The solid electrolyte interphase (SEI) on the negative electrode is known to play a key role on cell safety. Namely, if the SEI layer is damaged at high temperatures, electrode/electrolyte decomposition is induced along with cell heating, which can trigger thermal runaway.
Lithium hexafluorophosphate (LiPF6) is known be thermally decomposed (LiPF6 → LiF + PF5) over 70oC. Here, the as-generated phosphorus pentafluoride (PF5) is known to react with organic solvents such as ethylene carbonate (EC) at high temperatures.1 It is thus very likely that PF5 can attack the organic and/or inorganic components of SEI layer to deteriorate the passivating ability of SEI layer.
In this work, to confirm the SEI damage by PF5 attack, the open-circuit voltage (OCV) was monitored while the Li/SiO cells, wherein the concentration of PF5 precursor (LiPF6) was varied, were stored at 85oC. A more serious SEI damage is expected with a larger LiPF6 loading.
As a PF5 scavenger, tris(2,2,2-trifluoroethyl) phosphite (TTFP) was added into the electrolyte solution. The former (PF5) is a strong Lewis acid, while the latter is Lewis base, such that a strong complex formation is expected from the two.2
Experimental
Two different experiments were performed.
(1) The 2032-type coin cells with SiO working electrode, Li counter electrode and electrolyte (1.3 M LiPF6 EC:DEC (3:7, v/v)) were assembled, and cycled 5 times to obtain a stable SEI layer on the SiO electrode. The cells were then dismantled and re-assembled by adding different electrolyte (0.0 M, 0.7 M and 1.3 M LiPF6 in EC:DEC (3:7, (v/v)). The re-assembled cells were stored at 85oC for 1 day, while the OCV was monitored.
(2) The 2032-type coin cells with SiO working electrode, Li counter electrode and two different electrolyte solution (1.3 M LiPF6 in EC:DEC (3:7, v/v) and 1.3 M LiPF6 in EC:DEC (3:7, v/v) + 5% TTFP)) were assembled, and cycled 5 times to obtain a stable SEI layer on the SiO electrode. The same storage experiment was performed.
The passivation ability of SEI layer was assessed by monitoring the OCV values and the chemical composition of SEI layer was analyzed by using XPS.
Results and Discussion
Fig. 1 (a) shows the OCV profiles obtained from the Li/SiO cells during the 85oC storage. As is seen, the OCV values increase at a faster rate with an increase in LiPF6 concentration. The OCV increase signifies the oxidation (self-discharge) of SiO electrode, which in turn indicates the damage and loss of passivating ability of SEI layer.3 The SEI damage is resulted from PF5 attack, which is generated by thermal decomposition of LiPF6.
Fig. 1 (b) compares the OCV change for two Li/SiO cells. The cell containing TTFP shows a much slower OCV increase, illustrating that the SEI layer is rather intact even upon 85oC exposure. The SEI damage by PF5 attack was marginal due to the scavenging action of TTFP for PF5.
Reference
1. S. E. Sloop, J. B. Kerr, and K. Kinoshita, J. Power Sources, 119-121, 330 (2003).
2. S. S. Zhang, K. Xu, and T. R. Jow, Electrochemical and Solid-state Letters, 5, A206 (2002).
3. H. Park, T. Yoon, J. Mun, J. H. Ryu, J. J. Kim, and S. M. Oh, J. Electrochem. Soc., 160, A1539 (2013).
Figure 1. The OCV profiles obtained from the Li/SiO cells that were stored at 85oC. (a) The concentration of LiPF6 was varied in the electrolyte solutions, (b) TTFP was added into the electrolyte solution in one cell.