Rational Design And Performance Of Protein Based Functional Separator Materials For Lithium-sulfur Batteries

Environment and energy issues have emerged as a serious threat with the ever-growing development of the economy nowadays.Lithium-sulfur(Li-S)battery is one of the most promising candidates for the next generation high-performance secondary batteries,with the advantages of high specific capacity,high energy density,and sulfur is low-cost and eco-friendly.However,the shuttling of polysulfides and uncontrollable growth of lithium dendrites remain the most critical obstacles deteriorating the performance and safety of lithium-sulfur batteries.Rationally choosing the polysulfide-anchoring materials and designing separator structure is a simple and practical strategy to improve the electrochemical performance of Li-S batteries.Herein,we utilize proteins as multifunctional agents to design and fabricate protein-based composites for the application in separator engineering.Besides,the contribution of protein structure to the polysulfide-anchoring capability is also discussed.The specific research contents and results are as follows:(1)Due to the lack of cost-efficient strategies to characterize the comprehensive properties of interlayers and understanding of the roles that the interlayer properties play in the battery performance,we design a surfactant-controlled strategy to tune the structures/properties of the interlayers and establish the relationships among structure,property and performance through comprehensive characterizations.It is surprisingly found that only with a specific surfactant,gelatin protein,a robust and self-assembled porous graphite nanoplatelets(GNPs)interlayer can be achieved.Meanwhile,benefiting from the rich functional groups of protein,the protein-functionalization of GNPs not only leads to good adhesion to a separator for the GNP interlayer,but also strong polysulfide-trapping ability.As a result,by adding the protein-functionalized GNP interlayer to a Li-S battery,the electrochemical performance is obviously enhanced,owing to the abilities of the interlayer for trapping polysulfides and facilitating ion-transport simultaneously.The initial discharge capacity is increased from 881 m Ah g^-1 to 1200 m Ah g^-1 with GNP/gelatin coating added.At a high current density of 0.5 A g^-1,the capacity of the cell with GNP/gelatin is even increased by 62%compared with the cell without any coating.(2)Uncovering the key contributions of molecular details to capturing polysulfides is important for applying suitable materials that can effectively restrain the shuttle effect in advanced lithium-sulfur batteries.This is particularly true for natural biomolecules with substantial structural and compositional diversities strongly impacting their functions.Here,natural gelatin and zein proteins are first denatured and then adopted for fabrication of nanocomposite interlayers via functionalization of carbon nanofibers(CNFs).From the results of experiment and molecular dynamic simulations,we find that the lengths of the sidechains on the two proteins play critical roles.The short-branched gelatin shows significantly stronger adsorption of polysulfides,as compared with zein comprising many long-chain residues.The gelatin-based interlayer,along with its good porous structures/electrical conductivity,greatly suppresses the shuttle effect and yields exceptional electrochemical performance(discharge capacity of553 m Ah g^-1 after 500 cycles at 1 A g^-1).Furthermore,the implementation of proteins as functional binder additives further supports the finding that gelatin enables stronger polysulfide-trapping.As a result,high-loading sulfur cathodes(9.4 mg cm^-2)are realized,which deliver a high average areal capacity of 8.2 m Ah cm^-2 over 100 cycles at0.1 A g^-1.This work demonstrates the importance of sidechain length in capturing polysulfides and provides a new insight in selecting and design of desired polysulfide-binding molecules.(3)Separator plays a key role in molecule diffusion and ion transport kinetics;thus,endowing the separator with functions to address the above two issues is in urgent need.Herein,a protein-based,low-resistance Janus nanofabric is designed and fabricated for simultaneously trapping polysulfides and stabilizing lithium metal.The Janus nanofabric is achieved via combining such two functional nanofabric layers:gelatin-coated conductive nanofabric(G@CF)as a polysulfide-blocking layer and gelatin nanofabric(G-nanofabric)as an ion-regulating layer into a heterostructure.The gelatin-coating of G@CF effectively enhances the polysulfide-trapping ability owing to strong gelatin-polysulfide interactions.The G-nanofabric with exceptional wettability,high ionic conductivity(4.9 x 10^-3 S cm^-1)and high lithium-ion transference number(0.73)helps stabilize ion deposition and thus suppresses the growth of lithium dendrites.As a result,the Li/Li symmetric cell with G-nanofabric delivers ultra-long cycle life for over 1000 h with very stable performance.Benefiting from the synergistic effect from the two functional layers of the Janus nanofabric,the resulting Li-S batteries demonstrate excellent capacity,rate performance and cycle stability(e.g.initial discharge capacity of 890 m Ah g^-1 with a decay rate of 0.117%up to 300 cycles at 0.5A g^-1).