Structure and Electrochemistry of Li[Ni[sub x]Co[sub 1−2x]Mn[sub x]]O[sub 2] (0≤x≤1/2)

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Abstract
In a recent paper, Lu et al. showed that Li[Ni0.25Co0.5Mn0.25]O2Li[Ni0.25Co0.5Mn0.25]O2 and Li[Ni0.375Co0.25Mn0.375]O2Li[Ni0.375Co0.25Mn0.375]O2 are attractive electrode materials for Li-ion cells. Most interesting, they appear to be much less reactive with electrolyte at high temperatures than LiCoO2.LiCoO2. Since these two materials are part of the solid-solution series Li[NixCo1−2xMnx]O2Li[NixCo1−2xMnx]O2 with 0⩽x⩽1/2,0⩽x⩽1/2, which also contains LiCoO2LiCoO2 when x=0,x=0, it is possible to study the reasons for the safety advantage by a careful study of electrochemical, structural, and thermal properties as a function of x.x. This paper reports the structural and electrochemical properties of Li[NixCo1−2xMnx]O2Li[NixCo1−2xMnx]O2 for 0⩽x⩽1/2.0⩽x⩽1/2. The materials all show good specific capacity (between 110 and 130 mAh/g between 3.0 and 4.2 V vs. Li at 40 mA/g) and very little capacity loss over 50 cycles. Careful consideration of differential capacity measurements proves that Ni2+Ni2+ and Mn4+Mn4+ do, indeed, substitute for Co3+,Co3+, at least for x<0.15,xLiCoO2,LiCoO2, then saturates as xx in Li[NixCo1−2xMnx]O2Li[NixCo1−2xMnx]O2 increases. Materials with x⩾0.075x⩾0.075 show approximately equal thermal stability. The reason for the variation of the thermal stability with xx is the subject of a companion paper. © 2002 The Electrochemical Society. All rights reserved.