Elsevier

Electrochimica Acta

Volume 426, 10 September 2022, 140786
Electrochimica Acta

Microwave irradiation suppresses the Jahn-Teller distortion in Spinel LiMn2O4 cathode material for lithium-ion batteries

https://doi.org/10.1016/j.electacta.2022.140786Get rights and content

Highlights

  • The electrochemistry of commercial LiMn2O4 (LMO) for lithium-ion batteries is constantly challenged by the so-called Jahn-Teller distortion.

  • Microwave irradiation (a technique which is fast, low-cost, and environmentally benign) mitigates the Jahn-Teller distortion in LMO.

  • Microwave irradiation improves the specific capacity by about 21% and capacity retention (0.022% capacity loss per cycle for 100 cycles).

  • The enhanced electrochemistry of microwave-treated LMO is due to the tuning of the physico-chemical properties, including the increased ratio of the Mn(IV)/Mn(III) to greater than 50%, decreased lattice parameter, enhanced BET surface area, enhanced thermal stability, and enhanced structural crystallinity.

Abstract

Jahn-Teller (J-T) distortion remains the key challenge to realizing the full electrochemistry potential of the commercialized LiMn2O4 cathode material for lithium-ion batteries. In this work, the application of microwave irradiation to suppress the limiting J-T distortion in pristine LMO (LMO-p) materials and improve the electrochemistry of lithium-ion batteries has been reported. Unlike thermal heating, microwave irradiation inhibits J-T distortion. To establish the underlying science behind the impact of microwave irradiation, both the pristine (LMO-p) and the microwave-treated counterpart (LMO-m) are subjected to several characterization techniques, including synchrotron XRD, XPS, powder neutron diffraction (PND), MAS 7Li NMR, Raman and BET analysis. These techniques prove that LMO-m possesses the needed physico-chemical properties that explain: i) suppressed J-T distortion (such as an increased average Mn oxidation state (i.e., nMn ≥ 3.5+ or Mn4+/Mn3+ > 50%)), decreased lattice parameter, enhanced BET surface area, and enhanced structural crystallinity), and ii) improved electrochemical performance (such as increased specific discharge capacity, electronic and Li-ion transfer kinetics, and Coulombic efficiency). Microwave irradiation improves the specific capacity by about 21% and capacity retention (0.022% capacity loss per cycle for the LMO-m, ∼ 0.036% capacity loss per cycle for LMO-p for 100 cycles). The findings in this work are relevant for the realization of low-cost and environmentally benign scale-up modification processes for LMO spinel and related cathode materials for the development of high-performance lithium-ion batteries.

Introduction

Among the cathode materials for rechargeable lithium-ion batteries (LIBs), the spinel LiMn2O4 (LMO) remains one of the most widely researched cathode materials that meet the requirements for high specific capacity, high working voltage, low cost, low toxicity, thermal stability/high safety, and long cycle life for electric vehicles (EVs) and portable electronics. LMO has continued to attract research attention with the view to mitigating its key structural and physico-chemical challenges (i.e., severe capacity and voltage fading) that conspire against its widespread commercial utilization, which are the Jahn-Teller (J-T) distortion, Mn dissolution and /or disproportionation reaction [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. J-T lattice distortion and all other limiting processes take place in normal and elevated temperature conditions for the operation of the LMO.

The LMO has a cubic spinel structure with crystallographic space group symmetry Fd-3 m and a 3D network for Li+ insertion. The model crystal lattice: Li(8a)[Mn2](16d)O4 has Li and Mn occupying the 8a tetrahedral and 16d octahedral sites respectively in a close-packed array of oxygen atoms at the 32e sites, thus the model LMO has the structural formula: ([Li+](8a)[Mn3+, Mn4+](16d)O4). The cubic spinel LMO constitutes 50% Mn3+ (t2g3eg1) and 50% Mn4+ (t2g3) in the octahedral sites, thus an average Mn oxidation state (nMn) of 3.5+, which results in the remarkable effect of J-T distortion. The high spin Mn3+: d4 results in a cubic (i.e., regular octahedron) to tetrahedral (i.e., elongated octahedron) phase transition. Indeed, it is well-established that in LMO spinel, when the amount of Mn4+ is equal to Mn3+, many adjacencies of Mn3+– Mn4+ are created, which cause internal strains that ultimately results in the elevation of the diffusion barrier, thereby making the Li-ion diffusion difficult and less stable. However, increasing the amount of Mn4+ increases ionic conductivity by lowering the local Li diffusion activation barriers.

This cubic-to-tetragonal transition (J-T distortion) leads to an increase of 6.5% in the unit cell volume and the Li insertion capacity is decreased [13]. The J-T distortion results in capacity loss and/or fading of the cathode material during cycling [14,15]. Consequently, to improve the electrochemical performance of LMO it is imperative to suppress the J-T distortion. Previous reports [8,16,17] stated that the proportions of Mn3+ and Mn4+have great consequences on the electrochemical properties of the LMO. Therefore, an appropriate increase in the ratio r = [Mn4+]/[Mn3+] can suppress the J-T distortion. Aside from J-T distortion, the Mn dissolution into the electrolyte is another big challenge of LMO, and it has recently been found by the Aubauch group [18] that Mn3+ is the main soluble Mn-ion species in the electrolyte. Thus, the need to reduce the Mn3+ content in LMO cannot be over-emphasized.

To reduce the Mn3+ content in the spinel LMO cathode materials and suppress the J-T distortion, several workers have proposed various strategies, including i) cation (M)-doping of the Mn sites (LiMyMn2-yO4) [6,12] ii) defect-engineering to generate cation-deficient spinel (Li1-yMn2–2yO4) [19]; iii) surface modification [20]; and iv) microwave-assisted synthesis [21,22]. Unfortunately, despite the advantages, these efforts usually result in a significant loss in the specific capacity of the LMO cells [19,23]. Although dopants improve the electrochemical performance of the cathode materials, their use has the possibility of increasing the cost of materials and introduce impurities that can negatively impact the electrochemistry of the final product.

The environmental protection awareness has advanced the Sustainable (Green) chemistry that synthetic chemists in academic and industrial institutions have considered synthesis methods that are Green and viable with great interest [24], [25], [26]. Sustainable chemistry stipulates the design of products and processes that are efficient and environmentally friendly [25]. Thus, the need to explore efficient synthesis method that requires low energy without the use of toxic reagents and organic solvents. Therefore, this study explores the use of microwave and water as microwave absorber. Bilecka et al. [27] cited previous reports to show that there has been a rapidly increasing number of published papers involving the microwave synthesis of inorganic nanomaterials because of its sustainable features (short reaction time, efficient heating and environmentally friendlier). Microwave irradiation synthesis method is currently considered as an innovative technology that is evolving fast in the chemistry world of synthesis [25]. While Zhu et al. reported that water is a good sustainable option because of it is cheap, nontoxic, non -flammable, noncorrosive, polar and a good microwave absorber [24]. Dallinger et al. [28] cited previous reports showing that the combination of microwave and water has received enormous attention as the two prominent sustainable chemistry principles. It is also common knowledge that LMO is insoluble in water and thus suggests no lithium loss during the microwave irradiation with water as absorber. Hence the use of microwave irradiation and water as the absorber in this study follows the sustainable chemistry.

To date, there is no report on the effect of microwave irradiation on laboratory-ready-to-use or commercial LMO samples. Indeed, all known reports on the effects of microwave are focused on precursor LMO materials. Thus, without question, it is highly desirable to introduce a simple, low-cost and environmentally benign technique that will efficiently suppress J-T distortion in LMO and increase its specific capacity (rather than decrease it). Importantly, such a technique should be able to confer desirable physico-chemical properties on the LMO spinel: enhanced interfacial charge (both electron and Li-ion) transport, reduce impedance, good cycling stability and rate capability.

Microwave irradiation chemistry is based on efficiently heating material(s) by dielectric heating. This is the ability of reagent(s) or absorber and reagent(s) to absorb microwave irradiation and convert it to heat. The two main microwave heating mechanisms are the dipolar polarization and ionic conduction [26,29]. The Sun et al. [30] report can be used to describe these two mechanisms in this study as the irradiation microwave on the well dispersed LMO in water cause dielectric heating due to the dipolar nature of water (dipolar polarization). These polarized dipolar molecules rapidly arranged in the direction of the electric field and cause internal friction within the molecules that results in the volumetric heating of the LMO-water mixture. Furthermore, microwave irradiation onto dispersed LMO in water prompts high frequency oscillating electric field that caused oscillatory migration of ions in the LMO which results in friction within the LMO and thus heat was generated (ionic conduction). Consequently, this process generates heat by both dipolar polarization and ionic conduction which provided efficient (energy and time saving) heat to the mixture from the interior. This is contrary to the microwave irradiation of only LMO (without absorber) [21,22] and conventional heating method where heat is transferred from the surface to the interior which are relatively inefficient to the method used in this research work. Therefore, it is logical to suggest that this method can provide better heating strategy for use in nanomaterials synthesis.

This work introduces the use of microwave irradiation to reverse the J-T lattice distortion that conspires against the electrochemical performance of commercial LMO. From extensive microscopic and spectroscopic characterization, including Synchrotron and solid-state NMR analyses, and electrochemistry, it is proven that microwave irradiation can transform Mn3+-rich J-T lattice distorted LMO spinel to an electrochemically high-performing Mn4+-rich non-J-T active LMO spinel (Fig. 1). From detailed characterization, we correlate the impact of microwave irradiation on the mitigation of the J-T distortion on the LMO spinel. Interestingly, Mn4+-rich non-J-T active LMO spinel (i.e., nMn >3.5+ or Mn4+/Mn3+ > 50%) has increased specific capacity. This is in excellent agreement that increased amount of Mn4+ could lower the Li diffusion activation barriers in LMO, thereby enhancing the ionic conductivity and general electrochemical performance [31]. It should be noted that most literature have failed to clarify that the electrochemistry of LMO is dependent on the average Mn oxidation state (thereby leaving many researchers confused) but only indicate that increased Mn4+ content increases the conductivity and general electrochemical performance [31]. The finding in this work permits for cost-effective, environmentally friendly, and highly scalable modification strategy of commercial LMO for the development of longer life lithium-ion batteries.

Section snippets

Synthesis of LMO-microwave (LMO-m)

Microwave-irradiated lithium manganese oxide (abbreviated herein as LMO-m) was obtained from its pristine commercial LMO material (abbreviated herein as LMO-p, donated by the University of Western Cape ESIL, Cape Town, South Africa). In a typical microwave treatment process, 100 mg LMO-p was mixed with distilled water (50 ml) and then subjected to microwave irradiation (using an in-house, custom-built laboratory microwave, 230 Volts, 10 Amps minimum, 50 Hertz, single phase dedicated circuit) at

Physico-chemical characterization

The SEM images of the LMO-p and LMO-m (Fig. 2) show a similar morphology for the two samples, meaning that microwave treatment did not show any observable effects on the morphology of the commercial LMO. Single-point Brunauer-Emmett-Teller (BET) surface area data indicate that the LMO-m exhibits a larger surface area (31.0 m2 g  1), larger pores volume (0.029 cm3 g  1) and larger pore size (3.76 nm) than the corresponding values for the LMO-p (i.e., 29.0 m2 g  1, 0.026 cm3 g  1 and 3.59 nm

Conclusions

In summary, the unique approach of microwave irradiation has been used to modify commercial LMO by suppressing the limiting Jahn-Teller lattice distortion and immensely improving the electrochemical properties of LMO-based lithium-ion batteries. Using some of the state-of-the-art characterization techniques, it was shown that the enhancement of the electrochemical properties of the LMO was achieved by tuning the physico-chemical properties, including the increased ratio of the Mn4+/Mn3+ to

CRediT authorship contribution statement

A.B. Haruna: Conceptualization, Synthesis, Characterization, Experimentation, Writing; D. Barrett: Synchrotron experimentation, and Writing; C. Rodella: Synchrotron experimentation and Writing; R.M. Erasmus: Raman experimentation, Writing; A.M. Venter: PND experimentation and Writing; Z. Sentsho: PND experimentation and Writing; K.I. Ozoemena: Conceptualization, Methodology, Writing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

AB Haruna thanks the University of the Witwatersrand (Wits) for postdoctoral fellowship. The authors are grateful to the support of DSI-NRF-Wits SARChI Chair in Materials Electrochemistry and Energy Technologies (MEET) (UID No.: 132739). We thank Dr. K Eid of the Qatar University for assisting us to run the conventional XPS. This research used facilities of the Brazilian Synchrotron Light Laboratory (LNLS), part of the Brazilian Centre for Research in Energy and Materials (CNPEM), a private

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