A composite electrolyte structure (18, 28) comprising: a protective layer (14, 24, 34) having a Young's modulus of 10 6 pascals or greater and comprising a first particle (17, 27, 37), the first particle comprising an organic particle, an inorganic particle, an organic-inorganic particle, or a combination thereof, wherein the first particle (17, 27, 37) has a particle size of greater than 1 micrometer to 100 micrometers; and a solid electrolyte layer (13, 23) comprising a first electrolyte comprising a second particle (16, 26), the second particle (16, 26) comprising an organic particle, an inorganic particle, an organic-inorganic particle, or a combination thereof, wherein the second particle (16, 26) has an average particle size of greater than 1 micrometer to 100 micrometers, a first lithium salt, and a polymer; wherein the first particle (17, 27, 37) and the second particle (16, 26) are the same or different; wherein the protective layer (14, 24, 34) is on the solid electrolyte layer (13, 23); wherein the first particle (17, 27, 37) in the protective layer (14, 24, 34) and the second particle (16, 26) of the solid electrolyte layer (13, 23) comprises a homopolystyrene, a copolymer comprising a styrene repeating unit, a poly(styrenedivinylbenzene) copolymer, a poly(methyl methacrylate-divinylbenzene), a cross-linked product of poly(styrene-divinylbenzene) copolymer, a cross-linked product of a poly(methyl methacrylate-divinylbenzene), or a combination thereof; wherein the protective layer (14, 24, 34) further comprises a crosslinked polymerization product of a polymerizable compound (14a, 24a, 34a) in an amount of 10 parts by weight to 50 parts by weight, based on 100 parts by weight of the first particle (17, 27, 37) in the protective layer (14, 24, 34); wherein the Young's modulus of the protective layer (14, 24, 34) is measured according to ASTM D412; wherein the particle size is the average particle diameter when the first or second particle (16, 26)is of a spherical shape, or is a length of the major axis when the first or second particle (16, 26) has a rod shape, and wherein the "average particle size" refers to a particle diameter corresponding to 50% of the particles in a distribution curve in which particles are accumulated in the order of particle diameter from the smallest particle to the largest particle and a total number of accumulated particles is 100%. a protective layer (14, 24, 34) having a Young's modulus of 10 6 pascals or greater and comprising a first particle (17, 27, 37), the first particle comprising an organic particle, an inorganic particle, an organic-inorganic particle, or a combination thereof, wherein the first particle (17, 27, 37) has a particle size of greater than 1 micrometer to 100 micrometers; and a solid electrolyte layer (13, 23) comprising a first electrolyte comprising a second particle (16, 26), the second particle (16, 26) comprising an organic particle, an inorganic particle, an organic-inorganic particle, or a combination thereof, wherein the second particle (16, 26) has an average particle size of greater than 1 micrometer to 100 micrometers, a first lithium salt, and a polymer; a first particle (17, 27, 37), the first particle comprising an organic particle, an inorganic particle, an organic-inorganic particle, or a combination thereof, wherein the first particle (17, 27, 37) has a particle size of greater than 1 micrometer to 100 micrometers; and a solid electrolyte layer (13, 23) comprising a first electrolyte comprising a second particle (16, 26), the second particle (16, 26) comprising an organic particle, an inorganic particle, an organic-inorganic particle, or a combination thereof, wherein the second particle (16, 26) has an average particle size of greater than 1 micrometer to 100 micrometers, a first lithium salt, and a polymer; wherein the first particle (17, 27, 37) and the second particle (16, 26) are the same or different; wherein the protective layer (14, 24, 34) is on the solid electrolyte layer (13, 23); wherein the first particle (17, 27, 37) in the protective layer (14, 24, 34) and the second particle (16, 26) of the solid electrolyte layer (13, 23) comprises a homopolystyrene, a copolymer comprising a styrene repeating unit, a poly(styrenedivinylbenzene) copolymer, a poly(methyl methacrylate-divinylbenzene), a cross-linked product of poly(styrene-divinylbenzene) copolymer, a cross-linked product of a poly(methyl methacrylate-divinylbenzene), or a combination thereof; wherein the protective layer (14, 24, 34) further comprises a crosslinked polymerization product of a polymerizable compound (14a, 24a, 34a) in an amount of 10 parts by weight to 50 parts by weight, based on 100 parts by weight of the first particle (17, 27, 37) in the protective layer (14, 24, 34); wherein the Young's modulus of the protective layer (14, 24, 34) is measured according to ASTM D412; wherein the particle size is the average particle diameter when the first or second particle (16, 26)is of a spherical shape, or is a length of the major axis when the first or second particle (16, 26) has a rod shape, and wherein the "average particle size" refers to a particle diameter corresponding to 50% of the particles in a distribution curve in which particles are accumulated in the order of particle diameter from the smallest particle to the largest particle and a total number of accumulated particles is 100%.