A lithium-ion secondary battery, comprising a bare battery cell accommodating cavity in which a bare battery cell group comprising one or more bare battery cells A and one or more bare battery cells B is arranged, wherein the bare battery cell A comprises a first positive electrode sheet comprising a first positive-electrode active material selected from a single crystal or single crystal-like low-nickel ternary positive electrode material A1, wherein the molecular formula of the single crystal or single crystal-like low-nickel ternary positive electrode material A1 is LiNi x1 Co y1 Mn 1-x1-y1 O 2 , wherein 0 < x1 ≤ 0.65, 0 < y1 < 0.35, 0 < 1-x1-y1 < 0.35, and the volume distribution ratio of secondary particles is < 5% in the single crystal low-nickel ternary positive electrode material A1 and the volume distribution ratio of secondary particles is 10 % to 35 % in the single crystal-like low-nickel ternary positive electrode material A1; the bare battery cell B comprises a second positive electrode sheet comprising a second positive-electrode active material and/or a third positive-electrode active material, the second positive-electrode active material is selected from a polycrystalline high-nickel ternary positive electrode material B1, wherein the molecular formula of the polycrystalline high-nickel ternary positive electrode material B1 is LiNi x2 Co y2 Mn 1-x2-y2 O 2 , wherein 0.7 ≤ x2 < 1, 0 < y2 < 0.3, 0 < 1 - x2 - y2 < 0.3, and the third positive-electrode active material is selected from a polycrystalline low-nickel ternary positive electrode material B2, wherein the molecular formula of the polycrystalline low-nickel ternary positive electrode material B2 is LiNi x3 Co y3 Mn 1-x3-y3 O 2 , wherein 0 < x3 ≤ 0.65, 0 < y3 < 0.35, 0 < 1 - x3 - y3 < 0.35. wherein the bare battery cell A comprises a first positive electrode sheet comprising a first positive-electrode active material selected from a single crystal or single crystal-like low-nickel ternary positive electrode material A1, wherein the molecular formula of the single crystal or single crystal-like low-nickel ternary positive electrode material A1 is LiNi x1 Co y1 Mn 1-x1-y1 O 2 , wherein 0 < x1 ≤ 0.65, 0 < y1 < 0.35, 0 < 1-x1-y1 < 0.35, and the volume distribution ratio of secondary particles is < 5% in the single crystal low-nickel ternary positive electrode material A1 and the volume distribution ratio of secondary particles is 10 % to 35 % in the single crystal-like low-nickel ternary positive electrode material A1; the bare battery cell B comprises a second positive electrode sheet comprising a second positive-electrode active material and/or a third positive-electrode active material, the second positive-electrode active material is selected from a polycrystalline high-nickel ternary positive electrode material B1, wherein the molecular formula of the polycrystalline high-nickel ternary positive electrode material B1 is LiNi x2 Co y2 Mn 1-x2-y2 O 2 , wherein 0.7 ≤ x2 < 1, 0 < y2 < 0.3, 0 < 1 - x2 - y2 < 0.3, and the third positive-electrode active material is selected from a polycrystalline low-nickel ternary positive electrode material B2, wherein the molecular formula of the polycrystalline low-nickel ternary positive electrode material B2 is LiNi x3 Co y3 Mn 1-x3-y3 O 2 , wherein 0 < x3 ≤ 0.65, 0 < y3 < 0.35, 0 < 1 - x3 - y3 < 0.35. <EOS>