The mixed transition metal layered compound, LiNi 0.4 Mn 0.4 Co 0.2 O 2 , with the a-NaFeO 2 layer structure has been synthesized and characterized. The optimum temperature of synthesis was found to be 800-900 uC. Rietveld refinement showed that cobalt suppresses transition metal ion migration into the Li sites whereas nickel promotes the migration. XPS analysis shows that the Co and about 20% of the Ni and Mn are in the 31 oxidation state, while 80% of the Ni and Mn are in the 21 and 41 oxidation states, respectively. LiNi 0.4 Mn 0.4 Co 0.2 O 2 shows Curie-Weiss paramagnetic behavior above 150 K, and the value of the Curie constant is consistent with the above oxidation states. In lithium electrochemical cells the composition LiNi 0.4 Mn 0.4 Co 0.2 O 2 gave the highest reversible capacity among the studied compositions. It shows excellent rate capability, giving reversible capacities ranging from 180 to 155 mA h g 21 at current densities from 0.1 to 2.0 mA cm 22 . 2 1 4 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 1 4 -2 2 0 T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4 View Online 2 1 8 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 1 4 -2 2 0

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The magnetic properties of layered LiNi y Mn y Co1-2 y O2 (y = 0.5, 0.45, 0.4, and 1/3) compounds are studied in order to understand the transition metal ion distributions via their magnetic interactions. In LiNi0.5Mn0.5O2, an increase of magnetization is found below 100 K with ac magnetic susceptibility revealing broad peaks at 96, 40, 13, and 7 K. The low-temperature neutron diffraction and heat capacity studies do not reveal long-range magnetic ordering; the magnetic component of heat capacity shows a broad peak at 10 K. This behavior is explained by assuming a nonrandom distribution of transition metals. The 96 K transition is attributed to the ordering of clusters of Ni2+ spins in the transition metal and lithium layers, which are coupled by a 180° superexchange mechanism. The wide 40 K peak is explained by an increase of the cluster size due to intralayer Ni and Mn spin ordering, by analogy with antiferromagnetic ordering transitions in Li2MnO3 at 36.5 K and in NaNi0.5Mn0.5O2 at 55 K. The continuing increase of net magnetization in this temperature range indicates at least partial ferromagnetic interlayer ordering in LiNi0.5Mn0.5O2 as opposed to Li2MnO3 and NaNi0.5Mn0.5O2, which is caused by Ni2+ ions in the lithium layer. The 7−13 K anomalies are ascribed to the freezing of cluster magnetic moments. With increasing Co content, the amount of Ni2+ in the transition metal layer decreases, the cluster ordering transitions disappear, and only the spin-glass freezing is observed in LiNi0.4Mn0.4Co0.2O2 and LiNi1/3Mn1/3Co1/3O2 at 10 and 7 K, respectively. This is consistent with the lack of long-range ordering of the transition metal ions in these compounds. The evolution of the magnetic properties upon electrochemical cycling of LiNi0.5Mn0.5O2 is studied. Oxidation of Ni2+ (S = 2) to Ni3+ (S = 1/2) to Ni4+ (S = 0) is observed upon lithium removal as well as breakage of the partial magnetic ordering when 0.3 Li is removed. The latter is explained by the preferential oxidation of the Ni ions in the transition metal layers involved in the 180° magnetic exchange.

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The use of lithium titanate (Li4Ti5O12, LTO) for the negative electrode in lithium ion batteries has attracted enormous attention owing to its fast charging capability, high power, safe operating voltage window and stable structure (“zero strain”) during cycling. Researchers have investigated the formation of the solid electrolyte interface (SEI) of the LTO electrode, which prevents gassing issue and leads to longer cycle life. In this study, the solid-state diffusion property of LTO at room temperature was characterized using AC impedance spectroscopy at different states of charge (SOC) during charge and discharge to reveal the dependency of the lithium diffusion coefficient on SOC. Meanwhile the formation and growth of the solid electrolyte interface (SEI) on the LTO electrode using an electrolyte containing Silatronix OS3® additive were investigated using X-ray photoelectron spectroscopy (XPS). The composition of the SEI and its evolution due to cycling with the OS3® additive was compared to that with a commercial electrolyte. Half-cell coin cells of LTO vs lithium metal were formed and cycled at room temperature for over 200 cycles, where the resistance increase, as measured by impedance spectroscopy, is correlated to the SEI growth. Electrode samples were analyzed in the pristine state, after formation, and after 200 cycles. XPS results showed that a thin layer of SEI is formed during the first two formation cycles and the composition of the SEI on the surface of the LTO electrode varied with increasing cycle number. Based on the escape depths of Ti 3 s and Ti 2p regions, the SEI after formation is thicker than 5.5 nm but is less than 7.0 nm for both the OS3® and A7 electrolytes. Based on Ar-ion depth profiling, the SEI thickness in terms of the equivalent thickness of SiO2 after 200 cycles in coin cell configuration is estimated to be near 14 nm for both the OS3® and A7 electrolytes. A much higher fluorine content (F 1s peak) was found in the SEI formed with the OS3® electrolyte than the SEI formed with the commercial A7 electrolyte.

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