Cs₄PbBr₆ Combined with Graphite as Anode for High-Performance Lithium Batteries

Abstract

Cs₄PbBr₆ quantum dots are glass-based materials. The perovskite structural material of Cs₄PbBr₆ quantum dots has shown an unexpected electronic performance. However, the glass-based Cs₄PbBr₆ quantum dots’ capacity becomes weaker when running in charge/discharge. Here, graphite was introduced to Cs₄PbBr₆ quantum dots using the grinding method to enhance the cycling stability of Cs₄PbBr₆ quantum dots. The 10%, 25%, 35%, 40%, 75% content Cs₄PbBr₆ quantum dots were added to graphite (CQDs/G) and CQDs/G as an active material for lithium anode in electronic testing. The test results displayed 35% Cs₄PbBr₆ quantum dots content in CQDs/G, showing an excellent cycle performance (136.5 mAh g⁻¹ after 1000 cycles at 0.5 A g⁻¹ current density) and good rate ability. Graphite protected the CQDs in the long term, and has high potential economic value.

1. Introduction

Advanced energy storage is an intrinsic driving force for daily life [1]. There is a large spectrum of storage technologies with wide variation in terms of energy and power density, service life, and cost. As cathode material, graphite have achieved many triumphs on batteries. Because of they can be utilized conveniently and low cost [2,3]. From the C/LiCoO₂ rocking-chair cell to the first commercialized battery [4], The graphite leads to Lithium-ion battery cathode material with 25 years’ favor for consumers, even in today’s vehicles [5]. However, although the carbon electrode partly guarantees safe operation, the theoretic capacity is only 372 mAhg⁻¹ [6]. Graphite cannot meet the ever-increasing demands of higher-density batteries [7]. Moreover, graphite glass materials have gradually been used in the battery as an anode due to their network microcrystalline structure [8]. This network microcrystalline structure can help to accommodate their volumetric expansion and maintain the anisotropic particle arrangement, with a better distribution during lithium insertion and extraction [9].

The glass powder matrix battery has been reported in the energy area. Haodong Liu et al. prepared disordered rock salt Li_3+xV₂O₅, which can be used as a fast-charging anode and can reversibly cycle two lithium ions at an average voltage of about 0.6 volts versus a Li/Li⁺ reference electrode [10]. The Li_3+xV₂O₅, when used as an anode material, exhibits a long cycle performance, and can grow in a direction that can build a stable structure for high current–discharge cycling. Glassy materials possess unique and favorable physical-chemical, mechanical and textural properties that allow for them to suppress huge volumetric changes during the charge–discharge processes, avoiding electrode pulverization, and maintaining good connectivity and electron mobility through the negative electrode. Cs₄PbBr₆ quantum dots (CQDs) have perovskite structure material and grown on a glassy based matrix [11]. This is similar to the degradation of antibiotics in the preparation of luminescent materials. However, this is rarely utilized in anode energy storage. CQDs are 3D structures born from glass-based materials and are used as one of the glass-based materials. CQDs as lithium anode also showed an extraordinary electronic performance. However, after 500 cycles it reduced to 40 mAhg⁻¹ [11]. Graphite has tremendously been used as an anode in lithium battery factories [12]. In the X-C dual-phase, free carbon can not only function as a structural buffering matrix to dilute the volume change, but also provides a conductive network to promote the electrode reaction [13]. Therefore, as a negative electrode material, it has the characteristics of high conductivity, a large lithium ion diffusion coefficient, high lithium intercalation capacity, and low lithium intercalation potential [14].

In recent research work, to suppress the change in alloy volume during charging and discharging [15], the overall electrochemical performance of the alloy is improved by the design of effective nanostructures and introduction of a conductive carbon host. The working group of Chen J prepared bismuth-intercalated graphite (Bi@Graphite) materials [16]. The excellent rate ability and cycling stability of the as-prepared Bi@Graphite material can be attributed to its unique sandwich structure, with Bi nanoparticles uniformly distributed among the conductive protective graphite. Improving the electrochemical performance of the anode material by compounding graphite provides a new direction for the exploration of electrode materials for next-generation rechargeable batteries.

Here, we report on CQDs mixed with graphite anode material by grinding methods. CQDs were prepared using a conventional melt-quenching (M-Q) technique. We put different CsPbBr₆/CsPbBr₃ quantum dot contents mixed with graphite. The graphite provides a layered structure to prevent CQDs from running away and, at the same time, the topography changed from abrupt to smooth during the cycling due to the presence of CQDs. As the content of CsPbBr6 quantum dots reached 35%, a stable cycling performance and rate abilities were achieved.

2. Experimental Section

2.1. Synthesis of CQDs and CQDs/G

The synthesis of CQDs was the same as in the article. The glasses were prepared using the conventional melt-quenching technique [17]. Reagent grades of GeO₂-B₂O₃-PbO-Cs₂CO₃-PbBr-NaBr powder were used as the starting materials. After mixing, the batches were placed into an alumina crucible and melted in an electric furnace at 800 °C for 30 min, and then the melt was poured onto a brass plate with 300 °C annealing. The CQDs/G materials were prepared using grind methods and 10%, 25%, 35%, 40%, and 75% CQDs/G were used as active materials (AC).

2.2. Electrochemical Evaluation

We used CQDs/G as active materials (AC). First, CQDs/G powder was mixed with Super P carbon and polyvinylidene fluoride (PVDF) at a mass ratio of 80:10:10 in NMP solvent. The resultant slurry was cast on a Cu foil, followed by solvent evaporation in a vacuum. The film was punched out to obtain a working electrode with a diameter of 16 mm and an active mass of 2.0 mg. The electrochemical analysis was carried out in 2025 coin-type cells, using Li foil as a counter-electrode, Celgard 2500 discs as the separator, and 1.0 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 volume ratio) as the electrolyte. The electrochemical performance was evaluated in the voltage range of 0.01–3 V (vs. Li/Li+) using a galvanostatic cycler (4300 K, Maccor, Oakhurst, OK, USA). Cyclic voltammetry (CV) was tested in the same voltage range at a scan rate of 0.2 mV s⁻¹.

3. Result and Analysis

The CQDs glass shown in Figure 1a–c shows the Cs₄PbBr₆ quantum dot. This shows a glass bulk. After the Cs₄PbBr₆ quantum dot glass-ceramic is ground into powder, its surface morphology has a dendritic distribution, and the particle size is about 8 um, which is in the range of quantum dots [18]. Figure 1b–d show the morphologies after 35 wt% CQDs/65 wt% G. The Cs₄PbBr₆ quantum dot is dispersed on the surface of graphite and is fluffy and porous. The distribution of the Cs₄PbBr₆ quantum dot was further analyzed through the element distribution diagram, and the results are shown in Figure 1e–g. A Cs₄PbBr₆ quantum dot glass–ceramic is uniformly dispersed on the surface of graphite.

When we make it for a lithium anode (60% active material, 30% acetylene black, 10% PVDF), as Figure 2c shows, the 35% CQDs/G exhibited a good electronic performance (158.6 mAh g⁻¹) compared with 10%, 25%, 40%, 75% CQDs/G capacity. The capacities of 10%, 25%, 40%, 75% CQDs were 65.4 mAhg⁻¹, 103.4 mAhg⁻¹, 103.2 mAhg⁻¹, 60.1 mAhg⁻¹, respectively. As Figure 2a,b show, 10%, 25%, 35%, 40%, 75% CQDs/G had the same CV curve and charge–discharge curve. This shows that the 10%, 25%, 35%, 40%, 75% CQDs/G have the same charge/discharge mechanisms: 10%, 25%, 40%, 75% CQDs/G.

Moreover, Figure 2a certifies the electronic reaction. First, Li-ion de-embeds in the Cs₄PbBr₆ structure as Pb²⁺ changes to Pb⁴⁺. Second, GeO₂ changes to Ge at 1.3 eV, which can be attributed to the alloying reaction. As Cs₄PbBr₆ content increases, the CV curve first expands before gradually shrinking after 35%. CQDs/G have a stable electronic performance. The CV curves has shown in Figure 2a, at 0.25 eV, the steps coincided with Figure 2b, showing the cathodic oxidation. At 0.47, 0.68 eV, the peaks represnets Li-Pb and Li-Ge reaction, while the peak at 1.0–1.3 eV shows Ge converting to GeO₂. At anodic curves at 1.5 V, GeO₂ turned to Ge, while the curves 0.46 and 0.26 V represented Pb-Li [19] and Ge-Li [20]. The formula is as follows:

$${\mathrm{Pb}}^{2+}+{\mathrm{xLi}}^{+}+{\mathrm{xe}}^{-}=\mathrm{LixPb}$$

(1)

$${\mathrm{GeO}}_2+4{\mathrm{LI}}^{+}+4{\mathrm{e}}^{-}=\mathrm{Ge}+2{\mathrm{Li}}_2\mathrm{O}$$

(2)

The cycle times were then increased to 1000 and taking the rate test at different current densities. As Figure 2a shows, after 1000 cycles, 35% CQDs/G remained with 158 mAh g⁻¹, and exhibited significant stability. The EC of CQDs/G is 82.4% was better than the other CQDs/G contents. We also tested the rate of 10%, 25%, 35%, 40%, 75% CQDs/G. As Figure 3b–f shows, at 0.05 Ag⁻¹, 0.1 Ag⁻¹, 0.25 Ag⁻¹, 0.5 Ag⁻¹, 1 Ag⁻¹, 0.5 Ag⁻¹, 0.25 Ag⁻¹, 0.1 Ag⁻¹, 0.05 Ag⁻¹. At 1 Ag⁻¹ current density, all of them has showed excellent rate capability. Obviously, the 35% CQDs/G capacity remains at 80 mAhg⁻¹ in Figure 3d, and exhibits brilliant rate stability at different rates. The other CQDs/G contents also showed a stable rate performance; at 75%, the capacity in the initial cycle quickly declines. When CQDs increases to a certain degree, the graphite needs time to combine with CQDs.

To further illustrate graphite’s impact on the overall material, we tested the SEM, IR, and XRD of 35% CQDs/G before and after 1000 cycles. As shown in Figure 4a, we grind CQDs with graphite and, after coating the CQDs/G on the rough surface of copper foil. And assembled as cions for the electronic test. From the Figure 4a, after cycling 1000ths the SEM of CQDs/G to become smoother, hinting that the CQDs helped the surface stress reduce and promoted the transportation of electrons. When the content of CQDs/G increased from 10% to 75%, Figure 4b shows that the IR of 10%, 35%, 75% CQDs/G the 1012 and 782 bring up new peaks. CQDs/G did not show a weak peak at 1502; The new peaks was Br-H, which may be part of the Br- run-off from Cs₄PbBr₆. From the XRD of 10%, 25%, 35%, 40%, 75% CQDs/G, we can see that, from 10% to 75%, the (100) peak become more obvious [11]. After 1000 cycles, both CQDs/G flattened out. The 35% CQDs/G mixed very well in terms of cycling, neglecting the Cu diffraction peak that corresponds to PDF# 97-004-3493. As the content of Cs₄PbBr₆ reaches 75%, the peak in GeO₂ (110) became more obvious. After 1000 cycles, the peak in GeO₂ (110) declined due to the transfer of GeO₂- LiGe (111) [20].

After 1000 cycles, the internal tends to smoothen, as shown in Figure 4a. The SEM of the electronic disk, which showed a smooth, raised surface was stable for long-term cycling. The cathodic from 0.2 V corresponded to Li-Pb [21]. The charge–discharge from 1st to 1000th became narrow and from Figure 4c after 1000 times cycles the peaks of the Cs₄PbBr₆ has diappear. At the same time the capacity of cathode has droped,. it influenced by the Cs₄PbBr₆ structure and during the cycle the Cs₄PbBr₆ structure leads to cracks.

As the EIS of 10%, 25%, 35%, 40%, 75% CQDs/G after cycles hows in Figure 5, the semicircle prioritize smaller and then larger. The low-frequency region slope is the same as the high-frequency slope. From EIS, we can see that, when the content of up to a 35% semicircle turned smaller, as well as from 35% to 75% the semicircle gradually increased. This is related to the SEI of Cs₄PbBr₆ [22], when Cs₄PbBr₆ reaches a certain value, it slowly starts to accumulate on the surface. We also conducted a resistivity test at 10%, 25%, 35%, 40%, 75%, which can be find in

Table 1. Resistivity of 10%, 25%, 35%, 40%, 75% CQDs/G before and after cycle.

	10%	25%	35%	40%	75%
Before ρ (mΩ/cm2)	101.3	125.8	93.1	138.8	158.8
After ρ (mΩ/cm2)	120.7	138.2	113.5	151.5	167.2

. Table 1 shows the resistivity reached 101.3 Ω/cm², 125.8 Ω/cm², 93.1 Ω/cm², 138.8 Ω/cm², 158.8 Ω/cm². After 1000 cycles, the resistivity reached up to 120.7 Ω/cm², 138.2 Ω/cm², 113.5 Ω/cm², 151.5 Ω/cm², 167.2 Ω/cm². It because of SEI film formation leads to this phenomenon [23].

4. Conclusions

Cs₄PbBr₆ as a perovskite structure, has unstable statements. It can easily be transferred into CsPbBr₃. We prepared the Cs₄PbBr₆ quantum dots using melting methods and mixed them with graphite, showing an excellent cycle performance. Graphite can protect Cs₄PbBr₆ during the charge–discharge programs. When used as quantum dots, Cs₄PbBr₆ can reduce surface stress and make the surface smoother. This is good for electronic movements. Ge and Pb, as the main form of storage in glassy-based coins, have a stable environment, which could cycle 1000 s at 0.5 Ag⁻¹ current density. To enhance the perovskite stable ability, doping homogeneous elements could be used to replace the position of lead-ions in the lattice, enhance its stability or increase the content of the glass foundation to prevent the lattice from collapsing during the charge–discharge process. This still needs further research.