Decreasing Capacity Fade of LFP/ Graphite Pouch Cells Using New Additive Combinations and Mixed Salts

Abstract

Reliable grid energy storage is essential to energy electrification. Current batteries cannot be produced to meet carbon neutral goals. One method to reduce battery emissions is to improve battery lifetime. Using Lithium Iron Phosphate (LFP) cells, the market dominant battery chemistry for Li-Ion, we attempt to improve battery lifetime by changing additive compounds and salt mixtures in the cell’s electrolyte. Cells were received dry as 40-20-35 mm sized pouch cells, and were filled with electrolyte, containing several electrolyte additive formulations, including novel, unpublished additives. Cells underwent a first formation cycle at 40C, and were then subjected to a reference performance test (RPT) which involved short-term cycling at 20C and long-term storage at 55C. Several cells were also tested using long-term cycling at 70C. Temperature is variable in this study as low temperatures typically cause lithium plating, whereas high temperatures cause internal resistance growth as a result of solid electrolyte interphase growth. Non-destructive measurements of resistance to charge transfer (Rct, i.e., internal resistance) were taken in between each RPT and storage cycle. These data were used to determine correlation between capacity fade and increasing Rct. Additives were then compared to benchmark cells of well-studied electrolyte composition; both in terms of capacity retention and Rct growth. Successful additive combinations retain 2–4% more normalized capacity over comparator cells, increase absolute capacity by 5–15 mAh per cell, and decrease initial Rct in well tested solvent blends. Specifically, Di-fluoroethylene carbonate (DiFEC) proved to be an excellent co-additive to vinylene carbonate (VC) at high temperature cycling retaining more normalized capacity and having more absolute capacity than VC alone, while 4,5-bis(2,2-dioxo-1,3,2-dioxathiolan-4-yl)-1,3-dioxolan-2-one (ECBisDTD) proved to be excellent on storage conditions, retaining ~98% capacity after 1,512 hours of storage. The additive 4-(2,2-dioxo-1,3,2-dioxathiolan-4-yl)-1,3,2-dioxathiolane 2,2-dioxide (BisDTD) proved to be both excellent at high temperature cycling and RPT, retaining ~96% capacity over RPT, and substantially increasing the absolute capacity of cells with VC as the sole additive. Thus, several additives have the potential to increase efficiency and energy density of battery systems by improving both their lifetime and lifetime energy output.