Study On The Sodium Storage Properties Of Phenolic Resin-based Hard Carbon By Curing Mechanism

Sodium-ion battery(SIB)may be the most promising energy storage device after lithium-ion batteries because of the similar working principle and abundant sodium.The commercialization of SIB is inseparable from anode materials with excellent performance.Hard carbon(HC)with a low working platform,high reversible capacity and stable structure is considered the most promising one,but its initial Coulombic efficiency(ICE),reversible specific capacity and rate performance need to be further improved.In this thesis,phenolic resin was chosen as the precursor and different curing mechanisms were adopted to obtain HC to solve the above-mentioned problems pointedly.In addition,the relationships between the HC structure and electrochemical performance were explored systematically.The low ICE of HC is mainly attributed to the large specific surface area caused by the unstable structure of the precursor.In view of this,we employed air pre-oxidation to cure the spray-dried phenolic resin.Extended the pre-oxidation time can increase the crosslinking degree of the precursor,thereby taking the release of generated small molecules slow during the high-temperature carbonization and obtaining the HC with a low specific surface area.TG,FTIR and BET conducted showed that there is a negative correlation between the crosslinking degree of the precursor and the specific surface area of the derived HC.The PFHC-20 sample had the lowest specific surface area(35.3 m² g^-1),and accordingly achieved the highest ICE of 84.7%and reversible specific capacity of 334.3 m Ah g^-1 at 20 m A g^-1.Subsequently,the pre-oxidation temperature was further optimized,and the study showed that the microstructure and reversible capacity for these samples had little difference when the pre-oxidation temperature was between 120-180?C.Therefore,the pre-oxidation temperature can be continuously reduced to save production costs.In order to enhance the reversible specific capacity,especially the plateau capacity(voltage<0.1 V),we used the sol-gel method combined with normal temperature and pressure drying and high-temperature carbonization to prepare phenolic resin-based carbon aerogels.The microstructure of HC was adjusted by tuning the carbonization temperature,and the relationships between the microstructure and electrochemical performance in the ester-based electrolyte were studied roundly.The experimental results showed that the micropores of aerogel gradually closed with the increase of carbonization temperature,resulting in a lot of sodium storage sites.Therefore,RCA-1650 had the highest reversible specific capacity(380.6 m Ah g^-1)and plateau capacity(283.6 m Ah g^-1).Matching it with the cathode material O3-Na Ni_1/3Fe_1/3Mn_1/3O₂ to assemble a sodium-ion full battery,which achieved a high energy density of 281.8 Wh kg^-1,as well as excellent rate and cycle performance.The carbon aerogels were further combined with the ether-based electrolyte,the rate performance,ICE and cycle performance are significantly improved due to a thinner SEI and a smaller electrochemical polarization.Comparing the above two curing mechanisms,we conclude that the HC under air pre-oxidation can obtain high reversible specific capacity and ICE,while the HC obtained by the sol-gel method has better rate performance.