Tuning the Crystal Structure of A₂CoPO₄F (A = Li, Na) Fluoride‐Phosphates: A New Layered Polymorph of LiNaCoPO₄F

Abstract

Co-containing fluoride-phosphates are of interest in sense of delivering high electrode potentials and attractive specific energy values as positive electrode materials for rechargeable batteries. In this paper we report on a new Co-based fluoride-phosphate, LiNaCoPO 4 F, with a layered structure (2D), which was Rietveld-refined based on X-ray powder diffraction data [P2 1 /c, a = 6.83881(4) Å, b = 11.23323(5) Å, c = 5.07654(2) Å, = 90.3517(5)°, V = 389.982(3) Å 3 ] and validated by electron diffraction and high-resolution scanning transmission electron microscopy. The differential scanning calorimetry measurements revealed that 2D-LiNaCoPO 4 F forms in a narrow temperature range of 520-530°C and irreversibly converts to the known 3D-LiNaCoPO 4 F modification (Pnma) above 530°C. The non- [a] 4365 carbon-coated 2D-LiNaCoPO 4 F shows reversible electrochemical activity in Li-ion cell in the potential range of 3.0-4.9 V vs. Li/Li + with an average potential of ≈ 4.5 V and in Na-ion cell in the range of 3.0-4.5 V vs. Na/Na + exhibiting a plateau profile centered around 4.2 V, in agreement with the calculated potentials by density functional theory. The energy barriers for both Li + and Na + migration in 2D-LiNaCoPO 4 F amount to 0.15 eV along the [001] direction rendering 2D-LiNaCoPO 4 F as a viable electrode material for high-power Li-and Na-ion rechargeable batteries. The discovery and stabilization of the 2D-LiNaCoPO 4 F polymorph indicates that temperature influence on the synthesis of A 2 MPO 4 F fluoride-phosphates needs more careful examination with perspective to unveil new structures. trode potential to much higher values in comparison to oxides. Moreover, a better ionic transport is expected owing to a lower affinity of alkali metals towards fluoride than oxide anions. A much richer structural diversity of fluoride-phosphates offers manifold options for tuning the electrochemical properties. [4] Among fluoride-phosphates, the A 2 MPO 4 F (A = Li, Na; M = Mn, Fe, Co, Ni) family provides one of the largest playgrounds for searching new cathode materials. [5] From the electrochemical standpoint, this class also regains interest due to a theoretical possibility of multi-electron redox transitions enabling reversible de/intercalation of more than one alkali ion per transition metal center.The wide structural variety of A 2 MPO 4 F is primarily originated from multiple options in playing with the chemical composition of the cation sublattice. Depending on the nature of the A and M metals, MO 4 F 2 octahedra (key building blocks) constitute various types of linkage: from corner-sharing to facesharing and their combinations. Since A and M sites can be populated by more than one element type, altering their average ionic radius by substitutions might also influence the preferable MO 4 F 2 connectivity thus preserving a specific structural type for new chemical compositions or even giving rise to new structures.Corner-shared MO 4 F 2 octahedra are only characteristic to Na 2 MnPO 4 F [6,7] and its limited solid solu...

Mineral-Inspired Materials: Synthetic Phosphate Analogues for Battery Applications

“…One more illustration may be given in connection with the rare minerals grandidierite, MgAl 3 (SiO 4 )(BO 3 )O 2 , and ominelite, FeAl 3 O 2 (SiO 4 )(BO 3 ). Synthetic fluoride phosphates with the common formula A 2 M(PO 4 )F, where A = Li, Na and M = Ni, Fe, Co, Mn, Mg, extensively studied as promising cathodes for LIBs and NIBs (see, for instance one of the latest papers: [122]) are structural analogues of the mentioned natural silicate-borates [123].…”