Co-Substitution Enhances the Rate Capability and Stabilizes the

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Energy, Environmental, and Catalysis Applications

Co-Substitution Enhances the Rate Capability and Stabilizes the Cyclic Performance of O3-Type Cathode NaNi0.45xMn0.25Ti0.3CoxO2 for Sodium-Ion Storage at High Voltage Chaojin Zhou, Lichun Yang, Chaogang Zhou, Bin Lu, Jiangwen Liu, Liuzhang Ouyang, Renzong Hu, Jun Liu, and Min Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17945 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Co-Substitution Enhances the Rate Capability and Stabilizes the Cyclic Performance of O3-Type Cathode NaNi0.45-xMn0.25Ti0.3CoxO2 for Sodium-Ion Storage at High Voltage Chaojin Zhou,a Lichun Yang,a * Chaogang Zhou,b Bin Lu,a Jiangwen Liu,a Liuzhang Ouyang,a Renzong Hu,a Jun Liu,a and Min Zhu a a

School of Materials Science and Engineering, Guangdong Provincial Key

Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou 510641, China. b

College of Metallurgy and Energy, Key Laboratory of the Ministry of Education for

Modern Metallurgy Technology, North China University of Science and Technology, Tangshan 063009, China. * Corresponding author. E-mail address: [email protected]

Keywords: Co-substitution; sodium-ion battery; O3-type cathode material; high voltage cathode; sol-gel method

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Abstract O3-type NaNiO2-based cathode materials suffer irreversible phase transition when they are charged to above 4.0 V in sodium-ion batteries. To solve this problem, we partially

substitute

Ni2+

in

O3-type

NaNi0.45Mn0.25Ti0.3O2

by

Co3+.

NaNi0.45Mn0.25Ti0.3O2 with Co-substitution possesses an expanded interlayer and exhibits higher rate capability, as well as cyclic stability, compared with the pristine cathode in 2.0-4.4 V. The optimal NaNi0.4Mn0.25Ti0.3Co0.05O2 delivers discharge capacities of 180 and 80 mA h g-1 at 10 and 1000 mA g-1. At 100 mA g-1, NaNi0.4Mn0.25Ti0.3Co0.05O2 exhibits 152 mAh g-1 in the initial cycle and maintains 91.4 mAhg-1 after 180 cycles. Through ex situ X-ray diffraction, Co-substitution is demonstrated to be effective in enhancing the reversibility of P3-P3" phase transition from 4.0 to 4.4 V. Electrochemical impedance spectroscopy indicates that higher electronic conductivity is achieved by Co-substitution. Moreover, cyclic voltammetry and the galvanostatic intermittent titration technique demonstrate faster kinetics for Na+ diffusion due to the Co-substitution. This study provides a reference for further improvement of electrochemical performance of cathode materials for high-voltage sodium-ion batteries.

2


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1. Introduction Sodium resources are more widely distributed and cheaper than lithium resources. Therefore, it is important to develop sodium-ion batteries (SIBs) as energy storage system for large-scale application.1,2 In recently reported cathode materials, O3-type nickel-based layered oxides and their binary and ternary derivatives have received particular attention because they have low cost and potentially high capacity (> 200 mA h g-1).3,4 In order to promote the practical application of O3-type NaNiO2-based cathode materials, increasing their energy density to the commercial level of lithium-ion batteries is necessary. It is effective to enhance the energy density of SIBs by expanding their operating voltage.5,6 However, poor reversibility of phase transition at high cut-off voltage (above 4 V) can easily lead to severe capacity fading.7,8,9 Recent reports show that partial element substitution is effective at enhancing the reversibility of phase transition.10-12 For example, Ti-substituted Na0.9Ni0.4Mn0.4Ti0.2O2 with more reversible O3-P3 phase transition from 2.5 to 4.2 V shows higher capacity and stabilized cyclic performance compared to its Ti-free counterpart.13 Fe-substituted NaFe0.2Mn0.4Ni0.4O2 shows stable phase transition in the range of 4.0 to 4.3 V, which contributes extra reversible capacity.14 Li-substitution depresses the phase transition of Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 from O3 to P3ʹ in 1.75-4.4 V, thereby improving the rate capability and cycle stability.15 Co-doping has been widely used for improving the reversible Na+ storage performance of NaNiO2-based cathode materials,16,17,18,19,20 but the effects and role of Co at high cut-off voltage (above 4 V) 3



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have not been thoroughly elucidated to date. In

this

study,

to

enhance

the

electrochemical

stability

of

O3-type

NaNi0.45Mn0.25Ti0.3O2 (denoted as NMT) above 4 V, we partially substitute Ni2+ with Co3+ and investigate the effects of Co-substitution on improving the reversibility of P3-P3"

phase

transition

at

high

voltage

(4.0-4.4

V).

The

prepared

NaNi0.45-xMn0.25Ti0.3CoxO2 (denoted as NMTCox, x=0.05 and 0.1) processes expanded interlayer space owing to Co-substitution, which facilitates Na+ diffusion kinetics.21 Moreover, the high voltage durability of the material can be improved without sacrificing capacity by the substitution of the electrochemically active Co3+. This study not only reveals that improved cycling and rate performance under high voltage can be achieved by Co-substitution but also helps to elucidate the design of high-voltage cathode materials for SIBs. 2. Experimental section 2.1 Synthesis of O3-type layered oxides Figure 1schematically illustrates the experimental procedure O3-type layered oxides. NMTCox was prepared via sol-gel method. Firstly, stoichiometric amounts of NaNO3 (5 % excess), Mn(NO3)2, Ni(NO3)2·6H2O, C16H36O4Ti, and Co(NO3)2·6H2O (where the total transition metal amount was 0.02 mol) were dissolved in 100 ml dilute nitric acid (35 wt.%), and 0.02 mol citric acid as a chelating agent was added. Then, the solution was stirred at 75 °C for 12 h. Gel formed when the solvent was evaporated. The gel was dried overnight at 125 °C and then grounded. The raw material was calcined in air. The calcination temperature was set 500 °C firstly for 5 h 4


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and then elevated to 900 °C for 10 h.

Figure 1. Schematic illustration of the preparation procedure of O3-type layered oxides. 2.2 Characterization The phase structures of the collected products were characterized by X-ray diffraction (XRD) which was performed on PANalytical Empyrean diffractometer, using Cu Kα radiation. Morphology and microstructure were investigated by scanning electron microscope (SEM, Zeiss Gemini DSM 982) and transmission electron microscope (TEM, JEOL JEM-2100). Raman spectra were performed on a Raman microspectrometer (Jobin-Yvon Labor Raman HR-800, Villeneuve d'Ascq) with an excitation laser wavelength of 514.5 nm. X-ray photoelectron spectra (XPS) with an Al K achromatic X-ray source were used to analyze the valence states and binding energies. 2.3 Electrochemical measurements The working electrode preparation process is as follows. First, active material, super P, and polyvinylidene fluoride (PVDF) (80/10/10, w/w/w) were mixed 5



uniformly in N-methyl 1-2-pyrrolidone (NMP). Next, the slurry was coated onto Al current collector and dried in vacuum at 80°C. The coated Al foil was cut into round working electrodes with mass loadings of approximately 3.5 mg cm-2. Coin cells (CR2025) were assembled in a glove box filled with Ar (contents of H2O and O2 lower than 1 ppm). In coin cells, sodium plates were utilized as negative and reference electrodes, glass fiber membranes (GF/D-125R, Whatman) were used as separators, and 1 mol l-3 NaClO4 dissolved in diethyl carbonate (DEC) and ethylene carbonate (EC) (1/1, w/w) was utilized as an electrolyte. Battery tester (LAND, CT2001A) was used for the galvanostatic discharge/charge cycles. The half cells were tested in 2.0–4.4 V vs. Na/Na+ at various current. And electrochemical work station (Gamry, Interface 1000) was used to record electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The Nyquist plots were tested by applying a 5 mV amplitude signal in the frequency of 1 MHz to 0.01 Hz. The half cells were also subjected for galvanostatic intermittent titration technique (GITT) analysis. During the first cycle, the cells were charged at 4 mA g-1 in 2.0-4.4 V. For each applied galvanostatic current, the duration time is 0.05 h and the rest time was 0.5 h. 3. Results and discussion The phase components of NMTCox (x = 0, 0.05, 0.1) were analyzed by XRD, as shown in Figure 2. The main phase of NMTCox (x = 0, 0.05) is consistent with the α-NaFeO2 phase (JCPDS NO.54-0887, space group R 3 m). The sharp and strong diffraction peaks of the sample indicate good crystallinity. NiO impurity emerges in NMTCo0.1, indicating that excessive addition of Co3+ ions reduces the solubility of Ni 6


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in the O3 phase. In addition, with the increase of Co-substitution, the (003) peak shifts to lower angle (Figure 2b), suggesting the d-spacing expansion (inset of Figure 2b), which can enhance the kinetics for Na+ de-/intercalation therefore improve the rate performance of the cathode.12 To determine the lattice parameters of NMTCox (x = 0, 0.05, 0.1), the Rietveld method is used to refine the XRD patterns (Figure 2d, e, and f), and the refined crystallographic parameters are summarized in Table S1-3. As shown in Figure 2 g, with the increase of Co content, the lattice parameter a and the crystal cell volumes decrease, whereas the lattice parameter c increases. In the basal plane of the layered structure, the lattice parameter a is sensitive to (Mn, Ni Ti, Co)-O bond length changes.22 The value of a decreases because partial high spin Ni2+ ions are substituted by Co3+ ions with smaller size (rCo3+ = 0.545 Å, rNi2+ = 0.69 Å). The value of c increases because the electrostatic repulsions between the oxygen atoms in consecutive (Mn, Ni, Ti, Co) layers increase.18,22,23

7



Figure 2. (a) XRD patterns, (b) (003) peaks, and (c) d-spacing of (003) planes in NMT, NMTCo0.05 and NMTCo0.1; Rietveld refinement of XRD patterns for (d) NMT, (e) NMTCo0.05, and (f) NMTCo0.1; (g) relationship between the lattice parameters and Co-substitution. The inset of panel g is the illustration for the crystal structure of material, which was created using VESTA24. Figure 3a shows the Raman spectra of NMTCox (x = 0, 0.05, 0.1). In the spectrum of NMT, the strong peaks located at 579 cm-1 and 468 cm-1 are attributed to A1g and E1g modes of O vibrations, respectively. In comparison, the E1g peak exhibits blue shift, and the peak corresponding to E2g mode of Na vibrations emerges at 383 cm-1,25 8


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which indicate local structure distortion of TMO6 octahedron originating from Co-substitution. Figure 3b and 3c show the high-resolution XPS spectra for Co3+ and Ni2+. The existence of Co3+ in NMTCo0.05 and NMTCo0.1 can be identified by the Co 2p3/2 and 2p1/2 peaks at approximately 780.1 eV and 795.6 eV, respectively (Figure 3b).26,27 The Ni 2p1/2 peak shifts to higher binding energy with the increase of Co content (Figure 3c), which is observed because Co3+ has stronger electronegativity than Ni2+. The Raman and XPS spectra demonstrate that Co3+ ions enter in the lattice and partially substitutes Ni2+.

Figure 3. (a) Raman spectra, and XPS spectra for (b) Co3+, and (c) Ni2+ of NMTCox (x = 0, 0.05 and 0.1). SEM and TEM were used to investigate the morphologies and microstructures of NMTCox (x = 0, 0.05, 0.1). As shown in Figure 4a, NMT is a mixture of microparticles (1-2 μm) and nanoparticles (200-500 nm). In comparison, NMTCo0.05 and NMTCo0.1 exhibit microparticles with more even size (Figure 4b and 4c), indicating

Co-substitution

effectively

promotes

the

crystal

growth.

The

high-resolution TEM image of NMTCo0.05 (Figure 4d) indicates its high crystallinity. The lattice fringes with planar interspace of 2.52 Å and 2.15 Å correspond to (101) and (104) planes, respectively. The SAED pattern (Figure 4e) shows clear diffraction 9



spots without an impurity phase, indicating a well-crystallized structure of hexagonal NMTCo0.05. The element mapping (Figure 4f) indicates an even distribution of Na, Ti, Mn, Co, and Ni in NMTCo0.05.

Figure 4. SEM images of (a) NMT, (b) NMTCo0.05, and (c) NMTCo0.1; (d) HRTEM image, (e) SAED pattern, and (f) element mapping of NMTCo0.05. The electrochemical performance of NMTCox (x = 0, 0.05, 0.1) is evaluated while it acts as a cathode of SIBs. Figure 5a displays the typical charge/discharge profiles of NMTCox (x = 0, 0.05, 0.1) at 0.05 C (10 mA g−1) in 2.0-4.4 V. In the charge/discharge profiles, there are two plateaus at 2.7-3.5 V and 4.0-4.4 V, ascribing to the phase 10


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transition from O3 to P3 and P3 to P3", respectively, indicated by ex situ XRD as discussed later. The first reversible capacities of the NMTCox (x = 0, 0.05, and 0.1) samples are 164 mAh g-1, 182 mAhg-1, and 154 mAhg-1, respectively. With the increase of Co content, the discharge plateaus at 4.0-4.4 V become more pronounced and contribute higher reversible capacity: 46 mAh g-1 for NMT, 80 mAh g-1 for NMTCo0.05, and 75 mAh g-1 for NMTCo0.1, indicating Co-substitution is beneficial to enhance the capacity delivery in 4.0-4.4 V. Figure 5b compares the rate performance of NMTCox (x = 0, 0.05, 0.1). NMTCo0.05 exhibits reversible capacities of 180 mAh g-1, 150 mAh g-1, 140 mAh g-1, 135 mAh g-1, 130 mAh g-1, 110 mAh g-1 and 80 mA h g−1 at discharge rates of 0.05 C, 0.1 C, 0.25 C, 0.5 C, 1 C, 2.5 C and 5 C, respectively. Clearly, the rate capability of NMTCo0.05 is higher than that of NMT. At a lower rate, the capacity of NMTCo0.1 is lower than that of NMT, because of the precipitation of NiO impurities in the preparation process. However, the capacity of NMTCo0.1 is still more stable and higher than that of NMT at higher rate. The comparisons indicate Co-substitution enhances the rate capability of NMTCox. The energy density of NMTCo0.05 is outstanding even compared with those of other reported O3-type cathode materials (Figure 5c).12,14,28-30 The cycling performance of NMTCox (x=0, 0.05, 0.1) at 0.5 C is shown in Figure 5d. NMTCo0.05 exhibits a reversible capacity of 152 mAhg-1 in the first cycle, and maintains 91.4 mAh g-1 even after 180 cycles, with a capacity retention of 60%. Although NMTCo0.1 exhibits slightly lower initial capacity than NMT, its capacity retention (63%) is much higher than that of NMT (50%). The Na+ storage performance of NMTCox (x = 0, 0.05, 0.1) 11



indicates Co-substitution is effective to enhance the capacity delivery in 4.0-4.4 V and improve the cyclic stability of NMTCox.

Figure 5. Electrochemical performance of NMTCox (x=0, 0.05 and 0.1). (a) charge/discharge profiles at 0.05 C, (b) rate capability, (c) Ragone plots of typical reported cathode materials for SIBs and the NMTCo0.05 cathode in this work, and (d) cycle performance at 0.5 C. Ex situ XRD was investigated to reveal the structural evolution of NMTCo0.05 and NMT in 2.0-4.4 V (Figure 6b and 6d). As shown in Figure 6b, the pristine O3 phase is 12


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observed in the fresh electrode of NMTCo0.05. When the electrode was charged, the diffraction peaks of (003)O3, (006)O3, (101)O3, (102)O3 and (104)O3 become weak, and the peaks of (003)P3, (006)P3, (101)P3, (102)P3 and (104)P3 emerge simultaneously, verifying the phase transition from O3 to P3 accompanied with the expansion along c-axis.28,31-33 When the electrode is charged above 4.2 V, peaks corresponding to P3 phase gradually disappear, while new peaks, such as (003)P3", (101)P3", (102)P3", (104)P3" and (107)P3", appear,34,35 revealing the phase transition from P3 to P3". Overall, during the charge process, the phase transition of NMTCo0.05 is in the sequence of O3-P3-P3". In the discharge process, NMTCo0.05 undergoes phase transition of P3"-P3-O3. Note that when NMTCo0.05 is discharged to 2.0 V, the O3 phase is completely recovered, indicating good reversibility of NMTCo0.05 for Na+ storage in 2.0-4.4 V. In comparison with NMT, (003)O3, (104)O3 and (018)O3 peaks still exist in NMTCo0.05 in the charge state of 3.9 V but disappear in NMT (Figure 6d). The difference suggests that the phase transition of O3-P3 extends to higher voltage in NMTCo0.05. Moreover, when the electrodes are charged to 4.4 V, the diffraction peaks of P3" are more distinct in NMTCo0.05 than in NMT, indicating a more thorough transition from P3 to P3" in NMTCo0.05. These comparisons explain the higher capacity delivery of NMTCo0.05 during the charge process due to the Co-substitution.

13



Figure 6. Charge/discharge profiles of (a) NMTCo0.05 and (c) NMT electrodes, and the corresponding ex situ XRD patterns of (b) NMT and (d) NMTCo0.05 electrodes at various charge/discharge states. The insets of panels a and c are the illustrations of the crystal structures, which were created using VESTA24. To evaluate the kinetics of Na+ diffusion in NMT and NMTCo0.05, CV and GITT analysis were performed. Figure 7a and 7b show CV curves of NMT and NMTCo0.05 at different scan rates. The redox peaks between 2.4 V and 3.3 V can be associated with the Ni2+/Ni3+ redox process18,36,37, and the redox peaks between 3.3 and 4.4 V can be associated with the Ni3+/Ni4+ (Figure S1a).29,38,39 The XPS peaks of Mn and Ti do not show obvious change when NMTCo0.05 is charged to 3.5 V and 4.4 V (Figure 14


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S1b and c), indicating that the valence states of Mn and Ti do not change during the charging process.36,40,41 The apparent Na+ diffusion coefficient DNa+ can be determined based on the Randles–Sevcik Equation:42 ip=2.69×105n3/2A DNa+1/2v1/2C0

(1)

where ip is the peak current, n denotes the electron number, A represents the electrode area, v denotes the scan rate, and C0 is the Na+ concentration. By plotting ip vs. v1/2 (Figure 7c), the apparent Na+ diffusion coefficient of NMT and NMTCo0.05 are calculated as 1.01×10-11 cm2 s-1 and 1.52×10-11 cm2 s-1, respectively. The GITT is also used to calculate DNa+ of NMTCo0.05 and NMT. Figure 8a shows a typical τ vs. E profile of NMTCo0.05 for a single titration. By plotting E vs. τ1/2 (Figure 8b), DNa+ can be determined on the basis of eq. 2: 38,43 DNa +

4  mBVM  =   M B S

  

2

 E s   E

  

2

(  L / D ) 2

Na +

(2)

where τ denotes the constant current pulse time. MB and mB are the molecular weight and the mass of active material, respectively. VM is the molar volume of active material, S represents the electrode surface area, L is the particle radius of active material, Es is the change of the steady-state voltage, and Eτ is the total change of the cell voltage during the current pulse.44 Figure 8c shows GITT curves of NMT and NMTCo0.05 in the initial charge process and Figure 8d shows the corresponding DNa+. The average values of all DNa+ calculated in each interval are determined as 1.03 × 10-11 cm2 s-1 and 2.36 × 10-12 cm2 s-1 for NMTCo0.05 and NMT, respectively. CV and GITT analysis both demonstrate faster kinetics of Na+ diffusion in NMTCo0.05. The faster kinetics of Na+ diffusion in NMTCo0.05 is associated with the 15



expanded interlayer due to the Co-substitution and results in higher rate capability. 45,46

Furthermore, the Nyquist plots of NMT and NMTCo0.05 were measured, as shown

in Figure 7d. By fitting the Nyquist plots using the equivalent circuit, the charge transfer resistance for NMT and NMTCo0.05 can be determined as 870 Ω and 460 Ω, respectively, which suggest higher electronic conductivity of NMTCo0.05. Faster kinetics for Na+ diffusion and electronic transfer result in the improved rate performance.

Figure 7. CV curves of (a) NMT and (b) NMTCo0.05 at various scan rates, (c) the relationship between ip and v1/2, and (d) Nyquist plots of NMT and NMTCo0.05. The inset of Panel d is the equivalent circuit used to fit the Nyquist plots. Rs represents the ohmic resistance (including the electrolyte resistance and contact resistance), CPE represents the constant phase element, Rct denotes the resistance of charge transfer, and W corresponds to the Warburg diffusion resistance. 16


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Figure 8. (a) Single GITT titration curve of NMTCo0.05, (b) linear fitting of E vs. τ1/2 in panel a, (c) GITT curves, and (d) the calculated DNa+ of NMT and NMTCo0.05 in the first charge process. Current density = 0.02 C, time interval τ = 180 s. 4. Conclusions The effects of Co-substitution on the capacity, cycling stability and rate performance of NMTCox (x = 0, 0.05 and 0.1) are investigated in 2.0-4.4 V. NMTCo0.05 with the optimal Co content shows the best electrochemical performance and delivers reversible capacities of 180 mAh g-1 and 80 mAh g-1 at 0.05 C and 5 C, respectively. At 0.5 C, NMTCo0.05 exhibits a reversible capacity of 152 mAh g-1 in the first cycle, and maintains 91.4 mAh g-1 after 180 cycles. Ex situ XRD characterizations demonstrate that the reversible phase transition of P3-P3" is promoted by Co-substitution in 4.0-4.4 V. CV and GITT analysis demonstrate faster kinetics for Na+ diffusion due to the Co-substitution. This study provides a reference for further improving the electrochemical performance of high voltage cathode 17



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materials for SIBs. Supporting information Selected Rietveld refinement data from the X-ray diffraction pattern of NMTCox (x =0, 0.05 and 0.1) at room temperature, and XPS spectra of NMTCo0.05 at different voltages in the first charge process. Acknowledgments We thank the financial support from the National Natural Science Foundation of China (Grant No. 51671089), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. NSFC51621001), Guangdong Natural

Science

2017B030306004),

Funds

for

Guangdong

Distinguished Special

Young

Support

Scholar

Program

(Grant

No.

(2017TQ04N224),

Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme, and the Fundamental Research Funds for the Central Universities.

18


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Sodium-Ion

Batteries:

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to

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Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 710-721. (2) Nayak, P. K.; Yang, L.; Brehm, W.; Adelhelm, P. P. From Lithium-Ion to Sodium-Ion Batteries: Advantages, Challenges, and Surprises. Angew. Chem. Int. Ed. 2018, 57, 102-120. (3) Li, W.-J.; Han, C.; Wang, W.; Gebert, F.; Chou, S.-L.; Liu, H.-K.; Zhang, X.; Dou, S.-X. Commercial Prospects of Existing Cathode Materials for Sodium Ion Storage. Adv. Energy Mater. 2017, 7, 1700274. (4) Deng, J.; Luo, W.-B.; Chou, S.-L.; Liu, H.-K.; Dou, S.-X. Sodium-Ion Batteries: From Academic Research to Practical Commercialization. Adv. Energy Mater. 2018, 8, 1701428. (5) Hou, P.; Yin, J.; Ding, M.; Huang, J.; Xu, X. Surface/Interfacial Structure and Chemistry of High-Energy Nickel-Rich Layered Oxide Cathodes: Advances and Perspectives. Small 2017, 13, 1701802. (6) You, Y.; Manthiram, A. Progress in High-Voltage Cathode Materials for Rechargeable Sodium-Ion Batteries. Adv. Energy Mater. 2018, 8, 1701785. (7) Han, M. H.; Gonzalo, E.; Casas-Cabanas, M.; Rojo, T. Structural Evolution and Electrochemistry of Monoclinic NaNiO2 upon the First Cycling Process. J. Power Sources 2014, 258, 266-271. (8) Vassilaras, P.; Ma, X.; Li, X.; Ceder, G. Electrochemical Properties of Monoclinic 19



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NaNiO2. J. Electrochem. Soc. 2013, 160, A207-A211. (9) Wang, L.; Wang, J.; Zhang, X.; Ren, Y.; Zuo, P.; Yin, G.; Wang, J. Unravelling the Origin of Irreversible Capacity Loss in NaNiO2 for High Voltage Sodium Ion Batteries. Nano Energy 2017, 34, 215-223. (10) Xu, J.; Lee, D. H.; Clément, R. J.; Yu, X.; Leskes, M. Identifying the Critical Role of Li Substitution in P2-Nax[LiyNizMn1-y-z]O2 (0

Co-Substitution Enhances the Rate Capability and Stabilizes the

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