Typical commercial batteries use lithium oxides or phosphates, and graphite as the positive and negative electrodes, respectively. These electrodes are reaching their intrinsic limits in terms of energy storage capacity. The next battery generation requires increased energy density and/or reduced production costs. They must also sustain multiple cation insertions/de-insertions without sacrificing their structural integrity. The structural evolution of the electrode with electrochemical cycling must also be fully understood. Our research investigates new and existing electrode materials prepared using different techniques (solid-state, hydrothermal, or sol-gel). We use X-ray diffraction/scattering to identify the structural/morphological effects (single vs. two phases, volume changes) of the electrode materials on electrochemical properties.
Vital information about the chemical composition’s impact on the cationic arrangements of newly developed Li-rich layered oxides (Li1+xM1-xO2) and spinels is sought (X-ray diffraction/scattering and solid-state NMR) to assess both short- and long-range orders. XPS is also used to find crucial information about the role of the electrode/electrolyte interfaces, as a function of the electrolyte composition and oxidation state of the different elements in the materials. We correlate the XPS results by analysis of the gaseous products formed at high voltage.
Solid and polymer electrolytes
Li/Na glassy electrolytes are solid superionic conductors who are investigated by our team. These electrolytes present some advantages: they are more conductive than their crystalline counterparts, form no grain boundaries, and form films easily. However, accurate structural and morphological characterization of these electrolytes is extremely challenging and requires the highly specialized infrastructure requested. Short- and mid-range ordering in non-periodic systems of glassy electrolytes is evaluated (PDF and NMR). The specific atomic environments will be evaluated through IR and Raman spectroscopies, and compared to the homologous crystal structures.
Owing to its conductivity, polyethylene oxide (PEO) doped with lithium salt is the polymer electrolyte benchmark. However, its mechanical behavior is not entirely satisfactory for use in efficient batteries. The mechanical strength of PEO can be increased through block copolymerization with a rigid monomer. Block copolymers are advantageous by virtue of being less crystalline, which result in consistently higher conductivities. We are exploring triblock polymers consisting of a PEO core flanked by two partially-ionized polystyrene lithium sulfonate blocks. We want to exploit controlled radical polymerization methods to prepare BAB triblocks to accurately evaluate the effect of degrees of polymerization on conductivity, Tg, crystallinity, and morphology. Our NMR, DSC and AFM are valuable tools to advance in this research axis.
Passivation of electroactive materials
A passivation layer is known to form in liquid electrolytes at the surface of electroactive materials operating in the electrochemical instability windows of these electrolytes. The resulting layer, often called solid electrolyte interphase (SEI), is known to play an important role in battery operation, cyclability/life, and device safety.
Details of SEI layer formation and evolution, composition, and stability of emerging systems like superconcentrated aqueous or organic electrolytes and new active materials are ill-defined. The ISIO platforms unlock this opportunity for us by providing the means to accurately evaluate the SEI under battery operating conditions. In fact, advanced techniques at MAPLES like SECM, EQCM-D, and ESPR, are helping us to better understand the mechanistic aspects of SEI formation. The gaseous products associated with the SEI formation is addressed through atmospheric pressure ionization gas chromatograph. SEI formation at the interfaces of more complex thin nanocomposite films of metal oxide formed by atomic layer deposition and pulsed layer deposition are of interest for future-generation batteries and pseudocapacitors. The tangible outcome will be controlled SEI growth.
Binders for next-generation batteries
Currently, polyvinylidene difluoride (PVDF) is the most-commonly used binder in the formulation of Li-ion composite electrodes. However, PVDF requires the use of a toxic solvent to be processed. In light of the significant growth in Li-ion batteries anticipated in the near future, there is an urgent need for alternative clean and solvent-free processes. PVDF is also known to be inappropriate for the development of silicon electrodes, which is a promising potential negative electrode to replace graphite in next-generation Li-ion batteries. A variety of strategies, including the covalent grafting of binder-like polymers to silicon, have resulted in breakthroughs in improving the cycling performance of silicon-based anodes. Nonetheless, substantial work is still required to optimize the electrode and electrolyte formulations, and raise silicon-electrodes performance to meet stringent commercial performance requirements.
We are working to enhance the cycling stability and performance of silicon-based anodes by covalently grafting reversible mechanically deformable polymers on silicon particles. The morphology examined by SEM and porosity by Hg porosimeter of the resulting grafted polymer may be controlled by using well-developed diazonium grafting chemistry and controlled polymerization of polyacrylic acid. The stability of the grafted polymers can be assessed by AFM-IR and any morphological changes upon cycling and mechanical resistance can be evaluated by AFM.
All solid-state batteries (ASSBs)
Although liquid electrolytes are used in current Li-ion batteries, their SEI side reactions represent a major performance issue. Substantial gains may be achieved by replacing the liquid electrolytes with solid electrolytes. Tackling the solid/solid interface issue must go beyond the conventional inclusion of powdered electrolytes in the composite electrode preparation process. New chemical approaches (thin layer coating) are explored to engineer the interfaces for compatibility with electronic conductors and electrolytes. Our research is focusing at coating strategies in order to improve lithium ion conductivity which is crucial for enhancing the charge-discharge capacity of the cells under high current densities.
Two coating processes are explored: a physical (atomic layer deposition) and a chemical (sol-gel) both by using carefully-selected precursors. The coatings’ electrochemical efficiency is determined in a standard liquid electrolyte cell via impedance and cycling measurements, and through comparison with uncoated materials by using our XRD and XPS.
Cell polarization is also caused by grain boundary resistance between the different compounds as a result of the sintering process. Alternative techniques such as spark plasma sintering is explored to reduce the electrochemical blocking interfaces. The interfacial phenomena upon cycling at different temperatures is also probed by following the structural evolution by XRD and redox reactions (XPS under voltage bias, which is possible since ASSBs do not operate with volatile solvents).