Reaction Network of Layer-to-Tunnel Transition of MnO₂

Abstract

As a model system of 2-D oxide material, layered δ-MnO2 has important applications in Li ion battery systems. δ-MnO2 is also widely utilized as a precursor to synthesize other stable structure variants in the MnO2 family, such as α-, β-, R-, and γ-phases, which are 3-D interlinked structures with different tunnels. By utilizing the stochastic surface walking (SSW) pathway sampling method, we here for the first time resolve the atomistic mechanism and the kinetics of the layer-to-tunnel transition of MnO2, that is, from δ-MnO2 to the α-, β-, and R-phases. The SSW sampling determines the lowest-energy pathway from thousands of likely pathways that connects different phases. The reaction barriers of layer-to-tunnel phase transitions are found to be low, being 0.2-0.3 eV per formula unit, which suggests a complex competing reaction network toward different tunnel phases. All the transitions initiate via a common shearing and buckling movement of the MnO2 layer that leads to the breaking of the Mn-O framework and the formation of Mn(3+) at the transition state. Important hints are thus gleaned from these lowest-energy pathways: (i) the large pore size product is unfavorable for the entropic reason; (ii) cations are effective dopants to control the kinetics and selectivity in layer-to-tunnel transitions, which in general lowers the phase transition barrier and facilitates the creation of larger tunnel size; (iii) the phase transition not only changes the electronic structure but also induces the macroscopic morphology changes due to the interfacial strain.

Rapid mass production of two-dimensional metal oxides and hydroxides via the molten salts method

“…It is agreed that birnessite-type MnO 2 is the kinetically favoured product and is always formed first, while tunnelled-structure MnO 2 species, such as α phase and β phase, are the thermodynamically favoured products and can be formed from δ -MnO 2 (ref. 40). After the reaction time was extended to 10 min, the crystal structure and 2D morphology features of δ -MnO 2 remain unchanged (Supplementary Fig.…”

Electrochemically Derived Graphene‐Like Carbon Film as a Superb Substrate for High‐Performance Aqueous Zn‐Ion Batteries

“…The projector augmented wave (PAW) pseudopotentials were selected to describe core and valence electrons, and the generalized gradient approximation based on the Perdew–Burke–Ernzerhof (GGA‐PBE) functional with on‐site Coulomb repulsion (PBE+U) was selected to describe electron exchange and correlation. The effective U–J term ( U eff ) as determined by linear response theory was set as 4.0 eV for Mn . The monolayer supercells of 3 × 3 × 1 MnO 2 unit cells adsorbed with Zn 2+ and Na + ions were constructed to simulate their electrostatic binding behavior.…”

Salt‐Assisted Synthesis of 2D Materials

“…However, Li + ‐intercalated MnO 2 cannot be synthesized using a similar molten salt method with LiNO 3 , possibly because Li + (76 pm) has a relatively smaller radius than Na + (102 pm) and K + (138 pm); the small radius of Li + cannot maintain the layered structure, which turned into β‐MnO 2 with a (1 × 1) tunnel‐like structure. [ 139 ] Thus, ion species should be considered when designing the synthesis of target products. 2) Ion states can affect the growth rate of different directions.…”