The Taxonomy of Grid-Scale Energy Storage
Grid storage splits into four families by how energy is held: electrochemical (real batteries: lithium-ion, flow, iron-air, sodium-sulfur), mechanical (pumped hydro, compressed air, liquid air, CO2 systems, flywheels), thermal (molten salt, sand batteries, hot rocks), and chemical fuels (hydrogen, ammonia, methane). Strictly only the electrochemical family is a 'battery'; 'battery' has become loose marketing shorthand for anything that takes electricity in and gives it back later.
Grid Energy Storage technologies are best organized by the physical form in which energy is held, which cuts across the marketing label of "battery." **Electrochemical (true batteries):** energy is stored in chemical bonds inside cells. This family includes the Lithium-Ion Battery, the Flow Battery (vanadium redox is the classic, iron flow is cheaper), the Iron-Air Battery (controlled rusting of iron, championed by Form Energy), sodium-sulfur battery (deployed at grid scale in Japan, runs near 300 degrees Celsius), liquid metal battery (molten electrodes, developed by Ambri), and sodium-ion battery (lower energy density than lithium-ion but cheaper, fine when weight and volume do not matter). **Mechanical:** energy is stored as pressure, height, or rotation. This includes Pumped-Storage Hydroelectricity, Compressed-Air Energy Storage, liquid air energy storage, the Energy Dome CO2 Battery, flywheel energy storage, and gravity storage (such as Energy Vault stacking composite blocks, or weighted pistons in mineshafts). All work by using cheap electricity to do work against a pressure or height gradient, then recovering it through a turbine later. **Thermal:** energy is stored as heat and recovered via a steam turbine or used directly. See Thermal Energy Storage, which covers molten salt (used in concentrated solar power), sand battery systems (the Finnish Polar Night Energy silos), and hot-rock stores. Systems that store electricity as heat and convert it back are called Carnot battery designs. **Chemical fuels:** energy is stored in a fuel you can later burn or run through a fuel cell, including hydrogen, ammonia, and synthetic methane. These suit seasonal storage but have low round-trip efficiency. Strictly, a battery is electrochemical cells. Energy Dome calling its system a "CO2 Battery" and Polar Night Energy calling a hot sand silo a "sand battery" are the same liberty: accurate in spirit, loose by definition. "Battery" has become shorthand for anything that takes electricity in and gives electricity out later. The practical takeaway: once you drop the requirement to carry storage around and accept filling a field with it, the whole non-electrochemical world opens up.