1. Field of the Invention
The present invention relates to the field of Lithium ion batteries.
2. Related Art
Rechargeable lithium-ion batteries hold great promise as energy storage devices to solve the temporal and geographical mismatch between the supply and demand of electricity, and are therefore critical for many applications such as portable electronics and electric vehicles. Electrodes in these batteries are based on intercalation reactions in which Li+ ions are inserted (extracted) from an open host structure with electron injection (removal). However, the current electrode materials have limited specific charge storage capacity and cannot achieve the higher energy density, higher power density, and longer lifespan that all these important applications require. Silicon (Si) as an alloying electrode material is attracting much attention because it has the highest known theoretical charge capacity (4200 mA h g−1). However, it is challenging to overcome the issues associated with alloying and conversion reactions, which involve large structure and volume changes (400% volume expansion for Si) during Li+ ion insertion and extraction. These issues can cause large hysteresis in the charge and discharge potentials, low power rate, and short cycle life, due to material instability, and poor electron and ion conduction.
Recently, Si nanostructures have been intensively explored to attack the volume expansion and fracture problem. For example, many Si nanostructures, such as Si nanowires, carbon/Si spheres, Si nanotubes, core-shell crystalline/amorphous Si nanowires, Si nanotubes, have also shown initial capacity close to the theoretical limit, good (>90%) capacity retention over a large number of cycles. However, low cost and fast throughput processes with great mass and morphology control are still desirable to reach the full potential for commercialization.
The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings.
In the discussions that follow, various process steps may or may not be described using certain types of manufacturing equipment, along with certain process parameters. It is to be appreciated that other types of equipment can be used, with different process parameters employed, and that some of the steps may be performed in other manufacturing equipment without departing from the scope of this invention. Furthermore, different process parameters or manufacturing equipment could be substituted for those described herein without departing from the scope of the invention.
These and other details and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.
Various embodiments of the present invention describe a robust and scalable electrochemical process to fabricate electrodes comprising nanoporous Si network thin films with controllable porosity, which also demonstrate a high initial discharge capacity of 2570 mA h g−1 and 200 cycles in electrochemical tests. These nanoporous Si network thin films also show a capacity of above 1000 mAhg−1 beyond 200 cycles. Excellent rate capability is disclosed as well.
Nanoporous Si network thin films with controllable porosity and thickness are fabricated by a robust and scalable electrochemical process, and then released from Si wafers and transferred to flexible and conductive substrates. These nanoporous Si network thin films serve as high performance Li-ion battery electrodes, with an initial discharge capacity of 2570 mAh/g, above 1000 mAh/g after 200 cycles without any electrolyte additives. An embodiment also demonstrates that a certain oxide coating can be used to further improve the performance.
In one embodiment, a porous silicon thin film fabrication process is described as follows. To achieve high pore densities and uniform layers, a highly doped p-type wafer is selected. The wafers used have a resistivity approximately between 0.01-0.02 Ω cm which corresponds to a boron impurity concentration approximately between 3×1018-8×1018 cm−3. These wafers may be diced into one-inch chips and then loaded into a Teflon etching cell with an 18 mm diameter. The etching cell is held closed with three nylon screws, and sealed with a chemical resistant O-ring. The chamber is then filled with 49.5% hydrofluoric (HF) acid and ethanol (EtOH) in a ratio of 1:1 as shown in
As current flows through the cell, holes are driven to the surface of the silicon at the semiconductor/electrolyte interface. When these holes reach the surface, they effectively oxidize the surface silicon atoms by removing electrons, at which point the silicon atom(s) is dissolved into a silicon hexafluoride ion by the surrounding fluoride in a reaction given by:
2 h++6 HF+Si→SiF62−+H2+4 H+
This reaction proceeds in such a way that the holes are preferentially driven towards the existing etch sites, resulting in the creation of pores. For a typical etching current of 280-600 mA/cm2 the etching process is allowed to proceed for 15-120 seconds, depending on the desired thickness. For high etching currents (>1300 mA) the maximum etching time is greatly reduced to prevent the destruction of the porous network. After etching, the cell is rinsed 4 times with ethanol to remove the residual HF, and then dipped in hexane and air-dried. The hexane reduces the surface tension during drying and reduces the risk that the porous network will collapse. While critical point drying can be used, hexane drying provides comparable results with significantly less time required.
If we apply this concept to our porous silicon layers, we can see that by adjusting the HF concentration, we can switch between pore formation and electropolishing. The result of this is that a thin film of porous silicon can be undercut and freed from the silicon substrate, and then transferred to any type of receiver substrate. This process is shown schematically in
One large advantage of the porous layer transfer technique is that the silicon wafer can be reused many times. After the electropolishing step (which only removes a trivial amount of silicon) the wafer is ready for another porous layer to be formed.
Once the porous layer has been transferred to an appropriate substrate, drying is the most critical procedure. Depending on the amount of adhesion between the substrate and the porous layer, allowing the layer to air dry may result in peeling or cracking. To prevent this, a superior solvent such as hexane can be used, or a supercritical drying step may be required. In most cases where the receiver substrate is clean and smooth, the adhesion is sufficient to prevent the porous layer from being removed under any normal stresses.
In an alternative embodiment,
Silicon wafer chips (0.01-0.02 Ω-cm p-type) are secured in a teflon etch cell filled with a 1:3 hydroflouric acid and ethanol mixture. A nanoporous Si thin film is etched under constant current and light illumination.
Subsequently the solution is exchanged for 1:20 HF:EtOH and etched at 30 V to undercut the porous network and release it from the silicon substrate. Finally the etch cell is flushed with pure ethanol, and then the thin layer of nanoporous Si is transferred to Cu foil by slowly flowing ethanol over the chip while holding it in contact with the foil. The porous layer on Cu foil is then rinsed with hexane and allowed to air dry. The silicon substrate can be reused many times, ensuring efficient use of the Si source material.
The porosity of these nanoporous Si network thin films can be controlled by adjusting the etching current and light illumination intensity (
We assume the porosity is uniform along the thickness direction, which can be seen in
To investigate the electrochemical performance of these nanoporous Si network thin films, two-electrode 2032 coin cells with these nanoporous Si network thin films (˜20 nm pore size) on the Cu substrate were fabricated with Li metal as the counter electrode. As the volume of Si will expand upon the full lithiation to 400% of the original, samples with 80% porosity were used in this study. To understand the intrinsic properties of these nanoporous Si network thin films, galvanostatic cycling was used with voltage cutoffs of 0.01 and 1V vs Li/Li+. The charge capacity referred to here is the total charge inserted per unit mass of the nanoporous Si network thin films during Li insertion, whereas the discharge capacity is the total charge removed during Li extraction.
The structural changes of the nanoporous Si network thin films before and after lithiation were studied using scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The as-fabricated nanoporous Si network thin films have an average pore size of 20 nm and 1 um thickness as revealed in SEM images (
The first discharge capacity was 2570 mAh/g at the C/10 rate, or 10h per half-cycle (
The voltage profiles of the different cycles are shown in
High rate capabilities were also observed in the nanoporous Si network thin films (FIG. 6c). Charging/Discharging at C/20, C/10, C/5, C/2, 1C and 2C revealed good cyclability. The Coulombic efficiency of 99.7% was also quite high, indicating excellent reversibility. The capacity was very stable at the high rates, indicating good Li diffusivity in the Si. Although the capacity dropped at the 1 C rate to 1200 mAh/g, it was still much higher than the theoretical capacity of graphite (372 mA âh/g). Also it is found that addition of binder (CMC binder) can improve the performance, which has been reported before. In the case of Si-C composite, it is reported that the addition of a binder can improve the performance by holding the active materials together. However, in this study, since the nanoporous Si network thin films did not pulverize after electrochemical cycling, it is believed that the binder improves the performance by improving the electrical contact between active materials and current collectors.
In conclusion, various embodiments have shown that nanoporous Si network thin film anodes with 80% porosity have a high specific capacity (2570 mAh/g) and excellent cycling performance (>200 cycles) without any electrolyte additives. Our nanoporous Si anode design is easy to fabricate and has good electronic contact between the network and the current collector. Thus, nanoporous Si network thin films can be a promising, higher-capacity alternative for the existing graphite anode in Li ion batteries.
This U.S. Utility Application claims priority to U.S. Provisional Application Ser. No. 61/904,944 filed Nov. 15, 2013, which application is incorporated herein by reference as if fully set forth in their entirety.
The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 between the U.S. Department of Energy and the Regents of the University of California for the management and operation of the Lawrence Berkeley National Laboratory. The government has certain rights in this invention.
Number | Date | Country | |
---|---|---|---|
61904944 | Nov 2013 | US |