Sodium-Ion Batteries: Materials, Characterization, and Technology, 2 Volumes

Book Preface

Research studies on Na-ion batteries (abbreviated as NIBs or SIBs) began at the same time as Li-ion batteries (LIBs), during the late 1970s. However, due to less promising results, NIB research was largely abandoned for a few decades. This has changed in the late-2000s, and today research on NIBs is a very active and dynamic field. The main motivation beyond the development of new NIBs chemistries and optimizing their manufacturing is to advance batteries based on abundant and noncritical ele-ments. The working principle of (nonaqueous) LIBs and NIBs is similar, i.e. solid intercalation/insertion electrodes and liquid organic electrolytes are used. A lot of “know-how” could be directly transferred from LIBs to NIBs which is an impor-tant reason for the rapid development in NIB’s materials research. Another major advantage is that NIBs can be produced using the same manufacturing technology as LIBs. This substantially decreases the technological barrier for large-scale pro-duction which is a major hurdle for other alternative batteries. Na is more than 1000 times more abundant than Li and geographically evenly distributed which facili-tates the supply of noncritical battery materials unlike Li which is listed as a critical element according to the EU. It is important to realize, however, that replacing Li by Na is not sufficient to realize sustainable batteries as the alkali metals only con-tribute to a small fraction to the total battery weight. A more generalized approach is therefore needed which includes completely avoiding other critical metals such as cobalt. The Cu current collector in LIBs can be replaced by cheaper, abundant, and easy recyclable Al. Fortunately, the chemistry of Na cathode materials is richer than LIBs, which opens many avenues for more abundant alternative elements such as Fe, Mn, and Cu to Co and Ni used in LIBs (i.e. Co is also a critical material according to the EU, while the demand for Ni is predicted to increase by a factor of 25 by 2030 according to McKinsey). The recent economic turmoil due to the COVID-19 and the microchip crises is a sad but unquestionable example on how fragile our econ-omy is and how it strongly depends on the materials supply. More diverse battery chemistries will help circumvent such problems in the future in relation to supply chain fluctuations.
Compared to LIBs, the energy density of NIBs is generally around 20% lower due to the lower cell voltage, although the difference strongly depends on the exact cell
chemistry. NIBs have a similar performance to LiFePO4-based LIBs, for example, but it is important to note that phosphate rock representing phosphorus in any form is also a critical material according to the EU. Compared to lead-acid battery, which is, next to LIBs, the second major rechargeable battery technology currently in use, NIBs show a manifold improvement in energy density. For this reason, the field of applications of NIBs will be most likely stationary energy storage and small vehicles for which “premium” Li-ion batteries are not needed. It is quite encouraging that several companies around the world are active in developing materials and technol-ogy for SIBs. The companies Faradion (Reliance), AMTE, Tiamat, or HiNa battery showed various prototypes and demonstrators for different applications, and Natron Energy sells NIBs for high-power applications. For example, HiNa Battery Technol-ogy recently demonstrated a mini-electric vehicle powered by an 80-Ah pack in 2018, as well as a 100- kWh and 1-MWh grid storage systems in 2019 and 2021. A picture of the 1-MWh battery system is shown at the end of the preface and is an illustrative example on what can be already realized with NIBs. The same company reported that their pouch cells show an average voltage of 3.2 V, a specific energy of 145 Wh kg−1, ≥83% capacity retention after 4500 cycles at a 2C charge–discharge rate, and a working temperature down to −40 ∘C. A very recent announcement came from CATL, a large producer of LIBs for electric vehicles. Despite some previous believe that the main market of NIBs would be stationary applications, CATL suggested to use them in tandem with LIBs in a battery pack for electric vehicles. While the NIBs provide energy at high power and low temperatures, LIBs would guarantee an overall high energy density of the battery pack. This way, specific advantages of two technologies would be combined. All companies announced producing NIBs in the coming years, so it will be intriguing to see whether the potential of NIBs can be materialized in a competitive environment. The picture at the end of this preface gives an impression on what is already possible with NIBs.
As previously mentioned, the abundance of Na is a major driver for the develop-ment of NIBs. From a scientific point of view, the replacement of Li by Na in elec-trochemical cells raises fundamentally intriguing questions as the Na ion is approx. 30% larger. The difference in size and hence polarizability has a strong impact on the diffusion properties, the phase behavior, the solvation, the Gibbs energy, and the charge transfer. Whether the larger ion size of Na+ leads to less or more favorable properties compared to Li+ is of key relevance.
This book intends to provide an up-to-date overview on materials development for Na-ion batteries. Part 1 deals with anode materials in which properties of graphite, hard carbon, and alloys are discussed. Part 2 summarizes important cathode mate-rials, among layered oxides, polyanion compounds, and Prussian blue materials. Part 3 contains information on advanced characterization tools and methods used in NIB research, among X-ray/neutron scattering methods, NMR spectroscopy, pair distribution function analysis, and computational studies. Electrolytes are discussed in Part 4, which includes information on carbonate- and ether-based electrolytes, ionic liquids, polymer electrolytes as well as oxidic solid-state electrolytes. Part 5 addresses failure mechanisms and safety, technological, and environmental aspects of NIBs. Related technologies are discussed in Part 6, which includes chapters on high-power devices, seawater batteries, and solid-state batteries.

Picture of a 1-MWh NIB battery installed in 2021 in Taiyuan (China) as part of a micro-grid system (photo: HiNa battery/IOP, Yong-Sheng Hu).
We heartily thank all the authors who contributed their time and expertise to make this book reality. Hopefully, this book will be of help and support to the research community. We all hope that NIBs will face a bright future.

Philipp, Magda, Yong-Sheng