The High-Temperature Polymorph of LiBF₄.

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• Lithium tetrafluoroborate (LiBF₄) is a crucial component in electrolytes for lithium-ion batteries.
• Previous research has been ambiguous regarding the existence of a high-temperature polymorph of LiBF₄.
• Understanding phase transitions is vital for optimizing material performance and stability.

Purpose of the Study:

• To definitively determine the single-crystal-to-single-crystal phase transition of LiBF₄.
• To resolve longstanding ambiguities concerning the existence of a high-temperature polymorph of LiBF₄.
• To characterize the structural changes associated with the phase transition using X-ray crystallography.

Main Methods:

• Differential Scanning Calorimetry (DSC) to determine phase transition thermodynamics.
• Single-crystal X-ray diffraction at low (200 K) and high (313 K) temperatures.
• Analysis of crystallographic data to determine crystal systems and twin laws.

Main Results:

• LiBF₄ exhibits an endothermic phase change at 28.2 °C with ΔH = 1180 J mol⁻¹ and ΔS = 3.92 J mol⁻¹K⁻¹.
• The low-temperature phase (200 K) is a twinned trigonal P system.
• The high-temperature phase (313 K) is a C-centered orthorhombic system, representing an interconversion of low-temperature twin geometries.

Conclusions:

• The study confirms the existence of a high-temperature polymorph of LiBF₄ through single-crystal X-ray diffraction.
• The observed structural changes are consistent with calorimetric and previous NMR findings.
• This resolves ambiguity and provides critical structural insights into LiBF₄ phase transitions.