Investigation of Ionic Conductivity of Electrolytes for Anode-Free Lithium-Ion Batteries by Impedance Spectroscopy

Abstract

Anode-free lithium-ion batteries offer a volumetric energy density approximately 60% higher than that of conventional lithium-ion cells. Despite this advantage, they often experience rapid capacity degradation and a limited cycle life. Optimizing electrolyte formulations—particularly through the use of specific additives, solvents, and lithium salts—is essential to improving these systems. This study explores electrolytes composed of fluorinated and carbonate-based solvents applied in anode-free lithium-ion cells featuring copper as the anode substrate and Li_1.05Ni_0.33Mn_0.33Co_0.33O₂ as the cathode. In the present work, the ionic conductivity of electrolytes was studied by impedance spectroscopy, and the electrochemical parameters of anode-free lithium-ion cells were compared using these electrolyte solutions: lithium difluoro(oxalato)borat (LIDFOB) salts were used in a mixture of solvents such as fluoroethylene carbonate (FEC) and dimethoxyethane (DME) in a ratio of 3:7 and in a mixture of propylene carbonate (PC) and dimethoxyethane in a ratio of 3:7. Enhanced performance was observed upon the substitution of conventional carbonates with fluorinated co-solvents. The findings suggest that LiDFOB is a thermostable salt, and its high conductivity contributes to the formation and stabilization of the interface of solid electrolytes. The results indicate that at low temperature conditions, a double salt should be used for lithium current sources, for example, 0.4 M LiDFOB and 0.6 M LiBF₄, as well as electrolyte additives such as fluoroethylene carbonate and lithium nitrate.

1. Introduction

The growing global demand for energy, combined with the need to mitigate the effects of climate change, has led to a sharp increase in the use of renewable energy technologies [1,2]. An important addition to this is the need for more energy storage devices, the main component of which are chemical current sources [3]. The key criterion for rechargeable batteries is the service life, i.e., how many cycles a battery can withstand before there is a significant loss in its performance. All batteries lose performance during cyclic use [4]. The reason may be related, among other things, to the degradation of the electrolyte [5,6], and the battery performance can be influenced by the choice of carbon nanostructured material [7].

Electrochemical impedance spectroscopy (EIS) has emerged as a powerful, non-invasive diagnostic tool that facilitates real-time evaluation of battery behavior across a broad frequency range without disrupting cell operation [8]. It enables both in situ monitoring during cycling and ex situ measurements at defined charge states. EIS involves the application of a low-amplitude alternating current across a broad frequency spectrum, followed by measurement of the system’s response. This technique is highly effective for characterizing systems with multiple impedance contributions, including both bulk phases and interfacial regions, making it particularly suitable for analyzing complex electrochemical devices such as batteries. Due to the varying time constants of different electrochemical processes within the cell, EIS enables their separation in the frequency domain, allowing for detailed analysis of individual components and interactions [9].

The essence of the impedance spectroscopy method consists of applying a disturbing low-amplitude sinusoidal signal to the system under study and studying the output response signal caused by it.

Anode-free lithium-ion batteries represent a promising class of energy storage systems, distinctively lacking a pre-deposited lithium-metal anode. Instead, these cells rely exclusively on lithium extracted from the cathode during charging. This design offers significantly enhanced energy density and improved safety by eliminating metallic lithium from the anode, thereby reducing the risks of fire and short-circuiting [10,11]. Anode-free lithium-metal cells are capable of delivering approximately 60% higher specific energy compared to conventional lithium-ion cells [12]. For instance, cells employing a Li[Ni_0.5Mn_0.3Co_0.2]O₂ (NMC532) cathode utilize lithium exclusively from the cathode itself without relying on a graphite-based anode [13]. In contrast, conventional lithium-metal cells often require a substantial excess of lithium, which decreases their overall energy efficiency [14]. Furthermore, the use of thick lithium foils (greater than 60 μm) exacerbates the reduction in specific energy due to the increased mass of inactive materials [15]. Recent advancements suggest that avoiding lithium-metal foils in cell architecture may substantially lower manufacturing expenses while improving integration with established lithium-ion battery production processes [16]. Therefore, refining anode-free cell designs by selecting high-performance electrolytes and minimizing lithium consumption is essential for the development of advanced lithium-based energy storage systems.

In the absence of excess lithium, anode-free cells are prone to rapid capacity degradation. Typically, their operational lifespan does not exceed 20 cycles, with up to 80% of the initial capacity being lost [17]. This degradation is primarily attributed to lithium consumption through parasitic reactions with the electrolyte, which results in the formation of a solid electrolyte interphase (SEI) and electrically isolated lithium deposits [18]. These phenomena contribute to the accumulation of “dead” lithium and are exacerbated by the high-surface-area morphology that commonly develops in conventional electrolytes [19]. To mitigate these effects and prolong cycle life, approaches such as electrolyte system refinement [20,21] and the application of external pressure [22,23] have been explored, as both promote more compact lithium deposition. Beyond longevity, the safety of electrochemical cells also remains a critical challenge. Notably, concerns regarding the flammability and thermal instability of lithium-metal batteries have been under investigation since the 1980s. Safety is often discussed in the literature but rarely tested in real-world conditions. In some works, the use of “non-flammable” electrolytes is mentioned, since electrolyte-impregnated separators do not ignite when exposed to flame.

When designing a liquid electrolyte, three main components can be adjusted: additives, solvents, and salts. However, the boundaries between these categories are often blurred, as the same chemical compound can belong to multiple categories.

Electrolyte solvents play a crucial role in lithium-ion batteries by facilitating ion transport between the anode and cathode during charging and discharging. Solvents are chosen based on their ability to dissolve lithium salts, ensure good ionic conductivity, and maintain stability across the operational voltage and temperature ranges. Traditional electrolyte solvents for LIBs are carbonate-based solvents: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate and linear carbonates: methyl acetate, propylene methyl carbonate. Modern research on electrolytes for LIBs focuses on developing new solvents, as ether-based solvents: dimethoxyethane, 1,3-dioxolane and tetraethylene glycol dimethyl ether. Fluorinated carbonates—FEC’s unique properties allow the formation of a stable solid electrolyte interphase (SEI). It is particularly effective when used with high-voltage cathodes and at low temperatures is used as a co-solvent. FEC is particularly effective in combination with cathodes operating at voltages above 4.2 V due to its oxidation resistance and ability to reduce side reactions on the cathode. The use of FEC requires mixing with low-viscosity solvents (e.g., EMC or DMC) to ensure high ionic conductivity. Traditional carbonate solvents, such as diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate, limit battery performance at high temperatures (60 °C) due to their high volatility and flammability [24,25]. Various strategies have been developed to address these issues, including electrolyte additives [26,27], new types of lithium salts [28], thermally stable and non-flammable solvents [29], and solid electrolytes [30]. The use of novel lithium salts, including combinations such as lithium difluoro(oxalato)borate (LiDFOB) and lithium bis(fluorosulfonyl)imide (LiFSI), is an effective method to prevent side reactions on the cathode and create a stable SEI [31,32]. LiDFOB salt stabilizes the cathode at elevated temperatures. In a LiFePO₄|Li₄Ti₅O₁₂ cell, a comparison between an LiPF₆-based electrolyte and a LiDFOB-based electrolyte demonstrated that the new salt significantly improves cycling stability [33].

In addition to the solvent, the lithium salt is also a key component of the electrolyte. The traditional and widely commercially available carbonate-based electrolyte typically contains lithium hexafluorophosphate (LiPF₆), which possesses well-balanced properties. Furthermore, lithium perchlorate (LiClO₄), lithium tetrafluoroborate (LiBF₄), and lithium bis(fluorosulfonyl)imide (LiFSI) are also used as lithium salts in LIBs.

Conventional lithium-ion battery electrolytes begin to decompose due to the low stability of LiPF₆ salt at temperatures above 55 °C [34,35], and the solid electrolyte interphase (SEI) on the anode surface tends to dissolve at an elevated temperature of 65 °C [34,36], leading to increased resistance and rapid capacity degradation. In study [37], the use of a thermally stable electrolyte based on a dual lithium salt system (LiDFOB and LiFSI) and two solvents with high boiling/flash points (FEC and TEGDME) addressed the issue of unstable interfaces on both the cathode and anode, ensuring the safety of lithium-ion batteries at high temperatures. The addition of LiDFOB effectively suppresses metal dissolution at the cathode, while LiFSI and FEC contribute to the formation of a thermally stable SEI on the anode surface, particularly at high temperatures. Fluoroethylene carbonate (FEC) is also one of the most widely studied additives for lithium-based batteries due to its excellent SEI-forming properties [38,39,40]. Given the critical role of interphase chemistry in the performance of LIBs, FEC has been successfully incorporated into low-temperature electrolyte formulations. Organophosphites have also been proposed as LIB electrolyte additives for various purposes, including reducing flammability [41] and improving high-voltage stability [42]. Among them, tris(trimethylsilyl)phosphite (commonly referred to as TMSP or TMSPi) has gained recent attention for its passivating effect on metal oxides through the formation of a stable cathode electrolyte interphase (CEI) layer [43]. The function of tris(trimethylsilyl)phosphite (TMSPi) is acting as a passivating agent for metal oxides and participates in SEI formation. This additive reduces overpotential in full cells, stabilizes cathode electrolyte interphase (CEI), and helps maintain performance at low temperatures. In a 2019 report, Liu et al. identified TMSPi as one of the most effective additives for reducing the overpotential of Gr‖NCA full cells at −40 °C, thereby increasing capacity. Interestingly, only 0.5 wt% was sufficient for these improvements, while increasing the concentration to 1 wt% resulted in a negative effect [44]. Another study from the same year introduced an electrolyte formulation containing TMSPi, enabling a 5 V-class MCMB‖LiNi_0.5Mn_1.5O₄ (LNMO) cell to function at temperatures as low as −60 °C, with repeated charge/discharge cycles at −5 °C and a 0.3 °C rate without capacity degradation [45].

Organosulfur compounds with different structures and oxidation states have been extensively studied as additives for lithium-ion battery electrolytes that alter the interphase, particularly in low-temperature cells. Butyl sultone (BuS) was identified for this application as early as 2006 [46]. The role of this additive is to enhance thermal stability and decrease flammability, thereby improving safety and thermal management in battery systems.

High-temperature-resistant ionic liquids and non-flammable phosphates can enhance the thermal stability and fire resistance of conventional electrolytes. However, they exhibit high catalytic activity toward carbon electrodes and fail to form a stable SEI, leading to electrode (anode) degradation. While solid electrolytes provide reliable performance at elevated temperatures, they often suffer from low ionic conductivity or poor interfacial compatibility with electrodes. Therefore, developing thermally stable liquid electrolytes with high electrode compatibility, capable of stabilizing cathodes and forming a robust SEI under extreme conditions, would be a significant step toward improving the safety of lithium-ion batteries designed for high-temperature operation.

The choice of electrolyte type depends on the specific requirements of the project, including safety, energy density, cost, and operating temperature range. Electrolytes play an important role in the operation of lithium-ion batteries, and their development and improvement are ongoing in order to improve the characteristics and reliability of these batteries.

2. Materials and Methods

2.1. Preparation of Electrolytes

Electrolytes were prepared based on lithium salts including lithium difluoro(oxalato)borate (LiDFOB), lithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), and lithium tetrafluoroborate (LiBF₄). Solvent systems consisted of propylene carbonate (PC) and dimethoxyethane (DME); fluoroethylene carbonate (FEC) and DME in a 3:7 volume ratio; and a ternary solvent mixture of PC, DME, and ethylene acetate (EA). FEC and lithium nitrate (LiNO₃) were used as additives.

The electrolyte solutions were mechanically stirred for 12 h using a magnetic stirrer at room temperature. All preparation steps were performed in an argon-filled glovebox to prevent moisture or air contamination.

Electrochemical impedance spectroscopy (EIS) measurements were conducted under controlled temperature conditions using the EC-Lab software (version 11.43, Bio-Logic Science Instruments, Claix, France) in conjunction with a climate chamber (BINDER MK 56, BINDER GmbH, Tuttlingen, Germany).

2.2. Assembly of Samples of the 2032 Format (Cells)

The cells used for ionic conductivity measurements differed from standard electrochemical cells in that they contained no electrodes. Instead, polished spacers were used. All cells were assembled inside an argon-filled glovebox.

Seven coin cells (2032 format) were assembled using the following electrolyte compositions:

• LiDFOB in FEC:DME (3:7)
• LiClO₄ in FEC:DME (3:7)
• LiPF₆ in FEC:DME (3:7)
• 0.4 M LiDFOB + 0.6 M LiBF₄ in PC:DME (3:7)
• LiDFOB in PC:DME (3:7)
• LiDFOB in PC:DME (3:7) + 3% FEC + 1% LiNO₃
• LiDFOB in PC:DME:EA (3:7:3) + 5% FEC + 3% LiNO₃

Ionic conductivity was measured at various temperatures using the EC-Lab system.

The following materials were used to fabricate anode-free lithium-ion cells: N-methyl-2-pyrrolidine, polyvinyldenafluoride (Solef 5130, Solvay Company, Brussels, Belgium) lithium nickel manganese cobalt oxide (NMC111) (GN-P198-H, Gelon LIB Group, Jinan, China) carbon black Timcal Super C45, carbon nanotubes Tuball Coat_E H₂O 0.4% (Oxidized, OCSiAl, Novosibirsk, Russian Federation), Copper foil (China).

The cathode was based on Li_1.05Ni_0.33Mn_0.33Co_0.33O₂ (NMC111). The electrode slurry preparation steps were as follows:

• Polyvinylidene fluoride (Solef 5130) was dissolved in N-methyl-2-pyrrolidone under mechanical stirring at 60 °C for 2 h;
• Carbon nanotubes were dispersed in N-methyl-2-pyrrolidone using an ultrasonic homogenizer Bandelin Sonopuls HD 3100 (Bandelin electronic GmbH & Co. KG, Berlin, Germany) in 5 min intervals for 15 min;
• The polymer solution and nanotube dispersion were combined and sonicated for an additional 5–10 min;
• Carbon black and NMC111 active material were added under overhead stirring without heating for 40 min;
• The mixture was homogenized to obtain a uniform paste;
• Final mechanical stirring was carried out for at least 12 h at 1500 rpm, followed by 20 min of vacuum mixing to remove residual gases.

The cathode paste was applied using a doctor blade coater with a wet film thickness of 800 µm.

The negative electrode consisted of plasma-treated copper foil. Contaminants were removed using a Diener Atto low-pressure plasma cleaner (Diener electronic GmbH & Co. KG, Ebhausen, Germany). To eliminate residual moisture, the cut copper disks were dried in a BINDER VD-23 vacuum (BINDER GmbH, Tuttlingen, Germany) oven at 120 °C and ~1 mbar for 2 h, then transferred to the glovebox without exposure to air.

The separator used was Celgard 2300 (Celgard LLC, Charlotte, NC, USA). Coin cell assembly (2032 format) was performed in a SPEKS GB02M glovebox (SPEKS LLC, Moscow, Russian Federation) with oxygen and moisture levels below 1 ppm. Cells were sealed using an MTI-MSK-110 hydraulic press (MTI Corporation, Richmond, CA, USA).

Galvanostatic charge/discharge cycling was performed using an MTI BST8-MA (MTI Corporation, Richmond, CA, USA) eight-channel battery analyzer over a voltage range of 3.0 to 4.5 V. This equipment is intended for evaluating portable battery systems and investigating the longevity of electrode materials. Voltage values were referenced to the Li⁺/Li redox couple, using calibration data obtained from lithium-metal half-cell experiments. Specific capacity values were normalized to the mass of the active material.

3. Results

The ionic conductivity of the electrolyte was investigated at various temperatures using impedance spectroscopy. The results showed that lithium-ion conductivity depends significantly on electrolyte composition, particularly the presence of binary salts and functional additives. Temperature also plays a critical role: the highest ionic conductivity was observed at 30 °C and 60 °C, while conductivity at −20 °C did not exceed 5 mS/cm.

Among all salts tested, LiDFOB demonstrated the highest thermal stability and ionic conductivity. Its favorable transport properties contributed to the formation and stabilization of the solid electrolyte interphase (SEI). The data suggest that at low temperatures, binary salt systems such as 0.4 M LiDFOB + 0.6 M LiBF₄, along with additives like fluoroethylene carbonate (FEC) and lithium nitrate (LiNO₃), are more suitable for lithium-based power sources.

To evaluate conductivity, 2032-format coin cells were assembled with each electrolyte and tested at −20 °C, 30 °C, and 60 °C. Ionic conductivity was calculated using Equation (1):

$$\sigma ={\displaystyle \frac{\mathrm{l}}{\mathrm{R}·\mathrm{S}}}$$

(1)

where

• σ is the specific ionic conductivity;
• l is the spacer distance (70 µm, based on two 35 µm cardboard rings);
• R is the resistance value obtained from Nyquist plots;
• S is the cross-sectional area (0.79 cm²).

Since the thickness of the cardboard ring is 35 microns, and there are two of them in the cell, the distance between the spacers (l) will be 70 microns. In turn, the area S is 0.79 cm². For example, for the point in the figure, the specific ionic conductivity will be calculated as

$$\sigma ={\displaystyle \frac{70 · 1000}{\mathrm{10,000} · 0.79 · 1.4}}$$

For instance, a representative cell demonstrated a conductivity of 6.3 mS/cm under these conditions.

Analysis of the ionic conductivity data obtained at various temperatures suggests that certain electrolyte formulations are better suited for application in lithium-based energy storage systems. The maximum ionic conductivity was recorded at 60 °C and 30 °C, whereas at −20 °C, the conductivity values remained below 5 mS/cm as shown in Figure 1.

Different lithium salts, including LiDFOB, LiClO₄, and LiPF₆, were dissolved in various solvent mixtures. Among them, LiDFOB exhibited the highest ionic conductivity, attributed to its excellent thermal stability and widespread use in advanced lithium-ion battery (LIB) technologies. The high ionic conductivity of LiDFOB promotes the formation and stabilization of a robust solid electrolyte interphase (SEI). The incorporation of LiDFOB into electrolytes has been associated with several benefits, including improved cycling stability, enhanced low-temperature performance, and increased charge/discharge rates. As a result, LiDFOB is widely employed as a modern electrolyte additive in next-generation LIBs. Across all tested temperatures, electrolytes containing LiDFOB demonstrated superior ionic conductivity, maintaining values as high as 5 mS/cm at −20 °C when dissolved in a PC:DME (3:7) solvent system. Notably, LiDFOB-based electrolytes performed better at 30 °C and −20 °C in PC:DME, while at 60 °C, superior performance was observed in FEC:DME mixtures, likely due to the favorable properties imparted by the fluorinated co-solvent molecules.

The use of a dual-salt electrolyte composed of 0.4 M LiDFOB and 0.6 M LiBF₄ resulted in a decrease in ionic conductivity compared to single-salt systems. However, LiBF₄ offers superior thermal and moisture stability compared to conventional lithium hexafluorophosphate (LiPF₆). Recent studies have explored the application of LiBF₄ in highly concentrated electrolytes for high-voltage lithium-ion batteries, demonstrating enhancements in capacity retention and cycling performance. Moreover, the incorporation of LiBF₄ has been shown to improve the low-temperature electrochemical characteristics of electrode materials, as illustrated in Figure 2 Despite the slight reduction in overall conductivity, the lithium-ion transport properties of this electrolyte at low temperatures remain satisfactory for practical applications.

Electrolytes based on LiDFOB with additives such as fluoroethylene carbonate (FEC) and lithium nitrate (LiNO₃) exhibited moderate ionic conductivity, reaching approximately 10 mS/cm at 30 °C and 60 °C when dissolved in a PC:DME:EA (3:7:3) solvent mixture. The relatively low conductivity observed is likely attributed to the limited solubility of LiNO₃ across a range of temperatures. Although the inclusion of LiNO₃ has been shown to enhance various electrochemical properties, its poor solubility remains a significant challenge. Ethylene acetate was incorporated to improve low-temperature performance; however, due to the presence of 3% lithium nitrate, the ionic conductivity at −20 °C was limited to approximately 3.6 mS/cm. As a result, these electrolyte systems are primarily suitable for operation under ambient conditions rather than low-temperature environments.

To investigate the performance of anode-free lithium-ion cells, 2032-format Cu‖NMC111 cells were assembled. The configuration employed a copper current collector as the anode and a Li_1.05Ni_0.33Mn_0.33Co_0.33O₂ (NMC111) cathode with a nominal areal capacity of 1.6 mAh/cm². Two electrolyte formulations were evaluated: 1 M LiDFOB dissolved in PC:DME (3:7, v/v) and 1 M LiDFOB dissolved in FEC:DME (3:7, v/v). Both cells exhibited an initial charge capacity of approximately 1.71 mAh, corresponding to a specific capacity of 148 mAh/g, close to the theoretical value for NMC111 (170 mAh/g). This indicates efficient lithium extraction from the cathode during the initial charge, independent of the electrolyte formulation. However, notable differences were observed during discharge: in the cell with the PC:DME-based electrolyte, only ~25% of the deposited lithium was recovered, indicating poor reversibility. Such low reversibility in carbonate-based electrolytes has been extensively reported in the literature. For example, cells based on Cu‖LiNi₁/₃Mn₁/₃Co₁/₃O₂ cathodes using 1.2 M LiPF₆ in EC/EMC (3:7 w/w) electrolyte recovered only ~23% of the initially plated lithium. This substantial capacity loss is primarily attributed to the degradation of active lithium and the formation of a high-impedance interfacial layer resulting from side reactions between the plated lithium and both the carbonate solvents and LiPF₆ salt. In carbonate-based electrolytes, lithium often deposits in dendritic or mossy structures with high surface-area-to-volume ratios. During the subsequent stripping process, lithium trapped within the solid electrolyte interphase (SEI) becomes electrically isolated and, thus, electrochemically inactive, forming “dead lithium”. The accumulation of dead lithium leads to a progressive increase in cell impedance and ultimately results in complete cell failure after a limited number of cycles. To further elucidate the effect of electrolyte composition and temperature on the cycling stability of anode-free lithium-ion cells, galvanostatic charge–discharge experiments were conducted at 30 °C and 60 °C. Figure 2 presents the discharge voltage profiles during the initial three cycles, while Figure 3 summarizes the evolution of coulombic efficiency as a function of cycle number. These data provide important insights into the impact of electrolyte formulation on lithium reversibility and capacity retention under different thermal conditions.

Experimental results demonstrate that cells utilizing an electrolyte composed of 1 M LiDFOB dissolved in FEC:DME (3:7, v/v) exhibit superior cycling stability, maintaining a discharge capacity of approximately 200 mAh/g. In contrast, cells employing the same salt in a PC:DME (3:7) solvent mixture achieved a lower discharge capacity of around 170 mAh/g. The improved electrochemical behavior associated with FEC-based electrolytes is primarily linked to the development of robust and stable surface layers on cathode particles. These layers arise from specific interfacial interactions promoted by the presence of fluorinated solvent molecules. The protective layers effectively isolate the highly reactive lithium transition metal oxide cathode surfaces from parasitic side reactions with electrophilic alkyl carbonate solvents. Coulombic efficiency (CE), a critical parameter for anode-free lithium batteries, reflects the proportion of lithium that remains active after a complete charge–discharge cycle. The studied cells achieved a CE of approximately 96% for the FEC:DME-based electrolyte and 92% for the PC:DME-based electrolyte after 50 cycles. These results highlight the superior lithium utilization and interfacial stability imparted by the FEC-containing electrolyte system.

4. Conclusions

The ionic conductivity of various electrolyte formulations was systematically evaluated using impedance spectroscopy under different temperature conditions. These electrolytes were subsequently applied in anode-free lithium-ion battery configurations to assess their practical performance. The results demonstrated a strong dependence of lithium-ion conductivity on the electrolyte composition, particularly the presence of dual salts and functional additives. Significant differences in conductivity values were observed across different temperatures, with the highest ionic conductivities recorded at 60 °C and 30 °C. At −20 °C, the ionic conductivity of all tested electrolytes did not exceed 5 mS/cm. Among the salts studied, lithium difluoro(oxalato)borate (LiDFOB) exhibited superior thermal stability and ionic conductivity, contributing to the effective formation and stabilization of a robust solid electrolyte interphase (SEI). Furthermore, the findings suggest that under low-temperature operating conditions, the use of dual-salt systems—such as 0.4 M LiDFOB combined with 0.6 M LiBF₄—and the inclusion of additives such as fluoroethylene carbonate (FEC) and lithium nitrate (LiNO₃) are effective strategies for enhancing the performance of lithium-based energy storage systems.