The effect of AlF3 Coating on the High-Voltage Cycling Behavior of Li[Ni1/3Co1/3Mn1/3]O2 Cathode

Abstract

Introduction Li[Ni1/3Co1/3Mn1/3]O2 has been studied extensively as a promising cathode material for lithium secondary batteries. It has many advantages such as a high discharge capacity, excellent thermal stability, and good structural stability. In attempts to increase the reversible capacity of the cathode material, the upper cutoff voltage limit has been gradually increased. The increased upper voltage limit resulted in a moderate increase in the specific discharge capacity as expected. However, the improved discharge capacity was accompanied by unstable cycling performance when cycled up to 4.6 V. The origin of this capacity fading was mainly attributed to gradual decaying of electroactive Co as reported by Shaju et al.[1]. In our previous study, we reported that a fluorinesubstituted Li[Ni1/3Co1/3Mn1/3]O2 would exhibit an improved cycleability in the 3.4.6V range [2]. In this work, we first report the effect of surface modification of Li[Ni1/3Co1/3Mn1/3]O2 by AlF3 on electrochemical performance at high cut-off voltage

Experimental To prepare AlF3-modified Li[Ni1/3Co1/3Mn1/3]O2, ammonium fluoride (Aldrich) and aluminum nitrate nonahydrate (Aldrich) was separately dissolved in distilled water. After Li[Ni1/3Co1/3Mn1/3]O2 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 0.5 mol % of the Li[Ni1/3Co1/3Mn1/3]O2 powders. After the solution containing the cathode powders was constantly stirred at 80℃ for 5hr and then filtered by distilled water. The obtained Li[Ni1/3Co1/3Mn1/3]O2 powders were heated at 400℃ for 5 hr in nitrogen flowing. X-ray diffraction (XRD, Rigaku Rint-2000) and Scanning electron microscope(SEM, JSM 6400) were employed to characterize the prepared powders. Cell tests were done using the 2032 coin-type cell with Li-metal as the negative electrode. The cells were charged and discharged between 3.0 and 4.5V.

Results and discussion Figure 1 presents SEM and EDX elemental mapping images of the AlF3-coated Li[Ni1/3Co1/3Mn1/3]O2 powders. Al and F were well distributed throughout the Li[Ni1/3Co1/3Mn1/3]O2. This indicates that AlF3 was homogeneously covered on the surface of Li[Ni1/3Co1/3Mn1/3]O2. Figure 2 shows cycle performance of pristine and AlF3- coated Li[Ni1/3Co1/3Mn1/3]O2 cells during cycling in the range of 3.0.4.5 V. The pristine and AlF3-coated Li[Ni1/3Co1/3Mn1/3]O2 delivered discharge capacity of 172.1 and 173.4 mAh/g, respectively. The pristine Li[Ni1/3Co1/3Mn1/3]O2 showed gradual capacity loss with cycling, showing 75 % of capacity retention at 50 cycles. Meanwhile, the AlF3-coated Li[Ni1/3Co1/3Mn1/3]O2 showed remarkably enhanced capacity retention(93 %) during 50 cycles.