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The operation of lithium-ion batteries involves electron removal from and filling into the redox orbitals of cathode materials, experimentally probing the orbital electron population thus is highly desirable to resolve the redox processes and charge compensation mechanism. Here, we combine quantitative convergent-beam electron diffraction with high-energy synchrotron powder X-ray diffraction to quantify the orbital populations of Co and O in the archetypal cathode material LiCoO₂. The results indicate that removing Li ions from LiCoO₂ decreases Co t_2g orbital population, and the intensified covalency of Co–O bond upon delithiation enables charge transfer from O 2p orbital to Co e_g orbital, leadingmore » to increased Co e_g orbital population and oxygen oxidation. Theoretical calculations verify these experimental findings, which not only provide an intuitive picture of the redox reaction process in real space, but also offer a guidance for designing high-capacity electrodes by mediating the covalency of the TM–O interactions.« less

Layered Na-based oxides with the general composition of Na_xTMO₂ (TM: transition metal) have attracted significant attention for their high compositional diversity that provides tunable electrochemical performance for electrodes in sodium-ion batteries. The various compositions bring forward complex structural chemistry that is decisive for the layered stacking structure, Na-ion conductivity, and the redox activity, potentially promising new avenues in functional material properties. In this work, we have explored the maximum Na content in P2-type layered oxides and discovered that the high-content Na in the host enhances the structural stability; moreover, it promotes the oxidation of low-valent cations to their high oxidationmore » states (in this case Ni²⁺). This can be rationalized by the increased hybridization of the O(2p)-TM(3d-e_g*) states, affecting both the local TM environment as well as the interactions between the NaO₂ and TMO₂ layers. These properties are highly beneficial for the Na storage capabilities as required for cathode materials in sodium-ion batteries. It leads to excellent Na-ion mobility, a large storage capacity (>100 mAh g^–1 between 2.0-4.0 V), yet preventing the detrimental sliding of the TMO₂ layers (P2–O2 structural transition), as reflected by the ultralong cycle life (3000 (dis)charge cycles demonstrated). These findings expand the horizons of high Na-content P2-type materials, providing new insights of the electronic and structural chemistry for advanced cathode materials.« less

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Abstract Na‐ion batteries have experienced rapid development over the past decade and received significant attention from the academic and industrial communities. Although a large amount of effort has been made on material innovations, accessible design strategies on peculiar structural chemistry remain elusive. An approach to in situ construction of new Na‐based cathode materials by substitution in alkali sites is proposed to realize long‐term cycling stability and high‐energy density in low‐cost Na‐ion cathodes. A new compound, [K _0.444(1) Na _1.414(1) ][Mn _3/4 Fe _5/4 ](CN) ₆ , is obtained through a rational control of K ⁺ content from electrochemical reaction. Resultsmore » demonstrate that the remaining K ⁺ (≈0.444 mol per unit) in the host matrix can stabilize the intrinsic K‐based structure during reversible Na ⁺ extraction/insertion process without the structural evolution to the Na‐based structure after cycles. Thereby, the as‐prepared cathode shows the remarkably enhanced structural stability with the capacity retention of >78% after 1800 cycles, and a higher average operation voltage of ≈3.65 V versus Na ⁺ /Na, directly contrasting the non‐alkali‐site‐substitution cathode materials. This provides new insights into alkali‐site‐substitution constructing advanced Na‐ion cathode materials.« less

Abstract Na super ion conductor (NaSICON), Na _{1+ n} Zr ₂ Si _n P _{3– n} O ₁₂ is considered one of the most promising solid electrolytes; however, the underlying mechanism governing ion transport is still not fully understood. Here, the existence of a previously unreported Na5 site in monoclinic Na ₃ Zr ₂ Si ₂ PO ₁₂ is unveiled. It is revealed that Na ⁺ ‐ions tend to migrate in a correlated mechanism, as suggested by a much lower energy barrier compared to the single‐ion migration barrier. Furthermore, computational work uncovers the origin of themore » improved conductivity in the NaSICON structure, that is, the enhanced correlated migration induced by increasing the Na ⁺ ‐ion concentration. Systematic impedance studies on doped NaSICON materials bolster this finding. Significant improvements in both the bulk and total ion conductivity (e.g., σ _bulk = 4.0 mS cm ⁻¹ , σ _total = 2.4 mS cm ⁻¹ at 25 °C) are achieved by increasing the Na content from 3.0 to 3.30–3.55 mol formula unit ⁻¹ . These improvements stem from the enhanced correlated migration invoked by the increased Coulombic repulsions when more Na ⁺ ‐ions populate the structure rather than solely from the increased mobile ion carrier concentration. The studies also verify a strategy to enhance ion conductivity, namely, pushing the cations into high energy sites to therefore lower the energy barrier for cation migration.« less

The aqueous sodium-ion battery system is a safe and low-cost solution for large-scale energy storage, due to the abundance of sodium and inexpensive aqueous electrolytes. Although several positive electrode materials, e.g., Na_0.44MnO₂, were proposed, few negative electrode materials, e.g., activated carbon and NaTi₂(PO₄)₃, are available. Here we show that Ti-substituted Na_0.44MnO₂ (Na_0.44[Mn_1-xTi_x]O₂) with tunnel structure can be used as a negative electrode material for aqueous sodium-ion batteries. This material exhibits superior cyclability even without the special treatment of oxygen removal from the aqueous solution. Atomic-scale characterizations based on spherical aberration-corrected electron microscopy and ab initio calculations are utilized to accuratelymore » identify the Ti substitution sites and sodium storage mechanism. Ti substitution tunes the charge ordering property and reaction pathway, significantly smoothing the discharge/charge profiles and lowering the storage voltage. Both the fundamental understanding and practical demonstrations suggest that Na_0.44[Mn_1-xTi_x]O₂ is a promising negative electrode material for aqueous sodium-ion batteries.« less