Structural and Electrochemical Studies of Rhombohedral Na₂TiM(PO₄)₃ and Li_1.6Na_0.4TiM(PO₄)₃ (M = Fe, Cr) Phosphates

The crystal structures of NASICON compositions Z 2 TiM(PO 4 ) 3 (Z = Na, Li; M = Cr, Fe) were investigated through powder neutron diffraction on the beam lines D1A (λ = 1.9104 Å) and D2B (λ = 1.5924 Å) at ILL, Grenoble, France. Na 2 TiCr(PO 4 ) 3 and Na 2 TiFe(PO 4 ) 3 are isostructural. They crystallize in the space group R 3̄ c, with the sodium ions being located in the M1 (τ ∼ 1) and M2 sites (τ ∼ 1 / 3 ). NASICON Li 1.6 Na 0.4 TiCr(PO 4 ) 3 and Li 1.6 Na 0.4 TiFe(PO 4 ) 3 were prepared through ion exchange from the sodium analogues. They crystallize in the space group R 3̄, 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 Na 2+ x TiM(PO 4 ) 3 and Li 1.6+ x Na 0.4 TiM(PO 4 ) 3 (0 ≤ x ≤ 1 for M = Cr, 0 ≤ x ≤ 2 for M = Fe), the insertion proceeds through a single-phase mechanism.