Lithium-ion battery cathodes: Structural and chemical stabilities of layered cobalt and nickel oxides

As the demand for portable and light-weight power sources is increasing, lithium-ion batteries are finding widespread use due to their higher energy density compared to other traditional rechargeable systems. Commercial lithium-ion cells presently use LiCoO₂ as the cathode. However, with a reversible capacity of 140 mAh/g, only 50% of the theoretical capacity could be utilized. In addition, cobalt is expensive and toxic. Therefore, there is an enormous interest to develop alternate cathodes. In this regard, LiNi_0.85Co _0.15O₂ has become attractive as it offers 180 mAh/g, which corresponds to 65% of the theoretical capacity, and nickel is less expensive and less toxic than cobalt. However, the reason for the difference in capacity between the two systems and the long-term stability of LiNi_0.85Co _0.15O₂ during cycling remain to be established.; With an aim to address these issues, this dissertation focuses on the structural and chemical stabilities of chemically delithiated layered cobalt and nickel oxides. Lithium is chemically extracted from LiNi_1−y Co_yO₂ (0 ≤ y ≤ 1) in aqueous and nonaqueous media. Complete extraction of lithium could be achieved with a stronger oxidizer NO₂PF₆ in nonaqueous medium. The nickel-rich end members Ni_1−yCo_yO_2−δ are found to maintain the initial O3 (CdCl₂) type structure with oxygen contents close to 2 while CoO_2−δ is found to consist of a mixture of P3 (Na_0.6CoO₂ type) and O1 (CdI₂) type phases with a significant amount of oxygen vacancies. In fact, Li_1−x CoO₂ is found to be intrinsically unstable due to the loss of oxygen from the lattice for (1 − x) < 0.5, which appears to limit its practical capacity. A better stability of LiNi_0.85Co_0.15 O₂ towards oxygen loss allows a higher capacity for this system.; However, the delithiated Li_1−xNi_0.85Co _0.15O₂ cathodes are found to experience structural instability under mild heat (T > 50°C). A migration of Ni3+ ions from the nickel layer to the lithium layer causes the structural instability. Co 3+ does not migrate under similar conditions. The differences in the chemical and structural stabilities between the cobalt-rich and nickel-rich oxides is explained on the basis of a qualitative band diagram.; Additionally, extraction of lithium under alkaline conditions in aqueous medium is found to give various P3 type phases with different c parameters.