The synthesis, structure, and electrochemical performance of normalLifalse[NixLi1/3−2x/3Mn2/3−x/3false]O2 for 0

Rechargeable lithium-ion batteries that use an aqueous electrolyte have been developed. Cells with LiMn(2)O(4) and VO(2)(B) as electrodes and 5 M LiNO(3) in water as the electrolyte provide a fundamentally safe and cost-effective technology that can compete with nickelcadmium and lead-acid batteries on the basis of stored energy per unit of weight.

LiNixMn2−xO4 has been synthesized using sol‐gel and solid‐state methods for 0 < x < 0.5. The electrochemical behavior of the samples was studied in normalLi/LiNixMn2−xO4 coin‐type cells. When x = 0, the capacity of normalLi/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 at 4.7 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 Li+1Nix+2Mn1−2x+3Mn1+x+4O4−2 . The 4.1 V plateau is related to the oxidation of Mn3+ to Mn4+ and the 4.7 V plateau to the oxidation of Ni2+ to Ni4+ . The effect of synthesis temperature, atmosphere, and cooling rate on the structure and electrochemical properties of LiNi0.5Mn1.5O4 is also studied on samples made by the sol‐gel method. LiNi0.5Mn1.5O4 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. LiNi0.5Mn1.5O4 samples heated above 650°C suffer due to disproportionation into LiNixMn2−xO4 with x < 0.5 and LizNi1−zO with z ≈ 0.2, which occurs above about 650°C. Pure LiNi0.5Mn1.5O4 materials can be made by extended heatings near 600°C or by slowly cooling materials heated at higher temperatures. LiNi0.5Mn1.5O4 made at 600°C has demonstrated good reversible capacity at 4.7 V in excess of 100 mAh/g for tens of cycles.

The electrochemical behavior of normalLi/normalLifalse[NixLifalse(1/3−2x/3false)Mnfalse(2/3−x/3false)false]O2 cells for x=1/6, 1/4, 1/3, 5/12, and 1/2 is reported. normalLifalse[NixLifalse(1/3−2x/3false)Mnfalse(2/3−x/3false)false]O2 is derived from Li2MnO3 or normalLifalse[Li1/3Mn2/3false]O2 by substitution of Li+ and Mn4+ by Ni2+ while maintaining all the remaining Mn atoms in the 4+ oxidation state. Conventional wisdom suggests that lithium can be removed from these materials only until both the Ni and Mn oxidation states reach 4+, giving a charge capacity of 2x. We show that normalLi/normalLifalse[NixLifalse(1/3−2x/3false)Mnfalse(2/3−x/3false)false]O2 cells give smooth reversible voltage profiles reaching about 4.45 V when 2x Li atoms per formula unit are removed, as expected. If the cells are charged to higher voltages, surprisingly, they exhibit a long plateau of length approximately equal to 1−2x in the range between 4.5 and 4.7 V. Subsequent to this plateau, the materials can reversibly cycle over 225 mAh/g (almost one Li atom per formula unit) between 2.0 and 4.8 V. In situ X-ray diffraction and differential capacity measurements are used to infer that irreversible loss of oxygen from the compounds with x<1/2 occurs during the first charge to 4.8 V. This results in oxygen deficient layered materials with stoichiometry approximately equal to false[false]false[NixLifalse(1−2xfalse)/3Mnfalse(2−xfalse)/3false]O1.5+x at 4.8 V. These oxygen deficient materials then react reversibly with lithium. © 2002 The Electrochemical Society. All rights reserved.

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