Claudia Buhrmester

High-Rate Overcharge Protection of LiFePO4-Based Li-Ion Cells Using the Redox Shuttle Additive 2,5-Ditertbutyl-1,4-dimethoxybenzene

LiFePO 4 /Li 4 / 3 Ti 5 / 3 O 4 Li-ion cells have been investigated by many groups and their behavior in standard electrolytes such as 1 M LiPF 6 ethylene carbonate: diethyl carbonate (EC:DEC) is well known. Here we report on the behavior of these cells with 2,5-ditertbutyl-l,4-dimethoxybenzene added to the electrolyte as a redox shuttle additive to prevent overcharge and overdischarge. We explore methods to increase the current-carrying capacity of the shuttle and explore the heating of practical cells during extended overcharge. The solubility of 2,5-ditertbutyl-l,4-dimethoxybenzene was determined as a function of salt concentration in lithium bis-oxolatoborate-(LiBOB) and LiPF 6 -containing electrolytes based on propylene carbonate (PC), EC, DEC, and dimethyl carbonate (DMC) solvents. Concentrations of 2,5-ditertbutyl-l,4-dimethoxybenzene up to 0.4 M can be obtained in 0.5 M LiBOB PC:DEC (1:2 by volume). Coin-type test cells were tested for extended overcharge protection using an electrolyte containing 0.2 M 2,5-ditertbutyl-1,4-dimethoxybenzene in 0.5 M LiBOB PC:DEC. Sustained overcharge protection at a current density of 2.3 mA/cm 2 was possible and hundreds of 100% shuttle-protected overcharge cycles were achieved at current densities of about 1 mA/cm 2 . The diffusion coefficient of the shuttle molecule in this electrolyte was determined to be 1.6 X 10 - 6 cm 2 /s from cyclic voltammetry and also from measurements of the shuttle potential vs. current density. The power produced during overcharge was measured using isothermal microcalorimetry and found to be IV as expected, where I is the charging current and V is the cell terminal voltage during shuttle-protected overcharge. Calculations of the temperature of 18650-sized Li-ion cells as a function of time during extended shuttle-protected overcharge at various C-rates are presented. These show that Li-ion cells need external cooling during extended shuttle-protected overcharge if currents exceed about C/5 rates.

Chemical Overcharge and Overdischarge Protection for Lithium-Ion Batteries

Rechargeable lithium-ion batteries suitable for the mass consumer market require robust safety and tolerance to repeated overdischarge and overcharge to avoid costly charge control circuitry and to allow simple replacement of individual cells by consumers. A chemical redox shuttle additive to the electrolyte is shown to provide this protection. The molecule, 2,5 ditertbutyl-1,4-dimethoxybenzene, provides overcharge and overdischarge protection for hundreds of charge-discharge cycles in both single cells and series-connected batteries.

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.

Calculations of Oxidation Potentials of Redox Shuttle Additives for Li-Ion Cells

The oxidation potentials of seventeen molecules used as candidate shuttle additives in Li-ion cells were calculated using density functional theory and compared with experiment. The root-mean-square deviation between the calculated and measured oxidation potentials of these seventeen molecules is 0.15 V with the maximum deviation being 0.25 V, indicating that the ab initio calculation is in good agreement with the experiment. Neglecting thermal contributions in the calculation of standard oxidation potentials at ambient conditions does not lead to significant errors. An empirical relation between the oxidation potentials and the orbital energies of these molecules in solution is presented. The oxidation potential of a molecule could be estimated based on the orbital energy of the molecules highest occupied molecular orbital or its cations lowest unoccupied molecular orbital in solution with an error less than 0.3 V for most molecules reported.

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.

Triphenylamines as a Class of Redox Shuttle Molecules for the Overcharge Protection of Lithium-Ion Cells

The number of molecules that have been proposed as shuttle molecules has been steadily growing as research in the area continues. However, the number of molecules that provide lengthy overcharge protection remains small. A class of molecules that provides better than average overcharge protection in lithium-ion coin cells is the triphenylamine class. With the charge spread throughout the three aromatic rings and the central nitrogen, these molecules are stable and have a tuneable oxidation potential, as shown when electron-withdrawing bromines are added to the rings. The oxidation potential of triphenylamine is raised by approximately 0.11 V per bromine added. Also, the addition of bromine can increase the electrochemical stability of triphenylamine, as seen by an increased number of overcharge cycles in coin cells containing tris(4-bromophenyl)amine instead of triphenylamine as the redox shuttle. This stability comes from the bromine preventing the formation of the dimer of triphenylamine, tetraphenylbenzidine.

Spectroelectrochemical Studies of Redox Shuttle Overcharge Additive for LiFePO4-Based Li-Ion Batteries

2,5-Di-tert-butyl-l,4-dimethoxybenzene has recently been shown to behave as an electrochemical shuttle for overcharge and overdischarge protection of LiFePO 4 /graphite and LiFePO 4 /Li 4 / 3 Ti 5 / 3 O 4 lithium-ion batteries. The redox chemistry of this molecule is investigated here using a spectroelectrochemical method inside modified UV-vis cells as well as voltammetry inside a custom-made cell using an Ag/AgCl reference electrode. The redox potential of 2,5-di-tert-butyl-l,4-dimethoxybenzene is about 3.92 V vs. Li/Li + . When clear solutions of 2,5-di-tert-butyl-1,4-dimethoxybenzene are oxidized, the radical cation is yellow in low concentrations (about 0.001 M) and yellow-green in solutions of higher concentration, making UV-vis studies of the stability of the molecule straightforward. In this paper, we show that reversible oxidation and reduction of this molecule is possible, provided it is not exposed to potentials above 4.2 V vs. Li/Li + . If the potential is raised above 4.2 V, further oxidation followed by complete decomposition to another species occurs.