Tsinghua, MIT, Argonne team discovers lithium titanate hydrates for superfast, stable cycling in Li-ion batteries
An international research team from Tsinghua University, MIT and Argonne National Laboratory has discovered a series of novel lithium titanate hydrates that show better electrochemical performances compared to all the Li2O–TiO2 materials reported so far—including those after nanostructuring, doping and/or coating.
Reported in an open access paper in the journal Nature Communications, the novel lithium titanate hydrates show a specific capacity of about 130 mA h g−1 at ~35 C (fully charged within ~100 s) and sustain more than 10,000 cycles with capacity fade of only 0.001% per cycle.
Compounds on the binary Li2-TiO2 composition line, such as Li4Ti5O12 (2Li2O•5TiO2, LTO) and various TiO2 polymorphs (TO), are generally considered the most promising anode materials for Li-ion batteries in terms of rate capability and cycling stability, as well as the improved safety over graphite anode. Producing nanostructured materials on the Li2O–TiO2 composition line often requires water-based synthesis such as hydrothermal or sol-gel approaches, and thus one often deals with reaction intermediates that contain water (lithium titanate hydrates, LTHs) in the Li2O–TiO2–H2O ternary composition space.
Because water is considered “harmful” in high-voltage window aprotic electrolytes (free water can be highly reactive to LiPF6, lithium metal anode and lithium alkyl carbonates), most researchers calcine the nanostructured LTHs to completely remove all water by raising temperature to above 500 °C. However, this can cause an unwanted side effect of coarsening and aggregation of the structure.
Herein, we demonstrate that the high-temperature calcining may not be necessary. One may only need to remove the more loosely bound water (such as adsorbed and crystallographic water) by heating to a much lower temperature of <260 °C, which does not induce significant coarsening of the nanostructure. The deeply trapped water inside LTHs, or pseudohydrates (i.e., hydroxide or hydroxonium ions or as –OH and –H groups), does not necessarily degrade stability or performance in aprotic electrolytes, even with H2O:TiO2 molar ratio as high as 0.41. Indeed, the trapped water can promote structural diversity and nanostructuring that could be highly beneficial for battery performance in aprotic electrolytes.—Wang et al.
The team called its approach to the discovery of these materials optimized dehydration induced nanostructuring (ODIN).
When the scientists tested the materials in the laboratory, cycling stability improved and capacity degraded only slightly over 10,000 cycles. The material also charged very quickly—within less than two minutes—the team found. As noted by Jun Lu, Argonne battery scientist and co-author, “Most of the time, water is bad for non-aqueous lithium-ion batteries. But in this case, it can be downright good.”
The research team tracked how material composition and structure changed when heated by using various advanced characterization techniques, including x-ray diffraction provided by the Advanced Photon Source, a DOE Office of Science User Facility located at Argonne. When analyzing the combined characterization data, the team reported that the trapped water in the anode material improved performance by promoting structural diversity and forming nanostructures.
|The video shows the change in composition and structure as the starting material is heated and water is expelled to form a new layered structure (LS), then the desired hydrated nanostructure (HN), then beyond the desired structure all the way to a completely dehydrated nanostructure (DN).|
Looking to the future, Jun Lu observed that, because water is everywhere in nature and common in chemical synthesis, the fabrication approach reported in this research could open the door to discovery of other high-performance electrode materials.
The research was funded by DOE’s Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office, the National Science Foundation, the National Natural Science Foundation of China and the Ministry of Education of the People's Republic of China. The scientists used resources of the Advanced Photon Source, a DOE Office of Science User Facility.
Shitong Wang, Wei Quan, Zhi Zhu, Yong Yang, Qi Liu, Yang Ren, Xiaoyi Zhang, Rui Xu, Ye Hong, Zhongtai Zhang, Khalil Amine, Zilong Tang, Jun Lu & Ju Li (2017) “Lithium titanate hydrates with superfast and stable cycling in lithium ion batteries” Nature Communications 8, Article number: 627 doi: 10.1038/s41467-017-00574-9