Study On Electrode Process And Degradation Mechanism Of Phosphorus-based Anode For Lithium Ion Battery

Recent research on lithium-ion batteries focuses on high energy density electrode materials due to the fact that the driving mileage of electric vehicle is currently limited by the energy density of batteries.Red phosphorus exhibits high theoretical specific capacity of 2596 mAh g^-1and low cost,which is considered to be a promising anode material.However,the poor cycling performance,caused by large volume expansion in the process of alloying with lithium ion and low conductivity of elemental phosphorus,restricts its use as anode material for lithium-ion batteries.In order to improve the cycleablity of red phosphorus,the preparation of red phosphorus-carbon?P/C?composite material was investigated,the studies on electrode process,capacity fading and improvement by electrolyte modification for P/C composite material were carried out in this paper.The main results are as follows:1)Studies on preparation of P/C composite material show that the sample prepared by chemical vapor deposition method display better electrochemical properties than other samples prepared by microemulsion method and physical ball milling method.Its reversible specific capacity is up to 1450 mAh g^-1,and the efficiency is more than 80%in the first cycle.72%of its initial capacity can retain after 50 cycles.That attributes to the fact that the chemical vapor deposition method is beneficial for red phosphorus to deposit more uniform on the surface and pores of the activated carbon,which suppresses the powdering of the active particles and improves the utilization efficiency of red phosphorus.2)Investigation on the electrode process of P/C composite material reveals that the water-based binder such as polyacrylic acid?PAA?,guar gum?GG?and xanthan gum?XG?,with low solubility and electrolyte adsorption rate in electrolyte,ensures good contact of P/C composite material with conductive carbon and current collector,and then provides better charge-discharge performance.Crosslinking esterification reaction of PAA binder with sodium carboxymethyl cellulose?CMC?can enhance the viscosity of the polymer to improve the adhesion of P/C active particles with conductive carbon and the collector,which leads to remarkable improvement of electrochemical property of P/C composite electrode.Its reversible specific capacity reaches 1553 mAh g^-1,and the efficiency is up to 87.63%in the first cycle.85%of its initial capacity can retain after 50 cycles.In addition,the mixture of graphene and carbon black used as conductive agent in electrode,which makes up the voids of the other side,ensure the conductive network is more compact,and thus the electrode not only has the highest conductivity,but also higher compaction density than that used only carbon black as conductive.3)Studies on mechanism of capacity fading and improvement by electrolyte modification for P/C composite material display that the decrease in cyclic capacity of P/C composites can be attributed to the material powdering caused by the substantial expansion and contraction of the phosphorus negative electrode,and the increase in the impedance caused by the decomposition of the surface electrolyte into the SEI film and the active lithium loss.For P/C composite material,adding film-forming agent such as fluorinated ethylene carbonate?FEC?,succinonitrile?SN?into conventional electrolyte?such as 1mol LiPF₆/EC+DEC+EMC?can increase the initial capacity and efficiency in the first cycle,and the capacity retention up to 92%after 50 cycles,showing a significantly improved cycling performance.The modification mechanism was studied by adding 10%FEC electrolyte into electrolyte composed of 1mol LiPF₆/EC+DEC+EMC,and the results indicate that the addition of FEC not only improve the composition of SEI film,improving the transmission rate of lithium ion and inhibiting the rapid polarization growth of the battery,but also suppress the electrolyte decomposition of the loss of active lithium,reducing the battery capacity decay rate.In short,the capacity fading of phosphorus-based anode material is caused mainly by the powdering of the active particles and the decomposition of electrolyte.It is improved remarkably by composing phosphorus into the porous carbon,modifying the binder and adding the film-forming agent in the electrolyte in this study.The above research results have important theoretical and practical value for the development,production and application of high specific energy lithium ion batteries.