Enhancing Electrochemical Performance of Co-Free Cathode Materials for Lithium-Ion Batteries Via Surplus Lithium Incorporation
Authors: Ko Kunhee , Kang Kisuk 
Published: February 2024

Interest in electric vehicles has grown in recent years due to increased government subsidies due to environmental problems, improvements in infrastructure such as electric vehicle charging stations, and increased mileage. As interest in electric vehicles grows, not only the small battery market that used to enter mobile phones, but also the medium and large battery markets are growing at a rapid pace every year. As the medium and large battery market grows. Among the many sections of LIBs, the most utilized material in the cathode material is a layered cathode material, with representative examples being LiNi1-x-yCoxMnyO2 (NCM), LiNi1-x-yCoxAlyO2 (NCA), and LiCoO2 (LCO). Currently, the biggest roadblock in reducing the price of commercially used batteries is related to transition metal, the main raw material for cathode materials, especially cobalt, which is highly volatile in price. Since more than half of the world's cobalt deposits are in Democratic Republic of the Congo (DRC), the price of cobalt is very fluctuating and, in extreme instances, may reach four times that of Ni, the second-most expensive transition metal. In addition to economic factors, the usage of cobalt in Africa is becoming less desirable for political and ethnic reasons. Recent attempts have been undertaken to produce low Co, Co-free layered cathode materials for these reasons, but now, only Li-and Mn-rich materials and high voltage spinel LiNi0.5Mn1.5O4, disorder rock-salt (DRX) are being studied as materials for low Co, Co-free.
In the layered structure, which is a representative cathode material, the biggest reason why Co-free materials have difficulty in commercialization lies in the various important roles that Co plays in layered materials. Most well-known function of using cobalt is the suppression of cation disorder within the layered structure. Due to the comparable ionic sizes of Ni2+ (0.69A) and Li+ (0.74A), cation disorder arises when Ni2+ occupies the octahedral site of Li+ (0.74A) in the Li layer. Ni2+ ion in the Li layer inhibits Li ion diffusion in the Li layer during charging and discharging. In addition, since the Ni-O bond is shorter than the Li-O bond, the Li layer slab distance near where cation disorder occurs is shorter, and Li ion experiences a higher energy barrier during diffusion. The cation disorder negatively impacts the reversible specific capacity and rate capability for these reasons. If Co is substituted with Ni, the Li+/Ni2+ cation disorder is minimized as the oxidation state of Ni approaches Ni3+; nevertheless, as shown in several Ni-rich materials, the battery life performance and thermal stability worsen rapidly. If Co is replaced with Mn, since Mn is present as Mn4+ in the layered material, the oxidation state of Ni is close to Ni2+ to meet charge neutrality, and the Li+/Ni2+ cation disorder is enhanced further. For these reasons, in order to commercialize the Co-free layered structure, it is important to find a material that replaces Co but does not bring about a trade off in battery performance and structure.
In this paper, we provide a Li-surplus method for reducing cation disorder caused by the direct substitution of Co with Li (Fig. 1A). Li doping decreases the cation disorder ratio from 6.9% when x=0 to 1.84% when x=0.1 in Li1+x(Ni0.75Mn0.25)1-xO2 (Li-surplus NM75 or LS-NM75). As the ratio of cation disorder dropped, both the specific reversible capacity and rate capability, which were recognized as cation disorder's largest problem, improved. In fact, studies on improving the performance of materials using excess Li have already been conducted in several studies. However, in the case of these papers, only the predictable level of performance improvement, in which the rate capability is improved due to the reduction of cation disorder, was shown. However, in this study, in addition to the previously known diffusivity characteristic performance improvement, we determined that surplus Li decreased internal cracks in secondary particles and validated that surplus Li inhibited the creation of nanopores in primary particles, which are the origin of particle cracks. In particular, improvements in mechanical properties and cycle life were demonstrated due to these effects when surplus Li was employed. In addition, while synthesizing a layered material, the intercalation phenomena of Li into the layered structure was enhanced, and the synthesis process optimization minimized the cation disorder ratio and increased performance.

Figure 1