Toward a Stable Sodium Metal Anode in Carbonate Electrolyte: A

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Toward Stable Sodium Metal Anode in Carbonate Electrolyte: A Compact, Inorganic Alloy Interface Xueying Zheng, Haoyu Fu, Chenchen Hu, Hui Xu, Ying Huang, Jiayun Wen, Huabin Sun, Wei Luo, and Yunhui Huang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03536 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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The Journal of Physical Chemistry Letters

Toward Stable Sodium Metal Anode in Carbonate Electrolyte: A Compact, Inorganic Alloy Interface Xueying Zheng, Haoyu Fu, Chenchen Hu, Hui Xu, Ying Huang, Jiayun Wen, Huabin Sun; Wei Luo*, Yunhui Huang*

Institute of New Energy for Vehicles, School of Materials Science and Engineering, Tongji University, Shanghai 201804, China

Keywords: sodium metal anode; electrolyte additive; alloy interface; facile ion transport, dendrite-free deposition

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Abstract Development of the next generation high-energy-density, low-cost batteries will likely be fueled by sodium (Na) metal batteries due to their high capacity and earth-abundancy. However, their practical application is significantly plagued by the hyper-reactivity of Na metal, unstable solid electrolyte interphase (SEI) and dendritic Na growth, leading to continuous electrolyte decomposition, low Coulombic efficiency (CE), large impedance and safety concerns. Herein, we add a small amount of SnCl2 additive in a common carbonate electrolyte so that the spontaneous reaction between SnCl2 and Na metal enables in-situ formation of a Na-Sn alloy layer and a compact NaCl-rich SEI. Benefitted from this design, rapid interfacial ion transfer is realized and direct exposure of Na metal to the electrolyte is prohibited, which jointly achieve a non-dendritic deposition morphology and a markedly reduced voltage hysteresis in Na/Na symmetric cell for over 500 hours. The Na/SnCl2-added electrolyte/Na3V2(PO4)3 full cell exhibits high capacity retention over cycling and excellent rate capability (101 mAh/g at 10 C). This work can provide an easily-scalable and cost-effective approach for developing high-performance Na-metal batteries.


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Introduction Recent years have witnessed an unprecedented increase in the demand of advanced energy storage systems towards low cost and high-energy density for electric vehicles or smart grid.1-2 Due to earth-abundancy of Na resource, Na-ion batteries (SIBs) are deemed as one of the most promising alternatives to lithium-ion batteries (LIBs). Of various explored anode materials for SIBs, Na metal stands out with its lowest potential (2.71 V versus the SHE) and unparalleled high theoretical capacity (1165 mAh/g).3-4 However, strong reactivity of Na metal triggers continuous reaction with organic liquid electrolytes and contributes to thick, inhomogeneous SEI, incurring uneven or impeded ion transport through the interface.5-7 Additionally, large volume change of the “hostless” Na metal anode upon repetitive plating/stripping fractures the intrinsically fragile SEI that leads to uncontrolled side reactions and continuous re-formation of the SEI, resulting in low CE, dendritic Na growth and eventually the electrolyte depletion.89 Consequently, battery explosion could be triggered by dendrite-induced internal short-

circuit.10 In tackling these challenges, various strategies have been proposed. One is to regulate Na+ deposition in the nucleation stage using modified current collectors, such as porous Al foil,11 3D porous Cu12-13, functional carbon materials9, 14, or Mxene.15 Confining Na metal in a designed matrix with high surface area is also a viable approach, aiming at accommodating its large volume change and lowering the local current density.16-20 Building an artificial SEI (ASEI) with great homogeneity and stability on Na metal presents another way.21-22 To construct an ASEI, methods like atomic layer


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deposition (ALD), molecular layer deposition (MLD), and magnetron sputtering have been employed.23-25 However, these methods typically involve complex coating procedures or strict controlling parameters. To date, the most facile route to form a favorable SEI is to adjust the electrolyte formulation, where the spontaneous reaction between Na metal and the modified electrolyte plays a critical role. Pioneering work by Cui’s group demonstrated that the SEI composed of inorganic Na2O and NaF induced by NaPF6 decomposition in ether solvents can effectively passivate Na metal anode, where they achieved high average CE of 99.9% for more than 300 cycles in Na/Cu cell at 0.5 mA/cm2.26 Later, addition of KTFSI and Na2S6 additives in ether electrolytes was successively proposed.27-28 These electrolyte additives enable the formation of SEI with favorable compactness, high-modulus and rapid ion transport ability. In spite of the progress made in ether electrolytes, achieving stable cycling of Na metal anode in conventional carbonate electrolytes is still challenging owing to its high corrosivity towards Na metal (Table S1).5,

29-31

Due to the fact that ether solvents are endowed with high cost, high

flammability and low oxidation potential, developing carbonate electrolytes for Na metal anodes that are cheaper and more compatible with high-voltage cathodes becomes imperative.16, 32 Recently, Archer and Nazar both proposed that lowering Li ion diffusion barrier through its alloy phase could improve the Li metal stability.7, 33-34 Such electrochemically active alloy phase allows fast and uniform ion transfer, thereby lowering the interfacial impedance and eliminating local accumulated metal deposition.



Inspired by their works, an alloy phase developed on Na metal should have similar effects for Na metal stabilization. In this work, we adopted SnCl2 as additive to enable the in-situ formation of NaSn alloy phase and realized long-term cycling of Na metal anode in common carbonate electrolytes. Upon contacting with Na metal, SnCl2 additive is chemically reduced into Sn due to the hyper-reductivity of Na metal and Sn subsequently alloys with bulk Na to form an alloy interlayer.35-36 Meanwhile, anion Cl- contributes to a desirable NaClrich SEI, which further passivates Na metal against the corrosive electrolyte. During electrochemical plating/striping, the in-situ formed layers allow fast and uniform Na plating underneath, effectively inducing a dendrite-free morphology rather than a dendritic and mossy morphology for Na metal cycled in blank carbonate electrolyte (Figure 1). As a result, the overpotential of Na/Na symmetric cells kept low and stable for over 500 hours with only 50 mM SnCl2 added in carbonate electrolytes. Superior performance in both cycling stability and rate capability were also demonstrated in Na/SnCl2-added electrolyte/Na3V2(PO4)3 full cells.

Figure 1. (a) Formation of typical mosaic SEI on Na metal anode cycled in regular carbonate electrolyte, which drives uneven interfacial ion transfer and Na dendrite. (b) Fast and uniform ion transport benefitted from in-situ formation of Na-Sn alloy layer


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plus a NaCl-rich SEI, leading to a uniform plating/striping and dendrite-free morphology.

Results and Discussion Figure 2a exhibits an optical image of bare Na metal, which gives a silver metallic cluster. After dropping with SnCl2 added electrolyte (50 mM SnCl2 in 1.0 M NaClO4 in ethylene carbonate (EC)/propylene carbonate (PC)), an instant color change from silver to black occurred, indicating a spontaneous reaction between SnCl2 and Na metal (Figure 2b and Movie S1). Scanning electron microscopy (SEM) image in Figure 2c demonstrates a dense particle-agglomerated morphology of Na metal surface after pretreatment with SnCl2 added electrolyte. Diameter of the particles reaches approximately 3.5 μm (Figure 2d). From the cross-sectional SEM image, we can clearly spot an additional particle-packed layer uniformly covering on Na metal, as indicated by the dotted line in Figure 2e. Energy dispersive X-ray spectroscopy (EDX) mappings confirmed strong and conformal distribution of Na, Sn, Cl elements in both top-view and cross-sectional images within the as-formed layer (Figure 2f-h, Figure S1). Further, X-ray photoelectron spectroscopy (XPS) measurements were conducted to elucidate the chemical composition of SnCl2 treated Na metal. Figure S2 confirms the existence of Na 1s, Sn 3d, Cl 2p peaks in the full survey XPS spectra in consistence with the EDX results. As shown in Figure 2i, a sharp peak at 497.6 eV is derived from the overlapping region of Na KLL,37-38 whereas two split peaks of Sn 3d3/2 and Sn 3d5/2 spectra correspond to metallic Sn (494.1, 485.6 eV) and NaxSny alloy (492.2, 483.6 eV)



phases.39-41 The XPS results were confirmed by X-ray diffraction (XRD) measurement, where peaks related to metallic Sn and Na15Sn4 were detected (Figure S3). Moreover, evident XPS peaks corresponding to NaCl can be observed,42 suggestive of the presence of NaCl on the surface (Figure 2j). Overall, the above findings suggest uniform formation of Na-Sn alloy and NaCl layers on Na metal surface after reacting with SnCl2 added electrolyte, which may generate positive impact on the stability of Na metal as anode in batteries.

Figure 2. Optical photos of Na metal (a) before and (b) after reaction with SnCl2 added carbonate electrolyte. (c, d) Top-view SEM images of SnCl2 treated Na metal. (e-h) Cross-sectional SEM and EDX mapping images of SnCl2 treated Na metal with (f) Na,


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(g) Sn, (h) Cl elemental signals. High-resolution XPS (i) Sn 3d and (j) Cl 2p spectra of SnCl2 treated Na metal.

Na/Na symmetric cells were then assembled with SnCl2 added electrolyte to evaluate the formation of Na-Sn and NaCl-rich layers and their effects on electrochemical performances. After three cycles tested at 0.25 mA/cm2 (1 h for each plating or striping step), Na-Sn and NaCl-rich layers have been in-situ formed with conformal distribution of Sn, Cl element (Figure 3a-c). Compared with the pretreated Na metal anode (Figure 2), the in-situ formed layer on Na metal in cells is thinner, which corresponds to several microns (Figure S4). We further implemented cyclic voltammetry (CV) scan to evaluate the electrochemical process of Na metal electrode within SnCl2 added electrolyte. As shown in Figure 3d, two peaks near 0.15 and -0.15 V can be attributed to the sodiation/desodiation process of Sn.7 What’s more, a pronounced high current is obtained as compared to the control sample (Figure 3e), suggestive of favorable interfacial kinetics within the formed layers in SnCl2 added electrolyte.43



Figure 3. (a) Top-view SEM image with the corresponding EDX mapping of (b) Sn, (c) Cl of the Na metal electrode after 3 cycles tested in Na/Na symmetric cell with 50 mM SnCl2 added electrolyte. CV curves of Na/Na symmetric cells with (d) SnCl2 added electrolyte and (e) blank electrolyte scanned at 1 mV/s.

Figure 4a demonstrates the voltage profiles of Na/Na symmetric cells upon repeated plating/stripping at 0.5 mA/cm2 with a capacity of 1.0 mAh/cm2. The initial overpotential of the control cell exhibits obvious fluctuations, implying the morphological variation of Na metal electrode incurred by uneven deposition or electrolyte corrosion,24, 44 which typically happens in carbonate electrolytes.5, 30 With only 10 mM SnCl2 added in the electrolyte, the initial overpotential was effectively lowered, revealing a reduced energy barrier for Na deposition. However, its overpotential gradually increased and finally caught up with the control cell after 200 h, which may possibly originate from formation of a non-uniform or gradually-faded


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Na-Sn layer due to inadequate additive applied. When we increased the SnCl2 concentration to 50 mM, its voltage profile was substantially flattened from the beginning and manifested only slight overpotential increase even after 500 h. Such stable and small overpotential is a good indicator of the stabilized interface with facile ion transfer.45-46 Increasing the additive concentration to 100 mM did not give further improvement, which indicates that increasing SnCl2 concentration beyond 50 mM would not further benefit to Na+ diffusion or dendrite suppression within the cell. We then fixed the SnCl2 concentration at 50 mM to accomplish the subsequent characterizations. Facilitation in the interfacial ion transport upon SnCl2 addition was studied using electrochemical impedance spectroscopy (EIS). Nyquist plot shown in Figure 4d verifies that the interfacial ion transport has been evidently enhanced at the initial cycle upon SnCl2 addition. The semicircle can be ascribed to the interfacial resistance involving ion migration through the surface layer.47 Rs, Rct and CPE in equivalent circuit (inset of Figure 4e) each represents the resistance of electrolyte, the resistance of charge transfer and the adsorption effect of surface layer. A drastically decreased Rct upon SnCl2 addition is a direct evidence of the improved interfacial ion transport. Moreover, Rct of the control cell has increased to 216 Ω after 30 cycles, indicating thickened SEI and “dead Na” generation.44 Such continuous formation of SEI inevitably impedes ion transfer and ultimately depletes the electrolyte.16, 48 As opposed, the overall impedance of the cell with SnCl2 added electrolyte gets slightly decreased after 30 cycles (Figure 4e), which can be attributed to a gradually improved interface after initial multiple



cycles, suggestive of an activation process upon sodiation/desodiation.49-51 Three reasons can be accountable for the low impedance enabled by SnCl2 additive: i) fast and uniform surface diffusion is enabled through the Na-Sn layer,7, 34 ii) presence of compact NaCl-rich SEI exhibits less resistivity to Na+ transport,42 iii) the combination of as-formed Na-Sn and Na-Cl layers further passivate Na metal against electrolyte permeation, hence interrupting deleterious side reactions at the interface.

Figure 4. (a) Cycling performance of Na/Na symmetric cells with various concentrations of SnCl2. (b, c) The enlarged voltage profiles marked in (a). (d) Nyquist plots of cells with blank electrolyte and 50 mM SnCl2 added electrolyte after 1 cycle


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and 30 cycles. (e) Enlarged view of the region labelled in the green box in (d) with the equivalent circuit shown.

XPS measurements were performed with depth profiling to analyze the surface composition of the cycled Na metal electrode. Figure 5a-c demonstrate the XPS spectra of C 1s, Na 1s and O 1s on the surface of Na metal anode in SnCl2 added electrolyte after 20 cycles. The C 1s spectra can be deconvoluted into 4 peaks with binding energies of 289.9 eV (Na2CO3), 288.5 eV (ROCO2Na), 286.5 eV (-C–O-), 284.8 eV (-C–C-), which involve typical decomposition products of cyclic carbonate electrolytes.52-53 Peaks observed at 1072.9, 1072.1, 1071.5 eV from Na 1s spectra can be assigned to NaOH, NaCl and Na2CO3, respectively (Figure 5b).16, 22, 42 The O 1s spectrum shows binding energies at 537.1, 532.9, 532.2, and 531.2 eV, which corresponds to Na auger, -C-O-, NaOH, Na2CO3, respectively (Figure 5c).16, 38 In case of Cl 2p spectra, NaCl and organo-chloride composition were both found. Existence of organo-chloride is derived from the interaction between Cl- anion and solvent species in the electrolyte.39 Ar sputtering (3 min) was then implemented to detect the depth profile of Cl species. Only inorganic NaCl was detected, indicative of dominant presence of NaCl in the inner layer of SEI (Figure 5d). Overall, it can be deduced that species derived from electrolyte decomposition (Na2CO3, ROCO2Na, NaOH) inevitably exist throughout the surface layer, along with dominant presence of NaCl, especially in the inner SEI layer. It should be noted that this inorganic NaCl is Na+-conductive and resistant towards electrolyte permeation.42 The Sn 3d spectrum is relatively weak on the surface of Na metal before



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Ar sputtering, which is resulted from the coverage by SEI in the very uppermost layer.33, 39

After sputtering, strong peaks related to Sn species appeared, revealing that Na-Sn

layer locates beneath organic SEI (Figure 5e). The presence of NaxSny after 20 cycles also proves that the Na-Sn layer is capable of withstanding the volumetric variation of Na metal electrode upon repetitive plating/stripping. Stability of the as-formed layers were also monitored using post-mortem SEM characterization. As shown in Figure 5f, surface of Na metal cycled in blank electrolyte for 30 cycles became quite rough with observable fractures. Such porous structure with an increased surface area further exacerbates the reaction with electrolyte.49,

54

In

contrast, Na metal in 50 mM SnCl2 added electrolyte maintained the dense particle-like surface morphology without detectable dendritic growth or fractures after cycling (Figure 5g), which can be attributed to the homogeneous interfacial ion transport and effective surface protection. What’s more, EDX analysis verifies the presence of Sn, Cl coverage on Na metal anode (Figure 5h-j), indicative of its validity to stand unperturbed upon cycling.


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Figure 5. High resolution XPS spectra of Na metal cycled in 50 mM SnCl2 added electrolyte for 20 cycles. (a) C 1s, (b) Na 1s, (c) O 1s and (d) Cl 2p, (e) Sn 3d5/2 spectra. SEM images of Na metal after cycling in Na/Na symmetric cell for 30 cycles with (f) blank electrolyte and (g) 50 mM SnCl2 added electrolyte. (h) SEM image with the corresponding EDX mapping of (i) Sn, (j) Cl of the Na metal electrode after 30 cycles in 50 mM SnCl2 added electrolyte.

As a proof of concept, full cells were fabricated to investigate the practicality of SnCl2 added electrolyte with Na metal anode and Na3V2(PO4)3 (NVP) cathode. NVP cathode material was prepared by solid-state reaction based on our previous work (see details in Experimental Section). The full cells were subjected to cycling at 1 C in the



voltage range between 2.5 V and 4.0 V. Although similar discharge capacities were delivered at the beginning in both electrolytes, capacity of the cell with blank electrolyte presents a low capacity retention of 75%. In contrast, when using the 50 mM SnCl2 added electrolyte, the cell achieves a substantially improved capacity retention of 87% under the same testing condition (Figure 6a). Voltage profiles also show smaller polarization upon SnCl2 addition, originating from the facile ion transport through the Na/electrolyte interface (Figure 6c,d).43 Additionally, negligible fading in the discharge capacity is observed when increasing the rate from 0.5 to 1 C, and the discharge capacity still reaches 101 mAh/g even at a high rate of 10 C (Figure 6b). Its voltage profiles at various rates are shown in Figure S5, further validating the constrained voltage gap at high rates. Similar stable operation can be extended to the electrolyte system of 1.0 M NaClO4 in EC/diethyl carbonate (DEC) upon addition of 50 mM SnCl2, suggesting the versatility of SnCl2 additive in carbonate solvents. In a Na/Na symmetric set-up, the cell with SnCl2 added electrolyte operated stably over a prolonged period of 450 h without observable voltage fluctuations. As opposed, a sudden voltage divergence occurred in the control cell at 250 h (Figure S6). For full cell, SnCl2 additive enables a high CE over 250 cycles, in contrast to the control cell that faded quickly after 40 cycles (Figure S7a). Improved kinetic is also evident with higher discharge capacity delivered at various rates (Figure S7b), which hereby highlights the determinant role of anode protection in full cells.


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Figure 6. (a) Cycling performance and (b) rate behavior of Na/NVP full cells with blank or 50 mM SnCl2 added electrolyte. Voltage profiles of full cells tested at 1 C with (c) 50 mM SnCl2 added and (d) blank electrolyte.

Conclusion In summary, particle-agglomerated Na-Sn and NaCl-rich layers were in-situ constructed on Na metal anode upon adding tiny amount of SnCl2 (50 mM) in common carbonate electrolytes. Without any complex procedures, the layers simultaneously offer rapid interfacial ion transport and suppress parasitic reactions by insulating direct contact between Na metal and liquid electrolyte. As expected, Na/Na symmetric cells presented stable cycling with low voltage hysteresis in the SnCl2 added carbonate electrolyte over 500 h, far exceeding the performance of the cell with blank electrolyte. Such improved performances were extended to full cells that exhibit a high capacity retention along with restricted cell polarization at high rates. This approach sheds light on in-situ formation of a thin alloy-metal composite layer in enabling dendrite-free



metal anodes, which paves the way for practical applications.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Details for experimental and characterizations; Table S1; Movie S1; Figures (S1-S7) of EDX mapping images, XPS survey and XRD patterns of SnCl2-treated Na metal, crosssectional SEM and EDX elemental mapping images of Na electrode after 3 cycles’ test, detailed voltage profiles of Na/NVP full cell using SnCl2 additive in electrolyte, cycling performance of Na/Na symmetric cell and Na/NVP full cell using SnCl2 added NaClO4EC-DEC electrolyte.

Corresponding Author *Email: [email protected]; [email protected]

Notes The authors declare no competing financial interest.

Acknowledgement The authors acknowledge the financial support by National Natural Science Foundation of China (No. 51802224). This work was also supported by the Fundamental Research Funds for the Central Universities.

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Toward a Stable Sodium Metal Anode in Carbonate Electrolyte: A

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