High-performance Sodium Metal Anode Developed Using Mineral Matrix

As a novel type of secondary battery with high resource abundance,low cost,long life and high safety performance,sodium ion batteries are expected to considered as a complementary or effective alternative to lithium ion batteries in applications such as portable electronics and electric vehicles.Recently,the National Development and Reform Commission as well as the Ministry of Science and Technology clearly proposed to support the research of high energy density sodium ion batteries energy storage technology in the"14th Five-Year"Renewable Energy Development Plan.Sodium metal is regarded as the ultimate option for the anode of sodium ion batteries due to the high theoretical specific capacity(1166 mAh g^-1),low redox potential（-2.71 V vs.standard hydrogen electrode）.However,the formation and uncontrolled growth of sodium dendrite can puncture the separator and lead to short-circuiting of the battery,raising safety issues.Moreover,sodium metal has high reactivity and a tendency to react with electrolyte,while it causes large volume expansion during plating/stripping and tends to generate unstable solid electrolyte interface（SEI）,leading to low coulombic efficiency and poor cycling performance.In order to construct sodium metal batteries with excellent performance,it is required to improve the sodium deposition kinetics by homogeneous electric field distribution on the surface,starting from the plating/stripping morphology of sodium anodes to achieve dendrite free sodium anodes.Based on the Engineering Research Center of the Ministry of Education for Nano Mineral Materials and Applications of China University of Geosciences,this thesis utilizes the unique structure of minerals（montmorillonite and gypsum）to optimize the sodophilic properties,and constructs mineral/sodium metal composite anode and mineral-modified separators,which can reduce the nucleation potential and uniform nucleation of sodium metal,thus improving the coulombic efficiency and cycling performance,and finally assembling high-performance sodium metal batteries.The research for this thesis is summarized as follows:（1）The Na@Na-MMT composite anode is prepared by directly embedding the highly sodophilic Na⁺-intercalated montmorillonite（Na-MMT）into the sodium metal through a simple mechanical-rolling method.In this composite electrode,the Na-MMT is highly dispersed into the whole sodium electrode,which affords a stable sodium-ion-conductor interface to provide faster diffusion channels and effectively lower the nucleation potential of Na,thus guiding the uniform deposition of sodium.By optimizing the mass ratio between sodium metal and montmorillonite,the as-prepared Na@Na-MMT anode exhibits superior plating/stripping reversibility(over 600 h at 1 mAh cm^-2)and fast electrochemical kinetics(200 h at 4 mA cm^-2 with a capacity of 4 mAh cm^-2)in a carbonate-based electrolyte,which outperforms the performance of Na@Ca-MMT and bare Na anodes.In addition,Na@Na-MMT//Na₃V₂（PO₄）₃ sodium metal batteries exhibit excellent long cycling stability（86%capacity retention for 1000 cycles at 2 C）and good rate capability(providing a specific capacity of 45 mAh g^-1 at 30 C).（2）A simple method of methanol-assisted ball-milling is proposed to transform micron-sized gypsum（CaSO₄·2H₂O）into bassanite nanocrystal（CaSO₄·0.5H₂O,designated as Nano BA）.This material has been confirmed to possess excellent sodophilic properties,which can reduce the nucleation and deposition potential of sodium and effectively improve the ions migration kinetics.Based on this,the Na@Nano BA electrode prepared by mechanical complexation of Nano BA with sodium metal exhibited superior cycling performance(1200 h of stable cycling at 1 mA cm^-2,1 mAh cm^-2 and2000 h of stable cycling at 4 mA cm^-2,4 mAh cm^-2).Finally,the sodium metal battery assembled with Na@Nano BA as the anode and NVP as the cathode can stabilize 800cycles at 3 C with no capacity decay.（3）Based on the sodium-ion-conductor properties of Na-MMT and the sodophilic functional group of polyacrylonitrile（PAN）,we combined the two materials through an electrospinning technology,which is expected to yield a highly oriented and bifunctional composite nanofibrous separator（PAN/MMT-A）.This separator exhibits high electrolyte adsorption,ionic conductivity,abundant pores and thermal stability,and this unique highly aligned composite nanofibrous structure facilitates the regulation of Na⁺flux distribution and directs the uniform sodium deposition,while blocking sodium dendrite punctures at the physical level.The PAN/MMT-A separators-based symmetrical cells exhibit ultra-high fast kinetics by maintaining a stable cycling of 10.2 mV overpotential at a low current density of 1 mA cm^-2 for 2400 h and at a high current density of 4 mA cm^-2 for 1600 h.In addition,the corresponding sodium metal batteries exhibit long cycling performance（600 cycles at 3 C without capacity decay）and excellent rate performance（stable cycling at 30 C）.