Stable sodium metal anode enabled by interfacial room‐temperature liquid metal engineering for high‐performance sodium–sulfur batteries with carbonate‐based electrolyte

Sodium (Na) metal is a competitive anode for next‐generation energy storage applications in view of its low cost and high‐energy density. However, the uncontrolled side reactions, unstable solid electrolyte interphase (SEI) and dendrite growth at the electrode/electrolyte interfaces impede the practical application of Na metal as anode. Herein, a heterogeneous Na‐based alloys interfacial protective layer is constructed in situ on the surface of Na foil by self‐diffusion of liquid metal at room temperature, named “HAIP Na.” The interfacial Na‐based alloys layer with good electrolyte wettability and strong sodiophilicity, and assisted in the construction of NaF‐rich SEI. By means of direct visualization and theoretical simulation, we verify that the interfacial Na‐based alloys layer enabling uniform Na+ flux deposition and suppressing the dendrite growth. As a result, in the carbonate‐based electrolyte, the HAIP Na||HAIP Na symmetric cells exhibit a remarkably enhanced cycling life for more than 650 h with a capacity of 1 mAh cm−2 at a current density of 1 mA cm−2. When the HAIP Na anode is paired with sulfurized polyacrylonitrile (SPAN) cathode, the SPAN||HAIP Na full cells demonstrate excellent rate performance and cycling stability.

Nevertheless, Na metal anode is highly susceptible to side reactions with the electrolyte and form thick solid electrolyte interphase (SEI) due to its ultrareactive nature, which resulting in uneven Na + flux and Na dendrite growth. [18,19]Cui et al. [20] reported that the SEI layer is significantly less stable in carbonate-based electrolytes than ether-based electrolytes.Worse still, the solubility of Na dendrites in the carbonate-electrolyte is much higher than that of Li dendrites, generating "dead Na," which accompanied by a decrease in Coulomb efficiency and an increase in impedance. [21,22]Various strategies were employed to solve the issues of dendrite growth in ester-based electrolyte, including construction of a threedimensional (3D) porous host of Na, [13,[23][24][25] interfacial engineering, [26][27][28][29] optimizing electrolyte and separator, [30][31][32][33][34] and so forth.Undoubtedly, the above modification strategies have a significant effect in reducing the local current density, relieving volume expansion, and promoting a homogeneous Na + flux, and is capable of alleviating Na dendrite growth to a certain extent. [35]evertheless, the electrolyte additives are continuously consumed during the cycle, and the large exposed surface area of the 3D current collectors exacerbates electrode surface side reactions.By contrast, interface engineering, including deposition of electronic insulating passivation layer (e.g., Al 2 O 3 ), [36,37] construction of sodiophilic alkali metal-Na alloy protective layer (e.g., Na-Sn and Na-Bi) [38][39][40] and artificial SEI layer (e.g., NaF, NaBr, and NaTe), [41][42][43] appears to be an effective way to solve the above issues.Among them, sodiophilic alkali metal-Na alloys have both electronic and ionic conductivity, and can be formed by direct alloying with Na metal, thus it is considered to be a simple and effective solution.
Room-temperature liquid metal (RLM) usually refers to a class of metals or alloys such as Ga metal, Hg metal, Ga-based alloy (GaIn, GaInSnZn), and liquid Na-K alloy, and so forth. [44][47][48][49][50] For instance, Zhang et al. [51] reported a simple strategy to prepare alloy-based Li-ion battery anode via 3D MXene conductive skeleton in-situ encapsulated eutectic GaIn liquid metal (LM-MXene).The results show that the prepared LM-MXene exhibits an excellent cycling performance (409.8 mAh g −1 at 5 A g −1 after 4500 cycles with a capacity retention of 90.8%).Impressively, the liquid metal undergoes spontaneous solid-liquid interconversion upon Li ion alloying/dealloying process, allowing the crushed structure to self-healing and maintaining the structural integrity, while MXene skeleton buffers the volume expansion during alloying and prevented the liquid alloy from agglomerating.Fan et al. [52] designed a Li-Hg interface with "self-healing" property through the in situ reaction of spreading mercury droplet onto the Li metal surface at room temperature.This self-healing interface improves the compatibility of the lithium metal anode with the solid electrolyte and significantly reduces the anode/electrolyte interfacial impedance.Wei et al. [53] developed a novel method for inducing homogeneous deposition of Li via amorphous liquid metal nucleation seeds.The results demonstrate that Li metal symmetric cells have significantly improved electrochemical performance in ether-based and carbonate-based electrolytes.However, there are still fewer reports of liquid metal in Na metal anode.
Herein, we report a one-step strategy for the suppression of Na dendrite growth by heterogeneous Na-based alloys interfacial protective layer.Roomtemperature mobility and high reactivity provide the possibility for the GaInSn liquid metal (5°C, Ga, 62 at%; In, 25 at%; Sn, 13 at%,) to achieve self-diffusion on the Na metal surface, and in-situ alloying with Na metal to generate interfacial Na-based alloys layer.This interfacial Na-based alloys layer exhibits good sodiophilicity and electrolyte wettability, reduces the Na nucleation potential, and enables uniform Na ion deposition.Furthermore, the interfacial Na-based alloys layer alleviates the occurrence of side reactions at the Na metal/electrolyte interfaces and constructs a NaF-rich SEI after cycle, ensuring the stability of the cycles.Symmetric cells assembled using the as-prepared HAIP Na electrode demonstrate excellent cycling stability for 650 h at a current density of 1 mA cm −2 with a capacity of 1 mAh cm −2 in carbonate-based electrolyte.Likewise, the full cells composed of the HAIP Na anode coupling with sulfurized polyacrylonitrile (SPAN) cathode (labeled as SPAN||HAIP Na) exhibit an excellent rate performance (892.9 mAh g −1 at 5C) and superior long-term cycling performance (912.2 mAh g −1 after 1000 cycles at 3C with 87.4% capacity retention).

| RESULTS AND DISCUSSION
The heterogeneous Na-based alloys interfacial protective Na (HAIP Na) is synthesized based on the gravitational self-diffusion and high reactivity of liquid metal at room temperature.It should be noted that the melting point of the liquid metal depends on the content of each constituent element, [54] here we choose a GaInSn liquid metal with a melting point of 5°C.As we can see from Supporting Information S1: Figure S1A in supporting information, GaInSn is a liquid with a silvery-white appearance and fluidity property.Moreover, GaInSn liquid metal presents to be spherical in shape on the surface of Na foil due to the native oxide layer at the interface, which determining it does not wet for Na foil in the absence of external forces. [45]XRD pattern in Supporting Information S1: Figure S1B shows that GaInSn liquid metal is an amorphous state at room temperature.Figure 1A  the Na foil surface by brushing inside a glove box filled with argon gas, and in situ forming a stable heterogeneous Na-based alloys interfacial protective layer on the Na foil surface through the alloying reaction of Na metal and liquid metal.Obviously, the color changes on the surface of Na foil reflect an alloying reaction between liquid metal and Na metal, from silvery gray to grayblack (Supporting Information S1: Figure S2A-D, the images are taken with a phone).Moreover, scanning electron microscope (SEM) images also record the changes of the surface morphology of Na foil.As shown in Figure 1B, fresh Na foil has a smooth and flat surface.After GaInSn liquid metal plastering, GaInSn liquid metal is unevenly distributed on the Na foil (Figure 1C).Over time, GaInSn liquid metal uniformly spreads the surface of the Na foil through its fluidity (Figure 1D).Note that alloying process occurs when the liquid alloy comes into contact with the surface of Na foil.Finally, the liquid metal disappears on the Na metal surface, indicating that the alloying process is completed (Figure 1E).Morphologically, because the alloying process is along with volume changes, small pores appear on the surface of the HAIP Na foil, and the surface roughness increases, the cross-section SEM image in Supporting Information S1: Figure S3 exhibits that the interfacial Na-based alloys layer owns a thickness of approximately 50 μm.
X-ray diffraction (XRD) patterns are employed to reveal the formation processes of Na-based alloys.As shown in Figure 1F, Na foil displays a series of diffraction peaks at approximately 29°, 42°, 52°, 61°, and 69°, which can be indexed to (110), ( 200), (211), (220), and (310) crystal planes of Na metal (JCPDS No. 22-0948), and GaInSn liquid metal with a broad peak at around 34°.When the liquid metal is applied to the surface of the Na foil, the Na characteristic peaks disappear, and only amorphous peak of liquid metal is found.Notable, the diffraction peak at approximately 18.5°is produced by the protective tape (Supporting Information S1: Figure S4).As the alloying processes proceeding, the amorphous peak belonging to GaInSn liquid metal is reduced gradually, while the Na characteristic peaks reappear.Meanwhile, the peaks of Na-based alloys such as InNa, Na 4 Sn 9 , and Na 22 Ga 39 alloys are detected, respectively. [46]With the alloying time increases, the characteristic peaks gradually become stronger.24 h later, XRD patterns are basically consistent, indicating that the alloying processes of Na metal and GaInSn liquid metal are completed.HAIP Na is a complex mixture composed of Na metal, Na-Ga, Na-In, and Na-Sn alloys.Energy-dispersive spectrometer (EDS) element mapping of SEM image (Supporting Information S1: Figure S5) shows a uniform distribution of Na, Ga, In, and Sn elements on the Na foil surface (Figure 1G-J).In addition, the impact of interfacial Na-based alloys layer on the wettability of electrolyte is evaluated by observing the electrolyte spreading on the electrode surface.As shown in Supporting Information S1: Figure S6A, the electrolyte has an evident contact angle on the Na foil surface, while the HAIP Na is completely infiltrated by the electrolyte, indicating that the interfacial Na-based alloys greatly improve the wettability of the electrode, which could reduce the local current densities and achieve uniform Na plating/stripping.Furthermore, the interfacial Na-based alloys layer prepared by in-situ alloying has a good adhesion with the Na metal and does not fall off even under high bending condition (Supporting Information S1: Figure S6B).
To assess the advantages of artificial heterogeneous Nabased alloys interfacial protective layer, the Na plating/ stripping behaviors of Na||Na and HAIP Na||HAIP Na symmetric cells are studied in a cheap carbonate-based electrolyte.The voltage-capacity curves in Figure 2A investigate the electrochemical deposition behaviors of Na and HAIP Na electrodes, where the η n and η p represent the values of nucleation and plateau overpotential, respectively. [55]Obviously, the HAIP Na||HAIP Na symmetric cell exhibits both the smaller η n and η p values than that of Na|| Na symmetric cell, indicating that the interfacial Na-based alloys layer has a better sodiophilicity property, which could facilitate a uniform Na deposition.As shown in Figure 2B, the exchange current density (j 0 ) of the HAIP Na is 0.680 mA cm −2 , which is about twice higher than bare Na (0.292 mA cm −2 ), suggesting that the interfacial Na-based alloys layer significantly enhances the diffusion kinetics of Na + . [46]Figure 2C displays the electrochemical impedance spectroscopy (EIS) spectrum for the Na||Na and HAIP Na|| HAIP Na symmetric cells, and the inset in the figure shows the simulated equivalent circuit.For Nyquist plots, the semicircles in the high-frequency and low-frequency regions correspond to the SEI resistance (R SEI ) and charge transfer resistance (R ct ) separately.Clearly, the HAIP Na|| HAIP Na symmetric cell has a lower combination of R SEI and R ct than the Na||Na symmetric cell, being at 72 Ω versus 1624 Ω, certifying that the interfacial Na-based alloys layer has a crucial role in suppressing side reactions and building thin SEI at the electrode/electrolyte interfaces.
Given the merits mentioned above, the electrochemical performance are evaluated.Figure 2D and Supporting Information S1: Figure S7A,B present the long-term voltage-time profiles for the two sets of symmetric cells.Remarkably, HAIP Na||HAIP Na symmetric cells exhibit a more stable and lower initial voltage hysteresis than that of Na||Na symmetric cells at all current density.In addition, there are fluctuations in the voltage hysteresis of the Na||Na symmetric cell, which may be due to the repeated destruction and reconfiguration of the SEI during Na plating/stripping.As a result, the HAIP Na||HAIP Na symmetric cells show extremely long cycle life of 750 h at a current density of 0.5 mA cm −2 with capacity of 0.5 mAh cm −2 and 650 h at a current density of 1 mA cm −2 with capacity of 1 mAh cm −2 , significantly better than that of bare Na symmetric cells (145 h and 320 h, respectively).Even if the current density increases to 2 mA cm −2 , the HAIP Na||HAIP Na symmetric cell still exhibits a better cycling stability.Figure 2E shows the rate performances of Na||Na and HAIP Na||HAIP Na symmetric cells at current densities ranging from 0.5 to 3 mA cm −2 with a fixed capacity of 1 mAh cm −2 , among them, the plating/stripping time is varied accordingly to achieve 1 mAh cm −2 capacity each cycle.It is noteworthy that a short circuit occurs in Na||Na symmetric cell at a current density of 2 mA cm −2 , while for HAIP Na||HAIP Na symmetric cell, it is equally stable throughout the current density.The cycling performance of symmetric cells (i.e., initial overpotential and cycling stability) is bound up with the stability of the SEI.Therefore, we investigate the changes in resistance by the EIS impedance curves before and after cycling (Supporting Information S1: Figure S8).In consequence, the HAIP Na||HAIP Na symmetric cell is almost constant before and after cycling, indicating that HAIP Na electrode has an exceptionally stable SEI film.While for Na||Na symmetric cell, it undergoes a huge decay as cycling proceeding, probably due to the thick SEI formed initially breaking up and generating "dead Na" during the cycling.In addition, the XRD pattern of the Na-based alloy phase after five cycles is carried out to prove the stability of the interfacial protective layer.As demonstrated in Supporting Information S1: Figure S9, compared with the initial interfacial Na-based alloy phase, the composition of the interfacial protective alloy layer after 5 cycles is consistent with its initial composition, suggesting the good chemical and structural stability of the interfacial alloy layer.
The morphology of Na deposition on bare Na and HAIP Na electrodes in symmetric cells are explored.As plotted in Figure 3A-D, 3D rod-like interlaced dendrites cover the surface of bare Na with a large number of holes, which may be due to the difference in local current density caused by surface defects of Na foil, coupling with poor sodiophilicity, leading to uneven Na deposition.Compared to the bare Na electrode, HAIP Na electrode shows homogeneous Na deposition, even after 100 cycles of Na plating/stripping processes, there are still no visibly dendrites on the surface of HAIP Na electrode (Figure 3E-H), indicating that the HAIP Na electrode with a good sodiophilicity and able to induce uniform deposition of Na + ions on the electrode surface.X-ray photoelectron spectroscopy (XPS) measurements are further characterized to investigate the surface composition of SEI layers.Figure 3I-K demonstrate the high-resolution XPS spectra of C 1s, O 1s, and F 1s of the cycled Na and HAIP Na electrode in symmetric cells for 10 times, respectively, and the integral area of each characteristic peaks are employed to estimate the normalized content of each components (Figure 3L).Notably, the SEI of both electrodes mainly consists of Na 2 CO 3 , Na 2 O, NaF, and other typical decomposition products of carbonate-based electrolyte, suggesting that the interfacial Na-based alloys layer does not change the components of SEI.The binding energy of 284.8, 286.6, 288.1, and 290.1 eV in C 1s spectrum are corresponding to C-C, C-O, ROCO 2 Na, and Na 2 CO 3 , respectively.The peaks of O 1s XPS spectrum with binding energy located at 530.8, 531.9, 533.2, and 536.8 eV ascribe to carbonate, Na 2 O, Na 2 CO 3 , C-O and Na auger, respectively.From F 1s spectra, two peaks detected at 687.2 and 684.4 eV can be assigned to −CF 3 and NaF.Surprisingly, the ratio of the NaF component in HAIP Na−SEI is strikingly higher than that of Na−SEI, which is known to be conductive to constructing a stable SEI. [33]Thus, the good cycling performance of HAIP Na symmetric cells may be due to the construction of NaF-rich SEI by an interfacial Na-based alloy layer, which is consistent with the results of EIS measurements.
To deeply reveal the large differences of Na deposition behaviors during the electrochemical process, dynamic simulation of electric field intensity, current density, and electrolyte concentration on bare Na and HAIP Na electrodes are simulated by carring out the finite element method on COMSOL Multiphysics.As shown in Figure 4A-C and Supporting Information S1: Figure S10, the bare Na electrode exhibits uneven current density, distribution of electric field intensity, and severe electrolyte concentration polarization, which is due to the defective surface and uneven nucleation sites formed in the initial stage, eventually resulting in significant dendrite growth.By contrast, owing to the heightened SEI and Na sodiophilicity, HAIP Na electrode remains uniform current density, homogeneous electric field strength and Na + concentration gradient (Figure 4D-F and Supporting Information S1: Figure S11), as well as flat Na deposition morphology.The evolution of the Na deposition morphology is further in situ traced using optical microscopy in electrolytic cells.As shown in Figure 4G, the Na deposition is uneven.Na is first deposited at the edge positions and some of the places on the electrode surface due to its defective surface, these locations of priority deposition will be preferentially nucleated in subsequent deposition and exacerbate the rate of dendrite growth.In addition, we also observed Na dendrites fracture, these so-called "dead Na" will reduce the Coulombic efficiency and increase the impedance.On the contrast, the HAIP Na electrode exhibits a homogeneous Na deposition surface and without preferential deposition sites and any dendrites (Figure 4H), which is agreement with the SEM results in the disassembled symmetric cells and simulation results.
As a proof of concept, the full cells matched with SPAN cathode are constructed to analyze the enhanced electrochemical performance of HAIP Na anode in Figure 5A.Among them, the SPAN powders show a bulk-like morphology that consist of the nanospheres with a dimeter of 200 nm, the crystal structure is consistent with that previously reported (Supporting Information S1: Figure S12), and the SPAN mass loading is about 1 mg cm −2 (0.42 mg cm −2 -sulfur ).The zoomed anode/electrolyte interfaces emphasize the significance  5F, exhibiting a reversible capacity of 912.2 mAh g −1 with a capacity retention of 87.4% after 1000 cycles, and the Coulombic efficiency remains constant close to 100%.The Nyquist plots in Figure 5G compares the impedance of SPAN||HAIP Na full cells before and after 200 cycles at 3C.In consequence, the total resistance almost remains unchanged, meanwhile, SEM images of the cycled HAIP Na anode exhibit a relatively smooth surface (Supporting Information S1: Figure S14), suggesting that the interfacial Na-based alloys layer is able to form a stable SEI film and effectively inhibits parasitic side reactions of anode with electrolyte.
Figure 6 reveals the principle that the artificial heterogeneous Na-based alloys protective layer inhibits the growth of Na dendrites.As we all know, typical Na electrodeposition on substrate including Na nucleation as well as subsequent Na growth.Among, the Na nucleation process plays a decisive role in determining the electrodeposition of Na.For bare Na, when Na with an irregular surface comes in contact with the carbonatebased electrolyte, the high reactivity characteristic leads to spontaneous side reactions between Na metal and electrolyte at open circuit voltage, which causing the formation of inhomogeneous and unstable SEI.Subsequently, the Na + flux becomes more concentrated under the action of the uneven SEI, causing preferential nucleation of Na + flux and dendrite growth.Worse still, the poor sodiophilicity exacerbates the selective Na deposition and inhomogeneous plating.Conversely, the interfacial Na-based alloys layer constructed on the surface of Na foil prevents the continuation of side reactions at the electrode/electrolyte interface and generates uniform NaF-rich SEI.Meanwhile, the interfacial Na-based alloys layer enhances the electrolyte wettability and sodiophilicity of the electrode at the whole electrode level.All advantages are oriented to "tendency-free" nucleation of Na + flux and uniform plating/stripping process, which fundamentally inhibits the growth of dendrites.

| CONCLUSION
In summary, we reported a heterogeneous Na-based alloys protective layer designed by GaInSn liquid metal in-situ alloying route to protect Na metal anode at room temperature.The interfacial Na-based alloys layer with a good sodiophilicity and electrolyte wettability, hence reducing the Na nucleation overpotentials and enabling uniform Na plating/stripping.Furthermore, the asprepared interfacial Na-based alloys layer minimizes the parasitic side reaction of Na metal with electrolyte, preventing SEI film thickening and promoting the construction of NaF-rich SEI.As expected, the HAIP Na||HAIP Na symmetric cells indicate an excellent cycling performance (more than 600 h at a current density of 1 mA cm −2 with a capacity of 1 mAh cm −2 ) in carbonate-based electrolyte.Besides, the HAIP Na|| SPAN full cells demonstrate a reversible capacity of 912.2 mAh g −1 and with a high capacity retention of approximately 87.4% at 3C over 1000 cycles.This paper provides a facile, novel and efficient method to construct interfacial protective layer to solve the electrode/electrolyte interface issues of Na metal anode, and applicable to other metal electrode.
illustrates the preparation schematic diagram of HAIP Na.In brief, HAIP Na is synthesized by repeatedly shelling-out liquid metal on F I G U R E 1 (A) Illustration for the fabrication of HAIP Na.SEM images for the corresponding alloying process (B) fresh Na, (C) 1 h, (D) 3 h, and (E) 24 h.The scale bar is 50 µm.(F) XRD patterns of the phase transformation during the alloying processes.Elemental mapping images of (G) Na, (H) Ga, (I) In, and (J) Sn for the SEM image.The scale bar is 10 µm.HAIP Na, heterogeneous Na-based alloys interfacial protective Na; SEM, scanning electron microscope; XRD, X-ray diffraction.

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I G U R E 2 (A) Voltage-capacity curves of Na nucleation and deposition behaviors of bare Na and HAIP Na electrode.(B) The fitted Tafel points of Na||Na and HAIP Na||HAIP Na symmetric cells.(C) EIS plots of Na||Na and HAIP Na||HAIP Na symmetric cells before cycles.The inset is the equivalent circuit model.(D) Voltage-time profiles of Na||Na and HAIP Na||HAIP Na symmetric cells at 0.5 mA cm −2 to capacity of 0.5 mAh cm −2 .(E) Rate performances at 0.5, 1, 2, and 3 mA cm −2 with a fixed capacity of 1 mAh cm −2 .EIS, electrochemical impedance spectroscopy; HAIP Na, heterogeneous Na-based alloys interfacial protective Na.

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I G U R E 3 (A-D) SEM images of Na metal electrode cycling in symmetric cells after 1, 30, 50, and 100 cycles.(E-H) SEM images of HAIP Na electrode cycling in symmetric cells after 1, 30, 50, and 100 cycles.The scale bar is 10 µm.(I-K) XPS spectra of C 1s, O 1s, and F 1s of Na−SEI (below) versus HAIP Na−SEI (above) layer after 10 cycles Na plating/stripping.(L) The relative percentage of different species in the SEI.All tests with a plating of 1 mAh cm −2 at a current density of 1 mA cm −2 in 1 M NaPF 6 in EC/DEC (1:1 wt%) with 5 wt% FEC electrolyte.DEC, diethyl carbonate; EC, ethylene carbonate; FEC, fluoroethylene carbonate; HAIP Na, heterogeneous Na-based alloys interfacial protective Na; SEI, solid electrolyte interphase; SEM, scanning electron microscope; XPS, X-ray photoelectron spectroscopy.

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I G U R E 4 COMSOL simulations of the distribution of current density, electric field intensity, and electrolyte concentration on bare Na (A-C) and HAIP Na (D-F).In situ observations of the electrochemical deposition of Na on (G) bare Na and (H) HAIP Na electrode/ electrolyte interfaces by optical microscopy.Images of Na plating.The scale bar is 500 µm.The deposition current density is 1 mA cm −2 .HAIP Na, heterogeneous Na-based alloys interfacial protective Na. of interfacial Na-based alloys layer in the Na deposition processes.CV curves of the HAIP Na||SPAN full cells in Figure 5B show characteristic anodic and cathodic peaks at a scan rate of 0.1 mV s −1 .The rate performance of HAIP Na||SPAN full cells and corresponding charge/discharge profiles at various current rates are displayed in Figure 5C and Supporting Information S1: Figure S13, delivering a discharge of 1351.5, 1322.4,1235.1, 1034.7,892.9, and 596.8 mAh g −1 (based on the weight of sulfur) at 0.3C, 0.5C, 1C, 3C, 5C, and 10C respectively (1C = 1675 mAh g −1 ). Figure 5D presents the cycling behavior of the HAIP Na||SPAN full cells, the full cells deliver a stable discharge capacity of 1186 mAh g −1 from the second cycle, maintained during 200 cycles.The typical charge and discharge curves in Figure 5E obtained for the first, 10th, 30th, 50th, and 200th cycle are almost constant.Furthermore, the long-term cycle stability of HAIP Na||SPAN cell at 3C is tested in Figure

F I G U R E 5
Electrochemical performances of HAIP Na||SPAN full cells.(A) Schematic illustration of HAIP Na||SPAN full cell.(B) CV curves at a scan rate of 0.1 mV s −1 .(C) Rate capability at various rates from 0.2C to 3C. (D) Cycling performance at 1C and (E) corresponding voltage profiles.(F) Cycle performance at 3C. (G) EIS plots before and after 200 cycles.CV, cyclic voltammetry; EIS, electrochemical impedance spectroscopy; HAIP Na, heterogeneous Na-based alloys interfacial protective Na.