TYPE II CLATHRATES FOR RECHARGEABLE BATTERY ANODES

Abstract

An anode for a rechargeable battery includes a Type II clathrate having the formula MxX136, where a cage structure is formed by X, M represents one or more guest ions, and 0

Description

FIELD OF THE INVENTION

This invention relates to Type II clathrates for rechargeable battery anodes and rechargeable battery anodes including Type II clathrates.

BACKGROUND

High capacity and high rate rechargeable batteries with low cost and improved safety characteristics constitute a major requirement for electric vehicles, portable electronics, and other energy storage applications. Year-to-year electrochemical performance improvements in lithium-ion batteries (LIBs) are typically limited to 3-4%, with a major bottleneck being the lack of appropriate materials to satisfy the energy and power density requirements. Progress in nanostructured anodes has improved the potential of the practically achievable capacity and rates. For example, high capacity anodes such as silicon, which have been studied since the 1980s, have been found to overcome structural degradation problems through the use of nanowire morphologies. However, batteries utilizing silicon anodes can still only achieve a 30% gain in energy density due to the low capacity of the cathode: current cathodes have practical capacities of 150-180 mAh/g.

Both diamond cubic and amorphous silicon can reversibly alloy with Li electrochemically, making silicon a promising high energy density anode for Li-ion batteries. The theoretical charge storage capacity for silicon is about 4000 mAh/g, more than an order of magnitude higher than for graphite, the existing Li-ion battery anode. However, the structural changes that occur during this process results in the formation of a great deal of stresses than can lead to pulverization of the silicon. This is attributed to a 300% change in volume between the unlithiated and lithiated phases. The use of nanostructuring has been applied to allow the silicon to undergo this volume change without fracturing or pulverizing. This concept has been demonstrated with various silicon nanostructures. However, this strategy relies on the use of engineered space within or in between the nanostructured silicon, which effectively gives it space to expand and contract without impinging on itself. Agglomeration and degradation of the engineered structure after many lithiation/delithiation cycles may lower the effectiveness of this strategy. The nanostructuring also does not address the volume expansion of silicon upon lithiation.

SUMMARY

In one aspect, an anode for a rechargeable battery includes a Type II clathrate having the formula M_xX₁₃₆, where a cage structure is formed by X, M represents one or more guest ions, and 0≦x≦24. In some cases, x=0 and no guest ion is present.

In another aspect, forming an anode for a rechargeable battery includes preparing a composition including a Type II clathrate having the formula M_xX₁₃₆, where the cage structure is formed by X, M represents one or more guest ions, and 0≦x≦24. The composition is contacted with a current collector to form the anode.

In yet another aspect, a guest ion Y is electrochemically inserted into a cage structure of a Type II clathrate having the formula M_xX₁₃₆ to yield a Type II clathrate having the formula M_xYX₁₃₆, where the cage structure is formed by X, M represents one or more guest ions, and 0≦x≦24. Y is electrochemically removed from the Type II clathrate. When Y is a lithium ion, electrochemically inserting and removing comprise lithiation and delithiation, respectively.

Implementations may include one or more of the following features.

X may be selected from the group consisting of Si, Ge, and Sn. M may be an ion selected from the group consisting of ions of Na, K, Rb, Cs, Ba, Sr, Ca, Cl, Br, I, Eu, P, Te, Li, and Mg. In some cases, M_xX₁₃₆ represents A_yB_zX₁₃₆, wherein A and B represent different guest ions, 1≦y+z≦24, y≧1, and x≧1. A and B may be independently selected from the group consisting of ions of Na, K, Rb, Cs, Ba, Sr, Ca, Cl, Br, I, Eu, P, Te, Li, and Mg. Y may be selected from the group consisting of sodium ions, magnesium ions, and lithium ions.

An anode may be formed by one or more of the above aspects. A rechargeable battery may include the anode of any of the above aspects, the rechargeable battery further including a cathode and an electrolyte in contact with the anode and the cathode. The rechargeable battery may be a rechargeable sodium-ion battery, a rechargeable magnesium-ion battery, or a rechargeable lithium-ion battery. A device may include the rechargeable battery of any of the above implementations.

The anodes, batteries, and devices described herein provide advantages including insertion of multiple guest ions (e.g., up to 16 or 24) into the anode material without significant changes in lattice constant or clathrate structure, thus allowing extended cycling including, for example, lithiation and delithiation without structural damage of the anode material.

These general and specific aspects may be implemented using a device, system or method, or any combination of devices, systems, or methods. The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts a rechargeable battery. FIG. 1B depicts a device including a rechargeable battery.

FIGS. 2A and 2B depict the crystal structure of Type I and Type II clathrates, respectively.

FIG. 3 shows an X-ray diffraction (XRD) pattern of the clathrate prepared as described in the Example, as well as reference XRD patterns for Na₈Si₄₆ and Na₂₄Si₁₃₆. Peaks from cubic Si (c-Si) are marked with asterisks.

FIG. 4A shows voltage versus capacity determined by potentiodynamic cycling of a clathrate film prepared according to the Example with a 25 μA mg⁻¹ threshold current. FIG. 4B shows capacity and Coulombic efficiency of the clathrate film.

FIGS. 5A-5D show differential charge plots of the clathrate compared to c-Si and amorphous Si (a-Si) using a 25 μA mg⁻¹ (FIGS. 5A and 5B) and a 5 μA mg⁻¹ (FIGS. 5C and 5D) threshold current. FIGS. 5A and 5C are the first cycle and FIGS. 5B and 5D are the second cycle. The values of the differential charge were scaled to facilitate comparison.

FIGS. 6A-6C show XRD patterns of clathrate films on Cu prior to cycling (FIG. 6A), after lithiation (FIG. 6B), and after lithiation and delithiation (FIG. 6C). The insets in FIGS. 6B and 6C show the corresponding galvanostatic cycling curves.

FIGS. 7A-7E depict structures of Li₁₆Na₂₄Si₁₃₆ calculated by density functional theory (DFT). FIGS. 7A-7C depict Li₁₆Na₂₄Si₁₃₆ with Li in 16d sites. FIG. 7C depicts a unit cell for Li₁₆Na₂₄Si₁₃₆ showing arrangement of Li atoms on hexagon faces separating the Si₂₈ cages. Several cages were deleted for clarity. FIG. 7D depicts a unit cell for Li₁₆Na₂₄Si₁₃₆′ and FIG. 7E depicts Li₁₆Na₂₄Si₁₃₆″ showing all eight Si₂₈ cages. Si₂₀ cages were deleted for clarity. Na atoms are labeled and Na—Li bonds indicated by dashed lines.

DETAILED DESCRIPTION

As used herein, “rechargeable batteries” generally refers to sodium-ion, magnesium-ion, and lithium-ion batteries. As an example of a rechargeable battery, FIG. 1A depicts lithium-ion battery (LIB) 100 having anode 102 and cathode 104. Anode 102 and cathode 104 are separated by separator 106. Anode 102 includes anode collector 108 and anode material 110 in contact with the anode collector. Cathode 104 includes cathode collector 112 and cathode material 114 in contact with the cathode collector. Electrolyte 116 is in contact with anode material 110 and cathode material 114. Anode collector 108 and cathode collector 112 are electrically coupled via closed external circuit 118. Anode material 110 and cathode material 114 are materials into which, and from which, lithium ions 120 can migrate. During insertion (or intercalation) lithium ions move into the electrode (anode or cathode) material. During extraction (or deintercalation), the reverse process, lithium ions move out of the electrode (anode or cathode) material. When a LIB is discharging, lithium ions are extracted from the anode material and inserted into the cathode material. When the cell is charging, lithium ions are extracted from the cathode material and inserted into the anode material. The arrows in FIG. 1A depict movement of lithium ions through separator 106 during charging and discharging. FIG. 1B depicts device 130 including LIB 100. Device 130 may be, for example, an electric vehicle, an electronic device (e.g., a portable electronic device such as a cellular telephone, a tablet or laptop computer, etc.), or the like.

As described herein, anode material 110 for LIB anode 102 includes Type II clathrate materials having the general formula M_xX₁₃₆, in which the cage structure is formed by X, and M represents one or more guest ions, and 0≦x≦24. When x=0, no guest ion is present in the cage structure. X is selected from Si, Ge, Sn, or a combination thereof, and M represents one or more ions such as, for example, ions of Na, K, Rb, Cs, Ba, Sr, Ca, Cl, Br, I, Eu, P, Te, Li, or Mg. Examples include M_xX₁₃₆ (i.e., one type of guest ion) as well as A_yB_zX₁₃₆ (i.e., two different guest ions A and B, with 2≦y+z≦24, y≧1, and z≧1).

Type I clathrates of the form M₈Si₄₆, where M is a guest ion intercalated into the structure, are made of two pentagonal dodecahedra (Si₂₀) cages and six tetrakaidecahedra (Si₂₄ cages) per unit cell and crystallize in the Pm3n space group. Unit cell 200 of a Type I clathrate is depicted in FIG. 2A, with cage atoms 202 and guest ions 204. Type I clathrates are described in U.S. Patent Publication Nos. 2012/0021283, 2012/0295160, and 2013/0280609, all of which are incorporated herein by reference.

Type II clathrates of the form M_xSi₁₃₆ (0≦x≦24), are made of sixteen pentagonal dodecahedra plus eight hexakaidecahedra (Si₂₈ cages) per unit cell and crystallize in the PdJm space group. FIG. 2B depicts a Type II clathrate unit cell 210 of the form M_xX₁₃₆, with cage atoms 212 and guest ions 214. Guest ions 214 typically occupy interstitial areas in the polyhedral, but also may be weakly bound to the cages. The cage structure of Type II clathrates is advantageous for rechargeable battery electrodes, since ions (e.g., lithium, magnesium, sodium) can move in and out of the structure.

Type II clathrates described herein are able to withstand structural changes that accompany ion intercalation and de-intercalation with each charge/discharge cycle. The cage size in these clathrates is believed to be influenced more by the cage atoms 202 than the guest ions 204. For instance, there is only a 0.5% increase in the unit cell edge when increasing x from 4 to 23 in Na_xS₁₃₆, suggesting that a large number of guest ions can be inserted and removed from Type II clathrates with very little structural change and corresponding stresses. Silicon clathrates are also stable under pressure and have a large energy barrier for transformation to other phases. Since the compressive and tensile stresses experienced by rechargeable battery electrodes are high, structural stability is advantageous. In addition, with the guest atoms trapped inside the center of the cages, the Type II clathrates are relatively insensitive to moisture, water, and most acids. Mixed guest ions for Type II clathrates of the form A₈B₁₆Si₁₃₆ have been synthesized by using the concept that the cages that house A and B are different dimensions, and stabilization of the structure requires different guest ions. Tuning the physical and electrochemical properties of Type II clathrates may be achieved by the use of different guest ions in different cages.

Type II clathrates depicted in FIG. 2B have been synthesized via the decomposition of Zintl compounds and spark plasma treatment of sodium monosilicide. The electronic properties of these clathrates can be changed by doping with various metal ions, including alkali metals. Type II clathrates with mixed guest ions have also been made (e.g., Na₁₆Ba₈Si₁₃₆, T. Rachi et al., Chem. Phys. Lett. 409, 48-51 (2005)), facilitated by the difference in cavity sizes for the Si₂₀ and Si₂₈ cages.

As shown in Table 1 below, the Type II clathrates shown in FIG. 2B have larger cavity sizes than these Type I clathrates, which may allow for the incorporation of more guest ions than Type I clathrate based, for example, on ion sizes shown in Table 2. The structural differences between Type I and Type II clathrates may affect the electrochemical properties and storage capacities for guest ions. Unlike Type I clathrates, empty Type II clathrates are known (i.e., x=0). From an energy density standpoint, Type II clathrates may be better for energy storage applications since the mass of inactive material is lowered. More so than in Type I clathrates, the large difference in the cage sizes in Type II clathrates can allow for more diverse structures.

TABLE 1
Cavity sizes in silicon clathrates (S. Bobev et al., J. Am. Chem. Soc.
1999, 121, 3795-3796).
	Cavity size (Å) of Si cage
	Clathrate Type	Si20	Si24	Si28
	Type I (Si46)	1.18	1.34	—
	Type II (Si136)	1.048	—	1.845

TABLE 2
Ionic Radii of alkali and alkaline ions (Bobev et al.).
Alkali ion	Ionic Radius (Å)	Alkaline ion	Ionic Radius (Å)
Li+	0.60	Mg2+	0.65
Na+	0.95	Ca2+	0.99
K+	1.33	Sr2+	1.13
Rb+	1.48	Ba2+	1.35
Cs+	1.69

Application of a Type II clathrate-based anode in lithium-ion batteries may provide a much higher energy density (4161 mAh/g, 460 Wh/kg, 2100 Wh/L) when paired with a LiMn₂O₄ cathode) than graphite (372 mAh/g, 386 Wh/kg, 1441 Wh/L), but may demonstrate less lithiation strain and volume changes, which could lead to improved cycle life and capacity retention. The presence of Na guest ions in the Type II clathrate may also be exploited in anodes for sodium and magnesium batteries.

Example

Si clathrate with Na guest ions was synthesized from the thermal decomposition of NaSi Zintl compound as described, for example, in G. K. Ramachandran et al., J. Solid State Chem. 1999, 145, 716-730, which is incorporated herein by reference. NaSi was purchased from SiGNa Chemistry and dried at 100° C. to remove adsorbed moisture. The NaSi was loaded into a h-BN crucible, then placed in a quartz tube and heated in a tube furnace under continuous vacuum evacuation (˜10⁻⁴ Torr). The sample was heated to 340° C. (ramp rate of 10.5° C./min) and held for 30 minutes, then ramped using the same rate to 420° C. and held for 18 hours. After 18 hours, the furnace was turned off while the sample remained under continuous vacuum for 6 hours to remove Na vapor and enable formation of more Type II clathrate. After the synthesis, the powder was quenched successively with toluene, isopropanol, ethanol, and water to remove any unreacted Na. To remove unreacted silicon, the powder was treated with 1 M NaOH for 8 hours.

Powder X-ray diffraction (XRD) was performed using CuKα radiation on a PANalytical X′Pert Pro diffractometer. Structure refinement of the XRD data was performed using Panalytical's X′Pert HighScore Plus software. XRD patterns were indexed to structures for Type I and Type II clathrate from Ramachandran et al. and P. Melinon. et al., Phys. Rev. B 1999, 59, 10099. XRD measurements on electrodes after electrochemical cycling were performed by washing the electrode with hexanes after cell disassembly. Peak positions and intensities were normalized to the reflections originating from the Cu foil substrate. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were performed on a FEI XL30 ESEM-FEG. EDS was performed on 14 different particles in order to obtain an average composition.

Clathrate powder was mixed with carbon black (Timcal Super C45) and polyvinylidene difluoride (PVDF) in an 80:10:10 weight ratio and dissolved in N-methyl-2-pyrrolidone. This slurry was coated onto copper foil with a Meyer rod and dried at 100° C. The mass of the coating was determined using a microbalance (Mettler-Toledo, UMX2). Pouch cells were assembled in an Ar-filled glovebox using Li metal foil as the counter electrode, Celgard 3401 separator, and 1 M LiPF₆ in 1:1 mixture of ethylene carbonate and diethylecarbonate (Novolyte). The cells were tested using potentiodynamic cycling to perform electrochemical potential spectroscopy from 2-0.01 V vs. Li/Li⁺ with a 5 mV potential step amplitude and threshold current of either 5 or 25 μA/mg based on the mass of the clathrate. For preparing samples for ex situ XRD analysis after lithiation and delithiation, galvanostatic testing using a 25 μA/mg current density was performed.

The upper pane of FIG. 3 shows the powder XRD pattern of the as-made product, indicating a mixture of Type I (Na₈Si₄₆) and Type II (Na_xSi₁₃₆ 1≦x≦24) silicon clathrate, plus unreacted diamond cubic Si (c-Si). The middle and lower panes show reference patterns for Na₈Si₄₆ (Ramachandran et al.) and Na₂₄Si₁₃₆ (Melinon et al.), respectively. After treatment with NaOH, the c-Si peaks were absent from the XRD pattern, leaving only the peaks from the clathrates. The phase fraction as determined using Rietveld analysis showed the powder was composed of about 80% Type II clathrate and 20% of Type I.

Scanning electron microscopy (SEM) imaging revealed that the particles were approximately 1-5 μm in diameter. Energy-dispersive X-ray spectroscopy (EDS) analysis showed an average composition of 14.4 at % Na and 85.6 at % Si with a standard deviation of 3.2%. This corresponds to a Na:Si ratio of 0.169, which is in close agreement with the ratios in Na₈Si₄₆ and Na₂₄Si₁₃₆, suggesting that the Na sites in the clathrates were almost fully occupied. The powders were mixed into slurries with carbon black and polyvinylidene difluoride (PVDF) binder, then coated onto Cu foil. SEM images of the coated films showed that the carbon black formed a conducting network around the clathrate particles.

Potentiodynamic cycling of the clathrate films was performed in order to understand the basic electrochemical processes occurring in the electrodes during lithium insertion (charge) and de-insertion (discharge) in half-cells with Li metal counter electrodes. The cycling results using a 25 μA/mg threshold current are shown in FIG. 4A. The first charge curve 400 was characterized by a flat plateau at around 0.120 V vs. Li/Li⁺. However, since the XRD patterns showed that c-Si was etched away, and no features consistent with lithiation of a-Si were observed, these electrochemical features are likely from the reaction of Li with the clathrates. The discharge curve 402 showed a flat plateau at 0.425 V and was a similar shape to that observed in delithiation of c-Li₁₅Si₄, although at a slightly lower voltage. As shown in FIG. 4B, the first charge capacity was 2431 mAh/g (cycle number 1 in charge curve 420) and the discharge capacity was 1213 mAh/g (cycle number 1 in discharge curve 422), resulting in a Coulombic efficiency (CE) of about 50% (cycle number 1 in CE curve 424). These results show discharge of lithiated clathrate.

To better understand the lithiation and delithiation features of the Na-filled silicon clathrate, the differential charge (dQ) vs. potential plots were obtained and compared to those for c-Si, with plots 500 and 502 showing delithiation and lithiation, respectively, for the clathrate, and plots 504 and 506 showing delithiation and lithiation, respectively, for c-Si/c-Li₁₅Si₄. FIGS. 5A and 5B show the first and second cycle, respectively, for the electrode cycled using a 25 μA/mg threshold current. Due to the differences in observed capacities, the values of the dQ were scaled in order to clearly distinguish the differences between the two materials. Overall, the dQ plots of the clathrates look very similar to those for c-Si. Delithiation of c-Li₁₅Si₄ occurs as a two-phase reaction to form amorphous Si (a-Si) at the end of the discharge. These two-phase reactions are observed as peaks in the dQ vs. potential plots.

The similar shape of the dQ plots for the clathrates suggest a similar reaction mechanism. One difference between the clathrate dQ plots and those for delithiation of c-Li₁₅Si₄ is that the discharge was observed over a broader range of potentials for the clathrate. There is also a notable peak at 0.3 V vs. Li/Li⁺ in the discharge which is not seen in delithiation of c-Li₁₅Si₄. The second charge of the clathrate (FIG. 5B) showed two broad peaks, which are similar to those seen in lithiation of a-Si although at slightly lower potentials. When performing cycling with a 5 μA/mg threshold current (FIGS. 5C-5D), the voltage hysteresis between the charge and discharge decreased from 0.28 V when using 25 μA/mg to 0.25 V, but the other features remained substantially the same.

To confirm the amorphization process, ex situ XRD measurements were performed on the cells after galvanostatic cycling using a current of 25 μA/mg. The peaks attributed to the unlithiated clathrate (FIG. 6A) were observed to decrease in intensity after lithiation to about 1300 mAh/g (FIG. 6B), confirming the amorphization process. This capacity corresponds to about 184 Li inserted into the clathrate (or ˜1.6 Li/Si), assuming a composite anode of Type I and Type II clathrate with total molecular weight of approximately 3790 g/mol based on the phase fraction obtained from the Rietveld analysis. Several reflections corresponding to c-Li₁₅Si₄ were also observed (FIG. 6B).

For a sample that was lithiated to 1770 mAh/g and then partially discharged, the c-Li₁₅Si₄ peaks disappeared and no new reflections were observed (FIG. 6C), indicating that the material transformed into an amorphous phase during delithiation of c-Li₁₅Si₄. These results are consistent with the observed electrochemical characteristics. SEM imaging of samples after 10 cycles showed the clathrate particles were covered with an SEI layer, but no obvious pulverization or cracks were observed in the particles.

To better understand the amorphization process, first principles density functional theory (DFT) was used to calculate the lattice constant and energy of formation for several structures using the VASP code (G. Kresse et al., Phys. Rev. B 1996, 54, 11169; J. P. Perdew et al., Phys. Rev. Lett. 1996, 77, 3865). The Perdew, Burke and Ernzerhof (PBE) functional and projector-augmented wave (PAW) potentials were used along with the plane wave basis sets (G. Kresse et al., Phys. Rev. B 1999, 59, 1758-1775). The energy cutoff for the plane wave basis set was 300 eV. The convergence criteria for energy and forces were set to be 0.01 meV and 0.1 meV respectively. The Si 3s3p, Na 2p3s, Li 1s2s electrons were treated as valence electrons. The formation energies were calculated by subtracting the total energies of the elements from the energy of the structure, then dividing by the total number of atoms. For example, the formation energy for Si₁₃₆ was calculated using Eqn. 1 and that for Li₁₆Na₂₄Si₁₃₆ was calculated using Eqn. 2, where E(Si), E(Li), and E(Na) are the energies per atom for c-Si, Li and Na metals, respectively.

$\begin{array}{cc}{E}_{\mathrm{form}}=\frac{E\text{?}}{\text{?}}& \left(1\right)\\ E_{\mathrm{form}}=\frac{E\left({\mathrm{Li}}_{16}{\mathrm{Na}}_{24}{\mathrm{Si}}_{136}\right)-\text{?}}{\text{?}}\text{?}\text{indicates text missing or illegible when filed}& \left(2\right)\end{array}$

The results for Si₁₃₆, Na₂₄Si₁₃₆, Na₈Si₄₆, Li₂₄Si₁₃₆, Li₁₆Na₂₄Si₁₃₆, and Li₁₅Si₄ are shown in Table 3. Although the calculated formation energies for Na₈Si₄₆ and empty Si₁₃₆ were slightly positive, both structures have been experimentally synthesized. Note that the calculated formation energies are based on the total energies predicted by DFT at the ground state, i.e. 0 K, so the calculated values are reasonable. The formation energy for fully-filled Na₂₄Si₁₃₆ was −0.005 eV/atom, suggesting it is the more stable structure compared to empty Si₁₃₆.

TABLE 3
Calculated properties using DFT
		Lattice constant	Formation Energy
	Compound	(Å)	(eV/atom)
	Si136	14.74	0.052
	Na8Si46	10.24	0.002
	Na24Si136	14.78	−0.005
	Li24Si136	14.68	0.030
	Li16Na24Si136	14.95	0.012
	Li16Na24Si136′	14.90	−0.002
	Li16Na24Si136″	14.90	−0.003
	Li15Si4	10.54	−0.239

To model lithiated clathrate, the Type II structure was considered since the synthesized powders were predominately this phase. Type II clathrate crystallizes in the Ram space group, with Si occupying the 8a, 32e, and 96g sites and Na in the 8b and 16c sites, as described with Wyckoff symmetry notation. Replacing the Na with Li to form Li₂₄Si₁₃₆ resulted in a formation energy of 0.030 eV/atom, which is higher than that for Na₂₄Si₁₃₆. This is likely due to the ability of Na to better stabilize the clathrate structure due to its larger size. Despite this slightly positive formation energy, electrochemical Li insertion into the empty Na sites of type II clathrate has been confirmed with NMR, as described by T. Langer et al., J. Electrochem. Soc. 2012, 159, A1318-A1322.

Lithiation of Na-filled Type II clathrate was first modeled by placing Li 700 into the 16d sites, which are typically unoccupied in Type II clathrate, to form Li₁₆Na₂₄Si₁₃₆ (FIGS. 7A and 7B). The Si₂₈ polyhedra in Type II clathrate are composed of 12 pentagonal and 4 hexagonal faces. Occupancy of the 16d sites by Li 700 places them on the faces of the hexagons, equidistant from each Si 702, such that they are bisected and shared by neighboring Si₂₈ clusters (FIG. 7C). In this configuration, the Li are located in channels along the <110> directions parallel to the rows of Na 704 ions occupying the 16c sites. The Li—Si and Na—Li bond lengths were 2.454 and 3.237 Å, respectively, in this structure. The formation energy for this structure was 0.012 eV/atom, lower than that for Li₂₄Si₁₃₆. These results suggest that lithiation into Na-filled clathrate is not less favourable than lithiation into empty clathrate.

Next, the positions of the Li were displaced slightly from the 16d sites so that they were no longer sitting in the hexagonal faces, but rather inside the Si polyhedra. In the first structure, referred to as Li₁₆Na₂₄Si₁₃₆′, 2 Li atoms were placed inside the Si₂₈ cages so they shared the interstitial space with the Na atoms (FIG. 7D). The second structure, referred to as Li₁₆Na₂₄Si₁₃₆″, had the Li atoms distributed amongst the eight Si₂₈ cages of the unit cell in the following manner: two cages had 1 Li, two cages had 3 Li, and the remaining 4 cages had 2 Li sharing the cage volume with the Na (FIG. 7E). The calculated Na atomic coordinates for both structures showed that the Na positions were shifted away from the centres of the cages in order to accommodate the Li atoms. Both structures had negative formation energies (Table 3) and shorter bond lengths than in the original Li₁₆Na₂₄Si₁₃₆ structure, with the shortest bond length of 2.837 Å calculated in Li₁₆Na₂₄Si₁₃₆″, as shown in Table 4.

TABLE 4
DFT calculated Na—Li bond distances associated with Si26 cages
in different Li16Na24Si136 structures.
			Bond length
	Structure	Atom pair	(Å)
	Li16Na24Si136	Na—Li	3.237
	Li16Na24Si136′	Na1—Li12	2.939
		Na1—Li14	2.928
		Na2—Li1	2.960
		Na2—Li2	2.954
		Na3—Li3	2.935
		Na3—Li8	2.951
		Na4—Li4	2.928
		Na4—Li15	2.932
		Na5—Li5	2.934
		Na5—Li10	2.939
		Na6—Li6	2.942
		Na6—Li7	2.924
		Na7—Li9	2.934
		Na7—Li11	2.930
		Na8—Li13	2.937
		Na8—Li16	2.927
	Li16Na24Si136″	Na1—Li12	3.099
		Na2—Li1	2.929
		Na2—Li2	2.887
		Na3—Li3	2.899
		Na3—Li8	2.899
		Na3—Li13	2.837
		Na4—Li4	2.920
		Na4—Li15	2.927
		Na5—Li10	3.110
		Na6—Li6	2.946
		Na6—Li7	2.926
		Na7—Li9	2.924
		Na7—Li11	2.920
		Na8—Li5	2.886
		Na8—Li14	2.884
		Na8—Li16	2.896

These results suggest that it is energetically feasibly for more than one guest atom to occupy the space inside the Si₂₈ cage of Type II clathrate while maintaining the overall Si clathrate framework structure. The lattice constants for all of the lithiated Type II clathrate structures increased very slightly compared to the empty Si₁₃₆. This suggests that at the initial stages of Li insertion, the clathrate structure may be maintained. However, with more Li insertion, the experimental results show that transformation into an amorphous lithium and sodium containing silicide is more favorable. The formation energy of c-Li₁₅Si₄ was calculated to be −0.239 eV/atom. The very low energy for this phase may explain why lithiation of both empty and Na-filled clathrate eventually result in transformation to c-Li₁₅Si₄.

In summary, electrochemical and XRD analysis of lithium insertion into a mixture of Na-filled Type I and Type II clathrates showed a similar reaction mechanism in these materials as is found in diamond cubic silicon. Upon electrochemical lithiation, the clathrates became amorphous and transformed to c-Li₁₅Si₄ at low potentials, then remained an amorphous silicide after delithiation. DFT was used to calculate the lattice constants and formation energies for various Type II clathrate structures. The results showed that 16 Li atoms could be inserted per unit cell with very little increase in lattice constant. The DFT-predicted structures for Li₁₆Na₂₄Si₁₃₆ containing multiple-guest atoms inside the larger Si₂₈ cages shows that these configurations are energetically favorable and suggests that such lithiation processes can occur without significant changes in lattice constant or clathrate structure.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. An anode for a rechargeable battery, the anode comprising a Type II clathrate having the formula MxX136, wherein: a cage structure is formed by X,M represents one or more guest ions, and0≦x≦24.

2. The anode of claim 1, wherein X is selected from the group consisting of Si, Ge, and Sn.

3. The anode of claim 1, wherein M is an ion selected from the group consisting of ions of Na, K, Rb, Cs, Ba, Sr, Ca, Cl, Br, I, Eu, P, Te, Li, and Mg.

4. The anode of claim 1, wherein MxX136 represents AyBzX136, wherein A and B represent different guest ions, 1≦y+z≦24, y≧1, and x≧1.

5. The anode of claim 4, wherein A and B are independently selected from the group consisting of ions of Na, K, Rb, Cs, Ba, Sr, Ca, Cl, Br, I, Eu, P, Te, Li, and Mg.

6. A rechargeable battery comprising the anode of claim 1.

7. The rechargeable battery of claim 6, further comprising: a cathode; andan electrolyte in contact with the anode and the cathode.

8. A device comprising the rechargeable battery of claim 7.

9. A method of forming an anode for a rechargeable battery, the method comprising: preparing a composition comprising a Type II clathrate having the formula MxX136, wherein the cage structure is formed by X, M represents one or more guest ions, and 0≦x≦24; andcontacting the composition with a current collector to form the anode.

10. The method of claim 9, wherein X is selected from the group consisting of Si, Ge, and Sn.

11. The method of claim 9, wherein M is an ion selected from the group consisting of ions of Na, K, Rb, Cs, Ba, Sr, Ca, Cl, Br, I, Eu, P, Te, Li, and Mg.

12. The method of claim 9, wherein MxX136 represents AyBzX136, A and B represent different guest ions, 1≦y+z≦24, y≧1, and x≧1.

13. The method of claim 12, wherein A and B are independently selected from the group consisting of ions of Na, K, Rb, Cs, Ba, Sr, Ca, Cl, Br, I, Eu, P, Te, Li, and Mg.

14. An anode for a rechargeable battery formed by the method of claim 9.

15. A rechargeable battery comprising the anode formed by the method of claim 9.

16. The rechargeable battery of claim 15, wherein the rechargeable battery is selected from the group consisting of a rechargeable sodium-ion battery, a rechargeable magnesium-ion battery, and a rechargeable lithium-ion battery.

17. A device comprising the rechargeable battery of claim 15.

18. A method comprising: electrochemically inserting a guest ion Y into a cage structure of a Type II clathrate having the formula MxX136 to yield a Type II clathrate having the formula MxYX136, wherein: the cage structure is formed by X,M represents one or more guest ions, and0≦x<24; andelectrochemically removing Y from the Type II clathrate.

19. The method of claim 18, wherein Y is selected from the group consisting of sodium ions, magnesium ions, and lithium ions.

20. The method of claim 18, wherein X is selected from X is selected from the group consisting of Si, Ge, and Sn.

21. The method of claim 18, wherein M is an ion selected from the group consisting of ions of Na, K, Rb, Cs, Ba, Sr, Ca, Cl, Br, I, Eu, P, Te, Li, and Mg.

22. The method of claim 18, wherein Y is a lithium ion, and electrochemically inserting and removing comprise lithiation and delithiation, respectively.