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Atomic layer deposition has proven to be a particularly attractive approach for decorating mesoporous carbon substrates with redox active metal oxides for electrochemical energy storage. This study, for the first time, capitalizes on the cyclic character of atomic layer deposition to obtain a highly conformal and atomically controlled decoration of carbon onions with alternating stacks of vanadia and titania. The addition of 25 mass% TiO2 leads to an expansion of the VO2 unit cell, thus greatly enhancing lithium intercalation capacity and kinetics. Electrochemical characterization revealed ultrahigh discharge capacity of up to 382 mAh[middle dot]g-1 of the composite electrode (554 mAh[middle dot]g-1 per metal oxide) with an impressive capacity retention of 82 mAh[middle dot]g-1 (120 mAh[middle dot]g-1 per metal oxide) at a high discharge rate of 20 A[middle dot]g-1 or 52 C. Rigorous stability benchmarking showed superior stability over 3,000 cycles when discharging to a reduced potential of -1.8 V vs. carbon. These capacity values are among the highest reported for any metal oxide system, while in addition, supercapacitor-like power performance and longevity are achieved. On a device level, high specific energy and power of up to 110 Wh[middle dot]kg-1 and 6 kW[middle dot]kg-1, respectively, were achieved when employing the hybrid material as anode versus activated carbon cathode.
Nb2O5 has been explored as a promising anode material for use as lithium-ion batteries (LIBs), but depending on the crystal structure, the specific capacity was always reported to be usually around or below 200 mAh/g. For the first time, we present coarse-grained Nb2O5 materials that significantly overcome this capacity limitation with the promise of enabling high power applications. Our work introduces coarse-grained carbide-derived Nb2O5 phases obtained either by a one-step or a two-step bulk conversion process. By in situ production of chlorine gas from metal chloride salt at ambient pressure, we obtain in just one step directly orthorhombic Nb2O5 alongside carbide-derived carbon (o-Nb2O5/CDC). In situ formation of chlorine gas from metal chloride salt under vacuum conditions yields CDC covering the remaining carbide core, which can be transformed into metal oxides covered by a carbon shell upon thermal treatment in CO2 gas. The two-step process yielded a mixed-phase tetragonal and monoclinic Nb2O5 with CDC (m-Nb2O5/CDC). Our combined diffraction and spectroscopic data confirm that carbide-derived Nb2O5 materials show disordering of the crystallographic planes caused by oxygen deficiency in the structural units and, in the case of m-Nb2O5/CDC, severe stacking faults. This defect engineering allows access to a very high specific capacity exceeding the two-electron transfer process of conventional Nb2O5. The charge storage capacities of the resulting m-Nb2O5/CDC and o-Nb2O5/CDC are, in both cases, around 300 mAh/g at a specific current of 10 mA/g, thereby, the values are significantly higher than that of the state-of-the-art for Nb2O5 as a LIB anode. Carbide-derived Nb2O5 materials also show robust cycling stability over 500 cycles with capacity fading only 24% for the sample m-Nb2O5/CDC and 28% for o-Nb2O5/CDC, suggesting low degree of expansion/compaction during lithiation and delithiation.
Inorganic-organic hybrid materials with redox-active components were prepared by an aqueous precipitation reaction of ammonium heptamolybdate (AHM) with para-phenylenediamine (PPD). A scalable and low-energy continuous wet chemical synthesis process, known as the microjet process, was used to prepare particles with large surface area in the submicrometer range with high purity and reproducibility on a large scale. Two different crystalline hybrid products were formed depending on the ratio of molybdate to organic ligand and pH. A ratio of para-phenylenediamine to ammonium heptamolybdate from 1 : 1 to 5 : 1 resulted in the compound [C6H10N2]2[Mo8O26] ⋅ 6 H2O, while higher PPD ratios from 9 : 1 to 30 : 1 yielded a composition of [C6H9N2]4[NH4]2[Mo7O24] ⋅ 3 H2O. The electrochemical behavior of the two products was tested in a battery cell environment. Only the second of the two hybrid materials showed an exceptionally high capacity of 1084 mAh g−1 at 100 mA g−1 after 150 cycles. The maximum capacity was reached after an induction phase, which can be explained by a combination of a conversion reaction with lithium to Li2MoO4 and an additional in situ polymerization of PPD. The final hybrid material is a promising material for lithium-ion battery (LIB) applications.
Molybdenum carbides, oxides, and mixed anionic carbide–nitride–oxides Mo(C,N,O)x are potential anode materials for lithium-ion batteries. Here we present the preparation of hybrid inorganic–organic precursors by a precipitation reaction of ammonium heptamolybdate ((NH4)6Mo7O24) with para-phenylenediamine in a continuous wet chemical process known as a microjet reactor. The mixing ratio of the two components has a crucial influence on the chemical composition of the obtained material. Pyrolysis of the precipitated precursor compounds preserved the size and morphology of the micro- to nanometer-sized starting materials. Changes in pyrolysis conditions such as temperature and time resulted in variations of the final compositions of the products, which consisted of mixtures of Mo(C,N,O)x, MoO2, Mo2C, Mo2N, and Mo. We optimized the reaction conditions to obtain carbide-rich phases. When evaluated as an anode material for application in lithium-ion battery half-cells, one of the optimized materials shows a remarkably high capacity of 933 mA h g−1 after 500 cycles. The maximum capacity is reached after an activation process caused by various conversion reactions with lithium.
