Understanding Interfacial Reactions in Lithium Batteries Using Cryogenic and In-situ Transmission Electron Microscopy

School of Energy and Chemical Engineering (Energy Engineering (Battery Science and Engineering))The excessive release of harmful materials from using fossil fuels into atmosphere has promoted serious environmental concerns and causes a critical threat to human species. Among the emitted harmful materials, carbon dioxide is regarded as a major greenhouse gas that directly contributes to global warming. It is urgent to convert dependence on fossil fuels into renewable energy sources and develop alternative devices using renewable energy. The Li-ion battery system is considering as a proper device for storing and using renewable energy efficiently. It is widely used for diverse portable electronic devices after introducing the first Li-ion batteries in 1991, however, new types of batteries still need to meet the growing requirement for high energy dense devices such as electric vehicles and large grid energy storage. Since current Li-ion batteries are approaching the maximum state of electrochemical performance including their capacities, it is necessary to attempt developing materials to break through the limit. Developing battery anode materials which have high specific capacity and low electrochemical potential is actively conducting to increase the energy density of batteries. Li metal is considered as one of possible candidates to replace conventionally used carbonaceous materials because it has the highest theoretical capacity (~ 3,860 mAh g-1) and shows the lowest electrochemical potential (~ -3.040 V vs. SHE) among anode materials. Tin oxide (SnO2) is also attracted a lot of attention as an anode material for the next generation battery system due to its resource abundance, low cost, and eco-friendly properties. It also shows a high theoretical specific capacity (~ 782 mAh g-1) and a relatively low electrochemical potential which are suitable for anode materials. To commercialize these two materials, it needs to resolve their intrinsic problems. Formation of dendrite, low Coulombic efficiency of Li metal anode and large volume expansion, low conductivity of SnO2 anode should be improved through engineering the materials. Most of the expressed problems appear at the interface which is a starting point of the reactions. If we can control the interfacial reactions by tuning of the materials to suppress the formation of dendrite or large volume expansion, we can take a further step to commercialize the next-generation materials. To engineer the interface of materials, it is important to understand the interfacial reactions even specific reactions at the tiny regions which can contribute to failure of the entire system. Transmission electron microscopy (TEM) analysis is necessary to observe the reactions occurred locally because X-ray photoelectron spectroscopy (XPS) or X-ray diffraction (XRD) which are widely used for the surface analysis give ensemble average data of the entire surface. Specifically, cryogenic TEM (cryo-TEM) analysis can give an opportunity to analyze the Li metal anode which is hard to observe by conventional TEM technique due to its sensitive property to air, moisture, and high energy of electron beam. Solid electrolyte interphase (SEI) that can be formed on the surface of Li metal anode is also possible to be observed by cryo-TEM technique. Furthermore, structural and morphological changes during reaction with Li ions can be monitored by in-situ TEM analysis. It is a powerful tool to understand degradation mechanism of SnO2 anode including volume expansion during the lithiation and delithiation process. First, with the cryo-TEM technique, I tried to reveal an exact nanostructure of SEI layer on Li metal anode depending on the different liquid electrolyte system. The electrical performance of Li metal batteries is affected a lot by the SEI layer that derived from diverse components of electrolyte. I would like to establish a proper way to design stable Li metal batteries by elucidating the correlation between the nanostructure of SEI layer and the performance of battery system. Second, with the in-situ TEM technique, I would like to compare electrochemical and mechanical properties of two specific coating layers which are polypyrrole (ppy) organic layer and manganese oxide (MnO2) inorganic layer for SnO2 anode. This result can provide options for choosing optimized coating layers to enhance the electrochemical performance of alloying anode materials which are suffered by low conductivity and large volume expansion. My research goal is to understand the different reaction mechanism depending on interfacial properties of high energy density battery materials. Revealing the exact effect of different interface layers on anode materials through the advanced TEM analysis can provide a new knowledge to design the superior battery system. Deep understanding and improving interfacial properties of high energy density materials that have underlying problems is essential to take full advantage of the electrochemical performance of the materials.ope

Unveiling the Role of PEO-Capped TiO2 Nanofiller in Stabilizing the Anode Interface in Lithium Metal Batteries

Lithium metal batteries (LMBs) will be a breakthrough in automotive applications, but they require the development of next generation solid-state electrolytes (SSEs) to stabilize the anode interface. Polymer-in-ceramic PEO/TiO2 nanocomposite SSEs show outstanding properties, allowing unprecedented LMBs durability and self-healing capabilities. However, the mechanism underlying the inhibition/delay of dendrite growth is not well understood. In fact, the inorganic phase could act as both a chemical and a mechanical barrier to dendrite propagation. Combining advanced in situ and ex situ experimental techniques, we demonstrate that oligo(ethylene oxide)-capped TiO2, although chemically inert toward lithium metal, imparts SSE with mechanical and dynamical properties particularly favorable for application. The self healing characteristics are due to the interplay between mechanical robustness and high local polymer mobility which promotes the disruption of the electric continuity of the lithium dendrites (razor effect)

The Role of Polymer and Inorganic Coatings to Enhance Interparticle Connections Diagnosed by In Situ Techniques

Surface coating on alloy anodes renders an effective remedy to tolerate internal stress and alleviate the side reaction with electrolytes for long-lasting reversible lithium redox reactions in lithium-ion batteries. However, the role of surface coating on the interparticle connections of alloy anodes remains not fully understood. Herein, we exploit real-time lithiation and mechanic measurement of SnO2 nanoparticles via in situ TEM with different coating layers, including conducting polymer polypyrrole and metal oxide MnO2. As a result, polypyrrole is more flexible to accommodate the volume expansion issue. More importantly, the polypyrrole coating layers offer a large contact area and strong adhesion force between the SnO2 nanoparticles, ensuring fast lithiation kinetics and high cycling stability. These observations provide new insight into how the interparticle connections of alloy anodes with diverse coating approaches can impact battery performance, shedding light on the practical processing of the alloy anode materials for high-energy Li-ion batteries

Electrochemo-Mechanical Properties of Red Phosphorus Anodes in Lithium, Sodium, and Potassium Ion Batteries

Red phosphorus (RP) is a promising anode material for alkali-ion batteries due to a high theoretical capacity at low potentials when alloying with lithium, sodium, and potassium. Most alloy anode materials display large volume changes during cycling, which can lead to particle fracturing, low Coulombic efficiency, loss of electrical contact, and ultimately poor cycle life. In this paper we outline, through comprehensive electrochemo-mechanical characterization and modeling of the cycling stresses, why RP can be cycled at high current densities without fracture. Application of in situ nanoindentation and powder compression allows for measurement of the elastic, plastic, and fracture properties of RP. In situ transmission electron microscopy observation with extreme conditions (anisotropic ion diffusion and high current density) was used to validate the model, observing no catastrophic failure of RP particles. Electrochemo-mechanical characterization with geometry and stress modeling allows for predictions to be made for application of RP in alkali-ion batteries

Na/Al Codoped Layered Cathode with Defects as Bifunctional Electrocatalyst for High-Performance Li-Ion Battery and Oxygen Evolution Reaction

The rational design of bifunctional electrocatalyst through simple synthesis with high activity remains a challenging task. Herein, Na/Al codoped Li-excess Li-Ru-Ni-O layered electrodes are demonstrated with defects/dislocations as an efficient bifunctional electrocatalyst toward lithium-ion battery (LIB) and oxygen evolution reaction (OER). Toward LIB cathode, specific capacity of 173 mAh g(-1) (0.2C-rate), cyclability (>95.0%), high Columbic efficiency (99.2%), and energy efficiency (90.7%) are achieved. The codoped electrocatalyst has exhibited OER activity at a low onset potential (270 mV@10 mA cm(-2)), with a Tafel slope 69.3 mV dec(-1), and long-term stability over 36 h superior to the undoped and many other OER electrocatalysts including the benchmark IrO2. The concurrent doping resides in the crystal lattice (where Na shows the pillaring effect to improve facile Li diffusion), Al improves the stabilization of the layered structure, and defective structures provide abundant active sites to accelerate OER reactions

Unveiling interfacial dynamics and structural degradation of solid electrolytes in a seawater battery system

Understanding the interfacial reaction between a solid electrolyte and reactive species is of vital importance for the development of various battery systems. In particular, the interaction between the solid electrolyte, aqueous media, and dissolved gaseous phases at catalysts affects reaction kinetics and hybrid battery performance. These effects are concentrated at the interfaces that the three kinetic fluxes simultaneously encounter, which we refer to as a "triple-flux interface (TFI)". The TFI consists of a Na+ super ionic conductor as a solid electrolyte, a seawater catholyte, and dissolved gaseous phases on a catalytic carbon felt. Here, we provide insights into the interfacial dynamics during the operation of a hybrid Na-seawater battery under harsh operating conditions, including at high current densities and in acidified seawater solutions. We discover that high current densities and the permeation of acid compounds during the charge process cause the irreversible extraction of Na+, crevasse-like brokenness, and unavoidable phase distortion from the surface that is in contact with seawater. The investigation of the factors in terms of chemical potentials of the charge carriers would provide insight into designs for high-powered seawater battery systems

Near zero-strain silicon oxycarbide interphases for stable Li-ion batteries

We investigate silicon oxycarbide nanotubes that incorporate Si, SiC, and silicon oxycarbide phases, which exhibit near zero-strain volume expansion, leading to reduced electrolyte decomposition. The composite effectively accommodates the formation of c-Li15Si4, as validated by in situ TEM analyses and electrochemical tests, thereby proposing a promising solution for Li-ion battery anodes. Silicon oxycarbide, as confirmed by in situ TEM, exhibits near-zero volume expansion strain during lithiation, resulting in reduced electrolyte uptake