A research team led by Prof. Won-Sub YOON, Department of Energy Science (DOES) at Sungkyunkwan University, has lifted the fundamental restriction on the breaking point of the lattice and unraveled the enigma of nickel effect on layered cathode materials that has existed for decades in the battery field.
Prof. YOON, WON SUB
A research team led by Prof. Won-Sub YOON, Department of Energy Science (DOES) at Sungkyunkwan University, has lifted the fundamental restriction on the breaking point of the lattice and unraveled the enigma of nickel effect on layered cathode materials that has existed for decades in the battery field. Consequently, they discovered the possibility of developing a high-performance battery which can travel up to 400km by one charge.
While the technology of lithium ion batteries has been greatly successful since its advent in powering portable electronic devices, further advancements are insatiably demanding for wider applications such as in electric vehicles and grid-power storages. One of the key areas in these efforts is development of new positive electrode ‘cathode’ materials with higher energy densities to replace the lithium cobalt oxide that is currently prevailing as the cathode material. The research is very focused on increasing the amount of Li-ions ‘inserted’ in the electrode material, which affects the charge storage capacity, the speed of Li-ion movement within the crystal lattice of the electrode material, which affects the battery power, and the structural stability of the material upon in-and-out transport of Li-ion, which affects the battery life. Compared to other material families, the ‘layered’ materials are the most attractive in the sense of the three attributes listed above, and layered lithium transition metal oxides containing nickel (Ni), cobalt (Co) and manganese (Mn) have recently emerged as a promising family of cathode materials.
Aside from lithium ions, other elements play a role as building blocks forming a host structure for Li-ions (guests) to be inserted or extracted. Depending on the properties of the host structure, its electrochemical performance as a battery material is determined. For these multi-component layered systems, the current trend moves toward increasing the content of Ni in layered systems (known as Ni-rich layered materials) since Ni is capable of uptaking and delivering twice the charge, i.e., Li-ions of the other two. As Ni atoms occupy a large part of the transition metal layer in the host structure, it becomes a major factor in determining the overall properties of the host structure. Therefore, understanding the effect of increasing Ni content on the layered structure is important to designing high-energy electrode materials.
This series of materials containing Ni and other elements appear to inevitably have so-called ‘cation disorder,’ a phenomenon in which some of the Li-ions and Ni atoms switch positions from their own layers. This happens due to the fact that some of the Ni atoms exist in the valence state 2+ lower than Co or Mn as synthesized. The presence of Ni atoms in the Li-ion layer adversely affects the Li-ion movement in the Li-ion layer. In contrast to such a general perception, the study finds that the degree of cation disorders is mitigated upon increasing the Ni content in the lattice up to a certain concentration, and also reports that the oxidation state of Ni in the pristine compounds contributes significantly to cation disorder. Moreover, it is demonstrated that the extent of cation disorder critically affects the phase transition behavior during charging or discharging, and as a result, the phase transition becomes smoother with increasing Ni content. This smooth phase transition reduces the strain on structural behavior during cycling; consequently, it enhances the cycle performance of the electrode material.
In addition to the relationship between the Ni content and the phase transition, it was discovered that the actual environment in which Li-ions are situated is not directly linked to the total height, a sum of the Li-ion layer and the transition metal layer (c-axis). The height of the lithium layer becomes larger with increasing Ni content, even though the c-axis decreases. More importantly, it is shown that the lithium ion channel retains the environment where lithium ions can visit or leave, even if the c-axis shrinks from the initial dimension.
The results for the Ni-rich layered materials that are counter-intuitive account for the superior electrochemical performance, and address the misconception of Ni element in Ni-rich layered systems. Furthermore, this article provides a new perspective on the role of Ni in layered systems and disputes the conventional view concerning the c-axis parameter that has been considered a key factor in interpreting the behavior of Li-ion movement and the corresponding electrochemical performance. Hence, these results may suggest some aspects to consider in the design of high-energy electrode materials for next-generation batteries.