The phosphate lithium-ion conductor Li1.5Al0.5Ti1.5(PO4)3 (LATP) is an economically attractive solid electrolyte for the fabrication of safe and robust solid-state batteries, but high sintering temperatures pose a material engineering challenge for the fabrication of cell components. In particular, the high surface roughness of composite cathodes resulting from enhanced crystal growth is detrimental to their integration into cells with practical energy density. In this work, we demonstrate that efficient free-standing ceramic cathodes of LATP and LiFePO4 (LFP) can be produced by using a scalable tape casting process. This is achieved by adding 5 wt % of Li2WO4 (LWO) to the casting slurry and optimizing the fabrication process. LWO lowers the sintering temperature without affecting the phase composition of the materials, resulting in mechanically stable, electronically conductive, and free-standing cathodes with a smooth, homogeneous surface. The optimized cathode microstructure enables the deposition of a thin polymer separator attached to the Li metal anode to produce a cell with good volumetric and gravimetric energy densities of 289 Wh dm-3 and 180 Wh kg-1, respectively, on the cell level and Coulombic efficiency above 99% after 30 cycles at 30 °C.

Various compositions of the series Li1+xMx3+Zr2-x(PO4), where M3+ = Al3+, Sc3+, Y3+ were prepared by solution-assisted solid-state reaction, since they could have a higher reduction stability as solid electrolytes in lithium batteries than in germanium- or titanium-containing materials. The influence of substitution on crystallographic parameters, density, and ionic conductivity were investigated. The cation substitution of M3+ (M = Al, Sc, Y) for Zr4+ in LiZr2 (PO4)(3) stabilizes the rhombohedral NaSICON structure (space group R (3) over barc) at room temperature and increases the ionic conductivity significantly. Here, at 25 degrees C and with a consistent relative density of 94%-96%, an ionic conductivity of 2.7 x 10(-5) S cm(-1), 6.7 x 10(-5) S cm(-1) , and 3.6 x 10(-6) S cm(-1) was achieved with the compositions Li1.2Sc0.2Zr1.8(PO4)(3), LI1.2Y0.2Zr1.8(PO4)(3), and Li1.2Al0.2Zr1.8(PO4)(3), respectively In compar- ison with Li1+xScxZr2-x(PO4)(3), the Y3+ substitution in LiZr2(PO4)(3) enhanced the ionic conductivity slightly and denoted the maximum Li+ ionic conductivity obtained at room temperature. However, substitution with Al3+ decreased the ionic conductivity. For the first time, this work provides a complete overview of three series of solid Li-ion conductors in the Li2O-M2O3-ZrO2-P2O, system where M = Al, Sc, Y. Noticeable differences in the chemistry of resulting compounds were observed, which likely depend on the ionic radius of the cations being substituted. The series with Sc showed complete miscibility from x = 0 to x = 2 with a continuous change of the NaSICON polymorphs. The series with Y showed a solubility limit at about x = 0.3 and higher substitution levels led to the increasing formation of YPO4. The series with Al exhibited continuously decreasing ionic conductivity until x = 1, whereupon the investigation was terminated due to its very low conductivity of about 10(-10) S cm(-1).

The Cover Feature shows the kosnarite structure from an unusual perspective and with reminiscence of its volcanic origin. The analogous man-made and so-called NaSICON materials have been optimized with advanced processing methods. More information can be found in the Research Article by A. Loutati, F. Tietz and co-workers.

Polymer-ceramic composites combine the benefits of polymers and ceramics. In particular, the infiltration of the ceramic cathode with a Li-ion-conducting polymer in an all-solid-state Li battery enhances the utilization of the cathode active material (CAM) and enables the application of thicker cathodes with higher storage capacity. This concept has already been validated in our earlier work, in which a porous LiCoO2-Li6.45Al0.05La3Zr1.6Ta0.4O12 (LLZO:Al:Ta) composite cathode was fabricated by spark plasma sintering (SPS) technique. However, its performance stability was low. In the present work, the concept is modified using an LFP-LATP cathode with LiFePO4 as the CAM, Li1.5Al0.5Ti1.5(PO4)(3) as the ion-conducting phase, and tape-casting with free sintering instead of SPS. Both tape-casting and free sintering are more relevant for large-scale production. The sintered LFP-LATP cathode is infiltrated with the MEEP polymer and LiC2NO4F6S2 ion-conducting salt. A full cell with the polymer-infiltrated cathode, LLZO:Al:Ta separator, and Li anode shows nearly full LFP utilization in the 100 mu m thick cathode with an excellent area-specific storage capacity of above 3 mAh cm(-2). However, after a few dozen cycles, a Li dendrite penetrates the separator leading to abrupt capacity fading. The prevention of Li dendrite formation remains a challenge for our future work.

Phosphate lithium-ion conductors are outstanding electrolyte materials for solid-state lithium batteries. As polycrystalline ceramics, they must be sintered at high temperatures. Lithium tungstate Li2WO4 (LWO) is reported for the first time as an effective sintering aid to reduce the sintering temperature for one of the most common solid-state lithium-ion conductors, Li1.5Al0.5Ti1.5(PO4)(3) (LATP). While densification of LATP without sintering aids requires temperatures of at least 950 degrees C to obtain a relative density of 90 %, here relative densities of 90-95 % are achieved even at 775 degrees C when 5 wt.% of LWO are added. At 800 degrees C the LATP containing 5-7 wt.% LWO densifies to a relative density of 97.2 %. The ionic conductivity of LWO containing LATP is generally higher than that of pure LATP sintered at the same temperature. LATP containing 7 wt.% LWO shows high ionic conductivity of 4.4x10(-4) S/cm after sintering at 825 degrees C. A significant reduction in sintering temperature, an increase in density and in the ionic conductivity of LATP as well as its non-toxicity render LWO a very promising sintering aid for the development of LATP-based solid state batteries.