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Electronically conductive phospho-olivines as lithium storage electrodes

Abstract

Lithium transition metal phosphates have become of great interest as storage cathodes for rechargeable lithium batteries because of their high energy density, low raw materials cost, environmental friendliness and safety. Their key limitation has been extremely low electronic conductivity, until now believed to be intrinsic to this family of compounds. Here we show that controlled cation non-stoichiometry combined with solid-solution doping by metals supervalent to Li+ increases the electronic conductivity of LiFePO4 by a factor of 108. The resulting materials show near-theoretical energy density at low charge/discharge rates, and retain significant capacity with little polarization at rates as high as 6,000 mA g−1. In a conventional cell design, they may allow development of lithium batteries with the highest power density yet.

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Figure 1: Doped olivines of stoichiometry Li 1− x M x FePO 4 show electrical conductivity at room temperature that is a factor of 10 8 greater than in undoped LiFePO 4 , and absolute values >10 −3 S cm −1 over the temperature range −20 °C to +150 °C of interest for battery applications.
Figure 2: Ordered-olivine structure of LiFePO4: Pmnb space group, with Li in M1 site and Fe in M2 site.
Figure 3: Morphologies of undoped (insulating) and doped (conductive) powders.
Figure 4: Elemental mapping by scanning TEM (STEM).
Figure 5: X-ray diffraction of various powders showing the effect of cation stoichiometry on dopant solid-solubility.
Figure 6: Electrochemical test data for electronically conductive olivine.

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Acknowledgements

We thank W. D. Moorehead, P. Limthongkul and B. P. Nunes for assistance. This research was supported by the US Department of Energy, Basic Energy Sciences, Grant No. DE-FG02-87-ER45307, and used Shared Experimental Facilities at MIT supported by NSF Grant No. 94004-DMR.

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Correspondence to Yet-Ming Chiang.

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The authors declare personal financial interests in a commercial entity to which patent filings based on certain of the subject matter in this publication have been licensed by Massachusetts Institute of Technology.

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Chung, SY., Bloking, J. & Chiang, YM. Electronically conductive phospho-olivines as lithium storage electrodes. Nature Mater 1, 123–128 (2002). https://doi.org/10.1038/nmat732

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