Electrochemical reactions during battery charge and discharge cycles involve complex multi-step processes that determine energy storage capabilities, efficiency, and longevity. These reactions become particularly intricate in systems with conversion electrodes, where materials undergo significant phase transformations. Unlike intercalation-based mechanisms, conversion reactions often involve bond breaking and reformation, leading to substantial structural changes that impact performance metrics such as capacity retention and cyclability.

In lithium-sulfur (Li-S) batteries, the discharge process begins with the reduction of elemental sulfur (S₈) to form higher-order lithium polysulfides (Li₂Sₙ, where 4 ≤ n ≤ 8). These soluble intermediates migrate within the electrolyte, contributing to the "shuttle effect," where they undergo further reduction to insoluble lower-order polysulfides (Li₂S₂ and Li₂S). The final discharge product, lithium sulfide (Li₂S), precipitates as a solid phase on the electrode surface. During charging, the reverse process occurs: Li₂S is oxidized back to polysulfides and eventually to sulfur. Each step involves distinct electrochemical and chemical reactions, with phase transitions between solid, liquid, and dissolved species complicating the reaction pathway.

Similar complexities arise in other conversion-type systems. For instance, transition metal oxides (e.g., Fe₂O₃, Co₃O₄) in lithium-ion batteries undergo reduction to metallic nanoparticles embedded in a Li₂O matrix during discharge. The reverse reaction during charging often suffers from incomplete reconversion due to kinetic limitations or side reactions. These processes are typically accompanied by large volume changes, particle fracturing, and loss of electrical contact, which degrade cycle life.

Phase transformations in conversion electrodes are governed by nucleation and growth kinetics. In Li-S batteries, the precipitation of Li₂S requires supersaturation of polysulfides, followed by nucleation of solid particles. The morphology of these deposits—whether they form dense films or porous structures—affects reaction rates and reversibility. In situ studies using X-ray diffraction and spectroscopy reveal that the nucleation barriers for Li₂S are influenced by electrolyte composition, overpotential, and electrode surface properties. For example, conductive scaffolds or catalysts can lower nucleation barriers, promoting uniform deposition and improving cycling stability.

Multi-electron reactions further complicate the picture. A single sulfur molecule can accept up to two electrons per atom during full reduction to Li₂S, but intermediate steps involve single-electron transfers that generate radical species. These intermediates may participate in side reactions, such as electrolyte decomposition or corrosion of conductive additives. Similarly, metal oxide conversions often proceed through intermediate phases (e.g., LiFeO₂ in Fe₂O₃ reduction) that exhibit different electrochemical activities compared to the initial or final states.

The role of the electrolyte is critical in mediating these reactions. In Li-S systems, the electrolyte must balance polysulfide solubility to facilitate redox kinetics while minimizing shuttle effects. Ether-based electrolytes are commonly used due to their compatibility with sulfur species, but they are susceptible to oxidative degradation at high voltages. Additives such as LiNO₃ can form protective layers on the lithium anode, mitigating parasitic reactions. In contrast, aqueous electrolytes for metal-air batteries face challenges from oxygen evolution and hydrogen evolution reactions, which compete with the desired discharge/charge processes.

Electrode design strategies aim to manage phase transformations and their consequences. Porous carbon matrices in Li-S batteries physically confine sulfur and polysulfides, while polar surfaces chemically adsorb intermediates to reduce shuttling. In conversion electrodes for lithium-ion batteries, nanostructuring mitigates mechanical strain from volume changes, and conductive coatings maintain electronic pathways during cycling. However, these approaches often trade off between energy density and stability—nanostructured materials may improve kinetics but reduce volumetric capacity due to excess inactive components.

Quantitative analysis of these reactions relies on techniques like cyclic voltammetry, which identifies redox potentials and reaction reversibility, and galvanostatic intermittent titration, which measures phase transformation kinetics. For example, Li-S discharge profiles typically exhibit two voltage plateaus: a higher plateau (~2.3 V vs. Li⁺/Li) corresponding to the reduction of S₈ to long-chain polysulfides, and a lower plateau (~2.1 V) for further reduction to Li₂S. The relative lengths of these plateaus reflect the efficiency of each step, with capacity fading often linked to incomplete conversion at the lower plateau.

Degradation mechanisms in conversion reactions include not only mechanical failure but also chemical passivation. In Li-S batteries, accumulation of insulating Li₂S on the electrode surface increases impedance over time. In metal oxide systems, repeated conversion cycles may lead to aggregation of metal nanoparticles, reducing their catalytic activity for the reverse reaction. Strategies like redox mediators or hybrid electrodes combining conversion and intercalation materials are being explored to address these issues.

Understanding these multi-step reactions is essential for optimizing battery performance. While computational models can predict thermodynamic pathways, experimental validation is needed to account for kinetic bottlenecks and side reactions. Advances in operando characterization techniques, such as transmission electron microscopy and synchrotron X-ray imaging, provide real-time insights into phase evolution and interfacial phenomena. These tools enable targeted improvements in electrode architectures and electrolyte formulations, moving conversion-based systems closer to practical viability.

The challenges posed by phase transformations underscore the need for holistic approaches that integrate materials science, electrochemistry, and engineering. By elucidating the fundamental steps in these complex reactions, researchers can design systems that harness conversion mechanisms without sacrificing cycle life or efficiency. Future developments may focus on controlling nucleation processes, stabilizing intermediates, and tailoring interfaces to achieve reversible multi-electron transfer at practical rates.