pubs.acs.org/cm Published on Web 11/12/2009 r 2009 American Chemical Society 1180 Chem. Mater. 2010, 22, 1180–1185 DOI:10.1021/cm902627w Influence of Manganese Content on the Performance of LiNi 0.9-y Mn y Co 0.1 O 2 (0.45 e y e 0.60) as a Cathode Material for Li-Ion Batteries † Jie Xiao, ‡ Natasha A. Chernova, and M. Stanley Whittingham* Department of Chemistry and Materials Science and Engineering Program, State University of New York at Binghamton, Binghamton, New York, 13902-6000. ‡ Current address: Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99354 Received August 25, 2009. Revised Manuscript Received October 21, 2009 The layered oxide cathode material LiMO 2 , where M = Ni 0.9-y Mn y Co 0.1 and 0.45 e y e 0.60, was synthesized by a coprecipitation method. X-ray diffraction analysis shows that the maximum manganese content in the stoichiometric material, i.e. with Li:M = 1, cannot exceed 50%; otherwise, a second phase is formed. Rietveld refinement reveals that increasing manganese content suppresses the disorder between the lithium and nickel ions. Magnetic measurements show that part of the Mn 4þ ions in the manganese rich compounds is reduced to Mn 3þ ; this results in a larger hysteresis loop due to the increased magnetic moment of the resulting ferrimagnetically ordered clusters. LiNi 0.4 Mn 0.5 Co 0.1 O 2 and LiNi 0.45 Mn 0.45 Co 0.1 O 2 show similar electrochemical capacities of around 180 mAh/g (between 2.5 and 4.6 V at 0.5 mA/cm 2 ) for the first discharge. However, subsequent cycling of LiNi 0.4 Mn 0.5 Co 0.1 O 2 results in faster capacity loss and poorer rate capability indicating that manganese rich compounds, with Li:M = 1:1, are probably not suitable candidates for lithium batteries. 1. Introduction Layered materials have attracted extensive attention since the original work on TiS 2 1,2 and LiCoO 2 . 3-5 The majority of commercialized cathode materials for lithium ion bat- teries are based on LiCoO 2 . However, the scarcity and high price of cobalt limit its use for large scale systems such as those required for electric vehicles or for utility load-level- ing. Although there has been much effort on the related LiMnO 2 because of its lower cost and the environmentally- benign properties of manganese, LiMnO 2 with the R-Na- FeO 2 structure is metastable. Thus, it cannot be formed using traditional high temperature methods but must be formed at lower temperatures, for example, by ion exchange from R-NaMnO 2 6,7 or hydrothermally. 8 Even then, it trans- forms to the spinel structure upon Li cycling. 6-10 This conversion does not require oxygen ion rearrangement as both structures are cubic close-packed (ccp). Two ap- proaches to stabilize LiMnO 2 have been proposed. In the geometrical approach, a non-ccp, for example tunnel, structure is considered or “pillars” as in (VO) y MnO 2 are placed between the layers, which prevent conversion to the spinel phase. 11 In the electronic stabilization approach, Mn is partially substituted by other transition metals such as the more electronegative Ni. When there are equal amounts of Ni and Mn, the Ni is found in the 2þ state and the Mn in the 4þ state eliminating any Jahn-Teller Mn 3þ which can cause structural instability. Thus, the most successful elec- tronically stabilized layered oxides contain equal amounts of Ni and Mn. 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