Lithium And Electrolyte Distribution In Lithium-Ion Cells

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Rechargeable batteries are used everywhere but their capacity drops after a while due to uneven distribution of lithium

Lithium-ion cell in the slide of the STRESS-SPEC diffractometer at FRM II, which investigates questions in materials science. © A. Heddergott / TUM

The outstanding characteristics of the lithium-ion battery have hugely impacted our everyday lives. However, various effects that occur over time gradually reduce the great storage capacity of these batteries.

At the Technical University of Munich’s Research Neutron Source Heinz Maier-Leibnitz (FRM II), Dr Anatoliy Senyshyn, an instrument scientist at the high-resolution powder diffractometer SPODI, used neutron scattering to investigate the cause of these effects in cylindrical lithium-ion batteries and find answers to fundamental questions concerning capacity drop over time, lithium distribution and the structure and behaviour of rechargeable lithium-ion batteries. 

Until now theoretical models, calculations and measurements usually assume an even distribution of lithium in these batteries. However, the investigations indicate that the lithium is very unevenly distributed from the very beginning; and the inhomogeneity rises over time. 

The reason why battery capacity sinks

Lithium-ion cells produce electricity as lithium atoms release electrons, which then flow through a connected device. At the same time, inside the cell, a lithium-ion moves from one electrode to the other, which means there are always only as many electrons available as there are lithium ions. When the rechargeable battery loses capacity, it can also be said that the lithium “gets lost”. 

The products resulting from the electrolyte decomposition trap lithium atoms, which are then no longer available as mobile lithium to be exchanged between the two electrodes. As a result, the battery loses capacity. It ages.

Using neutrons to investigate a rechargeable battery

To observe a lithium-ion cell’s interior during charging and discharging, for example, the decomposition of the electrolyte or the distribution of the lithium is very difficult from outside of the cell due to the high reactivity of the cell components with oxygen and air humidity. 

Neutrons are particularly sensitive to lighter elements such as hydrogen and lithium. Therefore they can render the lithium inside a cell visible, making investigations possible under actual operational conditions.

The neutron scattering experiments at the instruments STRESS-SPEC and SPODI indicated a linear relationship between the loss of mobile lithium ions and the decomposition of the electrolyte, which occurs for example as an unwanted side reaction during charging. 

Neutrons make it possible to look non-destructively inside the lithium-ion batteries and see the distribution and amount of lithium in the graphite anode. In the background is the structure of the graphite that has incorporated lithium. © Reiner Müller

Another advantage of working with neutrons is that they can be used in non-destructive testing. Thus for example the researchers can observe internal battery processes from outside without intervening in the delicate system.

Modelling of lithium-ion cells can be significantly improved when developers take into account the uneven lithium distribution. 

These results are an important basis for making more efficient rechargeable batteries with longer service lives and higher performance in the future.