Recently Li–rich cathode materials such as layered LiNi_1−xM_xO₂ (0.1 < × < 0.5, M = Mn, Co., Al, etc.) have been widely investigated owing to their high specific capacity, excellent rate capability, low cost, and high output voltage (Hu et al., 2013; You and Manthiram, 2017; Nayak et al., 2018; Sun H. H et al., 2021). Generally, layered structured cathodes can be charged to 4.5 V and deliver an improved specific discharge capacity (Shi et al., 2018). However, the severe capacity decay and safety issues, especially at elevated temperature and high upper cutoff voltage, hindered their commercial application. Many mechanisms elucidating the deterioration of electrochemical performance during charge/discharge for layered structured cathodes have been proposed (Jung et al., 2014; de Biasi et al., 2019; Liu et al., 2020; Xu et al., 2020; Wang Y et al., 2020). The inherent defects, dissolution of Mn⁴⁺/Ni⁴⁺ transition metals, gas release, phase transitions, volume change, electrolyte decomposition and corrosion, and formation of inactive interphases have imposed a negative impact on the electrochemical performance of the cathode materials. Notably, most decay mechanisms emphasize the unstable interface of the cathode and electrolyte. For instance, the voltage drop is associated with the formation of an insulating solid electrolyte interphase due to the oxidation decomposition of liquid electrolyte; the residual LiOH or Li₂CO₃ on the particle surface will lead to a deterioration of the specific discharge capacity, especially at high rates. Additionally, phase transition combined with volume contraction and expansion during (de) lithiation, the reduction of Ni⁴⁺ in a highly delithiated state, and oxygen loss may cause cracking and destroy the surface morphology of the cathode material (de Biasi et al., 2019; Sharifi-Asl et al., 2019; Sun J et al., 2021). Consequently, interior energy density and cycle life were obtained.

To solve these issues, surface modifications, including element doping and constructing a coating for the cathode material, have been widely employed to enhance the structure stability (Zhang et al., 2015; Kalluri et al., 2017; Xia et al., 2018; Nisar et al., 2020; Herzog et al., 2021a; Lin et al., 2021; Yan et al., 2021). Coatings such as Al₂O₃, TiO₂, Li₃PO₄, ZnO, AlPO₄, LiAlO₂, and Li₂ZrO₃ are an effective protective layer on the cathode particles for minimizing the surface side reactions and improving the cycle stability of Ni–rich layered cathodes. However, coatings with poor electric/ionic conductivities often impose an additional electrical and ionic transport resistance to cathode materials, which is detrimental to the Coulombic efficiency, specific charge–discharge capacity, and rate capability at high temperatures/upper cutoff voltages.

Li₄Ti₅O₁₂, having a high Li⁺ ionic conductivity of 10⁻⁶ S cm⁻¹ and superb structural stability, has been investigated as a desirable functional surface coating to facilitate Li⁺ transport and enhance the electrochemical properties of electrode materials (Yan et al., 2021). Li₄Ti₅O₁₂ has the identical structure of spinel LiMn₂O₄, showing superior ability in the balance of surface protection and charge transfer during charge/discharge cycles (Yi et al., 2015; Zhou et al., 2016; Jia et al., 2018).

Effective strategies used to construct the surface coating layer on the cathode materials include subjecting the cathode materials to atomic layer deposition, radio–frequency (RF) magnetron sputtering, wet chemistry methods, chemical vapor deposition, followed by subsequent heating process (Liu et al., 2019). However, these methods are excessively subjected to expensive deposition instruments and low-yielding products, which show apparent disadvantages of high cost. Cost–effective and environmental–friendly dry powder coating approach is one of the widely studied methods and has been successfully used in industry to deposit Al₂O₃, TiO₂, LiAlO₂, and Li₄Zr₃O₈ coatings (Wang et al., 2018; Jiang et al., 2020; Herzog et al., 2021b). Ionic conductive Li₄Ti₅O₁₂ layer on the LiCoO₂ and LiNi_xMn_yCo_zO₂ surface showed excellently high–voltage stability, effectively suppressing structural degradation and facilitating lithium–ion diffusion for the LiCoO₂ cathodes (Wang C.-W et al., 2020; Herzog et al., 2021c). In this work, Li₄Ti₅O₁₂ coating on the Li_1.1Ni_0.35Mn_0.55O₂ particles was fabricated via a dry powder coating method, followed by a heating treatment during which nanosize TiO₂ in situ reacts with residual Li₂CO₃ on the surface of the as–prepared Li_1.1Ni_0.35Mn_0.55O₂ particles. Electrochemical performances are studied in both liquid and solid–state lithium batteries. It is found that the Li₄Ti₅O₁₂ coating enhances the rate capability and capacity retention of the Li_1.1Ni_0.35Mn_0.55O₂ electrodes in LiPF₆ electrolytes at various upper cutoff voltages and solid–state batteries at elevated temperatures.

Figure 1 shows the SEM images of Li_1.1Ni_0.35Mn_0.55O₂ before and after the coating treatment, respectively. The obtained Li_1.1Ni_0.35Mn_0.55O₂ particles show spherical morphology with diameters ranging from 3 to 9 μm. The surface of the secondary particles is rough and consists of highly agglomerated primary crystals with sizes of ∼300 nm. The morphology of Li_1.1Ni_0.35Mn_0.55O₂ particles maintains in good spherical shape after coating treatment, and no noticeable difference can be seen from the low–magnification, whereas, in the high–magnification SEM image (Figures 1B,D), the surface of the Li_1.1Ni_0.35Mn_0.55O₂ particles was smoother, and a thin of amorphous coating could be observed. ICP-OES results indicate that the Ti content in Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode is approximately 1.2 wt%. The calculated weight percentage of Ni and Mn of as-prepared Li_1.1Ni_0.35Mn_0.55O₂ cathode is 21 and 31 wt%, respectively.

To investigate the influence of the coating materials on the cycling performance of Li_1.1Ni_0.35Mn_0.55O₂. The cells using liquid electrolytes were assembled to evaluate the rate capabilities first. As shown in Figure 2, rate measurement is performed at different rates; as the rate increases, the capacities of the samples decrease. The initial discharge capacities of pristine Li_1.1Ni_0.35Mn_0.55O₂ cathode at the incremental rates are 116.5, 115.9, 107.4, 97.7, 84.4 and 76.1 mAh g⁻¹. The Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode delivers a capacity of 123.7, 124.8, 117.2, 109.9, 97.2 and 88.1 mAh g⁻¹ at 0.1, 0.2, 0.5, 1, 2 and 3 C, respectively. This reduced polarization was beneficial from the conductive Li₄Ti₅O₁₂ layer coated on the surface of LiNi_0.5Co_0.2Mn_0.3O₂ particles, which promoted the kinetics of Li⁺ extraction/insertion. (Thackeray and Amine, 2021). Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode exhibits a superior long–term cycling performance for 100 cycles at the 1 C rate, the specific capacity of Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode shows an increase in the first several cycles. This is beneficial from the protective layer that provides an activation of the cathode materials. The Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode shows a specific discharge capacity of 113.5 mAh g⁻¹ after 100 cycles. However, the specific discharge capacity of pristine Li_1.1Ni_0.35Mn_0.55O₂ cathode was 95.2 mAh g⁻¹ after the identical operation process. It is considered that the coating layer of spinel Li₄Ti₅O₁₂ provides stable interfacial reaction kinetics for Li_1.1Ni_0.35Mn_0.55O₂ cathode, which decreases the loss of irreversible capacity.

It is known that the layered Li–rich cathode materials exhibit a large capacity fade at high voltages because of dissolution of the transition metals and the attack by liquid electrolytes. In order to study the effect of coating for the high–voltage stability of the cathode materials, the electrochemical performance of pristine Li_1.1Ni_0.35Mn_0.55O₂ and Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode at 0.1 C with various upper cutoff voltages were also evaluated. As shown in Figure 3, higher discharge capacities were obtained for all samples with increased upper cutoff voltages. For instance, in the charge/discharge voltage range of 2.0–4.4 V (Figure 3A), the discharge capacities of the Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ and pristine Li_1.1Ni_0.35Mn_0.55O₂ are 141.9 and 133.6 mAh g⁻¹, respectively. The discharge capacities of 177.1, 208.6, 214.2, and 228.6 mAh g⁻¹ are obtained with upper cutoff voltages of 4.5, 4.6, 4.7, and 4.8 V for Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode, whereas the pristine Li_1.1Ni_0.35Mn_0.55O₂ cathodes only deliver 163.0, 195.7, 200.0, and 211.7 mAh g⁻¹, respectively. The initial irreversible capacities of Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode were reduced in comparison with the pristine Li_1.1Ni_0.35Mn_0.55O₂ under increased upper cutoff potential. These results demonstrated that the Li₄Ti₅O₁₂ coating on the Li_1.1Ni_0.35Mn_0.55O₂ coating plays a barrier to protect the cathode materials from the transition metal dissolution at a high delithiation state; in addition, electrolyte corrosion and the volume expansion/contraction of the cathode were alleviated owing to the superb integrity of the Li₄Ti₅O₁₂ coating material.

In Figure 4A, the Nyquist plots of the cells at the 2nd and the 50th cycle are compared. The semicircle in high frequency represents the solid electrolyte interface resistance (R_sf), which includes the lithium ions transfer through the electrode material, the coating layer, and solid electrolyte interphase during cycling. The semicircle in low frequency is assigned to the charge transfer resistance at the interface of cathode/electrolyte (R_ct) (Reddy et al., 2007). Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode shows initial smaller resistances than the counterpart; after the 50th cycle, all the resistances grow as a result of cycling degradation. The smaller increment of Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode than pristine cathode reflects less side reaction at the electrode/electrolyte interface, reflecting the inhabitation of the excessive formation of solid electrolyte interfaces (SEI) and cathode electrolyte interfaces (CEI) between the electrolyte and the electrodes. These results demonstrated that the coating inhibits the decomposition of electrolyte by the reductive lithium metal and high oxidative cathode during cycling (Zha et al., 2022). The ionic conductive Li₄Ti₅O₁₂ alleviates the increase in impedance and substantial cycling deterioration.

The ex–situ XRD patterns of the Li_1.1Ni_0.35Mn_0.55O₂ electrodes after cycles were recorded. The main diffraction peaks can be indexed into the layered α–NaFeO₂ structure with a space group of R3̅m. No new phases were identified for the sample with lithium–and titanium–containing coating, which may be attributed to the small amount of lithium–and titanium–containing coating or amorphous state under the detection revolution of XRD (Zhang et al., 2013; Ahaliabadeh et al., 2021). As shown in Figures 4C,D, the (003) and (101) peaks of the pristine sample shifted slightly toward the lower angle, reflecting the destruction of the crystal structure of the pristine sample during the charge and discharge process. In contrast, the Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ cathode restrained good structure stability during cycling.

SEM images of the Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ and Li_1.1Ni_0.35Mn_0.55O₂ cathodes after cycling are performed and shown in Figure 5. The surface crack could be seen from the Li_1.1Ni_0.35Mn_0.55O₂ cathodes after cycling, while the Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ presents the original spherical morphology (Figures 5A,B). The elemental mapping displayed in Figure 5E confirms that Ti is uniformly dispersed on the particles. The results demonstrated that the Li₄Ti₅O₁₂ plays a significant role in protecting the Li_1.1Ni_0.35Mn_0.55O₂ particles from the structure cracks and the electrolyte corrosion.

In addition, the cycling stability of the Li_1.1Ni_0.35Mn_0.55O₂ electrodes with/without coating were investigated in the all–solid–state lithium batteries with polymer electrolytes. The Nyquist plots in Figure 6A exhibit a smaller total resistance of the all–solid–state using Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrode. Figure 6B shows the initial galvanostatic curves of Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes in all–solid–state batteries cycling at 0.05, 0.1, 0.2 mA cm⁻² in the voltage range of 2.8–4.3 V at 60°C. The initial discharge capacities of Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes are 151, 138, and 123 mAh g⁻¹, respectively.

The rate performances of pristine and Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes are compared in Figure 6C. It reveals that the Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes delivers higher discharge capacities compared with those of pristine electrode at 0.05, 0.1, 0.2 mA cm⁻² for 10 cycles, indicating that Li₄Ti₅O₁₂ coating effectively improves the rate performance of Li–rich material. The pristine and Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes exhibit the Coulombic efficiency of 98.5 and 99.4%, respectively. Furthermore, the discharge capacity can be recovered entirely when the current density is back to 0.1 mA cm⁻², implying that the Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrode has desirable electrochemical reversibility and structural stability in all–solid–state lithium batteries. The capacity retention of Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes in the all–solid–state battery 89.3% at a current density of 0.1 mA cm⁻² for 60 cycles. The improved discharge capacity and cycle stability of Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes in the solid–state batteries can be ascribed to the high Li⁺ ionic conductive Li₄Ti₅O₁₂ coating.

The electrochemical properties of pristine and Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes at high upper cutoff voltages and solid–state batteries were compared at room temperature and 60°C. The experimental results demonstrate that the Li₄Ti₅O₁₂ coating layer is effective in stabilizing the Li_1.1Ni_0.35Mn_0.55O₂ crystal structure and providing fast lithium transfer at the electrode/electrolyte interface during charge/discharge cycles. The improved structural integrity and enhanced ionic conductivity of the Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrodes show improved cycling performance at high upper cutoff voltages. When assembled in an all–solid–state battery with polymer electrolyte, the Li₄Ti₅O₁₂–coated Li_1.1Ni_0.35Mn_0.55O₂ electrode still exhibits a better cyclability than the pristine Li_1.1Ni_0.35Mn_0.55O₂ electrode, demonstrating attractive application in electrode material design and optimization for LIBs.