This investigation elucidates the electrochemical reaction process occurring within lithium-sulfur battery cells in detail, which has been unclear even after a half century of study primarily due to the very high reactivity of the polysulfide species. The polysulfide intermediates were deactivated by organic conversion - benzylization, and LC/MS and NMR analyses were first applied. The results demonstrate that the second voltage plateau in the discharge profile, which is the most important step in practical use because of its constant voltage, is dominated by the reduction of the Li2S3 intermediate. The first voltage plateau and the transition state between the plateaus, in which the voltage varies with the capacity, are associated with multiple reactions including the decomposition of S8 into Li2Sx (x = 1 to 7) and the transformation of Li2Sy (y = 4 to 8) into Li2Sz (z = 1 to 3). It is also revealed that longer polysulfide species, Li2Si (i = 6 to 8), are responsible for the redox shuttle phenomenon, which causes serious capacity degradation.
Cationic polymer can capture polysulfide ions and inhibit polysulfide shuttle effect in lithium sulfur (Li-S) rechargeable batteries, enhancing the Li-S battery cycling performance. The cationic poly[bis(2-chloroethyl) ether-alt-1,3-bis[3-(dimethylamino) propyl]urea] quaternized (PQ) with a high density quaternary ammonium cations can trap the lithium polysulfide through the electrostatic attraction between positively charged quaternary ammonium (RN) and negatively charged polysulfide (S). PQ binder based sulfur electrodes deliver much higher capacity and provide better stability than traditional polyvinylidene fluoride (PVDF) binder based electrodes in Li-S cells. A high sulfur loading of 7.5 mg/cm is achieved, which delivers a high initial areal capacity of 9.0 mAh/cm and stable cycling capacity at around 7.0 mAh/cm in the following cycles.
The X-ray absorption spectroscopy technique has been applied to study different stages of the lithium/sulfur (Li/S) cell life cycle. We have investigated how speciation of S in Li/S cathodes changes upon the introduction of CTAB (cetyltrimethylammonium bromide, CH3(CH2)15N+(CH3)3Br−) and with charge/discharge cycling. The introduction of CTAB changes the synthesis reaction pathway dramatically due to the interaction of CTAB with the terminal S atoms of the polysulfide ions in the Na2Sx solution. For the cycled Li/S cell, the loss of electrochemically active sulfur and the accumulation of a compact blocking insulating layer of unexpected sulfur reaction products on the cathode surface during the charge/discharge processes make the capacity decay. A modified coin cell and a vacuum-compatible three-electrode electro-chemical cell have been introduced for further in-situ/in-operando studies.
We have investigated the chemical bonding interaction of S in a CTAB(cetyltrimethylammonium bromide, CH 3 (CH 2 ) 15 N + (CH 3 ) 3 Br -)-modified sulfur-graphene oxide (S-GO) nanocomposite used as the cathode material for Li/S cells by S K-edge X-ray absorption spectroscopy (XAS). The results show that the introduction of CTAB to the S-GO nanocomposite and changes in the synthesis recipe including alteration of the S precursor ratios and the sequence of mixing of ingredients lead to the formation of different S species. CTAB modifies the cathode materials through bonding with Na 2 S x in the precursor solution, which is subsequently converted to C-S bonds during the heat treatment at 155°C. Moreover, GO bonds with CTAB and acts as the nucleation center for S precipitation. All these interactions among S, CTAB and GO help to immobilize the sulfur in the cathode, and may be responsible for the enhanced cell cycle life of CTAB-S-GO nanocomposite-based Li/S cells.
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