Predicting the Electrochemical Properties of MnO₂ Nanomaterials Used in Rechargeable Li Batteries: Simulating Nanostructure at the Atomistic Level

Abstract

Nanoporous beta-MnO2 can act as a host lattice for the insertion and deinsertion of Li with application in rechargeable lithium batteries. We predict that, to maximize its electrochemical properties, the beta-MnO2 host should be symmetrically porous and heavily twinned. In addition, we predict that there exists a "critical (wall) thickness" for MnO2 nanomaterials above which the strain associated with Li insertion is accommodated via a plastic, rather than elastic, deformation of the host lattice leading to property fading upon cycling. We predict that this critical thickness lies between 10 and 100 nm for beta-MnO2 and is greater than 100 nm for alpha-MnO2: the latter accommodates 2 x 2 tunnels compared with the smaller 1 x 1 tunnels found in beta-MnO2. This prediction may help explain why certain (nano)forms of MnO2 are electrochemically active, while others are not. Our predictions are based upon atomistic models of beta-MnO2 nanomaterials. In particular, a systematic strategy, analogous to methods widely and routinely used to model crystal structure, was used to generate the nanostructures. Specifically, the (space) symmetry associated with the nanostructure coupled with basis nanoparticles was used to prescribe full atomistic models of nanoparticles (0D), nanorods (1D), nanosheets (2D), and nanoporous (3D) architectures. For the latter, under MD simulation, the amorphous nanoparticles agglomerate together with their periodic neighbors to formulate the walls of the nanomaterial; the particular polymorphic structure was evolved using simulated amorphization and crystallization. We show that our atomistic models are in accord with experiment. Our models reveal that the periodic framework architecture, together with microtwinning, enables insertion of Li anywhere on the (internal) surface and facilitates Li transport in all three spatial directions within the host lattice. Accordingly, the symmetrically porous MnO2 can expand and contract linearly and crucially elastically under charge/discharge. We also suggest tentatively that our predictions for MnO2 are more general in that similar arguments may apply to other nanomaterials, which might expand and contract elastically upon charging/discharging.

Hierarchical mesoporous urchin-like Mn3O4/carbon microspheres with highly enhanced lithium battery performance by in-situ carbonization of new lamellar manganese alkoxide (Mn-DEG)

“…Among the various transition metal oxides, manganese oxides (MnO [5,6], MnO 2 [7,8], Mn 2 O 3 [9,10], Mn 3 O 4 [11,12]) have attracted considerable attention because of their excellent electrochemical properties. As one of the manganese oxides, hausmannite Mn 3 O 4 has high theoretical capacity, low toxicity, low price, thermal stability and natural abundance, which is a promising anode material for LIBs [13][14][15].…”

TiO2 Nanotube Arrays Grafted with MnO2 Nanosheets as High-Performance Anode for Lithium Ion Battery

“…However, due to volume expansion, aggregation and intrinsically low electronic conductivity, MnO 2 suffers from rapid capacity fade during cycling process. Some methods have been used to solve the problem and nanostructured MnO 2 materials, including nanowires, nanotubes, and nanoflakes etc, have been developed to enhance its performance [8,16,[20][21][22][23]. Besides, MnO 2 nanoparticles loaded onto a conductive layer using deposition technique has been reported to show high electrochemical performance as well [24][25][26].…”

‘Breathing-crystals’ the origin of electrochemical activity of mesoporous Li–MnO2

“…The parameters are available in ref [ 14 ]. The MD simulations were performed using the DL_POLY code.…”