Jun-Yu Piao

Copper-substituted Na0.67Ni0.3−xCuxMn0.7O2 cathode materials for sodium-ion batteries with suppressed P2–O2 phase transition

P2-type sodium layered oxides NaxMO2 (M = transition metal) are considered as one kind of promising cathode material for sodium-ion batteries because of their known structures, superior electrochemical properties, and their ease of synthesis. The Ni2+/Ni3+ and Ni3+/Ni4+ redox reactions endow the P2–Na2/3Ni1/3Mn2/3O2 electrode with a relatively high operating voltage and high specific capacity. However, the phase transition from P2 to O2 and Na+/vacancy ordering make P2–Na2/3Ni1/3Mn2/3O2 susceptible to severe voltage and capacity decay. Herein, we propose to employ the electrochemically active copper(II) as a unique substituent to stabilize the P2 phase, forming Na0.67Ni0.3−xCuxMn0.7O2 (x = 0, 0.1, 0.2 and 0.3). Our work highlights the importance of Cu(II) in the structural engineering of high performance cathode materials, whose existence can not only stabilize the P2 phase against the notorious phase transition, but also contribute to the rechargeable capacity due to the high potential Cu2+/Cu3+ redox. We identified that the cathode formulated as P2-type Na0.67Ni0.1Cu0.2Mn0.7O2 shows favorable battery performance with much-alleviated structural degradation.

Controlling the Compositional Chemistry in Single Nanoparticles for Functional Hollow Carbon Nanospheres

Hollow carbon nanostructures have inspired numerous interests in areas such as energy conversion/storage, biomedicine, catalysis, and adsorption. Unfortunately, their synthesis mainly relies on template-based routes, which include tedious operating procedures and showed inadequate capability to build complex architectures. Here, by looking into the inner structure of single polymeric nanospheres, we identified the complicated compositional chemistry underneath their uniform shape, and confirmed that nanoparticles themselves stand for an effective and versatile synthetic platform for functional hollow carbon architectures. Using the formation of 3-aminophenol/formaldehyde resin as an example, we were able to tune its growth kinetics by controlling the molecular/environmental variables, forming resin nanospheres with designated styles of inner constitutional inhomogeneity. We confirmed that this intraparticle difference could be well exploited to create a large variety of hollow carbon architectures with desirable structural characters for their applications; for example, high-capacity anode for potassium-ion battery has been demonstrated with the multishelled hollow carbon nanospheres.

Construction of uniform transition-metal phosphate nanoshells and their potential for improving Li-ion battery performance

The construction of uniform core–shell nanostructures using transition-metal phosphates as the shell has been a long-standing challenge in the field of nanotechnology. Due to their extremely low solubility constants, metal phosphates are prone to precipitate independently in solution, making a heterogeneous growth around the preexisting seeds extremely hard to achieve. Here, we demonstrated that it is possible to overcome the hurdles arising from their intrinsic growth habit, and form uniform metal phosphate nanoshells with their thickness tuned with nanometer accuracy. Particularly, for the formation of different nanoshells including Ni3(PO4)2, Co3(PO4)2, and Mn3(PO4)2, it has been found that a cooperative effort to control both the solvent environment and the precipitating agent is critical to tuning the growth kinetics of these metal phosphates, making it convenient for us to grow uniform nanoshells around a large variety of seeds. The application of this synthetic protocol for the surface treatment of LiNi0.5Mn1.5O4, a well-known high voltage cathode material in lithium ion batteries, demonstrates that a 4 nm coating layer of Co3(PO4)2 can be achieved as a protective shell, which provides a much improved cycling stability to the electrode and holds promising potential for its application as a high energy cathode.

Controlling the Reaction of Nanoparticles for Hollow Metal Oxide Nanostructures

Hollow nanostructures of metal oxides have found broad applications in different fields. Here, we reported a facile and versatile synthetic protocol to prepare hollow metal oxide nanospheres by modulating the chemical properties in solid nanoparticles. Our synthesis design starts with the precipitation of urea-containing metal oxalate, which is soluble in water but exists as solid nanospheres in ethanol. A controlled particle hydrolysis is achieved through the heating-induced urea decomposition, which transforms the particle composition in an outside-to-inside style: The reaction starts from the surface and then proceeds inward to gradually form a water-insoluble shell of basic metal oxalate. Such a reaction-induced solubility difference inside nanospheres becomes highly efficient to create a hollow structure through a simple water wash process. A following high temperature treatment forms hollow nanospheres of different metal oxides with structural features suited to their applications. For example, a high performance anode for Li-ion intercalation pseudocapacitor was demonstrated with the hollow and mesoporous Nb2O5 nanospheres.

Controlled synthesis of hierarchically-structured MnCo 2 O 4 and its potential as a high performance anode material

To satisfy the upsurging demand for energy storage in modern society, anode materials which can deliver high capacity have been intensively researched for the next generation lithium ion batteries. Typically, the binary MnCo2O4 with a characteristic coupled metal cations showed promising potential due to its high theoretical capacity and low cost. Here, by means of a well-designed synthesis control, we demonstrated a scalable process to achieve a hierarchical structure of MnCo2O4, which existed as uniform microspheres with embedded mesopores, showing favorable structural characters for high performance during a fast charge/discharge process. Our synthesis highlighted the importance of sodium salicylate as an essential additive to control the precipitation of the two involved metal cations. It was proved that a dual role was played sodium salicylate which cannot only facilitate the formation of microspheric shape, but also act as an effective precursor for the creation of inner mesopores. We confirmed that the hierarchically-structured MnCo2O4 showed outstanding performance when it was tested as an anode material in lithium ion batteries as revealed by its extraordinary cycling stability and high rate capability.

A facile template free synthesis of porous carbon nanospheres with high capacitive performance

Porous carbon nanospheres have been widely used in different fields such as electric devices, catalysts, and water treatment. Here we will introduce a template-free process for the preparation of porous carbon nanospheres starting from a direct 3-aminophenol formaldehyde polymerization in a mixed solution. We identify that the addition of different alcohols, particularly ethanol and n-butanol, is able to change the growth habit of the polymer nanospheres and introduce a favorable inner compositional homogeneity for the preparation of porous structure. After the carbonization of the polymer nanospheres, the obtained porous carbon exhibits promising electrochemical performance when used as electrode material in super capacitor.

Construction of Uniform Cobalt-Based Nanoshells and Its Potential for Improving Li-Ion Battery Performance

Surface cobalt doping is an effective and economic way to improve the electrochemical performance of cathode materials. Herein, by tuning the precipitation kinetics of Co2+, we demonstrate an aqueous-based protocol to grow uniform basic cobaltous carbonate coating layer onto different substrates, and the thickness of the coating layer can be adjusted precisely in nanometer accuracy. Accordingly, by sintering the cobalt-coated LiNi0.5Mn1.5O4 cathode materials, an epitaxial cobalt-doped surface layer will be formed, which will act as a protective layer without hindering charge transfer. Consequently, improved battery performance is obtained because of the suppression of interfacial degradation.