The electrochemical storage mechanism in oxy-hydroxyfluorinated anatase for sodium-ion batteries

International audienceReplacing lithium ions with sodium ions as the charge carriers in rechargeable batteries can induce noticeable differences in the electrochemical storage mechanisms of electrode materials. Many material parameters, such as particle size, morphology, and the presence of defects, are known to further affect the storage mechanism. Here, we report an investigation of how the introduction of titanium vacancies into anatase TiO2 affects the sodium storage mechanism. From pair distribution function analysis, we observe that sodium ions are inserted into titanium vacancies at the early stage of the discharge process. This is supported by density functional theory calculations, which predict that sodium insertion is more favourable at vacancies than at interstitial sites. Our calculations also show that the intercalation voltage is sensitive to the anion coordination environment of the vacancy. Sodiation to higher concentrations induces a phase transition toward a disordered rhombohedral structure, similar to that observed in defect-free TiO2. Finally, we find that the X-ray diffraction pattern of the rhombohedral phase drastically changes depending on the composition and degree of disorder, providing further comprehension on the sodium storage mechanism of anatase

Combination of solid state NMR and DFT calculation to elucidate the state of sodium in hard carbon electrodes

We examined the state of sodium electrochemically inserted in HC prepared at 700–2000 °C using solid state Na magic angle spinning (MAS) NMR and multiple quantum (MQ) MAS NMR. The 23Na MAS NMR spectra of Na-inserted HC samples showed signals only in the range between +30 and −60 ppm. Each observed spectrum was ascribed to combinations of Na+ ions from the electrolyte, reversible ionic Na components, irreversible Na components assigned to solid electrolyte interphase (SEI) or non-extractable sodium ions in HC, and decomposed Na compounds such as Na2CO3. No quasi-metallic sodium component was observed to be dissimilar to the case of Li inserted in HC. MQMAS NMR implies that heat treatment of HC higher than 1600 °C decreases defect sites in the carbon structure. To elucidate the difference in cluster formation between Na and Li in HC, the condensation mechanism and stability of Na and Li atoms on a carbon layer were also studied using DFT calculation. Na3 triangle clusters standing perpendicular to the carbon surface were obtained as a stable structure of Na, whereas Li2 linear and Li4 square clusters, all with Li atoms being attached directly to the surface, were estimated by optimization. Models of Na and Li storage in HC, based on the calculated cluster structures were proposed, which elucidate why the adequate heat treatment temperature of HC for high-capacity sodium storage is higher than the temperature for lithium storage

NMR study for electrochemically inserted Na in hard carbon electrode of sodium ion battery

The state of sodium inserted in the hard carbon electrode of a sodium ion battery having practical cyclability was investigated using solid state 23Na NMR. The spectra of carbon samples charged (reduced) above 50 mAh g−1 showed clear three components. Two peaks at 9.9 ppm and 5.2 ppm were ascribed to reversible sodium stored between disordered graphene sheets in hard carbon because the shift of the peaks was invariable with changing strength of external magnetic field. One broad signal at about −9 to −16 ppm was assigned to sodium in heterogeneously distributed closed nanopores in hard carbon. Low temperature 23Na static and magic angle spinning NMR spectra didn't split or shift whereas the spectral pattern of 7Li NMR for lithium-inserted hard carbon changes depending on the temperature. This strongly suggests that the exchange of sodium atoms between different sites in hard carbon is slow. These studies show that sodium doesn't form quasi-metallic clusters in closed nanopores of hard carbon although sodium assembles at nanopores while the cell is electrochemically charged

A Reversible Phase Transition for Sodium Insertion in Anatase TiO₂

International audienceAnatase TiO2 is a potential negative electrode for sodium-ion batteries. The sodium storage mechanism is, however, still under debate, yet its comprehension is required to optimize the electrochemical properties. To clarify the sodium storage mechanism occurring in anatase, we have used both electrochemical and chemical routes from which we obtained similar trends. During the first discharge, an irreversible plateau region is observed which corresponds to the insertion of Na + within the interstitial sites of anatase and is accompanied by a drastic loss of the long-range order as revealed by x-ray diffraction, high resolution of high angle annular dark field scanning transmission electron microscope (HAADF-STEM) and pair distribution function (PDF) analysis. Further structural analysis of the total scattering data indicates that the sodiated phase displays a layered-like rhombohedral R-3m structure built from the stacking of Ti and Na slabs. Because of the initial 3D network of anatase, the reduced phase shows strong disorder due to cationic inter-mixing between the Ti and Na slabs and the refined chemical formula is (Na0.43Ti0.57)3a(0.22Na0.39Ti0.39)3bO2 where refers to vacancy. The presence of high valence Ti ions in the Na layers induces a contraction of the c-parameter as compared to the ordered phase. Upon de-sodiation, the structure further amorphized and the local structure probed by PDF is shown to be similar to the anatase TiO2 suggesting that the 3D network is recovered. The reversible sodium insertion/de-insertion is thus attributed to the rhombohedral active phase formed during the first discharge, and an oxidized phase featuring the local structure of anatase. Due to the amorphous nature of the two phases, the potential-composition curves are characterized by a sloping curve. Finally, a comparison between the intercalation of lithium and sodium into anatase TiO2 performed by DFT calculations confirmed that for the sodiated phase, the rhombohedral structure is more stable than the tetragonal phase observed during the lithiation of nanoparticles. In many areas of modern life, lithium-ion batteries are ubiquitous as energy-storage solutions. The growing demand for higher energy density and lower cost of electro-chemical energy storage devices, however, has motivated a search for auxiliary technologies based on alternative chemistries. 1,2 One possible candidate is the sodium-ion battery, which is attractive because of the high earth– abundance of sodium, and lower cost versus lithium-ion batteries, due to compatibility with aluminum as the an-odic current collector. 3-5 Development of sodium-ion batteries has been largely stimulated by knowledge of lithium-ion analogues. The intercalation of Na + or Li + ions into a host lattice can, however, give qualitatively different voltage profiles, corresponding to different intercalation mechanisms. For example, lithium insertion in Li4Ti5O12 is accompanied by a spinel to rock-salt phase transition. 6,7 The equivalent sodium insertion, however, proceeds via a complex three-phase–separation mechanism (spinel to two rock-salt phases of Li7Ti5O12 and Na6LiTi5O12). 8 Such differences in intercalation behaviour can often be attributed to different properties of Li versus Na, such as ionic radius and polarizability. 9, 10 In general, however, the performance of electrodes in sodium-ion batteries cannot be understood by simply extrapolating from their behaviour versus lithium, when it is necessary to carefully reexamine the sodium-intercalation behaviour

2023 roadmap for potassium-ion batteries

The heavy reliance of lithium-ion batteries (LIBs) has caused rising concerns on the sustainability of lithium and transition metal and the ethic issue around mining practice. Developing alternative energy storage technologies beyond lithium has become a prominent slice of global energy research portfolio. The alternative technologies play a vital role in shaping the future landscape of energy storage, from electrified mobility to the efficient utilization of renewable energies and further to large-scale stationary energy storage. Potassium-ion batteries (PIBs) are a promising alternative given its chemical and economic benefits, making a strong competitor to LIBs and sodium-ion batteries for different applications. However, many are unknown regarding potassium storage processes in materials and how it differs from lithium and sodium and understanding of solid–liquid interfacial chemistry is massively insufficient in PIBs. Therefore, there remain outstanding issues to advance the commercial prospects of the PIB technology. This Roadmap highlights the up-to-date scientific and technological advances and the insights into solving challenging issues to accelerate the development of PIBs. We hope this Roadmap aids the wider PIB research community and provides a cross-referencing to other beyond lithium energy storage technologies in the fast-pacing research landscape

NaFe0.5Co0.5O2 as high energy and power positive electrode for Na-ion batteries

O3-type NaFeO2 is a promising candidate as positive electrode materials for rechargeable Na batteries. However, its reversible range of sodium extraction is relatively narrow (x = ca. 0.4 in Na1 − xFeO2) because of the irreversible structural change, presumably associated with the iron migration into the adjacent tetrahedral sites in Na layers as we recently reported. Herein, we report that cobalt-substituted NaFeO2 demonstrates excellent electrode performance in a non-aqueous Na cell at room temperature. NaFe0.5Co0.5O2 delivers approximately 160 mAh g−1 of reversible capacity with relatively good capacity retention and excellent rate-capability in a voltage range of 2.5–4.0 V, resulting in reversible formation of Na0.3Fe0.5Co0.5O2. The partial substitution of metal element for Fe in O3-type NaFeO2-based materials is proved to be the important strategy to suppress the irreversible phase transition, and thus improving the reversibility of sodium removal/insertion as the electrode materials. Keywords: Sodium, Intercalation, Na-ion battery, NaFeO2, NaCoO