Construction Of Cellulose Microspheres And Their Performance Regulation In Lithium Battery Separators

A typical lithium battery comprises a positive electrode,negative electrode,separator,and electrolyte.The main roll of the separator is to prevent short circuits within the battery while enabling the migration of lithium ions.Conventional separator materials are made from petroleum-based material,such as polyethylene and polypropylene.However,these materials tend to curl and shrink at high temperatures,leading to the risk of battery short-circuit,explosion,and environmental harm.Cellulose stands as the preferred alternative material to polyolefin separators in lithium batteries,owing to its excellent thermal dimensional stability,exceptional mechanical strength,and superior wetting properties with the electrolyte.Nevertheless,cellulose-based battery separators encounter challenges including low porosity,reduced lithium ion migration efficiency,and elevated internal resistance.To achieve these challenges,N-methylmorpholine N-oxide(NMMO)/H2O as a solvent system was applied for the dissolution of cellulose in this study.It delves into the construction mechanisms underlying cellulose microsphere regeneration by strategically designing the composition of the coagulation bath.The study investigated the performance of cellulose microspheres as reinforcing materials for lithium battery separators and elucidates the structure-property relationships and mechanisms of functional groups in cellulose-based battery separators.This research provided theoretical support for the application of cellulose in ecofriendly lithium battery separators and energy storage materials.The specific research content is as follows:(1)To investigate the controllability of cellulose bead surface and internal morphology,the influence of coagulation bath composition on the physicochemical properties of regenerated cellulose beads was studied.Cellulose solutions were prepared using the NMMO/H2O system.Various coagulation baths,including water,alcohol solutions,and acid solutions,were employed to regulate the coagulation bath composition.This enabled the fabrication of regenerated cellulose beads with precisely controlled morphology and structure using a straightforward drop-ball method.The solidification mechanism of regenerated cellulose beads was confirmed through fluorescence labeling,and the physicochemical properties were thoroughly analyzed.Results indicated that dissolution and regeneration process resulted in the crystalline structure of cellulose from cellulose Ⅰ to cellulose Ⅱ.The composition of the coagulation bath composition could control the regeneration rate of cellulose beads,consequently enabling the manipulation of their structure and performance.The solidification of cellulose beads occurred from their outer layers to the inner core.Methanol(MT)as the coagulation bath yielded MT-cellulose beads with the highest porosity(90.51%).Water absorption testing demonstrated the exceptional water absorption capacity of MT-cellulose beads,recording uptake percentages of 603.47%at 0.5 minutes and 830.50%at 90 minutes.Upon reaching water absorption equilibrium,the internal network structure of the cellulose beads and the presence of hydrogen bonding contributed significantly to their integrity.(2)A controlled morphology of regenerated cellulose microspheres(RCM)was obtained by combining the reverse suspension method with solvent/nonsolvent exchange.Polyvinyl alcohol(PVA)serves as the base structure,while RCM served as the network skeleton and reinforcing material.The non-solventinduced phase separation(NIPS)method was employed to produce RCM/PVA composite separators with a cross-linked network structure.Microscopic morphology and structure of RCM were characterized using scanning electron microscopy and specific surface area measurements.RCM exhibited a distinctive nano-scale porous structure and a uniform spherical morphology.Discrepancies in morphology and porosity of RCM were attributed to variations in the coagulation bath composition.Especially,methanol and hydrochloric acid solutions resulting in higher specific surface areas,denoted as RCMMT and RCMHCl.The physical and chemical properties as well as electrochemical performance of the composite separators were investigated in this study.Results indicated that various RCM exhibited varying separator performance attributed to their surface charge,crystallinity,and porosity.Among them,RCMHCL/PVA separators exhibit the highest mechanical properties and electrolyte absorption,measuring 13.37 MPa and 428.79%,respectively.,the separator exhibited an ionic conductivity of 1.03 mS·cm-1 and an ion migration coefficient of 0.51 at room temperature.Moreover,it demonstrated good interfacial compatibility and a wide electrochemical window(>4.34 V).Microspheres,as reinforcing materials,could significantly enhanced the physical and chemical properties of the separator.(3)According to the enhancement mechanism of RCMHCl,RCMHCl/PVA separators with dual network architecture were prepared.Hydrogen bonding and network entanglement were formed between RCMHCl and PVA polymers.The uniform distribution and embedded structure of RCMHCl in the separator were confirmed.The cross-linking of RCMHCl and PVA led to the formation of a dualnetwork structure in the separator.The impact of RCMHCl content on the physical and electrochemical properties of the separator was investigated in this study.Results revealed that the separator with 3%RCMHCl exhibited good thermal stability(no significant curling or shrinking at 160℃)and mechanical properties(tensile strength of 15.53 MPa).The 3%RCMHCl imparted excellent electrolyte absorption(481.25%),providing efficient transport pathways for lithium ions.The 3%RCMHCl/PVA separator exhibited the best electrochemical performance and cycling stability with an ionic conductivity of 1.43 mS·cm-1,lithium-ion migration coefficient of 0.54,a discharge capacity of 152.60 mAh·g-1 after 200 cycles(capacity retention of 91.87%),and a coulombic efficiency of 99.43%.(4)A complete-cellulose-based lithium-ion battery separator was performed through a layered filtration method.Cellulose’s hydroxyl(-OH)functional groups were replaced with carboxyl(-COOH)functional groups through an oxidation reaction.Lithium hydroxide(LiOH)was employed for lithium modification of RCMHCl(RCM-Li),introducing-Li functional groups on the microspheres.The chemical structure of RCM-Li/MFC separators confirm the successful introduction of both-COOH and-Li functional groups into the separator.A sandwich structure was observed from the resulting separator,where RCM-Li served as support and reinforcement in the middle layer.The impacts of RCM-Li and MFC porous network structures as well as functional groups on the separator’s mechanical,and wetting properties,as well as electrochemical performance were analyzed in this study.Results indicated that the synergistic effects of-OH,COOH,and-Li functional groups rendered the separator compatible with the electrolyte,facilitating the mobility of additional Li+ions and enhancing interface stability.The 10%RCM-Li/MFC separator exhibited an electrolyte absorption rate of 496.32%,the highest ionic conductivity(1.95 mS·cm-1),excellent lithium-ion migration coefficient(0.65),and good cycling stability.Explored the effect of functional groups on the interface stability of batteries.By observing the surface of lithium metal and conducting molecular dynamics simulations of the battery system,indicating the introduction of functional groups effectively inhibits the growth of lithium dendrites and improves the diffusion rate of lithium ions.This research addressed the issues related to inadequate interface stability among separators,electrolytes,and electrodes through the controlled regulation of cellulose microsphere morphology and surface functionalization.The mechanisms of cellulose functional groups in lithium batteries were investigated,providing technical support and theoretical foundations for the application of cellulose-based materials in lithium battery separators.