A multifunctional separator with Mg(OH)2 nanoflake coatings for safe lithium-metal batteries

Dendrite growth and thermal runaway induce serious safety hazards,impeding the practical applications of lithium metal batteries(LMBs).Although extensive advances have been attained in terms of LMB safety,most work only focus on a single aspect at a time.This paper reports a multifunctional separator coated by Mg(OH)2 nanoflakes with various excellent properties including electrolyte wettability,ionic conductivity,Li+ transference number,puncture strength,thermal stability and flame retardance.When used in LMBs,the Mg(OH)2 nanoflake coatings enable uniform Li+ distributing,which makes it homogeneous to deposit lithium,realizing effective dendrite suppression and less volume expansion.Meanwhile,Mg(OH)2 coatings can ensure LMBs are in normal conditions without thermal runaway until 140 ℃.A part of lithium can be converted into Li+ ions by Mg(OH)2 during repeated charge/discharge cycles,not only reducing the risk of separator damage and consequent short circuit,but also replenishing the capacity loss of LMBs.The Mg(OH)2 nanoflakes can coat on all kinds of commercial separators to improve their performances,which offers a facile but effective strategy for fabricating multifunctional separators and a comprehensive insight into enhancing LMB safety.

“Three‐in‐one”strategy:Heat regulation and conversion enhancement of a multifunctional separator for safer lithium-sulfur batteries

The safety problems encountered with lithium–sulfur batteries(LSBs)hinder their development for practical applications.Herein,a highly thermally conductive separator was constructed by cross‐weaving super‐aligned carbon nanotubes(SA‐C)on super‐aligned boron nitride@carbon nanotubes(SA‐BC)to create a composite film(SA‐BC/SA‐C).This separator was used to fabricate safe LSBs with improved electrochemical performance.The highly aligned separator structure created a uniform thermal field that could rapidly dissipate heat accumulated during continuous operation due to internal resistance,which prevented the development of extremely high temperatures.The array of boron nitride nanosheets endowed the composite separator with a large number of adsorption sites,while the highly graphitized carbon nanotube skeleton accelerated the catalytic conversion of high‐valence polysulfides into low‐valence polysulfides.The arrayed molecular brush design enabled the regulation of local current density and ion flux,and considerably alleviated the growth of lithium dendrites,thus promoting the smooth deposition of Li metal.Consequently,a battery constructed with the SA‐BC/SA‐C separator showed a good discharge capacity of 685.2 mAh g−1 over 300 cycles(a capacity decay of 0.026%per cycle)at 2 C and 60°C.This“three‐in‐one”multifunctional separator design strategy constitutes a new path forward for overcoming the safety problems of LSBs.

Rational design and low-cost fabrication of multifunctional separators enabling high sulfur utilization in long-life lithium-sulfur batteries

The lithium-sulfur(Li-S)battery with an ultrahigh theoretical energy density has emerged as a promising rechargeable battery system.However,the practical applications of Li-S batteries are severely plagued by the sluggish reaction kinetics of sulfur species and notorious shuttling of soluble lithium polysulfides(LiPSs)intermediates that result in low sulfur utilization.The introduction of functional layers on separators has been considered as an effective strategy to improve the sulfur utilization in Li-S batteries by achieving effective regulation of LiPSs.Herein,a promising self-assembly strategy is proposed to achieve the low-cost fabrication of hollow and hierarchically porous Fe_(3)O_(4)nanospheres(p-Fe_(3)O_(4)-NSs)assembled by numerous extremely-small primary nanocrystals as building blocks.The rationally-designed p-Fe_(3)O_(4)-NSs are utilized as a multifunctional layer on the separator with highly efficient trapping and conversion features toward LiPSs.Results demonstrate that the nanostructured p-Fe_(3)O_(4)-NSs provide chemical adsorption toward LiPSs and kinetically promote the mutual transformation between LiPSs and Li_(2)S_(2)/Li_(2)S during cycling,thus inhibiting the LiPSs shuttling and boosting the redox reaction kinetics via a chemisorption-catalytic conversion mechanism.The enhanced wettability of the p-Fe_(3)O_(4)-NSs-based separator with the electrolyte enables fast transportation of lithium ions.Benefitting from these alluring properties,the functionalized separator with p-Fe_(3)O_(4)-NSs endows the battery with an admirable rate performance of 877 mAh g^(−1)at 2 C,an ultra-durable cycling performance of up to 2176 cycles at 1 C,and a promising areal capacity of 4.55 mAh cm^(−2)under high-sulfur-loading and lean-electrolyte conditions(4.29 mg cm^(−2),electrolyte/ratio:8μl mg^(−1)).This study will offer fresh insights on the rational design and low-cost fabrication of multifunctional separator to strengthen electrochemical reaction kinetics by regulating LiPSs conversion for developing efficient and long-life Li-S batteries.

Decorating ketjen black with ultra-small Mo_(2)C nanoparticles to enhance polysulfides chemisorption and redox kinetics for lithium-sulfur batteries

The low sulfur utilization and fast capacity fading resulting from the sluggish redox reaction and abominable polysulfides shuttle greatly hinder the practical applications of lithium-sulfur(Li-S) batteries.Herein, we develop a facile "in-situ growth" method to decorate ultra-small Mo2 C nanoparticles(USMo2 C) on the surface of Ketjen Black(KB) to functionalize the commercial polypropylene(PP) separators,which can accelerate the redox kinetics of lithium polysulfides conversion and effectively increase the utilization of sulfur for Li-S batteries. Importantly, the US-Mo2 C nanoparticles have abundant sites for chemical adsorption towards polysulfides and the conductive carbon networks of KB have cross-linked pore channels, which can promote electron transport and provide physical barrier and volume expansion space for polysulfides. Due to the combined effects of the US-Mo2 C and KB, Li-S cells employing the multifunctional PP separators modified with KB/US-Mo2 C composite(KB/US-Mo2 C@PP) exhibit a high specific capacity(1212.8 mAh g^(-1) at 0.2 C), and maintain a reversible capacity of 1053.3 m Ah g^(-1) after 100 cycles.More importantly, the KB/US-Mo2 C@PP cells with higher sulfur mass loading of 4.9 mg cm^(-2) have superb areal capacity of 2.3 mAh cm^(-2). This work offers a novel and promising perspective for high-performance Li-S batteries from both the shuttle effect and the complex polysulfides conversion.

MOFs derived ZnSe/N-doped carbon nanosheets as multifunctional interlayers for ultralong-Life lithium-sulfur batteries

The shuttle effect and slow conversion rate of lithium polysulfides(LiPSs)have become the main obstructs to the development of lithium-sulfur(Li-S)batteries.Herein,the low cost metal-organic frameworks derived nitrogen-doped carbon nanosheets embedded with zinc selenide nanoparticles(ZnSe/NC nanosheets)were designed and synthesized for Li-S batteries.As the LiPSs trapping-layer,these nanocomposites provide some key benefits:(1)The nitrogen doping changes local electron distribution in the carbon nanosheets,thus the electrical conductivity is greatly improved for facilitating the transport of electrons/ions.(2)Nitrogen atoms and ZnSe nanoparticles play an important role in anchoring the LiPSs via chemical interactions.(3)The remarkable catalytic activity of ZnSe nanoparticles can accelerate the redox kinetics of LiPSs.As a result,the Li-S battery with the ZnSe/NC nanosheets modified separator exhibits ultralong lifespan over 1500 cycles with a small capacity loss of only 0.046%per cycle at 1 C,which is superior over those reported values.Furthermore,the Li-S battery with a high sulfur loading of 4.71 mg cm^(-2) can still maintain a high areal capacity of 4.28 mAh cm^(-2) after 50 cycles.This work provides a new route to the design of multifunctional low cost and high-performance separators for remarkably stable Li-S batteries.

Formation of hierarchically 3D cactus-like architecture as efficient Mott-Schottky electrocatalyst for long-life Li-S batteries

Searching for new promising electrocatalysts with favorable architectures allowing abundant active sites and remarkable structure stability is an urgent task for the practical application of lithium-sulfur(Li-S)batteries.Herein,inspired by the structure of natural cactus,a new efficient and robust electrocatalyst with three-dimensional(3D)hierarchical cactus-like architecture constructed by functional zero-dimensional(0D),one-dimensional(1D),and two-dimensional(2D)components is developed.The cactus-inspired catalyst(denoted as Co@NCNT/NCNS)consists of N-doped carbon nanosheets(NCNS)and standing Ndoped carbon nanotubes(NCNT)forest with embedded Co nanoparticles on the top of NCNT,which was achieved by an in situ catalytic growth technique.The unique structure design integrates the advantages of 0D Co accelerating catalytic redox reactions,1D NCNT providing a fast electron pathway,and 2D NCNS assuring strong structure stability.Especially,the rich Mott-Schottky heterointerfaces between metallic Co and semiconductive NCNT can further facilitate the electron transfer,thus improving the electrocatalyst activity.Consequently,a Li-S battery with the Co@NCNT/NCNS modified separator achieves ultralong cycle life over 4000 cycles at 2 C with ultralow capacity decay of 0.016%per cycle,much superior over that of recently reported batteries.This work provides a new strategy for developing ultra-stable catalysts towards long-life Li-S batteries.

Synergistic engineering of cobalt selenide and biomass-derived S,N,P co-doped hierarchical porous carbon for modulation of stable Li-S batteries

Hierarchical porous carbon co-doped with heterogeneous atoms has attracted much attention thanks to sizable internal void space accommodating electrolyte,high-density microporous structure physically con-fining polysulfides(LPS),and heterogeneous atoms serving as active sites to capture LPS.However,solely relying on carbon material defects to capture LPS proves ineffective.Hence,metal compounds must be introduced to chemisorb LPS.Herein,cobalt ions are in-situ grown on the polydopamine layer coated on the surface of biomass-derived S,N,P co-doped hierarchical porous carbon(SNP-PC).Then a layer of nitrogen-doped porous carbon(MPC)dotted with CoSe nanoparticles is acquired by selenizing.Thus,a strong-polar/weak-polar composite material of SNP-PC studded with CoSe nanoparticles is obtained(SNP-PC@MPC@CoSe).Button cells assembled with SNP-PC@MPC@CoSe-modified separator enable superb long-cycle stability and satisfactory rate performance.An excellent rate capacity of 796 mAh g^(−1)at a high current rate of 4 C with an ultra-low capacity fading of 0.06%over 700 cycles can be acquired.More impressively,even in a harsh test condition of 5.65 mg cm^(−2)sulfur loading and 4μL mg^(−1)ratio of electrolyte to active materials,the battery can still display a specific capacity of 980 mAh g^(−1)(area capacity of∼5.54 mAh cm^(−2))at 0.1 C.This work provides a promising route toward high-performance Li-S batteries.