The diaphragm warehouse provides a steady stream of LiNO3 for stable lithium metal batteries

The lithium ions of the zero-excess lithium-metal battery (ZELMB) are initially stored in the positive active material (CAM), and there is no excess lithium metal on the negative electrode, so the CE requirements for the battery are high.

Figure 1 SEM plots of the original A) CG 2500 and B) FS 2190 diaphragms. CEM plots of CG 2500 and B) FS 2190 septum after adding 120 L of LiNO3 and drying.

The effect of LiNO3 as an electrolyte additive on the performance of lithium metal batteries (LMBs) and the mechanism by which it builds a protective layer on the surface of lithium metal anodes (LMAs) have been well studied. Although Lino3 is able to decompose on the surface of lithium metal and further form a protective layer composed of inorganic organic components, it is considered to be one because it reacts with deposited lithium metal in each cycle"Sacrificial electrolyte additives"。This means that as long as the salt Lino3 is present in the electrolyte, the beneficial effects will be maintained, and once depleted, the electrochemical performance will deteriorate. In this study, an improved LiNO3-containing separator was used to investigate how LiNO3 affects the electrochemical performance of ZELMB in carbonate electrolytes, with the expectation that the additive will be released slowly over a longer period of time, thereby prolonging its beneficial effects over multiple cycles. Figure 1 shows the SEM diagram of the improved anterior and posterior diaphragm. By comparing the original sem diagram of the diaphragm (Fig. 1A, B) with EDX, it was confirmed that Lino3 particles were present on the diaphragm (Fig. 1c, D) and that there were no other changes in the morphology of the diaphragm.

Figure 2 a) cell voltage and b) CE for unmodified and modified separators. c) None and d) voltage curves of cells with Lino3 modified separators on selected cycles.

By applying this modified diaphragm to Cu||In the case of LI battery, the effect of LiNO3 modification on electrical properties was studied, and the overpotential of electrodeposition dissolution of CE and lithium and its cycle life were evaluated. Compared to batteries with no modified separator, the modified separator makes Cu ||The cycle life of the LI battery increased by more than 400 hours (Figure 2A). cu ||The CE value of the LI cell (Figure 2B) provides a visual description of the irreversible reaction. The first charge and discharge CE value of the improved separator battery reached 797%, which is more than 69 of the original separator battery0%。The efficiency of the improved separator cell reached 95 after a long cycle0%。The performance of the battery without the improved separator began to deteriorate after 130 hours of cycling, and after 50 cycles before the battery failed, the CE value was 569% (Figure 2b). Figure 2c,d illustrates the evolution of the dissolution overpotential of lithium battery deposition in the first cycle. The overpotential of the battery of the original separator continued to rise, and the overpotential of the battery of the Lino3 modified separator was relatively low and did not fluctuate significantly. These experimental data show that LiNO3 in the electrolyte contributes to the efficient formation of the SEI layer.

Fig. 3 Top view (a, b) of lithium deposits after the first electrodeposition and SEM images (c, d) of the cross-sectional morphology of lithium deposits after 20 cycles

The Cu electrodes after cycles 1 and 20 were collected and analyzed using SEM and cryogenic FIB-SEM (Figure 3). The first electrodeposition of lithium from a battery using an unmodified separator exhibits an inhomogeneous morphology (Figure 3A), which is consistent with the observed low CE values. Cells using a modified separator produced smooth, dendride-free lithium (Figure 3b). Reducing the surface area of lithium deposits to form low surface area lithium (LSAL) can provide better safety, higher CE values, and longer cycle life. After 20 cycles, lithium deposited with unmodified separator cells showed high surface area licheny and loosely packed lithium deposits, while cells with modified separators formed tight lithium deposits. These results show that the nature of SEI and the lithium speciation are critical for the high CE of the lithium electrodeposition dissolution process, and CE is a key parameter to characterize the performance of each ZELMB.

Figure 4 a) at 0v e 2CV in the potential range of 5V. b) Enlarged plot of the potential of LiNO3 reduction. c) In OCV u -0CV in the voltage range of 5 V. d) Symmetrical Lithium||The Tafel diagram of a lithium battery by LSV, and the calculated corresponding exchange current density.

In order to study the electrochemical stability of LiNO3, a three-electrode Cu ||, with Li metal as the reference electrodeLI battery at 2CV tests were performed in the potential range of 5-0 V. In this potential range, LiNO3 can undergo reductive decomposition without parallel lithium metal electrodeposition (Figure 4a,b). The CV curve of the modified separator battery ** is now one from 18 V to the 15 V reaches the maximum current reduction peak. The battery with an unmodified separator is only in about 1A small reduction peak at 25 V may be due to the reduction of carbonate-based solvents, the formation of SEI, and side reactions on the copper surface. Constant current tests on cells containing separators without LiNO3 show the reduction reaction during the first electrodeposition process on a copper collector. In the first 15 seconds, both batteries showed a similar voltage drop. However, a battery with a modified separator of Lino3 is at about 18V shows a small plateau, which is caused by the lino3 restoration. This result is consistent with the CV measurements in Figure 4A,B. When the potential expands to -0At 5 V, the current peak of the lithium battery deposition dissolution reaction in the modified separator battery was sharper (Figure 4c). The modified separator battery undergoes more charge during the oxidation and reduction process, and the CE is higher, indicating that its initial capacity is higher. The higher oxidation current in the CV when LiNO3 is present in the electrolyte solution also means that the lithium deposition dissolution process is more dynamical. This is further confirmed by the exchange current density (I0) calculated from the LSV measurement (Figure 4D).

Fig. 5 A) LP57 with LiNO3 saturated and Cu || with modified diaphragmComparison of nitrate concentrations after battery cycling. b) Measure the relative atomic concentration of the deposited layer on the copper electrode after 20 cycles by EDX. c) Tof-SIMS spectra of the deposited layer on the copper electrode after 20 cycles.

Based on the above results, the authors concluded that modifying the separator with LiNO3 could maintain a constant concentration of LiNO3 in the electrolyte, which could be close to the saturation level. Further analytical studies were carried out to confirm the proposed mechanism of Lino3 dissolving into the electrolyte and its role as a sacrificial additive. Ion chromatography with conductivity detection was used to determine the nitrate concentration in the electrolyte after different cycles. The concentration of Lino3 in the saturated LiNO3 electrolyte solution gradually decreases with the number of cycles (Fig. 5a black line), suggesting that Lino3 is a sacrificial additive in lithium metal batteries. However, the concentration of LiNO3 in the electrolyte in the battery with modified separator is basically stable during cycling. This means that the LiNO3 stored in the diaphragm is continuously dissolved into the electrolyte. Therefore, the beneficial effects of the SEI protective layer on LMA can be maintained for a long time. As a result, the LiNO3-rich diaphragm acts like a reservoir that releases Lino3 steadily. During cycling, LiNO3 forms different decomposition products, resulting in an increase in the nitrogen content in the SEI, resulting in different chemical properties on the LMA surface. The elemental composition of the copper electrode surface after 20 cycles was investigated by EDX (Figure 5B), including whether the separator was Lino3 modified (Figure 5C). Cells with LiNO3-modified separators form nitrogenous compounds on the electrode surface. The chemical composition of the nitrogen compounds deposited on the copper electrode after 20 cycles was analyzed using TOF-SIMS (Figure 5C). The reduced nitrate species was detected in the cells with the modified separator, a result that supports the idea that nitrate is reduced on the electrode surface. Li3N produced by nitrate reduction exhibits high Li+ conductivity, which helps to reduce interfacial phase-to-phase resistance in cycling and accelerates the rate of Li+ migration through SEI.

Fig.6 NCM622 ||a) cycling performance and b) coulombic efficiency of Cu cells.

Three different types of NCM622 ||are usedCu cells, to study the effect of LiNO3 on the ZELMB cycle (Figure 6). All electroactive lithium in ZELMB is derived exclusively from CAM. This means that any non-reversible side reactions will deplete these electrically active lithium, resulting in a rapid degradation of the battery's capacity. When the cell contains only a carbonate-based electrolyte (LP57), 98% of the capacity has been lost after ten cycles (Figure 6A). With the addition of LiNO3, the rate of capacity decay slows down significantly. In batteries with improved separators, the tendency for capacity to decay is slower. Batteries with modified separators have higher CE fluids (Figure 6B).

Fig.7. Schematic diagram of the lithium deposition dissolution process. a) in LiNO3-free LP57 conventional carbonate electrolyte;b) Lino3 is added to the diaphragm to keep the concentration of Lino3 in the electrolyte constant.

The addition of LiNO3 to the separator can maintain a constant concentration of LiNO3 in the electrolyte, thereby improving the SEI of lithium deposits and reducing the formation of dead lithium and high surface area lithium (HSAL) (Figure 7). The absence of LiNO3 in the electrolyte can lead to premature failure of the ZELMB (Figure 7A). The separator continuously supplies LiNO3 to the electrolyte, which maintains a stable SEI at the negative electrode, which is beneficial for maintaining stable battery capacity, optimizing CE, and generating dense lithium deposits (see Figure 7B).

In this study, we explored the use of CU || in ZELMB with LP5LI and NCM622 ||LI battery, in the case of a commercial separator modified with LINO3. The improved diaphragm shows better performance. In cu ||In LI batteries, the improved separator enhances cycle life, reduces overpotential, and increases CE values. Lithium deposition in batteries without modified separators appears as mossy and porous. However, the lithium deposits of batteries with modified separators are dense and orderly. at NCM622 ||In the Cu battery test, the cycle life and capacity retention rate of the battery were improved by adding LiNO3 to the separator. Based on the findings of this study, we summarized the following ideas: a) LiNO3 in the electrolyte plays a positive role in the morphology of lithium deposition, which in turn contributes to the improvement of CE;b) The addition of LiNO3 to the diaphragm instead of the carbonate electrolyte can prolong the persistence of LiNO3 in the electrolyte;c) Lino3 in NMC ||The presence in CU batteries confirms its important role in the ZELMB cycling performance of liquid electrolytes.