The high-nickel content of ternary lithium batteries is becoming an important way to improve battery energy density. In addition, in the lithium-ion battery field, giants such as Guoxuan High-Tech and CATL are targeting research in cutting-edge areas such as solid-state electrolytes and lithium metal anodes. Competition in the battery industry is shifting upstream, and this field is also highly likely to produce disruptive key technologies. Recently, a research team of Chinese scientists published their latest research on battery materials in the prestigious academic journal *SCIENCE*. Their results show a major breakthrough in cathode materials (battery cathodes), a field where lithium-ion batteries are severely constrained by resources. This breakthrough deserves the attention of those working in the lithium-ion battery industry.
On October 9, 2017, a groundbreaking research finding was published in the journal *Nature Energy*. A materials science research team led by renowned Chinese-American materials scientists Zhenan Bao and Yi Cui successfully developed a novel cathode material for sodium-ion batteries. This material boasts extremely high battery capacity and significantly increased cycle life, and holds promise for replacing the expensive lithium-ion batteries, which are limited by their mineral reserves.
Photo | Renowned Chinese-American materials scientists, Stanford University professors Zhenan Bao (left), Yi Cui (center), and Minah Lee, the first author of this paper and a postdoctoral researcher at Stanford University (right).
This new material uses a completely new approach, which greatly improves the performance of sodium-ion batteries—its cycle capacity reaches 484 mAh/g, and its cathode energy density is as high as 726 Wh/kg.
Minah Lee, the first author of the paper and a postdoctoral researcher at Stanford University, said: "Our novel cathode is composed of oxygen and sodium and has an energy density comparable to that of conventional lithium cathodes, making it a reliable cathode for sodium-ion batteries to replace lithium-ion batteries."
Even more remarkably, due to the extremely abundant reserves of sodium on Earth, the mining and production costs of sodium-ion battery cathode materials are only 1/100th that of lithium-ion batteries, thus controlling the overall cost of sodium-ion batteries to about 80% of that of lithium-ion batteries. This breakthrough technological advancement represents another solid step forward for humanity on the path to large-scale energy storage.
The chart shows that as global demand for lithium-ion batteries continues to grow, lithium mining is facing a supply shortage, driving up prices. As reserves are depleted, prices may rise further.
In fact, as the most reliable battery for mobile devices, lithium-ion batteries dominate most rechargeable battery applications, including mobile phones, computers, and electric vehicles, thanks to their high energy density and deep charge/discharge capabilities. Furthermore, with increased lithium-ion battery production and economies of scale, their prices have been declining for many years, further solidifying their competitive advantage over other battery technologies.
Some scientists even believe that no other battery will be able to replace the lithium-ion battery's dominant position until all lithium resources on Earth are exhausted.
However, the seemingly far-fetched scenario of "reserves running out" is a real concern for many industry insiders. While global lithium-ion battery production continues to reach new highs and overall prices have fallen significantly, the prices of some raw materials used to produce lithium-ion battery electrodes have actually surged. This is because the mineral resources (lithium ore, cobalt ore, etc.) available on Earth for producing the cathode materials needed for lithium-ion batteries are actually quite scarce.
To meet current lithium-ion battery production demands, mines worldwide are already operating at their limits, making further increases extremely difficult. Moreover, accelerated mining would prematurely deplete these limited mineral resources, further driving up prices. Therefore, lithium-ion batteries face a challenge that most commodities never encounter: as production increases, prices not only fail to decline but may instead rise sharply.
To solve this problem, scientists turned their attention to sodium, an element located right next to lithium on the periodic table with very similar properties. Compared to lithium resources, the Earth's sodium reserves are so abundant that they are "inexhaustible": sodium chloride—table salt—is found everywhere, from the vast oceans to every household's dining table. Compared to the price of up to $15,000 per ton for lithium-ion battery materials, using sodium ions as an electrode material would cost only $150 per ton, a difference of 100 times.
Image | Compared to lithium, sodium resources on Earth are far more abundant. From the ocean, salt lakes, and salt mines, sodium accounts for over 2.7% of the Earth's crust by mass. Therefore, batteries using sodium as a material will be far cheaper than lithium-ion batteries.
However, despite its promising application prospects, research on sodium-ion batteries has yet to achieve a decisive breakthrough.
In fact, research on sodium-ion batteries began at the same time as that on lithium-ion batteries. Unlike other batteries that require redox reactions, these two types of batteries are "rocking chair batteries"—the ions themselves shuttle back and forth between the cathode and anode to achieve charging and discharging. In other words, the cathode and anode are used to collect, store, and release the ions that generate the electric current.
Image | Many elements are used in battery manufacturing. Considering various performance factors, lithium is currently the best option. However, the scarcity of mineral resources for lithium-ion battery electrode materials poses a hidden risk to its future development.
In the 1980s, breakthroughs were first achieved in the research of cathode materials for lithium-ion batteries. Cathode materials, represented by lithium cobalt oxide, combined with anode materials typically made of graphite, enabled lithium-ion batteries to achieve optimal performance, thus replacing the previous nickel-metal hydride rechargeable batteries and entering thousands of households. However, research on electrode materials for sodium-ion batteries was far less successful.
In fact, for sodium-ion batteries to operate efficiently, they must simultaneously meet the following two conditions. However, previous studies have shown that cathode materials for sodium-ion batteries either have high energy density but short cycle life, or long cycle life but low energy density.
With sufficient energy density, a unit mass of battery can supply enough electrical energy;
It has a long cycle life, and the battery capacity does not decrease significantly with the increase of charge-discharge cycles.
This time, the Stanford University team broke away from the previous mindset of using transition element oxides or polyanions as cathode materials, and used a brand-new organic material, "inositol," to combine with sodium ions.
You may not have heard of this tongue-twisting name, but this organic compound, whose structure is very similar to glucose, is widely found in plants and animals. It is a growth factor for animals and microorganisms, and a common nutrient in food. As an organic compound well-known in industry, inositol has mature processing and wide applications, which is crucial for controlling the cost of sodium-ion batteries.
Sodium and inositol can combine to form Na2C6O6, a compound that is an ideal cathode material. Theoretically, it can carry four sodium ions at a time, thus enabling the battery to have an extremely high capacity of 501 mAh/g.
In fact, before Bao Zhenan's team, others had also tried using Na2C6O6 as an electrode material to produce sodium-ion batteries. However, the theoretical maximum transport capacity of four sodium ions is actually difficult to achieve in practice, resulting in the energy density of Na2C6O6 batteries being far lower than expected.
Furthermore, after just one charge-discharge cycle, the energy density of the battery drops drastically in the second cycle, making it completely unsuitable for practical use. In real-world applications, batteries should retain a relatively high level of charge even after hundreds or even thousands of charge-discharge cycles.
Image | The novel sodium-ion battery cathode material used by Bao Zhenan's team. In the right image, yellow represents sodium ions, embedded in inositol, which is indicated by red and gray. A single Na2C6O6 molecule can carry up to four sodium ions at a time, resulting in extremely high energy density.
Minah Lee said, "The biggest obstacle in this study was that this compound could only store fewer than two units of sodium and electrons in previous studies, which was insufficient to compete with the energy density of lithium-ion battery cathodes. But here, by understanding and addressing the phase transition kinetics limitations in the redox reaction, we were able to enable this compound to store four units of sodium."
In this study, the Stanford team conducted a very in-depth exploration of the mechanism of Na₂C₆O₆ batteries. Through meticulous analysis of the forces acting at the atomic level, they successfully revealed the secret behind the material's actual energy density being lower than the ideal energy density: It turns out that during the binding and deintercalation of sodium ions with the electrode, only when the material undergoes a reversible phase change can all four sodium ions participate in the reaction. In previous studies, without special treatment, the material only underwent irreversible phase changes, resulting in fewer than four sodium ions participating in the reaction, thus lowering the ideal energy density.
After understanding the principle, they successfully transformed the irreversible process into a reversible one by reducing the volume of the active particles and selecting a suitable electrolyte, thereby increasing the cyclic capacity of the Na2C6O6 battery to nearly the theoretical upper limit of 484 mAh/g. Moreover, the rate of decrease in maximum battery capacity was significantly reduced compared to before, and the cathode energy conversion efficiency reached 87%.
This is the best achievement to date in the research of sodium-ion battery cathode materials, representing a significant breakthrough. For the first time, sodium-ion batteries have achieved both high energy density and near-perfect cycle stability. Furthermore, due to the use of inexpensive sodium and inositol, and with an energy density significantly higher than lithium-ion batteries, researchers claim that the cost of this battery is expected to be less than 80% of that of lithium-ion batteries with equivalent capacity, a truly remarkable advancement.
Image | Na2C6O6 nanoparticles before charging (left), which can bind a large number of sodium ions after full charging (right).
However, this is only a preliminary research result, and it is still some distance from practical application.
First, Bao Zhenan's team has only initially solved the cycle life problem of cathode materials. After 50 cycles, the capacity of the Na2C6O6 electrode has decreased by about 10%. Although this is a remarkable achievement compared to previous research, it is still far from meeting the requirement of hundreds of cycles in actual use.
Secondly, they have not yet researched anode materials that can be industrialized. Research on anode materials for sodium-ion batteries is equally challenging. Despite the research team's confidence, sodium ions are much larger than lithium ions (about 50% larger in diameter), making them unabsorbable by graphite, a material commonly used in lithium-ion battery anodes. To date, no anode material that is both effective and inexpensive (like graphite) has been developed. This will be the team's future research direction. Minah Lee explained that this research shows phosphorus is a good candidate material, but mass production remains difficult, so they are also exploring simpler ways to process this material.
Regarding the team's next steps, Minah Lee revealed, "Currently, our full cell energy density is limited by the anode (higher operating potential), so we are working on creating a better anode."
In short, this is a technology that has achieved a major breakthrough, but is still far from industrial application. However, any technology is very immature in its earliest stages. Similarly, in the field of materials science, the now ubiquitous hard drive, when it first achieved technological breakthroughs and realized MB-level data storage, weighed approximately 1 ton.
But it was precisely this "behemoth" that had absolutely no connection to portability that laid the foundation for today's portable hard drives with capacities often reaching several TB (1TB=1024GB) yet only the size of a pocket. It is quite possible that the Na2C6O6 material, which still appears rudimentary today, is a foundational precursor to future large-scale grid-level power storage technology.
