Li₃VP₃O₉N as a Multielectron Redox Cathode for Li-Ion Battery

Li₃VP₃O₉N was for the first time synthesized from its sodium analogue Na₃VP₃O₉N using a solid-solid Li⁺/Na⁺ ion-exchange method. This lithium variant of nitridophosphate is found to possess similar crystal structure (space group P2₁3) as its sodium analogue Na₃VP₃O₉N (a = 9.4507(1) Å) but with much smaller lattice parameter (a = 9.1237(1) Å). The crystal structure of Li₃VP₃O₉N was solved and refined using combined synchrotron X-ray and time-of-flight neutron powder diffraction data, allowing the three distinct lithium-ion sites to be identified. A lithium bond valence sum difference map calculation suggests the existence of isotropic three-dimensional lithium-ion-conducting pathways with a minimum valence threshold |ΔV| of 0.02. Li₃VP₃O₉N behaves as a promising reversible cathode material for rechargeable lithium-ion batteries with an average V³⁺/V⁴⁺ redox potential of 3.8 V (vs Li⁺/Li). Both cyclic voltammetry tests and chemical delithiation (using NO₂BF₄) indicate it is possible to partially remove the second lithium from the structure, though only at very high potentials (>4.9 V vs Li⁺/Li). It is also found that the unit cell volume of this compound expands instead of shrinking upon lithium removal, a rare phenomenon for polyanion-based cathodes. This abnormal volume expansion is found to be associated with the drastic expansion of the Li1(O1)₃N tetrahedral site after removing lithium from this site.