English

• INTRODUCTION

  Along with the successful implementation of the two industrial revolutions, human beings have made rapid progress in the development of science and technology, especially in the field of energy. At the same time, the development and consumption of lots of non-renewable energy sources have produced irreversible damages to the ecological environment of nature to a certain extent. People are increasingly aware of the importance of protecting, maintaining and improving the ecological environment: on one hand, reducing the development and utilization of non-renewable resources such as oil and coal; on the other hand, continuously accelerating the development and use of new and clean energy.^[1-4] Lithium-ion batteries (LIBs) take the lead in the 3C market by their high discharge capacity, wide operating temperature range, excellent cycle stability, portability and environmental friendliness.^[5-6]

  To a certain extent, the cathode material determines the performance of LIBs. At present, the most widely used cathode materials include lithium iron phosphate (LFP), lithium cobaltate (LCO) and ordinary ternary materials (NCM). Among them, the low vibration density of LFP is its biggest shortcoming; high cost and poor safety are fatal drawbacks of LCO; the safety of NCM is also a concern.^{[1, 3, 7-9]} To pursue materials with more excellent performance, researchers focus on lithium-rich manganese base.^[10-11] As shown in Table 1, compared with the above materials, LR has the advantages of high specific capacity.^[12-15] The chemical formula of LR is xLi₂MnO₃·(1-x)LiMO₂ (M = transition metal). The remarkable electrochemical feature of LR lies in its two-stage voltage plateau during the first charge, the voltage plat-form of which is between 3.8 and 4.4 V provided by Li⁺ extracted from the LiMO₂ phase. The voltage plateau of the second stage is about 4.5 V, which is provided by Li⁺ extracted from the Li₂MnO₃ phase.^{[4, 16]} The reason for the high capacity of LR cathode material may be the redox reaction of ions. This redox reaction is much more complicated than the conventional cathode materials. By comparison with traditional cathode materials, the oxygen vacancies generated by the loss of oxygen can lead to irreversible structure transition from the outside to the inside of LR. The specific manifestations are the low Coulombic efficiency (ICE) during the first charge-discharge process and the continuous decay of the discharge voltage and capacity during the cycling process. In addition, the migration, segregation, and continuous reduction of the average valence state of transition metal elements in the cathode material are also the reasons for the loss of discharge capacity and voltage decay during cycling.^{[4, 16]} Therefore, it is inevitable to modify LR. At first, researchers only performed a single modification of the material to propel the application of materials: element doping, surface coating, structural design, etc.^[17-20] Element doping helps to improve the rate capability of LR, surface coating can suppress the capacity decay during cycling, and structural changes are beneficial to the transport of ions in the material. The organic combination of single modification can realize the superposition of modification results. We summarize some of the multiple modifications that have yielded better electrochemical performance.^[21-23]

  Table 1

  

  Table 1.  Electrochemical Properties of Common Lithium-Ion Battery Cathode Materials^[2]

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  Abbreviation	Representation	Voltage (V)	Theoretical energy density (mAh g-1)
  LFP	LiFePO4	2.3-2.5	170
  LCO	LiCoO2	3.7-3.9	200
  NCM	LiNiCoMnO2	3.8-4.0	200
  LR	Li2MnO3·LiMO2	3.8-4.0	250

  Herein, six kinds of multiple modifications of LR with many current applications, easy experimental operation and ideal results are reviewed and summarized. Reasonable conjectures and pro-spects are put forward for the future modification direction of LR. The advantages and disadvantages of LR determine that this material is bound to have an inseparable relationship with modification.

  MULTIPLE MODIFICATION

  Multi-element Doping. Multi-element doping plays a key role in the performance improvement of LR at high rate, but is not very prominent in improving the cycling stability. The voltage decay of LR is related to the loss of lattice oxygen, which can be partially suppressed by element doping.^[24-25] Wenjun Jiang et al. modified LR by simultaneously doping two cations, Nd and Al. Co-precipitation method and high-temperature solid-state method were used to prepare modified materials doped with both Nd and Al, and the original LR without doped elements and the control samples doped with Nd and Al respectively were made to maintain the rigor of the experiment.^[26] The structure is shown in Figure 1-(a). Interestingly, the double-doped modified material performs outstandingly at high rate, with 149.3 mAh g^-1 at 10 C. Compared with 76.4% of LR, after 200 cycles the modified material still has 82% capacity remained at 1 C. The evolution of lattice oxygen was inhibited effectively by the combined effect of Nd and Al double cation doping. This multi-element doping reduces the dissolution of TM ions and improves the conduction of charges. Synergy between multiple elements boosts the electrochemical performance of LR. Therefore, this double-ion doping modification method can occupy an important position in the modification of cathode materials.

  Figure 1

  

  Figure 1.  Schematic diagram of multi-element doping modification. (a) of Nd/Al co-doping; (b) of V/Mo co-doping; (c) of Na/P co-doping; (d) of Na/F co-doping.^[26-29]

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  Similar to the above method of simultaneously doping Nd and Al elements, Yingying Sun et al. first prepared V-doped LR materials by co-precipitation method, as shown in Figure 1-(b), and then added Mo in the subsequent experiments.^[29] Electrochemical tests show that the doping of V significantly improves the cycling stability of LR under the premise of ensuring the rate performance. After 50 cycles, the modified sample still has a capacity of 190 mAh g^-1, which is 99.8% of the initial at 0.2 C when doping 3% V. After 300 cycles the capacity is 142 mAh g^-1, which is even slightly higher than the initial when doping 2% V. The doping of Mo greatly improves the discharge capacity of the cathode material at high rates. After doping 3% V and Mo, the discharge capacity of the modified material at 5 C is 100 mAh g^-1, 52.7% of that at 0.1 C. The unmodified material has about 80 mAh g^-1 capacity, which is only 39.8% of that at 0.1 C. The excellent cycling stability of the co-doped modified material is due to the doping of V, and the performance at high rate is attributed to Mo. The two elements work together to suppress the phase conversion of LR, which significantly improves the conductivity of LR. The modification method of successively doping two different elements may take an enlightening location in the modification of cathode materials for LIBs.

  Co-doping of anions and cations will also have a huge impact on LR materials.^[30-31] Hao Liu et al. prepared Na-doping, PO₄^- doping and Na⁺ and PO₄^- double-doping, respectively. The LMR-NPs are shown in Figure 1-(c).^[28] The rate performance of LMR-NP is excellent. At a high rate of 5 C, the modified sample has a discharge capacity of 153 mAh g^-1. After 150 cycles, it still has 86.7% of the initial at 1 C. High-temperature calcination makes the lattice of LMR-NPs uniform, in which Na⁺ plays the role of broadening the Li⁺ transport channel, accelerating the conduction of Li⁺, and preventing the migration of TM ions, thus inhibiting the phase evolution of LR. The doping of PO₄^- to occupy the O site reduces the oxidation at high temperature, and also helps to stabilize the crystal structure during cycling to some extent due to the larger volume of PO₄^-. The synergistic effect of Na⁺ and PO₄^- boosts the performance at high rate of LR and also enhances its cycle performance. This modification idea is suitable for a variety of LIBs cathode materials.

  Others have also done some researches on anion-cation co-doping, as shown in Figure 1-(d). Dongming Liu et al. proposed a strategy of anion-cation co-doping to modify LR by co-precipitation and high-temperature solidification. Na & F-LR co-doped with Na, F was prepared by phase method.^[27] Na and F occupied the Li and O sites, respectively. Compared with single doping, the modified sample is further improved. The discharge capacity of Na & F-LR at 5 C is 167 mAh g^-1, and after 100 cycles the modified sample still remains 100% capacity at 0.2 C. Na⁺ doping stabilizes the crystal structure of LR, inhibits its phase transition, and thereby effectively improves its cycling stability. The doping of F significantly increases the ionic and electronic conductivities in LR, resulting in a marked improvement in the electrochemical performance of the modified sample at high rates. The co-doping of Na and F preserves their respective good effects on LR, which further improves the electrochemical performance. The multi-element doping method adjusts the crystal structure with stabilizing interlayer spacing and maintaining the microscopic form, which will also be instructive to study other cathode materials.^[32-34]

  Multi-surface Coating. Multi-surface coating can well maintain the crystal structure of LR during cycling, but it is not obvious for improving the discharge capacity. In addition to the co-doping of the previous part, multiple cladding layers are also a good means of multiple modification. Kailing Sun et al. prepared LiV₃O₈/C co-coated LR by heat treatment, and the structure is shown in Figure 2-(a).^[35] ICE of the modified material reached 94%, but as for LR it was only 73%. After 50 cycles, the capacity retention of LR was 70% and that of the modified material was 86% at 1 C. At 5 C, the modified material had 176 mAh g^-1 capacity, meanwhile the LR was only 108 mAh g^-1. The enhanced discharge capacity is credited to the lithium host properties of LiV₃O₈ and the electronic conductivity of C. C coating layer effectively drops the resistance to charge transport and reduces the electrode polarization. The cycle performance of the cathode material is improved. The synergistic effect of the two different coating layers enhances the electron transfer capability of the modified sample.

  Figure 2

  

  Figure 2.  Schematic diagram of multi-surface coating modification. (a) of LiV₃O₈/C co-coating; (b) of LATP/PANI co-coating; (c) of Al₂O₃/SiO₂ co-coating; (d) of Li₄Mn₅O₁₂/MgF₂ co-coating.^[35-38]

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  Hybrid surface modification significantly improves the electrochemical performance of cathode materials. Xiangwan Lai et al. prepared Li_1.4Al_0.4Ti_1.6(PO₄)₃(LATP) and polyaniline (PANI) hybrid modified LR by sol-gel method. Figure 2-(b) shows the specific process.^[36] At 5 C LRNCM@3wt.%LATP@1wt.%PANI has 87 mAh g^-1 capacity, while unmodified LR only has 76 mAh g^-1. After 200 cycles, the capacity of LRNCM@3wt.%LATP@1wt.%PANI decayed from 238 to 139 mAh g^-1 at 1 C with 79% capacity remained. The capacity retention rate under the same conditions of LR is 60%. The successfully deposited hybrid coating obviously enhanced the electrochemical performance of LR, and the hybrid layer well protected the cathode material from side reactions and improved the kinetic performance of the cathode material. This hybrid modification layer can effectively maintain the LR crystal structure, which is helpful for guiding the performance improvement of LR and other materials for LIBs.

  The difference between the multi-layer coating and the hybrid surface modification is that the multi-layer coating can better maintain the structure of LR and improve the cycle stability of the cathode material.^[37] Wenxu Zhang et al. chose a simple method to coat Al₂O₃ and SiO₂ on the LR surface, as shown in Figure 2-(c). When the total coating amount was 2wt.% with the ratio of Al₂O₃ to SiO₂ being 2:1, the electrochemical performance of the modified material is the best. Compared with original LR, the ICE of the modified material increased from 81.42% to 84.55%. The discharge capacity at 5 C reaches 146.9 mAh g^-1. After 100 cycles the capacity at 1 C is greatly increased from 93.9 to 170.3 mAh g^-1. The multilayer coating of Al₂O₃ and SiO₂ forms a protective Li_x[Al_ySi_zO₄] layer and spinel structure layer and so on. The spinel phase significantly hinders the lattice oxygen escaping, which hinders the harmful phase evolution and effectively improves the cycle performance of LR. In addition, the coating layer also prevents the side reactions of LR by the electrolyte and inhibits the occurrence of side reactions. Al₂O₃ and SiO₂ coating together enhance the electrochemical performance of LR, and this modification method of multilayer coating provides a reference direction for the commercialization of LR in the future.

  The low ICE is a major shortcoming of LR. As mentioned, the loss of irreversible lattice oxygen seriously jeopardizes the cycling stability of LR. As shown in Figure 2-(d), Wei Zhu et al. used liquid-phase etching and liquid-phase deposition to coat Li₄Mn₅O₁₂ and MgF₂ in the outer layer of LR to shape a double coating layer.^[38] The ICE of the modified one is astonishing 96.4%, while that of the original LR is only 77.3%. After 300 cycles, the capacity retention rate of modified sample reaches 80% at 1 C, compared to LR with only 51% capacity remained. Li₄Mn₅O₁₂ has three-dimensional channels for Li⁺ conduction. They protect LR from side reactions and reduce the evolution of lattice oxygen. The MgF₂ coating also plays a protective role. The combined effect of the double coating inhibits the harmful transformation of the cathode material, improves the dynamic performance of the electrode material, and enhances the ICE of the modified LR. The efficiency reaches an astonishing 96.4%, while also significantly improving the cycling performance of the material and suppressing voltage decay. This modification method provides a competitive reference for the practical application of LR materials.

  Element Doping and Surface Coating. Element doping and sur-face coating can combine their advantages, but this multiple modification is not easy to handle. In order to avoid the limitation of a single coating or single doping, Min Li et al. used fluorite-structured CaF₂ as a coating material to limit the erosion of the cathode material by the electrolyte, and at the same time doped La³⁺ into the bulk phase of the material, ^[39] inhibiting the local phase transition of active materials from layered to spinel structures. As depicted in Figure 3-(a), they synthesized La-doping layered LR@CaF₂ by combining solvothermal method and high-temperature solid-phase calcination method. By comparison with the unmodified material, the electrochemical performance of the material doped with La³⁺ and coated with CaF₂ is greatly improved. At 0.5 C it provided 227.1 mAh g^-1 capacity, and 93.9% left after 100 cycles. However, LR presented only 217.3 mAh g^-1, and 71.9% remained under the same conditions. In addition, La-doping layered LR@CaF₂ decayed by 0.203 V, while LR decayed by 0.409 V after 100 cycles at 0.5 C. For the sake of the rigor of the experiment, a comparative experiment of only La³⁺ doping and CaF₂ coating modified materials was also performed, finding although there was some improvement in electrochemical performance, it was not as good as La-doping layered LR@CaF₂. They attribute this improvement in performance to the combined effect of La³⁺ doping and CaF₂ coating.

  Figure 3

  

  Figure 3.  Schematic diagram of doping & coating modification. (a) of La doping and CaF₂ coating; (b) of Cr doping and Li₃PO₄ coating; (c) of B doping and C coating; (d) of La and Zr co-doping and LLZO coating.^[39-42]

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  Similar to Min Li et al., Zige Tai et al. also used the same method to prepare LR with simultaneous element doping and surface coating.^[42] They prepared Cr-doped and Li₃PO₄-coated modified materials which were characterized by a series of tests, such as ICP, XRD, XPS, SEM, TEM, etc. As shown in Figure 3-(b), Cr³⁺ occupied the Co³⁺ site for doping into the material, and Li₃PO₄ effectively coated the surface of the material. Compared with LR, the new sample had a significant improvement in cycle stability. After 200 cycles the capacity remain boosted from 59.0% to 76.5% at 0.5 C. The voltage decay was reduced from 0.84 to 0.39 V. They attributed the upgrade of the cycle steadiness of LR to the synergistic effect of Cr³⁺ doping and Li₃PO₄ coating. The redox reaction of Cr ions suppressed the voltage decay to a certain extent and limited the phase transition of LR. The Li₃PO₄ coating provided Li⁺ transport channels and suppressed the release of lattice oxygen, resulting in an increase in the battery capacity retention. Regrettably, they did not conduct a comparative experiment of single Cr³⁺ doping and single Li₃PO₄ coating on the original sample. If there are two comparative experimental data mentioned above, the persuasiveness of the experimental conclusion will be significantly improved.

  The study of the modification mechanism is a particularly important experimental process, and the elucidation of the mechanism means universality.^[43-45] As shown in Figure 3-(c), Shiyou Li et al. doped B into LR by co-precipitation method, and continued to coat C on the surface of the material by liquid phase method. They formed a composite phase such as LR@spinel phase@C coating layer, and obtained the final modified material LRM-B/C.^[40] It is undeniable that LRM-B/C shows an outstanding discharge capacity of 243.6 mAh g^-1 compared to LR's of 200.9 mAh g^-1 at 0.1 C. After 100 cycles LRM-B/C retained 15.52% more capacity than LR. In addition, LRM-B/C exhibited excellent electrochemical performance in a wide temperature range. Under similar conditions, LRM only had 53.39% capacity, while LRM-B/C still held 83.30% capacity at 0.1 C for 100 cycles at 45 ℃. At -20 ℃, LRM-B/C exhibited an unexpected capacity of 108.9 mAh g^-1. In their article, they elaborated the mechanism of B doping and C coating synergistically modifying LR. B doping played a role in suppressing the loss of lattice oxygen, and C coating on the surface effectively protected the battery material and hindered the occurrence of side reactions and the deleterious phase transition of LR. There is no doubt that this modification method will play a certain guiding role in the research of other researchers in the field of battery materials in the future.

  Simple and efficient experimental procedures are what every scientific researcher wants.^[46-49] Chao Shen et al. prepared La, Zr ion co-doped, Li₇La₃Zr₂O₁₂ (LLZO)-coated LR cathode material S-LLNCM through a simple one-bath treatment method.^[41] As shown in Figure 3-(d), the biggest feature of this modification method is that in the process of LLZO coating, the La and Zr elements in the coating material realize the doping process. Although the experimental steps are simple, the doping of La and Zr ions and the coating of LLZO are achieved at the same time. The simple and refined experiment brings amazing changes, which is the biggest shining point of this experiment. Compared with the improvement of electrochemical performance at room temperature, S-LLNCM had more surprising performance at low temperature. 212.8, 173.8 and 134.1 mAh g^-1 capacities were obtained at 0, -10 and -20 ℃, respectively. Meanwhile, LR had only 170.2, 98.8, 50.0 mAh g^-1. Large particle size ion doping expands the Li⁺ transport channel and accelerates the Li⁺ transfer; the LLZO coating effectively suppresses the voltage decay and the harmful phase transition, improving the cycling stability. This modification method is of great significance to the modification of LR and even cathode materials for LIBs. This article proposes a universal modification idea.

  Element Doping and Structure Design. Element doping and structural design modification can obtain LR with excellent electrochemical performance, but the cost is slightly higher. Electrochemical performance is under the influence of structure. Dong Luo et al. prepared the precursor MCO₃ first, and then used the high-temperature solid-phase synthesis method to continue to prepare LR modified material CG.^[50] The innovation of this modified material lies in that the structures of the particles are changed on the basis of ion doping. LR microsphere particles with dual concentration gradients were constructed using a solvothermal method to control the concentration of CO₃^2-. The designed microsphere particles have a double concentration gradient of Ni/Mn and Al: as shown in Figure 4-(a), from the outside to the inside of the particle cross section, the content of Al element first decreases and then increases, while the contents of both Ni and Mn elements decrease. In the range of 2.0-4.8 V, the average voltage decay per cycle is 0.97 mV at 0.4 C. Meanwhile, capacity retained 84.1% after 400 cycles at 1.2 C. The doping of Al element effectively suppressed the occurrence of side reactions and increased the structural stability of LR microsphere particles; the novel LR particle structure (double concentration gradient) prevented the structural evolution and hindered the generation of low-valent Mn⁺ ion redox couples during cycling. The joint effect of element doping and the structural design of new LR particles improves the capacity retention rate of the material, and significantly improves the problems of rapid voltage decay and poor cycle stability of LR itself. This modification idea has a huge impact in the modification research of LR.

  Figure 4

  

  Figure 4.  Schematic diagram of doping & structure design modification. (a) of Al doping and structure design; (b) of Nb doping and structure design; (c) of Na doping and structure design; (d) of Ti doping and structure design.^{[50, 54-56]}

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  The loss of lattice oxygen and structural transformation have been the main reasons for the poor cycling stability of LR.^[51-53] Chunxiao Zhang et al. proposed doping Nb to modify LR, and the doping of Nb changed the surface structure of the material.^[54] The carbonate precursor and pristine LR cathode material were prepared in advance, and then the Nb-doped modified material was synthesized by liquid-phase method. As shown in Figure 4-(b), the doping of Nb promotes the structural change of the LR surface, forming a Ni-rich layered/rock-salt inhomogeneous interface. The effect of modification is particularly significant on the basis of the original LR, and the cycle stability is greatly improved. The Nb-doped material still has 181.7 mAh g^-1 capacity remained after 200 cycles, which is 85.5% of the initial at 1 C; while LR only remains less than 100 mAh g^-1. In addition, the modified material still performs well under high temperature conditions. The reason is that the doping of Nb replaces part of the position of Ni ions to form a more stable Nb-O bond, inhibiting the breakage of the TM-O bond, which effectively inhibits the crystallinity. During battery cycling, the migration of transition metal ions was promoted by the escape of lattice oxygen. On the other hand, the layered/rock-salt heterointerface generated on the LR surface due to Nb doping plays an important role in hindering the structural degradation during charge-discharge. This modification method of element doping to positively affect the material structure can provide guidance for the modification of cathode materials.

  Doping with morphological design is a good modification idea for battery cathode materials.^[57] As shown in Figure 4-(c), Qian Wang et al. first prepared pristine LR, and then obtained Na-doped sodium citrate and polyethylene glycol as chelating agents.^[56] Modified material SCLR has good layered structure. The rigor of this experiment is that not only a blank sample for adding a chelating agent but also a control sample using citric acid or sodium acetate as comparison was prepared. After 200 cycles, it still has a high capacity of 166 mAh g^-1 at 5 C, with 90.1% of the original. After 200 cycles, the voltage retention rate is 94.1% at 10 C, and the intermediate voltage is 3.37 V. The doping of Na ions makes LR have a steadier structure and keep better structural stability during long cycles; in addition, the particles designed by sodium citrate have a structure more suitable for charging and discharging. The diffusion of Li⁺ is promoted, and the rate performance of the cathode material is well improved. This modification method, which simultaneously performs Na⁺ doping and controls the morphology of cathode material particles, not only effectively improves the cycle stability of LR, but also ensures the specific capacity of the material. It has a profound impact on the application of LIBs.

  The combination of bulk doping and structural design can indeed boost the electrochemical performance of LR.^[58] As shown in Figure 4-(d), Dong Luo et al. prepared Ti-treated modified LR materials by lava-assisted solvothermal method.^[55] Compared with the original LR, the modified material has an amazing improvement in the cycle performance: after 500 cycles, the capaci-ty retention rate of the Ti-treated material is 85% at 1 C. Capacity decays 0.72 mV in every cycle between the 30th and 500th cycles, while the blank control sample has only 63% capacity retention after 200 cycles. The doping of Ti forms stronger Ti-O bonds to inhibit the migration of TM ions; the lava-assisted solvothermal method generates a surface Ti-based integrated layer, which forms a uniform and continuous protective layer to prevent Li vacancies from being generated on the surface. This modification method of element doping and structural design shines bright in improving the cycling stability and rate performance of LR.

  Surface Coating and Structure Design. Surface coating and structural design can effectively improve the electrochemical performance of LR, but the cost is high. Under some conditions, the surface coating will have some influence on the structure of LR particles and enhance the electrochemical performance of LR.^[63-64] Jiliang Wu et al. prepared pristine LR materials by co-precipitation and high-temperature solid-phase methods. They prepared LR with stepped HEPES molarity by reflux method, and successfully obtained lithium ion-deficient LR materials.^[60] Among them, at the concentration of 10%, HEPES still has 211.4 mAh g^-1 capacity at 0.5 C, with 91.6% of cycle retention after 200 cycles. As shown in Figure 5-(a), when the concentration of HEPES is 15%, its capacity is 149.1 mAh g^-1 at 10 C. Oxygen vacancies and spinel phase coatings occupy a crucial role in the modification. The former facilitates charge transfer and accelerates Li⁺ diffusion, and the latter creates conditions for the rapid diffusion of Li⁺ on the LR surface, which enhances the rate performance of the modified LR.

  Figure 5

  

  Figure 5.  Schematic diagram of coating & structural design modification. (a) of HEPES coating and structure design; (b) of MoO_x coating and structure design; (c) of Li(Ni_1/6Co_1/6Mn_4/6)₂O₄ and LiCoMnO₄ coating and structure design; (d) of P2-type Na_0.66MnO₂ coating and structure design.^[59-62]

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  Despite the high specific capacity of LR, the evolution of oxygen during the first charge-discharge severely compromises their electrochemical performance.^[65-67] Zhe Yang et al. first prepared LR by co-precipitation method, and then prepared MoO_x-modified LR by hydrolysis method, ^[61] with the structure shown in Figure 5-(b). The electrochemical test found that when the MoO_x coating amount was 3wt.%, the modified material performed best. After 100 cycles, it still had 224.2 mAh g^-1 capacity at 0.5 C, and the cycle maintained 85.8%. Under the same conditions as LR, the capacity was only 187.4 mAh g^-1 and the capacity only remained 75.1%. Modified material performs well at high rate, having 192.0 mAh g^-1 capacity at 5 C. The existence of MoO_x coating obviously inhibits the irreversible loss of oxygen during the first cycle of LR. At the same time, LR is protected from side reactions. The spinel phase induced by MoO_x provides the rapid diffusion of lithium ions. It also provides a channel for the charge transfer process, improving the rate performance of LR. The modified method, inducing spinel phase on LR surface while coating modification, is very effective for boosting the electrochemical performance of LR.

  Shenghua Yuan et al. designed a novel double spinel shell structure to better the defects of LR.^[62] As shown in Figure 5-(c), LR was synthesized first, and modified material LSS was formed subsequently. When the concentration of cobalt acetate and manganese is controlled at 5%, LSS-0.05 has the best electrochemical performance. At 5 C it has a capacity of 145 mAh g^-1. At 0.2 C it has 232 mAh g^-1 after 100 cycles, and the retention rate reaches 92.7%. The capacity of the original LR at 5 C is less than 100 mAh g^-1. The reason for the superior rate performance of LSS is spinel phase which provides a large number of three-dimensional diffusion channels of Li⁺, thus accelerating the diffusion of Li⁺. The good cycle performance is attributed to the coating layer which effectively prevents side reactions with electrolyte, enhances the structural stability and cycling stability, and thereby improves the capacity retention rate during charging and discharging. The combined effect of surface coating and structure design enhances the electrochemical performance after LR modification.

  LR has low ICE cause of irreversible evolution of lattice oxygen, which also leads to rapid voltage decay and poor cycling stability.^[68-69] Chao Huang et al. prepared a P2-NMO phase-coated LRMO/P2 modified material with Na vacancies by co-precipitation and high-temperature solid-phase sintering, as shown in Figure 5-(d).^[59] The ICE of LRMO is 83.8%, compared with 92.3% of LRMO/P2, which is a great improvement. The high ICE is due to the fact that the P2-NMO phase provides more vacancies, and the ingress of Li⁺ offsets the capacity loss to a certain extent. LRMO/P2 has a capacity of 150 mAh g^-1. Meanwhile, the capacity of LRMO is only 93 mAh g^-1 at 5 C. After 240 cycles at 1 C, LRMO/P2 still retains 80% of its capacity, while LRMO only has 65.3% of its capacity remaining. The P2-NMO phase acts as a coating to protect LRMO and inhibits side reactions and detrimental phase transitions. In addition, this Na⁺ vacancy-containing phase coating also effectively improves the ICE of the cathode material and provides channels for Li⁺ to transport, thereby enhancing the discharge capacity of LR at a high rate.

  Element Doping, Surface Coating and Structure Design. Element doping, surface coating and structure design can significantly improve the electrochemical performance of LR, but it requires high experimental process. Structural stability, ionic and electronic conductivity are critical properties of LIBs.^[74-76] As shown in Figure 6-(a), Junxin Chen et al. prepared Zr bulk-doped and Li₂ZrO₃-coated hollow-structure LR by co-precipitation and high-temperature solid-phase methods.^[70] After 100 cycles at 1C, the LR capacity decayed from 170 to 124 mAh g^-1 with 71% capacity remained, while the Li₂ZrO₃-coated LR capacity decreased from 177 to 143 mAh g^-1 with 81% capacity reserved. The excellent electrochemical performance of LR, especially the cycle stability, is attributed to this composite modification method. The Li₂ZrO₃ coating layer protected LR from the side reactions and hindered the harmful phase transformation. In addition, the gradient distribution of Zr within LR during sintering contributes to the structural stability of LR. The concentration gradient distribution of Zr enhances the electrochemical performance. The simple operation obtained multiple modification results, which significantly improved the electrochemical performance of LR. This experimental idea is beneficial to the study on cathode materials for LIBs.

  Figure 6

  

  Figure 6.  Schematic diagram of doping & coating & structural design modification. (a) of Zr doping, Li₂ZrO₃ coating and structure design; (b) of Ce doping, Li₂CeO₃ coating and structure design; (c) of Nb/Al co-doping, Al₂O₃ coating and structure design; (d) of Co doping, Li₂MoO₄ coating and structure design.^[70-73]

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  This method of multiple modification has been favored by many researchers.^[77-79] Jun Chen et al. modified LR by using the strate-gy of simultaneous lithium oxidation, coating Li₂CeO₃ and performing bulk doping of Ce.^[72] Spinel phase was induced on the particle surface by the impurity successfully. Modified sample has excellent electrochemical performance, with more than 170 mAh g^-1 capacity at 5 C, while the original LR only has about 130 mAh g^-1 capacity. In terms of cycle performance, the modified one shows outstanding cycling performance, with decaying 0.013% capacity and 1.76 mV voltage per cycle under the condition of 300 cycles at 2 C. Figure 6-(b) shows a schematic diagram of the decay of the particle cycling process. The doping of Ce element plays a pseudo-bonding effect, which reduces the evolution of lattice oxygen, and hinders harmful phase transformation of LR. Li₂CeO₃ effectively protects LR from the corrosion of the electrolyte and enhances the structural stability. The spinel phase Li₄Mn₅O₁₂ induced on the surface provides a three-dimensional channel for the rapid conduction of Li⁺ and promotes the transfer of charges. This experiment opens up novel views for the research of high-energy-density LR.

  Different from the experiments mentioned above, although Zhepu Shi et al. also adopted a composite modification of element doping, surface coating and structure design, there were more than one doped elements.^[73] Like most researchers, pristine LR and Nb co-doped, Al₂O₃-coated, and designed composite modified materials with spinel-phase shell structures were prepared by coprecipitation and high-temperature solid-state methods. Cycle performance of the modified material is significantly improved, and 57% of the capacity is maintained after 1000 cycles at 0.1 C. The 20 Ah pouch battery made with the SGC/Gr anode has a capacity of 345 Wh kg^-1, and after 340 cycles it still has 77.9% capacity retained at 0.2 C. In this experiment, through excellent design, a variety of different modification methods were integrated on the same material to construct a well-configured surface, as shown in Figure 6-(c). The existence of multi-layer composite surface effectively reduces the escape of lattice oxygen, decreases the discharge of gas, and enhances the cycle performance of LR. In addition, the excellent performance of the modified material on pouch cells is of great significance for the future commercialization of LR.

  As a major problem of LR cathode materials, lithium-nickel hybrid arrangement seriously damages the electrochemical performance of LR, which has always been a difficult problem for researchers to overcome. In order to slow down Li/Ni disorder, Junxin Chen et al. used Li₂MoO₄ coating, transition metal doping and surface structure to design a composite modification strategy.^[71] The synthesis idea is shown in Figure 6-(d). Li/Ni mixing phenomenon of the modified material is improved, and the electrochemical performance is obviously boosted. 75.9% of the capacity is maintained after 200 cycles at 1 C, and the capacity exceeds 150 mAh g^-1 at 2 C. LR has less than 70 mAh g^-1 capacity at 2 C. The Li₂MoO₄ surface coating constructs a special spinel phase layer on the LR surface to accelerate the conduction of ions and electrons, and forms a Li₂MoO₄ coating to reduce side reactions and enhance the structural stability of LR. The doping of Co inhibits Li/Ni disorder to a certain extent and improves the discharge capacity. Overall, the enhance of the electrochemical performance of the modified materials is the result of the combined effect of the above composite modification methods.

  CONCLUSION

  In summary, the characteristics of the LR material itself limit its application as a cathode material to a certain extent, and the limitations of some LR materials, such as the loss of lattice oxygen, structural decay, voltage decay, etc., can be overcome by modifi-cation. When a single modification cannot obtain the desired re-sults, multiple modification can combine the advantages of a single modification, and it naturally enters people's field of vision. We reviewed and summarized 6 different multiple modifications: doping + coating, doping + structure design, coating + structure design, multi-element doping, multi-surface coating, doping + coating + structure design. Although various multiple modifications can improve the electrochemical performance of LR, the simpler the modification process, the better the electrochemical performance is. The more modification is not the better, and the multiple modification achieved by simple operation is the optimal solution. Based on the above summary, we make a reasonable prediction for the future direction of LR modification. The modification direction of LR should be a multiple modification that integrates multiple modification methods while simplifying the modification experimental method.

  
  ACKNOWLEDGEMENTS: This work was supported by the Natural Science Foundation of Hunan Province (Nos. 2021JJ30823 and 2020JJ2048) and National Natural Science Foundation of China (No. 51974368). The authors declare no competing interests.
  COMPETING INTERESTS
  Full paper can be accessed via http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0170
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• 1. [1]

     Cai, M.; Zhang, H.; Zhang, Y.; Xiao, B.; Wang, L.; Li, M.; Wu, Y.; Sa, B.; Liao, H.; Zhang, L.; Chen, S.; Peng, D. -L.; Wang, M. -S.; Zhang, Q. Boosting the potassium-ion storage performance enabled by engineering of hierarchical MoSSe nanosheets modified with carbon on porous carbon sphere. Sci. Bull. 2022, 67, 933-945. doi: 10.1016/j.scib.2022.02.007

  2. [2]

     Camargos, P. H.; Santos, P. H. J.; Santos, I. R.; Ribeiro, G. S.; Caetano, R. E. Perspectives on Li-ion battery categories for electric vehicle applications: a review of state of the art. Int. J. Energ. Res. 2022, er, 7993.

  3. [3]

     He, W.; Guo, W.; Wu, H.; Lin, L.; Liu, Q.; Han, X.; Xie, Q.; Liu, P.; Zheng, H.; Wang, L.; Yu, X.; Peng, D. L. Challenges and recent advances in high capacity Li-rich cathode materials for high energy density lithium-ion batteries. Adv. Mater. 2021, 33, e2005937.

  4. [4]

     Zhao, S.; Guo, Z.; Yan, K.; Wan, S.; He, F.; Sun, B.; Wang, G. Towards high-energy-density lithium-ion batteries: strategies for developing high-capacity lithium-rich cathode materials. Energy Storage Mater. 2021, 34, 716-734. doi: 10.1016/j.ensm.2020.11.008

  5. [5]

     Li, C.; Wang, Z. -Y.; He, Z. -J.; Li, Y. -J.; Mao, J.; Dai, K. -H.; Yan, C.; Zheng, J. -C. An advance review of solid-state battery: challenges, progress and prospects. Sustain. Mater. Techno. 2021, 29, e00297.

  6. [6]

     Li, J.; Zhou, Z.; Luo, Z.; He, Z.; Zheng, J.; Li, Y.; Mao, J.; Dai, K. Microcrack generation and modification of Ni-rich cathodes for Li-ion batteries: a review. Sustain. Mater. Techno. 2021, 29, e00305.

  7. [7]

     Jiang, B.; Li, J.; Luo, B.; Yan, Q.; Li, H.; Liu, L.; Chu, L.; Li, Y.; Zhang, Q.; Li, M. LiPO₂F₂ electrolyte additive for high-performance Li-rich cathode material. J. Energy Chem. 2021, 60, 564-571. doi: 10.1016/j.jechem.2021.01.024

  8. [8]

     Ke, C.; Shao, R.; Zhang, Y.; Sun, Z.; Qi, S.; Zhang, H.; Li, M.; Chen, Z.; Wang, Y.; Sa, B.; Lin, H.; Liu, H.; Wang, M. S.; Chen, S.; Zhang, Q. Synergistic engineering of heterointerface and architecture in new-type ZnS/Sn heterostructures in situ encapsulated in nitrogen-doped carbon toward high-efficient lithium-ion storage. Adv. Funct. 2022, 2205635.

  9. [9]

     Li, L.; Fu, L.; Li, M.; Wang, C.; Zhao, Z.; Xie, S.; Lin, H.; Wu, X.; Liu, H.; Zhang, L.; Zhang, Q.; Tan, L. B-doped and La₄NiLiO₈-coated Ni-rich cathode with enhanced structural and interfacial stability for lithium-ion batteries. J. Energy Chem. 2022, 71, 588-594.

  10. [10]

      Liu, J.; Wang, J.; Ni, Y.; Zhang, K.; Cheng, F.; Chen, J. Recent breakthroughs and perspectives of high-energy layered oxide cathode materials for lithium ion batteries. Mater Today 2021, 43, 132-165.

  11. [11]

      Zuo, W.; Luo, M.; Liu, X.; Wu, J.; Liu, H.; Li, J.; Winter, M.; Fu, R.; Yang, W.; Yang, Y. Li-rich cathodes for rechargeable Li-based batteries: reaction mechanisms and advanced characterization techniques. Energ. Environ. Sci. 2020, 13, 4450-4497.

  12. [12]

      Chen, H.; Hu, Q.; Huang, Z.; He, Z.; Wang, Z.; Guo, H.; Li, X. Syn-thesis and electrochemical study of Zr-doped Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ as cathode material for Li-ion battery. Ceram 2016, 42, 263-269.

  13. [13]

      He, Z.; Li, J.; Luo, Z.; Zhou, Z.; Jiang, X.; Zheng, J.; Li, Y.; Mao, J.; Dai, K.; Yan, C.; Sun, Z. Enhancing cell performance of lithium-rich manganese-based materials via tailoring crystalline states of a coating layer. ACS Appl. Mater. 2021, 13, 49390-49401.

  14. [14]

      He, Z.; Ping, J.; Yi, Z.; Peng, C.; Shen, C.; Liu, J. Optimally designed interface of lithium rich layered oxides for lithium ion battery. J. Alloys Compd. 2017, 708, 1038-1045.

  15. [15]

      He, Z.; Wang, Z.; Chen, H.; Huang, Z.; Li, X.; Guo, H.; Wang, R. Electrochemical performance of zirconium doped lithium rich layered Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ oxide with porous hollow structure. J. Power Sources 2015, 299, 334-341.

  16. [16]

      Zhao, S.; Yan, K.; Zhang, J.; Sun, B.; Wang, G. Reaction mecha-nisms of layered lithium-rich cathode materials for high-energy lithium-ion batteries. Angew. Chem. Int. Ed. 2021, 60, 2208-2220.

  17. [17]

      Luo, Z.; Zhou, Z.; He, Z.; Sun, Z.; Zheng, J.; Li, Y. Enhanced electrochemical performance of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode by surface modification using La-Co-O compound. Ceram 2021, 47, 2656-2664.

  18. [18]

      Wang, R.; Sun, Y.; Yang, K.; Zheng, J.; Li, Y.; Qian, Z.; He, Z.; Zhong, S. One-time sintering process to modify xLi₂MnO₃·(1-x)LiMO₂ hollow architecture and studying their enhanced electrochemical performances. J Energy Chem. 2020, 50, 271-279.

  19. [19]

      Wei, H. -X.; Huang, Y. -D.; Tang, L. -B.; Yan, C.; He, Z. -J.; Mao, J.; Dai, K.; Wu, X. -W.; Jiang, J. -B.; Zheng, J. -C. Lithium-rich manganese-based cathode materials with highly stable lattice and surface enabled by perovskite-type phase-compatible layer. Nano. Energy 2021, 88, 106288.

  20. [20]

      Wei, H. -X.; Tang, L. -B.; Huang, Y. -D.; Wang, Z. -Y.; Luo, Y. -H.; He, Z. -J.; Yan, C.; Mao, J.; Dai, K. -H.; Zheng, J. -C. Comprehensive understanding of Li/Ni intermixing in layered transition metal oxides. Mater Today 2021, 51, 365-392.

  21. [21]

      Yang, S. -Q.; Wei, H. -X.; Tang, L. -B.; Yan, C.; Li, J. -H.; He, Z. -J.; Li, Y. -J.; Zheng, J. -C.; Mao, J.; Dai, K. Fast Li-ion conductor Li_1+yTi_2-y Al_y(PO₄)₃ modified Li_1.2[Mn_0.54Ni_0.13Co_0.13]O₂ as high performance cathode material for Li-ion battery. Ceram 2021, 47, 18397-18404.

  22. [22]

      Zheng, J. C.; Yang, Z.; Wang, P. B.; Tang, L. B.; An, C. S.; He, Z. J. Multiple linkage modification of lithium-rich layered oxide Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ for lithium ion battery. ACS Appl. Mater. Interfaces 2018, 10, 31324-31329.

  23. [23]

      Zhou, Z.; Luo, Z.; He, Z.; Zheng, J.; Li, Y.; Yan, C.; Mao, J. Suppress voltage decay of lithium-rich materials by coating layers with different crystalline states. J. Energy Chem. 2021, 60, 591-598.

  24. [24]

      Hou, X.; Yang, Y.; Mao, Y.; Song, J.; Yang, J.; Li, G.; Pan, Y.; Wang, Y.; Xie, J. Improved electrochemical performance of Li_1.15Ni_0.17Co_0.11Mn_0.57O₂ by Li₂O cathode additive. J. Electrochem. 2019, 166, A3387-A3390.

  25. [25]

      Huang, J.; Zhu, F.; Shu, Y.; Hu, G.; Peng, Z.; Cao, Y.; Jiang, J.; Zhang, S.; Wang, X.; Du, K. Enhanced electrochemical performance of O3-type Li_0.6[Li_0.2Mn_0.8]O₂ for lithium ion batteries via aluminum and boron dual-doping. Ceram 2021, 47, 34611-34618.

  26. [26]

      Jiang, W.; Zhang, C.; Feng, Y.; Wei, B.; Chen, L.; Zhang, R.; Ivey, D. G.; Wang, P.; Wei, W. Achieving high structure and voltage stability in cobalt-free Li-rich layered oxide cathodes via selective dual-cation doping. Energy Storage 2020, 32, 37-45.

  27. [27]

      Liu, D.; Fan, X.; Li, Z.; Liu, T.; Sun, M.; Qian, C.; Ling, M.; Liu, Y.; Liang, C. A cation/anion co-doped Li_1.12Na_0.08Ni_0.2Mn_0.6O_1.95F_0.05 cathode for lithium ion batteries. Nano Energy 2019, 58, 786-796.

  28. [28]

      Liu, H.; He, B.; Xiang, W.; Li, Y. C.; Bai, C.; Liu, Y. P.; Zhou, W.; Chen, X.; Liu, Y.; Gao, S.; Guo, X. Synergistic effect of uniform lattice cation/anion doping to improve structural and electrochemical performance stability for Li-rich cathode materials. Nanotechnology 2020, 31, 455704.

  29. [29]

      Sun, Y.; Wu, Q.; Zhao, L. A new doping element to improve the electrochemical performance of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ materials for Li-ion batteries. Ceram 2019, 45, 1339-1347.

  30. [30]

      Peng, Z.; Mu, K.; Cao, Y.; Xu, L.; Du, K.; Hu, G. Enhanced electrochemical performance of layered Li-rich cathode materials for lithium ion batteries via aluminum and boron dual-doping. Ceram 2019, 45, 4184-4192.

  31. [31]

      Peralta, D.; Salomon, J.; Reynier, Y.; Martin, J. -F.; De Vito, E.; Colin, J. -F.; Boulineau, A.; Bourbon, C.; Amestoy, B.; Tisseraud, C.; Pellenc, R.; Ferrandis, J. -L.; Bloch, D.; Patoux, S. Influence of Al and F surface modifications on the sudden death effect of Si-Gr/Li_1.2Ni_0.2Mn_0.6O₂ Li-ion cells. Electrochimi. Acta 2021, 400, 139419.

  32. [32]

      Sun, Y.; Zhang, L.; Dong, S.; Zeng, J.; Shen, Y.; Li, X.; Ren, X.; Ma, L.; Hai, C.; Zhou, Y. Improving the electrochemical performances of Li-rich Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ through cooperative doping of Na⁺ and Mg²⁺. Electrochimi. Acta 2022, 414, 140169.

  33. [33]

      Tang, Y.; Chen, S. Enhanced electrochemical performance of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ for lithium-ion batteries co-modified by lanthanum and aluminum. Ionics 2021, 27, 935-948.

  34. [34]

      Zhu, H.; Xiao, Y.; Dong, J.; Zhang, D.; Chang, C. New composite Li_1.4Mn_0.61Ni_0.18Co_0.18Al_0.03O_2.4 and LiNi_0.5Mn_1.5O_3.9F_0.1 cathode material with higher specific capacity and better capacity retention. Ionics 2020, 26, 3749-3760.

  35. [35]

      Sun, K.; Peng, C.; Li, Z.; Xiao, Q.; Lei, G.; Xiao, Q.; Ding, Y.; Hu, Z. Hybrid LiV₃O₈/carbon encapsulated Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ with improved electrochemical properties for lithium ion batteries. RSC Advances 2016, 6, 28729-28736.

  36. [36]

      Lai, X.; Hu, G.; Peng, Z.; Tong, H.; Lu, Y.; Wang, Y.; Qi, X.; Xue, Z.; Huang, Y.; Du, K.; Cao, Y. Surface structure decoration of high capacity Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode by mixed conductive coating of Li_1.4Al_0.4Ti_1.6(PO₄)₃ and polyaniline for lithium-ion batteries. J. Power Sources 2019, 431, 144-152.

  37. [37]

      Zhang, W.; Liu, Y.; Wu, J.; Shao, H.; Yang, Y. Surface modification of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material with Al₂O₃/SiO₂ composite for lithium-ion batteries. J. Electrochem. 2019, 166, A863-A872.

  38. [38]

      Zhu, W.; Tai, Z.; Shu, C.; Chong, S.; Guo, S.; Ji, L.; Chen, Y.; Liu, Y. The superior electrochemical performance of a Li-rich layered cathode material with Li-rich spinel Li₄Mn₅O₁₂ and MgF₂ double surface modifications. J. Mater. 2020, 8, 7991-8001.

  39. [39]

      Li, M.; Zhou, Y.; Wu, X.; Duan, L.; Zhang, C.; Zhang, F.; He, D. The combined effect of CaF₂ coating and La-doping on electrochemical performance of layered lithium-rich cathode material. Electrochimi. Acta 2018, 275, 18-24.

  40. [40]

      Li, S.; Fu, X.; Liang, Y.; Wang, S.; Zhou, X. A.; Dong, H.; Tuo, K.; Gao, C.; Cui, X. Enhanced structural stability of boron-doped layered@spinel @carbon heterostructured lithium-rich manganese-based cathode materials. ACS Sustain. 2020, 8, 9311-9324.

  41. [41]

      Shen, C.; Liu, Y.; Li, W.; Liu, X.; Xie, J.; Jiang, J.; Jiang, Y.; Zhao, B.; Zhang, J. One-pot synthesis and multifunctional surface modification of lithium-rich manganese-based cathode for enhanced structural stability and low-temperature performance. J. Colloid Inter. Sci. 2022, 615, 1-9.

  42. [42]

      Tai, Z.; Zhu, W.; Shi, M.; Xin, Y.; Guo, S.; Wu, Y.; Chen, Y.; Liu, Y. Improving electrochemical performances of lithium-rich oxide by cooperatively doping Cr and coating Li₃PO₄ as cathode material for lithium-ion batteries. J Colloid Interface Sci. 2020, 576, 468-475.

  43. [43]

      Li, W.; Zhao, B.; Bai, J.; Ma, H.; Li, K.; Wang, P.; Mao, Y.; Zhu, X.; Sun, Y. Rate performance modification of a lithium-rich manganese-based material through surface self-doping and coating strategies. Langmuir 2021, 37, 3223-3230.

  44. [44]

      Niu, B.; Li, J.; Liu, Y.; Li, Z.; Yang, Z. Re-understanding the function mechanism of surface coating, modified Li-rich layered Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathodes with YF₃ for high performance lithium-ions batteries. Ceram 2019, 45, 12484-12494.

  45. [45]

      Ren, X.; Li, D.; Zhao, Z.; Chen, G.; Zhao, K.; Kong, X.; Li, T. Dual effect of aluminum doping and lithium tungstate coating on the surface improves the cycling stability of lithium-rich manganese-based cathode materials. Acta Chim. Sinica 2020, 78, 1268-1274.

  46. [46]

      Xie, Y.; Chen, S.; Lin, Z.; Yang, W.; Zou, H.; Sun, R. W. -Y. Enhanced electrochemical performance of Li-rich layered oxide, Li_1.2Mn_0.54Co_0.13Ni_0.13O₂, by surface modification derived from a MOF-assisted treatment. Electrochem. Commun. 2019, 99, 65-70.

  47. [47]

      Zhai, Y.; Zhang, J.; Zhang, H.; Liu, X.; Wang, C. -W.; Sun, L.; Liu, X. The synergic effects of Zr doping and Li₂TiO₃ coating on the crystal structure and electrochemical performances of Li-rich Li_1.2Ni_0.2Mn_0.6O₂. J. Electrochem. 2019, 166, A1323-A1329.

  48. [48]

      Zhang, C.; Feng, Y.; Wei, B.; Liang, C.; Zhou, L.; Ivey, D. G.; Wang, P.; Wei, W. Heteroepitaxial oxygen-buffering interface enables a highly stable cobalt-free Li-rich layered oxide cathode. Nano Energy 2020, 75, 104995.

  49. [49]

      Zhang, J.; Zhang, H.; Gao, R.; Li, Z.; Hu, Z.; Liu, X. New insights into the modification mechanism of Li-rich Li_1.2Mn_0.6Ni_0.2O₂ coated by Li₂ZrO₃. Phys. Chem. Chem. Phys. 2016, 18, 13322-13331.

  50. [50]

      Luo, D.; Ding, X.; Hao, X.; Xie, H.; Cui, J.; Liu, P.; Yang, X.; Zhang, Z.; Guo, J.; Sun, S.; Lin, Z. Ni/Mn and Al dual concentration-gradients to mitigate voltage decay and capacity fading of Li-rich layered cathodes. ACS Energy Lett. 2021, 6, 2755-2764.

  51. [51]

      Cao, F.; Zeng, W.; Zhu, J.; Xiao, J.; Li, Z.; Li, M.; Qin, R.; Wang, T.; Chen, J.; Yi, X.; Wang, J.; Mu, S. Inhibiting Mn migration by Sb-pinning transition metal layers in lithium-rich cathode material for stable high-capacity properties. Small 2022, 18, e2200713.

  52. [52]

      Li, H.; Jian, Z.; Yang, P.; Li, J.; Xing, Y.; Zhang, S. Niobium doping of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials with enhanced structural stability and electrochemical performance. Ceram 2020, 46, 23773-23779.

  53. [53]

      Liu, H.; Xiang, W.; Bai, C.; Qiu, L.; Wu, C.; Wang, G.; Liu, Y.; Song, Y.; Wu, Z. -G.; Guo, X. Enabling superior electrochemical performance of lithium-rich Li_1.2Ni_0.2Mn_0.6O₂ cathode materials by surface integration. Ind. Eng. Chem. Res. 2020, 59, 19312-19321.

  54. [54]

      Zhang, C.; Wei, B.; Jiang, W.; Wang, M.; Hu, W.; Liang, C.; Wang, T.; Chen, L.; Zhang, R.; Wang, P.; Wei, W. Insights into the enhanced structural and thermal stabilities of Nb-substituted lithium-rich layered oxide cathodes. ACS Appl. Mater. Interfaces 2021, 13, 45619-45629.

  55. [55]

      Luo, D.; Ding, X.; Fan, J.; Zhang, Z.; Liu, P.; Yang, X.; Guo, J.; Sun, S.; Lin, Z. Accurate control of initial coulombic efficiency for lithium-rich manganese-based layered oxides by surface multicomponent integration. Angew. Chem. Int. Ed. 2020, 59, 23061-23066.

  56. [56]

      Luo, D.; Cui, J.; Zhang, B.; Fan, J.; Liu, P.; Ding, X.; Xie, H.; Zhang, Z.; Guo, J.; Pan, F.; Lin, Z. Ti-based surface integrated layer and bulk doping for stable voltage and long life of Li-rich layered cathodes. Adv. Funct. 2021, 31, 2009310.

  57. [57]

      Xu, L.; Meng, J.; Yang, P.; Xu, H.; Zhang, S. Cesium-doped layered Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathodes with enhanced electrochemical performance. Solid State Ionics 2021, 361, 115551.

  58. [58]

      Wang, Q.; He, W.; Wang, L.; Li, S.; Zheng, H.; Liu, Q.; Cai, Y.; Lin, J.; Xie, Q.; Peng, D. -L. Morphology control and Na⁺ doping toward high-performance Li-rich layered cathode materials for lithium-ion batteries. ACS Sustain. Chem. Eng. 2020, 9, 197-206.

  59. [59]

      Guo, H.; Jia, K.; Han, S.; Zhao, H.; Qiu, B.; Xia, Y.; Liu, Z. Ultrafast heterogeneous nucleation enables a hierarchical surface configuration of lithium-rich layered oxide cathode material for enhanced electrochemical performances. Adv. Mater. Interfaces 2018, 5, 1701465.

  60. [60]

      He, W.; Zhang, C.; Wang, M.; Wei, B.; Zhu, Y.; Wu, J.; Liang, C.; Chen, L.; Wang, P.; Wei, W. Countering voltage decay, redox sluggishness, and calendering incompatibility by near-zero-strain interphase in lithium-rich, manganese-based layered oxide electrodes. Adv. Funct. 2022, 32, 2200322.

  61. [61]

      Wu, J.; Li, H.; Liu, Y.; Ye, Y.; Yang, Y. In situ reconstruction of the spinel interface on a Li-rich layered cathode material with enhanced electrochemical performances through HEPES and heat treatment strategy. ACS Sustain. Chem. Eng. 2022, 10, 6165-6180.

  62. [62]

      Ku, L.; Cai, Y.; Ma, Y.; Zheng, H.; Liu, P.; Qiao, Z.; Xie, Q.; Wang, L.; Peng, D. -L. Enhanced electrochemical performances of layered-spinel heterostructured lithium-rich Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode materials. Chem. Eng. 2019, 370, 499-507.

  63. [63]

      Park, K.; Kim, J.; Park, J. -H.; Hwang, Y.; Han, D. Synchronous phase transition and carbon coating on the surface of Li-rich layered oxide cathode materials for rechargeable Li-ion batteries. J. Power Sources 2018, 408, 105-110.

  64. [64]

      Ran, X.; Tao, J.; Chen, Z.; Yan, Z.; Yang, Y.; Li, J.; Lin, Y.; Huang, Z. Surface heterostructure induced by TiO₂ modification in Li-rich cathode materials for enhanced electrochemical performances. Electrochimi. Acta 2020, 353, 135959.

  65. [65]

      Yang, Z.; Zhong, J.; Li, J.; Liu, Y.; Niu, B.; Kang, F. Li-rich layered oxide coated by nanoscale MoO_x film with oxygen vacancies and lower oxidation state as a high-performance cathode material. Ceram 2019, 45, 439-448.

  66. [66]

      Yuan, S.; Guo, J.; Ma, Y.; Zhou, Y.; Zhang, H.; Song, D.; Shi, X.; Zhang, L. Improving the electrochemical performance of a lithium-rich layered cathode with an in situ transformed layered@spinel@spinel heterostructure. ACS Appl. Energy Mater. 2021, 4, 11014-11025.

  67. [67]

      Yang, J.; Li, P.; Zhong, F.; Feng, X.; Chen, W.; Ai, X.; Yang, H.; Xia, D.; Cao, Y. Suppressing voltage fading of Li-rich oxide cathode via building a well-protected and partially-protonated surface by polyacrylic acid binder for cycle-stable Li-ion batteries. Adv Energy Mater. 2020, 10, 1904264.

  68. [68]

      Yang, K.; Liu, Y.; Niu, B.; Yang, Z.; Li, J. Oxygen vacancies in CeO₂ surface coating to improve the activation of layered Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material for Li-ion batteries. Ionics 2018, 25, 2027-2034.

  69. [69]

      Huang, C.; Wang, Z.; Wang, H.; Huang, D.; Zhao, J. -W.; Zhao, S. -X. In-situ construction of extra ion-store sites and fast ion-diffusion channels for lithium-rich manganese-based oxides cathode. J. Power Sources 2022, 535, 231437.

  70. [70]

      Ding, X.; Li, Y. X.; He, X. D.; Liao, J. Y.; Hu, Q.; Chen, F.; Zhang, X. Q.; Zhao, Y.; Chen, C. H. Surface Li⁺/K⁺ exchange toward double-gradient modification of layered li-rich cathode materials. ACS Appl. Mater. Interfaces 2019, 11, 31477-31483.

  71. [71]

      Li, Q.; Zhou, D.; Zhang, L.; Ning, D.; Chen, Z.; Xu, Z.; Gao, R.; Liu, X.; Xie, D.; Schumacher, G.; Liu, X. Tuning anionic redox activity and reversibility for a high-capacity Li-rich Mn-based oxide cathode via an integrated strategy. Adv. Funct. 2019, 29, 1806706.

  72. [72]

      Liu, Q.; Xie, T.; Xie, Q.; He, W.; Zhang, Y.; Zheng, H.; Lu, X.; Wei, W.; Sa, B.; Wang, L.; Peng, D. L. Multiscale deficiency integration by Na-rich engineering for high-stability Li-rich layered oxide cathodes. ACS Appl. Mater. Interfaces 2021, 13, 8239-8248.

  73. [73]

      Chen, H.; He, Z.; Huang, Z.; Song, L.; Shen, C.; Liu, J. The effects of multifunctional coating on Li-rich cathode material with hollow spherical structure for Li ion battery. Ceram 2017, 43, 8616-8624.

  74. [74]

      Meng, J.; Xu, L.; Ma, Q.; Yang, M.; Fang, Y.; Wan, G.; Li, R.; Yuan, J.; Zhang, X.; Yu, H.; Liu, L.; Liu, T. Modulating crystal and interfacial properties by W-gradient doping for highly stable and long life Li-rich layered cathodes. Adv. Funct. 2022, 32, 2113013.

  75. [75]

      Mezaal, M. A.; Qu, L.; Li, G.; Zhang, R.; Xuejiao, J.; Zhang, K.; Liu, W.; Lei, L. Promoting the cyclic and rate performance of lithium-rich ternary materials via surface modification and lattice expansion. RSC Advances 2015, 5, 93048-93056.

  76. [76]

      Chen, J.; Huang, Z.; Zeng, W.; Ma, J.; Cao, F.; Wang, T.; Tian, W.; Mu, S. Surface engineering and trace cobalt doping suppress overall Li/Ni mixing of Li-rich Mn-based cathode materials. ACS Appl. Mater. Interfaces 2022, 14, 6649-6657.

  77. [77]

      Phattharasupakun, N.; Geng, C.; Johnson, M. B.; Väli, R.; Liu, A.; Liu, Y.; Sawangphruk, M.; Dahn, J. R. Impact of Al doping and surface coating on the electrochemical performances of Li-rich Mn-rich Li_1.11Ni_0.33Mn_0.56O₂ positive electrode material. J. Electrochem. 2020, 167, 120531.

  78. [78]

      Chen, J.; Zou, G.; Deng, W.; Huang, Z.; Gao, X.; Liu, C.; Yin, S.; Liu, H.; Deng, X.; Tian, Y.; Li, J.; Wang, C.; Wang, D.; Wu, H.; Yang, L.; Hou, H.; Ji, X. Pseudo-bonding and electric-field harmony for Li-rich Mn-based oxide cathode. Adv. Funct. 2020, 30, 2004302.

  79. [79]

      Shi, Z.; Gu, Q.; Yun, L.; Wei, Z.; Hu, D.; Qiu, B.; Chen, G. Z.; Liu, Z. A composite surface configuration towards improving cycling stability of Li-rich layered oxide materials. J. Mater. 2021, 9, 24426-24437.

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  Figure 1  Schematic diagram of multi-element doping modification. (a) of Nd/Al co-doping; (b) of V/Mo co-doping; (c) of Na/P co-doping; (d) of Na/F co-doping.^[26-29]

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  Figure 2  Schematic diagram of multi-surface coating modification. (a) of LiV₃O₈/C co-coating; (b) of LATP/PANI co-coating; (c) of Al₂O₃/SiO₂ co-coating; (d) of Li₄Mn₅O₁₂/MgF₂ co-coating.^[35-38]

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  Figure 3  Schematic diagram of doping & coating modification. (a) of La doping and CaF₂ coating; (b) of Cr doping and Li₃PO₄ coating; (c) of B doping and C coating; (d) of La and Zr co-doping and LLZO coating.^[39-42]

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  Figure 4  Schematic diagram of doping & structure design modification. (a) of Al doping and structure design; (b) of Nb doping and structure design; (c) of Na doping and structure design; (d) of Ti doping and structure design.^{[50, 54-56]}

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  Figure 5  Schematic diagram of coating & structural design modification. (a) of HEPES coating and structure design; (b) of MoO_x coating and structure design; (c) of Li(Ni_1/6Co_1/6Mn_4/6)₂O₄ and LiCoMnO₄ coating and structure design; (d) of P2-type Na_0.66MnO₂ coating and structure design.^[59-62]

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  Figure 6  Schematic diagram of doping & coating & structural design modification. (a) of Zr doping, Li₂ZrO₃ coating and structure design; (b) of Ce doping, Li₂CeO₃ coating and structure design; (c) of Nb/Al co-doping, Al₂O₃ coating and structure design; (d) of Co doping, Li₂MoO₄ coating and structure design.^[70-73]

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  Table 1.  Electrochemical Properties of Common Lithium-Ion Battery Cathode Materials^[2]

  Abbreviation	Representation	Voltage (V)	Theoretical energy density (mAh g-1)
  LFP	LiFePO4	2.3-2.5	170
  LCO	LiCoO2	3.7-3.9	200
  NCM	LiNiCoMnO2	3.8-4.0	200
  LR	Li2MnO3·LiMO2	3.8-4.0	250

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