Time-Dependent Solid-State Polymorphism of a Series of Donor–Acceptor Dyads

In order to exploit the use of favorable electrostatic interactions between aromatic units in directing the assembly of donor–acceptor (D–A) dyads, the present work examines the ability of conjugated aromatic D–A dyads with symmetric side chains to exhibit solid-state polymorphism as a function of time during the solid formation process. Four such dyads were synthesized, and their packing in the solid state from either slower (10–20 days) or faster (1–2 days) evaporation from solvent was investigated using single crystal X-ray analysis and powder X-ray diffraction. Two of the dyads exhibited tail-to-tail (A–A) packing upon slower evaporation from solvent and head-to-tail (D–A) packing upon faster evaporation from solvent. A combination of single-crystal analysis and XRD patterns were used to create models, wherein a packing model for the other two dyads is proposed. Our findings suggest that while side chain interactions in asymmetric aromatic dyads can play an important role in enforcing segregated D–A dyad assembly, slowly evaporating symmetrically substituted aromatic dyads allows for favorable electrostatic interactions between the aromatic moieties to facilitate the organization of the dyads in the solid state

Time-Dependent Solid-State Polymorphism of a Series of Donor–Acceptor Dyads

In order to exploit the use of favorable electrostatic interactions between aromatic units in directing the assembly of donor–acceptor (D–A) dyads, the present work examines the ability of conjugated aromatic D–A dyads with symmetric side chains to exhibit solid-state polymorphism as a function of time during the solid formation process. Four such dyads were synthesized, and their packing in the solid state from either slower (10–20 days) or faster (1–2 days) evaporation from solvent was investigated using single crystal X-ray analysis and powder X-ray diffraction. Two of the dyads exhibited tail-to-tail (A–A) packing upon slower evaporation from solvent and head-to-tail (D–A) packing upon faster evaporation from solvent. A combination of single-crystal analysis and XRD patterns were used to create models, wherein a packing model for the other two dyads is proposed. Our findings suggest that while side chain interactions in asymmetric aromatic dyads can play an important role in enforcing segregated D–A dyad assembly, slowly evaporating symmetrically substituted aromatic dyads allows for favorable electrostatic interactions between the aromatic moieties to facilitate the organization of the dyads in the solid state

Synthesis of high-density olivine LiFePO4 from paleozoic siderite FeCO3 and its electrochemical performance in lithium batteries

The lithium-ion cathode material olivine LiFePO4 (LFP) has been synthesized for the first time from natural paleozoic iron carbonate (FeCO3). The ferrous carbonate starting material consists of the mineral siderite at about 92 wt. % purity. Because FeCO3 has divalent iron, the reaction with lithium dihydrogen phosphate (LiH2PO4) provides a unique method to develop iron-(II) containing LFP in an inert atmosphere. Since siderite FeCO3 is a common mineral that can be directly mined, it may, therefore, provide an inexpensive route for the production of LFP. After carbon-coating, the LFP yields a capacity in the range of 80–110 mAh g−1LFP (in one chosen specimen sample), which is lower than commercially available LiFePO4 (150–160 mAh g−1LFP). However, the tap density of LFP derived from siderite is noticeably high at 1.65 g cm−3. The material is likely to be improved with powder purification, nanosized processing, and more complete carbon-coating coverage with increased optimization

Auger Electrons as Probes for Composite Micro- and Nanostructured Materials: Application to Solid Electrolyte Interphases in Graphite and Silicon-Graphite Electrodes

In this study, Auger electron spectroscopy (AES) combined with ion sputtering depth profiling, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) have been used in a complementary fashion to examine chemical and microstructural changes in graphite (Gr) and silicon/graphite (Si/Gr) blends contained in the negative electrodes of lithium-ion cells. We demonstrate how AES depth profiling can be used to characterize morphology of the solid electrolyte interphase (SEI) deposits in such heterogeneous media, complementing well-established methods, such as XPS and SEM. In this way we demonstrate that the SEI does not consist of uniformly thick layers on the graphite and silicon; the thickness of the SEI layers in cycle life aged electrodes follows an exponential distribution with a mean of ca. 13 nm for the graphite and ca. 20–25 nm for the silicon nanoparticles (with a crystalline core of 50–70 nm in diameter). A “sticky-sphere” model, in which Si nanoparticles are covered with a layer of polymer binder (that is replaced by the SEI during cycling) of variable thickness, is introduced to account for the features observed

Functionality Selection Principle for High Voltage Lithium-ion Battery Electrolyte Additives

A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li⁺/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis results. The rational pathway of a cathode-electrolyte-interface formation was proposed for this type of additive. Both experimental results and the mechanism hypothesis suggest the effectiveness of the additive stems from both the polymerizable cyclic ring and the electron-withdrawing fluorinated alkyl group in the phosphate molecular structure. The successful development of cyclic fluorinated phosphate additives demonstrated that this new functionality selection principle, by incorporating useful functionalities of various additives into one molecule, is an effective approach for the development of new additives

Chemical “Pickling” of Phosphite Additives Mitigates Impedance Rise in Li Ion Batteries

The use of high-voltage, high-capacity positive electrodes in lithium ion batteries presents a challenge, given their tendency to degrade organic electrolytes. To prevent this damage, electrolyte additives modifying the cathode surface are required. Tris­(trimethylsilyl) phosphite (TMSPi) is one such electrolyte additive. However, the mechanism for its protective action (similar to other phosphite, borate, and boroxane compounds) remains not completely understood. In LiPF₆ containing carbonate electrolytes, TMSPi undergoes reactions yielding numerous products. Here we demonstrate that one of these products, PF₂OSiMe₃, is responsible for mitigation of the impedance rise that occurs in aged cells during charge/discharge cycling. This same agent can also be responsible for reducing parasitic oxidation currents and transition metal loss during prolonged cell cycling. Mechanistic underpinnings of this protective action are examined using computational methods. Our study suggests that this beneficial action originates mainly through inhibition of catalytic centers for electrolyte oxidation that are present on the cathode surface, by forming capping ligands on the transition metal ions that block solvent access to such centers

Chemical “Pickling” of Phosphite Additives Mitigates Impedance Rise in Li Ion Batteries

The use of high-voltage, high-capacity positive electrodes in lithium ion batteries presents a challenge, given their tendency to degrade organic electrolytes. To prevent this damage, electrolyte additives modifying the cathode surface are required. Tris­(trimethylsilyl) phosphite (TMSPi) is one such electrolyte additive. However, the mechanism for its protective action (similar to other phosphite, borate, and boroxane compounds) remains not completely understood. In LiPF₆ containing carbonate electrolytes, TMSPi undergoes reactions yielding numerous products. Here we demonstrate that one of these products, PF₂OSiMe₃, is responsible for mitigation of the impedance rise that occurs in aged cells during charge/discharge cycling. This same agent can also be responsible for reducing parasitic oxidation currents and transition metal loss during prolonged cell cycling. Mechanistic underpinnings of this protective action are examined using computational methods. Our study suggests that this beneficial action originates mainly through inhibition of catalytic centers for electrolyte oxidation that are present on the cathode surface, by forming capping ligands on the transition metal ions that block solvent access to such centers