Studies on Electrochemical Properties and Capacity Decay Mechanisms of Surface-Coated and Uncoated High Voltage Spinel LiNi0.5 Mn1.5O4

Abstract

The spinel structured LiNi0.5Mn1.5O4 (LNMO) is a very promising candidate for next-generation lithium-ion batteries cathode material for fast charge−discharge applications. Still, its capacity decay mechanisms and rate-limiting process are not yet well understood. In this study, LNMO was synthesized by the sol-gel method and then electrochemical impedance spectroscopy with galvanostatic intermittent titration, and cycling aging techniques were employed to measure key parameters, such as ionic diffusivity and exchange current density and to investigate the nature of capacity decay in disordered-phase LNMO. Different resistive components were separated after every 10 cycles. Cell overvoltage (ΔV) due to Ohmic conduction, charge transfer (CT), and concentration polarization (CP) were individually determined. Results revealed that the cell exhibited a higher ΔV at a higher discharged state, and LNMO/ electrolyte interface played a major role in the rate-determining step. The outcomes of the analysis showed that the creation of a solid electrolyte interface (SEI) from the residual product of oxidative electrolyte decomposition and growing thickness resulted in an ongoing capacity loss upon battery cycling. Battery life was estimated based on the obtained results, where battery calendar life was found to be more vulnerable than cycle life. Results also indicated that the working SOC range could be optimized based on the resistance analysis by avoiding those SOCs that have the most detrimental impact (e.g., heat generation and fire hazard). To improve the electrochemical performance of the bare LNMO, the LNMO samples were coated with different amounts (1.0 and 1.5 wt. %) of silica (SiO2) using a cost-effective and scalable ball milling process. The advantages of this coating were demonstrated by the improved electrochemical performance at ambient and elevated temperatures (25 and 55 °C) using half- and full-cell configurations with conventional liquid electrolytes. The SEI and coating properties have been highlighted by ex-situ TEM analysis, which indicated the close attachment and good wetting of the SiO2 layer with the LNMO active particles. Importantly, the 1 wt. % SiO2-coated material cycled at an extremely high C-rate of 10, and 40 for 400 cycles exhibited excellent cycling stability with capacity retentions of 97 and 88 %, respectively. Moreover, the 1 wt. % SiO2-coated material showed excellent cycling stability when charged at 6 C (10 min.) and discharged at C/3 for 500 cycles as per electric vehicle charging protocol. The interfacial resistance of the SiO2-coated LNMO is found to be much lower compared to the bare LNMO and did not considerably increase with the amount of coating.

Date of Award	2020
Original language	American English
Awarding Institution	HBKU College of Science and Engineering