Local Structures of Niobium-Containing Cation-Disordered Rocksalt Materials: Cobalt-Free Cathodes for Lithium-Ion Batteries
Electrochemical capacity improvements on current state-of-the-art cathode materials for Li-ion batteries such as LiCoO2 are becoming limited, and new families of materials with inherently greater capacities are required to more effectively meet the world’s energy storage demands. Li-excess and cation-disordered materials have shown promise as materials with significantly higher capacities, supposedly achieved in part through reversible anionic redox, in addition to traditional transition metal redox. They also allow for a broad compositional range, enabling the substitution of expensive, toxic elements such as Co for more abundant, cheaper, and safer transition metal species. However, the physical mechanisms controlling their structure, redox behaviour and ionic mobility are not yet well understood. Hence, this vast and largely untapped phase space is currently the subject of extensive investigation. This thesis presents the motivations, aims and findings of a project exploring the structural properties and electrochemical performance of the Ni and Mn members of the promising family of cathode materials Li1+xNbyMzO2 (x + y + z = 1, M = Mn, Ni). A range of complementary techniques are employed, including X-ray diffraction, neutron total scattering, solid state nuclear magnetic resonance, X-ray absorption spectroscopy and magnetic susceptibility measurements. These techniques are used in combination with electrochemical cycling to probe the local and long-range structures of these compounds, and their evolution with charge and discharge. The occurrence of short-range cation order in Li1.25Nb0.25Mn0.5O2 is reported and rationalised based on the principle of electroneutrality, combined with the accommodation of Mn Jahn-Teller and Nb second order Jahn-Teller distortions within the material. This cation ordering is found to be highly sensitive to the synthesis conditions and to have significant implications for the electrochemical performance of the material. It is demonstrated that cation order can be both beneficial and detrimental depending on the strength and nature of the ordering present. This can be understood by considering the effect of order on the 0-TM and 1-TM networks in the structure, which are crucial for Li diffusion. Following this, focus is turned toward the Ni-containing analogue of the same family: Li1.3Nb0.43Ni0.27O2. Based on the observations for the Mn-containing material, the preference for cation order is examined. Little evidence for cation order is observed in the Ni-based system, which can be rationalised based on the relative concentrations of the cation species and their charges, which makes cation ordering less viable. The electrochemistry of Li1.3Nb0.43Ni0.27O2 is examined and found to demonstrate significant voltage hysteresis, purportedly due to O-to-Ni charge transfer on charge. 17O NMR spectra of pristine and delithiated LNbNO are examined for signatures of O redox, however the spectra are complicated by the incomplete oxidation of Ni to Ni4+, meaning any possible O redox signal is obscured by unpaired spin density from Ni.
Batteries, Li-ion Batteries, Disorder, Order, Diffraction, Nuclear Magnetic Resonance, Pair Distribution Function
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