Mechanical Stresses in Lithium Phosphorus Oxynitride-Coated Germanium Thin-Film Electrodes A. Al-Obeidi, D. Kramer, R.

icon Mechanical Stresses in Lithium Phosphorus Oxynitride-Coated Germanium Thin-Film Electrodes

A. Al-Obeidi, D. Kramer, R. Moenig, C. V. Thompson.
Sponsorship: Singapore-MIT Alliance for Research and Technology

In the electronics and health industry, there has been a strong trend toward miniaturized devices for use in wearable electronics, medical implants, and wireless communication. The reduced energy consumption of microsystems makes it possible to integrate microbatteries directly onto electronic chips. Solid-state microbatteries are ideally suited for such applications since they can be integrated on microchips while offering improved safety (no liquid electrolytes and thermal runaway), performance (higher voltages, wider range of operating temperatures), and lifetime. The simplest and most common form of a solid-state battery is a planar solid-state thin-film battery. For the anode, germanium is an ideal candidate since it offers large volumetric capacities (7366 A h l−1) compared to lithium (2065 A h l−1) while being compatible with conventional semiconductor processing techniques. However, use of germanium is limited by the significant volumetric and structural changes that occur during cycling. In order to explore the relationship between electrochemistry and the mechanical stresses, in situ stress measurements on germanium thin-film electrodes coated with lithium phosphorus oxynitride (LiPON) were performed.

 

 

 

 

It was found that LiPON, a rigid solid electrolyte, suppresses morphological evolution and results in reproducible cycle-to-cycle stress behavior. The repeatable behavior observed in LiPON-coated films allows more direct characterization of electrochemical processes governing lithiation and delithiation. Cycling at various rates (Figure 1) revealed that the lithiation capacity of coated electrodes increased at slower cycling rates, saturating at about 1200 A h kg-1 when using rates slower than 1 C. Cycling below 100 mV resulted in the formation of c-Li15Ge4, which appeared as a sharp drop in the compressive nominal stress to values close to zero (Figure 2, points 1-2). Overlithiation of this phase resulted in a linear compressive increase in stress (Figure 2, points 2-3). These results indicate that cLi15Ge4 has a higher density than its a-LixGe precursor. Delithiation of c-Li15Ge4 seems to consist of two successive events: the formation of an intermediate phase followed by a rapid release of lithium from this intermediate phase, which resulted in the amorphization of the electrode. While crystalline Li15Ge4 develops a lower maximum nominal tensile stress than its amorphous counterpart, extraction of lithium from cLi15Ge4 requires more energy and therefore reduces the energy efficiency of a cell.

 

capacity plots

Nominal stress-capacity plots

FURTHER READING
  • Z. Choi, D. Kramer, and R. Mönig, “Correlation of stress and structural evolution in Li4Ti5O12-based electrodes for lithium ion batteries,“ Journal of Power Sources, vol. 240, p. 245, 2013.
  • A. Al-Obeidi, D. Kramer, R. Mönig, and C. V. Thompson, “Mechanical stresses and crystallization of lithium phosphorous oxynitride-coated germanium electrodes during lithiation and delithiation,“ Journal of Power Sources, vol. 306, p. 817, 2016.