English

• 1. INTRODUCTION

  Lithium-ion batteries surpassing other electrochemical energy storage systems in terms of energy density and lifetime have achieved unprecedented success in commercialization, specifically in the application of electric vehicles and portable devices^{[1, 2]}. With the ever-growing demand for batteries technology, further development toward higher energy density is dominantly limited by cathode materials^[3]. Layered lithium transition metal oxides Li[Ni_xMn_yCo_z]O₂ (NMC) have been considered as the most practicable cathode material for next-generation batteries, achieving widespread concerns both in industry and academia^[4]. However, existing NMC cathodes are currently challenging with an underachieved specific capacity, cycling stability, and thermal stability^[5]. Considerable effort has been put into investigating the relationship between composition and structural/electrochemical properties under realistic battery conditions. Solid evidence reveals that the electrochemical and thermal properties of Li[Ni_xMn_yCo_z]O₂ are strongly dependent on its composition. The specific capacity shows a linear increase with Ni contents but the corresponding capacity retention and safety gradually decrease (Fig. 1)^[6]. Undoubtedly, there is an irreconcilable contradiction between high capacity and structural/thermal stability.

  Figure 1

  

  Figure 1.  (a) The triangle shows the existing NMC materials; (b) A map of relationship between discharge capacity, and thermal stability and capacity retention of different compositions of Li[Ni_xMn_yCo_z]O₂

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  Given the significance of commercialization, global efforts have been continuously devoted to exploring effective approaches to enhance the overall performance of Li[Ni_xMn_yCo_z]O₂. Among these, a significant breakthrough has been achieved by controlling the local composition of the bulk and the surface of a single NMC particle to simultaneously improve capacity and structure/thermal stability^{[7, 8]}. Several novel structures including core-shell structure and concentration gradient structure were designed to push NCM cathodes towards higher energy density and better structure/thermal stability^{[9, 10]}. In this perspective, we reviewed the recent progress on advanced cathode materials regarding concentration gradient with composition and morphology controls. Furthermore, we forecast the promising prospects of the concentration gradient in developing extreme fast charging and Co-free cathode materials.

  2. CORE-SHELL STRUCTURE LAYERED OXIDE CATHODES

  The core-shell structure is an effective strategy by combining the advantages of different compositions to counter the trade-off between capacity and battery safety. Theoretically, the core-shell structure is characterized by the particle core with a Ni-rich composition (such as Li[Ni_0.8Mn_0.1Co_0.1]O₂ or even higher Ni content) to provide a high capacity, while the particle surface is designedly arranged with low Ni and high Mn contents (e. g. Li[Ni_1/3Mn_1/3Co_1/3]O₂ or Li[Ni_0.5Mn_0.5]O₂) to enhance chemical stability. Sun and Amine et al^[11] for the first time reported the core-shell structure of NMC cathode via co-precipitation method, wherein the particle is composed of a Li[Ni_0.8Mn_0.1Co_0.1]O₂ core encapsulated with a Li[Ni_0.5Mn_0.5]O₂ shell. Fig. 2a and 2b show that the structure of the particle has the core homogeneously protected by an optimized thickness of the shell with a clear core-shell boundary. The thickness of the shell is controllably optimized by the synthesis process to properly balance the thermal stability and capacity delivery.

  The core-shell structure of NMC expectedly showed significantly improved thermal stability and capacity retention, which is attributed to the effective protection of the shell composition. Despite lowering capacity, the shell composition of LiNi_0.5Mn_0.5O₂ blocks the parasitic reaction between Ni-rich core and electrolyte and suppresses the irreversible phase transition from layered structure to the rock-salt phase. This is dominantly beneficial to significantly improve the structural stability during cycling. Additionally, the thermally stable outer Li[Ni_0.5Mn_0.5]O₂ shell suppresses the oxygen release from the highly delithiated Li_1-x[Ni_0.8Co_0.1Mn_0.1]O₂, thereby improving the thermal stability of the core-shell structure.

  Figure 2

  

  Figure 2.  (a) Schematic drawings of a core-shell structure. (b) SEM image of core-shell Li[(Ni_0.8Co_0.1Mn_0.1)_0.8(Ni_0.5Mn_0.5)_0.2]O₂

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  The synergetic effects of the core-shell structure cathode materials shed new light on the development of advanced Li-ion batteries with a high energy density, long cycle life, and safety. Nevertheless, unsynchronized volume changes of the core and the shell layer, specifically during fast charging, trigger mechanical strain at the boundary, further result in the generation of gaps between the core and the shell layer. The physical disconnection between the core and the shell layer directly cuts off the Li⁺ diffusion and electron conduction, eventually causing the capacity degradation. Therefore, the further advance of core-shell structure cathode is required to eliminate the palpable boundary between the core and the shell layer.

  3. CONCENTRATION GRADIENT STRUCTURE LAYERED OXIDE CATHODES

  To enhance the chemical bonding between the core and the shell layer, Sun and Amine et al^[9] further developed core/concentration gradient shell structure cathode materials. It is characterized by the shell layer designed with a concentration gradient to achieve a smooth transition in composition from the core edge to the outer surface (Fig. 3a). The core/concentration gradient shell cathode materials not only eliminate the boundary of core-shell structure but also increase the capacity delivery and rate performance thanks to the improved electrochemical activity of the shell layer via a concentration gradient.

  Figure 3

  

  Figure 3.  (a) Schematic diagram of positive-electrode particle with Ni-rich core surrounded by concentration-gradient outer layer, (b) Schematic diagram of the full concentration gradient cathode with the nickel concentration decreasing from the centre towards the outer layer and the concentration of manganese increasing accordingly

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  The structural advantages and superior performance of the core/concentration gradient shell structure motivated the continuous exploration of concentration gradient cathode materials. Sun and Amine et al^[10] successfully introduced the concentration gradient structure into the entire particle, achieving a full concentration gradient NMC cathode material. As shown in Fig. 3b, the compositions of Ni and Co linearly decreased from the center, while the Mn concentration smoothly increased so that the even higher capacity and the Mn-enriched surface can strengthen the safety protection. In particular, the full concentration gradient cathode consists of highly oriented needle-like primary particles. The compact particle morphology and the strong [001] crystallographic texture facilitated Li⁺ transport and minimized contact with the electrolyte, additionally stabilizing the surface chemistry of the FCG electrode upon cycling and suppressing the generation of micro-cracks. As a result, the FCG cathode exhibits a convincingly superior electrochemical performance and thermal stability.

  Considering the stupendous potential of FCG cathode materials, numerous efforts are continuously invested to preserve its supremacy by further improvements in energy density and sustainability. Recently, Sun et al^[12] developed a two-slope full concentration gradient structure to maximize the average Ni concentration and simultaneously enhance surface protection. For example, a Li[Ni_0.85Co_0.05Mn_0.1]O₂ with a two-slope concentration gradient structure has successfully produced an extremely high energy density and excellent capacity retention^[13]. In addition, Al was introduced into the two slopes concentration gradient cathodes in an attempt to further optimize the energy density and cycling stability^[14].

  This approach provides opportunities to optimize NCM battery performance by selectively distributing each transition metal within a single particle to meet the demands of capacity, rate capability, and safety for next-generation EVs.

  4. CONCLUSION AND PERSPECTIVE

  Layered oxide cathodes, specifically Ni-rich cathodes, are currently considered as the most practicable candidates for the next-generation batteries. To further improve performance, the controllable concentration design in form of core-shell and concentration gradient structures has successfully pushed Li[Ni_xCo_yMn_z]O₂ toward higher energy density, longer lifetime and safety. In addition, the morphology control of concentration gradient cathodes with highly oriented primary particle microstructure is showing huge potentials in developing extremely fast-charging batteries through accelerating Li⁺ intercalation kinetics. On the other hand, transitioning away from the highly-priced Co has been a major challenge for the battery research communities and the prerequisite for the grant mission of achieving complete vehicle electrification. In the future arena, a developing trend on the design of advanced Co-less and Co-free cathodes with controllable concentration and morphology designs will spark the light for future innovations within the field. In summary, we believe that this novel approach will lead to a new era in the design and development of a wide range of functional cathode materials.

  
  
• 1. [1]

     Choi, J. W.; Aurbach, D. Promise and reality of post-lithium-ion batteries with high energy densities. Nat. Rev. Mater. 2016, 1, 16013–16029. doi: 10.1038/natrevmats.2016.13

  2. [2]

     Liu, T. C.; Lin, L. P.; Bi, X. X.; Tian, L. L.; Yang, K.; Liu, J. J.; Li, M. F.; Chen, Z. H.; Lu, J.; Amine, K.; Xu, K.; Pan, F. In situ quantification of interphasial chemistry in Li-ion battery. Nat. Nanotechnol. 2019, 14, 50–56. doi: 10.1038/s41565-018-0284-y

  3. [3]

     Liu, T.; Dai, A.; Lu, J.; Yuan, Y.; Xiao, Y.; Yu, L.; Li, M.; Gim, J.; Ma, L.; Liu, J.; Zhan, C.; Li, L.; Zheng, J.; Ren, Y.; Wu, T.; Shahbazian-Yassar, R.; Wen, J.; Pan, F.; Amine, K. Correlation between manganese dissolution and dynamic phase stability in spinel-based lithium-ion battery. Nat. Commun. 2019, 10, 4721–4732. doi: 10.1038/s41467-019-12626-3

  4. [4]

     Zheng, J.; Ye, Y.; Liu, T.; Xiao, Y.; Wang, C.; Wang, F.; Pan, F. Ni/Li disordering in layered transition metal oxide: electrochemical impact, origin, and control. Acc. Chem. Res. 2019, 52, 2201–2209. doi: 10.1021/acs.accounts.9b00033

  5. [5]

     Zheng, J.; Liu, T.; Hu, Z.; Wei, Y.; Song, X.; Ren, Y.; Wang, W.; Rao, M.; Lin, Y.; Chen, Z.; Lu, J.; Wang, C.; Amine, K.; Pan, F. Tuning of thermal stability in layered Li(Ni_xMn_yCo_z)O₂. J. Am. Chem. Soc. 2016, 138, 13326–13334. doi: 10.1021/jacs.6b07771

  6. [6]

     Noh, H. J.; Youn, S.; Yoon, C. S.; Sun, Y. K. Comparison of the structural and electrochemical properties of layered Li[Ni_xCo_yMn_z]O₂ (x = 1/3, 0. 5, 0. 6, 0. 7, 0. 8 and 0. 85) cathode material for lithium-ion batteries. J. Power Sources 2013, 233, 121–130. doi: 10.1016/j.jpowsour.2013.01.063

  7. [7]

     Myung, S. T.; Maglia, F.; Park, K. J.; Yoon, C. S.; Lamp, P.; Kim, S. J.; Sun, Y. K. Nickel-rich layered cathode materials for automotive lithium-ion batteries: achievements and perspectives. ACS Energy Lett. 2016, 2, 196–223.

  8. [8]

     Sun, Y. K.; Myung, S. T.; Park, B. C.; Prakash, J.; Belharouak, I.; Amine, K. High-energy cathode material for long-life and safe lithium batteries. Nat. Mater. 2009, 8, 320–324. doi: 10.1038/nmat2418

  9. [9]

     Kong, D.; Hu, J.; Chen, Z.; Song, K.; Li, C.; Weng, M.; Li, M.; Wang, R.; Liu, T.; Liu, J.; Zhang, M.; Xiao, Y.; Pan, F. Ti-gradient doping to stabilize layered surface structure for high performance high-Ni oxide cathode of Li-ion battery. Adv. Energy Mater. 2019, 9, 1901756. doi: 10.1002/aenm.201901756

  10. [10]

      Sun, Y. K.; Chen, Z. H.; Noh, H. J.; Lee, D. J.; Jung, H. G.; Ren, Y.; Wang, S.; Yoon, C. S.; Myung, S. T.; Amine, K. Nanostructured high-energy cathode materials for advanced lithium batteries. Nat. Mater. 2012, 11, 942–947. doi: 10.1038/nmat3435

  11. [11]

      Sun, Y. K.; Myung, S. T.; Kim, M. H.; Prakash, J.; Amine, K. Synthesis and characterization of Li[(Ni_{0. 8}Co_{0. 1}Mn_{0. 1})_{0. 8}(Ni_{0. 5}Mn_{0. 5})_{0. 2}]O₂ with the microscale core-shell structure as the positive electrode material for lithium batteries. J. Am. Chem. Soc. 2005, 127, 13411–13418. doi: 10.1021/ja053675g

  12. [12]

      Lim, B. B.; Yoon, S. J.; Park, K. J.; Yoon, C. S.; Kim, S. J.; Lee, J. J.; Sun, Y. K. Advanced concentration gradient cathode material with two-slope for high-energy and safe lithium batteries. Adv. Funct. Mater. 2015, 25, 4673–4680. doi: 10.1002/adfm.201501430

  13. [13]

      Lee, J. H.; Yoon, C. S.; Hwang, J. Y.; Kim, S. J.; Maglia, F.; Lamp, P.; Myung, S. T.; Sun, Y. K. High-energy-density lithium-ion battery using a carbon-nanotube–Si composite anode and a compositionally graded Li[Ni_{0. 85}Co_{0. 05}Mn_{0. 10}]O₂ cathode. Energy Environ. Sci. 2016, 9, 2152–2158. doi: 10.1039/C6EE01134A

  14. [14]

      Lim, B. B.; Myung, S. T.; Yoon, C. S.; Sun, Y. K. Comparative study of Ni-rich layered cathodes for rechargeable lithium batteries: Li[Ni_0.85Co_0.11Al_{0. 04}]O₂ and Li[Ni_{0. 84}Co_{0. 06}Mn_{0. 09}Al_{0. 01}]O₂ with two-step full concentration gradients. ACS Energy Lett. 2016, 1, 283–289. doi: 10.1021/acsenergylett.6b00150

• 

  Figure 1  (a) The triangle shows the existing NMC materials; (b) A map of relationship between discharge capacity, and thermal stability and capacity retention of different compositions of Li[Ni_xMn_yCo_z]O₂

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  Figure 2  (a) Schematic drawings of a core-shell structure. (b) SEM image of core-shell Li[(Ni_0.8Co_0.1Mn_0.1)_0.8(Ni_0.5Mn_0.5)_0.2]O₂

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  Figure 3  (a) Schematic diagram of positive-electrode particle with Ni-rich core surrounded by concentration-gradient outer layer, (b) Schematic diagram of the full concentration gradient cathode with the nickel concentration decreasing from the centre towards the outer layer and the concentration of manganese increasing accordingly

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