Because of their high theoretical energy density and low cost, lithium–sulfur (Li–S) batteries are promising next-generation energy storage devices. The electrochemical performance of Li–S batteries largely depends on the efficient reversible conversion of Li polysulfides to Li2S in discharge and to elemental S during charging. Here, we report on our discovery that monodisperse cobalt atoms embedded in nitrogen-doped graphene (Co–N/G) can trigger the surface-mediated reaction of Li polysulfides. Using a combination of operando X-ray absorption spectroscopy and first-principles calculation, we reveal that the Co–N–C coordination center serves as a bifunctional electrocatalyst to facilitate both the formation and the decomposition of Li2S in discharge and charge processes, respectively. The S@Co–N/G composite, with a high S mass ratio of 90 wt %, can deliver a gravimetric capacity of 1210 mAh g–1, and it exhibits an areal capacity of 5.1 mAh cm–2 with capacity fading rate of 0.029% per cycle over 100 cycles...

Cobalt in Nitrogen-Doped Graphene as Single-Atom Catalyst for High-Sulfur Content Lithium–Sulfur Batteries

Lithium-sulfur (Li-S) battery has emerged as one of the most promising next-generation energy-storage systems. However, the shuttle effect greatly reduces the battery cycle life and sulfur utilization, which is great deterrent to its practical use. This paper reviews the tremendous efforts that are made to find a remedy for this problem, mostly through physical or chemical confinement of the lithium polysulfides (LiPSs). Intrinsically, this "confinement" has a relatively limited effect on improving the battery performance because in most cases, the LiPSs are "passively" blocked and cannot be reused. Thus, this strategy becomes less effective with a high sulfur loading and ultralong cycling. A more "positive" method that not only traps but also increases the subsequent conversion of LiPSs back to lithium sulfides is urgently needed to fundamentally solve the shuttle effect. Here, recent advances on catalytic effects in increasing the rate of conversion of soluble long-chain LiPSs to insoluble short-chain Li2S2/Li2S, and vice versa, are reviewed, and the roles of noble metals, metal oxides, metal sulfides, metal nitrides, and some metal-free materials in this process are highlighted. Challenges and potential solutions for the design of catalytic cathodes and interlayers in Li-S battery are discussed in detail.

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Catalytic Effects in Lithium-Sulfur Batteries: Promoted Sulfur Transformation and Reduced Shuttle Effect.

Li-ion batteries (LIBs), commercialized in 1991, have the highest energy density among practical secondary batteries and are widely utilized in electronics, electric vehicles, and even stationary energy storage systems. Along with the expansion of their demand and application, concern about the resources of Li and Co is growing. Therefore, secondary batteries composed of earth-abundant elements are desired to complement LIBs. In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to LIBs. Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Thus, besides being free from any scarce/toxic elements, the low standard electrode potentials of K/K+ electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K+ ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than that of Li+ ions; this is essential to realize high-power KIBs. This review comprehensively covers the studies on electrochemical materials for KIBs, including electrode and electrolyte materials and a discussion on recent achievements and remaining/emerging issues. The review also includes insights into electrode reactions and solid-state ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs compared to those of LIBs and NIBs.

Research Development on K-Ion Batteries.

A Review of Precious-Metal-Free Bifunctional Oxygen Electrocatalysts: Rational Design and Applications in Zn−Air Batteries

Multi-functional separator/interlayer system for high-stable lithium-sulfur batteries: Progress and prospects

Lithium–sulfur batteries are promising next-generation energy storage devices due to their high energy density and low material cost. Efficient conversion of lithium polysulfides to lithium sulfide (during discharge) and to sulfur (during recharge) is a performance-determining factor for lithium–sulfur batteries. Here we show that MoS2−x/reduced graphene oxide (MoS2−x/rGO) can be used to catalyze the polysulfide reactions to improve the battery performance. It was confirmed, through microstructural characterization of the materials, that sulfur deficiencies on the surface participated in the polysulfide reactions and significantly enhanced the polysulfide conversion kinetics. The fast conversion of soluble polysulfides decreased their accumulation in the sulfur cathode and their loss from the cathode by diffusion. Hence in the presence of a small amount of MoS2−x/rGO (4 wt% of the cathode mass), high rate (8C) performance of the sulfur cathode was improved from a capacity of 161.1 mA h g−1 to 826.5 mA h g−1. In addition, MoS2−x/rGO also enhanced the cycle stability of the sulfur cathode from a capacity fade rate of 0.373% per cycle (over 150 cycles) to 0.083% per cycle (over 600 cycles) at a typical 0.5C rate. These results provide direct experimental evidence for the catalytic role of MoS2−x/rGO in promoting the polysulfide conversion kinetics in the sulfur cathode.

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Electrocatalysis of polysulfide conversion by sulfur-deficient MoS2 nanoflakes for lithium–sulfur batteries

The reversible intercalation of K+ into graphite has spurred interest in new rechargeable battery chemistries based on potassium ions. The viability of commercializing potassium-ion batteries (KIBs) is critically dependent on the availability of pre-potassiated cathode materials, and the compatibility between the electrolyte and electrode components; which have yet to be fully explored. Herein, we report a Fe and Mn based K-rich Prussian Blue analogue, which can be used as the cathode of rechargeable KIBs to deliver 115 mAh/g of discharge capacity with two voltage plateaus at 3.9 and 4.1 V vs. K+/K in an electrolyte with fluoroethylene carbonate additive. In half-cell studies, the cyclability of this Prussian Blue analogue was found to depend on the formation of a passivation layer on the anode and the decomposition of the active electrode material at the cathode. This study demonstrates the importance of electrode−electrolyte compatibility in the development of new rechargeable battery chemistries, especially when the evaluation is based on half-cell studies where potassium metal is used.

A Fe/Mn-Based Prussian Blue Analogue as a K-Rich Cathode Material for Potassium-Ion Batteries

Rechargeable lithium sulfur batteries have been regarded as one of the most promising power source systems for next generation EVs or HEVs. However, the low utilization of active materials, rapid capacity degradation, and poor rate capability seriously restrict their large-scale applications in commercial markets. Herein, a novel strategy, using a hydrophilic separator which is prepared by auto-oxidization and self-polymerization of a dopamine monomer onto the surface of conventional hydrophobic separators, is reported to improve the electrochemical performance of Li–S batteries. The cells with the hydrophilic separator show significantly enhanced cycle performance. At a rate of 0.2 C, the battery demonstrates an initial capacity of 1271 mA h g−1, and the capacity can still remain at 1020.3 mA h g−1 after 30 cycles, which improves 77% compared with the cells using conventional separators.

A hydrophilic separator for high performance lithium sulfur batteries

This study introduces an improved design of the interlayer between the cathode and separator of rechargeable lithium–sulfur batteries to mitigate the polysulfide crossover problem of the latter The design involves integrating carbon nanotubes with titanium dioxide by a facile room-temperature hydrolytic method to form a titanium dioxide coated carbon nanotube composite (CNT@TiO2) with customizable TiO2 content The CNT@TiO2 composite was then coated on a separator to form an interlayer much thinner than other standalone interlayers The TiO2 coating on the CNT surface provides the facility for lithium polysulfides (LiPS) interception by chemisorption, and the underlying CNT core renders the intercepted LiPS electrochemically viable in charging and discharging A good balance between the chemisorption properties of TiO2 and the charge transport properties of the CNTs is required to deliver a good interlayer performance because of the complementarity of these functions Consequently, a battery with an optimized CNT@TiO2 interlayer composition could deliver a high initial capacity of 1351 mA h g−1 and a discharge capacity of 803 mA h g−1 after 200 cycles at 01C, for less than half of the thickness of a typical standalone interlayer (12 μm)

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Balancing the chemisorption and charge transport properties of the interlayer in lithium–sulfur batteries

Layered two-dimensional (2D) early transition metal carbides (MXenes) are a new class of 2D materials with manifested activity for electrochemical energy conversion and storage. The utilization of MXenes is expected to increase by reducing their lateral size to the nanoscale. Herein, using Ti3C2Tx (T= O, OH, F) MXenes as an example, we show that µm-sized MXenes can be size-reduced to ~6 nm nanodots by ball-milling with red phosphorous (P). It is remarkable almost all starting MXenes were converted to nanodots with this method. The Ti3C2Tx surface O groups interacted with P strongly and chemically to drive the nanodots formation. Ti3C2Tx nanodots (TNDs) with different sizes could also be obtained by ball-milling Ti3C2Tx with other solid-state elements such as carbon, sulfur and silicon. The as-prepared TNDs and P composite was evaluated as a sodium-ion battery anode material. A good sodiation capacity (~600 mAh g−1 at 100 mA g-1) and good cycle stability (150 cycles) were shown.

Guochun Li

Papers

Electrocatalysis of polysulfide conversion by sulfur-deficient MoS2 nanoflakes for lithium–sulfur batteries

A Fe/Mn-Based Prussian Blue Analogue as a K-Rich Cathode Material for Potassium-Ion Batteries

A hydrophilic separator for high performance lithium sulfur batteries

Balancing the chemisorption and charge transport properties of the interlayer in lithium–sulfur batteries

A Red Phosphorous-Assisted Ball-Milling Synthesis of Few Layered Ti3C2Tx (MXene) Nanodot Composite