Suppressing the P2‐O2 phase transition and Na+/vacancy ordering in Na0.67Ni0.33Mn0.67O2 by a delicate multicomponent modulation strategy

P2‐type Na0.67Ni0.33Mn0.67O2 is a promising cathode for sodium‐ion batteries with features of high specific capacity and air resistance, whereas its cycling stability and rate performance are dissatisfactory suffering from the disastrous P2‐O2 phase transition and Na+/vacancy ordering during sodium‐ion de/intercalation, which makes it an obstruction for future practical applications. Herein, a delicate multicomponent modulation strategy is proposed to tackle these two issues simultaneously, in which Li+ and Ti4+ are introduced to replace the Ni2+ and Mn4+, respectively, whereas the Na+ content is also designed according to the principle of charge balance. Consequently, the designed cathode (Na0.72Ni0.28Li0.05Mn0.57Ti0.10O2) can deliver an enchanting cycling stability of 80% at 1 C after 200 cycles along with a considerable rate performance of 82.7 mAh g−1 at 5 C. In situ X‐ray diffraction measurement demonstrates the destructive P2‐O2 phase transition is suppressed and converted into a P2‐Z phase transition with superior reversibility as well as smooth charge/discharge curves with better Na+/vacancy disordering. In addition, the full cell matched with hard carbon anode delivers an excellent energy density of 263.4 Wh kg−1 at 37.3 W kg−1, exhibiting great practicality. Our work presents a mean to rationally design the component of layered oxide cathode and achieve fabulous performance for sodium ion batteries.

5][6] Lithium-ion batteries (LIBs) have been extensively used in diverse scenarios by virtue of gorgeous energy/power density and prominent lifespan. 5,7,8However, the limitation of lithium resources and soaring cost hinder the application of LIBs, compelling the development of cost-effective alternative techniques. 91][12] Recently, sodium-ion batteries (SIBs) have received significant attention by similar working mechanisms with LIBs as well as the abundance of sodium resources.][15][16] Up to now, various kinds of materials have been intensively researched as possible cathodes for SIBs, including layered transition metal oxides (LTMOs), polyanionic compounds, Prussian blue analogs (PBAs) and organic cathodes. 17,180][21] Generally speaking, Na x TMO 2 can be classified into P2 (Na + occupy the prismatic sites while oxygen atoms stacking sequences are ABBA) and O3 (Na + occupy the octahedral sites, whereas oxygen atoms stacking sequences are ABCABC) structures according to the different coordination environment of sodium ions and the stacking sequences of oxygen atoms. 22,23Noteworthily, P2-type materials possess spacious and direct sodium diffusion paths, as well as better air stability, exhibiting better rate and cycling performance.2][33] Therefore, it is of great significance to develop the cathode material with suppressed phase transition and Na + /vacancy ordering, thus achieving better electrochemical performance and practicality.
Herein, we propose a high voltage stable material Na 0.72 Ni 0.28 Li 0.05 Mn 0.57 Ti 0.10 O 2 (denoted as NNLMTO) with delicate multicomponent modulation to address the phase transition and Na + /vacancy ordering problems simultaneously.Specifically, the Li + and Ti 4+ are introduced to replace the Ni 2+ and Mn 4+ , respectively, whereas the Na + content is also designed for the principle of charge balance.Although single Li + or Ti 4+ doping has been reported, but the Li + , Ti 4+ , and Na + comodulation have not been reported.According to previous reports, Li + substitution can help to build a high sodium content P2 phase, thus increasing the theoretical capacity, and due to the unequal charge between Li + and Ni 2+ , the Na + content is increased, which will reduce the electrostatic repulsion between adjacent O layer and stabilize the Na layer.2][43] Consequently, the NNLMTO cathode delivers an excellent capacity of 128 mAh g −1 with a high operating voltage of 3.64 V at 0.1 C (1 C = 173 mAh g −1 ), as well as an enchanting cycling stability of 80% after 200 cycles at 1 C. Besides, it can also deliver a considerable rate performance of 82.7 mAh g −1 at 5 C and recover 96% when the current density is reduced to 0.1 C. In situ X-ray diffraction (XRD) demonstrates that the destructive P2-O2 phase transition is suppressed and converts into a P2-Z phase transition with superior reversibility, which eminently improves the stability of the crystal structure.Smooth charge/discharge curves along with density functional theory (DFT) calculations reveal that better Na + /vacancy disordering is realized thanks to the delicate multicomponent modulation strategy.Commendably, beyond the gorgeous performance in half cell, the coin-type full cell based on NNLMTO cathode and hard carbon (HC) anode delivers an excellent energy density of 263.4 Wh kg −1 at 37.3 W kg −1 , and a pouchtype full cell can provide with a fabulous capacity retention of 81.2% after 100 cycles at 0.1 C, exhibiting great potential in practical application for SIBs.
The NNLMTO and control samples of NNMO, Na 0.67 Ni 0.33 Mn 0.52 Ti 0.15 O 2 (denoted as NNMTO), and Na 0.82 Ni 0.18 Li 0.15 Mn 0.67 O 2 (denoted as NNLMO) are synthesized by a typical sol-gel method (see Section 4 for detail).The XRD and corresponding Rietveld refinement (Figure 1A,B and Supporting Information: Figure S1) are carried out to investigate the detailed crystallographic structure of the samples.All the main peaks can be assigned to the hexagonal P2-type structure with the space group of P6 3 /mmc for the four samples, excepting a trace amount of NiO impurities in the NNMO and NNMTO, which can be attributed to the limited solubility of Ni 2+ in a layered structure, 44 indicating the good crystallinity for all samples.Noteworthily, two superstructure peaks located at 27.2°and 28.3°are mainly originated from the in-plane Na + / vacancy ordering, which is observed in the NNMO sample. 45,46However, these superlattice peaks disappear in the other three samples (NNLMTO, NNLMO, and NNMTO), indicating that the Na + /vacancy ordering can be effectively inhibited through Li + and Ti 4+ substitution.The XRD refinement results (Supporting Information: Tables S1 and S2) show that Li + and Ti 4+ are successfully incorporated into the layered structure.In addition, after the multicomponent modulation, the lattice parameter of an increased due to the larger ionic radius of Li + and Ti 4+ compared with Ni 2+ and Mn 4+ , whereas the lattice parameter of c decreased, which may be attributed to more Na + content in the sodium layer, which is helpful for the stability under deep desodiation state.Besides, the Na e /Na f ratio increases from 1.71 to 1.83 in NNLMTO after multicomponent modulation, indicating the Na + rearrangement occurs in the sodium layer and realizes better Na + /vacancy disordering. 47The detailed P2-type structure can be schematically described in Figure 1C, in which the transition metal-oxygen (MO 6 ) octahedrons share edges with each other to form the transition metal layer (TMO 2 ), whereas Na + occupies the prismatic sites between two adjacent TMO 2 .The morphologies of the synthesized samples are surveyed by field-emission scanning electron microcopy, as shown in Figure 1D and Supporting Information: which all the samples show typically plate-like morphology of P2-type structure as previously reported while the morphology of NNLMTO, NNLMO, and NNMTO samples become smoother due to the Li + and Ti 4+ substitution. 48The transmission electron microscope (TEM) image (Figure 1E) further validates the platelike feature of NNLMTO.The high-resolution TEM (HRTEM) is used to survey the microstructure of NNLMTO.As shown in Figure 1F, the HRTEM image exhibits a clear lattice fringe with an interlayer spacing of 2.27 Å that can be indexed to the (012) plane of the P2type structure, which confirms the well-crystalline P2type structure.The elemental mapping results (Figure 1G) illustrate the elements of Na, Ni, Mn, Ti, and O distributed uniformly throughout the whole particle of NNLMTO.In brief, the structural and morphological analyses illustrate that NNLMTO and control samples are successfully synthesized and better Na + /vacancy disordering is achieved through such a multicomponent modulation strategy.
X-ray photoelectron spectroscopy (XPS) is executed for the sake of investigating the chemical state of NNLMTO samples.The survey spectrum (Supporting Information: Figure S3A) shows that the elements of Na, Ni, Mn, Ti, and O exist, whereas the Li signal is hard to detect in the survey spectrum and Li 1s high-resolution spectrum (Supporting Information: Figure S3E), which is ascribed to the small photoionization cross section and low content of Li. 49 The Mn 2p high-resolution spectrum is shown in Supporting Information: Figure S3B, in which two peaks located at 653.9 and 642.4 eV attributed to the Mn 2p 1/2 and Mn2p 3/2 , respectively, can be observed, indicating that the chemical state of Mn is tetravalent. 50,51Ni 2p high-resolution spectrum (Supporting Information: Figure S3C) shows four peaks, in which the characteristic peaks at the binding energy of 872.2 and 854.7 eV can be attributed to the Ni 2p 1/2 and Ni 2p 3/2 , and their corresponding satellite peaks are located at 878.8 and 861.1 eV, respectively, indicating the chemical state of Ni is divalent. 50,52Supporting Information: Figure S3D shows the Ti 2p high-resolution spectrum and two peaks at 463.7 and 457.9 eV corresponding to the Ti 2p 1/2 and Ti 2p 3/2 , respectively, can be identified, indicating that Ti is tetravalent.
The comparison investigation of electrochemical performance on NNLMTO and NNMO is performed to demonstrate the effects caused by multicomponent modulation.Figure 2A shows the first three cycle charge/discharge curves of P2-NNMO at 0.1 C (1 C = 173 mA g −1 ), which delivers a specific capacity of 149.9 mAh g −1 along with three obvious plateaus.The apparent plateaus observed in charge/discharge curves can also be indicated in cyclic voltammetry (CV) curves (Figure 2C), in which the redox peaks below 4 V are attributed to the sequential oxidation of Ni 2+ -Ni 3+ -Ni 4+ and Na + /vacancy ordering in the sodium layer, and the anodic peak above 4 V is relative to the P2-O2 phase transition. 24,26,53However, the charge/discharge curves of NNLMTO (Figure 2B) become smoother and the plateaus under 4 V almost disappear along with the shortened plateau above 4 V, which is also confirmed by CV curves (Figure 2D), indicating that the Na + /vacancy ordering and P2-O2 phase transition is inhibited after multicomponent modulation. 28,32In addition, the NNLMTO cathode can also deliver a decent specific capacity of 128 mAh g −1 at 0.1 C, whereas the other two control samples of NNLMO and NNMTO can deliver a specific capacity of 107.9 and 123.9 mAh g −1 , respectively, and their charge/discharge curves are shown in Supporting Information: Figure S4.For the purpose of studying the rate performance, both P2-NNMO and P2-NNLMTO samples are subjected to current densities ranging from 0.1 C to 5 C for testing, as depicted in Figure 2E,F.It can be intuitively observed that NNLMTO exhibits better rate performance than that of NNMO, in which an excellent specific capacity of 82.7 mAh g −1 can be achieved at 5 C and recover 96% when the current density back to 0.1 C. The elevated rate performance may be attributed to the enhanced Na + diffusion rate due to the suppressed Na + /vacancy ordering and P2-O2 phase transition, which is universally recognized unfavorable to the Na + kinetic behaviors, 26,33,47 revealing that such a multicomponent modulation strategy can effectively promote Na + diffusion.The comparison of cyclic stability between NNMO and NNLMTO is also carried out in half cells to investigate the positive effect of structure stability induced by multicomponent modulation, as shown in Figure 2G,H.Specifically, the NNLMTO cathode can deliver excellent specific capacity retentions of 85.3% after 50 cycles at 0.1 C and 80% after 200 cycles at 1 C, which is overwhelming compared to the NNMO sample with only 38.2% retention at 0.1 C and 25.2% at 1 C, respectively, revealing the structural stability is enhanced after multicomponent modulation.Moreover, to assess long life-span cyclic stability, the NNLMTO cathode is tested at 5 C (Supporting Information: Figure S5), and such a multicomponent modulation cathode can deliver a gorgeous capacity retention of 83.9% after 500 cycles and 76.3% after 1000 cycles, demonstrating great potential in large-scale energy storage field.In addition, the comparison cyclic stability performance of NNMO, NNLMO, NNMTO, and NNLMTO is also provided in Supporting Information: Figure S6, in which the NNLMTO sample exhibits better performance than other samples, indicating that the multicomponent modulation strategy is superior to single Li + or Ti 4+ doping.
Moreover, the evolution of average operating potential between NNMO and NNLMTO is shown in Supporting Information: Figure S7, in which the NNLMTO can deliver an outstanding average operating potential of 3.64 V at 0.1 C with a gorgeous retention of 95.9% after 50 cycles, which is intuitively better than NNMO and superior to most cathode materials up to now.
To investigate the structural evolution during the electrochemical measure process, in situ XRD is carried out on the NNLMTO sample during the charge/discharge process.As per our and other previous reports, the P2-O2 phase transition with large volume change occurs in P2-NNMO when charged beyond 4.2 V, leading to severe structure destruction and dramatic capacity decay. 26,54owever, such a P2-O2 phase transition is successfully suppressed in the NNLMTO cathode characterized by the in-situ XRD measurement.Figure 3A,B reveals the evolution process of XRD patterns for NNLMTO during the first charge/discharge process.Upon the charging process, the (002) peak shifts to a lower angle gradually while the (100) peak moves to a higher angle, which reflects an expanded lattice parameter c and the concentration of ab plane during Na + deintercalation.This phenomenon can be attributed to the increasing electrostatic repulsive force between TMO 2 layers and the oxidation of transition metal ions when Na + content is decreased, respectively. 54,55In addition, a new peak located at about 16.3°appears when charged beyond 4.2 V, which could be attributed to the emergency of Z phase. 56,57Z phase is an intergrowth structure of P-type layers and O-type stacking faults, which is much closer to the P2 structure with fewer stacking faults compared with the O2 phase or OP4 phase. 54,56Moreover, during the discharge process, all the peaks move back to their original position gradually, indicating the great reversibility of such a P2-Z phase transition.Besides, the detailed lattice parameters evolution during the charge process is shown in Figure 3C, the lattice parameter c declines beyond 4.2 V due to the emergency of the Z phase.However, the calculated volume change during the whole charge process is only 2.87%, which is significantly lower than NNMO with a volume change about 23%, indicating the number O-type stacking faults is much less and contributing to a robust structure.In brief, through in situ XRD analysis of NNLMTO, the destructive P2-O2 phase transition is well suppressed and transformed into a P2-Z phase transition with less stacking faults and better reversibility, which is account for the enhanced structural stability and electrochemical performance after the delicate multicomponent modulation strategy.
To investigate the kinetic behavior related to the multicomponent modulation, galvanostatic intermittent titration technique (GITT) measurement is performed on NNLMTO and NNMO samples.The GITT curves of NNLMTO and NNMO are shown in Supporting Information: Figure S8A,B, respectively.It is generally accepted that the Na + diffusion coefficient can be calculated by the followed simplified Equation 1 under condition of the functional relationship between E and τ 1/2 is linear 58,59 : in which D Na + (cm 2 s −1 ) is the diffusion coefficient of Na + in cathode, V m and M B are the molar volume and molar weight of the cathode materials, m B and S are the mass loading of the active materials and the surface area of the electrode (1.13 cm −2 in this work), respectively. 60Supporting Information: Figure S8C demonstrates the parameters of a single step.The D Na + in NNLMTO sample can be calculated based on Equation 2 due to the good linear functional relationship between E and τ 1/2 , as shown in Supporting Information: Figure S8D.It can be intuitively observed that the D Na + in NNLMTO is much higher than that of NNMO in both charge and discharge process (Supporting Information: Figure S8E,F), indicating better Na + intercalation/deintercalation kinetic behavior of NNLMTO.In addition, electrochemical impedance spectroscopy (EIS) is also utilized to analysis the Na + diffusion kinetic behavior, an equivalent circuit is employed to describe the interface, as shown in Supporting Information: Figure S9.The Nyquist plots of NNLMTO and NNMO are shown in Supporting Information: Figure S10A,B, both samples show a semicircle in the high frequency region and an oblique line in the low frequency region, which can be attributed to the charge transfer resistance R ct and Warburg resistance Z w , respectively. 61R ct represents the Na + charge transfer resistance on the corresponding interface between the electrode and the electrolyte whereas the Z w depends on the diffusion of Na + in the bulk of the electrode.It can be observed that NNLMTO possesses a smaller R ct (302.9Ω) than that of NNMO (556.9Ω), indicating optimized interface between electrode and electrolyte after multicomponent modulation.Moreover, the Na + diffusion coefficient can be calculated through EIS according to the Equation 2 61,62 : in which D Na + represents the the diffusion coefficient of Na + in cathode, R is the molar gas constant, T is the Kelvin temperature, A is the surface area of the electrode, F is the Faraday's constant, σ ω is the slope of the functional relationship between Z re and ω −1/2 , and C is the molar concentration of Na + in the cathode material.
In addition, the Na + diffusion coefficients calculated by Equation 2for NNLMTO and NNMO (Supporting Information: Figure S10C,D) are 2.02904 × 10 −14 and 0.84841 × 10 −14 , respectively, which also indicates the boosted kinetic behavior after muticomponent modulation, and is consistent with the GITT results.
The charge compensation mechanism of NNLMTO during the charging and discharging process is investigated by ex situ XPS.In the Ni 2p spectrum (Figure 4A), the binding energy of Ni shifts to higher energy during the charging process, indicating the oxidation of Ni 2+ into Ni 3+ and Ni 4+ , whereas during the discharging process, the binding energy moves back to lower energy, indicating that the reduction of Ni.It is worthy of note that when discharged to 2.5 V, the Ni element cannot totally recover to the pristine state due to the fact that Ni 3+ cannot be fully reduced to Ni 2+ beyond 2.5 V, which is consistent with the previous report. 63Figure 4B,C exhibit the Mn 2p and Ti 2p spectrum, it can be observed that the binding energy of Mn and Ti are almost unchanged during the charging and discharging process, indicating the electrochemical inertness of Mn and Ti in the NNLMTO sample.Figure 4D shows the O 1s spectrum of NNMLTO, in the pristine state, the peak located at 529.9 eV can be attributed to the lattice oxygen of divalent O 2− , and the other peaks located at the higher binding energy are ascribed to the oxygenated deposits. 64hen charged at 4.35 V, a peak related to the peroxo species (O − ) is observed with a binding energy of 530.5 eV, indicating the oxidation of O 2− into O − . 52,65,66urthermore, during the discharge process, the peroxo species peak disappears, indicating that the O element participates in the charge compensation during the charge and discharge process, which is consistent with other works. 67In addition, the galvanostatic charge/ discharge curves of NNLMTO from the first to the fiftieth cycle at the current density of 0.1 C and corresponding dQ/dV curves are presented in Figure 4E,F.After 50 cycles, the plateau above 4 V in charge/discharge curves and corresponding redox peaks in dQ/dV curves maintains well, 68,69 while the plateau and corresponding peaks in the NNMO sample (Supporting Information: Figure S11) almost disappear, indicating the promoted reversibility of lattice oxygen redox reaction after multicomponent modulation.
DFT calculations are employed to provide theoretical support for such a delicate multicomponent modulation strategy.Figure 5A shows the comparison of charge/ discharge curves of NNMO and NNLMTO.According to previous reports, 33 there are two main major intermediate phases in NNMO, one is Na 0.5 Ni 0.33 Mn 0.67 O 2 at about 3.5 V corresponding to an ordering structure where one row of Na f and two rows of Na e alternate arrangement in the plane.The other is Na 0.33 Ni 0.33 Mn 0.67 O 2 at about 4 V corresponding to an ordering structure where either Na f or Na e arrange in rows within a single layer.In addition, the plateaus at 3.3 V and 3.7 V can be concluded attributed to the Na + /vacancy ordering reactions between NNMO to the intermediate phase Na 0.5 Ni 0.33 Mn 0.67 O 2 and Na 0.5 Ni 0.33 Mn 0.67 O 2 to Na 0.33 Ni 0.33 Mn 0.67 O 2 .It can be seen that the two plateaus disappear in the charge/discharge curves of NNLMTO, indicating suppressed Na + /vacancy ordering reactions can be achieved after such a multicomponent modulation strategy.Moreover, the formation energies of Na sites in NNMO and NNLMTO are also calculated to underlying the mechanism of suppressed Na + /vacancy ordering reactions, as shown in Figure 5B.The energy difference of NNMO is 5.96 eV, whereas in NNLMTO is 2.57 eV, indicating that multicomponent modulation strategy effectively deceases the energy difference between Na e and Na f , which can prohibit the formation of intermediate phase with ordering structure, thus suppressing the Na + /vacancy reactions and promote solid solution reaction over a wider voltage range during Na + de/intercalation. 31,33The electronic structure is also investigated by DFT calculations in terms of NNMO and NNLMTO, as shown in Figure 5C,D.It can be intuitively seen that no density of states can be observed at the Fermi level, indicating a semiconductor feature.However, NNLMTO shows an obvious density of states at the Fermi level for spin-up states and a band gap of 0.426 eV for spindown states, indicating its half-metallic oxide feature, which is superior in conductivity compared with semiconductor. 48,70Thus, the electronic conduction is also enhanced thanks to such a multicomponent modulation strategy.
It is universally believed that only the evaluation of electrochemical performance from half cells is not sufficient to demonstrate the practical application potential of the cathode materials.Therefore, the full cell based on the NNLMTO cathode and commercial HC anode (denoted as NNLMTO//HC) is assembled in both coin-type and pouch-type batteries, and the reaction mechanism of charge/discharge process is schematically demonstrated in Figure 6A.During the charge process, Na + extracts from the NNLMTO cathode and inserts into HC anode accompanied by the electron migrating into the anode in the external circuit, and vice versa.Figure 6B,C shows the electrochemical performance at different current densities and the corresponding charge/ discharge curves of NNLMTO//HC coin-type full cell, in which the NNLMTO//HC can deliver a decent specific capacity of 122.4,120.4,99.0, 83.9, and 70.7 mAh g −1 (based on the cathode mass) at the discharge rate of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C within the voltage range of 2.2-4.35V, respectively.Moreover, the corresponding Ragone plot calculated based on the total mass of cathode and anode active materials is presented in Figure 6D, where an outstanding energy density of 263.4 Wh kg −1 at the power density of 37.3 W kg −1 can be achieved.In addition, it can still deliver a decent energy density of 131.2 Wh kg −1 even at an immense power density of 640.3 W kg −1 .Meanwhile, such a full cell device can deliver an excellent output operating voltage of 3.56 V at the power density of 37.3 W kg −1 with an insignificant drop when the power density increases (Supporting Information: Figure S12).This well-designed full cell device is competitive when compared with previously reported sodium ion full cells (Supporting Information: Table S3).Cycling performance is also a crucial indicator for commercialization, thus the cycling stability evaluation of NNLMTO//HC coin type full cell device is also performed after the rate capability test, as shown in Figure 6E,F.Such a coin-type full cell device can deliver a decent capacity retention of 77.5% after 30 cycles at 0.1 C and 78.6% after 100 cycles at 1 C. To further demonstrate the potential in practical application, the NNLMTO//HC full cell is charged into 4.35 V and used to lighten on a circuit board with a "USTC" logo composed of 38 green light-emitting diodes (Supporting Information: Figure S13).What's more, a pouch-type battery with a single layer of NNLMTO cathode and HC anode is assembled to better illustrate the practical application potential, as schematically shown in Figure 6G.In addition Supporting Information: Figure S14 provides the digital image of the real pouchtype full cell with a capacity of 17.7 mAh. Figure 6H shows the galvanostatic charging/discharge curves of such a pouch-type battery, in which a specific capacity of 113.4 mAh g −1 based on the cathode mass can be achieved in the voltage range of 1.2-4.35V.It is worthy to note that the specific capacity of pouch-type full cell is lower than that of the coin-type full cell, this phenomenon may be attributed that the HC electrode in coin-type full cell has been pretreated via a precycling procedure in half cells, which can improve the initial Coulombic efficiency.In contrast, due to the limitation of assembling machines, the HC anode is not pretreated in pouchtype full cell.Moreover, as shown in Figure 6I, such a pouch-type full cell can deliver an excellent capacity retention of 81.2% after 100 cycles at 0.1 C. In general, the good results of the electrochemical evaluation of NNLMTO//HC full cell device illustrate its large potential in practical application in terms of such a multicomponent modulation layered oxide cathode.

| CONCLUSION
In summary, we propose a delicate multicomponent modulation strategy to address the P2-O2 phase transition and Na + /vacancy ordering simultaneously for NNMO.The well-designed NNLMTO cathode material exhibits excellent electrochemical performance with a high capacity of 128 mAh g −1 and an operating voltage of 3.64 V within the voltage of 2.5-4.35V in half cell configuration.In addition, it also displays a considerable cycling stability of 80% after 200 cycles at 1 C, as well as a brilliant rate performance of 82.7 mAh g −1 at 5 C. Through in situ XRD measurement, the multicomponent modulation cathode NNLMTO shows a P2-Z phase transition instead of a P2-O2 phase transition, which extremely enhances structure stability.Furthermore, the smooth charge/discharge curves combined with smaller energy differences between Na e and Na f calculated by DFT indicate better Na + /vacancy disordering is realized after multicomponent modulation.Moreover, inspired by the gorgeous performance in half cell, the full cell device based on NNLMTO cathode and commercial HC is assembled in both coin-type and pouch-type, in which the coin-type full cell can deliver an excellent energy density of 263.4 Wh kg −1 at 37.3 W kg −1 , whereas a pouchtype full cell exhibits a fabulous capacity retention of 81.2% after 100 cycles at 0.1 C, demonstrating significant practicality.Our work demonstrates the positive effect of multicomponent modulation in sodium-based layered oxide design and provides insights to develop highperformance cathodes for sodium-ion batteries.

| EXPERIMENTAL SECTION
Experimental details are provided in the Supporting Information.

F I G U R E 2
Electrochemical properties of half-cells: the first three cycles of charge/discharge curves of (A) Na 0.67 Ni 0.33 Mn 0.67 O 2 (NNMO) and (B) Na 0.72 Ni 0.28 Li 0.05 Mn 0.57 Ti 0.10 O 2 (NNLMTO) at 0.1 C; corresponding cyclic voltammetry curves of (C) NNMO and (D) NNLMTO.(E) The comparison of rate performances between NNMO and NNLMTO with the current density ranging from 0.1 C to 5 C and (F) corresponding charge/discharge curves at different rates.(G, H) Comparison of cyclic performances between NNMO and NNLMTO at the current density of 0.1 C and 1 C.

F
I G U R E 3 (A) In situ X-ray diffraction patterns of the Na 0.72 Ni 0.28 Li 0.05 Mn 0.57 Ti 0.10 O 2 (NNLMTO) cathode collected during the first charge/discharge process with a current density of 0.2 C. (B) Corresponding intensity contour maps concerning the evolution of the main characteristic peaks of (002) and (100).(C) Corresponding lattice parameters evolution during the charge process.

F
I G U R E 4 Charge compensation mechanism analysis: (A-D) Ex situ X-ray photoelectron spectroscopy analysis of Ni 2p, Mn 2p, Ti 2p, and O 1s, respectively.(E) Galvanostatic charge/discharge curves of Na 0.72 Ni 0.28 Li 0.05 Mn 0.57 Ti 0.10 O 2 (NNLMTO) from the first to 50th cycle at the current density of 0.1 C and (F) corresponding dQ/dV curves.

F
I G U R E 5 Density functional theory (DFT) calculation: (A) Comparing charging and discharging curves of Na 0.67 Ni 0.33 Mn 0.67 O 2 (NNMO) and Na 0.72 Ni 0.28 Li 0.05 Mn 0.57 Ti 0.10 O 2 (NNLMTO).(B) Calculated formation energy of Na e and Na f in NNMO and NNLMTO.Partial density of states (PDOS) of (C) NNMO and (D) NNLMTO.

F
I G U R E 6 Evaluations of the full cell device based on Na 0.72 Ni 0.28 Li 0.05 Mn 0.57 Ti 0.10 O 2 (NNLMTO) cathode and hard carbon anode.(A) Schematic diagram of the working principle in full cell configuration.(B) Rate performance and (C) corresponding charge/discharge curves of the coin type full cell.(D) Ragone plot of the relationship between energy density and power density.(E, F) Cycling stability of coin type full cell device at 0.1 C and 1 C rate, respectively.(G) Schematic diagram of NNLMTO//HC pouch type full cell.(H) Charge/discharge curves of pouch-type full cell.(I) Cycling stability of pouch-type full cell.