Spatially confined transition metals boost high initial coulombic efficiency in alloy anodes

Abstract

Alloy-type materials hold significant promise as high energy density anodes for lithium-ion batteries. However, the initial coulombic efficiency (ICE) is significantly hindered by the poor reversibility of the conversion reaction and volume expansion. Here, the NiO/SnO₂ multilayers with a hybrid interface of alloy and transition metal oxides are proposed to generate Ni nanoparticles within confined layers, catalyzing Li₂O decomposition and suppressing the coarsening of Sn or Li₂O particles. Supported by density functional theory (DFT) calculations and revealed by operando magnetometry, the spatially confined, well maintained Ni active sites lower the energy barrier for Li–O bond rupture and enhance the migration dynamics of Li⁺. The enhanced reaction kinetics lead to achievement of an impressive ICE of 92.3% and a large capacity of 1247 mA h g⁻¹ with 97% retention after 800 cycles. Furthermore, the NiO/SnO₂ anode exhibits excellent electrochemical performances in both Na/K-ion batteries. Notably, when constructed with the same framework, SiO₂ also delivers significantly improved lithium storage properties with ultra-high ICEs. This work paves the way for advanced designs of alloy-type anodes that satisfy both ICE and overall electrochemical performance.

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Introduction

The rapid growth of new energy vehicles and large-scale energy storage systems has increased the demand for energy storage devices with lithium-ion batteries (LIBs) bearing the majority of this demand.^1–3 Among well-studied LIB anode materials, alloy-type materials (e.g., Sn, Si, Ge, etc.) based on alloying/de-alloying reactions have emerged as a promising candidate anode alongside commercial graphite materials due to their high theoretical capacity, low working potential and cost-effectiveness.^4–11 These alloy-type anodes typically suffer from drastic volume expansion (>300%), leading to poor electronic/ion conductivity, continuous growth of the solid electrolyte interphase (SEI), and significant performance degradation.^12–14 Recently, MO_x anodes (M = Sn, Si, etc.) have played a crucial role in advancing energy storage solutions and often exhibit better cycling stability, as the Li₂O matrix generated from the conversion reaction (xLi⁺ + MO_x + xe⁻ → M + xLi₂O) alleviates expansion stress and reduces volume fluctuations.^15–17 However, the poor reversibility of the conversion reaction results in low initial coulombic efficiency (ICE), a critical factor for anode materials as it dictates the ability to mitigate irreversible capacity loss.¹⁸ A low ICE reduces energy density of a battery, significantly limiting its practical applicability.¹⁹ Therefore, improving the ICE of alloy-type anodes is essential for successful commercial implementation.

Two main factors have been identified as the primary determinants of low ICE in alloy-type anodes. First, as discussed above, the irreversible transformation of Li₂O results in poor reversibility for the conversion reaction.^20,21 Meanwhile, the volume expansion of M (e.g., Sn and Si) during the alloying reaction causes continuous SEI growth, ultimately leading to partial lithium depletion.¹⁸ To achieve a high ICE, researchers have attempted to reduce the electrode size and enhance the contact area between M and Li₂O.^22,23 For example, Park et al. proposed high-entropy silicon anodes by the element screening method, facilitating mass and charge transport and achieving a high ICE of 90.3%.^24,25 Li et al. synthesized alloy anodes with a disordered lattice and liquid metal phase to achieve higher metallic conductivity, improving the ICE up to 91%.²⁶ However, the ongoing coarsening of grains, along with the associated volume expansion and electrode fractures, further accelerates the capacity decay. Additionally, transition metals (TMs) could catalyze the decomposition of Li₂O and facilitate the phase transition, and are commonly utilized in the construction of alloy-type anodes. However, building alloy-type anodes with small-sized, highly active TMs to achieve an ICE over 90% remains challenging.^27–31 Early research demonstrated that small-sized, highly active TMs can be generated in situ from transition metal oxides (TMOs) during the lithiation process.^17,32 In this context, hybridizing alloy-type metals with TMOs in a confined space may offer a new approach for achieving high ICE.

Herein, we report magnetron-sputtered NiO/SnO₂ multilayers (NSMs) capable of achieving an ultra-high ICE up to 92.3% and a remarkable capacity retention of 97% after 800 cycles. As a self-supporting electrode material, the prepared nanocomposite films have been successfully utilized in quasi-solid state batteries. Operando magnetometry tracks the phase evolution during the charge–discharge process. The tightly wrapped nanolayer structure provides a confined space to stabilize reduced Ni particles, suppressing the coarsening of Sn and Li₂O particles. Density functional theory (DFT) calculations revealed that the free energy of Li–O bonds and the migration dynamics of Li⁺ are significantly influenced by active Ni nanoparticles (NPs), thus facilitating the subsequent delithiation process of Li₂O. Furthermore, the NiO/SnO₂ anode exhibits decent electrochemical performance in Na/K-ion batteries (SIBs/PIBs), demonstrating great promise as advanced anode materials for next-generation batteries. More interestingly, the superior ICE improvement strategy is universal in NiO/SiO₂, CoO/SnO₂, and Fe₂O₃/SnO₂ alloy-type anode materials.

Results and discussion

The schematic illustration of the NiO/SnO₂ multilayers fabricated by magnetron sputtering is shown in Fig. 1a. Layers of 5 nm NiO and 10 nm SnO₂ are alternately sputtered onto the copper electrode to form well-confined NiO/SnO₂ interfaces. The synthesized multilayer films are directly used as binder-free anodes for lithium-ion batteries (Fig. 1a). All tested film materials and their preparation conditions are summarized in Table S1.† The X-ray diffraction pattern demonstrates the amorphous nature of the NSM (Fig. S1a†), which is further confirmed by the twisted and disordered lattice pattern of NiO and SnO₂ in the corresponding TEM image (Fig. S1b†). The X-ray photoelectron spectroscopy (XPS) survey spectrum of the NSM (Fig. S2a†) confirms the existence of C, O, Sn and Ni. And high-resolution XPS validates the dominant valences states of Ni²⁺ (Fig. S2b†) and Sn⁴⁺ (Fig. S2c†) in the NSM, representing the original high purity film. The cyclic voltammograms (CVs) of the samples versus Li⁺/Li recorded in the range of 0–3 V at a scan rate of 0.5 mV s⁻¹, are shown in Fig. 1b. In the first negative scan for NSM, SnO₂ or NiO combined with Li⁺ ions shows a reduction peak at around 1.2 V (i.e., SnO₂ + 4Li⁺ + 4e⁻ → Sn + 2Li₂O and NiO + 2Li⁺ + 2e⁻ → Ni + Li₂O, respectively).³³ The cathodic peaks at around 0.55 V and 0.25 V are attributed to the formation of the solid-electrolyte interface (SEI) layer and the alloying reaction (Sn + xLi⁺ + 2e⁻ → Li_xSn).^33,34 In the first positive scan, the peaks detected at 0.5 V and 2.2 V are attributed to the dealloying of Li_xSn alloys and the oxidation of Ni to NiO, respectively.³⁵ Notably, the NSM shows stronger decomposition peaks of Li₂O (Sn + 2Li₂O → SnO₂ + 4Li⁺ + 4e⁻) compared to pure SnO₂ between 1.0 V and 2.0 V, indicating a significant improvement in the reversibility of the conversion reaction.

Fig. 1 (a) Schematic illustration of the NSM preparation process. (b) CV curves at 0.5 mV s⁻¹ for the first cycle. (c) Initial charge–discharge curves at 1 A g⁻¹. (d) ICE comparison with reported SnO₂-based anodes. (e) Rate performance. (f) Long-term cycle performances at 1 A g⁻¹.

The electrochemical performance of the NiO/SnO₂ multilayer electrode was investigated by using coin-type half cells (LIBs). As shown in Fig. 1c, the specific capacities of the NSM anode for the first charge and discharge are 1543.1 and 1671.9 mA h g⁻¹ respectively, corresponding to a high initial coulombic efficiency (ICE) of 92.3%, as one of the best SnO₂-based electrode performances reported in the literature (Fig. 1d). In contrast, the ICEs of pure NiO (Fig. S3†) and SnO₂ are only 63.2% and 71.9%, respectively. The rate capability of NSM and SnO₂ electrodes at current rates ranging from 0.5 to 5 A g⁻¹ is shown in Fig. 1e. The NSM anode delivers a reversible capacity of 1453.2 mA h g⁻¹ at a current density of 1 A g⁻¹. When the current rate returns to 0.5 A g⁻¹, 91.8% of the original capacity (1333.7 mA h g⁻¹) is retained. In contrast, the SnO₂ electrode only exhibits 1038.5 mA h g⁻¹ at 1 A g⁻¹ and retained only 60.6% of its capacity at 0.5 A g⁻¹. Long-term cycling stability was evaluated at a current density of 1 A g⁻¹.Remarkably, the NSM anode exhibits excellent cycling stability (Fig. 1f) with nearly 100% coulombic efficiency and retains 97% of its reversible specific capacity even after 800 deep cycles.

To further understand the remarkable ICE stemming from the high reversibility of the conversion reaction (Li₂O + Sn → SnO₂ + Li⁺), ex situ XPS characterization was conducted at different potentials. As shown in Fig. 2a, when gradually charged to 3 V, Sn⁴⁺ fully occupies the valence state in the NSM, whereas in bare SnO₂ (Fig. 2b), 17% of Sn remains in the Sn⁰ state along with Sn⁴⁺.³⁶ After 50 cycles (Fig. S4†), the Sn⁰ content increases to 68% in bare SnO₂ (Fig. S4a†), while the NSM still shows Sn⁴⁺ without any impurities (Fig. S4b†), indicating Sn and Li_xSn coarsening in bare SnO₂ and the high reversibility of the conversion reaction in the NSM electrode. This result is consistent with the differential charge capacity plots (DCPs) observed in the NSM (Fig. 2c). The shift of the dealloying peak (Li_xSn → Sn) towards higher potential suggests increasing polarization due to the coarsening of Sn and Li_xSn. Meanwhile, the conversion reaction peaks from 1.0 to 2.0 V disappear after 100 cycles in bare SnO₂, indicating the failure of conversion reaction reversibility. Notably, the dealloying peak of the NSM remains stable even after 100 cycles (Fig. 2d), confirming the high reversibility and stability of the conversion/alloying reaction.^37,38

Fig. 2 Ex situ XPS spectra of Sn 3d for (a) NSM and (b) SnO₂ at four different charge states. Differential charge capacity versus voltage plots for (c) SnO₂ and (d) NSM electrodes from the 1st to the 100th cycles.

To explore the underlying reaction mechanism of the highly reversible and stable conversion reactions in the NSM structure, the phase evolution process during cycling was investigated using operando magnetometry.^39–44Fig. 3a and b show CV curves and time-sequenced magnetization of the NSM electrode, along with the potential response during the first three cycles. The periodic magnetization variation reflects the reversible and steady electrochemical processes. The second discharge/charge cycle response is magnified and discussed in detail (Fig. 3c). The peaks and valleys at different voltages in the real-time magnetization variation are labeled as V1 to V9. The lack of magnetization variation indicates that no reaction of NiO occurs from V1 to V2. The pronounced increase in magnetization from V2 to V3 is likely due to the reduction from NiO to Ni. The metallic Ni⁰ continues to store spin-polarized electrons between V3 and V4, thereby reducing the slope of the magnetization increase.^45,46 Followed by the formation and decomposition of the SEI, the magnetization increases (V4 to V5) and then falls (V6 to V7) corresponding to electron transfer.⁴⁷ As spin-polarized electrons are released from metallic Ni⁰, a significant increase in magnetization occurs from V7 to V8. The oxidation from Ni to NiO from V8 to V9 induces a rapid decline in magnetization. Thus, the in situ phase evolution of Ni particles in the lithiation process is successfully described by the advanced operando monitoring magnetization, addressing the limitations of the ex situ technique.^29,35,48 The size of Ni particles was later quantified using Langevin fitting, as shown in Fig. 3d. The fitted curve matches perfectly with the experimental data, and the calculated diameter of the Ni particle is 1.2 nm. The result proves that the confined multilayers restrict the Ni particle size to the nanoscale, ensuring their uniform dispersion in layers, which provides more electron transport channels. Furthermore, DFT calculations (Fig. 3e) were used to investigate the relationship between Ni NPs and the decomposition of inactive Li₂O at a potential of 1.2 V. As shown in Fig. 3e, the Li–O bond breaking process at the Ni/Li₂O interface is an exothermic reaction with a total free energy of −2.85 eV, while the Li₂O interface exhibits an energy barrier of 1.07 eV. This confirms that the Ni NPs accelerate the delithiation of Li₂O, consistent with the highly reversible conversion reactions in the NSM.

Fig. 3 Analysis of the reaction mechanism of the NSM anode in LIBs using operando magnetometry. (a) CV curves for the first three cycles at a scan rate of 0.5 mV s⁻¹ over a potential window from 0.01 V to 3 V. (b) Operando magnetic monitoring as a function of CV scanning under an applied magnetic field of 3T, and (c) the corresponding magnified view of the dotted area. (d) Langevin fitting curve for the hysteresis loop at a charge of 1.0 V. (e) Free energy for breaking Li–O bonds at the Ni/Li₂O interface and Li₂O surface at 1.2 V.

The overall morphology and surface microstructure of NSM and SnO₂ electrodes were examined after 50 discharge–charge cycles. In the SEM image, the SnO₂ electrode exhibits significant volume expansion and a fractured surface (Fig. S5a†). The corresponding staggered lattices observed in STEM (Fig. S6a and b†) are identified as unreacted Sn, SnO₂ and Li₂O particles. In contrast, the NSM electrode (Fig. S5b†), comprising SnO₂ and NiO composites, largely maintains its original structure with minimal changes (Fig. S6d and e†).⁴⁹ EDX mappings show a uniform distribution of C, O and Sn elements in the NSM electrode after 50 cycles, suggesting no significant atom migration or aggregation during the cycling process. In contrast, the SnO₂ electrode shows evident particle aggregation due to the poor distribution of Sn (Fig. S7†).⁵⁰ Overall, these observations strongly support our experimental results, such as those of operando magnetometry, validating the reversible structural transformation catalysed by Ni NPs in the NSM electrode. Additionally, compared with the thicker SEI film in the SnO₂ electrode (Fig. S6c†), the NSM electrode possesses a more uniform SEI film (Fig. S6f†), which reduces the impact of electrolyte corrosion and surface side reactions. Thus, such a synergistic interaction between the internal reversible structure and external SEI protection significantly improves both the cycle stability and rate performance.

The kinetic behavior of a lithium-ion battery in the whole cycle was investigated using the galvanostatic intermittent titration technique (GITT) method (Fig. 4a).^4,51 The diffusion coefficient (log D) of the NSM is significantly higher than that of SnO₂, indicating a faster ion diffusion rate. Electrochemical impedance spectroscopy (EIS) was used to detect changes in internal resistance during the electrochemical process. Fig. 4b illustrates the impedance from the 1st to the 200th cycles. Notably, by the 100th cycle, the impedance of the SnO₂ electrode increased fourfold compared to the initial cycle (Table S2†). This drastic rise indicates the electrode's failure to maintain structural integrity due to Li₂O deposition and Sn particle agglomeration during the process (Fig. S8†).⁵² Conversely, the impedance of the NSM shows minimal change even after 200 cycles, implying high structure reversibility during the reaction. For detailed insights, we conducted EIS of both NSM and SnO₂ electrodes in the alloying/dealloying voltage range and during the decomposition of Li₂O (discharge to 0.25 V and charged to 0.5 V, 1.2 V or 1.8 V, respectively) in a single cycle (Fig. 4c). The equivalent circuit fitting is shown in Fig. S9.† The NSM electrode exhibits lower charge transfer resistance (R_ct) in smaller semicircles at high-to-medium frequencies and various potentials compared to bare SnO₂, suggesting superior electrical conductivity of the NSM electrode.⁴ The density of states (DOS) curves further emphasize the conductivity difference between SnO₂ and the NSM (Fig. 4d and e). Above the conduction band, the DOS intensity of the NSM is much higher than that of SnO₂, indicating a lower energy demand for the electron transition in the NSM, which corresponds to higher conductivity.⁵³ These results demonstrate that the NSM structure significantly enhances Li⁺ diffusion and charge transfer, thereby markedly improving rate performance.

Fig. 4 Kinetic analysis of the electrochemical behavior of SnO₂ and NSM electrodes. (a) GITT analysis of SnO₂ and the NSM. (b) Impedance at different cycles. (c) Electrochemical impedance spectroscopy (EIS) of SnO₂ and NSM electrodes at discharge to 0.25 V, charge to 0.5 V, charge to 1.2 V and charge to 1.8 V. (d) DOS curves of the SnO₂ model and (e) the NSM model.

The schematic diagram of the high electrochemical performance mechanism, particularly in achieving ultra-high ICE for the NSM configuration, is summarized as follows (Fig. 5a): (1) Ni nanoparticles are first in situ generated from NiO and widely distributed in SnO₂ multilayers to catalyze the decomposition of Li₂O and promote the conversion reaction from Sn/Li₂O to SnO₂. (2) The multilayer structure effectively stabilizes Ni NP active sites and inhibits the coarsening of Sn and Li₂O particles by creating a confined environment. Additionally, it prevents the formation of a thicker and uneven SEI layer, thereby alleviating volume expansion. (3) The nanosized Ni particles derived from the well-designed NSM provide more electron transport channels and facilitate Li⁺ diffusion and charge transfer, thereby elevating reaction kinetics. The same strategy has been extended to NiO/SiO₂, CoO/SnO₂ and Fe₂O₃/SnO₂ multilayers to explore their applicability and effectiveness in lithium batteries, as shown in Fig. S10–S14.† It is found that the nanostructure NiO/SiO₂ electrode exhibits excellent comprehensive electrochemical performance, achieving an ultrahigh average ICE of 81.2%, significantly surpassing that of pure SiO₂. Additionally, the CoO/SnO₂ and Fe₂O₃/SnO₂-based electrodes also exhibited remarkable performance, delivering an ultra-high ICE of 91.1% and 90.5% at 500 mA g⁻¹, with high capacities and rate performance, respectively. Moreover, leveraging the structural benefits, the NSM electrode achieves one of the best electrochemical performances reported for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs), (Fig. S15 to S17; see the ESI† for details). The flexibility and adaptability of this structure make it suitable for various types of batteries, including lithium-ion, sodium-ion, or potassium-ion batteries, enabling outstanding performance across different applications.

Fig. 5 (a) Schematic diagram of the electrochemical performance mechanism of NSM and SnO₂ electrodes. (b) Structure model of a thin film battery and photographs of an electrochromic watch powered by a flexible battery. (c) Open circuit voltage and (d) an LED logo of SnO₂ powered by a flexible quasi-solid-state pouch cell at varied folding levels.

To evaluate the practical application prospects of the NSM, quasi-solid-state pouch flexible cells (NSM∣P(VDF-HFP)∣LiCoO₂) were assembled as shown in Fig. 5b.^54,55 The full battery features an equal size 40 mm cathode and anode. The open circuit voltage of the battery device remains almost unchanged (approximately 3.0 V) at different folding states (Fig. 5c). As another demonstration, a “SnO₂” logo comprising 90 LEDs is well illuminated by the flexible battery in both flat and various bending states (Fig. 5d). These electrochemical measurements highlight the key characteristics of flexibility and adaptability, providing a continuous and stable power supply despite changes in its physical shape or bending states. We also assembled coin-type full cells with the NSM and LiNi_0.8Co_0.1Mn_0.1O₂ as the anode and cathode separately, demonstrating similarities in the ultra-high capacity and ICE (Fig. S18†). On this basis, the “small and powerful” flexible battery is expected to be a competitive candidate for energy storage and conversion components in next-generation microelectronic devices, as well as in future integrated electronic devices.

Conclusion

In summary, a NiO/SnO₂ multilayer anode with confined transition metal oxide (TMO) layers has been developed for Li-ion batteries using an alternating magnetron sputtering method. The designed hierarchical structure of the NSM exhibits an ultrahigh ICE of 92.3% and a large capacity of 1247.6 mA h g⁻¹, retaining ∼97% after 800 cycles. Operando magnetometry directly evidences the evolution of Ni NPs from NiO, and the DFT calculations revealed that the Ni NPs significantly lower the energy barrier for breaking Li–O bonds, thereby facilitating the conversion reaction from Sn/Li₂O to SnO₂. Additionally, the confined environment in the NSM helps inhibit the coarsening of Sn and Li₂O by maintaining the nanoscale of Ni active sites, effectively alleviating volume expansion. Moreover, the well-dispersed Ni NPs profoundly enhance the conductivity and reaction kinetics by providing more electron transport channels and shortening the diffusion distance of Li⁺ and electrons, respectively. As expected, in a broader range of interfacial systems, we found that NiO/SiO₂, CoO/SnO₂, and Fe₂O₃/SnO₂ anodes all show significant performance improvements with an ultrahigh ICE of 81.2%, 91.1% and 90.5%, respectively. The multifunctionality and universality of NiO/SnO₂ anodes in Na/K-ion batteries and flexible quasi-solid-state full cells highlight their widespread feasibility, offering a promising path towards high-performance energy storage solutions.

Data availability

Essential data are provided in the main text and the ESI.† Additional data are available from the authors upon reasonable request.

Author contributions

H. Y. Fu: conceptualization, data curation, formal analysis, investigation, methodology, validation, visualization, writing – original draft. F. C. Gu: conceptualization, data curation, formal analysis, investigation, methodology, validation, visualization. Y. Z. Niu, S. X. Liao, Z. Y. Bu, H. N. Wang, and D. Yang: data curation, formal analysis, investigation, visualization. X. S. Wang: funding acquisition, supervision, writing – review & editing. Q. Li: funding acquisition, project administration, resources, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22179066, 92372127 and 22408192), the Natural Science Foundation of Shandong Province (ZR202210060028), the Postdoctoral Fellowship Program of CPSF under Grant GZC20231188, and the Shandong postdoctoral innovation program (SDCX-ZG-202400262).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4sc06323f

‡ Co-first-author.

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