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

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, LiNaCoPO₄F, with a layered structure (2D), which was Rietveld‐refined based on X‐ray powder diffraction data [P2₁/c, a = 6.83881(4) Å, b = 11.23323(5) Å, c = 5.07654(2) Å, β = 90.3517(5) °, V = 389.982(3) ų] and validated by electron diffraction and high‐resolution scanning transmission electron microscopy. The differential scanning calorimetry measurements revealed that 2D‐LiNaCoPO₄F forms in a narrow temperature range of 520–530 °C and irreversibly converts to the known 3D‐LiNaCoPO₄F modification (Pnma) above 530 °C. The non‐carbon‐coated 2D‐LiNaCoPO₄F 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‐LiNaCoPO₄F amount to 0.15 eV along the [001] direction rendering 2D‐LiNaCoPO₄F as a viable electrode material for high‐power Li‐ and Na‐ion rechargeable batteries. The discovery and stabilization of the 2D‐LiNaCoPO₄F polymorph indicates that temperature influence on the synthesis of A₂MPO₄F fluoride‐phosphates needs more careful examination with perspective to unveil new structures.