Time resolved impedance spectroscopy analysis of lithium phosphorous oxynitride - LiPON layers under mechanical stress

In this paper we present investigations on the morphological and electrochemical changes of lithium phosphorous oxynitride (LiPON) under mechanically bent conditions. Therefore, two types of electrochemical cells with LiPON thin films were prepared by physical vapor deposition. First, symmetrical cells with two blocking electrodes (Cu/LiPON/Cu) were fabricated. Second, to simulate a more application-related scenario cells with one blocking and one non-blocking electrode (Cu/LiPON/Li/Cu) were analyzed. In order to investigate mechanical distortion induced transport property changes in LiPON layers the cells were deposited on a flexible polyimide substrate. Morphology of the as-prepared samples and deviations from the initial state after applying external stress by bending the cells over different radii were investigated by Focused Ion Beam- Scanning Electron Microscopy (FIB-SEM) cross-section and surface images. Mechanical stress induced changes in the impedance were eva luated by time-resolved electrochemical impedance spectroscopy (EIS). Due to the formation of a stable, ion-conducting solid electrolyte interphase (SEI), cells with lithium show decreased impedance values. Furthermore, applying mechanical stress to the cells results in a further reduction of the electrolyte resistance. These results are supported by finite element analysis (FEA) simulations

Investigations on morphological and electrochemical changes of all-solid-state thin film battery cells under dynamic mechanical stress conditions

To design and manufacture high-performance energy storage devices with real mechanical flexibility is one of the main advantages of the solid-state battery technology. Mechanically flexible thin film, all solid-state Li-ion batteries are supposed to be the main power sources in emerging technologies such as flexible electronics, wearables, etc. However, if a flexible solid-state device is exposed to repeated external mechanical load, introducing additional aging mechanisms might be expected. In addition, externally introduced stress and strain to the battery functional components could influence lithiation kinetics of the respective electrode material. In the present study, the effect of the external mechanical load on the lithiation kinetics and the collateral mechanical fatigue of the full battery cell during dynamic bending were investigated in detail. Therefore, mechanically flexible, all solid-state MoO3/LiPON/Li battery cells were fabricated on a polymer substrate. Battery cells were exposed to static convex bending and it was ascertained that the bulk resistance of the positive electrode is largely dependent on the depth-of-discharge as well as mechanical stress state, while other processes such as charge transfer and electrolyte bulk resistance are less affected. Furthermore, battery cells were cycled galvanostatically, while they were bent repeatedly using different bending scenarios. Below a threshold bending frequency (f = 1/360 Hz), stable battery function was found, however mechanical aging of the battery cell was observed. As it was demonstrated, the metal current collector/positive electrode interface is highly prone to the physical degradation upon dynamic bending. As a result, delamination of the electrode and contact loss occur, causing capacity fading, accordingly. The present study shed light on the joint mechanical-electrochemical aging of mechanically flexible all solid-state Li-ion batteries

Flame aerosol deposited Li4Ti5O12 layers for flexible, thin film all-solid-state Li-ion batteries

Flexible thin film all-solid-state Li-ion batteries are considered as promising candidates to power a multitude of flexible and miniaturized electronic devices. The production of crystalline battery active materials generally involves high process temperatures above 500 °C. One current challenge in mechanically flexible thin film electrode fabrication is the direct deposition of such crystalline active materials onto temperature sensitive substrates. In the current work we have made a paradigm shift depositing highly pure crystalline Li4Ti5O12nanoparticles onto a flexible polyimide foil in a single step using flame spray pyrolysis technique. The Li4Ti5O12films were mechanically compressed at room temperature to 0.55 μm thin layers, to enhance their adhesion to the substrates, i.e. to increase mechanical stability. The smooth Li4Ti5O12 electrodes were covered with a solid electrolyte and tested against lithium metal electrodes. Stable electrochemical cycling behavior of the battery cells demonstrated the feasibility of the proposed technique for LTO thin film electrode fabrication on temperature sensitive and mechanically flexible polyimide substrates. Fundamental data on possible electrode cyclability upon electrode bending was obtained by successful cycling of LTO flex-TFBs in statically bent condition. This study could initialize a new branch for facile manufacturing of flexible thin film battery cells

Fabrication and performance of Li4Ti5O12/C Li-ion battery electrodes using combined double flame spray pyrolysis and pressure-based lamination technique

Reduction of lithium-ion battery (LIB) production costs is inevitable to make the use of LIB technology more viable for applications such as electric vehicles or stationary storage. To meet the requirements in today's LIBcost efficiency, our current research focuses on an alternative electrode fabrication method, characterized by a combination of double flame spray pyrolysis and lamination technique (DFSP/lamination). In-situ carbon coatednano-Li4Ti5O12 (LTO/C) was synthesized using versatile DFSP. The as-prepared composite powder was then directly laminated onto a conductive substrate avoiding the use of any solvent or binder for electrode preparation. The influence of lamination pressures on the microstructure and electrochemical performance of the electrodes was also investigated. Enhancements in intrinsic electrical conductivity were found for higher lamination pressures. Capacity retention of highest pressurized DFSP/lamination-prepared electrode was 87.4%after 200 dis-/charge cycles at 1C (vs. Li). In addition, LTO/C material prepared from the double flame spray pyrolysis was also used for fabricating electrodes via doctor blading technique. Laminated electrodes obtained higher specific discharge capacities compared to calendered and non-calendered blade-casted electrodes due to superior microstructural properties. Such a fast and industrially compelling integrative DFSP/lamination tool could be a prosperous, next generation technology for low-cost LIB electrode fabrication