Electrochemical and Impedance Investigation of Ni-Rich LiNi1-x-YCoxAlyO2 Cathode Material for Lithium-Ion Batteries

Abstract

Recently, the lihtium-ion batteries (LIBs) are applied from the power sources of portable electronic devices to large-scale energy storage devices such as electrical vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and energy storage system (ESS), based on the cost, safety, and energy density. LiNiO2 has been considered as a promising cathode material for large-scale LIBs due to its low cost and high specific capacity (275 mAh/g) [1]. However, it has several problems, such as difficult synthesis, low thermal stability in chaged state, and capacity fading upon long-term cycling. In order to overcome these problems, several partial substitutions (Co, Mn, Fe, Al, etc.) for nickel have been investigated [2-5]. Among those partial subtitutions, Co has been proven to be a promising cadidate for LIB due to the improved thermal stability of LiNiO2 in the delithiated state. In addition, it was reported that a small amount of Al3+ doping leads to retain Ni in 3+ state and as a result, the stabilization of layerd structure and electrochemical properties of LiNi1-xCoxO2 is improved [6]. In this work, LiNi1-x-yCoxAlyO2 (0.08≤x≤0.15, 0.15≤y≤0.05) powders were prepared by a co-precipitation method in aqueous solution to employ as a cathode material for Li-S batteries. Ni1-xCox(OH)2 precursor was firstly prepared by co-precipitation and then aqueous solution containing Al was added in Ni1-xCox(OH)2 precursor. The pH of Al solution was controlled in a range of 8-10. Final solution was filtered, washed and dried. LiNi1-x-yCoxAlyO2 cathode materials were obtained after calcining Ni1-x-yCoxAly(OH)2 with LiOH (Li/M=1~1.1) under air atmosphere at 700 oC for 20 h. For electrode fabrication, LiNi1-x-yCoxAlyO2 cathode materials, super-P carbon black, and polyvinylidene fluoride (PVdF, Mw: ~ 400,000, Sigma Aldrich) were ground with N-methyl-2-pyrrolidone (NMP, SAMCHUN PURE CHEMICAL) as a solvent. Lithium foil, polyethylene (PE, W-SCOPE, KOREA), and 1.15M LiPF6/EC-EMC (3:7) were used as a counter electrode, separator, and electrolyte, respectively. The morphology of LiNi1-x-yCoxAlyO2 cathode material was characterized using a field emission-scanning electron microscopy (FE-SEM, Carl Zeiss, LEO-1530) and focused ion beam milling (FIB, Carl Zeiss, 1540 EsB). The particle size of LiNi1-x-yCoxAlyO2 synthesized in this work ranged at 5~10 um. The electrical conductivity of the LiNi1-x-yCoxAlyO2 cathode materials were also analyzed by a powder resistivity measurement system (HAN TECH, HPRM-M2). Cycle performance tests were carried out using the 2030 coin type cell by CC/CV method in a potential range of 3.0~4.3 V at 0.1 C rate. Cyclic voltammetry (CV, WonATech, WBCS 3000) was conducted at a scanning rate of 0.1 mV s-1. In addition, electrochemical impedance spectroscopy (EIS, Metrohm Autolab B.V., PGSTAT302N) was performed with a frequency range of 0.5 mHz to 100 kHz at an amplitude of 10 mV. The prepared LiNi1-x-yCoxAlyO2 cathode materials exhibit an excellent performance with a high specific capacity of over 180 mAh g-1 at the first cycle and with a cycling retention higher than 80%.