Enabling the use of lithium bis(trifluoromethanesulfonyl)imide as electrolyte salt for Li‐ion batteries based on silicon anodes and Li(Ni0.4Co0.4Mn0.2)O2 cathodes by salt additives

Addition of LiDFOB to an LiFSI based electrolyte enables cycling of full cells with the cathode at voltages up to 4.5 V, without corrosion of the aluminum current collector. The LiDFOB additive aids the formation of a passivating SEI at the surface of the silicon anode. HF-forming additives, like LiBF₄, appear to ruin the excellent SEI forming properties of the LiFSI based electrolytes.

Abstract

Lithium bis(trifluoromethanesulfonyl)imide (LiFSI) is a promising alternative salt for Li-ion batteries. Unlike the conventional LiPF₆, it is not prone to HF formation, and thus resistant to moisture. However, for cell voltages relevant for high energy cathodes (>4.2 V), the aluminium current collector will corrode in electrolytes based on this salt, and mitigation strategies are needed. Here, the use of Lithium tetrafluoroborate (LiBF₄) and Lithium difluoro(oxalato)borate (LiDFOB) salts as additives is investigated, in order to enable the use of LiFSI-based electrolytes. The performance of the electrolytes is evaluated separately for high content silicon anodes, (NMC442) cathodes and the aluminium current collector by electrochemical methods and post mortem analysis by SEM imaging and X-ray photoelectron spectroscopy (XPS). Electrolytes with LiDFOB as additive showed the best performance for all components, and were therefore selected for cycling in full cells, composed of silicon anodes and NMC442. Results show that LiFSI-based electrolytes with LiDFOB additive has an electrochemical performance similar to conventional electrolytes, and is thus a competitive, alternative electrolyte with a low fluorine content. Furthermore, it is verified that the good SEI forming properties of LiFSI based electrolytes known from cycling in half cells, is also preserved during cycling in full cells

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