As the 4th Industrial Revolution rapidly progresses and the convergence of technologies is increasing, digital devices increasingly rely on energy solutions. Over the past decades, energy storage devices such as batteries and capacitors have been developed, and the scope of energy storage has expanded significantly, from many mobile devices to electric vehicles and grid-scale energy storage. In the future, it is necessary to develop high-energy-density and high-efficiency energy storage devices to form a new digital ecosystem by analyzing the huge amount of data accumulated each day. Therefore, in this study, through a preliminary study of the polymer materials used in secondary batteries, we sought a way to significantly improve the energy density of energy storage system using functional polymers.
In the first part, the study aimed to improve the energy density of the cathode. In lithium-sulfur batteries, sulfur turns into lithium polysulfide when discharging, which dissolves in the liquid electrolyte and causes the sulfur to escape from the cathode. This eventually leads to a loss of energy that the battery can generate. Therefore, we proposed to synthesize a new cross-linker with four functional groups (two double bonds, two aldehyde groups) and polymerize it with sulfur to retain the sulfur at the cathode. Qualitative analysis of the new cross-linker was performed using NMR and IR spectroscopy, and the battery was prepared by using it in inverse vulcanization reaction with sulfur to prepare the cathode. The prepared battery was compared to a conventional sulfur battery by galvanostatic charge and discharge test, demonstrating improved capacity retention. As another way to increase energy, we hypothesized that the polymer binder could also act as a cathode active material. A polymer binder containing a sulfur component, i.e., a disulfide, which is the active material of lithium-sulfur batteries, was prepared and the batteries were manufactured without any cathode active material. Through the use of electrochemical analysis of various battery combinations, we were able to calculate the specific capacity of the sulfur (disulfide group) contained in the polymer binder and confirmed that it can act as a cathode active material.
Since the polymer network used in this study is a PEM having high ionic conductivity and a wide electrochemical window, it was fabricated as an electric double layer capacitor and tested for another high-power density application. The electrochemical performance was observed according to various variables such as active material, conductive material, and plasticizer, and the energy density and power density were evaluated, and the long-term stability was tested to provide a reference for not only high energy devices but also high-power device applications.
In the second part, we looked to improve the energy density of the overall battery by developing a silicon anode active material. Although silicon has a high specific capacity, it is difficult to use as a cathode material due to its volume expansion, which causes the particles to pulverize and become useless within a few cycles. Therefore, we assumed that it is necessary to stably control the expansion and contraction of silicon, so silicon particles were enclosed inside and outside by carbon materials and monitored its electrochemical performance. Galvanostatic charging and discharging tests were conducted to verify the effect of the novel silicon-carbon composite on capacity retention. In another approach, it was assumed that the stable expansion and contraction of the silicon could be controlled by external factors. To validate this assumption, we designed a new liquid crystalline elastomer that shrinks with increasing temperature and applied it as a polymer binder to increase the contractility of the electrode.
In this dissertation, a cross-linker and a polymer binder for anode active materials were developed to improve the energy density, and the new polymer binder was applied to the application of electric double layer capacitors to investigate its feasibility. In addition, a new formation of silicon-carbon composite was synthesized as a solution to suppress the volume expansion of silicon, the next generation anode material, and a liquid crystalline elastomeric binder was examined as another possible solution. The research described in this thesis is the first proposal of solutions to move from current energy storage to the next generation of storage, and is expected to open a new horizon for the development of high-power, high-energy energy storage.