Phase Transformations and Phase Segregation during Potassiation of Sn x P y Anodes.

K-ion batteries (KIBs) have the potential to offer a cheaper alternative to Li-ion batteries (LIBs) using widely abundant materials. Conversion/alloying anodes have high theoretical capacities in KIBs, but it is believed that electrode damage from volume expansion and phase segregation by the accommodation of large K-ions leads to capacity loss during electrochemical cycling. To date, the exact phase transformations that occur during potassiation and depotassiation of conversion/alloying anodes are relatively unexplored. In this work, we synthesize two distinct compositions of tin phosphides, Sn4P3 and SnP3, and compare their conversion/alloying mechanisms with solid-state nuclear magnetic resonance (SSNMR) spectroscopy, powder X-ray diffraction (XRD), and density functional theory (DFT) calculations. Ex situ 31P and 119Sn SSNMR analyses reveal that while both Sn4P3 and SnP3 exhibit phase separation of elemental P and the formation of KSnP-type environments (which are predicted to be stable based on DFT calculations) during potassiation, only Sn4P3 produces metallic Sn as a byproduct. In both anode materials, K reacts with elemental P to form K-rich compounds containing isolated P sites that resemble K3P but K does not alloy with Sn during potassiation of Sn4P3. During charge, K is only fully removed from the K3P-type structures, suggesting that the formation of ternary regions in the anode and phase separation contribute to capacity loss upon reaction of K with tin phosphides.

Potassium Fluoride and Carbonate Lead to Cell Failure in Potassium-Ion Batteries.

While Li-ion is the prevailing commercial battery chemistry, the development of batteries that use earth-abundant alkali metals (e.g., Na and K) alleviates reliance on Li with potentially cheaper technologies. Electrolyte engineering has been a major thrust of Li-ion battery (LIB) research, and it is unclear if the same electrolyte design principles apply to K-ion batteries (KIBs). Fluoroethylene carbonate (FEC) is a well-known additive used in Li-ion electrolytes because the products of its sacrificial decomposition aid in forming a stable solid electrolyte interphase (SEI) on the anode surface. Here, we show that FEC addition to KIBs containing hard carbon anodes results in a dramatic decrease in capacity and cell failure in only two cycles, whereas capacity retention remains high (> 90% over 100 cycles at C/10 for both KPF6 and KFSI) for electrolytes that do not contain FEC. Using a combination of 19F solid-state nuclear magnetic resonance (SSNMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS), we show that FEC decomposes during galvanostatic cycling to form insoluble KF and K2CO3 on the anode surface, which correlates with increased interfacial resistance in the cell. Our results strongly suggest that KIB performance is sensitive to the accumulation of an inorganic SEI, likely due to poor K transport in these compounds. This mechanism of FEC decomposition was confirmed in two separate electrolyte formulations using KPF6 or KFSI. Interestingly, the salt anions do not decompose themselves, unlike their Li analogues. Insight from these results indicates that electrolyte decomposition pathways and favorable SEI components are significantly different in KIBs and LIBs, suggesting that entirely new approaches to KIB electrolyte engineering are needed.

Reversible Deposition and Stripping of the Cathode Electrolyte Interphase on Li2RuO3.

Performance decline in Li-excess cathodes is generally attributed to structural degradation at the electrode-electrolyte interphase, including transition metal migration into the lithium layer and oxygen evolution into the electrolyte. Reactions between these new surface structures and/or reactive oxygen species in the electrolyte can lead to the formation of a cathode electrolyte interphase (CEI) on the surface of the electrode, though the link between CEI composition and the performance of Li-excess materials is not well understood. To bridge this gap in understanding, we use solid-state nuclear magnetic resonance (SSNMR) spectroscopy, dynamic nuclear polarization (DNP) NMR, and electrochemical impedance spectroscopy (EIS) to assess the chemical composition and impedance of the CEI on Li2RuO3 as a function of state of charge and cycle number. We show that the CEI that forms on Li2RuO3 when cycled in carbonate-containing electrolytes is similar to the solid electrolyte interphase (SEI) that has been observed on anode materials, containing components such as PEO, Li acetate, carbonates, and LiF. The CEI composition deposited on the cathode surface on charge is chemically distinct from that observed upon discharge, supporting the notion of crosstalk between the SEI and the CEI, with Li+-coordinating species leaving the CEI during delithiation. Migration of the outer CEI combined with the accumulation of poor ionic conducting components on the static inner CEI may contribute to the loss of performance over time in Li-excess cathode materials.

Universal chemomechanical design rules for solid-ion conductors to prevent dendrite formation in lithium metal batteries.

Dendrite formation during electrodeposition while charging lithium metal batteries compromises their safety. Although high-shear-modulus (Gs) solid-ion conductors (SICs) have been prioritized to resolve the pressure-driven instabilities that lead to dendrite propagation and cell shorting, it is unclear whether these or alternatives are needed to guide uniform lithium electrodeposition, which is intrinsically density-driven. Here, we show that SICs can be designed within a universal chemomechanical paradigm to access either pressure-driven dendrite-blocking or density-driven dendrite-suppressing properties, but not both. This dichotomy reflects the competing influence of the SIC's mechanical properties and the partial molar volume of Li+ ([Formula: see text]) relative to those of the lithium anode (GLi and VLi) on plating outcomes. Within this paradigm, we explore SICs in a previously unrecognized dendrite-suppressing regime that are concomitantly 'soft', as is typical of polymer electrolytes, but feature an atypically low [Formula: see text] that is more reminiscent of 'hard' ceramics. Li plating (1 mA cm-2; T = 20 °C) mediated by these SICs is uniform, as revealed using synchrotron hard X-ray microtomography. As a result, cell cycle life is extended, even when assembled with thin Li anodes (~30 µm) and either high-voltage NMC-622 cathodes (1.44 mAh cm-2) or high-capacity sulfur cathodes (3.02 mAh cm-2).