Magnesium Phosphorus Oxynitride with High Mg2+ Ionic Conductivity as a Novel Thin-film Magnesium Electrolyte
Authors: Nishitani Yu , Shibata Satoshi , Tsujita Takuji , Tetsuyuki Okano , Omote Atsushi 
Published: February 2020

Rechargeable magnesium ion batteries (MIBs) have attracted much attention as a possible replacement of Li-ion batteries, because of the natural abundance of magnesium and the high volumetric energy capacity of magnesium-based anode materials [1]. One of the problems MIBs face is the lack of suitable liquid electrolytes that are compatible with the high-capacity cathode materials. All reported liquid electrolytes for MIBs that are capable of Mg deposition/dissolution have low anodic stability. Since many lithium solid-state electrolytes (SSEs) are more stable than liquid electrolytes in Li-ion batteries, there have been several reports of magnesium SSEs for MIBs [2,3]. However, their ionic conductivities, the most important property for Mg SSEs, are not high enough for application in MIBs [2-4]. Meanwhile, lithium phosphorous oxides after nitriding, known as LiPONs, are known to display ionic conductivities more than 10 times higher than those before nitriding [5]. This motivated us to apply the same concept to develop a novel phosphorous-based magnesium SSE.
In this study, a novel magnesium SSE with high ionic conductivity, magnesium phosphorous-oxynitride (MgPON), was successfully fabricated. The atomic layer deposition (ALD) technique was used to control the degree of nitriding. For the precursors of MgPON in the ALD reaction, we used bis(ethylcyclopentadienyl)magnesium (Mg(EtCp)2) for magnesium and tris(dimethylamino)phosphine (P(N(CH3)2)3) for phosphorus. NH3 and O2 gases were used for the nitriding and oxidation, respectively. Approximately 2-μm-thick MgPON films were deposited on glass substrates with Au interdigitated electrodes, where the substrate heater was set to about 450 °C. The bonding state of the deposited film was investigated by X-ray photoelectron spectroscopy. The spectrum showed peaks originating from the P-N bonding similar to that in LiPON films [6]. This result indicates that nitrogen atoms are placed at the O-substituted sites in the phosphorous glass network. The ionic conductivity of MgPON was measured by in-plane AC impedance in a vacuum (~ 10-2 Pa) at different temperatures. The obtained impedance was 1.3×10-6 S/cm with an activation energy of 1.3 eV at 210 °C. To identify the conduction species, we investigated the in-plane distribution of magnesium by energy dispersive X-ray spectroscopy, after applying a DC voltage to the interdigitated electrodes. The detected Mg content in the vicinity of the electrode with an applied negative voltage was higher than the other areas, meaning that the ionic conduction was carried out by Mg2+. This result indicates the potential of SSEs using thin-film technology. This study contributes to the development of not only high-capacity cathode materials, but also all-solid-state MIBs.

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