Abstract

This study examined lithium-ion-extracted lithium nickel manganese cobalt oxide as a cathode for aqueous zinc batteries. Lithium-ion was electrochemically extracted from the interlayer of the material in 1 M potassium hydroxide (KOH) solution. Simultaneously, insertion of water molecules and potassium ions led the interlayer spacing to expand significantly. The lithium-ion-extracted material, LixKyNi1/3Mn1/3Co1/3O2·nH2O, exhibited reversible charge-discharge behavior as a cathode for zinc battery in zinc oxide-saturated 1 M KOH. Interlayer spacing change during charge-discharge cycling revealed that the cathode operates through an insertion and extraction. Additionally, inductively coupled plasma optical emission spectrometry, X-ray photoelectron spectroscopy, and electrochemical quartz crystal microbalance analysis indicated that the energy storage of this cathode is predominantly governed by proton insertion/extraction.

In recent years, zinc secondary batteries, which utilize a water-based electrolyte and offer high safety, have attracted attention as post-lithium-ion batteries. Zn has a high specific capacity (820 mAh/g) and a redox potential of -0.76 V (versus the standard hydrogen electrode) as a cathode. Furthermore, combining it with new cathode materials could significantly enhance performance. In particular, layered compounds containing manganese are inexpensive, widely used in industry, and considered promising candidates. This study synthesized calcium manganese oxide with a layered structure and investigated its potential as a cathode material for zinc secondary batteries. It is already known that Ca₂Mn₃O₈ has a layered structure and can be synthesized with a Mn/Ca atomic ratio ranging from 1.5 to 2.5. This research examined the effect of adding Fe and Al to this calcium manganese oxide on battery performance. When Fe was added, the battery capacity increased by 20%, reaching 177 mAh/g compared to the sample without Fe. This increase is believed to result from an increased interlayer distance, promoting the incorporation of structural water and enhancing ion conversion reactions during charge and discharge. However, adding Al was found to have no beneficial effect on battery performance.

Synthesis and CO₂ adsorption ability of calcium-deficient tobermorite were investigated. The calcium-deficient tobermorites with Ca/(Al+Si) atomic ratios lower than that of the theoretical composition, 0.83, were prepared by the reaction in the system CaCl₂-Na₂SiO₃-AlCl₃-H₂O under atmospheric pressure. The calcium-deficient tobermorites were characterized by means of X-ray diffraction, thermal analysis (TG-DTA), infrared spectroscopy and chemical analysis. The calcium-deficient tobermorite was formed from starting solutions with Ca/(Al+Si) atomic ratios lower than 0.95. Most Ca²⁺ ions existing in interlayers of the tobermorite, 20% of total Ca²⁺, were removed successively by decreasing the initial Ca/(Al+Si) atomic ratio from 0.95 to 0.70, and then the basal spacing of the calcium-deficient tobermorite expanded by replacing one Ca²⁺ ion site with two Na⁺ ions until the atomic ratio of 0.7. However, the layer structure of the tobermorite was destroyed at the atomic ratios below 0.7. The amount of Na⁺ ions to compensate for charge deficiency in calcium-deficient tobermorites obtained at an initial Ca/(Al+Si) atomic ratio of 0.7 and initial Al/(Al+Si) atomic ratio of 0.10 was changeable by number of washings with pure water. The composition after washing was expressed by the formulas of Ca₄Na_0.4[(Al+Si)₆O₁₁H₂]⋅4H₂O including ion defects after ten times washings (5g/dm³ H₂O) and Ca₄Na_2.1[(Al+Si)₆O₂₂H₂]⋅4H₂O after one time washing (5g/100cm³ H₂O). The maximum exchange capacity for Na⁺→K⁺ of Na⁺ present in the calcium-deficient tobermorite was 95meq/100g irrespective of the amount of Na⁺. The adsorption capacity of CO₂ at 60°C in the tobermorite increased with increasing Na/(Al+Si) atomic ratio and reached a maximum value of 3.0mmol/g at the atomic ratio of 0.36. The desorption of CO₂ and adsorption of Na⁺ progressed simultaneously on washing the tobermorite which adsorbed CO₂ with 1N NaCl solution. Accordingly the tobermorite washed with NaCl solution was available as CO₂ adsorbent. when the adsorption and desorption were repeated many times, the formation of CaCO₃ originated from partial carbonation in the tobermorite was unavoidable, and consequently, the adsorption capacity of CO₂ decreased successively with increasing recycle number. On the other hand, when the calcium-deficient tobermorite after desorption of CO₂ was heated at 200°C, its adsorption capacity was kept at 2.0mmol/g even after using five times.