Cellulosic separator applications: New and improved separators for alkaline rechargeable cells

A study has been established at Naval Surface Warfare Center, Crane Division (NAVSURFWARCENDIV) and Rutgers University to obtain material analysis data on cellulosic separator properties such as tensile strength and swelling of alkaline-soaked film samples, degree of polymerization and crystallinity, cationic and anionic diffusion and penetration rates in alkaline media, and X-ray diffraction for crystallinity. These data are then related to cycle- and wet-life information from model electrochemical cells as a function of separator composition on an alkaline chemistry rechargeable cell set. The first examples used in this program are silver-zinc rechargeable cells of 28 Ah capacity, identical in every respect except for the separator composition, which are being tested in statistically significant numbers under identical temperature and relative humidity conditions, with 45% KOH as the electrolyte. The cycle-life test regime of continuous cycling is: C/5 discharge to 1.30 V, and C/30 and C/60 charge to 2.03 V, while the wet-life test regime includes a 30-day wet-stand at full charge between cycles. At the outset of the cell testing, a baseline cell was selected from each set in the matrix after the so-called formation cycling was complete, and the physical properties of crystallinity, tensile strength, and degree of polymerization were re-measured. Then, at intervals during cycle- and wet-life cycling, and as cells fail the life tests, these properties will be measured again for selected cells. In this way a correlation will be established between separator life under charge/discharge conditions in actual cells and the critical physical properties of each separator film. Eight separator compositions, all cellulose-based, are being evaluated. The purpose of the study is to utilize the cycle- and wet-life data which are a function of separator composition and physical properties in the alkaline chemistry rechargeable cell set, to designate a 'best' separator for incorporation into actual production cells by the manufacturing community for silver/zinc rechargeable cells. The recommendations will take the form of minimum separator physical properties which are beneficial to cell performance and long life, resulting in an improvement in the assets available for Fleet use in the US Navy. This paper discusses the data available to date on cycle- and wet-life, and their relationship to separator physical properties before and at several stages during cycling.