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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.