Anisotropic Li diffusion in pristine and defective ZnO bulk and (100) surface
Introduction
Artificial coating on the surface of Li-ion battery (LIB) cathode is an effective method to enhance the cathode stability at high operating voltage. Various materials such as binary metal oxides [[1], [2], [3], [4]], phophates [[5], [6], [7]], fluorides [[8], [9], [10]] have been reported as potential protective coating materials for LIB cathode. Those previous studies have further shown that the enhanced stability of the cathode structure increase as well the capacity retention of LIB over prolonged charging-discharging cycles. The following mechanisms have been proposed as the physical origin of the aforementioned enhanced performance: (1) the coating shields the active cathode material from parasitic decomposition of the electrolyte on the cathode surface [11,12] (2) the coating scavenges HF molecule which can decrease local acidity at the cathode/electrolyte interface area, hence decreasing the electrolyte degradation [13,14] and (3) the coating mitigates structural deformation on the cathode surface, e.g., surface oxygen loss, transition metal dissolution and surface cation migration [[15], [16], [17], [18], [19]].
Based on previous studies there are two forms of coating in general. The first is nano particles coating which commonly covers the surface of the cathode partially. The second is thin film layer which encapsulates all of the cathode surface. In our present work, we will assume that all cathode area is covered by the thin film layer uniformly. In such scenario, direct contact between electrolyte and cathode surface which possibly lead to electrolyte decomposition as well as transition metal dissolution (i.e. corrosion) can be suppressed maximally. As a further consequence the coating ability to transport Li is likely to play an important role. Previous studies have investigated several important aspects that influence the Li transport inside the coating materials such as coating thickness, atomic structure, coating/cathode interfacial structure and defect chemistries [[20], [21], [22]]. The thickness of the coating layer is perhaps one of the most studied due to the easiness to control coating deposition relative to the other aforementioned aspects. For instance, by using Magnetron Sputtering Lai et al synthesized ZnO coated Ni-rich Li(Ni0.8Co0.15Al0.05)O2 (NCA81505) cathode. They observed correlation between ZnO film thickness as a function of sputtering time with the electrochemical performance of the LIB. They found that the electrochemical performance increases when relatively thin ZnO film (∼20 nm) was deposited onto the cathode surface but then reduces when the ZnO film became thicker [23]. Similar insight was also obtained by Riley et al in which Atomic Layer Deposition was used to grow an Al2O3 film on Li(Ni0.33Co0.33Mn0.33)O2. They observed that Li transport is significantly hindered as the Al2O3 coating thickness exceeds ∼12 A [24].
While studies related to coating thickness and Li transport have been conducted intensively, study related to the role of defect on Li transport in coating materials in the context of LIBs application is relatively scarce. Herein, we focus on the ZnO one of the widely used surface protective material for Li-ion battery cathode. While previous studies have shown that ZnO indeed can increase the capacity retention of the overall Li-ion battery cycles [20,23,[25], [26], [27]] the quantitative knowledge related to how point defects affect the Li diffusion in ZnO remains unclear. Our results clarify that while some certain point defects hinder the Li diffusion in ZnO, oxygen vacancy promotes faster Li diffusion as shown by relatively low diffusion energy barrier.
Section snippets
Computational setup
We carried out spin-polarized density functional theory (DFT) calculations as implemented in quantum ESPRESSO package [28]. Ultrasoft pseudopotentials were used to parametrize the core region of each elements used in the present work. The Generalized Gradient Approximation of Perdew-Burke-Ernzerhof (GGA-PBE) was used to describe the exchange interactions and electronic correlations [29]. We applied 42 Ry kinetic cut-off energy for the expansion of Kohn-Sham wave functions. Brillouin zone
Results and discussion
Based on the ZnO structure, we considered two different hollow sites occupied by the interstitial Li (Lii), i.e., the octahedra and the tetrahedra sites. The octahedra site is a hollow site surrounded by six oxygen ions whereas the tetrahedra is surrounded by four oxygen ions. Geometry optimizations show that Lii located on the octahedra site (Lii-oct) is thermodynamically more stable as compared to the tetrahedra site (Lii-tet) by 1.04 eV. We also find that further geometry optimization if
Conclusion
By means of first-principles density functional theory (DFT) calculations coupled with nudged elastic band (NEB) method we have investigated the effect of two point and one complex defects on Lii diffusion barrier inside ZnO bulk. Our results show that Ovac lower the Lii diffusion energy barrier on both OO and O-T(O) paths as compared to the pristine ZnO case. For the case of Znvac, the Lii mobility also can increase as shown by the lowered diffusion barrier on O-T diffusion path but the with
CRediT authorship contribution statement
Ganes Shukri: Writing – original draft, Conceptualization, Investigation, Visualization, Methodology. Adhitya G. Saputro: Funding acquisition, Supervision, Writing – review & editing, Investigation, Conceptualization, Methodology. Poetri S. Tarabunga: Investigation. Febriyanti V. Panjaitan: Investigation. Mohammad K. Agusta: Methodology. Ahmad Nuruddin: Methodology. Hermawan K. Dipojono: Resources, Methodology.
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.
Acknowledgement
This research is financially supported by the Indonesian Ministry of Education, Culture, Research and Technology (KEMENDIKBUD-RISTEK) through the “Penelitian Dasar Unggulan Perguruan Tinggi (PDUPT) 2019” research funding scheme. GS would like to acknowledge financial support granted by the Faculty of Industrial Technology of Bandung Institute of Technology under the "Program Penelitian, Pengabdian kepada Masyarakat dan Inovasi ITB (P3MI) 2019" scheme.
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2023, AIP Conference Proceedings