Micro-stress pump with stress variation to boost ion transport for high-performance sodium-ion batteries

Abstract

Sluggish kinetics limit the practical application of sodium-ion batteries (SIBs); thus, innovative strategies and the design of materials with fast reaction kinetics are important for the development of SIBs. To solve these issues, the innovative strategy of using a micro-stress pump to boost Na⁺ transport by simulating rhythmic cardiac blood pumping has proposed for the first time. A smart material with cardiac-like behavior promotes the electrochemical kinetics through the self-regulation of stress under the variation of voltage in the redox reaction. Under the micro-stress field, a half-cell demonstrates a capacity of 119.1 mA h g⁻¹ at 35 A g⁻¹, and a 1.04 A h pouch cell shows an excellent energy density of 317.2 W h kg⁻¹ (the retention is 90.2% after 500 cycles at 1C). Via further analysis of physicochemical characterizations and the sensor signal, the signal correlation of the mechanism between electrochemistry and stress was obtained. This work provides a strategy for accelerating the transmission rate of Na⁺ based on a stress field.

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Broader context

Sodium-ion batteries (SIBs), which are based on abundant resources and have low cost, are likely to become the first choice for large-scale energy storage. However, the sluggish Na⁺ kinetics restrain the rate of ion transport, limiting the practical applications of SIBs. Thus, a rational strategy is needed to enhance the kinetics of Na⁺ at this stage. Here, we simulate the rhythmic cardiac blood pumping and propose a strategy to boost Na⁺ transport using a micro-stress pump for the first time. The carbon-coated liquid metal (LM) has a smart response with the variation of voltage to realize a reversible micro-stress driver. The micro-stress pump achieves the reversible deformation of the materials, which promotes the kinetics of Na⁺ by through the micro-stress driver to enhance the rate of ion transport. The mechanism relating the stress field and electrochemical field is analysed using in situ measurement and optical fiber sensors, and the micro-stress pump can be applied in high-rate batteries. This work provides a new strategy for rapid kinetics of ion transport using a stress field.

Introduction

Sodium-ion batteries (SIBs) have shown great possibilities in the energy storage applications on account of their low cost.^1–3 However, SIBs present some challenges, such as low capacity, slow dynamics and rapid capacity fading. These problems are usually caused by Na⁺, which has a large volume, moves slowly and destroys the structure in the electrochemical reactions.^4,5 Therefore, a novel materials design strategy is required to realize fast kinetics and compensate for the low energy efficiencies when the batteries are discharged and charged at a high current.

As is well known, the electrode–electrolyte interfacial chemistry and the body structure of the anode determine the process of ions moving from the electrolyte to the anode.^6,7 In attempting to accelerate the rate of the anode, a large number of research works have focused on ion solvation/desolvation and the micro-nano-structure with a short ion transport distance. For instance, a step-by-step desolvation strategy has been proposed to reduce the energy of the desolvation process.^8–10 Additionally, the single-layer-particle vertical electrode has been used to minimize the ion transfer distance and realize fast-charging capability.^11,12 Moreover, a suitable electrode structure with multichannel and electrolyte collocation can avoid the slow desolvation process to achieve good kinetics.¹³ Although electrode–electrolyte interfacial chemistry and the body structure of the anode have been consistently optimized, it is difficult to develop a matching electrode and electrolyte through the conventional methods. A fresh perspective and innovative approaches that are not limited to the electrode and electrolyte are needed to realize rapid charge/discharge at this stage.^14–16

Liquid metals (LMs) are recognized as smart materials with an innate large capacity that can respond to external stimuli and possess liquid deformability.^17–20 LMs have low chemical potential, high theoretical specific capacity and excellent electrical conductivity.²¹ On the other hand, liquid fluidity can reduce the electrode/electrolyte interface impedance and provide superior dynamics for Na⁺ transport.^22,23 Thus, LMs show great potential in terms of improving the transmission rate of Na⁺. Besides, the surface tension of the LMs can be regulated by applying a voltage, which can correspond to different stages of redox reactions in the batteries.^24–26 However, LMs have been used to improve the electrochemical transport kinetics in various studies without explaining the impact of the smart response on the metal ion transport kinetics. A serious problem in previous works is that the changes in the physical fields of the LMs caused by the smart response (including electrical field, force field and chemicals, etc.) in the electrochemical reaction has been neglected. Meanwhile, the dynamic reaction mechanism of Na⁺ transport coupled with multiple physical fields (electricity, magnetism, force, heat, light, etc.) in the anodes has not been reported. Although the excellent kinetics of LMs have attracted notice, the beneficial influences of stress fields have not been investigated. Thus, the Na⁺ transport in batteries with stress fields needs to be comprehensively demonstrated.

In this work, we define the useful behavior of inward stress in LMs to enhance the rate of Na⁺ diffusion (Fig. 1). The rate of Na⁺ insertion is accelerated by increased inward stress during the reduction process. Besides, the rate of Na⁺ extraction is accelerated by decreased inward stress during the oxidation process. The improvement of ion transport kinetics is realized through the change in inward stress throughout the various stages of the redox reaction. Meanwhile, we propose the use of carbon-coated GaInSn liquid metal nanoparticles (LMNCs) derived from the pyrolysis of an in situ polymerized GaInSn-based polymer to investigate the influence of stress on the electrochemical behavior. LMNCs with a carbon layer can encapsulate GaInSn nanoparticles and suppress particle agglomeration, which is beneficial to the ion transport rate under the influence of a stress field. To the best of our knowledge, there has been no previous work exploring the mechanism of Na⁺ transport under a stress field in the SIBs. We monitored the change in the stress field in the self-varying stress materials using in situ optical fiber sensors and found a close connection between the stress and the electrochemical behavior. The discovery of a new multi-field mechanism is beneficial to the design of high-rate anode materials. The structure shows excellent capacity at 35 A g⁻¹ and was successfully applied in a pouch cell. The results provide a new reference for the design and testing of SIBs driven by micro-stress.

Fig. 1 Mechanism of micro-stress pump to promote Na⁺ transport in the half-cell, full-cell and pouch cell.

Results and discussion

Rational design and properties of the electrode

To enhance the rate of Na⁺ transport by means of a stress field and to investigate the behavior of Na⁺ transport in the stress field, an LM electrode with a carbon shell and carbon spheres (NCs) was designed and synthesized (Fig. 2a). Fig. 2b illustrates how the stress changes of the LM correspond to different stages of redox reactions. Meanwhile, the increase of the stress field can promote the Na⁺ kinetics. In this strategy, the dissociated Na⁺ moves efficiently in the LM with the help of stress changes. Specifically, the rate of Na⁺ intercalation will be increased when the inward stress rises in the reduction stage. In contrast, the rate of Na⁺ de-intercalation will be increased when the inward stress falls in the oxidation stage. Fig. 2c shows the XRD patterns of the GaInSn, NCs and LMNCs. As shown in the TEM image (Fig. S1, ESI†), we can observe the morphology of LMNCs, LM and NCs. In Fig. S2 (ESI†), carbon layer prevents the aggregation of the liquid metal. To analyze the effect of LM on the carbon structures, the high-magnification TEM images and Raman spectra of NCs and LMNCs were measured (Fig. 2d and Fig. S3, ESI†), and it is clear that their ID/IG ratios are different, which indicates that LM induces the conversion of graphitic structures and widens the graphite distance; a more long-range ordered graphitic carbon layer provides active sites for Na⁺ storage.^27,28 The nitrogen adsorption/desorption isotherms demonstrate that the carbon-coated GaInSn nanostructure possesses a large surface area (111.7 m² g⁻¹) and an average pore diameter of 2.8 nm (Fig. 2e). Compared with the carbon nanostructure (surface area of 46.5 m² g⁻¹ and average pore diameter of 3.7 nm), the introduction of LM generates more pore structures to realize the deep penetration between the electrode and electrolyte.^29,30 Besides, a wider range of pore distribution was formed under the action of LM (Fig. S4, ESI†). As shown in Fig. 2f, the samples are in the stable stage before 250 °C and show certain decomposition from 250–600 °C. With a small amount of GaInSn, the LMNCs have the most residue among the samples. Based on the differential scanning calorimetry testing, the phase change behavior of the LMNCs and NCs are similar, and there is no obvious difference in the temperature of the phase shift (Fig. S5, ESI†).

Fig. 2 Synthesis principles and the mechanism by which the stress-electrochemical field promotes Na⁺ transport. (a) Schematic of the carbon-coated GaInSn liquid metal nanoparticles. (b) Mechanism by which the stress-electrochemical field improves the transmission rate of Na⁺. (c) X-ray diffraction (XRD) patterns of materials. (d) Raman spectra of different materials. (e) Characterization of the pore structure in the materials. (f) Thermal gravimetric analysis (TGA) of materials. (g) and (h) X-ray photoelectron spectroscopy of C 1s, Ga 2p, In 3d and Sn 3d.

We focused particularly on the C–C (284.8 eV) and C–O (286.4 eV) peaks in the C 1s spectra (Fig. 2g), as a high C content is important to ensure that GaInSn is prevented from agglomerating by the carbon layer. Ga and Ga₂O₃ peaks occurred at 1144.6 eV (1117.7 eV) and 1145.9 eV (1119.1 eV) in the Ga 2p spectra; the results demonstrate that Ga³⁺ interacts with –O groups and forms a small amount of an oxide layer.³¹ Besides, the binding energies of In 3d were 452.6 eV and 445.4 eV, corresponding to the In₂O₃ and In, respectively.³² Similarly, there were Sn 3d peaks at 495.45 eV (487.8 eV) and 492.45 eV (485.2 eV), corresponding to the SnO and Sn, respectively.²⁰ These results are in agreement with the formation of an oxide layer on the LM interface (Fig. 2h).

Deformation and stress measurement of materials under the electrochemical field

An optical fiber sensor was used to test the shape change and stress change of the LM in the primary battery model (Fig. 3a).^33,34 We attached a fiber Bragg grating (FBG) on the LM surface and applied a voltage to simulate the redox reaction environment. Fig. S6 (ESI†) presents the images of the FBG from the side view and top view. The FBG not only responds to stress, but also to temperature. Using constant temperature test conditions, the effect of temperature can be excluded. Therefore, the stress can be obtained from formula (1).where Δε represents the change in the microstrain, Δλ refers to the change in the reflection wavelength and k_ε refers to the coefficient related to the optical fiber, whose value is 1.2 pm με⁻¹. After formulaic signal processing, the trend in the variation of the microstrain can be calculated (Fig. 3b). During the charging process, the oxidation behavior becomes increasingly obvious, which reduces the surface tension and causes the generation of strain. On the contrary, the reduction behavior boosts the surface tension and promotes the deformation retraction during the discharging process.

σ = E × Δε (2)

Fig. 3 Measurement of stress-electrochemical field in the electrode materials. (a) Schematic of the stress-electrochemical measurement in the LM using a fiber Bragg grating (FBG) under the model of primary battery. (b) and (c) Diagram of the stress response in the LM observed using the sensor. (d)–(g) Atomic force microscopy (AFM) of the LMNCs of the pouch cell at different voltages. (h)–(k) Force–displacement curve of the LMNCs at different voltages.

The change in the strain can be converted to the stress according to formula (2), in which σ refers to the stress of the materials and E refers to the elastic modulus of the LM, which is about 10 Pa. The trend in the stress matches with that for the strain, and the redox reaction drives the change in the stress (Fig. 3c). These results demonstrate that the redox reaction can induce the deformation and stress of the LM in the battery.

Using electrochemical Kelvin probe force microscopy (KPFM), it was demonstrated that different voltages could induce morphological changes in the LM. Interestingly, expansion and contraction of the shape can be observed during the charge process at 0.3 V, 0.6 V, 0.9 V and 1.2 V (Fig. 3d–g).³⁵

The self-stress at the different voltages shows the most obvious the shape changes at 1.2 V, 2.0 V and 2.5 V (Fig. S7, ESI†). Therefore, the electrode materials can influence the Na⁺ diffusion through self-adapting structure regulation in the charging and discharging processes. Considering that the interface of the liquid materials is flowing, the stability of it is needed to be investigated. Therefore, the phase state of the sample can be determined by the electrochemical “force–displacement” curve (F–D curve) of AFM measurement (Fig. 3h–k). According to the F–D curve, the tip of AFM goes through five processes: (i) approach, (ii) contact, (iii) infiltration, (iv) retraction and (v) separation. First, the tip gradually approaches the electrode materials until the tip is close enough to create an attractive force in step (i). Secondly, the repulsive force increases linearly when the tip attaches to the LMNCs and infiltrates them continuously. Finally, the attractive force of the LMNCs increases during step (iii) until AFM tip retracts from the LMNCs in step (iv).³⁶ In addition, step (v) indicates the appearance of a tiny attractive force and tiny viscosity during the departure of the tip from the LMNCs. These results indicate the liquid feature and contact state of the electrode materials.³⁷ Based on this test model, the F–D curve shows that the viscosity length of the LMNCs increases as the voltage increases from 0.3 V to 1.2 V, indicating that oxidation induces a decrease in the surface stiffness of the LMNCs. As the oxidation of the electrode materials continues, the surface stiffness becomes smaller, accompanied by a decline in the stress. Upon contraction, the stress increases with the decline of the surface stiffness of the liquid materials in the reduction process. The F–D curve suggests that the stress of the materials correlates with the redox reaction. Moreover, the change in the nanoscale dynamic morphology provides highly active sites and good conductivity for Na⁺ storage. On this basis, the liquid state of materials has been maintained in a stable state with equally distributed Ga, In and Sn, which demonstrates the high stability. In conclusion, the electrode material exhibits a dynamic change in morphology and stress with voltage variation in the redox reaction.

Electrochemical performance and kinetic analysis of half-cell

The dynamic heterogeneous structure possesses unique open active sites, and the stress induces more diffusive pathways for Na⁺ transport and charge transfer, making this a promising smart dynamic structure for rapid Na⁺ storage. The cyclic voltammetry (CV) plot displays the initial three cycles of the half-cell from 0.01–3.0 V at 0.2 mV s⁻¹. The formation of a solid electrolyte interface (SEI) causes the irreversible capacity of the first cycle, and two pairs of redox peaks appear at 0.01–0.25 V (Fig. 4a).³⁸ From the galvanostatic curves of the LMNCs at 0.05 A g⁻¹, the half-cell shows initial discharge/charge capacities of 585.6/364.3 mA h g⁻¹, respectively (Fig. 4b). The LMNCs also deliver superior rate capacities of 348.7, 321.6, 289.1, 237.6, 204.3, 184.5, 166.2, 148.2, 132.7, and 119.1 mA h g⁻¹ at 0.5, 1, 4, 8, 12, 15, 20, 25, 30, and 35 A g⁻¹, respectively (Fig. 4c). The rate capacities of state-of-the-art anodes with high rates are listed in Fig. 4d for comparison.^38–44 The comparison indicates that the electrochemical performance is better than many of the works that have been reported. The cycling performance of the LMNCs is presented in Fig. 4e. After 1800 cycles, the LMNCs present a good capacity of 125.6 mA h g⁻¹ at 10 A g⁻¹.

Fig. 4 Electrochemical measurement of the half-cell with a stress-electrochemical field. (a) CV curves of the LMNCs at 0.2 mV s⁻¹. (b) First/second/third cycle of the discharging/charging curves of the LMNCs at 50 mA g⁻¹. (c) Electrochemical rate performance of the LMNCs. (d) Comparison of the electrochemical rate performance reported previously. (e) Cycling performance of the LMNCs, NCs and LM at 10 A g⁻¹. (f) Different proportions of capacity contribution in the materials. (g) Cycling performances of the LMNCs, NCs and LM at 15 A g⁻¹.

The excellent electrochemical performance is inseparable from the capacitive contribution, and the diffusion-controlled/capacitive contribution ratio can be observed in Fig. 4f. The ratio of the capacitive contribution of the LMNCs is 73.9% at 0.2 mV s⁻¹. At 15 A g⁻¹, the LMNCs also present an excellent capacity of 100 mA h g⁻¹ after 1000 cycles. Moreover, the most abundant elements in the LM are Ga and In. The Ga 2p and In 3d spectra of the LM anode demonstrate that these elements exist stably in the anode materials after cycling (Fig. S8 and S9, ESI†). The Ga and In of the LMNCs were also present in the anode after 1500 cycles, showing the stability of the elements during cycling (Fig. S10, ESI†).⁴⁵ The variation in the thickness of the LMNC electrode sheet is smaller than that in the LM electrode sheet (Fig. S11 and S12, ESI†). The alloying reaction emerges in the redox reaction process. We can observe Na⁺ transport into the core of the LMNCs, forming the Na-based alloy and the liquid-core structure. The flocculent composition increased, and a progressive color difference is observed inside the LMNC particles. The color difference disappears with the continuous extraction of Na⁺, demonstrating a stable reversible process (Fig. S13 and S14, ESI†).

The CV plot of the LMNCs in the half-cell exhibits three redox couples, corresponding to the occurrence of redox reactions on the carbon layer (0.01/0.08, 0.73/1.52 V vs. Na/Na⁺) and LM materials (0.02/0.2 V vs. Na/Na⁺), as shown in Fig. 5a. Each redox peak of the LMNCs can be attributed to the peaks in the NCs and LM (Fig. S15a and S16a, ESI†). To understand the differences in rate performance among the materials based on the redox potential, the CV peaks of the anode and cathode were employed to analyze the kinetics of Na⁺. As shown in Fig. S15b and c (ESI†) and Fig. S16b (ESI†), the slope of redox peak in the LM is larger than for the carbon material, which means that the capacitive contribution represents the main part of Na⁺ storage, accounting for the superior Na⁺ transport in the LM under the stress. The b value corresponds to the influence of stress on the LM at the different voltages in the previous structural characterization; the materials with liquidity exhibit a variable specific surface area and more active sites, indicating faster kinetics and an extremely high capacitive contribution.^46,47 Besides, the carbon materials with no morphological change show lower capacitive Na⁺ storage compared with the LM. Moreover, the LMNC shows the fluidity characteristics under stress, indicating a superior Na⁺ diffusion rate at the redox potential (Fig. 5b).^48,49 As shown in Fig. 5c, the introduction of LM obviously enhanced the b value. The effect of stress on the LMNCs is obvious within the range of 1.5–2.5 V, and the electrochemical fitting results are consistent with the previous AFM characterization.

Fig. 5 Electrochemical kinetic analysis of the half-cell with a stress-electrochemical field. (a) CV curves of LMNCs between 0.01 V and 3.0 V vs. Na/Na⁺. (b) Value of b in the LMNCs at the redox peak. (c) Value of b in the LMNCs at different potentials vs. Na/Na⁺. In situ EIS measurement of (d) LMNCs and (e) LM. (f) and (g) GITT curves of LMNCs. (h) and (i) Warburg impedance coefficient (σ) of LMNCs and LM in the discharging/charging process.

To study the effect of material deformation on the charge transfer in the redox reaction, in situ EIS measurements were performed as shown in Fig. 5f (Fig. S17, ESI†). As the potential decreases, interface the charge-transfer resistance (R_ct) gradually decreases in the discharging process. During the reduction process, the oxide film protective layer of the LMNCs is reduced, inducing more alloying reactions to improve the kinetics of charge transfer with the decaying R_ct (value decreasing from 35.25 to 18.75) from 0.6 V to 0.01 V (Table S1, ESI†). Furthermore, the weakening inward stress prompts the core of LMNCs to spread out to form more oxide film protective layer during the oxidation during charging process, resulting in reduced alloying electrochemical reaction and increasing R_ct (value increasing from 22.05 to 58.29) within the range of 0.01 V to 1.2 V (Table S2, ESI†). On the whole, the role of stress in the core of LMNCs improves the kinetics. Besides, EIS measurements of fresh LM also exhibit similar results to LMNCs, suggesting the promoting effect of stress on the kinetics. To explore the kinetics of Na⁺ transport, the galvanostatic intermittent titration technique (GITT) was conducted. We can observe that the trend in the stress corresponds with the Na⁺ diffusion coefficient (Fig. S18, ESI†). Increasing the inward stress can accelerate Na⁺ intercalation, and decreasing inward stress can accelerate Na⁺ de-intercalation (Fig. S19, ESI†). The D_Na⁺ of the LMNCs of charging exhibits a slight fluctuation within the low potential range of 0.01–1.3 V and an obvious increase in D_Na⁺ at 1.3–3.0 V (Fig. 5d and e). Besides, the D_Na⁺ continues to rise during the discharging process from 3.0 V to 0.01 V. The kinetic analysis indicates that the increasing inward stress improves the rate of Na⁺ intercalation during the reduction process. On the contrary, the decreasing inward stress promotes Na⁺ de-intercalation. These results demonstrate that inward stress promotes the ion dynamics at different redox stages. To further explore the tiny fluctuations in the kinetics at low potential, the Warburg impedance coefficient (σ) was used to reflect the ion diffusion rate.^36,50 We can observe that a negligible diffusion kinetics decline exists within the low potential range of 0.01–1.3 V. The rate of diffusion is overall increasing during subsequent charging from 1.3 V to 3.0 V. Besides, the Na⁺ diffusion rate shows an overall upward trend without the tiny downward fluctuations in the discharging process at low potential. The results verify that the emergence of increasing inward stress promotes Na⁺ transport in same direction and decreasing inward stress can improve transport in the opposite direction.

The full-cell devices were assembled with a NaFe_1/3Ni_1/3Mn_1/3O₂ cathode and LMNC anode, and the possibility of practical application was examined through the performance of the full-cell devices. In order to explore the durability for the application, the full-cell is denoted as LMNCs‖NaFe_1/3Ni_1/3Mn_1/3O₂, and the electrochemical performance of half-cells are presented in Fig. 6a and b. Besides, the charging/discharging curves of the full-cell are clear (Fig. S20, ESI†). In the voltage window from 2.0 V to 4.0 V, the full-cell shows an excellent capacity of 69.23 mA h g⁻¹ after 200 cycles at 8C with a retention of 92.1%. Moreover, the cells can deliver a capacity of 60.91 mA h g⁻¹ and an energy density of 158.3 W h kg⁻¹ after 200 cycles at 10C with a retention of 85.9%.

Fig. 6 Electrochemical performance and kinetic analysis of the full-cell with a stress-electrochemical field. (a) and (b) Cycling performance of LMNCs‖NaFe_1/3Ni_1/3Mn_1/3O₂ full-cell at 8C and 10C. (c) In situ EIS measurement of LMNCs‖NaFe_1/3Ni_1/3Mn_1/3O₂ full cell. (d) and (e) σ value of the LMNCs‖NaFe_1/3Ni_1/3Mn_1/3O₂ full-cell in the discharging/charging process.

The kinetics analysis can be seen from the in situ EIS plot (Fig. 6c and Table S3, ESI†); the trends in R_ct reflect the beneficial smart response of the stress field in the electrochemical kinetics analysis of the full-cells, which is different from that of the half-cells. As the working voltage decreases, the R_ct of the full-cell increases gradually throughout the whole discharging process. The oxidation reaction induces the emergence of an oxide layer in the anode and a continuous decrease in the inward stress of core in the LMNCs. The reduced inward stress induces the generation of a larger surface area protected by an oxide layer, resulting in the absence of alloying reactions and a slow rate of charge transfer in the oxidation. As the working voltage increases, the role of reduction becomes dominant, and the inward stress is enhanced. The inward stress exhibits an enhancement trend throughout the whole charging process. Along with this trend, the resistance to charge transfer decreases continuously, indicating that the reduction reaction removes the oxide layer and promotes the alloying reaction under inward stress. To sum up, the results demonstrate that inward stress could accelerate the electrochemical kinetics. In order to further explore the kinetics of Na⁺, the value of σ was used to analyze the diffusion rate (Fig. 6d and e).⁵¹ The core in the anode materials is deformed through oxidation and reduction, and the variable electrochemical paths are connected with the σ in the discharging and charging. Specifically, reduction enhances the inward stress to improve the σ and oxidation reduces the stress to decrease the σ when the voltage is less than 3.2 V. The fluctuating value of σ demonstrates effect of inward stress caused by redox on the charge transfer at the different voltages.

To further explore the commercial prospects and relationship between the stress field and electrochemical field of the LMNCs, pouch cells with an LMNC anode and NaFe_1/3Ni_1/3Mn_1/3O₂ cathode were assembled and are illustrated in Fig. S21 (ESI†).^52–54 To realize stable in situ monitoring of the stress field and electrochemical field, the FBG was embedded into the electrode and assembled into the pouch cells.⁵⁵ By coating the FBG inside the electrode rather than simply plugging into the pouch cell, it could provide feedback regarding the stress signal inside the materials instead of the integral stress of the pouch cell. The thickness of the electrode sheet in the pouch cell is between 0.13 mm and 0.15 mm; this thickness is sufficient to cover the FBG. As shown in Fig. S22 (ESI†), the surrounding environment of the sensor was closely observed, which allowed monitoring of the stress changes of the electrode material in the electrochemical reaction. The FBG of the pouch cells was connected to the cell testing system, and the pouch cell was connected to the optical spectrum analyzer (Fig. S23, ESI†).

As shown in Fig. 7a, the stress evolution of the anode in the pouch cell is monotonic and regular, which can be connected with the physical field mechanism. The stress evolution signal is presented in the relationship between wavelength and voltage; the growth of Δλ indicates increasing inward stress and a drop in Δλ indicates a decline in the inward stress.^33,34 Moreover, the voltage signal corresponds to the wavelength signal in a one-to-one fashion, and the inside stress of the LM core can be fitted (Fig. 7b).

Fig. 7 Smart monitor measurement of the stress-electrochemical field and electrochemical performance in the pouch cell. (a) Stress-electrochemical signal curve obtained by the FBG sensor and matched with the discharging/charging curve. (b) Strain and stress fitting curve of the pouch cell under the stress-electrochemical field. (c)–(e) In situ stress-EIS measurement of the pouch cell. (f) and (g) AFM of LMNCs in the pouch cell during the process of discharging/charging. (h) Upper limit and lower limit of bearing stress in the pouch cell. (i) Cycling performance of 1.04 A h pouch cell without optical fiber sensor.

During charging, the reduction reaction plays a dominant role to enhance the inward stress of the anode materials until the end of charging. During discharge, the oxidation reaction has an effect on the material deformation; the enhancement of oxidation weakens the inward stress of the anode material. The in situ monitoring technology realized the stress field to boost the Na⁺ transport and feedback the signal successfully. Additionally, the potential harmful influence of the FBG on the pouch cell was evaluated. Fig. S24 (ESI†) demonstrates that the pouch cell with the sensor has a capacity of 1.04 A h and the influence of sensor on the cell is limited, and the electrochemical galvanostatic charge/discharge curve of the cell with/without the sensor is basically the same. Furthermore, the cycling performance of the pouch cell with the sensor is presented in Fig. S25 (ESI†), which indicates that the sensor has no harmful influence on the cell.

The relationship between the stress field and electrochemical field caused by the smart response in the pouch cell can be obtained from the Fig. 7c. From 2.0 V to 3.6 V, the stress variation curve is consistent with the voltage variation curve. As the voltage rises, the reduction reaction promotes an increase in the stress of the LMNCs. Meanwhile, with the increase in stress inside the anode, R_ct shows a decreasing trend and demonstrates faster charge transfer and Na⁺ transport (Fig. S26, ESI†). During charging, the characterization of the LMNCs can be obtained from the AFM picture (Fig. 7f); the solid–liquid coexistent electrode material particle exhibits obvious roughness, indicating that a strong inward stress exists within the material. Besides, the stress rises rapidly and shows a linear relationship with time from 3.6 V to 4.0 V (Fig. 7d). As shown in Fig. 7e and the AFM image (Fig. 7j), the oxidation reaction reduces the inward stress of the LMNCs to show the rather low roughness; the core of material starts to spread out and has more surface space with the oxide layer during the discharging.³⁵ In the process of the oxidation reaction, the presence of more oxide layer could hinder the alloying reaction to reduce the value of R_ct. The trend in the stress per gram of active materials reflects the change of physical field (Fig. 7h). In a common pouch cell, the maximum stress is 7.59 N g⁻¹ inside the LMNCs during charging, and the minimum stress is 1.20 N g⁻¹ inside the materials; the change in the signal corresponds with the electrochemical signal. Furthermore, the 1.04 A h pouch cell was assembled with a NaFe_1/3Ni_1/3Mn_1/3O₂ cathode and LMNC anode to verify the practicality of the material (Fig. S27, ESI†). Importantly, the pouch cell exhibits good cycling stability at a current of 1C with a retention of 90.2% after 500 cycles. The cycling measurement of pouch cell was carried out as shown in Fig. 7i, indicating that the capacity is 1.04 A h and the energy density of total active materials is 280.3 W h kg⁻¹ after 500 cycles. Therefore, the LMNCs‖NaFe_1/3Ni_1/3Mn_1/3O₂ has great potential in energy storage applications.

Conclusions

In summary, the Na⁺ transport rate was optimized using a micro-stress pump via the simulation of rhythmic cardiac blood pumping for the first time. More importantly, this novel strategy was applied to a 1.04 A h pouch cell, and the mechanism of the correlation between the electrochemical signal and mechanical signal was analyzed using FBG. Specifically, the half-cell delivers 119.1 mA h g⁻¹ at 35 A g⁻¹. Besides, the full-cell shows an energy density of 160 W h kg⁻¹ at 8C after 200 cycles, and the 1.04 A h pouch cell shows an initial energy density of 280.3 W h kg⁻¹ at 1C after 500 cycles with a retention of 90.2%. (The highest energy density of pouch cell is 317.2 W h kg⁻¹, based on the anode and cathode.) The redox reaction rhythmically induces a stress change to accelerate the Na⁺ transport rate. This work offers a strategy for rapid ion transport via a stress field and is expected to inspire the optimization of sodium-ion batteries.

Methods

Materials

2,6-Diaminopyridine (DAP, 99.9%) was purchased from Macklin. The polymer polyvinylpyrrolidone (PVP K-30) and ammonium persulfate (APS, AR, 98%) were purchased from Aladdin. The liquid metal (GaInSn) was purchased from Dongguan Dingguan Metal Technology Co., Ltd, China. All the materials were used in the experiment without further purification.

Synthesis of the carbon nanoparticles

In a typical synthesis, 2.7 g DAP and 1.8 g PVP K-30 were dispersed in 100 ml deionized water. The above solution was subjected to vigorous stirring for 30 min. After stirring, 0.81 g of NaOH particles was dispersed in the above solution. Then, 14.4 ml ammonium peroxydisulfate (1.46 M APS) was added to the above mixture and stirred vigorously for 5 min. After finishing the synthesis step, the polymer precursor was collected by centrifugation and drying. Finally, the carbon nanoparticles were obtained by high-temperature calcination for 2 h at 600 °C at the rate of 2 °C min⁻¹.

Synthesis of dispersed liquid metal (LM) nanoparticles

In the preparation process of LM nanoparticles, 2.4 g GaInSn LM was dispersed into 200 ml deionized water. Then, the above solution was placed in a vacuum oven and under vacuum extraction for 10 min to ensure that a small amount of oxygen remained inside the solution. Finally, the LM was dispersed into tiny particles in the solution using an 800-W ultrasonication machine, yielding the nanoparticle dispersion liquid.

Synthesis of carbon-coated GaInSn liquid metal nanoparticles

First, 2.7 g DAP was fully dispersed into 100 ml deionized water containing 1.8 g PVP K-30. 5 ml of the LM nanoparticles dispersion liquid was added to the above solution. Then, 0.81 g of NaOH particles was dissolved in the mixed solution and stirred for 30 min. After agitation, the polymer precursor containing LM was obtained using 1.46 M APS and stirred for 5 min. Finally, the polymer precursor was collected by centrifugation and then calcinated for 2 h at 600 °C with a rate of 2 °C min⁻¹.

Coin-cell assembly process

CR2032 coin-type cells were used. The active materials of the anode were LMNCs, NCs and LM. The active material of the cathode was NaFe_1/3Ni_1/3Mn_1/3O₂. The separator in the coin-cell was glass microfiber GF/F. In the preparation of the half-cell, the electrode materials (LMNCs, NCs, LM, 70 wt%) were mixed with 20 wt% PVDF and 10 wt% Super C with N-methyl-2-pyrrolidone (NMP). A Cu collector was evenly coated with the agent. Then, the collector was placed into an oven at 60 °C until the agent was dry. The electrode sheets had a diameter of 11 mm (the loading of active materials was 0.8–1.5 mg cm²) and the Na sheets had a diameter of 13 mm, and they were assembled with 1.0 M NaPF₆ in 100% DME into the half-cell in a glovebox. In the assembly process of the full-cell, the anode sheets underwent pre-sodiation through 3 discharging/charging cycles in a half-cell. Then, the anode agent and cathode agent were both coated on the Al collector and punched into an 11 mm diameter. To ensure the capacity ratio of the negative to positive electrode (N/P ration) of the coin cells was 1.4–1.5, the loadings of the cathode and anode were 0.8–1.5 mg cm⁻² and 1.5–2.1 mg cm⁻², respectively. The full-cells were assembled with 1.0 M NaPF₆ in 100% DME. The performance of the half-cell and full-cell were measured. The voltages of the half-cell and full-cell were 0.01–3.0 V and 2.0–4.0 V, respectively.

Fabrication of the pouch cell with FBG

The optical fiber sensor was pasted on the Al collector of the anode, and the FBG part was coated by the active materials of the pouch cell to ensure that the active materials covered the FBG. The conductivity agent was Super C, and binder was PVDF. Then, the cathode side was prepared using NaFe_1/3Ni_1/3Mn_1/3O₂ (active materials : conductivity agent : binder = 7 : 2 : 1 in weight). To ensure the N/P ratio of the pouch cells was 1.4–1.5, the electrode sheets were cut to 6 × 8 cm, and the loadings of the cathode and anode were 5.45–6.29 mg cm⁻² and 7.1–8.6 mg cm⁻², respectively. The pouch cell was assembled in the laminated Al film with 1.0 M NaPF₆ in 100% DME. The type of separator in the pouch cell was Celgard 2400 membrane. After aging for 48 h at 40 °C, the pouch cell underwent electrochemical measurement within the voltage range of 2.0–4.0 V at 40 °C with external pressure using a stainless steel test fixture (120 mm × 160 mm).

Fabrication of pouch cell without FBG

The pouch cells were assembled with the LMNCs (anode) and NaFe_1/3Ni_1/3Mn_1/3O₂ (cathode), and the agent was prepared by mixing the active materials (70 wt%), conductivity agent (20 wt%), binder (10 wt%). The conductivity agent was Super C and the binder was PVDF. To ensure the N/P ratio of the pouch cells was 1.4–1.5, the electrode sheets were cut to 6 × 8 cm, and the loadings of the cathode and anode were 5.45–6.29 mg cm⁻² and 7.1–8.6 mg cm⁻², respectively. The pouch cells were assembled in laminated Al film with 1.0 M NaPF₆ in 100% DME and a Celgard 2400 membrane in a glovebox. For the pre-sodiation process, the anode sheet was discharged/charged for 3 cycles in a half-pouch cell. Using a stainless steel test fixture (120 mm × 160 mm), the pouch cell was placed in a constant temperature environment at 40 °C for 48 h.

Materials characterization

The XRD measurements of the samples were conducted using Cu Kα in a D/Max-2400 Rigaku instrument. The pore structure was analysed using an Autosorb iQ machine from the isotherms of nitrogen sorption/desorption. The relevant TGA data were recorded using an SDTA851e in the range of 0–800 °C at a rate of 10 °C min⁻¹. Elemental analysis was conducted using an ESCALAB 250 with Al Kα X-rays. To observe the structure of the samples, SEM and TEM were conducted using an SU8220 and Eindhoven instrument, respectively. The AFM measurement was carried out using a Bruker Dimension Icon. The optical fiber sensor was connected to a demodulation system in the pouch cell. In the electrochemical measurement, the stress signal and electrochemical signal were received at the same time.

Electrochemical measurement

The electrochemical voltage of the half-cell was 0.01–3.0 V, and the voltage of the full-cell and pouch cell were 2.0–4.0 V. The measurements were conducted using a CT2001A and Biologic VMP3. The cycling and rate performance of the pouch cells were conducted using a CT3002A Land system. EIS plots of the coin cells and pouch cells were tested at frequencies from 10⁵ to 10⁻² Hz. The electrochemical AFM was conducted on the anode materials after the pouch cells had finished cycling at different voltage. In situ stress-electrochemical measurements were carried out with an optical fiber sensor inside the anode materials; the electrochemical signal and stress signal were received at the same time. The calculation of energy density is the integral of (current × voltage) over time (the current and voltage are the actual sampled values) in a step with the unit W h in current recording.

Author contributions

X. J. contributed the “conceptualization, methodology, validation, writing – original draft, writing – review & editing”. M. F. P. and X. J. analysed the fiber signals. D. M. L. and X. J. conducted the experiments and measurements. Z. H. S., W. Y. J. and X. J. reviewed the manuscript. R. Y. M. contributed the “investigation”. B. R. L. purchased the raw materials. X. G. J. contributed the “conceptualization, resources, project administration and funding acquisition”. F. Y. H. contributed the “conceptualization, resources, supervision, project administration and funding acquisition”.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

The authors acknowledge the support from National Outstanding Youth Science Fund (52222314), Near Space Technology and Industry Guidance Fund Project (LKJJ-2023010-01), CNPC Innovation Fund (2021DQ02-1001), Dalian Outstanding Youth Science and Technology Talent Project (2023RJ006), Dalian Science and Technology Innovation Project (2022JJ12GX022), Xinghai Talent Cultivation Plan (X20200303).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ee00282b

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