STRUCTURE AND ELECTROCHEMISTRY OF NEW LITHIUM INTERCALATION COMPOUNDS PREPARED VIA LOW TEMPERATURE TECHNIQUES

摘要

The development of secondary lithium batteries involves the use of high performance cathodic materials and many works are devoted to the research of new or modified host structures as insertion compounds. In fact the electrochemical properties of numerous Li insertion materials strongly depend on the synthesis conditions or sample treatment and this point is now largely addressed in recent papers. Therefore there has been considerable interest in developing low temperature techniques because of their ability to produce materials with different local structure, powder morphology, bulk density, stoichiometry and especially attractive new structures. Ten years ago our group started investigations in this field especially using the sol-gel processes. The present work exemplifies the interest in using the soft chemistry route for preparing new or modified host structures for Li insertion, mainly vanadium, manganese, cobalt and nickel oxides. But we will also outline some specific drawbacks encountered. Two main synthesis ways are developed: the sol-gel process and precipitation reactions. The electrochemical behaviour of Li intercalation compounds is studied and discussed in relation with the specific chemical and structural properties induced by the synthesis way. In a first part we will report results obtained for the V_2O_5 xerogel as well as the hydrated sodium bronze Na_(0.33)V_2O_5, 1.6H_2O. Then we will demonstrate the interest of a new group of intercalation compounds, i.e., vanadium mixed oxides M_yV_2O_(5.16) characterized by a specific and attractive electrochemical behaviour. M is a divalent or trivalent cation: M~(2+) = Mg~(2+), Mn~(2+), Ni~(2+) with y = 0.16; M~(3+) = Al~(3+), Ga~(3+), Fe~(3+), Cr~(3+) with y = 0.11. Their synthesis, structural characterization as well as their electrochemical properties for the Li insertion reaction (thermodynamics, kinetics, cycle life) will be developed here. The results are compared with that known for the parent oxide V_2O_5. In particular, the presence of chromium ions is shown to induce a notable improvement of the capacity retention. Second, we will focus on the electrochemical features of various MnO_2 oxides prepared through the reduction of AMnO_4 (A= Li, Na, K) aqueous solution with reducing agents in alkaline, neutral or acidic medium. Of the compounds evaluated in this work, the sol-gel birnessite as well as the Bi-doped oxide Bi_(0.105)Mn_(2.12), 0.96 H_2O exhibit the most attractive properties with for instance more than 160 Ah/kg after 30 cycles at C/20 rate for the sol-gel oxide in the potential window 4.3/2V. The last part of our contribution will be devoted to high voltage cathode materials, such as LiCoO_2, and LiNiO_2. In the latter case we will focus our attention on the controle of the lithium deficiency in Li_(1-x)Ni_(1+x)O_2 and the correlative structural and electrochemical responses. We propose a reproducible way for preparing quasi-stoichiometric compounds by using an optimized value of Li excess (molar ratio Li/Ni = 1.07). We focus on the synthesis procedure of Li_(1-x)Ni_(1+x)O_2, the quantification of x and we will compare the efficiency of different methods. The electrochemical properties of some compounds are discussed in relation with structural data in order to understand the decay of capacity observed for various LiNiO_2 samples.