Improving the Performance of Titania Nanotube Battery Materials by Surface Modification with Lithium Phosphate

Self-organized TiO₂ nanotubes ranging from amorphous to anatase structures were obtained by anodization procedures and thermal treatments at 500°C. Then electrolytic Li₃PO₄ films were successfully deposited on the nanotube array by an electrochemical procedure consisting in proton reduction with subsequent increase in pH, hydrogen phosphate dissociation and Li₃PO₄ deposition on the surface of the cathode. The Li₃PO₄ polymorph (γ or β) in the deposit could be tailored by modifying the electrodeposition parameters, such as time or current density, as determined by X-ray patterns. The morphological analysis evidenced the formation of a 3D nanostructure consisting of Li₃PO₄ coating the TiO₂ nanotube array. The anode–solid electrolyte stacking was tested in lithium half cells. Interestingly, the electrochemical performances revealed a better cycling stability for samples containing low amount of lithium phosphate, which is deposited for short times and low current densities. These results suggested the possibility of fabricating 3D Li-ion batteries. nt-TiO₂/γ-Li₃PO₄/LiFePO₄ full cells were cycled at different rates in the C/5-5C range. This cathode-limited microbattery delivered a reversible gravimetric capacity of 110 mA h g^–1 and a capacity retention of 75 % after 190 cycles at 5C

Electrochemical Interaction of Few-Layer Molybdenum Disulfide Composites vs Sodium: New Insights on the Reaction Mechanism

The direct observation of real time electrochemical processes is of great importance for fundamental research on battery materials. Here, we use electron paramagnetic resonance (EPR) spectroscopy to monitor the electrochemical reaction of sodium ions with few-layer MoS₂ and its composite with carbon nanotubes (CNTs), thereby uncovering new details of the reaction mechanism. We propose that the sodiation reaction takes place initially in structural defects at the MoS₂ surface that have been created during the synthetic process (ultrasonic exfoliation), leading to a decrease in the density of Mo⁵⁺ at low symmetry sites that can be related to the electrochemical irreversibility of the process. In the case of the few-layer MoS₂/CNTs composite, we found metallic-type conduction behavior for the electrons associated with the Mo paramagnetic centers and improved electrochemical reversibility. The reversible nature of the EPR spectra implies that adsorption/desorption of Na⁺ ions occurs on the Mo⁵⁺ defects, or that they are neutralized during sodiation and subsequently created upon Na⁺ extraction. These effects help us to understand the higher capacities obtained in the exfoliated samples, as the sum of electrosorption of ions and faradaic effects, and support the suggestion of a different reaction mechanism in the few-layer chalcogenide, which is not exclusively an insertion process

Insight into the Electrochemical Sodium Insertion of Vanadium Superstoichiometric NASICON Phosphate

A slight deviation of the stoichiometry has been introduced in Na_{3–3<i>x</i>}­V_2+<i>x</i>(PO₄)₃ (0 ≤ <i>x</i> ≤ 0.1) samples to determine the effect on the structural and electrochemical behavior as a positive electrode in sodium-ion batteries. X-ray diffraction and XPS results provide evidence for the flexibility of the NASICON framework to allow a limited vanadium superstoichiometry. In particular, the Na_2.94V_2.02(PO₄)₃ formula reveals the best electrochemical performance at the highest rate (40<i>C</i>) and capacity retention upon long cycling. It is attributed to the excellent kinetic response and interphase chemical stability upon cycling. The electrochemical performance of this vanadium superstoichiometric sample in a full sodium-ion cell is also described

An Unnoticed Inorganic Solid Electrolyte: Dilithium Sodium Phosphate with the Nalipoite Structure

Solid electrolytes are crucial in the development of advanced lithium batteries and related technologies. Orthorhombic Li₂NaPO₄ (nalipoite) was synthesized, and its ionic conductivity compared very favorably with that of Li₃PO₄. The potential applicability of Li_3–<i>x</i>Na_<i>x</i>PO₄ as a lithium ion solid electrolyte was investigated for first time. First-principles DFT calculations were used to evaluate the possibilities of preparing other crystal structures

Improved Surface Stability of C+M_<i>x</i>O_<i>y</i>@Na₃V₂(PO₄)₃ Prepared by Ultrasonic Method as Cathode for Sodium-Ion Batteries

Coated C+M_<i>x</i>O_<i>y</i>@Na₃V₂(PO₄)₃ samples containing 1.5% or 3.5% wt. of M_<i>x</i>O_<i>y</i> (Al₂O₃, MgO or ZnO) have been synthesized by a two-step method including first a citric based sol–gel method for preparing the active material and second an ultrasonic stirring technique to deposit M_<i>x</i>O_<i>y</i>. The presence of the metal oxides properly coating the surface of the active material is evidenced by XPS and electron microscopy. Galvanostatic cycling of sodium half-cells reveals a significant capacity enhancement for samples coated with 1.5% of metal oxides and an exceptional cycling stability as evidenced by Coulombic efficiencies as high as 95.9% for ZnO@ Na₃V₂(PO₄)₃. It is correlated to their low surface layer and charge transfer resistance values. The formation of metal fluorides that remove traces of corrosive HF from the electrolyte is checked by XPS spectroscopy. The feasibility of sodium-ion batteries assembled with C+M_<i>x</i>O_<i>y</i>@Na₃V₂(PO₄)₃ is further verified by evaluating the electrochemical performance of full cells. Particularly, a Graphite//Al₂O₃@ Na₃V₂(PO₄)₃ battery delivers an energy density as high as 260 W h kg^–1 and exhibits a Coulombic efficiency of 89.3% after 115 cycles

Lithium Storage Mechanisms and Effect of Partial Cobalt Substitution in Manganese Carbonate Electrodes

A promising group of inorganic salts recently emerged for the negative electrode of advanced lithium-ion batteries. Manganese carbonate combines low weight and significant lithium storage properties. Electron paramagnetic resonance (EPR) and magnetic measurements are used to study the environment of manganese ions during cycling in lithium test cells. To observe reversible lithium storage into manganese carbonate, preparation by a reverse micelles method is used. The resulting nanostructuration favors a capacitive lithium storage mechanism in manganese carbonate with good rate performance. Partial substitution of cobalt by manganese improves cycling efficiency at high rates