The role of defects on the Jahn-teller effect and electrochemical charge storage in nanometric LiMn₂O₄ material

Reducing the particle size to improve the electrochemical properties of LiMn₂O₄ has been a common practice along the last years claiming that, by doing this, the Jahn-Teller (JT) distortion in its 3 V region is reduced. However, only a few papers have proved actual links between the mitigation of the JT effect and particle size; therefore, the JT effect is often entangled with mechanical and kinetic improvements, which are also associated to diminishing the particle size. Regardless that, there is a consensus that reducing particle size positively impacts Li⁺ insertion into LiMn₂O₄, particularly in its 3 V plateau. On the other hand, defects have emerged as factors determining the electrochemical behaviours of LiMn₂O₄. Several works pointed out that certain defects such as Mn_i^tet, F and V colour centres, dislocations or voids, could improve the 3 V performance of the LiMn₂O₄ by decreasing the JT distortion. Nevertheless, the preponderant defect causing this enhancement has not yet been established. The present paper aims to determine, among Mn_i^tet, F and V centres as well as dislocations, which defect impacts the most on the JT transition, specific capacity and stability of nanometric LiMn₂O₄. The aforementioned defects were detected by HRTEM, XRD, UV–Vis as well as magnetic measurements. Their implications on the potential drop caused by the JT effect, specific capacity and stability were recorded by potentiometric and galvanostatic charge/discharge measurements. The results indicate that only Mn_i^tet-related defect reduces the JT distortion, increasing the stability and capacity within the 3 V region, especially when particle are small.