Poor cycling stability and safety concerns regarding lithium (Li) metal anodes are two major issues preventing the commercialization of high-energy density Li metal-based batteries. Herein, a novel tri-layer separator design that significantly enhances the cycling stability and safety of Li metal-based batteries is presented. A thin, thermally stable, flexible, and hydrophilic cellulose nanofiber layer, produced using a straightforward paper-making process, is directly laminated on each side of a plasma-treated polyethylene (PE) separator. The 2.5 µm thick, mesoporous (≈20 nm average pore size) cellulose nanofiber layer stabilizes the Li metal anodes by generating a uniform Li flux toward the electrode through its homogenous nanochannels, leading to improved cycling stability. As the tri-layer separator maintains its dimensional stability even at 200 °C when the internal PE layer is melted and blocks the ion transport through the separator, the separator also provides an effective thermal shutdown function. The present nanocellulose-based tri-layer separator design thus significantly facilitates the realization of high-energy density Li metal-based batteries.

The application of lithium−sulfur (Li−S) batteries is severely hampered by the shuttle effect and sluggish redox kinetics. Herein, amorphous cobalt phosphide grown on a reduced graphene oxide-multiwalled carbon nanotube (rGO-CNT-CoP(A)) is designed as the sulfur host to conquer the above bottlenecks. The differences between amorphous cobalt phosphide (CoP) and crystalline CoP on the surface adsorption as well as conversion of lithium polysulfides (LiPSs) are investigated by systematical experiments and densityfunctional theory (DFT) calculations. Specifically, the amorphous CoP not only strengthens the chemical adsorption to LiPSs but also greatly accelerates liquidphase conversions of LiPSs as well as the nucleation and growth of Li 2 S. DFT calculation reveals that the amorphous CoP possesses higher binding energies and lower diffusion energy barriers for LiPSs. In addition, the amorphous CoP features reduced energy gap and the increased electronic concentrations of adsorbed LiPSs near Fermi level. These characteristics contribute to the enhanced chemisorption ability and the accelerated redox kinetics. Simultaneously, the prepared S/rGO-CNT-CoP(A) electrode delivers an impressive initial capacity of 872 mAh g −1 at 2 C and 617 mAh g −1 can be obtained after 200 cycles, exhibiting excellent cycling stability. Especially, it achieves outstanding electrochemical performance even under high sulfur loading (5.3 mg cm −2 ) and lean electrolyte (E/S = 7 μL E mg −1 S ) conditions. This work exploits the application potential for amorphous materials and contributes to the development of highly efficient Li−S batteries.

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However, the development of Li-S batteries is still facing many challenges, mainly including the insulating properties of S/Li 2 S 2 /Li 2 S, the huge volume changes during cycles as well as the dissolution and shuttle of lithium polysulfides (LiPSs) in the electrolyte. Additionally, the complex multi-electron and multi-phase reactions of sulfur species cause sluggish redox kinetics, which severely limits the battery performance. [2] Furthermore, for the purpose of taking full advantages of high-energy-density of Li-S batteries in practical applications, high sulfur loadings and low amount of electrolyte are necessary. It further increases the electrochemical polarization and aggravates the loss of LiPSs, leading to the inferior cycling stability and low specific capacity. [3] Nowadays, many researches concentrate on the design and optimization of sulfur host to solve the above problems. [4] Carbon materials and metal compounds are generally combined as the sulfur host to construct channels for transferring electrons and ions, adsorb LiPSs and catalyze the conversions of LiPSs. [5] Among the metal compounds, transition metal phosphides (TMPs) have drawn increasing interests because of their excellent electronic conductivity, easily adjustable electronic structure and high catalytic activity. [6] Some studies have shown that TMPs can chemically immobilize LiPSs through M-S and PLi bonds to restrain the shuttle effect and expedite redox kinetics in electrochemical conversions of LiPSs. Chen et al. found that CoP nanoparticles can effectively capture LiPSs and reduce the overpotential of Li 2 S nucleation. [7] Wang et al. demonstrated that MoP nanoparticles can inhibit the formation of "dead sulfur" under lean electrolyte conditions. [8] Qian et al. revealed the best catalytic behavior of CoP among several cobalt-based metal compounds (Co 3 O 4 , CoS 2 , Co 4 N, and CoP). [9] Although some progress has been made in the application of TMPs in Li-S batteries, it is still a challenge to further restrain the shuttle effect and improve the electrochemical kinetics through regulating its electronic structures.Since defect engineering has been shown to effectively tailor the electronic structure of metal compounds, the possibility of tuning the adsorption and electrochemical conversions of LiPSs on the surface of sulfur hosts through anion vacancy Lithium-sulfur batteries have aroused great interest in the context of rechargeable batteries, while the shuttle effect and sluggish conversion kinetics severely handicap their development. Defect engineering, which can adjust the electronic structures of electrocatalyst, and thus affect the surface adsorption and catalytic process, has been recognized as a good strategy to solve the above problems. However, research on phosphorus vacancies has been rarely reported, and how phosphorus vacancies affect battery performance remains unclear. Herein, CoP with phosphorus vacancies (CoP-Vp) is fabricated to study the enhancement mechanism of phosphorus vacancies in Li-S chemistry...