To overcome environmental issues such as global warming and air pollution, the development of sustainable energy technologies that are clean, affordable, and effectively infinite is required.^1-3) Energy produced using renewable energy resources such as solar and wind energies is appealing; however, efficient energy storage systems (ESSs) are needed to compensate for the resulting intermittent energy production.^1-5) To cope with the ever-increasing demands for the power sources to be used in grid-scale ESSs as well as portable electronic devices and electric vehicles (EVs), there have been many efforts to develop an efficient rechargeable battery system with high energy and power densities.^1-12) Among known rechargeable batteries, Li-ion batteries (LIBs) offer the highest gravimetric and volumetric energy densities, which makes them the optimal choice for portable electronic devices and an appealing option for EVs and grid-scale ESSs.^3-11) LIBs have dominated the portable electronic device market in the past two decades, and various types of EVs using LIB as power sources as well as prototype grid-scale ESSs have come to market during the past several years. In the pursuit of extended driving distances for EVs and lowered costs and long-lasting cycle performance for grid-scale ESSs, extensive efforts have been made to find potential candidates for next-generation cathode materials for LIBs.^13-27)

Since Sony succeeded in commercializing the LIB system based on a LiCoO₂ cathode and carbon anode, various types of cathode materials (e.g., layered, spinel, Li-rich, and polyanionic compounds) have been introduced and commercialized for LIBs. Among them, layered-type cathode materials, LiMO₂ (M = Co, Mn, Ni, Ni_xCo_yMn_1−x−y, or Ni_xCo_yAl_1−x−y ), have received great attention for EVs and grid-scale ESSs because of their high gravimetric energy density (> 500 Wh kg⁻¹) and good cycling performance.^19,28-36) However, these electrode materials generally suffer from structural instability in the charged state or at high temperature due to transition metal migration and high oxygen activity, which can result in severe safety issues in practical applications. These safety concerns triggered the exploration of various polyanionic compounds (i.e., LiMPO₄, Li₂MPO₄F, LiMSO₄F, Li₂MP₂O₇, where M = Fe, Mn, or Co) as cathode materials for LIBs because of the strong covalent bond of the polyanion, which stabilizes the oxygen ion within the crystal structure.^23-25,37-57) However, these electrodes exhibit low energy density and low ionic and electronic conductivities, which limit their further development and application. Spinelstructured LiMn₂O₄ has been the focus of much attention because of its high structural stability, low cost, low toxicity, and relatively high energy density.^58-67) LiMn₂O₄ has a facecentered cubic structure (space group Fd3m), with Mn and Li located at 16d octahedral and 8a tetrahedral sites, respectively. The spinel structure provides three-dimensional (3D) Li conduction pathways, which enable fast Li de/insertion upon electrochemical cycling. However, the electrochemical reaction of LiMn₂O₄ in Li-ion cells involves severe Jahn-Teller distortion and disproportionation due to existing Mn³⁺, resulting in poor cycle performance.^64-67) Many previous studies have suggested that the electrochemical performance of LiMn₂O₄ can be improved by doping with different cations and anions.

Among doped LiMMn_2−xO₄ (M = Co, Fe, Cr, Cu, and Ni) materials, high-voltage spinel LiNi_0.5Mn_1.5O₄, which utilizes the redox couples of Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺, is considered a promising cathode for next-generation LIBs for EV and grid-scale ESS applications because of its high operating voltage of ~ 4.7 V (vs. Li⁺/Li), low cost, and good cycle and high-rate performances.^68-83) However, several issues and challenges must be addressed in LiNi_0.5Mn_1.5O₄: i) the unstable solid electrolyte interphase (SEI) formation at high operating voltage (vs. Li⁺/Li), ii) the Mn dissolution upon electrochemical cycling, and iii) the structural transition from the LiM₂O₄-type spinel structure to M₃O₄-type spinel and rock-salt structures at the particle surfaces. Many efforts have been made to enhance the electrochemical properties of LiNi_0.5Mn_1.5O₄ by cation doping, applying electrolyte additives, controlling the particle size and morphology, controlling the degree of cation ordering, and surface modification.^34,68-117) Among these solutions, the most efficient and simple way to prevent deleterious side reactions upon electrochemical cycling of LiNi_0.5Mn_1.5O₄ is surface modification, which can enhance the overall capacity and cycle life.^34,68-106) Efficient surface passivation prevents the direct contact of active particles with the electrolyte solution and suppresses the phase transition and side reactions at the particle surface, resulting in marked improvements in the electrochemical performance of electrode materials.

In this review, we will introduce the various surface modification techniques as well as the detailed synthesis methods used to stabilize the surface of the high-voltage spinel LiNi_0.5Mn_1.5O₄ electrode material for LIBs. This review also covers the current state-of-the-art coating techniques based on metal oxides (i.e., ZnO, ZnAl₂O₄, Al₂O₃, TiO₂, MgO, Fe₂O₃, CuO, RuO₂, SiO₂, SnO₂, etc.), polyanionic compounds (i.e., Li₄P₂O₇, Li₃PO₄, LiPO₃, Li_0.1B_0.967PO₄, LiFePO₄, LiCoPO₄, FePO₄, Li₂SiO₃, LiAlSiO₄, etc.), polymers (polyimide (PI), polyacrylate, polypyrrole (PPy), poly(ethyl α-cyanoacrylate)), and carbon-based materials (amorphous carbon, graphene oxide, carbon nanotubes (CNTs)). In addition, various characterization techniques used to evaluate coating layers on electrodes such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy will be discussed. Finally, we will provide perspectives on the outlook of surface modification of LiNi_0.5Mn_1.5O₄ materials.

Various metal oxides have received great attention as coating materials for spinel LiNi_0.5Mn_1.5O₄. Several studies have reported on the use of ZnO coatings to protect the surface of LiNi_0.5Mn_1.5O₄ against HF attack. Sun et al. reported that ZnO coating could improve the electrochemical properties of LiNi_0.5Mn_1.5O₄.⁸⁴⁾ 1.5 wt% ZnO-coated LiNi_0.5Mn_1.5O₄ delivered a charge capacity of 137 mAh g⁻¹, and its capacity was maintained without any significant capacity degradation over 50 cycles at 55°C. Arrebola et al. also reported the effect of ZnO on nanosized spinel LiNi_0.5Mn_1.5O₄. LiNi_0.5Mn_1.5O₄ was easily wrapped by ZnO by mixing Zn(CH₃COO)₂·2H₂O and LiNi_0.5Mn_1.5O₄ in ethanol followed by heat treatment.⁸⁵⁾ The capacity of LiNi_0.5Mn_1.5O₄/ZnO at the current rate of C/4 was ~ 128 mAh g⁻¹ and was stably retained over 50 cycles. Sun et al. reported further enhancement of the electrochemical properties of LiNi_0.5Mn_1.5O₄ through improvement of the electrical properties of ZnO by Al doping.⁸⁶⁾ The presence of the Al-doped ZnO coating was verified through TEM analyses, and the thickness of the coating was determined to be approximately 1 - 2 nm. The discharge capacity of the Aldoped ZnO-coated LiNi_0.5Mn_1.5O₄ at 0.1C was ~ 128.9 mAh g⁻¹ and, surprisingly, that at 10C was ~ 114.3 mAh g⁻¹, which is ~ 77% of its theoretical capacity. Electrochemical impedance spectroscopy (EIS) analyses revealed that the Al-doped ZnO-coated LiNi_0.5Mn_1.5O₄ had a lower charge transfer resistance than bare LiNi_0.5Mn_1.5O₄ and ZnO-coated LiNi_0.5Mn_1.5O₄, which might affect the improvement of the power capability of LiNi_0.5Mn_1.5O₄. Furthermore, up to ~ 99% of the initial capacity of the Al-doped ZnO-coated LiNi_0.5Mn_1.5O₄ was maintained for over 50 cycles at 50°C as well as at 25°C. Lee et al. modified the surface of LiNi_0.5Mn_1.5O₄ through coating with ZnAl₂O₄, which provides low surface acidity, high thermal and chemical stability, and high mechanical resistance.⁸⁷⁾ For the ZnAl₂O₄ coating, LiNi_0.5Mn_1.5O₄ particles were added to a solution in which Zn(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O were dissolved, and the solution was stirred at 80°C to form a gel. The gel was heated at 180°C for further esterification followed by annealing at 900°C for 12 h. TEM analysis revealed the formation of a coating layer with a thickness of less than 100 nm on the surface of the LiNi_0.5Mn_1.5O₄ particles, and the presence of Zn and Al in the coating layer was confirmed through elemental analyses using TEM coupled with energy-dispersive X-ray spectroscopy (EDS). The capacity of the ZnAl₂O₄-coated LiNi_0.5Mn_1.5O₄ at 1C was ~ 130 mAh g⁻¹, which is ~ 88% of its theoretical capacity. In addition, the ZnAl₂O₄-coated LiNi_0.5Mn_1.5O₄ delivered a capacity of ~ 97.3% of its initial capacity after 100 cycles at the current rate of 1C, whereas the pristine LiNi_0.5Mn_1.5O₄ only provided ~ 89.0% retention of the initial discharge capacity. The thermal stability of the fully charged state of LiNi_0.5Mn_1.5O₄ was also enhanced with the addition of the ZnAl₂O₄ coating. The exothermic reaction temperatures of the pristine cathode were ~ 181.8°C and ~ 267.1°C, whereas the exothermic reactions of the ZnAl₂O₄-coated LiNi_0.5Mn_1.5O₄ occurred at ~ 192.1°C and 273.8°C, respectively.

Aluminum oxide (Al₂O₃) has also been used as a coating material for LiNi_0.5Mn_1.5O₄. Wang et al. reported Al₂O₃-modified LiNi_0.5Mn_1.5O₄ thin-film electrodes prepared by pulsed laser deposition (PLD).⁸⁸⁾ The LiNi_0.5Mn_1.5O₄ thin-film electrodes contained grains smaller than 200 nm covered with extremely small Al₂O₃ particles. The Al₂O₃-modified LiNi_0.5Mn_1.5O₄ exhibited enhanced cyclability with high coulombic efficiencies compared with pristine LiNi_0.5Mn_1.5O₄, which means that the surface modification using Al₂O₃ resulted in improvement of the structural stability of LiNi_0.5Mn_1.5O₄. Whereas the discharge capacity of the pristine LiNi_0.5Mn_1.5O₄ at the 50^th cycle was ~ 13.1 mAh g⁻¹, which is ~ 10.6% of its initial capacity at 55°C, LiNi_0.5Mn_1.5O₄ coated with 20-nm-thick Al₂O₃ exhibited ~ 89.3% capacity retention under the same conditions. Kim et al. also reported improvement of the electrochemical properties of LiNi_0.5Mn_1.5O₄ by coating with Al₂O₃.⁸⁹⁾ They used atomic layer deposition (ALD) to achieve a homogenous coating of Al₂O₃ on the surface of LiNi_0.5Mn_1.5O₄ particles. The deposition of an ~ 2-nm-thick amorphous layer on the particles was confirmed by TEM analysis. The SEI layer on the surface of the Al₂O₃-coated LiNi_0.5Mn_1.5O₄ was much thinner and contained fewer organic species than that on the surface of the pristine LiNi_0.5Mn_1.5O₄ because of the prevention of side reactions at high voltage provided by the Al₂O₃ coating. Thus, the Al₂O₃-coated LiNi_0.5Mn_1.5O₄ exhibited significantly improved capacity retention compared with the pristine LiNi_0.5Mn_1.5O₄. The coulombic efficiency of the Al₂O₃-coated LiNi_0.5Mn_1.5O₄ after the 80^th cycle was ~ 99.5%, whereas that of the pristine LiNi_0.5Mn_1.5O₄ was 97.9%. Furthermore, it was verified that the Al₂O₃ coating could prevent the dissolution of transition metals in LiNi_0.5Mn_1.5O₄. The Mn amount dissolved from the Al₂O₃-coated LiNi_0.5Mn_1.5O₄ was ~ 6.6 times less than that from the pristine LiNi_0.5Mn_1.5O₄, indicating the successful protection from HF attack provided by the Al₂O₃ coating. It has also been reported that a combination of nanostructuring and Al₂O₃ coating could improve the properties of LiNi_0.5Mn_1.5O₄. Cho et al. prepared a LiNi_0.5Mn_1.5O₄ nanowire electrode using the sol-gel-based template method. The average diameter and length of the LiNi_0.5Mn_1.5O₄ nanowires were ~ 140 nm and ~ 13 μm, respectively.⁹⁰⁾ As observed in Fig. 1(a), the LiNi_0.5Mn_1.5O₄ nanowires were uniformly coated by Al₂O₃ using ALD, and the thickness of the coating layer was ~ 1.2 nm. The ALD coating on the nanowire electrode enhanced the electrochemical properties of LiNi_0.5Mn_1.5O₄. Fig. 1(b) shows that whereas the capacity of the uncoated LiNi_0.5Mn_1.5O₄ nanowire was less than 10 mAh g⁻¹ at a current rate of 1C and was significantly degraded over prolonged cycles, the Al₂O₃-coated LiNi_0.5Mn_1.5O₄ nanowire exhibited a high coloumbic efficiency of ~ 98.8% over 25 cycles with a capacity at 1C of ~ 99 mAh g⁻¹ (10 times that of the uncoated LiNi_0.5Mn_1.5O₄ nanowire).

Titanium-based compounds have also been used as coatings for LiNi_0.5Mn_1.5O₄. Tao et al. reported the improvement of the electrochemical stability of LiNi_0.5Mn_1.5O₄ by surface modification with TiO₂.⁹¹⁾ The crystal structure of LiNi_0.5Mn_1.5O₄ was not affected by the TiO₂ coating, and no XRD peaks related to TiO₂ were observed, which indicates the low loading amounts and thin layer of TiO₂. Protective TiO₂ layers were chemically coated on the surface of LiNi_0.5Mn_1.5O₄ particles using a simple wet chemical method. The thickness of the coating layer of TiO₂ was ~ 2 nm, and titanium and oxygen were homogenously distributed throughout the TiO₂-coated LiNi_0.5Mn_1.5O₄ particles. The TiO₂-coated LiNi_0.5Mn_1.5O₄ delivered a high discharge capacity of 74.5 mAh g⁻¹ at 15C, which is a very fast current rate, and up to ~ 88.5% of its initial capacity was maintained over 500 cycles. In contrast, the capacity retention of the pristine LiNi_0.5Mn_1.5O₄ was only ~ 33%. Li₂TiO₃ has also been considered as a coating material for LiNi_0.5Mn_1.5O₄. Deng et al. reported the highly enhanced electrochemical performance of LiNi_0.5Mn_1.5O₄ resulting from coating with Li₂TiO₃.⁹²⁾ As observed in Fig. 1(c), Li₂TiO₃ has received great attention because of its 3D Li diffusion path and great structural stability in organic electrolytes. The Li₂TiO₃ coating did not affect the structural deformation of LiNi_0.5Mn_1.5O₄ or the formation of impurities. The thin and uniform Li₂TiO₃ coating on the surface of LiNi_0.5Mn_1.5O₄ particles was verified by TEM coupled with elemental mapping. Elemental Ti was homogeneously distributed throughout the particles, and the thickness of the coating layer was ~ 8 nm. At a fast current rate of 5C, 3 wt% Li₂TiO₃-coated LiNi_0.5Mn_1.5O₄ exhibited a discharge capacity of 95 mAh g⁻¹, which is ~ 65% of its theoretical capacity, whereas the capacity of pristine LiNi_0.5Mn_1.5O₄ was 70 mAh g⁻¹ under the same conditions. Furthermore, 3 wt% Li₂TiO₃-coated LiNi_0.5Mn_1.5O₄ exhibited better capacity retention than pristine LiNi_0.5Mn_1.5O₄. As shown in Fig. 1(d), at 55°C, up to ~ 94.1% of the initial capacity of Li₂TiO₃-coated LiNi_0.5Mn_1.5O₄ was maintained over 50 cycles. In contrast, the pristine LiNi_0.5Mn_1.5O₄ only exhibited 77.1% capacity retention under the same conditions.

Alva et al. studied the surface modification of LiNi_0.5Mn_1.5O₄ using MgO.⁹³⁾ Elemental Mg was primarily applied as an inhomogeneous MgO coating on the surface of LiNi_0.5Mn_1.5O₄ particles via heat treatment at 500°C and was then doped into the crystal structure at the subsurface of the particles at 800°C. Using EDS elemental mapping coupled with SEM, the uniform encapsulation of each particle of LiNi_0.5Mn_1.5O₄ by Mg was clearly verified. XPS analyses also supported the presence of Mg on the surface of the particles. At 50°C, the capacity of the Mg-coated LiNi_0.5Mn_1.5O₄ was ~ 70 mAh g⁻¹ at 5C, whereas that of pristine LiNi_0.5Mn_1.5O₄ was ~ 50 mAh g⁻¹.

Wang et al. reported the surface coating of Fe₂O₃ for LiNi_0.5Mn_1.5O₄.⁹⁴⁾ The LiNi_0.5Mn_1.5O₄ was modified by Fe₂O₃ through a simple chemical precipitation method using polyvinyl pyrrolidone polymer as a surfactant. The existence of elemental Fe in the Fe₂O₃-coated LiNi_0.5Mn_1.5O₄ was confirmed through atomic absorption spectroscopy (AAS) and XPS. No Fe₂O₃ peaks were detected in the XRD pattern of the Fe₂O₃-coated LiNi_0.5Mn_1.5O₄, which indicated that the Fe₂O₃ coating layer might be amorphous or that the amount of Fe₂O₃ was too small to be detected. Although the initial discharge capacity of the Fe₂O₃-coated LiNi_0.5Mn_1.5O₄ was ~ 126 mAh g⁻¹, which was slightly lower than that of pristine LiNi_0.5Mn_1.5O₄ (~ 131 mAh g⁻¹), the Fe₂O₃-coated LiNi_0.5Mn_1.5O₄ exhibited better cycle performance than pristine LiNi_0.5Mn_1.5O₄. Fe₂O₃-coated LiNi_0.5Mn_1.5O₄ exhibited excellent capacity retention of ~ 98.6% at room temperature (RT) over 100 cycles and ~ 92% capacity retention at 50°C over 50 cycles, whereas the capacity retention of pristine LiNi_0.5Mn_1.5O₄ was ~ 86.5% at RT and 79% at 50°C. Additionally, Fe₂O₃-coated LiNi_0.5Mn_1.5O₄ also exhibited excellent power capability, delivering ~ 67% of its theoretical capacity at 10C.

CuO has also been considered as a potential coating material for LiNi_0.5Mn_1.5O₄. Li et al. reported enhanced properties of LiNi_0.5Mn_1.5O₄ resulting from CuO coating.⁹⁵⁾ For the CuO coating, pristine LiNi_0.5Mn_1.5O₄ and Cu(H₃COO)₂·H₂O were mixed in distilled water with vigorous stirring, and the mixture was heated at 500°C for 6 h in air. TEM analyses revealed the uniform coating of 3 wt% amorphous CuO on the surface of the LiNi_0.5Mn_1.5O₄ particles with a coating thickness of ~ 3 nm. The CuO-coated LiNi_0.5Mn_1.5O₄ exhibited great power capability compared with that of pristine LiNi_0.5Mn_1.5O₄. The capacity of the 3 wt% CuO-coated LiNi_0.5Mn_1.5O₄ was 98.7 mAh g⁻¹ at 10 C, which was ~ 1.7 times higher than that of pristine LiNi_0.5Mn_1.5O₄ and was stably retained over 100 cycles without any significant capacity degradation.

Coating of LiNi_0.5Mn_1.5O₄ with RuO₂ has been studied by several researchers because of the high electronic conductivity and catalytic functions for electrochemical oxidation processes of RuO₂. Hong et al. reported a mechanism to improve the cycle performance of LiNi_0.5Mn_1.5O₄ by RuO₂ surface modification.⁹⁶⁾ The electrochemical properties of pristine LiNi_0.5Mn_1.5O₄ and RuO₂-modified LiNi_0.5Mn_1.5O₄ were evaluated at various cut-off potentials ranging from 3.0 to 4.5 V. The specific capacity of pristine LiNi_0.5Mn_1.5O₄ at a cut-off voltage of 3 V was severely degraded to 18.0 mAh g⁻¹ after 1000 cycles, which is only 13.8% of its maximum specific capacity, with ~ 59 mAh g⁻¹ retained at the cut-off voltage of 4.5 V over 1000 cycles, which represents ~ 50.9% of its maximum capacity. The cyclability of RuO₂-modified LiNi_0.5Mn_1.5O₄ was significantly improved when the cut-off potential was 4.5 V, with RuO₂-modified LiNi_0.5Mn_1.5O₄ delivering a specific capacity of 77.3 mAh g⁻¹ after 1000 cycles, corresponding to ~ 70.1% of its maximum capacity. Jung et al. also reported on RuO₂-coated LiNi_0.5Mn_1.5O₄ prepared by a wet chemical route with heat treatment at 450°C.⁹⁷⁾ The existence of elemental Ru was identified through SEM elemental mapping. In addition, TEM analyses confirmed that the LiNi_0.5Mn_1.5O₄ particles were coated by polycrystalline RuO₂ with a coating thickness of ~ 10 nm. The cyclabilities of the pristine and RuO₂-coated LiNi_0.5Mn_1.5O₄ electrodes were evaluated through charge/discharge tests at 0.1C for the first two cycles and then at 0.5C for subsequent cycles. The capacity retention of RuO₂-coated LiNi_0.5Mn_1.5O₄ with a loading weight of 9 mg cm⁻² was ~ 96.1%, which was higher than the ~ 85.7% capacity retention of pristine LiNi_0.5Mn_1.5O₄ measured under the same test conditions.

Several researchers have used SiO₂ for improvement of the electrochemical performances of LiNi_0.5Mn_1.5O₄. Fan et al. prepared SiO₂-coated LiNi_0.5Mn_1.5O₄.³⁴⁾ No significant structural change was observed with the SiO₂ coating in the XRD analysis, and the TEM analyses confirmed that a porous nanostructured SiO₂ layer with a thickness of ~ 30 nm wrapped the LiNi_0.5Mn_1.5O₄ particles. The capacity retention of the pristine sample and the 0.5, 1.0, and 3.0 wt% SiO₂-coated samples over 100 cycles was ~ 59%, 69%, 86%, and 87%, respectively, which indicated that the SiO₂ coating improved the cycle stability of LiNi_0.5Mn_1.5O₄. To verify the effect of the SiO₂ coating on the electrochemical performance of LiNi0_.5Mn_1.5O₄, the XPS spectra of the pristine and 1.0 wt% silica-coated samples measured at 55°C over 100 cycles were compared. The amount of LiF on the surface of the SiO₂-coated sample was much lower than that on the surface of the pristine sample because of the reaction between SiO₂ and HF. Because the LiF resulted in high impedance, preventing fast Li-ion diffusion, the SiO₂-coated LiNi_0.5Mn_1.5O₄ with the smaller LiF content exhibited lower polarization during charge/discharge as well as better cycle performance than the pristine LiNi_0.5Mn_1.5O₄. Shin et al. prepared LiNi_0.5Mn_1.5O₄ electrode materials encapsulated by a cross-linked composite polymer layer containing SiO₂ nanoparticles through a chemical cross-linking reaction.⁹⁸⁾ Fig. 2(a) shows the synthesis process for SiO₂-coated LiNi_0.5Mn_1.5O₄. Whereas the pristine LiNi_0.5Mn_1.5O₄ exhibited an initial coulombic efficiency of ~ 85.2%, the 3 wt% SiO₂-coated LiNi_0.5Mn_1.5O₄ delivered better coulombic efficiency of ~ 90.7%. As observed in Fig. 2(b), at 55°C, the pristine LiNi_0.5Mn_1.5O₄ electrode suffered from severe capacity degradation, and its capacity after 100 cycles was ~ 55% of its initial capacity. However, the 3 wt% SiO₂-coated LiNi_0.5Mn_1.5O₄ electrode exhibited improved capacity retention of ~ 81% over 100 cycles. Furthermore, the pristine LiNi_0.5Mn_1.5O₄ electrode experienced higher Mn and Ni dissolution, whereas no significant Mn and Ni dissolution from the SiO₂-coated LiNi_0.5Mn_1.5O₄ was detected. This finding indicates that the cross-linked composite polymer layer containing SiO₂ particles provided effective protection from HF attack.

Ma et al. prepared SnO₂-modified LiNi_0.5Mn_1.5O₄ synthesized via a facile synthesis method.⁹⁹⁾ The average particle size of the SnO₂-modified LiNi_0.5Mn_1.5O₄ was smaller than that of LiNi_0.5Mn_1.5O₄ because of the inhibition of crystal growth resulting from the formation of a thin SnO₂ layer on the surface of the LiNi_0.5Mn_1.5O₄ particles. EDS elemental mapping of the SnO₂-modified LiNi_0.5Mn_1.5O₄ revealed a homogeneous distribution of Ni, Mn, and Sn on the particles. As observed in Fig. 2(c), the capacity of the SnO₂-modified LiNi_0.5Mn_1.5O₄ at a current rate of 20C was ~ 112.2 mAh g⁻¹, which is ~ 77.2% of its theoretical capacity, whereas the capacity of the pristine sample at 20C was ~ 64.3 mAh g⁻¹. Over 500 cycles, the capacity of the SnO₂-modified LiNi_0.5Mn_1.5O₄ was maintained up to ~ 103 mAh g⁻¹, representing retention of ~ 75% of its initial capacity, whereas that of the pristine sample decreased to 55 mAh g⁻¹, which is only ~ 50% of its initial capacity. These electrochemical results indicated the improved power capability and greater cyclability of the SnO₂-modified LiNi_0.5Mn_1.5O₄ compared with those of the pristine sample.

Y₂O₃ coating has also been used for improvement of the electrochemical performance of LiNi_0.5Mn_1.5O₄. Wen et al. prepared Y₂O₃-coated LiNi_0.5Mn_1.5O₄ using a heterogeneous nucleation method.100) No significant structural differences were observed in the XRD patterns of the Y₂O₃-coated LiNi_0.5Mn_1.5O₄ and pristine samples. The elemental mapping performed using SEM/EDS analyses revealed a homogenous distribution of Y on the surface of LiNi_0.5Mn_1.5O₄. The thickness of the coating layer was determined to be ~ 6 - 8 nm based on TEM analyses. The Y₂O₃-coated LiNi_0.5Mn_1.5O₄ exhibited better power capability than the pristine LiNi_0.5Mn_1.5O₄. Whereas the capacity of the pristine LiNi_0.5Mn_1.5O₄ was 56 mAh g⁻¹ at 10C rate, the Y₂O₃-coated LiNi_0.5Mn_1.5O₄ exhibited a remarkable capacity of 102 mAh g⁻¹ at the same current rate, which corresponds to ~70% of its theoretical capacity despite the fast current rate. Furthermore, the capacity retention of the Y₂O₃-coated LiNi_0.5Mn_1.5O₄ was ~ 91.6% over 100 cycles at 55°C, which is much greater than that of the pristine LiNi_0.5Mn_1.5O₄, which was only ~ 77.9%.

Mou et al. reported the improved electrochemical performance of BiFeO₃-coated LiN_i0.5Mn_1.5O₄.101) The coating of 1.0 wt% BiFeO₃ on the surface of the particles was shown to considerably enhance the power capability and cyclability of LiN_i0.5Mn_1.5O₄. The 1.0 wt% BiFeO₃-coated LiN_i0.5Mn_1.5O₄ delivered discharge capacities of ~ 85.8 and ~ 74.8 mAh g⁻¹ at current rates of 5C and 10C, respectively, whereas the pristine LiN_i0.5Mn_1.5O₄ delivered discharge capacities of 77.5 and 60.9 mAh g⁻¹, respectively, under the same conditions. The capacity retention of the 1.0 wt% BiFeO₃-coated LiN_i0.5Mn_1.5O₄ over 50 cycles at 55°C (~ 92.9%) was higher than that of the pristine LiN_i0.5Mn_1.5O₄ (~ 80.3%).

Surface modification using a LiCoO₂/Co₃O₄ composite has also been considered for the enhancement of the properties of LiN_i0.5Mn_1.5O₄ because of the fast Li-ion transport into the LiCoO₂ structure and the cubic spinel structure of Co₃O₄, which is similar to that of LiN_i0.5Mn_1.5O₄. Qiao et al. prepared LiCoO₂/Co₃O₄-modified LiNi_0.5Mn_1.5O₄ using a simple liquid-phase process.102) No LiCoO₂ or Co₃O₄ XRD peaks were detected, which indicates the absence of any structural deformation resulting from the surface modification. TEM analyses revealed that the thickness of the coating layer was ~ 10 - 60 nm, and the d-spacing values measured in the coating layer were ~ 0.200 and ~ 0.243 nm, corresponding to the (104) plane of LiCoO₂ and (311) plane of Co₃O₄, respectively. The power capability and cyclability of LiN_i0.5Mn_1.5O₄ were significantly improved after coating with LiCoO₂/Co₃O₄. Up to ~ 85.5% of the theoretical capacity of the LiCoO₂/Co₃O₄-modified LiNi_0.5Mn_1.5O₄ was maintained at 5C, and its capacity over 200 cycles was ~ 97.8% of the maximal attainable capacity.

Various polyanion compounds have been used as coating materials for spinel LiN_i0.5Mn_1.5O₄. Many researchers have reported on the coating effect of phosphate-based compounds for LiN_i0.5Mn_1.5O₄. Chong et al. reported improvement of the electrochemical performance of Li₄P₂O₇-coated LiNi_0.5Mn_1.5O₄ resulting from the high ionic conductivity of Li₄P₂O₇.103) The Li₄P₂O₇-coated LiNi_0.5Mn_1.5O₄ was synthesized using the conventional solid-state method, and using XRD analysis, the main phase of this compound was identified as LiNi_0.5Mn_1.5O₄ with the cubic spinel structure with the coexistence of Li₄P₂O₇. The TEM analyses confirmed that the thickness of the Li₄P₂O₇ coating on the surface of LiNi_0.5Mn_1.5O₄ was ~ 8 - 10 nm. The Li₄P₂O₇-coated LiNi_0.5Mn_1.5O₄ retained ~ 86.7% of its initial capacity at the current rate of 40C, whereas pristine LiNi_0.5Mn_1.5O₄ only retained ~ 34.4% of its initial capacity at 40C. Furthermore, the cycle retention of LiNi_0.5Mn_1.5O₄ was significantly improved by coating with Li₄P₂O₇.

Li₃PO₄ has also been considered as a coating material for LiNi_0.5Mn_1.5O₄. Konishi et al. reported the effect of Li₃PO₄ coating on LiNi_0.5Mn_1.5O₄ using PLD.104) In the XRD analysis, besides for LiNi_0.5Mn_1.5O₄, other peaks related to Li₃PO₄ were not detected, which indicates that the coating consisted of the amorphous phase of Li₃PO₄. X-ray reflectometer (XRR) analysis verified that the thickness of the Li₃PO₄ coating layer was ~ 0.7 - 3.6 nm. The Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ exhibited a larger specific capacity and reduced capacity degradation compared with pristine LiNi_0.5Mn_1.5O₄. Chong et al. also prepared Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ as a stable high-voltage cathode material for LIBs.105) An ~ 5 - 6-nm-thick Li₃PO₄ coating was homogenously applied on the surface of LiNi_0.5Mn_1.5O₄ using a solidstate synthesis method. The Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ exhibited much more stable cyclability than the uncoated LiNi_0.5Mn_1.5O₄. The Li₃PO₄-coated LiNi_0.5Mn_1.5O₄ retained 80% of its initial capacity (~ 122 mAh g⁻¹) over 650 cycles. To further characterize the phase change after long cycles, dQ/dV plots of the Li₃PO₄-coated and uncoated LiNi_0.5Mn_1.5O₄ at the first cycle and 200th cycle were compared. The Li_3-PO₄-coated LiNi_0.5Mn_1.5O₄ did not experience a phase change, which indicates that the redox reactions of Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ were well retained even after 200 cycles.

LiPO₃ has also been used as a coating material for LiNi_0.5Mn_1.5O₄ because of its good ionic conductivity and chemical passivation properties. Chong et al. prepared LiNi_0.5Mn_1.5O₄ coated with 1-nm-thick LiPO₃ using the solidstate synthesis method.106) As shown in Fig. 3(a), the LiPO₃ coating provided many advantages for improvement of the electrochemical performance of the LiNi_0.5Mn_1.5O₄. The existence of the LiPO₃ coating layer verified using XRD and Raman spectroscopy. Two Raman shifts were detected at 1077 and 1172 cm⁻¹, representing the P-non-bridging oxygen in the LiPO₃ phase. Fig. 3(b) shows that with increasing C rate, the LiPO₃-coated LiNi_0.5Mn_1.5O₄ exhibited significantly improved power capability compared with the pristine LiNi_0.5Mn_1.5O₄. Furthermore, the LiPO₃-coated LiNi_0.5Mn_1.5O₄ exhibited 77% capacity retention at the fast current rate of 30C. These results indicate that the enhanced ionic conductivity resulting from the LiPO₃ coating could efficiently lower the cell impedance and result in the superior power capability of LiNi_0.5Mn_1.5O₄.

Li_0.1B_0.967PO₄, which has been investigated as a solid electrolyte for LIBs because of its high Li ionic conductivity, was also applied as a coating material for LiNi_0.5Mn_1.5O₄. Yang et al. prepared Li_0.1B_0.967PO₄-coated and Cr-doped LiNi_0.45Cr_0.1Mn_1.45O₄ using the soft combustion reaction.⁶⁸⁾ No significant structural change was detected with the addition of the Li_0.1B_0.967PO₄ coating, which indicated the presence of a separate layer of Li_0.1B_0.967PO₄ on the surface of the LiNi_0.45Cr_0.1Mn_1.45O₄ particles. The Li_0.1B_0.967PO₄-coated and Cr-doped LiNi_0.45Cr_0.1Mn_1.45O₄ exhibited outstanding cyclability with a capacity retention of ~ 85.4% over 400 cycles and excellent power capability with a capacity of ~ 90.7 mAh g⁻¹ at 20C. These results indicate that the Li_0.1B_0.967PO₄ coating layer provided a fast Li⁺ diffusion channel with low charge transfer resistance and served as a protective layer to achieve a more stable surface chemistry.

LiCoPO₄ has also been applied as a coating material for spinel LiNi_0.5Mn_1.5O₄ using a hydrothermal reaction synthesis method. Li et al. showed that the olivine LiCoPO₄-shellcoated structure could significantly enhance the electrochemical performance by inducing the production of an appropriate amount of Mn³⁺.⁶⁹⁾ The synthesis process for the LiCoPO₄-coated LiNi_0.5Mn_1.5O₄ is illustrated in Fig. 4(a). The XRD peaks of the prepared samples were well indexed to the cubic spinel structure of the Fd3m space group with no impurity phase. It was identified through inductively coupled plasma-atomic emission spectrometry (ICP-AES) that increasing the coating amount of LiCoPO₄ on the LiNi_0.5Mn_1.5O₄ particles led to an increase in the Mn³⁺ content in the LiNi_0.5Mn_1.5O₄. As observed in Fig. 4(b), the 5 wt% LiCoPO₄-coated LiNi_0.5Mn_1.5O₄ exhibited outstanding cyclability with 98.5% charge/discharge efficiency after 100 cycles and great power capability, delivering 130 mAh g⁻¹ at a high current rate of 20C, compared with pristine LiNi_0.5Mn_1.5O₄.

Jang et al. attempted to prepare LiFePO₄-modified LiNi_0.5Mn_1.5O₄ via a scalable sol-gel technique.⁷⁰⁾ No structural change occurred with the LiFePO₄ coating, and the existence of LiFePO₄ in the compound was verified through elemental mapping based on TEM measurements. At elevated temperature conditions of 55°C, the LiFePO₄-modified LiNi_0.5Mn_1.5O₄ delivered excellent cyclability with ~ 89% retention of its initial capacity after 50 cycles, whereas the pristine LiNi_0.5Mn_1.5O₄ experienced severe capacity degradation under the same measurement conditions. The authors reported that this outstanding improvement of the electrochemical performances of LiNi_0.5Mn_1.5O₄ resulted from the protective coating of LiFePO₄, which prevented severe HF attack, because at high temperature, the reaction is prone to be accelerated compared with that at ambient conditions. EIS analysis revealed the lower charge-transfer resistance of the LiFePO₄-modified LiNi_0.5Mn_1.5O₄ than the pristine LiNi_0.5Mn_1.5O₄ after 50 cycles at 55°C, demonstrating the benefit of the LiFePO₄ coating for enhanced electrochemical activity of LiNi_0.5Mn_1.5O₄ at high temperature.

Amorphous FePO₄ was also considered as a coating material for LiNi_0.5Mn_1.5O₄. Xiao et al. proposed the use of a novel ALD-derived ultrathin amorphous FePO₄ coating.⁷¹⁾ After applying the FePO₄ coating using ALD, no peaks of FePO₄ were detected in the XRD pattern because of the ultrathin and amorphous nature of the FePO₄ phase. The intensity of the P 2p spectra decreased during ALD cycling, indicating that the amount of elemental P on the surface of LiNi_0.5Mn_1.5O₄ increased with increasing number of ALD cycles. After 40 ALD cycles, no P 2p spectra was observed because of the surface saturation. HRTEM images revealed that the ultrathin FePO₄ coating layer was ~ 2-nm thick, and the lattice fringe of ~ 0.24 nm distance was consistent with the (222) spacing of spinel LiNi_0.5Mn_1.5O₄. The FePO₄-coated LiNi_0.5Mn_1.5O₄ prepared using 10 ALD cycles exhibited great electrochemical The performance, including a capacity of more than 80 mAh g⁻¹ at 5C and retention of ~ 91.96% of the initial capacity after 100 cycles. Whereas pristine LiNi_0.5Mn_1.5O₄ suffers from side reactions upon direct contact with the electrolyte, such as Mn dissolution and continuous electrolyte decomposition, the resistivity of the FePO₄-coated LiNi_0.5Mn_1.5O₄ prevented the direct contact of LiNi_0.5Mn_1.5O₄ with the electrolyte, helping to prevent the reduction and dissolution of Mn ions caused by oxidation. Yi et al. also prepared FePO₄-coated LiNi_0.5Mn_1.5O₄ via a sol-gel method.⁷²⁾ XRD measurements indicated that the FePO₄ coating did not change the structure of LiNi_0.5Mn_1.5O₄. EDS mapping based on SEM imagining revealed the presence of Fe and P as well as Ni, Mn, and O, indicating the presence of FePO₄ in the coated LiNi_0.5Mn_1.5O₄. TEM analyses verified that LiNi_0.5Mn_1.5O₄ particles were uniformly encapsulated by the ~ 1-nm-thick FePO₄ layer, which indicates that FePO₄ might have acted as an effective coating layer between the electrolyte and surface of LiNi_0.5Mn_1.5O₄. In addition, 1 wt% FePO₄-coated LiNi_0.5Mn_1.5O₄ exhibited great power capability with a capacity of ~ 120 mAh g⁻¹ at the fast current rate of 2C. After 80 cycles, the reversible discharge capacity of the 1 wt% FePO₄-coated LiNi_0.5Mn_1.5O₄ was ~ 117 mAh g⁻¹, whereas that of the pristine sample was only ~ 50 mAh g⁻¹.

Li₂SiO₃, a novel and typical Li⁺-ion conductor, has also been considered as a coating material for LiNi_0.5Mn_1.5O₄. Deng et al. reported novel composites of (1−x)LiNi_0.5Mn_1.5O₄· xLi₂SiO₃ prepared via a citric acid-assisted sol-gel method.⁷³⁾ The XRD peaks of the layer-structured Li₂SiO₃ and spinel LiNi_0.5Mn_1.5O₄ were simultaneously detected in all the (1−x)LiNi_0.5Mn_1.5O₄·xLi₂SiO₃ composites, indicating the coexistence of Li₂SiO₃ and LiNi_0.5Mn_1.5O₄. The HRTEM images and selected area diffraction (SAED) patterns revealed that all the samples exhibited clear lattice fringes, indicating the good crystallinity of the materials. In addition, a thin layer was observed on the edge of the grains, which indicates the existence of amorphous Li₂SiO₃ on the surface of the LiNi_0.5Mn_1.5O₄ particles. The Li₂SiO₃ coating could enhance the cyclic performance and power capability of the spinel LiNi_0.5Mn_1.5O₄. The composite with x = 0.1 exhibited excellent power capability with a capacity of 107 mAh g⁻¹ at the fast current rate of 2C and a capacity retention of ~ 85.3% over 50 cycles at 1C.

Deng et al. reported the coating of microporous LiAlSiO₄ with good Li ionic conductivity on the surface of LiNi_0.5Mn_1.5O₄ particles using a citric-acid-based sol-gel method.⁷⁴⁾ Fig. 5(a) presents a schematic illustration of the synthetic route used to produce the LiAlSiO₄-coated LiNi_0.5Mn_1.5O₄ particles. The XRD peaks of LiAlSiO₄ were not detected for coatings containing 0 - 5 wt% LiAlSiO₄. HRTEM analyses revealed that all of the samples had lattice fringes of ~ 0.48 nm, corresponding to the d-spacing of the (111) plane of spinel LiNi_0.5Mn_1.5O₄. In addition, the samples except pristine LiNi_0.5Mn_1.5O₄ were encircled by the coating layer of LiAl-SiO₄. The 3 wt% and 5 wt% LiAlSiO₄-coated LiNi_0.5Mn_1.5O₄ displayed outstanding cyclability of 96.7% and 97.1% over 150 cycles at 25°C, respectively. Furthermore, the capacity of the 3 wt% LiAlSiO₄-coated LiNi_0.5Mn_1.5O₄ was maintained up to ~ 94.3% over 150 cycles at 55°C, whereas the pristine LiNi_0.5Mn_1.5O₄ only exhibited capacity retention of ~ 76.5% under the same conditions (Fig. 5(b)).

Cho et al. proposed coating LiNi_0.5Mn_1.5O₄ particles with a polyimide (PI) gel polymer electrolyte (GPE) to prevent serious interfacial side reactions at high voltage, as shown in Fig. 6(a).⁷⁵⁾ The surface of the LiNi_0.5Mn_1.5O₄ particles was encircled with PI via thermal curing of a four-component (pyromellitic dianhydride (PMDA)/biphenyl dianhydride (BPDA)/phenylenediamine (PDA)/oxydianiline (ODA)) polyamic acid. TEM measurements indicated that the LiNi_0.5Mn_1.5O₄ particles were well encapsulated by an ~ 10-nm-thick PI layer. Fourier-transform infrared (FT-IR) spectroscopy peaks corresponding to the C=O bond of the PI layer were observed at 1723 cm⁻¹, whereas the peak of the C=O bond of the pristine polyamic acid appears at 1650 cm⁻¹. This shift of the C=O bond indicated that the polyamic acid was successfully imidized to form the polyimide wrapping the surface of the LiNi_0.5Mn_1.5O₄ particles. No substantial differences were detected in the XRD patterns of the PI-coated and pristine samples. Compared with the pristine LiNi_0.5Mn_1.5O₄, the PI-coated LiNi_0.5Mn_1.5O₄ exhibited significantly improved cyclability. Up to ~ 120 mAh g⁻¹ of the capacity of the PI-coated LiNi_0.5Mn_1.5O₄ was retained after 50 cycles, whereas that of the pristine LiNi_0.5Mn_1.5O₄ was only ~ 98 mAh g⁻¹ after 50 cycles. Pang et al. also reported enhanced electrochemical properties of LiNi_0.5Mn_1.5O₄ with PI coating.⁷⁶⁾ The PI-coated LiNi_0.5Mn_1.5O₄ was prepared via a co-precipitation method and exhibited a spinel structure with P4332 symmetry, indicating that no structural change occurred as a result of the coating process. The PI coating prevented severe capacity fade of LiNi_0.5Mn_1.5O₄ at the high temperature of 55°C, and the capacity retention of the PIcoated LiNi_0.5Mn_1.5O₄ over 180 cycles was ~ 85%, which indicates the excellent thermal stability, good chemical resistance, and mechanical properties of the PI.

A polyacrylate coating was also applied to improve the electrochemical properties of LiNi_0.5Mn_1.5O₄. Zhang et al. prepared lithium polyacrylate (PAALi)-coated LiNi_0.5Mn_1.5O₄.⁷⁷⁾ The LiNi_0.5Mn_1.5O₄ particles were uniformly surrounded by a thin disordered 1% PAALi layer, unlike the pristine LiNi_0.5Mn_1.5O₄ particles without any coating layers. The 1% PAALi-coated and pristine samples retained up to ~ 90.0% and ~ 78.2% of their initial capacities over 200 cycles at 1C, respectively, which indicates the highly enhanced electrochemical performances of LiNi_0.5Mn_1.5O₄ resulting from the PAALi coating. Furthermore, SEM analysis of the cycled electrodes of the 1% PAALi-coated and pristine samples showed that PAALi could encourage the formation of a SEI layer, which was compatible between LiNi_0.5Mn_1.5O₄ and the electrolyte.

Conductive PPy coatings have also been applied to improve the electrochemical properties of LiNi_0.5Mn_1.5O₄. Gao et al. prepared conductive PPy-coated LiNi_0.5Mn_1.5O₄ composites using simple chemical oxidative polymerization in an aqueous solution.⁷⁸⁾ No structural differences were detected in the XRD patterns of the pristine and PPy-coated LiNi_0.5Mn_1.5O₄ composites. SEM analysis indicated that the size of the LiNi_0.5Mn_1.5O₄ particles was ~ 200 - 500 nm. TEM and EDS analyses revealed that the ~ 3-nm-thick porous PPy layer containing N well wrapped the LiNi_0.5Mn_1.5O₄ particles. As observed in Fig. 6(b), whereas the pristine sample only exhibited capacity retention of 76.7% over 300 cycles, the 5% PPy-coated sample retained up to ~ 91% of its initial capacity after 300 cycles.

Poly(ethyl α-cyanoacrylate) (PECA) was also used as a coating material for LiNi_0.5Mn_1.5O₄. Liu et al. prepared surface-modified LiNi_0.5Mn_1.5O₄ particles via in situ polymerization of ethyl α-cyanoacrylate (ECA) to provide continuous surface coverage and a Li-ion transport channel.⁷⁹⁾ The XRD pattern of the PECA-coated LiNi_0.5Mn_1.5O₄ was identical to that of pristine LiNi_0.5Mn_1.5O₄, indicating that the polymer layer coating process did not affect the crystal structure of the LiNi_0.5Mn_1.5O₄. The LiNi_0.5Mn_1.5O₄ particles with a particle size of ~ 100 nm were encircled by an ~ 10-nm-thick PECA layer. Whereas the pristine LiNi_0.5Mn_1.5O₄ delivered ~ 74.7% retention of its initial capacity over 100 cycles, the PECA-coated LiNi_0.5Mn_1.5O₄ retained up to ~ 92% of its initial capacity under the same conditions. The polarization of the PECA-coated LiNi_0.5Mn_1.5O₄ after 100 cycles was smaller than that of the pristine LiNi_0.5Mn_1.5O₄, which demonstrates the effectiveness of the PECA layer in preventing the formation of the interface layer that hindered the charge transport at the interface between LiNi_0.5Mn_1.5O₄ and the electrolyte.

Carbon-based materials are known as excellent coating materials for electrode materials for LIBs because of their high electronic conductivity. Yang et al. reported enhanced electrochemical performance of carbon-coated LiNi_0.5Mn_1.5O₄ prepared by a sol-gel method.⁸⁰⁾ The XRD pattern of the carbon-coated sample was characteristic of spinel LiNi_0.5Mn_1.5O₄, and its particles were homogeneously wrapped by the carbon. The capacity of the carbon-coated LiNi_0.5Mn_1.5O₄ modified with the addition of 1 wt% sucrose (sample c) remained up to ~ 130 mAh g⁻¹ at 1 C and ~ 114 mAh g⁻¹ at 5C. The carbon-coated sample also exhibited a high capacity retention of 92% after 100 cycles.

Graphene oxide was also applied as a coating material for LiNi_0.5Mn_1.5O₄. Fang et al. synthesized graphene-oxidecoated LiNi_0.5Mn_1.5O₄ through simple mixing and annealing.⁸¹⁾ The graphene-oxide-coated sample had a pure spinel crystal structure without any contamination, and the existence of graphene oxide was verified through elemental mapping based on SEM. As shown in Fig. 7(a), TEM analyses revealed that graphene oxide with a thickness of ~ 5 nm was coated on the outer surface of LiNi_0.5Mn_1.5O₄. The graphene-oxide-coated LiNi_0.5Mn_1.5O₄ at large current densities of 5C, 7C, and 10C retained up to ~ 77%, ~ 66%, and ~ 56% of the capacity measured at 1C (Fig. 7(b)), and its capacity after 1000 cycles was ~ 61% of its initial capacity, indicating the enhanced electrochemical performances of LiNi_0.5Mn_1.5O₄ resulting from the graphene-oxide coating.

Wang et al. synthesized a carbon-coated LiNi_0.5Mn_1.5O₄ composite via pyrolysis of polyethylene glycol-400 (PEG) at high temperature.⁸²⁾ The carbon coating did not affect the crystal structure of LiNi_0.5Mn_1.5O₄. A distinct coating layer was identified on the surface of the LiNi_0.5Mn_1.5O₄ particles through TEM observation. The Raman spectra of carbon in the carbon-coated LiNi_0.5Mn_1.5O₄ composite clearly showed a disordered (D) band at 1366 cm⁻¹ and the ordered graphite (G) band at 1575 cm⁻¹, verifying the existence of carbon in the composite. Over 500 cycles, the carbon-coated LiNi_0.5Mn_1.5O₄ maintained up to ~ 71% of its initial capacity, whereas the pristine LiNi_0.5Mn_1.5O₄ only retained ~ 55% of its initial capacity.

Surface modification of LiNi_0.5Mn_1.5O₄ using CNTs has also been attempted. Hwang et al. prepared CNT-coated LiNi_0.5Mn_1.5O₄ and oxidized CNT (OCNT)-coated LiNi_0.5Mn_1.5O₄ composites via a mechano-fusion method to prevent unfavorable carbothermal reduction.⁸³⁾ The D and G peaks were verified in each Raman spectra of the CNT-LiNi_0.5Mn_1.5O₄ and OCNT-LiNi_0.5Mn_1.5O₄, verifying the existence of CNT and OCNT in each composite. The structures of the two composites were identical to that of the pristine LiNi_0.5Mn_1.5O₄ without any structural changes. TEM observation revealed that the ~ 10 - 15-nm thick CNT or OCNT layers encircled the surfaces of the LiNi_0.5Mn_1.5O₄ particles. The CNT-coated LiNi_0.5Mn_1.5O₄ and OCNT-coated LiNi_0.5Mn_1.5O₄ delivered higher energy densities than the pristine LiNi_0.5Mn_1.5O₄ until the power density reached ~ 1260 W kg⁻¹ at 2C.

Because of environmental problems such as air pollution and the greenhouse effect, considerable efforts have been made to design eco-friendly EVs, and LIBs have been considered one of the most attractive energy storage systems (ESSs) for EVs. To apply LIBs to these large-scale ESSs, however, their energy density must be improved. Thus, the development of electrode materials with theoretically high energy densities and high operation voltages is one of the major issues for LIBs in the world. Spinel LiNi_0.5Mn_1.5O₄ has received great attention as an attractive cathode material for LIBs because of its high operating voltage of ~ 4.7 V vs. Li⁺/Li based on the Ni²⁺/Ni⁴⁺ redox reaction and relatively high theoretical capacity of ~ 146.7 mAh g⁻¹. Recent studies on spinel LiNi_0.5Mn_1.5O₄ have focused on surface coating using various compounds such as metal oxides, polyanionbased compounds, polymers, and carbon-based materials to improve the power capability and cyclability of LiNi_0.5Mn_1.5O₄.

The desired coating material should exhibit high stability to prevent side reactions between LiNi_0.5Mn_1.5O₄ and the electrolyte as well as the HF attack accompanying Mn dissolution from LiNi_0.5Mn_1.5O₄. Even though the cyclability of coated LiNi_0.5Mn_1.5O₄ at room temperature is not significantly enhanced compared with that of pristine LiNi_0.5Mn_1.5O₄, coated LiNi_0.5Mn_1.5O₄ exhibits higher stability than pristine LiNi_0.5Mn_1.5O₄ during high-temperature cycle tests, which activate the side reactions and HF attack, because of the existence of the buffer layer for LiNi_0.5Mn_1.5O₄. Furthermore, coating of compounds containing Li ions in the structure or carbon-based materials simultaneously enables the achievement of great power capability and outstanding cyclability of LiNi_0.5Mn_1.5O₄ because of the high ionic or electronic conductivity of the coating material as well as the encapsulation of the particle surfaces.

Various experimental processes have been applied to achieve uniform and homogenous coatings of LiNi_0.5Mn_1.5O₄ particles. Many researchers have prepared coated LiNi_0.5Mn_1.5O₄ via heat treatment after mixing the LiNi_0.5Mn_1.5O₄ with a precursor containing the element desired in the solution base. Some researchers have attempted to apply deposition methods such as PLD or ALD to obtain more uniform and homogenous coatings on LiNi_0.5Mn_1.5O₄. The thickness of the coating layer is controlled by modifying the amount of reactants or the reaction time, and the existence of the coating layer in the composite is verified using SEM, TEM, XPS, or Raman spectra. It has been confirmed that the coating process does not affect the structure of LiNi_0.5Mn_1.5O₄ regardless of the type of coating material.

To apply LiNi_0.5Mn_1.5O₄ as a cathode material for LIBs, more intensive research on the stability of electrolytes for LIBs is required despite numerous reports on the stable cyclability of LiNi_0.5Mn_1.5O₄ over several hundred cycles. Previous studies on LiNi_0.5Mn_1.5O₄ have generally used halfcell tests based on small-scale coin cells. However, for the practical application of LiNi_0.5Mn_1.5O₄ in commercialized LIBs, it is essential to perform full-cell tests of LiNi_0.5Mn_1.5O₄ based on large-scale pouch cells. Furthermore, various analyses of the modified electrolytes in the full-cell test should be performed to verify the stability of the LIBs at high operation voltage resulting from the redox reaction of LiNi_0.5Mn_1.5O₄.