Synthesis and conductivity of LiFePO4 and solid solutions LiFe1–xМxPO4 (M – Zn, Mg, Cu, x<0.2)

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We studied the influence of reagents mixture composition on the formation of LiFePO4 under the conditions of low-temperature solid state synthesis by interaction of NH4FePO4 with molten LiCH3COO and by crystallization from nitrate melt. The interaction of the reagents mixture components was investigated for the systems NH4FePO42H2O–А (where А – LiNO3, Li2CO3, LiCH3COO) and FeC2O42H2O–(NH4)2HPO4)/NH4H2PO4–А (where А – Li2CO3, LiF). A stability of Fe(II) to oxidation under the synthetic conditions was established depending on the reaction mixtures composition. It was found that Fe(II) is the most stable to oxidation under the conditions of solid state synthesis from the system Li2CO3–FeC2O42H2O–(NH4)2HPO4)/NH4H2PO4. For the synthesized solid solutions with partial isovalent substitution of iron LiFe0.8M0.2PO4 (M – Mg, Cu), a unit cell volume, lattice parameters а and с and conductivity increase as compared with undoped LiFePO4, while the unit cell volume, the lattice parameters а and с and conductivity of LiFe0.8Zn0.2PO4 decrease. The obtained results indicate possibility to influence the electrical conductivity of materials based on LiFePO4 by isovalent substitution of iron.