A battery converts chemical energy into electrical energy through spontaneous redox reactions. The cell consists of three components: an anode (negative electrode), a cathode (positive electrode), and an electrolyte. A separator keeps the electrodes from shorting while allowing ion transport. During discharge, the anode oxidizes, releasing electrons to the external circuit. The cathode reduces, accepting electrons. Ions flow through the electrolyte to balance the charge. The difference in electrode potentials determines the cell voltage; a lithium-ion cell delivers about 3.7 V nominal. However, internal resistance causes a voltage drop under load, reducing available energy. The capacity, measured in ampere-hours, reflects the total charge stored. Energy density (Wh/kg) varies with chemistry. In rechargeable batteries, applying an external voltage reverses the reactions, restoring the original materials. Primary batteries are single-use because their reactions are not easily reversed. For example, alkaline batteries are primary, while lead-acid and lithium-ion are secondary.
The lithium-ion battery uses a reversible intercalation mechanism. During discharge, lithium ions (Li+) deintercalate from the graphite anode, travel through the liquid electrolyte, and intercalate into the cathode, typically lithium cobalt oxide (LiCoO2). Electrons flow through the external circuit, providing power to the device. During charging, an external voltage forces ions back to the anode and electrons back, reversing the reaction. This shuttle can be repeated hundreds to thousands of times. On the first charge, the electrolyte forms a thin solid-electrolyte interphase (SEI) on the graphite, which prevents further decomposition while conducting lithium ions. The SEI gradually thickens over cycles, consuming lithium and increasing internal resistance, which contributes to capacity fade. The choice of cathode material affects performance: LiCoO2 offers high energy density but limited cycle life due to structural changes and cobalt expense; lithium iron phosphate (LFP) provides longer life, better safety, and lower cost at the expense of voltage (3.2 V) and energy density. The electrolyte, a lithium salt in organic carbonates, supports high voltage but is flammable, linking safety to chemistry.
Battery capacity depends on the mass of active material and its utilization. Practical lithium-ion cells achieve up to 250 Wh/kg. Cycle life typically ranges from 500 to 1,000 cycles before capacity degrades to 80% of initial. Degradation results from loss of active lithium, electrolyte decomposition, and structural changes in electrodes. Loss of active lithium occurs through SEI growth and lithium plating during fast charging or at low temperatures. Cathode materials may exhibit phase transitions that reduce reversible capacity. Depth of discharge affects cycle life: shallower discharges extend longevity. Safety risks include thermal runaway, triggered by overcharging, internal shorts, or external heating. When internal temperature exceeds about 150°C, exothermic reactions accelerate, leading to fire or explosion. Dendrites—microscopic lithium filaments—can grow from the anode during fast charging, penetrate the separator, and cause short circuits. Battery management systems monitor voltage, current, and temperature to prevent unsafe operation. Pack-level cooling and venting further mitigate hazards.
Emerging technologies aim to improve safety and performance. Solid-state batteries replace the liquid electrolyte with a ceramic or polymer electrolyte that is non-flammable and mechanically blocks dendrite growth. Prototypes have achieved energy densities above 300 Wh/kg. Practical challenges include high interfacial resistance between solid electrolyte and electrodes and manufacturing scalability. Sodium-ion batteries use abundant sodium instead of lithium, lowering cost and improving safety. Their energy density is currently lower (100–160 Wh/kg) but suitable for grid storage. Sodium's larger ionic radius causes slower diffusion and more structural stress during cycling. Lithium-sulfur and lithium-air batteries offer higher theoretical energy densities but face cycle life and efficiency challenges due to intermediate species dissolution and side reactions. These advancements are shaping the next generation of rechargeable power.
