coating of nickel-rich cathode materials for lithium-ion batteries

Journal Pre-proof A review on doping/coating of nickel-rich cathode materials for lithium-ion batteries Wuwei Yan, Shunyi Yang, Youyuan Huang, Yong Yang, GuohuiYuan PII:

S0925-8388(19)34294-X

DOI:

https://doi.org/10.1016/j.jallcom.2019.153048

Reference:

JALCOM 153048

To appear in:

Journal of Alloys and Compounds

Received Date: 23 July 2019 Revised Date:

13 November 2019

Accepted Date: 15 November 2019

Please cite this article as: W. Yan, S. Yang, Y. Huang, Y. Yang, GuohuiYuan, A review on doping/ coating of nickel-rich cathode materials for lithium-ion batteries, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.153048. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

A review on doping/coating of nickel-rich cathode materials for lithium-ion Batteries Wuwei Yana, c, Shunyi Yanga, Youyuan Huanga*, Yong Yangb, GuohuiYuanc*, a

Shenzhen BTR Nanotechnology Co., Ltd, Shenzhen 518106, P. R. China

b

Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing

University of Science and Technology, Nanjing 210094, P. R. China c

Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, P. R. China

a*Corresponding author: E-mail: [email protected] c*Corresponding author: E-mail: [email protected] ABSTRACT Nowadays, lithium-ion batteries (LIBs) are widely applied in many fields, in order to reduce the material cost, increase volumetric/gravimetric energy density, raise safety performance and so on, nickel-rich cathode materials have gained much attention. Besides the technique for preparation of precursors and corresponding cathode materials, bulk doping and surface coating are very important to strengthen the performance of nickel-rich cathode materials. However, the intrinsic mechanism of doping and coating for cathode materials is not fully understood, further extensive and intensive investigation is needed. In this article, the varieties of doping and coating are reviewed, and research status, recent progresses and their underlying mechanism are presented. The paper suggests that combination of coating and doping at the same time is crucial to effectively improve the internal and external structure as well as electrochemical performance of nickel-rich cathode materials. Moreover, the present problems and challenges to nickel-rich cathode materials are put forward, the issues of safety and capacity fade of nickel-rich cathode materials are urgently need to resolve, which are closely related to the interface damage, volumetric change, lattice internal strain and gas release during cycling process.

Keywords: Cathode materials, Doping, Coating, Lithium-ion batteries 1. Introduction Facing global warming and depletion of fossil fuels, it is extremely valuable to research and develop renewable production and storage of energy technology in the twenty-first century. Meanwhile, it is a great challenge to meet the ever-increasing demand for reliable, safety, high 1 / 38

power and highly energetic LIBs from portable electronic devices, smart power grids and electric vehicles (EVs) [1-8]. What is more, the 18650-type cylindrical battery has been successfully applied in EVs of Tesla Model S, whose material is LiNi0.8Co0.15Al0.05O2 (NCA), providing a driving range of 270 miles per charge [9]. The amazing success of Tesla EVs stimulates more and more automotive companies to follow. Compared with graphite anode materials with a capacity over 350mAh/g, capacity of cathode materials limits the energy densities of LIBs and is considered as the bottleneck of driving range. Consequently, it is necessary to intensively explore and develop advanced cathode materials with higher energy density. Among the many cathode materials, high nickel or nickel-rich LiNixCoyM1-x-yO2 (0.8≤x＜1, 0＜y＜0.20, M=Mn, Al, etc.) [10-14] layered oxide cathode materials with superior discharge capacity (>200mAh/g) and voltage platform (3.6V) are deemed to be one of the most competitive candidates to traditional LiCoO2 and LiFePO4. For nickel-rich cathode materials, two very hot and typical varieties, named lithium nickel cobalt aluminum oxide (NCA, LiNixCoyAl1-x-yO2, 0.8≤x＜1, 0＜y＜0.20) and lithium nickel cobalt manganese oxide (NCM, LiNixCoyMn1-x-yO2, 0.8≤x＜1, 0＜y＜0.20) [15-20], are extensively investigated by researchers and experts. These nickel-rich cathode materials possess the similar structure (R-3m) as well as LiNiO2 [21-25], because they originated from partial cation substitution for Ni atoms in LiNiO2. The more nickel contents in NCA and NCM, the more discharge capacity and similar properties of LiNiO2, which are more difficult to be overcame for commercial application in future. Recently, these kinds of nickel-rich cathode materials exhibit some drawbacks for the commercialization, including capacity fade during long-term cycling, poor rate capacity, thermal instability, short storage lifetime at elevated temperature, high residual lithium, gas evolution, serious safety concern, Li/Ni cation mixing and so on [26-29]. Thereby, some crucial mechanisms are proposed to explain these problems in literature: (1) side reaction of electrolyte catalyzed by the delithiated NCA or NCM at voltages above 4.3 V with a concomitant oxygen release [30]; (2) dissolution of the transition-metal ions corroded by HF acid from the electrolyte [31]; (3) layered-to-spinel phase transformation and occurrence of NiO-type phase [31-33]; (4) microcrack formation and particle fracture, originating from internal strain, expansion and contraction of lattice volume during repeat charging/discharging. As shown in Fig. 1-3, these microcracks enlarged along the parasitic reaction area between active materials and the electrolyte, and 2 / 38

accelerated the fracture of particles and decomposition of the electrolyte. Finally, the pulverization of bulk cathode particles occurred [34-38]. The pulverized and separated particles cannot participate in electrochemical reaction, resulting in the capacity decay. Lately, Amhed et al. reported that defects of intragranular nanopores were observed in the primary particle of pristine nickel-rich cathode material, and their morphology with a diameter of 10-50 nm changed during cycling, accompanied by growth of a rock-salt like regions. These nanopores induced oxygen loss, phase transformation, and crack formation during lithiation/delithiation process, leading to cycling deterioration of high-Ni NCM. Besides, oxygen release was often found at the end of charge of nickel-rich cathode materials, accompanied by the phase transition, such as near 4.2 V for NCM811[39]. Oxygen release also resulted in depletion of electrolyte solvents, which caused the structural deterioration and safety issue. Therefore, it was used by cation doping, surface coating, and non-flammable or oxygen-scavenging electrolytes in order to decrease the oxygen loss and oxygen reaction. Moreover, Ni content and solid-state electrolyte interface (SEI) of nickel-rich cathode materials also caused the cycling deterioration. Fig. 4 shows that more contents in nickel-rich cathode materials easily can result in higher capacity decay, which is associated with the volume expansion of lattice cells and surface structural disability. Additionally, several types of compounds, such as LiF, Li2CO3, and LixPFyOz, appearing on the surface of nickel-rich cathode materials, can form a SEI layer (Fig. 5), which influenced Li+ removal and lower capacity retention. Thus, relieving oxygen loss, ameliorating electrode/electrolyte interface, maintaining internal structure, and decreasing the intragranular defects are very significant to improve cycling performance and safety behavior for LIBs. A very effective strategy to overcome these disadvantages is lattice/bulk doping and surface coating/modification for nickel-rich cathode materials. Therefore, in this review, we will make the statements on both of doping and coating, and analyze reasons and mechanisms of remarkable improvement on the electrochemical performance and structural stability for nickel-rich cathode materials.

3 / 38

Fig. 1. Capacity fading scheme of Ni-rich cathode materials. Reproduced with permission from Ref. [38].

Ref.

Fig. 2. SEM images of Li1-δ[NixCoyMn1-x-y]O2 at charging to 4.3 V in the first cycle: (a) x=0.6, (b) x=0.8, (c) x=0.9, and (d) x=0.95. Reproduced with permission from Ref. [38].

Fig. 3. Schematic illustration of the structure and characteristics of (a) C-NCM (commercial NCM) and (b) RASC-NCM materials. Reproduced with permission from Ref. [39]. 4 / 38

Fig. 4. The relationship of thermal stability and capacity retention of nickel-rich cathode materials. Reproduced with permission from Ref. [40].

Fig. 5. The microstructure and composition of the Ni-rich cathode surface. Reproduced with permission from Ref. [41]. 2. Doping elements The commonly used dopants for nickel-rich cathode materials are Al, Ti, Zr, Mg, B, F, W, Mo, Ga, Nb, Ca, etc (Fig. 6), including single or multiple doping [42-55]. Based on the special properties of doping elements, the electrochemical performance and structural stability of cathode materials can be adjusted. The function of doping can be summarized as follows: (1) forming an aliovalent substitution and multivalent substitution, owning electrochemically inactive property, and increasing the electronic and ionic conductivity; (2) restraining migration of Ni2+ ions to the Li layers, decreasing the cation mixing, and maintaining the stable structure; (3) suppressing oxygen release by strengthening transition metal (TM) oxide bond, and impacting the phase transformation during cycling. The doping elements often formed a concentration-gradient distribution, namely, enriched on the surface and decreased at the core, which facilitated the 5 / 38

structural stability. Moreover, the proper doping amount should be considered since higher or less content may result in capacity decay and cycling deterioration of nickel-rich cathode materials. Meanwhile, some factors such as the selection of dopants, doping method, doping depth and doping-treatment parameters, should be optimized in order to obviously improve the electrochemical performance of nickel-rich cathode materials, which has drawn the attention of many researchers and technicians at present. Herein, some important doping elements were introduced and summarized.

Fig. 6. Capacity retention of doped and primary LiNi0.8Co0.1Mn0.1O2 (NCM811) at 45℃. Reproduced with permission from Ref. [42]. 2.1 Al Doping Usually, doping elements have electrochemically inactive, high polarity, possess large ionic radii and strong binding capacity with oxygen. However, Al3+ with small ionic radii can strengthen the intercalation voltage, not oxidized in high voltage electrolyte. Al-doped NCM is extensively researched and demonstrates as a stable lattice dopant. Al3+ is electrochemically inactive, and the small doping amount (5%) can enhance capacity retention due to its stably primary phase structure [53-55]. Moreover, in terms of the structural stability, Al incorporation effectively prevented the formation of oxygen vacancies during delithiation process and suppressed oxygen evolution from the host structure, so inhibited the nickel migration from TM sites to the lithium sites, stabilizing the host structure. In comparison with their undoped counterpart, Al doping restricted the capacity decay during cycling and extended aging lifespan of the positive electrodes at the charged voltage of 4.3 V at 60℃, which was likely attributed to the structural stability of the interface between electrode and electrolyte. 6 / 38

Some studies revealed that Co3+ may cause a strained TM layer, but Al doping can change the strain of the edge by sharing MO6 octahedra. For example, Al doping relieved the lattice strain in LiNi0.45Mn0.45Co1-yAlyO2 because the small ionic radius of Al resulted in a more ordered TM structure. Moreover, the thermal stability of nickel-rich cathode materials was enhanced by addition of Al3+, because Al3+ tetrahedral sites can prevent Ni3+ migration and restrain the transformation from layered to a spinel-like structure. Guilmard et al. [56] demonstrated that Al3+ ions migrated to tetrahedral sites, and retained layered-to-spinel transformation in LixNi0.89Al0.16O2. Park et al. [57] suggested that the improvement of cycle performance through Al doping at high temperature was ascribed to stronger Al-O bonds than Ni-O bonds. Dixit et al. [43] unraveled that the structural stability of the Al-doping cathode material was related with a strong Al-O ion-covalent bond and charge transfer capability. However, Al doping can bring about the barrier of Li+ diffusion. Such as, Dianat et al. [58] revealed that Al dopant increased the thermal stability of cathode during the charging/discharging, but led to the irreversible capacity loss. Certainly, many test results from coin and full batteries also demonstrated that NCA showed less discharge capacity and rate behavior compared with NCM in the same Ni contents.

Fig. 7. Comparison of NCMA and NCM89/90 in physicochemical properties. Reproduced with permission from Ref. [59]. Additionally, the ratio of Al doping was often limited below 5 mol%, and a homogeneous distribution of Al3+ ions exhibited less detrimental surface reaction with the electrolyte. For this purpose, it is a smart design to obtain a uniform Al3+ region by introducing Al element into precursor preparation. Al and Ni, Co, Mn ions kept homogeneous distribution in spherical particles during precipitating process, thus it can also be called NCMA quaternary system. Al 7 / 38

doping can reduce volume contraction and expansion during deintercalation and intercalation, subsequently, the intrinsicly mechanical strain change, microcrack appearance and extension can be

effectively

suppressed.

Kim

et

al.

[59]

made

a

comparison

between

Li[Ni0.89Co0.05Mn0.05Al0.01]O2 (NCMA), NCM90 and NCA89, which were similar with Ni contents. They demonstrated that NCMA exhibited a better cycling stability and thermal stability than others, as shown in Fig. 7. For instance, NCMA delivered a high capacity of 228 mAh·g-1 during 4.3–2.7 V at a current density of 18 mA·g-1 in coin cell, and capacity retention of 85% was obtained after 1000 cycles in the corresponding full battery. 2.2 Zr Doping The valence state of Zr4+ remains unchanged and does not participate in electrochemical reaction in charge-discharge cycles. In order to maintain charge balance during Zr doping, some Ni3+ were reduced to Ni2+, and the charge capacity and discharge capacity increased. Also, Zr4+ ions can occupy Li+ sites and restrain the structural transformation from layered to spinel through suppressing diffusion of Ni+ ions into the Li+ sites. Furthermore, it was demonstrated that the Zr4+ doping could also decrease the cationic mixing and maintained the stability of originally hexagonal structure during cycling [59]. For example, Schipper et al. [60] compared the pristine with Zr-doped LiNi0.56Zr0.04Co0.2Mn0.2O2 materials, they indicated that the Zr-doping decreased Li/Ni cationic mixing, stabilized Ni tetrahedral sites and decreased the concentration of Jahn-Teller active Ni3+ ions. Sivaprakash et al. [61] considered that Zr4+ ions could replace lithium sites, act as “pillars”, maintain the stable structure, improve Li+ ion removal and rate capacity in LiNi0.8Co0.15Zr0.05O2, meanwhile, small amounts of Zr atoms may move to the cathode surface and form a protective layer, so that the reduction of Ni3+ to Ni2+ can be prevented. Moreover, Oh et al. [62] proposed that doping with high-valent cation, such as Zr4+, could hinder Ni2+ migration. Besides, Zr doping was also deemed to improve the electrochemical behavior at high temperature and high cut-off voltage. Therefore, until nowadays, Zr doping was widely investigated and used in nickel-rich cathode materials. 2.3 Ti Doping Ti doping can inhibit the formation of structural distortions during cycling since Ti4+ owns relative larger cation radius than Co3+, which decreased the lithium intercalation voltage and promoted more lithium deintercalation at a fixed overpotential. Kam et al. [63] demonstrated that 8 / 38

the partial substitution of Co with Ti in Li1+z(NixMnxCo1-2x-yTiy)O2 (x=0.4, y=0.04-0.07) increased the discharge capacity over 15% and showed excellent capacity retention. XRD patterns showed that substitution of Ti4+ for Co3+ retained the volume change and enhanced cyclability during delithiation/lithiation process. Furthermore, magnetic measurements revealed that Ti-doping balanced the charge by part reduction of Mn4+ to Mn3+. Markus et al. [64] analyzed the computational and experimental results, identified that Ti-doping could stabilize the NCM structure by strengthening the formation energy of the rock salt as well as binding capacity with oxygen. On the other hand, Ti-doping can improve the discharge capacity and capacity retention at high voltage condition. Wolff-Goodrich et al. presented that LiNi0.4Mn0.4Co0.2O2 and LiNi0.4Mn0.4Co0.18Ti0.02O2 exhibited similar discharge capacity and capacity retention in the voltage of 2.0-4.55 V [65]. Nevertheless, in the high voltage window of 2.0-4.7 V, the Ti-doped sample showed discharge capacity of 243 mAh/g, which was higher than that of the undoped sample (205 mAh/g), meanwhile, the capacity retention was also improved. Hu et al. [66] investigated the mechanical properties of NCA by Ti doping. The hardness (H) and fracture toughness (KIC) of NCA without doping decreased by 38-50% at delithiation state, however, the Ti-doped sample kept a stable value. 2.4 B Doping Polyanion doping, such as (BO3)3-/(BO4)5- [67], PO43- [68], SiO44- [69], etc., delayed the capacity decline in the Li-rich cathode materials by forming the stable oxygen close-packed structure. Moreover, it was reported that boron (B) strengthened TM-O bonding and maintained the stably layered structure, because B can make lattice parameters expend with an increase of doping amount. For example, Pan et al. [70] explained that B3+ ion with smaller ionic radius occupied the tetrahedral interstices of the oxygen ions within the TM and Li layers, so that enlarging volume of the unit cell.

9 / 38

Fig. 8. Schematic illustration of the effect of boron-doping on the NCM90 during cycling. Reproduced with permission from Ref. [71]. Sun’s team reported that Li[Ni0.90Co0.05Mn0.05]O2 (NCM90) with doping 1 mol% B exhibited discharge capacity of 237 mAh g-1 from 4.3 to 2.7 V, the capacity retention was 91% after 100th cycle at 55 ℃, exceeding 15% higher than that of the undoped sample [71]. Moreover, the undoped sample exhibited microcracks at interparticle boundaries, and the primary particles were separated from each other. These were attributed to the formation of NiO-like phase with thickness of 10nm as well as electrolyte penetration into the particle interior along the interparticle boundary. However, the doped sample did not present any obvious microcracks, as shown in Fig. 8. This phenomenon was proved to be associated with dense packing along the radial orientation, which can restrain internal strain originated from the change of lattice volume during cycling. Chen

[69]

proposed that LiNi0.8Co0.15Al0.05O2 with boracic polyanion doping exhibited less capacity degeneration compared with the undoped one, the XPS analysis demonstrated that B atoms entered into the bulk structure and formed a gradient boracic polyanion-doping layer. Meanwhile, some Ni3+ ions were reduced to Ni2+ ions on the surface of active material as result of the B doping. Thus, B doping stabilized the host structure and enhanced the performance of nickel-rich cathode materials, processing a stable characteristic similar to Al element. 2.5 Multiple doping Multiple doping is widely reported, and shows combination function. Li et al. [72] revealed that La-Al doped NCM811 displayed high capacity retention of 80.0% after 480th cycle at 10 C, La and Al functioned as pillars and enlarged c-axis direction in lattice cell and decreased cation mixing, so that increased Li+ diffusion rate and restrained the phase transformation. Qiu et al. [73] 10 / 38

revealed that Mn4+ and PO43− co-doped LiNi0.80Co0.15Al0.05O2 decreased the cationic disorder, enlarged the Li+ diffusion channel, and restrained the structural transformation during cycling. Consequently, the capacity retention of 85.5% at 1 C was obtained after 100 cycles. Both of high valence Mn4+ and PO43− polyanion with the strong covalent bond between the P and O, facilitated to enhance the layered structural stability. It is well known that cationic doping can stabilize the phase structure of nickel-rich cathode materials. Such as, F element with the strong electronegativity inhibited the loss of O through the relatively stronger F-metal bond, moreover, whose doping can expand the crystal lattice, promote the fast Li+ removal. The Zr and F co-doped LiNi0.8Co0.15Al0.05O2 displayed cycling performance of 90.5% after 200th cycle at 1 C, exceeding the bare sample with only 75.8% [74]. The Al-Mg doped Li[Ni0.8Co0.1Mn0.1]O2 exhibited low first discharge capacity, but the electrochemical cycling behavior, structural stability and thermal stability were improved [75]. Therefore, the constitution of doping elements in multiple doping need to be considered in order to obtain the excellent electrochemical performance and stable structure. 2.6 Other doping Sasaki et al. [76] revealed that the capacity retention of NCA with Mg substitution could be improved during cycling at 60 ℃, and the internal resistance of battery was decreased due to reduction of the charge-transfer resistance of NCA by Mg doping. Later, they reported that numerous cracks occurred in grain boundaries and the primary particle in the sample without Mg substitution after overcharge of 5.0 V. Furthermore, the cracks extended in the interface region between the active material and the electrolyte, resulting in fragment and formed isolation without electrochemically active feature during repeatedly cycling. These cracks were verified via in situ synchrotron XRD measurements, relating with the drastic shrinking of the c-axis during overcharge. Nevertheless, the c-axis change and the cracking appearance in the particles were relieved in the Mg-substituted cathode materials. Kim et al. [77] demonstrated that LiNO2 doped with 1mol% W greatly increased the cycling performance as well as the thermal stability. The assembled Li-ion battery delivered a discharge capacity of 247 mAh·g-1 between 2.7-4.3 V at 30 ℃ and a capacity retention ration of 90% after 100 cycles. The improvement resulted from the less phase transition which was from layered to cubic (rock salt). 11 / 38

3. Coating materials Generally speaking, it is believed that the coating influenced interface performance more than

internally

structural

stabilization

for

nickel-rich

cathode

materials.

Surface

coating/modification can prevent the cathode materials from direct contact with the electrolyte and avoid decomposition or oxidation of electrolyte. Thus, the coating strategy increased the cycling behavior and storage life, and reduced the heat release of nickel-rich cathode materials. Especially, the physical parameters of coatings, such as coating materials, size, thickness, uniformity, density and conductivity, etc., have a significant influence on the electrochemical behavior of nickel-rich cathode materials. The commonly used coating materials for nickel-rich cathodes are metal oxides (Al2O3, ZrO2, TiO2, B2O3, MoO3, WO3), phosphates, fluorides and conducting polymers [78-85]. However, aforementioned coating materials always restricted the migration of lithium ion and electron through the coating layer, and led to an increase on polarization and decrease on battery capacity. In order to overcome these problems, accordingly, some conductive materials and fast ionic conductors, such as LiAlO2, Li3ZrO2, Li2O-2B2O3, Li3PO4, Li2WO4 [86-91], have been applied as surface coating layer on nickel-rich cathode materials. 3.1 Al2O3 coating Al2O3 possesses high melting point and hardness, and it cannot be easily corroded in acid or alkali condition. Moreover, Al2O3 can form special interface for lithium-ion migration by reacting with LiOH or Li2CO3 on nickel-rich cathode materials at high temperature [92]. LiAlO2 that generated from the above compounds promoted ionic conductivity superior to the bulk structure, therefore, LiAlO2-coated interface provided more Li+ ions than that of Al2O3, and the improvement

on

ionic

conductivity

mediated

the

increase

on

resistance

during

charging/discharging process. Additionally, the nanoscale layer of LiAlO2 also stabilized the layered structure and restrained the strain deformation in the intercalation and extraction of the fast Li+ ion.

12 / 38

Fig. 9. Schemes of the Al2O3 coating layer evolution on (a) NCM523 and (b) LCO, under different annealing temperatures Reproduced with permission from Ref. [87]. The thicker Al2O3 coating usually caused the barrier for Li+ transportation, decreased the initial capacity and capacity retention. However, when treating on high temperature by 600 to 800 ℃, Al reacted with surface residual lithium and resulted in a γ-LiAlO2 phase in nickel-rich cathode materials, so the interface and coating layer were comprised by the mixture of γ-LiAlO2 and Al2O3 , as shown in Fig. 9. XRD analysis revealed that the diffusion of Al3+ ions from surface to the TM layer caused the expansion of the unit cell. It was verified that Al2O3 coating on NCM523 formed closer and more uniform layer at high sintering temperature, manifesting a better cycle performance [87]. Therefore, the increase on heating temperature made Al2O3 ions form a homogeneous surface coating and bulk doping for nickel-rich cathode materials at the same time. 3.2 Li2ZrO3 coating The capacity fade is usually related to lattice strain from phase transformation during de-intercalation and intercalation of lithium ions in cathode materials. ZrO2-coated LiCoO2 had been revealed a zero strain or zero expansion, which inhibited capacity degradation during electrochemical cycle [88, 89]. Furthermore, the Li2ZrO3 coating was extensively studied, whose conductivity is 3.3×10-5 S·m-1 at 573 K and belongs to Li-ion conductor with a monoclinic phase of C2/c symmetry [90]. Li2ZrO3 coating also decreased the dissolution of TM and restricted the side reaction between cathode and electrolyte, and the ion-transportation characteristic on the particle surface was improved. When Li2ZrO3 coating was annealed at temperature of 650℃, it formed a composite structure consisting of crystalline phase and amorphous, the former was 13 / 38

encapsulated by the latter. The configuration was proposed to facilitate fast Li+ deintercalation/intercalation in nickel-rich cathode materials. [91]. The decay on the electrochemical performance was induced by the surface reaction between the active material with the electrolyte as well as the cationic Li/Ni mixing. Nevertheless, the combination of a thin protective layer (like Li2ZrO3 and ZrO2) and Zr doping by partial substitution for the lithium sites or Ni sites could stabilize the surface and interior structure [91, 92]. Yoon et al. [93] demonstrated that Zr-doped LiNiO2 exhibited the excellent electrochemical performance by Zr substitution for Li sites or Ni sites, meanwhile, a thin protective layer of ZrO2 or Li2ZrO3 with a thickness of 5-10 nm was obtained after annealing at 700℃ in Fig. 10. Zr dopant played a role of pillar in the lithium layers and strengthened Li+ rate diffusion, moreover, Zr and Li atoms entered into NiO phase, reacted with electrolyte and formed SEI layer, which maintained the surface structure and decreased the Rct of coated Ni-rich cathode materials. Therefore, cooperation of Li2ZrO3 coating and Zr doping can effectively enhance the electrochemical performance and structural stability of nickel-rich cathode materials in terms of discharge capacity, rate performance and capacity retention, especially at high temperature of 60 ℃.

Fig. 10. Comparison of the capacity vs cycling stability of Zr-LNO to NCM, NCA and LNO with different contents. Reproduced with permission from Ref. [93]. 3.3 Li3PO3 coating Although the excessive amount of lithium is necessary to prepare highly ordered structure for nickel-rich cathode materials, some unreacted lithium always existed in form of main Li2O on the 14 / 38

surface of cathode materials. Moreover, the percentage of residual lithium increased with amount of Ni contents on the surface of nickel-rich cathode materials, which stopped the lithium ion diffusion [94], and easily reacted with moisture in the air, producing LiOH. In turn, LiOH transformed to Li2CO3 when contacted with CO2 in the air [95]. LiOH, Li2CO3 and other lithium-ion compounds on nickel-rich cathode materials were often called residual lithium. The excess residual lithium of nickel-rich cathode materials with Ni≥0.8 was usually diminished by washing procedure to satisfy the slurry preparation and electrode coating, so that decrease the interfacial resistance and gas release of cells during cycling. However, the washing procedure was not easy to control and keep homogeneity in each batch since the chemical delithiation loss in water was difficult to maintain the same level. Thus, it is necessary to develop the unwashing process. For example, some acid coating materials, H3PO4 [96], M3(PO4)2 (M=Ni, Co, Mn, Mg) [97], MPO4 (M=Al, Fe, Ce, Sr) [98], (NH4)10H2(W2O7)6 [99], (NH4)2MoO4 [100] can convert residual lithium to a conductive and protective nanolayer by solid-phase sintering treatment, which improve the process ability, storage life and electrochemical performance of cells. H3PO4 can decrease the amount of LiOH and Li2CO3 by forming Li3PO4, so that reduced the content of residual lithium. Moreover, H3PO4 behaved as a HF scavenging and reduced the HF content in the electrolyte, inhibiting serious side reactions between active materials and the electrolyte [101, 102]. Because Li3PO4 is an ionic conductor with a conductivity of ~6 × 10–8 S·cm-1 at 237 k [103, 104]. it is widely used as an effective coating material. Such as, Ryu et al. demonstrated that the capacity retention of Li3PO4-coated NCM811 was 96% after 100 cycles, which was higher than that of the uncoated sample with 69% in same cycle [105]. Although metal phosphates possess the strong bond energy of polyanion (PO43-), they formed a surface coating accompanied by bulk doping phase when heating at 600℃. Yan et al. [106] discovered that Li3PO4-coating agent diffused into the grain boundaries of the secondary particles in Ni-rich NCM annealed at 600°C for 2h, and led to an interface of solid-state electrolyte, offering a fast channel for Li-ion removal and prohibited penetration of liquid electrolyte into the boundary of primary particles. Consequently, Li3PO4 coated NCM displayed a long-term cycling stability and less voltage fade. 3.4 B2O3 coating B2O3 is a hot coating candidate due to the same feature with Al2O3 and strong B-O bond with 15 / 38

high chemical stability. Moreover, B2O3 is believed to be an ion-conductor with 3D networks. Such as, Zhou et al. [107] reported that the high-voltage capacity retention of LiCoO2 coated by B2O3 was improved after 100 cycles at 1 C during 3.0–4.5V. The coated B2O3 glass and partially diffused Li+ together formed an ion-conductive lithium boronoxide (LBO) interphase, acting as SEI as shown in Fig. 11, which facilitated the Li+ migration through solid-liquid interface upon further cycling. While the B2O3-coated layer played a physicochemical protection against electrolyte decomposition and active material dissolution. Electrochemistry tests suggested that the LBO-coated sample showed higher discharge capacity and more cyclic performance than that of the bare sample at low temperature. Additionally, XPS analysis revealed that BO33- had the stronger electronegativity compared to that of O2-, led to form a more stable interface, which decreased the interface resistance of Li+ removal through the interface, accelerated the charge transfer on the surface of cathode materials and enhanced the low-temperature performance [108].

Fig. 9. Schematic illustration of bare (a) and LBO-coated LiNi0.5Co0.2Mn0.3O2 sample (b), vacancy conducting mechanism in LBO glass (c). Reproduced with permission from Ref. [109]. Lithium boron oxide (Li2O-2B2O3, or LBO) has been used as a coating agent for lots of nickel-rich cathode materials [110–114] due to its advantage including excellent wetting property, low viscosity with easy process, high oxidation potential and Li-ion conductivity. Lim et al. [115] discovered that sample coated with LBO restrained the structural disability as well as TM dissolution at high temperature. A 2 wt.% LBO-coated sample exhibited an evident capacity retention of 94.2% after 100 cycles, which was more than that of the uncoated one with the 16 / 38

capacity retention of 75.3%. 3.5 Fluoride coating Metal oxides can react with HF in electrolyte and form stably protective layer containing metal fluorides (such as AlF3, MgF2, CeF2, CaF2) [116], thus, reduce the corrosion of particle surface from electrolyte acidity and alleviated the degradation of host materials. Additionally, AlF3 is more stable than Al2O3 due to a lower Gibbs free energy of formation, and fluorides are thought to be more stable in ambient air compared to oxides [117]. Kim et al. [118] reported that AlF3-coated Li[Ni0.8Co0.15Al0.05]O2 exhibited a capacity retention of 96% after 50 cycles than that the pristine sample with capacity retention of 86.5%. The fluoride coating layer inhibited the surface damage of cathode materials from HF in the electrolyte, suppressed the formation of LiF, and stabilized the electrode/electrolyte interface. Moreover, it prevented cathode particles from pulverizing during cycling due to the suppression of volume changes. Therefore, fluoride coating resulted in a lower impedance and more stable charge-discharge behavior of nickel-rich cathode materials. 3.6 Polymer Coating Polymer coating was easily coated on the surface of nickel-rich cathode materials and had some particular function, such as alleviating the residual lithium, increasing conductivity, inhibiting side reaction between cathode and electrolyte interface. Doo et al. [119] reported that a hydrophobic cathode material was obtained by coating hydrophobic polydimethylsiloxane. The coating layer prevented the contact between moisture and the particle surface of nickel-rich cathode material, so that relieved the increase of residual lithium of NCM811 in humid air, and kept excellent cycling performance as well as rate capacity for 2 week storage. Gan et al. [120] reported that PANI-PVP (Polyaniline-polyvinylpyrrolidone) coated LiNi0.8Co0.1Mn0.1O2 showed excellent capacity retention of 88.7% after 100 cycles and great rate performance. The conductive PANI coating layer acted as a rapid channel for electron conduction, and kept interface contact from electrode/electrolyte, alleviating the side reaction. Despite the above advantages, there are still some problems need further study, such as uniform coating layer, high temperature resistance and process technology for nickel-rich cathode materials, all these must be resolved before realizing the commercial application. 3.7 Other coating There was small capacity decay for the nickel-rich cathode materials by TiO2 coating, but the 17 / 38

structural stability and cyclability at 60℃ were significantly improved. It was reported that cycled cathode materials coated with TiO2 showed less formation of NiO phase than the uncoated one at 4.3 V, indicating less oxygen loss in the spinel LiNi2O4 phase [121]. Liu et al. [122] found that Nb5+ entered into the Li position of Li interclub and Li3NbO4 coating layer, strengthened the structural and interfacial stability, so that LiNi0.6Co0.2Mn0.2O2 by niobium compound modification brought in the superior electrochemical behavior. For example, the assembled Li-ion battery delivered a discharge capacity of 152 mAh·g-1 at 5 C, and the capacity retention of 91% after 100 cycles. Some coating agents were easily isolated by traditionally mechanical mixture, resulted in a nonuniform coating interface after annealing, which may lead to the short cycle life and much gas release. Furthermore, the partial coating layer under certain regions was too thick to promote lithium diffusion, and generated more impedance, which reduced discharge capacity. Therefore, it is necessary to develop a new coating technique to construct a homogenous and controllable modification layer. Atomic layer deposition (ALD) is a gas-phase method for thin-film growth, and can precisely control the coating thickness under atomic level, which is facilitated to alleviate electrolyte decomposition as well as TM dissolution [123-126]. Cui’s team [127] reported that NCM811 coated by LiAlF4 ion conductive thin film using ALD kept a capacity over 140 mAh/g after 300 cycles, which was higher than that of the pristine NCM811 with a capacity below 140 mAh/g after 113 cycles at a voltage window of 2.75-4.50 V. This result demonstrated that a stably inert protective layer and lithium-ion conductive interface are beneficial to improve the long-term capacity retention for nickel-rich cathode materials. The cathode-electrolyte interphase (CEI) with SOx functional group could only make Li+ ion remove while preventing the electron migrate. For example, Chae et al. [128] devised a CEI layer on NCM811 by thermal treatment from sodium dodecyl sulfate (SDS) and NCM811, the analyses of SEM, EIS and XPS demonstrated that the sulfonate-based artificial CEI layer on the NCM 811 surface effectively suppressed the electrolyte decomposition and reduced the interfacial resistance between electrode and electrolyte. Thus, the 0.5% SDS-treated NCM811 displayed the capacity retention of 91.9%, exceeding that of 69.6% for bare sample after 100 cycles. 4. Conclusions and perspectives The capacity fade of nickel-rich cathode materials is originated from the interface damage, 18 / 38

volumetric changes and lattice internal strains during repeated delithiation/lithiation process. The surface structure was damaged during the reduction of reactive Ni4+ to Ni2+ and oxygen release in a highly delithiated state, and the resultant NiO-like phase occurred on the surface of nickel-rich cathode materials, caused the capacity decay [129-131]. Certainly, it is very important to develop the electrolyte with functional additives, which formed uniform cathode-electrolyte interphase layers and prevented the corrosion of surface structure from HF in the electrolyte. In general, the active Ni4+ in Ni-rich cathode materials was easily reduced at high charge voltage, consequently, the electrolyte decomposed and formed non-conducting on the particle surface, such as LiF, Li2CO3 and ROCO2Li [132], which restricted Li+ removal and decayed the cycling behavior of battery. In addition, these lithium compounds and other by-products generated gases at the high cut-off charge, led to the battery swelling and safety problem. To resolve these problems, some additives were commonly put into electrolyte solutions and formed a stable passivation film, which facilitated the excellent electrochemical properties of nickel-rich cathode materials in LBs during long-term cycles. To our knowledge, lots of electrolyte additives have been reported, such as 1, 1, 2, 2-tetrafluoroethyl-2, 2, 3, 3-tetrafluoropropyl ether (TTE), triphenyl borate (TPB), lithium bis(oxalato)-borate (LiBOB), trimethylene sulfate (TMS), tris (trimethylsilyl) phosphate (TTSP) [133-136]. The added amount of electrolyte additives was little, but they obviously improved the coulombic efficiency, internal resistance, storage life-span, thermal stability and high-temperature cycle of full cells. Luo et al. [137] reported that allylboronic acid pinacol ester (ABAPE) was acted as a film forming additive, preventing the electrolyte decomposition and stabilizing the electrode-electrolyte interface. The LiNi0.8Co0.15Al0.05O2 pouch cells with 1.5% ABAPE exhibited the capacity retention of 87.7%, more than that of 71.8% with bare electrolyte during 500 cycles. However, electrolyte additives maybe not effectively influence phase transformation. For nickel-rich cathode materials, the occurrence of phase transformation was extremely obvious, from monoclinic (M) to rhombohedral (R2) near 4.0 V, again to another rhombohedral phase (R3) around 4.2 V [138-141] during cycling. It can easily destroy the core region within particle, resulting in appearance of microcrack and the penetration into the particle interior. Furthermore, these repeated cycles caused the pulverization of cathode materials, which detached from the original cathode, lost the participation in electrochemical reactions, and decreased the capacity retention. It seemed that it was a feasible method to suppress the 19 / 38

microcrack and structural evolution by doping and coating, which extended the cycling stability and safety of nickel-rich cathode materials. First of all, it is a key to decrease the cation-ion mixing during the synthesis process of nickel-rich cathode materials. Besides the proper sintering temperature and Li/M ratio, element doping is also a well method to decrease cation-ion mixing and stabilize the layered structure. Moreover, it should be focused on some important conditions about doping for the Ni-rich cathode materials,

such

as

dopant

species,

adding

amounts,

homogeneous distribution

and

heating-treatment temperature [142-145]. A valid doping may improve the stability of internal structure, restrict formation of micocracks and transformation of Ni2+ ions to Li layer, resulting in excellent thermal stability. Nevertheless, Doping cannot remarkably restrain oxygen loss, improve the long cycle life and safety performance, and resolve the surface parasitic reaction between cathode and electrolyte related with gas release, which may make battery swell and lead to potential security risk. Thus, it may limit to cylindrical battery, like 18650 and 21700, but not for square battery and soft battery. Nowadays, it is generally believed that the coating is more efficient than the bulk doping, because the coated cathode materials greatly increased cycle life, safety performance and energy density for battery. However, it is difficult to coat for NCA, because it requires much activation energy to break the bond energy between Al and O, compared with NCM. What is more, some special coating materials were used to improve the process ability. Although a large number of nickel-rich cathode materials with high Ni contents were researched and applied in order to satisfy higher energy density and lower battery cost. These materials often suffered from high sensitivity to moisture and easy gelation in polyvinylidene difluoride (PVDF) binder when exposed in high humidity atmosphere [146, 147]. Nevertheless, some acid coating agents can prevent these phenomena. For example, phosphoric acid and boric acid were demonstrated to neutralize the residual lithium and resist water, displaying better electrochemical performance. It is very important to improve performance of nickel-rich cathode materials combined surface coating and outer doping by a certain post-treatment temperature for coated materials. Bulk doping often diminished Li+/Ni2+ cation mixing and oxygen loss, furthermore, dopants easily occupied the Li+ and TM sites as supporting “pillars effect”. Consequently it stabilized the phase structure and enhanced the thermal security. Surface coating can suppress surface deterioration 20 / 38

related with side reactions between electrolytes and active materials, extending the cycling life and reducing the gas release. Hence, combination of doping and coating in the same time always generated multifunctional effects to greatly improve the electrochemical performance and security behavior [148-150]. Such as, the coating layer with nanometer thickness as well as micro doping depth in nickel-rich cathode materials often exhibited superior electrochemical performance [92]. For instance, some Ce4+ in Ce0.8Dy0.2O1.9 coated NCM811 diffused into the bulk structure and enlarged lattice parameter, suppressed the cation disorder and increased the first-cycle efficiency. Besides, Ce0.8Dy0.2O1.9 coating layer is a solid electrolyte with oxygen vacancies [151], captured oxygen release (such as O2-, O-) during charging. Hence, it enhanced the structural stability. The coating and doping of vanadium element in LiNi0.815Co0.15Al0.035O2 (NCA) exhibited discharge capacity of 202.6 mAh·g-1 at 0.1 C and 147mAh·g-1 at 5 C, which exceed that the pristine NCA (92.1 mAh·g-1 at 5 C) [152]. The residual lithium on NCA surface was removed by the reaction with a vanadium source, which relieved the HF formation in electrolyte. In addition, vanadium doping decreased the cation disorder and stabilized the layered structure, meanwhile, coating layer of vanadium compounds (V2O5 and Li3VO4) increased the Li+ diffusion and prevented the dissolution of the NCA cathode material. Partial coating agents often diffused into crystal lattice of nickel–rich materials when post-treatment temperature was elevated, and the coating interface changed more uniform and more stable. So the proper annealing temperature must be adopted to improve structural characteristics and electrochemical performance. Table 1 summarized the representative doping elements and coating materials of nickel-rich cathode materials. Doping and or coating can be obtained by varying the treatment temperature. Table 1 Effects of doping and coating on nickel-rich cathode materials No.

Type

Capacity

Initial efficiency

Cycling

Treatment temperature

1

Al doping

↓

↓

↑

≥700℃

2

Al2O3 coating

↓

↓

↑

500-700℃

3

Zr doping

↑

↑

↑

≥650℃

4

ZrO2 coating

↓

↓

↑

≥500-650℃

5

B doping

↓

↓

↑

≥500℃

6

B2O3 coating

↓

↓

↑

300-500℃

7

Ti doping

↑

↑

↑

≥600℃

8

TiO2 coating

↓

↓

↑

400-600℃

↓=Decrease, ↑=Increase. 21 / 38

Researches on doping and coating of nickel-rich cathode materials for lithium-ion batteries have been performed for many years, but it is lack of deeper comprehension and more extensive theoretical studies. Therefore, more intrinsic mechanisms need to be systemically investigated and analyzed in order to disclose the relationship between electrochemical behavior and doping/coating. These are related with batteries on capacity, cycle, safety, storage and so on. Especially, developing high-performance batteries with lifespan above ten years for EVs and energy storage device are urgently needed in future.

Acknowledgements This work was financially supported by the Postdoctoral Research Foundation of China (2018M633151) and the National Key Research and Development Program "New Energy Vehicle" Key Specialities (2016YFB0100300).

22 / 38

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Compd. 644 (2015) 223-227. [52] M. Chen, E. Zhao, D. Chen, M. Wu, S. Han, Q. Huang, L. Yang, X. Xiao, Z. Hu, Decreasing Li/Ni

disorder

and

improving

the

electrochemical

performances

of

Ni-rich

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1. Main dopants are summarized and analyzed, which decrease the cation mixing, remain phase structure and reduce oxygen release. 2. Coating is more efficient than bulk doping due to greatly increase cycle life and safety performance for battery. 3. Coating agents on decreasing the residual lithium need to be intensively researched to avoid the unwashing process. 4. Combination of the nano-thickness coating and micro-depth doping facilitates the multifunctional improvements to nickel-rich cathode materials.

Declaration of Interest Statement We declared that we have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the manuscript entitled, “A Review on Doping/Coating of Nickel-Rich Cathode Materials for Lithium-ion Batteries”