Mass Spectrometric Study on Volatilization Behavior of Electrolyte Solvents of Lithium-Ion Batteries

Abstract

For the distillation separation of organic solvents from lithium-ion battery (LIB) cells and their recycling, the volatilization behavior characteristics of lithium hexafluorophosphate (LiPF₆) with several types of alkyl carbonates in open cells were studied using mass spectrometry from 296 to 773 K. Similar volatilization behaviors were observed for three different dialkyl carbonates, i.e., dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and diethyl carbonate (DEC). When the electrolytic solution from single dialkyl carbonate with LiPF₆ was heated, solution mass loss occurred in three steps. In the first step near the boiling points of the dialkyl carbonates, it is expected that dialkyl carbonates (with trace POF₃ for DMC and EMC) can be recovered. The residues after heating to 773 K contained LiF and Li₃PO₄. For electrolytic solution from mixed alkyl carbonates with LiPF₆, dialkyl carbonates with higher volatilities could be recovered near their boiling points (with trace POF₃ for EMC + ethylene carbonate (EC)). EC with lower volatilities than the dialkyl carbonates could be recovered at higher temperatures near the EC boiling point with trace PF₅ and POF₃. The residues after heating to 773 K contained LiF and Li₃PO₄. With moisture, POF₃ generation, which must accompany hydrofluoric-acid generation, was observed from a lower temperature than that without moisture. Moisture also affected the chemical form of the residues. Lithium salts, such as Li₄P₂O₇, LiPO₃, LiF, or Li₃PO₄ were observed. The presence of water should be avoided when recovered electrolytic solution from LIBs is separated and recycled by distillation. Oxygen in air does not affect the volatilization behavior of LIB electrolytic solutions. These insights will be of practical importance to consider electrolytic solution recovery from LIB cells and its recycling by distillation without incineration to avoid damage by hydrofluoric-acid generation.

1. Introduction

Lithium-ion batteries (LIBs) have many attractive characteristics and are used in various applications, including hybrid electric vehicles, plug-in hybrid electric vehicles, and electric vehicles. The percentage of these next-generation cars as a proportion of newly sold cars has increased. Because of this increasing trend, it is expected that the number of end-of-life LIBs that are generated from waste vehicles will increase in future.

LIBs contain various metal elements, such as Co, Ni, and Li, and thus, a variety of processes have been investigated for their recycling, as summarized in recent reviews.^1–3) However, complex treatment costs for LIBs with multiple steps, such as incineration and solvent extraction, are often higher than the values of the contained valuable resources. Thus, end-of-life LIBs are often incinerated, and the resultant slags are used as road-bed materials without any valuable resource recovery. Simpler and more effective recycling processes for LIBs are required.

Honda Motor Co., Ltd.; Japan Metals & Chemicals Co., Ltd.; and Tohoku University investigated an incineration-less recycling system for LIBs with an effective disassembly of LIB units.⁴⁾ In this recycling system, electrolytic solution, i.e., lithium salt (typically lithium hexafluorophosphate, LiPF₆) with organic solvent (alkyl carbonates) inside the LIB cell was drained from the cell in a sealed condition before opening the cell for disassembly. In the drainage step, cleaning solution was injected to rinse the inside of the cell. To minimize environmental impact by additional solvent introduction, a typical electrolyte solvent, such as dimethyl carbonate (DMC), was used as the cleaning solution. Nickel–cobalt alloy was produced from extracted cathodes after disassembly, and it was confirmed that the produced alloy could be used as a metal hydride. Avoidance of electrolytic solution incineration limits corrosive gas production, such as hydrogen fluoride (HF), and avoids severe damage to components inside the LIB cells and to the treatment equipment.

Recovered electrolytic solution with cleaning solvent, which is typically a mixture of LiPF₆ and several organic carbonate types, could be separated by distillation for reuse.

A similar concept to recover electrolytic solution by depressurized vaporization with heating has been proposed in the literature;^5,6) however, fluorine in LiPF₆ is recovered as HF with water addition. To consider the recovery of these electrolyte solvents without HF generation, the vaporization behavior characteristics of this mixture and the influence of humidity must be investigated.

Similar research on the thermal behavior of electrolytic solution, i.e., a mixture of lithium salt and solvents in LIBs cell, has been conducted to study the performance change of batteries at elevated temperatures or its behavior during serious safety problems that are caused by flammable organic solvents.⁷⁾ Ravdel et al.⁸⁾ investigated the thermal decomposition of LiPF₆ at 358 K as solutions in alkyl carbonates, i.e., ethylene carbonate (EC), DMC, diethyl carbonate (DEC), and ethylmethyl carbonate (EMC), and mixed carbonates. The LiPF₆ decomposed to lithium fluoride (LiF) and phosphorus pentafluoride (PF₅), and the generated PF₅ reacted with the alkyl carbonates to form a variety of decomposition products, including carbon dioxide (CO₂), ethers (R₂O), alkyl fluorides (RF), phosphorus oxyfluoride (POF₃), and fluorophosphates (POF₂OR, POF(OR)₂). Kawamura et al.⁹⁾ investigated the thermal stability of alkyl-carbonate mixed solvent with LiPF₆ with/without water and found exothermic peaks between 503 and 553 K, which suggests that LiPF₆ is related to an exothermic decomposition reaction. They proposed that the decomposition of DEC solvent with LiPF₆ occurs as follows:   

\begin{equation} \text{LiPF$_{6}$ (s)} \leftrightarrow \text{LiF (s)} + \text{PF$_{5}$ (g)} \end{equation} (1)

  

\begin{align} &\text{C$_{2}$H$_{5}$OCOOC$_{2}$H$_{5}$} + \text{PF$_{5}$} \\ &\quad \rightarrow \text{C$_{2}$H$_{5}$OCOOPF$_{4}$} + \text{C$_{2}$H$_{4}$} + \text{HF} \end{align} (2)

  

\begin{equation} \text{C$_{2}$H$_{5}$OCOOC$_{2}$H$_{5}$} + \text{PF$_{5}$} \rightarrow \text{C$_{2}$H$_{5}$OCOOPF$_{4}$} + \text{C$_{2}$H$_{5}$F} \end{equation} (3)

  

\begin{equation} \text{C$_{2}$H$_{5}$OCOOPF$_{4}$} \rightarrow \text{POF$_{3}$} + \text{CO$_{2}$} + \text{C$_{2}$H$_{4}$} + \text{HF} \end{equation} (4)

  

\begin{equation} \text{C$_{2}$H$_{5}$OCOOPF$_{4}$} \rightarrow \text{POF$_{3}$} + \text{CO$_{2}$} + \text{C$_{2}$H$_{5}$F} \end{equation} (5)

  

\begin{equation} \text{C$_{2}$H$_{5}$OCOOPF$_{4}$} + \text{HF} \rightarrow \text{PF$_{4}$OH} + \text{CO$_{2}$} + \text{C$_{2}$H$_{5}$F} \end{equation} (6)

  

\begin{equation} \text{C$_{2}$H$_{5}$OH} + \text{C$_{2}$H$_{4}$} \rightarrow \text{C$_{2}$H$_{5}$OC$_{2}$H$_{5}$} \end{equation} (7)

Campion et al.¹⁰⁾ reported the thermal decomposition of lithium-ion battery electrolytic solution, 1.0 mol/L LiPF₆ in one or more carbonate solvents (combination of EC, DEC, and DMC). Thermal decomposition products included CO₂, ethylene (CH₂CH₂), dialkyl ethers (R₂O), alkyl fluorides (RF), POF₃, fluorophosphates (POF₂OR, POF(OR)₂), fluorophosporic acids (POF₂OH, POF(OH)₂), and oligoethylene oxides. They considered that trace protic impurities lead to POF₂OR generation, which decomposes LiPF₆ and carbonates autocatalytically. Yang et al.¹¹⁾ investigated the thermal stability of LiPF₆ salt and LiPF₆ in EC, propylene carbonate (PC), DMC, or EMC up to 573 K in the absence of any purported catalytic agents. Pure LiPF₆ salt was thermally stable up to 380 K in a dry inert atmosphere, and then dissociated to solid LiF and gaseous PF₅. With water in carrier gas (300 ppm), the decomposition onset temperature was lowered because of the direct thermal reaction between LiPF₆ and water vapor to form POF₃ and HF. Wang et al.¹²⁾ reported the thermal stability of LiPF₆ in EC + DMC + EMC using a microcalorimeter. They reported various exothermic or endothermic reactions of LiPF₆ with these alkyl carbonates. Handle et al.¹³⁾ studied LiPF₆ decomposition with thermal aging at 333 K. With proton impurities, the decomposition of LiPF₆ was enhanced at high temperatures. The main decomposition product of LiPF₆ in a mixture of EC and DEC at 333 K was difluorophosphoric acid (POF₂OH). Wilken et al.¹⁴⁾ reported the thermal stability of LiPF₆ in organic solvents at 358 K. For LiPF₆ in EC/DMC stored at 358 K up to 160 h, significant amounts of CO₂ were evolved. The evolution of POF₃, POF(OH)₂, POF₂(OCH₂CH₂)_nF, and POF₂OCH₃ as reactive decomposition products was also observed. EC was less reactive compared with DMC.

Although a variety of investigations have been conducted, these experiments have been conducted in a closed system, e.g., in sealed pans, because they focused on the performance change of batteries at elevated temperatures or its behavior during serious safety problems. Thus, in this study, for the distillation separation of organic solvents recovered from LIB cells and their recycling, a basic study of the volatilization behavior characteristics of LiPF₆ with several alkyl carbonate types was conducted in open cells using mass spectrometry from 296 to 773 K. Water and oxygen, which may affect the volatilization behavior and are important for recycling, were also investigated.

2. Experimental

2.1 Materials

Table 1 summarizes the typical electrolytic solution components of the LIBs that were used in the study, i.e., LiPF₆, DMC, EMC, DEC, and EC. Battery grade reagents for these alkyl carbonates were from the Wako Pure Chemical Industries Ltd. (Osaka, Japan). Water contents in the DMC, EMC, DEC, and EC were reported by the company to be 12, 16, 10, and 27 ppm, respectively. Battery grade electrolytic solutions of 1.0 mol/L of LiPF₆ in DMC/EMC/DEC/DMC + EC: 50/50 (v/v)/EMC + EC: 50/50 (v/v)/DEC + EC: 50/50 (v/v) were from Sigma-Aldrich Co. LLC. (St. Louis, MO, US). Water contents in the electrolytic solution measured by the Karl–Fischer method were reported by the company to be 10, 10, 9, 9, 11, and 5, respectively.

Table 1 Summary of chemicals in electrolytic solution.

2.2 Procedure for volatilization experiments

A skimmer-MS system (STA409CD/3/403/5/G, Netzsch GmbH, Selb, Germany, MS: QMG422, Inficon, Bad Ragaz, Switzerland) was used. The skimmer-MS system allows for simultaneous thermal analysis and mass spectrometry with ideal vapor gas transfer into the mass spectrometer under homogenous heating conditions,^18,19) and has been used extensively for vapor gas analyses.^20–23) Electrolytic solution samples were heated in an Al open cell (φ 6.8 mm, 85 µL) under near-atmospheric-pressure conditions (non-vacuum conditions), and the evaporating gas was analyzed with the mass spectrometer. Changes in electrolytic solution mass were measured during heating. DMC, EMC, and DEC are liquid at room temperature, and 70–85 µL of the electrolyte solvent was collected in a glove box in an Ar atmosphere, and introduced into the Al cell. EC is solid at room temperature. The EC was heated and melted in the Ar atmosphere, and 70–85 µL was collected and introduced into the Al cell. For single- or mixed-electrolyte solvents with LiPF₆, 70–85 µL of the electrolytic solution was collected in a glove box in an Ar atmosphere, and introduced into the Al cell. Gas replacement was conducted for 15 min under an Ar flow of 70 mL/min after Al cell setting in the skimmer-MS system. The heating temperature range for the electrolytic solution samples was 296 to 773 K, at 10 K/min. The Ar gas flow rate was set at 70 mL/min. The gas flow was regulated at room temperature, and then introduced to the system in all the experiments. For mass spectrometry, the m/z range was 1 to 200. The ionization voltage was set to 70 eV. The voltage of the secondary electron multiplier was 2000 V. The pressure in the mass spectrometer system was ∼8 × 10⁻⁴ Pa. Solid residues in Al cells after heating were analyzed by X-ray diffractometry (XRD, D2 PHASER, Bruker AXS GmbH, Karlsruhe, Germany Radiation source: Cu-Kα).

To investigate the influence of H₂O, the volatilization behavior of alkyl carbonates with and without moisture content was examined. In the series of experiments, moisture was introduced by Ar gas bubbling into water. To decrease the background noise from H₂O addition, the Ar flow rate was set to 250 mL/min. The estimated introduced H₂O was ∼12 g-H₂O/m³-Ar from temperature and humidity data that were obtained by a thermohygrometer and the state and Antoine equations. To investigate the O₂ influence, air was used instead of Ar, and the gas flow rate was set to 70 mL/min.

3. Results and Discussion

3.1 Volatilization behavior of alkyl carbonate solvents without LiPF₆

When each solvent, i.e., DMC, EMC, DEC, and EC, was heated in an open cell from room temperature to ∼573 K, a uniform mass loss was observed for all solvents near each boiling point until volatilization was completed. Figure 1 shows one of the typical results, namely, the volatilization behavior of DMC. The DMC mass in an open cell decreased uniformly around its boiling point (363.7 K). The solvent volatility and mass losses were not negligible even at room temperature, and thus, it was impossible to determine the initial sample mass accurately in an open cell. Therefore, the vertical axis in Fig. 1 shows mass changes from the beginning of the measurements. Figure 2 shows a typical result, which is the mass spectrum of the DMC vapor at ∼333 K. The spectrum is like that for DMC, which is reported in the mass spectra database.²⁴⁾ No obvious change in mass spectra for the DMC vapor was observed for all studied temperatures. The results were similar for all other solvents, i.e., EMC, DEC, and EC. Therefore, no significant thermal decomposition of the single solvent during its evaporation occurred for the studied temperature range.

Fig. 1

Volatilization behavior of DMC (boiling point: 363.65 K).

Fig. 2

Mass spectra of DMC vapor at ∼333 K.

When mixed solvents, i.e., EC + DMC, EC + EMC, and EC + DEC, were heated in an open cell from room temperature to ∼573 K, a simple overlap of mass losses and mass spectra by volatilization of each mixed solvent was observed (data not shown). Therefore, it is considered that any interactions between each solvent, such as the reaction between the solvents and their decomposition, did not occur for the studied temperature range.

3.2 Volatilization behavior of single alkyl carbonate with LiPF₆

Similar volatilization behaviors have been observed for all single dialkyl carbonates, i.e., DMC, EMC, and DEC with LiPF₆. All mixtures lost mass in three steps and the volatilization mechanism was the same. Table 2 summarizes the volatilization behaviors for these three carbonates with LiPF₆. The results for DMC with LiPF₆ are described in detail as representative of all dialkyl carbonates. Figures 3 and 4 show the volatilization behavior of DMC with LiPF₆ and the ion current for a dominant ionization product of DMC (m/z = 15), PF₅ (m/z = 107), and POF₃ (m/z = 104) as a function of temperature.

Table 2 Volatilization behavior of single dialkyl carbonates with LiPF₆.

Fig. 3

Volatilization behavior of DMC with LiPF₆ and ion current for a dominant ionization product of DMC (m/z = 15) as a function of temperature.

Fig. 4

Volatilization behavior of DMC with LiPF₆ and ion currents for a dominant ionization product of PF₅ (m/z = 107) and POF₃ (m/z = 104) as functions of temperature.

This mixture lost mass in three steps. First, a large mass loss occurred from room temperature to 391 K. This temperature range corresponds to the boiling point of DMC (363.7 K). The volatilization species in the first step were estimated to be DMC from mass spectra data. Volatilization of trace amounts of POF₃ were also observed for DMC and EMC. The ion current for POF₃ was lower than those for DMC and EMC. No significant ionization products that can be assigned to other chemicals were observed in the mass spectra data. Ionization products with a m/z larger than 108 were negligible for all samples and the temperature ranges studied. The mass spectra for HF cannot be measured because its mass (m/z = 20) overlapped with that for divalent Ar (m/z = 40/2 = 20), which was used as an atmospheric gas. The generation of POF₃ at elevated temperatures was reported in previous research in a closed system.^8–11,14) The generation of POF₃ is attributed to the following reaction derived from trace water impurity in the electrolytic solution:   

\begin{equation} \text{LiPF$_{6}$} + \text{H$_{2}$O} \rightarrow \text{LiF} + \text{POF$_{3}$} + \text{2HF}. \end{equation} (8)

The second mass loss occurred from 413 to 453 K. The volatilization species are DMC with trace amounts of PF₅ and POF₃. The generation of trace PF₅ is derived from the thermal decomposition of LiPF₆ (eq. (1)). Trace POF₃ may be generated from the following reaction derived from gaseous PF₅ and water impurity:   

\begin{equation} \text{PF$_{5}$ (g)} + \text{H$_{2}$O (g)} \rightarrow \text{POF$_{3}$ (g)} + \text{2HF (g)}. \end{equation} (9)

Generation of HF by eq. (8) could be considered. Reactions that are like eqs. (2) to (5) without water may also be possible, however, any evidence for the generation of plausible chemicals from DMC, e.g., specific m/z numbers for the mass of CH₃OCOOPF₄, CH₄, CH₃F, CO₂, or CH₃OCH₃ were not found in the observed mass spectra. For DMC volatilization, the temperature range was higher than the boiling point of DMC (363.7 K). Thus, part of the DMC was stabilized with LiPF₆, and the DMC was released and vaporized with LiPF₆ decomposition.

A smaller third mass loss occurred from 523 to 623 K. The volatilization species here may be POF₃. Further investigation is required; however, the mass loss may be derived from Li salt decomposition.

Figure 5 shows the XRD patterns of the residues after heating. For all heating residues, peaks that can be assigned to LiF and Li₃PO₄ were observed. The peaks that were assigned to Li₃PO₄ were smaller than those for LiF. LiF generation can be explained from the simple thermal decomposition of LiPF₆ as eq. (1). The generation of Li₃PO₄ has not been reported elsewhere.

Fig. 5

XRD patterns of residues after heating for single dialkyl carbonates, symbol ● (LiF, PDF No. 01-070-1934) and △ (Li₃PO₄, PDF No. 00-025-1030).

If we consider the recycling of recovered electrolytic solution, i.e., a mixture that consists of single dialkyl carbonate (DMC, EMC, or DEC) with LiPF₆, more than half of the dialkyl carbonate with trace POF₃ can be recovered by volatilization around their boiling points. The rest of the dialkyl carbonate with trace PF₅ and POF₃ can be recovered in the second volatilization step. However, a heating temperature above the boiling point is required. The third vaporization step produces trace POF₃, thus it is considered that the volatilization step must be avoided. The composition of the final solid residue at 773 K was LiF and Li₃PO₄.

3.3 Volatilization behavior of mixed alkyl carbonates with LiPF₆

Similar volatilization behaviors were observed for all mixed alkyl carbonates, i.e., DMC + EC, EMC + EC, and DEC + EC with LiPF₆. All mixtures lost mass in three steps and the volatilization mechanism was the same. Table 3 summarizes the volatilization behaviors for mixed carbonates with LiPF₆. Here, results for DMC + EC with LiPF₆ are described in detail as an example. Figures 6 and 7 show the volatilization behavior of DMC + EC with LiPF₆ and the ion current for a dominant ionization product of DMC (m/z = 59), EC (m/z = 88), PF₅ (m/z = 107), and POF₃ (m/z = 104) as a function of temperature. DMC and EC have mass fragments for m/z = 15 and 29. Thus, mass fragments with m/z = 59 and 88 were selected for DMC and EC instead of m/z = 15 and 29, respectively.

Table 3 Volatilization behavior of mixed alkyl carbonates with LiPF₆.

Fig. 6

Volatilization behavior of DMC + EC with LiPF₆ and ion current for a dominant ionization product of DMC (m/z = 59) and EC (m/z = 88) as a function of temperature.

Fig. 7

Volatilization behavior of DMC + EC with LiPF₆ and ion currents for a dominant ionization product of PF₅ (m/z = 107) and POF₃ (m/z = 104) as functions of temperature.

This mixture lost mass in three steps. The first mass loss occurred from room temperature to 378 K. This temperature range corresponds to the DMC boiling point (363.7 K). The estimated volatilization species in the first step was DMC. Volatilization of trace amounts of POF₃ was also observed for EMC + EC with LiPF₆. No significant ionization products that can be assigned to other chemicals were observed in the mass spectra data. This result indicates that an azeotrope did not form around this temperature.

The second large mass loss occurred from 378 to 523 K. This range corresponds to the EC boiling point (521 K). The volatilization species here can be considered as EC with trace amounts of PF₅ and POF₃. For single dialkyl carbonates with LiPF₆, the dialkyl carbonates volatilized in two steps; however, they volatilized in a single step with EC. The observed peak for the EC-related ion product (m/z = 88) at ∼500 K in Fig. 6 overlapped two peaks with peaks at 503 K and 513 K. A similar result was also observed for DEC + EC with LiPF₆. Like the volatilization behavior of single DMC, EMC, or DEC with LiPF₆ as described in the former section, EC stabilization with LiPF₆ occurred, and the stabilized EC was released with LiPF₆ decomposition with an increase in temperature. These results are consistent with previous studies that reported strong solvation of EC with lithium ions.^25,26)

The generation of trace PF₅ and POF₃ may be derived as shown in eqs. (1), (8), and (9).

A smaller third mass loss occurred from 523 to 623 K. The volatilization species here may be POF₃. Further investigation is required; however, the mass loss may be derived from the decomposition of residue that was derived originally from LiPF₆.

Figure 8 shows XRD patterns of the residues after heating. For all heating residues, peaks that can be assigned to LiF were observed. Small peaks that were assigned to Li₃PO₄ were observed. LiF generation can be explained from the simple thermal decomposition of LiPF₆ as eq. (1).

Fig. 8

XRD patterns of residues after heating for mixed alkyl carbonates, symbol ● (LiF, PDF No. 01-070-1934) and △ (Li₃PO₄, PDF No. 00-025-1030).

If we consider the recycling of recovered electrolytic solution, i.e., the mixture of mixed alkyl carbonate (DMC + EC, EMC + EC, or DEC + EC) with LiPF₆, dialkyl carbonates with lower boiling points, i.e., DMC, EMC, and DEC, can be recovered by volatilization around their boiling point. The remaining EC evaporated, probably in two steps. In the latter volatilization step, EC was recovered with trace PF₅ and POF₃. Further heating above 523 K produced trace POF₃ vapor. The composition of the final solid residue at 773 K was LiF with trace Li₃PO₄.

3.4 Influence of H₂O on volatilization behavior of alkyl carbonates

The volatilization behaviors of DMC + LiPF₆ with/without moisture were compared at the increased Ar gas flow rate of 250 mL/min. Figure 9 shows the results. Although the observed peak shape and ion current values changed slightly from the former results shown in Figs. 3 and 4, a similar DMC volatilization behavior was observed without moisture. With moisture, a large mass loss occurred between room temperature to 393 K, a slight mass loss occurred at ∼580 K, and a modest mass decrease up to 773 K was observed. The mass loss that corresponds to the second large mass loss that was derived from the second DMC volatilization without moisture was not observed. Therefore, DMC was vaporized in a single step with moisture. Ionic currents from PF₅-related products were not observed. The POF₃-related mass showed an increase in ionic strength from lower temperatures compared with no H₂O addition, i.e., ∼390, 580, and 710 K. It is speculated that the HF is generated with POF₃ generation as shown in eqs. (8) and (9). Thus, with moisture, HF can be generated from a lower temperature range. Figure 10 shows the XRD patterns of the residue after heating. The residue contains Li₄P₂O₇, LiPO₃, and LiF. Li₃PO₄, which was observed for the residue without moisture, was not detected. Thus, moisture can affect the residual solid composition.

Fig. 9

Volatilization behavior of DMC with LiPF₆ and ion current for a dominant ionization product of (a) DMC (m/z = 15), (b) PF₅ (m/z = 107), and POF₃ (m/z = 104) with/without H₂O as a function of temperature (solid lines are with H₂O and dotted lines are without H₂O).

Fig. 10

XRD patterns of residues after heating for DMC + LiPF₆ with/without H₂O; symbol ● (LiF, PDF No. 01-070-1934), △ (Li₃PO₄, PDF No. 00-025-1030), ○ (LiPO₃, PDF No. 01-070-5642), and ▲ (Li₄P₂O₇, PDF No. 01-077-1045).

Figure 11 shows the volatilization behaviors of DMC + EC + LiPF₆ with moisture. Data are not shown, but the similar volatilization behaviors of DMC and EC were observed without moisture as shown in Figs. 6 and 7 (the Ar flow rate of the experiment without moisture was 250 mL/min to decrease the background noise from H₂O addition, and was different from that of 70 mL/min for the experiments shown in Figs. 6 and 7). A large mass loss between room temperature and 383 K and further mass loss between 383 and 493 K were observed. In addition to the large mass losses, a slight mass loss between 558 and 598 K and a subsequent modest mass decline up to 773 K were observed. The ion current for a dominant ionization product of DMC (m/z = 59) shows that the DMC vaporized in a single step around its boiling point (364 K) as occurred without moisture. The ion current peak for a dominant ionization product of EC (m/z = 88) appeared at ∼483 K, but the peak separation was unclear compared with the case without moisture. Ionic currents that were derived from PF₅-related products were not observed. An increase in the POF₃-related ionic current was observed at ∼460, 580, and 730 K. The temperature range for the first peak at ∼460 K was much lower than that without moisture. Therefore, HF generation occurred from a lower temperature range with moisture.

Fig. 11

Volatilization behavior of DMC + EC with LiPF₆ and ion current for a dominant ionization product of (a) DMC (m/z = 59), (b) EC (m/z = 88), (c) PF₅ (m/z = 107), and (d) POF₃ (m/z = 104) with/without H₂O as a function of temperature (solid lines are with H₂O and dotted lines are without H₂O).

These results show that moisture does not affect the volatilization behavior of DMC with a lower boiling point, but it affects the EC with a higher boiling point. Moisture enhanced the reaction of eq. (8), and POF₃ was generated before LiPF₆ pyrolysis as shown in eq. (1). POF₃ generation at ∼580 K may result from Li salt decomposition. Figure 12 shows the XRD patterns of the residue after heating. The residue contains Li₄P₂O₇, LiPO₃, and LiF. Li₃PO₄ in the residue without moisture remained undetected. Thus, moisture can affect the residual solid composition.

Fig. 12

XRD patterns of residues after heating for DMC + EC + LiPF₆ with/without H₂O; symbol ● (LiF, PDF No. 01-070-1934), △ (Li₃PO₄, PDF No. 00-025-1030), ○ (LiPO₃, PDF No. 01-070-5642), and ▲ (Li₄P₂O₇, PDF No. 01-077-1045).

With moisture, POF₃ generation, which must accompany HF generation, was observed from a lower temperature than for that without moisture. High-purity solvent recovery becomes impossible and HF damage becomes possible. Moisture also affects the chemical form of the residual lithium salts. Thus, water should be avoided for recovered electrolytic solution recycle from LIBs by distillation.

3.5 Influence of O₂ on volatilization behavior of alkyl carbonates

The volatilization behaviors of DMC/DMC + EC with LiPF₆ with/without oxygen were investigated. Figures 13 and 14 show that no significant differences were observed. Thus, the influence of oxygen in air on the volatilization behavior of alkyl carbonates is small.

Fig. 13

Volatilization behavior of DMC with LiPF₆ and ion current for a dominant ionization product of (a) DMC (m/z = 15), (b) PF₅ (m/z = 107), and POF₃ (m/z = 104) with/without O₂ as a function of temperature (solid lines are for Air (with O₂) and dotted lines are for Ar (without O₂)).

Fig. 14

Volatilization behavior of DMC + EC with LiPF₆ and ion current for a dominant ionization product of (a) DMC (m/z = 59), (b) EC (m/z = 88), (c) PF₅ (m/z = 107), and (d) POF₃ (m/z = 104) with/without O₂ as a function of temperature (solid lines are with O₂ and dotted lines are without O₂).

Figure 15 shows the XRD patterns of residues after heating for DMC + LiPF₆/DMC + EC + LiPF₆ in air (see Figs. 5(a) and 8(a) for residues from the same targets without O₂). Even with O₂ in air, the chemical form of the residual lithium salts was not influenced.

Fig. 15

XRD patterns of residues after heating for DMC + LiPF₆/DMC + EC + LiPF₆ in air, symbol ● (LiF; PDF No. 01-070-1934) and △ (Li₃PO₄; PDF No. 00-025-1030).

Therefore, oxygen in air does not affect the volatilization behavior of the LIB electrolytic solutions extensively, and operation of the recovery process in air is possible.

4. Conclusions

For the distillation separation of organic solvents recovered from LIB cells and their recycling, we conducted a basic study of the volatilization behavior characteristics of LiPF₆ with several types of alkyl carbonates in open cells with mass spectrometry over a wide temperature range.

Similar volatilization behaviors were observed for three different dialkyl carbonates, i.e., DMC, EMC, and DEC. When electrolytic solution from a single dialkyl carbonate with LiPF₆ was heated, the solution lost mass in three steps. In the first step near the boiling points of the dialkyl carbonates, it is expected that dialkyl carbonates (with trace POF₃ for DMC and EMC) can be recovered. The residues after heating to 773 K contained LiF and Li₃PO₄. When the electrolytic solution of mixed alkyl carbonates with LiPF₆ was heated, dialkyl carbonates with higher volatilities (with trace POF₃ for EMC + EC) could be recovered near their boiling points. EC with lower volatilities than DMC, EMC, and DEC could be recovered at a higher temperature near the EC boiling point with trace PF₅ and POF₃. The residues after heating to 773 K contained LiF and Li₃PO₄. In both cases, carbonate stabilization with lower volatilities with lithium salts was observed.

With moisture, POF₃ generation, which accompanies HF generation, was observed from a lower temperature compared with that without moisture. Moisture also affects the chemical form of the residues, and lithium salts, such as Li₄P₂O₇, LiPO₃, LiF, and Li₃PO₄ were observed. Water should be avoided in the recycling of recovered electrolytic solution from LIBs by distillation. Oxygen in air does not affect the volatilization behavior of electrolytic solutions of LIBs significantly.

These insights are of practical importance to consider electrolytic solution recovery from LIB cells and recycling by distillation without incineration to avoid damage by HF generation.

Acknowledgments

This research was supported by the Environment Research and Technology Development Fund (3K152013, principal research institution: Honda Motor Co., Ltd.) of the Ministry of the Environment, Japan. The skimmer mass system used in the study was shared equipment from the High Efficiency Rare Elements Extraction Technology Area in the Tohoku Innovation Materials Technology Initiatives for Reconstruction from the Ministry of Education, Culture, Sports, Science and Technology in Japan.

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