Morphological Challenges at the Interface of Lithium Metal and Electrolytes in Garnet-type Solid-State Batteries

Abstract

To achieve a fast transition to renewable energies and electrification of vehicles, developing safe, high energy and power density storage is detrimental. The use of lithium metal anodes in combination with a solid electrolyte in solid-state batteries could offer exactly this combination. Traditional liquid electrolyte-based batteries soon reach their physicochemical limit in terms of energy and power density, as side reactions and dendrites limit these cells to graphite as the anode instead of lithium. Unlike solid-state batteries, lithium-ion batteries furthermore pose greater safety concerns due to the risk of leaking and high flammability. However, while promising, ensuring a safe implantation of electrode|solid electrolyte interfaces is still regarded as the key challenge to overcome. Whereas highly resistive interfaces are the major concern for the cathode|solid electrolyte interface, morphological issues such as dendrites and contact loss limit the anode|solid electrolyte interface. It was shown that a slow vacancy diffusion within lithium metal inherently limits the applicable current density for discharging a lithium metal anode. For every lithium atom stripped, an electron and a vacancy are left behind within the lithium metal anode. If the vacancy injection rate due to the discharge current is higher than the rate of lithium replenishment by diffusion or plastic deformation, the vacancies will accumulate at the interface and form resistive pores. Another challenge is the control of lithium morphology upon deposition, be it either on a lithium reservoir or on a metal current collector. Frequent issues include the penetration of lithium into the solid electrolyte by the formation of dendritic structures or a very heterogeneous island-like growth, drastically limiting cell cyclability. Therefore, this dissertation focuses on understanding and mitigating the morphological issues linked to the use of metal anodes and their impact on battery operation. First, however, the interfacial degradation of the used model system of Li|Li6.25Al0.25La3Zr2O12|Li was investigated using X-ray photoelectron spectroscopy and impedance spectroscopy. After finding a negligibly thin interphase, several strategies were employed and investigated regarding their success in compensating or even suppressing pore formation during anodic lithium dissolution. This includes altering the lithium metal grain structure, dispersing carbon nanotubes into lithium and using ionic liquids as pore filling agents. Especially the latter two methods yield a strong improvement in dissolution capacity to > 20 mAh cm-2 . Moreover, the lithium morphology was investigated during deposition on a metal current collector in dependence of the applied current density and metal thickness by developing a novel technique. A direct operando visualization of lithium growth below a thin metal current collector using an electron microscope allowed the observation of the lithium nucleation density as a function of current density. Overall this dissertation expands the knowledge on morphological challenges occurring at the Li|solid electrolyte interface during discharge and lithium deposition at metals during charge without the presence of a lithium reservoir. Based on this knowledge, several mitigation strategies were developed and investigated, paving the way for future optimization to mitigate and compensate morphological instabilities during operation inherent for lithium metal anodes. For example, it could be shown that it might be necessary to shift away from pure lithium metal to anode composites, as a means to tailor both the anode’s electrochemical and mechanical properties to the desired application.