Influence Of Molecular Structu Re Of Carboxymethyl Cellulose On High Performance Silicon Anode In Lithium-Ion Batteries

Silicon-based anode have been considered as the next generation of anode materials in lithium-ion battery to replace the traditional graphite anodes due to their high specific capacity,low discharge potential,environmental pollution and safety.However,the large volume change(400%)of the silicon-based material during the insertion and deintercalation of lithium-ions leads to the dest:ruction of the structure of the electrode material,resulting in rapid decay of the capacity of silicon-based anodes.As a common binder for silicon-based anodes,carboxymethyl cellulose has a stable interaction with silicon due to its abundant carboxymethyl group,and exhibits superior bonding performance compared with conventional PVDF binders.The aim of this study was to prepare carboxymethyl cellulose binders with different molecular structures and to explore the effect of the molecular structure of carboxymethyl cellulose on the electrochemical performance of silicon electrodes.Firstly,cellulose with different degree of polymerization of 845,697,501 and 215 was successfully prepared by controlling the acid hydrolysis conditions,and their crystal forms and crystallinity were charactaerized.Subsequently,12 kinds of carboxymethyl cellulose with diifferent molecular structures(different DP and DS)were successfully prepared.The prepared carboxymethyl cellulose was tested by Infrared Spectroscopy.Two observed absorption bands of-COO-groups correspond to the presence of abundant carboxymethyl groups on the modified cellulose,indicating the successful preparation of carboxymethyl cellulose.Carboxymethyl celluloses with different molecular structure were applied as a binder to the silicon electrode.Electrochemical impedance spectroscopy of silicon anodes prepared by different carboxymethyl cellulose showed that the increase of the DS of carboxymethyl cellulose was beneficial to reduce the internal resistance of the electrode and promote the conduction of lithium ions.The constant current charge and discharge test showed that the carboxymethyl cellulose CMC-215L with lower degree of substitution and lower degree of polymerization exhibited the best bonding performance(1656 mAh g-1 of specific capacity,71%of capacity retention rate after the 100th cycle).This study explains the mechanism of the influence of the molecular structure of carboxymethyl cellulose on the electrochemical performance of silicon-based electrodes:the combination of silicon particles and carboxymethyl cellulose with high DS and high DP is unstable,leading to the deterioration of cyclic performance of silicon anodes.On the contrary,the carboxymethyl cellulose with low DP and low DS can maintain the interaction with silicon in the volume expansion of silicon,which helps to maintain the structure of the electrode and improve the cycle performance of the silicon negative electrode.The results of this study provide more theoretical basis for the subsequent preparation of binders more suitable for silicon anodes.