What is claimed is:1. A coin-shaped lithium secondary battery comprising:a positive electrode plate which is a lithium complex oxide sintered plate;a negative electrode plate which is a titanium-containing sintered plate;a separator interposed between the positive electrode plate and the negative electrode plate;an electrolytic solution with which the positive electrode plate, the negative electrode, and the separator are impregnated; andan exterior body comprising a closed space, the closed space accommodating the positive electrode plate, the negative electrode plate, the separator, and the electrolytic solution,the lithium secondary battery having a thickness of 0.7 to 1.6 mm and a diameter of 10 to 20 mm;wherein the separator is made of sintered ceramics selected from the group consisting of MgO, Al2O3, ZrO2, SiC, Si3N4, AlN, and cordierite.2. The coin-shaped lithium secondary battery according to claim 1, wherein the lithium secondary battery has an energy density of 35 to 200 mWh/cm3.3. The coin-shaped lithium secondary battery according to claim 1, wherein the lithium secondary battery has a battery capacity of 1.8 to 45 mAh.4. The coin-shaped lithium secondary battery according to claim 1, wherein a value obtained by dividing the battery capacity of the lithium secondary battery by the thickness of the positive electrode plate is 1.8 to 28.1 mAh/mm.5. The coin-shaped lithium secondary battery according to claim 1, wherein the positive electrode plate has a thickness of 60 to 450 μm.6. The coin-shaped lithium secondary battery according to claim 1, wherein the lithium complex oxide is lithium cobaltate.7. The coin-shaped lithium secondary battery according to claim 1, wherein the positive electrode plate has a porosity of 20 to 60%.8. The coin-shaped lithium secondary battery according to claim 1, wherein the positive electrode plate has a mean pore diameter of 0.1 to 10.0 μm.9. The coin-shaped lithium secondary battery according to claim 1, wherein the positive electrode plate is an oriented positive electrode plate containing a plurality of primary grains composed of the lithium complex oxide, the plurality of primary grains being oriented at an average orientation angle of over 0° and 30° or less with respect to the plate face of the positive electrode plate.10. The coin-shaped lithium secondary battery according to claim 1, wherein the negative electrode plate has a thickness of 70 to 500 μm.11. The coin-shaped lithium secondary battery according to claim 1, wherein the titanium-containing sintered body comprises lithium titanate or niobium titanium complex oxide.12. The coin-shaped lithium secondary battery according to claim 1, wherein the negative electrode plate has a porosity of 20 to 60%.13. The coin-shaped lithium secondary battery according to claim 1, wherein the negative electrode plate has a mean pore diameter of 0.08 to 5.0 μm.14. The coin-shaped lithium secondary battery according to claim 1, wherein the electrolytic solution is a solution containing lithium borofluoride (LiBF4) in a non-aqueous solvent composed of at least one selected from the group consisting of γ-butyrolactone (GBL), ethylene carbonate (EC), and propylene carbonate (PC).15. The coin-shaped lithium secondary battery according to claim 1, further comprising:a positive electrode current collector; anda negative electrode current collector.16. The coin-shaped lithium secondary battery according to claim 1, wherein the exterior body comprises a positive electrode can, a negative electrode can, and a gasket, and the positive electrode can and the negative electrode can are crimped via the gasket to form the closed space.17. An IoT device comprising:a circuit board; andthe coin-shaped lithium secondary battery according to claim 1.18. The IoT device according to claim 17, wherein the coin-shaped lithium secondary battery is mounted on the circuit board by reflow solder.