Voltage-Sensitive Polytriphenylamine-Modified Separator for Over-Charge Protection in Li-S Batteries

In the past decade, lithium-sulfur batteries have attracted increasing attention owing to their high energy density and are considered to be one of the key options for the next generation of commercial high energy density batteries. However, for a practical battery system, both high energy density and good safety are important. The safety shortcomings of lithium-sulfur batteries have hindered their development and commercial application. Overcharging is a common battery safety problem. In the case of lithium-sulfur batteries, overcharging triggers the rapid growth of lithium dendrites, which can break through the separator and cause internal short-circuiting, leading to dangerous accidents such as thermal runaway and explosions. In practice, an electronic control device is typically installed in a battery to monitor its charging voltage and avoid overcharging avoid overcharging. However, this method increases the cost, weight, and size of the battery system, and reduces the energy density. For the self-protection of lithium-sulfur batteries in the case of overcharging, many polymerizable aromatic compounds are used as additives to improve the overcharge tolerance of lithium batteries. When the electrode surface is covered by a polymer film formed by employing electropolymerization, the cell dies permanently; thus the overcharge protection works only once. In contrast, electroactive polymers having reversible electrochemical doping/dedoping properties can be used to inhibit the overcharging of lithium-sulfur batteries is a more attractive approach. In this study, voltage-sensitive polytriphenylamine (PTPAn) was prepared by the chemical oxidation of triphenylamine as a raw material and successfully applied to lithium-sulfur battery separator. The conductivity test results showed that the PTPAn/polypropylene (PP) separator has an ionic conductivity of 1.56 mS·cm⁻¹ . The cyclic voltammogram (CV) test results showed that the PTPAn/PP separator has a redox peak in the range of 3.5–4.2 V. At a charge/discharge rate of 0.1C, the lithium-sulfur batteries with the PTPAn/PP separator and blank PP separator had a discharge specific capacity of 424.8 and 407.2 mAh·g⁻¹, respectively after 200 cycles, with Coulombic efficiencies of 99.38% and 98.59%, respectively. Further, the rate (0.1C, 0.2C, 0.5C, 1C) tests showed that the lithium-sulfur batteries with PTPAn/PP separator had higher discharge specific capacities at different rates than the lithium-sulfur batteries with the blank PP separator. Moreover, when the lithium-sulfur battery with the PTPAn/PP separator was overcharged at the 4th cycle, the charge specific capacity was 843.1 mAh·g⁻¹ and the discharge specific capacity was 839.8 mAh·g⁻¹ . The charging specific capacity was 690.2 mAh·g⁻¹ and the discharging specific capacity was 669.2 mAh·g⁻¹ at the 10th cycle of overcharging. At the 16th cycle of overcharging, the battery had a charge specific capacity of 538.7 mAh·g⁻¹ and a discharge specific capacity of 512.9 mAh·g⁻¹ . The overcharge test showed that lithium-sulfur batteries with the PTPAn/PP separator continued to work well after different overcharge rates. At an overcharging rate of 1C, the battery voltage remained stable at 3.9 V, with a charge specific capacity of 349.8 mAh·g⁻¹ and a discharge specific capacity of 328.7 mAh·g⁻¹ .