By Kyle Proffitt
February 28, 2024 | A group from Cornell has reported lithium ion coin cell batteries that charge in five minutes and can cycle at least 1,000 times while providing energy density on par with commercial products. Their work deals with optimizing the kinetics of lithium ion diffusion and reaction in the anode, and it led to the identification of a lithium-indium anode that can support ultrafast charging (sometimes measured in seconds) and is primarily limited by other battery components. The work was published earlier this year in Joule (DOI: 10.1016/j.joule.2023.12.022).
During lithium ion battery charging, the lithium ions must relocate from a cathode material, through a liquid electrolyte, and into the anode. There are intricate and fundamental physical processes that determine how quickly the lithium ions can a) diffuse through each distinct material, b) undergo ion solvation and desolvation in liquid electrolyte, and c) react with or embed within material at the anode. There is great interest in accelerating this charging process, especially for electric vehicles, where rapid charging is seen as part of the cure for range anxiety. As recharging time decreases, it can become quite similar to the familiar gas station visit on a long trip. You probably wouldn’t mind if your smartphone or laptop fully recharged in just a few minutes either.
To work toward this faster charging, Professor Lynden Archer’s group focused in on the kinetic parameters in the anode. They recognized that for rapid charging, lithium ions need to deeply penetrate and disperse throughout the anode material before they “settle in”. For this to be possible, the rate of reaction needs to be low relative to the rate of diffusion. That ratio is referred to as the second Damköhler number (Da_II), and the researchers believed that with a low Da_II, even in the face of high current densities (fast charging), more of the anode material would be made available for accepting Li+. Instead of plating unevenly and encouraging dendritic growth, the Li+ would be better able to diffuse throughout the anode and then react in an orderly manner. Based on this understanding, the group focused on low Da_II (LDA) anodes, hypothesizing that LDA materials could be used to create batteries better suited to rapid charging.
Materials Search
To identify the best LDA materials for use as anodes, the researchers first assessed diffusion energy barriers—how hard it is for lithium to migrate into and through the gaps of crystalline structures—for various anode materials, relying on prior experimental and computational results. They built on a prior study from Archer’s group in which they discovered the “unusually high diffusivity of lithium within indium”. There have since been a number of studies “in which In and Li-In alloys were used to create solid-state batteries,” Archer told Battery Power Online in an email. However, he said, “these cells suffered from a range of problems during cycling even at moderate rates, which are typically attributed either to the poor interfaces formed between the solid-state electrolyte and the Li metal anode or to Li’s intrinsic tendency to deposit during the battery recharge in non-planar mossy/dendritic structures.”
In the current study, indium showed one of the lowest Li+ diffusion barriers of available materials (0.16 eV, about 3 times lower than graphite), and lithium-indium alloys also showed very low diffusion barriers, with some an order of magnitude lower. The group then performed cyclic voltammetry experiments to assess how quickly Li+ became reduced in different materials, finding a rate four orders of magnitude lower for indium anodes compared with traditional graphite. With its combined low barrier to lithium ion diffusion (high diffusivity) and low speed of ion reduction, they deemed indium an inherently LDA material with promise for use as a fast-charging anode. Other tested materials showed a range of higher Da_II values.
LDA Anodes Charge Faster and Permit Energy Recovery
Further exploring their hypothesis that the LDA materials would correspond to improved fast charging, they tested half-cell batteries formed using the different anode materials, including indium, graphite, Li4Ti5O12 (LTO), and alloys of Sn, Al, and Cu. The reversibility of lithium storage in these anode materials was assessed, first pushing lithium into the anode with increasing current density and then measuring how much energy could be recovered on slow discharge.
At a high current density of 40 mA/cm2, for instance, graphite does not display any reversibility, whereas indium maintained 95.1% coulombic efficiency (CE, how much charge is recovered) at an even higher current density of 100 mA/cm2. “We discovered that the source of the failure [for graphite] is a mismatch between the electrode reaction kinetics and Li-ion transport at the electrodes,” Archer explained. No other tested material was competitive with indium; aluminum was next best with only 50% CE. Looking closely at the anode with scanning electron microscopy, the researchers saw a planar surface, negligible change in size or shape, and no apparent dendritic growth.
The researchers also discovered during the half-cell experiments that indium anode performance was improved by first cycling at low current density to establish lithium-indium alloy interphases that would then perform even better with the fast-charging current densities, aligning with the lower diffusion energy barriers of some alloys.
Onward to Full Batteries
At this point the researchers created complete cells, starting with LiFePO4 (LFP) cathodes and indium anodes, and they performed the slow-charge cell conditioning to establish lithium-indium interphases. Subsequent experiments demonstrated that the Li-In anode in this setup could reproducibly accept a high flux of energy, up to 70 mA/cm2, and slowly release that energy. At this charge density, the measured C-rate was 1,440C, a 2.5-second charge. However, lead author Shuo Jin explained by email that this high speed occurred at the expense of total capacity. “At a current density of 10 mA/cm2, it takes 6 minutes to charge the battery to 3.8V (10C). However, at a current density of 70 mA/cm2, it only takes 2.5 seconds (1,440C).
It is important to note that at this higher current density, the battery’s capacity is only 10 mAh/g, while the theoretical capacity is 170 mAh/g. The overpotential is extremely high, causing the full battery voltage to rise to 3.8V.” In other words, the battery very rapidly reached a “full” state with superfast charging at this higher current density, but this full state was not actually allowing complete utilization of lithium from the LFP cathode. The experiment was important, however, in demonstrating that the anode could reliably handle this current density.
Believing that the limitation was now the cathode, Archer and group employed a unique battery chemistry consisting of an I2 conversion cathode. This battery system also relies on lithium ion transfer, but the lithium ions oxidize I− to I3− by the reaction 6I−→2I3−+4e− as opposed to intercalating into LFP. Importantly, this chemistry enables a more rapid lithium ion release during charging, and the researchers demonstrated success with an even higher charge flux of 80 mA/cm2.
Additional experiments revealed that ether-based electrolytes enabled the formation of “thin, stable interphases at high reduction potentials”, and the researchers ultimately demonstrated the compatibility of Li-In anodes with a range of cathode materials, including LFP, NMC, I2, and O2.
Balancing Act
Archer explained the relationships among cathode material, capacity, and charge speed more completely. “We encountered a challenge in finding cathode materials that can match the rapid charging capabilities of the indium anode while providing sufficient capacity. Cathode materials like LFP or I2 exhibit very high charging rates but offer relatively low capacities. For instance, LFP typically provides a capacity of only about 120 mAh/g at 30C. On the other hand, cathodes like O2 can offer significantly higher capacities (around 1,000 mAh/g) but may compromise some of the fast-charging capabilities. Hence, the selection of the cathode becomes a matter of striking a balance between fast-charging capabilities and capacity.”
Balancing out proof-of-concept exceptional charging speed with reasonable energy density, the group demonstrated LFP-based batteries with more LFP loaded in the cathode. This setup allowed reproducible 12C (5-minute) charging over 1,200 cycles and an overall energy density of 145 Wh/kg, matching the energy density of other LFP-based lithium-ion batteries while allowing for much faster charging.
Still to Come
Despite the promising characteristics of indium for fast charging, there are significant hurdles, Archer says, including “the high cost of In, the greater mass of a Li-ion cell that uses Li-In in place of graphite, and the presently unexplored supply chain, manufacturing, and charging infrastructure challenges.”
Increased weight seems like a particular concern for use in mobile applications. However, the effects on total battery weight are not entirely straightforward. Indium is about 10 times heavier than graphite, but graphite typically encompasses only 10-15% of total battery weight. The researchers’ demonstration of at least one complete fast-charging battery design with total energy density similar to that of a commercial LFP-based lithium ion batteries indicates some practical feasibility for Li-In anodes and helps pave a path toward faster charging.
The group is also looking beyond pure indium and exploring blends with lighter metals to offset the weight issue. With aluminum, for example, even small additions of indium significantly reduced Da_II, converting it from a poor fast-charging anode material (50% CE at 40 mA/cm2) into one with promising characteristics (93% CE). According to the report, “these findings, although limited, suggest that the inherently low Da_II characteristics of In can be leveraged in multi-component alloying electrodes with fast-charge capabilities.”
Looking forward, Archer highlighted this research direction, saying, “We have conducted extensive research on high-capacity fast charging with In-Al blends. There will be quite a bit more to report soon.”
