Expanding the Material Search Space for Multivalent Cathodes

Abstract

Multivalent batteries are an energy storage technology with the potential to surpass lithium-ion batteries; however, their performance have been limited by the low voltages and poor solid-state ionic mobility of available cathodes. A computational screening approach to identify high-performance multivalent intercalation cathodes among materials that do not contain the working ion of interest has been developed, which greatly expands the search space that can be considered for material discovery. This approach has been applied to magnesium cathodes as a proof of concept, and four resulting candidate materials [NASICON V2(PO4)3, birnessite NaMn4O8, tavorite MnPO4F, and spinel MnO2] are discussed in further detail. In examining the ion migration environment and associated Mg2+ migration energy in these materials, local energy maxima are found to correspond with pathway positions where Mg2+ passes through a plane of anion atoms. While previous studies have established the influence of local coordination on multivalent ion mobility, these results suggest that considering both the type of the local bonding environment and available free volume for the mobile ion along its migration pathway can be significant for improving solid-state mobility.

Novel Structural Motif To Promote Mg-Ion Mobility: Investigating ABO4 Zircons as Magnesium Intercalation Cathodes

“…Furthermore, the distortion of the octahedral interstitial site in the zircon structure reduces the preference of the Mg-ion for this site. Minimizing the change in coordination of the migrating ion along the diffusion pathway correlates with smaller site energy differences, resulting in favorable, lower migration barriers because of the resulting flatter energetic landscape. , Large changes along the path to lower coordination numbers, such as 2 and 3, have been shown to correspond to the most unfavorable sites along a diffusion pathway for multivalent ions in a variety of materials. ,, This makes overlapping distorted octahedral and tetrahedral interstitial sites of zircon particularly well suited for Mg-ion transport. The interlocked interstitial sites of the one-dimensional zircon diffusion channels result in a “6-5-4” change in coordination, which corresponds to significantly less coordination change as compared to the typical “6-3-4” change in coordination found in diffusion pathways composed of face-sharing tetrahedral and octahedral sites (see Figure ).…”

“…The interlocked interstitial sites of the one-dimensional zircon diffusion channels result in a “6-5-4” change in coordination, which corresponds to significantly less coordination change as compared to the typical “6-3-4” change in coordination found in diffusion pathways composed of face-sharing tetrahedral and octahedral sites (see Figure ). The intermediate coordination of 5 in the zircon structure is much more favorable than 3 because migrating ions avoid squeezing through a plane of anions, which usually corresponds to higher energies …”

“…High energy sites along the diffusion pathway in a material can also correspond to the mobile cation passing through points of lower coordination. These lower coordination points may represent a position where the mobile cation passes through a plane of neighboring anions . For example, in the structural motif where a diffusion pathway is composed of edge-sharing octahedra as illustrated in Figure a, the lowest coordination occurs when the mobile cation passes through a triangular plane of 3 anions.…”

“…Multivalent batteries are one of several emerging “beyond Li-ion” battery energy storage technologies that aim to enable large-scale renewable energy. − Of the multivalent battery chemistries (Mg 2+ , Ca 2+ , Zn 2+ , Al 3+ , etc. ), the most progress has been made with magnesium batteries since the first lab-scale prototype magnesium cell was reported in 2000 using a Mg x Mo 6 S 8 Chevrel cathode .…”

“…), the most progress has been made with magnesium batteries since the first lab-scale prototype magnesium cell was reported in 2000 using a Mg x Mo 6 S 8 Chevrel cathode . Research efforts continue to focus on improving Mg cathodes in order to identify materials with suitable energy density and rate capability for high-performance batteries. − While progress has been made, the best available Mg cathodes exhibit inferior voltages as compared to state-of-the-art Li cathodes, and poor solid-state mobility, which results in insufficient rate capability . The identification of high-performance Mg cathodes is an issue that must be overcome in order to realize a Mg battery chemistry that can outperform Li-ion batteries and warrant commercialization. − …”