Treatment of carbon electrodes with Ti3C2Tx MXene coating and thermal method for vanadium redox flow batteries: a comparative study.

One of the significant challenges of vanadium redox flow batteries is connected to the negative electrode where the main reaction of V(ii)/V(iii) and the side reaction of hydrogen evolution compete. To address this issue, we used titanium carbide (Ti3C2Tx) MXene coating via drop-casting to introduce oxygen functional groups and metals on the carbon electrode surface. Characterization through scanning electron microscopy and X-ray photoelectron spectroscopy confirmed the even distribution of Ti3C2Tx MXene on the electrodes and the presence of titanium and termination groups (-O, -Cl, and -F). The cyclic voltammetry analysis of MXene-coated electrodes showed more sharp electrochemical peaks for the V(ii)/V(iii) reaction than thermal-treated electrodes, even at relatively high scan rates. Notably, a relatively high reaction rate of 5.61 × 10-4 cm s-1 was achieved for the V(ii)/V(iii) reaction on MXene-coated electrodes, which shows the competitiveness of the method compared to thermal treatment (4.17 × 10-4 cm s-1). The flow battery tests, at a current density of 130 mA cm-2, using MXene-coated electrodes showed pretty stable discharge capacity for over 100 cycles. In addition, the voltage and energy efficiency were significantly higher than those of the system using untreated electrodes. Overall, this work highlights the potential application of MXene coating in carbon electrode treatment for vanadium redox flow batteries due to remarkable electrocatalytic activity and battery performance, providing a competitive method for thermal treatment.

Bio-based anode material production for lithium-ion batteries through catalytic graphitization of biochar: the deployment of hybrid catalysts.

Producing sustainable anode materials for lithium-ion batteries (LIBs) through catalytic graphitization of renewable biomass has gained significant attention. However, the technology is in its early stages due to the bio-graphite's comparatively low electrochemical performance in LIBs. This study aims to develop a process for producing LIB anode materials using a hybrid catalyst to enhance battery performance, along with readily available market biochar as the raw material. Results indicate that a trimetallic hybrid catalyst (Ni, Fe, and Mn in a 1:1:1 ratio) is superior to single or bimetallic catalysts in converting biochar to bio-graphite. The bio-graphite produced under this catalyst exhibits an 89.28% degree of graphitization and a 73.95% conversion rate. High-resolution transmission electron microscopy (HRTEM) reveals the dissolution-precipitation mechanism involved in catalytic graphitization. Electrochemical performance evaluation showed that the trimetallic hybrid catalyst yielded bio-graphite with better electrochemical performances than those obtained through single or bimetallic hybrid catalysts, including a good reversible capacity of about 293 mAh g-1 at a current density of 20 mA/g and a stable cycle performance with a capacity retention of over 98% after 100 cycles. This study proves the synergistic efficacy of different metals in catalytic graphitization, impacting both graphite crystalline structure and electrochemical performance.

Fluorine-Substituted Halide Solid Electrolytes with Enhanced Stability toward the Lithium Metal.

The high ionic conductivity and good oxidation stability of halide-based solid electrolytes evoke strong interest in this class of materials. Nonetheless, the superior oxidative stability compared to sulfides comes at the expense of limited stability toward reduction and instability against metallic lithium anodes, which hinders their practical use. In this context, the gradual fluorination of Li2ZrCl6-xFx (0 ≤ x ≤ 1.2) is proposed to enhance the stability toward lithium-metal anodes. The mechanochemically synthesized fluorine-substituted compounds show the expected distorted local structure (M2-M3 site disorder) and significant change in the overall Li-ion migration barrier. Theoretical calculations reveal an approximate minimum energy path for Li2ZrCl6-xFx (x = 0 and 0.5) with an increase in the Li+ migration energy barrier for Li2ZrCl5.5F0.5 in comparison to Li2ZrCl6. However, it is found that the fluorine-substituted compound exhibits substantially lower polarization after 800 h of lithium stripping and plating owing to enhanced interfacial stability against the lithium metal, as revealed by density functional theory and ex situ X-ray photoelectron spectroscopy, thanks to the formation of a fluorine-rich passivating interphase.

Pyrophosphate Na2CoP2O7 Polymorphs as Efficient Bifunctional Oxygen Electrocatalysts for Zinc-Air Batteries.

Developing earth-abundant low-cost bifunctional oxygen electrocatalysts is a key approach to realizing efficient energy storage and conversion. By exploring Co-based sodium battery materials, here we have unveiled nanostructured pyrophosphate Na2CoP2O7 polymorphs displaying efficient bifunctional electrocatalytic activity. While the orthorhombic polymorph (o-NCPy) has superior oxygen evolution reaction (OER) activity, the triclinic polymorph (t-NCPy) delivers better oxygen reduction reaction (ORR) activity. Simply by tuning the annealing condition, these pyrophosphate polymorphs can be easily prepared at temperatures as low as 500 °C. The electrocatalytic activity is rooted in the Co redox center with the (100) active surface and stable structural framework as per ab initio calculations. It marks the first case of phospho-anionic systems with both polymorphs showing stable bifunctional activity with low combined overpotential (ca. ∼0.7 V) comparable to that of reported state-of-the-art catalysts. These nanoscale cobalt pyrophosphates can be implemented in rechargeable zinc-air batteries.

Potassium Cobalt Pyrophosphate as a Nonprecious Bifunctional Electrocatalyst for Zinc-Air Batteries.

Economic and sustainable (ecological) energy storage forms a major pillar of the global energy sector. Bifunctional electrocatalysts, based on oxygen electrolysis, play a key role in the development of rechargeable metal-air batteries. Pursuing precious metal-free economic catalysts, here, we report K2CoP2O7 pyrophosphate as a robust cathode for secondary zinc-air batteries with efficient oxygen evolution and oxygen reduction (OER||ORR) activity. Prepared by autocombustion, nanoscale K2CoP2O7 exhibited excellent oxygen reduction and evolution reactions among all phosphate-based electrocatalysts. In particular, the OER activity surpassed that of commercial RuO2 with low overpotential (0.27 V). First-principles calculations revealed that the bifunctional activity is rooted in the Co active site with the CoO5 local coordination in the most stable (110) surface. This nanostructured (tetragonal) pyrophosphate can be harnessed as an economic bifunctional catalyst for zinc-air batteries.

Non-flammable liquid electrolytes for safe batteries.

With continual increments in energy density gradually boosting the performance of rechargeable alkali metal ion (e.g. Li+, Na+, K+) batteries, their safe operation is of growing importance and needs to be considered during their development. This is essential, given the high-profile incidents involving battery fires as portrayed by the media. Such hazardous events result from exothermic chemical reactions occurring between the flammable electrolyte and the electrode material under abusive operating conditions. Some classes of non-flammable organic liquid electrolytes have shown potential towards safer batteries with minimal detrimental effect on cycling and, in some cases, even enhanced performance. This article reviews the state-of-the-art in non-flammable liquid electrolytes for Li-, Na- and K-ion batteries. It provides the reader with an overview of carbonate, ether and phosphate-based organic electrolytes, co-solvated electrolytes and electrolytes with flame-retardant additives as well as highly concentrated and locally highly concentrated electrolytes, ionic liquids and inorganic electrolytes. Furthermore, the functionality and purpose of the components present in typical non-flammable mixtures are discussed. Moreover, many non-flammable liquid electrolytes are shown to offer improved cycling stability and rate capability compared to conventional flammable liquid electrolytes.

Na2MnP2O7 polymorphs as efficient bifunctional catalysts for oxygen reduction and oxygen evolution reactions.

In order to design earth-abundant low cost electrocatalysts, this communication exploits polymorphism in Na2MnP2O7 pyrophosphate sodium insertion materials. Two polymorphs of Na2MnP2O7 have been prepared with a short annealing duration of 30 minutes. These scalable materials exhibit efficient bifunctional electrocatalytic activity stemming from the Mn redox centre and robust structural framework.

Sodium Cobalt Metaphosphate as an Efficient Oxygen Evolution Reaction Catalyst in Alkaline Solution.

Sodium cobalt metaphosphate [NaCo(PO3 )3 ] has CoO octahedra (CoO6 ) and shows superior oxygen evolution reaction (OER) activity in alkaline solution, comparable with the state-of-the-art precious-metal RuO2 catalyst. OER catalysts of this metaphosphate are prepared by combustion (Cb) and solid-state (SS) methods. The combustion-assisted method offers a facile synthesis and one-step carbon composite formation. Unusually high catalytic activity was observed in NCoM-Cb-Ar and could be due to chemical coupling effects between NaCo(PO3 )3 and partially graphitized carbon. This novel electrocatalyst exhibits very small overpotential of 340 mV with high mass activity of 532 A g-1 . Good charge transfer abilities and chemical coupling between NaCo(PO3 )3 and amorphous carbon gives the OER activity in NCoM-Cb-Ar.

Cubic Sodium Cobalt Metaphosphate [NaCo(PO3)3] as a Cathode Material for Sodium Ion Batteries.

Cubic-framework sodium cobalt-based metaphosphate NaCo(PO3)3 was recently demonstrated to be an attractive Na+ cationic conductor. It can be potentially used in the next-generation rechargeable Na ion batteries. The crystal structure and electrical conductivity were studied and found to have a three-dimensional framework with interconnecting tunnels for Na+ migration ( J. Solid State Electrochem. , 2016 , 20 , 1241 ). This inspired us to study the electrochemical (de)intercalation behavior of Na+ in the NaCo(PO3)3 assuming a cubic Pa3̅ framework. Herein, synergizing experimental and computational tools, we present the first report on the electrochemical activity and Na+ diffusion pathway analysis of cubic NaCo(PO3)3 prepared via conventional solid-state route. The electrochemical analyses reveal NaCo(PO3)3 to be an active sodium insertion material with well-defined reversible Co3+/Co2+ redox activity centered at 3.3 V (vs Na/Na+). Involving a solid-solution redox mechanism, close to 0.7 Na+ per formula unit can be reversibly extracted. This experimental finding is augmented with bond valence site energy (BVSE) modeling to clarify Na+ migration in cubic NaCo(PO3)3. BVSE analyses suggest feasible Na+ migration with moderate energy barrier of 0.68 eV. Cubic NaCo(PO3)3 forms a 3.3 V sodium insertion material.

Enabling the Electrochemical Activity in Sodium Iron Metaphosphate [NaFe(PO3)3] Sodium Battery Insertion Material: Structural and Electrochemical Insights.

Sodium-ion batteries are widely pursued as an economic alternative to lithium-ion battery technology, where Fe- and Mn-based compounds are particularly attractive owing to their elemental abundance. Pursuing phosphate-based polyanionic chemistry, recently solid-state prepared NaFe(PO3)3 metaphosphate was unveiled as a novel potential sodium insertion material, although it was found to be electrochemically inactive. In the current work, employing energy-savvy solution combustion synthesis, NaFe2+(PO3)3 was produced from low-cost Fe3+ precursors. Owing to the formation of nanoscale carbon-coated product, electrochemical activity was enabled in NaFe(PO3)3 for the first time. In congruence with the first principles density functional theory (DFT) calculations, an Fe3+/Fe2+ redox activity centered at 2.8 V (vs Na/Na+) was observed. Further, the solid-solution metaphosphate family Na(Fe1-xMnx)(PO3)3 (x = 0-1) was prepared for the first time. Their structure and distribution of transition metals (TM = Fe/Mn) was analyzed with synchrotron diffraction, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy. Synergizing experimental and computational tools, NaFe(PO3)3 metaphosphate is presented as an electrochemically active sodium insertion host material.

Na2.32Co1.84(SO4)3 as a new member of the alluaudite family of high-voltage sodium battery cathodes.

Electrochemical energy storage has recently seen tremendous emphasis being placed on the large-scale (power) grid storage. Sodium-ion batteries are capable of achieving this goal with economic viability. In a recent breakthrough in sodium-ion battery research, the alluaudite framework (Na2Fe2(SO4)3) has been reported, with the highest Fe3+/Fe2+ redox potential (ca. 3.8 V, Barpanda, et al., Nat. Commun., 2014, 5, 4358). Exploring this high-voltage sodium insertion system, we report the discovery of Na2+2xCo2-x(SO4)3 (x = 0.16) as a new member of the alluaudite class of cathode. Stabilized by low-temperature solid-state synthesis (T ≤ 350 °C), this novel Co-based compound assumes a monoclinic structure with C2/c symmetry, which undergoes antiferromagnetic ordering below 10.2 K. Isotypical to the Fe-homologue, it forms a complete family of solid-solution Na2+2x(Fe1-yCoy)2-x(SO4)3 [y = 0-1]. Ab initio DFT analysis hints at potential high voltage operation at 4.76-5.76 V (vs. Na), depending on the degree of desodiation involving a strong participation of the oxygen sub-lattice. With the development of safe organic electrolytes, Na2+2xCo2-x(SO4)3 can work as a cathode material (∼5 V) for sodium-ion batteries.