Influence of Stacking on H⁺ Intercalation in Layered ACoO₂ (A = Li, Na) Cathode Materials and Implications for Aqueous Li-Ion Batteries: A First-Principles Investigation

Abstract

Li- and Na-ion batteries are effective energy storage technologies. Nonetheless, currently used organic-electrolyte batteries present well-known safety problems. Therefore, the research community is intensively looking for potential alternatives. Aqueous batteries based on low-cost salts in water could be an interesting choice since they are safe and environmentally benign. However, working with aqueous electrolytes brings new detrimental mechanisms such as proton intercalation. Understanding the (de)­intercalation chemistry of protons and alkali is one of the keys for designing cathode materials in such aqueous electrochemical cells. In this work, we carry out density functional theory calculations to investigate the H+/alkali exchange in layered LiCoO2 and NaCoO2 materials. By computing the grand potential phase diagram and voltage–composition plots, we determine the relative stability of several orderings of protons, alkali, and vacancies. The fully protonated CoO2 lattice (CoO­(OH)) is revealed to be the most stable insertion product due to the formation of interlayer hydrogen bonds. Our computations demonstrate the key role of layer stacking: H+ insertion is favored in prismatic (P) stacking, while Li favors octahedral (O) stacking. While the fully protonated layered cobalt oxide is the thermodynamically favored product when protons and alkali compete, we show that mixing protons and lithium is energetically disfavored because of the different stacking preferences. We suggest that the kinetic difficulty in nucleating fully protonated phases in the layered oxide prevents proton insertion when cycling LiCoO2 in an aqueous electrolyte. The good cyclability and lack of proton insertion in LiCoO2 are, therefore, a result of the slow kinetics of protonation in partially lithiated cobalt oxide. On the other hand, we demonstrate that NaCoO2 is prone to proton and alkali mixing due to the different stacking preferences for sodium. We hypothesize that this could lead to proton intercalation and poor performances in aqueous batteries for NaCoO2 cathodes.

Effect of Aqueous Electrolytes on LiCoO2 Surfaces: Role of Proton Adsorption on Oxygen Vacancy Formation

“…The intercalated H + on (012) can hinder the Li + insertion/extraction process and promote the phase transition toward cobalt oxide spinels. 22,28,31 The OH vac formation is not energetically favored on the fully lithiated (104) configuration, as already observed on the clean surface. Notwithstanding, H + can facilitate the O vac formation on partially lithiated (104) surfaces.…”

“…As a result, the Co atoms located on the second layer move upward between the first and second layers, becoming 4-fold coordinated (Figure a). It is proved that the presence of H + is preferable with respect to Li + due to the stable O–H bond formation, and it negatively affects the Li + (re)­insertion/extraction, which is detrimental for the cathode performance. , Thus, (104) termination facilitated the H + intercalation on the material at voltages superior to 3.7 V versus Li + /Li, where the half-delithiated surface predominates.…”

“…Nevertheless, (012) might play a detrimental role in the cathode performance since H + can intercalate on the subsurface layer and it is energetically unfavored to remove it. The intercalated H + on (012) can hinder the Li + insertion/extraction process and promote the phase transition toward cobalt oxide spinels. ,, …”

“…Consequently, LCO and materials of the same family have been extensively investigated using aqueous electrolytes. Hydrothermal experiments in acidic media corroborated by computational studies revealed that LCO is very prone to accommodate H + on Li + sites, which degrades the cathode due to the difficulty to (re)­insert Li ions. − Nevertheless, cyclability experiments in aqueous electrolytes have shown a similar behavior as for organic electrolytes with excellent stability of LCO. , Though LCO bulk degradation was not observed during battery operations, a deterioration of the surface was detected due to O loss and cobalt oxide (CoO) formation …”

Carbon-encapsulated V2O3 nanorods for high-performance aqueous Zn-ion batteries

“…However, further penetration of organic LIBs into the large-scale energy-storage (LSES) market is hindered by concerns over operational safety and cost, primarily due to the use of flammable organic electrolytes and expensive electrode materials (Tarascon and Armand, 2001;Yang et al, 2011). In contrast, aqueous LIBs attract much attention in terms of non-flammability, high ionic conductivity, and low cost, although they are characterized by low energy density and poor stability due to the narrow voltage window, high Li + diffusion barriers, and the interference of unfavorable H + intercalation (Bin et al, 2018;Posada-Pérez et al, 2021). As a promising alternative, rechargeable aqueous Zn-ion batteries (AZIBs) based on safe, non-toxic, and low-cost aqueous electrolytes are ideally suitable for LSES applications (Ming et al, 2019;Zhang N. et al, 2020;Chao et al, 2020).…”