[Paper Controlling the Activation Energy for Single-Ion Diffusion through a Hybrid Polyelectrolyte Matrix by Manipulating the Central Coordinate Semimetal Atom. Victoria C. Ferrari, Raphael S. Alvim, Thiago B. de Queiroz, Gustavo M. Dalpian, Flavio L. Souza. J. Phys. Chem. Lett. 2019, 10, 24, 7684-7689. https://doi.org/10.1021/acs.jpclett.9b02928.]
Solid electrolyte for safer and faster-to-charge batteries
Our cell phones, laptops and tablets, as well as the electric cars that are beginning to transit planet Earth, would not exist without rechargeable lithium-ion batteries. These devices were the subject of the 2019 Nobel Prize in Chemistry, which recognized the work done in the United States, United Kingdom and Japan by three scientists in the 1970s and 80s, mainly focused on the development of the materials that compose the electrodes of these batteries.
However, there are still challenges to continue improving the performance and safety of lithium-ion batteries and to adapt this technology to new applications. One of these challenges refers to the development of solid materials for the electrolyte of these batteries, as an alternative to the liquid or gel-like materials which are currently widely used, which present a greater risk of causing accidents, such as the explosions of smartphones, widespread in the media. Located in the middle of the electrodes, the electrolyte has an important function of promoting the displacement of the lithium ions (only them, not the electrons) in their back and forth between the electrodes. For this reason, the electrolyte material must be a good ionic conductor – a condition that can be more difficult to achieve in solid materials.
In an article recently published in The Journal of Physical Chemistry Letters (impact factor = 7,329), a Brazilian scientific team presented an important advance in the development of solid materials for electrolytes that can be used in lithium-ion batteries and other electrochemical devices (those that produce electricity from chemical reactions and vice versa) and electrochromic devices (those that have a color or opacity change when voltage is applied to a material, such as smart windows). Using a simple and economical manufacturing method, which can be carried out at an industrial scale (the hydrolytic sol-gel), the researchers produced a solid polymer-based material that demonstrated exceptionally good performance as an ionic conductor. “The low amount of energy required to activate the ion in this material to move and its high ionic conductivity at room temperature may drastically reduce the charging time of the batteries,” specifies Professor Flavio Leandro de Souza, professor at the Brazilian Federal University of ABC (UFABC) and leader of the work.
This Brazilian electrolyte is a light and flexible film from the polyethylene family, with an aspect very similar to the material of the transparent films and polyethylene bags used on a daily basis. “From an aesthetic point of view, this material can provide lighter devices with different shapes,” explains Professor Souza. “In terms of safety, it brings unprecedented improvement, as it does not contain toxic materials in its composition and, because it is in a solid state, there is no risk of leakage in the event of breakage or fracture, also avoiding explosions usually observed nowadays, causing many devices to be banned in air travel.”
The secret behind the good performance of this electrolyte regards the presence of a germanium atom in the center of the polymeric structure, called the “coordination atom.” In fact, this metallic atom modifies the polymeric chain, reducing its spontaneous vibrations and thus attacking the main disadvantage of polymers as ionic conductors: the coupling of the movement of the lithium ion to the movement of the polymeric chain.
Beginning of the story: an off-plan experiment
The initial idea of the work dates back to the years 2001 to 2006, when Flavio Souza was a master’s student and later in his doctorate in Materials Science and Engineering at the Brazilian Federal University of São Carlos (UFSCar). During this period, under the guidance of Professor Edson Leite, Souza was trying to produce a silicon matrix with metallic nanoparticles, through a process that had the formation of a polymer as an intermediate step which final destination was the burning stage in a common oven. When Souza observed the solid, transparent and easy-to-manipulate polymer that had formed, he decided, out of sheer curiosity, to save the material and subject it to electrical characterization to check whether it was able to conduct nickel. “Nothing happened, but I showed it to my advisor, who suggested replacing the nickel with a lithium salt. To my surprise, this was a conducting material. And that is when it all started,” reports the scientist. This first material, a polymer that contained a silicon atom in the center of its structure, allowed lithium ions to move through its structure without much interference from the movements of the polymer chain, and for this reason it was classified as a fast ion conductor.
Years later, as a professor at UFABC and coordinator of the Laboratory of Alternative Energy and Nanomaterials, Souza decided to return to this subject and propose a challenge to a young student of Energy Engineering, Victória Castagna Ferrari, who had sought him out for undergraduate research. “The challenge proposed and accepted was to try to further improve this type of material for application in lithium ion batteries and electrochromic windows and to answer some scientific questions,” says Professor Souza. “Victória is a brilliant student, quickly showing she could take this challenge to a very high level,” he says.
The work was developed over two years of scientific initiation for Victória as a UFABC scholarship holder and two more years as a master’s student in Nanoscience and Advanced Materials with a CAPES scholarship, always under the guidance of Professor Souza.
During this period, Souza and his student wanted to answer a series of scientific questions. This included using several experimental and theoretical techniques and relied on the collaboration of other UFABC researchers: Professor Thiago Branquinho de Queiroz in experiments of solid state nuclear magnetic resonance, and professor Gustavo Martini Dalpian together with postdoctoral fellow Raphael da Silva Alvim in computer simulations.
The team first investigated whether the replacement of the silicon atom by another element (in this case, germanium) would influence the mobility of lithium ions in the material. The results were exceptional. “This substitution increased the conductivity by two orders of magnitude and reduced the activation energy by 50%,” says Souza. In fact, the experiments showed that the energy needed to set the lithium ion in motion was 0.27 eV (electron volts) in the silicon polymer and 0.12 eV in the germanium polymer. “This value undoubtedly sets a record as the lowest obtained for a solid polymeric electrolyte in the literature,” says Souza. In the scientific literature, Souza contextualizes, the value oscillates between 1 and 0.5 eV.
Further research efforts were then made to understand why germanium had made the polymer a better ionic conductor. The team was able to understand in detail the structure of polymers coordinated by silicon and germanium, the movement of the polymeric matrix, the movement of lithium ions and the interaction between them. The experiments and simulations confirmed that the exchange of silicon for germanium does not change the type of polymer (the fundamental nature of the structure is the same), but it does change the electronic structure of the polymer chain, changing the location of the most relevant orbitals and further reducing its spontaneous vibrations, which affects the interaction of lithium ions with the polymer chain.
This work was supported by Brazilian agencies Capes and CNPq (federal) and Fapesp (state), and used multi-user equipment from UFABC and the National Laboratory for Scientific Computing (LNCC).
To understand in detail how lithium ion batteries work, we recommend this video: