by Chenglin Zhang, Huaping Zhao and Yong Lei
Group of Applied Nanophysics, Institute of Physics & IMN MacroNano®, Technische Universität Ilmenau, Ilmenau 98693, Germany
Rechargeable batteries are one of the key parts of energy storage and delivery. Particularly in the boost of renewable energy utilization for the global mission of carbon neutrality, the grid-level energy storage of electricity generated from intermittent renewable sources (e.g., solar and wind) requires amounts of low-cost and high-capacity rechargeable batteries. Currently, lithium-ion batteries (LIBs) are one of the most attractive candidates owing to their outstanding energy density. But with the increasingly growing demands of LIBs for electric vehicles, portable electronics, and power grids, there has been raising concerns about the sustainability and cost of both lithium and cobalt which are indispensable internal components employed in LIBs. Especially, an ever-growing electric vehicles market might not leave enough lithium and cobalt for on-grid LIBs to satisfy large-scale energy storage applications. Therefore, there are pressing needs for alternative rechargeable batteries that are based on naturally abundant elements. In the last decade, considerable research efforts have been devoted to developing metal-ion batteries beyond lithium, such as sodium and potassium.
In addition to the sodium’ high abundance in the Earth’s crust, sodium-ion batteries (SIBs) offer reduced costs due to cheaper sodium-salt-containing electrolytes and economical aluminum current collectors, instead of copper current collectors in LIBs. Potassium is also abundant and potassium-salt-containing electrolytes are also cheaper than lithium-salt-containing electrolytes. More importantly, potassium-ion batteries (PIBs) are particularly promising for high power density owing to the high ionic conductivity and small Stokes radius of potassium ions. Moreover, both SIBs and PIBs offer improved operational safety attributing to the better heat dissipation properties of both sodium and potassium than that of lithium and the higher flashing point of both sodium- and potassium-salt-containing electrolytes than that of lithium-salt-containing electrolytes. Furthermore, SIBs and PIBs processing systems are also identical as that of LIBs, making SIBs and PIBs production a ‘drop-in’ technology.
In view of the great potentials of SIBs and PIBs ascost-effective and sustainable successors to LIBs, the Group of Applied Nanophysics at TU Ilmenau started to work on the development of SIBs in 2012 and PIBs in 2015, respectively, covering a broad range of challenging topics from materials and electrodes to prototype full cell. The researchers from the Group of Applied Nanophysics put much efforts into improving the energy storage performance of SIBs and PIBs with emphasis on electrodes design and materials engineering.
Electrodes Design. SIBs and PIBs share a similar electrochemical energy storage mechanism with LIBs, but the radius of sodium- and potassium-ions are much larger than that of lithium ion (i.e., 1.02 Å for sodium ion and 1.38 Å for potassium ion vs. 0.76 Å for lithium ion), which makes it more difficult for sodium- and potassium-ions to insert into and extract from electrode materials. And at the same time, the insertion/extraction of large sodium- and potassium-ions will cause a significant volume change of electrode materials, which could potentially lead to the pulverization of electrode materials and their exfoliation from the current collector after repeated insertion/extraction cycles, resulting in a fast performance decay 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 electrode during the ionic insertion/extraction. As a proof of concept, highly-ordered antimony (Sb) nanorod arrays were fabricated and studied as the anode of SIBs. Unlike the conventional nanoarrays, such highly-ordered Sb nanorod arrays realized a perfect vertical alignment and concomitant large interval spacing such that they offer direct channels for sodium ions to fully contact with Sb nanorods without dead angles and a sufficient free volume as buffer space to release strains and prevent pulverization (Figure 1). The Sb nanorod arrays anode showed a high capacity of 620 mAh g-1 at the 100th cycle with a capacity retention of 84% up to 250 cycles at a rate of 0.2 A g-1 and superior rate capability of delivering the capacities of 579.7 and 557.7 mAh g-1 at rates of 10 and 20 A g-1, respectively. Furthermore, a full-cell SIBs coupling with the P2-Na2/3Ni1/3Mn2/3O2 cathode was assembled and exhibited good cycling life up to 250 cycles and high energy density up to 130 Wh kg-1. This result highlights that well-defined electrode architectures enable to significantly improve the performance of SIBs, bringing out higher capacity, better rate capability, and longer cycling life.
Materials Engineering. Another point of paramount importance in developing SIBs and PIBs is to improve the energy density. For example, sodium is three times heavier than lithium and the redox potential of sodium is about 300 mV lower than that of lithium, which all inherently reduces the energy density of SIBs. Exploring new electrode materials with high specific capacity is one of the effective strategies to improve the energy density of SIBs and PIBs. The researchers from TU Ilmenau reported for the first time the utilization of an extended π-conjugated system to design organic SIB electrode materials for improving the capacity. As shown in Figure 2a, the π-conjugated system of SBDC (sodium benzene-dicarboxylate) is extended through the introduction of C=C bond to finally obtain SSDC (sodium 4,4’-stilbene-dicarboxylate). The extension of the aromatic core is conducive to fast charge transport and collection and to improve the toleration to the fast insertion/extraction of Na+ ions during the fast-charge and -discharge process. The reason should be the smaller energy gap of SSDC compared to that of SBDC, arising from the larger π-surface of the former, which results in higher electric stability and conductivity. Simultaneously, extending the aromatic surface is also helpful to strengthen the intermolecular interactions (e.g., π-π and C-H···π interactions), which leads to a layer-by-layer molecular arrangement with an efficient ion diffusion between two layers (Figure 2b). Remarkably, the SSDC electrodes exhibited much better rate performance than SBDC (Figure 2c). It delivers a capacity higher than 100 mAh g-1 even at a 2 A g-1 rate. The capacities at 5 A g-1 and 10 A g-1 are still as high as 90 and 72 mAh g-1, respectively. Moreover, the cyclability of the SSDC electrodes is also much better than that of SBDC (Figure 2d). This work was highlighted by the well-known scientific platform Phys.Org in USA with a title of ‘Na-ion batteries get closer to replacing Li-ion batteries’ (https://phys.org/news/2015-03-na-ion-batteries-closer-li-ion.html), and won the Publikationspreis 2018 der TU Ilmenau.
With respect to electrode materials engineering for PIBs, the researchers from TU Ilmenau developed highly nitrogen-doped carbon nanofibers (NCNFs) as the anode of PIBs (Figure 3a). As illustrated in Figure 3b, pyrrolic N (N-5), pyridinic N (N-6), and quaternary N (N-Q) were found in NCNFs, which create additional defects in the graphene layers. Such moieties (especially, N-5 and N-6) and associated defects can enhance the capacity by reversibly binding with the charge carriers and exhibiting fast kinetics. As a result, the NCNFs anode delivered reversible capacities of 248 mAh g-1 at 25 mA g-1 and 101 mAh g-1 at 20 A g-1, respectively, and retained 146 mAh g-1 at 2 A g-1 after 4000 cycles. Furthermore, they also realized the first PIB full cell in the world based on NCNFs anode and Prussian blue cathode. The full cell delivered a reversible capacity of 195 mAh g-1 that was the highest value for the PIB full cells at that time and its energy density of 130 Wh kg-1 was comparable to commercial LIBs. This work won the Publikationspreis 2020 der TU Ilmenau.
Inspired by these research achievements, the researchers from the Group of Applied Nanophysics plan to further improve the energy storage performance of both SIBs and PIBs using the electrodes design and materials engineering strategies and other innovative methods.
 Y. Xu, M. Zhou, Y. Lei, Nanoarchitectured array electrodes for rechargeable lithium- and sodium-ion batteries. Adv. Energy Mater.2016, 6, 1502514.
 L. Liang, Y. Xu, C. Wang, L. Wen, Y. Fang, Y. Mi, M. Zhou, H. Zhao, Y. Lei, Large-scale highly ordered Sb nanorod array anodes with high capacity and rate capability for sodium-ion batteries. Energy Environ. Sci.2015, 8, 2954.
 C. Wang, Y. Xu, Y. Fang, M. Zhou, L. Liang, S. Singh, H. Zhao, A. Schober, Y. Lei, Extended pi-conjugated system for fast-charge and -discharge sodium-ion batteries. J. Am. Chem. Soc.2015, 137, 3124.
 Y. Xu, C. Zhang, M. Zhou, Q. Fu, C. Zhao, M. Wu, Y. Lei, Highly nitrogen doped carbon nanofibers with superior rate capability and cyclability for potassium ion batteries. Nat. Commun.2018, 9, 1720.
Prof. Yong Lei
Group of Applied Nanophysics
Technische Universität Ilmenau