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Designing high-performance batteries

Source: EnvatoElements. LSQNYH8DAK

Aenert. Research Laboratory news
Developing high-energy and efficient battery technologies is one of the main aspects when it comes to advancing the electrification of transportation and aviation. However, sometimes it can take years before battery innovations are ready for large-scale application.
One of the most important aspects of a battery is the electrolyte. Electrolytes are the battery components which can exchange ions back and forth between the two electrodes in a battery thus producing electricity. This causes it to charge and discharge. For today’s lithium-ion batteries, electrolyte chemistry is relatively well-defined.  There are several generic types of electrolytes, which engineers tweak to suit particular applications. They usually comprise soluble salts, acids, or other bases.  Alternatives battery systems can contain liquid gel, or dry formats. Some types may also be polymers, solid ceramics, or molten salts.

Now (2023), a team of scientists at the Argonne National Laboratory discovered a sort of​“cooperative” behaviour which can occur among complex mixtures of components in battery electrolytes. They found that combining two different types of anions with cations can significantly improve the overall battery performance and hoped that careful selection of ion mixtures would help battery developers to precisely tailor their devices to produce desired performance characteristics.

The scientists focused on a type of battery called the multivalent battery. These batteries use cations such as zinc, magnesium and calcium having a charge of +2 as opposed to +1 for lithium ions. As they can move more charge, multivalent batteries can also store and release more energy, which makes them attractive for use in electric vehicles. However, nowadays most multivalent batteries do not perform well, as they are unstable and degrade. As a result, the electrolytes are unable to efficiently transport cations, diminishing the battery’s ability to generate and store electricity.

As one of the main candidates for a multivalent battery the scientists looked at one that consisted of zinc metal.  Their aim was to characterise how the components interacted and what structures formed when zinc cations were combined with two different types of anions in the electrolyte. They also wanted to find out how key aspects of battery performance was impacted by interactions such as metal deposition and stripping at the anode.

Therefore, a laboratory-scale battery system was designed comprising an electrolyte and zinc anode. The electrolyte initially contained zinc cations and an anion, called TFSI, with a very weak attraction to the cations. Chloride anions were then added to the electrolyte.

The researchers analysed the interactions and structures among these ions with the use of three complementary techniques: X-ray absorption spectroscopy, involving probing the electrolyte with synchrotron X-ray beams and measuring the absorption of the X-rays; Raman spectroscopy, which illuminates the electrolyte with laser light and evaluates the scattered light: Density functional theory which can simulate and calculate the structures formed by the interactions among the ions in the electrolyte.

The team found that in the presence of chloride TFSI anions would readily pair with zinc cations. This can affect the rate at which the cation can be deposited as metal on the anode during charging or subsequently stripped back into the electrolyte during discharge. Faster electrode reactions which need less energy can convert chemical energy into electricity more efficiently.

The team repeated these experiments with two other ion mixtures. One of them contained bromide ions instead of chloride, and the other, iodide ions instead of chloride. The outcome was similar to what happened with chloride: bromide and iodide induced TFSI anions to pair with zinc cations. With all three combinations of ions, the researchers measured the electrochemical activity at the interface between the electrolyte and the anode. Bromide and iodide were more active than chloride because they held zinc cations less strongly. This can enable a zinc-ion battery to charge and discharge more quickly.

Image: Molecular dynamics simulations and characterisation of electrolytes. Snapshots of local structure evolution for a 1 m Ca(NO3)2 electrolyte, b saturated Ca(NO3)2 electrolyte, and c aqueous gel electrolyte based on MD simulation at 10 ns. d The hydrogen bonds and the percentage of water molecular coordinated with Ca2+ for three electrolyte samples based on MD simulation at 10 ns. The hydrogen bond between the Ca2+–H2O complex and PVA repetitive unit is shown in the inset. The green, red, white and grey balls represent Ca, O, H and C, respectively. e Raman spectra of the 1 m, 2 m, 5 m and saturated Ca(NO3)2 aqueous electrolytes, and aqueous gel electrolyte.

Source: Xiao Tang, Dong Zhou, Bao Zhang, Shijian Wang/ Universal strategy towards high–energy aqueous multivalent ion batteries/ Research Square January 2021/ DOI:10.21203/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

What fascinated the scientists most in the course of the experiment was the cooperation that occurred among different types of ions in an electrolyte. The presence of the weakly attracting anions seemed to reduce the amount of energy needed to pull zinc metal out of solution. On the other hand, the presence of the strongly attracting anions was found to decrease the amount of energy needed to put the zinc back in solution. In total, less energy was needed to drive this back-and-forth process and enable a constant flow of electrical current.

Scientists are constantly working on inventing new battery technologies. In 2021, an aqueous multivalent ion battery by using wide–window super–concentrated aqueous gel electrolytes, high–capacity sulfur anodes, and high–voltage metal oxide cathodes was developed. This aqueous battery chemistry was responsible for long–lasting multivalent ion batteries with increased high energy density, reversibility and safety. As a demonstration model, a calcium ion−sulfur||metal oxide full cell was built which showed a high energy density of 110 Wh kg –1 and excellent cycling stability. Molecular dynamics modelling and experimental investigations showed that side reactions could be reduced through suppressing water activity and formation of a protective inorganic solid electrolyte interphase in the aqueous gel electrolyte. The unique redox chemistry could also be extended to aqueous magnesium ion and aluminum ion−sulfur||metal oxide batteries.

In 2023, scientists developed an ion‐percolating electrolyte membrane which could act as a stable Li ⁺ reservoir to ensure a near‐single Li⁺ transference number (0.78) and homogenise Li⁺ migration to reduce dendrite growth. This afforded the Li//LFP cell an ultrahigh average Coulombic efficiency (ca. 99.97%) after cycling for nearly half of a year and excellent cycling stability when pairing with LiCoO2 with limited Li amount and LiNi0.8Mn0.1Co0.1O2 . These characteristics demonstrated significant potential of utility value for the ion‐percolating electrolyte.

Image: (A) Schematic diagram for the structure of ion-percolating electrolyte membrane. (B) Photographs of IPS at flat and bending conditions. (C) Side-view SEM image of IPS (D) FTIR spectrum of PVDF, IPS and attapulgite. (E) Heat shrinkage tests of PE and IPS before and after treating at 1400C. (F) TGA curves of PE and IPS

Source: Yu-Ting Xu, Sheng-Jia Dai,  Xiao-Feng Wang, Yu-Guo Guo, Xian-Xiang Zeng, Xiong-Wei Wu/ An ion-percolating electrolyte membrane for ultrahigh efficient and dendrite-free lithium metal batteries/ InfoMat, November 2023/ DOI:10.1002/inf2.12498/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) 

The recent discoveries in the field of multivalent battery science have several potential benefits: Multivalent batteries use abundant elements supplied through stable, domestic supply chains. The same cannot be said of lithium which is less abundant and has an expensive, volatile international supply chain. With the discovery of this behavior in multivalent batteries, many of the aforementioned problems found in this battery type can be eradicated and more stable electrolytes for advanced batteries designed. With more precise control of component interactions, battery developers can enhance cation transport, increase electrode stability and activity, and enable faster, more efficient electricity generation and storage.

The next step in the research will be to investigate how other multivalent cations like magnesium and calcium interact with various anion mixtures. Another new line of research will deal with the use of machine learning to rapidly calculate the interactions, structures and electrochemical activity that occur with many different ion combinations. This would accelerate the selection of the most promising combinations which could then be analysed in the laboratory.

By the Editorial Board