Role of PF 6 À in the radiolytical and electrochemical degradation of propylene carbonate solutions
Résumé
h i g h l i g h t s g r a p h i c a l a b s t r a c t The presence of LiPF 6 in PC significantly affects the decomposition pathways. CO 2 production is doubled in irradiated PC/LiPF 6 1 M as compared to irradiated PC. This effect is specific of LiPF 6 and is not observed in other salts such as LiClO 4. A high reaction rate constant between the electron and PF 6 À in PC is measured. Radiolysis accelerates aging and enables the description of reaction mechanisms. a b s t r a c t The behavior under irradiation of neat propylene carbonate (PC), a co-solvent usually used in Li-ion batteries (LIB), and also of Li salt solutions is investigated. The decomposition of neat PC is studied using radiolysis in the pulse and steady state regime and is assigned to the ultrafast formation, in the reducing channel, of the radical anion PC À by electron attachment, followed by the ring cleavage, leading to CO. In the oxidative channel, the PC(eH) radical is formed, generating CO 2. The CO 2 and CO yields are both close to the ionization yield of PC. The CO 2 and CO productions in LiClO 4 , LiBF 4 and LiN(CF 3) 2 (SO 2) 2 solutions are similar as in neat PC. In contrast, in LiPF 6 /PC a strong impact on PC degradation is measured with a doubling of the CO 2 yield due to the high reactivity of the electron towards PF 6 À observed in the picosecond range. A small number of oxide phosphine molecules are detected among the various products of the irradiated solutions, suggesting that most of them, observed in carbonate mixtures used in LIBs, arise from linear rather than from cyclical molecules. The similarity between the degradation by radiolysis or electrolysis highlights the interest of radiolysis as an accelerated aging method.