1 / 19

Conductive Lithium Iron Phospho-olivines for Improved Electrochemical Performance

This study focuses on the synthesis and characterization of electronically conductive lithium iron phospho-olivines (Li1+xFe1-xPO4) with enhanced electrochemical performance. The approach involves increasing the electronic conductivity through the preparation of mixed valence iron. The results show that the doped Li1+xFe1-xPO4 samples exhibit p-type conductivity and improved rate capability compared to undoped LiFePO4.

afitch
Download Presentation

Conductive Lithium Iron Phospho-olivines for Improved Electrochemical Performance

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Li1+xFe1-xPO4: Electronically ConductiveLithium Iron Phospho-olivines with Improved Electrochemical Performance

  2. Outline • Background • Approach to improved electronic conductivity • Synthesis • Conductivity Measurements • Discussion of doping mechanism • Electrochemical study • Conclusions

  3. LiFePO4 Discharge reaction should have large negative Gibbs free energy: high discharge voltage—3.4 V  Host structure must be light and intercalate large amounts of Li high energy capacity—170 mAh/g Host structure must be lithium ion conductor  Structural modification during intercalation/deintercalation must be minimal—LiFePO4 and FePO4 are isotypic  Stable, non-toxic, inexpensive Electronic conductor

  4. Electronic Conductivity  Mixed ionic / electronic conductivity is needed  Diffusion will be rate-limited by slower species Electronic conductivities: LiCoO2 ca. 10-3 S cm-1 LiMn2O4 ca. 3 x 10-5 S cm-1 LiFePO4 1 x 10-9 to 1 x 10-10 S cm-1 Research Objective: Raise electronic conductivity of LiFePO4

  5. Approach Increase number of charge carriers through preparation of mixed valence iron σ = n e µ where σ conductivity n number of charge carriers e charge µ mobility of charge carriers Metals: lots of charge carriers, increase conductivity by lower T Semiconductors: few charge carriers, increase conductivity with higher T or doping

  6. Approach Substitution of lithium onto iron site may create mixed valence iron:Li1+xFe1-2x Fe3+(PO4) In point defect notation: 2+ x [ ] ] [  ' Li = h Fe

  7. LiFePO4 showing Fe, P polyhedron PO4 Li M1 site (Li) M2 site (Fe) Both are Octahedral MO6 polyhedron FeO6

  8. Synthesis of doped LiFePO4 Synthesis of Li1+xFe1-xPO4 x = 0.02, 0.035, 0.05 and 0.07) x/2 Li2CO3 + (1-x) FeC2O4.2H2O + NH4H2PO4 Li1+xFe1-xPO4 1. Jar Mill to mix and pulverize Heat at 350oC for 12 h under N2 atmosphere 2. Mix again in jar mill Pelletize Heat at 800oC for 14 h under 2.5%H2 / N2 atmosphere

  9. XRD of Li1+xFe1-xPO4 x = 0.02, 0.035, 0.05

  10. Lattice Constants: Slight Contraction of a lattice Constant Coordination Number 6: Li+ (0.90Å), Fe2+(0.92Å RD. Shannon, Acta Crystallogr. A32, 751 (1976)

  11. Doped Samples Are Black Li1.05Fe0.95PO4 LiFePO4

  12. Conductivity Measurment Direct measurement of resistivity leads to error owing to contact resistances R1 and R2 R1 R3 R2 In a 4-probe measurement, current is forced across probe 1 to probe 4 and the voltage drop is measured across probes 2 and 3. The resistance is is calculated from Ohm’s law

  13. Conductivity as a Function of Doping

  14. Determination of the Sign of Charge Carrier cold hot Thomson Effect n-type: cold end becomes negatively charged p-type: cold end becomes positively charged-as more electrons are promoted from the valence band to the acceptor levels at the hot end than the cold end and some of these flow to the cold end making it positively charged Observation: positive voltage at cold end—p-type

  15. Doping Mechanism • Dopant concentration dependency • p-type conductivity • Nominal stoichiometry Li1+xFe1-xPO4 Observations are in agreement with • Li1+xFe1-2x Fe3+(PO4) 2+ x

  16. Typical Charge/ Discharge Curves

  17. Voltage vs. capacity of Li/ Li1.05Fe0.95PO4 as a function of rate

  18. Capacity as Function of Composition and Rate Enhanced Rate Capability Relative to Undoped LiFePO4

  19. Conclusions Li1+xFe1-xPO4 samples have been prepared Evidence suggests p-type conductivity in agreement with lithium substitution onto the iron site Enhanced Rate Capability was shown relative to undoped LiFePO4

More Related