Lithium hexafluorophosphate (LiPF6) salt in carbonate-based solvents have been widely used as an electrolyte in commercial lithium-ion batteries (LIBs) for over three decades. The poor thermal stability of these electrolytes limits high temperature operation of LIBs. Studies have shown that the electrolyte decomposition at elevated temperature is initiated by the reaction: LiPF6 → LiF + PF5. However, the underlying process that triggers thermal decomposition of PF6– at elevated temperatures is still unknown and impedes our ability to design optimal electrolytes. Here, we report that the presence of urea (2 wt%) suppresses the decomposition reaction in 1 M LiPF6 dissolved in propylene carbonate (PC) at elevated temperatures. We employed nuclear magnetic resonance (NMR) techniques: spin-lattice (T1) relaxation time measurement and pulsed-field gradient (PFG) NMR in conjunction with computational simulations to elucidate the dynamic and structural properties of the 1 M LiPF6/PC electrolytes without (E1) and with (E2) urea additive and correlated them with the stability of the electrolyte at elevated temperatures. Our results showed thermally induced molecular level ion/solvent dynamics. In pristine electrolyte: i) PF6- and PC undergo a change in the mode of rotational motion from low-temperature small-angle librational motion to high-temperature overall reorientational motion; and ii) the Li+ transport mechanism changed from vehicular to a hopping mechanism at elevated temperature (~80 °C). In the presence of urea, the hopping motion of Li+ was suppressed due to preferential solvation with urea leading to capture of Li+ within a solvation cage, while minimally affecting the dynamic properties of PC and PF6-. AIMD simulation at 100 °C reveals an elongated P-F bond when it binds to Li+ which may prompt the PF6– decomposition. In the presence of urea, a shorter Li+- Ourea bond distance relative to Li+-FPF6 and Li+-OPC bond was observed, indicating a stronger interaction of urea with Li+. Based on these observations, we inferred that the first decomposition reaction is accelerated due to the presence of both free moving Li+ and thermally activated dynamics of PF6-. In this presentation, we will discuss the correlation between macroscopic thermal stability and molecular level ion/solvent dynamics of the LiPF6-based carbonate electrolytes.

Thermal decomposition of lithium hexafluorophosphate in a carbonate-based electrolyte

ACS Fall 2021

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