Regulating surface chemistry of separator with LiF for advanced Li-S batteries

Abstract

Lithium-sulfur (Li-S) batteries have attracted intensive attention owing to their ultrahigh theoretical energy density. Nevertheless, the practical application of Li-S batteries is prevented by uncontrollable shuttle effect and retarded reaction kinetics. To address the above issues, lithium fluoride (LiF) was employed to regulate the surface chemistry of routine separator. The functional separator demonstrates a great ability to suppress active S loss and protect lithium anode. This work provides a facile strategy for the development of advanced Li-S batteries.

Graphical abstract

Keywords

Li-S batteries / LiF / functional separator

1 Introduction

The advanced battery system is highly required to meet the increasing energy demand for electrical vehicles (EVs) and portable electronics [1,2]. Lithium-sulfur (Li-S) batteries have attracted intensive attention due to its ultrahigh theoretic energy density (2600 Wh/kg), low cost, and environmental benignity [3,4]. However, Li-S batteries often suffer from the notorious shuttle effect, which results in active materials loss [5]. In addition, the sluggish reaction kinetics due to the poor electron conductivity of S and Li₂S [6], and uncontrollable Li dendrite growth with high risk of short circuit and safety hazard [7,8] would further hinder the development of Li-S batteries. Therefore, effective strategies are in urgent need to address the above issues to realize practical Li-S batteries.

Numerous methods have been employed to tackle the problems in Li-S batteries. For example, the application of lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide (LiHFDF) as a new Li salt can form LiF-rich SEI and CEI layers, which not only inhibit the growth of lithium dendrite but also avoid active material losses at cathode, leading to an improved cell performance in Li-S batteries [9]. Besides, the addition of redox mediators [10,11] and rational design of cathode materials [12] are considered as effective strategies to suppress the shuttle effect. Recently, the shift from liquid electrolyte to polymer electrolyte provides high Li ion conductivity and favorable mechanical stabilities, benefiting to block polysulfides shuttle and suppress Li dendrite growth in Li-S batteries [13,14]. However, it is very difficult to completely restrain the active sulfur loss, especially under the high sulfur loading condition [15]. Additionally, the separator configuration demonstrates a great potential for blocking polysulfides diffusion and attracting extensive attention in the research of Li-S batteries [16,17]. The pioneering works by Manthiram introduced microporous carbon paper (MCP) to modify the separator, which not only inhibited the diffusion of soluble polysulfides, but also decreased the charge transfer resistance, further improving battery performance [18]. Notedly, carbon materials cannot provide an adequate chemisorption toward polysulfides owing to its non-polarity [19]. To increase the affinity toward polysulfides, various polar inorganic materials, including metal oxides [20], metal sulfides [21–24], etc., have been applied to absorption polysulfides. Therefore, rational design on the separator is promising to suppress the shuttle effect and improve the cycling performance in Li-S batteries.

Lithium fluoride (LiF) has shown a great ability to control Li deposition behaviors owing to its chemical and mechanical stability. Therefore, it is regarded as one of the most crucial components in the SEI layer [25–28]. Tao and coworkers have imaged the mosaic interface between Li and poly(ethylene oxide) (PEO) by advanced cryo-TEM, and demonstrated that the formation of LiF nanocrystals could be promoted by the introduction of Li₂S [29,30], and the uniform LiF can decrease the reactions between the Li metal and PEO. However, the role of LiF in protecting S cathode has not been fully understood. In this work, LiF is employed to regulate the surface chemistry of the routine separator for advanced Li-S batteries. The functional separator not only effectively suppresses the shuttle effect, but also promotes reaction kinetics and protects Li anodes from severe corrosion of polysulfides.

2 Materials and methods

2.1 Materials

Carbon paper (GDS 3260) was used as received. Commercial Li-S electrolyte (DodoChem) was composed of DOL/DME (V/V1/1) with 1 mol/L LiTFSI and 2% mass fraction LiNO₃ dissolved. S powder was received from Sinopharm, and Li₂S was purchased from Macklin.

2.2 Separator modification

LiF and PVDF (1:4 by mass) were dispersed in 10 mL NMP. The mixture was filtrated through the PE separator. Then, the separator obtained was dried under vacuum at 80°C overnight.

2.3 Li₂S₆ catholyte preparation

0.5 mol/L of Li₂S₆ catholyte was prepared according to Ref. [31]. 2.5 mmol of S and 0.5 mmol of Li₂S powders were added into 1 mL commercial Li-S electrolyte in a glove box. The mixture was stirred at 60°C overnight. The catholyte prepared was dark yellow and no particle was observed.

2.4 Li₂S₆ permeation experiment

U-shaped glass devices were divided into two chambers by a piece of separator. Each chamber was filled with 12 mL of fresh DME. 100 µL of prepared catholyte was added into the left chamber. After polysulfides diffusion for 40 min, optical photos were taken to compare the polysulfides inhibition effect of the separators. Then, 300 µL of solution in the right chamber was diluted with 1.7 mL of fresh DME, and the mixture (2 mL) was used for the UV-vis experiment.

2.5 Electrochemical measurements and characterization

2032-type coin cell was used to test the battery performance. Carbon paper was used as the cathode and the lithium foil with a thickness of approximately 450 µm was used as the anode. 13 µL of 0.5 mol/L Li₂S₆ catholyte was added in the cathode, and the typical S loading was 1.25 mg/cm². PE with a diameter of 19 mm was used as the separator and 20 µL of blank electrolyte was added on the opposite side of the separator. For the cell with the functional separator, the functional layer faced the cathode side. For SEM characterization of cycled Li anode, the cells were stopped after 200 cycles and then disassembled in the glove box. The cycled lithium metal was washed thoroughly with fresh DME. SEM images were obtained with a scanning electron microscope (FEI APREO).

2.6 Li⁺ transference number test

Li⁺ transference number test was conducted with Li-Li symmetry cells. The symmetry cells were previously treated with the following procedures to establish a steady interface, including charge at 0.01 mA/cm² for 4 h before a rest for 0.5 h, and discharge at 0.01 mA/cm² for 4 h before a rest for 0.5 h. The procedures were repeated for 4 cycles, and the steady-state current was obtained at the polarization of 10 mV.

3 Results and discussion

The functional separator was prepared by filtrating the NMP solution with LiF and PVDF on the PE separator. As a result, LiF particles with the size of approximately 100 nm were uniformly distributed on the surface of the routine PE separator, and the thickness of the LiF layer was approximately 13 µm (see Electronic Supplementary Material (ESM), Figs. S1 and S2). The XRD peaks of the functional layer were located at 38.6°, 45.0°, and 65.6°, which can be assigned to (111), (200), and (220) planes of LiF (PDF# 45–1460) (ESM, Fig. S3). The functional separator demonstrates a good capability to inhibit polysulfides diffusion in the permeation experiments with the U-shaped device, which is divided into two chambers by a piece of separator (ESM, Fig. S4). The UV-vis adsorption spectra confirmed that Li₂S₆ had a strong adsorption at approximately 400 nm, as shown in Fig. 1(a) [32]. The adsorption peak decreased obviously due to the inhibition effect of the functional separator, suggesting that the LiF layer could effectively suppress the shuttle effect.

Detailed electrochemical measurements were conducted to verify the advantages of the functional separator in Li-S batteries. The batteries were assembled with carbon paper as the cathode materials, and the typical sulfur loading was 1.248 mg/cm². The charge/discharge curves were obtained at a current density of 0.25 mA/cm². As depicted in Fig. 1(b), the potential hysteresis (ΔE) was a key indicator of the reaction kinetics. The potential hysteresis in the cell with the modified separator decreased by 39 mV, compared with that of the routine separator. The parallel results were obtained from cyclic voltammetry (CV) curves. As shown in Fig. 1(c), the reduction peaks were located around 2.3 V and 2.0 V, which can be assigned to the reduction reaction from S to Li₂S₄ (R₁) and then to Li₂S (R₂) [33]. Moreover, the oxidation peak (approximately 2.3 V) was attributed to the oxidation of Li₂S (O₁). With the LiF modified separator, the overpotential of R₁ and R₂ decreased by 30 mV and 20 mV, respectively, and the current peak of O₁ downshifted by 20 mV. In addition, Li-S cell with the functional separator demonstrated a higher peak current. Therefore, all the above results indicate that the LiF functional separator has a positive effect on promoting redox reaction kinetics, which further improves the cycling performance of Li-S batteries. Surprisingly, the specific capacity reached to 1230 mAh/g after 36 cycles as exhibited in Fig. 1(d), while the value of 1085 mAh/g was reported with routine separators. It is noted that a higher coulombic efficiency can be achieved in the cell with the LiF modified separator (ESM, Fig. S5).

To further understand the mechanism of reaction kinetics with the functional separator, the Li⁺ diffusion coefficient was investigated by CV. Figures 2(a) and 2(b) depicted CV curves at different scan rates, and the values of the peak current of different separators have been collected (ESM, Table S1). The diffusion coefficient of Li⁺ was calculated based on the Randles-Sevcik equation,

I_{\mathrm{p}}=2.69\times {10}^5{n}^{1.5}A{D}_{{\mathrm{L}\mathrm{i}+}^{0.5}}{C}_{{\mathrm{L}\mathrm{i}+}^{v^{0.5}}},

where I_p stood for the peak current, n was the number of electrons involved in the reaction, A was electrode area, D_Li+ and C_Li+ represented Li⁺ diffusion coefficient and Li⁺ concentration, respectively, and v was the scanning rate. Because n, A, and C_Li+ were constants, the slope of the liner fitting curve (I_p/v^0.5) was proportional to the Li⁺ diffusion rate (D_Li+). As revealed in Figs. 2(c) and 2(d), I_R1 and I_R2 represented the peak current (I_p) of the reduction reaction R₁ and R₂, and the slope values of the fitting line with the functional separator and the routine separator were displayed in Table S2 (ESM). The higher value of the functional separator implied an enhanced Li⁺ diffusion coefficient for both R₁ and R₂ in Li-S batteries.

Li⁺ transference number was also an important parameter to reflect the Li⁺ diffusion behavior. Li/Li symmetry cells were employed to evaluate the Li⁺ transference number. The cells established a steady interface according to Ref. [34]. Then, the cells were polarized at 10 mV to measure the response current (Fig. 3(a) and 3(b)). The transference number was calculated by utilizing the Bruce and Vincent method,

t+={\displaystyle \frac{I_{\mathrm{t}}(\mathrm{\Delta }V-I_0{R}_0)}{I_0(\mathrm{\Delta }V-I_{\mathrm{t}}{R}_{\mathrm{t}})}},

where t₊ was the Li⁺ transference number; I₀ and I_t were the initial current and steady-state current, respectively; R₀ and R_t represented the initial resistance and the steady-state resistance; the polarization of 10 mV was denoted as ΔV; the Nyquist plots of R₀ and R_t were provided in Fig. S6 (ESM); and the t₊ of the cell with the functional separator was 0.42, which was higher than that with the routine separator (t₊=0.27). The higher t₊ represented the enhanced Li⁺ diffusion. The higher Li⁺ transference number and Li⁺ diffusion coefficient are favorable to improve the conversion kinetics and battery performance.

The LiF modified separator not only tackled the challenges of S cathode, but also protected lithium anode from severe corrosion (ESM, Fig. S7). SEM images in Fig. 4 after 200 charge/discharge cycles showed different morphology of cycled lithium metal with the modified separator. Lithium metal was like pancake in shape with the protection of the LiF layer, and its diameter ranged from 5 µm to 20 µm (Figs. 4(a) and 4(b)). In contrast, lithium metal anode suffered from severe pulverization by polysulfides corrosion with the routine separator (Figs. 4(c) and 4(d)). The higher surface area of lithium dendrite formation exacerbated the electrolyte consumption, leading to a rapid cell failure, especially under lean electrolyte conditions [35].

To further understand the protection of the LiF functional layer on lithium anode, the Nyquist plots of Li-S cells after 5 cycles were received. As depicted in Fig. S8 (ESM), the semicircle in the medium frequency represented the charge transference resistance (R_ct) in Li-S batteries. The 17% decrease of R_ct demonstrated that less insulating Li₂S was formed on lithium anode surface owing to the polysulfides inhibition by the LiF functional layer, which also contributed to the improved cycling performance of Li-S batteries. Li-Li symmetry cells were assembled to investigate the protection of lithium by the LiF functional layer. As shown in Fig. S9 (ESM), Li-Li symmetry cells were operated at the current density of 1 mA/cm², and the areal capacity of charge and discharge were 1 mAh/cm². The voltage polarization of the symmetry cell with the routine separator was 103.9 mV, and the polarization increased dramatically after 95 h. In sharp contrast, the voltage polarization was 29.5 mV owing to the separator modification by LiF, and displayed highly stable voltage curves during 200 h cycling, suggesting that a more stable interface was established between Li and the electrolyte with the LiF functional layer.

4 Conclusions

In summary, the surface chemistry of the routine separator was regulated with LiF modification. The functional separator not only suppressed the shuttle effect, but also enhanced the reaction kinetics of the Li-S battery. Further mechanism analysis indicated that the improved Li⁺ diffusion coefficient and Li⁺ transference number are responsible for the fast kinetics process and the great electrochemical performance in Li-S batteries. In addition, the functional separator could effectively protect lithium metal anode. Therefore, this work provides a facile strategy for protecting both sulfur cathodes and lithium anodes in Li-S batteries.

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