ABSTRACT
The structural failure of Na2Mn[Fe(CN)6] could not be alleviated with traditional modification strategies through the adjustable composition property of Prussian blue analogues (PBAs), considering that the accumulation and release of stress derived from the MnN6 octahedrons are unilaterally restrained. Herein, a novel application of adjustable composition property, through constructing a coordination competition relationship between chelators and [Fe(CN)6]4- to directionally tune the enrichment of elements, is proposed to restrain structural degradation and induce unconventional energy coupling phenomenon. The non-uniform distribution of elements at the M1 site of PBAs (NFM-PB) is manipulated by the sequentially precipitated Ni, Fe, and Mn according to the Irving-William order. Electrochemically active Fe is operated to accompany Mn, and zero-strain Ni is modulated to enrich at the surface, synergistically mitigating with the enrichment and release of stress and then significantly improving the structural stability. Furthermore, unconventional energy coupling effect, a fusion of the electrochemical behavior between FeLS and MnHS, is triggered by the confined element distribution, leading to the enhanced electrochemical stability and anti-polarization ability. Consequently, the NFM-PB demonstrates superior rate performance and cycling stability. These findings further exploit potentialities of the adjustable composition property and provide new insights into the component design engineering for advanced PBAs.
ABSTRACT
Layered sodium transition-metal oxides generally encounter severe capacity decay and inferior rate performance during cycling, especially at a high state of charge. Herein, defect concentration is rationally modulated to explore the impact on electrochemical behavior in NaNi1/3Fe1/3Mn1/3O2 layered oxides. Bulk vacancies are increased through annealing in an oxygen-rich atmosphere, demonstrated by electron paramagnetic resonance measurement. It is found that the cathode with enriched oxygen vacancies exhibits significantly enhanced reversibility of redox reactions with a higher initial Coulombic efficiency of 90.0%. Furthermore, the reduced volume variations during the initial charge/discharge process are also confirmed by in situ X-ray diffraction. As a result, the oxygen-vacancy-rich cathode shows great cycling stability and superior rate performances. Also, full cells deliver a specific capacity of approximately 145.2 mAh g-1 at 0.5 C, with a high capacity retention of 78.3% after 100 cycles. This work presents a viable strategy for designing Na+ intercalated cathodes with a high-energy density.
ABSTRACT
Na4Fe3(PO4)2(P2O7) (NFPP) is currently drawing increased attention as a sodium-ion batteries (SIBs) cathode due to the cost-effective and NASICON-type structure features. Owing to the sluggish electron and Na+ conductivities, however, its real implementation is impeded by the grievous capacity decay and inferior rate capability. Herein, multivalent cation substituted microporous Na3.9Fe2.9Al0.1(PO4)2(P2O7) (NFAPP) with wide operation-temperature is elaborately designed through regulating structure/interface coupled electron/ion transport. Greatly, the derived Na vacancy and charge rearrangement induced by trivalent Al3+ substitution lower the ions diffusion barriers, thereby endowing faster electron transport and Na+ mobility. More importantly, the existing Al-O-P bonds strengthen the local environment and alleviate the volume vibration during (de)sodiation, enabling highly reversible valence variation and structural evolution. As a result, remarkable cyclability (over 10,000 loops), ultrafast rate capability (200 C), and exceptional all-climate stability (-40-60 °C) in half/full cells are demonstrated. Given this, the rational work might provide an actionable strategy to promote the electrochemical property of NFPP, thus unveiling the great application prospect of sodium iron mixed phosphate materials.