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I5.04 - Fabrication of an Embedded Free-Standing Anode by Structure-Controlled Carbon Nanofibers for Solid-State Li-ion Batteries 
April 8, 2015   2:30pm - 2:45pm

Stress originated from volumetric expansion (~300 %) during cycling is notorious in metal alloy based anode materials including Sn (994 mAh/g). Nanoscale Sn embedded buffer matrix is an ideal structure for high performance anode of solid state Li-ion batteries. There were previous attempts to utilize 1D carbon (C) structures as a buffer matrix with transition metal alloying or atomic layer deposition (ALD) coating. However, the drawbacks of them were requirement of electrochemically unreactive materials related with low capacity and additional post treatments. Here, we developed a novel fabrication method of ideal Sn/C 1D hybrid nanostructures via porosity controlled C nanofibers. For precise modulation of Sn size and dispersion, we considered Sn diffusion originated from thermal expansion coefficient (CTE) difference between Sn (23.5�10-6 /�C) and C (1.5�10-6 /�C) during calcination. Through this fabrication method, superior anode performance was realized without current collector, binder and conducting additives. Electrospinning proceeded with a solution of Sn acetate (SnAc) as metal precursor and polyacrylonitrile (PAN) as polymer matrix. Calcination proceeded at 700 oC, 5 h with the ambient controlled by high vacuum (HV) and Ar gas. The porosity was evaluated by BET specific surface area (SSA) and total pore volume of N2 adsorption/desorption isotherm. Structures were analyzed by FE-SEM and TEM. Electrochemical performance of Sn/C nanofibers was measured in all solid-state Li-ion cell. We modulated Sn diffusion by controlling the porosity of C nanofiber matrix and designed 3 calcination schemes as stabilization + Ar (SA) calcination, HV calcination, and Ar calcination. Stabilization accelerated the decomposition of polymer matrix via dehydration. Furthermore, HV and Ar calcination showed differences in gas-solid reactions between CO (g), CO2 (g) and C nanofibers. In porosity measurement, SA calcination showed the highest BET SSA as 98.97 m2/g and Ar calcination showed the lowest as 10.46 m2/g. In accordance with the porosity tendency, Sn size and dispersion were controlled well. SA calcination with highest porosity formed a structure of all Sn agglomerates outside C nanofibers. HV calcination with moderate porosity induced coexistence of outside Sn agglomerates and inside Sn nanoparticles. Finally, Ar calcination with lowest porosity induced 15 nm sized Sn nanoparticles fully embedded C nanofibers. For the first time, we successfully revealed a key parameter of controlling the structures of Sn/C nanofibers. Interestingly, all Sn/C nanofibers showed ohmic contact behavior, and we demonstrated all solid-state Li-ion cell with Sn/C nanofibers as an anode without current collector. In charge/discharge cycling performance, Sn fully-embedded C nanofibers showed superior capacity of 762 mAh/g with Coulombinc efficiency over 99.5 % during 50 cycles. Intimate relationship between Sn structures and electrochemical performances is discussed.

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