Electronic Properties of LiFePO4 and Li doped LiFePO4
The potential use of different iron phosphates as cathodematerials in lithium-ion batteries has recently been investigated.1 Oneof the promising candidates is LiFePO4. This compound has severaladvantages in comparison to the state-of-the-art cathode material incommercial rechargeable lithium batteries. Firstly, it has a hightheoretical capacity (170 mAh/g). Secondly, it occurs as mineraltriphylite in nature and is inexpensive, thermally stable, non-toxic andnon-hygroscopic. However, its low electronic conductivity (~;10-9 S/cm)results in low power capability. There has been intense worldwideresearch activity to find methods to increase the electronic conductivityof LiFePO4, including supervalent ion doping,2 introducingnon-carbonaceous network conduction3 and carbon coating, and theoptimization of the carbon coating on LiFePO4 particle surfaces.4Recently, the Li doped LiFePO4 (Li1+xFe1-xPO4) synthesized at ARL hasyield electronic conductivity increase up to 106.5 We studied electronicstructure of LiFePO4 and Li doped LiFePO4 by synchrotron based soft X-rayemission (XES) and X-ray absorption (XAS) spectroscopies. XAS probes theunoccupied partial density of states, while XES the occupied partialdensity of states. By combining XAS and XES measurements, we obtainedinformation on band gap and orbital character of both LiFePO4 and Lidoped LiFePO4. The occupied and unoccupied oxygen partial density ofstates (DOS) of LiFePO4 and 5 percent Li doped LiFePO4 are presented inFig. 1. Our experimental results clearly indicate that LiFePO4 has wideband gap (~; 4 eV). This value is much larger than what is predicted byDFT calculation. For 5 percent Li doped LiFePO4, a new doping state wascreated closer to the Fermi level, imparting p-type conductivity,consistent with thermopower measurement. Such observation substantiatesthe suggestion that high electronic conductivity in Li1.05Fe0.95 PO4 isdue to available number of charge carriers in the material. Furthermore,Hall effect measurement on Li doped sample confirmed presence of freecharge carriers, which are responsible for the observed electronicconductivity increase in Li doped LiFePO4. There is no evidence that Fe3+valence is created by doping with excessive Li+ in Li1.05Fe0.95PO4, asshown by Fe-edge XAS. (Fig.2) Instead, charge-carrier holes resideprimarily in unoccupied O 2p states, which compensate for the chargedeficiency from Li+ substitution for Fe2+. The increased conductivity inLi1.05Fe0.95PO4 is attributed to the new charge carriers (doped holes)and the strong electron correlation between O 2p and Fe 3dstates.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director. Office of Science. Office of Basic EnergySciences
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 927241
- Report Number(s):
- LBNL-57929-Ext.-Abs.; R&D Project: 673601; BnR: KC0302040; TRN: US0803167
- Resource Relation:
- Conference: 208th Meeting of the Electrochemical Society, LosAngeles, CA, Oct. 16-21, 2005
- Country of Publication:
- United States
- Language:
- English
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