"The future energy storage market will be diverse"
Interview with Prof. Yong Lei about promising new energy storage technologies for the battery of the future | December 2021
Since joining TU Ilmenau with an outstanding research record in the area of nanostructuring as the Head of the Applied Nano-Physics Group at the Institute of Physics and the IMN MacroNano® in 2011, Prof. Yong Lei and his team members have been working tirelessly on the development of different functional nanostructures for the next generation of high-performance devices for electrochemical energy conversion and storage. His main research topics include sodium- and potassium-ion batteries – so called ‘post lithium-ion batteries’ – with high energy density, as well as supercapacitors with high power outputs. Very recently Prof. Lei and his team have been developing an interesting new energy storage technology, hybrid-ion capacitors. We talked to him about the challenges and future prospects of this research.
Prof. Lei, most people are familiar with the currently dominating battery system, lithium-ion batteries (LIBs). The sodium-ion batteries and potassium-ion batteries you and your team are looking into are less known. Could you please explain the similarities and differences of these battery systems?
Actually, sodium-ion batteries (SIB) and potassium-ion batteries (PIBs) are similar to lithium-ion batteries. They all consist of three main components: cathode, anode and electrolyte, and their working principles are similar. But there are some small differences between them in chemistries. For examples, the cathodes of lithium-ion batteries usually contain cobalt or nickel which you don’t have to use in sodium- and potassium-ion batteries. Graphite can be used as the anode of lithium- and potassium-ion batteries, while sodium-ion batteries usually use hard carbon as the anode.
How many years have you been studying these new types of batteries?
We started working on sodium-ion batteries nine years ago. At that time, sodium-ion batteries were a burgeoning research field in order to deal with the concern about scarce lithium and cobalt resources. I think we are one of the earliest and dominating research groups to engage in this field in Europe. Regarding the research of potassium-ion batteries, my group was a pioneer research group in the world to start working on it from about five years ago.
What are your motivations for researching these sodium- and potassium-ion batteries?
The biggest motivation for developing sodium- and potassium-ion batteries are to find low-cost alternative energy storage systems to replace lithium-ion batteries. The largest advantage of sodium- and potassium-ion batteries is that they are much cheaper than lithium-ion batteries. The abundant sodium and potassium resources can meet the fast-increasing demand of cost-effective and large-scale energy storage applications in the future.
Are there some other advantages of these two types of batteries compared to lithium-ion batteries?
As far as I can see, there are at least three other advantages. First, the cost effectiveness of SIBs and PIBs is not just realized by replacing lithium with sodium or potassium. For the electrode materials, some scarce transition-metal elements that are used in the cathodes of LIBs like cobalt and nickel can be replaced with iron, manganese, magnesium in SIBs and PIBs. Moreover, in contrast to LIBs, the cheaper Al foil can be used as both anodic and cathodic current collectors for sodium- and potassium-ion batteries, while Al cannot be used as anodic current collector for LIBs because of Al-Li alloying chemistry.
The second advantage is the much better heat dissipation capability of SIBs and PIBs that might solve the safety issue of LIBs that is caused by poor heat dissipation (especially after dendrite-caused short-circuit within the battery) and sometimes even results in explosion of LIBs.
Third, lithium resources in the earth crust are low and highly uneven distributed, which means that supply is unstable and prices are rising. It is even predicted that lithium reserves could be short from 2025. In contrast, the resources of sodium and potassium are much higher and widely distributed around the world, making SIBs and PIBs much more sustainable battery systems for the future.
What are the main application fields of LIBs and PIBs?
In terms of current development, although SIBs and PIBs have the above-mentioned advantages, the performance of SIBs and PIBs – especially their energy density – are still not completely comparable with LIBs, which is mainly caused by the larger ionic radius of sodium and potassium – i.e., more difficult and slower ionic insertion/extraction – compared to lithium. This means that, if SIBs and PIBs are to provide the same energy storage performance as LIBs, SIBs and PIBs must have a larger volume (or mass) than LIBs. Therefore, SIBs (and PIBs) have so far been used mainly in areas where low cost is very important while energy density is not, such as solar cell plants, wind power plants, large-scale power grids, electric bicycles, and low-speed electric vehicles. But I believe in the future, with further development of SIBs and PIBs, they could replace LIBs in many more application areas.
One of your current projects from DFG concerning theses battery types is a project entitled ‘Sodium- and potassium-ion batteries based on highly ordered electrode architectures’. What are the research focuses of this project?
In this project, we focus on the electrode structure design of SIBs and PIBs. As mentioned above, the sodium and potassium ions are larger than the lithium ions, which leads to slow diffusion kinetics and poor electrode structure stability of SIBs and PIBs. Well-defined electrode architectures can counter and solve these problems. Designing such an advanced electrode can not only enhance the reaction activity between the electrode and electrolyte ions, but also buffer the volume variation of the electrode during the ionic insertion/extraction.
What kind of technology and equipment do you use for this research?
In my group, we have all the technologies and lab facilities we need for this kind of battery research, including different film-coating techniques like ALD, PVD and CVD to synthesize electrode structures, glove-boxes and other punching machines to assemble batteries as well as a full electrochemical testing station and a Landian system to assess battery performances. Furthermore, we use SEM, TEM, AFM, XRD, Raman and XPS measurements in the Center of Micro- and Nanotechnologies (ZMN) to characterize the chemical and physical properties of the electrodes and their electrochemical processes.
What results have you been able to achieve within this research so far?
Within the past years we have made quite a few important achievements regarding SIBs and PIBs: First, SIBs using Sb nanorod arrays show high capacities of 620 mAh g-1 at 0.5 A g-1 and 557.7 mAh g-1 at 20 A g-1, remaining the best Sb half-cell performance even three years after the work was published (Energy Environ. Sci. 2015, 8, 2954). Furthermore, SIBs using π-conjugated organic electrodes show one of the best energy densities reported for both organic SIBs and LIBs (J. Am. Chem. Soc. 2015, 137, 3124), a finding which was highlighted by the well-known scientific platform Phys.Org in the USA under the title ‘Na-ion batteries get closer to replacing Li-ion batteries’ (phys.org/news/2015-03-na-ion-batteries-closer-li-ion.html). Finally, as a pioneer group for PIB research, we have realized the first PIB full battery cell in the world and further developed it with a high energy density (130 Wh kg-1). This research result was published in Nature Communications (2018,9(1), 1720, 2018) and has received very high attention (594 citations after three years of its publication).
What challenges still remain in the research field of sodium- and potassium-ion batteries?
As mentioned above, the main challenge of SIBs and PIBs is to improve their energy density while keeping their advantages such as low-cost. As two highly promising alternatives to LIBs, SIBs and PIBs have been intensively investigated within the past years by both scientists in research institutions and engineers from industry. I think that in the future, with further research efforts in these fields, relevant technical developments and engineering solutions, the energy density of SIBs and PIBs shall be certainly improved.
Are there any other types of innovative energy storage devices that you’d consider promising or worth looking into?
I think that the future energy storage market will be diverse. Besides the sodium- and potassium-ion batteries, I would first recommend hybrid ion capacitors with a capacitor-type cathode and a battery-type anode, which can deliver both high energy density like a battery and high power density like a supercapacitor. Another very important type of energy storage devices could be micro-supercapacitors and micro-batteries that are of great significance as miniaturized power source for microelectronics. We have started this research topic with some very promising results such as the high-energy micro-supercapacitor based on microminiaturized honeycomb nanoscaffold (Nature Communications (2020, 11 (1), 299).
Prof. Dr. Yong Lei
Head of Applied Nanophysics Group
+49 3677 69-3748