Single-Ion Lithium Conducting Polymers with High Ionic Conductivity Based on Borate Pendant Groups.

A family of single-ion lithium conducting polymer electrolytes based on highly delocalized borate groups is reported. The effect of the nature of the substituents on the boron atom on the ionic conductivity of the resultant methacrylic polymers was analyzed. To the best of our knowledge the lithium borate polymers endowed with flexible and electron-withdrawing substituents presents the highest ionic conductivity reported for a lithium single-ion conducting homopolymer (1.65×10-4  S cm-1 at 60 °C). This together with its high lithium transference number t Li + =0.93 and electrochemical stability window of 4.2 V vs Li0 /Li+ show promise for application in lithium batteries. To illustrate this, a lithium borate monomer was integrated into a single-ion gel polymer electrolyte which showed good performance on lithium symmetrical cells (<0.85 V at ±0.2 mA cm-2 for 175 h).

Absence of Nanostructuring in NaPb mSbTe m+2: Solid Solutions with High Thermoelectric Performance in the Intermediate Temperature Regime.

Thermoelectric devices directly convert heat into electrical energy and are highly desired for emerging applications in waste heat recovery. Currently, PbTe based compounds are the leading thermoelectric materials in the intermediate temperature regime (∼800 K); however, integration into commercial devices has been limited. This is largely because the performance of PbTe, which is maximized ∼900 K, is too low over the temperatures of interest for most potential commercial applications (generally under 600 K). Improving the low temperature performance of PbTe based materials is therefore critical to achieve usage outside of existing niche applications. Here, we provide an in-depth study of the cubic NaPb mSbTe m+2 system of compounds ( m = 1-20) and report that it is an excellent class of low- to medium-temperature thermoelectrics when m = 10-20. We show that the as-cast polycrystalline ingots exhibit degenerate p-type conduction and high maximum ZTs of 1.2-1.4 at 650 K when m = 6-20. Because the ingots are found to be extremely brittle, we utilize spark plasma sintering (SPS) to prepare more mechanically robust samples, and surprisingly, find that SPS results in an undesired change in charge transport toward n-type behavior. We show this unanticipated transition from p-type behavior as ingots to n-type after SPS is due to dissolution of secondary phases that are present in the ingots into the primary matrix during the SPS process, resulting in a transformation from an inhomogeneous state to a solid solution without any observable evidence of nanoscale precipitation. This is in sharp contrast to the seemingly similar AgPbmSbTe m+2 (LAST) system, which is heavily nanostructured. The SPSed NaPb mSbTe m+2 is doped p-type by tuning the cation stoichiometry, i.e., Na1+ xPb m- xSb1- yTe m+2. The optimized compounds have very low lattice thermal conductivities of 1.1-0.55 W·m-1·K-1 over 300-650 K, which enhances the low-intermediate temperature performance and gives rise to maximum ZT values up to 1.6 at 673 K as well as an excellent ZTavg of 1.1 over 323-673 K for m = 10, 20, making Na1+ xPb m- xSb1- yTe m+2 among the highest performing PbTe-based thermoelectrics under 650 K.

Role of self-organization, nanostructuring, and lattice strain on phonon transport in NaPb(18-x)Sn(x)BiTe(20) thermoelectric materials.

The composition and microstructure of five thermoelectric materials, PbTe, SnTe, Pb(0.65)Sn(0.35)Te and NaPb(18-x)Sn(x)BiTe(20) (x = 5, 9), were investigated by advanced transmission electron microcopy. We confirm that the pure PbTe, SnTe, and Pb(0.65)Sn(0.35)Te have a uniform crystalline structure and homogeneous compositions without any nanoscale inclusions. On the other hand, the nominal NaPb(9)Sn(9)BiTe(20) phase contains extensive inhomogeneities and nanostructures with size distribution of 3-7 nm. We find that the chemical architecture of the NaPb(13)Sn(5)BiTe(20) member of the series to be more complex; besides nanoscale precipitates, self-organized lamellar structures are present which were identified as PbTe and SnTe by composition analysis and transmission electron microscopy image simulations. Density functional theory calculations suggest that the arrangement of the lamellar structures conforms to the lowest total energy configuration. Geometric-phase analyses revealed large distributed elastic strain around the nanoscale inclusions and lamellar structures. We propose that interface-induced elastic perturbations in the matrix play a decisive role in affecting the phonon-propagation pathways. The interfaces further enhance phonon scattering which, in turn, reduces the lattice thermal conductivity in these systems that directly results directly in improvement in the thermoelectric figure of merit.