ELECTROLYTIC DEPOSITION OF PURE PHASE SnSb FROM EUTECTIC SOLUTION ETHALINE FOR LITHIUM-ION BATTERY ANODE

Abstract

To provide a method for forming a highly stable lithium-ion battery anode.SOLUTION: A method including a process for preparing a solution containing an Sn(II) salt and an Sb(III) salt in a non-aqueous solvent, a process for inserting a conductive metal substrate into the solution, a process for applying a potential to the conductive metal substrate at room temperature, which is more negative than -0.55 V vs. Ag / Ag+, for a sufficient time to form a thin film of SnSb thereon, and a process for heating the solution before the process for applying the potential is provided.SELECTED DRAWING: Figure 1D

Description

[Technical Field]

[0001] [Government Assistance Clause] This invention was made with government support under NSF-SSMC-1710672 awarded by the National Science Foundation. The government has certain rights in this invention.

[0002] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 023,144, filed May 11, 2020, entitled "Electrodeposition Of Pure Phase SnSb From Eutectic Ethaline Solution," which is specifically incorporated by reference herein for all that it discloses and teaches. [Background technology]

[0003] With the growing demand for electric vehicles and automakers' commitment to all-electric vehicles, energy storage will remain important for the foreseeable future. As the world slowly transitions from nonrenewable energy sources like fossil fuels to cleaner, more sustainable storage methods, secondary batteries will play a major role in energy storage. Currently, graphite is used commercially for the anodes of most lithium-ion batteries, and while silicon is gradually becoming more practical, the market is saturated. These anodes have a low gravimetric capacity because they can only hold one lithium atom for every six carbon atoms. Therefore, current battery technology could be improved by replacing current graphite anodes. Alloy materials have been investigated as replacements for commercially available graphite anodes due to their higher volumetric and gravimetric capacities compared to graphite. Alloy anodes have the ability to react with multiple lithium atoms per metal center, resulting in a theoretical gravimetric capacity two to ten times that of graphite. One such alloy material, SnSb, has been extensively studied and shown to be a promising alternative to graphite.

[0004] Among secondary battery technologies, lithium-ion batteries are currently used in a wide range of applications, from portable devices to electric vehicles. In recent years, research into sodium-ion batteries, which have large-scale power storage capacity and are an alternative to lithium-ion batteries in certain technological fields, has been active. This is due to the fact that sodium is more abundant and cheaper. Graphite, the negative electrode material of conventional lithium-ion batteries, is anode material that is made of sodium. + The ion is Li + Because sodium ions are larger than sodium ions, they do not function well as sodium ion anodes, and graphite is not a suitable host. Therefore, a high-capacity, long-life sodium ion anode is needed. Currently, most research on sodium anode materials uses hard carbon. However, hard carbon has poor cycling performance and can lead to sodium metal plating and dendritic growth on its surface, posing safety concerns. Summary of the Invention

[0005] In accordance with the objectives of the present invention, as embodied and broadly described herein, one embodiment of a method for electrodepositing SnSb onto a substrate comprises preparing a solution containing a Sn(II) salt and an Sb(III) salt in a non-aqueous solvent, inserting a conductive metal substrate into the solution, and applying a voltage of -0.55 V vs. Ag / Ag to the conductive metal substrate for a time sufficient to form a SnSb thin film thereon. + and applying a potential more negative than

[0006] In another aspect of the present invention, and in accordance with its purposes, as embodied and broadly described herein, an embodiment of a method for forming a high stability sodium-ion battery anode comprises providing a solution including a Sn(II) salt and an Sb(III) salt in a non-aqueous solvent; inserting a conductive metal substrate into the solution; and maintaining a voltage of -0.55 V vs. Ag / Ag relative to the conductive metal substrate for a time sufficient to form a thin film of SnSb thereon. + applying a potential more negative than

[0007] In yet another aspect of the present invention, and in accordance with its purposes, as embodied and broadly described herein, an embodiment of a method for forming a high stability lithium ion battery anode comprises providing a solution including a Sn(II) salt and an Sb(III) salt in a non-aqueous solvent; inserting a conductive metal substrate into the solution; and applying a voltage of -0.55 V vs. Ag / Ag relative to the conductive metal substrate for a time sufficient to form a thin film of SnSb thereon. + applying a potential more negative than

[0008] The benefits and advantages of the present invention include, but are not limited to, providing a method for electrodepositing phase-pure SnSb onto a conductive metal substrate using ethaline solution for use as a sodium-ion or lithium-ion battery anode without the need for binders, carbon additives, or post-treatments.

[0009] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. [Brief explanation of the drawings]

[0010] [Figures 1A-1C] Figure 1A shows the cyclic voltammogram of 50 mM SnCl2·2H2O in ethaline. Figure 1B shows the cyclic voltammogram of 50 mM SbCl3 in ethaline. Figure 1C shows the cyclic voltammogram of the combination of 50 mM SnCl2·2H2O and 50 mM SbCl3 in ethaline. An Ag wire reference electrode and a Pt working electrode were used at a scan rate of 50 mV / s. [Figure 1D] FIG. 1D is an X-ray diffraction (XRD) scan of SnSb electrodeposited on a nickel substrate. [Figure 2A] Figure 2A shows a comparison of Sn and Sb electrodeposited from ethaline solution with SnSb electrodeposited under similar conditions, where the half-cell was cycled at a C / 2 rate from 0.01 V to 1.5 V vs. Na / Na+. [Figure 2B] FIG. 2B is a graph showing the electrochemical performance of electrodeposited SnSb as demonstrated by electrochemical cycling at a C / 2 rate over a potential window of 0.01 V to 1.5 V in a sodium half-cell. [Figure 2C] FIG. 2C is a graph showing the rate capability of electrodeposited SnSb at various current rates in a sodium half-cell for eight cycles at each rate over the potential range of 0.01 V to 1.5 V. [Figure 3A] FIG. 3A is a graph of the differential capacity of electrodeposited Sb, showing the first two cycles. [Figure 3B] FIG. 3B is a graph of the differential capacity of electrodeposited Sn, showing the first two cycles. [Figure 3C] FIG. 3C is a graph of the differential capacity of electrodeposited SnSb showing the first two cycles. [Figure 3D] FIG. 3D is a graph of the differential capacity of electrodeposited SnSb between the 1st, 2nd, 5th, 10th, 50th, 100th, and 150th cycles obtained by galvanostatic cycling at a C / 2 rate within the range of 1.5 V to 0.01 V vs. Na / Na+. [Figure 3E] FIG. 3E is a graph of the differential capacity of electrodeposited 50:50 Sn:SnSb during the 1st, 2nd, 5th, 10th, 50th, 100th, and 150th cycles obtained by galvanostatic cycling at a C / 2 rate within the range of 1.5 V to 0.01 V vs. Na / Na+. [Figure 4] FIG. 4 is a graph of ex-situ XRD of electrodeposited SnSb galvanostatically cycled at a C / 2 rate within the range of 1.5 V to 0.01 V vs. Na / Na+ for the indicated number of cycles. [Figure 5A] FIG. 5A is a graph of galvanostatic cycling of electrodeposited SnSb, Sn, and Sb from 0.01 V to 1.5 V at a C / 2 rate. [Figure 5B] FIG. 5B is a graph of the rate capability of electrodeposited SnSb in lithium half-cells at various current rates. [Figure 6A]FIG. 6A is a graph of the differential capacity during the first two cycles of Sb. [Figure 6B] FIG. 6B is a graph of the differential capacity during the first two cycles of SnSb. [Figure 6C] FIG. 6C is a graph of the differential capacity during the first two cycles of Sn. [Figure 6D] FIG. 6D is a graph of the differential capacity of SnSb galvanostatically cycled at a C / 2 rate after 1, 2, 5, 10, 50, 100, and 150 cycles. [Figure 7] FIG. 7 shows the XRD patterns of the Ni electrode surface of electrodeposited SnSb galvanostatically cycled at a C / 2 rate. [Figure 8A] FIG. 8A shows the differential capacity XRD patterns of precipitates with various amounts of Sn:SnSb. [Figure 8B] FIG. 8B shows the differential capacity XRD patterns of precipitates with various amounts of Sn:SnSb. [Figure 8C] FIG. 8C is a graph of galvanostatic cycling of deposits with various amounts of Sn:SnSb in a lithium half cell at potentials between 0.01 V and 1.5 V. [Figure 8D] FIG. 8D is a graph summarizing galvanostatic cycling data for electrodes losing 80% or less of their retention capacity for deposits with various amounts of Sn:SnSb, based on the second cycle. [Figure 9] FIG. 9 shows XRD patterns of SnSb electrodeposited from ethaline solution on a Cu foil electrode surface after preparation, heating at 110° C., and after water was injected into the solution. DETAILED DESCRIPTION OF THE INVENTION

[0011] The research goal of alloy anode materials is to achieve higher volumetric and gravimetric capacities compared to hard carbon. Silicon is promising for lithium-ion batteries due to its high theoretical capacity, but its ability to store sufficient amounts of sodium has been shown to be insufficient. Sn (847 mAh / g) and Sb (660 mAh / g) have been actively investigated as alloy anode materials due to their high theoretical capacity. Intermetallic compounds such as SnSb exhibit reduced volume expansion upon sodiation, resulting in longer cycle life and improved rate capability. Previous studies of SnSb have suggested that polycrystalline SnSb reforms during cycling in lithium-ion batteries, potentially offering advantages in terms of mechanical properties. Due to the lack of direct transfer of knowledge from lithium-ion to sodium-ion technology, i.e., silicon and graphite, SnSb has become a prime candidate for potential applications as a sodium-ion battery anode. Ultimately, the goal is to develop a simple, scalable synthesis method for SnSb anode materials while controlling their composition, morphology, and surface area, and improving their performance.

[0012] SnSb bimetallic alloys have been synthesized using methods such as ball milling, electrospinning, hot injection, chemical reduction, solvothermal synthesis, and conventional solid-state synthesis. Many of these methods require high temperatures, which can result in inhomogeneous products due to the difference in vapor pressure between the two elements. All of these synthesis methods produce powders, which require a binder for electrochemically testing the active material in batteries. Embodiments of the present invention describe the direct electrodeposition of SnSb onto conductive substrates at room temperature without the need for additional binders or post-treatment. Electrodeposition allows for control of the material's composition, particle size, and thickness. While the first reported Sn-Sb lithium anode was synthesized by electrodeposition, phase-pure SnSb has never been reported using this process. The most common impurities reported to date are Sn, Cu2Sb, Cu3Sn, and SnO. All of these impurities are electrochemically active. Therefore, electrodeposition of pure-phase SnSb is desirable to study how the material interacts with other components in the battery during electrochemical cycling. Pure-phase SnSb was synthesized by electrodeposition using a deep eutectic solvent. Pure SnSb electrodes exhibit gravimetric capacities comparable to those of carbon with binder additives while maintaining long cycle life in sodium-ion and lithium-ion batteries.

[0013] Electrodeposition is a scalable process utilized to synthesize materials for a variety of applications, including energy storage in batteries and capacitors, energy production using photoelectrochemical and thermoelectric conversion, and through the fabrication of coatings for corrosion resistance. This process is particularly attractive for battery applications because its ability to grow on non-planar substrates allows for the growth of high-surface-area, 3D-structured electrodes. Electrodeposition was initially reported as a synthesis used to produce Sn-Sb alloy systems for testing as secondary battery materials, but the synthesis of phase-pure SnSb via electrodeposition has only recently been reported.

[0014] Previously reported electrodeposition using aqueous solutions resulted in tin-rich products with crystalline tin impurities present in the product. The synthesis of SnSb in this invention uses ethaline, a deep eutectic solvent, a eutectic mixture with a lower melting point than the individual components. This solvent is attractive due to its favorable electrochemical window for synthesis, the inexpensive and readily available choline chloride and ethylene glycol components, and its biodegradability. Ethaline also minimizes side reactions within the solution, allowing for the direct, single-step electrodeposition of the target species.

[0015] Other suitable non-aqueous solvents may include ethylene glycol, choline chloride, urea, thiourea, glycerol, malonic acid, benzoic acid, and oxalic acid, and mixtures thereof.

[0016] Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. It will be understood that these drawings are provided for the purpose of illustrating particular embodiments of the invention and are not intended to limit the invention thereto.

[0017] Thin films of SnSb were synthesized using solutions containing Sn(II) and Sb(III) chlorides as precursors. The solvent for the electrodeposition solution was ethaline (choline chloride (>98% purity, both from Aldrich and VWR, high-purity grades) to ethylene glycol (Fisher Scientific) in a 1:2 weight ratio). The baths used for electrodeposition of Sn and Sb contained 50 mM SbCl3 (Sigma Aldrich, ACS reagent grade) and 50 mM SnCl2·2H2O (Sigma Aldrich, ACS reagent grade), respectively, dissolved in ethaline. Choline chloride was recrystallized in absolute ethanol (Pharmaco, 200 proof) by refluxing on a hot plate prior to use. The crystals were then filtered and dried in a vacuum oven at 100 °C. A solution containing 50 mM SbCl3 and 50 mM SnCl2·2H2O in ethaline was typically heated at 70 °C until the solution became homogeneous and clear. Electrodeposition was performed from the bath in a three-electrode setup using a Chi 660C potentiostat (CH Instruments) with a platinum mesh counter electrode (100 mesh, Alfa Aesar), a silver wire reference electrode, and nickel foil as the working electrode. The nickel foil was washed in 0.1 M nitric acid for 30 seconds before deposition. Kapton (copyright) tape was placed on the backside of the various foils to ensure deposition occurred on only one side of the film with controlled surface area. Single-step electrodeposition was performed at -0.7 V vs. Ag / Ag + The electrochemical tests were carried out at a constant potential of 0.05 V and 25° C. The electrochemical tests were carried out using electrodepositions carried out for 60 seconds.

[0018] Other suitable precursors containing tin or antimony include tin and antimony sulfates, antimony oxide (Sb2O3), tin and antimony citrates, and salts of the reaction of tin and antimony with gluconic and tartaric acids, and mixtures thereof.

[0019] Cyclic voltammetry (CV) was performed using a three-electrode setup consisting of a platinum working electrode (BASi, 1.6 mm diameter), a platinum mesh counter electrode (100 mesh, Alfa Aesar), and a silver wire reference electrode. CV was performed between -1.2 V and 1.2 V vs. Ag / Ag. + The scan rates were varied within the range of 100 s. The solutions were examined by direct analysis using real-time mass spectrometry (DART-MS; Broker MaXis QTOF with IonSense DART-SVP and Agilent TOF LCMS with Ion Sense DART-1001). The DART-MS was prepared in negative ion mode, and spectra were calibrated using Fombin Y as a standard. The electrode surfaces were examined by scanning electron microscopy (SEM; JOEL JSM-6500F, operated at 15 kV). To complement this technique, spectra were acquired using energy-dispersive X-ray spectroscopy (EDS) and analyzed using Aztec software (Oxford Instrument) to calculate the composition and identify the atomic configuration of the electrodeposited films. The crystalline structure present on the electrodeposited electrodes was identified using conventional X-ray diffraction (XRD; Broker D8 Discover DaVinci, Cu Kα radiation, λ = 1.54184 Å) and glancing angle X-ray diffraction (GAXRD; Broker D8 Discover Series 1, Cu Kα radiation, λ = 1.54184 Å).

[0020] Turning now to Figures 1A and 1B, the cyclic voltammetry of individual tin and antimony ethaline solutions is shown, respectively, and Figure 1C shows the cyclic voltammetry of a solution containing both tin and antimony in ethaline, which was investigated to identify the potential at which codeposition of the two metals should occur. The cyclic voltammetry of a solution containing both tin(II) chloride and tin(III) chloride in ethaline shows two reduction peaks in the first sweep that can be identified based on the cyclic voltammetry of the individual solutions. In the first cathodic sweep, Sb 3+From Sb 0 The reduction to Sn occurs at -0.3V. 2+ From Sn 0 Reduction to -0.55V vs. Ag / Ag + In the reverse scan, the value is -0.3V vs. Ag / Ag + Sn 0 From Sn 2+ The oxidation peak is 0.2V vs. Ag / Ag + Sb 0 From Sb 3+ There is an oxidation peak at about 0.9 V vs. Ag / Ag. + The more positive potential peak of Sb 3+ / Sb 5+ Sb at -0.3V is thought to be related to the couple 3+ From Sb 0 and -0.55 V vs. Ag / Ag + DeSn 2+ From Sn 0 The reduction potential of SnSb is important data for the electrodeposition of SnSb. Since tin has a more negative reduction potential, the codeposition of Sb and Sn is -0.55 V vs. Ag / Ag. + The cyclic voltammetry data shows that the potential is -0.3V to -0.45V vs. Ag / Ag. + This suggests that Sn peeling occurs while Sb is being deposited, so the deposition potential is -0.3 V vs. Ag / Ag. + This oxidation current cannot be 2+ Sn 0 Using this data and optimizing the electrodeposition conditions, SnSb on a nickel substrate was found to exhibit a tensile strength of -0.7 V vs. Ag / Ag. +The SnSb substrate was electrodeposited with a 1000 volt (V) electrodeposit. The X-ray diffraction (XRD) pattern confirms the formation of only the SnSb phase on the nickel substrate (Figure 1D). The peaks are identified in the X-ray diffraction pattern below. Due to the possible detrimental migration effect of copper on the active material layer observed in previous studies, a nickel substrate was used instead of copper. A scanning electron microscope equipped with an energy dispersive spectroscopy (SEM-EDS) was used to confirm the presence of SnSb.

[0021] A. Electrochemical performance of pure SnSb as a sodium ion anode: The electrochemical performance of thin-film SnSb as a sodium-ion anode was investigated using Swagelok half cells. All test cells used a standard electrolyte: 1 M NaPF8 in dimethyl carbonate (DMC) containing 5 wt% fluoroethylene carbonate (FEC). Life cycle studies exceeding 200 cycles are rare in the Sn-Sb literature and are not indicative of the material's overall performance or deficiencies. As demonstrated by galvanostatic cycling at a C / 2 rate, electrodeposited SnSb has higher stability and better performance than both pure Sn and Sb electrodeposited from ethaline (see DOI:10.1039 / c9cc00001a). During the first discharge, a significant amount of irreversible capacity loss was observed, which may be related to the formation of a solid electrolyte interface (SEI) layer between the electrode surface and the degradable electrolyte. However, after the first cycle, the material's performance began to stabilize, and a discharge capacity of 548.2 mAh / g was obtained on the second cycle. Consistent with previous reports, the capacities obtained near the C / 2 rate ranged from 400 to 600 mAh / g, significantly lower than the theoretical capacity (752 mAh / g). This is speculated to be due to the inability of tin to be fully sodiated in SnSb due to the shift to higher overpotentials at this rate. Based on the second-cycle capacity, electrodeposited SnSb was found to maintain remarkable stability during cycling, with 95% capacity retention (520.8 mAh / g) after 300 cycles and 90.5% capacity retention (496.1 mAh / g) after 500 cycles (Figure 2). The SnSb anode maintained this stability over many cycles, never dropping below 80% capacity retention until 800 cycles. This slow capacity decline can be associated with continuous volume expansion and pulverization, resulting in the separation of the active material and the growth of an SEI on the newly exposed surface. This SEI growth would result in slower kinetics and ultimately lead to sudden cell death. This can be supported by ex-situ SEM images of the SnSb electrode taken after cycling in a sodium ion half-cell, which show the buildup of an organic-like SEI on the electrode surface at later cycles.

[0022] In Figure 2A, SnSb electrodeposited under similar conditions, all from ethaline solution, is compared with electrodeposited Sn and Sb. The half-cells were measured at 0.01 V to 1.5 V vs. Na / Na. + The SnSb samples were cycled at a C / 2 rate of 1000 sq. m. Over 1,000 cycles, SnSb exhibited an overall higher capacity than both the Sb and Sn samples. SnSb appears to combine the long-term cycling stability of Sb with the high theoretical capacity of Sn. Rate capability tests were also conducted to test the performance of pure SnSb at various current densities (Figures 2B and 2C). Figure 2B is a graph showing the electrochemical performance of electrodeposited SnSb, shown by galvanostatic cycling at a C / 2 rate over a potential window of 0.01 V to 1.5 V in a sodium half-cell. Figure 2C is a graph showing the rate capability of electrodeposited SnSb at various current rates, shown by eight cycles at each rate over a potential range of 0.01 V to 1.5 V in a sodium half-cell. Electrodeposited SnSb exhibits specific gravimetric capacities of 720 mAh / g, 620 mAh / g, 540 mAh / g, 500 mAh / g, 472 mAh / g, and 440 mAh / g at C / 10, C / 5, C / 2, 1C, 2C, and 4C, respectively. After cycling at the higher rates, cycling at the C / 10 rate shows the electrode fully recovers without any significant loss in capacity, confirming the electrode's stability. While Figure 2A depicts both lithiation and delithiation with filled and unfilled squares, it should be noted that due to the large number of data points, the squares become indistinguishable for SnSb after approximately 50 cycles. A similar problem exists in Figure 5A below.

[0023] As the charge / discharge rate of the electrode increases, sodium or lithium reaching the electrode may become diffusion-limited, resulting in lower capacity stored at higher rates. Figure 2C shows the amount of energy stored in SnSb as a sodium-ion and lithium-ion anode at varying charge / discharge rates, respectively, for sodium and lithium-ion anodes. The current is increased after several cycles to observe how the electrode responds to faster rates. Fast charge / discharge rates can also stress the electrode material, and in some cases, electrode shattering and capacity loss are observed at high rates. A few "slow" charge / discharge cycles after the fast rate can demonstrate whether the capacity loss at high rates is related to the electrode's energy storage capability or whether it is due to mechanical breakdown of the electrode, resulting in less electrical contact with the active material and less charge being stored. If mechanical breakdown of the electrode occurs, the slower cycles will not exhibit the capacity expected for the "slow" cycle, but will instead show a continuous decline in stored capacity over those final few cycles. The graphs indicate that mechanical breakdown of the electrode is minor.

[0024] Using the C / 2 rate galvanostatic cycling data, differential capacity plots were extracted for the pure Sn, Sb, and SnSb phase samples (Figures 3A-3C). Unlike previous reports of SnSb in lithium-ion batteries, the electrochemistry involved in the sodiation of SnSb is not simply the addition of individual Sn and Sb sodiation events. The sodiation events for SnSb occur at different potentials compared to electrodes electrodeposited with Sn and Sb separately. Similarly, as in previous reports, sodiation events for SnSb are observed at 0.64 V, 0.46 V, and 0.28 V, with complementary desodiation events occurring at 0.6 V, 0.8 V, and 0.95 V. The sodiation event observed at 0.01 V in SnSb compared to a pure Sn electrode supports the hypothesis from previous reports that the inability of tin to completely sodiate in SnSb may be the reason SnSb cannot reach its theoretical capacity at this rate.

[0025] The differential capacity plot also confirms the absence of a Sn impurity phase present as a result of this synthesis. Figure 3B shows a sodiation event at approximately 0.01 V and a desodiation event at approximately 0.2 V, demonstrating the sharp and distinct features observed when cycling at this current density in the presence of Sn but not in pure-phase SnSb. This observation is confirmed by intentionally introducing Sn impurities into a 50:50 Sn:SnSb electrode (Figure 3E), where the differential capacity plot shows a distinct desodiation event at 0.2 V that is absent when cycling pure-phase SnSb at C / 2 (Figure 3D). This suggests that the introduction of Sn impurities would significantly increase the desodiation activity of Sn at 0.2 V to a more distinct peak when cycled at this rate due to the presence of more Sn in the system. Galvanostatic cycling of Sn-impregnated SnSb films also suggests that Sn impurities are detrimental to the electrode's cycle life.

[0026] Ex-situ XRD data (Figure 4) show that crystalline SnSb reappears after the first cycle (charge / discharge) at a C / 2 rate. Ni and SnSb features in the XRD are clearly marked below the data, and the major peaks are depicted in vertical boxes. Previous literature has reported the disappearance of peaks and the formation of amorphous features immediately after the first cycle, indicating that the electrodeposited SnSb does indeed reform, however, gradually losing long-range order and transforming from crystalline SnSb to the amorphous phase with each successive cycle, unlike the immediate transformation observed in previous reports. This observation is similar to the results of a previous study of SnSb in lithium-ion batteries, where SnSb reforms but loses long-range order over time. Extended-area X-ray fine structure (EXAFS) analysis in a recent report also suggests the reformation of the same SnSb phase with a similar Sn and Sb environment after desodiumation, but with a loss of long-range order based on their data. Recent studies using transmission electron microscopy (TEM) and electron diffraction also support the reformation of SnSb. Additional experiments were performed to verify the reformation of crystalline SnSb after electrochemical cycling in this system.

[0027] B. Electrochemical performance of pure SnSb as a lithium-ion anode: To understand the capacity, rate capability, and stability of the material compared to other Sn, Sb, or impure Sn-Sb electrodeposited films, electrochemical tests were performed in half-cells versus lithium. Because of our previous work demonstrating detrimental migration of copper to the active material, SnSb electrodeposited on Ni foil was used as the electrode for these electrochemical tests instead of using a copper substrate. The electrochemical performance of the SnSb film as a sodium ion anode was investigated using a Swagelok half-cell. The standard electrolyte, 1 M LiPF6 in dimethyl carbonate (DMC) with 5 wt% vinylene carbonate (VC), was used in all tested cells. To investigate the cycling characteristics of the electrodeposited SnSb, a 0.01 V to 1.5 V vs. Li / Li solution was used, as shown in Figures 5A and 5B. +The half-cells were cycled at a C / 2 rate for 1000 cycles. Sn and Sb electrodeposited from ethaline solution under similar conditions were also electrochemically tested for comparison with synthesized SnSb. Over 1000 cycles, SnSb exhibited higher total capacity than both Sb and Sn samples. Comparing the first cycle to the second, SnSb exhibited an initial irreversible capacity loss of 35%, which is widely attributed to the initial formation of a solid electrolyte interface (SEI). The retained capacity was greater than 97% on subsequent cycles. The SnSb electrode did not lose 20% of its initial capacity until after 400 cycles (603 mAh / g at the 401st cycle), compared to a gravimetric capacity of 757 mAh / g at the second cycle.

[0028] Rate performance tests were performed to determine the capacity at which the material could operate at various charge densities. Electrodeposited SnSb was capable of lithium cycling with gravimetric capacities of 693 mAh / g at a C / 2 rate, 690 mAh / g at a 1C rate, 675 mAh / g at a 2C rate, 660 mAh / g at a 5C rate, 630 mAh / g at a 10C rate, and 600 mAh / g at a 20C rate. Cycling from higher cycling rates back to a C / 2 rate demonstrated full capacity recovery and the material's stability during cycling at higher rates.

[0029] Differential capacity graphs were obtained from the galvanostatic cycling data. Differential capacity graphs are important for studying the electrochemical lithiation and delithiation steps of electrodeposited SnSb. Figures 6A-6D show differential capacity graphs for electrodeposited SnSb, Sn, and Sb. The reaction of Sb with lithium to form Li3Sb is observed between 0.8 V and 1.1 V vs. Li / Li + Li 4.4 The reaction between Sn and lithium to produce Sn is 0.4V~0.8V vs. Li / Li +The lithiation and delithiation of SnSb involves the individual lithiation and delithiation of Sn and Sb. Overall, cycling of SnSb results in a broadening of the electrochemical profile at later cycles, associated with increased resistance due to the non-uniform formation of the SEI on the electrode surface.

[0030] The surface morphology during cycling, as revealed through ex-situ SEM images, shows the distortion expected from multiple lithiation and delithiation events (Figures 7A and 7B). The surface of the pristine sample showed grains 150 nm in size. After the first lithiation and delithiation cycle, the SEM images show the resulting smaller grain sizes of less than 100 nm. After 10 cycles, the surface of the electrode begins to show cracks less than 100 nm wide. After 50 cycles, the surface shows more cracks present on the electrode. However, a different, less crystalline morphology was observed to form on the surface of the electrode, suggesting the formation of a strongly adherent SEI on the surface of the electrode.

[0031] To examine the changes in crystallinity with cycling, ex-situ XRD was performed simultaneously with ex-situ SEM (Figure 7). Feature identification in the XRD is shown below the data. After the first lithiation and delithiation, the presence of crystalline SnSb was confirmed, confirming the reformation of SnSb after delithiation of the electrode material, as seen in previous SnSb studies. Further cycling of the electrode shows a rapid decrease in crystallinity, although reformation still occurs up to 50 cycles. This decrease in crystallinity is attributed to the loss of long-range order in the crystalline material and the formation of smaller crystallites as a result of cycling.

[0032] To compare the performance of pure SnSb produced according to embodiments of the present invention with the impurity production seen in previous reports, a controlled method for forming tin-rich SnSb was developed. Using the same ethaline solvent, impure and tin-rich SnSb samples were intentionally synthesized containing concentrations equal to the target Sn:Sb ratio. Increasing the Sn content beyond a 1:1 Sn:Sb ratio resulted in the production of crystalline tin, as shown by the XRD in Figure 8A, the characteristics of which are identified below the data. Also evident through the XRD patterns is a continuous increase in crystalline intensity as deposition from more concentrated tin solutions occurs. Using SEM-EDS, the composition of the fabricated electrodes was confirmed to be similar to the target solution concentrations. The incorporation of tin into the system can be monitored by differential capacity plots within the tin and antimony lithiation range (Figure 8B). As discussed above, the lithiation and delithiation of SnSb is a combination of various phases that both the individual Sn and Sb components undergo within a lithium cell. Increasing the Sn content to artificially mimic the formation of impurities results in a clear increase in the activity for Sn lithiation and delithiation. Conversely, decreasing the Sb content throughout the cell results in a noticeable decrease in the dQ / dV associated with Sb lithiation and delithiation. The purity of the SnSb and the successful incorporation of tin into the system are also confirmed in the differential capacity plots shown above for SnSb as a Na-ion anode.

[0033] Testing the tin-rich SnSb electrode is important because it demonstrates the importance of SnSb purity to the stability of the system. Figure 8C shows the relationship between the SnSb content and the Li / Li charge at a rate of 0.01 V to 1.5 V vs. Li / Li at a rate of C / 2. +Figure 1 shows galvanostatic cycling of electrodeposited Sn:SnSb electrodes with various ratios during galvanostatic cycling. The artificially impure SnSb samples with high Sn content experienced a large capacity loss over the first 200 cycles (more than 10% capacity loss within 200 cycles). Pure tin exhibited the highest initial capacity loss of 94% at 200 cycles, while SnSb exhibited the lowest initial capacity loss of 7% after 200 cycles. Among the tin-rich SnSb electrodes, 13% capacity loss was observed for 1:3 Sn:SnSb, 19% capacity loss for 1:1 Sn:SnSb, and 26% capacity loss for 3:1 Sn:SnSb at 200 cycles. This large initial capacity loss can be attributed to the large volume expansion and instability of the crystalline Sn domains present in the film. Consequently, the tin-rich samples experienced less capacity loss during subsequent cycles (>200), and their coulombic efficiency increased with each cycle. The later, more stable cycling of the tin-rich SnSb electrode may be related to the remaining SnSb domains. The slow decrease in capacity during cycling in this state may be related to the slow SEI growth on the newly exposed surface. Figure 8D summarizes at what point each electrode falls below 80% capacity retention for the second cycle. With increasing Sn impurities, the Sn-Sb electrode begins to fall below this benchmark at earlier cycles. Reformation of SnSb in lithium systems has been shown in previous literature. Although the phase during intercalation is different, recent studies of SnSb reformation when tested against sodium confirmed the synergistic effect of SnSb reformation on the cycle life of the material. SnSb reformation may have benefits in alleviating the overall stress and expansion during lithiation / delithiation that are not present in pure Sn.

[0034] In summary, electrodeposition of phase-pure SnSb has been demonstrated using an ethaline-based solution. Electrodeposited SnSb without binder or carbon additives was tested as a negative electrode in sodium-ion batteries. While long-term cycling tests beyond 200 cycles have rarely been reported in previous SnSb literature, we report 1000 cycles to better understand the lifespan and long-term cycling stability of our SnSb electrodes. Although capacities below the theoretical maximum were obtained from galvanostatic cycling at the C / 2 rate, SnSb exhibited high cycling stability, with capacity retention dropping below 80% after 800 cycles. Rate capability tests demonstrated the cycling stability of SnSb at higher current densities. Differential capacity plots and ex-situ XRD revealed that the electrodeposited SnSb exhibited a gradual transition from crystalline SnSb to the amorphous phase during C / 2 cycling, instead of the immediate change seen in previous reports. The differential capacity plot also demonstrates the purity of the SnSb, with no specific electrochemical events of Sn. Along with the reformation of SnSb, some structural integrity was observed, which may be related to the observed electrochemical stability. If reformation is important for the material's stability, then material purity is crucial for obtaining a long cycle life of the material. While the films reported here are thin, the electrochemical performance of a similarly thick material, ~200 nm of Sn, was recently demonstrated by electrodepositing it onto a 3D scaffold architecture, with the electrode having three times the surface loading of current slurry-based anodes.

[0035] This electrodeposited SnSb, used as an anode in a lithium-ion battery, demonstrated stability, retaining only less than 80% of its capacity after 400 cycles. To explore the necessity of a pure-phase electrodeposition process, we created tin-rich samples to mimic impurity formation. Electrochemical data demonstrated the negative impact of increasing Sn content on the electrode's cyclability, highlighting the importance of purity on SnSb lifetime. While Sn ideally provides high energy density throughout the battery, stress from lithiation of the Sn impurity region in our electrodeposition adversely affects overall battery performance and lifetime. Essentially, we discovered that SnSb helps access the higher capacity of Sn while minimizing the stress and swelling associated with this process.

[0036] C. Effect of water on electrodeposition products: The presence of water appears to affect the chemical reaction since both the choline chloride and ethylene glycol precursors are hygroscopic.

[0037] Figure 9 shows the results of the electrochemical analysis of Ag / Ag from 50 mM SnCl2·2H2O and 50 mM SbCl3 in ethaline solution for 300 seconds. + Figure 1 shows the XRD patterns of a Cu foil electrode surface after conditioning, heating at 110 °C, and introducing water into the solution for SnSb electrodeposited from ethaline solution at 100 °C. The species present are identified below the XRD data.

[0038] Thus, electrodeposition products from as-prepared solutions containing Sb and Sn chlorides dissolved in ethaline solution were produced using the resulting mixture of choline chloride and ethylene glycol, producing crystalline SnSb, with Sn present as a minor crystalline phase. Heating the ethaline solution at 110°C prior to electrodeposition to drive off any water present results in the room-temperature electrodeposition of pure crystalline SnSb.

[0039] To support the hypothesis that the Sn phase is caused by hygroscopic ethaline rather than by water collected by the pristine, water was introduced into the previously heated solution, resulting in the reappearance of a minor crystalline Sn phase. However, the chemistry within the ethaline solvent does not immediately change when exposed to ambient room humidity and other conditions, so minimal humidity may be acceptable.

[0040] To support the hypothesis that heating the solution merely removes water and does not alter the reaction components, additional control experiments were performed. While unwanted water in the solution can be reduced by heating the ethaline solution to 110 °C, prolonged excessive heating adversely affects the electrodeposited product. X-ray diffraction (XRD) studies showed that electrodeposition at -0.7 V onto nickel substrates using solutions heated at 110 °C for 3 days resulted in an amorphous product with an antimony-rich Sn to Sb composition. EDS (Electron Dynamics Spectroscopy) showed that after heating at 110 °C for 3 days, the atomic ratio of the precipitates decreased from 1:1 to 2.5:1. Heating the solution for an additional 4 days reduced the tin content of the precipitates to 4.5:1.

[0041] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed; obviously, many modifications and variations are possible in light of the above teachings. The embodiments have been chosen and described to best explain the principles of the invention and its practical application, and thereby enable others skilled in the art to best utilize the invention, with various embodiments and various modifications as suited to the particular uses contemplated. It is intended that the scope of the invention be defined by the appended claims.

Claims

1. 1. A method for forming a high stability lithium ion battery anode, comprising: providing a solution containing a Sn(II) salt and an Sb(III) salt in a non-aqueous solvent; inserting a conductive metal substrate into the solution; and -0.55 V vs. Ag / Ag at room temperature relative to a conductive metal substrate for a time sufficient to form a thin film of SnSb thereon. + applying a potential more negative than heating the solution prior to applying the potential; A method comprising:

2. 10. The method of claim 1, wherein the non-aqueous solvent comprises a mixture of choline chloride and ethylene glycol.

3. 3. The method of claim 2, wherein the mixture of choline chloride and ethylene glycol is a deep eutectic mixture.

4. 2. The method of claim 1, wherein Sn(II) and Sb(III) are present in equimolar amounts.

5. 2. The method of claim 1, wherein the Sn(II) salt and the Sb(III) salt comprise Sn(II) chloride and Sb(III) chloride.

6. The method of claim 1 , wherein the conductive metal substrate comprises a nickel substrate.

7. The applied potential is about −0.7 V vs. Ag / Ag + The method according to claim 6, wherein

8. 10. The method of claim 1, wherein the non-aqueous solvent is selected from ethylene glycol, choline chloride, urea, thiourea, glycerol, malonic acid, benzoic acid, and oxalic acid, and mixtures thereof.

9. 2. The method of claim 1, wherein the Sn(II) and Sb(III) salts are selected from stannous and antimony sulfates, stannous and antimony citrates, and salts of the reaction of stannous and antimony with gluconic and tartaric acids, and mixtures thereof.

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