An interface, where two surfaces meet, is a fundamental device with its inherent complexity. Nobel laureate Prof. Herbert Kroemer also stated the same in his Nobel prize (2000) lecture to explain the phenomenal success in the design and application of semiconductor heterojunction devices (such as transistors, lasers) in microelectronics. The degree of interfacial complexity (Figure 1) depends on the interatomic bonding, lattice (or surface cell) mismatch, and atomic coordination at the interface, which usually different than its bulk phase. Understanding and tuning the interfacial complexity can lead to the design and tailoring of composite material properties for various applications, such as solid oxide fuel cells, nanocatalysts, thermal barriers, corrosion of materials, and structural materials for nuclear reactors.
A proper description of the interface, especially in atomic-scale is equally difficult for both theoreticians and experimentalists. Theoretical challenges include the size of the atomistic simulation cell. Based on the present computational resources, it is difficult as well as computationally expensive to treat more than a 1000 atoms cell. In addition, it is challenging to assign the same level of theory to treat the electronic properties of both sides of the interface. For example, additional term “Hubbard-U” in density functional theory (DFT) is required when the same metal form its oxide, to describe the localized (strongly correlated) d-electrons correctly in oxide systems. But this additional term is not essential for metal system. Therefore in a metal-oxide interface, where we have both metal and oxide system, it is criticial to bring a single theory which could correctly and seamlessly depict electronic properties of entire system. It is also experimentally challenging to map the particular interface by using various characterizing tools, such as atomic force microscopy (AFM), etc.
Scientists of the FIRST EFRC are addressing the challenges of interface research as they study how an interface’s design and its mechanical integrity alters the performance of batteries. For example, imagine that your designed interface is slowing down the Li-ion transport (Figure 2) from one electrode to another electrode through the electrolyte in the battery . One of the direct consequences of this incompatible interface will be having a poor charge-discharge cycle of your battery, means your battery could either discharge quickly or charge slowly.
A battery is a store-house of various interfaces, one crucial interface is electrode-electrolyte interface. An all-solid-state battery (ASSB), is formed by solid electrode-solid electrolyte. Engineering this interface is a crucial challenge toward realizing the ASSB. Here it raises the questions, what is ASSB and why do we want that kind of battery? As noted and as the name suggests, "all-solid-state", means all electrochemical components will be in a solid phase, which is the main difference with a conventional battery, where a liquid organic electrolyte is usually used. The high flammability and volatility of the liquid electrolyte are also responsible for any combustion risk, which is why we sometimes hear the news of the explosion of mobile phone batteries, electric vehicles, etc. To overcome this issue as well as to acquire its other advantages such as wide operating temperature range (−30 to 100 °C), promising energy densities, ASSB is a great choice for next-generation battery systems.
In the FIRST EFRC Hatzell's team has studied solid electrode-hybrid solid electrolyte interface design and its mechanical properties, as well as its role in the electrochemical performance of ASSB to make it reliable in practical application.
The hybrid solid electrolyte is a combination of salt (inorganic oxide) and polymer ion-conductor, which benefits from both the excellent transport properties of the salt and higher production scalability of the polymer. Hatzell's team considered three different molecular weight (300,000 g mol–1, 1,000,000 g mol1, and 5,000,000 g mol1) polymers (poly(ethylene oxide) [PEO]) and Li7La3Zr2O12 (LLZO) and Lithium perchlorate (LiClO4) as salts to construct a series of the hybrid solid electrolyte. Observed transport properties in each electrolyte are independent of the polymer molecular weight but the electrochemical properties (such as stability and capacity, especially the cell performance) differ significantly with the molecular weight. This variation either arises from the electrode (as well as electrolyte) microstructure or the electrode-electrolyte interfacial properties. Since various characterization techniques have confirmed that microstructure is independent of MW, the electrode-electrolyte interface must be responsible for differences in performance. Electrochemical impedance spectroscopy (EIS) observed an inverse relationship between interfacial resistance and molecular weight. Moreover, the properties that can describe an interface are roughness, adhesion strength, and Young’s modulus (a measure of elasticity). Mapping of these properties at the nanoscale by AFM suggested respectively, higher roughness and higher Young’s modulus for higher and lower molecular weight electrolyte. Improved Young’s modulus lowers the dendrite (needle or whisker type structure at the interface which could be responsible for catastrophic failure of the interface) formation, increases the stability and adhesion properties of the interface.
Based on the experimental characterizations and theoretical model, Hatzell's team elucidated the correlation between the distribution of inorganic salt (LLZO) particles in the hybrid electrolytes and sub-surface microstructure and its role on interfacial mechanical properties. Therefore, this study unveiled that proper control on the micro and nanostructure is possible by controlling the materials processing and chemistry, which eventually tailor the interfacial properties. This control is essential for the practical realization of long-lasting ASSB.