Battery Day

charging port on hood of electric vehicle - Credit: Andrii Malkov

Battery Day
Communications of the ACM, May 2021, Vol. 64 No. 5, Pages 52-59
By Jessie Frazelle

“Batteries are a part of everyday life; without them, the world would be a much different place. Your cellphone, flashlight, tablet, laptops, drones, cars, and other devices would not be portable and operational without batteries.”


Tesla held its first Battery Day on September 22, 2020. What a fantastic world we live in that we can witness the first Applelike keynote for batteries. Batteries are a part of everyday life; without them, the world would be a much different place. Your cellphone, flashlight, tablet, laptops, drones, cars, and other devices would not be portable and operational without batteries.


At the heart of it, batteries store chemical energy and convert it into electrical energy. The chemical reaction in a battery involves the flow of electrons from one electrode to another. When a battery is discharging, electrons flow from the anode, or negative electrode, to the cathode, or positive electrode. This flow of electrons provides an electric current that can be used to power devices. Electrons have a negative charge; therefore, as the flow of negative electrons moves from one electrode to another, an electrolyte is used to balance the charge by being the route for charge-balancing positive ions to flow.


Let’s break this down a bit and uncover the chemical reactions at play within batteries. An electrical current requires a flow of electrons. Where do those electrons come from?


Electrons in the anode are produced by a chemical reaction between the anode and the electrolyte. Simultaneously, another chemical reaction occurs in the cathode, enabling it to accept electrons. These chemical reactions create the flow of electrons, resulting in an electric current.


A chemical reaction that involves the exchange of electrons is known as a reduction-oxidation reaction, or redox reaction.


Reduction refers to a gain of electrons. Thus, half of this reaction—the reduction—occurs at the cathode because it gains electrons. Oxidation refers to a loss of electrons. Therefore, the other half of this reaction—oxidation—occurs at the anode because it loses electrons to the cathode. Each of these reactions has a particular electric potential. An electrochemical cell can be made up of any two conducting materials that have reactions with different standard potentials, since the more robust material, which makes up the cathode, will gain electrons from the weaker material, which makes up the anode.


Batteries can be made up of one or more electrochemical cells, each cell consisting of one anode, one cathode, and an electrolyte, as described earlier. The electrodes and electrolyte are generally made up of different types of metals or other chemical compounds. Different materials for the electrodes and electrolyte produce different chemical reactions that affect how the battery works, how much energy it can store, and its voltage.


New Technologies

Solid-state batteries. Instead of the liquid or polymer-gel electrolyte found in batteries today, a solid-state battery uses a solid electrolyte and solid electrodes. Recall that positive ions flow through the electrolyte to balance the electrons’ negative charge. Today, batteries are quite efficient at transferring positive ions since a liquid electrolyte is in contact with the electrodes’ entire surface area. Using a solid makes this a bit harder. Imagine the difference between dipping a chip in soup and dipping it into chopped tomatoes. The soup would cover more of the chip’s surface area than would the chopped tomatoes.



Nuclear batteries. So far we have discussed only batteries powered by chemical reactions, such as those in flashlights, phones, and other gadgets. Chemical batteries, also known as galvanic cells, discharge in a given amount of time and need to be either thrown away or recharged. Is there a type of battery that could last long term?



Silicon anode. Today the material typically used for the anode is graphite because it is economical, reliable, and relatively energy dense, especially compared with current cathode materials. The limiting factor of lithium-ion batteries is the amount of lithium that can be stored in the electrodes. Using silicon as the material for the anode, rather than graphite, allows around nine times more lithium ions to be held in the anode.


Tesla’s Battery Day

At Tesla’s Battery Day event, the company announced many changes to its battery that encompass more than just the materials used. Tesla has on staff the renowned battery scientist Jeff Dahn. His most recent papers, “A Wide Range of Testing Results on an Excellent Lithium-Ion Cell Chemistry to be Used as Benchmarks for New Battery Technologies” and “Is Cobalt Needed in Ni-rich Positive Electrode Materials for Lithium-Ion Batteries?”help give some insight into what Tesla has been working on.


The Battery Day outcomes could increase Tesla vehicles’ range while being more economical; the plan is to halve the cost per kilowatt-hour. Most startups (except for Sila Nanotechnologies, which seems to be most closely aligned with Tesla’s methodology) in this space tend to take a single design decision into account for their products—for example, anode material—and focus on that. Tesla, on the other hand, took a broader approach. It took into account not only the materials for the cathode and anode, but also the cell design, factory, and integration with the vehicle, illustrated in Figure 1. (Tesla claimed in its presentation there were more aspects not mentioned that it could improve in the future.)

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About the Author:

Jessie Frazelle is the cofounder and chief product officer of the Oxide Computer Company. Before that, she worked on various parts of Linux, including containers, as well as the Go programming language.