Potentiostatic intermittent titration technique (PITT) was applied to FeF2, FeF3, and FeO0.67F1.33 to gain insight into the transport-related aspects of the conversion reaction by quantitative analysis of Li(+) diffusion and hysteresis. PITT derived diffusion coefficient measurements were benchmarked relative to values extracted by electrochemical impedance spectroscopy (EIS). A reverse-step PITT methodology was used to evaluate true hysteresis by eliminating nucleation induced overpotentials. This method evaluates the minimum potential hysteresis and allowed an accurate representation of the potential required to move conversion reactions forward at C/1000 rates in both lithiation and delithiation. The high resolution PITT data were also used to gain further insight into reaction mechanisms involved in the reversible conversion reactions. Physical evidence, based on pair distribution function (PDF) structural analysis, and electrochemical evidence are presented regarding a new step in the reaction during the rutile FeF2 reconversion reaction.

It is demonstrated that Na ions are mobile at room temperature in the nitridophosphate compound Na 3 TiP 3 O 9 N, with a diffusion pathway that is calculated to be fully three-dimensional and isotropic. When used as a cathode in Na-ion batteries, Na 3 TiP 3 O 9 N has an average voltage of 2.7 V vs Na + /Na and cycles with good reversibility through a mechanism that appears to be a single solid solution process without any intermediate plateaus. X-ray and neutron diffraction studies as well as first-principles calculations indicate that the volume change that occurs on Na-ion removal is only about 0.5%, a remarkably small volume change given the large ionic radius of Na + . Rietveld refinements indicate that the Na1 site is selectively depopulated during sodium removal. Furthermore, the refined displacement parameters support theoretical predictions that the lowest energy diffusion pathway incorporates the Na1 and Na3 sites while the Na2 site is relatively inaccessible. The measured room temperature ionic conductivity of Na 3 TiP 3 O 9 N is substantial (4 × 10 −7 S/cm), though both the strong temperature dependence of Na-ion thermal parameters and the observed activation energy of 0.54 eV suggest that much higher ionic conductivities can be achieved with minimal heating. Excellent thermal stability is observed for both pristine Na 3 TiP 3 O 9 N and desodiated Na 2 TiP 3 O 9 N, suggesting that this phase can serve as a safe Na-ion battery electrode. Moreover, it is expected that further optimization of the general cubic framework of Na 3 TiP 3 O 9 N by chemical substitution will result in thermostable solid state electrolytes with isotropic conductivities that can function at temperatures near or just above room temperature.

In some applications, such as military or back-up energy applications, lithium-ion batteries can undergo storage for multiple years without use. If the batteries are not properly maintained, the pack voltage can decrease over time due to cell self-discharge, battery management system power requirements, and parasitic loads. However, lithium-ion batteries have a recommended discharge voltage limit corresponding to a nominal 0% state of charge, and if discharged below this limit, they will experience an overdischarge condition which can lead to dissolution of the copper current collector and introduce potential safety and performance issues. This paper investigates the nature of copper dissolution in overdischarged lithium-ion batteries including the relative concentration and chemical state of the copper found in overdischarged batteries through characterization by X-ray photoelectron spectroscopy and X-ray absorption fine structure spectroscopy.

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XRD and XAS were used to characterize the bulk structure, while XPS was used to characterize the surface structure, of commercially obtained nominally K4Fe(CN)6·3H2O, K3Fe(CN)6 and our synthesized Prussian Blue (PB) material. K4Fe(CN)6·3H2O was found to consist of a fully hydrated phase, which crystallizes in the monoclinic form and a less hydrated or anhydrous phase which crystallizes in the orthorhombic form. K3Fe(CN)6 was found to consist of the well-established orthorhombic form rather than the monoclinic form. The structure of our synthesized Prussian Blue (PB) was found to be consistent with that reported for (KOH)0.7Fe(III)1.33Fe(II)(CN)6·4.0H2O which crystallizes in the cubic form. XPS and XAS confirmed the presence of ferrous Fe(II) at the surface and bulk levels in K4Fe(CN)6·xH2O. However, XPS revealed the presence of Fe(II) (∼30%) and Fe(III) (∼70%) in the surface region of K3Fe(CN)6 while XAS confirmed the presence of mostly Fe(III) at the bulk level. Both XPS and XANES confirmed the presence of Fe(II) and Fe(III) in the surface and bulk regions of PB. This ex situ XAS study will be used to support the analysis of an in situ XAS data collected on a PB containing supercapacitor to understand the mechanistic origin of pseudocapacitance in these devices.