Dual-Functional Electrolyte Additive for Lithium–Sulfur Batteries Limits Lithium Dendrite Formation and Increases Sulfur Utilization Rate

Abstract

Lithium–sulfur batteries (LSBs) have received great attention as promising candidates for next-generation energy-storage systems due to their high theoretical energy density. However, their practical energy density is limited by a large electrolyte-to-sulfur (E/S) ratio (>10 µL _electrolyte/mg _s), and their cycle performance encounters challenges from electrode passivation and Li dendrite formation. In this work, a dual-functional electrolyte additive of tetraethylammonium nitrate (TEAN) is presented to address these issues. NO₃⁻ as a high-donor-number (DN) salt anion can promote polysulfide dissolution, increase sulfur utilization, and alleviate electrode passivation. The tetraethylammonium cation can adsorb around Li protrusions to form a lithiophobic protective layer to inhibit the formation of Li dendrites. TEAN LSBs show improving capacity, cycling stability, and higher coulombic efficiency under lean electrolyte (5 μL _electrolyte/mg _s) conditions.

1. Introduction

With the development of electric vehicles, the demand for rechargeable batteries with high energy density and long cycle life is strongly expected. LSBs are generally accepted as promising candidate storage devices due to their high theoretical specific capacity (1675 mAh g⁻¹), high natural abundance of sulfur, as well as low toxicity [1,2,3,4]. However, the development of LBSs still suffers from the formation and growth of lithium dendrites, the shuttle effect of polysulfides, and large electrolyte usage (electrolyte/sulfur ratio > 20 µL mg⁻¹) [5,6,7,8]. In particular, the large electrolyte usage greatly diminishes the actual energy density of LSBs [9,10,11], and therefore a low value electrolyte usage is essential to achieve a competitive energy density for LSBs. However, reducing the electrolyte usage not only reduces the sulfur utilization but also exacerbates the anode stability.

Using high DN value anions as additives for LSBs is one effective approach to enable the efficient use of sulfur [12,13]. NO₃⁻, as a high-DN-value anion, can promote the dissolution of polysulfide and thus increase sulfur utilization under lean electrolyte conditions [14]. Moreover, it can decompose and form solid-electrolyte interphase (SEI) on the lithium anode surface to inhibit the formation of Li dendrites [15]. However, the large volume change of a lithium layer during the discharging–charging process would cause the SEI to crack or break, resulting in non-uniform lithium deposition and further aggravating the growth of lithium dendrites [16]. Therefore, limiting the growth of lithium dendrites after SEI cracks is essential to ensure high coulombic efficiency and long cycle life in LSBs. Previous research has reported cationic surfactant-based electrolyte additives can limit Li dendrite formation. In the conventional electrolyte, Li protrusions can be unavoidably formed in the initial Li⁺ plating processes. Electric charges will accumulate over these protrusions, and Li⁺ will be continuously plating at the tips of these protrusions and result in lithium dendrite formation. After adding cationic surfactant-based electrolyte additive, more cations can adsorb around the Li protrusions under the action of electrostatic interactions. Meanwhile, the surface free energy of the Li protrusions is larger than that of the smooth electrode, and surface tension will also cause cations to adsorb on the surface of Li protrusions. And then a lithiophobic protective layer forms on the surface of the Li protrusions, and this lithiophobic protective layer will drive the Li⁺ away from the Li protrusion regions and limit Li dendrite formation [17]. The synergetic effect of the cationic surfactant and NO₃⁻ prevents lithium dendrite growth more effectively.

In this study, we present a novel approach to enhance the performance of LSBs operating under lean electrolyte conditions. We have developed a groundbreaking cationic surfactant that incorporates the NO₃⁻, serving as an electrolyte additive. This innovation has proven to be instrumental in elevating the overall performance of LSBs, particularly when electrolyte availability is limited. The electrolyte additive, with its unique formulation, demonstrates dual capabilities that contribute to the enhancement of LSB performance. First and foremost, it effectively curbs the growth of lithium dendrites, a notorious concern that can compromise battery safety and efficiency. Additionally, this novel additive holds the capacity to optimize sulfur utilization, particularly under the challenging conditions of reduced electrolyte content. The experimental findings highlight the exceptional performance of the developed electrolyte formulation. Li||Li symmetric cells serve as a reliable platform for assessing the cyclability of the system. Over a staggering 2200 h duration, the overpotential associated with Li plating/stripping remains remarkably stable, even when the cell is polarized at a demanding 1 mA cm⁻² current density. This exceptional stability underscores the effectiveness of the developed electrolyte additive in maintaining consistent performance over prolonged operational periods. Moreover, the application of the novel electrolyte formulation in practical LSBs, known as TEAN LSBs, has demonstrated encouraging results. Under the constrained condition of 5 µL _electrolyte/mg _s, TEAN LSBs exhibit an impressive enhancement in initial discharge capacity. Specifically, at a charge–discharge rate of 0.1 C, the TEAN LSBs showcase an enhanced initial discharge capacity of 1100 mAh g⁻¹.

2. Materials and Methods

2.1. Preparation of Electrolyte

Tetraethyl ammonium nitrate (TEAN) (purity > 99%) was purchased from Aladdin. Conventional electrolyte of 1.0 M Bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) in a mixture of 1,3-Dioxolane (DOL)/1,2-Dimethoxyethane (DME) (1:1 by volume) with 4 wt% LiNO₃ was provided by New Zebang Technology Co., Ltd (Hangzhou, China). TEAN electrolyte was obtained by adding 1 wt% TEAN into conventional electrolyte. Electrolyte preparation was completed in an argon-filled glove box (Super2440 Mikrouna) in which water and oxygen values were controlled below 0.01 PPM.

2.2. Cathode Preparation

Briefly, 1 g Kochen black and 3 g sulfur were mixed by milling for 1 h. The carbon–sulfur mixture was transferred to a tube furnace and treated at 155 °C under argon atmosphere for 12 h. The resulting mixture was cleaned with deionized water and ethanol three times successively. Finally, the C/S composites were obtained after being dried in a vacuum drying oven at 60 °C for 12 h.

Electrode slurry was prepared by mixing 90 wt% C/S composites and 10 wt% Polyvinylidene fluoride (PVDF) in N-pyrrolidone (NMP). Working electrodes were obtained by coating the slurry on an aluminum foil and then drying it in a vacuum drying oven at 60 °C for 12 h. The sulfur electrodes were punched into 1.2 cm diameter disk with sulfur loading of 1.0–1.5 mg cm⁻² and sulfur content of 17.3–21.0 wt% based on the consideration of the total mass of the cathode, and 3D current collector electrodes with high sulfur loading of about 5 mg cm⁻² and sulfur content of 29.9 wt% based on the consideration of the total mass of the cathode were prepared by mixing the slurry on the carbon cloth.

2.3. Electrochemical Measurements

The experimental setup and testing procedures were meticulously executed to assess the performance of Coin CR2025 LSBs under specific conditions. The assembly of these batteries took place within an argon-filled glovebox, ensuring an inert environment. The tests were conducted at a temperature of 30 °C, providing controlled conditions for evaluation. The electrolyte was added to the batteries at two different quantities: 5 μL/mg or 10 μL/mg. The operating voltage window for the LSBs was set at 1.7–2.8 V. Two types of galvanostatic tests were conducted to evaluate the battery performance:

Li Plating/Stripping Test Using Symmetric Lithium–Lithium (Li-Li) Cells: These cells were assembled using lithium metal foils (Φ14 mm) as both the negative electrode and the counter electrode. The Li-Li cells with 40 μL of electrolyte underwent cycling with a fixed capacity of 1 mAh cm⁻² at a current density of 1 mA cm⁻².

Li Plating/Stripping Test Using Lithium–Copper (Li-Cu) Cells: In this case, copper (Cu) current collectors were cut into disk shapes (Φ18 mm) and utilized as the positive electrode. Lithium metal foils (Φ16 mm) were employed as the negative electrodes. These Li-Cu cells were assembled with 40 μL of electrolyte and underwent cycling within a voltage range of 0–1.0 V. Cycling was performed at both 0.5 mA cm⁻² and 1 mA cm⁻² current densities, with fixed capacities of 0.5 mAh cm⁻² and 1 mAh cm⁻², respectively.

In all cells, Celgard 2400 PP membranes (Φ19 mm) served as separators, maintaining physical separation between the positive and negative electrodes. The experimental setups were tested on a NEWARE cell test system, which allowed for accurate monitoring and control of the testing conditions.

These systematic procedures and standardized testing environments ensure reliable and consistent data collection, enabling a thorough evaluation of the performance of the Coin CR2025 LSBs under varying conditions and using different testing approaches.

3. Results and Discussion

To assess the impact of TETA on enhancing the stability of lithium metal anodes, a series of experiments were conducted using Li-Li symmetric cells. The primary objective was to compare the voltage profiles of these cells when employing TEAN (TETA-based) electrolytes versus conventional electrolytes under specific conditions: a constant current density of 1 mA cm⁻² and a deposition capacity of 1 mAh cm⁻². The resulting data are displayed in Figure 1a. In the case of TEAN electrolytes, it was observed that the overpotential remained consistently stable even after an extended duration of 2200 h. This indicates that the TETA-based electrolyte effectively maintained the cell’s performance over an extended period of operation. On the contrary, the Li-Li symmetric cell operating with the conventional electrolyte exhibited a different behavior. The overpotential of this cell aggravated rapidly after only 1000 h of operation. This deterioration in performance suggests that the conventional electrolyte did not provide sufficient stability during cycling. The increasing overpotential was caused by the growth of lithium dendrites [18,19], which led to the rapid thickening of the SEI on the electrode surface and caused an increasing overpotential [20,21]. The results demonstrate that TEAN can effectively inhibit the growth of lithium dendrites. To further evaluate the effect of TETA on the stability of the lithium metal anode, Li-Cu symmetric cells were tested at the current density of 0.25 and 0.5 mA cm⁻² plating/stripping for 1 h (Figure 1b,c). The conventional Li-Cu asymmetric cell failed within 150 cycles at a current density of 0.25 mA cm⁻² with a fixed capacity of 0.25 mAh cm⁻²; in sharp contrast, the TEAN Li-Cu asymmetric cell survived 500 cycles without failing under the same conditions. TETA electrolytes show improving stability of Li||Cu asymmetric cells, and the same results can be obtained from Li-Cu asymmetric cell tests performed at a higher current density of 0.5 mA cm⁻² (Figure 1c). The results demonstrate that TEAN is critical to the stabilization of the Li-metal anode [22,23].

To further validate the crucial role of TEAN in stabilizing the Li-metal anode, scanning electron microscopy (SEM) was employed to examine the surface of the Li-metal electrodes after cycling in both TEAN and conventional electrolytes. After subjecting the Li-metal electrodes to 100 cycles in TEAN electrolyte (Figure 2a), the SEM images revealed a smooth and undisturbed surface. Notably, there were no observable lithium dendrites present, which is a highly desirable outcome, as lithium dendrites can cause safety issues and hinder the performance of the battery. The positive results were maintained even after 200 cycles (Figure 2b), confirming the long-term effectiveness of TEAN in preventing dendrite growth on the Li-metal anode. In stark contrast, the Li-metal surface in the conventional electrolyte exhibited an unsmooth appearance (Figure 2d) after 100 cycles and continued to worsen after 200 cycles (Figure 2e). This rough surface and the absence of TEAN likely contributed to the growth of lithium dendrites, which can lead to reduced battery performance and potential safety hazards. Similar observations were made when examining the surface of lithium electrodes in TEAN and conventional lithium–sulfur batteries (LSBs) after 150 cycles. In the TEAN electrolyte, the deposition of Li on the lithium anode surface was uniform, with no apparent large particles or dead lithium formations (Figure 2c). On the other hand, in the conventional LSBs, the lithium metal electrode surface showed loose deposition (Figure 2f), further indicating the beneficial effect of TEAN in limiting dendrite growth. To gain real-time insights into the generation of lithium dendrites during cycling, an in situ optical microscope was utilized. The Li||Cu asymmetric cell was assembled in a quartz cuvette and discharged at a current density of 1 mA cm⁻² for 1 h, and the surface morphology was observed [24]. As shown in Figure 2g,h, images show the enlarged cross-sections of the positive copper foil of the Li||Cu asymmetric cell with the base electrolyte control without additive at 0 min and 20 min of discharge, respectively, and Figure 2i,j images show the enlarged cross-sections of the positive copper foil of the Li||Cu asymmetric cell with TEAN additive at 0 min and 20 min of discharge, respectively. The comparison images (Figure 2i,j) show that many irregularly deposited lithium dendrites appeared in the cross-section of the copper foil after 20 min of discharge in the control group, and the lithium dendrite generation was serious, while the lithium deposition on the cross-section of the positive copper foil was uniform and dense without obvious dendrite generation in the experimental group during 20 min of discharge. The results consistently supported the advantages of using TEAN, as it demonstrated effective dendrite suppression during the cycling process. In conclusion, the SEM and in situ optical microscope analyses provide strong evidence that TEAN plays a critical role in stabilizing the Li-metal anode by maintaining a smooth surface and inhibiting the formation of lithium dendrites. These findings reinforce the importance of TEAN as a viable solution to enhance the cycling stability and safety performance of lithium-based batteries.

To comprehensively assess the impact of TEAN on the electrochemical performance of lithium–sulfur batteries (LSBs), a comparative study was conducted involving two sets of batteries: one utilizing the TEAN electrolyte and the other employing a conventional electrolyte. The cycling performance of these batteries was documented and is illustrated in Figure 3a. Upon initial discharge, the LSBs with TEAN electrolyte exhibited a remarkable discharge capacity of 1505 mAh g⁻¹. As the cycling progressed, these TEAN LSBs demonstrated a notable capacity retention rate of 50% after undergoing 150 cycles at a charge–discharge rate of 0.1 C. Additionally, the average coulombic efficiency (CE) of the TEAN LSBs stood at an impressive 98% over the course of these cycles. In contrast, the conventional LSBs, which were assembled with the conventional electrolyte, displayed an initial discharge capacity of 1079 mAh g⁻¹. This value was notably lower than the discharge capacity achieved by the TEAN LSBs. Throughout 150 cycles under the same conditions, the average CE of the conventional LSBs remained relatively high, at 97%. However, the lower initial discharge capacity in combination with the slightly lower average CE highlighted the advantages of the TEAN electrolyte in terms of overall battery performance. The outcomes of this comparative study undeniably underline the favorable influence of TEAN on the electrochemical properties of LSBs. The TEAN electrolyte contributed significantly to enhancing both the capacity and coulombic efficiency of the batteries, resulting in improved cycling performance and potentially extending the overall lifespan of the battery system. These findings underscore the potential of TEAN as a valuable component in enhancing the electrochemistry of lithium–sulfur battery systems.

To conduct a more comprehensive exploration into the influence of TEAN additive on the electrochemistry of LSBs, a series of experiments were conducted under lean electrolyte conditions, as depicted in Figure 3b. When utilizing the TEAN electrolyte, the LSBs exhibited an initial discharge capacity of 1100 mAh g⁻¹. As the cycling proceeded, these TEAN LSBs demonstrated a capacity retention rate of 50% and an average CE of 98% after undergoing 90 cycles at a charge–discharge rate of 0.1 C. Notably, even in this lean electrolyte environment of 5 µL _electrolyte/mg _s, the TEAN LSBs consistently showcased superior performance compared to their conventional electrolyte counterparts. Conversely, the LSBs assembled with the conventional electrolyte under lean conditions displayed an initial areal capacity of 811 mAh g⁻¹, which declined rapidly to 400 mAh g⁻¹ after only 30 cycles. This stark discrepancy in performance further emphasizes the beneficial impact of TEAN on the electrochemistry of LSBs, particularly under challenging lean electrolyte conditions. To delve deeper into the enhancement of cycle performance of LSBs under lean electrolyte conditions, a high-loading C/S complex cathode was developed [25,26,27,28]. When subjected to the same conditions, the TEAN LSBs consistently exhibited significantly improved capacity and coulombic efficiency compared to the conventional LSBs, as demonstrated in Figure 3c. This outcome reaffirms the effectiveness of TEAN in enhancing both the capacity retention and coulombic efficiency of LSBs, even in scenarios with constrained electrolyte availability. The charge/discharge profiles of the high-loading Li-S cell under lean electrolyte conditions (5 µL _electrolyte/mg _s) further supported these findings. The TEAN LSBs displayed lower polarization voltage and higher capacity compared to the conventional electrolyte LSBs, as depicted in Figure 3d. These improvements could potentially be attributed to TEAN’s ability to mitigate electrode passivation, thereby contributing to the observed enhanced electrochemical performance. In summary, the experimental results obtained under lean electrolyte conditions strongly reinforce the positive impact of TEAN additive on the electrochemistry of LSBs. Through improved capacity retention, coulombic efficiency, and alleviation of electrode passivation, TEAN emerges as a promising solution for enhancing the performance and stability of lithium–sulfur battery systems, even in scenarios with limited electrolyte availability.

To investigate the effect of TEAN on the passivation of the LSB electrode surface, electrochemical impedance spectroscopy (EIS) spectra of LSB tests were employed, and the results are shown in Figure 4a,b. TEAN LSBs show lower interface impedance. This is probably because TEAN can suppress the passivation of electrode and increase the ionic conductivity of the electrolyte [17,29], thus significantly reducing the charge transfer impedance [30,31]. To investigate the effect of TEAN on the ionic conductivity of the electrolyte, the ionic conductivity of the two electrolytes was tested at different temperatures, and the results are shown in Figure 4c. The ionic conductivity of TEAN electrolyte was increased at different temperatures from 20 °C to 60 °C. The increasing ionic conductivity can reduce electrode polarization and increase sulfur utilization [32]. Chronoamperometry (CA) tests also confirmed TEAN can limit the LSB electrode passivation (Figure 4c,d). According to the previous research [33,34,35], the maximum current (I_m) and its corresponding time (t_m) can be used to interpret the relative speed of passivation layer formation. In conventional electrolytes, t_m is about 1000 s, while in TEAN electrolytes, t_m is extended to about 1500 s. The results indicate that TEAN can effectively suppress the passivation of electrodes.

The surface chemistry of the cathode after cycling was analyzed by XPS. The sulfur electrode of conventional electrolyte shows relatively strong peaks at 161.5 and 162 eV corresponding to S-Li bonds of Li₂S [36,37], indicating more Li₂S deposition on the surface, which can deteriorate the interface and cause electrode passivation [38,39]. In addition, TEAN can significantly increase the ratio of Li₂SO_X to Li₂S/Li₂S₂ (Figure 5a,b). Indicating insoluble insulating Li₂S/Li₂S₂ can be oxidized to sulfate components on the cathode surface by TEAN, while the reduction of short-chain Li₂S/Li₂S₂ can expose more reaction sites of S cathode active material, effectively improving the utilization of singlet sulfur and then improving the cycling stability performance of Li-S batteries [37,40].

In order to comprehensively investigate the intricate effects of TEAN on the electrochemical reaction dynamics within LSBs, the study employed UV-visible spectroscopy (UV-VIS) as a characterization tool. The utilization of UV-VIS spectroscopy allowed for the examination of substances within the battery system based on the absorption peaks and intensity of distinct ions. The experimental setup involved the assembly of LSBs in an H-type cell configuration, with subsequent UV-VIS spectroscopic analysis performed after a 2 h discharge period. Figure 5c illustrates the UV-VIS spectra obtained from the two different electrolytes after the discharge process. In the spectra, various absorption peaks are observed at distinct wavelengths. Notably, the absorption peaks located at 420 nm, 470 nm, and 617 nm have been attributed to specific sulfur-related ions. Specifically, these peaks correspond to S₄²⁻, S₆²⁻, and S₃⁻, respectively, as documented in previous research [41,42]. The UV-VIS spectra provide valuable insights into the electrochemical reaction process occurring within the LSBs. By analyzing the absorption peaks and their intensities, it becomes possible to discern variations in the concentration and behavior of different sulfur species during the battery’s discharge phase. The presence of TEAN in the electrolyte may influence the formation, transformation, or distribution of these sulfur species, leading to potential modifications in the overall electrochemical behavior of the battery system. From Figure 5c, one can find that S₆²⁻ is the main product in conventional electrolytes, and S₄²⁻ is the main discharge product in TEAN electrolytes. The color of the two electrolytes is shown in Figure 5d. In the H-type cells, the conventional electrolyte exhibits light red corresponding to S₆²⁻ ions, while the TEAN electrolyte presents yellow corresponding to S₃²⁻/S₄²⁻ ions [43], indicating TEAN can promote the reduction of S₆²⁻ to S₃²⁻/S₄²⁻. The validity of this outcome is further corroborated through the distinctive absorption peaks observed in the UV-visible spectroscopy (UV-VIS) analysis. Previous research findings [40] have lent support to the interpretation of these results. Based on this collective body of knowledge, the reaction pathway of the cathode within a conventional electrolyte environment can be elucidated as follows:

$${\mathrm{S}}_8^{}{+2\mathrm{e}}^{-}\to {\mathrm{S}}_8^{2-}$$

(1)

$${\mathrm{S}}_8^{2-}\to {\mathrm{S}}_6^{2-}+1/4S_8$$

(2)

$${2\mathrm{S}}_{\mathrm{n}}^{2-}{+\mathrm{xe}}^{-}\to {\mathrm{nS}}_2^{-}$$

(3)

The main product of S₆²⁻ in conventional electrolyte after discharge for 2 h indicating a sluggish reduction process of long-chain S_n²⁻ (8 ≥ n ≥ 4) to short-chain S_n²⁻ (4 ≥ n ≥ 3) [44], which is disadvantageous for the electrochemical reaction of LBSs [45]. However, in TEAN electrolyte, the main product is S₄²⁻, and a stable S₃^•− intermediate was present in electrolyte after the same discharge processes, indicating that TEAN can promote the reduction of long-chain S_n²⁻ (6 ≤ n ≤ 8) to short-chain S_n²⁻ (2 < n ≤ 4) (Figure 5e) [12], which can greatly enhance the efficient utilization of sulfur [13]. Drawing from prior research insights [40], the potential reaction pathways of LSBs operating within a TEAN electrolyte environment can be conceptualized as follows:

$${\mathrm{S}}_8^{}{+2\mathrm{e}}^{-}\to {\mathrm{S}}_8^{2-}$$

(4)

$${\mathrm{S}}_8^{2-}{+2\mathrm{e}}^{-}\to {2\mathrm{S}}_4^{2-}$$

(5)

$${2\mathrm{S}}_{\mathrm{n}}^{2-}{+\mathrm{xe}}^{-}\to {\mathrm{nS}}_2^{-}$$

(6)

$${\mathrm{S}}_4^{2-}+\frac{1}{4}{\mathrm{S}}_8^{}\to {2\mathrm{S}}_3^{•-}$$

(7)

$${\mathrm{S}}_3^{•-}{+\mathrm{e}}^{-}\to {\mathrm{S}}_3^{2-}$$

(8)

$${\mathrm{S}}_2^{-}+\frac{1}{4}{\mathrm{S}}_8^{}\to {\mathrm{S}}_3^{2-}$$

(9)

In addition, the stable S₃^•− intermediate in TEAN electrolyte can guide the 3D deposition of Li₂S/Li₂S₂, which can reduce electrode passivation caused by the 2D Li₂S/Li₂S₂ deposition, improve the utilization of Li₂S during charging, and enable the stable cycling of Li-S batteries under lean electrolyte conditions [46,47,48]. The schematic illustration of TEAN effect on the electrochemical reaction process is shown in Figure 5e.

4. Conclusions

In conclusion, the present study introduces a novel electrolyte additive, TEAN, with significant implications for enhancing the performance of LSBs. The outcomes of this investigation demonstrate a clear advantage of employing TEAN in LSBs compared to traditional electrolyte formulations. Notably, TEAN-equipped LSBs consistently exhibit enhanced capacity, extended cycling stability, and elevated coulombic efficiency, all of which contribute to bolstering the overall effectiveness of the battery system. The significance of TEAN becomes particularly pronounced under lean electrolyte conditions (5 µL _electrolyte/mg _s), where the scarcity of electrolyte can challenge battery performance. In such scenarios, TEAN continues to excel, enabling LSBs to maintain their superior capacity, stability, and efficiency even in challenging operating environments characterized by limited electrolyte availability. TEAN opens new avenues for the development of more reliable and efficient energy storage systems based on lithium–sulfur chemistry. In essence, this study not only contributes to the understanding of TEAN’s role in LSBs but also paves the way for the future design and optimization of next-generation lithium–sulfur battery technologies that are poised to offer improved performance and reliability in diverse applications.