Li-ion batteries represent the most dynamic and fastest growing battery market with total sales of $10 billion in 2015¹.
Historically, the primary challenge for battery manufacturers has been the continuous demand for greater energy density, both volumetric and gravimetric.
The Li-ion cell market started in 1991 with the introduction of the Sony 18650 battery in consumer electronic applications. Since then, Li-ion battery energy density has grown by 3-4% every year, resulting in an approx. 2.5x overall increase in energy density over the past 25 years. A substantial amount of this improvement has been achieved by optimizing components of the battery other than the active material: the thicknesses of the aluminum foil, copper foil and separator have each reduced from about 50 microns to 7-10 microns over time, the cell packaging material thickness has decreased, and the mechanical design has been optimized.
Among other improvements, the amount of inactive material in the battery electrodes has steadily been reduced through the development of better binders and conductive additives. Over time, high-performance carbon black and multi wall carbon nanotubes (MWCNT) have replaced regular carbon black and graphite. Some battery electrode formulations now contain as much as 96-98% of active material. Even minor improvements in this area are very much sought after as these provide companies with a competitive edge in a highly competitive market.
Single wall carbon nanotubes (SWCNT) are an exceptional conductive additive that form conductive networks at very low concentrations and allow the amount of active material in the battery electrode to be increased up to 99.5%.
SWCNT replaces carbon black, graphene and MWCNT in battery electrodes
In comparison with traditionally used conductive additives such as carbon black, graphene and MWCNT, SWCNT enables conductivity starting from a concentration of just 0.001%. Low-rate systems, such as batteries for cell phones and laptops based on lithium cobalt oxide (LCO) chemistry, have been shown to work well with 0.02-0.06% of SWCNT. Recent trials in high-rate automotive batteries conducted by Aleees, the world’s largest manufacturer of lithium iron phosphate (LFP) powder, showed that a significant reduction in the amount of binder and a remarkable 10% increase in energy density can be achieved just by replacing the carbon black and graphite in LFP cathodes with 0.1% of SWCNT. It is worth pointing out that SWCNT provides mechanical reinforcement in a variety of materials, ranging from polyurethane glues and automotive paints to battery electrodes. This feature is of great interest particularly to manufacturers of lithium energy sources as it further increases the energy density and improves electrode flexibility and manufacturing yields.
The study by Aleees was carried out in 10 Ah pouch cells. The control recipe used 90.5% LFP, 4% Super P, 2% KS6 graphite and 3.5% PVDF. The recipe with OCSiAl’s TUBALL™ brand of SWCNT contained 98.4% LFP, 0.1% SWCNT and 1.5% PVDF, thus increasing the amount of active material by 7.9% points. In addition, the electrodes with nanotubes had better compressibility and were calendared to a density of 2.4 g/cc, which is 10% higher than the density of the control group electrodes. These improvements decreased the cathode thickness by about 18% and resulted in an increase of cell volumetric energy density by 10% on a cell level. This is a very significant improvement achieved just by changing the electrode conductive additive.
The rate capability of the SWCNT formulation was also better, clearly outperforming the control formulation at both 15C and 19C (Figures 2 and 3).
Silicone materials with SWCNT provide significantly longer cycle life
The use of nanotubes is, however, not only limited to the cathode. At an international seminar, that was devoted to the use of SWCNT in electrochemical power sources and was held in Shenzhen in June 2016, BAK, a large manufacturer of Li-ion batteries in China, shared their results with using SWCNT in silicon anodes. The cycle life of the 3.5 Ah 18650 cells with silicon anodes was improved from 350 to 500 cycles when <0.1% of nanotubes were added to the anode formulation in the form of TUBALL™ BATT H2O, a water-borne dispersion of SWCNT.
OCSiAl, the largest global producer of single wall carbon nanotubes, has shown through lab studies that a new composite material made of silicon layers deposited on SWCNT in sandwich-like structures can achieve remarkably high capacities, over 2500 mAh/g, and a long cycle life (Figures 4 and 5)².
The problem with silicon materials is that their volume changes up to several times during charge and discharge, and it leads to a fast degradation of the electrode structure and a short cycle life. It was observed that the SWCNT layers in this sandwich successfully accommodated volume changes during Li intercalation into silicon and prevented degradation of the material. SWCNT-based composites could decrease current source weights by 20% and sizes by 25%.
SWCNT-coated foil outperforms carbon-coated foil in key parameters
During the battery manufacturing process, anode and cathode materials are cast on aluminum and copper foils. These foils act as the battery’s conductive substrate, and the battery performance and cycle life depend on their properties. The thickness of currently available carbon coatings on foil is 1 micron or more, which negatively affects battery energy density.
Coatings with SWCNT provide manufacturers with aluminum and copper foil with a nanotube film of less than 50 nanometers on its surface, improving the cell impedance and adhesion.
Pilot line trials of SWCNT-coated foil at Aleees have shown that the discharge performance of an iron phosphate-based battery can be greatly improved, with the amount of power delivered at high rates increased by a factor of two. Figure 3 above shows the rate capability curves for 10 Ah LFP cells, with regular 20 µm aluminum foil, while Figure 6 below shows the cells made with the SWCNT-coated foil. As the differences in energy delivered between a regular 20 µm aluminum foil and a SWCNT-coated foil are provided in Table 1, with the performance improvement becoming very noticeable at above 5C rates.
Reduced cell impedance in the SWCNT-coated foil also causes lower cell heating. At the 15C rate, regular cell temperature reached 64°C, while the cells with nanotube-coated foils only reached 56°C.
This is particularly important in the field of lithium-ion cells for hybrid and electric vehicles. The use of copper foil coated with nanotubes can increase the charge acceptance of lithium-ion batteries at low temperatures and reduce battery degradation during cycling.
In the future, metal current collectors may be replaced by thinner conductive paper composed of SWCNT or SWCNT-metal composites. This could increase both volumetric and gravimetric energy density up to 15% like some calculations suggest.
Challenges for industrial usage of SWCNT in lithium batteries
More than 25 years of studying SWCNT has generated a large number of promising results on their superiority over other carbon-based additives. This unique material has, however, not been available in the volumes required or with low enough costs for mass adoption in battery manufacturing due to the difficulties of the industrial-scale synthesis of SWCNT. Recently, this problem was solved by the global nanomaterial manufacturer OCSiAl, which can produce 10 tonnes per year of high-quality SWCNT, with plans to increase production capacity to 60 tonnes in 2017.
The technology for introducing SWCNT into manufacturing processes is just as important as the nanotube synthesis itself. Today, more and more companies are beginning to make dispersions of SWCNT in water and NMP, the two solvents most often used in battery manufacturing, with Lanxess, Evermore and Duksan announcing the launch of SWCNT-based conductive masterbatches and dispersions that are ready for industrial applications.
¹Energy Storage, Frost & Sullivan (2009)
²O. Bobrenok et al, High Capacity Hybrid Si-SWCNT Anode Structures for Li-Ion Batteries, 228th ECS Meeting, Phoenix, AZ, October 11–16, 2015
Vice President, CTO
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