Synthesis, Structure, and Electrochemical Properties of the Layered Sodium Insertion Cathode Material: NaNi_¹/₃Mn_¹/₃Co_¹/₃O₂

A layered phase, NaNi_¹/₃Mn_¹/₃Co_¹/₃O₂ (NaNMC), isostructural to NaCoO₂ has been synthesized. Stoichiometric NaNMC crystallizes in a rhombohedral R3̅m space group where Na is in an octahedral environment (O3-Type). Galvanostatic cycling on NaNMC vs Na cell indicated a reversible intercalation of 0.5 Na, leading to a capacity of 120 mAh·g^–1 in the voltage range of 2–3.75 V and indicating its possible application in Na-ion batteries. The electrochemically driven Na insertion/deinsertion in NaNMC is associated with several phase transitions and solid solution regimes which are studied by <i>in situ</i> X-ray diffraction. Sodium deinsertion in Na_<i>x</i>NMC resulted in sequential phase transitions composed of biphasic and monophasic domains. The composition driven structural evolution in Na_<i>x</i>NMC follows the sequence O3 ⇒ O1 ⇒ P3 ⇒ P1 phases with an increased ‘<i>c</i>’ parameter, while the ‘<i>a</i>’ parameter remains almost unchanged

Titanium(III) Sulfate as New Negative Electrode for Sodium-Ion Batteries

Titanium(III) Sulfate as New Negative Electrode for Sodium-Ion Batterie

Low-Potential Sodium Insertion in a NASICON-Type Structure through the Ti(III)/Ti(II) Redox Couple

We report the direct synthesis of powder Na₃Ti₂(PO₄)₃ together with its low-potential electrochemical performance and crystal structure elucidation for the reduced and oxidized phases. First-principles calculations at the density functional theory level have been performed to gain further insight into the electrochemistry of Ti­(IV)/Ti­(III) and Ti­(III)/Ti­(II) redox couples in these sodium superionic conductor (NASICON) compounds. Finally, we have validated the concept of full-titanium-based sodium ion cells through the assembly of symmetric cells involving Na₃Ti₂(PO₄)₃ as both positive and negative electrode materials operating at an average potential of 1.7 V

Diglyme Based Electrolytes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are currently being considered for large-scale energy storage. Optimization of SIB electrolytes is, however, still largely lacking. Here we exhaustively evaluate NaPF₆ in diglyme as an electrolyte of choice, via both physicochemical properties and extensive electrochemical tests including half as well as full cells. Fundamentally, the ionic conductivity is found to be quite comparable to carbonate based electrolytes and to obey the fractional Walden rule with viscosity. We find Na metal to work well as a reference electrode and the electrochemical stability, evaluated potentiostatically for various electrodes and corroborated by DFT calculations, to be satisfactory in the entire voltage range 0–4.4 V. Galvanostatic cycling at C/10 of half and full cells using Na₃V₂(PO₄)₃ (NVP) or Na₃V₂(PO₄)₂F₃ (NVPF) as cathodes and hard carbon (HC) as anodes indicates rapid capacity fading in cells with HC anodes, possibly originating in a lack of a stable SEI or by trapping of sodium. Aiming to understand this capacity fade further, we conducted a GC/MS analysis to determine electrolyte reduction products and to propose reduction pathways, concluding that oligomer and/or alkoxide formation is possible. Overall, the promising results should warrant further investigations of diglyme based electrolytes for modern SIB development, albeit avoiding HC anodes

High Capacity Na–O₂ Batteries: Key Parameters for Solution-Mediated Discharge

The Na–O₂ battery offers an interesting alternative to the Li–O₂ battery, which is still the source of a number of unsolved scientific questions. In spite of both being alkali metal–O₂ batteries, they display significant differences. For instance, Li–O₂ batteries form Li₂O₂ as the discharge product at the cathode, whereas Na–O₂ batteries usually form NaO₂. A very important question that affects the performance of the Na–O₂ cell concerns the key parameters governing the growth mechanism of the large NaO₂ cubes formed upon reduction, which are a requirement of viable capacities and high performance. By comparing glyme-ethers of various chain lengths, we show that the choice of solvent has a tremendous effect on the battery performance. In contrast to the Li–O₂ system, high solubilities of the NaO₂ discharge product do not necessarily lead to increased capacities. Herein we report the profound effect of the Na⁺ ion solvent shell structure on the NaO₂ growth mechanism. Strong solvent–solute interactions in long-chain ethers shift the formation of NaO₂ toward a surface process resulting in submicrometric crystallites and very low capacities (ca. 0.2 mAh/cm²_(geom)). In contrast, short chains, which facilitate desolvation and solution-precipitation, promote the formation of large cubic crystals (ca. 10 um), enabling high capacities (ca. 7.5 mAh/cm²_(geom)). This work provides a new way to look at the key role that solvents play in the metal–air system