Surface modification of LiCoO2 with Li3xLa2/3−xTiO3 for all-solid-state lithium ion batteries using Li2S–P2S5 glass–ceramic
Introduction
Lithium ion batteries have been widely developed for portable electronic devices because they have a high energy density and a long life cycle. Recently, large-scale lithium-ion batteries for electric vehicles have attracted much attention, but deep concern over the safety of battery systems employing conventional organic liquid electrolytes hampers commercialization. The safety problems of conventional battery systems are mainly caused by intense chemical reactions with active electrode materials under elevated temperatures, leakage, and the narrow electrochemical window of liquid electrolytes [1]. Therefore, considerable efforts have been made on developing all-solid-state lithium ion batteries with improved safety.
For all-solid-state batteries, solid electrolytes with high ionic conductivity, comparable to that of liquid electrolytes, are required to achieve good electrochemical performance. Among solid electrolytes, sulfide-based electrolytes are believed to be the next generation of electrolytes for all-solid-state lithium batteries because of their high lithium ion conductivity as well as their wide electrochemical window [2], [3], [4]. However, when sulfide-based solid electrolytes and oxide cathode materials come into contact, a highly resistive layer is generated at the interface, resulting in the degradation of electrochemical performance during cycling [5], [6].
To reduce the large charge-transfer resistance by suppressing the formation of the highly resistive layer, surface modification of active materials with various oxides such as Li4Ti5O12, LiNbO3 and Li2O–SiO2 has been suggested. These materials can successfully prevent direct contact but simultaneously maintain the lithium ion path between the sulfide-based solid electrolytes and oxide cathode materials [1], [7], [8]. However, these coating materials generally have low lithium ion conductivities, which hinder the movement of the Li ion at the interface of the sulfide-based solid electrolyte and oxide cathode materials, despite their positive functionality.
Thus, in this study, lithium lanthanum titanate (Li3xLa2/3−xTiO3; LLTO), which is known to have a high conductivity of over 10−4 S cm−1 [9], [10], is firstly applied as a coating material for all-solid-state lithium batteries using sulfide electrolytes. The composition and coating amount of LLTO are changed to enhance the electrochemical properties of all-solid-state cells employing LiCoO2. In addition, the effects of the surface modification on charge–discharge behavior and charge-transfer resistance of the all-solid-state cells are discussed.
Section snippets
Experimental
The surface of LiCoO2 (99.8%, Sigma-Aldrich) was modified with various compositions of lithium lanthanum titanate (LLTO), Li3xLa2/3−xTiO3 (3x=0.33, 0.5, and 0.75), via the citric acid-assisted sol–gel method, while the coating amounts of Li3xLa2/3−xTiO3 were also changed, maintaining the Li content at 0.75. First, LLTO sols were prepared from lithium nitrate (LiNO3, 98%, JUNSEI), lanthanum (III) nitrate hydrate (LaN3O9·xH2O, 99.9%, Sigma-Aldrich) and titanium (IV) isopropoxide (Ti[OCH(CH3)2]4,
Results and discussion
The XRD patterns of pristine LiCoO2 (abbreviated as P-LCO) and LiCoO2 modified with 0.1 wt% of Li3xLa2/3−xTiO3 (3x=0.33, 0.5, and 0.75) particles (abbreviated as L0.33LTO–LCO, L0.5LTO–LCO and L0.75LTO–LCO, respectively) are shown in Fig. 1. The XRD patterns of all LLTO coated LiCoO2, as well as pristine LiCoO2, index to the stoichiometric rhombohedral structure of the R3̄m space group. No LLTO peaks are found because the amount of coating material is too small to be detected, and heat-treated
Conclusions
A Li3xLa2/3−xTiO3 coating on the surface of LiCoO2 was firstly conducted via the sol–gel process to enhance the cycle performance of all-solid-state cells using sulfide electrolytes. It is identified that Li3xLa2/3−xTiO3 is an effective material to increase reversible capacity and reduce capacity fading. The coating material Li3xLa2/3−xTiO3 should contain a high lithium content to have sufficient ionic conductivity, and the amount of Li3xLa2/3−xTiO3 covering the surface of LiCoO2 should not
Acknowledgment
This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Korean Ministry of Knowledge Economy (Grant no. 10037233).
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