solid-state battery

solid-state battery, device that converts chemical energy into electrical energy by using a solid electrolyte to move lithium ions from one electrode to the other. Solid electrolytes are materials, typically composite compounds, that consist of a solid matrix with relatively high ionic conductivity. Solid-state batteries differ from lithium-ion batteries, which are the most common type of rechargeable battery and use liquid or gel electrolytes. Relative to lithium-ion batteries, solid-state batteries have various advantages, including greater durability, a higher energy capacity, a faster charging rate, a longer life span, and a greater variation in shape.

Overview

Both solid-state and lithium-ion batteries are composed of a cathode—i.e., a positive pole, which is made of a cathodic material (e.g., lithium iron phosphate [LiFePO₄])—and an anode—i.e., a negative pole, which is made of an anodic material (e.g., carbon). The poles are separated by an electrolyte, a medium through which ions move.

When electrons move from the cathode to the anode, the chemical potential energy of the battery increases, giving the battery charge. When electrons move in the other direction, the chemical potential energy is converted into electricity, which is discharged from the battery. While the battery is charging or discharging, the oppositely charged ions move through the electrolyte inside the battery to balance the charge of the electrons, which makes the battery rechargeable.

There are two major differences between solid-state and lithium-ion batteries. First, solid-state batteries use a solid (rather than liquid or gel) electrolyte, such as lithium phosphorus oxynitride (LiPON). Second, because lithium-ion batteries use liquid or gel electrolytes, they require a separator between the cathode and the anode to prevent the electrolyte on one side of the battery from mixing with the electrolyte on the other. In solid-state batteries, the electrolyte itself separates the two poles.

Solid-state batteries have certain advantages over lithium-ion batteries. Inorganic solid electrolytes are unlikely to catch fire. Solid-state batteries are therefore safer to use in high-temperature environments compared with lithium-ion batteries. They are also more resistant to cold temperatures. In liquid and gel electrolytes, lower temperatures cause the ions to move more slowly, which causes battery performance to decrease, an effect that is reduced in solid electrolytes.

Increased density allows solid-state batteries to store more energy. A solid-state battery can power a device for a longer period of time than a lithium-ion battery of the same size. Alternatively, a smaller, lighter solid-state battery can power a device for the same amount of time as a larger lithium-ion battery. Another useful aspect of solid-state batteries is their ability to be cast in a variety of shapes. Typical lithium-ion batteries must be formed in such a way that prevents liquid leakage, whereas solid-state batteries, in which leakage is not a concern, can be made smaller or thinner and can even be curved.

Further, inorganic solid electrolytes deteriorate more slowly than their lithium-ion counterparts. Lithium-ion batteries have a limited number of charge cycles before becoming unusable. On the other hand, solid-state batteries can be recharged for many more charge cycles and maintain their capacity for far longer than their lithium-ion counterparts. In addition to providing better service, solid-state batteries exert less of a strain on the environment than lithium-ion batteries. A solid-state battery stores more energy with less material and has a longer life span than a lithium-ion battery, both of which help reduce its carbon footprint. While the manufacture of solid-state batteries requires more lithium, less graphite and cobalt are required in the process. Extracting graphite and cobalt from the earth is connected to environmental issues, particularly deforestation, water contamination, and air pollution.

History

English scientist Michael Faraday, who coined the term electrolyte, discovered two solid electrolytes, silver sulfide and lead fluoride (Ag₂S and PbF₂), between 1831 and 1834. His breakthroughs laid the groundwork for the study of solid-state ionics (the passing of ions through solid electrolytes).

Scientific research in the field of solid-state ionics increased in the following century, as more solid electrolyte materials were discovered and made available for experimentation. In 1916 another solid electrolyte, beta-alumina, was discovered. However, it did not gain widespread popularity among scientists, chemists, and manufacturers until the 1970s. In the 1950s a number of electrochemical systems that used solid electrolytes were developed, though none of them were in the form of batteries. Building on beta-alumina research, in 1966 Ford Motor Company developed the sodium sulfur (NaS) battery. While it was not a literal solid-state battery (because the electrode materials were in a molten state), the NaS battery was an important step toward the invention of the solid-state battery, presenting advantages such as low cost and high energy density. The discovery of solid-polymer electrolytes based on lithium salt-poly(ethylene oxide) complexes in the 1970s likewise contributed heavily to the development of true solid-state batteries.

In 1983 scientists at Oak Ridge National Laboratory in Tennessee discovered lithium phosphorus oxynitride, which led to the development of the thin-film solid-state battery, a solid-state battery with a thin-film electrolyte stacked on the cathode and anode in a vacuum. In the following decades, solid-ion conductors with even greater conductivity were discovered, leading to the development of a solid-state battery capable of technologically competing with a lithium-ion battery by 2011, when scientists discovered the lithium superionic conductor LGPS (Li₁₀GeP₂S₁₂). Since 2011 researchers at universities and for-profit companies have rushed to produce more powerful and economical solid-state batteries, which have found their way into many devices, including phones, computers, and cars.