The role of tungsten‑related elements for improving the electrochemical performances of cathode materials in lithium ion batteries Tungsten

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Tungsten (2021) 3:245–259

https://doi.org/10.1007/s42864-021-00083-9 REVIEW PAPER

The role of tungsten‑related elements for improving

the electrochemical performances of cathode materials in lithium ion batteries

Yong‑Qi Sun^1,2 · Weng Fu² · Yu‑Xiang Hu¹ · James Vaughan² · Lian‑Zhou Wang¹

Received: 23 November 2020 / Revised: 23 December 2020 / Accepted: 28 December 2020 / Published online: 24 May 2021

Abstract

Lithium ion batteries using Ni–Co–Mn ternary oxide materials (NCMs) and Ni–Co–Al materials (NCAs) as the cathode materials are dominantly employed to power the electric vehicles (EVs). Increasing the driving range of EVs necessitates an increase of Ni content to improve the energy densities, which, however, degrades the cycle stability. Here we review the doping/coating of tungsten and related elements to improve the electrochemical performance of these cathodes especially the cycle stability. The selection of tungsten and related elements is based on their special properties including the high valence state, strong bonding with oxygen and the large ionic radius. The improvement of cycle stability mainly results from two features: (1) the enhancement of bulk structure stability upon doping (Mo, W, Ta, Nb) and (2) the resistance of side reactions of electrode/electrolyte by the surficial layer induced by direct coating (V, W, Nb) or bulk doping. For the recent high Ni materials, the formation of Ni²⁺ and its migration to the Li layer induced by these doped/coated tungsten-related elements, and the presence of spinel or rock-salt phase before cycling contributes to improving the cycle stability. The key challenges are the selection of an optimized additive concentration and the fundamental understanding of the reaction mechanism, which will provide insightful guidance for maximizing the electrochemical performance of the state-of-the-art lithium-ion batteries at minimal additional process costs.

Keywords Nickel–cobalt–manganese oxide cathodes · Electrochemical performances · Tungsten and related elements · Doping/coating

1 Introduction

Concerns about global warming due to greenhouse gas emis- sion have rapidly expanded the demand for electric vehicles (EVs). For example, the global fleet of plug-in hybrid electric vehicles and battery electric vehicles increased by 160%, from 2.1 million in 2017 to 5.4 million in 2018 [1, 2]. Cur- rently, the Ni–Co–Mn ternary oxide materials (NCMs) and

Ni–Co–Al materials (NCAs) are considered as the most ideal cathode materials to meet the strict requirements of lithium ion batteries (LIBs) in the short- to mid-term because of their high energy density, good cycling performance, safety performance and relatively low costs [1–3]. Both NCMs and NCAs exhibit similar layered structure as LiCoO₂ (LCO, R3m space group with the space group number of 166), which was commercialized for LIBs in 1992 [2, 3]. Although LCO-based LIBs remain the main choice in the market of portable electronic devices, they are not normally used for the EVs due to the low energy density and relatively high cost of cobalt [4–6].

There is a “range anxiety” for the development of EVs:

expanding the driving range of EVs is one of the main incen- tives for the development of high-performance battery materials. Since NCM system was reported by Ohzuku et al. [5]

and Lu et al. [6], it was quickly developed to increase the capacity and lowering the material costs by increasing the Ni content, i.e., from LCO, to LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111),

Tungsten

www.springer.com/42864

* James Vaughan

james.vaughan@uq.edu.au

* Lian-Zhou Wang l.wang@uq.edu.au

1 Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering

and Nanotechnology, The University of Queensland, Brisbane, QLD 4072, Australia

2 School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

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LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523), LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622), LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) and ultrahigh Ni (x > 0.8 in Li[Ni_xCo_yMn_1−x−y]O₂). LiNiO₂ (LNO) would be the ultimate endpoint of this strategy with a theoretical specific capacity of 275 mAh·g⁻¹. However, increasing Ni content will considerably degrade other battery properties such as cycle stability (especially for long-term cycling) and thermal stability [4, 7, 8]. Another significant strategy to improve the specific capacity is to increase the cut-off voltage [2], which, however, will enhance the side reactions of electrode/electrolyte due to the high valence state of the transition metals (TMs) at the high-charge state, and therefore, this approach becomes challenging in terms of the compat- ibility issues with the electrolytes.

In NCM cathodes, Ni mainly contributes to the increase of specific capacity. Co³⁺ has the best structure stability because of its lowest octahedral-site stabilization energy (OSSE) compared with Mn³⁺ and Ni³⁺ [9]. However, there is overlap between the Co^3+/4+ band and the O^2–: 2p band, which causes an oxygen release issue. In comparison, the Mn^3+/4+ band does not show overlap with the O^2–: 2p band, which will not cause the oxygen release issue directly, while it is easy for the Mn ion to migrate from the octahedral site to a neighbouring tetrahedral site due to its highest OSSE.

These are some widely reported results, but the effect of Co and Mn on the structure and chemical stability of the NCM system is still the subject of ongoing investigations.

For example, using the direct Co/Mn exchange, recently, Liu et al. [10] found that Co had a more prominent effect than Mn in suppressing oxygen release and structure degradation because of the impeding effect. At a deeply charge state, Co⁴⁺ was reduced prior to Ni⁴⁺ and the formed Co²⁺ occu- pied the tetrahedral sites. This prolonged further Ni migration, resisted the phase transition from layered structure to rock-salt phase, postponed oxygen release and, therefore, improved the materials structure and associated thermal stabilities.

To deal with the issues introduced by increasing Ni content and the accompanied low Co/Mn contents in the NCM system, such as deteriorated cycle and rate performance, thermal instability, the doping and coating are for two main strategies from the basic targets to improve the bulk structure stability and to reduce the side reactions of electrode/electrolyte. In addition to NCM, NCA accounts for another important choice for the high Ni strategy and the lines between NCM and NCA have increasingly blurred [2, 11], since Mn, Al and Co were less used and replaced by more alternative dopants, like V, Nb, Ta, Cr, Mo, W, Ti and B, to address the high-Ni issues. There are several good reviews and perspectives [2, 12–15] on the doping/coating of layered cathode materials for improving the performance, but still no systemic review on the utilization of tungsten and related elements especially their universal mechanisms. In

this article, we reviewed the recent advances on coating and doping using tungsten and related elements including W, V, Nb, Ta and Mo to improve the electrochemical performances of layered cathode materials including NCM, NCA and ultrahigh Ni systems.

The fundamental principles for the selection of tungsten and related elements were first clarified, followed by the detail discussion of various layered cathode materials, and finally we summarized the key perspectives of this strategy.

2 Tungsten and related elements

The use of tungsten and related elements, the W-triangle in the periodic table as shown in Fig. 1 [16], to improve the electrochemical performance of NCM system is due to several fundamental principles. First, the oxides of these tungsten-related elements are normally present high valence states compared with other elements like Zr⁴⁺, Ti⁴⁺, Zn²⁺, Ca²⁺, and Mg²⁺: there are no electrons in the d orbital of these high valence cations (W⁶⁺, Mo⁶⁺, V⁵⁺, Nb⁵⁺ and Ta⁵⁺). Therefore, the M–O bonds (M represents W, V, Nb, Ta and Mo, while Re was not discussed due to the limited use) normally has a higher bond strength than the TM–O bonds, as indicated by the much lower Gibbs free energy of the oxides of these elements, which was calculated and summarized in Table 1 [17–19]. The stronger binding of M–O contributes to the overall TM–O covalence, inhibit oxygen release and, therefore, improve the structure stability, as schemed in Fig. 2.

Second, compared with Co³⁺ (0.0545 nm), Ni³⁺

(0.056 nm) and Mn⁴⁺ (0.053 nm), these tungsten-related cations normally have a larger ionic radius (0.064 nm for Nb⁵⁺, 0.064 nm for Ta⁵⁺, 0.059 nm for Mo⁶⁺ and 0.060 nm for W⁶⁺) [20]. Once doped into the crystal lattice, the slab spacing increases, which may enhance the charge transfer and Li⁺ diffusion during the lithium de-intercalation process and, therefore, improve the rate performance. Therefore, the expanded lattice parameters could be observed upon doping these tungsten-related elements.

Third, because the tungsten-related cations usually have high valence states (5+ or 6+), they can enhance the transition from Ni³⁺ to Ni²⁺ due to the charge compensation effect.

Formation of more Ni²⁺ could have multiple effects [3, 4, 8, 21]. It could increase the migration of Ni²⁺ from the TM layer to the Li layer, and the Li/Ni cation mixing due to the similar ionic radii of Ni²⁺ (0.069 nm) and Li⁺ (0.076 nm).

Increasing Li/Ni cation mixing could decrease the irrevers- ibility of Li⁺ during charge/discharge process and, therefore, sacrifice the specific capacity. Also, the transformation from Ni³⁺ to Ni²⁺ could increase the average lattice parameters, due to the larger ionic radius of Ni²⁺ (0.069 nm) compared with Ni³⁺ (0.064 nm). Furthermore, the formation of more

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Fig. 1 Tungsten-related elements in the periodic table [16]

Table 1 Gibbs free energy of formation and ionic radii of metal cations

a Most of the data were calculated using FactSage software [17], while Ni₂O₃ and Co₂O₃ were from the Refs. [18, 19]

Oxide V₂O₅ Nb₂O₅ Ta₂O₅ MoO₃ WO₃ Ni₂O₃ Co₂O₃ MnO₂ Li₂O Gibbs free energy (kJ·mol⁻¹)^a − 1070 − 1390 − 1534 − 458 − 544 − 485 − 490 − 465 − 443 Ionic radius (nm) [20] 0.054 0.064 0.064 0.059 0.060 0.056 0.0545 0.053 0.076

Fig. 2 Schematic diagram of the utilization of tungsten and related elements to improve the electrochemical performances of layered structure cathode materials

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Ni²⁺ could induce the formation of rock salt from layered structure [4, 8]. The effect of rock salt could have quite different effects. It could first enhance the structure stability of the materials and enhance the cycle stability if it is formed before cycling [4, 8], as displayed in Fig. 2. The formation of rock salt, on the other hand, the result of long-term cycling or high-temperature treatment, which could cause a reduced capacity retention due to the large lattice strain between these two phases [22]. In fact, tungsten-related elements are widely used to improve the cathode materials for LIBs [2, 12–15], and in this study only the NCM, NCA and ultrahigh- Ni layered-structure materials were analyzed.

3 Effect of tungsten and related elements

3.1 NCM system

In this section, the effects of doping/coating of tungsten and related elements on the NCM system were first detailed including NCM111, NCM523, NCM622 and NCM811.

Regarding the electrochemical performances, the analysis focuses on the cycle stability especially the long-term capacity retention, the rate performance, the high cut-off voltage performance and the thermal stability.

3.1.1 Effect of tungsten

The effect of tungsten-related elements was first summarized, and in addition to the foregoing three main roles, the

use of tungsten has its special advantages. First, Li₂WO₄ or LixWO₃ is a good Li⁺ conductor [3, 23] and the use of W-coating is expected to form a thin layer of Li₂WO₄ on the material surface, which can potentially facilitate Li⁺ diffusion, reduce electrochemical polarization and, therefore, improve the rate performance [3, 24–26]. Moreover, direct corrosion experiments proved that WO₃ and Ta₂O₅ had stronger resistances against HF attacking [27] compared to other oxides such as MoO₃ and Nb₂O₅. Therefore, use of WO₃ for doping and coating is expected to form a W-rich layer, which will reduce the side reactions of electrode/electrolyte. This can improve the structure stability and thus the cycle stability especially during long-term cycling, as summarized in Table 2. As could be observed, the synthesis methods could be overall classified into three types, i.e., adding the elements to precursor, adding the elements to active and other special methods. The former could be sim- ply divided into doping method, while adding the elements to active normally requires a second heat-treatment step at a relatively lower temperature, taken as a coating strategy.

For NCM111, Luo et al. [24] used tungsten to modify the material surface (Fig. 3a, b) and X-ray photoelectron spectroscopy (XPS) results showed that the surface of the W-coated sample had a high concentration of O²⁻ than the pristine one. Cyclic voltammetry (CV) curves showed that W-coating induced a smaller potential polarization, which agreed with the charge/discharge curves, where a more stable voltage plateaus of W-coated sample was observed (Fig. 3c, d). The electrochemical impedance spectrum (EIS) results showed that the charge transfer resistance (R_ct) of 3 wt%

Table 2 Effect of W-doping/coating on the electrochemical performances of NCM system

NCM ratio Method Concentration (best level) Electrochemical performance References

111 Adding (NH₄)₂WO₄ to active and 800 °C

treatment 2, 3, 4 wt% (3 wt%) Cycle stability was improved at 1 C and

4.5 V; rate capability was improved at 0.1–10 C and 4.5 V

[24]

111 Adding Li₄MgWO₆ during cathode

preparation 1 wt% Cycle stability was improved at 1 C and

4.2 V; slight decrease of initial capacity [23]

523 Adding (NH₄)₆H₂W₁₂O₄₀ to precursor 0.68 wt% Cycle stability was improved for half-cell test, while not in the full-cell test [21]

622 Adding H₂WO₄ to active and 500 °C

treatment 1 wt% Cycle stability was improved at 1 C and

4.3 V; slight increase of initial capacity [28]

622 Adding (NH₄)₂WO₄ to precursor 0.4, 0.8 and 2 wt% (0.8 wt%) Cycle stability was improved at 8 C and 4.6 V; rate capability was improved at 0.5–8 C

[3]

811 Adding WO₃ to active and 500 °C treat-

ment 0.25, 0.5 wt% (0.25 wt%) Cycle stability was improved at 1 C and

4.3–4.5 V; rate capability was improved at 0.2–7 C and 4.3 V

[26]

811 Adding WO₃ to precursor 0.25, 0.5, 1, 2 mol% (0.5 mol%) Cycle stability was improved at 1 C and 4.5 V; rate capability was improved at 0.2–10 C and 4.5 V

[25]

811 Adding (NH₄)₁₀H₂(W₂O₇)₆ to active and

450 °C treatment 1 wt% Long-term cycle stability was improved

at 0.1 C and 4.3, 4.5 V; thermal stability was improved

[29]

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W-coated sample decreased to 73 Ω after 100 cycles from 731 Ω of pristine sample (Fig. 3e). Due to the reduced side reactions of electrode/electrolyte associated with HF corrosion upon W-coating, the cycle stability of the cathode materials was greatly increased (Fig. 3f). Similar positive effects, like improved cycle and rate performances, were found by Yoshinaga et al. [23] with 1 wt% addition of Li₄MgWO₆. The Li₄MgWO₆ addition suppressed the imbalanced state of charge (SOC) between the anode and cathode, which resulted in the formation of microcracks (Fig. 3g, h) and thus caused capacity degradation over long-term cycling.

For NCM523, Yang et al. [21] tried W-doping and they found a contradiction between the half-cell and full-cell tests. W-doping had a positive effect on the cycle stability of half-cell tests, while for full-cell tests, there was not much difference. This was caused by the difference of active Li⁺ amount: there was infinite Li⁺ for the half-cell tests using

a Li metal anode, while the active Li⁺ was limited for the full-cell graphite anode. This showed that for the battery tests, full-cell tests should be conducted to prove the cycle stability.

For NCM622, Song et al. [28] used WO₃ for coating and they found 1 wt% coating had the best performance (cycle stability). X-ray diffraction (XRD) results showed that with increasing W amount, the values of c/a (crystal parameters of layered structure) and I₀₀₃/I₁₀₄ (intensity ratio of two dominant crystal facets in layered structure) both decreased, which indicated the Li/Ni mixing increased, because W⁶⁺

induced the transition from Ni³⁺ to Ni²⁺. There was an optimal coating amount to remove the residual Li⁺ on the material surface but not caused severe local segregation. There- fore, there was a stable and uniform WO₃ film, ~ 3.2 nm in thickness, when the amount was 1 wt%. This film effectively suppressed the side reactions of electrode/electrolyte, which

Fig. 3 Mechanisms for the utilization of W-coating to improve the performances of NCM materials. a, b Morphology difference, c, d CV curves, e charge transfer resistance and f cycle stability of the materials without and with W-coating. Reproduced with permission from Ref. [24], Copyright 2019 Elsevier; g, h Cross-sectional SEM

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greatly improved the cycle stability: the 1 wt% W-coated sample had the highest capacity of ~ 140 mAh·g⁻¹ after 200 cycles at 1 C (~ 120 mAh·g⁻¹ for pristine). Li et al. [3] further found that part of W atoms can migrate into TM layer, while the excess W atoms could form a W-rich layer on the surface, finally constructing a core–shell structure. 0.8 wt% of W-doping was the best level, which was close to the results by Song et al. [28]. At 8 C/4.6 V, the retention was improved from 57% (pristine) to 87% with 0.8 wt% W-doping after 100 cycles. Moreover, it was found that the Li⁺ diffusion coefficient was significantly improved, which could result from the high Li⁺ conductivity of Li₂WO₄-enriched layer on the surface.

For NCM811, Shang et al. [25] tried WO₃-doping and they found the best level was 0.5 mol%. XRD results showed an extension of lattice parameters upon W-doping, which agreed with the transmission electron microscope (TEM) results that the interplanar spacing of the pristine sample was 0.205 nm, while that of the 0.5 mol% W-doped sample was 0.207 nm. The cycle stability was greatly improved: after 100 cycles at 1 C, the retention of the 0.5 mol% W-doped sample was 92%, while that of pristine sample was 85%. The formation of surficial rock-salt phase was observed, and the W-doped sample had a much thinner rock-salt layer (3 nm) than the pristine one (10 nm). This suggested that W-doping suppressed the transition between layered structure and rock salt and, therefore, improved the capacity retention. Becker et al. [29] and Gan et al. [26] used WO₃ to coat NCM811.

Gan et al. [26] found the best concentration of W-coating was 0.25 wt%. For the cycle stability, the improvement at high voltage of 4.5 V was more remarkable than that at low voltage of 4.3 V. Morphology analysis showed that there were many cracks in the pristine sample after 100 cycles, but no cracks in the W-coated samples. This directly proved that the Li₂WO₄-rich layer resisted the side reaction of electrode/

electrolyte, contributing to a higher cycle stability. Simi- lar results were found by Becker et al. [29] using full-cell tests. Moreover, an improved thermal stability was observed upon W-coating (Fig. 3i): Differential Scanning Calorim- etry (DSC) analysis of the delithiated sample showed that the pristine sample had a low-temperature exothermic peak (210 °C) and a larger heat generation (1.96 kJ·g⁻¹), compared to the W-coated sample (225 °C, 1.72 kJ·g⁻¹).

3.1.2 Effect of molybdenum

Like W, Mo behaves similar to improving the electrochemical performances of NCM batteries due to their similar valence states, electron structures and ionic radius, as summarized in Table 3. For NCM111, Park et al. [30] first tried Mo doping and they found the best level was 1 mol% (175 mAh·g⁻¹). An increasing Mo amount increased the lattice constants and the Li/Ni mixing. In a further study by Wang

et al. [31], similar results were obtained that the best level was 1 mol%. The valence state of Mo was measured using XPS, where both Mo⁶⁺ and Mo⁴⁺ were observed, which proved the existence of the Mo⁶⁺/Mo⁴⁺ redox pair. CV curves showed that the potential polarization decreased with increasing Mo amount. Interestingly, it was found that for the pristine samples, an increasing calcination temperature resulted in an increased particle size due to more extensive particle agglomeration at high temperatures.

For NCM523 with Mo doping, Li et al. [32] found that 1 mol% was also the best level. Zhang et al. [33] found similar results that 1.0 wt% Mo-doped sample had the best performance. A new finding was that the partial Mo atoms intercalated into the crystal lattice, while excess Mo atoms formed a Mo-enriched layer on the surface, namely Mo segregation, and the latter could impede the side reactions of electrode/electrolyte. The authors tried different cut-off volt- ages and interestingly found that at 4.3 V the 1.0 wt% Mo- doped sample improved the cycle stability, while at 4.6 V, the Mo-modified samples had a degraded effect after 200 cycles. This degraded effect could result from that a higher cut-off voltage could increase the side reactions between Mo-modified materials and electrolyte, which could generate more non-electroactive impurities [33]. Additionally, high cut-off voltage enhanced the formation of NiO rock salt, because more Ni²⁺ was activated. The Mo segregation on the surface was further confirmed by Breuer et al. [34]. The extended X-ray absorption fine structure (EXAFS) results showed that the average coordination number of Mo–O was 1.3, which was much lower than 6, indicating that ~ 78%

Mo was segregated on the surface. Another important result was that spinel phase was formed at the periphery of both undoped and cycled samples, proving the phase transformation from layered structure to spinel normally begins on the grain surface.

For NCM622, Xue et al. [19] found that 0.7 mol% of Mo-doping improved the cycle stability. A capacity of 203 mAh·g⁻¹ at 1 C was obtained at 4.6 V and the capacity retention of Mo-doped sample was higher than that of the pristine one. A new finding was that Mo-doping remarkably suppressed the particle pulverization and thus, reduced R_ct. Liu et al. [35] further found 1 mol% Mo-doping was the best level, which was in consistent with previous studies on NCM111 [30, 31] and NCM523 [32, 33]. This study clearly proved that the primary particle size decreased with Mo doping amount increasing, i.e., 800, 400, 300, 200 and 150 nm with the doping amounts of 0, 0.5, 1, 1.5 and 2 mol%, respectively. Smaller particle increased the surface areas and shortened the Li⁺ transmission path. One issue with these studies [19, 35, 39, 40] could be the limited cycles tested (< 200 cycles), since the negative effect of Mo-doping on the cycle stability may not appear unless a long-term cycling test was performed at high cut-off voltage [33].

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For NCM811, Konishi et al. [36] tried Mo-doping to modify the thermal stability, where Mn was substituted by Mo. DSC results proved that the exothermic reaction of electrode/electrolyte was reduced, agreeing with the results of NCM523 [34]. Thermal desorption spectrometry–mass spectrometry (TDS–MS) was employed to measure the gases and O₂, HF and CO₂ were detected. The oxygen release of the Mo-doped sample was much less than the pristine one, demonstrating the improved thermal stability. XRD patterns further indicate that Mo-doping suppressed the phase transformation from spinel to rock salt at 200–350 °C. However, the Mo-doped samples had a lower discharge capacity than that of the pristine one. This could result from the high content of Mo (2 and 4 mol%), because previous studies proved normally 1 mol% Mo was the best level [31–33, 35]. Su et al. [37] found that a moderate Mo-doping could promote the formation of surficial rock salt because of the increased Ni²⁺ concentration induced by Mo⁶⁺. The formed rock salt before cycling would alleviate the further phase transformation from layered structure to rock salt during cycling. The

optimal Mo content was 1 wt%, which had a high capacity of 184 mAh·g⁻¹ at 1C with the capacity retention of ~ 92%

after 100 cycles. Susai et al. [38] also found that Mo-doping (1–3 mol%) could substantially improve the electrochemical performances. The main advance was the theoretical expla- nation that Mo⁶⁺ would preferably substitute Ni sites as density functional theory (DFT) calculations proved that Ni/Mo exchange showed the lowest substitution energy compared with Li, Co, and Mn. The replacement of Ni cations by Mo⁶⁺

and the increased Ni²⁺ concentration could also enhance the relative enrichment of Ni²⁺ in local sites of the particle, inducing the formation of rock-salt phase [36].

3.1.3 Effect of vanadium

Use of vanadium oxide or its lithium salt has its special advantages. First, LiVO₃ or Li₃VO₄ is a fast Li⁺ conductor with an excellent conductivity (10^–4 S·m⁻¹) [41], which could work as a fast-charging anode for LIBs [42, 43].

Therefore, the use of LiVO₃ or Li₃VO₄ coating is expected

Table 3 Effect of Mo-doping/coating on the electrochemical performances of NCM system

111 Adding MoO₃ to precursor 1, 2, 5 mol% (1 mol%) Cycle stability was improved at 20 mA·g⁻¹ and 4.4 V; slight increase of initial capacity

[30]

111 Adding MoO₃ to precursor 0.5, 1, 2 mol% (1 mol%) Cycle stability was improved at 20 mA·g⁻¹ and 4.6 V; increase of initial capacity; Rate capability was improved at 30–360 mA·g⁻¹ and 4.6 V

[31]

523 Adding MoO₃ to precursor 0.5, 1, 2 mol% (1 mol%) Cycle stability was improved at 0.5 C and 4.3 V; rate capability was improved at 0.5–8 C and 4.3 V

[32]

523 Adding (NH₄)₆Mo₇O₂₄ to precursor 0.2, 1, 2, 5 wt% Cycle stability was improved at 1 C and 4.3 V, while it was degraded at 4.6 V;

Rate capability was improved at 0.1–5 C and 4.3 V

[33]

523 Adding Li₂MoO₄ to precursor 1 mol% Cycle stability was improved at 1/3 C and 4.3–4.6 V; Rate capability was improved at 0.1–4 C and 4.3 V; Thermal stability was improved

[34]

622 Adding MoO₃ to precursor 0.7 mol% Cycle stability was improved at 4.6 V; Rate capability was improved at 0.5–8 C and 4.6 V; Thermal stability was improved

[19]

622 Adding ammonium molybdate to precursor 0.5, 1, 1.5, 2 mol% (1 mol%) Cycle stability was improved at 0.2 C and 4.6 V; Rate capability was improved at 0.2–2 C and 4.6 V

[35]

811 Solid state reaction using MoO₃ 2, 4 mol% Slight increase of initial capacity; Thermal

stability was improved [36]

811 Adding (NH₄)₆Mo₇O₂₄ to precursor 0.5, 1, 2 wt% (1 wt%) Cycle stability was improved at 1 C and 4.3, 4.5 V; Slight decrease of initial capacity at 4.5 V

[37]

811 Solution combustion method 1, 2 mol% (1 mol%) Cycle stability was improved at 1/3 C and 4.3 V; Rate capability was improved at 1/15–4 C and 4.3 V

[38]

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to improve the rate performance by lowering the Li⁺ diffusion resistance. Second, V₂O₅ has a quite high electronic conductivity of (10⁻³–10⁻² S·cm⁻¹), which is at least 1–2 orders of magnitude higher than those of NCM materials (< 10⁻⁴ S·cm⁻¹) [44, 45]. Thus, the introduction of V₂O₅ is expected to promote the electronic conductivity upon coating. In the NCM system, V-coating could present both roles, since V₂O₅ could react with the surficial residual Li⁺ or Li₂O. Kim et al. [46] further theoretically explained that V can act as a promising electron donor, because V³⁺ has two possible donor electrons in t_2g band, showing a stronger electronegativity, which will induce stable coexistence of Ni²⁺ and V⁵⁺. Inducing more Ni²⁺ could have multiple effects. Different from W and Mo, research on the use of V has mainly focused on coating but not doping, as summarized in Table 4.

For NCM111, Liu et al. [47] tried V₂O₅ coating and they found upon V-coating, the particle surface became rougher and more ambiguous due to the presence of this 5–8 nm layer, with the best level of 3 wt%. Both V⁵⁺ (90%) and V⁴⁺

(10%) were observed by the XPS. V₂O₅ coating remarkably improved the cycle stability. The capacity of 3 wt% V-coated sample was higher than the pristine one after 40 cycles at

1C: the coated samples had a better capacity retention. This could be caused by the V₂O₅ layer effectively resisting the side reactions of electrode/electrolyte. In a further study by Onodera et al. [42], a novel all-solid-state LIB was prepared using NCM111–LiVO₃ mono-particle as layered cathode and LiVO₃ as anode, which showed a high cycle stability.

For NCM523, Huang et al. [50] tried Li₃VO₄ for coating and they found that 3 wt% was the best level. By XPS, they found the binding energy of oxygen increased by 0.21–0.37 eV, indicating the improved stability of oxygen.

Upon high Li₃VO₄ coating to 3–5 wt%, the small Li₃VO₄ phase was directly observed by XRD. In a further study by Lu et al. [48], ultrathin LiV₂O₄ layer instead of V₂O₅ and Li₃VO₄, because of its stronger structure stability, was coated. Similar results including the improvement of cycle and rate performances were obtained, while the best level was 0.3 wt%, lower than the previous studies [47, 50]. In addition to coating, vanadium oxide could account for a good dopant, as calculated by Kim et al. [46] Zhu et al. [49]

tried V-doping and found that 3 wt% accounted for the best concentration. Interestingly, they found the impure Li₃VO₄ phase as the doping amount was 3 ~ 5 wt% by XRD, which agreed with those on vanadium coating by Huang et al. [50],

Table 4 Effect of V-doping/coating on the electrochemical performances of NCM system

111 Adding V₂O₅ to active and 350 °C

treatment 1, 2, 3, 4, 5 wt% (3 wt%) Cycle stability was improved at 300 mA·g⁻¹ and 4.5 V; rate capability was improved at 75–750 mA·g⁻¹ and 4.5 V

[47]

523 Adding V-bearing material to precur-

sor 1, 3, 5 mol% (3 mol%) Cycle stability was improved at

1 C and 4.4 V; rate capability was improved at 0.1–5 C and 4.4 V

[48]

523 Adding LiOH·H₂O/V₂O₅ to active and

700 °C treatment 1, 2, 3, 4 wt% (3 wt%) Cycle stability was improved 4.6 and 4.8 V; Rate capability was improved at 0.5–20 C and 4.6 V

[49]

523 Adding VOSO₄·3.6H₂O to active and

800 °C treatment 0.2, 0.3, 0.5 wt% (0.3 wt%) Cycle stability was improved at 1 C and 4.5 V; Rate capability was improved at 0.1–10 C and 4.5 V

[50]

424 Adding NH₄VO₃ to active and 300 °C

treatment 2.5 wt% Cycle stability was improved at 2, 10

C and 4.3 V; thermal stability was improved

[44]

700 °C treatment 3 wt% Cycle stability was improved at 0.5

C and 4.5 V; rate capability was improved at 0.5–10 C and 4.5 V

[51]

622 Adding CeVO₄ to active and 500 oC

treatment 1, 3, 5 wt% (3 wt%) Cycle stability was improved at 1 C

and 4.3 V; slight decrease of initial capacity; thermal stability was improved

[52]

700 °C treatment 1, 3, 5 wt% (3 wt%) Cycle stability was improved at 1 C and 4.3 V; rate capability was improved at 0.1–10 C and 4.3 V

[53]

811 Adding V₂O₅ to precursor 0.005, 0.01, 0.02 mol% (0.005 mol%) Cycle stability was improved at 0.5 C and 4.3 V; rate capability was improved at 0.1–2 C at 4.3 V

[54]

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indicating a moderate V amount should be used. A lower capacity was induced by vanadium coating at 0.1C due to the decrease of active materials, while the cycle stability got remarkably improved upon 3 wt% doping. For NCM622, Ran et al. [51] used Li₃VO₄ for coating and found the improved cycling performance. By analyzing the chemical compositions of the electrolyte after cycling by inductively coupled plasma–mass spectrometry (ICP–MS), they found V-coating significantly reduced the dissolution of metal ions (Ni, Co, Mn) into the electrolyte. This directly proved that the thin layer could effectively suppress the side reactions of electrode/electrolyte. Jiang et al. [52] further tried CeVO₄ and similar positive effects were observed; the best level of CeVO₄ was 3 wt%, similar to other studies [47–50].

For NCM811, Zhang et al. [53] tried Li₃VO₄ to coat the samples, and they also found 3 wt% was the best concentration. At 0.1C, the initial capacity decreased upon V-coating, because the active amount decreased. 3 wt% coated sample had the best cycle stability, the capacity remained 84% at 1 C

after 100 cycles. CV curves presented the smaller potential polarization and EIS results showed that after 100 cycles, the R_ct of pristine sample increased from ~ 63 to ~ 313 Ω; while for the 3 wt% V-coated sample, it only increased to ~ 203 Ω. This indicated the side reactions of electrode/electrolyte was suppressed, improving the cycle performance. Regard- ing the V-doping of NCM811, both experiments [54] and theoretical calculations [55] proved that an optimum level should be designed since too much doping concentration will sacrifice the capacity.

3.1.4 Effect of niobium and tantalum

Regarding the use of Nb for material modification, as summarized in Table 5, for NCM 111, Iwasaki et al. [56] tried nano-coating of Nb₂O₅ to construct film layers of 10, 20, 30 and 40 nm. This coated material was further fabricated to an all-solid-state battery, which had a discharge capacity of 152 mAh·g⁻¹ at 3–4.2 V, 0.025 C and 50 °C. Recently, Lv et al.

Table 5 Effect of Nb and Ta-doping/coating on NCM system

111 Nano-coating by a tumbling fluidized

bed (Nb) 10, 20, 30, 40 nm (30 nm) Increase of initial capacity; thermal stability was improved [56]

111 1D doping nanostructures (Nb) 1, 2, 3 mol% (2 mol%) Cycle stability was improved at 1, 2, 5 C and 4.3 V; increase of initial capacity at 0.1 C

[57]

523 Adding Nb₂O₅ to precursor (Nb) 0.5, 1, 2 mol% (1 mol%) Cycle stability was improved at 1 C and 4.3 V; rate capability was improved at 0.1–5 C; Thermal stability was improved

[58]

622 Adding Nb₂O₅ to precursor (Nb) 0.5, 1, 3 mol% (1 mol%) Cycle stability was improved at 2 C and 4.1 V; thermal stability was improved

[59]

622 Nb₂O₅ was coated by atomic layer

deposition (Nb) N/A Corrosion resistance at 4.2–4.6 V [60]

703 Adding Nb₂O₅ to precursor (Nb) 1, 2, 3, 4 mol% (2 mol%) Cycle stability was improved at 0.2 C and 4.35 V; rate capability was improved at 0.2–5 C

[61]

811 Li − Nb − O coating (Nb) 1, 2, 3 wt% (3 wt%) Cycle stability was improved at 1/3 C and 4.6 V; rate capability was improved at 0.1–2 C

[62]

811 Adding niobium oxalate to precursor

(Nb) 0.5, 1, 1.5, 2 mol% (1 mol%) Cycle stability was improved at 0.2

C and 4.6 V; rate capability was improved at 0.2–5 C

[63]

111 Atomic layer deposition (ALD) of

LiTaO₃ (Ta) 0, 2, 5, 10, 20 ALD cycles (5 ALD

cycles) Cycle stability was improved;

rate capability was improved at 100–700 mA·g⁻¹; thermal stability was improved

[64]

622 Adding Ta₂O₅ to precursor (Ta) 0.1, 0.25, 0.5 mol% (0.25 mol%) Cycle stability was improved at 1 C and 4.5 V; Slight increase of initial capacity

[65]

811 Adding Ta(OC₂H₅)₅ to precursor (Ta) 1.5 mol% Cycle stability was improved at 1/3 C and 4.3 V; rate capability was improved at 1/15–4 C; Thermal stability was improved

[66]

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[57] tried Nb-doping in one-dimensional (1D) nanostruc- tured NCM111. The Nb-doped sample showed a capacity of ~ 200 mAh·g⁻¹ at 0.1 C/2.7–4.3 V, while the pristine one only presented a capacity of ~ 152 mAh·g⁻¹.

For NCM523, Yang et al. [58] tried Nb doping and they found that 1 mol% accounted for the best level. The Nb doped sample had a high capacity retention of ~ 94%

after 100 cycles at 1C, outperforming the pristine sample, due to the reduced potential polarization and suppressed R_ct increase. At 55 °C, Nb-doping showed a more pro- nounced effect on the structure stabilization and for the 1 mol% Nb-doped sample, the capacity remained ~ 89%, which was ~ 20% higher than that of the pristine one. For NCM622, Kaneda et al. [59] found 1 mol% Nb-doping improved the cycle stability and particularly, the thermal stability was enhanced, indicated by a reduced oxygen release measured by gas chromatography–mass spectroscopy (GC–MS). Li₃NbO₄ on the surface suppressed the side reactions of electrode/electrolyte, which agreed with the results by Karayaylali et al. [60]. Li et al. [61] tried Nb-doping for NCM703 system and they found Nb-doping slightly increased the lattice constants, as indicated by both XRD refinement and TEM, which contributed to the improvement of rate performance. For NCM811, Xin et al. [62] tried Nb- coating and Lei et al. [63] tried Nb-doping, and the improved cycle stability of materials was obtained.

Among these tungsten-related elements (V, Nb, Ta, Mo and W), Ta⁵⁺ had the largest ionic radius and Ta₂O₅ showed the lowest Gibbs free energy, as shown in Table 1. This sug- gests that Ta-doping/coating could have a more prominent potential to modify the material structure and associated battery performance, as summarized in Table 5. Li et al. [64]

tried to use LiTaO₃ to coat NCM111 using an atomic layer deposition (ALD) method. They found a thicker coating resisted the dissolution of cathode materials into the electrolyte to maintain the microstructures, which contributed to the improved cycle and thermal stability. Chu et al. [65]

used Ta to dope NCM622, and they found Ta⁵⁺ were mostly in Li sites to play a pillar role to inhibit the cation mixing.

In a further study on NCM811, Weigel et al. [66] compared the doping effect of various cations including Mg²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺, and Ta⁵⁺. It was found that Ta-doping had the best performance to improve the cycle stability (at 1/3C and 45 °C), which could account for a promising dopant for the high Ni layered materials.

3.2 Ultrahigh‑Ni and NCA systems

In addition to the foregoing NCM systems like 111, 523, 622 and 811, recently more researches tried to use tungsten and related elements to improve the electrochemical performances of ultrahigh-Ni system, namely Li[Ni_xCo_yMn_1−x−y]

O₂ with x over 0.8, such as NCM955 and pure LiNiO₂ [4, 7, 8, 67, 68], as summarized in Table 6.

Park et al. [7] used W to dope the NCM955 materials and they found 1 mol% doping significantly improved the cycling stability (Fig. 4a). A spinel phase (20 nm) was observed on the surface of the 1 mol% W-doped sample, which was an intermedia phase between the layered phase and the rock salt. This spinel formed before cycling would stabilize the structure during cycling, quite different from those formed during cycling. At 4.4 V, the doped sample had a much better cycle stability (95% for doped vs. 81% for pristine), which resulted from the resistance of the formation and propaga- tion of the microcracks in the secondary particles (Fig. 4b, c). In-situ XRD proved that the volume contraction of the doped sample during the H2–H3 transition step (H in H2 and H3 denotes a hexagonal phase) was reduced to − 4.4%

(− 5.2% for pristine) (Fig. 4d). This suggested W-doping dis- sipated the local stress during H2–H3 transition, decreased the lattice strain and, therefore, reduced the formation of microcracks. This agreed with a recent study [22] that the formation of lattice strain between surficial rock salt and bulk layered phase was the main fatigue mechanism during long-term cycling. In addition, the microcracks propagated to the particle surface could open up microchannels for electrolyte infiltration and in this case, W doping could resist the HF attack. Sim et al. [67] used WO₃ to coat the NCM955. A protective layer of LiWO₃/Li₂WO₄, ~ 5 nm for 0.5 wt% coating, was formed on the surface, which resisted the electrolyte corrosion and, therefore, improve the cycle stability.

In 2018, Kim et al. [4] used W to dope a series of ultrahigh-Ni NCM materials, Li[NixCoyMn_1−x−y]O₂ with x = 0.8, 0.89, 0.9 and 1.0. It was found that 1 mol% doping could significantly improve the cycle and thermal stability due to the primary phase transformation from layered phase to rock salt on the surface before cycling (Fig. 4e). This two-phase structure, composed of a bulk layered phase and a surface rock salt, was normally observed during heat treatment at high temperatures or after long-time cycling for NCM materials but not before cycling. This rock-salt phase was a phase segregation due to the increased Li/Ni mixing. Theoretical calculations further proved that W doping increased the surface energy of the layered phase and decreased the surface energy of the rock salt: the formation of W-doped rock-salt surfaces was energetically favoured compared with the bare rock-salt surfaces and the layered surfaces. In a further study by Ryu et al. [8], the effect of W doping amount on the LiNiO₂ cathode was detailed, where the improved cycle and thermal stability were clearly observed (Fig. 4f). They found that W doping suppressed the detrimental phase transition of H2–H3 because of the reduction of the lattice collapse/

expansion and, therefore, structural stress during the phase transition using various techniques, similar to the results on NCM955 system [7].

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Table 6 Ultrahigh Ni and NCA systems

Ni ratio Method Concentration (best level) Electrochemical performance References

NCM955 W doping 1, 2 mol% (1 mol%) Long-term cycle stability was improved

at 1 C and 4.3 V; thermal stability was improved

[7]

NCM955 W coating 0.1, 0.5, 1 wt% (0.5 wt%) Cycle stability was improved at 0.5 C and 4.3 V; rate capability was improved at 0.5–5 C and 4.3 V

[67]

NCM, Ni/(Ni + Co + Mn) > 0.8, mole W doping 1 mol% Long-term cycle stability was improved at 4.2 and 4.3 V; thermal stability was improved

[4]

NC91 Different dopants 1 mol% Ta and W had the best long-term cycle

stability. Ta-doped sample remained 90%

capacity after 2000 cycles

[68]

LiNiO2 W doping 1, 1.5, 2 mol% (1 mol%) Cycle stability was improved at 0.5 C and 4.3 V; thermal stability was improved [8]

NCA, Ni/(Ni + Co + Al) = 0.92, mole W doping 1 mol% Cycle stability was improved at 1 C and 4.3 V; rate capability was improved at 0.2–10 C and 4.3 V

[69]

NCA, Ni/(Ni + Co + Al) = 0.88, mole Ta doping 0.5, 1, 2 wt% (1 wt%) Cycle stability was improved at 0.1, 1 C and 4.3, 4.5 V; rate capability was 0.1–10 C and improved

[70]

NCA, Ni/(Ni + Co + Al) = 0.85, mole Solid state reaction using Nb₂O₅

3, 5, and 10 mol% (5 mol%) Cycle stability was improved at 0.5 C and 4.5 V; rate capability was improved at 0.1–5 C

[71]

NCA, Ni/(Ni + Co + Al) = 0.8, mole V coating 0.4, 0.8 mol% Cycle stability was improved at 1 C and 4.3 V; rate capability was improved at 0.1–5 C; Thermal stability was improved

[72]

Fig. 4 Mechanisms for the utilization of W-doping to improve the performances of ultrahigh Ni system. a Cycle stability, b, c mor- phology difference, d unit cell volume changes of the materials without and with W-doping. a–d Reproduced with permission from Ref. [7], Copyright 2019 Elsevier; e Formation of surficial rock-salt

phase induced by W-doping. Reproduced with permission from Ref.

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In a recent study, Kim et al. [68] compared the doping of Mn, Al, B, W and Ta to adjust the cycle stability of ultrahigh Ni system (Li[Ni_0.9Co_0.09X_0.01]O₂). It was found Ta and W were the most effective in improving the cycle and thermal stabilities, while the widely used Al and Mn for NCM or NCA systems had very limited effect, which is corroborated by the results of Weigel et al. [66]. The synthe- sized Li[Ni_0.90Co_0.09Ta_0.01]O₂ exhibited exceptionally ~ 90%

capacity retention after 2,000 cycles. Most importantly, it was found the doping of Ta and W could refine the grains and dissipate the internal strain by modifying the grain dis- tributions from non-uniform to radially aligned primary particles. Moreover, the Li/Ni order (primary Li/Ni mixing before cycling) in the Li slab stabilize the delithiated structure and impeded the further Ni migration from the TM slab to the Li slab during cycling. This further contributed to the cycle stability.

NCA accounts for another typical high Ni system rapidly developed in the family of layered cathode materials. Lee et al. [72] tried V-coating (Ni/(Ni + Co + Al) = 0.8, mole) and it was found that a 17 nm surface layer was formed by the reaction between NH₄VO₃ and residual LiOH and Li₂CO₃. Working at 60 °C, the V-coated sample had excellent cycla- bility with ~ 90% retention after 200 cycles at 3–4.3 V, which was 18% higher than that of the pristine one. Regarding the

doping strategy of the NCA system, researchers have tried W [69], Ta [70] and Nb [71] and similar positive effects on the cycle stability were observed due to the improved structure stability caused by the stronger bond between these elements and oxygen. The key point was to select an appropriate concentration, as summarized in Table 6, since normally excess content showed negative roles to reduce the capacity especially at low charge/discharge rates [70, 71].

4 Discussion

The foregoing discussions demonstrated that use of tungsten and related elements for doping/coating is a promising strategy to improve the cycle stability of the layer-structure cathode materials including NCM, NCA and ultrahigh Ni materials. The improvement was ascribed to the special properties of tungsten and related elements. First, the strong bonding between these elements and oxygen contributes to a higher bulk structure stability upon doping. Second, either direct coating or elemental segregation caused by doping will construct an enriched layer on the material surface, which will suppress the side reactions of electrode/

electrolyte. Third, the high valence (5+ or 6+) tungsten and related elements can induce the formation of Ni²⁺ due to the

Fig. 5 Summary of the effect of tungsten and related elements on the cycle stability of NCM materials a W, b Mo, c V and d Nb and Ta

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charge compensation, Li/Ni mixing, surficial phase transformation from layered structure to spinel and further to rock salt. This structure and phase variations before cycling will help to maintain the structure stability during cycling, because the further ion migration can be resisted, which is especially important for high-Ni materials. Fourth, bulk doping of tungsten and related elements can expand the lattice parameter due to their relatively larger ionic radius, while surficial coating can create a high ion-conductivity layer, both of which contribute to the rate performance improvement. All these effects will reduce the structure degradation during cycling and thus improve the cycle stability, gener- ally indicated by a lower potential polarization, a smaller increase of charge transfer resistance and less microcracks in the cycled cathodes.

Concentration control is the most important issue regarding the use of tungsten and related elements for doping/coating. Most research reveals that an appropriate and moderate level should be selected, because too little addition cannot modify the structure thoroughly while too much addition will degrade the capacity, because most of the oxides of these elements are electrochemically inactive. The best levels for these elements using varying methods could be quite different, as summarized in Fig. 5. From the respects of materials and process costs, the use of these elements should also be optimized due to the varying extraction cost of these elements and the different difficulties to introduce these elements into the cathode materials. In this case, a multiple-element strategy may be the best option, as shown by some studies [51, 55, 69], although this will make things more complex and more research is required. Some researchers showed why it is necessary to use full-cell tests and the long-term cycling tests [21, 33]. There could be contradic- tory results between the half-cell tests and full-cell tests due to the different conditions like the active Li⁺ amount [21]

and in some cases, the negative effect of doping/coating cannot be reflected in the short-term cycling such as less than 100 cycles, for example [33].

5 Conclusion

In summary, doping/coating of tungsten and related elements shows great potential to improve the electrochemical performances of layered structure cathode materials (NCM and NCA) in lithium ion batteries especially the long-term cycle stability. This could be increasingly important along with the development of lithium ion batteries, based on high- Ni cathode materials and the next-generation lithium-rich cathode materials with higher energy densities, to power the electric vehicles toward sustainable development. For these cathode materials, the structure degradation during cycling and the accompanied capacity decrease are more

severe issues to be addressed, compared with the traditional NCM and NCA materials with high contents of Co and Mn.

From this respect, the doping/coating of tungsten and related elements, based on optimized process design and concentration selection, could provide significant strategies for the development and commercialization of these novel cathode materials for the state-of-the-art lithium ion batteries.

Acknowledgements This work was financially supported by the Aus- tralian CRC-P project “Value-added cobalt refining technologies pow- ering advanced batteries”, administered by Pure Battery Technologies Pty Ltd, and Australian Research Council through its Laureate Fellow- ship and Linkage Projects.

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