Hierarchical Interface Modeling of All-Solid-State Lithium Metal Battery Based on Energy Band Theory

Development of practical all-solid-state Li-metal batteries (ASSLB) is essential for a breakthrough in battery technology. Compared to conventional lithium-ion batteries using liquid electrolyte, ASSLBs have higher energy density, superior safety, and wider operating temperature range. However, fabrication of all-solid-state battery with Li-metal anode (LMA) and inorganic solid-state electrolyte (SSE) is a far challenge. Two critical issues are hindering the development of ASSLBs as following: large impedance of non-conformal electrode-electrolyte contact and electrochemical instability at the interface. Solid materials, being composed into all-solid-state battery, possess their own distinctive electrochemical and physical properties. When these materials in the forms of electrode or electrolyte are assembled into an electrochemical cell configuration, solid–solid interfaces are subsequently built inside the energy storage system. For realization of practical ASSLBs, interfacial stability at the LMA–SSE contact must be assured because the direct contact causes interfacial reactions leading to battery failure. Therefore, intrinsic instability of inorganic SSE against LMA is required to be prevented. Preceding researches have focused on developing the interface modification methods to enhance ASSLB's long-term cycling stability. But it is confined to a phenomenological solution if modification strategy is established without in-depth understanding of the interface. Also, the fact that rechargeable batteries are operated by behavior of charges inside electric field is often overlooked. The LMA-SSE interface of ASSLB is under in situ electric field during charging process, and the SSE possesses semiconducting properties unlike liquid electrolytes. Consequently, governing mechanism of interface stabilization cannot be identified by using only the conventional electrochemical techniques or by considering solely the material science perspective. For these reasons, solid-state physics concepts of electrical contacts and conduction are necessary to be discussed in ASSLB research.<br/><br/>This study demonstrates the significance of interface stabilization strategy via energy band alignment. The intrinsically unstable LMA–SSE interface is effectively improved by nanoscale titanium deposition on the electrolyte’s surface. And the pure transition metal nanolayer is transformed into an ion-conducting crystal structure through electrochemical interactions with Li-metal. The Li symmetric cell with titanium compound self-induced interlayer (TSI) has successfully maintained its constant overpotential over 1000 cycles and the significantly reduced impedance, whereas the cell having no interface modification exhibits erratic voltage profiles and is easily failed by repetitive charge–discharge process. Subsequently, the cell components can preserve their functions during the repetitive charge–discharge process. Such improvements are achieved by the formation of TSI functioning as stable Li-ion conductor and electron buffer at the anode-electrolyte interface, which is demonstrated by in-depth electrochemical analyses and is fully supported by widely accepted energy band theory. XRD analysis has confirmed the formation of self-induced interlayer, EDS analysis has shown its structural robustness and uniform elemental distribution, and depth-profiling XPS measurement has confirmed the effect of electron blocking phenomenon across the interface. The revealing results provide not only comprehensive understanding of the interfacial phenomena, but also fundamental interface modification method applicable to various types of all-solid-state battery. Furthermore, rigorous stability requirements of automotive applications are expected to be satisfied by the innovative interface modeling.