Prof. Minhaeng Cho and his team identifies the core principle of
next-generation water-based batteries
Lithium-ion transport mechanism is newly defined using cutting-edge spectroscopy.
The resulting safer batteries are expected to be widely applied.
▲ First author, PhD candidate, Joonhyung Lim (left); corresponding author,
Minhaeng Cho (middle); corresponding author, Kyungwon Kwak (right)
Research team at the Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (led by Prof. Minhaeng Cho, Korea University) revealed the core principle that will accelerate the commercialization of next-generation batteries. In detail, the team identified the mechanism of non-flammable and environmentally friendly lithium-ion batteries (LIBs) that use water as electrolytes1).
1) electrolyte: a substance that contains dissociated ions and enables current flow by transporting ions
LIBs are widely used in most of the electronic devices we carry every day, including smart phones and laptops. LIBs repeat the process of charging and discharging, using an organic solvent as electrolyte for travel between the anode and cathode. Such batteries are preferred because of their light weight coupled with high energy density. The problem is that the organic solvent used as an electrolyte is highly flammable, and is thus associated with environmental risk.
Consequently, investigations into next-generation batteries featuring improved stability are necessary, and secondary batteries using water-based electrolytes have received attention as useful alternatives. The role of electrolytes in secondary batteries is to transport lithium ions (Li+) between the anode and cathode. For the electrolyte to function stably, it must endure electrolysis under a high voltage environment while promptly transporting Li+. However, obstacles have prevented the clear identification of the roles of water as a solvent for transporting Li+.
Previous studies have proved that super-concentrated salt2) is required within the water-based electrolyte in order for it to endure electrolysis and remain stable. Paradoxically, concentrated salt increases the viscosity of the electrolyte, which acts as a barrier to Li+ transport. Thus, researchers have been studying the conditions under which not only is concentrated salt maintained, but also Li+ is promptly transported.
2) salt: a compound of an anion and a cation held together by the electrostatic forces; the salt used in Li+ batteries include lithium as the cation
The research team consisting of Prof. Minhaeng Cho, Prof. Kyungwon Kwak, and PhD candidate Joonhyung Lim, Department of Chemistry, College of Science, found a solution to this paradox using femtosecond IR pump−probe spectroscopy3) and two-dimensional IR spectroscopy4). As a result of observing the microscopic structure of the water-based electrolyte, the team discovered that the salt and water is not evenly distributed, as might be generally expected. Within the electrolyte, salt is concentrated in some regions and layers of water flow through them, like tidal currents flowing through an island. Furthermore, the research team conducted computer simulations to identify the role of water. As a result, the layers of water within the electrolyte were found to play two important roles in Li+ transport. First, water surrounding the salt clusters mitigates the electrostatic interactions between salt anions (TFSI-) and lithium ions (Li+). Second, water from other areas form nanometric channels to accelerate the transport of Li+. In short, water becomes the barrier against the electrostatic interactions, which disturb the transport of Li+, and at the same time, act as wires to facilitate its rapid transport.
3) femtosecond IR pump−probe spectroscopy: a type of non-linear spectroscopy usually used to observe the rotational motion or energy transport process of within a molecule or among molecules
4) two-dimensional IR spectroscopy: a spectroscopic device that provides 2D images of correlated molecular vibrations; offers structural information on real-time chemical dynamics with femtosecond time resolution
It is anticipated that these research findings will serve as guidelines for solving the paradoxical issues regarding the development of next-generation secondary batteries. In particular, the team discovered that certain anions (salt) form clusters in highly concentrated aqueous solutions and create “ion network structures”. Such findings are expected to guide the development of water-based lithium secondary batteries.
“Unlike previous studies, which focus on lithium secondary batteries only from a macroscopic perspective, this research successfully identifies the relationship between microscopic ion network structures and the macroscopic features of electrolytes.” said Prof. Minhaeng Cho, the head of Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science, Department of Chemistry, College of Science. He added, “Through this research, we discovered that the performance of lithium secondary batteries may be greatly influenced by microscopic molecular structures.”
Currently, the Center for Molecular Spectroscopy and Dynamics is preparing equipment to observe the microscopic molecular structure of electrolytes under high-voltage environments, that is, under conditions similar to those in actual working batteries. This upcoming project is anticipated to provide details of Li+ transport structures and the molecular structures of electrolytes in the operating environment.
The research results were published on Oct. 25 in the world-leading chemistry journal, Journal of the American Chemical Society (JACS, IF 14.357).
[Figure 1] Spectroscopic test of water-based lithium-ion electrolyte
As a result of the two-dimensional IR spectroscopy of the structure and dynamics of water-based electrolyte, it was observed that layers of pure water (shown in blue) and salt ion networks (shown in red) coexist and form an “ion network structure”. Through this research, it was concluded that the microscopic structure greatly affects Li+ transport.
[Figure 2] Structure of water-based electrolyte
The results of the spectroscopy proved that water molecules (shown in blue) and salt anions (shown in red) do not mix within water-based electrolytes but form an “ion network structure”. This structure remains unchanged regardless of increased concentration levels (a, b). Due to such a heterogeneous structure, nanometric water channels are formed within electrolytes (c). Li+ (shown in gray) is much more mobile through the water channel than near the salt clusters (d, e).