Solid-State Anode-Free Sodium-Ion Batteries Are Here

By Kyle Proffitt

April 29, 2024 | “Today will be the first time we release information about sodium all-solid-state batteries in anode-free setup,” Shirley Meng told the audience at last month’s International Battery Seminar & Exhibit.

Meng, University of Chicago, delivered a featured presentation at the event in which she discussed her latest research in anode-free battery designs, ultimately presenting some groundbreaking research that produced successful anode-free solid-state sodium-ion batteries.

She summarized battery evolution, tracing it all back to “Gen Zero”, when Stanley Whittingham first created a lithium-ion battery using a titanium cathode and a thick lithium metal anode, achieving 100-200 Wh/kg energy density. Switching to cathode materials containing lithium then allowed for progression through 3 more generations of anode evolution using carbon (Gen 1, where most commercial batteries still are), silicon composite (Gen 2), and finally, ultrathin lithium metal (Gen 3). These Gen 3 batteries push the energy density limit to 400-500 Wh/kg.

At 2024 Florida Battery, Meng added a potential fourth generation, the anode-free design. (She mentioned this work at the 2023 event). “If you have an electrolyte that is 100% perfect, you should be able to completely eliminate the seed layer of the lithium metal,” she said. The idea here is that instead of starting with lithium metal at the anode, you start with nothing between the electrolyte and the negative current collector (typically copper foil). Normally during charging, lithium ions move from cathode to anode and intercalate or plate there. This is no different. However, as Meng indicated, with an anode-free approach, there is no thin seed layer of lithium metal, and the anode is effectively built upon first charge.

The apparent simplicity of the design reveals its allure, but “Your volume change is going to be very significant,” she said. That extra 20 micrometers would represent a 20% expansion of the entire battery pack. In any case, Meng said, “most of the time people think of this [anode-free idea] as just an academic, scientific interest.”

She’s trying to change that. Beyond the simplicity of an anode-free design, there’s another driving motivation. “The production costs of alkaline metal sheets like lithium and sodium remains high, and the quality control remains low,” Meng said. She listed several characteristics that should be monitored, such as uniformity, surface composition, thickness, and microstructure. She applauded the scientists that have recently developed quantification techniques and tools, but added, “we don’t even have the proper tools to measure the grain size, the grain boundary impurities in the lithium metal.”

Lose the Anode

Meng says electrodeposition (which takes place upon charging when there’s no anode to start with) produces some of the most pure and dense lithium metal, addressing some of these concerns, but it requires “very interesting stack pressure engineering” (more on this later). Even then, we don’t have complete control. Meng was incredulous about the state of our knowledge in this area. “In the semiconductor industry with the Damascene process for depositing copper [in integrated circuits], the crystal orientations are strictly controlled. In lithium metal I cannot believe that we actually do not control crystal orientations. We have absolutely no control.” And the comparison gets even worse. “In the Damascene process, you only deposit copper [once]. With lithium metal we must repeat the process for hundreds or thousands of cycles, yet we do not know the crystallographic orientations.”

Meng touched on the significant discussion around —typically aluminum and copper foils that transfer charge into or out of the battery. Current collectors with a 3D structure instead of foil are being explored, but in Meng’s opinion, “not a single 3d current collector actually worked to deposit lithium metal.”

External pressure, however, is quite useful. “Lithium metal and sodium metal are very sensitive to pressure control,” Meng said. She showed data where only a few atmospheres of stack pressure really improved plating. With that pressure, “you can grow very dense and good microstructure lithium,” she said. These results all involved liquid ether electrolyte.

Meng also had some practical advice for cycling conditions in an anode-free setup: “You never ever strip all the way down to the current collector… if you always strip down to the current collector, mossy lithium will be generated between current collector and lithium metal.” Images of cells that underwent complete stripping showed irregularities in plated material after just 2 cycles.

Referring to these liquid-electrolyte, anode-free lithium-ion cells, Meng said they can work for “a few hundred cycles without trouble.” But she added: “We’re still trying to figure out if fast charging is possible.”

The Solid State

But perhaps a solid electrolyte would be more effective. One of the issues with liquid electrolyte and why 3d current collectors fail, Meng said, is that the electrolyte always finds its way into porosities. “SEI [solid electrolyte interphase] formation inside those pores can be very detrimental,” she said. Meng showed that the theoretical energy density gains for solid-state batteries when moving from all-silicon anode or lithium metal anode to the anode-free design exist but are not dramatic.

She showed an example with a silicon anode solid-state cell, saying it can accomplish “complete solid-state lithiation of silicon without interference from any SEI.” She added, “The key here is to use metallurgical grade silicon; you need a mixed conductor to facilitate the reaction.” LG Energy Solutions is supporting research to move these solid-state silicon anode batteries from laboratory scale to proper pouch cells. “It is really not a very easy task in an academic institution, but it is doable,” Meng said. Work in this area was published in early April in Nature Communications. In that work, Meng’s group showed the prelithiation of silicon in solid-state cells enabling up to 10 mAh/cm2 areal capacity.

After telling us that she’d be divulging results with anode-free solid-state sodium-ion cells, Meng indicated this was not her first goal. “I will be very upfront; it’s because in lithium chemistry, we did not succeed.” The efforts were not in vain though. Her group actually has succeeded in creating anode-free all-solid-state lithium ion cells, even ones with quite high energy density, and have published this work prior. Her group has made inroads toward anode-free all solid state lithium batteries using lithium phosphorus oxynitride (LiPON) film and very thin (10 micron) stainless steel current collectors from Nippon steel (who also presented research at the conference).

“Any time we think about anode-free configurations, the current collector has to come into play,” she said. “It only makes sense anode free if your current collector is very very thin.” This is because a thick current collector significantly reduces the total energy density by driving up the weight. Using this setup, Meng’s group has collaborated with Ensurge to develop microbatteries that have very high volumetric energy density of around 1200 Wh/L. The catch is that this research has been limited to the production of microbatteries because of the volumetric changes that occur at the anode. “We tried to make lithium anode-free work in the large scale for solid state. Because of the volume changes we couldn’t really make it work in considerable charge rate or area,” she said.

Sodium Becomes Practical

Unthwarted, Meng turned her attention to sodium. “Sodium has always been downplayed because of its energy density problems,” Meng began. However, when her group ran the numbers for theoretical energy densities, “this anode-free configuration actually did become automotive relevant.” Her group calculated energy densities up to about 700 Wh/L (350 Wh/kg). It made sense to press on. Of course, a major advantage of sodium ion batteries is significant cost savings.

In pre-print work  on ChemRxiv, Meng’s group has now reported the successful creation of all-solid-state anode-free sodium-ion batteries. Meng said her group identified a number of critical design factors for making functional solid-state sodium ion batteries, including electrochemically stable electrolyte, intimate interface contacts (because metals don’t flow through voids), a dense solid electrolyte, and a dense current collector. Sodium borohydride checks the box for stability in the presence of sodium metal. Pelletized aluminum allows intimate contact with sodium borohydride electrolyte, and it can be densified, unlike copper or titanium. With this design, “we can grow up to 60 microns of sodium metal without any problems,” Meng reported from half-cell experiments, corresponding to 7 mAh/cm2 current density.

Complete cells were then created and demonstrated to cycle 400 times, operating with 10 MPa stack pressure (about 100 atmospheres). There are still kinks to be worked out. “I don’t have a good sodium catholyte to build a very thick sodium cathode.” Her group used NaCrO2 cathode material and Na0.625Y0.25Zr0.75Cl4.375 catholyte. Because of this, the batteries have limited energy density (a firm number was not given). She says the field really needs to consider how to improve ion conductivity for sodium catholyte.

Pressure Release

Responding to an audience question about the stack pressures used in experiments, Meng said of potential commercial partners, “They all ask the same question, can we go below 1 megapascal?” She’s hopeful that they can. “We know now where the problem is. When you lower the pressure, there are voids that show up between interfaces. It’s probably about time to add in some molecules to fill those voids… polymers, organic molecules, or liquids, but they must be precisely controlled to fill in this gap.” She made a strong prediction: “Give us 12-24 months, it will be demonstrated that we can absolutely limit the pressure.”

Meng believes the initial success using sodium should spur greater efforts. “We still have a lot of work to do because everyone wants to replicate this in the lithium metal case. All of us should be more creative now that we have seen in the sodium case, this can be achieved.”