The Effect of Cellulose Separator Water-Scavenging on Cycle Life in Lithium-Ion Batteries

Abstract

The presence of water inside a lithium ion (Li-ion) battery causes several interconnected chemical mechanisms that lead to material degradation including transition metal dissolution. [1-4]. As a result, cell performance is reduced, and the cell capacity rapidly fades.To mitigate transition metal dissolution caused by trace water, research groups have proposed various approaches to scavenge and neutralize the water within different components of the cell [4-6]. These methods include using a dehydratable molecular sieve within the cathode active material powder [6], direct dosing of the electrolyte with a water scavenging additive [5], and introducing a metal organic framework with water scavenging properties by mixing it with a polymer binder to create a film for use as a separator. Results from these studies show promise in improving the cycling stability of cells under abuse conditions, such as elevated temperature and high water content in the electrolyte. While these water scavenging techniques show clear benefits to cell capacity retention, they come with the trade-off of higher material cost, more complex production processes, and lower energy density, which have limited their widespread adoption in Li-ion batteries.This study focuses on the use of “cellulose”, a cheap, abundant and naturally dehydrating biopolymer, as a separator material. Cellulose-based separators have been used in Li-ion batteries and shown to be advantageous for capacity retention of the cells. The benefits of the cellulose separators have been attributed to their superior wettability, uniform pore size distribution, high porosity, and low electrical resistance [7-10]. Despite their well-known hydrophilicity, their water scavenging capabilities have not been thoroughly evaluated.In this work, we present new insights into the interaction of water with cellulose-based separator. The water scavenging properties of the cellulose separator are investigated both outside of the battery using the Karl-Fischer Coulometric Titration technique, and inside of the battery through cycling tests. As shown in Figure 1, replacing the conventional polymer-based separator with a cellulose-based nonwoven separator resulted in a significant improvement in cycle life. Furthermore, the water scavenging mechanism of cellulose-based nonwoven separator is studied using surface chemistry characterizations, suggesting water scavenging by the naturally occurring hydrogen bonding sites of cellulose. Additional discussion on drying conditions and the impact of other fiber types are also provided.