Lithium secondary battery having improved electrochemical properties, and method for manufacturing same

1. A lithium secondary battery comprising a positive electrode, a negative electrode, a separation film, and an electrolyte, wherein the negative electrode includes a silicon-carbon composite as a negative active material,

wherein the electrolyte is obtained by adding fluoroethylene carbonate (FEC) and any one selected from the group consisting of vinyl ethylene carbonate (VEC), vinylene carbonate (VC), ethylene carbonate (EC), difluoroethylene carbonate (DFEC), t-butylbenzene, and t-pentylbenzene to a mixture of a lithium salt and an organic solvent,

wherein the silicon-carbon composite includes carbon nano-fibers and silicon particles, the silicon particles coated with amorphous SiO 2 ,

wherein the silicon particles are scattered between the carbon nano-fibers,

wherein a content of the silicon particles is in a range from 3 volume % to 40 volume %,

wherein a weight ratio of the silicon particles to the amorphous SiO 2 is in a range from 1:0.2 to 1:20,

wherein the silicon-carbon composite is obtained by producing a one-dimensional composite by performing electrospinning a mixed solution of a polymer and silicon particles and performing heat treatment on the one-dimensional composite, and

wherein the polymer includes any one or more selected from the group consisting of polyacrylonitrile (PAN), polyvinylalcohol (PVA), polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylic acid, and polyurethane.

2. The lithium secondary battery of claim 1 , wherein the electrospinning is performed at a humidity of 36% or less and at a temperature of 34° C. or less.

3. The lithium secondary battery of claim 1 , wherein the electrospinning is performed using a 17-25 gauge nozzle applied with a voltage of 0.5 kV/cm to 3.9 kV/cm.

4. The lithium secondary battery of claim 1 , wherein the heat treatment includes first heat treatment that is performed at a temperature of 230° C. to 300° C. for one hour to ten hours and second heat treatment that, after the first heat treatment, is performed at a temperature of 500° C. to 900° C. for one hour to seven hours.

5. The lithium secondary battery of claim 4 , wherein the second heat treatment is performed in an atmosphere of a mixed gas of an inert gas and a reducing gas.

6. The lithium secondary battery of claim 1 , wherein the lithium salt is LiPF 6 or LiBF 6 .

7. The lithium secondary battery of claim 1 , wherein the organic solvent includes PC (propylene carbonate), EC (ethylene carbonate), DMC (dimethylene carbonate), DEC (diethylene carbonate), or a mixed solution thereof.

8. An external device including the lithium secondary battery of claim 1 .