Atomic Level Fabrication of Oxychloride Interface for High‐Rate and High‐Voltage Lithium‐Ion Batteries

Abstract

In recent years, oxychloride‐based materials have emerged as a promising solid‐state electrolyte (SSE) candidate owing to its ultrahigh ionic conductivity and decent cathode compatibility. Although the fabrication of SSE coatings for cathode materials has been recognized as a promising strategy, a precise synthesis of oxychloride‐based SSE coating is still not realized due to the lack of appropriate preparation method. As a proof of concept, we propose a superior lithium‐ion conductive aluminum‐based oxychloride (LAOC) coating synthesized by atomic level fabrication strategy with unique self‐limiting reaction mechanism. The LAOC modified lithium cobalt oxide (LCO) cathode exhibits a high capacity retention of 86.4% after 500 cycles at 5 C and significantly improved high‐voltage cycling stability. The outstanding performance is ascribed to the high interfacial ionic conductivity and construction of robust cathode electrolyte interphase. The ionic conductivity of LCO increased from 1.785 × 10⁻⁷ to 2.823 × 10⁻⁶ S cm⁻¹ after LAOC coating. Scanning transmission X‐ray microscopy and transmission electron microscopy reveal that the LAOC coating suppresses interfacial degradation and mitigates the structural collapse of LCO. This study offers great opportunity for the atomic level fabrication of superior ionic conductive oxychloride thin films to realize high performance lithium‐ion batteries.

Synthesis of a lithium‐ion conductive aluminum‐based oxychloride (LAOC) thin film is realized by a facile atomic‐level chemical fabrication method. LAOC coating enables lithium cobalt oxide (LCO) cathode superior cycling performance at high rates and elevated voltages, owing to the fast interfacial ionic transport and excellent interfacial stability.

Introduction

Solid‐state electrolytes (SSBs) are believed to be promising materials that offer improved durability, stability, energy density for next‐generation batteries. ¹ , ² , ³ Recently, a series of oxychloride materials have been reported as superior solid ionic conductors which attract great research interests due to high ionic conductivity and excellent electrochemical stability with cathode materials. A Panasonic research team reported LiMOCl₄ (M = Nb, Ta) SSEs in 2023, achieving high ionic conductivities over 10 mS cm⁻¹, which exceeds the value of all the previously reported halide‐based SSEs. ⁴ Besides, oxychloride‐based SSEs show outstanding electrochemical compatibility with oxide cathode materials, with an 84.9% capacity retention for 1600 cycles when paired with Ni‐rich layered cathode. ⁵ The development of oxyhalide SSEs provides unlimited potential to lead the direction for safer and more efficient energy storage solutions.

Nevertheless, oxychloride SSEs do not demonstrate sufficient compatibility with anode materials, thus bi‐layer SSEs which adopt a layer of sulfide SSE on the anode side are still required for the normal operation of oxyhalide based all‐solid‐state batteries (ASSBs). ⁶ , ⁷ , ⁸ , ⁹ The interface mismatch of oxychlorides poses challenges when compared to their SSE counterparts including sulfides and halides. ¹⁰ , ¹¹ If oxychloride materials can be fabricated as a coating layer for cathode materials, a single sulfide SSE layer in ASSBs will be achievable, providing great advantages in terms of higher energy density and excellent cathode interfacial stability. Oxyhalide coatings may also broaden the application of oxyhalide materials to a wider range of battery systems, such as liquid cells, to boost the performance of cathode materials including rate capability and high‐voltage performance. Recently, an oxyfluoride (Li‐Ta‐O‐F) coating on nickel‐rich layered oxide cathode via gas‐solid reaction was reported, demonstrating excellent high‐voltage cycling performance at 4.8 V owing to superior interfacial oxidation stability. ¹² Due to the strong ionic bonding between F and Li that may hinder the ion mobility in oxyfluoride, ⁷ increased ionic conductivity could be more possibly achieved by oxychloride, thus potentially offering distinct advantages such as rate performance. Unfortunately, a precisely controlled fabrication of oxychloride SSE coating has not yet been reported, which severely limits its application in a variety of battery systems.

Atomic layer deposition (ALD) is a unique chemical phase vapor deposition method, which is widely adopted for the surface coating of particles. Its alternative precursor pulsing characteristics and self‐limiting reaction manner enable distinguished strengths including accurate control over thickness and composition, mild deposition temperature, and conformal coverage. ¹³ , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ The fabrication of ALD oxide‐based SSE coatings, including lithium niobium oxide, ¹⁸ lithium silicate, ¹⁹ and lithium lanthanum zirconium oxide, ²⁰ have been reported in recent years and demonstrated decent ionic conductivities. Precise synthesis and fabrication are attracting great attention nowadays driving by the intensive exploration of reaction thermodynamics and kinetics. ²¹ Considering the unique features of precise compositional control, ALD may provide opportunities for the successful fabrication of oxyhalide SSE coating.

In this work, an amorphous aluminum‐based oxychloride (Li‐Al‐O‐Cl, abbreviated as LAOC) coating layer on LCO via ALD technique has been reported for the first time. This superior ionic conductive LAOC coating enables LCO with excellent rate performance at 5 C with a capacity retention of 86.4% in 500 cycles. Moreover, outstanding cycling stability is achieved at the cut‐off voltage of 4.6 V owing to the excellent electrochemical compatibility of oxyhalide when coupling with high‐voltage cathode materials. The interfacial stability has been effectively tuned by the LAOC coating, resulting in a thinner cathode electrolyte interphase (CEI) and smaller charge‐transfer resistance due to the mitigated side reactions. Synchrotron‐based X‐ray absorption spectroscopy (XAS) measurement reveals that Co can maintain its local environment unchanged during the cycling, thus obtaining an excellent cycling stability under long‐term charge/discharge. Our study provides a promising strategy for fabricating highly ionic conductive and high‐voltage tolerant robust artificial CEI for cathode materials. The atomic level fabrication method with great universality offers huge potential for practical applications to achieve next‐generation lithium‐ion batteries.

Results and Discussion

LAOC is chosen as a typical SSE material for coating design via atomic level fabrication method owing to the low cost and abundance of Al element. Synchrotron‐based X‐ray diffraction was performed on both pristine LCO and LAOC coated LCO (LCO@LAOC) to understand the crystal structure before and after ALD surface chemistry (Figure 1a). No differences were observed from the spectra between LCO and LCO@LAOC. Both the features of two samples are well matched with a hexagonal α‐NaFeO₂ structure in the $R\overline{3}m$ space group, illustrating mild ALD procedure has no impact on the crystal structure of LCO. Synchrotron X‐ray pair distribution function (PDF) analysis was performed to further understand the local structures of LCO and LCO@LAOC (Figure 1b). The features at ∼1.9 and ∼2.8 Å correspond to Co–O distance and Co–Co distance, respectively. Also, PDF results show that the structure of LCO is not changed upon surface coating procedure. Synchrotron‐based XAS was applied to investigate the chemical state and local environment of Co. In the Co K‐edge X‐ray absorption near edge spectroscopy (XANES) spectrum of pristine LCO, a pre‐edge feature at around 7711 eV is assigned to the transition of the 1s electron to an unoccupied 3d orbital of the Co³⁺ with a low‐spin electronic configuration (Figure S1). The shoulder at around 7720 eV corresponds to the electron transition from 1s orbital to unoccupied 4p orbital with a shakedown process, whereas the feature at around 7730 eV corresponds to the 1s to 4p transition without the shake down process. Identical features were also presented in the Co K‐edge spectrum of LCO@LAOC, suggesting that the local structure and chemical state are not changed through ALD procedure. The overlapped first derivative spectra for LCO and LCO@LAOC also verifies the unchanged chemical state of Co (Figure S2). The local environment can be further revealed from the Co extended X‐ray absorption fine structure (EXAFS) spectra (Figure S3), the features at around 1.4 and 2.4 Å correspond to Co–O distance and Co–Co distance, respectively. According to the EXAFS spectra of LCO and LCO@LAOC, there is no difference regarding the Co–O and Co–Co distance after ALD procedure.

Figure 1.

Physical characterization of LCO@LAOC. a) XRD patterns of LCO and LCO@LAOC. b) PDF curves of LCO and LCO@LAOC. c) SEM image for LCO@LAOC. d) SEM‐EDX mapping of LCO@LAOC. e) TEM image of LCO@LAOC. f) TEM‐EDX mapping of LCO@LAOC. g) Cl K‐edge XANES spectra of LCO@LAOC, LiCl, and AlCl₃. h) XPS Cl 2p spectrum of LCO@LAOC. i) XPS Al 2p spectrum of LCO@LAOC.

Scanning electron microscopy (SEM) image shows crystal size of several to decades of microns for LCO (Figure S4). The morphology of LCO@LAOC was also examined under SEM (Figures 1c and S5). Comparing LCO with and without coating layer, the original surface morphology of LCO has no obvious difference upon ALD procedure. Energy‐dispersive X‐ray spectroscopy (EDX) mapping shows that Al and Cl elements are evenly distributed on the surface of LCO, confirming the presence of LAOC coating (Figure 1d). To verify the uniformity of coating procedure, another area of LCO@LAOC sample was also examined by EDX, showing similar distribution for the O, Al, Cl, and Co elements (Figure S6). The LCO@LAOC was further observed by transmission electron microscopy (TEM), showing a conformal ∼3 nm amorphous coating which covers the surface of LCO (Figure 1e). TEM‐EDX mapping reveals the distribution of Al and Cl, which covers beyond the edge of Co distribution and suggests the coating coverage (Figure 1f). Cl K‐edge XANES measurement was conducted to investigate the chemical state of Cl of LAOC coating in the LCO@LAOC (Figure 1g). Compared with AlCl₃ precursor, the white line feature from LCO@LAOC shifted toward higher energy, which is more assemble to the feature of LiCl (Figure 1g). The energy shift of LCO@LAOC and the similarity between LCO@LAOC and LiCl are also verified from the first derivative spectra (Figure S7). The XANES Cl K‐edge results indicate that the AlCl₃ precursor in the ALD procedure participate in the surface chemistry reaction thus the local environment of Cl has been converted to be surrounded by Li. Furthermore, the features of Cl K‐edge XANES spectrum from LCO@LAOC matched well with the previously reported aluminum‐based oxychloride materials. ²² X‐ray photoelectron spectroscopy (XPS) was performed to understand the surface chemistry of LCO@LAOC. Al 2p spectrum suggests Al is in the valance state of +3, which suggests that the chemical valence of Al remains from the AlCl₃ precursor (Figure 1i). Two peaks in the Cl 2p spectrum are attributed to the spin orbital splitting of 2p_1/2 and 2p_3/2 (Figure 1h). The features in Cl 2p for LCO@LAOC correspond to metal chloride.

To understand the interfacial ion transport of LAOC coating and its effects toward LCO cathode, a special set‐up was designed to measure the ionic conductivity of LCO@LAOC. A direct measuring method of ionic conductivity of LAOC coating has not been identified because the coating thickness is only several nanometers. Alternatively, we measured the powdery cathode samples to understand the influence of LAOC coating toward LCO. For comparison, Li‐Al‐O coated LCO (LCO@LAO) was also prepared by ALD method (procedures are described in the supporting information). Direct current (DC) polarization method is employed for the ionic conductivity testing, in which a series of DC voltages is applied successively and the generated current is recorded to calculate the resistance of cathode layer following ohm's law. Then, the ionic conductivity of cathode was calculated by Pouillet's law. Using polyether ether ketone (PEEK) die and stainless‐steel rods, the cathode disk was prepared by cold pressing the cathode powder in the PEEK die by stainless steel pushing rods on each side of the cathode powder. Then, two identical layers of halide solid‐state electrolyte Li₃InCl₆ (LIC) powder were pressed on each side of the cathode disk to block the electron transport. Prior to attaching two pieces of Li metal foils on each side, two identical layers of Li₆PS₅Cl (LPSC) were also pressed on each side to prevent the reaction between Li and LIC. The resistance of this set‐up was measured by biasing voltages according to ohm's law. Another cell was also prepared to calculate the additional resistance of the cell without cathode, following the same procedure except that there was no cathode powder in the beginning (Figure S8). The additional resistance was also calculated following the ohm's law. Therefore, the resistance of cathode layer can be obtained by using the initial resistance to subtract the additional resistance. Accordingly, the ionic conductivity can be calculated by the following equation:

$${\sigma }_{ic}=\frac{l}{RS}$$

where l is the thickness of the cathode material disk, S is the area of the disk, and R is the remaining resistance of the disk after subtracting the additional resistance. Obtained from this method, the ionic conductivities of LCO and LCO@LAO were 1.785 × 10⁻⁷ S cm⁻¹ and 2.448 × 10⁻⁷ S cm⁻¹, respectively (Figures S9 and S10). It can be seen that after applying a layer of Li‐Al‐O coating, the ionic conductivity has not been changed significantly from the testing results. Promisingly, the calculated ionic conductivity of LCO@LAOC is 2.823 × 10⁻⁶ S cm⁻¹, which shows an order of magnitude improvement to the pristine LCO and LCO@LAO (Figure S11). The improved ionic conductivity might contribute to excellent cycling performance and rate performance in the electrochemical testing.

To understand the coating effect toward electronic conduction of cathodes, conductivity was measured by both direct current (DC) polarization and four‐point probe (Figure S12). The DC polarization result shows that the electronic conductivity is 4.388 × 10⁻⁵ S cm⁻¹ for LCO and 2.148 × 10⁻⁵ S cm⁻¹ for LCO@LAOC, respectively. The decreased conductivity after coating may be owing to the electron blocking nature of oxychloride SSE thin film. Four‐point probe measurements show conductivities of 4.645 × 10⁻⁵ and 2.253 × 10⁻⁵ S cm⁻¹ for LCO and LCO@LAOC, respectively. The subtle higher conductivity values from four‐point probe measurements might be owing to the stack pressure during tesing, which possibly promotes more intimate contact between particles.

2032‐type coin cells were assembled to evaluate the electrochemical performances of LCO, LCO@LAO, and LCO@LAOC. The cathode composite is composed of 90% LCO or LCO@LAOC, 5% PVDF, 2.5% CB, and 2.5% VGCF in weight percentages for performance testing. All the cells were cycled at 0.1 C for 3 cycles for formation, and then were tested at 1 C in the voltage range of 2.8–4.2 V (versus Li/Li⁺) for evaluation of long cycling performance. Cycling at a rate of 1 C, the capacity retention of LCO@LAOC and LCO@LAO in 500 cycles were 75.5% and 47.8%, respectively, whereas the capacity dropped more quickly blow 50% in 400 cycles for pristine LCO (Figure 2a). The high capacity retention in long‐term cycling indicates that LAOC coating can maintain an excellent interphase stability. When increasing the cut‐off voltage toward 4.5 V, the capacity retention for LCO@LAOC and LCO@LAO were 87.6% and 73.1% in 100 cycles, suggesting oxychloride coating with enhanced high‐voltage stability (Figure 2b). Moreover, the average discharge voltage of LCO@LAO dropped faster than LCO@LAOC (Figure 2g), which suggests higher degree of polarization for LCO@LAO. In the high current measurement at 5 C, LCO@LAOC can maintain as high as 86.4% of its initial capacity in 500 cycles, whereas only 44.7% and 34.2% were achieved for LCO@LAO and LCO (Figure 2c). The voltage profiles of LCO@LAOC in Figure 2f demonstrate similar features from the 1^st cycle to 500^th cycles, indicating excellent cycling reversibility. To assess the performance of the LCO cathodes at different current densities, the rate performance at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, 5 C, and 10 C was also tested for LCO, LCO@LAO, and LCO@LAOC (Figure 2d). LCO@LAOC exhibited the best performance at higher rates, especially at 10 C, among all the three cathodes. As shown in Figure 2e, the voltage profiles of LCO@LAOC at different current rates can maintain obvious discharge plateaus with minimal polarization at each rate, indicating excellent ionic conduction at the interphase owing to high ionic conductivity of LAOC coating.

Figure 2.

Electrochemical performance of LCO@LAOC, LCO@LAO, and LCO. a) Cycling performance in the voltage range of 2.8–4.2 V (versus Li⁺/Li) at 1 C (1 C = 140 mA g⁻¹). b) Cycling performance in the voltage range of 2.8–4.5 V (versus Li⁺/Li) at 1 C (1 C = 185 mA g⁻¹). c) Cycling performance in the voltage range of 2.8–4.2 V (versus Li⁺/Li) at 5 C (1 C = 140 mA g⁻¹). d) Rate performance at a variety of current rates (1 C = 140 mA g⁻¹) in the voltage range of 2.8–4.2 V (versus Li⁺/Li). e) Discharge voltage profiles of LCO@LAOC at a variety of current rates. f) Discharge voltage profiles of LCO@LAOC for different cycles at 5 C. g) Average discharge voltages in the voltage range of 2.8–4.5 V (versus Li⁺/Li).

To investigate the effects of oxychloride coating toward the cycling performance under higher cut‐off voltage, commercial high‐voltage LCO (HVLCO), which has trace doping of Al and Mg, is selected as the base cathode material to mitigate the adverse high‐voltage phase transitions. Li‐Al‐O coated HVLCO (HVLCO@LAO), Li‐Al‐O‐Cl coated HVLCO (HVLCO@LAOC), and HVLCO were tested at a rate of 0.5 C (1 C = 200 mA g⁻¹) at a cut‐off voltage of 4.6 V (Figure S13). HVLCO delivers an initial discharge capacity of 190.76 mAh g⁻¹ and retains 96.39% of its capacity after 100 cycles, whereas HVLCO@LAO exhibits slightly higher initial capacity of 192.53 mAh g⁻¹ and higher retention of 97.31%. Promisingly, HVLCO@LAOC delivers the highest capacity of 198.22 mAh g⁻¹ at the first cycle and still deliver 201.91 mAh g⁻¹ after 100 cycles, achieving the highest capacity retention of 98.46%. In the voltage profiles, significant voltage fading and capacity dropping are presented in HVLCO, whereas the fading trend is mitigated in HVLCO@LAOC. The exceptional performance of HVLCO@LAOC can be ascribed to the stable CEI and fast interfacial ionic transport. Compared to cycling data of LCO summarized from literatures (Table S1), our oxychloride modification strategy enables superior performance for both LCO and HVLCO cathodes. Comparing electrochemical results of LCO@LAO and LCO@LAOC, the unique role of Cl can be summarized according to several aspects: 1) Improved interfacial ionic conductivity which promotes superior performance at high rates. 2) Excellent interfacial robustness that guarantees higher capacity retention during long cycling. 3) High‐voltage stability to withstand charging at higher cut‐off voltage at 4.5 and 4.6 V.

We also applied this ALD oxychloride coating strategy on Ni‐rich cathode to evaluate its universality for other types of cathode materials. Specifically, two cycles of ALD LAOC have been deposited on LiNi_0.83Co_0.11Mn_0.06O₂ (Ni83) cathode, while two cycles of ALD Li‐Al‐O has also been deposited on Ni83 for comparison. All the cells were cycled at 0.1 C (1 C = 200 mA g⁻¹) for three cycles for formation prior to cycling at 1 C. As shown in Figure S14, pristine Ni83 exhibited poor capacity retention after 300 cycles, whereas Li‐Al‐O coated Ni83 (Ni83@LAO) showed tiny improvement compared to Ni83, possibly due to the stabilized interface. Promisingly, LAOC coated Ni83 (Ni83@LAOC) exhibited excellent capacity retention with significant improvement of cycling stability compared to Ni83@LAO after 300 cycles. The extraordinary long‐term cycling stability is attributed to the excellent electrochemical stability of LAOC coating at the cathode interface and decent ionic conductivity at the interface. To further verify the role of interfacial ionic conductivity, the cells have also been tested under a high current rate of 5 C as well as different rates (Figures S15 and S16). The cells were sequentially cycled at 0.1 C and 1 C each for 5 cycles prior to 5 C cycling for the transition to a high current rate. The high cycling rates such as 5 C generally lead to more serious capacity fading owing to severe interfacial instability, more pronounced localized internal strain, structural inhomogeneity, and mechanical failure. After 500 cycles at 5 C, both the capacity of Ni83 and Ni83@LAO dropped quickly to almost no residual capacity, while the capacity retention showed dramatic increase for Ni83@LAOC, which also exhibited excellent performance for different rates. From the ionic conductivity testing results, Ni83 and Ni83@LAO showed similar ionic conductivity in the range of 10⁻⁷ S cm⁻¹, which is one order of magnitude lower than Ni83@LAOC (Figure S17). The low ionic conductivity may explain the reason for poor cycling performance at higher current rates. The electrochemical testing results demonstrate the universality of the ALD oxyhalide coating strategy for realizing high‐rate and high‐voltage electrochemical performance. We also believe that LAOC strategy could be extended to other SSE systems which suffer from unsatisfactory high‐voltage stability, such as polymer and sulfide‐based SSEs.

To elucidate the structural change of the LCO cathodes, synchrotron XAS measurements were conducted on LCO, cycled LCO (Cyc LCO), and cycled LCO@LAOC (Cyc LCO@LAOC). As shown in the Co K‐edge XANES spectra (Figure 3a), the peak positions of cycled LCO and cycled LCO@LAOC has no obvious changes compared with LCO. This observation is further verified by the first derivatives of the three Co K‐edge XANES curves (Figure 3b). EXAFS measurement was performed to further understand the local environments of LCO, cycled LCO, and cycled LCO@LAOC (Figure 3c). For LCO, two features at around 1.4 and 2.4 Å correspond to Co–O distance and Co–Co distance, respectively. After cycling, both Co–O distance and Co–Co distance are extended, which is indicative of the local distortion of Co environment. By contrast, the Co–O distance and Co–Co distance remain same for cycled LCO@LAOC compared to LCO, demonstrating high structural stability of LCO@LAOC during the charge/discharge cycling. In addition, the wavelet transform (WT) results further confirm the local distortion of Co in cycled LCO and highly stable Co in cycled LCO@LAOC (Figure 3d–f). The cycling degradation of LCO cathodes was also investigated by O K‐edge fluorescence yield (FY) spectra of LCO, cycled LCO, and cycled LCO@LAOC (Figure 3g). The pre‐edge shoulder feature at around 529 eV for both cycled LCO and cycled LCO@LAOC suggested the existence of re‐hybridization between Co 3d and O 2p orbitals after repeated lithium deintercalation‐intercalation. The intensity of the feature at 531 eV for cycled LCO was decreased significantly compared with LCO, which is indicative of the decreased Co─O bonding covalency. ²³ This observation also agrees well with the local structure distortion of CoO₆ octahedra found in the EXAFS spectra in Figure 3c. The intensity of the feature at 531 eV for cycled LCO@LAOC is slightly lower than LCO but significantly higher than cycled LCO, suggesting the suppressed local distortion of CoO₆ octahedra owing to the coating protection effect. To alleviate the effects of self‐absorption from FY mode, inverse partial fluorescence yield (IPFY) under O K‐edge was adopted to analyze the cycled LCO cathodes for Co L_2,3‐edge (Figure 3h). Two features (a and b) and their relative ratios are indicative of the oxidation states of Co. ²⁴ As shown in the Figure 3i, cycled LCO@LAOC remain a similar a/b ratio, whereas cycled LCO has a significantly higher ratio compared with LCO. The ratio results clearly suggests that LCO@LAOC is much more reversible than LCO during the charge/discharge, demonstrating the effectiveness of ionic conductive LAOC coating.

Figure 3.

Synchrotron X‐ray absorption characterization of LCO and LCO@LAOC after cycling. a) Co K‐edge XANES spectra of LCO, Cyc LCO, and Cyc LCO@LAOC. b) First derivatives of Co K‐edge XANES spectra. c) Co K‐edge EXAFS R space curves of LCO, Cyc LCO, and Cyc LCO@LAOC. d) EXAFS WT images of LCO. e) EXAFS WT images of Cyc LCO. f) EXAFS WT images of Cyc LCO@LAOC. g) O K‐edge XAS FY spectra of LCO, Cyc LCO, and Cyc LCO@LAOC. h) Co L‐edge XAS IPFY spectra of LCO, Cyc LCO, and Cyc LCO@LAOC. i) The values for the ratio of “feature a” over “feature b”.

To understand the evolution of surface and interphase upon cycling, XPS measurement was carried out for cycled LCO and cycled LCO@LAOC. XPS Cl 2p and Al 2p spectra of cycled LCO@LAOC exhibited same features compared with uncycled LCO@LAOC and suggests that the coating layer still presents after electrochemical cycling (Figures S18 and S19). From O 1s spectrum of cycled LCO (Figure 4a), the organic and inorganic species as common CEI species can be distinguished, including organic R─OLi, O─C═O, and C─O species and inorganic P─O─F, P─O, and carbonate species. Compared to cycled LCO, the common CEI species were also found in cycled LCO@LAOC, except for lattice O which is exclusively identified from cycled LCO@LAOC (Figure 4d). The presence of lattice O from XPS indicates that the CEI layer of cycled LCO@LAOC is much thinner than cycled LCO. ²⁵ In terms of F 1s spectrum for cycled LCO in Figure 4b, a major feature corresponding to PVDF and a minor feature corresponding to LiF were detected, which are associated with the binder and major inorganic CEI component. By contrary, a significant higher amount of LiF species was found for cycled LCO@LAOC (Figure 4e). Therefore, it is evident that there is a higher portion of inorganic components in the cycled LCO@LAOC than cycled LCO. We attribute the formation of LiF to the possible reaction between a partial fraction of oxychloride and trace amount of HF in the electrolyte. The wide electrochemical stability window of LiF would offer good interfacial passivation effect for cathodes. There were not any evident features from the Co 2p spectrum of cycled LCO (Figure 4c), indicating a thick CEI layer which buried bulk LCO underneath. Possibly owing to the thin thickness of CEI layer, Co 2p_3/2 and 2p_1/2 peaks and their satellites were presented in the Co 2p spectrum for cycled LCO@LAOC (Figure 4f). The difference of CEI thickness found in Co 2p spectra agrees well with the results from O 1s spectra, both illustrating that cycled LCO@LAOC has a thinner CEI than cycled LCO.

Figure 4.

Surface chemistry and impendence measurement upon charge/discharge cycling. XPS O 1s a), F 1s b), and Co 2p c) spectra for cycled LCO. XPS O 1s d), F 1s e), and Co 2p f) spectra for cycled LCO@LAOC. EIS measurement for g) LCO and h) LCO@LAOC. i) Values for R _ct+R _CEI after 50 and 100 cycles.

To further understand the influences of interfacial properties toward cycling performance, electrochemical impedance spectroscopy (EIS) was performed for LCO and LCO@LAOC after 50 cycles and 100 charge/discharge cycles. The quantitative analysis was conducted by fitting the EIS spectra with an equivalent circuit model which is inset of Figure 4g. The high‐to‐medium frequency region was associated to the charge transfer resistance and CEI (R _ct + R _CEI), while the low frequency region was associated to the lithium‐ion diffusion. As shown in the Figure 4g, the resistance in high‐to‐medium frequency region of LCO increased dramatically to nearly 500 Ω between 50^th cycle and 100^th cycle. Promisingly, within the same charge‐discharge cycles this resistance of LCO@LAOC remained stable below 50 Ω (Figure 4h). The fitting results of EIS spectra is shown in Figure 4i, which suggests the formation of a highly stabilized interphase of LCO@LAOC for long‐term cycling. To visually observe the morphology of CEI for the cycled cathodes, TEM characterization was performed for both LCO and LCO@LAOC after cycling (Figure S20). By carefully examining the edge area of the cathode particles, a thin and smooth CEI layer with a thickness of ∼6 nm on the surface of LCO@LAOC is clearly observed. However, the cycled LCO exhibits a significantly thicker CEI layer ranging from 20 to 60 nm with a dramatic variation of uniformity. From TEM analysis, it is sufficiently revealed that oxychloride surface modification enables the LCO cathode with a uniform thin CEI, suggesting enhanced interfacial stability which may contribute to the outstanding cycling performance.

The electrochemical reaction kinetics are investigated by the galvanostatic intermittent titration technique (GITT) to understand the effect of LAOC coating. Figure S21a shows the GITT profiles of LCO and LCO@LAOC at room temperature. The GITT calculation results are illustrated in Figure S21b–c. Notably, LCO@LAOC exhibits smaller polarization in the GITT voltage profiles, with higher calculated diffusion coefficient at nearly all the stages throughout charge and discharge. The average diffusion coefficient of LCO@LAOC is 3.625 × 10⁻¹¹ and 3.630 × 10⁻¹¹ cm² s⁻¹ for charge and discharge, whereas LCO exhibits lower values of 2.962 × 10⁻¹¹ cm² s⁻¹ for charge and 2.996 × 10⁻¹¹ cm² s⁻¹ for discharge. During charge and discharge, there are two significant dips at 4.07 and 4.18 V for both LCO and LCO@LAOC owing to the H2/M1 and M1/H3 phase transition in the plots of calculated diffusion coefficients, indicating the LAOC coating does not alter the lithium insertion/desertion mechanism. The enhanced electrochemical kinetics may be ascribed to the higher interfacial ionic conductivity, which reduces the charge transfer energy barrier at the interface, accelerating the ion extraction and insertion.

In situ XRD measurements are carried out to investigate the structural evolution of LCO and LCO@LAOC during the electrochemical operation (Figure S22). The typical (003) diffraction peaks are selected to probe the variation of lattice parameter of c value. Both LCO and LCO@LAOC exhibit similar structural evolution behavior in the charging stage. For both cathodes, the (003) peaks initially shift to a smaller angle and then shift back to a higher angle, indicating the lattice expansion in the low‐voltage range and lattice contraction afterward in the high‐voltage range. However, the variation amplitude of (003) peaks in the high‐voltage range for LCO@LAOC is lower than that of LCO, suggesting a reduced volume change and suppressed irreversible phase transition. The in situ XRD measurement provide solid evidence for the excellent structural stability and outstanding electrochemical performance of LCO@LAOC, in which the oxychloride modification layer can effectively mitigate the substantial internal volume change without generating any additional structural damages.

To further investigate the chemical information of the cathode after charge/discharge, scanning transmission X‐ray microscopy (STXM) was conducted for the cycled LCO particles and cycled LCO@LAOC particles. Detailed chemical information for any specific positions for the samples can be obtained from the STXM due to the combination of both imaging and spectroscopy techniques, as shown from Figure 5a. A series of different locations on a single particle for cycled LCO (Figure 5b) and cycled LCO@LAOC (Figure 5c) are selected from both near surface and bulk of the particle. From the Co L_2,3‐edge spectra for cycled LCO in Figure 5d, it is found that at different locations the intensity for the feature at ∼777 eV significantly increased after cycling compared with the LCO spectrum in Figure 3h. The Co L_2,3‐edge spectra in pristine LCO suggested serious irreversible phase transformation after charge/discharge cycling. By contrast, the intensity of this feature does not show obvious changes for cycled LCO@LAOC for both near‐surface and bulk areas (Figure 5e). Therefore, the excellent structural stability for both interface and bulk of LCO@LAOC cathode is verified by STXM characterization. TEM is also performed to understand the near‐surface evolution of LCO and LCO@LAOC after electrochemical cycling. Cycled LCO exhibited extensive near‐surface reconstruction with large area of Co₃O₄‐like structures and mixed phase structures observed from the FFT patterns (Figure S23). The irreversible structural degradation agrees well with the STXM results and leads to serious capacity decay. By contrast, $R\overline{3}m$ layered structure still dominates in cycled LCO@LAOC, indicating the oxyhalide coating can effectively mitigate the structural failure of LCO (Figure S24).

Figure 5.

Investigation on the chemical information of a single particle for cycled LCO and cycled LCO@LAOC. a) Schematic of STXM experimental set‐up. b) STXM optical density imaging for cycled LCO. c) STXM optical density imaging for cycled LCO@LAOC. d) Co L_2,3‐edge spectra for cycled LCO. e) Co L_2,3‐edge spectra for cycled LCO@LAOC.

Conclusion

In summary, a type of amorphous oxychloride‐based coating material (LAOC) has been successfully deposited on LCO using highly controllable ALD method. Precise deposition of oxychloride coating is achieved by manipulating vapor phase self‐limiting surface chemistry in the atomic level. Owing to the superior ionic conductive nature of amorphous oxychloride material, the surface modified LCO exhibited outstanding cycling stability with a capacity retention of 86.4% in 500 cycles at a high charge/discharge rate of 5 C, demonstrating promising potential for high power density and fast‐charging application. The great high‐voltage stability for LAOC coated cathode is also demonstrated in the voltage range of 3.0–4.6 V. The oxyhalide coating has enabled an ultra‐stable interphase, suggested by a thin CEI thickness and low impedance values. Moreover, it is found that the local environment of Co was well maintained after cycling, revealed by synchrotron‐based XAS measurement and agreed well with the high capacity retention. This work proposes a universal strategy for fabricating oxyhalide coating by designing atomic layer deposition chemistry and opens up great opportunities for high‐rate and stable cathode materials for long‐term cycling.

Author Contributions

X.S. and C.W. supervised the whole project. Y.S. performed the ALD procedure for cathodes and wrote the manuscript. J.M. tested electrochemical performance and processed the synchrotron‐based measurement data. X.Y. conducted the TEM characterization. H.R. conducted the SEM characterization. W.Z. conducted GITT measurement. L.F. performed in situ XRD. H.H. tested electronic conductivity. X.L., Y.G., and Y.G. participated in the data analysis. All authors have discussed the results and contributed to the final manuscript.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

Supporting Information

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.