Japanese scientists have synthesized two crystal materials that show great promise as solid electrolytes. All-solid-state batteries built using the solid electrolytes exhibit excellent properties, including high power and high energy densities, and could be used in long-distance electric vehicles.
High power batteries are desirable for numerous applications, including the electric vehicles of the future. These batteries must be rechargeable, remain safe to store and use at variable temperatures, and retain charge for a considerable length of time. Now, Yuki Kato and Ryoji Kanno in collaboration with colleagues from Toyota Motor Corporation, Tokyo Institute of Technology and High Energy Accelerator Research Organization (KEK) in Japan, have successfully designed and conducted trials on novel, high power all-solid-state batteries with promising results.
Most traditional batteries rely on the flow of ions through a liquid electrolyte between two electrodes; lithium-ion batteries used in mobile phones would be one example of this type of battery. However, batteries incorporating a liquid electrolyte are prone to problems, including low charge retention and difficulties in operating at high and low temperature. Previous designs for solid electrolytes have shown promise, but have proven expensive and some have exhibited problems with electrochemical stability.
Kato and his team synthesized two new lithium-based 'superionic' materials based on the same crystal structure previously discovered by the same team. They studied these crystal structures using Synchrotron X-ray diffractometer, BL02B2, at SPring-8 and neutron diffractometer iMATERIA(BL20) at J-PARC. Superionic materials are solid crystal structures through which ions can 'hop' easily, essentially maintaining a flow of ions similar to that which occurs inside a liquid electrolyte. They showed how the lithium ions move fast in the structure of their compounds even at room temperature.
Both superionic materials developed by the team showed extremely high ionic conductivity and high stability. The researchers used their two new solid electrolytes to create two battery cell types; one high-voltage cell and one cell designed to work under large currents. Both all-solid-state cell types exhibited superior performance compared with lithium ion batteries, operating very well at temperatures between -30 and 100°C. Kato's team found that the cells provided high power density, with ultrafast charging capabilities and a longer lifespan than existing battery types.
Although the technology requires further development before it is commercially available, these promising results indicate that all-solid-state batteries may soon provide a much-needed boost to applications requiring stable, long-life energy storage.
A need for solid electrolytes
Most batteries and capacitors we use in daily life are powered by liquid electrolytes. Rechargeable lithium ion batteries, for example, work by maintaining a flow of ions from the negative electrode to the positive electrode during use, and the ion flow is reversed during charging. Although lithium ion batteries are useful for these purposes, there is still strong demand for new devices with higher power and energy densities. All-solid-state batteries are the most promising candidates for future battery systems, due to the high energy density attainable by direct-series-stacking of battery cells. However, the low power characteristics of all-solid-state batteries, due to their higher solid electrolyte-resistivity than conventional liquid electrolyte, still remain unsolved.
The search for materials suitable for creating solid electrolytes has already produced some prototypes. So far, these 'superionic' materials, which allow ions to move quickly and freely through their crystal structure, have been developed using the expensive element germanium – researchers are therefore keen to find alternative superionic conductors that could provide the basis for all-solid-state batteries.
The development of two new lithium-based superionic conductor materials (structures: Li9.54Si1.74P1.44S11.7Cl0.3 and Li9.6P3S12 ) by Yuki Kato and his team represents a leap forward in the creation of useable solid-state batteries. Their two cells based on the novel solid electrolytes performed very well in trials in comparison with lithium ion batteries. The cells remained stable and operated consistently at a range of temperatures between -30 and 100°C. They exhibited high energy and high power densities, and very small internal resistance levels. Their properties would allow the cells to be stacked close together without interference.
Further, the cells exhibited ultrafast charging, retained their charge for lengthy periods, and appeared to have a long lifespan with excellent cycling ability (after over 500 cycles, the cells retained around 75% of their initial discharge capacity).
These promising results require further investigation prior to commercialization. The addition of high energy electrodes into the solid-state cells could enhance the power of the batteries still further. Also, processing technology to complement the batteries that would allow for battery stacking is required before such configurations could be fully tested. Kato and his team are hopeful that their new materials will pave the way for all-solid-state batteries for multiple applications, including long-distance electric vehicles, in future.
Solid electrolyte interphases on lithium metal anode
The prestigious Advanced Science journal has just published a review paper on solid electrolyte interphases of lithium metal anodes contributed by Prof. Qiang Zhang in Tsinghua University, China and Ji-Guang Zhang in Pacific Northwest National Laboratory.
"Lithium (Li) metal is regarded as the 'Holy Grail' of rechargeable battery technologies due to the high theoretical specific capacity, 3860 mA h g-1, 10 times that of commercial graphite anode, and the lowest redox potential, -3.040 V vs. the standard hydrogen electrode. Therefore, lithium metal batteries, such as lithium-sulfur and lithium-oxygen batteries with the theoretical energy density of 2600 and 3400 Wh kg-1, could be promising candidates in next-generation energy storage devices," said Dr. Qiang Zhang, an associate professor at Department of Chemical Engineering, Tsinghua University, Beijing, China.
"However, the safe use of lithium metal as an anode is still a great challenge, for the dendritic and mossy metal deposits are very easily obtained on working lithium metal anode. Lithium dendrites induce a low Coulombic efficiency and severe safety risk, hindering the practical demonstration of high-energy-density lithium metal batteries. The dendrite nucleation and growth are closely related to the surface layer between the electrolyte and anode, called the solid electrolyte interphase (SEI). The surface component and structure of the SEI layer play an extremely important effect on the morphology of lithium deposits and decide the cycling performance of lithium metal anode. Consequently, it is of great importance to have a deep understanding on the SEI layer."
"The recent achievements on the SEI layer of the lithium metal anode are highlighted in my manuscript," said Xin-Bing Cheng, a graduate student and the first author of the review, "We have briefly summarized the mechanisms of SEI formation and models of SEI structure. The analysis methods to probe the surface chemistry, surface morphology, electrochemical property, dynamic characteristics of the SEI layer are emphasized. The critical factors affecting the SEI formation, such as electrolyte component, temperature, current density, are comprehensively debated. The paper summarizes efficient methods to modify the SEI layer with the introduction of a new electrolyte system and additives, ex-situ formed protective layer, and electrode design."
Although these works afford new insights into SEI research, a robust and precise route for SEI modification with a well-designed structure, as well as the relationship between structure, properties, and electrochemical performance, is still inadequate. More studies on SEI layer building require collaborative works from physics, chemistry, nanomaterials, and engineering communities.
"Through further investigation on the science and engineering of SEI on lithium metal, the use of lithium metal as a superior anode in a rechargeable cell is quite promising. The ultra-stable and robust SEI will enable broad applications of rechargeable Li metal in advanced Li–S batteries, Li–air batteries, and other advanced Li batteries," Qiang told Phys.org.
Going solid-state could make batteries safer and longer-lasting
If you pry open one of today's ubiquitous high-tech devices—whether a cellphone, a laptop, or an electric car—you'll find that batteries take up most of the space inside. Indeed, the recent evolution of batteries has made it possible to pack ample power in small places.
But people still always want their devices to last even longer, or go further on a charge, so researchers work night and day to boost the power a given size battery can hold. Rare, but widely publicized, incidents of overheating or combustion in lithium-ion batteries have also highlighted the importance of safety in battery technology.
Now researchers at MIT and Samsung, and in California and Maryland, have developed a new approach to one of the three basic components of batteries, the electrolyte. The new findings are based on the idea that a solid electrolyte, rather than the liquid used in today's most common rechargeables, could greatly improve both device lifetime and safety—while providing a significant boost in the amount of power stored in a given space.
The results are reported in the journal Nature Materials in a paper by MIT postdoc Yan Wang, visiting professor of materials science and engineering Gerbrand Ceder, and five others. They describe a new approach to the development of solid-state electrolytes that could simultaneously address the greatest challenges associated with improving lithium-ion batteries, the technology now used in everything from cellphones to electric cars.
The electrolyte in such batteries—typically a liquid organic solvent whose function is to transport charged particles from one of a battery's two electrodes to the other during charging and discharging—has been responsible for the overheating and fires that, for example, resulted in a temporary grounding of all of Boeing's 787 Dreamliner jets, Ceder explains. Others have attempted to find a solid replacement for the liquid electrolyte, but this group is the first to show that this can be done in a formulation that fully meets the needs of battery applications.
Solid-state electrolytes could be "a real game-changer," Ceder says, creating "almost a perfect battery, solving most of the remaining issues" in battery lifetime, safety, and cost.
Costs have already been coming down steadily, he says. But as for safety, replacing the electrolyte would be the key, Ceder adds: "All of the fires you've seen, with Boeing, Tesla, and others, they are all electrolyte fires. The lithium itself is not flammable in the state it's in in these batteries. [With a solid electrolyte] there's no safety problem—you could throw it against the wall, drive a nail through it—there's nothing there to burn."
The proposed solid electrolyte also holds other advantages, he says: "With a solid-state electrolyte, there's virtually no degradation reactions left"—meaning such batteries could last through "hundreds of thousands of cycles."
The key to making this feasible, Ceder says, was finding solid materials that could conduct ions fast enough to be useful in a battery.
"There was a view that solids cannot conduct fast enough," he says. "That paradigm has been overthrown."
The research team was able to analyze the factors that make for efficient ion conduction in solids, and home in on compounds that showed the right characteristics. The initial findings focused on a class of materials known as superionic lithium-ion conductors, which are compounds of lithium, germanium, phosphorus, and sulfur, but the principles derived from this research could lead to even more effective materials, the team says.
The research that led to a workable solid-state electrolyte was part of an ongoing partnership with the Korean electronics company Samsung, through the Samsung Advanced Institute of Technology in Cambridge, Massachusetts, Ceder says. That alliance also has led to important advances in the use of quantum-dot materials to create highly efficient solar cells and sodium batteries, he adds.
This solid-state electrolyte has other, unexpected side benefits: While conventional lithium-ion batteries do not perform well in extreme cold, and need to be preheated at temperatures below roughly minus 20 degrees Fahrenheit, the solid-electrolyte versions can still function at those frigid temperatures, Ceder says.
The solid-state electrolyte also allows for greater power density—the amount of power that can be stored in a given amount of space. Such batteries provide a 20 to 30 percent improvement in power density—with a corresponding increase in how long a battery of a given size could power a phone, a computer, or a car.