Thickness variation of the solid electrolyte interphase (SEI) produced during charge-discharge cycling is investigated to analyze the effect of SEI on the electrochemical properties of LiMn 2 O 4 . Atomic force microscopy (AFM) is used to measure the SEI thickness and elastic modulus on the LiMn 2 O 4 surface. The SEI shows a broad thickness distribution due to the random nature of the LiMn 2 O 4 electrode surfaces, while the average SEI thickness increases with cycling and stabilizes after the 20 th cycle. Formation of a relatively thin SEI on the LiMn 2 O 4 surface accompanies low Coulombic efficiency at early cycling stages. The SEI produced in the early stages of cycling is vulnerable to capacity fading due to inefficient surface protection against possible side reactions. A fully-grown stable SEI after 20 cycles shields the cathode surface from the electrolyte, minimizing capacity fading. © The Author Solid electrolyte interphase (SEI) is the layer produced on the surface of active materials during charge-discharge cycling of a battery. SEI affects electrochemical properties of active materials, such as capacity retention and rate performance, and a stable SEI is known to protect the active materials from electrolyte side reactions. A wellknown example is the excellent cycling performance of graphite due to the thin SEI produced on the surface. SEI control for cathode materials is, therefore, important to improve the electrochemical performance of Li-secondary batteries. In particular, much interest has been given to the study of SEI formation on LiMn 2 O 4 particles, as its SEI is relatively unstable and ineffective in protecting the electrode surface from side reactions with a protic electrolyte, causing capacity fading due to Mn 2+ ion dissolution.
3-5Although LiMn 2 O 4 has been considered as a promising cathode material for lithium ion secondary batteries due to its stability and high discharge voltage, as well as its non-toxicity and low cost, its capacity fading is known to be a part of the reason for its limited commercial application. Many researchers have studied the effect of SEI on electrochemical properties; however, the transient nature of SEI formed during cycling, and their nanoscale dimension made detailed analyses difficult. 6,7 Transmission electron microscopy (TEM) has been used for the observation of SEI,8,9 however, this has inherent shortcomings since SEI, which is composed of organic and inorganic materials (LiF, Li 2 CO 3 , R-CO 3 Li), can be decomposed at ultra-high vacuum conditions and by the high energy focused electron beam. 10 Other techniques, such as spectroscopic ellipsometry 11 and X-ray reflectivity analysis, 12 were also used to analyze SEI. They found that the thickness of the SEI layer was less than 5 nm in the early stages of cycling.10-12 Atomic force microscopy (AFM) has been used to examine electrode surfaces, while most AFM research has been focused on the change in topography, particle morphology, and electrical conductivity of the electrode surface.13-15 Recentl...
