A cycle-stable sulfur cathode with a solid-phase conversion mechanism is developed by building a SEI layer on S/C particles.

No abstract

cycling performance still remains a challenge, even though great progresses have been made in the past decade. [2] Besides the poor rechargeability of Li metal anode, the low capacity utilization and inferior cycle stability of the present sulfur cathodes are also troublesome problems that hinder commercial development of Li-S batteries. [3] In general, the charge/discharge of sulfur cathodes undergo through a "dissolution-precipitation" redox reaction mechanism in the ether-based electrolytes, leading to the generation and dissolution of lithium polysulfide intermediates (LiPSs) during discharge, and the precipitation of solid oxidation products of LiPSs during charge. [4] This reaction mechanism brings about several challenges for stable cycling of sulfur cathodes. Firstly, the dissolution of LiPSs will cause the loss of active material, corrosion of Li anode, as well as shuttle effect, resulting in rapid capacity decay and low Coulombic efficiency (CE). [5] Secondly, the random and inhomogeneous precipitation of solid oxidation products of the dissolved LiPSs on the cathode surface will result in the blocking of Li-ion transport channels and the electrochemical deactivation of sulfur, leading to a low capacity utilization and even "sudden death" of sulfur cathode. [6] Moreover, the LiPSs dissolution will cause a substantial viscosity increase and significant ionic-conductivity decrease of electrolytes. [7] To ensure sufficient redox kinetics of sulfur cathode, a high electrolyte/sulfur ratio (E/S) is frequently needed, which will decrease considerably the energy density of Li-S batteries. [7b,8] To tackle these issues, various strategies have been proposed to depress the dissolution of LiPSs, such as utilizing porous carbon matrixes to absorb or immobilize the LiPSs, [3b,9] tailoring electrolytes to reduce the solubility or mobility of LiPSs, [10] designing interlayers to block the LiPSs diffusion [11] and applying catalysts to promote the LiPSs conversion. [12] However, due to the high solubility of LiPSs in ether-based electrolytes, these strategies can only alleviate but not completely inhibit the LiPSs dissolution. This means that, under high sulfur loading and lean electrolyte conditions that are essential for battery application, the soluble LiPSs in the electrolyte Solid phase conversion sulfur cathode is an effective strategy for eliminating soluble polysulfide intermediates (LiPSs) and improving cyclability of Li-S batteries. However, realizing such a sulfur cathode with high sulfur loading and high capacity utilization is very challenging due to the insulating nature of sulfur. In this work, a strategy is proposed for fabricating solid phase conversion sulfur cathode by encapsulating sulfur in the mesoporous channels of CMK-3 carbon to form a coaxially assembled sulfur/carbon (CA-S/C) composite. Vinyl carbonate (VC) is simultaneously utilized as the electrolyte cosolvent to in-situ form a dense solid electrolyte interface (SEI) on the CA-S/C composite surface through its nucleophilic reactio...

No abstract

Iron sulfides with high theoretical capacity and low cost have attracted extensive attention as anode materials for sodium ion batteries. However, the inferior electrical conductivity and devastating volume change and interface instability have largely hindered their practical electrochemical properties. Here, ultrathin amorphous TiO2 layer is constructed on the surface of a metal–organic framework derived porous Fe7S8/C electrode via a facile atomic layer deposition strategy. By virtue of the porous structure and enhanced conductivity of the Fe7S8/C, the electroactive TiO2 layer is expected to effectively improve the electrode interface stability and structure integrity of the electrode. As a result, the TiO2‐modified Fe7S8/C anode exhibits significant performance improvement for sodium‐ion batteries. The optimal TiO2‐modified Fe7S8/C electrode delivers reversible capacity of 423.3 mA h g−1 after 200 cycles with high capacity retention of 75.3% at 0.2 C. Meanwhile, the TiO2 coating is conducive to construct favorable solid electrolyte interphase, leading to much enhanced initial Coulombic efficiency from 66.9% to 72.3%. The remarkable improvement suggests that the interphase modification holds great promise for high‐performance metal sulfide‐based anode materials for sodium‐ion batteries.