Nanostructured Two-Component Composite Approach for Better High-Rate Capability of Cathode Materials
Authors: Kosova Nina V. , Devyatkina Evgeniya T. , Bobrikov Ivan A. , Balagurov Anatoly M. 
Published: February 2020

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→Li2.5V2(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