The recent discovery of the isostructrual cubic Na3PS4 and Na3PSe4 as fast Na-ion conductors provided a general structural framework for the exploration of new sodium superionic conductors. In this work, we systematically investigated the structures and ionic conduction characteristics of a series of compounds with the general chemical formula of Na3PSxSe4-x. Synthesis of Na3PS4 under different conditions (e.g., temperature, reaction vessel, mass of the precursors) reveals the reactivity of the precursors with the reaction tubes, producing different polymorphs. X-ray diffraction studies on the solid solution phases Na3PSxSe4-x identified a tetragonal-to-cubic phase transition with increasing Se concentration. This observation is consistent with the computed stability of the tetragonal and cubic polymorphs, where the energy difference between the two polymorphs becomes very close to zero in Se-rich compositions. Furthermore, ab initio molecular dynamic simulations suggest that the fast Na-ion conduction in Na3PSxSe4-x may not be causally related with the symmetry or the composition of these phases. The formation of defects, instead, enables fast Na-ion conduction in this class of materials.

All-solid-state batteries show great potential for achieving high energy density with less safety problems; however, (electro)chemical issues at the solid electrolyte/electrode interface may severely limit their performance. In this work, the electrochemical stability and chemical reactivity of a wide range of potential Na solid-state electrolyte chemistries were investigated using density functional theory calculations. In general, lower voltage limits are predicted for both the reduction and oxidation of Na compounds compared with those of their Li counterparts. The lower reduction limits for the Na compounds indicate their enhanced cathodic stability as well as the possibility of stable sodium metal cycling against a number of oxides and borohydrides. With increasing Na content (or chemical potential), improved cathodic stability but also reduced anodic stability are observed. An increase in the oxidation voltage is shown for Na polyanion systems, including borohydrides, NaSICON-type oxides, and aluminates, due to the covalent stabilization of the anions. In addition, the oxides exhibit remarkable chemical stability when in contact with various cathode materials (layered transition metal oxides and fluorophosphates), whereas the chalcogenides predictably display narrow electrochemical windows and high chemical reactivity. Our findings indicate some promising candidates for solid-state conductors and/or protective coating materials to enable the operation of high-energy-density all-solid-state Na batteries.

O3 layered sodium transition metal oxides (i.e., NaMO2, M = Ti, V, Cr, Mn, Fe, Co, Ni) are a promising class of cathode materials for Na-ion battery applications. These materials, however, all suffer from severe capacity decay when the extraction of Na exceeds certain capacity limits. Understanding the causes of this capacity decay is critical to unlocking the potential of these materials for battery applications. In this work, we investigate the structural origins of capacity decay for one of the compounds in this class, NaCrO2. The (de)sodiation processes of NaCrO2 were studied both in situ and ex situ through X-ray and electron diffraction measurements. We demonstrate that NaxCrO2 (0 < x < 1) remains in the layered structural framework without Cr migration up to a composition of Na0.4CrO2. Further removal of Na beyond this composition triggers a layered-to-rock-salt transformation, which converts P′3-Na0.4CrO2 into the rock-salt CrO2 phase. This structural transformation proceeds via the formation of an intermediate O3 NaδCrO2 phase that contains Cr in both Na and Cr slabs and shares very similar lattice dimensions with those of rock-salt CrO2. It is intriguing to note that intercalation of alkaline ions (i.e., Na+ and Li+) into the rock-salt CrO2 and O3 NaδCrO2 structures is actually possible, albeit in a limited amount (∼0.2 per formula unit). When these results were analyzed under the context of electrochemistry data, it was apparent that preventing the layered-to-rock-salt transformation is crucial to improve the cyclability of NaCrO2. Possible strategies for mitigating this detrimental phase transition are proposed.

Inorganic solid-state ionic conductors with high ionic conductivity are of great interest for their application in safe and high-energy-density solid-state batteries. Our previous study reveals that the crystal structure of the ionic conductor Li7P3S11 contains a body-centered-cubic (bcc) arrangement of sulfur anions and that such a bcc anion framework facilitates high ionic conductivity. Here, we apply a set of first-principles calculations techniques to investigate A7P3X11-type (A = Li, Na; X = O, S, Se) lithium and sodium superionic conductors derived from Li7P3S11, focusing on their structural, dynamic and thermodynamic properties. We find that the ionic conductivity of Na7P3S11 and Na7P3Se11 is over 10 mS cm-1 at room temperature, significantly higher than that of any known solid Na-ion sulfide or selenide conductor. However, thermodynamic calculations suggest that the isostructural sodium compounds may not be trivial to synthesize, which clarifies the puzzle concerning the experimental problems in trying to synthesize these compounds.

Solid-state batteries (SSBs) using a solid electrolyte show potential for providing improved safety as well as higher energy and power density compared with conventional Li-ion batteries. However, two critical bottlenecks remain: the development of solid electrolytes with ionic conductivities comparable to or higher than those of conventional liquid electrolytes and the creation of stable interfaces between SSB components, including the active material, solid electrolyte and conductive additives. Although the first goal has been achieved in several solid ionic conductors, the high impedance at various solid/solid interfaces remains a challenge. Recently, computational models based on ab initio calculations have successfully predicted the stability of solid electrolytes in various systems. In addition, a large amount of experimental data has been accumulated for different interfaces in SSBs. In this Review, we summarize the experimental findings for various classes of solid electrolytes and relate them to computational predictions, with the aim of providing a deeper understanding of the interfacial reactions and insight for the future design and engineering of interfaces in SSBs. We find that, in general, the electrochemical stability and interfacial reaction products can be captured with a small set of chemical and physical principles.

Close-packed chalcogenide spinels, such as MgSc2Se4, MgIn2S4, and MgSc2S4, show potential as solid electrolytes in Mg batteries, but are affected by non-negligible electronic conductivity, which contributes to self-discharge when used in an electrochemical storage device. Using first-principles calculations, we evaluate the energy of point defects as a function of synthesis conditions and Fermi level to identify the origins of the undesired electronic conductivity. Our results suggest that Mg-vacancies and Mg-metal antisites (where Mg is exchanged with Sc or In) are the dominant point defects that can occur in the systems under consideration. While we find anion-excess conditions and slow cooling to likely create conditions for low electronic conductivity, the spinels are likely to exhibit significant n-type conductivity under anion-poor environments, which are often present during high-temperature synthesis. Finally, we explore extrinsic aliovalent doping to potentially mitigate the electronic conductivity in these chalcogenide spinels. The computational strategy is general and can be easily extended to other solid electrolytes (and electrodes) to aid the optimization of the electronic properties of the corresponding frameworks.

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All-solid-state Na-ion batteries that operate at or close to room temperature are a promising next-generation battery technology with enhanced safety and reduced manufacturing cost. An indispensable component of this technology is the solid-state electrolyte that allows rapid shuttling of the mobile cation (i.e., Na+) between the cathode and anode. However, there are very few fast Na-ion conductors with ionic conductivity approaching that of the liquid counterparts (i.e., 1 mS cm-1). In this work, we present the synthesis and characterization of a fast Na-ion conductor, cubic Na3PSe4. This material possesses a room-temperature ionic conductivity exceeding 0.1 mS cm-1 and does not require high-temperature sintering to minimize grain boundary resistance, making it a promising solid-state electrolyte candidate for all-solid-state Na-ion battery applications. On the basis of density functional theory, nudged elastic band, and molecular dynamics investigations, we demonstrate that the framework of cubic Na3PSe4 only permits rapid Na+ diffusion with the presence of defects, and that the formation of the Na vacancy (charge-balanced by slight Se2- oxidation) is more energetically favorable among the various defects considered. This finding provides important guidelines to further improve Na-ion conductivity in this class of materials.

K-ion batteries are a potentially exciting and new energy storage technology that can combine high specific energy, cycle life, and good power capability, all while using abundant potassium resources. The discovery of novel cathodes is a critical step toward realizing K-ion batteries (KIBs). In this work, a layered P2-type K0.6CoO2 cathode is developed and highly reversible K ion intercalation is demonstrated. In situ X-ray diffraction combined with electrochemical titration reveals that P2-type K0.6CoO2 can store and release a considerable amount of K ions via a topotactic reaction. Despite the large amount of phase transitions as function of K content, the cathode operates highly reversibly and with good rate capability. The practical feasibility of KIBs is further demonstrated by constructing full cells with a graphite anode. This work highlights the potential of KIBs as viable alternatives for Li-ion and Na-ion batteries and provides new insights and directions for the development of next-generation energy storage systems.

Magnesium oxide and sulfide spinels have recently attracted interest as cathode and electrolyte materials for energy-dense Mg batteries, but their observed electrochemical performance depends strongly on synthesis conditions. Using first-principles calculations and percolation theory, we explore the extent to which spinel inversion influences Mg2+ ionic mobility in MgMn2O4 as a prototypical cathode, and MgIn2S4 as a potential solid electrolyte. We find that spinel inversion and the resulting changes of the local cation ordering give rise to both increased and decreased Mg2+ migration barriers, along specific migration pathways, in the oxide as well as the sulfide. To quantify the impact of spinel inversion on macroscopic Mg2+ transport, we determine the percolation thresholds in both MgMn2O4 and MgIn2S4. Furthermore, we analyze the impact of inversion on the electrochemical properties of the MgMn2O4 cathode via changes in the phase behavior, average Mg insertion voltages and extractable capacities, at varying degrees of inversion. Our results confirm that inversion is a major performance limiting factor of Mg spinels and that synthesis techniques or compositions that stabilize the well-ordered spinel structure are crucial for the success of Mg spinels in multivalent batteries.