A lithium-ion battery is a rechargeable battery that stores and releases energy by shuttling lithium ions back and forth between two electrodes. It’s the dominant battery technology in smartphones, laptops, electric vehicles, and power tools, largely because it packs more energy into less space and weight than older rechargeable batteries. Since Sony commercialized the first lithium-ion cell in 1991 with an energy density of about 80 Wh/kg, that figure has climbed to 360 Wh/kg in today’s mass-produced cells.
How a Lithium-Ion Battery Works
Every lithium-ion cell has five basic parts: two electrodes (called the anode and cathode), a separator between them, a liquid or polymer electrolyte, and metal current collectors on each side. The anode and cathode both store lithium, but the magic happens when ions move between them.
When the battery discharges (powering your device), the anode releases lithium ions. Those ions travel through the electrolyte, pass through the separator, and land at the cathode. That movement frees up electrons at the anode, which can’t pass through the separator. Instead, the electrons flow through an external circuit, through your phone or laptop, and back to the cathode. That flow of electrons is the electrical current you use.
Charging reverses the process. When you plug in, energy from the charger pushes lithium ions out of the cathode and back to the anode, resetting the battery for another cycle. The separator is critical here: it allows ions through while blocking electrons, preventing an internal short circuit.
What’s Inside the Cell
The anode in most commercial lithium-ion batteries is made of graphite, a form of carbon with a porous structure that can absorb and release lithium ions efficiently. Some newer designs use lithium titanium oxide or tin-based alloys instead, trading some energy capacity for faster charging or longer lifespan.
The cathode is typically a metal oxide. The exact chemistry varies by application. Cobalt oxide cathodes are common in phones and laptops. Iron phosphate cathodes show up in many electric vehicles and home energy storage systems because they’re more thermally stable and last longer, though they store slightly less energy per kilogram. Nickel-manganese-cobalt blends try to balance energy density, longevity, and cost.
The electrolyte is a lithium salt dissolved in an organic solvent. It’s the medium that carries ions between electrodes. Some batteries use a polymer-based electrolyte instead of a liquid one, which can make the cell thinner and more flexible.
Voltage and Energy Density
A single lithium-ion cell has a nominal voltage of 3.6V, though some chemistries sit at 3.7V or 3.8V. For context, a single alkaline AA battery is 1.5V, so lithium-ion cells deliver roughly twice the voltage per cell. A fully charged cell reaches about 4.2V, and it’s considered empty around 2.8 to 3.0V. Iron phosphate cells run a bit lower, with a nominal voltage of 3.2V.
Energy density is where lithium-ion batteries really stand out. Today’s best mass-produced cells hit about 360 Wh/kg, meaning each kilogram of battery stores 360 watt-hours of energy. That’s more than four times the energy density of those first commercial cells from 1991. In lab settings, experimental cells using lithium metal anodes have reached 711 Wh/kg, though those aren’t in consumer products yet. This high energy density is the reason lithium-ion batteries made modern smartphones and long-range electric vehicles possible.
How Charging Works
Lithium-ion batteries charge in two stages. First, the charger pushes a steady current into the cell (called constant-current charging). This phase does the heavy lifting, bringing the battery from empty to nearly full. At a typical charging rate, this stage takes roughly 50 minutes and brings the cell voltage up to 4.2V.
Once the cell hits that voltage ceiling, the charger switches to constant-voltage mode. It holds the voltage at 4.2V and lets the current gradually taper off. This slower phase tops off the last portion of capacity without overstressing the cell. Pushing too much current at high voltage can cause lithium to plate onto the anode as metal rather than being absorbed properly, which permanently damages the battery and can create safety risks. The two-stage approach prevents that.
What Affects Battery Lifespan
A lithium-ion battery doesn’t last forever. Each charge-discharge cycle causes small, irreversible changes inside the cell that gradually reduce its capacity. After enough cycles, the battery holds noticeably less charge than it did when new. Several factors speed up or slow down this degradation.
Temperature is the biggest one. Heat accelerates chemical side reactions inside the cell that consume active lithium and degrade the electrodes. Leaving a phone on a hot dashboard or charging a laptop on a soft surface that traps heat will shorten battery life faster than normal use. Extreme cold isn’t great either, as it increases internal resistance and can cause lithium plating during charging.
How deeply you discharge the battery also matters. Regularly draining a cell to near-zero and then charging it to 100% stresses the electrode materials more than keeping it between, say, 20% and 80%. High charging and discharging currents (fast charging, heavy gaming on a phone) generate more heat and mechanical stress on the electrodes, which compounds the effect.
The state of charge during storage plays a role too. A battery sitting at full charge for weeks degrades faster than one stored at a moderate level. If you’re putting a device away for a long time, charging it to around 50% first is a practical way to slow aging.
Why Lithium-Ion Dominates
Lithium-ion displaced older rechargeable technologies like nickel-cadmium and nickel-metal hydride for several practical reasons. It stores significantly more energy per unit of weight and volume, so devices can be smaller and lighter. It has no “memory effect,” meaning you don’t need to fully discharge it before recharging. And its self-discharge rate is relatively low, so a charged battery sitting on a shelf loses its charge slowly compared to older chemistries.
The tradeoffs are real but manageable. Lithium-ion cells need protection circuitry to prevent overcharging, over-discharging, and overheating, all of which can cause thermal runaway (a dangerous chain reaction of heat buildup). That’s why every lithium-ion battery pack in a phone, laptop, or EV includes a small circuit board that monitors voltage, current, and temperature. Manufacturing costs remain higher than simpler battery types, though prices have dropped dramatically over the past decade as production has scaled up for electric vehicles and grid storage.