Encapsulating Battery Components with Melting Gels

Abstract

For forming hermetic seals, melting gels are being studied as an alternative to epoxy. A typical melting gel is prepared by mixing together methyltnethoxysilane (MTES) and dimethyldiethoxysilane (DMDES), precursors with one or two methyl groups substituted for ethoxy. The methyl groups do not hydrolyze, which limits the network-forming capability of the precursors and contributes to the unusual softening behavior of the gels. The hermeticity of melting gels has been evaluated, both in contact with ionizing solvents used in electrolj^s and in contact with metallic lithium. Melting gels limit gas transport and protect battery components. INTRODUCTION Batteries, namely lithium-ion batteries, by design are electrochemically active systems'* . This poses a challenge to the materials that are used to seal battery components, because the components, especially the anode and electrolyte, are reactive. The encapsulating materials have to protect the components from environmental degradation without interfering with the fimclioning of the battery, The best seals would be inoi^anic glasses that seal at greater than 350'C. When high temperature sealing is not practical, organic polymers often are used, but their hermeticity is never as good as that of inorganic sealing glasses^. WTiat melting gels offer are nanocomposites of organic and inorganic components that can be formulated (a) to be more hermetic than polymers and (b) to soften and seal at temperatures below 200'C. To understand the role of sealing materials, hermeticity and permeation need to be defined. Measures of hermeticity are conftjsing and hard to compare direcdy, because the units, such as grams per meter squared per hour, often are related to test set-up. Hermeticity varies with temperature, ambient pressure and relative humidity. By any measure of hermeticity, it is clear that water vapor transport and oxygen transport have to be extremely low. Permeation is defined as the mass transfer of a gas through a solid The permeation rate is dependent on the solubility of gas into the solid and the difftisivity of gas through a solid. Another way to penetrate through a barrier is permeation via microscopic defects in the coatings. These defects may arise fi'ora impurities on the substrate surface or stress during fibn deposition, and these defects have to be avoided. Sealing materials can be made thicker to limit transport, but this is constrained by other design or geometry considerations. In addition to hermeticity, sealing materials have to be compatible in terms of thermal expansion and adhesion**. Looking further ahead, it is also necessary to design for batteries that operate with ions other than lithium, for example sodium*.