Approaching the Downsizing Limit of Maricite NaFePO₄ toward High‐Performance Cathode for Sodium‐Ion Batteries

Abstract

Maricite NaFePO 4 nanodots with minimized sizes (≈1.6 nm) uniformly embedded in porous N-doped carbon nanofibers (designated as NaFePO 4 @C) are first prepared by electrospinning for maximized Na-storage performance. The obtained flexible NaFePO 4 @C fiber membrane adherent on aluminum foil is directly used as binder-free cathode for sodium-ion batteries, revealing that the ultrasmall nanosize effect as well as a high-potential desodiation process can transform the generally perceived electrochemically inactive maricite NaFePO 4 into a highly active amorphous phase; meanwhile, remarkable electrochemical performance in terms of high reversible capacity (145 mA h g −1 at 0.2 C), high rate capability (61 mA h g −1 at 50 C), and unprecedentedly high cyclic stability (≈89% capacity retention over 6300 cycles) is achieved. Furthermore, the soft package Na-ion full battery constructed by the NaFePO 4 @C nanofibers cathode and the pure carbon nanofibers anode displays a promising energy density of 168.1 Wh kg −1 and a notable capacity retention of 87% after 200 cycles. The distinctive 3D network structure of very fine NaFePO 4 nanoparticles homogeneously encapsulated in interconnected porous N-doped carbon nanofibers, can effectively improve the active materials' utilization rate, facilitate the electrons/Na + ions transport, and strengthen the electrode stability upon prolonged cycling, leading to the fascinating Na-storage performance.

Hierarchical Engineering of Porous P2‐Na2/3Ni1/3Mn2/3O2 Nanofibers Assembled by Nanoparticles Enables Superior Sodium‐Ion Storage Cathodes

“…Accordingly, the Na + diffusion coefficients (D Na ) are calculated to be ≈10 −11 cm 2 s −1 (Figure 4b), which are higher than those of the previously developed Na 2/3 Ni 1/3 Mn 2/3 O 2 -based cathode materials for SIBs (mostly at magnitudes of ≈10 −12 -10 −14 cm 2 s −1 ). The D Na can also be determined from the CV study according to the Randles-Sevcik formula, that is, i p = 2.69 × 10 5 n 3/2 AD Na 1/2 C Na v 1/2 , [23,50] where i p (A) is the peak current, v (V s −1 ) is the scan rate, C Na is the Na ion concentration in the electrode (≈2.7 × 10 −2 mol cm −3 , calculation details are given in the Supporting Information), A (cm 2 ) is the contact area between the electrode and electrolyte, and n is the transferred electron number per molecule (n = 2/3 in this system). The D Na can also be determined from the CV study according to the Randles-Sevcik formula, that is, i p = 2.69 × 10 5 n 3/2 AD Na 1/2 C Na v 1/2 , [23,50] where i p (A) is the peak current, v (V s −1 ) is the scan rate, C Na is the Na ion concentration in the electrode (≈2.7 × 10 −2 mol cm −3 , calculation details are given in the Supporting Information), A (cm 2 ) is the contact area between the electrode and electrolyte, and n is the transferred electron number per molecule (n = 2/3 in this system).…”

“…[28,36,46] Figure 4c presents the CV curves collected at various sweep rates ranging from 0.1 to 1.0 mV s −1 ; with the scan rate increasing, the redox currents increase drastically, while the potential intervals between the oxidation and reduction peaks enlarge slightly, reflecting the minor polarization of the nanofiber cathode. The D Na can also be determined from the CV study according to the Randles-Sevcik formula, that is, i p = 2.69 × 10 5 n 3/2 AD Na 1/2 C Na v 1/2 , [23,50] where i p (A) is the peak current, v (V s −1 ) is the scan rate, C Na is the Na ion concentration in the electrode (≈2.7 × 10 −2 mol cm −3 , calculation details are given in the Supporting Information), A (cm 2 ) is the contact area between the electrode and electrolyte, and n is the transferred electron number per molecule (n = 2/3 in this system). Figure 4d plots the fitting curves between i p and v 1/2 of the four main redox peaks; as calculated, the D Na values for the O1, O2, R1, and R2 peaks are 3.40 × 10 −11 , 8.19 × 10 −11 , 4.96 × 10 −11 , and 2.65 × 10 −11 cm 2 s −1 , respectively, agreeing well with the GITT results.…”

“…[44] Recently, Chou's team tested the cycling performance of P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 in the voltage range of 2.0-4.0 V; the electrode maintained 89 mA h g −1 at 0.1 C and exhibited a significantly enhanced capacity retention of 71.2% even after 1200 cycles at 10 C. [36] Though these are very encouraging results, it should be noted that metal doping and surface modification cannot preclude the phase transition intrinsically, and the cyclic stability is still limited; additionally, narrowing the electrochemical window would sacrifice a large amount of capacity. [49,50] Accordingly, nanoengineering may provide an opportunity to tackle the aforementioned limitations facing the P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 cathode. [37] Nanotechnology has been extensively applied to boost electrode performance by virtue of the increased reactivity, the shortened ionic diffusion paths, and the strengthened ability to endure deformation stress.…”

“…[37] Nanotechnology has been extensively applied to boost electrode performance by virtue of the increased reactivity, the shortened ionic diffusion paths, and the strengthened ability to endure deformation stress. [49,50] Accordingly, nanoengineering may provide an opportunity to tackle the aforementioned limitations facing the P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 cathode. However, to the best of our knowledge, nanostructured Na 2/3 Ni 1/3 Mn 2/3 O 2 has scarcely been achieved up to date.…”

The Cathode Choice for Commercialization of Sodium‐Ion Batteries: Layered Transition Metal Oxides versus Prussian Blue Analogs

“…[2] The reversible capacity of ≈120 mAh g −1 , while LiCrO 2 is electrochemically inactive. [11] Different cathode materials, including transition metal (M) oxides (Na x MO 2 , x ≤ 1), [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] hexacyanoferrates (HCF) or Prussian blue and its analogs (PBAs), [27][28][29][30][31][32] polyanionic compounds, [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47] and organic compounds [48][49][50][51][52][53][54][55][56][57] have been widely studied for SIBs. The substantial growth of exploration on full cell systems, which serve as a bridge between laboratory studies and practical applicatio...…”