Study on the Mechanism of the Influence of Doping on the Properties of Cathode Materials of Sodium Ion Batteries

Jingjing Li; Hongji Li; Qiang Huang; Zhe Chen

doi:10.7536/PC210441

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (4) : 857-869 DOI: 10.7536/PC210441

research-article

Scan to read & Share

Study on the Mechanism of the Influence of Doping on the Properties of Cathode Materials of Sodium Ion Batteries

Author information +

History +

PDF

Abstract

The abundance of sodium salt in the earth’s crust is 1000 times higher than that of lithium. At the same time, low-cost aluminum foil can be used as the anode of sodium ion battery instead of copper foil, and the low-temperature characteristics are more excellent, which has a good application prospect in energy storage and standby energy storage scenarios. Therefore, sodium ion battery is considered one of the ideal choices for the next generation of large-scale energy storage technology. However, compared with lithium ion, the large ion radius and mass of sodium ion greatly limit its reversible deintercalation in electrode materials, resulting in relatively low working voltage and energy density of the battery. In the sodium ion battery materials system, the research of cathode materials needs great progress. In this paper, the existing typical cathode materials for sodium ion batteries are reviewed, including layered metal oxides, polyanions and Prussian blue compounds. The effect of doping on the performance of cathode materials for sodium ion batteries is analyzed. The cycling reversibility, reversible capacity and diffusion kinetics of sodium ions can be improved by element doping, which can change the properties of the crystal lattice to a certain extent, and enhance the stability, electronic conductivity and intercalation kinetics of sodium ions. In this paper, the achievements of doping application in the existing materials are summarized, and the future research direction and development prospect of cathode materials are put forward.

Contents

1 Research background

2 Doping modification of cathode materials for sodium ion batteries

2.1 Modification of layered metal oxides by doping

2.2 Modification of Prussian Blue by doping

2.3 Modification of polyanionic compounds by doping

3 Doping modification principle of cathode materials for sodium ion batteries

3.1 Restrain phase transition and stabilize structure

3.2 Increase the layer spacing and improve the dynamics

3.3 Improving the discharge capacity of cathode materials

3.4 Improve the electronic conductivity and ionic conductivity of materials

3.5 Inhibition of Na⁺Vacancy ordered structure

4 Conclusion and outlook

Graphical abstract

Keywords

sodium ion battery / positive material / doping

Cite this article

Download citation ▾

Jingjing Li, Hongji Li, Qiang Huang, Zhe Chen. Study on the Mechanism of the Influence of Doping on the Properties of Cathode Materials of Sodium Ion Batteries. Progress in Chemistry, 2022, 34(4): 857-869 DOI:10.7536/PC210441

登录浏览全文

4963

注册一个新账户忘记密码

1 Research background

As early as the 1970s and 1980s, sodium-ion batteries and lithium-ion batteries were widely studied by scientists. With the rapid commercialization of lithium-ion batteries, the development of sodium-ion batteries has gradually lagged behind^[1⇓⇓~4]. However, the distribution of lithium resources in the earth's crust is low, and it mainly depends on imports from South America, and the price is high, which restricts the development of lithium-ion batteries in large-scale energy storage systems. Sodium and lithium belong to the same main group element and have similar physical and chemical properties. In contrast, sodium is abundant, cheap and widely distributed in the earth's crust. Therefore, sodium-ion batteries are considered as a new generation of energy storage technology.In particular, the consumption of renewable energy, one of the important options for future energy storage grids, is of great significance to carbon peaking and carbon neutralization, and has ushered in a new research boom in recent years^[5,6]^[7⇓~9]^{[10⇓⇓~13]}. However, the radius of sodium ions is larger than that of lithium ions, and the molecular weight is also larger, which leads to slow migration kinetics and restricts the ability of rapid charge and discharge. At the same time, in the process of sodium storage, larger sodium ions are easy to cause larger volume changes of electrode materials.It even induces irreversible structural phase transition, which affects the cycle stability of the battery, and the potential of sodium is higher than that of lithium, which will reduce the working voltage and energy density of the battery^[5,14]. Therefore, the development of efficient, fast and stable electrode materials for sodium-ion batteries has become a top priority^[15]. In this paper, the existing problems of various cathode materials for sodium-ion batteries at present are discussed, and the effect of doping on the conductivity of cathode materials is emphatically discussed. The research progress of the corresponding modification is systematically reviewed, and the mechanism of doping modification is summarized. At present, there are relatively few review articles on doping modification at home and abroad, so this paper will play a certain role in promoting the future research work on doping modification of sodium-ion batteries.

Positive and negative electrode materials are the key factors affecting the working voltage, energy density, cycle performance and rate performance of sodium-ion battery system. At present, the anode materials for sodium-ion batteries mainly include carbon materials, metal oxide materials, phosphorus-based materials, intercalation compounds, organic compounds, alloys and conversion materials, which have been widely studied and can basically meet the requirements of industrial applications of sodium-ion batteries^[16,17]. Cathode material is an important functional part of sodium-ion battery, which is responsible for providing active sodium ions and high potential redox couple, and directly affects the reversible capacity and working voltage of the battery. The cycle life and high current rate performance of the battery are also closely related to the cathode materials. Therefore, finding suitable cathode materials is the key to make breakthroughs in sodium-ion batteries.

The redox potential of suitable cathode materials for sodium-ion batteries should be as high as possible, and the change of potential with sodium content should be small. Reversible deintercalation of sodium ions should be allowed as much as possible to obtain a higher reversible specific capacity; Reversible deintercalation of as many sodium ions as possible should be allowed to ensure rapid deintercalation of sodium ions; It should have good electronic conductivity and ionic conductivity, reduce polarization as much as possible, and facilitate large current charging and discharging; In the process of deintercalation of sodium, the volume change should be as small as possible, with sufficient ion diffusion channels and ion diffusion rates, and high structural, chemical and electrochemical stability^[18⇓~20]. It should have the characteristics of simple preparation method, abundant raw materials and environmental friendliness. At present, the commercialization of sodium-ion batteries requires high specific capacity and power density, good cycle life, safety, non-toxicity and low cost. Finding and developing suitable high-performance cathode materials has become the key to the further development of sodium-ion batteries^[21⇓~23].

2 Doping Modification of Cathode Materials for Sodium Ion Batteries

At present, the cathode materials that have attracted wide attention of researchers mainly include layered metal oxides, Prussian blue compounds and polyanionic compounds. However, each type of material has some characteristic disadvantages, such as the structural variability of layered metal oxides and the poor structural stability; Prussian blue has poor cycle stability and is easy to decompose at high temperature, which is potentially dangerous. Polyanionic compounds have low capacity and poor conductivity^[8,24,25]. Therefore, researchers have carried out a lot of modification studies on materials by hydrothermal method, solid phase method, sol-gel method and precipitation method to improve the electrochemical properties of materials, including element doping, particle nanocrystallization and surface material coating^[26⇓~28]. Table 1 summarizes some doping modifications reported in the literature on cathode materials for sodium-ion batteries^{[29⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~49]}.

2.1 Study on the Modification of Layered Metal Oxides by Doping

Layered metal oxides are a class of typical cathode materials for sodium-ion batteries in the general form of NaMO₂,M common transition metals such as Fe, Mn, Ni, Co, Ti and Cr^[50]. The layered metal oxide is a layered structure formed by alternately arranging transition metal layers and alkali metal layers, wherein the transition metal layers are formed by edge-sharing connection of repeated MO₆ octahedrons, and the Na⁺ is positioned between the transition metal layers to form the alkali metal layer. Delmas et al. Divided layered oxides into On and Pn phases according to the arrangement of Na⁺ between transition metal layers, mainly P2, P3, O2 and O3 phases, as shown in Fig. 1^[51]. The alkali atoms in the O-phase material are in octahedral positions, and the alkali atoms in the P-phase material are in triangular prismatic positions. N, which can be equal to 2 or 3, is determined by the packing of the oxygen atoms, represents the number of different sites occupied by the transition metal atoms, and also represents the number of distinguishable sodium layers. Generally speaking, in the cathode materials of sodium ion batteries, with the increase of the content of sodium ions in the alkali metal layer, the materials are easy to form sodium octahedral coordination. On the contrary, when the content of sodium ion is small, it is easy to form sodium prism coordination. Different types of layered oxides have different structures, so their electrochemical properties are also different. Among them, O3 and P2-type layered oxides are the main cathode materials.

The O3-type NaMO₂ is based on a hexagonal close-packed array of oxygen atoms, and the sodium and transition metal ions are located in different octahedral interstices according to their different ionic radii. In the O3 phase structure, the sodium ion migrates through a narrow tetrahedral center position, and its diffusion barrier is often large.

There is a sodium ion diffusion channel in the P2 structure, and the layer spacing is larger, so the energy barrier for sodium ion diffusion is lower than that of the O3 phase, and the sodium ion migrates from a triangular prism site to an adjacent site through a rectangular narrow channel of four oxygen atoms, which is less affected by the repulsion of the oxygen atoms. Therefore, sodium ions are less resistant to intercalation and deintercalation, which is more conducive to the intercalation and deintercalation of sodium, and may have a larger specific capacity. In addition, when the P2 phase structure transforms to other phase structures, the M — O bond needs to be broken, which can only occur at high temperature, and the phase transformation is very difficult, so it is more stable than other phase structures. Moreover, the conductivity of the P2 structure is better than that of the O3 structure at similar chemical composition^[52].

Layered oxides have promising applications, but there are still some problems. Due to the large radius of Na⁺, the de-intercalation of Na⁺ in the material will cause the slippage of the oxygen layer, resulting in a series of phase transitions^[53,54]. The larger Na⁺ will cause the volume contraction and expansion, which will greatly change the crystal structure of the material and induce the structural collapse^[55]. In addition, layered oxides have strong air sensitivity, which may react with water in the air and lead to performance degradation. In order to solve these problems, researchers usually use doping to modify the layered oxide electrode materials. Doped metal ions can not only provide a certain capacity for the material through their own valence changes, but also stabilize the structure of the material, which plays an important role in improving the capacity, cycle stability and rate performance of the material. For example, the Na(NiCoFeTi)_1/4O₂ prepared by doping Ti element is beneficial to stabilizing the structural change of the material during charging and discharging, reducing the irreversible capacity caused by the structural collapse of the material, inhibiting the Jahn-Teller effect of Ni and Co, and stabilizing the structure during the migration of sodium ions^[56,57]. It has been reported that the specific capacity of the material can be improved by doping elements with more valence States, such as Ni and Cr^[34]. Doping Mn and Fe is beneficial to improve the conductivity of the material^[30].

Mu et al. Used Na₂CO₃, CuO, Fe₂O₃, and Mn₂O₃ to prepare a cobalt and nickel-free layered metal oxide O3-Na_0.9[Cu_0.22Fe_0.30Mn_0.48]O₂ by a simple solid state reaction at 850 ° C; cobalt and nickel are toxic and expensive, so this material is not only environmentally friendly but also has a lower cost, making it more suitable for practical applications^[33]. The addition of Cu to the Fe/Mn transition metal layer can improve the stability, reversibility and kinetics of the material. This material is capable of storing a reversible capacity of about 100 mAh/G at an average storage voltage of 3.2 V and has a long cycle life, as shown in fig. 2. When assembled into a full cell in combination with a hard carbon negative electrode, the prototype rechargeable Na-ion battery delivers an energy density of 210 Wh/kg with 90% energy efficiency, high rate capability, and excellent cycling stability. At a high charge-discharge current of 5 C, the battery was able to deliver 74% of the initial capacity.

Wang et al. Used the sol-gel method, adding citric acid, magnesium nitrate and other nitrates to the solution and heating to obtain the gel, and the obtained gel was heated at 450 and 900 ° C to obtain Mg-doped Na_0.67Mn_0.67Ni_0.33-_xMg_xO₂^[35]. As shown in fig. 3, Mg can achieve a good balance between the discharge specific capacity and the capacity retention when the doping amount is 0.05, providing a reversible capacity of 123 mAh/G. The inactive Mg²⁺ ions in the transition metal layer allow more Na⁺ ions to reside at the prismatic sites upon charging, which effectively suppresses the sliding of the transition metal layer and the phase transition of P2-O2, thus stabilizing the overall charge balance of the compound and improving the cycling performance.

In addition, the doped elements can be one, two or even more^[58⇓~60]. Carlier et al. Used the co-precipitation method to add transition metal salts into the mixed solution of sodium hydroxide and ammonia, and the obtained precipitate was calcined at 950 ° C to obtain Na_2/3Co_2/3Mn_1/3O₂ layered metal oxides with P2-type structure^[61]. In this structure, the low-spin Co³⁺ ion and the Mn⁴⁺ ion are stabilized. Co³⁺ ions and Mn⁴⁺ ions are randomly distributed in the transition metal layer, and the P2-Na_2/3Co_2/3Mn_1/3O₂ phase exhibits excellent Na⁺ intercalation reversibility between 1.5 and 4.0 V,The good reversibility of the Na⁺ intercalation and delamination process determines the performance of the P2-NaCo_2/3Mn_1/3O₂ phase as an electrode material for sodium-ion batteries.

Cao et al. Calcined a mixture of Na₂O₂, Fe₂O₃, Mn₂O₃, and Cr₂O₃ at 900 ° C in argon to prepare a novel O3-type transition metal oxide NaCr_1/3Fe_1/3Mn_1/3O₂ as a cathode material for sodium-ion batteries by solid state synthesis^[34]. The material is a multi-element NaMO₂ with a layered structure containing chromium, and the research on the material is less at present, and the effect of a three-electron redox couple of Cr³⁺/Cr⁶⁺ on the material needs to be further discussed. As shown in Fig. 4, the material has a high reversible capacity of 186 mAh/G in the voltage range of 1.5 – 4.2 V, and the substitution of Fe³⁺ and Mn⁴⁺ can effectively suppress the disproportionation reaction of Cr⁴⁺ with Cr³⁺ and Cr⁶⁺,During the charge-discharge process, the material can achieve reversible multi-electron redox of Cr ions with CrO₆ octahedral distortion and reduction.

2.2 Study on Modification of Prussian Blue Materials by Doping

Prussian blue has a chemical formula of Fe₄[Fe(CN)₆]₃ and a cubic structure. A series of new compounds obtained by substitution and interstitial modification of Prussian blue are usually called Prussian blue compounds. The compound is a compound with a three-dimensional open structure, and its general structural formula is A_xM₁[M₂(CN)₆]_y_□1-_y·nH₂O, where A represents alkali metal ions, such as K or Na;M₁, and M₂ represents transition metals, such as Mn, Fe, Co, Ni, Cu, Zn; □ represents cyanide vacancy defects, and H₂O represents crystal water as well as zeolite water. Prussian blue analogs usually have a typical double perovskite structure, as shown in Fig. 5, which is characterized by two transition metal ions M₁, M₂ arranged alternately and connected with — C ≡ N — to form a rigid open framework structure.The interstitial sites in the open structure can accommodate a certain amount of alkali metal ions and water molecules, and can form a large three-dimensional ion channel to facilitate the rapid insertion and extraction of sodium ions^[62].

The application of Prussian blue materials in sodium-ion batteries has been widely concerned, mainly due to the research of Cui et al. In 2011. The prepared Prussian blue material K_0.71Cu[Fe(CN)₆]_0.72 has small internal stress in lattice and stable structure, and its theoretical specific capacity is 62 mAh/G. The material has excellent cycle stability, and the capacity retention rate is about 83% after 10,000 charge-discharge cycles at 17 C current density^[63]. Goodenough et al. First investigated the application of a series of Prussian blue compound KMFe(CN)₆(M=Mn,Fe,Co,Ni,Zn) in organic system sodium-ion batteries, and the specific discharge capacity was higher than 70 mAh/G at low rate^[62]. They prepared rhombohedral phase Na_1.72MnFe(CN)₆ and cubic phase Na_1.40MnFe(CN)₆ by controlling the concentration of substances in the solution, and the initial reversible specific capacity was 134 and 123 mAh/G, respectively. The study showed that the material had good rate performance, but the cycle performance needed to be improved^[64].

Prussian blue analogues as electrode materials for sodium-ion batteries have the following advantages: (1) they have a three-dimensional open structure and large ion channels, which facilitate the rapid intercalation and deintercalation of sodium ions^[65⇓~67]; (2) Some Prussian blue analogues have two electrochemical reaction active sites and high specific capacity^[68]; (3) high charging and discharging platform and high energy density; The characteristics of easy access to raw materials, easy preparation, low cost and environmental friendliness make the material very suitable for large-scale energy storage^[69⇓~71]. However, the Prussian blue analogs prepared by the traditional rapid precipitation process will have a large number of Fe(CN)₆ vacancies occupied by coordinated water in the crystal framework, and the presence of vacancies and water will damage the performance of the battery. The increase of Fe(CN)₆ vacancies reduces the content of sodium ions, the lattice will introduce more water molecules into the material skeleton due to charge balance, which reduces the initial charge capacity of the battery, and the decomposition of H₂O will cause more side reactions. Secondly, in order to adapt to the interstitial space, crystal water molecules may compete with alkali metal ions to block the intercalation reaction of sodium ions and reduce the capacity utilization of the material. Thirdly, the randomly distributed Fe(CN)₆ vacancies make the material framework easy to collapse in the process of sodium ion intercalation and deintercalation, which leads to structural instability and poor cycle life, and the sodium storage performance is seriously reduced^[39,72].

The electrochemical performance of Prussian blue can be significantly improved by doping^[73,74]. Gong et al. Milled MnCl₂·4H₂O and Na₄Fe(CN)₆·10H₂O at high rotation speed for 12 H, then washed and dried the products, and synthesized Fe-doped ultrafine Na_yFe_xMn_1-x[Fe(CN)₆] by ball milling^[36]. This experimental method is relatively simple and provides a good idea for large-scale synthesis of materials for practical applications. The Mn³⁺ in Mn-based Prussian blue has strong Jahn-Teller activity, which can cause structural changes in the electrochemical process. The electrochemical activity of Jahn-Teller distorted Mn³⁺ is lower than that of Jahn-Teller released, which can lead to the slow diffusion of Na⁺. Fe doping can suppress the Jahn-Teller distortion of Mn³⁺ to activate the Mn reduction reaction and suppress the structural changes during the insertion and extraction of Na⁺, thus improving the structural stability of the material. In addition, the conductivity of Mn-based Prussian blue can be improved by doping Fe, and the electrochemical stability and kinetic performance of the material are greatly improved. Fe-MnPBA/0.4 can provide a specific capacity of 119 mAh/G at 1 C, which is equivalent to an energy density of about 380 Wh/kg in a half cell. Even at a current density of 30 C, its specific capacity remains at 86 mAh/G.

Fu et al. Mixed and reacted Na₄Fe(CN)₆, Fe(NO₃)₃ and Ni(CH₃COO)₂ solution, and the obtained product was centrifugally dried to synthesize nickel-doped Prussian blue by precipitation method, revealing the effect of the substitution of transition metal ions on its electrochemical performance^[37]. The addition of a small amount of nickel ion can replace the high-spin Fe ion, improve the sodium ion storage capacity of the low-spin Fe²⁺/Fe³⁺ redox couple to 50 mAh/G, and promote the rapid diffusion of Na in the lattice. The doping of nickel ions also helps to improve the stability of the Prussian blue lattice, thereby improving the rate and cycling performance, even at high potentials. The improved electrochemical performance of Prussian blue doping at high potential can be attributed to the weaker crystal field splitting of iron ions after nickel doping, which makes it easier for electrons to transfer from the e_g orbital of Fe³⁺ ions to the t_2g orbital during discharge. As shown in Fig. 6, the study shows that 3% Ni doping can achieve the best electrochemical performance with a discharge capacity of 117 mAh/G.

Xie et al. Mixed the solution of sodium citrate, FeCl₂, MnCl₂, CoCl₂ and NiCl₂ with the solution of Na₄Fe(CN)₆, and added NaCl solution and polyvinylpyrrolidone dropwise to the mixed solution.Mn, Co, Ni Co-doped Prussian blue analogs with high purity and high crystallinity were prepared by a co-precipitation method using a citrate-assisted controlled crystallization process, which provides a simple method to improve the crystallinity of Prussian blue by balancing multiple transition metal lattice substitutions^[38]. The material can deliver a capacity of 111 mAh/G at 1 C and retain a capacity retention of 78.7% after 1500 cycles in a sodium-ion battery. Mn, Co, and Ni Co-doping can activate Fe under the low spin structure, thereby improving its capacity in the high pressure plateau, and the doping of large ionic radius Mn²⁺ can expand the lattice, which is beneficial to the rapid transfer of Na⁺ with less distortion. The doping of inert Ni can reduce the lattice stress and strain related to the volume change during cycling during the phase transformation process, thus playing an important role in stabilizing the framework structure. Co doping can alleviate the stress concentration caused by the increase of redox potential and improve the cycling stability of the material.

Yang et al. Mixed Na₄Fe(CN)₆, NiCl₂ and MnCl₂ solutions in proportion and synthesized Na₂Ni_xMn_yFe(CN)₆(PBMN) by co-precipitation method^[39]. As shown in Fig. 7, the initial discharge capacities of PBN, PBM and PBMN were 62.2, 128.4 and 123.3 mAh/G, respectively, and the initial charge-discharge coulombic efficiency reached 93.25%, 88.86% and 92.71%, respectively. The capacity retention of PBM, PBN, PBMN was 63.4%, 89.5% and 83.8% after 800 cycles. The charge-discharge capacity is determined by the redox sites in the crystal framework. The PBM sample has two redox sites of Mn ions and Fe(CN)₆ groups, so its capacity is high. In the deintercalation process of Na⁺, the lattice constant does not change when the Fe(CN)₆^4-/Fe(CN)₆^3- group is reduced/oxidized, while the lattice constant changes when the redox reaction of Mn²⁺/Mn³⁺ occurs, so the cycle stability of PBM is the worst. Ni is inert, so there is only one redox site of Fe(CN)₆ group in the PBN framework, and the material has the lowest capacity although it has better stability. In PBMN, both Mn and Ni exist, and Ni doping into the structure can balance the structural change caused by the redox reaction of Mn²⁺/Mn³⁺, so that the material can achieve high capacity and good cycling stability at the same time.

2.3 Study on the Modification of Polyanionic Compounds by Doping

The general chemical formula of polyanionic cathode materials is A_xM_y[(XO_m)_n]_z, where A is generally sodium or lithium, M is generally a transition metal group element, and X is phosphorus, sulfur, and other elements. At present, the most studied polyanionic materials include phosphate, pyrophosphate, fluorophosphate, sulfate, etc. In these compounds, the oxygen polyhedra of M and X form stable framework structures by sharing points or edges, and A can be stored in these stable frameworks^[75⇓~77]. Compared with layered oxides, the strong X — O bond in polyanionic compounds can produce a stronger degree of ionization in the M — O bond, while the weaker ionic bond in the M — O increases the antibonding orbital bonding distance of the Na/Na⁺, thus giving it a higher redox potential, which is the "induction effect" in polyanionic compounds^[78]. In addition, the strong X-O covalent bond greatly increases the stability of oxygen in the lattice and improves the safety of these materials. These excellent characteristics make polyanionic cathode materials a hot topic in the field of cathode materials for sodium-ion batteries. However, in the structural framework of polyanionic compounds, transition metal ions are separated by polyanionic groups that do not conduct electrons, and the isolated electron cloud is difficult to exchange charges, resulting in extremely low intrinsic conductivity of the material^[79,80]. In addition, the large molar mass of the bulky anionic group in polyanionic compounds leads to low specific capacity and low volumetric energy density. In view of the above problems, researchers often use doping to improve the electrochemical performance of materials. The structure of polyanionic compounds determines their lower electronic conductivity. In order to improve the electronic conductivity and promote the internal charge transport of materials, researchers have conducted in-depth research on the structure of materials. The structure of the polyanion compound for the positive electrode material of the sodium ion battery is composed of three elements: an alkali metal ion, a transition metal ion and a polyanion group. During the charge-discharge process of cathode materials, charge exchange is accompanied by the redox of transition metal ions in the structure, so the conduction path of electrons depends on the interconnection between transition metal-oxygen polyhedra. However, there are electron-insulating polyanionic groups in polyanionic compounds, which are bonded to the transition metal-oxygen polyhedron and block the electron transport path. Therefore, to upgrade the intrinsic electronic conductivity of materials, additional charge transfer pathways must be fabricated in the structural framework. There are two main methods reported in the literature, one is doping metal ions, in which the same valence metal ions can create metal polyhedral bond contacts, and the different valence ions can improve the electronic conductivity through charge compensation mechanism. The second method is by heterovalent substitution of nonmetallic sites. Heterovalent substitution can break the anion-oxygen bond, or introduce a charge exchange pathway into the anion group to improve the electron conduction ability of the polyanion group, thereby improving the intrinsic conductivity of the material^[81].

LiFePO₄ has been the most widely used in lithium-ion batteries due to its moderate potential, low cost, and long cycle life^[82⇓~84]. The success of lithium iron phosphate makes people turn their attention to sodium iron phosphate NaFePO₄, which is also orthophosphate, when studying sodium-ion batteries. It has two different structural types: natroxenite phase and lithiophosphate phase. Wang et al. compared the thermodynamic and kinetic behaviors of NaFePO₄ and LiFePO₄. Through the comparison of equilibrium potential, ion diffusion coefficient and reaction resistance, it was found that the diffusion of sodium ions in NaFePO₄ was slow and the contact and charge transfer resistance were larger.As a result, its rate performance is poor, and its charge transfer impedance is higher, so its actual specific capacity is lower than the theoretical specific capacity, and its electrochemical performance can not meet expectations at this stage^[85]. Researchers have found that doping is an effective way to improve the electrochemical properties of NaFePO₄. Liu et al. Added NaH₂PO₄, citric acid and Fe(NO₃)₃ into a polyvinylpyrrolidone solution to obtain an electrospinning precursor solution, obtained a fiber membrane through an electrospinning device, and then carbonized the obtained fiber at high temperature in a nitrogen atmosphere to prepare a ferric sodium phosphate type NaFePO₄ with uniformly embedded porous N-doped carbon nanofibers^[43]. In this experiment, the smallest size of NaFePO₄ nanodots was uniformly embedded into porous N-doped carbon nanofibers by electrospinning for the first time, and the prepared NaFePO₄@C fiber membrane was closely attached to aluminum foil, which could be directly used as an unbonded positive electrode for sodium-ion batteries. As shown in fig. 8, the material can deliver a reversible capacity of 145 mAh/G at a rate of 0.2 C, with a capacity retention of up to 89% after 6300 cycles. At the same time, the sodium-ion full battery composed of the doped composite and pure carbon nanofibers has a high mass energy density of 168 Wh/kg and a capacity retention of 87% after 200 cycles. Nitrogen doping and carbon nanofiber coating can effectively improve the utilization of sodium iron phosphate materials, promote the transmission of electrons and ions, and enhance the stability of the electrode structure.

For other polyanionic compounds, researchers have also modified them by doping. Wang et al. Prepared Br and N co-doped Na₃V₂(PO₄)₃/C multilayer nanocomposite by sol-gel assisted hydrothermal method^[44]. Cetyltrimethylammonium bromide and urea were prepared into solution A,NH₄VO₃ and C₂H₂O₄, which were dissolved in water, and then NH₄H₂PO₄ and trisodium citrate were added to prepare solution B under stirring at 80 ℃, and solution A was dropwise added to solution B, which was transferred to a reaction kettle for hydrothermal reaction for 12 H after uniform stirring, and the precipitate was calcined at high temperature in nitrogen atmosphere to obtain the final composite. The sodium superionic conductor structure of Na₃V₂(PO₄)₃ can provide three-dimensional structure and channels to improve the conductivity and structural stability of Na⁺. However, the electronic conductivity of Na₃V₂(PO₄)₃ is too low, and halogen doping can improve the electronic conductivity and structural stability. Br doping helps to reduce the activation energy of V⁴⁺/V⁵⁺ redox reaction in Na₃V₂(PO₄)₃, and N doping provides electrons to the carbon matrix, improves the conductivity of amorphous carbon, and provides a channel for the transfer of sodium ions. As shown in Fig. 9, Br and N co-doping improved the rate capability, providing a prospect for the fabrication of battery electrodes with high electrochemical performance.

Chen et al. Dissolved citric acid and NH₄H₂PO₄ in water, added CH₃COONa, Mn(CH₃COO)₂ and FeSO₄ to the solution in turn and heated to 80 ° C and stirred uniformly, the gel obtained after water evaporation was calcined at high temperature, and a carbon-coated Fe-doped manganese-based polyanion compound Na₃Mn_2-_xFeP₃O₁₁@C was synthesized by sol-gel method and carbothermal reduction method^[45]. The material not only has low cost, but also is more environment-friendly. The Jahn-Teller effect of Mn³⁺ produced by the pure phase Na₃Mn₂P₃O₁₁ compound during the charge process can make the electrochemical performance of the positive electrode poor. Doping Fe can prevent the growth of Na₃Mn_2-_xFeP₃O₁₁ particles during sintering, and the material can obtain higher specific surface area and smaller pore size, and can also provide convenience for the penetration of electrolyte and the rapid diffusion of Na⁺; It can also reduce polarization and shorten the migration distance of sodium ions, thus obtaining excellent rate performance and improving conductivity and electrochemical performance. When the iron doping amount is 0.4, the Na₃Mn_1.6Fe_0.4P₃O₁₁ shows the best cycle and rate performance.

Gao et al. Dissolved NH₄VO₃, NaF, NH₄H₂PO₄, and Co(NO₃)₂·6H₂O in water, heated and stirred, dried the water to obtain the precursor, calcined it at high temperature in argon atmosphere, and first prepared Co-doped Na₃V_2-_xCo_x(PO₄)₂F₃ as a cathode material for sodium-ion batteries by sol-gel method^[46]. The effects of Co doping on the structure and electrochemical properties of the Na₃V₂(PO₄)₂F₃ materials were analyzed using XRD, FE-SEM, galvanostatic charge-discharge and AC impedance measurements. The study shows that the substitution of Co²⁺ for V³⁺ can produce a mixed valence state of V^3+/4+ within the Na₃V₂(PO₄)₂F₃ lattice, thereby improving the electronic conductivity of Na₃V₂(PO₄)₂F₃ materials. The replacement of V³⁺ by Co²⁺, which has a larger ionic radius, can increase the unit cell volume of Na₃V₂(PO₄)₂F₃, broaden the sodium ion transport channel, and thus improve its ionic conductivity. In addition, Co doping can effectively reduce the charge transfer impedance of the Na₃V₂(PO₄)₂F₃ electrode. The electrochemical test results show that the Na₃V_1.9Co_0.1(PO₄)₂F₃ electrode with X = 0. 1 has the most excellent electrochemical performance, with the initial discharge specific capacity of 111. 3 mAh/G at 0. 1 C, the first reversible capacity of 91. 9 mAh/G at 5 C, and the capacity retention rate of 70% after 80 cycles.

Li et al. Used NaOH, NH₄VO₃, NH₄H₂PO₄, Mg(CH₃COO)₂ and citric acid as raw materials, mixed the solution uniformly, heated it at 80 ° C to form gel, and calcined the gel at high temperature to obtain Mg-doped Na₃V_2-_xMg_x(PO₄)₃/C by sol-gel method^[86]. The material has good electrochemical performance, as shown in Fig. 10, when the current density increases from 1 C to 30 C, the specific capacity only decreases from 112.5 mAh/G to 94.2 mAh/G, showing good rate performance. At high current densities of 10 and 20 C, the initial capacities were 108.4 and 106.4 mAh/G, respectively, and after 50 cycles, the discharge capacities stabilized at 104.6 and 86.2 mAh/G, respectively. The study shows that the morphology with smaller particle size can be obtained by doping Mg in Na₃V₂(PO₄)₃, which can increase the contact area between electrode and electrolyte and accelerate the charge transfer between interfaces. In the Mg²⁺ doped sample, the Mg²⁺ in the lattice can act as a pillar to prevent the crystal from collapsing during cycling, thereby improving the cycling stability of Mg-doped Na₃V₂(PO₄)₃. Doping Mg can also reduce the charge transfer resistance of the material and improve the sodium ion conductivity, and the presence of Mg²⁺ can also cause some local defects in the crystal structure and improve the electronic conductivity, thereby improving the rate performance.

3 Principle of Doping Modification of Cathode Materials for Sodium Ion Batteries

At present, element doping has made great progress in the field of sodium-ion batteries. Different valence, type, size, doping amount and location of doping ions will have different effects on electrode materials, and doping into different materials will also have different effects. The element doping of cathode materials can be followed by rules, and the principles of doping modification of different elements are also different. The main principles can be divided into the following.

3.1 Inhibit phase transition and stabilize structure

Some cathode materials will have structural distortion in the process of charge and discharge, resulting in rapid attenuation of the capacity and voltage of the materials. The doped metal ions can inhibit the phase change of the material in the cycling process, play a supporting role in the structure of the material in the charging and discharging process, prevent the structural collapse caused by the shuttle of sodium ions, inhibit the structural distortion of the material, and improve the stability of the materials, thereby improving the capacity retention rate of the materials. Wu et al. Systematically studied the effect of Zn doping on the structural evolution of P2-Na_0.66Ni_0.33Mn_0.67O₂ materials during cycling^[87]. As shown in Fig. 11, the transition metal ions present at the center of the Na_0.66Ni_0.33Mn_0.67O₂ tetrahedron will hinder the access of sodium ions to the center of the adjacent octahedron, thus preventing the deintercalation of sodium ions in the interlayer. In situ HEXRD spectra show that the peak of the material shifts during charging, and only the P2 diffraction peak of the Zn-doped material reappears during discharging, indicating that Zn doping can enhance the reversibility of P2-O2 phase transition in the Na_0.66Ni_0.26Zn_0.07Mn_0.67O₂, reduce the distortion degree of Ni-O octahedron in the Na-Ni-Mn-O structure, and then improve the electrochemical performance of the material.

3.2 Increasing the layer spacing, lifting the dynamics

Doping ions with larger radius can expand the interlayer spacing or increase the volume of the crystal, improve the diffusion space of sodium ions, and facilitate the deintercalation of Na⁺, thereby improving the rate performance. Wang et al. Studied the electrochemical performance of Nb-doped P2-Na_0.67Co_0.25Mn_0.75O₂, and the study showed that Nb doping could enhance the interplanar spacing of (002) peak, increase the lattice parameter and lattice volume, and the c-axis value of P2-Na_0.67Co_0.25Mn_0.75-_xNb_xO₂(x=0,0.015,0.030,0.045) increased from 11.199 199Å (X = 0) to 11.271 271Å (X = 0.030), which was beneficial to broaden the Na⁺ migration channel and reduce the charge/electron migration energy barrier, thus realizing an efficient Na⁺ diffusion mechanism and improving the rate performance of P2-type Mn-based cathode materials^[88]. In addition, the bond energy of Nb — O is higher, which can enhance the binding force of M — O bond and the stability of M-O2 layer, and can effectively promote the stability of crystal structure.

3.3 Improve the discharge capacity of cathode material

Ions that can undergo redox reactions are doped, and the ions can provide a certain capacity for the material through their own valence changes. Man et al. Introduced a proper amount of Co into the Prussian blue cathode to replace Ni in Prussian blue compounds to prepare a Na₂Ni_xCo_1-_xFe(CN)₆(PBN_xC_1-_x),Co^2+/3+ pair, which can provide active sites^[40]. The PBC curve shows a pair of weak peaks at 3.73/3.90 V, corresponding to the redox reaction of the Co^2+/3+ couple. The two small peaks of PBN_0.4C_0.6 around 3.70/3.80 V indicate that the Co^2+/3+ provides partial capacity, which possesses an initial discharge capacity of 90 mAh/G, higher than that of PBN (63 mAh/G).

3.4 Improve the electronic and ionic conductivity of the material

Doping with high-valence ions can improve the electronic conductivity, and it is also beneficial to improve the conduction of Na⁺, thus improving the rate performance of materials. Li et al. Synthesized Zr⁴⁺ doped cathode materials for P2-Na_0.75Mn_0.55Ni_0.25Co_0.05Fe_0.10Zr_0.05O₂ sodium-ion batteries^[89]. They used the two-point DC method to measure the resistance of the doped and undoped samples. As shown in Table 2, the resistance of the sample doped with Zr element is smaller, and the substitution of Zr ion can effectively improve the conductivity of the material, thus improving the rate performance of the material.

3.5 Suppressed Na⁺ vacancy ordered structure

In sodium-ion batteries, the vacancy ordering of Na⁺ is an important reason for capacity fading. Na⁺ vacancy ordering is ubiquitous in transition-metal layered oxide materials, and eliminating the ordered structure of Na⁺ vacancies can, to some extent, inhibit the capacity fading of materials during cycling, enhance the diffusion ability of Na⁺, and improve the rate performance of materials. By doping some ions with similar radius and different Fermi energy levels, the charge ordering in the transition metal oxide layer can be destroyed, and the generation of the Na⁺ vacancy ordered structure can be inhibited^[90,91]. Wang et al. Synthesized Na_2/3Ni_1/3Mn_1/3Ti_1/3O₂ cathode materials with Na⁺ site disorder structure by doping Ti, which has a similar radius to Mn⁴⁺ but a large difference in Fermi level^[92]. Analyzing the apparent diffusion coefficient of Na obtained by Galvanostatic intermittent titration technique (GITT), the mobility of Na ions of the material doped with Ti increased by two orders of magnitude, and the diffusion coefficient was Galvanostatic intermittent titration technique, while that of the undoped material was 10^-12~10^-11cm²/s. As shown in Fig. 12, Na⁺ migrates through two-dimensional channels characterized by interconnected diffusion pathways, which favor rapid ion migration. The calculated diffusion energy barrier is a very low barrier of 0.17 eV, which is in good agreement with the GITT results. It indicates that the doping of Ti reduces the excitation energy of Na⁺ transition between adjacent prismatic sites of Na layer, promotes the diffusion of Na⁺, reduces the potential energy difference between Na_e and Na_f, and inhibits the formation of ordered phase. The reversible specific capacity of the material reaches 88 mAh/G at 0. 1 C, and the capacity retention rate reaches 83. 9% after 500 cycles at 1. 0 C.

4 Summary and Prospect

(1) In this paper, the research progress of three kinds of cathode materials, including layered metal oxides, Prussian blue and polyanionic compounds, is reviewed. Layered metal oxides have high theoretical capacity and ionic conductivity, but poor cycling performance. Prussian blue cathode materials have relatively excellent electrochemical performance due to their three-dimensional ion diffusion channels and large specific surface area, but their thermal stability and cycle performance are poor. Polyanionic compounds have good thermal stability and structural stability, but their theoretical capacity and conductivity are low.

(2) In this paper, the effect of doping on the performance of cathode materials for sodium-ion batteries was summarized, and the principle of doping modification was summarized. Larger Na⁺ are prone to phase transition and structural collapse in the process of de-intercalation, and the presence of coordination water will produce side reactions, which lead to poor electrochemical performance of materials and rapid capacity decay. It is found that these problems can be effectively solved by doping modification. Doping with appropriate ions can inhibit the phase transition and stabilize the structure, increase the kinetics by expanding the interlayer spacing, and also improve the discharge capacity and conductivity of the material, thereby improving the rate performance and cycle stability of the material.

And (3) the cathode material of the sodium-ion battery obtained at present still cannot be applied on a large scale, and the next step is to regulate and control the doping ions to improve the performance, carry out co-doping of various ions in the material, and study the synergistic effect of various ions, so as to obtain the electrode material with more excellent performance. Doping, coating and morphology control can also be combined to make full use of the advantages of various modification methods to improve the electrochemical performance of the material. Finally, it is hoped that the summary of this paper can provide some reference for the research of cathode materials, and realize the large-scale industrial application of sodium-ion batteries as soon as possible.

References

Publishing order | Descend order by publishing year | Descend order by cited within

[1]	Nagelberg A S, Worrell W L. J. Solid State Chem., 1979, 29(3): 345.

[2]	Whittingham M S. Prog. Solid State Chem., 1978, 12(1): 41.

[3]	Abraham K M. Solid State Ion., 1982, 7(3): 199.

[4]	Johnson W B, Worrell W L. Synth. Met., 1982, 4(3): 225.

[5]	Hwang J Y, Myung S T, Sun Y K. Chem. Soc. Rev., 2017, 46(12): 3529.

[6]	Slater M D, Kim D, Lee E, Johnson C S. Adv. Funct. Mater., 2013, 23(8): 947.

[7]	Kundu D P, Talaie E, Duffort V, Nazar L F. Angew. Chem. Int. Ed., 2015, 54(11): 3431.

[8]	Yabuuchi N, Kubota K, Dahbi M, Komaba S. Chem. Rev., 2014, 114(23): 11636.

[9]	Ning Z, Liu Y, Chen C, Tao Z, Chen J. Chin. J. Inorg. Chem., 2015, 31(9): 1739.

[10]	Armand M, Tarascon J M. Nature, 2008, 451(7179): 652.

[11]	Dunn B, Kamath H, Tarascon J M. Science, 2011, 334(6058): 928.

[12]	Pan H L, Hu Y S, Chen L Q. Energy Environ. Sci., 2013, 6(8): 2338.

[13]	Yang Z G, Zhang J L, Kintner-Meyer M C W, Lu X C, Choi D, Lemmon J P, Liu J. Chem. Rev., 2011, 111(5): 3577.

[14]	Kim H, Kim H, Ding Z, Lee M H, Lim K, Yoon G, Kang K. Adv. Energy Mater., 2016, 6(19): 1600943.

[15]	Ong S P, Chevrier V L, Hautier G, Jain A, Moore C, Kim S, Ma X H, Ceder G. Energy Environ. Sci., 2011, 4(9): 3680.

[16]	Tang Y C, Zhao Z B, Wang Y W, Dong Y F, Liu Y, Wang X Z, Qiu J S. Electrochimica Acta, 2017, 225: 369.

[17]	Zhao C T, Yu C, Zhang M D, Sun Q, Li S F, Norouzi Banis M, Han X T, Dong Q, Yang J, Wang G, Sun X L, Qiu J S. Nano Energy, 2017, 41: 66.

[18]	Qian J F. Doctoral Dissertation of Wuhan University, 2012.

[19]	Zhao L W. Master Dissertation of Soochow University, 2013.

[20]	Qi Y R. Doctoral Dis sertation of University of Chinese Academy of Sciences. 2019.

[21]	(戚钰若. 中国科学院大学博士论文. 2019.).

[22]	Xiang X D, Zhang K, Chen J. Adv. Mater., 2015, 27(36): 5343.

[23]	Li W J, Han C, Wang W L, Gebert F, Chou S L, Liu H K, Zhang X H, Dou S X. Adv. Energy Mater., 2017, 7(24): 1700274.

[24]	Cai Y, Cao X, Luo Z, Fang G, Liang S. Adv Sci, 2018, 5(9), 1800680.

[25]	Lin C, Fiore M, Ji E W, Ruffo R, Do-Kyung Kim, Longoni G. Adv. Sustainable Syst., 2018, 2(3): 1700153.

[26]	Skundin A M, Kulova T L, Yaroslavtsev A B. Russ. J. Electrochem., 2018, 54(2): 113.

[27]	Liang Y R, Lai W H, Miao Z C, Chou S L. Small, 2018, 14(5): 1702514.

[28]	Kubota K, Dahbi M, Hosaka T, Kumakura S, Komaba S. Chem. Rec., 2018, 18(4): 459.

[29]	Wang Y, Liu W, Guo R, Luo Y, Xie J. Chem. Ind. Eng. Prog., 2018, 37(8): 3056.

[30]	Bucher N, Hartung S, Franklin J B, Wise A M, Madhavi S. Chem. Mater., 2016, 28(7): 2041.

[31]	Yabuuchi N, Kajiyama M, Yamada Y, Komaba S. Nature Mater., 2012, 11(6): 512.

[32]	Lee D H, Xu J, Meng Y S. Phys. Chem. Chem. Phys., 2013, 15(9): 3304.

[33]	Ramasamy H V, Kaliyappan K, Thangavel R, Seong W M, Kang K, Chen Z W, Lee Y S. J. Phys. Chem. Lett., 2017, 8(20): 5021.

[34]	Mu L Q, Xu S Y, Li Y M, Hu Y S, Li H, Chen L Q, Huang X J. Adv. Mater., 2015, 27(43): 6928.

[35]	Cao M H, Wang Y, Shadike Z, Yue J L, Hu E Y, Bak S M, Zhou Y N, Yang X Q, Fu Z W. J. Mater. Chem. A, 2017, 5(11): 5442.

[36]	Wang P F, You Y, Yin Y X, Wang Y S. Angew Chem., 2016, 55(26): 7445.

[37]	Gong W Z, Zeng R, Su S, Wan M, Rao Z X, Xue L H, Zhang W X. J. Nanoparticle Res., 2019, 21(12): 1.

[38]	Fu H Y, Liu C F, Zhang C K, Ma W D, Wang K, Li Z Y, Lu X M, Cao G Z. J. Mater. Chem. A, 2017, 5(20): 9604.

[39]	Xie B X, Zuo P J, Wang L G, Wang J J, Huo H, He M X, Shu J, Li H F, Lou S F, Yin G P. Nano Energy, 2019, 61: 201.

[40]	Yang D Z, Xu J, Liao X Z, He Y S, Liu H M, Ma Z F. Chem. Commun., 2014, 50(87): 13377.

[41]	Man X, Xu M, Huang Y, Chen R, Feng W. Electrochem Commun., 2015, 59: 91.

[42]	Wu X, Wu C, Wei C, Ling H, Yang H. ACS Appl. Mater. Interfaces, 2016, 8(8): 5393.

[43]	Yu S L, Li Y, Lu Y H, Xu B, Wang Q T, Yan M, Jiang Y Z. J. Power Sources, 2015, 275: 45.

[44]	Liu Y C, Zhang N, Wang F F, Liu X B, Jiao L F, Fan L Z. Adv. Funct. Mater., 2018, 28(30): 1801917.

[45]	Wang Z Y, Liu J M, Du Z J, Tao H Z, Yue Y Z. Inorg. Chem. Front., 2020, 7(5): 1289.

[46]	Chen L, Jin S, Liu H, Chen S, Chen L. J. Alloys Compd., 2019, 821: 153206.

[47]	Gao F, Yang K, Lv Y Y, Zhao L N, Fan M S, Liu H, Geng M M, Zhang M J, Wang K F. Synthetic Materials Aging and Application, 2019, 48 (3): 54.

[48]	(高飞, 杨凯, 吕扬阳, 赵丽娜, 范茂松, 刘皓, 耿萌萌, 张明杰, 王凯丰. 合成材料老化与应用, 2019, 48 (3): 54.).

[49]	Zhu T, Hu P, Wang X P, Liu Z H, Luo W, Owusu K A, Cao W W, Shi C W, Li J T, Zhou L, Mai L Q. Adv. Energy Mater., 2019, 9(9): 1803436.

[50]	Zhang J, Liu Y, Zhao X, He L, Chen J. Adv. Mater., 2020, 32(11):1906348.1.

[51]	Kim H, Yoon G, Park I, Park K Y, Lee B, Kim J, Park Y U, Jung S K, Lim H D, Ahn D, Lee S, Kang K. Energy Environ. Sci., 2015, 8(11): 3325.

[52]	Mu L Q, Qi X G, Hu Y S, Li H, Chen L Q, Huang X J. Energy Storage and Technol, 2016, 5(3): 324.

[53]	Delmas C, Fouassier C, Hagenmuller P. Phys. B+C, 1980, 99(1/4): 81.

[54]	Xia X, Dahn J R. Electrochem. Solid-State Lett., 2012, 15(1): A1.

[55]	Sun Y, Guo S H, Zhou H S. Energy Environ. Sci., 2019, 12(3): 825.

[56]	Yang L F, Li X, Liu J, Xiong S, Ma X T, Liu P, Bai J M, Xu W Q, Tang Y Z, Hu Y Y, Liu M L, Chen H L. J. Am. Chem. Soc., 2019, 141(16): 6680.

[57]	Komaba S, Yabuuchi N, Nakayama T, Ogata A, Ishikawa T, Nakai I. Inorg. Chem., 2012, 51(11): 6211.

[58]	Yue J L, Zhou Y N, Yu X Q, Bak S M, Yang X Q, Fu Z W. J. Mater. Chem. A, 2015, 3(46): 23261.

[59]	Guo H, Wang Y S, Han W Z, Yu Z X, Qi X G, Sun K, Hu Y S, Liu Y T, Chen D F, Chen L Q. Electrochimica Acta, 2015, 158: 258.

[60]	Wang H, Yang B J, Liao X Z, Xu J, Yang D Z, He Y S, Ma Z F. Electrochimica Acta, 2013, 113: 200.

[61]	Xu J, Lee D H, ClÉment R J, Yu X Q, Leskes M, Pell A J, Pintacuda G, Yang X Q, Grey C P, Meng Y S. Chem. Mater., 2014, 26(2): 1260.

[62]	Kataoka R, Mukai T, Yoshizawa A, Sakai T. J. Electrochem. Soc., 2013, 160(6): A933.

[63]	Carlier D, Cheng J H, Berthelot R, Guignard M, Yoncheva M, Stoyanova R, Hwang B J, Delmas C. Dalton Trans., 2011, 40(36): 9306.

[64]	Lu Y H, Wang L, Cheng J G, Goodenough J B. Chem. Commun., 2012, 48(52): 6544.

[65]	Wessells C D, Huggins R A, Cui Y. Nat. Commun., 2011, 2: 550.

[66]	Wang L, Lu Y H, Liu J, Xu M W, Cheng J G, Zhang D W, Goodenough J B. Angew. Chem., 2013, 125(7): 2018.

[67]	Matsuda T, Takachi M, Moritomo Y. Chem. Commun., 2013, 49(27): 2750.

[68]	Zhou M, Qian J F, Ai X P, Yang H X. Adv. Mater., 2011, 23(42): 4913.

[69]	Lee H, Kim Y I, Park J K, Choi J W. Chem. Commun., 2012, 48(67): 8416.

[70]	Okubo M, Asakura D, Mizuno Y, Kim J D, Mizokawa T, Kudo T, Honma I. J. Phys. Chem. Lett., 2010, 1(14): 2063.

[71]	Pasta M, Wessells C D, Huggins R A, Cui Y. Nat. Commun., 2012, 3: 1149.

[72]	Mizuno Y, Okubo M, Kagesawa K, Asakura D, Kojima N. Inorg Chem, 2012, 51(19): 10311.

[73]	Mizuno Y, Okubo M, Hosono E, Kudo T, Zhou H S, Oh-Ishi K. J. Phys. Chem. C, 2013, 117(21): 10877.

[74]	Minowa H, Yui Y, Ono Y, Hayashi M, Hayashi K, Kobayashi R, Takahashi K. Solid State Ion., 2014, 262: 216.

[75]	Moritomo Y, Urase S, Shibata T. Electrochimica Acta, 2016, 210: 963.

[76]	Jiang X L, Liu H J, Song J, Yin C F, Xu H Y. J. Mater. Chem. A, 2016, 4(41): 16205.

[77]	Shen C, Long H, Wang G C, Lu W, Shao L, Xie K Y. J. Mater. Chem. A, 2018, 6(14): 6007.

[78]	Shi Z C, Yang Y. Progress in Chemistry, 2005, 17(4): 604.

[79]	Chen J. Doctoral Dissertation of Jilin University, 2013.

[80]	Padhi A K, Manivannan V, Goodenough J B. J. Electrochem. Soc., 1998, 145(5): 1518.

[81]	Barpanda P, Lander L, Nishimura S I, Yamada A. Adv. Energy Mater., 2018, 8(17): 1703055.

[82]	Masquelier C, Croguennec L. Chem. Rev.. 2013, 113(8):6552.

[83]	Pan W L, Guan W H, Jiang Y Z. Acta Phys-Chim Sin, 2020, 36(5): 1905017.

[84]	Padhi A K. J. Electrochem. Soc., 1997, 144(4):1188.

[85]	Yamada A, Chung S C, Hinokuma K. ChemInform, 2010, 32(29):17.

[86]	Huang H, Yin S C, Nazar L F. Electrochem. Solid-State Lett., 2001, 4(10): A170.

[87]	Zhu Y J, Xu Y H, Liu Y H, Luo C, Wang C S. Nanoscale, 2013, 5(2): 780.

[88]	Li H, Yu X Q, Bai Y, Wu F, Wu C, Liu L Y, Yang X Q. J. Mater. Chem. A, 2015, 3(18): 9578.

[89]	Wu X H, Xu G L, Zhong G M, Gong Z L, McDonald M J, Zheng S Y, Fu R Q, Chen Z H, Amine K, Yang Y. ACS Appl. Mater. Interfaces, 2016, 8(34): 22227.