Abstract
The electrochemical cycling and storage characteristics of 
and 
nanoparticle-coated 
cathode materials 
were compared at room temperature and 
between 3 and 
. The doped cathodes showed degraded electrochemical performance at room temperature compared to the uncoated cathode. The first discharge capacities of the uncoated and the doped cathodes were 186 and 
, respectively. The doped cathodes showed 
after 30 cycles, while the uncoated cathode showed 
after 50 cycles at a 
rate. 
and 
-coated 
showed discharge capacities of 179 and 
, respectively, and hadsignificantly improved capacity retention, showing 133 and 
, respectively, after 50 cycles. After storage at 
, in the electrolytes using 
charged electrodes, the doped cathodes showed both greatly decreased side reactions with the electrolytes and formation of 
and 
phases from Li and Co dissolution. However, the coated cathodes did not show either structural transformation into the 
and 
phases or side reactions with the electrolytes.
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At present, 
powders are the most widely used cathode material in Li-ion batteries for mobile electronics, which alter the charge cutoff voltage from 
(vs graphite) (
vs lithium metal) to 
(vs graphite) (
vs lithium metal). By increasing the cutoff voltage, the energy density of the cell can be increased by 
. However, due to side reactions with the electrolytes on the particle surface and continuous structural destruction from the dissolution of Li and Co ions, the capacity of 
rapidly the fades. Previous studies have focused on improvements of electrochemical performances at room temperature using doped 
1, 2 or coatings.3–14 Here, coatings may be an attractive method for improving such problems, as it applies only to final 
materials. For doped cathodes, depending on the dopants (M) in 
and the value of 
, the specific capacity varies. In addition, uniform substitution of dopants into the Co 
sites in the 
are difficult to achieve in mass-production processes that use solid-state reactions.
Only a small number of storage studies at elevated temperatures (e.g., 
) have been reported15, 17 thus far. 
coatings 
have been reported to be effective in improving not only thermal stability but also electrochemical properties, depending on the M factor.15 In a 
coating, Fe ions were observed to be dissolved into the electrolytes during 
storage. For an 
coating, despite improving the structural stability at 
, it has been observed that the capacity decreases at a 1C rate as the electrode density increases.16 Recently, the authors investigated a
coating on 
and 
cathodes, which resulted in better rate capabilities compared to an 
coating at higher C rates.16, 17 However, thus far, a systematic comparison of coated and doped cathodes in terms of cycling and structural stability at 
has not been reported.
In this study, a comparison of 
and 
coatings on 
vs doped 
(
and Zn) cathodes is made regarding the electrochemical properties at room temperature and the structural stability when in storage at 
after charging at 
in electrolytes.
Experimental
was prepared by stoichiometric mixing of 
(99.99%, average particle size 
) and 
and firing at 
for 
. 
(
and Mg) was prepared by stoichiometric mixing of 
or 
with 
by dry milling at 
for 
. The mixture was then thoroughly mixed with 
by dry milling at 
, followed by firing at 
for 
. The cooled powders were remixed and fired at 
for 
. The average particle size of 
uncoated and doped 
were used for the electrochemical tests (Brunaner–Emmett–Teller surface area of the samples was 
). For scientific accuracy, same uncoated 
was used for coatings. Inductively coupled plasma-mass spectroscopy (ICP-MS) results of lithium stoichiometry 
in the uncoated 
showed 1.01. To prepare the coating solution, 
of Mg or Zn nitrates and 
of 
were dissolved in 
of distilled water, and 
particles that were pink in color instantly precipitated in the solution, which was stirred for 
at pH 
. This coating concentration was a 
coating concentration (the weight of cathode was fixed at 
). For instance, a 
coating concentration was a coating concentration that used 
of zinc nitrate and 
of 
per 
of cathode powders. The coated cathodes were annealed at 
for 
in air. For the storage test at 
, the charged cathodes at 
were dissembled from coin-type half cells in a glove box and were immersed in electrolyte and tightly sealed in vials. The vials were kept at 
for 1 day and cooled to room temperature.
The field-emission transmission electron microscope (FE-TEM) (JEOL 2100F operating at 
) samples were prepared by evaporation of the dispersed particles in acetone or hexane on carbon-coated copper grids. Powder X-ray diffraction (XRD) measurements were carried out using a Rigaku D/Max2000 with a Cu target tube. ICP-MS (ICPS-1000IV, Shimadzu) was used to determine the metal contents. The electrochemical properties were tested in a coin-type 2016R cell with lithium metal as the anode. The cathode consisted of 
active material, 
Super P carbon black, and 
polyvinylidene fluoride binder. The electrolyte for the coin-type half-cells (2016 type) was 
with ethylene carbonate/diethylene carbonate/ethyl-methyl carbonate 
(Cheil Ind. Korea). The capacity and capacity retention were measured to be between 3 and 
at a rate of 0.1, 0.2, 0.5, and 1C 
. Specific capacities of the cathodes were calculated using only the active material. The HF level in the electrolyte solution was determined by gas chromatography.
Results and Discussion
Figure 1 shows XRD patterns of the coated 
and doped 
(
and Zn) powders. Characteristic lines of 
structure can be observed. Single-phase oxide was obtained, for which Miller indexes 
are indexed for a hexagonal setting. For the 
powders, the lattice constants 
and 
values increase while those of the coated cathodes are identical to those of uncoated sample, as shown in Table I. The increase in lattice constants in the doped cathodes may be due to the larger size of 
and 
compared with that of the 
ions 
, 
, 
. Figure 2 shows TEM images of the 
(
and Mg) and the coated samples. The doped samples show typical lattice fringes of the (003) plane corresponding to 
. In the case of the 
-coated sample, its low-magnification image (Fig. 2c) does not show any coating layer, but its high-resolution image (2d) shows an amorphous coating layer with a thickness of 
. It was found that, depending on the metal (M) in 
, the crystallinity and coating thickness varied. For instance, for the 
-coated sample, the 
coating layer became olivine 
as a result of a reaction with Li in 
during the annealing process.16, 17 For the 
coating, the coating layer was amorphous and its thickness was 
to 
.6 For the 
-coated sample, nanoparticles 
were coated on the 
(Fig. 2e), and the amorphous coating layer that consisted of many pores and rough surfaces with a coating thickness of 
was also observed (Fig. 2f). In contrast to 
coating that exhibited the disappearance of the coating layer due to its complete reaction with the bulk surface, 
-coated 
cathodes (
and Mg) showed the clearly distinguishable coating layer on the cathode surface, consisting of amorphous phase. Thus, it is believed that 
nanoparticles had a partial reaction of bulk 
during an annealing process at 
.
Figure 1. XRD patterns of the 
(
and Mg) and 
and 
nanoparticle-coated 
powders.
Table I. XRD lattice constants 
and 
and 
ratio for 
(
and Mg) and 
and 
nanoparticles-coated 
.
| Material |
|
|
|
|---|---|---|---|
| 2.816 | 14.034 | 4.984 |
| 2.819 | 14.059 | 4.987 |
| 2.819 | 14.060 | 4.988 |
 coating | 2.816 | 14.034 | 4.984 |
 coating | 2.816 | 14.035 | 4.984 |
Figure 2. TEM images of the (a) 
and (b) 
cathode particles (c and d) 
nanoparticle-coated 
particle [(d) is a magnified image of (c)] and (e and f) 
nanoparticle-coated 
particle [(f) is a magnified image of (e)].
Figure 3 shows the charge and discharge curves of the uncoated and doped cathodes for each first cycle at 0.1, 0.2, 0.5, and 1C rates 
and after 30 or 50 cycles in coin-type half cells. For an uncoated cathode, the first charge and discharge capacities are 190 and 
, respectively. However, for the Mg-doped cathode, the first charge capacity is similar to that of uncoated cathode at 
, but its discharge capacity decreases to 
. The Zn-doped cathode shows charge and discharge capacities of 188 and 
, respectively. A decrease in the discharge capacities of the doped cathodes is associated with a decrease in the concentration of active 
ions and 
ions substituted into Co sites, leading to the creation of an equal number of 
ions for the charge balance. Both 
and 
exhibit a capacity retention rate of 57% after only 30 cycles, compared with the first discharge capacity at 1C. This retention value is inferior to that of the uncoated cathode with capacity retention of 63% after 50 cycles.
Figure 3. Plots of the cycling curves of coin-type half cells containing uncoated and 
(
and Mg) cathodes between 3 and 
at different C rates from 0.1 to 1C 
and after 30 and 50 cycles at a 1C rate. The same C rates were used during the charge and discharge processes.
The 
and 
-coated samples (
coating concentration) show first discharge capacities of 178 and 
, respectively, as shown in Fig. 4. Differential voltage profiles for the uncoated 3, and 
-coated cathodes were added in Fig. 5. The presence of the plateaus between 4 and 
, indicative of order/disorder phase transition,18 depends on the coating concentration, and only the 
coated sample shows the plateau. Our previous study on the 
coating concentration effect on 
also showed a similar result.19 At this time, we do not know the detailed mechanism for such a phenomenon but doubt it is related to Li stoichiometry difference near the cathode surface. ICP-MS results of lithium stoichiometry 
in the uncoated 
showed 1.01. Pereira et al. reported that order–disorder reactions disappeared when the Li stoichimetry 
was 
.20 Accordingly, higher 
coating concentration may induce the increased difference in Li concentration at the surface and inward, leading to more developed plateaus.
Figure 4. Plots of cycling curves of coin-type half cells containing uncoated and 
and 
nanoparticle-coated 
cathodes between 3 and 
at different C rates from 0.1 to 1C 
and after 30 and 50 cycles at a 1C rate. The same C rates were used during the charge and discharge processes. For the 
nanoparticle-coated 
cathodes, 1.5 and 
coating concentrations are compared.
Figure 5. Differential voltage profiles for the uncoated 3, and 
-coated cathodes in coin-type half cells.
The capacity retention of the 
and 
-coated samples (
coating) was 84 and 90%, respectively, after 50 cycles. The decreased capacity retention with the 
coating relative to 
coating occurred because the nonuniform coating layer with coated nanoparticles 
in size impedes Li-ion diffusivity into the bulk particle. A similar result was observed in the 
and 
-coated 
cathodes.15 When the coating concentration of the 
coating decreases to 
, the first discharge capacity is slightly larger than that of the 
coating, at 
. However, the capacity retention at 1C rate cycling decreases to 
, which is comparable with that of 
-coated 
. Table II summarizes the capacities of the uncoated, coated, and doped cathodes at different rates and after 30 and 50 cycles of the samples. Overall, the coated samples demonstrate enhanced capacity retention at 1C rate cycling. It is believed that 
nanoparticles exist as an amorphous solid solution on the particle surface with a partial reaction of bulk 
during an annealing process at 
, which may enhance both Li-ion and electronic conductivities. However, in the case of the 
-coated cathode consisting of both a thin coating layer 
and abnormally large nanoparticles with size of 
, these large nanoparticles may serve as abso lute insulators, leading to decreased conductivities. The 
-coated cathode 
does not show such large particles on the surface and has a uniform coating thickness of 
, although the reason for more a uniform coating layer formation than 
coating needs further investigation. The enhancement of electronic conductivities of the coated samples may be due to the formation of electronic holes in the solid solution near the particle surfaces, according to the reaction 
.21, 22 Based upon the above results, a more uniform coating is beneficial for both Li ion diffusivity and electronic conductivity. It is well known that large internal strains and subsequent mechanical degradation of the cathode materials originate from the dissolution of 
and 
ions, and that the formation of by-products between reaction-dissolved ions and electrolytes at the interface enhances the structural degradation of the cathodes.20, 21 This effect becomes evident during storage at elevated temperatures.16 A previous TEM result showed that an uncoated cathode charged at 
showed a structural transformation into the spinel phase at the particle surface after storage at 
for 
.16, 17
Table II. Comparison of the first discharge capacities of 
(
and Mg) and 
and 
nanoparticles-coated 
at 0.1, 0.2, 0.5, and 1C and at 1C after 30 and 50 cycles 
. Units are 
.
| Material | 0.1C | 0.2C | 0.5C | 1C | 1C, 30 cycles | 1C, 50 cycles |
|---|---|---|---|---|---|---|
| 186 | 183 | 174 | 163 | 125 | 100 |
| 175 | 164 | 151 | 139 | 79 | |
| 175 | 169 | 155 | 141 | 79 | |
 coating | 179 | 174 | 166 | 159 | 143 | 133 |
 coating 
| 187 | 178 | 162 | 156 | 140 | 131 |
 coating 
| 187 | 182 | 176 | 171 | 159 | 153 |
Storage behavior of the cathode at 
, especially at the charged state, is different from that at room temperature due to accelerated reactions between the cathode surface and the electrolytes. Many Li-ion manufacturers have regarded the storage test at 
as the most severe test condition among the various test methods to verify the structure stability of the cathode. At this test condition, cathode material is believed to play a major role in swelling of the Li-ion cells. Accordingly, it is important to provide the storage results of the cathode materials such as XRD and dissolved metal contents. Figure 6 shows XRD patterns of the uncoated sample after charging to 
as well as that after 
storage at 1 day. After storage at 
for 1 day, peaks assigned to 
, 
spinel phases, and 
phases are observed; these had significantly increased in size. Moreover, unknown phases with strong intense peaks are observed, and the development of similar peaks was observed in the 
cathode after storage at 
.17 At a higher voltage, the 
ions in the 
attack and oxidize the carbonate groups of the solvent molecules due to their acidic/nucleophilic properties and are reduced to 
.22 In addition, the formation of 
simultaneously accompanies the formation of 
, leading to a continuous increase of HF content, according to the reaction 
.23 The 
source is the cathodes, and uncoated and coated cathodes had 120 and 
, respectively, as measured by a Karl–Fisher moisture titrator. Hence, it is important to blacken the 
dissolution initially. In addition, the dissolved 
ions are solvated and combine with 
-oxidized solvent molecules to form lithium-containing organic products. As shown in Table III, the Co content in the uncoated sample after storage was 
(before storage its amount was negligible). The 
content of the uncoated sample after storage increases to 
, compared to 
before storage. In consequence, the charged uncoated cathode undergoes severe structural instability at 
. The low-magnification TEM images of the uncoated cathode after storage at 
in Fig. 7a show the presence of rough surfaces and different contrasts near the surfaces, which indicates the presence of a highly defected phase. Essentially, the high-resolution image in Fig. 7b shows the formation of the amorphous phase that resulted from the severe dissolution of Li and Co at 
.
Figure 6. XRD patterns of a uncoated 
electrode after a 
charge and after storage at 
for 
. After charging the cell, the electrode was separated from the cell and kept in the electrolyte in a vial.
Table III. Amounts of the dissolved 
ions into the electrolytes and 
contents in 
(
and Mg) and 
and 
nanoparticle-coated 
during storage at 
after 1 day after charging to 
vs lithium metal.
| Material | Co | HF before storage (ppm) | HF after storage (ppm) |
|---|---|---|---|
| 3000 | 200 | 900 |
| 270 | 190 | 350 |
| 900 | 210 | 500 |
 coating 
| 70 | 90 | 30 |
 coating | 90 | 100 | 50 |
Figure 7. TEM images of uncoated electrodes after storage at 
for 1 day at 
.
XRD patterns of the doped cathodes are, however, quite different from those of the uncoated cathode after storage at 
(Fig. 8). XRD patterns of the charged 
electrode stored at 
show that the peak intensities of unknown phases are significantly decreased, although 
and 
peaks continue to be present. For 
, unknown peaks including 
and 
phases are not observed, except for a slightly increased peak-broadening effect, compared to that of the charged cathode. This result indicates that Zn or Mg doping into the 
causes an improvement in structural stability and reduces surface reactivity with the electrolyte, although Zn doping leads to a better result. This result is further supported by the amount of dissolved Co content in the electrolyte at 
. The 
contents of the Mg- and Zn-doped cathodes after storage were 900 and 
, respectively. Similarly, the 
content decreased in the doped cathodes; they were measured at 500 and 
, respectively, as shown in Table III. This result agrees with the XRD patterns of the doped cathodes kept at 
, as shown in Fig. 6. The TEM image (Fig. 9) of the 
electrode shows the formation of the 
phase at the particle surface, while that of the 
electrode shows a pristine layered phase.
Figure 8. XRD patterns of the 
(
and Mg) and 
and 
nanoparticle-coated 
electrodes after a 
charge and storage at 
for 
. After charging the cell, the electrode was separated from the cell and kept in the electrolyte in a vial.
Figure 9. TEM images of (a) 
and (b) 
electrodes, after storage at 
for 1 day at 
and (c) 
and (d) 
nanoparticle-coated 
electrodes, after storage at 
for 1 day at 
.
As shown in Fig. 8, XRD patterns of the coated cathode after storage exhibit no peak spectral changes relative to those of the cathodes charged to 
, indicating that the surface coating layer effectively reduces the reactions with the electrolytes. It has been reported that the coating layers act as an 
scavenger.24 As shown in Table III, the 
content after storage was significantly decreased to 30 and 
for the 
and 
coatings, respectively. This result is consistent with other studies that concluded that the surface coating layer gettered 
ions from 
.24–27 TEM images of the stored samples support the XRD spectral results in Fig. 8, and neither coated sample shows the formation of the spinel phase while maintaining the original layered phase. TEM images of both coated samples are similar to the Zn-doped cathode, with a coating layer that does not show damage. In contrast to the uncoated cathode, the TEM image of the 
-coated cathode shows a smooth surface, which is indicative of the absence of a development of defect phases.
Conclusions
In terms of electrochemical cycling and storage characteristics at 
, coated samples showed much-improved results relative to doped cathodes. In contrast to the doped cathodes, the coated samples were effective in retarding direct reactions between the electrolyte and the cathode surface that consisted of highly oxidized 
ions. Overall, 
-coated cathodes showed a first discharge capacity of 
and excellent capacity retention at 1C rate after 50 cycles, as shown by a 
measurement. This result indicates that the amorphous-like coating layer was effective for improving electrochemical activity and that it decreased side reactions with the electrolytes at 
as well.
Acknowledgments
This work was supported by Ministry of Information and Communication (MIC) and Institute for Information and Technology Advancement (IITA) through the Information Technology (IT) Leading Research and Development Support Project.
Kumoh National Institute of Technology assisted in meeting the publication cost of this article.