Patent Application
20220090313
The present invention relates to a method of manufacturing thermal insulation sheets to be used as measures for thermal insulation.
In recent years, needs for energy saving have been increased. Among the ways to satisfy such needs are measures for increase in energy efficiency by keeping equipment warm. In secondary battery in which battery cells are combined, there are requests for thermal insulation between the battery cells in order that one battery cell having become hot is prevented from affecting neighboring battery cells. As a measure for this, thermal insulation sheets having an excellent thermal insulation effect may be adopted between the battery cells.
Such a thermal insulation sheet is disclosed in, e.g. PTL 1.
PTL 1: Japanese Patent Laid-Open Publication No. 2011-136859
A fiber sheet having first and second surfaces and spaces therein is prepared. The spaces of the fiber sheet are impregnated with silica sol containing water glass and ethylene carbonate. Silica gel is formed by causing the silica sol with which the spaces of the fiber sheet is impregnated to gel while a difference between respective temperatures at the first and surfaces of the fiber sheet is equal to or larger than 50° C. The silica gel is hydrophobized, thereby providing a thermal insulation sheet.
In the thermal insulation sheet, compressibilities of the first and second surfaces for a predetermined pressure applied thereto are different from each other. The thermal insulation sheet may be disposed between two battery cells so as to prevent one sell from influencing the other even if the one expands.
Next, a preparation of impregnation of spaces 21q in the inside of fiber sheet 21 with silica gel 31 constituting a silica xerogel is made. As a material of silica gel 31, silica sol 41 is prepared by adding about 6% ethylene carbonate, as a catalyst, to about 20% water glass. Fiber sheet 21 is immersed in silica sol 41, thereby impregnating spaces 21q in the inside of fiber sheet 21 with silica sol 41 to produce material sheet 201.
Next, material sheet 201 impregnated with silica sol 41 is pressed to have a uniform thickness. The uniform thickness may be obtained by another method, such as roll pressing. In order to reinforce its gel skeleton, material sheet 201 with the uniform thickness is cured while the sheet is sandwiched by films 202, thereby causing silica sol 41 to gel to change into silica gel 31 being silica xerogel. During the curing, material sheet 201 is left at a constant temperature such that silica sol 41 gels while silica sol 41 is held in spaces 21q of fiber sheet 21, thereby causing the resulting gel to grow further. In addition, material sheet 201 sandwiched by the films prevents evaporation of silica sol 41. In the gelation, material sheet 201 is left for about 10 minutes in the following conditions: surface 111 of fiber sheet 21 is directed upward in the vertical direction; surface 211 is directed downward in the vertical direction, i.e. is directed in the direction of gravity; surface 111 is kept at about 90° C.; and surface 211 is kept at about 20° C. Since the ethylene carbonate is added as a catalyst to the water glass, the hydrolysis reaction rapidly proceeds when the temperature exceeds 85° C., the gelation of silica sol 41 proceeds while part of the silica is eluted. For this reason, the content of silica gel in a portion of silica sol 41 with a higher temperature decreases more than in a portion of silica sol 41 with a lower temperature, resulting in an increase in the compressibility of the portion of silica gel 31 with the higher temperature for a pressure applied thereto. On the contrary, dehydration condensation of the portion of silica sol 41 with the lower temperature proceeds more than that of the portion of silica sol 41 with the higher temperature, hence causing silica sol 41 to gel as it is, resulting in a decrease in the compressibility of the portion of silica gel 31 with the lower temperature.
Next, silica gel 31 is hydrophobized by the following procedure. Fiber sheet 21 impregnated with silica gel 31 is immersed in hydrochloric acid 6N for about 30 minutes, thereby causing silica gel 31 to react with the hydrochloric acid. After that, fiber sheet 21 impregnated with silica gel 31 is immersed in silylation solution that is mixture solution of silylating agent and alcohol, and then, stored in a constant temperature bath at about 55° C. for about 2 hours. Through the procedure, the mixture solution of the silylating agent and the alcohol permeates into silica gel 31. When trimethylsiloxane bonds start to form as the reaction proceeds, the hydrochloric acid water is discharged to the outside from fiber sheet 21 impregnated with silica gel 31. After the completion of the silylation, silica gel 31 is dried in a constant temperature bath at about 150° C. for about 2 hours, thereby providing thermal insulation sheet 101.
Respective temperatures at surfaces 111 and 211 of fiber sheet 21, i.e. material sheet 201, may be differentiated from each other by the following procedure. For example, the fiber sheet is held for a predetermined period of time with surface 211 facing downward, i.e. facing in the direction of gravity, while surface 211 of material sheet 201 impregnated with silica sol 41 is placed on a cooling plate kept at a low temperature and surface 111 contacts a heating plate kept at a high temperature. Alternatively, surface 111 may be heated by irradiating surface 111 with infrared ray.
In this way described above, the gel skeleton is reinforced by causing silica sol 41 to gel while the difference of the temperatures at surfaces 111 and 211 is equal to or larger than 50° C., thereby providing a large difference in compressibility between respective portions of the fiber sheet near surfaces 111 and surface 211.
Material sheet 201 is preferably cured while surface 111 is directed upward in the vertical direction and the temperature at surface 111 is higher than the temperature at surface 211. Surface 111 having a higher temperature than surface 211 accelerates the hydrolysis reaction near surface 111 more than near surface 211, causing a part of the silica to be eluted, followed by travelling toward surface 211 by gravity. This configuration produces a large difference in compressibility between respective portions of thermal insulation sheet 101 near surfaces 111 and surface 211.
In the gelation, the temperature at surface 111 is preferably equal to or higher than 85° C. and is equal to or lower than 135° C. The temperature of surface 111 lower than 85° C. less proceed the hydrolysis reaction. The temperature of surface 111 higher than 135° C. excessively rises the reaction rate, causing larger variations in the reaction.
In thermal insulation sheet 101 obtained in this way described above, the portion of the sheet near surface 111 which have been kept at the high temperature exhibits high compressibility, and the portion thereof near surface 211 which have been kept at the low temperature exhibits low compressibility.
Toward the end of life of a secondary battery, the central portions of the battery cells expand due to, e.g. gases generated inside the battery cells. In conventional thermal insulation sheets in each of which silica xerogel is supported at uniform density in a fiber sheet, in the case where such thermal insulation sheets are too hard, the sheets cannot sufficiently absorb the cells' swelling. On the contrary, in the case where such thermal insulation sheets are too soft, compressing the sheets causes a decrease in their heat insulating properties. This causes a possible problem that, when a certain battery cell becomes hot, such a cell affects the neighboring battery cell.
In contrast, thermal insulation sheet 101 according to the embodiment used in secondary battery 301 prevents an influence of heat from one battery cell 302 caused by the heat and expansion of the cell to the neighboring battery cell 302, thereby preventing thermal runaway.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-050576 | Mar 2019 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/039947 | 10/10/2019 | WO | 00 |
