Synthesis and Electrochemistry of LiNiMn2_O4

LiNiMn2_104 has been synthesized using sol-gel and solid-state methods for 0 <x c 0.5. The electrochemical behav-ior of the samples was studied in Li/LiNiMn5_O4 coin-type cells. When x =0, the capacity of Li/LiMn2O4 cells appears at 4.1 V. As x increases, the capacity of the 4.1 V plateau decreases as 1-2x Li per formula unit, and a new plateau at4.7 V appears. The capacity of the 4.7 V plateau increases as 2x Li per formula unit, so that the total capacity of the samples(both the 4.1 and 4.7 V plateaus) is constant. This is taken as evidence that the oxidation state of Ni in these samples is +2, and therefore they can be written as The 4.1 V plateau is related to the oxidation of Mn " to Mn " and the 4.7 V plateau to the oxidation of Ni " to Ni". The effect of synthesis temperature, atmosphere, and cooling rate on the structure and electrochemical properties of LiNi55Mn15O4 is also studied on samples made by the sol-gel method. LiNi05Mn,504 samples made by heating gels at temperatures below 600°C in air are generally oxygen deficient, leading to Mn oxidation states significantly less than 4. LiNi55Mn,504 samples heated above 650°C suffer due to dispro-portionation into LiNiMn2_O4 with x < 0.5 and Li2Ni1_0 with z 0.2, which occurs above about 650°C. Pure LiNi55Mn, 504 materials can be made by extended heatings near 600°C or by slowly cooling materials heated at higher temperatures. LiNi9 5Mn1504 made at 600°C has demonstrated good reversible capacity at 4.7 V in excess of 100 mAh/g for tens of cycles. Infroductio