Although lithium-ion batteries, or LIBs, largely power the world as we now know it and are included in everything from electric vehicles to smartphones, the pressing need for grid-scale energy storage and other applications has researchers frantically searching for cheaper alternatives to LIBs that are more readily available.
As the subject of extensive ongoing research, batteries that use abundant multivalent metals are thought to be capable of revolutionising energy storage. Now, in a new study, the UH researchers have reviewed the current state of multivalent metal-ion battery research efforts. In doing so, they have devised a ‘roadmap’ for future works in this area and have stated what they believe to be the top candidate materials—aluminium, zinc, magnesium, and calcium—all of which have great potential but also plenty of challenges at the same time.
Multivalent Metal-based batteries
In the UH study, the researchers considered the key strengths as well as the common misconceptions of multivalent metal-based batteries. Multivalent metal-ion batteries are seen as an alternative solution for large-scale energy storage as energy density targets continue to rise. However, they should not, the research notes, be viewed as a direct replacement of or competitor to lithium-based batteries, such as LIBs.
The research also looked at the growth behaviour of metal anodes. For example, while magnesium is a promising material, it is not universally applicable. Rather, it is only suitable for use in selected electrolyte solutions where there are no side reactions and where the active metal surface is free of passivation.
Large-scale energy storage is a top priority for researchers and companies like Tesla. Pictured: Tesla’s ‘Megapack’ battery. Image Credit: Tesla.
As Yanliang Liang, the study’s co-first author explains: “We also discuss design strategies to enable genuine multivalent metal-ion-based energy storage materials with competitive performance”.
The University of Houston researchers identified ‘prime candidates’ for potential lithium alternatives, which include magnesium, calcium, zinc, and aluminium. They say that all these materials are known multivalent metals or metals that have several valence electrons to donate. They also share many similarities in working principles with lithium-ion batteries, suggesting that they could find a wide range of applications in industry.
Other key points include:
Direct use of metals as anodes is an important aspect for the safety and energy density promises of these batteries, but there are uncertainties surrounding the viability of these anodes.
Electrolyte solutions and the industry’s understanding of the associated interfacial phenomena are improving but still far from established.
Engineers who are seeking efficient cathode materials must consider design aspects that are uncommon in traditional battery studies. After all, the ion storage mechanism of multivalent battery cathodes is much more complicated than its lithium-ion counterpart.
To ensure future research is directed at improving the batteries, the UH researchers recommend the adoption of practices to comprehensively assess the compatibility of metal anodes with electrolyte solutions. They also recommend that further work should be done to i) better understand the growth behaviour of the metal anode so that safety promises can be delivered and ii) altogether design better cathode materials.
At present, the University of Houston researchers claim that none of the multivalent metal options are ready to replace lithium. However, some are further along than others.