat strongly
E: Li/polymer
0
depends on the charged state. The reaction heat is very
low for the composite anode charged up to 0.3 V and
100
200
300
400
500
appreciable for that charged up to 0.1 V, as shown inoFig.8a. An endothermic peak in the anode charged to 0.1
Temperature / C
V is observed at 180oC in the DSC curve, whichcorresponds to the fusion of lithium metal. That is, theFig.8. DSC curves of the composite anodes of a)high level insertion of lithium into the Li2.6Co0.4N-SnSb
SnSb-Li2.6Co0.4N and b) Li2.6Co0.2Cu0.2N-Co3O4
composite exhibits a lithium metal deposition. On thewith various Li intercalation states (Heating rate:
other
hand,
the
reaction
heat
of
the
o
5 C/min).
Li2.6Co0.2Cu0.2N-Co3O4 anode shows exothermic peaks at
250~320 oC, which do not depend on the charged
5000
6.0
state, as shown in Fi.8b. Fig.9 shows the reaction heatab
The practical reaction
5.5
4500
heat for Li/polymer(a) and specific reaction heat (b) of the different
5.0
4000
anodes at various charged states. The reaction heat of
4.5
3500
The theoretical reaction
4.0
the anode should be compared in terms of energy per
heatforLi/polymer
Metallic lithium
3000
3.5
discharged capacity, i.e., J mAh-1 (a specific reaction
Co3O4-Li2.6Co0.2Cu0.2N
2500
3.0
SnSb-Li2.6Co0.4Nheat). Specific reaction heats for the Li2.6Co0.2N-SnSb
2.5
2000
anode
charged up to 0.1V vs. Li/Li+ and 0.01 V vs.
2.0
1500
+
Li/Li are 0.82 J mAh-1 and 2.75 J mAh-1, respectively.
1.5
1000
1.0
Those for the Li2.6Co0.2Cu0.2N-Co3O4 anode are500
0.5
estimated to be 1.1 J mAh-1 compared to that of 1.24 J00.0
mAh-1 in lithium metal and PEO electrolyte (1:1
0.3
0.1
0
0.6
0
0.6
0.3
0.1
+
weight ratio). In practice, in the case of a lithium
Charged state vs. Li/Li / V
metal anode, an excess amount of lithium metal, atleast four times compared to the cathode capacity,
Fig.9. Reaction heat vs. charged state by a) J g-1 and should be used because of the dendrite formation on
the anode. Therefore, the reaction heat could beb) J mAh-1 for different anodes at 65℃
estimated to reach 4.96 J mAh-1 or higher.
Fig.10 further shows the typical charge and discharge profiles of the full cells based on the LiNi0.8Co0.2O2
cathode and the different anodes of lithium metal, SnSb-Li2.6Co0.4N and Li2.6Co0.2Cu0.2N-Co3O4, respectively,
while operating with the PEO electrolytes at 65 ℃. The LiNi0.8Co0.2O2/Li cell demonstrates a high working
potential of 3.5-4.0 V with a capacity of 120 mAh g-1, as shown in Fig.10a. Replacing the lithium metal with theinsertion anodes could result into a loss in the working potential but an improvement in the safety. In general, forfull utilization of lithium storage capacity of the composite anodes, the weight of cathodes has to be much overthat of anodes at about 4~5 times. As can be seen from Fig.10b, the high similarity in the potential-capacity trends
for the LiNi0.8Co0.2O2/Li2.6Co0.2Cu0.2N-Co3O4 cell indicates that the amorphous lithium metal nitrides remain high7

All solid-state Li-ion batteries based on intercalation electrodes and poly (ethylene oxide)-LiX electrolytes

5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0

a

Voltage / V

0
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0

Voltage cutoff: 3.0-4.1 V
20

40

b
0

5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0

60

80

100

120

140

Voltage cutoff: 2.1-3.7 V
100

200

c
0

stability from cycle to cycle. On the other hand, thevoltage plateau of the LiNi0.8Co0.2O2/SnSb-Li2.6Co0.4N
cell slightly increases from the first to the following
cycles, which probably is attributed to the two-phasetransformation of Li2.6Co0.4N between the crystal and theamorphous, as shown in Fig.10c. The high initialcoulombic efficiency of over 95 % and the large reversible
anode capacity above 450 mAh g-1 are very remarkablefor both the composite anodes, which might be highly
favorable for realizing a rather high energy density andcan be considered as an ideal power source for HEV & EV.
However, bearing in mind that the morphologicalinstability from the Li-alloy host is still questionable,
continuous research would be pursued to develop thelithium metal nitrides-Co3O4 based composite electrodes
proposed with low volume effects.

300

400

500

600

Voltage cutoff: 2.2-3.7 V
100

200

300

400

500

600

-1

Capacity / mAh g

Fig.10. Charge and discharge profiles of the fullcells based on LiNi0.8Co0.2O2 cathode and anode ofa) Li, b) Co3O4-Li2.6Co0.2Cu0.2N and c)
SnSb-Li2.6Co0.4N. Electrode compositions of b) andc) are the same as in Fig.9.

3.1.2 Modified graphitic carbon as insertion anodes.
From the application point of view, the first
approach for lithium metal-free anode is to use graphite,
as is accomplished in lithium-ion batteries with organic
solvent electrolytes. The charge and discharge profile ofmesocarbon microbead (MCMB, morphology as shown in
Fig.11), a typical graphitic carbon used in the current
lithium-ion cells, is shown in Fig.12. The very low charge
efficiency of 29.7 % in the first cycle indicates that side
reactions occur both in the reduction and oxidation
processes which lead to some uncertainty in thedetermination of the maximum reversible capacity of thegraphitic carbon. Consequently, its reversible capacity is
2.5a) η1= 29.7%

Voltage / V

2.0

Cycle 1
Cycle 2
Cycle 10
Cycle 20
Cycle 30

1.5
1.0
0.5
0.0
0

100

200

300

400

-1

Capacity / mAh g(Left) Fig.11 SEM images of the MCMB particles.
(Right) Fig.12. Charge and discharge profiles of the MCMB electrode with PEO electrolytes at 70 ℃
below to 150 mAh g-1 and the charge and discharge efficiency inthe first several cycles is extremely low. Poor solid/solidinterface of the graphite with the PEO electrolytes indicates thatstructural and interfacial performance has to be improved.
To make the graphitic carbon operate well with PEO
electrolytes, we design an ionic conducting LiCx layers bydirectly reacting the opened C-atom with lithium under special
conditions (such as high pressure and low temperature) on thesurface of the graphitic carbon that can prevent the side reaction
Fig.13. Structural images of the modified graphitic carbon.

8

Y. Liu, Y.Ono, T. Matsumura, A. Hirano, T. Ichikawa, N. Imanishi and Y. Takeda

4000

2:1(C:Li,mol ratio)
8:1
10:1
20:1
24:1
naturalIntensity / a.u.

3000

2000

1000

0
10

20

30

40

50

60

70

80

2θ/θ

Fig.14 XRD of the modified MCMB under
different Li/C ratio.

in the interface between the electrode and the PEO
electrolyte, as shown in Fig.13. Due to highLi-reactivity of its surface, MCMB is chosen andtreated by high energy ballmilling with lithium metal by
the aid of dodecane ({CH3(CH2)10CH3}) under Ar. The
high energy balmilling can make the surface of MCMB
become disordered and therefore surface C-atom
become open and further can react with Li.
Fig.14 shows that the typical crystalline structureof the treated MCMB remains to be unchanged which
can permit lithium reversible insertion and extraction.
XPS results as shown in Table 1 confirmed the formed
LiCx layer on the graphite surface. TEM observation
shown in Fig.15 further reveals that the MCMB loss
crystal on its surface upon treatment, indicating that theformed LiCx is prevailed by amorphous or
micro-crystalline phase. Cyclic voltammogrammeasurement as shown in Fig.16 indicates that the side

Fig.15. TEM images on the surface of the MCMB at left) without modification and right)
after modification.
irreversible reaction in the first cycle is highly suppressed after the treatment, which is favorable for theelectrochemical kinetics during lithium insertion into and extraction from the layered carbon. Finally, Fig.17
shows that the modified MCMB with PEO electrolytes presents an attractive behavior similar to that based on thecommon liquid electrolytes. Li intercalation into the graphitic hosts possesses several stages below to 200 mV.
However, in the real battery testing, only one potential plateau between 0.2-0 V can be distinguished, which iscorrespond to the LiCx compound reaction in an equilibrium phase. The capacity as a function upon the cycles
also demonstrates high stability, as shown in Fig.18. The reversible capacity is measured to be about 290 mAh g-1,
which is very comparable and close to the theory data (LiC6). Furthermore, we compared the thermal stability of
the MCMB with metallic lithium while operating with the PEO electrolytes via DSC measurement, as shown in
Fig.19. MCMB shows very low reaction heat under high Li utilization with the PEO electrolytes in comparison
with lithium metal anode. Such a thermal behavior is highly opposite to the one based on liquid electrolytes, inwhich the heat generation of graphite is very significant, especially, that of the reaction heat of PVDF and LiCx[11]. This result indicates that graphitic carbon might be a suitable anode candidate in large size rechargeable
lithium-ion batteries based on PEO electrolytes for EV in terms of structural stability and thermal reliancestability.
Table.1. XPS results of the MCMB with and without treatment.
C1s
Li1s
Without
Binding energy
0285 ev,treatment
Chemical composition
097.81%
With
Binding energy
56.10 ev,
291 ev,treatment
Chemical composition
2.23 %
37.50 %

F1s
0
0
0
0

O1s
533.00 ev,
2.19%
532.80 ev
60.20 %

All solid-state Li-ion batteries based on intercalation electrodes and poly (ethylene oxide)-LiX electrolytes

3.2 CathodeThe selection for proper cathodes is according to
the following requirements, such as inexpensive,
environmentally benign, wide operating temperature,
good capacity/cycling performance, extremely safe.
Transition metal oxides, such as LiCoO2, layered
LiNiO2 and LiMn2O4 have found application as positiveelectrode materials for high power applications. Thesematerials provide high potentials (about 4 V vs. Li) andgood reversible capacities (about 120 mA h g-1). The
use of LiMn2O4 solves problems related to cost and Co
and Ni toxicity, but the disproportionation of Mn3+
remains a severe problem. Pervious research reveals
that most of the current cathodes suffers from side
reaction with PEO electrolytes and detrimental phasetransitions above 70 ℃ that inevitably cause to capacity
loss and fade while being oxidized at a high potential.
In this work, in comparison with several cathodematerials, such as LixCoO2 (0.5<x<1, 0.5-electron
reaction, Cap=137 mAh g-1), LixMn2O4 (0<x<1,

Fig.16. Cyclic voltammogram of the electrodebased on a) no modified MCMB, b)
Li2.6Co0.2Cu0.2N and c) Co3O4-Li2.6Co0.2Cu0.2N at
the scan rate of 0.05 mV S-1, voltage cutoff: 1.4-0.
V, vs. Li/Li+.

2.5

2.5

2.0

2.0

liquid electrolyte

Voltage / V

Voltage / V

liquid electrolyte

1.5

1.5

1.0

1.0

0.5

0.5

0.0

0.0
0

50

100

150

200

250

300

350

0

400

50

100

150

200

250

300

350

400

-1

Capacity / mAhg

-1

Capacity / mAhg

Fig.17. Comparison in the charge and discharge profiles of the MCMB basedon liquid electrolytes and the modified MCMB based on PEO electrolytes.
450

Li metal with PEO:
-1
-14600 J g , 1.24 J mAh

20

liquidelectrolytes

400

300

After modification
basedon PEO

200

Without modification
based on PEO

Heat Flow / mW mg-1

Capacity / mAhg-1

350

250

15

10

graphite with PEO:
-1
-1
160 J g , < 0.8 J mAh

5

150
100

0

50

100

200
o

Temperature / C

0
0

5

10

15

20

25

Cycling number

30

35

40

300

400

500

graphite with liquid electrolytes-1-11500 J g , 4.3 J mAh(Left) Fig.18. Cycling performance of different MCMB electrodes with liquid and PEO
electrolytes.
(Right) Fig.19. DSC curves of different electrode with PEO and liquid electrolytes.
o9Y. Liu, Y.Ono, T. Matsumura, A. Hirano, T. Ichikawa, N. Imanishi and Y. Takeda

0Weight / mg

-2
-4
-6
-8

2C-LiFePO4

-10
100

200

300

400

500

600

2600
2400
2200

Heat Flow / μ V

90
80
70
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100

TG
DTA

40

-4

-20
-40

-6

-60

Heat Flow / ƒÊV

Weight / mg

200

301

1000
800

C-LiFePO4102,
121 221,401 112,410
202321210
011

2 θ / deg.

60

0

Fig.21. XRD of the resulting LiFePO4 and LiFePO4/C
powders.

-80

LiFePO4
200

Carbon-200
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44

80

100

1200

0 LiFePO420-81400 Amorphous

200

TG
DTA

-2

311
211
020

1600

400

Temperature / Ž
0

101

1800

600

700

111
201

2000

intensity / a.u.

10

-100
300

400

500

600

700

Temperature / ℃

Fig.20. TG/DTA curves of the LiFePO4 with andwithout PVC during heating to 700 ℃.
0.5-electron reaction, Cap=148 mAh g-1) and LixFePO4(0<x<1, 1-electron reaction, Cap=170 mAh g-1),
olivine type LiFePO4 is very attractive due to its
capability to operate within a very flat voltage plateau(around 3.5 V), high thermal stability, high working
temperature performance similar to PEO electrolytes,
low lattice volume changes, low cost and benign
environmental properties [12].
However, a significant drawback of the LiFePO4
is its poor rate capability caused by poor electron
conductivity. In this work, we use poly (vinyl chloride)
Fig.22. EPMA images of elemental distribution
as carbon sources to produce the LiFePO4/C compositeof the resulting materials.
by means of two pyrolysis reactions. The electronic
conductivity of the LiFePO4 can be substantiallyenhanced by dispersing fine particle size of the active
hosts in the carbon matrix. With this modification, the
LiFePO4 is capable of providing high capacity andhigh charge rates. TG/DTA analysis conducted on the
product upon the heating process shown in Fig.20
reveals that the LiFePO4 possesses several
decomposition reaction related to the starting materials.
The PVC does not cause to a reaction with the
LiFePO4. The carbon contents of the LiFePO4/C wasconfirmed to be about 5-7.5 wt% with the TG analysis.
XRD technique confirms the crystalline structure of
the composite proposed, as shown in Fig.21. As well,pyrolyzed carbon that is prevailed with disorderedstructure can be found in the product. Carbon coating Fig.23. SEM images of the resulting LiFePO /C
4
process does not bring a obvious change in the lattice powders.
parameters of the LiFePO4, as shown in table.2,
indicating the thermal pyrolysis mainly causes to a modification in the morphology. The homogeneous
distribution of the active hosts within the carbonaceous matrix is further evidenced by the elemental distribution

All solid-state Li-ion batteries based on intercalation electrodes and poly (ethylene oxide)-LiX electrolytes

11

mapping from Electron Probe Micro-Analysis (EPMA) shown in Fig.22. SEM observation shown in Fig.23
further reveals that the material possesses high porosity which is favorable for increasing the reactive area andtherefore the electrochemical kinetics. It suggests that the introduction of highly dispersed PVC has an effect on
preventing the single LiFePO4 growth during heating process. Meanwhile, the possible aggregation of the active
hosts during high temperature could be highly barred, resulting in fine grains of the active hosts which favorable
for the electrochemical kinetics. Finally, Fig.24 and 25 show that the LiFePO4/C electrode presents a stableworking potential at about 3.5 V vs .Li/Li+ and a capacity of 120 mAh g-1, as well as excellent cycling stability,
probably due to the low lattice volume changes with Li insertion/extraction and good interfacial compatibility
with the solid PEO electrolytes. The primary results suggest that the modified olivine type LiFePO4 might be agood cathode candidate for the PEO electrolytes.
Table.2. Lattice parameters of the resulting compounds.
a-axisb-axisc-axis
LiFePO4
10.380
6.023
4.714
2C-LiFePO4
10.381
6.015
4.711

Cell volume
294.777294.203

Crystal structure
Orthorhombic
Orthorhombic

140

5000

120
4000

Capacity / mAhg

Voltage / V

-1

100
3000

1 cycle
2
3
5
10
15
20
25

2000

1000

80

60

40

charge capacity
discharge capacity

20

0

0
0

20

40

60

80

Capacity / mAhg

100
-1

120

0

5

10

15

20

25

Cycling number(Left)Fig.24. Typical charge and discharge profiles of LiFePO4/C with PEO electrolytes at 65 oC (2.8-4.1 V).
(Right) Fig.25. Cycling performance of the LiFePO4/C with PEO electrolytes at 65 oC.
4. Conclusions
1) Composite anodes based on hexagonal lithium metal nitrides have been developed to show larger capacity
and higher first cycle efficiency over the current graphitic anodes. However, the performance of the composite
anodes containing Li-alloy hosts is still in doubt due to that the Li-alloy has a morphological instability along withan increased heating generation dependent on Li-insertion. In contrast, the composite anodes containing lithiumtransition-metal nitrides and Co3O4 demonstrate high first charge efficiency of 100 % and large capacity of 500
mAh g-1, as well as high cycling stability and low reaction heating under high Li-utilization with the PEO
electrolytes, indicating that they possess potential applications in large size rechargeable lithium-ion batteries for
EV in terms of high first-cycle charge efficiency, large capacity and high thermal reliance;2) Graphitic carbon that is currently used in commercial Li-ion batteries can be successfully operated with the
PEO electrolytes by designing an amorphous LiCx layer on its surface which can prevent the side reaction. The
material treating process is effective and comparable in production. The modified graphitic carbon demonstrates
good electrochemical behavior similar to that in liquid electrolytes. Furthermore, high thermal stability of thegraphitic carbon with PEO electrolytes under deep Li utilization suggests that the graphitic carbon might besuitable anode candidates in large size rechargeable lithium-ion batteries for EV based on PEO electrolytes interms of structural stability and thermal reliance;3) LiFePO4/C composite has been prepared by two pyrolysis processes using poly (vinyl chloride) as carbon
sources. The material was confirmed to be of high porosity and homogeneous distribution of active hosts within
the carbonaceous matrix. Primarily electrochemical results indicate that the olivine type LiFePO4/C is a promisingcathode candidate in terms of stable working potential, acceptable capacity, high charge/discharge efficiency andexcellent cycle life with the PEO electrolytes at the elevated temperature.

12

Y. Liu, Y.Ono, T. Matsumura, A. Hirano, T. Ichikawa, N. Imanishi and Y. Takeda

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