Bifunctional electrolyte additive for lithium-ion batteries☆
Introduction
Overcharge is one type of abuse to lithium-ion cells by forcing a current through to charge the cell when the lithium-ion cell is already fully charged. Overcharging a lithium-ion cell can generally result in the failure of a battery pack or a fire hazard. In order to maintain the normal operation of a battery pack, overcharge protection mechanisms need to be incorporated into each cell to prevent the unexpected overcharge of one or more lithium-ion cells. A redox shuttle is one of the proposed protection mechanisms for lithium-ion batteries [1], [2], [3], [4], [5], [6], [7], [8]. The redox shuttle is an electrolyte additive that can be reversibly oxidized/reduced at a characteristic potential and provides an intrinsic overcharge protection. Once extra charge is forced through a lithium-ion cell containing the redox shuttle, the redox shuttle is oxidized on the positive electrode; the oxidized specie diffuses back to the negative electrode and is reduced to its original state. Under this mechanism, the extra charge can be shuttled through the lithium-ion cells by the redox reaction of the redox shuttle without causing any damage to the lithium-ion cells.
The redox shuttles reported in the literature include ferrocene [1] and aromatic compounds [2], [3], [4], [5]. Due to the low redox potential of ferrocene (generally less than 3.5 V vs. Li+/Li), it is not practical to incorporate it into state-of-the-art lithium-ion batteries. The aromatic compounds, whose redox potential can be as high as 4.2 V vs. Li+/Li, were first patented by Sony corporation [2], [3]. Lately, it was reported that the compounds proposed by Sony corporation were not stable at all in the lithium-ion-battery environment [4]. Alternatively, Chen et al. [4] reported a stable redox shuttle, 2,5-ditertbutyle-1,4-dimethoxybenzene, that can sustain more than 200 cycles with 100% overcharge for each cycle. However, 2,5-ditertbutyl-1,4-dimethoxybenzene has a relatively low redox potential (∼3.9 V vs. Li+/Li), and is only practical for overcharge protection of LiFePO4. A desired redox shuttle is expected to have a redox potential between 4.3 V and 4.5 V vs. Li+/Li, to be compatible with the working potential of major positive electrode materials like LiCoO2 and Li[NixMnyCo1−x−y]O2 (0 ⩽ x, y, x + y ⩽ 1). The upper limit potential of 4.5 V is desired to ensure that the overcharge protection mechanism be activated before the decomposition of the electrolyte components, such as carbonates that can be oxidized at 4.8 V vs. Li+/Li.
In this work, 2-(pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole was reported as a stable redox shuttle with a redox potential of 4.43 V vs. Li+/Li. Moreover, the novel additive is a strong Lewis acid, and can act as an anion receptor as well. The impact of an anion receptor on the capacity retention and the power capability of lithium-ion cells has been reported previously [9]. In this paper, we discuss the potential of using 2-(pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole as a bifunctional additive for overcharge protection and improving the life of lithium-ion cells.
Section snippets
Experimental
The bifunctional additive investigated is 2-(pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole (PFPTFBB), whose molecular structure is shown in Fig. 1. The synthesis route for PFPTFBB is summarized in what follows. 0.91 g 3,4,5,6-tetrafluoro-1,2-dihydroxybenzene and 1.05 g 2,3,4,5,6-pentafluorobenzenbornic acid were mixed in 10 mL anhydrous toluene. The mixture was then heated and refluxed for more than 5 h. The water generated during the condensation reaction was removed by azeotropic
Results and discussion
Fig. 2 shows cyclic voltammograms of 0.05 M PFPTFBB and 1.2 M LiPF6 in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7, by weight). Both the counter and reference electrodes were lithium foils, and the working electrode was a platinum disk (Φ = 1 mm). Fig. 2 shows that PFPTFBB has a reversible redox reaction at about 4.43 V vs. Li+/Li. The onset potential of PFPTFBB is about 4.3 V vs. Li+/Li, which is high enough to provide overcharge protection for most state-of-the-art positive electrode
Conclusion
2-(Pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole was investigated as both a redox shuttle and an anion receptor for lithium-ion batteries. As redox shuttles, both PFPTFBB and PFPTFBB–F− show a reversible redox reaction at 4.43 V vs. Li+/Li, which is high enough for overcharge protection of state-of-the-art positive electrode materials. The redox potential of 4.4 V is very suitable for repeating cell overcharging without degrading the electrolyte system. This can also ease the cell
Acknowledgement
The authors acknowledge the financial support of the US Department of Energy, Freedom CAR and Vehicle Technologies Program, under Contract No. DE-AC02-06CH11357.
References (10)
- et al.
J. Electrochem. Soc.
(1990) - M. Adachi, US Pat. No. 5,763,119...
- et al.
J. Electrochem. Soc.
(1999) - et al.
Electrochem. Solid-State Lett.
(2005) - et al.
J. Electrochem. Soc.
(2006)
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2019, Nano EnergyCitation Excerpt :And in 2013, they also reported a new phosphate ester named as resorcinol bis(diphenyl phosphate) which also showed a bifunction of flame retardant and overcharge protection [204]. For the improvement on cycle performance of overcharge protection additives, Chen et al. [205] also reported redox shuttles additive named as 2-(Pentafluorophenyl)-tetrafluoro-1,3,2-benzodioxaborole which has a boron center showing Lewis acid property to dissolve LiF to improve capacity retention. Besides, in recent years, some bifunctional additives for 5 V class battery systems have been reported, such as phenoxy pentafluorocyclotriphosphazene and its derivatives [197,206,207].
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The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a US Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The US Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.