AbstractsChemistry

The objective of the synthesis of a novel conducting salt for lithium-ion battery cells based on weakly coordinating anions was tackled with the use of the difluorophosphato ligand. In initial electrochemical measurements, the stability of the ligand at the conditions prevailing in lithium-ion batteries was assured. Subsequently, the ligand was used to synthesize improved conducting salts on the basis of homoleptic aluminate species. The preparation of the initial target structure Li[Al(O2PF2)4] failed due to the remaining Lewis acidic character of the central aluminum atom. Instead, the formation of Li3[Al(O2PF2)6] and Al(O2PF2)3 was observed with hexacoordinate aluminum atoms. A possible mechanism towards these compounds was postulated in the solvent induced dismutation of the tetracoordinate Li[Al(O2PF2)4]. The electrochemical properties of the lithium aluminate were tested resulting in unsatisfactory electrical conductivities mainly due to the poor solubility in the conventional battery electrolyte solvents. The necessity of the realization of singly charged anions was accommodated by the exchange of the central aluminum atom for boron. The preparation of the tetrakis(difluorophosphato)borate anion [B(O2PF2)4]– succeeded using different routes and a comprehensive spectroscopic characterization was managed comprising several compounds of the [B(O2PF2)4]–-anion. The synthesis of the lithium salt Li[B(O2PF2)4] was established in multi-gram scale and the conducting salt was investigated electrochemically in different electrolyte systems up to the assembly of full cells and subsequent cycling experiments. The insufficient mobility of the voluminous [B(O2PF2)4]–-anion led to poor performances at elevated C rates. Heteroleptic boron species were prepared in order to improve the properties of the conducting salt. Target structures were the (oxalato)borates [(C2O4)BF(O2PF2)]– and [(C2O4)B(O2PF2)2]–, which were experimentally accessible by the use of two complementary routes. The comproportionation reaction of Li[B(O2PF2)4] and Li[BF4] and the successive reaction with ((H3C)3Si)2C2O4 led to a variety of products. The synthesis starting from lithium difluoro(oxalato)borate and (H3C)3Si(O2PF2) delivered similar results comprising up to seven different boron species. The mixtures showed acceptable stability and compatibility with the prevailing conditions in lithium-ion battery cells, but the lack of a defined and stable equilibrium state imposes a large handicap towards industrial application of the material. Nevertheless, the spectroscopic characterization of the (difluorophosphato)/fluoro(oxalato)borates was managed successfully. Das Ziel der Synthese neuartiger Leitsalze für Lithium-Ionen-Batterien auf der Grundlage von schwach koordinierenden Anionen wurde mit Hilfe des Difluorophosphato-Liganden verfolgt. Einleitenden elektrochemischen Messungen stellten die Stabilität des Liganden bei den in Lithium-Ionen-Batterien vorherrschenden Bedingungen sicher. Deshalb wurde der Ligand nachfolgend verwendet, um verbesserte…