Tuning the Porous Structure in PMMA-Templated Mesoporous MoO₂ for Pseudocapacitive Li-Ion Electrodes

Abstract

MoO2 is an exciting candidate for next-generation energy storage. It can be used for fast-charging applications in nanoscale form, but its kinetic performance is often limited by insulating MoO3 surface oxide layers. Here, we developed methods to produce polymer-templated porous MoO2 powders where electrical conductivity was well-maintained throughout the structure, even in the presence of some surface oxidation. Porosity, pore size, and crystallite size were controlled by varying the amount and size of the colloidal templates and through calcination temperature. The electrochemical performance was correlated with nanoscale structure: samples with high porosity, medium pore sizes, and good crystallinity display optimal rate capabilities, with over 100 mAh/g delivered in 3 min and 93% capacity retention after 1000 cycles. Kinetic studies were performed on samples with the largest and smallest crystallite sizes to understand the charge storage mechanism. In the sample with the smallest crystallite size, 85% of the total stored charge was capacitive, compared to 60% for the largest crystallite size. Sloping voltage profiles in materials with smaller domain sizes further suggest suppression of intercalation-induced phase transitions. This work thus provides insights into the mechanisms of charge storage in nanoscale MoO2 and design parameters for the production of fast charging materials.

Understanding How the Suppression of Insertion-Induced Phase Transitions Leads to Fast Charging in Nanoscale LixMoO2

“…The other option for the increased capacity in the nanoscale materials would be double layer capacitance. Based on previous work, we expect the surface area of the nanostructured MoO 2 to be in the range of 30−40 m 2 /g, 17,45,52 which would correspond to an electric double layer capacitance of 9−12 F/g (5−6 mAh/g for this voltage window), which alone does not account for the higher capacity. While the nanoporous MoO 2 has a similar surface area to the MoO 2 nanocrystals, the relatively low temperature used during the nanocrystal synthesis likely leads to a larger amount of undercoordinated surface sites and results in the significantly higher capacity from surface redox reactions compared to the other samples.…”

“…Between these chains are tunnels of interstitial sites for Li + diffusion and storage, while overlap of partially filled d orbitals between neighboring Mo atoms along the chains provides metallic electrical conductivity . Despite these advantages, micron-scale MoO 2 shows rather slow (de)­insertion kinetics, due to its need for a first-order phase transition during Li + insertion and its considerable 13% increase in unit cell volume at full lithiation. , More recently, nanostructured architectures of MoO 2 have shown much higher rate capability, ,− with limited ex situ evidence for changes in their phase transition mechanism during lithiation.…”

Ceramic Foams with Controlled Geometries Through 3D‐Printed Sacrificial Templates

“…Poly(methyl methacrylate) (PMMA) is frequently used because of its lowtemperature burn-out, low ash content, and biocompatibility. [24][25][26] Porous materials achieved using PMMA with the sacrificial templating method have yielded a wide range of nanoscale pores, finding applications in catalyst materials, [27][28][29][30][31][32][33] films, [34,35] membranes, [36,37] oxides, [38][39][40] carbon-based structures, [41][42][43][44] bioactive glass, [45] electrode materials, [46,47] and scaffolds and foams. [48,49] In addition, microscale pores have been successfully fabricated using packed PMMA beads (5-800 μm) as pore-forming agents, resulting in porosities ranging from 7% to 80%.…”

Ultrafast Sodium Intercalation Pseudocapacitance in MoS2 Facilitated by Phase Transition Suppression

“…Materials that store charge primarily through intercalation pseudocapacitance, however, do not undergo a first-order phase transition. One method to suppress these first-order phase transitions is to reduce the size of the primary particles, as has been shown in TiS 2 , 4 MoS 2 , 7,9 MoO 2 , 6,20 and LiFePO 4 . 21 Although several Li + systems (viz., Nb 2 O 5 and MoO 3 ) have demonstrated high levels of intercalation pseudocapacitance, 6,22−24 fewer materials have been explored for high-rate Na + ion intercalation pseudocapacitance.…”

“…Materials that store charge primarily through intercalation pseudocapacitance, however, do not undergo a first-order phase transition. One method to suppress these first-order phase transitions is to reduce the size of the primary particles, as has been shown in TiS 2 , MoS 2 , , MoO 2 , , and LiFePO 4 …”