Fluoroethylene Carbonate as Electrolyte Additive in Tetraethylene Glycol Dimethyl Ether Based Electrolytes for Application in Lithium Ion and Lithium Metal Batteries

Abstract

Solid electrolyte interphase (SEI) forming electrolyte additives are able to improve the performance of lithium ion and lithium metal batteries. In this work, the electrochemical performance of graphite and lithium metal, when using 1 M LiTFSI (lithium bis(trifluoro-methanesulfonyl)imide) in tetraethylene glycol dimethyl ether (TEGDME) with and without the addition of different amounts of fluoroethylene carbonate (FEC), is compared. It is shown that 1 M LiTFSI in TEGDME without additive is not able to form an effective SEI on graphite and that this electrolyte is also continuously decomposing on lithium metal. By the addition of > 2 wt% FEC, an effective SEI is formed on both, lithium metal and graphite, enabling good cycling stability. Furthermore, 1 M LiTFSI in TEGDME with FEC as additive is a suitable electrolyte for lithium iron phosphate (LFP) based lithium ion batteries.In analogy to metallic lithium and lithium-rich "lithium-alloys", lithiated (charged) graphite/carbon is thermodynamically unstable in the typically used organic solvent-based electrolytes. 1 Therefore, the carbon surfaces, which are exposed to the electrolyte, have to be kinetically protected by an solid electrolyte interphase (SEI). 2 Nevertheless, there are significant differences in the SEI formation process between metallic lithium and graphite/carbon. 3 Film formation on metallic lithium takes place right upon contact with the electrolyte. The various electrolyte components decompose spontaneously with low selectivity on the Li metal surface and parts of the decomposition products form the SEI. Due to an increase in IR drop across the SEI, with SEI growth, the reactivity of metallic lithium electrode vs. electrolyte decreases. As a consequence, the reduction of the electrolyte becomes more and more selective. The number of electrolyte components, which are still sensitive to reduction vs. the (now partially electronically "passivated") lithium electrode are limited. On the contrary, SEI formation on carbonaceous lithium storage materials takes place as a charge consuming side reaction in the first few cycles, especially during the first reduction (charge reaction). The electrolyte components, which are the least stable toward reduction, selectively react first. This makes SEI forming electrolyte additives particularly attractive for the use with carbonaceous anodes. When the electrolyte additive forms an effective SEI and is sensitive to reduction, the additive is reduced first and forms an initial SEI before reduction reactions of the other (main) electrolyte components takes place. 4 With the addition of suitable electrolyte additives, the initial SEI can be tailored and appropriate cell formation can be achieved. As a result of the different SEI formation processes, the SEI compositions on lithium metal and on carbonaceous anodes are different. 3,5 In addition to the above mentioned differences, the surface of metallic lithium is periodically renewed during cycling, causing formation of a new SEI in each following cycle (Figure 1). ...

A Sustainable Solid Electrolyte Interphase for High‐Energy‐Density Lithium Metal Batteries Under Practical Conditions

“…Among various approaches, LiF‐rich SEI (F‐SEI) and LiN x O y ‐contained SEI (N‐SEI) have arisen as exemplary methods to stabilize Li metal under mild conditions recently. F‐SEI is generated by fluorinated solvents or Li salts and N‐SEI is constructed by LiNO 3 additives in carbonate or ether electrolyte, respectively. Generally, F‐SEI and N‐SEI formed by additives as SEI precursors (<5 %, by weight or volume) cannot maintain sustainable due to irreversible exhaustion of limited additives under practical conditions .…”

Fabrication of Hybrid Silicate Coatings by a Simple Vapor Deposition Method for Lithium Metal Anodes

“…[1][2][3][4] In addition, nonuniform deposition of lithium onto LMA often leads to the formation of lithium dendrites, which is the primary cause of thermal runaway [5][6][7] and internal short circuit. [11,12] Given that these additives exhibit higher reduction potentials than those of electrolytes, LMA preferentially reacts with these additives to form solid electrolyte interphase (SEI). One strategy is to employ functional additives, such as lithium nitrate, [9] lithium polysulfide, [10] and fluoroethylene carbonate.…”

Key Aspects of Lithium Metal Anodes for Lithium Metal Batteries

“…A passivating film is immediately formed on the Li metal surface upon contact with the electrolyte due its high reactivity . The film that is formed after assembling the cell is known as an in situ SEI . However, the in situ SEI is weak and can develop cracks during stripping/plating.…”

“…However, the in situ SEI is weak and can develop cracks during stripping/plating. Therefore, suitable electrolyte additives are used to increase its stability . These additives usually have a strong affinity for Li metal and are used to strengthen the SEI film.…”