The energy density of battery systems is limited largely by the electrochemical window of the electrolyte. Herein, the combined thermodynamic and kinetic effects of mechanically induced metastability are shown to greatly widen the operational voltage window of solid‐state batteries based on ceramic‐sulfide electrolytes. Solid electrolyte voltage stability up to 10 V is achieved with minimal degradation, far beyond the capability of organic liquid electrolytes. Furthermore, combined experiment, ab initio computation, and theoretical modeling identify the nature of mechanically constrained Li10GeP2S12 decomposition both within the bulk and at interfaces with cathode materials at very high voltages. Previously unclear kinetic processes are identified that, when properly implemented, can potentially allow solid‐state full cells with remarkably high operational voltages.
Among the families of solid electrolytes, sulfides retain the highest ionic conductivity. [10][11][12][13][14] Sulfide glass solid electrolytes [15,16] and glass-ceramic solid electrolytes [17][18][19] have demonstrated ionic conductivities on the order of 0.1 − 1 mS cm −1 and above 1 mS cm −1 , respectively. The ceramic-sulfide electrolytes, most notably Li 10 GeP 2 S 12 (LGPS) and Li 10 SiP 2 S 12 (LSPS), are particularly promising as they maintain exceptionally high ionic conductivities.LGPS was one of the first solid electrolytes to reach ionic conductivities comparable with liquid electrolytes [20] at 12 mS cm −1 , only to then be displaced by LSPS [10] which achieved the highest reported ionic conductivity of 25 mS cm −1 . However, despite these superior ionic conductivities, the ceramicsulfide family has been plagued by reports of narrow electrochemical stability windows [11,13,21] and interfacial reactions with common electrode materials. [4,11,22] Moreover, the reported electrochemical stability windows of ceramic-sulfides suffer from substantial inconsistencies. Several works, both computational and experimental, have shown that the ceramic-sulfides are only stable in the narrow voltage window on the order of 1.7-2.1 V versus lithium, [11][12][13]21] which is the correct general thermodynamic prediction. Many others, however, have experimentally found that the upper voltage limit can reach in excess of 4-6 V versus lithium. [10,20,23,24] A consolidated understanding of these findings is needed in order to establish design principles for practical ceramic-sulfide batteries.In this work, we develop a generalized thermodynamic theory that unifies these disparate findings and, hence, provides the unique design principle through mechano-electrochemical effect for ceramic-sulfide-based solid-state batteries. Expanding upon our previous work, [25] in which core-shell morphologies were used to widen the voltage window of LSPS, we derive a generalized strain stabilization model that indicates at which voltages strain-induced stabilization can lead to metastability of the ceramic-sulfide phases. A mean-field solution to our generalized strain model recovers our previous model [25] and is shown to be a lower limit on the strain induced stability. The second solution we explore, a nucleation or inclusion decay, is shown to provide a greater capability for stabilization. Note that our current and previous [25] understanding forms a general theoretical framework for the design of ceramic electrolyte with widened voltage stability, which is not limited to any particular design strategy, such as the core-shell morphology of Ceramic-sulfide solid electrolytes are a promising material system for enabling solid-state batteries. However, one challenge that remains is the discrepancy in the reported electrochemical stability. Recent work has suggested that it may be due to the sensitivity of ceramic sulfides to mechanically induced stability. Small changes in ceramic-sulfide microstructure, for example, have been shown to c...
Stable photoelectrochemical solar fuel production requires protective coatings to achieve effective charge separation, transport, and injection at the semiconductor–liquid interfaces, implying that the coating should energetically align its intermediate band (IB) with both the photoabsorber's band edge and co‐catalyst's potentials. Yet approaches to adjust coating IB positions to accommodate various semiconductor light absorbers for constructing efficient and stable photoelectrodes have not been developed. Herein, three types of transition metal (M = Mn2+, Mn3+, and Cr3+ ions) alloyed TiO2 coatings are discovered using atomic layer deposition (ALD). The IB energetics of these coatings are characterized by X‐ray photoelectron spectroscopy and are found to be tunable inside the TiO2 bandgap, through varying ALD growth conditions. By applying these coatings to n‐type GaP and integrating with IrOx co‐catalysts, the water‐oxidation J–E performance is comparable to an uncoated corroding GaP photoanode. It reaches the bulk recombination limit of the GaP and achieves ≈28% absorbed photon to current efficiency under 475‐nm light excitation (6.48 mW cm−2) and 100‐h stable water oxidation. The outstanding performance and stability are attributed to the efficient charge separation and hole transport, as allowed by the energy alignment of the coating IB and the GaP valence band edge.
Numerous efficient semiconductors suffer from instability in aqueous electrolytes. Strategies utilizing protective coatings have thus been developed to protect these photoabsorbers against corrosion while synergistically improving charge separation and reaction kinetics. Recently, various photoelectrochemical (PEC) protective coatings have been reported with suitable electronic properties to ensure low charge transport loss and reveal the fundamental photoabsorber efficiency. However, protocols for studying the critical figures of merit for protective coatings have yet to be established. For this reason, we propose four criteria for evaluating the performance of a protective coating for PEC water-splitting: stability, conductivity, optical transparency, and energetic matching. We then propose a flow chart that summarizes the recommended testing protocols for quantifying these four performance metrics. In particular, we lay out the stepwise testing protocols to evaluate the energetics matching at a semiconductor/coating/(catalyst)/liquid interface. Finally, we provide an outlook for the future benchmarking needs for coatings.
Photocatalytic CO2 reduction (CO2R) in ∼0 mM CO2(aq) concentration is challenging but is relevant for capturing CO2 and achieving a circular carbon economy. Despite recent advances, the interplay between the CO2 catalytic reduction and the oxidative redox processes that are arranged on photocatalyst surfaces with nanometer-scale distances is less studied. Specifically, mechanistic investigation on interdependent processes, including CO2 adsorption, charge separation, long-range chemical transport (∼100 nm distance), and bicarbonate buffer speciation, involved in photocatalysis is urgently needed. Photocatalytic CO2R in ∼0 mM CO2(aq), which has important applications in integrated carbon capture and utilization (CCU), has rarely been studied. Using 0.1 M KHCO3 (aq) of pH 7 but without continuously bubbling CO2, we achieved ∼0.1% solar-to-fuel conversion efficiency for CO production using Ag@CrOx nanoparticles that are supported on a coating-protected GaInP2 photocatalytic panel. CO is produced at ∼100% selectivity with no detectable H2, even with copious protons co-generated nearby. CO2 flux to the Ag@CrOx CO2R sites enhances CO2 adsorption, probed by in situ Raman spectroscopy. CO is produced with local protonation of dissolved inorganic carbon species in a pH as high as 11.5 when using fast electron donors such as ethanol. Isotopic labeling using KH13CO3 was used to confirm the origin of CO from the bicarbonate solution. We then employed COMSOL Multiphysics modeling to simulate the spatial and temporal pH variation and the local concentrations of bicarbonates and CO2(aq). We found that light-driven CO2R and CO2 reactive transport are mutually dependent, which is important for further understanding and manipulating CO2R activity and selectivity. This study enables direct bicarbonate utilization as the source of CO2, thereby achieving CO2 capture and conversion without purifying and feeding gaseous CO2.
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