Abstract: To provide a positive electrode mixture layer that can enhance the charge/discharge cycle life, and a lithium ion secondary battery having the positive electrode mixture layer. The positive electrode mixture layer includes an active material, a solid electrolyte, and a binder. The active material contains a compound including Li, Ni, Mn, Co, and O, and the solid electrolyte contains an oxide having a garnet structure which contains Li, La, and Zr. The positive electrode mixture layer further contains an ionic liquid which contains an imidazolium cation and a sulfonylimide anion. The ratio of the number of Ni atoms present in one molecule of the compound to the total number of Li, Ni, Mn, Co, and O atoms present in the molecule is 12.5% or less. The relative amount of oxide with respect to the positive electrode mixture layer is 1 to 25 vol. %.

Abstract: A solid electrolyte having a garnet-type structure which contains Li, La, Zr, and O, in which, with respect to particles of the solid electrolyte having a particle size equal to or greater than the particle size at which the cumulative value of frequency in a volume-based particle size distribution reaches 10%, a number average of degrees of envelope of particles is 0.8 or greater, the degree of envelope being defined as a ratio (area within a contour of a particle/area within an envelope of the particle).