Sodium-ion batteries (SIBs) have emerged as a compelling alternative to lithium-ion batteries (LIBs), exhibiting comparable electrochemical performance while capitalizing on the abundant availability of sodium resources. In SIBs, P2/O3 biphasic cathodes, despite their high energy, require furthur improvements in stability to meet current energy demands. This study introduces a systematic methodology that leverages the meta-heuristically assisted NSGA-II algorithm to optimize multi-element doping in electrode materials, aiming to transcend conventional trial-and-error methods and enhance cathode capacity by the synergistic integration of P2 and O3 phases. A comprehensive phase analysis of the meta-heuristically designed cathode material Na0.76Ni0.20Mn0.42Fe0.30Mg0.04Ti0.015Zr0.025O2 (D-NFMO) is presented, showcasing its remarkable initial reversible capacity of 175.5 mAh g-1 and exceptional long-term cyclic stability in sodium cells. The investigation of structural composition and the stabilizing mechanisms is performed through the integration of multiple characterization techniques. Remarkably, the irreversible phase transition of P2→OP4 in D-NFMO is observed to be dramatically suppressed, leading to a substantial enhancement in cycling stability. The comparison with the pristine cathode (P-NFMO) offers profound insights into the long-term electrochemical stability of D-NFMO, highlighting its potential as a high-voltage cathode material utilizing abundant earth elements in SIBs. This study opens up new possibilities for future advancements in sodium-ion battery technology.

Sodium-ion batteries (SIBs) are potential candidates for large energy storage usage because of the natural abundance and cheap sodium. Nevertheless, improving the energy density and cycling steadiness of SIB cathodes remains a challenge. In this work, F-doping Na3Al2/3V4/3(PO4)3(NAVP) microspheres (Na3Al2/3V4/3(PO4)2.9F0.3(NAVPF)) are synthesized via spray drying and investigated as SIB cathodes. XRD and Rietveld refinement reveal expanded lattice parameters for NAVPF compared to the undoped sample, and the successful cation doping into the Na superionic conductor (NASICON) framework improves Na+ diffusion channels. The NAVPF delivers an ultrahigh capacity of 148 mAh g-1 at 100 mA g-1 with 90.8% retention after 200 cycles, enabled by the activation of V2+/V5+ multielectron reaction. Notably, NAVPF delivers an ultrahigh rate performance, with a discharge capacity of 83.6 mAh g-1 at 5000 mA g-1. In situ XRD demonstrates solid-solution reactions occurred during charge-discharge of NAVPF without two-phase reactions, indicating enhanced structural stability after F-doped. The full cell with NAVPF cathode and Na+ preintercalated hard carbon anode shows a large discharge capacity of 100 mAh g-1 at 100 mA g-1 with 80.2% retention after 100 cycles. This anion doping strategy creates a promising SIB cathode candidate for future high-energy-density energy storage applications.

To understand inhomogeneous reactions perpendicular to the current collector in an electrode for batteries, a method combining operando synchrotron X-ray diffraction and two-layer electrodes with different porosities is developed. The two layers are built using two different active materials (LiNi0.80Co0.15Al0.05O2 and LiMn2O4), therefore, tracing each diffraction pattern reveals which active material is reacting during the electrochemical measurement in transmission mode. The results demonstrate that the active material close to the separator is obviously more active than that one close to the current collector in the case of low porosity electrodes. This inhomogeneity should be due to the rate-limitation and especially to low average ionic conductivity of the electrolyte in the porous electrode because the current flows first mainly into the electrode regions close to the separator. The inhomogeneity is found to be mitigated by the adjustment of the electrode density and thus porosity. Hence, the novel operando method reveals a clear inhomogeneous reaction perpendicular to the current collector.