Effects of Particle Size on Mg²⁺ Ion Intercalation into λ-MnO₂ Cathode Materials

Abstract

An emergent theme in mono-and multivalent ion batteries is to utilize nanoparticles (NPs) as electrode materials based on the phenomenological observations that their short ion diffusion length and large electrode−electrolyte interface can lead to improved ion insertion kinetics compared to their bulk counterparts. However, the understanding of how the NP size fundamentally relates to their electrochemical behaviors (e.g., charge storage mechanism, phase transition associated with ion insertion) is still primitive. Here, we employ spinel λ-MnO 2 particles as a model cathode material, which have effective Mg 2+ ion intercalation but with their size effect poorly understood to investigate their operating mechanism via a suite of electrochemical and structural characterizations. We prepare two differently sized samples, the small nanoscopic λ-MnO 2 particles (81 ± 25 nm) and big micron-sized ones (814 ± 207 nm) via postsynthesis size-selection. Analysis of the charge storage mechanisms shows that the stored charge from Mg 2+ ion intercalation dominates in both systems and is ∼10 times higher in small particles than that in the big ones. From both X-ray diffraction and atomic-resolution scanning transmission electron microscopy imaging, we reveal a fundamental difference in phase transition of the differently sized particles during Mg 2+ ion intercalation: the small NPs undergo a solid-solution-like phase transition which minimizes lattice mismatch and energy penalty for accommodating new phases, whereas the big particles follow conventional multiphase transformation. We show that this pathway difference is related to the improved electrochemical performance (e.g., rate capability, cycling performance) of small particles over the big ones which provides important insights in encoding within the particle dimension, that is, the single-phase transition pathway in high-performance electrode materials for multivalent ion batteries.

Manganese‐Based Materials for Rechargeable Batteries beyond Lithium‐Ion

“…www.advancedsciencenews.com particle size (814 ± 207 nm) follows a conventional multiphase transformation pattern. [114] This work offers a deep insight to the impact of particle size on kinetics, proving that the singlephase transition pathway is crucial to the improved rate capability (Figure 10g) and cycling performance of λ-MnO 2 .…”

“…g) Rate performance of nanoscopic (S-LMOP) and micron-sized (B-LMOP) Li 0.19 Mn 2 O 4 upon Mg 2+ ion intercalation. Reproduced with permission [114]. Copyright 2019, America Chemical Society.…”

Hydrated MgxV5O12 Cathode with Improved Mg2+ Storage Performance

“…Remarkably, the Mg 2+ intercalation is highly reversible at a high average operating voltage of 2.1 V versus Mg/Mg 2+ (Figure 2a) at 0.05 A g −1 , along with a small hysteresis between the galvanostatic charge‐discharge (GCD) profiles (Figure S4, Supporting Information). Compared to the classical cathodes (e.g., Chevrel phases, MnO 2 , VOPO 4 ∙ x H 2 O, V 2 O 5 ) of MIBs, [ 13,17,18,23,26–29 ] increasing the current density has a significantly lower impact on the discharge capacities of the MgVOH electrode (Figure 2b), pointing to the fact that our MgVOH electrode possesses a flexible and ion‐conducting framework for Mg 2+ intercalation. Increasing the current densities from 0.1 to 0.2, 0.5, 1, 2, and to 4 A g −1 leads to the reversible and stable capacities of 143, 127, 102, 84, 68, and 35 mAh g ‐1 , respectively.…”

“…This is strongly related to the large polarization of anionic frameworks of electrodes and more intense repulsions among the intercalated divalent Mg ions due to a higher charge‐to‐radius ratio of Mg 2+ compared to Li + . [ 16–18 ] Because of heavy anionic frameworks and moderate polarity of anions, the classical Mo 6 S 8 Chevrel phases and transition metal dichalcogenides show an Mg 2+ screening effect, [ 19,20 ] mitigating the solid‐state diffusion difficulty of Mg 2+ to some extent. However, the facilitation of Mg 2+ diffusion within these electrode frameworks comes at the cost of average operating potentials (≈1 V vs Mg/Mg 2+ ) and gravimetric capacities, thus leading to low energy densities.…”

Effective 3D open-channel nanostructures of a MgMn2O4 positive electrode for rechargeable Mg batteries operated at room temperature

“…The other method is to control the nanostructure of the positive electrode pasted with the spinel nanoparticles to ease the insertion/extraction of the Mg ions. 22 We have designed an MMO powder with nanometerscale particles in a double-tiered open-channel network, which improves Mg 2+ diffusion from the electrolyte to the surface of the MMO nanoparticles. 23 Here, electrochemical battery operations were investigated in a three-electrode beaker cell heated to 100 C. However, the effects of the MMO powder characteristics, such as the specic surface area and porosity, on the room-temperature operation in a conventional coin cell remained unclear.…”