Fe-Si networks and charge/discharge-induced phase transitions in Li2FeSiO4 cathode materials.

Structural phase transitions of electrode materials are responsible for poor reversibility during charge/discharge cycling in Li-ion batteries. Using previously developed structural databases, we investigate a structural landscape for LixFeSiO4 systems at x = 1. Starting with low-energy Li2FeSiO4 crystal structures, we explore the crystal structures of the material in different states of charge. The as-prepared Li2FeSiO4 materials adopt low energy structures characterized by two-dimensional (2D) Fe-Si networks. After the removal of one Li per formula unit to form LiFeSiO4, the structures with three-dimensional (3D) diamond-like Fe-Si networks become more energetically favorable without a significant impact on the charge capacity, which agrees with previous experimental and theoretical work. However, we reveal that the structure with a 3D diamond-like Fe-Si network can further transform into a new structure at x = 1. And the Li atom is hard to reinsert into these new structures. Consequently the system is prevented from returning to the Li2FeSiO4 state. We believe that the formation of this new structure plays an important role in the loss of reversible capacity of Li2FeSiO4 electrode materials.

Rapid screening of abused drugs by direct analysis in real time (DART) coupled to time-of-flight mass spectrometry (TOF-MS) combined with ion mobility spectrometry (IMS).

Increasing in cases involving drugs of abuse leads to heavy burden for law enforcement agencies, exacerbating demand for rapid screening technique. In this study, atmospheric pressure ionization technologies including direct analysis in real time (DART) ion source coupled to a time-of-flight mass spectrometer (DART-TOF-MS)as well asdopant-assisted positive photoionization ion mobility spectrometry (DAPP-IMS) without radioactivity were utilized together as the powerful analytical tool for the rapid screening and identification of 53 abused drugs.The limits of detection (LOD) were 0.05-2µg/mL when using DART-TOF-MS and 0.02-2µg when using DAPP-IMS which could satisfy the actual requirement in forensic science laboratory. The advantages of this method included fast response, high-throughput potential, high specificity, and minimal sample preparation. A screening library of reduced mobility (K0), accurate mass of informative precursor ion ([M+H]+) and fragment ions was established respectively by employing a bench-top DAPP-IMS and TOF-MS in-source collision induced dissociation (CID) mode. Then the standardized screening procedure was developed with criteria for the confirmation of positive result. A total of 50 seized drug samples provided by local forensic laboratory we reanalyzed to testify the utility of the method. This study suggests that a method combing DART-TOF-MS and DAPP-IMS is promising for the rapid screening and identification of abused drugs with minimal sample preparation and absence of chromatography.

Exploration of tetrahedral structures in silicate cathodes using a motif-network scheme.

Using a motif-network search scheme, we studied the tetrahedral structures of the dilithium/disodium transition metal orthosilicates A2MSiO4 with A = Li or Na and M = Mn, Fe or Co. In addition to finding all previously reported structures, we discovered many other different tetrahedral-network-based crystal structures which are highly degenerate in energy. These structures can be classified into structures with 1D, 2D and 3D M-Si-O frameworks. A clear trend of the structural preference in different systems was revealed and possible indicators that affect the structure stabilities were introduced. For the case of Na systems which have been much less investigated in the literature relative to the Li systems, we predicted their ground state structures and found evidence for the existence of new structural motifs.