Hydrogen bond-induced configuration and electron transfer on layered double hydroxide for high-performance sodium-iodine batteries

Sodium-iodine (Na-I2) batteries are appealing electrochemical energy storage devices owing to the low cost and high energy density. However, they are constrained by shuttle effect due to the high solubility of iodine/iodide in electrolyte and the sluggish reaction kinetics of I2 ↔ 2I−. Accordingly, Na-I2 batteries with superficial redox mechanism and superior rate capability almost seem impossible. Herein, a layered double hydroxide (LDH) formulated as Ni2Al(OH)4.2(CO3)1.4·2H2O (NiAl-LDH) and an iodine-decorated LDH (NiAl-LDH-I2) were synthesized. When used as the Na-I2 battery cathode in the NaClO4 electrolyte, NiAl-LDH-I2 exhibits an increasing capacity, accompanied with a transformation from the (de)intercalation to the surface-controlled (pseudo)capacitive behavior. This is ascribed to irreversible reduction reaction of ClO4− → Cl− and the formation of I+-Cl− on the LDH surface over an overcharge process. Density functional theory (DFT) calculations reveal that the adsorbed I2 and I-Cl both exhibit zigzag chain-like configurations, which are not easily intercalated into the interlayer space of NiAl-LDH, but can be anchored on the NiAl-LDH surface via multiple hydrogen bonds. These hydrogen bonds can stabilize I+ to boost the multi-electron redox reaction of 2I− ↔ I2 ↔ 2I+. The adsorbed I2 is activated by the electron transfer and the elongation of I-I bond, then fasten the reaction kinetics of I2 ↔ 2I−. Therefore, NiAl-LDH-I2 shows outstanding rate capability and cycling performance with a large capacity of 310/210 mAh/g at 0.3/ 1.5 A/g and excellent capacity retention over 2700 cycles.