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X-Ray Technology help improve batteries

In search of longer-lasting batteries for electric vehicles, scientists have turned their attention to pure lithium metal as replacement for the currently-used graphite-based anodes. Pure lithium could contribute to the reduction of battery weight and also extend the driving range of a vehicle. The only problem is that the lifetime of batteries with this metal is still shorter than that of commonly-used batteries.

Now (2022), scientists at Brookhaven National Laboratory analysed lithium metal deposition and removal from a battery anode during cycling to research possible sources of battery failure. They conducted the study using intense x-rays and tracked lithium on its way from the cathode to the anode and back in the course of a complete charge and discharge cycle. To address the problem of tracking lithium atoms whose weak signal can get obscured by signals emitted by the other materials constituting the battery, they used a battery cell with a bare anode, which makes it easier to measure the signal of the moving lithium ions. Then, they conducted a study where two different anode materials, copper and molybdenum, were compared. They deposited pure lithium metal on the anode after it had been extracted from the cathode material during operation of these batteries. This made it possible to analyse how uniformly lithium metal was added to and removed from the anode surfaces. Using this method, the team could find out how much lithium was contained across the electrode while the cell was kept at various levels of charge and discharge. In the copper anode, half the lithium capacity was deposited on the anode up to the half-charged state, and all possible lithium was deposited by the full charge state. Upon discharge, first problems arose. In some parts of the anode, the lithium was removed proportionally to the discharge, in others there was a lag in lithium removal, where stripping went slowly during the first half of discharge, but then picked up speed to complete the process when the battery had been fully discharged. Still in other parts, the lagging was so heavy that the greater part of the lithium stayed on the anode even after the battery had been fully discharged.

The mapping data showed that the regions of poor performance occurred in spots that were about five millimeters in diameter. The scientists assumed that poor spreading of the liquid electrolyte throughout the battery cell might be responsible for the loss of capacity in those areas.

Scientists have long tried to find the problems lying at the bottom of reduced battery life. In 2016, lithium growth in a glass capillary cell was analysed during which a change of mechanism from root-growing mossy lithium to tip-growing dendritic lithium when electrolyte diffusion limitation started was detected. Researching sandwich cells, the scientists showed that mossy lithium could be blocked by nanoporous ceramic separators, while dendritic lithium could easily penetrate nanopores and short the cell. The results of the research demonstrated that metal batteries were constrained by their design (“Sand's capacity”), which, however, could be remediated by using concentrated electrolytes with stiff, permeable, nanoporous separators for improved safety.

Image: In situ observations of lithium electrodeposition in a glass capillary filled with an electrolyte solution consisting of 1 M LiPF6 in EC/DMC. (a) Photo of the capillary cell, whose middle part was pulled thinner for easier optical observation. (b) Voltage responses of the capillary cell at a deposition current density of 2.61 mA cm−2. (c–g) In situ snapshots of the growth of lithium during the electrodeposition. Red arrow in (e) points to the emergence of dendritic lithium. Red dash line in (g) labels the clear morphological difference between the pre- and post-Sand's time lithium deposits. (h) Theoretical interpretation of the growth mechanisms of lithium electrodeposition during concentration polarization

Source: Peng Bai, Ju Li, Fikile R. Brushetta and  Martin Z. Bazant/ Transition of lithium growth mechanisms in liquid electrolytes/ Energy and Environmental Science Issue 10, 2016/ Open Access This article is licensed under a
Attribution-NonCommercial 3.0 Unported (CC BY-NC 3.0)

In 2019, scientists analysed the performance and safety implications of charging a 2.6 Ah 18650 Li-ion battery when exposed to a temporally transient thermal condition. If transferred from a warmer chamber to a colder chamber, the cells showed plating dominant charging behaviour which was not present in the ones equilibrated to their respective thermal environment. To analyse which safety implications the exposing of cells to a thermal transient had, an end of life safety assessment was performed on three different cells. The scientists started with completely new cells in a pristine initial condition, discharged and cycled the cells. The transient cell started with a discharge capacity of 1.98 Ah and was stopped at 1.50 Ah. The equilibrium cell started with a discharge capacity of 2.08 Ah and was stopped at 1.57 Ah. The equilibrium 20°C cell needed many cycles to reach equivalent energy loss, so this cell was cycled 100 times and exhibited ~6% capacity fade.

Image: (A) Schematic diagram of the cell transfer process used to create a thermal transient condition and (B) experimentally measured and theoretically predicted cell temperature upon transfer to the 0°C chamber

Source: Rachel Carter, Emily J. Klein, Todd A. Kingston and Corey T. Love/ Detection of Lithium Plating During Thermally Transient Charging of Li-Ion Batteries/ Front. Energy Res., 05 December 2019/ Open Access This article is licensed under a
Attribution 4.0 International (CC BY 4.0)

The findings of this experiment will have many potential advantages for the future design of batteries: the results show that lithium was not removed equally in all places but that it was more difficult to remove lithium at certain places. Through identifying and analysing the causes of the problems, solutions may be found to these problems and make better batteries with higher capacities and longer lifetimes.

The next steps in the research will comprise distinguishing between metal and solvent effects as well as testing the effectiveness of strategies for mitigating potential problems such as electrolyte inhomogeneity. This may lay the foundation for developing high-capacity lithium metal anode batteries with a longer lifetime.