Phospholyte? Slows Carbonate Degradation in PF6-based non-aqueous electrolytes

This article was written by New Dominion's Chief Scientist and co-founder, Dr. Mason Harrup and comments on an article that appeared in the Journal of Power Sources a few years ago, but which we only became aware of recently. (https://www.sciencedirect.com/science/article/abs/pii/S0378775319313564). Dr. Harrup will respond to any technical questions.

Water has a dynamic, significant role in the stability of PF6--based non-aqueous electrolytes for lithium and sodium ion batteries. ?These electrolytes are highly susceptible to the formation of HF in the presence of water. ?Most remarkably, significant HF formation can even be observed in battery grade electrolytes which contain <20 ppm H2O.? Testing results (via 1H and 19F NMR) demonstrate an autocatalytic hydrolysis pathway of hexafluorophosphate to form HF in the electrolyte. Byproducts of the salt degradation, PO3F2- and PO2F2-, are also confirmed with NMR. It is acknowledged that these species play a dramatic role in the stability of the electrolyte. The most vulnerable carbonate solvent is shown to be cyclic EC. In comparison, PC clearly shows a slower rate of degradation. The rate of decarboxylation of the acyclic carbonate species depends on the length of the aliphatic chain (i.e., DMC > EMC > DEC).?

Electrolytes with and without added water have also been tested in full-cell batteries with a high nickel cathode and a hard carbon anode with the electrolyte stabilizing cosolvents Phospholyte? and FEC. Results indicate that Phospholyte? acts as a “scavenger” and inhibits the formation of HF. This Phospholyte? cosolvent has also been used in other studies to stabilize LiPF6-based electrolytes at high temperatures. ?Intrinsically, Phospholyte? has mitigated the complete formation of HF. We believe that this result stems from the structure of the phosphazene, which has a cyclic ring with resonating p bonds between P and N.? For comparison, the commonly used FEC additive exhibits only a minimal effect in mitigating the degradation products.?

In the continuing development of lithium and sodium ion battery technologies, electrolytes should be closely monitored for degradation. Similarly, cosolvents such as Phospholyte? should be employed to increase the stability of the electrolyte. Stabilizing the electrolyte and mitigating the degradation of the electrolyte, primarily initiated by PF6- degradation and the formation of HF enhances the shelf life and performance of both lithium and sodium ion batteries.? In full cells with aged (stored at 52 °C for 31 days) electrolyte containing Phospholyte? shows comparable performance to pristine electrolyte exhibiting a capacity of 91 ± 0.5 mAh/g. ?In comparison, the full cell with only FEC demonstrates a dramatic decrease in initial capacity. When water is added (100 ppm), the capacity drastically decreases to 14 ± 2 mAh/g, similar to the Phospholyte?-free electrolyte. Remarkably, when Phospholyte? is present in aged electrolyte with added water the full cell has an initial capacity of ~44 ± 3 mAh/g and an overall capacity retention of 62% at 100 cycles. This verifies that the salt degradation byproducts are detrimental to the performance of the battery – a condition that Phospholyte? significantly inhibits.

In summary, recent studies on the effect Phospholyte? has on carbonate-based electrolytes is profound. ?Not only are the carbonates by themselves susceptible to degradation – especially as the temperature increases – but the degradation of the PF6- anion catalytically accelerates this degradation.? In the presence of Phospholyte?, the PF6- anion and its degradation fragments are sequestered within the core ring structure, essentially preventing, or at least dramatically slowing, the action of these species, thereby providing the carbonates with significant protection from degradation.? This lengthens battery life – both shelf life and cycle life – all while providing superior resistance to the deleterious effects that heat has on all carbonate electrolytes.