Antimony based anode material for rechargeable batteries and preparation method

Abstract

An antimony based anode material for a rechargeable battery includes nanoparticles of composition SbM.sub.xO.sub.y, where M is an element selected from the group consisting of Sn, Ni, Cu, In, Al, Ge, Pb, Bi, Fe, Co, and Ga, with 0x<2 and 0y2.5+2x. The nanoparticles form a substantially monodisperse ensemble with an average size not exceeding a value of 30 nm and by a size deviation not exceeding 15%. A method for preparing the antimony based anode material is carried out in situ in a non-aqueous solvent and starts by reacting an antimony salt and an organometallic amide reactant and oleylamine.

Claims

1. An antimony based anode material for a rechargeable battery, the anode material comprising nanoparticles of composition SbM.sub.xO.sub.y, where M is an element selected from the group consisting of Sn, Ni, Cu, In, Al, Ge, Pb, Bi, Fe, Co, and Ga, with 0<x<2 and 0y2.5+2x, wherein the nanoparticles are monodisperse nanocrystals of SbM.sub.xO.sub.y having an average size between 5 nm and 30 nm and a size deviation not exceeding 15%.

2. The material according to claim 1, wherein the nanoparticles are coated with a capping species.

3. The material according to claim 1, wherein the nanoparticles are composed of SbSn.sub.x.

4. The material according to claim 3, wherein x is 1.5.

5. The material according to claim 1, wherein the average size of the nanoparticles is between 10 nm and 25 nm.

6. The material according to claim 5, wherein the average size of the nanoparticles is between 15 nm and 25 nm.

7. The material according to claim 1, wherein the full width at half aximum (FWHM) of a size distribution of the average size is below 11%.

8. The material according to claim 1, wherein the full width at half maximum (FWHM) of a size distribution of the average size is below 10%.

9. The material according to claim 1, wherein the full width at half maxinmm (FV/FIM) of a size distribution of the average size is in a range of 7% to 11%.

10. The material according to claim 1, wherein the full width at half maximum (FWHM) of a size distribution of the average size is in a range of 7% to 10%.

11. A rechargeable battery comprising an anode and a cathode, the anode comprising an antimony based anode material, which comprises nanoparticles of composition SbM.sub.xO.sub.y, where M is an element selected from the group consisting of Sn, Ni, Cu, In, Al, Ge, Pb, Bi, Fe. Co, and Ga, with 0<x<2 and 0y2.5+2x, wherein the nanoparticles are monodisperse nanocrystals of SbM.sub.xO.sub.yhaving an average size between 5 nm and 30 nm and a size deviation not exceeding 15%.

12. The rechargeable battery according to claim 11, wherein said battery is a sodium battery or a lithium ion battery.

13. The material according to claim 1, wherein the nanoparticles are substantially spherical.

14. The material according to claim 1, wherein the nanoparticles have centrosymmetric polyhedral shapes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above mentioned and other features and objects of this invention and the manner of achieving them will become more apparent and this invention itself will be better understood by reference to the following description of various embodiments of this invention taken in conjunction with the accompanying drawings, wherein are shown:

(2) FIG. 1 Schematic illustration of synthesis of Sb NCs with corresponding TEM images and structure;

(3) FIG. 2A HRTEM of one Sb NP [viewed along (012) direction];

(4) FIG. 2B SAED pattern;

(5) FIG. 2C PXRD pattern of Sb NPs with varying size (10, 14, and 20 nm);

(6) FIGS. 3A and 3B TEM images of Sb NPs synthesized using OLA solvent;

(7) FIG. 4A TEM image of Sb NPs synthesized without LiDPA for Sb(DMA).sub.3 based synthesis;

(8) FIG. 4B TEM image of Sb NPs synthesized without LiDPA for SbCl.sub.3 based synthesis;

(9) FIGS. 5A, 5B, 5C, and 5D TEM images of Sb NPs for Sb(DMA).sub.3 based synthesis: time dependent reaction (corresponding to Table 1);

(10) FIGS. 6A, 6B, 6C, and 6D TEM images of Sb NPs for Sb(DMA).sub.3 based synthesis: temperature dependent reaction (corresponding to Table 2);

(11) FIGS. 7A, 7B, 7C, and 7D TEM images of Sb NPs for Sb(DMA).sub.3 based synthesis: different precursor amount (corresponding to Table 3);

(12) FIGS. 8A, 8B, 8C, and 8D TEM images of Sb NPs for Sb(DMA).sub.3 based synthesis: different DIBAH amount (corresponding to Table 4);

(13) FIGS. 9A, 9B, 9C, 9D, and 9E TEM images of Sb NPs for SbCl.sub.3 based synthesis: time dependent reaction (corresponding to Table 5);

(14) FIGS. 10A, 10B, and 10C TEM images of Sb NPs for SbCl.sub.3 based synthesis: temperature dependent reaction. (corresponding to Table 6);

(15) FIGS. 11A, 11B, 11C, and 11D TEM images of Sb NPs for SbCl.sub.3 based synthesis: different precursor amount. (corresponding to Table 7);

(16) FIG. 12 TEM images of synthesized Sn.sub.3Sb.sub.2 NPs;

(17) FIG. 13A Galvanostatic discharge profiles for 10 nm Sb anodes;

(18) FIG. 13B CVs for the initial five charge-discharge cycles for electrode containing 10 nm Sb NPs (scan rate=0.1V/s);

(19) FIGS. 13C, 13D, and 13E Reversible discharge capacities of Sb and Sn.sub.3Sb.sub.2 based anodes (normalized by the content of Sb or SnSb) at different current densities (0.3, 4.8 and 12.2 A/g);

(20) FIG. 13F Rate capability of Sb based electrodes at different current densities; all measurements performed in Li-batteries;

(21) FIG. 14A CVs for the initial five charge-discharge cycles for electrode containing 10 nm Sb NPs (scan rate=0.1V/s);

(22) FIGS. 14B and 14C Rate capability of Sb based electrodes at different current densities; all measurements performed in Na-batteries; and

(23) FIG. 15 TEM image of synthesized CoSb NPs.

DETAILED DESCRIPTION OF THE INVENTION

(24) In the description above and in the following description of the invention and of exemplary embodiments, any statements relating to possible explanations or interpretations of observations and results shall not be construed as binding the invention to a particular theory.

(25) General Procedure:

(26) Synthesis of Monodisperse Sb NCs. 10-20 nm Sb NCs with a narrow size distribution of 7-11 A can be obtained by injecting Sb precursor, diluted with toluene or octadecene, into a hot (preferably 160-200 C.) solution containing mixture of trioctylphosphine (TOP), Lithium diisopropylamide (LiN(iPr).sub.2, LDA) and oleylamine (OLA). Two precursors, tris(dimethylamino)antimony (III) [Sb(NMe.sub.2).sub.3 or simply Sb(DMA).sub.3] and inexpensive antimony(III) chloride, yielded NCs of very similar quality under nearly identical reaction conditions. Detailed structural characterization can be made using e.g. high-resolution transmission electron microscopy (HR-TEM), electron diffraction, powder X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). Using these methods, the particles were confirmed to be chemically pure, highly-crystalline rhombohedral Sb NCs (space group N166, R.sup.33m, a=b=0.4307 nm, c=1.1273 nm, JCPDS 35-0732).

(27) A suitable synthesis for Sn NCs is via formation of amides in-situ, as intermediates during a one-pot synthesis, thereby eliminating their elaborate synthesis and purification, as well as enabling highly unstable amides to be used as precursors. In a first step a neutral amine (OLA, an acid) and an amide base (LiDPA) were mixed in order to establish an acid-base equilibrium due to higher basicity of secondary amide according to
(LiDPA):Oleyl-RNH.sub.2+LiN(iPr).sub.2Oleyl-NHLi+HN(iPr).sub.2.

(28) It is assumed that in a second step, transient and unstable Sb(III) oleylamide is formed in-situ by the reaction of Li oleylamide with Sb(DMA).sub.3 or SbCl.sub.3. The decomposition of Sb(III)-oleylamide occurs instantly due to known instability of bulky Sb(III) amides. It is assumed that highly unstable transient precursors may in fact be far superior due to enhanced nucleation and growth kinetics controlled by the speed and sequence of the mixing, and through concentrations of starting reagents etc. It was found that in the absence of LiDPA, Sb(DMA).sub.3 and SbCl.sub.3 were reduced by OLA much slower, yielding poorly defined, sub-100 nm precipitates. Importantly, the combination of TOP and OLA ligands significantly improved uniformity of the Sb NCs as compared to pure OLA system. In the absence of OLA, the reaction proceeded uncontrollably due to boththe fast formation of highly unstable Sb(III)-DPA species and the lack of stabilization provided by OLA. The size-tuning in 10-20 nm range in SbCl.sub.3 and Sb(DMA).sub.3-based systems can be conveniently achieved by varying the reaction time, temperature, precursor amounts and, as an option, by adding diisobutylaluminum hydride (DIBAH) as a reducing agent. It should be noted that addition of DIBAH also allows increase of reaction yield for smallest NCs, from 15% to >30% for 10 nm Sb NCs. In both systems, as the reaction proceeds, nearly spherical 10 nm Sb NCs evolve into more faceted 20 nm-large NCs.

(29) The NCs and NPs can be decapped using solutions of hydrazine in acetonitrile. Briefly, a powder of organic-capped Sb NCs was three times stirred with 1M N.sub.2H.sub.4/CH.sub.3CN solution, rinsed with CH.sub.3CN. Then the NCs were re-dispersed in water shortly before the preparation of battery anodes. The removal of the initial organic ligands has been estimated by integrating the intensities of aliphatic CH stretching modes in FTIR spectra and was found to amount to about 93%.

(30) Materials

(31) Chemicals and Solvents:

(32) Cobalt (II) chloride (Aldrich), Oleylamine (OLA, tech., TCI), Toluene (99.9%, Sigma-Aldrich), Toluene dried (99.9%, max. 0.005% H.sub.2O, VWR Merck), Ethanol (99.9%, Scharlau), Oleic acid (OA, 90%, Aldrich), Octadecene (ODE, 90%, Sigma-Aldrich), Antimony (III) chloride (SbCl.sub.3, 99.999%, ABCR), lithium diisopropylamide (LiDPA, 97%, Sigma-Aldrich), 1-dodecanethiol (Sigma-Aldrich), Trioctylphosphine (TOP, STREM), Diisobutylaluminum hydride solution (DIBAH, 1M in Toluene, Sigma-Aldrich), Tris(dimethylamino) antimony (III) [Sb(DMA).sub.3, Sigma-Aldrich]. Hydrazine (Gerling Holz+Co), Acetonitrile (Sigma-Aldrich). Dried oleylamine (OLA) and octadecene (ODE) were prepared by drying OLA and ODE at 100 C. under vacuum overnight.

(33) Battery Components:

(34) Antimony (99.5%, Alfa-Aesar), Carbon black (Super C65, provided by TIMCAL), 1 M solution of LiPF.sub.6 in ethylene carbonate/dimethyl carbonate (EC/DMC, Novolyte), NaClO.sub.4 (98%, Alfa Aesar), propylene carbonate (BASF), ethylene carbonate (Novolyte), Diethyl carbonate (>99%, Aldrich), 4-Fluoro-1,3-dioxolan-2-one (FEC, >98.0%, TCI), Celgard separator (Celgard 2320, 20 m microporous trilayer membrane (PP/PE/PP), Celgard Inc. USA) and glass-fiber separator (EUJ-grade, Hollingsworth & Vose Company Ltd., United Kingdom), Carboxymethyl cellulose (CMC, Grade: 2200, Lot No. B1118282, Daicel Fine Chem Ltd).

(35) Methods

(36) 1. Synthesis of Monodisperse Sb NPs/NCs and SbSn NPs/NCs

(37) 1.1 General Information about Synthesis of Monodisperse Sb NCs

(38) Monodisperse 10-20 nm Sb NCs (FIG. 1) with standard size distributions of 10% or below were obtained by injecting Sb precursor, diluted with toluene or octadecene, into hot solution containing mixture of trioctylphosphine (TOP), lithium diisopropylamide (LiN(iPr).sub.2, LDA) and oleylamine (OLA). Two precursors, tris(dimethylamino) antimony (III) [Sb(NMe.sub.2).sub.3 or simply Sb(DMA)3] and inexpensive antimony(III) chloride, yielded NCs of very similar quality. Detailed structural characterization with high-resolution transmission electron microscopy (TEM), electron and X-ray powder diffraction (FIG. 2) confirmed the formation of chemically pure, highly-crystalline Sb NCs (space group R.sup.33m, a=b=0.4307 nm, c=1.1273 nm, JCPDF 35-0732). FIG. 2C illustrates a single crystalline nature of Sb NCsa finite size effect on the broadening of XRD reflections.

(39) 1.2 Synthesis of Antimony Nanoparticles from Sb(DMA).sub.3 Precursor

(40) In a typical synthesis of 17 nm Sb NPs, 8 mL of Trioctylphosphine (TOP, 8 ml) was added into 50-mL three neck flask and dried under vacuum for 45 minutes at 100 C. under constant stirring. In parallel, LiDPA (3.6 mmol, 0.38 g) and Sb(DMA).sub.3 (0.39 mmol, 75 L) were separately dissolved under inert atmosphere in 2 mL of dried OLA and ODE, respectively. The temperature of TOP was rised to 170 C. under argon, followed by the injection of LiDPA/OLA solution, and, in 30 s, Sb(DMA).sub.3/ODE solution. The solution color appeared dark brown immediately, indicating the formation of antimony nanocrystals. After 30 seconds, the reaction solution was quickly cooled down to room temperature using water-ice bath. Upon cooling, at about 160 C., 12 ml of anhydrous toluene were added. Then, 0.4 mL of 1-dodecanthiol were added at 50 C. After cooling, Sb NCs were precipitated by 60 mL of ethanol following by their centrifugation at 8500 rpm for 4 min. Then, Sb NCs were redispersed in toluene solution of oleic acid (6 mL, 1 ml OA/50 ml Toluene) to replace the weakly bonded surface capping ligands. Finally, the Sb NCs were precipitated second time by 6 ml of ethanol, centrifuged and redispersed in common nonpolar solvents such as TCE, chloroform or toluene forming long-term stable solutions.

(41) 1.3 Synthesis of Antimony Nanoparticles from SbCl.sub.3 Precursor

(42) The synthetic procedure is the same as the procedure for Sb(DMA).sub.3 precursor. For dissolution of SbCl.sub.3, dried toluene solution was used instead of ODE.

(43) 1.4 Further Remarks on the Synthesis

(44) (i) Similar results, in terms of the size and quality of Sb NCs, were obtained from the SbCl.sub.3 precursor using OLA solvent (FIG. 3).

(45) (ii) No NCs could be formed without LiDPA: only polydisperse large particles are formed upon the injecting DIBAH into SbCl.sub.3 or Sb(DMA).sub.3-TOP solution directly (FIG. 4).

(46) (iii) Size of Sb NPs synthesized from Sb(DMA).sub.3 precursor can be controlled by the variation of reaction time (Table 1, FIG. 5), temperature (Table 2, FIG. 6), amount of Sb(DMA).sub.3 precursor (Table 3, FIG. 7) or amount of DIBAH (DIBAH have to be added in 30 sec after injection of LiDPA followed by injection of Sb(DMA).sub.3/ODE solution, Table 4, FIG. 8).

(47) (iv) Size of Sb NPs synthesized from SbCl.sub.3 precursor can be controlled by the variation of reaction time (Table 5, FIG. 9), temperature (Table 6, FIG. 10) or different amount of SbCl.sub.3 precursor (Table 7, FIG. 11).

(48) (v) As comparative example, the size was measured for commercial Sb: the diameter was 3.97 m with standard size distribution of 6.3%.

(49) TABLE-US-00001 TABLE 1 Experimental conditions for Sb(DMA).sub.3 based synthesis: time dependent reaction. Reac- LiDPA(in 2 tion Mean TOP, mL OLA), Sb(DMA).sub.3, T(red), time, size, S, # mL mmol mmol C. sec nm % A 8 3.6 0.39 160 5 10 7.8 B 8 3.6 0.39 160 10 12 8.4 C 8 3.6 0.39 160 30 17 8.4 D 8 3.6 0.39 160 60 19 10.1

(50) TABLE-US-00002 TABLE 2 Experimental conditions for Sb(DMA).sub.3 based synthesis: temperature dependent reaction. Reac- LiDPA(in 2 tion Mean TOP, mL OLA), Sb(DMA).sub.3, T(red), time, size, S, # mL mmol mmol C. sec nm % A 8 3.6 0.39 150 30 14 8.1 B 8 3.6 0.39 160 30 17 8.4 C 8 3.6 0.39 170 30 18 12.1 D 8 3.6 0.39 180 30 20 11.4

(51) TABLE-US-00003 TABLE 3 Experimental conditions for Sb(DMA).sub.3 based synthesis: different precursor amount. Reac- LiDPA(in 2 tion Mean TOP, mL OLA), Sb(DMA).sub.3, T(red), time, size, S, # mL mmol mmol C. sec nm % A 8 3.6 0.13 160 30 11 15.0 B 8 3.6 0.26 160 30 16 12.8 C 8 3.6 0.39 160 30 17 8.4 D 8 3.6 0.52 160 30 21 12.7

(52) TABLE-US-00004 TABLE 4 Experimental conditions for Sb(DMA).sub.3 based synthesis: different DIBAH amount. Reac- LiDPA(in 2 tion Mean TOP, mL OLA), DIBAH, Sb(DMA).sub.3, T(red) time, size, S, # mL mmol mL mmol C. sec nm % A 8 3.6 0 0.39 160 30 17 8.4 B 8 3.6 0.1 0.39 160 30 13 9.5 C 8 3.6 0.2 0.39 160 30 12 11.5 D 8 3.6 0.3 0.39 160 30 11 9.1

(53) TABLE-US-00005 TABLE 5 Experimental conditions for SbCl.sub.3 based synthesis: time dependent reaction. Reac- LiDPA(in 2 tion Mean TOP, mL OLA), SbCl.sub.3, T(red), time, size, S, # mL mmol mmol C. sec nm % A 8 3.6 0.125 160 10 8 8.9 B 8 3.6 0.125 160 30 11 8.1 C 8 3.6 0.125 160 45 14 8.2 D 8 3.6 0.125 160 60 18 11.8

(54) TABLE-US-00006 TABLE 6 Experimental conditions for SbCl.sub.3 based synthesis: temperature dependent reaction. Reac- LiDPA (in 2 tion Mean TOP, mL OLA), SbCl.sub.3, T(red), time, size, S, # mL mmol mmol C. sec nm % A 8 3.6 0.125 160 10 8 8.9 B 8 3.6 0.125 180 10 10 12.3 C 8 3.6 0.125 200 10 12 18.6

(55) TABLE-US-00007 TABLE 7 Experimental conditions for SbCl.sub.3 based synthesis: different precursor amount. Reac- LiDPA(in 2 tion Mean TOP, mL OLA), SbCl.sub.3, T(red), time, size, S, # mL mmol mmol C. sec nm % A 8 3.6 0.03125 160 30 15 16.7 B 8 3.6 0.0625 160 30 12 11.0 C 8 3.6 0.125 160 30 11 8.1 D 8 3.6 0.1875 160 30 10 9.2
1.5 Synthesis of SnSb (Sn.sub.3Sb.sub.2) NPs

(56) Monodisperse 20 nm SnSb NPs (FIG. 12) with standard size distributions of 10% were obtained by injecting SbCl.sub.3 precursor, diluted with toluene, into hot solution containing mixture of oleylamine (OLA), lithium diisopropylamide (LiN(iPr).sub.2, LDA) and DIBAH. Based on chemical analysis data, SnSb NCs correspond to the Sn.sub.3Sb.sub.2 composition.

(57) In a typical synthesis of SbSn.sub.x NPs, OLA (10 mL) and SnCl.sub.2 (0.25 mmol, 0.048 g), were added into 50-mL three neck flask and dried under vacuum for 45 minutes at 100 C. under constant stirring. In parallel, LiN(SiMe.sub.3).sub.2 (3.6 mmol, 0.6 g) and SbCl.sub.3 (0.25 mmol, 0.055 g) were separately dissolved under inert atmosphere in 1 mL of anhydrous toluene. The temperature of SnCl.sub.2/OLA was rised to 210 C. under argon, followed by the injection of Li[N(SiMe.sub.3).sub.2/toluene solution, then 0.6 mL of 1M DIBAH solution, and, finally, SbCl.sub.3/toluene solution. After 6 hours, the reaction solution was quickly cooled down to room temperature using water-ice bath. Upon cooling, at about 160 C., 12 ml of anhydrous toluene were added. Then, 0.5 mL of 1-dodecanthiol and 0.5 mL of oleic acid were added after synthesis at 50 C. After cooling, SnSb NCs were precipitated by 60 mL of ethanol followed by their centrifugation at 8500 rpm for 4 min. Then, SnSb NCs were redispersed in toluene solution of oleic acid (12 mL, 1 ml OA/50 ml Toluene). Finally, the SnSb NCs were precipitated second time by 12 ml of ethanol, centrifuged and redispersed in common nonpolar solvents such as TCE, chloroform or toluene forming long-term stable solutions.

(58) 1.6 Ligand Removal

(59) For typical ligand removal procedure, 25 mL of acetonitrile and 0.8 mL of hydrazine were mixed with Sb or SnSb NPs which were collected after their precipitation and centrifugation. The mixture was stirred for 2 h at room temperature and then centrifuged at 8000 rpm for 4 min. Then, 20 mL of acetonitrile were mixed with precipitated Sb or SnSb NPs, in order to remove excess of hydrazine and centrifuged at 8000 rpm for 4 min. This procedure was repeated three times. Finally, centrifuged Sb or SnSb NPs were dried under vacuum at room temperature for 12 h.

(60) 1.7 Characterization

(61) Transmission Electron Microscopy (TEM) images were obtained with Philips CM30 TEM microscope at a voltage of 300 kV. Carbon-coated TEM grids from Ted-Pella were used as substrates. Aberration-corrected HD-2700CS (Hitachi; cold-field emitter) at a voltage of 200 kV were used for Scanning transmission electron microscopy (STEM) investigations. Wide-angle powder X-ray diffraction (XRD) spectra were collected on STOE STADI P powder X-ray diffractometer.

(62) Size of NCs was determined by PEBBLES software in automatic mode for at least 100 NCs per each sample. Size deviations of NCs were calculated via formula (1) using Gaussian fit of measured NCs,

(63) $\begin{array}{cc}S=\frac{\mathrm{FWHM}}{2d}*100\%,& \left(1\right)\end{array}$
where FWHM is fullwidth at half maximum of the Gaussian fit; d is diameter of NCs at maximum of the Gaussian fit.

(64) The tin and antimony content of SbSn.sub.x was determined by d induced coupled plasma (ICP) spectroscopy. Two ICP measurements of two samples independently gave a ratio of Sb to Sn of 1:1.5.

(65) 1.8 Synthesis of CoSb NPs

(66) In a typical synthesis of monodisperse 7.3 nm CoSb NCs (FIG. 15) with standard size distribution of 14.9%, dried OLA (13 mL) was mixed with of CoCl.sub.2 (0.065 g, 0.5 mmol) in a 50-mL three neck flask, additionally dried under vacuum (45 minutes at 120 C.), heated to 270 C. under nitrogen, followed by the injection of LiN(iPr).sub.2 solution (3.6 mmol, i.e. 0.38 g, in 2 mL of OLA) and, after 30 s, 0.5 mmol of SbCl.sub.3 (0.114 g, in 0.2 mL of toluene and 0.8 mL of ODE). The reaction was held for 16 h. Finally, the reaction was quickly quenched by cooling with an ice-water bath together with the injection of anhydrous toluene (12 mL) at around 150 C. Upon cooling, oleic acid (0.4 mL) was added at around 50 C. After cooling, CoSb NCs were precipitated by adding ethanol (60 mL), followed by centrifugation at 8500 rpm for 4 min. Then CoSb NCs were redispersed in toluene (12 mL) containing oleic acid (0.25 mL), and then precipitated by ethanol (12 mL) and centrifuged at 8000 rpm for 1 min. Finally, CoSb NCs were dispersed in common nonpolar solvents such as chloroform or toluene and stored in the fridge.

(67) Based on chemical analysis data, CoSb NCs correspond to a ratio of Co:Sb of 1:1.08.

(68) 2. Preparation of Sb or SnSb-Based Electrodes and Electrochemical Testing.

(69) 2.1 Introduction

(70) Since the rate-performance is a complex function of the electrode formulation (chemistry and amounts of binder and conductive additive), porosity, electrode thickness, electrolyte, temperature etc. these parameters were kept identical and unchanged for all cells in the experiments performed, allowing to focus on the lithiation and sodiation kinetics instrinsic to the electrode material.

(71) There are several factors which are most plausible contributors to the fast alkali-ion insertion and extraction in Sb electrodes. First, the layered crystal structure of Sb (FIG. 1) with low atomic packing factor of just 39% [4] allows for a fast diffusion of Li and Na ions, while electronic conductivity is sufficiently high due to semimetallic nature of this element.

(72) Second reason lies in the chemical transformation occurring during the insertion and extraction of alkali metal ions. While Sn shows up to 6 intermediate crystalline phases, pure Sb as electrode and using in-situ XRD pointed to only one intermediate crystalline phase (Li.sub.2Sb) upon lithiation, while de-lithiation is likely to proceed directly from Li.sub.3Sb towards crystalline Sb [29], [30]. For Na-(de)insertion, only one crystalline phase, Na.sub.3Sb (mixture of cubic and hexagonal) has been reported [22].

(73) Li-ion data for 20 nm and bulk Sb closely resembled the data from literature, namely two main peaks at 0.88V and 0.84V corresponding to the two-step lithiation (via Li.sub.2Sb), while the main de-lithiation feature is located at 1.05V. Partial amorphization of Li.sub.3Sb during Li-insertion can possibly take place, as suggested by the third, weak feature at 0.75V for bulk Sb and by an overall merging of both peaks into a continuous wave for 20 nm Sb. Na-ion data for bulk Sb fully reproduced the work of Darwiche et al. [22], who also for the first time correlated electrochemical data with in-situ XRD for Na-ion cells with bulk Sb electrodes. Three well separated Na-ion insertion features at 0.3-0.75 V may be correlated to the following sequence of transitions
amorphous Sb.fwdarw.amorphous Na.sub.3Sb.fwdarw.Na.sub.3Sb.sub.hex/Na.sub.3Sb.sub.cub.fwdarw.Na.sub.3Sb.sub.hex.[22].

(74) De-insertion primarily occurs as Na.sub.3Sb.sub.hex.fwdarw.amorphous Sb transition (peak at 0.8V), with the additional broad feature at ca. 0.88V, which may correspond to partial crystallization of Sb.

(75) Cycling Stability:

(76) While in Li-ion cells excellent cycling stabilities with capacities above 600 mAh g.sup.1 (e.g. 90% or more of theoretical) at 0.5 and 1C-rates were observed for all Sb-electrodes, a clear difference is observed at 4C-rate, at which nanoscopic Sb remains stable, while the capacity of bulk Sb quickly fades. A consistently higher performance of 20 nm Sb NCs was found as compared to 10 nm and bulk Sb, in both Li-ion and Na-ion cells. The difference between 10 nm and 20 nm Sb NCs is most pronounced for Na-cells, where 10-20% lower charge storage capacities are seen at all cycling rates. This difference might be explained by instabilities caused by large proportion of the surface atoms for 10 nm or smaller particles.

(77) Volumetric changes are generally considered as a major problem for obtaining stable cycling in alloying anode materials. These values can be estimated from the difference of the molar volumes (% V.sub.m) between the final (Li.sub.xM) and initial metallic (M) phases: % V=100%[V.sub.m(Li.sub.xM)V.sub.m(M)]/V.sub.m(M). In the case of Sb, much smaller increase of volume by only 135% (meaning that the final volume is 2.35 times greater), upon full lithiation is expected, as compared to Si (310%) and Sn (260%), and may indeed explain much greater cycling stability of Li-ion cells. At the same time, much greater % V of 290% is expected for Na-ion cells upon sodiation to hexagonal Na.sub.3Sb [22]. Despite much greater volumetric changes, electrochemical cycling with Na-ions involves only one crystalline (Na.sub.3Sb) and several amorphous phases, including Sb (contrary to Li-ion cells!). More isotropic expansion/contraction of amorphous phases can reduce the amount of anisotropic mechanical stress [23]. Furthermore, a thinner or more stable solid-electrolyte interface (SEI) layer may be formed in Na-ion cells.

(78) 2.2 Electrode Preparation and Measurement Set-Up

(79) In a typical electrode preparation, Carbon black, CMC binder and dried Sb or SnSb NCs were ball-milled for 1 h. Obtained homogeneous suspension was pasted on Ti current collector (13 mm diameter) to provide load of approximately 3 mg/cm.sup.2. All electrochemical measurements were conducted in homemade, reusable and air-tight coin-type cells. Test cells were assembled in an argon-filled glovebox with water and oxygen content below 1 ppm. Lithium or sodium metals were used as both reference and counter electrodes. An active electrode was covered with Celgard separator membrane and a glass fiber separator of 1 mm thickness was placed between working and reference electrode. As an electrolyte, mixture of ethylene carbonate and dimethyl carbonate 1:1 (w/w), with 1M LiPF.sub.6 was used for Li-ion batteries and 1 M NaClO.sub.4 in propylene carbonate for Na-ion batteries. Additionally, 3% flouroethylene carbonate (FEC) was added to improve cycling stability. All charge-discharge galvanostatic tests and cyclic voltammetry scans were conducted on MPG2 multi-channel workstation (Bio Logic).

(80) 2.3 Electrochemical Results

(81) Electrochemical behavior of Sb NPs of different sizes was tested using CMC binder and carbon black as additives (63.75 wt % of Sb, 21.25 wt % of CB and 15 wt % of CMC). The same composition was used for anodes based on SnSb NPs and commercial Sb NPs. All synthesized Sb or SnSb NPs were treated with hydrazine as described above. For better cyclic stability, fluoroethylene carbonate (FEC) was used as a electrolyte additive in all prepared batteries. In order to improve cyclic stability, in this work we used high content of binder and carbon black what allows partially decrease mechanical deformation of Sb or SnSb NPs caused by huge volume changes during lithiation and delithiation processes.

(82) All batteries were cycles in the 20 mV-1.5V potential range. The obtained capacities were normalized by the mass of Sb or SnSb, not taking into account the mass of carbon black which also can be considered as an active material in this potential range.

(83) Li-Ion Batteries

(84) FIG. 13A shows discharge-charges curves of Sb based anodes obtained by galvanostatic cycling. The first charge and discharge capacity were 1400 and 600 mAh/g, respectively, which correspond to 43% of initial Coulombic efficiently. Such low efficiently could be related to the formation of large amount of SEI coursed by small size of Sb NPs and their partial oxidation. Namely, first discharge plateau at 1.4-1.2V is related to irreversible the formation of SEI. Cyclic voltammetry measurements have shown that formation of SEI takes place indeed at 1.4V (FIG. 13B). Formation of Sb from oxide and all litigation steps take place at 0.5-0.9 V following by delithiation at 1.1V. The second and the following discharge cycles have shown separation of two lithiation processes, with formation of two different phases at 0.9 and 0.8V, respectively. At these potentials, these phases can be assigned to the formation of Li.sub.2Sb and Li.sub.3Sb alloys. Delithiation process takes place in one step with formation of Sb NPs. Thus, lithiation and dilithation processes of Sb NPs after first cycle can be described using the following questions:
Sb+2Li.fwdarw.Li.sub.2Sb(1)
Li.sub.2Sb+Li.fwdarw.Li.sub.3Sb(2)

(85) FIGS. 13C, D, E show cyclic stability of synthesized Sb, SnSb and commercial Sb NPs at different current densities. As it can be seen from the figure, at very low currents (FIG. 13C), the capacity level of all Sb NPs is lower than for SnSb NPs with excellent capacity retention over 100 cycles even for commercial antimony. However at higher currents (4.8 and 12.2 A/g), cyclic stability is different. 10 nm Sb NPs have showed the best cyclic stability in comparison with 20 nm and commercial Sb NPs.

(86) More detailed picture of capacity retention at different current densities is shown on the Fig. 13F. At low current densities (up to 0.6 A/g) capacity is similar for all samples. However, the difference could be seen at higher current density. The higher is current density, the bigger is difference. Thus, small Sb NPs have strong advantage in term of using them at high current densities. Decreasing of Sb NPs size can drastically improve stability of Sb based anode at high discharge-charge rates. The best results of the cyclic stability were obtained for 10 nm NPs which are inch better than commercial Sb NPs and the results reported so far in the literature. 10 nm Sb NPs can be considered as alternative anode materials to Sn and Si or as an additive to improve cyclic stability of last ones at high charge-discharge rates.

(87) Na-Ion Batteries

(88) FIG. 14A shows the cyclic voltammogram for the first five cycles obtained for Sb NPs. During the first discharge cycle a peak in the range of 1.0 V is observed, which can be assigned to the formation of the SEI. Sodiation takes place below 0.7 V with the main sodiation process at 0.4 V. For the second and subsequent cycles a second peak at 0.7 V appears. These two peaks can be assigned to the formation of NaSb and Na3Sb phases according to the reaction scheme:
Sb+Na.sup.++e.sup..fwdarw.NaSb(3)
NaSb+2Sb+2Na.sup.++2e.sup..fwdarw.Na.sub.3Sb(4)

(89) For the desodiation reaction one peak at 0.9-0.8V appears, which involves the reformation of the Sb NPs.

(90) FIG. 14B shows the electrochemical performance of electrodes composed of either 10 nm Sb NPs or commercial Sb cycled galvanostatically at different C-rates. For electrodes based on commercial Sb at low currents capacities close to the theoretical maximum at 660 mAh/g were obtained. However, with increasing C-rate the capacities drop significantly below 100 mAh/g. In striking contrast to that electrodes composed of 10 nm Sb NPs show excellent rate capability with capacities above 300 mAh/g even at 20C. The fact that electrodes based on 10 nm Sb NPs show do not reach capacities close to the theoretical maximum even at low currents is most likely an intrinsic property of the NPs, since the same observation was made using 1 M NaPF.sub.6 in EC:DEC (1:1) instead of 1 M NaClO.sub.4 in PC (FIG. 14C).

(91) Conclusions

(92) Sb has considerably higher theoretical gravimetric storage capacity (660 mAh g.sup.1) [4] than graphite. There is no substantially important difference in the volumetric charge-storage capacity: 1890 mAh cm.sup.3 for Sb, 2200 mAh cm.sup.3 for Si and 2000 mAh cm.sup.3 for Sn, still much higher than that of Graphite (843 mAh g.sup.1). From economic view-point based on the raw material costs Sb is viable for large-scale battery production.

(93) Electrochemical performance, in terms of cycling stability and rate-capability, of Na-ion cells is as good as for Li-ion cells, despite the fact that sodiation leads to much greater volumetric changes. As a main conclusion, 20 nm Sb NCs exhibit an overall much better performance as compared to bulk Sb and 10 nm Sb NCs, with the greatest difference seen for Na-ion cells for which 20 nm Sb exhibit up to 20% higher capacities as compared to 10 nm NCs. With capacity retention of at least 80% at high current rates of 20C, nanoscopic Sb electrodes is possibly the best-performing Na-ion anode material, and is comparable to fastest Li-intercalation materials such as graphite and lithium titanate.

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