Evolution of Solid Electrolyte Interphase during Cycling and Its Effect on Electrochemical Properties of LiMn₂O₄

Abstract

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...

Bimodal AFM-Based Nanocharacterization of Cycling-Induced Topographic and Mechanical Evolutions of LiMn2O4 Cathode Films

“…Two voltage plateaus can be seen at ∼4.0 and ∼4.15 V, which correspond to the Li-ion intercalation and deintercalation processes during reversible electrochemical reactions and are in agreement with the reported results. 10,33 Figure 2c shows the capacity retention and Coulombic efficiency. The capacity retention decreases to 88% during the first 10 cycles and then decreases to 84% at 50 cycles and 70% at 100 cycles, respectively.…”

Electrochemical Surface Analysis of LiMn₂O₄ Thin-film Electrodes in LiPF₆/Propylene Carbonate at Room and Elevated Temperatures

“…Thus far, degradation mechanisms of LiMn 2 O 4 have been extensively studied for over two decades, and various degradation phenomena such as Jahn-Teller distortion, [6][7][8] dissolution of manganese ion, 1-5,9-13 structural changes [3][4][5]10,[14][15][16][17] and surface film formation 15,[17][18][19][20][21][22][23][24][25][26][27][28][29] have been reported. In particular, the dissolution is the most severe problem for LiMn 2 O 4 .…”

Investigation on Surface-Film Formation Behavior of LiMn2 O4 Thin-Film Electrodes in LiClO4 /Propylene Carbonate

“…However, LiMn 2 O 4 suffers from severe capacity degradation during charge–discharge cycles especially at an elevated temperature such as 55 °C . So, far, LiMn 2 O 4 degradation mechanisms have been widely studied, and it has been suggested that Jahn–Teller distortion, dissolution of manganese ion, structural changes,, and surface‐film formation occur during cycling. Among these degradation factors, the dissolution of manganese ion has been extensively studied.…”