Synergistic Optimization of LiNi_0.5Mn_1.5O₄ Thin Films Deposited by RF Reactive Sputtering at Various Ar/O₂ Flow Ratios for High Performance All-Solid-State Thin-Film Battery

The recent 4th industrial revolution is having a great impact on our society. Next-generation electronic devices for the Internet of Things (IoT), robotics, and driverless vehicle systems such as wireless sensors, actuators, displays, and computer devices are powered by energy storage devices (i.e. lithium-ion batteries). (LIB)). It has become very important to make the next generation of electronic devices flexible and compact in a limited space. However, conventional LIBs using liquid electrolytes (LEs) have limited size and safety issues related to flammability and volatility, which can make it difficult to miniaturize LIBs for electronic devices. One of the methods to solve the above problems is to use a thin-film all-solid-state battery (thin-film ASSB) based on solid-state electrolyte (SSE). For high performance ASSBs, thin-film SSEs play a pivotal role in increasing capacity and cycle performance, but the lack of high-energy-density ASSBs persists primarily in thin-film cathodes. Therefore, we need to develop a next-generation thin-film cathode material with high energy density and long-term stability.<br/>The LiNi_0.5Mn_1.5O₄ (LNMO) in which Ni is substituted by 25% at the Mn site of the LMO has been developed to compensate for the limitations of the LMO. The LNMO exhibits a high theoretical capacity of 147 mAh g^–1 calculated from a high operating voltage (~ 4.7 V vs. Li⁺/Li). However, LNMO still suffer from unsatisfactory electrochemical performances in the shape of thin-film, because the electrochemical reactivity of thin-film LNMO with Li⁺ depends on a variety of physical and chemical parameters such as crystal facet, structure, composition, morphology, and surface area.<br/>In this study, we investigated the electrochemical properties of LiN_0.5Mn_1.5O₄ (LNMO) thin films based on the correlations between the ordering of cations (Ni and Mn) and oxygen vacancy (V_O). We found that the cation order (Ni and Mn) of LNMO changed from a disordered structure (d-LNMO) to an ordered structure (o-LNMO) and the amount of V_O and Mn³⁺ decreased in the LNMO thin films with an increased oxygen flow rate during sputtering deposition. In addition, we elucidate using first-principles density functional theory calculations that V_O in the LNMO structure hinders the diffusion of Li atoms. It was determined that the o-LNMO structure has lower activation energy than the d-LNMO structure through the climbing image nudged elastic band (CI-NEB), thus, higher Li diffusivity. Therefore, the main findings relate to LNMO thin films with the fewest V_O and Mn³⁺ ions showing the highest rate capability and cycle performance.<br/>Moreover, we succeeded in fabricating an all-solid-state thin-film battery (ASSTFB) cell composed of an LNMO (100 nm)|LiPON (500 nm)|Al₂O₃ (1 nm)|Li (1 μm) on flexible stainless steel (SS) substrate. The flexible metal substrate such as SS was used instead of the rigid single-crystal substrate used in other studies, and this is the first report on flexible ASSTFB with LNMO thin-film. We believe that our study makes a significant contribution to the literature because it has formed the basis for the potential development of newly developed high-performance energy storage devices.