Ruediger Oesten

Lithium fluoroalkylphosphates: a new class of conducting salts for electrolytes for high energy lithium-ion batteries

The effort to develop improved electrolytes that satisfy the requirements of lithium rechargeable batteries has intensified the search for new conducting salts having an improved chemical and electrochemical stability. With lithium fluoroalkylphosphates, we introduce a new class of conducting salts for electrolytes for high energy lithium-ion batteries. The results of electrochemical studies of Li[(C2F5)3PF3] in organic carbonates in comparison to LiPF6 including electrochemical stability and charge–discharge efficiency are reported. In addition, the influence of perfluorinated alkyl groups on stability towards hydrolysis is demonstrated.

HIGH-PERFORMANCE GEL-TYPE LITHIUM ELECTROLYTE MEMBRANES

Abstract Battery-grade solution products have been used for the synthesis of new types of poly(acrylonitrile) PAN-based polymer electrolyte membranes. Basically, two classes of membranes have been prepared differing by the type of lithium salt in the ethylene carbonate–dimethyl carbonate (EC–DMC) solution trapped in the PAN matrix, i.e. LiPF 6 or LiC(CF 3 SO 2 ) 3 lithium methide salt, respectively. The results demonstrate that both classes of membranes have high conductivity and very good chemical and electrochemical stability. These unique characteristics make the membranes suitable for applications in high-voltage, rechargeable lithium batteries.

Electrochemical and in-situ XRD characterization of LiNiO2 and LiCo0.2Ni0.8O2 electrodes for rechargeable lithium cells

Abstract The effect of Co⇒Ni substitution on the structural and electrochemical properties of Li x Co 1− y Ni y O 2 (0 x y =l or y =0.8) was studied by a combination of slow scan rate cyclic voltammetry (SSCV) and in-situ XRD techniques. It was shown that LiCo 0.2 Ni 0.8 O 2 exhibits a single phase region upon oxidation up to 4.08 V (Li/Li + ), while LiNiO 2 undergoes two phase transitions in the same potential range. At potentials higher than 4.08 V, both compounds exchange lithium via phase transitions associated with a drastic shrinkage of the interlayer distance in their structures. Long-term cycling between 3.0 and 4.2 V, at full utilization of the active masses, show that the performance of the LiCo 0.2 Ni 0.8 O 2 electrode is lower than that of LiNiO 2 , in spite of the homogeneous character of the solid-state reaction in the former compound. As was shown by SSCV and XRD analysis, the decrease of capacity upon cycling in these compounds can be attributed to the loss of the electrode ability of full Li deintercalation. This seems to relate to an appearance of structural defects, such as cation mixing in the Li layers.

In situ XRD study of Li deintercalation from two different types of LiMn2O4 spinel

Abstract A mechanism of the capacity loss in LiMn2O4 was studied by comparison of two types of spinel with different morphologies and specific surface areas (2.9 and 0.8 m2 g−1). The combination of SSCV and in situ XRD techniques shows that the irreversible structural conversion occurs partially in spinel of the low specific surface area during the first Li deintercalation at a potential 4.0 V. This conversion is mainly responsible for the capacity loss in this type of spinel. It seems that these peculiarities of the electrochemical behavior are related to the presence of the layered component in this material. In contrast, spinel of the high specific surface area presents an example of a stable material without considerable variation of its capacity and XRD patterns after long-term cycling. This type of spinel demonstrates two distinct phase transitions in the vicinity of 4.0–4.1 V, corresponding to two SSCV peaks.