A novel morphology-controlled synthesis of Na+-doped Li- and Mn-rich cathodes by the self-assembly of amphiphilic spherical micelles
Graphical abstract
Introduction
Nowadays, the lithium-ion batteries (LIBs) have grabbed widespread attention in electronic world, however, with the rapid popularization of portable mobile electronic devices (e.g., mobile phones, laptops, digital cameras, ect.), electric vehicles and large-scale energy storage grids, researching for higher capacity and ultra-long lifespan electrode materials especially cathode materials are increasingly urgent [[1], [2], [3]]. So far, there are a lot of cathode materials have been successfully commercialized and get a quite significant improvement on the energy density, such as layered LiCoO2 (~ 140 mAh g−1) [4,5], spinel LiMn2O4 (~ 100 mAh g−1) [6], olivine LiFePO4 (~ 160 mAh g−1) [7] and the layered ternary material LiNi1/3Co1/3Mn1/3O2 (~ >160 mAh g−1) [[8], [9], [10]], but the development of cathode materials still couldn't meet the increasing energy storage requirements. Besides, the specific capacities of these traditional cathode materials showing in practical application are close to their theoretical limits with no further increase potential. Therefore, the Li- and Mn-rich (LMR) cathode materials with a chemical formula of (1-x)Li2MnO3∙xLiMO2 (M = 3d or 4d transition metal; TM) and a high theoretical specific capacity (≥250 mAh g−1) are regarded as the most promising candidates for next-generation cathode materials of LIBs [[11], [12], [13]]. In spite of the high energy density, LMRs always suffer from disgusting voltage decaying and capacity fading during prolonged cycles which have limited their practical use [[14], [15], [16]]. Previous researches have demonstrated that the morphology of LMRs plays a vital role in the realization of their high performance [17,18]. Such as, the nano-micro hierarchical structure could shorten the distance of lithium-ions diffusion, increase the contact area between electrode materials and the electrolyte which is crucial to the rate capability [19,20]. The internal hollow structure with hollow microsphere architecture can alleviate the volume change of the electrode materials during cycles which contribute to the structure stability [[21], [22], [23]]. The morphology evolution, from 1D structure (wire-like) [24] to 2D (rod/plate-like) [[25], [26], [27], [28]] and 3D (cube/sphere/flower-like) structures [[29], [30], [31]] or from solid to porous and hollow structures [23,32], is a process of in-depth research on the effect of morphological regulation on electrochemical performance. Also, ions doping is recognized as one of the most effective methods on the inhibition of voltage fading. Hence, it is also an effective strategy to improve the electrochemical properties of LMRs cathode materials by controlling the product morphology and, at the same time, introducing ions doping. However, the commonly used methods for designing and regulating the morphology of LMRs electrode materials are too cumbersome and complex, or costly [33]. It is extremely necessary to achieve the purpose by controlling the morphology in a simple, cheap and easy way.
The self-assembly behaviors of amphiphiles (especially the low-cost surfactants) in solution have been widely investigated for a long time both experimentally and theoretically. Such as Nagarajan et al. [34] and Wang et al. [35] had systematically summarized the principles of self-assembly for both amphiphilic classical surfactants and block copolymers, provided professional support for the practical utilization of self-assembled surfactants in morphology design. The amphiphilic molecular compounds or large polymeric amphiphiles always holding both a hydrophilic moiety and a hydrophobic moiety, will be ionized in aqueous solution. The self-assembly behaviors usually lead to the formation of simple spherical or rod-like micelles, single-chamber or multi-compartment vesicles and so on [[36], [37], [38]]. In the past few years, these charged micelle groups with a multi-dimensional structure contributed enormously to the surface modification and structure-directing in fabricating various inorganic or organic materials [39,40]. Furthermore, the micelles formed by self-assembly of small molecule surfactants can also be used as soft templates to construct complex hierarchical or porous structured materials through a simple wet chemical process [[41], [42], [43]]. For instance, Zhou et al. [44] reported that onion-like metal-organic colloidsomes were achieved successfully by coupling the assembly of the anionic surfactant sodium dodecyl benzene sulfonate (SDBS). The formed multi-shelled M(OH)x(DBS)y (M = Co or Ni) colloidosomes are sufficiently stable and also show a remarkable catalytic activity in oxygen evolution reaction (OER). It follows that the micelles or vesicles formed by amphiphilic surfactants, especially the conventional low molecular weight surfactants, could be practically applied in the morphologies control of micro/nanoscale multidimensional materials.
Based on the above viewpoint, herein, we introduced the self-assembly behaviors of low molecular weight and cheap anionic surfactants, sodium dodecyl sulfate (SDS), into the synthesis and regulation of the precursor for LMRs (denoted as SDS-LMR hereafter). SDS has many isomers, and the linear structure is selected here. The 3D porous skeleton–supported core and hierarchically structured electrode material assisted by Na+ doping was successfully realized finally by a typical co-precipitation method with the subsequent calcination process. The novel structured SDS-LMR shows excellent electrochemical performance and the voltage fading was also effectively suppressed, the corresponding discharge specific capacity retains 231 mAh g−1 after 100 cycles at 0.5C rate with 93.1% capacity retention and also retains a high capacity of 190.8mAh g−1 at 5C rate due to Na+ ions doping. The voltage decay is a critical issue for the commercialization of LMR materials. And whether it is the rate test or the long cycle test, the discharge average voltage or the mid-point voltage, the performance of SDS-LMR is significantly better than that of Pristine-LMR. The in situ X-Ray diffraction (XRD) and the first principle calculation based on density function theory (DFT) are employed to clarify the process of self-assembly and the mechanism of performance improvement by Na+ doping. Also, to verify the extensibility of this method, the other surfactant, SDBS, was used to carry out the same experiment and similar structures also have been successfully obtained (denoted as SDBS-LMR hereafter). Additionally, this method is simple, low cost and has the potential to be extended to industrial production.
Section snippets
Materials preparation
SDS/SDBS-LMR and Pristine-LMR samples were synthesized by co-precipitation and subsequent calcination. The various ionic salts used here, the TM chlorates (Mn, Co, Ni), the carbonate of sodium and lithium, were purchased from Xilong Scientific Co., Ltd. (Shantou, China) and without further treatment. Firstly, the appropriate amount of SDS was added into deionized water to form spherical micelles which are resulted from its self-assembly behavior. Secondly, the stoichiometric amount of TM
Self-assembly behavior analysis of SDS
For most linear anionic surfactants, their self-assembly behaviors are much the same. Taking a low molecular weight surfactant, the anionic surfactant SDS that is adopted in our work, as an example, in the SDS aqueous solution (Fig. 1a), ionization occurs firstly, the SDS molecules separate into negatively charged DS− ions (CH3-(-CH2-)11-O-SO3−) and counterions (the positively charged Na+ ions). As is known to all, the water-soluble ions are always hydrophilic, whereas organic groups are
Conclusion
In summary, by introducing and utilizing the self-assembly behaviors of low molecular weight anionic surfactants, SDS and SDBS, into the synthesis process, a novel 3D porous skeleton–supported core and hierarchical structure of Li1.2Ni0.13Co0.13Mn0.54O2 cathode material is successfully synthesized. The Na+ was also successfully introduced in the structure during the synthetic process. Benefitting from the synergy between unique structure design and Na+-doping, the samples demonstrate excellent
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
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51931006, 51871188 and 51701169), the National Key R&D Program of China (No. 2016YFA0202602), Guangdong Basic and Applied Basic Research Foundation (No. 2019A1515011070) and the Fundamental Research Funds for the Central Universities of China (Xiamen University: No. 20720190013). This work was also supported by the “Double-First Class” Foundation of Materials and Intelligent Manufacturing
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