The loss of sulfur cathode material as a result of polysulfide dissolution causes significant capacity fading in rechargeable lithium/sulfur cells. Here, we use a chemical approach to immobilize sulfur and lithium polysulfides via the reactive functional groups on graphene oxide. This approach enabled us to obtain a uniform and thin (around tens of nanometers) sulfur coating on graphene oxide sheets by a simple chemical reaction-deposition strategy and a subsequent low-temperature thermal treatment process. Strong interaction between graphene oxide and sulfur or polysulfides enabled us to demonstrate lithium/sulfur cells with a high reversible capacity of 950-1400 mA h g(-1), and stable cycling for more than 50 deep cycles at 0.1C (1C = 1675 mA g(-1)).

Sulfur (S) encapsulated in porous carbon nanofibers (CNFs) was synthesized via electrospinning, carbonization and solution-based chemical reaction-deposition method. The chemical reactiondeposition strategy provides intimate contact between the S and the CNFs. This would not necessarily be the case for other reported methods, such as ball milling and thermal treatment. These novel porous carbon nanofiber-sulfur (CNF-S) nanocomposites with various S loadings showed high reversible capacity, good discharge capacity retention and enhanced rate capability when they were used as cathodes in rechargeable Li/S cells. We demonstrated here that an electrode prepared from a porous CNF-S nanocomposite with 42 wt% S maintains a stable discharge capacity of about 1400 mA h g À1 at 0.05 C, 1100 mA h g À1 at 0.1 C and 900 mA h g À1 at 0.2 C. We attribute the good electrochemical performance to the high electrical conductivity and the extremely high surface area of the CNFs that homogeneously disperse and immobilize S on their porous structures, alleviating the polysulfide shuttle phenomenon. SEM measurements showed that the porous CNF structures remained nearly unchanged even after 30 cycles' discharging/charging at 0.05 C.

Using ab initio calculations combined with experiments, we clarified how the kinetics of Li-ion diffusion can be tuned in LiNixMnyCozO2 (NMC, x + y + z = 1) materials. It is found that Li-ions tend to choose oxygen dumbbell hopping (ODH) at the early stage of charging (delithiation), and tetrahedral site hopping (TSH) begins to dominate when more than 1/3 Li-ions are extracted. In both ODH and TSH, the Li-ions surrounded by nickel (especially with low valence state) are more likely to diffuse with low activation energy and form an advantageous path. The Li slab space, which also contributes to the effective diffusion barriers, is found to be closely associated with the delithiation process (Ni oxidation) and the contents of Ni, Co, and Mn.

energy density, high voltage, and long cycle life. [ 2 ] As one of the most widely used cathode materials, LiNi x Mn y Co z O 2 (labeled as NMC) has been investigated extensively, due to their high reversible capacity, good environmental compatibility, and relatively high Li-ion diffusivity. In the previous works, different kinds of NMC materials with different content ratio of Ni, Co, and Mn have been developed, and their electrochemical properties have also been studied, such as Li(Ni 1/3 Mn 1/3 Co 1/3 ) O 2 (111), [ 3,4 ] Li(Ni 0.4 Mn 0.4 Co 0.2 )O 2 (442), [ 5 ] Li(Ni 0.42 Mn 0.42 Co 0.16 )O 2 (552), [ 6 ] Li(Ni 0.5 Mn 0.3 Co 0.2 )O 2 (532), [ 7 ] Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 (622), [ 8 ] and Li(Ni 0.7 Mn 0.15 Co 0.15 )O 2 (71515). [ 9 ] For example, Noh et al. compared the electrochemical properties including the Li-ion diffusion coeffi cient, capacity retention, and electrochemical stabilities (25 to 55 °C) of layered NMC cathode materials ((111), (532), (622), (71515), (811) and Li(Ni 0.85 Mn 0.075 Co 0.075 )O 2 ) at room temperature and found that the Ni content had a great infl uence on the electrochemical properties. [ 10 ] Solid phase diffusion coeffi cient ( D s ) is one of the most important parameters for the active materials of the LIBs, as it determines the charge and discharge rate capability directly. In particular, for high power density applications, fast Li-ion transport in cathode materials is a key factor and must be needed. As a result, many experimental and theoretical works have been devoted to investigating the Li-ion diffusion properties in layered cathode materials. [ 11,12 ] However, to the best of our knowledge, there is little work reported to study the relationship between the layer distance and kinetics of Li-ion diffusion in different temperatures of layered NMC cathode materials systematically, which is important for LIBs applied in multitemperature environments.At the same time, in order to measure D s accurately, many methods such as galvanostatic intermittent titration technique (GITT), [ 3,[13][14][15][16] potentiostatic intermittent titration technique (PITT), [ 14,17 ] electrochemical impedance spectroscopy, [ 18 ] and cyclic voltammetry [ 19 ] have been developed in the past decades. Although factors such as the inaccuracy of the assumptions, Understanding and optimizing the temperature effects of Li-ion diffusion by analyzing crystal structures of layered Li(Ni x Mn y Co z )O 2 (NMC) ( x + y + z = 1) materials is important to develop advanced rechargeable Li-ion batteries (LIBs) for multi-temperature applications with high power density. Combined with experiments and ab initio calculations, the layer distances and kinetics of Li-ion diffusion of LiNi x Mn y Co z O 2 (NMC) materials in different states of Li-ion de-intercalation and temperatures are investigatedsystematically. An improved model is also developed to reduce the system error of the "Galvanostatic Intermittent Titration Technique" with a correction of NMC particle size distribution. The Li-ion diff...

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Understanding and further designing new layered Li(NiMnCo)O (NMC) (x + y + z = 1) materials with optimized thermal stability is important to rechargeable Li batteries (LIBs) for electrical vehicles (EV). Using ab initio calculations combined with experiments, we clarified how the thermal stability of NMC materials can be tuned by the most unstable oxygen, which is determined by the local coordination structure unit (LCSU) of oxygen (TM(Ni, Mn, Co)-O-Li): each O atom bonds with three transition metals (TM) from the TM-layer and three to zero Li from fully discharged to charged states from the Li-layer. Under this model, how the lithium content, valence states of Ni, contents of Ni, Mn, and Co, and Ni/Li disorder to tune the thermal stability of NMC materials by affecting the sites, content, and the release temperature of the most unstable oxygen is proposed. The synergistic effect between Li vacancies and raised valence state of Ni during delithiation process can aggravate instability of oxygen, and oxygen coordinated with more nickel (especially with high valence state) in LSCU becomes more unstable at a fixed delithiation state. The Ni/Li mixing would decrease the thermal stability of the "Ni═Mn" group NMC materials but benefit the thermal stability of "Ni-rich" group, because the Ni in the Li layer would form 180° Ni-O-Ni super exchange chains in "Ni-rich" NMC materials. Mn and Co doping can tune the initial valence state of Ni, local coordination environment of oxygen, and the Ni/Li disorder, thus to tune the thermal stability directly.

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