There has been considerable research and development activity targeted towards the commercial manufacture of battery systems based on lithium insertion cathodes. The emphasis is placed on the comparison between specific capacities and rechargeablility of lithium-containing high voltage cathode materials such as manganese oxides and $LiMO_2$ compounds, where M is Co or Ni. Lithiated transition metal oxides, particularly $LiNiO_2$ and $LiNi_{0.7}Co_{0.3}O_2$ are technically important materials for Li-ion battery applications. The former oxide, $LiNiO_2$ has attracted considerable attention over the last few years since it is economically favorable. Moreover, nickel is potentially less toxic and exhibits a higher discharge capacity in comparison to $LiCoO_2$. However, the difficulties encountered in stabilizing the higher oxidation state of Ni during heat treatment makes the control of defect concentration in this material quite a formidable task. According to previous studies, $LiNiO_2$ has been $[Li^+_{1-z}Ni^{2+}_z]_{3a}[Ni^{3+}_{1-z}Ni^{2+}z]_{3b}[O_2]_{6c}$ (0<z<0.2) depending on preparation conditions. A small amount of structural disorder due to the displacement of nickel and lithium ions in $LiNiO_2$ strongly affects the electrochemical properties such as the working voltage and rechargeable capacity. These problems have rendered the oxide less favorable for practical applications in rechargeable batteries. These considerations have led to the electrochemical studies of $LiNi_{1-y}CoyO_2$ series. On the other hand, in comparison to $LiNiO_2, LiNi_{0.7}Co_{0.3}O_2$ is easier to synthesize as reported by others researcher. There have been several reports in the literature on the synthesis of $LiNiO_2$ using the solid-state process employing various precursors and heat treatment conditions, involving different combinations of time, temperature and atmospheres. The vast disparity in the electrochemical properties of the synthesized materials using different precursor reveals the importance of the nature of the precursors on the synthesis of the oxide.
Solid-state processes are the preferred traditional methods used for synthesizing lithiated transition metal oxides. The electrochemical performance of the oxide is largely governed by the chemical stoichiometry, homogeneity, crystallite and particle sizes. Most solid-state processes used for synthesizing $LiNiO_2$ generally require prolonged heat treatment time at the high temperature. These high temperature treatments generally dont yield materials exhibiting good electrochemical properties. This is true particularly for $LiNiO_2$ due to its decomposition and loss of lithium thereby leading to cation disorder. Furthermore, the high temperature treatments induce growth of crystallites, which adversely affect the rate capacity due to enhanced polarization losses during cycling. An increase in the crystallite size combined with the cation disorder both contribute to lowering of kinetics of diffusion. There is therefore a need to identify approaches that can yield the stoichiometric oxide with good control of the crystal chemistry, particle size and morphology. The use of solution chemistry based on techniques for synthesizing materials has several advantages. All these approaches yield molecularly homogenous intermediate precursors. The precursor structure varies depending on the starting materials employed and the ensuing reaction chemistry. Despite the variation in the reaction chemistry, the precursor can be designed and synthesized to suitablely consist of molecular structures closely resembling the final desired oxide. As a result, moderate subsequent heat treatment is required for a short time, which lead to the formation of the crystalline oxide. Furthermore, the reduced diffusion distances in comparison to the solid-state approaches provide the ability to generate fine grained materials with limited coarsening and growth of the particles. Recently, there has been considerable activity in identifying the new low temperature based sol-gel approaches for synthesizing $LiNiO_2$.
In present study, a modified route called the emulsion-drying approach has been developed for synthesizing these lithiated transition metal oxides. The emulsion-drying approach is based on using metal nitrate hydrates that are forced to undergo complete hydrolysis in a mixture of aqueous-oil solvents. The emulsion-drying process offers the advantages and characteristics of the solution chemistry, albeit without the use of the unstable and expensive metal alkoxides. In the present paper, the emulsion-drying process has been described for synthesizing $LiMO_2$, M = Ni, Ni_{0.7}Co_{0.3}$, powders. Detailed phase evolution studies of precursors during heat treatment have also been conducted, while finally assessing the electrochemical properties of the synthesized oxides.
In particular, the thermal stability of the $LiNiO_2$ has been considered as a one of the most important properties which must be improved in order to apply this material to the realistic lithium rechargeable batteries because the thermal instability of delithiated $Li_{1-x}NiO_2$ causes safety hazard of the cell. Furthermore, amount of lithium available in $LiNiO_2$, i.e. specific capacity, is limited by the thermal properties of $LiNiO_2$ because the thermal stability of delithiated $Li_{1-x}NiO_2$ is significantly degraded by lithium deintercalation. The delithiated $Li_{1-x}NiO_2$ is metastable and liberates oxygen at elevated temperatures and that the temperature at which the oxygen evolution takes place depends on x in $Li_{1-x}NiO_2$. As part of a larger study to develop nickel-based lithium transition metal oxides with improved thermal stability as cathode materials for lithium secondary batteries, we examined the thermal behavior and decomposition mechanism of the electrochemically delithiated $Li_{1-x-y}Na_yNiO_2$ using thermogravimetry (TG), differential scanning calorimetry (DSC), high temperature x-ray diffraction (XRD) and XANES measurements. 4The delithiated $Li_{1-x-y}Na_yNiO_2$ was thermally decomposed to a spinel phase (Fd3m) at around 220℃. On further heating, delithiated $Li_{1-x-y}Na_yNiO_2$ of all compositions turned into a rock salt phase (Fm3m) with NiO structure. And We observed that powders substituted Na ions is more stable in the charged state against the elevated temperature.