Nano-network electronic conduction in iron and nickel olivine phosphates

doi:doi:10.1038/nmat1063

The provision of efficient electron and ion transport is a critical issue in an exciting new group of materials based on lithium metal phosphates that are important as cathodes for lithium-ion batteries. Much interest centres on olivine-type LiFePO₄, the most prominent member of this family¹. Whereas the one-dimensional lithium-ion mobility in this framework is high², the electronically insulating phosphate groups that benefit the voltage also isolate the redox centres within the lattice. The pristine compound is a very poor conductor ( $approx$ 10^-9 S cm^-1), thus limiting its electrochemical response. One approach to overcome this is to include conductive phases, increasing its capacity to near-theoretical values^3,^4,^5,⁶. There have also been attempts to alter the inherent conductivity of the lattice by doping it with a supervalent ion. Compositions were reported to be black p-type semiconductors with conductivities of $approx$ 10^-2 S cm^-1 arising from minority Fe³⁺ hole carriers⁷. Our results for doped (and undoped) LiMPO₄ (M = Fe, Ni) show that a percolating nano-network of metal-rich phosphides are responsible for the enhanced conductivity. We believe our demonstration of non-carbonaceous-network grain-boundary conduction to be the first in these materials, and that it holds promise for other insulating phosphates.

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