Improved Stability of LiCoO₂ Positive Electrode with Kosmotropic Anion in Aqueous Lithium-Ion Batteries

Aqueous rechargeable lithium (Li) batteries (ARLB) were recently highlighted owing to low cost and safety issues. However, the narrow electrochemical potential window of water and instability of the electrode surface remained significant challenges. One of the promising solutions to mitigate these challenges is the use of extremely high concentrations of electrolyte salts, called water-in-salt electrolyte (WiSE). The predominant aggregated-ion pairs in the WiSE widened the potential window to ~3.0 V and protected the electrode surface by forming solid-electrolyte interphase (SEI). However, the possible precipitation and the high cost of the salts may lose the advantages of using the aqueous medium.<br/>In this presentation, I focus on the interfacial reaction of positive electrodes with a low concentration of lithium salt (0.5~3 mol kg^-1(m)) to improve the performance of the aqueous Li-ion cells. The LiCoO₂ (LCO) with the layered structure was used, which typically showed rapid deteriorations in an aqueous medium. It was attributed to the dissolution of cobalt ion in the presence of O₂ gas and the intercalation of H⁺ through the water dissociation.^1-3 Soft X-ray absorption near-edge structure (XANES) spectroscopy exhibited a distorted oxide layer of LCO with 1 m LiTFSI (Li(NSO₂CF₃)₂) (aq) after 30 cycles tested in an argon-filled glove box. It indicated the irreversible Li⁺ (de)intercalation due to the inserted H⁺ effect. In sharp contrast, the deterioration of the LCO electrode was considerably suppressed by using 0.5 m Li₂SO₄ (aq). Since there was no evidence of SEI formation and negligible pH effect in the mild alkaline solutions, we attributed the different cell performances to the role of the anion. In addition, 1 m NO₃^–, and ClO₄^– showed better performances than TFSI^-. Interestingly, the order of capacity retentions, SO₄^2- &gt; NO₃^- &gt; ClO₄^- &gt; TFSI^-, was related to the strength of kosmotrope, the water-structure-making property of ion. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra revealed the increased ice-like O-H bond vibration at 3200 cm^-1 with increasing SO₄^2– concentration, as the hydrogen-bonding network nearby SO₄^2– becomes rigid. In contrast, NO₃^–, ClO₄^–, and TFSI^– showed the intense signals at 3600 cm^-1 with increasing anion concentrations, indicating enlargements of the disordered hydrogen bond and the free water. Therefore, superior cyclability with SO₄^2–was caused by the strong hydrogen-bonding network, which harnessed the intercalation of H⁺ into the LCO electrode at the interface. In addition, molecular dynamics (MD) simulation envisioned the locally concentrated SO₄^2– paring with Li⁺ at the interfacial region, suggesting the predominant ice-like water at the LCO surface. The LCO cathode coupled with Li_xNb_2/7Mo_3/7O₂ anode showed 74% capacity retention for 500 cycles with 3 m Li₂SO₄(aq) for full cells. By comparison, 3 m LiTFSI and 9 m LiNO₃ only maintained &lt;40% capacity. I will discuss the anions’ propensity according to the strength of kosmotrope and their correlation with cell performances in the presentation.<br/>References<br/>1. Ramanujapuram, A.; Gordon, D.; Magasinski, A.; Ward, B.; Nitta, N.; Huang, C.; Yushin, G., Degradation and stabilization of lithium cobalt oxide in aqueous electrolytes. Energy & Environmental Science 2016, 9 (5), 1841-1848.<br/>2. Yong-gang Wang, J.-y. L., Cong-xiao Wang, and Yong yao Xia, Hybrid Aqueous Energy Storage Cells Using Activated Carbon and Lithium-Ion Intercalated Compounds II. Comparison of LiMn₂O₄, LiCo_1/3Ni_1/3Mn_1/3O₂, and LiCoO₂ Positive Electrodes. Journal of Electrochemical Society 2006, 153 (8), A1425-A431.<br/>3. Byeon, P.; Bae, H. B.; Chung, H.-S.; Lee, S.-G.; Kim, J.-G.; Lee, H. J.; Choi, J. W.; Chung, S.-Y., Atomic-Scale Observation of LiFePO₄ and LiCoO₂Dissolution Behavior in Aqueous Solutions. Advanced Functional Materials 2018, 28 (45).