Correlation Study of Surface Energy of Electrodes and Electrolytes with Lithium Ion Battery Performance

Abstract

Despite the huge success of the lithium-ion batteries (LIBs) in the portable electronic devices and electric vehicles (EVs) applications, the fundamental understanding on the electrode/electrolyte interface still remains challenging. The interfacial phenomena are governed by the physico-chemical properties of the electrode surface as well as the nature of electrolyte components. At the first part of this work, the surface free energy (SFE) analysis is performed for various commercial grade LiMn2O4 (LMO) powders and the three SFE components, Lifshitz van der Waals (γ𝑠LW), acid(γ𝑠+), and base(γ𝑠−), are obtained based on the van Oss-Chaudhary-Good (vOCG) theory. It is revealed that Mn dissolution is strongly correlated with the Lewis acid-base component (γ𝑠AB = 2√γ𝑠+ ∙ γ𝑠− ), which is attributed to the short-range columbic interactions between the Lewis acidic site of LMO surface (γ𝑠+) and the basic electrolyte species (e.g., solvents, anions), and between the Lewis basic site (γ𝑠−) and the acidic electrolyte species (e.g., HF). At the second part, the SFE analysis is performed to shed some light on surface chemical properties of graphite anode and the solid-electrolyte interphase (SEI) layer formed on it. The edge and basal planes of pristine graphite show relatively high γ+ and γ-, respectively. The presence of SEI layer brings dramatic difference in the SFE properties of the graphite electrodes. In particular, the γ- values becomes one order of magnitude higher. In addition, the SFE values also depend on the types of Li salt employed for SEI formation. LiPF6 and LiFSI solutions form inorganic-rich SEI layer, and thus higher total SFE than the organicrich SEI formed in a LiClO4 solution. At the last part, various polymers are examined to search a suitable probe solid triplet with a low condition number, which is mandatory to determine the three SFE components of liquid samples. Among the tested combinations, PE/PVF/PMMA set is found to have the lowest condition number, which is rather high compared to that of probe liquid set. Further exploration for better probe solid triplet is needed. ⓒ 2016 DGIST