Copper Current Collector With Tin-Based Protective Layer For Sodium Metal Anode

Lithium-ion batteries have been developed rapidly in recent years,but limited lithium resources and the steadily rising price of lithium battery components will restrict the lager scale application of Li-ion batteries in near future.In the meantime,Na batteries have aroused great interest due to the natural abundance and wide availability of sodium element.Na metal anode is one of the most promising anodes for sodium batteries,owing to its low electrochemical potential and high theoretical specific capacity.However,nonuniform Na ion flux induced by inhomogeneous solid electrolyte interface usually leads to the formation of mossy or dendritic Na,which gives rise to the low Coulombic efficiency and potential safety hazards.In this work,in order to inhibit the dendrite formation of sodium metal anode and to improve the cycle stability and safety of the batteries,copper current collector with tin-based protective layer was fabricated and its electrochemistry performances of sodium metal batteries were investigated.Herein,a well-designed artificial protective layer consisting of PVDF and Sn nanoparticles was constructed on Cu current collector?PSN@Cu current collector?by a scalable and facile doctor blade coating technology.Flexible PVDF matrix in the protective layer can accommodate interface fluctuation,while sodiophilic Sn nanoparticles significantly reduce nucleation overpotential for uniform Na nucleation,and provide high ionic conductivity for uniform deposition/stripping and high mechanical modulus against potential dendrite growth.In this paper,we firstly optimized the proportions of Sn and PVDF,and found that the optimal ratio of Sn to PVDF is 4:6.Subsequently,detailed physical and electrochemical performance tests were conducted on the PSN@Cu current collector.These results show that an PSN protective layer with enhanced Young's modulus?5.48 GPa?was achieved due to the high mechanical strength of Sn?⁵0 GPa?,which could mechanically suppress the formation of sodium dendrites..In addition,the interface resistance of Cu current collectors?256.6??is more than ten times that of PSN@Cu current collectors?24.48??after the 100th cycles.In the Coulomb efficiency test,a high average CE of 99.73%for 2800 h at 2 mA cm^-2 was achieved on PSN@Cu current collector.Furthermore,the PSN@Cu current collector can deliver a stable cycling lifetime for 2300 h at 1 mAh cm^-2,which increases tenfold compared to that of bare Cu current collectors?²20 h?.The PSN@Cu current collector exhibits excellent Coulombic efficiency,and its long cycle stability has outperformed those reported in the literature.This approach highlights the significance of hybrid protective layer with synergistic properties and opens a new opportunity to stabilize Na metal anodes.At the end of this paper,we investigated the Na deposition behavior of PSN@Cu current collectors in order to further explore the reasons for the excellent performance of the Na metal anode.The PSN@Cu current collector has a very small sodium nucleation overpotential?9 mV?originating from the sodiophilic nature of Sn at the first sodium deposition cycle.Also,Sn nanoparticles can bring a higher Na ion diffusion coefficient to the PSN@Cu current collector,which is 39.9 times that of commercial Cu current collectors,thus leading to a smooth Na nucleation and deposition.At the same time,the top surface of SEI on PSN@Cu current collector consists of RCOONa,RCH₂ONa,Na₂O and NaF,while the interior SEI is dominated by PSN layer,Na₂O,Na_2-xO₂ and NaF.Abundant inorganic products make SEI more compact and impenetrable,favorably protecting the underlying Na metal anode from further reacting with electrolytes.These result fully that the modification of the copper current collector by the PSN can effectively protect the sodium metal anode,which corresponds to excellent battery performance.