High Capacity Graphite Anodes for Li-Ion battery applications using Tin microencapsulation Basker Veeraraghavan, Anand Durairajan, Bala Haran Ralph White and Branko Popov University of South Carolina, Columbia, SC and Ronald Guidotti Sandia National Laboratories Albuquerque, NM

Introduction  Graphite has good cycle life but low theoretical capacity (372 mAh/g)  Tin has high theoretical capacity (991 mAh/g)  Tin based anodes have poor cycling characteristics due to density changes of Tin  Reducing the Sn particle size may mitigate the problem

Objectives  To obtain an anode material with high specific capacity, better rate capability and good cycle life  To use electroless deposition for preparing Sn-C composites and to optimize the deposition conditions  To optimize the Sn loading on graphite based on discharge characteristics  To study the effect of Sn loading on the electrochemical performance of the composite

 Preparation of Sn/Graphite composites  Electroless deposition of Sn using hypophosphite bath  pH-10 (using NaOH) and T-50  C  Cell Preparation for testing  1/2” T-cells used for electrochemical testing  Electrodes prepared by cold rolling using PTFE binder (10wt%)  Whatman fiber used as separator and Li-foil used as counter and reference electrode  1M LiPF 6 in EC/DMC (1:1 v/v) used as electrolyte Experimental

Experimental (Cont’d.)  Electrochemical characterization  Charge-discharge and cycling behavior  Cycling was performed between 2V and 5 mV at C/15 rate (0.1mA/cm 2 )  Electrochemical Impedance Spectroscopy (EIS)  100kHz to 1mHz with 5mV sinusoidal signal  Cyclic Voltammetry  CVs were performed in the potential range 1.6V to 0.01V at 0.05 mV/s  Physical characterization  SEM, EDAX and XRD

SEM images of bare and 15% sn-coated SFG10 samples 15% SnBare 10  m

EDAX studies of bare and 15% sn-coated SFG10 samples Bare 15% Sn

XRD analysis of 15% sn-coated SFG10 samples as a function of heat treatment temperature

Charge discharge studies of 15% sn-coated SFG10 samples as a function of heat treatment temperature

Comparison of charge-discharge curves of bare and 15 wt% sn-coated graphite.

Charge-Discharge curves of bare and sn-coated SFG10 samples

Percentage increase in reversible capacity as a function of composition of sn

Utilization of sn in the coated samples as a function of the composition of tin SampleReversible Capacity (mAh/g) Capacity due to Sn (mAh/g) Utilization of Sn 1 (%) Specific Surface area (m 2 /g) Volumetric Surface area (m 2 /cm 3 ) Volumetric Capacity (mAh/cm 3 ) Bare 5% Sn 10% Sn 15% Sn 20% Sn Utilization of tin = (Capacity due to tin/weight of tin in the composite)/Theoretical capacity of tin (991 mAh/g)*100

Impedance plots for the bare and sn-coated SFG10 samples at fully discharged state

Cyclic Voltammograms of bare and 15% sn coated SFG10 samples for the reversible cycle

Cycle life studies of bare and 15% sn coated SFG10 samples at C/15 rate

Rate Capability studies of bare and 15% sn coated SFG10 samples

Conclusions  Tin encapsulation on SFG10 graphite results in high performance anodes for use in Li-ion batteries  Reversible capacities are improved upto 15% Sn, relative to bare graphite  Cycle life of the bare graphite is improved on Sn-encapsulation  The optimum heat treatment temperature was found to be 200  C  Crystallinity increases with temperature  Sn-C based anodes show better conductivity and lower polarization resistance compared to virgin carbon  Addition of Polypyrrole reduces irreversible capacity and further studies need to be done to optimize the amount of polypyrrole

Acknowledgements This work was funded by the Dept. of Energy division of Chemical Science, Office of Basic Energy Sciences and, in part, by Sandia National Laboratories (Sandia National Laboratories is a multi- program laboratory operated by Sandia corp., a Lockheed Martin Company, for the U.S. Dept. of Energy under Contract DE-AC04- 94AL85000.)