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Rechargeable alkali batteries (RABs) are a key technology for alleviating global energy demands, operating via reversible electrochemical reactions at the positive electrode (cathode) and negative electrode (anode). Current state-of-the-art lithium-ion batteries (LIBs) achieve a high coulombic efficiency (CE) of more than 99.98%, meaning only 0.02% of reactions are undesired, known as side reactions. This high CE is partly due to the presence of a passivating layer at the anode known as the solid electrolyte interphase (SEI) that protects the electrolyte from being reductively decomposed. However, there are still unwanted side reactions occurring at the cathode that cause long term capacity fade. This thesis presents the development and application of advanced methodologies for studying degradation processes at the cathode across various battery types. The operando gas analysis technique known as Online Electrochemical Mass Spectrometry (OEMS) was used throughout the thesis. Three variants of OEMS are explored: purging OEMS (POEMS), closed leak OEMS (CLEMS) and intermittently closed OEMS (ICEMS) of which the latter was developed in this work. A means of interfacing large-format prismatic and cylindrical cells with ICEMS & CLEMS was additionally developed, enabling the investigation of gas evolution in commercially relevant cells. POEMS was primarily used to study laboratory scale model systems. Four cathode materials (CAMs) are studied: two state-of-the-art (lithium nickel cobalt aluminium oxide, NCA, and lithium nickel manganese cobalt oxide, NMC) and two next-generation (sodium (II) hexacyanoferrate (Prussian White, PW) and lithium nickel manganese oxide, LNMO). Each CAM exhibited distinct degradation behaviours, with NMC, NCA and PW suffering from structural degradation and LNMO from electrolyte oxidation. It was found that the lattice oxygen release from NMC and NCA (which reacts with the electrolyte forming CO₂) affects commercial cells. An analogous process was observed in PW, where reconstruction at the surface resulted in CN ligand release at high potentials, which react with the electrolyte forming both CO₂ and (CN)₂. A comprehensive reaction pathway for electrolyte decomposition on LNMO positive electrodes was proposed, highlighting how oxidation products (namely protons) autocatalyze decomposition of multiple components in the electrode. Additionally, strategies to mitigate these processes were explored; including forced surface reconstruction on PW, Ta-doping in NMC, and use of cathode electrolyte interphase (CEI) forming additives on LNMO. Furthermore, a novel method for screening for effective CEI forming additives was developed using POEMS. These findings highlight the versatility of OEMS as a powerful tool for understanding and mitigating degradation in RABs.