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Since the state-of-the-art Li-ion batteries are close to reaching their theoretical limit in energy density, it becomes crucial to develop next-generation batteries that enable better safety, higher energy density, and longer lifetime. One such next-generation technology is solid-state batteries, employing solid-state electrolytes. Both polymer and inorganic electrolytes are well-explored in this context. While polymers are flexible and easily processable, their ionic conductivities are generally insufficient. Inorganic ceramics can be good ionic conductors, but display interfacial issues. Therefore, combining polymeric and ceramic material in composites polymer electrolytes (CPEs) can – in principle – be beneficial to merge the advantages of both categories. However, it is still unclear how to best construct such systems, and how the ions are actually transported in them. 

This thesis explores ionic transport in CPEs, both with ion-conducting (“active”) and non-ion-conducting (“passive”) ceramic fillers. The focus is on the amorphous polymer material poly(trimethylene carbonate) (PTMC), the active ceramic filler Li₇La₃Zr₂O₁₂ (LLZO), and the passive ceramic fillers LiAlO₂ (LAO) and NaAlO₂ (NAO). The ionic transport mechanism in PTMC:LLZO CPEs is determined to be dependent on two main factors: particle loading and surface chemistry. An increase in ionic conductivity up to 30 wt% of Li₇La₃Zr₂O₁₂ is seen due to formation of additional transport pathways along the polymer-ceramic interfaces, while higher loadings affect the ionic conductivity negatively. While this can partly be explained by particle agglomeration, the presence of Li₂CO₃ on the Li₇La₃Zr₂O₁₂ surface also contributes to retard the ionic movement along the interfaces. Therefore, boric acid treatment is explored as a strategy to enable a Li₂CO₃-free surface of Li₇La₃Zr₂O₁₂ particles, which renders improved ionic transport and battery performance. Boron-treated Li₇La₃Zr₂O₁₂ shows formation of LiBO₂, which yields a negative zeta-potential, indicative of interactions between the ceramic particles and Li⁺ ions. That the surface chemistry – rather than the bulk – of the ceramic filler ultimately controls the overall transport, opens the door towards employment of passive fillers. It is shown that LiAlO₂  particles can increase the ionic conductivity by one order of magnitude and the Li⁺ transference number to almost 1, effectively rendering the LiAlO₂-based CPE a single-ion conductor. These enhanced ionic transport properties can be explained by the ability of LiAlO₂  particles to promote better ion-ion separation through the attraction of negatively charged TFSI anions to the surface. This renders considerably improved battery performance, enabling cycling in Li||NMC cells. Similar effects are also seen for the analogous Na-ion battery system. 

Thereby, considering that the bulk conductivity of active fillers does not contribute to the overall ionic conduction in CPEs, and that passive fillers such as LiAlO₂  can greatly enhance the ionic transport because of its surface chemistry enabling greater ion-ion separation and favorable transport pathways, this thesis provides guidelines for future design of solid-state conductors for Li- and Na-batteries. 

This thesis explores the anode-less battery concept, where the anode is created by lithium plating during charging. Such devices face problems with electrochemical stability, specifically at the anode interface, where electrolyte decomposition occurs. One strategy for implementing anode-less concepts is using solid polymer electrolytes (SPEs). To make SPEs a viable option, an understanding of the solid electrolyte interphase (SEI) formation at the anode interface is necessary. Conventional post-mortem photoelectron spectroscopy (PES) is challenging for SPEs, thus, herein, two novel PES-based methods are evaluated and used for investigating the decomposition products making up the initial SEI in SPE-based anode-less batteries.

The first method is in situ deposition photoelectron spectroscopy (ISDPES), in which lithium is deposited by evaporation onto the SPE surface, intended to emulate the plating of lithium during charging. The second method is operando PES, in which a custom battery cell is cycled and the interface is characterised during operation. Both methods are shown to be highly useful in providing information about SPE decomposition and SEI formation in SPE-based batteries. However, the ISDPES method is limited to emulating only the first charging cycle, and gives no information about reactions at stages other than lithium plating. This limitation is overcome by operando PES, which, while time consuming, allows for the sequential lowering of the potential until lithium plating is reached.

In general, these methods show two kinds of compounds forming from the decomposition of the polymers in the studied SPEs. The first is lithium alkoxides, still attached to the polymer chain. The other is hydrocarbons, suggested to take the shape of polyethylene segments or oligomers (the form of which depend on the polymer they originate from). For the salts and additives it depends more on the nature of the salt/additive, but in general they involved far more inorganic products. 

In the interplay between SPE components (polymers, salts, and additives), it is observed that the presence of one component in an SPE influences the decomposition of the others. It is also found that the stability of the polymer is less important than the stability of the decomposition layer when optimising for coulombic efficiency. The sequential lowering of potential using operando PES shows another dimension to improving the SEI: the order of decomposition. This thesis thereby contributes to the understanding of the SEIs of SPE-based anode-less batteries, where a functional SEI is one necessary part of a viable cell chemistry.