Preventing energy loss in Li-ion batteries

Researchers at the Korea Institute of Science and Technology (KIST) have developed a solution to prevent the initial loss of lithium (Li) ions in supposedly fully charged batteries, which should enable the mileage of electric vehicles (EVs) and usage time of smartphones to be drastically increased.

When its batteries are fully charged, an electronic device will normally indicate that they are at 100% capacity. However, this value only represents 70–90% of the theoretical energy density that can be stored in the batteries, owing to the permanent loss of Li ions that occurs during the initial charge in the stabilisation (formation) stage of battery production.

In an effort to overcome this issue, KIST researchers created an electrode pre-treatment solution capable of minimising this initial Li ion loss in graphite–silicon oxide (SiO_x) composite anodes. After being dipped in the solution, the anode, which was composed of 50% SiO_x, demonstrated negligible Li loss, enabling a full cell to exhibit near-ideal energy density. The team’s work has been published in the Journal of the American Chemical Society.

While most commercial Li batteries utilise a graphite anode, SiO_x has been garnering significant attention as a next-generation material for anodes due to its high capacity, which is 5–10 times greater than graphite. Yet SiO_x also irreversibly consumes three times as much active Li as graphite. As a result, a composite electrode, consisting of a mixture of graphite and SiO_x, is now gaining recognition as an alternative for practical next-generation anodes.

However, while there was a corresponding increase in the capacity of graphite-SiO_x composite electrodes at higher ratios of SiO_x, there was also an increase in loss of initial Li. Consequently, the ratio of SiO_x content in a graphite–SiO_x composite electrode was limited to 15%, as increasing the ratio to 50% would result in an initial Li loss of 40%.

To simultaneously achieve high capacity and high initial efficiency, scientists proposed various prelithiation methods involving the pre-doping of extra Li into the anode. Dr Minah Lee’s research team at KIST developed a process wherein the electrode is dipped into a unique solution to mitigate Li consumption by the SiO_x electrode. The team then applied this process to a graphite–SiO_x composite material with significant commercialisation potential.

The research team found that the pre-treatment solutions developed previously would cause the unintended insertion of solvent molecules with Li ions into the graphite, owing to the graphite’s versatile intercalation capability. This intercalation of large solvent molecules resulted in the structural breakdown of the graphite–SiO_x composite electrode. To prevent electrode failure, the researchers developed a new solution using a weakly solvating solvent to reduce the interaction between the solvent and the Li ions. This solution enabled the selective insertion of Li ions into the active materials, ensuring a stable supply of additional Li to the graphite–SiO_x composite electrode.

The initial Li consumption was completely prevented after the graphite–SiO_x electrode was immersed in the solution developed by the research team for approximately one minute, even at a 50% SiO_x ratio. Consequently, the electrode showed a high initial efficiency of nearly 100%, indicating negligible Li loss (≤1%) in the initial charge. Electrodes developed through this process had a capacity 2.6 times higher than conventional graphite anodes, while also maintaining 87.3% of the initial capacity after 250 charge/discharge cycles.

“As a result of this study, we should be able to increase the SiO_x content in graphite–SiO_x composite anodes to over 50%, as opposed to the 15% ratio allowed by conventional materials, making it possible to produce lithium-ion batteries with a greater capacity and improve the mileage of future EVs,” Dr Lee said.

Dr Jihyun Hong, a co-researcher at KIST, added, “The technology is safe and suitable for mass production, and therefore is likely to be commercialised.”

Image caption: A KIST researcher conducts an experiment to evaluate high-capacity batteries following prelithiation. Image credit: Korea Institute of Science and Technology (KIST).

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