Design principles for heterointerfacial alloying kinetics at metallic anodes in rechargeable batteries.

How surface chemistry influences reactions occurring thereupon has been a long-standing question of broad scientific and technological interest. Here, we consider the relation between the surface chemistry at interfaces and the reversibility of electrochemical transformations at rechargeable battery electrodes. Using Zn as a model system, we report that a moderate strength of chemical interaction between the deposit and the substrate-neither too weak nor too strong-enables highest reversibility and stability of the plating/stripping redox processes. Focused ion beam and electron microscopy were used to directly probe the morphology, chemistry, and crystallography of heterointerfaces of distinct natures. Analogous to the empirical Sabatier principle for chemical heterogeneous catalysis, our findings arise from competing interfacial processes. Using full batteries with stringent negative electrode-to-positive electrode capacity (N:P) ratios, we show that such knowledge provides a powerful tool for designing key materials in highly reversible battery systems based on Earth-abundant, low-cost metals such as Zn and Na.

Multimodal electrochemistry coupled microcalorimetric and X-ray probing of the capacity fade mechanisms of Nickel rich NMC - progress and outlook.

Lithium nickel manganese cobalt oxide (NMC) is a commercially successful Li-ion battery cathode due to its high energy density; however, its delivered capacity must be intentionally limited to achieve capacity retention over extended cycling. To design next-generation NMC batteries with longer life and higher capacity the origins of high potential capacity fade must be understood. Operando hard X-ray characterization techniques are critical for this endeavor as they allow the acquisition of information about the evolution of structure, oxidation state, and coordination environment of NMC as the material (de)lithiates in a functional battery. This perspective outlines recent developments in the elucidation of capacity fade mechanisms in NMC through hard X-ray probes, surface sensitive soft X-ray characterization, and isothermal microcalorimetry. A case study on the effect of charging potential on NMC811 over extended cycling is presented to illustrate the benefits of these approaches. The results showed that charging to 4.7 V leads to higher delivered capacity, but much greater fade as compared to charging to 4.3 V. Operando XRD and SEM results indicated that particle fracture from increased structural distortions at >4.3 V was a contributor to capacity fade. Operando hard XAS revealed significant Ni and Co redox during cycling as well as a Jahn-Teller distortion at the discharged state (Ni3+); however, minimal differences were observed between the cells charged to 4.3 and 4.7 V. Additional XAS analyses using soft X-rays revealed significant surface reconstruction after cycling to 4.7 V, revealing another contribution to fade. Operando isothermal microcalorimetry (IMC) indicated that the high voltage charge to 4.7 V resulted in a doubling of the heat dissipation when compared to charging to 4.3 V. A lowered chemical-to-electrical energy conversion efficiency due to thermal energy waste was observed, providing a complementary characterization of electrochemical degradation. The work demonstrates the utility of multi-modal X-ray and microcalorimetric approaches to understand the causes of capacity fade in lithium-ion batteries with Ni-rich NMC.

Structural and electrochemical investigation of crystallite size controlled zinc ferrite (ZnFe2O4).

Zinc ferrite, ZnFe2O4(ZFO), is a promising electrode material for next generation Li-ion batteries because of its high theoretical capacity and low environmental impact. In this report, synthetic control of crystallite size from the nanometer to submicron scale enabled probing of the relationships between ZFO size and electrochemical behavior. A facile two-step coprecipitation and annealing preparation method was used to prepare ZFO with controlled sizes ranging ∼9 to >200 nm. Complementary synchrotron and electron microscopy techniques were used to characterize the series of materials. Increasing the annealing temperature increased crystallinity and decreased microstrain, while local structural ordering was maintained independent of crystallite size. Electrochemical characterization revealed that the smaller sized materials delivered higher capacities during initial lithiation. Larger sized particles exhibited a lack of distinct electrochemical signatures above 1.0 V, suggesting that the longer diffusion length associated with greater crystallite size causes the lithiation process to proceed via non discrete lithium insertion, cation migration, and conversion processes. Notably, larger particles exhibited enhanced electrochemical reversibility over 50 cycles, with capacity retention improving from <20% to >40% at C/2 cycling rate. This intriguing result was probed through x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) measurements of the cycled electrodes. XAS revealed that the larger crystallite size materials do not completely convert to Fe0during the first lithiation and that independent of size, delithiation results in the formation of nanocrystalline FeO and ZnO phases rather than ZnFe2O4. After 20 cycles, the larger crystallites showed reversibility between partially oxidized FeO in the charged state and Fe0in the discharged state, while the smaller crystallite size material was electrochemically inactive as Fe0. XPS analysis revealed more significant solid electrolyte interphase (SEI) formation on the cycled electrodes utilizing ZFO with smaller crystallite size. This finding suggests that excessive SEI buildup on the smaller sized, higher surface area ZFO particles contributes to their reduced electrochemical reversibility relative to the larger crystallite size materials.

Syntheses and spectroscopic, structural, electrochemical, spectroelectrochemical, and theoretical studies of osmium(II) mono- and bis-alkynyl complexes.

The syntheses of trans-[Os(C≡C-4-C(6)H(4)X)Cl(dppe)(2)] [X = Br (3), I (4)], trans-[Os(C≡C-4-C(6)H(4)X)(NH(3))(dppe)(2)](PF(6)) [X = H (5(PF(6))), I (6(PF(6)))], and trans-[Os(C≡C-4-C(6)H(4)X)(C≡C-4-C(6)H(4)Y)(dppe)(2)] [X = Y = H (7), X = I, Y = C≡CSiPr(i)(3) (8)] are reported, together with improved syntheses of cis-[OsCl(2)(dppe)(2)] (cis-1), trans-[Os(C≡CPh)Cl(dppe)(2)] (2), and trans-[Ru(C≡C-4-C(6)H(4)I)(NH(3))(dppe)(2)](PF(6)) (9(PF(6))) (the last-mentioned direct from trans-[Ru(C≡C-4-C(6)H(4)I)Cl(dppe)(2)]), and single-crystal X-ray structural studies of 2-4, 5(PF(6)), 6(PF(6)), and 7. Ammine complexes 5(PF(6))/6(PF(6)) are shown to afford a facile route to both symmetrical (7) and unsymmetrical (8) osmium bis(alkynyl) complexes. A combination of cyclic voltammetry, UV-vis-NIR spectroelectrochemistry, and time-dependent density functional theory (TD-DFT) has permitted identification and assignment of the intense transitions in both the resting state and the oxidized forms of these complexes. Cyclic voltammetric data show fully reversible oxidation processes at 0.32-0.42 V (3, 4, 7, 8) (with respect to ferrocene/ferrocenium 0.56 V), assigned to the (formal) Os(II/III) couple. The osmium(III) complex (di)cations 5(2+) and 7(+) were obtained by in situ oxidation of 5(+) and 7 using an optically transparent thin-layer electrochemical (OTTLE) cell. The UV-vis-NIR optical spectra of 5(2+) and 7(+) reveal low-energy bands in the near IR region, in contrast to 5(+) and 7 which are optically transparent at frequencies below 22,000 cm(-1). TD-DFT calculations on trans-1, 2, 5(+), and 7 and their oxidized forms suggest that the lowest-energy transitions are chloro-to-metal charge transfer (trans-1), chloro-to-phenylethynyl charge transfer (2), and metal-to-phenylethynyl charge transfer (5(+), 7) in the resting state and chloro-to-metal charge transfer (trans-1(+)), phosphorus-to-metal charge transfer (5(2+)), alkynyl-to-metal charge transfer (7(+)), or phenylalkynyl-centered π → π* (2(+)) following oxidation. The presence of intense CT bands in the resting states and oxidized states and their significantly different nature across the two states, coupled to their strong charge displacement suggest that these species have considerable potential as electrochemically switchable nonlinear optical materials, while the facile unsymmetrical bis(alkynyl)osmium(II) construction suggests potential in construction of multistate heterometallic modular assemblies.