J. R. Dahn

Electrochemical and In Situ X‐Ray Diffraction Studies of Lithium Intercalation in Li x CoO2

Electrochemical properties of Li x CoO 2 are studied as Li is deintercalated from LiCoO 2 . High precision voltage measurements and in situ x-ray diffraction indicate a sequence of three distinct phase transitions as x varies from 1 to 0.4. Two of the transitions are situated slightly above and below x=1/2 and are caused by an order/disorder transition of the lithium ions. The order/disorder transition is studied as a function of temperature allowing the determination of an order/disorder phase diagram

Studies of Lithium Intercalation into Carbons Using Nonaqueous Electrochemical Cells

Li/graphite and Li/petroleum coke cells using a in a 50:50 mixture of propylene carbonate (PC) and ethylene carbonate (EC) electrolyte exhibit irreversible reactions only on the first discharge. These irreversible reactions are associated with electrolyte decomposition and cause the formation of a passivating film or solid electrolyte interphase on the surface of the carbon. The amount of electrolyte decomposition is proportional to the specific surface area of the carbon electrode. When all the available surface area is coated with the film of decomposition products, further decomposition reactions stop. In subsequent cycles, these cells exhibit excellent reversibility and can be cycled without capacity loss.

Thermal stability of LixCoO2, LixNiO2 and λ-MnO2 and consequences for the safety of Li-ion cells

LiCoO2, LiNiO2 and LiMn2O4 are all stable in air to high temperature. By contrast, LixCoO2, LixNiO2 and LixMn2O4 (x<1) are metastable and liberate oxygen when they are heated in air or in inert gas. The temperature at which oxygen evolution occurs depends on x and on the material. Using thermal gravimetric analysis and mass spectrometry, we have studied the thermal decomposition of these materials in inert gas. We find that the nickel materials are least stable, the manganese compounds are most stable, and that the cobalt compounds show intermediate behaviour. These results have important consequences for the safety of Li-ion cells, and suggest that cells using LiMn2O4 as the cathode should be safer than those using LiNiO2 or LiCoO2.

Synthesis and Characterization of Li1 + x Mn2 − x O 4 for Li‐Ion Battery Applications

Several series of Li{sub 1+x}Mn{sub 2{minus}x}O{sub 4} samples prepared at different temperatures and in different oxygen environments were studied in order to further understand the dependence of the electrochemical performance on the sample heating temperature, on the amount of excess lithium x, and on the oxygen stoichiometry. The authors show that a peak in the differential capacity of Li/Li{sub 11x}Mn{sub 2{minus}x}O{sub 4} electrochemical cells at 3.3 V on discharge always appears with the 4.5 V peak previously reported by Tarascon et al. However X-ray diffraction and Rietveld refinement show that the mixing of Li and Mn cations in 8a sites is not the origin of these capacity peaks. Samples heated to 900 C and samples that re more oxygen deficient exhibit larger 3.3 and 4.5 V peaks. Samples made at 750 C in air or oxygen with x = 0.1 show almost no evidence for the 3.3 and 4.5 V peaks. Furthermore, these samples do not degrade even after cells are charged to 4.9 V, suggesting these samples should be excellent for Li-ion battery applications. A simple way of determining x with thermal gravimetric analysis is also demonstrated.

Lithium Insertion in Carbons Containing Nanodispersed Silicon

Graphite and pregraphitic carbons are intercalation hosts commonly used in Li ion cells. Using chemical vapor deposition of benzene and of silicon-containing precursors, the authors have prepared carbons containing nanodispersed silicon. The silicon resides within the unorganized regions in the pregraphitic carbons. Materials with up to 11% atomic silicon have been prepared. These materials reversibly react with lithium in electrochemical cells and the reversible specific capacity increases from {approximately}300 mAh/g, in the absence of silicon, to near 500 mAh/g as silicon is added. For silicon content < 6 atomic percent, the reversible capacity increases linearly with a slope of approximately 30 mAh/g per percentage point silicon. This suggests that each silicon atom can reversibly bond with {approximately}1.5 lithium atoms. The increased capacity due to the silicon appears as a broad feature in the differential capacity between 0.1 and 0.6 V vs. Li metal. The large reversible capacities are maintained over many charge/discharge cycles. The carbonaceous matrix provides a pathway for diffusion of Li to the nanodispersed silicon atoms, while it can still intercalate a substantial amount of lithium. Nanodispersions of other lithium alloying atoms in carbon probably can be prepared.

The Effect of Boron Substitution in Carbon on the Intercalation of Lithium in Li x ( B z C 1 − z ) 6

Boron-substituted carbons, B z C 1-z , have been produced by chemical vapor deposition from benzene and boron trichloride precursors at 900 o C. The voltage and reversible capacity of Li/Li x (B z C 1-z ) 6 cells were measured for the range of boron concentrations 0 0 showed greater reversible capacities than Li/coke cells, and for z>0.10 the capacities exceeded that of graphite

Lithium Intercalation from Aqueous Solutions

Lithium can be intercalated into a wide variety of materials using nonaqueous electrochemical cells. The use of aqueous methods is less common because of the reactivity of many lithium intercalation compounds with water. Here the authors show that lithium can be intercalated into host compounds from aqueous LiOH solution, provided the chemical potential of the intercalated lithium is sufficiently lower than the chemical potential of lithium in lithium metal. Using LiMn[sub 2]O[sub 4] as the host, the authors formed Li[sub 2]Mn[sub 2]O[sub 4] by intercalating Li from LiOH solution in an aqueous cell. This method may prove to be an economical way of preparing lithium transition metal oxides with high lithium contents for lithium-ion cell cathodes.

Electrochemical Lithium Intercalation in VO[sub 2](B) in Aqueous Electrolytes

Electrochemical lithium intercalation in VO{sub 2}(B) electrodes in aqueous electrolytes has been studied by means of electrochemical methods (such as cyclic voltammetry and constant current discharge and charge) as well as atomic absorption spectrophotometry. Experiments were conducted in various buffer electrolytes having a range of pH from 6.0 to 11.3. Voltammetry clearly reveals current peaks related to lithium intercalation and deintercalation in the whole pH range investigated. Electrolyte pH plays a very important role in the performance of VO{sub 2}(B) electrodes. When pH is higher than about 10, the capacity involved in the lithium intercalation in VO{sub 2}(B) electrodes shows a rapid decline with repetitive cycling, which is suggested to be the result of the dissolution of the VO{sub 2}(B) electrodes into the bulk electrolytes. Decreasing the pH tends to reduce the dissolution of VO{sub 2}(B) and thereby gives better cycling behavior of VO{sub 2}(B) electrodes. On the other hand, it also leads to increased hydrogen evolution that might affect the lithium intercalation reaction. Thus, the optimum pH range for the lithium intercalation reaction is found to be between 8 and 10. VO{sub 2}(B) electrodes cycled in this pH range demonstrate very good capacity retention.

The effect of boron substitution in carbon on the intercalation of lithium in Li[sub x](B[sub z]C[sub 1[minus]z])[sub 6]

Boron-substituted carbons, B z C 1-z , have been produced by chemical vapor deposition from benzene and boron trichloride precursors at 900 o C. The voltage and reversible capacity of Li/Li x (B z C 1-z ) 6 cells were measured for the range of boron concentrations 0 0 showed greater reversible capacities than Li/coke cells, and for z>0.10 the capacities exceeded that of graphite

High‐Capacity Carbons Prepared from Phenolic Resin for Anodes of Lithium‐Ion Batteries

Carbons were made from resol and novolak resins at temperatures from 700 to 1,100 C by pyrolysis in inert gas. Using electrochemical methods, the authors have studied the insertion of lithium within these materials. Resol resin, heated to 1,000 C, has a reversible capacity of about 550 mAh/g, shows little hysteresis, and exhibits good cycling performance. Since these resins cost about 2.2