Nanostructured Two-Component Composite Approach for Better High-Rate Capability of Cathode Materials

Many efforts have been made to turn insulating compounds into attractive electrode materials, including nanosizing, carbon nanopainting and metal doping. A new approach to enhance high-rate capability of materials with poor electric conductivity is based on the development of two-component composites comprising of low-conductive and high-conductive electrode materials. Monoclinic Li₃V₂(PO₄)₃ is a promising cathode for high-power lithium-ion batteries because of its open three-dimensional framework allowing fast transport of lithium ions. Its theoretical capacity is 132 and 197 mAh×g^-1 upon cycling to 4.3 and 4.8 V, respectively. Recently, the enhanced electrochemical performance of nanostructured LiVPO₄F/Li₃V₂(PO₄)₃ composites has been reported [1].

                The aim of the present work was to prepare (1-x)LiMPO₄/xLi₃V₂(PO₄)₃ (M=Mn,Fe) nanocomposites with 0.05£x£0.50 by mechanochemically assisted solid state synthesis and to investigate their structural features and electrochemical performance.

                The composites were synthesized by the joint carbothermal reduction of MnO₂ (Fe₂O₃) and V₂O₅ in the mixtures with Li₂CO₃ and (NH₄)₂HPO₄ using AGO-2 planetary mill. Crystal structure was studied by X-ray and neutron powder diffraction (NPD) using Rietveld refinement. NPD spectra were obtained at IBR-2 reactor of JINR (Dubna). Particle size and morphology were studied by SEM and TEM. The local structure of the LiFePO₄/Li₃V₂(PO₄)₃ composites was investigated by Mössbauer spectroscopy. The samples were cycled in Swagelok-type half-cells with Li anode, Whatman GF/C separator and LiPF₆-based electrolyte in the 2.0-4.8 V (Mn) and 2.5-4.3(4.8) V (Fe) range at 0.1-10C cycling rates. Additionally, GITT method was applied for the evaluation of Li ionic diffusion coefficient in the composites upon cycling.

                Two phases were present on XRD and NPD patterns of all samples: orthorhombic LiMPO₄ (space group Pnmb) and monoclinic Li₃V₂(PO₄)₃ (space group P2₁/n). The refined cell parameters of LiMPO₄ and Li₃V₂(PO₄)₃ in the composites slightly deviate from those of pure components (Fig. 1). Mössbauer spectroscopy of the 0.75LiFePO₄/0.25Li₃V₂(PO₄)₃ composite reveals the presence of tiny amounts of Fe³⁺ ions.  According to SEM and TEM, the average particle size of the composite decreases as compared with pure components, indicating that Li₃V₂(PO₄)₃ and LiMPO₄ phases hinder the crystal growth of each other.

             In the 2.5-4.8 V range, Li₃V₂(PO₄)₃ exhibits four redox plateaus corresponding to a sequence of phase transitions: Li₃V₂(PO₄)₃→Li_2.5V₂(PO₄)₃→Li₂V₂(PO₄)₃→ Li₁V₂(PO₄)₃→V₂(PO₄)₃. On the dQ/dU=f(U) plots of the LiMPO₄/Li₃V₂(PO₄)₃ composites, some additional peaks are also observed corresponding to two redox pairs: Mn³⁺/Mn⁴⁺ (at 4.4 V) and Fe³⁺/Fe²⁺ (at 3.45 V). Charge-discharge profiles of the composites and the variation of specific discharge capacity of the composites vs. cycling rate are shown in Fig. 2. When the voltage cut-off increases to 4.8 V for 0.75LiFePO₄/0.25Li₃V₂(PO₄)₃, its capacity increases and discharge profile gets a sloping form. The enhancement of high-rate electrochemical performance of LiMPO₄ in the composites is considered to be a result of reduction in particle size and the nucleation energy for the formation of the MPO₄ phases. GITT measurements confirm the increase of D_Li and the improvement of Li diffusion in the composites.

                Thus, the nanostructured two-component composite approach (combination of low- and high-conductive electrode materials) supported by mechanochemically assisted solid state synthesis is shown to be applicable and very promising. The increase in the number of voltage plateaus and the mean intercalation voltage should be advantageous for improvement of cell performance.

[1] N.V. Kosova, E.T. Devyatkina, A.B. Slobodyuk, A.K. Gutakovskii // doi 10.1007/s10008-013-2213-1