Structural variations and corrosion of lithium cobalt spinel for lithium-ion battery cathode materials

Abstract

The operation capacity and retention can be used to assess the electrochemical performances of lithium ion battery, and these performances are highly dependent on the crystal structure and compositions of electrode materials. This study discusses the optimized feasibility of a typical cathode substance (lithium cobalt spinel) for further enhance the cycling stability by applying solid state method. The crystal compositions, structures, morphologies of the as-synthesized products were evaluated by a number of advanced characterization technologies. The doping effects on evaporation, annealing and calcination treatments with low-cost and attainable metal precursors (i.e. tetraethyl orthosilicate, Al2O3 and Fe2O3) and other chemical materials have been investigated. Some preliminary works of quantitative X-ray diffraction technique were first used to quantify the content of impurity phase (Co3O4) in the samples.

The corrosion performance of one representative spinel cathode material (LiFe0.5Co1.5O4) can reflect its properties in the simulated electrolyte solution. The LiFe0.5Co1.5O4 powder was dissolved in 1 mol/L HCl, and the concentrations of lithium, cobalt and iron ions were measured via ICP-OES. The results showed the Li: Co ratios of two parallel groups are higher than the 1: 1.5. The pH values of acid solution showed a downward trend, which indicates the interactions between the metal ions and hydrogen ions.

The works introduce carbon element from activated carbon (AC) for inorganic and salicylic acid (SA), oxalic acid (OA) and citric acid (CA) for organic substances into the experimental system as well. The carbon conversion rate results via total organic carbon tests showed that the OA group (51.96 %) is higher than that of SA group (29.05 %). After evaporation step, citric acid with the mixture in ethanol changed to a kind of very hard glaze, which is difficult to remove from the container and grind to the powder for next step. Meanwhile, this CA-modified LiCo2O4 powder cannot keep dry in a dryer with room temperature and became a viscous substance. The SA group needs extra energy consumption to change into dried state for evaporation. So that, the mentioned conditions limit the application possibilities of SA and CA routes, and the utilization potential of OA group is relatively higher than these two routes. Furthermore, Co2+ and Co3+ ions in cobalt oxygen bonds were revealed by FT-IR spectroscopy. In addition, the results of activated carbon (AC) as an extra carbon supplement to modify LiCo2O4 highlighted the increased thermal treatment temperature and time impacts for different intensities changes of (003) and (104) crystal planes of LiCo2O4 phase presented opposite rise and fall trends in X-ray diffraction patterns, as well as AC-modified products showed more dispersed in SEM images than bare materials.

Overall, this work has explained the generations of lithium cobalt spinel products by thermal treatment via solid state method which is an easy and reachable approach. Three schemes containing the inorganic metal doped and activated carbon modified as well as organic acid modified routes. To enhance the purity of aiming phase, five possible reasons have been discussed about the impurity phases removing (i.e. Co3O4, Fe2O3 and Fe3O4).