Aqueous Li-ion batteries (ALIBs) are an important class of battery chemistries owing to the intrinsic non-flammability of aqueous electrolytes. However, water is detrimental to most cathode materials and could result in rapid cell failure. Identifying the degradation mechanisms and evaluating the pros and cons of different cathode materials are crucial to guide the materials selection and maximize their electrochemical performance in ALIBs. In this study, we investigate the stability of LiFePO4(LFP), LiMn2O4(LMO) and LiNi0.8Mn0.1Co0.1O2(NMC) cathodes, without protective coating, in three different aqueous electrolytes, i.e., salt-in-water, water-in-salt, and molecular crowding electrolytes. The latter two are the widely reported “water-deficient electrolytes.” LFP cycled in the molecular crowding electrolyte exhibits the best cycle life in both symmetric and full cells owing to the stable crystal structure. Mn dissolution and surface reduction accelerate the capacity decay of LMO in water-rich electrolyte. On the other hand, the bulk structural collapse leads to the degradation of NMC cathodes. LMO demonstrates better full-cell performance than NMC in water-deficient aqueous electrolytes. LFP is shown to be more promising than LMO and NMC for long-cycle-life ALIB full cells, especially in the molecular crowding electrolyte. However, none of the aqueous electrolytes studied here provide enough battery performance that can compete with conventional non-aqueous electrolytes. This work reveals the degradation mechanisms of olivine, spinel, and layered cathodes in different aqueous electrolytes and yields insights into improving electrode materials and electrolytes for ALIBs.
Most research on the electrochemical dynamics in materials for high‐energy Li‐ion batteries has focused on the global behavior of the electrode. This approach is susceptible to misleading analyses resulting from idiosyncratic kinetic conditions, such as surface impurities inducing an apparent two‐phase transformation within LiNi0.8Co0.15Al0.05O2. Here, nano‐focused X‐ray probes are used to measure delithiation operando at the scale of secondary particle agglomerates in layered cathode materials during charge. After an initial latent phase, individual secondary particles undergo rapid, stochastic, and largely uniform delithiation, which is in contrast with the gradual increase in cell potential. This behavior reproduces across several layered oxides. Operando X‐ray microdiffraction (‐XRD) leverages the relationship between Li content and lattice parameter to further reveal that rate acceleration occurs between Li‐site fraction (
- NSF-PAR ID:
- 10455451
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Energy Materials
- Volume:
- 13
- Issue:
- 37
- ISSN:
- 1614-6832
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
-
more » « less
Abstract Vanadium multiredox‐based NASICON‐Na
z V2−y My (PO4)3(3 ≤z ≤ 4; M = Al3+, Cr3+, and Mn2+) cathodes are particularly attractive for Na‐ion battery applications due to their high Na insertion voltage (>3.5 V vs Na+/Na0), reversible storage capacity (≈150 mA h g−1), and rate performance. However, their practical application is hindered by rapid capacity fade due to bulk structural rearrangements at high potentials involving complex redox and local structural changes. To decouple these factors, a series of Mg2+‐substituted Na3+y V2−y Mgy (PO4)3(0 ≤y ≤ 1) cathodes is studied for which the only redox‐active species is vanadium. While X‐ray diffraction (XRD) confirms the formation of solid solutions between they = 0 and 1 end members, X‐ray absorption spectroscopy and solid‐state nuclear magnetic resonance reveal a complex evolution of the local structure upon progressive Mg2+substitution for V3+. Concurrently, the intercalation voltage rises from 3.35 to 3.45 V, due to increasingly more ionic VO bonds, and the sodium (de)intercalation mechanism transitions from a two‐phase fory ≤ 0.5 to a solid solution process fory ≥ 0.5, as confirmed by in operando XRD, while Na‐ion diffusion kinetics follow a nonlinear trend across the compositional series. -
more » « less
Abstract The deposition of protective coatings on the spinel LiMn2O4(LMO) lithium‐ion battery cathode is effective in reducing Mn dissolution from the electrode surface. Although protective coatings positively affect LMO cycle life, much remains to be understood regarding the interface formed between these coatings and LMO. Using operando powder X‐ray diffraction with Rietveld refinement, it is shown that, in comparison to bare LMO, the lattice parameter of a model Au‐coated LMO cathode is significantly reduced upon relithiation. Less charge passes through Au‐coated LMO in comparison to bare LMO, suggesting that the reduced lattice parameter is associated with decreased Li+solubility in the Au‐coated LMO. Density functional theory calculations show that a more Li+‐deficient near‐surface is thermodynamically favorable in the presence of the Au coating, which may further stabilize these cathodes through suppressing formation of the Jahn–Teller distorted Li2Mn2O4phase at the surface. Electronic structure and chemical bonding analyses show enhanced hybridization between Au and LMO for delithiated surfaces leading to partial oxidation of Au upon delithiation. This study suggests that, in addition to transition metal dissolution from electrode surfaces, protective coating design must also balance potential energy effects induced by charge transfer at the electrode‐coating interface.
-
more » « less
Abstract Ni‐rich LiNi0.8Co0.1Mn0.1O2(NCM811) has been considered as a promising cathode material for high energy density lithium‐ion batteries. However, it experiences undesirable interfacial side‐reactions with the electrolyte, which lead to a rapid capacity decay. In this work, a homogeneous precipitation method is proposed for forming a uniform silicon dioxide (SiO2) coating on the NCM811 surface. The strong Si−O network provided a stable protective layer between the NCM811 active material and electrolyte to improve the electrochemical stability. As a result, the NCM811@SiO2cathode showed superior cycling stability (84.9 % after 100 cycles at 0.2 C) and rate capability (142.7 mA h g−1at 5 C) compared to the pristine NCM811 cathode (56.6 % after 100 cycles, 127.9 mA h g−1at 5 C). Moreover, the SiO2coating effectively suppressed voltage decay and pulverization of the NCM811 particles during long term cycling. This uniform coating technique offers a viable approach for stabilizing Ni‐rich cathode materials for high‐energy density lithium‐ion batteries.
-
more » « less
Abstract With the rapid growth of the lithium‐ion battery (LIBs) market, recycling and re‐use of end‐of‐life LIBs to reclaim lithium (Li) and transition metal (TM) resources (e.g., Co, Ni), as well as eliminating pollution from disposal of waste batteries, has become an urgent task. Here, for the first time the ambient‐pressure relithiation of degraded LiNi0.5Co0.2Mn0.3O2(NCM523) cathodes via eutectic Li+molten‐salt solutions is successfully demonstrated. Combining such a low‐temperature relithiation process with a well‐designed thermal annealing step, NCM523 cathode particles with significant Li loss (≈40%) and capacity degradation (≈50%) can be successfully regenerated to achieve their original composition and crystal structures, leading to effective recovery of their capacity, cycling stability, and rate capability to the levels of the pristine materials. Advanced characterization tools including atomic resolution electron microscopy imaging and electron energy loss spectroscopy are combined to demonstrate that NCM523's original layered crystal structure is recovered. For the first time, it is shown that layer‐to‐rock salt phase change on the surfaces and subsurfaces of the cathode materials can be reversed if lithium can be incorporated back to the material. The result suggests the great promise of using eutectic Li+molten–salt solutions for ambient‐pressure relithiation to recycle and remanufacture degraded LIB cathode materials.
