Improved electrochemical performance of LiMn1.5M0.5O4 (M=Ni, Co, Cu) based cathodes for lithium-ion batteries

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Highlights

  • LiMn1.5M0.5O4 cathode active materials were doped (M) with Ni, Co and Cu.

  • Ni shows a more efficient substitution of the Mn3+ ion, improving the poor cycling behavior of LiMn2O4.

  • Higher thermal stabilization and high crystallinity is obtained by doping.

  • Low capacity fade is obtained for LiMn1.5Ni0.5O4.

Abstract

LiMn2O4 and LiMn1.5M0.5O4 (M: Ni, Cu, Co) doped particles have been synthetized by sol-gel. Particles between 50 and 200 nm were obtained with the cubic spinel structure of the LiMn2O4. Ni doping shows a more efficient substitution in the octahedral 16d site, replacing the Mn3+ ion, improving the important drawback of poor cycling behavior of LiMn2O4. The average pore size decrease with the addition of the doped elements in the LiMn2O4 structure from 2.9 to 2.6 nm. Thermal analysis shows that the doped particles present higher thermal stability that the undoped ones. Electrochemical behavior of the cathodes prepared with each of the active materials show that the doping influenced the electrochemical performance of the active material. Thus, a specific capacity of 33, 74, 44 and 53 mAh g−1 (at C) and 74, 89, 59 and 69 mAh g−1 (at C/10) were obtained for LiMn2O4, LiMn1.5Ni0.5O4, LiMn1.5Cu0.5O4 and LiMn1.5Co0.5O4 cathodes, respectively. All cathodes present good electrochemical stability with low capacity fade of 0.5 and 3.1% for LiMn1.5Ni0.5O4 and LiMn1.5Cu0.5O4, respectively, after 50 cycles. These results show an improvement of electrochemical performance for LiMn2O4 doped with Ni, Cu and Co, demonstrating their suitability for lithium-ion battery systems.

Introduction

The increase of portable electronic devices and hybrid electric vehicles in recent years raised the demand of high energy density energy storage systems. Lithium-ion batteries (LIBs) were first reported in 1970s and after commercialization by Sony in 1991, became the central pillar of energy storage systems because of its long lifespan, low self-discharging, high energy/power density and low memory effect. LIBs are composed by three main components: anode as a negative electrode, cathode as positive electrode and separator/electrolyte between both electrodes [1,2]. The cathode active materials, through their characteristics lattice sites/spaces, are responsible for the lithium-ion storage/release, via electrochemical intercalation. Active cathode materials with a robust crystal structure are responsible for the cycling stability and specific capacity of the battery. Also, cathodes with high electrochemical intercalation potential and anodes, lead to high energy density batteries [3].

Cathode active materials such as Li2MnO3, LiCoO2, LiFePO4, Li(NiCoAl)O2 and LiNiO2, among others, are used for Li-ion rechargeable battery applications (both, commercially and under investigation) [4,5]. One of the promising cathode active material for LIBs is spinel LiMn2O4 (LMO), that can intercalate two lithium ions in its structure. The first intercalate at 4 V (vs Li+/Li) and the second at 3 V (vs Li+/Li). Advantages are environmental benignity, high rate capability, high cell potential, low toxicity and low cost are associated to this structure. Nevertheless, the applicability of this structure in LIBs is limited to a capacity of 148 mAh g−1 at 4 V region, due to the volume expansion at 3 V region [6,7]. This active material presents some drawbacks as a practical capacity of 120 mAh g−1, reached by the ≈80% of the lithium ions deintercalation. The fracture of the structure by the repeated cycles and the decomposition of the electrolyte (at high voltages) represent relevant disadvantages of this active material [8]. Further, this active material presents poor cycling behavior due to their fast capacity fading in the 3 V range originated by the Mn3+ dissolution during the lithium-ion deintercalation/intercalation and the Jahn-Teller distortion of MnO6 octahedron at 4 V. Thus, the reduction of the Mn3+ ion in the LiMn2O4 spinel structure will improve the cycling performance of the batteries assembled with this active material [9]. Efforts to improve the cyclability and performance of such cathodes involve morphology control [10], coating/functionalization [11,12], dimension reduction [13], oxygen stoichiometry [14] and doping [15]. The substitution of the Mn3+ ion by other ions are the most efficient improvement method and doping metals including Zn, Co, Fe, Cu, Ti, Mg, Al, Ni or Cr, came as a solution to improve this drawback [8,16,17]. Furthermore, the doping with transition metals in some active materials promotes the change in the oxidation state of oxygen due to the deintercalation/intercalation of lithium ion into cathode during the charge/discharge process. The addition of elements with multiple oxidation states should balance the charge in the cathode structure during electrochemical cycling, increasing the stability and specific capacity of the material [18].

Previous work [19] have demonstrated that the addition of rare earth elements as Gd, Nd or Dy into LiMn1.5Ni0.5O4 influence the electrochemical behavior of the active material. LiMn1.48Ni0.5Gd0.02O4 shows excellent applicability in LIBs, compared with the others studied active materials, at high scan-rate (C-rate) with good electrochemical performance (104 mAh g−1 after 55 cycles at C-rate). LiNi0.5Mn1.5O4 active material surface-treated with cobalt at 500 and 700 °C allow to increase the electrochemical response of LIBs [20]. Transmission electron microscopy (TEM), Raman and X-ray photoelectron spectroscopy (XPS) results show that the samples treated at 700 °C creates a surface layer on the active material that enhances the long cycle and high-temperature cycling performance. The reason for this excellent performance is due to the cobalt layer that decreases the nickel concentration and also increases the oxidation state of nickel on the surface. The sample shows a capacity of 93 mAh g−1 after 2000 cycles at 5C-rate with a capacity retention of 81%.

Compared with rare earth elements, the use of transition metal elements more abundant in the Earth crust and less expensive, to improve the LMO drawbacks should be explored. Therefore, Cu and Co doping of LiMn2O4, that are poorly studied, were achieved and compared with 0.5 ratio Ni doping, which is more studied and leads to high specific capacity value. Further, a sol-gel synthesis method for Ni, Cu and Co doping of LiMn2O4 is presented in order to develop LIB cathode active materials for energy storage systems. With the present study, the different contributions of each element have been studied and their electrochemical performance evaluated, showing their potential in the energy field.

Section snippets

Particles synthesis and characterizations

LiMn2O4 and Ni, Cu and Co doped (LiMn1.5Ni0.5O4, LiMn1.5Cu0.5O4 and LiMn1.5Co0.5O4, respectively) particles were synthetized via sol-gel method using precursors of lithium acetate – LiCH3COO (99.9%), manganese acetate – Mn(CH3COO)2 (99.5%), nickel acetate – Ni(CH3COO)2 (99.9%), copper acetate – Cu(CH3COO)2 (99.9%) and cobalt acetate – Co(CH3COO)2 (99.9%) (all from Alfa Aesar, USA), in their stoichiometry ratio. To reach homogeneous solutions, the precursors were dissolved in 2-ethyl hexanoic

Results and discussion

The LMO doped with nickel, copper, cobalt, and pure LMO particles were studied to characterize the doping influence on particles main characteristics and their performance as active material in lithium-ion battery systems.

Conclusions

A sol-gel synthesis was successfully used to produce LiMn2O4 and LiMn1.5M0.5O4 (M: Ni, Cu, Co) doped particles with a size range between 50 and 200 nm. Particles show similar stoichiometric ratio of the doped elements and lattice fringes between 0.47 and 0.48 nm for the single crystalline domains doped particles. Doped elements also occupy the octahedral 16d site, where the Ni shows a more efficient substitution, replacing the Mn3+ ion and therefore improving the drawback of poor cycling

CRediT authorship contribution statement

Renato Gonçalves: Conceptualization, Methodology, Investigation, Validation, Writing - original draft, Writing - review & editing. Poonam Sharma: Investigation, Writing - original draft. Pura Ram: Investigation, Writing - original draft. Stanislav Ferdov: Investigation, Writing - original draft. M. Manuela Silva: Resources, Writing - review & editing, Funding acquisition. Carlos M. Costa: Conceptualization, Methodology, Validation, Investigation, Writing - original draft. Rahul Singhal:

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.

Acknowledgment

Work supported by the Portuguese Foundation for Science and Technology (FCT) funds strategic funding UID/FIS/04650/2020 and UID/QUI/0686/2020, project PTDC/FIS-MAC/28157/2017, and Grants CEECIND/00833/2017 (R.G.) and SFRH/BPD/112547/2015 (C.M.C.). Financial support from the Basque Government Industry Department under the ELKARTEK and HAZITEK programs is also acknowledged. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, EGEF and ESF) is gratefully acknowledged.

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    R.G. and P.S. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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