Analysis of Deterioration Mechanism of Al-Pouch Film Used As Packaging Materials for Lithium Ion Batteries

Abstract

Generally, lithium ion batteries (LIBs) can be divided into three different types: cylindrical, prismatic, and pouch-type LiBs with regard to the packaging form. Contrary to cylindrical or prismatic type batteries, pouch-type battery uses a pouch as an exterior material to protect inside of the cell from external influences, and have a metal lead-taps to receive and deliver electricity from external source. To safeguard the inside of the lithium ion battery, the pouch must have high resistance to gas and moisture permeability and excellent corrosion resistance to electrolytes.However, Al-pouch can be prone to contamination from electrolyte containing lithium salt during the electrolyte injection process, a key part of their manufacturing process. This electrolyte contamination can be fatal to the long-term reliability or durability of pouch-type LiBs. In this study, we investigate deterioration behavior of aluminium pouch film due to the electrolyte contamination. In order to clarify deterioration mechanism of Al-pouch film, we prepared exposure test of Al-pouch film and dummy cell samples which were stained by electrolyte droplet (LiPF6 in EC:EMC=1:2 (v/v) + 2 % VC). The change of micro-structure of the samples was tracked during 20 weeks by storing them under accelerated test conditions (RH 95 %, 60 ℃). As a result, we find out severe defect of the samples of Al-pouch film by damage to outermost nylon film and aluminium layer, and confirmed the effects on the battery after the defects occurred.To understand such phenomena, the micro-defect of Al-pouch films was observed by characterization techniques (XRD, FT-IR, SEM, EDX). We demonstrate that outermost layer of nylon film is damaged when it reacts with an electrolyte containing LiPF6 salt such as decreasing crystallinity, surface crazing, forming microdefects. The degradation of nylon film leads to severe surface cracking, followed by nylon locally peeling off from the Al-pouch. Peeling of the nylon film causes to expose the underlying thin aluminum film that constitute the middle layer of Al-pouch. Once partially exposed to the atmosphere, severe pitting corrosion of the aluminum film occurs due to atmospheric moisture and acids produced by LiPF6. Finally, a large amount of moisture to easily penetrate pouch-type LiBs through these corroded pits, showing the detrimental effects of electrolyte contamination on pouch-type LiBs’ long-term durability and reliability. Figure 1