Recent Progress of High Voltage Spinel LiMn_1.5Ni_0.5O₄ Cathode Material for Lithium-Ion Battery: Surface Modification, Doping, Electrolyte, and Oxygen Deficiency

Abstract

High voltage spinel LiMn_1.5Ni_0.5O₄ (LMNO) is a promising energy storage material for the next generation lithium batteries with high energy densities. However, due to the major controversies in synthesis, structure, and interfacial properties of LMNO, its unsatisfactory performance is still a challenge hindering the technology’s practical applications. Herein, this paper provides general characteristics of LiMn_1.5Ni_0.5O₄ such as spinel structure, electrochemical properties, and phase transition. In addition, factors such as electrolyte decomposition and morphology of LMNO that influence the electrochemical performances of LMNO are introduced. The strategies that enhance the electrochemical performances including coating, doping, electrolytes, and oxygen deficiency are comprehensively discussed. Through the discussion of the present research status and presentation of our perspectives on future development, we provide the rational design of LMNO in realizing lithium-ion batteries with improved electrochemical performances.

1. Introduction

Renewable electricity has received great attention due to the lack of traditional fossil fuels and the need to preserve the environment.¹ In the past few decades, there has been immense research and progress in developing sustainable energy, with increasing demands for eco-friendly energy storage systems (ESSs). Among these energy storage methods, electrochemical energy storage devices have come to the fore due to their convenience and energy storage performance.^2,3 Currently, lithium-ion batteries (LIBs) are widely used in a variety of applications, including electric devices, electric vehicles (EVs), and grid energy storage systems.⁴⁻⁶

Lithium-ion batteries (LIBs) have gained interest due to their high energy and power densities. So far, various types of oxide cathode materials have been investigated, which all exhibit different characteristics.^7,8 Generally, there are three types of structure with different oxide cathodes: layered (LiMO₂, M = metal element), spinel (LiM₂O₄), and olivine structure (LiMPO₄). The layered structure, LiCoO₂ (LCO) has a theoretical specific capacity of 274 mAh g^–1, high theoretical volumetric capacity of 1363 mAh cm^–3, low self-discharge, and high discharge voltage. However, its high cost, low thermal stability, and fast capacity fade at high current rates are major limitations.⁹ LiNiO₂ (LNO) has similar theoretical capacity to LCO, but its thermal stability and blocking of lithium diffusion pathways are major limitations.¹⁰ Spinel structure, LiMn₂O₄ (LMO), has a theoretical specific capacity of 147 mAh g^–1 and has the advantage of lower cost and safety compared to LCO and LNO.¹¹ Olivine structure, LiFePO₄ (LFP), is famous for its high power capability, good safety features, no thermal runaway, and chemical stability with a capacity of 170 mAh g^–1.¹² LiMnPO₄ (LMP) is a favorable cathode candidate for its high energy density, low toxicity, safe operation, low-performance cost, and adequate thermal electrochemical stability with high redox voltage (4.4 V versus Li⁺/Li).¹³ Among these structures, spinel structure LiMn_1.5Ni_0.5O₄ (LMNO) has been considered as one of the most promising cathode materials that not only operates at high voltage (4.7 V vs Li/Li⁺) but also has a theoretical capacity of 147 mAh g^–1, which is 25% higher than spinel LiMn₂O₄ structure. Additionally, its energy density (658 Wh kg^–1, specific energy density = specific capacity * average operating voltage) is higher than other cathode materials such as LiCoO₂ (518Wh kg^–1), LiMn₂O₄ (440Wh kg^–1), LiFePO₄ (591Wh kg^–1).¹⁴ In addition, several anode materials have been paired with LMNO to study the full cell characteristics.¹⁵⁻¹⁸ Among these materials, Li₄Ti₅O₁₂ (LTO), operating at 1.5 V, has gained interest because when it is paired with LMNO cathode, it exhibits a safer and higher power battery with superior electrochemical performance at a voltage window of 3.2 V.^17,19−21 Likewise, in recent years, optimization of LMNO has been studied, such as defect studies by using density functional theory (DFT) to understand the structure of ordered (P4₃32) and disordered (Fd3m) phase. These studies focus on understanding the effects of defects on the ordered and disordered phases.²²⁻²⁴ Moreover, LMNO has the advantages of being inexpensive, easy to synthesize, and having 3D spinel structures that allow rapid Li-ion diffusion.^25,26

Despite these attractive properties, there are some bottlenecks that hinder the industrialization of LMNO. These limitations mainly arise from the incompatibility of the electrolyte with cathode materials. Because of operating at high redox voltage (∼4.9 V), carbonate compounds in the electrolyte decompose and make byproducts, which in turn form a cathode-solid electrolyte interphase (CEI). Creating a continuous CEI layer results in severe polarization and negatively affects electrochemical performance. In addition, Mn dissolution during electrochemical cycling and the formation of impurity (NiO, Li_xNi_1–xO) phases are also one of the main problems. These disadvantages lead to the continuous growth of CEI, oxygen deficiencies in high temperature, and structure instabilities, which increase the polarization and expedite capacity decay.^5,27 When it comes to full cell, optimization of cell design including choice of electrolyte and anode material needs to be considered.

Tremendous strategies have been developed to deal with the aforementioned bottlenecks. For example, researchers have studied which phase of LMNO exhibits different characteristics. When it is F-type, it helps to exhibit better structural reversibility during both lithiation and delithiation, but it exhibits poor cycling performances due to Jahn–Teller distortion, which induces Mn dissolution into the electrolyte.²⁸ Also, using surface modification methods are being investigated in which coating acts as a protective layer for high voltage working potential electrodes, which reduces the capacity losses and increases the cycle life.²⁹ In terms of applying dopant, it helps to increase the structural stability of the LNMO lattice.³⁰ Moreover, integrating electrolyte additives to prevent undesired side reaction and avoiding the dissolution of transitional metal ions that reduce capacity fading^31,32 and oxygen controlling with temperature that prevents LMNO from structural degradation and enhances the capacity are being investigated.^14,33

Herein, the purpose of this Review is to comprehensively summarize four aspects to improve the electrochemical performance of LMNO cathode materials: (i) effects of coating, (ii) effects of doping, (iii) effects of electrolyte, and (iv) oxygen deficiency. Detailed information about the factors that affect electrochemical performance of LMNO is provided and help improving strategies that have great potential to be useful for further commercialization will also be proposed. Furthermore, conclusion and perspectives are listed for the possible solutions that are valuable to designing a practicable battery.

2. Structure of LiMn_1.5Ni_0.5O₄

Knowing the spinel structure of LMNO (Figure 1a) is critical in enhancing electrochemical performances. Due to the influence of ordering Ni and Mn at two octahedral sites, LMNO has two different structures: ordered and disordered. In the stoichiometric ordered (P4₃32) phases, Li is in 8c sites, Ni in 4a sites, Mn in 12d sites, and O in 8c and 24e sites. In ordered (P4₃32) phase, Mn and Ni are ordered regularly. On the other hand, in the nonstoichiometric disordered (Fd3m) phases, Li is on 8a sites, Ni and Mn were randomly distributed on 16d sites, and O is on 32e sites.³⁴ In the ordered phase, the ordering of Ni and Mn exists without Mn³⁺ ions owing to annealing process below 700 °C that leads to the ordering of Ni and Mn on two distinct octahedral sites and the oxidation of Mn³⁺ ions into Mn⁴⁺ ions. In contrast, the disordered spinel shows different features that include the disordering of Ni and Mn and the presence of Mn³⁺ ions. The correlation of the disordering of Ni and Mn with the existing of Mn³⁺ ions is because of the synthesis conditions, such as the sintering temperature, postannealing and speed of cooling.³⁵ In addition, a superlattice can be seen only at the ordered phase, which can be detected by X-ray diffraction (XRD) and Raman spectrum.^36,37 In the XRD, two small peaks are observable at P4₃32 phase, while it is absent at Fd3m phase. This structure difference is also detectable through Raman spectrum, showing more peaks are observable at P4₃32 phase. As shown in Figure 1b, during the charge/discharge process, when LMNO possesses a superlattice (P4₃32 phase), two plateau are present at 4 V (Mn redox) and 4.7 V (Ni redox). On the other hand, at Fd3m phase, only one peak is observable at 4.7 V (Ni redox).

Figure 1.

(a) Crystal structure of LMNO and (b) electrochemical charge and discharge profiles for disordered and ordered LMNO structures, reprinted with permission from ref (41). Copyright 2014 Royal Society of Chemistry.

It has been reported that the electrochemical performance of disordered spinel is better than ordered spinel at high current densities, and its electrochemical performance of LMNO is closely related to structure.^35,38 The presence of Mn³⁺ ions in the spinel plays an important role in the spinel. The impurity of Li_xNi_1–xO that reduces the capacity accompanies the disordered formation. In order to maintain charge neutrality in disordered spinel, inactive Mn⁴⁺ ions are reduced to Mn³⁺ ions.³⁸ Mn³⁺ ions can increase the electronic conductivity in the disordered spinel, so that Mn³⁺ can improve electrochemical performance. However, existing Mn³⁺ can worsen the electrochemical performances of LMNO due to transforming into Mn²⁺ ions through disproportion reaction, and these Mn²⁺ ions are easily dissolved into electrolyte at high operating temperature and potential.³⁹ As a result, the presence of Mn³⁺ can also negatively influence the electrochemical performances such as shortening cycle life. Furthermore, disordering Ni and Mn sites in disordered spinel also affect electrochemical performance. Delithiation appears during phase transformation from P4₃32 structure to structure and leads to disordering Ni and Mn sites.⁴⁰ Likewise, understanding the correlation between structure and property relationship is essential to increase electrochemical performances of LMNO.

3. Factors Affecting Electrochemical Performances of LiMn_1.5Ni_0.5O₄

LiMn_1.5Ni_0.5O₄ has a theoretical capacity of 147 mAh g^–1 due to redox reaction of intercalation of one Li-ion per formula unit owing to the oxidation of Ni²⁺/Ni⁴⁺ at around 4.7 V. In ordered P4₃32 phase only one plateau can be seen in around 4 V that shows absence of Mn³⁺. However, in disordered, there are two plateaus which are corresponding to the Ni²⁺/N³⁺ and Ni³⁺/Ni⁴⁺ redox reaction around 4.7 V and Mn⁴⁺/Mn³⁺ redox reaction at 4 V.^29,41 As mentioned earlier, due to unwanted side reactions and dissolution of Mn ions, many investigations have been conducted in electrolyte decomposition, oxygen deficiency, morphology and phase transition.

3.1. Electrolyte Decomposition

It is critical to have appropriate electrolyte with high stability especially in high voltage LMNO cathode materials to deliver full capacity and for long cycle life. Normally, conventional electrolytes composed of LiPF₆ salt dissolved in organic carbonate esters such as ethylene carbonate (EC) mix with dimethyl carbonate (DMC) and/or diethyl carbonate (DEC) and/or ethyl methyl carbonate (EMC) are used in LIBs. However, conventional electrolytes have redox reaction which generates acidic species (HF or PF₅) from the decomposition of LiPF₆ (Figure 2).²⁹ In general, the surface film of Li₂CO₃ or LiOH covers the cathode materials, which is formed by reaction of metal oxides with Co₂ and H₂O during the cycle.^42,43 Li₂CO₃ existing on the surface is originated from incomplete conversion of the carbonate precursors.⁴⁴ This Li₂CO₃ reacts with electrolyte conducting salts like LiPF₆ and LiBF₄ and cause the decomposition of electrolyte.⁴⁵

Figure 2.

Electrolyte decomposition reactions that occur at high voltage lithium-excess metal oxides, reprinted with permission from ref (46). Copyright 2012 American Chemical Society.

In terms of decomposition of electrolyte, there are three types of reaction pathways: (i) chemical (non-Faradaic) reduction and oxidation, (ii) electrochemical (Faradaic) reduction and oxidation, and (iii) nonredox reactions. (Figure 3) Through these reactions, it can influence on forming interphase layer. In high voltage cell, LMNO, when chemical oxidation is dominant decomposition process, it affects the surface reactivity of positive electrode and electrolyte reaction.⁴⁴

Figure 3.

General oxidation pathways for organic species (RX), reprinted with permission from ref (44). Copyright 2022 Wiley-VCH.

To protect the electrode surface, there has been extensive research and development on electrolytes which also enhance the electronic performances on liquid, solid, and polymer electrolytes. In consequence, it is undeniable that electrolyte plays critical role in batteries and affects electrochemical performances. Electrolytes used in LMNO are shown in Table 1.

Table 1. Electrolytes Used in LMNO.

Cell configuration	Electrolyte compositions	Remarks	ref
LMNO/Li	1 M LiPF6 in EC/EMC (3:7) + 1 wt % HFiP		(47)
	LAGP	solid electrolyte	(48)
	40PIL-IL (1 M LIFSI in PYR13FSI)	solid electrolyte	(56)
	Li6.4La3Zr1.4Ta0.6O12 (LLZTO)	solid electrolyte	(57)
	1.3 M LiPF6 in EC/EMC/DEC (3:2:5)		(58)
	1 M LiPF6 in EC/DEC (3:7) + ETFEC		(49)
	1 M LiPF6 in EC/EMC (3:7) + 0.1 wt % TPFPS		(50)
	1 M LiPF6 in EC/EMC (3:7) + 1 wt % DMMP		(51)
	1 M LiPF6 in EC/EMC (3:7) + 2.5 wt % LiBOB		(59)
	1 M LiPF6 in EC/DMC/DEC (1:1:1)		(60)
	1 M LiPF6 in EC/DEC/DMC (1:1:1) + 1 wt % THB		(52)
	1 M LiPF6 in EC/DEC (1:1) + PAMM		(53)
LMNO/graphite	1.2 M LiPF6 in EC/EMC (3:7) + 0.1 wt % DMF-SO3		(61)
	1.2 M LiPF6 in EC/EMC (3:7) + lithium borate	4 types of lithium borate	(54)
	1 M LiPF6 in FEC/F-EMC/F-EPE (3:5:2)		(62)
	1 M LiPF6 in EC/EMC/DMC (3:4:3) + 1 wt % TMSP		(63)
	1.2 M LiPF6 in EC/EMC (3:7) + 0.5 wt % LiCDMB		(64)
LMNO/Li4Ti5O12 (LTO)	1 M LiPF6 in EC/DEC (1:2) + 1 wt % LiO-t- C4F9 and Al(HFiP)3		(55)

Cresce et al. studied highly fluorinated phosphate triester based additive electrolyte. This additive improved anodic stability and provided protective SEI chemistry on graphite anode.⁴⁷ Robinson et al. investigated lithium-ion battery with using solid state electrolytes Li_1+xAl_xGe_2–x(PO₄)₃ (LAGP). Despite resulted in good mechanical connection with materials while sintering, Li/LAGP/LMNO exhibited low capacity and high voltage reaction was not reversible on discharge.⁴⁸ Zheng et al. investigated ethyl-(2,2,2-trifluoroethyl) carbonate (ETFEC) additive with different ratio. Adding ETFEC as an additive can improve cycle performance, owing to high stability of ETFEC which prevents large oxidation between electrode and electrolyte, especially at high temperature.⁴⁹ Lee et al. employed tris(pentafluorophenyl)silane (TPFPS) as an electrolyte additive. This additive showed better Coulombic efficiency compared to TPFPS free electrolyte due to stability to migrate the electrolyte decomposition on the LMNO cathode.⁵⁰ Xu et al. investigated the effect of dimethyl methylphosphonate (DMMP) additive. Addition of DMMP exhibited better capacity retention, owing to suppress the decomposition between electrode which contribute to improve electrochemical performances.⁵¹ Perea et al. studied the effect of adding additive 1,3,5-trihydroxybenzene (THB). Addition of THB improved the capacity retention by inhibiting electrolyte decomposition on the cathode surface.⁵² A cross-linking polymer network of acrylic anhydride-2-methyl-acrylic acid-2-oxirane-ethyl ester-methyl methacrylate (PAMM) based electrolyte is introduced by Ma et al. Performances such as electrochemical stability, mechanical strength, flame resistance, interphase compatibility, and suppressing Mn dissolution, are increased by using PAAM based electrolyte.⁵³ Four types of lithium borate electrolyte (LFPTB, LPTB, LTSTB, LPrTB) is presented by Xu et al. Among those, lithium 4-pyridyl trimethyl borate (LPTB) exhibited the best performance improvement. LPTB enables generation of a cathode passivation film and acts as a functional group delivery which results in improved capacity retention and efficiency.⁵⁴ Two additives, LiO-t-C₄F₉ and Al(HFiP)₃, to the electrolyte are studied in LMNO/LTO cell. LiO-t-C₄F₉ additive improves capacity retention by increasing electrolyte oxidation at the cathode. For the Al(HFiP)₃ additive, it reduces parasitic reaction at LMNO electrode but does not affect capacity retention.⁵⁵

3.2. Oxygen Deficiency

Oxygen deficiency is one of the key factors that should be considered when using LMNO as cathode material. As aforementioned, LMNO oxygen deficiency occurs during structural change from low temperature (below 700 °C) ordered structure, P4₃32 to high temperature (over 700 °C) disordered structure, Fd3m phase. This disordered structure is accompanied by a rock salt impurity phase such as Ni_x, Li_xNi_1–xO, and (LiNiMn)O₄ and with oxygen deficiency within the spinel.⁶⁵ To be more specific, a correlation between structure and temperature with oxygen content is shown in Figure 4. A temperature below 683 °C shows a single phase; a temperature between 683–805 °C shows a single phase with an increase in unit cell volume; a temperature between 805–950 °C shows two phases coexist; a temperature above 950 °C shows a single cubic rock salt phase.⁶⁶ Also, oxygen deficiency in LMNO can be shown as LiMn_1.5Ni_0.5O_4–x. When x is below 0.05, it has single spinel phase (P4₃32); when x is between 0.05 and 0.18, it has a single spinel phase (Fd3m); when x is between 0.18 and 0.7, it has two phases, spinel phase (Fd3m) and rock salt phase (Fm3m); when x is over 0.7, it has two phases, rock salt phase (Fm3m) and secondary spinel phase (Fm3m). During this phase transition, oxygen is released from the cathode material, Mn⁴⁺ changes to Mn³⁺ in order to satisfy the charge balance. Also, as temperature increases, Kunduraci et al. showed that surface area, lattice parameters, and weight loss depend on the temperature.⁶⁷

Figure 4.

(a) Correlation between phase fraction and temperature and (b) correlation between phases and oxygen content, reprinted with permission from ref (66). Copyright 2008 Electrochemical Society.

Oxygen deficiency can also occur through various ways such as delithiation, side reaction of electrolytes. When transitional metal Mn³⁺ changes to Mn⁴⁺ during the charging process (delithiation), LMNO become more oxidative and releases oxygen.⁶⁸ Charging induced oxygen loss is exacerbated when cutoff voltages above 4.4 V are adopted. On the other hand, voltages under 4.3 V results in reduced kinetics of oxygen loss.³³ In the voltage region of 3–4.4 V, delithiation reaction is related to the oxidation of Ni and voltage region around 4.5 V is related to Mn sites during charging.⁶⁹ Likewise, voltage plateau ∼4.5 V is attributed to an oxygen loss and structural change. This voltage plateau does not show up in the layered cathodes other than LMR cathodes, primarily because they do not have such pronounced oxygen loss in the first cycle.

Unwanted side reaction from the electrolytes can cause the oxygen loss. Due to oxygen’s high potential property, oxygen from cathode is highly oxidative and the reduction of oxygen forms a free oxygen radical in the electrolyte as shown in Figure 2.^70,71 These oxygen radicals yield a byproduct such as H₂O and CO₂. The existence of H₂O can lead to the production of impurities of Li₂O or by diffusion of Li and O from the cathode and affects electrochemical performances by producing LiF.^33,72 In terms of CO₂, it reacts with Li₂O to form Li₂CO₃. And Li₂CO₃ along with LiF can form a SEI layer coated on the surface on the particle, which decreases the conductivity.⁴⁶

In order to compensate for this oxygen loss from the particle, surface coating, increasing lithium content, and controlling cooling rate have been used. As mentioned above, oxygen loss and structural degradation occur on the surface of the particle. Therefore, surface coating acts as a protection barrier that induces an improvement electrochemical performance in LIBs.⁷³ In terms of doping, the latest report was shown by Liang et al., who presented new orbital hybridization.⁷⁴ They found Ge (at 16c site) with valence state of +3 in 4s orbital that enables to interact with oxygen electrons in the 2p orbital. This 4p-2s orbital hybridization with Ge gives LMNO enhanced stability for the metal oxygen framework. Also, it contributes to stable thin CEI and prevents LMNO from structural collapse.

As mentioned above, oxygen from the cathode can react with Li, forming Li₂O. The equation will be 2Li⁺ + O^2– = Li₂O, and from this equation, the equilibrium K_c will be K_c = [Li₂O]/([Li⁺]² * [O^2–]). Hence, increasing the lithium salt will reduces the concentration of oxygen radical. Therefore, excess lithium will reduce the migration of lattice oxygen from the cathode into the electrolyte.³³ However, when the concentration of LiPF₆ increases, there are both advantages and disadvantages. For the advantages, when the concentration of LiPF₆ increases, the value of radical distribution function increases which improves stability and reduces the interaction with electrode and oxygen loss (Figure 5a). On the other hand, for the disadvantages, when the concentration of LiPF₆ increases, ionic conductivity decreases due to a decrease of diffusivity (Figure 5b).⁷⁵

Figure 5.

(a) Radical distribution function value and (b) ionic conductivity with different concentrations of LiPF₆, reprinted with permission from ref (75). Copyright 2018 American Chemical Society.

In terms of electrochemical performance depending on the temperature, Pasero et al. introduced compared 4 different samples: quenched from 855 °C, 805 °C, 743 °C, HOP (slow cooling under pressure to room temperature). These samples’ discharge capacity varies from 120–140 mAh g^–1. With increasing quench temperature, the plateau begins to appear at 4 V. Also, HOP sample shows the best capacity retention and sample quenched from 855 °C shows the worst capacity retention.⁶⁶ Tong et al. also compared 4 different samples which quenched from 750 °C (S750), 700 °C (S700), 650 °C (S650), and SRT (slow cooling to room temperature). Besides SRT sample (ordered sample), they all exhibits two plateaus at 4 and 4.7 V which corresponds to Mn and Ni redox. Under the rate of 5C, S750 and S700 achieves 80% capacity retention after 1000 cycles, but S650 and SRT retain only 54% and 24% of capacity.⁷⁶ Sun et al. heated samples with 5 different temperatures (650 °C, 750 °C, 850 °C, 950 °C, 1000 °C). Samples heated up to 650 and 750 °C (ordered phase) show only one plateau at 4.7 V and the others all exhibit two plateau at 4 and 4.7 V. Discharge capacity of these samples show 125.2, 121.3, 134.5, 126.8, 106 mAh g^–1, respectively. They show 86.27, 84, 99.32, 91.76, 88.52% capacity retention from 10th cycle to 50th cycle at 1C.⁷⁷

3.3. Morphology

Different morphology is studied to enhance the electrochemical performances including cycling and rate performances. After been through different approach (synthesis, doping, surface modification, and etc.) morphology changes from spinel structure to different structure. Kebede reported LMO@LMNO having polyhedral morphology which enhanced rate capability. This product shows 123 mAh/g discharge capacity at 0.2C and shows 96.2% retention after 120 cycles.⁷⁸ Tong et al. introduced microrods morphology through metal oxalate precursor. The resulting product showed excellent capacity retention up to 20C. Plus LNMO microrods quenched at 700 °C deliver a capacity of 116 mA h g^–1 even at a discharging rate of 50 C, and the electrode maintains 80% of its capacity after 1000 cycles at 5 C.⁷⁶ Karunawan et al. proposed truncated octahedral shape LMNO with plausible crystal growth mechanism via solid state method. The resulting product exhibited 128.53 mAh g^–1 with high initial Coulombic efficiency (ICE) with superior cycling stability which retained 90.32% after 250 cycles.⁷⁹ And the others are mentioned in section 4.

3.4. Phase Transition

Phase transition is also one of the factors that influence electrochemical performances of LMNO. As shown in Figure 1b, two different phases have different capacity. Likewise, knowing a phase transition is one of the key factors to understand the LMNO. Through various studies, it is reported that LMNO has been through different phases: (i) from disordered phase (Fd3m) to ordered phase (P4₃32). In this phase transition, there are three characteristics: cationic ordering at 4a and 12d sites, anionic ordering at 8c and 24e sites, and displacement of all atoms; (ii) from spinel (Fd3m) phase to rock salt (Fm3m) phase. In this phase transition, one-half of O₂ is released by the spinel to yield the rock salt phase; and (iii) from cubic (Fd3m) phase to tetragonal I4₁/ and spinel structure.

4. Methods Improving Electrochemical Performances of LiMn_1.5Ni_0.5O₄

As mentioned above, many cathode materials are undergoing different problems. In order to improve electrochemical performances of LMNO, many researchers used different approaches to solve the problems. Generally, three different techniques are introduced in this paper: surface modification, doping, and CEI modification.

4.1. Surface Modification

Surface modification has been reported to have advantages of suppressing the side reaction that decreases the overall capacity and cycle ability and dissolution of transition metal ions. The main effects of surface coating can be the following: (i) enables the charge transfer at the surface of the particle, (ii) changes the particle’s morphology, (iii) suppresses the metal dissolution and side reaction.⁷³ Also, it has been found to provide a stable interface between the electrolyte and the active material which derived higher electrochemical performances with various coating methods: Chemical vapor deposition process (CVD), dry coating, atomic layer deposition (ALD), coprecipitation, radio frequency magnetron sputtering, sol–gel, solvothermal (Figure 6).²⁸

Figure 6.

Coating methods (a) CVD, (b) Ball mill, (c) Sol–gel, (d) Solvothermal, (e) Coprecipitation, (f) ALD methods, reprinted with permission from ref (28) Copyright 2022 Elsevier.

4.1.1. Chemical Vapor Deposition (CVD)

Chemical vapor deposition (CVD) is a coating method that uses chemical reactions at the surface of a heated substrate, with reagents supplied in gas phase precursors. CVD is known to offer an advantage by depending on chemical reactions which allow tunable deposition rates and high quality products.⁸⁰

There are essential sequential phases in every CVD process: reactants are transferred convectively and diffusively from gas inlets to reaction zone, generation of new active species through gas phase chemical process, transportation of starting reactants and their products to the target substrate, chemical/physical adsorption and diffusion of substances on the substrate surface, film formation through surface-catalyzed heterogeneous processes, desorption surface reaction through volatile derivatives, byproducts are transferred convectively and diffusively in the reaction away from the reaction zone.²⁸ Characteristics of different coatings are shown in Table 2.

Table 2. Characteristic of Different Coatings Using CVD Method.

Synthesis Method	Particle size	Lattice constant (Å)	Morphology	Surface Modification	Capacity (mAh g–1)/ temperature	Capacity retention (Rate/Cycle)	Remarks	ref
CVD	-	-	truncated octahedron	SnO2	135 at 0.5C/25 °C	95.21% at 1C/100	-	(81)
	-	-	spherical	Al	131.6 at - /25 °C	81% at - /400	10 nm Al coated	(82)

Lee et al. used tin oxide by employing electron cyclotron resonance metal–organic chemical vapor deposition to prepare the SnO₂-LMNO cathode. It not only performed with better rate capability at both room temperature and 60 °C, but also effectively suppressed the charge transfer resistance.⁸¹ Sun et al. employed an electron-beam vapor deposition approach to make Al coating on LMNO. Al-LMNO showed low impedance owing to the presence of a conductive Al layer, which performed with better rate capability. They also presented that Al-coated LMNO can be turned to Al³⁺ doped LMNO at high temperature (Figure 7).⁸²

Figure 7.

SEM images of (a) SnO₂ coated LMNO, reprinted with permission from ref (81). Copyright 2015 Elsevier. SEM images of (b) Al coated LMNO, reprinted with permission from ref (82). Copyright 2016 Elsevier.

4.1.2. Dry Coating

In the dry coating method, larger particles are directly coated using smaller fine particles by external mechanical forces without using any solvents and binders.⁸³ Larger particles that create engineered particulates with tailored properties can be changed in property or function.⁸⁴ It can be considered that the intimate electrode–electrolyte contacts are built by dry coating of the electrode particles with the solid electrolyte particles.⁸⁵ Dry coating has the advantage of improving the dispersion and adhesion onto the cohesive particles.⁸⁶ Characteristics of different coatings are shown in Table 3.

Table 3. Characteristic of Different Coatings Using Dry Coating Method.

Synthesis method	Particle size	Lattice constant (Å)	Morphology	Surface modification	Capacity (mAh g–1)/ temperature	Capacity retention (Rate/Cycle)	Remarks	ref
dry coating	2 μm	-	truncated octahedron	SiO2	122 at 40C/55 °C	82.4% at 80C/400	1 wt % SiO2	(87)
	-	-	truncated octahedron	SiO2	127.2 at 1C/25 °C	89%% at 1C/100	3 wt % SiO2	(88)
	-	-	truncated octahedron	Al2O3	116.6 at 1C/25 °C	91.5% at 1C/100	3 wt % Al2O3	(88)
	-	-	truncated octahedron	TiO2	119.9 at 1C/25 °C	91.6% at 1C/100	3 wt % TiO2	(88)
	-	-	truncated octahedron	MgCO3	121.2 at 1C/25 °C	95.4% at 1C/100	3 wt % MgCO3	(88)
	1–2 μm	8.1651	octahedron	Ta2O5	131.5 at C/10/55 °C	97% at 0.2C/100	2 wt % Ta2O5	(89)
	1 μm	-	polyhedron	ZrO2	110 at 40C/25 °C	85.6% at 40C/1200	1 wt % ZrO2	(93)
	0.53 μm	-	polyhedron	Li4P2O7	123.8 at 0.5C/25 °C	74.3% at 0.5C/893	0.05 mol ratio	(90)
	100 nm–1 μm	-	octahedron	Li0.1B0.967PO4	137.1 at 1C/25 °C	91.3% at 1C/400	Cr doped	(91)
	-	-	spherical	C-LiFePO4	82 at 1C/25 °C	74.5% at 1C/140	20 wt % LFP (2 wt % C)	(92)

Nisar et al. used ball milling process to coat LMNO with silica nanoparticles. It exhibited better cycle and rate capability with high retention owing to high charge transfer kinetics and low interfacial charge transfer resistance.⁸⁷ Cho et al. presented three different coating materials: SiO₂, Al₂O₃, TiO₂, and MgCO₃. These coated LMNO exhibited structural stability and suppressed Mn dissolution with high capacity retention.⁸⁸ Ben et al. employed Ta₂O₅ as a coating material. Ta₂O₅ coated LMNO exhibited high capacity retention and better cycling performances at room temperature and 55 °C. It has advantage of high resistance against hydrofluoric acid (HF) attack.⁸⁹ Chong et at., presented Li₄P₂O₇ coated LMNO, which exhibited better rate capability and cycling capability. Li₄P₂O₇ coating layer acts as a solid electrolyte or artificial SEI layer that prevents Ni redox couple from decomposing the electrolyte.⁹⁰ Yang et al. employed a Cr doped Li_0.1B_0.967PO₄ (LBPO) coated material on LMNO. Cr³⁺ doping reduces NiO impurity and enhances structural stability, and LBPO can enhance Li⁺ transference and suppress the unwanted side reaction. Overall, Cr doped Li_0.1B_0.967PO₄ exhibited better cycle stability and rate performances (Figure 8).⁹¹ Liu et al. employed C-LiFePO₄ as a coating material on LMNO. C-LiFePO₄ coated LMNO exhibited better capacity retention (at high rate) and cyclability in room temperature. This coated LMNO showed high surface conductivity and prevents the reaction that occurs in the electrolyte.⁹²

Figure 8.

SEM images of (a) MgCO₃-LMNO, reprinted with permission from ref (88). (b) Al₂O₃-LMNO (dry coating), reprinted with permission from ref (88). Copyright 2019 Springer Nature. (c) Ta₂O₅-LMNO, reprinted with permission from ref (89). Copyright 2018 American Chemical Society. (d) Cr doped Li_0.1B_0.967PO₄-LMNO, reprinted with permission from ref (91). Copyright 2014 Royal Society of Chemistry.

4.1.3. Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD) is a self-limiting half reaction that comes along with surface-controlled and iterative monolayer-by-monolayer process relied on two sequential.⁹⁴ Most of the ALD processes have binary reaction sequences where two surface reactions occur and deposit a binary compound film. Due to a finite number of surface sites, a finite number of surface species can be deposited.⁹⁵ ALD enables the deposition of material precise angstrom-level thickness with excellent consistency and has the advantages in the application of surface coatings on LIB cathode and anode materials.⁹⁴ Characteristics of different coatings are shown in Table 4.

Table 4. Characteristic of Different Coatings Using ALD Method.

Synthesis method	Particle size	Lattice constant (Å)	Morphology	Surface modification	Capacity (mAh g–1)/ temperature	Capacity retention (Rate/Cycle)	Remarks	ref
ALD	-	-	nanowire	TiO2	103.17 at C/7.5/25 °C	-	-	(96)
	-	-	nanowire	Al2O3	105.72 at C/7.5/25 °C	-	-	(96)
	-	-	-	Al2O3	115 at 0.5C/25 °C	92% at 0.5C/200	4 cycles of ALD	(97)
	-	-	-	Al2O3	130 at 0.5C/25 °C	91% at 0.5C/200	-	(102)
	-	-	-	FePO4	112 at 0.5C/25 °C	91.96% at 0.5C/100	10 cycles of ALD	(98)
	-	-	-	Li3PO4–TiO2	122 at 0.5C/25 °C	81.2% at 0.5C/300	-	(99)
	-	-	-	AlPO4	100.6 at 0.5C/25 °C	94% at 0.5C/100	10 cycles of ALD	(100)
	-	-	-	LiAlO2	92 at C/3/25 °C	-	-	(101)

Cho et al. employed TiO₂ and Al₂O₃ as a thin coating material on LMNO nanowire through ALD synthesis method. This coated LMNO enables decreasing irreversible capacity fade and diminishing the Mn dissolution, which led to acceleration of Li ion migration and reduction in resistance.⁹⁶ Song et al. presented Al₂O₃ coated LMNO by ALD process. Al₂O₃ coated LMNO exhibited reduced side reactions involving organic components of the electrolyte decomposition, which results in high capacity retention.⁹⁷ Xiao et al. presented ultrathin FePO₄ coated LMNO. FePO₄ layer acts as Li ions reservoir and active buffer that leads to high capacity. With 10 ALD cycles of FePO₄ coated LMNO, there was stabilized capacity retention and suppressed Mn dissolution, along with the advantages on the electron/ion diffusion on the surface.⁹⁸ Deng et al. designed a hybrid Li₃PO₄–TiO₂ coated LMNO. The Li₃PO₄–TiO₂ coating layer helps to inhibit the side reactions and enhance interfacial ionic and electronic conductivities. As a result, Li₃PO₄–TiO₂ coated LMNO exhibited improved rate capability and cyclic stability.⁹⁹ They also designed AlPO₄ coated LMNO, which exhibited a high capacity retention with long cycle life (Figure 9).¹⁰⁰ Park et al. used the LiAlO₂ coating material on LMNO with improved electrochemical stability.¹⁰¹

Figure 9.

SEM images of (a) FePO₄-LMNO, reprinted with permission from ref (98). Copyright 2015 Wiley-VCH. (b) LPO-TiO coated LNMO, reprinted with permission from ref (99). Copyright 2019 Elsevier. (c) ALP −50 coated LMNO, reprinted with permission from ref (100) Copyright 2017 Elsevier.

4.1.4. Coprecipitation

Coprecipitation is considered an efficient, simple, scalable, and tunable technique.¹⁰³ Especially, coprecipitation reaction has been widely used in high volumetric energy-density cathode materials.¹⁰⁴ Usually, there are two main steps using coprecipitation process to synthesize battery active materials. The first step involves formation of particles from a coprecipitation reaction (precursors). These precursors are dried to get rid of residual water or other solvents. And then, the precursor particles are mixed with a lithium source and calcined to produce the final active materials that is used in battery electrodes.¹⁰⁵ There are many factors that can influence the composition and morphology of the particles such as temperature, pH, concentration, stirring rate and mixing method, rate of reactant feed, and the use of additives. Characteristics of different coatings are shown in Table 5.

Table 5. Characteristic of Different Coatings Using Coprecipitation Method.

Synthesis method	Particle size	Lattice constant (Å)	Morphology	Surface modification	Capacity (mAh g–1)/ temperature	Capacity retention (Rate/Cycle)	Remarks	ref
coprecipitation	-	8.174	spherical	CeO2	138 at 0.1C/25 °C	96.8% at 0.1C/100	1 wt % CeO2 wrapped in graphene	(106)
	-	-	octahedron	MoO3	128 at 0.1C /-	94.6% at 0.1C/100	2 wt % MoO3	(107)
	-	-	spherical	FeF3	132 at C/6/25 °C	100% at -/200	2 wt % FeF3	(108)
	10 μm (sphere size)	-	spherical	Fe2O3	126 at 1C/25 °C	98.6 at 1C/100	-	(110)
	700 nm (polygon size)	-	polyhedron	CuO	131.5 at 0.5C/25 °C	96.5% at 0.5C/100	3 wt % CuO	(111)
	-	-	polyhedron	Co3O4	126 at 1C/25 °C	96.8% at 1C/300	5 wt % Co3O4	(112)
	-	-	spherical	Co3O4	127.7 at 0.2C/25 °C	81% at 5C/2000	-	(113)
	-	-	crystalline	ZrO2	118 at 1C/25 °C	96% at 1C/150	1 wt % ZrO2	(109)
	-	8.173	spherical	Y2O3	127.2 at 2C/55 °C	97.7% at 1C/300	-	(114)
	-	-	octahedron	RuO2	131.7 at 0.5C/25 °C	97.7% at 0.5C/100	2 wt % RuO2	(115)
	200 nm	8.175	polyhedron	YPO4	138 at 0.1C/25 °C	77.5% at 0.1C/240	3 wt % YPO4	(116)
	300 nm	8.1782	polyhedron	YF3	108 at 0.1C/25 °C	84% at 0.1C/100	2.8 wt % YF3	(117)
	-	-	octahedron	AlF3	103.6 at 0.1C/25 °C	93.6% at 0.1C/50	1 wt % AlF3	(118)
	200 nm	8.1614	polyhedron	MgF2	115.3 at 0.1C/25 °C	89.9% at 0.1C/100	5 wt % MgF2	(119)

Qureshi et al. designed CeO₂ coated LMNO wrapped in graphene. CeO₂ coating layer helps to suppress the reaction at the electrolyte/electrode and Mn dissolution. In terms of graphene wrapping, it helps to reduce material aggregation and improves conductivity which results in improvement of electrochemical performances. Overall, this coated LMNO exhibited better cyclability and capacity retention.¹⁰⁶ Wu et al. employed MoO₃ as a coating material. MoO₃ coating layer not only enables to improve the high rate charge/discharge profiles and cycle life but also suppress the side reactions at electrode and in electrolyte. Thus, MoO₃ coated LMNO displayed better cyclic retention and electrochemical performances (Figure 10).¹⁰⁷ Luo et al. used FeF₃ as a coating material on LMNO by precipitation method. FeF₃ coated LMNO displayed enhanced electrochemical properties and thermal stability.¹⁰⁸ Wu et al. designed ZrO₂ and ZrP₂O₇ coated LMNO. ZrO₂ coated LMNO by coprecipitation method, exhibited improved cycling stability with high capacity retention. On the other hand, ZrP₂O₇ coated LMNO by the sol–gel method, exhibited lower capacity retention than ZrO₂ coated LMNO.¹⁰⁹

Figure 10.

SEM images of (a) 2 wt % MoO₃-LMNO, reprinted with permission from ref (107). Copyright 2022 MDPI. (b) Y₂O₃-LMNO, reprinted with permission from ref (114). Copyright 2015 Springer Nature (c) 2 wt % RuO₂-LMNO, reprinted with permission from ref (115). Copyright 2015 Elsevier. (d) 3 wt % YPO₄-LMNO, reprinted with permission from ref (116). Copyright 2018 American Chemical Society. (e) 1 wt % AlF₃-LMNO, reprinted with permission from ref (118). Copyright 2015 Elsevier. (f) 5 wt % MgF₂-LMNO, reprinted with permission from ref (119). Copyright 2015 Royal Society of Chemistry.

4.1.5. Radio Frequency Magnetron Sputtering

Radio frequency (RF) magnetron sputtering is the technique that argon ions are accelerated by radio frequency (RF) electric field in order to hit a target to sputter. It enables to produce even films and has the advantage of low costs, high efficiency, easy control, and high deposition rates.¹²⁰ RF sputtering is widely used in thin film of LIBs for surface treatment of active materials, anodes and cathodes, current collectors, and components of battery materials.¹²¹

4.1.6. Sol–Gel

The sol–gel method is a wet-chemical process that involves the formation of an inorganic colloidal suspension (sol) and gelation of the sol in a continuous liquid phase (gel) to form a three-dimensional network structure.¹²² Using the sol–gel method, a solid phase is generated by gelation of a colloidal solution. Then gel is dried to form a “dry gel” (xerogel), and in order to stabilize, densify, and eliminate unreacted organic residues, heat treatment is used.²⁸ The sol–gel method is well-known as one of the promising thin-film preparation methods, which has been utilized to prepare various kinds of transition metal oxide thin films to construct thick film rechargeable lithium batteries with low cost and high deposition.^123,124 This sol–gel method can provide thin film with high electrochemical performance and crystal phases involved in the thin film that can be controlled by changing the chemical compositions of the sol.¹²⁴ Characteristics of different coatings are shown in Table 6.

Table 6. Characteristic of Different Coatings Using Sol–Gel Method.

Synthesis method	Particle size	Lattice constant (Å)	Morphology	Surface modification	Capacity (mAh g–1)/ temperature	Capacity retention (Rate/Cycle)	Remarks	ref
sol–gel	865.84 nm	8.17029	polyhedron	CNT	127.8 at 0.5C/25 °C	95.77% at 0.5C/100	-	(125)
	-	8.1746	spherical	F-doped Li4SiO4	142.5 at 0.1C/25 °C	97.8% at 4C/100	2 wt % Li4SiO4	(126)
	5 μm	8.187–8.189	polyhedron	TiO2	113 at 1C/25 °C	99% at 1C/100	-	(127)
	-	-	polyhedron	La2CO3	126.5 at 1C/25 °C	-	1.5 wt % La2CO3	(129)
	-	-	octahedron	La2CO3	112.2 at 0.1C/25 °C	90.5% at 1C/100	2 wt % La2CO3	(130)
	10 nm	-	amorphous	ZrP2O7	120 at 1C/25 °C	80% at 1C/150	1 wt % ZrP2O7	(109)
	200–500 nm	8.1882	polyhedron	FePO4	114.5 at 0.1C/25 °C	102% at 2C/80	1 wt % FePO4	(131)
	-	-	truncated octahedron	YBa2Cu3O7	116.7 at 2C/20 °C	87% at 2C/100 (at 60 °C)	5 wt % YBCO	(132)
	10 nm	-	polyhedron	La0.7Sr0.3MnO3	124 at 2C/25 °C	90% at 2C/100	-	(128)
	-	-	polyhedron	Li4Ti5O12	99 at 0.5C/25 °C	99% at 0.5C/100	3 wt % LTO	(133)
	-	-	polyhedron	LiCoO2	133.6 at 0.1C/25 °C	94.35% at 2C/194	1 wt % LiCoO2	(134)
	-	-	polyhedron	LiNbO3	100 at 10C/25 °C	96.1% at 0.5C/250	1 wt % LiNbO3	(135)
	5 μm		octahedron	Li1.4Al0.4Ti1.6(PO4)3	132.3 at 0.C/25	98.1% at 1C/100	0.5 wt % Li1.4Al0.4Ti1.6(PO4)3	(136)

Ding et al. employed carbon nanotube (CNT) coated on LMNO with a two-step calcinated process. During the synthesis process, CNT plays a crucial role: a physical barrier, size tailoring, adjusting Mn, and crystal forming assistance. This CNT coated LMNO exhibited better cycling stability and rate capability, which results in improved electrochemical performances.¹²⁵ Xu et al. designed F doped Li₄SiO₄ coated LMNO by sol–gel synthesis method. Fluorine dopant in LMNO enables increasing the bonding that can stabilize the structure and improve rate capability and cycle stability. Also, Li₄SiO₄ coating layer performs as a protective layer and superionic conductor. As a result, F doped Li₄SiO₄ coated LMNO significantly displayed better electrochemical performances.¹²⁶ LMNO coated with TiO₂ better capacity retention and smaller polarization gap due to inhibition of SEI formation. At a high temperature after cycling, TiO₂ coated LMNO maintained structural integrity.¹²⁷ Zhao et al. employed La_0.7Sr_0.3MnO₃ as a coating material on LMNO, which exhibited lower surface and charge/discharge resistance and higher lithium diffusion rate. It also prevents active material from HF in the electrolyte during the cycle (Figure 11).¹²⁸

Figure 11.

SEM images of (a) CNT-LMNO, reprinted with permission from ref (125). Copyright 2022 Springer Nature. (b) 5 wt % YBCO-LMNO, reprinted with permission from ref (132). Copyright 2014 Elsevier. (c) La_0.7Sr_0.3MnO₃-LMNO, reprinted with permission from ref (128). Copyright 2012 Elsevier. (d) 3 wt % LTO-LMNO, reprinted with permission from ref (133). Copyright 2013 Elsevier. (e) 1 wt % LiCoO₂-LMNO, reprinted with permission from ref (134). Copyright 2017 RSC. (f) 0.5 wt % Li_1.4Al_0.4Ti_1.6(PO4)₃-LMNO, reprinted with permission from ref (136). Copyright 2020 Elsevier.

4.1.7. Solvothermal

Solvothermal synthesis is a chemical reaction that occurs in a solvent at a temperature higher than the boiling temperature of the solvent in a sealed vessel. Solvothermal can control over the particle shape and size through adjusting the reaction parameters. In this process, if the solvent is water, then it can be called a hydrothermal process.

This process is implemented in a sealed vessel, autoclave, offering a product which can be washed and filtered to acquire the coated sample. This coating method is used in coating morphologies: bulk powders, single and nano crystals, and thin films. In terms of process, the precursors for the coating material used by organic or nonaqueous solvent, are dissolved in solution. In addition, the cathode material that needs to be coated is added as well. The mixture is then heated at a high temperature. Types of solvent, time, temperature, concentration of precursor material and OH^– are key factors that should be considered.^28,137,138 Characteristics of different coatings are shown in Table 7.

Table 7. Characteristic of Different Coatings Using Solvothermal Method.

Synthesis method	Particle size	Lattice constant (Å)	Morphology	Surface modification	Capacity (mAh g–1)/ temperature	Capacity retention (Rate/Cycle)	Remarks	ref
solvothermal	-	-	polyhedron	TiO2	136 at 1C/25 °C	88.5% at 2C/500	-	(143)
	0.5–3 μm	8.168	octahedron	MgO	118 at C/10/25 °C	62% at C/10 to 5C/24	5 wt % MgO	(139)
	-	-	-	graphene	144.8 at 0.1C/25	98.5% at 0.1C/100	2.5 wt % GNR	(140)
	-	-	polyhedron	ZnO	137 at C/3/55 °C	99% at C/3/50	1.5 wt % ZnO	(144)
	0.5–1, 1–3 μm	8.161	-	ZnO	126 at 0.1C/25 °C	92% at 0.1C/20	1.5, 5 wt % ZnO	(141)
	-	8.1614	spherical	CeO2	129.7 at 0.2C/-	98.3% at 1C/106	3 wt % CeO2	(145)
	-	8.1657	polyhedron	SnO2	145.4 at 0.2C/25 °C	75% at 2C/500	2 wt % SnO2	(146)
	-	-	polyhedron	Mn3O4	108 at 10C/55 °C	78% at 1C/100	2.6 wt % Mn3O4	(147)
	-	-	octahedron	V2O5	131.5 at 1C/25 °C	92.2% at 1C/100	5 wt % V2O5	(148)
	-	8.177	spherical	Co3O4	120 at 10C/25 °C	95.8% at 10C/300	0.8 wt % Co	(149)
	7–10 μm	8.1713	spherical	CdO	133.3 at 1C/25 °C	95.2.% at 1C/300	0.4 wt % Cd	(150)
	-	8.16676	octahedron	Al2O3	129 at 1C/25 °C	92.6% at 1C/200	0.5 wt % Al2O3	(151)
	-	-	amorphous	SiO2	130 at 0.5C/55 °C	86% at 0.5C/100	1 wt % SiO2	(152)
	0.5–1 μm	8.1771	spherical	SiO2	133.3 at 0.1C/25 °C	97.5% at 0.1C/100	-	(153)
	-	-	octahedron	RuO2	100 at 1C/25 °C	96.1% at 0.5C/150	0.57 wt % RuO2 with 1 wt % carbon additives	(154)
	1.5 μm	-	spherical	Li4Ti5O12	112.8 at 0.5C/25 °C	93.6% at 0.5C/100	5 wt % LTO	(142)
	-	-	spherical	La2O3/Al2O3(LAO)	124 at 1C/25 °C	94% at 1C/200	4.0 wt % (mLa2O3/mAl2O3 = 3:1)	(155)
	-	-	spherical	Fe2O3	132 at 0.5C/25 °C	98.6% at 1C/100	-	(110)

MgO coated LMNO was presented by Alva et al. They showed MgO coated LMNO exhibited better capacity, Coulombic efficiency, and rate capability at 50 °C, especially MgO calcinated at 800 °C.¹³⁹ Xiong et al. employed sandwich structured graphene sheets (GNR) coated LMNO (GNRs@LMNO@GNRs). They mentioned GNRs enables the fast diffusion of Li ions, decreases electrochemical reaction resistance, improves the conductivity, and suppresses the volume changes, which are the factors that contribute to the electrochemical performances. The design of this LMNO exhibited higher capacity, Coulombic efficiency, and better rate capability and cycle performances.¹⁴⁰ Alcántara et al. showed ZnO coated LMNO using different ratio of Zn (1.5%, 5%) and preparing temperature (700 °C, 800 °C). In consequence, material prepared at 800 °C ZnO coated LMNO improves electrochemical performances. Also, they concluded losing ZnO contributes to incorporation of Zn in the spinel structure and the formation of two cubic phases with different lattice parameters are led by electrochemical extraction of lithium.¹⁴¹ Zhao et al. employed Li₄Ti₅O₁₂ (LTO) as a coating material on LMNO. LTO coating layers helps to suppress the reaction in electrolyte and contribute to small polarization. LTO coated LMNO exhibited high capacity retention and increased Li ion mobility, and it also showed high cyclability at high temperature (Figure 12).¹⁴²

Figure 12.

SEM images of (a) TiO₂-LMNO, reprinted with permission from ref (143). Copyright 2017 Elsevier. (b) CeO₂-LMNO, reprinted with permission from ref (145). Copyright 2017 Elsevier. (c) 5 wt % V₂O₅-LMNO, reprinted with permission from ref (148). Copyright 2015 Elsevier. (d) Fe₂O₃-LMNO, reprinted with permission from ref (110). Copyright 2016 Elsevier. (e) SiO₂-LMNO, reprinted with permission from ref (153). Copyright 2017 American Chemical Society. (f) 0.57 wt % RuO₂-LMNO, reprinted with permission from ref (154). Copyright 2017 Elsevier.

4.2. Effect of Doping

Like other methods to improve electrochemical performances, doping also has been found to be an effective in improving cycling and rate performances. Also, it enables to modify the properties of electrode materials such as changing the structure, composition, morphology and phase transition. It is crucial to select which doping material to use, owing to the substitution of nickel and manganese sites by dopants that promote electronic conductivity. Depending on which material to use as a dopant, LMNO can form an oxygen deficiency (LiNi_0.5Mn_1.5O_4–x) or a nickel deficiency (LiNi_0.5–xMn_1.5+xO_4-δ). In order to compensate for these oxygen and nickel deficiencies, reduction of Mn⁴⁺ to Mn³⁺ occurs. By replacing nickel ions to dopant ions, Mn ions can help to offset the capacity loss. In addition, Mn³⁺ ions act as internal carriers, assisting the hopping conduction mechanism and enhancing the rate of charge transfer.

Several dopants have been reported that they have been shown that it enhances the cycling stability and rate performances by using metal ion (Figure 13), nonmetal ion (Figure 14), and multi metal doping (Figure 15). As a result, the doping of ions affect the cycling stability and electrochemical performances of LiMn_1.5Ni_0.5O₄. Characteristics of different dopants are shown in Table 8.

Figure 13.

SEM images of (a) 0.2Fe-LMNO, reprinted with permission from ref (164). Copyright 2016 Elsevier. (b) Mg-LMNO, reprinted with permission from ref (158). Copyright 2014 Elsevier. (c) 0.06Si-LMNO, reprinted with permission from ref (166). Copyright 2020 Elsevier. (d) 0.03Ti-LMNO, reprinted with permission from ref (168). Copyright 2020 Elsevier. (e) 0.05Na-LMNO, reprinted with permission from ref (170). Copyright 2014 Elsevier. (f) 0.05Ru-LMNO, reprinted with permission from ref (173). Copyright 2015 Elsevier.

Figure 14.

SEM images of (a) 0.01P-LMNO, reprinted with permission from ref (159). Copyright 2015 American Chemical Society. (b) 0.1Cl-LMNO, reprinted with permission from ref (175). Copyright 2014 Elsevier. (c) F-LMNO, reprinted with permission from ref (160). Copyright 2017 Elsevier.

Figure 15.

SEM images of (a) 0.03Cu 0.01Al-LMNO, reprinted with permission from ref (176). Copyright 2020 Elsevier. (b) 0.01Mg 0.02Y-LMNO, reprinted with permission from ref (161). Copyright 2021 Elsevier. (c) 0.03Mg 0.03Si-LMNO, reprinted with permission from ref (177). Copyright 2018 Elsevier. (d) 0.2La 1Ti-LMNO, reprinted with permission from ref (178). Copyright 2019 Elsevier. (e) 0.025Cr 0.025Ti-LMNO, reprinted with permission from ref (179). Copyright 2015 RSC. (f) 0.03Cu 0.03Al 0.03Ti-LMNO, reprinted with permission from ref (162). Copyright 2016 Elsevier.

Table 8. Characteristic of Different Dopants.

Property	Particle size	Lattice constant (Å)	Morphology	Dopants	Capacity (mAh g–1)/ temperature	Capacity retention (Rate/Cycle)	Remarks	ref
metal	3–6 μm	8.1730	octahedron	Co	130.1 at 0.1C/25 °C	92.8 at 1C/100	0.08 Co-LMNO	(156)
	1 μm	8.1750	octahedron	Cr	139.7 at 0.2C/25 °C	97.08% at 0.2C/40	0.15 Cr-LMNO	(163)
	0.737 μm	8.1760	octahedron	B	136.1 at 0.2C/25 °C	83.3% at 3C/500	0.01 B-LMNO	(157)
	-	8.2129	polyhedron	Fe	134 at 0.5C/25 °C	92% at 0.5C/300	0.2 Fe-LMNO	(164)
	-	8.1752	octahedron	Al	125 at 1C/55 °C	84% at 20C/600	0.06 Al-LMNO	(165)
	15 μm	8.1696	spherical	Mg	121.5 at 0.1C/25 °C	92% at 0.1C/80	-	(158)
	-	8.1711	octahedron	Si	135.7 at 0.2C/25 °C	91.5% at 0.2C/200	0.06 Si-LMNO	(166)
	3–5 μm	8.1860	polyhedron	Mo	135.6 at 0.1C/25 °C	90.5% at 0.1C/80	0.05 Mo-LMNO	(167)
	-	8.1880	polyhedron	Ti	127.3 at 10C/25 °C	91.7% at 1C/200	0.03 Ti-LMNO	(168)
	-	-	octahedron	Zr	138.4 at 1C/25 °C	95% at 1C/200	post calcined	(169)
	100–300 nm	8.1630	polyhedron	Na	125 at 1C/25 °C	93% at 1C/100	0.05 Na-LMNO	(170)
	-	8.1820	polyhedron	Cu	137.2 at 1C/25 °C	98% at 10C/100	0.05 Cu-LMNO	(171)
	-	8.1777	octahedron	Y	121.3 at 0.2C/25 °C	-	0.01 Y-LMNO	(172)
	600 nm–1 μm	8.170	octahedron	Ru	133.4 at 1C/25 °C	99.4% at 1C/150	0.05 Ru-LMNO	(173)
	-	8.181	octahedron	Zn	140.4 at 1C/25 °C	95% at 1C/400	0.05 Zn-LMNO	(174)
	-	8.167	polyhedron	Ge	133.4 at 1C/25 °C	84.9% at 1C/1000	0.02 Ge-LMNO	(74)
nonmetal	200–500 nm	8.1854	octahedron	P	142.7 at 0.1C/25 °C	Below 88.5% at 3C/300	0.01 P-LMNO	(159)
	5 μm	8.1913	octahedron	Cl	125.75 at 0.2C/30 °C	99% at 0.2C/40	0.1 Cl-LMNO	(175)
	-	8.2037	polyhedron	F	124.7 at 1C/25 °C	92.4% at 1C/300	F-LMNO	(160)
multimetal	-	8.1781	octahedron	Cu, Al	103.4 at 5C/25 °C	96.55 at 0.25C/100	0.03 Cu 0.01Al-LMNO	(176)
	-	8.1684	truncated octahedron	Mg, Y	133 at 1C/25 °C	98.49% at 1C/100	0.01 Mg 0.02Y-LMNO	(161)
	-	8.1803	truncated octahedron	Mg, Si	122.4 at 0.5C/25 °C	98.86 at 0.5C/100	0.03 Mg 0.03 Si-LMNO	(177)
	3 μm	8.1681	polyhedron	La, Ti	138.1 at 0.1C/25 °C	91% at 0.1C/300	0.5 La 1 Ti-LMNO	(178)
	-	8.1590	octahedron	Cr, Ti	118.7 at 10C/25 °C	102.1% at 1C/100	0.025 Cr 0.025 Ti-LMNO	(179)
	1 μm	8.1761	truncated octahedron	Cu, Al, Ti	137 at 0.1C/25 °C	97.9% at 0.1C/120	0.03 Cu 0.03 Al 0.03 Ti-LMNO	(162)

Yang et al. employed Co as a dopant. Co dopant enables to completely remove NiO impurity and results in better rate capability and capacity retention.¹⁵⁶ A different ratio of boron was used as a dopant where 0.01 ratio of boron doped LMNO showed the best electrochemical performances. Boron doping improves Mn³⁺ ions and improves structural stability, which all contribute to enhance electrochemical performances.¹⁵⁷ Liu et al. investigated different mole percent of Mg dopant in LMNO. Among those, Mg gradient-doped (GD) LMNO exhibited superior electrochemical performances with better discharge capacity and retention rate. It also suppresses the reaction between the electrolyte and cathode and improves Li ion diffusion.¹⁵⁸ Liang et al. designed Ge doped with 4s-2p orbital hybridization LMNO that exhibited long-lasting battery performance. They showed orbital hybridization of Ge 4s and O 2p orbitals contributes to strengthen the oxygen lattice.⁷⁴ Deng et al. introduced nonmetal P doped LMNO. They showed octahedral shape is maintained when less amount of P is doped. On the other hand, when excess amount of P is doped, it has truncated octahedral shape. 0.04 mol ratio of P doped LMNO showed the highest discharge capacity, and 0.01 mol ratio of P doped LMNO exhibited high retention ability. The degree of disordered transitional metal ions in LMNO is increased by the effect of P doping.¹⁵⁹ Nonmetal F gradient doped LMNO heated at different temperature is introduced by Luo et al. Heated at 400 °C F doped LMNO exhibited the best performance with enhanced cycling stability and high rate.¹⁶⁰ Co-doped LMNO with Mg and Y resulted better electrochemical performances presented by Lin et al. Mg ion helps to increase the stability of particle structure, and Y ion helps to broaden the diffusion channel of Li ions and increases the stability of lattice structure. This codoped synergistic effect made LMNO improve electrochemical performance.¹⁶¹ Deng et al. employed Cu, Al, Ti tridoped LMNO. This tridoping suppresses impurity phase, enhances structural stability, and increases electronic conductivity. Ti, Al doping enhances structural stability and Al, Cu doping contributes to electronic conductivity.¹⁶²

4.3. Cathode-Electrolyte Interface (CEI)

The film formed on surface of a cathode is called cathode-electrolyte interface (CEI) which results in a loss of contact between cathode particles.¹⁸⁰ This CEI is formed at end of the first charge and discharge steps of the battery cell.¹⁸¹ To be specific, when cathode electrochemical potential is below the energy level value of the electrolyte’s highest occupied molecular orbital (HOMO), then oxidation of the conventional LiPF₆/organic carbonate-based electrolyte occurs. CEI can be formed through the electron transfer from the HOMO to the cathode.⁴⁴ Throughout other studies, initial CEI formation can be summarized as the following: (i) Carbonyl group of EC first absorbs on surface of the transition metal at the cathode surface. (ii) CEI formation occurs at uncovered transition metal atoms (iii) CEI formation is provoked especially at high cell potential. (iv) Transition metal coordination additives and cathode coatings can suppress the electrolyte decomposition on the cathode surface.^44,182−184 Formed CEI has a characteristic of enabling lithium-ion transfer, suppress transition metal dissolution, reduce the interfacial resistance, and inhibit structural change and side reaction between electrode and electrolyte.

Numerous researches have been conducted to figure out the CEI’s chemical composition and determination through various investigation methods such as XPS^{58,180,185,186} and EIS.^58,187 Duncan et al. used XPS surface measurements to study CEI in LMNO/Li₄Ti₅O₁₂ (LTO) cell with using conventional carbonate electrolytes (LiPF₆/LiBF₄) (Figure 16). They found the following: (i) All electrodes are covered with salt and organic species. (ii) Alkyl carbonates (ROCO₂Li) and polyethers compose the organic layer. Also, Li₂CO₃ was detected in the electrode that was stored at 60 °C without cycling, whereas polycarbonate, represented as –[OCO2-]_x–, was found on the electrode that was stored or cycled at 60 °C. (iii) Depending on which electrolyte to use, Li_xPF_yO_z or Li_xBF_yO_z composes the salt layer, along with LiF that is present on all cells.¹⁸⁷ In another study, Yoon et al. used Al₂O₃ coating as a HF scavenger that made CEI relatively stable. In this study, EIS spectra showed that resistance of the CEI on the LMNO, coated with Al₂O₃, was relatively stable, but uncoated LMNO displayed sharp decrease (Figure 17).⁵⁸

Figure 16.

(Color online) F 1s and P 2p spectra of as-synthesized LMNO, EC:DEC 3:7 + 1 M LiPF₆ electrolyte, a) stored 24 h at 60 °C, b) stored at 0% SOC for 60 days at 60 °C, c) stored at 100% SOC for 60 days at 60 °C, d) cycled 100 times at room temperature, e) cycled 100 times at 60 °C f) cycled 100 times at room temperature, LTO negative electrode, reprinted with permission from ref (187). Copyright 2011 Electrochemical Society.

Figure 17.

(a) A representative EIS data and the fitting curve. (b) EIS spectra obtained at 4.2 and 4.8 V for both the Al₂O₃ coated and uncoated LNMO electrodes. (c) Variations in CEI resistance with increasing potential, reprinted with permission from ref (58). Copyright 2021 Elsevier.

5. Conclusion and Perspectives

This Review summarizes the four major aspects of LiMn_1.5Ni_0.5O₄ (LMNO) cathode material which all influence the characteristics and electrical performances: (i) coating strategy (ii) doping strategy, (iv) electrolyte, and (v) oxygen deficiency. As discussed earlier, LMNO has various spinel phases that have different characteristics, which depend on different temperature or oxygen deficiency, and it transforms from ordered phase, P4₃32, to disordered phase, to rock salt phase. Furthermore, during cycling, intercalation/extraction of lithium ions can be detected and leads to unwanted side reaction at the LMNO/electrolyte interface. Therefore, different methods have been proposed to suppress the unwanted side reaction through both surface modification and doping. By using various synthesis method (i.e., chemical vapor deposition process (CVD), dry coating, atomic layer deposition (ALD), coprecipitation, radio frequency magnetron sputtering, sol–gel, solvothermal) with different materials, using different dopants, adding additive in electrolyte, and controlling oxygen radicals, it all made LMNO possible to enhance the electrochemical performances. Likewise, these factors have been proven to play a crucial role in LMNO. For the further development, herein we propose our perspectives:

• (1)Through surface modification, doping, surface control in the half-cell configuration, electrochemical performance improvements are shown. However, when it comes to full-cell configuration with graphite anode, capacity fade still remains as a problem. To be specific, gradual consumption of Li ions in SEI layer on the graphite anode affects capacity fade. Therefore, a different approach is needed to design the cell, and which materials should be used as an anode should be considered.
• (2)Through many studies, it has proven that LMNO becomes stable with variety of methods in the room temperature. However, when the half cell is tested at low or high temperature, it becomes very unstable. As a result, studying different characteristics of the element is necessary.
• (3)According to the experimental results done in electrolytes, electrolyte decomposition is one of the problems that has to be considered. Especially, since LMNO operates at high voltage (5 V), it is necessary to find the proper electrolyte that corresponds with the LMNO. For instance, studying alternative liquid electrolytes that have less reaction with metal ion dissolution or solid electrolytes with high lithium ion conductivity and compatible interface is needed.
• (4)For the stability of LMNO, understanding the phase transition is necessary. Through different formation of LMNO, it has different characteristics. By understanding these factors, it is possible to control the LMNO phases. For example, since LMNO is sensitive to temperature, deciding on which temperature to sinter can affect the stability. Another factor is proton intercalation which affects the stability. During the decomposition reaction, intercalation of Li ion decreases and triggers proton intercalation and surface reconstruction in spinel structure. Therefore, a proton free mechanism should be considered.

In conclusion, among the many strategies that led to better electrochemical performances for LMNO, we believe that the doping strategy is more likely to be commercialized. Besides using either metal/nonmetal or multimetal ions as mentioned in section 4.2, high entropy cathodes are emerging as a new class of material. High entropy, using multiple element (at least five) to make a single phase system, is used in various fields, such as catalysis, energy storage, and others.¹⁸⁸ Owing to its outstanding properties, such as structure stability and strength, in recent years, this strategy has been extended to oxide cathode materials.^189,190 Even though there are few reported studies about high entropy cathode materials, we believe there is no doubt that this strategy will not only contribute to optimized electrochemical performances but also makes feasible the commercialization of batteries in the next generation.

The authors declare no competing financial interest.