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Unravelling Lithium Pileups in Battery Charging

For many years, scientists have been looking for ways to improve the life cycle of commercial lithium-ion batteries as despite the fact that they are a commonly-used and indispensable part of many technical applications, there are still issues concerning their operational capacity. There exist several materials which might lead to improved efficiencies. Metal oxides, for example, have been shown to be a suitable candidate as conversion-type electrode materials for lithium batteries because have great potential due to their high storage capacity.

Commercial Li-ion batteries are usually based on a mechanism called intercalation where lithium is reversibly inserted into and extracted from electrode materials without damaging their crystal structure. Intercalation only takes place during the charging and discharging process. Although these materials are extremely stable, only a limited number of lithium ions can take part in the reaction. As a result, their capacity is lower than conversion-type materials. In contrast, using metal-oxide electrode materials enables the participation of more lithium ions in intercalation processes, which can increase battery capacity. However, the crystal structure of these materials completely changes from its original state, causing instabilities over multiple charge-discharge cycles.

Now (2021), scientists at Brookhaven National Laboratory have a found a method to improve lithium-metal batteries by removing kinetic barriers in iron-oxide electrodes which they had found to be responsible for a capacity decrease during long-term cycling in a previous study. At high current, the battery had charged and discharged relatively fast, which is normal for real batteries. However, if this cycling had occurred too fast, a lithium gradient had arisen across the electrode material.

In the current follow-up study these kinetic barriers were removed by operating the batteries at milder conditions of low current and constant voltage after charge and discharge. Two nontoxic and abundant metal oxides, nickel oxide and iron oxide, were examined in lithium-ion half-cell batteries. The electrochemical tests revealed significant differences in battery voltage profiles and capacity over 10 cycles. To characterize these changes, the team performed experiments using quick X-ray absorption and scattering (QAS), pair distribution function (PDF), and X-ray powder diffraction (XPD). The QAS beamline yielded chemical information, such as oxidation states, on each metal at different states of charge and discharge, whereas the PDF and XPD beamlines helped analyse the crystal structures. Performing the x-ray synchrotron studies, the team learnt that the reduction and oxidation reactions of nickel in nickel oxide and iron in iron oxide did not appear to be very reversible. However, they did not know the reason for the incomplete reconversion reactions and capacity fade. With the help of transmission electron microscopes (TEMs), high-resolution images were made which showed the intermediate phases of lithium metal oxides after charging. It was found that these intermediate phases were responsible for lithium being not fully extracted during charging which makes it accumulate over time and decrease the amount of available lithium ions for subsequent cycles, causing a drop in capacity. In view of these results, the scientists believed that charging and discharging take place by means of different asymmetric reaction processes. Energy is required for lithium-ion extraction during charging, which is why this reaction is based on energy transfer. On the other hand, the insertion of lithium ions during discharging takes place spontaneously, and this fast lithium diffusion is driven by kinetics.

Because of their overall good capacity characteristics, lithium-metal batteries have been at the centre of scientific attention for some time. In 2020, scientists used graphite anode to host lithium ions during charging. This greatly reduced the risk of dendrite formation. However, the graphite took up nearly half the volume of the cell without improving energy storage, which caused the battery work safely but decreased its performance. Therefore, a solid electrolyte material was created. This replaced the flammable component of the battery and also reduced unwanted chemical reactions between the electrolyte and other battery materials which could cause degradation over time. Pouch-style cells with a single-sheet cathode layer were created as well as an anode contact foil. The cathode consisted of a nickel-manganese-cobalt cathode material. Tests showed that the cells could operate safely at temperatures ranging from -30°C to 45°C, but they did lose some operating capacity in the coldest temperatures.

In 2021, scientists researched the processes taking place inside a lithium-metal coin battery. The team repeatedly charged and discharged lithium coin cells with the same high-intensity electric current that electric vehicles need to charge. Some cells went through a few cycles, while others went through more than a hundred cycles. Apart from lithium spike formation, the scientists also found an unexpected second culprit: a hard build-up formed as a by-product of the internal chemical reactions of the battery. Every time the battery was recharged, the by-product, called solid electrolyte interphase, grew. If they capped the lithium, it tore holes in the separator, creating openings for metal deposits to spread and form a short.

There are several benefits to using lithium-metal batteries over conventional ones: they can be counted among the high-performance storage cells which can hold 50% more energy than lithium-ion ones. However, higher failure rates and safety issues like have impeded commercialization efforts. An understanding of pileups during battery charging could help scientists increase the total amount of energy stored by next-generation lithium-ion batteries with metal-oxide electrodes over multiple charge-discharge cycles.

The next step in the research will be to analyse other conversion-type electrode materials such as metal sulphides and perform studies during battery cycling. This work will hopefully make a valuable contribution to creating more efficient reliable and safe lithium batteries.