Preparation And Electrochemical Properties Ofgraphene-based Lithium Iron Phosphate

Lithium ion battery has been used in almost all digital 3C products and some new energy vehicles,which is the backbone force to promote energy transformation.The choice of cathode material for lithium ion battery determines the discharge capacity,service life and cost of the battery.However,after lithium iron phosphate（LFP）entered the market,its development was restricted by its poor conductivity,low migration rate of lithium ions and poor performance of high power and low temperature performance.In this paper,aiming at the disadvantages of lithium iron phosphate,the influence of surfactant on the morphology and electrochemical properties of lithium iron phosphate was firstly investigated,and the carbon coating method was optimized.After the lithium iron phosphate is grown in situ on the graphene,the carbon coating is carried out to improve the electrical conductivity to improve the high power performance.In situ growth of lithium iron phosphate was performed on nitrogen-doped graphene to explore the influence,and the optimal material was compared with commercial lithium iron phosphate.The specific research contents are as follows:（1）The hydrothermal method and N₂high temperature calcining method were used to prepare lithium iron phosphate positive electrode materials,and the influence of different SDS additives on morphology and electrochemistry properties was investigated:with the increase of SDS additives,the peak intensities of XRD decreased,and the electrochemical performance was the best when the SDS added amount was 2.5wt%.Subsequently,the influence of the amount of ethylene glycol（EG）on the lithium iron phosphate electrode material was investigated,and it was found that due to its high viscosity,ethylene glycol could prevent the excessive growth and agglomeration of particles during the hydrothermal process,acting as a liquid surfactant.As the amount of ethylene glycol increased,the particles became smaller and thinner,and the crystallinity decreased.The cyclic performance is the best when the amount of ethylene glycol is 10ml（14.3 vol%）.On this basis,ball grinding and wet deposition carbon coating were carried out.The results show that the coating effect of wet deposition is more obvious,the particles are smaller and the charging and discharging platform is longer,which indicates that the material structure obtained by this method is more stable.After wet deposition of carbon coating,the cycle performance and the ratio performance are improved.The capacity of 0.2C is 150.2m Ah/g,the capacity retention rate after 50 cycles is 102%,and the specific discharge capacity under 5C is 68.5m Ah/g.After carbon coating,the resistance decreases,the degree of polarization decreases,and the structure reversibility is greatly improved during charging and discharging.（2）In situ growth of lithium iron phosphate on reduced GO（RGO）:the specific discharge capacity of graphene based lithium iron phosphate（LFP+RGO）without carbon coating is only about 40m Ah/g,which requires carbon coating protection.Using ethylene glycol as dispersant,C@LFP was loaded on GO and hydrothermal reduction was performed to prepare C@LFP+RGO（EG）positive electrode material.The high power performance was greatly improved,with a capacity of 93.3m Ah/g under 10C.In addition,the high conductivity C@LFP+RGO（EGL）resistance was further reduced by adding ascorbic acid during the above process,and the lithium ion migration rate was increased seven fold.The specific discharge capacity under 10C is 103.3m Ah/g,which is about 10%higher than that of C@LFP+RGO（EG）,and the capacity retention rate after 50 cycles is 94.7%,which is also significantly higher than that of materials without ascorbic acid added.（3）The above prepared C@LFP+RGO（EGL）was doped with different contents of nitrogen,and melamine was used as nitrogen source.Nitrogen doping on RGO was successfully realized and C@LFP+NRGO（EGL）was prepared.SEM and TEM results show that the addition of nitrogen increases the defects on the surface of particles,and the defects also increase with the increase of doping amount.The optimal nitrogen doping amount was2wt%,and its specific capacity reached about 130m Ah/g under the condition of 10C.The capacity retention rate was 98.8%after 60 cycles at different multiples.The mobility of lithium ions was 27 times higher than that of unadulterated C@LFP+RGO（EGL）.However,after 100 laps,both the resistance increase and the lithium ion migration rate decrease obviously,which also provides the direction for further improvement.Comparing C@LFP+2wt%NRGO（EGL）with commercial lithium iron phosphate,the discharge specific capacity of commercial lithium iron phosphate is very high under the condition of low multiplier,but the capacity attenuation of high multiplier is relatively serious,and the high-multiplier performance of C@LFP+2wt%NRGO（EGL）is better.