No abstract

Among lithium transition metal oxides used as intercalation electrodes for rechargeable lithium batteries, LiCoO 2 is considered to be the most stable in the ␣-NaFeO 2 structure type. It has previously been believed that cation ordering is unaffected by repeated electrochemical removal and insertion. We have conducted direct observations, at the particle scale, of damage and cation disorder induced in LiCoO 2 cathodes by electrochemical cycling. Using transmission electron microscopy imaging and electron diffraction, it was found that (i) individual LiCoO 2 particles in a cathode cycled from 2.5 to 4.35 V against a Li anode are subject to widely varying degrees of damage; (ii) cycling induces severe strain, high defect densities, and occasional fracture of particles; and (iii) severely strained particles exhibit two types of cation disorder, defects on octahedral site layers (including cation substitutions and vacancies) as well as a partial transformation to spinel tetrahedral site ordering. The damage and cation disorder are localized and have not been detected by conventional bulk characterization techniques such as X-ray or neutron diffraction. Cumulative damage of this nature may be responsible for property degradation during overcharging or in long-term cycling of LiCoO 2-based rechargeable lithium batteries.

LiAlyCO1−yO2 solid solutions of α‐NaFeO2 structure type have been synthesized from homogeneous hydroxide precursors. X‐ray powder diffraction and scanning transmission electron microscopy show that single‐phase solid solutions are formed up to y ≈ 0.5 at 800°C in air. Electrochemical tests using nonaqueous liquid electrolyte cells shows that the open‐circuit voltage and working voltage increase with Al content as predicted by ab initio calculations [Ceder et al., Nature, 392, 694 (1998)], while capacity fade was significant during cycling at room temperature. However, both the absolute capacity and cycleability were improved at 55°C. © 1999 The Electrochemical Society. All rights reserved.

For nearly 20 years, poly(ethylene oxide)-based materials have been researched for use as electrolytes in solid-state rechargeable lithium batteries. Technical obstacles to commercialization derive from the inability to satisfy simultaneously the electrical and mechanical performance requirements: high ionic conductivity along with resistance to flow. Herein, the synthesis and characterization of a series of poly(lauryl methacrylate)-b-poly[oligo(oxyethylene) methacrylate]-based block copolymer electrolytes (BCEs) are reported. With both blocks in the rubbery state (i.e., having glass transition temperatures well below room temperature) these materials exhibit improved conductivities over those of glassy-rubbery block copolymer systems. Dynamic rheological testing verifies that these materials are dimensionally stable, whereas cyclic voltammetry shows them to be electrochemically stable over a wide potential window, i.e., up to 5 V at 55ЊC. A solid-state rechargeable lithium battery was constructed by laminating lithium metal, BCE, and a composite cathode composed of particles of LiAl 0.25 Mn 0.75 O 2 (monoclinic), carbon black, and graphite in a BCE binder. Cycle testing showed the Li/BCE/LiAl 0.25 Mn 0.75 O 2 battery to have a high reversible capacity and good capacity retention. Li/BCE/Al cells have been cycled at temperatures as low as Ϫ20ЊC.

Mass transport and thermodynamic properties of Li x CoO 2 were studied by the potentiostatic intermittent titration technique ͑PITT͒. We determined the chemical diffusion coefficient (D Li ) and the thermodynamic factor ͑⌰͒ of Li in the region 0.45 Ͻ x Ͻ 0.7, where Li x CoO 2 exists as a single phase having either a rhombohedral or a monoclinic structure. Solid-state thin-film batteries were used in order to ensure a well-defined diffusion geometry. Both D Li and ⌰ have minima at the phase boundaries of the Li vacancy ordered phase Li 0.5 CoO 2 . The self-diffusion coefficient of Li (D Li ) has a minimum at x ϭ 0.5 associated with the Li vacancy ordering. As the degree of ordering increases, nonmonotonic variations of D Li , ⌰, and D Li become more pronounced when approaching x ϭ 0.5 in Li x CoO 2 .High power applications of rechargeable Li batteries require fast Li ion mobility within intercalation compounds. The ␣-NaFeO 2 structure of LiCoO 2 ensures a high chemical diffusion coefficient of Li (D Li ) and in turn, a high rate capability. 1 The structure is based on a nearly cubic close-packed arrangement of O 2Ϫ with the Li ϩ and Co 3ϩ alternatively occupying the octahedral sites between adjacent oxygen ion layers ͑space group R3 m). 2 According to previous theoretical studies, Li diffusion in the Li layer occurs via tetrahedral sites by a divacancy mechanism. 3-5 The values of D Li in the literature vary from 10 Ϫ13 to 10 Ϫ7 cm 2 /s. 6-20 This large difference is attributed to different assumptions for the geometrical factors ͑diffu-sion length and cross-sectional surface area͒ used in the calculation of D Li . 17,18 In the case of conventional composite powder electrodes, for example, when the diffusion length is erroneously identified as the thickness of the electrode, the value of D Li will be overestimated particularly for thick electrodes. Single-crystal electrodes would provide a well-defined geometry, but they are not easily available. 19 In terms of both geometry and feasibility, thin-film electrodes combined with solid electrolytes are ideal for diffusion studies. Thin-film electrodes with liquid electrolytes were employed in previous diffusion studies; 10,14,18 however, because permeation of liquid electrolytes into cracks in thin-film cathodes cannot be ruled out, use of a solid electrolyte is preferrable.Extraction of Li from LiCoO 2 occurs via several phase transitions. In Li x CoO 2 , two rhombohedral phases coexist for 0.75 р x р 0.93, 21 which has been attributed to the insulator metal transition upon Li extraction. 22,23 D Li is not defined as a single value in the two-phase region. At compositions near Li 0.5 CoO 2 , Li ions are ordered in rows separated by rows of vacancies within the Li layers, 21 which is accompanied by a lattice distortion to a monoclinic symmetry ͑space group P2/m). 4,21,24 The degree of ordering is sensitive to the sample purity. 25 Recently, Van der Ven and Ceder predicted that D Li and the self-diffusion coefficient of Li (D Li ) have a minimum at x ϭ 0.5 due to Li or...