Unblocking Oxygen Charge Compensation for Stabilized High‐Voltage Structure in P2‐Type Sodium‐Ion Cathode

Abstract Layered transition‐metal (TM) oxides are ideal hosts for Li+ charge carriers largely due to the occurrence of oxygen charge compensation that stabilizes the layered structure at high voltage. Hence, enabling charge compensation in sodium layered oxides is a fascinating task for extending the cycle life of sodium‐ion batteries. Herein a Ti/Mg co‐doping strategy for a model P2‐Na2/3Ni1/3Mn2/3O2 cathode material is put forward to activate charge compensation through highly hybridized O2 p —TM3 d covalent bonds. In this way, the interlayer O—O electrostatic repulsion is weakened upon deeply charging, which strongly affects the systematic total energy that transforms the striking P2–O2 interlayer contraction into a moderate solid‐solution‐type evolution. Accordingly, the cycling stability of the codoped cathode material is improved superiorly over the pristine sample. This study starts a perspective way of optimizing the sodium layered cathodes by rational structural design coupling electrochemical reactions, which can be extended to widespread battery researches.

All the cells were fabricated in an Ar-filled glove box. The Na metal and glass fiber were adopted as anodes and separators, respectively. NaClO 4 in propylene carbonate (PC)/ ethylene carbonate (EC) (1:1 in volume) was employed as electrolyte. The galvanostatic charge/discharge tests was performed on the NEWARE battery system. For the long cyclic stability evaluation, the cells were firstly activated at constant current density of 0.1 C (1 C =170 mA g -1 ) in the initial three cycles, and evaluated at current density of 1 C in the following cycles.
Collection and analysis of in situ synchrotron XRD. The in situ synchrotron XRD data were collected at 11-ID-C beamline at the APS of ANL with the X-ray wavelength of 0.1173 Å. The coin cells for the in situ XRD measurements were specially designed with holes on both sides of cases sealed with Kapton films as X-ray transparent windows. The fabrication processes of the in situ cells were similar to those used in the electrochemical testing. In a typical in situ data collection, transmission geometry was applied with a Perkin-Elmer detector to record twodimensional (2D) diffraction patterns. A standard CeO 2 powder sample was adopted to calibrate sample-to-detector distance, detector tilt angles and the instrumental resolution function. The Fit2D software was used to integrate and calibrate the collected 2D patterns, and the lattice parameters were extracted from the integrated XRD patterns using Fullprof software.
In situ PDF measurements. The in situ synchrotron PDF measurements were carried out using 11-ID-C beamline at APS of ANL (~105.7 keV, λ = 0.1173 Å). The electrodes were prepared by mixing P-or D-NNM cathode materials with super-P carbon and PVDF at a weight ratio of 6:2:2, and compressed into pellets. The electrode pellets were then fabricated into a specially designed cell suitable for in situ PDF measurements (the AMPIX cell) with a glass fiber as separator, Na metal as anode and NaClO 4 in PC/EC (1:1 in volume) as electrolyte 4 . The collection of raw scattering data was similar to that of the in situ XRD, except that the sample-todetector distance is closer for a high value of momentum transfer (Q max ~ 22 Å). The collected 2D scattering images were reduced to one-dimensional data using Fit2D software. The resulted one-dimensional data was corrected with PDFgetX3 software for background and Campton scatterings, and to compute the G(r) functions by Fourier transform. The PDF refinements were carried out using PDFgui software.
First-principles calculations. First-principles density functional theory (DFT) calculations reported in this study were conducted with the Vienna Ab-initio Simulation Package (VASP) (5)(6)(7)(8) with the projector augmented wave (PAW) potentials 9 and the Perdew-Becke-Ernzerhof (PBE) exchange-correlation 10 . A plane wave basis with a cutoff energy of 520 eV and -centered k-meshes with a density of 8000 k-points per reciprocal atom were used for all calculations. For the structural model, binary Na-Ni 1/4 Mg 1/12 Mn 7/12 Ti 1/12 O 2 ground-state convex hulls were constructed using the structures with the lowest energy for each composition during the desodiation process of Na 2/3-x Ni 1/4 Mg 1/12 Mn 7/12 Ti 1/12 O 2 (0 < x < 2/3). Then, the stable phases were located by using the convex hull, that is, the set of compounds that have an energy lower than that of any other compound or linear combination of compounds at that composition during the desodiation process. All calculations were spin-polarized, with all transition metal atoms initialized in a high-spin configuration and relaxed to self-consistency. The DFT + U method was used to treat the localized 3d electrons of Ni and Mn with a U of 6.4 and 3.8 11 , obtained by fitting it to experimental and calculated formation enthalpies in a previous study 12 .
Voltage profile calculations. The average sodiation/desodiation voltage (relative to Na/Na + ) can be computed using the negative of the reaction free energy per Na added/removed, as shown in where F is the Faraday constant, ∆N Na is the amount of Na added/removed and ∆G f is the (molar) change in free energy of the reaction 13 . Considering a two-phase reaction between Na x MO and Na y MO : Na x MO + (y -x)Na ➝ Na y MO, ∆G f can be approximated by the total internal energies from DFT calculations neglecting the entropic contributions (0 K), where E(Na x MO) and E(Na y MO) are the DFT energies at the respective compositions. The        Supplementary Tables   Table S1. The unit cell parameters of P-NNM extracted from XRD Rietveld refinement.