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

Abstract

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:DEC͒ is well known. Here we report on the behavior of these cells with 2,5-ditertbutyl-1,4-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-1,4-dimethoxybenzene was determined as a function of salt concentration in lithium bis-oxolatoborate-͑LiBOB͒ and LiPF 6 -containing electrolytes based on propylene carbonate ͑PC͒, EC, DEC, and dimethyl carbonate ͑DMC͒ solvents. Concentrations of 2,5-ditertbutyl-1,4-dimethoxybenzene up to 0.4 M can be obtained in 0.5 M LiBOB PC:DEC ͑1:2 by volume͒. 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 ϫ 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.

New class of nonaqueous electrolytes for long-life and safe lithium-ion batteries

“…A large amount of heat is generated when the redox shuttle is activated to provide overcharge protection for lithium-ion batteries 20 . However, the potential impact of the heat generation on the cell has not been well studied.…”

Redox Shuttle Additives for Lithium-Ion Battery

“…DDB is electrochemically reversible at 3.9 V vs. Li/Li + and can provide over 300 cycles of 100% overcharge per cycle for lithium-ion cells with LiFePO 4 as the cathode. The DDB was evaluated to confirm its superior performance in terms of high-rate overcharge protection 19 and overdischarge protection, 17 and a theoretical and mechanistic analysis for its high stability was also conducted. 18,20,21 Dahn's group later discovered another two stable redox shuttle systems, which are phenothiazine derivatives (12 in Figure 2) and 2,2,6,6-tetramethylpiperinyloxide (TEMPO) (11 in Figure 2) derivatives.…”

“…Finally, some other factors, such as a good solubility 19,30 and high diffusion coefficient of the redox shuttles in non-aqueous electrolytes, are also highly desired to maximize the shuttle molecule's mobility through the cell, therefore delivering high-current overcharge protection.…”

“…However, DDB is not quite compatible with the lithium-ion cell system; specifically, it is not soluble with the conventional carbonate-based electrolytes. 48 30 have investigated an asymmetric structure design (ANL-1 in Figure 10) to create intramolecular dipole moments that facilitate the dissolution of the redox shuttle in the electrolyte. However, while successful in improving solubility, those asymmetric redox shuttles appear to be less electrochemically stable because the electronic structure of the redox center is disturbed and certain chemical bonds are weakened within the structure, which, as a result, does not provide long enough overcharge protection.…”

Vehicle Battery Safety Roadmap Guidance

“…153,159 The limitations on the use of shuttle additives often are due to limited charge rate, stability at high potentials, reduction of cell performance, and heat generation during shuttle activation. 156 One promising shuttle is based on electrolyte solutions containing the StabiLife electrolyte salt Li 2 B 12 F 12 .…”