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論文題目「Synthesis of Amorphous Oxide Nanoparticles by RF Thermal Plasma for Electrolyte in Solid State Batteries」

Du Xiangyu

1．Introduction
Lithium-ion conducting glasses have been widely studied due to their potential application as solid-state amorphous electrolytes in secondary batteries. Since amorphous electrolytes could increase lithium ion conductivity and have more contact area with electrode. LiPO3 stands out due to high Li-ion conductivity. Furthermore, adding other lithium metal oxide could increase ion conductivity. Meanwhile, other lithium metal oxide could decrease the glass transition temperature thus electrolyte has larger contact area with electrode during working. Hence Li3BO3 and Li4SiO4 are two candidates for improving LiPO3 battery characters.
Radio Frequency (RF) thermal plasma can be characterized as: a high enthalpy flame with extremely high temperature fields, low velocity (10~20 m/s) resulting long reaction time, absence of electrodes thereby avoids contamination, and rapid quenching. Therefore, RF plasma has been widely applied in production of nanoparticles which are difficult to synthesized by other methods. In present study, LiPO3, Li3BO3 and Li4SiO4 are tired to be synthesized by RF thermal plasma respectively.

2．Experimental
The setup mainly consists of a powder feeder, a plasma torch, a reaction chamber, a quenching tube, and a particle collection filter. Quenching tube was set under the torch with a distance of 15 cm. Raw materials were fed into thermal plasma and instantaneously evaporated due to the high enthalpy and homogeneous nucleation, condensation, coagulation will happen in the chamber. Synthesized nanoparticles were collected from the inner wall of the reaction chamber and the filter.
Experiments were operated at a condition of 4MHz, 20 kW, atmospheric pressure. Ar was introduced as carrier gas (3 L/min), inner gas (5 L/min). Sheath gas was set at 60 L/min with 0~5 L/min comes from O2, 55~60 L/min comes from Ar. Quenching gas was injected with different flow rate from 20~40 L/min. Raw material was mixed with stoichiometric ratio and the feed rate changes from 200~1000 mg/min.
Prepared nanoparticles were characterized for phase identification by X-Ray Powder Diffraction (XRD). Amorphization degree was calculated semi-quantitatively by JADE 7.0.
Morphology of the particles, diffraction patterns and size distributions were observed by Transmission Electron Microscopy (TEM) and Scanning Electron Microscope (SEM) observation. RAMAN was used to identify the structure of the constituent molecules. Element mapping and relative abundance were observed by STEM-EDS.

3．Results and Discussion
(a) Li-P-O system
In Li-P-O system, amorphous LiPO3 nanoparticles were synthesized successfully regardless of the oxygen gas flow rate and feed rate change. Amorphization degrees are almost 100wt%. Since necessary quenching rate for LiPO3 is smaller (16.6 K/s) than quenching rate estimated in plasma (10^4 K/s), therefore, amorphous LiPO3 nanoparticles were synthesized effortlessly. However, mean diameter of the nanoparticles increases as the feed rate increases due to the longer growth time and larger growth rate.

(b) Li-B-O system
In Li-B-O system, amorphous high-temperature phase Li3BO3, β-Li3BO3, was synthesized. This unstable component was preserved because of the high quenching rate in RF thermal plasma. Amorphization degree of β-Li3BO3 nanoparticles increased notably from 60wt% to 71wt% due to the increase of quenching gas. Since quenching rate is estimated to increase from 104 K/s to above 105 K/s due to existence of quenching gas, thus amorphization degree was increased.

(c) Li-Si-O system
Diffraction peaks of XRD correspond to Li4SiO4 were observed. All the peaks become broader when quenching gas was injected. Li2CO3 and Li2SiO3 peaks were also detected. Li2CO3 peaks come from synthesized Li2O particles reacted with CO2 in atmosphere after the experiments. Li2SiO3 was considered as by-products.
The amorphization degree increased from 52wt% to 70wt% after quenching gas was injected. The reason is the same with Li-B-O system. Since plasma plume becomes compressed in axial direction when quenching gas was injected. In compressed area, quenching rate was estimated to be higher than 10^5 K/s even 10^6 K/s according to numerical analysis while quenching rate is 10^4 K/s without quenching gas. Therefore, amorphization degree was increased.
Halo diffraction patterns in TEM images suggest amorphous state exists in all the products. Spherical particles were observed in no quenching gas condition with a mean diameter at 93 nm. Spherical and irregular nanoparticles were observed when quenching gas at 20 L/min and 40 L/min. Since the less long-range order in material, the higher amorphization degree would be. Therefore, the particles become irregular and hard to clarify.
Formation mechanism of Li4SiO4 nanoparticles was investigated. Si has the highest nucleation temperature then nucleates firstly. Li oxide and silica vapor co-condense on Si then Li4SiO4 was synthesized.

4. Conclusion
In present work, amorphous nanoparticles of LiPO3, Li3BO3, and Li4SiO4 were synthesized by RF thermal plasma successfully and all the formation mechanisms were investigated. LiPO3 could be synthesized even at 1000 mg/min. Quenching gas could increase the amorphization degree of Li3BO3, Li4SiO4 nanoparticles. Future work is to decrease the Li2CO3 content in products and mix the Li3BO3 or Li4SiO4 with LiPO3.

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