Carbon-based materials for lithium-ion capacitors

Abstract

Lithium-ion capacitors (LICs) can deliver high energy density, large power density and excellent stability since they possess a high-capacity battery-type electrode and a high rate capacitor-type electrode. Recently, great efforts have been devoted to fabricating carbon-based electrodes for LICs, which can effectively enhance their electrochemical performance due to the high electrochemical activity of carbon materials. Here, we present the recent developments of carbon-based materials as electrodes of LICs. Carbon-based battery-type materials, which can be either anodes or cathodes, highlighted here include carbonaceous materials, carbon/transition metal compounds, carbon/polyanion composites, carbon/metalloid and metal materials as anodes and carbon/metal compound composites as cathodes. Apart from battery-type electrodes, carbon-based materials also play an important role in the design of capacitor-type electrodes of LICs, which focus on carbonaceous materials as cathodes. The prospects and challenges in this field are also discussed.

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Zhiqiang Niu

Zhiqiang Niu is a Professor at the College of Chemistry, Nankai University. He received his PhD degree from the Institute of Physics, Chinese Academy of Sciences, in 2010. After his postdoctoral research at the School of Materials Science and Engineering, Nanyang Technological University (Singapore), he started his independent research career at Nankai University in 2014. He has been awarded the National Youth Thousand Talents of China (2015). He has published more than 70 peer-reviewed journal papers. His research interests include nanocarbon materials and advanced energy storage devices.

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1. Introduction

Lithium-ion batteries (LIBs) and supercapacitors (SCs) are considered as the two most promising energy storage systems.^1–4 Typically, LIBs possess high energy density (>150 W h kg⁻¹) but low power density (<1 kW kg⁻¹) and inferior cycling stability (usually <4000 cycles).^5–7 In contrast, SCs can provide large power density (>10 kW kg⁻¹) as well as long cycle life (usually >10⁵ cycles). However, the relatively poor energy density (5–10 W h kg⁻¹) limits their applications in some fields.^8,9 To bridge the gap between LIBs and SCs, lithium-ion capacitors (LICs) that can combine the merits of both LIBs and SCs were developed.^10–14 A LIC device generally employs a high capacity battery-type electrode and a high rate capacitor-type electrode in combination with an appropriate electrolyte.^15,16 During the charge–discharge process, charges are simultaneously and asymmetrically stored in the LIC by surface ion adsorption/desorption on the capacitor-type electrode and Li⁺ intercalation/de-intercalation in the battery-type electrode, respectively.^17,18 In LICs, the capacitor-type electrode can be either the anode or cathode, while the battery-type electrode serves as the counter electrode. It is worth mentioning that the battery-type electrode and capacitor-type electrode perform in different potential windows, which can enlarge the operating voltage range of LICs and lead to high energy density.

The energy and power densities of LICs mainly depend on the design of electrode materials in the devices.^17,19,20 Until now, a variety of materials, such as metal compounds, polyanions and metalloid/metal compounds, have been fabricated for battery-type electrodes in LICs due to their high gravimetric specific capacity and excellent electrochemical activity.²¹ However, the low conductivity, large volume variation and high polarization of these active materials limit their further development in LICs.¹⁶ To solve these problems, carbon materials were often incorporated into the electrodes based on them due to their large specific surface area, high conductivity and outstanding electrolyte accessibility.^22,23 In addition, carbon materials can also serve as the active materials of battery-type electrodes in LICs directly because they possess active sites for Li⁺ intercalation/de-intercalation.^24–26 Different from battery-type electrodes, the counter electrodes of LICs are capacitor-type. As a result, various porous carbon materials with large specific surface area, such as activated carbon (AC), graphene and biomass-derived carbon, are promising candidates for capacitor-type electrodes of LICs.^27,28 Their capacitances mainly depend on the ion adsorption/desorption on the surface of carbon-based electrodes.²⁹ Thus, porous structures with an appropriate pore-size distribution play an important role in the electrochemical performance of carbon materials in LICs.^30,31 To further enhance the capacitance of the capacitor-type electrode, functional groups are usually introduced into the carbonaceous materials to provide pseudo-capacitance.^32,33 As discussed above, progress towards advanced LICs mostly benefits from the continuous development of carbon materials.

LICs were first developed by Amatucci et al. in 2001.³⁴ Since then, the studies on LICs have flourished, as shown in Fig. 1. Although some reviews of energy-storage systems have mentioned the progress in LIC systems,^{3,23,25,35–38} a systematic review focusing on carbon-based materials for LICs has not been conducted. In this review, we will describe the fundamental principle of LICs and discuss the carbon-based battery-type electrode/capacitor-type electrode materials and their renaissance over several decades. Then we highlight the major roles of carbon, either as active materials or as conductive materials, in the applications of LICs by focusing on the most representative examples (Fig. 2). After a brief summary, the prospects and challenges associated with LICs are also discussed, which provide a deep insight into the carbon-based electrodes for future research of LICs.

Fig. 1 Carbon-based LIC-related papers year by year in Web of Science Core Collection, date: 2019 March 16, search strategy: TOPIC: “lithium ion hybrid capacitor*” or ‘‘hybrid supercapacitor*’’ or ‘‘Li ion capacitor*’’ or ‘‘Li ion hybrid capacitor*’’ or ‘‘hybrid battery capacitor*’’ or “lithium ion hybrid capacitor*”. Timespan: 2001–2019.

Fig. 2 Schematic illustration of carbon-based battery-type electrodes and capacitor-type electrodes of LICs.

2. The bases of lithium-ion capacitors

It is noteworthy that the lithium-ion capacitor (LIC) and the lithium-ion battery-type capacitor are collectively called a lithium-ion hybrid capacitor. LICs are electrochemical energy storage devices that combine the advantages of high power density of a supercapacitor and high energy density of a Li-ion battery. Before understanding the basic energy storage mechanism of LICs, it is necessary to identify the components of LICs. As a kind of asymmetric supercapacitor, LICs are usually composed of a battery-type electrode with the insertion/extraction of lithium ions and a capacitor-type electrode with pseudo-capacitance or adsorption/desorption of ions.^37,39,40 Since the battery-type electrode can not only act as an anode, but also serve as a cathode, the LICs can be divided into two types:

(1) the battery-type electrode acts as the anode and the capacitor-type electrode serves as the cathode, such as a Li₄Ti₅O₁₂//AC system. Typically, during the charge process, anions are absorbed on the porous surface or defects of the cathode, while Li⁺ ions are intercalated into the active material of the anode. During discharging, the adsorbed anions are released from the cathode and the Li ions are de-intercalated from the anode, as illustrated in Fig. 3a.^41,42

Fig. 3 The charge storage mechanisms of LICs.

(2) the capacitor-type electrode acts as the anode and the battery-type electrode serves as the cathode, such as an AC//LiFePO₄ system. Typically, during the charge process, Li⁺ de-intercalates from the cathode material and enters the electrolyte. At the same time, Li⁺ in the electrolyte migrates and adsorbs on the anode. The discharge process will be the reverse case, as shown in Fig. 3b.

Significantly, a battery-type electrode often undergoes a faradaic reaction with a relatively constant potential, which will provide a large specific capacitance (Fig. 4).⁴³ When the battery-type electrode works as an anode, it usually has to be pre-lithiated before assembling a LIC, which could maintain the potential of the anode much closer to that of Li (∼0.1 V vs. Li/Li⁺).^44,45 As a result, the working voltage of LICs can increase and even reach up to 4.5 V. The energy density of LICs can be calculated by the following equation:

where V is the working voltage and q is the capacity. According to eqn (1), the energy density is related to their capacitance and the operating voltage.^37,46 Owing to the high capacity and wide working voltage of battery-type electrodes, the energy density of LICs can be greatly improved. However, the faradaic reaction of the battery-type electrode is often accompanied by a phase transformation, which leads to poor rate performance, inferior cycle life and sluggish dynamics.^46–49 To solve these problems, in addition to developing nanoscale-structured electrodes, the introduction of highly conductive carbonaceous additives, such as graphene, carbon nanotubes (CNTs) and AC, is also an effective strategy.^50–53

Fig. 4 Schematic of voltage changes of asymmetrical cells with double-layer and intercalation electrodes and a capacitor-type anode during the charge–discharge process. The electrolytes with salts of LiPF₆ are assumed to be used for asymmetrical cells, respectively. Reproduced with permission.⁴³ Copyright 2009, Elsevier.

Different from the battery-type electrode, a reversible ion adsorption or fast redox reaction occurs at the surface of the capacitor-type electrode, which offers a possibility that LICs possess comparable power density to that of SCs by combining a suitable battery-type electrode with fast kinetics.^54,55 The cycling performance of LICs is also important for their practical application. In addition to the intrinsic behaviour of the battery-type electrode and capacitor-type electrode, the mass ratio between the battery-type electrode and capacitor-type electrode also plays a vital role in the cycle life of LICs. For an asymmetric cell, the charges at both electrodes should be balanced (Q_anode = Q_cathode). The stored charges are related to the specific capacity (C) and mass (m) of the electrode (Q = C × m).^56,57 Therefore, the optimized mass ratio between the battery-type electrode and capacitor-type electrode can be calculated according to the following equation:⁴

where m, C and ΔE are the mass of the electrode, specific capacitance and voltage range in the charge/discharge process for the anode and cathode, respectively.⁵⁸ Nevertheless, the capacitance of the capacitor-type electrode is relatively lower compared with that of the battery-type electrode, such as AC.^28,59 Therefore, the fabrication of carbon materials with high capacity is the primary aim in the design of advanced LICs. Two effective strategies are usually used to enhance the specific capacitance of capacitor-type electrodes: designing the structure of capacitor-type electrodes with large specific surface area and introducing pseudocapacitive materials or heteroatom doping.^60,61 As mentioned above, owing to the good intra- or inter-particle conductivity and outstanding electrolyte accessibility of the intra-pore space, carbon materials, either as active materials or as conductive additives, play irreplaceable roles in the applications of LICs.^38,62–64

3. Carbon-based battery-type electrodes

3.1 Anodes

3.1.1 Carbonaceous materials. Graphite possesses a planar layered structure and its individual layer (called graphene) has a hexagonal mesh structure.^65,66 In graphite, the distance between neighboring graphene is 0.335 nm, which ensures that Li⁺ ions can be inserted in between the layers of graphite.⁶⁷ Owing to the flat and low Li-insertion potential of ∼0.1 V as well as the high theoretical capacity of 372 mA h g⁻¹, graphite is considered to be an ideal anode material for LICs.^68–73 LICs based on graphite film anodes can be optimized for working in a wide voltage range from 1.5 up to 4.5 V, with the corresponding energy density as high as 103.8 W h kg⁻¹.⁷⁰ Apart from conventional film electrodes, graphite can also be designed into interdigital electrodes to achieve micro-LIC devices (Fig. 5a), where AC serves as the cathode.⁷³ Such a micro-LIC delivers a high energy density of ∼4.865 W h cm⁻² at a charge/discharge current of 0.5 mA cm⁻².

Fig. 5 (a) Schematic of an on-chip LIC device with an AC/graphite configuration. Reproduced with permission.⁷³ Copyright 2015, Elsevier. (b) SEM and (c) 3D mapping chemical intensity images of HOG. (d) The corresponding model of transformation kinetics in depth of lithium diffusion. Reproduced with permission.⁷⁸ Copyright 2016, American Chemical Society. (e) Synthesis procedure of 3D carbon/rGO foam. (f) SEM images of neat melamine and composite foam samples. Reproduced with permission.⁸¹ Copyright 2018, Elsevier. (g) Schematic illustration of the fabrication process of BNC (B and N co-doped carbon nanofibers), and (h) corresponding cycle performance of the BNC//BNC LIC at a current density of 2 A g⁻¹. Reproduced with permission.²⁹ Copyright 2017, Wiley-VCH.

Graphene is a single atomic plane of graphite.⁷⁴ Owing to its unique two-dimensional (2D) structure, graphene displays excellent conductivity, high theoretical surface area (2630 m² g⁻¹) and good mechanical strength with a remarkable Young's modulus (∼1.0 TPa).⁷⁵ However, the hydrophobic surface of pristine graphene sheets often leads to agglomeration in solvents. As a result, large-scale preparation of pristine graphene architectures by a solution-based process is not feasible. Therefore, graphene oxide (GO) usually acts as the precursor of graphene to rationally construct reduced graphene oxide (rGO) architectures (e.g., layered films and 3D porous architectures), that can be used as the anodes of LICs.^76–81 For instance, highly oriented rGO (HOG) sponge was obtained by lyophilization and subsequent thermal reduction of GO sponge.⁷⁸ The unique structure of HOG sponge could be beneficial to the fast Li ion transportation as the anodes for LICs (Fig. 5b–d). As a result, the cell based on HOG delivers excellent energy and power densities of 231.7 W h kg⁻¹ and 57 W kg⁻¹, respectively. In addition to the high electrochemical activity, compressible and flexible mechanical properties would also be achieved by rationally preparing the rGO-based foam electrodes (Fig. 5e and f).⁸¹ Furthermore, the conductivity of such foam can be enhanced by N doping that originates from melamine. The LIC devices based on such foam display an energy density of 40 W h kg⁻¹, which can be maintained for 800 charge/discharge cycles. To further improve the conductivity and reduce the aggregation of rGO nanosheets, single-walled carbon nanotubes (SWCNTs) were incorporated into the rGO-based architecture.⁷⁹ The LICs based on a pre-lithiated SWCNTs/rGO composite maximize the operable voltage window of 0.01–4.1 V, thus exhibiting a high energy density of 222 W h kg⁻¹ at a power density of 410 W kg⁻¹.

Unlike graphite and graphene, graphdiyne possesses both sp- and sp²-hybridized carbon atoms. The structure of graphdiyne is similar to the case of graphene but contains extra uniformly arranged 18C cavities, which are beneficial to the storage of Li ions. In addition, graphdiyne can be assembled into bulky architectures with a highly porous nanostructure, which can be directly used as the electrode of LICs. Their unique porous structure results in short transport pathways for ions. Li's group explored bulk graphdiyne powder as an anode material for high performance LICs. Bulk graphdiyne, featuring both micropore and mesopore morphologies, was prepared through a facile cross-coupling reaction. LICs that were constructed with a graphdiyne anode and an AC cathode exhibited higher energy and power densities, as well as excellent cyclic performance.

Compared with graphene, carbon nanotubes (CNTs) possess higher conductivity and could be assembled into freestanding porous films.⁸² Furthermore, functional groups can also be incorporated into CNT-based structures to enhance their performance by the synergistic effects from CNTs and functional groups.^83–85 For instance, a N-doped CNT/amorphous carbon (CNT/C) composite was fabricated by carbonizing a CNT/polyaniline mixture.⁸³ This composite possesses a mesoporous CNT architecture acting as the backbone and a thin N-rich amorphous carbon layer is coated on the CNT surface. Owing to such a unique structure, it delivers a high rate capability of 81 mA h g⁻¹ at a high charge/discharge rate of 60C (1C = 1 h charge/discharge).

Biomass-derived carbon is usually prepared by thermally treating agricultural and forest biomass at high temperature. It is a kind of friendly electrode material due to its pollution free, large specific surface area as well as high electrochemical activity.^29,86–89 For example, sisal fibers were utilized to prepare graphitic carbon through the pre-carbonization and catalytic graphitization process.⁸⁹ The graphitic structure of this graphitic carbon contributes to high plateau capacity and electrical conductivity, and the porous structure supplies a smoother pathway for Li⁺ ion diffusion and protects the graphitic structure from etching by the electrolyte. Therefore, LICs based on graphitic carbon display a high energy density of 104 W h kg⁻¹ at a power density of 143 W kg⁻¹. Furthermore, heteroatom doping, which usually comes from the precursors, is also used to enhance the performance of biomass-derived carbon.²⁹ B and N co-doped carbon nanofibers (BNC) can serve as both the anodes and cathodes of LICs due to their superior electrochemical activity in a wide voltage range (Fig. 5g). Therefore, the resultant “dual carbon” BNC//BNC LIC delivers outstanding energy (220 W h kg⁻¹) and power densities (22.5 kW kg⁻¹, Fig. 5h).

3.1.2 Carbon/transition metal compounds. Traditional transition metal oxides, including Li₄Ti₅O₁₂, TiO₂, Nb₂O₅, Fe₂O₃, SnO₂ and so on, have been extensively explored as insertion/conversion-type anodes of LICs due to their abundant reserves in the Earth's crust and high gravimetric specific capacity.^90–94 However, most of the transition metal oxides, such as Fe₂O₃ and SnO₂, usually undergo a phase transformation during the charge/discharge process, which is demonstrated by the plateaus in the galvanostatic charge–discharge curves and sharp peaks in the CV profiles. The phase transformations will result in large volume expansion, leading to a negative effect on the integrity of electrodes, and thus the cycling performance of LICs will be poor. Phase transformation generally involves nucleation and growth processes, which directly lead to sluggish kinetics for battery-type electrodes. Different from the above mentioned materials, the structure of TiO₂ and Li₄Ti₅O₁₂ is more stable with negligible strain during reversible lithium intercalation/extraction. Specifically, Li₄Ti₅O₁₂ is a relatively rare zero-strain material, which has a volume change of less than 1% during the process of charge and discharge. Unfortunately, their low conductivity, which directly leads to high polarization, remains a short slab for quick charge and discharge of LICs.^95–101 The incorporation of these transition metal oxides into porous carbon materials to achieve hybrid architectures would overcome or relieve the above issues.^102–108 The hybrid architectures could combine the exceptional merits of nanostructured transition metal oxides with those of carbon-based materials.^109–112 The nanosized structure of transition metal oxides provides a short ion diffusion pathway as well as large interfacial surface and the carbon materials offer a fast transmission route for electrons.

CNTs is one of the most common carbon materials used for supporting transition metal oxides. Moreover, CNTs are easily interconnected into a 3D conductive network, thus presenting good conductivity and flexibility.^{101,112,114–116} For example, nanosized TiO₂ (B)/MWCNTs (multi-walled carbon nanotubes) composite films, in which TiO₂ (B) nanocrystals were uniformly dispersed on a MWCNT matrix, were fabricated by an ultracentrifugation process followed by a hydrothermal treatment.¹⁰⁷ Such a structure can effectively avoid the aggregation of TiO₂ nanoparticles and enhance the power capability by enabling ultrafast Li⁺ de/intercalation ability (235 mA h g⁻¹ at 100.5 A g⁻¹) (Fig. 6a and b). Moreover, a RuO₂/MWCNTs nanocomposite, where nanosized RuO₂ was highly dispersed on MWCNTs, was also confirmed to be a promising anode material for LICs.¹¹⁷ Furthermore, the classical 1D structure of CNTs endows the composite films with the ability of acting as flexible binder-free anodes of LICs. Freestanding and flexible CNTs/transition metal oxide composite films could be prepared by different methods.^96,109,118 For instance, a flexible a-Fe₂O₃/MWCNTs composite was prepared by a scalable spray deposition technique.¹¹⁸ Based on such a composite, the LIC device delivers an outstanding energy density of 50 W h kg⁻¹ at a power density of 1000 W kg⁻¹. This also demonstrates that the incorporation of CNTs into the transition metal oxide-based anode could lead to a decrease of internal resistance and enhancement of ion diffusion behavior of LICs.

Fig. 6 (a) SEM and HRTEM images of the (uc)-TiO₂(B)/MWCNT (70/30) composite. (b) The rate capability of S-TiO₂ (B) (red) and L-TiO₂ (B) (green). Reproduced with permission.¹⁰⁷ Copyright 2016, Wiley-VCH. (c) Schematic illustration of the fabrication of the G-Li₄Ti₅O₁₂ nanocomposite and (d) Ragone plots for the LIC devices. Reproduced with permission.¹⁰² Copyright 2013, Springer. (e) The fabrication of graphene wrapped iron oxide (CG@SF) under HI/HCl treatment. (f) The LIC based on a CG@SF anode and a fully etched highly CG cathode. (g) Specific capacitance and rate capability of the CG@SF//CG hybrid full cell. Reproduced with permission.⁹⁴ Copyright 2016, Wiley-VCH. (h) TEM image of the porous TiO₂ hollow microspheres wrapped with graphene nanosheets. Reproduced with permission.¹¹³ Copyright 2016, Wiley-VCH. (i) SEM image of the Li₄Ti₅O₁₂/carbon-textile electrode (inset: a digital photograph of the flexible Li₄Ti₅O₁₂/CT electrode). (j) Stability of the flexible LIC measured at 0.5 A g⁻¹ under different bending conditions (insets: digital photographs of the fully packaged LIC under different bending states). Reproduced with permission.¹⁰⁹ Copyright 2017, Springer.

Compared with CNTs, graphene possesses higher surface area, as a result, the loading of transition metal oxides on graphene will be increased.^119,120 Therefore, various graphene/transition metal oxide composite architectures were designed by different strategies.¹²¹ For instance, Li₄Ti₅O₁₂ nanoparticles were uniformly coated by graphene sheets through a solvothermal method.¹⁰² The assembled LIC based on this graphene/Li₄Ti₅O₁₂ anode displays an ultrahigh energy density of 95 W h kg⁻¹ at 0.4C (2.5 h, Fig. 6c and d). Different from the insertion reaction of Ti-based oxides (Table 1) and the nearly zero-strain of Li₄Ti₅O₁₂, the conversion reaction of other oxides (Nb₂O₅, Fe₂O₃, SnO₂) would display a faster decrease in capacity due to the severe volume changes during the charge/discharge process. Thus the fabrication of integrated graphene/metal oxide composite electrodes for LIC is highly desirable (Table 2).^91,122,123 For instance, a Nb₂O₅/graphene composite with an orthorhombic phase, which presents outstanding pseudocapacitive behavior, was also synthesized by a simple method and further served as the anodes of LICs.^122,123 Furthermore, crumpled graphene (CG) coated with spiky Fe₂O₃ nanoparticles (CG@SF) was prepared by combining etching and reduction processes (Fig. 6e and f).⁹⁴ Since the porous particles tightly attach onto the crumpled graphene, the CG@SF offers low contact resistance and structural stability during the repetitive charge/discharge processes. Therefore, the LIC devices based on CG@SF anodes display a maximum energy density of up to 121 W h kg⁻¹ (Fig. 6g).

Table 1 A summary of the performance of carbon/Ti-based composite anodes of LIHCs

Device configuration (anode//cathode)	Voltage range	Cycling stability	Maximum energy density	Maximum power density	Ref.
TiO2@mesoporous carbon//AC	0–3.0 V	80.5% after 10 000 cycles	67.4 W h kg^−1	5000 W kg^−1	108
TiO2 hollow spheres@graphene//graphene	0–3.0 V	65% after 1000 cycles	72 W h kg^−1	2000 W kg^−1	113
TiO2 nanobelt//graphene hydrogels	0–3.8 V	73% after 600 cycles	82 W h kg^−1	19 000 W kg^−1	126
TiO2@rGO//AC	1.0–3.0 V	—	42 W h kg^−1	8000 W kg^−1	90
H-TiO2/PPy/SWCNTs//AC	1.0–3.0 V	∼77.8% after 3000 cycles	31.3 W h kg^−1	4000 W kg^−1	112
TiO2-CNT//activated carbon	1.0–3.0 V	∼	59.6 W h kg^−1	13 900 W kg^−1	127
Li4Ti5O12/CNT//AC	1.5–2.8 V	92% after 6000 cycles	84.2 W h kg^−1	12 652.5 W kg^−1	128
Li4Ti5O12 nanowires//AC	0–2.5 V	84% after 1000 cycles	18.44 μW h cm^−2	—	106
100-Li4Ti5O12-G-600C//AC	1.5–3.0 V	97% after 2000 cycles	52 W h kg^−1	14 400 W kg^−1	129
Li4Ti5O12/CT//NGC	1.0–3.0 V	79% after 5000 cycles	2 mW h cm^−3	185 mW cm^−3	109
Li4Ti5O12–graphene//AC	1–2.5 V	—	30 W h kg^−1	1000 W kg^−1	104
TiO2-coated Li4Ti5O12//AC	0.5–2.5 V	83% after 5000 cycles	74.85 W h kg^−1	7500 W kg^−1	130
Graphene-wrapped Li4Ti5O12//AC	1.0–2.5 V	75% after 1000 cycles	50 W h kg^−1	2500 W kg^−1	131
Spheres Li4Ti5O12//AC	1.0–3.5 V	93% after 500 cycles	74.3 W h kg^−1	468.7 W kg^−1	132
Li4Ti5O12//N-doped porous carbon	1.0–3.0 V	—	63 W h kg^−1	5000 W kg^−1	133
Graphene-Li4Ti5O12//graphene-sucrose	0–3.0 V	94% after 500 cycles	95 W h kg^−1	3000 W kg^−1	102

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Table 2 A summary of the performance of carbon/transition metal oxide-based LIHCs

Device configuration (anode//cathode)	Voltage range	Cycling stability	Maximum energy density	Maximum power density	Ref.
Nb2O5 film//AC	1.0–3.5 V	87% after 1000 cycles	95.55 W h kg^−1	5350.9 W kg^−1	93
Nb2O5-carbide-derived carbon//AC	1.0–2.8 V	—	30 W h kg^−1	5000 W kg^−1	144
T-Nb2O5@C//AC	1.0–3.5 V	75% after 1000 cycles	63 W h kg^−1	6500 W kg^−1	145
Mesoporous Nb2O5–C//AC	1.0–3.5 V	—	74 W h kg^−1	12 137 W kg^−1	146
T-Nb2O5–graphene//AC	0.8–3 V	93% after 2000 cycles	47 W h kg^−1	18 000 W kg^−1	147
Nb2O5–CNT//AC	0.5–3 V	—	33.5 W h kg^−1	4000 W kg^−1	148
Graphene wrapped Fe2O3//graphene	1.0–4.0	87% after 2000 cycles	121 W h kg^−1	18 000 W kg^−1	94
Fe2O3@C//N-HPC	1.0–4.0 V	84.1% after 1000 cycles	65 W h kg^−1	9200 W kg^−1	149
FexO@graphene//porous graphene	0–3.5 V	75% after 3000 cycles	129.6 W h kg^−1	3500 W kg^−1	150
Fe2O3//AC	0–3.5 V	55% after 2500 cycles	90 W h kg^−1	—	151
Fe3O4 in graphene//3D graphene	1.0–4.0 V	70% after 1000 cycles	204 W h kg^−1	2650 W kg^−1	152
AC:SnO2/Cu/CNT//AC	—	81% after 200 cycles	90 W h kg^−1	—	116

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Significantly, the composites of carbon and metal oxides can not only fabricate traditional-shaped LICs, but also construct fiber-shaped LICs. For example, lightweight and portable fiber-shaped LICs were designed by using Li₄Ti₅O₁₂ as anodes and AC as cathodes.¹⁰⁶ This research would give more suggestions to develop fiber-shaped energy storage devices with high electrochemical performance. To further improve the safety of LICs, quasi-solid-state LICs based on TiO₂@graphene anodes were also fabricated (Fig. 6h).¹¹³ Such quasi-solid-state LICs can achieve a maximum energy density of 72 W h kg⁻¹ based on the total mass of electrode materials. Furthermore, graphene encapsulated Fe₃O₄ cube composite films (rGO@Fe₃O₄) were also fabricated as the anodes of flexible quasi-solid-state LICs.¹³⁴ In this composite, ultrathin and flexible graphene shells uniformly enwrapped the Fe₃O₄ nanocube, which could effectively accommodate the volume variation of Fe₃O₄ and also suppress the aggregation of Fe₃O₄ during the cycling process. In addition to CNTs or graphene, there are still many carbon materials that can be used as conductive/supportive agents, such as AC,¹³⁵ PPy,¹¹² CMK-3,¹³⁶ carbon nanofibers,⁹⁶ carbon cloth,⁹³ and so on. For example, Li₄Ti₅O₁₂ nanoplates were coated on carbon textile (LTO/CT) to serve as the anode of flexible LICs.¹⁰⁹ Owing to the excellent mechanical strength of LTO/CT composite films (Fig. 6i), the flexible LICs present extremely small capacity fluctuation under different bending states (Fig. 6j). Recently, Jiang et al. found that Li₄Ti₅O₁₂/AC composite anodes present increased capacity retention as the AC content increases in the composite even though their overall specific capacity decreases gradually.¹³⁵ These results demonstrate that it is of great significance to improve the electrochemical properties and mechanical strength of metal oxide-based anodes by incorporating carbon materials into the composite anodes.

Compared to transition metal oxides, transition metal carbonitride has excellent chemical stability, good electrical conductivity and outstanding oxidation/corrosion resistance (Table 3).^137–139 Owing to the high theoretical specific capacity (∼1043 mA h g⁻¹) and obvious pseudocapacitive characteristics, VN is a popular candidate for anodes of LICs.^124,140 Nevertheless, pure VN nanowires deliver only a capacity of 400 mA h g⁻¹ at 0.1 A g⁻¹ and thus their charge storage ability has to be improved for fabricating LICs with high performance. Fortunately, creating a 3D porous structure that combines VN nanowires with graphene nanosheets could effectively enhance the charge-storage ability because of the formation of abundant pore channels (Fig. 7a and b).¹²⁴ As a result, VN–RGO//carbon-based LIC devices exhibit a high energy density of ∼162 W h kg⁻¹ at a power density of 200 W kg⁻¹ as well as a high power density of 10 kW kg⁻¹ at an energy density of ∼64 W h kg⁻¹ (Fig. 7c). Similarly, graphene can also be added to a NbN-based electrode to further enhance the charge storage ability of Li⁺ ions in NbN. A NbN/N-doped graphene nanocomposite was controllably prepared by combining a simple hydrothermal process with ammonia annealing (Fig. 7d).¹²⁵ The fabricated nanocomposite delivers a high energy density of 122.7–98.4 W h kg⁻¹ at 100–2000 W kg⁻¹ (Fig. 7e and f). The enhanced electrochemical performance is mainly due to the size of the as-obtained NbN nanoparticles of about 10–15 nm and the addition of graphene nanosheets, whose structure could improve the utilization of active materials and relieve the aggregation of nanoparticles. Importantly, owing to the flexibility of graphene, a freestanding layer-stacked composite architecture, integrating 0D NbN nanoparticles with 2D graphene nanosheets (GNS), was also fabricated.¹⁵⁴ Based on the free-standing NbN/GNSs anode, the LIC delivers outstanding energy (136 W h kg⁻¹) and power (25 kW kg⁻¹) densities in the voltage range of 1.0–4.0 V. Similar to nitrides, carbides, such as TiC, also have excellent chemical stability and good electrical conductivity. Fan's group rationally designed a TiC nanoparticle chain anode by a carbothermal conversion of graphene/TiO₂ hybrid aerogels with a 3D conductive network.¹³⁸ Owing to the unique structure of the TiC electrode, the assembled PHPNC//TiC LIC shows excellent potential for bridging-the-gap between conventional LIBs and SCs.

Table 3 A summary of the performance of carbon/transition metal carbonitride-based LIHCs

Device configuration (anode//cathode)	Voltage range	Cycling stability	Maximum energy density	Maximum power density	Ref.
CTAB–Sn(IV)@Ti3C2//AC	1.0–4.0	71% after 4000 cycles	239.50 W h kg^−1	10 800 W kg^−1	139
TiC//N-doped porous carbon	0–4.5 V	82% after 5000 cycles	101.5 W h kg^−1	67 500 W kg^−1	138
VN–rGO//activated carbon	0–4 V	83% after 1000 cycles	162 W h kg^−1	10 000 W kg^−1	124
NbN/nitrogen-doped graphene//AC	2.0–4.0 V	81% after 1000 cycles	122.7 W h kg^−1	2000 W kg^−1	125
NbN//PANI-derived carbon	0–4 V	95% after 15 000 cycles	149 W h kg^−1	45 000 W kg^−1	153

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Fig. 7 (a) Schematic of the fabrication of a 3D VN–RGO composite. (b) TEM, HRTEM and selected area electron diffraction (SAED) patterns of the 3D VN–RGO composite. (c) Cycle stability for 1000 cycles at a current density of 2 A g⁻¹. Reproduced with permission.¹²⁴ Copyright 2015, Wiley-VCH. (d) Low-magnification TEM image of the NbN/NG-50% hybrid material. (e) Cycle performance of the NbN/NG-50% based LIC at 500 mA g⁻¹, and (f) Ragone plot of NG and NbN/NG-X based LICs (X = 75%, 50%, 25%). Reproduced with permission.¹²⁵ Copyright 2015, Wiley-VCH.

3.1.3 Carbon/polyanion composites. Li₃V₂(PO₄)₃, which is constructed with corner-sharing VO₆ octahedra and PO₄ tetrahedra, usually crystallizes in a monoclinic structure. The relatively large void space of such crystals allows various lithium sites with fast diffusion and reaction kinetics. Since Li₃V₂(PO₄)₃ material is an amphoteric oxide that can be either reduced via lithium insertion or oxidized via lithium removal, the electrochemical performance of both low (1.0–3.0 V) and high (3.0–4.3 V) potential applications for Li₃V₂(PO₄)₃ was investigated.¹⁴¹ Based on this, two types of LIC devices (LVP-C//AC and AC//LVP-C) were constructed (Fig. 8a). Fig. 8b reveals that the LVP-C//AC and AC//LVP-C devices deliver maximum energy densities of ∼27 and ∼25 W h kg⁻¹, respectively. The LVP-C//AC device performs better than AC/LVP-C at similar power ratings, which provides a further direction in improving the energy density of LICs. Apart from carbon addition, doping of transition metal (Ni) ions is also an effective route to improve the conductivity of Li₃V₂(PO₄)₃. For example, Li₃V_1.95Ni_0.05(PO₄)₃/C was fabricated by a carbon-thermal reduction method.¹⁵⁵ Different from Li₃V₂(PO₄)₃, LiTi₂(PO₄)₃ possesses a NASICON-type framework structure, which is built of PO₄ tetrahedra linked by the corners of TiO₆ octahedral units. Each PO₄ tetrahedron is connected with four TiO₆ octahedral units and in turn a TiO₆ unit is connected with six PO₄ tetrahedra, thus allowing numerous ionic substitutions at different lattice sites. A Pechini type polymerizable decomposition strategy was employed to prepare LiTi₂(PO₄)₃ at 1000 °C in air. After further coating with a carbon layer through ball milling, carbon-coated LiTi₂(PO₄)₃ nanoparticles (80–85 nm) were achieved.¹⁵⁶ A two-phase reaction mechanism during Li-insertion at 2.38 V and extraction at 2.60 V was observed by cyclic voltammetry measurement. As a result, the LICs based on carbon-coated LiTi₂(PO₄)₃ anodes exhibit ultrahigh energy and power densities of 14 W h kg⁻¹ and 180 W kg⁻¹, respectively.

Fig. 8 (a) Schematic representation of the charge storage mechanism of LICs based on LVP-C/AC and AC/LVP-C. (b) Ragone plots showing the performance of LVP-C/AC and AC/LVP-C. Reproduced with permission.¹⁴¹ Copyright 2015, Elsevier. (c and d) SEM images of the TiNb₂O₇ MWs. (e) Ragone plots of the TNO@C//CFs LIC, CFs//CFs and CMK-3//CMK-3 symmetric SCs. Reproduced with permission.¹⁴² Copyright 2015, Elsevier. (f) Morphology of the Sn@NRT (N-rich nanotube) and the NRT structure with illustration showing the ion penetration process. (g) The rate capability of the Sn@NRT||NRT LIC at different current densities. Reproduced with permission.¹⁴³ Copyright 2017, Wiley-VCH.

TiNb₂O₇ can act as an alternative candidate for Li₄Ti₅O₁₂ as the anodes of LICs. The monoclinic crystal structure of TiNb₂O₇ is formed by disordered Nb and Ti atoms, which could provide two dimensional interstitial spaces for fast Li ion insertion. TiNb₂O₇@C microwires were synthesized by an electrospinning method, followed by carbon coating at high temperature (Fig. 8c and d).¹⁴² The as-fabricated LICs based on TiNb₂O₇@C//CFs exhibit an energy density of 110.4 W h kg⁻¹ at 99.58 W kg⁻¹ (Fig. 8e). Furthermore, a TiNb₂O₇/holey graphene (TNO/HG) nanocomposite with superior rate capability and long-term cycling performance was prepared through in situ anchoring of a TiNb₂O₇ network nanostructure onto 2D holey graphene.¹⁵⁷ Importantly, the Li insertion behavior of the TiNb₂O₇@C electrode was studied in depth and the intercalation pseudo-capacitive reaction mechanism was achieved.

In addition to the above mentioned materials, nano-TiP₂O₇,¹⁵⁸ NASICON-type LiTi_1.5Zr_0.5(PO₄)¹⁵⁹ and LiSn₂(PO₄)₃¹⁶⁰ were also reported for application in LICs. Because they were not composites with carbon, we cannot review those materials in detail.

3.1.4 Carbon/metalloid and metal materials. Because of its low lithiation potential (<0.5 V) and high theoretical specific capacity (4200 mA h g⁻¹), Si is also a promising substance for the anodes of high-performance LICs.^161,162 Nevertheless, its large volume expansion (>300%) that would cause pulverization is very serious. Moreover, owing to its intrinsic semi-conductive nature, the low conductivity could also limit its charge/discharge efficiency at a high current density.^163–165 The incorporation of Si into carbon materials to form hybrid composites would relieve the volume variation of Si, build a stable electrode–electrolyte interface and enhance the conductivity and mass diffusion simultaneously. For example, a porous Si@C ball-in-ball hollow structure was designed, in which porous Si hollow yolks were fully encapsulated in carbon shells with well-defined internal void spaces.¹⁶⁶ Owing to the unique structure, the LICs based on Si@C anodes show a stable cycling performance for 15 000 cycles at 6.4 A g⁻¹. Besides, heteroatom B-doping is also a facile way to modify Si electrodes. For instance, B-Si/SiO₂/C used as the anode of LICs delivers a high energy density of 128 W h kg⁻¹ at 1229 W kg⁻¹ and a high power density of 9704 W kg⁻¹ at 89 W h kg⁻¹.¹⁶⁵ To further improve the safety of LICs, a solid-state LIC was fabricated using a Si-VACNFs (vertically aligned carbon nanofibers) anode in conjunction with a flexible, solid-like gel polymer film as the electrolyte and separator.¹⁶⁷ Due to the synergistic effect of Si-VACNFs and gel polymer, the LIC could stably operate in the temperature range from −20 to 60 °C with a very low self-discharge rate and an excellent shelf life.

Compared to Si, Sn has a superior electronic conductivity, leading to the fact that Sn anodes would display a better rate capability. Nevertheless, Sn also suffers from a large volume change during the charge/discharge process, which can result in severe polarization of the electrode material.^168–170 Currently, the design of small Sn nanoparticles with a coupling carbon substrate is an effective approach to overcome the above issues. For example, a Sn-C nanocomposite was synthesized by a facile confined growth method,¹⁷¹ and the composite possessed multiple structural advantages including well-dispersed ultrafine Sn nanoparticles, an interconnected spherical network and a large specific surface area. As a result, the LICs exhibit high power densities of 731.25 W kg⁻¹ and 24 375 W kg⁻¹ at energy densities of 195.7 W h kg⁻¹ and 84.6 W h kg⁻¹, respectively. To further motivate the ion diffusion kinetics and accommodate Sn nanoparticle aggregation, Kang's group¹⁴³ fabricated an Sn@NRT (N-rich nanotube) anode with open mesoporous channels, where the ultrasmall Sn particles (∼3.8 nm) were embedded in the 1D CNTs (Fig. 8f). Indeed, the LIC device based on the Sn@NRT||NRT anode was found to exhibit a high energy density of 274 W h kg⁻¹ at 153 W kg⁻¹ and a high power density of 22 800 W kg⁻¹ at 127 W h kg⁻¹ (Fig. 8g).

3.2 Pre-lithiation

Pre-lithiation is usually used in a system with neither electrode containing lithium atoms.⁵⁴ Such a procedure is an effective strategy to reduce the potential of the anode to bring it close to that of Li/Li⁺ and to compensate for the ion loss during solid electrolyte interface/cathode electrolyte interface layer formation. The former one can broaden the working voltage and improve the energy density of LICs, while the latter one can avoid the irreversible capacity loss and improve the cycling stability of LICs.^45,172 Significantly, pre-lithiation is also a popular technique to increase the Li⁺ concentration in the electrolyte.

The most popular method for pre-lithiation is an electrochemical strategy. Similar to the assembly method of a half-cell, such a method through the direct electrochemical charge/discharge process achieves Li⁺ ion doping in the anode material.¹⁷³ The primary merit of such an approach is the good controllability of applying a programmable procedure. Nevertheless, the assembly/disassembly cell process could limit its large scale application in LICs.⁵⁰ Compared to the electrochemical strategy, a chemical charging method, also called an external short circuit, is a more scalable approach via a chemical reaction between the lithium metal and the electrode material in the Li-containing electrolyte.^47,174 However, the amount of Li ions embedded in electrode material cannot be understand clearly during the pre-lithiation process, since it is impossible to control or monitor the state of electric charge.

Besides, direct contact between the active material and metal Li is also used to realize Li⁺ doping.^44,46 Park et al. studied the lithiation process of graphite anodes via the direct contact method using in situ synchrotron wide-angle X-ray scattering for LICs.⁷¹ But the uncertainty of the amount of pre-embedded lithium is still a challenge for the direct contact method. Although pre-lithiation has certain merits while applying to LIC systems, some of the issues caused by pre-lithiation cannot be ignored.⁴⁶ For instance, SEI formation results in an increase in the internal resistance, thus hindering the attainment of a higher power density.

3.3 Cathodes

3.3.1 Carbon/metal compound composites. Owing to the low cost, high theoretical capacity of 170 mA h g⁻¹ and excellent electrochemical and thermal stabilities, LiFePO₄ is a promising battery-type electrode for LICs. During the lithium insertion/extraction process, LiFePO₄ undergoes a two-phase reaction and presents a flat plateau at 3.4 V. However, the poor diffusion kinetics of Li⁺ together with the poor electronic conductivity of pristine olivine-LiFePO₄ (10⁻¹⁰ to 10⁻⁷ Ω⁻¹ cm⁻¹) limit the power capability of such electrodes. Coating carbon on their surface would enhance the power density.^175–179 For example, highly dispersed LiFePO₄ nanoparticles embedded in hollow structured graphitic carbon were synthesized using an ultracentrifugation method.¹⁷⁹ The resultant architecture had double cores that included crystalline LiFePO₄ (core 1) covered by amorphous LiFePO₄ (core 2), which were encapsulated in graphitic carbon (shell). These core–shell LiFePO₄ nanocomposites showed enhanced diffusion of Li ions due to the presence of the amorphous LiFePO₄ phase. Moreover, nano-LiFePO₄ particles embedded in a porous carbon matrix as battery-type cathodes of LICs were also prepared by combining a sol–gel procedure with a solid-state-reaction.¹⁸⁰ Because of the good conductivity of the carbon matrix and porous structure of the composite, the LICs display superior rate properties. Similar to LiFePO₄, Li₃V₂(PO₄)₃ was also combined with carbon to overcome its low intrinsic electronic conductivity.^181,182 For instance, Li₃V₂(PO₄)₃ nanoparticles coated with a ∼8 nm thin carbon layer were fabricated through a carbon thermal reduction method.¹⁸³ With Li₃V₂(PO₄)₃/C as the cathode and AC as the anode, the fabricated LICs deliver an energy density of 35 W h kg⁻¹ at a power density of 102 W kg⁻¹. It is noted that when battery-type electrodes were used as the cathodes of LICs, capacitor-type electrodes would serve as the anodes of LICs.

4. Carbon-based capacitor-type electrodes

4.1 Cathodes

4.1.1 Carbonaceous materials. AC was a dominating cathode material in the early research of LICs based on the energy-storage mechanism of surface adsorption, since it exhibits high surface area (∼3000 m² g⁻¹), excellent conductivity (∼60 S m⁻¹) and good chemical stability. The energy storage ability of an AC cathode often delivers a capacity of about 50 mA h g⁻¹. However, it is relatively lower compared with anode materials. For charge-balance, the mass loading of the cathode is usually two to four times that of the anode according to the specific capacitances of the cathode and anode. Amatucci's group fabricated a LIC device by using AC as the cathode and nanostructured Li₄Ti₅O₁₂ (LTO) as the anode, which was the first application of AC in LICs.³⁴ Following this work, a series of LICs based on AC cathodes were reported to optimize their electrochemical properties. The voltage window of LICs based on AC cathodes in an organic electrolyte was optimized in the range of 1.5–4.5 V for guaranteeing high energy density and maintaining long cycle stability of LICs.⁷⁰ To solve the imbalances of electrochemical capacity and kinetics between the capacitive cathode and Li⁺ storage anode of LICs, a full carbon-based LIC that employed N-rich mesoporous carbon spheres (ANCS) as both cathode and anode was developed (Table 4).³⁰ The preparation of ANCS was realized by an aerosol-spaying process, which could control the pore structure and the degree of N doping simultaneously (Fig. 9a). High-level N-doping leads to more capacitive charge-storage contributions to both cathode and anode, obtaining high energy and power densities (Fig. 9b). In most cases, AC is only a counter electrode to measure the electrochemical performance of anode materials in LICs, such as LiCrTiO₄//AC,¹⁸⁵ TiP₂O₇//AC,¹⁵⁸ Li₄Ti₅O₁₂//AC,¹⁰⁶ Fe₃O₄-G//AC,¹⁸⁶ graphdiyne//AC,¹⁸⁷ LiNi_0.5Mn_1.5O₄//AC¹⁸⁸ and so on.

Table 4 A summary of the performance of carbonaceous material-based LIHCs

Device configuration (anode//cathode)	Voltage range	Cycling stability	Maximum energy density	Maximum power density	Ref.
ANCS//ANCS	0–4.5 V	88% after 9000 cycles	206.7 W h kg^−1	22 500 W kg^−1	30
N-Doped carbon nanopipes//reduced graphene oxides	0–4.0 V	91% after 4000 cycles	262 W h kg^−1	9000 W kg^−1	189
BNC//BNC	0–4.5 V	81% after 5000 cycles	220 W h kg^−1	22 500 W kg^−1	29
Hard carbon//bio-derived mesoporous carbon	1.7–4.2 V	81% after 8000 cycles	121 W h kg^−1	9000 W kg^−1	88
Graphdiyne//AC	2.0–4.0 V	94.7% after 1000 cycles	110.7 W h kg^−1	1000.35 W kg^−1	187
Graphene//armored graphene	0–4.3 V	89% after 1000 cycles	160 W h kg^−1	19 000 W kg^−1	190
Oriented rGO sponge//AC	1.5–4.0 V	84.2% after 1000 cycles	231.7 W h kg^−1	2800 W kg^−1	78
Reduced GO//resin-derived carbon combined with GO	0–4.0 V	79% after 3000 cycles	148.3 W h kg^−1	6500 W kg^−1	77
Microcrystalline graphite//mesoporous carbon nanospheres/graphene	2.2–4.2 V	93% after 4000 cycles	80 W h kg^−1	11 600 W kg^−1	191
N-Doped hard carbon//activated carbon	2.0–4.0 V	97% after 5000 cycles	28.5 W h kg^−1	6940 W kg^−1	192
Graphene//activated carbon	2.0–4.0 V	74% after 300 cycles	95 W h kg^−1	222.2 W kg^−1	193
Graphite//graphene	2.0–4.0 V	97% after 3500 cycles	135 W h kg^−1	1500 W kg^−1	194
Soft carbon//activated carbon	0–4.4 V	63% after 15 000 cycles	115 W h kg^−1	15 000 W kg^−1	195

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Fig. 9 (a) Schematic illustration of the ANCS formation process. (b) Ragone plots of a LIC based on the bifunctional ANCS electrode. Reproduced with permission.³⁰ Copyright 2018, American Chemical Society. (c) The fabrication process of the G@HMMC material. (d) The SEM and TEM images of the G@HMMC850 material. (e) Long cycling performance of a LIC composed of G@HMMC850 and pre-lithiated graphite. Reproduced with permission.¹⁸⁴ Copyright 2018, Elsevier.

Owing to the unsatisfactory specific capacitance of commercial AC, many researchers have shifted attention to various biomass-derived AC materials recently. Biomass-derived carbon can exhibit nanotubular, nanofibrillar, and lamellar structures. Importantly, the hierarchical pore structure of biomass-derived carbon is easy to achieve during the high-temperature calcination process, which is beneficial to electrolyte infiltration. The advantages of the controllable microstructure and tunable surface functional groups make the biomass-derived AC materials more attractive than commercial AC for LICs.⁵⁹ For instance, the AC derived from egg white shows a high surface area of 3250 m² g⁻¹ with micropore size from 0.8 to 1.4 nm, which contributes to the high capacitance and excellent rate performance in LICs.⁸⁶ As a result, the optimized LICs based on this AC cathode deliver a high energy density of 257 W h kg⁻¹ at 867 W kg⁻¹. Recently, hierarchical porous carbon with a larger specific surface area of 3898 m² g⁻¹ was rationally synthesized by using egg white biomass as a precursor and NaCl as a template, and was then used as the cathodes of LICs.¹⁹⁶ In general, owing to the complexity of components, biomass-derived carbon is usually doped with heteroatoms, such as S, N, P and so on. Heteroatom doping can effectively change the electron distribution of carbon materials, which can improve the electron conductivity of active materials. Furthermore, surface functionalization can further improve the capacitive performance of carbon materials since doping can lead to redox reactions, electron donor capability, or/and electrode wettability. For example, N-doped AC (NAC)⁸⁶ with a moderate N content of 4 wt% was prepared utilizing agricultural waste (corncob) as a precursor.^197,198 The obtained AC with a narrow micro-/meso-pore distribution has a high specific surface area of up to 2859 m² g⁻¹. The LIC using NAC as the cathode shows an energy density as high as 230 W h kg⁻¹ at a power density of 1747 W kg⁻¹. Moreover, coatings with non-carbon films to enhance the electrochemical performance of carbon materials have recently begun to gain attention. For instance, a thin TiO₂/C composite film coated on porous biomass-derived biocarbon (PBC) (denoted as PBC@TC) was fabricated by a facile sol–gel strategy.¹⁹⁹ The results show a distinct Li⁺ storage capacity improvement for the PBC@TC electrode compared with the uncoated electrode, demonstrating the significant positive effect of TiO₂/C ultrathin-films on improving the electrochemical performance of the carbon matrix. However, high temperature calcination could lead to high energy consumption, which would increase the production cost.

In addition, metal organic framework (MOF)-derived carbon with various architectures has also been widely used in LICs. For example, high surface area (2714 m² g⁻¹) carbon cuboids were synthesized by pyrolysing the zinc based MOF-5, which exhibits a unique crumpled-sheet assembled porous morphology with the desired levels of micro and mesoporosity.²⁰⁰ Based on the advanced structure, the MOF-based LIC delivers a maximum specific energy density of 65 W h kg⁻¹ with excellent power capability. Similarly, MOF-derived polyhedral hollow carbon was also fabricated and used in LICs.²⁰¹ It is worth mentioning that removal of metal ions is a necessary process during the preparation of MOF-derived carbon due to the existence of metal ions in the MOF precursor.

The unique 2D structure and physical properties of graphene and its derivatives make them distinctive building blocks for fabricating various 3D porous architectures. Such 3D architectures exhibit excellent chemical stability and high specific surface area as well as fast electron transport kinetics due to the combination of porous structures and the excellent intrinsic properties of graphene and its derivatives. Thus, these 3D architectures will be the promising cathodes of LICs with high performance. Various strategies have been reported to construct graphene-based architectures for LICs.²⁰² For example, micron-sized porous graphene belts are fabricated by a chemical vapor deposition (CVD) approach.²⁰³ Owing to the high ratio of length to diameter (up to 100), good structural integrity and few-layered structure, the obtained graphene belts exhibit good durability and satisfactory Ragone performance (8044 W kg⁻¹ at 51 W h kg⁻¹ and 120 W h kg⁻¹ at 503 W kg⁻¹). Moreover, activated and expanded graphite oxide was synthesized by a micro-wave exfoliation method.⁵³ Using such activated graphite oxide as the cathode, the packaged LIC delivers a gravimetric energy density of 53.2 W h kg⁻¹ at an operating voltage of 4 V. Importantly, the incorporation of microporous carbon with surface functionalization into graphene architectures is an effective strategy to further improve the power and energy densities of LICs. A graphene@meso-/microporous carbon with rational oxygen containing groups was prepared by combining carbonization and activation processes.¹⁸⁴ In the rationally designed G@HMMC electrode (Fig. 9c and d), the hierarchical meso-/microporous carbon offers abundant active sites and the graphene network provides more channels for electrons. Therefore, the LIC with the G@HMMC cathode displays ultrahigh energy densities of 233.3–143.8 W h kg⁻¹ at 450.4 to 15 686 W kg⁻¹ (Fig. 9e).²⁰⁴

Owing to their unique hollow architecture, appropriate pore size and ability to form nanoscale network structures, CNTs are considered as one of the most popular cathode materials for LICs. A flexible nanostructured LIC using thin film MWCNTs as the cathode and a-Fe₂O₃/MWCNT composite as the anode was fabricated.¹¹⁸ On the basis of the total weight of the anode and cathode, the LIC displays a very high specific energy density of 50 W h kg⁻¹ at a specific power density of 1000 W kg⁻¹ in the voltage range of 0–2.8 V. In cathodes based on CNTs, the good electrical conductivity of the MWCNTs is beneficial to improve Li-ion transport kinetics and cycling performance. Although CNTs have good rate characteristics, the complex purification processes and high production costs would hinder their practical application. Similar to CNTs, one-dimensional carbon nanofibers possess superior kinetic properties due to their high aspect ratio and oriented electronic/ionic transport paths, making them promising candidates for LIC electrodes. Shi et al. constructed a highly graphitized carbon fiber with a hierarchical porous structure by electrospinning followed by carbonization and chemical activation.²⁰⁵ Owing to the synergistic effect of large specific surface area (up to 2157.4 m² g⁻¹), hierarchical porous structure, as well as greatly improved electrical conductivity, the as-assembled LIC with carbon fiber cathode and Fe₃O₄ anode delivers a maximum energy density of 124.6 W h kg⁻¹ with excellent rate capability, demonstrating a promising approach for developing advanced activated carbon electrodes for high-performance LICs.

5. Conclusion and perspective

The progress of LICs is mostly benefited from the development of advanced carbon-based electrodes. Based on diverse carbon-based materials, LIC devices with high energy and power densities were designed and assembled. This review summarizes recent developments of carbon-based materials as the anodes and cathodes of LICs. The carbon-based anode materials focus on carbonaceous materials, carbon/transition metal compounds, carbon/polyanion composites, carbon/metalloid and metal materials. In addition to anodes, carbon-based materials also play an important role in the design of cathodes of LICs, where carbonaceous materials and carbon/metal compound composites are highlighted.

Although great efforts have been devoted to the fabrication of carbon-based electrodes and the design of LICs based on them, much work still remains to be done. The inherent electrochemical performance of the carbon materials and the growth of active materials on the carbon materials depend on the porous structure and the surface properties of carbon materials. Various carbon materials have been developed to serve as the electrodes directly or to support other active materials. However, in most cases, their microstructures are disordered and random and their surface properties are not controlled well. Furthermore, some pores of these materials are not electrochemically accessible during the contact with the electrolyte, limiting the electrochemical performance of carbon-based electrodes. Thus rational design of carbon materials with precisely controllable and novel microstructures, sizes, shapes, and surface area should be further considered. Besides, the morphology, distribution and mass loading of active materials on carbon materials could be rationally controlled to enhance the synergistic effects between active materials and carbon materials.

The recent boom in electronic devices with different functions has increased the demand for flexible energy storage devices. The design of flexible LICs mainly depends on the fabrication of cathodes and anodes with both electrochemical and mechanical properties. Although some flexible cathodes and anodes have been achieved based on carbon materials such as CNTs and graphene by different methods, the specific surface area, conductivity, and mechanical properties of CNTs and graphene are not fully utilized in these flexible electrodes. As a result, the electrochemical properties of LICs based on them often degrade under repeatedly bending states. Therefore, cathodes and anodes with enhanced electrochemical and mechanical properties should be further developed for practical applications. Besides LICs, such guidance can also be applied to sodium-ion hybrid capacitors, Li–S hybrid capacitors, magnesium ion hybrid capacitors, lithium–air capacitor-battery and so on. Carbon-based materials will continue to drive the innovation in science and technology in the field of LICs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21573116, 51822205, 21875121 and 51602218), Ministry of Science and Technology of China (2017YFA0206701), Ministry of Education of China (B12015), Tianjin Basic and High-Tech Development (18JCQNJC02400), the Fundamental Research Funds for the Central Universities and the Young Thousand Talents Program.

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