Surface Science and Engineering for Electrochemical Materials

ConspectusIn electrochemical energy storage systems, the reversible storage capacity of battery materials often degrades due to parasitic reactions at the electrode-electrolyte interface, transitional metal dissolution, and metallic dendrite growth at the surface. Surface engineering techniques offer the opportunity to modify the composition and structure of a surface, thereby enabling control over chemical reactions occurring at the surface and manipulating chemical interactions at the solid-solid or solid-liquid interface. These modifications can help stabilize the surface of electrode materials and prevent unwanted reactions with electrolytes without changing the original properties of the bulk structure. This allows for achieving full theoretical capacity and maximizing battery material capacity retention with minimal overpotentials. In the past decade, our teams have been working on developing a variety of surface engineering techniques. These include applying atomic and molecular layer deposition (ALD and MLD), templating, doping, and coating via wet-chemical processes to stabilize the surfaces of electrode materials. The aim is to mitigate parasitic side-reactions without impeding charge transfer kinetics, suppress dendrite growth, and ultimately improve the electrode performance.This Account summarizes the research conducted in our research laboratory with an aim to improve battery cycling durability and efficiency by modifying electrode surfaces. We have employed techniques such as ALD, MLD, templating, and wet-chemical processes to illustrate how the stabilized surface improves the performance of lithium-ion (Li-ion), solid-state electrolytes and magnesium-metal (Mg-metal) batteries. For instance, by applying ultrathin layers of inorganic (e.g., Al₂O₃) or organic-inorganic coatings (e.g., alucone, lithicone, and polyamides) to the surface of LiNi_<i>x</i>Mn_<i>y</i>Co_<i>z</i>O₂ (<i>x</i> + <i>y</i> + <i>z</i> = 1, NMC) and silicon (Si) electrodes─usually just a few angstroms or nanometers thick─we have observed notable improvements in cycling efficiency and durability. When using ultrathick electrodes, the traditional electrode fabrication has a problem with high tortuosity, which hinders both rate capability and long-term cycling. To solve this issue, three-dimensional templates have been employed to reduce electrode tortuosity, enabling high-rate performance and long-term cycling. In the case of Mg-metal batteries, the buildup of an insulating MgO layer due to side reactions with electrolytes blocks Mg²⁺ ion transport, which can ultimately cause the battery to fail. To address this issue, we have developed an artificial solid-electrolyte interface using cyclized polyacrylonitrile and magnesium trifluoromethanesulfonate. This interface prevents the reduction of the carbonate electrolyte while allowing Mg²⁺ diffusion, ultimately boosting overall cell performance.This Account also discusses the significance of choosing suitable materials and effective surface engineering methods with the objective of enhancing surface properties while preserving the bulk properties of the electrodes. It is believed that surface modification and engineering can not only significantly improve the electrochemical performance of existing battery materials but also facilitate the development of new battery materials that were previously incompatible with current electrolytes. By highlighting these aspects, this Account underscores the transformative potential of surface modification and engineering in battery technology, paving the way for future innovations in energy storage solutions.