Preparation of Amorphous Lipon Electrolyte Particles Using the Polygonal-Barrel Plasma Treatment System
Authors: Nishizawa Hideki , Iwane Hiroaki , Tasaki Yuzo , Kikuyama Hideshi , Honda Yuji , Abe Takayuki 
Published: February 2020

Introduction
Bulk-type all-solid-state lithium ion batteries consisting of electrode and electrolyte particles are expected to find application in electric vehicles. The systems using oxide-based electrolyte particles can make batteries safer, because they do not produce toxic gases such as H2S, which can be generated in sulfide-based systems by a reaction between electrolyte particles and air moisture [1]. However, the charge capacity of oxide-based systems is too small for the application in electric vehicles. To improve this capacity, it is important to increase lithium ion-conduction paths such as electrode particle‒electrolyte particle interfaces in the cathode. Effective lithium ion-conduction paths can be produced in the cathode by using small electrolyte particles with high ion conductivity in a mixture with electrode particles. However, to the best of our knowledge, there have been no oxide-based electrolyte particles that can be used effectively for this purpose. We, therefore, focused our attention on amorphous lithium phosphorus oxynitride (Lipon) electrolytes, which have been used practically in film-type all-solid-state lithium ion batteries [2,3]. While Lipon particles are conventionally prepared using a solid-state reaction [4], this method causes crystallization of the obtained Lipon particles due to heating at 950ºC, resulting in low ion conductivity. In contrast, the novel system we developed recently for the N2 plasma treatment of powder samples, called the “polygonal-barrel plasma treatment system” [5], achieves the nitridation of the particles without heating [5]. In this study, the preparation of amorphous Lipon particles with a small size and high ion conductivity was investigated by treating Li3PO4 particles with N2 plasma using the polygonal-barrel plasma treatment system.

Experimental
Samples were prepared by treating Li3PO4 particles (Toshima Manufacturing Corporation, 29 µm and 77 nm average particle sizes) with N2 plasma. The N2 plasma treatment was performed at an RF power of 100 W and an N2 gas pressure of ca. 13 Pa for 30-180 min without heating. During treatment, the hexagonal barrel containing the powdery sample was oscillated at 5 rpm over an angular range of ± 75º.
The obtained samples were characterized by X-ray photoelectron spectroscopy (XPS) and cross-sectional observation using a transmission electron microscope (TEM). The lithium-ion conductivity of the samples was determined by impedance measurements (frequency range: 5 Hz-1 MHz).

Results and discussion
The color of the samples changed from white to light brown after N2 plasma treatment. To clarify the cause of this change, the as-received Li3PO4 and the plasma-treated samples with a particle size of 29 µm were examined by XPS. The results showed the presence of the N1s peak only for the plasma-treated samples. The N1s spectrum of the sample treated for 60 min, shown in Figure 1 (I), had an asymmetric peak, which was deconvoluted into the peaks for the N-P=N bond (peak position: 397.9 eV) and the N-P<N bond (399.3 eV) occurring in the Lipon structure [2]. On the other hand, an amorphous layer was seen near the particle surface in the cross-sectional TEM image of this treated sample, indicating the formation of amorous Lipon. Furthermore, the observed amorphous layer had a thickness of 320 nm, implying that nano-sized amorphous Lipon particles can be obtained when 77-nm Li3PO4 particles are employed.
The ion conductivity of the samples (particle size: 77 nm) was measured by impedance measurements.
In all cases, the Cole-Cole plot of a sample had a semicircular shape, and the magnitude of the semicircle was decreased by N2 plasma treatment. Figure 1 (II) shows the ion conductivity determined from the Cole-Cole plots versus sample treatment time. Ion conductivity was increased by N2 plasma treatment, and the maximum ion conductivity after 90-min treatment (8.80 × 10-6 S cm-1) was higher than the values reported for Lipon films (e.g., 6.4 × 10-6 S cm-1 [3]). These results demonstrate that our plasma treatment system allows the preparation of small, highly ion conductive amorphous Lipon electrolyte particles, which will be useful to improve the charge capacity of bulk-type all-solid-state lithium ion batteries.
References
[1] T. Ohtomo, A. Hayashi, M. Tatsumisago, K. Kawamoto, J. Mater. Sci. 48 (2013) 4137.
[2] Z. Hu, D. Li, K. Xie, Bull. Mater. Sci. 31 (2008) 681.
[3] N. Suzuki, T. Inaba, T. Shiga, Thin Solid Films 520 (2012) 1821.
[4] K. Senevirathne, C.S. Day, M.D. Gross, A. Lachgar, N.A.W. Holzwarth, Solid State Ionics 233 (2013) 95.
[5] K. Matsubara, M. Danno, M. Inoue, Y. Honda, N. Yoshida, T. Abe, Phys. Chem. Chem. Phys. 15 (2013) 5097.

Figure 1