Sodium Electrochemical Deintercalation and Intercalation in O3-NaRhO2 and P2-Na xRhO2 Layered Oxides

Electrochemical Behaviors and Doping Rules of NaRhO2 Cathode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have great advantages for energy storage and conversion due to their low cost and large storage capacity. Currently, NaRhO2 is used as an electrode material for sodium-ion batteries. Doping first- and second-row transition metals has been carried out to comprehensively assess NaRhO2 as a cathode material. The geometric and electronic structures and electrochemical and doping behaviors of NaRhO2 cathode materials for SIBs have been investigated using density functional theory calculations. The results show that the bond lengths of Rh-O in NaRhO2 decrease during sodium deintercalation. The band gap of NaRhO2 with sodium extraction gradually reduces. The density of states of NaxRhO2 shows that the interaction between the Rh-4d and O-2p orbitals increases and the orbitals shift toward the right. The average intercalation voltage of NaxRhO2 cathode material increased from 2.7 to 3.9 eV. After doping with first- and second-row transition metal elements from Sc to Zn and Y to Cd, the changes in the band gaps of the doped NaRhO2 materials exhibit a W-type rule. In contrast, their magnetic moments show a reverse W-type rule. These findings on the pristine and doped NaRhO2 can provide theoretical guidance for the preparation of novel electrode materials suitable for sodium-ion batteries.

Active material and interphase structures governing performance in sodium and potassium ion batteries

Development of energy storage systems is a topic of broad societal and economic relevance, and lithium ion batteries (LIBs) are currently the most advanced electrochemical energy storage systems. However, concerns on the scarcity of lithium sources and consequently the expected price increase have driven the development of alternative energy storage systems beyond LIBs. In the search for sustainable and cost-effective technologies, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have attracted considerable attention. Here, a comprehensive review of ongoing studies on electrode materials for SIBs and PIBs is provided in comparison to those for LIBs, which include layered oxides, polyanion compounds and Prussian blue analogues for positive electrode materials, and carbon-based and alloy materials for negative electrode materials. The importance of the crystal structure for electrode materials is discussed with an emphasis placed on intrinsic and dynamic structural properties and electrochemistry associated with alkali metal ions. The key challenges for electrode materials as well as the interface/interphase between the electrolyte and electrode materials, and the corresponding strategies are also examined. The discussion and insights presented in this review can serve as a guide regarding where future investigations of SIBs and PIBs will be directed.

Comparison of Layered Li(Li0.2Rh0.8)O2 and LiRhO2 upon Li Removal: Stabilizing Effect of Li Substitution

Phase transformations upon delithiation in layered oxides with the NaCrS₂ structure type are widely studied for numerous combinations of 3d transition metals because of the application of LiCoO₂ and its derivatives as cathode materials in rechargeable Li-ion batteries. However, complete replacement of 3d by 4d transition metals still yields phenomena never seen in compounds containing 3d metals only. In the present work, the structural evolution of Li-rich O3-Li(Li_0.2Rh_0.8)O₂, having a mixed occupancy of 20% Li and 80% Rh in the metal-O slabs, was studied during electrochemical Li removal and insertion and compared with the isostructural stoichiometric LiRhO₂. The latter compound undergoes a transformation from the layered NaCrS₂ to the tunnel-like rutile-ramsdellite intergrowth structure of the γ-MnO₂ type. Partial replacement of Rh by Li, in contrast, completely prevents this transition, resulting in a reversible cell expansion and shrinkage within the layered structure upon (de)lithiation. Moreover, no anomalously short Rh-O and O-O distances were observed in Li_x≈0(Li_0.2Rh_0.8)O₂ with the Rh^4.75+ intermediate valence state at 4.8 V, in contrast to Li_x≈0RhO₂ with Rh⁴⁺ at 4.2 V, as confirmed by operando synchrotron X-ray diffraction and extended X-ray absorption fine structure studies. We believe that the difference in the Li-O and Rh-O covalency is responsible for the observed structural stabilization. The longer and more ionic Li-O bonds in the (Li,Rh)O₂ layers impede the shortening of O-O distances needed for transformation to the γ-MnO₂ type because of a higher negative charge on O anions connected to Li cations and the stronger electrostatic repulsion between them.