Crystal Structure, Physical Properties, and Electrochemistry of Copper Substituted LiFePO₄ Single Crystals

Copper substituted lithium iron phosphate, with the olivine structure, has been synthesized hydrothermally as dark-brown single crystals with 15% of the iron sites occupied by cupric ions. Single crystal X-ray diffraction and magnetic susceptibility,combined with Li solid state NMR studies performed on powder sample, showed that 5% of the lithium ions also reside on the iron site, giving the formula Li0.95[(Fe2+)0.70(Fe3+)0.10(Cu2+)0.15Li0.05)]PO4. The compound exhibits anisotropic magnetic susceptibility and orders antiferromagnetically at 48 K. The magnetic moments are aligned along [010] as in LiFePO4. The optical measurements indicate that the dark brown color originates from interionic d-d transitions. A part of the lithium ions can be cycled in an electrochemical cell, corresponding to the oxidation/reduction of the iron ions. An onset of a second electrochemical process is observed at about 4.1 V, tentatively attributed to the removal of Li ions from the Fe site. The first principles calculations indicate above 5 V redox potential for the Cu2+/Cu3+ pair in the olivine structure, much higher than experimentally observed. The diffusion coefficient is determined from galvanostatic intermittent titration data, as part of the delithiation process proceeds as a single-phase reaction in this disordered compound.