Stability investigations of composite solid electrolytes based on Li₇La₃Zr₂O₁₂ in contact with LiCoO₂

Highlights

• Chemical reaction stability of composites based on Li₇La₃Zr₂O₁₂ to LiCoO₂ was studied.

• Composites are stable in contact with LiCoO₂ up to 900 °C.

• Glass addition increased stability of tetragonal Li₇La₃Zr₂O₁₂ in contact with LiCoO₂

Abstract

LiCoO₂ was synthesized by the sol-gel method. Using the proposed synthesis method, a cathode was obtained at the final annealing temperature of 700 °C with a grain size of 40–80 nm. The chemical reaction stability of composite solid electrolytes based on Li₇La₃Zr₂O₁₂ in contact with LiCoO₂ was studied using differential scanning calorimetry, Raman spectroscopy, and X-ray diffraction. The composite solid electrolytes based on tetragonal and cubic modification of Li₇La₃Zr₂O₁₂ with the addition of Li₂O-Y₂O₃-SiO₂ glass are stable in contact with LiCoO₂ over a wide temperature range (35–900 °C). Tetragonal Li₇La₃Zr₂O₁₂ solid electrolyte is stable in contact with the LiCoO₂ cathode due to the introduction of a glassy additive. Therefore, the possibility and prospects of using composite solid electrolytes based on Li₇La₃Zr₂O₁₂ in lithium and lithium-ion power sources with LiCoO₂ as a cathode are shown.

Introduction

Li₇La₃Zr₂O₁₂ (LLZ), as well as compounds based on it, are promising solid electrolytes for lithium and lithium-ion power sources [[1], [2], [3]]. LLZ has two structural modifications with lithium-ion conductivity values that differ significantly. The cubic modification has higher total conductivity values (~10⁻⁴ S·cm⁻¹ at 25 °C) than the tetragonal one (10⁻⁶–10⁻⁷ S·cm⁻¹ at 25 °C). However, stabilization of highly conductive cubic modification requires the introduction of some dopants, such as Al, Ga, Y, Ta, Nb, etc. [1]. LLZ possesses not only high conductivity values, but also stability in contact with lithium metal, the most promising anode material [4]. LiCoO₂ and compounds based on it [[5], [6], [7]] are widely used as cathode materials in lithium-ion batteries due to their high electrochemical characteristics and good cycling [5]. Moreover, LiCoO₂ is also promising for creating modern high-energy lithium power sources paired with lithium anodes. The stability of solid electrolyte in contact with cathode material should be studied for evaluating the possibility of such power sources.

One of the simplest ways to determine the stability of solid electrolyte in contact with electrode is to heat their mixture. The temperature of interaction and phase composition of the studied mixture after heating at different temperatures can be investigated using differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis. The stability of solid electrolytes with the garnet-like cubic structure doped with various elements (Ba, Ta, Ca, Nb, Al, etc.) in contact with LiCoO₂ has been shown by some researchers [[8], [9], [10]]. For example, a mixture of Li₆BaLa₂Ta₂O₁₂ solid electrolyte and cathode material (1:1) was heated in the temperature range of 400–900 °C for 24 h by Thangadurai et al. [8]. According to the XRD analysis, the solid electrolyte was chemically stable in contact with LiCoO₂ up to 900 °C. Meanwhile, the interaction of other cathode materials (LiNiO₂, LiMn₂O₄ and Li₂MMn₃O₈; M = Fe, Co) with the solid electrolyte and formation of reaction products with a perovskite-like structure at temperatures above 400 °C was observed. Thermal stability of LiCoO₂ in contact with Al-doped LLZ between 300 and 800 °C was shown by Wakasugi et al. [10]. The formation of interaction products between the solid electrolyte and LiMn₂O₄ at 600 °C and LiFePO₄ at 400 °C was established using XRD analysis. However, after heating the mixture of tetragonal LLZ with LiCoO₂ to 600 °C and higher, the appearance of an impurity phase in the form of La₂Li_0.5Co_0.5O₄ was observed in our previous work [11].

Thermodynamic simulation is another widely used method for determining the stability of electrolyte in contact with electrode materials [[11], [12], [13]]. In our previous work [11], the stability of the tetragonal modification of LLZ to LiCoO₂ in the temperature range of 25–527 °C was investigated. According to the obtained data, these compounds interact in the studied temperature range with the formation of Li₈ZrO₆, La₂Zr₂O₇, and La₂CoO₄. The possible reaction of Li₇La₃Zr₂O₁₂ and Li₅La₃Ta₂O₁₂ electrolytes with LiCoO₂, LiMnO₂, and LiFePO₄ cathodes was studied using density functional theory by Miara et al. [12]. LiMnO₂ and LiFePO₄ were found to react with garnet-like solid electrolytes, while LiCoO₂ was relatively stable.

Transport properties of lithium-ion solid electrolytes can be improved using different techniques, such as doping [1], modification of the synthesis method [1,[14], [15], [16]], or introduction of some additives [[17], [18], [19], [20], [21], [22], [23]]. Currently, significant research has been devoted to composite solid electrolytes based on lithium-ion conductive electrolytes [1,17,24]. The introduction of sintering additives (Li₃BO₃, LiPO₃, Li₃PO₄, Li₄SiO₄, etc.), including glasses (Li₂O-Al₂O₃-SiO₂, Li₂O-B₂O₃-SiO₂, Li₂O-Y₂O₃-SiO₂, etc.) is one promising direction for reducing the time and temperature of synthesis, as well as increasing the lithium-ion conductivity of the solid electrolyte with the garnet-like structure [[17], [18], [19], [20], [21], [22], [23]]. For example Rosero-Navarro et al. [19] showed, that the introduction of Al₂O₃ and Li₃BO₃ into Li_7-xLa_2.95Ca_0.05ZrTaO₁₂ solid electrolyte leads to a reduction in the final annealing temperature from 1200 to 900 °C, while high values of lithium-ion conductivity were retained (1·10⁻⁴ S·cm⁻¹ at 32 °C). Moreover, composite solid electrolytes based on highly conductive electrolytes with garnet-like structures makes it possible to solve the stability problem of ceramic electrolytes in air, which complicates work with them and their continued use [1,23,25]. For example, the addition of Li₃BO₃ to LLZ-based electrolyte improves its stability in air; and a decrease in conductivity of the composite electrolyte after air exposure for several days is not observed [25]. As shown in our previous work [23], the introduction of Li₂O-Y₂O₃-SiO₂ glass in the cubic modification of LLZ leads to a decrease in surface degradation of samples in air. However, the effect such additives have on the stability of LLZ-based solid electrolyte to cathode materials has not been investigated.

The aim of the work was to study the chemical reaction stability of composite solid electrolytes based on tetragonal and cubic modifications of LLZ with the addition of lithium yttrium-silicate glass in contact with the LiCoO₂ cathode material.