Richard L. Wang

Studies of Aromatic Redox Shuttle Additives for LiFePO4-Based Li-Ion Cells

Fifty eight aromatic organic molecules were screened as chemical shuttles to provide overcharge protection for LiFePO 4 /graphite and LiFePO 4 /Li 4 / 3 Ti 5 / 3 O 4 Li-ion cells. The majority of the molecules were based on methoxybenzene and on dimethoxybenzene with a variety of ligands added to explore their effect. The added ligands affect the redox potential of the molecules through their electron-withdrawing effect and affect the stability of the radical cation. Of all the molecules tested, only 2,5-di-tert-butyl-1,4-dimethoxybenzene shows an appropriate redox potential of 3.9 V vs Li/Li + and long-term stability during extended abusive overcharge totaling over 300 cycles of 100% overcharge per cycle. The reasons for the success of this molecule are explored.

Phenothiazine Molecules Possible Redox Shuttle Additives for Chemical Overcharge and Overdischarge Protection for Lithium-Ion Batteries

The molecules 10-methylphenothiazine, 10-ethylphenothiazine. 3-chloro-10-methylphenothiazine, 10-isopropylphenothiazine, and 10-acetylphenothiazine are shown to be stable redox shuttle additives in LiFePO 4 /graphite and LiFePO 4 /Li 4 / 3 Ti 5 / 3 O 4 Li-ion coin cells to protect against overcharge and overdischarge. The diffusion constant of 10-methylphenothiazine was measured using cyclic voltammetry to be 1.5 X 10 - 6 cm 2 /s, which translates to maximum shuttle-protected overcharge current densities near 2 mA/cm 2 in practical cells. Although the redox potentials of these molecules (near 3.5 V) are somewhat low for LiFePO 4 , their stability over repeated overcharge and overdischarge cycles is about equal to that of 2,5-di-tert-butyl-1,4-dimethoxybenzene that has been shown to provide protection for over 200 cycles of 100% overcharge at C/10. Given the stability of these oxidized molecules, we believe that phenothiazine represents an attractive core for ligand substitution to adjust the redox potential to more practical values.

The Use of 2,2,6,6-Tetramethylpiperinyl-Oxides and Derivatives for Redox Shuttle Additives in Li-Ion Cells

The stable radical, 2,2,6,6-tetramethylpiperinyl-oxide (TEMPO), is shown to be a stable redox shuttle in Li 4 / 3 Ti 5 / 3 O 4 /LiFePO 4 Li-ion coin cells providing over 120 cycles of shuttle-protected overcharge. Derivatives of TEMPO, such as 4-methoxy-TEMPO and 4-cyano-TEMPO are also stable. Relatives of TEMPO, having a five-membered ring, such as 3-cyano-2,2,5,5-tetramethyl-l-pyrrolidinyloxy (3-cyano-PROXYL) show similar stability. One disadvantage of these molecules is their relatively low oxidation potentials, which are too close to that of LiFePO 4 for commercial applications. Ab initio calculations show that the redox potential of these molecules can be tailored by substitutions of fluorine for the hydrogen atoms in the methyl groups.

A Combined Computational/Experimental Study on Tertbutyl- and Methoxy-Substituted Benzene Derivatives as Redox Shuttles for Lithium-Ion Cells

Calculations were performed on a group of 43 t-butyl- and methoxy-substituted benzenes to assess their suitability as redox shuttle additives for lithium-ion cells. One molecule of this type has been previously shown to be excellent for overcharge protection of mesocarbon microbead (MCMB)/LiFePO 4 and Li 4/3 Ti 5/3 O 4 /LiFePO 4 lithium-ion cells. These calculations showed that 2,5-di-t-butyl-1,4-dimethoxybenzene was the best molecule in this group, as far as oxidation potential and stability are concerned. Other molecules that showed promise were 2-t-butyl-l,4-dimethoxybenzene, 4-t-butyl-1,2-dimethoxybenzene, and 3,5-di-t-butyl-l,2-dimethoxybenzene. These molecules were synthesized and tested in coin-type cells to confirm the results expected based on the calculations. The results of the calculations show that the substitution pattern on the molecule can be used to predict the most reactive sites on the oxidized molecule. For the dimethoxy-substituted molecules, the calculations predict that the ortho and para substitution pattern results in more stable oxidized molecules than the meta pattern.