One of the interesting technological changes from the late 1980s to the present day is how much more we collectively interact with batteries. The first solid-state batteries, developed in the 1800s, were a scientific curiosity: the handiwork of one Michael Faraday. Thirty years ago, you needed C-size batteries for flashlights, AAs for a Walkman or Discman, and maybe a few AAAs for a remote control—but everything significant had to stay plugged into the wall. Today, batteries are an integral part of life. They power the phones and laptops we rely on for work and leisure and increasingly power our vehicles.
One type of battery chemistry often referenced is a so-called “solid-state battery.” So what makes a solid-state battery different from a “regular” battery, such as the alkaline batteries in a flashlight or the lead-acid batteries in our cars? A battery is an energy storage device with positively and negatively charged terminals that connect internally through a conductive medium called an electrolyte. Solid-state batteries use a solid or semi-solid electrolyte, such as an alloy, polymer, paste, or gel, in contrast to the liquid electrolyte bath found in most conventional battery chemistries.
Of Potatoes and Power Cells
In battery chemistry, there are solids, and then there are solids. Not all electrolytes are created equal; they all permit the passage of electrons but do it differently. Some are crystalline solids, such as metallic lithium alloys, while others are sheets of plastic or ceramic, and still others are made of a polymer gel.
The simplest example of a solid electrolyte might be the potato in a potato battery. Potatoes seem pretty dry, but they contain enough moisture to permit the conduction of electrons and ions through the protein-and-starch matrix of the potato. Stick a penny and a galvanized screw into a potato, connect them to an LED, and the LED will light. Wire a thousand potato batteries together, and you can play Doom (sort of).
Potatoes are also a great example of a quasi-solid-state battery. Some solid-state batteries use a solid matrix suffused with a conductive solution: so-called “soggy sand” electrolytes. The cross-linked proteins and starch polymers in a potato form a matrix through which ions percolate.
Lithium Battery Chemistries
Lithium is the metal of choice for many solid-state batteries due to the element’s high energy density and low binding energy. Structurally, these widely used batteries use lithium ions (Li+) in their cathode and electrolyte, while their anode is often made of graphite or silicon. Why lithium? Lithium atoms are tiny and pack tightly, so in a sample of metallic lithium; there are a lot of resident electrons and lithium ions per unit mass. However, because lithium has such a low atomic weight, lithium nucleons are easily separated from their electrons.
Lithium-ion
Lithium-ion batteries have the greatest energy density per unit mass of any solid-state battery chemistry, up to 1.6 kilowatt-hours per kilogram. They’re also usually rechargeable. These two perks leave lithium batteries at the top of the heap. Lithium-ion batteries are the portable power source for various consumer electronics, such as laptops and cell phones.
Many lithium-ion batteries now use a polymer gel or membrane, although some still use a liquid electrolyte. Some designs, such as those in the first and second generations of the Tesla Powerwall, use lithium combined with nickel, manganese, and cobalt (so-called NMC batteries) in their construction. However, concerns over the toxicity of nickel and cobalt were factors in the development of lithium-iron-phosphate (lithium ferrophosphate, or LFP) battery chemistry. Similar concerns may be part of a rumored change to LFP chemistry for the forthcoming Powerwall 3.
Just like gels themselves, lithium batteries have one foot (terminal?) on the “solid-state” side of the line and the other on the “liquid electrolyte” side. Not all solid-state batteries use lithium, but most do; not all lithium batteries are solid-state, but many are. Some batteries use a polymer like polyethylene as the electrolyte, which we call lithium-polymer batteries.
All-solid-state
Some lithium battery designs use not a solution of lithium ions as an electrolyte but a solid lithium alloy, frequently a ceramic. Similar to graphene, the idea is that electrons can flow freely through the crystal matrix of ceramics, as they do through graphene’s wandering resonance bonds. These “pure” solid-state batteries (that is, ones that use a solid electrolyte as well as a solid anode and cathode) enjoy a few advantages over chemistries that use a liquid or gel as their electrolyte.
Perhaps most important is the safety hazard of liquid electrolytes. Lead-acid batteries, such as those in automobiles, are a great illustration. Car batteries have stickers and embossing all over them, warning the user not to open up the battery casing nor touch a leaking or damaged battery, lest the sulfuric acid electrolyte inside the battery cause permanent injury or even death.
However, all-solid-state battery chemistries have some intrinsic issues that prevent them from simply stealing the limelight.
First, it takes much more pressure to mash two sheets of ceramic together hard enough to get a reasonable reaction rate than it does to dunk two electrodes in an electrolyte bath and wait. As a result, it’s difficult to make all-solid-state batteries that operate at reasonable pressures.
Second, solid-state batteries that get overcharged are vulnerable to crystal growth called dendrites that permanently and irreversibly damage the battery. Dendrites will form in any solution with dissolved ions, which is bad enough when it’s just between a battery’s electrodes, as crystal growth through a liquid electrolyte (between the positive and negative electrodes) will short them and destroy the battery. But solid electrolytes rely on the free flow of electrons through the electrolyte’s crystal matrix. Displacing the electrolyte to deposit it in crystals elsewhere disrupts that electrical flow, and so does coronal discharge: the strange behavior of electrons around corners and sharp edges, such as those of dendrite crystals.
