High-Performance Na-O2 Batteries Enabled by Oriented NaO2 Nanowires as Discharge Products

Alkalized MXene/carbon nanotube composite for stable Na metal anodes

Ti3C2 MXenes are emerging 2D materials and have attracted increasing attention in sodium metal anode fabrication because of their high conductivity, multifunctional groups and excellent mechanical performances. However, the severe self-restacking of Ti3C2 MXenes is not conducive to dispersing Na+ and limits the function of regulating sodium deposition. Herein, an alkalized MXene/carbon nanotube (CNT) composite (named A-M-C) is introduced to regulate Na deposition behavior, which consists of Na3Ti5O12 microspheres, Ti3C2 MXene nanosheets and CNTs. Ti3C2 MXene nanosheets with large interlayer spaces and "sodiophilic" functional groups can provide abundant active sites for uniform nucleation and deposition of Na. Plenty of nanosheets are grown on the surface of the microsphere, thereby reducing the local current density, which can guide initial Na nucleation and promote Na dendrite-free growth. Furthermore, CNTs increase the electrical conductivity of the composite and achieve fast Na+ transport, improving the cycling stability of Na metal batteries. As a result, at a capacity of 1 mA h cm-2, the A-M-C electrode achieves a high average coulombic efficiency (CE) of 99.9% after 300 cycles at 2 mA cm-2. The symmetric cells of A-M-C/Na provide a long cycling life of more than 1400 h at 1 mA cm-2 with a minimal overpotential of 19 mV at an areal capacity of 1 mA h cm-2. The A-M-C/Na//NVP@C full cell presents a high coulombic efficiency of 98% with 100 mA g-1 in the first cycle. The strategy in this work provides new insights into fabricating novel MXene-based anode materials for dendrite-free sodium deposition.

Fabricating Na/In/C Composite Anode with Natrophilic Na-In Alloy Enables Superior Na Ion Deposition in the EC/PC Electrolyte

In conventional ethylene carbonate (EC)/propylene carbonate (PC) electrolyte, sodium metal reacts spontaneously and deleteriously with solvent molecules. This significantly limits the practical feasibility of high-voltage sodium metal batteries based on Na metal chemistry. Herein, we present a sodium metal alloy strategy via introducing NaIn and Na₂In phases in a Na/In/C composite, aiming at boosting Na ion deposition stability in the common EC/PC electrolyte. Symmetric cells with Na/In/C electrodes achieve an impressive long-term cycling capability at 1 mA cm^-2 (> 870 h) and 5 mA cm^-2 (> 560 h), respectively, with a capacity of 1 mAh cm^-2. In situ optical microscopy clearly unravels a stable Na ion dynamic deposition process on the Na/In/C composite electrode surface, attributing to a dendrite-free and smooth morphology. Furthermore, theoretical simulations reveal intrinsic mechanism for the reversible Na ion deposition behavior with the composite Na/In/C electrode. Upon pairing with a high-voltage NaVPOF cathode, Na/In/C anode illustrates a better suitability in SMBs. This work promises an alternative alloying strategy for enhancing Na metal interfacial stability in the common EC/PC electrolyte for their future applications.

3D Ag@C Cloth for Stable Anode Free Sodium Metal Batteries

While sodium metal anodes (SMAs) feature many performance advantages in sodium ion batteries (SIBs), severe safety concerns remain for using bulk sodium electrodes. Herein, a 3D Ag@C natrophilic substrate prepared by a facile thermal evaporation deposition method, which can be employed as a much safer "anode-free" SMA, is reported. Initially, there is no bulk sodium on the Ag@C substrate in the assembled SIBs. Upon charging, sodium will be uniformly deposited onto the Ag@C substrate and afterwards functions as a real SMA, thus inheriting the intrinsic merits of SMA and enhancing safety simultaneously. While cycling, the as-synthesized substrate demonstrates superior sodium plating/stripping cycling stability at 1, 2 and 3 mA cm^-2 with a capacity of 2 mAh cm^-2 . Theoretical simulations reveal that Na ions prefer to bind with Ag and form a Na-Ag network, thus clearly revealing uniform sodium deposition on the Ag@C substrate. More importantly, a full battery based on Ag@C and Prussian white with impressive Coulomb efficiency (CE), high rate capability (from 0.1 C to 5 C) and long-term cycling life is illustrated for the first time, thus making Ag@C feasible for the establishment of "anode-free" SIBs with reduced cost, high gravimetric/volumetric energy density and enhanced safety.

Nano Polymorphism-Enabled Redox Electrodes for Rechargeable Batteries

Nano polymorphism (NPM), as an emerging research area in the field of energy storage, and rechargeable batteries, have attracted much attention recently. In this review, the recent progress on the composition and formation of polymorphs, and the evolution processes of different redox electrodes in rechargeable metal-ion, metal-air, and metal-sulfur batteries are highlighted. First, NPM and its significance for rechargeable batteries are discussed. Subsequently, the current NPM modulation strategies of different types of representative electrodes for their corresponding rechargeable battery applications are summarized. The goal is to demonstrate how NPM could tune the intrinsic material properties, and hence, improve their electrochemical activities for each battery type. It is expected that the analysis of polymorphism and electrochemical properties of materials could help identify some "processing-structure-properties" relationships for material design and performance enhancement. Lastly, the current research challenges and potential research directions are discussed to offer guidance and perspectives for future research on NPM engineering.

Can Hybrid Na-Air Batteries Outperform Nonaqueous Na-O2 Batteries?

In recent years, there has been an upsurge in the study of novel and alternative energy storage devices beyond lithium-based systems due to the exponential increase in price of lithium. Sodium (Na) metal-based batteries can be a possible alternative to lithium-based batteries due to the similar electrochemical voltage of Na and Li together with the thousand times higher natural abundance of Na compared to Li. Though two different kinds of Na-O2 batteries have been studied specifically based on electrolytes until now, very recently, a hybrid Na-air cell has shown distinctive advantage over nonaqueous cell systems. Hybrid Na-air batteries provide a fundamental advantage due to the formation of highly soluble discharge product (sodium hydroxide) which leads to low overpotentials for charge and discharge processes, high electrical energy efficiency, and good cyclic stability. Herein, the current status and challenges associated with hybrid Na-air batteries are reported. Also, a brief description of nonaqueous Na-O2 batteries and its close competition with hybrid Na-air batteries are provided.

An all-nanosheet OER/ORR bifunctional electrocatalyst for both aprotic and aqueous Li-O2 batteries

Rechargeable lithium-oxygen (Li-O2) batteries are receiving intense interest because of their high energy density. A highly efficient catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a key factor influencing the performance of Li-O2 batteries. In this work, a facile synthesis of an all-nanosheet architecture electrocatalyst constructed from a monolayer ruthenium dioxide (RuO2) nanosheet with a nitrogen doped sulfonated graphene nanosheet (RuO2-NS-GNS) has been developed for Li-O2 batteries. This complex catalyst displays excellent activity towards the ORR and OER in both aprotic and aqueous Li-O2 batteries. A low overpotential around 1.0 V during the discharge/recharge process is obtained for the aprotic Li-O2 battery with RuO2-NS-GNS. Meanwhile, linear sweep voltammetry curves show that the OER and ORR potentials are 1.45 V and 0.81 V in an alkaline solution (1 M LiOH-5 M LiNO3) for RuO2-NS-GNS, respectively. Both aprotic and aqueous Li-O2 batteries with RuO2-NS-GNS exhibit stable cyclability and low round-trip overpotential without obvious degradation at a limited specific capacity of 1000 mA g-1. The advanced electrochemical performance of RuO2-NS-GNS in both aprotic and aqueous Li-O2 batteries can be attributed to the increased catalytic sites and synergistic effect arising from RuO2 and NS-GNS nanosheets.

MOF-derived NiO–NiCo2O4@PPy hollow polyhedron as a sulfur immobilizer for lithium–sulfur batteries

A MOF-derived NiO–NiCo₂O₄@PPy hollow polyhedron is prepared as a sulfur host to effectively enhance cell performance. S/NiO–NiCo₂O₄@PPy displays a high initial discharge capacity of 963 mA h g⁻¹ with a high initial coulombic efficiency of 95.2% at 0.2C.

Defective glycerolatocobalt(ii) for enhancing the oxygen evolution reaction

Ar plasma was adopted to create a defective glycerolatocobalt(ii) with rich coordinatively unsaturated metal sites as an effective OER catalyst.