Realizing High Voltage Lithium Cobalt Oxide in Lithium-ion Batteries

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Realizing High Voltage Lithium Cobalt Oxide in Lithium-ion Batteries Xiao Wang, Xinyang Wang, and Yingying Lu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01236 • Publication Date (Web): 27 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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Industrial & Engineering Chemistry Research

Realizing High Voltage Lithium Cobalt Oxide in Lithium-ion Batteries Xiao Wang, † Xinyang Wang† and Yingying Lu* State Key Laboratory of Chemical Engineering, Institute of Pharmaceutical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China † These authors contributed equally to this work Abstract

The combination of high voltage cathode and metal or graphite anodes provides a feasible way for future high-energy batteries. Among various battery cathodes, lithium cobalt oxide is outstanding for its excellent cycling performance, high specific capacity and high working voltage, and has achieved great success in the field of consumer electronics in the past decades. Recently, demands for smarter, lighter and longer standby-time electronic device have pushed lithium cobalt oxide-based batteries to their limits. To obtain high voltage batteries, various methods have been adopted to lift the cut-off voltage of the batteries above 4.45 V (vs Li/Li+). This review summarized the mechanism of capacity decay of lithium cobalt oxide during cycling. Various modifications to achieve high voltage lithium cobalt oxide, including coating and doping, are also presented. We also extend the discussion of popular modification methods for electrolytes including electrolyte additives, quasi-solid electrolyte, and electrode/electrolyte interface. Introduction Lithium-ion batteries have attracted wide attention since 1970s. In the research field of lithium-ion batteries, the cathode is a key component since its properties can largely influence the energy density and lifetime of the battery. To obtain high energy density batteries, various cathode materials have been studied, such as lithium cobalt oxide (LCO), lithium iron phosphate, manganesebased cathode and so on. 1-10 Figure 1 shows that different cathode materials have different volumetric energy density with different anodes, among which LCO shows higher volumetric energy density after Li-rich.

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Figure.1 Calculated volumetric energy densities of lithium batteries.10 Reproduced from ref. 10 with permission from the Royal Society of Chemistry, copyright 2018.

In the field of consumer electronics products, because of the pursuit of lighter and thinner products, there is an increasing demand for the higher volume energy density and working voltage of batteries. To meet these requirements, it is necessary to make a proper selection for cathode materials with higher compaction density and satisfactory working voltage range. Among several cathode materials, LCO has good compaction density (4.1 - 4.3 g cm-3) and high working voltage range (3.7 – 3.9 V). To make use of the capacity of LCO, researchers have studied it for decades and the cutting voltage has been lifted from 4.2V to 4.45V, with the volumetric energy density of LCO-based batteries improved from 200Wh/L to 700Wh/L. Although lithium-rich materials have higher capacity and voltage, they have serious oxygen evolution and irreversible voltage and capacity attenuation after the first cycle, making it difficult to be widely applied in the market. Therefore, in the foreseeable future, LCO cathode are still the mainstream of cathode materials of Li-ion battery for consumer electronic products. Nickel-cobalt-manganese ternary material (NCM), more likely to be used in electrical vehicles because of its cost-effectiveness, is a competing cathode material compared with LCO. It has different ratios, such as NCM111, NCM523, NCM811 and so on. Although it has a good capacity density (3.6-3.8 g cm-3), the ternary material is not comparable to LCO in compaction density. This means that ternary materials have unsatisfactory volume energy density. In addition, the performance of ternary materials on the working voltage is not as high as that of LCO. Although ternary materials with high nickel ratio deliver high capacity density, the cycling performance of ternary materials is relatively poor due to the cationic rearrangement, thus they are not suitable for 3C market with high requirements for battery life.11-13 Therefore, in general, LCO as a high voltage cathode material still has its advantages. The cut-off voltage of current LCO electrode in practical use is only 4.2 V, that is, 50% lithium ions are extracted. This is because when lithium ions in LCO are deintercalated more than 50%, the structure of LCO will be unstable and the capacity fading occurs. At the same time, at high voltage, a series of side reactions will occur at the interfaces between LCO particles and electrolyte, resulting in irreversible element dissolution. LCO can be stabilized by modifications of the structure of LCO particles, such as surface modification or element doping. Electrolytes is another bottle-neck for practical application of high voltage LCO (HVLCO). Conventional non-aqueous electrolytes which mainly refers to the carbonate solvent-based electrolytes exhibit inferior anodic stability for high-voltage operation above 4.3 V vs. Li/Li+. Developing solid electrolytes toward an all-solid-state battery is considered as the ultimate solution whereas they still suffer from the challenges such as low ionic conductivity, poor interfacial contact between electrolyte and electrode, and oversensitivity, which indicate that there is still a long way to go. Here, we highlight the recent progress in the use of surface modification, elements doping and electrolyte additives for HVLCO and the quasi-solid electrolytes as an appreciate alternative at the present stage. Challenges for LCO LCO was first discovered as a viable layered rock-salt cathode material by Goodenough in 1980.14, 15 After that, Sony Corporation applied it into the portable device in 1991. The structure of LCO is isostructural with α-NaFeO2. In the structure, Li+ and Co3+ ions are ordered on alternating (111) planes. This (111) ordering introduces a distortion of the lattice to hexagonal symmetry, —

which leads to the crystallization of LCO in the space group R3m with cell constants a=2.816Å and c=14.08Å. With the deintercalation of Li+ ions from bulk LCO, the phase transition occurs from H1 to the H2, M1, H3, M2, and O1 phases with different amounts of extraction of Li+ ions showed in Figure 2 a and b. In terms of Figure 2, it is obvious that there exists a reduction of c lattice around 50% of Li+ extraction, which happens when overcharged above 4.2 V. Most groups2, 7, 11, 16-38 believe that the decreased c lattice leads to the capacity fading because the changes of c lattice but almost no changes of a lattice will induce nonuniform strain in particles and causes particle fractures. Some researchers thought that the ultrahigh voltage cannot be


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used as cycling cut-off voltage higher than 4.6 V. The transition from H3 to H1-3 near 4.55 V can greatly change the oxygen layer arrangement. Other than the structure changes during phase transition, there also exists Co dissolution which greatly lead to capacity fading. Amatucci et al.20 tested the Co dissolution, and they found that the Co dissolution was detected above 4.2 V. Chebiam et al.25 attributed it to the loss of oxygen from lattice at deep lithium extraction. After studying the LCO structure at high voltage operation, researchers believe that the unstable structure and the losses of active elements will lead to the incompatibility between host LCO and other battery component (such as electrolyte, binders, conductive additives, etc.), which deteriorates the electrochemical performance and causes safety issues at high voltages. Besides the intrinsic instability of LCO at high voltage, the interface stability between LCO and other battery component is another challenge. It is believed32 that the μc of the cathode should be higher than the highest occupied molecular orbital of the electrolyte unless the electrolyte will be oxidized. Thus, it is important to design a proper electrolyte to adjust to the high voltage. The LiPF6 which is the widely used as electrolyte salt has low thermal stability and is sensitive to hydrolysis. It will decompose and cause thermal runaway from side reactions. Cycling on high voltage will exacerbate the side reactions between electrolyte and cathode. So, how to design electrolytes and use additives to inhibit the side reactions in LCO batteries, especially at high voltages, is worth studying. Researchers have found a lot of approaches to solve the problems when the LCO is cycled above the 4.2 V such as doping and coating. Doping can effectively change the structure of LCO and stabilize it. Coating can stabilize the electrolyte/electrode interface and prevent the side reactions between LCO and other battery components. All the modification methods will be extensively discussed below.

Figure 2. Variation of lattice a and c during electrochemical lithium extraction of LiCoO2.19 Reproduced from ref. 19 with permission from the The Electrochemical Society, copyright 1996.

DOPING Doping is an effective way to improve the capacity and electrochemical stability window of the LCO. Different element doping has different effect. For example, transition metal ions can increase the capacity while non-transition metal ions can increase the delithiation potential2. The element doping can change the physical properties of the material’s crystal lattice. Since the high potential or capacity LCO batteries have a lot of benefits, many research groups have investigated the doping effects through different non-transition ions, such as Mg, Ca, Al, B, et al., and transition metal ions, such as Ni, Zn, Cu, Rh et al. Some others



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developed mixed elements doping like Ni + Mn, Li + Mn, Mn + Mg, Mg / Al / Zr + F, and La + Al.

Non-transition elements Non-transition ions dopants used in LCO include Mg, Al, Ca, B, et al. The ionic radii of the ionic elements are 0.72Å (rMg2+), 1.00 Å (rCa2+), 0.535 Å (rAl3+), 0.27Å(rB3+). Mg is a nontoxic, light and cheap element, which is regarded as a promising substituent for Co in LCO. Tukamoto et al.39 found that Mg doped LCO has a better electronic conductivity without impurity phase. Though the capacity of Mg doped LCO is slightly lower, the stability of cycling at 4.3 V voltage can be achieved. Zou et al.40 synthesized LiMg0.05Co0.95O2 which had a charge/discharge capacity of around 160 mAh g-1 at 0.2 C rate over 50 cycles. This is the first time that the stable capacity was obtained from doped LCO at high voltage (4.5 V). Sathiyamoorthi et al. 41studied the different amounts of Mg in LiCo1-xMgxO2 and found that the LiMg0.2Co0.8O2 has highest discharge capacity of 191.3 mAh g-1 at voltage of 4.5 V. Mladenov et al.42 found that the replacement of Co by Mg contributes to the enhancement of capacity retention, while the Mg dopants in LiO2-layers has no effect. Since the Mg doped LCO exhibits better electrochemical performance than bare LCO, many groups try to find how the Mg2+ lead to the enhanced electrical performance of LCO. Tukamoto et al.39 thought because of the presence of a small amount of Co4+. LCO exhibits p-type semiconductivity. They believed that the undoped LCO is Li-deficient with a small concentration of Li vacancies and equal number of Co4+. The low concentration of Mg will lead to occupation of Li vacancies by Mg, which causes the reduced concentration of Co4+ and the decrease of the conductivity. The high concentration of Mg will substitute the Co sites, which can contribute to the creation of Co4+ for charge balance. Levasseur et al.43 agreed with it, and suggested that intermediate spin Co3+(IS) ions are trapped in a square-based pyramidal environment due to the presence of oxygen vacancies. Kim et al.44 analyzed the crystal structure of LiCo1-xMgxO2 with X-ray diffraction and Rietveld refinement, showed in Figure.3. They found that the material displays a single phase when x ≦ 0.11, and second phase of MgO will occur when x ≥ 0.13. The higher the Mg concentration is, the larger distance between CoO2 slabs are. Besides, they believed that the increased Mg content will increase the carrier concentration while carrier mobility is nearly unchanged, which leads to the enhancement of the electrical conductivity (Figure 3 a-d). The electrochemical performance of Mg doped LCO is showed in Figure 3 e and f.45 Ca is another popular element used as a dopant for LCO. Sathiyamoorthi et al.46, 47 synthesized Ca doped LCO materials by low temperature solid state method. The lattice parameters a and c increase with the Ca content increases, which is resulted from the bigger radius of Ca. The capacity of the doped LCO reached 178.5 mAh g-1 after 25 cycles at 4.5 V. Other than the divalent non-transition elements mentioned above, the trivalent non-transition elements were used as dopants as well, such as Al, B et al. Ceder et al.2 suggested that the Al substitution can increase the intercalation potential but decrease the capacity. This was later proved by several other groups.48 Huang et al.48 synthesized the solid solution LiAlxCo1-xO2 and found that when x=0.15, the initial capacity can reach 160 mAh g-1 and no major changes could be observed through XRD. Jang et al.49 synthesized LiAl0.25Co0.75O2 and showed that the doped LCO had a better performance when cycled at 55℃. They also found that the c/a ratio increases with the Al content but the volume of the cell remains unchanged. Yoon et al.50 concluded that Al substitution for Co leads to larger local structure distortion, and the capacity fading is associated with the existing Co4+ and the structure distortion. Besides, they found the temperature of LiAl0.25Co0.75O2 synthesis has an influence on the local structure distortion during cycling51. Myung et al.52 studied relationships among the Al content, Co dissolution, capacity and the variation in c-axis, and concluded that the capacity will decrease when Al replace the Co, but the cycling stability can be effectively enhanced.


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Figure 3. a and b: Structure and electrical properties of LiCo1-xMgxO2; c and d: Calculated CoO2 slab and inter-slab distances from Rietveld refinement of LiCo1-xMgxO2.44, e and f: The initial charge capacity and cycling stability of LiCo1-xMgxO2.45 Reproduced from ref. 44 and 45 with permission from Elsevier and Elsevier, copyright 2006 and 2014.

Yoon et al.53 manufactured the Al doped LCO by sol-gel process, and the DSC result showed that the Al dopants can enhance the thermal stability of the bulk LCO and reduce the heat generation when reacting with electrolytes, which was later proved by Zhou et al.54 as well. They55 concluded that the substitution of Al for Co will increase the O2 participation in the charge compensation process during charging.



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Besides the Al, B was used as an effective trivalent dopant as well. Alcantara et al.56 suggested that the boron dopants can improve the reversibility of the material and contribute to the adaption of lattice to lithium order-disorder in the depleted LiO2 layers, preventing the onset of the structural first-order transition related to the Verwey transition in Li0.5CoO2.57 Julien et al.58 realized that the boron doped LCO provides low polarization but the capacity is not satisfying (only 130 mAh g-1 at 4.3 V).

Transition metal elements Transition metal elements used as LCO dopants include Ni, Zn, Cu, Rh et al. The radii of the ionic elements are 0.74Å (rZn2+), 0.73 Å (rCu2+), 0.56 Å (rNi3+), 0.665Å(rRh3+). Zou et al.59 synthesized Cu, Zn and Mn doped LCO and showed that the Cu and Zn doped LCO provided great initial capacity of around 165-170 mAh g-1 in the range of 3.5 V-4.5 V, but faded slowly and retained ~130 mAh g-1 and ~110 mAh g-1, respectively. The Mn doped LCO showed better capacity stability, which delivered ~158 mAh g-1 after 50 cycles at 4.5 V. Nithya et al.60 used microwave method to synthesize Cu doped LCO and found that the LiCu0.2Co0.8O1.9 delivered a capacity of around 150 mAh g-1 in the range of 2.7 V-4.6 V (Figure 4 a, b). Valanarasu et al.61 found that the 3 mol% Zn doped LCO delivered about 178mAh g-1 after 30cycles between 3.0 V-4.5 V (Figure 4 c, d). The trivalent ions used as dopants can be Ni3+, Rh3+ and Fe3+ et al. Madhavi et al.62 found that the Rh can suppress the phase transition when 50% Li deintercalates and the voltage of the deintercalation decreases. Reddy et al.63 studied two kinds of LCO substituted by Ni: LiNi0.3Co0.7O2 and LiNi0.7Co0.3O2 and showed that the latter delivered better capacity of 165 mAh g-1 after 60 cycles at 4.3 V. Liang et al.64 found that the Ni located in Li sites can support the layered structure, which allows the LCO to intercalate / deintercalate more Li ions (Figure 4 e, f).


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Figure 4 Initial capacity and cycling performance of LiCuxCo1-xO2 (a and b)60, Zn doped LCO (mol%)61 and Ni doped LCO64. Reproduced from ref. 60, 61 and 64 with permission from Elsevier, Springer and Elsevier, copyright 2011, 2012 and 2014.

Mixed elements Although single element doping can improve the electrochemical performance of LCO to some extent, its effectiveness is not comprehensive and is only beneficial for one or two aspects, such as Mg mainly improves the conductivity of lithium cobalt oxide, Al can play a pillar effect to stabilize the material structure, Ni can make the material release more lithium ions at a specific



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voltage, and so on. To improve the electrochemical properties of materials more comprehensively, researchers tried to dope lithium cobalt oxide with two different elements. Compared with single element doping, binary doping can improve more properties of the materials, which is a good reference for future research on high voltage LCO.

Ni+Mn Researchers tried to take Mn and Ni as codopants because LiNiO2 and LiMnO2 are isostructural with LCO. The redox of Ni2+/Ni4+ is lower than that of Co3+/Co4+, which can improve the capacity of the LCO and the Mn4+ can show pillar effects to stabilize the structure. Lu et al.65 reported the electrochemical performance and thermal stability of LiNixCo1-2xMnxO2 for the first time. When the x=0.25 or 0.375, the materials delivered 160 mAh g-1 at 4.4V and showed better thermal stability. Ohzuku et al.66 synthesized LiNi1/3Co1/3Mn1/3O2 (NMC111), which delivered 150 mAh g-1 at 3.5V-4.2 V or 200 mAh g-1 at 3.5 V-5.0 V. Shaju67 and Hwang68 studied the structure change of NMC111 and found that the redox process around 3.8V and 4.6V were ascribed to Ni2+/4+ and Co3+/4+, respectively. Kim et al.69 found that the great cycling stability of NMC111 can be attributed to no structural variation and volume change during cycling, and the cation mixing effect was great undermined low. Li et al.70 tested two different methods to synthesize NMC111 and found that the sample prepared by spray dry method exhibited better performance. Other methods to synthesize the NMC111 include ultrasonic spray pyrolysis method71, solid-state reaction of metal oxide and lithium hydroxide72, co-precipitation73. The electrochemical performance and structure are showed in Figure 5 a and b. Besides the NMC111, researchers studied different proportions of Ni, Mn and Co as well. Lee et al.74 synthesized material with concentration gradient of metal elements inside particles. The electrochemical performance demonstrated continuously cycling for over 2500 cycles with only 16.7% capacity fading but the capacity was not high, which suggests that the improved capacity can be reached through optimizing the concentration gradient to reduce the lattice mismatch. Lin et al.75 prepared LiNi0.4Mn0.4Co0.2O2 by spray pyrolysis method. The prepared material exhibited elements segregation - Ni-poor and Mn-rich surface show great resistance against surface reconstruction - which contribute to the superior electrochemical performance. Shen et al.76 investigated the LiNi0.5Co0.2Mn0.3O2 and detected the formation of NiMn2O4 and LiOH after charged at 4.5 V. The lithium ion mobility is limited, the vulnerable surface film leads to the cracks on the surface and the stable NiMn2O4 generates after cycling, causing structural damage and bringing down the electrochemical performance. Hwang et al.77 found that the discharge capacities of the LiNi0.8Co0.1Mn0.1O2 (NMC811) are low at high rate charge and discharge, which is resulted from the severe structural inhomogeneity and instability of NMC811. They discovered that the structural degradation owes to the reduction of the Ni ions and the consequent decrease in Li ions conductivity. And these problems lead to the capacity fading as well. Kim et al.78 designed and synthesized a stable Ni-rich cathode by a polymeric-bead assisted coprecipitation method. The prepared material showed a transition metal gradient with reduced nickel ion and the internal pores in the secondary particles, which stabilizes structure and accommodates the strain generated during cycling.

Li+Mn Other than the Ni and Mn, researchers also studied Li and Mn as codopants. Because the Li2MnO3 is isostructural with LCO, the codoping is easy with the two elements.79 Numata et al.80, 81 synthesized the LiCo1-xLix/3Mn2x/3O2 and showed that the Co3+ ions located near the Mn4+ requires a higher oxidation voltage, which decreases the capacity. Thackeray et al.79 discovered that the xLiMnO3 (1-x) LiCoO2 can be activated by the Li2O and Li from the structure and delivered capacity of more than 250mAh g-1 at high voltage. Rana et al.82 studied the 0.5Li2MnO3 0.5LiCoO2 and found that the Li2MnO3 and LiCoO2 exit as separate domains. The randomly distributed Li2MnO3 acts as excess lithium source, and leads to higher capacity and upper operating voltage. Sun et al.83 discovered that the discharge capacity decreases with the increasing Li2MnO3 content and the layered structure can be remained unchanged during charge process, which enhances cycling performance. After studied the electrochemical performance


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of the Li + Mn codoped LCO, the further studies regarding the structure changes were conducted. Xiang et al.84 found that the initial capacity changes with the different x of xLiMnO3 (1-x) LiCoO2. The results showed that when x=0.4/0.5, the initial capacity reaches the highest. The larger the x, the higher the oxygen loss. Though the modified LCO can deliver high capacity and work at high cut-off voltage, the irreversible capacity loss, which is caused by the loss of the oxygen and lithium, is still a problem for applying this modification method. Furthermore, the elements loss can result in the safety issues which should be taken into consideration. The electrochemical performance and structure were showed in Figure 5 c and d.

Other mixed dopants Jung et al.85 used metal and F as codopants. They found that the Mg and F co-substituted the Co performed better cycling stability and the phase transition was not observed through differential capacity vs. voltage profiles. The initial capacity of LiMg0.05Co0.95O1.95F0.05 (LMgCOF) was almost the same as the LCO. They believed that the metal ions could promote the Li+ diffusion and the F could modify the surface of the material and prevent the HF attack. The effect of metal ions doped in LCO have been studied thoroughly, but the specific effect of F ion substituted for O sites still needs further study. Liu et al.86 used La and Al as codopants to improve the LCO and showed that the phase transition between 4.1 V-4.2 V was inhibited (Figure 5 e and f) and the material could deliver 190 mAh g-1 at 4.5 V with 96% capacity retention after 50 cycles. They thought that the La works as a pillar and the Al as a positively charged center, which stabilizes structure, enhances Li+ diffusion and suppresses phase transition. As discussed above, elements doped LCO performed better than the pristine material, and the effects of different elements have been studied for a long time, such as Mg, Al, Ni, Mn, et al. They can enhance the electronic conductivity, structure stability and structure reversibility. Researchers also tried to combine two elements to dope the LCO and the results are satisfying. But there are still some problems to be studied, such as the actual effect of F substitution for O.



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Figure.5 Structure model and cycling performance of LiNi1/3Mn1/3Co1/3O2 codoped LCO (a and b)73, 87 and xLi2MnO3⋅(1−x) LiCoO2 (c and d)88, 89; electrochemical properties of La + Al codoped LCO (e and f)86. Reproduced from ref. 73, 87, 88, 89 and 86 with permission from Elsevier, Elsevier, Elsevier, The Royal Society of Chemistry and Nature copyright 2004, 2003, 2015, 2012 and 2018.

COATING Doping is a great approach to improve the structure stability, electrochemical performance and thermal stability. But it is hard to suppress the reactions between electrolyte and cathode surface. Surface reaction is a main cause of the capacity decay. For example, the reaction of HF with the hydroxylated LCO results in the formation of H2O and LiF. The LiF, which is insoluble in the electrolyte, will inhibit the migration of Li+. The resultant H2O can react with PF5 in electrolyte, which produces extra HF.90 Considering the shortcomings of doping methods, coating as an effective way to protect the surface of LCO particles. The coating materials include oxides, fluorides, phosphates and others.

Oxides Due to the great thermal stability and electrochemical stability, oxides were first used materials as coatings to modify the surface of the LCO. Researchers believed that the oxides can act as a protect layer to prevent the HF reacting with the bulk cathode. Cho et al.23, 91 synthesized the cathode with SnO2 and Al2O3 coating. The former delivered capacity of 166 mAh g-1 with 86% capacity retention after 47 cycles between 2.75 V-4.4 V at 0.5C rate, and the latter delivered capacity of 174 mAh g-1 with 97% after 50 cycles between 2.75 V-4.4 V. It is believed that the high concentration of Sn or Al on surface can prevent the phase transition from monoclinic phase, which results in the better electrochemical performance. Scott et al.92 coated the LCO with Al2O3 by


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atomic layer deposition and showed that the coated cathode delivered 133 mAh g-1 at 7.8 C, which was a great improvement compared with the pristine LCO. This ultrathin film can protect the LCO particles and enable it to be cycled at high rates. Liu et al.93 studied Al2O3 coated LCO by coprecipitation method, and found that amorphous Al2O3 can prevent Co dissolution from the LCO, which decreases capacity loss. Oh et al.94 thought that there exists a thin layer of Li-Al-Co-O, which protects the structure from corrosion of electrolyte. The results suggested that the dissolution of Co decreases and the thermal stability is enhanced. After that, the same group95 observed that the AlF3 ` 3H2O was formed from the reaction of the Al2O3 coating with the HF and H2O. They believed that the coating can scavenge the H2O impurities, which prevents the corrosion and decreases the Co dissolution. As a consequent, the electrochemical performance was promoted. Daheron et al.96 studied the LiCo1-xAlxO2 solid solution formed between bulk LCO and Al2O3 coating via XPS. They found that the compound can make the LCO less sensitive to the acidic attack and prevents the Co dissolution, which enhances the cycling stability. Jung et al.97 found that Li3.4Al2O3 is the energetically most favorably composition of the lithiated oxide alumina. They found that the Al in the Al2O3 coating layer can accept the electrons from the Li atoms resulting in formation of different Al structures, and Li can diffuse faster than the Al and O atoms when concentration is optimized, leading to deeper insight of the Li diffusion in the coating layer. Han et al.98 compared the annealing temperature when coating LCO with Al2O3. They found that the Al can diffuse into LCO lattice and the modified material showed better initial capacity but worse cyclability at high temperature. They concluded that the worse cyclability is resulted from the intercalation of surface alumina into bulk LCO (Figure 6 b, c). The coating layer of Al2O3 on LCO is generally believed to be effective to inhibit the Co dissolution, as well as preserve the variation range of the lattice parameters during cycling, which greatly improved the capacity retention of cathode. Other than the Al2O3, there are other oxides used as coating materials. Mladenov et al.42 reported a MgO coated LCO, which showed good cyclability with 135 mAh g-1 after 30 cycles at 4.3V. Wang et al.99 showed that the MgO-coated LCO showed good electrochemical stability up to 4.7 V, which may be resulted from the diffusion of metal ions from the coating layer and occupied Li vacancies preventing the vacancy ordering (Figure 6 a). Shim et al.100 studied the effect of different annealing temperature on MgO-LCO and showed that the sample annealed at 810℃ led to high doping level of Mg2+ in Li sites, which improved cycling stability through pillar effect, and it was later proved by Orikasa101 as well. (Figure 6 d, e). ZrO2 102-104, ZnO105-108, CuO109, and Fe2O3110 were also used as the coating materials. Though most oxides coatings can enhance the cycling stability, the mechanisms for the enhancement is controversial. Some researchers believed that the coating can stabilize the structure of the LCO. They thought the coating can suppress the phase transition from H2 to M1, which is the main cause of the structural degradation. However, other researchers thought that the coating materials cannot suppress phase transition but make the LCO more reversible. In conclusion, the mechanisms of effectiveness of coatings need to be studied further as well as the effect of solid solution formed between coating materials and bulk LCO.



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Figure 6. a: Cycling performance of MgO-coated LCO and Al2O3-coated LCO99; b: Scheme of the Al2O3 coating layer evolution on LCO under different annealing temperature98; c: The near-0-ppm region of Al MAS NMR results of as-coated and annealed LCO samples, compared to LiAlCoO2 solid solution98; d: Schematic view of the electronic structure at MgO-LCO/electrolyte interface upon electrolyte immersion101; e: Schematic view of the surface of MgO-LCO during lithium ion extraction.101 Reproduced from ref. 99, 98 and 101 with permission from The Electrochemical Society, American Chemical Society and Wiley-VCH, copyright 2002, 2017 and 2014.

Fluorides Apart from the oxides, fluorides are also effective coating materials. In the studies of the oxide coating, researcher found that the surface composition of modified LCO contains F- ions, which arouses the interest about studying the fluoride coating materials. Since the Al2O3 performed well, AlF3 was widely studied by researchers. Sun et al.111, 112 coated the LCO with AlF3, and the 0.5 mol% AlF3-coated LCO showed better electrochemical performance with the capacity retention of 97.7% at 4.5 V and 94.3% at 4.54 V after 50 cycles respectively (Figure 7 a and b). They found that the coating layers can stabilize the interface between the cathode and electrolyte, preventing the attack from the HF, and the Co dissolution, which delays the phase transition to spinel phase during cycling. They also found that using more AlF3 will lead to thicker inhomogeneous layer, which is not good for electrochemical performance. Lee et al.113 blended AlF3-coated LCO and AlF3-coated LiNi1/3Co1/3Mn1/3O2, which delivered capacity of ~185 mAh g-1 at 4.5 V and showed 95% capacity retention after 50 cycles. Zhou et al.114 coated the LCO with AlF3 by atomic layer deposition. The fabricated electrode had a high active material loading of 20 mg cm-2 and delivered capacity of 216 mAh g-1 at 4.7 V and preserved 75.7% of its initial capacity after 100 cycles. Yang et al.115 synthesized LaF3-coated LCO, which had a discharge capacity of 177.4 mAh g-1 at 4.5 V and showed 90.9% of initial capacity retention after 50 cycles. The obtained material also exhibited a good overcharge potential of 4.3 V, 4.5 V and 4.7 V. Abaolaich et al.116 coated LCO with CeF3,


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which showed a reversibility capacity of 160 mAh g-1. Qian et al.117 synthesized Li-Al-F coated LCO, which delivered initial capacity of 216.2 mAh g-1 and showed capacity retention of 81.8% at 4.6 V in Figure 7 c-f. They created a stable conductive solid solution layer via hydrothermal-assisted surface treatment. Through density function theory calculation, they predicted the most possible structure of the solid solution layer to be Li1/3Al1/3Co2/3O4/3F2/3. The layer retarded the reaction occurred at high voltage and alleviated the phase transition from O3 to H1-3. Even though the coatings with fluorides show better electrochemical performance, the Li diffusivity is likely to be low which may limit the rate capability of modified cathode. Hao et al.118 studied the Li transport in LCO and found that the diffusivity of Li is low for α-AlF3 (α-Al2O3) due to unfavorable Li binding sites and relatively high diffusion barriers. This diffusivity is lower than those in the benchmark materials, Li-β-alumina and LiFePO4. Therefore, it is also a challenge for researchers to discover how to enhance the Li diffusivity with fluoride coatings.

Figure 7. a and b: Electrochemical performance of AlF3 coated LCO at different voltage112; c: The formation and synthetic process for constructing LAF-LCO; d: Electrochemical performance of LAF-LCO; e-f: Electrochemical analyses of 2% LAF-LCO.117 Reproduced from ref. 112 and 117 with permission from The Electrochemical Society and Nature, copyright 2009 and 2018.

Phosphates Phosphates are used as coating material for better thermal stability and can be applied in harsh conditions because of the strong P=O bonds and strong covalent bonds formed between polyanions and metal ions.37 Cho et al.119-126 firstly showed that coating with AlPO4 enhanced the thermal stability and cycling stability of the bulk LCO. The modified LCO retained 135 mAh g-1 after 200 cycles at 60 ℃.119 After that, they studied120 the correlation between the thickness of coating layer and thermal stability. The results showed that the thicker the coating, the better the thermal stability. However, the Li diffusivity decreased with the thicker layer. They concluded that the nanoscale coating layer (~20 nm) was believed to be the optimized condition for modified LCO. The tests of overcharge behavior were also conducted and showed that the coated LCO did not exhibit thermal runaway even at 3C rate, which enhanced safety of Li-ion cells.123, 124They122 compared the P2O5coated LCO and AlPO4-coated LCO, and showed that the thermal stability was enhanced by coating, while the AlPO4-coated LCO delivered more capacity during cycling. Compared with the Al2O3-coated LCO, AlPO4 could protect LCO even at 4.8V. The irreversible capacity of Al2O3-coated LCO (~34 mAh g-1) was larger than the AlPO4-coated LCO (~24 mAh g-1).125 Kim et al. found that the AlPO4 nanoparticles could effectively inhibit the non-uniform distribution of local strain, which greatly enhanced



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the cycling performance at ultrahigh voltage of 4.6V. Lee et al.127 studied the concentration of coated AlPO4, and the 1 wt% AlPO4 showed better electrochemical performance at 4.8 V (Figure 8 a). Kim et al.128 studied the size of particles, and found that the particles smaller than 20nm resulted in complete encapsulation while those with size bigger than 150nm formed partial encapsulation. The electrochemical performance was better with complete encapsulation. They concluded that the AlPO4 coating was better with particle size smaller than 3 nm and thickness smaller than 10-15 nm. Appapillai et al.129 studied the microstructure change occurred at surface of AlPO4-coated LCO. They found the AlPO4 absent and formation of LIAlyCo1-yO2 and Li3PO4. The former was formed by the migration of Al3+ ions to lattice of LCO, and the latter was formed through reaction between LiCO3 and AlPO4. LIAlyCo1-yO2 was believed to stabilize the structure of LCO and Li3PO4 was believed to act as a Li diffusion channel which enhanced Li ions conductivity. Besides, Li3PO4 reduced the potential experienced by LixCoO2 and inhibited the decomposition of electrolyte (Figure 8 b). Lu et al.130 found a thin layer formed on the surface of pristine LCO during cycling containing LiF and LixPFyOz, while the layer formed on the surface of AlPO4-coated LCO included Co- and Al- containing fluorides, oxyfluorides and PFx(OH)y-like species. They speculated that the coated LCO particles promoted the formation of CoAl-O-F species which prevented the side reaction with electrolyte and reduced impedance growth when cycling at high voltage (Figure 8 c). Lee et al.131 synthesized LiCoPO4-coated LCO through reaction between Co3(PO4)2 and surface of LCO. The modified material showed better thermal stability than AlPO4-coated LCO. Li et al.132 used FePO4 as coating material and the 3 wt.% FePO4-coated LCO showed good electrochemical performance with initial capacity of 146 mAh g-1 at 4.3V and 88.7% capacity retention after 400 cycles. Morimoto et al.133 studied LiMgPO4-coated LCO which showed initial capacity of ~200mAh g-1 and capacity of ~175 mAh g-1 after 50 cycles at 4.5 V (Figure 8 d). After that, they synthesized Li1+xAlxTi2-x(PO4)3 (LATP) through Li2O-Al2O3-TiO2P2O5 system. The LATP-coated LCO exhibited high capacity of 180 mAh g-1 at 4.5 V. The high Li diffusivity of the LATP inhibited the increase in charge transfer resistance and decreased the activation energy.134 Since most coating materials have poor ionic mobility, Zhou et al135 used Li3PO4 (LPO)-coated LCO to improve the performance. The LPO acted as an efficient channel for Li+, which enhanced the rate performance of LCO. Shen et al. synthesized LiAlPO3.93F1.07 (LAPF)-coated LCO. It delivered capacity of 206 mAh g-1 up to 0.5C and showed great capacity retention of 91.7% after 50 cycles (Figure 8 e). From the coating with phosphate, researchers found an interesting phenomenon. Bai et al.136 used YPO4 as coating layer, and found that the YPO4 increased acidity of electrolyte by converted Lewis acid, which enhanced the conductivity of SEI layers. They believed the effect could be realized through adding YPO4 or Al2O3 into electrolyte and concluded that the Lewis acids purified the surface of LCO, increased the conductivity of formed SEI and promoted the formation of more stable surface solid solution with the substitution for Co and Li ions with metal ions from these additives.137


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Figure 8. a: Electrochemical performance of AlPO4 coated LCO127; b: Cross-sectional TEM images of the AlPO4 coated LCO129; c: Proposed working mechanism of AlPO4 coated LCO130; d: cycling performance of LiMgPO4 coated LCO133; e: Cycling performance of LiAlPO3.93F1.07 coated LCO138. Reproduced from ref. 127, 129, 130, 133 and 138 with permission from The Electrochemical Society, American Society Chemistry, American Society Chemistry, Elsevier and Elsevier, copyright 2004, 2007, 2009, 2012 and 2017.

Coating materials have been studied for a long time and many groups found various chemicals for coating. The solid solution between coating layer and bulk LCO is considered as the protective layer for inhibition of side reactions between LCO and electrolyte. It is common that solid solutions form during the annealing at high temperature. However, the exact properties and composition are still not clear meaning that more in-depth understanding is needed. Additionally, the different coating materials showed different improvement of LCO. For example, fluorides can enhance the stability and phosphates can tolerate the harsh conditions. Therefore, selecting suitable coating materials in different conditions needs to be considered. Besides, the binary coatings such as LiMn2O4139, 140, MgAl2O4141, LiMgPO4133 et al. showed better electrochemical performance than the unitary coatings, but there are few studies about it. Furthermore, polymers have also been used as coating materials, but the studies and their working mechanism are rare.

Electrolyte additives



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As discussed above, many doping and coating methods have been used to improve the structural stability and the interfacial compatibility of LCO at high voltages. However, conventional carbonate-solvent-based electrolytes such as ethylene carbonate exhibit inferior anodic stability of lower than 4.3 V vs. Li/Li+, which becomes another obstacle to hinder HVLCO for further commercialization. Moreover, the high flammability of non-aqueous electrolyte solvents is a big potential safety hazard. To overcome these annoying issues, many efforts based on developing advanced electrolytes with unique characteristics such as fast ion transportation, less side reactions, higher operation voltage ranges and working temperature limits have been devoted. In terms of solvents, sulfone-based solvents142-144, room temperature ionic liquids145-147, dinitrile solvents148 and plenty of functional groups modified carbonates146, 149 with high anodic stabilities have been investigated. Unfortunately, these solvents still suffer from some issues including large viscosity, high price, low cathodic stability combining carbonaceous cathode and high toxicity. On the other hand, adding just a small amount (usually less than 5 %) of functional additives, which can form a stable artificial solid electrolyte interphase layer on the cathode (CEI) and improve the anodic stability of the electrolyte, have been recently recognized as a promising means for HVLCO due to the lower cost and high effectivity. Nanocompound additives: Different from a surface coating/modification strategy using metal oxides, Liu et al.150 proposed a novel solid superacid model (Figure 9a) by soaking the nano - Al2O3 in common LiPF6/EC/DMC electrolyte and then removing the nano-Al2O3. They found that the added nano-Al2O3 scavenges the HF which is recognized to lead to capacity leakage and converts to solid superacids AlF3/ Al2O3 and LiAlF6/AlF3 in the presence of water residue. Furthermore, these solid superacid can corrode the insulating alkaline species such as Li2CO3, LiOH, etc. on LiCoO2 and enhance the ionic conduction of the CEI, thus 85 % of initial discharge capacity (192 mAh g-1) was remained after 100 cycles between 3.3 and 4.5 V at 0.15 mA cm-2 using the nano-Al2O3 soaked electrolyte, as shown in Figure 9b. Considering various superacids can be formed by the similar mechanism, the group speculated that other metal oxides such as MgO, ZnO, ZrO2, TiO2 and SiO2 can also work by forming solid superacids.. Recently, our group also have studied some issues of SiO2 used as the electrolyte additive. We151, 152 developed an organic-inorganic hybrid electrolyte by grafting SiO2 nanoparticles with ionic liquid 1-methy-3-propylimidazolium bis(trifluoromethanesulfone)imide in a conventional PC/LiTFSI liquid electrolyte (SiO2-IL-TFSI) (Figure 9c). The nanoparticles can serve as reservoirs and immobilize the anions, thus significantly retard Li dendrite growth and delay a cell-short-circuit by one order of magnitude in comparison to that using a pure PC electrolyte according to the space charge model. Besides, the room-temperature electrochemical stability window of the silica nanoparticles modified electrolyte was proved stable in the voltage of 0—7 V. Inspired by the solid superacid model, the SiO2 combined nanoparticle additive may also act as a precursor of superacid and be an appropriate HVLCO alternate, in the meanwhile, avoid the dendrites generated from lithium metal anode.


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Figure 9. (a) Scheme of the formation solid superacids on LiCoO2 in LiPF6-based electrolyte150. (b) Discharge capacity retention Li/LiCoO2 cell cycled between 3.3 and 4.5 V at 0.15 mA cm-2 using different electrolytes150. (c) Schematic of SiO2-IL-TFSI/PC as additives in 1M LiTFSI hybrid electrolyte system151. Reproduced from ref. 150 and 151 with permission from The Electrochemical Society and Wiley-VCH, copyright 2007 and 2014.

Benzene derivatives and aromatic heterocyclic additives Biphenyl (BP) was first chosen to be an overcharge indicator, as the H2 evolution would generate at higher voltage and stop the battery operation by opening the safety cap at a certain pressure. But the thick, non-Li+- conductive decomposition generated from too much BP will cover the cathode surfaces and make it inert. Replaced by forming a very thin Li+-conductive polymer films on the surface of LCO using just a tiny amount (e.g.,0.1-0.2 wt%), Abe et al.153, 154 first presented the effects of a series of benzene derivatives (BP and o-terphenyl) and heterocyclic compounds (furan, thiophene, N-methylpyrrole and 3,4ethylenedioxyiophene) additives on improving the cycle performance of HVLCO above 4.2 V. Because these additives process higher HOMO energy levels than those of carbonate solvents, they can be oxidized ahead of the electrolytes. Also, thanks to the conjugated structure of the polymer species, these films are electro-conductive and named as electro-conducting membrane (ECM). After this initial study, Takeuchi et al.155, 156 examined OT, N, N-dimethyl-aniline, anisole, thioanisole, o/m/p-(1Adamantyl) toluene, furan, 2-methyl furan, 2,5-dimethyl furan, thiophene, 2-methyl thiophene and 2,5-dimethyl thiophene for potential additives. They obtained the exact oxidation potentials (Eox) by linear scanning of voltage method with Li/Pt cells at a scan rate of 5 mV/s and proposed the relatively lower oxidation potentials, making these additives quickly oxidize than the base electrolyte solution, thereby covering the catalytic and oxidative sites of LCO. The stabilized cathode/electrolyte interface allows excellent cycling performance up to 4.4 V. Xia et al.157 further found 2,2’-Bithiophene (2TH) and 2,2’:5’,2’’-Terthiophene (3TH) can be electrochemically polymerize prior to the base electrolyte and even the monomolecular thiophene on the cathode surface, which blocks off severe electrolyte decomposition at high voltages. The discharge capacity retention is 50% in the base electrolyte, while the electrolyte containing 0.1 wt% 3TH displays a high retention of 84.8% in the voltage range of 3.0-4.4 V.



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Phosphorus-based additives It is generally recognized that the highly combustible carbonated-based organic electrolyte used in the current lithium based battery technology exists potential hazards of firing and explosion under hostile conditions such as short-circuit, high thermal impact and especially electrical overcharged. A variety of phosphides have been confirmed useful as flame-retardant solvents or additives to improve the safety of batteries. Also, some of them such as (Phenoxy)Pentafluorocyclotriphosphazene N3P3(OPh)F5 (PFPN)158, 159, N-(triphenylphosphoranylidene) aniline (TPPA)160, Tri(β-chloromethyl) phosphate (TCEP)161, Poly[bis(ethoxyethoxyethoxy)phosphazene] (EEEP)162 etc. have been reported as electrolyte additives to improve the performances of HVLCO. Xia et.al159 found that an addition of only 5% PFPN can make the electrolyte thoroughly non-flammable, as shown in Figure 10a. The discharge capacity of the cell with PFPN additive in the electrolyte only showed a 10 % capacity fade from 188.1 mAh g-1 to 169 mAh g-1 for 60 cycles in contrast to a 34.6 % fade of using a blank electrolyte at a high cut-off voltage of 4.5 V (Figure 10b). However, since the PFPN is difficult to commercialize for its low volatilization temperature, very recently, the group162 synthesized and explored the EEEP additive. They found that a 5 % EEEP containing electrolyte made the capacity retention of LCO cathode elevate to 89.9 % from 51.2 % after 100 cycles at a high cutoff voltage of 4.4 V, in the meanwhile, obviously reduce the flammability of electrolyte. They ascribed the enhanced performance to a stable solid interface forming on the cathode which is related to the P-O bond of molecules based on extended analyses of SEM and XPS (Figure 10c).

Figure 10. (a) An illustration of the flammability testing of the electrolytes159. (b) Cycle performance of LiCoO2/Li half cells in electrolytes without and with 5 wt% PFPN at current density of 40 mA g-1 and cut-off voltage of 4.5-3.0 V159. (c) The C 1s, O 1s, P 2p and N 1s XPS spectra of LCO using


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the electrolyte without/with the EEEP additive after 50 cycles162. Reproduced from ref. 159 and 162 with permission from Elsevier and Elsevier, copyright 2015 and 2017.

Sulfur-based additives and other additives Sulfur-based additives, such as Diphenyl disulfide (DPDS)163, methylene methanedisulfonate (MMDS)164, 1,3,2-dioxathiolane2,2-dioxide (DTD), 1,3-propanediol cyclic sulfate (TMS) and propylene sulfate (PLS)165 have also been used in HVLCO cathode not only because they have a similar effect to the additives but also benefit the graphite or lithium metal anode owing to the sulfurbased functional groups. With the guidelines that SO2, vinylene trithiocarbonate or butylene sulfite could be reduced prior to the carbonate solvent and form a positive SEI film on the graphite anode, Zhao et al.163 evaluated the DPDS which consists both conjugated benzene component and sulfur atoms as a new bifunctional electrolyte additive. Linear sweep voltammograms proved the oxidation of DPDS (Figure 11a) while XRD, FTIR and SEM analysis demonstrated that the SEI film is produced primarily on the graphite anode via the decomposition of DPDS at normal voltage and the CEI can be formed simultaneously on the LCO at higher potentials. The cell with 1.0 wt% DPDS exhibited the best capacity retention with a value of 83% of the initial discharge capacity, and still retained 130 mAh g-1 after 200 cycles in the range of 3.0 V- 4.4V (Figure 11b). Zuo et al.164 found the capacity retention of LCO cells in 3.0-4.5 V is significantly increased from 32.0% to 69.6% after 150 cycles with the addition of 0.5 wt% MMDS into the base electrolyte, as shown in Figure 11c. They ascribed this improvement to the modification of components of cathode surface layer in the presence of MMDS. TEM images clearly demonstrated a thin surface layer which has a thickness of approximately 3-5 nm (Figure 11d, e and f) would effectively decrease the interfacial resistance thus improving the cycling performance. Boron-containing additives including lithium bis(oxalate)borate166, tris (2H-hexafluoroisopropyl) borate167 and trimethylboroxine168 have been found to be effective electrolyte additives for the formation of protective films on the HVLCO. Some nitrile-based electrolytes, such as (fumaronitrile169, heptyl cyanide170, suberonitrile, succinonitrile (SN)171 and 4(trifluoromethyl) benzonitrile172 have also been used as HVLCO additives because the electrochemically and thermally stable – CN group of the decomposition has a lone pair of two electrons leading to a strong bond with Co ions on the LCO surface.

Figure 11. (a) Linear sweep voltammograms of LiCoO2/Li163. (b) Cycle performance of LiCoO2/graphite cells with different amounts of DPDS in voltage ranges of 3.0-4.4 V163. (c) Discharge capacity of cells as a function of cycle number obtained at different cut-off voltage: 3.0-4.2 V (up), 3.0-



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4.5 V (down)164. TEM images of LiCoO2 electrode: (d) fresh, (e) without MMDS after 150 cycles, and (f) with MMDS after 150 cycles164. Reproduced from ref. 163 and 164 with permission from Elsevier and Elsevier, copyright 2016 and 2012.

Mixed additives It is noted that single electrolyte additives, which are the focus of many academic studies, are unlikely to ever be competitive with multiple additives chosen to act synergistically. Wang et al.173 studied over 55 combinations of additives in which vinylene carbonate (VC), fluoroethylene (FEC), vinyl ethylene carbonate (VEC), LiBOB and LiTFSI were highlighted. The group verified VC as a promising additive and set the performance of cells with 2 wt.% VC as a baseline to evaluate the effectiveness of additive blends. The combination of VC, VEC and FEC, 2 wt.% for each component, was shown to have the highest coulombic efficiency and the second lowest charge slippage but with the highest impedance and recognized suitable for cells destined only for low rate or long lifetime applications. Kim et al.171 found that the addition of both 2 wt% VC and 2 wt% SN leads to intermediate behavior while single 2 wt% VC /SN respectively suppressed/increased the impedance growth of cathode during cycling. Ji et al.174 revealed that LiBOB can be oxidized prior to the decomposition of base electrolytes but does not work well with Co-based LCO while the use of SUN tends to form too thick CEI containing a large amount of LiF. However, the interface impedance was greatly suppressed thus enhancing the cycling performance of LCO when using a combination of additives containing both. Recently, Cui’s group167 developed a novel tree-component functional additive using tris(2H-hexafluoroisopropyl) borate (THFPB), adiponitrile (AND) and cyclohexylbenzene (CHB). By systematic analysis, it is demonstrated to be a synergistic effect, that THFPB can suppress the appearance of needle-shaped crystals by the addition of the dinitrile of AND and reduce the polarization and interfacial resistance by partially dissolving resistant components such as LiF. Furthermore, the addition of CHB which consisted benzene groups participates in CEI modification at the high voltage by polymerization as mentioned above. The cells containing this three-component additive showed a higher capacity retention of 87.6, 79.6 and 60.0% than base electrolytes at cut-off voltages of 4.4, 4.45 and 4.5 V.

Quasi-solid electrolytes Adding electrolyte additives is a facile and effective way to enhance the performance of LCO at a high cut-off voltage, but liquid electrolytes essentially still exist potential safety concerns of electrolyte leakage, depletion, limited working temperature, negligible mechanical strength as well as the potential internal short circuit caused by dendrites while combining lithium metal anode. The current solid electrolytes which recognized as an ultimate alternative toward an all-solid state battery can be divided into two categories: polymer and inorganic ceramic. Usually, polymer electrolytes exhibit better flexibility whereas relatively low ionic conductivity and a narrow voltage window while the latter possess a desirably high mechanism strength and a higher ionic conductivity but is very fragile. One strategy is to make a polymer/ceramic composite solid electrolyte, but unfortunately, things are still far from satisfactory because of some annoying issues especially the sluggish/unstable interfaces and low bulk ionic conductivity problems. Quasi-solid electrolytes are some normally solid composite systems with liquid electrolytes. Combining the advantages of both liquid and solid electrolytes, the quasi-solid electrolytes may become an appropriate choose of transitional period to all-solid-state high-voltage LCO batteries and worth noted. Gel polymer electrolytes Gel polymer electrolytes （GPEs） are formed by adding plasticizers or solvents into solid polymer electrolytes and has a special phase between solid and liquid. Ether solvents, liquid oligomers and ionic liquids are in common use as the plasticizers. The transport of Li+ is mainly supported by the solvation of plasticizers while polymers play a holder role. Plenty of works have demonstrated that GPEs have higher acceptable ionic conductivity than solid electrolytes at ambient temperature and higher


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thermal and mechanical stability than liquid electrolytes. A series of polymer hosts, such as poly(ethylene oxide) (PEO)175, poly(propylene oxide) (PPO)176, poly(acrylonitrile) (PAN)177, poly(methyl methacrylate) (PMMA)178, poly(vinylidene fluoride) (PVdF)179,

poly（vinylidene fluoride-co-hexafluoro propylene）(PVdF-co-HFP)180 and poly(vinylpyrrolidone) (PVP)181 have

been developed for many years. Though considering relatively better surface contact due to the flexibility of polymer, few of them are compatible with high-voltage cathodes causing by the limited voltage window. For example, the widely studied PEO holder will be slowly oxidized at voltage over 3.9 V, which has restricted their use. Normally, blending, copolymerization, crosslinking and surface modification are the main strategies to improve the high voltage stability, ionic conductivity and mechanical stability. Kim et al.182 developed a homogeneous gel polymer electrolyte consisting of 25 wt.% PVdf-co-HFP, 65 wt.% EC-PC and 10 wt.% LiTFSI using a solvent-casting technique and its ionic conductivity can reach 1.2×10-4 S cm-1. While combining the LCO cathode and Li metal anode, the cell delivers a specific discharge capacity of 147.6 mAh g-1 and a coulombic efficiency of 99.8 % at the initial cycle at 4.2 V. Recently, Cui’s group183 demonstrated for the first time an bacterial cellulose supported copolymer poly(methy vinyl ether-alt-maleic anhydride) (P(MVE-MA) combining PC/LiODFB liquid electrolytes as gel polymer electrolyte (PMM-GPE) for a 4.45 V LCO lithium metal battery (the highest voltage application in LCO for GPE as we know), the fabrication process is shown in Figure 12a. The produced P(MVE-MA) membrane consists a sandwich cross-linking architecture of which the bacterial cellulose can work as a stress contributor while the P(MVE-MA) consisting an ultra-high Young’s modulus of 6.9 GPa works as strain contributor so that a flexible electronic can be achieved (Figure 12b). Electrochemical measurements further demonstrated that the PMM-GPE has a wide voltage window over 5.2 V and a high Li+ transference number of 0.54. Based on all the superiority of components, the LCO/Li cell with PMM-GPE exhibits an ultrahigh discharge capacity retention of 85 % of the initial capacity (174.4 mAh g-1) after 700 cycles even at 60 ℃ (Figure 12c). By contract, the basic battery based on the LiPF6/EC-DMC electrolyte only remain 1% of the initial capacity at same test conditions. Rao et al.184 developed a GPE using an electro-spun polymer blending membrane of PAN and PMMA incorporating with Nmethy-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquids. The electro-spun membrane has 3D interconnected fibers with a porosity of 86 % and a reversible cathodic stability up to 5 V. Inspired by additives used in liquid electrolytes, Choi’s group185 first studied the influence of electrolyte additives including EDOT, TH and BP on the cycling performance of LCO based on GPE. PVdF-HFP, dibutyl phthalate and acetone were mixed, ball milled and then cast to a thin film of 500 μm. After immersed in methanol to remove the plasticizer, homogeneous pores can be realized to contain a certain amount of liquid electrolytes without leakage. Their research demonstrated that the thiophene additive led to the best performance with a capacity retention and a high initial discharge capacity of 136 mAh g-1 between 2.8 and 4.3 V. Recently, bilayer electrolytes186 in a single battery strategy was also suggested in which the high-voltage electrolyte near the cathode can protect the low-voltage electrolytes near anode from oxidation. In summary, some efforts have made to improve the performance of high voltage, but the GPEs are still limited. It is suggested that novel high-performance GPEs and more effective bulk or interfacial modification methods should be developed.



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Figure 12. (a) A schematic representation of the PMM-CPE fabrication process. (b) Schematic illustration of flexible LiCoO2/Li metal cell in a bent state. (c) Cycle performance (1C) of the 4.45 V LiCoO2/Li cell with LiODFB/PC electrolyte and PMM-GPE at 60 ℃183. Reproduced from ref. 183 with permission from Wiley-VCH, copyright 2018.

Electrolytes combining ceramic nanoparticles To achieve a quasi-solid state, ceramic nanoparticles are conveniently adapted to the electrolytes with an amount ≥ 50%. These nanoparticle fillers can improve the bulk conductivity while retaining the mechanism strengths and flexibility. Furthermore, liquid electrolytes with ceramic fillers display wider electrochemical window which offer the possibility of applying HVCLO and enabling good long-term cycling performance. Choi et al.187 developed a novel hybrid quasi-solid electrolyte composed an ionic liquid (LITFSI/Pyr14TFSI) and BaTiO3 ceramic nanoparticles without a polymer. Dispersing an amount of BaTiO3 ceramic powders in the liquid ionic electrolyte system can develop space charge layers on the boundaries of ceramic particles which tend to overlap, yielding favorable pathways for Li+ transport (Figure 13a, b). The hybrid quasi-solid electrolyte exhibits excellent flame retardancy (Figure 13c), an excellent ionic conductivity of 1.3×10-3 S cm-1 at 30 ℃ and a high anodic stable voltage up to 6.0 V. Also, a stable discharge capacity of 128 mAh g-1 (0.1 C) at 80 ℃ (Figure 13d) was available due to the high thermal stability of this electrolyte. Appetecchi et al.188 reported the adding of selected Al2O3 ceramic powders can also solve the liquid leakage problem. Li et al.189 used the insulting 2D graphene-analogues boron nitride (g-BN) Nano sheets as an electrolytes host. The rich porosity enables g-BN to incorporate a large amount of ionic liquids as much as ten times of its initial volume. The hybrid electrolytes can achieve a high ionic conductivity of 3.85×10-3 S cm-1 at 25℃（even 2.32×10-4 S cm-1 at -20℃） due to the smooth Li+ transport pathway leading by uniform and wide channels between and outside g-BN layers. Tsao et al.190 prepared a novel organic-inorganic hybrid gel polymer electrolyte with immobilized ionic liquid components by a combination of sol-gel method and a chemical cross-linking reaction. This quasi solid electrolyte exhibits a low glass transition temperature, high ionic conductivity of 5.9 mS cm-1 with the lithium ion transference number creasing from 0.28 to 0.57 and a high electrochemical window of 5.0 V. In general, ceramic nanoparticles combined quasi-solid electrolytes are very promising candidates for LCObased batteries with higher energy density, but the choices are still limited. Furthermore, the conductivity and oxidation stability should be further enhanced to match HVLCO.


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Figure 13. (a) and (b) Model of the Lithium-ion conducting paths arising from the formation of the charge space. (c) Flammability testing of hybrid solid electrolyte. (d) Cycling performance for Li/LiCoO2 cells with liquid electrolyte and hybrid solid electrolyte at 80 ℃187. Reproduced from ref. 187 with permission from Springer, copyright 2017.

Electrolyte wetting on the interface Ceramic electrolytes usually possess higher modulus and ionic conductivity. However, the intrinsic rigidity always makes an inferior interfacial contact known as a heterogeneous solid-solid contact, thus leading to sluggish ion transport. Some study also showed the major voltage drop from cathode to anode is mainly derived from the huge interfacial impedance. Moreover, a floating of interface during charging and discharging could make voids at interface intensifying the bad contact. Adding modified electrolytes is recognized as an effective method to improve the interfacial performance. Xu et al.191 used n-BuLi to construct a stable interface between Li metal anode and Li7La3Zr1.5Ta0.5O12 ceramic solid electrolytes. The modified cells had been cycled for 400 cycles at 100 and 200 μA cm-2 with a constant resistance while the interface resistance of control group increasing from 1056 to 2419 Ω cm2 after only 10 cycles. Wang et al.192 revealed a small amount of liquid electrolyte (2 μ L LiPF6 in EC/DMC/DEC) at interface can eliminate the large interfacial resistance while preventing the reduction of Li1.4Al0.4Ti1.6(PO4)3 by lithium metal. Kitaura et al.193 fabricated LCO interfaces by using a super-cooled liquid state of glassy electrolytes. A method of LiNbO3 coating was used to further suppress the side reaction of LCO and electrolytes. The Li4Ti5O12/80Li2S·20P2S5 glass/LiNbO3-coated LiCoO2 prepared showed a reversible capacity of 120 mAh g-1 at 0.064 mA cm-2.

Conclusion and perspective



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To meet the demand of high energy density batteries in portable electronic device, charging LCO to higher voltage is a satisfying choice. However, due to the intrinsic properties of LCO and instability of interfaces between LCO particles and other battery components, researchers have studied LCO and found a lot of strategies to solve the problem, including doping, coating and modification of electrolyte. Doping was the earliest proposed and effective way to stabilize the LCO particles. Through element doping, such as Al, Mg, Ca etc., has been proved by many groups. The widely-agreed mechanism of dopants is pillaring effect meaning that the elements have larger ionic radii than Co-ions so that they can stabilize the layered structure during the process of lithium ions intercalation/deintercalation. Some elements like Mg were also believed to change the electronic conductivity by increasing the amounts of Co4+. Though doping method have been studied for a long time, there are still a lot of unknown areas remain to be studied, such as the effects of F-, effects of multiple elements doping and mechanism of phase transition inhibition, etc. So, for doping, more studies are needed to understand the effects of different elements deeply especially the mechanism of inhibition of phase transition, and the synergetic effects of two or more elements should be studied as well, especially for combination of anion and cation. Coating is another effective way to enhance the electrochemical performance of HVLCO. It can construct a protective layer to prevent corrosion from electrolyte which can produce HF by trace of water. The coating layer is believed to make the phase transition more reversible rather than inhibit the phase transition, which is different from doping. It is also widely agreed that there exists a solid solution between bulk LCO and coating layer. The solid solution was believed to be synthesized through transition metals diffusing to the surface of LCO. However, there is no direct evidence to prove it or characterize it. Thus, finding a way to detect the properties of solution layer is a focus of researchers. Since the lithium ion mobility should be considered when designing the coating materials, more studies should focus on lithium ion conductive materials, instead of stability of materials only. Therefore, polycation substance with Li ions can be considered as a great candidate. And polyanion may be effective as well. In terms of liquid electrolytes, adding electrolyte additives is generally more attractive for its tiny amount of use and great effectiveness of enhancing the high voltage performance of LCO through the modified cathode/electrolyte interface and the improved cathodic stability of the electrolyte. Additives with a high HOMO can be preferentially oxidized or electrochemically oxidized to form functional CEI films, which covers the active sites and prevents further decomposition of the electrolytes. Multifunctional phosphorous-based additives can largely improve the thermal stability of HVLCO and lead flame-retardant electrolytes. Sulfide-based additives can simultaneously enhance the anodic stability of graphite or lithium metal anodes. Mixed additives which are chosen to work synergistically and usually outperform a single additive, should be the focus of future study. The quasi-solid electrolyte which is recognized to be an appropriate alternative nowadays displays higher safety and mechanical stability than liquid electrolyte, but it still suffers from poor interface stability. So, strategies about stable and compatible interface constructions are still needed and the loading of liquid electrolyte should be further reduced. In this review, most of common modification strategies are summarized. Since many groups have studied the LCO for decades, the LCO cathode have been widely used in lithium-ion batteries for portable electronic devices. HVLCO-based batteries have advantages in terms of capacity, voltage and compaction density comparing other cathode materials.

AUTHOR INFORMATION Corresponding Author


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*Tel.: +86-0571-87953906. E-mail: [email protected]. ORCID Yingying Lu: 0000-0001-9713-8441 Notes The authors declare no competing financial interest.

Acknowledgements X.W and X.Y. contributed equally to this work. X.W wrote the introduction, challenges of LCO, DOPING, COATING while X.Y wrote the introduction, electrolyte additives and conclusion. We acknowledge financial supports from the National Key R&D Program of China (2018YFA0209600,2016YFA0202900) and the Natural Science Foundation of China (NSFC, grant number 21878268, 21676242).

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