Study On The Preparation Of Cobalt-based Chalcogenides And Their Sodium Storage Performance

Cobalt has variable valence,so the variety of cobalt-based materials is diverse.Cobalt sulfides/selenides are widely regarded to be promising anode materials for next-generation rechargeable SIBs owning to its competitive theoretical specific capacity.Nevertheless,the severe volume expansion of cobalt sulfides/selenides materials during the process of repeated sodium ion insertion/extraction results in the electrode material structure to be destroyed or even pulverized,which accelerates the rapid decay of battery capacity and further impedes their practical applications in energy storage.To tackle these issues,three methods mainly have been developed,such as morphology control,composite modification and heteratom doping.At present,there are still some problems in the research progress of cobalt sulfides/selenides materials.For one thing,the synthetic condictions are harsh including high energy consumption;needing special atmosphere;high requirements for technical equipment;complex synthetic process.For another,many defects exist in the product itself including coated cobalt sulfides/selenides with larger size;wide size distribution;uneven distribution with carbon materials.In order to solve the above problems,this paper has carried out the following three works.（1）By directly grinding o-phenylenediamine,o-vanillin,cobalt acetate tetrahydrate and selenium powder at ambient temperature,a self-assembly solid state reaction happened to give rise to a bis-Schiff base complex with cobalt（II）,which was evenly distributed in the selenium powder surroundings.After subsequent annealing,ultrafine Co_0.85Se nanoparticles encapsulated within the irregular N-doped carbon scaffold(denoted as Co_0.85Se?NC)were readily in-situ fabricated.Through the solid-state reaction,the Schiff base formed by o-vanillin and o-phenylenediamine is complexed with cobalt ions to achieve atomic level confinement and uniform dispersion,which is beneficial to execute subsequent in-situ thermal reduction process and obtain ultrafine Co_0.85Se nanoparticles uniformly embedded in N-doped carbon materials.The Co_0.85Se?NC composite exhibits longer cycle life and higher rate performance.The reversible specific capacity was 194.3 m A h g^-1,after 500 cycles at the current density of 0.1 A g^-1,and the capacity retention rate was 62.9%.The average charge capacities at current densities of 5 A g^-1 was 154.3 m A h g^-1,and the average capacity retention at5 A g^-1 vs 0.1 A g^-1 is as high as 50.6%.（2）By directly grinding o-phenylenediamine,o-vanillin,cobalt acetate tetrahydrate and selenium powder at ambient temperature,a self-assembly solid state reaction happened to give rise to a bis-Schiff base complex with cobalt（II）,which was evenly distributed in the sulfur powder surroundings.Then,the Co S nanoparticle was uniformly implanted in N,S-doped carbon matrix（denoted as Co S?NSC）acquired by subsequent sulfurization and carbonization processes.The active materials of the composite obtained by this work was also uniformly encapsulated within the irregular heteroatom-doped carbon scaffold.During the heat treatment,nitrogen atoms contained in the precursor are doped into the carbon skeleton.Meanwhile,part of the sulfur atoms are also doped into the carbon skeleton,providing more heteroatom doping.The doping of the sulfur atom not only further improves the electronic conductivity of the conductive material,but also provides more active sites for sodium ions storage,causing the Co S?NSC material to exhibit better cycle performance and rate performance.Benefiting from the advantageous nanoparticles embedded architecture,the Co S?NSC composite delivers long-term cycling stability(306.5 m Ah g^-1 after 400cycles with a capacity retention of 63.7%at 0.1 A g^-1)and excellent rate performance(the average charge capacities of 464.7 m A h g^-1 at 0.1 A g^-1 and 263.3 m A h g^-1 even at 5 A g^-1 and the average capacity retention at 5 A g^-1 vs 0.1 A g^-1 is as high as 56.7%).（3）The target materials produced in the first two works were hard,which made it difficult for grinding,fabrication of electrodes and the penetration of the electrolyte.In addition,cobalt source was rare and cobalt salts was relatively high-cost.In order to address the problems,we partially replaced cobalt acetate tetrahydrate with anhydrous ferric acetate and kept the same feed ratio of other materials.A mixture of Schiff base complex of cobalt/iron and sulfur powder was obtained by solid-state reaction.N,S-codoped carbon material uniformly coated Fe Co S₂ nanoparticle composite material was obtained by subsequent in-situ thermal reduction（denoted as Fe Co S₂?NSC）.Half of Fe²⁺replaces half of Co²⁺in Co S to form a new complete solid solution Fe Co S₂.Compared with single metal sulfides,double metal sulfides have complementary synergistic effects.Fe Co S₂?NSC composite material has a softer texture,which is beneficial for making electrode materials,and also for the electrolyte materials to fully wet.Fe Co S₂?NSC,as an anode material for sodium ion batteries,shows high specific capacity and good rate performance.In the initial cycle,the charge and discharge capacity of the Co_0.85Se?NC composite at current densities of 0.1 A g^-1 are determined to be 486.4/739.9 m A h g^-1,corresponding to an initial coulombic efficiency（ICE）of65.7%.The average charge capacities at current densities of 0.1 and 5 A g^-1 are 415.7and 146.0 m A h g^-1.Compared with the specific capacity at a current density of 0.1 A g^-1,the average charge capacity retention at a current density of 5 A g^-1 was 35.1%.Moreover,the reversible specific capacity was 310.4 m A h g^-1,after 300 cycles at the current density of 0.1 A g^-1,and the capacity retention rate was 69.3%.