Preparation Of Cellulose Sulfate Lithium Binder And Its Performance In Lithium Ion Batteries

Cellulose sulfate（CS）is a kind of cellulose inorganic ester derivatives formed by the sulfation reaction.The preparation of CS is mainly through esterification with concentrated sulfuric acid,chlorosulfonic acid,and other methods to synthesize products in the form of low molecular weight and sodium salts.Cellulose sulfates in the form of lithium salts have not been reported or applied.1.Cellulose sulfate lithium（CSL）was prepared by using refined cotton with high polymerization degree as cellulose raw material,sulfur trioxide pyridine complex as esterification reagent,and being alkalized with lithium hydroxide.Through exploring and optimizing the reaction temperature,reaction time,and amount of esterification reagent during the esterification reaction,CSL products with excellent water solubility were prepared.Sufficient structural characterization and property analysis were conducted for the products.The results showed that the introduction of sulfuric acid groups improved the water solubility of the material.More importantly,the original hydrogen bond structure was destroyed and the material changed from a crystalline structure to an amorphous structure.The microscopic morphology also changed significantly.2.Using the synthesized high molecular weight CSL product with excellent water solubility as a binder,it was applied to the lithium iron phosphate（LFP）cathode to conduct electrochemical performance tests and mechanism analysis.Compared to commercial polyvinylidene fluoride（PVDF）binder,electrodes prepared by using CSL as the binder exhibited better adhesion and electrochemical performance.At 1 C,the LFP-CSL electrode still had a specific discharge capacity of 133.4 m Ah g^-1 and a capacity retention rate of 93.1%after 300 cycles,while the LFP-PVDF electrode was only 108.0 m Ah g^-1 and 75.2%,respectively.Furthermore,the mechanism of different binders on the cycling performance was explored through the measurement of ion conductivity,lithium ion diffusion coefficient,electrochemical impedance during cycling,and electrode surface micromorphology before and after cycling.The results showed that sulfation and the introduction of Li⁺made CSL have higher ionic conductivity.This promoted the diffusion and migration of Li⁺during the charging and discharging process,and displayed lower electrochemical polarization and better rate performance.More importantly,CSL consistently provided excellent adhesion performance and led to stable long cycle performance.3.The influence of CSL as the binder on graphite（Gr）anode was investigated.Compared to PVDF binder,CSL exhibited excellent adhesion and electrochemical performance.At 0.5 C,Gr-CSL still had a specific discharge capacity of 312.1 m Ah g^-1 and a capacity retention rate of 95.5%after 300 cycles,while PVDF only had 171.5 m Ah g^-1and 69.7%.Furthermore,by testing the first charge discharge curve,cyclic voltammetry curve,electrochemical impedance before and after cycling,X-ray photoelectron spectroscopy after cycling,and electrode SEM analysis before and after cycling,the mechanism of different binders influencing the cycling performance of graphite electrodes was explored.The results showed that CSL could inhibit the decomposition of electrolytes and improve the rate performance.The stable and excellent long cycle performance was attributed to the fact that CSL cloud always provide excellent adhesion performance during the long cycle process and ensure the integrity of the electrode.