Toward a Synergistic Optimization of Porous Electrode Formulation and Polysulfide Regulation in Lithium-Sulfur Batteries.

The use of functional materials is a popular strategy to mitigate the polysulfide-induced accelerated aging of lithium-sulfur (Li-S) batteries. However, deep insights into the role of electrode design and formulation are less elaborated in the available literature. Such information is not easy to unearth from the existing reports on account of the scattered nature of the data and the big dissimilarities among the reported materials, preparation protocols, and cycling conditions. In this study, model functional materials known for their affinity toward polysulfide species, are integrated into the porous sulfur electrodes at different quantities and with various spatial distributions. The electrodes are assembled in 240 lithium-sulfur cells and thoroughly analyzed for their short- and long-term electrochemical performance. Advanced data processing and visualization techniques enable the unraveling of the impact of porous electrodes' formulation and design on self-discharge, sulfur utilization, and capacity loss. The results highlight and quantify the sensitivity of the cell performance to the synergistic interactions of catalyst loading and its spatial positioning with respect to the sulfur particles and carbon-binder domain. The findings of this work pave the road for a holistic optimization of the advanced sulfur electrodes for durable Li-S batteries.

Solution-gel-based surface modification of LiNi0.5Mn1.5O4-δ with amorphous Li-Ti-O coating.

LNMO (LiNi0.5Mn1.5O4-δ) is a high-energy density positive electrode material for lithium ion batteries. Unfortunately, it suffers from capacity loss and impedance rise during cycling due to electrolyte oxidation and electrode/electrolyte interface instabilities at high operating voltages. Here, a solution-gel synthesis route was used to coat 0.5-2.5 µm LNMO particles with amorphous Li-Ti-O (LTO) for improved Li conduction, surface structural stability and cyclability. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis coupled with energy dispersive X-ray (EDX) showed Ti-rich amorphous coatings/islands or Ti-rich spinel layers on many of the LTO-modified LNMO facets, with a thickness varying from about 1 to 10 nm. The surface modification in the form of amorphous islands was mostly possible on high-energy crystal facets. Physicochemical observations were used to propose a molecular mechanism for the surface modification, combining insights from metalorganic chemistry with the crystallographic properties of LNMO. The improvements in functional properties were investigated in half cells. The cell impedance increased faster for the bare LNMO compared to amorphous LTO modified LNMO, resulting in Rct values as high as 1247 Ω (after 1000 cycles) for bare LNMO, against 216 Ω for the modified material. At 10C, the modified material boosted a 15% increase in average discharge capacity. The improvements in electrochemical performance were attributed to the increase in electrochemically active surface area, as well as to improved HF-scavenging, resulting in the formation of protective byproducts, generating a more stable interface during prolonged cycling.

A limitation map of performance for porous electrodes in lithium-ion batteries.

Driven by expanding interest in battery storage solutions and the success story of lithium-ion batteries, the research for the discovery and optimization of new battery materials and concepts is at peak. The generation of experimental (dis)charge data using coin cells is fast and feasible and proves to be a favorite practice in the battery research labs. The quantitative interpretation of the data, however, is not trivial and decelerates the process of screening and optimization of electrode materials and recipes. Here, we introduce the concept of polarographic map and demonstrate how it can be leveraged to quantify the contribution of different non-equilibrium phenomena to the performance limitation and total polarization of a lithium-ion cell. We showcase the accuracy and diagnostic power of this approach by preparing and analyzing the electrochemical performance of 54 sets of LiNixMnyCo1-x-yO2 electrodes with different formulations and designs discharged in a range of 0.2C-5C.

Dielectric Barrier Discharge (DBD) Plasma Coating of Sulfur for Mitigation of Capacity Fade in Lithium-Sulfur Batteries.

Sulfur particles with a conductive polymer coating of poly(3,4-ethylene dioxythiophene) "PEDOT" were prepared by dielectric barrier discharge (DBD) plasma technology under atmospheric conditions (low temperature, ambient pressure). We report a solvent-free, low-cost, low-energy-consumption, safe, and low-risk process to make the material development and production compatible for sustainable technologies. Different coating protocols were developed to produce PEDOT-coated sulfur powders with electrical conductivity in the range of 10-8-10-5 S/cm. The raw sulfur powder (used as the reference) and (low-, optimum-, high-) PEDOT-coated sulfur powders were used to assemble lithium-sulfur (Li-S) cells with a high sulfur loading of ∼4.5 mg/cm2. Long-term galvanostatic cycling at C/10 for 100 cycles showed that the capacity fade was mitigated by ∼30% for the cells containing the optimum-PEDOT-coated sulfur in comparison to the reference Li-S cells with raw sulfur. Rate capability, cyclic voltammetry, and electrochemical impedance analyzes confirmed the improved behavior of the PEDOT-coated sulfur as an active material for lithium-sulfur batteries. The Li-S cells containing optimum-PEDOT-coated sulfur showed the highest reproducibility of their electrochemical properties. A wide variety of bulk and surface characterization methods including conductivity analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and NMR spectroscopy were used to explain the chemical features and the superior behavior of Li-S cells using the optimum-PEDOT-coated sulfur material. Moreover, postmortem [SEM and Brunauer-Emmett-Teller (BET)] analyzes of uncoated and coated samples allowed us to exclude any significant effect at the electrode scale even after 70 cycles.

Atomic Layer Deposition of Nitrogen-Doped Al Phosphate Coatings for Li-Ion Battery Applications.

In situ nitrogen doping of aluminum phosphate has been investigated in two different plasma-enhanced atomic layer deposition (PE-ALD) processes. The first method consisted of the combination of trimethyl phosphate plasma (TMP*) with a nitrogen plasma and trimethyl aluminum (TMA), that is, TMP*-N2*-TMA. The second method replaces TMP* with a diethylphosphoramidate plasma (i.e., DEPA*-N2*-TMA) of which the amine group could further aid nitrogen doping and/or eliminate the need for a nitrogen plasma step. At a substrate temperature of 320 °C, the TMP*-based process showed saturated growth (0.8 nm/cycle) of a nitrogen-doped (approximately 8 atom %) Al phosphate, while the process using DEPA* showed a similar amount of nitrogen but a significantly higher growth rate (1.4 nm/cycle). In the latter case, nitrogen doping could also be achieved without the nitrogen plasma, but this leads to a high level of carbon contamination. Both films were amorphous as-deposited, while X-ray diffraction peaks related to AlPO4 appeared after annealing in a He atmosphere. For high coating thickness (>2 nm), a significant increase in the Li-ion transmittance was found after nitrogen doping, although the coating has to be electrochemically activated. At lower thickness scales, such activation was not needed and nitrogen doping was found to double the effective transversal electronic conductivity. For the effective transversal ionic conductivity, no conclusive difference was found. When a lithium nickel manganese cobalt oxide (NMC) powder is coated with one ALD cycle of N-doped Al phosphate, the rate capability and the energy efficiency of the electrode improves.

In Situ Mechanical Analysis of the Nanoscopic Solid Electrolyte Interphase on Anodes of Li-Ion Batteries.

The interfacial decomposition products forming the so-called solid-electrolyte interphase (SEI) significantly determine the destiny of a Li-ion battery. Ultimate knowledge of its detailed behavior and better control are required for higher rates, longer life-time, and increased safety. Employing an electrochemical surface force apparatus, it is possible to control the growth and to investigate the mechanical properties of an SEI in a lithium-ion battery environment. This new approach is here introduced on a gold model system and reveals a compressible film at all stages of SEI growth. The demonstrated methodology provides a unique tool for analyzing electrochemical battery interfaces, in particular in view of alternative electrolyte formulations and artificial interfaces.

Mesoscopic modeling of Li insertion in phase-separating electrode materials: application to lithium iron phosphate.

A simple mesoscopic model is presented which accounts for the inhomogeneity of physical properties and bi-stable nature of phase-change insertion materials used in battery electrodes. The model does not include any geometric detail of the active material and discretizes the total active material domain into meso-scale units featuring basic thermodynamic (non-monotonic equilibrium potential as a function of Li content) and kinetic (insertion-de-insertion resistance) properties. With only these two factors incorporated, the model is able to simultaneously capture unique phenomena including the memory effect observed in lithium iron phosphate electrodes. The analysis offers a new physical insight into modeling of phase-change active materials which are of special interest for use in high power Li-ion batteries.