Plasma Enhanced Atomic Layer Deposition of Sodium Phosphorous Oxynitride

The combination of a high demand for energy storage materials and limited supply of Li on Earth’s crust for Li-ion batteries (LIBs) is a major driving force in the search for an alternative solution to battery materials. A more sustainably sourced and abundant material such as Na can lower the resource and operating costs associated with large-scale energy storage and offload the demand for Li in battery materials. Thus, interest in Na-ion batteries (SIBs) continues to rise as does the development of Na-ion conducting materials for solid state electrolytes (SSEs) and SIB protection layers. Atomic layer deposition (ALD) is a key technique in processing new materials compatible with complex architectures. Thermal ALD and plasma enhanced ALD (PEALD) lithium phosphorous oxynitride (LiPON) processes as well as analogous thermal sodium phosphorous oxynitride (NaPON) have been previously developed as conformal ALD SSEs. In this work, we demonstrate process development and characterization of an analogous low temperature (250 ^oC) PEALD of NaPON. Though it was expected that both PEALD NaPON and PEALD LiPON would behave similarly, the seemingly simple substitution of Na for Li source precursors led to distinct properties not observed in the previously mentioned processes. The NaPON process in this work demonstrates significant tunability of N coordination states by varying plasma nitrogen (^PN₂) exposure time. Electrochemical characterization showed an ionic conductivity of 8.2Χ10^-9 S/cm at 80 ^oC, activation energy of 1.03 eV, and reversibility of PEALD NaPON vs Na metal upon cycling. This first instance of low temperature NaPON deposition by PEALD shows promise for further development and understanding of more versatile processing of Na SSE thin film materials.