Abstract
The crystal structures of NASICON compositions Z2TiM(PO4)3 (Z = Na, Li; M = Cr, Fe) were investigated through powder neutron diffraction on the beam lines D1A (λ = 1.9104 A) and D2B (λ = 1.5924 A) at ILL, Grenoble, France. Na2TiCr(PO4)3 and Na2TiFe(PO4)3 are isostructural. They crystallize in the space group R3c, with the sodium ions being located in the M1 (τ ∼ 1) and M2 sites (τ ∼ 1/3). NASICON Li1.6Na0.4TiCr(PO4)3 and Li1.6Na0.4TiFe(PO4)3 were prepared through ion exchange from the sodium analogues. They crystallize in the space group R3, with the lithium ions being located in the M3 tetrahedral site. The remaining, unexchanged, sodium ions are still located on the M1 site. Electrochemical insertion of sodium or lithium into these structures was followed through slow intermittent titration techniques coupled with in situ X-ray diffraction. We show that for both Na2+xTiM(PO4)3 and Li1.6+xNa0.4TiM(PO4)3 (0 ≤ x ≤ 1 for M = Cr, 0 ≤ x ≤ 2 for M = Fe), the insertion proceeds through a single-phase mechanism.
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