Modification Of Functional Groups For Graphene Oxide And Its Application In Silicon Anode For Lithium-ion Batteries

Lithium-ion batteries are widely used as efficient and portable energy storage devices,but currently,lithium-ion batteries using graphite as the anode material cannot meet the application requirements.Silicon（Si）is considered the most promising anode material for lithium-ion batteries owing to its ultrahigh theoretical capacity,low working potential,and abundant reserves.However,the poor conductivity and volumetric expansion of Si usually leads to rapid capacity decays.The Si/rGO composite formed by Si and flexible reduced graphene oxide（rGO）with high conductivity can effectively buffer the volumetric expansion of Si.During continuous charging and discharging processes,the volumetric expansion of Si leads to the cracking of the rGO coating layer and the shedding of the Si particles,which in turn seriously impacted affected the cycle life of the battery.Therefore,improving the composite ability of rGO coating layer on Si is of great significance for the practical application of Si anode materials.In this thesis,water（H₂O）and boric acid（H₃BO₃）were used to regulate the functional groups of graphene oxide（GO）prepared using the Hummers method.The obtained GO with more hydroxyl and epoxy functional groups could well combine with Si particles,which improving the electrochemical performance of the Si/rGO composite.At the same time,a highly stable Si/rGO-b composite of sandwich structure and Si/C/rGO composite of honeycomb structure were designed using improved GO（GO-3）and Si nanoparticles（NPs）,respectively.When used as electrodes,the composites showed an excellent cycling performance.The main work and research results of this thesis are as follows:（1）The carbon/oxygen ratio（C/O）,specific surface area and defect structure of the synthesized rGO have a significant impact on the conductivity of the Si/rGO composite,the transport of lithium ions and the formation of solid electrolyte interface（SEI）film.These properties of rGO are mainly determined by the pristine GO.So H₂O was introduced into the reactions to regulate the oxidation of oxidant,and H₃BO₃was introduced to protect the hydroxyl functional groups of GO from further oxidation.The hydroxyl and epoxy functional groups in GO-3 synthesized by co-regulation of H₂O and H₃BO₃were significantly increased,and the prepared rGO-3 had more defective structures and better conductivity.（2）To investigate the effect of GO with different functional groups on the electrochemical performance of the Si/rGO composites,Si/rGO-1,Si/rGO-2,and Si/rGO-3composites were synthesized using Si NPs with GO-1（with Hummers method as reference）,GO-2（with additional H₂O）,and GO-3（with H₂O and H₃BO₃）,respectively.As an electrode,the initial coulombic efficiency（ICE）of Si/rGO-1,Si/rGO-2,and Si/rGO-3was 75.7%,79.5%,and 82.4%,respectively.At a current density of 1 A/g,after charging and discharging for 45,60 and 88 cycles,respectively,the remaining specific capacity exceeded 1000 m Ah/g.The Si/rGO-3 composite exhibited the best electrochemical performance.（3）A sandwich structure of silicon/reduced graphene oxide composite（Si/rGO-b）was formed by a combination of ammonia modified Si NPs with GO-3.Firstly,GO combined with Si NPs to form p-Si/rGO-b with a core-shell structure.The core-shell structure limited the movement and aggregation of Si NPs,effectively buffering the volumetric expansion of Si.Then,the p-Si/rGO-b composite recombined with GO again to form eventually a sandwiched structure of Si/rGO-b,which improving the conductivity of the composite.At the same time,the outer coating layer of rGO could continue to collect the Si NPs falling off the core-shell structure,preventing the capacity of the composite from decaying.When used as an electrode,the Si/rGO-b composite exhibited excellent cycling performance（operated stably for more than 800 cycles at high capacity retention rate of 82.4%）and superior rate capability（300 m A h/g at 5 A/g）..（4）A composite of silicon/carbon/reduced graphene oxide（Si/C/rGO）of honeycomb structure was constructed with GO-3,ACET and Si NPs.The Si/C composite particles were embedded in the honeycomb holes formed by rGO,and the flexible ACET provided the buffering space for volumetric expansion of Si.The outer layer of rGO maintained the structural integrity and conductivity of the composite.When used as an electrode,the specific capacity of the Si/C/rGO composite was 1004 m Ah/g after 270 cycles at a current charging density of 1 A/g,and the ICE was 86.3%.In the rate cycle test,the reversible capacity was up to 447 m Ah/g at 5 A/g.