Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/extraction of charge carriers (for example, lithium ions). Common synthesis methods can introduce large cations such as potassium, barium and ammonium ions into the tunnels, but how these cations affect charge storage performance is not fully understood. Here, we report the role of tunnel cations in governing the electrochemical properties of electrode materials by focusing on potassium ions in α-MnO2. We show that the presence of cations inside 2 × 2 tunnels of manganese dioxide increases the electronic conductivity, and improves lithium ion diffusivity. In addition, transmission electron microscopy analysis indicates that the tunnels remain intact whether cations are present in the tunnels or not. Our systematic study shows that cation addition to α-MnO2 has a strong beneficial effect on the electrochemical performance of this material.

While 3D printing of rechargeable batteries has received immense interest in advancing the next generation of 3D energy storage devices, challenges with the 3D printing of electrolytes still remain. Additional processing steps such as solvent evaporation were required for earlier studies of electrolyte fabrication, which hindered the simultaneous production of electrode and electrolyte in an all-3D-printed battery. Here, a novel method is demonstrated to fabricate hybrid solid-state electrolytes using an elevated-temperature direct ink writing technique without any additional processing steps. The hybrid solid-state electrolyte consists of solid poly(vinylidene fluoride-hexafluoropropylene) matrices and a Li -conducting ionic-liquid electrolyte. The ink is modified by adding nanosized ceramic fillers to achieve the desired rheological properties. The ionic conductivity of the inks is 0.78  × 10 S cm . Interestingly, a continuous, thin, and dense layer is discovered to form between the porous electrolyte layer and the electrode, which effectively reduces the interfacial resistance of the solid-state battery. Compared to the traditional methods of solid-state battery assembly, the directly printed electrolyte helps to achieve higher capacities and a better rate performance. The direct fabrication of electrolyte from printable inks at an elevated temperature will shed new light on the design of all-3D-printed batteries for next-generation electronic devices.

Correctly establishing a structure-property relationship is necessary to rationally develop energy materials for performance optimization. Bulk characterizations fall short of deciphering localized structural features at nanoscale and atomic scale. This work atomically resolves structural heterogeneity existing in single MnO 2 nanoparticles and demonstrates its significant effect on energy storage property, which was neglected by traditional bulk characterizations. Attention should thus be paid to controllable synthesis toward structural homogeneity with predictable/ tunable energy storage property and the proper choice of structural characterization tools. SUMMARY[MnO 6 ] octahedra are the structural units for a large family of manganese dioxides (MnO 2 ) possessing one-dimensional tunnel structures with extensive applications in catalysis and energy storage. Despite the long-range [MnO 6 ] ordering confirmed by conventional diffraction tools, surprisingly, the functional properties of a specific MnO 2 tunnel phase still vary significantly in literature with unclear structural origins. Here, we demonstrate the existence of tunnel heterogeneity featuring localized tunnel intergrowths within single MnO 2 nanoparticles via atomically resolved imaging. The degree of tunnel heterogeneity increases with the size increase of tunnels from b-MnO 2 (1 3 1 tunnel) to a-MnO 2 (2 3 2 tunnel), and to todorokite MnO 2 (3 3 3 tunnel). Furthermore, the tunnel heterogeneity within one MnO 2 nanoparticle significantly affects the energy storage kinetics even down to sub-nanometer scale. These findings are expected to call for renewed attention to the controlled synthesis of homogeneous tunnel-specific phases with predictable properties and to yield a more precise structure-property relationship in polymorphic materials.

Thermal runaways triggered by the oxygen release from oxide cathode materials pose a major safety concern for widespread application of lithium ion batteries. Utilizing in situ aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) at high temperatures, we show that oxygen release from LiCoO cathode crystals is occurring at the surface of particles. We correlated this local oxygen evolution from the LiCoO structure with local phase transitions spanning from layered to spinel and then to rock salt structure upon exposure to elevated temperatures. Ab initio molecular dynamics simulations (AIMD) results show that oxygen release is highly dependent on LiCoO facet orientation. While the [001] facets are stable at 300 °C, oxygen release is observed from the [012] and [104] facets, where under-coordinated oxygen atoms from the delithiated structures can combine and eventually evolve as O. The novel understanding that emerges from the present study provides in-depth insights into the thermal runaway mechanism of Li-ion batteries and can assist the design and fabrication of cathode crystals with the most thermally stable facets.

To treat impairments in hard tissues or overcome pathological calcification in soft tissues, a detailed understanding of mineralization pathways of calcium phosphate materials is needed. Here, we report a detailed mechanistic study of hydroxyapatite (HA) mineralization pathways in an artificial saliva solution via in situ liquid cell transmission electron microscopy (TEM). It is found that the mineralization of HA starts by forming ion-rich and ion-poor solutions in the saliva solution, followed by coexistence of the classical and nonclassical nucleation processes. For the nonclassical path, amorphous calcium phosphate (ACP) functions as the substrate for HA nucleation on the ACP surface, while the classical path features direct HA nucleation from the solution. The growth of HA crystals on the surface of ACP is accompanied by the ACP dissolution process. The discoveries reported in this work are important to understand the physiological and pathological formation of HA minerals, as well as to engineer the biomineralization process for bone healing and hard tissue repairs.