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Supercharge your battery research – Part 2

Supercharge your battery research – Part 2

Battery technology has come a long way since the rudimentary voltaic pile was developed over two centuries ago. The breakthrough innovation of the lithium ion battery and its subsequent improvements has increased the use and accessibility of electronics, particularly in the consumer market. Electronics are more portable, affordable, and thanks to rechargeable or secondary batteries, they are becoming more sustainable.

Expansion of application possibilities is another reason that energy storage research, particularly batteries, is currently a hot topic. For example, only a decade ago drones were the domain of the military industrial complex, and now a drone with a camera is a standard part of nearly any successful photographer or influencer’s gear. Thanks to an improved battery life and more cost-efficient materials, a drone now has an affordable price tag for a larger segment of the civilian population.

This kind of disruption is happening in larger, more profitable markets as well. Tesla, a newsworthy brand thanks to their technological innovations and public relations, still has a small, albeit increasing, market share of the overall automotive market. Their success has challenged other established brands to recognize that a change from conventional combustion engines can be lucrative. Volvo and Ford are committed to be «fully electric» by 2030 [1]. General Motors (GM) has committed to not only be electric by 2035, but for their business to be carbon neutral by 2040 [2].

The automotive market is a high profile example of an industry that will drastically retool their sector—from manufacturing to sales—and this will happen across many other industries as there is a greater focus on climate change and renewable energy sources from governments and consumers alike. Accurate and scalable R&D will be required to make these transformations possible, and the hunt for improved energy storage solutions is at the heart of these changes.

Electrochemistry was the key to the discovery of energy storage and is the natural technique of choice for future innovations.

Electrochemical characterization techniques for lithium ion batteries

In part one of this series, we introduced various techniques to analyze the composition and purity of electrode materials and lithium salts, in addition to accurate water content determination in the battery materials.

In this article we highlight techniques that will allow the characterization of multiple attributes of the electrochemical behavior of Li-ion batteries using a high precision potentiostat/galvanostat. In some cases, the difference between techniques is due to performing the experiment in a different mode (i.e., potentiostatic or galvanostatic), and the additional information gathered provides a more complete picture of battery behavior.

Galvanostatic Intermittent Titration Technique (GITT)

One of the first techniques available to researchers exploring the properties of battery electrode materials is the Galvanostatic Intermittent Titration Technique (GITT). Usually conducted on a half-cell, this technique is a series of current perturbations followed by a relaxation time, which provides information about the thermodynamic properties and electrode materials including the critical diffusion co-efficient. All of this information gives a better understanding of the electrochemical behavior that can be expected by the materials.

If you’re looking for more information on this subject, download our free application note AN-BAT-003.

Potentiostatic Intermittent Titration Technique (PITT)

Potentiostatic Intermittent Titration Technique (PITT) is similar to the GITT technique detailed above, but the PGSTAT is operated in potentiostatic mode. A series of potential step perturbations is applied to the system, and current is measured as a function of time. Both GITT and PITT are capable of accurately determining the diffusion coefficient.

When using a PGSTAT in galvanostatic mode you can also characterize the performance of Li-ion batteries by using different current rates and charging and discharging during various cycles, known colloquially as «cycling». With this technique researchers can understand the rate performance of the Li-ion battery, its capacity, and the associated power and energy density. This is the most commonly used technique in battery research. A constant current constant voltage (CCCV) procedure is usually applied in order to make sure that a battery is fully charged, while avoiding any battery overcharge.

Learn more about characterizing the performance of lithium ion batteries with cycling by downloading our free application note AN-BAT-002.

CCCV is the industry standard for Li-ion battery charging, and PGSTAT operates in both galvanostatic and potentiostatic mode for this measurement. Galvanostatic cycling is performed within a safe potential window at which the electrolyte is stable. Any slight deviation from the potential cutoff may result in poor cycle life.

Voltage profile of a 18650 Li-ion battery, cycled at ~ C/15 (left), and its corresponding dQ/dV versus V plot (right). The corresponding peaks and plateaus are marked in the figures.

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical Impedance Spectroscopy (EIS) provides additional data and therefore greater insight into the battery’s behavior and potential performance by using galvanostatic charge/discharge cycling and then adding in the most powerful technique that is being used extensively in current battery research. With EIS, the highly dynamic behavior of a battery and the diffusion of ions at the interfaces can be characterized. In a single experimental procedure encompassing a broad range of frequencies, the influence of the governing physical and chemical phenomena may be isolated and distinguished at a given frequency range and state of charge. With EIS it is possible to measure the internal impedance of the battery and model it using the equivalent circuit and understand the contribution of the battery components to the total impedance of the cell.

For EIS determination of batteries it is important to use 4-terminal sensing to avoid the contribution of wires to overall impedance. This is important for any low impedance electrochemical system. Learn more about this research by downloading our free Application Notes AN-EC-013 and AN-BAT-008.

With EIS, it is possible to determine the through-plane tortuosity of battery electrodes, which along with overall electrolyte conductivity, the transference number of a Li-ion of battery electrolyte, and diffusion coefficient of electrolyte gives a good indication of the practicality of certain battery chemistry for high power applications. In addition, the mass transport limitation of the battery separator and its ionic conductivity plays a crucial role in overall performance of the batteries.

By determining the MacMullin number, researchers can determine the quality of the separators for their application in certain Li-ion cells.

Nyquist plot: Negative imaginary part of impedance as a function of the real part of impedance for an 18650 cell.

Download our free Application Notes below for more information on these subjects.

Download our free white paper «A Guide to Li-ion Battery Research and Development» written by electrochemical instrument innovators at Metrohm Autolab. This white paper provides additional information about applicable electrochemical techniques and provides useful definitions to terminologies that are relevant to Li-ion battery research and development.

 

Free white paper:

A Guide to Li-ion Battery Research and Development

Post written by Dr. Reza Fathi, Product Specialist at Metrohm Autolab, Utrecht, The Netherlands.

Supercharge your battery research – Part 2

Supercharge your battery research – Part 1

Replacing traditional fuel-powered vehicles with battery-powered options is essential to reduce carbon dioxide (CO2) emissions. This greenhouse gas results from the combustion of fossil fuels, therefore limiting its input into the atmosphere will also influence global warming. Battery research therefore focuses on discovering new materials with higher energy and power density as well as a more efficient energy storage.

Various critical parameters need to be determined to develop viable new batteries. In this first of two blog posts, I want to highlight a few of the analytical parameters which can be determined using high precision analytical instruments from Metrohm and provide some free downloads in this research area.

What’s in a lithium battery?

Today, lithium ion batteries are the most common rechargeable batteries available on the market. A battery consists of an anode (negative pole) and cathode (positive pole). An electrolyte facilitates charge transfer in the form of lithium ions between these two poles. Meanwhile a separator placed between anode and cathode prevents short-circuits. An example cross section can be seen in Figure 1.

Figure 1. Cross-section illustration of a lithium ion battery. While the battery is being charged, lithium ions migrate from the cathode to the anode (from right to left), and during discharging they move from the anode to the cathode (from left to right).

The anode is made from graphite containing intercalated lithium applied to a copper foil, while the cathode consists of metal oxides dotted with lithium ions applied to an aluminum foil. The most common transition metals used in cathode materials are cobalt, nickel, manganese, or iron. The electrolyte is an anhydrous aprotic solvent containing a lithium salt (e.g., lithium hexafluorophosphate) to facilitate charge transfer. The separator is prepared from a porous material, acting as an insulator to prevent short-circuits. The composition of all of these components has a significant influence on the battery characteristics.

After this brief overview about the composition of a lithium battery, let’s take a look at selected key parameters and how they can be analyzed.

Water content in battery raw materials

Lithium-ion batteries should be free of water (concentration of H2O less than 20 mg/kg), because water reacts with the conducting salt (e.g., LiPF6) to form toxic hydrofluoric acid. Sensitive coulometric Karl Fischer titration is the ideal method for determining water content at trace levels. Water determination for solids is carried out using the Karl Fischer oven method – the residual moisture in the sample is evaporated and transferred to the titration cell where it is subsequently titrated. The working principle and advantages of the KF oven method are described in more detail in our blog post «Oven method for sample preparation in Karl Fischer titration».

For more details on how to carry out the water determination in one of the following battery components, download our free Application Bulletin AB-434:

 

  • raw materials for the manufacture of lithium-ion batteries
  • electrode coating preparations (slurry) for anode and cathode coating
  • the coated anode and cathode foils as well as in separator foils and in packed foil layers
  • electrolytes for lithium-ion batteries

Transition metal composition of cathode materials

The cathode of a lithium-ion battery is usually made from metal oxides derived from cobalt, nickel, manganese, iron, or aluminum. To produce the cathode, solutions containing the desired metal salts are used. For an optimized production process, the exact content of the metals present in the solution must be known. Additionally, the metal composition within the obtained cathode material should be determined. Potentiometric titration is a suitable technique to determine the metal content in starting solutions and the finished cathode materials.

The following mixtures of metals or metal oxides can be analyzed potentiometrically:

  • Nickel, cobalt, and manganese in solutions
  • Nickel, cobalt, and manganese in cathode materials such as cobalt tetraoxide (Co3O4), lithium manganite, or lithium cobaltite

For more details about the potentiometric analysis of a mixture of nickel, cobalt, and manganese download our free Application Note AN-T-218.

Analysis of lithium salts

Potentiometric titration is also ideally suited for determining the purity of lithium salts. For lithium hydroxide and lithium carbonate, the purity is determined using an aqueous acid-base titration. It is also possible to determine carbonate impurity within lithium hydroxide using this method.

For more details about performing the assay of lithium hydroxide and lithium carbonate, download our free Application Note AN-T-215.

For the assay of lithium chloride and lithium nitrate, the lithium is directly titrated using the precipitation reaction between lithium and fluoride in ethanolic solutions. For more details about how to carry out the assay of lithium chloride, download Application Note AN-T-181 and for lithium nitrate download AN-T-216.

The knowledge of other cations which might be present in lithium salts (and their concentration) is also of interest. Various cations (e.g., sodium, ammonium, or calcium) can be determined using ion chromatography (IC). IC is an efficient and precise multi-parameter method to quantify anions and cations over a wide concentration range.

The chromatogram in Figure 2 shows the separation of lithium, sodium, and calcium in a lithium ore processing stream.

Figure 2. Ion chromatogram of the lithium ore processing stream (1: lithium, 23.8 g/L; 2: sodium, 1.55 g/L; 3: calcium, 0.08 g/L).

For more information on how this analysis was carried out, download our free Application Note AN-C-189.

Eluated ions and decomposition products

In the development and optimization of lithium-ion batteries, one of the items of special interest is the content of ions (e.g., lithium, fluoride, and hexafluorophosphate) in the electrolyte or in eluates of different components. Ion chromatography allows the determination of decomposition products in electrolyte, or anions and cations eluated for example from finished batteries. Additionally, any sample preparation steps that might be required (e.g., preconcentration, dilution, filtration) can be automated with the Metrohm Inline Sample Preparation («MISP») techniques.

For more detailed information about selected IC applications for battery research, check out our Application Notes:

  • Cations in lithium hexafluorophosphate (AN-C-037)
  • Trace cations in lithium hexafluorophosphate (AN-CS-011)
  • Anions in electrolyte (AN-N-012)
  • Decomposition products of lithium hexaflurophosphate (AN-S-372)

Summary

This blog post contains only part of the analyses for battery research which are possible using Metrohm’s analytical instruments. Part 2 discusses the electrochemical characterization of batteries and their raw materials. Click below to read it!

Battery research

Positive experiences with top quality Metrohm equipment!

Post written by Lucia Meier, Technical Editor at Metrohm International Headquarters, Herisau, Switzerland.