A battery contains two electrodes (an anode and a cathode), a liquid or paste called an electrolyte, a thin separator between the electrodes, and an outer casing that holds everything together. These components work as a team: the electrodes store chemical energy, the electrolyte shuttles charged particles between them, and the separator prevents the two sides from touching while still letting those particles through. The exact materials vary depending on the battery type, but every battery from a tiny coin cell to a car battery follows this same basic architecture.
How the Parts Work Together
The anode is the negative side of the battery, and the cathode is the positive side. Both store reactive chemicals. When you connect a battery to a device, a chemical reaction at the anode releases electrons, tiny charged particles that carry electrical energy. Those electrons can’t travel through the electrolyte (the separator blocks them), so they’re forced to flow out through a metal current collector, through the wire in your device, and back into the battery on the cathode side. That flow of electrons is the electrical current that powers your phone, flashlight, or remote.
Meanwhile, inside the battery, positively charged ions travel through the electrolyte from one electrode to the other, completing the circuit internally. The electrolyte’s entire job is to carry those ions while forcing the electrons to take the long way around, through your device. When the reactive chemicals are used up, the battery is dead. In rechargeable batteries, plugging in a charger reverses the chemical reaction, pushing everything back to its starting position.
What’s Inside an Alkaline Battery
The standard AA, AAA, C, and D batteries you buy at a grocery store are alkaline batteries. The negative electrode is made of powdered zinc formed into a gel. The positive electrode is manganese dioxide, a dark compound packed around the inside of the steel casing. The electrolyte between them is potassium hydroxide, a strong alkaline solution (which is where the battery gets its name).
When the battery discharges, the zinc reacts with the potassium hydroxide, releasing electrons. Those electrons flow through your device and return to the manganese dioxide side, where a second reaction absorbs them. Once the zinc is fully reacted, there’s nothing left to give, and the battery dies. Standard alkaline batteries are single-use because these chemical reactions can’t be cleanly reversed.
What’s Inside a Lithium-Ion Battery
The rechargeable batteries in phones, laptops, electric cars, and power tools are almost all lithium-ion. The negative electrode is typically graphite, a form of carbon arranged in thin layers that lithium ions can slip in and out of. The positive electrode is where things get more varied. Early rechargeable lithium-ion batteries, first commercialized by Sony in 1991, used lithium cobalt oxide. Today, two cathode chemistries dominate: lithium iron phosphate (common in electric vehicles from some manufacturers) and lithium nickel manganese cobalt oxide (used in many other EVs and consumer electronics). Each blend trades off between energy density, cost, and lifespan.
The electrolyte in a lithium-ion battery is a lithium salt dissolved in organic solvents like ethylene carbonate and dimethyl carbonate. Unlike the water-based electrolyte in an alkaline battery, these organic solvents are flammable, which is one reason lithium-ion batteries can catch fire if punctured or badly damaged.
Sitting between the two electrodes is a thin, microporous separator made of polyethylene or polypropylene plastic. Its pores are large enough for lithium ions to pass through but small enough to physically block the electrodes from touching. If the separator fails, the battery short-circuits and can overheat rapidly. Many separators are engineered with a thermal shutdown feature: if the battery gets too hot, the plastic softens and its pores close, cutting off ion flow before things get dangerous.
What’s Inside a Car Battery
Traditional car batteries use lead-acid chemistry, one of the oldest rechargeable battery designs. Inside the plastic case, you’ll find a series of lead alloy plates submerged in sulfuric acid. Pure lead is too soft to hold its shape, so small amounts of antimony, calcium, tin, or selenium are mixed in to strengthen the plates. One set of plates is coated with lead dioxide (the positive electrode), and the other set is spongy pure lead (the negative electrode).
A full-size car battery can contain up to 18 pounds of lead and roughly a gallon of sulfuric acid. Both are hazardous. The acid is highly corrosive, and lead is a well-known toxic metal, which is why car batteries should always be recycled through designated programs rather than thrown in the trash.
Button Cells and Coin Batteries
The small, flat batteries used in watches, hearing aids, and key fobs come in several chemistries packed into a tiny metal disc. Some use silver oxide paired with zinc. Others use a zinc-air design, where oxygen from the atmosphere enters through small holes in the casing and reacts with zinc inside. Older button cells historically contained mercury or cadmium, though U.S. law has phased out mercury-containing batteries and pushed for recycling of cadmium-based cells.
The Outer Casing
Everything described above sits inside a protective shell. Alkaline batteries use a steel can. Lead-acid car batteries have a thick polypropylene plastic case. Lithium-ion batteries come in three main shapes: cylindrical metal cans (like the 18650 cells inside many laptop battery packs), rigid prismatic boxes made of aluminum or steel, and flexible pouches.
Pouch-style lithium-ion cells, common in phones and tablets, use a layered film made of aluminum sandwiched between polymers. The inner layer of polypropylene seals against heat, the aluminum sheet blocks moisture and light, and the outer layer of polyamide or polyester provides mechanical toughness. This design keeps the cell thin and lightweight while preventing electrolyte from leaking out.
Hazardous Materials to Be Aware Of
Batteries can contain mercury, lead, cadmium, nickel, and silver, all of which pose risks to health and the environment if they end up in landfills. Lead-acid batteries are the most obviously hazardous given the volume of lead and corrosive acid inside. Nickel-cadmium rechargeables contain cadmium, a toxic heavy metal. Even common lithium-ion batteries carry flammable organic solvents that can release toxic fumes if the cell is damaged.
Most of these metals are recoverable through recycling. Many retailers and municipal waste programs accept used batteries, and in the case of lead-acid car batteries, recycling rates are already very high because the lead has significant scrap value.
Solid-State Batteries: What Changes Inside
The next major shift in battery design replaces the liquid electrolyte with a solid material. Solid-state batteries use ceramics, polymers, resins, or glass composites to move ions between electrodes. Removing the flammable liquid electrolyte makes the battery safer and potentially allows it to store more energy in the same space, since the solid electrolyte can also serve as the separator. These batteries are not yet widespread in consumer products, but several automakers and electronics companies are investing heavily in bringing them to market.