# New Sodium Intercalation Cathode Prepared by Sodiation of Delithiated Host LiNi1/3Mn1/3Co1/3O2

Authors: Van Hoang Nguyen; Le Minh Nguyen; Tuyen Thi Kim Huynh; Van Man Tran; My Loan Phung Le
Journal: Advances in Materials Science and Engineering
 (2021)

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## Abstract

In this work, the layered structure LiNi1/3Mn1/3Co1/3O2 (LiNMC) served as a host to enable sodium-ion intercalation. LiNMC was initially charged in Li-half-cell at C/25 rate up to 4.5 V to extract maximum of Li+ ions and then discharged at the same rate in Na-half-cell down to 2 V for full sodiation to form NayNMC phase. The electrochemical characteristics of the new sodium phase NayNMC were evaluated by cyclic voltammetry (CV), galvanostatic cycling, and electrochemical impedance spectroscopy (EIS). On the CV curve, the featured peaks of phase transition induced by Na+ intercalation into NayNMC host could be distinguished from the couple peak located at 3.8 V upon the Li+ intercalation into LixNMC. The high uniformity and crystallinity of the NayNMC phase enable delivering a good initial capacity of about 120 mAh g−1 with high rate capability up to 5 C rate. Energy-dispersive X-ray spectroscopy (EDS) confirms the presence of sodium element in the sodiated NayNMC. It was also noticed that the pristine O3-type layered structure remained unchanged after ion exchanging but the lattice parameters increased due to the large size of sodium-ions inserted into the structure.


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## Body

## 1. Introduction

Switching from fossil fuel to green renewable energy has been forcing the significant development of electrical power sources technology due to its versatility and mobility usage. Among them, Li-ion batteries are the best choice for its powerful storage capability and lightweight. However, commercial Li-ion batteries are facing the challenge of lithium depletion for the next few decades as forecasted elsewhere [1–3]. In this context, the emerging of alternative candidates such as Na-ion batteries is unavoidable because of the predictable low fabrication cost of the batteries and the abundance of raw materials used to prepared electrodes [1, 4–6].NaNi1/3Mn1/3Co1/3O2 (NaNMC) with a layered structure and chemical composition inspired by the commercial analog LiNMC has been considered as the promising cathode for Na-ion batteries [7]. Indeed, its high capacity and high operation voltage are beneficial from the robust transition elements Ni, Mn, and Co in equal ratio. This cathode material could be prepared by several reaction ways including the sol-gel process, solid-state reaction, or coprecipitation. Hwang et al. synthesized NaNMC material via coprecipitation of metal salts and calcinated at a low temperature of about 750°C in 24 hours. The material exhibited a high capacity of 140 mAh g−1 and relatively superior performance in half-cell compared to different ratio of Ni/Mn/Co in the synergistic research [8]. Sathiya and colleagues are pioneers in synthesizing the material NaNMC via a sol-gel process following a calcination step at 900°C in 12 hours [9]. The material showed a discharge capacity of 120 mAh g−1 and very stable cycling. Unfortunately, moisture-sensitive hindered the application ability of NaNMC in practical batteries. Besides, it might be quite difficult to obtain single-phase NaNMC by sintering in the air due to the complexity of composition [10]. On the contrary, the moisture issue is tolerant in the synthesis of LiNMC, whereas an O3 structure with a low percentage of cation disordering could be obtained by simple calcination [11, 12].The isotropic structure and high crystallization degree of lithium layered compounds are promising to provide a uniform diffusion pathway for sodium-ions [13]. It has been proved that Li-doped sodium layered structures enhance the structural stability and the capacity of the electrode materials [14, 15]. Reversely, due to the large spacing slab of sodium layered compounds, the sodium compounds demonstrated their potential to intercalate lithium-ions at higher capacity and rate capability [7, 16]. Therefore, sodiation in delithiated host with the lowest remained lithium-ions is another way to enhance the electrochemical properties of the sodium cathode. Recent studies have reported the use of a delithiated cathode phase as a sodium intercalation host for Na-ion batteries, including layered oxide, phosphates, and Li-rich cathodes [13, 17–20]. Previously, we studied the full sodiation into LiNMC synthesized via sol-gel reaction [13]. The number of Na+ ion inserted into the delithiated host is comparable to the number of lithium extracted at C/10 rate in the voltage range 2–4 V. The diffusion coefficient of Na+ ion into the solid structure calculated from Galvanostatic Intermittence Titration Technique (GITT) is relatively stable with the value range of 1 × 10−9–7 × 10−10 cm2 s−1. However, the structure of the sodiated cathode was incompletely investigated. Hence, in this work, the high crystallinity LiNi1/3Mn1/3Co1/3O2 layered cathode (LiNMC) was subjected to sodium-ion-exchange and evaluated the performance in sodium half-cell. The lithium extraction was limited by a cutoff voltage of 4.5 V to evaluate whether there is more vacancy to uptake sodium-ion. In addition, the structure evolution was further explored after the initial sodiation and consecutive cycling.


## 2. Experiment

### 2.1. Electrode Preparation

Commercially, LiNi1/3Mn1/3Co1/3O2 (LiNMC) powder was supplied by MTI Corp. Electrode slurry was made from a mixture of LiNMC, carbon C65, and polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP) in the weight ratio of 94 : 3:3 in N-methyl pyrrolidone (NMP) solvent and then ball-milled for thoroughly mixing before coating on Al foil. The coated Al foil was dried at 110°C in a vacuum for 12 hours and punched into 12 mm round shapes with mass loading about 3-4 mg cm−2.


### 2.2. Electrochemical Ion Exchange to Synthesize NazNMC and Electrochemical Characterization

The scheme in Figure1 demonstrates the experimental procedure of ion-exchange. Lithium half-cell, consisting of the LiNMC as a cathode, lithium metal as an anode, and Whatman microfiber GF/C separator wetted by LiPF6 1M/EC-DMC (1 : 1) electrolyte, was assembled in the glovebox. The cell was charged by the rate of C/25 (1 C corresponding to the extraction of 1 ion per molecular in 1 hour) from the open circuit potential (OCP) to 4.5 V versus Li+/Li then and kept for at least 12 hours to fully extract lithium-ions. Next, the delithiated cathode was taken out in glovebox and soaked many times with dimethyl carbonate (DMC) solvent and finally dried at 100°C on the hot plate.Figure 1
Scheme of the electrochemical ion-exchange process to prepare a new sodium insertion compound from a lithium insertion compound. Lithium-ions extracted from LiNMC crystal structure during charging in Li-half-cell then sodium-ions intercalated to the structure during discharging in Na-half-cell.The delithiated cathode was placed again in sodium half-cell using sodium metal as anode and Whatman microfiber GF/C wetted by NaClO4 1M/PC-FEC (98 : 2) electrolyte. The charge–discharge rate of the cell was controlled in galvanostatic mode. The cells were firstly discharged by the rate of C/25 from OCP to 2.0 V versus Na+/Na to insert Na+ ion into the delithiated cathode. Cyclic voltammetry (CV) measurement was performed in the voltage range of 2.0–4.0 V versus Na+/Na. EIS was conducted before and after cycling by applying the alternative voltage with 10 mV amplitude and the frequency in a range from 1 MHz to 10 mHz. All the electrochemical characterization was conducted on MPG-2 and/or VSP high accuracy apparatus (Biologic-France). The Swagelok-type cell was used for ex situ structural characterization to prevent short circuits while coin-cell type CR2032 was used for cycling tests.X-ray diffraction analysis of the cathode after casting on Al foil and the cycled cathode on lithium/sodium cells was performed using CuKα (λ = 1.508 Å) with the scan rate of 0.02o/step/0.25 s on D8 Advance Diffractometer (Bruker). Scanning Electron Microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) (S4800 FESEM, Hitachi) was used to evaluate the morphology and element composition of the cathode.


## 2.1. Electrode Preparation

Commercially, LiNi1/3Mn1/3Co1/3O2 (LiNMC) powder was supplied by MTI Corp. Electrode slurry was made from a mixture of LiNMC, carbon C65, and polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP) in the weight ratio of 94 : 3:3 in N-methyl pyrrolidone (NMP) solvent and then ball-milled for thoroughly mixing before coating on Al foil. The coated Al foil was dried at 110°C in a vacuum for 12 hours and punched into 12 mm round shapes with mass loading about 3-4 mg cm−2.


## 2.2. Electrochemical Ion Exchange to Synthesize NazNMC and Electrochemical Characterization

The scheme in Figure1 demonstrates the experimental procedure of ion-exchange. Lithium half-cell, consisting of the LiNMC as a cathode, lithium metal as an anode, and Whatman microfiber GF/C separator wetted by LiPF6 1M/EC-DMC (1 : 1) electrolyte, was assembled in the glovebox. The cell was charged by the rate of C/25 (1 C corresponding to the extraction of 1 ion per molecular in 1 hour) from the open circuit potential (OCP) to 4.5 V versus Li+/Li then and kept for at least 12 hours to fully extract lithium-ions. Next, the delithiated cathode was taken out in glovebox and soaked many times with dimethyl carbonate (DMC) solvent and finally dried at 100°C on the hot plate.Figure 1
Scheme of the electrochemical ion-exchange process to prepare a new sodium insertion compound from a lithium insertion compound. Lithium-ions extracted from LiNMC crystal structure during charging in Li-half-cell then sodium-ions intercalated to the structure during discharging in Na-half-cell.The delithiated cathode was placed again in sodium half-cell using sodium metal as anode and Whatman microfiber GF/C wetted by NaClO4 1M/PC-FEC (98 : 2) electrolyte. The charge–discharge rate of the cell was controlled in galvanostatic mode. The cells were firstly discharged by the rate of C/25 from OCP to 2.0 V versus Na+/Na to insert Na+ ion into the delithiated cathode. Cyclic voltammetry (CV) measurement was performed in the voltage range of 2.0–4.0 V versus Na+/Na. EIS was conducted before and after cycling by applying the alternative voltage with 10 mV amplitude and the frequency in a range from 1 MHz to 10 mHz. All the electrochemical characterization was conducted on MPG-2 and/or VSP high accuracy apparatus (Biologic-France). The Swagelok-type cell was used for ex situ structural characterization to prevent short circuits while coin-cell type CR2032 was used for cycling tests.X-ray diffraction analysis of the cathode after casting on Al foil and the cycled cathode on lithium/sodium cells was performed using CuKα (λ = 1.508 Å) with the scan rate of 0.02o/step/0.25 s on D8 Advance Diffractometer (Bruker). Scanning Electron Microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) (S4800 FESEM, Hitachi) was used to evaluate the morphology and element composition of the cathode.


## 3. Results and Discussion

O3 represents the layered structure where the alkaline ions and transition ions occupy sequentially the octahedral sites between the adjacent arrays of hexagonal closed packed oxygens, as seen inside Figure1. Additionally, there are three repeated layers built of the octahedral of transition metal connected through edges and corners in the unit cell that classified it into O3-type [21]. The O3-type structure could accept almost one alkaline ion per molecular formula.In this work, the commercial power LiNMC with a high crystallinity degree and uniform grain size distribution was used so that the results are not affected by structure default or particle size distribution. The first charge–discharge of LiNMC in Li-half-cell given in Figure2(a) exhibits the S-shape sloping feature of LiNMC with plateaus at about 3.8 V due to lithium extraction in solid solution [11, 22]. The number of lithium extracted from the process was calculated from the number of charge input divided by the theoretical maximum charge that gives a value of about 0.69 lithium-ion at a high cutoff voltage of 4.5 V at C/25 rate, corresponding to a specific capacity of 190 mAh g−1. The high specific charge capacity obtained at rate C/25 was comparable to our synthesized LiNMC [13] but it exceeded the expected value of LiNMC which is about 160–180 mA g−1 because of the small charging current. The side reactions such as phase transition of O3-phase due to excessive release of Li+ ion and releasing of lattice oxygen [13, 23, 24] that contributed to structuring destruction and electrolyte composition are probably small contributions to the faradic reaction since the current decreases quickly to the minimum after holding at 4.5 V.Figure 2
(a) Initial charge curve (oxidation state) of LiNMC in half-cell Li | LiPF6 1M/EC-DMC (1 : 1) | LiNMC at C/25 rate. (b) First charge-discharge curve of NazNMC in half-cell Na | NaClO4/PC-FEC (98 : 2) | NazNMC.
(a)(b)Later, delithiated LiNMC was subjected to be discharged in sodium half-cell and the first voltage profile was given in Figure2(b). Interestingly, at a low rate of C/25, an S-shape discharge curve appears with an average voltage of about 300 mV lower than that of LiNMC in lithium half-cell followed by flat plateaus in the lower voltage region. The S-shape curve is due to the insertion/extraction of the remained lithium in the delithiated host phase. Additionally, a few lithium-ions must be retained to prevent the O3-type layered structure from collapsing. Comparing to the first discharge, in the next charge step, more additional ions including sodium and the remaining lithium-ions could be extracted from the sodiated cathode because of higher cutoff charge voltage than initial OCV. Once the steady charge–discharge state is reached, the maximum ion reversibly exchanged into delithiated LiNMC is about 0.53 ion at a rate of C/25 which is closed to the maximum value reported for NaNMC in previous studies [8, 9]. Therefore, the highest value was obtained in the initial intercalation step due to the main contribution of sodium-ion. The amount of sodium-ion intercalated, however, is not equal to the lithium-ion extracted during the oxidation process of LiNMC. In fact, the sodium-ion is bigger than lithium-ion and the electrostatic repulsion between them and the host phase obstructed its replacement for all Li-vacancies. Regarding the first discharge at C/10, the S-shape feature is replaced by discharge curves with a sloping region at high voltage while two short plateaus at 2.6 V and 2.4 V due to structural rearrangement during sodium intercalation [9] have remained. The staircase-like voltage profile with voltage jump at a high voltage of NaNMC was replaced by a smooth curve revealing the role of Li+ ion as pillar ion to prevent the slab gliding [14, 15, 25, 26]. The local structure of Li+ ion studied by 7NMR [14] revealed that Li+ migration between the sodium-ion slab and the transition metal slab is relatively reversible and is because Li+ incapability stabilizes in the prismatic sites instead of the octahedral sites when slabbing gliding during sodium extraction . So, the remaining Li+ ion was strapped in the structure to enhance the stability of the sodiated phase.The electrochemical properties were evaluated after three charges/discharge cycles in the sodium half-cell. In Figure3(a), CV curves of new cathode exhibited the features differing from LiNMC in lithium half-cell [27] and the highest intensity peak shifted below 2.5 V. According to the voltage profile, this redox couple represents doubling plateaus that could be observed on the curve and its position coincides with the highest potential of couple peak previously reported for the single-phase NaNMC [9]. In addition, several peaks appeared on the oxidation and reduction curves due to sodium intercalation into the host structure. After the first cycle, the consecutive CV curves were almost superposition indicating the steady-state was gained.Figure 3
Electrochemical behavior of the sodiated NazNMC in half-cell of Na || NaClO4/PC-FEC (98 : 2) || NazNMC. (a) CV curves at a scan rate of 0.1 mV s−1. (b) The galvanostatic charge–discharge profiles at the rate of C/25 and C/10. (c) The specific capacity and Coulombic efficiency versus cycles and (d) rate capability test.
(a)(b)(c)(d)The voltage profile along with sodium intercalation is given in Figure3(b) and the discharge capacity obtained at each cycle is shown in Figure 3(c). It could be seen that the shape of the curve did not completely change but the capacity gradually decreased during cycling. The stable capacity was obtained of about 117 mAh g−1 (C/10) at the 50th cycle, corresponding to roughly 85% capacity retention calculated from the highest capacity of 137 mAh g−1 (C/25). At the same time, high Coulombic efficiency of almost 100% remains during cycling.The sodiated electrode displayed an excellent rate capability (Figure3(d)). The capacity slightly decreased when the rate changes from C/10 to 2 C were applied. At the rate of 2 C, the capability remained 105 mAh g−1 corresponding to 74.0% of its initial capacity at the rate of C/10. At a rate of 5 C, the capacity starts to drop faster but remains at a value of 90 mAh g−1. When it turned back to the original rate C/10, the capacity delivered is about 96.5% of its initial capacity.The sodiated electrode showed a capacity decrease with the cycle at a normal rate of C/10, as could be seen in Figures3(c) and 3(d). The structure of the new sodiated cathode was investigated to correlate with its electrochemical performance. As seen in Figure 4(a), XRD patterns of the pristine electrode LiNMC exhibited the peaks of the O3-layered structure (LiCo0.3Ni0.7O2 as a reference, PDF#01-087-1563). After the first discharge at 2 V, XRD patterns revealed that peaks belonging to the O3-type layered structure of the O3-NayNMC phase [9] (NaNi0.5Mn0.5O2, PDF#054-0887 as a reference, Figure 4(b)) existed together with peaks of the original O3-layered LiNMC. Indeed, the peaks of the new phase shifted to lower 2θ degrees demonstrated the increase of distance between adjacent slabs due to the insertion of bigger Na+ ions (ionic radius of Na+ ion is 1.02 Å, compared to 0.76 Å of Li+ ion [28]). The lattice parameters were calculated using hexagonal symmetry and R-3 m space group, typically, a = b = 2.8555 Å; c = 14.2109 Å; and V = 100.35 Å3 for the pristine LiNMC and a = b = 2.8886 Å; c = 16.0428 Å; V = 115.93 Å for the sodiated NayNMC. The values of the electrochemically synthesized NayNMC are lower than those of the chemically synthesized NaNMC [8, 9], possibly because of the introduction of lithium-ion trapped in the slab which has a shorter metal-oxygen bond length than that of sodium analog.Figure 4
(a) XRD of pristine LiNMC, 2nd discharge, and 20th discharge of new sodium cathode; (b) XRD of the new sodium cathode at the 20th discharge; and (c) EIS spectra of the LiNMC cathode in Li-cell and the new sodium cathode at various stages in Na-cell.
(a)(b)(c)The surface roughness (as seen in Figure5) combining with lattice strain induced by the insertion of bigger size sodium-ion contributed to the broadening of collected diffraction peaks after sodiation. Furthermore, the XRD pattern collected after few cycles (the highest pattern in Figure 4(a)) demonstrates only the peaks of O3-NayNMC, which is similar to the pattern of the first sodiation stage; hence, it revealed that the O3-type structure remained stable after cycling. The XRD results indicated that the concentration of lithium-ion decreased to a minimum after the second discharge leading to the vanishing of the LixNMC phase.Figure 5
SEM images of the pristine electrode LiNMC (a-c) and the EDX spectrum (d). SEM images of the sodiated NayNMC after 50 cycles in sodium half-cell (e-g) and the EDX spectrum (h). EDX results for elements percentage were put inside the corresponding EDX spectrums.EIS spectra of the electrode at a different stage was investigated (Figure4(c)). The first assemble electrode in lithium half-cell and sodium half-cell (the small figure inside Figure 4(c)) is characterized by two depressed semicircles of charge transfer processes and a long-sloping tail of ionic diffusion. The charge transfer process could be divided by small charge transfer resistance of SEI (Solid Electrolyte Interface) layered appearing at high frequency and another charge transfer resistance of the electrode at the medium-to-low frequency. The diameter of the semicircles is equal to the resistance values of the corresponding process. It is observed that the lower specific frequency at the top of semicircles and the higher charge transfers were obtained in sodium half-cell possibly indicating slower kinetic of sodium intercalation compared to lithium intercalation into the same delithiated host [20]. EIS spectrum after sodiation exhibits three semicircles as previously reported on NayNMC electrode [29]. The first and second semicircles observed at high frequency represent SEI and charge transfer of the electrode, respectively. Regarding the diameter of the first semicircle, the resistance of SEI layered did not change much after the 1st cycle indicating a stable formation of the SEI layer before and after cycling. However, the diameter of the second semicircle changed with cycling. The EIS spectra obtained at the 1st discharge showed that the charge transfer resistance obviously reduces due to electrolyte permeability and increases gradually during cycling. However, the charge transfer resistance enlarges by cycle and the value obtained at the 30th cycle is approximately 1.5 times higher than that obtained at the 1st cycle. Since the specific frequency could be an indicator for the change of chemical composition at the surface [30], the significant reduction of a specific frequency at the 30th cycle presumably coincides with the change of electrode composition and phase component given by the XRD result, Figure 4(a), and the observed degradation of the electrode.It is noticeable that the diffusion tail of the initial EIS changing into an elongated semicircle after the first discharge (1st sodiation) indicated a change in diffusion mode from semi-infinite to finite diffusion. This result could be explained by the restriction of sodium-ions diffusion in the sodium-rick phase outside the particles and/or in the porous structure of the cathode after sodiation.SEM images of the pristine electrode LiNMC and the sodiated NazNMC are given in Figure 5. LiNMC is crystallized in spherical shape particles with a diameter of about 500 nm agglomerated in macrosize spherical shape clusters (Figures 5(a)–5(c)). The particles are covered by small carbon beads with 50 nm diameters. Overall, the LiNMC cathode has a uniform distribution of components facilitating their electrochemical performance. After the sodiation process, SEM images in Figures 5(e) and 5(g) displayed several cracks and the round shape particles become rough. Carbon layered (red circled in Figure 5(e)) seemed to be removed from the surface of NayNMC; thus, NayNMC particles mostly are exposed in SEM images. The change of particle size could not be observed but the lattice parameters increased as mentioned. Besides, the needle-like crystals present in SEM images might be residual sodium salt remaining after washing but not likely the product of the reaction between the electrode with moisture and air as [31].The SEM results revealed that the change of electrode’s morphology, leaching of carbon coating, contributed to the increase of resistance and gradual degradation of electrode performance during the cycle. Moreover, this allowed side reactions such as electrolyte decomposition to occur that consumed the electron and reduce the current efficiency resulting in fast degradation of capacity at low current density as seen in Figures3(c) and 3(d).EDX results for elements analysis are also inserted in Figures5(e) and 5(f). Medium and heavy elements (transition metals) could be mostly detected for LiNMC and the sodiated NayNMC; only lithium cation is negligible due to the lightweight. The composition of the electrode seems to change after cycling in sodium half-cell. EDX results of the new sodiated cathode displayed a relatively high concentration of sodium as expected, but a slight decrease in the concentration of transition metals was obtained due to sodium salt residue in the cathode surface.Although the sodium-ion migration in the oxide depends on the concentration of sodium-ion in the host and the potential and is affected by the phase transition [32], the apparent diffusion coefficient of Na+ ion intercalated into the sodiated structure was investigated as an indicator for kinetic of the electrode reaction. According to XRD results, the insertion of Na+ into lithiated LiNMC leads to the formation of an O3-like layered structure after the first discharging. It is well known that the intensity of oxidation/reduction peak is proportional to the square root of scan rate demonstrated by the Randles–Sevcik equation [33–36]:(1)I=2.69×105n3/2AD1/2Cv1/2,where I (A), n, A (cm2), D (cm2 s−1), C (mol cm−3), and ν (V s−1) are the current at peak, electrode area, diffusion coefficient, and concentration of sodium-ion at full discharge state at specific scan rate, respectively. The concentration of sodium-ion could be calculated as a ratio between the number of intercalated sodium-ion at the equilibrium discharge (0.53 ion Na+) and the volume cell of the O3-NaxNMC sodiated phase. As could be seen in Figure 6(a), the shape of the CV curves did not change after several scan rates. Moreover, one pair of redox peaks at 2.7 V and 2.2 V corresponding to the phase transition from O3 to P3 phase during the sodium extraction/insertion process could be seen. The peak currents at each scan rate were plotted as a function of scan rate square root (in Figure 6(b)). The linear relationship of both oxidation and reduction peaks indicates that equation (1) is satisfied with highly acceptable R2 values > 99.8%.Figure 6
CV curve of sodium intercalation/deintercalation into LixNMC at various scan rates (a). The plot of peaks current as a function of the square root of the scan rate (b).
(a)(b)The diffusion coefficient could be deduced from the slope of the linear lines, as equation (1). The diffusion coefficients calculated are 6.75 × 10−11 cm2 s−1 and 4.0 × 10−11 cm2 s−1 for oxidation and reduction scan, respectively. The values of diffusion coefficients are quite close to the value obtained by GITT technique in the biphasic region of the sol-gel LiNMC [13]. There are many reasons to explain for relatively low diffusion coefficient including cathode damage due to conductive carbon removing so as to prevent the transportation of sodium to the surface.


## 4. Conclusion

The new sodium insertion host was prepared by the sodiation of LiNMC and evaluated the sodium intercalation behaviors. It was demonstrated that the O3-type structure of the pristine LiNMC remained after the insertion of Na+ ion. Basically, the delithiated host NayNMC could receive a proportional number of sodium-ions intercalated into the structure as most lithium-ion extraction to give higher capacity than the synthesized NaNMC so a high charge cutoff voltage was applied to extract more lithium-ions, but its capacity is incomparable to lithium analog capacity due to bigger sodium-ions than lithium ones. Sodiated NazNMC cathode displays a potential performance with 85% capacity retention after 50 cycles. However, the particle cracking, cathode damage due to cell disassemble, electrolyte decomposition, and conductive carbon removing might be responsible for the structural deterioration and should be resolved to enhance the stability of Na+ ion intercalation. Once this obstacle can be overcome, the ion-exchange method gives a direct, one-step, facile way to prepare the cathode NayNMC for high-performance Na-ion batteries.



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