Exploring the Electrochemical Properties of Ultrathin LiMn1.5Ni0.5O4 Films at High and Low Current Rates for Lithium-Ion Micro Battery Applications

Abstract

The development in the miniaturized - smart electric and electronic gadgets with the advancement in technologies has dramatically enhanced the demand for cobalt free-high energy density batteries. Among various choice of high-performance cathode materials Ni substituted LiMn2O4 spinel of composition LiMn1.5Ni0.5O4 (LMNO) has proved to be one of the solutions to yield a high-performance high-energy density battery which can operate at a higher potential window. Ultra - thin films of LMNO fabricated by Pulsed laser deposition method at 600 °C substrate temperature using 2000 pulses. LMNO films fabricated on SrTiO3 (STO) and stainless steel (SS) substrates are employed for a detailed structural and electrochemical performance analysis. Disordered phase corresponding to fd3m space group of LMNO thin film exhibits better electrochemical performance compared to the ordered phase. Phase formation of LMNO thin films are studied employing X-ray diffraction and Raman spectroscopy methods. The morphology obtained for the film of thickness 41.63 nm identified from the scanning electron microscopy (SEM) images clearly reveals the effect of growth conditions such as substrate temperature and base pressure on the growth behavior of these films. Since the electrochemical data obtained for the films are area limited, the capacity of the ultra-thin film is significantly low and can be ameliorate by controlling the area. Kinetic study of LMNO film reveals that the lithium diffusion during the charge storage mechanism resembles to a diffusion-limited process similar to a faradic process. The cycling stability and rate capability at different current densities of LMNO thin film electrodes are further investigated. Charge discharge profile for both higher (25, 50, 75, 100 and 125 mA) and lower (5, 10, 15, 20, and 25 mA) current densities are obtained and subjected for 500 cycles. The thin film electrode shows a stable profile for 500 cycles. Areal discharge capacity of 1.3 μAh/cm2 is observed at a higher current density while at lower current density an areal discharge capacity obtained is 2.13 μAh/cm2 with a coulombic efficiency of around 80% and 96% respectively. Electrochemical impedance spectroscopy studies carried out on LMNO films before and after cycle shows a strong contribution of charge-transfer resistance along with film resistance and solution resistance and the equivalent circuit model for the same is evolved. Present studies indicates that LMNO ultra-thin films can be employed as a cathode layer for fabricating the all-solid thin film micro-batteries for energy applications. Figure 1