Normally you cannot use single crystalline silicon as a battery component on its own because it expands considerably when implemented with lithium, starts to crack and is gradually destroyed. We use the doped semiconductor silicon of the chip directly. However, it is first carefully microstructured using a knowledge of the crystal axes and then electrochemically specially activated.

—Michael Sternad, researcher at the Christian Doppler Laboratory for Lithium Batteries at TU Graz

Possible applications are small self-powered, data-transmitting sensor chips for cars, airplanes (temperature, engine/chassis vibrations, tire pressure sensors) or medical purposes (sticker to measure body temperature, contact lenses that measure blood sugar concentrations).

The main issue that inhibits so far the real-life application of silicon as practicable anode in LIBs is the remarkable, to some extent inhomogeneous, increase in volume during lithiation. This increase amounts up to 300% if we refer to amorphous silicon (a-Si) and complete electrochemical lithiation (0 mV vs Li/Li⁺). Both internal mechanical stress and the dramatic expansion leads to structural damage especially after several charge-discharge cycles have been completed. To deal with this issue and to diminish dilatation one may take advantage of thin Si films and nanostructured or porous materials with their ‘free’ volume, i.e., to make use of small, nm-sized Si particles. Moreover, it is favorably to enduringly keep Si in the amorphous (a-Si) rather than in the crystalline state (c-Si) while the battery is subject to charging and discharging. The use of nanostructured Si buffers not only volume expansion but also helps shorten diffusion paths for both Li ions and electrons. The large surface area of active material, however, may result in unwanted side reactions and heavy formation of passivating interphases. These so-called solid-electrolyte interphases (SEI) form because of the electrochemical instability of bare Si surfaces being in contact with the commonly used liquid electrolytes.

In contrast to these approaches, the present study proposes the direct use of single crystalline acceptor-doped Si … Against possible objectives regarding the electrochemical activity of monocrystalline Si, we will show how single crystalline Si in a well-defined microstructured form can serve as powerful, long-lasting Si electrode that does not need any binders or conductive additives. For this purpose, μm-sized towers, with dimensions being larger than 10 μm, were fabricated and their electrochemical performance tested. Although the monocrystalline towers cannot benefit from nanosize effects, several outstanding properties make them superior to nanostructured Si especially if microbatteries are considered.

As has been shown recently via in situ atomic scale imaging, during the first lithiation process m-Si transforms to an a-Li_xSi-phase via a so-called “ledge-mechanism”, i.e., by peeling off Si layer after Si layer. Fortunately, Li ion diffusivity in a-Li_xSi is rather high making it a convenient active material. Subsequent delithiation of a-Li_xSi yields the desired amorphous form of Si being characterized by the advantages sketched above. Hence, if we could make use of a-Si electrochemically obtained from wafer-grade Si directly, the well-established manufacturing methods of semiconductor industry can be utilized to pre-fabricate structured Si anodes with crystal orientations perfectly supporting the preferred lithiation pathway. Such implemented batteries may be produced in quantities of up to 5000 cells per 8-inch wafer, i.e., in a massively-parallel way. The low effort would result in low unit costs.

—Sternad et al.

The TU Graz team surface-structured a boron-doped 8-inch silicon wafer, producing towers with a base area of 50 × 50 μm² and a height of 32 μm; the distance between the towers was 17.5 μm. To complete the electrode, sputter-coated the backside of the silicon wafer with a copper layer and sawed the wafer to produce 4 x 4 mm² electrode units.

The m-Si electrode. (a) top view (4 × 4 mm2), (b) cross section and (c) the 3D patterned surface in detail. Sternad et al. Click to enlarge.

Cycling experiments showed specific discharge capacities as high as 980 mAhg^–1 could be reached during the 1^st cycle; the Coulombic efficiency was 89%. After the 1^st cycle the capacity slightly increases reaching 1093 mAhg^–1 (5^th cycle, 98.9% Coulombic efficiency).

They also tested the anode in a full cell configuration using Li(Ni_0.80Co_0.15Al_0.05)O₂ (NCA) as cathode. The full cell can be charged and discharged in a stable way for more than 100 times with only a marginal capacity fade. The Coulombic efficiencies are as high as 99.7%.

Using scanning electron microscopy (SEM) the team was able to characterize the changes in the anode structure during Li uptake. While the native anode shows an even geometry with the 17.5 μm gaps separating the towers, in charged state, expanded towers are almost in direct contact to each other. Discharging the anode does not re-establish the original tower-to-tower distances. A larger distance between the towers would cause mechanical instability because the expanded towers would not sustain each other.

SEM images of the Si towers. High-angle view of (a) the native silicon towers, (b) the charged ones (5 cycles) and (c) the towers discharged again. (d,e) the same images but tilted by 60 degrees. Sternad et al. Click to enlarge.

The whole system is able to reversibly endure lateral dilatations of up to 37% and perpendicular dilatations of up to 31%.

The excellent cycling performance at high energy densities combined with, in respect of Si, only moderate dilatation and morphology changes emphasizes monocrystalline silicon as a highly practicable anode material. The patterned electrode, whose tower structure is scaled such that it optimally withstands volume expansion, is expected to perform even several hundreds of cycles in full cell configuration as preliminary tests have shown. Considering micro-applications, such as sensors, an extraordinary high cycle number is, however, not always required. As the energy density is rather high, in many cases the lifetime of the battery, if charged and discharged just a few times, would exceed that of the whole sensor.

—Sternad et al.

The Christian Doppler Laboratory for Lithium Batteries at the Institute for Chemistry and Technology of Materials at TU Graz was established in 2012 and is committed to developing new concepts for lithium batteries. In addition to Si micro batteries, solid-state Li batteries are also being investigated. The corporate partners of the CD Laboratory are AVL List GmbH and Infineon Technologies Austria AG.

Resources

• M. Sternad, M. Forster & M. Wilkening (2016) “The microstructure matters: breaking down the barriers with single crystalline silicon as negative electrode in Li-ion batteries” Scientific Reports 6, Article number: 31712 doi: 10.1038/srep31712