Developing a new generation of lithium-ion batteries

The global proliferation of portable electronic devices and consumer demand for higher performance has put pressure on companies to accelerate innovation. This pace of innovation depends largely on battery performance, however, in order to develop batteries with superior performance, it is necessary to understand the fundamental chemistry of their materials. To achieve this goal, advanced in situ measurement techniques are required, and the development of magnetic resonance spectroscopy, including nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy, as well as imaging techniques such as magnetic resonance imaging (MRI), is paving the way for this process.

Lithium Ion Battery

Lithium's high energy density and high electrochemical potential make lithium-ion batteries (LIBs) one of the most popular choices in the world. Since their development in the 1970s, LIBs have achieved significant technological innovations, with Sony launching the first rechargeable battery in 1991.

Rechargeable batteries rely on electrochemical reactions, where chemical energy is converted into electrical energy, and vice versa, through the movement of ions and electrons in an electrolyte between two electrodes, the positive and negative electrodes.

Figure 1: Schematic diagram of the working principle of lithium-ion batteries

(Figure Note: Charge Discharge Electrolyte Electrode Separator)

During the first charging cycle of a LIB, as lithium ions flow through the electrolyte toward the anode, some of them react with degradation products of the electrolyte to form insoluble deposits on the anode. These deposits form a solid electrolyte interphase (SEI) that prevents the anode material from decomposing and is critical for the long-term operation of the battery. The formation of a stable SEI that conducts ions but insulates electrons determines many performance parameters and is therefore extremely attractive for LIB research.

Studying LIBs using NMR

NMR technology can be used to study the detailed structural information (including electronic structure) of various battery systems, such as identifying intermediates, studying the kinetic properties of battery materials, etc. NMR is particularly suitable for studying the kinetic properties of alkali metal ions, which are important components of battery materials. Even in highly disordered systems, solid-state NMR can be used to characterize the local structure of LIB materials and clarify the signal transformation of various chemical substances in the materials. Lithium has two NMR active isotopes (6Li and 7Li), so the kinetic properties of lithium can be directly studied and the movement of lithium ions can be quantitatively analyzed.

The development of NMR technology has helped improve the understanding of SEI, allowing researchers to separate and quantify SEI films from multiple aspects. For example, using 7Li and 19F magic angle spinning (MAS) NMR techniques, changes in lithium fluoride (LiF) in the SEI film between the anode and the electrolyte of recharged LIBs can be identified and quantified. 1NMR methods can also monitor and quantify dendrite growth. The changes in the intensity of the Li spectrum peak during cyclic charge and discharge are related to the growth of dendrite tissue and the smooth deposition of metal. Studies have found that in situ NMR can determine that up to 90% of the lithium deposited during the slow charging of Li/LiCoO2 batteries is dendritic. 2NMR can be used to systematically test methods such as electrolyte additives, advanced separators, battery pressure, temperature and electrochemical cycling conditions to inhibit dendrite growth. 3 Coupled with in situ quantitative monitoring of SEI and new battery materials, NMR has played a key role in promoting the design of innovative LIBs.

Is EPR a complementary technology?

Measuring dendrite formation during battery operation is challenging but necessary for the ongoing research of alternative LIB designs and materials. In addition to NMR, EPR spectroscopy is also well suited for studying the evolution of metallic lithium species in situ. EPR spectroscopy has also been used for semi-quantitative detection of deposited lithium metal in LIBs with metallic lithium anodes and LiCoO2 cathodes.

EPR imaging is being used to study the formation and disappearance of radical oxygen species in new batteries as a function of current, potential, resting time, electrolyte or temperature.

Using MRI to obtain spatial information

In addition to spectroscopy, MRI is also a powerful non-invasive technique that can provide time-resolved and quantitative information on changes in the electrolyte and electrodes of LIBs. Similar to NMR, MRI can detect and locate the microstructure of lithium, and has the unique advantage of providing spatial information, so that specific structural changes can be located. The advantages of MRI technology in studying new battery materials and battery design are increasingly recognized. Other applications include LIB capacity decay studies, battery testing after a large number of cycles, high stress and accelerated aging tests.

All-solid-state battery

One of the most cutting-edge areas of LIB research is the transition from liquid to solid electrolytes. The flammability of liquid electrolytes represents a safety concern given the potential for short circuits in LIBs. For many years, researchers have been investigating the use of solid electrolytes as an alternative to liquid electrolytes, which not only improves safety but also offers the potential to resist dendrite formation on lithium metal anodes, thereby increasing energy density. Although all-solid-state batteries are not a new concept, their progress has been hampered to date by their poor rate and cycling performance, likely due to the high internal resistance to lithium ion transfer at the solid-solid electrode-electrolyte interface [4]. 5 Therefore, studying interfacial reactions and charge transport is critical to realizing the potential of these batteries, and NMR is ideal for this purpose.

NMR is also useful for characterizing potential solid electrolyte materials such as ceramics that rely on ion transport. NMR combined with conductivity measurements can be used to analyze ion kinetics and help elucidate the relationship between local structure and kinetic parameters.

Batteries of the future

It is clear that the development of analytical techniques over the past 40 years has had a significant impact on the battery industry. High-resolution imaging provided by techniques such as electron microscopy and optical microscopy is often limited to surface imaging and difficult to interpret quantitatively. NMR and EPR spectroscopy are both non-invasive methods with quantitative capabilities and are currently being further investigated to improve their sensitivity and resolution.

A deeper understanding of possible alternative electrode materials, electrolyte compositions (lithium salts, solvents, and additives), and process control of SEI and dendrite formation are paving the way for safer LIBs with higher energy density. The rapid development of new materials such as high-capacity, high-operating-voltage cathodes is challenging electrolytes and interphase chemistry. This innovation is being combined with advanced analytical techniques such as EPR, NMR spectroscopy, and MRI to ensure that LIB research continues to provide future energy storage solutions.