Abstract
The structure, synthesis, and electrochemical behavior of layered 
for 
5/12, and 1/2 are reported for the first time. 
is derived from 
or 
by substitution of 
and 
by 
while maintaining all the remaining Mn atoms in the 4+ oxidation state. 
with 
can deliver steady capacities of 150 and 160 mAh/g at 30 and 55°C, respectively, between 3.0 and 4.4 V using a current density of 30 mA/g. Differential scanning calorimetry experiments on charged electrodes of 
for 
indicate that this material should be safer than 
with 
5/12, and 1/2 can be cycled between 2.0 and 4.6 V to give capacities of about 200, 180, and 160 mAh/g, respectively, at 30°C. 
with 
gives a capacity of 220 mAh/g at 55°C between 2.0 and 4.6 V using a current density of 30 mA/g. © 2001 The Electrochemical Society. All rights reserved.
Export citation and abstract BibTeX RIS
Cathode materials help to determine the performance and the safety of Li-ion batteries. The search for a cheaper, higher capacity, and safer layered cathode material than 
or 
is still an ongoing task. Many recent reports address this problem.1
2
3
4
5
6
7
8
9
10
11
12 One very promising cathode material is 
7
8 
can deliver a capacity of about 160 mAh/g at ambient temperature and more than 200 mAh/g at 55°C.7 It has a very smooth discharge profile with an average voltage of 3.5 V vs. Li/Li+. Paulsen et al.8 pointed out that 
can be regarded as a solid solution between the layered end members 
and 
In 
Cr is in the +3 oxidation state while Mn is in the +4 oxidation state. When Li is extracted electrochemically from the lattice of 
during the charge process, 
is oxidized to 
since Mn cannot, normally, be oxidized beyond 4+ in these materials.
Numata et al.11
12 reported a layer-structure solid-solution cathode material, 
between 
and 
has the same structure as 
except that there is a superlattice ordering of 
and 
(1:2) in the transition metal layer, which reduces the symmetry of 
from 
to 
13
14 Both 
and 
have 
or 
partially replacing 
and 
in 
, respectively, while maintaining the remaining Mn atoms in the 4+ oxidation state. Therefore, 
is a member of the series 
with 
while 
is equivalent to 
We found that 
can coexist with 
in the transition metal layers of the layered materials 
and 
6
15
16
17 We show here that 
can replace 
and 
in 
to make the layered solid-solution series 
with all Mn atoms expected to be in the 4+ oxidation state. When Li is deintercalated from 
should be oxidized to 
Here, we report the structure, electrochemical, and safety behavior of 
(
and 1/2).
Experimental
(98%+, Aldrich), 
(98%+, Fluka), and 
(97%+, Fluka) were used as the starting materials. The samples 
(
1/12, 1/6, 1/4, 1/3, 5/12, and 1/2) were prepared by the "mixed hydroxide" method.17 A 50 mL aqueous solution of the transition metal nitrates was slowly dripped (1 to 2 h) into 400 mL of a stirred solution of LiOH using a buret. This causes the precipitation of 
with a homogeneous cation distribution. The buret was washed three times to make sure that all the transition metal nitrates were added to the LiOH solution. The precipitate was filtered out and washed twice with additional distilled water to remove the residual Li salts (LiOH and the formed 
). The precipitate was dried in air at 180°C overnight. The dried precipitate was mixed with the stoichiometric amount of 
and ground in an automatic grinder. Pellets about 5 mm thick were then pressed. The pellets were heated in air at 480°C for 3 h. Tongs were used to remove the pellets from the oven and sandwich them between two copper plates in order to quench the pellets to room temperature. The pellets were ground and new pellets made. The new pellets were heated in air at 900°C for another 3 h and quenched to room temperature in the same way.
X-ray diffraction (XRD) was made using a Siemens D500 diffractometer equipped with a Cu target X-ray tube and a diffracted beam monochromator. Profile refinement of the collected data was made using Hill and Howard's version of the Rietveld program Rietica.18
"Bellcore-type" electrodes were prepared for the electrochemical tests. z grams of the sample is mixed with ca. 0.1z (by weight) super S carbon black and 0.25z Kynar 2801 [poly(vinylidene fluoride)-hexafluoropropylene, Elf-Atochem]. This mixture was added to 3.1z acetone and 0.4z dibutyl phthalate (DBP, Aldrich) to dissolve the polymer. After several hours of stirring and shaking, the slurry was then spread on a glass plate using a notch bar spreader to obtain an even thickness of 0.66 mm. When the acetone evaporated, the dry films were peeled off the plate and punched into circular disks of 12 mm diam. The punched electrode was washed several times in anhydrous diethyl ether to remove the DBP. The washed electrode was dried at 90°C overnight in air before use. Using the above positive electrodes, 2325 type coin-cells (23 mm diam, 2.5 mm thick) were assembled in an argon glove box (
) with lithium as the anode, Celgard 2502 membrane as the separator, and 1 M 
in 33% ethylene carbonate 
diethyl carbonate (DEC) (Mitsubishi Chemical) as the electrolyte. Usually, the cathode mass was around 20 mg. The cells were tested using constant charge and discharge currents between the desired potential limits.
Results and Discussion
Figure 1 shows typical X-ray diffraction (XRD) patterns of 
(
5/12, and 1/2) with Miller indices indicated. All of the peaks can be indexed based on a hexagonal 
structure (space group: 
166), except for a very broad peak between 20 and 25°. This broad peak is thought to be caused by short-ranged superlattice ordering of the Li, Ni, and Mn atoms in the transition metal layers. The broad peak has also been observed in examples from Ref. 8, 11, and 12. The superlattice peak is difficult to detect for the sample 
since there is very little Li in the transition metal layers. For convenience, the structure of 
is considered to be isostructural to 
Figure 2 shows the lattice parameters (a and c), and the 
ratio as a function of x in 
Both a and c increase linearly with Ni content but the 
ratio decreases. Such a behavior suggests a substitution of 
for 
and 
in a solid solution.
Figure 1. XRD profiles of 
with 
5/12, and 1/2 as indicated. The Miller indices of the peaks are indicated in the lower panel. The arrows designate the position of a short-range order superstructure peak.
Figure 2. Lattice constants and 
ratio of 
vs. x.
Figure 3a shows the voltage vs. capacity for 
cells between 3.0 and 4.4 V vs. 
using a specific current of 30 mA/g. This corresponds to a current density of about 0.5 mA/cm2. 
with 
has a very smooth charge/discharge curve and an average potential of about 3.85 V. The irreversible capacity loss is only about 15 mAh/g, which is about 10% of the first charge capacity. Figure 3b shows the capacity vs. cycle number for a 
cell 
at 30°C and then at 55°C. 
can deliver a capacity of 150 mAh/g at 30°C and a capacity of about 160 mAh/g at 55°C.
Figure 3. (a) Voltage vs. capacity of a 
cell with 
(b) Capacity vs. cycle number for a 
cell with 
Figure 4 shows voltage vs. capacity for a 
cell discharged at a variety of currents. The cell was charged using a current of 5 mA/g before each discharge test. A capacity of over 140 mAh/g at an average voltage of about 3.5 V is delivered at 400 mA/g. After the rate capability testing, the electrode was charged to 4.4 V at 5 mA/g and discharged to 2.0 V at 5 mA/g. The cell still had a capacity of 171 mA/g, the same as at the start of the experiment. 
with 
has very good rate capability.
Figure 4. Voltage vs. capacity of a 
cell with 
discharged at various currents.
Figure 5 shows the voltage vs. capacity of 
cells with 
5/12, and 1/2 between 2.0 and 4.6 V. These electrodes were first charged to 4.8 V using a specific current of 5 mA/g (see dashed line), then were cycled between 2.0 and 4.6 V. The current density was initially 5 mA/g (2 cycles, dashed line), then 10 mA/g (2 cycles, thin solid line), and finally, 30 mA/g (50 cycles, thick solid line). There is an irreversible plateau around 4.5 V (marked by the arrows in Fig. 5) during the first charge which disappears during the following cycles. The plateau length decreases as x increases. It is believed that the plateau is not due to the decomposition of the electrolyte. The capacity before the plateau (up to 4.45 V) during the first charge is very close to the theoretical capacity of 
assuming that all the 
is oxidized to 
when the Li is extracted. We speculate that during the plateau electrons are removed from the oxygen atoms in the structure leading to a partial loss of oxygen from the electrode. When lithium is inserted back into the structure of the electrode (subsequent to the charge to 4.8 V), after all 
is reduced into 
a portion of the 
is apparently reduced to 
by the further inserted Li. The capacity due to the Mn reduction is shown as a plateau near 3.3 V in the voltage profile.
Figure 5. Voltage vs. capacity of 
cells having 
5/12, and 1/2 as indicated. The cells were first charged to 4.8 V during the first cycle and then cycled between 2.0 and 4.6 V using currents as indicated by the legend.
Figure 6 shows the capacity vs. cycle number for 
cells with 
5/12, and 1/2. The electrodes with 
5/12, and 1/2 can deliver stable capacities of about 200, 180, and 160 mAh/g, respectively, between 2.0 and 4.6 V. Figure 7 shows the capacity vs. cycle number for the electrode with 
at 55°C after the cell had been cycled 50 times first at 30°C. The electrode with 
can still deliver a steady capacity of about 220 mAh/g.
Figure 6. Capacity vs. cycle number for 
cells having 
5/12, and 1/2 as indicated.
Figure 7. Capacity vs. cycle number for a 
cell with 
as indicated in the legend.
Figure 8 shows the differential scanning calorimetry (DSC) profiles of wet electrodes of 
with 
which was charged to 4.2, 4.4, 4.6, or 4.8 V in a lithium coin cell with an electrolyte of 1 M 
(33:67%). The DSC experiments were made in welded sealed stainless steel tubes so that no leaking of pressurized electrolyte is possible.19 The capacity indicated in each panel of Fig. 8 indicates the amount of Li extracted from the electrode. The heat associated with each exothermic peak is also indicated on the panels of Fig. 8. The onset temperature of the main exothermic peak is above 260°C provided that the charge potential is less than 4.4 V vs. Li. When the charge potential is 4.4 V, there is a small peak between 240 and 260°C, but it only produces 205 J/g of heat. When the electrode is charged above 4.5 V, the thermal properties are quite different from those when the charge potential is less than 4.4 V. First, there is a lower temperature exothermic peak between 200 and 230°C. Second, the strongest exothermic peak becomes broad and is distributed between 240 and 280°C (for 4.6 V) or its onset temperature moves to a lower temperature (for 4.8 V).
Figure 8. DSC profiles (2°C/min) of charged 
electrodes at the indicated potentials. Some experiments were duplicated. The experiments were conducted using welded stainless steel pans to eliminate all leaking. Comparison results for 
electrodes charged to the same potentials is given as the dashed curves where data is available. The Brunauer, Emmett, Teller surface area of the 
sample was 5.0 m2/g and that of the 
was 0.2 m2/g.
As a comparison, battery grade 
(FMC) was charged to 4.2 V and delivered a capacity of about 130 mAh/g. The onset temperature of the exothermic peak of the DSC curve of the wetted 
electrode (charged to 4.2 V) measured by the same methods is 185°C20 and produced a total heat of about 700 J/g.20 Therefore, 
with 
is safer than 
if the charge potential is less than 4.4 V vs. 
Conclusions
with 
5/12, and 1/2 can be synthesized by the mixed hydroxide method. This solid solution represents 
substitution for 
and 
in 
while maintaining all the remaining Mn atoms in the 
oxidation state. The electrochemical performance of 
with 
at both 30 and 55°C is excellent. DSC results for wetted and charged electrodes of 
with 
indicates this material would be at least as safe as 
if its charge potential were below 4.4 V. 
with 
5/12, or 1/2 can be cycled between 2.0 and 4.6 V to give a stable capacity of about 200, 180, or 160 mAh/g, respectively, at 30°C. During the first charge to 4.8 V of these materials, an irreversible plateau was observed around 4.5 V. 
with 
can give a stable capacity of 220 mAh/g at 55°C between 2.0 and 4.6 V using a specific current of 30 mA/g.
Clearly, these materials warrant further study in prototype Li-ion cells. They contain no cobalt, do not convert to the spinel phase, and are easily prepared. Their electrochemical and safety characteristics are comparable or better than 
Acknowledgment
The authors acknowledge the support of NSERC, 3M Co., and 3M Co., Canada, for the funding of this work.
Dalhousie University assisted in meeting the publication costs of this article.