In-Situ Cycling in Liquid Electrochemical TEM of FIB Prepared Semi All Solid State 2-D Micro-Battery

Abstract

All solid state Li-ion micro-battery is a promising candidate to power miniaturized sensors for Internet of things (IOT) and other electronic devices. The LiNi0.5Mn1.5O4 spinel (LNMO) is a potential positive electrode material for Li-ion micro-batteries offering a theoretical capacity of 147 mAh/g (65 µAh/cm2/µm for a bulk density of 4.47 g/cm3) and operates up to now at the highest potential (around 4.7 V vs. Li/Li+) [1]. Fabricated micro-battery has a stacking of 4 layers sandwiched on each other [from bottom to top-Si (0.385mm)/ Al2O3 (100 nm)/ Pt (30 nm) / LNMO (400 nm)] where Al2O3 and Pt are deposited by ALD and LNMO is deposited by PVD (Fig1a). 100% theoretical capacity was achieved on the first discharge (till 4.4V vs. Li+/Li) and 80% capacity still retained even after 300 cycles. Further thin film is characterized by Raman and XRD to confirm the phase of LNMO.Charging and discharging of Li-ion batteries can result in significant transformation of the cathode structure. It is crucial to know which structural changes occur at the atomic scale to understand the evolution of the electrochemical performance and degradation routes of the different cathode materials. The in situ cycling in liquid TEM has given us an opportunity to enter into the battery so as to spot the slightest textural, chemical or structural modifications of the electrode materials resulting in important advances in knowledge on electrochemical energy storage [2-3].Here, our approach is based on the cycling a semi all-solid-state battery Si/Al2O3/Pt/LNMO inside the TEM using liquid-electrochemical cell (Fig1b) with conventional liquid electrolyte (LiClO4, EC: DMC). The Pt current collector of semi all solid state micro-battery is connected to the Pt part of reference electrode on the e-chips used for TEM study (Fig 1c). First, using FIB preparation technique, we sliced a full 2-D “micro-battery” making it as thin to observe/analyse by TEM techniques. Then, we modified the micro-battery FIB lamellar design using FIB-SEM tool to get good electrical contact and reduced polarisation. Several technological problems have to be overcome in the process. For the instance, it is mandatory to obtain a good electrical contact between the Pt reference electrode of e-chip and Pt of FIB lamellar, which is achieved by making a Pt bridge between them. 4-Probe electrical conductivity performed locally confirms the good electrical conductivity between the different parts of FIB lamellar and Pt reference electrode of e-chip.The final modified version of FIB lamellar of semi all solid state 2D micro-battery [from bottom to top-Si (0.385mm)/ Al2O3 (100nm)/ Pt (630nm) / LNMO (400nm)/ Pt (2µm)] is then cycled inside the liquid electrochemical TEM holder in potential window of 4.1 V-4.8 V Vs Li+/Li. The flow of the electrolyte (LiClO4 EC:DMC 1:1) inside the TEM holder was further controlled by microfluidic controller with the flow rate of 2 µL/min. CV was recorded at a sweep rate of 0.1mV/s and two plateaus at 4.4 V and 4.6 V was observed corresponding to Ni2+/3+ and Ni3+/4+ oxidation respectively (Fig 1d).The semi all solid 2-D micro-battery was further studied using ASTAR (Automatic crystal orientation and phase mapping) device [5] to perform the phase mapping of two possible crystalline structures i.e. ordered phase and disordered phase of LiMn1.5Ni0.5O4 spinel material along with some Ni rich impurities LixNi1-xO [4].The orientation mapping was also performed to gain some information regarding grain boundaries (Fig 1e-insert electron diffraction pattern), grain size distribution (maximum grains of less than 10 nm in size) and disorientation between the grains ( disorientation of 55o between the two selected grains) etc. A region of interest of 1.2 µm2 was chosen for the scanning area with a step size of 5 nm. Several TEM analyses were also made such as HRTEM, EELS, EDX and STEM-HAADF at high resolution on pristine sample as well as post-mortem sample (after cycling). Thanks to ASTAR technique to provide us information regarding the epitaxial effect between the stacking of different layers (observed till few nm of thickness), porosity , morphological and structural changes between pristine and after cycling.