Applying KPFM on all solid-state battery interfaces research
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Abstract
All-solid-state batteries as potential next generation batteries are attracting increasingly more attention. Compared to liquid electrolyte-based lithium-ion batteries, all-solid-state batteries have obvious advantages, including non-flammability and higher energy density. However, the complex interfacial problems in all-solid-state batteries limit their further applications. Among different kinds of interfacial problems, the most challenging ones are lithium dendrites originating from anode lithium electrode – solid electrolyte interface, grain boundaries in solid electrolytes, and space charge layers at electrode – solid electrolyte interfaces. The former leads to short-circuiting of batteries while the latter adds interfacial resistance. More importantly, compared to other interfacial problems, those two problems are less studied because of a lack of suitable methods. In recent years, the development of Kelvin probe force microscopy (KPFM), which is a non-destructive technology with high spatial resolution, has provided us with new possibilities to study those interfacial problems. Using KPFM, I compare changes in measured contact potential difference (CPD) in batteries under different working condition. In my PhD thesis, I apply advanced operando KPFM to track the origin of those interfacial problems in all-solid-state batteries.
For metals and semiconductors, the CPD signal measured by KPFM and its change are known to be related to the work function and to changes in the work function of the sample locally. However, for mixed-ionic-electronic conductor (MIEC) such as solid electrolyte in all-solid-state batteries, the measured CPD signal and its changes require deeper understanding because of the existence of mobile ions and a poor electron conductivity. In Chapter 3, I theoretically outline that the measured CPD change on MIEC in a non-equilibrium state results from Galvanic (electrical) potential change. I verify this conclusion by KPFM measurements on a Hebb-Wagner cell under different polarization states.
In Chapter 4, I explore the origin of the formation of lithium dendrites at Li6.25Al0.25La3Zr2O12 (LLZO) grain boundaries. LLZO is a solid electrolyte with a high lithium ionic conductivity. Operando KPFM reveals a drop of electrical potential at grain boundaries close to the lithium anode (negative) electrode under working conditions. I further use time-resolved electrostatic force microscopy (tr-EFM) and electron beam irradiation on LLZO solid electrolyte. Those results reveal that the electrical potential drop at grain boundaries results from electrons being trapped at grain boundaries. The different electric conduction property at LLZO grain boundaries compared with LLZO bulk explains the preferential lithium dendrites formation at LLZO grain boundaries.
In Chapter 5, I quantitatively reveal the role of a space charge layer on the all-solid-state battery interfacial resistance. Space charge layer can only be investigated for thin film battery with well-defined interfaces. The thin film battery consists of metallic lithium as the anode, Li3PO4 (LPO) as the solid electrolyte and LiCoO2 (LCO) as the cathode. Operando KPFM measurements on the thin film battery cross-section show that a space charge layer only exists at the LPO|LCO interface, which changes with battery voltage. With the help of operando nuclear analysis reaction (NRA), I prove that the space charge layer evolution at LPO|LCO interface results from lithium ion gradually accumulating on the LCO side while lithium ion gradually depletes on the LPO side, with an increase in battery voltage. Finally, based on those findings and in-situ electrochemical impedance spectroscopy (EIS), I calculate that space charge layer resistance at the LPO|LCO interface has a maximum value around 33.82 – 38.29 Ω cm2 when the battery voltage is 4.3 V, in the working voltage window of 3.0 V to 4.3 V. That accounts for approximately 80% of the whole battery interfacial resistance.