High-energy, long-cycle-life secondary battery with electrochemically pre-doped silicon anode and rubeanic acid cathode

概要

1. Introduction
In order to design lithium-ion batteries (LIBs) as power sources for automotive application and transportation, low cost and high energy density are the issues.1 For solution, it is necessary to increase the capacity of the battery both negative electrode and positive electrode. Because the battery application was applied by Liebig's law of the minimum, which means that the extrinsic capacity of a battery rely on the smaller side of electrode capacities. In my Ph.D. thesis, it will overcome technical challenges 2-5 so that can provide feasible technique of applyinglithium doped silicon as negative electrode active agent and rubeanic acid as positive electrode active agent, both of them can happen multi-electron. The compounds and properties of pre-dopedsilicon and redox reaction mechanism of rubeanic acid were investigated for enlarging the capacity of united Si-RA full cell 4 time larger than that of existing LIB

2. Electrochemically pre-doped silicon anode under pressure
A slurry of anode was prepared by mixing silicon powder as the negative electrode active material, acetylene black as the conductive material and polyacrylic acid as the binder ina mass ratio of 7: 1.5: 1.5 by adding water. The film-packed cells were sealed under vacuum, andconsisted of a stacked silicon electrode, lithium-coated copper foil, a separator film, and anelectrolyte solution of 12.1 wt % LiPF6 in a mixture of ethylene carbonate (24.0 wt %), diethyl carbonate (55.9 wt %), and fluoroethylene carbonate (8.0 wt %). Electrochemical pre-doping was performed with a bipotentiostat/galvanostat (Wave Driver 20, Pine Research) in a voltage range from open circuit voltage (~3.2 V) to 10 mV at a current rate of 125 mA/g-silicon (0.05 C) or 50mA/g-silicon (0.02 C) at 25 ℃ under a pressure of 50 kPa. Moreover, the electrodes were reassembled into a coin-type half cell and full cell, and the charge and discharge test was performed at a constant current of 0.2 C at 25℃.

Figure 1 shows a charge and discharge curves of Silicon❘LiNCM (ternary cathode material) full cell using an electrochemically pre-doped silicon anode and an untreated pristine silicon anode. The anode and cathode electrode balance of the full cell was set to 1.0. The capacity of the cell with pre-doped silicon anode was 150 Ah / kg, which was close to the theoretical capacity of the LiNCM. In contrast, the untreated pristine silicon anode lost 15% of its capacity and was considered to be consumed as irreversible capacity. Furthermore, the cycle degradation was also remarkable.

A TEM image of electrochemically pre-doped silicon is shown in figure 2. In the pre-doped silicon anode under pressure, sea-island-like Li2CO3 particles were distributed in the SEI layer, whereas Li2O was observed in silicon anode pre-doped without pressure. Li2CO3 is thought to be a byproduct during SEI formation by reductive reaction of electrolyte, and to suggest the promotion of SEI formation by pressure. In 7Li MAS NMR and XRD, formation of Li15Si4 was observed in the electrochemically pre-doped silicon anode under pressure. This indicates that the lithiation of silicon is unevenly distributed, it is considered to make anode difficult to degrade even if the charge and discharge cycle is repeated.

3. Elucidation of rubeanic acid cathode reaction mechanism
A slurry of cathode was prepared by mixing crushed rubeanic acid powder as the positive electrode material, acetylene black and vapor grown carbon fiber (VGCF-H) as the conductive materials and carboxymethyl cellulose sodium salt (CMC) and ethylene-vinyl chloride copolymer resin (SE) as the binder in a mass ratio of 74: 16: 2: 4: 4 by adding water. The mass ration was aiming at practical application so that it was set to a high value for competitiveness. The slurry was coated on aluminium foil and dried at 80 ℃ to get electrode. The rubeanic acidwould decompose in carbonate based solvent according to previous research, therefore it was needed to prepare other electrolyte solution of 2M LiTFSI dissolved in sulfolane. Electrochemical properties were measured during galvanostatic cycling with a charge–discharge test system(TOSCAT-3100, TOYO SYSTEM Co., Ltd.) at a constant current of 90 mAh g −1 per rubeanic acidactive agent (0.2C) and voltage range of1.2–4.2 V

The first cycle of general charge and discharge measurement starts fromcharge process. Because the rubeanic acid itself is an insulator, thin electrode (25 μm) with smaller internal impedance. However, when the electrode is made thicker to practical scale of 80 μmwith the same electrode compositions, there occurs a phenomenon that the discharge capacity get decreaseddramatically. the discharge capacity of practical scale of 80 µm thickness rubeanic acid cathode was increased by using pre-treatment with start electrochmical measurement from discharge order.

Based on the results of Raman mapping method measurements, it is predicted that (a) (b) Reductant Oxidant rubeanic acid reacts during charge process of electrode and polymerized together to become another substance. The redox reaction mechanism of the rubeanic acid cathode was characterizedby using cyclic voltammetry (CV) and Raman mapping method. As the polymerization progresses in electrode, the amount of C-S and S-S increases relative to C=S. Because the terminate C=Sbond was redox reaction centers and also the as the polymerization proceeds, the amount of C=Sdecreases accordingly. In other words, the C=S bonds that can be charged and discharged are consumed more and more due to polymerization progressing, and the appearance of the outside is the phenomenon of capacity reduction or degradation.

4. Trial for Si-RA full cell and initiatives for practical use
The purpose of this study is to realize a battery with lighter weight and higher energy density through the application of silicon anode and rubeanic acid cathode. A new organic secondary that finally reaches the energy density approximately 2 to 3 times larger than that of the conventional LIBs. We have succeeded in producing a prototype full cell consisting of electrochemically pre-doped silicon anode under pressure and rubeanic acid cathode to achieve high energy density that is far from conventional LIBs, even at the laboratory level. Based on the Figure 3, the energy density can be calculated according to :

Energy density = Capacity × averaged working potential After substituting corresponding values, we get :
184 Ah/kg×1.84 V = 339 Wh/kg

In comparison, the parameter of conventional LIB ranges from 150 to 200 Wh/kg. Therefore, the prototype full cell have shown enough potential possibility for battery application.

5. Conclusion
I performed electrochemical pre-doping under pressure to improve the properties of a silicon anode and activation process to apply rubeanic acid cathode in practical scale thickness simultaneously as characterization about its reaction mechanism.

I have succeeded in producing a prototype full cell consisting of electrochemicallypre-doped silicon anode under pressure and rubeanic acid cathode to achieve high energy densitythat is far from conventional LIBs, even at the laboratory level. The prototype full cell have shownenergy property 1.7 times ~ 2.3 times enlarged than that of conventional LIB.

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