There are many types of battery materials, but they can be broadly classified into four categories: metals, intercalation compounds, polymers, and nanomaterials. Metals are the simplest and oldest type of battery materials, such as lead-acid, nickel-cadmium, and nickel-metal hydride batteries. They have low energy density, high power density, moderate cycle life, low safety, low cost, and high environmental impact. Intercalation compounds are materials that can reversibly insert and extract ions between their layers, such as lithium-ion, lithium-sulfur, and sodium-ion batteries. They have high energy density, moderate power density, high capacity, high cycle life, moderate safety, high cost, and moderate environmental impact. Polymers are materials that can conduct ions or electrons through their chains, such as lithium-polymer, polyacene, and organic radical batteries. They have moderate energy density, high power density, low capacity, low cycle life, high safety, moderate cost, and low environmental impact. Nanomaterials are materials that have at least one dimension in the nanoscale, such as carbon nanotubes, graphene, and quantum dots. They have very high energy density, very high power density, very high capacity, very high cycle life, very high safety, very high cost, and unknown environmental impact.