Elsevier

Nano Energy

Volume 86, August 2021, 106096
Nano Energy

The surface triple-coupling on single crystalline cathode for lithium ion batteries

https://doi.org/10.1016/j.nanoen.2021.106096Get rights and content

Highlights

  • A surface triple-coupling is exploited on single crystalline LiNi0.5Co0.2Mn0.3O2 cathode by the hydrolysis of NaPF6.

  • The modulation process constructs a “sandwich” configuration from surface to bulk: rock salt - mixing zone - layered phase.

  • The as-prepared sample exhibits a significantly improved cycle stability at 4.5 V and 45 °C.

Abstract

Single crystalline (SC) cathode materials, which are less susceptible to micro/nano-cracks formation and offer better structure stability compared to the polycrystalline counterpart, have attained great attention. However, the parasitic side reactions at the cathode-electrolyte interface induces the loss of active species, which consequently leads to continual degradation of the electrochemical performances. Herein, a triple coupling of concentration-gradient Na+, F- co-doping and surface NaF coating are exploited for the first time on SC LiNi0.5Co0.2Mn0.3O2 cathode by the hydrolysis of NaPF6. This process regulates the external structure of materials by constructing a “sandwich” configuration from surface to bulk: rock salt - mixing zone - layered phase. The detailed interface transformation mechanism is revealed by Neutron powder diffraction (NPD), spherical aberration corrected high-resolution scanning transmission electron microscopy (HR-STEM), electron energy loss spectroscopy (EELS), and Ar+ sputtering assisted X-ray photoelectron spectroscopy (XPS). The synergistic effects endow the SC cathode with outstanding capacity retentions: 91.3% at 25 °C and 85% at 45 °C, after 500 cycles at 5 C between 3.0 and 4.5 V. In addition, a high full-cell reversible capacity of 168.9 mAh g−1 with a capacity retention of 92.4% is achieved after 300 cycles at 1 C. Multiple characterizations further indicate that these superior results are mainly ascribed to the overall structure integrity of SC material, the thin cathode electrolyte interface, high content of lithium fluoride, and the low solubility of transition metal ions. This work opens a new avenue to construct a benign interface towards high-performance lithium ion batteries.

Graphical Abstract

Stabilization of single-crystalline cathode offers a great potential to conquer the challenges presented in its polycrystalline counterpart. The triple-coupling of Na+ and F- co-doping together with NaF coating is boosted on single-crystalline particles, which induces its surface phase reconstruction to endow the material with appealing electrochemical properties.

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Introduction

Demands for clean, dense energy storage provide a huge incentive for the around-the clock development of rechargeable lithium ion batteries (LIBs) [1], [2], [3], [4], [5], [6], [7]. Advanced cathode materials are highly considered as the hard-core components in the next generation LIBs, which dominate their output energy density and manufacturing cost. LiNixCoyMn1−x-yO2 (0 < x < 1, 0 < y < 1) layered oxide materials, as one of the most competitive candidates, have attracted growing interest on account of their compelling energy storage ability, appealing structure stability, and low production cost [4], [8], [9], [10], [11], [12], [13]. Compared with conventional polycrystalline species, single crystalline (SC) particles display a superior structural stability during sustainable long-term cycling of batteries, as the formation of micro/nano-cracks within the particles is alleviated [7], [10], [14], [15], [16], [17], [18], [19]. The structural integrity of SC particles avoids the undesired electrode/electrolyte side-reactions, which effectively prevents the irreversible phase transformation from layered to rock-salt structure [17], [18]. As recently reported, SC primary particles displayed satisfying electrochemical properties compared to polycrystalline cathode during in-depth charge-discharge process [11], [18], [19]. However, the intrinsic degradation of SC material is still inevitable due to the corrosion of electrolyte and dissolution of transition metal (TM) ions, which hinders the complete improvement of LIBs.

Currently, many efforts have dedicated to the study of SC materials by optimizing the synthetic methods [4], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. However, few reports concentrate upon its surface nanostructure reconstruction. In order to further ameliorate the electrochemical behaviors of SC particles, it is imperative to seek out an effective regulation strategies to endow them with a well defined unique texture with high structure stability and benign interface towards electrolytes [21], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]. Among the potential dopants, F- clearly improves the cathode/electrolyte interface integrity due to the enhanced resistance against continuous hydrogen fluoride attack from the electrolytes and the stronger F-metal (Li, TM) bonds [34], [35], [36], [37], [38]. Another candidate dopant Na+ (rNa+ = 1.02 Å), with a higher radius than that of Li+ (rLi+ = 0.76 Å), expands the transport path of Li+ when occupying the Li+ position [22], [30], [31], and thus improves the Li+ diffusivity and electrochemical performances of layered LiNi1/3Co1/3Mn1/3O2 and Li1.1Ni0.2Co0.3Mn0.4O2 [32], [33]. Besides, an ultrathin surface NaF coating is another effective approach to ameliorate the notorious interfacial issues by building a physical isolation between the electrolyte and the cathode [21], [22]. However, the triple-coupling effect of relative merits is still not reported and the corresponding regulation mechanism is unclear. Therefore, a concentration-gradient Na+ and F- co-doping together with NaF coating on SC LiNi0.5Co0.2Mn0.3O2 (NCM) is prepared by hydrolysis of NaPF6. The surface transformation mechanism is thouroughly elucidated by Neutron powder diffraction (NPD), high-resolution transmission electron microscopy (HRTEM) together with focused ion beam (FIB) technology, selected area electron diffraction (SAED), energy-dispersive spectroscopy (EDS) mapping, electron energy loss spectroscopy (EELS) and Ar+ sputtering assisted X-ray photoelectron spectroscopy (XPS). NPD indicates the concentration of the antisite defects is reduced in the modified samples than that of the pristine counterpart. HRTEM results suggest that the as-prepared materials experience a structure evolution: a surface rock salt phase, an intermediate mixing zone and the bulk layered phase. Hence, the optimized sample (NFNCM-2) exhibits outstanding cycle stabilities in the voltage range of 3–4.5 V with a capacity retention up to 94% at 1 C after 300 cycles, as well as retentions of 91.3% and 85% at 5 C after 500 cycles at 25 °C and 45 °C, respectively. An extraordinary full cell performance is also achieved in the voltage range of 2.7–4.4 V with a capacity retention of 92.4% at 1 C after 300 cycles. Post-mortem examinations further indicate that these prominent results are mainly ascribed to the thin cathode electrode interface (CEI) film, high content of lithium fluoride, and the low solubility of TM ions as demonstrated by the time-of-flight secondary ion mass spectroscopy (TOF-SIMS), XPS, HRTEM, XRD and scanning electron microscope (SEM). Thus, this work promotes a further research on the performance improvement of SC-based LIBs.

Section snippets

Results and discussion

To investigate the hydrolysis effect of NaPF6 on the crystal structure of the cathode materials, neutron powder diffraction (NPD) was carried out on both NCM and NFNCM-2 as depicted in Fig. 1a and b. The experimental results are well matched with the calculated data accompanying with low Rwp by refining the background, lattice parameters, as well as atomic position, sample absorption and thermal parameters. All of the observed reflections belong to the R-3m space group with a layered α-NaFeO2

Conclusions

Na+ and F- co-doping and NaF coating on the single crystalline LiNi0.5Co0.2Mn0.3O2 particle surface has been constructed by hydrolysis of NaPF6. The unique approach induces the co-enrichment of phase reconstruction to endow the materials with appealing electrochemical properties. The corresponding surface phase transition phenomenon has been clearly illustrated by high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and Ar+ sputtering assisted X-ray

CRediT authorship contribution statement

Q.Q.Z. and K.L. designed the experiments. Q.Q.Z. conducted the experiments and wrote the paper. L.L. supplied the NCM cathode material. K.L., X.G.S. and S.D. revised the manuscript and discussed the experimental results. S.T. performed the HRTEM characterization and analysis. C.L. carried out NDP test and analyzed the results. X.J.L. participated in the discussions of the results. J.L.Z. and W.L. supervised the work.

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.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 21476158, 21621004), and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT_15R46). X.S. and S.D. were supported by the U.S. Department of Energy’s Office of Science, Office of Basic Energy Science, Materials Sciences and Engineering Division. A portion of this research used the NOMAD instrument at the Spallation Neutron Source, a DOE Office of Science User Facility

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