{ "sample": "02-jechem-2021-review", "threshold": 0.35, "cache_status": { "exists": true, "usable": true, "reason": "cache_page_count_ok", "expected_pdf_pages": 18, "cached_docling_pages": 18, "cached_docling_max_page": 18, "formula_enrichment_enabled": false, "formula_enrichment_scale": 3.0, "formula_nodes": 0, "recognized_formula_nodes": 0 }, "candidate_blocks": [ { "page": 1, "page_count": 18, "bbox": [ 202.17, 310.38, 557.65, 413.29 ], "text": "Li and Mn rich (LMR) layered oxides, written as xLi2MnO3\u0001(1 \u0003 x)LiMO2 (M = Mn, Ni, Co, Fe, etc.), have been widely reported in recent years due to their high capacity and high energy density. The stable struc- ture and superior performance of LMR oxides make them one of the most promising candidates for the next-generation cathode materials. However, the commercialization of these materials is hindered by several drawbacks, such as low initial Coulombic efficiency, the degradation of voltage and capacity dur- ing cycling, and poor rate performance. This review summarizes research progress in solving these con- cerns of LMR cathodes over the past decade by following three classes of strategies: morphology design, bulk design, and surface modification. We elaborate on the processing procedures, electrochemical per- formance, mechanisms, and limitations of each approach, and finally put forward the concerns left and the possible solutions for the commercialization of LMR cathodes. \u0001 2021 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published", "category": "metadata", "coverage": 0.774, "words": 163 }, { "page": 2, "page_count": 18, "bbox": [ 389.08, 323.94, 557.66, 424.63 ], "text": "Mingjian Zhang got his Ph.D. degree from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences in 2013, then worked there as an assistant research fellow for one year. From 2014 to 2018, he was a postdoc at School of Advanced Materials, Peking University, and became an assistant research professor since 2018. Meanwhile, he was a research scholar in Brookhaven National Lab from 2016 to 2019, then in the University of Chicago since 2019. He has been engaged in the fields of electrode materials for Li- ion batteries, crystal growth and structure analysis of nonlinear optical crystals.", "category": "metadata", "coverage": 0.0, "words": 101 }, { "page": 2, "page_count": 18, "bbox": [ 306.6, 452.98, 557.65, 562.22 ], "text": "Feng Pan, founding Dean of School of Advanced Mate- rials, Peking University Shenzhen Graduate School, got B.S. from Dept. Chemistry, Peking University in 1985 and Ph.D. from Dept. of P&A Chemistry, University of Strathclyde, Glasgow, UK, with ‘‘Patrick D. Ritchie Prize” for the best Ph.D. in 1994. With more than a decade experience in large international incorporations, Prof. Pan has been engaged in fundamental research and product development of novel optoelectronic and energy storage materials and devices. As Chief Scientist, Prof. Pan led eight entities in Shenzhen to win 150 million RMB grant for the national new energy vehicles (power battery) innovation project since 2013.", "category": "metadata", "coverage": 0.0, "words": 108 }, { "page": 4, "page_count": 18, "bbox": [ 37.59, 440.42, 288.71, 514.77 ], "text": "tively in NMC cathode: Ni2+/3+ is responsible for providing capacity, Mn4+ is for maintaining the structural stability, and Co3+ could pro- mote the diffusion of Li+ ions [13–16]. However, the traditional NMC cathode only offers limited improvements in terms of practi- cal capacity (<200 mA h g\u00031) [17]. The key benchmark for the next- generation cathode material is a higher energy density than all cur- rent materials.", "category": "scientific_body", "coverage": 0.758, "words": 66 }, { "page": 4, "page_count": 18, "bbox": [ 37.59, 517.18, 288.72, 744.88 ], "text": "Since the report by Dahn et al. in 2001, Li and Mn rich (LMR) layered oxides xLi2MnO3\u0001(1 \u0003 x)LiMO2 (M = Mn, Ni, Co, Fe, etc.), have been regarded as the next-generation cathode material due to the high specific capacity (>250 mA h g\u00031) [22,23]. As depicted in the formula, LMR layered oxides contain two components, trig- onal LiMO2 and monoclinic Li2MnO3. The Li2MnO3 component could be activated at the first charging above 4.5 V, providing a high capacity coupling with oxygen redox. In addition, manganese is much cheaper and less toxic than cobalt, which makes LMR oxi- des more economically attractive compared to LCO [24]. Neverthe- less, LMR oxides also have a few of drawbacks. (1) The large irreversible capacity loss during the first cycle, which leads to the low initial Coulombic efficiency. (2) Voltage and capacity decay during cycling. (3) Poor rate performance due to the poor elec- tronic conductivity of manganese-based oxide [9,25,26]. In this review, we focus on these challenges for the commercialization of LMR layered oxides and the recent progress to overcome them. In Section 2, the basic crystal structure and the electrochemical behavior of LMR oxides are introduced. In Section 3, the recent pro- gress on overcoming challenges for LMR layered oxides are sum- marized. In Section 4, promising developments and future directions for LMR oxides are suggested.", "category": "scientific_body", "coverage": 0.879, "words": 227 }, { "page": 5, "page_count": 18, "bbox": [ 37.59, 67.44, 288.71, 127.78 ], "text": "independent Fe and Mn rich nanodomains in Li1.2Fe0.4Mn0.4O2 were observed by electron energy-loss spectroscopy (EELS) mea- surements [32]. Besides, the composite model was also supported by Li magic angle spinning (MAS) NMR, extended X-ray absorption fine structure (EXAFS) spectroscopy, and in situ surface enhanced Raman spectroscopy [33–35].", "category": "scientific_body", "coverage": 0.896, "words": 52 }, { "page": 5, "page_count": 18, "bbox": [ 306.6, 105.7, 557.68, 202.33 ], "text": "As shown in Fig. 2(a), about 100 mA h g\u00031 of irreversible capac- ity could be observed during the first cycle for Li1.2Ni0.2Mn0.6O2, leading to a low initial Coulombic efficiency of 72.3%, much lower than that of NMC and LCO cathodes. The low initial Coulombic effi- ciency of LMR cathode is largely attributed to the electrode/elec- trolyte reaction in the first cycle, especially the oxygen loss. When the charge voltage is below 4.6 V, ethylene carbonate (EC), dimethyl carbonate (DMC) and etc. in the solvent would decom- pose into CO2 and other gases due to the oxidizing O2", "category": "scientific_body", "coverage": 0.78, "words": 104 }, { "page": 5, "page_count": 18, "bbox": [ 37.59, 213.87, 288.68, 274.28 ], "text": "In summary, there is no unified conclusion about the crystal structure of LMR oxides (composite or solid-solution) till now. One possible reason is that, the actual crystal structure of LMR oxi- des varies with the elemental compositions (especially the content of lithium) and the synthetic conditions (oxygen partial pressure, cooling rate, etc.) [38,39].", "category": "scientific_body", "coverage": 0.882, "words": 55 }, { "page": 5, "page_count": 18, "bbox": [ 306.6, 482.31, 557.68, 546.23 ], "text": "z\u0003 [52]. These decomposition products would erode the electrode–electrolyte interphase, result- ing in the dissolution of the transition metal ions. Continuous dis- solution of the metal ions from the material further leads to a rugged surface and increased interphase impedance, which would cause rapid degradation of the electrode.", "category": "scientific_body", "coverage": 1.0, "words": 47 }, { "page": 8, "page_count": 18, "bbox": [ 37.59, 67.44, 288.69, 117.35 ], "text": "capacity loss in the first cycle could be recovered by applying a constant-voltage step during discharge. Li+ would occupy the tetra- hedral sites to form a new P’’ phase under a harsh reductive condi- tion (<1.4 V), and those Mn ions in Li layers triggered by oxygen redox would serve as ‘‘pillars” to stabilize the structure.", "category": "scientific_body", "coverage": 0.904, "words": 56 }, { "page": 8, "page_count": 18, "bbox": [ 306.59, 67.43, 557.67, 127.78 ], "text": "a Coulombic efficiency of 93%. After 200 cycles at 1C, it still has an excellent capacity retention of 92%. Similarly, Xu et al. synthesized a hierarchical quasi-spherical Li1.2Ni0.2Mn0.6O2 oxide with active (010)-oriented surface [65]. Combining the hierarchical structure and active (010)-planes, this material exhibits efficient transports of both Li+ ions and electrons.", "category": "scientific_body", "coverage": 0.821, "words": 60 }, { "page": 8, "page_count": 18, "bbox": [ 37.59, 303.67, 288.68, 374.51 ], "text": "To accelerate the commercialization of LMR cathodes, research- ers devoted a lot of efforts to optimize the electrochemical perfor- mance through different methods. Fortunately, the three concerns discussed above have been resolved to some extent in recent years. In this section, we will elaborate on these effective methods in three categories: morphology design, bulk design, and surface modification.", "category": "scientific_body", "coverage": 0.962, "words": 57 }, { "page": 8, "page_count": 18, "bbox": [ 37.59, 412.64, 288.68, 493.9 ], "text": "Various morphologies, such as 1D nanowires, 1D nanotubes, 2D nanoplates, 3D porous morphology and other hierarchical nano morphologies, have been widely reported in other layered oxides, including LCO and NMC [60,61]. It is also an effective way to opti- mize the electrochemical performance of LMR oxides, such as mit- igating the capacity and voltage degradation during cycling. The detailed electrochemical performance of LMR cathodes with differ- ent morphologies reported in recent years are shown in Table 2.", "category": "scientific_body", "coverage": 1.0, "words": 75 }, { "page": 9, "page_count": 18, "bbox": [ 37.59, 380.04, 288.7, 471.79 ], "text": "volume change caused by phase transformation, enhancing the mechanical stability, and contributing to the cycling stability. However, there are also some disadvantages using this strategy, such as complicated preparation, high cost, low tap density and etc., which greatly hinder the practical application in scaling up the cathode materials. In addition, it is difficult to achieve consis- tent morphology in different batches of materials, while even a slight difference in the morphology might lead to a big difference in the performance.", "category": "scientific_body", "coverage": 0.921, "words": 80 }, { "page": 10, "page_count": 18, "bbox": [ 306.59, 704.89, 557.68, 754.81 ], "text": "(Fig. 5h-i) [80]. Owing to the gradient distribution of Ni and Mn elements, the Ni/Mn ratio is much higher at the surface. Therefore, TM migration was suppressed and the voltage and capacity decay during long-term cycling decreased. The method has been extended to other LMR oxides [81,82]. Core-shell structural design", "category": "scientific_body", "coverage": 1.0, "words": 55 }, { "page": 10, "page_count": 18, "bbox": [ 37.59, 704.89, 288.67, 754.81 ], "text": "Besides, gradient cathode materials have become a hot topic in recent years [79]. Ju et al. designed a gradient oxide Li1.2Mn0.44- Co0.04Ni0.32O2 (Fig. 5g), in which Ni element increases and Mn ele- ment decreases from the bulk to the surface, which could be proved by Energy dispersive X-ray spectroscopy (EDS) line scan", "category": "scientific_body", "coverage": 0.885, "words": 56 }, { "page": 11, "page_count": 18, "bbox": [ 306.59, 662.43, 557.71, 754.18 ], "text": "In summary, various structure design methods, including intro- ducing porous structure, structural defects, elemental gradient dis- tribution and O2-type structure in LMR cathodes could distinctly promote the electrochemical performance. The detailed electro- chemical properties of LMR cathodes with different structure designs reported recently are listed in Table 3. Correspondingly, there are some limitations with them. Porous structure would decrease the tap density of cathode materials, thus the volumetric energy density. Structural defects always require the precise pro-", "category": "scientific_body", "coverage": 0.986, "words": 75 }, { "page": 11, "page_count": 18, "bbox": [ 37.59, 683.35, 288.7, 754.18 ], "text": "In addition, since O3-type LMR cathodes suffer severe voltage and capacity decay, synthesizing O2-type LMR cathodes is also an effective remedy. Xia et al. prepared an O2-type Li-rich material with a single-layer Li2MnO3 superstructure via ion exchange [83]. Owing to this novel structure, this oxide could maintain stable oxy- gen redox reactions and small structural changes during cycling, and delivered an extraordinary reversible capacity of 400 mA h g\u00031.", "category": "scientific_body", "coverage": 1.0, "words": 74 }, { "page": 12, "page_count": 18, "bbox": [ 306.59, 704.44, 557.65, 755.66 ], "text": "In addition, two or more ions together may play different roles in multiple elemental doping, which make it possible for them to work synergistically to produce better electrochemistry than the single elemental doping, namely multi-ion co-doping. Liu et al. doped Na cation and F anion in Li1.2Ni0.2Mn0.6O2 cathode, as shown", "category": "scientific_body", "coverage": 1.0, "words": 55 }, { "page": 13, "page_count": 18, "bbox": [ 306.59, 705.51, 557.68, 755.43 ], "text": "3.3.1. Surface coating To reduce the side reactions at the electrode/electrolyte inter- face, it is straightforward to build an electrochemically-inert pro- tective layer, or a surface coating, on the surface of the cathodes [99–101]. For LMR cathodes, metal oxides, phosphate, fluoride,", "category": "scientific_body", "coverage": 0.85, "words": 44 }, { "page": 14, "page_count": 18, "bbox": [ 37.59, 339.39, 288.69, 441.63 ], "text": "Phosphates has also been regarded as effective candidates for surface coating. Xiao et al. coated aluminum phosphate (AlPO4) at the surface of LMR through atomic layer deposition (ALD) method [104]. They found a spinel structure formed at the surface during ALD process, and this spinel layer effectively inhibited the oxygen release during the first charging-discharging, as shown in Fig. 7(d). Therefore, the initial Coulombic efficiency of the LMR cathode was promoted distinctly. When the cathode was coated with 5 ALD cycles of AlPO4, it exhibited the best initial Coulombic efficiency (86.2%), as shown in Fig. 7(e). In addition to the pro-", "category": "scientific_body", "coverage": 0.825, "words": 107 }, { "page": 15, "page_count": 18, "bbox": [ 37.59, 527.67, 288.71, 692.67 ], "text": "Although surface coating is one effective method to promote the electrochemistry, there are still some concerns during practical operations. (1) The uniformity of the coating. A uniform and thor- ough coating can effectively protect the active materials. Unfortu- nately, processing routes which can guarantee the uniformity, like ALD, can complicate the synthesis procedures and increase the cost. (2) The thickness of the coating materials. In consideration of the intrinsic inertness, a thick coating may inhibit the electron and Li+ transport, and decrease the capacity and rate. (3) The bind- ing force between the coating layer and the active material. The coating layer may fall off due to the volume change of cathode materials during cycling if the binding force is weak. High temper- ature calcination usually can improve the binding force. In brief, an ideal surface coating is a uniform coating with an optimized thick- ness and excellent binding force, and it should be implemented through a facile route.", "category": "scientific_body", "coverage": 1.0, "words": 154 }, { "page": 16, "page_count": 18, "bbox": [ 306.6, 89.15, 557.65, 139.06 ], "text": "This work was financially supported by the National Key R&D Program of China (2016YFB0700600), the Soft Science Research Project of Guangdong Province (No. 2017B030301013), and the Shenzhen Science and Technology Research Grant (ZDSYS201707281026184).", "category": "back_matter", "coverage": 0.0, "words": 34 }, { "page": 16, "page_count": 18, "bbox": [ 37.59, 303.67, 288.69, 395.43 ], "text": "In conclusion, LMR layered oxide as one of the most promising next-generation cathodes, still suffers some disadvantages, includ- ing the low initial Coulombic efficiency, poor rate performance, and severe voltage and capacity decay. In order to accelerate its commercialization, researchers have developed various effective methods in recent years. We divide these methods into three cat- egories (Fig. 11), and introduce them through various cases in details. Moreover, the issues and limitations for each method are also discussed.", "category": "scientific_body", "coverage": 0.918, "words": 77 }, { "page": 16, "page_count": 18, "bbox": [ 37.59, 575.62, 288.69, 677.87 ], "text": "5. Compared with LCO, LMR cathode has a low tap density due to the polycrystalline morphology consisting of nanosized primary particles. Therefore, the tap density of LMR cathode should be pro- moted a lot before the commercialization. One effective method to promote the tap density of the layered materials is to synthesize single crystal layered materials, which has been utilized in LCO and Ni-rich NMC cathode [138,139]. The single-crystal layered cathodes can achieve a larger tap density compared to secondary-particle layered cathodes, because the density of a sin- gle crystal particle is close to the theoretical value.", "category": "scientific_body", "coverage": 0.989, "words": 99 } ], "low_coverage_blocks": [ { "page": 2, "page_count": 18, "bbox": [ 389.08, 323.94, 557.66, 424.63 ], "text": "Mingjian Zhang got his Ph.D. degree from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences in 2013, then worked there as an assistant research fellow for one year. From 2014 to 2018, he was a postdoc at School of Advanced Materials, Peking University, and became an assistant research professor since 2018. Meanwhile, he was a research scholar in Brookhaven National Lab from 2016 to 2019, then in the University of Chicago since 2019. He has been engaged in the fields of electrode materials for Li- ion batteries, crystal growth and structure analysis of nonlinear optical crystals.", "category": "metadata", "coverage": 0.0, "words": 101 }, { "page": 2, "page_count": 18, "bbox": [ 306.6, 452.98, 557.65, 562.22 ], "text": "Feng Pan, founding Dean of School of Advanced Mate- rials, Peking University Shenzhen Graduate School, got B.S. from Dept. Chemistry, Peking University in 1985 and Ph.D. from Dept. of P&A Chemistry, University of Strathclyde, Glasgow, UK, with ‘‘Patrick D. Ritchie Prize” for the best Ph.D. in 1994. With more than a decade experience in large international incorporations, Prof. Pan has been engaged in fundamental research and product development of novel optoelectronic and energy storage materials and devices. As Chief Scientist, Prof. Pan led eight entities in Shenzhen to win 150 million RMB grant for the national new energy vehicles (power battery) innovation project since 2013.", "category": "metadata", "coverage": 0.0, "words": 108 }, { "page": 16, "page_count": 18, "bbox": [ 306.6, 89.15, 557.65, 139.06 ], "text": "This work was financially supported by the National Key R&D Program of China (2016YFB0700600), the Soft Science Research Project of Guangdong Province (No. 2017B030301013), and the Shenzhen Science and Technology Research Grant (ZDSYS201707281026184).", "category": "back_matter", "coverage": 0.0, "words": 34 } ], "high_risk_body_missing_blocks": [], "front_summary_missing_blocks": [], "back_matter_low_overlap_blocks": [ { "page": 16, "page_count": 18, "bbox": [ 306.6, 89.15, 557.65, 139.06 ], "text": "This work was financially supported by the National Key R&D Program of China (2016YFB0700600), the Soft Science Research Project of Guangdong Province (No. 2017B030301013), and the Shenzhen Science and Technology Research Grant (ZDSYS201707281026184).", "category": "back_matter", "coverage": 0.0, "words": 34 } ] }