shuttle effect of polysulfide. [3] The suppression of the polysulfide shuttle effect is the primary challenge that has hindered the development of lithium-sulfur batteries. The ideal sulfur cathode host should have: (i) a highly porous structure with interconnected architecture to encapsulate sulfur, (ii) strong capability to restrain soluble polysulfides, (iii) high electronic conductivity, and (iv) flexible but robust mechanical properties.Carbon materials with well-designed pore structures, such as mesoporous carbon [4] and carbon nanotubes, [5] have been utilized to restrain the migration of soluble polysulfides from the cathode. Carbon materials can effectively improve the electronic conductivity of sulfur cathodes, and the as-fabricated lithium-sulfur cells showed high specific capacities during the initial few cycles. However, there has been a serious decay of capacity during long-term cycling. It is mainly ascribed to the weak intrinsic interactions between nonpolar carbon and polar polysulfides intermediate, and the large volume expansion/extraction of sulfur compounds during the discharge/ charge processes. [6] The physical barriers provided by sequestration and adsorption in carbon materials can only slow down diffusion of lithium polysulfides out of cathodes in the shortto medium-term of cycling (<100 cycles). Additionally, weak interaction can cause the detachment and separation of lithium sulfides (Li 2 S x , 1 < x < 2, the fully discharged products), from carbon matrix, which will induce irreversible active mass loss and isolation of electrical contacts. Therefore, strong physical and chemical interactions between sulfur/lithium polysulfides and the host materials are crucial to suppress polysulfides shuttling effects and capacity decay. It has been discovered that surface functionalized host materials, such as reduced graphene oxides, [1] polar metal oxides (MnO 2 , [7] Ti 4 O 7[8] ), metal-organic frameworks (MOFs), [9] and metal carbide MXene, [10] showed much better properties because of their hydrophilic surfaces that can bind lithium polysulfides via polar-polar interactions.Recently, a large family of ternary metal carbides, nitrides, or carbonitrides has been successfully prepared. These are termed as "MXene" and are denoted as M n + 1 X n T x , where M is an transition metal such as Ti or V, X is C and/or N, and T is a surface termination group (e.g., O, OH, and F). [11] They have been emerged as brand-new 2D materials with potential applications as electrode materials. Owing to layered structure, high electronic conductivity, remarkable chemical durability, Crumpled nitrogen-doped MXene nanosheets with strong physical and chemical coadsorption of polysulfides are synthesized by a novel one-step approach and then utilized as a new sulfur host for lithium-sulfur batteries. The nitrogendoping strategy enables introduction of heteroatoms into MXene nanosheets and simultaneously induces a well-defined porous structure, high surface area, and large pore volume. The as-prepared nitrogen-dope...

A physics-based side-reaction coupled electrochemical model for capacity fade of a graphite/LiMn 2 O 4 cell is developed by including the key degradation mechanisms in both anode and cathode. The side reactions considered in this study include 1) solid electrolyte interphase (SEI) growth and manganese deposition on the anode and 2) manganese dissolution, electrolyte oxidation and salt decomposition on the cathode. Our study reveals three stages of capacity fade upon long term cycling: acceleration, stabilization, and saturation. In the acceleration stage, capacity fade is due mainly to the cyclable lithium loss induced by the anode SEI growth. In the stabilization stage, the anode SEI growth slows down as it gets thicker, the cathode Mn dissolution-induced capacity loss outpaces cyclable lithium loss, and the cathode becomes more intercalated at the end of discharge. In the saturation stage, cathode capacity degrades further and becomes the limiting factor, the cyclable lithium is shifted to the anode and the cathode reaches end-of-discharge saturation due to the severe cathode capacity fade. This study shows that the cyclable lithium loss and the cathode capacity loss are the two major contributors to the cell capacity fade, and the interaction between them determines the cell capacity.Energy security and climate change challenges provide a strong impetus for the development of electric vehicles (EV) and hybrid electric vehicles (HEV). 1,2 Lithium ion batteries are one of the most important components of the EVs and HEVs. One of the most important topics is the degradation, or aging process, of the cells during operation. A substantial amount of work has gone into understanding capacity fade through experiments and theoretical/numerical studies. Ramadass et al. 3 carried out a complete capacity fade analysis for the Sony 18650 cells after hundreds of cycles. They divided the capacity fade into rate capability loss as well as primary and secondary active material losses. However, they proposed no model to quantitatively explain the capacity fade due to different mechanisms. Safari et al. 4 proposed a multimodal physics-based aging model to predict the capacity fade for Li-ion batteries. They assumed that capacity fade stems mainly from solid electrolyte interphase (SEI) growth on the anode, and that lithium ion consumption during SEI growth was the main contributor to cycling degradation. Zhang et al. 5 developed a single-particle model which they calibrated against the cycling data to study the parameter trajectories. They suggested that there were probably different stages of capacity fade in the lithium ion battery. However, they proposed no degradation model to quantitatively study the degradation process.The studies above have attempted to elucidate capacity fade by focusing on specific mechanisms, not by regarding all the key mechanisms together. Because each mechanism is coupled to each other, it is important to consider all of them and their interactions simultaneously. In this way, the whole life of a ba...

In article number https://doi.org/10.1002/aenm.201702485 Dan Wang and Guoxiu Wang and co‐workers design a novel strategy for doping heteroatomic nitrogen into MXene frameworks. The resultant nitrogen‐doped MXene nanosheets demonstrate a well‐defined porous structure, a high surface area and large pore volume. The nitrogen‐doped porous MXene nanosheets are successfully used for strong physical and chemical co‐absorption of polysulfides. Lithium‐sulfur batteries, based on porous N‐doped MXene nanosheets/sulfur composites, exhibit an outstanding electrochemical performance.