The safety problems of lithium ion batteries (LIBs) have been the main obstacles that hinder their broad applications in portable electronic devices, electric vehicles, and energy storage. Such problems originate from flammable solvent-containing liquid electrolytes that could be easily oxidized upon excessive heat, leading to further heat accumulation and, subsequently, thermal runaway. The design strategies of a safe electrolyte could control the flammability and volatility of the liquid electrolyte, might prevent the thermal runaway, and ultimately ensure the risk-free and fire-free operation of LIBs. This work is to explore the mechanism of thermal runaway and review the state-of-the-art of the designs of a safe electrolyte for LIBs, including the additions of flame retardant additives, overcharge additives, and stable lithium salts and the adoption of solid-state electrolytes, ionic liquid electrolytes, and thermosensitive electrolytes. The features, advantages, and drawbacks of these strategies are systematically summarized, compared, and discussed, while the development direction of a safer electrolyte for future LIBs is proposed in the end.

Lithium−sulfur batteries are considered as the most promising candidate for nextgeneration energy storage devices. However, they are subjected to the "shuttle effect" of soluble lithium polysulfides (LiPSs). Herein, a free-standing membrane composed of two-dimensional MXene material (Ti 3 C 2 T x ) and graphene oxide (GO) is synthesized by a simple vacuum-filtration method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy are carried out to determine structure, morphology, and composition of the Ti 3 C 2 T x /GO composite membrane, respectively. As a functional layer of trapping LiPS species, the Ti 3 C 2 T x /GO composite membrane and commercial polypropylene (PP) are successfully assembled to be a hybrid separator, Ti 3 C 2 T x /GO@ PP, to suppress the shuttle effect of LiPSs. The porous and rough surface of the Ti 3 C 2 T x /GO composite membrane is beneficial to improve the wettability of the commercial separator in an etherbased electrolyte. The cells with the Ti 3 C 2 T x /GO@PP hybrid separator exhibit a low polarization potential of 0.26 V in the conversion from Li 2 S 4 to Li 2 S 2 /Li 2 S and deliver a discharge capacity of 640.0 mA h g −1 for 5 C rate, indicating that the hybrid separator benefits the rate performance. According to the results of electrochemical impedance spectroscopy, increased discharge capacity is attributed to the reduced internal resistance and intensified Li + diffusion. The results of X-ray photoelectron spectroscopy focusing on the surfaces of both sides of the hybrid separator indicate that the shuttle effect of LiPSs is suppressed through a coefficient of the terminated groups' catalytic conversion on long-chain LiPSs and the titanium-reactive centers' Lewis acid−base pairs on short-chain LiPSs. Combining with digital photographs of the H-type electrolytic cell, the results of UV−visible absorption spectroscopy suggest that the concentration of long-chain polysulfides declines instantly under the redox effect of the terminated groups on Ti 3 C 2 T x surfaces and then infiltrate through the hybrid separator by virtue of concentration difference impetus. Generally, a Ti 3 C 2 T x /GO@PP hybrid separator restrains LiPS diffusion and improves the rate performance of Li−S batteries.

A pomegranate-like cathode, VN/S@G, is synthesized according to a simple principle of electrostatic attraction through a controllable Zeta potential method, establishing a hierarchicalstructured VN/S nanoclusters encapsuled with graphene nanosheets. Internal VN nanoparticles trap lithium polysulphides (LiPSs) and catalyse them transforming from long-chain to short-chain species; whereas the external cladding layers of graphene nanosheets confine the transformations in a nanoscale-catalysis reactors. VN/S@G cathode exhibits excellent long-cycling life at 2 C rate during the 2000 cycles, corresponding to 0.038 percent of capacity fade per cycle. According to insitu Raman and electrochemical impedance spectroscopies, VN catalyst accelerates chemical transformations of liquid-state LiPSs to solid-state Li 2 S 2 /Li 2 S and graphene intensifies Li + diffusion behaviour. Improvement of electrochemical performance of the VN/S@G cathode depends on a coefficient of physical and chemical interactions between VN catalyst and LiPSs species.[a] Dr.

As a unique branch of Li−S batteries, solid-phase sulfur conversion polymer cathodes have shown superior stability with fast ion-transfer kinetics and high discharge capacities owing to the mere existence of short-chain sulfur species during charging/ discharging. However, representative compounds such as sulfurized polyacrylonitrile (SPAN) and polyaniline (SPANI) suffer from low sulfur contents and poor cycling performances under large current densities due to the sulfurization occurring only on polymers' surface. Here, a graphdiyne-like porous organic framework, denoted as GPOF, is synthesized and used as a host for enabling solid-phase sulfur conversion. Plenty of unsaturated bonds in GPOF provide sufficient reaction sites to bind sulfur chains, resulting in a high active sulfur content in the cathode. Moreover, the microporous GPOF possesses suitable cavities to accommodate the volume expansion, leading to favorable long-term cycling stability. As a result, the sulfurized GPOF cathode (SGPOF-320) displays outstanding electrochemical stability with negligible capacity decline after 250 cycles at 0.2 C with an average discharge capacity of 925 mA h g −1 . Our work applies a facile procedure to produce sulfur conversion porous polymer cathodes, which could provide a proper way for exploring more suitable cathode materials for high-performance Li−S batteries.

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LithiumÀ sulfur (LiÀ S) batteries with ultrahigh theoretical specific capacity (1675 mAh g À 1) have become a research hotpot, which focuses on the solution of shuttle effect of soluble lithium polysulfides (LIPS). Chemical immobilization of LIPS and encapsulation structure of the sulfur cathode are effective strategies to suppress the shuttle effect. Herein, high entropy metal nitride (HEMN) was prepared by mechanochemicalassisted synthesis as an innovative anchor to restrain LIPS via the chemical bonding interaction between HEMN and LIPS. Furthermore, the composite of HEMN and S was encased by graphene (GR) through electrostatic attraction caused by opposite zeta potential, thus as the effective cathode (HEMN/ S@GR) of LiÀ S batteries. HEMN has plentiful active metal sites which can chemically adsorb LIPS, enhancing the cycle life of LiÀ S batteries. Graphene can accelerate surface charge transfer of sulfur cathode to reduce the interface resistance because of its high conductivity. The initial specific capacity of HEMN/S@GR cathode is 1193 mAh g À 1 and still maintains 695 mAh g À 1 after 100 cycles at 0.1 C. When increased to 1.0 C, the initial specific capacity is 556 mAh g À 1 and exhibits remarkable stability in long cycles. This study presents the synergetic effect of HEMN and graphene on LIPS in LiÀ S batteries, which introduces a new application way of high entropy materials in the field of energy storage devices.

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