Development of Fluorine-Free Electrolytes for Aqueous-Processed Olivine-Type Phosphate Cathodes

1. Introduction

Transitioning towards a carbon-neutral society presents various challenges, including those in electrochemical energy storage. Since the 1990s, lithium-ion batteries have been the most common commercial energy storage device. However, current commercial lithium-ion batteries (LIBs) use fluorinated salts, solvents, and electrolyte additives. The main hazards of fluorinated compounds include potential toxicity, a negative environmental impact, and complicated recycling [1,2,3].

The most commercial electrolyte uses the fluorinated salt lithium hexafluorophosphate (LiPF₆), which contains 75 wt.% fluorine, where the PF₆⁻ anion can release toxic substances like HF and POF₃ in the presence of moisture, high temperatures, or acids [4,5,6]. Although fluorine-containing electrolytes are essential for optimal performance [1,7], alternatives without fluorine have been explored. Salts such as lithium perchlorate (LiClO₄), which offers high solubility in aprotic solvents, high conductivity, and good electrochemical stability, have been explored. However, the high oxidation state of Cl^VII makes it highly explosive in organic solutions [8,9]. Lithium tetrachloroaluminate (LiAlCl₄) is another fluorine-free lithium salt that has been extensively studied, but side reactions forming LiCl can block electrode reactions [10]. Additionally, AlCl₄⁻ oxidizes at 4 V, low for many applications. For the half-cell configuration for Li/LFP at 1C in the potential range of 0–2 V, the specific capacity obtained for LiAlCl₄ 3SO₂ is 148 mAh·g⁻¹.

Lithium bis(oxalate)borate (LiBOB) has high electrochemical (>4.5 V vs. Li⁺/Li) and thermal stability, but its solubility in the EC:EMC mixed solvent system is below 0.9 mol kg⁻¹ solvent and its conductivity is low, being 2–3 mS cm⁻¹ at 25 °C [11,12,13,14,15]. The specific capacity of LiBOB was found to be in the range of 90–120 mAh·g⁻¹ in GBL and 135 mAh·g⁻¹ in PC:EMC:DMC at room temperature. Adding to that, in the half-cell Li/LFP cell configuration at C/10 in the range potential of 2.5–4.4 V and at room temperature, a specific capacity of 165 mAh·g⁻¹ in LiDCTA:PC, 158 mAh·g⁻¹ in LiDCTA:PEGDME, and 165 mAh·g⁻¹ in LiB(CN)₄:PEGDME was found [6]. Therefore, despite some previously reported fluorine-free lithium salts, most do not meet the commercial electrolyte requirements [16,17]. On the anion side, weakly coordinating FSI (FSO₂NSO₂F), TFSI (CF₃SO₂NSO₂CF₃), and “Hückel-type anions” like 2-trifluoromethyl-4,5-dicyanoimidazole (TDI) have been developed for Li-ion cells, yet all still contain fluorine [18,19].

This work presents a novel fluorine-free alternative to existing salts for Li-ion battery electrolytes, lithium 1,1,2,3,3-pentacyanopropenide (LiPCP, Figure 1). We found that 1,1,2,3,3-pentacyanopropenide (PCP) is among the most stable carbanions. Its remarkable stability is attributed to the inductive effect of the five electron-withdrawing cyano groups. Consequently, the negative charge is delocalized across the central propenide structure [20,21]. R. H. Boy reported that no changes in the spectra of PCP were noted up to 11 M perchloric acid or ~85% sulfuric acid, indicating that the free anion may still exist even in this highly acidic environment [22], making this salt stable and with no side reactions in contact with air, water, and even highly acidic solutions. Furthermore, this novel lithium salt presents high thermal stability up to >300 °C [23].

The new generation of lithium-ion batteries targets sustainable and greener chemistries, with a special focus on cobalt-free cathode materials. Critical raw materials such as cobalt (Co) and nickel (Ni) are classified as carcinogenic, mutagenic, and toxic to reproduction (CMR) [24]. Cobalt, in particular, is a major cost driver and raises significant moral and environmental concerns due to questionable mining conditions [25,26,27,28]. There are, however, alternative electrode materials that do not include critical resources. Phospho-olivine-type cathode materials like LiFePO₄ (LFP) and LiMn_xFe_1−xPO₄ (LMFP) with 0 ≤ x ≤ 1 are considered eco-friendly, since they consist solely of abundant elements [29,30]. Furthermore, the high structural stability of the polyanionic phosphate network and the relatively low operational voltage window, which prevents unwanted parasitic reactions of the battery electrolyte, allow a long cycle and span life for olivine-based LIBs [31,32,33].

Fluorinated compounds are not only in electrolytes, but also in electrodes, with fluorinated binders like polyvinylidene fluoride (PVdF), which involves the use of N-methyl-2-pyrrolidone (NMP) as a solvent, both used in most commercial electrodes. The binder plays an important role in the performance, cost, environmental impact, and recycling possibility of the battery [34,35]. At present, standard electrode processing uses PVdF as a binder. Despite its good electrochemical stability and adhesion ability [36,37,38,39], it is expensive and makes the battery recycling process problematic. In fact, during electrode manufacturing, this binder is dissolved into the hazardous, teratogenic, and irritating solvent N-methyl-2-pyrrolidone (NMP), [40,41,42]. For this reason, water-soluble binders are gaining more attention. Among these, sodium carboxymethyl cellulose (CMC-Na) has been considered as a potential binder for lithium-ion batteries. CMC provides several advantages, including being economically available, hydrophilic, environmentally friendly, biodegradable, and compatible with potential electrode materials to develop green and sustainable LIBs [43,44,45,46].

A SEI-stabilizing electrolyte additives, vinylene carbonate (VC) and acetonitrile (AN), both fluorine-free, are proposed as alternatives to other popular additives like fluoroethylene carbonate (FEC), which is the most widely used commercial electrolyte additive [47,48]. VC is reductively decomposed or oxidized prior to other electrolyte components [49,50]. AN has a low cost, and its low viscosity and very high dielectric constant make it a good option for dissolving salts [51,52].

Additionally, the growing awareness and concern about the environmental and health impacts of per- and polyfluoroalkyl substances (PFAS) have led to significant regulatory actions [53]. For instance, the European Union has proposed a comprehensive ban on all non-essential PFAS by 2025 and across all uses by 2030 [54]. Similarly, the United States Environmental Protection Agency (EPA) has been working on regulations to limit PFAS in industrial uses. Canada is also following suit, with initiatives to restrict these substances under the Canadian Environmental Protection Act. These bans highlight the urgent need to develop sustainable alternatives in various industries, including energy storage. In this context, the development of fluorine-free batteries and components becomes not only a technological advancement, but also a crucial step towards a safer and more sustainable society.

Therefore, in this work, we report the viability of this new fluorine-free lithium salt, LiPCP with fluorine-free electrolyte additives working with aqueous-processed LFP and LMFP cathodes. For this purpose, we report the electrolyte conductivity at different concentrations and temperatures, an analysis of the electrolyte stability, and an analysis covering voltammetry and galvanostatic cycling, demonstrating the capability of fluorine-free electrolytes for aqueous-processed olivine-type cathode materials. These findings represent the initial phase of investigating safer and more environmentally friendly electrolytes with aqueous-processed electrodes for lithium-ion batteries.

3. Materials and Methods

3.1. Reagents

Ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl-methyl carbonate (EMC), diethyl carbonate (DEC), and vinylene carbonate (VC) battery-grade solvents were purchased from BASF (Ludwigshafen am Rhein, Germany). Lithium hexafluorophosphate (LiPF₆, battery grade) and acetonitrile (AN, anhydrous, 99.98%) were obtained from Sigma Aldrich (Saint Luis, MO, USA). A total of 1 mol·kg⁻¹ LiPF₆ in EC:DMC (30:70 wt.%) was purchased from E-Lyte Innovations (battery grade). The metallic lithium foil was purchased from Honjo Metal. The lithium iron phosphate (LFP, LiFePO₄, 1.45 wt.% carbon) and lithium iron manganese phosphate (LMFP, LiMn_0.6Fe_0.4PO₄, 1.5~2.5 wt.% carbon) powdered materials were acquired from MTI (Richmond, CA, USA) and MSE Supplies, respectively. Sodium carboxymethyl cellulose (CMC, average M_w of 250,000, degree of substitution equal to 0.9) was purchased from Sigma Aldrich. Super-P and Ketjenblack conductive carbons were procured from Alfa Aesar and MSE Supplies, respectively. The carbon-coated aluminum foil (15 µm thickness of aluminum, 1 μm of conductive carbon) was purchased from MSE Supplies.

3.2. Electrolyte Preparation

The LiPCP salt was synthesized in the laboratory at the Warsaw University of Technology in a two-step procedure, as reported elsewhere [23]. First, a stoichiometric amount of water and excess pyridine was added to tetracyanoethylene to obtain pyridinium pentacyanopropenide. Afterwards, tetrahydrofuran (THF) and lithium hydride (LiH) were added as the monovalent metal donor. After filtration and washing with ether at room temperature, LiPCP was obtained. LiPCP salt has a purity above 99%, and the characterization by nuclear magnetic resonance (NMR) is shown as follows:

¹³C NMR (125 MHz; acetone–d₆) δ/ppm: 135,7 (C1–C2–C3), 117,0 (2C, C1–CN/C3–CN (the side group of the three cyano groups that are on one side of the molecule)), 114,6 (C2–CN), 113,9 (2C, C1–CN/C3–CN (two cyano groups on one side of the molecule)), 57,8 (2C, C1–C2–C3), [23].

The fluorine-free electrolyte solutions were prepared in an argon-filled glovebox (LabStar, MBraun, H₂O < 1 and O₂ < 10 ppm, Garching, Germany)) by dissolving the LiPCP salt in different solvents. For solvent screening, electrolyte solutions of 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%), EC:EMC (30:70 wt.%), and EC:DEC (30:70 wt.%) were prepared. For comparison, the commercial standard 1 mol·kg⁻¹ LiPF₆ electrolyte solutions were prepared in the same solvent mixtures. Additionally, for molality screening, solutions from 0.1 to 1.2 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%) were prepared.

3.3. Ionic Conductivity Measurements

The ionic conductivity of the LiPCP-based electrolytes was obtained via electrochemical impedance spectroscopy (EIS) using a VMP3 potentiostat-galvanostat (VMP3, Bio-Logic, Seyssinet-Pariset, France) in the temperature range of 0 °C to 50°C. The measurements were carried out in the frequency range of 500 kHz to 1 Hz with 10 points per decade and a signal amplitude of 5 mV. The measurement at each frequency was repeated 6 times. The conductivity micro cells consisted of an electrolyte placed between two stainless steel electrodes. A cryostat-thermostat (Haake K75), (Vreden, Germany) with a temperature controller (DC50) was used for the thermoset samples.

The conductivity (σ) in mS·cm⁻¹ was calculated according to Equation (1), as follows:
where k is the cell constant (0.3–0.7 cm⁻¹, determined with ± 0.3% precision) for each conductivity microcell and R is the bulk resistance in ohms (Ω) obtained from the respective Nyquist plot.

3.4. Linear Sweep Voltammetry

Linear sweep voltammetry (LSV) was used to evaluate the electrochemical stability window of the following electrolytes: 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%), 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%) + 5 wt.% VC, 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%) + 5 wt.% VC + 5 wt.% AN, and 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%) + 5 wt.% VC + 10 wt.% AN. The tests were conducted at room temperature in a two-electrode Swagelok-type cell configuration at a scan rate of 0.5 mV s⁻¹. For the set-up, a Pt disc was used as the working electrode and a Li metal disc as the counter and reference electrode.

3.5. Lithium Passivation Measurement

Lithium passivation was investigated with electrochemical impedance spectroscopy (EIS) for the following electrolytes: 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%) + 5 wt.% VC, 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%) + 5 wt.% VC + 10 wt.% AN, and 1.0 mol·kg⁻¹ LiPF₆ in EC:DMC (30:70 wt.%). The measurements were carried out in the Swagelok-type cells with two lithium electrodes (discs) with a Celgard 2400 separator in between of them. For the measurement, a VMP3 potentiostat-galvanostat (VMP3, Bio-Logic) was used. The measurements were carried out in the frequency range of 500 kHz to 1 Hz, with 10 points per decade and signal amplitude of 5 mV. The measurement at each frequency was repeated 6 times. The impedance spectra were repeatedly measured in a period of 6 h. The Impedance spectra were analyzed using RelaxIS 3 software.

3.6. Electrode Preparation

LFP and LMFP were used independently as the active materials (AM). CMC was employed as the binder. Super-P and Ketjenblack conductive carbon black were used independently as the conductive additives (CMs) for electrode preparation. The components were mixed following different AM:CM:binder ratios (as indicated where appropriate throughout the manuscript) by using magnetic stirring (IKA-type RH Basic Magnetic Stirrer, Staufen im Breisgau, Germany) at 500 rpm for 24 h, and distilled water as the solvent. The obtained slurry was then cast onto carbon-coated aluminum foil by using a blade coater (Doctor Blade, ZEHNTNER Testing Instruments, ZAA 2300, and ZEHNTNER film applicator ZUA 2000, Sissach, Switzerland). All electrodes were first dried at 80 °C for 2 h, then, subsequently, they were vacuum dried at 120 °C overnight (Memmert VO 400, Schwabach, Germany)).

3.7. Cyclic Voltammetry Measurement

To evaluate the electrode/electrolyte compatibility, cyclic voltammetry (CV) measurements were conducted in a potentiostat-galvanostat (VMP3, Bio-Logic). LFP and LMFP electrodes of 11 mm in diameter were punched and assembled in two-electrode Swagelok-type cell configuration using a disc of Li metal as a counter electrode. Celgard 2400 microporous polypropylene was used as the separator, 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%) was used as the electrolyte, and 1 mol·kg⁻¹ LiPF₆ in EC:DMC (30:70 wt.%) was used as the reference electrolyte. The scan rate was set at 0.5 mV·s⁻¹ in 2.5–4.0 V vs. Li⁺/Li potential limits for the LFP cathode and 2.5–4.5 V vs. Li⁺/Li potential limits for the LMFP cathode.

3.8. Galvanostatic Cycling

For galvanostatic cycling, LFP and LMFP electrodes of 15 mm in diameter were punched and assembled in CR2032 coin cells, using a Li metal disc as the negative electrode and Celgard 2400 as the separator. The electrolyte solution was 0.8 mol·kg⁻¹ LiPCP in EC:DMC (30:70 wt.%) + 5 wt.% VC. The tests were performed in a SOLLICH potentiostat-galvanostat (SOLLICH 2061 MPG&T, Multichannel Potentiostat-Galvanostat and Battery Tester) in 2.5–3.9 V vs. Li⁺/Li potential limits for the LFP cathode and 2.4–4.2 V vs. Li⁺/Li for the LMFP cathode. The protocol consisted of one formation cycle at C/25 (where C = 170 mAh·g⁻¹ in the case of LFP and 160 mAh·g⁻¹ in the case of LMFP) followed by C/10 cycling. In addition, the rate capability of the LFP and LMFP cathodes with the LiPCP-based electrolyte was screened. To this purpose, 1 cycle at C/25, 5 cycles at C/20, 5 cycles at C/10, 5 cycles at C/5, 5 cycles at C/2, and 5 cycles at 1C were performed.

Table 4 shows the various electrolyte compositions used in the different experiments throughout this research.

4. Conclusions

In summary, the electrochemical performance of aqueous-processed olivine-type LFP and LMFP electrodes was systematically investigated using a fluorine-free LiPCP-based electrolyte. The precise optimization of the electrolyte composition revealed that an EC:DMC (30:70 wt.%) mixture provided the highest ionic conductivity. The optimal concentration of LiPCP was found to be 0.8 mol·kg⁻¹, achieving a conductivity of 9.6 mS·cm⁻¹ at 20 °C. This electrolyte exhibited electrochemical stability up to 4.4 V vs. Li⁺/Li when VC was utilized as a solid electrolyte interphase (SEI)-stabilizing additive, resulting in a fully fluorine-free electrolyte with favorable characteristics.

A comprehensive screening of various compositions for aqueous-processed LFP and LMFP electrodes was conducted. The optimal component ratio was found to be 87:10:3 (AM:CM:binder). Two conductive additives, Super P and Ketjenblack, were evaluated, with Ketjenblack consistently delivering a superior electrochemical performance. The concentration of CMC in the binder solution during electrode preparation was varied, and a concentration of 1% yielded the most favorable electrode properties. The selected electrode formulation proved suitable, achieving specific capacities of 150 mAh·g⁻¹ (LFP) and 125 mAh·g⁻¹ (LMFP) with the reference LiPF₆ electrolyte.

LiPCP-based electrolytes, incorporating fluorine-free linear carbonates as solvents and a stabilizing additive, exhibited fast kinetics and stable reversible cycling. Stable galvanostatic cycling of half-cells confirmed the compatibility between the LiPCP and olivine-type cathodes. The electrochemical performance of LFP with LiPCP was comparable to that with LiPF₆. On the contrary, the performance of LMFP with LiPCP was inferior, attributed to increased cell polarization, which hindered the full utilization of the high-potential plateau of this cathode. While LiPCP is compatible with LMFP, further optimization of the electrolyte composition for this system is necessary.

In conclusion, lithium-ion batteries devoid of critical and problematic elements such as cobalt, nickel, and fluorine were engineered and exhibited a satisfactory performance. These findings represent the initial phase of a groundbreaking investigation into developing safer and more environmentally friendly electrolytes compatible with aqueous-processed electrodes for lithium-ion batteries. This first stage lays the foundation for a novel approach, aiming to innovate the battery industry by reducing its dependence on hazardous materials and assisting in improving the overall sustainability and safety of energy storage systems.

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Claudia Limachi www.mdpi.com

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Development of Fluorine-Free Electrolytes for Aqueous-Processed Olivine-Type Phosphate Cathodes

1. Introduction

3. Materials and Methods

3.1. Reagents

3.2. Electrolyte Preparation

3.3. Ionic Conductivity Measurements

3.4. Linear Sweep Voltammetry

3.5. Lithium Passivation Measurement

3.6. Electrode Preparation

3.7. Cyclic Voltammetry Measurement

3.8. Galvanostatic Cycling

4. Conclusions

Greenberg

1. Introduction

3. Materials and Methods

3.1. Reagents

3.2. Electrolyte Preparation

3.3. Ionic Conductivity Measurements

3.4. Linear Sweep Voltammetry

3.5. Lithium Passivation Measurement

3.6. Electrode Preparation

3.7. Cyclic Voltammetry Measurement

3.8. Galvanostatic Cycling

4. Conclusions

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