| Due to the finite reserves and industrial pollution of traditional fossil energy,human beings have been seeking new energy sources to meet the increasing demand for energy.As one of the important contemporary new energy resources,there are numbers of researches regarding anode,cathode,electrolyte,and separator of lithium ion battery.Considerable progress has been made on increasing the specific capacity,power,charge and discharge performance,and cycling stability of the battery.Separator provides insulation between electrodes and micro-pathway for ion transportation.Therefore,its structural stability,morphological integrity,and mechanical strength are significantly related to the safety performance of the battery.In order to make up for the shortcomings of commercial separators,such as insufficient hydrophilic property,poor thermal stability,and inferior porosity,it is proposed to develop a new type of cellulose separator.Multi-scales modelling and mechanical properties of cellulose separator are studied in this thesis,which holds great significance for improving safty performance of lithium-ion batteries.Modelling and mechanical calculation of the materials and structures of cellulose separator at different scales are carried out,where the cross-scale transfering mechanism of elastic properties is revealed.The research mainly includes molecular simulation at the micro-scale,microfibril unit calculation and single fiber modelling at the meso-scale,and RVE modelling of fibrous network at the macro-scale.Based on the multi-scale modelling method,the mechanical properties of cellulose separator in working conditions are studied,concerning the influence of electrolyte immersion and elevated temperature.Finally,combining the findings of multi-scale modelling and working condition mechanics research,the separator preparation,microstructure regulation,and mechanical properties improvement tests are carried out to accomplished the integrated research of separator materials,microstructure,and macroscopic properties.The work of this thesis mainly includes several parts:(1)Study on mechanical behavior of cellulose-based material system at micro-scale.At the micro-scale,the mechanical behavior of the cellulose separator materials is studied.On the one hand,the molecular models of cellulose crystallization zone,cellulose amorphous molecular chain,and lignin are constructed on Materials Studio software,and the mechanical properties are calculated to obtain the elastic modulus of the molecular models.On the other hand,mechanical tensile tests are carried out on the prepared samples of cellulose,hemicellulose,and lignin.In addition,molecular simulation method is applied to study the blending performance and interface property between separator materials and electrolyte solvents(DMC and EC organic solvents).The elastic properties of materials calculated by molecular simulation at micro-scale will be applied as the input for material properties of single fiber at meso-scale.(2)Structural modelling and mechanical behavior study of single fiber at meso-scale.At the meso-scale,the structural modelling and mechanical behavior of the single fiber of cellulose separator are studied.Firstly,the material composition and multi-layer structure of single fiber plant cell wall are analyzed,and the elastic modulus of the microfibril unit inside layers S1 to S3 is calculated.Then,a multi-layer concentric ellipse model is established for single fiber,and the simplified model is used to simulate the elastic mechanical properties of single fiber.Meanwhile,an elliptical model with S2 layer microfibril layup is built to explore the influence of microfibril on the stress distribution of the fiber,and the elastic properties of single plant fiber are measured by AFM microprobe test.The simulated property of single fiber will be applied for the fibrous network,serving as the elastic properties input for the beam elements.(3)Research on RVE modelling and simulation of separator at macro-scale.At the macro-scale,representative volumetric element(RVE)modelling and simulation on microstructure,i.e.,fibrous network of cellulose separator are conducted.Two structural modelling methods are proposed for the fibrous network,which are,structural reconstruction methods based on image structure morphology skeleton and random fibrous network model based on image structure information statistics.Significantly,an innovative one-step programming modelling method combining image processing,structural reconstruction,finite element model construction,and elastic behavior simulation based on Inp file is proposed.In addition,Taufactor plugin of Matlab is utilized to simulate the ion diffusion ability of the fibrous network model,and to explore the effect of structural parameters.The RVE model can be used to predict the elastic mechanical properties of the macro-scale separator,optimize the design of the microstructure parameters,and then guide the determination of parameters for preparation process.(4)Study on mechanics of separator in working condition based on multi-scale model.Based on the established multi-scale structural modelling and mechanical property calculating method,the elastic mechanical properties of cellulose separator in the actual working conditions inside the battery are studied.Firstly,multi-scale study of electrolyte immersion factor is conducted.Swelling effect of the cellulose material system is studied using molecular simulation at micro-scale.Swelling and weakening of single fiber are discussed at meso-scale,combining microfibril unit calculation and single fiber elliptic model simulation.The wet elastic modulus of macro separator is calculated at macro-scale through fibrous network RVE model.Secondly,the influence of temperature on the elastic stiffness of cellulose separator is discussed at the micro-and macro-scales.At the micro-scale,the molecular dynamics simulation of materials under different temperatures is carried out.At the macro-scale,the thermal stability experiment and the elastic tensile test after heating of the separator are carried out.Finally,the elastic constitutive relation is studied for wet mechanical properties of cellulose separator,where the influence of wet swelling and stiffness weakening should be fully considered.(5)Study on preparation and mechanical improvement of cellulose-based separator.Based on the guidance of multi-scale modelling research and combined with the wet mechanical properties study of cellulose separator,it is proposed to treat the cellulose pulp with appropriate process and attempt to prepare cellulose separator.TEMPO oxidation process and high pressure homogenization process are applied to achieve the regulation of cellulose separator microstructure(i.e.fiber morphology).Cellulose separator is prepared by the green process of vacuum filtration method,and its basic properties,pore properties,surface properties,mechanical properties,and electrochemical performances are comprehensively tested and characterized.Inspired by the advantages of the natural plant material system,lignin material is introduced into the cellulose separator as a wet strength enhancer,and its mechanism is explored.The multi-scale study can provide valuable guidance for the macroscopic preparation experiments,and construct the integrated transfer relationship of "material-structure-property" of cellulose separator. |
PDF Full Text Request