What a cell is built from
A battery cell is the smallest sealed unit that stores charge. The Congressional Research Service, the research arm of the United States Congress, states that every cell contains 2 electrodes, a cathode and an anode, an electrolyte, a chemical solution that lets electricity flow between them, and a separator, a physical barrier that keeps the 2 electrodes from touching. The cathode is the positive terminal and the anode is the negative terminal. During use, electrons flow from the anode to the cathode, and charging the cell reverses that flow.
Which way the charge moves
Inside the cell, it is lithium ions, not electrons, that cross the electrolyte and the separator. They move from the anode to the cathode while the cell is powering something, and back from the cathode to the anode while it charges. Outside the cell, in the wire and the device it powers, electrons make the same trip in the same direction, anode to cathode during use, because that electron flow through the outside circuit is what actually does the work. The electrolyte itself is a lithium salt dissolved in an organic solvent, a liquid the ions can move through on their way between the 2 electrodes.
How the chemistry chosen changes what a cell can do
Not every lithium ion cell performs the same. Battery University compared 5 named lithium chemistries in a single table, and nominal voltage, how much electrical pressure a cell delivers, ranges from 3.20 volts for lithium iron phosphate up to 3.70 for lithium manganese oxide.
Where the source lists 2 nominal figures for a chemistry, the lower figure is used here.
Show the numbers
| Lithium cobalt oxide | 3.60 |
| Lithium manganese oxide | 3.70 |
| Nickel manganese cobalt | 3.60 |
| Lithium iron phosphate | 3.20 |
| Nickel cobalt aluminium | 3.60 |
Cycle life, how many times a cell can charge and discharge before its capacity falls to 80% of what it held when new, ranges further still. Lithium iron phosphate reaches 2,000 cycles or more, the highest of the 5, while nickel cobalt aluminium oxide manages 500. That figure moves with how deep and how hard the cell is used, not a fixed number. Specific energy, how much energy a cell packs into a kilogram of its own weight, ranges from 90 to 120 watt hours per kilogram for lithium iron phosphate up to 200 to 260 for nickel cobalt aluminium oxide, predictable to 300 in that chemistry.
Lithium cobalt oxide reaches 240 watt hours per kilogram in specialty cells, and nickel cobalt aluminium oxide is predictable to 300, both above the ranges drawn here.
Show the numbers
| Lithium cobalt oxide | 150 to 200 watt hours per kilogram |
| Lithium manganese oxide | 100 to 150 watt hours per kilogram |
| Nickel manganese cobalt | 150 to 220 watt hours per kilogram |
| Lithium iron phosphate | 90 to 120 watt hours per kilogram |
| Nickel cobalt aluminium | 200 to 260 watt hours per kilogram |
The same 4 parts hold more energy than they used to
The standard cylindrical cell used in laptops and some electric cars has kept the same 4 part structure for decades while its capacity kept growing. It held 1,100 milliamp hours in 1994. By 2001 that reached 1,900. Today a high energy version of that same cell delivers over 3,000, nearly 2.7 times the 1994 figure.
This describes the standard 18650 cylindrical cell format specifically, not every lithium ion cell shape. Source 2 states the current figure as a floor, over 3,000 milliamp hours, not an exact reading, so the bar and its display round to that floor.
Show the numbers
| 1994 | 1,100 |
| 2001 | 1,900 |
| Today | 3,000 |
A cell also loses a small share of its charge just sitting unused. The Clean Energy Institute at the University of Washington puts that self discharge rate at under 2% a month.