Modeling the kinetics of lithium ion incorporation into tunnel vacancies of spinel-type manganese dioxide λ-MnO2 from aqueous solutions

Spinel-type manganese dioxide, lambda-MnO2, has a three dimensional network of tunnels connecting vacant sites in the oxide lattice. This oxide can preferentially incorporate lithium ions into the lattice vacancies since lithium ions fit the tunnels in size. With the incorporation, the lattice Mn(IV) reduces to Mn(III) maintaining electric neutrality. The rate of incorporation decreased with time, suggesting inhibition of the reaction due to reaction products as pre-formed Mn(III) in the oxide decreased the incorporation rate. The incorporation rate increased with increasing pH, temperature, the mass concentration and specific surface area of the lambda-MnO2 samples, and the amount of a radical scavenger added. A kinetic model is proposed by considering the following reaction processes: (1) an Mn(IV)-vacancy pair on the oxide surface oxidizes a hydroxide ion in solution and forms a free vacancy, lattice Mn(III), and a hydroxyl radical. A part of the products reacts backward because the reaction products do not satisfy electric neutrality; (2) the free vacancy remaining takes up the lithium ion from the solution to reestablish electric neutrality; (3) the formed Mn(III)lithium ion pair reduces the Mn(IV)-vacancy pair in the oxide bulk and the lithium ion transfers to the bulk through the tunnel, regenerating the Mn(IV)-vacancy pair on the surface; and (4) the hydroxyl radical decomposes to oxygen and water. The steps (1) and (2) were assumed to be rate determining, and the rate equation was derived. The model rate equation reproduced the observed results, and the lithium ion incorporation properites of lambda-MnO2 could be evaluated with the model parameters.