Identification of Li-Ion Battery SEI Compounds through ⁷Li and ¹³C Solid-State MAS NMR Spectroscopy and MALDI-TOF Mass Spectrometry

Solid-state ⁷Li and ¹³C MAS NMR spectra of cycled graphitic Li-ion anodes demonstrate SEI compound formation upon lithiation that is followed by changes in the SEI upon delithiation. Solid-state ¹³C DPMAS NMR shows changes in peaks associated with organic solvent compounds (ethylene carbonate and dimethyl carbonate, EC/DMC) upon electrochemical cycling due to the formation of and subsequent changes in the SEI compounds. Solid-state ¹³C NMR spin–lattice (T₁) relaxation time measurements of lithiated Li-ion anodes and reference poly­(ethylene oxide) (PEO) powders, along with MALDI-TOF mass spectrometry results, indicate that large-molecular-weight polymers are formed in the SEI layers of the discharged anodes. MALDI-TOF MS and NMR spectroscopy results additionally indicate that delithiated anodes exhibit a larger number of SEI products than is found in lithiated anodes

Improving the Capacity of Sodium Ion Battery Using a Virus-Templated Nanostructured Composite Cathode

In this work we investigated an energy-efficient biotemplated route to synthesize nanostructured FePO₄ for sodium-based batteries. Self-assembled M13 viruses and single wall carbon nanotubes (SWCNTs) have been used as a template to grow amorphous FePO₄ nanoparticles at room temperature (the active composite is denoted as Bio-FePO₄-CNT) to enhance the electronic conductivity of the active material. Preliminary tests demonstrate a discharge capacity as high as 166 mAh/g at C/10 rate, corresponding to composition Na_0.9FePO₄, which along with higher C-rate tests show this material to have the highest capacity and power performance reported for amorphous FePO₄ electrodes to date

Engineering the Transformation Strain in LiMn_<i>y</i>Fe_1–<i>y</i>PO₄ Olivines for Ultrahigh Rate Battery Cathodes

Alkali ion intercalation compounds used as battery electrodes often exhibit first-order phase transitions during electrochemical cycling, accompanied by significant transformation strains. Despite ∼30 years of research into the behavior of such compounds, the relationship between transformation strain and electrode performance, especially the rate at which working ions (e.g., Li) can be intercalated and deintercalated, is still absent. In this work, we use the LiMn_<i>y</i>Fe_1–<i>y</i>PO₄ system for a systematic study, and measure using operando synchrotron radiation powder X-ray diffraction (SR-PXD) the dynamic strain behavior as a function of the Mn content (<i>y</i>) in powders of ∼50 nm average diameter. The dynamically produced strain deviates significantly from what is expected from the equilibrium phase diagrams and demonstrates metastability but nonetheless spans a wide range from 0 to 8 vol % with <i>y</i>. For the first time, we show that the discharge capacity at high C-rates (20–50C rate) varies in inverse proportion to the transformation strain, implying that engineering electrode materials for reduced strain can be used to maximize the power capability of batteries

Accommodating High Transformation Strains in Battery Electrodes via the Formation of Nanoscale Intermediate Phases: Operando Investigation of Olivine NaFePO₄

Virtually all intercalation compounds exhibit significant changes in unit cell volume as the working ion concentration varies. Na_<i>x</i>FePO₄ (0 < <i>x</i> < 1, NFP) olivine, of interest as a cathode for sodium-ion batteries, is a model for topotactic, high-strain systems as it exhibits one of the largest discontinuous volume changes (∼17% by volume) during its first-order transition between two otherwise isostructural phases. Using synchrotron radiation powder X-ray diffraction (PXD) and pair distribution function (PDF) analysis, we discover a new strain-accommodation mechanism wherein a third, amorphous phase forms to buffer the large lattice mismatch between primary phases. The amorphous phase has short-range order over ∼1nm domains that is characterized by <i>a</i> and <i>b</i> parameters matching one crystalline end-member phase and a <i>c</i> parameter matching the other, but is not detectable by powder diffraction alone. We suggest that this strain-accommodation mechanism may generally apply to systems with large transformation strains