Insights into the Enhanced Cycle and Rate Performances of the F‐Substituted P2‐Type Oxide Cathodes for Sodium‐Ion Batteries

Abstract

A series of F‐substituted Na2/3Ni1/3Mn2/3O2−xFx (x = 0, 0.03, 0.05, 0.07) cathode materials have been synthesized and characterized by solid‐state 19F and 23Na NMR, X‐ray photoelectron spectroscopy, and neutron diffraction. The underlying charge compensation mechanism is systematically unraveled by X‐ray absorption spectroscopy and electron energy loss spectroscopy (EELS) techniques, revealing partial reduction from Mn4+ to Mn3+ upon F‐substitution. It is revealed that not only Ni but also Mn participates in the redox reaction process, which is confirmed for the first time by EELS techniques, contributing to an increase in discharge specific capacity. The detailed structural transformations are also revealed by operando X‐ray diffraction experiments during the intercalation and deintercalation process of Na+, demonstrating that the biphasic reaction is obviously suppressed in the low voltage region via F‐substitution. Hence, the optimized sample with 0.05 mol f.u.−1 fluorine substitution delivers an ultrahigh specific capacity of 61 mAh g−1 at 10 C after 2000 cycles at 30 °C, an extraordinary cycling stability with a capacity retention of 75.6% after 2000 cycles at 10 C and 55 °C, an outstanding full battery performance with 89.5% capacity retention after 300 cycles at 1 C. This research provides a crucial understanding of the influence of F‐substitution on the crystal structure of the P2‐type materials and opens a new avenue for sodium‐ion batteries.

Restraining Oxygen Loss and Boosting Reversible Oxygen Redox in a P2-Type Oxide Cathode by Trace Anion Substitution

“…into the TM or Na sites − and microstructure tailoring have been adopted for modifying P2-Na x TMO 2 . However, to our knowledge, anionic substitution such as O-site F-substitution has rarely been reported for ameliorating the electrochemical performance of P2-Na x TMO 2 to date. , For instance, F-substituted P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 – x F x ( x = 0–0.07) cathodes were designed recently by Dai’s group and they found that F-substitution enables partial reduction of Mn 4+ to Mn 3+ in the pristine state, which hampers the Ni 2+ /Mn 4+ cation ordering, and thus the biphasic reaction during the low voltage region is clearly suppressed. As a consequence, the F-substituted P2-Na 2/3 Ni 1/3 Mn 2/3 O 1.95 F 0.05 shows much improved cycle and rate performance as compared to P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 …”

Three‐Layer Structured SnO2@C@TiO2 Hollow Spheres for High‐Performance Sodium Storage

“…The above results signify that the Na storage process in SnO 2 @C@TiO 2 and SnO 2 @C is mainly capacitive controlled. [ 33,34 ]…”

Regulating Surface and Local Chemistry in High Na-content P2-type Cathode to Achieve Ultrahigh Power and Low Temperature Sodium Storage

“…3a). For P2-NaMNNb, three pairs of redox peaks were found at 3.12/3.2, 3.28/3.34 and 3.56/3.64 V, respectively corresponding to the Ni 2+ /Ni 3+ /Ni 4+ redox couples respectively [14][15][16][17] . It is noted that, Nb doping can efficiently reduce the polarization voltage (ΔV) between adjacent anodic and cathodic peaks is reduced from 97 to 61 mV, which agrees well with the charge/discharge profiles of the P2-NaMNNb and P2-NaMN (Supplementary Fig.…”

“…Among the layered structure (mainly P2, O3, according to the count of edge-sharing TMO 6 octahedra with the oxygen piling in ABBA or ABCABC stacking 12 as displayed in Supplementary Fig. 1), P2-Na 2/3 Mn 2/3 Ni 1/3 O 2 , with classic composition has been extensively investigated, which displays a striking specific capacity of 160 mAh g -1 within 2.0-4.5 V based on Ni 2+/3+/4+ redox couples [13][14][15][16][17] . Nevertheless, such a structure suffers from irreversible structural changes or phase transitions (e.g.…”