In large-scale power transportation, water-based lithium-ion batteries have received widespread attention because of their inherent safety, low cost and economy.
Recently, a research team from the University of Chinese of Hong Kong has developed a new type of hydrogen bond-anchored electrolyte, which uses low-cost cyclobutyl sulfone as a hydrogen bond receptor, which can not only limit the activity of free water, but also inhibit the hydrogen evolution reaction, providing a reasonable and effective development strategy for stabilizing high-pressure aqueous lithium-ion batteries.
"The hydrogen bond-anchored electrolyte salt has a concentration of 3.6 m/kg, which greatly reduces the production cost compared to the highly concentrated electrolyte." Dr. Wang Yu of the University of Chinese hong Kong said.
On November 10, 2021, the research paper was published on Matt under the title "Enabling high-energy-density aqueous batteries with hydrogen bond-anchored electrolytes", with Dr. Yu Wang as the first author and the corresponding author being Professor Lu Yijun of the University of Chinese, Hong Kong[1].
Figure | Related research papers (Source: Matter)
With high energy density and stable cycle life, non-aqueous lithium-ion batteries have been occupying a key position in the energy storage device market for small consumer electronics and large electric transportation.
It should be noted that excessive use of such flammable organic electrolytes will bring certain safety risks. Using water as an electrolyte is not only safe, low-cost, but also affordable, and is seen as a promising strategy.
Due to the decomposition reaction of water, the voltage window of the traditional water electrolyte is narrow, and the energy density of the waterborne lithium-ion battery is limited, which further affects the choice of high energy density electrode materials.
In addition, the electrocatalyst hydrogen evolution reaction (HER) associated with negative water reduction is also one of the challenges for aqueous lithium-ion batteries, which typically occurs above most energy-intensive operating potentials of the negative electrode.
Therefore, for water-based lithium-ion batteries, in order to have a high energy density, reducing the potential of the water electrolyte and broadening its voltage window is the top priority.
Previously, to solve these problems, researchers proposed the following two strategies.
Figure | Schematic diagram of the problems faced by waterborne lithium-ion batteries (Source: Matter)
One is the use of high concentrations of lithium fluoride salts, including bis (trifluoromethane) sulfonimide lithium (LiTFSI), bis (pentafluoroethanesulfonyl) lithium imide (LiBETI), lithium trifluoromethanesulfonate (LiOTf) and lithium trifluoromethanesulfonimide (LiPTFSI).
Under this strategy, the voltage window will be significantly improved in two ways. On the one hand, free water in the electrolyte is minimized by Li+ dissolution; on the other hand, an anion-derived solid electrolyte interface (SEI) is formed on the anode surface.
Due to the cost and potential toxicity of high concentrations of lithium fluoride salts, the researchers attempted to replace fluorinated salt-rich electrolytes with other lithium-based water electrolytes, such as 32m KAc–8m LiAc and "ionic hydrogels" (50% LiPAA–50% H2O).
However, due to the lack of reliable SEI, the stability and cycle life of batteries developed based on these electrolytes are not ideal.
Another strategy is to limit the activity of free water. Using polyethylene glycol (PEG) as a crowding agent, the researchers prepared a dilute molecularly congested electrolyte with a voltage window of 3.2V at low salt concentrations, which can support the Li4Ti5O12(LTO)/LiMn2O4 (LMO) full battery to cycle 300 times at 1C.
Free water is confined to the PEG network by forming hydrogen bonds with the PEG, thereby significantly reducing its own activity. However, the long chain of pegasus greatly limits the conductivity of the electrolyte.
The team's hydrogen bond anchored electrolyte successfully overcame the problems of the above two strategies.
It is understood that this hydrogen bond-anchored electrolyte can not only significantly inhibit the hydrogen evolution reaction, but also form a hierarchical anode electrolyte interface with a voltage window of 3.4V.
As a result, the team brought a water-based Li4Ti5O12/LiMn2O4 full battery with stable long cycle life and extremely high coulomic efficiency.
The paper mentions, "The battery achieves an energy density of 141 W h/kg in 300 cycles at 1C and 125Wh/kg in 1000 cycles at 5C, with a coulomb efficiency of up to 99.5%–99.9%." ”
Figure | Structure of hydrogen-bonded anchored electrolytes and their relationship to SEI formation processes (Source: Matter)
According to on-line electrochemical mass spectrometry, the release of hydrogen/oxygen during cycling is negligible, further confirming the stability of the electrolyte designed by the team.
In addition, the team designed a hydrogen-bonded anchored electrolyte with higher conductivity than the "molecularly crowded electrolyte" PEG.
Dr. Wang Yu said that the conductivity of the hydrogen bond-anchored electrolyte reached 2.5mS/cm, which is about three times that of the molecularly crowded electrolyte. At this conductivity, waterborne lithium-ion batteries can achieve the superior performance of fast charge and fast discharge, and can be charged in less than 12 minutes.
It is worth mentioning that this hydrogen bonding electrolyte provides a general strategy for designing high energy density waterborne lithium-ion batteries.
In addition, the strategy confirms that under the interaction between hydrogen bonds, researchers can try to adjust the water reactivity of the charged interface from the perspective of thermodynamics and kinetics, which brings new ideas for the study of water electrolyte design and water decomposition to produce hydrogen.
In the future, the team will further reduce the melting point of this hydrogen bond-anchored electrolyte to improve its low temperature performance. For example, adding a co-solvent with a lower melting point to the electrolyte, adjusting the salt concentration, etc.
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reference:
1.Yu Wang et al. ”Enabling high-energy-density aqueous batteries with hydrogen bond-anchored electrolytes.“ Matter(2021)
https://doi.org/10.1016/j.matt.2021.10.021