A Facile Li2TiO3 Surface Modification to Improve the Structure Stability and Electrochemical Performance of Full Concentration Gradient Li‐Rich Oxides

Full concentration gradient lithium‐rich layered oxides are catching lots of interest as the next generation cathode for lithium‐ion batteries due to their high discharge voltage, reduced voltage decay and enhanced rate performance, whereas the high lithium residues on its surface impairs the structure stability and long‐term cycle performance. Herein, a facile multifunctional surface modification method is implemented to eliminate surface lithium residues of full concentration gradient lithium‐rich layered oxides by a wet chemistry reaction with tetrabutyl titanate and the post‐annealing process. It realizes not only a stable Li2TiO3 coating layer with 3D diffusion channels for fast Li+ ions transfer, but also dopes partial Ti4+ ions into the sub‐surface region of full concentration gradient lithium‐rich layered oxides to further strengthen its crystal structure. Consequently, the modified full concentration gradient lithium‐rich layered oxides exhibit improved structure stability, elevated thermal stability with decomposition temperature from 289.57 °C to 321.72 °C, and enhanced cycle performance (205.1 mAh g−1 after 150 cycles) with slowed voltage drop (1.67 mV per cycle). This work proposes a facile and integrated modification method to enhance the comprehensive performance of full concentration gradient lithium‐rich layered oxides, which can facilitate its practical application for developing higher energy density lithium‐ion batteries.

Full concentration gradient lithium-rich layered oxides are catching lots of interest as the next generation cathode for lithium-ion batteries due to their high discharge voltage, reduced voltage decay and enhanced rate performance, whereas the high lithium residues on its surface impairs the structure stability and long-term cycle performance.Herein, a facile multifunctional surface modification method is implemented to eliminate surface lithium residues of full concentration gradient lithium-rich layered oxides by a wet chemistry reaction with tetrabutyl titanate and the post-annealing process.It realizes not only a stable Li 2 TiO 3 coating layer with 3D diffusion channels for fast Li + ions transfer, but also dopes partial Ti 4+ ions into the sub-surface region of full concentration gradient lithium-rich layered oxides to further strengthen its crystal structure.Consequently, the modified full concentration gradient lithium-rich layered oxides exhibit improved structure stability, elevated thermal stability with decomposition temperature from 289.57°C to 321.72 °C, and enhanced cycle performance (205.1 mAh g −1 after 150 cycles) with slowed voltage drop (1.67 mV per cycle).This work proposes a facile and integrated modification method to enhance the comprehensive performance of full concentration gradient lithium-rich layered oxides, which can facilitate its practical application for developing higher energy density lithium-ion batteries.
the LLOs crystal structure. [27]Consequently, the Li 2 TiO 3 modification remarkably alleviates the detrimental influence of lithium surface residues, improves the crystal structure stability, facilitates the Li + ions transfer of LLOs, and eventually enhances the comprehensive electrochemical performance and thermal stability of LLOs.This study offers an effective and facile strategy to enhance the stability of Ni-rich surface structure of LLOs, which fulfills the improvement of electrochemical performance of LLOs and facilitates the practical application of high energy density cathode materials.

Morphology and Crystal Structure
The scanning electron microscopy (SEM) images of LLO precursor and the as-prepared LLO cathode powders are shown in Figure S1, Supporting Information.The precursor possessed the spheroid morphology with a particle size of ~10 μm (Figure S1a, Supporting Information).After lithiation, the LLO was still composed of nanosized primary grains and inherited the similar particle size of precursor, but exhibited a porous and loose morphology due to the gas generation during the high-temperature annealing process, as shown in Figure S1b, Supporting Information.Figure 1a,b displays the crosssection line profiles of LLO-T2, where the Ni element was increased from the inner core to out shell, while the Mn, Co elements were gradually decreased.The Ti element was not detected probably due to the trace content, indicating that the surface modification had no influence on the TM ions concentration gradient.Figure 1c-g further displays the morphology and transition metal elements distribution of LLO-T2.It shows that LLO-T2 particles turned to be dense and compact after surface modification, and the corresponding energy dispersive spectroscopy (EDS) maps manifested the Ni, Co, Mn, and Ti elements that were uniformly distributed on the surface of LLO-T2.
The crystal structure of LLOs is characterized by X-ray diffraction (XRD) in Figure S2a, Supporting Information.It was shown that all materials possessed the high crystallinity with strong layered structure diffraction peaks (R-3m space group) and weak monoclinic structure diffraction peaks (C2/m space group) at 20-25°. [28]Compared with LLO, several weak diffraction peaks locating at ~35.86°and ~43.59°w ere detected after surface modification, and the peak intensity gradually enhanced as the coating content increased, which was indexed to the Li 2 TiO 3 phase (C2/c space group, PDF#33-0831), [26] manifesting the Li 2 TiO 3 was successfully coated on the surface of LLO.The enlarged patterns of (003) diffraction peak are displayed in Figure S2b, Supporting Information, which slightly shifted to a lower angle position after Li 2 TiO 3 modification, indicating that partial Ti 4+ ions were doped into the LLO structure for the enlarged lithium interlayer spacing. [29,30]The high-resolution transmission electron microscopy (HRTEM) characterization results of LLOs before and after Li 2 TiO 3 modification are displayed in Figure 1h,i.Clearly, both the surface region and bulk region of LLO possessed layered structure with interplanar crystal spacing of 4.71 Å, corresponding to (003) plane, [26,31] as shown in Figure 1h.After surface modification, the bulk region (ii) of LLO-T2 still kept the same crystal structure but with a slightly enlarged lattice of 4.76 Å, further confirmed the partial Ti 4+ ions doping to lithium layer at the subsurface region, as shown in Figure 1i.In addition, a ~6 nm uniform coating layer with an interplanar crystal spacing of 2.50 Å was detected in the surface region (i), which was indexed to the (−131) plane of monoclinic structure (C2/c space group).The corresponding Fast Fourier Transform (FFT) patterns also confirmed the existence of Li 2 TiO 3 phase on the surface of LLO-T2.Besides, the scanning transmission electron microscope (STEM) characterization with selected area electron diffraction (SAED) of LLO-T2 was also performed as shown in Figure S3, Supporting Information, in which the bulk region of LLO-T2 was indexed into rhombohedral layered phase (PDF#00-009-0063), while at the surface region, beside the rhombohedral layered phase, a monoclinic phase with crystal planes of (1, −3, −5), The surface chemical environment variation of LLOs was further analyzed by X-ray photoelectron spectroscopy (XPS) spectra.Figure 2a displays the O1s spectra, the peaks at ~529.5 and 532.0 eV are the characteristic feature of Metal-O and oxygenated deposited species (LiOH/Li 2 CO 3 ), respectively. [7,32]After modification, the surface lithium residues content was decreased from 48.59% to 21.07%.The residual oxygenated deposited species on the LLO-T2 electrode surface was due to the interaction between the active material, PVDF, NMP, and Super P during the cathode slurry preparing process.The C1s spectra of LLO and LLO-T2 were compared as shown in Figure 2b.The C-C peak (~284.7 eV) was related to conductive carbon Super P, the C-O (~285.9eV) and C=O peaks (~288.0eV) were corresponded to the surface carbonate species, like Li 2 CO 3 or ROCO 2 Li, while the C-F peak (~290.7 eV) was originated from the binder agent of PVDF. [32,33]learly, the surface carbonate species content was lower in LLO-T2, for the decreased intensity of C-O and C=O peaks of LLO-T2 when compared with that of LLO, indicating the enhanced air-surface stability after Li 2 TiO 3 modification.Figure 2c shows the Ti 2p spectra of LLO-T2 sample with detection depth of 0, 7, and 14 nm.The main peaks locating at ~459 and 464.6 eV were assigned to Ti 2p 3/2 and Ti 2p 1/2 , respectively, in good line with the Ti 4+ of Li 2 TiO 3 . [24,26]The decreased intensity of Ti 2p spectra with etching depth illustrated the Ti element mainly distributed on the surface region of LLO-T2.Ti 2p signal appeared at the depth of 14 nm, which exceeded the thickness of Li 2 TiO 3 coating layer (~6 nm as shown in the HRTEM image), suggesting that the Ti 4+ ions were successfully doped into the sub-surface region of LLO-T2.Besides, the spectra of Mn 2p and Ni 2p are further displayed in Figure S4, Supporting Information, in which the Mn 2p spectra of both samples were only consistent with the Mn 4+ spectra peaks. [34]The Ni 2p spectra were composed of both Ni 3+ spectra peaks and Ni 2+ spectra peaks, [34,35] but the LLO and LLO-T2 samples possessed the same Ni 2+ (61%) and Ni 3+ (39%) content.Thus, we could conclude that the surface modification by Li 2 TiO 3 did not change the valence state of Mn and Ni ions on the surface.Furthermore, the surface lithium residues content detected by the acid-base titration method is presented in Table 1.The surface lithium residues content was reached up to 8931 ppm for LLO, but decreased to 936 ppm for LLO-T2 after surface modification.Therefore, it can be concluded that the Li 2 TiO 3 modification strategy not only effectively eliminated the surface lithium surface residues but also simultaneously realized the Li 2 TiO 3 surface coating and Ti 4+ sub-surface doping to LLOs.

Electrochemical Performance
The electrochemical performance of LLOs was evaluated by LLO||Li coin cells.Figure 3a,b and Figure S5, Supporting Information show the charge-discharge profiles of LLOs from the 1st to the 150th cycle (25 °C).During the first charging process (0.1 C), the slope region I below 4.4 V was related to the de-intercalation of Li + from LiTMO 2 structure accompanied by the TM ions oxidation (Ni 2+ /Ni 4+ , Co 3+ / Co 4+ ). [15]The region II (~4.4 V plateau) was related to Li 2 MnO 3 activation process together with the irreversible Li + loss and oxygen evolution. [36,37]Specifically, the LLO exhibited an initial discharge capacity of 249.2 mAh g −1 with the initial Coulombic efficiency (ICE) of 72.94%, while the LLO-T2 displayed an elevated discharge capacity of 256.0 mAh g −1 with a slight lower ICE of 71.46%.After 150 cycles at 0.5 C rate, the average discharge voltage decay of LLO was 1.74 mV per cycle, but decreased to 1.67 mV per cycle for LLO-T2.Figure 3c,d shows the CV curves in the range of 2.0-5.0V with a scan rate of 0.5 mV s −1 .During the first cycle, the O1 peak at ~4.2 V was attributed to the oxidation of Ni 2+ to Ni 4+ and Co 3+ to Co 4+ , and the O2 peak at ~4.7 V was related to the Li 2 MnO 3 activation process, which were related to regions I and II as shown in Figure 3a,b, respectively. [38]oticeably, after surface modification, the peak intensity of O2 was obviously reduced, indicating the limited irreversible oxygen loss.During the reverse process, the R1 peak at ~3.0 V was attributed to the reduction of Mn 4+ to Mn 3+ ions. [38]The intensity of R1 peak was much stable after Li 2 TiO 3 modification, indicating the enhanced structure stability of LLO-T2 for the restrained John-Teller effect of Mn 3+ . [39,40]The R2 peak at ~3.70 V was corresponded to the reduction of Ni 4+ to Ni 2+ , while the R3 peak at ~4.20 V was attributed to the reduction of Co 4+ to Co 3+ . [15,38] The enhanced cycle stability and rate capacity of LLO-T2 were ascribed to the positive surface modification of Li 2 TiO 3 , which not only formed the Li 2 TiO 3 coating layer to protect the fragile interface of LLOs from being electrolyte corrosion but also built 3D channels to facilitate the Li + ion diffusion.Besides, the partial penetrated Ti 4+ ions in the sub-surface region of LLOs also stabilized the crystal structure for the strong Ti-O band and enlarged Li + interlayer spacing, thus benefiting the cycle stability and rate performance of materials. [27]he electrochemical impedance spectra (EIS) test was conducted to explore the impedance and electrochemical kinetics variation of LLOs, as shown in Figure 4d.The R b , R f , R ct , and W o in the equivalent circuit represented the bulk, surface film, charge transfer, and Warburg resistance respectively.The corresponding EIS results are summarized in Figure 4e and  Table S1, Supporting Information.During the 3rd cycle, the R f and R ct dominated the resistance together.The R ct became the main contributor after 50 cycles and turned into the sole dominator after 100 cycles for all LLOs due to the gradually increased detrimental side-reaction between cathode and electrolyte.The R ct for LLO gradually increased from 51.53 (3rd cycle) to 75.99 (50th cycle) and 109.90Ω (100th cycle), while the LLO-T2 only increased from 39.23 (3rd cycle) to 52.3 (50th cycle) and 80.37 Ω (100th cycle), indicating the enhanced interface stability of LLO due to Li 2 TiO 3 layer.The Li + ion diffusion coefficient (D Li þ ) was also calculated according to the Equations (S1)-(S3), Supporting Information. [15,20]The calculated D Li þ shown in Figure 4f displays that LLO-T2 exhibited higher Li + diffusion coefficient (D Li þ = 1.77 × 10 −14 cm 2 s −1 ) than others (D Li þ = 3.02 × 10 −15 cm 2 s −1 , 1.03 × 10 −14 cm 2 s −1 , and 6.89 × 10 −15 cm 2 s −1 , for LLO, LLO-T1, and LLO-T3, respectively).

Materials Structure Stability
The in situ XRD experiments of LLO and LLO-T2 were performed in the first two cycles in the working voltage of 2.0-4.7 V to study the structure stability of cathodes during electrochemical process, as shown in Figure 5.[43] At the charging plateau (~4.4 V), the (003) peak and intensity changed mildly, indicating the slight variation of c-parameter during the oxygen activation process.When the voltage increased to 4.7 V, the (003) peak position gradually shifted to the high angle with decreased peak intensity, indicating the drastic drop of c-parameter for the irreversible oxygen loss and TM migration.In the first discharge process from 4.7 to 3.6 V, the (003) peak position shifted quickly to low-angle region, corresponding to the reinsertion of Li + to TM layers.When further discharged to 2.0 V, the (003) peak reversely back to higher angle position, related to the decreased electrostatic repulsion for the Li + insertion to lithium layers again.For the (101) peak, it was gradually shifted to the higher angle position during the initial charge process, then kept relatively steadily for the following charge process.In the discharge process, the (101) peak reversely shifted back to lower angle position again.This change was reported to be related to the reversible Li + ions extraction/insertion process in the LLO structure. [43]A similar change tendency of (003) and (101) peak was also observed for the LLO-T2 as in Figure 5b, but the peak position and intensity variation was obviously limited, manifesting the enhanced structural stability of LLO during the charging-discharging process after Li 2 TiO 3 surface modification.Figure 5c,d explores the thermal stability of LLOs before and after Li 2 TiO 3 surface modification.Before testing, all samples were charged to 4.8 V with constant current (0.1 C) mode and followed constant voltage (4.8 V) charging mode to set the LLOs at an unstable status.Then, the differential scanning calorimetry (DSC) testing was conducted with a heating rate of 10 °C min −1 from 30 °C to 375 °C under the N 2 atmosphere.The DSC results indicated that all LLOs showed the exothermic peaks between 200 °C and 350 °C, which was related to the thermal decomposition of charged LLOs.It was noticed that the onset temperature of exothermic peaks gradually elevated while the thermal release was reduced as the Li 2 TiO 3 content was gradually increased.Specifically, the onset temperature was increased from 289.568 °C (LLO) to 311.056 °C (LLO-T1), 321.723 °C (LLO-T2), and 332.194 °C (LLO-T3), but the corresponding thermal release decreased from 255.844 to 173.222, 156.877, and 124.471J g −1 .

Materials Postmortem Analysis
Figure 6a-c shows the XPS characterization of LLO and LLO-T2 electrodes after 150th cycles.When compared with the samples before cycling, the O1s XPS results of both cycled LLO and LLO-T2 displayed the decreased metal-O intensity but increased oxygenated deposited species intensity for the accumulated side reaction between the cathode and liquid electrolyte.It has to be noted that the LLO-T2 cathode still possessed a higher metal-O content but lower oxygenated deposited species content on the surface, indicating the restrained interface reaction and stabilized LLO surface structure after Timodification.The lower Mn 3+ and higher Ni 2+ content of cycled LLO-T2 than that of cycled LLO also manifested the sluggish surface structure degradation of LLO after Ti-modification.Figure 6d further shows the Fourier transform infrared spectrometer (FTIR) characterization results of LLO and LLO-T2 electrodes before and after 150th cycles.For both samples before cycled, the bands at 540 and 613 cm −1 were assigned to the TM-O stretching in MO 6 octahedrons in cathode.After 150th cycled, the TM-O bands intensity was gradually decreased, implying the irreversible crystal structure transformation.The bands at ~839 and ~878 cm −1 existed in four samples, which were assigned to the stretching of CO 2À 3 derived from the trace carbonate species on the surface. [15,16]The band locating at ~1632 cm −1 was attributed to the O-H stretching of H 2 O, [44] probably derived from the residual water on the cathode surface.Nevertheless, after Li 2 TiO 3 surface modification, the intensity of O-H and CO 2À 3 bands were both reduced.After 150th cycles, the bands locating at ~1085 and 1124 cm −1 of both samples were ascribed to the C-O stretching of (CH 2 OCO 2 Li) 2 and the CO 2À 3 stretching of ROCO 2 Li, respectively, [15,44] indicating the generation of side-reaction products between cathode and electrolyte.While after Li 2 TiO 3 surface modification, these band peaks intensity was limited, manifesting the enhanced interface stability of LLO.Besides, the limited band intensity locating at ~1768 and 1804 cm −1 of C=O stretching of carbonyl from cathode electrolyte interface (CEI) products further manifested the positive function of Li 2 TiO 3 surface modification. [15,44]igure S6, Supporting Information shows the typical SEM images of LLO and LLO-T2 electrodes after 150th cycles.It was observed that the spherical morphology of the LLO was destroyed and broken after longterm cycles due to the drastic stress variation during the continuous Li + insertion/de-insertion process, as marked by the red dotted cycles in Figure S6a, Supporting Information.Besides, a thick layer of electrolyte by-products was also generated and covered on the surface of LLO, as marked by the red-dotted rectangles.In contrast, the LLO-T2 could maintain the integrated spherical morphology with restrained byproduct generation as shown in Figure S6b, Supporting Information.Figure S7, Supporting Information compares the XRD patterns of LLO and LLO-T2.Both the fresh electrodes were well indexed into typical LLOs structure with Al diffraction peaks (black clovers) derived from the Al current collector.While after 150th cycles, the peak intensity of both electrodes was decreased with enlarged full-width at half maximum (FWHM), indicating the gradually deteriorated layered structure.Furthermore, the LLO-T2 still exhibited the elevated structure stability than the bare material, as confirmed by the enlarged patterns of (003) and (104) diffraction peaks in Figure S7b,c, Supporting Information, in which the (003), (104) peaks position shift of LLO-T2 was smaller (Δ = 0.13°, 0.3°) than the LLO (Δ = 0.19°, 0.43°). [45]he detailed structure evolution of LLOs after long cycles was further explored by HRTEM analysis, as shown in Figure 7. Before characterizing, the cycled cathode electrodes were immersed into dimethyl carbonate (DMC) solution for 2 h and washed several times.For LLO, the layered crystal structure with lattice fringe of 4.71 Å corresponding to (003) plane was still maintained in the bulk region (iii), while the phase transformation occurred intensively in the sub-surface region (ii) with the lattice fringe of 2.32 Å related to the (222) plane of spinel phase. [46]What is more, the packed rock-salt phase with lattice fringe of 2.4 Å related to (111) plane was also observed in the surface region (i) in Figure 7a. [36]The heterostructure in the above three regions manifested the LLO underwent a gradual phase transformation from layer to spinel and rock structure.As a stark contrast, LLO-T2 (Figure 7b) still kept the integrated layered structure in the sub-surface region (ii) with the compact Li 2 TiO 3 coating layer in the surface region (i), indicating the enhanced structure stability after Li 2 TiO 3 surface modification.

Conclusion
In summary, a multi-functional Li 2 TiO 3 modification layer was introduced to LLOs by a facile method, which not only eliminated the detrimental lithium residues but also realized the in situ forming of Li 2 TiO 3 surface coating with partial Ti 4+ ions dopants into the subsurface region of LLOs.The formed Li 2 TiO 3 coating was perfectly compatible with the crystal structure of LLOs, protected the fragile surface structure and facilitated the fast Li + migration at the surface region.Meanwhile, the partial doped Ti 4+ in the subsurface region formed more stable TiO 6 octahedron with strong ionic Ti-O band and acted as the pillar in Li + ions layers to further reinforce the crystal structure of LLOs.As a result, the structure stability, cycling stability, and rate performance of LLOs was effectively elevated.Furthermore, the thermal stability at charged state was also enhanced after Li 2 TiO 3 surface modification.Therefore, the multifunctional Li 2 TiO 3 modification obtained by a facile and green method could facilitate the practical application of LLOs and offer an enlightenment for researchers to further improve the performance of this material.

Experimental Section
Synthesis of Li 2 TiO 3 surface-modified FCG-LLOs materials: The FCG-LLOs were synthesized by the wet chemical co-precipitation reaction and the post-hightemperature calcination process as reported in previous works. [20,22]The chemical composition of FCG-LLO was gradually changed from Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 to To prepare the Li 2 TiO 3 -modified LLO, 2.0 g LLO was added into the mixtures of 60 mL ethyl alcohol and 1 mL deionized water, then stirred for 1 h at 25 °C.Next, the desired amount of Ti(OC 4 H 9 ) 4 ethyl alcohol solution (TBT: 86.1, 173.9, and 263.6 mg, corresponding to 1, 2, and 3 wt% of Li 2 TiO 3 to LLO, respectively) was slowly added into the above mixtures drop by drop and stirred for another 3 h, in which the TBT was hydrolyzed to TiO 2 and coated on the surface of LLO.Subsequently, the acquired precipitates were rotary evaporated at 38 °C and dried at 100 °C for 12 h.Finally, the materials were calcined at 850 °C for 6 h, wherein the TiO 2 transformed into Li 2 TiO 3 by consuming the surface lithium residues and a portion of Ti 4+ ion was permeated into the sub-surface region of LLO under the driving force of concentration difference.Finally, the Li 2 TiO 3 -modified LLO was successfully synthesized.The modified products with the Li 2 TiO 3 weight percent of 1, 2, and 3 wt% were denoted as LLO-T1, LLO-T2, and LLO-T3, respectively.
Material characterization: The materials crystal structure was characterized by X-ray diffraction (XRD) spectroscopy (Smartlab-9, Cu Ka radiation source, 0.01°/step, 10°min −1 , 5-90°).Scanning electron microscopy (SEM; Hitachi, SU8020, 5 kV) and high-resolution transmission electron microscopy (TEM; JEOL, JEM-2100F, 200 kV) were applied to characterize the morphologies and nanocrystal structures.Scanning transmission electron microscope (STEM; Thermo Fisher Scientific, MA, USA, 300 kV) was applied to characterize the crystal structure at atomic dimension.The STEM images were performed in a CEOS probe corrected FEI Themis TEM at an electron accelerating voltage of 300 kV with a probe convergence angle of 17.8 mrad, spatial resolution of 0.08 nm, and probe current of ~20 pA.The inner semiangular angle for the highangle annular dark field (HAADF) detector are 45 mrad.HAADF STEM images were filtered by the standard high-pass filtering method to reduce noise.X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific, ESCALAB 250Xi) was used to study the surface chemical valence state of materials.Fourier transform infrared spectrometer (FTIR; Thermo Fisher Scientific, Nicolet380) was used to study the inorganic and organic group variation of LLOs before and after cycled.Differential scanning calorimetry system (DSC; Mettler-Toledo) was adopted to test the thermal stability of highly charged LLOs.
Electrochemical measurements: The electrochemical performance of cathodes was evaluated using CR2025 coin cells.The electrodes with mass loading of 3-5 mg cm −2 were fabricated by blending cathode active materials, conductive agent (Super P), and binder agent (PVDF) with a mass ratio of 8:1:1 in the N-methyl-2-pyrrolidone (NMP) solvent, then coating on the Al foil and drying at 100 °C in a vacuum oven for 24 h.Li disk and Celgard 2400 membrane were used as the counter electrode and separator, respectively.The electrolyte was composed of 1 M LiPF 6 salt dissolving in ethylene carbonate (EC)/fluoroethylene carbonate (FEC)/ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) with the volume ratio of 2:3:4:1.The cells were assembled in an Ar-filled glove box with water and oxygen contents both below 0.1 ppm.The cells were cycled on the battery test system (LAND-CT2001A) between 2.0 and 4.7 V versus Li + /Li (similarly hereinafter) at 0.5 C (25 °C) after activated at 0.1 C (25 mA g −1 ).Cyclic voltammograms (CV) tests were conducted on the electrochemical workstation (CHI 660e; Shanghai Chenhua) with a scan rate of 0.5 mV s −1 between 2.0 and 5.0 V. Electrochemical impedance spectra (EIS) test was carried out from 0.01 to 10 5 Hz at the charged state of 3.8 V using the same electrochemical workstation.
Thermal stability measurement: The coin cells were charged by constant current (0.1 C) to a higher voltage of 4.8 V followed by a constant voltage (4.8 V) charge for another 10 h to extract more Li + ions from LLO and situated the LLO at an unstable status.After that the highly charged cells were disassembled in the glove box, then the electrodes were washed with dimethyl carbonate (DMC) several times to remove the electrolyte residuals and dried in the glove box.Finally, the cathode electrodes were sealed into the Al pan and executed the DSC test with a rate of 10 °C min −1 from 30 °C to 375 °C under the N 2 atmosphere.
Surface lithium residues content detection: The surface lithium residues content was tested by the acid-base indicator method.Firstly, 1.0 g LLO and LLO-T2 was added into the 100 mL deionized water in a beaker and stirred for 3 min separately.After that 20 mL limpid filter solution was taken out and added into two drops of phenolphthalein solution (color indicator) as the detected solution.Then, 0.05 M hydrochloric acid solution (HCl) was slowly dropped into the above solutions until the color changed from the amaranthine to the colorless state.According to the consumed volume of 0.05 M HCl, the surface lithium residues content was calculated for LLO and LLO-T2.

Figure 1 .
Figure 1.a, b) Typical SEM image and the corresponding line scan profiles of Ni, Co, and Mn elements for LLO-T2, c-g) Typical SEM image and the corresponding EDS mapping of Mn, Co, Ni, and Ti elements for LLO-T2.Typical HRTEM and the corresponding FFT images for h) LLO and i) LLO-T2, respectively.

Figure 2 .
Figure 2. XPS spectra of a) O1s and b) C1s for LLO and LLO-T2 samples.c) XPS spectra of Ti 2p with different etch depth for LLO-T2.

Figure
Figure 4a,b shows the cycle performance of LLOs at 25 °C.It was observed that the discharge capacity of LLO was improved after Li 2 TiO 3 surface modification, and exhibited the highest discharge capacity with enhanced cycle stability for LLO-T2.Specifically, the LLO only maintained a capacity of 189.1 mAh g −1 with retention of 87.6% after 150 cycles, while LLO-T2 exhibited 205.1 mAh g −1 with a retention of 95.3%. Figure 4c shows the rate performance of LLOs from 0.1 to 5 C rate.The Li 2 TiO 3 modification also enhanced the rate capacity of LLOs and exhibited the best rate performance for LLO-T2.When the current density increased from 0.1 to 5 C, the discharge capacity of LLO-T2 was 258.4,249.3, 219.8, 193.2, 168.2, and 116.4 mAh g −1 , superior to the LLO of 241.2, 224.8, 197.8, 176.1, 155.2, and 107.8 mAh g −1 .The enhanced cycle stability and rate capacity of LLO-T2 were ascribed to the positive surface modification of Li 2 TiO 3 , which not only formed the Li 2 TiO 3 coating layer to protect the fragile interface of LLOs from being electrolyte corrosion but also built 3D channels to facilitate the Li + ion diffusion.Besides, the partial penetrated Ti 4+ ions in the sub-surface region of LLOs also stabilized the crystal structure for the strong Ti-O band and enlarged Li + interlayer spacing, thus benefiting the cycle stability and rate performance of materials.[27]The electrochemical impedance spectra (EIS) test was conducted to explore the impedance and electrochemical kinetics variation of LLOs, as shown in Figure4d.The R b , R f , R ct , and W o in the equivalent circuit represented the bulk, surface film, charge transfer, and Warburg resistance respectively.The corresponding EIS results are summarized in Figure4e and

Figure 4 .
Figure 4. a-c) The cycling performance of LLOs at 25 °C, 0.5 C. d, e) The EIS results of LLOs after 3rd, 50th, and 100th cycles at a fixed charged state of 3.8 V. f) The relationship between the Z re and the square root of frequency (ω −1/2 ) in the low-frequency region and the calculated Li + diffusion coefficient.

Figure 5 .
Figure 5.In situ XRD characterization of a) LLO and b) LLO-T2 for the initial two cycles.c, d) The DSC results for LLO, LLO-T1, LLO-T2, and LLO-T3 at the fully charged state.

Table 1 .
The surface lithium residues content detected by the acid-base titration method.