Synthesis of cobalt-free low-cost cathode electrodes for lithium-ion batteries

The layered LiCoO2 compound, which has a high theoretical capacity (274 mAh/g) and theoretical volume capacity (1363 mAh/cm3), was the first cathode material in Li-ion batteries to be commercially used. Another appealing aspect of this material is its high discharge voltage and good cycle performance. However, this cathode material has major drawbacks. Among the commercially available cathode materials, LiCoO2 has the lowest thermal stability, leading to the release of oxygen under operating conditions over a particular temperature, causing unexpected cell reactions. However, the toxicity of cobalt and its high cost have prompted a quest for alternative elements that can replace cobalt. Given its similar crystal structure, LiNiO2 may be an adequate alternative to LiCoO2. This cathode compound is inexpensive and has a large capacity. However, this cathode material is not commonly used commercially. This occurs because Ni+ ions block lithium diffusion pathways during the intercalation process in pure LiNiO2, resulting in thermal instability and decomposition at high temperatures. Another cathode compound used in LIBs is LiNi0.8Co0.15Al0.05O2 (NCA), which has a high reversible specific capacity of 265 mAh/g compared with LiCoO2. The most important manufacturers of this cathode compound are Panasonic and Tesla. When the NCA x value in the LiNixCoyAlzO2 formulation is greater than or equal to 0.8, it is considered nickel-rich. Reducing the amount of cobalt in the compound provides an advantage in terms of cost, and the amount of energy stored in the battery is also increased with an increase in nickel content. However, it should be noted that an increase in nickel content also facilitates thermal degradation. LiMnO2 cathode compounds, which are more cost-effective and less toxic than cobalt and nickel elements synthesized in the past, are widely used. However, these compounds still have several issues, such as the tendency to revert to the spinel structure during the intercalation process and the leaching of manganese ions from the layered structure as a result of electrochemical cycles. Researchers have developed a Li(Ni0.5Mn0.5)O2 cathode compound to address these issues by doping nickel ions into the LiMnO2 structure. This improves the mobility of both lithium and nickel ions and has an energy density to LiCoO2. In addition, researchers have observed that stability is further increased by adding cobalt to this compound, resulting in the formation of LiNixCoyMnzO2 (NMC) which is still in commercial use today. In the cathode compound known as NMC, lithium ions are located in the 3a region, whereas nickel, manganese, and cobalt ions, which have oxygen atoms in the 3b and 6c regions, are in a mixed valency state. The cation mixture of Li+ (0.76 Å) and Ni2+ (0.69 Å) occurs because of the close proximity of their ionic radii, with nickel occupying regions typically occupied by lithium in the 3a regions. There have been studies on the synthesis and electrochemical properties of Li(Ni1/3Mn1/3Co1/3)O2 cathode electrodes, such as those conducted by Dahn and Ohzuku. Subsequently, research has focused on new synthesis methods and surface coatings that incorporate NMC species in different stoichiometric ratios, resulting in the synthesis of higher-capacity cathode materials by varying the ratios of transition metals. This type of cathode material exhibits polymorphic transformations that occur when it changes from a layered structure to a spinel structure during certain cycles. This transformation is caused by the migration of manganese atoms from tetrahedral positions in the lithium layers during intercalation and the migration of lithium atoms to octahedral positions, leaving the manganese atoms in empty positions. These atom migrations negatively affect the electrochemical properties of the structure and decrease its performance. To solve this problem, researchers have used alkali metals such as sodium, potassium, and magnesium to fill the octahedral cavities and prevent ion migration to the tetrahedral cavities. Another issue that occurs in these cathode compounds is the irreversible loss of lithium and oxidation of the electrolyte, which occurs when the Li2MnO3 phase is activated when the voltage exceeds 4.4 V during the first charge. This activated phase releases Li2O into the electrolyte, preventing lithium ions from returning to the cathode and causing oxygen to react with the electrolyte, forming an oxide layer. These issues lead to capacity losses and performance drops in subsequent cycles. In this study, we introduced an innovative approach, which involved the replacement of cobalt with iron in cathode electrodes. Specifically, we utilized NMC chemistry to synthesize NMF cathode electrodes. The production of NMF particles was achieved using the sol-gel method, incorporating metal salts as key components for structure formation. We meticulously examined the structure of NMF particles and electrodes through X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), revealing the absence of impurities in the NMF powders. Our FESEM analysis further determined particle sizes ranging from 200 to 950 nm. To evaluate the performance of NMF cathodes, we conducted galvanostatic charge and discharge tests at a rate of 1C within the potential range of 2.0 to 4.6 volts. The prepared cathode electrodes underwent 500 cycles at a 1C rate, and the results indicated that approximately 85% of the initial capacity was retained at the end of this extensive testing. Electrochemical impedance tests indicated that all resistance values increased with the number of cycles. This exceptional cycle performance and stability were achieved, providing a promising outlook for NMF cathode material. In contrast to the limited attention NMF cathode material has received in prior literature, this study contributes valuable insights. It not only supports the potential of NMF material in influencing future research but also highlights the development of a competitive cathode material capable of challenging NMC chemistry. This strategic substitution not only addressed the issue of toxicity associated with cobalt, a heavy metal commonly found in lithium-ion batteries but also delivered significant cost advantages. Cobalt typically constitutes a substantial portion of cell manufacturing costs, making the use of iron a more cost-effective alternative. Our primary objective was to develop cathode electrodes that not only showcased superior cyclic performance but also enhanced stability. In the light of the data obtained as a result of the study, we proved that NMF cathode materials, in which cobalt is replaced by iron in the structure but the NMC crystal structure is still preserved, can be commercialized instead of NMC cathode material. The socio-economic reasons brought by cobalt and the prevention of the use of such a harmful metal have heralded significant developments in the lithium-ion battery industry. Ultimately, our aim was to increase the commercial viability of this innovative approach, which holds promise for the future of lithium-ion battery technology. Moreover, there is promising potential for enhancing cell capacity further by incorporating various elements or carbon-based materials into the NMF cathode material