An Effective Way to Stabilize Ni-Rich Layered Cathodes

Stable cycling with a high energy density at an affordable cost is a key challenge for the prevailing cathode material, Ni-rich layered oxides, to power the development of long-range electric vehicles. Now, Sun and co-workers introduced a Li[Ni0.90Co0.09Ta0.01]O2 cathode in Nature Energy, demonstrating great potential to overcome this challenge. Stable cycling with a high energy density at an affordable cost is a key challenge for the prevailing cathode material, Ni-rich layered oxides, to power the development of long-range electric vehicles. Now, Sun and co-workers introduced a Li[Ni0.90Co0.09Ta0.01]O2 cathode in Nature Energy, demonstrating great potential to overcome this challenge. Since the first commercialization in 1991 by Sony, lithium-ion batteries (LIBs) have been fueling the rapid development of modern electrical vehicles (EVs), drones, and various consumer electronics. The booming market has been pressing for batteries with longer lifetime and a higher energy density at a lower cost, which depend largely on the cathode.1Li W. Erickson E.M. Manthiram A. High-nickel layered oxide cathodes for lithium-based automotive batteries.Nat. Energy. 2020; 5: 26-34Crossref Scopus (262) Google Scholar Among various cathode materials, layered oxide (LiTMO2, 3d transition metals [TM]) is the most widely studied. For example, LiCoO2 (LCO), the cathode material in Sony’s first commercial LIBs, offers a good balance of energy density and lifetime; however, the increasing demand in energy density and surging price of Co make LCO not very appealing to modern EVs. Plenty of alternative layered oxides have been explored. By partial substitution of Ni in LiNiO2 with Co/Mn and Co/Al, LiNixCoyMn1-x-yO2 (NCM) (0 < x, y < 1) and LiNixCoyAl1-x-yO2 (NCA) (0 < x, y < 1) can be obtained, respectively. NCM is now widely used to power various EV models, and NCA is mainly used by Tesla Motors.1Li W. Erickson E.M. Manthiram A. High-nickel layered oxide cathodes for lithium-based automotive batteries.Nat. Energy. 2020; 5: 26-34Crossref Scopus (262) Google Scholar Both NCM and NCA are rich in Ni as the key redox active species; thus, a higher Ni content with less Co can increase the specific capacity and reduce the battery cost. However, these advantages come at the expense of the cathode stability, given that the capacity fades rapidly over the cycles.2Noh H.J. Youn S. Yoon C.S. Sun Y.K. Comparison of the structural and electrochemical properties of layered Li[NixCoyMnz]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: 121-130Crossref Scopus (1047) Google Scholar For a typical Ni-rich layered cathode, the culprits to its degradation mainly involve the irreversible structural change,3Märker K. Reeves P.J. Xu C. Griffith K.J. Grey C.P. Evolution of structure and lithium dynamics in LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes during electrochemical cycling.Chem. Mater. 2019; 31: 2545-2554Crossref Scopus (80) Google Scholar,4Märker K. Xu C. Grey C.P. Operando NMR of NMC811/graphite lithium-ion batteries: structure, dynamics, and lithium netal deposition.J. Am. Chem. Soc. 2020; 142: 17447-17456Crossref PubMed Scopus (19) Google Scholar crack formation, and electrolyte attack.5Pender J.P. Jha G. Youn D.H. Ziegler J.M. Andoni I. Choi E.J. Heller A. Dunn B.S. Weiss P.S. Penner R.M. Mullins C.B. Electrode degradation in lithium-ion batteries.ACS Nano. 2020; 14: 1243-1295Crossref PubMed Scopus (125) Google Scholar The migration of Ni4+ to Li+ sites leads to the transformation of layered to a rock-salt or spinel structure, which impedes the migration of Li+ ions.6Zheng S. Hong C. Guan X. Xiang Y. Liu X. Xu G.L. Liu R. Zhong G. Zheng F. Li Y. et al.Correlation between long range and local structural changes in Ni-rich layered materials during charge and discharge process.J. Power Sources. 2019; 412: 336-343Crossref Scopus (49) Google Scholar Anisotropic lattice expansion and contraction during the cycling build up strain at local regions and initiate the formation of microcracks.5Pender J.P. Jha G. Youn D.H. Ziegler J.M. Andoni I. Choi E.J. Heller A. Dunn B.S. Weiss P.S. Penner R.M. Mullins C.B. Electrode degradation in lithium-ion batteries.ACS Nano. 2020; 14: 1243-1295Crossref PubMed Scopus (125) Google Scholar,7Xu C. Märker K. Lee J. Mahadevegowda A. Reeves P.J. Day S.J. Groh M.F. Emge S.P. Ducati C. Layla Mehdi B. et al.Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries.Nat. Mater. 2020; 24https://doi.org/10.1038/s41563-020-0767-8Crossref Scopus (65) Google Scholar Furthermore, at highly charged states (e.g., at 4.3 V), strongly oxidative Ni3+ or Ni4+ ions decompose the electrolyte, forming a surface reaction layer that can further hinder the transport of Li+ ions, which escalates upon particle pulverizations.6Zheng S. Hong C. Guan X. Xiang Y. Liu X. Xu G.L. Liu R. Zhong G. Zheng F. Li Y. et al.Correlation between long range and local structural changes in Ni-rich layered materials during charge and discharge process.J. Power Sources. 2019; 412: 336-343Crossref Scopus (49) Google Scholar Tremendous efforts have been devoted to stabilizing the Ni-rich cathodes, mainly via structural engineering and chemical treatment. Radial partitioning in secondary particles, surface coating, and infusion grain boundaries with solid electrolytes have been shown to improve the stability.8Zhou L. Cao Z. Wahyudi W. Zhang J. Hwang J.-Y. Cheng Y. Wang L. Cavallo L. Anthopoulos T. Sun Y.-K. et al.Electrolyte engineering enables high stability and capacity alloying anodes for sodium and potassium ion batteries.ACS Energy Lett. 2020; 5: 766-776Crossref Scopus (62) Google Scholar Chemical doping of foreign ions is another strategy. So far, although various metal ions (Mg2+, Al3+, Zr4+, etc.) and nonmetal ions (F−, B3+, Si4+, etc.) have been explored to dope the NCM cathode, the actual effect of individual dopants is highly complex.9Zhang S.S. Problems and their origins of Ni-rich layered oxide cathode materials.Energy Storage Mater. 2020; 24: 247-254Crossref Scopus (124) Google Scholar Recently in Nature Energy, Sun and co-workers reported a very stable Ni-rich layered cathode Li[Ni0.90Co0.09Ta0.01]O2 (NCTa90).10Kim U.-H. Park G.-T. Son B.-K. Nam G.W. Liu J. Kuo L.-Y. Kaghazchi P. Yun C.S. Sun Y.-K. Heuristic solution for achieving long-term cycle stability for Ni-rich layered cathodes at full depth of discharge.Nat. Energy. 2020; 21https://doi.org/10.1038/s41560-020-00693-6Crossref Scopus (51) Google Scholar The authors compared several dopants, namely aluminum (Al), boron (B), tungsten (W), and tantalum (Ta), and found that the Ta-doped cathode (i.e., NCTa90) shows the best cycling stability (Figure 1). Specifically, full cells composed of a NCTa90 cathode and a graphite anode exhibit 90% capacity retention after 2,000 cycles at full depth of discharge. In comparison, the Al-doped material has a capacity retention of only 48%. The authors employed a wide range of characterization tools to unravel the mechanisms behind the excellent performance, and they proposed two possibilities: (1) controlled radially aligned fine structures or (2) ordered occupation of TM ions in Li sites. First, they found that the morphology of doped particles depends strongly on the dopant. For example, Al doping only leads to randomly oriented NCA90 particles, whereas Ta doping leads to radially aligned needle-like fine structures, as compared in Figure 1. The authors performed the density functional theory (DFT) calculations and revealed that the large Ta ionic radius strains (104) planes preferentially with an increased surface energy, leading to the [003] texture to the particle, which was verified by transmission electron microscopy (TEM) observations. As a result, Ta doping hinders the particle coarsening at high lithiation temperatures, allowing optimal size refinement among the dopants investigated in the study. The radially aligned fine structures in NCTa90 offer several merits. Unlike usual randomly oriented NCA90 particles, where highly anisotropic strains accumulated in local regions trigger the formation of microcracks, radially aligned structures with crystallographic texturing in NCTa90 can convert these randomly oriented local strains into circumferential strains. This allows uniform expansion and contraction of the spherical particle, dramatically improving the particle’s mechanical stability. Moreover, these needle-like structures are aligned with (003) planes along the length direction; the wide d-spacing of (003) planes provides an avenue for rapid transportation of Li-ion in the particle, which is advantageous to improve the cathode’s electrochemical stability and rate performance. Second, Ta-doping leads to the formation of an ordered crystal via the cation mixing, i.e., alternating occupation of Ta ions in Li slabs, as supported by observation of the superlattice structures via the TEM and selected area electron diffraction (SAED) analyses. Structural transformation of layered to spinel- and/or rock-salt structures is a widely observed phenomenon in Ni-rich layered cathodes, and it is generally considered to be a detrimental process, because (1) this leads to a loss of active material, and (2) it can result in fatigue degradation7Xu C. Märker K. Lee J. Mahadevegowda A. Reeves P.J. Day S.J. Groh M.F. Emge S.P. Ducati C. Layla Mehdi B. et al.Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries.Nat. Mater. 2020; 24https://doi.org/10.1038/s41563-020-0767-8Crossref Scopus (65) Google Scholar as well as impede the Li-ion transport. The authors have, on the other hand, proposed that this ordered structure does not hinder the Li-ion transport; instead, it can stabilize the delithiated structure and protect the cathode from the electrolyte attack, and it is also believed to be a reason for the improved thermal stability observed in the Ta-doped cathodes. In summary, the radially aligned fine structures and the ordered occupation of Ni ions in Li slabs make the Ta doping exceptionally effective to stabilize Ni-rich layered cathodes. This strategy is potentially applicable to other dopants of high oxidation states (above 3+), as the authors have demonstrated a comparable effect with the W doping. This work opens a door to effectively improve the cathode’s stability. Further optimizations might release its full potential to address the range-anxiety and boost the development of the EV industry. J.L. thanks the Research Startup Fund from Harbin Institute of Technology (Shenzen), China. An Effective Way to Stabilize Ni-Rich Layered CathodesLu et al.ChemJanuary 14, 2021In Brief(Chem 6, 3165–3167; December 3, 2020) Full-Text PDF