Positive and negative electrodes and Li-ion batteries
Positive Electrodes
The positive electrode research focusses on new efficient materials like i) fluorinated pyroborates which is stimulated by the lightness of borate and the possible analogy with pyrophosphates which have higher potentials than phosphates or ii) iron oxalates in which the redox properties of iron are studied. Full in situ Mössbauer spectrometry and TEM studies are also used to follow the Li insertion/extraction from defective Li(Fe,Mn)PO4 prepared by co-precipitation/annealing in air. Work is also directed towards high capacity layered Li2MO3 oxides such as Li2Ru1-ySnyO3. These materials show greatly reduced voltage decay compared to the widely studied Li(LixNiyCozMn1-x-y-z)O2 phases and will aid the fundamental understanding of these important compounds.
The positive electrode research focusses on new efficient materials like i) fluorinated pyroborates which is stimulated by the lightness of borate and the possible analogy with pyrophosphates which have higher potentials than phosphates or ii) iron oxalates in which the redox properties of iron are studied. Full in situ Mössbauer spectrometry and TEM studies are also used to follow the Li insertion/extraction from defective Li(Fe,Mn)PO4 prepared by co-precipitation/annealing in air. Work is also directed towards high capacity layered Li2MO3 oxides such as Li2Ru1-ySnyO3. These materials show greatly reduced voltage decay compared to the widely studied Li(LixNiyCozMn1-x-y-z)O2 phases and will aid the fundamental understanding of these important compounds.
Fig. 1. Load curves for several cycles for a Li/Li2Ru0.75Sn0.25O3 cell together with (lower left inset) its capacity retention and (top right inset) its power rate. Cells cycled at C/10.
A full electrochemical/TEM study (Fig.2) aims to understand the real role of water on the Si surface, by comparing oxidation by H2O and by O2. Various electrolytes and additives are tested.
The study aims to protect the silicon surface against further oxidation due to the water of the electrode formulation through a preoxidation in air.
Fig2: TEM characterisation of the oxidation of Si electrodes
The effects of relaxation on materials are studied in TiSnSb, a model conversion electrode material, by using 119Sn Mössbauer and 7Li MAS NMR spectoscopies. Specifically designed in situ and ex situ experiments are used to follow the chemical changes which occur in the electrode during cycling and during the relaxation.
Solid-state Li-ion batteries
From the literature, while Li4SiO4 and Li3PO4 themselves show bad conductivity, the Li4-xSi1-xPxO4 solid-solution gives excellent conductivity (10-2 S/cm) at 300°C, and is a potential solid electrolyte. Li4-xSi1-xPxO4 samples with x = 0.25, 0.5 and 0.75 are studied to try to reach such high conductivities. Modelling is in progress to determine the Li-ion migration pathways in these materials.
A full all-solid-state TiO2-nt/polymer electrolyte/LNMO microbattery is investigated. Conformal polymer coating is performed on TiO2-nt by electropolymerisation from monomers and LiTFSI salt, and appears beneficial for electrochemical performance.
