The Perfect Battery?
The Perfect Battery?
Top 6 chemistry challenges to make the best Li-ion batteries.
Hello,
Since Li-ion batteries dominate your life whether you know it or not, there are certain characteristics, and many hurdles that had to be overcome in order for them to reach such mainstream adoption. There is an increasing demand for Li-ion batteries as seen below [1].
Today I will share with you a list of features that makes Li-ion batteries unique and scalable across phones, computers, and even cars. These features are fundamental to how they work as well as the practical applicability.
This list goes into the chemistry in particular which all aided in the success of Li-ion battery commercialization.
While not a comprehensive list, here are some of the most important features for Li-ion batteries
Reversibility: for a better understanding of how Li-ion batteries work you should check out the twitter thread in my last post.
We need to recharge our batteries at the fundamental level. Since many chemical reactions are not easily reversibly (burning firewood can’t be reversed), a way to reversibly store chemical energy was a huge discovery.
-Dr. Stanly Whittingham, who was a part of three individuals who won the Nobel prize in chemistry in 2019, first discovered the reversible intercalation reaction of lithium in titanium disulfide in 1977. In this reaction, Li ions are able to insert into and subsequently extract out of the titanium disulfide structure reversibly. This paved the way for rechargeable Li-ion batteries as we know them today.
Capacity: Simply the amount of charge stored in a cell. Just as we need a reversible reaction, we need a reaction of a meaningful amount of Li, as we need enough “juice” to power your phone for the whole day. You may have noticed that as new generations of phones come out, the tech has gotten better, thus your phone battery lasts much longer than it used to.
-Due to this fact, materials that can hold more lithium are more desirable. Lithium iron phosphate(LFP), and lithium transition metal oxides (TMO’s, also known as layered oxides) are very strong options on the cathode side [2]. These two materials (and their optimized derivatives) alone account for the overwhelming majority of commercial batteries today.
-There are plenty of materials that perform better than LFP and TMOs along this single dimension, but they do not supersede the overall performance.
Energy Density: What good is a long lasting car battery if you can only drive 20mph top speed? For this reason, the selected materials must operate at a reasonably high voltage. Based on the chemical properties of the materials, higher energy density allows the battery to output more energy in the course of its discharge.
-High energy density cells is what makes powering an electric vehicle possible. Fun fact: when your phone gets low on battery, sometimes it doesn’t work as well if intensive programs are running. One reason is due to the chemistry, as the operating voltage is actually slightly lower at low battery than at full battery.
Power Density: What good is a car that can drive 100mph if it takes 10min to get up to speed? You also don’t want to wait forever to recharge your car right? So, we would like to make a battery that can exert the most power as well, thus how fast can we discharge and charge the battery.
-There is a tradeoff between energy density and power density between various energy storage technologies. Capacitors can exert more power than batteries, but cannot store as much energy overall as seen in this figure.
-The rate at which the materials can handle the lithium ions being moved back and forth needs to be high. The problem is that the faster we charge/discharge, the more likely other detrimental processes occur [3]. Optimizing this the cell for these conditions is thus vital for a functional cell.
Capacity Retention: Also known as cyclability, batteries need to be recharged over and over again to be useful, it would be no use having to replace your phone and laptop batteries every month. It is for this reason that the long term health of a cell is one of the most important parameters that researchers and engineers look at.
-For those of you worried about your phone battery, capacity loss can be mitigated by less exposure to extreme temperature, less time at low and deep states of charge(completely full or completely dead), and slower rate of charging/discharging(less important in phones). This graph shows the decrease in capacity of LFP as the cells are cycled as well as how charging rate (10C faster than 1C) and temperature are important [4].
Stability: Lastly, both the cathode and anode materials need to be compatible with the electrolyte material that facilitates the transfer of lithium ions to and from. Again based on the chemistry, certain materials will be outside the stability window, thus unwanted chemical reactions will occur.
-No matter how much we tinker, nature does what it wants and the materials will take the reaction of least resistance. This could mean the difference between the formation of a healthy protective layer called the SEI [5], or reactions that block the lithium movement altogether or even cause short circuits.
There you have it, hope you enjoyed this list of the top 6 chemistry challenges that impact Li-ion batteries.
-Grayson
If you found this interesting, consider giving me a follow on twitter.
[1] Ding, Y., Cano, Z.P., Yu, A. et al. Automotive Li-Ion Batteries: Current Status and Future Perspectives. Electrochem. Energ. Rev. 2, 1–28 (2019). https://doi.org/10.1007/s41918-018-0022-z
[2] NMC: lithium 0.33nickel 0.33manganese 0.33cobalt oxide, and 80NCA: lithium 0.8nickel 0.1cobalt 0.1aluminum oxide. These materials are called layered due to their unique structure of alternating layers of lithium and metals. They are like a sandwich where lithium is the meat and cheese, and the bread is the transition metals(Ni, Mn, and Co for example).
[3] Faster charging/discharging can lead to reduced battery life and reduction of capacity. More technically, some of these adverse effects can be caused by voltage hysteresis, microcracking, and structural damage.
[4] Y. Zhang, C.-Y. Wang, X. Tang. Cycling degradation of an automotive LiFePO4 lithium-ion battery. J Power Sources, 196 (3) (2011), pp. 1513-1520
[5] Solid electrolyte interface: solid layer that forms on the electrode/electrolyte interface that is of different composition than the electrode itself. These reactions are carefully studied to determine whether they help or hinder the battery cell health.