Layered Li(Ni, M)O 2 Systems as the Cathode Material in Lithium-Ion Batteries
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Layered Li(Ni, M)O
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Systems as the Cathode Material in Lithium-Ion Batteries C. Delmas and L. Croguennec
Abstract Compared with LiCoO2, the dominant cathode material in today’s lithium batteries, lithium nickel oxide derivatives [Li(Ni, M)O2, where M Co, Fe, Al, Mg] offer a higher specific energy at a lower cost. The synthesis and structure of these materials are described. The electrochemical performances of the pure nickel compound and a number of multicomponent systems are assessed. The goals of these fundamental studies are to optimize the synthesis conditions and material composition to achieve good electrochemical reversibility, decrease capacity loss upon cycling, and enhance thermal stability in the deintercalated state in order to improve cell safety. Keywords: cationic substitution, diffraction, intercalation, layered oxides, lithium nickelate, rechargeable lithium batteries, redox processes.
Introduction Lithium nickel oxide derivatives are considered next-generation positive-electrode materials for high-energy lithium-ion batteries, particularly for powering electric or hybrid vehicles.1 In comparison with LiCoO2 , which until now has been used in almost all types of portable batterypowered devices, the main advantages of LiNiO2 (lithium nickelate) are its lower cost and its higher specific energy. Nevertheless, this material has many drawbacks to overcome (e.g., capacity loss upon cycling and poor thermal stability) before it can compete in practical devices. To this end, a great number of fundamental studies have been carried out to explain the structural modifications that occur during the electrochemical cycling of lithium nickelate as well as the effects of cationic substitution on its structural and electrochemical properties. Over the last ten years, the studies that have been the most relevant from an application point of view have attempted to optimize the synthesis conditions and material composition to achieve good electrochemical reversibility, decrease capacity loss upon cycling, and
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enhance thermal stability in the deintercalated state in order to improve cell safety.
The Li1zNi1zO2 System: Synthesis and Structure From the earlier magnetic-property studies of Goodenough et al. on lithium nickel oxides,2 the departure from the ideal stoichiometry, leading to the overall Li1zNi1zO2 formula, is well known. Nevertheless, only recent studies, carried out to optimize the lithium nickelate synthesis in order to improve its electrochemical properties, have enabled the preparation of near-stoichiometric materials.3,4 Structural characterization by x-ray and neutron diffraction has shown that the structure consists of NiO2 slabs made of edgesharing NiO6 octahedra with lithium ions inserted between the slabs in octahedral sites. The z nickel ions in excess are found in the lithium site. The charge balance, which requires that 2z Ni2 ions are in the divalent state, leads to the crystallographic formula (Li1zNiIIz) (NiIIzNiIII1z)O2. Contrary to what is claimed in some papers, there
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