Research On Modification Of Metal Sodium Anode By Self-supported Carbon Fiber Matrix Composites

The sodium metal anode has high theoretical specific capacity(1166 mAh g^-1)and low electrode potential（-2.714 V vs.Standard hydrogen electrode）.Compared with lithium metal,sodium metal has abundant natural resources.Because of its low price,it is attractive to become a high-performance negative electrode for the next generation of metal batteries.Secondary sodium metal batteries matching sodium metal cathode and converted cathode materials have higher theoretical energy density.Conversion-type cathode materials include sulfur,oxygen,bromine,and iodine.In particular,conversion-type cathode materials are mostly cheaper than intercalated cathode materials,which will reduce the production cost of batteries and provide a more advanced solution for large-scale energy storage.However,a series of problems caused by sodium metal anode in the cycle process can’t be ignored.First of all,sodium metal has high reducibility.It is easy to react with the organic electrolyte,forming a solid electrolyte interface（SEI film）,reducing the efficiency of the battery.Secondly,in the process of sodium metal deposition/stripping,uneven electrodeposition will cause volume expansion,resulting in continuous rupture and regeneration of SEI film,and the formation of new SEI film accompanied by continuous participation of electrolyte reaction,which causes the electrolyte to be consumed continuously,reducing the coulombic efficiency of the battery,and eventually causing the battery to fail.Moreover,dendrites generate during the electrodeposition of metallic sodium may puncture the separator,leading to short circuit and serious safety concerns.It is important that restricting the sodium dendrites growth.Based on the above challenges,we carried out the following works:（1）The polymer polyacrylonitrile as carbon source was dissolved in N,N-dimethylformamide solvent.After being prepared into spinning solution,self-supporting NCF carbon fiber material was prepared by the process of electrostatic spinning and high-temperature calcination.Carbon fiber material（NCF）has been used as current collector,guiding Na uniform plating and achieving the effect of inhibiting the growth of sodium dendrites.Within the operable range of the experiment,through the comparison of different calcination temperatures,the material calcined at 900 ^oC is named NCF900,which has the best restriction effect on dendrite growth.High reversibility with an average CE of 99.81%over1200 cycle has been achieved at the current density of 1 mA cm^-2 and specific capacity of 0.5mAh cm^-2.The nucleation overpotential is 10.2 mV.Due to the sodium-affinity of carbon materials,sodium tends to nucleate and grow on NCF900 during electrodeposition.In addition,the three-dimensional network framework can provide enough space for sodium metal deposition,thus reducing the volume expansion concern of sodium anode.The NCF900 has good electrical conductivity,reducing the local current density and improving even sodium deposition.（2）The polyacrylonitrile as carbon source was dissolved in N,N-dimethylformamide solvent,with the addition of appropriate amount of stannous chloride dihydrate metal salt.After the solution was fully dissolved,it was prepared into electrospinning solution.Then,after high temperature calcination,a self-supporting carbon fiber material containing metal nanoparticles was prepared,named Sn/NCF900.Sn/NCF900 combines the good flexibility and mechanical properties of the NCF900 carbon fiber material,while dispersing tiny metal nanoparticles evenly within the carbon fiber.In addition,the Sn metal particles in Sn/NCF900 can form alloy with sodium during the process of sodium electrodeposition,which has a better sodiophilic effect and further reduces the nucleation barrier of sodium.After assembling into a half battery,Sn/NCF900 can cycle for nearly 2000 h under the condition of current density of 1 mA cm^-2and specific capacity of 0.5 mAh cm^-2,with an average coulomb efficiency of 99.85%and a lower nucleation overpotential of 4.9 mV.It is suggested Sn/NCF900 has good inhibition effect on sodium dendrites.（3）Similarly,the polyacrylonitrile as carbon source was dissolved in N,N-dimethylformamide solvent,with the addition of two metal salts,stannous chloride dihydrate and cobalt acetate tetrahydrate.After being prepared into spinning solution,self-supporting Sn-Co/NCF900 carbon fiber material containing metal nanoparticle,was prepared by the process of electrostatic spinning and high-temperature calcination.The Sn-Co/NCF900 carbon fiber is evenly dispersed with tiny Sn,SnO₂ and Sn₂Co metal nanoparticles.Sn metal nanoparticles have good sodiophilic properties.Co dispersed around Sn nanoparticles plays a fence role,which inhibits the agglomeration of tin in the battery cycle and maximizes the sodiophilic effect of tin.Sn-Co/NCF900 can be cycled for more than 3000h at current density of 1 mA cm^-2 and specific capacity of 0.5 mAh cm^-2,with an average coulomb efficiency of 99.91%and the lowest nucleation overpotential of 4.4 mV.Sn-Co/NCF900 has an excellent effect of inhibiting sodium dendrites,which greatly improves the stability of the battery.