Elsevier

Electrochimica Acta

Volume 282, 20 August 2018, Pages 30-37
Electrochimica Acta

Origin of storage capacity enhancement by replacing univalent ion with multivalent ion for energy storage

https://doi.org/10.1016/j.electacta.2018.06.032Get rights and content

Highlights

  • Capacity of α-MnO2 can be doubled by replacing Na+ with Ca2+ as charge carriers.

  • Mechanism for the enhanced capacity is revealed via First-principle calculations.

  • Ca2+ causes doubled electron transfer than Na+ under the same concentration in α-MnO2.

  • Multivalent cations are promising candidates for future energy storage devices.

Abstract

The storage capacity or capacitance of a material could be enhanced significantly by replacing univalent ion with multivalent ion in the energy storage field. However, the mechanism of the enhancement is unknown. Here, we dedicate to understand the origin of the enhancement on the storage capacity of multivalent ions over univalent ions. The experimental results show that the specific capacitance and charge-discharge rate of α-MnO2 are doubled by using Ca2+ cation to replace Na+ cation in the electrolyte as the energy storage medium. The First-principles calculations are used for a further understanding for the enhancement on the capacity, charge rate and the insertion mechanism. The given number of cations (two Na+ or Ca2+ ions) can be preferably stabled in one α-MnO2 unit cell to decrease the irreversible tetragonal-orthorhombic deformation caused by John-Teller effect. Because the insertion of Ca2+ triggers double electron transfer than Na+, the capacity and charge-discharge rate of α-MnO2 using Ca2+ cation as storage medium are doubled. The result pave a path to understand the enhancement on the storage capacity by replacing the univalent ions (such as Li+, Na+, K+, etc.) with multivalent ions (such as Ca2+, Mg2+, Zn2+, Al3+, etc.).

Introduction

Manganese dioxide (MnO2) has been widely investigated in electrochemical applications, such as Li-ion batteries [1,2], Li-O2 batteries [3,4], Na-ion batteries [5,6], supercapacitors [7,8], etc., due to its low cost, nontoxicity and high capacity [[9], [10], [11], [12], [13]]. α-MnO2 is one of the polymorphs which are able to accommodate protons [14,15], univalent cations [16,17] and multivalent cations [18,19] in 2 × 2 tunnel with appropriate size and stable structure [[20], [21], [22], [23]]. The storage capacity or capacitance of α-MnO2 could be enhanced significantly by replacing univalent ion with multivalent ion in the energy storage field.

First-principle density functional theory (DFT) calculation is widely accepted and utilized to verify and predict the property of electrode materials in experiment, such as Li-ion, Na-ion, Mg-ion battery, etc [17,18,24]. For instance, it is reported that α-MnO2 can achieve the initial capacity as high as 360 mAh g−1 in rechargeable Li-ion batteries [25]. The calculated charge-discharge process and potential profile through First-principle DFT calculation agree well with the experimental data [16,17]. With the development and demand of large scale systems such as electric vehicles and grid systems, the Na-ion batteries have gained considerable renewed interest due to their low-cost advantage [26,27]. The insertion of Na ion into MnO2 delivers a capacity over 200 mAh g−1 [5]. The insertion of Na ion in Na0.44MnO2 was studied by using DFT calculations for Na-ion batteries, in which seven intermediate phases were found and the calculated potential plateaus agreed well with experiments [24]. α-MnO2 has also been used as cathode materials in Mg-ion batteries with the initial capacity of 280 mAh g−1 [28], and its magnesiation has been studied recently [18]. In addition, because of the high pseudocapacitance of α-MnO2, it is a promising electrode material for the high energy density of supercapacitors. The amorphous α-MnO2 nanosheets of controllable width could achieve a specific capacitance over 500 F g−1 [7,8,[29], [30], [31]].

However, compared to the extensively theoretical study of lithium capture in α-MnO2 for Li-ion batteries, the structure of α-MnO2 when used as supercapacitor electrodes is rarely characterized. In our previous works, charge storage mechanism of α-MnO2 was explored in the aqueous electrolytes containing univalent and bivalent cations [32,33].

In this work, we explored the charge storage mechanism of α-MnO2 in electrolytes containing Na+ and Ca2+ cations by both experiment and DFT calculations. The experimental results show that the capacity and charge-discharge rate of α-MnO2 is doubled by simply replacing the electrolyte containing Na+ cations with the electrolyte containing Ca2+ cations with the similar cation activity. Meanwhile, the DFT calculations prove that two Na+ or Ca2+ cations can be stored in one unit cell of α-MnO2. But more cations than two in the tunnel will lead to strong John-Teller effect, which will cause irreversible distortion of the structure. The calculated results compensate for and agree well with the experimental findings from the atomic scale, which enlighten us to understand the enhancement on the storage capacity by replacing the univalent ions (such as Li+, Na+, K+, etc.) with multivalent ions (such as Ca2+, Mg2+, Zn2+, Al3+, etc.).

Section snippets

Experimental

MnO2 powder was prepared by self-reacting micro-emulsion method. The details of preparation can be referred to in our previous work [34]. Electrodes were prepared by mixing as-prepared MnO2 powder (the active material), acetylene black and polytetrafluoroethylene (PTFE) with weight ratio of 7:2:1. 70 mg MnO2 powder and 20 mg acetylene black were first mixed and dispersed in ethanol for 30 min under ultrasound atmosphere. Then the ink was dried at 80 °C for 4 h to get dark mixed powder and 10 mg

Experimental

Fig. 1a shows the XRD pattern of the as-prepared MnO2 sample. The broad and low-intensity peaks indicate the amorphous nature and small size of particles. Broad peaks at 2θ = 37.0° and 65.8° can be indexed to characteristic peaks of α-MnO2 (JCPDS No. 44-0141). According to our previous work [34], the as-prepared α-MnO2 presents as individual particle with size of about 4 nm. In this work, this amorphous nano-sized α-MnO2 is used in the following electrochemical tests. Fig. 1b shows the SEM

Conclusions

In summary, we studied the charge storage mechanism of the insertion of Na+ and Ca2+ cations in α-MnO2. The capacity of α-MnO2 depends logarithmically on the cation activity in the electrolyte, and the formulas of the relationship between capacity and pA (i.e. logaA, aA is the cation activity and A is Na+ or Ca2+) are proposed. The capacity and charge-discharge rate of α-MnO2 are able to be doubled by replacing the electrolyte containing Na+ with Ca2+ cations under the similar cation activity.

Acknowledgements

This work was financially supported by NSFC (No. 51102139), the National Key Basic Research (973) Program (No. 2014CB932400), Shenzhen Technical Plan Projects (No. JC201105201100A and JCYJ20160301154114273). We also appreciate the financial support from CERC-CVC (2016YFE0102200).

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