Cathode Materials for Lithium-ion Batteries: A Brief Review

In recent years, a significant research effort has been devoted to developing high-performance cathode materials for next-generation LIBs in satisfying stringent requirements of electric vehicles and large-scale energy storage. Amongst them are lithiated transition metal oxides, vanadium pentoxides, and polyanion-type materials.

2.1 Lithiated transition metal oxides

The lithiated transition metal oxides as cathode materials for LIBs are reviewed under two main categories: lithium-rich layered oxides and nickel-rich layered oxides.

2.1.1 Lithium-rich layered oxides

Among the prospective cathode materials for LIBs, lithium- rich layered oxide materials xLi₂MnO₃·(1−x)LiMO₂ (M = Mn, Ni, Co, Fe, Cr, etc.) have attracted much attention in recent years due to their large capacities (>280 mAhg⁻¹) with 3.6 V or larger operating voltages [63]. Some of the numerous challenges facing the utilization of these materials in commercial LIBs include low initial coulombic efficiency, poor rate capability, and voltage degradation during cycling [63]. However, in recent years, considerable progress has been reported in overcoming these challenges [63].

Layered-structure LiNi_0.5Co_0.2Mn_0.3O₂ has been a promising cathode material for rechargeable lithium batteries due to their relatively low cost, less toxicity and high discharge capacity (＞200 mAh g^-1) above a voltage of 4.5 V [63]. However, the large-scale application of LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials has been hindered by the severe capacity fading and poor rate performance at high voltage or high current density over numerous cycles [63]. One facile approach to combat these shortcomings is a surface modification using fluorides, phosphates, and metal oxides to suppress the side reactions and the phase transformation at the interface, thus slowing the capacity degradation and strengthening the structural stability [63]–[72]. Wang et al. [68] reported that zirconium doped LiNi_0.5Co_0.2Mn_0.3O₂ materials were prepared by solid state method. The zirconium doping reduced the cation mixing degree and enlarged the interplanar spacing, which enabled the lithium ion to migrate easily in the crystal structure of the doped cathode materials. Hence, the zirconium doped samples, Li(Ni_0.5Co_0.2Mn_0.3O₂)_0.99Zr_0.01O₂, exhibited a capacity retention of 83.78% at 1 C after 100 repeated cycle in a high voltage of 4.6 V, while that of bare sample retained only 69.35%. Additionally, the rate capability of Li(Ni_0.5Co_0.2Mn_0.3O₂)_0.99Zr_0.01O₂ was also significantly reinforced, especially at high current density. The improved electrochemical performances of the zirconium doped samples were attributed to the less cation mixing degree, better lithium transportation kinetic, and lower impedance.

Niobium dopants have been reported as the most attractive candidates with the mutual effect of consolidating structure stability and boosting electrical conductivity of electrode materials for LIBs [73]–[77]. For instance, Wu et al. [78] synthesised Nb-doped LiNi_1/3Co_1/3Mn_1/3O₂ with improved capacity retention, resulting from the more stable structure and lower resistance. Yang et al. [73] also investigated the effect of niobium (Nb) doping on the structure, and electrochemical performance of LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials for LIBs was reported. A controlled Nb doping strategy was adopted to conquer the drawbacks of LiNi_0.5Co_0.2Mn_0.3O₂, including the unstable structure and restricted lifespan. The spherical precursor  Ni_0.5Co_0.2Mn_0.3(OH)₂ and Li(Ni_0.5Co_0.2Mn_0.3O₂)_1-xNb_xO₂ (x=0,   0.005,   0.01,   0.02) were prepared by hydroxide co-precipitation and solid-state method. The Li⁺/Ni²⁺ cation mixing degree was reduced with Nb doping. Besides, the rate capability and cycling stability of LiNi0.5Co0.2Mn0.3O2 were improved by Nb doping. The experimental results showed that Li(LiNi_0.5Co_0.2Mn_0.3O₂)_0.99Nb_0.01O₂ sample with appropriate Nb doping amount exhibited high capacity retention of 93.77% after 100 cycles at 1.0 C and enhanced rate performance with 125.5 mAh g^-1 at 5.0 C, which is more superior to that of LiNi_0.5Co_0.2Mn_0.3O₂ sample. Additionally, at elevated temperature (55 ℃), the Nb doping displayed further remarkable effect on stabilizing the structure, and 88.63% of the initial reversible capacity could be retained, which is ~20% higher than that of the LiNi_0.5Co_0.2Mn_0.3O₂. Therefore, controlled Nb doping can improve the structural stability and electrochemical performance of advanced LiNi0.5Co0.2Mn0.3O2 cathode materials to develop high energy density LIBs.

Zhou et al. [79] synthesised Sn-doped Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials for LIBs with enhanced electrochemical performance using a sol-gel method. Doping with an appropriate amount of Sn⁴⁺, the electrochemical performance of Li_1.2Mn_0.54-xNi_0.13Co_0.13Sn_xO₂ cathode materials was remarkably improved at x = 0.01. The substitution of Sn⁴⁺ for Mn⁴⁺ widens the Li⁺ diffusion channels owing to its larger ionic radius compared to Mn⁴⁺ and boosts the structural stability of Li-rich oxides, leading to an improved electrochemical performance in the Sn-doped Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials. The Li_1.2Mn_0.54Ni_0.13Co_0.13Sn_0.01O₂ electrode delivered a high initial discharge capacity of 268.9 mAh g⁻¹ with an initial coulombic  efficiency of 76.5% and  a  reversible  capacity of 199.8 mAh g⁻¹ at 0.1 C with capacity retention of 75.2% after 100 cycles. The rate performance of Li_1.2Mn_0.54-xNi_0.13Co_0.13Sn_xO₂ (LMNCS) electrodes compared to Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ (LMNC) electrodes between 2.0 and 4.8 V is illustrated in Figure 3a, while Figure 3b displays the variations of discharge capacities of the electrodes with discharge current density. The Li_1.2Mn_0.54Ni_0.13Co_0.13Sn_0.01O₂ (LMNCS1) electrode exhibited the superior rate capability with discharge capacities of 239.8, 198.6, 164.4, 133.4, and 88.8 mAh g⁻¹ at 0.2, 0.5, 1, 2, and 5 C, respectively, which are much higher than those of LMNC (196.2, 153.5, 117.5, 92.7, and 43.8 mAh g⁻¹ at 0.2, 0.5, 1, 2, and 5 C, respectively) as shown in Figure 3. The excellent rate performance of the Sn- doped electrodes is attributed to the enlargement of Li⁺ diffusion channels caused by the doping of Sn⁴⁺, which is favourable to the intercalation/deintercalation of Li⁺ [80],[81]. Besides, the discharge capacities for all electrodes decrease with the increasing discharge rate, as depicted in Figure 3b. This can be credited to the increased polarization at high rates [82].

3.png

Figure 3. (a) Rate capability of the LMNC and LMNCS electrodes between 2.0 and 4.8 V; (b) Variations of the highest discharge capacity with discharge rate [79]

Furthermore, Yang et al. [83] demonstrated the use of Li₄V₂Mn(PO₄)₄ (labelled as LVMP) in coating Li-rich Mn- based cathode materials (Li[Li_1.2Mn_0.54Ni_0.13Co_0.13]O₂) (labelled as LNCMO) to reduce the residual Li compounds on the surface and prevent the dissolution of transition metals during cycling. LVMP is a composite cathode material with three-dimensional Li-ion diffusion channels and a stable structure at high operating voltages (> 4.5 V). In addition, it has been reported that LVMP has excellent electrochemical performance due to the “mutual doping” between Li₄V₂Mn(PO₄)₃ and LiMnPO₄ [83]–[85]. Theoretical calculations proved that LVMP reduces the Li-ion diffusion energy barrier and provides efficient diffusion pathways to enhance the kinetic performance of the LNCMO. The as-synthesised LNCMO@LVMP compounds delivered a discharge capacity of 300 mAh g⁻¹ with a high initial coulombic efficiency (84.2%) and excellent cycling stability (capacity retention of 82% after 50 cycles at 2C and 78.1% after 200 cycles at 1 C) coupled with a good rate performance (157.5 mAh g⁻¹ at 2 C). Therefore, the phosphate-based materials provide a new structural design strategy towards achieving excellent energy and power density in Li-rich materials.

Layered LiNi_1-x-yCo_xAlyO₂ (denoted as NCA, 1-x-y>0.5) materials have attracted great interest because of their encouraging features such as appealing capacity (>200 mAh g^-1), good cycling performance, and low cost of raw materials [86]–[90]. For instance, LiNi_0.8Co_0.15Al_0.05O₂ has been recommended as cathode material of LIBs for electric vehicles in Tesla Model S due to its highly reversible capacity and modest cyclic stability as compared with LiCoO₂ and LiFePO₄ [91]– [93]. However, LiNi_0.8Co_0.15Al_0.05O₂ still suffers from poor cycling stability and thermal instability during cycling. To overcome this shortcoming, LCNA materials with high capacity, excellent cycling life, and superior thermal stability have been exploited to deliver better performance for LIBs [86]. A practical approach for increasing the capacity of LNCA materials is by raising the electrochemically active Ni content as demonstrated for LiNi0.85Co0.10Al0.05O2, which delivered a higher capacity (233 mAh g^-1 at 0.1 C) than LiNi0.81Co0.10Al0.09O2 (227 mAh g^-1) [88]. However, the LiNi_0.85Co_0.10Al_0.05O₂ with higher Ni content exhibits severe cyclic deterioration and thermal instability compared to LiNi0.81Co0.10Al0.09O2 [88],[94]. To mitigate the adverse effects of increased Ni content, Zhou et al. [86] proposed an easy co-precipitation synthesis of Ni-rich microspherical Ni0.9Co0.07Al0.03(OH)2 precursor with uniform particle size and large Brunauer-Emmett-Teller (BET) specific surface area of 173.8 m² g^-1 employing AlO^2-    as   Al  source.  The co-precipitation technique combined the advantages of high tap density (>2 g cm^-3) with stable reactivity of microspheres and high activity with short paths for ion/electron diffusion of nanoparticles [86]. The uniform and dense LiNi_0.9Co_0.07Al_0.03O₂ microspheres with well-assembled nanoparticles and a low degree of Ni²⁺/Li⁺ mixing were synthesised by optimizing calcination conditions. The as- prepared LiNi_0.9Co_0.07Al_0.03O₂ displayed a remarkable high initial reversible capacity of 236 mA h g^-1 at 0.1 C and good cycling performance  (with capacity retention of 93.1%  after 100 cycles at 1 C and 86.3% after 200 cycles at 2 C). Additionally, the sample also demonstrated superior cyclic stability at elevated temperatures of 45, 55, and 60 ^∘C. The experimental results obtained confirmed that LiNi0.9Co0.07Al0.03O2 possessed good thermal stability (heat generation of 517.5 J g^-1 at 4.3 V), fast lithium-ion diffusion (~7 x 10^-10 cm² s^-1), and high electronic conductivity (~5.4 x 10^-4 S cm^-1). Moreover, the authors reported that the as- synthesised LiNi_0.9Co_0.07Al_0.03O₂ exhibited superior capacity, better cycling performance, and higher rate capability than other reported Ni-based LiNi_1-xU_xO₂ materials. The impressive electrochemical performance of the cathode material was attributed to the combination of the high Ni component, layered structure with a low degree of Ni²⁺/Li⁺ mixing, and uniform microspheres with homogenous distribution of Ni, Co, and Al. Additionally, the assembled LiNi_0.9Co_0.07Al_0.03O₂/KS₆ full cell delivered high capacities of 210 and 180 mA h g^-1 at 0.1 and 1C, respectively, as depicted in Figure 4.

4.jpg

Figure 4. Electrochemical properties of LiNi_0.9Co_0.07Al_0.03O₂/KS₆ full cell. (a) Charge and discharge profiles at selected cycles at a rate of 0.1 C. (b) Cycling performance at a rate of 1 C. Inset: discharge profiles at selected cycles [86]

The full cell also exhibited capacity retention of 91.7% after 100 cycles. Therefore, the as-prepared LiNi_0.9Co_0.07Al_0.03O₂ is a potential candidate for cathode material of advanced LIBs owing to its prominent stable structure, excellent electrochemical performance, and thermal stability.

Wan et al. [95] reported the effects of Mn or Ti doping on the crystalline structure and electrochemical properties of NCA (LiNi_0.85Co_0.10Al_0.05−xM_xO₂, M=Mn or Ti, x<0.01) cathode materials. The NCA cathode material was prepared by one step of hydrothermal reaction at 170 ^∘C and doped with either Mn or Ti since they have similar atomic radii as that of Ni and, in coordination with oxygen, can replace the transition metal atoms in the octahedral interstitial sites. This process can enhance thermal and structural stability and, in some cases, decrease electronic and ionic resistance [96],[97]. Mn and Ti doping resulted in cell volume expansion. This larger volume also improved the electrochemical properties of the cathode materials because Mn⁴⁺ and Ti⁴⁺ were introduced into the octahedral lattice space occupied by the Li-ions to expand the Li layer spacing and, thereby, improved the lithium diffusion kinetics. Consequently, the NCA-Ti electrode demonstrated superior performance with a high discharge capacity of 179.6 mAh g⁻¹ after the first cycle, approximately 23 mAh g−1 higher than that obtained with the undoped NCA electrode, and 166.7 mAh g⁻¹ after 30 cycles. A good coulombic efficiency of 88.6% for the NCA-Ti electrode was observed in the first charge and discharge capacities. Besides, the NCA-Ti cathode material displayed outstanding cycling stability of 93% up to 30 cycles.

2.1.2 Nickel-rich layered oxides

Ni-rich cathode materials LiNi_xCo_yMn_zO₂ (x+y+z=1, x≥0.6), labelled as Ni-rich NCM, have higher gravimetric energy density than LiNi_xCo_yMn_zO₂ (x+y+z=1, x≤0.5), which makes them one of the most promising cathode materials for high-energy LIBs [98],[99]. However, their structural and thermodynamic stability, cycle, and rate performances require further improvement before their full commercialisation as cathode materials in LIBs [100]. For instance, it is still a challenge to acquire stable high energy density from Ni-rich cathode materials under high operating voltage [101] because high operating voltages facilitate the dissolution of transition- metal ions into the electrolyte and damage the surface layered structure [102]. Besides, the structural transformation (layered → spinel →rock salt) of Ni-rich cathode materials at the surface is aggravated by the migration of Ni²⁺ ions with a low energy barrier into Li slabs owing to their similar ionic radius to Li⁺ [103],[104]. This surface phase transformation, together with the degree of Li⁺/Ni²⁺ mixing is worsened by increasing the Ni content in LiNi_xCo_yMn_zO₂ cathode materials and especially under high operating voltage (4.5 V) [105]–[107]. In recent years, remarkable efforts have been focussed on improving the structural stability of Ni-rich materials by decreasing cationic mixing and inhibiting interfacial side reactions, such as lattice doping [108],[109] and surface reconstruction [110]–[113]. Specifically, lattice doping can stabilize the bulk structure, whereas surface modifications can reduce the side reactions on the electrode/electrolyte interface and inhibit the dissolution of transition metal ions. Wu et al. [98] demonstrated the use of rare earth element Ce (obtainable from the economically and environmentally friendly raw material, cerous nitrate) to reinforce the interface of Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ cathode materials for LIBs under high operating voltage. The authors synthesised a series of modified samples through the reaction between cerous nitrate and LiNi_0.8Co_0.1Mn_0.1O₂ at different calcination temperatures, as depicted in Figure 5. At the calcination temperature of above 500 ^∘C, a CeO₂ coating layer is formed to protect the electrode from erosion by the electrolyte and alleviate the increasing resistance during cycling [98],[114],[115]. During the calcination process, Ce⁴⁺ ions with strong oxidizing properties promote the oxidation of Ni²⁺ to Ni³⁺, which effectively reduces the degree of Li⁺/Ni²⁺ mixing in the surface lattice [116]. Additionally, the inert Ce³⁺ ions in transition metal slabs with strong Ce-O bonds help maintain the layered structure of Ni-rich cathode materials at high delithiation state [117]–[119]. Consequently, the modified Ni-rich materials fabricated with an erosion-resistant CeO₂ layer outside and stronger Ce-O bonds inside with reduced Li⁺/Ni²⁺ mixing exhibited excellent electrochemical properties, especially at high operating voltages. For example, the 50th capacity retention at 0.2 C within 2.75–4.5 V was improved from 178.7 mAhg^-1 (89.8%) to 196.8 mAhg^-1 (99.2%) after Ce salt modification at 600 ^∘C.

5.jpg

Figure 5. Schematic diagrams of the synthesis process and the structure of modified Ni-rich cathode materials with a CeO₂ coating layer outside and Ce³⁺ doping inside [98]

However, it has been reported that the negative effect of residual lithium species on NCM can be overcome during manufacturing by coating the Ni-rich NCM with the Li₃PO₄ protective layer to vastly enhance the cycling stability and rate capability of the cathode material [120],[122],[123]. Fan et al. [120] synthesised a unique synergistic multifunctional coating layer on the surface of LiNi_0·8Co_0·1Mn_0·1O₂ (NCM811) by transforming the residual lithium species into Li₃PO₄ and attaching graphene fragments. According to the microscopy studies performed by the authors, a 10 nm amorphous Li₃PO₄ layer together with nanosized Li₃PO₄ particles uniformly covered the surface of NCM811 while the graphene fragments connected the NCM spherical particles to form electronic network. Owing to the synergistic effect of the Li₃PO₄ (LPO) coating layer and the graphene network (GN), the modified sample (GN-LPO-NCM811) exhibited high-capacity retention of 94.3% after 150 cycles at 0.5 C between 3.0 and 4.3 V, while the pristine material displayed considerably lower retention of only 88.1%. Besides, the GN-LPO-NCM811 also demonstrated improved cycling stability at an elevated temperature of 55 °C; the half-cell of GN-LPO-NCM811 achieved high-capacity retention of 80% after 400 cycles at 20 C. As part of its outstanding electrochemical performance, the GN-LPO-NCM811 delivered 70% of its initial capacity at an extremely high rate of 10 C, while the pristine NCM811 only supplied 50% of the capacity. The outcome of the EIS (electrochemical impedance spectrum) studies conducted by the authors confirmed that the Li₃PO₄ apart from protecting the materials from side reactions with the electrolytes also enhances the Li⁺ transportation at the electrode/electrolyte interface. Similarly, the electronic network bridged by graphene fragments provides fast electron transfer. Additionally, the thermal stability of the GN-LPO-NCM811 was significantly improved due to the protection from the mixed-conducting coating layer. The GN-LPO-NCM811 with high ionic conductivity and excellent electronic conductivity offers a template for engineering the surface of Ni-rich NCM cathode materials of the future [120].

To overcome some of the challenges responsible for poor electrochemistry performances of Ni-rich materials,  Su et al. [124] designed and synthesised a structure-gradient LiNi_0.8Co_0.1Mn_0.1O₂ (APS-NCM) cathode material with a high compaction density and superior rate performance using a secondary co-precipitation method. The structure comprised closely staked nanoparticles as the core and directionally aligned nanosheets with exposed active planes as the shell. Besides, the structure-gradient design combined the advantages of high compacted density from the core and fast Li⁺ transport from the shell. Consequently, the APS-NCM possessed a high compaction density with excellent electrochemical performances, especially at high rates. APS- NCM delivered 160 mAh g^-1 discharge capacity even at 10 C, while that of original LiNi_0.8Co_0.1Mn_0.1O₂ material  was  only 99.1 mAh g^-1 within 2.75 – 4.3 V (vs. Li⁺/Li). In addition, the 100th cycling capacity retention also increased from 84.9% to 95.9% at 0.2 C. Although, the compaction density of APS- NCM decreased a little, its volumetric energy density was significantly improved. The proposed strategy for designing and  synthesizing  layered  cathode  materials  with  exposing {010} active planes and satisfied compaction density, can be developed further to meet different application requirements.

Moreover, Chen et al. [125] investigated the effect of dual- conductive layers on layered Ni-rich cathode material. Li₃PO₄ combined with conducting polypyrrole (PPy) formed dual- conductive coating layers on LiNi_0.8Co_0.1Mn_0.1O₂ cathode material to improve cycling and rate performance. These dual- conductive coating layers offered high ionic and electronic conductivity to the Ni-rich cathode materials. The Li₃PO₄ coating layer remarkably improved the ionic conductivity of the cathode materials by removing the lithium residual on the particle surface and reducing the generation of HF in the electrolyte. On the other hand, the PPy layer prevents the direct contact between active cathode materials and electrolyte; thus, reducing the side reactions and dissolution of transition metals. Besides, the stretchy PPy capsule shell could reduce the generation of internal cracks by resisting the internal pressure. Therefore, the conductive PPy polymer enhanced the electrical conductivity of the cathode materials. The outcome of the electrochemistry tests revealed that the modified cathodes displayed much-improved cycling stability and rate capability. The modified cathode material retained 95.1% of its initial capacity at 0.1 C after 50 cycles, while the bare sample retained only 86% and delivered 159.7 mAh g^-1 at 10C compared with 125.7 mAh g^-1 for the bare sample. Hence, the dual-conductive coating layers composed of Li₃PO₄ and PPy were very effective for improving the discharge capacity, rate capability, and capacity retention compared to raw or single coated Ni-rich cathode materials.

2.2 Vanadium pentoxide

Vanadium pentoxide (V₂O₅), as a promising cathode material, has been intensively studied in LIBs for many years because of its natural abundance, low cost, typical layered crystal structure, good lithium-ion de-intercalation or intercalation properties, and high theoretical capacity of 294 mAh g⁻¹ (corresponding to the insertion of two Li⁺ ions) [126]–[131]. However, the application of V₂O₅ in LIBs is limited by poor diffusion coefficient, worse cycle stability, and low electronic conductivity [132]. Construction of nanostructure and preparation of carbon-V₂O₅ composite materials are generally considered as two practical approaches to shorten the transfer path for electrons and ions and to increase the electrode conductivity, respectively [133]–[136].

2.2.1 Micro- and nano-structured vanadium pentoxide

Several V₂O₅-based nanostructures such as nanowire [137]–[139], nanobelt [133],[140],[141] nanosheet [142]–[144], hollow nanosphere [145]–[148] and nanoflower [149] have been synthesised to improve the electrochemical performances of V₂O₅-based electrodes. Ramasami et al. [139] synthesised V₂O₅ nanowire clusters (V₂O₅NWC) for applications in LIBs by a gel-combustion method using bio-fuel cassava starch (Manihot esculenta). The V₂O₅NWC exhibited a discharge capacity of 188 mAh g-1 with better capacity retention of 90% at the end of the 50th cycle, corresponding to a coulombic efficiency of about 99%. The scanning electron microscopic (SEM) images of V₂O₅NWC revealed both nanorod and nanowire morphologies which contributed to the good rate capability and cycling stability of the nanostructure.

Wu et al. [133] examined hierarchical sisal-like V₂O₅ microstructures consisting of primary one-dimension (1D) nanobelt with [001] facets orientation growth and rich oxygen vacancies as cathode materials for LIBs. The microstructures were synthesised through a facile hydrothermal process using polyoxyethylene-20-cetyl-ether as a surface control agent and followed by calcination. The sisal-like V₂O₅ delivered excellent electrochemical properties, including a longer cycling lifespan, higher reversible capacity, and higher rate capability. The as-synthesised microstructures benefitted from the synergistic effects of the primary 1D nanobelt and the stable [001] facets. The primary 1D nanobelt shortens the transfer path of electrons and ions, while the stable [001] facets could reduce the side reaction at the interface of electrode/electrolyte [133],[140]. Besides, the 1D nanobelts structure also alleviates the strain of volume expansion and offers excellent electronic conductivity along the longitudinal direction [133]. Moreover, the oxygen vacancies generated by the low valence state vanadium can facilitate the diffusion of lithium ions, thereby enhancing the conductivity and cycling stability of the V₂O₅-based materials [133]. The experimental results revealed that the sisal-like V₂O₅ exhibited markedly higher rate performance than the bulk V2O5 counterpart, which was synthesised by direct pyrolysis of NH₄VO₃ under the same sintering processes. The sisal-like V₂O₅ delivered the stable discharge  capacities of 297, 261, 235, 211, 190,160 and  105 mAh g^-1 at 0.1, 0.2, 0.5, 1, 2, 5, 10 C (1 C = 300 mA g^-1), respectively. However, the corresponding capacity of bulk V₂O₅ was only 269, 244, 225, 175, 143, 92, and 38 mAh g^-1, respectively.  With a  remarkably high discharge  capacity  of 297 mAh g-1  at 0.1 C and excellent cyling stability, the  sisal-like V₂O₅ microstructures are top candidates for advanced cathode materials for LIBs.

Porous hollow V₂O₅ nanotubes (VNTs), apart from possessing the advantages of 1D nanostructure, also offer pores with different aperture and hollow structures [150]. Some of the merits of ID nanostructure include shortened Li⁺ diffusion pathway, increased contact area between the active material and the electrolyte, the reduced charge transfer resistance, and the structural stability, which could improve the strain relaxation to withstand the volume expansion during Li⁺ insertion/extraction process [150]. Hence, VNTs as cathode materials for LIBs can improve the penetration of electrolytes, increase the contact surface area between the electrolyte and active material and accommodate the volume variations via additional void space during cycling [150]. Liu et al. [150] fabricated VNTs using electrospinning and successive sintering process. By controlling the calcination time, the cathode materials comprising V₂O₅ were synthesised with good electrochemical performance and nanotubes morphology after sintering at 400 °C for 2 hours in the air. The optimized VNTs electrode displayed enhanced electrochemical performance with good specific discharge capacities, cycling durability (capacity retaining 72.5% at 100 cycles), and improved high-rate performance (186 mAh g^-1 at 1000 mA g^-1). This can be ascribed to the superior nanostructure with pores and hollow morphology.

Jiang et al. [151] reported a structural design and engineering of 1-D rod-like V₂O₅ cathode materials for LIBs using polyphenol-grafted collagen fiber CF as a unique bio- template. The collagen fiber was modified by bayberry tannin, a natural plant polyphenol, to enhance its affinity for vanadium anions. After simple calcination in air, the organic CF template was removed entirely, and simultaneously the rod-like V2O5 was prepared. The as-prepared 1D rod-like V₂O₅ displayed enhanced electrochemical performance as cathode materials for LIBs due to its unique structural merits. The SEM images of the natural collagen fiber and V₂O₅ synthesised at a different temperature ranging from 450-650 °C are presented in Figure 6.

6.jpg

Figure 6. SEM images of natural CF (a-b) and 1D rod-like V₂O₅ synthesised at different temperature ranging from 450- 650 °C (c-l) [151]

Like the natural CF (Figure 6a-b), the V₂O₅-450 (i.e., V₂O₅ prepared at 450 °C) retain 1D ordered morphology of fibrous bundles, which comprise numerous V₂O₅ particles (Figure 6c- f). However, those samples prepared at high temperatures over 500 °C show quite different morphology, which was constructed by the assemblage of primary V₂O₅ crystals to form a long-rod-like architecture (Figure 6g-l). This phenomenon could be attributed to the crystal growth of V₂O₅ along the direction of [001] during the annealing process above 500 °C. Amongst them, the V₂O₅-600 exhibits a relatively well-crystallized structure and uniform morphology than other samples [151]. Moreover, the transfer pathway of electrons and ions was shortened by the novel 1D rod-like structure. Also, the lithium-ion diffusion was enhanced by the oxygen vacancies generated by low valence state vanadium (V⁴⁺). Consequently, the 1D rod-like V₂O₅ delivered a high specific capacity of 255 mAh g^-1 at 0.1 C, and excellent rate capability of 100 mAh g^-1 at 10 C. The method adopted by the authors provides a new strategy for the environmentally friendly and size-controlled synthesis of 1D rod-like V₂O₅- based cathode for efficient lithium storage.

2.2.2 Vanadium pentoxides carbon composite

Hydrated vanadium pentoxide (V₂O₅∙nH2O) compared with crystalline V₂O₅ exhibits better chemical stability and can reversibly absorb more than two lithium ions due to its enlarged interlayer spacing [126]. However, the exploration of V₂O₅∙nH2O cathode is limited by its inherently low conductivity and slow electrochemical kinetics, leading to a significant decrease in capability [126]. Several attempts have been made to improve the electrical conductivity and rate performance of V₂O₅∙nH2O cathode by incorporating carbon materials such as graphene [129],[152],[153], graphene oxide (GO) [129],[154], and carbon nanotubes (CNTs) [155]–[157], into the cathode material owing to their extensive distribution, low density, high specific surface area, excellent electrical and thermal conductivity, mechanical stability, and environmentally friendly components [158]. Apart from effectively facilitating the charge transport, the insertion of carbon materials also accommodates the volume change caused by the intercalation and de-intercalation of lithium ions [126]. Although significant improvement in rate performance and electrical conductivity has been reported [159],160], this was often achieved at the cost of a large amount of carbon coating (from 20 wt.% to 60 wt.%). The negative implication of the excessive amount of carbon materials is the consumption of lithium ions, which can affect cathode materials' capacity and even narrow the voltage window [126]. Therefore, research effort is being focussed on exploring the V₂O₅∙nH2O/carbon composite containing low carbon content without damaging the capability of LIBs. Zhang et al. [126] fabricated a crystalline V₂O₅∙nH₂O nanobelts/reduced graphene oxide (rGO) composite with 8 wt.% carbon using a simple but effective dual electrostatic assembly strategy. The V₂O₅∙nH₂O/rGO cathode material for LIBs displayed a high reversible capacity of 268 mAh g-1 at 100 mA g-1 and especially an excellent rate capability (196 mAh g-1 at 1000 mA g-1 and 129 mAh g^-1 at 2000 mA g^-1), surpassing those of the V₂O₅/carbon composites reported in the literatures [159],[160]. The outstanding performance of V₂O₅∙nH₂O/rGO is attributed to the synergetic effects between one-dimensional V₂O₅∙nH₂O nanobelts and two-dimensional rGO nanosheets, which provide a short transport pathway and enhanced electrical conductivity.

Furthermore, hierarchical nanoarchitecture has been proven to be highly effective in expanding accessible active sites and providing a fast ion diffusion pathway in energy storage materials [161],[162]. Nak et al. [163] demonstrated the construction of a hierarchically open-porous V₂O₅/rGO composite microball for high-performance LIB cathodes using spray frozen technique combined with thermal reduction process. The open-porous surface of rGO microballs offers the advantages of immobilizing V₂O₅ particles, enhancing the electronic conductivity, and stabilizing active V₂O₅ crystals [163]. The radial orientation of the composite microballs effectively provides a fast diffusion pathway for Li-ion. Besides, V₂O₅/rGO composite microballs delivered initial discharge capacity of 273 mAh g^-1 at 100 mA g^-1, which is greater than those of rGO (78 mAh g^-1) and V₂O₅ particles (214 mAh g-1). Additionally, the composite microballs displayed a better rate capability of 51.3% (compared to 36.4% for V₂O₅ particles) and the capacity retention of 80.4% (two times greater than V₂O₅ particles) Coulombic efficiency of 97.1% over 200 cycles, depicting enhanced cyclic stability. Hence, the unique composite microstructure architecture offers an excellent chemical approach to enhance the electrochemical performance of cathode materials for LIBs.

Various attempts have been made by researchers to synthesise porous carbon-based on natural porous structures of plants for the growth of V₂O₅ nanostructure arrays. However, most of the achieved carbons have irregular microporous structures and rough surfaces, such as bamboo [164],[165], wood [166],[167], and lotus seed-pod shells [168], which are not suitable for the growth of V₂O₅ nanostructures arrays inside them. In combating this challenge, Tan et al. [169] fabricated carambola-like V₂O₅ nanoflowers arrays on the surface of natural microporous reed carbon (PRC) through a convenient high temperature carbonization process and hydrothermal method. The natural oxygen-containing groups formed on the carbon surface permit the direct growth of V₂O₅ nanoflowers arrays on the PRC surface after a one-step hydrothermal process [169]. Owing to the novel composite structures, the as-prepared cathode material delivered an excellent discharge capacity of 273 mAh g^-1 at 0.2 C after 100 cycles and significant capacity retention of 96.2%. The discharged capacity decreased to 180 mAh g^-1 after 500 cycles at 2.0 C, with a capacity decay of 0.025% per cycle, and the average Coulombic efficiency of the battery was about 100% throughout the long-term cycles. Therefore, the as-synthesised V₂O₅ nanoflowers arrays on PRC have demonstrated high-rate capability and excellent cycling stability, thereby qualifying them as high-performance cathode materials for LIBs.

Liang et al. [170] reported V₂O₅/CNTs composites prepared through a facile hydrothermal method. The composites as cathode materials for lithium-ion batteries exhibited improved electrochemical performance compared to electrode materials free of CNTs. The cycling performance of the V₂O₅/CNTs composites at a current density of 100 mA g⁻¹ between 2–4 V is shown in Figure 7a. The V₂O₅-60CNT composites (containing 60-mg functional CNTs) delivered a high initial capacity of 305 mA h g⁻¹, and an excellent discharge capacity of 196.8 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 50 cycles. However, the electrode materials free of CNTs (V2O5) delivered an initial capacity of 263 mA h g⁻¹, and a discharge capacity of 171 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 50 cycles (Figure 7a). The rate capability of V₂O₅/CNTs composites at various current densities within the potential window of 2–4V is depicted in Figure 7b. V₂O₅-60CNT displayed a higher discharge capacity (69.5 mA h g⁻¹) than V₂O₅ (55 mA h g⁻¹) at a rate of 10 C (1000 mA g⁻¹). After the high-rate cycling, V₂O₅-60CNT electrode delivered a discharge capacity of 236 mAh g⁻¹ when the current density was returned to 1 C (100 mA g⁻¹). V₂O₅-30CNT (containing 30-mg functional CNTs) and V₂O₅ delivered capacities of 195 and 156 mAh g⁻¹ under the same conditions, respectively. Thus, the V₂O₅-60CNT electrode exhibited outstanding rate capability and capacity performance. The enhanced electrochemical performance can be attributed to the fact that the CNTs in the V₂O₅/CNTs composites can effectively facilitate ionic diffusion by raising the electrical conductivity.

7.png

Figure 7. (a) Cycling performance of V₂O₅, V₂O₅-30CNT, and V₂O₅-60CNT at a current density of 100 mA g⁻¹ ; (b) rate capability of V₂O₅, V₂O₅-30CNT, and V₂O₅-60CNT at different current densities [170]

2.2.3 Doped vanadium pentoxide thin film

Most of the methods proposed for improving the electrochemical performance of V₂O₅ are usually time- consuming and expensive owing to the demands for the synthesis of complex process [171]. A facile and rational synthetic approach is the doping of V₂O₅ with transition metal ions since the electronic conductivity and cycle stability can be increased by one order of magnitude compared with undoped cathode materials. Hu et al. [172] reported a porous Cu-doped V₂O₅·nH₂O thin film electrode, directly synthesised via low-temperature annealing and simple drop-casting method from V₂O₅/H₂O₂ sol with various concentration copper ion (Cu²⁺). The experimental results confirmed that the mass fraction of Cu²⁺ influenced the morphologies, valence state of vanadium, and the electrochemical performance of Cu- doped V₂O₅·nH₂O thin film. For instance, when the mass fraction of Cu2+ was 1 wt.%, the prepared Cu-doped V₂O₅·nH₂O thin film electrode displayed porous network nanostructure (Figure 8b) in contrast to the homogeneous smooth and dense surface of pure V₂O₅·nH₂O film (Figure 8a). Thus, 1 wt.% Cu-doped V₂O₅·nH₂O thin films displayed greater lithium-ion diffusion coefficients, better cycling stability, excellent electrochemical reversibility, and higher specific capacity than the pure V₂O₅·nH₂O thin film [172]. The Cu-doped V₂O₅·nH₂O thin film cathode delivered a high discharge specific capacity of 344 mAh g⁻¹ at a current density of 250 mA g⁻¹. Excellent cycling stability with only 0.14% per cycle degradation during 104 cycles was obtained even at a high current density of 550 mA g⁻¹. Hence, 1 wt.% Cu-doped V₂O₅·nH₂O thin films hold great potentials as future cathode material for high-performance LIBs.

8.png

Figure 8. (a) SEM images of pure V2O5·nH2O (Cu = 0 wt.%) and (b) Cu-doped V2O5·nH2O (Cu = 1 wt.%) thin films [172]

2.3 Polyanion-type materials

Several researchers have investigated the positive effects of polyanion doping in improving the electrochemical performance and cycling stability of Ni-rich layered LiNi_{1-x- y}Co_xMn_yO₂ cathode materials for LIBs. Generally, ion doping reduces cation mixing and widens the spacing between the lithium and transition metal layers to enhance the electrochemical performance [173]–[175]. In addition, polyanions consist of tetrahedral or octahedral anionic units, which stabilise the crystal structure [176]. For instance, Zhao et al. [177] reported that the gradient polyanion phosphorus doping into Li_1.17Mn_0.5Ni_0.17Co_0.16O₂ enhanced the discharge capacity and cycling stability of the cathode material. The improved electrochemical performance of the doped cathode material can be attributed to the polyanion dopants which stabilised the oxygen close-packed structure and protected the cathode material from corrosion induced by electrolytes [177]. Similarly, Ma et al. [176] proposed that the excellent cycling performance and structural stability are improved by BO33- substitution for O in the Li_1.16(Ni_0.25Mn_0.75)_0.84O₂ system. Moreover, Zhang et al. [178] modified LiNi_0.6Co_0.2Mn_0.2O₂ (NCM) cathode materials by substituting the boracic polyanion to suppress capacity degradation and enhance power performance. The boracic polyanion-doped samples possessed outstanding electrochemical performance, high lithium-ion diffusion coefficient rate, and small charge transfer impedance due to the stabilisation of the crystal structure by the boracic polyanion [178]. The modified NCM cathode materials exhibited superior rate property at high current density (131.02 mAh g^-1 at 5 C) compared to the bare NCM (115.96 mAh g^-1 at 5 C) and higher capacity retention (76.07% after 100 cycles at 1 C) than the bare NCM (59.15% after 100 cycles at 1 C). Also, the cation mixing declined from 4.65% to 3.21%. Therefore, the boracic polyanion doping can significantly improve the rate capabilities and cycling stability of the NCM cathode materials [178].

Xie et al. [179] synthesised and characterised boron-based polyanion-tuned ultrahigh-Ni layered cathodes (Ni content > 90%) for high-energy-density LIBs. The modified cathode materials B₂O_3-LiNi_0.94Co_0.06O₂ (B-NC) delivered a discharge capacity of 223 mAh g^-1 at C/3 with 80% capacity retention after 400 cycles in full cells with graphite anodes, superior to 61% retention for the bare LiNi_0.94Co_0.06O₂ (NC). The improved cyclability was credited to the development of a well-passivated boron/phosphorus-rich cathode-electrolyte interphase in B-NC. Besides, B-NC exhibited a markedly enhanced air exposure and thermal stability owing to the presence of the doped boron polyanion (atomic boron) in its lattice structure which advances the oxygen structure stability. A 30-day air-stored B-NC delivered a discharge capacity of 125 mA h g^-1 at 10 C rate, in sharp contrast to 65 mAh g-1 for the stored NC.

Even though polyanion electrode materials are superior in thermal and structural stability compared to layered basic oxides such as LiCoO₂, Li(Ni,Mn,Co)O₂ and Li(Ni,Co,Al)O₂, their commercialisation is seriously hampered by the low energy density. To overcome this shortcoming, Kim et al. [180] synthesised a novel Li-rich fluorophosphate compound, Li₅V₂PO₄F₈, capable of delivering high energy density that originated from both multi-electron reactions of vanadium and the high voltage induced by fluorine. The novel cathode material possessed a new crystal structure comprising a robust 3-D framework of corner-sharing octahedra VO₂F₄ with tetrahedra PO₄ and 3-D Li-ion diffusion pathways can facilitate the (de)intercalation of Li ions. The outcome of the electrochemical test showed that the as-prepared cathode material delivered the highest redox voltage, ~ 4.4 V (vs. Li/Li⁺), among V³⁺/V⁴⁺ redox reactions with 111 mAh g^-1 of reversible capacity. During the first charge only, the cathode displayed an active redox reaction of the V⁴⁺/V⁵⁺ at ~ 4.9 V (vs. Li/Li⁺) with 228 mAh g-1 of charge capacity. Also, stable capacity retention and good rate capability at both charging and discharging rates up to 2 C rate were observed during the vanadium deficient phase. In addition, vanadium deficient Li₅V₂PO₄F₈ phase exhibited stable capacity retention for 100 cycles at 1 C and fast rate capability of both charging and discharging at 2 C. Thus, the new Li-rich polyanion material, Li₅V₂PO₄F₈ with high voltage and high energy density, is a very promising cathode for LIBs, and further investigation on chemical substitution would undoubtedly enhance its electrochemical performance.

Furthermore, Qiu et al. [181] demonstrated the co-doping of PO₄^3- polyanion and Mn⁴⁺ cation into Ni-rich LiNi_0.80Co_0.15Al_0.05O₂ cathode in enhancing the structural stability and electrochemical performance of the cathode material for LIBs. The Mn⁴⁺ cation and PO₄^3- polyanion co- doped Ni-rich LiNi_0.80Co_0.15Al_0.05O₂ cathode materials were synthesised by high-temperature solid-phase reaction using Ni_0.80Co_0.15Al_0.05(OH)₂ precursor and dopants (Figure 9). The modified materials offered reduced Li⁺/Ni²⁺ cationic mixing, expanded thickness of lithium layered interslab, suppressed structural degradation, enhanced electrochemical reaction kinetic and cycling stability than the pristine cathode. In the cell potential of 2.7 – 4.3 V, the 3% PO₄^3- and Mn⁴⁺ co-doped cathode delivered a reversible discharge capacity of 204 mAh g⁻¹ at 0.1 C, outstanding cycling stability with a capacity of 174 mAh g^-1 and capacity retention of 85.5% at 1 C after 100 cycles, especially, and a superior discharge capacity of 157.8 mAh g-1 at 5 C. Even at an elevated temperature of 55 °C, the cathode retained 80.9% of initial capacity (195 mAh g^-1) at 1 C after 100 cycles. Therefore, owing to the stabilisation role of Mn⁴⁺ and Mn⁴⁺, the modified materials with a moderate number of dopants possessed improved structural stability coupled with enhanced electrochemical performance, especially at elevated temperature and high cut-off cell potential. Moreover, the authors' polyanion and cation co- doping strategy serves as a template for the modification of Ni-rich layered oxides cathode materials.

9.jpg

Figure 9. The schematic illustration for the synthesis of modified material and the corresponding crystal structure [181]

Compared with layered oxides, polyanionic compounds can provide longer cycling life and greater safety due to their stable polyanionic frameworks, characteristics that have helped olivine LiFePO₄ secure great success in the market [182],[183]. However, the lower capacity arising from heavier and larger polyanionic groups is a disadvantage to their use. Therefore, anion engineering is considered essential for designing polyanionic compounds with improved cathode performance. Lu et al. [183] adopted a polyanion substitution strategy to explore a new series of polyanionic solid-solution alluaudite-type cathode materials, Na_xFe_y(PO₄)_3-z(SO4)_z(0 ≤ z ≤ 3), that might exhibit enhanced electrochemical properties. The electrode properties of Na_xFe_y(PO₄)_3-z(SO4)_z(z = 0, 1, 1.5, 2, 3) were measured at a C/50 current rate in the voltage range 2.0 – 4.5 V. The discharge capacity profiles of the polyanionic solid-solution cathodes are shown in Figure 10a. The phosphate Na₂Fe₃(PO₄)₃ delivered a reversible capacity of approximately 80 mAh g^-1 at a current rate of C/50 while  the voltage profile displays several distinct voltage plateaus. However, the voltage profiles become smooth, slopy curves after the substitution of polyanion SO₄ into the structure, signifying single-phase reaction mechanisms. Interestingly, the average voltage increased  markedly from 3.19 (z = 0) to 3.36 (z = 1.5) and, finally, to 3.72 V (z = 3) by SO₄ substitution as illustrated in Figure 10b. The elevated voltage is attributed to the inductive effect of the highly electronegative SO4 polyanion [183],[184]. Moreover, superior capacity can be acquired by the polyanionic solid-solution system after mixing two polyanions (PO₄ and SO₄) in the structure. For instance, Na_xFe_y(PO₄)_3-z(SO4)_zwith z = 1.5 (Na/Fe = 1), delivered a larger reversible capacity (113.2 mAh g^-1) than the two end- members: Na₂Fe₃(PO₄)₃ with z = 0 (82.1 mAh g^-1) or Na_2.56Fe_1.72(SO4)₃ with z = 3 (102.9 mAhg^-1) as depicted in Figure 10c. The poor measured capacity exhibited by Na₂Fe₃(PO₄)₃, extremely lower than its theoretical capacity of 107.4 mAh g^-1, may be ascribed to its larger particle size (0.5 – 2 um) as compared with that (below 200 nm) of the sulphate end member with z = 3, where phosphates tend to crystallise more facilely than sulphates under the identical synthetic conditions. Additionally, the as-synthesised cathode materials exhibited tunable voltage and superior capacities depending on the mixing ratio of the two polyanions. The polyanion substitution strategy reported by the authors can contribute immensely to the development of new cathode materials for both Li-ion and Na-ion batteries.

10.jpg

Figure 10. Electrochemical performance of Na_xFe_y(PO₄)_3-z(SO4)_z(z = 0, 1, 1.5, 2, 3). (a) Discharging profiles cycled between 2.0-4.5 V at current rate C/50. (b) Variance of average discharging voltages. (c) Variance of theoretical and reversible capacities during SO₄ substitution. Coloured areas and the values in brackets represent the reversible capacities [183]