5359 www.MaterialsViews.com wileyonlinelibrary.com density, which is approximately 6.6 times larger than that of the current intercalationbased LIBs (≈387 Wh kg −1 ). [5][6][7][8] Remarkably, recent research efforts have been successful in resolving the chronic technical challenges associated with sulfur electrodes, especially dissolution of lithium polysulfi des and low electric conductivity of elemental sulfur. [9][10][11][12] The representative solutions along this direction include sulfur-carbon composite structures that minimize sulfur exposure to electrolyte, [ 5,13 ] surface coating of active particles, [ 7,[14][15][16] engagement of allotropic sulfur, [ 17,18 ] and use of solid-state electrolytes [ 19 ] and electrolyte additives. [ 20,21 ] Although the improved cycle lives based on these approaches represent considerable progresses in the area of Li-S batteries, it should be noted that the most critical issue related to Li metal anodes still remains unaddressed: upon repeated charge and discharge, Li dendrites grow on the surface of the Li metal anode, which triggers multiple mechanisms for rapid capacity fading. The electrolyte becomes decomposed successively along the Li dendrite surfaces, which destabilizes the electrode-electrolyte interface and also increases the resistance (or overpotential) leading to continuous capacity decay. The electrolyte could be eventually exhausted. The dendrite growth could also promote short circuits between both electrodes, and thus resulting in severe safety hazard. [ 22 ] An additional but very critical hurdle related to the Li dendrite growth is that the dendrite growth becomes accelerated with areal current density. [22][23][24][25][26] Hence, the increase in the areal capacity of sulfur electrode (or the areal mass loading of sulfur active material) would amplify the problems originating from Li dendrites. Likewise, the dissolution of the fatal lithium polysulfi des would also be amplifi ed under the increased areal capacity. The demonstration of improved cycle lives from most of recent sulfur electrode designs, in turn, implies that the cycling tests were conducted with moderate mass loadings of the sulfur active materials. Therefore, in order to exalt Li-S batteries to more practical technology, more systematic approaches need to be engaged to resolve the issues from both sides of electrodes.With purpose of developing Li-S cells with high areal energy densities, herein, we have adopted or discovered the key cell components (sulfur electrode, separator, and electrolyte) in a way that Li dendrite formation and polysulfi de dissolution are minimized even at a practically viable loading of sulfur active material (≈17 mg cm −2 , the mass of sulfur-carbon composite). The synergistic outcomes from smart engineering of eachThe battery community has recently witnessed a considerable progress in the cycle lives of lithium-sulfur (Li-S) batteries, mostly by developing the electrode structures that mitigate fatal dissolution of lithium polysulfi des. Nonetheless, most of the ...

Despite the unparalleled theoretical gravimetric energy, Li-O 2 batteries are still under a research stage because of their insuffi cient cycle lives. While the reversibility in air-cathodes has been lately improved signifi cantly by the deepened understanding on the electrode-electrolyte reaction and the integration of diverse catalysts, the stability of the Li metal interface has received relatively much less attention. The destabilization of the Li metal interface by crossover of water and oxygen from the air-cathode side can indeed cause as fatal degradation for the cycle life as the irreversibility of the air-cathodes. Here, it is reported that cheap poreless polyurethane separator can effectively suppress this crossover while allowing Li ions to diffuse through selectively. The polyurethane separator also protects Li metal anodes from redox mediators used for enhancing the reversibility of the air-cathode reaction. Based on the Li metal protection, a persistent capacity of 600 mAh g −1 is preserved for more than 200 cycles. The current approach can be readily applicable to many other rechargeable batteries that suffer from similar interfacial degradation by side products from the other electrode.

Despite the attractive theoretical capacity of sulfur over 1600 mA hg À1,l ithium-sulfur (Li-S) batteries suffer from insufficient cycle lives mainly due to fatal polysulfide dissolution. This chronic drawback becomes amplified at highareal capacities, especially close to commercial levels (i.e., > 3mAhcm À2 ). Here, we introduce an integrated approacho fa dopting poreless urea-urethane copolymer (spandex) separator and cesium nitrate (CsNO 3 )e lectrolyte additive. The spandex separator prevents soluble polysulfides from reaching Li metal anode and also suppresses Li dendrite growth via its intrinsic wet adhesion. When combined with as ulfur-carbon composite cathode, the cell based on the spandex separator and the electrolyte additive delivers ah igh areal capacity of 4mAhcm À2 with decent cycling performance, such as 79.2 %c apacity retention after 200 cycles with respect to the capacity in the second cycle.

can deliver superior rate performance despite its inherently low electric conductivity. Especially, modifications of the intrinsic phase via doping [17] with aliovalent elements and incorporating off-stoichiometry [18] improved the rate performance remarkably. Also, the formation of metastable structures that allows the nucleation of a second phase to bypass was revealed [19,20] as the origin of the exceptional rate capability of LFP.Besides the tuning of active material, various polymeric binders were lately investigated [21][22][23][24][25] for LFP electrodes in both organic and aqueous media. Aqueous binders were particularly highlighted because of their conspicuous advantages, including low cost and easy disposal of waste solvents after processing. [21,22,24,26] In spite of these advantages, currently, LFP electrodes are mostly manufactured via N-methyl-2pyrrolidone (NMP)-based slurry process because the use of aqueous media causes Li ion extraction from active powder and corrosion of aluminum (Al) current collector. [27][28][29] As in most LIB cathodes, polyvinylidene difluoride (PVDF) dispersed in NMP has been mainly used for LFP electrodes, to take advantages of the properties of PVDF, such as high electrochemical stability, high specific dielectric constant, and decent Li ion conductivity. However, PVDF binders operate mainly based on van der Waals interaction, leading to weak adhesion of the electrode to the current collector. Also, PVDF used for most commercial LIBs has high molecular weights (MWs) of around 1 000 000 and therefore often suffers from binder aggregation during slurry preparation as well as in the final electrode film. Herein, we introduce an unconventional binder, namely spandex, with relatively low molecular weight of around 300 000. Compared to the commercial PVDF binders with both high and low MWs, the spandex binder exhibited superiority in uniformness of electrode morphology, adhesion of electrode to an Al current collector, conservation of solvent during slurry preparation, and rate capability in battery operation. Furthermore, the enhanced adhesion of electrode renders the spandex binder suitable for 3D porous electrodes that are considered for future flexible battery applications. Moreover, we can take advantage of the long research and industrial experience of spandex; spandex is usually produced by step-growth polymerization and used for various applications, including clothes, daily supplies, biomedical devices, etc. [30,31] This investigation conveys a useful lesson that although occupying a small content in the electrode, binder can considerably improve the performance of established commercial LIB electrodes, and unexplored polymers can be good candidates for such opportunities. Figure 1 schematically illustrates the electrode morphologies when high MW PVDF (H-PVDF) and low MW spandex (L-spandex) are adopted as binders. H-PVDF often causes Lithium-ion batteries (LIBs) have been successfully developed as power sources for mobile information technology (IT) devices and hybrid...

The highest areal energy density to date of a lithium-sulfur battery is demonstrated on page 5359 by J. W. Choi and co-workers through the combination of smart engineering of the key cell components (electrode, electrolyte, and separator). The integrated strategy suppresses both lithium polysulfide dissolution from the sulfur cathode and lithium dendrite growth from the lithium anode, leading to the highest areal capacity of 9 mAh cm −2 while preserving stable cyclability. AREAL ENERGY DENSITY