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Considerations for Producing Nano-sized Materials for Anode and Cathode Coatings in Lithium Ion Batteries

Lithium ion batteries are the power source for most of today’s electronics, but the race is on to enhance these batteries enough so they can be the future of electric vehicles. Researchers and manufacturers are working every day to alter the anode and cathode coatings in order to improve conductivity and increase capacity. The quality of the coating starts early in the grinding and dispersing process. Nano World News spoke with Stephen Miranda of NETZSCH Premier Technologies LLC about what special considerations manufacturers must take when producing nano-sized particles and dispersions for anode and cathode coatings.

Considerations for Producing Nano-sized Materials for Anode and Cathode Coatings in Lithium Ion Batteries

Lithium ion batteries are the power source for most of today’s electronics, but the race is on to enhance these batteries enough so they can be the future of electric vehicles. Researchers and manufacturers are working every day to alter the anode and cathode coatings in order to improve conductivity and increase capacity. The quality of the coating starts early in the grinding and dispersing process. Nano World News spoke with Stephen Miranda of NETZSCH Premier Technologies LLC about what special considerations manufacturers must take when producing nano-sized particles and dispersions for anode and cathode coatings.

Nano World News: Please start with a brief overview of the benefits of using nano-sized particles in the production of lithium ion (Li-ion) batteries.

Stephen Miranda: Battery manufacturers are always looking to increase overall efficiency by improving conductivity, lengthening charge life and shortening recharge time. All three are needed to make a commercially viable battery solution for advanced technology like cell phones, laptops and even vehicles. Second generation lithium batteries require a precise and repeatable manufacturing approach that relies on advances in nanotechnology. Anode and cathode coatings with nano-sized particles help manufacturers produce a better performing, more efficient battery.

NWN: The market is changing every day. What materials are people currently using as anodes and cathodes, and what challenges do they present in the manufacturing process?

SM: The most popular cathode material today is lithium cobalt oxide (LiCoO2). However, this material is expensive and offers very low charge/discharge current, around 150mAh/g, and it can be flammable over 4.3 volts. So, many researchers and manufacturers are developing substitutes that are safer and offer higher capacities — lithium manganese oxide spinel (LiMn2O4), lithium nickel oxide (LiNiO2), lithium iron phosphate (LiFePO4), lithium nickel manganese oxide (LiNiMnO2), lithium nickel manganese cobalt oxide (LiNiMnCoO2), and others.

Graphite is the most common anode material with a charge/discharge capacity of 372 mAh/g. This capacity is really unrivaled by any other material at this time, but there are some other materials that are in development to replace graphite. For example, tin-silicone has an expected charge/discharge capacity of more than 500 mAh/g, which will result in longer battery life. The main challenge with graphite is that it is very difficult to grind down to the nano-range.

The materials selected as anodes and cathodes will affect a battery’s voltage, capacity and battery life. So will their quality, which is where nanotechnology comes into play.

NWN: If graphite is so difficult to grind, what type of mills are ideal?

Wet-grinding graphite is quite difficult, and people don’t often realize that you can also have great success, and much less contamination, with dry-grinding mills, specifically fluidized bed jet mills designed for use with superheated steam. These jet mills can be used for both anode and cathode materials and are especially effective for graphite. With the steam powered fluidized bed jet mills, graphite can be ground to less than 0.5 micron. Dry grinding with this type of jet mill can actually result in zero metal contamination and offer lower energy use than wet media mills.

NWN: Is contamination of the coating dispersion a major concern for manufacturers?

Quality, in general, is a major concern. The coating must be a homogenous, consistent dispersion and, most importantly, free from contamination in order for the lithium ions to penetrate the graphite anode effectively. The quality of the coating dispersion can result in varying rates of conductivity, battery life and recharge time.

Metal grinding equipment can slough off metal particles leading to contamination that will have to be removed later in the manufacturing process. This requires more time and resources in production. You can prevent this contamination up front by using ceramic wet-grinding mills, which are free of metal grinding shafts and chambers. Mills constructed with Yttrium-stabilized zirconium oxide and silicon carbide rotors, shafts and chambers offer minimum wear and high-efficiency grinding.

NWN: You briefly mentioned homogenous, consistent dispersions — please expand on that.

Generally, the particles must remain free from agglomerates. Micronized air bubbles within the mixture can impact conductivity of coatings and, ultimately, a Li-ion battery’s overall performance.

Mixing equipment is available that automatically de-aerates while wetting powders into a liquid solution by using macrocavitation and pressure wetting. This breaks down agglomerates and removes micronized air with a vacuum, resulting in repeatable and optimal dispersions.

Grinding media size can also affect the coating. Today, many wet-grinding mills work with grinding media as small as 90 microns. As we are able to decrease the size of the grinding media, we are able to proportionally increase the surface area of the ground particles, which leads to a more homogenous, consistent coating and even faster production times.

NWN: Is the speed of producing these coatings imperative to battery manufacturers?

SM: Of course. Time is money. But, it’s also imperative that they understand the pitfalls of increasing the speed and energy of the mill just to speed up production times. As we discussed earlier, contamination is one of the major issues with grinding dispersions, and this is directly related to the wear on the grinding equipment. So, it’s important they understand that energy input is a square function of speed; therefore, two times rotor tip speed results in an exponential increase in kinetic energy in the grinding media. Wear rate of the grinding beads and mill components is a cubic function of speed; therefore, two times the speed is equal to eight times the wear.

Contamination can be increased if you increase the mill speed and resulting grinding energy beyond the kinetic energy or stress intensity required to disperse or even reduce the primary particle size of the material. High stress intensity increases wear on the mill, raising the chance for materials to slough off into the dispersion. I highly recommend manufacturers work with their grinding partner to determine the most optimal amount of energy to maximize efficiency and reduce wear.

NWN: We learned here a bit more about the concerns of the industry. So, what’s the future of the lithium-ion battery market?

SM: Manufacturers will continue to push the envelope to find the most efficient solution. They will continue to look for safer cathode materials that have higher capacities and greater potential. For anode materials, we’ll see more developments in intermetallic materials to also increase capacity.

One thing is for sure, the market share for Li-ion batteries is only increasing. For example, we are seeing the market share for Korean companies rising to what we expect will eventually overcome the Japanese market.

Look out for further developments in what’s being called “advanced Li-ion batteries.” In the next 10 to 15 years there will also be developments that lead to new battery systems, such as lithium air and zinc air batteries that will likely be adopted for consumer products rather quickly.

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