Elsevier

Journal of Power Sources

Volume 364, 1 October 2017, Pages 121-129
Journal of Power Sources

Unraveling the effect of exposed facets on voltage decay and capacity fading of Li-rich layered oxides

https://doi.org/10.1016/j.jpowsour.2017.07.078Get rights and content

Highlights

  • Two nanoplates with exposed (101) or (001) facets are prepared.

  • Effect of exposed facets on voltage decay and capacity fading of LLOs is unraveled.

  • Large area of active surface will accelerate electrochemical corrosion of LLOs.

  • Phase transformation of LLOs can be mitigated by increasing the area of inactive surface.

  • We discover Co and Ni are easier to react with electrolyte than Mn.

Abstract

In this work, the effect of exposed facets on voltage decay and capacity fading of Li-rich layered oxides (LLOs) is unraveled via two nanoplates which are prepared by very similar co-precipitated-precursor method. The top-bottom surface of one nanoplate is (101) facet (S101 sample, active plane) and the other is exposed (001) facet (S001 sample, inactive plane). Although S101 sample delivers an excellent rate capability (149 mA h g−1 at 2000 mA g−1), both its voltage and capacity decrease faster than S001 sample. TEM, HRTEM, XPS and XRD of cycled samples demonstrate: (1) Large area of active surface will accelerate electrochemical corrosion and phase transformation of LLOs and (2) electrochemical corrosion and phase transformation of LLOs with large area of inactive surface can be mitigated, leading to slow voltage decay and capacity fading. In addition, we discover Co and Ni are easier to react with electrolyte than Mn. Therefore, suitable facets and compositions may be more useful for improving the electrochemical performance of LLOs.

Graphical abstract

Voltage decay and capacity fading of Li-rich layered oxides can be mitigated by large area of exposed inactive surface.

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Introduction

The mass application of Li-ion batteries in the electric vehicle and large-scale energy storage is greatly limited by the availability of high-energy density cathodes. Therefore, the development of cathodes with high performance is urgently needed [1]. In this regard, Li-rich layered oxides (LLOs, xLi2MnO3·(1-x)LiMO2 or Li1+xM1-xO2, M = Mn, Co, Ni) have been widely investigated due to their high specific capacity (ca. 250 mA h g−1) and low cost [2], [3]. The high specific capacity of LLOs originates from the electrochemical activation of Li2MnO3 component over 4.5 V [4], [5]. However, the activation of Li2MnO3 component leads to high irreversible capacity loss and surface rearrangement of LLOs [6]. In LLO cathodes, Li-ions can only transport along the plane groups paralleled to the (001) facet [7]. It makes many characters of LLOs, such as rate capability and element segregations, have a close relation with exposed facets.

Up to now, literature have demonstrated that high percentage of active exposed planes, which have unimpeded pathways for Li-ions transportation, can help to optimize the rate capability of LLOs [8], [9]. For instance, Wu et al. and Sun's group have shown that the rate capability of LLO nanoplates can be greatly improved by increasing the percentage of {010} active exposed planes [10], [11]. In addition, Wang's group and Nanda et al. have found that both element segregations and phase transformation of LLOs are facet-dependent [12], [13], [14]. For element segregations and phase transformation, they are the most undesirable phenomena since they will result in voltage decay and capacity fading of LLOs [15], [16], [17]. However, the effect of inactive and active facets on element segregations and phase transformation of LLOs is far from clear. This is because LLOs with exposed inactive or active planes used to do the above study should be prepared by very similar method to ensure a single variable. It greatly enhances the difficulty of the study on the role of inactive and active facets in element segregations and phase transformation of LLOs.

In this work, two nanoplates are prepared by very similar co-precipitated-precursor method. The top-bottom surface of one nanoplate is (101) facet (S101 sample, active plane) and the other is exposed (001) facet (S001 sample, inactive plane). Electrochemical test suggests the rate capability of S101 sample is better than that of S001 sample. It is due to the active exposed plane which can shorten the transport distance of Li-ions in electrode materials. However, both the capacity and voltage of S101 sample decrease much faster than those of S001 sample. This is because large area of inactive surface can mitigate electrochemical corrosion and phase transformation of LLOs in terms of the discussion of TEM, HRTEM, XPS and XRD for cycled samples. In addition, we discover Co and Ni are easier to react with electrolyte than Mn.

Section snippets

Sample synthesis

Two LLO nanoplates were prepared by very similar co-precipitated-precursor method. Analytical grade chemicals of nickel sulfate (NiSO4·6H2O), manganese sulfate (MnSO4·H2O), cobalt sulfate (CoSO4·7H2O), sodium hydroxide (NaOH), diglycol (NMP: N-methyl pyrrolidinone), and ammonium hydroxide (NH4OH) were chosen as the starting materials. For synthesis of the nanoplate with exposed (101) plane, 1825.4 mg MnSO4·H2O, 1209.1 mg NiSO4·6H2O, 1293.1 mg CoSO4·7H2O, and 50 mL NMP were dissolved in 100 mL

Structure and morphology of the nanoplates

The crystal structure of the as-prepared nanoplates is analyzed by XRD, as shown in Fig. 1. All the strong diffraction peaks for the two XRD patterns can be indexed to a typical hexagonal α-NaFeO2-type structure with a space group R-3m [18], except for the weak peaks in the 2θ range of 20–25°. These weak peaks are caused by lithium-cation ordering in the transition-metal layers. It is the features of the integrated monoclinic Li2MnO3 phase (C2/m) [19]. In addtion, both the pair reflections

Conclusions

In this work, two nanoplates are prepared by very similar co-precipitated-precursor method. The top-bottom surface of one nanoplate is (101) facet (S101 sample) and the other is exposed (001) facet (S001 sample). The two nanoplates have similar chemical composition, particle size, and surface oxides etc., and their difference is mainly concentrated in exposed facet. This difference results in the following facts: the rate capability of S101 sample is better than that of S001 sample, and the

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (No. 21373136) and the Natural Science Foundation of Shanghai (No. 16ZR1416800).

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