New Fluorine-Free Polycyano Fully-Conjugated Aliphatic Anions for Applications in Battery Electrolytes

Abstract

Lithium-ion batteries are nowadays the most popular energy storage technology. A raising number of research investigates the development of modern batteries, while the market of secondary lithium batteries is still growing. This creates problems with disposal of expired batteries, as many electrolyte components comprise fluorine. Most of lithium-ion batteries research focuses on development of electrode materials. However, apart from electrodes, a key component of Li-ion batteries is electrolyte. Electrolyte limits critical battery properties, such as safety, thermal stability, battery lifetime and internal resistance of a cell. Conventional electrolytes consist of lithium salt, solvent (or solvents mixture) and additives. Since both solvents and additives are well developed and not problematic, in this work we have focused on lithium salts. Among all lithium salts that can be used in large scale manufacturing, LiPF6 is used almost exclusively. Even considering all its advantages, LiPF6has numerous drawbacks, too. These include low thermal stability, moisture sensitivity and toxic and corrosive products evolution upon decomposition [1,2]. In the recent years many studies on new lithium salts have been published, however none of them has been introduced widely to the market so far [3-7]. Therefore, there is a substantial need for new ideas in this field. Our approach was to design new anions for lithium salts tailored particularly for Li-ion battery electrolyte. The concept that we have chosen were anions based on aliphatic chain with conjugated double bonds, that allows π-electrons as well as negative charge delocalization. The chain was fully substituted with nitrile electron withdrawing groups. Application of this ideas should lead to a highly stable and weakly-coordinating anion. As a result we have synthesized two novel lithium salts: lithium pentacyanopropenide (LiPCP) and lithium hexacyano-3-azapentadienide (LiHCAP). Both salts show excellent thermal stability (up to 280 and 350 °C respectively) as proven with means of TGA and DSC. Electrochemical stability tests have shown 4.5 V window width for all salts. Conductivity of the electrolytes containing those new salts have been examined in propylene carbonate and typical battery mixtures. In both cases outstanding conductivity values has been reached, with 10 mS cm-1 have been observed at the room temperature. Lithium cation transference numbers have been investigated as well, giving satisfactory results. Finally, to evaluate compatibility with electrode materials, cycling experiments have been performed both with commercial and custom made electrodes. In conclusion, we have synthesized and thoroughly tested two members of a new class of lithium salts for applications in non-aqueous electrolytes for modern batteries. Obtained results exhibited clearly that LiPCP and LiHCAP would be successfully applied in lithium-ion batteries improving their performance, safety and environmental friendliness.