Application of Dreamweaver Separators to Lithium Ion Batteries: Quick Start Guide

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1 Application of Dreamweaver Separators to Lithium Ion Batteries: Quick Start Guide Drying: Preparing the separators for use Cell winding and stacking, Hipot testing Electrolyte: wetting and amount Formation: Understanding the initial charge cycles

2 Drying Dreamweaver separators are made from materials that are far more polar than conventional polyolefin separators. They therefore retain more water, and need to be dried before use. 6.00% 5.00% Percent of roll weight lost Drying curves for several rolls of Dreamweaver Silver are shown at the right in a 120 C vacuum oven. Drying recommendations are in the table to the right. Smaller rolls will dry more quickly, while larger rolls can take up to 48 hours to remove the moisture. Rolls should be stored in a dry room after drying. Cell drying can occur at the same temperature as separator drying. Dreamweaver has prepared an Application Note to help assist a battery manufacturer in developing its own drying specifications, which may be found at /dwi_an0002r1.0.pdf 4.00% 200mmx1000m 3.00% 200mmx400m 60mmx1000m 60mmx400m 2.00% 1.00% 0.00% Separator Drying Temp Small Rolls Large Rolls Titanium 130 C 24 hrs 48 hrs Silver 110 C 24 hrs 48 hrs Gold 140 C 24 hrs 48 hrs

3 Cell Winding and Stacking Winding/Stacking: Dreamweaver separator looks and feels like paper, and can be processed at high speeds. However, because it does not stretch like conventional polyolefin separators, the equipment that was designed for those separators can sometimes need to be adjusted. Care must be taken to eliminate out of square rollers, which can cause breaks. The frictional properties of DWI separators are different from conventional separators, so appropriate grippers and friction points may need to be used. Sensors and detectors that are set up to detect polyolefin separators may also need to have parameters reset. Dreamweaver has prepared an Application Note for winding and stacking, which can be found at /dwi_an0001r1.0.pdf

4 Hipot Testing Hipot testing: DWI separators have higher porosity which will require different voltages for hipot testing. To determine proper voltages for different cell configurations, three cell types have been tested: Open cells: An electrode stack under minimal pressure. Close cells: an electrode stack in a sealed pouch with welded tabs. Closed cells, vacuum degassed: and electrode stack in a sealed pouch with welded tabs, vacuum degassed. Each of these configurations required lower voltages to achieve a relevant hipot test procedure. Cell Configuration Relevant Cell Geometries Hipot Voltage Open Cells Small pouch cells with low V pressure Closed, multilayer cells Larger pouch cells with V low pressure Closed multilayer cells, vacuum degassed Prismatic or cylindrical cells with pressure applied between layers V The voltage required for your cell will have to be determined through experimentation. However, relevant ranges can be taken from the DWI experiments listed in the table to the right.

5 Electrolyte: Wetting dynamics and amount Electrolyte Ratio: DWI separators are significantly more polar than normal polyolefin separators. Because of this, they wet out much faster and do not require long waiting periods for the electrolyte to saturate the cell. The electrolyte may also disperse itself differently in the cell, as it will have much more affinity for the separator than for the electrodes. Inadequate electrolyte quantity may leave dry regions in the electrodes. This would be indicated in the performance as slightly lower capacity. Properly wetted cells with DWI separators generally have 1-3% higher capacity than similar cells build with polyolefin separators. If capacity appears low, it may be because of inadequate electrolyte amount. Another symptom of inadequate electrolyte quantity is the presence of soft shorts in the initial cycles. DWI Silver DWI Gold Polyolefin

6 Formation: understanding the initial charge cycles Cells with DWI separator will allow formation of an SEI layer on the anode as normal, but also may form SEI on the separator as well. This has the benefit of reducing the pore size. However, the cells can require more time in formation to complete the process, and coulombic efficiency of the first cycle will be lower. This is to complete the SEI formation and does not affect the final capacity of the cell (there is no permanent lithium loss). A suggested formation cycle is: CC rate of first charge: C/6 or slower CV cutoff of first charge: C/60 or slower This step may require more time and can depend strongly on the electrodes. Voltage: LFP: V, Voltage: NMC: V CC discharge to normal low voltage # cycles: normally only one required, but may require a second to complete the SEI. DWI Silver prior to formation DWI Silver after formation Prior to proper SEI formation, the cell will experience high selfdischarge as the SEI continues to form, but this will stop once the SEI is fully formed. It is also possible that gas may be formed, and this should be evacuated. Polyolefin formation DWI Gold formation