Abstract:
The growing demand for electric transportation and large-scale energy storage has accelerated the development of next-generation ion batteries with high energy density, high power capability, long cycle life, and improved safety. However, increasing electrode capacity often induces structural degradation, while replacing flammable liquid electrolytes with solid electrolytes introduces challenges related to ionic conductivity, interfacial resistance, and mechanical stability. In this presentation, entropy engineering and multielement substitution are explored as effective strategies for stabilizing high-energy sodium-ion cathodes and developing safe solid electrolytes for lithium- and sodium-metal batteries. Advanced synchrotron X-ray and neutron techniques, complemented by electrochemical characterization and density functional theory calculations, are employed to correlate composition, atomic structure, phase evolution, ion transport, and electrochemical performance.
For sodium-ion batteries, a high-entropy O3-type NaCu0.1Ni0.3Fe0.2Mn0.2Ti0.2O2 layered oxide cathode was developed to enhance structural stability during Na⁺ extraction and insertion. X-ray absorption fine structure analysis confirmed a single-phase structure with uniformly distributed transition-metal ions. Operando synchrotron X-ray diffraction revealed that the high-entropy effect stabilized the O3 framework and delayed phase transformation until the extraction of approximately 0.32 Na⁺, enabling a reversible capacity of 130 mAh g⁻¹ and stable cycling over 500 cycles.[1] Furthermore, a Li- and Co-free P2-type Ca-doped Na0.67[Mg2/9Cu1/9Mn2/3]O2 cathode was designed to exploit high-capacity oxygen redox. Synergistic Cu substitution and Ca doping stabilized lattice oxygen and suppressed detrimental phase transformations, delivering 205 mAh g⁻¹ and a full-cell energy density of 250.7 Wh kg⁻¹.[2]
To improve battery safety, high- and medium-entropy ceramic solid electrolytes with NASICON- and garnet-type structures were developed. High-entropy Li1.3Al0.4Ti0.5Zr0.5Sn0.5Ta0.1(PO4)3 exhibited enhanced electrochemical stability against lithium metal,[3] whereas medium-entropy Li6.5La3Zr0.5Ta0.5Nb0.5Y0.5O12 maintained stable ionic conductivity after 30 days of air exposure.[4] Incorporating high-entropy ceramic fillers into ionic-liquid-containing polymer electrolytes further enhanced ionic conductivity and enabled stable lithium-metal batteries over 400 cycles.[5] In parallel, an electrospun nanofiber-reinforced ceramic–polymer electrolyte provided continuous Na⁺-transport pathways and high mechanical strength, enabling stable sodium-metal batteries over 500 cycles.
These findings demonstrate that entropy engineering can simultaneously regulate electrode phase stability, ionic transport, and solid-electrolyte durability. Further advances in electrode–electrolyte interfacial compatibility, scalable manufacturing, cost reduction, and long-term reliability will be essential for translating these materials into commercially viable high-energy, high-power, and safe batteries.
Keywords – sodium-ion batteries, lithium-ion batteries, solid electrolyte