In sharp contrast to molecular synthesis, materials synthesis is generally presumed to lack selectivity. The few known methods of designing selectivity in solid-state reactions have limited scope, such as topotactic reactions or strain stabilization. This contribution describes a general approach for searching large chemical spaces to identify selective reactions. This novel approach explains the ability of a nominally "innocent" Na 2 CO 3 precursor to enable the metathesis synthesis of single-phase Y 2 Mn 2 O 7 : an outcome that was previously only accomplished at extreme pressures and which cannot be achieved with closely related precursors of Li 2 CO 3 and K 2 CO 3 under identical conditions. By calculating the required change in chemical potential across all possible reactant-product interfaces in an expanded chemical space including Y, Mn, O, alkali metals, and halogens, using thermodynamic parameters obtained from density functional theory calculations, we identify reactions that minimize the thermodynamic competition from intermediates. In this manner, only the Na-based intermediates minimize the distance in the hyperdimensional chemical potential space to Y 2 Mn 2 O 7 , thus providing selective access to a phase which was previously thought to be metastable. Experimental evidence validating this mechanism for pathway-dependent selectivity is provided by intermediates identified from in situ synchrotron-based crystallographic analysis. This approach of calculating chemical potential distances in hyperdimensional compositional spaces provides a general method for designing selective solid-state syntheses that will be useful for gaining access to metastable phases and for identifying reaction pathways that can reduce the synthesis temperature, and cost, of technological materials.

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While it is expected that inhomogeneity negatively affects battery performance, a quantitative understanding of the influence of inhomogeneity has remained elusive due to the difficulty of measuring it in a precise and rapid manner. Here, the ability of high-energy synchrotron X-rays to effectively probe the inhomogeneity in battery cathode films is demonstrated both for fundamental studies of single-layer cathode films and for improving manufacturing processes for industrially relevant multilayer stacks. High-energy lateral mapping studies were performed for very high energy density batteries (∼300 Wh/kg) made from NMC622 cathodes and Li metal anodes, where NMC622 denotes Li-(Ni 0.6 Mn 0.2 Co 0.2 )O 2 . It was first demonstrated for a multilayer pouch cell (7 layers, ∼3 mm thick) that both local and long-range variations in the NMC loading can be precisely quantified, allowing the quality of the coating process to be assessed. Next, it was shown that for a single cathode layer extracted from a pouch cell battery cycled to failure that local variations in the cathode state-of-charge (SOC) can be mapped with a sensitivity of about 0.1%. In this manner it was possible to identify three hot spots in which the local performance was much worse than for the rest of the cell as well as to gain insights into the specific failure mechanisms affecting both these local regions and the cell as a whole.

Wh kg −1 , and a life of 1000 cycles. [3] To meet these targets, large improvements in specific energy are critically needed. One promising route to increase the specific energy of batteries is to switch to Li metal anodes. Batteries with Li metal serving as the anode can deliver exceptionally high energy densities, [4] because of their advantages in having the lowest redox potential (−3.04 V vs standard hydrogen electrode) and a high theoretical specific capacity (3860 mAh g −1). Studies suggest that the energy density of batteries can be doubled or tripled relative to present technologies using Li metal anodes. [2] However, the practical applications of Li metal anodes are severely hindered by their rapid failure. This failure is primarily driven by parasitic reactions between Li metal and electrolyte. The common signatures of failure are the formation of dead/isolated Li, which is electrically disconnected from the bulk Li metal during uneven stripping on the anode surface, [5,6] the dendritic or mossy Li, which is induced by inhomogeneous distributions of space charge [7] on the anode surface which arise from uneven anode surfaces or cracks in the solid electrolyte interphase (SEI). [8] Rechargeable Li metal batteries (LMBs) cannot be deployed in automotive applications unless lifetime limitations are overcome. Numerous strategies have been explored to successfully extend the lifetime of LMBs. These include developing novel electrolytes that produce more stable SEI layers, creating artificial SEI layers on the Li metal surface, and applying external pressures. [9] While these and other methods provide pathways for improving the lifetime of batteries, none of them have succeeded in meeting industrial performance targets. In order to accelerate the development of viable LMB technologies, it is critically important to develop diagnostic techniques that can rapidly and easily identify the primary cause of battery failure and that can make useful predictions about the lifetime of a cell based on data collected early in its lifetime (instead of waiting long times for cells to reach their actual failure point before getting feedback). There are three commonly seen battery failure mechanisms for LMBs that reduce the capacity of a cell over its lifetime. [10] Lithium (Li) metal serving as an anode has the potential to double or triple stored energies in rechargeable Li batteries. However, they typically have short cycling lifetimes due to parasitic reactions between the Li metal and electrolyte. It is critically required to develop early fault-detection methods for different failure mechanisms and quick lifetime-prediction methods to ensure rapid development. Prior efforts to determine the dominant failure mechanisms have typically required destructive cell disassembly. In this study, non-destructive diagnostic method based on rest voltages and coulombic efficiency are used to easily distinguish the different failure mechanismsfrom loss of Li inventory, electrolyte depletion, and increased cell impedancewhich are deep...