Prof. Hiroshi Nishihara of the University of Tokyo Graduate School of Science announced in February 2020 that he had developed a pressurized electrolytic pre-doping technology that could increase the capacity of secondary batteries. Using the developed technology, the capacity of secondary batteries has increased by 20%. It was confirmed that the capacity reduction due to repeated charge and discharge was also suppressed.
In the conventional secondary batteries, side reactions related to the electrolytic solution and the additive occur at the first charge / discharge. This becomes an irreversible capacity, and there is a problem that the capacity of the battery becomes smaller than the calculated value. As a method of solving this, electrochemical pre-doping in which a negative electrode is reacted with lithium before assembling a secondary battery has been studied. However, it may take several hours for the electrolytic reaction, and it has not been practically used until now.
Nishihara’s laboratory has now demonstrated that lithium can be pre-added to large concentrations at high currents by performing electrochemical pre-doping of the negative electrode in a pressurized environment.
In the experiment, the capacity and voltage during charging and discharging were measured using a secondary battery consisting of an electrochemically pre-doped silicon anode and a cathode material (LiNMC). It was compared with the secondary battery.
As a result, the capacity of the secondary battery using the pre-doped silicon negative electrode was 150 Ah / kg, which was close to the design value of the active material. On the other hand, the capacity of the secondary battery using the silicon anode without pre-doping was 125 Ah / kg, and the capacity was reduced by about 20%.
When charge and discharge were repeated, the capacity decreased by 15 Ah / kg by the fifth cycle. From these facts, it was found that a high capacity and a long life of the secondary battery can be achieved by the pressurized electrolytic pre-doping.
Using the newly developed technology, a solid electrolyte interface (SEI) layer is formed on the surface of silicon particles by electrochemical pre-doping.
In particular, in a silicon negative electrode that has been pre-doped in a pressurized environment, an electrolyte solution and an additive react with each other to form island-like SEI containing Li2 CO3 . On the other hand, the silicon anode pre-doped in a non-pressurized environment mainly contained Li 2O.
Furthermore, ultra-high-speed MAS solid state nuclear magnetic resonance measurement and X-ray diffraction measurement confirmed that stable Li 15 Si 4 was formed on the electrode subjected to pressure electrolytic pre-doping.
From this, it was found that the electrode was hardly deteriorated even when charge and discharge were repeatedly performed.