The SEM-EDS analysis of the eruption parts due to the continuous E-ray exposure on the NSICON surface. The graphs of pH values of immersion solution (sea water & D.I water). a) The pH of pure D.I water and pH-adjusted D.I water b) The pH of pure seawater and pH-adjusted seawater. The SEM-BSE images of the deterioration parts of the bicycles NSICON. a) The BSE images of degradation and non-degradation parts.
The SEM images of loaded NASICON cross-section on the seawater side with separation of NASICON and carbon felt. a) 1mA/cm-2 for 24 hours charged. The quantitative values for each element of the NASICON surface (surface unetched, carbon etched).
Research background
Sodium-ion batteries
Because Na-based ion batteries have many advantages, such as low cost, easy availability, the sufficiency of Na ions, and suitable redox potential (ܧஷȀ ൌ ୁǤͳܸ 💏ܵܪܧờ5 to provide the most critical energy density for the research. unlimited amount of Na sources in the world Seawater in particular is one of the most abundant resources of Na ions on Earth, covering about 78% of the Earth's surface.
If we can extract Na ions from seawater at extremely low cost and use seawater as sources of SIBs, we need not worry about resource limitation. As the most abundant sources of Na ions on Earth, seawater contains a variety of salts and ions.
Operating principle of aqueous rechargeable sodium-ion batteries
During the discharge process, the Na metal anode is oxidized to Na+ ions that are transported to seawater via the NASICON and organic electrolyte. Because the discharge voltage of ORR reactions is theoretically higher than that of HER, we need to promote the dissolution of atmospheric oxygen gas into seawater with a flow system when seawater batteries are discharged, which can be methods to improve voltage performance.
The research necessities of the solid electrolyte
Many researchers have tried to investigate the stability of the NSICON-type ceramics in water. I focused on the direct influences of the D.I water and sea water on the NSICON. There has been no research on the degradation of the NSICON after charging and discharging the seawater batteries.
I hypothesized that there should be degradation of NASICON on the surface during the galvanostatic electrochemical process. Then we can conclude which one would affect the degradation of NASICON.
Characterization of the solid electrolyte in seawater batteries
Characteristic analysis of the NASICON
The electrochemical characteristics of the NASICON defined with fundamental equipment such as EIS (electrochemical impedance spectroscopy), LSV (linear sweep voltammetry). This main reaction of the NASICON could cause the performance degradation of the seawater batteries. The platinum treated side of the NASICON should face the open round cap (seawater side).
The cross section of NASICON was ion milled with IM-4000 plus (HITACHI, Japan) for 2 hours to be easily analyzed SEM-EDS (Energy Dispersive Spectrometer) and SEM-BSE (Back Scattering Electron mode). I focused on the fact that by-products (HCl & NaOH) would have an effect (degradation) on the surface of NASICON when seawater batteries were charged or discharged. The discharge voltage was reduced from 2.75V to 1.7V and there was further voltage drop from 1.7V to 1.3V. The abrupt voltage reduction during the discharge process should be revealed with supplementary experiments and analyses.
The seawater batteries could also be charged with separation of NSICON and carbon felt (Fig. 46 a). There was no degradation part when the seawater batteries were charged by separation of NASICON and carbon felt (Fig. 45). I found that the electrochemical process of the seawater batteries would damage the NSICON.
Immersion test of the solid electrolyte in the seawater & D.I water
- Experimental section
- Results and discussions
- Conclusions
Treatment of platinum for facilitation of OER/ORR
- Experimental section
- Results and discussions
- Conclusions
The Pt metal film was conglomerated into seed shapes on the surface of NASICON after 550oC 1h heat treatment (Figure 29 a). To observe the cross-section of the NASICON, we simply broke the NASICON with tweezers or small forceps. Seawater batteries were charged/discharged at a high current density of 2 mA/cm-2 to observe the degradation of NASICON.
But that of sea water side of the cycling NSICON was different with pristine NSICON and central part. All the Na ions from the seawater are normally reduced on the Ni mesh. The SEM images of the NSICON cross-section on the seawater side. a, b, c) With different charge current densities. d, e, f) different pH value of sea water.
The depth of degradation increased seriously with the decrease in seawater pH (Fig. 40 d e f). On the other hand, in the central part (non-degradation part), the grain boundaries of NSICON were not broken by the charging process. As the NASICON and the carbon felt were taken apart, the formed HCl on the carbon felt would directly damage the surface of the NASICON.
Therefore, I analyzed the degradation phenomenon of NASICON's solid electrolyte when the seawater batteries were charged/discharged under harsh conditions. The full size of the indium spacer mask (5 inches for a 4 inch wafer) was the same as the wet Si etching mask (Fig. 50 a). So there was a need for a zig-for liquid cell to hold the liquid cells on the TEM holder.
Before it was applied in seawater batteries, the properties of the NSICON itself were studied with a lot of analytical equipment. The analysis of the solid electrolyte and degradation will be fundamental for the development of seawater batteries.
Observation of the NASICON degradation
Si based membrane chips for in-situ TEM liquid cell
Background
- The need of silicon based membrane chips
- The principle of fabrication
In recent decades, with the interesting characteristics of the TEM techniques, People have been adapted to the many applications in TEM equipment. To solve the problems with the open chamber types, where the sample had to be embedded in a liquid layer, different types of closed system liquid cells were developed and adapted (Fig. 46 b). One of the most important things is to offer the possibilities of various experiments such as electrical bias58, 59, flow type60, heating61,62 and cooling63.
Since the NSICON worked in contact with seawater during the electrochemical process, the liquid environment should be essential to inspect the characteristics or degradation mechanism of the NSICON. As most papers mentioned about the minor phase solution of the NSICON36,65, in particular, the observation of the minor phase solution could be achieved by liquid cells. For example, when rectangular unprotected area of silicon wafer ((100) oriented) is etched, the flat inclined sidewalls ((111) oriented) and flat bottom ((100) oriented) are created (fig 50 a).
So by adjusting the size of the opening spot (non-protection area of layer), we could freely make the trenches on the surface of silicon wafer. The photomasks had a 5 x 5 inch square shape and patterned what the users wanted to lithograph on the 4 inch silicon wafer (Fig. 49 a). Pal et al hypothesized that the reason for the undercut was derived from the plane direction of the intersection of (111) planes.
If dicing after wet etching of silicon wafer would be necessary anyway, I can get rid of the convex corner in liquid cell patterns. Fig52 shows the Auto CAD designs of the masks for generation and elimination of convex corners and etched results of silicon wafers. Although the cross-cutting process must be necessary when the edge of pieces is encased with silicon nitride, there was no need for the compensation parts by adding the unnecessary shapes.
Fabrication procedure
To etch the silicon nitride on the wafer, optimization work is needed. The patterned silicon wafer was immersed in 25 wt % KOH etchant for 10 hours at 65oC (wet etching). Covered evaporation pan on the top of the small etchant tank to protect the evaporation of the etchant from the heat.
Once the silicon wafer was completely etched from the base solution, the Si wafer was cleaned with D.I water carefully. The PR would not be uniformly coated on the silicon wafer because there were some broken windows during the etching process. Indium metal was deposited on Si wafer photolithographed with E-beam evaporator, WC-4000 (Woosung hi-vac, Korea).
The indium-coated Si wafer was rinsed with acetone to remove PR (lift-off process). Since die tape was placed on the etched side of the wafer, there were no concerns about soft film residue on the Si3N4 windows. The bottom chip was placed on the container, then the top chip was placed on the bottom.
The bottom chip placed in the container and the chip assembly at the bottom of the container was cooled to seal the bottom. The UV epoxy was adhered to the top of the vessel liquid cell assembly and cured with UV light for 1 minute. The fluid cell containing the zig can be fixed in the TEM holder in situ with high vacuum grease.
Conclusions
The NSICON must have chemical and physical stability and high ionic conductivities acting as separator and electrolyte. As the solid electrolyte came into contact with seawater and was affected by electrochemical processes, the seawater and D.I water immersed the NSICON beads for a long time. The hydronium NASICON generated on the NASICON surface and small phase was dissolving out of the NSICON.
The platinum precious metal was effective in the OER/ORR process, which should be important factors to improve the performance of seawater batteries. These studies suggested not only the new insights into NASICON properties but also the structural breakdown of the solid electrolyte of NASICON. Na-ion batteries, recent developments and current challenges to become low-cost energy storage systems.
Handbook of methods for the analysis of the various parameters of the carbon dioxide system in seawater. Investigation of lithium/air secondary batteries - Stability of NASICON-type lithium ion conducting glass ceramics with water. Reactivity of NASICON with water and interpretation of the detection limit of a NASICON-based Na+ ion-selective electrode F.
Mechanical properties of the solid electrolyte Al-substituted Li7La3Zr2O12 (LLZO) using micro-pillar indentation test. Hybrid Na-airflow batteries using an acidic catholyte: effect of catholyte pH on cell performance. Anisotropic etching of silicon in alkaline solutions III: On the possibility of formation of spatial structures during anisotropic etching of Si (100) in KOH and KOH+IPA solutions.
