ADAPTIVITY APPLIED TO LITHIUM ION BATTERIES
I described in the earlier post how adaptive systems turned smartphones into great cameras. Let’s now talk about how adaptivity and adaptive charging can make a battery perform far better.
Let’s start briefly with the basic operation of a lithium ion battery. The early posts of this blog describe the operation of the lithium-ion battery in more detail. I will briefly recap here the basic operation and explain where its performance is limited. For the reader who wants to learn more, select “The Basics” category tag and feel free to review these earlier posts.
The figure below illustrates the basic structure of a lithium-ion battery. On the left hand side, one sees an electron microscope image of a battery showing the anode, the cathode and the separator, essentially the three basic materials that constitute the battery. On the right hand side, one sees a sketch illustrating the function of these materials during the charging process: The lithium ions, “stored” inside the individual grains of the cathode, move through the separator and insert themselves inside the grain of the graphite anode. If you are an engineer or physicist, you are asking, “where are the electrons?” A neutral lithium atom becomes an ion in the solution, travels through the separator to the anode. The electron travels in the opposite direction through the external circuitry from the Aluminum collector to the Copper collector, where then it is captured by a lithium ion to form a molecular lithium-carbon bond.
This seems simple enough, so what can go wrong? lots! I will focus here on a handful of mechanisms that become critical as the battery’s storage capacity and energy density increase. Looking at the diagram above, it is hopefully obvious that increasing energy density means to the reader packing more and more ions into this little sketched volume. It means reducing the dimensions of the anode, the cathode, the separator, and trying to saturate the capabilities of the anode grains to absorb ions. It’s like when you try to put as much water as possible inside a sponge. Now, in this process, small variations in manufacturing become really detrimental to performance. Look at the left photograph and observe the coarseness of the grain size for both electrodes. That means the uniformity of the ionic current is poor. As the energy density rises, a large number of ions are all rushing from the cathode to the anode. But this lack of uniformity creates stress points, both electrical and mechanical, that ultimately lead to failure: gradual loss of material, gradual loss of lithium ions, and gradual mechanical cracking, all leading in time to a gradual loss of capacity and ultimate failure.
I will jump to two key observations. First, it should be apparent that when energy density is low, these effects are benign, but when energy density is high, there are so many ions involved in the process that small manufacturing variations become detrimental. Second, it should be apparent too that faster charging results in the same effect, i.e., more ions are trying to participate in the process.
Clearly, battery manufacturers are trying to improve their manufacturing processes and improve their materials — but let’s face it, this is becoming an incredibly expensive process. Smartphone and PC manufacturers are not willing to pay for more expensive batteries. This is very similar to the earlier post about camera lenses. Make great lenses but they become very expensive, or shift the burden to computation and correct the errors dynamically and adaptively.
That’s precisely what adaptive charging does: Be able to measure the impact of the manufacturing variations, embedded defects, non-uniformity of material properties and what have you in real time, assess what these errors are and how they may be progressing in time, then adjust the voltage and current of the charging current in such a way to mitigate these “errors”….then keep doing it as long as the battery is in operation. This makes each battery unique in its manufacturing history, material properties, and performance, and lets the charging process get tailored in an intelligent but automated fashion to the uniqueness of the battery.
It’s a marriage of chemistry, control systems and software, that shifts the burden from expensive manufacturing to less expensive computation. But what is clear is that it does not make battery manufacturing any less important, and it does not replace battery manufacturing — it is complementary. It is no different that how adaptive algorithms in the camera are complementary to the lens, not replacing it. This is cool innovation!