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== Agenda ==
15:30 -15:35 Intro BATTERY 2030+ Erik Berg

15:35- 16:30 Presentation “Na-ion and K-ion chemistry for sustainable battery” Shinichi Komaba

16:30- 16:50 Q&A and Summery Erik Berg

== Recording ==
{{#ev:youtube|q_Hmvy9VQWg|||Na-ion and K-ion chemistry for sustainable battery - Shinichi Komaba - Battery 2030+ Excellence Seminar}}

== Transcript ==
Thank you very much for that kind introduction, Dr. Berg. 

It is my great pleasure and honor to give my talk for Europe’s largest project, 2030+. My name is Shinichi Komaba, and I am currently in Tokyo, where it is 11:30 PM. During my talk, I will see tomorrow, but for the European audience, I should say good afternoon. Today, I would like to describe and introduce our work on potassium and sodium ion chemistry for sustainable batteries.

When we look at the periodic table, group I elements are very important for electrochemical energy storage. Hydrogen is important not only for nickel-metal hydride batteries but also for fuel cells. Lithium is, of course, essential for lithium-ion batteries. Similarly, sodium and potassium ions are alternatives for lithium-ion batteries. Recently, we’ve even been looking at rubidium electrochemistry, although it is not for practical batteries. In the periodic table, we have many choices for metal ions and ionic carriers, but in my opinion, from lithium to potassium, these three alkali metals are the most advantageous for energy storage systems.

When comparing the ionic sizes, lithium is the smallest among the alkali metal ions. However, once lithium, sodium, and potassium ions are dissolved in a liquid electrolyte, their interactions with polar organic solvents or water molecules create a solvation shell. For instance, in propylene carbonate, we observe the Stokes radius, which is the size of the solvation shell. In water, the Stokes radius for each alkali ion with its hydration shell varies.

Interestingly, in cases like magnesium and aluminum ions, the solvation shell is much larger than for alkali metals. For lithium, the Stokes radius is the largest compared to sodium and potassium. Surprisingly, for potassium and rubidium, the Stokes radii are smaller than their crystalline ionic sizes because these large alkali metals disrupt the water or solvent’s Coulombic interaction network. As a result, the Stokes radii become smaller than the exact ionic size in solid ionic compounds. This smaller Stokes size, due to weaker Coulombic interactions, results in unique electrochemistry and ionic devices.

When we compare lithium, sodium, and potassium, it’s clear that sodium and potassium are heavier than lithium, which is a disadvantage. However, in terms of standard electrode potential (E₀), potassium has a lower potential than lithium, which can expand the potential window for potassium systems. Additionally, potassium and sodium have higher ionic conductivity in propylene carbonate compared to lithium, with lower desolvation energy at the interface. This means that desolvation during the interfacial process is easier for sodium and potassium.

Another important factor is the melting point: lithium has the highest melting point among alkali metals. Although metallic sodium and potassium are highly reactive with water, in the event of dendrite formation during overcharge, the lower melting points of sodium and potassium compared to lithium could allow Joule heating to melt dendrites, potentially improving safety during overcharge conditions.

We already use potassium ions in practical dry cells, such as manganese dry cells and nickel-metal hydride batteries. Most of the audience here are battery experts, and you know that potassium hydroxide is used in alkaline batteries because of its smaller hydrated ion size and faster mobility compared to sodium and lithium. This results in the highest ionic conductivity for potassium hydroxide in aqueous solutions, making it the best choice for alkaline batteries to minimize internal resistance.

Moving from aqueous to non-aqueous systems, alkali metals can also be used as electrode materials. A study from Professor Compton’s group reported the electrode potentials of group I alkali metals—lithium, sodium, potassium, rubidium, and cesium—in ionic liquids. If we refer all the potentials to lithium, we see that sodium’s E₀ is generally higher than lithium's, but potassium and rubidium can sometimes exhibit lower deposition potentials than lithium in propylene carbonate.

We also tested sodium, lithium, potassium, and rubidium by comparing their open circuit potentials after electrochemical plating on copper foil for 24 hours. We found that sodium's equilibrium potential is higher than lithium's, while potassium and rubidium exhibit lower potentials, suggesting that potassium and rubidium could offer a wider potential window for applications where the anodic limit is determined by the solvent species.

However, while sodium, potassium, and rubidium are highly reactive and have lower melting points than lithium, they must be handled with care due to their reactivity, especially after heat generation. Today, I will first introduce some recent work on rubidium in our lab, and then I will discuss sodium and potassium battery applications.

Starting with rubidium, we have recently achieved rubidium intercalation in graphite to form a graphite intercalation compound (GIC) and a layered rubidium oxide. Electrochemical insertion and extraction of rubidium into graphite were highly reversible, and the rubidium GIC exhibited a reversible capacity of around 280 mAh, compared to the 400 mAh of lithium in a stage 1 compound (LiC₆). Sodium, however, shows negligible electrochemical reactivity with graphite and is better suited to hard carbon electrodes for practical sodium-ion batteries.

We also studied alkali metal layered oxides, including rubidium cobalt, rhodium, and iridium compounds. One of our findings was that rubidium rhodium oxide forms a layered P2 structure and exhibits a beautiful charge-discharge curve in a rubidium half-cell, though the reversible capacity is quite limited. While this demonstrates the potential for rubidium-ion batteries, they are not practical or sustainable due to the limited energy storage potential.

Looking beyond rubidium, potassium and sodium are more attractive for sustainable battery technology. If we look at the elemental abundance of lithium, copper, and cobalt in the Earth's crust, these elements are quite limited. However, with sodium and potassium, we do not need to rely on these limited resources. Sodium and potassium intercalation chemistry can help us build sustainable batteries.

We have already published extensive reviews on sodium and potassium ion batteries in *Chemical Reviews*. While these elements are heavier than lithium, their diffusion rates and mobility are faster, and their working potentials are comparable to lithium-ion systems. In 2009, we demonstrated a 3-volt sodium-ion battery using nickel manganese oxide and hard carbon. At the time, no one believed that a solid electrolyte interphase (SEI) would form on the negative electrode or that sodium intercalation into layered sodium oxides would be reversible. However, after optimizing electrolyte and hard carbon materials, we achieved highly reversible sodium intercalation, proving the viability of sodium-ion batteries.

The choice of binders is also critical for improving the cycle performance of sodium-ion batteries. For instance, polyacrylate and carboxymethyl cellulose (CMC) binders outperformed polyvinylidene fluoride (PVDF), which tends to degrade due to incomplete SEI formation on hard carbon. By using these binders, we achieved stable sodium insertion and extraction, opening new avenues for understanding sodium insertion mechanisms in hard carbon.

We conducted several analyses using techniques such as X-ray diffraction, NMR, and neutron scattering to investigate sodium insertion mechanisms. We concluded that sodium can intercalate into stacked graphene layers in hard carbon and fill nanopores at lower potentials. This insight has allowed us to synthesize higher-capacity hard carbon materials, with reversible capacities reaching 480 mAh using a magnesium oxide template synthesis method.

In collaboration with Professor Kazuma Gotoh, we have also studied the safety of sodium deposition in hard carbon using operando NMR. Sodium deposition shows lower dendrite formation compared to lithium, suggesting improved safety for sodium-ion batteries. Additionally, we applied muon spin rotation and relaxation techniques to study sodium diffusivity in hard carbon, further expanding our understanding of sodium-ion batteries.

Next, let’s look at the positive electrode materials for sodium-ion batteries. Layered sodium cobalt oxides were first reported in the 1980s by Professor Hagenmuller and Dr. Delmas, but at the time, the upper cutoff voltage was limited due to electrolyte decomposition. Sodium layered oxides also experience multiple phase transitions during cycling, which causes lattice parameter changes and material degradation. These phase transitions present challenges for the long-term stability of sodium-ion batteries.

Manganese-based layered oxides, which suffer from Jahn-Teller distortion, are another option. By controlling the synthesis conditions, we have been able to stabilize both orthorhombic and hexagonal P2-type sodium manganese dioxide structures, achieving higher capacities and improved cycling stability.

The rich chemistry of sodium-layered oxides offers numerous possibilities. For example, sodium iron manganese oxides exhibit highly reversible charge-discharge properties, and cobalt-based systems show excellent electrochemical performance with smooth potential variations. One promising candidate for practical use is an iron-nickel-manganese-titanium layered oxide, which we optimized to achieve high reversible capacity and moderate working voltage.

Finally, let me introduce potassium-ion batteries. Potassium offers a wider potential window than lithium due to its lower redox potential, making it an attractive alternative for high-voltage battery systems. In recent studies, we found that manganese hexacyanoferrate paired with graphite electrodes is one of the best combinations for potassium-ion batteries, achieving energy densities comparable to some lithium-ion systems.

The number of scientific papers on potassium-ion batteries has been increasing rapidly, and I believe that in five to ten years, potassium-ion batteries will reach the same level of interest as sodium-ion batteries. In our comprehensive review published in *Chemical Reviews*, we highlighted the advantages of potassium-ion systems. For the negative electrode, graphite is the best choice, and we’ve shown that highly reversible potassium insertion and extraction can be achieved with reversible capacities over 250 mAh.

For positive electrodes, Prussian blue compounds based on manganese and iron are some of the most promising materials. They offer two voltage plateaus, and by replacing iron with manganese, we can maximize the energy density of the system. Potassium-ion

batteries also benefit from polyanion-based materials and metal-organic frameworks, which accommodate the larger ionic size of potassium.

We have also explored different electrolyte solutions, including ionic liquids and polymer electrolytes, for sodium and potassium-ion batteries. One promising additive is DMSF, which improves capacity retention in sodium and potassium cells, maintaining stable performance over 500 cycles. Given that these systems are compatible with existing lithium-ion manufacturing plants, the transition to sodium and potassium-ion batteries would be relatively seamless.

To conclude, sodium and potassium-ion batteries offer a sustainable alternative to lithium-ion batteries. Systematic studies of new materials and reactions will lead to the development of next-generation energy storage systems based on these abundant and environmentally friendly elements.

Some of my work was supported by projects like DX-GEM and Gtex, and I’d like to thank my laboratory members for their contributions. Thank you very much for your attention.