Lithium Hydroxide Based Performance Improvements for Nickel Rich Ncm Layered Cathode Material

Abstract

The increased adoption of electric vehicle technologies is driving the demand for higher energy density batteries. Since the lithium ion battery was first commercialized 25 years ago in 1991, significant improvements have been achieved through both battery design and materials innovations. However, even as these past achievements are celebrated, current research and industry targets are focused on higher energy and lower cost batteries. It is necessary to consider advanced materials on both the cathode and anode sides of the battery to achieve these goals. Commercially available electric vehicles have achieved driving ranges in excess of 200 miles (320 km) per charge (1). This has been made possible in part through the use of high nickel cathode materials. Nickel-rich layered cathode materials such as LiNi0.8Co0.15Al0.05O2 (NCA) and nickel-rich LiNi1-2xCoxMnxO2 (NCM) can deliver high specific capacities of over 200 mAh g-1. However, the challenges surrounding synthesis of these materials are well documented (2), (3), (4). It is established that synthesis of NCA requires use of lithium hydroxide as the lithium precursor. Additional research efforts have been focused on nickel-rich NCM materials (Ni ≥ 0.6 moles) in an effort to balance high specific capacity with battery safety. The choice of lithium precursor, LiOH.H2O or Li2CO3, can have a dramatic effect on the performance of these materials. Here we report the advantages of using lithium hydroxide on the performance of nickel-rich cathode materials. Superior physical properties, such as higher tap and packing density, can be achieved at lower synthesis temperatures for nickel-rich NCM materials when using lithium hydroxide as the lithium precursor. We will show that using lithium hydroxide allows for improved material crystallinity as observed by x-ray diffraction and that Rietveld refinement shows greater structural purity and less mixing of Li+ and Ni2+ in the lithium layer for lithium hydroxide versus lithium carbonate. X-ray diffraction results have shown that lithium is incorporated in the structure of the NCM when using lithium hydroxide, while use of lithium carbonate results in excess free lithium (unreacted lithium that could be present as Li2O, LiOH or Li2CO3 depending on handling of the cathode material post synthesis) leading to an increase in material pH that can cause jelling of the cathode slurry and swelling of the cell upon cycling. Furthermore, as x decreases in LiNi1-2xCoxMnxO2 the lithium hydroxide advantage becomes more apparent. We will also show electrochemical performance improvements at optimized synthesis conditions. References Straubel, J. B. "Driving Range for the Model S Family." Tesla Motors, 30 Dec. 2014. www.tesla.com. B. H. Kim, et al., The effect of oxygen pressure on the synthesis of LiNiO2. Solid State Phenomena, Vols. 124-126, (2007), p. 1043-1046 P. Kalyani, N. Kalaiselvi, Various aspects of LiNiO2 chemistry: A Review. Science and Technology of Advanced Materials, 6, (2005), p. 689–703 Kim, M.-H., Shin, H.-S., Shin, D. & Sun, Y.-K., Synthesis and electrochemical properties of Li[Ni0.8Co0.1Mn0.1]O2 and Li[Ni0.8Co0.2]O2 via co-precipitation. Journal of Power Sources, (2006), 159, p. 1328-1333 Figure 1