Polymer graphite composite anodes for Li-ion batteries

Transcription

1 Polymer graphite composite anodes for Li-ion batteries Basker Veeraraghavan, Bala Haran, Ralph White and Branko Popov University of South Carolina, Columbia, SC Plamen Atanassov University of New Mexico, Albuquerque, NM 87131

2 Problem Definition Electrolyte decomposition Solvated lithium intercalation and reduction Irreversible reactions lead to Losses in capacity / active lithium material Lowers cell energy densities, increases cell cost Previous approaches Modification to the electrolyte Addition of SO 2, CO 2 Other solvents like DMPC Modification to the electrode Mild oxidation Coating with Ni, Pd

3 Objectives To prepare PPy/C composite which will reduce the initial irreversible capacity To improve the conductivity and the coulombic efficiency of the electrode To obtain material with better rate capability and good cycle life Approach Produce a matrix of PPy which forms a conducting backbone for the graphite particles by in-situ polymerization

4 Experimental Preparation of PPy/Graphite composites Dropwise addition of pyrrole into aqueous slurry of graphite at 0 C with nitric acid acting as an oxidizer for 40 h Wash repeatedly with water and methanol and vacuum dried at 200 C for 24h Cell Preparation for 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

5 Experimental (Cont d.) Electrochemical characterizations Charge-discharge and cycling behaviors Arbin Battery test system used for the testing Cycling was performed between 2V and 5 mv at C/15 rate (0.25 ma/cm 2 ) Cyclic Voltammetry CVs were performed from 1.6V to 0.01V at 0.05 mv/s Electrochemical Impedance Spectroscopy (EIS) 100kHz to 1mHz with 5mV PP signal Physical characterizations SEM micrographs TGA and BET analysis

6 TGA analysis of polymer composite SFG10 samples Weight Percent (%) Bare 5% PPy 6% PPy 7.8% PPy 8.4% PPy PPy Temperature

7 Charge-discharge curves of polymer composite SFG10 samples 4.0 Potential (V vs Li/Li + ) Bare 5% polymer 6% polymer 7.8% polymer 8.4%polymer Specific Capacity (mah/g)

8 Change in irreversible capacity loss with PPy loading at C/15 rate Amount of PPy loading (wt%) Initial lithiation capacity (mah/g) Initial delithiation capacity (mah/g) Overall irreversible Capacity (%) Initial coulombic efficiency (%)

9 Comparison of surface area and capacity for polymer composite electrodes Amount of PPy loading (wt%) Reversible Capacity (mah/g) Specific Surface area (m 2 /g) Volumetric Surface area (m 2 /cm 3 ) Volumetric Capacity (mah/cm 3 )

10 Cyclic voltammograms of polymer composite SFG10 samples Specific Current (ma/g) Bare 5% PPy 6% PPy 7.8% PPy 8.4% PPy Potential ( V vs Li/Li + )

11 SEM pictures of polymer composite SFG10 samples 10 µm 10 µm Bare PPy/C

12 Impedance studies of polymer composite SFG10 samples Imaginary Z (Ω-g) Bare 5% polymer 6% Polymer 7.8% Polymer 8.4% Polymer Real Z (Ω-g)

13 Equivalent circuit used to fit the experimental data C 1 C 2 R Ω DPE 1 DPE 2 R 1 R 2 R 1 SEI layer resistance R 2 Polarization resistance R Ω ohmic resistance C 1 SEI layer capacitance C 2 Double layer capacitance

14 Equivalent circuit parameters for polymer composite electrode Sample R Ω (ohm) R 1 (ohm) C 1 (Farad) R 2 (ohm) C 2 (Farad) Bare x x10-6 5% PPy x x10-6 6% PPy x x % PPy x x % PPy x x10-6

15 Comparison of coulombic efficiencies for SFG10 samples Coulombic effieiency (%) % PPy Bare Cycle number

16 Rate capability studies of composite SFG10 samples 400 Specific Capacity (mah/g) % polymer Bare 0 C/15 rate C/6 rate C/3 rate C rate C/15 rate Cycle number

17 Cycle life studies of composite SFG10 samples 400 Specific Capacity (mah/g) % PPy Bare Cycle number

18 Charge-Discharge curves of polymer composite SFG10-15% sn samples 4.0 Potential (V vs Li/Li + ) SFG10-15%Sn 15% Sn-PPy Specific Capacity (mah/g)

19 Comparison of irreversible capacities for bare and polymer composite SFG10 samples Sample Initial lithiation capacity (mah/g) Initial delithiation capacity (mah/g) Irreversible capacity (%) Initial coulombic efficiency (%) Bare Bare-PPy % Sn % Sn-PPy

20 Conclusions Polypyrrole on SFG10 graphite results in high performance anodes for use in Li-ion batteries Irreversible capacity is reduced up to 7.8% PPy composite Charge discharge studies are supported by CV data Reduction in irreversible capacity seen during cathodic scan Polymer composite anodes show better conductivity and lower polarization resistance compared to virgin carbon Polymer composite anode show better rate capability and longer cycle life

21 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 multiprogram laboratory operated by Sandia corp., a Lockheed Martin Company, for the U.S. Dept. of Energy under Contract DE-AC04-94AL85000.)