Highly toxic Organofluorophosphates as Electrochemical Aging Products during Formation at 4.8 V in NCM622/SG-3 Lithium Ion Batteries

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Organofluorophosphates (OFPs) have been reported to pose substantial health hazards due to their structural similarities to pesticides and nerve agents. Formation of OFPs in lithium ion batteries (LIB) due to hydrolysis of the conducting salt lithium hexafluorophosphate (LiPF6) and thermal stress has been discussed in literature. The electrochemical formation of OFPs in commercially available as well as in in-house formulated electrolytes containing fluoroethylene carbonate (FEC) as a film-forming additive is presented in this study. Cycling of the self-assembled LIB coin cells (CR2032) with cut-off voltages of 4.8 V gave rise to 15 non-acidic and two acidic OFPs after 10 charge/discharge steps. Here, the quantity of FEC had an impact on the amount of OFPs formed during electrochemical cycling experiments, which raises concerns of the utilization of FEC-containing electrolytes for high voltage applications. Moreover, the formation pathway of OFPs through EC-polymerization proposed in literature is evaluated and an alternative mechanism with FEC as the carbonyl carbon-donor is presented. Structure elucidation and separation of the formed OFPs is achieved by utilization of a) hydrophilic interaction liquid chromatography (HILIC) for acidic OFPs and b) reversed-phase (RP) chromatography for non-acidic OFPs hyphenated to a high-resolution ion trap time-of-flight mass spectrometer (IT-TOF-MS). The findings presented in this study support further investigation of the formation of OFPs in FEC-containing electrolytes, quantitative approaches and toxicological assessments due to the highly toxic nature of OFPs.
In state-of-the-art LIBs three components, a cyclic carbonate (e.g. EC, PC) and a linear carbonate (e.g. DEC, DMC, EMC) together with a conducting salt, namely LiPF6, formulates the electrolyte. The possible reaction products of these base components arising due to thermal stress, overcharging and even regular battery operation are enormous. That being said, there are also reactions taking place in the LIB that are beneficial, even necessary, for long cycling stabilities. The solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) are essential electrochemical reaction products within the LIB and are usually formed during the first charge/discharge cycles of battery operation, the so-called formation of the cell. However, while the generation of the SEI and CEI are a welcome reaction for the performance of the LIB, the relative instability of the electrolyte results in a manifold of unwanted side reactions. Some of the limitations of EC-based electrolytes emerge with high cut-off voltages. Due to the poor oxidation stability of EC, operating at voltages higher than 4.3 V vs Li/Li+ leads to reactions resulting in rapid irreversible capacity loss. With the incorporation of the film forming additive fluoroethylene carbonate (FEC) better capacity retention during cycling at higher cut-off voltages were observed. While FEC is used to form a stable SEI while minimizing the consumption of the electrolyte components, namely, EC, the benefits at high voltage applications are a welcome side effect. However, recent studies by Lin and coworkers illustrated that FEC has higher reactivity towards oxidation reactions than EC on LixCoO2-surfaces indicating the impact of the reactive surface area of the cathodes. NCM622, which is used in this study, is also known for its high surface area reactivity. This can lead to a number of new side reactions when using FEC-based electrolytes. While there are numerous possible electrolyte-reactions the instability of LiPF6 towards moisture and thermal stress is still most problematic. The supposed LiPF6 and LiF/PF5 equillibrium is disrupted by trace amounts of water, reacting to hydrofluoric acid (HF) and POF3. This highly reactive POF3 is converted by further reactions with water and the organic components of the electrolyte to finally form organophosphates (OPs) and potentially highly toxic organofluorophosphates (OFPs). In this work, an established method of HPLC-IT-TOF-MS was used to elucidate the structures of electrochemically produced OFPs. Focusing on the reactions taking place during the formation period self-constructed coin cells were electrochemically aged via ten charge/discharge cycles before analysis. The impact of the cut-off voltage was investigated for different electrolytes. The electrolytes employed in this study contained EC and EMC (30/70 wt%) with 1 molL-1 LiPF6 as conducting salt as well as different amounts of FEC (2, 5 and 10 wt.%). Coin cells (CR2032) were assembled and used for the investigation after ten cycles of battery operation. Triple layered polypropylene separators (FS2190, Freudenberg & Co. KG, Weinheim, Germany) were soaked with 150 µL of a specific electrolyte. The coin cells were assembled in a dry room (dew point of -65 °C, H2O

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