Magnesium is one of the few elements on Earth that offers real long-term potential for technical mass applications. One reason for this is that reserves of Mg minerals are widespread and easy accessible and these reserves should last approximately 430 000 years (seawater not included), if the consumption remains constant. Another reason is that magnesium is recyclable and virtually non-toxic. Rather, it is an essential element in nature and central in photosynthesis and for metabolic functions in animals and humans. In batteries, the Mg ion can transport and store two charges per ion and, thus, offers the potential for higher storage capacities, which is complemented by the fact that a Mg metal anode can work safely in a cell with a liquid electrolyte. In contrast, Li ions are stored in graphite in the anode of current Li ion batteries, for safety reasons – and are thereby diluted by a factor of eight.

A good, working and rechargeable Mg battery would not only be technical progress, which could potentially improve the performance of battery-driven applications, it would also be a relief, because it opens up the possibility for the mass production of big batteries, which is, in the mid- to long-term, more difficult to achieve with current Li ion technology due to the limitation of certain raw materials and the expensive recycling process for lithium.

The work on Mg batteries started with some scattered publications a few decades ago. The feasibility was demonstrated in principle but it became clear that the classical concepts for electrolytes and electrodes cannot just be transferred or applied to Mg battery technology and more questions were raised than answered. Then, the field moved more in the focus of battery research in the 2000s, after a breakthrough had been achieved with the development of a new and effective electrolyte. However, there were still no more than 10 to 15 publications per year. This changed considerably in the 2010s when the number of publications steadily increased so that at the end of the decade, between 120 and 150 publications per year had been made on various topics of Mg batteries.

After this initial phase, with its quest for the first electrolytes, cathodes and anodes and after both progress and frustration, it is our aim to present here a first extensive summary of the principles, state-of-the art and prospects of Mg batteries.

The book will start with a general overview and motivation for a Mg battery by M. Fichtner, followed by an electrolyte part with a chapter on the development of non-aqueous systems by R. Mohtadi and O. Tutusaus and a chapter on solid-state magnesium ion conductors by C. Battaglia et al. Theoretical modelling of multivalent ions in inorganic hosts is critical for an understanding of the dynamics and storage process of Mg in materials and will be introduced by G. Gautam and P. Canepa. The anode side seems simple and straightforward because pure Mg metal has been claimed to be suitable as an anode material. However, the chapter by E.M. Sheridan et al. shows that there are many more options, which all have their prospects and drawbacks. Complementing work is presented by M. Matsui in a chapter where the electrochemical properties of magnesium metal and intermetallic anodes are discussed.

One of the most critical issues in the field is the development of effective insertion cathodes for magnesium batteries. Intercalation and conversion materials and related mechanisms are introduced by H.D. Yoo and S.H. Oh. B. Ingram will further expand upon this discussion with a contribution on high voltage cathodes, which includes a techno-economical evaluation based on a methodology developed in their laboratory. The part on cathode materials will be complemented by an overview on organic electrodes by J. Bitenc and R. Dominko and a further contribution from Z. Zhao-Karger, who gives an overview of the properties and status of Mg sulfur batteries.

An unusual but interesting concept is the dual-ion battery, which relies on the combination of the Mg ion plus a second ion, both reversibly stored in different electrodes. The concept, first results and interesting effects of the co-intercalation of cations and anions will be presented by H. Li et al. As aqueous (primary) Mg batteries are still one of the most widespread applications, D. Hoeche will present an overview on the status of this technology.

Finally, although the development of Mg batteries is still at an early stage and the performance of Mg batteries is not yet competitive, data already available from laboratory cells has been collected by C. Tomasini Montenegro et al. The data was evaluated for the first life-cycle analysis of Mg batteries and gives clear indications for further directions in the development of Mg battery cells.

At this point, I would like to cordially thank all of the authors and co-authors for their commitment and dedication and their contributions to all of the relevant topics of Mg battery technology, making “Magnesium Batteries – Research and Applications” a comprehensive reflection of the state-of-the art in the field.

Maximilian Fichtner

Helmholtz-Institute Ulm (HIU), Helmholtzstr. 11, Ulm, 89081, Germany