Significant improvement of AlF3-coated LiCoO2 cathode in high voltage cycling

Abstract

Introduction LixCoO2 enables to deliver a reversible capacity of around 140 mAh/g typically when charged to 4.2 V (x . 0.5), which is much lower than its theoretical capacity of 274 mAh/g. In attempts to increase the reversible capacity of LiCoO2, the positive electrode has to be charged above 4.2 V. However, the improved discharge capacity was accompanied by unstable cycling performance when cycled up to 4.5V. To maintain the high reversible capacity of LiCoO2 at high voltage, the substitution of Al, Mg, and Zn for Co to give LiCo1-xMxO2 have been intensively studied. However, capacity fading has been still observed even though some improvement has been achieved. Several surface modifications of LiCoO2 by metal oxide coating such as Al2O3, ZrO2, MgO, SnO2, TiO2, and SiO2 have been widely studied. It is well-known that a metal oxide coating could significantly improve the electrochemical properties. In this work, we first report the effect of surface modification of LiCoO2 by AlF3 on electrochemical performance at higher cut-off voltage of 4.5 V.

Experimental To prepare AlF3-modified LiCoO2, ammonium fluoride (Aldrich) and aluminum nitrate nonahydrate (Aldrich) was separately dissolved in distilled water. After LiCoO2 (Nippon Chemical Co.) powders were poured into the aluminum nitrate nonahydrate solution, the ammonium fluoride solution was slowly added to the solution. The molar ratio of Al to F is fixed to 3 and the amount of AlF3 in the solution corresponded to 2 mol % of the LiCoO2 powders. After the solution containing the cathode powders was constantly stirred at 80 ℃ for 5hr and then filtered by distilled water. The obtained LiCoO2 powders were heated at 400℃ for 5 hr in nitrogen flowing. Transmission electron microscopy(TEM, JEOL 2010) and Powder X-ray diffraction (XRD, Rigaku Rint-2000) employing Cu Kα radiation was used to characterize the prepared powders. Charge-discharge were performed with a coin type cell (CR2032) and laminated-type full cell wrapped with an Al pouch. AC impedance measurement were performed using a Zahner Elektrik IM6 impedance analyzer over the frequency range from 1 MHz to 1mHz with the amplitude of 10 mVrms.

Results and discussion The laminated-type full cell were charged and discharged for 500 cycles at the rate of 1 C between 3.0 V and 4.4 V(Figure 1). The capacity of the C/LiCoO2 cell rapidly decreased with cycling and reached to almost 0 after 500 cycles. On the other hand, the AlF3-coated LiCoO2 cell shows an excellent cycling performance and has capacity retention of 91% after 500 cycles. Figure 2 shows the discharge capacities for the Li/LiCoO2 and AlF3-coated LiCoO2 cells as a function of various C-rate (20 - 320 mA/g) between 3.0 and 4.5 V. At 2 C-rate (320 mA/g), the obtained discharge capacity of the AlF3-doped electrode is about 85 % of that of 0.13 Crate (20 mA/g), as compared with that of the pristine one showing only 35 %.