Energy Storage
Lithium-Ion Battery
A lithium-ion battery stores energy by shuttling lithium ions between electrodes through an electrolyte (intercalation). It has a nominal cell voltage around 3.6-3.7V, high energy density (roughly 100-265 Wh/kg, 250-700 Wh/L) and round-trip efficiency near 90%, which makes it dominant for phones and EVs. Its flammable electrolyte creates thermal-runaway risk, and cells use thin wound or stacked electrodes to keep ion paths short.
Pumped-Storage Hydroelectricity
Pumped-storage hydroelectricity (PSH) moves water between two reservoirs at different elevations: it pumps water uphill when power is cheap and releases it through reversible turbines to generate when demand peaks. With 70-80% round-trip efficiency and facility lifespans of decades to over a century, it is by far the world's largest grid storage technology, limited mainly by the need for suitable hilly terrain.
Grid Energy Storage
Grid energy storage stores electricity for later use to balance supply and demand, mainly to absorb the variability of solar and wind. It spans electrochemical, mechanical, thermal, and chemical families. Pumped hydro is by far the largest installed form; lithium-ion suits short durations under about eight hours, and long-duration storage is the open frontier.
Flow Battery
A flow battery stores energy in liquid electrolytes held in external tanks and pumped through a reaction cell stack. Its defining advantage is that energy capacity (tank size) is decoupled from power (stack size), so storage scales by enlarging the tanks. Vanadium redox is the common type, with 10,000-20,000+ cycle life but low energy density (roughly 20-50 Wh/L), suiting multi-hour grid storage.
Thermal Runaway
Thermal runaway is a self-reinforcing feedback loop in which rising temperature triggers reactions that release more heat, accelerating until failure. In lithium-ion batteries it can cause fires or explosions when a cell is overheated, overcharged, or damaged. Larger cells are more prone to it because heat generation scales with volume while cooling scales with surface area. Mitigations include cell chemistry, thermal management, and failure isolation.
Compressed-Air Energy Storage
Compressed-air energy storage (CAES) uses cheap electricity to compress air into underground caverns, then expands it through a turbine to generate later. Compression makes heat: diabatic plants waste it and reheat with gas (about 27-42% efficiency), while adiabatic designs store the heat and reach about 70%. Existing plants include Huntorf in Germany and McIntosh in Alabama.
Why Batteries Are Built From Many Small Cells Instead of One Giant One
Even a Tesla Powerwall or a grid-scale storage container is thousands of small cells inside, for four physical reasons: heat dissipation gets worse as cells grow (the square-cube law), thick electrodes slow ion transport, cell voltage is fixed by chemistry so you must wire cells in series, and many small cells isolate failures and ease manufacturing.
Thermal Energy Storage
Thermal energy storage holds energy as heat — via sensible heat (warming a material), latent heat (phase-change materials), or reversible chemical reactions — and recovers it through a steam turbine or direct use. Examples include molten salt in concentrated solar plants and 'sand batteries' that heat sand or rock. Systems that store electricity as heat and convert it back are called Carnot batteries.
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.
Iron-Air Battery
An iron-air battery stores energy through reversible oxidation and reduction of iron — essentially controlled rusting and un-rusting. Iron is cheap, abundant, and non-flammable, but energy density is low so cells are bulky. Targeting 100+ hour discharge, it is aimed squarely at multi-day grid storage; Form Energy is the leading commercial developer.
Why Pumped Hydro Still Dominates Grid Storage: The Trade-Off Axes
Pumped hydro is old and modest on most metrics, yet it remains by far the largest installed grid storage because it wins the axis that matters most: lifetime cost per kWh at multi-day timescales. Other technologies beat it on round-trip efficiency, response time, energy density, or geographic flexibility, but none yet matches its cost where geography permits. The real frontier is getting pumped-hydro economics without needing a mountain.
Energy Dome CO2 Battery
The Energy Dome CO2 Battery is a long-duration mechanical storage system, not an electrochemical battery. It compresses CO2 into liquid using surplus renewable power, storing the heat, then evaporates and expands the gas through turbines to generate for 8-24 hours in a closed loop. CO2 liquefies under modest pressure, giving higher density than compressed air. Google made it the focus of its first long-duration storage investment in 2025.