Nano-network electronic conduction in iron and nickel olivine phosphates.
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 LiFePO(4), the most prominent member of this family. Whereas the one-dimensional lithium-ion mobility in this framework is high, 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 (sigma approximately 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. 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 approximately 10(-2) S cm(-1) arising from minority Fe(3+) hole carriers. Our results for doped (and undoped) LiMPO(4) (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.
University of Waterloo, Department of Chemistry, 200 University Ave. West, Waterloo Ontario Canada N2L 3G1.
P Subramanya
PS
Herle
B
B
Ellis
N
N
Coombs
L F
LF
Nazar
D004553:Electric Conductivity; D058085:Iron Compounds / Q000737:chemistry*; D017616:Magnesium Compounds / Q000737:chemistry*; D008854:Microscopy, Electron; D036103:Nanotechnology; D009532:Nickel / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D017640:Silicates / Q000737:chemistry*; D014961:X-Ray Diffraction
D058085:Iron Compounds; D017616:Magnesium Compounds; D010710:Phosphates; D017640:Silicates; D009532:Nickel; C034475:olivine