(588cu) Electrochemical Li Extraction from Brine with Spinel MnO2-Based Materials

A growing market on energy storage technologies has led to an increase in demand for lithium. Traditional Li recovery is carried out through solar evaporation and precipitation from salt-rich lakes.¹ As an alternative, electrochemically switched ion exchange materials have demonstrated promising results in product purity and separation as Li has a low concentration (compared to Na or Mg) in the mentioned lakes.² The extraction mechanism for these systems is as follows: an electrical potential is applied to an electrode material to drive the ionic intercalation/deintercalation reaction.³ Spinel manganese dioxide (l-MnO₂) is a key electrode material due to its superior selectivity for Li over other ions in solution. It also has a higher separation factor compared to other systems and better stability in water than layered materials.⁴ Although l-MnO₂ presents the desirable Li selectivity, it suffers from structural deterioration due to the Jahn-Teller effect (JTE).⁵ As Li intercalates into the system, a reduction reaction occurs to the Mn atom, changing its oxidation state from 4+ to 3+, causing the JTE that yields a phase change due to a disproportionation reaction in Mn³⁺-rich zones.⁶ Through cycling, this leads to irreversible capacity loss and degradation of the l-MnO₂ lattice in the form of Mn dissolution; some studies report a capacity loss of over 50% after 30 cycles.⁷ To minimize the JT effect, numerous studies have opted for doping the structure with other metals. Brady et al. suggested in their study of hollandite that the dopant should facilitate the reduction M⁴⁺® M³⁺ more readily than Mn to mitigate the JT effect.⁸

Aiming to investigate the same properties in l-MnO₂, we used DFT implemented by the Vienna Ab Initio Simulation Package (VASP) to perform numerous simulations on the geometry optimization as well as doping this composition with other metals to study the structural stability during ionic intercalation. The carried-out calculations included the charge redistribution after the addition of a dopant. It was concluded that a stoichiometric amount of doping must be implemented to fully mitigate the JTE present in Mn. The solid-state nudged elastic band method⁹ was implemented to study the irreversible phase change present in the spinel MnO₂ electrodes upon cycling.¹⁰ These results elucidate how even small amounts of doping (³5%) can mitigate Mn dissolution and enhance the capacity upon cycling. This fundamental study was carried out for the application of l-MnO₂ as a working electrode in a flow cell to obtain a better means of extracting Li from brine.

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