The Structural Transition and Magnetic Properties of Lithium Deintercalation in LiFePO$_{4}$

Abstract

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Lithium deintercalation of <formula formulatype="inline"><tex Notation="TeX">$(1-x)$</tex> </formula> LiFePO<formula formulatype="inline"><tex Notation="TeX">$_{4}/x$</tex> </formula> FePO<formula formulatype="inline"><tex Notation="TeX">$_{4}(0\leq x \leq 1)$</tex></formula> have been studied with X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), and MÖssbauer spectroscopy. Lithium deintercalation to obtain <formula formulatype="inline"> <tex Notation="TeX">$(1-x)$</tex></formula> LiFePO<formula formulatype="inline"> <tex Notation="TeX">$_{4}/x$</tex></formula> FePO<formula formulatype="inline"> <tex Notation="TeX">$_{4}$</tex></formula> was done by chemical oxidation process using NO<formula formulatype="inline"><tex Notation="TeX">$_{2}$</tex> </formula>BF<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula>. The crystal structure was found to be an orthorhombic with space group <emphasis emphasistype="boldital">Pnma</emphasis> for each sample. There exists a mixture of LiFePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula> and FePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula> phase between <formula formulatype="inline"><tex Notation="TeX">$x = 0$</tex></formula> and <formula formulatype="inline"><tex Notation="TeX">$1$</tex> </formula>. The MÖssbauer spectra have been taken at various temperatures ranging from 4.2 K to room temperature. The magnetic NÉel temperature <formula formulatype="inline"><tex Notation="TeX">$(T_{N})$</tex></formula> was determined to be 51 and 114 K for LiFePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula> and FePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex> </formula>. Also, the iron ions were ferric and ferrous for LiFePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex></formula> and FePO<formula formulatype="inline"><tex Notation="TeX">$_{4}$</tex></formula>, both occupying the octahedral sites at various temperatures. We confirmed that the change of MÖssbauer spectrum shape as the evidence of two-valence state of iron ions coexists on lithium deintercalation in LiFePO<formula formulatype="inline"> <tex Notation="TeX">$_{4}$</tex></formula> from 4.2 K to room temperature. </para>