Design And Preparation Of High-performance Black Phosphorus-based Sodium Storage Material

Sodium-ion battery is expected to replace lithium-ion battery as a large-scale electrochemical energy storage device due to its abundant reserves of raw materials,low cost and high safety.The main bottleneck hindering the development of sodium-ion batteries is their low energy density.Black phosphorus(BP)is the most stable phosphorus allotrope,which has the highest theoretical specific capacity(2596 mAh·g1)and reasonable discharge/charge potential among the cathode materials of sodium ion batteries,and is expected to greatly improve the energy density of sodium ion batteries.However,single black phosphorus is prone to volume expansion in the process of sodium storage,which leads to the problems of poor cycling performance and large voltage hysteresis.The volume expansion of black phosphorus can be alleviated by nanocrystallization and composite with other highly conductive materials,so that it has high specific capacity and excellent cycling performance.However,most battery materials with high specific capacity have the phenomenon of voltage hysteresis,which will greatly reduce the energy storage efficiency of the battery and is difficult to be applied in practice.In this paper,a test method for sodium storage performance of black phosphorus based materials was first established.On this basis,by determining the better carbon source,regulating the chemical composition of composite materials and optimizing the structure,high performance black phosphorus based sodium storage anode materials were prepared.Detailed research contents are as follows:(1)The black phosphorus/graphite(BP/G)composite was prepared by the mineralized black phosphorus as raw material and the graphite ball grinding.Using polyvinylidene fluoride(PVDF)and sodium carboxymethyl cellulose(NaCMC)as binder,1MNaPF6/(EC:DEC=1:1 Vol%,10%FEC)and 1MNaPF6/(PC:DEC=1:1 Vol%,10%FEC)as electrolyte,BP/G composite materials were prepared into electrode sheets and assembled into button batteries.The effects of binder and electrolyte on sodium storage properties of BP/G composite were studied.The results show that,when NaCMC is used as binder,the dissolution of sodium polyphosphide can be effectively reduced,so as to improve the cyclic performance of black phosphorus based materials(After 50 cycles,the capacity retention rate is 64%).At the same time,as a result of the 1M NaPF6/(EC:DEC=1:1 Vol%,10%FEC)electrolyte can provide higher electrical conductivity,thus making the BP/G showed higher specific capacity(724 mAh·g-1).Therefore,a test method for the sodium storage performance of black phosphorous based materials using NaCMC as binder and 1M NaPF6/(EC:DEC=1:1 Vol%,10%FEC)as electrolyte was established.(2)Using graphite and carbon nanotubes as carbon sources,black phosphorus/graphite(BP/G)and black phosphorus/carbon nanotubes(BP/CNTs)composites were prepared by ball milling method.The mechanism of voltage hysteresis of black phosphorus based materials and its influencing factors were investigated.The results show that the voltage hysteresis of the black phosphorus based material is mainly caused by the asymmetric reaction path.This is because the sodium phosphide in the process of discharge will hinder ion transmission,so that the electrode has a serious polarization phenomenon,away from the original electrode potential,which leads to the reaction path asymmetry in the process of charge/discharge,resulting in voltage hysteresis.Under the condition of the same sodium storage mechanism,the ion and electron transport rate will also affect the voltage hysteresis of black phosphorus based materials.The slower the ion and electron transport rate is,the larger the voltage hysteresis is.In addition,the voltage hysteresis of black phosphorus based materials is mainly affected by phosphorus components,while carbon materials have little effect on the voltage hysteresis.Therefore,combining the market price of the two carbon materials and using graphite as carbon source with lower cost,it is more conducive to the large scale preparation of high performance black phosphorus based sodium storage materials.(3)In order to reduce the voltage hysteresis of BP/G binary material and improve the performance of sodium storage,a three-way battery material of black phosphorus/graphite/Sn24P19.3I8(BP/G/Sn24P19.3I8)was prepared.Firstly,BP/G composite with different structural morphology were prepared by ball milling method and their sodium storage properties were studied.The preparation condition of black phosphorus based materials with better performance was found to be 72 h under ball milling.On this basis,BP/G/Sn24P19.3I8 ternary composite was prepared by adding Sn24P19.3I8,and its sodium storage properties were studied.Results show that the ball mill of 72 h BP/G/Sn24P19.3I8 ternary material in sodium showed high capacity electric(919 mAh·g-1)and the small voltage hysteresis,this is because the Sn24P19.3I8 can modulate the reaction path of black phosphorus,form strong phosphorus tin bond at the same time,reduce the dissolution of sodium poly phosphide.However,the addition of tin will lead to the deterioration of its cycle performance.The new BP/G/BP/G/Sn24P19.3I8 terpolymer was prepared by extending the milling time to 120 h.Its cycling performance was significantly improved,and the capacity retention rate was 88%after 50 cycles at the current density of 0.5 A·g-1.This is attributed to the fact that the reduction of particle size can effectively alleviate the volume expansion phenomenon.