GU ET AL . VOL. 7 ’ NO. 1 ’ 760 –767 ’ 2013 www.acsnano.org 760 December 13, 2012 C 2012 American Chemical Society Formation of the Spinel Phase in the Layered Composite Cathode Used in Li-Ion Batteries Meng Gu, † Ilias Belharouak, ‡ Jianming Zheng, # Huiming Wu, ‡ Jie Xiao, # Arda Genc, § Khalil Amine, ‡ Suntharampillai Thevuthasan, † Donald R. Baer, † Ji-Guang Zhang, # Nigel D. Browning, ^ Jun Liu, ^ and Chongmin Wang †, * † Environmental Molecular Science Laboratory, ^ Fundamental and Computational Science Directorate, and # Energy and Environmental Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States, ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States, and § FEI Company, 5350 NE Dawson Creek Drive, Hillsboro, Oregon 97124, United States L i-ion batteries have been widely used as an energy storage device for mod- ern electric devices, grid application, and renewable energy. 1À7 Cathodes with a layered structure such as Li 1.2 Ni 0.2 Mn 0.6 O 2 (LNMO) and Li 1.2 Ni 0.1 Mn 0.525 Co 0.175 O 2 (LNMCO) can provide much higher capacity than the traditional cathode materials such as LiCoO 2 and LiMn 2 O 4 spinel. 8À12 Therefore, these layered structures are one of the most pro- mising candidates for future heavy duty applications such as hybrid and electric vehicles. However, the application of these materials faces three fundamental chal- lenges: (1) voltage instability, (2) capacity fad- ing, and (3) slow charge/discharge rate. Collective experimental observations indicate these challenges are closely related to the structural characteristics of these materials, such as the crystal structure, spatial distribution of cations, and phase stability upon lithium extraction and insertion. Structurally, these layered structures are often composed of the intergrowth of LiMO 2 R 3m and Li 2 MO 3 C2/m phases. 7,13 Nevertheless, it is not clear how each individual phase affects the per- formance of these materials. Further, the cations are not necessarily uniformly distrib- uted at nanometer scale. Recently, Gu et al. 7 reported a nanoscale phase separation caused by the preferential segregation of Ni atoms on the particle surface and bound- aries in the layered lithium nickel manga- nese oxide cathode materials. They further predicted that the formation of the Ni-rich surface layer would affect the diffusion of Li ions, and thus possibly have an impact on the rate performance of this cathode. 7 Accompanying the extraction of Li from the Li 2 MO 3 phase at the 4.5 V voltage plateau is * Address correspondence to Chongmin.Wang@pnnl.gov. Received for review October 31, 2012 and accepted December 13, 2012. Published online 10.1021/nn305065u ABSTRACT Pristine Li-rich layered cathodes, such as Li 1.2 Ni 0.2 Mn 0.6 O 2 and Li 1.2 Ni 0.1 Mn 0.525 - Co 0.175 O 2 , were identified to exist in two different structures: LiMO 2 R 3m and Li 2 MO 3 C2/m phases. Upon 300 cycles of charge/discharge, both phases gradually transform to the spinel structure. The transition from LiMO 2 R 3m to spinel is accomplished through the migration of transition metal ions to the Li site without breaking down the lattice, leading to the formation of mosaic structured spinel grains within the parent particle. In contrast, transition from Li 2 MO 3 C2/m to spinel involves removal of Li þ and O 2‑ , which produces large lattice strain and leads to the breakdown of the parent lattice. The newly formed spinel grains show random orientation within the same particle. Cracks and pores were also noticed within some layered nanoparticles after cycling, which is believed to be the consequence of the lattice breakdown and vacancy condensation upon removal of lithium ions. The AlF 3 -coating can partially relieve the spinel formation in the layered structure during cycling, resulting in a slower capacity decay. However, the AlF 3 -coating on the layered structure cannot ultimately stop the spinel formation. The observation of structure transition characteristics discussed in this paper provides direct explanation for the observed gradual capacity loss and poor rate performance of the layered composite. It also provides clues about how to improve the materials structure in order to improve electrochemical performance. KEYWORDS: lithium ion battery . layered structure . spinel formation . phase transformation ARTICLE