Patent Application
20120231326
Not applicable.
Not applicable.
This invention relates to method of making porous silicon, and its method of use as a rechargeable battery anode, and to batteries containing same.
In lithium ion batteries, the anode uptakes lithium ions from the cathode when the battery is being charged and releases the lithium ions back to the cathode when the battery is being discharged. One important parameter of the anode material is its capacity to retain lithium ions, since this will directly impact the amount of charge a battery can hold. Another important parameter is cyclability, which is the number of times the material can take up and release lithium ions without degradation or significant loss of capacity. This parameter will directly influence the service life of the battery.
Presently, carbon-based materials (e.g. graphite) are utilized as the anode material in rechargeable batteries.1,2 The theoretical capacity limit for intercalation of Li into the carbon is 372 mAh/g, which corresponds to the fully loaded material LiC6. However, the practical limit is ˜300-330 mAh/g. Consequently, to increase capacity and to meet higher power requirements anticipated for applications like electric vehicles, new materials with higher capacity are necessary. This is an area of active research directed towards new materials such as Si, Sn, Sb, Pb, Al, Zn and Mg etc. and new morphologies.3
Silicon has been widely studied as a promising material for next-generation anodes, due to its extremely high theoretical lithium ion capacity of 4200 mAh/g,4 which corresponds to the fully loaded material Li4.4Si. However, silicon has serious expansion/contraction problems during cycling, due to the volumetric change from silicon to lithiated silicon. This greatly increases stress in the crystal structure, leading to pulverization of the silicon. This pulverization leads to increased internal resistance, lower capacity, and battery cell failure.
A variety of silicon structures and silicon-based composites have been examined in order to reduce the lithiation-induced stress and suppress the structural destruction of silicon, which is believed to be the main cause for the loss of sustainability and the lack of capacity retention during charge/discharge cycling.5-11 Finding an optimal structure/composition of silicon or silicon based materials is a current challenge in the field of battery anode materials research.
One approach being taken by researchers is to consider nanostructured forms of silicon, which have been hypothesized to be more resistant to performance degradation. Others have used nanocomposites consisting of silicon powder and carbon black.12-15 These studies used micro-particulate Si or carbon coated silicon. Many of these approaches require expensive vacuum-based manufacturing techniques to create the silicon nanostructure or composite.
The work on Si nanoclusters16 and Si/graphite nanocomposites17 showed improvements in the cycle life and lithium capacity as compared to the silicon powder with binder. The improvement of cyclability is due to the nanosize Si particles and their uniform dispersion within the silicon oxide phase retained by the carbon matrix, which could effectively suppress the pulverizing of Si particles by the volume change during lithium insertion and extraction. Si-graphite composites have a higher capacity and cyclability than Si nanoclusters because the silicon particles are uniformly distributed in the graphite matrix resulting in each silicon particle becoming completely covered by multiple graphite layers.
Recent work on silicon nanowires (NWs) have shown improvement in silicon's performance as an anode material,18-21 and Si NWs were found to exhibit a higher capacity than other forms of Si.11 The observed charge discharge capacity18 remained nearly constant at 80% of theoretical value of Si, giving a Coulombic efficiency of 90% with little fading up to 10 cycles, which is considerably better than previously reported results.22-23 The fading response beyond 10 cycles was not reported, however. Other experiments using carbon-silicon nanowires21 show an increase in the cycle stability of the lithium-ion batteries as compared to silicon nanowires18 due to the carbon support. The carbon support allows very little structure or volume change to occur but there is a trade-off in capacity.
Another example of a silicon nanomaterial is porous silicon (“pSi”), which has been shown to be a promising anode for rechargeable batteries.24,25 In this work, the charge capacity is defined as the total charge inserted into the projected electrode surface area exposed to the electrolyte (this ignores any surface area due to structuring), given as μAh·cm−2. Unfortunately, these groups have not yet been able to successfully prepare pSi-based anodes with both high capacity and long cycle life. The few studies on pSi as a lithium-ion anode material do not report the high performance shown by our materials.
Thus, what is needed in the art is a porous silicon that is cost effective to make and has both high capacity and long cycle life.
The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims or the specification means one or more than one, unless the context dictates otherwise. The term “about” means the stated value plus or minus the margin of error of measurement or plus or minus 10% if no method of measurement is indicated. The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or if the alternatives are mutually exclusive. The terms “comprise”, “have”, “include” and “contain” (and their variants) are open-ended linking verbs and allow the addition of other elements when used in a claim.
When discussing pore width and depth herein, what is meant is an average pore width and depth, since there will typically be some variability in these measurements.
The present invention provides an improved anode material comprising coated porous silicon for lithium ion batteries; a lithium ion battery with improved cycling behavior and high capacity, which is 80% of theoretical capacity for 50+ cycles; a low cost method for manufacturing anodes for lithium ion batteries; a reproducible method for making battery anode materials; and a lithium ion battery having substantially higher discharge capacity than present day batteries.
In this invention, we also provide a method to calculate the mass of porous silicon as compared to the bulk silicon. The capacity definition used by prior work24-26 is the total charge inserted into the projected electrode surface area exposed to the electrolyte, given as μAhcm−2 (micro-Amp-hours-cm−2). This definition neglects the electrode surface area within the pores, however. In our work, we calculate the charge capacity as the total charge inserted into mass of the surface area, given as mAhg−1 (milli-Amp-hours/gram).
We provide herein a method of fabricating porous silicon by electrochemical etching process that can be done with either acid or plasma. Preferred acids include hydrofluoric acid (HF, usually about 49%), perfluoric, ammonium bifluoride, ammonium fluoride, potassium bifluoride, sodium bifluoride, hydrohalic acids nitric, chromic, sulferic, and the like, as well as mixtures thereof. Particularly preferred are acids such as HF in organic solvents such as DMF, as well as HF in ethanol and HF in acetic acid, etc. Preferred high density plasma's include the plasma gases of SF6, CF4, BCl3, NF3, XeF2, and the like as well as mixtures thereof. The etched silicon is then coated with a passivating agent, which appears to prevent silicon degradation on repeated use. A particularly preferred passivating agent is gold applied at 10-100 nm, preferably 20-50 nm, but other passivating agents may also be useful.
The resulting coated porous silicon material is capable of intercalating large amounts of lithium ions and retains this ability through a large number of charge/discharge cycles. We are thus able to significantly improve the anode material, achieving improved cycling behavior and lasting at least 50 cycles with high capacity of at least 1000 mAh/g. With certain pSi formulations, we were able to achieve capacities as high as 3400 mAh/g and a lifespan of at least 200 cycles. Further, it is shown how to maximum either of these important parameters by modifying etch conditions.
More particularly, a method making coated porous silicon is provided wherein flat (wafer) or other 3D forms of silicon are etched under current to produce porous silicon having pores from 10 nm to 10 μm in diameter with an pore depth of 5-100 μm, wherein the silicon is then coated with at least 1 nm of a passivating material to produce a coated porous silicon having a charge capacity of at least 1000 mAh/g for at least 50 cycles.
The silicon can be crystalline silicon, semicrystalline silicon, amorphous silicon, doped silicon, coated silicon, or silicon pretreated by coating with silicon nanoparticles. Current ranges from 1-20 mA, or even as high as 40 mA, and is applied for about 30-300 minutes. The current can be continuous or intermittent and both are exemplified herein. The porosity can be increased by decreasing the concentration of acid and/or increasing the current, and pore size and depth are shown herein to optimize either cycle life or capacity, as needed for the application. The etching can use a high density plasma gas or an acid, and preferably uses HF in DMF in a ratio ranging from 1:5 to 1:35, more particularly 1:5-1:25, or 1:5-1:10. In preferred embodiments, the coating is carbon or gold, preferably at least 5 nm, 10, or 20 nm of gold, or combinations of gold or carbon and other passivating agents can be used. In preferred embodiments the capacity is least 3000 mAh/g or 3400 mAh/g, and the lifespan is at least 100 cycles, 150 cycles, 200 cycles or 250 cycles.
Anodes made from the above etching and coating method are also provided, as are batteries comprising such anodes. The coated porous silicon can be crushed or otherwise comminuted, bound with a matrix material and shaped to form an anode. Alternatively, it can be used as is or be lifted off the bulk silicon and used on a optional substrate with an optional transition layer that is optionally doped. The substrate is selected from the group consisting of copper, bulk silicon, carbon, silicon carbide, carbon, graphite, carbon fibers, graphene sheets, fullerenes, carbon nanotubes, graphene platelets, and the like, and combinations thereof. A rechargeable battery comprising such anodes together with a separator and a cathode material can be packaged in a coil-cell, pouch cell, cylindrical cell, prismatic cell or any other battery configuration.
The following examples are exemplary only and not intended to be limiting of the various embodiments of the invention.
For all experiments, prime grade, boron doped, p-type and single-side polished silicon wafers from Siltronix™ and University™ wafer were used. All the wafers were 275±25 microns thick and had resistivities between 14-22 Ωcm and 10-30 Ωcm with face orientation of (100).
Porous silicon (pSi) was generated by etching crystalline silicon in aqueous hydrofluoric acid (HF) electrolytes in a standard electrochemical cell made out of Teflon.™ A Viton™ O-ring was used to seal the cell. The wafers were pressed against the gasket with an aluminum plate. Wire form platinum was immersed in the solution as the counter electrode. All etching was performed under constant current conditions, with proper current provided by an Agilent™ E3612A DC Power Supply. The unpolished side of the wafer was coated with aluminum to reduce the contact resistance to the aluminum back plate.
For all the results reported here, the etchings are performed using dimethylformamide (DMF) and a 49% HF solution at different volume ratios. The control of pores diameter, depth and spacing was achieved entirely through the variation of the etching conditions such as current density, etch time and wafer resistivity. Careful control of the various etching parameters is needed, as the pSi structure is very sensitive to processing conditions. After the reliability of the DMF etch was established, more than 40 samples were produced by using different etching conditions. Four sets of etching conditions are shown in Table (1).
After etching, the wafers were rinsed with methanol and water to take away the etching solution and by-products. The wafers were coated with a 20 nm gold coating, via E-Beam evaporation, to prevent surface oxidation.
A three-electrode electrochemical cell (Hosen Test™ cell, Hohsen™ Corp. Japan) was used for all electrochemical measurements. Porous silicon was used as a working electrode and lithium foil as counter electrode. The backside of the porous silicon was coated with aluminum or copper, but copper was preferred. Fiber glass was used as a separator, wetted with an electrolyte. The electrolyte was 1.0 M LiPF6 in 1:1 w/w ethylene carbonate: diethyl carbonate (Ferro™ Corporation).
All the cells are made in an Argon-filled glove box. All the experiments were performed using Arbin Instruments™ BT2000. Various pSi samples were cycled between 0.09 and 1.5 V versus Li/Li+ and other voltage with different current density.
The porosity and thickness of the pSi layer were among the most important parameters which characterize pSi.27 The porosity is defined as the fraction of void within the pSi layer and can be determined easily by weight measurements. The Siltronix™ and University™ wafers are first weight before anodisation (m1), then just after anodisation (m2), and finally after dissolution of the whole porous layer in a molar NaOH aqueous solution (m3). The porosity is simply given by this equation:
From the measured mass it is also possible to measure the thickness of the layer according to the following formula:
The thickness can also be directly determined by scanning electron microscopy (SEM). In Eq. (3), d is the density of bulk silicon and S is the wafer area exposed to HF during anodisation. Once thickness of porous, surface area and density of bulk silicon is known, the mass of porous area can be calculated by using Eq. (3).
The porous silicon was studied for reversible charge performance by incorporating into the test cell as shown in
a shows the voltage profiles (between 0.09 to 2 V, at a charge rate of 60 μA) of the pSi electrode (sample A) pictured in top and side cross-sectional view in
The structure morphology changes during Li insertion were studied to understand the high capacity and good cyclic stability of pSi electrode.
a show the voltage profiles of the pSi electrode (sample B) prepared at a higher current of 7 mA in a 5 cm2 etch cell with lower amounts of HF and DMF such that the HF:DMF ratio was increased from 8:100 to 10:100 (
a show the voltage profiles of the pSi prepared like sample B, except at a lower current of 5 mA in a 5 cm2 etch cell with longer etching time (
This improvement in capacity and cyclic stability may reflect a unique feature of the pSi nanostructure that is observable only after changing to the stable copper current collecting material. We speculate that the unusual capacity increase results from an increasing amount of amorphous LixSi formed per cycle, suggesting the Li is accessing some part of the pSi structure in increasing amounts until 80% of the pSi is participating in reversible Li storage. This high capacity is maintained with high Coulombic efficiency of 95-99% to at least 76 cycles, as shown in
a shows the voltage profiles of the pSi prepared like sample B, except with a slightly shorter etch time of 200 seconds (
The porosity, thickness, pore diameter and microstructure of porous silicon (pSi) depends on the anodization conditions. For a fixed current density, the porosity decreases as HF concentration increases. Additionally, the average depth increases and porosity decreases with increasing HF concentration (Table 2). Fixing the HF concentration and current density, the porosity increases with the thickness (Table 3). Increasing current density increases the pore depth and porosity (Table 4). This happens because of the extra chemical dissolution of the porous silicon layer in HF. The thickness of a porous silicon layer is determined by the time that the current density is applied, that is, the anodization times. Another advantage of the formation process of porous silicon is that once a porous layer has been formed, no more electrochemical etching occurs for it during the following current density variations.27
The cycle life and specific capacity of pSi structures with different porosities but the same average pore depth were compared. Etching parameters for creating same depth and different porosity of porous silicon (pSi) are given in (Table 5). Shown in the
The cycle life and specific capacity of pSi structures with almost same porosities but different average pore depth were compared. Etching parameters for creating same porosity and different depth of porous silicon (pSi) are given in (Table 6). Shown in
The cycle life and specific capacity of wider pSi structures etched at different conditions was tested. Etching parameters for creating wider pores are given in (Table 7). Shown in
Morphology changes during Li insertion were studied to understand the high capacity and good cyclic stability of the pSi electrode.
The cycle life and specific capacity of pSi structures etched after coating with Si nano-particles was tested. A 1M solution of Si particles in ethanol was spotted onto the silicon wafer before etching, dried overnight and etching was performed using the parameters of Table 8. Shown in
The structure morphology changes during Li insertion were studied to understand the high capacity and good cyclic stability of pSi electrode.
The cycle life and specific capacity of deeper pSi structures was also tested. Etching parameters for fabricating deeper pores are given in Table 9. Shown in the
The structure morphology changes during Li insertion were studied to understand the high capacity and good cyclic stability of pSi electrode.
A complete summary of the copper backed samples is presented in table 10:
Although we have exemplified the processes herein with the use of a macroscopically flat wafer, the porous silicon need not be flat, and can be applied to other Si structures, for example, pillars, thick or thin free-standing wires, and three-dimensionally porous Si, and supported on bulk Si or other substrates as needed for structural stability. Thus, the porous silicon need not be flat in macro- or microscopic dimension, but can have a variety of topologies. A commonality of these structures is they have higher surface area-to-volume ratios than that of bulk Si, and some of these Si structures have been shown to be effective battery anodes. A mixture of Si structures supported on bulk Si may be effective battery anodes also. Thus, existing pillars and wires can be further improved with the etching and coating technique as described herein. Alternatively, pillars can be produced by carrying on the etching until such point as pillars are formed by removal of sufficient silicon.
Bulk Si can provide structural support for the pSi and can further improve cycle life, with an optional transitional layer between the porous and bulk silicon being important in some applications. This transitional layer experiences decreasing lithiation based on distance from the bottom of the pores. The bulk silicon just beneath the porous silicon provides a good electrical conductivity path in the structure to the current collector, which can be doped to make it even more electrically conductive. This electrical conductivity can improve cell performance by reducing internal cell electrical resistance and consequent voltage losses. The transitional layer, which experiences decreasing lithiation as a function of depth, also functions as a stress gradient, enabling the cyclically lithiated and delithiated inter-pore silicon to stay physically attached to the bulk silicon substrate.
The electrochemical etch process can be applied to other substrates besides the prime grade, boron doped, p-type and single-side polished silicon wafers from Siltronix™ and University™ wafers used in Example 1. A silicon layer that has been deposited on another material, which can act as a current collector or a manufacturing structure, can be used as a substrate. This will enable further efficiencies in manufacturer of battery anodes with the pSi etched in place on a convenient substrate suitable to manufacturing processes. The substrate may be removable or it may be retained in the final anode structure. The substrate can have other functions, such as a structural part of the cell and/or as a current collector. This can be formed as a discrete substrate or in a continuous format, facilitating roll-to-roll manufacturing processes suitable for battery manufacture. An example would be deposition of silicon, in various possible forms (crystalline, polycrystalline, amorphous, silicon carbine, etc.) on a roll-to-roll copper substrate. This silicon would then be made porous. The copper/porous silicon structure could then be mated with other components of a secondary lithium battery cell in a continuous form.
The pSi structure can be also combined with a carbon material to improve cycle life. Possible carbon supports include, carbon fibers, graphene sheets, fullerenes, carbon nanotubes, and graphene platelets. Alternatively, any of these forms of carbon can contribute to the passivation coating.
The electrochemical etch process can proceed in other geometries besides a closed etch cell, for example, in a open system with the Si substrate immersed in containing the etch fluid. Thus, the invention is not limited to the way that the etch is performed.
Plasma etching, which does not involve the use of corrosive HF, can also generate pSi structures. There are examples of creating pSi structures using a variety of plasma gases, such as SF6, CF4, BCl3, NF3, and XeF2.
Porous silicon wafers can be subjected to a size reduction process such as roll or hammer crushing and ball-milling or attriting. The resultant powder-like material can then be used to manufacture Li-ion batteries by the processes typically used for making Li-ion batteries such as the known mixing, coating and calendaring processes. Thus, the coated porous silicon can be used as is, or ground and mixed with a matrix or other binding agent and formed into the desired anode shape.
A self-standing porous silicon layer is produced by modifying the electrochemical process. For a given silicon doping level and type, current density and HF concentration are the two main anodizing parameters determine the microstructure and porosity of layers. Keeping this in mind, a porous silicon layer can be separated from the substrate in a one step separation (OSS) or a two step separation (TSS) method.
The one step anodization lift-off procedure is driven by the dissolution of fluorine ions as the pores grow deeper. The dissolution of fluorine ions create high porosity layer (50-80% porous) below a less porous layer (10-30% porous). The pores then expand to overlap one another until the porous silicon breaks away from its substrate.
In order to perform the TSS, a silicon wafer is etched at a constant current density to create long; straight pores, and then a dramatic boost in the current density expands the pores rapidly to create an electro-polished layer that then allows the porous silicon to disconnect from the wafer.
The two step etch process was carried out successfully in organic solutions. The initial low porous layer was etched at room temperature with a current ranging from 5-12 mA for any where between 1-3 hours. This initial etching condition creates the main parts of the porous layer. Boosting the current density between 40-300 mA after the initial etching caused the base of the pores to expand and overlap and allowed the porous layer to separate from the substrate. This electropolishing lift-off step is carried out for 10 minutes to 1 hour. All of these parameters can be tuned to create porous structures of different sizes. A layer of lift-off self-standing porous silicon layer is directly put on the current collecting materials.
The following references are incorporated by reference herein in their entirety:
Negative Electrode. Journal of Power Sources 26, 545-551 (1989).
Nanowires as High Capacity Electrode for Lithium Ion Batteries. Nano Letters (2009).
This patent claims priority to U.S. Provisional Application No. 61/256,445, filed Oct. 30, 2009, and incorporated by reference herein in its entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US10/54577 | 10/28/2010 | WO | 00 | 4/27/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61256445 | Oct 2009 | US |
