Towards High Performance Lithium Sulfur Batteries: Surface Oxidation of Micro Porous Carbon for Better Sulfur Positive
Authors: Yoshida Luna , Ishikawa Masashi 
Published: December 2020

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