“Hydrotriphylites” Li1-xFe1+x(PO4)1-y(OH)4y as Cathode Materials for Li-ion Batteries

Abstract

Lithium iron phosphate LiFePO<sub>4</sub> triphylite is now one of the core positive electrode (cathode) materials enabling the Li-ion battery technology for stationary energy storage applications, which are important for broad implementation of the renewable energy sources. Despite the apparent simplicity of its crystal structure and chemical composition, LiFePO<sub>4</sub> is prone to off-stoichiometry and demonstrates rich defect chemistry owing to variations in the cation content and iron oxidation state, and to the redistribution of the cations and vacancies over two crystallographically distinct octahedral sites. The importance of the defects stems from their impact on the electrochemical performance, particularly on limiting the capacity and rate capability through blocking the Li ion diffusion along the channels of the olivine-type LiFePO<sub>4</sub> structure. Up to now the polyanionic (i.e. phosphate) sublattice has been considered idle on this playground. Here, we demonstrate that under hydrothermal conditions up to 16% of the phosphate groups can be replaced with hydroxyl groups yielding the Li<sub>1-x</sub>Fe<sub>1+x</sub>(PO<sub>4</sub>)<sub>1-y</sub>(OH)<sub>4y</sub> solid solutions, which we term “hydrotriphylites”. This substitution has tremendous effect on the chemical composition and crystal structure of the lithium iron phosphate causing abundant population of the Li-ion diffusion channels with the iron cations and off-center Li displacements due to their tighter bonding to oxygens. These perturbations trigger the formation of an acentric structure and increase the activation barriers for the Li-ion diffusion. The “hydrotriphylite”-type substitution also affects the magnetic properties by progressively lowering the Néel temperature. The off-stoichiometry caused by this substitution critically depends on the overall concentration of the precursors and reducing agent in the hydrothermal solutions, placing it among the most important parameters to control the chemical composition and defect concentration of the LiFePO<sub>4</sub>-based cathodes.

Exploring the Origin of the Superior Electrochemical Performance of Hydrothermally Prepared Li-Rich Lithium Iron Phosphate Li1+δFe1−δPO4

“…In that case, Fe excess and/or Li/ Fe antisite defects can be formed due to kinetic limitation of the reaction. 6,14,29 In contrast, the Li 3 PO 4 -based route is known to produce high-performance lithium iron phosphate free from Fe + Li defects. 30−34 It is obvious that insertion of Li + into the Fe position M2 of the LiFePO 4 crystal structure should result in partial oxidation of the Fe cations up to +3 state in order to maintain the charge balance.…”

“…The final stage of the reaction is chemical exchange of “0.5Fe” for 1Li. In that case, Fe excess and/or Li/Fe antisite defects can be formed due to kinetic limitation of the reaction. ,, In contrast, the Li 3 PO 4 -based route is known to produce high-performance lithium iron phosphate free from Fe + Li defects. − …”

Influence of Carbon Coating on Intercalation Kinetics and Transport Properties of LiFePO4

“…It should also be noted that LiFePO 4 demonstrates extremely rich defect chemistry including Li/Fe antisite mixing, partial Li for Fe or Fe for Li substitution, partial Fe oxidation, cation vacancies, substitution of phosphate with hydroxyl groups etc. which add an extra complexity to the problem dramatically altering the electrochemical behavior (for instance, suppressing phase separation) . Obviously, the understanding of the complex interplay of the intercalation rate‐controlling factors requires accurate experimental characterization of intercalation kinetics and quantitative estimation of the key kinetic parameters.…”

Hydroxyl Defects in LiFePO4 Cathode Material: DFT+U and an Experimental Study

“…The P-deficient LiFePO 4 was synthesized via a hydrothermal route at a low concentration of initial reagents . The prepared solutions of 7 LiOH (0.0075 mol, NCCP, 98%) and H 3 PO 4 (0.0025 mol, Reakhim, 98%) in D 2 O (Kurchatov Institute, 99.8%) were first mixed to form a Li 3 PO 4 precipitate.…”

“…Indeed, infrared spectroscopy showed absorption bands characteristic of the structural OH bonds, while thermogravimetric/mass spectrometry analysis confirmed the water loss of the LFP sample in two stages with onsets at 350 and 450 °C, which are much higher compared to the typical temperatures of the adsorbed water loss. Additionally, the density functional theory (DFT) calculations showed that the formation of PO 4 /O 4 H n ( n = 1–4) defects is feasible under the synthesis conditions …”