While the solid electrolyte and electrodes are the core components of solid-state batteries, a range of key auxiliary materials play critical roles in enhancing performance, ensuring stability, and enabling efficient manufacturing. These materials address challenges such as interface resistance, mechanical stress, and chemical compatibility, contributing to the overall functionality and reliability of the battery.

One essential category of auxiliary materials is interface modifiers. The interface between the solid electrolyte and electrodes is a major source of resistance in solid-state batteries, as poor contact and chemical reactions can hinder ion transport. Interface modifiers are designed to improve this contact and stabilize the interface. For example, buffer layers made from materials like lithium niobate (LiNbO₃) or lithium phosphate (Li₃PO₄) are often deposited between the solid electrolyte and cathode. These layers prevent direct reaction between the electrolyte and cathode, which can form resistive phases, while still allowing lithium ions to pass through. In the case of lithium metal anodes, artificial solid electrolyte interphases (SEIs) are used to modify the anode-electrolyte interface. These artificial SEIs, typically composed of lithium fluoride (LiF) or lithium carbonate (Li₂CO₃), prevent the formation of unstable native SEIs that can grow and block ion transport. They also help to evenly distribute lithium ions during plating and stripping, reducing the formation of dendrites that can short-circuit the battery.

Binder materials are another important class of auxiliary materials in solid-state batteries. Binders are used to hold electrode particles together and adhere them to current collectors, ensuring mechanical integrity and electrical conductivity. In traditional liquid batteries, polymeric binders like polyvinylidene fluoride (PVDF) are commonly used, but they are often incompatible with solid electrolytes due to poor chemical stability or insufficient adhesion. For solid-state batteries, researchers are developing new binders that can interact with both electrode materials and solid electrolytes. For example, polymer binders with polar functional groups (e.g., hydroxyl or carboxyl groups) can form hydrogen bonds with oxide or sulfide electrolytes, improving adhesion and reducing interface resistance. In some cases, solid electrolytes themselves are used as binders, creating a homogeneous electrode-electrolyte mixture that enhances ion transport. For sulfide-based batteries, which are often processed in powder form, binders like styrene-butadiene rubber (SBR) or carboxymethyl cellulose (CMC) are used to maintain the structure of the electrode composite during pressing and sintering.

Current collectors are also critical auxiliary materials, responsible for conducting electrons between the electrodes and the external circuit. In solid-state batteries, current collectors must be compatible with the electrode materials and solid electrolytes, as well as resistant to corrosion. For cathodes, aluminum (Al) is commonly used as a current collector, but it can react with certain solid electrolytes, such as sulfides, at high voltages, forming insulating aluminum sulfide phases. To prevent this, researchers are coating Al current collectors with a thin layer of titanium (Ti) or carbon, which acts as a barrier. For lithium metal anodes, copper (Cu) is often used as a current collector, but it can alloy with lithium at high temperatures, leading to a loss of conductivity. As a result, some designs use nickel (Ni) or stainless steel current collectors, which are more stable in contact with lithium metal. Additionally, porous current collectors are being explored to accommodate the volume expansion of lithium metal anodes, reducing mechanical stress and improving cycle life.

Additives are another group of auxiliary materials that play diverse roles in solid-state batteries. For example, conductive additives like carbon black or graphene are added to electrode composites to enhance electronic conductivity, especially for cathodes with low conductivity (e.g., LFP). Plasticizers may be used in polymer electrolytes to increase chain mobility and improve ionic conductivity. In sulfide-based systems, lithium halides (e.g., LiI) are sometimes added to reduce grain boundary resistance in the electrolyte. Additives can also be used to modify the mechanical properties of solid electrolytes; for instance, adding small amounts of polymers to oxide electrolytes can improve their flexibility and reduce brittleness. Each of these auxiliary materials contributes to overcoming specific challenges in solid-state battery design, and their optimization is crucial for achieving high performance and reliability.

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