Fabrication And Performances Of Chitin Nanofiber-based Lithium-ion Battery Separators

Separator,as a key component in lithium-ion battery to isolate cathode and anode,plays a significant role on enhancing the electrochemical performances and safeties of LIBs.Microporous polyolefin membranes are widely used as separators in current LIBs owing to their good strength,high electrochemical stabilities,and appropriate porosities.However,they also exist some serious problems:(1)Low thermal stabilities,the low melting point of polyolefin separators make them suffer from large shrinkages with the increase of battery temperature,which greatly restricted the working temperature of polyolefin based separators in LIBs and may lead to safety risk of LIBs.(2)Poor wettability,the polyolefin separators have poor wettability with the polarity liquid electrolyte due to their nonpolarity,which may partly lead to heterogeneous diffusion of liquid electrolyte in the separators and effect on the electrochemical performance of cells.(3)Environmental pollution,the sources of polyolefin based separators are mainly dependent on the ever-decreasing fossil oil,and they are not sustainable and non-biodegradable,which will lead to severe environmental pollution with the expanded market of LIBs.To address aforementioned issues of polyolefin based separators,natural polymer nanofiber based separators have received intensive attention due to their good electrolyte wettability,high thermal stabilities,excellent mechanical strength and low cost.However,they also exist some serious problems:(1)Low ionic conductivity,the rich hydroxyl groups on the surface of natural polymer nanofibers lead to the formation of dense and nonporous films due to the intensive hydrogen bonding of the hydroxyl groups among nanofibers,which results in low Li+ion transport in separators.(2)High moisture absorption,moisture from the atmosphere can be absorbed by natural polymer nanofiber based separators,due to the rich hydroxyl groups on the surface of natural polymer nanofibers.Therefore,the natural polymer nanofiber based separators should be treated by high temperature and long time before assembling the cells,which may increase the cost of preparation of cells.So,it becomes an emphasis and difficulty of the work to preparation high ionic conductive and low moisture absorption of natural polymer nano fiber membrane.In this project,we take chitin nanofiber as an example and propose the template method,surface chemical modification method and surface coating method to fabricate several chitin nanofiber membranes.at the time,we explore their performance as the lithium ion battery separators.The main research results are summarized as follows:1.We developed a new type of porous membranes for Li/Na-ion batteries consisted of chitin nanofibers derived from prawn shells.The porous membranes were prepared by a "salt templating" method and the nanopore structures were tuned by varying the composition ratio of chitin nanofibers and sodium dihydrogen citrate(SDCA).The porosities,electrolyte uptake capabilities,and ionic conductivities of as-fabricated separators were examined as a function of SDCA content in the chitin nanofiber suspension.The performances of chitin nanofiber membrane(CNM)separators were evaluated in LiFePO4/Li half cells,and their performances are comparable to or even better than commercialized PP separator.Our proposed fabrication of separators by using sustainable chitin nanofibers will shed light on the future sustainable development of Li/Na-ion batteries.2.Based on the research of chitin nanofiber porous membrane,we further developed a surface modification strategy to fabricate a cyanoethyl chitin nanofiber separator.The as-prepared cyanoethyl chitin nanofiber(CCN)separator can not only have a high tensile strength but also enable high Li+ ion transport in the membrane due to the special interaction between ions and cyano-groups.The ionic conductivity and Li+ ion transference number of CCN separators are as high as 0.45 mS/cm and 0.62,which is about 13 times and 2.5 times that of unmodified chitin nanofiber separators(0.035 mS/cm and 0.25),respectively.Considering the attractive advantages and the potentials of scaled up fabrication,the CCN separator is expected to be very promising for the application in advanced lithium-ion batteries with superior performance and high safety.3.Based on the cyanoethyl chitin nanofiber separator,we prepared CCN/PVDF-HFP composite membrane by a simple dip coating process.The as-prepared CCN/PVDF-HFP composite membranes can not only stable versus lithium or graphite anode but also partly decrease their moisture absorption due to the hydrophobicity of PVDF-HFP.This composite membrane provides a new strategy of commercial application for natural polymer nanofiber separators.