Sodium/Potassium (Na/K) metal anodes have been considered as the promising anodes for next-generation Na/K secondary batteries owing to their ultrahigh specific capacity, low redox potential and low cost. However, their practical application is still hampered due to unstable solid electrolyte interphase, infinite volume change, and dendrite growth. Herein, we design a 3D-Na3Bi/3D-K3Bi alloy host which enables the homogeneous and heterogeneous nucleation growth of Na/K metal. The unique structure with periodic alternating of electron and ion conductivity improves the mass transfer kinetics and prevents the volume expansion during cycling. Meanwhile, the sodiophilicity of Na3Bi/potassiophilicity of K3Bi framework can avoid dendritic growth. Cycling lifespans over 700 h with 1 mAh cm−2 for 3D-Na3Bi@Na electrode and about 450 h with 1 mAh cm−2 for 3D-K3Bi@K electrode are achieved, respectively. 3D-Na3Bi@Na||Na3V2(PO4)3 full battery shows sustainable cycle performance over 400 cycles. This design provides a simple but effective approach for achieving safety of sodium/potassium metal anodes.

Integration of homogeneous and heterogeneous nucleation growth via 3D alloy framework for stable Na/K metal anode

at an average storage voltage of 3.2 V with long cycle life. When coupled with hard carbon anode, a prototype rechargeable sodium-ion battery offers an energy density of 210 Wh kg −1 , a round-trip energy effi ciency of 90%, high rate capability, and excellent cycling stability. These desired performances make this system to be closer to the level of practical applications. The Na 0.9 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 (Na 0.9 [Cu II 0.22 Fe III 0.30 Mn III 0.16 Mn IV 0.32 ]O 2 ) material was synthesized by a simple solid-state reaction at 850 °C in air atmosphere using precursors of Na 2 CO 3 , CuO, Fe 2 O 3 , and Mn 2 O 3 . The crystal structure of the as-synthesized material was determined by X-ray diffraction (XRD) as shown in Figure 1 a together with its refi nement results by the Rietveld method (see Table S1, Supporting Information). It can be seen that all the Bragg diffraction peaks are in excellent agreement with the JC DS No. 01-0821495 (O3-type α-NaFeO 2 ) and can be indexed to a hexagonal layered structure with a space group of 3 R m − , indicative of a typical O3-type layered structure (note that the letter “O” refers to the Na coordination environment of octahedral site whereas the number “3” refers to the number of MO 2 slab according to Delmas’ notation. [ 13 ] A schematic illustration of the O3-type structure is also shown in Figure 1 b. The structure refi nement gives the lattice parameters a = 2.9587(7) A, c = 16.3742(6) A. The lattice parameter of c -axis is slightly larger than that of other O3-type materials [ 5f–h , 6b , 8a ] because the Na content is less than 1. The inductively coupled plasma (ICP) result confi rms the composition of Na 0.89 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 (see Table S2, Supporting Information). The morphology of the resulting sample is shown in Figure 1 c. The distribution of the particle size is in the range of 10–30 μm with about 3 μm sized primary particle agglomerations together (Figure 1 d). Most importantly, unlike other O3-type materials, [ 4–9,12 ] this material is very stable against water. In order to confi rm this, we intentionally design an accelerated aging experiment as described in the Experimental Section which was verifi ed by LiMO 2 as shown in Figure S1 (Supporting Information). We placed the as-synthesized material in deionized water for 3 d and then dried the material at 100 °C for overnight. The obtained material was checked by XRD again. It can be seen that the XRD pattern is nearly identical to that of the as-synthesized material, which is very different from other O3-type materials as shown in Figure S2 (Supporting Information). These results suggest that the O3-Na 0.9 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 is very stable against water. Furthermore, after the material was stored in air for one month Large-scale electrical energy storage systems are one of the core technologies in renewable energies and smart grid, among which sodium-ion batteries show great promise due to the abundant sodium resources. Layered metal oxides (of general formula: A x MO 2 , where A = Li, Na; M = Co, Ni, Mn, Cr, Fe, etc.) with alternating alkali metal layer and transition metal layer have long been of particular interest since the early 1980s as an important class of cathode materials for rechargeable batteries due to their easy synthesis and high energy density. [ 1 ] One of the most successful examples is LiCoO 2 , [ 1a ] which is commonly used as a cathode in lithium-ion batteries with the highest volumetric energy density for portable electronic devices. Its metal substituted materials (LiCo 1− x − y − z Ni x Mn y Al z O 2 ) are being used in power batteries for electric vehicles. In the case of sodiumion batteries operated at room temperature which are proposed for large-scale electrical energy storage owing to the naturally abundant sodium resources in recent years. [ 2,3 ] Na x CoO 2 that can electrochemically and reversibly intercalate Na is the fi rst example, [ 1b ] then a large number of layered metal oxides have been extensively exploited. [ 4–12 ] However, the practical applications have been hindered by two major challenges. First, unlike LiMO 2 , almost all the Na x MO 2 are not stable against moisture (either they can be oxidized by water or water/carbon dioxide molecules can be intercalated into alkali metal layer). [ 4–9,11,12 ]

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Prototype Sodium-Ion Batteries Using Air-Stable and Co/Ni-Free O3-Layered Metal Oxide Cathode

The sodium metal anode exhibits great potential in next-generation high-energy-density batteries due to its high theoretical capacity (1165 mA h g−1) at low redox potential (−2.71 V versus standard hydrogen electrode) as well as the high natural abundance and low cost of Na resources. However, its practical application in rechargeable batteries is hindered by uncontrollable dendrite growth that leads to poor coulombic efficiency, short lifespan, infinite volume change and even safety issues during plating/stripping processes. Among various strategies, the application of skeletons for Na metal anodes demonstrates a positive influence on reducing local current density, inhibiting dendrite growth, and alleviating volume expansion. This work reviews the research progress of various skeleton materials for sodium metal anodes in recent years, including carbon-based skeletons, alloy-based skeletons, metallic skeletons and MXene-based skeletons. Simultaneously, the recent technological advances and strategies are summarized and categorized. Finally, we discuss the development prospects and research strategies of skeleton materials in sodium metal anodes from the perspective of basic research and practical applications.

Recent advanced skeletons in sodium metal anodes

Standard approximations to the density functional theory exchange-correlation functional have been extraordinary successful, but calculating formation enthalpies of reactions involving compounds with both localized and delocalized electronic states remains challenging. In this work we examine the shortcomings of the generalized gradient approximation (GGA) and GGA+U in accurately characterizing such difficult reactions. We then outline a methodology that mixes GGA and GGA+U total energies (using known binary formation data for calibration) to more accurately predict formation enthalpies. We demonstrate that for a test set of 49 ternary oxides, our methodology can reduce the mean absolute relative error in calculated formation enthalpies from approximately 7.7–21% in GGA+U to under 2%. As another example we show that neither GGA nor GGA+U alone accurately reproduces the Fe-P-O phase diagram; however, our mixed methodology successfully predicts all known phases as stable by naturally stitching together GGA and GGA+U results. As a final example we demonstrate how our technique can be applied to the calculation of the Li-conversion voltage of LiFeF3. Our results indicate that mixing energies of several functionals represents one avenue to improve the accuracy of total energy computations without affecting the cost of calculation.

Qingpeng Yang

Papers

Enhanced processability and electrochemical cyclability of metallic sodium at elevated temperature using sodium alloy composite