All-Solid-State Lithium-Ion Microbatteries with a Silicon Negative Electrode for Microelectronic Systems

All-solid state microbatteries are innovative energy storage solutions for powering microelectronic devices such as ICs, real-time clocks, memories, sensors. Today, most microbatteries available on the market are based on the Li/LiPON/LiCoO₂ active core developed by Bates and al.[1]. Nevertheless this system is no longer adapted to emerging applications, which now require lower operating voltages (1 to 2 V) and the possibility to connect the microbattery as a conventional electronic component, i.e. using the reflow soldering (SR) process. Thus, two options are possible to get rid of metallic lithium which melts around 180°C: the development of Li-free cells [2] or efficient all-solid-state Li-ion cells. The difficulty to control the morphology of plated lithium in Li-free cells, which is often detrimental to the cycle life, promotes the choice of robust Li-ion cells.

This work reports on the development of ‘2 V’ lithium-ion cells, comprising an amorphous silicon film as the negative electrode and amorphous lithium titanium oxysulfide film as the positive electrode, both obtained by sputtering. Silicon thin film electrode was previously found to have excellent cycle life in all-solid-state half-cells [3]. As for TiO_yS_z, these materials exhibit high volumetric capacities, which may vary with composition. Their electrochemical activity involves highly reversible redox processes on both sulfur and titanium around 2V/ Li⁺/Li [4,5]. In order to obtain Li-ion cells, new lithiated titanium oxysulfide films Li_xTiO_yS_zwere synthesized and characterized.

The electrochemical behavior of electrode materials, especially the one of new positive electrode materials, and of the complete Li-ion cells was studied (figure 1). The positive material and the cells exhibit excellent cycle life at room temperature when operated up to 130 µA.cm^-2(figures 2 & 3).

A higher sulfur content in the positive electrode was found to enhance its specific capacity. EIS measurements showed it also slightly increases the electrode polarization. This polarization was found to sharply increase at the end of the charge for all Li_xTiO_yS_z compositions.   Depending on cycling conditions (voltage window, current density), reversible slow-fading phenomena, as the memory effect related to the Li_xSi negative electrode, were highlighted [6].

Finally, no detrimental effect of three successive SR thermal treatments at 260°C on the active core was evidenced.

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[2] B. J. Neudecker, N. J. Dudney, J. B. Bates, J. Electrochem. Soc., 147, 517 (2000)

[3] V.P. Phan, B. Pecquenard, F. Le Cras, Adv. Funct. Mater., 22, 2580 (2012)

[4] M.H. Lindic, H. Martinez, A. Benayad, B. Pecquenard, P. Vinatier, A. Levasseur, D. Gonbeau, Solid State Ionics, 176, 1529 (2005)

[5] B. Fleutot, B. Pecquenard, F. Le Cras, B. Delis, H. Martinez, L. Dupont, D. Guy-Bouyssou, J. Power Sources, 196, 10289 (2011)

[6] M. Ulldemolins, F. Le Cras, B. Pecquenard, Electrochem. Comm., 27, 22 (2013)