One of the most challenging goals in the field of Na-ion batteries is finding a proper anode material that yields high capacities and can withstand a large number of cycles without structure degradation. Among investigations, antimony stands out with a theoretical capacity of 660 mAh/g and relatively high electrical conductivity. The working mechanism is based on the alloying reaction which entails large volume changes of up to 293%. Consequently, it causes severe microstructure degradation and poor capacity retention during electrochemical cycling. The current challenge is to design materials with a specific microstructure that can endure large strains occurring during sodium incorporation. We took advantage of a solvothermal reaction to synthesize nanosized composite material. The main phase (79%) of the as-obtained sample was found to be rhombohedral antimony (R-3m space group), with the secondary monoclinic phase (21%) Sb₄O₅Cl₂ (P21/a space group). Scanning electron microscopy (SEM) revealed the presence of an extraordinary branch-like microstructure of antimony with sub-micrometer particle size. Subsequently, we assembled the cells with metallic sodium as a counter electrode. The two-phase material was characterized by a reversible capacity of 500 mAh/g after 100 cycles at the current density of 100 mA/g yielding great capacity retention equal to 89% (compared with 2nd discharge capacity). Ex-situ X-ray diffraction measurements and operando Raman spectroscopy suggest the reversible formation of NaSb and Na₃Sb phases during the sodiation process and the return of long-range order in antimony after desodiation. Finally, via SEM ex-situ measurements, we observed the return of microstructure not only after the first and second charge but even after five cycles of sodiation and desodiation. Tailoring the microstructure to the branch shape allowed for obtaining the high-capacity, stable anode. This project is supported by the National Science Center Poland (NCN) based on decision number 2019/35/O/ST8/01799.

One of the ways to develop new Li-ion batteries with an extended lifespan, improved safety, and higher energy and power density is to replace the conventional graphite anode, working already at its theoretical limits, with other, better compounds. Recently it has been proposed to combine conversion and alloying Li-storage mechanisms within a single compound, benefiting from their advantages and confining the disadvantages (conversion-alloying materials, CAMs). Despite the overall improved electrochemical properties of CAMs, they suffer from insufficient cycling stability. Until now, the only possibility of improving cyclability was to use complex and expensive synthesis methods. On the other hand, there is an intriguing group of anode materials, high-entropy oxides (HEOs), which show great cycling stability in Li-ion cells, regardless of the used synthesis method. The reasons for this behavior have not been fully understood so far. In this work, we resolve the problem of the capacity fade of CAMs by applying the high-entropy approach. We successfully synthesized a new spinel-structured anode material, Sn_0.80Co_0.44Mg_0.44Mn_0.44Ni_0.44Zn_0.44O₄. Importantly, it was obtained using a simple solid-state synthesis method without expensive additives. When tested against the metallic Li electrode, the HEO delivers a high reversible specific capacity of 600 mAh/g at 50 mA/g in the voltage range of 0.01-2.5 V, and great capacity retention reaching nearly 100% after 500 cycles under 200 mA/g, outperforming conventional CAMs. By using several operando and ex-situ characterization techniques, we found that the Li-storage mechanism in the HEO is different compared to the conventional oxides. We identified numerous interesting features, including the reversible lithiation of the amorphous multi-component matrix, electrochemical activation of typically inactive magnesium, and excellent mixing of all the elements at the atomic scale maintained during cycling. All those observations interpreted together can provide a detailed explanation of the origins of the great cycling stability.