Lepidocrocite-type Layered Titanate Structures: New Lithium and Sodium Ion Intercalation Anode Materials

The electrochemical characteristics of lepidocrocite-type titanates derived from K_0.8Ti_1.73Li_0.27O₄ are presented for the first time. By exchanging sodium ions for potassium, the practical specific capacity of the titanate in both sodium and lithium half cells is considerably enhanced. Although the gross structural features of the titanate framework are maintained during the ion exchange process, the symmetry changes because sodium occupies different sites from potassium. The smaller size of the sodium ion as compared to potassium and the change in site symmetry allow more alkali metal cations to be inserted reversibly into the structure during discharge in sodium and lithium cells than in the parent compound. Insertion of lithium cations takes place at an average of about 0.8 V vs Li⁺/Li while sodium intercalation occurs at 0.5 V vs Na⁺/Na, with sloping voltage profiles exhibited for both cell configurations, implying single-phase processes. Ex situ synchrotron X-ray diffraction measurements show that a lithiated lepidocrocite is formed during discharge in lithium cells, which undergoes further lithium insertion with almost no volume change. In sodium cells, insertion of sodium initially causes an overall expansion of about 12% in the <i>b</i> lattice parameter, but reversible uptake of solvent minimizes changes upon further cycling. In the case of the sodium cells, both the practical capacity and the cyclability are improved when a more compliant binder (polyacrylic acid) that can accommodate volume changes associated with insertion processes is used in place of the more common polyvinylidene fluoride. The ability to tune the electrochemical properties of lepidocrocite titanate structures by varying compositions and utilizing ion exchange processes make them especially versatile anode materials for both lithium and sodium ion battery configurations

Effect of Surface Microstructure on Electrochemical Performance of Garnet Solid Electrolytes

Cubic garnet phases based on Al-substituted Li₇La₃Zr₂O₁₂ (LLZO) have high ionic conductivities and exhibit good stability versus metallic lithium, making them of particular interest for use in next-generation rechargeable battery systems. However, high interfacial impedances have precluded their successful utilization in such devices until the present. Careful engineering of the surface microstructure, especially the grain boundaries, is critical to achieving low interfacial resistances and enabling long-term stable cycling with lithium metal. This study presents the fabrication of LLZO heterostructured solid electrolytes, which allowed direct correlation of surface microstructure with the electrochemical characteristics of the interface. Grain orientations and grain boundary distributions of samples with differing microstructures were mapped using high-resolution synchrotron polychromatic X-ray Laue microdiffraction. The electrochemical characteristics are strongly dependent upon surface microstructure, with small grained samples exhibiting much lower interfacial resistances and better cycling behavior than those with larger grain sizes. Low area specific resistances of 37 Ω cm² were achieved; low enough to ensure stable cycling with minimal polarization losses, thus removing a significant obstacle toward practical implementation of solid electrolytes in high energy density batteries

Crystal Chemistry and Electrochemistry of Li_<i>x</i>Mn_1.5Ni_0.5O₄ Solid Solution Cathode Materials

For ordered high-voltage spinel LiMn_1.5Ni_0.5O₄ (LMNO) with the <i>P</i>4₃2₁ symmetry, the two consecutive two-phase transformations at ∼4.7 V (<i>vs</i> Li⁺/Li), involving three cubic phases of LMNO, Li_0.5Mn_1.5Ni_0.5O₄ (L_0.5MNO), and Mn_1.5Ni_0.5O₄ (MNO), have been well-established. Such a mechanism is traditionally associated with poor kinetics due to the slow movement of the phase boundaries and the large mechanical strain resulting from the volume changes among the phases, yet ordered LMNO has been shown to have excellent rate capability. In this study, we show the ability of the phases to dissolve into each other and determine their solubility limit. We characterized the properties of the formed solid solutions and investigated the role of non-equilibrium single-phase redox processes during the charge and discharge of LMNO. By using an array of advanced analytical techniques, such as soft and hard X-ray spectroscopy, transmission X-ray microscopy, and neutron/X-ray diffraction, as well as bond valence sum analysis, the present study examines the metastable nature of solid-solution phases and provides new insights in enabling cathode materials that are thermodynamically unstable