Ni-rich layered oxides are significantly promising cathode materials for commercial high-energy-density lithium-ion batteries. However, their major bottlenecks limiting their widespread applications are capacity fading and safety concerns caused by their inherently unstable crystal structure and highly reactive surface. Herein, surface structure and bulk charge regulation are concurrently achieved by introducing high-valence Ta 5+ ions in Ni-rich cathodes, which exhibit superior electrochemical properties and thermal stability, especially a remarkable cyclic stability with a capacity retention of 80% for up to 768 cycles at a 1C rate versus Li/Li + . Due to the partial Ta enrichment on surface, the regulated surface enables high reversibility of Li + insertion/extraction by preventing surface Ni reduction in deep charging. Moreover, bulk charge regulation that boosts charge density and its localization on oxygen remarkably suppresses microcracks and oxygen loss, which in turn prevents the fragmentation of the regulated surface and structural degradation associated with oxygen skeleton. This study highlights the significance of an integrated optimization strategy for Ni-rich cathodes and, as a case study, provides a novel and deep insights into the underlying mechanisms of high-valence ions substitution of Ni-rich layered cathodes.
The atomic scale variation of tantalum ion doped Li 4 NbO 4 F ceramics is initially predicted through first principle calculations. Subsequently, Li 4 Nb (1- x ) Ta x O 4 F (0.00 ≤ x ≤ 0.10) ceramics are prepared using the solid phase reaction method. After sintering at 875 °C for 4 h, the Li 4 Nb 0.96 Ta 0.04 O 4 F ceramics exhibit optimum dielectric properties : ε r = 14.73 ± 0.19, Q × f = 83,439 ± 2810 GHz (@9.31 GHz, improved by 41 %), τ f = - 28.33 ± 1.36 ppm/°C. Both the increase in relative density from 95.9 ± 0.32 % to 96.2 ± 0.34 % and the increase in grain size from 14.9 μm to 15.6 μm are non-intrinsic factors that contribute to the optimization of the dielectric properties. Instead, the internal factors suggest that the primary cause of the gradual decrease in ε r is the redshift of the Raman wave number. The reduction in dielectric loss is attributable to the weakening of damping behavior, as well as the increase in lattice energy and electron cloud density. Finally, promising potential of Li 4 Nb 0.96 Ta 0.04 O 4 F ceramics for low-temperature co-fired ceramic (LTCC) applications is indicated by their favourable chemical compatibility with Ag electrodes.
Dan Liu,Penghui Ren,Di Zhao,Songyu Li,Jianqiao Wang,Hang Zhou,Wei Liu,Yuheng Zeng,Xuegong Yu,Peng Wang,Can Cui
Transition metal oxides (TMOs) with high work function (WF) are typically used for hole selective layers for crystalline silicon (c-Si) heterojunction solar cells. Among them, tungsten oxide (WO3−x) exhibits a wide bandgap and high thermal stability; however, the photoelectrical conversion efficiency (PCE) of solar cells based on WO3−x lags behind its counterparts (MoO3−x and V2O5−x). Moreover, the films are usually deposited by vacuum methods, which makes it difficult to modulate the WF or oxygen vacancies (VO) to improve the hole selectivity. In this work, a low cost, solution-processed WO3−x film with ion doping is deposited as a hole selective layer for p-Si heterojunction solar cells. Experimental results and density functional theory calculations demonstrate that Ta5+ tends to replace W5+ and strengthens the W–O binding, which facilitate the reduction of VO and increase the WF of WO3−x films, resulting in the increase of band bending of the c-Si surface and the improvement of hole selectivity. The solar cells with a structure of p-Si/Ta5+:WO3−x/Ag have achieved a remarkable PCE of 18.67%, which is the highest efficiency obtained for WO3−x/c-Si heterojunctions based on solution processing up to now. This work also paves the way for extending the doping strategy of WO3−x to other optoelectronic device applications.
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