Guozhe
Ma
,
Yu
Zheng
,
Fanbo
Meng
and
Renzong
Hu
*
School of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Advanced Energy Storage Mater., South China University of Technology, Guangzhou, 510640, China. E-mail: msrenzonghu@scut.edu.cn
First published on 22nd August 2023
High-entropy oxides (HEOs) have started to attract widespread interest as anode materials for lithium-ion batteries (LIBs), because of their high theoretical discharge capacities, fast ionic conductivity, and stable structure caused by the unique entropy stabilization effect and the “cocktail” effect. However, the lithiation mechanism of HEO anode is still conversional, which prevents its further development. Herein, we propose an Li2O-doping strategy for improving the lithium storage ability of HEO anodes. Li2O is successfully introduced into the spinel high-entropy oxide to obtain a (FeMgNiCrMnLi)3O4 anode (Li-SHEO) via a solution combustion synthesis and ball milling method. Experimental results show Li doping would induce the growth of oxygen vacancies and regulate the conversion reactions during the discharge process, leading to improved electrochemical performance. As a result, the lithiation process of an Li-SHEO anode includes an enhanced Li+ ion intercalation process and a typical conversion reaction. Compared with the (FeMgNiCrMnLi)3O4 (SHEO) anode, the Li-SHEO anode shows a high reversible discharge capacity of 850.7 mA h g−1 after 200 cycles under a large current density of 2.0 A g−1.
In 2015, inspired by the study of high-entropy alloys (HMAs) and the basic principles of thermodynamics, Rost et al. introduced the concept of high entropy to a multi-component oxide. A synthesized novel metal oxide containing five metallic elements demonstrates the structural stabilization effect driven by entropy, which sparked research into and development of HEOs.8 Similar to the classification of HMAs, materials with a configurational entropy (Sconfig) ≥ 1.5R can be classified as “high-entropy”, and in the case of an oxide system with a near-equimolar composition of five cations, Sconfig can reach 1.61R.9–11 Thus, HEOs can be defined as single-phase oxide solid solutions consisting of five or more different cations occupying the same Wyckoff position in the crystal with similar molar proportions to obtain a high Sconfig for the system.8,12,13 The high Sconfig endows HEOs with excellent physicochemical properties, such as the most typical entropy-induced phase stabilization effect, a sluggish diffusion effect, a lattice distortion effect, and a “cocktail” effect.14–17 These unique properties mean HEOs exhibit great research value in the fields of energy storage, catalysts, and magnetic materials.
Then, Sarkar et al. first introduced the concept of an HEO anode and developed a transition-metal-based HEO (TM-HEO). It is found that the stabilization effect of entropy brought significant benefits to the capacity retention of the HEO, resulting in greatly improved cycling stability and rate capability.18 The five-cation HEO anode material has been shown to have superior cycling stability compared to four-cation “medium-entropy oxides”, as the exclusion of one element from the system reduces the Sconfig from ∼1.61R to ∼1.39R depriving it of the unique properties of HEOs.8,18 Thus, it is the unique high-entropy effect that accounts for the exceptional performance of high-entropy oxide anodes. Inspired by this groundbreaking work, a variety of HEO anode materials, including spinel-type HEO (S-HEO), perovskite-type HEO (PE-HEO), and rock-salt-type HEO (RS-HEO), were developed.18–21 Compared with single transition metal oxide anodes, HEOs have been found to exhibit higher dielectric properties and ionic conductivity, while the entropy-stabilized single-phase crystal structure can be maintained during the cycling process, ensuring good reversible energy storage ability.22–25 For HEO anode materials, choosing appropriate elemental components is critical to achieving better performance. For example, using Co, Ni, Zn, Mn, etc. as active species can provide cycling capacity, while using non-active species such as Mg and Al as buffer materials can stabilize the lattice structure, suppress volume expansion, and ensure good cycling stability.13,26 The performance of an HEO is closely related to the metal cations; therefore, by adjusting the types of cations, it is possible to regulate the crystal structure and properties of the HEO anode, providing a modular approach for the design and performance regulation of anode materials in the field of LIBs.16,18,25
The aggregation of multiple cations in the crystal lattice and the oxygen defects caused by lattice distortion are among the reasons for the improved electrochemical performance of an HEO.27 It is reported that oxygen vacancies can improve electronic and ionic conductivity for anode materials with more additional active sites for the intercalation/deintercalation of Li+ ions. In addition, the charge distribution imbalance resulting from oxygen vacancies can provide additional Coulombic forces, leading to a rapid Li+ ion transfer rate. An aliovalent ion-doped HEO can exhibit interesting properties due to internal charge compensation and defect formation (e.g., oxygen vacancies).26,28–30 Berardan et al. investigated a series of Li+/Na+-substituted (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O HEOs and it was found that the Li-doped samples exhibited clear increased conductivities at room temperature.24 Previously, Ersu Lökçü et al. had introduced Li2O into a type of RS-HEO via a solid-state sintering method. As a result, the Li2O-doped HEO anode can deliver a higher capacity and rapid reaction kinetics. The enhanced performance can be attributed to the additional oxygen vacancies and charge distribution imbalance brought about by Li2O addition.26
Herein, a Li-doped (FeMgNiCrMn)3O4 (Li-SHEO) is prepared by an environmentally friendly and efficient solution combustion synthesis method (SCS) and applied as the anode material for LIBs. The Li-SHEO anode exhibits improved rate capability and cyclic properties. In particular, the Li-SHEO anode delivers a high reversible capacity of 850.7 mA h g−1 under a large current density of 2.0 A g−1 after 200 cycles. Characterization techniques show that the introduction of Li2O improves the valence state of Mn in SHEO and induces the formation of oxygen vacancies. During cycling, more Li+ ions can be embedded into the crystal structure of the SHEO anode, and the conversion reaction can occur over a broader voltage range. Finally, a two-step lithiation mechanism of the Li-SHEO anode is studied in detail and put forward.
Fig. 1 XRD patterns of (a) Li-SHEO material, and (b) SHEO and Li-SHEO materials for revealing the peak shift behaviour. |
TEM and TEM-EDS tests were performed to further confirm the phase structure, particle size distribution, and elemental distribution of Li-SHEO material. Fig. 2(a) shows the TEM image of Li-SHEO material, which reveals that the powder is composed of particles with sizes ranging from tens of nanometres, and the particle size distribution is relatively uniform. The HRTEM image of Li-SHEO powder in Fig. 2(b) exhibits a well-defined lattice fringe with d = 0.46 nm corresponding to the (111) plane of the spinel phase. Fig. 2(c) illustrates the EDS elemental scan analysis images of the Li-SHEO powder, in which Mn, Mg, Fe, Cr, O, and Ni elements are highly uniformly distributed, which is consistent with the characteristic random distribution of metal cations in lattice sites in an HEO.
Fig. 2 (a) TEM and (b) HR-TEM image, as well as (c) HADDF image with the related EDS mapping results of Li-SHEO material. |
To examine the chemical valence for all the metals and the oxygen vacancies of the Li-SHEO anode, an XPS test was performed on the Li-SHEO and SHEO materials and the results are exhibited in Fig. 3.
Fig. 3 High-resolution XPS spectra of Li-SHEO and SHEO: (a) Mn 2p, (b) Mg 1s, (c) Ni 2p, (d) Cr 2p, (e) Fe 2p, (f), (g) O 1s, and (h) Li 1s. |
As exhibited in Fig. 3(a), the two main peaks at binding energies of 652.7 eV (Mn 2p1/2) and 641.2 eV (Mn 2p3/2) are considered to be signals of Mn3+, while the peak at a binding energy of 643.3 eV is considered to be highly correlated with Mn4+.20,32 Compared with the SHEO material, the peak at 643.3 eV of the Li-SHEO material shows a higher intensity with a larger area, indicating a higher valence state of Mn in the Li-SHEO material, which can be attributed to charge compensation caused by the successful introduction of Li2O. Fig. 3(b)–(e) display that other transition metals, such as Mg 1s, Ni 2p, Cr 2p, and Fe 2p, show similar spectral features in both Li-SHEO and SHEO, which suggests that the introduction of Li2O does not cause valence changes in the transition metal elements except for Mn. It is evident that only the valence state of Mn changes due to charge compensation upon the introduction of Li2O, which is related to the multivariable valence nature of Mn.
Fig. 3(f) and (h) present the spectra of O 1s of Li-SHEO and SHEO, respectively, which can be deconvoluted into three peaks located at 529.8, 531.3, and 532.8 eV, which correspond to Me–O bonds, oxygen vacancies, and chemical oxygen, respectively.18,33,34 There is a significant difference in oxygen vacancy content between the two samples. Specifically, the XPS peak representing oxygen vacancies is significantly enhanced in Li-SHEO after the introduction of Li2O. The increase in oxygen vacancies can be explained from multiple perspectives. From the perspective of crystal structure integrity, the introduction of Li2O increases defects in the SHEO crystal structure, thereby promoting the generation of more oxygen vacancies. Furthermore, from the perspective of crystal charge neutrality principles, the introduction of low-valence Li also leads to the production of oxygen vacancies to maintain charge neutrality. It is reported that the introduction of oxygen vacancies can simultaneously enhance the ionic and electronic conductivity of the anode material,29 whilst introducing extrinsic Coulombic forces that facilitate the migration of Li+ ions.30 As shown in Fig. 3(h), the XPS peak located at 55.8 eV is related to the Li–O bond, indicating that the Li+ ion was successfully introduced as a Li2O component into the Li-SHEO material. Another 54.7 eV peak corresponds to LiOH, which is due to the reaction between Li2O and water in the air.
A CV test was further carried out to investigate the impact of Li2O-introduction on the lithiation process of the SHEO anode at a scan rate of 0.2 mV s−1 in the scan voltage range of 0–3 V. As can be seen in Fig. 4(b) and (c), the Li-SHEO anode shows high reduction peaks at 1.0 and 0.28 V, which are much higher than the 0.5 and 0.16 V of the SHEO anode, demonstrating a smaller electrochemical overpotential and fast electrochemical reaction kinetics in both the Li+ ion intercalation process and the metallic reduction reaction after the addition of Li2O. Both Li-SHEO and SHEO anodes exhibit a similar anodic peak observed at 1.6 V, representing the oxidation reaction of the metallic components in SHEO during the charge process. In the subsequent cycles, the CV curves of the Li-SHEO anode show better overlap and repeatability, illustrating better electrochemical reversibility of the Li-SHEO anode.35
To investigate more accurate electrochemical reaction plateaus of the Li-SHEO anode, the charge–discharge curves at 0.1, 0.2, and 0.5 A g−1 were studied, and the results are shown in Fig. 4(d). With the increase in current density, the two discharge plateaus exhibited a downward shift. Under 0.1 A g−1, the position of the first discharge plateau related to the lithium insertion reaction is higher and broader, which is attributed to a more complete electrochemical reaction process at a lower current density. Fig. 4(e) displays the rate capability of Li-SHEO and SHEO anodes. The Li-SHEO anode delivers high discharge capacities of 1242.7, 886.9, 880, 836.1, 764.8, and 658.7 mA h g−1 under 0.1, 0.2, 0.5, 1.0, 2.0, and 4.0 A g−1, which are higher than those of the SHEO anode, showing good lithiation ability under a large density current.
For further estimating the cyclic properties of the Li-SHEO anode, galvanostatic discharge–charge measurements were carried out under a large current density of 2.0 A g−1 within 0.01–3 V (vs. Li/Li+) for the Li-SHEO and SHEO anodes. As shown in Fig. 4(f), the Li-SHEO anode exhibits better cyclic properties with a 200th discharge capacity of 850.7 mA h g−1 with a capacity retention of 122.0%, while the SHEO anode can only deliver a low 200th discharge capacity of 460.6 mA h g−1 with a capacity retention of 73.9%. This excellent cycle stability can be attributed to the unique high-entropy effect of Li-SHEO. Notably, an increase in capacity can be observed. In the first 150th cycles, both Li-SHEO and SHEO anodes exhibit an increasing trend in specific capacity. This phenomenon of capacity enhancement is commonly observed in metal oxide anodes. The capacity rise can be attributed to the enhanced utilization of conversion reactions, the optimization of the electrolyte surface layer, and the changes in morphology.36 Specifically, the active material particles undergo reversible lithiation/de-lithiation through conversion reactions, leading to smaller particles. The reduction in particle size generates new active sites for lithium storage, resulting in a capacity increase. Moreover, the decomposition of the electrolyte and solid electrolyte interphase (SEI) under the catalytic effect of nano transition metal generated from the conversion reaction also contributes to the capacity increase.37 The difference can be attributed to the higher Sconfig of Li-SHEO. The stabilization effect of entropy ensures that the Li-SHEO anode remains stable under a high current density of 2.0 A g−1, while SHEO experiences a rapid capacity decrease due to phase structure degradation. Moreover, as displayed in Fig. 4(f), Li-SHEO shows higher Coulombic efficiency than the SHEO anode, indicating the fast electrochemical kinetics process caused by the successful introduction of Li2O, which is consistent with the former enhanced rate capability.
To investigate the phase transition behaviour during the lithiation process in the SHEO anode and Li-SHEO anode, ex situ XRD analysis was conducted on electrodes under different states of charge (SOC). As shown in Fig. 5(b), when first discharged to 1.0 V, Li-SHEO exhibited the characteristics of a spinel phase corresponding well to the Fe1.145Mn1.148Li0.706O4 phase (ICCD:01-089-7827). When further discharged to 0.8–0.5 V, the spinel phase still existed and it is noteworthy that a change had taken place in the peak intensity of the (311) and (222) planes, corresponding to the Fe2.74Li1.26O4 phase (ICCD:01-085-1992). It can be observed that there is no phase transition behavior in the SHEO anode as found in the Li-SHEO anode within the voltage range of 1.0 V–0.5 V. The phase transition from Fe1.145Mn1.148Li0.706O4 to Fe2.74Li1.26O4 indicates that the first discharge plateau belongs to the Li+ ion intercalation process of the Li-SHEO anode, while the SHEO anode exhibits only a conversion reaction process. During the late discharge and early charge stages, no distinct peaks can be observed in the Li-SHEO and SHEO anodes. When recharged to 3 V, the characteristic peak of the spinel phase of the (311) and (222) peaks can be restored in the Li-SHEO anode. However, no significant peaks can be observed in the recharged SHEO anode.
The disappearance of the spinel phase may be related to the conversion reaction of highly disordered atoms in the Li-SHEO and SHEO anodes. The special high-entropy effects (lattice distortion effect, slow diffusion effect) suppress the formation of the secondary phase of the conversion reaction, resulting in an amorphous state with a long-range disorder, which cannot be observed with distinct peaks in the XRD patterns. As the charging proceeds, some metal cations return to their original lattice sites, and the spinel structure is partially restored in the Li-SHEO anode.
Based on the experimental results, we propose the lithiation mechanism in SHEO and the reasons for its increased reversible capacity. The lithiation process of the Li-SHEO anode consists of two parts, which is the former fast Li+ ion intercalation process at high working potential and the subsequent conversion reaction at low working potential. The increase in the reversible capacity of the Li-SHEO anode results from the joint contribution of Li+ ion intercalation into the crystal lattice of the Li-SHEO anode and subsequent more complete transformation reactions for lithium storage. It was illustrated that the addition of Li2O can induce the formation of more oxygen vacancies, which can provide additional active sites for lithium storage.38 In particular, the higher plateau voltage and the more distinct plateau in the initial charge–discharge curve, and the higher reduction peak potential and intensity in the CV curve corresponding to the lithiation process also indicate that more Li+ ions are embedded into the crystal structure of Li-SHEO with a fast charge transfer rate. Upon comparing the charge–discharge curves and CV curves of the Li-SHEO and SHEO anodes, it was found that the voltage platform and reduction peak corresponding to the conversion reaction in the Li-SHEO anode are raised and widened. These observations provide evidence that the conversion reaction can occur over a broader voltage range in the Li-SHEO anode. Thus, the introduction of Li2O reduces the conversion reaction barrier and leads to a more complete conversion reaction in the Li-SHEO anode.
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