Plate batteries include virtually all the lead acid, lithium ion, nickel cadmium, nickel metal hydride and other batteries used today for large scale energy storage.  A plate type battery is a battery where the "active material" - the chemicals that actually store electrical energy, are pasted into a 'grid', to form the plate.   A batteries active material is generally a pasty, crumbly material, often of relatively high resistance, that is unfit to produce strong, rigid, low resistance electrodes.  Therefore the active material is pressed into spaces in the grid, which supplies the rigidity and current conductors to pass the electrical energy out of the battery.  Most batteries are constructed of many plates, to allow a large surface area between active material and the surrounding electrolytes, as is shown here.

The 'active material' of a battery changes composition upon discharge.  For example, the positive material of a standard lead acid battery is Lead Dioxide, PbO2.  During discharge the PbO2 reacts with the sulfuric acid in the electrolyte to form PbSO4, or Lead Sulfate.  

Now, PbO2 and PbSO4 are molecules of different sizes, such that the active material pockets within the grid literally swell and shrink with each charge and discharge cycle, loosening the active materials connection to the grid, among other issues.  In a Lithium Ion battery, the mechanism is in some ways different, as a grid type electrode is not required, however the insertion of Lithium Ions into the graphite electrode causes a 10% size change with each cycle, and this morphological change eventually damages the electrode.  These mechanical size changes, while not the only degradation mechanisms within a battery, are generally the most degrading, and limits the cycle life of almost all plate type batteries.

A few plate batteries have high enough cycle life to be of value in utility applications.  One example is the Nano Titanate technology championed by Altair-Nano.  In this technology, the active material is contained in a spherical crystalline form that does not change morphology (shape or size) during the charge discharge cycle, removing that important path for cycling degradation.  The result is a highly increased cycling capability.

For different (and somewhat arcane) reasons, the lithium chemistry of Lithium Iron Phosphate (LiFePO4) has a much higher cycle life than other lithium chemistries, providing for as much as 7000 100% DOD cycles.  The most well known manufacturer using this chemistry is A123 Systems, who is widely expected to be a major player in future electric vehicle battery, HEV, and PHEV markets.

Both of these technologies are relatively expensive, but because of their advantages over other plate batteries are being marketed for utility applications.  Altair Nano is actively marketing into frequency regulation utility applications, and A123 Systems has been funded by the DOE to provide a large scale 8MW/32MWh battery for Southern California Edison. 

 

As the price of battery energy storage plummets, the market for energy storage will soar.  Which battery design or set of designs will take advantage of this huge market opportunity is not known.  However, here are some of the contenders:


Sodium Beta / Sodium Sulfur / Aqueous Sodium
Imagine a battery that doesn't even start working until it is around 600F, hotter than the broiler in your oven.  Containing molten Sulfur and Sodium, this would not at first glance seem an ideal choice for battery technology, especially if you remember that Sodium metal will generally ignite spontaneously just from the water vapor in air.  However, Sodium Sulfur, or "NaS" batteries are the most successful of the new technology batteries, and are now being used in utilities in the USA and more in Japan, Europe and the Middle East.  Sold presently only by NGK Insulators of Japan, these batteries have a cycle life of 2500 100% DOD or 4500 80% DOD cycles.

Another high-temperature liquid Sodium battery is the Zebra battery manufactured by MES-DEA of Switzerland.  Using Nickel Chloride instead of Sulfur, the Zebra battery could be safer, run at a somewhat lower temperature, and, given all the valuable Nickel metal in it, would provide some revenue on recycling.  Presently, Zebra is only being sold into motive power applications.

A recent article on Ceramatec, describes how they are working on a 'warm' NaS battery, where the Sodium would stay solid at an operating temperature of a mere 200F.  This advance could make NaS batteries extremely competitive in the utility storage world.

Even more interesting is technology for a safe, low-cost sodium-ion battery system coming out of Carnegie Mellon University, and recently funded by the DOE at $10 million.  This technology is run at room temperature with an aqueous sodium (basically sea water) electrolyte, and, if high cycle life can be obtained, could represent a winning solution.


Zinc-Air  / Metal-Air Batteries
When you drive your car, you are combusting the hydrocarbons in your gas tank with oxygen in the outside air to produce CO2 and H2O. If you had to carry the oxygen around with you instead of getting it free from the outside air, your cars would have to be much, much bigger.  In fact, to combust the 20 gallons of gas in your gas tank requires the oxygen in more than 1.5 tons of air, as much air as fills (uncompressed) roughly 10 suburban houses. 

The potential advantage for Metal-Air batteries is similar to that for gasoline in that you only have to store one reactant.  The other reactant, Oxygen, you get for free from the air around.  In the case of Zinc-Air, however, no net gasses are produced as the battery stores and then releases Oxygen during the charge/discharge cycle. 

The reactant metal, Zinc for example, is plated or accumulates on an electrode in a Zinc-Air cell during the charging phase, and is turned back into Zinc ions in solution during the discharge phase.  Metal Air cells have at least the potential of virtually unlimited cyclability.

An important note is that IBM announced in August of 2009 a major effort to develop a Lithium-Air battery, which has the potential of producing 'serious' electric vehicle batteries, batteries with an energy density sufficient to power a car for 400 miles.  While it is unlikely that this will produce a cost effective utility scale battery, it is an effort worth following.

A major advance in Metal Air batteries is being funded by the DOE to Arizona State spin off Fluidic Energy.  The advance here is the potential use of Ionic Liquids as electrolyte. Ionic liquids, different from water, won't evaporate out the air cathode or air membrane that is required in a Metal-Air battery. They also don't electrolysize into Hydrogen and Oxygen as water does in the presence of high-voltage electrodes. This means that potentially higher-energy metals than Zinc may be used, increasing the energy density dramatically.


Flow Batteries
A flow battery cell is constructed of two halves, the cathode side and the anode side, each containing an electrode for conducting current in and out of the cell, with the halves being separated by a membrane.  Two tanks of electrolyte, the catholyte and the anolyte (you can guess which tank connects to which side of the cell) are connected to the cell such that the electrolytes are pumped into the cell during discharge and recharge. 

In a flow battery cell, the active material is floating in the electrolyte, not pasted into an electrode grid.  Flow batteries are known as "Redox" batteries, as during discharge, the reactant in one side of the cell is reduced (adds an electron), while the other is oxidized (loses an electron).  Since both reactions happen in solution, the physical change to reactant sizes has no effect on electrodes, and therefore flow battery cells could have extremely long or even unlimited cycle life.

In addition, power and energy are decoupled in certain types of flow cells.  The power (MW) rating being determined primarily by the amount of cell electrode surface area and associated power conditioning equipment, and the energy (MWh) rating being determined by the amount of electrolyte.  So, for certain types of flow batteries, more MWh is mostly a matter of larger electrolyte tanks. 

The following are different types of flow batteries, each of which has their own benefits and issues.

Iron Chromium Flow Battery
First developed by NASA during the Apollo Program, Iron Chromium (FeCr) has the benefit of using relatively benign chemicals, and could represent the safest type of flow battery available.  Also, electrolyte chillers are not necessarily required (as in Zinc Bromine), and the active materials are relatively inexpensive and widely available.  

Some primary issues with FeCr are the fact that there are different reactants in each half of the cell, such that transfer of one reactant (for example Chromium ions) through the membrane results in a permanent loss of capacity.  This transfer might happen due to inefficient or damaged membranes.  Another issue is that the energy density of FeCr is low, requiring much larger electrolyte tanks than some other flow battery systems.

Iron Chromium is presently being sold into mostly Telecom applications by Deeya Energy, and is also being developed for utility applications by various startups.

Zinc Bromine Flow Battery
Zinc Bromine is considered a "hybrid flow battery", as during the charging process, Zinc metal is directly plated on the anode electrode.  This means that the power/energy relationship in a Zinc Bromine flow battery is more fixed than some other flow battery systems, as the total energy available in a system is limited by the available electrode area for plating Zinc.  Zinc Bromine is, along with Vanadium Flow batteries, the most studied flow battery system for utility applications, and is actively being manufactured and sold into utility applications by two companies.  

Zinc Bromine has a relatively high energy density, and uses relatively inexpensive and widely available reactants.  Bromine gas is a serious health hazard, but in the most common designs the elemental Bromine is complexed during the charging process into a material with a much lower vapor pressure, making it much less hazardous. 

Issues with Zinc Bromine flow batteries include growth of dendrites on the anode during Zinc plating (tree like spikes of Zinc metal which can pierce membranes and cause shorting), and the need for large electrolyte chillers to improve efficiency and to reduce the potential of Bromine gas production.  Some manufacturers claim unlimited cyclability (30 year life), while others state that a small degradation of capacity happens with cyclability on the Bromine side electrode.

Zinc Bromine flow batteries are presently being produced in individual packages as large as 500KW/2.8MWh, and are being produced by ZBB Energy, and Premium Power.  Other companies in pre-production are also working on Zinc Bromine flow systems.

Vanadium Flow Battery
Vanadium flow batteries are another 'true' flow battery, which has been studied extensively and has been used in a few utility applications.  A Vanadium flow battery uses the same reactants on both sides of the cell, and so does not suffer from the ionic transfer issues of Iron Chromium.  It also has potentially the highest efficiency of any flow battery design, and a relatively high energy density as well.

The cost of the Vanadium based reactants is higher than that of FeCr or Zinc Bromine above, and there are questions as to safety.  While the Vanadium flow battery electrolyte itself is not terribly dangerous, the Vanadium PentaOxide used in one part of the electrolyte is very poisonous in powder form (should a spill occur that dries to powder). 

Utility scale Vanadium flow batteries are presently being offered by the Chinese company Prudent Energy, and are also being developed by several pre-production companies.