Synthesis And Electrochemical Characterization Of Li₂MnSiO₄@C Nano-composite

The performance of the cathode material is the key factor which effects the performance of lithium-ion batteries.Improving the performance of the cathode material and exploring new cathode materials are the main direction in the field of lithium-ion batteries research.Polyanion-type lithium orthosilicate cathode material Li₂MnSiO₄ has drawn widespread attention because of its advantages of high theoretical specific capacity(³33 mAh g^-1),low cost,stable chemical structure and environmental friendliness.However,disadvantages such as the problem in obtaining a pure product,poor electronic conductivity(¹0^-16 S cm^-1)and low lithium ion diffusion coefficient(¹0^-16 cm2 s-1)obstruct its application.To obtain a Li₂MnSiO₄ with excellent property,some modifications are necessary.In this paper,using oleic acid as both surfactant and carbon source,we prepared the ultrafine Li₂MnSiO₄@C nanoparticles by employing a simple method,the nonstoichiometric Li₂Mn_1-xSiO₄@C?x=0.05,0.1,0.2,0.25?and Li₂Mn_1-xMg_xSiO₄@C?x=0.1,0.2?.The main results are as follows:?1?Ultrafine Li₂MnSiO₄@C nanoparticles: by changing the amount of oleic acid?the molar ratio of oleic/Li₂MnSiO₄@C are 1:1,2:1 and 3:1?,we synthesized three Li₂MnSiO₄@C samples with different carbon contents.All the three samples were composed of 10-20 nm uniformly spherical-like particles.With the amount of oleic acid increasing,the particle size of Li₂MnSiO₄ and the carbon content gradually increased,but the amount of MnO impurity gradually decreased.Electrochemical test showed that,when the molar ratio of oleic and Li₂MnSiO₄@C was 2:1,the first discharge capacity reached 313 mAh g^-1 at a rate of 0.05 C,closing to the theoretical capacity.After 50 cycles,the discharge capacity of the sample remained 154.7 mAh g^-1,showing an excellent cycling performancerate and a good rate capability.So when the oleic acid is used as both surfactant and carbon source to synthesize Li₂MnSiO₄ nanoparticles,the best molar ratio of oleic acid and Li₂MnSiO₄ is 2:1.So in the following processes,the molar ratio of oleic acid and Li₂MnSiO₄ was fixed in 2:1.?2?Nonstoichiometric Li₂Mn_1-xSiO₄@C?x=0.05,0.1,0.2,0.25?: by changing the amount of Mn source,four samples,including Li₂Mn₀.95SiO₄@C,Li₂Mn₀.9SiO₄@C,Li₂Mn₀.8SiO₄@C and Li₂Mn₀.75SiO₄@C were synthesized by a simple method.XRD results showed that the main crystalline phase in Li₂Mn₀.95 SiO₄@C and Li₂Mn₀.9SiO₄@C was MnO.Li₂Mn₀.8SiO₄@C and Li₂Mn₀.75SiO₄@C were composed of 15-20 nm irregular spherical-like particles.The amount of Mn did not obviously influence the morphology,the particles size or the aggregation state of Li₂MnSiO₄ nanoparticles,but it could affect the preferential growth direction of the final production.Both the two samples showed mesoporous structures and contained a small amount of micropores,and Li₂Mn₀.75SiO₄@C had a larger specific area.The results displayed that it was more conducive to permeate electrolyte for Li₂Mn₀.75SiO₄@C,so it could achieve a better electrochemical performance.The results of electrochemical test showed that,the first discharge capacity of the two samples are all closed to their theoretical capacity.Though underdoing some capacity fading in the cycling,the two samples still remained high capacities,and especially the Li₂Mn₀.75SiO₄@C remained a discharge capacity of 163.4 mAh g^-1 after 50 cycles.Electrochemical impedance spectroscopy showed that Li₂Mn₀.75SiO₄@C had a lower charge transfer resistance.The better performance of Li₂Mn₀.75SiO₄@C could be attribute to its better crystallinity,more defect in its structure,larger specific area,smaller particle size,and lower inner resistance.?3?Mg doped Li₂Mn_1-xMg_xSiO₄@C?x=0.1,0.2?: by changing the amount of Mg,two samples including Li₂Mn₀.9Mg₀.1SiO₄@C and Li₂Mn₀.8Mg₀.2SiO₄@C were synthesized by a simple method.The XRD results showed that both the two samples were good crystallinity,but they both contained some MnO.When the amount of Mg doping were 0.2,the sample also had an impurity phase of Mg₁.03Mn₀.97SiO₄,so the amount of Mg should not be too much.The two samples were both composed of 15-25 nm particles.The mapping images showed that the element on the sample surface were uniformly distributed,and the molar ratio of Mg/Mn was close to theoretical value.The BJH result showed that the two samples were mainly mesoporous structure and contained a small amount of micropore simultaneously,and Li₂Mn₀.9Mg₀.1SiO₄@C had a larger specific area.The results of electrochemical test showed that,the first discharge capacities of the two samples were all closed to their theoretical capacity.After 50 cycles,the discharge capacities of Li₂Mn₀.9Mg₀.1SiO₄@C and Li₂Mn₀.8Mg₀.2SiO₄@C were 131.8 mAh g^-1 and 123.5 m Ah g^-1,respectively.