Here we report the LiδPyOz-coated LiNi0.5Co0.2Mn0.3O2 cathode materials doped with P and Si ions, which possess both higher speeds and thermal stability. The coated cathodes showed quite impressive results; The speed capability was improved by almost 100% at a speed of 7C compared to the pristine LiNi0.5Co0.2Mn0.3O2. Furthermore, the amount of heat generation at a 4.5 V load interruption due to oxygen evolution was reduced by 79% compared to the pure LiNi0.5Co0.2Mn0.3O2 sample.
Nano-SIMS of coated LiNi0.5Co0.2Mn0.3O2 cathode particle in cross-section (a: cross-sectional image, b: image of lithium, c: image of P, and d: image of Si). b) is a high-resolution TEM image of the dotted area of (a) and digitized Fourier transform images of red dotted areas of (b). Graphs of the operating voltage (cell potential at half value of discharge or charge capacity) in the pristine and coated LiNi0.5Co0.2Mn0.3O2 with increasing C rates from 0.2 to 7C at 21oC. Graph of discharge capacity as a function of cycle number in lithium half-cells containing pristine and coated LiNi0.5Co0.2Mn0.3O2 cathodes between 4.5 and 3V at 60oC.
Nyquist diagram Graphical representation of pristine and coated LiNi0.5Co0.2Mn0.3O2 before and after cycling at 60oC. Evolution of the chex/ahex ratio as a function of temperature for pristine and coated cathodes.
INTRODUCTION
REVIEW OF RELATIVE LITERATURE
Lithium-ion batteries
- Layered oxide cathodes
- LiNiO 2
- LiNi 1-y Co y O 2
- LiNi 1−x−y Co x Mn y O 2
- Coating effect
- Nanoscale Coatings on Bulk cathodes…
EXPERIMENT
Electrodes were prepared by coating a cathode slurry on an Al foil followed by drying at 130°C for 20 min, and finally followed by spin pressing. For the cycling tests, 3 identical cells were used and the standard deviation of the cells was ± 3 mAh/g. The surface of the uncoated and coated samples was observed using scanning electron microscopy (SEM) (JSM 6400, JEOL) and transmission electron microscopy (TEM) (JEOL 2010F).
TEM samples were prepared by evaporating the dispersed particles in acetone or hexane on carbon-coated copper grids. Spectra were recorded in the constant pass energy mode at 50.0eV, using a 30μm diameter analysis area. Before etching, the surface of the electrode was coated with the composition of binder and carbon black, Ni2+ and Ni3+ peaks could not be obtained.
The change of oxidation states of Si in SiO2 was estimated from the peak position, and the reference SiO2 thin film was used to estimate the etching depth. Nano Secondary Ion Mass Spectrometry (Nano-SIMS) analyzes were performed on the CAMECA Nano SIMS 50 instrument. A pre-spray of the surface was performed before the measurement to eliminate surface contamination.
Electrochemical impedance spectroscopy (EIS) data were collected before and after 40 cycles at 60oC with alternating current amplitude of 10 mV in the frequency range 0.5 MHz to 10 mHz by an Ivium impedance analyzer. EIS measurements of the cell before and after cycling were performed after cell assembly and after discharging to 3V, respectively. Differential scanning calorimetry (DSC) samples of the cathodes were prepared by charging the coin cells to 4.5 V at a 0.1 C rate, followed by holding them at this potential for 2 h.
The electrolyte-soaked electrodes were extracted in the dry room (moisture content was below 100 ppm) and transferred to aluminum pans. The cells were charged to 4.5 V and then separate electrodes were prepared in the dry room. The separated electrodes were washed with dimethyl carbonate (DMC) solvent for several times and dried in the dry room and then attached to the sample holder in the oven.
RESULTS AND DISCUSSIONS
Therefore, this result, together with such a decrease in lattice constants, also indicates the formation of a LiNiCo0.2Mn0.3O2 solid solution phase doped with Si and P ions, as a result of the reaction between Si2P2O7 and LiNi0.5Co0.2Mn0.3O2 at 800oC. The TEM image of the coated samples shows an amorphous coating layer < 2 nm thick (Fig. 11a), and we believe that this layer is composed of LiδPyOz. The inner parts of the coating layer consist of layered phase confirmed by and (2-21) planes from digitized Fourier-transformed images.
The decreased capacity of the sample coated at a rate of 0.2C is due to the decrease in the amount of electroactive Ni2+ or Ni3+ ions as a result of the increase of Si and P ions in the 3b sites. 12b shows the discharge capacity of pristine and coated cathodes as a function of cycle number under increasing C rates. In this regard, it can be concluded that the significantly improved rate capability of the coated cathode is due to the amorphous coating layer and is not related to the bulk phase.
In addition, we evaluate changes in operating voltages (operating voltage corresponds to the voltage of half the capacity value) at different speeds (Figure 13). During charge and discharge, the operating voltages of the coated samples are lower during charge and higher during discharge than that of the pristine sample. For example, the operating voltage of the coated sample was 3.44 V at 7C, which is much higher than that of the pristine sample (2.7 V).
To observe the changes of the electrode materials after cycling at 60oC, electrochemical impedance analysis and XPS were performed. Before cycling, the Rct value of the pristine cathode was smaller than that of the coated sample, which could be related to the presence of a clean ion-conducting coating layer similar to LixPyOz. After cycling, the RSEI and Rct values in the coated sample were found to be much smaller than those in the pristine sample, at almost 1/5 of the value.
According to the XPS spectra of the pristine and coated cathode before and after cycling at 60°C at depths of 15, 30, and 60 nm from the surface, the spectra of the coated cathode were quite similar. To evaluate the thermal stability of the cathode materials, a differential scanning calorimetry (DSC) analysis was used. The peak area was indicative of the total amounts of oxygen evolved from the cathode and this was estimated to be 580 J/g.
Such structural stability is supported by in situ XRD measurements of samples up to 325 °C (Figure 15). Both the pristine and coated cathodes do not completely transform to the spinel phase even at 250 oC, but the higher chex/ahex ratios of the coated cathodes than the pristine ones at both 200 and 250 oC indicate the preferential formation of layered phases.
CONCLUSION
ACKNOWLEDGEMENTS
This work was supported by the grant from the Technology Innovation Program of the Ministry of Knowledge Economy of Korea (Project No. 10032319). This research was also supported by MKE (The Ministry of Knowledge Economy), Korea, under the ITRC (Information Technology Research Center) support program under the supervision of NIPA (National IT Industry Promotion Agency) (NIPA-2011-C.
Assessment of selected electrode-solution interactions that determine the performance of Li and Li-ion batteries. A review of the characteristics and analyzes of the solid electrolyte interphase in Li-ion batteries. In situ transmission X-ray absorption fine structure analysis of the Li deintercalation process in Li(Ni0.5Co0.5)O-2.
Synthesis and structural characterization of layered Li Ni1/3Co1/3Mn1/3 O-2 cathode materials by ultrasonic spray pyrolysis method. Structural and electrochemical properties of layered Li Ni1-2xCoxMnx O-2 (x=0.1-0.3) positive electrode materials for Li-ion batteries. Effect of preparation methods of LiNi1-xCoxO2 cathode materials on their chemical structure and electrode performance.
Electrochemical analysis for cycle performance and capacity decay of a lithium-ion battery operated at elevated temperature. Contribution of the structural changes of LiNi0.8Co0.15Al0.05O2 cathodes to the exothermic reactions in Li-ion cells. Cation ordering in Li NixMnxCo(1-2x) O-2 layered cathode materials: a nuclear magnetic resonance (NMR), pair distribution function, X-ray absorption spectroscopy and electrochemical investigation.
XAS Investigation of Inhomogeneous Bulk and Surface Metal-Oxygen Covalent Bonding for Charge Compensation in Li-Ion Battery Cathode Material Li Ni1/3Co1/3Mn1/3 O-2. Self-Discharge of LiMn2O4/C Li-ion Cells in Their Discharged State - Understanding by Three-Electrode Measurements. Synthesis and electrochemical properties of ZnO-coated spinel LiNi0.5Mn1.5O4 as 5 V cathode material for lithium secondary batteries.
Effect of ZnO coating on the electrochemical cycling behavior of LiMn2O4 spinel cathode materials at elevated temperature. Structural characterization of surface-modified LixNi0.9Co0.1O2 cathode materials with MPO4 coatings (M = Al, Ce, SrH and Fe) for Li-ion cells. Controlled nanoparticle metal phosphate coatings (metal = Al, Fe, Ce and Sr) on LiCoO2 cathode materials.
