This Small Business Technology Transfer Phase I project will seek to improve the performance of the cathode materials used in lithium-ion (Li-ion) batteries. LiCoO2 is the primary cathode material, but it is relatively expensive, and has safety concerns. A safer alternative is LiMn2O4, but its utilization is limited by a relatively higher fade capacity; i.e. decrease in capacity with repeated cycling. The project will investigate the effects of nanoporous structure of ceramic coatings on the fade capacity of LiMn2O4. Different material composition and firing temperatures of thin-film coatings of titania and zirconia nanoparticles on LiMn2O4 will be studied. Fixed rate cycle tests (1C) will be performed at a temperature of 55C for different coating formulations to determine the number of cycles before the capacity decreases to 80% of the initial value. The best performing formulation will be tested further using both high-rate cycling for at least 200 cycles and progressive rate cycling. This formulation will also be tested on LiCoO2. The goal is to increase the life of the coated cathode material by an order of magnitude as compared to the uncoated material. This project is also expected to provide additional understanding of nanoparticle coating techniques that may be applicable to variety of energy storage applications.
The broader/commercial impact of this project, if successful, will result in improvements in the cycle life of Li-ion batteries and allow the use of safer materials. Although lithium-ion batteries have gained wide acceptance in consumer electronic products, their use in other markets, particularly transportation applications, has been limited by their lifetimes and safety concerns. Improving the lifetime of the safer materials used in these batteries can enhance Li-ion batteries' penetration into transportation and other large markets, enabling access to a $7 billion end user market. The successful outcome of this project will impact applications where there is a need for enhanced cycle life and thermal stability.
Intellectual Merit The primary focus for this project is to develop coatings of nanoparticle ceramic oxides that increase the cycle life of a commercial LiMn2O4 spinel cathode material used in lithium-ion batteries operated at relatively high temperatures (~50 °C). The studies performed under this STTR Phase I project focused on identifying the material formulations and annealing temperatures for the nanoporous coatings that provide the best increase in cycle life for the LiMn2O4 spinel. As shown in Figure 1, the coatings developed in this project significantly improved the cycling stability of the lithium manganese oxide spinel cathode material. In Phase 1 we were able to improve cycle life by a factor of three. While performing this research, we explored numerous fabrication and processing variables, including evaluating the effects of the composition of the ceramic coating on the performance of the cathode material. Coatings of TiO2 that were applied by heating while stirring and then fired at 400 °C were the most successful, yielding, on average, a factor of three increase in cycle life over uncoated spinel at test temperatures of 55 °C ± 3 °C. Spinel coated with stirred-filtered TiO2 and fired at 400 °C yielded approximately a factor of two increase in cycle life, as did spinel coated with ball-milled TiO2 and fired at 400 °C. Spinel coated with ball-milled TiO2 and fired at 500 °C did not display any improvement in cycle life over uncoated spinel. Coated cathode materials perform worse than uncoated materials when cycled at a high rate of 2C. Although in general the cells containing the coated spinel displayed a higher discharge capacity than the uncoated cells after the first charge-discharge cycle, the discharge capacity of the coated spinels decreased more rapidly than for the uncoated spinels. Our conclusion is that while the proper coatings can significantly improve performance at cycling rates less than or equal to 1C, at higher rates the added impedance of the coatings reduces performance. This information helps us to better target our market applications. We now know that the coatings are not suitable for applications requiring continuous cycling at high currents. In the future, SolRayo will target applications that require more modest cycling currents and only short-duration intermittent power demands in excess of 1C. Broader Impacts of the Work In this project, we successfully improved the high-temperature cycle life of a cathode material used in Li-ion batteries. This advance allows inherently safer materials to be employed in lithium-ion batteries, although at the cost of reducing the initial capacity of these batteries by a few percent. Although lithium-ion batteries have gained wide acceptance in consumer electronic products, their use in other markets has been limited by their lifetimes and safety concerns, particularly in applications at higher temperatures. Improving the lifetime and safety of the materials used in these batteries can enhance their penetration into transportation and other large markets, leading to licensing opportunities that could possibly reach $50 million annually in a $7 billion world market. Depending on the application, cathode materials account for some 5% to 15% of the total battery cost. It appears that the cost of applying our coatings might double the price of the cathode materials. Given that LiMn2O4 is less expensive than many competing cathode materials, the net increase in total battery cost may be approximately 2% to 10%. This may somewhat limit the market applications for our materials to areas where the need for enhanced cycle life and thermal stability can justify the increased cost of the coating. This work also benefits the nation by improving our understanding of nanoparticle coating techniques suitable for a variety of energy storage applications.
