Machine-Learning-Driven High-Throughput Screening for High-Energy Density and Stable NASICON Cathodes

The Na super ionic conductor (NASICON), which has outstanding structural stability and a high operating voltage, is an appealing material for overcoming the limits of low specific energy and larger volume distortion of sodium-ion batteries. In this study, to discover ideal NASICON cathode materials, a screening platform based on density functional theory (DFT) calculations and machine learning (ML) is developed. A training database was generated utilizing the previous 124 545 electrode databases, and a test set of 3126 potential NASICON structures [NaxMyM'1-y(PO4)3] with 27 dopants at the metal site and 6 dopants at the polyanion central site was constructed. The developed ML surrogate model identifies 796 materials that satisfy the following criteria: formation energy of <0.0 eV/atom, energy above hull of ≤0.025 eV/atom, volume change of ≤4%, and theoretical capacity of ≥50 mAh/g. The thermodynamically stable configurations of doped NASICON structures were then selected using machine learning interatomic potential (MLIP), enabling rapid consideration of various dopant site configurations. DFT calculations are followed on 796 screened materials to obtain energy density, average voltage, and volume change. Finally, 50 candidates with an average voltage of ≥3.5 V are identified. The suggested platform accelerates the exploration for optimal NASICON materials by narrowing the focus on materials with desired properties, saving considerable resources.

Pearl-Structure-Enhanced NASICON Cathode toward Ultrastable Sodium-Ion Batteries

Based on the favorable ionic conductivity and structural stability, sodium superionic conductor (NASICON) materials especially utilizing multivalent redox reaction of vanadium are one of the most promising cathodes in sodium-ion batteries (SIBs). To further boost their application in large-scale energy storage production, a rational strategy is to tailor vanadium with earth-abundant and cheap elements (such as Fe, Mn), reducing the cost and toxicity of vanadium-based NASICON materials. Here, the Na_3.05 V_1.03 Fe_0.97 (PO₄ )₃ (NVFP) is synthesized with highly conductive Ketjen Black (KB) by ball-milling assisted sol-gel method. The pearl-like KB branch chains encircle the NVFP (p-NVFP), the segregated particles possess promoted overall conductivity, balanced charge, and modulated crystal structure during electrochemical progress. The p-NVFP obtains significantly enhanced ion diffusion ability and low volume change (2.99%). Meanwhile, it delivers a durable cycling performance (87.7% capacity retention over 5000 cycles at 5 C) in half cells. Surprisingly, the full cells of p-NVFP reveal a remarkable capability of 84.9 mAh g^-1 at 20 C with good cycling performance (capacity decay rate is 0.016% per cycle at 2 C). The structure modulation of the p-NVFP provides a rational design on the superiority of others to be put into practice.

Na+-Activation Engineering in the Na3V2(PO4)3 Cathode with Boosting Kinetics for Fast-Charging Na-Ion Batteries

Na superionic conductor-structured phosphates have attracted wide interest due to their high working voltage and fast Na⁺ migration facilitated by the robust 3D open framework. However, they usually suffer from low-rate capability and inferior cycling stability due to the low intrinsic electronic conductivity and limited activated Na⁺ ions. Herein, a doping protocol with Na⁺ in the V³⁺ site is developed to activate extra electrochemical Na⁺ ions and expand the migration path of Na⁺, leading to the improvement of the electronic conductivity and diffusion kinetics. It is also disclosed that the generated stronger Na-O bonds with high ionicity significantly conduce to the enhanced structural stability in the Na⁺-substituted Na_3.05V_1.975Na_0.025(PO₄)₃/C cathode. The obtained composite can deliver an excellent rate capacity of 83.8 mA h g^-1 at 20 C and a moderate cycling persistence of 91.3% over 1500 cycles at 10 C with great fast-charging properties. The reversible structure evolution is confirmed by the ex situ XRD, XPS, and ICP characterization. This work sheds light on awakening electroactive Na⁺ ions and designing phosphates with superior electrochemical stability for practical Na-ion batteries.