Right now, you are probably reading this article on a device powered by a Li-ion battery. In the past decade, these energy storage devices have become ubiquitous in daily life. Improvements to Li-ion batteries have provided major boosts to portable electronics like laptop computers and smartphones. These electronics have evolved to use increasingly sophisticated computing technology that would never be portable without the significant amount of energy stored in the Li-ion batteries that power it. More recently, the research focus on batteries has shifted toward more efficient powering of electric vehicles, which show promise for helping reduce society’s reliance on traditional fossil fuels for transportation.
While significant progress has been made toward making electric vehicles perform just as well as those with combustion engines, further improvements in battery technology are becoming more difficult. The current materials used in most Li-ion batteries—LiCoO2 or LiNi0.6Mn0.2Co0.2O2 for the positive electrode, and graphite for the negative electrode—are reaching the limits of their capacities to store electricity. That means future breakthroughs can’t just involve optimizing the materials we currently use; they require development of new materials that use entirely different chemistries to store energy.
Recently, a team of researchers at the Synthetic Control Across Length-scales for Advancing Rechargeables (SCALAR) Energy Frontier Research Center (EFRC) did just that, by showing how two new positive electrode materials store up to 50% more charge than materials currently in use.
Li-ion batteries store electricity using chemical reactions
In all batteries, electricity is stored by using electrons to drive a particular chemical reaction inside the battery. When that electricity is needed to power a device, electrons are extracted from the battery and the chemical reaction inside is reversed. How much total charge a battery can store depends on how much reversible chemistry can happen. The key word here is reversible—injecting electrons often results in unwanted chemical reactions that can’t be reversed and decrease the battery’s efficiency.
As their name suggests, Li-ion batteries focus on one specific chemical reaction that is highly reversible: the intercalation, or storage, of Li+ ions inside a host material electrode. When the host material has space to accommodate the Li+ ions, the amount of structural change during charging and discharging is minimized, and the intercalation reactions become much more reversible than most other types of chemical reactions. As a result, Li-ion batteries can sometimes last for up to 1000 charge-discharge cycles, while other rechargeable batteries (lead-acid batteries, for example) rarely make it to 500 cycles.
Unfortunately, maintaining this high level of reversibility imposes limitations on how much charge Li-ion batteries can store. Most commonly used electrode materials can only store up to 1 Li+ ion per formula unit, which places an upper limit on the total capacity of the battery. To surpass the limit, researchers must turn to new types of chemical reactions, which can end up storing more, but also are less reversible, which means batteries that use them will degrade even more quickly.
Cathodes currently limit energy density
In a full Li-ion battery, there are two host materials: the positive electrode, or cathode, which contains Li+ before the battery is charged, and the negative electrode, or anode, which does not. Cathodes tend to be much more difficult to develop, because materials that initially contain Li+ tend to collapse once the Li+ is removed. As you might expect, this problem becomes much worse when the amount of Li is increased. Accordingly, cathodes that can reversibly remove and store >1 Li+ per formula unit are scarce.
Schematic of the redox processes and the resulting structural response during charging of Li2FeS2. Upon removal of Li (grey), both Fe (red) and S (yellow) are oxidized. S atoms form bonds between each other (far right) to stabilize additional capacity in this material.
The SCALAR researchers reported that Li2FeS2 and LiNaFeS2 are two cathode materials that store up to 1.5 Li+ per formula unit by using sulfur anions (S2-) instead of the usual oxygen anions (O2-), which are used in commercial LiCoO2 and LiNi0.6Mn0.2Co0.2O2 cathodes. Sulfur atoms are larger than oxygen atoms, meaning that using sulfur provides a structure with space for the extra Li+. More importantly, the researchers showed that the sulfur atoms in these two compounds form bonds with each other to share their electrons during charging. That is, when Li+ is removed, the pairing up of the sulfur anions (2 S2- → S22- + 2e-) helps to stabilize the material and prevent it from collapsing. Then, when Li+ is re-intercalated during discharge, the sulfur pair stops bonding with each other and the sulfur atoms return to their original configuration.
Paving the way for the next generation of batteries
Li-ion batteries that store more energy are essential for improving portable devices and for reducing the reliance on fossil fuel-powered cars for transportation. Beyond just these applications, more advanced energy storage technologies are needed to better use renewable energy sources like solar and wind energy, which cannot reliably supply energy on a daily basis without batteries.
To reach the goals we have for the next generation of batteries, using and understanding new battery chemistries is key. These sulfur-containing cathodes that show reversible chemistry past the 1 Li+ per formula unit limit offer examples of how different types of chemical reactions could work in a battery. Importantly, this work paves the way for batteries that can maintain high amounts of charge for longer periods of time.
