Elsevier

Applied Surface Science

Volume 483, 31 July 2019, Pages 270-277
Applied Surface Science

Full length article
Enhanced electrochemical performances of Li2MnO3 cathode materials via adjusting oxygen vacancies content for lithium-ion batteries

https://doi.org/10.1016/j.apsusc.2019.03.210Get rights and content

Highlights

  • A series of the oxygen-deficient Li2MnO3 are prepared by low temperature reduction process.

  • The contents of oxygen vacancies in the Li2MnO3 are controlled by adjusting the amounts of reducing agents.

  • The effects of oxygen vacancies contents on the electrochemical performance of Li2MnO3 are investigated.

  • Oxygen vacancies can significantly improve the cycling performance and rate capability.

Abstract

In this study, oxygen vacancies are successfully introduced into Li2MnO3 by low-temperature reduction. Electron paramagnetic resonance is used to detect oxygen vacancy and the results state clearly that oxygen vacancies indeed exist in the reduced samples, and R-LMO-40 shows the highest content. The relationships between oxygen vacancies content and the electrochemical performance of Li2MnO3 as cathodes for lithium ion batteries are also investigated. The oxygen-deficient samples show much better electrochemical performance compared to the pristine Li2MnO3, especially R-LMO-40 with an optimized content of oxygen vacancies. At the rates of 0.5 C, 1 C, 2 C and 5 C, R-LMO-40 delivers discharge capacities of 143.1, 134.0, 126.1 and 90.9 mAh g−1, respectively. After cycling for 50 cycles, there is nearly no capacity decay. These better electrochemical performances are believed to be related to the characteristics of the oxygen vacancies. Electrochemical impedance spectroscopy analysis clearly indicates that oxygen vacancies can suppress the charge transfer resistance and increase lithium ions diffusion capability. These findings reveal that oxygen vacancies can be utilized to enhance the electrochemical performances of Li-rich oxide cathode materials.

Introduction

With the extensive application of electronic portable devices and electric vehicles, rechargeable lithium ion batteries (LIBs) have been highlighted due to their high energy and power densities as well as long lifespan [1,2]. However, cathode materials as the vital component of LIBs determine the performance indexes of the battery, including specific capacity, rate capability and cycling performance. Among various cathode materials, lithium-rich layered oxides (LLOs) are gaining great attentions owing to high capacities (>200 mAh g−1) and high operating voltage (>4.5 V) [3]. The large specific capacity is mainly attributed to the reversible anionic redox process [[4], [5], [6], [7], [8]] and the formation of the localized electron holes on the oxygen atoms that are adjusted through transition metal and lithium ions [[9], [10], [11]].

The researchers take a great interest in Li2MnO3, which is well known as the structural unit of LLOs due to its higher specific capacity (459 mAh g−1), low cost and its more environment friendly characteristics [12]. Nevertheless, Li2MnO3 is still restricted for its practical applications. This is because material is considered to be electrochemically inactive as Mn is +4 to be further oxidized. However, it exhibits the characteristic first-charge plateau above 4.5 V. This voltage plateau is associated with the oxidation of O2– into O2, causing the irreversible release of O2 from the surface of the materials and generating some oxygen vacancies [13,14]. During the electrochemical activation process, these oxygen vacancies generated at high voltage can promote the migration and structural evolution of transition metal ion [15], resulting in the capacity and voltage decay.

Tremendous strategies have been proposed to optimize the electrochemical properties of Li2MnO3 [[16], [17], [18], [19], [20]]. An example is, the substitution of elements for Li and Mn in the material, like Mg-substituted Li2MnO3 (Li2-xMgxMnO3) [16] is capable to improve the cycling performance [17,18]. Mg-doped Li1.98Mg0.01MnO3 delivered the largest discharge capacity of 307.5 mAh g−1 at 0.1 C in the first cycle with capacity retention of 84.5% upon 30 cycles. The remarkably electrochemical property was ascribed to the introduction of Mn3+ by Mg-doping, which could facilitate the activation of Li2MnO3 and reduce oxygen release during the first cycle. Particle size reduction is another effective way to enhance the electrochemical performance of Li2MnO3. The nanosized cathode materials can enlarge the specific surface area and shorten the diffusion pathways of electrons and lithium ion [19,20]. About 10 nm of Li2MnO3 nanoparticles delivered a discharge capacity of 236 mAh g−1 in the first cycle, and exhibited excellent cycling performance within 2.0 and 4.9 V at 14.3 mA g−1 [19].

Oxygen vacancies can remarkably improve the electron and ion properties of the materials. Thus it has achieved the widespread application in the field of energy conversion and storage, for instance, sodium ion batteries, lithium-air batteries and lithium ion batteries [[21], [22], [23], [24], [25]]. The existence of oxygen vacancies in the anodes of LIBs can raise the intrinsically electronic conductivity of the materials [26,27]. For the cathode materials of LIBs, introducing oxygen vacancies can increase the electronic conductivity and lithium ion diffusion coefficients. For example, the electrochemical performances of LiV3O8 [28], Li1.2Mn0.54Ni0.13Co0.13O2 [29] and LiNi0.5Mn1.5O4 [30,31] have been greatly enhanced as the oxygen vacancies are introduced into the materials. Through the chemical method, the intentionally introduced oxygen vacancies for Li2MnO3 can retard the collapse of the structure, and improve the cycling stability during the cycling process. Kubota et al. introduced the oxygen vacancies into Li2MnO3 by CaH2 and LiH [32]. The results showed that the oxygen-deficient Li2MnO3-x delivered a large discharge capacity of 305 mAh g−1 at 10 mA g−1. Lim et al. synthesized Li2MnO3-x (x ≈ 0.071) by a carbon thermal reduction of Li2MnO3 at 850 °C [33]. They investigated the effect of oxygen vacancies on the cycling stability in Li2MnO3 through the combination of the experiments and first-principle calculations. Collectively, the above results showed that Li2MnO3-x exhibited the improved electrochemical activity without structural degradation. However, the relationships between the content of oxygen vacancies and electrochemical performance have seldom been systematically investigated before.

Recently, our previous research has demonstrated that low-temperature reduction could produce the oxygen vacancies in Li2MnO3 phase while the reduced sample as cathode material of Li-ion battery showed much better electrochemical performance than pristine Li2MnO3 [34]. In addition, Li2MnO3 was synthesized with various contents of oxygen vacancies by adjusting the amount of reducing agents, and compared the electrochemical properties of pristine Li2MnO3 (R-LMO-0) and the reduced samples (R-LMO-30, R-LMO-40 and R-LMO-50) as cathode materials for LIBs. The results showed that the oxygen-deficient sample with an optimized content of oxygen vacancies could deliver the highest charge-discharge capacity at different rates and develop the best cycling performance.

Section snippets

Materials synthesis

The Li2MnO3 with various amounts of oxygen vacancies were prepared by a simple rheological phase method while pristine Li2MnO3 nanobelts (R-LMO-0) were prepared by a molten-salt method using Na0.44MnO2 nanobelts precursor, which were synthesized according to published procedures [35].

To synthesize the R-LMO-0, 1 mmol as-prepared Na0.44MnO2 nanobelts, 4 mmol mixture molten salt with 88 mmol% LiNO3 and 12 mmol% LiCl were mixed and ground evenly in a mortar. The mixture was then transferred into a

Results and discussion

XRD pattern and SEM image of Na0.44MnO2 precursor are presented in Fig. S1. The main diffraction peaks in Fig. S1a match well with Na4Mn9O18 (JCPDS No. 27-0750). Fig. S1b shows that majority of Na0.44MnO2 nanobelts with diameters about 100–200 nm and lengths up to tens of microns were obtained.

The XRD patterns of pristine Li2MnO3 (R-LMO-0) and the Li2MnO3 reduced by low-temperature with different amounts of stearic acid (R-LMO-30, R-LMO-40 and R-LMO-50) are displayed in Fig. 1. All diffraction

Conclusion

In conclusion, pristine Li2MnO3 and Li2MnO3 with different amounts of oxygen vacancies were synthesized and characterized, and their electrochemical performance as LIB cathodes were measured. When used as cathode materials, the samples with various amounts of oxygen vacancies exhibited improved performances, and the sample with an optimized content of oxygen vacancies showed the best electrochemical performance. For R-LMO-40, specific discharge capacities of 173.9, 143.1134.0126.1 and 90.9 mAh g

Acknowledgements

We thank the National Natural Science Foundation of China (grant No. 21271145), the Natural Science Foundation of Hubei Province (grant No. 2015CFB537) and the Science and Technology Innovation Committee of Shenzhen Municipality (contract No. JCYJ20170306171321438) for the financial support for this investigation.

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