{ "sample": "10-elsevier-tiny-body", "threshold": 0.35, "cache_status": { "exists": true, "usable": true, "reason": "cache_page_count_ok", "expected_pdf_pages": 7, "cached_docling_pages": 7, "cached_docling_max_page": 7, "formula_enrichment_enabled": false, "formula_enrichment_scale": 3.0, "formula_nodes": 0, "recognized_formula_nodes": 0 }, "candidate_blocks": [ { "page": 1, "page_count": 7, "bbox": [ 202.01, 329.03, 561.57, 431.96 ], "text": "The practical application of Li-rich layered oxides is impeded by its cycle instability, poor rate capability and serious voltage decay. Here, nano-sized spinel Li 4 Ti 5 O 12 (LTO) is constructed on Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 (LLNCMO). The distinctly heteroepitaxial structure of LTO nanocoating on LLNCMO is corroborated by HAADF-STEM. The LTO coating with fast lithium ion diffusion kinetics acts as “lithium ion pump” when Li + -ions cross over it. The integrated structure can effectively retard oxygen evolution and phase transformation engendering higher capacity and voltage retention. LTO heteroepitaxially coated LLNCMO (LTO@LLNCMO) shows an improvement of initial coulombic efficiency to 74.3% and more stable life with a high capacity retention up to 96.9% at 2C after 500 cycles (40.5% for LLNCMO). LTO@LLNCMO demonstrates minimal voltage fading of 1.33 mV per cycle suggesting suppression of phase conversion to spinel-like structure. The epitaxial spinel modification strategy can be applied to control the surface stability of cathodes.", "category": "front_summary", "coverage": 0.0, "words": 177 }, { "page": 1, "page_count": 7, "bbox": [ 37.61, 476.06, 292.37, 661.99 ], "text": "With the rapid expansion of portable electronics, electric ve- hicles and power grid, the existing lithium-ion batteries (LIBs) as an important power source urgently need to be ameliorated. [ 1 , 2 ] Especially, the insufficient energy-density of the cathode materi- als restricts their large-scale deployment. [ 3 , 4 ] Among the cathode materials studied, Li-rich layered oxides (LLOs) might surmount such limitations due to the redox reaction at higher voltage and a higher capacity surpassing 250 mAh g −1 . [5–9] Nonetheless, the large initial irreversible capacity loss, poor cycling stability, inferior rate capability and the voltage fading jeopardize the practical ap- plication of LLOs cathodes. [10–14] These issues might be caused by the irreversible structural rearrangement and phase transfor- mation to spinel-like structure from surface to the interior. [15– 17] Consequently, the improvement of surface structural stability might effectively raise the capacity and mitigate voltage fading for LLO cathodes. It has been proved that surface coating is a simple and efficient remedy to reform the interfacial stability. [18–27] Although inert", "category": "front_summary", "coverage": 0.0, "words": 172 }, { "page": 4, "page_count": 7, "bbox": [ 37.6, 57.5, 292.29, 96.97 ], "text": "LLNCMO material. In the enlarged display of the (003) peaks ( Fig. 1 b), the peaks of the coated LTO@LLNCMO material obviously shifts to lower values illustrating the increase of c axis, which is beneficial for the Li-ion diffusion along the Li layer. [24]", "category": "scientific_body", "coverage": 0.0, "words": 46 }, { "page": 4, "page_count": 7, "bbox": [ 37.6, 99.34, 292.37, 306.2 ], "text": "The morphologies and microstructures of as-prepared materi- als were observed in SEM and TEM images to confirm the in- fluence of LTO surface modification on the morphology, they are shown in Fig. 1 c–h. SEM images demonstrate that the size of nanoparticles show no obvious variation after LTO surface modi- fication, which is further proved by TEM. The only difference is that nanoparticles of LLNCMO material are aggregated together but LTO@LLNCMO nanoparticles have better dispersion, which is ben- eficial for the infiltration of electrolyte leading to superior elec- trochemical performance. It is clearly observed in the TEM im- ages of LTO@LLNCMO materials that a uniform ultrathin LTO coat- ing layer ( Fig. 1 h marked with red dotted lines) on the surface of LLNCMO nanoparticles with the thickness of approximately 2 nm has been formed. Elemental mappings of bare and LTO-coated LL- NCMO materials shown in Fig. 2 were detected to illustrate the dis- tribution of transition metal elements. In both bare LLNCMO and surface coated LTO@LLNCMO materials, nickel, cobalt and man- ganese are distributed uniformly with no segregation. Moreover, Ti is homogeneously distributed on the surface of nanoparticles for LTO@LLNCMO material. [31]", "category": "scientific_body", "coverage": 0.0, "words": 195 }, { "page": 4, "page_count": 7, "bbox": [ 37.6, 308.56, 292.41, 745.55 ], "text": "Atomic scale HAADF-STEM was used to clarify the surface and interface structures before and after LTO-coating LLNCMO materi- als, results are shown in Fig. 3 . For bare LLNCMO material ( Fig. 3 a), it is demonstrated that the well crystallization extends from the inside to the surface of the nanoparticles with the identical d - spacing of 0.478 and 0.245 nm which can be assigned to (003) and (101) faces of LLNCMO, respectively. Furthermore, the sur- face structure of LTO@LLNCMO material is obviously different from that of LLNCMO. From the atomic scale HAADF-STEM images of LTO@LLNCMO material ( Fig. 3 b–f), the clear spinel LTO structure with a thickness of about 2 nm is found on the surface of lay- ered LLNCMO nanoparticles and the two phases form an integrated structure in one nanoparticle, which has been referred to as het- eroepitaxial structure due to the same oxygen arrays of layered LL- NCMO and spinel LTO phases. [ 29 , 31 , 32 ] The interplanar distances of LTO coating are measured to be 0.244 and 0.238 nm correspond- ing to the (400) and (222) planes of spinel LTO structure, respec- tively. In addition, LTO surface spinel phase with excellent lithium ion transmission features can supply effective 3D diffusion routes for lithium ions which play a similar role to the ion pumps in an- imals and plants, as shown in Fig. 3 g. LTO spinel phase on the surface of LLNCMO is expected to stabilize the structure, suppress irreversible oxygen release and voltage decay during Li + -ion inser- tion/extraction leading to more outstanding electrochemical perfor- mance. [ 29 , 31 ] XPS measurements were applied to characterize the surface compositions and chemical states of the transition metal ions and oxygen on the bare LLNCMO and LTO-coated LTO@LLNCMO mate- rials, as shown in Figs. S1 and 4. Compared to the sum XPS pattern of as-prepared LLNCMO (Fig. S1a), there appear two new peaks at 455 ~ 468 eV for LTO@LLNCMO which can be assigned to the bind- ing energy value of Ti2p. [ 28 , 30 ] It further testifies that spinel LTO is successfully constructed on the surface of LLNCMO nanoparti- cles. In the Ni2p and Co2p spectra in Fig. S1b and c, it can be found that the binding energy has no distinct variation before and after LTO coating and the chemical states of Ni and Co are + 2 and + 3, respectively. The peak locations of Mn2p 1/2 and Mn2p 3/2 are almost unchanged after LTO coated, which is further proved by the same level of Mn3s peak splitting (Figs. S1d and 4a). [ 27 , 29 ] The Mn3s peak splitting illustrates that the oxidation states of Mn ions in both LLNCMO and LTO@LLNCMO materials are slightly less than + 4. This resulted from Mn 2 + ions being oxidized insufficiently to", "category": "scientific_body", "coverage": 0.0, "words": 462 }, { "page": 4, "page_count": 7, "bbox": [ 306.6, 684.04, 561.44, 746.04 ], "text": "Mn 4 + ions or oxygen vacancies are engendered in the synthetic process. [27] After LTO-modification, the average oxidation states of Mn ions in LTO@LLNCMO material still maintain about + 3.77 with the same level in LLNCMO material. [ 27 , 28 ] The XPS of O1s in LTO@LLNCMO materials as shown in Fig. 4 b still retain two peaks at 529.33 and 531.64 eV which belong to lattice oxygen in the", "category": "scientific_body", "coverage": 0.0, "words": 74 }, { "page": 5, "page_count": 7, "bbox": [ 37.6, 494.19, 292.37, 566.59 ], "text": "crystal lattice and impurity lithium oxides on the surface of the layered cathode materials, respectively. [ 27 , 32 ] Compared to the O1s spectra in LLNCMO material, the peak at 533.97 eV disappears indicating more excellent surface stability in LTO coated material LTO@LLNCMO. From the Ti2p spectrum in LTO@LLNCMO material ( Fig. 4 c), the peaks at 458.33 and 463.94 eV can be attributed to Ti2p 3/2 and Ti2p 1/2 of Ti 4 + .", "category": "scientific_body", "coverage": 0.0, "words": 77 }, { "page": 5, "page_count": 7, "bbox": [ 306.6, 494.19, 561.4, 742.89 ], "text": "To examine the variation of cycling stability and the voltage de- cay of LLNCMO modified by ultrathin LTO heteroepitaxial coating layer at high current density, the charge and discharge of LLNCMO and LTO@LLNCMO cathodes are continued to 500 cycles at 2 C as shown in Fig. 5 c. The discharge capacities are 153.5 and 199.3 mAh g –1 for LLNCMO and LTO@LLNCMO cathodes at 2 C, respectively, il- lustrating heightened lithium storage capacity at large current den- sity. After 500 charge and discharge cycles, the discharge capacity of LLNCMO cathode rapidly deteriorates to 62.1 mAh g –1 . In con- trast, LTO@LLNCMO cathode preserves high discharge capacity with a capacity retention of 96.9 % during 500 prolonged cycles, reflect- ing the benefits of the LTO heteroepitaxial coating layer on the structural stability. [30] Furthermore, the average discharge volt- age of LLNCMO cathode quickly diminishes to 2.39 V in 500 charge and discharge cycles producing a strong voltage decay of 2.34 mV cycle −1 , much larger than 1.33 mV cycle −1 for LTO@LLNCMO cath- ode. It demonstrates that LTO heteroepitaxial coating layer can ef- fectively retard the structure transformation during repeated lithi- ation and delithiation, which is further certified by the TEM im- ages after 50 cycles (Fig. S2). [ 27 , 29 ] The rate capabilities of LL- NCMO and LTO@LLNCMO cathodes are also tested at different cur- rent densities, results are shown in Fig. 5 d. The average discharge capacities are 286.7, 283.4, 255.4, 223.8, 199.3, 148.7 and 104.7 mAh g –1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 C for LTO@LLNCMO cathode,", "category": "scientific_body", "coverage": 0.0, "words": 276 }, { "page": 6, "page_count": 7, "bbox": [ 306.6, 141.52, 561.29, 191.46 ], "text": "This work was supported by the National Natural Science Foun- dation of China ( 51702148 , 51802139 ), Henan Science and Tech- nology Research Program ( 182102410074 , 172102410047 ) and the Training Plan of Young Backbone Teachers in Colleges and Univer- sities of Henan Province ( 2019GGJS244 , 2018GGJS156 ) .", "category": "back_matter", "coverage": 0.0, "words": 39 }, { "page": 6, "page_count": 7, "bbox": [ 37.6, 57.5, 292.37, 735.08 ], "text": "respectively. Except for at 0.1 C, the average discharge capacities of the LTO-coated cathode are all enhanced at every current den- sity. Specifically, the average discharge capacity of LTO@LLNCMO cathode is 104.7 mAh g –1 at 10 C considerably higher than that of LLNCMO cathode. Compared with the electrochemical performance of bare LLNCMO cathode, LTO@LLNCMO evinces impressively opti- mum long cycle life and the charge/discharge ability at large cur- rent density due to the distinctive surface and interface structure. The integrated structure of LTO spinel heteroepitaxial coating layer and host LLNCMO guarantees the super stability and smoother channels for Li + ions insertion/extraction. In addition, ultrathin LTO with 3-dimensional diffusion path can further improve the kinetic process of lithium ion diffusion without reducing the electronic conductivity. To further understand the redox process, cyclic voltammetry (CV) was done between 2.0 and 4.8 V with 0.1 mV s −1 (Fig. S3). The redox peaks of LLNCMO and LTO@LLNCMO cathodes both are consistent with the reported Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 system in the literatures. [29–31] The anodic peaks above 4.5 V can be assigned to the extraction of lithium from Li 2 MnO 3 accompanied by oxy- gen release. [13] Another broad anodic peaks at about 4.1 V corre- sponds to the oxidation of the transition metal ions Ni 2 + and Co 3 + . [ 13 , 33 ] The cathodic peaks located at approximately 3.7 V belong to the reduction of the transition metal ions. The anodic peaks shift to lower potential and the cathodic peak shifts to higher po- tential for LTO@LLNCMO cathode compared with the bare LLNCMO, which suggests the improved reversibility of redox reaction after ultrathin LTO coated. The kinetics of electrochemical reaction in LLNCMO and LTO@LLNCMO cathodes can be surveyed by electrochemical impedance spectroscopy. The Nyquist plots before and after 50 cy- cles at 0.1 C fitted with equivalent circuit (Fig. S4–S6) are all com- posed of a semicircle and a sloped line, which are the impedance of the solid electrolyte interface ( R SEI ) and the charge transfer resis- tance ( R ct ) and the Warburg resistance ( W ) reflecting the lithium- ion diffusion process in the solid electrode, respectively. [34– 37] Before cycling, the resistance of R SEI and R ct displays 232.0 \u0003 for LTO@LLNCMO cathode, which is much smaller than 625.2 \u0003 for bare LLNCMO cathode. It illustrates that LTO can boost the charge transfer reaction. Although the values of R SEI and R ct of the two cathodes are both abated after 50 charge/discharge cycles, LTO@LLNCMO cathode still manifests a lower resistance than that of LLNCMO cathode. This implies that the formation of stronger SEI film on the surface of ultrathin LTO heteroepitaxial coating layer with prominent rate capability. In summary, we designed a spinel Li 4 Ti 5 O 12 nano-sized coat- ing on the surface of Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 material to alleviate the inferior electrochemical performances, especially serious volt- age loss. The heteroepitaxial structure is formed between nano- sized LTO coating and LLNCMO material because of the compatibil- ity of the lattice structures. As cathode for LIBs, LTO heteroepitax- ially coated LLNCMO (LTO@LLNCMO) cathode displays higher ini- tial coulombic efficiency of 74.3 % than that of 60.6 % for LLNCMO cathode. The cycle stability is also enhanced up to 96.9 % capacity retention at 2 C after 500 cycles (40.5 % for LLNCMO). Moreover, the voltage decay is reduced from 2.34 to 1.33 mV cycle –1 after coating with spinel LTO. The excellent electrochemical properties indicate that the epitaxial LTO coating could restrain the surface phase transformation into spinel-like structure for LLNCMO mate- rial. In addition, spinel LTO coating with 3D lithium ion diffusion channels and more prominent lithium ion diffusion kinetics can drive Li + ions quickly across the interface of LLNCMO and elec- trolyte acting as “lithium ion pump”. This spinel LTO heteroepitax- ially coated strategy can be extended to modify other LLOs for high performance next-generation LIBs.", "category": "scientific_body", "coverage": 0.0, "words": 684 } ], "low_coverage_blocks": [ { "page": 1, "page_count": 7, "bbox": [ 202.01, 329.03, 561.57, 431.96 ], "text": "The practical application of Li-rich layered oxides is impeded by its cycle instability, poor rate capability and serious voltage decay. Here, nano-sized spinel Li 4 Ti 5 O 12 (LTO) is constructed on Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 (LLNCMO). The distinctly heteroepitaxial structure of LTO nanocoating on LLNCMO is corroborated by HAADF-STEM. The LTO coating with fast lithium ion diffusion kinetics acts as “lithium ion pump” when Li + -ions cross over it. The integrated structure can effectively retard oxygen evolution and phase transformation engendering higher capacity and voltage retention. LTO heteroepitaxially coated LLNCMO (LTO@LLNCMO) shows an improvement of initial coulombic efficiency to 74.3% and more stable life with a high capacity retention up to 96.9% at 2C after 500 cycles (40.5% for LLNCMO). LTO@LLNCMO demonstrates minimal voltage fading of 1.33 mV per cycle suggesting suppression of phase conversion to spinel-like structure. The epitaxial spinel modification strategy can be applied to control the surface stability of cathodes.", "category": "front_summary", "coverage": 0.0, "words": 177 }, { "page": 1, "page_count": 7, "bbox": [ 37.61, 476.06, 292.37, 661.99 ], "text": "With the rapid expansion of portable electronics, electric ve- hicles and power grid, the existing lithium-ion batteries (LIBs) as an important power source urgently need to be ameliorated. [ 1 , 2 ] Especially, the insufficient energy-density of the cathode materi- als restricts their large-scale deployment. [ 3 , 4 ] Among the cathode materials studied, Li-rich layered oxides (LLOs) might surmount such limitations due to the redox reaction at higher voltage and a higher capacity surpassing 250 mAh g −1 . [5–9] Nonetheless, the large initial irreversible capacity loss, poor cycling stability, inferior rate capability and the voltage fading jeopardize the practical ap- plication of LLOs cathodes. [10–14] These issues might be caused by the irreversible structural rearrangement and phase transfor- mation to spinel-like structure from surface to the interior. [15– 17] Consequently, the improvement of surface structural stability might effectively raise the capacity and mitigate voltage fading for LLO cathodes. It has been proved that surface coating is a simple and efficient remedy to reform the interfacial stability. [18–27] Although inert", "category": "front_summary", "coverage": 0.0, "words": 172 }, { "page": 4, "page_count": 7, "bbox": [ 37.6, 57.5, 292.29, 96.97 ], "text": "LLNCMO material. In the enlarged display of the (003) peaks ( Fig. 1 b), the peaks of the coated LTO@LLNCMO material obviously shifts to lower values illustrating the increase of c axis, which is beneficial for the Li-ion diffusion along the Li layer. [24]", "category": "scientific_body", "coverage": 0.0, "words": 46 }, { "page": 4, "page_count": 7, "bbox": [ 37.6, 99.34, 292.37, 306.2 ], "text": "The morphologies and microstructures of as-prepared materi- als were observed in SEM and TEM images to confirm the in- fluence of LTO surface modification on the morphology, they are shown in Fig. 1 c–h. SEM images demonstrate that the size of nanoparticles show no obvious variation after LTO surface modi- fication, which is further proved by TEM. The only difference is that nanoparticles of LLNCMO material are aggregated together but LTO@LLNCMO nanoparticles have better dispersion, which is ben- eficial for the infiltration of electrolyte leading to superior elec- trochemical performance. It is clearly observed in the TEM im- ages of LTO@LLNCMO materials that a uniform ultrathin LTO coat- ing layer ( Fig. 1 h marked with red dotted lines) on the surface of LLNCMO nanoparticles with the thickness of approximately 2 nm has been formed. Elemental mappings of bare and LTO-coated LL- NCMO materials shown in Fig. 2 were detected to illustrate the dis- tribution of transition metal elements. In both bare LLNCMO and surface coated LTO@LLNCMO materials, nickel, cobalt and man- ganese are distributed uniformly with no segregation. Moreover, Ti is homogeneously distributed on the surface of nanoparticles for LTO@LLNCMO material. [31]", "category": "scientific_body", "coverage": 0.0, "words": 195 }, { "page": 4, "page_count": 7, "bbox": [ 37.6, 308.56, 292.41, 745.55 ], "text": "Atomic scale HAADF-STEM was used to clarify the surface and interface structures before and after LTO-coating LLNCMO materi- als, results are shown in Fig. 3 . For bare LLNCMO material ( Fig. 3 a), it is demonstrated that the well crystallization extends from the inside to the surface of the nanoparticles with the identical d - spacing of 0.478 and 0.245 nm which can be assigned to (003) and (101) faces of LLNCMO, respectively. Furthermore, the sur- face structure of LTO@LLNCMO material is obviously different from that of LLNCMO. From the atomic scale HAADF-STEM images of LTO@LLNCMO material ( Fig. 3 b–f), the clear spinel LTO structure with a thickness of about 2 nm is found on the surface of lay- ered LLNCMO nanoparticles and the two phases form an integrated structure in one nanoparticle, which has been referred to as het- eroepitaxial structure due to the same oxygen arrays of layered LL- NCMO and spinel LTO phases. [ 29 , 31 , 32 ] The interplanar distances of LTO coating are measured to be 0.244 and 0.238 nm correspond- ing to the (400) and (222) planes of spinel LTO structure, respec- tively. In addition, LTO surface spinel phase with excellent lithium ion transmission features can supply effective 3D diffusion routes for lithium ions which play a similar role to the ion pumps in an- imals and plants, as shown in Fig. 3 g. LTO spinel phase on the surface of LLNCMO is expected to stabilize the structure, suppress irreversible oxygen release and voltage decay during Li + -ion inser- tion/extraction leading to more outstanding electrochemical perfor- mance. [ 29 , 31 ] XPS measurements were applied to characterize the surface compositions and chemical states of the transition metal ions and oxygen on the bare LLNCMO and LTO-coated LTO@LLNCMO mate- rials, as shown in Figs. S1 and 4. Compared to the sum XPS pattern of as-prepared LLNCMO (Fig. S1a), there appear two new peaks at 455 ~ 468 eV for LTO@LLNCMO which can be assigned to the bind- ing energy value of Ti2p. [ 28 , 30 ] It further testifies that spinel LTO is successfully constructed on the surface of LLNCMO nanoparti- cles. In the Ni2p and Co2p spectra in Fig. S1b and c, it can be found that the binding energy has no distinct variation before and after LTO coating and the chemical states of Ni and Co are + 2 and + 3, respectively. The peak locations of Mn2p 1/2 and Mn2p 3/2 are almost unchanged after LTO coated, which is further proved by the same level of Mn3s peak splitting (Figs. S1d and 4a). [ 27 , 29 ] The Mn3s peak splitting illustrates that the oxidation states of Mn ions in both LLNCMO and LTO@LLNCMO materials are slightly less than + 4. This resulted from Mn 2 + ions being oxidized insufficiently to", "category": "scientific_body", "coverage": 0.0, "words": 462 }, { "page": 4, "page_count": 7, "bbox": [ 306.6, 684.04, 561.44, 746.04 ], "text": "Mn 4 + ions or oxygen vacancies are engendered in the synthetic process. [27] After LTO-modification, the average oxidation states of Mn ions in LTO@LLNCMO material still maintain about + 3.77 with the same level in LLNCMO material. [ 27 , 28 ] The XPS of O1s in LTO@LLNCMO materials as shown in Fig. 4 b still retain two peaks at 529.33 and 531.64 eV which belong to lattice oxygen in the", "category": "scientific_body", "coverage": 0.0, "words": 74 }, { "page": 5, "page_count": 7, "bbox": [ 37.6, 494.19, 292.37, 566.59 ], "text": "crystal lattice and impurity lithium oxides on the surface of the layered cathode materials, respectively. [ 27 , 32 ] Compared to the O1s spectra in LLNCMO material, the peak at 533.97 eV disappears indicating more excellent surface stability in LTO coated material LTO@LLNCMO. From the Ti2p spectrum in LTO@LLNCMO material ( Fig. 4 c), the peaks at 458.33 and 463.94 eV can be attributed to Ti2p 3/2 and Ti2p 1/2 of Ti 4 + .", "category": "scientific_body", "coverage": 0.0, "words": 77 }, { "page": 5, "page_count": 7, "bbox": [ 306.6, 494.19, 561.4, 742.89 ], "text": "To examine the variation of cycling stability and the voltage de- cay of LLNCMO modified by ultrathin LTO heteroepitaxial coating layer at high current density, the charge and discharge of LLNCMO and LTO@LLNCMO cathodes are continued to 500 cycles at 2 C as shown in Fig. 5 c. The discharge capacities are 153.5 and 199.3 mAh g –1 for LLNCMO and LTO@LLNCMO cathodes at 2 C, respectively, il- lustrating heightened lithium storage capacity at large current den- sity. After 500 charge and discharge cycles, the discharge capacity of LLNCMO cathode rapidly deteriorates to 62.1 mAh g –1 . In con- trast, LTO@LLNCMO cathode preserves high discharge capacity with a capacity retention of 96.9 % during 500 prolonged cycles, reflect- ing the benefits of the LTO heteroepitaxial coating layer on the structural stability. [30] Furthermore, the average discharge volt- age of LLNCMO cathode quickly diminishes to 2.39 V in 500 charge and discharge cycles producing a strong voltage decay of 2.34 mV cycle −1 , much larger than 1.33 mV cycle −1 for LTO@LLNCMO cath- ode. It demonstrates that LTO heteroepitaxial coating layer can ef- fectively retard the structure transformation during repeated lithi- ation and delithiation, which is further certified by the TEM im- ages after 50 cycles (Fig. S2). [ 27 , 29 ] The rate capabilities of LL- NCMO and LTO@LLNCMO cathodes are also tested at different cur- rent densities, results are shown in Fig. 5 d. The average discharge capacities are 286.7, 283.4, 255.4, 223.8, 199.3, 148.7 and 104.7 mAh g –1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 C for LTO@LLNCMO cathode,", "category": "scientific_body", "coverage": 0.0, "words": 276 }, { "page": 6, "page_count": 7, "bbox": [ 306.6, 141.52, 561.29, 191.46 ], "text": "This work was supported by the National Natural Science Foun- dation of China ( 51702148 , 51802139 ), Henan Science and Tech- nology Research Program ( 182102410074 , 172102410047 ) and the Training Plan of Young Backbone Teachers in Colleges and Univer- sities of Henan Province ( 2019GGJS244 , 2018GGJS156 ) .", "category": "back_matter", "coverage": 0.0, "words": 39 }, { "page": 6, "page_count": 7, "bbox": [ 37.6, 57.5, 292.37, 735.08 ], "text": "respectively. Except for at 0.1 C, the average discharge capacities of the LTO-coated cathode are all enhanced at every current den- sity. Specifically, the average discharge capacity of LTO@LLNCMO cathode is 104.7 mAh g –1 at 10 C considerably higher than that of LLNCMO cathode. Compared with the electrochemical performance of bare LLNCMO cathode, LTO@LLNCMO evinces impressively opti- mum long cycle life and the charge/discharge ability at large cur- rent density due to the distinctive surface and interface structure. The integrated structure of LTO spinel heteroepitaxial coating layer and host LLNCMO guarantees the super stability and smoother channels for Li + ions insertion/extraction. In addition, ultrathin LTO with 3-dimensional diffusion path can further improve the kinetic process of lithium ion diffusion without reducing the electronic conductivity. To further understand the redox process, cyclic voltammetry (CV) was done between 2.0 and 4.8 V with 0.1 mV s −1 (Fig. S3). The redox peaks of LLNCMO and LTO@LLNCMO cathodes both are consistent with the reported Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 system in the literatures. [29–31] The anodic peaks above 4.5 V can be assigned to the extraction of lithium from Li 2 MnO 3 accompanied by oxy- gen release. [13] Another broad anodic peaks at about 4.1 V corre- sponds to the oxidation of the transition metal ions Ni 2 + and Co 3 + . [ 13 , 33 ] The cathodic peaks located at approximately 3.7 V belong to the reduction of the transition metal ions. The anodic peaks shift to lower potential and the cathodic peak shifts to higher po- tential for LTO@LLNCMO cathode compared with the bare LLNCMO, which suggests the improved reversibility of redox reaction after ultrathin LTO coated. The kinetics of electrochemical reaction in LLNCMO and LTO@LLNCMO cathodes can be surveyed by electrochemical impedance spectroscopy. The Nyquist plots before and after 50 cy- cles at 0.1 C fitted with equivalent circuit (Fig. S4–S6) are all com- posed of a semicircle and a sloped line, which are the impedance of the solid electrolyte interface ( R SEI ) and the charge transfer resis- tance ( R ct ) and the Warburg resistance ( W ) reflecting the lithium- ion diffusion process in the solid electrode, respectively. [34– 37] Before cycling, the resistance of R SEI and R ct displays 232.0 \u0003 for LTO@LLNCMO cathode, which is much smaller than 625.2 \u0003 for bare LLNCMO cathode. It illustrates that LTO can boost the charge transfer reaction. Although the values of R SEI and R ct of the two cathodes are both abated after 50 charge/discharge cycles, LTO@LLNCMO cathode still manifests a lower resistance than that of LLNCMO cathode. This implies that the formation of stronger SEI film on the surface of ultrathin LTO heteroepitaxial coating layer with prominent rate capability. In summary, we designed a spinel Li 4 Ti 5 O 12 nano-sized coat- ing on the surface of Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 material to alleviate the inferior electrochemical performances, especially serious volt- age loss. The heteroepitaxial structure is formed between nano- sized LTO coating and LLNCMO material because of the compatibil- ity of the lattice structures. As cathode for LIBs, LTO heteroepitax- ially coated LLNCMO (LTO@LLNCMO) cathode displays higher ini- tial coulombic efficiency of 74.3 % than that of 60.6 % for LLNCMO cathode. The cycle stability is also enhanced up to 96.9 % capacity retention at 2 C after 500 cycles (40.5 % for LLNCMO). Moreover, the voltage decay is reduced from 2.34 to 1.33 mV cycle –1 after coating with spinel LTO. The excellent electrochemical properties indicate that the epitaxial LTO coating could restrain the surface phase transformation into spinel-like structure for LLNCMO mate- rial. In addition, spinel LTO coating with 3D lithium ion diffusion channels and more prominent lithium ion diffusion kinetics can drive Li + ions quickly across the interface of LLNCMO and elec- trolyte acting as “lithium ion pump”. This spinel LTO heteroepitax- ially coated strategy can be extended to modify other LLOs for high performance next-generation LIBs.", "category": "scientific_body", "coverage": 0.0, "words": 684 } ], "high_risk_body_missing_blocks": [ { "page": 4, "page_count": 7, "bbox": [ 37.6, 57.5, 292.29, 96.97 ], "text": "LLNCMO material. In the enlarged display of the (003) peaks ( Fig. 1 b), the peaks of the coated LTO@LLNCMO material obviously shifts to lower values illustrating the increase of c axis, which is beneficial for the Li-ion diffusion along the Li layer. [24]", "category": "scientific_body", "coverage": 0.0, "words": 46 }, { "page": 4, "page_count": 7, "bbox": [ 37.6, 99.34, 292.37, 306.2 ], "text": "The morphologies and microstructures of as-prepared materi- als were observed in SEM and TEM images to confirm the in- fluence of LTO surface modification on the morphology, they are shown in Fig. 1 c–h. SEM images demonstrate that the size of nanoparticles show no obvious variation after LTO surface modi- fication, which is further proved by TEM. The only difference is that nanoparticles of LLNCMO material are aggregated together but LTO@LLNCMO nanoparticles have better dispersion, which is ben- eficial for the infiltration of electrolyte leading to superior elec- trochemical performance. It is clearly observed in the TEM im- ages of LTO@LLNCMO materials that a uniform ultrathin LTO coat- ing layer ( Fig. 1 h marked with red dotted lines) on the surface of LLNCMO nanoparticles with the thickness of approximately 2 nm has been formed. Elemental mappings of bare and LTO-coated LL- NCMO materials shown in Fig. 2 were detected to illustrate the dis- tribution of transition metal elements. In both bare LLNCMO and surface coated LTO@LLNCMO materials, nickel, cobalt and man- ganese are distributed uniformly with no segregation. Moreover, Ti is homogeneously distributed on the surface of nanoparticles for LTO@LLNCMO material. [31]", "category": "scientific_body", "coverage": 0.0, "words": 195 }, { "page": 4, "page_count": 7, "bbox": [ 37.6, 308.56, 292.41, 745.55 ], "text": "Atomic scale HAADF-STEM was used to clarify the surface and interface structures before and after LTO-coating LLNCMO materi- als, results are shown in Fig. 3 . For bare LLNCMO material ( Fig. 3 a), it is demonstrated that the well crystallization extends from the inside to the surface of the nanoparticles with the identical d - spacing of 0.478 and 0.245 nm which can be assigned to (003) and (101) faces of LLNCMO, respectively. Furthermore, the sur- face structure of LTO@LLNCMO material is obviously different from that of LLNCMO. From the atomic scale HAADF-STEM images of LTO@LLNCMO material ( Fig. 3 b–f), the clear spinel LTO structure with a thickness of about 2 nm is found on the surface of lay- ered LLNCMO nanoparticles and the two phases form an integrated structure in one nanoparticle, which has been referred to as het- eroepitaxial structure due to the same oxygen arrays of layered LL- NCMO and spinel LTO phases. [ 29 , 31 , 32 ] The interplanar distances of LTO coating are measured to be 0.244 and 0.238 nm correspond- ing to the (400) and (222) planes of spinel LTO structure, respec- tively. In addition, LTO surface spinel phase with excellent lithium ion transmission features can supply effective 3D diffusion routes for lithium ions which play a similar role to the ion pumps in an- imals and plants, as shown in Fig. 3 g. LTO spinel phase on the surface of LLNCMO is expected to stabilize the structure, suppress irreversible oxygen release and voltage decay during Li + -ion inser- tion/extraction leading to more outstanding electrochemical perfor- mance. [ 29 , 31 ] XPS measurements were applied to characterize the surface compositions and chemical states of the transition metal ions and oxygen on the bare LLNCMO and LTO-coated LTO@LLNCMO mate- rials, as shown in Figs. S1 and 4. Compared to the sum XPS pattern of as-prepared LLNCMO (Fig. S1a), there appear two new peaks at 455 ~ 468 eV for LTO@LLNCMO which can be assigned to the bind- ing energy value of Ti2p. [ 28 , 30 ] It further testifies that spinel LTO is successfully constructed on the surface of LLNCMO nanoparti- cles. In the Ni2p and Co2p spectra in Fig. S1b and c, it can be found that the binding energy has no distinct variation before and after LTO coating and the chemical states of Ni and Co are + 2 and + 3, respectively. The peak locations of Mn2p 1/2 and Mn2p 3/2 are almost unchanged after LTO coated, which is further proved by the same level of Mn3s peak splitting (Figs. S1d and 4a). [ 27 , 29 ] The Mn3s peak splitting illustrates that the oxidation states of Mn ions in both LLNCMO and LTO@LLNCMO materials are slightly less than + 4. This resulted from Mn 2 + ions being oxidized insufficiently to", "category": "scientific_body", "coverage": 0.0, "words": 462 }, { "page": 4, "page_count": 7, "bbox": [ 306.6, 684.04, 561.44, 746.04 ], "text": "Mn 4 + ions or oxygen vacancies are engendered in the synthetic process. [27] After LTO-modification, the average oxidation states of Mn ions in LTO@LLNCMO material still maintain about + 3.77 with the same level in LLNCMO material. [ 27 , 28 ] The XPS of O1s in LTO@LLNCMO materials as shown in Fig. 4 b still retain two peaks at 529.33 and 531.64 eV which belong to lattice oxygen in the", "category": "scientific_body", "coverage": 0.0, "words": 74 }, { "page": 5, "page_count": 7, "bbox": [ 37.6, 494.19, 292.37, 566.59 ], "text": "crystal lattice and impurity lithium oxides on the surface of the layered cathode materials, respectively. [ 27 , 32 ] Compared to the O1s spectra in LLNCMO material, the peak at 533.97 eV disappears indicating more excellent surface stability in LTO coated material LTO@LLNCMO. From the Ti2p spectrum in LTO@LLNCMO material ( Fig. 4 c), the peaks at 458.33 and 463.94 eV can be attributed to Ti2p 3/2 and Ti2p 1/2 of Ti 4 + .", "category": "scientific_body", "coverage": 0.0, "words": 77 }, { "page": 5, "page_count": 7, "bbox": [ 306.6, 494.19, 561.4, 742.89 ], "text": "To examine the variation of cycling stability and the voltage de- cay of LLNCMO modified by ultrathin LTO heteroepitaxial coating layer at high current density, the charge and discharge of LLNCMO and LTO@LLNCMO cathodes are continued to 500 cycles at 2 C as shown in Fig. 5 c. The discharge capacities are 153.5 and 199.3 mAh g –1 for LLNCMO and LTO@LLNCMO cathodes at 2 C, respectively, il- lustrating heightened lithium storage capacity at large current den- sity. After 500 charge and discharge cycles, the discharge capacity of LLNCMO cathode rapidly deteriorates to 62.1 mAh g –1 . In con- trast, LTO@LLNCMO cathode preserves high discharge capacity with a capacity retention of 96.9 % during 500 prolonged cycles, reflect- ing the benefits of the LTO heteroepitaxial coating layer on the structural stability. [30] Furthermore, the average discharge volt- age of LLNCMO cathode quickly diminishes to 2.39 V in 500 charge and discharge cycles producing a strong voltage decay of 2.34 mV cycle −1 , much larger than 1.33 mV cycle −1 for LTO@LLNCMO cath- ode. It demonstrates that LTO heteroepitaxial coating layer can ef- fectively retard the structure transformation during repeated lithi- ation and delithiation, which is further certified by the TEM im- ages after 50 cycles (Fig. S2). [ 27 , 29 ] The rate capabilities of LL- NCMO and LTO@LLNCMO cathodes are also tested at different cur- rent densities, results are shown in Fig. 5 d. The average discharge capacities are 286.7, 283.4, 255.4, 223.8, 199.3, 148.7 and 104.7 mAh g –1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 C for LTO@LLNCMO cathode,", "category": "scientific_body", "coverage": 0.0, "words": 276 }, { "page": 6, "page_count": 7, "bbox": [ 37.6, 57.5, 292.37, 735.08 ], "text": "respectively. Except for at 0.1 C, the average discharge capacities of the LTO-coated cathode are all enhanced at every current den- sity. Specifically, the average discharge capacity of LTO@LLNCMO cathode is 104.7 mAh g –1 at 10 C considerably higher than that of LLNCMO cathode. Compared with the electrochemical performance of bare LLNCMO cathode, LTO@LLNCMO evinces impressively opti- mum long cycle life and the charge/discharge ability at large cur- rent density due to the distinctive surface and interface structure. The integrated structure of LTO spinel heteroepitaxial coating layer and host LLNCMO guarantees the super stability and smoother channels for Li + ions insertion/extraction. In addition, ultrathin LTO with 3-dimensional diffusion path can further improve the kinetic process of lithium ion diffusion without reducing the electronic conductivity. To further understand the redox process, cyclic voltammetry (CV) was done between 2.0 and 4.8 V with 0.1 mV s −1 (Fig. S3). The redox peaks of LLNCMO and LTO@LLNCMO cathodes both are consistent with the reported Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 system in the literatures. [29–31] The anodic peaks above 4.5 V can be assigned to the extraction of lithium from Li 2 MnO 3 accompanied by oxy- gen release. [13] Another broad anodic peaks at about 4.1 V corre- sponds to the oxidation of the transition metal ions Ni 2 + and Co 3 + . [ 13 , 33 ] The cathodic peaks located at approximately 3.7 V belong to the reduction of the transition metal ions. The anodic peaks shift to lower potential and the cathodic peak shifts to higher po- tential for LTO@LLNCMO cathode compared with the bare LLNCMO, which suggests the improved reversibility of redox reaction after ultrathin LTO coated. The kinetics of electrochemical reaction in LLNCMO and LTO@LLNCMO cathodes can be surveyed by electrochemical impedance spectroscopy. The Nyquist plots before and after 50 cy- cles at 0.1 C fitted with equivalent circuit (Fig. S4–S6) are all com- posed of a semicircle and a sloped line, which are the impedance of the solid electrolyte interface ( R SEI ) and the charge transfer resis- tance ( R ct ) and the Warburg resistance ( W ) reflecting the lithium- ion diffusion process in the solid electrode, respectively. [34– 37] Before cycling, the resistance of R SEI and R ct displays 232.0 \u0003 for LTO@LLNCMO cathode, which is much smaller than 625.2 \u0003 for bare LLNCMO cathode. It illustrates that LTO can boost the charge transfer reaction. Although the values of R SEI and R ct of the two cathodes are both abated after 50 charge/discharge cycles, LTO@LLNCMO cathode still manifests a lower resistance than that of LLNCMO cathode. This implies that the formation of stronger SEI film on the surface of ultrathin LTO heteroepitaxial coating layer with prominent rate capability. In summary, we designed a spinel Li 4 Ti 5 O 12 nano-sized coat- ing on the surface of Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 material to alleviate the inferior electrochemical performances, especially serious volt- age loss. The heteroepitaxial structure is formed between nano- sized LTO coating and LLNCMO material because of the compatibil- ity of the lattice structures. As cathode for LIBs, LTO heteroepitax- ially coated LLNCMO (LTO@LLNCMO) cathode displays higher ini- tial coulombic efficiency of 74.3 % than that of 60.6 % for LLNCMO cathode. The cycle stability is also enhanced up to 96.9 % capacity retention at 2 C after 500 cycles (40.5 % for LLNCMO). Moreover, the voltage decay is reduced from 2.34 to 1.33 mV cycle –1 after coating with spinel LTO. The excellent electrochemical properties indicate that the epitaxial LTO coating could restrain the surface phase transformation into spinel-like structure for LLNCMO mate- rial. In addition, spinel LTO coating with 3D lithium ion diffusion channels and more prominent lithium ion diffusion kinetics can drive Li + ions quickly across the interface of LLNCMO and elec- trolyte acting as “lithium ion pump”. This spinel LTO heteroepitax- ially coated strategy can be extended to modify other LLOs for high performance next-generation LIBs.", "category": "scientific_body", "coverage": 0.0, "words": 684 } ], "front_summary_missing_blocks": [ { "page": 1, "page_count": 7, "bbox": [ 202.01, 329.03, 561.57, 431.96 ], "text": "The practical application of Li-rich layered oxides is impeded by its cycle instability, poor rate capability and serious voltage decay. Here, nano-sized spinel Li 4 Ti 5 O 12 (LTO) is constructed on Li 1.2 Ni 0.13 Co 0.13 Mn 0.54 O 2 (LLNCMO). The distinctly heteroepitaxial structure of LTO nanocoating on LLNCMO is corroborated by HAADF-STEM. The LTO coating with fast lithium ion diffusion kinetics acts as “lithium ion pump” when Li + -ions cross over it. The integrated structure can effectively retard oxygen evolution and phase transformation engendering higher capacity and voltage retention. LTO heteroepitaxially coated LLNCMO (LTO@LLNCMO) shows an improvement of initial coulombic efficiency to 74.3% and more stable life with a high capacity retention up to 96.9% at 2C after 500 cycles (40.5% for LLNCMO). LTO@LLNCMO demonstrates minimal voltage fading of 1.33 mV per cycle suggesting suppression of phase conversion to spinel-like structure. The epitaxial spinel modification strategy can be applied to control the surface stability of cathodes.", "category": "front_summary", "coverage": 0.0, "words": 177 }, { "page": 1, "page_count": 7, "bbox": [ 37.61, 476.06, 292.37, 661.99 ], "text": "With the rapid expansion of portable electronics, electric ve- hicles and power grid, the existing lithium-ion batteries (LIBs) as an important power source urgently need to be ameliorated. [ 1 , 2 ] Especially, the insufficient energy-density of the cathode materi- als restricts their large-scale deployment. [ 3 , 4 ] Among the cathode materials studied, Li-rich layered oxides (LLOs) might surmount such limitations due to the redox reaction at higher voltage and a higher capacity surpassing 250 mAh g −1 . [5–9] Nonetheless, the large initial irreversible capacity loss, poor cycling stability, inferior rate capability and the voltage fading jeopardize the practical ap- plication of LLOs cathodes. [10–14] These issues might be caused by the irreversible structural rearrangement and phase transfor- mation to spinel-like structure from surface to the interior. [15– 17] Consequently, the improvement of surface structural stability might effectively raise the capacity and mitigate voltage fading for LLO cathodes. It has been proved that surface coating is a simple and efficient remedy to reform the interfacial stability. [18–27] Although inert", "category": "front_summary", "coverage": 0.0, "words": 172 } ], "back_matter_low_overlap_blocks": [ { "page": 6, "page_count": 7, "bbox": [ 306.6, 141.52, 561.29, 191.46 ], "text": "This work was supported by the National Natural Science Foun- dation of China ( 51702148 , 51802139 ), Henan Science and Tech- nology Research Program ( 182102410074 , 172102410047 ) and the Training Plan of Young Backbone Teachers in Colleges and Univer- sities of Henan Province ( 2019GGJS244 , 2018GGJS156 ) .", "category": "back_matter", "coverage": 0.0, "words": 39 } ] }