The Battery Cell — A Rocking Chair Between Two Electrodes

What Goes In When You Plug In a Charger

At night, we plug our phone into the charger and unplug it in the morning. The screen shows it is full, and we say we filled the battery. Hearing those words easily brings to mind a picture of electricity being poured like water into a flat lump overnight.

If you open up a battery, you will not see an empty container being filled with water. There are only two thin plates coated with different materials, a membrane keeping the two plates from touching, and a liquid soaking the space between them. What changes while charging is the chemical state of the materials coated on the two plates, and the energy is stored in those two plates as chemical energy (DOI: 10.1021/ja3091438). While you use your phone, that chemical change happens in reverse, sending an electric current flowing out through the external wires.

Very small particles travel back and forth between the two plates, and every time a particle crosses, electricity also makes a full circuit through the external wires. Battery researchers have given this round trip the name of the rocking chair (DOI: 10.1149/1.2068978). If you follow what goes where inside a single cell, you can see what a battery stores and what it gives out.

Four Things Inside a Single Compartment

Whether in a cell phone or an electric vehicle battery, the smallest unit is the same. A cell is the smallest compartment making up a battery, containing a positive electrode, a negative electrode, an electrolyte filling the space between them, and a separator dividing the two, all inside a single case.

A lithium ion is a lithium atom that has given up an electron and become a grain with a positive electrical charge. This is the grain that traveled back and forth between the two plates in the first section.

The positive electrode is the positive terminal of the cell, where lithium ions return and settle when discharging. The negative electrode is the negative terminal of the cell, where lithium ions cross over and stay when charging. The two plates from the first section are these two electrodes.

The electrolyte is a substance filling the space between the two electrodes that lets lithium ions pass through while preventing electrons from crossing. The liquid wetting the space between the plates is this.

The separator is a thin membrane dividing the positive and negative electrodes so they do not touch, with many tiny holes through which lithium ions pass. The membrane that divided the two plates is this.

Inside the cell, lithium almost always travels back and forth between the two electrodes in the form of ions. Lithium ions cross over to stay at the negative electrode when charged, and return to settle at the positive electrode when discharged. The two electrodes are also made of different materials. The positive electrode is a material like a metal oxide holding lithium, and the negative electrode is typically graphite, which is also the material used in pencil leads (DOI: 10.1002/9781118615515).

Because the electrolyte allows only ions to pass, among the reactions happening inside the cell, the ion portion passes through the inside of the cell while the electron portion is pushed out to the outside of the cell (DOI: 10.1021/ja3091438). This single property creates the two-forked road of the next section.

Although the separator does not directly participate in the battery's chemical reactions, it greatly determines the cell's performance and safety (DOI: 10.1002/eem2.12129). Because it must let ions pass well while being mechanically strong and withstanding heat, trade-offs between these properties are the challenge of separator design (DOI: 10.1016/j.rineng.2026.111067). What happens when the separator cannot endure is covered in another piece of this series, the one dealing with Thermal Runaway — A Staircase Climbed on Its Own Heat.

In an actual cell, besides these four, parts that collect and send out electricity, binders and additives that hold materials together, and a case wrapping the outside are included as well (DOI: 10.1016/j.sajce.2024.09.008). Still, the ones dividing up the work of the cell are the four above, and their jobs are each distinct. A review paper summarizing the history leading up to the lithium-ion battery evaluates that meshing chemically distinct parts—such as the negative electrode, positive electrode, and electrolyte—inside a single container was often a greater challenge than the pros and cons of any single part (DOI: 10.1021/acs.chemrev.8b00422). Making a cell is the work of choosing matching materials.

Discharge: One Journey, Two Roads

You unplug your charger in the morning and open a map app. The moment you do, lithium ions that were resting at the negative electrode and their partner electrons set off separately toward the positive electrode. Their starting point and destination are the same, but their path splits in two.

A circuit is a closed path with no breaks where electricity comes out of a cell, goes through a device, and returns to the cell. The path the electrons take is this circuit. Because electrons cannot cross the electrolyte, they leave the negative electrode, travel through the wire outside the cell, pass through the phone, and return to the positive electrode. The lithium ions pass through the electrolyte, slip through the pores of the separator, and go to the positive electrode. The two meet again at the positive electrode, and as the positive electrode material accepts the electrons, the lithium ions settle inside it.

The lithium ions go through the internal electrolyte, and the electrons go through the external wire. This separation is how a cell does work.

Current is the flow of electricity along a wire, measured in amperes by how much passes a single point in one second. Voltage is the difference that makes electricity flow between two points, measured in volts. What a discharging cell puts out is the external current flowing at a certain voltage for a set amount of time (DOI: 10.1021/ja3091438), and the phone uses that flow passing along the external path to light up the screen and send signals. While the separator and the electrolyte block a shortcut for the electrons, the device placed on the external path receives and uses that energy.

The two paths move by the exact same amount at the exact same moment. As one ion crosses inward, one electron flows across through the external wire, so even though electrons are flowing through the external wire, neither positive nor negative electricity piles up separately on either electrode. Before and after charging, the positive and negative electricity inside the cell are equal and balance each other out, and what changes is the energy contained in the cell. That energy depends on which electrode the lithium is sitting on. The chemical states of the two electrode materials differ when the lithium is at the negative electrode versus the positive electrode, and energy is contained in that difference. Discharge is the process of lithium moving its seat from the negative electrode to the positive electrode, and the current flowing outward is the other half of that movement.

Charging: Squeezing Between Graphite Layers

When you plug in a charger at night, the road runs in reverse. Charging is the process where an external power source pushes electrons to leave the positive electrode, travel through the external wire to the negative electrode, while lithium ions match that pair by moving from the positive to the negative electrode; and discharging is the process where those ions return spontaneously to the positive electrode and their paired electrons travel through the external circuit to the positive electrode. What the charger does is move electrons extracted from the positive electrode over to the negative electrode by that amount, while the lithium ions cross over to the negative electrode through the electrolyte and separator along the way. For a battery to be rechargeable, the reaction must be able to run in reverse like this when current and voltage are applied from the outside (DOI: 10.1021/ja3091438).

How the lithium ions that arrive at the negative electrode stay there is the core of this battery. Graphite is a material made of stacked thin sheets of connected carbon atoms, and lithium ions wedge themselves into the gaps between those sheets. Intercalation is when lithium ions wedge themselves into the gaps or between the atomic layers of an electrode material and stay there, while the skeleton of the electrode remains largely intact during this process. The positive electrode is also built with a skeleton that accepts lithium, so lithium goes in and out within a set range (DOI: 10.1021/ja3091438), and during charging and discharging, the processes of lithium entering and exiting occur as a paired set at the two electrodes (DOI: 10.1002/9781118615515).

Wedging in takes time. When charging quickly, the decisive bottleneck that ties down speed is the diffusion of lithium spreading into the electrode material, and the slow movement of lithium ions in the electrolyte within the small pores of the electrode also ties down the speed (DOI: 10.1002/aenm.202101126). The same review paper views these two slowness factors as the cause of the phenomenon where lithium piles up as metal on the surface of the graphite negative electrode (DOI: 10.1002/aenm.202101126). What that piled-up lithium causes is covered in another piece of this series, the one dealing with why batteries age. Because charging is the work of transferring lithium into the electrode material, there is a limit to how fast you can push it in.

The Rocking Chair, and Cells Born Empty

Researchers called batteries that replaced a solid lithium metal anode with a material that takes in and gives out lithium rocking chair batteries, and the idea to increase safety and lifespan that way dates back to the early 1980s (DOI: 10.1149/1.2068978). Like a chair that rocks back and forth, lithium goes to one electrode and comes back, and during normal round trips it always moves from a seat inside one framework to a seat inside the other. Unlike a chair, however, lithium goes from the negative to the positive electrode by itself, and a charger has to push it the other way. Today's lithium-ion batteries grew out of this rocking chair concept (DOI: 10.1002/anie.201913923). Formalizing the concept of electrochemical intercalation and sketching the rocking chair battery are credited to battery chemist Michel Armand (DOI: 10.1149/2.0072007jes).

Before the rocking chair, the negative electrode was lithium metal itself. When oil prices soared in the early 1970s, research surged to find new batteries that worked at room temperature, and rechargeable batteries pairing a lithium metal anode with a layered sulfide cathode went as far as production. However, as charging was repeated, lithium grew like branches across the electrolyte and stretched all the way to the cathode, and inside a flammable organic liquid electrolyte that became a dangerous short circuit (DOI: 10.1021/ar2002705).

The next step came from the voltage side. Changing the anode from lithium metal to another material lowered the cell voltage so that sulfide cathodes could not compete with existing batteries using water, and researchers soon dropped that path. Once it became known that metal oxide cathodes could produce a higher voltage compared to lithium, research shifted toward extracting lithium from layered oxides containing cobalt or nickel. Around the same time, it was also confirmed that lithium does not grow like branches in graphite but instead moves in and out repeatedly (DOI: 10.1021/ar2002705).

There was an unexpected stumbling block here. These oxide cathodes are made in a state containing lithium, meaning a discharged state. According to one memoir looking back at this period in the first person, battery manufacturers initially could not accept the idea of assembling cells with a discharged cathode (DOI: 10.1021/ar2002705). Lithium-ion cells are assembled in a discharged state, and the first charge performed after the cell is made in the factory finally moves the lithium to the negative electrode (DOI: 10.1021/ar2002705). For people who pictured batteries as containers sold pre-filled, this was an unfamiliar sequence. Seen through the rocking chair, the chair simply starts at one end. Japanese chemist Akira Yoshino, introduced as the inventor of the lithium-ion battery, looks back on the path from first making this battery to commercializing it, and points to the moment safety tests succeeded as the turning point of that work (DOI: 10.1002/anie.201105006).

During the first few charges after making the cell, another thing happens together. The SEI is a very thin film formed on the surface of the negative electrode, created as part of the electrolyte decomposes during the first few charges and continuing to grow little by little afterward. The discovery that a graphite anode can repeat charging and discharging in a suitable organic electrolyte and that a stable SEI forms on the graphite surface at that time led lithium-ion batteries to practical use (DOI: 10.1002/9781118615515). While this film is forming, some of the lithium that crossed over from the cathode gets trapped and cannot return, reducing the share available for use right from the start by that much (DOI: 10.1021/ja3091438). However, in experiments using non-carbonate solvents, results have also shown that the way lithium ions are bound to the solvent played a greater role than the SEI in protecting the graphite, so the exact part this film plays has not been fully sorted out yet (DOI: 10.1021/acsenergylett.7b01177).

Voltage Comes From the Difference Between Two Electrodes

A battery in a drawer sits at 1.5 V between its two ends even when connected to nothing at all. Voltage is the difference between two points. In a cell, those two points are the anode and the cathode, and what makes the difference is the materials of the two electrodes.

Electrode potential is a value written in volts of how strongly an electrode material pulls on electrons, measured against a defined reference electrode. A cell stores energy in these two electrodes: the anode, which wants to give up electrons, and the cathode, which wants to pull them in (DOI: 10.1021/ja3091438). The voltage of a cell comes from the difference in potential between the two electrodes. That is why changing even one electrode material changes the cell voltage. The history in the previous section is an example of this; the obstacle was that changing the anode from lithium metal to another material lowered the cell voltage, and research shifted in that direction once oxide cathodes capable of producing a higher voltage compared to lithium became known (DOI: 10.1021/ar2002705).

You cannot increase voltage endlessly. There is a range of voltage that the electrolyte filling the space between the two electrodes can withstand, and for a cell to last a long time, that range sets the maximum voltage. Electrolytes using water can sustain voltages up to 1.5 V, whereas lithium-ion cells use organic solvent electrolytes with a wider range to pack more energy into the same size (DOI: 10.1021/ja3091438). If an electrode's potential lies outside that range, the voltage can be raised further, but the price is that a membrane allowing lithium ions to pass must cover the electrode surface (DOI: 10.1021/ja3091438). The SEI from the previous section is the membrane that does this job at the anode (DOI: 10.1002/9781118615515).

Then who decides the current? The notion that a battery always outputs the same current is a persistent misconception that lingers even before learning direct-current circuits in college and long after learning them (DOI: 10.1080/03043799108939504). A systematic literature review gathering studies on electric circuit misconceptions notes in an ongoing interim analysis (17 out of 93 papers passing inclusion criteria) that most students still believe this (DOI: 10.1109/fie58773.2023.10343239), and a study surveying 107 college freshmen likewise found that most students believed the battery to be a source that always supplies the same current (DOI: 10.31571/saintek.v15i1.10012).

What the cell determines is the voltage between the two electrodes, and the magnitude of the current flowing under that voltage is determined jointly by the circuit connected to the cell. When you brighten the screen or open a heavy app, your phone draws more current, and inside the cell, that many more lithium ions cross over to the cathode every second. The voltage of the cell also moves slightly depending on the remaining amount and the intensity with which it is drawn, and how to read those numbers is in another part of this series, the one dealing with battery numbers.

Cathode Material: Same Seat, Different Framework

Articles introducing electric vehicle or mobile phone batteries often use words like "nickel" or "iron phosphate." What those words point to is the cathode material. The cathode material is the substance that accepts and releases lithium ions at the positive electrode, and it largely determines the cell's performance and safety. The development of cathode materials has driven safer and more efficient lithium-ion batteries (DOI: 10.1016/j.sajce.2024.09.008), and finding stable cathode materials while simultaneously increasing cell voltage and storage capacity is cited as the path to next-generation lithium batteries (DOI: 10.1002/anie.201902359).

Commercial cell cathode materials broadly split into three branches. They are layered metal oxides containing lithium (cobalt, nickel, manganese, or a mixture of them), manganese-centered spinel oxides, and iron-bearing phosphates (DOI: 10.1002/9781118615515). Here we look at the first and the third, namely the nickel-cobalt family and iron phosphate.

The nickel-cobalt family is the layered oxide introduced in the history section. Metals and oxygen form plate-like layers, and lithium moves in and out between those layers. According to a review paper's summary, in this family nickel increases the storable energy and electric vehicle driving range, manganese aids safety, and cobalt helps thermal stability. Research continues to reduce cobalt and increase nickel due to cost and ethical issues, but a cathode packed with high nickel has large energy while leaving cobalt dependence and stability as challenges (DOI: 10.1016/j.sajce.2024.09.008).

Iron phosphate is a cathode material with a different crystal structure woven from lithium, iron, and phosphate, namely the olivine structure. It puts safety and stability first while storing less energy (DOI: 10.1016/j.sajce.2024.09.008). Its advantages are a long lifespan and resistance to heat, and its weakness of poor electron conduction is made up for by thinly coating the powder surface with carbon and slightly adding other metal ions into the crystal to help lithium ions move easily (DOI: 10.3969/j.issn.2095-4239.2013.02.003). An article honoring Armand's achievements evaluates that the carbon coating process he led made iron phosphate the safest cathode material in 2002 (DOI: 10.1149/2.0072007jes).

Choosing a cathode material is a trade-off. Even for the same rocking chair, the energy and stability the cell holds vary depending on what the framework at one end is made of. An evaluation that something is resistant to heat does not mean thermal runaway will never happen, and the differences between the two families when exposed to heat are covered in the piece dealing with thermal runaway.

Following One Night and One Day, One Step

At eleven o'clock at night, you plug in your phone. The charger pulls electrons out of the positive electrode, pushes them through the external wire toward the negative electrode, and lithium ions sitting inside the positive electrode's framework leave their places to match them. The ions travel through the electrolyte, squeeze through the tiny pores of the separator, and wedge themselves between the plates of graphite. During fast charging, the movement of lithium inside the electrodes, including this wedging, becomes the bottleneck that limits the speed. Until morning, the plus electricity and minus electricity inside the cell balance out just the same, and what has changed is that a set range of lithium has left the positive electrode, crossed over to the negative electrode side, and energy has been stored in the cell.

At seven o'clock in the morning, you unplug the charger and open a map app. Now the reaction runs in reverse on its own. Because the electrons in the negative electrode cannot cross the electrolyte, they go around the external wire and the phone to the positive electrode, while the lithium ions pass through the internal electrolyte and separator to return to the positive electrode's framework. The voltage the phone receives comes from the difference between the two electrode materials, and the magnitude of the current flowing right now is as much as the screen and app pull and use.

When you plug the charger back in the next night, the lithium crosses over to the negative electrode once again. What goes back and forth every day is the position of the lithium. However, just as the SEI trapped some of the lithium during the first few charges, side reactions and changes occur in the battery materials depending on voltage and temperature while you use it, shrinking the amount it can hold and potentially leading to safety issues. Protective strategies to manage those reactions are also being studied together (DOI: 10.1002/adma.202401482). What those changes are continues in the part covering why batteries age ("Battery Aging — Fewer Marbles, Cracked Shelves").

Where to Look Next

To see the same story from the side of the external circuit, there is "Voltage, Current, Power — What Goes All the Way Around Undiminished". It covers how current does not decrease even after passing through a device and what voltage actually does. The scale for measuring the energy contained in a cell is found in "Watts and Watt-Hours — Reading Electricity's Two Numbers Like a Speedometer and an Odometer".

The other pieces of this series build upon this structure. "Ampere-hours and Watt-hours — How Much It Holds, How Fast It Gives It Back", which deals with battery numbers, reads numbers like mAh and Wh written on a mobile phone. "Battery Aging — Fewer Marbles, Cracked Shelves", which covers why batteries age, looks at the growth of the SEI and lithium accumulating on the anode surface, while "Thermal Runaway — A Staircase Climbed on Its Own Heat", which covers thermal runaway, deals with when the separator fails to hold up and differences according to cathode materials.

One unresolved problem also remains. The structure and composition of the SEI have long been a subject of debate, and the role played by even one of its main components is not yet clear (DOI: 10.1002/aenm.202100046).

Source Papers

• "The Li-Ion Rechargeable Battery: A Perspective" (2013) — DOI: 10.1021/ja3091438 — Cells store chemical energy in two electrodes (a reducing-agent anode and an oxidizing-agent cathode), while an electrolyte drives ionic components inward and electronic components outward. The cathode is a framework through which lithium moves reversibly within a certain range. The electrolyte window sets the maximum long-life voltage, with an aqueous window of 1.5 V. Potentials outside the window require a surface protective film, and that film's formation irreversibly locks up lithium during the first charge.
• "Lithium Rocking Chair Batteries: An Old Concept?" (1992) — DOI: 10.1149/1.2068978 — The rocking-chair battery, which replaced the metal anode with a lithium-supplying anode, and the ideas for improving safety and lifespan date back to the early 1980s.
• "From Solid‐Solution Electrodes and the Rocking‐Chair Concept to Today's Batteries" (2019) — DOI: 10.1002/anie.201913923 — A chronology from solid-solution electrodes and the rocking-chair concept to today's lithium-ion batteries.
• "Tribute to Michel Armand: from Rocking Chair – Li-ion to Solid-State Lithium Batteries" (2020) — DOI: 10.1149/2.0072007jes — Formalization of Armand's electrochemical intercalation concept and rocking-chair battery technology, with an evaluation that carbon coating made LiFePO4@C the safest cathode (tribute article).
• "Evolution of Strategies for Modern Rechargeable Batteries" (2012) — DOI: 10.1021/ar2002705 — 1970s Li/TiS2 dendrite growth and short-circuiting, voltage drop upon anode replacement, high voltage of oxide cathodes, dendrite-free reversible intercalation of graphite, and manufacturers' reluctance to assemble cathodes in a discharged state.
• "The Birth of the Lithium‐Ion Battery" (2012) — DOI: 10.1002/anie.201105006 — Inventor Yoshino's retrospective on development and commercialization, where successful safety testing served as the turning point.
• "Lithium Batteries" (2013) — DOI: 10.1002/9781118615515 — Complementary intercalation and deintercalation of a graphite anode and a lithiated metal oxide cathode; the discovery that a stable SEI forms on graphite in a proper organic electrolyte gave rise to lithium-ion batteries. Three categories of commercial cathode materials (layered, spinel, phosphate).
• "Lithium-ion battery fundamentals and exploration of cathode materials: A review" (2024) — DOI: 10.1016/j.sajce.2024.09.008 — Cell components, the roles of nickel, manganese, and cobalt in NMC, cobalt dependence and stability challenges in high-nickel cathodes, and olivine LFP's safety and stability versus low energy density.
• "Safer Lithium‐Ion Batteries from the Separator Aspect: Development and Future Perspectives" (2020) — DOI: 10.1002/eem2.12129 — Separators do not directly participate in electrochemical reactions, but they dictate performance and safety.
• "Required properties of separators for lithium-ion batteries: A systematic review" (2026) — DOI: 10.1016/j.rineng.2026.111067 — Trade-offs among ionic conductivity, mechanical strength, and thermal stability.
• "Before Li Ion Batteries" (2018) — DOI: 10.1021/acs.chemrev.8b00422 — Historical evaluation that interlocking disparate parts was a greater challenge than the pros and cons of individual components.
• "Fast Charging of Lithium‐Ion Batteries: A Review of Materials Aspects" (2021) — DOI: 10.1002/aenm.202101126 — The decisive speed limit in fast charging is lithium movement inside the electrodes, compounded by the slow ion mobility of the electrolyte within the porous electrodes; both effects cause lithium plating on graphite.
• "New Insights on Graphite Anode Stability in Rechargeable Batteries" (2018) — DOI: 10.1021/acsenergylett.7b01177 — In non-carbonate solvents, the lithium-ion solvation structure plays a larger role in graphite stability than the SEI does.
• "A Study of University Students' Understanding of Simple Electric Circuits Part 2" (1991) — DOI: 10.1080/03043799108939504 — The misconception that "batteries are a source of constant current" exists both before and after university classes.
• "A Systematic Review of Student Misconceptions about Electricity and Electric Circuit Concepts" (2023) — DOI: 10.1109/fie58773.2023.10343239 — Ongoing research analyzing a random sample of 17 out of 93 papers, showing that most students still believe batteries are constant current sources.
• "Analysis of first-year university students' conceptual understanding of direct current electric circuits" (2026) — DOI: 10.31571/saintek.v15i1.10012 — Technical survey of 107 students, where over half answered incorrectly, with most believing batteries are constant current sources.
• "Advances in the Cathode Materials for Lithium Rechargeable Batteries" (2019) — DOI: 10.1002/anie.201902359 — New cathode materials that increase cell voltage and capacity along with stability are the best path to next-generation batteries.
• "A review on lithium iron phosphate cathode materials" (2013) — DOI: 10.3969/j.issn.2095-4239.2013.02.003 — LFP's long lifespan and thermal stability, improved via carbon coating (electronic conductivity) and metal-ion doping (lithium-ion diffusion).
• "Side Reactions/Changes in Lithium‐Ion Batteries: Mechanisms and Strategies for Creating Safer and Better Batteries" (2024) — DOI: 10.1002/adma.202401482 — Side reactions and changes in battery materials produce capacity loss and safety issues.
• "A Growing Appreciation for the Role of LiF in the Solid Electrolyte Interphase" (2021) — DOI: 10.1002/aenm.202100046 — The structure and composition of the SEI have long been debated, and the role of its main component, LiF, remains unclear.