Structural transformation in a Li1.2Co0.1Mn0.55Ni0.15O2 lithium-ion battery cathode during high-voltage hold (original) (raw)

A decrease in the c-lattice parameter was observed in Li 1.2 Co 0.1 Mn 0.55 Ni 0.15 O 2 during constant voltage holding at 4.5 V by in situ X-ray diffraction. Comparison of magnetic susceptibility data before and after high-voltage hold reveals the change in average oxidation states of transition metal ions during highvoltage holding process. Transmission electron microscopy studies show the spinel reflections with fundamental trigonal spots from the particles after high-voltage hold indicating substantial structural modification. The structural transformation was believed to occur due to the oxygen release and/or the migration of transition metal cations to lithium layer during constant voltage holding. Introduction Rechargeable lithium-ion battery (LIB) technology is the leading candidate to power electric vehicles (EV) because of its high stored energy density, light weight, low maintenance, long service-life, and high efficiency as compared to internal combustion (IC) engines. 1-4 However, in order to realize the complete electrification in transportation systems, the performance of LIBs needs to be improved while maintaining maximum life and safety. 1 Recently, Li-rich materials, Li 1+y M 12y O 2 (M = Co, Mn, Ni) (Li-rich NMC hereafter) reveals promising high capacity when operated at high voltage. 5-7 Structurally integrated Li 2 MnO 3-stabilized 'layered-layered' xLi 2 MnO 3 ?(1 2 x)LiMO 2 (M = Mn, Ni, Co), (for example, 0.5Li 2 MnO 3 ?0.5LiNi 0.27 Mn 0.27 Co 0.27 O 2 or Li 1.2 Co 0.1 Mn 0.55 Ni 0.15 O 2 investigated in the present study) has shown improved electrochemical performance compared to stoichiometric LiMO 2 (M = Co, Mn, Ni) cathodes. 8-11 Most importantly, these structures deliver promising high capacity between 200-250 mAhg 21 within the voltage window of 2.0-4.8 V (vs. Li/Li +) which makes them excellent candidates for LIBs in EV applications. 6,8 The crystal structure of Li-rich NMC compounds is derived from the layered LiMO 2 with a-NaFeO 2 structure with trigonal symmetry (rhombohedral or hexagonal unit cell, R3m space group) and the excess lithium ion present in the structure occupies the transition-metal layer, filling all the octahedral sites of cubicclose-packed (ccp) oxygen arrays. 6 The presence of lithium ions in the transition-metal (TM) layer generates cation-ordering between TM ions, may lead to the formation of Li 2 MnO 3 phase (monoclinic unit cell, C2/m space group). 3 The presence of cation-ordering can be detected from X-ray diffraction (XRD) pattern, where the superlattice peaks appear at lower 2h angles (22u-26u with Cu-Ka radiation). However, LiMO 2 and Li 2 MnO 3 structures have similar ccp layers with interlayer spacing of y4.7 Å for (001) of layered monoclinic and (003) of layered trigonal (Fig. 1) phases, which allows perfect integration of these