Assembly Structure Design And Electrochemical Properties Of Cathode Electrode For High-performance Lithium-ion Battery

The development of advanced energy storage system fundamentally depends on the rational design of not only the new high-performance active materials,but also the effective control of the microstructure of the electrodes,which is supposed to support stable and efficient electron and ion transport.The purpose of this thesis is to study the relationship between the assembly structure and electrochemical performance of the electrodes,and to develop the new preparation technology of thick electrode with uniform and stable assembly structure.We have systematically studied the influence of molecular chain motion on the electron/ion transport path constructed by porous structure,conductive network nad interfaces in different electrode preparation methods.Finally,high-performance lithium-ion battery composite cathodes（lithium iron phosphate（LFP）or lithium nickel cobalt manganite（NCM）or lithium nickel cobalt aluminum acid（NCA））with uniform assembly structure（electron/ion transport network）and high-performance thick electrodes/high-performance flexible thick electrodes with high energy density were obtained.The research contents and conclusions are as follows:（1）For the first time,we prepare a functional nanocoating biobinder using the well-known poly（lactic acid）to address the needs.It is found that the composite electrodes with this nanocoating biobinder are upgraded with uniform and robust assembly structures,including the electron-transportation network,ion-transportation network,and interfaces.Importantly,the nanocoating finally works as a new type of polymeric artificial cathode electrolyte interphase（poly-CEI）to protect the active particles.Therefore,a remarkable improvement in the electrochemical performance has been achieved.In particular,the LFP cathode can deliver a high discharge capacity of 74.6 m A h g^-1 at 10 C and a high capacity retention of 95.5%even after 850 cycles at 2 C.In short,this study may reshape the essential roles of a binder in composite electrodes by highlighting its critical link to assembly structures.（2）Further,we prove that the functional PLLA biobinder can universally regulate the electrode structure in ghigh-capacity active materials systems（including NCA and NCM）which perform high surface activity and bad cycle stability.The rate performance and cycle stability of PLLA-based electrode are significantly higher than those of commercial PVDF-based electrode.In addition,we further explore the effectiveness of PLLA as a surface modification layer（poly-CEI）of active particles.The results show that PLLA can form nano-coating on the surface of Ni-rich NCA,which can significantly alleviate the occurrence of side reactions at the electrolyte/active particle interface,protect the active particles without hindering the transportation of Li⁺,and the modified particles show improved cycle stability.（3）The porous composite electrodes with high active materials loading are prepared by semi-dry method（wet slurry-hot pressing combined method）.Further,the thick electrodes with high LFP loading are prepared by melt blending method（dry method）for the first time.Compared with the traditional slurry method,the melt processing method can not only greatly increase the LFP load(>80 mg cm^-2)and the areal sprecific capacity(>10 m A h cm^-2),but also show significant advantages in the uniformity of electrode structure.The results show that the uniformity of the assembly structure of the electrode prepared by melt method is better than that obtained by slurry method,and the CE of the thick electrode is close to 100%.The importance of the uniform assembly structure for Li⁺diffusion transport kinetics and electron transport network is proved once again.The work provides a new idea for the scaled-up production of the thick electrode with high active material loading.（4）The methods and mechanism of structural regulation of porous thick electrode under the melt processing method is explored by using the classical polycrystalline PLLA system.The rheological behavior of the composites during processing is regulated by introducing PDLA into PLLA binder.The dispersion of the functional particles and the final porous structure of electrode are controlled by the different crystallization process and the various morphology of homocrystallites（hc）and stereocomplex crystallites（sc）.Finally,it is found that the electrode prepared with PLLA/PDLA blends as polymer scaffold has fast electron/ion transfer rate,and the highest areal specific capacity can reach,which is much higher than that of the PLLA-based electrode(<5 m A h cm^-2).Here,the fundamental scientific problem of assembly structure control in the preparation of thick electrode by melt processing method is explored from the polymer physics point of view.（5）A flexible self-supporting high LFP-loading electrode（LFP>90 wt%）with ultra-high aspect ratio bacterial cellulose（BC）and carbon nanotubes（CNT）as three-dimensional overlapping network is prepared by vacuum assisted self-assembly method,in order to fit well with flexible electronic devices.The results show that the introduction of BC can not only improve the ductility（elongation at break up to 13%）of the electrode,but also significantly improve the rate capability and cycle stability of the electrode.The flexible LFP cathode can deliver a high discharge capacity of 110 m A h g^-1 at 5 C and a high capacity retention of near 100%even after 100 cycles at 1 C.BC and CNT with high aspect ratio can form perfect electron/ion transport network in the composite electrode.This method has industrial feasibility in the preparation of flexible composite electrode with ultra-high mass loading of active materials.