Corresponding Authors: Liang Zheng, Yue Xinyang.
First Author: Ding Luoyi.
Correspondence unit: Shanghai Jiao Tong University.
Background] With the increasing demand for energy storage, there is an urgent need to develop high-energy-density batteries. However, the practical application of LMBS is greatly limited due to the uneven flux of lithium dendrites and lithium ions, resulting in a short cycle life and low coulombic efficiency (CE) of lithium metal batteries (LMBS). The formation of lithium dendrites can be divided into two processes: nucleation and growth, and according to the "sand's time" model, lithium nucleation is closely related to the migration number (TLI+) of lithium ions. Higher TLI+ can delay the occurrence of dendrite nucleation.
Therefore, many researchers have focused on the development of anion-anchored separators, that is, inhibition of lithium dendrite formation in LMBS by increasing TLI+, a strategy that can inhibit dendrite growth to a certain extent. However, this will inevitably reduce the contribution of anions to the construction of solid-state electrolyte boundaries** (SEIs), i.e., they will be detrimental to the construction of stable inorganic-rich SEIs.
Introduction] Zheng Liang's research group at the Center for Transformative Molecular Frontiers of Shanghai Jiao Tong University has taken a unique approach and adopted the strategy of "tandem separator" to confine free anions around the separator while constructing an excellent SEI structure, so that high-TLI+ and high-quality SEI with fast Li+ transmission can be obtained at the same time, and the "decoupling" of high-TLI+ and high-quality SEI can be realized.
Thanks to this, the growth of lithium dendrites has been greatly mitigated. Symmetrical cells with Z5P PE SBF tandem separator have long-term lithium plating peel stability (1000 cycles) at a current density of 10 mA2, assembled NCM811|LI and LFP|The electrochemical performance of the whole battery has been significantly improved. In particular, lini06co0.2mn0.Pouch cells with 2O2 (NCM622) cathode also exhibit excellent long-cycle performance at high rates.
The research results were published in Advanced Functional Materials under the title of "Tandem Design of Functional Separators for Li Metal Batteries with Long-term Stability and High-Rate Capability".
Figure 1Schematic diagram of a) PE separator, b) anion-anchored separator, and c) tandem separator during charging.
Description] The low cost of Z5P PE SBF tandem diaphragm and Z5 prepared by simple scraper coating method has the advantage of realizing large-scale industrial production. In addition, the thermal stability and electrolyte wettability of the "tandem separator" are significantly improved. And compared with traditional PE separators, it has high ionic conductivity and TLI+. The fitting curves of the variable-speed CV show that the cells of Z5P PE SBF show the most significant slope and therefore the most favorable Li+ diffusion kinetics, which is consistent with the enhanced ionic conductivity of the Z5P PE SBF separator. At the same time, the rapid Li+ conductivity of Z5P PE SBF-based cells allows more active lithium to shuttle back and forth in the SEI, resulting in a rapid reversible insertion of Li+ extraction. In addition, the activation energy of the Z5P PE SBF septum was lower, indicating that the SEI induced by Z5P was more conducive to the rapid transfer of Li+.
Figure 2Comparison of the physical and chemical properties of various separators.
Symmetrical battery cycle performance table shows that the polarization voltage generated by Z5P PE SBF-BASED batteries in the initial stage (95 mV vs Li+ Li) is much lower than that of Z5P PE and PE (320 mV and 358 mV, respectively), and the battery life of Z5P PE SBF is effectively extended compared with PE and Z5P PE. Similarly, the Z5P PE SBF has the lowest polarization voltage and the longest cycle life at a current density of 10 mA cm2. In addition, the excellent rate performance of Z5P PE SBF-BASED SYMMETRICAL BATTERIES MEANS THAT THE Z5P PE SBF separator can promote the rapid transfer of Li+ in SEI and alleviate the accumulation of dead lithium. At the same time, the CE results show that the Z5P PE SBF separator can significantly reduce the lithium capacity loss caused by the growth of lithium dendrites, which is essential for long-term cycling.
Figure 3Evaluation of the electrochemical properties of various separators.
Scanning electron microscopy (SEM) and in-situ optical testing were used to characterize the dendrite inhibition effect of the modified separator more intuitively. Through SEM**, it can be found that the lithium metal anode corresponding to the PE separator has a large number of rod-shaped lithium dendrites and loose deposited layers. For the Z5P PE separator, there are still more dendrites on the surface of the lithium anode. This will lead to a decrease in the electrochemical performance of the battery, with more harmful side reactions and the formation of additional SEIs. However, the lithium metal surface corresponding to the Z5P PE SBF separator is relatively smooth and shiny, and there is almost no lithium dendrite. This means that the "tandem separator" significantly inhibits the formation of lithium dendrites. At the same time, the results of in-situ light microscopy further confirm the above conclusions.
Figure 4Morphological analysis of symmetrical cells with different separator assemblies.
According to the time-of-flight secondary ion mass spectrometry (TOF-SIMS) results, a large number of LIF were observed in the outer and inner regions of the corresponding SEI for the Z5P PE SBF diaphragm. On the contrary, the SEI corresponding to the Z5P PE separator has only a trace amount of LIF on the surface. And as the sputtering deepens, the amount of LIF decreases. X-ray photoelectron spectroscopy (XPS) depth profiles showed similar results, with Li3SB, SBOX, and LIF compounds observed in both the inner and outer regions of the SEI. The SEI composition of the Z5P PE SBF diaphragm remains basically unchanged.
In stark contrast, the LIF content in the SEI of Z5P PE-based cells gradually increases as the etch depth deepens. However, the LIF content was still much lower than that in Z5P PE SBF, indicating that there were more organic components. In conclusion, combining the results of XPS and TOF-SIMS, we can confirm that Z5P PE SBF-based cells have a considerable amount of Li3SB and LIF components both inside and outside the SEI. In addition, according to the density functional theory (DFT) calculations, the strong adsorption of Li3SB to Li+ preferentially enriches Li+ on its surface, and the low diffusion barrier enables Li+ to migrate rapidly. These results suggest that it is feasible to incorporate the Li3SB component into the Lif-rich SEI, which is expected to achieve enhanced Li+ transport and reversible insertion extraction behavior, thereby constructing a long-term stable lithium metal anode.
Figure 5Study on the interface properties of Z5P PE SBF separator.
In contrast, the NCM811|, assembled with PE and Z5P PE diaphragmsLi full battery. Its electrochemical performance in terms of discharge rate and cycling stability is significantly improved. When the positive electrode load is increased to 10 mg cm2, Z5P PE SBF-based batteries can still exhibit the highest capacity retention. In addition, the Z5P PE SBF separator can further improve the rate performance of the battery. This is mainly due to its excellent Sei rich in Li3SB and LIF, which is conducive to the rapid migration of Li+.
Thermal accumulation during high-rate LMBS runs is a key killer of electrochemical performance. Especially under the condition of high current density, the presence of Joule heating leads to a serious uneven temperature distribution of the material, resulting in the formation of dendrites. Therefore, it is valuable to develop a functional diaphragm with uniform heat distribution. NCM622|Pursy pouch battery, charged tested at high magnification and recorded by an infrared camera. The results show that the Z5P PE SBF separator can achieve a faster and uniform temperature distribution at high magnification, while the battery using the conventional PE separator shows a significant local heat increase. This is because the inorganic layer applied on both sides of the diaphragm facilitates heat transfer. Therefore, the uniform distribution of temperature guarantees the safety of the battery at high speeds.
Figure 6Full electrochemical performance test.
Summary] In summary, a design concept of "tandem" separator with asymmetrical function on both sides is proposed for dendritic lithium metal anode and high-rate stable LMBS. On the positive side, the Z5P cell can limit the transport of anions, homogenizing the Li+ flux. On the side facing the lithium anode, the SBF unit can react with the Li metal to form Li3SB and LIF to construct a stable inorganic rich SEI.
As a result, this tandem separator avoids the drawbacks caused by anion anchoring, which would otherwise not be able to participate in SEI construction as traditional designs do. Thanks to this design, the NCM811|LI and LFP|The LI whole cell exhibits excellent cycling stability and excellent rate capability. What's more interesting is that the Z5P PE SBF tandem separator can also evenly distribute the temperature, which significantly improves the safety of the battery when working at high rates. This work clarifies the concept of functionalized tandem separators and allows for a wide selection of functional materials. Therefore, this concept can be applied not only to the long-term stability and high rate capability of LMBS, but also to many other battery systems.
Literature details] Luoyi Ding, Xinyang Yue*, Yuanmao Chen, Zhiyong Wang, Jijiang Liu, Zhangqin Shi, Zheng Liang*, Tandem Design of Functional Separators for Li Metal Batteries with Long-Term Stability and high-rate capability. adv. funct. mater. 2023, dol:10.1002/adfm.202304386