Boron in Batteries and Capacitors

POWERING ENERGY STORAGE

Global efforts to reduce emissions and the need for improved energy storage for mobile applications are promoting rigorous research efforts into new battery technologies. Commercial and experimental applications of boron-based materials to improve both anodes and electrolytes indicate substantial benefits, including for high-power applications like power tools and hybrid electric vehicles. Borates lead to improvements in a variety of battery applications, including surface treatment of graphite anodes, catalyzing the synthesis of graphite, and as electrolyte additives.

Borate compounds, including lithium bis(oxalato)borate (LiBOB), are widely used in commercial lithium-Ion batteries. Incorporating borate compounds into the solid electrolyte interphase (SEI) substantially decreases the decomposition reactions of the anode and cathode, greatly improving performance and safety. When applied to the anode or used in electrolytes, borates likely improve the surface interface, a critical layer that forms during cycling. When used to modify the surface of graphite, treatment with borates at less than 1000°C results in promising improvements in rate capability, life cycle, and capacity.

Where electrical capacitors—devices that store electrical energy—are concerned, borate chemicals are used in many formulations as well as in various parts of the manufacturing of aluminum-plate capacitors. And boron-containing electrolytes are found in supercapacitors—important energy devices that are enabling electric and hybrid electric vehicle technology.
FACT
Ionization energy of boron

SQ grade 20 Mule Team® Borax ammonium pentaborate is used in the preparation of both wet and dry electrolytic capacitors. It is a component of electrolytes developing a thin oxide film on aluminum foil when an electric current is applied, as well as for inserting into an aluminum container during the final assembly of capacitors. The purity of the borate components is essential to the production of high-quality capacitors. 

Boron has been a key element in commercial electrolytes for lithium-ion batteries, and Dr. Huang discovered a boron-containing compound that outperforms the commercial compounds as the electrolyte for the emerging sodium-ion batteries.

Batteries: Improving safety and efficiency

Lithium-ion batteries used in portable electronics, use graphite as an anode material. However, graphite anodes are susceptible to lithium deposition and dendrite formation at high charge rates. These issues can short out the battery cell and cause safety issues. Borate surface coating may protect against lithium deposition, leading to better safety characteristics and potentially improving the stability of the SEI, which forms a boundary between electrode and electrolyte. (Several studies have shown that borates are beneficial to the formation of the SEI.)

But borates can help save energy even earlier in the process. High-temperature heat treatment of the graphite used in batteries contributes to a highly ordered crystalline graphite structure. But high-temperature graphitization is an expensive and energy-intensive process that requires temperatures in the 3000°C range. Adding borate before graphitization improves electrochemical properties and lowers the temperatures required for graphitization. In addition to increasing crystallinity, boron is thought to incorporate into the lattice structure of graphite at higher temperatures, initiating greater alignment and changing the electronic structure. In the graphite lattice, boron can act as an electron acceptor, leading to a specific capacity of 437 mAh/g—higher than the theoretical maximum for pure graphite (372 mAh/g).

lithium polymer batteries
 

High-quality borates for high-quality capacitors

Electrical capacitors are constructed of two conducting surfaces, separated by an insulating or dielectric medium. In wet electrolytic capacitors, only one conducting surface is a metallic plate; the other is a chemical compound.

The dielectric medium is a very thin film of oxide of the metal in the metallic plate—typically a thin, chemically roughened or etched aluminum foil). This foil must be thoroughly cleaned before the oxide film is formed. Borax and boric acid are excellent cleaning agents for this purpose. The active dielectric oxide film is formed by immersing the foil in an aqueous electrolyte solution consisting of boric acid and either ammonium or sodium borate, and then applying an electrical voltage.

It is essential that the electrolytes be kept free of impurities such as chlorides, nitrates, sulfate, and iron. After formation of the film, the anode foil is rinsed clean often with a boric acid solution. The capacitor is assembled by inserting the anodes into their containers (usually aluminum cans in the case of the wet types) and adding the working electrolyte. The electrolyte is usually an aqueous solution of boric acid and ammonium borate.

In dry capacitors, the electrolyte is non‑aqueous and of relatively low conductivity. The etching, cleaning, and film-formation steps are generally the same as in wet capacitors. The electrolytes used range from viscous liquids to nearly solid masses. Some more commonly used mixtures include glycol‑ ammonium borates, ammonium acetates‑borates, and amine borates. Water‑soluble organic acids, alone or with associated salts such as ammonium borates, are frequently employed. The use of ammonium salts appears to be particularly advantageous for high-voltage applications.

In every case, purity of ingredients is essential to the production of high-quality capacitors. According to a technical report produced for Wright Air Development Center, “the ultimate useful life of an electrolytic capacitor depends on the complete elimination of the slightest trace of contaminants and the maintenance of the electrolyte in proper chemical composition,” To provide the highest standards of purity, U.S. Borax produces Special Quality (SQ) grades of 20 Mule Team Borax borax decahyrdrate, Optibor®, 20 Mule Team boric acid, and ammonium pentaborate.

Graphite anodes
 
“Boron can form many boron-hydrogen rich compounds that have very high hydrogen content,” Dr. Zhenguo Huang said. These types of compounds have been intensively studied for hydrogen storage, which remains as a key hurdle to the wide deployment of hydrogen fuel cell powered vehicles.

Borates, supercapacitors, and electric vehicles

Supercapacitors (also known as ultracapacitors, electric double-layer capacitors, power capacitors, and electrochemical capacitors) are electrochemical energy storage devices. These devices offer the charge and discharge rates of conventional capacitors at an energy density near that of batteries.

When a DC voltage is applied, supercapacitors electrostatically store the charge at the interface between a polarizable electrode and an electrolyte solution. The charge separation that results form the reversible adsorption of electrolyte ions into the electrode creates energy storage. Borates are used in the electrolyte solution.

Supercapacitors offer charge and discharge times of 30 seconds or less, compared with hours for a conventional lithium-ion battery. They also have a much higher cycle life and power density. They cannot be overcharged, contain no heavy metals, and have higher energy and power densities than conventional capacitors do. Though these devices have existed for decades, they are increasing in popularity as a component of portable consumer electronic goods, backup power supplies, and electric or hybrid vehicles.

Hybrid car
 

20 Mule Team Borax Products

These 20 Mule Team Borax products are developed for use in the manufacture of batteries and capacitors.

Ammonium Pentaborate

Whether inhibiting heat, flame, or corrosion, the purity of this alkaline salt makes it an excellent choice. Learn More

Borax Decahydrate

From detergents to dyes to adhesives, this mild alkaline salt does it all, particularly excelling as a buffering and fluxing agent. Learn More

Optibor

From reducing melting temperatures in fiberglass production to inhibiting corrosion in fuel additives, Optibor has a multitude of uses in numerous industries. Learn More

U.S. Borax, part of Rio Tinto, is a global leader in the supply and science of borates—naturally-occurring minerals containing boron and other elements. We are 1,000 people serving 500 customers with more than 1,700 delivery locations globally. We supply 30% of the world’s need for refined borates from our world-class mine in Boron, California, about 100 miles east of Los Angeles.  Learn more about Rio Tinto

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