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

Wei Wang,Jianguo Si,Liguo Yue,Weicai Zhang,Qingqing Chen,Haoliang Huang,Fei Lin,Yang Zhao,Sheng Meng,Lifeng Liu

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.