Ruibo Zhang

Understanding the stability of MnPO4

We have revealed the critical role of carbon coating in the stability and thermal behaviour of olivine MnPO4 obtained by chemical delithiation of LiMnPO4. (Li)MnPO4 samples with various particle sizes and carbon contents were studied. Carbon-free LiMnPO4 obtained by solid state synthesis in O2 becomes amorphous upon delithiation. Small amounts of carbon (0.3 wt%) help to stabilize the olivine structure, so that completely delithiated crystalline olivine MnPO4 can be obtained. Larger amount of carbon (2 wt%) prevents full delithiation. Heating in air, O2, or N2 results in structural disorder (<300 °C), formation of an intermediate sarcopside Mn3(PO4)2 phase (350–450 °C), and complete decomposition to Mn2P2O7 on extended heating at 400 °C. Carbon coating protects MnPO4 from reacting with environmental water, which is detrimental to its structural stability.

Structure, defects and thermal stability of delithiated olivine phosphates

Studies of thermal decomposition mechanism of olivine Fe1−yMnyPO4 are reported here for inert (He), oxidizing (O2) and oxidizing and moist (air) atmospheres using in situ X-ray diffraction and thermal gravimetric analysis with mass spectroscopy. The results indicate that the olivine structure is inherently stable up to at least 400 °C and y = 0.9 for particle size down to 50 nm. However, structural disorder and oxygen loss in the presence of reductive impurities, e.g. carbon and hydrogen, can occur as low as 250 °C for particles larger than 100 nm and at 150 °C for 50 nm particles. Fe1−yMnyPO4 is hygroscopic at high Mn contents, y ≥ 0.6, and moisture exposure is more detrimental to its thermal stability than carbon or small particle size. Nano-Fe1−yMnyPO4 (y > 0.7) with particle size about 50 nm, when exposed to moisture, disorders at 150 °C and transforms to sarcopside phase by 300 °C, no matter whether the delithiation was done electrochemically or chemically. Contrary, under inert atmosphere the sample produced by chemical delithiation is stable up to 400 °C.

What Happens to LiMnPO4 upon Chemical Delithiation

Olivine MnPO4 is the delithiated phase of the lithium-ion-battery cathode (positive electrode) material LiMnPO4, which is formed at the end of charge. This phase is metastable under ambient conditions and can only be produced by delithiation of LiMnPO4. We have revealed the manganese dissolution phenomenon during chemical delithiation of LiMnPO4, which causes amorphization of olivine MnPO4. The properties of crystalline MnPO4 obtained from carbon-coated LiMnPO4 and of the amorphous product resulting from delithiation of pure LiMnPO4 were studied and compared. The phosphorus-rich amorphous phases in the latter are considered to be MnHP2O7 and MnH2P2O7 from NMR, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analysis. The thermal stability of MnPO4 is significantly higher under high vacuum than at ambient condition, which is shown to be related to surface water removal.

Hydrothermal synthesis, structure refinement, and electrochemical characterization of Li2CoGeO4 as an oxygen evolution catalyst

Lithium cobalt germanate (Li2CoGeO4) has been synthesized for the first time by a hydrothermal method at 150 °C. Elemental composition, morphology, and crystal structure of this compound were characterized by various analytical techniques including SEM, TEM, ICP, and XRD analyses. Structure refinement and DFT calculation suggests the crystal structure of the resulting Li2CoGeO4 from hydrothermal synthesis is isostructural to Li2ZnGeO4, significantly different from previous reports. Electrochemical studies confirmed Li2CoGeO4 as an active catalyst for oxygen evolution reaction (OER). In alkaline electrolyte (0.1 M NaOH), rotating disk electrodes made with Li2CoGeO4 as catalyst have a Tafel slope of ca. 67 mV dec−1 and an overpotential of 330 mV at 50 μA cm−2cat, about 90 mV less than the electrodes containing another known OER catalyst, Co3O4. Further characterization with cyclic voltammetry, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and elemental analyses revealed the oxidation of Co from 2+ to 3+ during the reaction along with significant surface amorphization and loss of Li and Ge from the catalyst.