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A primary battery coupling lithium oxidation to reduction of fluorinated carbon, producing lithium fluoride and carbon.

A primary battery using metallic lithium and iron disulfide. The overall discharge conversion consumes four lithium atoms and releases four electrons per $\mathrm{Fe}{\mathrm{S}}_2$ formula unit. This is distinct from the rechargeable host-to-host transport in a conventional lithium-ion battery.

This lithium iron disulfide battery consumes a metallic lithium anode and converts iron disulfide into iron and lithium sulfide. Commercial cells of this chemistry are primary batteries. A conventional lithium-ion battery is designed for reversible cycling: lithium ions transfer between two host electrodes, commonly a carbon insertion anode and a transition-metal compound cathode, while electrons travel in the external circuit. Metallic lithium is not the intended normal anode reactant in that design. Primary Li/FeS₂ conversion is the key distinction from the rechargeable lithium-ion host-electrode system. The fact that both transport lithium ions does not make them the same battery chemistry, and a nominal 1.5 V is not a definition of either class.^[ref]

A primary battery is intended for a single discharge; a secondary battery is designed for repeated electrical recharge through sufficiently reversible reactions. A microbattery is defined by its small dimensions or microfabricated construction, not by rechargeability. Therefore microbatteries can belong to either class. A thin-film Li-ion microbattery with reversible insertion electrodes is secondary because lithium can shuttle back and forth during discharge and charging; a miniature cell using irreversible primary chemistry remains primary. Small size alone does not justify assigning every microbattery to the secondary class.

This is a primary battery with a strongly reconstructive conversion electrode. Discharge produces stable LiF and a carbon phase while breaking the original carbon–fluorine framework. Charging would have to separate and reorganize those products, restore the fluorinated carbon structure and recover the initial lithium electrode.

LiF is highly stable and poorly conducting, the phase distribution and electrical contacts have changed, and the reverse pathway can require a large overpotential. Electrolyte oxidation or other side reactions can occur before efficient reconstruction is achieved. Thermodynamic reversibility of an overall reaction equation therefore does not guarantee a practical rechargeable electrode.