Single crystals of (Ca2CoO3)0.7CoO2 were grown using a modified strontium chloride flux technique, and their electrical and thermal properties were determined. Growth conditions were established for obtaining plate-like single crystals of a relatively large size (5×10×0.05 mm3). At 973 K, thermoelectric power S and electrical resistivity ρ of the specimens are ≈240 μV K−1 and ≈2.3×10−5 Ω m, respectively, and their thermal conductivity κ is nearly 3 W m1 K−1, as determined by mathematical extrapolation. The figure of merit ZT=S2T/ρκ derived therefrom is ≈0.87 at 973 K. The relatively low κ is likely the result of a misfit structure between the CoO2 layer and the Ca2CoO3 slab.

Rechargeable potassium-ion batteries have been gaining traction as not only promising low-cost alternatives to lithium-ion technology, but also as high-voltage energy storage systems. However, their development and sustainability are plagued by the lack of suitable electrode materials capable of allowing the reversible insertion of the large potassium ions. Here, exploration of the database for potassium-based materials has led us to discover potassium ion conducting layered honeycomb frameworks. They show the capability of reversible insertion of potassium ions at high voltages (~4 V for K2Ni2TeO6) in stable ionic liquids based on potassium bis(trifluorosulfonyl) imide, and exhibit remarkable ionic conductivities e.g. ~0.01 mS cm−1 at 298 K and ~40 mS cm–1 at 573 K for K2Mg2TeO6. In addition to enlisting fast potassium ion conductors that can be utilised as solid electrolytes, these layered honeycomb frameworks deliver the highest voltages amongst layered cathodes, becoming prime candidates for the advancement of high-energy density potassium-ion batteries.

Improving costs and scale reflect looming opportunities

The degradation behaviors of bare and Al-oxide coated LiCoO 2 in the high-voltage phase transition region were investigated at the charge voltage of 4.7 V. In both materials, two voltage plateaus that indicate phase transitions from the O3 to H1-3 and O1 phases were observed in the first charge/discharge. Bare LiCoO 2 exhibited considerably decreased capacity, and increased polarization and charge transfer resistance in the cycle test, whereas these changes were remarkably suppressed in the coated LiCoO 2 . The phase transitions of the coated LiCoO 2 can be assumed to be fairly reversible, since the voltage plateaus remained even after 20 cycles. After the cycle tests, stacking faults were observed throughout the bare LiCoO 2 particle. Pitting corrosion occurred on the faults, and the formation of a spinel-like layer was observed on the surface of the cycled bare LiCoO 2 . The pitting corrosion caused intrinsic capacity fading by Co dissolution. The formation of the spinel-like layer also resulted in effective capacity fading due to the increased polarization. Both the pitting corrosion and the formation of the spinel-like layer were markedly suppressed by the surface coating. Therefore, a surface coating that stabilizes the electrode/electrolyte interface greatly affects the charge/discharge characteristics, even in the high-voltage phase transition region. Layered LiCoO 2 , which is the most common positive electrode material used in Li-ion batteries, has a high theoretical capacity of 274 mA h g −1 and high energy density. 1 Various researches have been in progress to improve its practical performance.2-23 However, its actual capacity in commercial Li-ion batteries is typically limited to approximately 60% of the theoretical value, while its charging voltage is no more than 4.4 V (vs. Li + /Li). This is because the LiCoO 2 electrodes rapidly degrade at higher charging voltages. Until now, many different mechanisms have been proposed for the degradation of LiCoO 2 , such as a transition to the monoclinic phase, 4,24,25 dissolution of Co,6,26 formation of the spinel phase, 27-29 attack by HF, 9,11,15,16 and reduction of Co. 20,29,30 Additionally, at charge voltages exceeding 4.5 V, LiCoO 2 exhibits transitions from the O3 phase to the H1-3 and O1 phases.31-33 Since these phase transitions involve rearranging the atomic stacking (i.e., change in the stacking order of the O-Co-O atomic sheets), the transitions are believed to cause irreversible degradation of the LiCoO 2 structure, resulting in poor charge/discharge reversibility in the highvoltage region.7,13 However, the detailed degradation mechanism has not been elucidated.It is known that the charge/discharge performance of positive electrode materials can be improved with surface coatings. Various coating materials have been studied. [3][4][5]7,8,10,11,17,[34][35][36][37] Recently, an oxidecoating containing Al and P has been reported to suppress the capacity fading in LiCoO 2 even at above 4.5 V. 12,23 This suggests that the surface structure of LiCoO 2 st...

Bi 2 Sr 2 Co 2 O y (BC-222) whiskers were grown with an excellent Seebeck coefficient (S) and electrical resistivity (ρ) for power generation applications at high temperatures in air. The S value of these long BC-222 whiskers reached almost 300 μV K−1 at 973 K. Thermal conductivity (κ) was measured and found to be suppressed to a low value of about 2.0 W m−1 K−1. This is considered due not only to a phonon–phonon interaction but also other scattering processes. The resulting dimensionless thermoelectric figure of merit ZT(=S2T/ρκ) was more than 1.1, which corresponds to a conversion efficiency of almost 10% at 973 K in air.