Imec proposes heterogeneous thin-film nanostructures for high-rate chip-compatible energy storage

(PresseBox) (Leuven, ) Using microelectronic-compatible fabrication techniques, the energy storage team at imec and the group of professor Detavernier of Ghent University have fabricated promising heterogeneous thin-film nanostructures suitable for energy storage in 3D thin-film Li-ion batteries that could potentially be used for on-chip energy delivery. The nanostructures consist of carbon nanosheets (CNS) with conformal deposited TiO2 thin films. These CNS/TiO2 thin-film anode structures have a large electrochemical window of operation (3 - 0.01V vs. Li+/Li), a discharge capacity of ~0.37Ah/cm3, and excellent cyclic stability with a capacity retention of 98%.

One of the challenges in reducing the size of autonomous microsystems, such as MEMS-based sensor systems, is to realize an on-board power source, preferably in combination with an energy harvester. However, such a microbattery can only be fabricated with microelectronic-compatible materials and techniques. A suitable high-rate material combination for Li-ion microbattery applications that we have identified are heterogeneous thin-film nanostructures consisting of CNS as highly conductive three-dimensional current collectors and amorphous TiO2 thin films as active electrode. For this purpose, we have used microelectronic fabrication techniques only: plasma-enhanced chemical vapor deposition (PECVD) to realize the vertically standing CNS on the silicon substrate and atomic layer deposition (ALD) for thin continuous layers of TiO2 deposited on top. The fast-charging CNS/TiO2 material forms only half of the battery cell, so further work is needed to complete it with a solid electrolyte and cathode layers.

The corrugated petal-like arrangement of the CNS leads to high surface-to-volume ratio (the surface area of a single graphene sheet is ~2630m2 g−1). This results in a much improved electrochemical performance because the lithium can insert into the TiO2 host at both sides of the CNS. The amorphous TiO2 thin-film coats the CNS surface as a closed and conformal layer and serves as an ideal host for fast and efficient lithium intercalation/deintercalation. The fact that the TiO2 film is very thin, typically 20nm, leads to fast Li-ion diffusion and to increased rate capability. As the area improvement of the CNS is around 30 times for a 1µm-high CNS coating, the battery capacity is the same as for a 600nm planar film. Therefore, the CNS/TiO2 combines the fast kinetics of thin films with the larger capacity of thicker films. In addition, we managed to store the Li+ by both insertion and by conversion reactions by using a wet Li-electrolyte. The conversion reaction for thin amorphous TiO2 proved to be totally reversible as the capacity remained nearly unchanged even after 30 cycles. The Li+ insertion reaction into TiO2 causes less than 4% volume change, which makes it a suitable candidate for all solid-state batteries. At this point we yet have to examine if the conversion reaction will be also possible in an all solid-state arrangement.

To demonstrate the high-rate capability of TiO2/CNS anode cycles, we did tests at various charge/discharge rates from C/10 to 100C (1C equals the current needed to fully charge the battery in 1 hour). For slow lithiation/delithiation rates, the discharge capacity contributions for intercalated Li and the TiO2 conversion reaction are seen in the two voltage plateaus between 0.2-1V and 1-2V. The shift in voltage plateau with discharging current is the result of IR drop, which is another issue which currently limits thin-film batteries. Concluding from these results, we believe using CNS with an optimum amount of TiO2 coating is a promising approach for the fabrication of electrodes of chip-compatible thin-film Li-ion batteries.

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