Battery Mechanisms & Materials Design

sarbajit.banerjee@psi.ch 
+41563103724

The ability to grow proteins as single crystals paved the way for the determination of high-resolution structures, enabling the development of detailed structure-function relationships and the design of targeted approaches to modulate protein function by binding of small molecules in specific pockets (drug design) or by specific modifications of amino acid sequences. We have adopted this powerful approach to intercalation electrodes used for energy storage. Much of the current knowledge of diffusion pathways is derived from mesoscale mapping of lithiation inhomogeneities using electron and X-ray microscopy at larger length scales (providing particle- and electrode-level information). We have developed single crystal intercalation chemistry and single crystal electrochemistry, as a transformative new lens that provides atomic resolution views of ion diffusion pathways in solids. Single-crystal diffraction data provide a clear view of Li-ion site preferences and occupancies as a function of discharge depth, which is further mapped using operando synchrotron X-ray diffraction.

We have demonstrated through multiple examples the spectacular implications of the reconfigured atomic connectivity accessible in metastable solid-state compounds. We have unlocked a distinctive new energy storage mechanism in which additional Li-ions are accommodated by cation rearrangement rather than by displacive phase transformations. We are interested in non-equilibrium physics approaches to surf free energy landscapes in search of bespoke new compounds that afford tunable ion diffusion pathways and new function.

Despite their rapid emergence as the dominant paradigm for electrochemical energy storage, the full promise of lithium-ion batteries has yet to be fully realized, in part due to challenges in adequately addressing common degradation mechanisms. Positive electrodes of Li-ion batteries store ions in interstitial sites based on redox reactions throughout their internal volume. However, variations in the local concentration of inserted Li-ions and inhomogeneous intercalation-induced structural changes generate significant stress. Such stresses can accumulate and cause significant delamination and transgranular/­intergranular fracture in typically brittle oxide materials under continuous electrochemical cycling. Through an extensive program of operando synchrotron X-ray diffraction, spectroscopy, and imaging and in collaboration with colleagues at TU Darmstadt, we have investigated the coupling between electrochemistry, mechanics, and geometry spanning surface reaction, solid-state diffusion, phase nucleation, and phase transformation processes in intercalation electrodes. We are seeking to identify design rules and tunable material design parameters that can be used to modulate the kinetics and thermodynamics of intercalation phenomena, ranging from atomistic and crystallographic material design principles (based on alloying, polymorphism, and pre-intercalation) to mesoscale structuring of electrode architectures (through control of crystallite dimensions and geometry, curvature, and external strain). Scale-bridging characterization and modeling, along with materials design, holds promise for deciphering design principles and modulating multiphysics couplings to substantially modify intercalation phase diagrams in a manner that unlocks greater useable capacity and enables alleviation of chemo-mechanical degradation mechanisms.

Magnesium batteries have attracted considerable attention owing to the possibility of accessing higher energy densities and for overcoming the safety problems of lithium batteries given the supposedly non-dendrite forming nature of magnesium. We are exploring galvanic replacement strategies to develop intermetallics and nanocomposites for magnesium batteries. 

Demand for lithium is expected to quadruple at the end of the decade. Traditional geological reserves are unlikely to meet the anticipated gap, thus unconventional sources of lithium will need to be utilized, setting the stage for fierce competition for perhaps the most critical of mineral resources required for the energy transition. Direct Lithium Extraction refers to the umbrella of technologies being developed to access lithium from unconventional sources. Electrochemical extraction offers significant promise for its selectivity and low operating cost when coupled with renewable energy. We are exploring materials and process design considerations for electrochemical extraction of lithium from aqueous sources. We are particularly interested in specific strategies for improving capacity and selectivity for electrochemical lithium extraction based on materials design across length scales. Strategies range from site-selective modification of insertion hosts to controlled tortuosity of ion diffusion pathways in porous electrode architectures. Electrochemical lithium extraction from unconventional sources stands poised to be a linchpin of a sustainable economy when coupled with cleaning of wastewater, hydrogen generation, and recovery of ancillary critical metals.

Building artificial neurons and synapses is critical to achieving the promise of energy efficiency and acceleration envisioned for brain-inspired information processing. Emulating the spiking behavior of biological neurons in physical materials requires precise programming of conductance non-linearities such as metal—insulator transitions. Strong correlated solid-state compounds exhibit pronounced nonlinearities arising from dynamic electron-electron and electron-lattice interactions. However, detailed understanding of atomic rearrangements and their implications for electronic structure remains obscure. To achieve ultra-fast and low-energy switching, we are exploring the mechanistic origins of metal-insulator transitions and instabilities in correlated transition metal oxides to find electronic transitions maximally disentangled from structural transformations. We seek to design, discover, and manipulate new correlated electron oxide compounds by controlling mechanisms that bring a correlated oxide to the edge of criticality and by learning to systematically couple the material's degrees of freedom with external fields to modulate these mechanisms.

Grid-level energy storage requires a diversity of battery chemistries so as to alleviate stresses on current mineral supply chains and to provide safer alternatives to Li-ion batteries. Anion batteries, which implement fluoride and chloride anions as charge carriers have received considerable attention as an orthogonal alternative with the potential to alleviate supply chain challenges and precluding the need for metal electrodeposition. Conversion electrodes have shown promise but challenges remain with regards to high operation temperatures, large volume changes, and sluggish anion diffusion, which result in overall poor cycle life and rate performance. We are focused on the design of insertion-based fluoride electrode materials where the integrity of the structural framework of insertion electrodes is preserved upon F-ion insertion/deinsertion and to understand anion diffusion mechanisms in general in electrodes and solid electrolytes.