Synthesis and characterisation of ternary system LiCoPO₄-LiNiPO₄-LiMnPO₄ cathode materials for Li-ION batteries
Tan, Tze Qing
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Electrochemical energy storage devices with high energy density are important in modern society. In order to obtain high energy density Li ion batteries, cathode materials with high discharge voltage and discharge capacity are required. Hence, high voltage cathode materials such as olivine LiMPO₄ (M = Mn, Co, and Ni) and spinel Li₂CoMn₃O₈ have been extensively studied by researchers. In this study, three analogous series with the formula of LiCo₁-x[Ni₀.₅Mn₀.₅]xPO₄, LiNi₁-x[Co₀.₅Mn₀.₅]xPO₄ and LiMn₁-x[Co₀.₅Ni₀.₅]xPO₄ (0 ≤ x ≤ 1) within the phase triangle of LiCoPO₄ – LiNiPO₄ –LiMnPO₄ were systematically studied as potential candidates for high voltage rechargeable lithium ion batteries. The samples were synthesized by conventional solid state route at temperature 750–1000 °C in air for 12 hours with two different cooling conditions (i.e. slow cooling and quenching). These compositions were characterised by using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Impedance Spectroscopy Analyser. Initially, LiCoPO₄, LiNiPO₄ and LiMnPO₄ were prepared as end members and were used as standard references. The prepared samples were single phase and structurally stable up to 1000 °C. All the XRD patterns could be indexed with the olivine structure and the space group of Pnma. Structural analysis using Rietveld refinement of conventional XRD data revealed that the estimated anti-site defects was comparably low which is less than ~5 %. The changes in lattice parameters across the series Mn, Co and Ni were in accordance with Vegard’s law. The structure and electrical properties of the slow-cooled and quenched samples were compared. The results showed that the anti-site defects in both the quench and slow-cooled samples have quite similar values. Nevertheless, all the samples exhibits low intrinsic electrical conductivities of about ~10-8 S cm-1 that were measured using a LCR meter at 300 °C. Hence, further modification were performed on complex olivine LiNi₁/₃n₁/₃Co₁/₃PO₄ in order to improve the conductivity. LiNi₁/₃Mn₁/₃Co₁/₃PO₄ was ball milled to reduce the particle size followed by ball milling with three different carbon sources: graphene nano-platelets (GNP), carbon nanotube (CNT) and carbon black (CB) to form composites. The results showed that these composites have exhibited relatively higher coin cell conductivity compare to the bare sample. Hence, it was believed that this processing route can probably be applied to improve the conductivity of the three analogous series. On the other hand, a small research was also carried out to study the effect of Zn doped into spinel Li₂CoMn₃O8, however, the result was not quite promising because the doped samples exhibited low discharge capacity.