Facile Synthesis of Nickel Phosphide @ N-Doped Carbon Nanorods with Exceptional Cycling Stability as Li-Ion and Na-Ion Battery Anode Material

Abstract

Nickel phosphide (Ni₂P), as an anode material for both lithium- and sodium-ion batteries, offers high theoretical specific and volumetric capacities. However, considerable challenges include its limited rate capability and low cycle stability arising from its volume change and degradation during cycling. To solve these issues, appropriate composite micro/nanoparticle designs can improve conductivity and provide confinement. Herein, we report a simple pyrolysis method to synthesize nitrogen-doped carbon-coated Ni₂P nanorod arrays (Ni₂P@NC) from nickel foam and an ionic resin as a source of carbon, nitrogen and phosphorus. The N-doped open-ended carbon shells provide Ni₂P containment, good electrical conductivity, efficient electrolyte access and the buffering of bulk strain during cycling. Consequently, as a LIB anode material, Ni₂P@NC has impressive specific capacity in long-term cycling (630 mAh g⁻¹ for 150 cycles at 0.1 A g⁻¹) and a high rate capability of 170 mAh g⁻¹ for 6000 cycles at 5 A g⁻¹. Similarly, as a SIB anode, Ni₂P@NC retains a sizable 288 mAh g⁻¹ over 300 cycles at 0.1 A g⁻¹, and 150 mAh g⁻¹ over 2000 cycles at 2 A g⁻¹. Furthermore, due to a sizable portion of its capacity coinciding with adequately low voltage, the material shows promise for high volumetric energy storage in full-cell format. Lastly, the simple synthesis method has the potential to produce other carbon-coated metal phosphides for electrochemical applications.

1. Introduction

Present-day commercial lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) employ graphitic carbon and hard carbon (HC) respectively as anode materials [1,2]. For practical applications, the overall merits of graphite and HC remain unmatched. This is due mainly to their combination of charge storage at very low voltages (at an average of 0.15 vs. Li⁺/Li for graphite and approximately 0.45 V vs. Na⁺/Na for HC) [3,4], moderate specific capacities (372 mAh g⁻¹ for graphite and up to ~250 mAh g⁻¹ HC), low material costs and adequate rate capability and cycling stability [1,3,5]. However, both materials are in fact quite low in specific and volumetric capacities, the latter owing to their low densities of ~2.26 and 1.5 g cm⁻³, respectively [6,7]. Moreover, the high specific energy contribution of graphite and HC comes mainly from their low voltage profiles rather than their storage capacity contribution. Therefore, from the perspective of increasing the specific and volumetric energy of LIBs and SIBs, alternative anode materials with particularly high specific and volumetric capacities are of investigative interest, provided that an adequately low anode voltage is also achievable [8,9,10,11].

Ni₂P has been widely researched as an electrocatalyst material but has only recently received attention as a LIB and SIB anode material [12,13,14]. As an anode in both the LIB and SIB cell formats, Ni₂P stores charge via the reversible conversion reaction of Ni₂P + 3Li/Na→Li₃P/Na₃P + 2Ni, which, as a three-electron process, offers a high theoretical specific capacity of 542 mAh g⁻¹ [15]. It also has favorable thermal stability (compared to low-voltage storage in carbons) owing to its higher average voltage profile, within which a sizable charge storage capacity can be practically accessed within an average voltage of ~0.5 V vs. Li⁺/Li and Na⁺/Na [15]. Furthermore, as a transition-metal phosphide with a high crystal density of 7.55 g cm⁻³ [16], Ni₂P has almost three times the crystal density of graphite and over five times that of HC [6,7], which is conducive to reaching higher volumetric energy storage in LIBs and SIBs.

However, typical of all conversion reaction materials, Ni₂P undergoes large volume changes during cycling that cause phase decomposition and resultant kinetic problems of poor electron transfer conductivity [17,18]. Solutions to these problems include the particle design of appropriate micro- and nanostructures and the use of carbon as a protective layer to contain material breakage during cycling [19,20]. For example, Park’s team successfully prepared amorphous and crystalline Ni₂P and self-assembled Ni₂P nanoparticles by thermal injection using an Ar atmosphere and standard Schlenk-line technology. By varying the quantity of the nickel acetylacetonate reactant, Ni₂P nanoparticles were produced with comparable shapes and sizes but with different crystalline states. Notably, the amorphous Ni₂P had greatly improved initial coulombic efficiency (ICE) and capacity retention, achieving a reversible capacity of 573.2 mA h g⁻¹ at 0.5 C and, in successive cycles, a superb CE of 99.1% in LIBs [21]. With the use of Ni₂P as a SIB anode material, Yin et al. successfully prepared Ni₂P@Carbon/graphene with three-dimensionally interconnected porous structures. The synergistic interaction between carbon layers and Ni₂P nanoparticles provided structural stability during cycling, maintaining a specific capacity of 124.5 mA h g⁻¹ at 1 A g⁻¹ for 2000 cycles. Such a synthesis approach combines solvothermal reactions and in situ phosphorylation procedures [22]. Overall, the carbon covering prevented the nanoparticles from clumping together, while it also improved the electron transfer rate and provided containment for volume changes.

The reported morphologies of synthesized Ni₂P in the literature include nanoparticles, hollow spheres, nanosheets and nanobelts [12,23,24,25,26,27,28]. The two main methods commonly used to synthesize Ni₂P are (1) the solvothermal/hydrothermal method (where it is not easy to manipulate the morphology or to remove the excess solvent [29,30]), and (2) chemical vapor deposition using hypophosphite as a phosphorus source with prepared Ni/NiO precursors of the designed morphology as the nickel source. However, in the latter method, the reaction releases toxic and flammable PH₃ gas and is quite labor-intensive (Table S1). Therefore, achieving an effective Ni₂P architecture via a convenient method with low toxicity remains an important goal for its realization as an anode material.

Herein, we report a simple and safe pyrolysis method to synthesize nitrogen-doped carbon-coated (NC) Ni₂P nanorods with structural features that enhance Ni₂P performance as a LIB and SIB anode. The synthesis method involves the pyrolysis of a novel nitrogen- and phosphorous-containing resin in the presence of nickel foam. The phosphatization growth/carbon deposition reaction occurs on the nickel metal surface to form self-assembling superstructure arrays comprising composite 100 nm diameter Ni₂P nanorods that develop a carbon shell throughout the synthesis process. The unique nanostructure and composition of Ni₂P@NC were determined by various physical characterization methods, followed by electrochemical testing. In both high-rate and long-term cycling in half-cell format, Ni₂P@NC had excellent performance, which can be attributed to its unique structure. For a comparison, a number of controls were also tested, which included a synthesized Ni₂P, NC, and Ni₂P/NC mixture as well as commercial Ni₂P (see Materials and Methods). Finally, the possible specific and volumetric energy contributions the material might facilitate in full-cell formats was calculated.

2. Materials and Methods

Ni₂P@NC was synthesized as follows: 2.5 g of a purchased nitrogen- and phosphorus-containing resin (Sunresin Materials Co., Ltd., Zhejiang, China) was first placed in a quartz boat to form a uniform depth layer. A nickel foam sheet (6 × 3 × 0.15 cm) was then positioned on top of the resin, and the assembly was heated for 60 min in a tube furnace at 1000 °C under a N₂ atmosphere at 10 °C min⁻¹. After cooling to room temperature, the Ni₂P@NC products were detached from the nickel foam surface via dry sonication. ICP analysis revealed the Ni₂P content to be approximately 85 wt.%. Nitrogen-doped carbon (NC) was made using the above procedure but with the omission of nickel foam. To make nickel phosphide nanopowder or micropowder (Ni₂P), red phosphorus and nickel powder were heated at 1000 °C for 2 h. Ni₂P/NC was made by combining Ni₂P and NC in a ball mill for two hours at a weight ratio of 85:15. Commercial Ni₂P (C-Ni₂P) was purchased from (Aldrich Chemical Corporation, Milwaukee, WI, 53201, USA).

Crystal structure determination was carried out using X-ray diffractometry (XRD) on a D/Max-III XRD diffractometer (Rigaku Co., Cu Kα, Akishima City, Tokyo, Japan). Structural, compositional and particle size analysis employed scanning electron microscopy (SEM) on a FESEM SU8220 (Hitachi Corp., Tokyo, Japan), and transmission electron microscopy (TEM) was performed using a Titan ETEM G2 80–300 (FEI Co., Hillsboro, OR, USA). Surface-state and chemical environment analyses were carried out using X-ray photoelectron spectroscopy (XPS) with Al K radiation on an ESCALAB 250 (Thermo-VG Scientific, New York, NY, USA). Pore structure characterization and specific surface area were determined by N₂ adsorption–desorption measurements (Micromeritics, ASAP 2420 instrument, Atlanta, GA, USA). For this purpose, a 100 mg sample was placed in a 77 K quartz glass tube and degassed in a Dewar bottle cooled with sufficient nitrogen, after which the sample was transferred to the analytical station of the adsorber for adsorption testing. The specific surface area and pore size distribution information of the material were calculated using the Barrett, Joyner and Halenda (BJH) method.

Electrodes were made using the slurry cast method, with slurries comprising 80 wt.% active materials (either Ni₂P@NC, Ni₂P, Ni₂P/NC, NC or C-Ni₂P), 10 wt.% acetylene black, 10 wt.% carboxymethyl cellulose and deionized water. The slurries were cast onto copper foil followed by drying at 85 °C for 12 h and cut into 14 mm diameter electrode discs. The active material mass loading for each electrode was approximately 2.1 mg (when testing the CV curve, the cell had an active material load of approximately 0.6 g). Half cells were constructed with Li or Na metal foil counter electrodes, and glass microfiber separators (Whatman GF/D) were used with an electrolyte of 1 M LiPF₆ dissolved in EC/DMC/DEC (1:1:1 (v:v:v)) or 1 M NaClO₄ dissolved in PC/EC (1:1 (v:v)) with 5% fluoroethylene carbonate (FEC) added, respectively. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on an electrochemical instrument (IM6, Zahner-Elektrik, Kronach, Germany). Charge/discharge cell cycling was carried on the Neware cell test system (Neware Battery, Shenzhen, China) set at a potential range of 0.01 to 3 V.

3. Results and Discussion

Figure 1 shows the XRD patterns of Ni₂P@NC, Ni₂P/NC, Ni₂P and NC powders. All samples containing Ni₂P produced sharp 2θ diffraction peaks at 40.7, 44.6, 47.4, 54.2, 55.0 and 74.7°, relating, respectively, to the (111), (201), (210), (300), (211) and (400) crystal planes of hexagonal Ni₂P (PDF#65-3544). Ni₂P@NC and Ni₂P/NC patterns show a poorly defined peak at around 26°, analogous to the strongly pronounced peak seen in the NC sample.

SEM imaging of the Ni₂P@NC sample under low magnification revealed a uniform growth distribution of Ni₂P@NC arrays formed on the nickel foam substrate. Under high magnification, the arrays can be seen to consist of nanorods with a consistent width and length, with no entanglement or agglomeration (Figure 2A–C). In contrast, Ni₂P (Figure S2) formed aggregated structures containing a large number of nanoparticles. In the Ni₂P/NC mixture (Figure S3), Ni₂P and NC particles appeared to be freely mixed with disorderly contact, providing only the smallest stabilized structure. Furthermore, the presence of irregular holes in NC suggests the loss of phosphorus during pyrolysis (Figure S4). TEM imaging of an individual Ni₂P@NC nanorod (Figure 2D) shows that the nanorods are roughly 100 nm in diameter and that carbon layers wrap uniformly around them, and the corresponding Fourier transform image (inset) also confirms good single-crystal characteristics. High-resolution TEM (HRTEM) shows that the carbon layer is ~12 nm in thickness, and the Ni₂P lattice spacing of 5.05 Å corresponds to the (100) crystal plane (Figure 2E,F). In the sonicated samples (Figure 2D), the nanorods can be seen to have open tips caused by their detachment from the nickel foam. The open-tip structure should facilitate good electrolyte access to the battery material, and Figure 2E,F show areas of close contact between the carbon and Ni₂P, as well as some void non-contact areas. These features could, respectively, provide buffering expansion space and support good electrical contact during cycling. Finally, elemental mapping (Figure 2G–L) shows a broad dispersion of the four elements in a single Ni₂P@NC nanorod and a uniform dispersion of N and P elements throughout the carbon layer (which may also promote electrochemical capacity).

Figure 3A shows the N₂ adsorption–desorption curve for Ni₂P@NC, which is clearly characterized by a hysteresis loop and type IV curve [31]. The tests reveal that Ni₂P@NC has a specific surface area of 272.3 m² g⁻¹ according to the Brunauer–Emmett–Teller (BET) analysis, and it is clear from the pore size distribution curve that the sample predominantly contains mesopores. The specific surface areas of Ni₂P, Ni₂P/NC and NC were 18.9, 32.5 and 308.9 m² g⁻¹, respectively (Figures S5–S7), showing an increasing trend with carbon content. Notably, the NC sample has a high specific surface area resulting from both the absence of Ni₂P and the formation of additional pores due to the removal of phosphorus from the carbon during pyrolysis.

The elemental composition and bond types of Ni₂P@NC were determined using XPS. Figure 3B shows the detection of four major elements in the broad-scan spectra. The Ni 2p spectra (Figure 3C) show six clear peaks assigned to Ni 2p_3/2 (853.2 eV) and Ni 2p_1/2 (869.9 eV) in Ni-P; Ni 2p_3/2 (856.9 eV) and Ni 2p_1/2 (874.9 eV) in Ni-O; and also two satellite peaks at 863.1 eV and 880.7 eV [8,32]. The P 2p spectra (Figure 3D) reveal the bond types of P-Ni (129.6 eV), P-C (133.3 eV) and P-O (134.2 eV) [33,34]. The C 1s spectra (Figure 3E) reveal C-C (284.6 eV), C-P (283.6 eV), C-N (285.7 eV), C-O (286.4 eV) and O-C=O (288.9 eV) [35,36]. The N 1s spectra (Figure 3F) reveal pyridinic N (398.9 eV), pyrrolic N (399.9 eV) and graphitic N (401.1 eV) [37,38], which indicates the N-doping of carbon. The detection of C-P- and C-N-type bonds indicates the N and P doping of carbon, both of which are known to have donor/acceptor properties, resulting in expanded lattice spacing and the formation of external defects, which can enhance Li⁺/Na⁺ adsorption/desorption, storage capacity, cycling stabilization and rate performance [35,39,40].

Figure S1 shows SEM images of the timeline of the progression of Ni₂P@NC nanorod growth from the Ni foam surface. After 5 min, the surface of the Ni foam appears quite rough (Figure S1C), which, under higher magnification, reveals the presence of short, finger-like projections of the budding nanorods (Figures S1D and S8). The core–shell growth pattern suggests that carbon (present in the gaseous hydrocarbons arising from the pyrolyzed resin) first dissolves in the molten Ni₂P and is then deposited as a carbon shell on the Ni₂P surface once its saturation point is reached. Similar “vapor-liquid-solid” growth mechanisms have been proposed for carbon deposition on transition-metal carbides and metal carbides [41,42]. Rod-like Ni₂P growth with carbon shell deposition is likely to involve capillary forces that draw molten nickel up the rods while the lower-density saturated carbon is pushed to the surface of the rods (Figures S1E,F and S9) to obtain carbon-coated Ni₂P nanorods (Figure S1H). Upon cooling from the liquid to the solid state, voids are generated between the carbon shell and Ni₂P core due to their different cooling coefficients.

Galvanostatic discharge/charge cycling curves of Ni₂P@NC at 0.1 A g⁻¹ as a LIB anode (in half-cell format against Li metal) are shown in Figure 4A. While the initial discharge capacity was 1058 mAh g⁻¹ and the initial charge capacity was 690 mAh g⁻¹ (i.e., 65.28% of ICE), a high CE was quickly reached by the 2nd and 3rd cycles that remained well above 90% by the 150th cycle at 630 mAh g⁻¹. Figure 4B shows the CVs of Ni₂P@NC in the LIB electrolyte over the first five cycles at 0.2 mV s⁻¹. During cathodic scanning, the first scan shows an extended cathodic peak relating to SEI film formation, which, by the 2nd to 5th cycle, was completely gone, showing that such an irreversible capacity component is restricted to the 1st cycle only. The anodic peaks at 1.08 V and 2.37 V correspond to the oxidation of Ni to NiP and Ni₂P, respectively, and consistently appeared in subsequent scans [43,44]. In contrast, Ni₂P powder (Figure S10) in the second and third cycles exhibited a relatively destabilized anomalous curve shape. To further examine the electrochemical behavior and stability of Ni₂P@NC, CV at 0.1 to 2 mV s⁻¹ was carried out after SEI film formation at 0.2 mV s⁻¹ (shown earlier in Figure 4C). The subsequent curves over increasing rates of voltage change remained largely unchanged, indicating that Ni₂P@NC has a stable reversible capacity as a LIB anode.

The rate capabilities of Ni₂P@NC as a LIB anode at 0.1 and 10 A g⁻¹ (Figure 4D) were 750 and 130 mAh g⁻¹, respectively, and recovered a high capacity of 670 mAh g⁻¹ when returned to 0.1 A g⁻¹, i.e., resuming at 89.3% retention of the original value. In comparison, the rate capabilities of the control materials (Figure 4D) at 0.1 and 10 A g⁻¹ were as follows: Ni₂P (407 and 13 mAh g⁻¹); NC (267 and 35 mAh g⁻¹); Ni₂P/NC (330 and 14 mAh g⁻¹); and the C-Ni₂P control (414 and 65 mAh g⁻¹, Figure S11). Therefore, Ni₂P@NC clearly had a notably higher rate capability than the four control materials. Figure 4E shows the long-term cycling tests of Ni₂P@NC as a LIB anode. Here, the initial first cycle’s irreversible capacity loss due to the formation of the SEI film is included in the plot [45]. During subsequent charge/discharge cycles, the CE holds at >90% at 630 mAh g⁻¹. Notably, this value considerably exceeds the 542 mAh g⁻¹ theoretical capacity of Ni₂P and the isolated NC sample. However, excess theoretical capacities for Ni₂P are commonly reported, particularly with combinations of carbon (see Table S2). In the present case, it is possible that the N-doped carbon shell is able to greatly boost the ion storage capacity of Ni₂P and that interfacial charge storage and the additional contribution of pseudo-capacitance also result in an increase in the material’s capacity [41,46]. Impressively, Ni₂P@NC retains a stabilized capacity of >600 mAh g⁻¹ after 150 cycles, unlike the three controls Ni₂P, NC and Ni₂P/NC, which dropped to 75.5, 103.7 and 84.9 mAh g⁻¹, respectively, by the 70th cycle. Furthermore, Ni₂P@NC cycled at 5 A g⁻¹ (Figure 4F) was also very stable, dropping from 226 to 170 mAh g⁻¹ in merely 6000 cycles, equating to a 0.00413% average capacity drop per cycle. Clearly, the unique structure and composition of Ni₂P@NC are conducive to steady long-term cycling and high-rate cycling. Table S2 shows a significant increase in cycling stabilization and the retention of the relative capacity of Ni₂P@NC over the previously reported Ni₂P-based materials as LIB anodes. The particularly high capacity retention of our Ni₂P@NC composites is evidently associated with the appropriate nanostructure combined with a novel nanorod morphology.

Figure 5A shows the galvanostatic discharge/charge cycling curves of Ni₂P@NC at 0.1 A g⁻¹ against a Na metal counter electrode in SIB half-cell format. Ni₂P@NC delivered 555.5 mAh g⁻¹ in the first discharge and 343.2 mAh g⁻¹ in the first charge for an ICE of 61.8%. A high reversible capacity of 290 mAh g⁻¹ was maintained with nearly 100% CE at the 300th cycle. Figure 5B shows CV curves obtained at 0.2 mV s⁻¹. In the cathodic scan, the intense reduction peak at 1.0 V corresponds to the formation of Na₃P. In the first scan, between 0.01 and 1.5 V, the formation of the SEI film generated a higher current, while in the following four scans, the tight overlap of the curves occurred. In the anodic scans, peaks at 2.0 V correspond to the oxidation of Ni to Ni₂P, and subsequent scans overlap well. In contrast, Ni₂P powder (Figure S12) produced less overlap in the second and third cycles, indicating lower stability. Over sweep rates of 0.1 to 1.5 mV s⁻¹ (Figure 5C), Ni₂P@NC generated constant curve shapes (showing little deviation in reduction–oxidation peak locations), which indicates that the material facilitates rapid ion mobility. Moreover, Ni₂P@NC as a SIB anode had a high rate performance between 0.1 and 5 A g⁻¹ (Figure 5D) of 343.2 and 115 mAh g⁻¹, respectively. On return to 0.1 A g⁻¹, the reversible capacity of 280.3 mAh g⁻¹ was restored to 81.7%, indicating excellent rate performance. The percentage recovery on return to 0.1 A g⁻¹ followed the order NC > Ni₂P@NC > C-Ni₂P > Ni₂P/NC > Ni₂P, which reveals the stability of NC and its stabilizing effect on cycling in both Ni₂P@NC and Ni₂P/NC. Notably, as with LIB cycling, Ni₂P@NC as a SIB anode produced the highest capacity at all rates tested, remaining 290 mAh g⁻¹ over 300 cycles at 0.1 A g⁻¹, compared to values < 100 mAh g⁻¹ for the other control materials after 150 cycles (Figure 5E). The related electrochemical properties of C-Ni₂P are shown in Figure S13. The long-term cycling of Ni₂P@NC at 2 A g⁻¹ (Figure 5F) had a minimal capacity loss from the 1st to 300th cycles at 179.6 to 151 mAh g⁻¹, equating to a 0.008% loss per cycle. Therefore, Ni₂P@NC has outstanding stable cycling performance as a SIB anode material at a high current density. Table S3 compares the cycling performance of Ni₂P@NC in this study to earlier studies on Ni₂P and Ni₂P-carbon combinations as a SIB anode. Similar to its behavior as a LIB anode, Ni₂P@NC as a SIB anode demonstrates exceptional cycling stability and a seemingly unmatched rate capability. This is clearly facilitated by the unique carbon-coated nanorod structure that enhances electrical conductivity and shortens the ion transport pathway.

Electrochemical impedance spectroscopy (EIS) revealed that the samples had an order of increasing electrode/electrolyte transfer resistance of Ni₂P > Ni₂P/NC > Ni₂P@NC > NC (Figure S14). This suggests that close carbon contact between the NC shell and Ni₂P in Ni₂P@NC is synergistic to charge transfer. This, in conjunction with its open tubular morphology, appears to account for its improved cycling stability and higher rate capability over those of Ni₂P and Ni₂P/NC. More specifically, the collective advantages of the Ni₂P@NC structure appear to include (1) the buffering of volume expansion via carbon shells that are open-ended with void spaces between the core–shell contact; (2) the N- and P-doped widening of pores in carbon, allowing rapid ion movement and high capacity; (3) a nanorod structure, facilitating fast ion and electron transfer.

Finally, we consider the possible energy storage contribution of Ni₂P@NC as a LIB and SIB anode material. Of notable concern is the high-voltage slope tail region in the Ni₂P@NC charge curve profiles (Figure 4A and Figure 5A), which appears detrimental to energy storage. However, it is also evident, owing to its high specific capacity, that a significant capacity still resides in the lower-voltage region of the curves. Therefore, it was of interest to determine whether restricting cycling to this region could theoretically produce higher cell specific energy than using its full capacity capability. The calculation process used is described in Section S2 of the SI (“Energy calculations for Ni₂P@NC as a LIB and SIB anode material”) in Figures S15–S17. Firstly, the average voltages preceding all possible voltage/charge cutoff points were calculated and plotted against the corresponding capacity–mass balancing of a typical LIB or SIB cathode for the range of all possible anode capacity cutoff points (Figures S15 and S16). This produced the range of all theoretical full-cell specific capacities, which were then multiplied by the corresponding average cell voltages to obtain the cell specific energy. Plotting the full range of specific energy data points revealed maxima (Figure S17). Notably, the maxima positions do not correspond to the use of the full-anode capacity at the 3 V cutoff. For example, the theoretical specific energy maximum (active material only) for Ni₂P@NC in full-cell LIB format as a Ni₂P@NC/NMC 333 cell was 435.4 Wh kg⁻¹ (Figure 6). This resulted from an anode-to-cathode mass balance ratio of 1:2.19 and a full-cell cutoff at 2.54 V. Here, the corresponding anode specific capacity cuts off at 438.4 mAh g⁻¹ and 1.2 V vs. Li⁺/Li at an average anode voltage of 0.63 V vs. Li⁺/Li. However, the calculated specific energy, unfortunately, still falls 8.3% short of graphite/NMC 333 active materials at an optimal mass balance of 1:1.86. Moreover, the result for the equivalent SIB full cell, compared to a hard carbon (HC) combination with a high-capacity cathode (Table S5), was even lower at ~24% short of the hard carbon equivalent.

Interestingly, while Ni₂P@NC clearly falls short of graphite and hard carbon in its specific energy contribution, its calculated volumetric energy contribution was found to be considerably higher than that of graphite and HC. The calculation of volumetric energy first required the calculation of the total densities of the materials in the cells that produced the highest specific energy (Table S4), followed by the multiplication of cell density by specific energy. Notably, these values were impressively high at 993.2 Wh L⁻¹ and 402 Wh L⁻¹ for Ni₂P@NC in LIB and SIB formats, respectively, regarding active materials. This corresponds to a ~25.2 and ~30.4% increase, respectively, compared to graphite and HC cells (Figure 6). Therefore, despite the considerably higher average voltage profile of Ni₂P@NC compared to both graphite and HC, the combination of its high specific capacity and high density with optimized cathode balancing revealed, via calculations, that it could achieve much higher volumetric energy than graphite and HC. Moreover, the above method provides a useful first step for predicting the energy contribution of materials based on half-cell specific capacity results prior to full-cell format testing. It should also be noted that previous studies using other forms of ab initio calculations have been extremely useful in the battery research field, e.g., for the prediction of a 5 V LIB employing Li₂CoMn₃O₈ cathode material in studies by Eglitis and Borstel [47,48]. Furthermore, from the perspective of developing and evaluating alternative electrode materials with high specific capacities, the calculated results show the importance of including maximum calculations in conjunction with material density determination.

4. Conclusions

In this study, an environmentally friendly solid ionic resin provided a carbon and phosphorus source to synthesize novel N-doped carbon-encapsulated Ni₂P nanorod arrays (Ni₂P@NC) on nickel foam via “pyrolytic growth/carbon deposition”. Structural and compositional analyses found that Ni₂P@NC comprises uniform N-doped carbon shells encasing highly crystalline Ni₂P nanorods. These features significantly improve the composite’s structural stability and electrical conductivity. Additionally, the open-ended nanorod structure allows Ni₂P to be in direct contact with the electrolyte, thus simplifying the ion transport path. Consequently, Ni₂P@NC cycled as a LIB anode material with an impressively high capacity, a high rate capability and high cycling stability (retaining 630 mAh g⁻¹ at 0.1 A g⁻¹ after 150 cycles and 170 mAh g⁻¹ at 5 A g⁻¹ after 6000 cycles with a capacity loss of only 0.00413% per cycle). Similarly, Ni₂P@NC as a SIB anode material had a strong performance of 343.2 mAh g⁻¹ at 0.1 A g⁻¹ and 151 mAh g⁻¹ at 2 A g⁻¹ after 2000 cycles with only 0.00796% capacity loss per cycle. Furthermore, it was determined by calculations that a sizable portion of the capacity coincides at a low enough average voltage to offer a potentially significant increase in volumetric energy over standard graphite and HC anodes. Lastly, the simple synthesis method has the potential to produce other carbon-coated metal phosphides for electrochemical applications.