Solid electrolytes are considered a necessary step for Li-ion ions to surpass the practical limit of 800Wh/L. Next to safety aspects, solid-state electrolytes could enable more compact batteries thus increasing their volumetric storage capacity. A main driver, however, is the potential for solid-electrolytes to increase the electrochemical window to accommodate high voltage positive electrodes (5V cathodes) and stability against metallic lithium as negative electrode. As such, solid-state batteries could enable high energy density through increased cell voltage. Numerous Li-ion conducting solids are known today, however most do not have the broad electrochemical window and stability against lithium yet. Nitrogen doped lithium phosphate glass or LiPON is a well known solid electrolyte material attributed with excellent electrochemical stability above 5V and stable against metallic lithium [1]. However, due to its limited ionic conductivity (~10^-6 S/cm) it is only applicable in thin-film batteries for use for medical implants and various sensor systems. It has also been evaluated as solid-electrolyte coatings on electrode particles as buffer layer between active material and the bulk electrolyte. In a thin-film stack, it is an ideal model system for electrochemical study.

In this paper, we report on the electrochemistry of RF-sputtered LiPON thin films and studied the out plating of lithium until break down. The LiPON thin-films had ionic conductivities ranging between 1x10^-7 and 1x10^-6 S/cm [2]. The decomposition potential was calculated from basic thermodynamic quantities and was linked to current-voltage (I-V) measurements with great consistency. The reported I-V measurements were conducted on metal-electrolyte-metal or MEM structures. The decomposition of LiPON was shown to occur at a potential of 4.8 V and proceeds in a diffusion limited way. It could be shown that LiPON breakdown is rather determined by the time needed for the decomposition reactions to complete, rather than being solely driven by the electric field. From the diffusion limited Li-ion current, the Li⁺ diffusion coefficient was extracted and found to be 2x10^-11 cm²/s. This value is in excellent agreement with literature reports [3]. Ultimately hard breakdown was shown by TOF-SIMS imaging experiments and occurs through the formation of metallic lithium filaments.

In the present work, we have for the first time formulated a breakdown mechanism for LiPON layers based on experimental measurements and thermodynamic considerations. The presented work provides a detailed evaluation of a solid electrolyte's breakdown and stability using I-V measurements. As such these measurements allow determination of ion transport phenomena as well as of the electrochemical stability window. It can also provide insight in the breakdown mechanism of RRAM devices.

References

[1] J. Bates; N. Dudney; G. Gruzalski; R. Zuhr; A. Choudhury; C. Luck; J. Robertson, Solid State Ionics, 52-53 (1992) 647-654.
[2] B. Put; P.M. Vereecken; J. Meersschaut; A. Sepúlveda; A. Stesmans, ACS Appl. Mater. Interfaces., 8 (2016) 7060-7069.
[3] X. Yu; J. Bates; G. Jellison; F. Hart, A stable thin-film lithium electrolyte: Lithium phosphorus oxynitride. J Electrochem Soc., 144 (1997), 524-532.