Preparative hydrophilic interaction liquid chromatography of acidic organofluorophosphates formed in lithium ion battery electrolytes

Highlights

• Two-step preparative HILIC fractionation of lithium ion battery electrolyte.

• Extraction of potentially hazardous organofluorophosphate decomposition products.

• Effective removal of electrolyte matrix for toxicological investigations.

Abstract

The expansion of lithium ion battery (LIB) application is accompanied by the growth of battery pack sizes. This progression emphasizes the consideration of electrolyte safety as well as environmental aspects in case of abuse, accident, or recycling. Hexafluorophosphate is one of the most commonly used conducting salt anions in electrolytes. It has great potential to degrade to various acidic and non-acidic organo(fluoro)phosphates with presence of water and during battery cell operation. Consequently, toxicological investigation on these organo(fluoro)phosphates has emerged because they either have structural similarities as chemical warfare agents or play a widespread physiological role as phosphates in the human body. This circumstance underlines the need of isolated examination of these compounds for safety assessment. In this work, we used hydrophilic interaction liquid chromatography for the extraction of acidic organofluorophosphates from thermally aged LIB electrolytes. The developed two-step fractionation method provided high separation selectivity towards acidic head groups, which allowed the separation of undesired matrix and target compounds. These findings facilitate isolated toxicological investigations on organofluorophosphates that are beneficial for environmental and safety research, the battery cell industry, and human safety surveillance in regard to aged LIB electrolytes.

Introduction

Since their commercialization in the 1990s, the fields of application of lithium ion batteries (LIBs) grew tremendously. Today, hand-held devices, fully electric vehicles (EVs), ships, or stationary energy storage systems (ESS) with hundreds of megawatt power for stabilization of the power grid are realized and planned [1,2]. Despite the enlargement of battery systems, the fundamental cell chemistry and composition remain mostly the same. The applied electrolyte is based on the ternary composition of lithium hexafluorophosphate dissolved in a mixture of linear and cyclic organic carbonates [3].

Since the electrolyte represents the best compromise regarding the demands of LIB operation, the hexafluorophosphate anion suffers from a certain redox instability. The reaction cascade, first described by Plakhotnyk et al., leads to a huge variety of organophosphates (OPs) and organofluorophosphates (OFPs). Reports by Campion et al., emphasizing the toxic potential of these compounds, raised interest in qualitative and quantitative investigations [[4], [5], [6], [7]]. Due to the acidic and non-acidic natures of these compound classes, the applied analytical techniques ranged from gas chromatography [8,9], ion chromatography [[10], [11], [12]], to other liquid chromatography [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22]] hyphenated with a variety of detection systems [23]. Especially, the use of larger LIB cell and battery systems and therefore resulting high amounts of electrolyte emphasize the importance of decomposition product investigation. While a high toxic potential from non-acidic structures can be assumed, attributable to the structural similarity to pesticides and chemical warfare agents, the effect of acidic molecules needs further investigations. Especially, fluorinated species are of importance, since the mode of action for irreversible binding to the acetylcholinesterase enzyme is based on a phosphorylation of the serine under fluorine cleavage at the enzymes active site [24]. Although two possible approaches by either synthesis or extraction can be used to obtain pure acidic OFPs. However, the synthesis of OFPs and related compounds is under strong restriction and surveillance (Chemical Weapons Convention). Moreover, the synthesis of every compound is very complex and costly due to the board structural diversity [25]. Therefore, an extraction method is depicted in this study. The challenge of high conducting salt concentration, as well as the separation of small, structural similar and ionic compounds can be solved by hydrophilic interaction liquid chromatography (HILIC). Particularly, the head group dependent separation mechanism of iHILIC Fusion(+) columns, demonstrated among others by Vosse et al., is beneficial for preparative application, hence a representative fraction of formed OFP species can be extracted at once [15,26].

This work presents a preparative HILIC method for the fractionation of acidic OFPs from LIB electrolytes. The method development performed with an iHILIC Fusion(+) column incorporates the consideration of (i) suitable chromatographic resolution, (ii) high organic solvent amount for evaporation (iii) minimum eluent salt content and (iv) variation of injection volume in order to find the best compromise between fractionation and chromatographic performance. The two main compounds (hexafluorophosphate, difluorophosphate) with the highest interfering effect due to high concentrations were monitored parallel to three OFP targets (Fig. 1). Subsequently, the successful two-step fractionation of OFPs from solvent, conducting salt and their decomposition products (gas chromatography – mass spectrometry; GC–MS, HILIC-MS) was demonstrated. The OFP content and acetonitrile residues were quantified by inductively coupled plasma – mass spectrometry (ICP-MS) and nuclear magnetic resonance spectroscopy (NMR), respectively.