Jisuk Kim

Synthesis, Thermal, and Electrochemical Properties of AlPO4-Coated LiNi0.8Co0.1Mn0.1 O 2 Cathode Materials for a Li-Ion Cell

Although LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode material has a larger specific capacity than LiCoO 2 , their thermal instability has hindered their use in Li-ion cells. An AlPO 4 coating on the LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode, however, noticeably diminished the violent exothermic reaction of the cathode material with the electrolyte, without sacrificing the specific capacity of the bare LiNi 0.8 Co 0.1 Mn 0.1 O 2 (188 mAh/g at 4.3 V charge cut off). The results were consistent with the thermal abuse tests using Li-ion cells; the AlPO 4 -coated LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode did not exhibit thermal runaway with smoke and explosion, in contrast to the cell containing the bare cathode. In addition, the AlPO 4 -coated LiNi 0.8 Co 0.1 Mo 0.1 O 2 cathode exhibited a superior cycle-life performance compared to the bare LiNi 0.8 Co 0.1 Mn 0.1 O 2 .

Controlled Nanoparticle Metal Phosphates (Metal = Al , Fe, Ce, and Sr) Coatings on LiCoO2 Cathode Materials

Despite the fact that the same coating concentration and annealing temperature are used for MPO 4 nanoparticle coatings (M = Al, Fe, Ce, and SrH) on a LiCOO 2 cathode, the extent of the coating coverage is influenced by the nanoparticle size or morphology. Nanoparticles (AlPO 4 or FePO 4 ) with a size smaller than 20 nm led to the complete encapsulation of LiCoO 2 , but those with sizes greater than 150 nm (CePO 4 ) or with whisker shapes (SrHPO 4 ) led to partial encapsulation. This difference affected the discharge capacity. The LiCoO 2 completely encapsulated with AlPO 4 or FePO 4 showed the highest discharge capacity of 230 mAh/g at 4.8 and 3 V at a rate of 0.1 C (=18 mA/g), which diminished with decreasing coating coverage in the order of Al ∼ Fe Ce > SrH > Fe > bare cathode. This is consistent with the capacity retention result obtained at 90°C storage for 4 h.

Washing Effect of a LiNi0.83Co0.15Al0.02O2 Cathode in Water

The formation of LiOH and Li 2 CO 3 impurities on high Ni content LiNi 0 . 8 3 Co 0 . 1 5 Al 0 . 0 2 O 2 powders due to H 2 O and CO 2 absorption from the air can be reduced without structural degradation by washing in water. Although the as-synthesized sample had a moisture content of 570 ppm immediately after firing, this level increased rapidly to 1270 ppm in air with a relative humidity of 50%. However, its content was decreased to 210 ppm after washing twice in water, followed by heat-treatment at 700°C. It is believed that this improvement was due to the decreased level of Li 2 CO 3 and LiOH impurities on the particles. This was highlighted by the decreasing swelling of the Li-ion cell at 90°C, and the thickness of the cell containing the washed samples was decreased by 50% compared with the bare sample.

Phase Transition of Bare and Coated Li x CoO2 (x = 0.4 and 0.24) at 300 ° C

The phase transitions of the bare and AlPO 4 -coated delithiated Li x CoO 2 (x = 0.4 and 0.24) according to the coating concentration (thickness) after heat-treatment at 300°C was investigated using nuclear magnetic resonance and X-ray absorption spectra. It was found that the bare and coated Li x CoO 2 predominantly decomposed into spinel Li x Co 2 O 4 or Co 3 O 4 phases depending on the charging voltage. As the charging voltage was increased from 4.3 to 4.6 V, the bare and 1 wt % AlPO 4 -coated Li x CoO 2 decomposed into the Li x Co 2 O 4 phase while the 2.4 wt % AlPO 4 -coated sample decomposed into the Co 3 O 4 phase. The improvement in the thermal stability of the 2.4 wt % AlPO 4 -coated Li x CoO 2 , compared to the bare and 1 wt % AlPO 4 -coated samples, could be explained by the dominant local formation of the Co 3 O 4 phase over the Li x Co 2 O 4 phase.