Oxygen vacancies with localized electrons direct a functionalized separator toward dendrite-free and high loading LiFePO4 for lithium metal batteries
Graphical abstract
Schematic illustrations of ionic transport regulation process on TiO2−x@PP and application for lithium metal battery.
Introduction
In recent years, the increasing demand for high-capacity and safe energy storage motivates the employment of Li metal-based anode with ultrahigh specific capacity (3860 mAh g−1) and low reduction potential (−3.04 V versus the standard hydrogen electrode) for the development of lithium metal batteries (LMBs), which are of great promises for next-generation energy storage [1], [2], [3]. However, several problems originating from the uneven deposition and the low mobility of Li+ result in undesirable cyclic lifespan and limited practical application [4], [5]. Particularly, the formation of adverse Li dendrites can lead to an internal impedance increase, fast capacity decay of cells as well as some safety hazards [6], [7], [8]. Inhibiting the generation of Li dendrites plays an important role in attaining the overall sustainability of future batteries. Massive efforts have been dedicated for that purpose, such as modifying Li metal anodes [9], [10], fabricating solid-electrolyte interphase (SEI) [11], [12], employing functionalized separators [13], [14] and selecting stable electrolyte [15], [16]. Among these, exploiting functionalized separators is a quite practical and adaptable way. However, the frequently-used polypropylene (PP) separator with disorder pore size has a limited regulation function on the migration of ions. Consequently, unregulated transport of Li+ gives rise to inhomogeneous Li deposition and thus the generation of dendrites, and Li+ migration number will be greatly reduced. Moreover, the free migration of anions brings many severe issues, including the formation of concentration polarization [17], joule heating, side reactions [18], and poor performance at high rates [19]. Therefore, an ideal separator can not only facilitate Li+ diffusion but also restrict anions transport.
Multifarious functional materials have been applied to modify the pp separator for suppressing the dendrites’ growth [20], [21], [22], [23], [24]. For example, the lithiophilic groups-such as CO [25], COOH [26]-have been designed to reduce the heterogeneous nuclear barrier. The nanostructured Al2O3 [27] or SiO2 [28] have been introduced to accelerate the uniform deposition of Li+. Although these strategies have obtained some success in inhibiting the formation of Li dendrites, the electrochemical performance of batteries is still far from satisfactory. The critical issue for the above-mentioned strategies is that they merely focus on the process of Li growth or the primal nucleation of Li, leading to that they are unable to concurrently resolve problems of ion uneven deposition and inferior mobility of Li+. Especially, the existence of the tip effect in the cyclic process also prevents the uniform deposition of Li+ [29]. Therefore, it is desirable to develop a functional material that can simultaneously regulate the diffusion of both cations and anions, and sustainably realize the uniform distribution of Li+ during the cycle process.
Among many materials, metal oxide materials are cost effective and easier to achieve large-scale production. Herein, by addressing issues of sluggish ions diffusion and low electrical conductivity with defect engineering, the metal oxides as functional material have provided an effective approach for the separator modification [30], [31], [32], [33]. The introduction of defects in metal oxides can change the local electron distribution and enhance ion migration. Semiconductor TiO2 is an ideal model material for such a study because it is commonly used as a stable protective layer. In TiO2−x (the TiO2 with oxygen atomic vacancies), the Ti atoms near the oxygen vacancy possess localized excess electrons [34], [35], [36]. The localized electrons have a strong attraction to lithium nuclei, which is favorable for the dissolution of lithium nuclei from lithium clusters into isolated individuals [37]. The result is that more lithium ions can be utilized and evenly deposited throughout the cycle. More importantly, the TiO2−x owns positively charged oxygen vacancies. Based on the Lewis acid–base interaction, the TiO2−x possesses a strong affinity with TFSI− anion. Thus, TiO2−x can suppress the diffusion of anions while synchronously facilitating the migration of cations.
Hence, we propose to adopt the TiO2−x with electronic localization as a functional layer for the separator modification in LMBs. As shown in Fig. 1(a), in terms of the common PP separator without available adsorption sites, the TFSI- anions can migrate toward the anode side through PP separator. Unfortunately, it is impossible to achieve the homogeneous distribution of Li+ flux, leading to the generation of Li dendrites. In sharp contrast, for TiO2−x@PP separator, the diffusion of TFSI- anions is blocked by the Lewis acid–base interaction between the TiO2−x and TFSI-. Correspondingly, uniform Li+ flux and good adhesion in Li metal anode can be effectively realized, which suppresses the formation of Li dendrites. The TiO2−x modified PP separator is assembled into a cell, which displays stable long-term cycling performance under different conditions. Notably, the Li|TiO2−x|LFP battery with high LiFePO4 (LFP) loading mass (9.24 mg cm−2) manifests stable cycling performance and extended cycle life over 900 cycles. Furthermore, this work offers a facile and fairly practical way to achieve long-life capability by overcoming some remaining challenges, imbuing the practical application of LMBs with effective support.
Section snippets
Results and discussion
The unregulated diffusion of ions can generate inhomogeneous Li deposition, resulting in possible induction of Li dendrites and short-circuiting. Therefore, the chemical characteristics of TiO2−x and TiO2 are first investigated by the Density function theory (DFT) calculations. The adsorption energy of Li on the TiO2 and TiO2−x surfaces are calculated to estimate the adsorption strength of Li on the TiO2 and TiO2−x surface. The adsorption energy of Li on the TiO2−x surface is −0.78 eV (Fig. 1
Conclusions
In summary, we demonstrate a TiO2−x material coated pp separator that can stabilize the lithium electrode in lithium metal batteries. After comprehensive DFT calculations and characterizations, it is indicated that the separator can effectively suppress dendrite growth and maintain a uniform ionic flux on the lithium metal surface. On the one side, the introduction of TiO2−x by defect engineering provides more diffusion pathways for Li+ while synchronously hindering anions shuttling to achieve
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.
Acknowledgments
The authors acknowledge financial support provided by the National Natural Science Foundation of China (52064049), the Key National Natural Science Foundation of Yunnan Province (2018FA028 and 2019FY003023), the International Joint Research Center for Advanced Energy Materials of Yunnan Province (202003AE140001), the Key Laboratory of Solid State Ions for Green Energy of Yunnan University (2019), the Analysis and Measurements Center of Yunnan University for the sample testing service, and the
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