K-Doping Suppresses Oxygen Redox in P2-Na0.67Ni0.11Cu0.22Mn0.67O2 Cathode Materials for Sodium-Ion Batteries.

In P2-type layered oxide cathodes, Na site-regulation strategies are proposed to modulate the Na+ distribution and structural stability. However, their impact on the oxygen redox reactions remains poorly understood. Herein, the incorporation of K+ in the Na layer of Na0.67Ni0.11Cu0.22Mn0.67O2 is successfully applied. The effects of partial substitution of Na+ with K+ on electrochemical properties, structural stability, and oxygen redox reactions have been extensively studied. Improved Na+ diffusion kinetics of the cathode is observed from galvanostatic intermittent titration technique (GITT) and rate performance. The valence states and local structural environment of the transition metals (TMs) are elucidated via operando synchrotron X-ray absorption spectroscopy (XAS). It is revealed that the TMO2 slabs tend to be strengthened by K-doping, which efficiently facilitates reversible local structural change. Operando X-ray diffraction (XRD) further confirms more reversible phase changes during the charge/discharge for the cathode after K-doping. Density functional theory (DFT) calculations suggest that oxygen redox reaction in Na0.62K0.03Ni0.11Cu0.22Mn0.67O2 cathode has been remarkably suppressed as the nonbonding O 2p states shift down in the energy. This is further corroborated experimentally by resonant inelastic X-ray scattering (RIXS) spectroscopy, ultimately proving the role of K+ incorporated in the Na layer.

Viable Synthesis of Porous MnCo2 O4 /Graphene Composite by Sonochemical Grafting: A High-Rate-Capable Oxygen Cathode for Li-O2 Batteries.

With an anticipation of their use in electric vehicles, Li-O2 batteries are found to be attractive despite their complex chemistry and drawbacks. To be successful, cathode materials that are robust enough to overcome the sluggish kinetics of the charge-discharge reactions are essential. This work reports sonochemically synthesized porous MnCo2 O4 /graphene (MCO/G) as a hybrid cathode material in nonaqueous Li-O2 batteries. The MCO/G hybrid is synthesized in less than four hours and offers a strong synergistic coupling between the MnCo2 O4 nanospheres and graphene sheets. It catalyzes the oxygen reduction through a three-electron-transfer process and initiates the oxygen evolution at 1.55 V vs. RHE in basic medium. A small charge-discharge voltage hysteresis of 0.8 V and a cycle life of 250 cycles at a limited capacity of 1000 mAh g-1 in a tetraglyme-based nonaqueous Li-O2 battery is demonstrated. The porous channels created on the sonochemically synthesized cathode facilitates easy oxygen adsorption onto the active sites to accommodate more discharge products following its decomposition. It exhibits a better rate capability in comparison to the widely used Vulcan carbon and benchmark Pt/C catalysts. The excellent cyclability, rate capability, and low overpotential demonstrates MnCo2 O4 /graphene composite as a promising cathode candidate for Li-O2 batteries. The porous nanosphere architecture with internal oxygen diffusion pathways and peripheral conductive graphene extensions fulfils the requirements that a robust cathode is expected to have to overcome the harsh Li-O2 battery conditions and to serve as a high-rate-capable cathode for Li-O2 batteries.

Long-Cycle-Life Calcium Battery with a High-Capacity Conversion Cathode Enabled by a Ca2+/Li+ Hybrid Electrolyte.

Calcium (Ca) batteries represent an attractive option for electrochemical energy storage due to physicochemical and economic reasons. The standard reduction potential of Ca (-2.87 V) is close to Li and promises a wide voltage window for Ca full batteries, while the high abundance of Ca in the earth's crust implicates low material costs. However, the development of Ca batteries is currently hindered by technical issues such as the lack of compatible electrolytes for reversible Ca2+ plating/stripping and high-capacity cathodes with fast kinetics. Herein, we employed FeS2 as a conversion cathode material and combined it with a Li+/Ca2+ hybrid electrolyte for Ca batteries. We demonstrate that Li+ ions ensured reversible Ca2+ plating/stripping on the Ca metal anode with a small overpotential. At the same time, they enable the conversion of FeS2, offering high discharge capacity. As a result, the Ca/FeS2 cell demonstrated an excellent long-term cycling performance with a high discharge capacity of 303 mAh g-1 over 200 cycles. Even though the practical application of such an approach is questionable due to the high quantity of electrolytes, we believe that our scientific findings still provide new directions for studying Ca batteries with long-term cycling.

Decoupling the Cumulative Contributions of Capacity Fade in Ethereal-Based Li-O2 Batteries.

In the loop of numerous challenges and ambiguities, Li-O2 batteries are crawling to reach their commercialization phase. To achieve the progressive milestones, along with the developments in the architecture of cathodes, anodes, and electrolytes, understanding its failure mode is equally important. Under an unrestricted charge-discharge protocol, cyclability of nonaqueous Li-O2 batteries are limited to only a few cycles. This report examines an additive-free ether-based Li-O2 battery in the perspective of identifying the origin of possible side reactions and their affiliations to integral components of the battery. Structural and compositional changes during every charge-discharge sequence are studied using bottom-up sequential tear-down analysis. The substantial increase in impedance and corresponding decrease in capacities after every cycle are interrelated to the amount of electrode passivation resulting from the discharge products and electrolyte decomposition. From the tear-down analysis, it is approximated that, among the total capacity loss, ≈55% is attributed to the cathode, ≈28% of the loss corresponds to the anode, and ≈17% is attributed to the electrolyte, given that battery failure instigates from the "reactive oxygen species". Electrochemically formed Li2O2 via the superoxide pathway induces large decomposition overpotentials up to 4.6 V versus Li/Li+ because of its overrated reactivity with electrolytes and carbon supports. On the contrary, efficient decomposition of chemically formed Li2O2 below 3.9 V proves that the extra charge potential observed for electrochemically formed Li2O2 is in fact consumed for the decomposition of irreversibly formed side products via the superoxide pathway. Spontaneous reactivity of Li2O2 and trivial reactivity of Li2O highlight the need of advanced strategies to maneuver oxygen red-ox in selective pathways that unalter the electrolyte and electrodes, and the necessity of their synchronized performance for the evolution of practical Li-O2 batteries.

Ultrasonochemically-induced MnCo2O4 nanospheres synergized with graphene sheet as a non-precious bi-functional cathode catalyst for rechargeable zinc-air battery.

Rechargeable zinc-air batteries are considered to be more sustainable and efficient candidates for safe, low-cost energy storage because of their higher energy density and the abundance of zinc resources. Recently Zn-air batteries have aroused significant research attention, however, because an unresolved impediment due to the notorious instability of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) kinetics of the cathode catalyst limit their practical application. Herein, we report the synthesis of non-precious MnCo2O4 nanospheres synergized with a graphene sheet as a bi-functional cathode catalyst for rechargeable Zn-air battery application using a one-pot probe sonochemical method. Structural characterization confirms that the MnCo2O4 nanospheres successfully anchored on graphene sheet. X-ray photoelectron spectroscopy revealed that the Mn and Co in the MnCo2O4 are in mixed valence states on the graphene sheet surface and the MnCo2O4-graphene sheet (MCO-GS) hybrid catalyst exhibits excellent OER and ORR activity compared with their individual counterparts. A rechargeable Zn-air battery using an MCO-GS catalyst reveals unique small charge-discharge overpotential, cycling stability and higher rate capability than a bare MnCo2O4 (MCO) catalyst. This superiority in electrocatalytic activity combined with simplicity of material synthesis, turn the MCO-GS hybrid into a promising catalyst for a rechargeable Zn-air battery.