Chemical “Pickling” of Phosphite Additives Mitigates Impedance Rise in Li Ion Batteries

The use of high-voltage,
high-capacity positive electrodes in lithium
ion batteries presents a challenge, given their tendency to degrade
organic electrolytes. To prevent this damage, electrolyte additives
modifying the cathode surface are required. Tris­(trimethylsilyl) phosphite
(TMSPi) is one such electrolyte additive. However, the mechanism for
its protective action (similar to other phosphite, borate, and boroxane
compounds) remains not completely understood. In LiPF6 containing
carbonate electrolytes, TMSPi undergoes reactions yielding numerous
products. Here we demonstrate that one of these products, PF2OSiMe3, is responsible for mitigation of the impedance
rise that occurs in aged cells during charge/discharge cycling. This
same agent can also be responsible for reducing parasitic oxidation
currents and transition metal loss during prolonged cell cycling.
Mechanistic underpinnings of this protective action are examined using
computational methods. Our study suggests that this beneficial action
originates mainly through inhibition of catalytic centers for electrolyte
oxidation that are present on the cathode surface, by forming capping
ligands on the transition metal ions that block solvent access to
such centers.