Single-Crystal Li1+x[Ni0.6Mn0.4]1-xO2 Made By All-Dry Synthesis

Abstract

Single-crystal LiMO2 (M = 3d transition metal such as Ni, Mn, Al, and Co) has received much attention as a positive electrode material over the last few years due to its superior cycling stability over conventional polycrystalline materials1. Moreover, Co has fallen out-of-favour as a transition metal of choice due to its relatively high cost2 and the human rights abuses associated with its mining3. As well, the synthesis of conventional LiMO2 materials requires complex co-precipitation equipment that not only increases the cost of manufacture, but also produces waste such as Na2SO4. Furthermore, coating W on the particles’ surface in a separate step after co-precipitation improves the electrochemical performance of these materials4. Therefore, Co-free, W coated, single-crystal LiMO2 materials, made in a simple synthesis process like LiNi0.6Mn0.4O2 (NM64) are of most interest.In this presentation, a simple, solvent- and waste-free synthesis method is shown to create NM64 materials with and without a W coating. This all-dry synthesis uses a mixture of metallic Ni, MnO2, LiOH·H2O, and an optional W precursor, along with two to three heating steps, and an agglomeration separation step to produce single-crystal NM64. The resulting material is R-3m phase pure with ≤4% Ni in the Li layer and contains only trace residual lithium. Additionally, the NM64’s grain size is between 2 to 5 µm, as shown in Fig. 1a), which can be tuned by the addition of W during the initial synthesis rather than with a separate coating step. While W inhibits grain growth during synthesis4, the Ni, Mn, and Li interdiffusion is largely unaffected according to the unit cell parameters and the Ni in the Li layer obtained from Rietveld refinement of their XRD patterns. Fig. 1b) illustrates that while NM64 materials without W retain 91% of their original capacity after 100 cycles at C/5, matching the vendor material, when W is added they outperform it with 93% retained capacity. It is believed that this incredibly simple process could be adopted relatively easily into current commercial positive electrode manufacturing facilities to reduce the complexity, cost, and time of manufacture. Figure 1. (a) shows SEM micrographs of the all-dry synthesized NM64 material without and with W. (b) shows half coin cell cycling of NM64 materials with and without W as compared to a vendor material. References J. Li et al., J Electrochem Soc, 164, A1534–A1544 (2017). Mining.com https://www.mining.com/markets/.J. P. Otamonga and J. W. Poté, J Geochem Explor, 208, 106394 (2020).D. Rathore et al., ACS Energy Lett, 7, 2189–2195 (2022). Figure 1