Preparation And Performances Of Regenerated Cellulose Separator For Lithium-ion Batteries

Owing to the advantages of high energy density,long cycle life and no memory effect,lithium-ion batteries（LIBs）have become the most widely used secondary energy storage battery.As one of the key components affecting the performance of LIBs,the separator is mainly used to isolate the positive and negative electrodes and provide a transport channel for Li⁺.Although the commercial polyolefin separator has the advantages of high strength and stable electrochemical properties,it has the defects of non degradability,poor hydrophilicity of electrolyte and low thermal dimensional stability,which is difficult to meet the needs of high-performance LIBs.Cellulose has the advantages of abundant reserves,low price,degradable and renewable.Its semi crystalline and polyhydroxy structure can give the separator excellent thermal dimensional stability and electrolyte hydrophilicity.Cellulose has become the best alternative material for preparing high-performance separators.At present,cellulose separators are mostly made of nano cellulose,which has the problems of high cost and energy consumption.In this paper,a new regenerated cellulose separator（RCS）for LIBs was prepared from cotton pulp by a simple and green non solvent induced phase separation method,and the performance of the separator was improved.（1）The RCS was prepared by non-solvent induced phase separation（NIPS）using cotton pulp as raw material,NaOH/Urea system as solvent and ethanol as non-solvent.It was found that ethanol has a strong ability to regenerate cellulose solution,which can cause the instantaneous liquid-liquid phase separation and form porous structure in RCS.Increasing the temperature of ethanol bath（15-55℃）would increase the porosity（51-61%）and electrolyte absorption（414-436%）of RCS,thus improving the electrochemical performance of RCS,but also significantly reducing its tensile strength（21.6 MPa to 13.4 MPa）.RCS had excellent ionic conductivity（0.90-1.25 m S/cm）and electrochemical stability window（4.7 V）,and its interface resistance was much lower than that of commercial Celgard separators.The first discharge capacity of the LIBs assembled with RCS was about 150 m Ah/g,then the discharge capacity was about 110 m Ah/g after 100 cycles of charge and discharge at 0.5 C.The high rate（5 C）discharge capacity can still be maintained at about 109 m Ah/g,which is much better than LIBs assembled with Celgard separators.（2）The all-cellulose separator（RCS/CBC）with cationic bacterial cellulose（CBC）as reinforcing phase was prepared by NIPS method.The positively charged CBC can generate charge adsorption with the negatively charged cellulose molecular,improving the bonding strength between the two phases,and then improve the strength of RCS/CBC.With the increase of CBC content,the strength of RCS/CBC increased first and then decreased.When the the content of CBC was 3.0 wt%,the obtained RCS/CBC-3 had the best tensile strength（27.2 MPa）,which was about 54.5%higher than RCS,and its porosity,thickness and electrolyte absorption were 49.2%,12.8μm and 424%respectively.Density functional theory（DFT）calculation and actual test could prove that the presence of CBC can reduce the interaction strength between the RCS/CBC and Li⁺,and enhance the binding force between the RCS/CBC and PF₆^-,thereby improving Li⁺migration number and ionic conductivity.Compared with Celgard separators,RCS/CBC possessed better electrochemical performance,and the assembled LIBs with RCS/CBC have better cycle stability and rate performance.（3）A dual network regenerated cellulose based LIBs separators（DN-RCS）with acrylic acid/acrylamide（AA/AM）enhanced network was prepared by UV polymerization.On the basis of optimizing the components of UV polymerization reaction solution,the performances of DN-RCS were studied.The introduction of covalently linked AA/AM reinforcement network could greatly improve the mechanical properties of RCS,but too high concentration of UV polymerization solution（≥15 wt%）damaged the toughness and pore structure of DN-RCS.When the concentration of UV polymerization solution was 10 wt%,the obtained DN-RCS-10 has the best comprehensive strength performance,with a tensile strength of 39.4MPa（121%higher than RCS）,a puncture resistance of about 267 g and good toughness.The pore size distribution of DN-RCS-10 was mainly 10-100 nm,the average pore diameter was17.2 nm,the porosity was 62.4%and the electrolyte absorption rate reached 575%.The ionic conductivity of DN-RCS-10 was about 1.03 m S/cm,while that of Celgard separator was only0.65 m S/cm.The LIBs assembled with DN-RCS-10 possessed excellent battery performance.The oxidation-reduction potential difference（0.47 V）,the first discharge capacity（155m Ah/g）,the discharge capacity retention rate after 100 charge and discharge cycles（89.5%）,and the discharge capacity at 5 C rate（101 m Ah/g）were far better than that of Celgard separators（0.64 V,141 m Ah/g,71.6%,85 m Ah/g）.（4）Ethylene vinyl acetate copolymer microspheres（EVAs）with different melting temperatures were prepared by solvent evaporation method,and then compounded with RCS to prepare regenerated cellulose based composite separators（RCS/EVAs）with hot pore closure function.The melting temperature of EVAs is negatively correlated with the vinyl acetate（VA）content.RCS/EVAs with required thermal closure temperature can be designed by adjusting VA content.The actual thermal closure temperatures of RCS/EVAs12（VA content is 12 wt%,the same below）,RCS/EVAs18 and RCS/EVAs25 were 103.7℃,88.4℃and 75.4℃respectively,while the hot closure temperature of Celgard separator was about138.2℃.RCS/EVAs had good electrolyte absorption（325-394%）,porosity（47-48%）,tensile strength（28.0 MPa）and thermal dimensional stability.In addition,the ionic conductivity of RCS/EVAs12,RCS/EVAs18 and RCS/EVAs25 were 0.89 m S/cm,1.04 m S/cm and 1.36m S/cm,and the electrochemical stability window reached 5.0 V,and the corresponding first discharge capacity were 146 m Ah/g,147 m Ah/g and 149 m Ah/g respectively.In general,the LIBs assembled with RCS/EVAs not only have good safety performance,but also have excellent cycle stability and magnification performance.