Supervalent doping of LiFePO₄ for enhanced electrochemical performance

Abstract

Vanadium and titanium doped orthophosphates LiFe 0.9 M 0.1 PO 4 with an olivinetype structure (space group Pnma) were prepared by mechanochemically assisted solid state synthesis using a high-energy AGO-2 planetary mill and post-annealing at 750 °C. It has been established that the V and Ti ions do not fully substitute for Fe 2+ in the LiFePO 4 structure. The other part of these ions participate in the formation of the secondary phases with the open Nasicon-type structures: monoclinic Li 3 V 2 (PO 4 ) 3 (space group P2 1 /n) and rhombohedral LiTi 2 (PO 4 ) 3 (space group R-3c). According to TEM, the average particle size of the nanocomposites is about 100-300 nm. EDX microanalysis reveals that the small particles of the secondary phases are segregated on the surface of the larger LiFePO 4 particles. On the charge-discharge profiles of LiFe0.9M0.1PO4, the plateaus corresponding to LiFePO 4 and the secondary phases are observed. V doping improves cycleability and rate capability of LiFePO 4 to a greater extent than Ti.

Anisotropic crystallite size distributions in LiFePO4 powders

“…10,11 Composite particles have mechanically stable non-coherent boundaries between misoriented mosaic blocks; 12 conglomerates are formed by ordered 13 or chaotically disordered nanocrystallites, 14 secondary phase particles are segregated on larger LiFePO 4 particles. 15 The conglomerates, even as large as 100 mm, may disintegrate and completely disappear upon additional chemically active annealing. 14 As a rule, a mosaic microstructure negatively affects the ionic conductivity.…”

“…(3) Presence of mosaic blocks and fused particles is obvious in some cases, [12][13][14][15][16][17][18][26][27][28][29][30][31] but detection of small-angle and other boundaries separating the coherent domains requires laborious (HR)TEM studies, which can hardly be compared in statistical reliability with XRD studies.…”