CO₂ Reactive Laser Sintering of Garnet-Type Li-Ion Conductors

In contrast to currently commercialized Li-ion batteries (LIBs), the most promising all-solid-state Li-ion battery (ASSLB) technology, with the integration of solid-state electrolytes (SSEs), has the potential to provide higher energy density, better safety, and longer cycle life. Over the decades, significant progress has been made for ASSBs by the development and optimization of SSEs that can provide efficient Li-ion conduction while mitigating the problems of leakage and high flammability associated with organic liquid electrolytes. The replacement of liquid electrolytes with safer, more reliable SSEs simplifies the battery design, alleviates safety concerns, and improves battery performance. Among the different developed chemistries of inorganic SSEs, e.g., sulfides and halides, oxides are considered promising SSE candidates for ASSLBs due to their chemical stability and ionic transport properties. Particularly, the garnet-like Li-ion conductive oxide Li_6.4La₃Z_1.4Ta_0.6O₁₂ (LLZTO) is one of the most promising SSEs due to its excellent chemical stability against lithium metal, high lithium ion conductivity (10^-3 S/cm at 25°C) and wide electrochemical window (5.6 V vs. Li⁺/Li).<br/>ASSLBs are difficult to process by traditional manufacturing strategies due to the brittleness of SSE ceramic materials, poor solid-solid contact (SSE/electrodes), and electrolyte-electrode stability issues; as a result, battery performance is far from ideal. Currently used SSEs manufacturing processes such as high temperature sintering and mechanical milling have a number of limitations including being time-intensive, energy consuming, and difficult to scale up. In contrast, reactive laser sintering, an additive manufacturing (3D printing) technique that uses a laser as the power source, has drawn increasing attention due to its speed, cost, simplicity, and capability of achieving one-step manufacturing of materials with desirable properties that are difficult to achieve by conventional processes.<br/>Herein, we utilized the promising reactive CO₂laser sintering technology to synthesize the LLZTO SSE in one-step process. The CO₂ laser sintering approach has been previously used for fuel cell applications; however, this is the first time that this technology is used to manufacture solid-state lithium-ion conductors. During the reactive laser synthesis of LLZTO SSE, the precursor mixture absorbs the laser energy directly, which leads to a rapid increase in temperature that accelerates the reaction and densification while reducing Li loss. A benefit of the inherent nature of ultrafast heating/cooling rates of reactive laser technology is the formation of non-stoichiometric materials with high concentration of defects. Allowing the development of highly disordered superionic conductive SSEs to enable high discharge/charge rate capabilities in ASSLBs. Our work successfully demonstrates that the cubic LLZTO phase, which has a high ionic conductivity (10^-4 S/cm), can be formed using the CO₂ laser reactive sintering approach. The simplified processing steps dramatically reduce the processing cost and the production time of the promising LLZTO SSE, making it an ideal synthesis technique for large-scale battery manufacturing market.