Nanoporous and lyophilic battery separator from regenerated eggshell membrane with effective suppression of dendritic lithium growth
Graphical abstract
Introduction
Since the application of batteries has been vigorously expanded into new fields, such as smart electronics, clean-energy vehicles and grid-scale storage, the search for portable, high capacity and safe electrical energy storage technologies has become one of the paramount motivators for battery material research [1], [2], [3], [4]. Lithium metal-based secondary batteries, including lithium–air, lithium–metal oxides and lithium–sulfur batteries (LSBs), are attractive alternatives to conventional lithium-ion batteries (LIBs), owning to the high theoretical capacity of lithium metal anode (3860 mA h g-1) and the lowest redox potential among all existing anodes [5], [6]. However, for the practical use of lithium metal anode, the severe dendritic lithium formation on the lithium metal surface should be suppressed [7], [8]. During the repeated charge-discharge cycling, the continuous uneven deposition and stripping of lithium induce uncontrollable growth of lithium dendrites, which can penetrate through the polymer separator and form micro-short circuits between the positive and negative electrodes, causing the serious safety issues including fire and even explosions [9], [10], [11], [12].
In recent years, although great efforts have been made on the optimization of electrolytes [13], [14], [15], [16], the achievements on suppressing dendritic lithium growth are still limited, because normally lithium metal cannot conformally contact the separator in microscale during charge/discharge processes. Separators with well-designed nanostructures hold the key for suppressing the growth of lithium dendrites. Traditionally, this problem was clumsily alleviated by using thicker and more tortuous separators. However, such separators normally lead to increased impedance loss, and still cannot fully suppress the dendritic lithium growth [17]. Currently, the most widely-used commercial separators (such as CelgardTM-2400 shown in Fig. S1) are made of polyolefin films [18], [19], [20], predominantly polyethylene (PE) or polypropylene (PP). Unfortunately, owning to the unsuitable morphology and pore size distribution of commercial separators, the capability for suppressing the formation and growth of lithium dendrites is very limited. Moreover, polyolefin films usually suffer from insufficient electrolyte wettability, low porosity and serious thermal shrinkage [21], [22], [23], [24], [25], which are partially responsible for the relatively poor electrochemical performance and poor safety of lithium metal-based batteries. Previous reports have explored the modification of commercial Celgard separator by coating with ceramic or polymer for improving the electrolyte affinity and resistance to thermal shrinkage [26], [27], [28]. It is ideal to design and fabricate novel separators that can solve all of the above issues.
Based on the systematic evaluation in lithium metal-based batteries, here we report that the pristine eggshell membrane (ESM) extracted from waste eggshell is a promising candidate of separator with remarkable properties. Moreover, to overcome the curved shape and limited size of raw ESM, we further develop a biomimetic and economic strategy to fabricate large-area and flat nanoporous regenerated ESM (RESM) film from pristine ESM. Interestingly, the nanoporous RESM film can well inherit and even significantly enhance the merits of raw ESM as battery separator. It should be noted that although ESM film was investigated for using in energy related applications, such as supercapacitor, synthesis template or starting material [29], [30], [31], [32], [33], [34], [35], [36], [37], but the detailed intrinsic properties, regeneration strategy and effects of ESM film for metal lithium-based secondary batteries still have not been investigated. With the good electrolyte wettability, high electrolyte uptake, good mechanical strength, high thermal stability and well-distributed porous structure of RESM film, the batteries with RESM film can exhibit greatly enhanced performances than those with commercial Celgard separator in terms of battery safety, reversible capacity, rate capability and long-term cycling stability under high rates. Impressively, the three-dimensional nanoporous and flexible RESM film can effectively suppress the dendritic lithium growth during charge/discharge processes and maintain a uniform ionic flux on the lithium metal surface.
Section snippets
Chemicals
The waste eggshells were collected from the canteen of Nanjing University. All other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd, and were of analytical grade and used without further purification.
Extraction of egg-shell membranes (ESM)
The waste eggshells were firstly cleaned with deionized water and then immersed into HCl solution (1.0 M) for 6 h to remove the hard CaCO3 outer shells. Then, the resultant ESM were washed with deionized water and dried at room temperature for 12 h.
Dissolution of ESM
The obtained ESM were cut into
Results and discussion
Fig. 1a shows the typical structure of bird or reptile eggs, which usually possess three protective layers: the hard eggshell, the outer ESM and the inner ESM. The hard eggshell is mainly composed of CaCO3 crystals stabilized by protein matrix, while the outer and inner ESM are primarily composed of a porous matrix of interwoven protein fibers and polysaccharides (glycans) [38], [39]. After the evolution by the Mother Nature for billions of years, ESM has developed numerous functions and merits
Conclusions
In summary, we show that the ESM and RESM acquired from abundant and renewable poultry sources can be used as close-to-ideal battery separators that fully outperforming the existing mainstream option. A key finding is the suppression of dendritic lithium growth when using lithium metal electrodes, solving a long standing problem in lithium metal based batteries. Finally, an effective method with high compatibility to current technologies was developed to prepare large-area and flat RESM from
Acknowledgements
This work is supported by National Key R&D Program of China (2017YFA0208200, 2016YFB0700600 and 2015CB659300), Projects of NSFC (21403105 and 21573108), Natural Science Foundation of Jiangsu Province (BK20150583 and BK20160647) and the Fundamental Research Funds for the Central Universities (020514380107).
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These authors contributed equally.