Mn‐Rich Phosphate Cathode for Sodium‐Ion Batteries: Anion‐Regulated Solid Solution Behavior and Long‐Term Cycle Life

Abstract

As a sodium superionic conductor, Mn‐rich phosphate of Na3.4Mn1.2Ti0.8(PO4)3 is considered as one of the promising cathodes for sodium‐ion batteries owing to its good thermodynamic stability and high working voltage. However, Na3.4Mn1.2Ti0.8(PO4)3 is faced with low electronic conductivity, poor cycling stability and complex phase transition caused by multi‐electron transfers, which limits its practical application. Herein, an anion‐regulated strategy is proposed to optimize the Mn‐rich Na3.4Mn1.2Ti0.8(PO4)3 phosphate cathode. After introducing F anions into the lattice, the rate performance is improved from 60.5 to 72.8 mAh g−1 at 20 C. Ascribed to unique structure design, the reaction kinetics of Na3.4Mn1.2Ti0.8(PO4)3 are significantly improved, as demonstrated by cyclic voltammetry at varied scan rates and galvanostatic intermittent titration technique. The generated M‐F bond inhibits Jahn–Teller effect with an improved cycle stability (85.8 mAh g−1 after 1000 cycles at 5 C with 94.3% capacity retention). Interestingly, reaction mechanism of Na3.4Mn1.2Ti0.8(PO4)3 with the complex two‐phase and solid solution reactions changes to the whole solid solution reaction after fluorine substitution, and leads to a smaller volume change of 5.41% during reaction processes, which is verified by in situ X‐ray diffraction. This anion regulation strategy provides a new method for designing the high‐performance phosphate cathode materials of sodium‐ion batteries.

Electronic States Tailoring and Pinning Effect Boost High-Power Sodium-Ion Storage of Oriented Hollow P2-Type Cathode Materials

“…As a typical P2-NMO cathode, the interlayer gliding of transition metal layers occurs when Na + ions are extracted in the charging process, resulting in an unwanted phase transition of P2–O2 combined with rapid capacity decline. ,, Due to the rational design of the chemical component and microstructure with abundant exposed facets for efficient ion transport, the NMMO-0.08 cathode exhibits excellent rate performance and prolonged cyclic stability. Therefore, in situ XRD is conducted to further monitor the structural evolution upon Na + extraction/insertion.…”

“…Layered sodium transition metal oxides (Na x TMO 2 , 0 < x ≤ 1), as the main cathode materials of SIBs, represent high theoretical capacities, flexible crystal structures, scalable synthesis process, and abundantly available transition metals. − According to the coordination environment of Na + and different stacking of oxygen layers, P2-type cathodes are given more expectation for scale development originating from more open prismatic paths for Na + transport with promising rate capability. − Meanwhile, their high tolerances against moist air will definitely reduce the difficulty of the preparation and storage . For the Na 2/3 Ni 1/3 Mn 2/3 O 2 cathode, typically, all Na + (2/3) can be reversibly extracted with a high specific capacity (161 mAh g –1 ) based on the Ni 2+ /Ni 3+ /Ni 4+ redox reaction in the voltage window of 2–4.5 V. However, unsatisfied P2–O2 phase transition associated with abrupt change (about 23.2%) in the length of the c -axis and oxygen framework shift when charging to 4.2 V, remarkably reduces the diffusion coefficient of Na + , slows down the transport kinetics and breaks down the cycle stability. − Moreover, the adverse TM slab sliding combined with structural changes leads to increased internal stress and collapsed layered structure with severe cracks.…”

SAXS unveils porous anodes for potassium-ion batteries: dynamic evolution of pore structures in Fe@Fe2O3/PCNFs composite nanofibers

“…11 However, the larger radius of Na + /K + compared to Li + leads to severely slow reaction kinetics, fragmented electrode structures, and much poorer rate capability and cyclability. 12–14 Therefore, there is an urgent need to investigate suitable anode materials for potassium and sodium ion batteries. In addition, potassium ions 15 are more readily and reversibly embedded in graphite materials compared to sodium ions.…”

Ultracapacity Properties of the Refined Structure in Na-Rich Na3.4V2(PO4)3/C as Sodium-Ion Battery Cathodes by Tapping the Na-Vacancy Potential

“…Most importantly, sodium has similar physical and chemical properties to lithium, and the working principle of SIBs is very similar to that of LIBs. , Until now, three traditional cathode materials, polyanionic frameworks, layered transition metal oxides, and ferrocyanide compounds, still dominate in the field of SIBs. − The open framework structure of polyanions enables them to have high structural stability, which can meet the deintercalation mechanism of Na + , and is expected to achieve ultrastable sodium-ion batteries . In particular, NASICON-type Na 3 V 2 (PO 4 ) 3 can well adapt to the rapid diffusion of Na + . − The open structure of Na 3 V 2 (PO 4 ) 3 can effectively improve the reaction kinetics of Na + with a large radius and weight during electrochemical migration. , Moreover, the strong P–O bond in Na 3 V 2 (PO 4 ) 3 ensures the high stability of the structure during electrochemical processes. − Nevertheless, such materials still need to be further studied for practical applications.…”