A lithium-ion battery. Image Credit: Janaka Dharmasena/Shutterstock.com
Lithium batteries play a major role in modern portable electronics, providing a reliable power source to operate electric vehicles, laptop computers, and mobile phones.
Researchers are now working to develop lithium technology on a large scale as an electricity storage option to support the National Grid during peak demand. However, energy density remains a major issue for lithium batteries and raises the question of how can more power be packed into a lighter and smaller space?
In the search of an improved lithium-ion battery, studies have demonstrated that cells consisting of a ‘lithium-rich’ cathode have roughly 50% higher energy density than traditional lithium cells.
The use of IR and Raman spectroscopy-based techniques has been useful in the analysis of the chemical processes in lithium batteries, specifically the role played by oxygen in cathode oxidation reactions.
Improving lithium-ion batteries is seen as a major challenge in achieving the mass uptake of electric car technology. Image Credit: Matej Kastelic/Shutterstock.com
Understanding Electrochemical Processes
An understanding of the internal processes that take place as part of electrochemical cycling is crucial to build a lithium battery that is truly efficient and safe. The electrode/electrolyte interface is especially important due to its ability to control the kinetics and stability of the cell affecting power, safety, and cycling stability.
Surface characterization techniques like IR spectroscopy are valuable due to the lack of adequate knowledge on the interfacial reactions of lithium-ion batteries.
In situ IR can be used to analyze the electrode/electrolyte interface during the batteries operation. This is considered as a powerful method because of its ability to provide data continuously during electrochemical cycles. This ability eliminates the generation of erroneous data from relaxation and contamination.
The use of an attenuated total reflectance (ATR) accessory featuring a reaction cell, such as the Golden Gate™ from Specac, allows in situ FT-IR to be performed in the laboratory. Vibrational spectroscopy (Raman and IR) is a robust analytical tool for the in situ analysis of surface processes taking place in lithium-ion batteries.
Using IR and Raman spectroscopy as complementary methods allows IR to be used to investigate the interface between the organic electrolyte and lithium and Raman to study structural changes in the electrode material.
An example of electrode analysis using in situ FTIR. In this experiment an electrochemical cell using a platinum electrode and grated graphene electrode is being analysed. Image Credit: Ya-Qing Bie/Nature Comms
Observing Dendrite Formation and Electrode Degradation
When a number of charge/discharge cycles is experienced by a lithium battery, especially at a fast rate, the lithium electrode surface degrades. This degradation leads to the formation of small lithium fibers called dendrites, which lower the capacity of the battery.
Dendrites continue to form over the service life of a battery, lithium fibers grow from the lithium electrode surface and extend across the electrolyte until they come into contact with the other electrode. This development leads to battery short-circuiting, which can result in overheating and could possibly cause a fire.
According to ongoing research, the formation of dendrites is due to seed crystal contaminants present in the electrolyte, offering a focus for the development of electrode subsurface structures that lead to dendrites. If it is possible to resolve the dendrite, lighter, higher energy density lithium batteries that are capable of using lithium anodes can be produced.
The solid–electrolyte interphase and the degradation that occurs following constant cycling and reformation are other issues that deteriorate battery performance.
The reaction cell accessory for Specac's Golden Gate™ allows for in situ electrochemical analysis
Using Infrared to Study Electrodes in Practice
Surface features on lithium and carbon electrodes can be effectively analyzed using surface sensitive FTIR spectroscopy. Providing specific data about chemical bonds and functional groups is the strength of this technology, which means that it can be employed for the determination of transient lithium species.
Also, this non-destructive technique can be employed for in situ electrode analysis when an extensive library of IR spectra for common lithium species is available. A recent study analyzed the process of dendrite formation with FTIR and proposed a solution to address dendrite formation and unstable electrolyte problems.
An ionic liquid, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide, consisting of a specific mixture of lithium salts was used in this study to treat the electrodes before charging and cycling. A durable and lithium ion-permeable solid–electrolyte interphase (SEI) was provided by this process, lasting for more than 1000 cycles with a Coulombic efficiency of over 99.5%.
The use of IR to investigate this method demonstrated low dendrite formation and a low rate of change in the electrolyte during the experiment. IR is an ideal tool to analyze the formation and subsequent evolution of the SEI layer in lithium batteries in situ. However, polished electrode surfaces are preferred in test cells because IR and Raman techniques are based on reflectance measurements.
A stable uncontaminated environment and constant temperature are crucial for IR spectroscopy in electrochemical cells and can be achieved using specially designed ATR cells, where the electrochemical experiment is carried out.
Infrared Accessories for Electrochemical Analysis
Specac offers a high performance single reflection monolithic diamond stage called the Golden Gate™ ATR accessory, which is regarded as the gold standard for application in spectroscopic electrochemical experiments due to the availability of a wide variety of sampling options for material analysis. With this feature, analysis can be carried out at various temperatures in a reaction cell.
Minimal sample preparation is required for the Golden Gate™ ATR. This accessory is suitable for high-throughput qualitative and quantitative study of electrolyte liquids, solids, and gels with transmission spectroscopy.
Specac also supplies a reaction cell variant for in situ electrode IR measurements at pressures of up to 3000 psi, and if needed, temperatures up to 200°C.
Find out more about the Golden Gate in situ Reaction Cell
The Golden Gate™ from Specac
Specac offer another ATR accessory, the Quest™ for more routine high throughput analysis over an extended wavelength. This ATR accessory is equipped with Synopti-Focal Array technology, all reflective gold-coated optics, and four interchangeable crystal pucks, making it ideal for the analysis of samples in the mid- and far-infrared range.
The Quest™ from Specac
This information has been sourced, reviewed and adapted from materials provided by Specac.
For more information on this source, please visit Specac.