Li₂B₆O₉F₂, a New Acentric Fluorooxoborate

Abstract

Novel Li2B6O9F2 can be either synthesized from LiBF4 and B2O3 or from LiB3O5 and LiBF4, at 673 K. The three dimensional anionic network is related to the polymeric anion of pentaborates MB5O8 (M: Rb, Cs); replacing one bridging oxygen atom per repetition unit of the latter by BF2O2 tetrahedron yields the again three dimensional fluorooxoborate anion of the title compound.The new solid electrolyte is stable up to 753 K and decomposes evolving BF3. It is a purely ionic conductor with conductivities of 1.3 × 10–8 and 2.2 × 10–10S·cm–1 at 573 and 473 K, respectively.

Full differentiation and assignment of boron species in the electrolytes Li2B6O9F2 and Li2B3O4F3 by solid-state 11B NMR spectroscopy

“…2. The 11 B spectra of both fluorooxoborates display complex line shapes, as they are composed of sub-spectra originating from the various boron species present in the crystal structure [2,3]. It can also be seen that use of fluorine decoupling causes intensity changes in some parts of the spectra, whereas other parts remain unaffected.…”

“…[3,2]. Magic-angle spinning (MAS) 11 B NMR spectra were acquired employing a 4 mm HXY probe, with the sample spinning at 12 kHz, and using a recycle delay of 10 s. All 11 B NMR spectra were referenced to the customary standard BF 3 Á OEt 2 , using B(OCH 3 ) 3 as a secondary reference, which is shifted by þ18.1 ppm to BF 3 Á OEt 2 [27].…”

“…The bulk ionic conductivity of LiB 6 O 9 F is 6:6 Â 10 À9 S cm À1 at 673 K, classifying it as a solid electrolyte, but is much lower at room temperature [1]. To improve conductivity, the compounds Li 2 B 3 O 4 F 3 [2] and Li 2 B 6 O 9 F 2 [3] with higher stoichiometric lithium content were synthesised. The fundamental building blocks of these compounds are shown in Fig.…”

“…Thus, the negative effective charges at the terminal oxygen or fluorine atoms will be reduced, enhancing lithium ion mobility. This approach entails the synthesis of an entirely new class of compounds, namely crystalline lithium fluorooxoborates, which was realised in our group using acid-base reactions in the solid state [1][2][3]. Three new lithium fluorooxoborates were discovered, namely LiB 6 O 9 F [1], Li 2 B 3 O 4 F 3 [2] and Li 2 B 6 O 9 F 2 [3].…”

“…This approach entails the synthesis of an entirely new class of compounds, namely crystalline lithium fluorooxoborates, which was realised in our group using acid-base reactions in the solid state [1][2][3]. Three new lithium fluorooxoborates were discovered, namely LiB 6 O 9 F [1], Li 2 B 3 O 4 F 3 [2] and Li 2 B 6 O 9 F 2 [3]. All of them have acentric structures and show non-linear optical properties, LiB 6 O 9 F with an efficiency comparable to KDP [4].…”