Aqueous Lithium-Iodine Battery

Abstract

Higher energy density and cycling efficiency are required for the next generation of batteries. Iodine is an attractive material towards the high energy density battery thanks to its high theoretical energy density of ~0.74 kWh kg–1 and 3.7 kWh L–1. However, due to the nature of low ionic conductivity of solid lithium iodide, the primary lithium-iodine (Li–I2) batteries can only support for extremely low current rate applications such as a pacemaker. To improve rechargeability and rate capability, we have proposed aqueous cathode with a triiodide/iodide (I3 –/I–) redox couple to construct a low-cost, non-flammable, and environmentally friendly Li–I2 battery.[1-4] The aqueous cathode can contain high concentration of I3 –/I– redox couple using I2 and I– ion, which are transformed to I3 – in an aqueous medium (I2(s) + I– ↔ I3 –, K (equilibrium constant) = 723). The electrochemical reaction of this I3 –/I– redox couple in the aqueous cathode leads the total cell reaction of Li–I2 battery: 2Li + I3 – ↔ 2Li+ + 3I–. This aqueous Li–I2 battery demonstrates high storage capacity (~98% of the theoretical capacity), Coulombic efficiency (>99.5%) and cycling performance (>99.5% capacity retention for 100 cycles), which are thus far one of the highest performance among current Li–I2 batteries, aqueous cathode batteries, and lithium-ion batteries using aqueous electrolyte. In addition, high solubility of the I3 –/I– redox couple and appropriate operating potential (~3.5 V vs. Li+/Li and ~0.5 V vs. SHE) but avoiding the electrolysis of water offer a high energy density of ~0.33 kWh kg–1. This energy density can be further improved when equipped with a flow device and aqueous electrolyte reservoir, which allows the aqueous Li–I2 battery to be grid-scale flow battery applications.