Marian Cristian Stan

Puzzling out the origin of the electrochemical activity of black P as a negative electrode material for lithium-ion batteries

Black phosphorus prepared via the mineralization concept displays promising characteristics with respect to Li-ion battery applications. Although the theoretical specific capacity of black phosphorus as a negative electrode material is 2596 mA h g−1, a good cycling stability at high capacities, however, is still missing. Even worse, a large capacity drop after the first cycle is present and, depending on the electrode preparation, a poor first cycle coulombic efficiency (only 8%) represents a huge challenge to overcome. Herein we report on the performances of black phosphorus as a negative electrode material and display the roles of Cu3P and the copper current collector in this context. Furthermore, a composite material prepared by reacting pristine black phosphorus particles with a copper solution under solvothermal conditions shows a first cycle coulombic efficiency of 43%. We show that the presence of copper is crucial for the electrochemical performance of black phosphorus and that also the particle size and the electrode preparation procedure play a crucial role.

O3-type Na[Fe1/3Ni1/3Ti1/3]O2 cathode material for rechargeable sodium ion batteries

A Na[Fe1/3Ni1/3Ti1/3]O2 cathode material for sodium-ion batteries has been synthesized by a solid-state reaction method. The obtained Na[Fe1/3Ni1/3Ti1/3]O2 shows an O3-type structure, and delivers a discharge capacity of 117 mA h g−1 at a current density of 10 mA g−1 in a range of 1.5–4.0 V at 20 °C. Furthermore, the Na[Fe1/3Ni1/3Ti1/3]O2 cathode material shows good rate capability and cycling stability. The working and structural transition mechanisms of the Na[Fe1/3Ni1/3Ti1/3]O2 material are examined by ex situ X-ray absorption spectroscopy (XAS) and in situ X-ray diffraction (XRD) methods. The valence state of Fe ions in the Na[Fe1/3Ni1/3Ti1/3]O2 material is estimated to be 2.67+. The main redox couple is Ni2+/Ni4+, but the Fe2+/Fe3+ contributes a little as well at voltages below 2.0 V. The original O3 phase transforms to a P3 phase during sodium extraction with good reversibility, but a slightly irreversible change of lattice parameters may lead to capacity decay during long-term cycling. Moreover, the gas evolution during the first charge/discharge process is analyzed by using an operando mass spectrometry technique. The obvious release of CO2 gas at the end of the charge process may be the other origin of the capacity decay. Nevertheless, the absence of O2 evolution indicates an improved safety of the Na/Na[Fe1/3Ni1/3Ti1/3]O2 cell.

A 3D porous Li-rich cathode material with an in situ modified surface for high performance lithium ion batteries with reduced voltage decay

High crystallinity Li-rich porous materials integrated with an in situ formed surface containing carbonaceous compounds are synthesized through a facile approach. The rationally designed procedure involves the formation of a specific morphology of a hydroxide precursor assisted by a self-made template and subsequent high temperature treatment to obtain a Li1.2Mn0.56Ni0.16Co0.08O2 target product. The porous morphology is investigated using field-emission scanning electron microscopy and its surface area is quantitatively examined by gas sorption analysis coupled with the Brunauer–Emmett–Teller method. The crystallinity is characterized by X-ray diffraction and high-resolution transmission electron microscopy. X-ray photoelectron spectroscopy, CHN elemental analysis and small angle neutron scattering confirm the presence of carbonaceous compounds in the surface composition. The prepared material exhibits superior discharge rate capability and excellent cycling stability. It shows minor capacity loss after 100 cycles at 0.5C and retains 94.9% of its initial capacity after 500 cycles at 2C. Even more notably, the “voltage decay” during cycling is also significantly decreased. It has been found that carbonaceous compounds play a critical role in reducing the layered to spinel structural transformation during cycling. Therefore, the present porous Li-rich material with surface modified carbonaceous compounds represents an attractive material for advanced lithium-ion batteries.