Improvement of electrode/electrolyte interfacial properties and electrochemical performance of secondary batteries through electrolyte additives

Abstract

With the expansion of global electric vehicles (EVs) and energy storage system (ESS) market, the demand for secondary batteries with high energy density, power density, and safety is growing more than ever before. The performance and safety of batteries largely hinge on the characteristics of the electrolyte, of which the characteristics change greatly, relying on the concentrations and types of constituents such as solvents, salts, and additives. In particular, electrolyte additives, which generally compose of less than 10% of the electrolyte, perform various roles such as overcharge protection, flame retardants, electrode/electrolyte interface properties etc., depending on their molecular structures. Since utilizing electrolyte additive is a simple and economical feasible way without the sacrifice of energy density, the development and discovery of electrolyte additives has been most actively adopted strategy for enhancing the battery performance in the battery industry. Following the development direction, in this thesis, two studies on improvement of electrode/electrolyte interfacial properties by electrolyte additives and in-depth analyses on the effect of additives are presented. First, CEI/SEI-forming additive diphenyl diselenide (DPDS) was introduced to the electrolyte for surface stabilization of high nickel-based cathode materials and the effect was investigated in LiNi0.8Mn0.1Co0.1O2 (NMC811)/graphite batteries. Since DPDS has a lower lowest unoccupied molecular orbital (LUMO) energy level and a highest occupied molecular orbital (HOMO) energy level compared to those of ethylene carbonate (EC) and diethyl carbonate (DEC), it can be oxidized on the cathode and reduced on the anode, thereby forming protective layer on the both electrodes. The formed DPDS-driven CEI/SEI effectively prevented electrolyte decomposition, micro-crack formation, Li/Ni cation mixing, and transition metal dissolution during cell operation. As a result, in the high voltage window of 2.8–4.5 V, DPDS enhances the capacity retention of NMC811/graphite batteries from 68.0% to 82.0% after 200 cycles at 1C. Second, electrostatic shield-forming additive (ESA) is introduced into the membrane-less and flow-less Zn- Br battery (MLFL-ZBB) to suppress dendrite formation. During the Zn2+ plating process, metal ions as ESAs form an efficient electrostatic shielding layer around the tip and repels the Zn2+ ions to adjacent flat regions of the electrode, resulting in uniform Zn plating. Herein, various metal ions, which has lower reduction potential compared to Zn2+/Zn are used as ESAs, and their effect on the cell performance and the requirements for ESA are explored. We here suggest the selection criteria of metal ions for efficient electrostatic shielding formation in FLML-ZBB system and achieve the high Coulombic efficiency of 98.5% over 700 cycles at a high current density of 20 mA cm−2 and areal capacity of 2 mAh cm−2 without short circuit, which validate the effectiveness of selected metal ions.