[Image above] Lithium iron phosphate batteries are used frequently in stationary energy storage applications. Understanding the defects that occur in the crystal structure of the lithium iron phosphate cathode can help researchers improve the battery’s electrochemical performance. Credit: Yo-Co-Man, Wikimedia (CC BY-SA 4.0)
When it comes to lithium-ion batteries, lithium nickel manganese cobalt oxide (NCM) is the standard material used for the cathode due to its quality uniformity and high energy density. In recent years, though, concerns over the use of cobalt in batteries has led to development of new cathode materials that do not contain this metal.
Lithium iron phosphate (LiFePO4, LFP) is a widely used cobalt-free cathode material known for high stability, safety, and affordability. It is used frequently in stationary energy storage applications but usually not in electric vehicles because of low energy density. (Tesla, however, is experimenting with LFP batteries in their Model 3.)
As with any material, LFP is not perfect—it is known to experience certain kinds of defects in its crystal structure, which can affect the material’s electrochemical performance. In particular, there generally is a considerable amount of lithium–iron antisite defects, a type of point defect when lithium and iron atoms exchange their positions in the crystal lattice.
Researchers traditionally have taken it for granted that the phosphate group in LFP does not experience defects due to its high stability, which comes from each phosphorus atom being covalently bonded to four oxygens. But that thinking recently changed when researchers in France and Slovenia synthesized LiFePO4 powders with a high number of phosphorus vacancies.
The presence of phosphorus vacancies in LFP was surprising because it meant the material contained a lot of undercoordinated oxygen atoms, thus throwing off charge distribution in the material. The LFP must have some mechanism to compensate for the unbalanced charge—but what was it?
In 2019, researchers led by the Skolkovo Institute of Science and Technology (Skoltech) synthesized the phosphorus-deficient LFP to investigate the charge compensation mechanism. Their analysis revealed that, contrary to belief, iron atoms did not appear to be part of the charge compensation mechanism. Instead, it appeared that charge balance was maintained by residual water in the form of OH groups inside the phosphorus vacancies.
The existence of OH defects in LFP is fascinating because it draws similarities to the olivine mineral group (Mg,Fe)SiO4, a mineral being considered for carbon capture storage. Studies have shown that almost any olivine mineral sample stores structural water in the form of point defects, including OH-stabilized silicon vacancies.
Because the silicon site in olivine is equivalent to the phosphorus site in LFP, “the discovery of P/4H defects in LFP suggests that there exist many commonalities between defective structures of the two crystal groups,” the Skoltech researchers write in a new paper.
The new paper published this April, and it follows up on the 2019 study by using knowledge of the olivine similarities to drive further investigation of the OH defects in LFP. In particular, the researchers were curious whether OH groups reside in the lithium and iron vacancies of LFP as well as the phosphorus vacancies, “as up to now the experimental methods had not been able to precisely localize and confirm the OH defects in the structure of either LFP or olivine minerals,” they write.
For the study, the researchers used a joint computational and experimental approach comprising density functional theory (DFT), molecular dynamics calculations, and X-ray and neutron diffraction.
The DFT-based thermodynamic model affirmed the feasibility of hydrogen incorporation into LFP. It found that substitution of four or five hydrogen atoms for phosphorus was the most favorable process due to it requiring the least amount of energy (0.44–0.6 eV per hydrogen atom). Substitution of lithium for hydrogen or iron for two hydrogens was theoretically possible (0.7–0.9 eV/atom), while formation of individual hydrogen defects appeared highly unlikely (3.1 eV/atom).
Molecule dynamic simulations revealed that the substitution of hydrogen for phosphorus occurs in several configurations that are readily affected by neighboring defects, making it difficult to observe the OH defects with diffraction-based methods. However, by predicting the hydrogen atom coordinates using DFT, the researchers successfully identified the location of hydrogen atoms with meaningful accuracy in the neutron diffraction data collected at low temperatures on physical LFP samples.
In an email, Skoltech assistant professor Stanislav Fedotov says their study demonstrates that the crystal structure of LiFePO4 “still conceals a lot more peculiar features than it was expected.” Their goal now is to monitor the temperature-dependent evolution of hydroxyl defects and dig deeper into the hydrothermal mechanism of their formation.
“A desired outcome of this research would be a strengthening of our perception of the LiFePO4 structure and mechanics to successfully cope with electrochemistry-related issues and improve the functional properties of the material,” Fedotov says.
The paper, published in Inorganic Chemistry, is “Hydroxyl defects in LiFePO4 cathode material: DFT+U and an experimental study” (DOI: 10.1021/acs.inorgchem.0c03241).