The solid-electrolyte interphase (SEI) layer is pivotal for the stable and rechargeable batteries especially under high rate. However, the mechanism of Li+ transport through the SEI has not been clearly...

despite all these promises, three intrinsic drawbacks need to be resolved before fulfilling the promise of the market potential.First, the most stable but electronically insulating S 8 (≈10 −14 S cm −2 ) with cyclic configuration is used as the starting material in Li-S cathode, significantly limiting the full utilization of the active materials to reach the theoretical capacity. Therefore, it is the first priority to design the cathode that ensures the maximum usage of the starting materials, which sets the upper limit of the capacity performance. Second, the muti-step reduction process releases highly soluble lithium polysulfides (LiPSs) intermediates (Li 2 S x , where x = 4-8) into the organic electrolyte. [6] Unlike the batteries based on ion-insertion mechanism, [7,8] Li-S battery possesses unique and complex electrochemical/chemical processes during operation. During galvanostatic discharge process, two distinct plateaus can be verified at about 2.4 and 2.1 V in the voltage profile, corresponding to the reduction of sulfur into long-chain polysulfides and subsequent reaction from short-chain polysulfides to Li 2 S, respectively. [9] Further investigations about the reaction mechanism of Li-S battery based on experimental and theoretical studies reveal that the existence of various intermediates during electrochemical processes, indicating much more complex battery chemistry compared to the simple stepwise reaction model. [10,11] As a result, the soluble intermediates can diffuse through the polymeric separator to the anode surface, causing the loss of active materials and degradation of anode. Third, the density difference between the starting material (sulfur, 2.07 g cm −3 ) and discharge product (Li 2 S, 1.66 g cm −3 ) causes significant volumetric change during continuous cycling, damaging the integrity of the cathode structure and leading to serious capacity fading. [12] Besides above problems concerning the cathode side of the Li-S battery, other issues arose on the anode side, such as the unstable solid-electrolyte interphase (SEI), surface passivation, and uncontrolled lithium dendrite growth. [13][14][15] Stemmed from the basic problems mentioned above from the very beginning of the designed Li-S battery systems, various derivative problems were gradually unraveled during persistent efforts for improving the battery performance to approach the ultimate goal for commercialization. In the past decade, fundamental studies about Li-S battery were carried in laboratories all over the world, and it gradually put the puzzle together while brought promising performance improvement. [11,[16][17][18][19][20] However, to date, most of the lab-scale progresses have been based on batteries with sulfur loading lower than 2 mg cm −2 , Lithium-sulfur (Li-S) batteries, due to the high theoretical energy density, are regarded as one of the most promising candidates for breaking the limitations of energy-storage system based on Li-ion batteries. Tremendous efforts have been made to meet the challenge of high-performan...

The lithium–sulfur battery is regarded as one of the promising energy‐storage devices beyond lithium‐ion battery due to its overwhelming energy density. The aprotic Li–S electrochemistry is hampered by issues arising from the complex solid–liquid–solid conversion process. Recently, tremendous efforts have been made to optimize the electrochemical reaction in Li–S batteries through rationally designing compositions and structures of cathodes. However, a deep and comprehensive understanding of the actual mechanisms of Li–S batteries and their impact on the performance is still insufficient. The vigorous development of various electrochemical analysis and in situ techniques establish a bridge between the microstructure of components and the macroscopic electrochemical performance, thus providing more scientific guidance for the optimal design of Li–S batteries. In this review, based on insights into the mechanism of aprotic Li–S electrochemistry with the aid of in situ characterization and electrochemical methods, the advanced innovations in optimizing Li–S batteries are systematically summarized, including the materials design, cathode configurations optimization, and electrolyte engineering, with the aim to gain a comprehensive understanding of cathodic redox processes and thus achieve high‐performance Li–S batteries. The current status and possible future directions of the field are accordingly outlined.

The aprotic lithium–oxygen (Li–O2) battery has triggered tremendous efforts for advanced energy storage due to the high energy density. However, realizing toroid-like Li2O2 deposition in low-donor-number (DN) solvents is still the intractable obstruction. Herein, a heterostructured NiS2/ZnIn2S4 is elaborately developed and investigated as a promising catalyst to regulate the Li2O2 deposition in low-DN solvents. The as-developed NiS2/ZnIn2S4 promotes interfacial electron transfer, regulates the adsorption energy of the reaction intermediates, and accelerates O–O bond cleavage, which are convincingly evidenced experimentally and theoretically. As a result, the toroid-like Li2O2 product is achieved in a Li–O2 battery with low-DN solvents via the solvation-mediated pathway, which demonstrates superb cyclability over 490 cycles and a high output capacity of 3682 mA h g–1. The interface engineering of heterostructure catalysts offers more possibilities for the realization of toroid-like Li2O2 in low-DN solvents, holding great promise in achieving practical applications of Li–O2 batteries as well as enlightening the material design in catalytic systems.

Albeit the effectiveness of surface oxygen vacancy in improving oxygen redox reactions in Li–O2 battery, the underpinning reason behind this improvement remains ambiguous. Herein, the concentration of oxygen vacancy in spinel NiCo2O4 is first regulated via magnetron sputtering and its relationship with catalytic activity is comprehensively studied in Li–O2 battery based on experiment and density functional theory (DFT) calculation. The positive effect posed by oxygen vacancy originates from the up shifted antibond orbital relative to Fermi level (Ef), which provides extra electronic state around Ef, eventually enhancing oxygen adsorption and charge transfer during oxygen redox reactions. However, with excessive oxygen vacancy, the negative effect emerges because the metal ions are mostly reduced to low valence based on the electrical neutral principle, which not only destabilizes the crystal structure but also weakens the ability to capture electrons from the antibond orbit of Li2O2, leading to poor catalytic activity for oxygen evolution reaction (OER).

The state-of-the-art design strategies toward highly active catalytic materials and cathode structures for Li–CO2 batteries are reviewed and discussed.

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