Performance of Si-Integrated Li-Ion Microbatteries with Side-By-Side Electrodes: A Geometry Study

Lithium-ion cell chemistries, with their quite high energy densities, are naturally interesting candidates for the application as buffers in future self-sufficient energy-harvesting microelectronic systems [1,2]. For these applications, thin film or 3D micro batteries could come into consideration, although they might be comparatively expensive and the specific capacity per area is limited [3,4]. Another promising approach is the combination of state-of-the-art lithium ion batteries materials and Si-processing, as in the present micro battery concept. Applying this concept, the limitations of thin film and 3D micro batteries can be overcome for a range of applications. Production cost is limited since a vast number of batteries can be batch-fabricated on one silicon wafer and conventional (i.e. non-thin-film) composite electrodes are used. The latter furthermore provides for a high specific capacity per area. However, for packaging reasons, it is of high advantage to have a coplanar arrangement of the electrodes [3-5].

We fabricated batteries on a silicon wafer with the anode and the cathode lying side by side, separated by a narrow silicon spacer. Such a design facilitates assembly and encapsulation considerably because no polymer separator is needed and both electrodes can be contacted from downside.

The production process for this kind of micro battery has been described in detail elsewhere [5]. Briefly, wet chemical etching of a Si-wafer of {110} crystal orientation is applied to form long and narrow cavities with vertical sidewalls at the long sides and a slope at the front sides. This ensures a good utilization of area (i.e. high specific capacity) at a high geometric accuracy as well as an easy contacting of the current collectors. The structured substrate is oxidized and passivated against Li-diffusion prior to the deposition of the current collectors by thin film sputtering and the Li intercalation electrodes by volumetric micro dispensing [6]. A lid with filling holes is attached and the electrolyte filled in. After formation, the device is almost hermetically sealed by the attachment of a second thin glass lid (fig 1.).

Fig. 1: Schematic profile of a battery with side-by-side electrode setup (arrows: paths of Li-ion movement)

We have applied lithium titanate as the anode material and nickel cobalt manganese oxide as the cathode material (Li₄Ti₅O₁₂ // Li_x(Ni_1/2Co_1/5Mn_3/10)O₂). The fabricated cells have shown a reproducible rate capability and cycling performance during continuous cycling between 1.8 and 2.8V. Due to the long paths for Li ion movement and the relatively thick electrodes (100…200 µm) the impedance of such cells is naturally higher as compared to cells with stacked electrodes. However, utilizing the special mechanical properties of silicon as a battery housing material it is possible to decrease the dimension of w_cav (width of the cavities for the electrodes) and especially the separator width (w_sep) down to dimensions where a considerable diminution in cell impedance and, accordingly, an improved rate capability can be expected.

We fabricated and tested side-by-side cells with different geometrical setups to investigte the influence of cell geometry on cell performance. The geometrical parameters (given in fig. 1) have been varied as follows:

parameter	dimension [µm]
wcav	300 … 1800
wsep	30 …   400
d2	75 …   600

Complete cells as well as cells filled with electrolyte only were investigated by means of measuring their 1 kHz impedance and impedance spectrometry. We found that for certain setups a specific impedance close to the impedance of state of the art lithium ion and lithium polymer batteries with stacked electrodes can be achieved. The rate capability, specific capacity per area and energy density were measured and compared for different cell geometries.

Based on the findings of our study it is possible to estimate the influence of a scaling of the investigated geometry parameters on the overall cell impedance.  This allows for a tailoring of micro batteries to meet the requirements of specific applications, e.g. specific current loads, times of operation, and form factors. Important examples for such applications are small energy autarkic sensor systems, e.g. implantable sensors for medical applications [7], or miniaturized sensor networks.

REFERENCES

[1] Belleville, M. et al., Microelectronics Journal 41, 11 (2010) pp.740-745.

[2] Hahn, R., Frequenz, 58,  3-4 (2004) pp. 87-91. DOI: 10.1515/FREQ.2004.58.3-4.87.

[3] Hahn, R. et al, GMM-Fachbericht Band 79 (2014), pp. 33-38.

[4] Ferrari, S. et al., J. Power Sources 286 (2015) 25-46.

[5] Hoeppner, K. et al., JPhys: Conf Ser 476 (2013), 012086. DOI:10.1088/1742-6596/476/1/012086.

[6] Hoeppner, K. et al., EnMat Conference, 2013, Abstr. No 3276.

[7] F. Albano et al., J. Power Sources, 185 (2008), pp. 1524–1532.