Atomic Layer Deposition of Sodium Phosphorus Oxynitride: A Conformal Solid-State Sodium-Ion Conductor

Abstract

The development of novel materials that are compatible with nanostructured architectures is required to meet the demands of next-generation energy-storage technologies. Atomic layer deposition (ALD) allows for the precise synthesis of new materials that can conformally coat complex 3D structures. In this work, we demonstrate a thermal ALD process for sodium phosphorus oxynitride (NaPON), a thin-film solid-state electrolyte (SSE), for sodium-ion batteries (SIBs). NaPON is analogous to the commonly used lithium phosphorus oxynitride SSE in lithium-ion batteries. The ALD process produces a conformal film with a stoichiometry of Na4PO3N, corresponding to a sodium polyphosphazene structure. The electrochemical properties of NaPON are characterized to evaluate its potential in SIBs. The NaPON film exhibited a high ionic conductivity of 1.0 × 10–7 S/cm at 25 °C and up to 2.5 × 10–6 S/cm at 80 °C, with an activation energy of 0.53 eV. In addition, the ionic conductivity is comparable and even higher than the ionic conductivities of ALD-fabricated Li+ conductors. This promising result makes NaPON a viable SSE or passivation layer in solid-state SIBs.

Atomic and Molecular Layer Deposition of Alkali Metal Based Thin Films

“…For the LiPON films (deposited at 300 °C), RT ionic conductivity values as high as 6–7 × 10 –7 S/cm were achieved. , Most interestingly, the use of the thin LiPON films as a solid electrolyte in a practical microbattery in a combination with ALD/MLD-grown Li–organic electrodes could be demonstrated . For the sodium-counterpart NaPON, slightly lower ionic conductivity values were recorded: 1.0 × 10 –7 S/cm at RT and up to 2.5 × 10 –6 S/cm at 80 °C . Besides these phosphate materials, competitive ionic conductivity values have been reported for “LiB x C y O z ” films (a mix of Li 3 BO 3 and Li 2 CO 3 ): up to 2.23 × 10 –6 S/cm at RT, by controlling the Li 2 CO 3 content .…”

“…The process appeared nonideal, though: The growth rate increased in tandem with deposition temperature, including a steep increase from 0.2 Å/c at 325 °C to 1 Å/c at 375 °C. Some incorporation of carbon was detected, likely due to precursor decomposition at the high deposition temperatures. , Additionally, NaF films have been deposited with HF-pyridine as the fluorine precursor. At 175–200 °C, 0.85 Å/c growth was achieved, and the roughness of thin (<10 nm) NaF films was reasonably low despite them being crystalline …”

“…Some incorporation of carbon was detected, likely due to precursor decomposition at the high deposition temperatures. 97,125 Additionally, NaF films have been deposited with HF-pyridine as the fluorine precursor. At 175−200 °C, 0.85 Å/c growth was achieved, and the roughness of thin (<10 nm) NaF films was reasonably low despite them being crystalline.…”

Nanoscale Li, Na, and K ion-conducting polyphosphazenes by atomic layer deposition

“…The resolution of As discussed previously, the ALD LiPON and NaPON films can both be classified as polyphosphazenes (N/P = 1). 19,38 Remarkably, while the NaPON process produces a film nearly structurally identical to the thermal LiPON ALD process, the resulting stoichiometry (Na 4 PO 3 N vs. Li 2 PO 2 N) does not indicate a simple replacement of Na with Li. Furthermore, when KO t Bu is reacted with DEPA in the same manner, similar growth characteristics are observed as in NaPON, but negligible nitrogen is incorporated into the film.…”

Recent Advances in Materials Design Using Atomic Layer Deposition for Energy Applications

“…fabricated NaPON SSEs for Na‐ion battery using sodium tert‐butoxide and diethyl phosphoramidate as precursors. [ 178 ] NaPON deposited at 375 °C exhibited high ionic conductivity of 1.0 × 10 −7 and 2.5 × 10 −6 S cm −1 at 25 and 80 °C, respectively. Moreover, LiPON has been validated as an SSE on various 3D solid‐state batteries.…”