User Stories

By Sarah Fields
July 2019

Lithium-ion batteries can come in the form of laminated lithium-ion batteries for mobile electronic devices, cylindrical batteries for industrial power tools, and other cylindrical batteries for energy storage systems. The R&D division of Murata Manufacturing Co., Ltd., is using multiphysics simulation to examine batteries using lithium metal as a negative electrode material.

Dendrites, needle-like growths, are a fierce antagonist to efficient lithium-ion battery functioning. Dendrites form when a current is applied to a lithium metal electrode and can cause unwanted side reactions that result in short circuiting, drastically limiting the life of the battery.

Mitigating dendrite formation is an active area of research for the entire battery industry. Most researchers approach the problem of safety hazards and life span due to dendrite formation by changing the chemistry in some way. However, gains in this area have been painstakingly slow, prompting some researchers to take an alternative path.

When examining batteries that use lithium metal as a negative electrode material, Jusuke Shimura, a R&D engineer at Murata, looked to investigate the effect of changing the charging current pattern on dendrite formation.

This approach is gaining traction in the battery and energy storage world as the industry ramps up to meet the needs of an era of electrification and renewable energy.

Using Multiphysics to Minimize Dendrites

Lithium dendrite occurs when current is applied to the lithium metal electrode, resulting in a short circuit. “In order to commercialize lithium-ion batteries with lithium metal electrodes, this problem must be solved,” says Shimura.

The key to his approach was identifying a current pattern for charging that would minimize the growth of lithium dendrites. This approach works because at the off time between pulses, the concentration gradient at the electrode interface decreases, minimizing dendrite buildup. Also, introducing reverse pulses in the current pattern plays an important role by repeatedly dissolving formed dendrites.

To capture the electrochemical effects over his geometry, Shimura enlisted the battery modeling capabilities of COMSOL Multiphysics®. He used a combination of experimental evidence and simulation to determine the best charging pattern. Many researchers have been exploring this challenge from a chemical and material perspective. To make strides in this area, Shimura wanted to establish a baseline understanding of his physical system experimentally. It was important for him to understand the shape of dendrite formation over time. To accomplish this, he created an X-ray CT-compatible laminated cell that contains a contrast agent in its electrolyte membrane, and visually measured the formation of dendrites over time (Figure 1).

“I created a laminated cell that could be imaged with X-ray computed tomography, so that I would know where the dendrites are forming. Then, I used COMSOL® to find the best pulse pattern of charging to limit dendrite growth based on the shape and the size of the formed dendrites,” explains Shimura.

With the data from the X-ray computed tomography, Shimura created a model of a lithium metal cell and analyzed the effect of changing the current pattern. The results showed how much lithium metal precipitated onto the dendrite (Figure 2).

Using multiphysics modeling, Shimura evaluated various current patterns to determine the current pattern with the slowest rate of dendrite formation (Figure 3). This method allowed him to examine which has more lithium deposition — the electrode surface with planar diffusion (bottom part of Figure 3) or the dendrite with spherical-like diffusion (left part of Figure 3) through one cycle of the pulse pattern.

He ultimately found that a repetition of reverse pulse for 20 seconds, off-time for 10 seconds, forward pulse for 20 seconds, and off-time for 10 seconds resulted in the least dendrite growth (Figure 4).

„Mit diesem Muster konnten wir die Wachstumsrate der Dendriten auf weniger als ein Drittel reduzieren. Wie wir erwartet hatten, wurde dies allein durch die Änderung des Lademusters erreicht - die Chemie blieb gleich“, erklärt Shimura.

Die Simulation von Shimura basierte auf der experimentell ermittelten Dendritengröße und nutzte die Akku-Modellierungsmöglichkeiten von COMSOL Multiphysics, die die konzentrationsabhängige Butler-Volmer-Gleichung zur Modellierung der Elektrodenreaktionen und gekoppelte Diffusions-Migrations-Gleichungen zur Modellierung des Lithium-Ionen-Transports verwenden.

Akkus für die Zukunft

Mithilfe von Simulationen fand Shimura das beste Pulsmuster zum Laden eines Lithium-Ionen-Akkus mit einer Lithium-Metall-Elektrode. Verglichen mit der Anwendung von Gleichstrom verbesserte dieser Ansatz die Lebensdauer des Akkus um mehr als das Dreifache. „Dank COMSOL konnten wir mit einer auf Grundsätzen basierenden Simulation zeigen, dass das optimierte Lademuster die Lebensdauer des Akkus verbessert“, sagt Shimura.

Shimura geht davon aus, dass die Multiphysik-Simulation auch in Zukunft eine wichtige Rolle bei der Aufrechterhaltung des hohen Forschungstempos spielen wird und schließt: „Wir freuen uns darauf, COMSOL weiterhin zu nutzen, um die Vorteile optimierter Lademuster für Akkus auf den Markt zu bringen.“