Electrochemical Activity of Positive Electrode Material of P2-Nax[Mg0.33Mn0.67]O2 Sodium Ion Battery

Author: ZHANG Xiaojun¹, LI Jiale^1,2, QIU Wujie^2,3, YANG Miaosen¹, LIU Jianjun^2,3,4

Abstract

With the advantages of low cost and wide distribution of raw materials, sodium-ion batteries are considered to be the best alternative materials for lithium-ion battery cathode materials. In the P2-phase NaMnO2 with layered structure, binary solid solution of the transition metal layer can effectively improve the electrochemical performance of the electrode material. In this study, the structural model of Nax[Mg0.33Mn0.67]O2 with Mg ion solid solution was constructed by using the Coulombic model. The first-principles calculations revealed that discharge voltage of Nax[Mg0.33Mn0.67]O2 reached 3.0 V at a sodium ion content of less than 0.67. Electronic density of states and charge population analysis showed that the solid solution of Mg motivated the anionic electrochemical activity of lattice oxygen in the P2-phase Nax[Mg0.33Mn0.67]O2, which transformed the electrochemical reaction mechanism of the system from cationic and anionic synergic redox reaction to reversible anionic redox reaction. This transformation provides a novel method for the design of electrode materials for Na ion batteries, as well as a new approach for the optimization and exploration of other ion batteries.

Keywords： sodium ion battery ; electrochemical activity ; first principle ; alkali metal doping

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Berthelot et al.[10]found that in the layered oxide NaTMO2 containing a single transition metal (TM), sodium ions and vacancies are ordered in the pure sodium layer, resulting in many voltage platforms for this type of oxide during the discharge process. . This results in a rapid decay of specific capacity and a significant reduction in cycle performance, so the energy conversion efficiency of this type of oxide is low. Solid solution elements are introduced into the transition metal layer to form a mixed arrangement of binary or even multi-element transition metals. The electrode material contains a large number of disordered charges, which can effectively suppress the above voltage platform and improve energy conversion efficiency. Yabuuchi et al.[11]used Na2CO3, (MgCO3)4Mg(OH)2·5H2O and MnCO3 as raw materials. A solid-state reaction was carried out at 900°C for 12 hours to obtain a binary disordered P2 phase Na2/3[Mg1/3Mn2/3]O2 electrode material with Mg solid solution. They found that at a current density of 10 mA/g, the initial specific capacity of the prepared P2 phase Na2/3[Mg1/3Mn2/3]O2 cathode material was approximately 150 mAh/g[11]. Slightly lower than the specific capacity of Na2/3MnO2 (184 mAh/g). Bruce et al.[12]found that although there was an electrochemical reaction of lattice oxygen in the P2 phase Na2/3[Mg1/3Mn2/3]O2, no oxygen precipitation was observed. It shows that the introduction of Mg improves the cycle reversibility and reversible specific capacity of the material. However, during the charge and discharge process, the microscopic electrochemical reaction mechanism of the lattice oxygen in this system is still unclear, and the mechanism by which Mg solid solution improves the stability of the system is also unclear.

Therefore, this work takes P2 phase Nax[Mg0.33Mn0.67]O2 as the research object and adopts the first-principles calculation method of density functional theory (DFT). A systematic study on the electrochemical activity and structural stability of the discharge performance of Nax[Mg0.33Mn0.67]O2 cathode material with solid solution of Mg ions was conducted. In order to clarify the microscopic mechanism of electrode materials in electrochemical reactions at the microscale of electrons and atoms, it will provide a reference for the understanding of electrochemical processes and the design of new materials.

1 Calculation method

The calculations in this work are based on the plane wave basis software VASP package[13,14]of density functional theory. The additive plane wave method is used[15], and the exchange correlation functional is the generalized gradient approximation (GGA) in the form of Perdew-Burker-Ernzerhof[13,16]. The Hubbard parameter U is introduced to correct the d electrons of Mn, and the effective U value is 3.9 eV[17,18]. The cutoff energy of the lower plane wave is 600 eV. When ion relaxation is completed, the forces on all atoms are less than 0.1 eV·nm-1. When optimizing the crystal structure, a 3×3×1 (72 atoms) supercell structure is used, the lattice constant is 0.874 nm×0.874 nm×1.056 nm, and the k-point grid of the Brillouin zone is 3×3× 3[19]. The frozen phonon method was used to calculate the lattice vibration spectrum in the Phonopy software package. In order to avoid the influence of periodic boundary conditions, a 3×3×1 supercell structure was used to calculate the force constants and phonon spectra of P2 phase NaMnO2 and Na[Mg0.33Mn0.67]O2. The point charge Coulomb model is used to quickly calculate the ion occupation of the desodium structure, and the Na occupation configuration with the lowest Coulomb energy is selected for more accurate first-principles calculations[20]. The discharge voltage of the electrode material can be expressed as[20]:

$V=-\frac{G(\text{N}{{\text{a}}_{{{x}_{2}}}}\text{M}{{\text{O}}_{2}})-G(\text{N}{{\text{a}}_{{{x}_{1}}}}\text{M}{{\text{O}}_{2}})-({{x}_{2}}-{{x}_{1}})G(\text{Na})}{({{x}_{2}}-{{x}_{1}}){{e}^{-}}}$

Where G is the total energy of the corresponding system, and e- is the element charge[21].

2 Results and discussion

2.1 Microstructural characteristics and structural stability

The space group of the P2 phase NaMnO2 structure is R$\bar{3}m (Fig. 1)[22,23]. The spatial configuration of the Mg solid solution Na0.67[Mg0.33Mn0.67]O2 structure is similar to that of NaMnO2. Mg ions replace 1/3 of the Mn ions in the transition metal layer. The theoretical ion ratio of Mg to Mn is 1:2. Experimental characterization found that at this ratio, Mg ions in the Nax[Mg0.33Mn0.67]O2 structure only form disordered arrangements with Mn, retaining the order of the Na layer[24]. When the ion ratio Mg:Mn>1:2, Mg, Na, and Mn will form a disordered arrangement of cations. As shown in Figure 1(A), the stacking mode of lattice oxygen is ABBA..., Mg and Mn respectively occupy the octahedral sites between the oxygen AB layers, and Na occupies the triangular prism sites between the oxygen AA and BB layers[ 25,26]. As shown in Figure 1(B), there is a honeycomb arrangement of Mg and Mn in the transition metal layer[27], which is similar to the arrangement between Li and Mn in lithium-rich compounds[28]. The [MgO6] octahedron is arranged with 6 [MnO6] octahedrons sharing edges[29,30]. In the alkali metal layer of the Na0.67[Mg0.33Mn0.67]O2 structure, there are two lattice sites for sodium ions. One is arranged with the upper and lower layers of [MgO6] or [MnO6] octahedrons sharing edges. The other is coplanarly arranged with upper and lower layers of [MgO6] or [MnO6] octahedrons.

Fig. 1   Schematic diagram of P2-Na2/3[Mg1/3Mn2/3]O2

In structures with different sodium ion contents, sodium ions are affected by the Coulomb interaction between Mn and Mg in the transition metal layer and Na ions in the alkali metal layer, showing two different occupying modes. Therefore, this work first uses the Coulomb model to quickly screen out the P2 phase Na0.67[Mg0.33Mn0.67]O2 configuration with the lowest Coulomb energy. In order to verify the rationality, we calculated and simulated the XRD patterns of these screened configurations and compared them with the measured results[11]. The results are shown in Figure 2. The calculated (016) and (110) are slightly shifted to the right compared with the experimental characterization, which is mainly due to the existence of amorphous and defective structures in some crystal planes of the experimentally prepared materials. The structure of the computational model is a perfect crystal structure, so there is a certain deviation between the XRD broadening and peak intensity of the computational simulation and the experimental results. In addition, there is an arrangement of Na ions in these two crystal planes, and the insertion and detachment of Na ions is another possible reason for the shift of the corresponding peak positions. After considering the above effects, the peak shape and intensity of the simulated XRD are consistent with the experimental results, and the constructed model can reproduce the microstructural information in the experiment, indicating that the theoretically screened structure is relatively accurate and reliable[31,32].

Fig. 2   Comparison of calculated and experimental XRD patterns of Na0.67[Mg0.33Mn0.67]O2

In order to study the effect of Mg solid solution on structural stability, we used first principles combined with the "frozen phonon method" to calculate the lattice vibration spectra of P2-NaMnO2 and P2-Na[Mg0.33Mn0.67]O2. As shown in Figure 3, the possessor wave has no imaginary frequency in the entire Brillouin zone, indicating that P2-Na[Mg0.33Mn0.67]O2 has dynamic stability. By comparing the phonon spectra of the two materials, it was found that Mg doping did not significantly change the vibration frequency range and had little impact on the lattice vibration. The Mg-doped structure also showed good dynamic stability. In addition, Bruce et al. successfully prepared P2 phase Na[Mg0.33Mn0.67]O2 with Mg solid solution, which further demonstrated that the material has additional thermodynamic stability. Therefore, it is not difficult to see that P2-Na[Mg0.33Mn0.67]O2 has good structural stability.

Fig. 3   Phonon dispersion curves of (A) NaMnO2 and (B) Na0.67[Mg0.33Mn0.67]O2

2.2 Analysis of electrochemical properties of P2 phase Nax[Mg1/3Mn2/3]O2

In order to study the effect of Mg doping on the electrochemical properties of materials, we calculated the discharge voltage of Mg solid solution structure P2-Nax[Mg0.33Mn0.67]O2 (Figure 4). The concentration range of Na ions is determined experimentally, that is, 0.11≤x≤0.66[11].Figure 4(A) shows the structural changes during the discharge process, and its corresponding voltage (Figure 4(B)) mainly includes three platforms: 3.4, 2.9 and 2.1 V. The predicted theoretical capacity is 152 mAh/g, which is basically consistent with the experimental results[11]. The discharge voltage curve calculated from the first principles is slightly higher than the actual measured result. The main reason is that the first principles calculation ignores the influence of experimental measurement conditions, such as electrolyte, lithium ion conductivity experimental measurement temperature, etc. Our previous research showed[33]that although the calculated discharge voltage curve is higher than the experimentally measured curve, the overall change trend is consistent. Therefore, it can be considered that during the entire discharge process, the voltage of Nax[Mg0.33Mn0.67]O2 is consistent with the experimental results[12,20]. When x<66%, Nax[Mg0.33Mn0.67]O2 has a high voltage of about 3.0 V, and there is no obvious additional voltage platform, indicating that the substitution of Mg2+ for Mn3+ has the effect of inhibiting sodium ion rearrangement and structural phase change. Previous charge and discharge studies on NaMnO2 and other systems have found that the orderly arrangement of transition metals is usually accompanied by more voltage platforms.

Fig. 4   (A) DFT-calculated structural changes and (B) discharge voltage curve of P2-Nax[Mg0.33Mn0.67]O2 during discharge

Under ideal circumstances, the valence states of Mg and Mn in Na2/3[Mg0.33Mn0.67]O2 are +2 and +4 respectively, and cannot continue to be oxidized to higher valence states. Therefore, there is no cation electrochemical activity in the system, and the charge and discharge process of the material is an anion electrochemical reaction. In Na0.67MnO2, the initial valence state of Mn ions is +3.33. During the charging process, Mn ions can transfer 0.67 electrons outward to reach a stable valence of +4. At this time, all Na+ has been released, and the lattice oxygen has never participated in the electrochemical reaction[34]. Therefore, the charge and discharge process of Na0.67MnO2 appears as a cationic electrochemical reaction. Many studies have shown that when the number of electrons lost by lattice oxygen is less than 0.33, the anionic electrochemical reaction has good reversibility[11-12,28]. The excessive oxidation of oxygen anions (the number of electrons lost is greater than 0.33) causes the electron configuration of oxygen to deviate from the stable eight-coupler rule, resulting in an irreversible transformation reaction and the formation of an O-O bond. It may even lead to oxygen evolution and irreversible charge and discharge of the electrode structure[27,35]. In Na0.67[Mg0.33Mn0.67]O2, if the limit state of charge loss is considered. That is, when Na ions are completely detached to form the Na0[Mg0.33Mn0.67]O2 structure, Mg and Mn always maintain +2 and +4 valences. The O anion is oxidized to -1.67 valence, losing 0.33 electrons, which is lower than the limit of irreversible anion electrochemical reaction. Therefore, in the entire charging reaction of Na0.67[Mg0.33Mn0.67]O2, the lattice oxygen does not need to be spatially reorganized, and the electrochemical reaction is reversible. The introduction of Mg2+ not only maintains the reversible specific capacity, but also increases the energy density of the material by increasing the discharge voltage.

In order to prove the electrochemical activity of oxygen in the Nax[Mg0.33Mn0.67]O2 material during the discharge process, we calculated the electronic density of states (Figure 5) for the initial and final discharge structures of the material. It was found that during the discharge process, Na ions were gradually embedded, the total number of electrons in the system increased, and the Fermi level moved to a higher energy level. The number of holes in the O2p orbit gradually decreases, indicating that the electrons entering the system are transferred to the empty orbits of lattice oxygen, and the lattice oxygen is reduced. During the discharge process of the electrode material, the lattice oxygen participates in the electrochemical reaction of anions. At this time, there is almost no change in the Mn-d orbital electrons, and there is no charge transfer, that is, the valence state of Mn does not change during the discharge process, proving that Mn is not electrochemically active[12,36]. However, during the discharge process of P2-NaxMnO2, electrons continue to fill the high-energy Mn and O empty orbitals, indicating that both Mn and O are electrochemically active and are a typical electrochemical reaction in which anions and cations cooperate.

Fig. 5   Electronic density of states of (A) P2-Nax[Mg0.33Mn0.67]O2 and (B) P2-NaxMnO2 under different Na ion contents during discharge

PDOS: projected density of states

A consistent conclusion can be drawn through charge population analysis (Figure 6). During the discharge process of Nax[Mg0.33Mn0.67]O2, the charge amount of Mn ions basically does not change, so it does not participate in the electrochemical reaction; in the process of increasing the Na content from 0.11 to 0.66, the O ions obtained about 0.2e- . Significant charge filling occurred, showing anionic electrochemical activity[37]. Through the charge population analysis of P2-NaxMnO2, it was found that as the Na content increases, Mn and O jointly participate in the electrochemical reaction. This result is consistent with the analysis of electronic density of states. It is proved that the solid solution of Mg changes the electrochemical reaction mechanism of the system from anion and cation cooperative electrochemical reaction to a reversible anion electrochemical reaction, and this process does not affect the charge and discharge reversibility of the material.

Fig. 6   Charge analysis of (A) Nax[Mg0.33Mn0.67]O2 and (B) P2-NaxMnO2 under different sodium ion content

3 Conclusion

This study used first-principles calculations to systematically study the microstructural characteristics, kinetic stability and electrochemical activity of the Mg2+ solid solution P2 phase Nax[Mg1/3Mn2/3]O2. The introduction of Mg2+ changes the electrochemical reaction type of the material from the anionic and cationic cooperative electrochemical reaction of NaxMnO2 to the reversible anionic electrochemical reaction of Nax[Mg0.33Mn0.67]O2. When the O anion in P2-Nax[Mg0.33Mn0.67]O2 participates in the electrochemical reaction, the charge gain and loss range is less than 0.33, which has good reversibility. The introduction of Mg2+ not only increases the discharge voltage of the material, but also maintains the reversible specific capacity of the material, and ultimately increases the energy density of the material.

In sodium ion electrode materials, introducing alkaline earth metals into the transition metal layer for cationic solid solution is a new material performance optimization strategy. Its basic mechanism is to trigger the electrochemical activity of anions by sacrificing the electrochemical activity of cations, changing the electrochemical reaction mechanism of the material, increasing the discharge voltage, and ultimately optimizing the energy density of the material. This strategy not only provides a new method for the design of electrode materials for sodium-ion batteries, but also provides new ideas for the optimization and exploration of other ion batteries.

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