The initial charge/discharge behavior of LiNi 0.5 Mn 0.5 O 2 for a Liion battery has been examined by XRD, in situ XAFS and electrochemical methods. The LiNi 0.5 Mn 0.5 O 2 ( R 3m : a0.28999(7) nm, c1.4330(7) nm in a hexagonal setting) was observed to retain its initial structure after charging to 4.5 V (vs. Li/Li { ). XAFS measurements have also revealed that the majority of the nickel and manganese, respectively, exists in a divalent and a tetravalent state. Upon charging LiNi 0.5 Mn 0.5 O 2 , Ni 2{ ions were consequently oxidized to Ni 4{ . A Liion cell constructed with LiNi 0.5 Mn 0.5 O 2 /graphite shows only a 15÷ capacity loss after 500 cycles at 60 C. [Received July 16, 2002; Accepted November 6, 2002] Keywords : In situ XAFS, Lithium nickel manganese oxide, Layered structure, Lithium ion battery, Cathode
IntroductionLayerstructured LiNiO 2 (space group R 3m) has been considered to be a very promising cathode material for 4V class lithium ion batteries because of its lower cost and higher capacity compared with LiCoO 2 . 1),2) However, when LiNiO 2 is in its charged state, its crystal structure shows peculiar behavior. For example, Ohzuku has indicated that LiNiO 2 (space group R 3m) was oxidized to NiO 2 (space grope R 3m) via Li 1|x NiO 2 (0.25x0.55) which has a monoclinic structure (C2/m). 3) Therefore, this crystal structural change leads to capacity fading of a lithium ion battery by repetition of expansion and contraction.To overcome this problem, many studies on LiNi 1|x Mn x O 2 have been carried out. For example, Nitta 4) has tried a mixed valence control by substituting a part of the Ni layer (3a site) with manganese, in order to stabilize Ni in a high order oxidation state and retain the basic structure of LiNiO 2 . They tried to substitute Mn(II) for a part of the Ni using MnO or MnCO 3 and showed that the majority of the manganese in LiNi 1|x Mn x O 2 existed in a divalent state, which is the same valence as that of the starting materials, MnO or MnCO 3 . To improve cycle performance, Yoshio 5) has synthesized LiNi 1|x Mn x O 2 with high crystallinity, using a trivalent manganese compound, gMnOOH, as a man ganese source, and showed that the LiNi 1|x Mn x O 2 obtained had excellent cycle performance.Ohzuku and Makimura 6) have obtained LiNi 0.5 Mn 0.5 O 2 , which is solid solution between LiNiO 2 and LiMnO 2 , by heating a mixture of LiOH and nickel manganese double hydroxide at 1000 C in air and revealed that the reversible capacity of the LiNi 0.5 Mn 0.5 O 2 was 190 mAh/g between 5.0 and 3.0 V (vs. Li/Li { ). In LiNi 0.5 Mn 0.5 O 2 it has been speculated 7),8) that the Ni and Mn ions, respectively, have valance {2 and {4, though earlier work by Sphar 9) as sumed both Ni and Mn to have valence {3 in the starting material. Because the capacity of LiNi 0.5 Mn 0.5 O 2 is 200 mAh/g, the assumption of Ni 2{ and Mn 4{ in the starting material requires that the nickel ion cycles between Ni 2{ and Li 4{ . Our study in this paper was focused on the behav ior of the LiNi 0.5 Mn 0.5 O 2 during charge...