<aside> 💡 The purpose of this page is to help guide you on your way to becoming a battery expert 🤓 🔋. It’s meant for anyone with an undergraduate engineering background, but not necessarily with any prior knowledge of batteries or electrochemistry. It’s not meant to be exhaustive, but merely provide a layout for how to approach and which resources to access for more thorough reading.
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Batteries have been around for over 100 years, but the modern lithium-ion battery wasn’t commercialized until 1991 (by Sony). At their core, batteries are electrochemical energy storage devices where chemical energy can be converted into electrical energy (and vice versa). More specifically, a voltage source provides an energy input that drives an electrochemical redox reaction. Rechargeable (secondary) batteries can charge and discharge multiple times (e.g. Lead-acid, Li-ion), whereas non-rechargeable (primary) batteries can only be discharged once (e.g. alkaline cells, such as your standard AA batteries from Duracell).
In 2024, the most exciting battery technology is the lithium-ion battery because of its importance for electric transportation as well as grid storage to balance intermittencies of renewables (i.e. the sun doesn’t always shine, the wind doesn’t always blow, so solar panels and wind power need excess energy stored for later use in order to be cost effective and fulfill electricity usage requirements at all times). The transportation and grid storage sectors presently result in significant carbon emissions (totaling more than 30% of global carbon emissions), so there are significant efforts to decarbonize these sectors. The lithium-ion battery is a good match for transportation applications (e-bikes, electric vehicles, buses, even small aircraft and marine vehicles) because of its high energy density (in units of watt-hours per kilogram, or per liter). Since all modes of transportation are limited in how much weight and volume they can carry, a battery needs to store as much charge as possible in the lightest or smallest possible volume. The high specific energy of lithium satisfies this condition.
<aside> 💡 Remember: when you plug in your phone to a charger, the applied voltage source creates a potential difference across the battery electrodes; electrons flow through the external circuit and ions flow from the positive electrode (the metal oxide cathode) to the negative electrode (the graphite anode) via the ionically conductive liquid electrolyte. The rate at which these electrons flow is the current (in amperes, as determined by your charging software), multiplied by how long you charge, is equal to the capacity (in ampere-hours). The capacity of the cell is limited by how many ions you can store in the electrodes, and directly translates to how long you can use your phone on a single charge, or how many miles your EV can drive on a single charge. The “specific” capacity is this capacity per weight (kg) or volume (L), usually with respect to an individual cell.
$$ Current\ [Amps]\ *\ Time\ [hours]\ / \ Weight\ [kg] = Amp-hours/kg $$
The specific capacity multiplied by the cell voltage is then in units of watt-hours per kilogram (or liter), which is the specific energy density of the cell:
$$ Specific\ Capacity\ *\ Volts\ =\ Watt-hours/kg $$
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Lithium-ion batteries, like any other battery, consist of a positive electrode, a negative electrode, and an electrolyte. On charge, the positive electrode undergoes an oxidative reaction (lithium ions are released from a metal oxide) and the negative electrode undergoes a reductive reaction (lithium ions are intercalated into graphite to form lithiated graphite). On discharge, the reverse happens (see Figure 1 for visualization of this process). By convention, the positive electrode is often called the “cathode”, and the negative electrode is often called the “anode.” This deviates from conventions used in classical electrochemistry, where the cathode is the electrode where reduction occurs, and the anode is the electrode where oxidation occurs (i.e. the battery terminology is always used in relation to discharge mode). In this article, I refer each as the positive electrode and negative electrode. The electrolyte is the medium which conducts ions between electrodes, but is insulating to electrons. The electrodes are porous composites, where individual particles are mixed with a polymer binder, and conductive carbon black. The liquid electrolyte is allowed to infiltrate into the porous electrodes during manufacturing, and charging and discharging occurs in each particle. Lastly, lithium-ion batteries are unique because there is a nanoscale passivation layer (about 10 nanometers thick, give or take) that forms on the graphite surface during initial charge. This passivation layer is called the “solid-electrolyte-interphase” and it forms from electrolyte reduction reactions (because we operate outside the thermodynamically stable voltage window of the non-aqueous electrolyte). However, such a battery still works because the solid-electrolyte-interphase (SEI) is kinetically stable. Once it forms, it’s there and prevents further electrolyte decomposition as long as it effectively passivates the surface. One of the key discoveries of the lithium-ion battery showed that while propylene carbonate as a solvent doesn’t quite work (leads to graphite exfoliation, since the solvent gets taken up into the graphite), adding a co-solvent mixture using ethylene carbonate works magically (the ethylene carbonate decomposes preferentially to passivate the graphite surface, leading to a stable interphase).
[Figure 1. Schematic of Li-ion battery charging and discharging in a 4680 cylindrical cell. Source: Youtube](https://s3-us-west-2.amazonaws.com/secure.notion-static.com/ceefd8db-d0f2-40ab-8a15-ddab2343fbc9/Battery_schematic_movie.mp4)
Figure 1. Schematic of Li-ion battery charging and discharging in a 4680 cylindrical cell. Source: Youtube
Lithium-ion batteries are now comprised of many separate global industries working together, ranging from the initial mining and refining of raw materials (e.g. cobalt from the Congo, nickel from Russia, China, Indonesia, Australia, lithium from the salt mines in Chile), synthesis and processing of electrode active materials (e.g. graphite, lithium cobalt oxide, nickel manganese cobalt oxide, lithium iron phosphate; mostly China), electrode and cell manufacturing (mostly China), pack assembly (mostly China), and finally recycling and second-use. Apart from battery engineering, a lot of focus needs to be placed on manufacturing optimization, such as the use of different form factors (pouch, prismatic, cylindrical), formation processes (the intricate cycling protocols and electrolyte wetting processes that occur before cells are ready for use), pack-level sizing (e.g. how many individual cells should be put together in what order to reach application voltage needs, etc.). This all results in a dramatically decreasing battery cost curve, from > $1000/kWh in 2010 to $139/kWh in 2023 (Figure 2).
Figure 2. Battery pack cost curve. $139/kWh in 2023. Source: Bloomberg NEF (https://www.bloomberg.com/news/articles/2023-11-26/battery-prices-are-falling-again-as-raw-material-costs-drop)
Much of what happens at each step currently occurs in China (CATL, BYD) as well as South Korea (Samsung, LG Chem), though the US and EU are developing domestic means of production, including lithium mining (Lilac Solutions), gigafactories by many of the large automobile companies (Tesla, General Motors, Ford, Mercedes-Benz North America, Toyota North America), and recycling/circular economy (Redwood Materials, Li-Cycle). To build millions of electric vehicles, billions of batteries are needed (equivalent to hundreds of GWh of batteries). A lot of this effort is also encouraged by recent policy, such as the US Department of Energy Bipartisan Infrastructure Law and the Inflation Reduction Act, which results in funding that trickles down into academic research groups, battery startups and electric vehicle companies. This means that for the next decades to come and beyond, there will be a significant need for a domestic battery research and engineering workforce, making a PhD in battery research highly valuable in ours and the next n generations (Figure 3).
[Figure 3. Tesla 4680 cell production video. Source: Youtube](https://prod-files-secure.s3.us-west-2.amazonaws.com/002f3fa7-7c37-4164-a59a-806dd3870b30/db5cdd24-bd28-4c9e-a34f-35fc7880a749/Making_Batteries.mp4)
Figure 3. Tesla 4680 cell production video. Source: Youtube
As for specific research fields within lithium-ion and lithium metal batteries, there are many…
Much of the academic effort in the past ~15 years has been devoted to increasing the energy density of lithium-ion batteries, given the importance for portable devices and transportation, including the following topics in brief (my research group is interested in most of these topics to some extent):