High-concentration lithium bis(fluorosulfonyl)imide/1,2-dimethoxyethane (LiFSI/DME) electrolytes are promising candidates for highly reversible lithium−metal anodes. However, the performance of lithium−sulfur (Li−S) batteries with a high concentration of LiFSI/DME declines because LiFSI reacts irreversibly with lithium polysulfide, which is formed during the charge−discharge process of Li−S batteries. Hence, to apply high-concentration LiFSI/DME to Li−S batteries, we investigated carbon with an appropriate pore size for use in a sulfur composite cathode and optimized the composition of high-concentration LiFSI/DME. The results showed that the combination of carbon with mesopores of 2−3 nm diameter and 3 M LiFSI in DME/ 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropylether (HFE) (1:1 by vol.) provided a high-rate capability (943 mA h g −1 at a rate of 2 C). Moreover, the ratio of the 50th discharge capacity to the 2nd discharge capacity (capacity retention) improved from 50.0 to 61.6% with HFE dilution of high-concentration LiFSI/DME. The improved performance was achieved by suppressing the dissolution of lithium polysulfide, decreasing the viscosity of the electrolyte, and forming a thin solid electrolyte interface on the lithium−metal anode due to HFE dilution.
1. Introduction Elemental sulfur has extensive attentions because of its high theoretical specific capacity (1672 mA h g-1), natural abundance and low law-material cost. Therefore, sulfur is a promising candidate for a cathode of next generation batteries. However, there are many issues in fully demonstrating the potential of sulfur-based cathode for Li-S cells: intrinsically insulating of sulfur, large volume change during cycles and dissoluble lithium polysulfide intermediates (Li2Sn, 4 ≤ n ≤ 8) in an electrolyte. In our previous study, we solved these problems by using micro porous carbon (AC) to accommodate sulfur and demonstrate its stable cycling.1 Moreover, we proved that oxidation of AC with dilute nitric acid is useful to enhance sulfur-utilization.2 In this work, we apply hydrogen peroxide (H2O2), concentrated nitric acid (HNO3) and potassium permanganate (KMnO4) to oxidizing agent for surface functionalization of AC. Oxygen atomic percentage of ACs increases by using stronger oxidizing agent. Electrochemical performance of KMnO4-oxidized AC-sulfur composite cathode shows the highest specific capacity (614.04 mAh g-1 at the 50th cycle). 2. Method 2.1. preparation of H2O2 AC To prepare H2O2 AC, AC was added into 30 wt.% H2O2 and stirred for 48 hrs. By vacuum filtration and washing with deionized water, black material was obtained. The carbon product was dried in vacuum at 80ºC overnight. 2.2. preparation of HNO3 AC To prepare HNO3 AC, AC was added into 69 wt.% HNO3 and refluxed at 120ºC for 2 hrs. By vacuum filtration and washing with deionized water, black material was obtained. The carbon product was dried in vacuum at 80ºC overnight. 2.3. preparation of KMnO4 AC To prepare KMnO4 AC, AC and KMnO4 powder were added into 98 wt.% H2SO4 and stirred for 2 hrs. Next, deionized water and citric acid were added into the solution. By vacuum filtration and washing with deionized water, black material was obtained. The carbon product was dried in vacuum at 80ºC overnight. 2.4. preparation of 1000ºC AC To prepare reduced AC for comparison, AC was thermally annealed at 1000ºC for 1 hr under Ar atmosphere. 2.5. Characterization To determine surface oxygen loading of each AC, X-ray photoelectron spectroscopy (XPS) was carried out. To prepare AC-sulfur (S) composites, each AC was mixed with S at a weight ratio of AC : S = 48 : 52. The mixture was thermally annealed at 155ºC for 5 hr, so that AC-S, H2O2 AC-S, HNO3 AC-S, KMnO4 AC-S, 1000ºC AC-S were obtained. Each AC-S cathode was prepared by mixing the AC-S, acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a respective weight ratio of 89 : 5 : 3 : 3 and then coating on an Al foil current collector. The cells with the AC-S electrode and Li metal foil as a counter electrode were assembled in a glove box filled with Ar. The solution, lithium bis(trifluorosulfonyl)imide (LiTFSI) : tetraglyme (G4) : hydrofluoroether (HFE) = 10 : 8 : 40 (by mol), was used for the electrolyte. A charge-discharge cycling test was carried out at a current density of 167.2 mA g-1 (0.1 C) with charge and discharge cutoff voltages of 3.0 and 1.0 V at 25ºC. 3. Major results and conclusion Table 1 shows the surface element ratio of each AC based on the area of C1s and O1s spectra. Each oxidized AC had increased surface oxygen loading compared with pristine AC. For oxidized ACs, surface oxygen increased in the order of H2O2 AC, HNO3 AC, KMnO4 AC. This order is consistent with the order of strength of oxidizing agents. Thus, oxygen atomic percentage of ACs was found to be higher by using stronger oxidizing agent. Contrary to such oxidation, 1000ºC AC had decreased surface oxygen loading compared with pristine AC. This is because surface oxygen-containing functional groups of AC are reduced by thermal treatment under an inert atmosphere. Fig. 1 shows discharge capacity for each AC-S cathode. The cathode with higher oxygen-containing AC exhibited higher discharge capacity. This suggests that affinity between Li2Sn and AC surface was enhanced by introducing polar functional groups. We will also report results of further characterization for oxidized ACs. This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteies (ALCA-SPRING [JPMJAL1301])” from JST. (1) T. Takahashi et al., Prog. Nat. Sci.: Mater. Int., 25, 612 (2015). (2) S. Okabe et al., Electrochemistry, 85, 671 (2017). Figure 1
1. Introduction Lithium-sulfur (Li-S) batteries are rechargeable devices assembled with a sulfur cathode and a lithium metal anode. Li-S batteries have twice the volumetric energy density and 5 times the gravimetric energy density of lithium-ion batteries (LIB). Hence, Li-S batteries are expected to be applied to stationary power sources and EV vehicles [1]. However, Li-S batteries have the following issues: ・Sulfur and the final discharge product (Li2S) are insulators. ・In the discharge process, sulfur expands up to 1.8 times, so the structure of batteries is unstable. ・Intermediate products (Li2Sx (x = 4 – 8)) dissolve in an electrolyte; Li2Sx (x = 4 – 8) diffused to an anode to provide an insulating layer at the anode surface. ・In the charge process, Li2Sx (x = 4 – 8) causes redox shuttling. As a result, Li-S batteries cannot charge and discharge stably. In order to deal with this problem, Nazar et al. reported the method that sulfur is confined in porous carbon [2]. This approach provides a cathode realizing good electronic conductivity, restriction of sulfur expansion, and suppression of Li2Sx (x = 4 – 8) dissolution. Although these improved characteristics allow Li-S batteries to operate, The discharge capacity of Li-S batteries is still not high enough and this needs to be addressed. In our previous study, we reported that oxidation treatment to microporous carbon (MC) with HNO3 improves Li-S batteries' discharge capacity [3]. Moreover, we clarified that the discharge capacity of Li-S batteries has an approximate proportional relation with the amount of oxygen-containing functional groups on the MC surface [4]. This work attempts to elucidate the mechanism of improved Li-S battery performance by oxidation treatment to MC. Our report would lead to the proposal of a novel strategy to improve the performance of Li-S batteries. 2. Method 2.1 Preparation of Oxidized MC-Sulfur composite (Ox MC-S) MC was added into 69 wt.% HNO3 and refluxed at 120ºC for 2 h. By vacuum filtration and washing with deionized water, Ox MC was obtained. Ox MC dried in vacuum at 80ºC overnight was mixed with sulfur at a weight ratio of Ox MC: S = 48: 52. The mixture was thermally annealed at 155ºC for 5 h (Ox MC-S). Untreated MC was also composited with sulfur by the same method (MC-S). 2.2 Assembling of Cells Each MC-S cathode was prepared by mixing the MC-S, acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a respective weight ratio of 89: 5: 3: 3 and coating the resulting aqueous slurry on an Al foil current collector. The cells with the MC-S electrode and Li metal foil as an anode were assembled in a glove box filled with Ar. Lithium bis(trifluorosulfonyl)imide (LiTFSI): tetraglyme (G4): hydrofluoroether (HFE) = 10: 8: 40 (by mol) was used as the electrolyte. 2.3 Electrochemical Impedance Spectroscopy (EIS) To elucidate the effect of oxidation treatment on the internal resistance of Li-S batteries, EIS was carried out at various potentials (Discharge 2.0 – 1.0 V and Charge 1.0 – 3.0 V). The obtained Nyquist plots were used for the evaluation of solid electrolyte interphase (SEI) resistance (Rsei), charge-transfer resistance (Rct), and Warburg impedance (Rw). Rw was investigated with the calculation of Warburg coefficient (σ). 3. Major results and conclusion Since oxidation treatment to MC significantly increased the discharge capacity of Li-S batteries [3][4], it was expected that oxidation treatment would lower the internal resistance of Li-S batteries. EIS of MC-S and Ox MC-S at various potentials showed that oxidation treatment reduced Rsei by an average of 12.2 Ω. This indicates that the SEI thickness was reduced, or the SEI was composed of highly ion-conductive components by the oxidation treatment. Rct decreased only at lower potentials, and the Warburg coefficient decreased except at the end of charge and discharge potential. These results suggest that the oxidation treatment decreases overall resistance, but especially SEI resistance and Warburg impedance, which may improve the discharge capacity of Li-S batteries. We will also report the activation energy of Rsei and Rct and mechanism analysis of decreasing Rsei by oxidation to MC. This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING [JPMJAL1301])” from JST. [1] Y. Guo et al., Angew. Chem. Int. Ed., 52 (2013) 13186. [2] X. Ji et al., Nat. Mater., 8 (2009) 500. [3] S. Okabe et al., Electrochemistry, 85 (2017) 671. [4] L. Yoshida et al., ECS 238th PRiME Meeting Abstracts (2020).
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