High-Performance All-Solid-State Li-S Batteries Through Advanced Carbon Fiber Decorated with 2D Transition Metal Sulfide

All-solid-state Lithium-sulfur batteries (ASSLSBs) are considered to concurrently deliver high energy density and safety. Considering the poor electronic conductivity of sulfur, porous carbon has been widely used as the host sulfur and serve as conductive additive to boost the sulfur mass loading and rate performance. However, the conventional porous carbon used in liquid cells exhibits low efficiency in ASSLSBs because it is challenging for the immobile solid electrolytes (SEs) to reach the sulfur confined in the pores distributed in the depth of carbon. The pore structure and distribution highly impact the electrochemical performance of ASSLSBs. Herein, for the first time, we designed a novel carbon fiber with a core-shell structure that processes a layer of micropores located only at the surface via activation by solid potassium hydroxide (KOH). The carbon fibers were fabricated through scalable electrospinning of polyacrylonitrile followed by carbonization and activation. The obtained polyacrylonitrile-derived porous carbon fibers (named PPCF) possessing an ultrahigh specific surface area of 1519 m²/g provide sufficient sites to host sulfur. Since the porous carbon layer is at the surface, the sulfur within the surface pores has excellent contact with SE; Meanwhile, the dense core provided excellent electron conduction. Therefore, this structurally designed carbon fiber boosted the utilization of sulfur due to enhanced electron and ion accessibilities, accelerated charge transfer, and dramatically promoted the reaction kinetic in the ASSLSBs.<br/>However, the employment of carbon additives can improve the electrical conductivity but accelerate the decomposition of SSEs. Herein, we designed a highly conductive carbon fiber decorated with hybrid 1T/2H MoS₂ nanosheets and applied it in ASSLSBs. The chemical and electrochemical compatibility among MoS₂, sulfur and sulfide SSE can greatly improve the stability of the cathode and therefore maintain pristine interfaces between the different compositions for stable ion and electron transport. The presence of electrical-conductive metallic 1T MoS₂ and its uniform distribution on carbon fiber without aggregation benefit the electron transfer between carbon and sulfur. As a result, our ASSLSB delivered an ultrahigh initial discharge and charge capacity of 1456 mAh g^-1 and 1470 mAh g^-1 at 0.05 C individually with ultrahigh initial coulombic efficiency and maintained high capacity retention of 78 % after 220 cycles. The extremely high initial coulombic efficiency is attributed to the elimination of shuttle effects through SSE and stable interface. The batteries also obtained a remarkable rate performance of 1069 mAh g^-1 at 1 C. This study pioneered the new idea that fabricating the high performance ASSLSBs through developing surface functionalized and stabilized conductive carbon additives in metal sulfides based ASSLSBs.