Aqueous rechargeable lithium-ion batteries have continued to attract growing research attention owing to their low solvent costs, non-flammability and high ionic conductivity of the electrolytes. However, the commercialization of these batteries remains a big challenge mainly due to the associated poor energy density owing to the electrochemical instability of water. Specifically, the most critical issue is the inadequate reductive stability of aqueous electrolytes against hydrogen evolution reactions (HER).
Using water-in-salt electrolytes has been identified as a promising approach for improving the electrochemical stability of the aqueous electrolytes, attributed to their expanded electrochemical stability. The electrochemical stability of water-in-slat electrolytes can be improved further by stabilizing their solid-electrolyte interphase (SEI) layers and reducing their water activity values. Unfortunately, the long-term stabilities of the resulting SEIs formed in the concentrated aqueous electrolytes fail to suppress HER, leading to decreased capacity retention. Consequently, excess lithium salts degrade the intrinsic properties of aqueous electrolytes. To this end, developing an alternative approach, besides increasing lithium salt concentration, to effectively suppress HER is highly desirable.
Previous findings revealed that adding divalent salts in water-in-salt electrolytes, such as li – Mg (LMH) and Li – Ca (LCH) binary salt hydrates, can expand their electrochemical window and improve their reductive stabilities. Although the formation of a less soluble SEI layer effectively minimized HER, it could not be completely suppressed due to the gradual capacity degradation of Li4Ti5O12 electrode. To optimize the less-soluble SEI designs and improve the cycle performance of Li4Ti5O12 electrode, dense SEI layers are required. Additionally, effective control of the initial SEI formation processes is vital in constructing durable SEI with no or minimal parasitic reactions.
Inspired by the previous revelations, PhD candidate Shinji Kondou, Yoshifumi Watanabe, Professor Kaoru Dokko, Professor Masayoshi Watanabe and Professor Kazuhide Ueno from Yokohama National University explored the feasibility of electrochemical pretreatment before charge-discharge cycling in LCH electrolyte in enhancing the formation of SEI and improving the cycle performance of Li4Ti5O12 electrode. Hydrate hydrate-melt electrolytes were also studied for comparison purposes. Their work is currently published in the journal, ChemElectroChem.
The research team reported the formation of robust SEI and a significant improvement in the cycle performance of the Li4Ti5O12 electrode. At 1 – C rate, it recorded a 95.5% capacity retention after 50 cycles. The Coulombic efficiency of the electrode increased from 82% before pretreatment to 96% after pretreatment at -1.2 V in LCH. The improvements were not that significant in hydrate melt electrolytes. These observations suggested the possible formation of a dense SEI layer before charge-discharge cycling, which is highly effective in suppressing HER. The LCH electrolyte provided less water-soluble Ca-based SEI components with smaller interfacial resistance and improved electrochemical stability than the hydrate-melt electrolyte. Thus, Ca-derived components of SEI did not suppress its Li-ion conductivity.
In summary, Yokohama National University scientists demonstrated the feasibility and effectiveness of using a combination of LCH electrolyte and optimized electrochemical pretreatment process in achieving stable SEI formation and improved cycle performance of Li4Ti5O12 electrode. The resulting SEI had a potential range -0.8 to -1.2 V. Pretreatment at more negative potential was characterized by a more compact electrode surface morphology and an increase in the SEI resistance value. In a statement to Advances in Engineering, Professor Kazuhide Ueno pointed out that their findings offer a promising method for designing stable SEI layers for practical application of low-potential anode materials in developing high-performance Aqueous rechargeable lithium-ion batteries.
Kondou, S., Watanabe, Y., Dokko, K., Watanabe, M., & Ueno, K. (2022). Electrochemical Pretreatment of Solid-Electrolyte Interphase Formation for Enhanced Li4Ti5O12 Anode Performance in a Molten Li – Ca Binary Salt Hydrate Electrolyte. ChemElectroChem, 9(11), E202200061.