Bimetallic active sites with tailored d-bands boost multistep sulfur redox reactions for enhanced lithium-sulfur battery performance

The shuttle effect and sluggish redox kinetics of lithium polysulfides (LiPSs) are generally regarded as main obstacles to practical applications of the promising high energy density lithium-sulfur batteries (LSBs). Using electrocatalysts to expedite LiPSs conversion during charge/discharge has recently become an effective approach to improving LSB performance, but developing highly efficient catalysts active for bidirectional LiPSs conversion both involving multiple electron transfer steps remains a great challenge. Herein, we report the synthesis of a new composite electrocatalyst consisting of bimetallic nickel/cobalt metal–organic frameworks (Ni/Co-MOF) anchored on highly dispersed amino-modified MXene (Ti 3 C 2 -NH 2 , i.e., TCN) supports through a simple one-step hydrothermal method. Density functional theory (DFT) calculations demonstrate that incorporating both Ni and Co species into MOF results in significant electronic coupling effect, making the d-band centers of Ni and Co sites in Ni/Co-MOF@TCN become comparable and the composite catalyst shows similar adsorption and dissociation capability toward LiPSs. Such a synergy between Ni and Co sites has been validated by comprehensive experimental characterization, which confirms that Ni/Co-MOF@TCN has moderate binding strength for LiPSs adsorption/dissociation, so that both Ni and Co can effectively boost the kinetics and thermodynamics of the bidirectional conversion of LiPSs. Consequently, LSBs containing Ni/Co-MOF@TCN as sulfur hosts exhibit a high specific capacity of 1465 mAh g −1 at 0.1 C and excellent stability at 1 C for 600 cycles with a low decay rate of 0.052 % per cycle. Even with a high sulfur loading of 6.8 mg cm −2 and a low electrolyte/sulfur (E/S) ratio of only 5 μL mg −1 , the Ni/Co-MOF@TCN-containing LSBs can still deliver an initial areal capacity of 6.81 mAh cm −2 at 0.1 C. Moreover, Ni/Co-MOF@TCN also shows outstanding performance in single-layer Li-S pouch cells.