Large-Scale Exploration of the LiSiPON System Using a High Throughput Experimental Method
Authors: Berthou William , Legallais Maxime , Yildirim Gunay , Bousquet Bruno , Sorieul Stéphanie , Motto Ros Vincent , Pecquenard Brigitte , Le Cras Frédéric 
Published: August 2024

Amorphous thin film materials in the LiPON(1) or LiSiPON(2) systems have been prepared for the first time in the early 1990s using magnetron sputtering. Since then, LiPON materials have been used as state-of-the-art solid electrolytes into all-solid-state thin film micro-batteries thanks to their outstanding properties (electrochemical stability vs Li°, wide electrochemical stability window of ~[4-0] V vs. Li+/Li0, isotropic and homogeneous medium, beneficial mechanical properties(3), very low electronic conductivity). The ionic conductivity of LiPON prepared from Li3PO4 targets remains rather moderate, reaching a maximum of 3.10-6 S.cm-1 at room temperature. However, it has recently been demonstrated that the introduction of SiO2 as a second glass former can increase this value up to 2.10-5 S.cm-1, and also that the ionic conductivity of these compounds does not increase steadily with their Li content.(4)This highlights the convoluted effects of mixed formers(5) (Si, P, B...), mixed anions(6) (O, N, S...) and Li concentration on the ionic conductivity. Nevertheless, the latter are usually observed on limited series of compositions, and barely not studied on large sets of samples, which can limit the understanding of composition-structure-conductivity relationships.

In this context the aim of our work was to build a High Throughput Screening (HTS) approach to explore the LixSiyPzOuNw system as a case study (Fig.1). Indeed, HTS approaches aim to accelerate material breakthrough discovery, and to understand beyond the material mechanisms. In material science, the goal is to accelerate the discovery of new keys organic, inorganic or composite materials. It may also bring beyond ‘classical’ iterative method regarding the study of complex systems (ternary, quaternary...)(7). To do so, wide ranges of material libraries are synthesized, prior to be tested through an automated characterization workflow.

Our specific approach starts with the preparation of material libraries by combinatorial synthesis using magnetron co-sputtering with tilted targets, then goes through automated and fast characterizations targeting specific properties, relevant to its application as ionic conductor.

Deposition of thin films displaying composition and thickness gradients at the surface of a 4’’ silicon substrate is achieved by co-sputtering three target materials in a pure Ar or N2 atmosphere. Deposition through a shadow mask allows to discretize the continuum and to prepare a library of 76 separate LixSiyPzOuNw samples in one experiment. Then, by tuning the sputtering parameters (i.e discharge gas, gas pressure, incident power, target tilt, target-to-substrate distance,...) different compositional domains can be explored.

Relevant automated characterization techniques are then applied to the material library spread on the 4’’ substrate. For the assessment of these amorphous ionic conducting films, dimensional (thickness), physical-chemical (composition & local structure) and functional properties (ionic/electronic conductivities) are studied.In this sequence, determining the composition of the thin film is a real challenge due to a number of requirements :(i) light elements analysis, especially lithium; (ii) need of localized analysis (mapping) on few mm² over large substrate (4” wafer); (iii) scarce material quantity available (~ μg) and finally (iv) fast analysis. Over lab-scale techniques available, only Laser Induced Breakdown Spectroscopy technique (LIBS) seems to fulfil all these requirements. To this purpose, LIBS is developed as a HTS mapping technique for chemical analysis. A sample calibration approach is implemented through coupling with robust chemical analysis techniques (RBS, NRA, ICP-OES, SEM-EDS).

In short, this specific HTS procedure will be discussed and results about composition-structure-conductivity relationships in the LiSiPON system will be presented (Fig.2).

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Figure 1