Diamond Annual Review 2020/21

84 85 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Fighting fatigue in lithium-ion batteries Related publication: Xu C., Märker K., Lee J., Mahadevegowda A., Reeves P. J., Day S. J., Groh M. F., Emge S. P., Ducati C., Layla Mehdi B., Tang C. C. & Grey C. P. Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries. Nat. Mater. 20 , 84–92 (2021). DOI: 10.1038/s41563-020-0767-8 Publication keywords: Lithium-ion batteries; Degradation; X-ray diffraction L ithium-ion (Li-ion) battery technology is the most popular choice for electrification of transport, an essential step in reducing fossil fuel consumption and developing a more sustainable society. Layered nickel-rich lithium transition metal oxides offer excellent energy densities when used as state-of-the-art cathodes for electric vehicle batteries. However, this group of materials suffers from rapid performance loss. It is essential to understand the degradationmechanisms at thematerial level, particularly during long-termageing. Researchers used the LongDuration Experiments (LDE) facility at Diamond Light Source’s High-ResolutionPowder Diffractionbeamline (I11) to track the evolution of the crystal structure of battery materials operando during their ageing. The LDE facility's unique capability, performing experiments over several months, is crucial to understanding battery degradation, which continues as the battery ages. The results showed that a portion of the cathode material cannot reach the fully charged state after ageing, i.e. becomes fatigued and this portion increases as the ageing progresses. Combining the results from multiple techniques, the team proposed a ’pinning’ mechanism, where the crystal lattice of the material is pinned by its surface, preventing the material from being fully charged. These layered nickel-rich lithiumtransitionmetal oxides are themost popular cathodematerials in the foreseeable future and the degradationmechanismproposed is expected to be universal for all such materials. These findings are, therefore, highly relevant to the practical development of Li-ion batteries. They also highlight the vital importance of surface protection tomitigate degradation. Layered nickel-rich lithium transition metal oxides (LiTMO 2 ,TM= Ni, Co, Mn, Al,etc.)arethestate-of-the-artcathodematerialsforLi-ionbatteriesforelectrical vehicle applications due to many of their merits, for instance, excellent energy densities and less usage of costly rawmaterials 1 . However, these Ni-richmaterials typically suffer from more rapid performance fading compared to the canonical LiCoO 2 and their lower Ni-content analogues and the reason, at least in part, is nested in their structural instability during electrochemical cycling 2 . Layered Ni-rich materials possess an α-NaFeO 2 type structure with TM and Li occupying alternating layers as illustrated in Fig. 1a. During delithiation, the structure exhibits anisotropic changes such that the lattice gradually contracts in the a direction while expanding initially but then contracts rapidly in the c direction. In addition, as the material approaches highly delithiated state, the structure is also thermodynamically less stable. Moreover, the large anisotropic lattice change, particularly the rapid lattice collapse at high state-of-charge (SoC), can lead to mechanical degradations, for instance, particle fracturing. In this study, a long-duration operando X-ray diffraction studywas conducted to monitor the structure evolution of NMC-811 (LiNi 0.8 Mn 0.1 Co 0.1 O 2 ) during its ageing 3 . Such long duration experiments possess significant technical challenges as many of the electrochemistry cells designed for in situ / operando X-ray diffraction studies often do not provide good air-tightness over the required timescale. Herein, a novel coin cell setup is reported for the first time with top and bottom casings thinned to ~50 μ m in thickness using laser technology, as illustrated in Fig. 1b. The NMC-811/graphite cell studied here achieved more >1,000 cycles (~6-month time), providing an excellent opportunity to study the structural evolution of the NMC-811 cathode over a long period of ageing. Our previous study showed that NMC-811 exhibits a solid-solution mechanism, that is, no phase transition, during initial lithiation and delithiation 4 . However, after ageing, evident peak broadening is observed at SoCs as shown by the(003)reflectioninFig.2,whichisindicativeofthepresenceofmultiplephases. In conjunction with ex situ diffraction results, a co-existence of a minimum of threeNMCphaseswhichhavediscernible latticeparameters, i.e.atdifferentSoCs, has been confirmed.The phase with the (003) peak at the lowest position (hence largest c -parameter) is attributed to the one that is least charged, i.e. fatigued. Moreover, the population increases as evidenced by the change in the line-shape as well as phase quantification obtained fromRietveld refinements. The lattice parameters of the fatigued phase in various aged samples, i.e. at different number of cycles, are found to be highly similar, which is a strong indication that this fatigue phenomenon is a structure-driven process. The fact that no long-range structure transformation is observed in the LDE results excludes the possibility that fatigue degradation is induced by bulk structure transformations. As for local environments, no obvious increase in the anti-site mixing (i.e. Ni present in the Li site, which is a known phenomenon in Ni-rich materials) nor drastic change in the lithium mobility (by solid-state Nuclear Magnetic Resonance (NMR) spectroscopy) has been observed. The possibilities that such heterogeneity in the SoC was introduced by intergranular cracking and kinetic limitation have also been excluded based on the fact that the fatigue phase is still detected in an electrochemically aged single-crystal NMC charged at an extremely slow rate. One common phenomenon observed on the surface of Ni-rich cathodes is structure transformation from layered to more densified, rock salt-like structure. Unlike the bulk which expands and contracts considerably during delithiation (illustrated in Fig. 3a), the crystal structure of the surface rock salt remains unchanged due to its electrochemically inactive nature. This will, however, Crystallography Group Beamline I11 generate substantial dynamic lattice mismatch at the interface between the surface rock salt layer and the bulk.We therefore proposed a‘pinning’mechanism thatthefatiguedegradation lies inthehigh latticestrainatthe interfacebetween surface and bulkwhen the NMC cathode is at SoCs above the threshold of ~75%. Without a surface layer, as illustrated in Fig. 3a, the initial delithiation leads to a small expansion in the c direction and further delithiation is accompanied by a rapid collapse (Fig. 3a) and the material is able to access the full SoC range. However, when a surface rock salt layer is present, the drastic lattice collapsed at high SoCs are obstructed due to the large lattice strain and therefore further delithiation is no longer achieved. In summary, we demonstrated a novel coin cell setup which is suitable for long-duration operando X-ray diffraction studies. The results obtained on an NMC-811/graphite cell shows that a proportion of the NMC-811 becomes fatigued after cycling and the fraction of this fatigued population increases as the ageingcontinues.Theoriginofsuchfatiguedegradation isproposedtostemfrom the high interfacial lattice strain between the reconstructed surface and the bulk layered structure. Our study provides novel insights into designing strategies to helpmitigate this degradation process. References: 1. LiW. et al. High-nickel layered oxide cathodes for lithium-based automotive batteries. Nat. Energy 5 , 26–34 (2020). DOI: 10.1038/s41560-019-0513-0 2. Xu C. et al. Phase Behavior during Electrochemical Cycling of Ni-Rich Cathode Materials for Li-Ion Batteries. Adv. Energy Mater. 11 , 2003404 (2021). DOI: 10.1002/aenm.202003404 3. Xu C. et al. Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries. Nat. Mater. 20 , 84–92 (2021). DOI: 10.1038/ s41563-020-0767-8 4. Märker K. et al. Evolution of Structure and LithiumDynamics in LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) Cathodes during Electrochemical Cycling. Chem. Mater. 31 , 2545–2554 (2019). DOI: 10.1021/acs.chemmater.9b00140 Funding acknowledgement: This work is supported by the Faraday Institution under grant no. FIRG001. Corresponding author: Dr Chao Xu, Department of Chemistry, University of Cambridge/The Faraday Institution, cx237@cam.ac.uk Figure 2: Evolution of the NMC (003) reflection at (a) cycle 348; (b) cycle 439; (c) cycle 915. The thickened line is the diffraction pattern collected at the end of the charging step. Reproduced with permission 3 . Copyright 2020, Nature Publishing Group. Figure 1: (a) Illustration of the crystal structure of the pristine NMC-811; (b) Schematic drawing of the laser-thinned coin cell. Reproduced with permission 3 . Copyright 2020, Nature Publishing Group. Figure 3: Illustrations of the structural evolution of (a) the active phase; (b) the fatigued phase during the delithiation process. The red, blue and green circles represent oxygen, transition metal and lithium, respectively. Reproduced with permission 3 . Copyright 2020, Nature Publishing Group.

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