Cation mixing in Li-based layered positive electrode materials has been reported to negatively affect the electrochemical performance and transport properties of intercalated Li. However, no previous reports have systematically correlated the impact of cation mixing (Ni atoms in the Li layer) on the electrochemical properties and Li transport. Herein, a series of Li-deficient LNO (Li1−xNi1+xO2) materials with different amounts of Ni in the Li layers ranging from ca. 1.5%–6.0% was intentionally prepared by varying the Li/Ni ratio during synthesis. An order of magnitude decrease in the Li chemical diffusion coefficient was found between samples with 1.5% and 6% Ni in the Li layer. A similar dependence of the diffusion constant on the amount of Ni in the Li layer was also observed in the Li-excess materials Li 1 + x Ni 0.5 Mn 0.5 1 − x O 2 for x = 0, 0.04, 0.08, 0.12, suggesting that, in general, larger amounts of Ni in the Li layer will lead to worse kinetics. This work quantitatively demonstrates that the amount of Ni in the Li layer needs to be carefully considered for the development of high-energy Ni-containing layered positive electrode materials as it directly affects overall electrochemical performance, phase transitions, and Li diffusion, leading to worse kinetics and seriously hindering rate capability.

With research continuing to push for higher Ni content in positive electrode materials, issues such as the 1st cycle irreversible capacity and kinetic hindrances related to Li diffusion become more significant. This work highlights the impact of various material parameters on electrochemical performances, specifically the kinetic hindrances to Li diffusion in the low voltage region. Increasing the amount of substituents, increasing the secondary particle size and increasing the primary particle size were all variables found to decrease capacity in the ∼3.4–3.6 V region at modest discharge rates and increase the 1st cycle IRC. The capacity in the ∼3.4–3.6 V region can be recovered when cycling at a higher temperature at similar discharge rates or when cycling to a low cut-off voltage of 2 V. Since these processes are related to the diffusion of Li in the positive electrode, analysis of the Li chemical diffusion coefficient, D c , is presented using a reinvented approach we call the “Atlung Method for Intercalant Diffusion.” The measured D c for the single crystalline LiNi0.975Mg0.025O2 materials were found to be about 2 orders of magnitude smaller compared to the polycrystalline materials if the secondary particle size was used in the calculation of D c for the polycrystalline samples. If the primary particle size of the polycrystalline materials was used, then D c was similar to the single crystal materials. These results demonstrate that lattice diffusion is much slower compared to grain boundary diffusion offering insight for optimizing material morphology for better rate performance.

Melt synthesis is a fast and simple process to make dense LiMnyFe1-yPO4 (LMFP with 0 ≤ y ≤ 1) from all-dry, low-cost precursors with zero waste. This study characterizes melt LMFP materials with 0-100% Mn after particle size reduction by planetary milling and carbon coating with glucose. The melt LMFP samples show higher electrical conductivity at similar pellet density than LFP (0% Mn) and LMFP (79% Mn) reference samples made by traditional methods. The melt LMFP samples exhibit higher crystallinity than the reference samples and show no crystalline impurities. Their unit cell volume and crystallographic density scale with Mn content; the percentage of Fe and/or Mn in Li positions is below 1.5%, which is comparable to reference samples. Crystallite sizes of at least 100 to 175 nm are observed for melt LMFP, which is larger than the fine ~50 nm crystallites of reference LMFP. Melt LFP shows specific discharge capacity and cycling stability comparable to reference LFP, but the melt LMFP samples with 25-100% Mn shows worse performance than reference LMFP (79% Mn). Part two of this study will quantify the solid-state lithium diffusion coefficient in melt LMFP materials and correlate it to their electrochemical performance.

Melt synthesis is a fast and simple process to make dense LiMnyFe1-yPO4 (LMFP with 0 ≤ y ≤ 1) from all-dry, low-cost precursors with zero waste. Part one of this study confirmed that highly crystalline and phase pure LMFP materials can be made by melt synthesis. This part shows that planetary milling can reduce the primary particle size of melt LMFP (0-75% Mn) to ~200 nm, which is smaller than the primary particles in commercial LFP reference material (0% Mn). However, further particle size reduction is needed to reach particle sizes below 70 nm observed in reference LMFP (79% Mn). Melt LFP shows almost identical specific capacity and charge/discharge voltage as reference LFP. Melt LMFP materials show a high voltage Mn plateau at ~4 V associated with the Mn2+/3+ redox, the length of which scales with Mn content. The Mn plateau raises the average discharge voltage of LMFP; hence a minimum specific discharge capacity between 160 mAh/g (0% Mn) and 145 mAh/g (80% Mn) is sufficient to match the volumetric energy density of LFP. The Atlung Method for Intercalant Diffusion provides the lithium diffusion coefficient in LFP and LMFP made by the melt synthesis method.