Improvement of the rate capability of LiMn2O4 by surface coating with LiCoO2

doi:10.1016/S0378-7753(01)00832-1

Journal of Power Sources

Volume 103, Issue 1, 30 December 2001, Pages 86-92

https://doi.org/10.1016/S0378-7753(01)00832-1 Get rights and content

Abstract

In order to use LiMn₂O₄ as a cathode material of lithium-secondary battery for an electric vehicle (EV), its rate capability should be improved. To enhance the rate capability of LiMn₂O₄ in this work, the surface of LiMn₂O₄ particle was coated with LiCoO₂ by a sol–gel method. Because LiCoO₂ has a higher electric conductivity than LiMn₂O₄, it is possible to improve the rate capability of LiMn₂O₄. After the surface coating, LiCoO₂-coated LiMn₂O₄ showed a higher discharge capacity of 120 mAh/g than as-received LiMn₂O₄ (115 mAh/g) because LiCoO₂ has a higher capacity than LiMn₂O₄. The rate capability of the coated LiMn₂O₄ improved significantly. While as-received LiMn₂O₄ maintained only 50% of its maximum capacity at a 20C rate (2400 mA/g), the LiCoO₂-coated LiMn₂O₄ maintained more than 80% of maximum capacity. LiCoO₂-coated LiMn₂O₄ with 3 wt.% conducting agent (acetylene black) showed the higher rate capability than as-received LiMn₂O₄ with 20 wt.% conducting agent. From electrochemical impedance spectroscopy (EIS) result that the first and second semicircles of coated LiMn₂O₄ were reduced, the improvement of rate capability is attributed to a decrease of passivation film that acts as an electronic insulating layer and a reduced inter-particle contact resistance. Accordingly, It is proposed that the surface coating of LiMn₂O₄ with LiCoO₂ improve the rate capability as well as the specific and volumetric energy density due to the decrease of conducting agent.

Introduction

Since the first commercialization by Sony Energytech in the early 1990s, the lithium-ion rechargeable battery (LIB) has become a major product to dominate the market for small rechargeable batteries [1], [2]. Furthermore, Li-ion batteries are expected to be used as a large-scale energy storage device for electric vehicles (EV) as well as for electric power load leveling system [3], [4], [5].

Though commercialized Li-ion batteries are currently using various types of cathode material such as LiCoO₂, LiNiO₂, LiMn₂O₄, and a substituted transition metal oxides (LiNi_1−xCo_xO₂), LiCoO₂ in these cathode materials is most widely used because it has an excellent cycle life and it is relatively more stable against thermal decomposition in charged state than nickel based materials [6], [7]. However, as cobalt is an expensive and relatively rare transition metal, attention has been paid to LiMn₂O₄ pursuing the advantages of low cost and environmental affinity. Especially, the good thermal behavior of LiMn₂O₄ is a positive factor for a large-scale battery to be used for EV because it is not necessary to equip the expensive safety devices [8].

However, the pure lithium manganese oxide has a poor cycle stability and an insufficient rate capability. It is reported that the capacity fading mechanism of the pure lithium manganese oxide at room temperature is related to the Jahn–Teller distortion caused by the presence of Mn³⁺ Jahn–Teller ions [9]. The Jahn–Teller distortion could be reduced with the cation substitution for Mn in the 16d sites. For instance, LiMn_2−yCo_yO₄ (y=0.1) or LiMn_2−yLi_yO₄ (y=0.05) were reported to have the excellent cycle stability at room temperature because of the decrease of Mn³⁺-ions [10], [11], [12].

In spite of the success in improving cycle stability, little has been reported on the enhanced rate capability of LiMn₂O₄. In particular, it is necessary to improve the rate capability of LiMn₂O₄-spinel in order to use it as a cathode material of LIB for EV. The reason for poor rate capability of LiMn₂O₄ is not clear but it may be attributed to its low electrical conductivity (10⁻⁶ S/cm) [13]. The low electric conductivity of LiMn₂O₄ can limit the current flow between particles, which may decrease its rate capability. Therefore, LiMn₂O₄ needs more conducting agent to obtain the sufficient rate capability property in comparison with LiCoO₂. Because large amount of conducting agent decreases the energy density of battery, the amount of conducting agent should be minimized. It indicates that the increase in electric conductivity of LiMn₂O₄ can improve its rate capability without decreasing energy density.

In the present study, the surface of LiMn₂O₄ was encapsulated with small LiCoO₂ particles, which has a higher electric conductivity (10⁻² S/cm) than LiMn₂O₄, in order to reduce the inter-particle resistance and improve its rate capability [13]. The effects of its surface coating with LiCoO₂ on rate capability of LiMn₂O₄ were also investigated.

Section snippets

Preparation of LiCoO₂-coating solution and coating of LiMn₂O₄ with LiCoO₂

The coating solution to encapsulate the surface of LiMn₂O₄ was prepared by the sol–gel process. Stoichiometric amounts of lithium acetate (Li(CH₃COO)·2H₂O, 98% Aldrich) and cobalt acetate (Co(CH₃COO)₂·4H₂O, 99% Aldrich) with a cationic ratio of Li:Co=1:1 were dissolved in distilled water and stirred at 50–60°C. An aqueous glycolic acid water solution (HOCH₂CO₂H, 70% Aldrich) as a chelating agent was then added to this mixture solution to produce a gel-type solution. The molar ratio of glycolic

Optimization of annealing condition for LiCoO₂-coated LiMn₂O₄

Sun et al. reported that a very small powder was synthesized by sol–gel method from aqueous solution of metal acetates containing glycolic acid as a chelating agent [14], [15]. In the current work, sol–gel method was used to coat the surface of LiMn₂O₄ with fine LiCoO₂ particulates. The gel precursor of LiCoO₂ as a coating solution surrounded the surface of LiMn₂O₄ before it was evaporated completely and became a powder. Afterwards, annealing was performed to crystallize its gel precursor which

Conclusions

For the improvement of the rate capability, the surface of LiMn₂O₄ was coated with very fine LiCoO₂ particulates prepared by sol–gel method. The LiCoO₂-coated LiMn₂O₄ was annealed at 800°C for 6 h to crystallize LiCoO₂. The amount of LiCoO₂ content in LiCoO₂-coated LiMn₂O₄ analyzed by the ICP was about 7 mol%. LiCoO₂-coated LiMn₂O₄ showed a higher discharge capacity (120 mAh/g) than as-received LiMn₂O₄ (115 mAh/g) maintaining the same excellent cycle stability. Especially, the rate capability of

Acknowledgements

The authors wish to express thanks to the Research Park of LG Chemical Ltd. for its partial financial support of this work.

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Improvement of the rate capability of LiMn2O4 by surface coating with LiCoO2

Abstract

Introduction

Section snippets

Preparation of LiCoO2-coating solution and coating of LiMn2O4 with LiCoO2

Optimization of annealing condition for LiCoO2-coated LiMn2O4

Conclusions

Acknowledgements

Solid State Ionics

J. Power Sources

J. Power Sources

J. Power Sources

Solid State Ionics

Electrochim. Acta

Prog. Solid State Chem.

Solid State Ionics

J. Power Sources

Improvement of the rate capability of LiMn₂O₄ by surface coating with LiCoO₂

Preparation of LiCoO₂-coating solution and coating of LiMn₂O₄ with LiCoO₂

Optimization of annealing condition for LiCoO₂-coated LiMn₂O₄