“Win‐Win” Scenario of High Energy Density and Long Cycling Life in a Novel Na3.9MnCr0.9Zr0.1(PO4)3 Cathode

The development of high‐energy and long‐lifespan NASICON‐type cathode materials for sodium‐ion batteries has always been a research hotspot but a daunting challenge. Although Na4MnCr(PO4)3 has emerged as one of the most promising high‐energy‐density cathode materials owing to its three‐electron reactions, it still suffers from serious structural distortion upon repetitive charge/discharge processes caused by the Jahn‐Teller active Mn3+. Herein, the selective substitution of Cr by Zr in Na4MnCr(PO4)3 was explored to enhance the structural stability, due to the pinning effect of Zr ions and the ≈2.9‐electron reactions, as‐prepared Na3.9MnCr0.9Zr0.1(PO4)3/C delivers a high capacity retention of 85.94% over 500 cycles at 5 C and an ultrahigh capacity of 156.4 mAh g−1 at 0.1 C, enabling the stable energy output as high as 555.2 Wh kg−1. Moreover, during the whole charge/discharge process, a small volume change of only 6.7% was verified by in situ X‐ray diffraction, and the reversible reactions of Cr3+/Cr4+, Mn3+/Mn4+, and Mn2+/Mn3+ redox couples were identified via ex situ X‐ray photoelectron spectroscopy analyses. Galvanostatic intermittent titration technique tests and density functional theory calculations further demonstrated the fast reaction kinetics of the Na3.9MnCr0.9Zr0.1(PO4)3/C electrode. This work offers new opportunities for designing high‐energy and high‐stability NASICON cathodes by ion doping.


Introduction
[3] Nonetheless, the larger cation radius of sodium than that of lithium induces irreversible structural changes of electrode materials during the (de)sodiation process, meanwhile, the higher redox potential of Na compared with Li (−2.71 V vs -3.04 V relative to the standard hydrogen electrode) lowers the working voltage and thus the energy density of SIBs. [4]The above puzzles seriously limit the widespread applications of SIBs.[22] Elevating working voltage by adopting highpotential redox couples and increasing capacity by achieving multielectron reactions have been recognized as two effective strategies to boost the energy density of NASICON cathodes.For example, Goodenough's group fabricated Na 4 VMn(PO 4 ) 3 through partial substitution of V by Mn, which slightly improved the output voltage to 3.5 V (vs Na + /Na) because of the higher potential of Mn 2+ /Mn 3+ center (3.6 V) than that of V 3+ /V 4+ (3.4 V). [23] Nearly at the same time, Na 3 MnTi(PO 4 ) 3 was reported and the activation of both Mn 2+ /Mn 3+ and Mn 3+ /Mn 4+ (4.1 V) redox couples further promoted the working voltage of such materials. [24]Later, Yamada et al. [25] obtained Na 3 Cr 2 (PO 4 ) 3 with a high operating voltage up to 4.5 V, while apparent capacity decay was observed for this compound during cycling.Notably, the number of electrons that participated in electrochemical reactions for these NASICON-type cathodes is restricted to 2 per formula unit, which severely limits the sodium-storage capacity.To address this issue, Masquelier et al. further raised the upper cutoff voltage of Na 4 MnV(PO 4 ) 3 to 4.3 V, leading to a high capacity of 156 mAh g −1 by activating the V 4+ /V 5+ (3.9 V) redox couple.However, irreversible structural changes resulted in only 2-Na + (de) The development of high-energy and long-lifespan NASICON-type cathode materials for sodium-ion batteries has always been a research hotspot but a daunting challenge.Although Na 4 MnCr(PO 4 ) 3 has emerged as one of the most promising high-energy-density cathode materials owing to its threeelectron reactions, it still suffers from serious structural distortion upon repetitive charge/discharge processes caused by the Jahn-Teller active Mn 3+ .Herein, the selective substitution of Cr by Zr in Na 4 MnCr(PO 4 ) 3 was explored to enhance the structural stability, due to the pinning effect of Zr ions and the ≈2.9-electron reactions, as-prepared Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C delivers a high capacity retention of 85.94% over 500 cycles at 5 C and an ultrahigh capacity of 156.4 mAh g −1 at 0.1 C, enabling the stable energy output as high as 555.2 Wh kg −1 .Moreover, during the whole charge/discharge process, a small volume change of only 6.7% was verified by in situ X-ray diffraction, and the reversible reactions of Cr 3+ /Cr 4+ , Mn 3+ /Mn 4+ , and Mn 2+ /Mn 3+ redox couples were identified via ex situ X-ray photoelectron spectroscopy analyses.Galvanostatic intermittent titration technique tests and density functional theory calculations further demonstrated the fast reaction kinetics of the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C electrode.This work offers new opportunities for designing high-energy and high-stability NASICON cathodes by ion doping.
intercalation after the first cycle. [26]Zhao's team and Mai's group realized the reversible three-electron reactions in Na 3 V 1.5 Al 0.5 (PO 4 ) 3 (V 2+ / V 3+ , V 3+ /V 4+ , and V 4+ /V 5+ couples) and Na 3 MnTi(PO 4 ) 3 (Ti 3+ /Ti 4+ , Mn 2+ /Mn 3+ , and Mn 3+ /Mn 4+ couples), respectively, by lowering the discharge cutoff voltage. [4,27]Nevertheless, a relatively low average voltage was observed, which was caused by the low-potential V 2+ /V 3+ (1.6 V) or Ti 3+ /Ti 4+ (2.1 V) couple.Standing on the shoulders of the above-mentioned pioneering studies, our group successfully prepared the Na 4 MnCr(PO 4 ) 3 /C cathode, where reversible three-electron reactions along with a high output voltage of ≈3.53 V were achieved, leading to the highest specific energy of 566.5 Wh kg −1 ever reported for phosphate cathodes. [28]The concern is the unsatisfactory cycling performance mainly resulted from the Jahn-Teller lattice distortions of Mn 3+ .Therefore, it is imperative to develop a synergic strategy to improve the cycling durability of Na 4 MnCr(PO 4 ) 3 and simultaneously maintain its high energy density, which, as far as we know, still remains a big challenge.
Herein, we report a novel Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C cathode where inert Zr 4+ can solidly rivet atoms together upon Na + insertion/ extraction and thus stabilize the crystal structure.Meanwhile, the substitution of Cr 3+ by larger-radius Zr 4+ effectively enlarges the unit cell volume and facilitates Na-ion diffusion.As a result, the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C cathode exhibits significantly improved cycling performance (85.94% capacity retention after 500 cycles at 5 C), outstanding rate capability (156.1 mAh g −1 at 0.1 C and 67.8 mAh g −1 at 15 C), and ultrahigh energy density up to 555.2 Wh kg −1 based on ≈2.9-electron transfer.The highly reversible solid-solution and bi-phase reactions of the cathode material and the Cr 3+ /Cr 4+ , Mn 3+ /Mn 4+ , and Mn 2+ /Mn 3+ charge compensation mechanisms upon charge/discharge were revealed by in situ X-ray diffraction (XRD) and ex situ X-ray photoelectron spectroscopy (XPS).In addition, the high ionic/electronic conductivity of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C was confirmed by multiple electrochemical measurements and density functional theory (DFT) calculations.Finally, the Naion full battery based on Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C cathode and hard carbon anode were constructed, demonstrating great prospects for practical applications.

Results and Discussion
The function of Zr doping is schematically illustrated in Figure 1a.The inert Zr 4+ ions in Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 can maintain stablility upon repeated Na + release/uptake and thus generate a pinning effect to effectively stabilize the NASICON framework, meanwhile, the larger ionic radius of Zr 4+ than that of Cr 3+ enlarges the unit cell volume, providing fast channels for Na-ion transportation.In the present study, a series of Na 4-x MnCr 1-x Zr x (PO 4 ) 3 /C (x = 0, 0.05, 0.1, and 0.15) materials were synthesized through a sol-gel method followed by a heat treatment, the XRD patterns of all the samples are well indexed to the rhombohedral NASICON structure with a R3c space group (Figure S1, Supporting Information).Figure 1b displays the XRD Rietveld refinement of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C, in which a good coincidence between the calculated and experimental data is obtained (R wp =2.22%).The unit cell parameters deduced from the structural refinement are as follows: a = b = 8.934 Å, c = 21.517Å, and V = 1487.265Å3 (Table S1, Supporting Information), which are larger than those of the pristine Na 4 MnCr(PO 4 ) 3 /C (a = b = 8.914 Å, c = 21.453Å, and V = 1476.139Å3 , Figure S2 and Table S2, Supporting Information) due to the larger radius of Zr 4+ (0.72 Å) than that of Cr 3+ (0.615 Å). [29,30] The scanning electron microscopy (SEM) image in Figure 1c shows the nanoparticles morphology of the asprepared Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C, and the particle sizes mainly range from 150 to 300 nm.The high-resolution transition electron microscopy (HRTEM) image further reveals that the phosphate nanograins are uniformly coated by the carbon layer with a thickness of approximately 3 nm (Figure 1d).It has been validated that the carboncoated nanostructures can effectively enhancing the electronic/ionic conductivity and improve the structural stability. [21]Moreover, the regular lattice fringes with an interplanar distance of 0.28 nm (Figure S3, Supporting Information) match well with the (116) plane of the NASI-CON structure.The corresponding fast Fourier transform (FFT, inset in Figure 1d) pattern with clear diffraction points also confirms the high crystallinity of the sample.Thermogravimetric (TG) and SEM analyses were performed to optimize the carbon content in the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C material (Figures S4 and S5, Supporting Information).It is demonstrated that an appropriate carbon content of 16.06 wt% can effectively inhibit particles agglomeration, while the particles obviously grow larger after reducing the carbon content (11.07 wt%), and the particles are seriously adhered together after increasing the carbon content (18.90 wt%).Raman spectrum (Figure S6, Supporting Information) of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C shows the two characteristic peaks of carbon at 1350 cm −1 (D-band) and 1590 cm −1 (G-band), and the intensity ratio of D band to G band (I D / I G ) is calculated to be 0.85.This indicates the partial graphitization of carbon, accounting for its favorable electronic conductivity. [31,32]nductively coupled plasma mass spectroscopy, survey X-ray photoelectron spectroscopy (XPS), and TEM energy dispersive spectroscopy (EDS) tests verify the existence of Na, Mn, Cr, Zr, P, O, and C elements in Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C, and the atomic ratio is very close to the designed stoichiometry (Figure S7 and Table S3, Supporting Information), suggesting the high purity of the as-prepared sample.SEM-EDS mapping images demonstrate the uniform distribution of these elements within the nanoparticles (Figure 1e).Additionally, Fourier transform infrared (FTIR) spectral analysis confirms the presence of the M-O (M = Mn/Cr/Zr) and P-O bonds in the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 / C sample (Figure S8, Supporting Information). [33,34]The core-level Zr 3d XPS spectrum shows two characteristic peaks at about 182.98 and 185.48 eV, corresponding to the Zr 3d 5/2 and Zr 3d 3/2 peaks of Zr 4+ (Figure S9a, Supporting Information). [29,35][38] Moreover, the N 2 adsorption-desorption curves manifest that the Brunauer-Emmett-Teller specific surface area of Na 3.9 Mn Cr 0.9 Zr 0.1 (PO 4 ) 3 /C is 91.11 m 2 g −1 , and the pore size distribution is centered at 3.6 nm (Figure S10, Supporting Information).
The electrochemical properties of the Na 4-x MnCr 1-x Zr x (PO 4 ) 3 /C (x = 0, 0.05, 0.1, and 0.15) materials were evaluated in coin-type Na half cells.Figure 2a plots the cyclic voltammetry (CV) curves of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C at 0.1 mV s −1 over a voltage range of 1.4-4.6V (vs Na + /Na), three distinct pairs of redox peaks located at 4.43/ 4.34 V, 4.28/4.07V, and 3.73/3.42V are attributed to the Cr 3+ / Cr 4+ , Mn 3+ /Mn 4+ , and Mn 2+ /Mn 3+ couples, respectively (as will be demonstrated later).The well-overlapped CV curves after the first charge process validate the high reversibility of the Na + insertion/extraction behavior.Consistent with the CV results, the galvanostatic charge/discharge profiles of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C at 0.1 C Energy Environ.Mater.2024, 7, e12519 2 of 8 (1 C = 160 mA g −1 , based on 2.9-electron transfer) between 1.4 and 4.5 V also display three voltage plateau regions of the stepwise Cr and Mn redox reactions (Figure 2b). [28]The trailing in the discharge curve below 2.0 V should be attributed to the residual Mn 3+ /Mn 2+ reduction reaction and the pseudocapacitive behavior (as will be discussed later). [28,30,39]Since the carbon matrix contributes inappreciable capacity to the composite (Figure S11, Supporting Information), the specific capacity was calculated based on the mass of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 in this study.The electrode can deliver an initial charge/discharge capacity of 170.8/156.4][41][42][43][44] The irreversible capacity can be attributed to the slight electrolyte decomposition at high potential (Figure S12, Supporting Information).Intriguingly, the combination of high capacity and high working voltage (≈3.55 V) endow the electrode with a remarkable energy density up to 555.2 Wh kg −1 .In addition, the electrochemical impedance spectroscopy (EIS) measurements demonstrate the favorable electronic conductivity and superior ionic diffusivity of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C with a 16.06 wt% carbon content (Figure S13, Supporting Information).For comparison, the charge/discharge profiles and CV curves of the Na 3.9 MnCr 0.95 Zr 0.05 (PO 4 ) 3 /C (Zr-0.05)and Na 3.9 MnCr 0.85 Zr 0.15 (PO 4 ) 3 /C (Zr-0.15)counterparts were also tested.Among these Zr-doped samples, the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 / C cathode featuring the second-highest capacity, the smallest capacity loss, and the highest overlapping degree of CV curves (Figure S14, Supporting Information), shows the optimum overall performance.
Since Zr content significantly affects the electrochemical performance, we further evaluated the cycling stability of all the Zr-doped samples in comparison with the pristine Na 4 MnCr(PO 4 ) 3 /C (Figure 2c).It is found that Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C exhibits the highest discharge capacity retention of 87.57% over 50 cycles at 0. These results indicate that Zr 4+ doping has a positive effect on stabilizing the NASICON structure.The inferior cycling performance of the Zr-0.05 sample is mainly caused by the less pinning effect, while the increased Na1 occupancy in the Zr-0.15 sample weakens the electrostatic repulsion between MO 6 octahedra and leads to the decrease of c parameter (Figures S4 and S15; Tables S1, Supporting Information), which limits the Na-ion diffusion and results in the performance degradation. [45]Moreover, excessive Zr doping would disrupt the long-range ordered NASICON structure to a certain extent (Figure S1, Supporting Information), resulting in unsatisfactory cycling performance.The post-mortem XRD and SEM examinations of the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C electrode after cycling show that the rhombohedral NASICON structure and the nanoparticles morphology (without obvious agglomeration) are well maintained (Figure S16, Supporting Information), accounting for its excellent cyclic stability. [46]igure 2d depicts the rate capability of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C, where discharge capacities of 156.1, 148.0, 137.0, 128.9, 117.3, 92.8, 79.8, and 67.8 mAh g −1 are obtained at 0.1, 0.2, 0.5, 1, 2, 5, 10, and 15 C, respectively.The superb rate performance could be attributed to the low electronegativity of Zr (1.33 for Zr vs 1.66 for Cr) and enlarged unit cell volume, which synergistically enhances the electronic and ionic conductivities of the cathode material (EIS spectra are given in Figure S17, Supporting Information). [47]][50] The extended durability of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C is demonstrated at a current density of 5 C, affording respectable capacity retentions of 91.05% and 85.94% after 200 and 500 cycles, respectively (Figure 2e).The cycling durability and rate capability of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C are summarized in Table S5 (Supporting Information), which outperform those of the reported Mn-based NASICON-structured cathodes for SIBs.
To investigate the structural evolution of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 / C upon Na + release/uptake, in situ XRD was conducted by employing Al foil as both current collector and monitoring window (Figure 3a,b).The ( 116) and (113) diffraction peaks move toward higher 2θ angles during the charge process, and restore to their initial positions at the end of discharge, corresponding to a solid-solution reaction accompanied by the lattice shrinkage/expansion upon sodium extraction/insertion.In contrast, the (211) peak firstly vanishes during charge and reappears in the following Na + insertion process.Furthermore, the (300) peak splits into two peaks and eventually coalesces into a single peak again.These phenomena indicate a typical bi-phase reaction.Hence, both solid-solution and bi-phase reactions are involved in the (de)sodiation process.All diffraction peaks undergo significant shifts and then recover to their pristine states, illustrating the high structural reversibility of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C.][52][53] Such a low volume change is mainly attributed to the mitigated structural distortion enabled by Zr doping, rendering the respectable cycling life of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C.
Thereafter, ex situ XPS was applied to shed more light on the valence variations of Mn and Cr in Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C during the initial Na + extraction/insertion process.At the open-circuit voltage (OCV) state, the Mn 2p 1/2 (653.9eV) and Mn 2p 3/2 (641.8 eV) peaks (accompanied by a satellite peak at 645 eV) in the core-level Mn 2p spectrum reveal the bivalence state of Mn. [34,46] Upon charging to 3.8 V, the binding energy of Mn 2p 3/2 shifts to 642.2 eV, corresponding to the oxidation from Mn 2+ to Mn 3+ .When further charged to 4.3 V, the Mn 2p 3/2 peak moves to 642.6 eV, manifesting the Mn 3+ to Mn 4+ transition (Figure 3d). [49]In contrast, the binding energies of Cr 2p peaks remain constant until charging to 4.3 V, then the peaks obviously shift to higher binding energies when charged to 4.5 V, indicating the oxidation from Cr 3+ to Cr 4+ (Figure 3e). [40]Therefore, the ultrahigh capacity of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C is ascribed to the ≈2.9-electronMn 2+ /Mn 3+ , Mn 3+ /Mn 4+ , and Cr 3+ /Cr 4+ reactions.When the electrode is subsequently discharged from 4.5 to 1.4 V, both Mn 2p and Cr 2p spectra recover to their pristine states, suggesting the highly reversible charge-compensation reactions in Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 / C. It is noteworthy that there are partial Mn 3+ species remained in the electrode when discharged to 2.0 V (Figure S18, Supporting Information), due to the slight voltage hysteresis.This is responsible for the discharge capacity below 2.0 V.
To probe the electrode process kinetics of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 / C, galvanostatic intermittent titration technique (GITT) and variablescan-rate CV measurements were performed.Figure 4a displays the GITT voltage profiles in the third cycle along with the corresponding Na + diffusion coefficients (D Na , calculation details are provided in Supporting Information).The insets of Figure 4a show that the cell underwent a constant current flux (0.1 C) for 30 min followed by a relaxing time for 120 min to allow the voltage to reach equilibrium.The calculated D Na values mainly vary from 10 −11 to 10 −10 cm 2 s −1 , which compare favorably with those of the reported Na 4 MnCr(PO 4 ) 3 and other Mn-based NASICON-type cathodes (Table S6, Supporting Information). [28,34,39,54]In addition, the pseudocapacitive contribution to the charge storage process of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C was identified by the CV profiles at different scan rates (Figure 4b), based on the relation of i p ¼ av b (logi p ¼ blogv þ loga), where i p is the peak current, v is the scan rate, a and b are variable parameters.When the b value is close to 0.5, the Na-storage reaction is mainly controlled by ion-diffusion process; when the b value approaches 1.0, pseudocapacitive behavior dominates the reaction.As calculated, the b values for the R1, R2, O1, and O2 peaks are all larger than 0.5 but lower than 1.0 (Figure S19a, Supporting Information), indicating the combination of diffusion-limited and capacitive behaviors in the Figure 3. a) In situ X-ray diffraction patterns of the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C electrode and the unit cell volume change during the first charge/discharge process at 0.2 C, as well as the corresponding b) contour plots.c) Comparison of volume variation (ΔV/V pristine ) between the present Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 and previously reported NASICON cathodes for SIBs.Ex situ XPS of core-level d) Mn 2p, and e) Cr 2p spectra of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C at various charged/discharged states.
Energy Environ.Mater.2024, 7, e12519 Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C electrode.The pseudocapacitive contribution is further quantified according to the equation of where k 1 v and k 2 v 1=2 represent the capacitive and diffusion-controlled currents, respectively. [43]Figure 4c presents the representative CV curve at 0.4 mV s −1 , displaying a 68.8% pseudocapacitive contribution (purple region) to the total current.The pseudocapacitive contribution gradually increases with the increase of scan rate (Figure S19b, Supporting Information).
The effect of Zr-doping on the electrode ionic/electronic conductivity was further investigated by DFT computations.The optimum Na + migration pathways in the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 framework following the Na1↔Na2 channels are simulated based on the climbing-image nudged elastic band (cNEB) method (Figure S20, Supporting Information). [28]As shown in Figure 4d, Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 possesses lower Na-migration energy barriers (0.25-0.33 eV) than that of the undoped Na 4 MnCr(PO 4 ) 3 (0.39 eV), confirming the improved sodium diffusion kinetics owing to the enlarged unit cell volume (broadened diffusion channels) by Zr doping.Additionally, the density of states (DOS) calculations reveals that both Na 4 MnCr (PO 4 ) 3 and Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 exhibit some metallic properties without energy band gap around the Fermi level (E f , Figure 4e), the higher states at E f of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 demonstrate the existence of more active electrons and thus the enhanced electronic conductivity after Zr-doping. [45]Therefore, the fast reaction kinetics and superior rate performance of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C can be attributed to the reduced ion-diffusion barrier and the accelerated electron transportation.
The practicality of the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C cathode was examined by assembling full batteries using the commercialized hard carbon (HC) anode (Figure 5a).The sodium-storage performance of the HC anode is provided in Figure S21 (Supporting Information), showing an operating potential of ≈0.1 V vs Na + /Na and a steady reversible capacity of ≈285 mAh g −1 .In a bid to balance the capacity, the active mass ratio of the anode to cathode was set as 1:1.7, and the voltage window of the full cell was optimized to be 1.2-4.5 V (Figure 5b). Figure 5c shows that the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C‖HC full cell delivers a first discharge capacity of 150.6 mAh g −1 at 0.1 C along with a working voltage of ≈3.45 V, leading to a remarkable energy density of 327.2 Wh kg −1 based on the total mass of anode and cathode active materials.The great superiority in capacity, voltage, and energy density of the present full cell compared with those of the state-of-the-art NASICON and layered oxide cathode-based Na-ion full cell systems is demonstrated in Figure 5d. [43,46,51,55,56]oreover, the full battery affords a high capacity retention of 91.03% after 50 cycles at 0.5 C (Figure 5e), even better than that in half cells.This phenomenon could be interpreted by the dendrite growth on sodium metal anode in half cells. [57]The rate performance of the full cell from 0.1 to 10 C is depicted in Figure 5f, a desirable discharge capacity of 70.9 mAh g −1 has still remained at 10 C. Intriguingly, the customized light-emitting diodes (LEDs) can be continuously lightened up by the prototype pouch cell (Figure 5f inset), signifying the bright future of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C for practical applications.

Conclusions
In summary, a novel NASICON-type Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C cathode with high energy density and long lifespan was developed for SIBs.The introduced Zr ions not only firmly support the NASICON framework against structural distortion, but also enlarge the unit cell volume to facilitate the reversible extraction/insertion of Na + .As a result, the tailored electrode achieves a high cycling stability (85.94% capacity retention over 500 cycles) and a high rate capability (156.1 mAh g −1 ) CV profiles at various scan rates ranging from 0.1 to 1.0 mV s −1 of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C and c) the capacitive contribution (purple area) to the total current at 0.4 mV s −1 .d) The Na-migration energy barriers along the Na1↔Na2 pathways and e) the total DOS of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 and Na 4 MnCr(PO 4 ) 3 .
Energy Environ.Mater.2024, 7, e12519 at 0.1 C and 67.8 mAh g −1 at 15 C).Meanwhile, the ≈2.9-electron reactions revealed by ex situ XPS at ≈3.55 V account for the ultrahigh energy density of the electrode (up to 555.2 Wh kg −1 ).In situ XRD demonstrates the highly reversible solid-solution and bi-phase reactions of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C with a small volume change of 6.7% during the complete charge/discharge process, contributing to the long-term cycling stability.Furthermore, the improved kinetic properties and enhanced electronic conductivity after Zr doping were verified by GITT, CV, EIS investigations, and DFT computations.As a proof of concept, the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C‖HC full battery displays a high capacity retention of 91.03% after 50 cycles along with an outstanding energy density of 327.2 Wh kg −1 , further proving the positive effect of Zr dopant on stabilizing crystal structure without sacrificing the specific energy.This work puts forward a "win-win" strategy to boost the energy density and cycling life of the Mn-based NASICON cathodes simultaneously.

Figure 1 .
Figure 1.a) Schematic illustration of the NASICON structure and the Na + -diffusion accelerating and pinning effects of Zr 4+ doping.b) Rietveld refined XRD pattern, c) SEM, d) HRTEM, inset is the corresponding FFT diffraction pattern, and e) SEM-EDS element mapping images of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C.

Figure 4 .
Figure 4. a) Charge/discharge GITT curves of the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C cathode at 0.1 C and corresponding D Na values.Insets show the representative steps from the GITT curves.b) CV profiles at various scan rates ranging from 0.1 to 1.0 mV s −1 of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C and c) the capacitive contribution (purple area) to the total current at 0.4 mV s −1 .d) The Na-migration energy barriers along the Na1↔Na2 pathways and e) the total DOS of Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 and Na 4 MnCr(PO 4 ) 3 .

Figure 5 .
Figure 5. a) Schematic diagram of the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C‖HC full battery.b) CV curves of the Na 3.9 MnCr 0.9 Zr 0.1 (PO 4 ) 3 /C cathode and HC anode.c) Galvanostatic charge/discharge profiles of the full battery at 0.1 C. d) Comparison of capacity, voltage, and energy density of the full battery in this work with those of state-of-the-art sodium-ion full cells.e) Cycling and f) rate performances of the full battery.Insert digital photo exhibits LEDs enlightened by the pouch cell.